Chemical Waves and Rate Oscillations in the H2 + O2 Reaction on a Bimetallic Rh(111)/Ni Catalyst

Abstract

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.