**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.

**Figure 21.**

*Comparison of mass and electron transfer within overlapped two photocatalysts between the illumination condition with UV light and without UV light.*

3:8, respectively. Therefore, this study assumes that the molar ratio of CO2/NH3/ H2O = 3:2:3 and 3:8:12 is theoretical molar ratio to produce CO and CH4, respectively. However, the molar ratio of CO2/NH3/H2O = 1:1:1 is not matched with these theoretical molar ratios to produce CO and CH4. Since the ionized Cu doped with TiO2 provides free electron for the reduction reaction process [39], the reductants of NH3 and H2O which are less than the values indicated in the theoretical scheme are enough for producing CO and CH4 in this study. The highest molar quantities of CO and CH4 per weight of photocatalyst in the reactor, which are obtained for the

*Comparison of concentration of formed CH4 among several molar ratios of CO2/NH3/H2O under the*

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

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

molar ratio of CO2/NH3/H2O = 1:1:1, are 10.2 and 1.8 μmol/g, respectively.

resulting that the production of CH4 starts later.

**Figure 23.**

**89**

*illumination condition with UV light.*

In addition, it is confirmed from **Figure 22** that the concentration of formed CO is increased from the start of illumination of Xe lamp and decreased after attaining the peak concentration. However, the concentration of formed CO increases again after 48 hours. It is believed that the decrease in the concentration of formed CO is resulted from the oxidization reaction between CO and O2 which is by-product as shown in Eq. (3) [40]. Since the produced CO might be remained near the photocatalyst due to high absorption performance of netlike glass fiber, this oxidization reaction is thought to be occurred. The increase in the concentration of formed CO after 48 hours might be due to the difference in reaction rates between CO2/H2O and CO2/NH3 condition. It is also revealed that the maximum concentration of formed CO is higher when the molar of NH3 is higher than that of H2O. Since the number of H<sup>+</sup> which can be provided is 3 and 2 for NH3 and H2O, respectively, it is considered that NH3 is effective for promoting the reduction performance of Cu/ TiO2. Furthermore, it is found from **Figures 22** and **23** that the concentration of formed CH4 starts to increase after the decreasing of CO concentration. According to the reaction schemes, the more H<sup>+</sup> and electron are needed to produce CH4,

**Figure 24** shows the concentration changes of formed CO along the time under the Xe lamp without UV light. In this experiment, CO is the only fuel produced from the reactions, that is, no CH4 was detected. 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 CO or CH4 was produced in the blank test as expected. According to **Figure 24**, the CO2 reduction performance is the best

#### **Figure 22.**

*Comparison of concentration of formed CO among several molar ratios of CO2/NH3/H2O under the illumination condition with UV light.*

According to **Figures 22** and **23**, the CO2 reduction performance is the highest for the molar ratio of CO2/NH3/H2O = 1:1:1.

According to the reaction scheme to reduce CO2 with H2O or NH3 as shown by Eqs. (1)–(5), (13)–(20), the theoretical molar ratio of CO2/H2O to produce CO or CH4 is 1:1 or 1:4, respectively, while that of CO2/NH3 to produce CO or CH4 is 3:2, *CO2 Reduction Characteristics of Cu/TiO2 with Various Reductants DOI: http://dx.doi.org/10.5772/intechopen.93105*

#### **Figure 23.**

*Comparison of concentration of formed CH4 among several molar ratios of CO2/NH3/H2O under the illumination condition with UV light.*

3:8, respectively. Therefore, this study assumes that the molar ratio of CO2/NH3/ H2O = 3:2:3 and 3:8:12 is theoretical molar ratio to produce CO and CH4, respectively. However, the molar ratio of CO2/NH3/H2O = 1:1:1 is not matched with these theoretical molar ratios to produce CO and CH4. Since the ionized Cu doped with TiO2 provides free electron for the reduction reaction process [39], the reductants of NH3 and H2O which are less than the values indicated in the theoretical scheme are enough for producing CO and CH4 in this study. 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.

In addition, it is confirmed from **Figure 22** that the concentration of formed CO is increased from the start of illumination of Xe lamp and decreased after attaining the peak concentration. However, the concentration of formed CO increases again after 48 hours. It is believed that the decrease in the concentration of formed CO is resulted from the oxidization reaction between CO and O2 which is by-product as shown in Eq. (3) [40]. Since the produced CO might be remained near the photocatalyst due to high absorption performance of netlike glass fiber, this oxidization reaction is thought to be occurred. The increase in the concentration of formed CO after 48 hours might be due to the difference in reaction rates between CO2/H2O and CO2/NH3 condition. It is also revealed that the maximum concentration of formed CO is higher when the molar of NH3 is higher than that of H2O. Since the number of H<sup>+</sup> which can be provided is 3 and 2 for NH3 and H2O, respectively, it is considered that NH3 is effective for promoting the reduction performance of Cu/ TiO2. Furthermore, it is found from **Figures 22** and **23** that the concentration of formed CH4 starts to increase after the decreasing of CO concentration. According to the reaction schemes, the more H<sup>+</sup> and electron are needed to produce CH4, resulting that the production of CH4 starts later.

**Figure 24** shows the concentration changes of formed CO along the time under the Xe lamp without UV light. In this experiment, CO is the only fuel produced from the reactions, that is, no CH4 was detected. 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 CO or CH4 was produced in the blank test as expected. According to **Figure 24**, the CO2 reduction performance is the best

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

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

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

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

i. Cu in Cu/TiO2 prepared by this study exists in the form of Cu<sup>+</sup> ion in Cu2O.

ii. Under the condition of CO2/H2/H2O, the highest concentrations of CO and CH4 produced as well as the highest molar quantities of CO and CH4 per weight of photocatalyst for Cu/TiO2 are obtained for CO2/H2/H2O ratio of 1:0.5:0.5. 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 of CO2/H2O and CO2/H2.

iii. Under the condition of CO2/H2/H2O, the highest concentration of CO in two discs case is 1.4 times as large as that in the single disc case, while the highest concentration of CH4 is 1.7 times with UV light illumination. Under the illumination condition without UV light, the highest concentration of CO with two Cu/TiO2 disc is 2.8 times as large as that with single Cu/TiO2

iv. Under the condition of CO2/H2/H2O, the highest molar quantity of CO per weight of photocatalyst with two Cu/TiO2 discs overlapped is 54% of that with single Cu/TiO2 disc with UV light illumination. The highest molar quantity of CH4 per weight of photocatalyst with two Cu/TiO2 discs

v. Under the condition of CO2/H2/H2O, the molar quantity of CO per weight of photocatalyst with two Cu/TiO2 discs overlapped is slightly (1.1 times) higher than that with single Cu/TiO2 disc without UV light illumination.

overlapped is 65% of that with single Cu/TiO2 disc.

[13, 42, 43].

solution into N2 and H2 [25].

The conclusions on this chapter are as follows:

**4. Conclusions**

disc.

**91**

#### **Figure 24.**

*Comparison of concentration of formed CO among several molar ratios of CO2/NH3/H2O under the illumination condition without UV light.*

for the molar ratio of CO2/NH3/H2O = 1:1:1. In addition, it is confirmed from **Figure 24** that the concentration of formed CO is increased from the start of illumination of Xe lamp and decreased after reaching the maximum concentration. However, the concentration of formed CO is increased gradually again after a while. It can be considered that the same reaction mechanism under the illumination condition with UV light as mentioned above occurred.
