Methanol Decomposition on TiO 2 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 TiO 2 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 TiO 2 (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.