Dissociative Adsorption and Hydrogenolysis of Ethane over Clean and Ni-Covered

The hydrogenolysis of ethane has been investigated on clean and Ni-covered Pt( 1 1 1) at reactant partial pressures between 0.5 and 4 Torr of ethane and between 80 and 270 Torr of hydrogen and for surface temperatures from 550 to 640 K. On clean Pt( 1 1 1) the hydrogenolysis reaction proceeds with an activation energy of 36.6 kcal/mol and reaction orders in C2H6 and H2 pressures of 1 and -1.8, respectively. The rate of ethane hydrogenolysis increases when Ni is added to the surface. The catalytic activity of a Pt( 11 1) surface with the equivalent of a monolayer of Ni is higher than that observed on Pt(l11) but lower than those reported for Ni( 100) and Ni( 11 1). An analysis of the increase in the rate of hydrogenolysis with Ni coverage suggests an ensemble requirement of two or three nickel atoms for this reaction. The dependence of the rate of ethane hydrogenolysis with respect to the partial pressures of the reactants was analyzed according to a kinetic model that involves a rate-limiting, irreversible C-C bond cleavage step. The results indicate that the hydrocarbon fragments preceding C-C bond breaking are C2H4 or C2H3 (perhaps ethylidyne) on clean Pt(l11) and C2H2 on Pt(l11) surfaces with Ni coverages of 1 and 1.3 monolayers (ML). Postreaction surface analysis by Auger electron spectroscopy showed the presence of a submonolayer carbonaceous residue, the amount of which increased with reaction temperature and Ni coverage. The kinetics of the ethane decomposition reaction on Pt( 1 1 I ) and Ni/Pt( 11 1) surfaces was measured under the high incident flux conditions of 0.01-0.001 Torr of ethane. The apparent activation energies for dissociation of ethane vary between 9 and 12 kcal/mol. The dissociative sticking probabilities for ethane on clean Pt(i11) fall in the range 10-7-106 for temperatures between 515 and 635 K. The presence of Ni adatoms on Pt( 11 1) enhances the capacity of this surface to dissociate ethane (C-H bond breaking). The dissociative sticking probabilities of ethane on a Pt( 11 1) surface covered with a monolayer of Ni are -20 times larger than those seen on clean pt( 1 1 I ) . Our results indicate that Ni/Pt( 11 1) surfaces are able to hydrogenate hydrocarbon fragments faster than Pt( 1 1 1).