Upside-Down Adsorption: The Counterintuitive Influences of Surface Entropy and Surface Hydroxyl Density on Hydrogen Spillover
Research Poster Physical Sciences & Mathematics 2025 Graduate ExhibitionPresentation by Kelle Hart
Exhibition Number 115
Abstract
Although hydrogen spillover is often invoked to explain anomalies in catalysis, spillover remains a poorly understood phenomenon. Hydrogen spillover (H*) is described as highly mobile H atom equivalents that arise when H2 migrates from a metal nanoparticle to an oxide or carbon support. In the 60 years since its discovery, few methods have become available to quantify or characterize H*-support interactions. We recently showed in-situ infrared spectroscopy and volumetric chemisorption can quantify reversible H2 adsorption on Au/TiO2 catalysts, where adsorbed hydrogen exists as H* and interacts with titania surface hydroxyl (TiOH) groups. Here, we report parallel thermogravimetric analysis and Fourier transform infrared spectroscopy methods for systematically manipulating the surface TiOH density. We examine the role of surface hydroxylation on spillover thermodynamics using van't Hoff studies to determine apparent adsorption enthalpies and entropies at constant H* coverage, which is necessary to maintain constant H* translational entropy. Although surface TiOH groups are the likely adsorption sites, the data show removing hydroxyl groups increases spillover. This surprising findingthat adsorption increases as the adsorption site density decreasesis associated with improved thermodynamics on dehydroxylated surfaces. A strong adsorption enthalpy–entropy correlation implicates the changing surface entropy of the titania support itself (i.e., an initial state effect) is deeply intertwined with the H* configurational entropy. These effects are surprising and should apply to all low-coverage adsorbates where entropy terms dominate more traditional enthalpic considerations. Moreover, this study points toward a kinetic test for invoking spillover in a reaction mechanism: namely, in situ dehydroxylation should enhance spillover processes.
Importance
Hydrogen spillover is a phenomenon that occurs primarily on heterogenous catalysts. Spillover is frequently invoked in many catalytic reaction mechanisms. These invocations commonly have little to no concrete evidence that the hydrogen spillover phenomenon is occurring during the reaction. Here we provide the first clear kinetic test for invoking hydrogen spillover in a catalytic reaction mechanism: namely, in-situ dehydroxylation (removing surface hydroxyls from the catalyst) should enhance the rate of catalytic reactions where H spillover has been invoked in a kinetically important step. We provide robust thermodynamic explanations for the inverse relationship between surface hydroxylation and hydrogen spillover. These explanations revolve around the intrinsic surface entropy of the catalyst support, of which is not commonly discussed in literature and should be considered in future studies.