Recent studies of the photocatalytic reduction of CO2 for the production of solar fuels will be described. Photocatalytic reduction of CO2 by rhenium and manganese bipyridine (bpy) complexes has been extensively studied. Dimerization of the singly reduced species prior to formation of the active catalyst poses a significant challenge since it results in irreversible loss of catalyst. Mechanistic studies of the cyano-substituted complex, Mn(CN)(bpy)(CO)3 (1), reveal that the cyano-group resists dissociation in the singly reduced state, and the anionic species [Mn(CN)(bpy)(CO)3]- (1-) disproportionates to the active CO2 reducing state, [Mn(bpy)(CO)3]- and 1, rather than forming the inactive Mn-Mn bonded dimer.1 In addition to bypassing dimer formation, the disproportionation process also provides a convenient path to obtain the two-electron reduced catalyst required for CO2 reduction from two singly reduced species (1-). This system has been studied photochemically with with [Ru(dmb)3]2+ (dmb = 4,4′-dimethyl-2,2′- bipyridine) as a photosensitizer and 1-benzyl-1,4-dihydronicotinamide (BNAH) in mixtures of dimethylformamide (DMF) /triethanolamine (TEOA) or acetonitrile (MeCN) / TEOA under irradiation of visible light.2 Upon 15 h of irradiation, the products CO and HCO2H were detected at different concentrations depending on the solvent system-induced selectivities. Further concentration studies show that upon single electron reduction, this catalyst prefers disproportionation rather than dimerization to form an Mn-Mn dimer as is observed in the Mn(Br)(bpy)(CO)3 catalysts. We have also examined another way to prevent dimerization of the singly reduced species, and that is to build Mn(bpydc)- (CO)3Br (bpydc = 5,5′-dicarboxylate-2,2′-bipyridine) into the struts of a robust Zr(IV)-based metal−organic framework (MOF).3 Under visible-light irradiation, this MOF exhibits high turnover numbers (TON) for CO2 reduction to HCO2H with [Ru(dmb)3]2+ as a photosensitizer and BNAH as a sacrificial reductant in DMF/TEOA. The higher photocatalytic performance than the homogenous reference is ascribed to the isolation of the monomeric singly reduced Mn species in the MOF framework.

Photocatalytic reduction of CO2

253rd National Meeting (2017)

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