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Nanoclocks in the catalytic formation of water on rhodium: A theoretical perspective

par McEwen, Jean-Sabin
Référence Nonlinear phenomena, complex systems and statistical mechanics (2008-12-03: Brussels, Belgium)
Publication Non publié, 2008-12-03

Communication à un colloque

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Résumé :

Despite the large progress made in the recent years to provide a sound understanding of the oscillatory behavior of various catalytic surface reactions, there still are a number of questions apparently unsolved [1]. One of these open questions concerns the gap between the choice of catalyst in surface science studies and those used in heterogeneous catalysis. Indeed, in surface science, oriented single crystals are mainly used [1] while multi-facetted metal particles are present in catalysis. A field emitter tip on the other hand can be regarded as a good approximation of a nanometer-sized metal particle. Moreover, the field ion microscope can be used to image surface reactions with near-atomic resolution under operating conditions in the presence of external fields (of the order of 10 V/nm). In this work, we will examine and develop a comprehensive model on the non-linear effects involved in the catalytic formation of water from H_2/O_2 on rhodium as observed in a field ion microscope [2], which reveals the existence of catalytic oscillatory patterns at the nanoscale. In our modeling, we construct a kinetic mean field model to understand the bistable behavior as well as the observed oscillations for this system. The kinetic parameters are obtained through experimental data and density functional theory calculations of O and H adsorbed on rhodium clusters and low Miller indexed rhodium surfaces as a function of coverage and the electric field [3]. The theory is consistent with the coverage and temperature dependence of the total sticking coefficient and the temperature programmed desorption rates of O_2 on Rh(111), Rh(011) and Rh(001). It is also consistent with the temperature-programmed desorption rates of H_2 on these three surfaces as well. Here, the anisotropy of the tip is also taken into account and the diffusion of hydrogen on the tip allows the various nanofacets to interact. The formation of subsurface oxygen is also incorporated into the model which is determined to be crucial in order to explain the experimental behavior. Our resulting model shows that the resulting non-equilibrium oscillatory patterns find their origin in the different catalytic properties of of various nanofacets that are simultaneously exposed at the tip's surface. These results suggest that the underlying surface anisotropy, rather than a standard reaction-diffusion mechanism, plays a major role in determining the self-organizational behavior of multi-faceted nanostructured surfaces. [1] R. Imbihl, Catal. Today 105, 206 (2005). [2] T Visart de Bocarmé et al., Surf. Interface Anal. 36, 533 (2004). [3] J.-S. McEwen et al., Chem. Phys. Lett. 452, 133 (2008).