Electrochemical promotion of oxidative coupling of methane on platinum/polybenzimidazole catalyst

The electrochemical promotion of catalytic methane oxidation was studied using a (CH4,O-2,Ar), Pt\polybenzimidazole (PBI)-H3PO4\Pt,(H-2,Ar) fuel cell at 135degreesC. It has been found that C2H2, CO2, and water are the main oxidation products. Without polarization the yield of C2H2 was 0.9% and the yield of CO2 was 7.3%. This means that C-2 open-circuit selectivity was approximately 11%. Open-circuit voltage was around 0.6 V. It has been shown that the CH4 --> C2H2 catalytic reaction can be electrochemically promoted at negative polarization and exhibits a clear "volcano-type'' promotion behavior, meaning that there was a maximum promotion effect at a polarization of -0.15 V, or 0.45 V catalyst potential vs. a hydrogen electrode (3.8% C2H2 yield). The catalytic rate enhancement ratio, r(C-2)/r(o)(C-2), at this maximum was 4.2. There was no C2H2 production at polarization greater than or equal to0.1 and less than or equal to-0.3 V. The yield of C2H2 decreased with decreasing temperature. Dependence of CO2 yield on polarization also showed a "volcano-type'' behavior with maximum yield of 8.3% at -0.15 V polarization. The catalytic rate enhancement ratio for CO2 production, r(CO2)/r(o)(CO2), at this maximum was 1.1, which means that this catalytic reaction is only slightly affected by the electrochemical polarization. This indicates that polarization especially affects the C-2 selectivity of the catalyst. The obtained data was explained by the electrochemical production of Pt-H active centers at the electrolyte-catalyst-gaseous reactant interface (lambda much greater than 1). (C) 2002 The Electrochemical Society.