THE ORIGIN OF PARTICLE-SIZE EFFECTS IN SUPPORTED METAL-CATALYSTS - PROPANE HYDROGENOLYSIS ON RU AL2O3 CATALYSTS/

The kinetics of the hydrogenolysis of propane on Ru/Al2O3 catalysts di ffering in particle size have been analysed according to a classical r ate equation which supposes the reactive species to be a dehydrogenate d form of the alkane. With small ruthenium particles there is a sharp maximum in rate as hydrogen pressure is increased; with large particle s the peak is broader and at higher hydrogen pressures. Published resu lts showing the latter to give higher turnover frequencies are only va lid at high hydrogen:alkane ratios. Ethane selectivity (S-2) is almost independent of hydrogen pressure on small particles, but on large par ticles it increases markedly as the hydrogen pressure is raised. The r esults are interpreted by supposing hydrogen chemisorption to be stron g on small ruthenium particles, and weaker on large ones; effects of t emperature on selectivities are also explained in this way. The geomet ric basis of structure-sensitivity is thereby called into question.