International Journal of Photoenergy
Volume 2017 (2017), Article ID 1625274, 11 pages
https://doi.org/10.1155/2017/1625274

Research Article

Effect of Fe Loading Condition and Reductants on CO₂ Reduction Performance with Fe/TiO₂ Photocatalyst

Akira Nishimura,¹ Noriaki Ishida,¹ Daichi Tatematsu,¹ Masafumi Hirota,¹ Akira Koshio,² Fumio Kokai,² and Eric Hu³

¹Division of Mechanical Engineering, Graduate School of Engineering, Mie University, 1577 Kurimamachiya-cho, Tsu, Mie 514-8507, Japan
²Division of Chemistry for Materials, Graduate School of Engineering, Mie University, Tsu, Mie 514-8507, Japan
³School of Mechanical Engineering, The University of Adelaide, North Terrace, Adelaide, SA 5005, Australia

Correspondence should be addressed to Akira Nishimura; nisimura@mach.mie-u.ac.jp

Received 23 March 2017; Revised 19 June 2017; Accepted 10 July 2017; Published 9 August 2017

Academic Editor: Elisa Isabel Garcia-Lopez

Copyright © 2017 Akira Nishimura et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Fe-doped TiO₂ (Fe/TiO₂) film photocatalyst was prepared by sol-gel and dip-coating process and pulse arc plasma method. The effect of pulse number on the CO₂ reduction performance with the Fe/TiO₂ was investigated in this study. In addition, the effect of reductants such as H₂O, H₂, and NH₃/H₂O on the CO₂ reduction performance with the Fe/TiO₂ photocatalyst was also investigated. The characteristics of the prepared Fe/TiO₂ film coated on a netlike glass fiber which is a base material were analyzed by SEM, EPMA, EDX, and EPMA. Furthermore, the CO₂ reduction performance of the Fe/TiO₂ film was tested under a Xe lamp with or without ultraviolet (UV) light. The results show that the CO₂ reduction performance with the pulse number of 100 is the best with H₂O and/or H₂ as reductant under UV light illumination, while that with the pulse number of 500 is the best when NH₃/H₂O is used as reductant. On the other hand, the CO₂ reduction performance with the pulse number of 500 is the best under every reductant condition without UV light illumination. The highest CO₂ reduction performance with the Fe/TiO₂ is obtained under H₂ + H₂O/CO₂ condition, and the best moral ratio of total reductants to CO₂ is 1.5 : 1.

1. Introduction

Due to mass consumption of fossil fuels, global warming and fossil fuel depletion have become serious global environmental problems in the world. After the industrial revolution, the averaged concentration of CO₂ in the world has been increased from 278 ppmV to 400 ppmV by 2015 [1]. Therefore, it is necessary to develop a new CO₂ reduction or utilization technology in order to recycle CO₂.

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].

TiO₂ is the principle catalyst for almost all types of photocatalysis reaction. It is well known that CO₂ can be reduced into fuels, for example, CO, CH₄, CH₃OH, and H₂ by using TiO₂ as the photocatalyst under ultraviolet (UV) light illumination [4–9]. If this technique could be applied practically, a carbon circulation system would then be established: CO₂ from the combustion of fuel is reformed to fuels again using solar energy, and true zero emission can be achieved. However, the CO₂ reduction rate using pure TiO₂ is very low, that is, the fuel concentration in the products is very low. In addition, pure TiO₂ is only photoactive at a wavelength below 400 nm due to its relatively large bandgap energy (~3.2 eV) [10].

Recently, studies on CO₂ photochemical reduction by TiO₂ have been carried out from the viewpoint of performance promotion by extending absorption range towards visible region [11–15]. Noble metal doping such as Pt, Pd, Au, and Ag [11], nanocomposite CdS/TiO₂ combining two different band gap photocatalysts [12], N₂-modified TiO₂ [13], light harvesting complex of green plants assisted Rh-doped TiO₂ [14], and dye-sensitized TiO₂ [15] have been attempted to overcome the shortcomings of pure TiO₂. They did improve the CO₂ reduction performance; however, the concentrations in the products achieved in all the attempts so far were still low, ranging from 10 ppmV to 1000 ppmV [4–6, 9] or from 1 μmol/g-cat to 100 μmol/g-cat [7, 8, 11–15]. Therefore, a big breakthrough in increasing the concentration level of products is necessary to advance the CO₂ reduction technology in order to make the technology practically useful.

It was reported that doping transition metal was a useful technique for extending the absorbance of TiO₂ into the visible region [16]. 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) [17] and can easily fit into the crystal lattice of TiO₂ [16, 18, 19]. 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 [16, 18, 19]. Due to easy availability as well as the above described characteristics, Fe is selected as the dopant in the present study.

In the present paper, TiO₂ film is coated by sol-gel and dip-coating process on net glass fiber (SILIGLASS U, Nihonmuki Co.). Netlike glass fiber is a net composed of glass fiber whose diameter is about 10 μm. The fine glass fibers are knitted, resulting in a diameter of the aggregate fiber of about 1 mm. According to manufacturer specifications of the netlike glass fiber, the porous diameter of glass fiber is about 1 nm and the specific surface area is about 400 m²/g. The netlike glass fiber consists of SiO₂ whose purity is over 96 wt%. The aperture of the net is about 2 mm × 2 mm. Since the netlike glass fiber has a porous characteristic, it is believed that the TiO₂ film is captured by the netlike glass fiber easily during a sol-gel and dip-coating process. In addition, it can be expected that CO₂ absorption performance of prepared photocatalyst is promoted due to the porous structure of the netlike glass fiber.

Then, Fe is loaded on the TiO₂-coated netlike glass fiber by pulse arc plasma method which can emit nanosized Fe particles by applying high electrical potential difference. Since the amount of loaded Fe can be controlled by pulse number, the present paper investigates the impact of the pulse number on characterization and CO₂ reduction performance of the prepared Fe/TiO₂.

To promote the CO₂ reduction performance of photocatalyst, it is important to select the optimum reductant which provides the proton for the reduction reaction. Generally speaking, H₂O is used as a reductant for CO₂ reduction by photocatalyst according to some review papers [20–23]. The reaction scheme of CO₂ reduction with H₂O is as follows [4, 24–26].

Photocatalytic reaction:

Oxidization:

Reduction:

H₂ is also a good candidate for the reductant since H₂ can be converted into a proton by photocatalyst [27–36]. The reaction scheme of CO₂ reduction with H₂ is as follows [30].

Photocatalytic reaction:

Oxidization:

Reduction:

Though there are some reports on CO₂ reduction with H₂ [27–36], there are few reports investigating the reaction with TiO₂ as the photocatalyst is a few [27–31]. Particularly, there is no study on CO₂ reduction with Fe/TiO₂. This paper investigates the reduction performance of Fe/TiO₂ for CO₂ with H₂ as well as CO₂ with H₂O. Furthermore, the present paper also investigates the effect of NH₃ + H₂O on CO₂ reduction performance with the Fe/TiO₂ as the photocatalyst since NH₃ is thought to be a good H₂ carrier. According to the authors’ review, there is no report on CO₂ reduction over Fe/TiO₂ with NH₃. NH₃ is thought to be used as reductant directly since NH₃ aqueous solution can decompose into H₂ and N₂ by photocatalyst [37]. After decomposition of NH₃, it is believed that the reaction scheme of CO₂ reduction with H₂ can be applied.

In this paper, TiO₂ film doped with Fe (Fe/TiO₂) was prepared and characterized by scanning electron microscope (SEM), electron probe microanalyzer (EPMA), transmission electron microscope (TEM), and energy dispersive X-ray spectrometry (EDX) analysis to clarify the optimum amount of loaded Fe. The CO₂ reduction characteristics of Fe/TiO₂ coated on net glass fiber with H₂O and/or H₂ and NH₃ under the condition of illuminating Xe lamp with or without UV light were investigated.

2. Experiment

2.1. Preparation of Fe/TiO₂ Film

Sol-gel and dip-coating process was used for preparing TiO₂ film. TiO₂ sol solution was made by mixing [(CH₃)₂CHO]₄Ti (purity of 95 wt%, Nacalai Tesque Co.) of 0.3 mol, anhydrous C₂H₅OH (purity of 99.5 wt%, Nacalai Tesque Co.) of 2.4 mol, distilled water of 0.3 mol, and HCl (purity of 35 wt%, Nacalai Tesque Co.) of 0.07 mol. The netlike glass fiber was cut to disc, and its diameter and thickness were 50 mm and 1 mm, respectively. The netlike glass fiber was dipped into TiO₂ sol solution at the speed of 1.5 mm/s and pulled up at a 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), resulting in the TiO₂ film fastened on the base material. FT and FD were set at 623 K and 180 s, respectively. Fe was loaded on the TiO₂ film by pulse arc plasma method. The pulse arc plasma gun device (ULVAC Inc., ARL-300) having an Fe electrode whose diameter was 10 mm was applied for Fe loading. After the netlike glass fiber coated with TiO₂ was set in the chamber of the pulse arc plasma gun device, which was vacuumed, the nanosized Fe particles were emitted from the Fe electrode applying an electrical potential difference of 200 V. The pulse arc plasma gun can evaporate an Fe particle over the target in the circle area whose diameter is 100 mm when the distance between the Fe electrode and the target is 160 mm. Since the difference between Fe electrode and TiO₂ film was 150 mm in the present study, Fe particle can be evaporated over the TiO₂ film uniformly. The amount of loaded Fe was controlled by pulse number. In the present paper, the pulse number was set at 100, 500, and 1000.

2.2. Characterization of Fe/TiO₂ Film

The structure and crystallization characteristics of Fe/TiO₂ film were evaluated by SEM (JXA-8530F, JEOL Ltd.), EPMA (JXA-8530F, JEOL Ltd.), TEM (JEM-2100F/HK, JEOL Ltd.), and EDX (JEM-2100F/HK, JEOL Ltd.). Since these measuring instruments use electron for analysis, the sample should be an electron conductor. Since the netlike glass disc is not an electron conductor, the carbon vapor deposition was conducted by the dedicated device (JEE-420, JEOL Ltd.) for Fe/TiO₂ coated on a netlike glass disc before analysis. The thickness of carbon deposited on samples was approximately 20–30 nm.

The electron probe emits the electrons to the sample under the acceleration voltage of 15 kV and the current of 3.0 × 10⁻⁸ A, when the surface structure of sample is analyzed by SEM. The characteristic X-ray is detected by EPMA at the same time, resulting in the concentration of chemical element analyzed according to the relationship between the characteristic X-ray energy and the atomic number. The spatial resolution of SEM and EPMA is 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 the sample is analyzed by TEM. The size, thickness, and structure of loaded Fe were evaluated. The characteristic X-ray is detected by EDX at the same time, resulting in the concentration distribution of chemical element toward thickness direction of the sample being analyzed. In the present paper, the concentration distribution of Ti and Fe were analyzed.

2.3. CO₂ Reduction Experiment

Figure 1 shows the experimental setup of the reactor consisting of a stainless pipe (100 mm (H.) × 50 mm (I.D.)), a netlike glass disc coated with Fe/TiO₂ film which is located on the Teflon cylinder (50 mm (H.) × 50 mm (D.)), a quartz glass disc (84 mm (D.) × 10 mm (t.)), a sharp cut filter which cuts off the light of wavelength below 400 nm (SCF-49.5C-42L, SIGMA KOKI CO. LTD.), a 150 W Xe lamp (L2175, Hamamatsu Photonics K. K.), a mass flow controller, a CO₂ gas cylinder, and a H₂ gas cylinder.

Figure 1: Schematic drawing of CO₂ reduction experimental setup.

The reactor volume available for CO₂ charge is 1.25 × 10⁻⁴ m³. The light of the Xe lamp, through the sharp cut filter and the quartz glass disc that are at the top of the stainless pipe, illuminates the netlike glass disc coated with Fe/TiO₂ film, which is located inside the stainless pipe. The wavelengths of light from the Xe lamp are ranged from 185 nm to 2000 nm. The Xe lamp can be fitted with a sharp cut filter to remove UV components of the light. With the filter, the wavelengths of light from the Xe lamp are ranged from 401 nm to 2000 nm [38]. The average light intensity of the Xe lamp on the photocatalyst without and with setting the sharp cut filter is 57.5 mW/cm² and 43.7 mW/cm², respectively.

In the CO₂ reduction experiment with H₂O or NH₃ + H₂O, after purging the reactor chamber with CO₂ gas of the purity of 99.995 vol% flowed through the reactor for 15 minutes, the valves located at the inlet and the outlet of reactor were closed. After confirming the gas pressure and gas temperature in the reactor at 0.1 MPa and 298 K, respectively, the distilled water of 100 μL or NH₃ aqueous solution (NH₃: 50 vol%) of 200 μL was injected into the reactor through gas sampling tap, and the Xe lamp illumination was turned on at the same time. The water and NH₃ aqueous injected vaporized completely in the reactor. Due to the heat of the Xe lamp, the temperature in the reactor was attained at 343 K within an hour and kept at approximately 343 K during the experiment.

In the CO₂ reduction experiment with H₂, CO₂ gas with the purity of 99.995 vol% and H₂ gas with the purity of 99.99999 vol% which were controlled by mass flow controller were mixed in the buffer chamber and introduced in the reactor which was prevacuumed by a vacuum pump. After that, Xe lamp illumination was turned on. The mixing ratio of CO₂ and H₂ was confirmed by TCD gas chromatograph (Micro GC CP4900, GL Science) before introducing into the reactor. In the CO₂ reduction experiment with H₂ + H₂O, the distilled water was injected into the reactor after charging CO₂ and H₂.

In the CO₂ reduction experiment with H₂O or H₂, the molar ratio of H₂O or H₂ to CO₂ was 1 : 1. In the CO₂ reduction experiment with NH₃ + H₂O, the molar ratio of NH₃ and H₂O to CO₂ was 0.7 : 1 : 1. In the CO₂ reduction experiment with H₂ + H₂O, the molar ratio of H₂ and H₂O to CO₂ was set at 0.5 : 0.5 : 1, 1 : 1 : 1, 2 : 2 : 1, 1 : 0.5 : 1 and 1 : 1 : 0.5.

The gas in the reactor was sampled every 24 hours during the experiment. The gas samples were analyzed by FID gas chromatograph (GC353B, GL Science) and methanizer (MT221, GD Science). Minimum resolution of FID gas chromatograph and methanizer is 1 ppmV.

3. Results and Discussion

3.1. Characterization of Fe/TiO₂ Film

Figures 2–5 show EPMA images of TiO₂, Fe/TiO₂(100), Fe/TiO₂(500), and Fe/TiO₂(1000) film, respectively, where Fe/TiO₂(100) means TiO₂ with Fe loaded by the pulse number of 100. EPMA analysis was carried out for 1500 times magnification SEM images. In EPMA image, the concentrations of each element in observation area are indicated by the different colors. Light colors, for example, white, pink, and red, indicate that the amount of element is large, while dark colors like black and blue indicate that the amount of element is small.

Figure 2: EPMA image of TiO₂ film.

From these figures, it can be observed that the TiO₂ film with teeth-like shape was coated on a netlike glass fiber. It is also seen that TiO₂ film builds a bridge among several glass fibers like reported also in [38]. During firing process, the temperature profile of TiO₂ solution adhered on the netlike glass disc was not even due to the different thermal conductivities of Ti and SiO₂. The thermal conductivities of Ti and SiO₂ at 600 K are 19.4 W/(m·K) and 1.82 W/(m·K), respectively [39]. Due to the thermal expansion and shrinkage around the netlike glass fiber, a thermal crack formed on the TiO₂ film. Therefore, the TiO₂ film on the netlike glass fiber was teeth like.

It is observed from Figure 3 that nanosized Fe particles loaded on TiO₂ uniformly. On the other hand, it is revealed that the amount of loaded Fe increases with increasing the pulse number from Figures 4 and 5. In other words, there is an Fe layer covering TiO₂ for Fe/TiO₂(500) and Fe/TiO₂(1000).

Figure 3: EPMA image of Fe/TiO₂(100) film.

Figure 4: EPMA image of Fe/TiO₂(500) film.

Figure 5: EPMA image of Fe/TiO₂(1000) film.

To evaluate the amount of loaded Fe within the TiO₂ film quantitatively, the observation area, which is the center of netlike glass disc, of diameter of 300 μm, is analyzed by EPMA. The ratio of Fe to Ti in this observation area is counted by averaging the data obtained in this area. Table 1 gives the weight percentages of elements Fe and Ti in the Fe/TiO₂ film. From this table, it can be seen that more Fe is contained in the Fe/TiO₂ film with increasing pulse number. From these results, it is proved that the amount of dopants on a photocatalyst can be controlled by pulse arc plasm method quantitatively.

Table 1: Weight ratio of elements Fe and Ti within prepared Fe/TiO₂ film.

Photocatalyst type

Weight ratio

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

Fe/TiO₂(100)
98.16
1.84
100.00

Fe/TiO₂(500)
79.27
20.73
100.00

Fe/TiO₂(1000)
46.47
53.35
100.00

Figures 6–8 show EDX images of Fe/TiO₂(100), Fe/TiO₂(500), and Fe/TiO₂(1000) film, respectively. EDX analysis was carried out using 150,000 times magnification TEM images. In these figures, the yellow circle indicates the existence of Fe.

Figure 6: EDX image of Fe/TiO₂(100) film.

According to Figure 6, it is observed that Fe particle is loaded on TiO₂ film. The average size of Fe particle for Fe/TiO₂(100) in the present study is approximately 28 nm where the longest length of Fe particle was used as a representative length. As to Fe/TiO₂(500) shown in Figure 7, there was an Fe layer covering the TiO₂ film. The average thickness of Fe layer for Fe/TiO₂(500) in the present study was found to be approximately 36 nm. For Fe/TiO₂(1000), the thickness of Fe layer was thicker than that on the Fe/TiO₂(500). The average thickness of Fe layer for Fe/TiO₂(1000) in the present study is approximately 230 nm. Since the pulse arc plasm was repeated continuously with such a large pulse number of 1000, the tip of Fe electrode was melted and a lump of Fe was emitted. Therefore, the thickness of Fe layer for Fe/TiO₂(1000) was remarkably longer than that for Fe/TiO₂(500).

Figure 7: EDX image of Fe/TiO₂(500) film.

Figure 8: EDX image of Fe/TiO₂(1000) film.

Figure 9: Comparison of CO₂ reduction performance of TiO₂ and Fe/TiO₂ with H₂O under illumination conditions with UV light.

Figure 10: Comparison of CO₂ reduction performance of TiO₂ and Fe/TiO₂ with H₂ under illumination conditions with UV light.

Figure 11: Comparison of CO₂ reduction performance of TiO₂ and Fe/TiO₂ with NH₃ + H₂O under illumination conditions with UV light.

Figure 12: Comparison of CO₂ reduction performance of TiO₂ and Fe/TiO₂ with H₂O under illumination conditions without UV light.

Figure 13: Comparison of CO₂ reduction performance of TiO₂ and Fe/TiO₂ with H₂ under illumination conditions without UV light.

Figure 14: Comparison of CO₂ reduction performance of TiO₂ and Fe/TiO₂ with NH₃ + H₂O under illumination conditions without UV light.

Figure 15: Impact of reductant combination on CO₂ reduction performance of Fe/TiO₂(100) under illumination conditions with UV light.

3.2. Effect of Amount of Fe on CO₂ Reduction Performance of Fe/TiO₂

Figures 9–11 show the comparison of molar quantity of CO per weight of photocatalyst along the time under the Xe lamp with the UV light on, for TiO₂ or Fe/TiO₂ film under H₂O/CO₂, H₂/CO₂, and NH₃ + H₂O/CO₂ condition, respectively. In this experiment, CO is the only fuel produced from the reactions. 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, that is, the by-product, Figures 9–11 only show the concentration up to 72 hours. Before the experiments, a blank test, that 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 Figures 9 and 10, the molar quantity of CO per weight of photocatalyst for Fe/TiO₂(100) is the highest among the prepared photocatalysts under H₂O/CO₂ and H₂/CO₂ conditions. Although the amount of Fe increases with increasing pulse number as shown in Table 1, Fe layer covers TiO₂ film according to EPMA and EDX analysis. When the metal covers TiO₂ surface, the light absorption ability is impaired and the contact between TiO₂ and CO₂ + reductant is blocked [40]. Consequently, the Fe/TiO₂(100) which has enough area absorbing light and contacting reactants has the best CO₂ reduction performance. On the other hand, the molar quantity of CO per weight of photocatalyst for Fe/TiO₂(500) is the highest among the prepared photocatalysts under NH₃ + H₂O/CO₂ condition. Since six electrons are necessary to decompose NH₃ into H₂ and N₂ by photocatalyst [37] and CO₂ reduction with H₂ also need two electrons after NH₃ decomposition as described in Section 1, more electrons are used under NH₃ + H₂O/CO₂ condition compared to H₂O/CO₂ and H₂/CO₂ conditions. Therefore, the Fe/TiO₂(500) which can provide more electrons is the best photocatalyst even though Fe layer covers TiO₂ film. However, the CO₂ reduction performance of Fe/TiO₂ under NH₃ + H₂O/CO₂ condition is lower compared to that under H₂O/CO₂ and H₂/CO₂ conditions due to complicity of the reactions. In addition, a very thick Fe layer covers TiO₂ film for the Fe/TiO₂(1000), resulting in a very low CO₂ reduction performance. The improvement of photocatalytic performance by Fe doping under the illumination condition with UV light is thought to be caused by the generation of shallow charge traps in the crystal structure which decreases the recombination rate of electron-hole pairs [18] except for Fe/TiO₂(1000).

Figures 12–14 show the comparison of molar quantity of CO per weight of photocatalyst along the time under the Xe lamp illumination without UV light, for TiO₂ or Fe/TiO₂ film under H₂O/CO₂, H₂/CO₂, and NH₃ + H₂O/CO₂ condition, respectively. 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 hours for most cases due to the reverse reaction by CO and O₂ which is by-product, Figures 12–14 only show the concentration up to 96 hours.

From Figures 12–14, it can be seen that the CO₂ reduction performance of TiO₂ is promoted by Fe doping due to extension of the photoresponsivity of TiO₂ [41] to the visible spectrum as well as decrease in the recombination rate of electron-hole pairs by the generation of shallow charge traps in the crystal structure. According to Figures 12–14, the molar quantity of CO per weight of photocatalyst for Fe/TiO₂(500) is the highest among the prepared photocatalysts, which indicates that the larger pulse number is suitable for the illumination condition without UV light. Under the illumination condition without UV light, the amount of doped Fe is important to absorb the visible light in order to perform the photocatalytic reaction [41]. As to the Fe/TiO₂(1000), since the too-high loading Fe covered the TiO₂ surface, the light absorption ability was impaired and the contact between TiO₂ and CO₂ + reductant was blocked [40], resulting in lower CO₂ reduction performance compared to the other pulse numbers.

3.3. Optimization of Reductant on CO₂ Reduction Performance of Fe/TiO₂

To investigate the combination effect of reductant, Figure 15 shows the comparison of molar quantity of CO per weight of photocatalyst along the time under the Xe lamp with UV light on, for Fe/TiO₂(100) film under H₂ + H₂O/CO₂ condition. Due to the UV light illumination condition, Fe/TiO₂(100) is selected as the best photocatalyst as described in Section 3.2. For comparison, the results under H₂O/CO₂ and H₂/CO₂ conditions are also shown in this figure. Since the present study wants to know the CO₂ reduction performance obtaining a larger amount of product, the results under the illumination condition with UV light are shown.

From Figure 15, it reveals that the molar quantity of CO per weight for Fe/TiO₂ under H₂ + H₂O/CO₂ condition is larger than the sum of that under H₂O/CO₂ and H₂/CO₂ conditions, which agrees with the report carried out by TiO₂ [30]. Simultaneous presence of H₂O and H₂ in the system have not changed the reaction pathway, but have accelerated the photoreduction of CO₂ with H₂, since H₂O donated electron to inhibit the recombination of electron and hole [30]. In addition, the molar ratio of H₂ and H₂O to CO₂ of 1 : 0.5 : 1 gives the best CO₂ reduction performance. However, the molar ratio of H₂ and H₂O to CO₂ of 0.5 : 1 : 1 shows the second best performance, which means the moral ratio of total reductants to CO₂ of 1.5 : 1 is optimal. According to reaction scheme of H₂O/CO₂ and H₂/CO₂ shown in Section 1, the theoretical molar ratio of CO₂ to H₂O or H₂ is 1 : 1. It is therefore believed that the excess amount of reductant is necessary for CO₂ reduction in practice.

Table 2 lists the maximum molar quantity of CO per weight of photocatalyst for each condition to summarize the CO₂ reduction performance of prepared photocatalysts in this study. According to Table 2, the molar ratio of H₂ and H₂O to CO₂ of 1 : 0.5 : 1 gives the best CO₂ reduction performance in this study as described above. Table 3 lists the initial rate of CO production for each condition to investigate the CO₂ reduction rate of prepared photocatalysts in this study. The initial rate of CO production is estimated by dividing the molar quantity of CO per weight of photocatalyst at 24 h by illumination time of 24 h. According to Table 3, the initial rate of CO production under H₂ + H₂O/CO₂ condition is larger than that under the other conditions. This indicates that the combination of H₂ and H₂O leads the fast reaction of prepared photocatalyst since H₂ and H₂O promote the reduction and oxidation reaction during photocatalysis reaction, respectively.

Table 2: Comparison of CO₂ reduction performance of prepared photocatalysts in this study.

TiO₂ (μmol/g)
Fe/TiO₂(100) (μmol/g)
Fe/TiO₂(500) (μmol/g)
Fe/TiO₂(1000) (μmol/g)

With UV

H₂O/CO₂
5.2
8.2
7.9
3.0

H₂/CO₂
5.8
7.2
6.4
1.5

NH₃ + H₂O/CO₂
2.6
2.6
3.0
1.7

H₂ : H₂O : CO₂ = 1 : 0.5 : 1
N/A
33
N/A
N/A

H₂ : H₂O : CO₂ = 0.5 : 1 : 1
N/A
29
N/A
N/A

H₂ : H₂O : CO₂ = 0.5 : 0.5 : 1
N/A
27
N/A
N/A

H₂ : H₂O : CO₂ = 1 : 1 : 1
N/A
23
N/A
N/A

H₂ : H₂O : CO₂ = 2 : 2 : 1
N/A
17
N/A
N/A

Without UV

H₂O/CO₂
5.3 × 10⁻²
1.3 × 10⁻¹
1.5 × 10⁻¹
7.2 × 10⁻²

H₂/CO₂
6.6 × 10⁻²
1.2 × 10⁻¹
1.4 × 10⁻¹
6.9 × 10⁻²

NH₃ + H₂O/CO₂
3.2 × 10⁻²
1.0 × 10⁻¹
1.3 × 10⁻¹
1.1 × 10⁻¹

Table 3: Comparison of initial rate of CO production of prepared photocatalysts in this study.

TiO₂ (μmol/(g·h))
Fe/TiO₂(100) (μmol/(g·h))
Fe/TiO₂(500) (μmol/(g·h))
Fe/TiO₂(1000) (μmol/(g·h))

With UV

H₂O/CO₂
1.6 × 10⁻¹
2.7 × 10⁻¹
2.2 × 10⁻¹
9.5 × 10⁻²

H₂/CO₂
2.2 × 10⁻¹
2.8 × 10⁻¹
2.4 × 10⁻¹
5.3 × 10⁻²

NH₃ + H₂O/CO₂
7.4 × 10⁻²
8.9 × 10⁻²
9.3 × 10⁻²
5.3 × 10⁻²

H₂ : H₂O : CO₂ = 1 : 0.5 : 1
N/A
3.4 × 10⁻¹
N/A
N/A

H₂ : H₂O : CO₂ = 0.5 : 1 : 1
N/A
6.7 × 10⁻¹
N/A
N/A

H₂ : H₂O : CO₂ = 0.5 : 0.5 : 1
N/A
4.0 × 10⁻¹
N/A
N/A

H₂ : H₂O : CO₂ = 1 : 1: 1
N/A
6.0 × 10⁻¹
N/A
N/A

H₂ : H₂O : CO₂ = 2 : 2 : 1
N/A
4.5 × 10⁻¹
N/A
N/A

Without UV

H₂O/CO₂
1.3 × 10⁻³
3.6 × 10⁻³
4.7 × 10⁻³
2.7 × 10⁻³

H₂/CO₂
1.7 × 10⁻³
3.1 × 10⁻³
4.3 × 10⁻³
2.9 × 10⁻³

NH₃ + H₂O/CO₂
6.7 × 10⁻⁴
2.9 × 10⁻³
3.7 × 10⁻³
3.6 × 10⁻³

Compared to the previous research on CO₂ reduction with H₂ + H₂O over pure TiO₂, the CO₂ reduction performance of photocatalysts prepared in this study is almost 100 times as large as that reported in [30], which is owing to Fe doping. Though reference [30] reported the CO production performance of ZrO₂ under H₂ + H₂O/CO₂ condition, the CO production performance of Fe/TiO₂ prepared by this study is approximately 30 times as large as that of ZrO₂. In addition, the other research [42] also reported the CO₂ reduction performance of TiO₂ under H₂ + H₂O/CO₂ condition. The CO production performance of Fe/TiO₂ prepared by this study is approximately 100 times as large as that reported in reference [42]. Since Fe doping provides the free electron preventing recombination of electron and hole produced as well as improving the light absorption effect, the big improvement of CO₂ reduction performance is obtained in the present study, which reaches almost the same level compared to the other studies [7, 8, 11–15], in terms of CO₂ reduction performance. One way to further promote the CO₂ reduction performance is double overlapping arrangement of Fe/TiO₂ coated on netlike glass disc. The authors have already reported that the double overlapping arrangement of Fe/TiO₂ coated on netlike glass disc was effective to improve CO₂ reduction performance of Fe/TiO₂ under H₂O/CO₂ condition since the electron transfer between two overlapped photocatalysts was promoted by overlapping [38]. In addition, the other dopants like Cu, which can absorb the longer wavelength light than Fe [19], should be used at lower positioned layers since the wavelength of penetrating light becomes long through higher positioned photocatalyst by losing energy [38]. This idea is similar to the concept of the hybrid photocatalyst using two photocatalysts having different band gaps [43–45].

In the rear future, this study would like to carry out the reusability evaluation on the structure characterization and photocatalytic activity, which are important to investigate the possibility of applying a photocatalyst in a practical way.

4. Conclusions

Based on the investigation in this study, the following conclusions can be drawn.

(1)TiO₂ could be coated on netlike glass fiber where it appears in teeth-like shape. Fe fine particles could be loaded onto the TiO₂ using the pulse arc plasma method with various pulse numbers. When the pulse number was over 500, there would be a Fe layer formed on the TiO₂.(2)With the UV illumination condition, the molar quantity of CO per weight of photocatalyst for Fe/TiO₂(100) is the highest among the prepared photocatalysts under H₂O/CO₂ and H₂/CO₂ conditions, while that for Fe/TiO₂(500) is the highest among the prepared photocatalysts under NH₃ + H₂O/CO₂ condition.(3)Under the illumination condition without UV light, the molar quantity of CO per weight of photocatalyst for Fe/TiO₂(500) is the highest among the prepared photocatalysts irrespective of reductant used, indicating that the larger pulse number is suitable for the illumination condition without UV light.(4)The CO₂ reduction performance of Fe/TiO₂ under NH₃ + H₂O/CO₂ condition is lower compared to that under H₂O/CO₂ and H₂/CO₂ conditions due to complicity of the reaction.(5)Since a very thick Fe layer covers TiO₂ film for the Fe/TiO₂(1000) and blocks light absorption and reactant contact with TiO₂, the CO₂ reduction performance with Fe/TiO₂(1000) is very poor.(6)In this study, the highest CO₂ reduction performance of Fe/TiO₂, that is, the molar quantity of CO per weight of photocatalyst for Fe/TiO₂(100), is obtained under H₂ + H₂O/CO₂ condition, and the best moral ratio of total reductants to CO₂ is found to be 1.5 : 1.

Conflicts of Interest

The authors declare that there is no conflict of interest regarding the publication of this paper.

Acknowledgments

The authors would like to gratefully thank JSPS KAKENHI (Grant no. 16K06970) and joint research program of the Institute of Materials and Systems for Sustainability, Nagoya University, for the financial support of this work.

References

World Data Center for Greenhouse Gases, http://ds.data.jma.go.jp/gmd/wdcgg/wdcgg.html.

S. Das and W. M. A. W. Daud, “Photocatalytic CO₂ transformation into fuel: a review on advances in photocatalyst and photoreactor,” Renewable and Sustainable Energy Reviews, vol. 39, pp. 765–805, 2014.

T. Sakakura, J. C. Choi, and H. Yasuda, “Transformation of carbon dioxide,” Chemical Reviews, vol. 107, no. 6, pp. 2365–2387, 2007.

K. Adachi, K. Ohta, and T. Mizuno, “Photocatalytic reduction of carbon dioxide to hydrocarbon using copper-loaded titanium dioxide,” Solar Energy, vol. 53, no. 2, pp. 187–190, 1994.

A. Cybula, M. Klein, A. Z. Jurek, M. Janczarek, and A. Zaleska, “Carbon dioxide photoconversion. The effect of titanium dioxide immobilization conditions and photocatalyst type,” Physicochemical Problems of Mineral Processing, vol. 48, no. 1, pp. 159–167, 2012.

G. R. Dey, A. D. Belapurkar, and K. Kishore, “Photo-catalytic reduction of carbon dioxide to methane using TiO₂ as suspension in water,” Journal of Photochemistry and Photobiology A, vol. 163, no. 3, pp. 503–508, 2004.

P. N. Paulino, V. M. M. Salim, and N. S. Resende, “Zn-Cu promoted TiO₂ photocatalyst for CO₂ reduction with H₂O under UV light,” Applied Catalysis B: Environmental, vol. 185, pp. 362–370, 2016.

L. Liu, C. Zhao, J. T. Miller, and Y. Li, “Mechanistic study of CO₂ photoreduction with H₂O on Cu/TiO₂ nanocomposites by in situ x-ray absorption and infrared spectroscopies,” The Journal of Physical Chemistry C, vol. 121, pp. 490–499, 2017.

S. Kaneco, H. Kurimoto, Y. Shimizu, K. Ohta, and T. Mizuno, “Photocatalytic reduction of CO₂ using TiO₂ powders in supercritical fluid CO₂,” Energy, vol. 24, no. 1, pp. 21–30, 1999.

M. M. Gui, S. P. Chai, B. Q. Xu, and A. R. Mohamed, “Enhanced visible light responsive MWCNT/TiO₂ core-shell nanocomposites as the potential photocatalyst for reduction of CO₂ into methane,” Solar Energy Materials & Solar Cells, vol. 122, pp. 183–189, 2014.

L. L. Tan, W. J. Ong, S. P. Chai, and A. R. Mohamed, “Noble metal modified reduced graphene oxide/TiO₂ ternary nanostructures for efficient visible-light-driven photoreduction of carbon dioxide into methane,” Applied Catalysis B: Environmental, vol. 166-167, pp. 251–259, 2015.

A. A. Beigi, S. Fatemi, and Z. Salehi, “Synthesis of nanocomposite CdS/TiO₂ and investigation of its photocatalytic activity of CO₂ reduction to CO and CH₄ under visible light irradiation,” Journal of CO₂ Utilization, vol. 7, pp. 23–29, 2014.

B. Michalkiewicz, J. Majewska, G. Kadziolka, K. Bubacz, S. Mozia, and A. W. Morawski, “Reduction of CO₂ by adsorption and reaction on surface of TiO₂-nitorogen modified photocatalyst,” Journal of CO₂ Utilization, vol. 5, pp. 47–52, 2014.

C. W. Lee, R. A. Kourounioti, J. C. S. Wu et al., “Photocatalytic conversion of CO₂ to hydrocarbons by light-harvesting complex assisted Rh-doped TiO₂ photocatalyst,” Journal of CO₂ Utilization, vol. 5, pp. 33–40, 2014.

O. Ozcan, F. Fukruk, E. U. Akkaya, and D. Uner, “Dye sensitized CO₂ reduction over pure and platinized TiO₂,” Topics in Catalysis, vol. 44, no. 4, pp. 523–528, 2007.

J. A. Wang, R. L. Ballesteros, T. Lopez et al., “Quantitative determination of titanium lattice defects and solid-state reaction mechanism in iron-doped TiO₂ photocatalysts,” The Journal of Physical Chemistry B, vol. 105, pp. 9692–9698, 2001.

L. Laokiat, P. Khemthong, N. Grisdanurak, P. Sreearunothai, W. Pattanasiriwisawa, and W. Klysubun, “Photocatalytic degradation of benzene, toluene, ethylbenzene, and xylene (BTEX) using transition metal-doped titanium dioxide immobilized on fiberglass cloth,” Korean Journal of Chemical Engineering, vol. 29, no. 3, pp. 377–383, 2012.

Z. Ambrus, N. Balazs, T. Alapi et al., “Synthesis, structure and photocatalytic properties of Fe(III)-doped TiO₂ prepared from TiCl₃,” Applied Catalysis B: Environmental, vol. 81, no. 1-2, pp. 27–37, 2008.

K. Nagaveni, M. S. Hegde, and G. Madras, “Structure and photocatalytic activity of Ti1-xMxO2±δ (M = W, V, Ce, Zr, Fe, and Cu) synthesized by solution combustion method,” The Journal of Physical Chemistry B, vol. 108, pp. 20204–20212, 2004.

S. Das and W. M. A. W. Daud, “Photocatalytic CO₂ transformation into fuel: a review on advances in photocatalyst and photoreactor,” Renewable and Sustaibnable Energy Reviews, vol. 39, pp. 765–805, 2014.

H. Sun and S. Wang, “Research advances in the synthesis of nanocarbon-based photocatalysts and their applications for photocatalytic conversion of carbon dioxide to hydrocarbon fuels,” Energy & Fuels, vol. 28, pp. 22–36, 2014.

L. Liu and Y. Li, “Understanding the reaction mechanism of photocatalytic reduction of CO₂ with H₂O on TiO₂-based photocatalysts: a review,” Aerosol and Air Quality Research, vol. 14, pp. 453–469, 2014.

Y. Sohn, W. Huang, and F. Taghipour, “Recent progress and perspectives in the photocatalytic CO₂ reduction of Ti-oxide-based nanomaterials,” Applied Surface Science, vol. 396, pp. 1696–1711, 2017.

Z. Goren, I. Willner, A. J. Nelson, and A. J. Frank, “Selective photoreduction of CO₂/HCO₃⁻ to formate by aqueous suspensions and colloids of Pd-TiO₂,” Journal of Physical Chemistry, vol. 94, no. 9, pp. 3784–3790, 1990.

I. H. Tseng, W. C. Chang, and J. C. S. Wu, “Photoreduction of CO₂ using sol-gel derived titania and titania-supported copper catalysts,” Applied Catalysis B, vol. 37, no. 1, pp. 37-38, 2002.

A. Nishimura, N. Sugiura, M. Fujita, S. Kato, and S. Kato, “Influence of preparation conditions of coated TiO₂ film on CO₂ reforming performance,” Kagaku Kogaku Ronbunshu, vol. 33, no. 2, pp. 146–153, 2007.

Y. Kohno, H. Hayashi, S. Takenaka, T. Tanaka, T. Funabiki, and S. Yoshida, “Photo-enhanced reduction of carbon dioxide with hydrogen over Rh/TiO₂,” Journal of Photochemistry and Photobiology a, vol. 126, no. 1–3, pp. 117–123, 1999.

K. R. Thampi, J. Kiwi, and M. Gratzel, “Methanation and photo-methanation of carbon dioxide at room temperature and atmospheric pressure,” Nature, vol. 327, pp. 506-507, 1987.

T. Abe, M. Tanizawa, K. Watanabe, and A. Taguchi, “CO₂ methanation property of Ru nanoparticle-loaded TiO₂ prepared by a polygonal barrel-sputtering method,” Energy & Environmental Science, vol. 2, pp. 315–321, 2009.

C. C. Lo, C. H. Hung, C. S. Yuan, and J. F. Wu, “Photoreduction of carbon dioxide with H₂ and H₂O over TiO₂ and ZrO₂ in a circulated photocatalytic reactor,” Solar Energy Materials & Solar Cells, vol. 91, no. 19, pp. 1765–1774, 2007.

G. Mahmodi, S. Sharifnia, M. Madani, and V. Vatanpour, “Photoreduction of carbon dioxide in the presence of H₂, H₂O and CH₄ over TiO₂ and ZrO photocatalysts,” Solar Energy, vol. 97, pp. 186–194, 2013.

Y. Kohno, T. Tanaka, T. Funabiki, and S. Yoshida, “Photoreduction of CO₂ with H₂ over ZrO₂. A study on interaction of hydrogen with photoexcited CO₂,” Physical Chemistry Chemical Physics, vol. 2, pp. 2635–2639, 2000.

K. Teramura, H. Tsuneoka, T. Shishido, and T. Tanaka, “Effect of H₂ gas as a reductant on photoreduction of CO₂ over a Ga₂O₃ photocatalyst,” Chemical Physics Letters, vol. 467, no. 1–3, pp. 191–194, 2008.

Y. Kohno, T. Tanaka, T. Funabiki, and S. Yoshida, “Identification and reactivity of a surface intermediate in the photoreduction of CO₂ with H₂ over ZrO₂,” Journal of the Chemical Society, Faraday Transactions, vol. 94, no. 13, pp. 1875–1880, 1998.

K. Teramura, T. Tanaka, H. Ishikawa, Y. Kohno, and T. Funabiki, “Photocatalytic reduction of CO₂ to CO in the presence of H₂ or CH₄ as a reductant over MgO,” The Journal of Physical Chemistry B, vol. 108, pp. 346–354, 2004.

Y. Izumi, “Recent advances in the photocatalytic conversion of carbon dioxide to fuels with water and/or hydrogen using solar energy and beyond,” Coordination Chemistry Reviews, vol. 257, pp. 171–186, 2013.

J. Nemoto, N. Goken, H. Ueno, and M. Kaneko, “Photodecomposition of ammonia to dinitrogen and dihydrogen on platinized TiO₂ nanoparticles in an aqueous solution,” Journal of Photochemistry and Photobiology A: Chemistry, vol. 185, no. 2-3, pp. 295–300, 2007.

A. Nishimura, X. Zhao, T. Hayakawa, N. Ishida, M. Hirota, and E. Hu, “Impact of overlapping Fe/TiO₂ prepared by sol-gel and dip-coating process on CO₂ reduction,” International Journal of Photoenergy, vol. 2016, Article ID 2392581, 12 pages, 2016.

Japan Society of Mechanical Engineering, Heat Transfer Hand Book, Maruzen, Tokyo, 1st edition, 1993.

C. Zhao, L. Liu, G. Rao et al., “Synthesis of novel MgAl layered double oxide grafted TiO₂ cuboids and their photocatalytic activity on CO₂ reduction with water vapor,” Catalysis Science & Technology, vol. 5, pp. 3288–3295, 2015.

Y. Wang, F. Wang, Y. Chen et al., “Enhanced photocatalytic performance of ordered mesoporous Fe-doped CeO₂ catalysts for the reduction of CO₂ with H₂O under simulated solar irradiation,” Applied Catalysis B: Environmental, vol. 147, pp. 602–609, 2014.

J. Jensen, M. Mikkelsen, and F. C. Krebs, “Flexible substrates as basis for photocatalytic reduction of carbon dioxide,” Solar Energy Materials & Solar Cells, vol. 95, no. 11, pp. 2949–2958, 2011.

G. Marci, E. L. G. Lopez, and L. Palmisano, “Photocatalytic CO₂ reduction in gas-solid regime in the presence of H₂O by using GaP/TiO₂ composite as photocatalyst under simulated solar light,” Catalysis Communications, vol. 53, pp. 38–41, 2014.

G. Song, F. Xin, J. Chen, and X. Yin, “Photocatalytic reduction of CO₂ in cyclohexanol on CdS-TiO₂ heterostructured photocatalyst,” Applied Catalysis A: General, vol. 473, pp. 90–95, 2014.

G. Song, F. Xin, and X. Yin, “Photocatalytic reduction of carbon dioxide over ZnFe₂O₄/TiO₂ nanobelts heterostructure in cyclohexanol,” Journal of Colloid and Interface Science, vol. 442, pp. 60–66, 2015.