Hydrogenolysis of ethane, propane, n-butane, and neopentane on the (111) and (110)-(1.times.2) surfaces of iridium

Abstract

The hydrogenolysis of ethane, propane, n-butane, and neopentane has been investigated on the (111) and (110)-(1{times}2) single-crystalline surfaces of iridium at reactant partial pressures between 0.2 and 5.0 Torr of hydrocarbon and between 20 and 500 Torr of hydrogen and for surface temperatures from 400 to 700 K. A coupled high-pressure reactor-ultrahigh vacuum analysis chamber was employed, which permitted both the measurement of the specific rates of reaction and in situ pre- and postreaction surface characterization. Both the apparent reaction kinetics (activation energies and preexponential factors) and the dependence of the rates of reaction on the reactant partial pressures (apparent reaction orders) were examined in detail. Postreaction surface analysis by Auger electron spectroscopy indicated the presence of a submonolayer carbonaceous residue, the coverage of which was nearly identical on both surfaces and essentially independent of reaction conditions, i.e. surface temperature and reactant partial pressures. Titration of these residues with hydrogen produced only methane in all cases, suggesting that the carbonaceous residue plays the role of a spectator. A mechanistic model involving a rate-limiting, irreversible, unimolecular C-C bond cleavage step has been employed to describe the variations in the specific activity and selectivity of hydrogenolysis with variations in both the temperaturemore » and the reactant partial pressures. The apparent kinetic parameters implicated by this model, which represent combinations of rate coefficients of several elementary reactions, have been found to be consistent with the expected values for the preexponential factors of the contributing elementary reactions. This model is applied to the data to describes the reaction for the different compounds. 71 refs., 17 figs., 5 tabs.« less