semiconductor and a molecular catalyst. Lim et al. reported the use of four types of semiconductors: p-Si, p-GaAs, p-InP, and p-GaP. Metallic molecular catalysts used are mostly based on nickel, cobalt, ruthenium, and rhenium, whereas enzymes (dehydrogenase and pyridine) are the non-metallic molecular catalysts (Lim et al., 2014). CO2 photoelectrocatalysis is feasible in gaseous phase or in aqueous solvent. In gaseous phase, the photocatalysts directly react on carbon dioxide and steam to produce synthetic or oxygenated hydrocarbons. Water dissociation and carbon dioxide reduction are realized at the same time. On the contrary, in the other system, water dissociation is physically separated from CO2 reduction because the photocatalysts are integrated in a photoelectrochemical reactor. CO2 and steam direct photocatalysis produces hydrocarbons and alcohols such as methanol, methane, carbon monoxide, and other compounds of alkene type or paraffin. Different photocatalytic CO2 reduction systems exist, one of them is the photoactive metal-organic frameworks (MOFs) (Wang et al., 2015). MOFs’ porosity facilitates CO2 adsorption and, thus, increases the photocatalytic efficiency. Different photocatalysts have been investigated for their performance in photocatalytic CO2 reduction (TiO2, BiVO4, BiWO6, Zn2GeO4). Present investigations are developing less toxic catalysts. The challenge is to develop efficient photocatalysts that can reduce CO2 under visible light, because currently most of them are only active in the UV region, and to design an efficient harnessing solar radiation reactor (optical fiber, monolith photoreactors, etc.) (Das and Wan Daud, 2014).
In atmospheric conditions, in a dielectric barrier discharge, syngas is produced, either by the conversion of a mixture of CO2 and CH4 or by CO2 reduction in water (Ozkan et al., 2015).
Solar energy could be stored as formic acid which is electrochemically generated from CO2; it is a viable solar fuel pathway. White et al. accomplished this transformation by semi-optimized indium-based electrolyser stack powered by a photovoltaic panel. They separated light absorption and CO2 reduction through the use of a commercial solar panel (White et al., 2014).
Another route to valorize the CO2 is the use of plasma. Plasma permits to reform CO2 by methane, providing a highly efficient fuel compared to thermocatalytic methods. But it is not commercially viable due to the higher energy plasma input. Carbon nanofibers along with syngas (H2 + CO), traces of methanol, and some hydrocarbons (C2H2, allene) were produced by CO2 reduction by plasma-assisted in situ decomposition of water. Mahammadunnisa et al. reported, for the first time, the simultaneous activation of carbon dioxide and water in a catalytic non-thermal plasma dielectric barrier discharge reactor operated under ambient conditions. Three different conditions were applied (plasma alone without catalyst, plasma with NiO/γAl2O3, or plasma with Ni/γ-Al2O3). The higher CO2 conversion was due to synergy between plasma excitation of CO2 molecule and catalytic action of NiO catalyst. Nickel-based catalysts are promising for H2 and syngas production. Plasma alone is good for syngas formation. But for the reduction of CO to methane, NiO catalyst facilitates the conversion of CO into methane (Mahammadunnisa et al., 2013).
Ionic Liquids
Ionic liquids’ (ILs) physicochemical properties make them very attractive for the CCS. Once separated from the atmosphere or exhaust flue, there are two options for the carbon dioxide: chemical transformation into useful products or into a form suitable for long-term storage. Long-term CO2 storage is not perfect because the CO2 absorption in ILs is reversible (increase in the viscosity leading to CO2 and absorption rate diffusion restriction). Electrochemical transformation of carbon dioxide is achieved by direct electroreduction at noble metals, or at carbon electrodes (with a substantial overpotential for the last one); thus, yielding carbon dioxide radical anion (CO2•−), which then typically dimerizes in C2O42− or disproportionates in CO and CO32−. Also electroreduction could be achieved in more active metals, such as transition-based metal centers (Ni, Fe, Pd, Ru, Re, Rh) complexed with various ligands such as porphyrins, bipyridines, or phosphines; thus, yielding a variety of reduction products, most commonly carbon monoxide and onward products such as formic and oxalic acid but also extending to methanol and methane (hydrogen evolves at the same potentials required for CO2 reduction). CO2 electroreduction in RTILs (room temperature ionic liquids) allows long-term storage and produces valuable useful chemicals. The choice of RTILs and reaction conditions greatly influences the reduction products. Also, lower potential routes are available for CCS, which are indirect electrochemical reduction routes. The formation of peroxodicarbonate C2O62- by oxygen reduction to superoxide in RTILs is an example. Among hydrogen, nitrogen, and carbon monoxide, carbon dioxide and water have the highest solubility in an ionic liquid (C4C1-iPF6) at 25°C. CO2 is the most soluble in fluorinated IL. CO2 can be collected from the ILs by pressure decreasing. Optimizing the design of the appropriate RTILs, as well as the electrode material, is compulsory to enhance the absorption and the reduction of CO2 (Rees and Compton, 2011).
CO2 Electrochemical Reduction in Aqueous Solutions
This process has been studied since the late 90s on metallic electrodes in aqueous solutions. It is depicted in Figure 2. The main products of CO2 reduction are methane, ethylene, formate, carbon monoxide, and some alcohols (methanol, ethanol, and propanol). Copper is the most studied electrode, able to give a large panel of products (hydrocarbons and other products such as alcohols at a high current efficiency) when compared with other metals. Indeed, metals are divided into four groups for the CO2 aqueous electrochemical reduction. Group 1 (Hg, Pb, Bi, In, Sn, Cd, Tl) binds the CO•− intermediates and gives formate (or formic acid) as a product. Group 2 (Au, Ag, Zn, Ga, Pd) yields CO as the major product. Group 2 binds the CO2•− to varying degrees, but cannot reduce CO. Copper is the only commonly studied (for the CO2 reduction) metal that falls into intermediate and reduces the group 3. Copper binds the CO2•− intermediate and reduces the CO to higher reduction products such as hydrocarbons (methane, ethylene, etc.) and alcohols. The last group (Ni, Fe, Pt, and Ti) does not reduce CO2 directly, strongly binding hydrogen to produce hydrogen only.
