Recent Progress in Materials Exploration for Thermocatalytic, Photocatalytic, and Integrated Photothermocatalytic CO2‐to‐Fuel Conversion

The excess depletion of carbon‐rich fossil fuels and agroforest biomass resources has aggravated the energy crisis and environmental pollution, causing increased CO2 emissions. Accordingly, the goal of “peak CO2 emissions and carbon neutrality” is proposed to alleviate global warming. CO2‐to‐fuel conversion is considered as a preferable move for reducing the atmospheric CO2 concentration and further upgrading to chemical feedstocks. However, the highly efficient CO2 conversion remains challenging due to the thermodynamic limits and kinetic barriers, which require high energy input through conventional thermocatalysis. Inspired by “artificial photosynthesis,” photocatalytic CO2 transformation has received tremendous attention and makes remarkable progress over the past decades, although it is still far from practical application. Recently, the integrated photothermocatalysis has emerged as an intelligent strategy to utilize solar energy to induce local heating and energetic hot carriers, which synergistically promote CO2‐to‐fuel conversion. The key to the success of CO2 upgradation is catalysts’ development with improved activity, selectivity, and stability. This review highlights the recent advancements in materials designing for practical CO2 conversion through thermocatalysis, photocatalysis, and photothermocatalysis during the past five years, emphasizing the reaction pathways and mechanism on the CO bond activation and intermediates formation. Finally, the current challenges and future opportunities are described.

The excess depletion of carbon-rich fossil fuels and agroforest biomass resources has aggravated the energy crisis and environmental pollution, causing increased CO 2 emissions. Accordingly, the goal of "peak CO 2 emissions and carbon neutrality" is proposed to alleviate global warming. CO 2 -to-fuel conversion is considered as a preferable move for reducing the atmospheric CO 2 concentration and further upgrading to chemical feedstocks. However, the highly efficient CO 2 conversion remains challenging due to the thermodynamic limits and kinetic barriers, which require high energy input through conventional thermocatalysis. Inspired by "artificial photosynthesis," photocatalytic CO 2 transformation has received tremendous attention and makes remarkable progress over the past decades, although it is still far from practical application. Recently, the integrated photothermocatalysis has emerged as an intelligent strategy to utilize solar energy to induce local heating and energetic hot carriers, which synergistically promote CO 2 -to-fuel conversion. The key to the success of CO 2 upgradation is catalysts' development with improved activity, selectivity, and stability. This review highlights the recent advancements in materials designing for practical CO 2 conversion through thermocatalysis, photocatalysis, and photothermocatalysis during the past five years, emphasizing the reaction pathways and mechanism on the C═O bond activation and intermediates formation. Finally, the current challenges and future opportunities are described.
an economical manner to convert CO 2 into high-value-added products, which can be manufactured into chemicals and fuels on an industrial scale, thus closing the carbon loop. However, the CO 2 molecule is kinetically inert and thermodynamically stable, which requires effective catalytic techniques to decrease reaction barriers in C═O activation and C─O cleavage. Thus, the CO 2 conversion requires substantial energy inputs, including heat and sunlight, which could achieve sufficient reaction rates for highly efficient and selective CO 2 transformation through thermocatalytic and photocatalytic approaches. [2a,b] Moreover, photothermocatalysis as the combination of two energy sources in one reaction process has been regarded as an advanced approach, which further enhances the catalytic performance of CO 2 -to-fuel conversion. [3] The chronology of the technologies for CO 2 conversion is presented in Figure 1, highlighting the milestones of the developments of catalysts, products, and techniques for their characterization.
Among these technologies, thermocatalytic transformations of CO 2 either via direct or indirect approaches have already been applied at the industrial level and have relatively higher product outputs. Four main classes of products can be obtained from CO 2 thermocatalysis, that is, carbon monoxide (CO), methanol (MeOH), methane (CH 4 ), and other higher hydrocarbons. [4] Cheap and clean hydrogen (H 2 ) from an economical and sustainable route is critical to have a positive environmental impact on the aforementioned CO 2 thermocatalytic processes. First, CO generated from reverse waterÀgas shift (RWGS, CO 2 þ H 2 ! CO þ H 2 O) has been considered as a valuable catalytic process for CO 2 hydrogenation as CO is an important feedstock for FischerÀTropsch (FT) and MeOH synthesis (CO 2 þ 3H 2 ! CH 3 OH þ H 2 O). Second, a large proportion of *CO ('*' represents "the surface adsorption site on catalyst") absorbed on the catalyst surface could suppress the competing RWGS route, thus generating a higher-value-added productmethanol. Third, the methanation of CO 2 , also called the Sabatier reaction (CO 2 þ 4H 2 ! CH 4 þ 2H 2 O), undergoes an eight-electron process for the formation of syngas and compressed natural gas. Finally, the direct hydrogenation of CO 2 into other hydrocarbons, including light alkanes and olefins, is similar to that of the FT process through the methanol-mediated or nonmethanol-mediated pathways.
However, CO 2 utilization through thermocatalytic processes requires high energy costs accompanied by additional CO 2 emission, as the heat was mainly obtained from the combustion of fossil fuels. By comparison, photocatalysis driven by renewable solar energy is an attractive approach and serves as an effective alternative for CO 2 transformation, which has already been at a stage of rapid development. [5] Under solar light irradiation, semiconductor photocatalysts could accelerate the C═O activation process, which could be subsequently reduced to hydrocarbons by photogenerated electrons. Unfortunately, the practical application of photocatalysis has yet reached an industrial scale due to the lack of highly effective photocatalysts, limited photocatalytic performance, suboptimal reactors, and the intermittent nature of solar light. Therefore, the integration of light and heat in CO 2 chemical transformations emerges as a promising approach to further reduce the consumption of fossil fuels and increase the conversion efficiency of atmospheric CO 2 . Photothermocatalysis absorbs the visible and near-infrared light  energy, which undergoes the dissipation into heat (temperature up to 300À500 C or even higher), thus inducing a process similar to conventional thermocatalysis. [6a,b] The photothermo effect can be ensured by the construction of efficient light-absorbing materials to capture heat from sunlight and raise the temperature of the whole reaction system. Another way is to synthesize plasmonic metals to cause surface plasmon resonance (SPR) effects, which could generate numerous hot electrons and holes. There are two categories of mechanisms and applications focused on plasmonic nanostructures: 1) photoinduced hot carriers (plasmonic photocatalysis) and 2) light-excited phonons (photothermal catalysis). [7] Accordingly, the plasmonic photothermocatalytic behavior provides a preferable understanding of electron-driven and temperature-driven catalysis over plasmonic materials, which is conducive to bridge the gap between the application of thermocatalysis and photocatalysis. Thereby, RWGS, methanation, methanol, and other hydrocarbons synthesis routes can be realized in a highly active and selective manner. Meanwhile, Ozin et al. proposed universal equations, which could describe net CO 2 emissions induced from (photo)thermocatalysis RWGS and methanation synthesis in batch and flow reactors. With archetype model systems, some factors of the final amount of effluent CO 2 can be concluded, which provided a promising guide for the rapid development of (photo)thermocatalytic CO 2 conversion using sunlight as the clean and sustainable energy source. [8] In Figure 2, we summarize the major properties of the technologies for thermocatalytic, photocatalytic, and photothermocatalytic CO 2 conversion, highlighting the differences in reaction routes, products, advantages, and disadvantages of each technology. Nowadays, the published reviews on CO 2 reduction mainly concentrated on a certain technology (e.g., thermocatalysis, photocatalysis), while a few have emphasized the integration of thermo-and solar energy in catalysis. Considering the rapid and fascinating scientific research that have been sprung up in these important and hot topics, we herein present a comprehensive and timely review of the recent advances in thermo-, photo-, and the integrated photothermocatalytic CO 2 conversion to highvalue-added chemicals and fuels during the past five years, aimed at highlighting the differences and similarities between solarand thermo-driven catalysis in depth. The motivation and fundamentals of each catalytic technology are first introduced, followed by the emphatic discussions on their recent progress toward small molecules (e.g., formate, CO, CH 4 , and alcohols) and fine chemical (e.g., C 2þ olefins and oxygenates) synthesis. It mainly provides objective assessment on the merits and demerits of catalysts, in terms of catalytic activity, selectivity, and stability. Then, distinct catalytic mechanisms for these technologies are summarized to provide a comprehensive understanding of C═O activation and intermediate adsorption, which has been realized by the advanced studies of in situ/operando characterizations and theoretical calculations. Finally, the current challenges and future perspectives for the efficient utilization and upgradation of CO 2 into value-added products are addressed in these flourishing fields. Thus, this review provides horizontal comparisons by discussing different catalysts or reaction pathways for the same product. In parallel, this review provides vertical comparisons of the recent advances in catalytic technologies for CO 2 conversion. As a result, this review is expected to be an essential work to provide important references related to the chemical transformation of CO 2 into chemical feedstocks with explicit directivity.
C═O activation in the chemical conversion of CO 2 -to-chemicals/ fuels. Thermocatalysis driven by heat energy performs high efficiency and applicability for the mass-produced synthesis of fine chemicals (hydrocarbons, olefins, and NH 3 , etc.) in conventional industrial manufacture. Accordingly, thermocatalytic CO 2 conversion has been an ideal candidate to generate high-valueadded carbonaceous compounds, which mainly involve hydrogenation reactions with a CO 2 flow over catalysts at controllable temperatures and pressures. In the CO 2 thermocatalysis process, CO 2 molecules first adsorb on the surface of catalysts, and then essential activation energy (E a ) is required to bend C═O chemical bonds. Subsequently, another activation energy barrier should be overcome for the formation process of intermediates, where H 2 is directly used as the reductive reagent and *H donor. Finally, the interactions between intermediates and the catalyst surface influence the desorption ability of product molecules, which would exhibit different selectivities for C 1 chemicals or more valuable C 2þ chemicals by regulating C─H bonding or CÀC coupling. As shown in Figure 3a, the reaction process of CO 2 hydrogenation should overcome excessive energy barriers without the presence of catalysts (solid line), which need enough thermal energy to match to overcome the activation energy E a1 of the elemental step. [9] Thus, the development and employment of highly efficient catalysts is the key to accelerate the CO 2 conversion rate by lowering the E a2 and E a3 values (dashed line). In this regard, four main reaction pathways on thermocatalytic CO 2 conversion have sprung up in the recent 5 years, especially in the field of heterogeneous catalysis.

Plausible Routes for CO 2 Hydrogenation
Hydrogen is a high-energy carrier, which can be used to hydrogenate CO 2 into the products of CO, MeOH, CH 4, and other hydrocarbons (Figure 3b). The related reaction pathways are described as follows.
2.2.1. CO 2 to CO: Redox versus Associative Pathway CO 2 -to-CO conversion via RWGS reaction has been regarded as a promising route for CO 2 hydrogenation, which is an endothermic reaction driven by certain temperatures and pressures. [10] CO 2 þ H 2 ↔ CO þ H 2 O, ΔH 298K ¼ 41:2 kJ mol À1 (1) The reaction mechanisms have been generally classified into redox and associative mechanisms. 1) In a redox reaction, H 2 acts as a reductive reagent instead of a participant for the generation of intermediates. For example, Cu species supported on transition metal oxides/carbides (TMCs) has always been the popular alternatives for highly active and selective RWGS reactions. Cu 0 is first oxidized by CO 2 molecules to form Cu þ and CO, and then Cu þ can be reduced by H 2 to generate H 2 O. The redox mechanism can be modeled as follows.
Inspired by the abovementioned redox process, Konsolakis et al. employed Cu/CeO 2 as the catalyst and conducted thermodynamic analysis, realizing high CO 2 conversion (52%) and CO selectivity (95%) at 380 C and ambient pressure. [11] Other Cu-based materials, such as Cu/ZnO [12] and Ru-decorated Cu/ZnO/Al 2 O 3 [13] catalysts, were also ideal alternatives for the activation and dissociation of CO 2 in the RWGS reaction. Compared with traditional metal oxide-supported Cu-based catalysts, Ma and co-workers synthesized highly dispersed Cu over β-Mo 2 C (1.3 wt% Cu/β-Mo 2 C) catalyst, which performed superior CO 2 conversion rate (47.7 Â 10 À5 mol CO2 g cat À1 s À1 ) and extraordinary CO selectivity (99.2%) during a 40 h test ( Figure 4). [14] The optimized activity and stability are attributed to the synergistic effect between Cu and β-Mo 2 C, which contributed to the high dispersion of active Cu sites. During a redox reaction, the CO products were generated from CO 2 dissociation, while the oxygen atoms extracted from the CO 2 molecules were finally reduced by H 2 to produce H 2 O. Consistent with the experimental results, the proposed reaction mechanism obtained via transient analysis [15] suggests that the CO 2 adsorption/activation Figure 3. a) Energy diagram of the thermocatalytic reaction process. E a1 , E a2 , and E a3 indicate the activation energy for reactants and intermediates during the different reaction progresses of CO 2 hydrogenation. The solid and dashed lines illustrate the progress of the chemical reaction without and with the presence of a catalyst, respectively. Reproduced with permission. [9] Copyright 2019, Wiley-VCH GmbH. b) Four main products from thermocatalytic CO 2 hydrogenation reactions.
on the Cu sites is a rate-determining step, while the subsequent reduction of the adsorbed oxygen atoms of *CO 2 or hydroxyls of *COOH finally produces CO or H 2 O molecules. Therefore, the introduction of metallic Cu is a promising way to lower the E a of the rate-limiting step, which is conducive to the redox process. [16] The catalytic performance of the RWGS reaction via the redox mechanism can be further improved by some other strategies, such as alloying and constructing high-surface-area or nanostructuring catalysts. Reina's group reported that saponite-supported NiCu catalysts exhibited preferable selectivity for the formation of CO (%80%) when compared with the saponite-supported NiCo and Ni catalysts. [17] As evidenced by X-ray photoelectron spectroscopy (XPS) results, an electronically rich NiÀCu interface contributed to the effective CO 2 activation process. Furthermore, inverse nanoporous Cr 2 O 3 /Cu, [18] nanotube-structured TiO 2 / Cu, [19] and double-shell nanofiber-structured CuSiO/CuO x [20] presented strong metalÀsupport interactions and offered abundant surface active sites for CO 2 activation and H 2 dissociation in CO 2 -to-CO conversion. 2) In an associative pathway, metal oxides-supported metal catalysts are extensively employed as the active materials for CO 2 hydrogenation into formate intermediate and the subsequent C═O bond cleavage, thus selectively producing CO. The associative mechanism can be modeled as follows.
For example, Au supported on γ-Al 2 O 3 catalyst achieves high selectivity for CO formation (>99%). [21] The hydrogenation of CO 2 to formate intermediates on Au sites and subsequent C═O bond breakage to CO product on γ-Al 2 O 3 support were confirmed by diffusive reflectance infrared Fourier transform spectroscopy (DRIFT) measurement. Other metals (Pd or Ni) decorated on Al 2 O 3 support catalyst [22] also display high activity as the formate intermediates can easily adsorb on these metal sites determined by the preferable H 2 dissociation and H adsorption capabilities. However, Ag exhibits inferior activity in RWGS reaction when compared with other metals. Hence, Kim et al. adopted in situ surface analysis and density functional theory (DFT) calculations to investigate the role of Ag sites. The results demonstrated that the absorbed carbonate species on the Ag surface hinders H 2 dissociation and H adsorption for CO production and thus hardly reach the thermodynamic conversion limit.
2.2.2. CO 2 to MeOH: CAMERE versus Formate Pathway CO 2 to MeOH is an ideal application for large-scale MeOH production, which is extensively used as transportation fuel or as a building block for other high-value-added liquid fuels. It is an exothermic reaction that can proceed at a relatively low temperature and a high pressure with the assistance of effective catalysts.
Two different mechanisms were proposed in the published literature, [23a,b] as illustrated in Figure 5a. One pathway that undergoes the RWGS reaction with CO acts as the reaction intermediate (CAMERE pathway) and then hydrogenates to MeOH via the conventional syngas-to-MeOH process (CO þ 2H 2 ! CH 3 OH). A recent study over nanointermetallic InNi 3 C 0.5 first converted CO 2 into CO*, [24] which was then selectively hydrogenated into CH 3 OH below 300 C through CAMERE pathway, evidenced by DRIFT analysis (Figure 5b). Meanwhile, investigated by DFT analysis, the adsorbed CO 2 * easily converted into CO* with a low E a (0.32 eV) through an exothermal process (E reaction ¼ À0.38 eV), which is preferable for the formate or carboxylate pathway. In addition, DFT calculations over Pd/TiO 2 catalyst also demonstrated that the CAMERE pathway is the primary route for MeOH production, and CO*-to- Figure 5. a) Reaction scheme for CO 2 -to-MeOH conversion via CAMERE (left, RWGS þ COÀhydro) and formate (right) pathways. Reproduced with permission. [23] Copyright 2014, Elsevier Ltd. In situ DRIFT measurements of CO 2 hydrogenation reaction against temperature on b) InNi 3 C 0.5 via CAMERE pathway. Reproduced with permission. [24] Copyright 2019, Cell Press.
www.advancedsciencenews.com www.advenergysustres.com HCO* conversion is the rate-determining step. [25] The other pathway proposes that the first step of MeOH synthesis is the direct CO 2 hydrogenation through formate (HCOO*) or carboxylate (COOH*) intermediate, and the subsequent formate/ carboxylate hydrogenation is regarded as the rate-determining step (formate pathway). [26] In situ DRIFT measurement confirmed that CO 2 was first activated and converted into HCOO* intermediate at a relatively low temperature in the presence of Co 4 N nanosheets and then generated CH 2 O* species with a further increase of reaction temperature. [27] Atomic Pt embedded in MIL-101 shows high turnover frequency (TOF) (117 h À1 ) and optimal selectivity for MeOH production (90.3%) at 150 C and 32 bar during CO 2 hydrogenation. [28] The Pt single atom and coordinated O atom synergistically contributed to H 2 dissociation and OÀH formation. Afterward, the H atom in the OÀH group combined with CO 2 to generate HCOO* intermediate. For neighboring Pt monomers, [29] CO 2 was first hydrogenated into formic acid (HCOOH) via the transformation of COOH* intermediate. The successive steps involve the HCOOH hydrogenation into CH(OH) 2 *, CHOH*, and CH 2 OH* intermediates and finally MeOH. The above reaction pathways effectively lower the activation energy and enhance the catalytic activity, which result in the preferable selectivity for MeOH production. When referred to catalysts, Cu-based materials and metal oxide supports have been commonly applied in MeOH synthesis. In these catalysts, Cu serves as the main active component, whose performance can be enhanced by the incorporation of other elements (Zn, Al, Mg, Ca, Sr, Ba, Zr, Ti, F, etc.). The commercial Cu/ZnO/Al 2 O 3 catalyst presented high selectivity and activity toward MeOH production by CO 2 hydrogenation, with the intermediates of HCOO, H 2 COO, H 2 COOH, CH 2 O, and CH 3 O. [30] However, the severe reaction conditions (200À400 C or 20À100 bar) largely impede the economic feasibility of this technology. Delightedly, a low-temperature (120À170 C) CO 2 -to-CH 3 OH conversion with nearly 100% yield was realized using tertiary amine and alcohol through the condensed-phase hydrogenation of alkylcarbonate, accompanied by the generation of ammonium formate and alkylformate intermediates. [31] The incorporation of alkaline metal oxides (e.g., MgO, CaO, SrO, and BaO) into Cu/Al 2 O 3 catalysts contributes to a higher Cu þ /Cu 0 ratio and surface area, which show a higher MeOH production rate and suppress the competing RWGS reaction. [32] Evidenced by DFT, DRIFT, and kinetic Monte Carlo (KMC) simulations, the reduced Zr 3þ sites at the interface of Cu/ZrO2 catalysts preferably absorb the *CO 2 , *CO, *HCO, and *H 2 CO intermediates, which moderately promote methanol synthesis on Cu. [33] In this work, TiO 2 can also enhance MeOH production on Cu via the RWGS pathway with carboxyl intermediates. Meanwhile, the formation rate of *H 3 CO is independent of the formation rate of *HCOO, suggesting that the CO 2 -to-MeOH conversion does not follow the formate pathway for both catalysts. As the synergetic effect of Cu and ZnO is non-negligible in CO 2 hydrogenation, the ternary Cu/ZnO/ZrO 2 catalyst system is a potential candidate for this reaction and can be further enhanced by oxidative F 2 -fluorination procedures. [34] With a mild difluorine treatment, the ZnO x defect sites and the E a for the RWGS reaction increase, while the E a for MeOH production from CO 2 hydrogenation is nearly unchanged, thus achieving optimized MeOH yield and selectivity. When compared with traditional Cu catalysts, many researchers have also engineered the d-band centers and the active center densities of Pd/Pt/Rh-based catalysts by incorporating non-noble metals to form an alloy and thus tuning their structures. Accordingly, an optimal Pd@Zn coreÀshell-structured catalyst was synthesized to further increase the kinetic barrier for CO formation, thus facilitating MeOH formation with the selectivity of 70% at relatively low pressure and temperature (2 MPa and 270 C). [35] Similarly, Díez-Ramírez et al. reported a PdÀCuÀZn/SiC composite catalyst, and PdZn served as the active site for highly efficient MeOH production from CO 2 hydrogenation (Figure 6a,b). [36] Zeng's group fabricated Pt 3 Co octopods [37] and Rh 75 W 25 nanosheets [38] to catalyze CO 2 hydrogenation with enhanced MeOH yields (Figure 6c-f ). The alloying and specific morphology synergistically bring about the electronic effects of heterogeneous catalysts, which contribute to higher negative charge density and abundant active sites for enhanced CO 2 activation and intermediate adsorption.

CO 2 to CH 4 : CAMERE versus Formate Pathway
CO 2 to CH 4 (Sabatier reaction) is thermodynamically favorable, whereas the conversion undergoes an eight-electron process with high kinetic barriers. Therefore, the development of highly efficient catalysts is demanded to achieve satisfactory rates and selectivities.
Many studies have sprung up for the Sabatier reaction based on the supported transition metal (e.g., Ni, Co, Fe, Ru, Pd, and Rh) catalytic systems. Meanwhile, metal oxides (e.g., SiO 2 , TiO 2 , Al 2 O 3 , ZrO 2 , CeO 2 ) and carbon-based materials are the most commonly used supports, which provide high surface areas and heat-resistant properties. Notably, CO 2 methanation involves two categories of mechanisms. 1) One involves CO 2 -to-CO conversion followed by CO methanation, which proceeds via a LangmuirÀHinshelwood mechanism. In detail, the dissociation of CO 2 into CO* first happens through an RWGS reaction. Subsequently, methane can be generated by direct hydrogenation of CO* intermediate or through a two-step pathway of dissociation of CO* to the C* intermediate and further hydrogenation to methane. 2) The other one involves the direct hydrogenation of CO 2 to CH 4 , and the formate species are suggested as the main reaction intermediates. [39] Based on the proposed mechanisms, commercialized Ni-based materials were widely studied for Sabatier reaction at relatively high temperatures to overcome kinetic limitations. Mutschler et al. reported that Al 2 O 3 -supported Ni catalyst showed the maximum CH 4 yield of 78% and selectivity of 98% at 623 K, accompanied with a small amount of gaseous CO (the highest CO yield of 18.2%). [40] To further increase the CO 2 conversion, Vita et al. synthesized Ni catalyst on gadolinium-doped ceria (Ni/GDC), which increased the amount of catalytic layer and thus achieved the highest CH 4 productivity (10.7 L CH4 g À1 h À1 ) at 400 C during a 200 h continuous test. [41] However, Ni-based catalysts show inactivation at a relatively high temperature as the interaction of Ni sites with CO intermediates could generate Ni carbonyl (Ni(CO) 4 ). Therefore, the key is to develop efficient Ni-based catalysts and optimize reaction conditions, which ensure effective activation of the reaction system below 300 C and finally achieve a high CH 4 yield. For example, the automethanation phenomenon that happened on Ni/metal oxide (CeO 2 , ZrO 2 , Y 2 O 3 , or Al 2 O 3 ) at room temperature and atmospheric pressure was reported to enhance the CO 2 -to-CH 4 performance by purging a raw material gas containing oxygen into the reactor. [42] The enhancement is mainly ascribed to the high thermal energy generated from H─O bond combustion under cofeeding with oxygen. Moreover, bulk LaNi 5 alloy is well known for its hydrogen storage capability, which was also selected to realize improved activity for the Sabatier reaction. [43] LaNi 5 supported on γ-Al 2 O 3 showed a high CO 2 conversion (75%) and a superior CH 4 selectivity (98%) at 1 atm and 300 C (Figure 7a). From the DRIFT analysis (Figure 7b), the reaction went through an RWGS mechanism, which is described in detail as follows: H 2 was first dissociated into *H on the surface of Ni sites and subsequently, CO 2 was activated and dissociated on Ni sites. Then, the generated *CO rapidly adsorbed on LaO x decorations, and part of *CO was successively decomposed into *C and *O. Finally, the hydrogenation of *CO or *C was conducted to generate CH 4 . When compared to Ni, some noble metals (e.g., Ru, Pd, Rh.) are ideal alternatives for CO 2 methanation at low temperatures (≤300 C). For instance, to reach the same CO 2 conversion (85%), the commercial Ru/Al 2 O 3 catalyst could proceed with the methanation reaction at 200 C, while Ni-based catalyst activates the reaction above 300 C. [44] The reaction is suggested to be dominated by Figure 6. a) CO 2 conversion and b) CH 3 OH selectivity of PdÀCuÀZn/SiC composite during CO 2 -to-MeOH conversion with only CO 2 and H 2 in the feed. Reproduced with permission. [36] Copyright 2018, Elsevier B.V. c) scanning transmission electron microscopy (STEM) image of a Pt 3 Co octopod. d) Time course of the CO 2 hydrogenation into MeOH catalyzed by Pt 3 Co octopods at 150 C with CO 2 (8 bar) and H 2 (24 bar). Reproduced with permission. [37] Copyright 2016, Wiley-VCH GmbH. e) STEM image of Rh 75 W 25 nanosheets. f ) Time course of CO 2 hydrogenation into MeOH catalyzed by Rh 75 W 25 nanosheets at 150 C under 32 bar of H 2 /CO 2 mixed gas (H 2 /CO 2 ¼ 3:1). Reproduced with permission. [38] Copyright 2017, American Chemical Society. the thermodynamic equilibrium, and thus the rapid reaction rate could be realized on Ru sites at a relatively low temperature without the requirement for an intermediate condensation step. Meanwhile, the metal loading of Ni is two orders of magnitude higher than that of Ru, indicating that Ru/Al 2 O 3 is the preferable catalyst for small-scale CO 2 methanation application. Sun et al. demonstrated that 5 wt% Ru/CeO 2 ÀMgO exhibited an efficient carbon capture and utilization (CCU) performance with a stable CO 2 conversion (79%) and CH 4 yield (3.36 mmol g À1 ) after 10-cycle CCU processes (Figure 7c,d). [45] Meanwhile, the introduction of oxygen vacancies into the Ru-oxide support interface is beneficial to improve the stability of the whole reaction system. The in situ DRIFT (Figure 7e) was further employed to explore the mechanism of the CO 2 methanation process under the synergistic effects of active Ru sites and oxygen vacancies. The proposed mechanism includes the following steps. 1) Adsorbed CO 2 molecules are activated by oxygen vacancies while CeO 2 support is oxidized, and H species are chemisorbed onto the active Ru sites. This step generates bidentate formate intermediate at the interface of metalÀsupport, which was then dissociated into metalÀcarbonyl (RuÀCO) and OÀH groups. 2) RuÀCO was hydrogenated by chemically adsorbed H species, which was finally converted into the CH 4 product and determined as the rate-determining step. In addition, strategies such as tailoring nanoparticle sizes, [46] optimizing structure and compositions, [47] altering reaction conditions (feed gas compositions, space velocity, temperature, and pressure), [48] and constructing intermetallics in alloys [49] have been adopted to develop efficient catalysts. By tailoring the suitable particle size of the Ru nanoparticles, the adsorption capacity of H species and the following Figure 7. a) CO 2 conversion and CH 4 selectivity of LaÀNi/γ-Al 2 O 3 in the Sabatier reaction. The inset in (a) shows the TEM image of the LaÀNi/γ-Al 2 O 3 catalyst reduced in H 2 at 700 C. b) The evolution of surface species as a function of time was analyzed by in situ DRIFT spectra of the LaÀNi/γ-Al 2 O 3 catalyst after H 2 injection over preadsorbed CO 2 at 400 C and 1 bar. Reproduced with permission under the terms of the CC-BY license. [43] Copyright 2016, The Authors. Published by MDPI. c) TEM image and d) CH 4 yield after 10-cycle CCU process of 5 wt% Ru/CeO 2 ÀMgO. e) In situ DRIFTS spectra during the CCU process of CeO 2 ÀMgO. Reproduced with permission. [45] Copyright 2020, Elsevier Ltd. formation of CH x O species by the consecutive hydrogenation of CO* intermediate were certainly enhanced. The above-mentioned steps could promote the cleavage of the C─O bond, which was regarded as the rate-determining step of the CO 2 methanation process. [46] Cored et al. [47] synthesized a coreÀshell-structured Ru 0 @RuC catalyst by a facile hydrothermal method, which showed superior activity for CO 2 -to-CH 4 conversion below 200 C with 100% selectivity (Figure 8a,b). Analyzed by isotopic 13 CO/ 12 CO 2 /H 2 experiments, the shell of RuC acted as the active site, which efficiently activates CO 2 and H 2 , and finally CH 4 generation occurs via a direct CO 2 hydrogenation mechanism. Although noble metals (e.g., Ru, Rh, and Pd) have exhibited remarkable catalytic performance in the Sabatier reaction, the high cost and low storage have hindered their large-scale applications. Therefore, alloying noble metals with cheap metals (e.g., Fe, Co, Ni, Mn, Cu) can reduce the system cost and moreover, bring about the electronic effect and geometric effect for the enhanced catalytic activity. Herein, Zeng's group demonstrated that face-centered tetragonal (fct)-PdFe intermetallic nanocrystals (NCs) presented high mass activity (5.3 mmol g À1 h À1 ) and stability for CO 2 methanation at 180 C under 1 bar, which was 1.6, 3.3, and 5.3 times as high as that of Ru, Ni, and Pd (Figure 8c,d). [49] Evidenced by DRIFT measurements and DFT calculations, the metallic Fe sites were demonstrated to play a vital role in fct-PdFe intermetallic NCs, which facilitated the direct conversion of CO 2 into CO* intermediates, thus resulting in the enhanced hydrogenation of *CO into CH 4 .

CO 2 to C 2þ Hydrocarbons: FT Synthesis versus Methanol-Mediated Pathway
CO 2 to hydrocarbons involves two different pathways, namely, the modified FT synthesis (FTS) and the methanol-mediated synthesis. The two pathways of thermocatalytic CO 2 hydrogenation can generate different products, which can be used as important industrial intermediates and high-value-added chemicals, etc. They are mainly alkanes (liquefied petroleum gas [C 3 ÀC 4 0 ] and gasoline [C 5 ÀC 11 ]), alkenes (lower olefins [C 2 ÀC 4 ═]), and aromatics.
1) The FTS pathway first generates CO from CO 2 hydrogenation via RWGS reaction and then obtains hydrocarbons through FTS (CO 2 þ H 2 ↔ CO þ H 2 O; n CO þ 2 m H 2 ↔ C n H 2m þ m H 2 O). The first RWGS step in modified FTS is an endothermic reaction, which requires high temperatures to activate CO 2 molecules and accelerate reaction rates. However, FTS is an exothermic reaction, which indicates that lower temperatures could lead to fast hydrogenation of CO to multicarbon products. [50] Meanwhile, FTS goes through multiple steps, which involve reactants adsorption, chain initiation, growth, and termination, as well as product desorption for CO hydrogenation into carbonaceous compounds. [51] Therefore, different catalysts and reaction conditions (temperatures, pressures, and CO 2 /H 2 ratios) could significantly influence the CO 2 conversion rate and product yield as well as selectivity. As the traditional RWGS catalysts, Fe-based materials have been applied in the modified FTS process owing to their low cost and flexibility. In general, modification of these Fe-based catalysts is a prerequisite to realize enhanced CO 2 Figure 8. a) High-resolution transmission electron microscopy (HRTEM) image and b) CO 2 conversion (left axis, black) and CH 4 STY (right axis, blue) of Ru 0 @RuC catalyst. Reproduced with permission. [44] Copyright 2019, American Chemical Society. c) high angle ring dark field (HAADF)ÀSTEM image, d) time versus CH 4 yield (left axis)/TON numbers (right axis) for fct-PdFe, face-centered cubic (fcc)-PdFe, Ru/C, Ni/C, and Pd/C. Reproduced with permission. [46] Copyright 2020, Wiley-VCH GmbH. hydrogenation performance. The modification strategies can be categorized into the following four groups: a) In situ structural transformation. Fe 2 O 3 was reported to prefer the formation of light olefins (C 2þ ) rather than paraffin and suppresses the selectivity toward CH 4 and CO production at 623 K under a total pressure of 15 bar in CO 2 hydrogenation. [52] Analyzed by the X-ray diffraction (XRD) pattern and Mössbauer spectrum, the characteristic peak of iron carbides (Fe 5 C 2 ) could be detected during CO 2 hydrogenation condition, which was obtained from the in situ transformation of Fe 2 O 3 . Importantly, Fe 5 C 2 plays a vital role in the high selectivity of light olefin production and suppresses the high yield of CH 4 as Fe 5 C 2 is an active phase to promote the FTS process and chain growth reaction. Accordingly, either the direct CO 2 methanation or CO-based FTS into CH 4 could be suppressed over the Fe 2 O 3 -based catalyst, and thus the selectivity of C 2 ÀC 4 hydrocarbons increases to a value of 37% with the olefin to paraffin ratio of 2.7 at CO 2 conversion of 40%. Meanwhile, the selectivity of C 5þ hydrocarbons reaches 36%, while the values of CH 4 and CO were only 12% and 15%. This work provides in-depth sight into the fabrication of Fe-based catalysts toward CO 2 hydrogenation in terms of facilitating short-chain olefin formation and suppressing undesired C 1 products. b) Alkali metals promotion. Alkali metals (e.g., Na and K) can act as promoters when introduced into Fe-based catalysts, which enhances the yield and selectivity for olefin production by accelerating CO dissociation and suppressing H 2 adsorption. In addition, the promoters significantly facilitate the formation of Fe carbides, which are well known as the active species for FTS reaction. Huang's group synthesized a series of Fe-based catalysts with different amounts of Na dopants for CO 2 hydrogenation. [53] When Na content increases, CO 2 conversion rate and alkenes selectivity are optimized, and the highest values can be up to 36.8% and 64.3%, respectively, with the Na content of 1% (Figure 9a). With the assistance of the comprehensive investigations on the physical and chemical properties of xNa/ Fe (x indicates the weight percent of Na in wt%) catalysts, the promotion effects of Na dopant were determined to be the following key points. First, the interaction between catalysts and CO 2 molecules can be further enhanced in the presence of Na, which makes easier CO 2 activation. Second, Na facilitates the carbonization of Fe 3 O 4 species in the presence of CO that is in situ generated from CO 2 hydrogenation, thus improving the formation and stability of Fe 5 C 2 active species. The increasing amount of Fe 5 C 2 species favors subsequent CÀC coupling and carbon chain growth for heavier olefins (C 2 ÀC 4 olefins and C 5þ hydrocarbons) production. Third, Na as a promoter could increase the CO dissociation and adsorption rate, considering that CO is an important reaction intermediate in the catalytic process of CO 2 hydrogenation into alkenes over Fe-based catalysts, which was also reported in a previous study. [54] Meanwhile, the Na promoter could inhibit the secondary hydrogenation of alkenes into alkanes as it can break the symmetry of Figure 9. a) Catalytic performance of xNa/Fe (x indicates the weight percent of Na in wt%) catalysts toward CO 2 -to-hydrocarbons conversion. b) The possible pathway of Na/Fe catalyst for CO 2 hydrogenation to olefins reaction. Reproduced with permission. [53] Copyright 2018, American Chemical Society. c) Synthetic scheme of Fe/C þ K catalyst. d) CO 2 conversion and product distribution of promoted and unpromoted Fe catalysts in CO 2to-hydrocarbons. Reproduced with permission. [55] Copyright 2018, American Chemical Society. olefins and thus accelerate the desorption of olefins from the surface of the catalysts. Therefore, 1Na/Fe catalysts exhibited preferable heavy alkenes generation, while the selectivity toward methane production decreased to 7.2%. The possible reaction pathway is described as follows (Figure 9b): following a modified FTS process, CO 2 and H 2 molecules are first adsorbed and activated on the surface of the 1Na/Fe catalyst, and subsequently, the CO* species and H 2 O are generated via RWGS reaction. Then, the CO* species are dissociated and further hydrogenated into CH x species and finally transformed into alkenes through a CÀC coupling reaction. Potassium is also a popular promoter for Fe catalysts in CO 2 hydrogenation, which facilitates CO 2 capture via a well-known "potassium carbonate mechanism." Moreover, K as an electron donor can accelerate the electron transfer from Fe-based catalysts to *CO intermediate, thus leading to higher selectivity toward olefins production via the FTS process. An active and stable Fe-based catalyst derived from the metalÀorganic framework (MOF) was reported by Ramirez et al. Potassium was found to be a great promoter, improving the spaceÀtime yields (STYs) toward C 2 ÀC 4 olefin (33.6 mmol g cat À1 h À1 ), which was significantly higher than other dopants (e.g., Cu, Mo, Li, Na, K, Mg, Ca, Zn, Ni, Co, Mn, Fe, Pt, and Rh) (Figure 9c,d). [55] The possible reasons for the optimized promotion effects of the K lie in the fact that element K maintains the stability of Fe 3 O 4 and Fe 5 C 2 , which act as essential active species in CO 2 hydrogenation. Followed by the proposed reaction pathway, CO 2 was first adsorbed on Fe 3 O 4 active sites and converted into CO via the RWGS reaction and then underwent the hydrogenation process of CO via FTS on Fe 5 C 2 sites to produce light olefins (C 2 ÀC 4 ). In addition, the K promoter can enhance the adsorption capability of CO 2 and CO molecules while weakening H 2 affinity, which causes a higher CO coverage and a lower H 2 coverage on Fe-based reaction centers for a higher selectivity to olefins. Olefin selectivity on Fe 5 C 2 sites is closely associated with the secondary hydrogenation of olefins, which results in further chain growth and an overhydrogenation reaction. Therefore, during the FTS process, the K promoter strengthens the Fe─C bonds rather than the Fe─H bonds, contributing to a higher CO coverage on the surface of Fe-based active species. The increase of the CO/H 2 ratio protects the catalyst surface from olefin readsorption and suppresses the formation of undesirable paraffin. Recently published studies demonstrate similar promoting effects of K element doping into FeÀAlÀO spinel catalyst toward CO 2 hydrogenation reaction. [56a,b] To conclude, the K promoter imposes critical impacts on the catalyst phases (i.e., Fe oxides or carbides), chemical compositions, electronic states, and stability of the resulting Fe-based catalysts. Other K phases have also been introduced into Fe-based catalysts to activate CO 2 and subsequently convert it into olefins through a tandem mechanism (RWGS þ FTS pathways). For example, Fe 2 O 3 @KO 2 and Fe 2 O 3 @K 2 CO 3 reported by Gascon's group yield similar productivities of olefins when compared with the commercial FTS catalysts. [57a,b] c) Other metals promotion. Some non-noble metals, such as Cu, Zn, Mn, Co, and Ti, have also been introduced into Fe-based catalysts for CO 2 hydrogenation. Cu promoter acted as a structural modifier that can increase Fe dispersion in the catalyst. Meanwhile, the incorporation of Cu into the Fe bulk lattice brought about optimized electronic effects, which facilitated CO 2 activation and hydrogenation. Moreover, the olefin product distributions can be affected by alloying Fe with Cu. Noted that the addition of alkali metal further improves the RWGS and FTS tandem reactions as either a metallic or metal carbide active species can be maintained due to the interactions between Cu and K promoters. Hwang et al. reported that FeÀCuÀK catalyst showed high activity for liquid hydrocarbons production toward CO 2 hydrogenation, [58] which exhibited a C 5þ yield of 18.1%, much higher than that of FeÀK (12.8%) and FeÀCu (2.3%) catalysts. The experimental results reveal that K is mainly responsible for CO formation; Cu-Fe species facilitates CÀC coupling; Fe-Cu-K catalyst promotes the chain growth reactions. Higher alcohol synthesis (HAS) has been regarded as a popular pathway to obtain high-value C 2þ chemicals from CO 2 hydrogenation. The catalysts applied in HAS process should keep a balance between alcohol formation (via dissociative CO activation) and olefin generation (via nondissociative CO activation) to achieve an optimal selectivity for C 2þ alcohols. Noble palladium has been used to achieve high selectivity for ethanol production, but the high cost impedes its large-scale application. [59] Xu et al. recently synthesized a CuFeZn catalyst with the addition of Cs promoter, [60] which exhibited a satisfactory CO 2 conversion efficiency (36.6%), C 2þ OH selectivity (19.8%), C 2þ OH STY (1.47 mmol g cat À1 h À1 ), and C 2þ OH/ROH fraction (93.8%) (Figure 10a,b). The DRIFT and temperature-programmed surface reaction (TPSR) results indicated that the synergistic effects of CuÀZnO and CuÀFe 7 C 3 active sites promoted a tandem reaction for the C 2þ alcohol synthesis. In detail, RWGS reaction first proceeded on CuÀZnO sites to form CO* intermediates, which were hydrogenated into C 2þ OH via surface alkyl intermediates over CuÀFe 7 C 3 sites, therefore simultaneously suppressing the direct CO 2 hydrogenation into methanol via formate intermediates ( Figure 10c). Meanwhile, higher CO 2 -to-alcohol conversion was achieved with the addition of 3 wt% Cs promoter as Cs addition weakens hydrogenation ability, similar to the role of K promoter played in CO 2 hydrogenation. Other transition metals like Mn and Co were also incorporated into Fe-based catalysts to obtain high CO 2 conversion and optimal selectivity for light olefins production. [61a,b] These studies prove that the Mn and Co can be preferable alternatives for enhanced CO 2 adsorption and activation while hindering the secondary hydrogenation to reach high olefin/paraffin (O/P) ratios. Compared with the bulk morphology of Fe, element Ti was introduced as a structural promoter into the Fe-based catalyst to form a FeÀTiÀO-layered oxide material, which results in higher dispersion of Fe active sites and more stable phrase structure for highly selective CO 2 -to-olefins conversion. [62] d) Supports optimization. Different supports have been applied to optimize the metal dispersity, size, and metalÀsupport interactions that resulted from their unique physicochemical properties. Metal oxides are supposed to be ideal candidates used in metallic catalysts with alkali promoters to synthesis high-value chemicals via CO 2 hydrogenation. For example, Shi et al. synthesized a series of TiO 2 -supported CoÀCu catalysts with a suitable amount of K promoter, [63] which could reach a maximum C 5þ yield with a moderate CO 2 conversion (13%), C 5þ selectivity (35.1 C mol%), and high stability under steam (200 h). Zeolites as another traditional support possess a unique structure, and acidity, which is in favor of oligomerization/aromatization/ isomerization of hydrocarbons. Accordingly, Wei et al. combined NaÀFe 3 O 4 catalyst with an HZSM-5 zeolite into a multifunctional catalyst, [64] which hydrogenated CO 2 into heavy hydrocarbons (C 5 -C 11 ) with a selectivity of 78% among all hydrocarbon products, whereas efficiently decreases methane yield. Meanwhile, the as-synthesized catalyst showed superior stability during a 1000 h test, which could be comparable with the industrially applied catalysts for CO 2 conversion into liquid fuels. The proposed pathways of Na-Fe 3 O 4 /zeolite-multifunctional catalyst for CO 2 -to-liquid fuels conversion are described as follows: CO 2 molecules are first activated and reduced to CO after hydrogenation via RWGS process on Fe 3 O 4 active species and then the hydrogenation of CO to olefins on in situ-formed Fe 5 C 2 sites via FTS reaction. Subsequently, the olefin intermediates could diffuse to zeolite acid sites and process a series of oligomerization/isomerization/aromatization reaction steps, and thus the heavy hydrocarbons, especially gasoline-range isoparaffins and aromatics, are formed after desorption from zeolite pores. In addition, a variety of carbon-based materials such as carbon nanotubes, porous carbons, and carbon nanofibers have also been selected as promising supports due to their thermal conductivity, high surface area, electron enriched surface, and high stability. Herein, single-walled carbon nanotubes [65] and mesoporous carbon [66a,b] are synthesized as supports for Fe-based materials, which realize enhanced CO 2 conversion efficiency, optimized selectivity of light olefins (C 2 ÀC 4 ), and O/P ratio at catalytically active Fe carbide sites through a tandem reaction combining RWGS with FTS processes. Figure 10. a) CO 2 conversion (yellow open squares) and selectivities of alcohol, olefin, C 2þ alkane, CH 4 , and CO in all carbon products. b) Distribution of alcohols in STY and weight fraction of C 2þ OH/ROH (brown solid circles) over x%CsÀCuFeZn catalysts. c) The possible pathway of CsÀCuFeZn in CO 2 hydrogenation. Reproduced with permission. [60] Copyright 2020, American Chemical Society.
www.advancedsciencenews.com www.advenergysustres.com 2) The methanol-mediated synthesis pathway for CO 2 -tohydrocarbons conversion also involves two steps, namely, CO 2 is first hydrogenated into MeOH and then reduced into multicarbon compounds. In recent five years, many studies focus on the MeOH-to-olefin conversion, which was thermodynamically initiated at high temperatures (>300 C). However, CO 2 -to-MeOH conversion is an exothermic step that is not favored at such high temperatures, and CO is produced as a byproduct through an endothermic RWGS process. Therefore, zeolite as a solid acid catalyst has been applied for CÀC coupling (≥350 C) and converts MeOH into olefins, which effectively suppresses CO production. Especially, the SAPO-34 and ZSM-5 zeolites are commonly used as active components for methanol conversion to olefin processes at a level of industrialization. Combining ZnOÀZrO 2 , CuOÀZnOÀZrO 2 , In 2 O 3 ÀZrO 2 , ZnGa 2 O 4 , or ZnOÀY 2 O 3 with SAPO-34 zeolite as composite catalysts have been applied to CO 2 hydrogenation. When referred to SAPO-34, the generated MeOH intermediate is first converted into active hydrocarbon pool species ((CH 2 ) n ) inside the channels. Then, MeOH reacts with (CH 2 ) n species to produce light olefins (C 2 ÀC 4 ) and is further converted into higher olefins, alkanes, and aromatic through condensation, alkylation, and cyclization processes. [67] Li's group reported a ZnOÀZrO 2 /SAPO tandem catalyst that successfully converted CO 2 into light olefins (C 2 ÀC 4 ) with high selectivity of 80À90% among hydrocarbon products (Figure 11a,b). [68] CO 2 hydrogenation proceeded on the surface of ZnOÀZrO 2 to generate CH x O intermediates (precursors of MeOH), and subsequently lower olefins were produced on the SAPO zeolite ( Figure 11c). To improve the catalytic activity of CO 2 -to-MeOH conversion of ZnÀZr composites, CuÀZnZr catalyst with coreÀshell structure was synthesized by Chen et al. [69] Under a low concentration of MeOH intermediate, the reduced contact interface and hydrogenation ability on the reaction centers successfully suppress CH 4 production and achieve a high olefin selectivity of 72% at 400 ºC and 2 MPa total pressure. The hybrid catalyst of In 2 O 3 ÀZrO 2 /SAPO-34 was also found to exhibit high selectivity of C 2 ÀC 4 olefins with an ideal CO 2 conversion efficiency at 400 C via a direct tandem catalysis process, [70] by combining a CO 2 -to-MeOH catalyst (In 2 O 3 ÀZrO 2 ) with a methanol-to-olefins catalyst (SAPO-34). In other words, methanol could be generated at the active sites (oxygen vacancies) of In 2 O 3 ÀZrO 2 in the first step, which subsequently went through the SAPO-34 channels and was converted into light olefins automatically. [71] The vital roles of the oxygen vacancies were also confirmed on ZnGa 2 O 4 catalyst toward the direct conversion of CO 2 to C 2 ÀC 4 olefins. [72] Analyzed from in situ infrared spectroscopic tests, the oxygen vacancies effectively activated CO 2 molecules, and simultaneously ÀGaÀOÀ and ÀZnÀOÀ pairs dissociated H 2 into H species. Then methoxide species (CH 3 O*) were formed upon prolonging the time, which were further hydrogenated into MeOH intermediates. Finally, MeOH diffused into the channels of SAPO-34 and thus selectively generated light olefins. As a bifunctional oxideÀzeolite catalyst could initiate CO 2 -to-MeOH and CÀC coupling on two different active sites, ZnOÀY 2 O 3incorporated SAPO-34 zeolite was reported as another alternative to realize a high selectivity of light olefins production (83.9%) with low CH 4 selectivity (1.8%) toward CO 2 -to-hydrocarbons. [73] ZSM-5 zeolites have been applied together with other metal oxides to enhance the selectivity of valuable C 5-11 fuels in CO 2 hydrogenation. The involved mechanism was the concept of the dual cycles: olefins-based cycle and aromatics-based cycle. [74] Methanol intermediates were first converted into olefinic species, and then propene and higher alkenes were obtained from alkenes methylation and cracking in the olefins-based cycle. Meanwhile, higher alkenes could be converted into toluene by oligomerization and cyclization. Furthermore, in the aromatics-based cycle, ethene was generated from methylation of toluene and side-chain elimination. Moreover, aromatics were simultaneously generated by successive oligomerization, cyclization, and dehydrogenation processes. Accordingly, an active catalyst composed of ZnOÀZrO 2 aerogel and H-ZSM-5 zeolite (ZnOÀZrO 2 /H-ZSM-5) realized CO 2 conversion of 16% with the aromatics selectivity of 76%, [75] while the selectivity of CH 4 was less than 1% at 340 C and 40 bars (Figure 11d,e). The MeOH intermediate was first generated over the oxygen vacancies on the surface of ZnO-ZrO 2 , and then the aromatic hydrocarbons were generated from MeOH species in H-ZSM-5 via the above mechanisms ( Figure 11f ). Similarly, the ZnCrO x -ZnZSM-5 compound was used in CO 2 -to-hydrocarbons conversion, which resulted in the highest aromatics selectivity of 81.1% in C 5þ hydrocarbons with 19.9% CO 2 conversion at 320 C. [76] 3. Photocatalysis

Motivation and Principles
Photocatalytic CO 2 reduction (PCR), the so-called artificial photosynthesis, takes advantage of solar energy to activate CO 2 molecules and subsequently convert them into different products such as HCOOH, CO, CH 4 , HCHO, or CH 3 OH over semiconductors (Figure 12a). PCR is a promising technology that can convert CO 2 waste into solar fuels, thus realizing the recycling of renewable carbon fuels and retaining carbon neutrality without secondary pollution. Though much efforts have been made to optimize CO 2 photoreduction to fuels under solar light, the overall photoconversion efficiencies are very limited for practical application and there is a lack of a standard for solar-driven CO 2 conversion efficiency that needs to be achieved for practicality. Therefore, if a CO 2 -to-fuel photoconversion technology is ever to become a reality, a significant advance is required. Recently, a rationally designed CuÀPt/TiO 2 photocatalyst reported by Sorcar et al. could achieve a satisfying photoconversion efficiency of 1.0% with an apparent quantum yield (AQY) of 86% over 6 h under artificial sunlight (AM1.5), which successfully converted CO 2 and H 2 O to a fuel compatible with our current energy infrastructure. [77] Nevertheless, the reaction mechanisms are required to be thoroughly explored, which can contribute to the optimization of the photocatalysts and photocatalytic processes.
In general, the PCR process involves four steps: 1) the formation of excitons within semiconductors upon the illumination with efficient energy larger than the bandgaps of the semiconductor photocatalysts; 2) excitons' dissociation into hot electrons to the conduction band (CB) and leaving holes in the valance band (VB) driven by the built-in electric field; 3) charge carriers migration to the surface active sites; and 4) transformation of CO 2 into carbonaceous compounds by the photogenerated electrons when the location of CB is more negative than the redox potentials of CO 2 reduction half reactions. Meanwhile, the holes in the VB of semiconductors should contribute to the oxidation of H 2 O to O 2 when the VB edge is more positive than the redox potential of H 2 O/O 2 . The H 2 O oxidation process can be replaced by organic upgradation to value-added chemicals with a matching oxidation potential so that the CO 2 reduction process can be integrated with the organic oxidations. Notably, the photocatalysts are the keys to the successful transformations and should be carefully designed as their bandgaps should be suitable to not only satisfy the large overpotentials of the abovementioned redox reactions but also broaden the utilization of solar energy. Moreover, the electronic and geometric structure of the photocatalyst should be optimized to suppress the recombination of e À /h þ pairs, aiming at improving the lifetimes of photogenerated carriers and quantum efficiency. Furthermore, the Figure 11. a) SEM image and schematic structure of ZnOÀZrO 2 /SAPO catalyst. b) CO 2 hydrogenation on ZnZrO/SAPO, ZnZrO, and SAPO and MeOH conversion on ZnZrO/SAPO and SAPO. c) Illustration of the possible pathway for ZnOÀZrO 2 /SAPO catalyst toward CO 2 -to-olefins conversion. Reproduced with permission. [68] Copyright 2017, American Chemical Society. d) SEM images of ZnOÀZrO 2 /H-ZSM-5 catalyst. e) Product distribution and CO 2 conversion of ZnOÀZrO 2 /H-ZSM-5 catalyst in direct CO 2 hydrogenation. f ) The possible pathway of CO 2 hydrogenation into aromatics over bifunctional ZnOÀZrO 2 /H-ZSM-5 catalyst. Reproduced with permission. [75] Copyright 2019, American Chemical Society.
www.advancedsciencenews.com www.advenergysustres.com modification and reconstruction of the surface structure of the employed photocatalysts to increase the number of active sites are vital to the adsorption and activation of CO 2 molecules, which can facilitate the electrons transfer from photocatalysts to CO 2 . In addition, the introduction of proper cocatalysts onto photocatalysts can provide an optimized interface to accelerate the photoinduced electron transfer rate and accumulate a large number of electrons for CO 2 reduction reaction (CO 2 RR), which can effectively prolong the lifetime of e À /h þ pairs and improve the overall quantum efficiency. Finally, the binding energies of reaction intermediates should be optimized to facilitate the desorption of intermediate species, thus avoiding the "poison" phenomenon of photocatalysts and effectively terminating the PCR reaction for the formation of high-value-added carbonaceous products. The extensively studied PCR systems can be divided into the following five categories (Figure 12b): one-step excitation semiconductor system, one-step excitation photosensitized semiconductor system, two-step excitation heterojunction system, two-step excitation direct Z-scheme system, and two-step excitation indirect Z-scheme system.
However, their wide bandgaps and rapid recombination of photoinduced electron/hole pairs restrict their utilization of solar light and minimize the lifetime of charge carriers, as well as the photoactivity, stability, and AQY of PCR.

One-Step Excitation Semiconductor
One-step excitation semiconductor system is a commonly employed candidate to drive PCR with the assistance of a cocatalyst (e.g., Pd, Pt, Ag, Au, Cu, and Ni). Once the semiconductor photocatalyst absorbs solar light, the photoinduced electrons could thermodynamically flux to the cocatalyst as the Femi level of the cocatalysts situates below the CB of semiconductor, which acts as dual roles of electron "accumulation pool" and active sites for CO 2 reduction. The loading and structural optimization of cocatalysts could enrich the number and intrinsic performance of active sites, thus facilitating the activity and selectivity of PCR. For instance, Bai and co-workers hybridized TiO 2 nanosheets with Pd cocatalyst to investigate the performance of the hybrid structures toward CO 2 -to-fuel conversion. [87] Note that the Pd cocatalyst with a small nanosheet structure provides abundant edge sites for the enhancement of charge kinetics, CO, and CH 4 formation rates toward CO 2 RR (Figure 13a-c), which adequately interpreted the edge-dependent photocatalytic performance. Meanwhile, Pd nanosheets could attract photoinduced electrons from TiO 2 to prolong the lifetime of electronÀhole pairs, which effectively enhanced the stability of the overall catalytic system. Furthermore, the plasmonic effect of the Pd metal induced by visible light could generate "hot" electrons, which were then injected into the CB of TiO 2 to improve the light absorption ability of TiO 2 , and thus realized the efficient CO 2 conversion under visible light irradiation. Yu's group deposited Pd cocatalyst onto the surface of graphitic carbon nitride (g-C 3 N 4 ) and explored the facet effect of Pd toward PCR performance. [88] The Pd nanoparticles broaden the light absorption range of bare g-C 3 N 4 , which is mainly attributed to a localized photothermal effect of Pd. Meanwhile, the interface between Pd with g-C 3 N 4 could form Schottky barriers, which effectively separated photogenerated carriers to facilitate the separation of electrons and holes. The electrons generated from the g-C 3 N 4 surface could flow and accumulate on the surface of Pd nanoparticles, which were responsible for the long-term Reproduced with permission. [87] Copyright 2017, The Royal Society of Chemistry. d) HRTEM image of 0.5%PtÀWO 3 . The e) CO and f ) CH 4 production versus reaction time over 0.5%PtÀWO 3 photocatalyst. Reproduced with permission. [90] Copyright 2019, Elsevier B.V. g) The adsorption and desorption thermodynamics of CH 3 OH@Cu/CN. h) The schematic illustration of CH 3 OH@Cu/CN toward CO 2 -to-CH 4 conversion in PCR. Reproduced with permission. [93] Copyright 2019, The Royal Society of Chemistry.
www.advancedsciencenews.com www.advenergysustres.com CO 2 hydrogenation and the formation of CH 3 OH. Moreover, the experimental results and DFT calculations verified that the optimized ratio of (111)/(100) facets of Pd could facilitate the interfacial charge carrier mobility, CO 2 adsorption ability, and CH 3 OH desorption. Metallic Pt has also been employed as a cocatalyst to improve the photoactivity of semiconductors in CO 2 RR. For example, Wang and co-workers decorated Pt nanoparticles onto the surface of SiC nanosheets to study the synergistic effects for the enhancement of activity and selectivity in CO 2 -to-CH 4 conversion. [89] The Pt nanoparticles as the cocatalysts could attract the photogenerated electrons from SiC, which increased the local electron densities around the active sites for the 8-electron CH 4 formation process. It should be mentioned that the generated CO intermediates could be well stabilized on Pt and were further reduced to CH 4 by the accumulated electrons on Pt "island." Similarly, Liu's group introduced different amounts of Pt on hexagonal WO 3 to enhance the mobility and lifetime of the photogenerated carriers. [90] Moreover, Pt adjusted CO adsorption ability and electronic localization, thus realizing highly selective CO and CH 4 formation in the PCR process (Figure 13d-f ).
TiO 2 is the most studied material in PCR during the past decades, and the incorporation of metals such as Ag and Au has been demonstrated to be an effective approach to improve the catalytic performance of TiO 2 . The particle sizes of the cocatalysts significantly influence the selectivity for CO 2 conversion. Hammad et al. reported Ag-loaded TiO 2 catalysts with different sizes of Ag nanoparticles via a photodeposition method in different atmospheres. [91] The study found that smaller Ag nanoparticles could be prepared on the surface of TiO 2 in the absence of O 2 ,\ and showed higher selectivity in CO 2 -to-CO conversion. The main reason for the enhanced PCR lies in the fact that smaller Ag nanoparticles could efficiently collect abundant photoexcited electrons from the CB of TiO 2 , which could accelerate CO 2 activation and CO desorption. Meanwhile, localized surface plasmon resonance (LSPR) of Ag nanoparticles broadened the photoabsorption range, which enhanced the utilization of solar light and photoactivity of the overall catalytic system. Au is also an ideal cocatalyst, [92] which imposes LSPR effects to broaden the sunlight absorption range to visible light regions. Moreover, the hot electrons generated from Au nanoparticles activated the adjacent electrons through energy transfer and increased local temperature, which could improve the catalytic kinetics and efficiency of PCR. Furthermore, the Schottky junction between Au and semiconductors promoted the migration of electrons from semiconductors to Au nanoparticles, which facilitated the separation of the photo-induced carriers, thus achieving long-term catalytic stability.
Except for noble metals, some non-noble metals such as Cu and Ni have also demonstrated their effectiveness as cocatalysts for the selective CO 2 RR in photocatalytic systems. Li et al. deposited Cu nanoparticles on CN, [93] which realized high selectivity for CH 4 formation. Demonstrated by the kinetics and thermodynamics results, the precisely tailored Cu/CN interfaces could establish binary electron transfer channels, which contributed to *CH 3 OH intermediate activation and further hydrogenation reaction for the final CH 4 product formation (Figure 13g,h). Xu and co-workers prepared Ni@graphitic carbon (GC) catalyst composed of Ni nanoparticles on the surface of a few-layered GC. [94] The presence of Ni cocatalyst could bind to GC via Ni─C bonds, which efficiently accelerated the mobility and separation of photoinduced carriers, and CO 2 adsorption, which resulted in high efficiency in PCR.
In addition, other materials like alloys, [95] metal oxides, [96] metal phosphates, [97] Mxenes, [98] graphdiyne, [99] and organics [100] have been considered as cocatalysts to improve the activity and selectivity of PCR. In general, the cocatalysts can not only accumulate electrons around them to increase local electron densities but also retard the recombination of photoexcited electron/hole pairs, thus facilitating multiple electron transfer processes for multicarbon compounds' formation and long-term stability in PCR. Meanwhile, the plasmonic effects of some metals could enhance the sunlight absorption ability, which effectively improves the AQE and solar-to-fuel efficiencies in CO 2 RR.

One-Step Excitation Photosensitized Semiconductor
One-step excitation photosensitized semiconductor system is another alternative for PCR. Currently, the photosensitizers that served as light-harvesting roles can be classified into the following categories, such as metal complexes, organic compounds, inorganic quantum dots (QDs)/nanosheets, etc. Metal complexes for PCR were mainly based on bipyridine, and porphyrin units, among which the RuÀbipyridine complex has been the most popular choice to be used as the photosensitizer due to its good stability and suitable energy level. For example, TiO 2 , [101] Co 3 O 4 , [102] α-Fe 2 O 3 , [103] NiAl-LDH (NiAl-LDH), [104] and Co 2 P [105] have been used as the catalysts with the combination of Ru(bpy) 3 Cl 2 ·6H 2 O as a photosensitizer to form a hybrid photocatalyst system. The photosensitizers first adsorb incident light to generate electrons, which transfer from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of photosensitizers. Subsequently, the electrons flow to the CB of semiconductors and perform a highly selective CO 2 -to-fuel conversion (e.g., formic acid, CO, and CH 4 ). Ir(pyr) is another commonly used photosensitizer that has a strong visible light absorption band, which participates in the PCR catalyzed by a Ru-based complex to realize CO 2 -to-formate conversion. [106] Non-noble metal-based photosensitizers have also attracted some attention for PCR. For example, Kang and co-workers prepared a series of Zn-porphyrins anchored on the TiO 2 surface to achieve a lower energy sensitization and increased catalytic stability. [107] The hybrid catalyst exhibited a high turnover number (TON) of %800 and a long lifetime of 90 h under red light irradiation (>550 nm) for CO 2 -to-CO conversion (Figure 14a,b). The introduction of functional groups (e.g., acetylene, -COOH) to ZnÀporphyrins could improve the energyharvesting ability, photoinduced electron mobility, and binding capability at TiO 2 . Recently, heteroleptic diamineÀdiphosphine Cu (Cu(dppb)) complexes were regarded as popular choices to explore their photosensitizing abilities in the PCR system. Tsubomura and co-workers found that the optimized performance would be achieved when the length of the carbon chains between the two phosphorous atoms in the diphosphine ligands is extended. [108] Using Re(bpy) as a catalyst and Cu(dppb) as a photosensitizer, a robust hybrid system was designed to realize visible light-driven CO 2 reduction to CO. The CoÀquaterpyridine www.advancedsciencenews.com www.advenergysustres.com molecule complex has also been regarded as a highly efficient and selective homogeneous sensitizer. Accordingly, Robert's group designed a novel moleculeÀmaterial hybrid by covalently immobilizing Coqpy-Ph-COOH onto mpg-C 3 N 4 through an amide linkage, [109] which exhibited high selectivity for CO production (98%) and long-term stability (no degradation after 4 days) upon a solar simulator. Finally, the same group reported a CoÀquaterpyridne complex, which has been covalently attached to graphene surface by carboxylic acid groups. [110] The as-prepared catalyst was proved to be a highly active material toward visible light-driven CO 2 conversion in acetonitrile, which showed extraordinary stability (>200 h irradiation) and high selectivity of carbonaceous products (>97%). Importantly, the product selectivity could be completely controlled by varying experimental conditions: CO as the only product was achieved in phenol or trifluoroethanol (a weak acid), while formate was obtained in a mixed solution of acetonitrile and triethanolamine. Organic dyes are another class of widely employed photosensitizers. Robert and co-workers reported that purpurin served as a simple organic photosensitizer to be used in PCR together with a molecular Fe(II) catalyst, [111] which realized highly selective CO 2 -to-CO conversion (FE CO ¼ 95%,) under visible light illumination (λ > 420 nm) in 1/9 (v/v) CH 3 CN/H 2 O aqueous solution. The main absorption peak of purpurin locates at 510 nm, which is conducive to broadening the light absorption range of the PCR system. Meanwhile, the readding of purpurin to the reaction solution mixture could restore the catalytic activity and selectivity, which exhibited a TON of 120 and 95% selectivity for CO production over 4 days. Except for the organic dyes, other organic compounds can be chosen for the application of PCR due to their visible absorption ability, good photophysical properties, and tunable redox potentials. Chao and co-workers reported an organic photosensitizer, the typical thermally activated delayed fluorescence (TADF) compound 4CzIPN, [112] for CO 2 reduction with a simple terpyridinneÀFe(III) complex catalyst under visible light irradiation. The photocatalytic system could realize an excellent selectivity for CO production of 99.3% with an enhanced TON and TOF of 2250 and 60 min À1 , respectively. Particularly, the TON of the PCR system containing 4CzIPN photosensitizer was around 100 times higher than ones containing [Ru(bpy) 3 ] 2þ . TiO 2 as an extensively used photocatalyst has been incorporated with an organic photosensitizer to realize enhanced CO 2 capture and conversion for visible light-driven PCR. For example, Liu and co-workers designed a novel four-site ionic liquid (IL)-photosensitized anatase TiO 2 catalyst to improve CO 2 conversion efficiency under visible light irradiation, [113] which exhibited the highest CO 2 absorption ability up to 3.26 mol mol IL À1 and a high rate of 3.52 μmol g À1 h À1 for CH 4 formation with a high FE CH4 value above 96%. The IL could absorb CO 2 molecules and form a stable structural complex to enhance electron delocalization. Moreover, the IL has strong interaction with TiO 2 nanoparticles, which is attributed to a redshift of the light absorption edge and a synergistically broadened light absorption range to visible light regions. In this work, the IL played dual roles toward CO 2 absorption and activation. In brief, the IL-nCO 2 complex would form once the ILs bond with the CO 2 molecules and then absorb visible light, which could generate abundant photoinduced electrons. Subsequently, the electrons Figure 14. a) Schematic illustration of the possible mechanism for ZnÀporphyrinsÀTiO 2 ÀReC hybrid system in PCR. b) Long-term photocatalytic CO 2to-CO conversion of ZnÀporphyrinsÀTiO 2 ÀReC hybrid system under light emitting diode (LED) lamp irradiation (>500 nm, 60 W). Reproduced with permission. [107] Copyright 2018, American Chemical Society. c) Illustration of the probable mechanism for mpgÀC 3  could transfer from the CB of IL-nCO 2 to the CB of TiO 2 and finally launch the visible light-driven CO 2 -to-CH 4 conversion over the TiO 2 surface. Employing inorganic materials as photosensitizers is beneficial to experimental studies and mechanism exploration. As low-cost, stable, and nontoxic light harvesters, carbon dots can be coupled with other photocatalysts, thus exhibiting a wide spectral response in PCR. Li's group proposed carbon dots-hybridized MOF photocatalyst, [114] which exhibited enhanced activity than the pristine MOFs in CO 2 reduction. Carbon dots serve as excellent photosensitizers for a highly efficient PCR system under long-wavelength light irradiation, which can be ascribed to the polyaromatic units and functional groups with strong πÀπ* and nÀπ* transitions of carbon dots. Mn-doped semiconductor QDs reported by Son et al. showed higher photostability and broadened the absorption range of photosensitization, [115] thus activating the Ni(cyclam)-based molecular catalyst for enhanced activity and highly selective CO 2 -to-CO conversion. This can be explained by the reason that the energetic hot electrons induced by Mn-doped QDs enable the long-range interfacial electron transfer in "nonlinked" QDs/molecular catalyst hybrids, which contributed to six times higher production amounts in CO 2to-CO conversion than the pristine Ni(cyclam) 2þ system. In addition, mesoporous graphitic CN (mpg-C 3 N 4 ) is also an ideal redox photosensitizer used in PCR due to its strong visible light response and high stability under sunlight illumination in various solvents, especially H 2 O. For example, Cao and co-workers fabricated a polymeric CN catalyst on the surface of carbon paper via a chemical vapor deposition method. [116] The addition of phosphate realized a high selectivity of 93.8% toward CO 2 photoreduction to carbonaceous chemicals with an AQY of 0.4%. This study opened a new era for designing efficient photocatalysts of CO 2 reduction in aqueous solution, which also found that phosphate plays a key role in the CO 2 conversion process. Robert et al. reported a hybrid photocatalytic system containing Fe-complex catalyst and mpg-C 3 N 4 photosensitizer for the first time, [117] which exhibited a high FE CO value of 97% and a TON of 155 with an AQY of %4.2% (Figure 14c-e). The mpg-C 3 N 4 possessed enough negative CB minimum (À1.35 V vs calomel electrode (SCE)), which could reduce the Fe I (qpy)(CO 2 ) adduct to achieve an enhanced CO formation under visible light irradiation. Halide perovskite colloidal NCs are also promising alternatives to replace the organic photosensitizers, which possess many advantages such as low nonradiative recombination rates, as well as tunable chemical composition and structures. Accordingly, Chen et al. prepared inorganic ligand-capped CsPbBr 3 NCs as visible light-harvesting photosensitizers, which exhibited lowcost, rich active sites, and a high molar extinction coefficient. [118] Furthermore, the [Ni(terpy) 2 ] 2þ was immobilized on the surface CsPbBr 3 NCs forming a hybrid catalyst, which showed a high yield of 1724 μmol g À1 for CO/CH 4 production in CO 2 photoreduction. The CsPbBr 3 NC photosensitizer offered abundant surface sites to tightly immobilize Ni-based complexes. Meanwhile, CsPbBr 3 NCs can broaden the light absorption range and provide a large number of photoinduced electrons to Ni-based catalytic sites for the PCR system. Some other metal-based compounds have also been applied as photosensitizers due to their strong light responses. For example, Bian and co-workers reported the use of Bi 2 WO 6 particles as photosensitizers and Mn complexes as the catalyst for the highly selective CO production toward PCR. [119] He's group proposed a promising strategy to achieve an efficient, selective, and stable PCR system, [120] in which Zn x Cd 1Àx S was used as the photosensitizer to couple with Fe-based molecular catalyst. The results showed that Zn x Cd 1Àx S facilitated charge separation and mobility, as well as visible light response. This group further combined Bi 2 S 3 with CdS as a hybrid photosensitizer in another work, [121] which not only exhibited enhanced visible light absorption due to its narrow bandgap but also improved the photoinduced electron transport efficiency between the Z-scheme CdS/Bi 2 S 3 photosensitizer and the Febased catalytic center. Thereby, the system achieved improved CO yield in CO 2 photoreduction. In summary, the one-step excitation (photosensitized) semiconductor system has commonly been conducted in organic solutions, which hardly exhibits satisfying photocatalytic performance, as H 2 O is essential for the significant promotion of catalytic activity and stability. Moreover, the use of organic solvents and sacrificial reagents hinders its practical application due to its deficient environmental protection, and thus exploring the hybrid systems with the absence of organic solvent conditions and sacrificial reagents will be more promising in the PCR field.

Two-Step Excitation Heterojunction
Two-step excitation heterojunction system has been regarded as a coupled system of two semiconductors used in photocatalytic reactions, where semiconductors are photoexcited to generate electrons and holes on their VB and CB, respectively, and then, the photoinduced carriers would flow within the hybrid materials depending on the relative energy potentials of CBs and VBs. In terms of the band structures of the coupled semiconductors, the heterojunction system has commonly been divided into three categories: straddling gap (type I), staggered gap (type II), and broken gap (type III) ( Figure 15). [122] In comparison, type II heterojunctions are considered the most popular choice to facilitate the separation of electron/hole pairs: the electrons would transfer to the semiconductors with a more positive CB position, while the holes flowed to the semiconductors with a more negative VB position. Accordingly, the flow directions of electrons and holes are opposite, which can significantly suppress the recombination of the photogenerated charge carriers and enhance the efficiency of the PCR system. Meanwhile, the different semiconducting components employed in heterojunctions can broaden the light absorption range due to their appropriate band structures. TiO 2 , carbon, g-C 3 N 4 , and other oxides/chalcogenides/ carbides/oxysalts-based materials are the most commonly applied components in type II heterostructures for highly efficient PCR. Wang and co-workers designed a paragenetic anatase/rutile TiO 2 heterostructure by a facile phase transformation method. [123] The two-phase TiO 2 heterojunction facilitated CO 2 absorption capacity and photoinduced carrier mobility and then activated the redox reaction of CO 2/ CO 2 À , thus realizing a better selectivity for CO production when compared with P25. In addition, some low-cost materials like metal oxides or sulfides with visible light response have been applied to realize efficient charge transfer and enhanced solar energy utilization of TiO 2 . For example, a well-connected TiO 2 /ZnO heterojunction achieved www.advancedsciencenews.com www.advenergysustres.com an improved photocatalytic yield of CO 2 reduction to CH 4 (2.56 μmol g À1 h À1 ) than pure TiO 2 (0.55 μmol g À1 h À1 ). [124] The optimal performance could be attributed to the rapid photoexcited carrier mobility and slow recombination of electron/hole pairs. Although the as-prepared heterojunctions could convert CO 2 into solar fuels, the light absorption regions still required to be broadened. Accordingly, WO 3-x nanowires were prepared onto the surface of mesoporous TiO 2 by Li's group, leading to a strong SPR which extended the light response range of the whole catalytic system to visible and near-infrared light. [125] Hence, the TiO 2 /WO 3Àx heterojunctions exhibited a high production rate of CH 4 (16.3 μmol g À1 h À1 ) during PCR than the pristine TiO 2 (3.5 μmol g À1 h À1 ) and WO 3Àx (8.0 μmol g À1 h À1 ) (Figure 16a,b). Within the heterojunction, TiO 2 provides continuous photoelectron injection, which plays an important role in the stable SPR of WO 3Àx . Subsequently, the plasmonic hot electrons induced from WO 3Àx nanowires could efficiently suppress hydrogen evolution reaction (HER) under UVÀvis light irradiation, thus resulting in CH 4 formation with impressive efficiency and high selectivity ( Figure 16c). Meanwhile, TiO 2 was also combined with CaTiO 3 [126] or Cu 2 ZnSnS 4 [127] to fabricate heterostructure composite, which possessed an intimate contact at the interface due to their appropriate band structures and matching crystal structures. The hybrids not only enhanced the surface basicity to provide abundant active sites for improved CO 2 absorption and activation but also improved the lifetime of photogenerated carriers to achieve an efficient and selective CO 2 conversion to gaseous hydrocarbons (e.g., CO, CH 4 ) than pristine TiO 2 . In addition, Lee and co-workers prepared a reduced form of black TiO 2 to widen the absorption region via H 2 -mediated reduction step. [128] A disordered structure of TiO 2 was developed with a blueshift of the XPS peak of TiÀO, which was caused by the occupation of the oxygen vacancies by electrons. Moreover, g-C 3 N 4 was introduced and wrapped with black TiO 2 to form a meta-free heterostructure. Engineering 2D/2D heterojunction photocatalysts provided a high interfacial contact area and high density of active sites, which contributed optimized physicochemical properties (e.g., charge carrier dynamics) and thus facilitated the employment of solar energy as a sustainable energy source. [129] Accordingly, the resulting g-C 3 N 4 / black TiO 2 showed increased light absorption at 470 nm. Moreover, the CBs and VBs of the heterojunction processed proper positions for the redox reactions of CO 2 /CH 4 and H 2 O/ O 2 , respectively, thus reaching improved yields and stability for both CO and CH 4 productions than the pure black TiO 2 . C 3 N 4 -based heterojunctions are another class of popular alternatives used in highly efficient PCR, which facilitates photocatalytic efficiency by enhancing light absorption, charge separation, and mass transport. Li and co-workers designed 2D g-C 3 N 4 nanosheets which acted as a soft template to immobilize Co 3 O 4 platelets through a facile method. [130] The hybrids achieved an enhanced photocatalytic performance which was 13.5-and 2.6fold higher than that of pure 2D g-C 3 N 4 and Co 3 O 4 . The synergistic effects of 2D g-C 3 N 4 and Co 3 O 4 not only facilitate the photoinduced carrier transportation due to the large specific surface area of 2D g-C 3 N 4 , but also contribute to the accumulation of electrons onto the surface of g-C 3 N 4 from the CB of Co 3 O 4 for the absorption and reduction of CO 2 À intermediate, thus improving photocatalytic properties (activity, selectivity, and stability) of the whole system under UVÀvis light irradiation. As a visible light transducer, CdS shows a wide light response range and has been frequently selected as a photoharvester in photoredox reactions. For example, Wang and-coworkers improved the PCR activity of boron-doped bulk CN (BCN) semiconductors by surface engineering with CdS, [131] which exhibited 10 times higher CO 2 reduction activity than that of BCN under visible light illumination (Figure 16d,e). Due to the inner electric field of CdS/BCN heterojunctions, CdS as sensitization material could first adsorb visible light and then transferred the photoinduced electrons to the CB of BCN and finally activated and reduced CO 2 molecules to CO. The improved activity is mainly due to the improved visible light absorption and facilitated transport/separation of electron/hole pairs (Figure 16f ). Interestingly, the ternary metal sulfides have also been used with g-C 3 N 4 to fabricate type-II heterojunction for CO 2 photoreduction. For example, CdIn 2 S 4 [132] or Zn 0.2 Cd 0.8 S [133] was prepared and combined with g-C 3 N 4 , which enhanced the CO 2 adsorption ability, charge mobility, and electron/hole pair separation efficiency and thus achieved higher photocatalytic activity and stability for the CO 2 -to-CH 3 OH conversion under visible light than pristine g-C 3 N 4 . In addition, with the appropriate band structures, C 3 N 4 could connect with Bi 4 Nb 8 OCl, [134] LaPO 4 , [135] BiFeWO x , [136] NiAl-LDH, [137] Mexene (e.g., Ti 3 AlC 2 ), [138] or MOFs (e.g., Zr-based MOF, NH 2 -MIL-101(Fe)). [139aÀc] The charge transfer and separation phenomenon can be observed in these type-II heterostructures  through a two-step excitation process. In detail, the photogenerated electrons could escape from the CB of C 3 N 4 under visible light illumination and then be transferred to the CBs of these semiconductors, while the holes induced from the VBs of these semiconductors could flow to the VB of C 3 N 4 , thereby launching the CO 2 reduction and water oxidation/sacrificial reagents on the CB of the additional semiconductor and VB of C 3 N 4 , respectively. Thanks to the synergistic effects between C 3 N 4 and these semiconductors, the separation efficiency of electron/hole pairs, electron transportability and lifetime, and the light response would be improved, thus realizing the enhanced activity, selectivity, and stability of the whole PCR systems. In addition, to further advance the photocatalytic activities of C 3 N 4 -based heterojunctions, the defect engineering of g-C 3 N 4 would play a significant role in the pristine counterpart for artificial photosynthesis, [140] which paves a new pathway of solar energy conversion, hence turning dreams into reality to facilitate CO 2 -to-fuel conversion toward a breakthrough in practical applications.
Carbon-based materials such as graphene, heteroatom-doped graphene, rGO, and carbon fibers are also employed as one of the components to construct heterojunctions for the enhanced PCR performance. As a 2D nanosheet-like material, graphene could act as an electron acceptor or transporter with a large specific surface area, which efficiently separated photogenerated charge pairs in a binary system. Ali et al. prepared a nanowire-like WSe 2 /graphene composite through an ultrasonication process, [141] which could reduce CO 2 into CH 3 OH when illuminated with UVÀvis light. The proposed mechanism suggests that the charge carriers are first generated on the surface of WSe 2 , and the photogenerated electrons transfer to the graphene nanosheets due to the high electron conductivity of the latter, thus The generation rates of CH 3 OH, CO, and CH 4 of different TiO 2 -based photocatalysts toward PCR. c) Possible mechanism of TiO 2 /WO 3-x heterojunctions used in photocatalytic CO 2 -to-chemicals conversion. Reproduced with permission. [125] Copyright 2019, Wiley-VCH GmbH. d) TEM image of CdS/BCN heterojunctions. e) PCR performance of different samples, including BCN, CdS/ BCN-30, CdS/BCN-60, and CdS/BCN-90. f ) Possible mechanism of CdS/BCN heterojunctions in PCR. Reproduced with permission. [131] Copyright 2018, American Chemical Society. g) Energy diagrams and schematic illustration of the photocatalytic mechanism of 2D/2D rGO/MOF heterojunctions for CO 2 reduction. Reproduced with permission. [143] Copyright 2019, Elsevier B.V.
www.advancedsciencenews.com www.advenergysustres.com suppressing the recombination of the electron/hole pairs. Finally, the photoinduced electrons could reduce the adsorbed CO 2 into CH 3 OH, while the holes stored on the WSe 2 were responsible for H 2 O oxidization, respectively. When compared with 2D graphene, 3D-hierarchical nanostructured graphene with interconnected networks could provide larger surface areas and more active sites for photoredox reactions. Moreover, heteroatoms (e.g., N, S) were doped into graphene, which has been regarded as a promising way to modify its electronic structure and the interactions between graphene and another semiconductor. Accordingly, Liu and co-workers fabricated a 2D/3D ZnIn 2 S 4 /N-doped graphene photocatalyst by hydrothermal synthesis, [142] which showed higher CO 2 photoreduction ability. The enhanced PCR performance mainly results from the broadened light absorption range from UV to near-infrared (NIR) region, rapid charge carrier separation, and selective CO 2 uptake capacity compared with the pristine ZnIn 2 S 4 . Importantly, the photogenerated electrons could transfer from the CB of ZnIn 2 S 4 and then accumulate onto the surface of N-doped graphene, owing to the strong electron affinity of the polar sites of N dopants, which thereby efficiently absorb CO 2 molecules and enable their reduction into products of CO, CH 4 , and CH 3 OH. Recently, Peng and co-workers designed a 2D/2D rGO/MOF heterojunction by cofiltering rGO nanosheets with conductive MOF nanosheets, [143] which exhibited a high CO yield rate of 3.45 Â 10 4 μmol g À1 h À1 and high selectivity of 91.74% for CO formation after absorbing visible light. The built-in electric field at the interface of heterojunctions improves the charge harvesting and transport, as well as CO 2 activation (Figure 16g). The abundant charges were generated on the rGO scaffold and then transferred to the conductive and intimate interface of heterojunctions. Finally, a high density of electrons was accumulated at the NiÀN 4 active sites in MOF, thereby achieving highly active and selective PCR. Electrospun carbon nanofiber with 1D nanostructure shows the high specific surface area, strong conductivity, and ample reaction sites and therefore is also a promising component for a heterojunction photocatalyst. For example, Dai and co-workers reported coreÀshellnanostructured carbon nanofibers@TiO 2 nanoparticles composites, [144] which exhibited good intimate contact at the interface of hybrids, thus facilitating the charge transfer capacity and separation of the photoinduced electron/hole pairs. Moreover, the 1D morphology of the photocatalyst could provide more active sites for CO 2 reduction. Furthermore, the black carbon nanofibers with high conductivity could not only promote light absorption to generate heat energy for rapid diffusion of reactants and intermediates but also facilitate the separation rate of electron/hole pairs. Therefore, the system exhibited 2.3 times higher PCR activity than the pristine TiO 2 , which convincingly revealed the important role of carbon nanofibers.
Other metal oxides, chalcogenides, carbides, and oxysaltsbased materials such as CuO/ZnO, ZnTe/ZnO, ZnS/CdS, BiFeWOx/In 2 S 3 , MoS 2 /BN, and Ti 3 C 2 QDs/Mexene QDs have also been designed to form heterojunctions for efficient charge separation and transportation in PCR system. For instance, CuO and ZnO nanospheres were combined to form a hierarchical pÀn heterojunction via a simple hydrothermal method. [145] The as-obtained photocatalyst facilitated the separation efficiency of photoinduced electron/hole pairs and prolonged the lifetime of electrons to realize a robust CO 2 -to-CH 3 OH conversion under visible light illumination. The plausible mechanism is that CuO is activated upon visible light irradiation, and electrons and holes would generate in the CB and VB of CuO, respectively. Then, the photoexcited electrons easily transferred to the CB of ZnO, which was responsible for the reduction of carbonate species into CH 3 OH product. ZnTe as a typical p-type semiconductor could also connect with n-type ZnO semiconductor, forming a pÀn junction due to their appropriate band structures through a facile electrochemical deposition method. [146] The resulting ZnO/ ZnTe heterostructure could realize an efficient and stable CO 2 photoreduction, which maintained a 0.04 μmol cm À2 CH 4 yield during a consecutive 8 h test. The formation of MottÀSchottky junctions at the interface of ZnO/ZnTe facilitated the charge kinetics processes (generation and transferability). Metal chalcogenides have also been chosen as promising alternatives to be used in photocatalytic reactions due to their outstanding activity and low-cost properties. For instance, Wang and co-workers reported branch-like ZnS/CdS hierarchical heterostructures as highly efficient photocatalysts that could absorb visible light and provide high specific surface area, [147] abundant disclosed active sites for strong CO 2 absorption/capture, and accelerate the charge carrier transfer and separation ability, thus exhibiting the optimal CO formation rate of 33.3 μmol h À1 , accompanied with high durability and reusability toward the PCR reaction. In detail, CdS first absorbed the energy of visible light and generated photoexcited electrons on its CB position, and then the electrons migrated to the CB of ZnS due to their large energy difference of the CB positions between CdS and ZnS. Finally, the electrons accumulated onto the surface of ZnS and activated CO 2 RR. Hierarchical In 2 S 3 with a narrow bandgap of 2.0À2.3 eV and appropriate band edges has been extensively studied as an intriguing photocatalyst. In recent work, Zhong and co-workers prepared a core@shell-structured BiFeWO x @In 2 S 3 by a facile hydrothermal approach. [148] The heterojunctions showed enhanced CO 2 photoconversion to CH 4 and CO than that of pure BiFeWO x and In 2 S 3 under visible light illumination as the hierarchical structure improved light harvest and enriched active sites. Meanwhile, the heterojunctions could accelerate the separation efficiency and transferability of photogenerated charge carriers. 2D nanostructural MoS 2 was synthesized and incorporated with h-BN nanoplatelets to design an h-BN-MoS 2 heterostructure, [149] which presented as an effective photocatalyst for CO 2 reduction into CH 3 OH under visible light irradiation. The maximum yield of CH 3 OH could reach up to 5994 μmol g cat À1 , which is 3.8 times higher than that of the pure MoS 2 . The increased PCR performance was caused by the effective separation of the photoinduced charge carriers at the interfaces between h-BN and MoS 2 2D/2D heterojunctions, which not only facilitated the carrier mobility for the highly active and selective CO 2 photoreduction but also improved the practicability of electrons for the enhanced reduction of CO 2 in the whole PCR system. MXenes as a new class of 2D metal carbides, nitrides, and carbonitrides have attained increasing attention in the catalytic fields due to their optimized electronic structures, rich active sites, and good chemical stability. Accordingly, Chen and co-workers fabricated hierarchical Ti 3 C 2 QDs/Cu 2 O nanowire heterostructures via a progressive electrostatic method to realize a highly efficient PCR. [150] The experimental results and

Two-Step Excitation Direct Z-Scheme
Two-step excitation Z-scheme systems involve two-step excitation and charge separation processes, in which the stronger reductive electrons of one photocatalyst and the stronger oxidative holes of the other one synergistically initiate the photoredox reactions. It should be noted that the main difference between Z-scheme and type-II heterostructures is the transfer directions of the photoinduced charges of the components with different CBs and VBs (Figure 12b). The Z-scheme systems can be divided into two categories, that is, direct and indirect Z-scheme systems, and the latter employs a conductive intermediate. In a direct Z-scheme system, the electrons generated from the CB of semiconductor I can directly combine with the holes generated from the VB of semiconductor II, and then the electrons occupied on the CB of semiconductor II with stronger reducibility can react with CO 2 to form carbonaceous compounds, while the holes on the VB of semiconductor I with a stronger oxidizability can promote H 2 O/O 2 half reaction or sacrificial reagents.
For example, α-Fe 2 O 3 /g-C 3 N 4 hybrid acted as a hierarchical direct Z-scheme system to exhibit improved photocatalytic performance toward CO 2 reduction, [151] which achieved a high CO production rate of 27.2 μmol g À1 h À1 without cocatalyst and sacrificial reagent (Figure 17a,b). The direct Z-scheme mechanism of α-Fe 2 O 3 /g-C 3 N 4 played in PCR is shown in Figure 17c. When these two semiconductors are in direct contact, the electrons will flow from the semiconductor with a higher Fermi level to the other one through the interface until their Fermi levels are approximately similar, thus forming a built-in electric field at the interface between α-Fe 2 O 3 and g-C 3 N 4 . The constructed electric field could cause the depletion or accumulation of free charge carriers near the semiconductor surface. Finally, the energy band edges in the semiconductor with higher Fermi levels are bent upward toward the interface, and the energy band edges in the semiconductor with lower Fermi levels are bent downward. [152] Therefore the CBs and VBs of both components could be bent after an intimate contact between α-Fe 2 O 3 and g-C 3 N 4 . The constructed internal static electric fields of α-Fe 2 O 3 promoted the photoexcited electrons transferring onto the interface and combining with the holes in the VB of g-C 3 N 4 , and thus the reducibility of electrons in the CB of g-C 3 N 4 and the oxidizability of holes in the VB of α-Fe 2 O 3 increased. The electron spin resonance (EPR) was further conducted to validate the direct Zscheme mechanism of α-Fe 2 O 3 /g-C 3 N 4 played in PCR (Figure 17d,e). The spin reactive ·OH and ·O 2 À species could absorb onto the surface of the photocatalyst and the Figure 17. a) TEM images of α-Fe 2 O 3 /g-C 3 N 4 Z-scheme system. b) The CO generation rates of different photocatalysts toward PCR. c) The band energy alignment of the direct Z-scheme photocatalytic system of α-Fe 2 O 3 /g-C 3 N 4 hybrids before and after intimate contact between the two components. DMPO spin-trapping ESR spectra tested for d) •OH and e) •O 2 À (e) under visible light of different photocatalysts. Reproduced with permission. [151] Copyright 2018, Wiley-VCH GmbH.
www.advancedsciencenews.com www.advenergysustres.com corresponding characteristic peaks of DMPOÀOH and DMPOÀO 2 À signals were detected. Meanwhile, the signals of the α-Fe 2 O 3 /g-C 3 N 4 sample were much stronger than that of either α-Fe 2 O 3 or g-C 3 N 4 . The results sufficiently demonstrated that α-Fe 2 O 3 /g-C 3 N 4 hybrid followed the Z-scheme system rather than the conventional heterojunction type, which was demonstrated to exhibit an enhanced separation efficiency of electro-nÀhole pairs than that of α-Fe 2 O 3 or g-C 3 N 4 . Therefore, the hierarchical photocatalyst with Z-scheme construction broadens light absorption, improves the binding energy of CO 2 molecules, facilitates the separation of electronÀhole pairs, and increases the reduction ability of photoinduced electrons, which synergistically enhance its performance toward PCR. Similarly, other photocatalysts like Co 3 O 4 , [153] WO 3 , [154] MoS 2 , [155] Bi 2 S 3 , [156] SnS 2 , [157] SnFe 2 O 4 , [158] NiMoO 4 , [159] FeWO 4 , [160] BiVO 4 , [161] and Cu 2 V 2 O 7 [162] have also been employed to match the energy bandgap of g-C 3 N 4 nanosheets, forming efficient visible lightdriven Z-scheme systems for PCR. As aforementioned, g-C 3 N 4 -based Z-scheme photocatalysts construct a close interfacial contact between g-C 3 N 4 and another semiconductor, which contributes to the improved CO 2 photoreduction into high-valueadded carbonaceous compounds including CO, CH 4 , CH 3 OH, and HCOOH, etc. Moreover, Fei et al. provided a theoretical study for a rational-designed catalyst via element doping and Z-scheme heterojunction construction. [163] The heterojunction was composed of BiOBr and g-C 3 N 4 with or without S doping, which was used for investigating the electronic effect on CO 2 reduction activity. Work function and charge density difference indicated the formation of a built-in electric field in BiOBr/g-C 3 N 4 Z-scheme heterojunction, which could effectively facilitate the separation of photoinduced electron/hole pairs. Moreover, the enhanced strength of the built-in electric field was accurately analyzed by Bader charge and electric field intensity. In addition, the effect of S doping could modulate the electronic structures and thus enhance the photocatalytic performance, which was evidenced by the change in absorption coefficient. Furthermore, analyzed by the in-depth investigation of rate-determining energy, the heterojunction with S doping preferred to generate CH 3 OH than CH 4 , which properly explained the selectivity of CO 2 -to-fuel conversion in this system. Similar to CNs, flavin has been also regarded as a metal-free and photoactive material. Savateev and co-workers reported a biomimetic coreÀshell structure composed of micrometer-sized flavins with potassium poly(heptazine imide), [164] which successfully converted CO 2 molecule into CO, CH 4 , MeOH, and EtOH in the water vapor atmosphere. The Z-scheme heterojunction possessed an overall wide bandgap, which significantly extended its excited states lifetimes, thus facilitating the photoredox processes and suppressing the interface charge recombination.
In addition, other direct Z-scheme systems such as ZnIn 2 S 4 / TiO 2 , [165,166] BiOBr/NiO, [167] Cu 2 O/TiO 2 , [168] rGO/TiO 2 , [169] NiFe LDH/Cu 2 O, [170] BiVO 4 /Cu 2 O, [171] WO 3 /Cu 2 O, [172] and Cu 2 ZnSnS 4 /ZnO [173] have been proposed to realize the enhanced activity for PCR. For example, Yu's group reported a step-scheme coreÀshell TiO 2 @ZnIn 2 S 4 heterojunction, [166] which exhibited much higher catalytic performance than that of pure TiO 2 or ZnIn 2 S 4 in CO 2 photoreduction. Due to the large specific surface areas, abundant active sites, and Z-scheme structure, the catalyst showed highly efficient conversion of solar fuels due to the improved separation of photogenerated charge carriers, which was evidenced by in situ XPS, work function calculation, and EPR measurements. The group also reported a novel Z-scheme BiOBr/NiO catalyst composed of two kinds of p-type semiconductors, [167] which could achieve enhanced CO 2 photoreduction activity than that of BiOBr or NiO. Analyzed by the in situ XPS and work function calculation, the photoinduced electrons transferred from BiOBr to NiO via a Z-scheme mechanism, which contributed to a strong redox ability and enhanced charge separation. Meanwhile, in situ DRIFT spectra revealed the formation of complex intermediates during the CO 2 photoreduction process. These results provided a new insight for the CO 2 photoreduction mechanism in Z-scheme heterojunctions constructed by p-type semiconductors. These newly developed Z-scheme photocatalytic systems realize efficient and stable CO 2 conversion into CH 4 or CO with a high yield and formation rate.

Two-Step Excitation Indirect Z-Scheme
Different from the direct Z-scheme system, a third component such as noble metal or graphene oxide has been incorporated as an indispensable conductive intermediate to facilitate the charge transfer and separation of electron/hole pairs in an indirect Z-scheme system. To this end, systems such as ZnO/Au/ g-C 3 N 4 , [174] ZnCdS/Au/g-C 3 N 4 , [175] SnS/Au/g-C 3 N 4 , [176] ZnFe 2 O 4 /Ag/TiO 2 , [177] Cu 2 O/Ag/ZnO, [178] AgPO 4 /Ag/CeO 2 , [179] α-Fe 2 O 3 /amine-rGO/CsPbBr 3 , [180] ZnFe 2 O 4 /rGO/In 2 O 3 , and [181] Bi 2 WO 6 /rGO/g-C 3 N 4 [182] have been constructed and investigated in PCR, where photoexcited electrons transfer from semiconductor I to conductive intermediate and then to semiconductor II, resulting in enhanced charge separation and improved redox reactions. Recently, Li et al. reported a Z-scheme ZnO/Au/g-C 3 N 4 film photocatalyst with an LSPR effect for CO 2 -to-CO conversion under UVÀvis light illumination. [174] Verified by DFT calculations and finite-difference time domain (FDTD) method, it can be concluded that the g-C 3 N 4 / ZnO interface builds the electric field to facilitate carrier mobility. Meanwhile, the Au nanoparticles not only acted as a suitable conductive intermediate for the vectorial electron transfer but also as the LSPR source to promote the separation efficiency of electronÀhole pairs and optical response range. Accordingly, the ZnO/Au/g-C 3 N 4 photocatalyst maintained a high CO formation rate of 689.7 μmol m À2 during a consecutive 8 h reaction time, which was significantly higher than that of pure ZnO film (155.5 μmol m À2 ). Kuang's group designed an all-solid-state Z-scheme α-Fe 2 O 3 /amine-rGO/CsPbBr 3 hybrid for highly efficient PCR (Figure 18a,b). [180] The photocatalyst showed a stable product (CH 4 þ CO) yield of 469.16 μmol g À1 , which was 8.3 times higher than that of pristine CsPbBr 3 (Figure 18c). The amine-rGO could effectively regulate the interfacial interaction between CsPbBr 3 and α-Fe 2 O 3 and then facilitate the transfer of photogenerated carriers from α-Fe 2 O 3 to CsPbBr 3 through amine-rGO, therefore contributing to the enhanced separation of electronÀhole pairs. Moreover, CsPbBr 3 and α-Fe 2 O 3 played the roles of photocatalyst I and photocatalyst II, respectively, which showed a wide light-absorption capability and suitable reduction and oxidation potential, leading www.advancedsciencenews.com www.advenergysustres.com to an improved catalytic activity toward CO 2 -to-CH 4 /CO conversion with an approximately stoichiometric amount of O 2 formation. The above research work demonstrated that the indirect Z-scheme composed of ternary materials could not only facilitate the transferability of charge carriers but also widen the light response and therefore promote the development of efficient PCR into high added-value carbonaceous chemicals and fuels.

Motivation and Principles
The conventional thermocatalytic CO 2 hydrogenations require high energy consumption and system pressure, while the PCR rarely provides high-energy photons to sufficiently activate CO 2 conversion toward multicarbon chemicals and fuels production. Accordingly, the integration of these two techniques in a hybrid system is regarded as a promising route for highly efficient CO 2 reduction, which simultaneously utilizes solar energy and heat energy to initiate the reaction. When an effective catalyst (especially the sample with black color) is irradiated upon light illumination, the light energy (e.g., visible and near-infrared light) can dissipate into heat energy, which can operate the conventional thermocatalytic reactions under mild conditions. Note that the surface temperature of the catalyst can reach up to 500 C or even higher, which offers efficient energy to conduct the thermodynamically unfavorable CO 2 reduction. The catalysts used in photothermocatalysis have been generally classified into two categories: plasmonic metals and other systems. The plasmonic metal system plays the LSPR effect, which contributes to elevated electric fields on the catalyst surface, and thereby resulted in a high density of energetic charge carriers, mainly hot electrons and holes after nonradiative decay through intraband or intraband excitations (Figure 19a). The photoexcited hot electrons could promote bond activation and intermediate conversion in CO 2 reduction. In detail, the hot electrons transform the reactant molecules along an exciting potential energy surface, which contributes to extra vibration energy to the reactant, thereby activating the molecular bonds (Figure 19b). When referred to the intermediate evolution, both direct and indirect routes play critical roles in optimizing the reaction selectivity and activity (Figure 19c,d). [9] Other systems without LSPR effects could also perform efficient activities in photothermocatalysis, mainly depending on the structural optimization of the catalysts. However, the mechanism of photothermocatalysis is complex due to unknown thermodynamics and kinetics, and the synergy between the photocatalytic and thermal effects remains unexplored. In general, the newly developed photothermocatalysis could overcome the difficulties of inferior activity in photocatalysis and excess reaction barriers in thermocatalysis, with abundant solar energy under lower temperatures and pressures.

Plausible Routes for CO 2 Hydrogenation
The primary reactions evolved in photothermocatalysis can be broadly classified into five groups: 1) RWGS reaction, 2) methanol and ethanol synthesis, 3) methanation reaction, and 4) CO 2 hydrogenation. Some aspects are similar to thermocatalysis, which have been reviewed in Section 2.1.

CO 2 to CO
1. In the thermocatalytic RWGS reaction, some transition metals and metal oxides have been studied to produce CO via CO 2 hydrogenation under certain temperatures and pressures. It should be noted that the photothermocatalytic RWGS reaction concentrates on nanostructured materials, which exhibited distinguished catalytic performance. For example, Ozin and coworkers designed a black indium oxide catalyst via hydrogenating In 2 O 3 NC, [183] thus forming amorphous domains of In 2 O 3Àx on Reproduced with permission. [180] Copyright 2020, Elsevier Inc.  Figure 19. a) The illustration of hot carriers generated through nonradiative decay that resulted from the LSPR effects. b) Bond activation over plasmonic metals. Immediate conversion after absorbing the plasmon energy from the metal in an c) indirect and d) direct route, respectively. Reproduced with permission. [9] Copyright 2019, Wiley-VCH GmbH.   (Figure 20a,b). The constructed nonstoichiometric/stoichiometric In 2 O 3Àx /In 2 O 3 heterojunction enabled high activity for RWGS reaction with 100% selectivity toward CO formation, which achieved an extremely high CO production rate of 1874.62 μmol h À1 m À2 (23882.75 μmol g À1 h À1 ) (Figure 20c), three orders of magnitude higher than pale yellow In 2 O 3 (19.64 μmol g À1 h À1 ) under 100 W LED white lamp. The significantly enhanced performance could be ascribed to the synergistic roles of stronger sunlight absorption ability and photothermal effects. The oxygen vacancies in In 2 O 3Àx promoted the insertion of protons to weaken the C─O bonds of the CO 2 reactant, which facilitated the CO formation (Figure 20d). Meanwhile, the In 2 O 3Àx /In 2 O 3 heterostructure enhanced the light absorption across the solar spectrum, which gave rise to local heating on the surface of the catalyst and then contributed to the thermocatalytic conversion of the RWGS reaction. Moreover, the heterostructure could effectively separate photoinduced electronÀhole pairs, which also contributed to the enhanced PCR performance. Therefore, the surface black In 2 O 3Àx layer acted as promising material to perform an improved activity in photothermal RWGS reactions. Recently, Ozin and Zhang and co-workers reported a supraphotothermal Ni@p-SiO 2 catalyst, which exhibited a satisfying catalytic activity and stability toward RWGS and methanation reactions due to the spatial confinement effects, heat insulation, and infrared shielding effects of catalysts. [184] Halas and co-workers reported the synthesis of Al/Al 2 O 3 /Cu 2 O heterostructures (Figure 20e), [185] which exhibited highly efficient and selective RWGS activity under light irradiation at room temperature than that in thermal conditions (Figure 20f ). To investigate the spatial temperature, light absorption ability, and integrated electric field of the heterostructural catalyst, the work demonstrated that the LSPR effect of Al@Cu 2 O antenna reactors could provide abundant hot carriers and broaden the light absorption range of Cu 2 O under visible light illumination, thereby achieving highly selective CO production in CO 2 conversion. Al@Cu 2 O acted as the active component exhibiting plasmon-enhanced CO 2 conversion, which was accompanied by heat generation from the thermalization of the hot carriers, thus resulting in an enhanced photothermocatalytic process (Figure 20g). The local temperature of Al/Al 2 O 3 /Cu 2 O surface could reach up to 175 C under visible light irradiation due to the photothermal heating from electro-nÀelectron scattering during the hot-carrier decay. The photoinduced hot carriers fluxed from Al core to Cu 2 O shell, which played the main role in C─O bond dissociation and prolonged the lifetime of excited states. Accordingly, the Al@Cu 2 O antenna reactor mechanism was proposed, which achieved higher activity and selectivity in the RWGS reaction than the traditional thermocatalysis, and thus showed an optimal CO formation rate of %360 μmol CO cm À2 s À1 during the light-excited process (10 W cm À2 ). Other catalysts have also been reported for efficient CO 2 photothermocatalysis, such as TiN@TiO 2 @In 2 O 3-x (OH) y , [186,187] 2D In 2 O 3Àx , [188] Cu hydroxyapatite, [189] W-doped TiO 2 , [190] Pd/ TiO 2 , [191] FeOÀCeO 2 , [192] MoO 3Àx , [193] Bi 4 TaO 8 Cl/W 18 O 49 , [194] Bi 2 S 3 /UiO-66, [195] Co@CoN&C, [196] Ni 12 P 5 , [197] and Pd nanoparticle (NPs)/barium titanate, [198] which reached satisfying CO formation rates upon solar light illumination.

CO 2 to Alcohols
In comparison with photothermocatalytic RWGS reaction, studies on methanol and ethanol synthesis are quite limited. Hong and co-workers designed a Pd/ZnO catalyst through a facial impregnation approach to realize industrial MeOH formation at relatively low pressure (12 bar) under light irradiation. [199] The enhanced MeOH yield was due to the synergy effect between photocatalysis and thermocatalysis, as well as a wide visible light absorption range induced by the LSPR effect of Pd nanoparticles (Figure 21a-c). The latter was confirmed to be the main contributor (>90%) to the enhanced photothermocatalytic MeOH formation. The interface of PdÀZnO would accept the abundant hot electrons and acted as the active site to promote CO 2 absorption and activation via electron injection to the antibonding orbital of CO 2 molecules, thus resulting in MeOH synthesis in a similar way to thermocatalytic CO 2 hydrogenation. Therefore, Pd/ZnO achieved a promotion of CO 2 conversion efficiency more than two times and enhanced yield of MeOH (1.5À3.0 times) under light irradiation in a flow fixed-bed plasmon-assisted photothermal reactor under relatively lower pressure. Some other catalysts including Cu 0 /Cu 2 O, [200] CoO/Co/ TiO 2 , [201] and LaCo x Fe 1Àx O 3 [202] have been reported to achieve efficient MeOH synthesis through plasmon-assisted photothermocatalysis or photoÀthermo coupling conditions ( Table 1). For example, Cui's group fabricated a complex composed of AuCu alloy nanoparticles and ultrathin porous g-C 3 N 4 nanosheets (Figure 21d), [203] and the integrated photocatalysis and thermocatalysis synergistically realized an enhanced yield (0.89 mmol g À1 h À1 ) and selectivity (93.1%) of ethanol formation at 120 C under visible light irradiation using 1.0 wt% AuCu/ g-C 3 N 4 composites (Figure 21e). The positive charges on Au surface facilitated CO 2 adsorption, while Cu with enriched negative charges improved the *CO 2 À and *CO intermediate generation. Meanwhile, the strong interaction between AuCu alloy and g-C 3 N 4 promoted photoinduced carrier transportation. Moreover, the increased temperature on the surface of the catalyst accelerated the dynamics and thermodynamics of reactant molecules, which resulted in the enhanced *CO dimerization and CÀC coupling reactions (Figure 21f ). Accordingly, the AuCu/g-C 3 N 4 catalyst exhibited 4.2-and 7.6-fold higher yield toward photothermocatalytic ethanol synthesis than that in photocatalytic and thermocatalytic processes, respectively.

CO 2 to CH 4
Methanation reaction is known as the Sabatier reaction, which can produce CH 4 via CO 2 hydrogenation. Ru-and Ni-based materials have been the most frequently used catalysts for CO 2 methanation to realize the satisfying CH 4 yield and selectivity. Ye's group constructed a hybrid catalyst with small Ru nanoparticles supported on ultrathin MgÀAl few-layer LDHs (MgÀAl FL-LDH) supports (Figure 22a,b), [204] which could reach a peak rate of CH 4 formation (277 mmol g À1 h À1 ) in a flow-type reactor (Figure 22c, d). The Ru nanoparticles played vital roles in H 2 activation and the subsequent CO 2 hydrogenation as metallic Ru could enhance the local temperature of the Ru@FL-LDHs surface after absorbing solar light. Meanwhile, ultrathin MgÀAl LDH possessed abundant active sites, which benefited the CO 2 activation and reactant absorption. A series of control experiments had been conducted to reveal the mechanism of photothermocatalytic processes in this work. Light illumination is regarded as a driving force to initiate the photo-to-thermal conversion, while photocatalysis played a negligible role in CH 4 production. In other words, the photothermal reaction is quite similar to that in the traditional thermocatalysis, where Ru@FL-LDHs reached a high reaction temperature of %350 C under continuous irradiation to activate H 2 molecules. Moreover, the close intact nature between Ru nanoparticles and FL-LDHs certainly contributed to the highly efficient CO 2 hydrogenation process. Therefore, Ru@FL-LDHs performed an efficient and stable methanation reaction toward CO 2 photothermocatalysis under mild reaction conditions. In addition to Ru, Ni as an abundant and low-cost material has been employed as an active catalyst for photothermocatalytic CO 2 methanation. Accordingly, Li et al. realized satisfying CO 2 methanation performance by developing a new photothermal reactor with the presence of a layer of absorber (Figure 22e). [205] Under solar light illumination (from 0.52 to 0.7 kW m À2 ), the catalyst that was composed of single Ni atoms, and ultrathin amorphous Y 2 O 3 nanosheets (the SA Ni/Y 2 O 3 nanosheets) (Figure 22f ) showed an optimal CH 4 formation yield up to 7.5 L m À2 h À1 with a high CO 2 conversion efficiency of 80% under elevated temperature (Figure 22g,h). The rational design of single-atom Ni coupled with the selective light absorber (here ultrathin amorphous Y 2 O 3 nanosheets) could realize a high and stable photothermocatalytic methanation reaction under solar light irradiation at a relatively lower initial reaction temperature using SA Ni/Y 2 O 3 composites.
The Co species also exhibits a high photothermo effect in CO 2 methanation as it can absorb the solar light and then convert it to thermal energy via the intra-and interband electron transition. It means that photoexcited electrons could recombine with the holes to generate phonons rather than photons, which increases the local temperature on the surface of Co-based catalysts and thereby performs good activity in photothermocatalytic CO 2 methanation under solar light irradiation. Herein, Jia's group fabricated highly dispersed Co nanoparticles on Al 2 O 3 support (Figure 23a), [206] which achieved an extraordinary CH 4 formation rate of 6036 μmol g À1 h À1 with high CH 4 selectivity of 97.7% during continuously catalytic testing (Figure 23b,c). The broad optical response range, the strong adsorption ability toward reactant molecules, and the large density of active sites synergistically contributed to the enhanced photothermocatalytic performance of Co/Al 2 O 3 . Moreover, the UVÀvis light irradiation promoted the formation of oxygen vacancies on the surface of Co/Al 2 O 3 , which improved the absorption ability of CO 2 molecules, Figure 21. a) HRTEM image of Pd/ZnO catalyst. b) The STY toward different products over Pd/ZnO catalyst in photothermo CO 2 hydrogenation under different reaction temperatures with or without light irradiation. c) The reaction mechanism of Pd/ZnO catalyst in SPR-mediated CO 2 -to-MeOH conversion. Reproduced with permission. [199] Copyright 2019, Wiley-VCH GmbH. d) TEM and HRTEM images of AuCu/g-C 3 N 4 composites. e) Photothermocatalytic CO 2 reduction over different catalysts under light irradiation at 120 C. f ) Schematic mechanism of AuCu/g-C 3 N 4 composites in photothermocatalytic CO 2 -to-ethanol conversion. Reproduced with permission. [203] Copyright 2020, Elsevier B.V.  Table 1. Performance comparisons of the representative catalysts used in photothermocatalytic CO 2 hydrogenation under light irradiation. [183,[185][186][187][188][189][190][191][192][193][194][195][196][197][199][200][201][202][203][204][205][206][207][208][209][210][211][212][213][214][215][216][217][218] Catalyst The equilibrium temperature values of the samples under focused light irradiation; b) "À" indicates not noted. Figure 22. a,b) TEM and HRTEM images of Ru@FL-LDHs catalyst. c) The CH 4 yield over Ru@FL-LDHs catalyst under 300 W Xenon lamp irradiation (light intensity, 10 Suns) at H 2 /CO 2 flow rates of 25.0 and 5.0 mL min À1 , respectively. d) Schematic illustration of photothermocatalytic CO 2 hydrogenation processes over Ru@FL-LDHs catalyst in a flow reactor. Reproduced with permission. [204] Copyright 2017, Wiley-VCH GmbH. e) Schematic illustration of the newly developed photothermocatalytic reactor system for CO 2 methanation with the selective absorber and the SA Ni/Y 2 O 3 nanosheets. f ) STEM image and the corresponding EDS mapping images of Ni, Y, and O of the SA Ni/Y 2 O 3 nanosheets. g) The spatial temperature mapping of the newly developed photothermocatalytic system under simulated solar light illumination using an infrared camera (intensity: 1.0 kW m À2 ). h) The performance of the SA Ni/Y 2 O 3 nanosheets catalyzed in photothermocatalytic CO 2 hydrogenation. Reproduced with permission. [205] Copyright 2019, Springer Nature.
www.advancedsciencenews.com www.advenergysustres.com thus facilitating the CO 2 conversion into CH 4 product. Furthermore, the Al 2 O 3 support with high surface area and porosity also played an essential role in improving the CH 4 production yield, selectivity, and stability. Except for metallic catalysts, other materials like boron, TiO 2, and their heterojunctions have also been investigated in photothermocatalytic process. For example, Ye's group reported a fascinating approach to design the elemental B particles for the first time (Figure 23d), [207] which showed high light-response ability and efficient photo-thermo conversion. The photothermal effect of B particles could elevate the local temperature of the catalysts to activate CO 2 molecules. Meanwhile, the self-healing ability of the B element under light irradiation could decompose H 2 molecules and provide an active proton source as well as electrons for CO 2 reduction. In addition, the in situ formation of boron oxides contributed to the enhanced CO 2 absorption. In conclusion, elemental B catalyst exhibited a CH 4 production rate of 2.5 μmol h À1 in the presence of water without any sacrificial agents or cocatalyst under UVÀvisÀIR light illumination (Figure 23e,f ), which opened up a new era toward highly efficient CO 2 conversion and solar light utilization. Yu's group employed a facile strategy to design TiO 2 photonic crystals (TiO 2 PCs) through an anodizationÀcalcination process (Figure 23g,h), [208] which possessed the satisfying heat radiation capture ability. With the specific PC morphology, the TiO 2 PC catalyst showed localized surface photothermo effect (Figure 23i), which not only improved light absorption but also increased the surface temperature to accelerate reaction kinetics and product yield rates, finally contributing to a high CO 2 methanation rate of 35.0 μmol g À1 h À1 , superior to that of commercial TiO 2 or TiO 2 nanotube arrays. Except for the aforementioned catalyst, some other materials such as Ru/iÀSiÀo, [209] Ni NPs/ BaTiO 3 , [210] Ni/CeÀTi, [211] TiO 2 NPs/graphene, [212] and Figure 23. a) TEM, b) product rate, and c) selectivity over Co/Al 2 O 3 catalyst in photothermal catalytic CO 2 conversion (green bars, CH 4 ; pink bars, CO). Reproduced with permission. [206] Copyright 2020, American Chemical Society. d) HRTEM images of the elemental B particles. e) Gas production of CO and CH 4 over elemental B particles under full arc irradiation. f ) The impact of adding H 2 O on the gas production of CO and CH 4 over elemental B particles in the photothermocatalytic CO 2 hydrogenation system. Reproduced with permission. [207] Copyright 2017, Wiley-VCH GmbH. g) Top-view and h) cross-sectional view SEM images of TiO 2 PCs. i) Schematic mechanism of TiO 2 PCs for the improved photothermocatalytic CO 2 hydrogenation performance. Reproduced with permission. [208] Copyright 2018, American Chemical Society.

CO 2 to C 2þ Hydrocarbons
The direct hydrogenation of CO 2 into hydrocarbons is a promising route to produce valuable CH x chemicals. Fe-based catalysts have been regarded as ideal alternatives used in photothermocatalytic CO 2 hydrogenation to produce multicarbon hydrocarbons under solar light illumination at atmospheric conditions. Zhang's group proposed a Fe-500 catalyst composed of Fe and FeO x nanoparticles on a MgOÀAl 2 O 3 support (Figure 24a-c), [214] which showed satisfying performance toward CO 2 -to-C 2þ hydrocarbon conversion with high CO 2 conversion (50.1%) and C 2þ selectivity (52.9%) when irradiated by UVÀvis light (Figure 24d). Evidenced by the experimental results and DFT calculations, the construction of Fe/FeO x heterojunctions suppressed the -CH 2 and -CH 3 hydrogenation to CH 4 on the active Fe 0 species. On the contrary, it facilitated the CO 2 hydrogenation into CO and the subsequent CÀC coupling (Figure 24e), thereby improving the selectivity of C 2þ hydrocarbons production. Meanwhile, the local surface temperature of Fe-500 could rapidly increase up to 252 C under Xe lamp irradiation, which achieved almost the same activity as that in the thermocatalytic CO 2 conversion. Moreover, the light density is demonstrated to show negligible impact on CO 2 conversion or Figure 24. a) TEM image, b) the corresponding EDS elemental mapping of Fe, Mg, Al, and O, and c) HRTEM image of Fe-500 catalyst. d) Product selectivities (i.e., CH 4 , C 2À4 , and C 5þ ) and CO 2 conversion over Fe-500 catalyst under UVÀvis illumination under a flow test. e) The energy profiles of the C-C coupling pathways on 3O/Fe (110) and Fe (110). The blue, black, red, and white spheres represent Fe, C, O, and H atoms, respectively. Reproduced with permission. [214] Copyright 2021, Wiley-VCH GmbH. f ) Schematic illustration of the photothermocatalytic CO 2 hydrogenation to CH 4 , C 2þ hydrocarbons, and ethanol using Na-Co@C catalyst. Reproduced with permission. [217] Copyright 2018, Elsevier B.V.
www.advancedsciencenews.com www.advenergysustres.com C 2þ selectivity, which indicated that the CO 2 -to-C 2þ conversion on Fe-500 catalyst followed a photothermocatalytic mechanism instead of photocatalysis. Other Fe-based catalysts (e.g., θ-Fe 3 C [215] and Fe 2 O 3 /Fe 3 O 4 [216] ) have also been investigated in photothermocatalytic CO 2 hydrogenation, which reached extraordinary selectivity for C 2þ hydrocarbons formation. Furthermore, metallic Co, and Ni have also been employed in the CO 2 hydrogenation to generate C 2þ hydrocarbons through the photothermocatalytic processes. For example, NaÀCo@C [217] (Figure 24f ) and Ni-or Co-doped BaZr 0.5 Ce 0.3 Y 0.2 O 3Àδ [218] have been employed as efficient materials to enhance the surface temperature, photoexcited electron mobility, CO 2 dissociation, absorption ability of CO, CÀC coupling, and the rapid desorption of reaction products onto the surface of catalysts, which synergistically contributed to the highly efficient photothermocatalytic CO 2 hydrogenation to C 2þ hydrocarbons. These studies could guide the design of efficient catalysts for artificial photosynthesis via photothermocatalytic CO 2 hydrogenation, which can finally produce high-value-added multicarbon chemicals and fuels from CO 2 .

Conclusion and Perspectives
The efficient utilization and chemical transformation of CO 2 into value-added chemicals and fuels have received increasing attention and been regarded as a promising approach to alleviate global warming, energy, and environmental crises. Notably, solar-driven photocatalysis has presented significantly enhanced activities and selectivities, which cater to the scientific and industrial communities and can be comparable with traditional thermocatalysis. This review primarily outlines the state of the art of the technologies toward thermocatalytic, photocatalytic, and integrated photothermocatalytic CO 2 conversion into products of CO, CH 4, formate, methanol, ethanol, and C 2þ hydrocarbons/ oxygenates in the past 5 years (Figure 25). Special focuses have been paid to the designing strategies for effective heterogeneous catalysts for selective CO 2 conversion. Although the processes of thermocatalytic CO 2 hydrogenation into C1 chemicals (e.g., MeOH, CH 4 ) have been commercialized, the achievements in CO 2 chemical transformations are still not satisfactory concerning the technical and economic viability and energy efficiency for large-scale employment. Accordingly, extensive investigations have been conducted to develop UV and/or visible light-sensitive semiconductors for photocatalytic and photothermocatalytic CO 2 conversion. Particularly, plasmonic photocatalysis and plasmonic photothermocatalysis not only occupy an important position in practical CO 2 -to-fuels conversion with a high utilization efficiency of solar energy but can also reference the selection of catalysts and reaction conditions from the conventional thermocatalysis, which builds a strong bridge between photocatalysis and conventional thermocatalysis. Moreover, the distinct pathways toward different products of these techniques have been comprehensively summarized to reveal the reaction mechanisms and key steps that determine the formation of the intermediates and final products, which can further serve as the guidance for the rational design of catalytic systems. Regardless of the aforementioned achievements, there is still plenty of room to improve the catalytic performance in the community of thermocatalysis and photo(thermo)catalysis.
More challenges should be overcome toward the following aspects. 1) It is highly demanded to develop highly active and cost-effective catalysts for large-scale synthesis, such as layer double hydroxides with tunable band structure and oxides/nitrides  with optimized geometric effect.
2) The yields and selectivities of C 2þ hydrocarbons and oxygenates are more desired, although most of the existing catalytic systems produce C 1 chemicals (e.g., CO, formate, CH 4 ). The synthesis of C 2þ hydrocarbons and oxygenates needs to overcome the thermodynamic limits and kinetic barriers during the multielectron transfer processes in CO 2 RR, which will relieve excessive energy consumption and improve the energy conversion efficiency of the current CO 2 conversion techniques. Importantly, CO 2 adsorption, activation, and *CO intermediate formation significantly determine the yield of C 2þ products. For example, the *CO dimerization affects the production rates of C 2 H 4 , ethanol, and n-C 3 H 7 OH. The *CH 2 CHO intermediate generated from the *COÀCO dimerization is the rate-determining step for C 2 H 4 and C 2 H 5 OH formation. Furthermore, n-C 3 H 7 OH can be produced by inserting CO into the stabilized *CH 3 CHO intermediate. Therefore, the selectivities of targeted C 2þ products have to be well controlled by the rational design of nanostructured catalysts. Intelligent strategies can be employed, such as morphology and structure optimization, alloying/dealloying, surface modification, and support introduction. Meanwhile, the structural integrity of catalysts and long-term durability of the overall catalytic systems still require to be improved for practical application as most durability tests have been conducted for a few hours and deactivation analyses are quite scarce. For example, thermal stability and recyclability of thermocatalysts have been tough problems to be resolved. Meanwhile, to suppress the photocorrosion and enhance the durability of (photo)thermocatalysts in the aqueous solution, coating the surface of the catalysts with protective layers through atomic layer deposition or electron beam evaporation methods may substantially improve the system stability for large-scale application. Accordingly, the structure of the catalysts, reaction conditions, and reactor configurations are required to be carefully designed for the improvement of the long-term catalytic system. 3) In terms of solar-driven photocatalysis, the intensity of the light source used in the lab (e.g., Xe lamp, Hg lamp) is much greater than sunlight. Moreover, the natural day/night cycle and narrow light absorption ability of most photo(thermo)catalysts not only bring challenges for industrial application but also reduce the utilization of solar energy. Accordingly, some ideal catalysts, such as rare-earth upconversion luminescent materials, photothermal semiconductors with a black color, or high density of defects/vacancies, can be designed to directly convert solar light to heat energy and applied in CO 2 hydrogenation, which not only elevates the photochemical contribution but also enhances the charge transfer efficiency and photoinduced carrier lifetimes, thus improving the catalytic activity. 4) Adequate photoreactors are required to effectively absorb and utilize sunlight for photo(thermo)catalysis, and therefore the continuous flow systems can be rationally designed to meet the requirements of industrial applications. 5) A standardized metrology method requires to be created to evaluate the solar-to-chemical conversion efficiency in photocatalytic and photothermocatalytic CO 2 reduction so that the comparison of the results from different research groups is convincing and fairly. Meanwhile, the intrinsic photocatalytic effect and photothermal effect should be identified. The key is to distinguish the thermal and nonthermal effects in photothermocatalysis, which can be measured by a precise instrument like nanoscale thermometers.
6) Computational analysis through DFT is of great significance to complement the experimental characterizations, which contributes to revealing the structureÀactivity relationship of the system and identifying the active sites, intermediate species, reaction pathways, and the roles of additives and water for CO 2 RR. Generally, as catalysts stand at the center to realize improved activity, selectivity, structure stability, and long-term durability, significant advances in designing effective heterogeneous catalysts are required for practical applications. The demonstrated strategies include structure optimization, crystal facets regulation, and defect engineering, thereby exposing increased density of active sites, high specific surface area, and closely conjoint interfaces to facilitate CO 2 adsorption and the subsequent transformations. Moreover, the reaction conditions and reactor design are always ignored, which need to be optimized to satisfy energy transfer (e.g., heat, light) and simplify equipment demand, aiming at constructing a well-established system for the enhancement of overall CO 2 RR performance. Furthermore, in situ/ operando techniques (e.g., XRD, TEM, Extended X-ray absorption fine structure (EXAFS), fourier transform infrared spectroscopy (FTIR), Raman) and computational chemistry (e.g., theoretical calculations, molecular dynamics modeling) are required to be carried out, which can not only assess the precise nature of the active sites and identify the intermediate species to reveal the reaction pathways, but also determine the key parameters of designing efficient catalysts for highly efficient CO 2 conversion. Machine learning is the practice of building models from data, which has attracted intensive attention in catalysis, which has been integrated into the computational and experimental research in recent years. It can analyze the atomistic potentials and predict the catalytic properties of the catalytic systems. For example, DFT has been widely employed as an important tool for potential pathway screening, free energy calculations, the d-band center prediction, and molecular dynamics simulations, thus obtaining effective information about the relevant surface structures and compositions of catalysts under reaction conditions. Accordingly, machine learning provides increased value to advance the catalysis research, which not only helps researchers get more valuable information with existing data but also provides more sophisticated methods to collect new and useful data. Therefore, it can contribute to gain deep insights into the synthetic parameters for efficient catalysts designing and optimal reaction conditions in thermocatalysis, photocatalysis, and photothermocatalysis. In conclusion, comprehensive measures of precise catalyst preparation, in-depth structural characterization, thorough CO 2 RR testing, and advanced computational study are indispensable to explore the full potentials of the technologies of thermo-, photo-, and photothermocatalytic CO 2 conversion.