DRIFTS STUDY OF ADSORBED FORMATE SPECIES IN THE CARBON-DIOXIDE AND HYDROGEN REACTION OVER RHODIUM CATALYSTS

Carbon dioxide methanation has been studied over Rh/Al2O3 and lutetium-promoted Rh/Al2O3 in a catalytic flow reactor. The results obtained have been related to the nature of the adsorbed species under the actual flow reaction conditions, studied in a Fourier-transform infrared spectrometer, operating in the diffuse reflectance mode (DRIFTS). From this, the presence of formate and carbon monoxide species at the catalyst surface during the reaction is proved. In the proposed reaction scheme, adsorbed carbon monoxide species hinder the dissociative chemisorption of hydrogen, thus justifying the lower activity of the rare-earth-promoted sample. Kinetic analysis of the DRIFT spectra, after adsorbing and decomposing formic acid, shows that formate species play a major role in the reverse water-gas shift equilibrium, which determines the rate of carbon dioxide methanation.