Alkane transformations on supported platinum catalysts .3. The stability of Pt/Al2O3 (EUROPT-3) and of PtRe/Al2O3 (EUROPT-4) during the hydrogenolysis of alkanes

The hydrogenolysis of ethane, propane and n-butane has been studied on Pt/Al2O3 (EUROPT-3) and PtRe/Al2O3 (EUROPT-4) catalysts between 520 and 660 K by a thermal cycling technique using a 10-fold excess of H-2. Analogous catalysts having higher metal concentrations have also been examined. With neither ethane nor propane does any progressive deactivation result from repeated thermal cycles, but with n-butane on Pt/Al2O3 catalysts initial loss of activity alters product selectivities, specifically by decreasing the probability of breaking the internal C-C bond. After this, they behave reproducibly. With PtRe/Al2O3 catalysts, product selectivities shown by n-butane are very different from those given by the Pt/Al2O3 catalysts but are unaltered by the somewhat greater initial deactivation; they show some sample-to-sample variation, which is attributed to differences in surface Re concentrations. These results are explained by a model in which sites on low-index planes in very small Pt particles are those that are first inactivated through the formation of strongly held dehydrogenated species; when Re atoms are present they preferentially occupy these sites and the operation of bimetallic centres explains why deeper hydrogenolysis occurs in this case. Reproducible rates of n-butane hydrogenolysis are obtained with Pt/Al2O3 below about 600 K, but not with PtRe/Al2O3; use of short (1 min) reaction pulses overcomes this difficulty. Partially deactivated catalysts can, however, be studied in the continuous-flow mode. (C) 1996 Academic Press, Inc.