DIRECT AND TRAPPING-MEDIATED DISSOCIATIVE CHEMISORPTION OF METHANE ONIR(111)

The initial probabilities of dissociative chemisorption (CH4)-C-13 and CD4 have been measured on the close-packed Ir(111) surface at both lo w and high total pressures. At pressures below similar to 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 (CH4)-C-13 and 13.7 kcal/mol for CD4 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-exponen tial factor for desorption relative to that of reaction is similar to 180 due to the larger phase space available to the molecule for desorp tion relative to reaction. The measured differences in C-H versus C-D bond activation for methane are attributed to zero-point energy differ ences between each isotopomer and point to classical over-the-barrier reaction dynamics as the reaction pathway for trapping-mediated chemis orption. The incident kinetic energy of the impinging methane was incr eased by diluting the methane in argon at a total pressure of 1 Torr w hich raised the gas temperature of the methane to the surface temperat ure. In this case, methane chemisorbs dissociatively via both trapping -mediated and direct C-H bond activation. Direct activation of (CH4)-C -13 is characterized by an activation energy of 17.4 kcal/mol and a pr e-exponential factor of similar to 0.61, while direct activation of CD 4 occurs with an activation energy of 18.1 kcal/mol and the same pre-e xponential factor. These activation energies are averages of a distrib ution of activation barriers which occur because of the nature of the multidimensional potential energy surface describing the interaction o f the methane with the surface. (C) 1997 Elsevier Science B.V.