Catalytic properties of well dispersed supported ruthenium alloys

Abstract

Small particles of Ru alloyed with Sn, Ph, Sb, Ge, or Si were prepared by reacting a well dispersed Ru/Al 20 3 parent sample with an organometallic compound of the desired modifier, by the so-called controlled surface reaction method. Two series of RuSn samples were thus obtained from parent Ru catalysts of 75% and 35% dispersion. The adsorption of hydrogen as a function of the amount of Sn shows a large initial decrease on the smaller particles, and a linear variation on the less dispersed catalyst. These catalysts were compared to Ru/Al 2O 3 catalysts of widely varying dispersion in the hydrogenolysis of n-hexane ( nH) and 2-methylpentane (2MP) at 458 K, and of 2,2,3,3-tetramethylbutane (TeMB) at 473 K. On pure Ru/Al 20 3 catalysts a strong effect of particle size is observed, both on turnover frequencies and on selectivities. When the dispersion is increased, turnover frequencies and deep hydrogenolysis decrease; the selectivity for TeMB hydrogenolysis is shifted from splitting of the central bond (2 iQ to demethylation (C 1 + iC 7), and that of nH from the cleavage of external to internal bonds. The addition of about 0.3 wt% of Sn or Pb to the well dispersed Ru sample shifts the catalytic properties towards those of large particles, while a similar amount of Ge stimulates an opposite behaviour and shifts the catalytic properties toward those of smaller particles. These effects can be attributed to the preferential occupancy of edge and corner sites by Sn or Pb in bimetallic catalysts, as predicted by the theory of topological segregation. Ge could either be localised at the dense planes or be randomly distributed. On larger Ru particles, the addition of Sn has only little effect on selectivity. In that case no topological segregation was expected and the catalytic results suggest the formation of Sn islands.