Theoretical Study on the Electro-Reduction of Carbon Dioxide to Methanol Catalyzed by Cobalt Phthalocyanine

Density functional theory (DFT) calculations have been conducted to investigate the mechanism of cobalt(II) tetraamino phthalocyanine (CoPc-NH2) catalyzed electro-reduction of CO2. Computational results show that the catalytically active species 1 (4[CoII(H4L)]0) is formed by a four-electron-four-proton reduction of the initial catalyst CoPc-NH2. Complex 1 can attack CO2 after a one-electron reduction to give a [CoIII-CO22-]- intermediate, followed by a protonation and a one-electron reduction to give intermediate [CoII-COOH]- (4). Complex 4 is then protonated on its hydroxyl group by a carbonic acid to generate the critical species 6 (CoIII-L•--CO), which can release the carbon monoxide as an intermediate (and also as a product). In parallel, complex 6 can go through a successive four-electron-four-proton reduction to produce the targeted product methanol without forming formaldehyde as an intermediate product. The high-lying π orbital and the low-lying π* orbital of the phthalocyanine endow the redox noninnocent nature of the ligand, which could be a dianion, a radical monoanion, or a radical trianion during the catalysis. The calculated results for the hydrogen evolution reaction indicate a higher energy barrier than the carbon dioxide reduction. This is consistent with the product distribution in the experiments. Additionally, the amino group on the phthalocyanine ligand was found to have a minor effect on the barriers of critical steps, and this accounts for the experimentally observed similar activity for these two catalysts, namely, CoPc-NH2 and CoPc.