Atomistic Adsorption of Oxygen and Hydrogen on Platinum Catalysts by Hybrid Grand Canonical Monte Carlo/Reactive Molecular Dynamics

Abstract

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.

DFT calculation of oxygen adsorption on platinum nanoparticles: coverage and size effects

“…The results obtained with the "Relaxing Pt" approach show the sinking of the O atoms into the Pt nanoparticle structure as we increase the O coverage. This result agrees with observations of subsurface O atoms with increasing oxygen coverage, already observed experimentally 40,41 and in computational studies for nanoparticles 39,42 and slabs 43 . To accurately detect subsurface oxygen, our simulations would have to use a different approach and sample different initial configurations for the oxygen atoms.…”

“…This effect increases the average bond strength, as the bridge sites in the edges between (111) and (100) facets have stronger binding energies than the initial HCP positions. The oxygen preference for adsorption sites in the edges and vertices of the nanoparticle with increasing O coverage was also previously observed with DFT 33,39 and MD 42 calculations, which similarly to our results, predicted a large concentration of oxygen in the edges of Pt nanoparticles and almost free terrace sites at (111) and (100) facets.…”

“…Subsurface oxygen with increasing O coverage was also observed experimentally 40,41 , and with MD calculations 42 , and with DFT calculations for Pt slabs 43 . The presence of subsurface oxygen is an important phenomenom on catalysis research, as it is associated with the Pt catalyst corrosion and changes in the catalyst surface reactivity 40,41,44 . Recently, Gai et al 42 studied the effects of oxygen coverage for Pt catalysts via MD calculations on Pt surfaces and nanoparticles of different sizes and shapes. For spherical Pt nanoparticles of various sizes, they observed that hollow sites are the first to adsorb O, followed by subsurface sites.…”

“…Smaller nanoparticles formed bulk oxides at lower pressures as compared with the larger clusters, which was explained due to the decrease on O adsorption surface sites on smaller nanoparticles and to the higher percentage of edge and corners sites which adsorb O more easily. The effect of the rate of edges and corners was also demonstrated by comparing different Pt nanoparticles shapes, where octahedron models with a higher percentage of edges and corners showed higher oxygen coverages when compared with cubic and cuboctahedral models 42 .…”

Exploring substrate/ionomer interaction under oxidizing and reducing environments

“…Ionomer thin-films cast onto a Pt surface can serve as model systems providing a focused glimpse into the catalyst layer. Although bulk, continuous polycrystalline Pt does not fully describe Pt nanoparticle phenomenon present in real catalyst layers, it can still elucidate surface specific interactions that impact ionomer properties and morphology [9], [10]. While impact of Pt substrate on ionomer performance have been shown [8], [11], efforts to clarify the source of this impact have been contradictory, especially in elucidating the role of water on oxidized and unoxidized Pt surfaces [12], [13].…”