A novel photoreactor for separating hydrogen and oxygen in photocatalytic water splitting

H2 and O2 were produced in a twin reactor with two compartments divided by a Nafion membrane. Ce3+ and Ce4+ are adopted as redox mediator, which transferred the electron and holes between H2-evolving catalyst(Pt/SrTiO3:Rh) and O2 evolving catalyst (BiVO4). The optimal pH in the twin reactor was 1. Water splitting under Ce3+/Ce4+ equilibrium concentrations between two compartments gave the highest initial and average H2 generation rates, 1.21 and 1.45 μmol/g-hr, respectively. Our results indicates that the rate limiting step falls on the catalyst itself rather than the diffusion resistance of Ce3+/Ce4+ by Nafion membrane. H2 and O2 were separated in water splitting thus preventing backward reaction. Introduction Hydrogen has been considered to be one of the potential energy sources because it is energy efficient, clean, and abundant in nature. A better way to produce cleaner hydrogen is by the so-called 'photocatalytic water splitting' which can utilize sunlight to decompose water into hydrogen and oxygen with the aid of photocatalyst. The H2 photocatalyst and O2 photocatalyst are separated in two vessels and are divided by a Nafion membrane. Redox mediators are used to balance the electrons and holes on two sides of the Nafion membrane in order to sustain the reaction. Therefore, the reverse reaction of H2 and O2 in a single reactor can be avoided and the photocatalytic efficiency is improved. The cost of separation is also saved. Experimental The H2 catalyst, Pt/SrTiO3:Rh, was prepared by a sol-gel method. The O2 catalyst was BiVO4. Figure 1 shows the schematics of twin photoreactor. First, 0.15 gram of Pt/SrTiO3:Rh and 0.15 gram of BiVO4 was added in 180 mL of 2mM Ce2(SO4)3 and Ce(SO4)2 solution respectively. Two compartments was divided by a Ce4+ pretreated Nafion membrane. The reaction was under 300W Xe lamp irradiation for 6 hr and the H2 and O2 evolved were collected respectively and analyzed by GC. The visible light intensity at 1.76 mW/cm2. Figure1: Schematics of twin photoreactor Results and Discussion 1. Characteristics of photocatalyst The H2 catalyst, Pt/SrTiO3:Rh, the absorption band at 450~600nm proves that Rh has been doped into SrTiO3 and therefore band position has blue shift. The band gap of the Pt/SrTiO3:Rh calculated from UV result is 2.31eV, which is narrower than that of SrTiO3, 3.26eV. Fig2 shows the XRD result of Pt/SrTiO3:Rh and BiVO4(monoclinic). The diffraction peaks of the loaded Pt and the doped Rh are absent due to the trace amount presented in the catalysts. Fig3 shows the SEM of Pt/SrTiO3:Rh and BiVO4. The round and plate-like morphology is observed and the particle size ranges from 0.7μm to 2μm. Pt/SrTiO3:Rh is observed to have round and cubic shape and its Catalyst Laboratory of National Taiwan University particle size is around 50nm to 100nm. The specific surface area of BiVO4 is 1.24 m2/g, while that of Pt/SrTiO3 is 6.49 m2/g. Figure2: XRD patterns of (a) Pt/SrTiO3:Rh (b) BiVO4 Figure3(a):The SEM images of Pt/SrTiO3:Rh Figure3(b):The SEM images of BiVO4 2. Photocatalytic water splitting As shown in Figure4, the initial H2 and O2 generation rate are 1.21 and 0.67 μmol/g-hr, respectively. The average H2 and O2 generation rate under equilibrium concentration are 1.45 and 0.71 μmol/g-hr, respectively. The photo quantum efficiency for H2 production under equilibrium Ce3+/Ce4+ concentration is calculated to be 0.445%. Figure4: The H2 and O2 evolving rate in water splitting reaction under equilibrium Ce3+/Ce4+ concentration Summary The twin reactor system can produce H2 and O2 separately with visible light driven photocatalysts, that can eliminate the potential explosion of H2/O2 mixture for industrial production. Furthermore, the twin reactor gave higher H2 generation rate than that in the single reactor, which is the result by preventing holes/electrons recombination and backward reaction.