**3. Results and discussion**

#### **3.1. Characterization of Fe/TiO2 film by SEM and EPMA**

**Figures 4** and **5** show SEM images of TiO<sup>2</sup> and Fe/TiO<sup>2</sup> film coated on netlike glass disc, respectively. The SEM images were taken at 1500 times magnification under the acceleration voltage of 15 kV and the current of 3.0 × 10−8 A.

**Figures 6** and **7** show EPMA images of TiO<sup>2</sup> and Fe/TiO<sup>2</sup> film coated on netlike glass disc, respectively. The EPMA analysis was carried out to obtain 1500 times magnified SEM images as shown in **Figures 4** and **5**. In EPMA image, the concentrations of each element in observation area are indicated by different colors. Dilute colors indicate that the amount of element is large, while dense colors indicate that the amount of element is small.

To identify the position of each element, the colored circles are added to these SEM and EPMA images. The red circles shown in SEM images of **Figures 4** and **5** indicate that the amount of Ti is large as pointed out in EPMA images of **Figures 6** and **7**. From these figures, it can be observed that TiO<sup>2</sup> film with teethlike shape is coated on netlike glass fiber. It is also seen that TiO2 film builds a bridge among several glass fibers. It is believed that the temperature profile of TiO<sup>2</sup> solution adhered on the netlike glass disc is not uniform because the thermal conductivities of Ti and SiO<sup>2</sup> are different during firing process. The thermal conductivities of Ti and SiO<sup>2</sup> at 600 K are 19.4 and 1.82 W/(mK), respectively [22]. Since the thermal expansion and shrinkage around netlike glass fiber occur, thermal crack of TiO<sup>2</sup> film is caused. Therefore, TiO2 film on netlike glass fiber is teethlike.

**Figure 5.** The SEM image of Fe/TiO<sup>2</sup>

**Figure 6.** The EPMA image of TiO<sup>2</sup>

film coated on netlike glass disc.

Effect of Overlapping Fe/TiO2 Coated on Netlike Glass Disc and Cu Disc on CO2 Reduction

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69

film coated on netlike glass disc.

The yellow circle shown in SEM image of **Figure 5** indicates the existence of metal particles as pointed out in EPMA images of **Figure 7**. It is observed from **Figure 5** that Fe particles adhere on the netlike glass fiber directly. According to **Figure 5**, the size of doped metal is below

**Figure 4.** The SEM image of TiO<sup>2</sup> film coated on netlike glass disc.

Effect of Overlapping Fe/TiO2 Coated on Netlike Glass Disc and Cu Disc on CO2 Reduction http://dx.doi.org/10.5772/intechopen.70389 69

**Figure 5.** The SEM image of Fe/TiO<sup>2</sup> film coated on netlike glass disc.

**3. Results and discussion**

68 Carbon Dioxide Chemistry, Capture and Oil Recovery

**3.1. Characterization of Fe/TiO2**

observed that TiO<sup>2</sup>

tivities of Ti and SiO<sup>2</sup>

**Figure 4.** The SEM image of TiO<sup>2</sup>

TiO2

SiO<sup>2</sup>

TiO2

of TiO<sup>2</sup>

**Figures 4** and **5** show SEM images of TiO<sup>2</sup>

voltage of 15 kV and the current of 3.0 × 10−8 A.

**Figures 6** and **7** show EPMA images of TiO<sup>2</sup>

film on netlike glass fiber is teethlike.

 **film by SEM and EPMA**

respectively. The SEM images were taken at 1500 times magnification under the acceleration

respectively. The EPMA analysis was carried out to obtain 1500 times magnified SEM images as shown in **Figures 4** and **5**. In EPMA image, the concentrations of each element in observation area are indicated by different colors. Dilute colors indicate that the amount of element is

To identify the position of each element, the colored circles are added to these SEM and EPMA images. The red circles shown in SEM images of **Figures 4** and **5** indicate that the amount of Ti is large as pointed out in EPMA images of **Figures 6** and **7**. From these figures, it can be

film builds a bridge among several glass fibers. It is believed that the temperature profile

at 600 K are 19.4 and 1.82 W/(mK), respectively [22]. Since the thermal expansion and

The yellow circle shown in SEM image of **Figure 5** indicates the existence of metal particles as pointed out in EPMA images of **Figure 7**. It is observed from **Figure 5** that Fe particles adhere on the netlike glass fiber directly. According to **Figure 5**, the size of doped metal is below

solution adhered on the netlike glass disc is not uniform because the thermal conduc-

large, while dense colors indicate that the amount of element is small.

shrinkage around netlike glass fiber occur, thermal crack of TiO<sup>2</sup>

film coated on netlike glass disc.

and Fe/TiO<sup>2</sup>

and Fe/TiO<sup>2</sup>

film with teethlike shape is coated on netlike glass fiber. It is also seen that

are different during firing process. The thermal conductivities of Ti and

film coated on netlike glass disc,

film coated on netlike glass disc,

film is caused. Therefore,

**Figure 6.** The EPMA image of TiO<sup>2</sup> film coated on netlike glass disc.

**Figure 7.** The EPMA image of Fe/TiO<sup>2</sup> film coated on netlike glass disc.

10 μm. Since the Fe particles used have diameters below 10 μm, it is confirmed that the Fe particles can be loaded without agglomeration by the sol-gel and dip-coating process.

**Figures 8** and **9** show SEM image of TiO<sup>2</sup> and Fe/TiO<sup>2</sup> film coated on Cu disc, respectively. The SEM images were taken at 1500 times magnification under an acceleration voltage of 15 kV and a current of 3.0 × 10−8 A.

**Figures 10** and **11** show EPMA images of TiO<sup>2</sup> and Fe/TiO<sup>2</sup> film coated on Cu disc, respectively. EPMA analysis was carried out to obtain 1500 times magnified SEM images as shown in **Figures 8** and **9**.

respectively [22]. Therefore, the thermal expansion around Fe particles and the ther-

**iii.** Because of thermal stress caused by the uneven distribution of temperature, the cracks

Therefore, a large amount of Cu pointed out by orange circles, which is an element of basis Cu disc, around Fe is observed in **Figure 11**. In addition, a large amount of Ti around

sol occur for Fe/TiO<sup>2</sup>

Effect of Overlapping Fe/TiO2 Coated on Netlike Glass Disc and Cu Disc on CO2 Reduction

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71

and Cu disc occur for nonmetal-doped TiO<sup>2</sup>

film around the cracks occur after firing process.

, while the thermal

mal shrinkage around the other areas of TiO<sup>2</sup>

film coated on Cu disc.

film coated on Cu disc.

expansion and shrinkage between TiO<sup>2</sup>

around Fe and the shrinkage of TiO<sup>2</sup>

Fe is also seen in the same figure.

film.

**Figure 9.** SEM image of Fe/TiO<sup>2</sup>

**Figure 8.** SEM image of TiO<sup>2</sup>

According to **Figures 9** and **11**, the TiO<sup>2</sup> film is contracted around Fe particles. The surface characteristics of TiO<sup>2</sup> or Fe/TiO<sup>2</sup> are explained as follows [23]:


Effect of Overlapping Fe/TiO2 Coated on Netlike Glass Disc and Cu Disc on CO2 Reduction http://dx.doi.org/10.5772/intechopen.70389 71

**Figure 8.** SEM image of TiO<sup>2</sup> film coated on Cu disc.

**Figure 9.** SEM image of Fe/TiO<sup>2</sup> film coated on Cu disc.

**Figure 7.** The EPMA image of Fe/TiO<sup>2</sup>

70 Carbon Dioxide Chemistry, Capture and Oil Recovery

and a current of 3.0 × 10−8 A.

in **Figures 8** and **9**.

characteristics of TiO<sup>2</sup>

**Figures 8** and **9** show SEM image of TiO<sup>2</sup>

According to **Figures 9** and **11**, the TiO<sup>2</sup>

**i.** Before firing process, TiO<sup>2</sup>

**ii.** The temperature profile of TiO<sup>2</sup>

**Figures 10** and **11** show EPMA images of TiO<sup>2</sup>

or Fe/TiO<sup>2</sup>

or Fe/TiO<sup>2</sup>

film coated on netlike glass disc.

particles can be loaded without agglomeration by the sol-gel and dip-coating process.

10 μm. Since the Fe particles used have diameters below 10 μm, it is confirmed that the Fe

and Fe/TiO<sup>2</sup>

SEM images were taken at 1500 times magnification under an acceleration voltage of 15 kV

tively. EPMA analysis was carried out to obtain 1500 times magnified SEM images as shown

are explained as follows [23]:

form during firing process since the thermal conductivities of Cu, Ti and Fe are different. Thermal conductivities of Cu, Ti and Fe at 600 K are 383, 19.4, and 54.7 W/(mK),

or Fe/TiO<sup>2</sup>

and Fe/TiO<sup>2</sup>

film coated on Cu disc, respectively. The

film is contracted around Fe particles. The surface

sol solution adhered on Cu disc is not uni-

sol solution is adhered on Cu disc uniformly.

film coated on Cu disc, respec-

respectively [22]. Therefore, the thermal expansion around Fe particles and the thermal shrinkage around the other areas of TiO<sup>2</sup> sol occur for Fe/TiO<sup>2</sup> , while the thermal expansion and shrinkage between TiO<sup>2</sup> and Cu disc occur for nonmetal-doped TiO<sup>2</sup> film.

**iii.** Because of thermal stress caused by the uneven distribution of temperature, the cracks around Fe and the shrinkage of TiO<sup>2</sup> film around the cracks occur after firing process. Therefore, a large amount of Cu pointed out by orange circles, which is an element of basis Cu disc, around Fe is observed in **Figure 11**. In addition, a large amount of Ti around Fe is also seen in the same figure.

**Figure 10.** EPMA image of TiO<sup>2</sup> film coated on Cu disc.

The center of netlike glass disc or Cu disc whose diameter is 300 μm is analyzed by EPMA to evaluate the amount of doped Fe within TiO<sup>2</sup> film quantitatively. The ratio of Fe to Ti is measured by averaging the data detected by EPMA.

**Table 1** lists weight percentages of elements Fe and Ti in the Fe/TiO<sup>2</sup> film coated on netlike glass disc or Cu disc. From this table, it can be seen that more Fe is contained in the Fe/TiO2 film on netlike glass disc than that on Cu disc, under the doping condition, that is, R of 10 wt%. As expected, the netlike glass fiber can capture the dopant metal powder well during dip-coating process due to the porous structure. However, Cu disc hardly captures the dopant metal powder during dip-coating process since the surface of Cu disc is smooth. From these results, it is clear that the amount of dopants that could be doped is influenced by the base material when the sol-gel and dip-coating process is adopted for metal doping.

**3.2. CO2**

Fe/TiO coated on netlike glass disc

**Figure 11.** EPMA image of Fe/TiO<sup>2</sup>

 **reduction characteristics of Fe/TiO2**

**Photocatalyst type Weight ratio**

**Table 1.** Weight ratio of elements Fe and Ti within the prepared metal-doped TiO<sup>2</sup>

Fe/TiO coated on Cu disc 98.15 1.85 100

film coated on Cu disc.

under the Xe lamp with UV light on, for TiO<sup>2</sup>

 **coated on netlike glass disc or Cu disc**

film.

film coated on netlike glass disc or

**Figure 12** shows the concentration changes of CO produced in the reactor along the time

Cu disc or their overlap. In this figure, a single overlapping means the photocatalyst coated on the upper surface of netlike glass disc is overlapped over the photocatalyst coated on Cu disc, while a double overlapping means that the photocatalyst coated on both the upper and lower surfaces of netlike glass disc is overlapped over the photocatalyst coated on Cu disc. In

or Fe/TiO<sup>2</sup>

**Ti (wt%) Fe (wt%) Total (wt%)**

Effect of Overlapping Fe/TiO2 Coated on Netlike Glass Disc and Cu Disc on CO2 Reduction

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73

74.76 25.24 100

Effect of Overlapping Fe/TiO2 Coated on Netlike Glass Disc and Cu Disc on CO2 Reduction http://dx.doi.org/10.5772/intechopen.70389 73

**Figure 11.** EPMA image of Fe/TiO<sup>2</sup> film coated on Cu disc.

The center of netlike glass disc or Cu disc whose diameter is 300 μm is analyzed by EPMA

like glass disc or Cu disc. From this table, it can be seen that more Fe is contained in the

is, R of 10 wt%. As expected, the netlike glass fiber can capture the dopant metal powder well during dip-coating process due to the porous structure. However, Cu disc hardly captures the dopant metal powder during dip-coating process since the surface of Cu disc is smooth. From these results, it is clear that the amount of dopants that could be doped is influenced by the base material when the sol-gel and dip-coating process is adopted for

film on netlike glass disc than that on Cu disc, under the doping condition, that

film quantitatively. The ratio of Fe to Ti is

film coated on net-

to evaluate the amount of doped Fe within TiO<sup>2</sup>

Fe/TiO2

**Figure 10.** EPMA image of TiO<sup>2</sup>

72 Carbon Dioxide Chemistry, Capture and Oil Recovery

metal doping.

measured by averaging the data detected by EPMA.

**Table 1** lists weight percentages of elements Fe and Ti in the Fe/TiO<sup>2</sup>

film coated on Cu disc.


**Table 1.** Weight ratio of elements Fe and Ti within the prepared metal-doped TiO<sup>2</sup> film.

#### **3.2. CO2 reduction characteristics of Fe/TiO2 coated on netlike glass disc or Cu disc**

**Figure 12** shows the concentration changes of CO produced in the reactor along the time under the Xe lamp with UV light on, for TiO<sup>2</sup> or Fe/TiO<sup>2</sup> film coated on netlike glass disc or Cu disc or their overlap. In this figure, a single overlapping means the photocatalyst coated on the upper surface of netlike glass disc is overlapped over the photocatalyst coated on Cu disc, while a double overlapping means that the photocatalyst coated on both the upper and lower surfaces of netlike glass disc is overlapped over the photocatalyst coated on Cu disc. In this experiment, CO is the only fuel produced from the reactions. The reaction scheme for CO production by the photochemical reaction with CO<sup>2</sup> and H<sup>2</sup> O is shown as follows [4, 7, 24–26]:

$$\text{TiO}\_2 + \text{hv} \rightarrow \text{ h}^+ + \text{e}^- \tag{1}$$

the crystal structure, which decreases the recombination rate of electron-hole pairs [20]. In

larger than that on Cu disc as shown in **Table 1**, the reaction surface area, which can receive the light, is small due to aperture of net. Therefore, the photocatalytic performance of Fe/

coated on netlike glass disc is lower than that on Cu disc. Moreover, the Fe/TiO<sup>2</sup>

coated on Cu disc. Although the present study expected the positive synergy effect of combination of two photocatalysts coated on different base materials, the positive effect observed

tion material, the electron transfer between the two overlapped photocatalysts might not be

performance is promoted, resulting that the peak concentration of CO is approximately 1.5

transfer between two overlapped photocatalysts is promoted, resulting that the synergy effect

**Figure 13** illustrates the difference of electron transfer phenomenon between single and double overlapping. In this figure, the hole produced by photochemical reaction is not shown mainly to clarify the electron transfer phenomenon. It is believed that the path for electron

**Figure 14** shows the comparison of molar quantities of CO per weight of photocatalyst among the prepared photocatalysts. These values are estimated based on the maximum CO obtained under the illumination condition with UV light up to 72 h. According to this figure, the molar

might not be activated well due to the lack of light illumination. Although it is believed that

of combination of two photocatalysts coated on different base materials is obtained.

(= 0.25 g-cat). Though the netlike glass fiber captures the large amount of TiO<sup>2</sup>

**Figure 13.** Comparison of electron transfer phenomena between single and double overlapping.

coated on Cu disc is higher than that

http://dx.doi.org/10.5772/intechopen.70389

coated on netlike glass disc and Fe/TiO<sup>2</sup>

double overlapping, the photocatalytic

single overlapping. The reason is thought to be that the electron

Effect of Overlapping Fe/TiO2 Coated on Netlike Glass Disc and Cu Disc on CO2 Reduction

coated on Cu disc shows the highest performance since

adhered in the pores of netlike glass fiber

coated on Cu disc (= 0.02 g-cat) is smaller than that on netlike glass disc

coated on netlike glass disc is

, which is an electrical insula-

single

75

sol and Fe

addition, the concentration of CO produced by Fe/TiO<sup>2</sup>

overlapping shows the small superiority over Fe/TiO<sup>2</sup>

realized well. However, in the experiment of Fe/TiO<sup>2</sup>

transfer is constructed by double overlapping.

particle during dipping process well, some Fe/TiO<sup>2</sup>

quantity of CO per weight of Fe/TiO<sup>2</sup>

the weight of Fe/TiO<sup>2</sup>

times as large as the Fe/TiO<sup>2</sup>

TiO2

on netlike glass disc. Though the weight ratio of Fe for Fe/TiO<sup>2</sup>

was very small. Since the netlike glass fiber consists of SiO<sup>2</sup>

$$\rm H\_2O + \rm h^\* \rightarrow \cdot OH + \rm H^\* \tag{2}$$

$$\cdot \text{OH} + \text{H}\_2\text{O} + \text{h}^\* \rightarrow \cdot \text{O}\_2 + 3\text{H}^\* \tag{3}$$

$$\rm{CO}\_2 + 2H^+ + 2e^- \rightarrow \rm{CO} + \rm{H}\_2O \tag{4}$$

In the first step, hole h<sup>+</sup> and electron e− are generated by illuminating light on photocatalyst as shown in Eq. (1). The generated hole reacts with H<sup>2</sup> O in the oxidation reaction step, resulting in proton H<sup>+</sup> produced as shown in Eqs. (2) and (3). The proton and electron react with CO<sup>2</sup> in the reduction reaction step, resulting that CO is produced as shown in Eq. (4).

Since the concentrations of CO in most experiments started to decrease after illumination of 48-72 hours for illumination conditions with UV light due to the reverse reaction by CO and O2 , which is a by-product as shown in Eq. (3). **Figure 12** shows only the concentration up to 72 h. 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.

According to **Figure 12**, the concentration of CO increases due to Fe doping irrespective of base material. The improvement of photocatalytic performance by Fe doping under the illumination condition with UV light can be caused by the generation of shallow charge traps in

**Figure 12.** Comparison of concentrations of produced CO among photocatalysts coated on different base materials under the illumination condition with UV light.

the crystal structure, which decreases the recombination rate of electron-hole pairs [20]. In addition, the concentration of CO produced by Fe/TiO<sup>2</sup> coated on Cu disc is higher than that on netlike glass disc. Though the weight ratio of Fe for Fe/TiO<sup>2</sup> coated on netlike glass disc is larger than that on Cu disc as shown in **Table 1**, the reaction surface area, which can receive the light, is small due to aperture of net. Therefore, the photocatalytic performance of Fe/ TiO2 coated on netlike glass disc is lower than that on Cu disc. Moreover, the Fe/TiO<sup>2</sup> single overlapping shows the small superiority over Fe/TiO<sup>2</sup> coated on netlike glass disc and Fe/TiO<sup>2</sup> coated on Cu disc. Although the present study expected the positive synergy effect of combination of two photocatalysts coated on different base materials, the positive effect observed was very small. Since the netlike glass fiber consists of SiO<sup>2</sup> , which is an electrical insulation material, the electron transfer between the two overlapped photocatalysts might not be realized well. However, in the experiment of Fe/TiO<sup>2</sup> double overlapping, the photocatalytic performance is promoted, resulting that the peak concentration of CO is approximately 1.5 times as large as the Fe/TiO<sup>2</sup> single overlapping. The reason is thought to be that the electron transfer between two overlapped photocatalysts is promoted, resulting that the synergy effect of combination of two photocatalysts coated on different base materials is obtained.

**Figure 13** illustrates the difference of electron transfer phenomenon between single and double overlapping. In this figure, the hole produced by photochemical reaction is not shown mainly to clarify the electron transfer phenomenon. It is believed that the path for electron transfer is constructed by double overlapping.

**Figure 14** shows the comparison of molar quantities of CO per weight of photocatalyst among the prepared photocatalysts. These values are estimated based on the maximum CO obtained under the illumination condition with UV light up to 72 h. According to this figure, the molar quantity of CO per weight of Fe/TiO<sup>2</sup> coated on Cu disc shows the highest performance since the weight of Fe/TiO<sup>2</sup> coated on Cu disc (= 0.02 g-cat) is smaller than that on netlike glass disc (= 0.25 g-cat). Though the netlike glass fiber captures the large amount of TiO<sup>2</sup> sol and Fe particle during dipping process well, some Fe/TiO<sup>2</sup> adhered in the pores of netlike glass fiber might not be activated well due to the lack of light illumination. Although it is believed that

**Figure 13.** Comparison of electron transfer phenomena between single and double overlapping.

**Figure 12.** Comparison of concentrations of produced CO among photocatalysts coated on different base materials

this experiment, CO is the only fuel produced from the reactions. The reaction scheme for CO

TiO2 + hv → h<sup>+</sup> + e<sup>−</sup> (1)

and H<sup>2</sup>

produced as shown in Eqs. (2) and (3). The proton and electron react with CO<sup>2</sup>

Since the concentrations of CO in most experiments started to decrease after illumination of 48-72 hours for illumination conditions with UV light due to the reverse reaction by CO and

, which is a by-product as shown in Eq. (3). **Figure 12** shows only the concentration up to 72 h. 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

According to **Figure 12**, the concentration of CO increases due to Fe doping irrespective of base material. The improvement of photocatalytic performance by Fe doping under the illumination condition with UV light can be caused by the generation of shallow charge traps in

O + h<sup>+</sup> → · OH + H<sup>+</sup> (2)

O + h<sup>+</sup> → O2 + 3 H<sup>+</sup> (3)

are generated by illuminating light on photocatalyst as

O is shown as follows [4, 7, 24–26]:

O (4)

in

O in the oxidation reaction step, resulting

production by the photochemical reaction with CO<sup>2</sup>

CO<sup>2</sup> + 2 H<sup>+</sup> + 2 e<sup>−</sup> → CO + H<sup>2</sup>

and electron e−

the reduction reaction step, resulting that CO is produced as shown in Eq. (4).

shown in Eq. (1). The generated hole reacts with H<sup>2</sup>

H<sup>2</sup>

74 Carbon Dioxide Chemistry, Capture and Oil Recovery

In the first step, hole h<sup>+</sup>

in the blank test as expected.

in proton H<sup>+</sup>

O2

·OH + H<sup>2</sup>

under the illumination condition with UV light.

**Figure 14.** Comparison of CO<sup>2</sup> reduction performances per weight of photocatalyst under the illumination condition with UV light.

shallow charge traps in the crystal structure. In addition, the maximum concentration of

**Figure 15.** Comparison of concentrations of produced CO among the photocatalysts coated on different base materials

coated on Cu disc, which has a different tendency from the results obtained under the UV illumination condition. Under the illumination condition without UV light, it is believed that the amount of doped Fe is important to absorb the visible light to perform the photo-

netlike glass disc is lower than that on Cu disc under the illumination condition with UV

Furthermore, according to **Figure 15**, the positive effect of overlapping is obtained, especially under the double overlapping condition. As mentioned before, since the netlike glass fiber

two overlapped photocatalysts might not be realized well. However, the photocatalytic performance is promoted up to approximately 1.1 times by double overlapping when the maximum concentrations of CO are compared between the single and the double overlapping condition. The electron transfer between the two overlapped photocatalysts is promoted by double overlapping. However, compared to the previous cases with UV illumination, the improvement effect by double overlapping is small. Since the wavelength of illuminating light penetrating through netlike glass disc becomes longer due to losing energy, the electron

decreases. The range of wavelength of light illuminating after penetrating through netlike glass disc is narrower than that in the UV illumination condition cases. Therefore, the effect of electron transfer promotion between the two overlapped photocatalysts is small compared

catalytic reaction. Since the amount of doped Fe for Fe/TiO<sup>2</sup>

coated on netlike glass disc is almost the same as that by Fe/TiO<sup>2</sup>

Effect of Overlapping Fe/TiO2 Coated on Netlike Glass Disc and Cu Disc on CO2 Reduction

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77

coated on Cu disc as shown in **Table 1**, the CO<sup>2</sup>

, which is an electrical insulation material, the electron transfer between the

coated on netlike glass disc equals that on Cu disc, while that on

coated on Cu disc, which is positioned under the netlike glass disc,

coated on netlike glass disc is

reduction

CO obtained by Fe/TiO<sup>2</sup>

performance of Fe/TiO<sup>2</sup>

produced by the Fe/TiO<sup>2</sup>

with that in the UV illumination cases.

light.

consists of SiO<sup>2</sup>

much larger than that for Fe/TiO<sup>2</sup>

under the illumination condition without UV light.

the Fe doping could assist CO<sup>2</sup> reduction if the light could illuminate inside the pore of netlike glass disc, the positive effect of Fe doping on CO<sup>2</sup> reduction performance per weight of photocatalyst base under the overlapping condition was not as significant as that in the case of Fe/TiO<sup>2</sup> coated on netlike glass disc. In addition, the mass transfer in the space between the two photocatalysts coated on netlike glass disc and Cu disc should be enhanced in order to meet the photoreaction rate under the overlapping condition. If the produced fuel remains in the space between two photocatalysts, the reactant of CO<sup>2</sup> and H<sup>2</sup> O could 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 the photocatalyst. Therefore, it is necessary to control the amount of doped Fe, which is coated on the surface and on the pore of the netlike glass fiber, as well as to optimize the aperture of netlike glass fiber.

**Figure 15** shows the concentration changes of CO produced in the reactor along the time under the Xe lamp illumination without UV light, for TiO<sup>2</sup> or Fe/TiO<sup>2</sup> film coated on netlike glass disc or Cu disc or their overlap. In this experiment, CO is the only fuel produced from the reactions. Since the concentration of CO almost started to decrease after illumination of 72–96 h for all cases due to the reverse reaction by CO and O<sup>2</sup> , which is the by-product as shown in Eq. (3), **Figure 15** shows the concentration only up to 96 h.

From **Figure 15**, it can be seen that the CO<sup>2</sup> reduction performance of TiO<sup>2</sup> is promoted by Fe doping due to extension of the photoresponsivity of TiO<sup>2</sup> to the visible spectrum as well as decrease in the recombination rate of electron-hole pairs by the generation of Effect of Overlapping Fe/TiO2 Coated on Netlike Glass Disc and Cu Disc on CO2 Reduction http://dx.doi.org/10.5772/intechopen.70389 77

**Figure 15.** Comparison of concentrations of produced CO among the photocatalysts coated on different base materials under the illumination condition without UV light.

shallow charge traps in the crystal structure. In addition, the maximum concentration of CO obtained by Fe/TiO<sup>2</sup> coated on netlike glass disc is almost the same as that by Fe/TiO<sup>2</sup> coated on Cu disc, which has a different tendency from the results obtained under the UV illumination condition. Under the illumination condition without UV light, it is believed that the amount of doped Fe is important to absorb the visible light to perform the photocatalytic reaction. Since the amount of doped Fe for Fe/TiO<sup>2</sup> coated on netlike glass disc is much larger than that for Fe/TiO<sup>2</sup> coated on Cu disc as shown in **Table 1**, the CO<sup>2</sup> reduction performance of Fe/TiO<sup>2</sup> coated on netlike glass disc equals that on Cu disc, while that on netlike glass disc is lower than that on Cu disc under the illumination condition with UV light.

the Fe doping could assist CO<sup>2</sup>

76 Carbon Dioxide Chemistry, Capture and Oil Recovery

**Figure 14.** Comparison of CO<sup>2</sup>

of Fe/TiO<sup>2</sup>

with UV light.

like glass disc, the positive effect of Fe doping on CO<sup>2</sup>

the space between two photocatalysts, the reactant of CO<sup>2</sup>

under the Xe lamp illumination without UV light, for TiO<sup>2</sup>

From **Figure 15**, it can be seen that the CO<sup>2</sup>

72–96 h for all cases due to the reverse reaction by CO and O<sup>2</sup>

shown in Eq. (3), **Figure 15** shows the concentration only up to 96 h.

by Fe doping due to extension of the photoresponsivity of TiO<sup>2</sup>

glass fiber, as well as to optimize the aperture of netlike glass fiber.

reduction if the light could illuminate inside the pore of net-

reduction performances per weight of photocatalyst under the illumination condition

and H<sup>2</sup>

or Fe/TiO<sup>2</sup>

reduction performance of TiO<sup>2</sup>

photocatalyst base under the overlapping condition was not as significant as that in the case

two photocatalysts coated on netlike glass disc and Cu disc should be enhanced in order to meet the photoreaction rate under the overlapping condition. If the produced fuel remains in

the surface of photocatalyst, resulting that the photochemical reaction could not be carried out well even though the light is illuminated for the photocatalyst. Therefore, it is necessary to control the amount of doped Fe, which is coated on the surface and on the pore of the netlike

**Figure 15** shows the concentration changes of CO produced in the reactor along the time

glass disc or Cu disc or their overlap. In this experiment, CO is the only fuel produced from the reactions. Since the concentration of CO almost started to decrease after illumination of

as well as decrease in the recombination rate of electron-hole pairs by the generation of

coated on netlike glass disc. In addition, the mass transfer in the space between the

reduction performance per weight of

O could be blocked to reach

film coated on netlike

is promoted

, which is the by-product as

to the visible spectrum

Furthermore, according to **Figure 15**, the positive effect of overlapping is obtained, especially under the double overlapping condition. As mentioned before, since the netlike glass fiber consists of SiO<sup>2</sup> , which is an electrical insulation material, the electron transfer between the two overlapped photocatalysts might not be realized well. However, the photocatalytic performance is promoted up to approximately 1.1 times by double overlapping when the maximum concentrations of CO are compared between the single and the double overlapping condition. The electron transfer between the two overlapped photocatalysts is promoted by double overlapping. However, compared to the previous cases with UV illumination, the improvement effect by double overlapping is small. Since the wavelength of illuminating light penetrating through netlike glass disc becomes longer due to losing energy, the electron produced by the Fe/TiO<sup>2</sup> coated on Cu disc, which is positioned under the netlike glass disc, decreases. The range of wavelength of light illuminating after penetrating through netlike glass disc is narrower than that in the UV illumination condition cases. Therefore, the effect of electron transfer promotion between the two overlapped photocatalysts is small compared with that in the UV illumination cases.

**Figure 16** shows the comparison of molar quantities of CO per weight of photocatalyst among the prepared photocatalysts. These values are estimated based on the maximum CO obtained under the no-UV illumination condition up to 96 h. It reveals that the molar quantity of CO per weight of Fe/TiO<sup>2</sup> under the double overlapping condition is the highest among all experimental conditions. In addition, it also reveals the positive effect of overlapping on CO<sup>2</sup> reduction performance in terms of molar quantity of CO per weight of photocatalyst is achieved in both single and double overlapping cases. It is believed that Fe/TiO<sup>2</sup> coated on Cu disc, which is positioned under the netlike glass disc, can utilize at least some of the light penetrating through the aperture of netlike glass disc for photochemical reactions, although the wavelength of the penetrating light becomes long.

The bigger synergy effect in terms of molar quantity of CO per weight of photocatalyst for two overlapped photocatalysts in no-UV cases is achieved when comparing with UV illumination cases. Since the photochemical reaction rate and the amount of produced fuel are small under the no-UV illumination condition compared to those under UV light, it would be beneficial to the mass transfer between produced fuel and reactant of CO<sup>2</sup> and H<sup>2</sup> O on the surface of photocatalyst in no-UV cases. As a result, the mass transfer and photochemical reaction are carried out effectively in no-UV cases. Therefore, the molar quantity of CO per weight of photocatalyst for overlapping cases is large in no-UV cases. According to previous reports [27, 28], the mass transfer is an inhibition factor to promote the CO<sup>2</sup> reduction performance of

> photocatalyst and it is necessary to control the mass transfer rate to meet the photochemical reaction rate. **Figure 17** illustrates the comparison of mass and electron transfer within two

> **Figure 17.** Comparison of mass and electron transfer within two overlapped photocatalysts between the illumination

Effect of Overlapping Fe/TiO2 Coated on Netlike Glass Disc and Cu Disc on CO2 Reduction

http://dx.doi.org/10.5772/intechopen.70389

79

tocatalysts prepared in the present study is almost at the same level. However, the present study clarifies that the double overlapping arrangement is effective in improving the CO<sup>2</sup>

ing an appropriate area of aperture can be a good base material for overlapping arrangement instead of netlike glass fiber since the former has a good electrical conductivity, light permeability, and gas diffusivity. In addition, the dopant like Cu, which can absorb the longer wavelength light than Fe [21], should be used in the layers at lower positioned photocatalysts in overlapping conditions. This proposal is similar to the concept of the hybrid photocatalyst

Based on the investigation into this study, the following conclusions can be drawn:

film with teethlike shape could be coated on netlike glass fiber, and Fe fine particles

reduction performance of pho-

film would contract around Fe particles

. It, therefore, proposes that the netlike porous metal hav-

overlapped photocatalysts in UV and no-UV illumination cases.

Compared to the previous researches [4, 6–9, 11–15], the CO<sup>2</sup>

using two photocatalysts having different band gaps [29–31].

are loaded without agglomeration. However, TiO<sup>2</sup>

was coated on the Cu disc.

reduction performance of Fe/TiO<sup>2</sup>

condition with UV light and that without UV light.

**4. Conclusions**

when Fe/TiO<sup>2</sup>

**1.** TiO2

**Figure 16.** Comparison of CO<sup>2</sup> reduction performances per weight of photocatalyst under the illumination condition without UV light.

Effect of Overlapping Fe/TiO2 Coated on Netlike Glass Disc and Cu Disc on CO2 Reduction http://dx.doi.org/10.5772/intechopen.70389 79

**Figure 17.** Comparison of mass and electron transfer within two overlapped photocatalysts between the illumination condition with UV light and that without UV light.

photocatalyst and it is necessary to control the mass transfer rate to meet the photochemical reaction rate. **Figure 17** illustrates the comparison of mass and electron transfer within two overlapped photocatalysts in UV and no-UV illumination cases.

Compared to the previous researches [4, 6–9, 11–15], the CO<sup>2</sup> reduction performance of photocatalysts prepared in the present study is almost at the same level. However, the present study clarifies that the double overlapping arrangement is effective in improving the CO<sup>2</sup> reduction performance of Fe/TiO<sup>2</sup> . It, therefore, proposes that the netlike porous metal having an appropriate area of aperture can be a good base material for overlapping arrangement instead of netlike glass fiber since the former has a good electrical conductivity, light permeability, and gas diffusivity. In addition, the dopant like Cu, which can absorb the longer wavelength light than Fe [21], should be used in the layers at lower positioned photocatalysts in overlapping conditions. This proposal is similar to the concept of the hybrid photocatalyst using two photocatalysts having different band gaps [29–31].
