**3.3 Effect of overlapping of Cu/TiO2 film with H2 and H2O on CO2 reduction characteristics**

**Figures 15** and **16** show the concentration change of CO and CH4 produced in the reactor under the illumination of Xe lamp with UV light, with two Cu/TiO2 films coated on netlike glass discs overlapped, respectively. The photocatalyst is coated on both upper and lower surfaces of the top disc and only the upper surface of the bottom disc.

**Figures 17** and **18** show the molar quantities of CO and CH4 per weight of photocatalyst in the reactor along the time under the Xe lamp with UV light, respectively. The total amount of Cu/TiO2 on two discs is 0.4 g.

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

#### **Figure 16.**

*Change of concentration of CH4 for Cu/TiO2 overlapped with time for several molar ratios of CO2/H2/H2O under illumination condition with UV light.*

#### **Figure 17.**

*Change of molar quantity of CO 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.*

#### **Figure 18.**

*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.*

According to **Figures 15**–**18**, the CO2 reduction performance is the highest for the molar ratio of CO2/H2/H2O = 1:0.5:0.5, the same as that in the case of single Cu/ TiO2 disc. In addition, the order of CO2 reduction performance of Cu/TiO2 overlapped is the same as that of single Cu/TiO2. However, comparing **Figures 15** and **16** with **Figures 9** and **10**, the concentrations of CO and CH4 for two Cu/TiO2 discs overlapped are higher than those for single Cu/TiO2 disc under every molar ratio of CO2/H2/H2O. The highest concentration of CO for Cu/TiO2 overlapped is 7273 ppmV, which is 1.4 times as large as that for single Cu/TiO2. On the other hand, the highest concentration of CH4 for Cu/TiO2 overlapped is 516 ppmV, which is 1.7 times as large as that for single Cu/TiO2. In the case of two discs overlapped, the following things are believed: (i) the amount of photocatalyst used for photocatalysis reaction is increased, (ii) the electron transfer between two Cu/TiO2 films promotes the activity of photocatalysis reaction, and (iii) the lower positioned Cu/TiO2 disc utilizes the light passing through the top disc.

However, comparing **Figures 17** and **18** with **Figures 11** and **12**, the molar quantities of CO and CH4 per weight of photocatalyst in two discs case are lower than those for single Cu/TiO2 disc case under every molar ratio of CO2/H2/H2O. The highest molar quantity of CO per weight of photocatalyst in two discs overlapped case is 82 μmol/g, which is 54% of that in single disc case. Similarly, the highest molar quantity of CH4 per weight of photocatalyst in two discs overlapped case is 5.8 μmol/g, which is 65% of that in single disc case. The reasons of this result are considered to be: (i) some parts of the Cu/TiO2 film on the lower positioned disc cannot receive the light, (ii) if the produced fuel remains in the space between two discs, the reactants of CO2, H2, and H2O would be blocked to reach the surface of photocatalyst, resulting that the photochemical reaction could not be carried out well even though the light is illuminated for photocatalyst.

that in the single disc case. The order of CO2 reduction performance in two discs is the same as that in the single disc case. However, comparing **Figure 19** with **Figure 13**, the concentrations in two discs case are higher than those in single case under every molar ratio of CO2/H2/H2O. The highest concentration of CO in two discs case is 271 ppmV, which is 2.8 times as large as that in single disc case. The same reasons explained in the case of illumination with UV light can be thought to

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

*molar ratios of CO2/H2/H2O under illumination condition without UV light.*

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

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

In addition, comparing **Figure 20** with **Figure 14**, the molar quantity of CO per weight of photocatalyst in two Cu/TiO2 discs overlapped case is singly higher than that in the single disc case under every molar ratio of CO2/H2/H2O. The highest molar quantity of CO per weight of photocatalyst is 3.1 μmol/g in two disc cases, which is 1.1 times as large at that in the single disc case. Though the effect of overlapping layout is not obtained under the illumination condition with UV light, the effect of overlapping layout is confirmed under the illumination condition without UV light. Since the photochemical reaction rate and the amount of produced fuel are small under the no-UV illumination condition compared to that with UV light, it would be beneficial to the mass transfer between produced fuels and reactants of CO2, H2, and H2O on the surface of photocatalyst in no-UV cases [36]. As a result, the mass transfer and photochemical reaction are carried out effectively in no-UV cases. Therefore, the effect of overlapping layout is obtained in no-UV cases. According to the previous reports [37, 38], the mass transfer is an inhibition factor to promote the CO2 reduction performance of photocatalyst, and it is necessary to control the mass transfer rate to meet the photochemical reaction rate. **Figure 21** illustrates the comparison of mass and electron transfer within overlapped two photocatalysts in UV and no-UV illumination cases [27].

**3.4 Effect of molar ratio of CO2, NH3 and H2O on CO2 reduction characteristics**

**Figures 22** and **23** show the concentration changes of formed CO and CH4, along the time under the Xe lamp with UV light, respectively. The amount of Cu/TiO2 on the netlike glass disc is 0.1 g. Before the experiments, a blank test, which 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.

cause the results.

**87**

**Figure 20.**

**Figures 19** and **20** show the concentration changes of CO produced and the molar quantity of CO per weight of photocatalyst in the reactor with two overlapped Cu/TiO2 film coated on netlike glass disc under the illumination of Xe lamp without UV light, respectively. In this experiment, CO is the only produced from the reactions.

According to **Figures 19** and **20**, the CO2 reduction performance in two discs case is the highest for the molar ratio of CO2/H2/H2O = 1:0.5:0.5 which is the same as

**Figure 19.**

*Change of concentration of CO for Cu/TiO2 overlapped with time for several molar ratios of CO2/H2/H2O under illumination condition without UV light.*

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

#### **Figure 20.**

*Change of molar quantity of CO per unit weight of photocatalyst for Cu/TiO2 overlapped with time for several molar ratios of CO2/H2/H2O under illumination condition without UV light.*

that in the single disc case. The order of CO2 reduction performance in two discs is the same as that in the single disc case. However, comparing **Figure 19** with **Figure 13**, the concentrations in two discs case are higher than those in single case under every molar ratio of CO2/H2/H2O. The highest concentration of CO in two discs case is 271 ppmV, which is 2.8 times as large as that in single disc case. The same reasons explained in the case of illumination with UV light can be thought to cause the results.

In addition, comparing **Figure 20** with **Figure 14**, the molar quantity of CO per weight of photocatalyst in two Cu/TiO2 discs overlapped case is singly higher than that in the single disc case under every molar ratio of CO2/H2/H2O. The highest molar quantity of CO per weight of photocatalyst is 3.1 μmol/g in two disc cases, which is 1.1 times as large at that in the single disc case. Though the effect of overlapping layout is not obtained under the illumination condition with UV light, the effect of overlapping layout is confirmed under the illumination condition without UV light. Since the photochemical reaction rate and the amount of produced fuel are small under the no-UV illumination condition compared to that with UV light, it would be beneficial to the mass transfer between produced fuels and reactants of CO2, H2, and H2O on the surface of photocatalyst in no-UV cases [36]. As a result, the mass transfer and photochemical reaction are carried out effectively in no-UV cases. Therefore, the effect of overlapping layout is obtained in no-UV cases. According to the previous reports [37, 38], the mass transfer is an inhibition factor to promote the CO2 reduction performance of photocatalyst, and it is necessary to control the mass transfer rate to meet the photochemical reaction rate. **Figure 21** illustrates the comparison of mass and electron transfer within overlapped two photocatalysts in UV and no-UV illumination cases [27].
