Water photolysis by the K$_{2}$Ln$_{2/3}$Ta$_{2}$O$_{7}$ and K$_{2}$LnTa$_{3}$O$_{10}$ (Ln$=$La, Pr, Nd) tantalates

Six compounds of the hydrated phase of K$_{2}$Ln$_{2/3}$Ta$_{2}$O$_{7}$ and K$_{2}$LnTa$_{3}$O$_{10}$ were prepared by the polymerizable complex method; these compounds were characterized in crystal structure, specific surface area (BET), optical band gap (DRS) and reactivity for water photolysis using a 300 W Hg-lamp. Tantalates containing La, Pr and Nd show a shift in the band gap value, from 3.8, 2.6 and 2.07 eV, respectively. Hydrogen production without co-catalyst has been observed, and no noticeable difference appears when NiO$_{x}$ was impregnated to powders. The hydrogen production notably increases when a sacrificial agent as methanol was used. In this work we compare the hydrogen production efficiencies for the bi-octahedral and three-octahedral tantalum hydrated compounds.