**5. Oxygenated alcohol selective electrocatalysts**

The wide range of theoretically possible products from CO to C2+ alcohols and hydrocarbons and fuels makes the recent research to put a lot of efforts on production of more valuable products like oxygenated alcohols. The major problem as discussed before is due to a very stable structure of CO2 molecule, very high activation energy needed to transform it to more attractive molecules. This high activation barrier would cause high over potentials and in case of oxygenated alcohols like methanol or ethanol high numbers of electrons (6 and 12 respectively) needed to reduce CO2 molecule to desired products. So far many different metallic and alloys have been used as electrocatalysts for this application [82]. Although the performance of other product formations such as CH3OH and C2H5OH were well below the target values, the market size of these chemicals was estimated to be much larger than those of HCOOH and CO [83]. Thus, the coproduction of economically viable HCOOH and CO with other products such as CH4,C2H4,CH3OH, and C2H5OH was suggested to cancel out the maximum voltage requirement [84].

### **5.1 Metal alloys**

Of all metals, Cu has been identified as unique in that it is able to produce a number of "beyond CO" products such as hydrocarbons and organic oxygenates such as aldehydes and alcohols [85]. Moreover, metal alloys can adjust the binding ability of active intermediates and thus are promising to enhance the reaction selectivity and kinetics. Lu et al. [21] have synthesized an aerogel with high porosity when [BMIM][BF4] and H2O with a molar ratio of 1:3 were selected as electrolytes, the faradaic efficiency (FE) and current density of CH3OH can be up to 80% and 31.8 mA/cm<sup>2</sup> , respectively, over the Pd83Cu17 aerogel which attributed to the valence states, ratios, and strong interaction of Pd and Cu [86]. Also, a Zn/Ag foam electrocatalyst was prepared by Low et al. The active sites in this electrocatalyst are the strained submicron Zn dendrites, resulting in a FE of 10.5% for producing CH3OH [87].

### **5.2 Metal oxides**

Metal oxide electrocatalysts have the merits of high selectivity and high energy efficiency [88]. Cuprous oxide/polypyrrole particles with octahedral and icosahedra structure (Cu2O(OL-MH)/Ppy) can achieve a ultrahigh CH3OH activity and selectivity with FE of 93 ± 1.2% and 1.61 ± 0.02 μmol/(cm2 ·s) formation rate at −0.85 V [89]. Albo and Irabien [90] used gas diffusion electrode loaded with Cu2O and achieved a FE of 42.3% for CH3OH formation, founding that Cu + can significantly affect the selectivity and activity toward CH3OH. Moreover, nano Cu2O has a higher stability and selectivity compared with Cu for CH3OH production. The result of more metallic alloys and metal oxide electrocatalysts for this application have been illustrated in **Table 4**.


#### **Table 4.**

*Oxygenated alcohol selective electrocatalysts.*

It is noteworthy to mention that there are different pathways suggested for methanol and ethanol production via electrochemical reduction of CO2. One possible pathway for methanol production is believed to be produced through hydrogenation of methoxy intermediate (\*OCH3) [44]. In detail, the \*CO species is formed first. Then, the \*OCH3 intermediate is made from the competition between desorption of formaldehyde and the proton electron coupled transfer to formaldehyde bonded on local surface. At least, another proton electron coupled transfer occurring on \*OCH3 species results in methanol [65]. This possible pathway has been illustrated in **Figure 4**. In addition, the plausible pathway for ethanol

**Figure 4.** *Schematic of possible pathways for methanol production.*

**Figure 5.** *Schematic of possible pathways for ethylene and ethanol CO production.*

*Heterogeneous Electrocatalysts for CO2 Reduction to Value Added Products DOI: http://dx.doi.org/10.5772/intechopen.97274*

production should be discussed alongside ethylene. Ethylene is generally believed to form through either dimerization of \*CH2 species or proton electron coupled transfer to the carbon site of the ethylene oxide intermediate (\*OCHCH2) that is derived from dimerization of \*CO [79]. Both routes might be the halfway leading to formation of ethanol by insertion of \* CO species into \* CH2 species or proton electron coupled transfer to the oxygen site of the \* OCHCH2 species, correspondingly [65], as illustrated in **Figure 5**.
