Impact of Rotation Speed of Ball Milling on P₄O₁₀ Size Thus on Promotion of CO₂ Reduction Performance with P₄O₁₀/TiO₂ Photocatalyst

Abstract

The aim of this study is to investigate the impact of the rotation speed of ball milling on the CO₂ reduction performance of P₄O₁₀/TiO₂. The rotation speeds studied were 600 rpm, 400 rpm and 200 rpm. It is revealed that the particle size of P₄O₁₀ within P₄O₁₀/TiO₂ prepared at the rotation speed of 600 rpm was the smallest among the investigated rotation speeds. It is revealed that the concentration of formed CO, as well as the molar quantity of CO per unit weight of photocatalyst P₄O₁₀/TiO₂, prepared at the rotation speed of 600 rpm in the case of CO₂:H₂O = 1:1 was the highest among the different molar ratios irrespective of light illumination condition. In the case of CO₂:H₂O = 1:1 under the light illumination conditions of UV and VIS and IR, VIS and IR, and IR, the following findings were obtained: The molar quantity of CO per unit weight of photocatalyst prepared at 600 rpm was 25.2 μmol/g under the light illumination condition of UV and VIS and IR, which was 20.3% more than that prepared at 400 rpm and 20.6% more than that prepared at 200 rpm. The molar quantity of CO per unit weight of rotation speed of 600 rpm under the light illumination condition of VIS and IR was 18.4 μmol/g, which was 21.3% more than that prepared at 400 rpm and 38.7% more than that prepared at 200 rpm. The molar quantity of CO per unit weight of photocatalyst prepared at 600 rpm was 11.9 μmol/g under the light illumination condition of IR, which was 1.8% more than that prepared at 400 rpm and 8.2% more than that prepared at 200 rpm.

1. Introduction

Photocatalysts are a promising technology for the utilization of solar light, i.e., renewable energy. The photocatalytic reduction reaction can convert CO₂ into fuel species such as CO, CH₄, CH₃OH, etc. [1,2,3]. TiO₂ is a popular photocatalyst used for CO₂ reduction. However, TiO₂ only works under ultraviolet (UV) light illumination, which accounts for only 4% of solar light [4]. On the other hand, visible light (VIS) and an infrared light (IR) account for 44% and 52% of the solar light energy reaching the Earth’s surface, respectively [4]. If a photocatalyst which can absorb VIS and IR could be developed, the performance of CO₂ reduction would be improved significantly.

Some previous studies have reported on extending the absorbed light to IR or near IR [5,6,7,8]. Under light wavelengths ranging from 200 nm to 2400 nm, a W₁₈O₄₉/g-C₃N₄ composite showed a production performance for CO of 45 μmol/g and CH₄ of 28 μmol/g [5]. It was reported that WS₂/Bi₂S₃ nanotubes exhibited a production performance for CH₃OH of 28 μmol/g and C₂H₅OH of 25 μmol/g, under light illumination conditions where the wavelength ranged from 420 nm to 1100 nm [6]. It was reported that CuInZnS decorated g-C₃N₄ exhibited a production performance for CO of 38 μmol/g with the illumination of light with wavelength ranges from 200 nm to 1000 nm [7]. It was reported that hierarichical ZnIn₂S₄ nanorods, which were prepared using the solvothermal method, exhibited a production performance for CO of 54 μmol/g and CH₄ of 9 μmol/g [8]. There is no previous study on TiO₂ absorbing IR except for the authors’ previous studies [9,10]. According to the authors’ previous study [9], the largest molar quantity of CO per unit weight of the photocatalyst for P₄O₁₀/TiO₂ film under IR light illumination conditions, in the case of CO₂/H₂O, was 2.36 μmol/g, and, in the case of CO₂/NH₃, was 33.4 μmol/g [9]. The authors’ previous studies [9,10] prepared P₄O₁₀ following [11].

The authors’ previous studies [9,10] set the rotation speed of ball milling to 600 rpm. This study proposes that the rotation speed of ball milling influences the particle size of P₄O₁₀. In addition, this study proposes that the particle size of P₄O₁₀ might depend on the performance of CO₂ reduction with H₂O. However, there is no studies investigating the impact of the rotation speed of ball milling on the CO₂ reduction performance of P₄O₁₀/TiO₂. Therefore, this study aims to investigate the impact of rotation speed of ball milling on the CO₂ reduction performance of P₄O₁₀/TiO₂. The reaction scheme of CO₂ reduction with H₂O [12,13,14] can be shown as follows:

<Photocatalytic reaction>

TiO₂ + hv → h⁺ + e⁻

(1)

<Oxidization reaction>

H₂O + h⁺ → OH + H⁺

(2)

·OH + H₂O + h⁺ → O₂ + 3H⁺

(3)

<Reduction reaction>

CO₂ + 2H⁺ + 2e⁻ → CO + H₂O

(4)

CO₂ + 8H⁺ + 8e⁻ → CH₄ + 2H₂O

(5)

2. Results and Discussion

2.1. Characterization of Prepared Photocatalyst

Figure 1 shows EPMA (electron probe microanalyzer) images of P₄O₁₀/TiO₂ film which is adhered to a netlike glass disk. We obtained the black and white SEM (scanning electron magnetron) images at 1500 times magnification, which were available for EPMA analysis. In Figure 1, the EPMA images with changing rotation speeds of 600 rpm, 400 rpm and 200 rpm are shown. On EPMA images, the concentrations of each element in the observation area are indicated by the diverse colors. If the amount of an element is high, light colors (e.g., white, pink and red) are used, while dark colors (e.g., black and blue), indicate low amounts of elements. We can see from Figure 1 that the TiO₂ film has a teeth-like shape when adhered to the netlike glass disk, regardless of rotation number. It is thought that the temperature distribution of TiO₂ solution adhered to the netlike glass disc was not even during the firing process because the thermal conductivity of Ti and SiO₂ at 600 K were 19.4 W/(m∙K) and 1.82 W/(m∙K), respectively [15]. Thermal expansion and shrinkage around the netlike glass fiber occurred. After that, a thermal crack formed within the TiO₂ film. Consequently, the TiO₂ film on the netlike glass disk was teeth-like.

Figure 2 shows SEM images at 7000 times magnification with varying rotation speeds of 600 rpm, 400 rpm and 200 rpm. Figure 3 shows a comparison of the distributions of particle size of prepared P₄O₁₀ for different rotation speeds. The average particle size diameter of P₄O₁₀ in the case of rotation speeds of 600 rpm, 400 rpm and 200 rpm was measured using SEM images. Since the particle shape was not always circular, this study measured the longest diameter and the shortest diameter for the ellipse-shaped P₄O₁₀ particles. The data shown in Figure 3 were obtained from the particle size measured by SEM image, where the rotation speed was 200 rpm. This study proposes that the measured numbers are reasonable according to the statistical analysis. The particles of P₄O₁₀ were suspended in TiO₂ sol in this study. The shape of P₄O₁₀ and the concentration of P₄O₁₀ in TiO₂ solution were maintained. Therefore, it can be claimed that the properties of TiO₂ sol were maintained after the introduction of P₄O₁₀ particles. In addition, the particles of P₄O₁₀ were inserted in TiO₂ sol to prepare the photocatalyst in this study. TiO₂ sol means the state of a colloid in a medium. Therefore, P₄O₁₀ particles can be distributed in TiO₂ sol. This study adopted the sol–gel process to prepare the photocatalyst. Consequently, the authors use the phrase “TiO₂ sol”. After that, the equivalent diameter had the same area as the ellipse.

The average diameter of the P₄O₁₀ particles in the case of the rotation speeds of 600 rpm, 400 rpm and 200 rpm were 0.523 μm, 0.762 μm and 0.746 μm, respectively. According to Figure 2 and Figure 3, the particle size of P₄O₁₀ in the case of the rotation speed of 600 rpm was the smallest. In addition, the particle sizes for the rotation speeds of 400 rpm and 200 rpm were similar. However, the ratio of particle size of P₄O₁₀ distributed from 0.1 μm to 0.4 μm in the case of 400 rpm was 35.5%, and in the case of 200 rpm was 27.0%. This meant that the smaller particles of P₄O₁₀ in the case of 400 rpm were larger than in the case of 200 rpm. This is because the share stress increases with the increase in the rotation speed, due to the increase in the rotation itself as well as the friction force [16]. In addition, the authors observed that the surface of a prepared photocatalyst particle becomes fine with the increase in the rotation speed. Moreover, the size of the prepared photocatalyst particles decreased with the increase in rotation speed.

This research studied the amount of loaded P₄O₁₀ within the TiO₂ film at the center of the netlike glass disk with a diameter of 300 μm. The ratio of P₄O₁₀ to Ti within P₄O₁₀/TiO₂ for the rotation speeds of 600 rpm, 400 rpm and 200 rpm was calculated at 3.21 wt%, 2.13 wt% and 1.94 wt%, respectively. From this result, we observe that the amount of loaded P₄O₁₀ increased with the increase in the rotation speed. The particle size of P₄O₁₀ decreases with the increase in the rotation speed due to the larger energy acting on the particle of P₄O₁₀. As a result, the number of deposited particles of P₄O₁₀ in the TiO₂ sol in the dipping process increased with the increase in rotation speed, indicating sufficient mixing of the particles of P₄O₁₀ with the TiO₂ sol.

To investigate the impact of rotation speed on the characteristics of prepared P₄O₁₀ particles, Figure 4 shows the data of XPS with changing rotation speed. According to Figure 4, the peak of detected intensity at approximately 135 eV is the highest, which indicates that the prepared P type material is P₄O₁₀ irrespective of the rotation speed, as the binding energy of P₄O₁₀ ranges from 135.0 eV to 135.9 eV [17].

Figure 5 shows all constituent element data from XPS analysis among different rotation speeds from this study. According to Figure 5, O, Ti and P are confirmed. Therefore, it can be said that TiO₂ and P₄O₁₀ have been prepared according to SEM, EPMA and XPS data obtained in this study.

2.2. CO₂ Reduction Performance of P₄O₁₀/TiO₂ with Changing Molar Ratio of CO₂/H₂O at the Rotation Speed of 600 rpm

Figure 6 shows a comparison of CO₂ reduction performance of P₄O₁₀/TiO₂ among different molar ratios of CO₂:H₂O under the light illumination conditions of UV, VIS and IR. In Figure 6, the rotation speed is 600 rpm. In addition, the concentration of formed CO and the formation rate of CO are shown. It is seen in Figure 5 that the concentration of formed CO, as well as the molar quantity of CO per unit weight of photocatalyst in the case of CO₂:H₂O = 1:1, is the highest among the different molar ratios. The concentration of formed CO is 539 ppmV. The molar quantity of CO per unit weight of photocatalyst is 25.2 μmol/g. This optimal molar ratio matches with the theoretical molar ratio to produce CO as explained by Equations (1)–(5). Therefore, it is revealed that the CO₂ reduction characteristics of P₄O₁₀/TiO₂ with H₂O performs with the same tendency of TiO₂. Figure 7 shows a comparison of formation rate of CO among different molar ratios. The formation rate of CO was saturated after 8 h of light illumination. We observed the peak of the production rate of CO at 2 h.

2.3. CO₂ Reduction Performance of P₄O₁₀/TiO₂ with Changing Rotation Speed and Light Illumination Conditions

Figure 8, Figure 9 and Figure 10 show a comparison of the rotation speed on the CO₂ reduction performance of P₄O₁₀/TiO₂ in the case of CH₄:CO₂ = 1:1 under the light illumination conditions of UV and VIS and IR, VIS and IR, and IR, respectively. According to Figure 8, Figure 9 and Figure 10, it is revealed that the concentration of formed CO, as well as the molar quantity of CO per unit weight of photocatalyst, is the highest for the rotation speed of 600 rpm, irrespective of light illumination condition. In Figure 8, the concentration of formed CO for the rotation speed of 600 rpm is 539 ppmV under the light illumination condition of UV and VIS and IR, which is 39.6% more than for 400 rpm and 44.5% more than for 200 rpm.

The molar quantity of CO per unit weight of photocatalyst for the rotation speed of 600 rpm is 25.2 μmol/g under the light illumination condition of UV and VIS and IR, which is 20.3% more than that prepared at 400 rpm and 20.6% more than prepared at 200 rpm. In Figure 8, the concentration of formed CO for the rotation speed of 600 rpm is 396 ppmV under the light illumination condition of VIS and IR, which is 41.4% more than that prepared at 400 rpm and 68.5% more than that prepared at 200 rpm. The molar quantity of CO per unit weight of photocatalyst for the rotation speed of 600 rpm is 18.4 μmol/g under the light illumination condition of VIS and IR, which is 21.3% more than that prepared at 400 rpm and 38.7% more than that prepared at 200 rpm. In Figure 10, the concentration of formed CO for the rotation speed of 600 rpm is 254 ppmV under the light illumination condition of IR, which is 9.8% more than that prepared at 400 rpm and 27.0% more than that prepared at 200 rpm. The molar quantity of CO per unit weight of photocatalyst for the rotation speed of 600 rpm is 11.9 μmol/g under the light illumination condition of IR, which is 1.8% more than that prepared at 400 rpm and 8.2% more than that prepared at 200 rpm.

The particle size of P₄O₁₀ decreases with the increase in the rotation speed according to Figure 3, resulting in the amount of P₄O₁₀ for the rotation speed of 600 rpm increasing compared to the other rotation speeds as described above. Therefore, it is thought that the CO₂ reduction performance for a rotation speed of 600 rpm exhibits a high CO₂ reduction performance. Moreover, it can be claimed that the CO₂ reduction performance is improved with the loading P₄O₁₀ under the light illumination conditions. Phosphorus has a layer structure that absorbs the light with wavelengths from UV to IR, according to the reference [18]. The layer thickness matches the wavelength of IR.

In addition, the observation area, the center of the netlike glass disk with a diameter of 300 μm, has been analyzed by EPMA to measure the amount of P within the TiO₂ film. We calculate the ratio of P to Ti by averaging the data detected in the observation area. The amount of element P within P₄O₁₀/TiO₂ film in the case of the rotation speed of 600 rpm, 400 rpm and 200 rpm is 3.21 wt%, 2.13 wt% and 1.94 wt%, respectively. Since the amount of P, i.e., P₄O₁₀ increases, it is thought that the performance of CO₂ reduction is promoted.

3. Experimental Procedure

3.1. Preparatioon Procedure of P₄O₁₀/TiO₂

The TiO₂ film was prepared using sol–gel and dip coating processes [19]. The preparation method of TiO₂ sol has previously been explained in the authors’ past studies [19,20]. This study prepared the P₄O₁₀/TiO₂ film using the sol–gel and dip coating process. Since the firing temperature of the photocatalyst is 343 K, the crystal type of TiO₂ prepared by this study was anatase. When the firing temperature is over 923 K, the crystal type of TiO₂ prepared by this study was rutile. Regarding P₄O₁₀ that is loaded on TiO₂, P₄O₁₀ was made from red P by means of mechanical synthesis [9]. The preparation method of P₄O₁₀ has been already explained in the authors’ previous study [9]. The rotation speed of a ball mill crusher was set at 600 rpm, 400 rpm and 200 rpm, though the authors’ previous study was set at 600 rpm [9]. Figure 11 shows the ball milling machine, Al₂O₃ balls for crushing red P and prepared P₄O₁₀.

3.2. Characterization Procedure of P₄O₁₀/TiO₂

The characteristics of the external and crystal structure of P₄O₁₀/TiO₂ film prepared were evaluated by SEM (JXA-8530F, JEOL Ltd., Akishima City, Japan) and EPMA (JXA-8530F, JEOL Ltd.) [21]. A netlike disk was adopted as a base material for adhering the P₄O₁₀/TiO₂ to before analyzing the characteristics. The thickness of the deposited carbon was approximately 2023 nm. The acceleration voltage and the current were set at 15 kV and 3.0 × 10⁻⁸ A, respectively, to analyze the external structure.

3.3. Experimental Procedure of CO₂ Reduction

Figure 12 shows the experimental apparatus: the reactor consisting of a stainless tube of 100 mm (H.) × 50 mm (I.D.); P₄O₁₀ coated on a netlike glass disk of 50 mm (D.) × 1 mm (t.) located on the Teflon cylinder of 84 mm (D.) × 10 mm (t.); a sharp cut filter that removed light with wavelengths below 400 nm (SCF-49.5C-42L, SIGMA KOKI Co., Ltf., Tokyo, Japan); a 150 W Xe lamp (L2174, Hamamatsu Photonics K. K., Hamamatsu City, Japan); a mass flow controller; and a CO₂ gas cylinder (purity: 99.995 vol%). The reactor chamber was 1.25 × 10⁻⁴ m³ in size. The light of the Xe lamp located on the stainless tube was directed toward the P₄O₁₀/TiO₂, passing through the sharp cut filter and the quartz glass disk positioned on the top of the stainless tube. The wavelength of the Xe lamp light ranged from 185 nm to 2000 nm. The sharp cut filter eliminated UV from the Xe lamp light. After the filter, the wavelength of light ranged from 401 nm to 2000 nm or 801 nm to 2000 nm, using a similar technique to the authors’ previous study [9].

The average light intensity from the Xe lamp without the cut filter for the rotation speeds of 600 rpm, 400 rpm and 200 rpm was 70.1 mW/cm², 70.3 mW/cm² and 70.0 mW/cm², respectively. The mean light intensity from the Xe lamp with the filter for the rotation speeds of 600 rpm, 400 rpm and 200 rpm, respectively, was 60.6 mW/cm², 60.7 mW/cm² and 60.4 mW/cm², respectively, for the wavelength between 401 nm and 2000 nm. In addition, the mean light intensity for the rotation speeds of 600 rpm, 400 rpm and 200 rpm, respectively, was 43.3 mW/cm², 40.3 mW/cm² and 39.9 mW/cm², respectively, for the wavelength between 801 nm and 2000 nm. We measured the light intensities using a light intensity meter located 55 mm away from the lamp, which was the distance between the Xe lamp and the prepared P₄O₁₀/TiO₂ photocatalyst during the CO₂ reduction experiment.

CO₂ gas with 99.995 vol% filled the vacuumed reactor chamber. After that, the valves at the inlet and the outlet of reactor were closed during the experiment. We set the pressure and the gas temperature at 0.1 MPa and 298 K, respectively, in the reactor. We injected liquid H₂O into the reactor by syringe via the gas sampling tap and turned the Xe lamp on simultaneously. We varied the amount of injected H₂O following the set molar ratio of CO₂:H₂O. The injected H₂O was vaporized due to the heat of the infrared light components from the Xe lamp. The temperature in the reactor was maintained at 343 K for an hour. Then, it was maintained at 343 K during the experiment. We varied the molar ratio of CO₂:H₂O to 1:0.5, 1:1, 1:2 and 1:4. The reacted gas in the reactor was extracted by a gas syringe via gas sampling tap. Then, it was analyzed by an FID gas chromatograph (GC353B, GL Science, Tokyo, Japan) and a methanizer (MT221, GL Science). The minimum resolution of FID gas chromatograph and methanizer was 1 ppmV. Regarding the recyclability of photocatalyst, the experiment was carried out three times for the P₄O₁₀/TiO₂ photocatalyst in this study.

4. Conclusions

This study has experimentally investigated the impact of the rotation speed of ball milling on the CO₂ reduction performance of P₄O₁₀/TiO₂. The rotation speeds studied were 600 rpm, 400 rpm and 200 rpm. As a result, the following conclusions are drawn:

(i)

It was revealed that the particle size of P₄O₁₀/TiO₂ prepared by the rotation speed of 600 rpm was the smallest among the investigated rotation speeds. The average diameter of P₄O₁₀ particles prepared at the rotation speeds of 600 rpm, 400 rpm and 200 rpm was 0.523 μm, 0.762 μm and 0.746 μm, respectively.

(ii)

It was revealed that the concentration of formed CO as well as the molar quantity of CO per unit weight of photocatalyst P₄O₁₀/TiO₂ prepared at the rotation speed of 600 rpm in the case of CO₂:H₂O = 1:1 is the highest among the different molar ratios, irrespective of light illumination condition.

(iii)

In the case of CO₂:H₂O = 1:1 under the light illumination condition of IR, the following findings were obtained: The concentration of formed CO for the rotation speed of 600 rpm was 254 ppmV, which is 9.8% more than that prepared at 400 rpm and 27.0% more than that prepared at 200 rpm. The molar quantity of CO per unit weight of photocatalyst for the rotation speed of 600 rpm was 11.9 μmol/g under the light illumination condition of IR, which is 1.8% more than that prepared at 400 rpm and 8.2% more than that prepared at 200 rpm.