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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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Surface Entropy Mediated Hydrogen Spillover on Au/TiO 2 : Influences of Strongly Adsorbed Water on H 2 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 in situ 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.
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- Award ID(s):
- 2102525
- PAR ID:
- 10641464
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- Journal of the American Chemical Society
- Volume:
- 147
- Issue:
- 33
- ISSN:
- 0002-7863
- Page Range / eLocation ID:
- 29908 to 29918
- Subject(s) / Keyword(s):
- Adsorption Entropy Hydrogen Oxides Thermodynamics
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1021/jacs.5c06813
