Converting carbon dioxide into valuable chemical fuels such as formic acid, methanol or long-chain alcohols can effectively combat problems such as global warming and fossil resource shortages. The reduction reaction of carbon dioxide by electrocatalysis is a very promising strategy. Finding efficient, safe, and highly selective electrocatalysts for CO2 reduction reactions is an important task. Transition metal complexes are common catalysts used for electrocatalytic CO2 reduction. This is mainly because they have two advantages. First, in the reduction process of metal complexes, there will be an empty coordination point conducive to the coordination and activation of CO2. Secondly, through the modification of ligands, the electronic and geometric structures of metal complexes can be changed, thereby regulating the catalytic activity of catalysts. Currently, most of the transition metal complexes, such as Re, Ru, Fe, Co, Ni, Mn, have been widely used in the electrocatalytic reduction of CO2.
The electrocatalytic reduction of CO2 by transition metal complexes generally uses heterogeneous catalysis, that is, the reaction occurs at the interface between the electrode and the electrolyte.
- The reduction process consists of the following three steps:
(1) CO2 is chemically adsorbed on the transition metal complex to activate CO2.
(2) The activated CO2 obtains electrons or protons (or simultaneously), the C-O bond breaks and/or forms a C-H bond to form a reaction intermediate (e.g., *COOH).
(3) Continue to obtain electrons to form the final product and then separate from the transition metal complex, the catalytic site enters the next reaction cycle.
- The reaction equation of CO2 electrocatalytic reduction to other products is as follows:
Transition metal complexes are good electron transport mediators and exhibit a variety of chemical properties. It has been reported in the early 1980s that cobalt-based macrocyclic complexes can effectively electro catalyze CO2 reduction to produce CO, HCOOH, methanol and methane, whether in solution or adsorbed on carbon electrodes. Shen's group  immobilized co-protoporphyrin molecular catalysts in pure electrolyte aqueous solution on pyrolytic graphite (PG) electrode and used electrocatalytic reduction to reduce CO2 to CO and methane, in addition to small amounts of HCOOH and methanol were produced. In addition to cobalt-based complexes, other transition metal complexes such as ruthenium, copper, palladium, and nickel are also irreplaceable for the electrocatalytic reduction of CO2.
Figure 1. Mechanistic scheme for the electrochemical reduction of CO2 on Co protoporphyrin
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- Shen, J.; et al. Electrocatalytic reduction of carbon dioxide to carbon monoxide and methane at an immobilized cobalt protoporphyrin. Nature Communications. 2015, 6.