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 conta ining WO3, Fe2+, and Fe3+ species was studied and a comparison of product y ields made between the inner- and outer-irradiation systems. Prolonged irra diation 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 prod uct yields showed a dependence on the duration of irradiation, illumination wavelength, WO3 concentration, concentration of Fe3+ and Fe2+ cations, typ e of iron (LII) salt, and pH. The optimum photoproduction for O-2 and H-2 w as Obtained under the following operating conditions: [WO3]; 8 g dm(-3), [F e3+]: 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 additio n to these conditions, a ratio of [Fe2+]:[Fe3+] = 2-4 was found to be suita ble 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 fro m an outer-irradiation photoreactor. When Fe-2(SO4)(3) was replaced by Fe(N O3)(3) or FeCl3 as a sourer of Fe3+ species, higher initial O-2 evolution r ates were obtained but the long term yields were lower than those from disp ersions containing Fe-2(SO4)(3). The studied system utilized WO3 to accompl ish the initial light absorption, charge separation and O-2 evolution in th e 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 react ion was photodecomposition of water into O-2 and H-2. Mechanistic implicati ons were considered to account for the probable reaction steps leading to t he products observed and the dependence of the yields on the examined react ion parameters. (C) 1999 Elsevier Science S.A. ALL rights reserved.