Why rough surfaces make good catalysts: Reaction mechanisms of hydrogen on the Pt(211) stepped surface - E. J. Baerends, R. A. Olsen, D. A. McCormack
Vrije Universiteit Amsterdam
Molecules impinging from the gas phase on a metallic surface may dissociate (reaction) or be backscattered in rotationally or vibrationally excited states. There has been a long-standing controversy on the relative importance of reaction on ideal low-index terraces on the surface versus reaction at steps. Already in the seventies, strong proponents of mostly or even exclusive dissociation at steps (Somorjai) and of overwhelming importance of the ideal terraces (Ertl) faced off. Today, first principles simulations, combining electronic structure theory and nuclear dynamics calculations can answer the question.
It has experimentally been established that the dissociative adsorption of gas molecules like H2 on rough metallic surfaces can exhibit an intriguing dependence on energy: reaction first declines with decreasing collision energy, but then at low energy (relevant at ambient temperature) starts to rise again. We study this effect by constructing an exhaustive density-functional model for the interaction between an H2 molecule and a stepped Pt(211) surface, explicitly including all six molecular degrees of freedom, and applying the corrugation reduction procedure.1 Using classical trajectories, we show that 1) virtually all reaction occurs at the step; 2) the terrace still plays an important role, the low-energy amplification of reaction arising from a precursor state on the terrace, where H2 gets dynamically trapped in a weak chemisorption well.2 Although the trapping occurs at an unreactive site, above the lower edge of the step, the time spent on the surface increases the likelihood that the molecules will migrate to the reactive upper edge of the step, where dissociation can proceed. The lower the energy of the H2 molecules, the longer they are trapped, and the more likely they will react.
The calculated reaction probabilities are in good agreement with experimental results for the related Pt(533) surface, and lead to new interpretations of the experimental findings. Quantum dynamics calculations refine but essentially confirm the picture.3
- R. A. Olsen, H. F. Busnengo,, A. Salin, M. F. Somers, G. J. Kroes, E. J. Baerends, J. Chem. Phys. 116 (2002) 3841
- R. A. Olsen, D. A. McCormack, E. J. Baerends, Surf. Sci. 571 (2004) L325; J. Chem. Phys. (2005) 122 (2005) 194708
- R. A. Olsen, D. A. McCormack, M. Luppi, E. J. Baerends, J. Chem. Phys. 128 (2008) 194715
Contact person: Dr. Matthe Uijttewaal - TU Clausthal