To commence our study, we carried out the photocatalytic reaction using various heterogeneous photo-catalysts in a handcrafted chamber illuminated by full-spectrum light (Fig. 2 and Table S1). The control experiments revealed that the combination of CH4 and CO2 at room temperature did not yield any detectable CH3OH in the absence of light or catalysts. Utilizing commercial GaN powder as a catalyst led to a higher methanol rate of 41 μmol g−1 h−1 than other semiconductors traditionally used for photo-CH4 dehydrogenation and CO2 reduction. This finding underscores GaN’s exceptional potential for efficient photocatalytic conversion of CH4 and CO2 into CH3OH.Fig. 2: Methanol production through co-conversion of CH4 and CO2 using an array of semiconductors and metal/GaN-cr interfacial catalysts.The rates of methanol production were assessed under initial reaction conditions using 1 mg of catalysts with irradiation from a 300 W full-spectrum Xenon lamp at room temperature and ambient pressure.To further boost the reaction efficiency, air-stable single-metal particles, including Ir, Ru, Pt, Rh, Pd, and Au, were in situ deposited on the GaN support via a universal chemical-reduction method (Fig. S1), forming an active interface denoted as metal/GaN-cr. Among these, monometallic interfaces featuring Au and Pd nanoparticles provided a notable increase in photocatalytic activity. In the field of both traditional thermal and sustainable-energy-driven reactions, bimetallic nanoparticles can remarkably augment catalytic performance in terms of conversion and selectivity, a phenomenon described as a 1 + 1 > 2 synthetic effect32,33. Motivated by this encouraging principle to improve reactivity, we experimented with 0.2 wt% metal loading of both physical mixture of Au with Pd and their alloy in varying weight ratios under standard conditions. Further, increasing metal loading to 0.5 wt% and even 1 wt% did not increase the CH3OH yield (Table S2). Thus, 0.2 wt% loading of alloyed AuPd/GaN interface with a 1-to-1 weight ratio eventually offers us the best CH3OH production rate among all tested samples.With this AuPd/GaN-cr catalyst in hand, we first characterized the nano-sized distribution of metal particles. Transmission electron microscopy (TEM) images of AuPd/GaN-cr demonstrate a suboptimal metal dispersion in bulky GaN support, a consequence of the in situ chemical reduction preparation (Figs. S2 and S3). To improve this apparent drawback of metal aggregation, we employed a soft colloid-immobilization process. This method enabled the assembly of the AuPd alloy with GaN, resulting in evenly distributed metal nanoparticles coupled with intact and highly active GaN (denoted as AuPd/GaN-ci)—as opposed to the etching by base or acid (Fig. 3a). As can be seen from a typical high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) image and its size population analysis of AuPd/GaN-ci (Figs. 3b–e and S4), the average diameter of 3.59 nm of metal colloid particles has been evenly immobilized on the GaN surface. The well-established bimetallic composition of supported AuPd alloy was also confirmed by the following energy dispersive X-ray (EDX) spectroscopy analysis. More importantly, despite successive load-wash-dry catalyst processing treatments, the composition and crystalline structure of the GaN host remained largely unchanged, as reflected by the X-ray diffraction (XRD) results (Fig. S5). This implies the great prospect of GaN powder as active support in photocatalysis industrial settings. The TEM and high-resolution TEM (HRTEM) results clearly illustrate the well-dispersion and formation of active metal/GaN interface, respectively (Fig. 3f, g). In addition, the enhanced UV–vis photo-adsorption property (Fig. S6) in UV and visible range of AuPd/GaN-ci might arise from interband absorption and plasmonic effect on metallic AuPd nanoparticles’ surface, which was further confirmed in the results of Au 4f and Pd 3d X-ray photoelectron spectroscopy (XPS) (Fig. S7).Fig. 3: Construction and characterization of AuPd/GaN-ci.a Schematic diagram of preparation for AuPd/GaN-ci. b HAADF-STEM, c–e elemental mapping, f TEM, and g HRTEM images of AuPd/GaN-ci. The inset of b and c shows the corresponding nanoparticle size distribution and selected area of EDX spectrum for AuPd/GaN-ci, respectively.Taking such an interfacial AuPd/GaN-ci as the best-in-class photocatalyst, we proceeded to evaluate its photocatalytic redox behavior relative to AuPd/GaN-cr under initial reaction conditions shown in Fig. 2. Notably, the CH3OH production rate over AuPd/GaN-ci can increase to a high value that is 2.6 times greater than that attained by AuPd/GaN-cr (Fig. 4a and Table S3). Detailed analysis further confirmed that CH3OH and CO are the predominant products in oxygenates and gas phases, respectively (Fig. S8 and Table S4). The distinctive disparity in reactivity between AuPd/GaN-ci and bare GaN, especially in CH3OH production estimated to be 34 times higher, affirms the critical role of AuPd/GaN interface in the photocatalytic co-conversion of CH4 with CO2 to CH3OH (Fig. 4b). Such inference is further consolidated by a giant leap in CH3OH yield from unsupported AuPd particles to AuPd/GaN-ci (Fig. 4b and Table S5). These findings inspired us to embark on trials with additional interfacial catalysts comprising alloy AuPd and various representative semiconductors. However, it did not lead to further enhancement of CH3OH yield, despite the wide-band structures of Ga2O3, ZnO, and TiO2 being known for direct methane activation (Fig. 4c and Table S6). These results suggest that the interfacial AuPd/GaN surface possesses a unique catalytic efficacy in converting CH4 and CO2 into CH3OH.Fig. 4: Catalysts and conditions optimization of photoinduced conversion of CH4 with CO2.a Methanol and CO yield rates over Au/GaN-ci, AuPd/GaN-ci, and Pd/GaN-ci under standard conditions. b The importance of AuPd/GaN-ci interface in photoconversion of CH4 and CO2. c The effect of different wide-band semiconductors on reactivity. d The methanol production of AuPd/GaN-ci under different ratios of CH4 and CO2 gas mixture, and e different pressures. f Performance comparison of AuPd/GaN-ci with the reported photocatalytic systems in the literature. (The references are listed in the Table S9) Standard reaction conditions: 2.5 mmol of CH4, 2.5 mmol of CO2, 1 mg of catalyst, full-spectrum Xenon lamp, 2 h.Considering the intrinsic nature of this novel redox transformation, involving CH4 and CO2 simultaneously, we investigated the impact of varying CH4/CO2 ratio on CH3OH production while maintaining total gas pressure of 1 bar (Fig. 4d and Table S7). Our data analysis revealed that an increase in the proportion of either CH4 or CO2 resulted in lower CH3OH yield and selectivity, demonstrating that a 1:1 ratio of CH4/CO2 is most favorable for our designed photocatalytic system. Moreover, when submitting this photoconversion to low pressure of 0.2 atm, it still achieved a CH3OH productivity of 435 μmol g−1 h−1, a rate that aligns with the record reported in prior studies for CH3OH (Fig. 4e and Table S8). More importantly, the yield rate of 1405 μmol g−1 h−1 obtained under ambient pressure exceeded those of other conventional photocatalytic syngas-based dry-reforming systems (Fig. 4f and Table S9). However, such active AuPd/GaN surface begins to give a reduced CH3OH yield after extensive reuse (Table S10), likely due to either unavoidable oxidation of metal cocatalyst with O species from CO2 (Figs. S9 and S10) or slow aggregation of metal (Fig. S11). In spite of this, the robustness and applicability of the photocatalytic system toward CH3OH production under batch reactor was demonstrated by reusing over 10 h with a total CH3OH yield of 13.66 mmol g−1 and a turnover number (TON) of 941, with no noticeable carbon depositions (Table S10 and Fig. S12).With such an improved CH3OH productivity, we move forward to understand the reaction mechanism under light-driven conditions, which appears to be very different from the well-known process of dry-reforming of CH4 and CO2. Overwhelming research on CH3OH synthesis finds that CO2, CO, and CH4 can each serve as the carbon source, reacting with appropriate reductants or oxidants2,34. In our analysis of gas products, CO was almost the exclusive product with negligible amounts of H2 and other hydrocarbons (Figs. S13 and S14 and Table S4). This observation effectively eliminates the significant impact of CH4 dehydrogenation as a side reaction35. Given the previous indirect pathway from CH4 (CO2) to CH3OH via intermediates (H2, CO, O2) (Fig. 5a inset and Table S11), we examined the reactivity of AuPd/GaN-ci in control experiments wherein identical ratios of H2 + CO2, H2 + CO, and CH4 + CO as reactants were used. As depicted in Fig. 5a, the control systems all showed much lower CH3OH yield. These experiments ruled out the potential pathway in which CO2 or CO, acting as the main carbon source, reacts with H2 – which could be possibly generated from CH4—to form CH3OH. Furthermore, replacing CH4 + CO2 with CH4 + O2 in a 10:1 ratio, where the number of oxygen atoms exceeds those theoretically generated from CO2 to CO (quantified as 6.6 μmol), did not prompt CH4 conversion to CH3OH. The result ruled out the CH4-oxidation with O2, possibly derived in situ from CO2 cleavage, as the primary mechanism for CH3OH production. On all accounts, it is induced that this unique photocatalytic CH3OH synthesis is mostly likely to undergo a direct transformation of CH4 and CO2 through surface catalysis rather than an indirect route mediated by intermediate such as CO, O2, or H2.Fig. 5: Mechanistic and kinetic study profiles for photoconversion of CH4 and CO2 over AuPd/GaN-ci.a Generation rates of methanol from photoconversion of 2.5 mmol H2 + 2.5 mmol CO2, 2.5 mmol H2 + 2.5 mmol CO, 2.5 mmol CH4 + 2.5 mmol CO, and 2.5 mmol CH4 + 0.25 mmol O2 over AuPd/GaN-ci. b GC–MS spectra of CO produced from photoconversion of 12CH4 + 12C16O2 and 12CH4 + 13C16O2. c GC–MS spectra of CH3OH produced from photoconversion of 12CH4 + 12C16O2, 13CH4 + 12C16O2, and 12CH4 + 12C18O2. d Kinetic isotope effects for photoconversion of CH4 with CO2. e The generation rate of methanol from CH4 photoconversion with CO2 over AuPd/GaN-ci under photo-driven and thermal catalytic conditions. The inset of e shows the Arrhenius plot of methanol photosynthesis over AuPd/GaN-ci.In the next study, the 13C-labeled experiment was implemented to further ascertain the carbon source of products, sketching the specific functions of the two gases in the overall reaction. When using 13CO2 instead of 12CO2, the characteristic peak of CO at m/z = 28 shifted up to m/z = 29 (Fig. 5b). In conjunction with the reductive control experiment discussed earlier, it is demonstrated that the majority of formed CO comes from CO2. In the presence of 13CO2, there is no detectable 13C labeled CH3OH (m/z = 33) by either NMR or GC–MS analysis, indicating that CO2 was not the carbon source of CH3OH (Fig. S15). Conversely, in the experiment with 13CH4, 13CH3OH signals were detected by both MS and NMR, thereby confirming the incorporation of CH4 as the primary carbon source and CO2 as the oxygen source (Figs. 5c and S16). To further trace the oxygen source in the methanol product, we conducted an O18 labeling isotopic experiment, where the generation of O18 labeled CH3OH coincides with the use of O18 labeled CO2 as the reactant. This evidence directly reveals the contribution of CO2 to the oxygen in CH3OH (Fig. 5c). Complementary to this synthesis approach, the determination of a high kinetic isotope effect (KIE) value of 3.79 and 4.88 for the CH3OH formation over AuPd/GaN-ci and GaN samples reveals that the C–H cleavage in CH4 is a critical component of the rate-determining step (Figs. 5d and S17). Based on all these findings, we proposed a distinctive transformation involving oxygen-atom-grafting (OAG) from CO2 to CH4, wherein CH4 mainly contributes to the carbon for CH3OH and CO2 to the oxygen for CH3OH in the reaction products.To understand the role of the AuPd/GaN-ci interface in the catalytic transformation, we collected and analyzed the UV-vis diffraction and Photoluminescence (PL) spectra of various samples, including GaN, Au/GaN-ci, AuPd/GaN-ci and Pd/GaN-ci (Figs. S6 and S18). As UV–vis comparative results showed, introducing mono- or bimetallic metal catalysts did not significantly improve light adsorption in pristine GaN. Furthermore, the only PL characteristic peak observed across all GaN samples with or without metal demonstrates that the photoabsorption at the metal/GaN interface mainly originates from GaN semiconductor, rather than the metal particles. This consistency in PL peaks and UV-vis results suggests a negligible contribution of light-excited electrons from the metal alloy, which might otherwise participate in the reactant’s activation36,37. On the other hand, the diminished absorption of GaN at wavelengths above 400 nm also explains the reduced reactivity under visible light conditions (Table S12). Meanwhile, thermographic photograph measurements demonstrated that this surface photocatalysis reaction occurred at 25 °C (Fig. S19). This result confirmed the absence of a concomitant photothermal effect upon light irradiation. All these analyses identify the synergy between photoexcited GaN and alloyed AuPd, forming an active photoelectron-transfer interface that potentially enables energy and electron transfer with CH4 and CO2. Such synergistic photo-active interface thus endows the OAG process with a lower activation energy (60 kJ mol−1) than GaN (72 kJ mol−1) alone (Fig. 5e inset and S20), as well as a higher apparent quantum efficiency (Table S13). Moreover, the negligible CH3OH yield under thermally driven conditions implies the AuPd/GaN-ci interface’s effectiveness in overcoming the thermodynamic challenges of traditional thermo-catalysis in OAG conversion (Fig. 5e and Table S14). The successful application of interfacial AuPd/GaN-ci in the OAG process introduces a new perspective on using photoexcited metal/semiconductor interfaces as a versatile toolbox in advancing sustainable chemical conversions.The further experimental findings that the molar ratio of CH3OH to CO is markedly increased on the combined AuPd/GaN-ci surface compared to the single GaN surface point toward the substantial synergistic effect of the AuPd/GaN-ci surface on activating both CH4 and CO2 (Fig. 6a inset). The interaction of CH4 and CO2 with the GaN surface has been recognized to effectively activate inert molecules for conversions19,28,30. To further elucidate this synergistic effect, density functional theory (DFT) calculations were thus performed to understand preliminary interaction behaviors that are critical for catalytic processes on GaN and AuPd/GaN-ci surfaces. The analysis commenced with the optimized configurations of the two surfaces interacting with CO2. Our DFT results show that the originally linear CO2 molecule adopts a bent structure with elongated C=O bonds upon adsorption on the polar GaN surface (Fig. 6b inset). This bending is essential for CO2 activation, with the O–C–O angle notably decreasing on the synergistic AuPd/GaN-ci surface. That significant variation in adsorption mode, compared to GaN alone, likely results from the apparent steric hindrance at the metal/GaN interface, as evidenced by the nonbonding interaction between the metal and oxygen in CO2 in both the adsorption model and corresponding differential electron density diagram (Tables S15 and S16). Consequently, the AuPd/GaN-ci surface exhibits more negative adsorption energy for CO2, indicating stronger affinity and enhanced CO2 activation. As a comprehensive result of such interaction, the evident electron transfer, particularly between CO2 and both metal and GaN surfaces, implies that adsorbed CO2 on the AuPd/GaN-ci surface is more likely to dissociate and release CO upon charge transfer (Fig. S21).Fig. 6: Calculation and experiment study profiles of pre-adsorption and activation processes for CH4 and CO2 on the surface of GaN and AuPd/GaN-ci samples.a The rates of CH4 and CO2 consumption and (inset of a) produced CH3OH-to-CO mole ratio for GaN and AuPd/GaN-ci. b The calculated adsorption energies (Eads) for adsorbed CO2 and c CH4 on GaN and AuPd/GaN-ci surfaces along with the corresponding optimized stereograms. Color code: C: dark gray; N: light gray; O: red; H: light pink; Ga: green; Au: orange; Pd: purple; d the C–H bond length of CH4 adsorbed on the comparative catalysts’ surface with the differential charge density stereograms (left, inset of d) for GaN and schematic of C–H bond activation in CH4 for AuPd/GaN-ci (right, inset of d). The cyan and yellow regions in the electron density cloud diagram indicate electron loss and gain.Based on the O18 labeling CO2 experiment discussed above, it is plausible that the remaining oxygen atom from CO2 is then positioned to potentially graft with activated CH4 on the AuPd/GaN-ci surface to form CH3OH. For the CH4 adsorption structures, CH4 preferentially locates closer to the synergistic AuPd/GaN-ci surface, resulting in lower adsorption energy for CH4 (Fig. 6c). This observation suggests a pronounced pre-adsorption characteristic of CH4 with low-polarizability over AuPd/GaN-ci. Even so, AuPd/GaN-ci surface with a considerable negative adsorption energy of CO2 reveals a higher affinity and easier activation for CO2 than CH4, which might explain the higher consumption rate of CO2 (Fig. 6a). Consistent with previous cases, our data show significant CH4 polarization on the GaN surface, evidenced by the disparity in the electron density cloud and the elongation of the C–H bond (Fig. 6d inset left and Tables S17 and S18). Importantly, the AuPd/GaN-ci surface further intensifies the polarity of the C–H bond in CH4, leading to even greater bond stretching compared to GaN alone (Fig. 6d inset right). These results support the hypothesis that the AuPd/GaN-ci surface amplifies the direct activation of CH4 compared to the single GaN surface. Considering all of the presented experimental and DFT data above, the synergistic surface of GaN with metal on AuPd/GaN-ci can facilitate strong enough interactions with both inert CH4 and CO2 at the same time. That multifunctional capability makes it possible to drive the surface-bound intermediates pathway rather than the gas phase for the oxygen-atom of CO2 grafting with CH4, leading to the formation of CH3OH and CO.Lastly, it is well known for its enhanced visible-light absorption capabilities when noble metals are incorporated into semiconductors38,39. For this reason, we decided to explore the potential of our synthesis for broader applicability in the visible light region, conducting this OAG reaction over GaN and interfacial AuPd/GaN-ci catalyst under the irradiation of a Xenon lamp with 435 nm wavelength of cut-off filter. Notably, the UV–vis absorption results confirm AuPd/GaN-ci’s better visible-light absorption compared to GaN (Fig. S6). As anticipated, the AuPd/GaN-ci gave a reappeared methanol yield rate in the OAG process while the unmodified GaN surface did not show reactivity (Table S19). Overall, this compelling observation presents the feasibility of our approach in CH3OH photosynthesis from CH4 and CO2 across a wide solar spectrum.In summary, we have discovered a direct and synergistic photocatalytic method for CH3OH synthesis by co-reforming two major greenhouse gases of CH4 and CO2 under mild conditions with very high selectivity. This method employs an interfacial metal/semiconductor as a photo-redox catalyst at room temperature. The photoexcited interface between AuPd nanoparticles and GaN support ensures simultaneous C–H activation of CH4 and CO2 reduction to produce methanol via an oxygen-atom-graft from CO2 to CH4. The scalability of this transformation was demonstrated in a batch reactor by recycling the catalyst multiple times. Adapting the herein OAG process to continuous flow operations, such co-conversions of CH4 and CO2 could revolutionize industrial CH3OH production, while also making a positive impact on climate change mitigation.
