**3.5 Proposal to improve the CO2 reduction performance with H2O and H2 or NH3**

Under the condition of CO2/H2/H2O, the highest molar quantity of CO per weight of photocatalyst is 153 μmol/g in a single disc case with UV light illumination. The CO production performance achieved in this study is approximately 500 times as large as that reported in [24, 26] which is owing to Cu doping. The CH4 production performance achieved in this study is almost the same as that reported in [24]. Since the doped Cu provides the free electron preventing recombination of electron and hole produced as well as the improvement of the light absorption effect, the big improvement of CO2 reduction performance is obtained in this study.

One way to further promote the CO2 reduction performance may be that different metals should be doped on the higher and the lower positioned photocatalysts discs. The co-doped such as PbS-Cu/TiO2, Cu-Fe/TiO2, Cu-Ce/TiO2, Cu-Mn/TiO2, and Cu-CdS/TiO2 would promote the CO2 reduction performance of TiO2 under the CO2/H2O condition [7, 9]. When the combination of CO2/H2/H2O is considered, the ion number of dopant is important to match the number of electron emitted from the dopant with H<sup>+</sup> as shown by the reaction schemes of CO2/H2O and CO2/H2. The same number of electron and H+ are necessary for fuel production. Though Cu<sup>+</sup> ion is applied to promote the CO2 reduction performance with TiO2 in this study, it is expected that the co-doping of Cu and the other metal having larger positive ion might have positive effect for CO2 reduction with H2 and H2O. In addition, the dopant like Fe, which can absorb the shorter wavelength light than Cu [17, 41, 42], should be used at the higher positioned layer. The wavelength of light becomes long *CO2 Reduction Characteristics of Cu/TiO2 with Various Reductants DOI: http://dx.doi.org/10.5772/intechopen.93105*

after penetrating the higher positioned photocatalyst [36]. Therefore, it may be an effective way for utilization of wide wavelength range light that the higher positioned Fe/TiO2 which absorbs the shorter wavelength light and the lower positioned Cu/TiO2 which absorbs the longer wavelength light are overlapped. This idea is similar to the concept of hybridizing two photocatalysts having different band gaps [13, 42, 43].

On the other hand, under the condition of CO2/NH3/H2O, the highest molar quantities of CO and CH4 per weight of photocatalyst in the reactor, which are obtained for the molar ratio of CO2/NH3/H2O = 1:1:1, are 10.2 and 1.8 μmol/g, respectively. Compared to the previous research on CO2 reduction with H2 and H2O over pure TiO2, the CO2 reduction performance of photocatalyst prepared in this study is approximately 35 times as large as that reported in Refs. [24, 39], which is owing to not only Cu doping but also the combination of NH3 and H2O. The CO production performance over the Cu/TiO2 prepared in this study is approximately 3 times as large as that reported in the reference [44]. However, the CH4 production performance of Cu/TiO2 prepared in this study is one twentieth as large as that of Cu/TiO2 reported in the other reference [45]. Therefore, it is necessary to promote the conversion from NH3 into H2 in order to improve the reduction performance according to the reaction scheme to reduce CO2 with NH3. One way to promote the conversion from NH3 into H2 is thought to be using Pt as a dopant. It was reported that Pt/TiO2 was effective to dissolve NH3 aqueous solution into N2 and H2 [25].
