Introduction

Direct conversion of methane (CH4) into value-added chemical products represents a cornerstone for advancing sustainable C1 chemistry. Formaldehyde (HCHO), as a high-value CH4 converting product, is widely used as a raw material for over 50 industrial processes1. Notably, HCHO is a promising hydrogen carrier, performing an important role in green energy. The hydrogenolysis of HCHO and water (HCHO + H2O → HCOOH + H2) meets the standards of “hydrogen economy”, and the theoretical hydrogen efficiency (8.4 wt.%) is higher than that of formic acid (4.4 wt.%)2. Additionally, oxygen effectively promotes the hydrogenolysis of HCHO and water on Ag/MgO, which can effectively solve the process of pre-eliminating molecular oxygen to avoid reverse hydrogen oxidation reactions3. However, current industrial HCHO production via methanol dehydrogenation at 500–600 °C remains plagued by multi-step complexity, excessive energy demands (40% methanol consumption), and substantial CO2 emissions4. This stark reality underscores the imperative for developing mild-condition one-step CH4-to-HCHO conversion systems to realize truly sustainable C1 chemistry5.

Photocatalytic CH4 oxidation to C1 oxygenates (CH3OOH, CH3OH, HCHO, etc.) has become a research hotspot, enabling ambient-condition CH4 activation using economical O2 oxidants6,7,8,9,10. The current photocatalytic selective oxidation of CH4 systems predominantly employ batch-type gas‒liquid‒solid reactors, which necessitate pressurized operation to enhance CH4 solubility in aqueous media11,12,13,14,15. More critically, these systems suffer from progressive product degradation during batch cycling, thereby reducing the selectivity. To improve the selectivity, it is often solved by increasing the amount of water and continuous flow of water11,16, which will cause problems in subsequent purification and separation. If gas‒solid photocatalytic selective oxidation of CH4 is achieved under atmospheric pressure, similar to photocatalytic oxidative coupling of CH4, it is expected to resolve these limitations through continuous product desorption.

ZnO is widely utilized as a semiconductor catalyst for the photocatalytic conversion of CH4 to HCHO14,17,18. Lattice oxygen can participate in the reactions to generate *OCH3 species and its further dehydrogenation to produce HCHO, and the consumed lattice oxygen is then replenished by dissociated  O218. Therefore, manipulating the lattice oxygen mobility of metal oxides can effectively enhance the HCHO selectivity. While thermal activation can significantly enhance the activity of lattice oxygen, it can also lead to overoxidation of CH4. Single atoms (SAs) can regulate the microenvironment of the catalyst surface, effectively suppressing overoxidation of the target product19,20,21. Therefore, metal SAs-modified ZnO might be an ideal photocatalyst to achieve high HCHO yield and selectivity with the assistance of heat.

In this work, we rationally designed Ag SAs-modified ZnO (Ag1-ZnO) for gas‒solid photothermal catalytic selective oxidation of CH4 to HCHO in a flow reactor under atmospheric pressure. The Ag SAs accumulate photogenerated holes and activate H2O to generate *OH species, which promote the C–H bond cleavage of CH4. In addition, Ag SAs alter the local electron distribution of ZnO, which is beneficial for the generation and desorption of *HCHO to generate gas-phase HCHO. Furthermore, the assistance of heat promotes the separation of photogenerated electrons and holes, and enhances the mobility of surface lattice oxygen, thus promoting the formation of HCHO. As a result, Ag1-ZnO catalyst demonstrates a HCHO production rate of 117.8 ± 1.7 μmol h–1 with 71.2 ± 0.8% selectivity. After 12 h of aqueous-phase absorption of the reaction effluents, the HCHO concentration reaches 514.2 ± 33.7 µmol mL−1 (1.54 ± 0.10 wt.%). The absorption trapping process also provides a co-benefit that the cogenerated CH3OH (28.1 ± 1.1 µmol mL−1) in the solution can effectively inhibit the polymerization of HCHO. This work establishes an environmentally benign and energy-efficient strategy for the practical industrial application of direct CH4 to HCHO under mild operating conditions.

Results

Catalyst screening and characterizations

To screen metallic SAs-modified ZnO (M1-ZnO) catalysts for the gas‒solid photothermal catalytic selective oxidation of CH4 to HCHO, density functional theory (DFT) calculations were performed. The key step in the photothermal catalytic selective oxidation of CH4 to HCHO is the effective desorption of HCHO from the catalyst to prevent its excessive oxidation11,22,23. Therefore, the ZnO (101) model is selected for modeling HCHO adsorption on metallic SAs modified catalysts. As shown in Fig. 1a, the adsorption energy of HCHO (Eads) on M1-ZnO follows the order: Ag1-ZnO (−1.10 eV) < Au1-ZnO (−1.41 eV) < Pt1-ZnO (−1.51 eV) < Pd1-ZnO (−1.56 eV), indicating Ag SAs incorporation can effectively promote HCHO generation. Based on theoretical simulation results, a series of metal SAs (Ag, Au, Pt, and Pd) modified ZnO (Supplementary Fig. 1) were synthesized for photothermal catalytic oxidation of CH4. It is found that Ag1-ZnO with the lowest HCHO adsorption energy shows the highest HCHO generation rate and selectivity (Fig. 1b). There is a good correlation between HCHO/(CO + CO2) in the photothermal catalytic oxidation of CH4 on M1-ZnO and the adsorption energy of HCHO on M1-ZnO in DFT calculations (Fig. 1c), indicating that metal SAs doping can effectively regulate the adsorption performance of HCHO to control the photothermal catalytic selective oxidation of CH4 to HCHO. Based on theoretical and experimental results, a specific study is then conducted on the photothermal catalytic selective oxidation of CH4 to HCHO on Ag1-ZnO samples in reference to pure ZnO and Ag nanoparticle-modified ZnO (AgNP-ZnO).

Fig. 1: Screening of M1-ZnO for photothermal catalytic selective oxidation of CH4 to HCHO.
figure 1

a Calculated adsorption energy of HCHO (Eads) on M1-ZnO. b Gas‒solid photothermal catalytic CH4 oxidation performance for M1-ZnO. c HCHO/(CO + CO2) of gas‒solid photothermal catalytic CH4 oxidation for M1-ZnO plotted against the HCHO adsorption on M1-ZnO. The reaction conditions in (b): CH4 (45 mL min−1) +  air (5 mL min−1), RH = 20%, external heating at 150 °C, Xe lamp with wavelength range of 200–1200 nm and 1000 mW cm–2 light intensity. Blue, red, black, white, dark gray, yellow, light gray and turquoise spheres in (a) represent Zn, O, C, H, Ag, Au, Pt and Pd atoms, respectively. Source data are provided as a Source Data file.

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The fabricated ZnO exhibits nanoparticles with an average size of approximately 22.7 nm (Supplementary Fig. 2a, b). The high-resolution transmission electron microscopy (HRTEM) image (Supplementary Fig. 2c) shows the lattice fringes of ZnO with an interplanar spacing of 0.247 nm, corresponding to the (101) plane of ZnO. The Ag1-ZnO and AgNP-ZnO samples were obtained through a simple 85 °C bath assisted NaH2PO2 reduction method by regulating the amounts of AgNO3 and NaH2PO2. The actual loading amounts of Ag in Ag1-ZnO and AgNP-ZnO are 0.18 wt.% and 0.96 wt.%, respectively, as measured by inductively coupled plasma–optical emission spectroscopy (ICP‒OES). The X-ray diffraction (XRD) spectra of all prepared samples (Supplementary Fig. 3) are well indexed to the typical hexagonal wurtzite structure of ZnO (JCPDS file no. 36-1451)24. No characteristic diffraction peaks of Ag can be observed on either Ag1-ZnO or AgNP-ZnO, demonstrating the small size and good dispersion of Ag species on ZnO. For Ag1-ZnO, the Ag species remain highly dispersed on the ZnO NPs from the energy-dispersive X-ray spectroscopy (EDS) elemental mapping images (Fig. 2a). The Ag atoms can be observed from the aberration-corrected high-angle annular dark-field scanning TEM (AC-HAADF-STEM) image due to its being much brighter than Zn atoms (Fig. 2b). Furthermore, a lattice spacing of 0.236 nm corresponding to the Ag (111) plane appears in the AgNP-ZnO sample (Supplementary Fig. 4a), and the EDS elemental mapping image (Supplementary Fig. 4b) further verifies the formation of Ag NPs in the AgNP-ZnO sample. And the average size of Ag NPs on AgNP-ZnO sample is around 12.9 nm (Supplementary Fig. 4c, d). Moreover, the ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS) (Supplementary Fig. 5) clearly shows that only the AgNP-ZnO sample exhibits a surface plasmon peak, which also proves the formation of Ag NPs and Ag SAs in AgNP-ZnO and Ag1-ZnO samples, respectively.

Fig. 2: Catalyst characterizations.
figure 2

a HAADF-STEM image and the corresponding EDS elemental mapping signals of Ag1-ZnO. b Magnified high-resolution AC-HAADF-STEM image of Ag1-ZnO and the corresponding intensity profile from the marked position. c The normalized XANES spectra at the Ag K-edge. d The k3-weighted Fourier transform EXAFS spectra at the Ag K-edge. e Wavelet transformation for the k3-weighted EXAFS signal at the Ag K-edge. Source data are provided as a Source Data file.

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To further explore the local structure of Ag species in Ag1-ZnO and AgNP-ZnO, X-ray photoelectron spectroscopy (XPS) and synchrotron X-ray absorption spectroscopy (XAS) were carried out. The X-ray absorption near-edge structure (XANES) spectrum at the Ag K-edge of AgNP-ZnO (Fig. 2c) shows a similar white-line intensity to that of the Ag foil, indicating that metallic Ag is the dominant Ag species in AgNP-ZnO. The white line for Ag1-ZnO is between Ag foil and Ag2O, indicating that the strong metal-support interaction leads to the formation of partially oxidized Ag species (Ag0 <Agδ+ <Ag+) (Fig. 2c). The states of Ag species of Ag1-ZnO and AgNP-ZnO are also confirmed by the XPS results (Supplementary Fig. 6). The Fourier-transformed k3 weighted extended X-ray absorption fine structure (EXAFS) spectra at the Ag K-edge of Ag1-ZnO (Fig. 2d) display a prominent Ag–O shell peak at 1.59 Å, and there is no Ag–Ag shell, confirming the atomic dispersion of Ag in Ag1-ZnO. The AgNP-ZnO sample shows a major peak at the Ag–Ag shell, indicating that Ag species appear as metallic NPs. Wavelet transform EXAFS analysis (Fig. 2e) further visualizes structural information. Furthermore, EXAFS fitting analysis was conducted to determine the coordination configurations of Ag1-ZnO. According to the best-fitted EXAFS (Supplementary Fig. 7, and Supplementary Table 1), the coordination number of Ag–O (2.06 Å) is estimated to be 2 for Ag1-ZnO. Then, DFT calculations were carried out to acquire the structure of Ag1-ZnO. In Ag1-ZnO, each Ag atom binds to two O atoms (Supplementary Fig. 7c), which is consistent with the EXAFS fitting result.

Photothermal catalytic selective oxidation of CH4 to HCHO

Photothermal catalytic oxidation of CH4 was investigated in a 50-mL gas–solid flow reactor with irradiation of a 300-W Xe lamp (λ = 200–1200 nm, 1000 mW cm−2) under external heating at 150 °C (Supplementary Fig. 8). The obtained HCHO reaches stability after 60 min of illumination (Supplementary Fig. 11 and Supplementary Table 3). For comparison, the products of each sample are analyzed after 180 min of light exposure. A mixture of 50 mL min−1 CH4/air (25 vol.% O2/N2) = 45:5 and 20% relative humidity (RH) was used as the feed gas after screening under preliminary conditions (Supplementary Fig. 12 and Supplementary Tables 4−7). As shown in Fig. 3a and Supplementary Table 8, the yield of HCHO over Ag1-ZnO sample is 117.8 ± 1.7 μmol h–1, which is approximately 2.2 times higher than that over pure ZnO. The introduction of Ag SAs increases the HCHO selectivity from 59.6 ± 0.9% to 71.2 ± 0.8%. And the valuable products (CO, CH3OH and HCHO) generated over Ag1-ZnO sample reach 136.8 ± 3.1 μmol h–1 with a selectivity of 82.7 ± 0.3%, and the CH4 conversion on Ag1-ZnO is 0.14 ± 0.003%. When the surface temperature of the catalyst is controlled at 228 °C, the apparent quantum yield (AQY) values of HCHO at 365, 400 and 420 nm on Ag1-ZnO are 20.19 ± 2.32%, 1.98 ± 0.31% and 0.05 ± 0.01%, respectively (Supplementary Fig. 13). The activity of ZnO sample treated with NaH2PO2 (ZnO(SM)) is basically consistent with that of pure ZnO, indicating that simple NaH2PO2 treatment has no effect on the activity of ZnO. Notably, the HCHO production rate of Ag1-ZnO can reach 11.8 ± 0.4 μmol h–1 with a selectivity of 72.1 ± 1.3% under simulated sunlight (AM 1.5 G, 100 mW cm−2) when the surface temperature of catalyst is the same as the above temperature (Supplementary Fig. 14 and Supplementary Table 9). However, a bare HCHO yield (8.1 ± 0.5 μmol h–1) and a large amount of produced CO2 (80.0 ± 1.6 μmol h–1) over AgNP-ZnO sample are achieved, indicating that the presence of Ag NPs leads to the overoxidation of CH4. The HCHO yield obtained from the gas‒solid photothermal catalytic oxidation of CH4 over Ag1-ZnO under atmospheric pressure is not only higher than all reported photocatalytic systems (Supplementary Table 14)11,12,13,14,15,17,18,22,23,25,26, but also still possesses superiority compare to traditional high-pressure and high-temperature catalytic reaction systems (Supplementary Table 15)27,28,29,30,31,32,33,34,35.

Fig. 3: Activity of photothermal catalytic selective oxidation of CH4 to HCHO.
figure 3

a Activity on ZnO, ZnO(SM), Ag1-ZnO, and AgNP-ZnO under external heating at 150 °C. b Activity on Ag1-ZnO under external heating at different temperatures. c Activity on Ag1-ZnO at a catalyst surface temperature of 228 °C under irradiation with different light intensities. d Continuous stability tests on Ag1-ZnO. e Concentration of the absorption solution (2 mL distilled water at 0 °C) for a 3, 6 and 12 h of reaction. (The insert is a photo of the chromogenic agent added to the absorption solution after dilution 1000 times). f Comparison of the catalytic activity for photooxidation of CH4 to HCHO over Ag1-ZnO with other photocatalysts. The reaction conditions in a–d: CH4 (45 mL min−1) +  air (5 mL min−1), and RH = 20%. Source data are provided as a Source Data file.

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The yield and selectivity of HCHO increase with the increasing of external heating temperature (<150 °C) under 1000 mW cm−2 light intensity irradiation, and the activity cannot be further increased at a higher temperature (Fig. 3b and Supplementary Table 10). The surface temperature of Ag1-ZnO sample reaches approximately 228 °C under external heating at 150 °C during the reaction (Supplementary Fig. 15). The HCHO yield is almost directly proportional to the light intensity on Ag1-ZnO when the surface temperature of the catalyst is controlled at 228 °C by heating (Fig. 3c and Supplementary Table 11). Only CO and CO2 are generated on 250 °C on Ag1-ZnO in the absence of light (Supplementary Fig. 14). These results fully demonstrate that the HCHO production is driven by light rather than heat. Heat input may promote the separation of photo-generated charge carriers and HCHO desorption, thereby promoting the generation of gas-phase HCHO. The results of comparative experiments demonstrate that no products are detected without photocatalysts or CH4 feed (Supplementary Fig. 14 and Supplementary Table 9), indicating that all products are derived from photothermal catalytic oxidation of CH4. Bare HCHO are obtained over Ag1-ZnO under visible (Vis)-light (λ > 420 nm) (Supplementary Fig. 14), indicating that the HCHO is mainly generated by UV excitation.

A long-term test was carried out to assess the stability of the Ag1-ZnO sample. The HCHO yield gradually decreases to 92.1 μmol h–1 after a long-term test of 12 h, and the activity of the catalyst reacted for 12 h can be restored after ultrasonic washing with ultrapure water (Fig. 3d and Supplementary Table 12). The HAADF-STEM image and the corresponding EDS elemental mapping, the magnified high-resolution AC-HAADF-STEM image, and XAFS of spent Ag1-ZnO (Supplementary Fig. 16) shows that Ag SAs are well retained after the long-term test. The XRD result confirms the stable crystalline structure of Ag1-ZnO after the long-term test (Supplementary Fig. 17). The XPS result of the spent Ag1-ZnO (Supplementary Fig. 18) indicates the consumption of lattice oxygen and surface carbon deposition after the long-term test. Additionally, the O2-TPO results show significant CO2 generation over the spent Ag1-ZnO when the temperature rises to around 130 °C (Supplementary Fig. 19). The above results indicate that the activity attenuation of Ag1-ZnO is mainly caused by carbon deposition and the inability of lattice oxygen regeneration in a timely manner.

To elucidate the advantages of gas‒solid photothermal catalytic selective oxidation of CH4 to HCHO in a flow reactor, the gas after reaction over Ag1-ZnO was absorbed for 3, 6 and 12 h with 2 mL of distilled water at 0 °C. The HCHO concentrations in the absorption solution at 3, 6 and 12 h are determined to be 160.9 ± 5.4, 297.1 ± 15.8 and 514.2 ± 33.7 µmol mL−1, respectively (Fig. 3e, Supplementary Fig. 20 and Supplementary Table 13). At the same time, there is a small amount of CH3OH in the absorption solution, which can effectively prevent the polymerization of HCHO. The 514.2 ± 33.7 µmol mL−1 (1.54 ± 0.10 wt.%) of HCHO solution obtained from the gas‒solid atmospheric pressure photothermal catalytic reaction over Ag1-ZnO outperforms all state-of-the-art photocatalysts (Fig. 3f and Supplementary Table 14)11,12,13,14,15,17,18,22,23,25, whether it is a gas–liquid–solid batch reaction or continuous flow reaction. It can be observed that this strategy can effectively solve the problems of products not being separated or concentrated in a timely manner in gas–liquid–solid reactions13,22. In addition, after 12-hour absorption of the gases from reactor, the absorption rates of HCHO and CH3OH can still reach 81.2 ± 5.1% and 91.1 ± 3.6%, respectively, indicating that it can be expected to synthesize industrial grade HCHO solutions with the extension of reaction time and the expansion of reactor scale. In summary, the gas–solid photothermal catalysis method for CH4 direct conversion to HCHO is of good industrial application prospects.

Insight into mechanism

To explore the origin of the high HCHO activity and selectivity over Ag1-ZnO, the charge separation efficiency of the catalysts was studied by photocurrent and electrochemical impedance measurements. ZnO has the lowest photocurrent density (22.5 μA cm−2), which is increased by approximately 2.3 and 2.9 times for Ag1-ZnO and AgNP-ZnO, respectively (Supplementary Fig. 21a). The electrochemical impedance results (Supplementary Fig. 21b) show that the electron transmission resistance decreases in the order of ZnO > Ag1-ZnO > AgNP-ZnO. Furthermore, in situ steady-state photoluminescence (PL) was carried out to explore the thermal assistant effect. The PL peak is significantly quenched with increasing temperature in all samples, indicating that heat can improve the internal charge separation of photocatalysts (Supplementary Fig. 22). The weaker PL peaks for Ag1-ZnO and AgNP-ZnO exhibit significantly suppressed charge recombination rates at both 25 and 250 °C (Supplementary Fig. 23), which is in good agreement with the photocurrent and electrochemical impedance analysis. The above results demonstrate that the introduction of Ag species and the heat input greatly improve the charge separation efficiency, which is conducive to the yield of oxygenates in photothermal catalytic CH4 oxidation.

In situ electron spin resonance (in situ ESR) was carried out to understand the charge transport behavior in the presence of H2O and O2 on all samples. The signal at g = 2.000 belonging to oxygen vacancies is only observed on AgNP-ZnO (Fig. 4a)36,37, indicating that Ag NPs contribute to the formation of oxygen vacancies on ZnO. Under illumination, the ESR signal at g = 1.959 (Zn+) in all samples increases rapidly37. The order of peak intensity is: Ag1-ZnO > AgNP-ZnO > ZnO (Fig. 4a), which is inconsistent with the photoelectrochemical results. It may be due to the fact that Ag SAs as a hole acceptor promote the transfer of electrons to Zn2+, while Ag NPs as an electron acceptor reduce the amount of Zn+. To confirm this speculation, in situ near-ambient pressure X-ray photoelectron spectroscopy (in situ NAP-XPS) was then performed. For the Ag1-ZnO sample, Zn 2p peaks exhibit a negative shift (Fig. 4c), indicating that Zn2+ works as the electron acceptor under irradiation. And the Ag 3d peaks also show a negative binding energy shift (Fig. 4d), indicating the increase in valence state of Ag, which is different from most of other elements38,39. The peak at 531.7 eV (Supplementary Fig. 25) attributed to the oxygen species near oxygen vacancies does not change much40. These results show direct evidence that Ag SAs act as the hole acceptor during photocatalytic reaction. For AgNP-ZnO sample, the signals for Ag 3d and Zn 2p separately exhibit a positive and a small negative shift (Supplementary Fig. 26a, c), and the peak at 531.7 eV is enhanced (Supplementary Fig. 26b), which support that the electrons on Zn+ can transfer to the Ag NPs and the holes transfer to the lattice oxygen for activation. Meanwhile, as temperature rising, the peak intensity of Zn+ increases on all samples (Fig. 4b and Supplementary Fig. 24a, b), indicating that heat input facilitates the separation of photogenerated electrons and holes. Compared with those obtained under Ar flow, the peak of Zn+ drops and the peak of oxygen vacancies disappears after O2 addition on Ag1-ZnO under illumination (Supplementary Fig. 24c), which is mainly due to O2 consuming the electrons of Zn+ and the activated O2 can fill oxygen vacancies. The intensity of Zn+ in the presence of H2O significantly increase compared with that obtained under Ar or O2 flow. The holes on Ag atoms are effectively consumed by H2O, promoting the separation of photo-generated electrons and hole, and thus achieving a large amount of Zn+ generation.

Fig. 4: Mechanistic studies.
figure 4

In situ ESR spectra of different samples at 75 °C (a), and Ag1-ZnO at different temperatures (b) in O2 + H2O in the dark (dashed line) and after 10 min of light irradiation (solid line). In situ NAP-XPS of Zn 2p (c) and Ag 3d (d) over Ag1-ZnO under dark and light irradiation. In situ DRIFTS of photothermal catalytic oxidation of CH4 over different samples under external heating at 150 °C (e) and over Ag1-ZnO under different external heating temperatures (f). g HCHO-TPD spectra of different samples. h GC-MS of HCHO produced by photothermal catalytic oxidation of CH4 with  H216O  +  16O2, H218O  +  16O2 or H216O  +  18O2 over Ag1-ZnO or Ag1-ZnO with 18O exchange. In situ ESR spectra of different samples in Ar at 75 °C (i), and Ag1-ZnO in Ar at different temperatures (j) in the dark (dashed line) and after 10 min of light irradiation (solid line). k COHP and ICOHP for adsorption of *CH2 intermediates on the lattice oxygen over ZnO and Ag1-ZnO slabs. The reaction conditions in (a), (b), (d): CH4 (45 mL min−1) +  air (5 mL min−1), and RH = 20%. Blue, red, black, white, and dark gray spheres in (k) represent Zn, O, C, H and Ag atoms, respectively. Source data are provided as a Source Data file.

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To verify the effect of Ag species on the gas‒solid photothermal catalytic oxidation of CH4, the intermediate steps were dynamically monitored on different samples by in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). For ZnO and Ag1-ZnO, the same characteristic peaks are observed. Upon light irradiation, the vibrational modes of *CH3 deformation (1156 cm–1)41, and the stretching vibrations of the C–H bond (2861 cm–1) and C–O bond (1075 cm–1) in the absorbed *OCH3 species appear immediately (Fig. 4e and Supplementary Fig. 27)42,43,44, indicating that CH4 is rapidly dissociated into *CH3 and exists in the form of *OCH3 over ZnO and Ag1-ZnO. Furthermore, the peaks of *CH2 stretching (1430 cm–1) and *HCHO species (2720 cm–1) gradually increase with the prolonged light irradiation45,46. Since the formation of HCHO via the conversion of CH3OH has been excluded (Supplementary Fig. 28), the results presented above suggest that the target product (HCHO) is generated from the conversion of the *CH2 intermediate, which comes from the further dehydrogenation of *CH3. Meanwhile, the growth of *HCOO (1878 and 1773 cm–1) and *CO32– (1539 and 1239 cm–1) species is assigned to the overoxidation of CH447,48,49. All peaks show stronger intensity on Ag1-ZnO, indicating that the doping of Ag SAs is beneficial for the dehydrogenation of CH4 to HCHO. In contrast, intense *HCOO and *CO32– species and weak *OCH3 and *HCHO intermediates can be observed for AgNP-ZnO (Fig. 4e, Supplementary Fig. 27), demonstrating that CH4 is more prone to overoxidation with the loading of Ag NPs. As the temperature increases, a significant increase in *OCH3 and *HCHO species can be observed over Ag1-ZnO, accompanied by the raise in the likelihood of excessive oxidation (Fig. 4f). Therefore, the selection of an appropriate reaction temperature can effectively promote the selective oxidation of CH4 to HCHO. In addition, the HCHO-TPD experiment was used to investigate the oxidation and desorption of HCHO on various catalysts. It can be seen that the desorption peak of HCHO located at 96 °C over Ag1-ZnO is much lower than that of ZnO (117 °C) and AgNP-ZnO (178 °C), indicating its easier HCHO desorption feature. HCHO begins to decompose into CO at approximately 40 °C and CO2 is released as the temperature increases over AgNP-ZnO, demonstrating that the loading of Ag NPs effectively promotes the decomposition of HCHO and the activation of lattice oxygen. Hence, the loading of Ag SAs on ZnO facilitates the formation and desorption of *HCHO; while the loading of Ag NPs on ZnO promotes *HCHO adsorption and activation of oxygen, leading to lower HCHO selectivity and the overoxidation of CH4.

To obtain an in-depth understanding of gas–solid photothermal catalytic selective oxidation of CH4 to HCHO over Ag1-ZnO, a series of isotope labeling experiments were carried out. The C–H bond fracture process of CH4 is elucidated in the reaction with 18O2 and D216O. As displayed in Supplementary Fig. 29, the generated water mainly exists in the form of HD16O, implying that hydrolyzed hydroxyl species mainly contribute to the dissociation of the C–H bond in CH4. The source of oxygen in the products during the reaction was also determined through isotope labeling experiments. All signals in CH3OH and HCHO remain basically unchanged with 16O2 and H218O (Supplementary Fig. 30a, c), while HCH18O and CH318OH are observed in the reaction with 18O2 and H216O (Supplementary Fig. 30e), indicating that O2 is the oxygen source for HCHO and CH3OH. The generated CO2 is mainly in the form of C16O18O with H218O and 16O2 or H216O2 and 18O2 (Supplementary Fig. 30b, d, f), demonstrating that H2O is involved in the overoxidation of HCHO and CH3OH. To clearly demonstrate that lattice oxygen can participate in the reaction, the reaction over Ag1-ZnO after 18O exchange was performed44,48. CH318OH and C16O18O are monitored over Ag1-ZnO after 18O exchange, confirming that the surface lattice O in the ZnO participates in the oxidation of CH4 (Supplementary Fig. 31). In addition, the GC-MS results of HCHO product in isotope experiments (Fig. 4h) further verify that the O in HCHO mainly comes from O2. And the detection of HCH18O using H216O + 16O2 as the feed gas over Ag1-ZnO with 18O exchange (Fig. 4h) also confirms the involvement of lattice oxygen in the generation of HCHO. Moreover, by comparing the effects of different O2 treatments on catalysts after the photothermal catalytic oxidation of CH4 under anaerobic conditions (Supplementary Fig. 32), the significance of lattice oxygen in the production of HCHO is confirmed, demonstrating that the photoactivation of O2 to replenish lattice oxygen is crucial for the reaction. Based on the above results, the photothermal catalytic oxidation of CH4 over Ag1-ZnO occurs via a Mars–van Krevelen mechanism. The lattice oxygen on ZnO participates in the reaction, and the consumed oxygen sites are filled with oxygen to continue the reaction.

Based on the analysis above, the activation process of lattice oxygen plays an important role in the gas‒solid photothermal catalytic selective oxidation of CH4 to HCHO, so the activation effect of heat and Ag SAs on lattice oxygen in ZnO was further investigated. In the dark, the ESR signal of oxygen vacancies disappears after ZnO loading with Ag SAs in Ar, while the ESR peak signal of oxygen vacancies increases after loading Ag NPs in Ar (Fig. 4i). Under light, the peak of oxygen vacancies in Ag1-ZnO appears clearly, while the signals of ZnO and AgNP-ZnO samples show almost no change compared to those in the dark, indicating that the loading of Ag SAs is beneficial for accelerating hole enrichment and thus facilitate the activation of surrounding lattice oxygen. At the same time, the increase in temperature also promotes the activation of lattice oxygen (Fig. 4j). The O2-TPD results (Supplementary Fig. 33) show that the peak at 153 °C attributed to the physisorption oxygen (100–200 °C) is observed in all samples, and the peak at 350–450 °C in all samples is attributed to the superficial lattice oxygen (350–600 °C)50. However, no obvious peak attributed to chemically adsorbed oxygen (200–350 °C) is observed in all samples. Moreover, for the Ag1-ZnO sample, the superficial lattice oxygen peak shifts to a lower temperature than that of the other samples, which proves that the loading Ag SAs on ZnO contributes to the mobility lattice oxygen (the capacity of lattice oxygen converting to the surface reactive oxygen species). The desorption peak of surface lattice oxygen on AgNP-ZnO locates at relatively higher temperature, which is due to the strong adsorption ability of Ag NPs towards oxygen species51,52. Then, crystal orbital Hamilton population (COHP) are used to analyze the covalent Zn–OCH2 interactions over ZnO and Ag1-ZnO. The positive and negative values in the COHP diagram correspond to bonding and antibonding contributions, respectively53. Compared with the ZnO slab, the integrated COHP (ICOHP) value of the Ag1-ZnO slab decreases and the Zn–O bond length increases, demonstrating that Ag SAs decrease the Zn–OCH2 bonding state occupancy (Fig. 4k and Supplementary Fig. 34). Therefore, the heat and the introduction of Ag SAs can promote the activation of lattice oxygen, thereby leading to the formation of HCHO.

To obtain a full image of the possible reaction pathways for the selective oxidation of CH4 to HCHO over Ag1-ZnO, DFT calculations were performed. ZnO (101) with an Ag atom (Supplementary Fig. 7c) is used for modeling, and the Gibbs free energy is corrected based on the experimental test temperature. *OH is a key species for the activation of the C–H bond in CH4 according to the isotope labeling experiment. For CH4 oxidation on the Ag1-ZnO system, as shown in Fig. 4a, H2O is absorbed (reaction energy (ΔE) = − 0.47 eV) and activated to form *OH (ΔE = 0.04 eV), and then the generated *OH interrupts the C–H bond in CH4 to form *(O)CH3 species and gas H2O (ΔE = − 1.17 eV). The calculation results of dissociation C–H in CH4 over Ag1-ZnO indicate that *OH prefers to break C–H in CH4 than that ZnO itself (Supplementary Fig. 35). After that, the *(O)CH3 can either couple with the adsorbed *H to form *CH3OH or directly continue dehydrogenation to form HCHO. The ΔE of direct *(O)CH3 dehydrogenation is −1.83 eV, and the competing pathway of *CH3OH formation must overcome a barrier (Ea) of 0.61 eV. The *CH3OH desorption to form CH3OH still requires a ΔE of 0.73 eV. That is, *(O)CH3 is more inclined to continue dehydrogenation. Then, the dissociated *(O)CH2 group was attached on the Ag atom with O–Ag bridge to form *HCHO (Ea = 0.20 eV, ΔE = − 0.54 eV). *HCHO undergoes desorption to generate HCHO (ΔE = 0.60 eV). Finally, O2 is activated and replenishes the consumed lattice oxygen. After comparing with the simulation results of the HCHO formation process on pure ZnO (Supplementary Fig. 37), it can be concluded that Ag SAs doping facilitates the desorption of HCHO and the replenishment of lattice oxygen, which agrees with the above characterization results. In addition, the Gibbs free energy results at different temperatures also indicate that an increase in reaction temperature can effectively promote the generation of HCHO (Fig. 5a, Supplementary Figs. 36 and 37).

Fig. 5: DFT calculations and the proposed reaction mechanism.
figure 5

a Calculated Gibbs free energy diagrams for CH4 oxidation to HCHO or CH3OH at 501.15 K on Ag1-ZnO. Proposed reaction mechanism for gas–solid photothermal catalytic selective oxidation of CH4 to HCHO on Ag1-ZnO from the energy band structure (b) and the molecular level (c). The inset images in (a) are the optimized configurations of the corresponding reaction intermediates. Blue, red, black, white and dark gray spheres in (a) and (c) represent Zn, O, C, H and Ag atoms, respectively. Source data are provided as a Source Data file.

Full size image

A reaction mechanism for the gas–solid photothermal catalytic selective oxidation of CH4 to HCHO on Ag1-ZnO is proposed in Fig. 5b, c. Under thermal action, photogenerated electrons and holes on ZnO rapidly separate to form Zn+O− active sites under irradiation. The holes on O− transfer to Ag SAs and then extract the H atom from CH4 to form *CH2 species under the action of the surface *OH species. Then, the activation of lattice oxygen is promoted under the joint action of heat and Ag SAs, thereby facilitating the formation and desorption of *HCHO to form HCHO. Finally, the electrons on Zn+ activate O2 to form O2−, replenishing the consumed lattice oxygen.

Discussion

To summarize, we propose a continuous-flow gas–solid photothermal catalysis strategy under ambient pressure to overcome application bottlenecks regarding products concentration of traditional gas‒liquid‒solid photocatalysis. The rationally designed Ag1-ZnO photocatalyst has been proven to be a suitable photocatalyst. The Ag SAs accumulate photogenerated holes, which promote the C–H bond cleavage of CH4 with the assistance of *OH. And Ag SAs alter the local electron distribution of ZnO, which is beneficial for the formation and desorption of *HCHO to generate gas-phase HCHO. Furthermore, the heat input promotes the separation of photogenerated electrons and holes, and enhances the mobility of surface lattice oxygen, thus promoting the formation of HCHO. An ultrahigh HCHO (514.2 ± 33.7 µmol mL−1; 1.54 ± 0.10 wt.%) concentration is obtained with water absorption of the gases after a 12-hour reaction. With rational catalyst design, optimization of catalytic systems, and understanding of the mechanism, this work has laid a solid foundation for the sustainable selective oxidation of CH4 into valuable chemicals in industry.

Methods

Materials

All chemicals were used without further purification. Oxalic acid dihydrate (H2C2O4•2H2O, 99.99%), zinc nitrate hexahydrate (Zn(NO3)2•6H2O) and ethanol (CH3CH2OH, 99.5%) were bought from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4•3H2O, ≥99.9%), silver nitrate (AgNO3, ≥99.98%), hexahydrate chloroplatinic acid (H2PtCl6•6H2O, ≥99.9%) and palladium chloride (PdCl2, ≥99.9%) were purchased from Aladdin Chemical Co. Ltd. (Shanghai, China). Deuterium water (D2O, 98%) and heavy oxygen water (H218O, 98%) were purchased from Shanghai Yuanye Bio-Technology Co., Ltd (Shanghai, China). Dimethyl sulfoxide (DMSO, 99.99%) was purchased from Beijing Vokai Biotechnology Co., LTD (Beijing, China). Methane (CH4, 99.999%), air (99.9995%), Argon (Ar, ≥99.999%) and Oxygen-18 (18O2, 99%) were purchased from Dalian Special Gas Co., Ltd. (Dalian, China).

Synthesis of ZnO NPs

ZnO NPs were synthesized as following. First, a solution of H2C2O4 (50 mM, 100 mL) was added dropwise to a stirred solution of Zn(NO3)2 solution (50 mM, 100 mL) to obtain a zinc oxalate precipitate. Then the precipitate was washed with deionized water several times and dried overnight at 80 °C. The obtained zinc oxalate powder (5.00 g) was calcined in a muffle furnace at 350 °C for 6 h with a heating rate of 5 °C min−1 to obtain ZnO NPs.

Synthesis of Ag1-ZnO and AgNP-ZnO

Ag1-ZnO and AgNP-ZnO were synthesized through a simple sodium hypophosphite (NaH2PO2) reduction method. Specifically, ZnO NPs (50.0 mg) were dispersed in deionized water (50 mL) by sonication (30 min), followed by adding 158.0 μL (Ag1-ZnO, 0.912 μmol Ag) or 789.9 μL (AgNP-ZnO, 4.56 μmol Ag) of AgNO3 solution (5.76 mM) and stirring for 1 h. Then, sodium hypophosphite (5.78 mM) with a molar weight of 5 times AgNO3 was added dropwise to the above solution at 85 °C and stirred for 1.5 h. The obtained photocatalysts were washed several times with deionized water and dried under vacuum at 80 °C overnight. For comparison experiments, Au1-ZnO, Pt1-ZnO and Pd1-ZnO containing the same metal mass were successfully synthesized using the above method in addition to using HAuCl4, H2PtCl6 and PdCl2 as the corresponding metal precursors.

Synthesis of Ag1-ZnO with 18O exchange

For the isotope labeling experiment, 0.1 g of Ag1-ZnO was treated under a flow of Ar gas (100 mL min−1) bubbling through H218O at 350 °C for 2 h to exchange the lattice 16O with 18O. This was followed by another 2 h of Ar treatment without H218O at 350 °C to remove the residual H218O or −18OH on the surface.

Photothermal catalytic activity measurements

Photothermal catalytic oxidation of CH4 was conducted in a 50-mL flow reactor (PLR-GPTR50, Beijing Perfectlight) at different temperatures under a 300-W Xenon lamp (PLS-SXE300+, Beijing Perfectlight). The light was at a wavelength of 200 nm < λ < 1200 nm and the light intensity was 1000 mW/cm2. The schematic diagram of the experimental device and the physical photo of the reaction device are shown in Supplementary Fig. 8. In a typical test, 1.0 mL of an aqueous photocatalyst dispersion (4 mg mL−1) after 30 min of ultrasound was uniformly dropped into a quartz vessel (diameter 4 cm, height 1 cm), and then the obtain quartz vessel was dried at 353 K and placed in the reactor. Then the reactor was heated to a certain temperature, and different highly pure CH4 (99.999%)/air (99.999%, O2/N2 = 25 vol%) values at a total flow rate of 50 mL min−1 and different RH values (controlling the amount of wet air prepared by bubbling the air through 25 °C water) passed through the reactor to replace the gas for 30 min. After that, the light was turned on for the reaction. The temperature of the catalyst surface was measured using a thermocouple at the bottom of the quartz vessel as shown in Supplementary Fig. 8b.

CO, CO2, C2H6 and CH3OH content were based on the calibration curve by detected via gas chromatography (GC, FULIGC9790II) equipped with a methanizer and a flame ionization detector using a column (Porapak Q) (Supplementary Fig. 9). The gaseous HCHO was collected by introducing it into 50 mL of water for 5 min at 0 °C, and the amount of HCHO in the absorption solution was determined by the colorimetric method (Hitachi U4100). The concentrations of HCHO and CH3OH in the 3 h and 12 h absorption solutions were quantified by colorimetric method and 1H NMR (WNMR-I400), respectively. For detecting HCHO, 5 mL of the absorption solution was mixed with as-prepared 1 mL 0.25% (v/v) acetylacetone solution and the mixed solution was kept in boiling water for 3 min. The HCHO content was based on the calibration curve by detecting the absorption intensity at 413 nm (Supplementary Fig. 10). For detecting CH3OH, 0.5 mL absorption solution was mixed with 0.1 mL D2O and 0.05 μL DMSO. The concentration of CH3OH was calculated based on the internal standard DMSO.

The selectivity of HCHO was defined as follows: HCHO selectivity = the number of moles of HCHO/the number of moles of (CO + CO2 + C2H6 + CH3OH + HCHO) × 100%. The selectivity of the CO + CH3OH + HCHO products was calculated as follows: CO + CH3OH + HCHO selectivity = the number of moles of (CO + CH3OH + HCHO)/the number of moles of (CO + CO2 + C2H6 + CH3OH + HCHO) × 100%.

AQY of HCHO over Ag1-ZnO was determined using the following equation:

$${{{\rm{AQY}}}}\left({{{\rm{HCHO}}}}\right)=\frac{N({{{\rm{electrons}}}})\,}{N({{{\rm{photons}}}})}\times 100\%$$
(1)

where N(electrons) and N(photons) represent the number of reacted electrons and the number of incident photons, respectively. In accordance with the chemical equation (\({{{{\rm{CH}}}}}_{4}{-4{{{{\rm{e}}}}}^{-}}{\to }{{{\rm{HCHO}}}}\))10,11,54, N(electrons) = 4 × N(HCHO) = 4 × M(HCHO)NA, where N(HCHO), M(HCHO) and NA represent the number of produced HCHO molecules, the number of HCHO moles and Avogadro’s constant, respectively. The number of incident photons is calculated as N(photons) = IAt/Eλ, where I, A, t and Eλ denote light intensity (W cm−2), irradiation area (cm−2), light incident time (s) and photon energy (J), respectively. Eλ=hc/λ (λ = 365, 400 or 420 nm). The light intensity I at different wavelengths (365, 400 and 420 nm) was 0.00764, 0.0879, and 0.110 W cm−2, and the irradiation area coated with 20 mg catalyst was A = 12.56 cm−2. The surface temperature of the catalyst was controlled approximately 228 °C under monochromatic light irradiation.

Characterizations

Powder XRD (BRUKER D8 Advance) was carried out with Cu Kα radiation to obtain the crystalline structure of the photocatalysts. TEM (JEOL JEM-F200) and STEM (JEOL JSM-7500F) were performed to observe the morphology of the photocatalysts. The contents of precious metals (Au, Ag, Pt, and Pd) in the samples were measured by ICP‒OES (Agilent 720). XPS (Escalab 250Xi) was used to detect the surface chemistry of photocatalysts. In situ NAP-XPS measurements were carried out on a SPECS NAP-XPS system. The Ag K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) of the samples were collected under ambient conditions, where a pair of channel-cut Si (111) crystals was used in the monochromator. The Ag1-ZnO data were obtained in the fluorescence mode. The reference Ag foil and AgNP-ZnO were measured in transmission mode. The energy of the storage ring was 2.5 GeV with an average electron current of <200 mA.

The optical properties of the photocatalysts were determined by UV–vis diffuse reflectance spectroscopy (Hitachi U4100). The PL (FLS 980), transient photocurrent (TPC), and electrochemical impedance (EIS) of the photocatalyst were obtained to evaluate its photoelectric activity. The TPC and EIS were performed in a 0.1 M Na2SO4 aqueous. A fluorine-doped tin oxide electrode coated with the photocatalyst, a platinum wire, and saturated calomel electrodes were tested under a 300-W xenon lamp as the working, counter, and reference electrodes, respectively. The working electrodes were prepared using the colloid drop coating method. 5 mg of photocatalyst was dispersed in a mixed solution (0.01 mL of 5% Nafion, 0.2 mL of isopropanol and 0.6 mL of water) with ultrasonication.

In situ ESR spectra were measured on a JES-FA 200 JOES spectrometer. In the test, the sample (20 mg) was placed in a nuclear magnetic resonance tube under different atmospheres for 20 min. The ESR data were then recorded under full-spectrum illumination at different temperatures. Due to the influence of temperature on the peak signal, the highest test temperature in this study was 75 °C.

In situ DRIFTS test for photocatalytic selective oxidation of CH4 was conducted on a Nicolet 6700 FTIR spectrometer. First, the samples were pretreated at 350 °C for 1 h using N2 with a flow rate of 50 mL min−1 to remove the residual substances on the photocatalyst surface. Then the CH4 mixed gas (CH4: Air = 45: 5, RH = 20%, flow rate = 50 mL min−1) was passed through the reaction chamber at different temperatures for 30 min to reach the gas–solid stable state. The background spectrum with the CH4 mixed gas flow was recorded at the different temperatures and was then subtracted from the total spectra. The samples were scanned 30 times (each time per 60 s) under 300-W xenon lamp irradiation.

In situ DRIFTS of CO adsorption was conducted on a Bruker Tensor 27 FTIR spectrometer. First, all photocatalysts were treated with pure He at a flow rate of 30 mL min−1 and a temperature of 150 °C for 1 h before cooling. After the samples were cooled to 30 °C, the background spectrum was recorded at a flow of 30 mL min−1 He, and CO/He (2 vol.%) was then introduced into the reaction cell at a flow of 30 mL min−1. Then, pure He was passed again through the reaction chamber at a flow of 30 mL min−1 to desorb the gaseous CO. Spectra were recorded during all the processes.

O2-TPO, O2-TPD and HCHO-TPD were conducted on a custom-made multipurpose adsorption instrument (DAS-7000, Guangzhou Huasi Co., Ltd., China). For O2-TPO, 50 mL min−1 O2 was introduced to the sample (0.1 g) at 30 °C, after which the temperature was increased from 30 to 800 °C at a rate of 10 °C min−1, and the data were recorded. For O2-TPD, the sample (0.1 g) was pretreated with pure He (flow rate = 50 mL min−1) at 150 °C for 1 h, the temperature was then reduced to 30 °C, and the sample was treated with a flow of O2 for 2 h. Then, the O2 flow was switched to a flow of He, after which the temperature was increased from 50 to 800 °C at a rate of 10 °C min−1, and the data were recorded. The method of HCHO-TPD is consistent with the testing method of O2-TPD, except for changing O2 adsorption to 50 ppm HCHO standard gas adsorption after He gas treatment at 350 °C.

Computational methods

The Vienna ab initio Simulation Package (VASP5.4.4) was used to perform all spin-polarized DFT simulations within the generalized gradient approximation (GGA) based on a Perdew−Burke−Ernzerhof (PBE)55,56. A generalized gradient approximation was chosen. The core-valence electron interactions were described using the projector augmented-wave method. The first Brillouin zone was sampled with a 3 × 1 × 1 Monkhorst–Pack grid57. The plane wave energy cutoff was set to 400 eV to optimize the geometric structures. The energy and electronic force convergence tolerances were below 1 × 10−5 eV and 0.02 eV/Å, respectively. The reaction pathway was explored with the NEB method, with Climbing image or Dimer method to locate the transition state. Based on the comparison between the calculated coordination and bond length of Ag single atoms on different facets (Supplementary Fig. 7c) and the EXAFS fitting results of Ag1-ZnO (Supplementary Table 1), ZnO (101) was used for DFT calculations. The optimized ZnO (101) supercell (p 2 × 5) parameters were a = 12.27 Å, b = 16.25 Å, c = 20 Å, α = β = 90°, and γ = 105°. The free energy of the reaction on the surface was calculated as G(T) = εele + Δ ZPE + ΔH – TΔS, where εele, ΔZPE, T, ΔH and ΔS were the total energy for the adsorption of intermediates on the model surface, the zero-point energy change, the reaction temperature, the enthalpy change, and the entropy change, respectively.