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

Abstract

Density functional theory (DFT) calculations have been conducted to investigate the mechanism of cobalt(II) tetraamino phthalocyanine (CoPc-NH 2 ) catalyzed electro-reduction of CO 2 . Computational results show that the catalytically active species 1 ( 4 [Co II (H 4 L)] 0 ) is formed by a four-electron-four-proton reduction of the initial catalyst CoPc-NH 2 . Complex 1 can attack CO 2 after a one-electron reduction to give a [Co III −CO 2 2− ] − intermediate, followed by a protonation and a one-electron reduction to give intermediate [Co II −COOH] − (4).Complex 4 is then protonated on its hydroxyl group by a carbonic acid to generate the critical species 6 (Co III −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-NH 2 and CoPc.

Mechanistic Insights into Photochemical CO2 Reduction to CH4 by a Molecular Iron–Porphyrin Catalyst

“…The detailed reaction mechanism for the transformation of CO 2 to CH 4 is displayed in Figures , , and . Meanwhile, to accurately estimate the barriers for the reduction of 3 to 4 and Int3 to Int5 , the Marcus theory (eq ) was used to calculate the corresponding reorganization energy and thus to obtain the related energy barrier . It is found that for the reduction of 3 to 4 , the reorganization energy and the related barrier are estimated to be +18.4 and +9.0 kcal/mol, respectively.…”

“…Meanwhile, to accurately estimate the barriers for the reduction of 3 to 4 and Int3 to Int5, the Marcus theory 129 (eq 9) was used to calculate the corresponding reorganization energy and thus to obtain the related energy barrier. 130 It is found that for the reduction of 3 to 4, the reorganization energy and the related barrier are estimated to be +18.4 and +9.0 kcal/mol, respectively. Likewise, for the reduction of Int3 to Int5, the reorganization energy and the related barrier are +7.6 and +13.9 kcal/mol, respectively.…”

Aqueous Photoelectrochemical CO2Reduction to CO and Methanol over a Silicon Photocathode Functionalized with a Cobalt Phthalocyanine Molecular Catalyst

“…mechanism of MeOH formation in the PEC reaction is likely to be the same as in electrocatalytic CO 2 reduction, and that CO is the key intermediate to the formation of MeOH. [17,[26][27][28] The optimal potential for FE MeOH of STA-GO/CoPc is about 0.4 V lower than that of CFP-GO/CoPc (Figure 4c), demonstrating the photovoltage contribution from the Si to the catalyst. The lower optimal FE MeOH on STA-GO/CoPc compared with CFP-GO/CoPc may be due in part to the planar structure of the Si substrate; that is, the planar electrode is not able to trap the electrogenerated CO intermediate near its surface for further reduction as effectively as the porous CFP electrode.…”

Aqueous Photoelectrochemical CO2Reduction to CO and Methanol over a Silicon Photocathode Functionalized with a Cobalt Phthalocyanine Molecular Catalyst

“…At −0.62 V, STA‐GO/CoPc produces MeOH with a TOF of 0.18 s −1 , which is similar to the TOF for MeOH of CFP‐GO/CoPc at −1.0 V measured to be 0.21 s −1 . We propose that the mechanism of MeOH formation in the PEC reaction is likely to be the same as in electrocatalytic CO 2 reduction, and that CO is the key intermediate to the formation of MeOH [17, 26–28] . The optimal potential for FE MeOH of STA‐GO/CoPc is about 0.4 V lower than that of CFP‐GO/CoPc (Figure 4c), demonstrating the photovoltage contribution from the Si to the catalyst.…”

“…Chemie Zuschriften mechanism of MeOH formation in the PEC reaction is likely to be the same as in electrocatalytic CO 2 reduction, and that CO is the key intermediate to the formation of MeOH. [17,[26][27][28] The optimal potential for FE MeOH of STA-GO/CoPc is about 0.4 V lower than that of CFP-GO/CoPc (Figure 4c), demonstrating the photovoltage contribution from the Si to the catalyst. The lower optimal FE MeOH on STA-GO/CoPc compared with CFP-GO/CoPc may be due in part to the planar structure of the Si substrate; that is, the planar electrode is not able to trap the electrogenerated CO intermediate near its surface for further reduction as effectively as the porous CFP electrode.…”