Fe-doped TiO₂ (Fe/TiO₂) film photocatalyst was prepared by sol-gel dip-coating, and pulse arc plasma process. The netlike glass fiber was used as a base material for the photocatalyst since it had a porous structure. The CO₂ reduction performance with H₂ and H₂O into CO over the Fe/TiO₂ photocatalyst was investigated. In addition, this study investigated overlapping two Fe/TiO₂ coated on netlike glass fiber in order to utilize the light more effectively as well as increase the amount of photocatalyst for CO₂ reduction. The characterization of prepared Fe/TiO₂ film coated on netlike glass fiber was analyzed by SEM, EPMA, TEM, EDX and EELS. Furthermore, the CO₂ reduction performance of the Fe/TiO₂ film was tested under a Xe lamp with or without ultraviolet (UV) light, respectively. The results show that the CO₂ reduction performance peaks under the condition of CO₂/H₂/H₂O=1:1:0.5 in both cases with UV light and without UV light illumination. The highest concentration of CO with the Fe/TiO₂ overlapped photocatalyst is 1.2 times of that with the single Fe/TiO₂ photocatalyst. On the other hand, results also show the highest molar quantity of CO per weight of photocatalyst for Fe/TiO₂ overlapped is almost half of single Fe/TiO₂.

Keywords: photocatalyst, Fe-doped TiO₂, CO₂ reduction, reductants, overlapping effect

Due to mass consumption of fossil fuels, global warming and fossil fuels 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 278ppmV to 403ppmV by 2016.¹ Therefore, it is necessary to develop a new CO₂ reduction or utilization technology in order to recycle CO₂. There are six vital CO₂ conversions mechanisms: 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 (e.g. CO) has become an attractive route due to its economically and environmentally friendly behavior.² TiO₂ is the principle catalyst used for almost all types of photocatalysis reaction. It is well reported that CO₂ can be reduced into fuels e.g. CO, CH₄, CH₃OH, and H₂ etc. with TiO₂ as the photocatalyst under ultraviolet (UV) light illumination.^2,4−7 However, the CO₂ reduction performance of TiO₂ is still low since TiO₂ has a large bandgap energy (~3.2 eV) and a rapid electron hole recombination rate.⁵ 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.⁸⁻¹⁵ N-doped reduced graphene oxide promotion,⁸ modified using Ce and La for TiO₂ nanoparticles⁹, heterostructured g-C₃N₄/Ag-TiO_2,¹⁰ noble metal doping such as Pt, Pd, Rh, 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 shortcoming of the pure TiO₂. They did improve the CO₂ reduction performance, however, the amount of the products achieved in all the attempts were still unsatisfactory, ranging from 1mmol/g-cat to 350 mmol/g-cat.⁸⁻¹⁵ Therefore, a 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⁺ = 78.5pm, Ti⁴⁺ = 74.5pm)¹⁷ and can easily fit into the crystal lattice of TiO_2.^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 The performance of Fe-doped TiO₂ (Fe/TiO₂) was reported for degradation of oxalic acid,²⁰ rhodamine-B²¹ and methyl orange,²² resulting that the performance is superior to pure TiO₂. Extending the absorbance of TiO₂ into the visible region such as 600nm was also reported.¹⁸⁻²⁰ However, the application of Fe/TiO₂ for CO₂ reduction is not investigated well compared to phtocatalytic degradation. Due to easy availability as well as above described characteristics, Fe is selected as the dopant in the present study.

Since a reductant is necessary for CO₂ reduction to produce fuel, H₂O and H₂ are usually used as reductant according to the review papers.^5,7 To promote the CO₂ reduction performance, it is important to select the optimum reductant which provides the proton (H⁺) for the reduction reaction. The reaction scheme of CO₂ reduction with H₂O is as follows:²³⁻²⁵

<Photocatalytic reaction>

TiO₂ + hn → h⁺ + e^-                                                                                                                                                                    (1)

<Oxidization>

H₂O + h⁺ → ∙OH + H⁺                                                                                                                                                               (2)

∙OH + H₂O + h⁺ → O₂ + 3H⁺                                                                                                                                                    (3)

<Reduction>

CO₂ + 2H⁺ + 2e^- → CO + H₂O                                                                                                                                                 (4)

The reaction scheme of CO₂ reduction with H₂ is as follows:²⁶

<Photocatalytic reaction>

TiO₂ + hn → h⁺ + e^-                                                                                                                                                                    (5)

<Oxidization>

H₂ + 2h⁺ → 2H⁺ + 2e^-                                                                                                                                                                (6)

<Reduction>

CO₂ + e^- → ∙CO₂^-                                                                                                                                                                 (7)

∙CO₂^- + H⁺ + e^- → HCOO^-                                                                                                                                                        (8)

HCOO^- + H⁺ → CO + H₂O                                                                                                                                                       (9)

Though there are some reports on CO₂ reduction with H₂O or H₂,^5,7 the effect of combination of H₂O and H₂ is not investigated well. Though a few studies using pure TiO₂ under CO₂/H₂/H₂O condition were reported,²⁶⁻²⁸ the combined effect of ratio of CO₂, H₂ and H₂O and metal doping on CO₂ reduction performance were not investigated. Though Nishimura et al.²⁹ reported the combined effect of ratio of CO₂, H₂ and H₂O over Fe/TiO₂ under the illumination condition with UV light, they did not investigate that under the illumination condition without UV light. To clarify the effect of doping Fe, it is important to examine the CO₂ reduction performance under the illumination condition without UV light. In addition, the previous study²⁹ did not investigate to promote the CO₂ reduction performance for different ratios of CO₂, H₂ and H₂O, e.g., by changing the amount of phototcatalyst or overlapping two photocatalysts. Therefore, the aim of the present study is to clarify the optimum ratio of CO₂/H₂/H₂O for promoting the CO₂ reduction performance with Fe/TiO₂ photocatalyst.

In this study, TiO₂ film is coated by sol-gel and dip-coating process on netlike glass fiber (SILIGLASS U, Nihonmuki Co.). The netlike glass fiber is a net composed of glass fiber whose diameter is about 10 mm. The fine glass fibers are knitted, resulting that the diameter of aggregate fiber is about 1mm. According to manufacturer’s specifications of netlike glass fiber, the porous diameter of glass fiber is about 1nm and the specific surface area is about 400m²/g. The netlike glass fiber consists of SiO₂ whose purity is over 96 wt%. The aperture of net is about 2mm×2mm. Since the netlike glass fiber has a porous characteristic, it is believed that TiO₂ film is captured by netlike glass fiber easily during sol-gel and dip-coating process. In addition, it can be expected a CO₂ absorption performance of prepared photocatalyst is promoted due to the porous structure of 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 uniformly by applying high electrical potential difference. The amount of loaded Fe can be controlled by pulse number. In this study, the pulse number is set at 100.

Since the netlike glass fiber has aperture of net, the light can pass through the netlike glass fiber. The present study has investigated overlapping two Fe/TiO₂ coated on netlike glass fiber in order to utilize the light more effectively as well as increase the amount of photocatalyst for CO₂ reduction. Though the effect of overlapping two Fe/TiO₂ coated on netlike glass fiber on CO₂ reduction performance with H₂O was reported by Nishimura et al.,³⁰ it was not investigated for CO₂/H₂/H₂O. In addition, Nishimura et al.³⁰ prepared Fe/TiO₂ by sol-gel dip-coating process only. Since it is difficult to dope a metal on TiO₂ uniformly by sol-gel dip-coating process,³⁰ this study selects the pulse arc plasma process to dope Fe. It is believed that electron transfer between overlapping two Fe/TiO₂ coated on netlike glass fiber is carried out.³⁰ Therefore, it is expected that the CO₂ reduction performance is promoted under many H⁺ production condition such as CO₂/H₂/H₂O due to matching the electron according to Eqs. (1)-(9).

In this study, Fe/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 H₂ and H₂O using Fe/TiO₂ coated on netlike glass fiber as photocatalyst under the condition of illuminating Xe lamp with or without UV light were investigated. The molar ratio of CO₂/H₂/H₂O was set at 1:0.5:0.5, 1:1:0.5, 1:0.5:1, and 1:1:1 to determine the optimum ratio of CO₂/H₂/H₂O. In addition, the effect of overlapping two Fe/TiO₂ coated netlike glass fiber on CO₂ reduction performance was investigated.

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 0.3mol, 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. Netlike glass fiber was cut to disc, and its diameter and thickness were 50mm and 1mm, respectively. The netlike glass disc was dipped into TiO₂ sol solution at the speed of 1.5mm/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), resulting that TiO₂ film was fastened on the base material. FT and FD were set at 623K and 180s, respectively. Fe was then loaded on TiO₂ film by pulse arc plasma method. The pulse arc plasma gun device (ULVAC, Inc., ARL-300) having Fe electrode whose diameter was 10mm 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, where was vacuumed, the nanosized Fe particles were emitted from Fe electrode with applying the electrical potential difference of 200V. The pulse arc plasma gun can evaporate Fe particle over the target in the circle area whose diameter is 100 mm when the distance between Fe electrode and the target is 160mm. Since the distance between Fe electrode and TiO₂ film was 150 mm, Fe particle can be evaporated over TiO₂ film uniformly. The amount of loaded Fe was controlled by pulse number. In this study, the pulse number was set at 100. To promote the CO₂ reduction performance, overlapping layout of two photoatalysts were investigated. The photocatalyst coated on both upper and lower surfaces of netlike glass disc was overlapped over the other photocatalyst coated on upper surface of netlike glass disc.

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.), EDX (JEM-2100F/HK, JEOL Ltd.) and EELS (JEM-ARM2007 Cold, JEOL Ltd.). Since these measuring instruments use electron for analysis, the sample should be an electron conductor. Since 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 netlike glass disc before analysis. The thickness of carbon deposited on samples was approximately 20-30nm.

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 that the concentration of chemical element is analyzed according to the relationship between the characteristic X-ray energy and the atomic number. The spatial resolution of SEM and EPMA is 10mm. 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 Fe 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 can be analyzed. In this study, the concentration distributions of Ti and Fe were analyzed.

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

CO₂ reduction experiment

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

Figure 1 Schematic drawing of CO₂ reduction experimental set-up.

The reactor has volume of 1.25× 10^-4m³, which is available for CO₂. The light of 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 wavelength of light from Xe lamp is ranged from 185 nm to 2000nm. The Xe lamp can be fitted with a sharp cut filter to remove UV components of the light. With the filter, the wavelength of light from Xe lamp is ranged from 401nm to 2000nm. Figure 2 shows the light transmittance data of the sharp cut filter to prove the removal of the light whose wavelength is below 400nm. The average light intensity of Xe lamp on the photocatalyst without and with setting the sharp cut filter is 75.3 mW/cm² and 68.2 mW/cm², respectively.

Figure 2 Light transmittance data of sharp cut filter.

In the CO₂ reduction experiment with H₂ and H₂O, CO₂ gas of 99.995 vol% purity and H₂ gas of 99.99999 vol% purity were mixed in the buffer chamber, and introduced in the reactor which was pre-vacuumed by a vacuum pump. The mixing ratio of CO₂ and H₂ was controlled by a mass flow controller and confirmed by a TCD gas chromatogragh (Micro GC CP4900, GL Science) before introducing into the reactor. After confirming the mixing ratio of CO₂ and H₂, the distilled water was injected into the reactor through gas sampling tap and Xe lamp illumination was turned on at the same time. The molar ratio of CO₂/H_2/H₂O was set at 1:0.5:0.5, 1:1:0.5, 1:0.5:1, and 1:1:1, respectively. The injected water vaporized completely in the reactor. Due to the heat of Xe lamp, the temperature in reactor was attained at 343K within an hour and kept at approximately 343K during the experiment.

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 1ppmV.

Characterization of Fe/TiO₂ film

Figures 3 & 4 show EPMA images of TiO₂ and Fe/TiO₂ film, respectively. 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.

From these figures, it can be observed that TiO₂ film with teeth like shape was coated on netlike glass fiber. It is also seen that TiO₂ film builds a bridge among several glass fibers like reported also in.³⁰ 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. Their thermal conductivities of Ti and SiO₂ at 600 K are 19.4 W/(mK) and 1.82 W/(mK), respectively.³¹ Due to the thermal expansion and shrinkage around netlike glass fiber, thermal crack formed on TiO₂ film. Therefore, TiO₂ film on netlike glass fiber was teeth like. As to Fe, it is observed from Figure 3 that nanosized Fe particles loaded on TiO₂ uniformly, resulting from that the pulse arc plasma method can emit nanosized Fe particles.

Figure 3 EPMA image of TiO₂ film.

Figure 4 EPMA image of Fe/TiO₂ film.

To evaluate the amount of loaded Fe within TiO₂ film quantitatively, the observation area, which is the center of netlike glass disc, of diameter of 300 mm is analyzed by EPMA. The ratio of Fe to Ti in this observation area is counted by averaging the data obtained in this area. As a result, the weight percentages of elements Fe and Ti in the Fe/TiO₂ film are 1.84 wt% and 98.16 wt%, respectively.

Figure 5 shows EDX images of Fe/TiO₂. EDX analysis was carried out using 160000 times magnification TEM images. In these figures, the yellow circle indicates the existence of Fe. From this figure, it is observed that Fe particle is loaded on TiO₂ film, and no Fe layer has formed. The average size of Fe particle for Fe/TiO₂ loaded is approximately 28 nm where the longest length of Fe particle was used as a representative length.

Figure 6 shows EELS spectra of Fe in Fe/TiO₂ film. From this figure, the peaks at around 708eV and 720eV can be observed. Compared to EELS data base,³² the EELS spectra of Fe₂O₃ (Hematite) matches with Figure 6. Therefore, Fe in Fe/TiO₂ prepared presents as Fe³⁺ ion in Fe₂O₃. Since the doped Fe emits the electron during the photochemical reaction and the electron plays a part in the fuel production from CO₂ under CO₂/H₂O condition²³⁻²⁵ and CO₂/H₂ condition,²⁶ the ion number of Fe is important. For Fe/TiO₂ photocatalyst, the incorporated Fe³⁺ ions can substitute the octahedrally coordinated Ti⁴⁺ ions in TiO₂ lattice, which extends the photo-response of TiO₂ to the visible light region and promotes the formation of electron-hole pair significantly.³³ Since the Fe³⁺ ion acts as an electron-trapped agent, Fe/TiO₂ exhibits a low recombination rate of electron-hole pairs and high separation efficiency.³³

Effect of molar ratio of CO₂, H₂ and H₂O on CO₂ reduction characteristics of Fe/TiO₂

Figures 7 and 8 show the concentration changes of CO produced and the molar quantity of CO per weight of photocatalyst in the reactor along the time under the Xe lamp with UV light on, respectively. The amount of Fe/TiO₂ on the netlike glass disc is 0.10g. In this experiment, CO is the only fuel produced from the reactions. 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.

Figure 5 TEM image of Fe/TiO₂ film.

Figure 6 EELS spectra of Fe in Fe/TiO₂.

Figure 7 Comparison of concentration of CO among several moral ratios of CO₂/H₂/H₂O under the illumination condition with UV light.

Figure 8 Comparison of molar quantity of CO per unit weight of photocatalyst among several moral ratios of CO₂/H₂/H₂O under the illumination condition with UV light.

According to Figures 7 & 8, the CO₂ reduction performance is the highest for the moral ratio of CO₂/H₂/H₂O=1:1:0.5. Since the reaction scheme of CO₂/H₂/H₂O is not clarified, this study refers to the reaction scheme of CO₂/H₂O and CO₂/H₂ as shown by Eqs. (1) – (9). It is known from Eqs. (1) – (9) that 2 H⁺ and 2 electrons are necessary for producing CO in the case of CO₂/H₂O as well as that of CO₂/H₂. In other words, the same number of H⁺ and electron is needed. According to Figure 6, Fe in Fe/TiO₂ prepared is in the form of Fe³⁺ ion in Fe₂O₃. Therefore, it is believed that 3 electrons are emitted per Fe, resulting that the electron rich state is formed. According to Eqs. (2), (3) and (6), H⁺ is produced by the oxidization of reductant. Comparing H₂O with H₂, H₂O is a weaker reductant,³⁴ resulting that larger amount of H₂ is necessary to match the electron rich state. On the other hand, the lowest CO₂ reduction performance is obtained for the moral ratio of CO₂/H₂/H₂O = 1:0.5:1 according to Figures 7 and 8. This result also shows that larger moral ratio of H₂ compared to H₂O is necessary for the better CO₂ reduction performance of Fe/TiO₂.

Figures 9 & 10 show the concentration changes of CO produced and the molar quantity of CO per weight of photocatalyst in the reactor along the time under the Xe lamp without UV light on, respectively. In this experiment, CO is the only fuel produced from the reactions.

According to Figures 9 & 10, the CO₂ reduction performance is the best for the moral ratio of CO₂/H₂/H₂O=1:1:0.5 in the case of illumination condition with UV light. It is considered that the same reaction mechanism as mentioned above is occurred. The order of CO₂ reduction performance of Fe/TiO₂ without UV light is the same as that with UV light. It is revealed that the wave length of light doesn’t influence on the CO₂ reduction characteristics of Fe/TiO₂ with H₂ and H₂O.

Figure 9 Comparison of concentration of CO among several moral ratios of CO₂/H₂/H₂O under the illumination condition without UV light.

Figure 10 Comparison of molar quantity of CO per unit weight of photocatalyst among several moral ratios of CO₂/H₂/H₂O under the illumination condition without UV light.

Effect of overlapping on CO₂ reduction characteristics of Fe/TiO₂

Figures 11 & 12 show the concentration changes of CO produced and the molar quantity of CO per weight of photocatalyst in the reactor along the time under the Xe lamp with UV light on, for Fe/TiO₂ film coated on netlike glass disc overlapped, respectively. The total amount of Fe/TiO₂ overlapped is 0.25g.

Figure 11 Comparison of concentration of CO for Fe/TiO₂ overlapped among several moral ratios of CO₂/H₂/H₂O under the illumination condition with UV light.

Figure 12 Comparison of molar quantity of CO per unit weight of photocatalyst for Fe/TiO₂ overlapped among several moral ratios of CO₂/H₂/H₂O under the illumination condition with UV light.

According to Figures 11 and 12, the moral ratio of CO₂/H₂/H₂O = 1:1:0.5 generates the best CO₂ reduction performance among various ratios, which is the like the case of the single Fe/TiO₂. In addition, the order of CO₂ reduction performance of Fe/TiO₂ overlapped is the same as that of single Fe/TiO₂. However, comparing Figure 11 with Figure 7, the concentration of CO for Fe/TiO₂ overlapped is higher than that for single Fe/TiO₂ under every molar ratio of CO₂/H₂/H₂O. The highest concentration of CO for Fe/TiO₂ overlapped is 993ppmV which is 1.2 times as large as that of single Fe/TiO₂. When overlapped, the following things may occur:

1. The total amount of photocatalyst used for photocatalysis reaction is increased.
2. The electron transfer between two Fe/TiO₂ films promotes the activity of photocatalysis reaction.
3. The lower positioned Fe/TiO₂ film utilizes the light passing through the higher positioned Fe/TiO₂

However, comparing Figure 12 with Figure 8, the molar quantity of CO per weight of photocatalyst for Fe/TiO₂ overlapped is lower than that for single Fe/TiO₂ under every molar ratio of CO₂/H₂/H₂O. The highest molar quantity of CO per weight of photocatalyst is 18mmol/g which is half of single Fe/TiO₂. The reasons of this result are considered as follows:

1. Some area of lower positioned Fe/TiO₂ film can’t receive the adequate light.
2. If the produced fuel CO remains in the space between two discs, the reactant of CO₂, H₂ and H₂O may be blocked to certain degree to reach the surface of photocatalyst, resulting that the photochemical reaction could not be carried out well even though the light is illuminated for photocatalyst. The mass and electron transfer mechanism for Fe/TiO₂ overlapped is illustrated in Figure 13.

From these results, the effective light utilization and mass transfer promotion are necessary for improvement of CO₂ reduction performance of Fe/TiO₂ overlapped.

Figures 14 &15 show the concentration changes of CO produced and the molar quantity of CO per weight of photocatalyst in the reactor along the time under the Xe lamp without UV light on, for Fe/TiO₂ film coated on netlike glass disc overlapped, respectively.

Figure 13 Mass and electron transfer mechanism for Fe/TiO₂ overlapped.

Figure 14 Comparison of concentration of CO for Fe/TiO₂ overlapped among several moral ratios of CO₂/H₂/H₂O under the illumination condition without UV light.

Figure 15 Comparison of molar quantity of CO per unit weight of photocatalyst for Fe/TiO₂ overlapped among several moral ratios of CO₂/H₂/H₂O under the illumination condition without UV light.

According to Figures 14 & 15, the moral ratio of CO₂/H₂/H₂O=1:1:0.5 again generates the best CO₂ reduction performance. The order of CO₂ reduction performance of Fe/TiO₂ overlapped is the same as that of single Fe/TiO₂. However, comparing Figure 14 with Figure 9, the maximum concentration of CO for Fe/TiO₂ overlapped is higher than that for single Fe/TiO₂ under every molar ratio of CO₂/H₂/H₂O. The highest concentration of CO for Fe/TiO₂ overlapped is 135ppmV which is about 1.2times as large as that for single Fe/TiO₂. The same reasons can explain this result as the illumination condition with UV light.

However, comparing Figure 15 with Figure 10, the molar quantity of CO per weight of photocatalyst for Fe/TiO₂ overlapped is lower than that for single Fe/TiO₂ under every molar ratio of CO₂/H₂/H₂O. The highest molar quantity of CO per weight of photocatalyst for overlapped case is only 3.0mmol/g which is almost half of single Fe/TiO₂.

In this study, the highest molar quantity of CO per weight of photocatalyst is 36mmol/g for single Fe/TiO₂ under the illumination condition with UV light. Compared to the previous research on CO₂ reduction with H₂ and H₂O over pure TiO₂, the CO production performance of photocatalysts prepared in this study is approximately 120 times as large as that reported in references,^26,28 which is owing to Fe doping. The CO production performance of Fe/TiO₂ prepared in this study is approximately 36 times as large as that of ZrO₂ reported in the reference.²⁶ This is due to that the doped Fe provides the free electron preventing recombination of electron and hole produced as well as improving the light absorption effect. Another way to further promote the CO₂ reduction performance may be using the netlike porous metal which has an appropriate area of aperture as a base material instead of netlike glass fiber. Since the netlike porous metal has good electrical conductivity, light permeability, and gas diffusivity, it can be expected that the CO₂ reduction performance can be further promoted. In addition, the other dopant 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.³⁰ This proposal is similar to the concept of the hybrid photocatalyst using two photocatalysts having different band gaps.³⁵⁻³⁷

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 teeth like shape. Fe fine particles could be loaded on TiO₂
2. Fe in Fe/TiO₂ prepared in this study is in the form of Fe³⁺ ion in Fe₂O₃.
3. Under the illumination condition with UV as well as without UV, the concentration of produced CO and the molar quantity of CO per weight of photocatalyst for Fe/TiO₂ are the highest with the ratio of CO₂/H₂/H₂O = 1:1:0.5. Since Fe in Fe/TiO₂ emits three electrons, the larger H₂ which is stronger reductant compared to H₂O is necessary to match the electron rich state.
4. Under the illumination condition with UV as well as without UV, the concentration of produced CO for Fe/TiO₂ overlapped is higher than that for single Fe/TiO₂. The highest concentration of CO for Fe/TiO₂ overlapped is 1.2 times as large as that for single Fe/TiO₂.
5. However, the molar quantity of CO per weight of photocatalyst for Fe/TiO₂ in overlapped case is lower than that for single Fe/TiO₂ The highest molar quantity of CO per weight of photocatalyst for Fe/TiO₂ overlapped is just half of single Fe/TiO₂.
6. The CO₂ reduction performance of prepared Fe/TiO₂ is superior to the previous studies with TiO₂ under CO₂/H₂/H₂O condition.

The authors would like to gratefully thank from JSPS KAKENHI Grant Number 16K06970, and joint research program of the Institute of Materials and Systems for Sustainability, Nagoya University for the financial support of this work.

The authors declare that there is no conflicts of interest.

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