Global warming and the decreasing supply of fossil fuels have brought increasing attention to the photocatalytic conversion of carbon dioxide (CO2) into chemicals or fuel precursors, e.g., carbon monoxide (CO) and hydrogen (H2) for the Fischer-Tropsch reaction. Complexes of the type Mn(bpy)(CO)3Br [bpy = 2,2′-bipyridine, dmb = 4,4′-dimethyl-2,2′-bipyridine and analogues thereof] have been extensively studied for photocatalytic and electrocatalytic reduction of CO2 to formic acid (HCO2H) and CO. It was reported previously that replacement of the bromide ligand with a cyanide ligand in Mn(bpy)(CO)3Br to yield Mn(bpy)(CO)3(CN), 1, altered the reduction mechanism during electrocatalysis. Herein, we report a series of studies on the potential use 1 as a photocatalyst using [Ru(dmb)3]2+ as a photosensitizer in dry N,N-DMF-triethanolamine (TEOA) or MeCN-TEOA system. The optimal photocatalytic conditions were found and the mechanism was elucidated via concentration studies. The formation of HCO2H and CO from CO2 decrease with increasing concentration of the Mn complex and increase with a higher Ru(dmb)32+ : 1 ratio up to 5 : 1. This supports the disproportionation mechanism as previously proposed in the electrochemical studies of this catalyst. As the concentration of 1 increases, the probability of disproportionation of two one-electron-reduced species reduces due to a higher population of unreduced Mn complex. Experimental controls showed that TEOA is necessary to donate a proton for HCO2H formation. The disproportionation mechanism for complex 1 was also validated by infrared spectroelectrochemistry (IR-SEC) studies in N,N-DMF-TEOA and MeCN-TEOA. These IR-SEC studies revealed that the stability of the singly reduced [Mn(bpy)(CO)3(CN)].– differed from the N,N-DMF-TEOA solvent system to the MeCN-TEOA system, which may contribute to the observed selectivity for HCO2H over CO production in the former.

Photochemical reduction of carbon dioxide using a CN-modified fac-Mn(bpy)(CO)3 catalyst

251st National Meeting (2016)

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