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Catalysts 2020, 10, 1287 6 of 25 Catalysts 2020, 10, x FOR PEER REVIEW 6 of 25 Figure 1. Band structures of some semiconductors with respect to CO2 reduction potentials for different Figure 1. Band structures of some semiconductors with respect to CO2 reduction potentials for products at pH = 7. different products at pH = 7. The reported photoelectrocatalytic CO2 reduction over electrodes to date mainly contain pure The reported photoelectrocatalytic CO2 reduction over electrodes to date mainly contain pure semiconductors, metal-modified semiconductors and semiconductor/metal hybrid catalysts [34–36]. semiconductors, metal-modified semiconductors and semiconductor/metal hybrid catalysts [34–36]. The incorporation of metal species into the semiconductor electrodes has intrigued considerable The incorporation of metal species into the semiconductor electrodes has intrigued considerable attention due to their large redox-active surface area, increased light-adsorbing surface area and attention due to their large redox-active surface area, increased light-adsorbing surface area and enhanced electron transport ability. The employed electrode systems in CO2 photoelectroreduction enhanced electron transport ability. The employed electrode systems in CO2 photoelectroreduction process can be mainly categorized into two kinds of configurations, those are half-cells with sacrificial process can be mainly categorized into two kinds of configurations, those are half-cells with reagents donating electrons for oxidation in anode electrode and full-cells with semiconductor sacrificial reagents donating electrons for oxidation in anode electrode and full-cells with photoanodes to compensate electrons for oxidation [14]. In photoelectrochemical reaction cells, semiconductor photoanodes to compensate electrons for oxidation [14]. In photoelectrochemical the semiconductor electrode is immersed in the electrolyte and is connected to a counter electrode reaction cells, the semiconductor electrode is immersed in the electrolyte and is connected to a (half-cell) or another semiconductor electrode (full-cell) via an external circuit. When the semiconductor counter electrode (half-cell) or another semiconductor electrode (full-cell) via an external circuit. electrode is illuminated under simulated solar light, its electrons can be excited from the VB to CB, When the semiconductor electrode is illuminated under simulated solar light, its electrons can be leaving an equal number of holes in the VB. Simultaneously, with the aid of extra electric field, excited from the VB to CB, leaving an equal number of holes in the VB. Simultaneously, with the aid the charge accumulating at the interface between semiconductor electrode and electrolyte will give rise of extra electric field, the charge accumulating at the interface between semiconductor electrode and to a perturbation of the energy levels of the semiconductor [37]. Inspired by this kind of perturbation, electrolyte will give rise to a perturbation of the energy levels of the semiconductor [37]. Inspired by the photogenerated electron–hole pairs are spatially separated and are injected into the electrolyte this kind of perturbation, the photogenerated electron–hole pairs are spatially separated and are at the respective electrodes to produce electrochemical CO2 reduction and water oxidation reactions. injected into the electrolyte at the respective electrodes to produce electrochemical CO2 reduction Through the conversion of CO2 to chemicals or fuels, the abundant electricity and solar energy can and water oxidation reactions. Through the conversion of CO2 to chemicals or fuels, the abundant thus be effectively converted into and stored as chemical energy. electricity and solar energy can thus be effectively converted into and stored as chemical energy. 2.4. Mechanisms of Ethanol Production from CO2 2.4. Mechanisms of Ethanol Production from CO2 CO2 reduction is actually a kind of multiple electron–proton-involved reaction whether by CO2 reduction is actually a kind of multiple electron–proton-involved reaction whether by electrocatalysis, photocatalysis or photoelectrocatalysis routes. Since CO2 is a linear molecule and electrocatalysis, photocatalysis or photoelectrocatalysis routes. Since CO2 is a linear molecule and one of the most thermodynamic stable carbon compounds, it demands input energy to activate CO2. one of the most thermodynamic stable carbon compounds, it demands input energy to activate CO2. Additionally, CO2 reduction to clean fuel ethanol using water as the reducing agent to supply protons Additionally, CO2 reduction to clean fuel ethanol using water as the reducing agent to supply and electrons, is an uphill reaction with a highly positive change in Gibbs free energy: 2CO2 + 3H2O protons and electrons,0is an uphill reacti−o1n with a highly positive change in Gibbs free energy: 2CO2 = C2H5OH + 3O2 (∆G = 1325.3 KJ·mol ). Thus, input energy such as electricity or solar energy is + 3H2O = C2H5OH + 3O2 (∆G0 = 1325.3 KJ·mol-1). Thus, input energy such as electricity or solar energy required to overcome this reaction barrier. The most energetically demanding and likely key step is the is required to overcome this reaction barrier. The most energetically demanding and likely key step activation of CO2 with one electron transfer from the catalyst to the lowest unoccupied molecular orbital is the activation of CO2 with·− one electron transfer from the catalyst to the lowest unoccupied (LUMO) of CO2 to form a CO2 species [38]. The adsorption of CO2 onto the catalyst surface can allow molecular orbital (LUMO) of CO2 to form a CO2·− species [3·−8]. The adsorption of CO2 onto the for both heterogeneous electron transfer and stabilization of CO2 species due to the decreased LUMO catalyst surface can allow for both heterogeneous electron transfer and stabilization of CO2·− species level of CO2 as the molecule bends [39]. Depending on the operating conditions, the type of reductant, due to the decreased LUMO level of CO2 as the molecule bends [39]. Depending on the operating the number and potential of the charge carriers involved in the reduction reaction, the following conditions, the type of reductant, the number and potential of the charge carriers involved in the multiple electron–proton transfer steps determine the distribution of products. As shown in Figure 2, reduction reaction, the following multiple electron–proton transfer steps determine t·−he distribution CO is formed by two electron–proton transfers through the intermediates of *CO2 and *COOH, of products. As shown in Figure 2, CO is formed by two electron–proton transfers through the and is considered as the key intermediate to produce a C2 product like ethanol [40]. Competing with intermediates of *CO2·− and *COOH, and is considered as the key intermediate to produce a C2 desorption and loss, a C1 intermediate like CO should be adsorbed on the catalysts surface firmly product like ethanol [40]. Competing with desorption and loss, a C1 intermediate like CO should be enough to persist until a second C1 intermediate is available for C–C coupling [41]. The surface-bond adsorbed on the catalysts surface firmly enough to persist until a second C1 intermediate is available C1 intermediate maybe couple with another surface-bond C1 intermediate or a nearby C1 intermediate for C–C coupling [41]. The surface-bond C1 intermediate maybe couple with another surface-bond C1 intermediate or a nearby C1 intermediate in the solution [42,43]. The key step of ethanol formationPDF Image | Advances in Clean Fuel Ethanol Production from CO2 Reduction
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