**1. Introduction**

After the industrial revolution, the averaged concentration of CO2, which causes the global warming in the world, has been increased from 278 to 400 ppmV by 2015 [1]. Therefore, it requested a new CO<sup>2</sup> reduction or utilization technology in order to recycle CO<sup>2</sup> .

According to the review of CO<sup>2</sup> conversion technologies [2], there are six vital CO<sup>2</sup> conversions: chemical conversions, electrochemical reductions, biological conversions, reforming,

#### © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

inorganic conversions, and photochemical reductions [3]. Recently, artificial photosynthesis or the photochemical reduction of CO<sup>2</sup> to fuel has become an attractive route due to its economically and environmentally friendly behavior [2].

TiO2 is the principal catalyst for almost all types of photocatalytic reaction. It is well known that CO<sup>2</sup> can be reduced into fuels such as CO, CH4 , CH<sup>3</sup> OH and H<sup>2</sup> , and so forth by using TiO2 as the photocatalyst under ultraviolet (UV) light illumination [4–9]. After practical application of this technology, a carbon circulation system would then be established: CO<sup>2</sup> from the combustion of fuel produced by TiO<sup>2</sup> is reformed to fuels again using solar energy, and true zero emission can be achieved. However, the CO<sup>2</sup> reduction performance of TiO<sup>2</sup> is still low. One of the barriers to realize a carbon circulation system utilizing solar energy is that TiO<sup>2</sup> is only photoactive at the wavelength below 400 nm due to its relatively large band gap energy (∼3.2 eV) [10].

Recently, studies on CO<sup>2</sup> photochemical reduction by TiO<sup>2</sup> have been carried out from the viewpoint of performance promotion by extending absorption range toward visible region [11–15]. Noble metal doping such as Pt, Pd, Rh, Au and Ag [11], nanocomposite CdS/TiO<sup>2</sup> combining two different band gap photocatalysts [12], N<sup>2</sup> -modified TiO<sup>2</sup> [13], light harvesting complex of green plant–assisted Rh-doped TiO<sup>2</sup> [14] and dye-sensitized TiO<sup>2</sup> [15] have been developed for this process. However, the concentration in the products achieved is still low, ranging from 10 to 1000 ppmV [4, 6–9] or from 1 to 100 μmol/g-cat [11–15]. Therefore, the big breakthrough for increasing the concentration level of products is necessary to advance the CO<sup>2</sup> reduction technology in order to make the technology practically useful.

that TiO<sup>2</sup>

promotion of CO<sup>2</sup>

CO<sup>2</sup>

coupling effect on CO<sup>2</sup>

**2. Experimental**

were added to TiO<sup>2</sup>

film. TiO<sup>2</sup>

**2.1. Procedure to prepare Fe/TiO2**

Tesque Co.) of 0.3 mol, anhydrous C<sup>2</sup>

In the present study, TiO<sup>2</sup>

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

sol solution consists of [(CH<sup>3</sup>

film and doped metal are captured by netlike glass fiber easily during sol-gel and dip-

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

reduction performance would be achieved. There is no previous study on the

photocatalyst is promoted due to the porous structure of netlike glass fiber. On the other hand, Cu disc is also adopted since the recombination of electron and hole produced by photochemical reaction can be prevented by a free electron emitted from Cu disc. The coupling effect of

performance is investigated. The illumination light is able to penetrate through the netlike glass fiber and reach on Cu disc. If the synergy effect between the photocatalyst coated on netlike glass fiber and that on Cu disc was obtained due to an active electron transfer between them, the

scanning electron microscope (SEM) and electron probe micro analyzer (EPMA) analysis. The

Sol-gel and dip-coating process, which is a general procedure, was selected to prepare Fe/TiO<sup>2</sup>

Co.) of 2.4 mol, distilled water of 0.3 mol and HCl (purity of 35 wt%, produced by Nacalai Tesque Co.) of 0.07 mol. Fe particles (produced by Merck KGaA, particle size below 10 μm)

ness were 50 and 1 mm, respectively. The Cu disc used has diameter and thickness of 50 and

1.5 mm/s and pulled up at the fixed speed of 0.22 mm/s. Then, it was dried out and fired under the controlled firing temperature (FT) and firing duration time (FD), with Fe/TiO<sup>2</sup>

sol solution. Netlike glass fiber was cut to disc, and its diameter and thick-

)2 CHO4

H5

1 mm, respectively. The base material was dipped into Fe/TiO<sup>2</sup>

prepared photocatalysts coated on overlapped netlike glass fiber and Cu disc on CO<sup>2</sup>

film.

reduction performance of metal-doped TiO<sup>2</sup>

film doped with Fe (Fe/TiO<sup>2</sup>

condition of illuminating Xe lamp with or without UV light were investigated.

 **film**

absorption performance of prepared

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

or nondoped TiO<sup>2</sup>

) was prepared and characterized by

coated on netlike glass fiber and/or Cu disc under the

] Ti (purity of 95 wt%, produced by Nacalai

sol solution at the speed of

film

OH (purity of 99.5 wt%, produced by Nacalai Tesque

reduction

65

.

coating process. In addition, it can be expected that a CO<sup>2</sup>

**Figure 1.** The base materials used for coating TiO<sup>2</sup>

In the present paper, TiO<sup>2</sup> sol-gel and dip-coating process with doping is adopted in order to extend its photoresponsivity to the visible spectrum to promote the CO<sup>2</sup> reduction performance. This process can incorporate dopants into TiO<sup>2</sup> lattice, resulting in better optical and catalytic properties [16]. In addition, the integration of dopants into the sol during the gelation process facilitates direct interaction between TiO<sup>2</sup> and dopants during the sol-gel process [17].

It was reported that doping transition metal was a useful technique for extending the absorbance of TiO<sup>2</sup> into the visible region [18]. For doping, various metal ions have been used, but among them, Fe3+ is considered as a strong candidate as it has a similar radius to Ti4+ (Fe3+ = 78.5 pm, Ti4+ = 74.5 pm) [19] and can easily fit into the crystal lattice of TiO<sup>2</sup> [18, 20, 21]. Moreover, the redox potential (energy differential) of Fe2+/Fe3+ is close to that of Ti3+/Ti4+, resulting in shifting its optical absorption into the visible region [18, 20, 21]. Due to easy availability as well as the above-described characteristics, Fe is selected as the dopant in the present study.

In the present study, a net glass fiber (SILIGLASS U, produced by Nihonmuki Co.) and Cu disc are used as base materials for coating TiO<sup>2</sup> film by sol-gel and dip-coating process. **Figure 1** shows the netlike glass disc, Cu disc, and two overlapped base materials. The netlike glass fiber is a net composed of glass fiber whose diameter is about 10 μm. The fine glass fibers are knitted, resulting in the diameter of aggregate fiber about 1 mm. According to the manufacture specifications of netlike glass fiber, the porous diameter of glass fiber is about 1 nm and the specific surface is about 400 m<sup>2</sup> /g. The netlike glass fiber consists of SiO<sup>2</sup> whose purity is over 96 wt%. The aperture of net is about 2 × 2 mm. Since the netlike glass fiber has a porous characteristic, it is believed Effect of Overlapping Fe/TiO2 Coated on Netlike Glass Disc and Cu Disc on CO2 Reduction http://dx.doi.org/10.5772/intechopen.70389 65

**Figure 1.** The base materials used for coating TiO<sup>2</sup> film.

inorganic conversions, and photochemical reductions [3]. Recently, artificial photosynthesis

is the principal catalyst for almost all types of photocatalytic reaction. It is well known

as the photocatalyst under ultraviolet (UV) light illumination [4–9]. After practical appli-

cation of this technology, a carbon circulation system would then be established: CO<sup>2</sup>

One of the barriers to realize a carbon circulation system utilizing solar energy is that TiO<sup>2</sup>

photochemical reduction by TiO<sup>2</sup>

reduction technology in order to make the technology practically useful.

to extend its photoresponsivity to the visible spectrum to promote the CO<sup>2</sup>

only photoactive at the wavelength below 400 nm due to its relatively large band gap energy

viewpoint of performance promotion by extending absorption range toward visible region [11–15]. Noble metal doping such as Pt, Pd, Rh, Au and Ag [11], nanocomposite CdS/TiO<sup>2</sup>

developed for this process. However, the concentration in the products achieved is still low, ranging from 10 to 1000 ppmV [4, 6–9] or from 1 to 100 μmol/g-cat [11–15]. Therefore, the big breakthrough for increasing the concentration level of products is necessary to advance the

catalytic properties [16]. In addition, the integration of dopants into the sol during the gelation

It was reported that doping transition metal was a useful technique for extending the

used, but among them, Fe3+ is considered as a strong candidate as it has a similar radius to Ti4+ (Fe3+ = 78.5 pm, Ti4+ = 74.5 pm) [19] and can easily fit into the crystal lattice of TiO<sup>2</sup> [18, 20, 21]. Moreover, the redox potential (energy differential) of Fe2+/Fe3+ is close to that of Ti3+/Ti4+, resulting in shifting its optical absorption into the visible region [18, 20, 21]. Due to easy availability as well as the above-described characteristics, Fe is selected as the

In the present study, a net glass fiber (SILIGLASS U, produced by Nihonmuki Co.) and Cu disc

the netlike glass disc, Cu disc, and two overlapped base materials. The netlike glass fiber is a net composed of glass fiber whose diameter is about 10 μm. The fine glass fibers are knitted, resulting in the diameter of aggregate fiber about 1 mm. According to the manufacture specifications of netlike glass fiber, the porous diameter of glass fiber is about 1 nm and the specific surface is

of net is about 2 × 2 mm. Since the netlike glass fiber has a porous characteristic, it is believed

, CH<sup>3</sup>

to fuel has become an attractive route due to its eco-

OH and H<sup>2</sup>

is reformed to fuels again using solar energy, and true

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


[14] and dye-sensitized TiO<sup>2</sup>

sol-gel and dip-coating process with doping is adopted in order

into the visible region [18]. For doping, various metal ions have been

, and so forth by using

have been carried out from the

lattice, resulting in better optical and

and dopants during the sol-gel process [17].

film by sol-gel and dip-coating process. **Figure 1** shows

whose purity is over 96 wt%. The aperture

[13], light harvesting

[15] have been

reduction perfor-

from the

is

is still low.

or the photochemical reduction of CO<sup>2</sup>

64 Carbon Dioxide Chemistry, Capture and Oil Recovery

combustion of fuel produced by TiO<sup>2</sup>

TiO2

TiO2

CO<sup>2</sup>

that CO<sup>2</sup>

(∼3.2 eV) [10].

Recently, studies on CO<sup>2</sup>

In the present paper, TiO<sup>2</sup>

absorbance of TiO<sup>2</sup>

about 400 m<sup>2</sup>

dopant in the present study.

are used as base materials for coating TiO<sup>2</sup>

/g. The netlike glass fiber consists of SiO<sup>2</sup>

nomically and environmentally friendly behavior [2].

zero emission can be achieved. However, the CO<sup>2</sup>

combining two different band gap photocatalysts [12], N<sup>2</sup>

mance. This process can incorporate dopants into TiO<sup>2</sup>

process facilitates direct interaction between TiO<sup>2</sup>

complex of green plant–assisted Rh-doped TiO<sup>2</sup>

can be reduced into fuels such as CO, CH4

that TiO<sup>2</sup> film and doped metal are captured by netlike glass fiber easily during sol-gel and dipcoating process. In addition, it can be expected that a CO<sup>2</sup> absorption performance of prepared photocatalyst is promoted due to the porous structure of netlike glass fiber. On the other hand, Cu disc is also adopted since the recombination of electron and hole produced by photochemical reaction can be prevented by a free electron emitted from Cu disc. The coupling effect of prepared photocatalysts coated on overlapped netlike glass fiber and Cu disc on CO<sup>2</sup> reduction performance is investigated. The illumination light is able to penetrate through the netlike glass fiber and reach on Cu disc. If the synergy effect between the photocatalyst coated on netlike glass fiber and that on Cu disc was obtained due to an active electron transfer between them, the promotion of CO<sup>2</sup> reduction performance would be achieved. There is no previous study on the coupling effect on CO<sup>2</sup> reduction performance of metal-doped TiO<sup>2</sup> or nondoped TiO<sup>2</sup> .

In the present study, TiO<sup>2</sup> film doped with Fe (Fe/TiO<sup>2</sup> ) was prepared and characterized by scanning electron microscope (SEM) and electron probe micro analyzer (EPMA) analysis. The CO<sup>2</sup> reduction characteristics of Fe/TiO<sup>2</sup> coated on netlike glass fiber and/or Cu disc under the condition of illuminating Xe lamp with or without UV light were investigated.
