Catalytic reforming of methane with carbon dioxide over nickel catalysts II. Reaction kinetics

Abstract

The reforming of methane with carbon dioxide was studied over nickel supported on SiO2, TiO2, MgO and activated carbon. Specific activities on a turnover frequency basis were in the order: Ni/TiO2 > Ni/C > Ni/SiO2 > Ni/MgO. Interestingly, a 2-fold increase in activation energy for this reaction was observed over Ni/TiO2 after several hours time on stream. The reverse water-gas shift reaction was found to be close to thermodynamic equilibrium over all catalysts. Partial pressure dependencies were obtained with the Ni/C and Ni/SiO2 catalysts at 723 K for comparative purposes only, but a more thorough kinetic analysis was made with the Ni/MgO and Ni/TiO2 catalysts, which were shown previously to strongly inhibit carbon deposition. Partial pressure dependencies were obtained at 673, 698, and 723 K for Ni/TiO2 and at 773, 798, and 823 K for Ni/MgO. In situ DRIFTS studies clearly showed the presence of both linear and bridged carbon monoxide adsorption on Ni/SiO2 under reaction conditions; however, adsorbed carbon monoxide could not be identified on Ni/TiO2. A reaction model for CH4-CO2 reforming, based on CH4 activation to form CHx and CHxO decomposition as the slow kinetic steps, successfully correlated the rate data.