The catalytic conversion of CO2 into valuable chemicals and fuels has attracted increasing attention, providing a promising route for mitigating the greenhouse effect of CO2 and also meeting the global energy demand. Among many homogeneous and heterogeneous catalysts for CO2 reduction, this mini-review is focused on heme and nonheme metal complexes that act as effective catalysts for the electrocatalytic and photocatalytic reduction of CO2. Because metalloporphyrinoids show strong absorption in the visible region, which is sensitive to the oxidation states of the metals and ligands, they are suited for the detection of reactive intermediates in the catalytic CO2 reduction cycle by electronic absorption spectroscopy. The first part of this review deals with the catalytic mechanism for the one-electron reduction of CO2 to oxalic acid with heme and nonheme metal complexes, with an emphasis on how the formation of highly energetic CO2 is avoided. Then, the catalytic mechanism of two-electron reduction of CO2 to produce CO and H2O is compared with that to produce HCOOH. The effect of metals and ligands of the heme and nonheme complexes on the CO or HCOOH product selectivity is also discussed. The catalytic mechanisms of multi-electron reduction of CO2 to methanol (six-electron reduced product) and methane (eight-electron reduced product) are also discussed for both electrocatalytic and photocatalytic systems.
Bibliographical noteFunding Information:
Fig. 7 Nickel(II) catalysts supported by non-macrocyclic and macrocyclic bipyridyl-NHC ligands. Reprinted with permission from ref. 117. Copyright 2018, Royal Society of Chemistry.
The authors gratefully acknowledge the contributions of their collaborators and coworkers cited in the listed references, and support through a SENTAN project (to S. F.) from the Japan Science and Technology Agency (JST), JSPS KAKENHI (No. 16H02268 to S. F.), the NRF of Korea through CRI (NRF-2012R1A3A2048842 to W. N.), GRL (NRF-2010-00353 to W. N.), and the Basic Science Research Program (2017R1D1A1B03029982 to Y. M. L., 2017R1D1A1B03032615 to S. F., and 2017R1C1B2011074 to H. S. A.).
All Science Journal Classification (ASJC) codes