Variation of catalyst selectivity by control of the environment of surface sites

Abstract

Previously published models, which have interpreted selectivity characteristics of metallic hydrogenation catalysts in terms of (i) molecular congestion at active sites, (ii) surface contamination and (iii) hydrogen occlusion in metals, are tested and elaborated.

But-1-ene isomerisation has been investigated at ruthenium sites congested by CO. The catalyst was prepared by impregnation of Ru₆C(CO)₁₇ into silica. After a minor displacement of CO it showed behaviour characteristic both of the parent cluster (activity in hydrogen–deuterium exchange) and of conventional supported ruthenium (product composition in the ethene–deuterium exchange) and of conventional supported ruthenium (product composition in the ethene–deuterium reaction). Preferential cis-but-2-ene formation from but-1-ene occurred at these CO-congested sites, whereas uncongested sites provided preferential trans-but-2-ene formation.

Adsorption of sulphur on catalytically active metal by decomposition of H₂S modifies selectivity in that it converts the major process in buta-1,3-diene hydrogenation from 1:2-addition to 1:4-addition. Adsorption of H₂S has been measured on evaporated films of Cr, Mn, Fe, Co, Ni, Mo, Pd, W, Re and Pt and on Co-powder and Ni/silica. The selectivity changes are recorded and interpreted in terms of the electronic effect of adsorbed sulphur on metal atoms remaining exposed at the surface and, for Fe, Co, Ni and Pd, the effect of subsequent incorporation of sulphur into the surface. The behaviour of Pt was anomalous, the effect of adsorbed sulphur on selectivity being less than expected.

Finally, the cavity theory of hydrogen occlusion in metals correctly predicts the formulation or conditions of preparation required to translate iridium from its traditional position as the least selective Group VIII metal for buta-1,3-diene hydrogenation (to butene) to a new position as one of the most selective.