Heather Abbott-Lyon - Microcanonical unimolecular rate theory at surfaces. III. Thermal dissociative chemisorption of methane on Pt(111) and detailed balance

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      Publication Details (including relevant citation   information):

      A. Bukoski, H.L. Abbott, and I. Harrison,   Journal of Chemical Physics, 123, 94707 (2005).

      Abstract:

      A local hot spot model of gas-surface reactivity is used to   investigate the state-resolved dynamics of methane dissociative   chemisorption on Pt(111) under thermal equilibrium conditions.   Three Pt surface oscillators, and the molecular vibrations,   rotations, and the translational energy directed along the   surface normal are treated as active degrees of freedom in the   16-dimensional microcanonical kinetics. Several energy transfer   models for coupling a local hot spot to the surrounding substrate   are developed and evaluated within the context of a master   equation kinetics approach. Bounds on the thermal dissociative   sticking coefficient based on limiting energy transfer models are   derived. The three-parameter physisorbed complex microcanonical   unimolecular rate theory (PC-MURT) is shown to closely   approximate the thermal sticking under any realistic energy   transfer model. Assuming an apparent threshold energy for CH4   dissociative chemisorption of E0=0.61 eV on clean Pt(111), the   PC-MURT is used to predict angle-resolved yield, translational,   vibrational, and rotational distributions for the reactive   methane flux at thermal equilibrium at 500 K. By detailed   balance, these same distributions should be observed for the   methane product from methyl radical hydrogenation at 500 K in the   zero coverage limit if the methyl radicals are not subject to   side reactions. Given that methyl radical hydrogenation can only   be experimentally observed when the CH3 radicals are kinetically   stabilized against decomposition by coadsorbed H, the PC-MURT was   used to evaluate E0 in the high coverage limit. A high coverage   value of E0=2.3 eV adequately reproduced the experimentally   observed methane angular and translational energy distributions   from thermal hydrogenation of methyl radicals. Although rigorous   application of detailed balance arguments to this reactive system   cannot be made because thermal decomposition of the methyl   radicals competes with hydrogenation, approximate applicability   of detailed balance would argue for a strong coverage dependence   of E0 with H coverage—a dependence not seen for methyl radical   hydrogenation on Ru(0001), but not yet experimentally explored on   Pt(111).

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