Propane Hydrogenolysis on Sulfur- and Copper-Modified Nickel Catalysts

The rates and selectivity of propane hydrogenolysis on Ni/MgAl2O4 catalysts modified by preadsorbed sulfur and on Ni/SiO2 catalysts modified by copper alloying have been measured. On both types of catalysts, the rate of ethane production goes through a maximum at high temperatures. For the NiCu/SiO2 catalysts, this behavior correlates with the onset of secondary hydrogenolysis of ethane, but not for the sulfur-modified Ni/MgAl2O4 catalysts. The selectivity for ethane formation on Ni/MgAl2O4 catalysts was higher with sulfur than without, but a decreasing function of sulfur at high coverage. The selectivity on NiCu/SiO2 catalysts was found to increase monotonically with the copper content. Comparisons with a previous propane hydrogenolysis study on nickel–copper catalysts with very different surface properties reveal surprising agreements. Our NiCu/SiO2 catalysts have been shown to be strongly enriched in copper in the surface of the alloy particles after reduction. This is similar to previous results for unsupported NiCu catalysts but in contrast to the NiCu/SiO2 catalysts used by Dalmon and Martin (J. Catal.66, 31 (1980)), reported to have bulk composition also in the surface after reduction. The hydrogenolysis results of the two NiCu/SiO2 catalyst studies are, nevertheless, in close agreement, but deviate strongly from the hydrogenolysis results for the unsupported NiCu particles. It is difficult to reconcile the various hydrogenolysis results with simple ensemble models. It is suggested that special sites may dominate hydrogenolysis activity on nickel catalysts.