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Abstract:
The reaction between NO and hydrogen on Pt{100} has been studied during transient changes of the partial pressure ratio of the reactants at constant substrate temperature using low-energy electron microscopy (LEEM), mirror electron microscopy (MEM) and selected-area low-energy electron diffraction (LEED). Surface steps and other mesoscopic defects are found to influence the nucleation and propagation of the reaction–diffusion fronts (RDFs) which develop when the NO/H2 partial pressure ratio is decreased. In particular, LEEM and MEM show that RDF propagation is hindered by surface steps which cause a laterally anisotropic and jump-like (‘stop-and-go’) propagation. The surface crystallography, on the other hand, has no influence on the RDF propagation characteristics. In LEED only two relatively diffuse patterns in the areas in front of and behind the RDF were observed. The actual border of the front has a width of less than the spatial resolution limit of the instrument, i.e. <100 nm. At T=395 K a switch from heterogeneous, defect-related front nucleation to homogeneous, defect-free nucleation was observed. The analysis of the RDF nucleation and propagation characteristics and their temperature dependence reveal how the diffusion of NO and hydrogen on the Pt{100} surface is dependent on the presence of steps and defects. Finally, we present a model for the NO+H2 reaction on Pt{100} which qualitatively explains the observed phenomena.
