X-ray absorption fine structure of Ru metal clusters supported on zeolite

Abstract

The physicochemical property of extremely small Ru clusters supported on Y zeolite has been investigated with X-ray absorption fine structure (XAFS). The Ru clusters were prepared according to the following procedure. First a Ru red complex ion was obtained in the supercage of zeolite by the reaction of $RuCl_3$ and $NH_3$ aqueous phase at 330 K. Ru red complex ion was decomposed under vacuum at 673 K, during which the Ru species was partially reduced spontaneously. Third, Ru species was fully reduced to form a cluster in the supercage by heating in $H_2$ flowing at 623 K. The resulting Ru cluster size has been controlled with heating rate for the complex decomposition, as indicated by the average number of Ru atoms per cluster which was obtained from the xenon adsorption measurements. Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) have been measured for the Ru cluster under He and $H_2$ at room temperature and 110 K. The coordination number of the nearest Ru-Ru pairs obtained from the standard curve fitting analysis of the EXAFS appeared to be underestimated at room temperature and 110 K, when compared with the cluster size estimated by the xenon adsorption method. This difference has been attributed to the static disorder, i.e., surface reconstruction in the small Ru cluster. Nevertheless, there was a good correlation between the coordination number from EXAFS and the results from the xenon adsorption measurement. The electronic structure of the Ru cluster probed by XANES under dihydrogen atmosphere at room temperature did not change over the range of cluster size corresponding to 20 - 60 Ru atoms per cluster. Hydrogen chemisorption and the activity of ethane hydrogenolysis on Ru cluster were not affected significantly by the change of Ru cluster size. This has been attributed to the same electronic structure of Ru clusters over the cluster size range. However, the results of $^129Xe$ NMR spectrum as a funct...