Hydrogen Production from Methane and Water on Platinum Loaded Titanium Oxide Photocatalysts

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.