Methanol Decomposition on TiO₂ Under Ambient Conditions: Cooperative Proton Transfer at Hydrated Interfaces

Methanol decomposition is a benchmark reaction for understanding surface-mediated redox chemistry and photocatalytic processes on oxide materials. While its mechanism on TiO₂ has been extensively studied under ultra-high-vacuum (UHV) conditions, the reaction pathway under ambient, liquid-phase environments remains largely unexplored. Here, we investigate the photocatalytic decomposition of methanol on rutile TiO₂(110) following direct adsorption from liquid solution by combining surface-sensitive sum frequency generation (SFG) spectroscopy with density functional theory (DFT) calculations. SFG measurements identify methoxy as the dominant surface species under these conditions. DFT calculations reveal that cooperative hydrogen-bonding interactions stabilize both the transition state and the dissociated configuration, leading to a systematic reduction in activation barriers. Concurrently, localized excess charge facilitates proton transfer through dynamic redistribution along the reaction coordinate, thereby lowering the barrier for O─H bond cleavage and stabilizing reaction intermediates. Under resonant bandgap excitation, methoxy undergoes structural reorientation followed by oxidation to form a stable surface hydroxymethyl species. DFT calculations further demonstrate that interfacial water and hydroxyl groups stabilize localized charge carriers, reduce activation barriers, and promote proton-coupled electron transfer. These findings provide molecular-level insight into the role of interfacial hydration in governing photocatalytic reaction pathways at oxide-liquid interfaces.