Hydrogen Chemisorption on Silica-Supported Pt Clusters:  In Situ X-ray Absorption Spectroscopy

Hydrogen chemisorption on small silica-supported Pt clusters was investigated using in situ extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray absorption near-edge structure (XANES) spectroscopy. The clusters were found to exhibit a bulklike Pt first nearest neighbor (NN) distan...

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Bibliographic Details
Published in:The journal of physical chemistry. B 1997-06, Vol.101 (25), p.4972-4977
Main Authors: Reifsnyder, Scott N, Otten, Mark M, Sayers, Dale E, Lamb, H. Henry
Format: Article
Language:English
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Summary:Hydrogen chemisorption on small silica-supported Pt clusters was investigated using in situ extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray absorption near-edge structure (XANES) spectroscopy. The clusters were found to exhibit a bulklike Pt first nearest neighbor (NN) distance (2.76 Å) and low disorder while covered by chemisorbed hydrogen. In contrast, bare Pt clusters produced by heating in vacuo at 300 °C are characterized by a contracted Pt NN distance (2.66 Å) and greater disorder. These effects are reversed by re-exposure of the bare Pt clusters to H2 at 25 °C. The metal−support interface is characterized by a short Pt−O distance, irrespective of the presence of chemisorbed hydrogen. An apparent L3 edge shift of 0.8 eV relative to bulk Pt is observed for the hydrogen-covered clusters. This shift is attributed to a decrease in the Pt L3 edge resonance (white line) intensity, as no corresponding shift is observed at the L2 edge. A hydrogen-related L2,3 XANES feature at 9 eV appears with nearly equal intensity at each edge. This peak is assigned to electronic transitions from Pt 2p levels to H 1s−Pt 5d antibonding states with mixed d3/2−d5/2 character. From the L2,3 XANES analysis, we find that the number of unoccupied d states in hydrogen-covered Pt clusters is 23% less than in bulk Pt. In contrast, the L2,3 XANES spectra of bare silica-supported Pt clusters are closely similar to those of bulk Pt; quantitative analysis reveals only a slight (4%) decrease in the number of unoccupied d states.
ISSN:1520-6106
1520-5207
DOI:10.1021/jp970244e