Photothermal Catalysis: An Emerging Green Approach to Upcycling Plastic Waste

The increasingly massive accumulation of plastic waste is triggering a global pollution crisis, causing severe economic and health issues. As an effective strategy to realize synchronous environmental remediation and value‐added chemical production, the catalytic upcycling of plastics has received extensive attention. Among the upcycling approaches, the emerging photothermal catalysis outstands with features of high conversion efficiency and mild reaction conditions. Herein, the advancements of photothermal catalysis in plastic waste upcycling are reviewed for the first time. The general limitations of the traditional thermocatalytic and photocatalytic upcycling of plastics are first discussed. Subsequently, the photothermal catalytic approaches to upcycling of plastics are classified into three categories depending on the catalytic mechanism, and discussed in depth. Finally, the current challenges in the field are appraised, and several suggestions concerning the investigation of the mechanisms and practical applications for the conversion of plastics into valuable chemicals are highlighted.

The increasingly massive accumulation of plastic waste is triggering a global pollution crisis, causing severe economic and health issues.As an effective strategy to realize synchronous environmental remediation and value-added chemical production, the catalytic upcycling of plastics has received extensive attention.Among the upcycling approaches, the emerging photothermal catalysis outstands with features of high conversion efficiency and mild reaction conditions.Herein, the advancements of photothermal catalysis in plastic waste upcycling are reviewed for the first time.The general limitations of the traditional thermocatalytic and photocatalytic upcycling of plastics are first discussed.Subsequently, the photothermal catalytic approaches to upcycling of plastics are classified into three categories depending on the catalytic mechanism, and discussed in depth.Finally, the current challenges in the field are appraised, and several suggestions concerning the investigation of the mechanisms and practical applications for the conversion of plastics into valuable chemicals are highlighted.
are already outstanding reviews providing comprehensive discussions on the important topic. [2,12]In this review, we focus on the catalytic strategy for upcycling plastic waste driven by heat and/or light.
The thermocatalytic technology can depolymerize waste plastics back into monomers, which can, in turn, be repolymerized into plastic products after purification. [13]This circular recycling strategy can maximize the carbon utilization efficiency in plastic management.Unfortunately, owing to the strong chemic inertia of polymers, the thermolysis process of plastics waste usually demands high temperatures (>300 °C) and costly metal complex catalysts to narrow the product distribution. [14]In this case, its large-scale exploitation and economic feasibility have not been demonstrated.Unlike thermolysis, photocatalytic recycling of polymers does not involve high heat and pressure, and thus has attracted wide attention in recent years. [15]In the photocatalytic process, tremendous electrons (e À ) and holes (h þ ) can be generated in the photocatalyst under light irradiation.The energetic photogenerated carriers can initiate various redox reactions under mild conditions. [16]A number of photocatalysts, including CdS/CdO x , [17] MoS 2 /CdS, [18] and g-C 3 N 4 /Ni 2 P, [19] have been developed to utilize PET as a hole scavenger to generate hydrogen from water under visible light.There are also reports that photocatalysis can upcycle various polyolefins into high-value carboxylic acids in a visible-light-driven aerobic oxidation process. [20]However, low capture efficiency of solar energy and recombination of photoinduced charge carriers limit the catalytic performance. [21]Especially for polyolefins, the plastics that account for the largest fraction of global polymer production and waste, the production rate of the upcycling products is usually at the level of μmol g À1 h À1 .In addition, the copious CO 2 emissions in the photo-oxidation reaction of plastics waste are causing concerns.
Recently, photothermal catalysis has emerged as a technique that integrates the merits of photocatalysis and thermolysis by synergistically utilizing the thermal and photo energies. [22]ompared with the energy-intensive thermocatalysis, the photothermal catalysis powered by renewable solar energy presents a green and sustainable way to drive catalytic reactions.Moreover, the photothermal process can directly convert the light into heat, which can maximize the solar energy utilization compared with the typical photocatalytic technologies.Hence, the photothermal catalysis can render excellent catalytic performance even under moderate conditions, which is highly attractive for industrialization.With the integration of the light and heat in the photothermal catalysis, the variation in the reaction pathway might transform the products into higher value chemicals, making this process even more intriguing.The unique advantages of photothermal catalysis have been investigated in the fields of water splitting, nitrogen fixation, and CO 2 reduction, but its application in upcycling of plastic wastes has not yet raised enough attention. [23]In fact, some recent works have demonstrated encouraging promoting effects and great potential of photothermal catalysis in the plastic upcycling, featuring excellent conversion efficiency and mild reaction conditions.In this context, it is necessary to summarize these advancements of photothermal catalysis for the progress of plastic waste upcycling techniques.In this work, we 1) briefly illustrate the traditional thermocatalytic and photocatalytic routes for upcycling of plastics and their limitations; 2) outline the fundamental reaction mechanisms and catalyst design principles of photothermal catalysis for upcycling plastic waste; 3) summarize and compare the recently reported photothermal catalytic systems for upcycling plastic waste; and 4) appraise current challenges and future trends in this field.

Thermocatalytic Plastics Upcycling
The thermocatalytic upcycling of plastics is generally carried out at high temperatures or high pressures, breaking down the stable framework of plastics into high-value products.9b] Notably, the repeat units and intermonomer linkages in the chain structures primarily determine the chemical properties of polymers, thus they reflect the difficulty of catalytic recycling.Polymers in which the monomers are linked by C─C bond, including polyethylene (PE), polypropylene (PP), and so on, constitute the largest part of polymer production and waste. [7,24]13a] Thanks to their high conversion efficiency, the thermocatalytic upcycling of plastic waste has been extensively reported and even industrialized. [25]Recently, Chen et al. [26] proposed the cracking of polyethylene into valuable hydrocarbon fuels over Fe-modified Pt/Al 2 O 3 at 330 °C and 24 h.Nakaji et al. [27] upcycled the low-density polyethylene (LDPE) into liquid fuel (C 5 -C 21 ) and wax (C 22 -C 45 ) over Ru/ CeO 2 with high yields.It was found that the β zeolite and silicalite-1-encapsulated Pt nanoparticles (Pt@S-1) could efficiently hydrocrack LDPE into naphtha with a yield of 89.5%. [28]Solid waxes and olefins were gained from the pyrolysis of polyolefins at 400-600 °C. [29]However, violent reaction conditions such as high temperatures and pressures are generally required to activate the extremely inert C─C bonds. [30]15a]

Photocatalytic Plastics Upcycling
Polyesters, most commonly referring to PET, are a class of widely used plastics formed by polar C-O linkages between monomers. [31]25a] Photocatalysis generally can be divided into three fundamental processes: 1) generation of electron-hole pairs via photoexcitation of semiconductor, 2) separation and transport of the photoexcited electrons and holes, and 3) surface chemical reactions driven by energetic charge carriers. [33]Photocatalytic plastic conversion reactions are usually classified according to their target products as photocatalytic degradation and photocatalytic upcycling.In photocatalytic degradation, the plastics serve as the waste to be mineralized into CO 2 and H 2 O in the presence of O 2 , yielding less economic benefits. [34]On the contrary, the photocatalytic upcycling highlights the potential value of plastic waste as a hydrocarbon resource to generate valuable fuels, organics, and materials.Reisner et al. [17] reported a photocatalytic system for coupling polyester photoreforming and H 2 evolution using a CdS/CdO x quantum dot catalyst.Various plastics including PLA, PET, and PUR are oxidized by excited holes to carbonate ions and organic products, while excited electrons reduce H 2 O to produce H 2 .Photoreforming of PET yielded H 2 with a respectable activity of 3.42 mmol 2 g CdS À1 h À1 .It is important to note that the polyesters are polar polymers linked by carbon-oxygen bonds, which facilitates the alkaline hydrolysis and the subsequent photoreforming reactions.With facile pretreatment by NaOH aqueous solution, the photoreforming performance for PET was increased fourfold to 12.4 mmol H 2 g CdS À1 h À1 .[19] Nevertheless, the photocatalytic conversion of nonpolar polyolefins remains a challenge owing to the inherent chemical inertness of C─C bond.The reported works about photocatalytic upcycling of polyolefins usually exhibit small product generation rate at the level of μmol g À1 h À1 and extremely low product yield (Table 1, entries 1, 2). [20]Besides, the photocatalytic plastics recycling usually requires a very positive potential of photogenerated holes and a very negative potential of photogenerated electrons to generate free radicals or drive the redox reaction. [35]herefore, the reported work usually employed wide bandgap semiconductors with absorption edges smaller than 500 nm.
As the dominating proportion of the light in the solar spectrum is in the visible (400-700 nm) and near-infrared (700-2500 nm) regions, the narrow-band absorption characteristics of photocatalysts have become an inherent limit to their catalytic reactivity. [21,36]

Photothermal Catalytic Upcycling of Plastic Wastes
The integration of heat and light in photothermal catalysis systems offers a new strategy to maximize solar energy utilization efficiency (Figure 1c).In the photothermal system, the catalyst can effectively capture full solar spectrum through a variety of ways: 1) Interband transition of semiconductor, which absorbs high-energy photons to generate excited charge carriers. [16]his strategy is also a fundamental process of photocatalysis.However, it is worth noting that the photogenerated charge carriers in the photothermal catalysis can not only directly drive the surface redox reactions, but also produce local heat through the nonradiative relaxation process. [37]; 2) Localized surface plasmonic resonance (LSPR) effect, which presents strong and broad absorption bands across the UV-vis-NIR region to generate hot electrons. [38]38b] Then, these hot electrons can inject into the electron-accepting orbitals of adsorbates via indirect or direct electron transfer.38c] 3) Photothermal effect, which transforms low-energy NIR photons into thermal energy and promotes the catalytic process. [39]When the incident photon energy matches the electronic transition energies of molecules, the electrons in the ground state can be excited into the higher energy orbitals.Most commonly, the electron excitation occurs from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) under illumination.The electrons then relax from the excited state to the ground state via the electron-vibration coupling, resulting in the heating of molecules.From the perspective of photothermal catalysis, the effect usually occurs in polymer and carbon materials (generally with black color).These materials have plentiful conjugated π bonds, which allow electrons to jump from π to π* orbitals even under low-energy NIR light. [37]Thus, they can efficiently transform the light energy into heat energy even with low irradiation energy.
In the photothermal catalysis, light and thermal effects can work separately or collectively.Generally, depending on the specific reaction pathway(s) of catalytic reaction, the photothermal catalysis systems can be classified into three categories, i.e., thermal-assisted photocatalysis, photo-assisted thermocatalysis, and photothermal coupling catalysis. [40]

Thermal-Assisted Photocatalysis
23c] The thermal energy introduced into the system primarily serves to decrease the reaction activation energy, accelerate the transfer rate of reactants/intermediate, and promote the transport of charge carriers.The heat could either be produced by light irradiation through the photothermal effect or supplied by the external heating.
The method of thermal-assisted photocatalysis via external heating has been executed to upcycling plastic waste.Soo et al. [41] designed a vanadium complex as photocatalyst to upcycle polymers through a photo-driven C─C bond cleavage reaction.Although the concept of photothermal catalysis was not mentioned, the thermal energy was introduced into the photocatalytic process to fully dissolve the polymers and speed up the reaction.With the photocatalyst and visible light irradiation for 7 days, the hydroxyl-terminated polyethylene can be completely converted at 85 °C, resulting in a formic acid yield of 5 AE 2%.For comparison, the typical photocatalytic upcycling of polyethylene presents extremely low product yield (Table 1, entries 1, 2).The result indicates the obvious merits of photothermal catalysis compared with the conventional photocatalysis in terms of efficiency of plastic upcycling.Pan et al. [42] realized an efficient and unique recycling of PE through a photothermal C-H activation and modification Thermo-a-Photo a) Vanadium complex [41]  Photo-a-Thermo Co-O 5 single-site [50] PET Simulated sunlight (740 mW cm À2 ) Thermocatalysis Co-O 5 single-site [50] PET / 180 3 18.2% BHET 12.7% Photothermal-co c) Ni-Ti-Al [53] LDPE 4 kW solar simulator 500 Thermo-a-Photo TBADT [42] LDPE Sunlight 110 48 16 600 3000 Thermo-a-Photo TBADT [42] HDPE Sunlight 110 48 18 600 1400 Thermo-a-Photo TBADT [42] Plastic film Sunlight 110 48 22 200 2100 Thermo-a-Photo TBADT [42] Trash Photo-a-Thermo AgNPs [48] PECA 445 nm (600 mW cm À2 ) 140 d) 1 %26.5 %10.5 a) Defined as thermal-assisted photocatalysis; b) Defined as photo-assisted thermocatalysis; c) Defined as photothermal coupling catalysis; d) Heated by the photothermal conversion from the incident light.
synergy (Figure 2a).They grafted the diisopropyl azodicarboxylate (DIAD) onto the PE backbone under sunlight at 110 °C using tetrabutylammonium decatungstate (TBADT) as the photocatalysts.The introduction of polar DIAD groups accelerated and assisted the chain scission of PE.Through optimizing the amount of DIAD or TBADT, the number-average molecular weight (M n ) of PE dramatically decreased after the photothermal treatment.Moreover, a variety of real-world PE wastes were successfully degraded into low molecular weight PE waxes with tunable polarity (Table 1, entries 12-15).The resulting PE-graft-DIAD wax was subsequently employed as a blending compatibilizer between PE/PP and starch (Figure 2b).Compared to typical thermal catalysis, the photothermal reaction allows milder temperature conditions of 110 °C and maintains high conversion efficiency.The above results suggest that thermal-assisted photocatalysis is an effective strategy to upcycle chemically inert plastic waste.Unfortunately, the thermal-assisted photocatalysis heated by the photothermal effect, which implies higher energy efficiency, has not been used in the upcycling of plastics.

Photo-Assisted Thermocatalysis
The second category is photo-assisted thermocatalysis, in which thermal catalysis primarily drives the whole reaction.The incident light is converted into heat via photothermal effect and thus raises the temperature of surroundings. [43]For metal nanoparticles, the hot charge carriers generated by the photoinduced LSPR effect can decay and release their energy to heat the surface adsorbates, thus accelerating the catalytic reaction. [44]Silver nanoparticles (NPs) are among the most extensively applied nanomaterials, and possess the advantages of facile fabrication, relatively inexpensive, and tunable size. [45]Moreover, the AgNPs typically absorb light in the range of 320-510 nm, containing about half of the visible light range, thereby ensuring effective capture of solar energy. [46]In 2019, Firestone et al. [47] embedded dilute AgNPs into LDPE, and performed the photothermaldriven thermal-oxidative degradation of plastic waste for the first time.The AgNPs were prepared by a reverse micellar reduction of silver nitrate, with an average particle size of 17 nm.As shown in Figure 3a, the strong peak centered at 430 nm is attributed to the plasmon-mediated processes of AgNPs.The cobalt (II) stearate was employed as the catalyst to degrade plastics.Subsequently, the AgNPs at a concentration of 0.002 wt% and catalyst at a concentration of 1 wt% were incorporated into the LDPE powder, and the mixture was melt-pressed to a polymer nanocomposite film (PE-AgNP-Co).The reaction pathway of thermooxidation of polyolefins has been established in a previous report (Figure 3b), [9a] in which various oxygen-containing groups, such as carbonyl groups, hydroxides, and backbone modified groups, are produced and monitored by Fourier transform infrared (FTIR) spectroscopy and UV-vis absorption spectroscopy.As shown in Figure 3c,d, under blue light irradiation, the PE-AgNP-Co sample exhibited stronger carboxylic acid FTIR signal and ketone UV-vis signal than other samples, indicating an increased degradation rate.This result verifies the photothermal effect of AgNPs forcefully boosted the thermal oxidation process of LDPE.The photothermal degradation rate of PE-AgNP-Co is equivalent to placing the sample into a 60 °C oven.Besides, this mechanical property of composites depolymerization decreased due to the photothermal catalytic depolymerization process (Table 1, entry 16).
In a follow-up work, [48] Huang and co-workers applied the photothermal effect of metal nanoparticles in the field of  [42] Copyright 2022, Wiley-VCH GmbH.polyethylcyanoacrylate (PECA) degradation.The photothermal heating via the LSPR effect of AgNPs was used to drive the thermo-oxidative depolymerization of PECA and starch composites.The depolymerization process occurs dominantly in the surrounding of AgNPs, which formed mesoscopic defect sites in the interior of PECA-starch composite film.Interestingly, these defects are associated with the decline of tensile strength (Table 1, entry 17).Therefore, this mechanical property of composites can be modulated through a chemical depolymerization process.The authors emphasized that through these relatively unusual properties, the composites are effective systems to confirm the existence of heterogeneous temperature field in the photothermal heating process.Besides, the heterogeneous heating phenomenon is significant for exploring new methods for management of plastic waste.
Beyond the photothermal degradation approach, the photothermal upcycling of plastics has gained recent scientific attention.The upcycling approach treats the plastic waste as neglected hydrocarbon resources to produce recoverable and valuable chemicals, rather than waste that needs to be mineralized into CO 2 .Liu et al. [49] reported a multiwalled carbon nanotubes premodified by polydopamine (CNT-PDA) material for the photothermal catalytic upcycling of PET.The CNT-PDA was used as light absorber to convert the photon energy to heat for driving the thermocatalytic transformation of PET to bis-2-hydroxyethyl terephthalate (BHET), mediated by a cholinium phosphate ([Ch] 3 [PO 4 ]) catalyst.The mechanism for the cleavage reaction of ester groups in the PET chains is proposed and displayed in Figure 4a.The ethylene glycol was activated by the [Ch] 3 [PO 4 ] catalyst and subsequently formed intramolecular hydrogen bonds.The carbon of the ester group was then attacked by the neighboring nucleophilic oxygen atom, thus promoting the glycolysis of PET.Benefiting the unique localized solar heating effect of CNT-PDA, the photothermal system allows a low depolymerization temperature (150 °C) and a higher upcycling efficiency than thermal catalysis (Figure 4b).The PET substrate was completely converted after 2.5 h of irradiation, and a remarkable yield of up to 80% for BHET product was reached after increasing the reaction time.Besides, the photothermal catalysis exhibited superior performance than conventional thermal catalysis with the same catalyst (Table 1, entries 4, 5).A large-scale attempt of the photothermal catalytic polyester upcycling was demonstrated under focused sunlight, and about 50 g of BHET was produced after 45 min of reaction (Figure 4c).Different from the photoreforming strategy, the photothermal system not only presents more excellent activity and selectivity, The "light" condition means the photothermal heating at an average temperature of about 55 °C.Reproduced with permission. [47]opyright 2019, IOP Publishing.
but also saves the effort of the alkaline pretreatment to hydrolyze the polymers.In comparison with conventional thermocatalysis, the solar energy-driven heating system can save the power consumption by 3.7 GJ and CO 2 emissions by 0.4 tons per ton of PET converted (Figure 4d,e).
In the very recent report, they deposited the cobalt single-site catalysts (Co SSCs) on the surface of CNT-PDA, and employed the composites to photothermally catalyze the upcycling of polyesters. [50]The PDA decorated on the surface of CNTs has a strong coordination ability to cobalt ions, thus rendering the Co species uniformly dispersed in the form of isolated individual atoms (Figure 5a,b).In the k 3 -weighted extended X-ray absorption fine structure (EXAFS) analysis (Figure 5c), the fitting results of samples presented that the coordination of Co atoms was unsaturated Co-O 5 form, and the bond lengths between Co and neighboring oxygen were 1.98 Å.In the typical photothermal catalytic PET depolymerization process, the unique Co-O 5 single sites in Co SSCs can coordinate with the carbonyl oxygen atoms of PET.The interaction will reduce the charge density of the carbonyl carbon atom, thus facilitating the nucleophilic attacks of oxygen in ethylene glycol (Figure 5d).Therefore, the space-time yield of Co single-site anchored CNT-PDA is dramatically higher than that of ordinary catalysts.Furthermore, the photothermal catalytic upcycling of PET showed 5.4 times higher conversion and 6.6 times higher yield than those of thermocatalysis at the same apparent temperature (Figure 5e and Table 1, entries 6, 7).UV light with a wavelength of 365 nm was introduced into the thermocatalytic system to study the effect of photogenerated carriers.As displayed in Figure 5e, the incident UV light did not promote the reaction activity, which indicated that the thermal catalytic mechanism dominated the photothermal catalysis process.The superior performance in efficiency of photothermal plastic upcycling is attributed to the unique localized heating effect of CNT-PDA materials.

Photothermal Coupling Catalysis
The last strategy is photothermal coupling catalysis, which unites heat and light to drive the reaction synergistically. [51]In order to achieve high energy conversion efficiency, the incident photons are captured in various pathways.40c] Moreover, the low-energy photons with long wavelengths can produce heat by photothermal effects, thereby accelerating the reaction rate of photocatalysis or triggering the thermal catalytic reaction. [52]22a] Luo et al. [53] performed the photothermal catalytic upcycling of polyolefin waste using concentrated solar energy for the first time.As displayed in Figure 6a, they constructed a photothermal catalytic pyrolysis equipment with temperature up to 500 °C using a solar simulator.A series of composites composed of NiO (denoted as Ni), TiO 2 (denoted as Ti), γ-Al 2 O 3 (denoted as Al), and g-C 3 N 4 (denoted as CN) were prepared as the photothermal catalysts (Figure 6b).The Ni-Ti-Al exhibits broad and strong absorption in the UV and visible light regions, implying the potentials of Ni-Ti-Al material for both thermocatalysis and photocatalysis (Figure 6c).Then, the LDPE was photothermally c) Setting of outdoor tests: 1, solar concentrator; 2, customized reactor; 3, temperature monitors; 4, solar power senor; 5, humidity sensor.d) The energy saving of photothermal catalysis approach by treating 100 000 tons of PET waste per year.e) Contribution of photothermal catalysis on CO 2 and total greenhouse gas (GHG) emission reduction as compared to thermal catalytic approach.Reproduced with permission. [49]Copyright 2022, Elsevier.
upcycled with the catalysts at 500 °C to produce high-value-added liquid and gas chemicals.It is found that H 2 was the main gas component for all samples, while the contents of CH 4 and C 2-5 were relatively high for Ti-Al and CN-Al (Figure 6d,e).Furthermore, the selectivity of Ni-Ti-Al catalyst to jet fuel (C 8 -C 16 aliphatics and aromatics) was as high as 81% among the liquid products (Figure 6f ), which may originate from the good carrier transfer between Ni and TiO 2 and the LSPR effect of Ni.As a result, a H 2 yield of 34.20 mol kg plastics À1 and a remarkable jet fuel yield of 39.10 wt% have been obtained in the photothermal upcycling using Ni-Ti-Al catalyst (Table 1, entries 8, 9).In their latest research, [54] the Ni-Ti-Al catalyst was used for the photothermal catalytic upcycling of LDPE to produce CNTs and H 2 .The Ni-Ti-Al exhibited superior photothermal catalytic performance with a H 2 output of 54 mol kg plastic À1 and the CNTs output of 287 g kg plastic À1 at 700 °C (Figure 6g and Table 1, entries 10, 11).As shown in Figure 5h, the resulting CNTs displayed a multiwalled structure with an inner diameter of about 9 nm and an external diameter of about 20 nm.The mechanism study demonstrated that the transfer of photogenerated electrons between Ni and TiO 2 significantly decreased the metal-support interaction in the material (Figure 6i).It consequently achieved the moderate interaction between Ni metal and the support, which boosted the production of CNTs and H 2 in the upcycling of LDPE.
As a green and sustainable approach, the photothermal upcycling of plastic waste has achieved great progress, and a wide range of valuable products have been yielded by the abovementioned three photothermal catalytic strategies.Unfortunately, due to the multiple different variables (e.g., reaction conditions, catalytic mechanism, types of plastics, and product) in the upcycling reaction, the standardized comparison of performance of photothermal catalytic systems in different studies is difficult.A summary containing key parameters of the reported photothermal catalytic systems for upcycling of plastic waste has been presented in Table 1.In addition, a concise summary diagram based on the published results is presented in Figure 7 to help understand and compare the main characteristics of the three photothermal categories.The thermal-assisted photocatalytic process mainly proceeds through photo-driven redox reactions.Benefiting from the powerful redox ability of photogenerated electrons and holes, this approach can upcycle plastics even in mild reaction conditions, but usually exhibits low conversion rates.The photo-assisted thermocatalytic process is primarily driven by heat, and the heat is converted from incident light through the LSPR effect and the photothermal effect of materials.This approach for upcycling polymers features moderate light intensity and operating temperature.Notably, the high-efficiency and high-value upcycling of polyesters has been achieved thanks to some breakthroughs in recent years.The reported photothermal coupling catalysis study for plastic upcycling usually operates under harsh reaction conditions with complex products.The approach can directly upcycle plastic waste, even extremely stable polyolefins, into valuable products with a high conversion rate by Reproduced with permission. [50]Copyright 2022, Wiley-VCH GmbH.
means of harsh conditions.However, these harsh conditions also create an obstacle to thoroughly investigating the catalytic pathways during the plastic conversion process.

Conclusion and Outlooks
In this review, we illustrate some of the latest strategies for converting plastic waste into high-value chemicals and fuels.With the synergistic effect of heat and light, the photothermal process allows milder reaction conditions with reduced energy consumption and CO 2 emissions relative to the conventional thermocatalytic process.More importantly, thanks to the efficient solar utilization and high-energy-driven pyrolysis process, the photothermal approach exhibited extremely boosted conversion efficiency over the photocatalytic approach.Multifarious mechanisms of photothermal catalytic upcycling of plastics were elaborated in detail, including thermal-assisted photocatalysis, photo-assisted thermocatalysis, and photothermal coupling catalysis.Nevertheless, the developments are still in the early stages, and great efforts are still needed to promote the activity, selectivity, stability, and cost of the technology for large-scale applications.Below, we highlight several suggestions for the future investigations of photothermal catalytic plastic upcycling.
Figure 6.a) Schematic of the photothermal catalytic pyrolysis system.b) X-ray diffraction patterns of the pristine catalysts.Reproduced with permission. [54]Copyright 2023, Elsevier.c) UV-vis spectra of the prepared catalysts.The products of photothermal catalytic upcycling of LDPE with different catalysts: d) gas yields, e) gas compositions, and f ) classified components of liquid products.Reproduced with permission. [53]Copyright 2022, Elsevier.g) CNTs conversions and carbon yields of the Ni-Ti-Al catalyst used at different temperatures.h) TEM images of the solid mixture of LDPE and Ni-Ti-Al catalysts after photothermal catalysis.i) The proposed mechanism of the photothermal upcycling of LDPE over Ni-Ti-Al catalysts.Reproduced with permission. [54]Copyright 2023, Elsevier.
The energy conversion efficiency in the photothermal process needs to be promoted.Photothermal upcycling of plastics requires a high light intensity, usually provided by focusing sunlight through a Fresnel condenser lens.In such cases, the area of the light illumination is extremely restricted, making it difficult for photothermal catalysis to reach a high conversion/production efficiency.It is necessary to explore novel photothermal systems with high performance under low light intensity.More specifically, this is a photon absorption and photothermal conversion challenge that boils down to the task of engineering highefficiency photothermal catalysts and reactors.From a catalyst perspective, integrating different photocatalysts in the same photothermal system can provide vast opportunities for advancing the photothermal conversion efficiency.For example, drawing on experience from the tandem solar cells, we can apply semiconductors with different bandgaps to absorb and convert different regions of the solar spectrum to maximize solar energy utilization.From a reactor perspective, we need to design a scalable photothermal reactor that best captures, utilizes, and reserves the light and heat in the catalytic processes.There is a lack of research on reactor engineering compared with the catalyst study for photothermal catalysis.However, some recently reported strategies, such as photon sponges, [55] optofluidics, [56] cascade devices, [57] and heterostructure devices [43b] have achieved encouraging progress for significantly advancing the photothermal catalytic efficiencies.
The photothermal catalytic conversion of plastics should produce more high-value-added products.Currently, the majority of products generated by the photothermal oxidative degradation of polyolefins are a combination of CO 2 and H 2 O, which create the most greenhouse gas emissions and the lowest economic benefits.It is promising to extend the oxidative products from CO 2 to other more valuable oxygenic chemicals such as alcohols, carboxylic acid, and esters.The design and optimization of photothermal catalysts with tailored and well-defined active sites are crucial for amplifying product activity and selectivity.To date, limited types of materials have been reported for photothermal catalytic plastic upcycling, with many materials yet to be explored.Predictably, the large library of photothermal catalysts developed in the environment and energy fields can offer immense opportunities for boosting the plastics valorization performance.The exploration of cocatalyst loading should be given particular attention, which is less studied but plays a vital role in product selectivity.In addition, the catalytic processes of plastic upcycling are like those of biomass conversion, as both substrates are obstinate polymeric materials.The experience in bond cleavage chemistry of biomass molecules can also enlighten the plastic conversion process.Therefore, the development of photothermal catalytic plastic upcycling can be accelerated by learning the success of biomass valorization.
In-depth mechanism investigations should be conducted.The mechanism understanding in the photothermal catalytic upcycling of plastic waste is a formidable task owing to the system's complexity, but important for the future design of photothermal materials and reactors.Advanced in situ techniques are helpful in offering critical insights into catalytic mechanisms and constructing the structure-mechanism-performance relationship.Some in situ characterizations such as FTIR, Raman spectroscopy, and X-ray absorption spectroscopy employed to monitor the reactive intermediates and identify the actual active sites for other photothermal catalytic systems should, in principle, be applicable to photothermal conversion of plastics as well, though the in situ cells may have to be reconfigured for some cases that require violent reaction conditions.The photothermal catalytic reaction can proceed through photochemical and/or thermochemical pathways.The two types of reaction pathways probably undergo different intermediates and can be ascribed to different reaction dynamical models.Therefore, the in situ techniques with the ability to trace the conversion procedure in real time are vital to investigate the detailed mechanism of photothermal plastic upcycling.The role of light (whether it is photochemical or just light-induced heating) can also be elucidated by conducting systematic studies, varying the illumination intensity, the light wavelength, and the heating temperatures for determination of the activation energy with light on and off.
Besides, computational approaches such as density functional theory (DFT) calculations can validate the experimental results to gain deeper understanding of reaction pathways.The application of theoretical analysis in the field is infrequent, probably attributed to the polytropic carbon skeleton of plastics and the intricate degradation process.Recently, machine learning has emerged as a forceful tool to predict high-performance catalytic systems under various harsh conditions.As for photothermal catalytic plastic upcycling, it is also reasonable to employ machine learning for screening more efficient catalysts and exploring the conversion mechanism.Overall, the development of photothermal catalytic plastic upcycling can be accelerated by the synergistic effects of in situ characterizations, experimental observations, and theoretical estimations.
Photothermal catalytic upcycling strategies that can treat polymer mixtures should be explored.Most real-world plastic wastes are mixtures of multiple plastics with widely varying chemical compositions.Therefore, from the perspective of large-scale application of the technology, photothermal catalytic systems should be able to convert a variety of plastics into a specific class of chemicals.Given the extreme complexity of multiple plastic systems, the photothermal catalysts face more severe challenges in selectivity, activity, and stability compared with single plastics.The coupling of photothermal catalysis with other catalytic technologies is worth considering.For example, plastic waste can be depolymerized into small molecules through microwave or enzymes processing, and then photothermal catalytic technologies come in to efficiently upcycle those small molecules.Moreover, plastics sorting appears to be a way to address the obstacle.The conventional manual sorting method is resultful but too inefficient.There is a demand to develop automated separation technologies based on the differences in the mechanical, optical, and electronic properties of plastic waste.
Plastic waste pollution is clearly a challenging global issue that will require relentless efforts.Nevertheless, drawing on the successful experience in other photothermal catalytic techniques (e.g., for CO 2 catalysis), we believe that with continued optimization of efficiency, durability, reactor design, and economic feasibility, the photothermal catalytic plastic upcycling approaches can play a pivotal role in tackling this problem.

Figure 1 .
Figure 1.Schematic diagram of the upcycling of plastic waste via different strategies.a) Thermocatalytic upcycling of polyolefins via cleavage of C─C bonds.b) Upcycling of polyesters coupling water splitting via photocatalysis.c) Upcycling of different plastic waste via photothermal catalysis.

Figure 2 .
Figure2.a) The proposed mechanism of the photothermal upcycling of PE using the TBADT as the photocatalysts.b) Schematic for upcycling PE waste into blending compatibilizers.Reproduced with permission.[42]Copyright 2022, Wiley-VCH GmbH.

Figure 3 .
Figure 3. a) UV-vis absorption spectra of AgNPs with a concentration of 0.5 nM in toluene (inset-top: transmission electron microscopy (TEM) image of AgNPs; bottom: the diameter distribution of AgNPs).b) A brief diagram of the reaction path and corresponding products of the thermo-oxidation of polyolefins.The intensity of c) carboxylic acid FTIR peak and d) ketone UV-vis peak for the different LDPE samples.The "oven" condition means external heating of 55-60 °C.The "light" condition means the photothermal heating at an average temperature of about 55 °C.Reproduced with permission.[47]Copyright 2019, IOP Publishing.

Figure 4 .
Figure 4. a) The proposed mechanism of the photothermal upcycling of PET.b) Conversions and yields in photothermal and thermal upcycling of PET.c) Setting of outdoor tests: 1, solar concentrator; 2, customized reactor; 3, temperature monitors; 4, solar power senor; 5, humidity sensor.d)The energy saving of photothermal catalysis approach by treating 100 000 tons of PET waste per year.e) Contribution of photothermal catalysis on CO 2 and total greenhouse gas (GHG) emission reduction as compared to thermal catalytic approach.Reproduced with permission.[49]Copyright 2022, Elsevier.

Figure 5 .
Figure 5. a) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and b) aberration-corrected HAADF-STEM image of Co SSCs.c) Fourier transformation of k 3 -weighted EXAFS of corresponding materials.d) Integrated features of Co SSCs and the proposed mechanism of the photothermal upcycling of PET over Co SSCs.e) Conversions and yields of upcycling of PET over Co SSCs under different reaction conditions.Reproduced with permission.[50]Copyright 2022, Wiley-VCH GmbH.

Figure 7 .
Figure 7. Functioning principles and main characteristics of three categories of photothermal catalytic systems for upcycling of plastic waste.

Table 1 .
Summary of catalytic performance of different strategies of plastic upcycling.