Direct and trapping-mediated dissociative chemisorption of methane on Ir(111)

Abstract

The initial probabilities of dissociative chemisorption 13CH 4 and CD 4 have been measured on the close-packed Ir(111) surface at both low and high total pressures. At pressures below ∼ 10 −3 Torr, trapping-mediated dissociative chemisorption is the dominant reaction mechanism since the gas temperature is equal to the wall temperature of 300 K, with activation energies for C H (C D) bond cleavage of 12.6 kcal/mol for 13CH 4 and 13.7 kcal/mol for CD 4 with respect to the gas phase energy zero of the methane infinitely far from the surface and at rest. For both methane isotopomers the ratio of the pre-exponential factor for desorption relative to that of reaction is ∼ 180 due to the larger phase space available to the molecule for desorption relative to reaction. The measured differences in C H versus C D bond activation for methane are attributed to zero-point energy differences between each isotopomer and point to classical over-the-barrier reaction dynamics as the reaction pathway for trapping-mediated chemisorption. The incident kinetic energy of the impinging methane was increased by diluting the methane in argon at a total pressure of 1 Torr which raised the gas temperature of the methane to the surface temperature. In this case, methane chemisorbs dissociatively via both trapping-mediated and direct C H bond activation. Direct activation of 13CH 4 is characterized by an activation energy of 17.4 kcal/mol and a pre-exponential factor of ∼ 0.61, while direct activation of CD 4 occurs with an activation energy of 18.1 kcal/mol and the same pre-exponential factor. These activation energies are averages of a distribution of activation barriers which occur because of the nature of the multidimensional potential energy surface describing the interaction of the methane with the surface.