Mechanism for catalytic partial oxidation of methane to syngas over a Ni/Al2O3 catalyst

The mechanism of catalytic partial oxidation of methane to syngas (POM) over a Ni/alpha-Al2O3 catalyst was studied by using a pulse reactor and temperature-programmed surface reaction (TPSR) techniques. Over a reduced nickel catalyst (Ni-0/Al2O3), methane activation follows the dissociation mechanism; while on oxidic nickel catalysts (NiO/Al2O3), methane is first oxidized to carbon dioxide and water, and simultaneously NiO is reduced to Ni-0. CH4 dissociation occurs over NiO active sites, generating hydrogen and surface C species. A transient process was observed during the CH4/O-2 reaction. The nickel valence was transformed from NiO to Ni-0 at a certain critical temperature and simultaneously, the reaction was transformed rapidly from the complete oxidation of methane to the partial oxidation of methane. It has been found that the POM reaction takes place over a thin layer of the catalyst bed. This reaction zone is nearly isothermal, over which almost 100% of oxygen and more than 90% of methane are converted. The temperature drop in the downstream of the catalyst bed does not imply that the steam or carbon dioxide reforming reaction occurs in the lower part of the bed. Ni-0 species constitute the active sites for the partial oxidation of methane to syngas, Both methane and oxygen are activated on Ni-0 sites, generating surface Ni ... C and Nidelta+... Odelta- species. These two kinds of intermediates have been proposed to account for the mechanism of methane partial oxidation. The Nidelta+... Odelta- species over Ni-0 catalyst surface is considered to be a kind of weakly bounded, mobile oxygen species. The reaction between Nidelta+... Odelta- and Ni ... C intermediates generate the primary product of CO. However, the presence of NiO over the catalyst surface significantly reduces the CO selectivity. Thus, the NiO species are not possible to be the intermediate for the POM reaction. The mechanism of partial oxidation of methane should follow the direct oxidation route. (C) 2000 Elsevier Science B.V. All rights reserved.