Heather Abbott-Lyon - Microcanonical unimolecular rate theory at surfaces. II. Vibrational state-resolved dissociative chemisorption of methane on Ni(100)

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      Publication Details (including relevant citation   information):

      H.L. Abbott, A. Bukoski and I. Harrison.   Journal of Chemical Physics, 121, 3792 (2004).


      A three-parameter microcanonical theory of gas-surface reactivity   is used to investigate the
      dissociative chemisorption of methane impinging on a Ni~100!   surface. Assuming an apparent threshold energy for dissociative   chemisorption of E0=65 kJ/mol, contributions to the dissociative   sticking coefficient from individual methane vibrational states   are calculated: (i) as a function of molecular translational   energy to model nonequilibrium molecular beam experiments and   (ii) as a function of temperature to model thermal equilibrium   mbar pressure bulb experiments. Under fairly typical molecular   beam conditions (e.g., Et>25 kJ/mol, Ts>475 K, Tn<400   K), sticking from methane in the ground vibrational state   dominates the overall sticking. In contrast, under thermal   equilibrium conditions at temperatures T>100 K the   dissociative sticking is dominated by methane in vibrationally   excited states, particularly those involving excitation of the   nu4 bending mode. Fractional energy uptakes f j defined as the   fraction of the mean energy of the reacting gas-surface collision   complexes that derives from specific degrees of freedom of the   reactants (i.e., molecular translation, rotation, vibration, and   surface) are calculated for thermal dissociative chemisorption.   At 500 K, the fractional energy uptakes are calculated to be f t   = 14%, f r = 21%, f v = 40%, and fs = 25%. Over the temperature   range from 500 K to 1500 K relevant to thermal catalysis, the   incident gas-phase molecules supply the preponderance of energy   used to surmount the barrier to dissociative chemisorption, fg =   ft + fr + fv ~ 75%, with the highest energy uptake always coming   from the molecular vibrational degrees of freedom. The   predictions of the statistical, mode-nonspecific microcanonical   theory are compared to those of other dynamical theories and to   recent experimental data.

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