Effects of Co3O4 Cocatalyst on InTaO4 for Photocatalytic Reduction of CO2 to CH3OH under Visible Light Irradiation

Pei-Wen Pan¹, Yu-Wen Chen^1*, Anton S. Brichkov², Vladimir V. Kozik²

¹Department of Chemical Engineering, National Central University, Taoyuan City.
²Department of Chemistry, Tomsk State University, Tomsk, Russia.

DOI: 10.4236/mrc.2019.84004                

Abstract

InTaO₄ was synthesized by a solid-state reaction method using metal oxide as the starting materials. Co was added by incipient-wetness impregnation. The sample was pretreated by H₂ (200 Torr) reduction at 500?C for 2 h and subsequent O₂ (100 Torr) oxidation at 200?C for 1 h. The core-shell structure of metallic Co and Co₃O₄ was formed by this reduction-oxidation procedure. The catalysts were characterized by powder X-ray diffraction, scanning electron microscope, and ultraviolet-visible spectroscope. The photocatalytic reduction was carried out in a Pyrex reactor with KHCO₃ or NaOH aqueous solution bubbled with ultra pure CO₂ gas under visible light illumination. SEM micrographs show many small Co₃O₄ particles on the surface of InTaO₄. The band gap of Co₃O₄-InTaO₄ was 2.7 eV, confirming that these catalysts have the ability to reduce CO₂ to methanol. The methanol yield increased with the amount of Co₃O₄ cocatalysts. The catalyst had a higher activity in KHCO₃ aqueous solution than in NaOH solution. The InTaO₄ catalyst with 1 wt% Co₃O₄ cocatalyst had the highest activity among all catalysts. Co₃O₄ was incorporate into the surface structure of InTaO₄ to form Co_xInTaO_4-x. It resulted in more defect sites on the surface of InTaO₄ and changed the valence band structure. It formed a Schottky barrier to suppress the electron-hole recombination.

Keywords

Carbon Dioxide, Utilization, Photoreduction, Methanol Formation, Visible Light Irradiation

1. Introduction

Photocatalytic reduction of carbon dioxide to methane and methanol has been extensively studied by many researchers [1] - [6]. Anpo et al. [7] carried out a series of research on Ti-zeolite and Ti-mesoporous materials since 1997. Several photocatalysts were reported, such as Ti-oxide/Y-zeolite [8], Ti-MCM-41 [9], Ti-MCM-48 [9], FSM-16 [10] [11], Ti-β zeolite [12], and self-standing porous silica thin films [13] [14] [15], etc. It is important to use the catalysts with low energy band gap, because the lower the band gap is, the easier the photon excited [16] [17] [18] [19].

InMO₄ (M = Ta, Nb, V) catalysts have been reported as photoactive for water splitting reaction under visible light [20] [21] [22] [23]. According to the band structures of InTaO₄, the photoreduction of carbon dioxide on InTaO₄ catalysts should be feasible. Our previous study [24] showed that NiO-InTaO₄ was active for photoreduction of CO₂ to produce methanol. It has been reported that other cocatalysts such as Co₃O₄ [25] [26] are effective. However, it has not been reported in literature for photoreduction of CO₂ [27] [28] [29].

In this study, the Co₃O₄-InTaO₄ with various Co₃O₄ contents was synthesized. The catalysts were characterized by powder X-ray diffraction, scanning electron microscope, and ultraviolet-visible spectroscopy. The photocatalytic activities of Co₃O₄-InTaO₄ photocatalysts for CO₂ reduction under visible light irradiation were investigated.

2. Experimental

2.1. Catalyst Preparation

The polycrystalline InTaO₄ was synthesized by a solid-state reaction method as reported in literature [24]. The pre-dried In₂O₃ and Ta₂O₅ were used as the starting materials. The stoichiometric amounts of precursors were mixed and reacted in an aluminum crucible in air at 1100˚C for 12 h. The material was stirred at least 3 times during preparation to ensure well mix of starting materials.

Co₃O₄-InTaO₄ samples with various Co₃O₄ cocatalyst (0.3 wt%, 0.5 wt% and 1 wt%, respectively) were prepared by incipient-wetness impregnation with aqueous solution of Co(NO₃)₂. After preparation, the sample was heated by a water bath at 100˚C. The dried powder was then calcined at 400˚C for 4 h in an oven. The sample was pretreated by H₂ (200 Torr) reduction at 500˚C for 2 h and subsequent O₂ (100 Torr) oxidation at 200˚C for 1 h. The core-shell structure of metallic Co and Co₃O₄ was formed by this reduction-oxidation procedure.

2.2. Catalyst Characterization

2.2.1 X-Ray Diffraction (XRD)

The XRD experiments were performed using a Siemens D-500 powder diffractometer with Cu-K_α radiation (40 kV, 41 mA), 0.024˚ step size and 1 sec step time from 5˚ to 90˚. The detailed experimental procedure has been reported in the previous literature [25]. The Bragg-Brentano focusing geometry was employed with an automatic divergence slit (irradiated sample length was 12.5 nm), a receiving slit of 0.1 nm, a fixed slit of 4˚ and a proportional counter as a detector.

2.2.2. Scanning Electron Microscopy (SEM)

The detailed experimental procedure has been reported in previous literature [25]. Briefly, the samples were placed on an aluminum stage specially made for SEM. The samples were sputter-coated with Au for 90 s before the experiment began. The microstructure and morphology of the materials were examined using a scanning electron microscope (Hitachi S-800) with a field gun. An accelerating voltage of 20 kV was used. The composition of the samples was determined by X-ray energy dispersion spectrum (EDS) with accelerating voltage of 20 kV.

2.2.3. Ultraviolet-Visible Spectroscopy (UV-vis)

The diffuse reflectance UV-vis was measured with a Cary 300 Bio UV-visible Spectrophotometer. Powder samples were loaded in a quartz cell with Suprasil windows, and the spectra were collected in the range from 300 nm to 800 nm against quartz standard.

2.3. Photocatalytic Reaction

Photocatalytic reactions were carried out in a continuous flow reactor. The detailed reaction procedure was described in previous literature [25]. The catalyst powder ( 0.14 g ) was dispersed in a reactant solution (50 mL) in a down-window type irradiation cell made of Pyrex glass (75 ml). 0.2 M Sodium hydroxide aqueous solution or 0.2 M potassium bicarbonate aqueous solutions were employed as an absorbent of carbon dioxide and the ultra pure CO₂ were added continuously into the reactor for 1 h to remove the oxygen in the water, and saturated carbon dioxide in the solution. Using the cooling system combined with water pump, the temperature of the reactor was maintained at room temperature. Light on to start the reaction, and the irradiation was continued for 20 h. The light source was a 500 W halogen lamp (Ever bright; H-500). After reaction for 20 h, the reaction solution was centrifuged for 10 min to separate the reaction products from the powder catalyst. 10 mL of the upper stratum was taken for analyzing the concentration of methanol. The amount of methanol was determined by a gas chromatography equipped with a flame ionization detector, using Poropack-QS column.

3. Results and Discussion

3.1. XRD

Figure 1 shows the XRD patterns of the Co₃O₄-InTaO₄ samples. The XRD patterns of InTaO₄ samples are well consistent with monoclinic InTaO₄ pattern and space group P2/c, indicating that the samples were fully crystallized in the wolframite-type structure. InTaO₄ has major peaks at around 2θ = 23.967˚ (−110),

Figure 1. XRD patterns of InTaO₄ with various amounts of Co₃O₄ cocatalysts. (a) InTaO₄, (b) 0.3 wt%, (c) 0.5 wt% and (d) 1.0 wt% Co₃O₄-InTaO₄.

29.356˚ (−111) and 29.899˚ (111) [20] [21] [22] [23] [24]. The lattice parameters of the crystal were refined as: a = 4.83300 (−1) Å, b = 5.77800 (1) Å, c = 5.15700 (1) Å and β = 91.380˚. The indexed results are in good agreement with those reported in the JCPDS database card (No. 25-0391). Zou and his coworkers [20] have reported the structural refinements of InTaO₄. The InTaO₄ compound belongs to a monoclinic system with space group P2/c, a = 5.1552 (1), b = 5.7751 (1), c = 4.8264 (1) Å and β = 91.373 (1)˚. The structure is composed of two kinds of octahedral: InO₆ octahedron and TaO₆ octahedron. The InO₆ octahedron connects to each other to form zigzag chains by sharing edge. These InO₆ chains are connected through TaO₆ octahedron to form the three dimensional network.

Figure 1 also shows that the characteristic XRD peaks corresponding to Co species such as Co₃O₄ were not observed in the XRD patterns, indicating that the Co species on InTaO₄ was too small to detect. There was no difference in XRD patterns between InTaO₄ and 1.0 wt% Co₃O₄-InTaO₄, indicating that the addition of Co₃O₄ cocatalyst on the surface of InTaO₄ did not change the bulk structure of InTaO₄. However, it could modify the surface of InTaO₄ as discussed in the latter section. One can conclude that Co₃O₄ nanoparticles were well dispersed on the surface of InTaO₄.

3.2. SEM

Figure 2 shows the SEM photographs of InTaO₄ samples with various amounts of Co₃O₄ cocatalysts. The particle size of InTaO₄ was about 0.5 μm. Many nano Co₃O₄ particles were present on InTaO₄ surface, in consistent with XRD results.

(a) (b) (c) (d)

Figure 2. SEM micrographs, (a) InTaO₄, (b) 0.3 wt% Co₃O₄-InTaO₄, (c) 0.5 wt% Co₃O₄-InTaO₄, and (d) 1.0 wt% Co₃O₄-InTaO₄.

3.3. UV-vis Spectroscopy

Photocatalytic activity is dependent on the band structure of semiconductor. The information of band structure is very important for understanding photocatalytic reaction. Figure 3 shows the diffuse reflectance spectra of InTaO₄ samples with various amounts of Co₃O₄ loading. It shows higher light absorption ability of Co₃O₄/InTaO₄ in the visible light compared with InTaO₄. The band gap of InTaO₄ was 3.0 eV. For 0.3, 0.5, and 1.0 wt% Co₃O₄-InTaO₄ catalysts after calcinations, an obvious absorption in the visible light region were observed on all catalysts. The absorbance of the sample increased with increasing the amount of Co₃O₄ cocatalysts. The band gap of 0.3, 0.5, and 1.0 wt% Co₃O₄-InTaO₄ were calculated to be 2.8 eV, 2.7 eV and 2.6 eV, respectively (Table 1). The results indicate that adding Co₃O₄ cocatalyst on InTaO₄ changed the band gap. The bangap of the catalyst decreased with an increase of Co₃O₄ loading.

Table 1. Band gap of Co₃O₄-InTaO₄.

Figure 3. UV-vis spectra, (a) InTaO₄, (b) 0.3 wt% Co₃O₄-InTaO₄, (c) 0.5 wt% Co₃O₄-InTaO₄, and (d) 1.0 wt% Co₃O₄-InTaO₄.

3.4. Photocatalytic Reaction

The activities of carbon dioxide reduction on InTaO₄ samples with various Co₃O₄ loadings are shown in Figure 4. All catalysts produced methanol from the photoreduction of CO₂ under visible light irradiation. No other products were detected in gas phase and liquid phase. The rate of the reaction product increased linearly with the visible light-irradiation time, and the reaction stopped immediately when the irradiation was ceased. The formation of the reaction product was not detected under dark conditions. The reaction rate varied with the smount of cocatalyst. The results in Figure 4 were obtained from the InTaO₄ catalyst with Co₃O₄ cocatalyst suspended in 0.2 M NaOH and 0.2 M KHCO₃ aqueous solution. The highest methanol yield of 1.0 wt% Co₃O₄-InTaO₄ was 1.150 μmol.h⁻¹ g catal.⁻¹. In the NaOH solution, 0.5 wt% Co₃O₄-InTaO₄ demonstrated the highest methanol yield, and the Co₃O₄ cocatalyst enhanced the production of methanol. The Co₃O₄ cocatalyst not only provides reaction centers, which effectively transfer the electrons from the surface of catalysts to Co, but also enhances the light absorbance.

The results showed that the photocatalytic reduction was induced by the visible light irradiation. The formation rate of methanol increased with the presence

Figure 4. Methanol yield of photoreduction of CO₂ on InTaO₄ with various amounts of Co₃O₄ cocatalysts after pretreatment in 0.2 M NaOH and 0.2 M KHCO₃ aqueous solutions, under visible light irradiation. Catalyst: 0.14 g; Volume of the solution: 50 ml.

of cocatalysts on InTaO₄ photocatalysts. The photocatalyst had a higher activity in KHCO₃ aqueous solution than in NaOH solution, in agreement with literature results [17]. The InTaO₄ catalyst with 1.0 wt% Co₃O₄ cocatalyst in KHCO₃ aqueous solution gave the highest yield of methanol among all catalysts.

In the case of 1.0 wt% Co₃O₄-InTaO₄, Co₃O₄ species were loaded on InTaO₄ as nanoparticles, which were not observed by SEM analysis. However, after pretreatment, there was a formation of bulky Co₃O₄ particles on InTaO₄ due to aggregation of Co₃O₄ nanoparticles, leading to low photocatalytic activity. Bulk Co₃O₄ is a p-type semiconductor, which induces the formation of positive holes. For photoreduction of CO₂, hole scavengers are necessary to facilitate photoreduction; consequently, bulk Co₃O₄ reduced the photocatalytic activity of 1.0 wt% Co₃O₄–InTaO₄ catalyst. Hence, it is necessary to avoid the formation of bulk Co₃O₄.

The characterization results of 0.5 wt% Co₃O₄-loaded InTaO₄ photocatalyst showed the presence of ultra-fine Co₃O₄ thin films on metallic cobalt particles. The high dispersion of Co₃O₄ particles on the surface and interface of InTaO₄ plays a major role in determining its photocatalytic activity. The metallic cobalt acts as a bias for electron transfer from InTaO₄ to Co₃O₄ layer and the excited electron can migrate easily to the surface to facilitate photoreduction of CO₂. Methanol acts as a hole scavenger to improve the yield.

It should be noted that for dry InTaO₄, all the donor states are occupied and no optical transitions from the valence band to the donor state occurs. Instead, when InTaO₄ is immersed in water, partial depletion of the donor states will occur. This leads to band bending and the formation of a depletion layer, as reported in literature [23] [27] [28] [29]. The ionized donor states can be filled through optical excitation of valence band electrons, which explains the sub-bandgap optical absorption of InTaO₄ and high photoactivity of InTaO₄ in liquid phase reduction of CO₂. Zou et al. [20] reported that the bottom of conduction band of Ta_3d is lower than conduction band level of Co₃O₄. Accordingly, the conduction band level of the Co₃O₄/InTaO₄ is not high enough for electrons transfer across InTaO₄ and Co₃O₄ interface. Co cations are presumably located on the Ta³⁺ sites, especially when one considers that the formation of singly charged acceptor defects is energetically much more favorable than the formation of the triply charged defects that would be formed if any Co would substitute for Ta⁵⁺. Since Co₃O₄ was added after formation of full crystallite of InTa₄. Co₃O₄ did not incorporate into bulk InTaO₄ crystal. Co₃O₄ was incorporate into the surface structure of InTaO₄ as Co_xInTaO_4-x. It resulted in more defect sites on the surface of InTaO₄ and changed the valence band structure and the surface became a Schottky barrier to suppress the recombination of electron and holes. The higher light absorption ability of Co₃O₄/InTaO₄ in the visible light compared with InTaO₄ was also responsible for the high activity of Co₃O₄/inTaO₄ catalysts.

4. Conclusion

InTaO₄ was synthesized by a solid-state reaction method using metal oxide as the starting materials. Various amounts of Co were added by incipient-wetness impregnation method. The catalysts were characterized by powder X-ray diffraction, scanning electron microscope, and ultraviolet-visible spectroscope. The photocatalytic reduction was carried out in a Pyrex reactor with KHCO₃ or NaOH aqueous solution bubbled with CO₂ gas under visible light illumination. SEM micrographs show the appearance of many small Co₃O₄ particles on InTaO₄. The band gap of Co₃O₄-InTaO₄ was 2.7 eV, showing that these catalysts have the ability to reduce CO₂ to methanol. The effect of adding various amounts of cocatalysts on the photocatalytic reduction was investigated. The methanol yield increased with the amount of Co₃O₄ cocatalyst. The photocatalyst had a higher activity in KHCO₃ aqueous solution than in NaOH aqueous solution. The reaction on InTaO₄ catalyst with 1.0 wt% Co₃O₄ cocatalyst had the highest yield of methanol among all catalysts. Co₃O₄ was incorporate into the surface structure of InTaO₄ as Co_xInTaO_4-x. It resulted in more defect sites on the surface of InTaO₄ and changed the valence band structure. The surface became a Schottky barrier to suppress the recombination of electron and holes. The higher light absorption ability of Co₃O₄/InTaO₄ in the visible light compared with InTaO₄ was also responsible for the high activity of Co₃O₄/inTaO₄ catalysts.

Acknowledgements

This research was supported by Ministry of Science and Technology, Taiwan.

Conflicts of Interest

The authors declare no conflicts of interest.

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