Efficient methane oxidation to formaldehyde via photon–phonon cascade catalysis

Methane oxidation to HCHO over Ru-ZnOFigure 1a shows the production rate and selectivity of HCHO from methane oxidation over different metal-loaded ZnO species. The setup of the reaction system is shown in Supplementary Fig. 2. HCHO and other liquid products were quantified using the calibration curves in Supplementary Fig. 3. For pure ZnO, HCHO is the primary product (302.1 μmol h−1), whereas CH3OH (31.5 μmol h−1) and CO2 (61.2 μmol h−1) rank as minor ones. Upon loading of Pd, Ag, Au and Pt, there was a gradual increase in the production rate of methanol, while the yield of CO2 also rose, leading to a lower HCHO selectivity. Ru generated more HCHO and reduced the production of CH3OH and CO2, exhibiting an impressive selectivity of 85.0% towards HCHO. Only minute quantities of HCOOH and CO were detected (Supplementary Figs. 3c and 4). Subsequently, the Ru loading amount was optimized; the optimized amount is 0.5 wt% (Supplementary Fig. 5a). High Ru loading amounts (>2 wt%) result in high selectivity of CH3OH. However, the main product is CH3OOH over 0.5Ru-ZnO at 30 °C (Supplementary Fig. 6), while the principal products remain CH3OH and HCHO over 2Ru-ZnO (Supplementary Fig. 5b). ZnO loaded with Au, Pd, Pt and Ag shows a similar product trend to 2Ru-ZnO (Supplementary Fig. 7). The variation in the product selectivity between low Ru and high Ru loading amounts indicates different functions of Ru species across the Ru loading range, which we discuss later.Fig. 1: HCHO production from methane oxidation by photon–phonon-driven cascade catalysis.a, The production rate of methane oxidation products over different cocatalyst-loaded ZnO species and their selectivity to HCHO operated at 150 °C. b, Influence of reaction temperatures on the production rate and selectivity of HCHO over 0.5Ru-ZnO. The following reaction conditions were used: 10 mg photocatalyst, 180 ml water, 20 bar CH4, 1 bar O2, 365 nm LED (75 mW cm−2, illumination area of 12.56 cm2), 1 h reaction. c, The effect of O2 partial pressure on the reaction over 0.5Ru-ZnO with a fixed CH4 pressure of 20 bar. The error bars for the 0.75 bar O2 condition were obtained from three independent reactions; those for the other conditions were obtained from two independent reactions. The data are presented as mean values ± standard error of the mean. d, The product yield over 0.5Ru-ZnO versus the reaction time with 0.75 bar O2 and 20 bar CH4. e, The stability test of the 0.5Ru-ZnO catalyst for 14 h with each cycle lasting 2 h. f, Summary of CH4 conversion and HCHO selectivity over the reported benchmark photocatalysts. The numbered data points are as follows: (1) Au1/In2O3 (ref. 16), (2) q-BiVO4 (ref. 12), (3) TiO2 (90%A + 10%R) (ref. 13), (4) WO3 (ref. 14), (5) Au/ZnO (ref. 15), (6) Cu-def-WO3 (ref. 17), (7) AuCu-ZnO (ref. 30) and (8) Pd-def-In2O3 (ref. 25).Source dataThe impact of temperature was examined over the selected 0.5Ru-ZnO catalyst (Fig. 1b). When the reaction temperature increased from 15 °C to 150 °C, the production rate of all products was enhanced roughly sixfold. Notably, the product distribution at temperatures below 100 °C differed from that at temperatures exceeding 100 °C (Supplementary Fig. 8). At 15 °C, the primary product was CH3OOH with a selectivity of 89%, while HCHO amounted to only 9%. When the temperature rose to 30 °C and 50 °C, the production rates of HCHO rose to 13.7 μmol h−1 and 40.8 μmol h−1, with selectivities of 11.5% and 21.0%, respectively. The production rate of HCHO reached 198.0 μmol h−1 with a selectivity of 66.0% at 100 °C, while the production rate of CH3OOH dwindled to 39.0 μmol h−1 with a selectivity of 14.4%. When the temperature increased to 150 °C, the production rate of HCHO further escalated to 448.5 μmol h−1 with a high selectivity of 85.0%. At 200 °C, the production rate of HCHO increased, but a dramatic rise in CO2 production (∼18%) resulted in a low HCHO selectivity of 71.7%. The optimal reaction temperature is thus 150 °C. This indicates that CH3OOH is transformed to HCHO at higher temperatures, suggesting that CH3OOH is prone to decomposing under higher-temperature conditions19,20,21.Following this, the O2-to-CH4 ratio was tweaked to increase HCHO selectivity (Fig. 1c). At lower O2 pressures (<0.75 bar), only a limited CO2 production rate was detected. The highest HCHO selectivity of 90.4% was achieved at an O2 pressure of 0.75 bar, with a production rate of 401.5 μmol h−1. Control experiments in the absence of light, methane or catalysts were conducted (Supplementary Fig. 9). Only trace quantities of HCHO and CO2 were detected under these conditions, substantiating the idea that HCHO originates from CH4 and that the process is predominantly induced by photocatalysis rather than thermocatalysis. We also investigated methane conversion and found that 0.5Ru-ZnO had a high methane conversion of 1.1% at 20 bar of methane after 1 h of reaction (Supplementary Fig. 10).Figure 1d shows the HCHO production over time. In the first 60 min, the HCHO production rate is very high (∼400 μmol h−1) because a high concentration of methane and a low concentration of products exist. The average HCHO production rate from 60 to 120 min drops to ∼100 μmol h−1 due to accumulated HCHO on the surface of the catalysts. Further extending the reaction time (>120 min), the production of formaldehyde nearly stops increasing, while the production of CO2 increases rapidly. This is because of the oxidation of HCHO when its concentration is rather high. Similar reaction trends were observed in the thermocatalytic oxidation of methane to methanol22. The results of HCHO oxidation in the presence and absence of methane confirm that the HCHO concentration in the reaction system notably influences the reaction kinetics (Supplementary Fig. 11). The stability of 0.5Ru-ZnO was also assessed (Fig. 1e, Supplementary Figs. 12 and 13, and Supplementary Tables 1 and 2). The 0.5Ru-ZnO catalyst displays constant activity over seven cycles, indicating its excellent stability. The used 0.5Ru-ZnO photocatalyst was subsequently characterized using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Both the phase structure of ZnO and the chemical valency of Ru remained consistent in 0.5Ru-ZnO after the long-term reaction (Supplementary Figs. 14 and 15). The performance of 0.5Ru-ZnO was compared with efficient catalysts previously reported for HCHO production from methane via either photocatalysis or thermocatalysis. Overall, 0.5Ru-ZnO showed a high methane conversion of 1.1% after 1 h of reaction, with a remarkable selectivity exceeding 90%. This represents a methane conversion that is an order of magnitude higher than those of previously documented photocatalysts with a high HCHO selectivity (Fig. 1f and Supplementary Table 3). Such high methane conversion is also comparable to that achieved by thermal catalysts operated at temperatures beyond 500 °C (Supplementary Fig. 16 and Supplementary Table 3), while the selectivity of HCHO (>90%) is somehow higher than that achieved by thermocatalysis (<65%).Characterization of photocatalystsZnO presents an absorption edge at 400 nm (Fig. 2a). Pronounced absorption throughout the visible spectrum is noticeable over 0.5Ru-ZnO and 2Ru-ZnO, probably due to the scattering effect of Ru. The XRD pattern of ZnO reveals that all peaks correspond to the hexagonal wurtzite phase (JCPDS no. 99-0111) of ZnO (Fig. 2b). No new peaks for Ru were discernible over 0.5Ru-ZnO and 2Ru-ZnO, due to the high dispersion or small particle sizes of Ru. XPS analysis was performed to further investigate the chemical states of Ru species. The C 1s spectrum of ZnO can be split into four peaks, corresponding to C=O, C–O, C–C and Zn–C, which is due to the residual C doping after the calcination of zinc oxalate at low temperatures23. An extra peak at 281.4 eV was identified as Ru4+ over 0.5Ru-ZnO, while two peaks of Ru4+ and Ru0 (280.5 eV) were observed in 2Ru-ZnO (Fig. 2c)24. These results suggest that the Ru species are in the Ru4+ state over 0.5Ru-ZnO and in a mixed state of Ru4+ and Ru0 over 2Ru-ZnO.Fig. 2: Characterization of the photocatalysts.a,b, UV–Vis spectra (a) and XRD patterns (b) of ZnO, 0.5Ru-ZnO and 2Ru-ZnO. c, Ru 3d and C 1s high-resolution XPS spectra of ZnO, 0.5Ru-ZnO and 2Ru-ZnO. Sat., satellite peak. d,e, High-resolution TEM images of 0.5Ru-ZnO (d) and 2Ru-ZnO (e). The yellow circles in d highlight the ZnO particles and in e highlight the Ru nanoparticles. f, The element mappings of 0.5Ru-ZnO. Scale bars in d–f, 10 nm. g, HAADF-STEM image of 0.5Ru-ZnO. Scale bar, 2 nm. The yellow circles highlight Ru single atom sites. h, Normalized Ru K-edge XANES spectra of 0.5Ru-ZnO with Ru foil and RuO2 as references. i, Fourier transform (FT) magnitudes of Ru K-edge EXAFS spectra in R space of 0.5Ru-ZnO and references.Source dataTransmission electron microscopy (TEM) shows that ZnO exhibits an average particle size of 10–20 nm (Supplementary Fig. 17a). No apparent Ru nanoparticles or clusters can be observed over 0.5Ru-ZnO, while obvious small dark dots can be observed over 2Ru-ZnO (Supplementary Fig. 17). The interplanar lattice spacing of 0.26 nm precisely corresponds to the (002) plane of ZnO (Supplementary Fig. 18). High-resolution TEM and scanning TEM (STEM) images also depict no discernible Ru species on 0.5Ru-ZnO, yet distinct 1–2 nm dark dots are observed on 2Ru-ZnO (Fig. 2d,e and Supplementary Fig. 19). Energy dispersive spectrometry mapping in Fig. 2f shows that Zn and Ru elements are uniformly dispersed over 0.5Ru-ZnO. Supplementary Fig. 20 shows that the weight percentage of Ru is 0.43 wt%. Aberration-corrected high-angle annular dark field STEM (HAADF-STEM) images were captured to further investigate Ru species over 0.5Ru-ZnO. Distinct bright spots were observed (Fig. 2g and Supplementary Fig. 21), and no obvious dimer structures or clusters were detected, suggesting that Ru is very likely dispersed in a monatomic state. Ru K-edge X-ray absorption near-edge structure (XANES) spectra and extended X-ray absorption fine structure (EXAFS) spectra were measured to investigate the coordination environment of 0.5Ru-ZnO. The Ru K-edge XANES spectra of 0.5Ru-ZnO show the oxidation state of Ru as between those of RuO2 and metallic Ru foil and closer to the former (Fig. 2h), indicative of Ru species in a partially oxidized state on the ZnO support. Fourier-transformed EXAFS at the Ru K-edge does not present a signal corresponding to Ru–Ru bonding (Fig. 2i), indicating that the atomically dispersed Ru are stabilized by the O atoms of 0.5Ru-ZnO. The coordination environment of Ru was delineated through curve-fitting analysis of the EXAFS spectra (Supplementary Fig. 22) and is quantitatively summarized in Supplementary Table 4. The curve-fitting results of 0.5Ru-ZnO elucidate a pronounced peak at a radial distance of 2.006 Å, with a coordination number of 4.6, attributable to the Ru–O bonding interaction, which originates from the backscattering between atomically dispersed Ru centres and the surrounding O atoms of the substrate. From these observations, Ru is dispersed as single atoms with a chemical state of Ru4+ in the low-Ru-loading-amount sample (0.5Ru-ZnO) and as nanoparticles in the high-Ru-concentration sample (2Ru-ZnO). These findings align with recent studies on other metal oxides loaded with single atoms16,25.Mechanism of methane oxidationElectrochemical characterizations were used to investigate O2 reduction and methane oxidation involved in methane oxidation by O2. A more negative current was achieved over 0.5Ru-ZnO than over ZnO (Fig. 3a), suggesting that Ru is an electron acceptor and promotes the oxygen reduction reaction. Moreover, 0.5Ru-ZnO exhibits a lower photocurrent density for methane oxidation than that of ZnO under a bias of 0.6 V versus reversible hydrogen electrode (RHE) in the CH4-saturated electrolyte (Fig. 3b). This is because Ru serves as an electron acceptor and competes with the small bias, thus reducing the electron transfer to the counter electrode via the external circuit (Supplementary Fig. 23).Fig. 3: Pathways of charge transfer.a,b, Electrochemical oxygen reduction curves (a) and transient photocurrent density curves (b) of ZnO and 0.5Ru-ZnO with a bias of 0.6 V versus RHE. c, PL spectra of ZnO and 0.5Ru-ZnO. d,e, Photo-induced light absorption of ZnO (d) and 0.5Ru-ZnO (e) in Ar, CH4 and air. f, Schematic illustration of the charge generation and transfer in 0.5Ru-ZnO.Source dataTo obtain more insights into the role of Ru in charge transfer and recombination, we used photoluminescence (PL) spectroscopy. The PL peak of Ru-ZnO is substantially diminished compared with that of ZnO, suggesting a notable reduction in the charge recombination (Fig. 3c and Supplementary Fig. 24). In situ photo-induced absorption (PIA) spectroscopy was employed to delve deeply into the charge transfer processes. An in situ ultraviolet–visible–near-infrared diffuse reflectance spectroscopy (UV–Vis–NIR DRS) system was developed to measure the reflectance of the photocatalyst under various lighting conditions and reaction atmospheres26. Essentially, a constant 365 nm LED light functions as a ‘pump’ light, triggering continuous electron transition from the valance to the conduction band of ZnO, thus creating free carriers. Photons from the probe lamp are absorbed by these free carriers, exciting electrons to higher energy states27. Through an analysis of the probe light absorption under dark (ground state) and light (excited state), PIA can be calculated by the following equation:$$\Delta {\rm{Abs}}=\frac{{R}_{{\mathrm{dark}}}-{R}_{{\mathrm{light}}}}{{R}_{{\mathrm{dark}}}}\times 100 \%$$
(1)
The calculated PIA spectrum shows absorption from 1,000 nm to 2,700 nm under Ar (Fig. 3d and Supplementary Fig. 25), which is attributed to free carrier absorption either in the conduction band or in the valence band of ZnO (ref. 27). The PIA of ZnO is quenched in air, attributable to the notable electron scavenging capacity of oxygen. The excited electrons in the conduction band are quickly consumed by oxygen, leaving less observable free carrier absorption. This supports the premise that the PIA of ZnO from 1,000 nm to 2,700 nm under Ar results from the excited electrons in the conduction band4,28. Upon the introduction of methane, a marginal increase in the PIA spectrum is noticeable compared with that in Ar, indicating that methane is a hole acceptor, allowing for more excited electrons in the conduction band. Following this, we explored the PIA of 0.5Ru-ZnO in an Ar environment to investigate the charge transfer between Ru and ZnO (Fig. 3e). In comparison with the PIA of ZnO, 0.5Ru-ZnO displayed a pronounced decrease, mirroring the phenomenon observed when air was introduced. This implies that Ru is capable of quenching the excited electrons in the conduction band of ZnO. The PIA absorption of 0.5Ru-ZnO under a methane atmosphere exhibited similar outcomes as ZnO. Ru therefore acts as an electron acceptor. The photogenerated charge transfer process of 0.5Ru-ZnO is illustrated schematically in Fig. 3f.The in situ electron paramagnetic resonance (EPR) spin-trapping technique was employed to investigate the mechanism of oxygen reduction, using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin trap15. 0.5Ru-ZnO generates more •OOH than ZnO (Fig. 4a), which is attributed to the superior separation efficiency of photoexcited carriers by the electron acceptor of Ru. However, an increase in Ru loading leads to a substantial drop in •OOH production due to the strong light scattering of Ru nanoparticles29. To quantify the concentration of •OOH radicals, we employed nitroblue tetrazolium (NBT) as a scavenger of superoxide radicals30. Pristine ZnO manifested a first-order kinetic constant of 0.035 min−1, whereas 0.5Ru-ZnO exhibited a higher value of 0.058 min−1 (Supplementary Fig. 26), which is consistent with the EPR results. The generation of •OH radicals was also analysed. 0.5Ru-ZnO produced the highest quantity of •OH, while 2Ru-ZnO generated even lower amounts than the pristine ZnO (Fig. 4b). In the meantime, the 7-hydroxy coumarin fluorescence signal at 454 nm also confirms that 0.5Ru-ZnO promotes the production of •OH, while 2Ru-ZnO produces a substantially lower amount (Supplementary Fig. 27), which is also due to the scattering effect of Ru nanoparticles29. To investigate whether methane is activated by photo-induced holes or •OH, we conducted control experiments with different solvents (Supplementary Figs. 28 and 29), together with adding sacrificial agents for •OH and h+ (Supplementary Fig. 30)31,32,33. All these clearly indicate the importance of •OH radicals in methane activation and high-value chemical synthesis, which should be produced by water oxidation by photoholes.Fig. 4: Mechanism of photocatalytic methane oxidation.a, EPR spectra of ZnO, 0.5Ru-ZnO and 2Ru-ZnO for the detection of •OOH radicals in air-saturated methanol solution under light irradiation. b, EPR spectra of ZnO, 0.5Ru-ZnO and 2Ru-ZnO for the detection of •OH radicals. c, GC–MS spectra of HCHO produced by CH4 oxidation over 0.5Ru-ZnO with 16O2 + H216O, 18O2 + H216O or 16O2 + H218O. d–f, In situ DRIFT spectra for photocatalytic CH4 conversion over ZnO (d), 0.5Ru-ZnO (e) and 2Ru-ZnO (f) in the presence of CH4, O2 and water vapour.Source dataTo confirm the oxygen source of HCHO, we conducted isotope experiments of methane oxidation over 0.5Ru-ZnO using 18O2 or H218O. The oxygen in HCHO is mainly from H2O rather than O2 (Fig. 4c), and the oxygen in HCHO produced by 0.5Au-ZnO and 2Ru-ZnO is also from H2O rather than O2 (Supplementary Figs. 31a and 32a). These results are consistent with previous isotopic experiments on HCHO produced by single-atom Cu-modified WO3 and ethane oxidation to acetaldehyde17,34. In contrast, the oxygen in CH3OH is from O2, as evidenced by the isotopic results over 0.5Au-ZnO and 2Ru-ZnO (Supplementary Figs. 31b and 32b), which is consistent with the isotopic experiments on CH3OH produced by Au-ZnO and AuNPs-In2O3 (refs. 15,16). These findings suggest that HCHO is produced by another reaction pathway rather than the oxidation of CH3OH.In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was employed to understand the photocatalytic oxidation process of methane. The peak at 1,304 cm−1 is ascribed to CH4 (Fig. 4d). 0.5Ru-ZnO shows a band at 2,854 cm−1, which is associated with the C–H stretching vibration of adsorbed •CH3 species (Fig. 4e)35. The intensity of this band increases with prolonged irradiation time. However, this feature was not observed over both ZnO and 2Ru-ZnO, strongly indicating the high methane activation efficiency of 0.5Ru-ZnO. The peaks at 1,335/1,350 cm−1, attributed to the vibration of adsorbed CH3OOH species36,37,38, were observable in 0.5Ru-ZnO but notably weaker in 2Ru-ZnO. Furthermore, vibrational peaks at 1,052 cm−1, 2,939 cm−1 and 2,971 cm−1, associated with methoxy and C–H stretching vibrations in CH3OH (refs. 39,40), are uniquely present in 2Ru-ZnO and 0.5Au-ZnO (Fig. 4f and Supplementary Fig. 33). This implies that methanol production is favourable over 2Ru-ZnO, aligning with the product selectivity of 0.5Ru-ZnO and 2Ru-ZnO in catalytic methane oxidation. Two peaks at 1,446/1,460 cm−1 and 1,594 cm−1 are attributed to HCOO• species35. The peak area of this band keeps increasing with time over ZnO, 0.5Ru-ZnO and 2Ru-ZnO, indicating that the consumption of HCOO• is much slower than its formation on the surface of the catalyst. The accumulation of HCOO• prevents further oxidation of HCOO• to CO2, consistent with the high selectivity of oxygenates over all catalysts41.To fully understand the reaction process of HCHO production from methane by 0.5Ru-ZnO, it is necessary to consider both photocatalysis and phonon energy. The methane oxidation performance by photocatalysis, thermocatalysis and photon–phonon-driven cascade catalysis is compared in Fig. 5a. Only a trace amount of HCHO was produced by thermocatalysis at 150 °C. A notable production rate of CH3OOH, accompanied by a modest quantity of HCHO, was observed by photocatalysis at 30 °C. CH3OOH is stable at room temperature (Supplementary Table 5). In contrast, the cascade of photocatalysis and phonon-driven decomposition generates a substantial quantity of HCHO (401.5 μmol h−1) with a high selectivity (>90%), highlighting the efficacy of phonon input in promoting the product yield. Compared with photocatalysis, the HCHO yield by photon–phonon-driven cascade catalysis increased by nearly 30-fold, and the selectivity improved almost eightfold. Moreover, CH3OOH, which is produced by the oxidation of methane through photocatalysis at 30 °C, is subsequently subjected to 150 °C without light irradiation (Fig. 5b). The secondary step leads to all CH3OOH molecules being transformed to CH3OH, HCHO and CO2, with HCHO as the primary product (∼80%). The transformation of CH3OOH over ZnO or in the absence of the catalyst shows similar results (Supplementary Figs. 34 and 35), suggesting that CH3OOH conversion to HCHO is a phonon-driven decomposition step.Fig. 5: Reaction pathways.a, Methane oxidation performance by photocatalysis, thermocatalysis and photon–phonon-driven cascade catalysis. The error bars were obtained from three independent reactions. The data are presented as mean values ± standard error of the mean. b, Photocatalytic methane conversion over 0.5Ru-ZnO first at 30 °C under light irradiation and then kept at 150 °C in the dark for 1 h. The following reaction conditions were used: 10 mg photocatalysts, 180 ml water, 20 bar CH4, 0.75 bar O2, 365 nm LED (75 mW cm−2, illumination area of 12.56 cm2)/without light, 1 h reaction. c, Reaction path for methane oxidation over RuSA-ZnO (red curve) and Ru-nanoparticle-decorated ZnO (RuNPs-ZnO, blue curve). The potential energy (eV) denotes the energy associated with different intermediates in the reaction. d, The proposed reaction pathways of photon–phonon-driven cascade catalysis for methane oxidation to HCHO. ad., adsorbed.Source dataTo explain the isotopic result and the high selectivity of CH3OOH over 0.5Ru-ZnO, we undertook theoretical modelling, and we propose the following reaction pathways (Fig. 5c,d and Supplementary Fig. 36). Upon photon irradiation, excited electrons in the conduction band of ZnO are transferred to Ru4+, reducing it to Ru(4−δ)+. Ru(4−δ)+ then reduces the adsorbed O2 to H2O. Methane is first activated by •OH radicals from water oxidation to produce adsorbed *CH3. The reaction energy barrier of *CH3 over RuSA-ZnO is lower than that over RuNPs-ZnO. Following this, the adsorbed *CH3 reacts with •OH species produced from the oxidation of adsorbed water to form adsorbed *CH3OH species. The adsorption energy of *CH3OH species on RuSA-ZnO (−1.30 eV) is lower than that on RuNPs-ZnO (−1.42 eV), which results in a lower reaction energy for the further reaction of *CH3OH species with •OH to form *CH3OOH on RuSA-ZnO. CH3OOH is therefore formed more easily on RuSA-ZnO, indicating that CH3OOH is the main intermediate on RuSA-ZnO, while there is less CH3OOH formed on RuNPs-ZnO. Subsequently, the adsorbed *CH3OOH desorbs to form CH3OOH or further undergoes dehydration to form the adsorbed *HCHO, which then desorbs in the form of HCHO. In addition, Ru with a high weight percentage (2Ru) and other metals (Au, Pd, Pt and Ag) were loaded onto ZnO in the form of nanoparticles (Supplementary Fig. 37), which probably creates a new reaction pathway for CH3OH production (Supplementary Fig. 38). It is proposed that photogenerated holes oxidize H2O to produce •OH, which then oxidizes CH4 to •CH3. In parallel, O2 is reduced by photoelectrons to generate H2O2 and then •OH. CH3OH is next produced from the combination of •CH3 and •OH radicals on the nanoparticle cocatalysts.