Hydrogen Recombination Rates on Silica from Atomic-Scale Calculations

A combined molecular dynamics (MD) and Monte Carlo approach was used to bridge time scales, enabling calculations of surface recombination rates for hydrogen on silica. MD was used for temperatures between 10 and 600 K at a high pressure of 10 atm, yielding recombination coefficients between 0.1 and 1. For the lower pressures more common in applications, low recombination rates make the corresponding calculations intractably expensive. A Monte Carlo technique, informed by the MD simulations, was designed to bridge the essential time scales. Distinct weak and strong surface binding sites for atomic hydrogen with densities of approximately 10 nm–2 were found using grand canonical Monte Carlo (GCMC) simulations, which, in turn, were used to obtain Eley–Rideal rate constants based on semiequilibrium theory. Monte Carlo variational transition state theory (MCVTST) was used to calculate Langmuir–Hinshelwood and thermal desorption rate constants for hydrogen atoms in strong and weak adsorption sites. Calculated reaction rates were used in a Langmuir kinetics model to estimate the recombination coefficient γ for T = 10–2000 K at gas-phase radical densities between 1012 and 1016 cm–3, yielding values of γ = 10–4–0.9.