Dynamics of CO interaction with Ru(0001): microscopical elucidation of the results of molecular beam experiments

Loncaric1, J.I.Juaristi1,2,3, G. Fuechsel4 and P. Saalfrank5

1.Centro de Física de Materiales CFM/MPC (CSIC-UPV/EHU), San Sebastián, Spain

2.Donostia International Physics Center DIPC, San Sebastián, Spain

3.Departamento de Física de Materiales, Facultad de Químicas, Universidad del País Vasco (UPV/EHU), San Sebastián, Spain

4.Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, Leiden, Netherlands

5.Institut für Chemie, Universität Potsdam, Potsdam, Germany

A longstanding goal in the field of heterogeneous catalysis is to understand surface reactions on a microscopical level, which would allow for construction of optimal catalysts. In this respect, the interaction of CO with transition metals is one of the most studied examples, both as a simple model system and due to its relevance in the first stage of the Fischer–Tropsch process.

Ruthenium is known to be a very good Fischer–Tropsch catalyst and for this reason the CO/Ru(0001) system has been particularly well studied experimentally. Molecular beam experiments [1,2] have established that at low incidence energies and regardless of surface temperature (in the range of 80-390 K) the sticking probability is close to unity and that it decreases slowly with increasing incidence energy. These results are explained by the existence of a deep chemisorption well. However, the measured scattering angle distributions and the angular dependence of the energy loss are not consistent with such deep chemisorption well.

We will present our recent molecular dynamics simulations on top of a density functional theory based six dimensional potential energy surface [3]. With our calculations we are able to reproduce all experimental observations, and thus give microscopical explanation for them. We will show that the dynamics and the reaction probabilities are dominantly influenced by the initial orientation of the molecule, i.e., whether the O atom or the C atom is closer to the surface. Due to this, scattered molecules, in contrast to adsorbed molecules, explore different regions of the potential energy surface which results in the unexpected scattering angle distributions observed in the molecular beam experiments.

By employing the local density friction approximation [4], we will also quantify for this system, the importance of the energy loss due to excitations of electron-hole pairs compared to the energy loss to phonon excitations.

[1] Kneitz et al., Surf. Sci. 440, 307 (1999)

[2] Riedmueller et al., Surf. Sci. 465, 347 (2000)

[3] Fuechsel et al., J. Chem. Phys. 141, 094704 (2014)

[4] Juaristi et al., Phys. Rev. Lett. 100, 116102 (2008)