Synthesis of α-Ga2O3 by Water Oxidation of Metallic Gallium as a Photocatalyst for CO2 Reduction with Water.

We have succeeded to synthesize gallium oxide consisting of α-phase (α-Ga2O3) with the calcination of GaOOH obtained by a direct reaction of liquid Ga metal with water for the first time and found that α-Ga2O3 exhibits photocatalytic activity for CO2 reduction with water and water splitting as well. The calcination above 623 K converted GaOOH to α-Ga2O3, and the samples calcined at 723-823 K were well crystallized to α-Ga2O3 and promoted photocatalytic CO2 reduction with water, producing CO, H2, and O2. This is observed for the first time that α-Ga2O3 without a cocatalyst has shown very high photocatalytic activity for the conversion of CO2 to CO.

Introduction

Gallium oxide (Ga2O3) has been investigated as a photocatalyst for the reduction of CO2 with water and water splitting as well. Among six polymorphs of Ga2O3, i.e., α-, β-, ε-, δ-, γ-, and κ-phases,[1−3] β-Ga2O3 has been widely investigated as the photocatalyst for CO2 reduction with water[4] owing to its thermodynamical stability. In addition, Ga2O3 consisting of two phases, α- and γ-phases or β- and γ-phases, shows the photocatalytic activity for CO2 reduction, for which their phase boundaries are reported to be active sites for CO2 reduction.[5−7] Precipitation of gallium oxide hydroxide (GaOOH) on α- and β-phases is also reported to increase the photocatalytic CO2 reduction.[8] In most cases, noble metals such as Ag[8−13] and Pt[14] have been employed as cocatalysts to enhance CO productivity in photocatalytic CO2 reduction. However, it has been reported that without a cocatalyst, Ga2O3 consisting of two phases, α- and β- or β- and γ-phases, exhibits high activity for the photocatalytic CO2 reduction.[5,15,16] It should be noted that except for the β-phase, no single phase of Ga2O3 is examined as a photocatalyst for CO2 reduction with water because of the lack of thermal stability and difficulties in their synthesis.

In this work, we have succeeded to synthesize α-phase Ga2O3 (α-Ga2O3) with calcination of GaOOH obtained by a direct reaction of liquid Ga metal with water (referred to as water oxidation of Ga) and examined the photocatalytic activity of the synthesized α-Ga2O3 for CO2 reduction with water. It should be mentioned that the formation of GaOOH by the water oxidation of liquid Ga metal was successfully done for the first time. Furthermore, the single phase of α-Ga2O3 was easily obtained by calcination of GaOOH and was found to show high catalytic activity for the photocatalytic reduction of CO2 with water. This is also observed for the first time.

Results and Discussion

Figure shows the scanning electron microscopy (SEM) images of samples (a) before and (b) after the calcination at 732 K for 2 h. After the water oxidation of Ga metal to GaOOH, particle sizes of the sample were widely distributed from a few micrometers to a few nanometers. By the calcination, no significant difference appeared in particle sizes, which was confirmed by the specific surface area determined by the Brunauer–Emmett–Teller (BET) method, showing around 7.0 m2/g with no appreciable difference before and after the calcination.

Figure 1

SEM images of samples (a) before and (b) after the calcination at 723 K for 2 h.

X-ray diffraction (XRD) patterns are given in Figure . The diffraction pattern for the sample just after the water oxidation shows that Ga metal was mostly converted to GaOOH with no trace of the diffraction peaks originated from Ga metal. As depicted in Figure , calcination above 623 K converted GaOOH to α-Ga2O3. However, the sample calcined at 623 K was not well crystallized, while the samples calcined above 723 K were well crystallized to α-Ga2O3 without any traces of other phases including GaOOH. UV–vis before calcination showed high intensity in the UV region caused by GaOOH, while after the calcination, that became Ga2O3 like and no particular structure related to the catalytic activity was observed. X-ray photoelectron spectra (XPS) for samples before and after calcination also show full conversion of GaOOH to α-Ga2O3. By comparing the SEM images before and after the calcination, it is revealed that no appreciable sintering occurred, indicating that GaOOH particles were simply converted to α-Ga2O3 particles. This would be the reason for a small difference in BET surface areas before and after the calcination.

Figure 2

X-ray diffraction patterns for the prepared samples.

CO2 reduction tests were conducted for all calcined samples and all of them showed the production of CO, H2, and O2. However, for samples calcined below 623 K, among the observed production rates of H2, CO, and O2, their stoichiometric mole ratios, i.e., ([H2] + [CO])/(2 × [O2]), were much higher than 1.0, indicating the existence of Ga metal or hyperstoichiometric Ga2O3. Therefore, the results for the samples calcined at 723 and 832 K are presented in Figure a,b, respectively. Both samples showed photocatalytic activity for CO2 reduction and water splitting. Except for the initial production rates at 1 h, the production rates of H2, CO, and O2 stayed nearly constant for up to 5 h, keeping the stoichiometric rates nearly 1, as indicated in Figure c. H2 production rates for both samples were nearly the same as high as 25 μmol/h. The CO production rates were around 4 and 2 μm/h for the sample calcined at 723 K for 6 h and 823 K for 2 h, respectively. Although the CO production rates are much less than the H2 production rate, referring to a sample mass of 100 mg, the present production rate of around 5 μmol/h is comparable or a little larger than that observed in previous works for the Ga2O3 photocatalyst using the same reaction system.[5−7] Hence, we can conclude that the synthesized α-Ga2O3 samples exhibit very high photocatalytic activity for the CO2 reduction with water.

Figure 3

Production rates (μm/h) for H2, O2, and CO for the samples calcined at (a) 723 K and (b) 823 K used for photocatalytic CO2 reduction tests and (c) the stoichiometric ratio for the two samples.

This is observed for the first time that the single phase of α-Ga2O3 shows photocatalytic activity for CO2 reduction with water and water splitting as well. Nevertheless, the selectivity of CO production compared with H2 production remained low. Because the activity for H2 production caused by water splitting and simultaneously occurring CO2 reduction was rather high, the optimization of microparticulation with homogeneous particle distribution would significantly improve the photocatalytic activity, which remains a future study. The Ag cocatalyst could further improve the selectivity.

The XRD patterns of the samples before and after used as photocatalysts are compared in Figure , which shows no appreciable difference before and after the use, except for the appearance of the traces of GaOOH in the sample calcined at 823 K. Ga2O3 could react with water as follows

Figure 4

Comparison of X-ray diffraction patterns before and after the use of samples exhibiting high photocatalytic activity.

As found by Kawaguchi et al.,[8] the existence of GaOOH could enhance the photocatalytic activity. However, it is difficult to mention the role of GaOOH for CO2 reduction because reaction produces neither H2 nor O2. The calcination of GaOOH proceeds to the opposite direction of eq . There could be some thermodynamic equilibrium as eq so that GaOOH could be produced in water during the use of α-Ga2O3 for the photocatalytic reduction of CO2. Since the dynamic equilibrium given by eq produces neither H2 nor O2, it would not directly contribute to the photocatalytic CO2 reduction. However, the intermediate state in the dynamic equilibrium could contribute to CO2 reduction on Ga2O3 and/or GaOOH. Still, the reaction mechanism of CO2 reduction with water over α-Ga2O3 is not clear and should be investigated in the future.

Conclusions

We have succeeded to synthesize α-phase Ga2O3 with the calcination of GaOOH obtained by the water oxidation of liquid Ga metal and examined the photocatalytic activity of the synthesized α-phase Ga2O3 for the CO2 reduction with water. α-Ga2O3 samples obtained by the calcination at 723 and 823 K exhibited the photocatalytic activity for CO2 reduction and water splitting. Compared to the H2 production rate, the CO2 production rate was rather small. Nevertheless, referring to a sample mass of 100 mg, we can conclude that α-Ga2O3 without a cocatalyst has very high activity for the photocatalytic reduction of CO2 with water.

It has been reported that without the cocatalyst, Ga2O3 consisting of two phases, α- and β-phases or β- and γ-phases, exhibits high activity for the photocatalytic CO2 reduction,[5,15,16] while the present study indicates that the single phase of α-Ga2O3 exhibits very high photocatalytic activity for the CO2 reduction with water and water splitting as well. At present, the reaction mechanism is not clear. However, GaOOH produced during the reaction could have a certain role. This suggests that the existence of two phases of α-Ga2O3 and GaOOH are required for the CO2 reduction as observed in previous works for the two phases, i.e., α- and β-phases or β- and γ-phases.

It should also be noted that the structure and particle sizes of α-Ga2O3 were not optimized. Microparticulation with homogeneous particle distribution would significantly improve the photocatalytic activity and remain as a future work together with clarification of the active sites and mechanism of the photocatalytic reaction.

Experimental Section

α-Ga2O3 was obtained by calcination of GaOOH produced by the water oxidation of liquid Ga, which is the first trial except for one report on the formation of GaOOH by ultrasonic irradiation of molten gallium in water.[17] Liquid Ga metal (4.5 g, Kojundo Chemical Laboratory Co., Ltd., purity 99.99%) and 20 mL of distilled water were placed in a plastic beaker and agitated in a stirred bath at 343 K until Ga was mostly dispersed in water as GaOOH, the reaction product with water. After filtered and dried in air, the product was calcined in air at temperatures ranging from 573 to 823 K for 2–12 h.

The photocatalytic CO2 reduction tests were carried out in a fixed-bed flow reactor cell. One hundred grams of the calcined sample was dispersed in an aqueous solution of 10 mL of NaHCO3 (1 M). The air in the cell was replaced with CO2 gas at a flow rate of 50 mL/min for 45 min. Then, the tests were conducted with UV irradiation under CO2 gas at a flow rate of 3.0 mL/min. A 300 W Xe lamp was used as a UV source and its light intensity was about 40 mW/cm2 in a wavelength range of 254 ± 10 nm. The reaction products (mainly CO, H2, and O2) were analyzed by a gas chromatograph equipped with a thermal conductivity detector.

The characterization of samples was performed by scanning electron microscopy (SEM), X-ray diffraction (XRD), XPS, and UV–vis diffuse reflectance spectroscopy. SEM images of the samples were acquired using a JSM-6500F, operating at 15 kV. XRD patterns of the samples were recorded on a MiniFlex600 (Rigaku) using Cu Kα as a radiation source with an operating voltage of 40 kV and a current of 15 mA. The XRD patterns were collected at a 2θ angle range of 20–70°. The 2θ step size was 0.02°, and the scanning rate was 5°/min. The specific surface areas (SSAs) of the samples were determined by Brunauer–Emmett–Teller (BET) SSA measurements at 77 K (liquid N2 temperature) using a Monosorb (Quantachrome). Before the BET measurements, the samples were heated at 573 K for 3 h in a N2 atmosphere as a pretreatment.