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 differing in particle size have been analysed according to a classical rate equation which supposes the reactive species to be a dehydrogenated form of the alkane. With small ruthenium particles there is a sharp maximum in rate as hydrogen pressure is increased; with large particles the peak is broader and at higher hydrogen pressures. Published results showing the latter to give higher turnover frequencies are only valid at high hydrogen:alkane ratios. Ethane selectivity (S-2) is almost independent of hydrogen pressure on small particles, but on large particles it increases markedly as the hydrogen pressure is raised. The results are interpreted by supposing hydrogen chemisorption to be strong on small ruthenium particles, and weaker on large ones; effects of temperature on selectivities are also explained in this way. The geometric basis of structure-sensitivity is thereby called into question.