Methane exists abundantly around Japan as methane hydrate. As the effective use of such methane, the conversion of methane into methanol has recently attracted much attention. Photocatalytic reaction is one of the methods which convert methane into methanol without using much energy. However, it is indispensable to improve the photocatalytic activity for their practical use. Our group has attempted to improve the activity of mesoporous tungsten trioxide and titanium dioxide (m-WO3 and m-TiO2) photocatalysts, which convert methane into methanol, by loading the ultrafine metal clusters as cocatalyst on the photocatalysts. As a result, we have succeeded in loading ultrafine metal-cluster cocatalysts onto m-WO3 and m-TiO2 and thereby improving their photocatalytic activity. Our study also demonstrated that the kind of metal element suitable for each photocatalyst depends on the kind of the photocatalysts, and thereby it is important to select the metal clusters suitable for each photocatalyst for improving its photocatalytic activity.
In recent years, the effective use of methane (CH4) has attracted attention owing to the large amount of methane hydrate which is estimated to exist under the sea around Japan (Figure 1(a)) . When CH4 is burnt for generating energy, it generates small amounts of compounds which cause environmental problems, such as carbon dioxide, nitrogen oxide, and sulfur oxide (Figure 1(b)), . Therefore, CH4 can be used as a comparatively clean energy source. However, since CH4 is a gas state at room temperature, it occupies a large volume and therefore costs for the transportation. On the other hand, if CH4 is converted to liquid methanol (CH3OH), the transportation costs could be reduced. Furthermore, the generated CH3OH could be effectively used as a raw material for producing various chemical compounds. Therefore, in recent years, considerable attention has been focused on the development of a methodology for efficiently converting CH4 into CH3OH.
Regarding such a conversion, the current main methods require extreme conditions such as high temperature of around 500°C and high pressure of 50 atm or more . Furthermore, since the reaction proceeds in two stages, the conversion consumes a large amount of energy. Therefore, many studies have been conducted to find a method for the direct conversion of CH4 to CH3OH under mild conditions. However, most direct reactions require also high temperature and pressure conditions together with the use of an oxidizing agent . Using a photocatalytic reaction for this conversion, the reaction can be conducted under moderate conditions . However, the catalytic activity is still low, and further improvement is required for practical application. Then, our group has attempted to improve the photocatalytic activity of the photocatalyst used in this conversion. Our strategy was to load the controlled metal clusters on the surface of the photocatalyst as active sites (cocatalyst).
2. Research examples
Both tungsten oxide (WO3) and titanium oxide (TiO2) have redox potentials that are sufficient to generate hydroxyl radicals (·OH) (Figure 2(a)), which is necessary for the reaction to proceed (Figure 2(b)) . Furthermore, in their mesoporous structures (m-WO3 and m-TiO2), surface area becomes large and thereby the adsorption of reactive molecules is enhanced. In addition, in m-WO3 and m-TiO2, since the regular arrangement of pores suppresses the recombination of electrons and holes generated by the light irradiation, electrons and holes can be efficiently used in the reaction . Then, in our study, we attempted to activate such m-WO3 and m-TiO2 by loading the ultrafine metal clusters as cocatalyst on the photocatalysts.
2.1 m-WO3 photocatalyst
2.1.1 Preparation and loading of cocatalyst
m-WO3 was prepared using a hard template method in the same manner as that reported in the literature . First, mesoporous silica (KIT-6) was prepared to be used as a template (Figure 3(a)). Then, the obtained KIT-6 was mixed with 12-tungsto(VI)phosphoric acid
Next, metal clusters were loaded onto the obtained m-WO3. Our previous research on water-splitting photocatalysts [9, 10, 11, 12, 13] revealed that when a chemically-synthesized ligand-protected metal cluster is adsorbed onto a photocatalyst and its ligand is removed by the calcination, the controlled metal clusters can be loaded on the photocatalyst. Then, in this study, each metal cluster was loaded onto m-WO3 using such a method (Figure 3(c)). Silver (Ag), nickel (Ni), and cobalt (Co) were chosen as metal element because they were expected to work as cocatalyst [14, 15]. Glutathionate (SG) was used as the ligand of the metal cluster. SG-protected silver clusters (Ag
2.1.2 Structural characterization
In the transmission electron microscope (TEM) image of m-WO3, pores of 10 nm or less were observed (Figure 4(a,b)). In the small angle X-ray diffraction (XRD) pattern of m-WO3 (Figure 4(c)), similar to the literature , a peak assigned to the (211) plane of KIT-6 was observed, in addition to the peak attributed to the (110) plane derived from cubic I4132 or I4332. These results indicate that the synthesized m-WO3 has an arranged structure with regular pores.
Figure 5(a,b) shows the TEM images of Co
2.1.3 Evaluation of photocatalytic activity
The photocatalytic activity was measured using an experimental apparatus built in-house consisting of a high-pressure mercury (Hg) lamp (400 W) and a quartz cell (Figure 6) . First, 300 mg of M
Figure 7 shows the rate of CH3OH evolution from an aqueous solution containing M
According to a previous study on m-WO3 , in the conversion of CH4 to CH3OH, first, the holes generated via photoexcitation oxidize the hydroxyl groups (OH−) on the photocatalyst surface and/or water (H2Oad) adsorbed on the surface to form ·OH radicals. The ·OH radicals have intense oxidizing power and attack CH4 to generate methyl radicals (·CH3). The ·CH3 radicals thus produced react with H2O to produce CH3OH (Figure 2(b)). It can be considered that loading M
2.2 m-TiO2 photocatalyst
2.2.1 Preparation and loading of cocatalyst
A m-TiO2 photocatalyst was prepared according to a method reported in the literature . Specifically, m-TiO2 photocatalyst was prepared by forming a photocatalyst around the micelle composed of surfactants. First, hexadecyltrimethylammonium bromide ([(C16H33)N(CH3)3]Br) was added to an aqueous solution of titanyl oxysulfate sulfuric acid hydrate (TiOSO4), and the solution was stirred at 60°C for 24 h. The obtained precipitate was washed with water and dried at 120°C for 10 h. Then, the dried sample was calcined at 450°C for 2 h to obtain m-TiO2 (Figure 8). The M
2.2.2 Structural characterization
Figure 9(a) shows a TEM image of the prepared m-TiO2. It can be confirmed from this image that the material has a mesoporous structure different from that of m-WO3 (Figure 4(b)). The diffuse reflection (DR) spectrum of m-TiO2 revealed that the obtained m-TiO2 has a band gap of around 3.24 eV, indicating that the obtained m-TiO2 has an anatase type structure .
Figure 10(a) shows a TEM image of the Ni
2.2.3 Evaluation of photocatalytic activity
The photocatalytic activity of a series of M
On the other hand, regarding the metal elements, Ni showed a particularly high cocatalytic effect in M
We thank Mr. Shun Yoshino for technical assistance. This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI (grant numbers JP16H04099 and 16 K21402), Scientific Research on Innovative Areas “Coordination Asymmetry” (grant number 17H05385), and Scientific Research on Innovative Areas “Innovations for Light-Energy Conversion” (grant number 18H05178). Funding from the Takahashi Industrial and Economic Research Foundation, Futaba Electronics Memorial Foundation, Iwatani Naoji Foundation, and Asahi Glass Foundation is also gratefully acknowledged.