**1. Introduction**

Paris Agreement adopted in 2015 sets the goal that the increase in average temperature in the world from the industrial revolution by 2030 should be kept less than 2 K. However, due to the increase in the averaged concentrations of CO2 in the atmosphere to 410 ppmV in December 2019 [1], CO2 reduction or utilization technologies to recycle CO2 are urgently required.

There are six vital CO2 conversions: chemical conversions, electrochemical reductions, biological conversions, reforming, inorganic conversions, and photochemical reductions [2, 3]. Recently, artificial photosynthesis or the photochemical reduction of CO2 to fuel has become an attractive route due to its economically and environmentally friendly behavior [2].

The application of CO2 as a raw material can produce chemicals and energy to diminish the CO2 accumulation in the atmosphere [2]. If we consider energy producing possibilities, one possibility is the photochemical conversion of CO2 into value-added chemicals which could be used as fuel [4].

The most widely used photocatalyst for the photocatalytic reactions is TiO2 due to its availability, chemical stability, low cost, and resistance to corrosion [5]. It is

well known that CO2 can be reduced into fuels, e.g., CO, CH4, CH3OH, H2, etc. by using TiO2 as the photocatalyst under ultraviolet (UV) light illumination [6–9]. However, pure TiO2 has the limitation. It is only active when irradiated by UV light, which is not effective under sunlight. Since the solar spectrum only consists of about 4% of UV light, sunlight is not able to active the TiO2 effectively for photocatalytic reaction. In addition, TiO2 has a high electron/hole pair recombination rate compared to the rate of chemical interaction with the absorbed species for redox reactions [10].

<Oxidization>

<Reduction>

below [24, 25]:

<Oxidization>

<Reduction>

investigated previously.

**75**

<Photocatalytic reaction>

H2 þ 2*h*<sup>þ</sup> ! 2H<sup>þ</sup> þ 2*e*

� ! �CO2

CO2 þ *e*

� þ H<sup>þ</sup> þ *e*

H<sup>þ</sup> þ *e*

The reaction scheme to reduce CO2 with NH3 can be summarized as shown

TiO2 þ *hν* ! *h*<sup>þ</sup> þ *e*

H2 ! 2H<sup>þ</sup> þ 2*e*

� ! �CO2

There are some reports on CO2 reduction with either H2O or H2 [7, 9]. However, the effect of using H2O and H2 or NH3 together as reductants is not investigated well. Though a few studies using pure TiO2 under CO2/H2/H2O condition were reported [24, 26], the effect of ratio of CO2, H2 and H2O or NH3 as well as the effect of Cu doping with TiO2 on CO2 reduction performance of photocatalyst were not

Consequently, the purpose of this chapter is to clarify the effect of molar ratio of CO2 to H2O and H2 or NH3 on the performance of CO2 reduction with Cu/TiO2. The CO2 reduction performance with H2O and H2 or NH3 using Cu/TiO2 coated on netlike glass fiber as photocatalyst under the condition of illuminating Xe lamp with or without UV light was investigated. Cu is loaded on TiO2-coated netlike glass fiber by pulse arc plasma method which can emit nanosized Cu particles by applying high electron potential difference. The amount of loaded Cu can be controlled by the pulse number. Cu/TiO2 prepared was characterized by Scanning Electron Microscope (SEM) and Electron Probe Micro Analyzer (EPMA), Transmission Electron Microscope (TEM), Energy Dispersive X-ray Spectrometry (EDX), and Electron Energy Loss Spectrum (EELS) analysis. The CO2 reduction performance with H2O and H2 or NH3 under the condition of illuminating Xe lamp with or without UV

H<sup>þ</sup> þ *e*

� þ H<sup>þ</sup> þ *e*

CO2 þ *e*

�CO2

CO2 þ 8H<sup>þ</sup> þ 8*e*

�CO2

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

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

CO2 þ 8*e*

� (7)

� (8)

� (13)

� (15)

� (17)

� ! �H (16)

� ! HCOO� (18)

� ! CH4 þ 2H2O (20)

� ! HCOO� (9)

� ! �H (11)

� þ 8 � H ! CH4 þ 2H2O (12)

2NH3 ! N2 þ 3H2 (14)

HCOO� þ H<sup>þ</sup> ! CO þ H2O (19)

HCOO� þ H<sup>þ</sup> ! CO þ H2O (10)

Recently, studies on CO2 photochemical reduction by TiO2 have been carried out from the viewpoint of performance promotion by extending absorption wavelength toward visible region. It was reported that a transition metal doping is useful technique for extending the absorbance of TiO2 into the visible region [11–15]. Noble metal doping such as Pt, Pd, Au and Ag [11], Au, Pd-three dimensionally ordered macroporous TiO2 [12], composition materials formed by GaP and TiO2 [13], nanocomposite CdS/TiO2 combining two different band gap photocatalysts [14], and carbon-based AgBr nanocomposited TiO2 [15], had been attempted to overcome the shortcomings of the pure TiO2. They could improve the CO2 reduction performance; however, the concentrations in the products achieved in all the attempts so far were still low, ranging from 1 to 150 μmol/g-cat [11–16].

Though various metals have been used for doping [11–16], Cu is considered as a favorite candidate. Cu can extend the absorption band to 600–800 nm [17, 18], which covers the whole visible light range. Cu-decorated TiO2 nanorod thin film performed 10 times yield as large as TiO2 for C2H5OH production [19]. Cu-loaded N/TiO2 also showed the good performance which yielded eight times as large as TiO2 for CH4 production [20]. Noble metals such as Pt and Au are too expensive to be used in industrial scale. Therefore, Cu is the best candidate because of its high efficiency and low cost compared to noble metals. Due to its availability as well as above described characteristics, Cu is selected as the dopant in this study.

Since a reductant is necessary for CO2 reduction to produce fuel; H2O and H2 are usually used as reductants according to the review papers [7, 9]. To promote the CO2 reduction performance of photocatalyst, it is important to select the optimum reductant which provides the proton (H<sup>+</sup> ) for the reaction scheme of CO2 reduction with H2O is as follows [21–23]:

<Photocatalytic reaction>

$$\text{TiO}\_2 + h\nu \to h^+ + e^- \tag{1}$$

<Oxidization>

$$\text{H}\_2\text{O} + \text{h}^+ \rightarrow \cdot \text{OH} + \text{H}^+ \tag{2}$$

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

<Reduction>

$$\text{CO}\_2 + 2\text{H}^+ + 2\text{e}^- \rightarrow \text{CO} + \text{H}\_2\text{O} \tag{4}$$

$$\text{CO} + 8\text{H}^+ + 8e^- \to \text{CH}\_4 \tag{5}$$

The reaction scheme of CO2 reduction with H2 is as follows [24]: <Photocatalytic reaction>

$$\text{TiO}\_2 + h\nu \to h^+ + e^- \tag{6}$$

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

<Oxidization>

$$\text{CH}\_2 + 2\text{h}^+ \rightarrow 2\text{H}^+ + 2\text{e}^- \tag{7}$$

<Reduction>

$$\cdot \text{CO}\_2 + e^- \rightarrow \cdot \text{CO}\_2^- \tag{8}$$

$$\cdot \text{CO}\_2^- + \text{H}^+ + e^- \rightarrow \text{HCOO}^- \tag{9}$$

$$\text{HCOO}^- + \text{H}^+ \rightarrow \text{CO} + \text{H}\_2\text{O} \tag{10}$$

$$\cdot \text{H}^+ + e^- \to \cdot \text{H} \tag{11}$$

$$\text{C}\text{O}\_2 + 8e^- + 8\cdot\text{H} \rightarrow \text{CH}\_4 + 2\text{H}\_2\text{O} \tag{12}$$

The reaction scheme to reduce CO2 with NH3 can be summarized as shown below [24, 25]:

<Photocatalytic reaction>

$$\text{TiO}\_2 + h\nu \to h^+ + e^- \tag{13}$$

<Oxidization>

$$\text{2NH}\_3 \rightarrow \text{N}\_2 + \text{3H}\_2\tag{14}$$

$$\text{H}\_2 \rightarrow 2\text{H}^+ + 2\text{e}^- \tag{15}$$

<Reduction>

$$\text{H}^+ + e^- \to \cdot \text{H} \tag{16}$$

$$\cdot \text{CO}\_2 + e^- \rightarrow \cdot \text{CO}\_2^- \tag{17}$$

$$\cdot \text{CO}\_2^- + \text{H}^+ + e^- \rightarrow \text{HCOO}^- \tag{18}$$

$$\text{HCOO}^- + \text{H}^+ \rightarrow \text{CO} + \text{H}\_2\text{O} \tag{19}$$

$$\text{CO}\_2 + 8\text{H}^+ + 8\text{e}^- \rightarrow \text{CH}\_4 + 2\text{H}\_2\text{O} \tag{20}$$

There are some reports on CO2 reduction with either H2O or H2 [7, 9]. However, the effect of using H2O and H2 or NH3 together as reductants is not investigated well. Though a few studies using pure TiO2 under CO2/H2/H2O condition were reported [24, 26], the effect of ratio of CO2, H2 and H2O or NH3 as well as the effect of Cu doping with TiO2 on CO2 reduction performance of photocatalyst were not investigated previously.

Consequently, the purpose of this chapter is to clarify the effect of molar ratio of CO2 to H2O and H2 or NH3 on the performance of CO2 reduction with Cu/TiO2. The CO2 reduction performance with H2O and H2 or NH3 using Cu/TiO2 coated on netlike glass fiber as photocatalyst under the condition of illuminating Xe lamp with or without UV light was investigated. Cu is loaded on TiO2-coated netlike glass fiber by pulse arc plasma method which can emit nanosized Cu particles by applying high electron potential difference. The amount of loaded Cu can be controlled by the pulse number. Cu/TiO2 prepared was characterized by Scanning Electron Microscope (SEM) and Electron Probe Micro Analyzer (EPMA), Transmission Electron Microscope (TEM), Energy Dispersive X-ray Spectrometry (EDX), and Electron Energy Loss Spectrum (EELS) analysis. The CO2 reduction performance with H2O and H2 or NH3 under the condition of illuminating Xe lamp with or without UV

light was investigated. The molar ratio of CO2/H2/H2O was changed for 1:1:1, 1:0.5:1, 1:1:0.5, 1:0.5:0.5 to clarify the optimum combination of CO2/H2/H2O for CO2 reduction with Cu/TiO2. According to the reaction scheme to reduce CO2 with H2O or NH3 as shown above, 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, 3:8, respectively. Therefore, this study assumes that the molar ratio of CO2/NH3/ H2O = 3:2:3 and 3:8:12 are theoretical molar ratio to produce CO and CH4, respectively. Moreover, the effect of overlapping two layers of Cu/TiO2-coated netlike glass fiber on CO2 reduction performance with H2 and H2O was investigated.

The electron probe emits the electrons to the sample under the acceleration voltage of 15 kV and the current of 3.0 <sup>10</sup><sup>8</sup> A, when the surface structure of sample is analyzed by SEM. The characteristic X-ray is detected by EPMA at the same time, resulting that the concentration of channel element is analyzed according to the relationship between the characteristic X-ray energy and the atomic number. The spatial resolutions of SEM and EPMA are 10 μm. The EPMA analysis helps not only to understand the coating state of prepared photocatalyst but also to measure the amount of doped metal within TiO2 film on the base material. The electron probe emits the electron to the sample under the acceleration voltage of 200 kV, when the inner structure of sample is analyzed by TEM. The size, thickness, and structure of loaded Cu were evaluated. The characteristic X-ray is detected by EDX at the same time, resulting that the concentration distribution of chemical element toward thickness direction of the sample is analyzed. In the present study, the concentration distribution of Ti and Cu were analyzed.

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

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

EELS can be applied not only for element detection but also determination of oxidization states of some transition metals. The EELS characterization was performed by JEM-ARM200F equipped with GIF Quantum having 2048 ch. The dispersion of 0.5 eV/ch can be achieved for the full width at half maximum of the zero loss peak.

**Figure 1** [27, 28] shows the experimental set-up of the reactor composing of stainless tube (100 mm (*H*.) 50 mm (*I*.*D*.)), Cu/TiO2 film coated on netlike glass

(*H*.) 50 mm (*D*.)), a quartz glass disc (84 mm (*D*.) 10 mm (*t*.)), a sharp cut filter cutting off the light whose wavelength is below 400 nm (SCF-49.5C-42 L, SIGMA KOKI CO. LTD.), a 150 W Xe lamp (L2175, Hamamatsu Photonics K. K.), mass flow controller, and CO2 gas cylinder. The volume of reactor to charge CO2 is

illuminates Cu/TiO2 film coated on the netlike glass disc through the sharp cut filter and the quartz glass disc that are at the top of the stainless tube. The wavelength of light from Xe lamp is ranged from 185 to 2000 nm. Since the sharp cut filter can remove UV components of the light from the Xe lamp, the wavelength of light from Xe lamp is ranged from 401 to 2000 nm with the filter. **Figure 2** [29] shows the performance of the sharp cut filter to cut off the wavelength is below 400 nm. The

*Schematic drawing of CO2 reduction experimental set-up (left: CO2/H2/H2O system; right: CO2/NH3/H2O*

. The light of Xe lamp which is located inside the stainless tube

disc (50 mm (*D*.) 1 mm (*t*.)) located on the Teflon cylinder (50 mm

**2.3 CO2 reduction experiment**

1.3 <sup>10</sup><sup>4</sup> <sup>m</sup><sup>3</sup>

**Figure 1.**

*system).*

**77**
