Kinetic and DRIFTS studies of the CO2 catalytic hydrogenation on Ru/SiO2: A comprehensive and strategic investigation of CO2 methanation mechanisms by kinetic modeling and data regression

The kinetics of the CO2 hydrogenation/methanation is investigated on a Ru/SiO2 catalyst. Diffuse reflectance infrared Fourier transform spectroscopy experiments attest the presence of linearly adsorbed and bridge-bonded carbonyls on the metallic sites, as well as bicarbonate and formate species. Kinetic assessments at different temperatures and partial pressures show a direct relationship between methane turnover frequency and H2 pressure, suggesting that H* is involved in the rate-determining step. Contrariwise, increases in CO2 pressure inhibited methane formation. Applying Langmuir-Hinshelwood kinetics and non-linear regression, strategic kinetic models for CH4 formation rate were conceived and tested. The best model indicates that methanation starts off with CO2 dissociation into CO*reactive, proceeding along a H*-assisted pathway. The model also points out that competing oxygenated species (O*, HO*), intrinsically generated by CO2 activation, are hydrogenated faster on key sites, hindering the hydrogenation of carbon intermediates, which explained the inhibitory effect observed when CO2 pressure was increased.