Methane oxidation by oxygen transported through solid electrolyte

Abstract

Solid oxide electrolytes such as yttria-stabilized zirconia, doped ceria, and {beta}-phase bismuth oxide are well known as good oxygen ion conductors. Application of this high ionic conductivity have been made to fuel cells, oxygen pumps, oxygen sensors, and reactors for various kinds of catalytic reaction. Recently, an attempt has been made to utilize oxygen ions transported through a solid electrolyte to a catalyst electrode for the oxidative coupling of methane. It has been reported that oxygen transported to Bi{sub 2}O{sub 3}-Ag catalyst at low transport rates showed higher activity and selectivity for production of C{sub 2} compounds (ethane and ethylene) than oxygen supplied from the gas phase. However, over an Ag electrode such high selectivity was not obtained. In the present work, the oxidation of methane was carried out over a single electrode in order to escape from the complexity which arises from the coexistence of electrode and catalyst. Silver pastes and lanthanum cobaltate powders were employed as electrode materials, because these materials are known to catalyze partial and deep oxidation, respectively. The solid electrolyte tubes were made of zirconia doped with 8 mol% yttria (yttria-stabilized zirconia (YSZ)). Reactants and products such as CO, H{sub 2}, CO{sub 2},C{sub 2}H{sub 6},more » C{sub 2}H{sub 4}, and C{sub 3}H{sub 8} were analyzed by a gas chromatograph with Unibeads 1S and Molecular Sieve 13X columns. A material balance within {+-}5% was achieved for carbon. To compare the reactivity of the transported oxygen with that of oxygen in the gas phase, the following three reaction systems were employed: (1) transport system, where the reaction is caused by oxygen transported electrochemically, (2) Mixture system, where the reaction is caused by oxygen premixed in the gas phase, and (3) Joint system, where both transported and premixed oxygen taken part in the reaction.« less