Fundamental studies of supported bimetallic catalysts by NMR spectroscopy

Abstract

<p>Characterization of a bimetallic catalyst in terms of its surface composition is important in understanding the mechanisms of reactions over such catalysts. Since catalytic surfaces are covered with adsorbates under reaction conditions, the influence of adsorbates on the surface compositions of bimetallic catalysts is also important. Hydrogen was found to influence the surface compositions of silica and alumina supported Pt-Rh catalysts to a certain extent as the surfaces of bimetallic catalysts were enriched in Rh under the influence of hydrogen. Although the extent of Rh enrichment of the surface was not large, the surface compositions in the presence of hydrogen were significantly different from those of an adsorbate-free Pt-Rh surface which is known to be enriched in Pt;Various hydrogenation reactions on transition metals are important commercially whereas certain hydrogenolysis reactions are useful from fundamental point of view. Understanding the hydrogen mobility and kinetics of adsorption-desorption of hydrogen is important in understanding the mechanisms of such reactions involving hydrogen. The kinetics of hydrogen chemisorption was studied by means of selective excitation NMR on silica supported Pt, Rh and Pt-Rh catalysts. The activation energy of hydrogen desorption was found to be lower on silica supported Pt catalysts as compared to Rh and Pt-Rh catalysts. It was found that the rates of hydrogen adsorption and desorption on Pt-Rh catalyst were similar to those on Rh catalyst and much higher as compared to Pt catalyst;The Ru-Ag bimetallic system is much simpler to study than the Pt-Rh system and serves as a model system to characterize more complicated systems such as the K/Ru system. Ag was found to decrease the amounts of adsorbed hydrogen and the hydrogen-to-ruthenium stoichiometry. Ag reduced the populations of states with low and intermediate binding energies of hydrogen on silica supported Ru catalyst. The rates of hydrogen adsorption and desorption were also lower on silica supported Ru-Ag catalyst as compared to Ru catalyst. Thus Ag influenced the kinetics and thermodynamics of hydrogen chemisorption on Ru particles and it was found that electronic and ensemble effects were not responsible for this influence of Ag. Instead, the effect of silver was due to the selective segregation of silver to the edge, corner and other defect-like sites which are proposed to be highly active for dissociative hydrogen adsorption. Hence hydrogen adsorption on Ru particles was found to be structure sensitive.</p>