Electrochemical CO2 reduction

Begum, Afroza (2011) Electrochemical CO2 reduction. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Electrochemical methods for CO₂ reduction have been receiving continuous attention for the last few decades mainly for the conversion of CO₂ to fuels as an alternative global energy source and the inexpensive production of carboxylated products for industrial applications. CO₂ is considered as a greenhouse gas, whose increasing concentration in the atmosphere is a growing environmental concern. The main obstacle for the direct electrochemical reduction of CO₂ is the requirements of very high negative standard potential as high as -2.21 V vs SCE. Several transition-metal based molecular catalysts, for example, ruthenium polypyridyl complexes, were reported to convert CO₂ to fuels, such as methanol, formic acid, oxalic acid, methane etc., at a lower cathodic potential. On the other hand, electrocatalytic conversions of CO₂ using aromatic ketones were also found to be useful in the syntheses of medicinally useful materials, such as 2-arylpropanoic acids and α-oxocarboxylic acids (as anti-inflammatory drugs), agricultural chemicals and perfumes. A number of monometallic and bimetallic ruthenium benzimidazole and benzothiazole based polypyridyl complexes were synthesized in this work that were found to be operate as electrocatalysts for CO₂ reduction at reasonable cathodic potentials. These complexes were characterized by cyclic voltammetry (CV), electronic absorption (UV-Vis) and emission, X-ray diffraction (XRD) techniques. -- Electrolysis of the complexes was performed to reveal their electrocatalytic effect on CO₂ reduction. Product analysis by high-performance liquid chromatography (HPLC)/UV-Vis methods revealed that formate and oxalate are detectable in addition to some other products. In this study, it was found that structural variations in the electrocatalyst could influence the activity, however all benzothiazole containing complexes exhibited much higher activities than those of the corresponding benzimidazole complexes. Formate and oxalate were produced and detected at moderately low cathodic overpotentials for [{(bpy)₂Ru}₂(py₂tbtz)]⁴⁺. This is an unprecedented result for benzothiazole complexes. In addition, the electrochemical reduction of CO₂ by fluorenone-9-one was found to produce 9-hydroxyfluorene and 9-hydroxyfluorene-9-carboxylic acid, in addition to formate and oxalate at a low cathodic overpotential.

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