No abstract

Photocatalytic reaction of CH4 gas with H2O vapor was examined over Pt/TiO2 around room temperature (ca. 323 K) in a flow reactor. H2 and CO2 were the main products, and only trace amounts of C2H6 and CO were also observed. After an induction period, the molar ratio of H2 to CO2 in the outlet gas became close to 4. Thus, the main reaction is suggested as the equation CH4 + 2H2O(g) → 4H2 + CO2, which can be referred to as photocatalytic steam reforming of methane (photocatalytic SRM), where the photoenergy is converted to chemical potential of products (= 113 kJ mol-1). The reaction would be promoted by photoexcited electrons and holes, which were generated by band gap photoexcitation of the TiO2 photocatalyst. The apparent quantum yield in the range 345−385 nm with high-intensity light (60 mW cm-2) was 0.6%, while the one in the range 240−270 nm with low-intensity light (2 mW cm-2) reached 2%. Highly active Pt/TiO2 photocatalysts consisted of anatase phase TiO2 having large surface area and enough loading amount (more than 0.1 wt %) of metallic Pt nanosized particles, which should be loaded by the photodeposition method, preferentially in the reaction condition. During the induction period, the photoirradiated side of the catalyst in the reaction cell turned pale brown, and the reaction was much accelerated. The colored surface substances accelerating the reaction rate, whose chemical formula can be described as [CH2O] n , were revealed as the reaction intermediates. The reaction mechanism was also investigated.

Photocatalytic steam reforming of methane (CH 4 + 2H 2 O f 4H 2 + CO 2 ) was examined over modified and unmodified sodium tantalate photocatalysts around room temperature. The activity of the photocatalysts was much influenced by metal cation doped into the bulk of NaTaO 3 , the crystallites size of NaTaO 3 , and metal nanoparticles loaded on the surface. The highest activity was obtained over the NaTaO 3 doped with La 3+ , where a moderate amount of La 3+ should substitute for Na + without distorting the crystal structure of NaTaO 3 . The large crystallites size was a more important factor than the high surface area for higher photocatalytic activity of NaTaO 3 :La. These factors such as doping and large crystallites should increase the density and the mobility of the photoexcited carriers to improve the intrinsic photocatalytic activity. A moderate amount of Pt nanoparticles loaded on NaTaO 3 :La most enhanced the activity among the examined metal particles, which would contribute to not only separation of excited electrons from holes and the successive hydrogen production but also methane activation during reaction with water or holes. The heat treatment after the addition of Pt, which probably formed the practical metal-semiconductor junction, was required for a high and stable photocatalytic activity. The best photocatalyst, Pt/NaTaO 3 :La(2%), exhibited more than two times higher activity than Pt/TiO 2 did. The photocatalytic reaction lasted for a long time without deactivation. The effect of reaction conditions such as reaction temperature, feed gas composition, and light intensity on the catalytic performance was systematically optimized, and the highest H 2 production rate reached 4.5 µmol min -1 (270 µmol h -1 ), corresponding to 0.6% methane conversion and 11% water conversion, which exceeded the equilibrium conversion. The apparent quantum yield in the range of 240-270 nm for the hydrogen production was estimated to be 30% when the irradiation light intensity was 2 mW cm -2 , which was higher than that of the photocatalytic water-splitting system employing NiO/NaTaO 3 :La.

Ga2O3 photocatalysts showed a high and stable activity for the photocatalytic steam reforming of methane (PSRM; 2H2O(g) + CH4 → 4H2 + CO2) around room temperature. The activity was much influenced by the cocatalyst and the crystal structure of Ga2O3; the highest activity was obtained over Pt-loaded β-type Ga2O3 with specific surface area of 10−20 m2 g−1. The addition of metal cations into the bulk and/or on the surface of Ga2O3 was also effective to improve the photocatalytic activity; metal cations having both a smaller oxidation number than that of Ga3+ and a similar ionic radius to that of Ga3+, such as Mg2+ and Zn2+, were effective as the dopant into the bulk of β-Ga2O3, while cations of the aluminum group such as In3+ and Al3+ were effective as the surface additives. When we compared the activity for the PSRM with those for the water decomposition (WD; H2O → H2 + 1/2O2) and the methane decomposition (MD; CH4 → x/2H2 + CH4−x ), it was revealed that the improvement of the bulk processes would mainly influence the water activation while that of the surface processes would affect the methane activation.

Photocatalytic steam reforming of methane (PSRM; 2 H2O (g) + CH4 → 4 H2 + CO2) was examined over metal-loaded K2Ti6O13 photocatalysts. Although the production rate was improved by loading Pt cocatalyst on the K2Ti6O13 photocatalyst, unfavorable formation of CO and gradual deactivation of photocatalyst were observed. On the other hand, a Rh-loaded K2Ti6O13 sample showed two times higher activity than the Pt-loaded one did, and promoted the PSRM selectively without deactivation for many hours. In the highly active Rh-loaded photocatalyst, the Rh cocatalyst existed as a mixture of small metallic rhodium and large rhodium oxide particles. The photocatalytic activity tests for hydrogen evolution and oxygen evolution from each aqueous solution of sacrificial reagent (methanol and silver nitrate, respectively) revealed that the metallic rhodium particles and the rhodium oxide particles could function as cocatalysts preferably for reduction and oxidation, respectively. Also on a Na2Ti6O13 photocatalyst, a mixture of rhodium metal and oxide similarly enhanced the photocatalytic activity. Thus, it is suggested that the Rh cocatalyst on these alkaline titanates bifunctionally promoted the PSRM.