X-ray-absorption study of the interaction of hydrogen with clusters of supported palladium

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Abstract
The extended x-ray-absorption fine structure (EXAFS) and x-ray-absorption near-edge structure (XANES) at the Pd K edge were used to observe the interaction of hydrogen with palladium clusters, 5 nm in average size, supported on alumina. A sample was reduced in flowing dihydrogen at 573 K and atmospheric pressure. This sample was then cooled to room temperature (RT) in dihydrogen, allowing for the formation of β-phase palladium hydride. Another sample was reduced in the same way, but then evacuated at 573 K to prevent formation of the hydride. This sample was cooled to RT in vacuo and contacted with helium at atmospheric pressure. Analysis of the Pd EXAFS recorded at RT showed a 3.7% increase in the Pd-Pd distance and greater disorder for the hydride compared to the sample free of hydride. The XANES results were consistent with a narrowing of the l=1 band features for the hydride sample due to the increased Pd-Pd distance. In addition, the line shape of the edge for the hydride may result from hydrogen lowering the number of unoccupied p-like states near the Fermi level of Pd. The observed lattice expansion and modified electronic structure have been reported for bulk palladium hydride.