Mechanistic investigations on the partial oxidation of methane to synthesis gas over a nickel-on-alumina catalyst

The mechanism of the partial oxidation of methane to synthesis gas (CO+H₂) was investigated over a nickel-on-alumina catalyst. The pulse reaction experiments combining XPS and TPR investigations were used to verify the active sites and selective O species present on the working catalyst. It was found that both, reduced Ni and NiO_x species are present on the catalyst surface during the reaction. Unlike the highly dispersed surface NiAl₂O₄ species on the fresh catalyst which are reducible at ca. 800°C, these NiO_x species present on the working catalyst can be reduced at 520-560°C, they are thus assigned to adsorbed O atoms. The comparisons of the pulse reactions of CH₄, CH₄/O₂ (2 : 1) and CH₄/CO₂(1 : 1) indicate that the reduced Ni sites play a role in dissociating methane into surface C atoms and H₂, while the direct formation of CO occurs via reacting surface C atoms with adsorbed O atoms present as NiO_x. We propose that the primary surface reaction of the partial oxidation of methane over Ni/Al₂O₃ proceeds via a CH₄ pyrolysis followed by the reduction of NiO_x by the surface C atoms (arising from CH₄ dissociation), which is reoxidized by incorporation of O₂ into the catalyst.