Light-Induced In Situ Oxidative Coupling Mediated by Triplet State Reactive Oxygen into Nanostructured Hexaniobate Photocatalyst

An eventual reduction of our reliance on fossil fuels naturally requires the engineering of advanced photocatalysts to convert light energy into chemical bonds. The selection of photon absorbing redox-active centers in transition metal oxides encloses M–O bond strength and localized acidity. Solid acid potassium hexaniobate (HxK4–xNb6O17·nH2O) contains a high concentration of Brønsted sites distributed along an excess number of bidimensional interlamellar spacing units. This structure has been fairly prospected for H2 production, but the ability to promote oxidative dehydrogenation (ODH) and coupling has not been announced since hydrolysis competes with polymerization. In this study, the catalytic activity of H2.8K1.2Nb6O17·H2O has been evaluated for the case of aniline (C6H5NH2) in the presence of O2 from atmospheric air. Two particle morphologies were examined: layered and nanotubular. Both presented considerable photoreactivity under UV-excitation with the solar conversion yield accounting for 17 wt %. Optical spectroscopy was applied to assess the extension of aniline partial oxidation. Density functional theory calculations have been performed to rationalize the observed properties and support describing a system that models the oligomerization of activated monomers at niobate acid sites. After irradiation, homolytic N–H bond dissociation is realized by triplet state reactive oxygen from labile Oox sites of Nb–O–Nb redox centers. Lattice oxygen is then restored by O uptake from molecular O2 according to the Mars–van Krevelen mechanism. The confinement of reactant molecules into nanometer scale reaction venues favors photopolymerization due to strong short-range interaction driving forces. The results open the door for the application of layered niobates in other photoinduced conversions. Alternatives include the synthesis of olefins and hydrocarbon fuels by the oxidative coupling of methane or alcohols dehydration.