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. Computa-tional 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 center dot--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 pi orbital and the low-lying pi* 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.

Automated summary

Density functional theory calculations were used to investigate the mechanism of cobalt(II) tetraamino phthalocyanine catalyzed electro-reduction of CO2. The results showed that the active species formed through reduction of the initial catalyst can attack CO2 to produce carbon monoxide as an intermediate and product. The redox properties of the ligand were found to be important in the catalysis. The higher energy barrier for hydrogen evolution reaction compared to carbon dioxide reduction was consistent with the experimental product distribution.