Structure-function relationship during CO2 methanation over Rh/Al2O3 and Rh/SiO2 catalysts at atmospheric pressure conditions

The effect of support material and chemical state of Rh for Rh/Al2O3 and Rh/SiO2 model catalysts during CO2 hydrogenation were studied by a combined array of in situ characterisation techniques including diffuse reflectance infrared Fourier transform spectroscopy, energy-dispersive X-ray absorption spectroscopy and high-energy X-ray diffraction at 250-350 \ub0C and atmospheric pressure. The CO2 methanation proceeds via intermediate formation of adsorbed CO species on metallic Rh likely followed by their hydrogenation to methane. Linearly-bonded CO species is suggested to be a more active precursor in the hydrogenation compared to the bridge-bonded species, which seems to relate to particle size effects: for larger particles mainly the formation of inactive bridge-bonded CO species takes place. Further, analysis of the chemical state of Rh during reaction conditions reveal a minor formation of RhOx from dissociation of CO2 , which is a consequence of the increased activity observed over Rh/Al2O3 catalyst