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Molecules 2020, 25, 3653 9 of 23 Table 1. Performance comparison of different photoelectrochemical CO2 reduction systems from recent literature. Photocathode a p+-n-n+-Si/TiO2 + Cu/Ag p-Si NWs + Sn CuO + Cu2O Si/GaN-NPhN4-Ru(CP)2+ RuCt 2 p-n+-Si + SnO2 NW Co3O4/CA + Ru(bpy)2dppz FTO/TiO2/Cu2O + Ru-BNAH p-Si + Bi Fe2O3 NTs + Cu2O FTO/CuFeO +CuO 2 Condition b 100 mW cm−2, 0.1 M CsHCO3 100 mW cm−2, 0.1 M KHCO3 70 mW cm−2, 0.1 M NaHCO3 100 mW cm−2, 0.05 M NaHCO3 100 mW cm−2, 0.1 M KHCO3 9 mW cm−2, 0.1 M NaHCO3 100 mW cm−2, 0.1 M KCl Efficiency c Ref. C2H4, 10–25%, −8 mA cm−2 at −0.4 V vs. reversible hydrogen electrode [94] (RHE) for 20 days HCOOH, 88%, 18.9 μmol h−1 cm−2, [95] −0.875 V vs. RHE for 3 h CH3OH, 95%, 85 mM at −0.2 V vs. standard hydrogen electrode (SHE) [96] after 1.5 h HCOOH, 35–64%, −1.1 mA cm−2 at [97] −0.25Vvs. RHEfor20h HCOOH, 59.2%, −18 mA cm−2 at −0.4 [98] Vvs. RHEfor3h HCOOH, 86%, 110 μmol h−1 cm−2 at −0.60 V vs. normal hydrogen electrode [99] (NHE) for 8 h HCOOH, NA, 409.5 umol at −0.9 V vs. [100] NHE after 8 h HCOOH, 70–95%, ~−4 mA cm−2 at [92] −0.32 V vs. RHE for 7 h CH3OH, 93%, 6 h, 4.94 mmol L−1 cm−2 at −1.3Vvs. SCE for 6 h [101] CH3COOH,80%,142μMat−0.4Vvs. [102] Ag/AgCl after 2 h 50 mW cm−2, 0.5 M KHCO3 −2 100 mW cm , 0.1 M KHCO3 100mWcm−2,0.1MNaHCO 3 a The configuration is described as “semiconductor + cocatalyst”. b The reaction conditions for photoelectrochemical (PEC) measurements include the light intensity of solar simulator and the electrolyte. c The PEC efficiency parameters include the product, faradaic efficiency/photocurrent density/production rate or yield/stability at a certain working potential. 2.5. Enzyme As is known, the catalyst is one of the key components in CO2 reduction systems including thermal catalysis, electrocatalysis, photocatalysis, and photoelectrocatalysis. However, a prominent problem associated with most catalytic systems is low product selectivity, where more than one product, including CO, formate, methane, ethylene, and other components, are usually observed in one catalysis reaction. By contrast, the reduction of CO2 via biocatalytic processes received particular attention because of their special substrate and product selectivity as well as high conversion efficiency. Enzymes are biocatalysts renowned for their high efficiency and selectivity. In living cells, different enzymes often work together or in a specific order to catalyze multi-step biochemical reactions, playing crucial roles in the synthesis of natural products and metabolism [103]. Inspired by the biocatalytic reaction, enzymes including enzyme cascades were explored in vitro to complete the conversion of CO2 to certain chemicals via a one-step or multi-step process. Figure 5 shows the approximate number of papers published in the past two decades using enzymes as catalysts in CO2 conversion. It is obvious that the research has presented an increasing tendency, especially in the recent 10 years, suggesting more and more attention was paid to the biocatalytic conversion of CO2. In 1993 and 1994, Yoneyam et al. [104,105] demonstrated that CO2 can be biocatalyzed into CH3OH in a CO2-saturated phosphate buffer solution, in which pyrroloquinoline quinone (or methyl viologen) was used as an electron mediator, and formate dehydrogenase, formaldehyde dehydrogenase, and alcohol dehydrogenase were used as biocatalysts. Subsequently, Obert [106] presented the reduction of CO2 to methanol using three different dehydrogenases in three consequent reductions, in which reduced nicotinamide adenine dinucleotide (NADH) molecules were required at each step. Such a multi-enzyme system was composed of three different dehydrogenases (Figure 6) that catalyze the conversion of CO2 to CH3OH in the presence of NADH. In this enzyme cascade, formate dehydrogenase (FDH) catalyzes the conversion of CO2 to formate, formaldehyde dehydrogenase (FaldDH) then catalyzes the formate to formaldehyde and, finally, alcohol dehydrogenase (ADH)PDF Image | Research Progress in Conversion of CO2 to Valuable Fuels
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