Ab initio based tight-binding molecular dynamics simulation of the sticking and scattering of O2∕Pt(111)

Computational approaches to dissociative chemisorption on metals: towards chemical accuracy

We review the state-of-the-art in the theory of dissociative chemisorption (DC) of small gas phase molecules on metal surfaces, which is important to modeling heterogeneous catalysis for practical reasons, and for achieving an understanding of the wealth of experimental information that exists for this topic, for fundamental reasons. We first give a quick overview of the experimental state of the field. Turning to the theory, we address the challenge that barrier heights (E_b, which are not observables) for DC on metals cannot yet be calculated with chemical accuracy, although embedded correlated wave function theory and diffusion Monte-Carlo are moving in this direction. For benchmarking, at present chemically accurate E_b can only be derived from dynamics calculations based on a semi-empirically derived density functional (DF), by computing a sticking curve and demonstrating that it is shifted from the curve measured in a supersonic beam experiment by no more than 1 kcal mol^-1. The approach capable of delivering this accuracy is called the specific reaction parameter (SRP) approach to density functional theory (DFT). SRP-DFT relies on DFT and on dynamics calculations, which are most efficiently performed if a potential energy surface (PES) is available. We therefore present a brief review of the DFs that now exist, also considering their performance on databases for E_b for gas phase reactions and DC on metals, and for adsorption to metals. We also consider expressions for SRP-DFs and briefly discuss other electronic structure methods that have addressed the interaction of molecules with metal surfaces. An overview is presented of dynamical models, which make a distinction as to whether or not, and which dissipative channels are modeled, the dissipative channels being surface phonons and electronically non-adiabatic channels such as electron-hole pair excitation. We also discuss the dynamical methods that have been used, such as the quasi-classical trajectory method and quantum dynamical methods like the time-dependent wave packet method and the reaction path Hamiltonian method. Limits on the accuracy of these methods are discussed for DC of diatomic and polyatomic molecules on metal surfaces, paying particular attention to reduced dimensionality approximations that still have to be invoked in wave packet calculations on polyatomic molecules like CH₄. We also address the accuracy of fitting methods, such as recent machine learning methods (like neural network methods) and the corrugation reducing procedure. In discussing the calculation of observables we emphasize the importance of modeling the properties of the supersonic beams in simulating the sticking probability curves measured in the associated experiments. We show that chemically accurate barrier heights have now been extracted for DC in 11 molecule-metal surface systems, some of which form the most accurate core of the only existing database of E_b for DC reactions on metal surfaces (SBH10). The SRP-DFs (or candidate SRP-DFs) that have been derived show transferability in many cases, i.e., they have been shown also to yield chemically accurate E_b for chemically related systems. This can in principle be exploited in simulating rates of catalyzed reactions on nano-particles containing facets and edges, as SRP-DFs may be transferable among systems in which a molecule dissociates on low index and stepped surfaces of the same metal. In many instances SRP-DFs have allowed important conclusions regarding the mechanisms underlying observed experimental trends. An important recent observation is that SRP-DFT based on semi-local exchange DFs has so far only been successful for systems for which the difference of the metal work function and the molecule's electron affinity exceeds 7 eV. A main challenge to SRP-DFT is to extend its applicability to the other systems, which involve a range of important DC reactions of e.g. O₂, H₂O, NH₃, CO₂, and CH₃OH. Recent calculations employing a PES based on a screened hybrid exchange functional suggest that the road to success may be based on using exchange functionals of this category.

Dynamics of O2 Chemisorption on a Flat Platinum Surface Probed by an Alignment-Controlled O2 Beam

O₂ adsorption on Pt surfaces is of great technological importance owing to its relevance to reactions for the purification of car exhaust gas and the oxygen reduction on fuel-cell electrodes. Although the O₂ /Pt(111) system has been investigated intensively, questions still remain concerning the origin of the low O₂ sticking probability and its unusual energy dependence. We herein clarify the alignment dependence of the initial sticking probability (S₀ ) using the single spin-rotational state-selected [(J,M)=(2,2)] O₂ beam. The results indicate that, at low translational energy (E₀ ) conditions, direct activated chemisorption occurs only when the O₂ axis is nearly parallel to the surface. At high energy conditions (E₀ >0.5 eV), however, S₀ for the parallel O₂ decreases with increasing E₀ while that of the perpendicular O₂ increases, accounting for the nearly energy-independent O₂ sticking probability determined previously by a non-state-resolved experiment.

Development of a ReaxFF potential for Pt–O systems describing the energetics and dynamics of Pt-oxide formation

A ReaxFF force field description of Pt–O systems has been developed, validated and applied to oxygen diffusion on Pt(111).

Tight-binding molecular dynamics simulation of ZnSe liquid within the local environment dependence

We investigate the structural, electronic and dynamical properties of ZnSe liquid using tight-binding molecular dynamics (TBMD) simulations. We report the TBMD calculations for the solid and liquid forms of the ZnSe compound. To produce more realistic results the TB model includes the local environment dependence in the Hamiltonian matrix at finite temperature for ZnSe. To further demonstrate the efficiency of the TBMD approach, we present results for finite temperature physical properties of ZnSe liquid. We are able to show good agreement with experiment for the atomic mean-squared displacement and melting point.

Kinetic Monte Carlo simulations of the interaction of oxygen with Pt(111)

The adsorption, dissociation, diffusion, and desorption of oxygen interacting with the Pt(111) surface have been studied using kinetic Monte Carlo simulations. This study has been motivated by uncertainties in the theoretical and the experimental derivations of O(2)Pt(111) reaction barriers. The simulations reproduce all known experimental data within basically one set of parameters, thus yielding microscopic insights into the elementary reaction steps occurring in the interaction of oxygen with Pt(111) and providing reliable estimates for adsorption energies and diffusion and desorption barriers. In particular, we confirm that the distance of oxygen atoms directly after dissociation is caused by ballistic hot atom motion rather than by diffusive motion. We address the equilibrium structure of oxygen atoms at high coverages. At low temperatures, chains of oxygen pairs are formed. We show that this mechanism can be explained by a lowered dissociation in the vicinity of already adsorbed atoms. Finally we discuss the role of the lateral interaction between the oxygen atoms in the oxygen desorption process.

Modified Shepard interpolation method applied to trapping mediated adsorption dynamics

The modified Shepard (MS) interpolation method is applied to H(2)/Pd(111) to investigate its performance for a system for which dissociative adsorption takes place through a direct as well as an indirect (i.e. dynamic trapping) mechanism. The input data were obtained from an available accurate potential energy surface (PES) interpolated by using the corrugation reducing procedure (CRP). Dissociation probabilities obtained from classical trajectory calculations with the MS-PES are in very good agreement with the results for the CRP-PES. Thus, this study confirms the MS method as a promising tool to tackle low energy adsorption dynamics of polyatomic molecules, usually dominated by trapping.