Reaction of cyclopropane, methylcyclopropane and propylene with hydrogen on the (111) and (110)-(1x2) surfaces of iridium

Abstract

The hydrogenation, isomerization, and hydrogenolysis of cyclopropane, methylcyclopropane, and propylene have been investigated on the (111) and (110)-(1X2) single-crystalline surfaces of iridiium at reactant partial pressures from 0.4 to 10 Torr of hydrocarbon (P_(HC)) and between 20 and 500 Torr of hydrogen (P_H2) and at surface temperatures from 375 to 700 K. Both the kinetics of the reaction (activation energies and preexponential factors) and the dependences of the rates of reaction on the reactant partial pressures (apparent reaction "orders") were examined in detail. Postreaction surface analysis revealed the presence of a carbonaceous residue, the coverage of which was found to vary with the reaction conditions. The reaction of cyclopropane and hydrogen resulted in both hydrogenation to propane and the hydrogenolysis to methane and ethane, with the hydrogenation channel dominating for temperatures below 500 K. The Ir(110)-(1X2) surface was found to be more active than the (111) surface for both the hydrogenation and the hydrogenolysis of cyclopropane. In both cases, the specific activity (per metal surface atom basis) on Ir(110)-(1X2) was greater by a factor between approximately 2 and 10. The hydrogenation of methylcyclopropane on both the Ir(111) and Ir(110)-(1X2) surfaces was found to be dominated by the production of n-butane. This result was interpreted qualitatively by invoking parallel reaction mechanisms for the production of n-butane and isobutane, with then-butane channel exhibiting a higher apparent activation energy, thus, dominating at the higher temperatures. The hydrogenolysis of methylcyclopropane was found to be similar to that of cyclopropane on both surfaces. However, a selectivity difference was observed between the two surfaces for hydrogenolysis, the product distributions for the major reaction channels were CH_4 + C_3H_8 on Ir(111) and CH_4 + C_2H_6 + C_3H_8 on Ir(110)-(1X2). The absence of ethane in the major hydrogenolysis channel on the Ir(111) surface can be explained on a stereochemical basis if formation of the appropriate intermediate proceeds through an "edge complex", the formation of which is forbidden sterically on the Ir(111) surface. The Ir(110)-(1X2) surface was found to possess a greater specific activity compared to the Ir(111) surface for the hydrogenation of propylene.

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