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The behavior of ultra-thin (\1 ML) films of vanadium oxide on Rh(111) during the H 2 ? O 2 reaction was studied in the 10 -6 -10 -4 mbar range with photoemission electron microscopy (PEEM). One observes that under reaction conditions the V-oxide condenses forming a stripe pattern of macroscopic dimensions on the surface. The pattern is characterized by an intrinsic wave length which obeys a power law dependence on the reactants pressure. The origin of the pattern is discussed within the concept of reactive phase separation, i.e. the strongly attractive interactions between V and O and/or changes in the interfacial energies caused by reduction are presumably the main driving forces for condensation.

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A new type of traveling interface modulation has been observed in the NH 3 + O 2 reaction on a Rh(110) surface. A model is set up which reproduces the effect, which is attributed to diffusional mixing of two spatially separated adsorbates causing an excitability which is strictly localized to the vicinity of the interface of the adsorbate domains.

Self-organization phenomena such as rate oscillations, chemical wave patterns, and precipitation of nanoparticles can be observed in the catalytic H2 + O2 reaction on a Rh(111) surface after alloying with Ni. The bimetallic Rh(111)/Ni surface has been studied in the 10–6–10–4 mbar range using PEEM (photoemission electron microscopy) and LEEM/SPELEEM (low energy electron microscopy and its spectroscopic variant) as the main analytical methods. The Rh(111)/Ni catalysts are prepared by thermal decomposition of Ni­(CO)4 on Rh(111), resulting in an alloyed surface with about 25% Ni in the topmost layers. One finds rate oscillations and chemical wave patterns comprising target patterns, pulse trains, and rotating spiral waves. The oscillatory behavior is attributed to periodic changes in the composition of the bimetallic surface alloy causing concomitant variations in catalytic activity. Under pattern-forming reaction conditions, three-dimensional NiO particles develop on top of the alloyed Rh/Ni surface, with dimensions ranging from <1 μm up to 50 μm. Their size which depends on the total pressure controls the Ni content in the surface alloy.