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The structural properties and oxidation state of vanadium in alumina-supported vanadium-phosphorous oxides at nearly monolayer coverage influence its catalytic performances. The population distribution VVO and VIVO species in the amorphous VPO phase strongly depends on the sample history. By using alumina-supported VPO instead of conventional bulk catalysts, we facilitate enhanced discrimination between active phase and bulk phase signals in the vanadium-phosphorous oxygen system. This brings light on the speciation of VPO catalysts in general. Our observations were achieved by using mainly in situ EPR operando Raman–GC spectroscopic techniques at relevant temperatures and gas conditions, showing that the (VivO)2P2O7 phase is formed during propane ammoxidation at 753K. Such transformation is less intense in inert atmosphere at 753K. Ammonia adsorption on the VPO catalyst surface leads to the formation of VO2+ sites, probably bearing Brønsted and/or Lewis acidity. Propane ammoxidation appears to promote the formation of mixed valence compounds. [ABSTRACT FROM AUTHOR]

Abstract: A series of experiments was conducted to reveal the impact on the catalyst and NH3-SCR activity of an industrial deNO x catalyst from Haldor Topsøe A/S (V2O5-WO3/TiO2) by exposure to H2 and H2/O2. Treatment of the SCR catalyst was done by exposure for 60min with three different concentrations of H2 (0–2%) in an 8% O2/N2 mixture, where the SCR activity was measured before and after the hydrogen treatment. The results show that the activity of the SCR catalyst is only negligibly affected during exposure to the H2/O2 gas and in all cases it returned reversibly to the initial NO x conversion rate after reexposure to the standard NH3-SCR gas. In situ electron paramagnetic resonance (EPR) and UV–vis spectroscopy at 250°C suggest that a significant part of V4+ present during SCR conditions was oxidized to V5+ during exposure to 2% H2 +8% O2. However, the distribution of vanadium in oxidation states V4+ and V5+ returned to the initial redox equilibrium in less than an hour after the hydrogen–oxygen treatment, emphasizing the reversibility of the V4+-V5+ system. [Copyright &y& Elsevier]