Methane oxidation over Fe-, Co-, Ni- and V-containing mixed conductors

Abstract

The catalytic oxidation of methane over mixed conducting ceramics, including perovskite-type SrFe0.7Al0.3O3−δ and La0.3Sr0.7Co0.8Ga0.2O3−δ, dual-phase composite (SrCo)0.5(Sr2Fe3)0.5O4.75±δ, La2Ni0.9Co0.1O4+δ with K2NiF4-type structure and zircon-type CeVO4+δ, is primarily governed by bonding energy between oxygen and transition metal cations, which leads to general correlations between the catalytic activity, oxygen desorption, oxygen ionic transport, thermal expansion, and, often, phase stability. The steady-state conversion of dry CH4 either by oxygen permeating through dense oxide ceramics in a membrane reactor or by atmospheric O2 (methane/air ratio of 30:70) in a fixed bed reactor with membrane material as catalyst results in high CO2 selectivity, increasing when the oxygen permeability of mixed conductors increases. The prevailing mechanism of total methane combustion makes it necessary to incorporate reforming catalysts in the membrane reactors for natural gas conversion to Synthesis gas (syngas). Dominant CO2 formation is also observed for the oxidation of CH4 pulses supplied in helium flow over the mixed conductor powders, except for SrFe0.7Al0.3O3−δ yielding synthesis gas with the H2/CO ratio close to 2, characteristic of the partial oxidation process. For a model reactor comprising one disk-shaped membrane and a catalyst both made of SrFe0.7Al0.3O3−δ, the methane conversion and CO selectivity at 1223 K achieved 65% and 48%, respectively.