Multi-electron reduction, carbon dioxide

M,S4 preparation, 38 28 (M4S31 inverted clusters, 38 28-29 M—S(thioether bond), 35 3 Multalin (software), 47 86 Multi-electron reduction, carbon dioxide, 43 423-426... 

The application of such systems to the multi-electron reduction of carbon dioxide to formate, methanol or methane is now under active investigation in our laboratory. 

Table 7.2 Standard electrode potentials for the single- and multi-electron reduction of carbon dioxide (in an aqueous solution at pH 7) [126]...

It was found, that also Ru and Os colloids can act as catalysts for the photoreduction of carbon dioxide to methane [94, 95]. [Ru(bpy)3]2+ plays a role of a photosensitizer, triethanolamine (TEOA) works as an electron donor, while bipyridinium electron relays (R2+) mediate the electron transfer process. The production of hydrogen, methane, and small amounts of ethylene may be observed in such a system (Figure 21.1). Excited [Ru(bpy)3]2+ is oxidized by bipyridinium salts, whereas formed [Ru(bpy)3]3+ is reduced back to [Ru(bpy)3]2+ by TEOA. The reduced bipyridinium salt R + reduces hydrogen and C02 in the presence of metal colloids. Recombination of surface-bound H atoms competes with a multi-electron C02 reduction. More selective reduction of C02 to CH4, ethylene, and ethane was obtained using ruthenium(II)-trisbipyrazine, [Ru(bpz)3]2+/TEOA/Ru colloid system. The elimination of hydrogen evolution is thought to be caused by a kinetic barrier towards H2 evolution in the presence of [Ru(bpz)3]2+ and noble metal catalysts [96]. 

Overall, the pyridinium catalyst enables the step-by-step reduction of carbon dioxide to highly reduced species. Hence, pyridinium salts alleviate the need for metal-based multi-electron transfer. 

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