Characterization of carbon-supported ruthenium–tin catalysts by high-resolution electron microscopy

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Abstract
Ruthenium and bimetallic ruthenium–tin catalysts supported on high-surface-area carbon are characterized by high-resolution electron microscopy (HREM) and CO chemisorption. The characterization results give important insights into the reasons for the remarkable catalytic properties of Ru–Sn bimetallic catalysts in selective hydrogenation reactions of α,β-unsaturated aldehydes. In the monometallic Ru/C sample, ruthenium is present as small particles, showing no variation in the crystalline structure and lattice parameters with respect to the bulk state. At low Sn : Ru ratios, the average metal particle size is smaller than for monometallic Ru/C, whereas at higher Sn: Ru ratios Ru particles agglomerate on the surface of the support. According to HREM observations, most metal particles in the bimetallic catalysts possess a structure consistent with that of elemental ruthenium. CO chemisorption results indicate that the surface of the ruthenium particles is partially covered by tin species. Only in the samples with high tin loading are a few Ru–Sn only particles found, in addition to a majority of particles having the structure of pure ruthenium. The high selectivity of Ru–Sn/C catalysts for selective hydrogenation of the CO group in α,β-unsaturated aldehydes can thus be attributed to the presence of small ruthenium particles whose surface is covered with tin species which are most likely in ionic form. Ru–Sn alloy particles do not appear to make a significant contribution to the catalytic activity.