CH4 Oxidative Coupling on Ag

The oxidative coupling of CH4 (OCM) in solid oxide fuel cells has attracted considerable attention in recent years because of the strong interest in the production of C2 hydrocarbons from natural gas. Work in this area utilizing solid electrolytes prior to 1999 has been reviewed.53 

It is known32 reported that the solid electrolyte itself, i.e. Y203-doped-Zr02, is a reasonably selective catalyst for CH4 conversion to C2 hydrocarbons, i.e., ethane and ethylene32 and this should be taken into account in studies employing stabilized Zr02 cells. At the same time it was found54 that the use of Ag catalyst films leads to C2 selectivities above 0.6 for low methane conversions. 

As shown on Fig. 8.49 one can influence dramatically both the total CH4 conversion as well as product selectivity by varying the Ag catalyst potential. Thus under open-circuit conditions (Uwr=U r ) the CH4 conversion is near 0.02 with a C2 selectivity (methane molecules reacting to form C2H4 and C2H6 per total reacting CH4 molecules) near 0.5. Increasing Uwr increases the methane conversion to 0.3 and decreases the selectivity to 0.23, while decreasing Uwr decreases the conversion to 0.01 and increases the 

The rates of C2H4, C2H6 and C02 formation depend exponentially on UWr and O according to equation (4.49) with a values of 1.0, 0.75 and 0.4, respectively, for I 0, and of 0.15, 0.08 and 0.3, respectively for I 0. Linear decreases in activation energy with increasing E have been found for all three reactions.54 It should be emphasized, however, that, due to the high operating temperatures, A is near unity and electrocatalysis, rather than NEMCA, plays the dominant role. 

Similar studies utilizing Au electrodes on YSZ showed again that the selectivity and yield of C2 hydrocarbons can be significantly affected by applying currents or potentials to the cell.40,41,53 The behaviour with Au appears to be qualitatively similar with that obtained with Ag electrodes although electrophilic behaviour is also reported.40,41 

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