**3.2 Effect of molar ratio of CO2, H2, and H2O on CO2 reduction characteristics**

**Figures 9** and **10** show the concentration changes of CO and CH4 produced in the reactor along the time under the illumination of Xe lamp with UV light,

**Figure 5.** *TEM image of Cu/TiO2 film.*

respectively. **Figures 11** and **12** show the molar quantities of CO and CH4 per weight of photocatalyst in the reactor along the time under the illumination of Xe lamp with UV light, respectively. The amount of Cu/TiO2 is 0.2 g. In this experiment, a blank test, that was running the same experiment without illumination of Xe lamp,

*Change of concentration of CO with time for several molar ratios of CO2/H2/H2O under illumination condition*

**Figure 7.**

**Figure 8.**

**Figure 9.**

**81**

*with UV light.*

*EELS spectra of Cu in Cu/TiO2.*

*EELS spectra of TiO2 referred from EELS data base [34].*

*CO2 Reduction Characteristics of Cu/TiO2 with Various Reductants*

*DOI: http://dx.doi.org/10.5772/intechopen.93105*

**Figure 6.** *EDX images of Cu/TiO2 film.*

*CO2 Reduction Characteristics of Cu/TiO2 with Various Reductants DOI: http://dx.doi.org/10.5772/intechopen.93105*

**Figure 7.** *EELS spectra of Cu in Cu/TiO2.*

**Figure 8.** *EELS spectra of TiO2 referred from EELS data base [34].*

#### **Figure 9.**

*Change of concentration of CO with time for several molar ratios of CO2/H2/H2O under illumination condition with UV light.*

respectively. **Figures 11** and **12** show the molar quantities of CO and CH4 per weight of photocatalyst in the reactor along the time under the illumination of Xe lamp with UV light, respectively. The amount of Cu/TiO2 is 0.2 g. In this experiment, a blank test, that was running the same experiment without illumination of Xe lamp, had been carried out to set up a reference case. No fuel was produced in the blank test as expected.

According to **Figures 9**–**12**, the CO2 reduction performance is the highest for the molar ratio of CO2/H2/H2O = 1:0.5:0.5. Since the reaction scheme of CO2/H2/H2O is not fully understood, this study refers to the reaction scheme of CO2/H2O and CO2/ H2 as shown by Eqs. (1)–(12). It is known from the reaction scheme that the theoretical molar ratio of CO2/H2O and CO2/H2 to produce CO is 1:1. On the other hand, the theoretical molar ratio of CO2/H2O and CO2/H2 to produce CO is 1:4. Since the molar ratio of CO2/H2/H2O = 1:0.5:0.5 can be regarded as the molar ratio of CO2/total reductants = 1:1, it is believed that the results of this study follow the reaction scheme presented in Eqs. (1)–(12). Comparing the CO production with the CH4 production, CO is produced first. According to Eq. (5), it is believed that some CO might be converted into CH4. Therefore, the start of CH4 production is slower than that of CO production. Producing CH4 needs four times H<sup>+</sup> and electrons as many as producing CO needs. Therefore, it is revealed that the optimum molar ratio

**Figure 10.**

*Change of concentration of CH4 with time for several molar ratios of CO2/H2/H2O under illumination condition with UV light.*

of CO2/H2/H2O is decided by the CO production scheme. Though CO decreases

*Change of molar quantity of CH4 per unit weight of photocatalyst with time for several molar ratios of*

According to Hinojosa-Reyes et al. [35], TiO2 and Cu2O formation leads to the photocatalytic activity since Cu2O is a semiconductor with small band gap energy. In addition, Cu performs to avoid the electron and hole recombination and promotes the charge transfer. In this study, it seems that the effect of Cu and Cu2O on

*Change of concentration of CO with time for several molar ratios of CO2/H2/H2O under illumination condition*

**Figures 13** and **14** show the concentration changes of CO produced and the molar quantity of CO per weight of photocatalyst in the reactor under the illumination of Xe lamp without UV light, respectively. In this experiment, CO is the only

According to **Figures 13** and **14**, the CO2 reduction performance is also the highest for the molar ratio of CO2/H2/H2O = 1:0.5:0.5 in this case. It is considered that the same reaction mechanism as mentioned above is conducted. The CO2 reduction performance of Cu/TiO2 under the illumination condition without UV

after reaching the peak, CH4 increases gradually.

*CO2/H2/H2O under illumination condition with UV light.*

*CO2 Reduction Characteristics of Cu/TiO2 with Various Reductants*

*DOI: http://dx.doi.org/10.5772/intechopen.93105*

photoactivity is performed.

**Figure 12.**

**Figure 13.**

**83**

*without UV light.*

fuel produced from the reactions.

**Figure 11.**

*Change of molar quantity of CO per unit weight of photocatalyst with time for several molar ratios of CO2/H2/H2O under illumination condition with UV light.*

*CO2 Reduction Characteristics of Cu/TiO2 with Various Reductants DOI: http://dx.doi.org/10.5772/intechopen.93105*

#### **Figure 12.**

*Change of molar quantity of CH4 per unit weight of photocatalyst with time for several molar ratios of CO2/H2/H2O under illumination condition with UV light.*

#### **Figure 13.**

*Change of concentration of CO with time for several molar ratios of CO2/H2/H2O under illumination condition without UV light.*

of CO2/H2/H2O is decided by the CO production scheme. Though CO decreases after reaching the peak, CH4 increases gradually.

According to Hinojosa-Reyes et al. [35], TiO2 and Cu2O formation leads to the photocatalytic activity since Cu2O is a semiconductor with small band gap energy. In addition, Cu performs to avoid the electron and hole recombination and promotes the charge transfer. In this study, it seems that the effect of Cu and Cu2O on photoactivity is performed.

**Figures 13** and **14** show the concentration changes of CO produced and the molar quantity of CO per weight of photocatalyst in the reactor under the illumination of Xe lamp without UV light, respectively. In this experiment, CO is the only fuel produced from the reactions.

According to **Figures 13** and **14**, the CO2 reduction performance is also the highest for the molar ratio of CO2/H2/H2O = 1:0.5:0.5 in this case. It is considered that the same reaction mechanism as mentioned above is conducted. The CO2 reduction performance of Cu/TiO2 under the illumination condition without UV

**Figure 14.**

*Change of molar quantity of CO per unit weight of photocatalyst with time for several molar ratios of CO2/H2/H2O under illumination condition without UV light.*

**Figure 16.**

**Figure 17.**

**Figure 18.**

**85**

*under illumination condition with UV light.*

*Change of concentration of CH4 for Cu/TiO2 overlapped with time for several molar ratios of CO2/H2/H2O*

*Change of molar quantity of CO per unit weight of photocatalyst for Cu/TiO2 overlapped with time for several*

*Change of molar quantity of CH4 per unit weight of photocatalyst for Cu/TiO2 overlapped with time for several*

*molar ratios of CO2/H2/H2O under illumination condition with UV light.*

*CO2 Reduction Characteristics of Cu/TiO2 with Various Reductants*

*DOI: http://dx.doi.org/10.5772/intechopen.93105*

*molar ratios of CO2/H2/H2O under illumination condition with UV light.*

**Figure 15.**

*Change of concentration of CO for Cu/TiO2 overlapped with time for several molar ratios of CO2/H2/H2O under illumination condition with UV light.*

light is lower than that under the illumination condition with UV light. Therefore, it can be claimed that Cu/TiO2 obtains the main photoenergy from UV light.
