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

Abstract

Carbon monoxide hydrogenation was studied over a γ-alumina-supported 1 wt% Rh catalyst by means of kinetic and in situ infrared measurements. The study was carried out at 200–300 °C, 0–22.5 kPa H₂ 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 H₂ 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 H₂ 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 α 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 °C and this CO* coverage decreases as temperature increases in the range 200–300 °C; it is also observed that the heat of CO adsorption on the Rh surface decreases with CO* coverage.