Hydrogen spillover involves the migration of H atom equivalents from metal nanoparticles to a support. While well documented, H spillover is poorly understood and largely unquantified. Here we measure weak, reversible H2 adsorption on Au/TiO2 catalysts, and extract the surface concentration of spilled-over hydrogen. The spillover species (H*) is best described as a loosely coupled proton/electron pair distributed across the titania surface hydroxyls. In stark contrast to traditional gas adsorption systems, H* adsorption increases with temperature. This unexpected adsorption behaviour has two origins. First, entropically favourable adsorption results from high proton mobility and configurational surface entropy. Second, the number of spillover sites increases with temperature, due to increasing hydroxyl acid–base equilibrium constants. Increased H* adsorption correlates with the associated changes in titania surface zwitterion concentration. This study provides a quantitative assessment of how hydroxyl surface chemistry impacts spillover thermodynamics, and contributes to the general understanding of spillover phenomena.
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Raw data are available through ScholarSphere, Penn State’s open access repository at https://scholarsphere.psu.edu/ or available from the authors upon reasonable request. The atomic coordinates of the models in DFT calculations of charge density difference and spin density are provided in Supplementary Data 1.
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The authors gratefully acknowledge the Department of Energy Basic Energy Sciences Program (DE-SC0022053 and DE-SC0016192) for primary support of this work. Preliminary experiments were supported by the National Science Foundation (CBET-1803769, 1803808 and 2102430) and the Research Corporation for Science Advancement. The computational work was completed with resources provided by the Research Computing Data Core at the University of Houston. We thank M. Janik at Penn State for invaluable discussions and T. Xie for his assistance in collecting TEM data.
The authors declare no competing interests.
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Mahdavi-Shakib, A., Whittaker, T.N., Yun, T.Y. et al. The role of surface hydroxyls in the entropy-driven adsorption and spillover of H2 on Au/TiO2 catalysts. Nat Catal 6, 710–719 (2023). https://doi.org/10.1038/s41929-023-00996-3