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

ESCOBAR, M.; GRACIA, F.; KARELOVIC, A.; JIMÉNEZ, R.
Catalysis Science & Technology 9 (2015) 4532-4541.

DOI: 10.1039/C5CY00676G

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 KCO 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−1In 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.

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