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Interaction of small gas phase molecules with alumina supported rhodium nanoparticles: an in situ spectroscopic study

Supported nanoparticulate Rh systems are studied as a model system for the important three way catalysts as used in the combustion engines of cars. Small Rh nanoparticles with a small particle size distribution can be easily synthesized and their morphology is studied using x-ray absorption fine structure (XAFS) spectroscopy. The interaction of the supported rhodium nanoparticles on γ-Al2O3 with small gas phase molecules like H2, O2, CO, NO, H2S and SO2 is investigated, in situ and time resolved, using a combination of techniques, i.e. XAFS, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry. The surface species formed upon exposure of the metal particles to the adsorbing molecules, and their sometimes disruptive interaction with the metal particles, are identified as a function of temperature and time. Dynamic equilibria are observed which change the oxidation state and the nuclearity of the supported rhodium particles under operational conditions. Rather than merely adsorb on a catalyst particle, these gases have a substantial role in defining the nature of the particle.