Niobate Nanosheets as Catalysts for Photochemical Water Splitting into Hydrogen and Hydrogen Peroxide

Tetrabutylammonium-stabilized Ca2Nb3O10 nanosheets catalyze photochemical splitting of water into hydrogen and hydrogen peroxide under UV irradiation. The peroxide forms on the surface of the catalyst and adsorbs to it, fully deactivating the catalyst after 48 h of irradiation. For the Pt-modified niobate, complete deactivation occurs within 24 h due to higher H-2 evolution rate with this system. The peroxide was identified via Raman and IR spectroscopy. In vibrational spectra, the irradiated niobate exhibits bands at 580, 778, and 940 cm(-1), which suggest that the peroxide is present as a side-on ligand coordinated to the Nb5+ ions on the nanosheet surface. The same species can be formed by treating the nanosheets with 30% aqueous H2O2. If the catalyst is irradiated in (H2O)-O-18, the bands shift to lower energy, indicating that the peroxide species is formed from water. Room-temperature storage of the nanosheets in (H2O)-O-18 also leads to isotopic exchange of all Nb-O atoms over the course of 24 h. Peroxide amounts measured by titration of the catalyst with o-tolidine after 6, 12, and 24 h of irradiation form in a 1: 1 stoichiometry with hydrogen. Infrared data shows that the peroxide is partially desorbed by evacuating the catalyst and by purging with argon gas. Under catalytic conditions, this treatment restores up to 60% of the original activity. For the H2O2-treated catalyst, near quantitative H2O2 desorption occurs within 20 min at 450 degrees C. Separate irradiation experiments of the catalyst in the presence of air reveal that H-2 evolution is diminished and that O-2 uptake occurs from the atmosphere. Most of the hydrogen peroxide formed under these conditions is subsequently detected in the supernatant and only small amounts on the catalyst. NMR spectra show that the tetrabutylammonium cations are decomposed. These findings suggest that there are two distinct pathways for peroxide formation, one involving one-electron reduction Of O-2 and one involving two-electron reduction of water (in the absence of O-2).