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4.2 Fundamental Electrochemistry and Materials Technology 21 confirmed with gas chromatography, FTIR, and infrared-spectroelectrochemistry (IR-SEC) during and after the experiment. Accumulation of the insoluble carbonate salt is the main problem for the described system. This issue can be overcome by utilizing a sacrificial Mg-anode to prevent precipitation (by creating soluble Mg carbonate salts) and to increase the catalytic turnover. However, such an approach is clearly not sustainable, and other solutions such as the use of soluble Lewis acids need to be identified. A broad variety of Lewis acids should be tested for use together with the Mn(mesbpy) catalyst to prevent formation of carbonate precipitates. Other CO2 reduction catalysts like Re(pby)(CO)3 (Hawecker et al., 1984) and Ni(cyclam) (Froehlich & Kubiak, 2015), are well known for their ability to reduce CO2 to CO in the presence of a proton source (H2O, TFE, PhOH) through a proton-coupled electron transfer mechanism. These molecular catalysts could also be used as potential candidates for CO and carbonate production under anhydrous conditions. 4.2.1.2 Heterogeneous Catalysis: CO2 Reduction by Lead Heterogeneous CO2 reduction in aqueous solutions on metal electrodes is thoroughly described by Hori (2008). Unfortunately, the majority of these electrodes (e.g., Cu) tend to have low selectivity towards CO and produce a mixture of products. Lead and mercury electrodes, however, can reduce CO2 to CO and an equimolar amount of carbonate. According to Gennaro et al. (1996), mixtures of CO and oxalate were formed during the direct electrolysis of CO2 at –2.21 V vs. SCE in DMF using a mercury electrode. It was shown that the yield of CO and carbonate depended on the concentration of CO2 in the reaction mixture as well as the operating temperature. The yield of CO decreased with the decreasing concentration of CO2, while steady increase of the CO yield was observed with the decreasing temperature (25◦C to –20◦C). Similar results were found using a lead electrode, which would be the more ideal device candidate. Coupling of two radical ions CO2•– under electrochemical reduction conditions resulted in the formation of oxalate. CO and carbonate formed during the electron transfer reaction between CO2 and CO2•–. 4.2.2 Anode Reaction: O2 Oxidation Electrochemical water oxidation on precious metals and metal oxide semiconductor surfaces (e.g., TiO2, NiOx, FeOx, IrOx, WO3) is a widely studied process and can be used as a platform for carbonate oxidation to diatomic oxygen. We note that the concept for the non-aqueous process was originally described by described by the Kubiak group (cf. Ratiff et al., 1986–1988; Lewis, 1993) as shown in Figure 4.4 (Breedlove et al., 2001). Although the Ni3 cluster catalyst shown is sub-optimal for the CO2 reduction process (being superseded now by the Mn-Mg species describedPDF Image | ISRU Challenge Production of O2 and Fuel from CO2
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