Hydrogen Recombination Rates on Silica from Atomic-Scale Calculations

Abstract

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 gasphase radical densities between 10 12 and 10 16 cm −3 , yielding values of γ = 10 −4 −0.9.

Single-Crystal Pt-Decorated WO3 Ultrathin Films: A Platform for Sub-ppm Hydrogen Sensing at Room Temperature

“…We fit the temperature dependence of the rate constants with an Arrhenius model, finding activation energies of 0.13 eV for k 1,i and 0.12 eV for k –1,d . These results can be compared with ab initio computations that find activation energies in the range 0.06–0.42 eV for the dissociative chemisorption on Pt surfaces, with lower values expected for smaller particles. , Similarly, recombinative desorption at oxide surfaces entails activation energies in the range 0.05–0.9 eV depending on the hydrogen binding energy at the adsorption site …”

“… 39 , 40 Similarly, recombinative desorption at oxide surfaces entails activation energies in the range 0.05–0.9 eV depending on the hydrogen binding energy at the adsorption site. 41 …”

“…39,40 Similarly, recombinative desorption at oxide surfaces entails activation energies in the range 0.05−0.9 eV depending on the hydrogen binding energy at the adsorption site. 41 The surface-to-bulk diffusion rate constants k 2,i/d show a weak temperature dependence (Figure 4f). The bulk-to-surface rates k −2,i/d in Figure 4g, instead, show a linear trend which allows to extract activation energies of 0.43 and 0.24 eV in the intercalation and deintercalation experiments, respectively.…”

Poisoning of Hydrogen Recombination on Silica Due to Water Adsorption

“…This reaction is expected to be dominant in the hightemperature regime, presumably after most of the molecules physisorbed in the shield layer have desorbed from the surface. Based on atomic-scale simulations of hydrogen recombination, 23 we set the activation energy for LH hydrogen recombination to 21.96 kcal/mol. 2.3.4.…”

“…The bottom layer represents chemisorbed species tightly bound to the surface, and the top layer represents relatively weakly bound physisorbed species. Similar models have been used to reproduce the non-Arrhenius recombination rates on silica surfaces seen in experimental and computational studies. , In our model, the adsorbed water molecules reside in the physisorption top layer and block radical species from reaching chemisorption sites on the surface in the bottom layer. This physisorbed shield layer of water molecules thus hinders adsorption and the Eley–Rideal mechanism, poisoning surface reactions.…”