Surface Entropy Mediated Hydrogen Spillover on Au/TiO₂: Influences of Strongly Adsorbed Water on H₂ Adsorption Thermodynamics

Hydrogen spillover, a poorly understood adsorption phenomenon, plays an important role in hydrogen storage, catalytic hydrogenation, and energy conversion processes. While widely invoked to explain anomalous observations, the fundamental mechanisms underlying spillover remain under debate, particularly regarding the influence of surface adsorbates, such as water. In this study, we investigate how strongly adsorbed water (SAW) impacts hydrogen spillover (H*) on Au/TiO₂ catalysts using <i>in situ</i> Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA). By carefully correlating IR and TGA data, we quantify the relationship between water coverage and spillover. At low to moderate temperatures (<200 °C), SAW resides primarily on Ti Lewis acid sites, while hydrogen spillover is associated with surface hydroxyl groups. Our findings reveal that even though H* and SAW do not directly compete for surface adsorption sites, SAW suppresses H*. Van't Hoff studies indicate SAW suppresses spillover by modifying the surface entropy of the titania, presumably by perturbing multiple proton transfer equilibria across the support surface. Maintaining constant water and hydroxyl coverage over a modest temperature range allowed for the determination of reliable thermodynamic parameters for hydrogen spillover on titania, yielding a slightly exothermic heat of adsorption (-7 ± 1 kJ/mol H*). These insights highlight the indirect role that surface water can play in catalytic reactions involving hydrogen spillover and offer a new perspective on catalyst design and optimization for hydrogen-involved reactions. This work also highlights the importance of considering the entropy of oxide surfaces in understanding catalysis over oxides.