Direct dissociative chemisorption of propane on Ir(110)

Abstract

We have employed molecular beam techniques to investigate the initial probability of direct dissociative chemisorption, Pd, and the intrinsic trapping probability, ξ, of C3H8, C3D8, and (CH3)2CD2 on Ir(110) as a function of beam translational energy, Ei, from 1.5 to 59 kcal/mol. For C3H8 and (CH3)2CD2, a measurable (≥ 0.02) initial probability of direct dissociative chemisorption is observed above a beam energy of approximately 7 kcal/mol. For C3D8 this energy is roughly 10 kcal/mol. Above these energies the initial probability of direct chemisorption of each of the isotopomers of propane increases nearly linearly with Ei, approaching a value of approximately Pd=0.48 at Ei=52 kcal/mol for C3H8 and (CH3)2CD2, and Pd=0.44 at Ei=59 kcal/mol for C3D8. This kinetic isotope effect for the direct chemisorption of C3D8 relative to C3H8 is smaller than that expected for a mechanism of H (or D) abstraction by tunneling through an Eckart barrier, suggesting a contribution of C–C bond cleavage to direct chemisorption. The lack of a kinetic isotope effect for the direct chemisorption of (CH3)2CD2 relative to C3H8 indicates that 1° C–H bond cleavage dominates over 2° C–H bond cleavage during the direct chemisorption of propane on Ir(110). The trapping behavior of each of these isotopomers of propane is approximately identical as a function of Ei, with ξ >0.9 at Ei=1.5 kcal/mol, ξ = 0.3 at Ei=20 kcal/mol, and ξ < 0.1 above Ei= 40 kcal/mol.