High-energy Charge Separation State for Photocatalytic Nonoxidative Methane Conversion

doi:10.21203/rs.3.rs-1398376/v1

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High-energy Charge Separation State for Photocatalytic Nonoxidative Methane Conversion

https://doi.org/10.21203/rs.3.rs-1398376/v1

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The nonoxidative coupling of methane (NOCM) under mild conditions is a promising technique for low-cost H₂ and carbon feedstock productions. Here, we fabricate a Ce-Zr solid solution photocatalyst containing nonstoichiometric CeO_2 − x nano-islands with stepped surfaces and abundant oxygen vacancies, enabling unprecedented adsorption of methane and ideal for the photocatalytic NOCM at room temperature. It is revealed that the Ce-Zr photocatalyst can undergo a cooperative and reversible photo-induced transition from the initial resting state to the charge-separation state that stores sufficient energy to dissociate the inert methane C-H bond, thereby resulting in excellent photocatalytic activity with long-term stability (> 40 h, > 20 cycles) and high selectivity, exceeding that of traditional Ce-based photocatalysts by two orders of magnitude. This work not only provides fresh insight into the catalyst design for methane conversion but also extends the boundaries of conventional photocatalysts with the unique ability of storage and delayed release of solar energy.

Methane, the main component of natural gas, shale gas, and biogas, is not only a widely used fuel but also a potential carbon feedstock with huge reserves which is projected to change the landscape of the current petrochemical industry.^1-4 The nonoxidative coupling of methane is one of the key reactions to exploit these abundant feedstocks by selectively activating the C-H bonds in methane (2CH₄ → C₂H₆ + H₂), which can offer value-added C2+ hydrocarbons and clean H₂ energy.^5-7 However, owing to the large positive Gibbs free energy, this reaction always suffers from severe thermodynamic limitations even at elevated temperatures. For instance, the maximum methane conversions are only 0.0004% and 1.0% in thermal equilibrium at 298 K and 723 K, respectively. Further increase in temperature will inevitably lead to the formation of aromatics and undesired coke deposition.^6,8

Photocatalytic NOCM is expected to be a sensible approach to achieve methane conversion under mild conditions and beyond the thermodynamic constrains.^9,10 Despite the potential, the efficiency and cost are still far from satisfactory. The first challenge is how to achieve well adsorption and polarization of methane molecules on the active sites of photocatalyst since the non-polar regular tetrahedral structure and strong C-H bonds lead to low polarizability and negligible electron affinity of CH₄. It has been found that methane can undergo highly facile C-H bond activation on the surface of noble metals.^11,12 Therefore, many attempts are based on noble metal or alloy co-catalysts, such as Au@ZnO,¹³ Pt@HGTS,¹⁴ AuPd@ZnO,¹⁵and Ag@HPW-TiO₂.¹⁶ In addition, the incorporation of some highly dispersed metal cations (Zn⁺, Ga³⁺, Ce⁴⁺, Nb⁵⁺) into the supports is also an effective strategy to promote the activation of methane.^17-20 Another challenge constraining the development of efficient photocatalytic NOCM is the instability of the catalysts.⁹ To date, only a few unsupported semiconductor photocatalysts can durably produce both ethane and H₂ under photocatalytic NOCM conditions.^21-23 In many cases, the H moieties detached from methane will combine with the lattice oxygen of catalysts and be released as water instead of H₂.^15-17 Therefore, the oxidative regeneration of the catalyst at intervals is indispensable in these reactions.

We recognized that the ready abundance of cerium would make it an ideal element to replace noble metals for the photocatalytic NOCM.^24,25 First, Ce-based oxides exhibit particularly low oxygen vacancy formation energies, providing abundant coordinatively unsaturated sites for methane chemisorption.^26,27 These oxygen vacancies can also act as defect centers to trap photogenerated electrons or holes to inhibit the charge recombination, building up an effective channel for electron transfer between the adsorbed CH₄ molecule and the catalyst, thereby weakening the inert C-H bond. Second, cerium compounds are able to store electrons as the oxidation state of Ce can be readily shifted between +4 and +3 by storing/releasing electrons.²⁸ Electron-rich (oxygen-deficient) photocatalysts are expected to facilitate the release of H₂, thereby preventing the catalyst from being reduced during the NOCM process. Although the efficiency of the current Ce-based photocatalysts is still very low, one can see that by fine-tailoring the nanomorphology of catalysts, such as increasing the content of edge sites, coupled with defect engineering, it is challenging but possible to achieve an efficient earth-abundant photocatalytic NOCM system.

In this work, we report a selective etching-reconstruction strategy to fabricate ultrasmall CeO_2-x islands on the surface of Ce-Zr solid solutions (C@CZ). It was found that magnesium powder can selectively reduce and etch the lattice Zr and O on the surface of CeZr solid solutions, and the residual Ce-rich surface will be reconstructed to form CeO_2-x nano-islands with stepped surfaces and oxygen vacancies. The C@CZ catalyst with abundant edge and defect structures exhibits a reversible and stable photoinduced charge-separated state and high photocatalytic activity in NOCM, far exceeding that of traditional Ce-based quantum photocatalysts.

Synthesis and characterization. The Ce-Zr solid solutions with various Ce/Zr mole ratios were prepared by sol-gel auto combustion method (designated as CZ when the Ce/Zr ratio is 1). As illustrated in Fig. 1a, the as-synthesized CZ was fully mixed with a certain amount of magnesium powder and treated in 5% H₂/Ar at 923 K for 4 h. The obtained samples were thoroughly washed with hydrochloric acid and water several times to completely remove unreacted Mg, and then dried in a vacuum. It was found that the nanostructure and catalytic activities of the samples highly depended on the amount of Mg added. If the molar ratio of Mg/(Ce + Zr) in the starting mixtures is 0.6, the finally obtained sample will be designated as CZ_1 − x. If the molar ratio is 2.0, the finally obtained sample will be designated as C@CZ. From the ICP-AAS measurements (inductively coupled plasma atomic absorption spectroscopy, Supplementary Table 1), the molar ratios of Ce:Zr in CZ, CZ_1 − x, and C@CZ can be determined to be 50:50, 63:37, and 65:35, respectively. Combined with the mass change and various characterizations of the solid solutions before and after the Mg treatment, a big picture can be drawn that the Mg treatment will preferentially reduce and etch away lattice Zr on the surface of the solid solution, thereby forming a Ce-rich surface (CZ_1 − x). Increasing the amount of Mg from 0.6 to 2.0 will further remove the excess surface O and promote the aggregation of Ce to form stepped CeO_2 − x nano-islands, which will be described in detail later.

As shown by powder X-ray diffraction (PXRD) (Supplementary Fig. 1), the as-synthesized CZ possesses a single fluorite phase structure with the lattice constants between pure CeO₂ and ZrO₂. The high-resolution TEM (HRTEM) image (Fig. 1b) shows that CZ is polycrystalline consisting of many small crystalline domains with diameters ranging from 5 to 10 nm. The lattice spacing of the (111) facets of CZ is 0.306 nm, which is smaller than that of pure CeO₂ (0.312 nm), due to the substitution of half Ce⁴⁺ ions (0. 97 Å) by smaller Zr⁴⁺ ions (0. 84 Å).²⁹ Energy dispersive x-ray spectroscopy (EDS) analysis in a transmission electron microscope (TEM) clearly indicates a large-scale homogeneous distribution of Zr, Ce, and O elements in CZ (Fig. 1c). Wavelet transformation (WT) of EXAFS was further employed to investigate the coordination environments of Ce atoms. The WT contour plot of Ce L₃-edge pure CeO₂ exhibited two intensity maximum at around 4 Å⁻¹ and 8 Å⁻¹, which can be assigned to the Ce-O and the Ce-Ce contributions, respectively (Fig. 1d). In contrast, the intensity maximum attributed to Ce-Ce vanished in the WT contour plot of CZ, confirming that Zr and Ce are highly uniformly dispersed in the CZ bulk.³⁰

For CZ_1 − x, the low amount of Mg treatment (Mg/(Ce + Zr) ≤ 0.6) did not lead to any structural change of the solid solutions (Supplementary Fig. 2) but only reduced the surface crystallinity by selectively removing Zr atoms (Fig. 1e). However, in the case of C@CZ, it was found that the high amount of Mg treatment (Mg/(Ce + Zr) ≥ 2.0) would shift the XRD peaks of the solid solutions to lower 2θ angles with the formation of many ceria domains on the surface (Fig. 1f). As further revealed by the spherical aberration-corrected TEM (ACTEM, Figs. 1g and 1h) images, the ceria domains existed in the form of nano-islands with surrounding step edges and irregular stepped surfaces. The presence of pure ceria nano-islands was also unambiguously confirmed by the ACTEM line scan (Fig. 1h). Of course, despite the formation of surface-localized ceria nano-islands, EDS analysis in scanning electron microscopy (SEM) clearly demonstrated that the Zr, Ce, and O elements are still uniformly distributed at a large scale in the bulk of C@CZ (Supplementary Fig. 3). Furthermore, by fitting the Ce L₃-edge extended X-ray absorption fine structure (EXAFS) spectra (Fig. 1i and Supplementary Table 2), it was found that the first shell coordination numbers (CN) of the Ce-O bond in CZ, CZ_1 − x, and C@CZ decreased from 8.0 in pure CeO₂ to 7.0, 6.8, and 7.2, respectively, indicating the existence of a large number of oxygen vacancies in these materials.

The electronic states of CeO₂, CZ, CZ_1 − X, and C@CZ were further revealed by X-ray photoelectron spectroscopy (XPS). The fitted Ce 3d and O 1s spectra in Figs. 2a and 2b indicate that Ce predominantly existed as Ce⁴⁺ in commercial CeO₂, while O mainly existed in the form of lattice oxygen (O_L) and adsorbed oxygen (O_C).^31,32 The incorporation of Zr and the treatment of Mg can significantly increase the concentrations of Ce³⁺ and oxygen vacancies (V_O). The relative contents of Ce³⁺ near the surface in CZ, CZ_1 − x, and C@CZ increased from 6% in CeO₂ to 15%, 27%, and 29%, respectively, accompanied by a sequential increase in V_O concentrations. C@CZ had the highest proportion of Ce³⁺ and V_O among all the samples, probably due to the lower V_O formation energy³³ and geometric distortions on the step edges of ceria nano-islands that can better accommodate more spacious Ce³⁺ ions (1.10 Å).³⁴

It is worth noting that the binding energies of Ce 3d, O1s, and Zr 3d (Supplementary Fig. 4) for CZ_1 − x were all shifted to higher values compared to those for CZ, which is caused by the selective formation of Zr cationic defects during the lower amount of Mg treatment.^35,36 Further increasing the amount of Mg will remove excess surface O and quench cationic defects, promoting surface reconstruction to form stepped ceria nano-islands. Therefore, the binding energies of Ce 3d, O1s, and Zr 3d for C@CZ shifted back to lower values. It is also found that with the increase of Mg dosage, the contents of adsorbed oxygen (O_C) on the surface of the solid solutions first slightly increased in CZ_1 − x and then largely decreased in C@CZ. All these results strongly support the aforementioned conclusion that the Mg treatment will preferentially reduce and etch lattice Zr, followed by the removal of excess surface O. More importantly, the surface O_C is too reactive to over-oxidize methyl radical intermediates in the NOCM reaction, resulting in undesired carbon oxides.³⁷ The low concentration of O_C in C@CZ is highly beneficial to improve the selectivity of C2 + hydrocarbons in NOCM.

Raman spectroscopy studies were conducted on catalysts under ambient conditions with an excitation laser line (514 nm) (Fig. 2c). The peak at around 464 cm^− 1 and 630 cm^− 1 can be assigned to the F_2g vibration mode and D band mode in the structure of ceria, respectively.32 The former represents the symmetrical stretching vibration of ceria, and the latter corresponds to defect sites. Obviously, C@CZ had the highest D/F_2g ratio, implying the most abundant V_O in C@CZ, which further confirmed the XPS results. On the other hand, besides V_O, the abundant lattice O²⁻ anions (O_L) coexisting at the step edges of CeO_2 − x nano-islands greatly enhanced the basicity of C@CZ,³⁸ as revealed by the temperature-programmed desorption (TPD, Fig. 2d). It is widely recognized that the basic surface can effectively inhibit the oligomerization of C2 + products to form coke,³⁹ thereby improving the durability and selectivity of the catalyst.

Photocatalytic NOCM. The catalysts were evaluated for the NOCM reaction in a batch reactor under a 300 W xenon lamp irradiation (200–780 nm) at 293 K. As shown in Supplementary Fig. 5, compared with pure CeO₂ and ZrO₂, the Ce-Zr solid solutions exhibited significantly enhanced photocatalytic NOCM activities. Among all the Ce-Zr solid solutions with various Ce/Zr mole ratios, CZ (Ce/Zr = 1) exhibited the highest activity, 7 times higher than pure CeO₂. Next, the effect of Mg treatment on the catalytic activity of CZ was tested. Most unexpectedly, the photocatalytic activity underwent a rare V-shaped reversal with the deepening of the reduction by Mg. As shown in Fig. 3a, initially, the catalytic activity of solid solutions would decrease with the increase of Mg dosage, reaching a minimum value when the Mg/(Ce + Zr) ratio is close to 0.6 (CZ_1 − x), which is only one-third of that of CZ. With further increasing the amount of Mg, the catalytic activity turned to increase sharply, reaching the highest point at Mg/(Ce + Zr) = 2. Therefore, C@CZ exhibited the highest photocatalytic activity for the NOCM reaction among all the samples. A high methane conversion rate of 52 µmolg^− 1h^− 1 with nearly stoichiometric amounts of H₂ and ethane was achieved over C@CZ under 2 h of light irradiation at 293 K (Supplementary Table 3). No reaction occurred in the absence of catalyst or light.

As shown in Fig. 3b, and Supplementary Table 4, fresh C@CZ provided a superior methane conversion rate and remained constant over 20 cycles during a 40-hour test. The ethane selectivity is no less than 94% for each cycle. After 8 hours of continuous reaction, the methane conversion exceeded 8% (Supplementary Fig. 6), while still maintaining high selectivity to ethane (92%) and low selectivity to CO_X (3%) and coke (5%). The excellent stability and selectivity are understandable due to the highly reduced and basic surface of C@CZ. Specifically, excessive Mg treatment and surface reconstruction can form abundant surface oxygen vacancies (high V_O) and remove reactive oxygen species (low O_C), thereby avoiding the participation of oxygen atoms in NOCM reaction to form CO_X by-products. Meanwhile, the basic surface can effectively inhibit the formation of coke. In contrast, other Ce-based semiconductor photocatalysts started to decrease in both activity and selectivity after a few cycles under the same reaction conditions. For example, the catalytic activity of pure CeO₂ almost vanished after ten cycles, while the ethane selectivity was greatly reduced, resulting in a large amount of coke (11%) and CO_X (23%) by-products.

The apparent activation energy (Ea) of photocatalytic NOCM over CeO₂, CZ, CZ_1 − x, and C@CZ were estimated by Arrhenius plots in Fig. 3c. Different from thermal reactions, the activation energy of photoreaction is generally small and mainly reflects the energy required for product desorption.⁴⁰ It is found that the Ea of C@CZ was only slightly higher than that of CZ and CeO₂, indicating that the reconstituted CeO_2 − x nano-islands on the surface of C@CZ did not restrict product desorption. In contrast, CZ_1 − x had a significantly larger Ea among all samples, about twice that of other samples, indicating the difficulty in the desorption of products on the surface of CZ_1 − x, which can lower the turnover frequency of the active sites. On the other hand, Fig. 3d showed that the reaction order for methane over C@CZ was only 0.24, a value much smaller than other Ce-based catalysts (0.41–0.47) and other traditional NOCM semiconductors such as ZnO (0.46) and GaN (0.61),²² suggesting that the surface of C@CZ can well adsorb methane, leading to very weak dependencies of the reaction rate on the gas-phase methane concentrations.

The intensity-dependent experiment was further tested to study the catalytic activity under light irradiation with different intensities. As shown in Fig. 3e, the apparent quantum efficiency (AQE, 325 nm) increased linearly with light intensity, reaching 1.04% at 10 mW⋅cm^− 2, while the photocatalytic reaction rate was proportional to the square of light intensity, suggesting that two photons are required to drive the rate-determining step (for a detailed explanation see Supporting Information). Consequently, an excellent methane conversion rate of 92 µmolg^− 1h^− 1 was achieved at 10 mW⋅cm^− 1, exceeding that of traditional Ce/SiO₂ quantum photocatalysts by a factor of 128.¹⁹

Mechanism study. As illustrated in Supplementary Table 1, the Brunauer-Emmett-Teller (BET) specific surface area of C@CZ is 50.9 m²⋅g^− 1, which is comparable to that of CeO₂ (47.4) and slightly higher than that of CZ (44.4) and CZ_1 − x (37.9). On the other hand, the bandgap of both C@CZ and CZ_1 − x is 3.17 eV, very close to that of CeO₂ (3.20) and slightly larger than that of CZ (3.02) (Supplementary Fig. 7). These results clearly demonstrate that the superior activity of C@CZ cannot be attributed to the much larger surface area or enhanced light absorption capacity, which usually play important roles in photocatalytic reactions. There must be some unique abilities in C@CZ that are essential for the activation of methane molecules.

The CH₄-TPD and in-situ Fourier-transform infrared (FTIR) results provided convingcing evdences for the excellent methane adsorption and polarization ability of C@CZ with stepped CeO_2 − x nanoislands. As shown in Figs. 4a, a prominent CH₄ desorption peak at lower temperatures and broad CH₄ desorption bands at higher temperatures were observed on the C@CZ photocatalyst, while CZ did not have any observable CH₄ desorption peak. And then, in-situ FTIR was applied to investigate the polarization of methane on the surface of C@CZ. As shown in Fig. 4b, upon the exposure of CH₄ on the surface of C@CZ, the IR peaks at 3017 and 1305 cm^− 1 were observed, which can be ascribed to the asymmetric stretching vibration and asymmetric deformation vibration of CH₄, respectively.⁴¹ The peak at 2906 cm^− 1 caused by the symmetric stretching vibration of CH₄ was also appeared, which is originally infrared-forbidden for the free CH₄ molecule, unambiguously confirming the efficient polarization of methane C-H bonds on C@CZ. Distinctively, the IR peaks remained intact even after vacuuming at room temperature, indicating a strong methane-catalyst interaction, which is in good agreement with the results of CH₄-TPD and low reaction order.

The as-synthesized C@CZ photocatalysts underwent a unique photoactivation process under light irradiations. Notably, an induction period was always observed when the light illumination started immediately upon the exposure of C@CZ to methane (Fig. 4c, bottom). The induction period persisted even when methane was pre-contacted with C@CZ under dark conditions (Fig. 4c, middle), indicating that the induction period was not caused by the diffusion or adsorption equilibrium of methane.²¹ Conversely, if C@CZ was photo-irradiated in vacuum for 30 min prior to the injection of methane, the photocatalytic NOCM reaction would start at the optimal reaction rate and remain constant without any induction period (Fig. 4c, top). This result implied that C@CZ could realize a photo-induced dynamic transition from a “resting state” to an “active state” during photocatalysis.

In-situ electron paramagnetic resonance (EPR) spectroscopic measurements further revealed the nature of this transition. As shown in Fig. 4d, the as-synthesized C@CZ had two EPR signals at g = 2.001 and g = 1.965, which can be assigned to unpaired electrons on oxygen vacancies (V_O⁺)⁴² and Ce³⁺,⁴³ respectively. The EPR signals of both V_O⁺ and Ce³⁺ would dramatically increase with illumination in vacuum, reaching the maximum at about 30 min and remaining unchanged even after stopping the illumination. This indicates that the photo-excited charge separation process could generate a large amount of stable V_O⁺ (V_O + h⁺ → V_O⁺) and Ce³⁺ (Ce⁴⁺ + e⁻ → Ce³⁺) in C@CZ. It is well known that both V_O and Ce³⁺ can easily diffuse on defective ceria substrates via O anion migration and polaron hopping.⁴⁴ Due to the low formation energy and reduced geometric distortions, the V_O and large Ce³⁺ are better accommodated at step edges than in bulk or ideal surfaces. Therefore, since the surface of C@CZ is rich in stepped CeO_2 − x nano-islands, it can provide a large number of sites for accommodating and stabilizing photogenerated V_O⁺ and Ce³⁺, thus avoiding the fast recombination in bulk. Furthermore, the doublet EPR signal of Ce³⁺ implied the existence of two magnetically inequivalent sites for Ce³⁺ in C@CZ.⁴⁵

Most impressively, upon exposure to methane, the EPR signal of V_O⁺ was largely reduced, while the Ce³⁺ signal was largely enhanced, demonstrating that C@CZ in the high-energy charge-separated state was able to abstract electrons from adsorbed methane molecules. Ionized methane molecule becomes reactive, followed by the facile dissociation to form a proton and a methyl radical.

Notably, this photo-induced dynamic transition was accompanied by significant color changes of C@CZ. As illustrated in Fig. 5a, the color of C@CZ changed from light yellow to dark blue after light irradiation in vacuum. The dark blue C@CZ maintained its color even after stopping the illumination and catalyzed the NOCM reaction efficiently under light irradiation. The color reverted to the original light yellow color within a few minutes upon exposure to O₂ without light irradiation.

The entire photocatalytic cycle was shown in Fig. 5b. Initially, the as-synthesized C@CZ is in a resting state, which is inactive. On the other hand, well-adsorbed methane molecules are polarized in an energetically more favorable “alkyl” pathway, forming CH₃^δ−-H^δ+.⁴⁶ The resting state becomes the active state under light irradiation through the successive photo-excited charge separation and diffusion (O migration & polaron hopping) process, which generates abundant and stable V_O⁺ and Ce³⁺ on the step edges of CeO_2 − x nanoislands. The high-energy C@CZ in the active state could directly extract electrons from polarized methane molecules, dissociating methane into methyl radicals and protons. Finally, the excess electrons stored on C@CZ were released by reducing protons to generate H₂ under light irradiation, while the catalyst reverted to the initial resting state, thereby completing the photocatalytic cycle.

In summary, our studies provide insight into how to design efficient earth-abundant photocatalytic NOCM systems by tailoring the nanostructure of catalysts. Using a novel selective etch-reconstruction strategy, the stepped CeO_2 − x nano-islands were fabricated on the surface of Ce-Zr solid solutions, which can undergo a dynamic photo-induced transition from the initial resting state to the active state that significantly enhances photocatalytic NOCM performance. This cooperative and reversible interaction between photocatalysts and photons promises to extend the boundaries of conventional photocatalysts. Furthermore, the stable high-energy charge separation states enable efficient solar energy harvesting and delayed charge storage and release.

Catalyst preparation.

Preparation of CZ: The Ce-Zr solid solutions with different Ce/Zr ratios were prepared by sol-gel auto combustion method. Typically, 0.04 mol of Ce(NO₃)₃·6H₂O and the corresponding amount of ZrO(NO₃)₂·xH₂O were accurately weighed in Ce/Zr molar ratios of 3:7, 4:6, 5:5, 6:4 and 7:3, and each was placed in an evaporation dish with 20 mL of the deionized water. Then 0.04 mol of glycine was added to the above solution under continuous stirring. After the formation of cerium zirconate gel, excess water was evaporated at 353 K, followed by an elevated temperature (453 K) to trigger auto combustion to obtain Ce-Zr solid solutions. Finally, the obtained Ce-Zr solid solutions were calcined in air at 923 K for 4 h to remove the incompletely reacted glycine. The as-synthesized samples were named Ce_0.3Zr_0.7O₂, Ce_0.4Zr_0.6O₂, Ce_0.5Zr_0.5O₂ (CZ), Ce_0.6Zr_0.4O₂ and Ce_0.7Zr_0.3O₂.

Preparation of CZ_1 − x and C@CZ: The as-synthesized CZ was fully mixed with a certain amount of magnesium powder (the molar ratio of Mg/(Ce + Zr) = 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.5, 2.0, and 4.0) and treated in 5% H₂/Ar at 923 K for 4 h. The obtained samples were thoroughly washed with hydrochloric acid (2 M) and water several times to completely remove unreacted Mg, and then dried in a vacuum. If the molar ratio of Mg/(Ce + Zr) is 0.6, the finally obtained sample was designated as CZ_1 − x. If the molar ratio is 2.0, the finally obtained sample was designated as C@CZ.

Photocatalytic tests.

The catalysts were evaluated for methane dehydrogenation in a batch-type system under a 300 W xenon lamp irradiation (200–780 nm) at 293 K. Specifically, a freshly prepared catalyst (3 mg) was spread evenly on the bottom of an air-tight quartz reactor (75 mL, 12.5 cm² ⋅ 6 cm) in vacuum (P < 1.0 Pa), followed by the introduction of 0.1 or 0.3 mmol of pure methane. The light intensity at the catalyst surface was monitored by using a power meter (CEL-NP2000-10, AULTT, China). A catalyst-contact thermometer was employed to measure the temperature of the catalyst surface in situ. To remove the photothermal effect, a cooling system was employed. After the reaction, the amount of hydrogen was measured by GC with a thermal conductivity detector (TCD). The organic products were analyzed by gas chromatography and gas chromatography-mass spectrometry. The carbon oxides (if any) products were evaluated using a gas chromatograph equipped with a flame ionization detector combined with a hydrogenation reactor (GC-2014, Shimadzu). For the recycling test, after each cycle, the samples were vacuumed at room temperature, followed by the introduction of pure methane (0.3 mmol).

Data availability

The data that support the findings of this study are available from the Article and Supplementary Information, or from the corresponding authors on reasonable request.

Acknowledgments

Funding: This work was financially supported by the National Natural Science Foundation (NSFC) of China (Grant No. 92061105 and 21875090), the Natural Science Foundation of Jilin Province (20210101121JC), and the Fundamental Research Funds for the Central Universities.

Author contributions: G. W. performed the catalyst synthesis, characterization, and catalytic tests. X. M. participated in the catalyst preparation and catalytic tests. J. L., Z. P. and Q. Z. participated in the characterization. R. T. and R. F. provided helpful discussion. L. L. designed the study, analyzed the data, and wrote the paper.

Competing interests

The authors declare no competing interests.

Supplementary information:

Materials and Methods

Supplementary Table 1 to 4

Supplementary Figures 1 to 7

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High-energy Charge Separation State for Photocatalytic Nonoxidative Methane Conversion

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