Atomistic adsorption of oxygen and hydrogen on platinum catalysts by hybrid grand canonical monte carlo/reactive molecular dynamics

The reactivity of a metal catalyst depends strongly on the adsorbate coverage, making it essential for the reactivity models to account for the in situ structures and properties of the catalyst under reaction conditions. The use of first principle based thermodynamic approaches to describe adsorbate-adsorbate interaction though attractive for its technical rigor is tedious and computationally demanding especially for metal nanoparticles. With the advent of empirical reactive force fields (ReaxFF), there is a great deal of interest to advance simulation approaches like hybrid grand canonical Monte Carlo reactive molecular dynamics (GCMC/RMD) that enable efficient use of ReaxFF to model the adsorptive states. The predictive ability of GCMC/RMD relies upon the quality of the force field, which in turn depends upon the training set used for its parametrization. To this end, we investigate the adsorption behavior of O and H over the Pt catalysts using the newly developed Pt/O/H ReaxFF. We assess the thermodynamic stability of Pt-adsorbates by GCMC/RMD and provide insight on the atomic composition of in situ catalysts. The theoretical adsorption isotherms of O and H are derived in many Pt surfaces over a wide range of reference gas pressures (e.g., 10^-20 atm to 10 atm) relevant to the observed real catalysis, including the Pt(111), unreconstructed and reconstructed Pt(110) surfaces, and even Pt nanoparticles of different sizes and shapes. The force field is further evaluated to predict the relative binding energies of O on Pt(321) surface, while it has not been trained for this kinked surface. For both oxygen and hydrogen atoms, adsorption occurs initially at the Pt surface, followed by subsurface and bulk. Examination of the equilibrated structures discloses the contribution of different sites on the surface, subsurface, and the bulk regions during adsorption at various applications. The adsorption behavior obtained in this paper agrees with the DFT and/or the experimental data reported in the literature, which validates the Pt/O/H ReaxFF and demonstrates its applicability in catalytic reactions coupled with time acceleration tools. Based on the derived adsorption isotherm, one can infer the relative affinity of O, H, or OH species, and thus prepare appropriate structures at the specified reaction conditions for further investigation of the catalytic reactions by molecular dynamics and for designing experimental conditions for optimal catalyst performance.