Efficient Capture of CO<sub>2</sub> and Its Selective Reduction to Formic Acid Using Tin-Based Nanomaterials

Abstract

CO2 emissions from the combustion of fossil fuels and other anthropogenic sources have become the main contributing factors to global warming. Chemical methods of absorbing/capturing CO2 from combustion flue gases have made it a sought-after approach in engineering emission solutions because of its simplistic and convenient operation and high absorption efficiency. The conversion of CO2 into renewable fuels and high energy density chemicals by clean and economic processes has drawn scientists' attention over the decades. The electrocatalytic conversion of CO2 using Sn-based materials has been demonstrated to be a promising method for producing formate, an important industrial chemical. Here, we attempt to capture CO2 using a simple counter-flow spray-based contactor and convert the dissolved CO2, using Sn and Cu-based bulk materials and SnO2-based nanomaterials, to selectively and efficiently produce formic acid by electrolysis. The approach explored in this thesis offers an alternate approach to building stable and efficient electrodes for the electrocatalytic reduction of CO2 and moving it closer to economic viability. The analysis of chronoamperometric data using the modified Cottrell equation allowed the fitting data to determine the rate constant, k, of the proton transfer step in the proposed three-step; electron, proton, and electron transfer steps, the so-called ECE model. Periodic sampling of formic acid concentration from the catholyte allowed independent measurement of k, which favorably compared with the one extracted from chronoamperometric data. Observed values of k range from 10-6 to 10-3 s-1 which appear to be several orders less than typical proton transfer reactions on heteroatoms such as oxygen and nitrogen. In light of Marcus' theory, theoretical modelling provided an estimate of the solvent's reorganization energy indicating potential additional contribution from the changes in the bending angle of the CO2 anion. The k value increased with increasing formate production, suggesting that the proton transfer step is probably the rate-determining step (RDS) in the overall reduction of CO2 to formate anion. The proton transfer reaction seems to transition from a kinetic to the diffusion-controlled limit at higher electrode potential, which became evident when a rechargeable Li-ion battery was used in place of the potentiostat. Under diffusion control, the electroreduction produced a comparable concentration of formate (about 2000 ppm) over 24 hours.