Kinetic and in situ FTIR study of CO methanation on a Rh/Al2O3 catalyst

Carbon monoxide hydrogenation was studied over a gamma-alumina-supported 1 wt% Rh catalyst by means of kinetic and in situ infrared measurements. The study was carried out at 200-300 degrees C, 0-22.5 kPa H-2 and 1-7.5 kPa CO. The in situ FTIR scrutiny of the catalyst surface shows that adsorbed CO* species and vacancies dominate the Rh surface, while no effect of H-2 and H2O pressures on surface coverage was observed under the conditions studied. The kinetic data are consistent with the mechanism in which the C-O bond dissociation is assisted by a double H-addition while H-2 dissociative adsorption, CO molecular adsorption and the HCO* formation are quasi-equilibrated steps. A two-parameter Langmuir-Hinshelwood rate expression is deduced for CH4 formation, in agreement with the proposed sequence of elementary steps and kinetic data. The effect of temperature on parameters a and K-CO leads to an apparent activation energy of 82.3 kJ mol(-1), an average CO adsorption enthalpy of -14.1 kJ mol(-1) and an entropy change of -17.9 J mol(-1) K-1. In situ FTIR experiments show a full coverage of the Rh surface with adsorbed CO below 200 degrees C and this CO* coverage decreases as temperature increases in the range 200-300 degrees C; it is also observed that the heat of CO adsorption on the Rh surface decreases with CO* coverage.