Dynamically biased microcanonical treatment of HCl dissociative chemisorption on Au(111) yields thermal associative desorption dynamics by detailed balance

A dynamically biased microcanonical transition state theory model of HCl dissociative chemisorption on Au(111) was recently developed for Shirhatti et al.'s [J. Phys. Chem. Lett. 7, 1346 (2016)] non-equilibrium supersonic molecular beam experiments where normal translational energies varied over a 50-250 kJ/mol range. Model parameters include a relatively low threshold energy for reaction, E0 = 30.88 kJ/mol; an efficiency, εn = 0.26, for molecular normal translational energy to contribute to the active exchangeable energy capable of promoting reaction; and one surface oscillator, s = 1, involved in the local gas-surface collision complex wherein energy was taken as microcanonically exchangeable, although potentially subject to dynamical bias through efficiencies. To validate the model, detailed balance was used to predict the desorbing HCl product state distributions from thermal associative desorption of H(c) + Cl(c) on Au(111) that have been measured by Rettner and Auerbach [Science 263, 365 (1994)] while impinging a H atom beam onto a chlorinated Au(111) surface. Modeling of Rettner's [J. Chem. Phys. 101, 1529 (1994)] rovibrationally resolved HCl product state distributions allowed for the additional establishment of a rotational efficiency of εr = 0.23 that led to consistent prediction of both the thermal associative desorption and the supersonic molecular beam experimental results. The HCl(g) ⇄ H(c) + Cl(c) reactivity on Au(111) is thereby identified as a new benchmark system for gas-surface reactivity where highly detailed experimental data are available coming from both sides of the reaction barrier. The activation energy for thermal associative desorption was estimated as Ea,assoc = 5.6 ± 1.3 kJ/mol based on four independent kinds of measurement. The effect of dynamics on the thermal dissociative sticking coefficient, S(T), of interest in practical catalysis, was quantitatively assessed. As compared to a statistically behaving HCl/Au(111) system with Ea = 31 kJ/mol, dynamics reduced S(T) by a factor of 10 at 800 K, equivalent to an increase in activation energy of ΔEa = 15 kJ/mol.