Abstract
The feasibility of determining the surface composition of a series of supported PtRu bimetallic cluster catalysts using infrared spectroscopy has been explored. The nominal bulk compositions of Ru in these samples were 10, 22, 38, 62, and 80 atom%; total metal content of each sample was 6% by weight. The infrared absorption bands of CO (g) selectively adsorbed on Pt sites and those of NO(g) selectively adsorbed on Ru sites were stable in vacuo at room temperature. As the concentration of Ru was increased, there was a gradual shift in the frequencies of these bands indicating the weakening of the CO bond and strengthening of the NO bond. Prolonged exposure of each bimetallic catalyst (fully covered by both CO and NO) to NO(g) resulted in reaction between NO(g) and CO (ads) yielding CO 2(g) and N 2O(g). As the content of Ru was increased, there was a corresponding increase in the NO-band intensity and a decrease in the CO-band intensity. If both extinction coefficients and stoichiometric coefficients are taken to be independent of surface composition, a simple linear correlation between the absorbance ratio and the bulk metal composition was obtained, suggesting that the composition of the surface changed in the same way as that of the bulk. Such a plot provides a quick and inexpensive method of determining surface compositions of supported PtRu cluster catalysts.
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