Modified cobalt catalysts in the partial oxidation of methane at moderate temperatures

Cobalt on alumina catalysts were prepared and tested in the partial oxidation of methane using dry feed at moderate gas temperatures (700-973 K) and atmospheric pressure. The effect of adding small amounts (1/15 molar ratio) of Ni, Fe, Cr, Re, Mn, W, Mo, V and Ta oxides was studied. The catalysts were prepared using incipient wetness impregnation followed by direct reduction of catalyst precursors, and characterized using H-2 chemisorption, N-2 Sorption, TPx, XRD and XPS. The catalytic testing was performed using air as oxidant with CH4/O-2 = 2 and a gas hourly space velocity of 15-150 Nl CH4/(g h). The product composition was compared to thermodynamic equilibrium at bed exit temperatures, and any deviation from equilibrium could be explained by temperature gradients. It was found that the presence of co-impregnated metals or metal oxides that may either form bimetallic or mixed oxide species with cobalt was detrimental to the catalyst performance. The most detrimental elements included Mo, W, V and Ta, and their poor activity for synthesis gas formation could be explained by the low cobalt metal surface area determined from hydrogen chemisorption, as compared to the unmodified catalyst. Results indicate that deactivation of cobalt involves oxidation of the active phase, and that carbonaceous deposits may be involved in the deactivation. When increasing the gas hourly space velocity the presence of a modifier had a dramatic effect on the catalyst performance, and only Ni improved catalyst stability and yields. All other modifications tested were detrimental to methane conversion at the levels investigated. Dissociation of methane was investigated by TPx, and the activation of C-H bonds was found to occur above the hcp to fcc phase transition temperature of cobalt, indicating that methane activation to synthesis gas could be structure sensitive on metallic cobalt. A 2-3 zone reaction mechanism involving pyrolysis, combustion and reforming is suggested. (C) 2008 Elsevier Inc. All rights reserved.