| A model of gas-surface reactivity is developed based on the ideas that (a) adsorbate chemistry is a local phenomenon, (b) the active system energy of an adsorbed molecule and a few immediately adjacent surface atoms suffices to fix microcanonical rate constants for surface kinetic processes such as desorption and dissociation, and (c) energy exchange between the local adsorbate-surface complexes and the surrounding substrate can be modeled via a Master equation (ME) to describe the system/heat reservoir coupling. The resulting microcanonical unimolecular rate theory (MURT) for analyzing and predicting both thermal equilibrium and nonequilibrium kinetics for surface reactions is applied to the dissociative chemisorption of methane on Pt(111) and Ni(100). Based on fitting experimental data to a truncated Master equation (PC-MURT) which neglects the effects of vibrational energy transfer the apparent threshold energy for C-H bond cleavage of CH4 on Pt(111) is 0.61 eV and on N(100) is 0.67 eV, down from 4.5 eV in the gas phase. Additionally, PC-MURT analysis of the thermal programmed desorption of methane on Pt(111) indicates the physisorption well depth is 0.16 eV.; For both the Pt(111) and Ni(100) surfaces, predictions are made for the initial methane dissociative sticking coefficient as a function of isotope, normal translational energy, molecular beam nozzle temperature, and surface temperature determined from nonequilibrium molecular beam experiments. The simple three-parameter PC-MURT is shown to predict experimental methane dissociative sticking coefficients on the Ni(100) surface over roughly ten orders of magnitude variation in both pressure and sticking, even at quantum state resolved levels of detail. Thermal equilibrium dissociative sticking coefficients for methane on Pt(111) are predicted for the temperature range from 250--2000 K and on Ni(100) from 350--500 K. Tolman relations for the activation energy under thermal equilibrium conditions, a variety of effective activation energies under nonequilibrium conditions, and expressions for the efficacy of sticking with respect to normal translational and vibrational energy are derived. Differential energy uptakes suggest that on Pt(111) under thermal equilibrium conditions relevant to catalysis, the incident gas molecules supply the preponderance of energy used to surmount the barrier to chemisorption. |
