Recent Advances (2012-2015) in the Photocatalytic Conversion of Carbon Dioxide to Fuels Using Solar Energy: Feasibilty for a New Energy

In this chapter, recent advances in photocatalytic CO2 conversion with water and/or other reductants are reviewed for the publications between 2012 and 2015. Quantitative comparisons were made for the reaction rates in umol h(-1) g(cat)(-1) to acertain the progress of this field although the rates depends on photocatalyst conditions and reaction conditions (temperature, pressure, and photon wavelength and flux). TiO2 photoproduced methane or CO from CO2 and water at rates of 0.1-17 mol h(-1) gcat(-1) depending on the crystalline phase, crystalline face, and the defects. By depositing as minimal thin TiO2 film, the rates increased to 50-240 mu mol h(-1) Gaseous water was preferred rather than liquid water for methane/CO formation as compared to water photoreduction to H-2 Pt, Pd, Au, Rh, Ag, Ni, Cu, Au3Cu alloy, I, MgO, RuO2, graphene, g-C3N4, Cu-containing dyes, and Cu -containing metal-organic frameworks (MOFs) were effective to assist the CO2 photoreduction using TiO2 to methane (or CO, methanol, ethane) at rates of 1.4-160 limol h-Igcat.Metals of greater work function were preferred. By depositingi as minimal thin photocatalyst film, the rates increased to 32-2200 limot h -I gcat-I. The importance of crystal face of TiO2 nanofiber was suggested. As for semiconductors other than TiO2, ZnO, Zn6Ti layered double hydroxide (LDH), Mg3In LDH, KTaO3, In(OH)(3), graphene, graphene oxide, g-C3N4, CoTe, ZnO, ZnTe, SrTiO3, ZnGa2O4, ZnzGeO(4), Zr Co Ir oxides, Nb2O5, HNbO3, NaNbO3, InNbO4, NiO, Co3O4, Cu2O, AgBr, carbon nanotube, and the composites of these were reported to form methane, CO, methanol, acetaldehyde from CO2 and water at rates of 0.15-300 mot h(-1) g(cat)(-1) that were comparable to rates using promoted TiO2. The band energy designs comprising appropriate conduction band for CO2 reduction and valence band for water oxidation were made progresses in these semiconductors and semiconductor junctions in the three years. If H-2 was used as a reductant, Ni/SiO2-Al2O3 formed methane at 423 K under pressurized CO2 + H-2 at a rate of 55 mmol h(-1) g(cat)(-1). This rate was not enabled by heating the system under dark, suggesting photoactivated reaction followed by thermally-assisted reaction(s) via Ni H species. As pure photocatalytic reactions from CO2 + Hz, methanol formation rates were improved up to 0.30 umol h-' goat' by the doping of Ag/Au nanoparticles, [Cu(OH)(4)](2-) anions, and Cu -containing dyes to Zn Ga LDH. Furthermore, sacrificial reductants, e.g. hydrazine, Na2SO3, methanol, triethanol amine, and triethyamine, were also utilized to form CO, formate, and methanol at rates of 20-2400 p.mol h(-1) g(cat)(-1) using semiconductor or MOF photocatalysts. Finally, similar to the integrated system of semiconductor photocatalyst for water oxidation and metal complex/enzyme catalyst for CO2 (photo)reduction, two semiconductors (WO3, Zn Cu Ga LDH) were combined on both side of proton-conducting polymer to form methanol at a rate of 0. 05 limo] h(-1) g(cat)(-1) from CO2 and moisture. These promotion of photoconversion rates of CO2 and new photocatalysts found in these three years have indicated the way beyond for a new energy.