Structure–function relationship for CO2 methanation over ceria supported Rh and Ni catalysts under atmospheric pressure conditions

Abstract

In situ structural and chemical state characterization of Rh/CeO₂ and Ni/CeO₂ catalysts during atmospheric pressure CO₂ methanation has been performed by a combined array of time-resolved analytical techniques including ambient-pressure X-ray photoelectron spectroscopy, high-energy X-ray diffraction and diffuse reflectance infrared Fourier transform spectroscopy. The ceria phase is partially reduced during the CO₂ methanation and in particular Ce³⁺ species seem to facilitate activation of CO₂ molecules. The activated CO₂ molecules then react with atomic hydrogen provided from H₂ dissociation on Rh and Ni sites to form formate species. For the most active catalyst (Rh/CeO₂), transmission electron microscopy measurements show that the Rh nanoparticles are small (average 4 nm, but with a long tail towards smaller particles) due to a strong interaction between Rh particles and the ceria phase. In contrast, larger nanoparticles were observed for the Ni/CeO₂ catalyst (average 6 nm, with no crystallites below 5 nm found), suggesting a weaker interaction with the ceria phase. The higher selectivity towards methane of Rh/CeO₂ is proposed to be due to the stronger metal–support interaction.