(541h) Non-Equilibrium Surface Pattern Formation During Catalytic Reactions with Nanoscale Resolultion

Heterogeneous catalysis by metals plays a fundamental role in modern industrial chemistry to ensure technical progress along with sustainability.  With this background, it is surprising that many catalytic reactions deemed to be simple have detailed mechanisms and underlying kinetics that are still not well understood.  To provide a sound understanding of such phenomena, surface science techniques may be employed in studies with oriented 2D single crystals.  However, it has been recognized that a 2D metal particle is morphologically a simplification as compared to the 3D nanosized particles usually applied in heterogeneous catalysis.  To approximate the 3D morphology of a single nanometer-sized metal particle the apex of a field emitter tip can be considered a most suitable approach [1-7].  In fact, near atomic resolution of the tip can be achieved under the operating conditions of a field ion microscope (FIM) where an external field on the order of 10 V/nm is applied to either image the surface structure at low temperatures or reacting adsorbates at higher temperatures.

Stunningly, a field emitter tip is also large enough to allow the emergence of regular oscillations from the molecular fluctuations. This is the case when a rhodium nanosized crystal is exposed to hydrogen and oxygen [1-5] or when a platinum nanosized crystal is exposed to H₂ and NO₂ [6]. Using density functional calculations and a kinetic mean field model, we show that the observed nonequilibrium oscillatory patterns find their origin in the different catalytic properties of all of the 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 multifaceted nanostructured surfaces.     

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[3]  J.-S. McEwen, P. Gaspard, T. Visart de Bocarmé, N. Kruse, Surf. Sci. 604 (2010) 1353.

[4]  J.-S. McEwen, P. Gaspard, F. Mittendorfer, T. Visart de Bocarmé, N. Kruse, Chem. Phys. Lett. 452 (2008) 133

[5]  J.-S. McEwen, A. Garcia Cantu Ros, P. Gaspard, T. Visart de Bocarmé and N. Kruse, Catal. Today 154 (2010) 75. 

[6]  J.-S. McEwen, Y. De Decker, P. Gaspard, C. Barroo, T. Visart de Bocarmé and N. Kruse, Langmuir 26 (2010) 16381.

[7]   A. Garcia Cantu Ros, J.-S. McEwen, P. Gaspard, Phys. Rev. E 83 (2011) 021604.