Catalysis of the cathodic reduction of carbon dioxide by transition metal complexes

Pearce, David John (1986) Catalysis of the cathodic reduction of carbon dioxide by transition metal complexes. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

The application of nickel and cobalt square planar complexes to the catalysis of the cathodic reduction of carbon dioxide has been studied in a variety of aprotic solvent/water mixtures. The complexes can have a high turnover number and, in terms of a reduction in overpotential, the complexes can also be effective catalysts; the presence of the catalyst may reduce the overpotential by more than 0.5V. The majority of complexes were soluble in the electrolytes used, e.g. Ni(teta)²⁺, Co(salen) where the maximum current density achieved has been less encouraging. However, a very fast catalytic cycle in which CO₂ diffusion to the electrode is rate limiting has been obtained using an insoluble complex, cobalt phthalocyanine, adsorbed on the electrode surface. It has been confirmed that the product of the reduction of CO₂ catalysed by such complexes is carbon monoxide, although bicarbonate is also formed due to the generation of base. It is proposed that CO is the major product because CO₂ is bound to the transition metal centre via a metal-carbon bond thus constraining protonation to occur at an oxygen atom. The influence of added proton donor, the choice of base electrolyte cation and the presence of heterocyclic compounds able to act as ligands to the octahedral sites of the complexes have also been investigated. The electrochemical techniques employed included cyclic voltammetry, constant potential electrolysis, steady state measurements and potential step methods. (D72241/87)

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