Reduction to CH4 and Hydrocarbons

Most reports of C02 reduction to methane and hydrocarbons have involved Cu electrode materials. Thus, Cu electrochemistry in relation to C02 reduction has been extensively studied, and the field widely reviewed [67]. Extensive information on C02 reduction to methane is available in several excellent reviews [41, 42, 67]. 

Hori et al. was one of the first to report on the reduction of C02 to methane in aqueous solution [49, 68]. Here, at a current density of 5 mAcnT2 in 0.5 M KHC03 electrolyte, a temperature dependence of faradaic efficiency was observed for the observed products such that, at 273 K, the efficiency for methane was 70%. Moreover, the efficiency was shown to decrease linearly as the temperature was raised, 

Other experiments showed that both selectivity and product distribution could be drastically affected by changes in the electrode potentials [70]. Similarly, surface roughness and pretreatment were also factors affecting methane production [41]. In addition, when the effect of crystal structure was examined, the rate of methane production in a C02-saturated 0.5 M KHC03 electrolyte was shown to be highest on Cu(lll), followed by Cu(110) and Cu(100) [41]. 

While the overpotentials are high for methane production, rather high current densities have also been achieved at Cu electrodes. For example, Cook et al. were able to reduce C02 to methane in aqueous 0.5 M KHC03 solutions at a current density of 38mAcm-2, with 33% faradaic efficiency [71], although the potential was -2.29V (versus SCE). Subsequently, it proved possible to increase the faradaic yields to 79% for methane and ethene together on Cu-coated glassy carbon electrodes from in situ Cu deposition, but in this case the potential was —2.0 V (versus SCE), in the same electrolyte with current densities of up to 25 mA cm 2. 

The mechanism of C02 reduction to methane at Cu electrodes has been proposed by various groups [72-74], most of which involved the splitting of adsorbed CO followed by the hydrogenation of surface C atoms. When DeWulf et al. used X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy to study the reaction [72], they observed surface-bound carbenes (Cu CH2) as an intermediate in the system. Likewise, others used both in situ infrared (IR) reflection absorption spectroscopy and surface-enhanced Raman spectroscopy to observe the initial product of C02 reduction on Cu [74]. Typically, two different linearly bound CO species were identified and attributed to adsorption on either surface defect sites or terraces. 

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