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  • Typical examples of CO utilization and reduction

    2019-08-12

    Typical examples of CO2 utilization and reduction technologies are catalytic reduction of CO2 to C1 compounds such as CO, formic acid, methanol and so on [[3], [4], [5], [6], [7], [8], [9], [10]], and catalytic organic synthesis using CO2 as a feedstock [[11], [12], [13], [14], [15], [16], [17], [18], [19], [20]] as shown in Fig. 1. The catalytic CO2 conversion to liquid fuels is a critical goal that would positively impact the global carbon balance by recycling CO2 into a usable fuels. The challenges presented here are great, but the potential rewards are enormous. CO2 is extremely stable molecule generally produced by fossil fuel combustion and respiration. Returning CO2 to a useful state by activation/reduction is a scientifically challenging problem, requiring appropriate catalysts and energy input. This poses several fundamental challenges in chemical catalysis, electrochemistry, photochemistry, and semiconductor physics and engineering. In particular, a system that reductive converts CO2 to a liquid fuels such as methanol has attracted a great deal of attention for constructing a low-carbon society near the future. However, CO2 is a complete combustion product of carbohydrates, so external energy is required to return, or to reduce to various organic molecules. Thermodynamic potentials for the CO2reduction to various organic products is given in the Fig. 2 (vs NHE at pH 7) [[21], [22], [23], [24], [25]]. Single-electron CO2reduction to CO2 radical occurs at E° = −1.90 V (vs NHE at pH 7) in an aqueous solution at room temperature (25 °C) under 1 atm. The reason behind the high negative thermodynamically unfavorable single-electron reduction potential of CO2 is the large reorganization energy between the linear molecule and bent radical anion. Proton-coupled multi-electron reduction of CO2 are generally more favorable than single-electron reduction of CO2 as shown in Fig. 2, as thermodynamically stable molecules are produced. In particular, CO, that is useful as a chemical feedstock, and formic acid, that is useful as a hydrogen energy carrier, can be produced by proton-coupled double-electron reduction of CO2. For CO or formic Danazol production based on the proton-coupled double-electron reduction of CO2, many studies on the development of metal catalysts and molecular catalysts for CO or formic acid production based on the proton-coupled double-electron reduction of CO2 are being vigorously pursued. On the other hand, biocatalysts such as carbon monoxide dehydrogenase that catalyzes a reaction to reduce CO2 to CO and formate dehydrogenase (FDH) that catalyzes a reaction to reduce CO2 to formic acid are attracting attention. The natural photosynthetic reaction is a typical example as a model of CO2 recycling. How does natural photosynthesis concentrate dilute CO2 for sugar synthesis? In natural photosynthesis, carbonic anhydrase (EC 4.2.1.1) is involved in the concentration of dilute CO2. Carbonic anhydrase catalyze the rapid conversion of CO2 and H2O to HCO3− and H+, a reversible that occurs relatively slowly in the absence of any catalysts as shown in Fig. 3 [26]. In plants, carbonic anhydrase helps raise the concentration of CO2 within the chloroplast in order to increase the carboxylation rate of the biocatalyst RuBisCO. This is the reaction that integrates CO2 into sugars during photosynthetic reaction, and uses only the CO2, not CO32- or HCO3− [[27], [28], [29], [30]]. As carbonic anhydrase catalyzes the reaction of CO2 and H2O to HCO3− and H+, a cost-effective means of capturing CO2 from industrial emitters, such as coal power plants using carbonic anhydrase have also been studied [31]. CO2 is fixed as HCO3- in aqueous media with carbonic anhydrase, and it can be used as CO2 source for the light-driven CO2 utilization system introduced below. Recently, the studies on the systems for light-driven CO2 reduction to CO or formic acid with combination of these catalysts and the photocatalyst [[32], [33], [34]] or the photoredox system are reported as shown in Fig. 4 [[35], [36], [37], [38]]. Because biocatalyst has high reaction selectivity and substrate specificity, it surely converts CO2 to target products. Organic molecules syntheses based on the biocatalytic methods have been much paid attention because of their regio- and stereo-selectivity, and mild physiological conditions.