Electroreduction of CO2 on Metallic Cathodes

The reduction of CO2 at metallic cathodes has been studied with almost every element in the periodic table °. This reaction can be driven electrochemi-cally or photochemically and semiconductors have been used as cathodic materials in electrochemical or photoelectrochemical cells . The aim of these studies has been to find cathodes that discriminate against the reduction of H2O to H2 and favor the reduction of CO2 and also to find a cathode selective for one product in the reduction of CO2. A fundamental requirement is that the latter process occurs at a lower overpotential on such electrodes. However the purposes mentioned before in metallic cathodes depends on a series of factors such a solvent, support electrolyte, temperature, pressure, applied overpotential, current density, etc. (we will see the same factors again in macrocyclic electro-catalysis). For instance when protons are not readily available from the solvent (e.g., A,A -dimethylformamide), the electrochemical reduction involves three competing pathways-oxalate association through self-coupling of COj anion radicals, production of CO via O-C coupling between and COj and CO2, and formate generation by interaction of C02 with residual or added water.  

Copper electrodes have shown interesting electrocatalytic properties towards the reduction of CO2 because they produce hydrocarbons such as CH4 and C-2 compounds and considering the high accessibility of this metal comparing with noble metals its use as a electrode material is an important topic of research. For this reason we include some of the current literature in this chapter .  

Another example using alloys was presented by Schrebler and coworkers where the electrocatalytic reduction of CO2 was studied in CuRe alloy highly dispersed in polypirrole films at Au electrodes (Au/PpyCuRe). These electrodes were stable over 30 hr of electrolysis and the applied overpotential was less than -1.5 V. The faradaic efficiency for methane formation was 31% on Au/PpyCuRe and the intermediate CH2 was detected . 

It has also been suggested that the loss of catalytic activity of copper electrodes depends on the crystallographic properties of the electrode, the surface characteristics and the morphology . The rate of methanol synthesis from a 1 1 mixture of CO2 and H2 at a Cu(lOO) single crystal has been measured and a kinetic model has been proposed This model correctly predicts the rates of methanol production in catalysts under industrial conditions. 

When the electrochemical reduction of CO2 was studied using singlecrystal Cu electrode, Cu(lll), Cu(lOO), Cu(s)-[n(100) x (111)] and Cu(s)-[ (100) X (110)] at constant current density of 5 mA cm in bicarbonate aqueous solutions, the Cu(lll) electrode yielded mainly CH4 and Cu(lOO) gave C2H4. Introduction of (111) steps to Cu(lOO) basal plane leading to Cu(s)-[n(100) x (111)] orientations promoted C2H4 formation and inhibited CH4 formation  

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