Photocatalytic reduction of carbon dioxide to methane and methanol has been extensively studied by many researchers  - . Anpo et al.  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 , Ti-MCM-41 , Ti-MCM-48 , FSM-16  , Ti-β zeolite , and self-standing porous silica thin films   , 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    .
InMO4 (M = Ta, Nb, V) catalysts have been reported as photoactive for water splitting reaction under visible light    . According to the band structures of InTaO4, the photoreduction of carbon dioxide on InTaO4 catalysts should be feasible. Our previous study  showed that NiO-InTaO4 was active for photoreduction of CO2 to produce methanol. It has been reported that other cocatalysts such as Co3O4   are effective. However, it has not been reported in literature for photoreduction of CO2   .
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.
2.1. Catalyst Preparation
The polycrystalline InTaO4 was synthesized by a solid-state reaction method as reported in literature . 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-Kα 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 . 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 . 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 . The catalyst powder (
3. Results and Discussion
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)     . 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  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.
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
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
of cocatalysts on InTaO4 photocatalysts. The photocatalyst had a higher activity in KHCO3 aqueous solution than in NaOH solution, in agreement with literature results . 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    . 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.  reported that the bottom of conduction band of Ta3d 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 CoxInTaO4-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.
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 CoxInTaO4-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.
This research was supported by Ministry of Science and Technology, Taiwan.
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