Enigmatic HCl + Au(111) Reaction: A Puzzle for Theory and Experiment

• Published in: Journal of physical chemistry. C 2016-11, Vol.120 (45), p.25760-25779
• Main Authors: Füchsel, Gernot, del Cueto, Marcos, Díaz, Cristina, Kroes, Geert-Jan
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.6b07453

The reactive scattering of HCl on Au(111) is currently one of the most peculiar reactions in the field of surface chemistry, as it so far eludes an accurate theoretical description. Possible reasons for the observed mismatch between theory and experiment that were not yet all considered in the computations are (i) dissipative effects due to the creation of electron hole pairs and excitations of surface atom motion that might inhibit reaction and (ii) use of an inappropriate density functional theory method or even its failure due to the occurrence of a charge transfer at the transition state. In this work, we address all of these possibilities and perform quasiclassical molecular dynamics simulations employing different methodologies. We use ab initio molecular dynamics simulations to account for surface atom motion and surface temperature effects, employing the PBE and the RPBE functionals. We also construct an accurate potential energy surface incorporating the six adsorbate degrees of freedom of HCl on Au(111) by using the neural network approach. In our molecular dynamics simulations, we model the experimental beam conditions to study the influence of the rovibrational state population distribution of HCl in the molecular beam on reaction. Likewise, molecular dynamics with electronic friction calculations based on the parameter-free local density friction approximation in the independent atom approximation (LDFA-IAA) are performed to get first insights into how electron hole pair excitation might affect the reactivity of HCl. Although satisfying agreement with the experiment could not yet be achieved by our simulations, we find that (i) RPBE yields larger reaction barriers than PBE and lower computed reaction probabilities, improving the agreement with experiment, (ii) the reactivity strongly depends on the rovibrational state population, (iii) surface atom motion and electronically nonadiabatic effects modeled with the efficient to use but approximate LDFA influence the reaction only modestly, and (iv) a moderate amount of charge is transferred from the surface to the dissociating molecule at the transition state. We suggest that the reported experimental reaction probabilities could be too low by a factor of about 2–3, due to potential problems with calibrating coverage of Au by Cl using an ill-defined external standard in the form of Auger peak ratios. However, taking this factor into account would not yet resolve the discrepancy between the theoret