The conversion of methane, a readily available carbon resource, under mild conditions presents a significant global challenge. Current industrial methods, such as steam methane reforming, require high temperatures (700-1100°C) and pressures (20-40 atm), making them energy-intensive. Developing on-site and on-demand methane conversion technologies at ambient conditions is crucial for sustainable methane utilization. Photocatalysis, using light to drive redox reactions, offers a promising approach for achieving this goal. However, despite various studies, the microscopic mechanism of non-thermal methane activation remains unclear. A comprehensive understanding of the underlying mechanism is essential for designing effective photocatalytic systems. Previous studies using ex-situ techniques, such as electron spin resonance and fluorescence spectroscopy, have hinted at the involvement of photogenerated holes and surface species, but definitive identification of the reactive species and intermediates under actual working conditions has been challenging. The current research addresses this gap by employing in-situ/operando techniques to elucidate the mechanism of non-thermal C-H activation and inform strategies for designing molecular-level interfacial chemical systems.

Extensive research has explored the photocatalytic conversion of methane, focusing on the role of photogenerated holes and electrons in driving oxidation and reduction reactions on photocatalyst surfaces. Various studies using techniques like electron spin resonance (ESR) and fluorescence spectroscopy have indicated the presence of photogenerated holes trapped at surface lattice oxygen sites or as surface hydroxyl radicals. However, challenges remain in definitively identifying the reactive species responsible for non-thermal methane activation due to limitations in ex-situ measurement techniques, which often involve conditions significantly different from those in actual photocatalytic reactions (e.g., liquid nitrogen temperatures or the addition of spin trap scavengers). This has limited our ability to fully grasp the mechanism of C-H bond activation and develop optimal strategies for efficient methane conversion. The lack of in-situ/operando studies capable of identifying reactive species and intermediates under realistic reaction conditions hinders the progress in designing efficient photocatalytic systems.

This study combined real-time mass spectrometry, operando infrared (IR) absorption spectroscopy, and ab initio molecular dynamics (AIMD) simulations to gain microscopic insights into photocatalytic methane conversion. Three metal oxides – Ga₂O₃, NaTaO₃, and TiO₂ – were employed as representative photocatalysts, representing both d¹⁰ and d⁰ configurations. The reaction activity was evaluated under both dry (PH₂O = 0 kPa) and wet (PH₂O = 2 kPa) conditions with varying methane pressures. The temperature increase under UV irradiation was minimal (~20 K). Isotopically labeled water (H₂¹⁸O) was used to investigate the role of interfacial water in the conversion process. Operando IR spectroscopy using isotope-labeled water (D₂O) was also performed to monitor the reaction mechanism in real-time. AIMD simulations, using density functional theory (DFT) calculations with a hybrid functional, were carried out to provide detailed molecular-level insights into the C-H activation kinetics on a β-Ga₂O₃ surface. The computational methods included investigating both neutral and charged states to adequately model the reaction environment. Detailed descriptions of the catalyst preparation methods (including Pt loading on Ga₂O₃, NaTaO₃, and TiO₂), reaction activity measurements, operando DRIFT spectroscopy setup, and AIMD simulation parameters are provided in the supplementary materials. The analysis of experimental results and simulation data involved fitting the experimental data using relevant kinetic models to determine rate constants and elucidate the reaction mechanism.

The key findings of the study are as follows: 1. **Interfacial Water's Crucial Role:** The study demonstrated a dramatic increase in methane conversion rates (more than 30 times) under wet conditions compared to dry conditions for all three photocatalysts (Pt/Ga₂O₃, Pt/NaTaO₃, and Pt/TiO₂). This highlights the critical role of interfacial water in photocatalytic methane conversion. 2. **Water-Mediated C-H Cleavage:** Experiments using isotopically labeled water (H₂¹⁸O) revealed that the interfacial water, rather than lattice oxygen, is the primary source of oxygen in CO₂ formation. Operando IR spectroscopy with D₂O provided direct evidence of hydrogen abstraction from methane by photoactivated OD radicals on the photocatalyst surface, confirming that interfacial water mediates the initial C-H bond cleavage (CH₄ + OD(ad) → CH₃(a) + HDO(ad)). 3. **Moderate Stabilization of Intermediates:** AIMD simulations showed that under dry conditions, the formation of CH₃ radicals is highly exothermic and leads to overstabilization on the Ga₂O₃ surface, hindering further reactions. However, under wet conditions, the presence of interfacial water moderates the stabilization of CH₃ radicals, enabling further reactions and improving the overall conversion rate. 4. **Non-Linear Methane Pressure Dependence:** The study observed non-linear dependence of methane conversion rates on methane partial pressure (PCH₄), indicating that the initial C-H activation step is not rate-limiting. Instead, subsequent reactions involving surface intermediates (such as CH₃) appear to control the overall reaction rate. 5. **Kinetic Preference for Water Oxidation:** While thermodynamically, methane oxidation is more favorable than water oxidation, the results show a kinetic preference for the oxidation of water by surface-trapped holes, followed by water-mediated methane activation. This suggests that the kinetic factors, rather than thermodynamics alone, govern the reaction pathway. 6. **Surface Homocoupling:** The experimental data and modeling suggest that the homocoupling reaction (2CH₃ → C₂H₆) occurs on the surface of the water-covered photocatalyst, further supporting the importance of the interfacial water layer in facilitating the reaction.

The findings of this study address the long-standing question of the microscopic mechanism behind photocatalytic methane activation. The dramatic enhancement of methane conversion under wet conditions, confirmed for different metal oxide photocatalysts, unequivocally demonstrates the crucial role of interfacial water. This goes beyond simply providing a reaction medium; interfacial water actively participates in the C-H bond cleavage process by acting as a hydrogen abstracting agent. The detailed mechanism revealed through operando spectroscopic measurements and AIMD simulations highlights the importance of balancing the stabilization of reaction intermediates. Overstabilization, as observed under dry conditions, inhibits further reactions and leads to low conversion rates. The presence of interfacial water mitigates this overstabilization, enabling efficient methane conversion. The non-linear dependence of the reaction rate on methane partial pressure indicates a complex reaction pathway with multiple rate-limiting steps involving surface intermediates. The observed kinetic preference for water oxidation over methane oxidation, despite thermodynamic considerations, underscores the importance of kinetic factors in photocatalytic reactions. This study provides valuable insights that can guide the rational design of highly efficient photocatalytic systems for methane conversion.

This research provides a comprehensive understanding of the role of interfacial water in photocatalytic methane conversion, showing that it significantly accelerates C-H activation at ambient conditions. The combination of experimental and computational techniques revealed a water-mediated mechanism where photoactivated interfacial water species facilitate C-H bond cleavage and prevent the overstabilization of intermediates. This results in a dramatic increase in conversion rates. This work lays the foundation for designing effective photocatalytic systems for sustainable methane utilization by emphasizing the importance of interfacial engineering. Future research could explore different water configurations, other photocatalyst materials, and alternative strategies to further enhance photocatalytic methane conversion efficiency.

The study primarily focused on three specific metal oxide photocatalysts. The generalizability of the findings to other photocatalyst materials needs further investigation. The AIMD simulations were conducted on a simplified β-Ga₂O₃ model, and more complex models incorporating defects and different surface terminations could provide a more complete picture. While operando IR spectroscopy provided valuable insights into reaction intermediates, the identification of all intermediates and their detailed reaction pathways could be further improved using more advanced characterization techniques.