The effect of selected reaction parameters on the photoproduction of oxygen and hydrogen from a WO3-Fe2+-Fe3+ aqueous suspension

The photoproduction of oxygen and hydrogen from an aqueous suspension containing WO3, Fe2+, and Fe3+ species was studied and a comparison of product yields made between the inner- and outer-irradiation systems. Prolonged irradiation of the WO3-H2O-Fe2+/Fe3+ system led to the generation of O-2 and H-2, and the yields could be sustained for a period of several days. The product yields showed a dependence on the duration of irradiation, illumination wavelength, WO3 concentration, concentration of Fe3+ and Fe2+ cations, type of iron (LII) salt, and pH. The optimum photoproduction for O-2 and H-2 was Obtained under the following operating conditions: [WO3]; 8 g dm(-3), [Fe3+]: 14-20 mol dm(-3), pH 1.5-2.5, and illumination wavelength: 200-500 nm. The illumination time needed to reach the stationary state was ca. 6 and 30 h for the inner- and outer-irradiation systems, respectively. In addition to these conditions, a ratio of [Fe2+]:[Fe3+] = 2-4 was found to be suitable for the simultaneous production of O-2 and H-2. The product yields from the annular photoreactor were usually about four times more than those from an outer-irradiation photoreactor. When Fe-2(SO4)(3) was replaced by Fe(NO3)(3) or FeCl3 as a sourer of Fe3+ species, higher initial O-2 evolution rates were obtained but the long term yields were lower than those from dispersions containing Fe-2(SO4)(3). The studied system utilized WO3 to accomplish the initial light absorption, charge separation and O-2 evolution in the presence of Fe(aq)(2+)species as an electron acceptor, and then relied on the Fe-aq(2+) photochemical process to produce hydrogen. The overall reaction was photodecomposition of water into O-2 and H-2. Mechanistic implications were considered to account for the probable reaction steps leading to the products observed and the dependence of the yields on the examined reaction parameters. (C) 1999 Elsevier Science S.A. ALL rights reserved.