Highly Selective Conversion of CH 4 to High Value-Added C 1 Oxygenates over Pd Loaded ZnTi-LDH

The selective oxidation of methane to high value-added liquid oxygenated compounds under mild conditions is of great signiﬁcance to promote the eﬃcient utilization of the carbon source, but it also faces the dilemma of low activity and over-oxidation. Here, ZnTi-layered double hydroxides (LDH)-A200 photocatalysts with Pd loading are prepared to achieve eﬃcient oxidation of methane, with O 2 as an oxidant under ambient condition. The highest generation rate of C 1 liquid products (methanol and formaldehyde) reaches 4924.47 μ mol g − 1 h − 1 with a selectivity close to 100% over 0.5Pd-ZnTi-LDH-A200, which is 20 times higher than that of bare ZnTi-LDH-A200. The photochemical results show that the modiﬁed photocatalysts present much higher generation and separation eﬃciency of electron-hole pairs. In situ X–ray photoelectron spectroscopy indicates that Pd nanoparticles are the hole acceptor, which is beneﬁcial to charge separation in the photocatalysis. Furthermore, electron spinresonance spectroscopy and temperature–programmed–desorption analysis prove that Pd loading is helpful to the adsorption of methane and oxygen on the surface of ZnTi-LDH-A200, promoting the production of reactive oxygen species and activation of methane. All these factors work together to promote the eﬃcient conversion of CH 4 to high value-added C 1 oxygenates.


DOI: 10.1002/aenm.202301118
to synthesize high value-added chemicals. [1][2][3] Traditionally, the industrial conversion of CH 4 generally through some indirect reforming methods, which are high-energy consumption due to the large C-H bond dissociation energy of CH 4 . [4][5][6] Considering the increasingly serious energy problems nowadays, developing direct and energy-saved methods to convert CH 4 to high value-added chemicals is of great significance. [7][8][9] Limited by excessive oxidation, it is difficult to achieve both high conversion rate and high selectivity of CH 4 conversion at the same time. [10,11] Therefore, direct oxidation of CH 4 to high value-added chemicals still faces great challenges, [12,13] and it is necessary to develop new methods to realize both high conversion rate and high selectivity simultaneously under mild conditions. By using the energy of sunlight, photocatalysis can activate inert molecules such as CH 4 under moderate conditions. [14,15] Theoretically, the excited photons can generate various reactive oxygen species (ROS), which also facilitates C-H dissociation of CH 4 molecules. [16][17][18] However, the photocatalytic activity for CH 4 conversion is very low because of the weak adsorption of CH 4 on the photocatalysts surface. With this consideration, layered double hydroxides (LDHs) attracted scientists' attention due to its distinctive 2D (2 dimensional) layered structure and large surface area, which may result in more active site exposure [19][20][21][22][23] and increase adsorption of small molecules. [24,25] As reported, LDHs have been widely used in photocatalytic ammonia synthesis and water splitting. [26][27][28][29] For example, the etched ZnCr-LDHs (ZnCr-1 h) was synthesized to be efficient for fixing N 2 and produce NH 3 at a rate of 33.19 μmol g −1 h −1 under ultraviolet and visible light radiation. [30] However, the efficiencies of LDHs are still very low in these reports because of the main disadvantages of low optical utilization and severe electron-hole pair recombination. [31,32] Hence, further improving the charge separation of LDHs to promote its performance is meaningful. Besides, increasing the adsorption of reactants by appropriate co-catalyst loading can also enhance the photocatalytic activities. Meanwhile, co-catalyst loading can promote the separation of photogenerated electrons and holes, which provides important guidance for the rational design of photocatalysts. [33][34][35]  In this work, the photocatalytic activity of ZnTi-LDH for CH 4 conversion to high value-added chemicals was investigated and its activity was further improved by Pd loading. Using O 2 as the oxidant in an aqueous solution, the highest yield of CH 4 conversion rate is achieved at 0.5Pd-ZnTi-LDH-A200, which is 4924.47 μmol g −1 h −1 with 100% selectivity to C 1 oxygenates, being 20 times higher than that of bare ZnTi-LDH-A200. The sample characterization and mechanism studies indicate that Pd loading significantly increases the light absorbance, as well as the generation and separation rates of photocarriers, which result in the improvement of photocatalytic generation of ROS. Besides, Pd loading also provides more exposed active sites for CH 4 and O 2 adsorption. All these factors synergistically improve the photocatalytic efficiency of ZnTi-LDH for CH 4 conversion.

Characterizations
The crystal structures of the prepared photocatalysts were analyzed by X-ray diffraction (XRD) patterns. The XRD spectra of ZnTi-LDHs are displayed in Figure 1a, which show similar diffraction patterns to the previous report. [36] The main diffraction peaks at 2 = 13.2°are attributed to (003) crystal plane of ZnTi-LDH, while the other peaks at 24.3°, 28.3°, 32.9°, and 36.2°a re assigned to (006), (012), (101), and (009) crystal planes, respectively. After being calcined at low temperature of 200°C, the XRD pattern of LDH are well maintained with the reduced peak intensity of 2 = 13.2°, which may be caused by the partial collapse of the crystal lattice along with the loss of some inter-layer small molecules. [36] By loading Pd on the ZnTi-LDHs with and without calcination, no new diffraction peaks appeared, indicating the low amount and good dispersion of Pd nanoparticles on the surface of ZnTi-LDH (Figure 1a). 2D-layered structure of ZnTi-LDH was observed by scanning electron microscope (SEM) image in Figure 1b and N 2 adsorption-desorption isotherms in Figure S1, Supporting Information. It can be seen that all the modified and unmodified samples show typical IV isotherms and H3-type hysteresis loops (P/P 0 > 0.4), indicating the mesoporous structures with slit like pores comprised of aggregates (loose assemblages) of platelike particles. [37] The specific surface area (S BET ) was calculated by Brunauer-Emmett-Teller (BET) equation. The S BET of ZnTi-LDH, ZnTi-LDH-A200, 0.5Pd-ZnTi-LDH, and 0.5Pd-ZnTi-LDH-A200 are 89.8, 97.7, 84.0, and 87.7 m 2 g −1 , and there is no significant differences, indicating that it's not the main factor to improve the catalytic performance of CH 4 transformation. [38] From Figure S2, Supporting Information, we can see that all ZnTi-LDHs samples retain the nanosheet structure with the size of about 300-500 nm. [31] Enlarging the transmission electron microscope (TEM) image of 0.5Pd-ZnTi-LDH-A200 in Figure 1c, the lattice fringes with d = 0.25 nm belong to the (009) crystal plane of LDHs can be clearly characterized by high-resolution transmission electron microscope (HRTEM) in Figure 1d. In addition, the elemental distribution of 0.5Pd-ZnTi-LDH-A200 was also analyzed by energy-dispersive spectroscopy mapping (EDSmapping) to indicate the uniform dispersion of Pd on the surface of ZnTi-LDH (Figure 1e). [39] The light absorption properties of the photocatalysts were studied by UV-vis diffuse reflectance spectra (UV-DRS), which is displayed in Figure 2a. The superimposed increment of the absorption at the range of 400-800 nm suggested the defective introduction and Pd loading for the sample of 0.5Pd-ZnTi-LDH-A200. Besides, according to the Tauc-plot curves, [40] the absorption band edge of ZnTi-LDH also shows some red shifts by the modification of single calcination, Pd loading or both ( Figure S3, Supporting Information). Then, the prepared ZnTi-LDHs were characterized by Fourier transform infrared (FT-IR) spectra, which clearly shows that there are no obvious differences between the spec-tra of unmodified and modified ZnTi-LDH (Figure 2b), meaning that the modification processes did not change the structure of ZnTi-LDH. The broad peaks at 3200-3400 cm −1 are originated by the bonded and free O-H vibration on the surface of laminar layers, [41] while the peaks at 1505 and 1381 cm −1 are attributed to the vibrations of carbonate ions between the layers. [42] The element components of Pd-ZnTi-LDH-A200 were analyzed by X-ray photoelectron spectroscopy (XPS), and the results are displayed in Figure S4, Supporting Information. The binding energies of photoelectrons of Ti 2p 1/2 and Ti 2p 3/2 in Figure S4a, Supporting Information, are located at the binding energies of 464.1 and 458.5 eV with the interval of 5.6 eV, and no obvious peaks belong to Ti 3+ are observed. [41] The binding energies of photoelectron of Zn 2p 1/2 and Zn 2p 3/2 in Figure S4b, Supporting Information, are located at 1044.4 and 1021.4 eV with the interval of 23.0 eV. [39] These results are in good agreement with previously reported values, indicating the successful preparation of ZnTi-LDH photocatalyst.
To further investigate the generation and separation of photo charge carriers, the photocurrent density and photoluminescence of the ZnTi-LDHs were performed. The positive photocurrent densities in Figure 2c mean that ZnTi-LDH is an n-type semiconductor. After modification, the photocurrent density of ZnTi-LDH was significantly improved, suggesting the increased generation of photocarriers. Meanwhile, the luminescence density of ZnTi-LDH was also remarkably depressed (Figure 2d), indicating the efficient reduction of charge recombination. Moreover, the transient fluorescence spectra of ZnTi-LDHs were also tested and fitted in Figure S5, Supporting Information, where 1 and 2 are the decay times of the photocarriers, 1 and 2 are the corresponding coefficients. The fast decay component 1 represents the radiation emission generated by direct inter-band exciton recombination, and the slow decay component 2 involves the radiation emission originated by indirect recombination of excited electrons and valence band holes. It could be obviously founded from Table S1, Supporting Information, that both 1 and 2 of 0.5Pd-ZnTi-LDH-A200 ( 1 = 1.18 ns, 2 = 6.19 ns) are slightly larger than those of ZnTi-LDH ( 1 = 1.04 ns, 2 = 5.01 ns). The average lifetime of 0.5Pd-ZnTi-LDH-A200 ( = 3.27 ns) calculated by = ( 1 1 2 + 2 2 2 )·( 1 1 + 2 2 ) −1 is also longer than that of ZnTi-LDH ( = 2.78 ns), suggesting more efficient separation of photocarriers by 0.5Pd-ZnTi-LDH-A200 than that by ZnTi-LDH. [43] The higher generation rate, lower recombination rate and longer lifetime of photocarriers over 0.5Pd-ZnTi-LDH-A200 proposes a potentially higher photocatalytic performance.

Photocatalytic CH 4 Conversion
The photocatalytic performance of Pd-ZnTi-LDHs-A200 was investigated by the conversion of CH 4 to C 1 oxygenates at ambient temperature and under 2.0 MPa pressure, with O 2 as the oxidant in water. In this work, the produced C 1 oxygenates with high added value from CH 4 conversion were detected by NMR and determined to be mainly CH 3 OH and HCHO, while CO 2 was considered as the by-product of excessive oxidation. Specifically, 13 CH 4 isotope labeling experiments were first performed to qualitatively confirm the liquid products. In 1 H-NMR spectrum of Figure 3a, the peak of chemical shift at 3.2 ppm is attributed to CH 3 OH, [33] and that at 2.5 ppm is the internal standard DMSO. [34] The 13 C-NMR spectrum was acquired by using 0.4 MPa 13 CH 4 as the reactant. The peaks in Figure 3b with the chemical shifts of 81.4 and 48.5 ppm are attributed to 13 CH 3 OH and HO 13 CH 2 OH (methylene diol, the main form of HCHO in aqueous solution), respectively. [44] The above NMR results not only determine that the liquid product of the reaction contains CH 3 OH and HCHO, but also confirm the carbon source of liquid products is mainly derived from CH 4 . Interestingly, the unstable product of CH 3 OOH were not detected by NMR, different to our previous reports. [33,45] This may be because of the strong adsorption of ·OOH or CH 3 OOH on the surface of Pd-ZnTi-LDH-A200, resulting in the fast reduction of them to ·OH or CH 3 OH. [13,46,47] The catalytic activities of the prepared photocatalysts were evaluated by quantitatively measuring the generation rates of C 1 oxygenate products. The generation of liquid products CH 3 OH and gas product CO 2 were quantified by GC equipped with TCD and FID detectors, while the generation of HCHO was quantified by a colorimetric method [48] with a standard curve ( Figure S6, Supporting Information). In Figure 3c, the original ZnTi-LDH without calcination and Pd loading shows very poor performance for CH 4 conversion with only a slow rate of HCHO production (299.95 μmol g −1 h −1 ). It is widely reported that co-catalyst loading is beneficial to improve the activity of photocatalysts, [40,49] herein Pd was photo deposited on the surface of the prepared ZnTi-LDH. However, the activity improvement for C 1 products is very limited by Pd loading, which is only 1.8 times higher (557.13 μmol g −1 h −1 ) than the unloaded ZnTi-LDH. This may be because that the hydroxyl group on the surface of ZnTi-LDH is not conducive to Pd loading, [38] so we calcined ZnTi-LDH at low temperature of 200°C in order to partially remove the surface hydroxyl group, and named the calcined sample as ZnTi-LDH-A200. Comparatively, the CH 4 conversion activity of the calcined ZnTi-LDH is not significantly improved. Then, Pd nanoparticles were photo deposited on the surface of ZnTi-LDH-A200 and obtained a surprising C 1 oxygenates generation rate (4924.47 μmol g −1 h −1 ) for CH 4 conversion, while tiny overoxidation production of CO 2 was produced (inset of Figure 3c). The experimental results in Figure 3d show a volcanic trend relationship between the loading amount of Pd and C 1 oxygenates production rate. The highest C 1 oxygenates generation rate is achieved at 0.5Pd-ZnTi-LDH-A200 with the selectivity of ≈100%, which is 20 times higher than that of bare ZnTi-LDH-A200. The C 1 oxygenates production rates of all the experiments are listed in Table S2, Supporting Information, as well as the control experiments.
Although other noble metal (Au, Ag, and Pt) loading can also improve the photocatalytic activity of ZnTi-LDH, there is still a big gap compared with Pd loading (Figure 3e). This may be because of different role of Pd for the photocatalytic activity enhancement. Here, Pd likely acts as the hole acceptor, [45,49] while Au, Ag and Pt act as electron acceptor, [50] which will be discussed later. Most importantly, the activity of used photocatalysts for highly selective CH 4 conversion can be recovered by simple calcination for 3 h at the temperature of 200°C. About 85% of the activity is kept after 4 cycles' CH 4 conversion reaction. And it can be seen from Figure S7, Supporting Information, that the XRD pattern of the selected photocatalyst does not change significantly after CH 4 conversion reaction, while its micro-structure remains the same after the CH 4 conversion in Figure S8, Supporting Information, indicating its good stability during the photocatalytic CH 4 conversion reaction. Furthermore, the photocatalytic experiments of CH 4 conversion under the light with different wavelength of 365, 400, 420, and 450 nm were performed to study the relationship between the photocatalytic activity and light wavelength. The results in Figure S9, Supporting Information, indicate that the photocatalytic activity of the selected catalyst is strongly related to the wavelength of the light we used.

Mechanism Study
CH 4 temperature-programmed desorption (CH 4 -TPD) experiments were carried out to evaluate the adsorption of CH 4 on the surface of different ZnTi-LDHs. Before TPD analysis, the TG spectrum were collected and shown in Figure 4a. The result represents that the layered structure of ZnTi-LDH would collapse [51] and metal oxide of TiO 2 and ZnO ( Figure S10, Supporting Information) would appear when the calcination temperature is higher than 250°C, which is consistent with the previous reports. [38,52] On this basis, the chemisorption of CH 4 is evaluated by the peak at 203°C in Figure 4b. To investigate the effect of the calcination temperature to the activity, we compared the photocatalytic activities of ZnTi-LDH calcined at different temperatures. The results in Figure S11, Supporting Information indicate that increasing the calcination temperature of ZnTi-LDH cannot improve the photocatalytic activity. However, the calcination of ZnTi-LDH is beneficial for the Pd loading ( Figure S12, Supporting Information), which is demonstrated to be the main reason of the photocatalytic activity enhancement in Figure 3c,d. Obviously in Figure 4b, the loaded Pd on the surface of ZnTi-LDH provides some adsorption sites of CH 4 molecules, resulting in the enhanced CH 4 conversion rate. The charge transfer from ZnTi-LDH-A200 to Pd was observed by in situ XPS spectra in Figure 4c. In dark, the peaks at 343.61 and 338.29 eV are at-tributed to 3d 32 and 3d 52 orbits of Pd 2+ . [53] Under full spectrum light irradiation, they shift to 343.74 and 338.52 eV, which confirm the hole acceptor role of Pd 2+ in the photocatalytic processes by ZnTi-LDH-A200. [45] The TPD and XPS results demonstrate that Pd acts not only as the adsorption site of CH 4 , but also as the hole acceptor of ZnTi-LDH-A200. In the EPR spectrum of 0.5Pd-ZnTi-LDH-A200 shown in Figure 4d, the signal peak at g = 2.076 is assigned to the adsorbed oxygen, indicating the efficient adsorption of O 2 molecules. [40,49] The signal peak at g = 2.028 is attributed to the oxygen radicals of O −· (Ti 4+ O 2− Ti 4+ O −· ), [42,54,55] Adv. Energy Mater. 2023, 13, 2301118 which was reported to be formed by the trapping of photoproduced holes. ERP tests indicate that Pd loading is conductive to the adsorption of O 2 molecules on the defective site of ZnTi-LDH, which may result in the generation of more reactive oxygen species and enhanced CH 4 oxidation.
Considering the generation of reactive oxygen species (ROS, i.e., O 2 −· and ·OH), the band structure of ZnTi-LDH and 0.5Pd-ZnTi-LDH-A200 was characterized by Tauc plot curve and Mott-Schottky plot curve. The band gaps (E g ) of ZnTi-LDH and 0.5Pd-ZnTi-LDH-A200 are calculated to be 3.54 and 3.23 eV from Figure S3, Supporting Information. Their conduction band potentials are obtained by the flat band potentials measured by Mott-Schottky plot curves in Figure S13, Supporting Information. The positive slopes of the samples indicate that the catalysts are n-type semiconductors, for which the flat band potential is located about 0.2 eV below the conduction band (E CB ). Accordingly, their conduction band potentials (E CB ) are determined to be −0.63 and −0.74 V (versus NHE), and their valence band potentials (E VB ) are calculated to be 2.91 and 2.49 V (versus NHE), respectively. These results imply that the band potentials of ZnTi-LDH and 0.5Pd-ZnTi-LDH-A200 are theoretically sufficient to generate ROS of O 2 −· and ·OH. [56,57] Under full spectrum light irradiation, the surface adsorbed O 2 on 0.5Pd-ZnTi-LDH-A200 can be effectively reduced to O 2 −· , which is further proved by in situ EPR study with DMPO as the radical trap in acetonitrile. As displayed in Figure 5a, no signal is detected in dark, while six distinct peaks (A H = 9.4 G, A N = 13.6 G) appear under light irradiation. The sextuple peaks are the characteristic peaks of DMPO-OOH, [58] indicating the formation of O 2 −· on the surface of 0.5Pd-ZnTi-LDH-A200 under light irradiation. What's more, the generation of O 2 −· were also quantitatively measured by NBT method. [56,59] According to the fitting results by first-order kinetics in Figure 5b, the k constant of 0.5Pd-ZnTi-LDH-A200 is determined to be 0.0354 min −1 , which is five times bigger than that of ZnTi-LDH (0.0065 min −1 ). Subsequently, the generation of ·OH by 0.5Pd-ZnTi-LDH-A200 were also measured by in situ EPR study and CA method. The ERP experiments were carried out by using DMPO as the radical trap in water. Similarly, no peaks is observed before illumination, and a quadruplex peaks (A H = 14.9 G, A N = 14.9 G) with a relative intensity of 1:2:2:1, belong to DMPO-OH, [60,61] appear after illumination (Figure 5c). The generation of ·OH is represented by the PL intensity of generated 7-OH-CA by the reaction between ·OH and CA. The PL intensity of 7-OH-CA generated by 0.5Pd-ZnTi-LDH-A200 is significantly higher than that by ZnTi-LDH, suggesting much more ·OH generation (Figure 5d).
Furthermore, isotope labeling experiments were carried out to determine the oxygen source in the products of C 1 oxygenate. The experimental results show that CH 3 16 OH and its fragment peaks (m/z = 31, 32) were detected when H 2 18 O and 16 O 2 were used (Figure 6a), while CH 3 18 OH and the fragment peaks (m/z = 33, 34) were detected when H 2 16 O and 18 O 2 were used (Figure 6b). [45] These results prove that the oxygen of the C 1 oxygenate products mainly come from O 2 , but not H 2 O. Based on the results and discussions above, the photocatalytic mechanism of selective oxidation of CH 4 on 0.5Pd-ZnTi-LDH-A200 is proposed in Scheme 1. [62] Under light illumination, electrons on the valence band of ZnTi-LDH-A200 are excited to the conduction band, forming the holes with positive charge on the valence band. One hand, the holes transfer to the loaded Pd, and react with the adsorbed H 2 O to produce ·OH, which continues to react with CH 4 , forming ·CH 3 radicals. On the other hand, the excited electrons react with the adsorbed oxygen to produce ·OOH. Then, the produced ·CH 3 radicals are oxidized to CH 3 OH and HCHO by ·OOH, realizing the highly selective CH 4 conversion to high value-added C 1 oxygenates. In this process, the loaded Pd on the surface of ZnTi-LDH-A200 plays the key roles of CH 4 adsorption sites and hole acceptor, which efficiently increases the chemisorption of CH 4 molecules and promotes the separation of photocarriers in the excited ZnTi-LDH-A200.

Conclusion
In summary, the loaded Pd nanoparticles on the 2D-layered ZnTi-LDH-A200 realize highly selective CH 4 conversion to high valueadded C 1 oxygenates. The highest production rate achieved is up to be 4924.47 μmol g −1 h −1 over 0.5Pd-ZnTi-LDH-A200 with the 100% selectivity to C 1 oxygenates, which is 20 times higher than that on the bare ZnTi-LDH-A200. In this reaction, O 2 was demonstrated to be the main oxygen source by isotope labeling experiments. During the photocatalytic process of CH 4 conversion to C 1 oxygenates, the loaded Pd nanoparticles act as the chemisorption sites of CH 4 and the hole acceptor. As the chemisorption site of CH 4 , Pd increases the CH 4 adsorption and as the hole acceptor, Pd promotes CH 4 oxidation to ·CH 3 radicals. Pd furthermore promotes the separation of photocarriers in the excited ZnTi-LDH-A200, facilitating ·OOH production. Two factors result in the significantly enhanced photocatalytic activity of ZnTi-LDH for highly selective CH 4 conversion.

Experimental Section
Synthesis of ZnTi-LDH Photocatalysts: ZnTi-LDHs were synthesized using the modified co-precipitation method according to the previous report. [31] The preparation process was as follows: After the dissolution of 1.190 g of Zn(NO 3 ) 2 ·6H 2 O and 0.900 g of urea in 50 mL deionized water, a designed amount of (0.22 mL) of TiCl 4 was added dropwise under vigorous stirring for 20 min. Then the mixed solution was transferred to an autoclave and aged at 130°C for 48 h. The precipitate was centrifuged and washed thoroughly with water and finally dried overnight at 60°C. At last, 1.0 g sample of the prepared ZnTi-LDH was placed in a porcelain boat and calcined in the muffle furnace at 200°C for 3 h with a heating rate of 2°C min −1 . [52] Metal co-Catalysts Loading on ZnTi-LDH: Metal co-catalysts (Au, Ag, Pd, Pt,) were loaded on the surface of ZnTi-LDH photocatalysts by a photo deposition method. [17] Typically, 200 mg photocatalyst was first dispersed into 30 mL methanol by ultrasonication. After being stirred for 5 min, a certain amount of metal precursor (HAuCl 4 , AgNO 3 , K 2 PdCl 4 , H 2 PtCl 6 ) aqueous solution was added into the suspension. By purging with argon for 20 min, the suspension was sealed and irradiated for 3 h at 25°C under stirring in a multi-channel reactor. Finally, the samples were collected by centrifugation, and washed with methanol and deionized water for three times and dried at 60°C for 12 h. The as-prepared photocatalysts were denoted as x% M-ZnTi-LDH (x = 0.1, 0.3, 0.5, 0.8, and 1.0, which represents the mass percentages of M to ZnTi-LDH; M = Au, Ag, Pd, Pt). As a comparison, M-ZnTi-LDH samples without calcination process were also prepared by the same method above.
Photocatalytic CH 4 Conversion: Direct CH 4 photocatalytic conversion reactions were conducted in a custom-built 200 mL top-irradiation high pressure reactor under full-spectrum irradiation. [44] A Xenon lamp (300 W) (PLS-SXE300D, Beijing Perfectlight Technology Co., Ltd) was employed as the light source. The light intensity was measured by the light intensity meter (PL-MW2000, Beijing Perfectlight Technology Co., Ltd) and determined to be 150 mW cm −2 . In a typical test, 20 mg of the photocatalysts was dispersed uniformly in 100 mL distilled water through ultrasonication. With the gas-tight seal, the suspension was purged with ultrapure O 2 (99.999 vol%) for 20 min to completely remove air. Then, 0.1 MPa O 2 was maintained in the reactor. Next, CH 4 (99.999 vol%) was injected to acquire the desired pressure (1.9 MPa). The reaction was conducted for 2 h at 25°C with a circulating cooling system. Gaseous products as well as CH 3 OH in liquid products were measured by the gas chromatograph (GC-2014, Shimadzu Co., Ltd) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). [33] Formaldehyde (HCHO) was quantified through the colorimetric method [48] on the UVvis absorbance spectrometer (UV-3600 Plus, Shimadzu Co., Ltd). CH 3 OOH and CH 3 OH were also analyzed by the nuclear magnetic resonance (NMR) spectra of 1 H and 13 C via an NMR equipment (AVANCE III JEOL Ltd).
Cyclic experiments were also carried out to investigate the stability of the optimal photocatalyst. The used photocatalysts were centrifuged to collect after the reaction, then washed with deionized water, dried in the oven at 60°C, and calcined at 200°C for 3 h to remove the surface adsorbed carbides.
Isotope Labeling Experiment: To explore the source of carbon in the products, 20 mg of the photocatalyst was dispersed in 3 mL H 2 O and degassed for 30 min to completely remove air. The reactor was then refilled with 0.5 MPa 13 CH 4 (99 vol%) or 0.5 MPa 12 CH 4 and 0.1 MPa O 2 (99.999 vol%), respectively. The reaction was also carried out at 25°C for 6 h. The products were measured by 13

Monitor of Hydroxyl Radicals (·OH) and Hydroperoxyl Radicals (·OOH):
In situ electron paramagnetic resonance (EPR) was used to monitor the generation of ·OH and ·OOH radicals before and after light irradiation. [56,60] For the detection of ·OH radicals, 10 mg of the photocatalysts was suspended in 5 mL water, with DMPO as the trapping agent. For detecting ·OOH radicals, 10 mg catalyst was suspended in 5 mL acetonitrile, with DMPO as the trapping agent as well.
Nitrotetrazolium blue chloride (NBT) was used as the probe molecule to measure the generation of O 2 −· radicals. [59] Typically, 10 mg of the photocatalysts was mixed with 100 mL NBT solution (0.02 mm) and stirred in dark for 15 min. After light irradiation, a certain amount of the reactants was sampled at every 10 min intervals. UV-vis spectra were then detected on UV-3600 Plus spectrofluorometer after being filtered.
The photoluminescence (PL) technique was used to test the ·OH generation because of the reaction between ·OH and coumarin (CA) to generate 7-hydroxycoumain (7-OH-CA), which could be detected by photoluminescence spectra at 450 nm. [61] Typically, 10 mg of the photocatalysts was dispersed in 100 mL CA aqueous solution (0.5 mm) and stirred in dark for 15 min. The light was turned on, then the suspension was filtered and sampled at every 10 min intervals. At last, the generation of 7-OH-CA was measured by F-4500 spectrofluorometer.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.