Alkane transformations on supported platinum catalysts .5. Effects of formation and removal of carbon deposits on alkane hydrogenolysis using EUROPT-1 (6.3% platinum/silica)

The carbonaceous deposit formed during hydrogenolysis of n-butane on EUROPT-1 (6.3% Pt/SiO2) at similar to 535 K using a 10-fold excess of H-2 causes an increase in isomerization selectivity S-i and a decrease in the probability of central C-C bond splitting F, while that formed during a thermal cycle between 523 and 633 K has the opposite effect: both lead to an similar to 50% loss of activity. We propose that the first kind resides mainly on sites comprising atoms of low coordination number (Type II sites) and the second kind mainly on sites in planar zones (Type I sites). Variation of H-2 pressure at 533 K using a short reaction-pulse technique shows that S-i decreases as H-2 pressure is raised; F also decreases, but to a lesser extent. Rates of each reaction have been fitted to a rate expression derived from a mechanism in which partially dehydrogenated intermediates enter the slow step. The intermediate for hydrogenolysis has lost two H atoms; that for isomerization, three or four. This explains the smaller value of the equilibrium constant K-A for dehydrogenation to the isomerization intermediate, and also the much higher apparent activation energy for this reaction. (C) 1997 Academic Press.