Thermodynamic Considerations on the Oxidation State of Co/γ-Al2O3 and Ni/γ-Al2O3 Catalysts under Dry and Steam Reforming Conditions

The oxidation state of the active metal is an important factor for catalyst stability under dry and steam reforming conditions. This work explores the correlation of the oxidation state of the active metal with the coking behavior of alumina-supported cobalt and nickel catalysts from a thermodynamic point of view. To this end, the thermodynamics of the oxidation of Co/gamma-Al2O3 and Ni/gamma-Al2O3 were investigated by using calculations at both standard and technical reforming conditions. It is shown that oxidation of nickel by water or CO2 cannot occur spontaneously under reforming conditions regardless of participation of the alumina support material because of the positive Gibbs reaction energies. Cobalt, in contrast, is more easily oxidized and may form CoAl2O4 through interaction with the support. This phase may react with surface carbon to regenerate the catalyst after carbon formation through thermal cracking of methane. A Mars-van Krevelen type reaction scheme is proposed to explain the higher coking resistance of cobalt compared to nickel.