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The orthorhombic phase of KNbO 3 perovskite has been applied for nitrogen (N 2 ) photoreduction to ammonia (NH 3 ). However, this material suffers from a low surface area and low ammonia production efficiency under UV light irradiation. To eliminate these barriers, we used a metal−organic framework (MOF), named as TMU-5 ([Zn(OBA)(BPDH) 0.5 ] n •1.5DMF, where H 2 OBA = 4,4′-oxybis-(benzoic acid) and BPDH = 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene), for the synthesis of the KNbO 3 @TMU-5 hybrid material. KNbO 3 @TMU-5 achieved a NH 3 production rate of 39.9 μmol•L −1 •h −1 •g −1 upon UV light irradiation, as compared to 20.5 μmol•L −1 •h −1 •g −1 recorded for KNbO 3 under similar experimental conditions. Using different characterization techniques especially gas adsorption, cyclic voltammetry, X-ray photoelectron spectroscopy, photocurrent measurements, and Fourier transform infrared spectroscopy, it has been found that the higher photoactivity of KNbO 3 @TMU-5 in ammonia production is due to its higher surface area, higher electron−hole separation efficiency, and higher density of negative charges on Nb sites. This work shows that hybridization of conventional semiconductors (SCs) with photoactive MOFs can improve the photoactivity of the SC@MOF hybrid material in different reactions, especially kinetically complex reactions like photoconversion of nitrogen to ammonia.

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Polyoxometalates (POMs) and in particularly Keggin heteropolytungstates are much studied and commercially important catalysts for dioxygen-based oxidation processes. The rate-limiting step in many POM-catalyzed O-based oxidations is reoxidation of the reduced POM by O. We report here that this reoxidation process, as represented by the one-electron-reduced Keggin complexes POM (α-PWO and α-SiVWO) reacting with O, is efficiently catalyzed by a combination of copper and iron complexes. The reaction kinetics and mechanism have been comprehensively studied in sulfate and phosphate buffer at pH 1.8. The catalytic pathway includes a reversible reaction between Cu(II) and Fe(II), followed by a fast oxidation of POM by Fe(III) and Cu(I) by O to regenerate Fe(II) and Cu(II). The proposed reaction mechanism quantitatively describes the experimental kinetic curves over a wide range of experimental conditions. Since the oxidized forms, α-PWO and α-SiVWO, are far better oxidants of organic substrates than the previously studied POMs, α-SiWO and α-AlWO, this synergistic Fe/Cu cocatalysis of reduced-POM reoxidation could well facilitate significant new O/air-based processes.