Upside-Down\nAdsorption: The Counterintuitive Influences\nof Surface Entropy and Surface Hydroxyl Density on Hydrogen Spillover

Although hydrogen spillover is often\ninvoked to explain anomalies\nin catalysis, spillover remains a poorly understood phenomenon. Hydrogen\nspillover (H*) is best described as highly mobile H atom equivalents\nthat arise when H<sub>2</sub> migrates from a metal nanoparticle to\nan oxide or carbon support. In the 60 years since its discovery, few\nmethods have become available to quantify or characterize H*-support\ninteractions. We recently showed <i>in situ</i> infrared\nspectroscopy and volumetric chemisorption can quantify reversible\nH<sub>2</sub> adsorption on Au/TiO<sub>2</sub> catalysts, where adsorbed\nhydrogen exists as H* and interacts with titania surface hydroxyl\n(TiOH) groups. Here, we report parallel thermogravimetric analysis\nand Fourier transform infrared spectroscopy methods for systematically\nmanipulating the surface TiOH density. We examine the role of surface\nhydroxylation on spillover thermodynamics using van't Hoff studies\nto determine apparent adsorption enthalpies and entropies at constant\nH* coverage, which is necessary to maintain constant H* translational\nentropy. Although surface TiOH groups are the likely adsorption sites,\nthe data show removing hydroxyl groups increases spillover. This surprising\nfindingthat adsorption increases as the adsorption site density\ndecreasesis associated with improved thermodynamics on dehydroxylated\nsurfaces. A strong adsorption enthalpy–entropy correlation\nimplicates the changing surface entropy of the titania support itself\n(i.e., an initial state effect) is deeply intertwined with the H*\nconfigurational entropy. These effects are surprising and should apply\nto all low-coverage adsorbates where entropy terms dominate more traditional\nenthalpic considerations. Moreover, this study points toward a kinetic\ntest for invoking spillover in a reaction mechanism: namely, in situ\ndehydroxylation should enhance spillover processes.