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Abstract: A periodic model has been selected to model the structure and reducibility of vanadia/titania catalysts and the effect of alkali doping for Li, Na and K. Alkali interact with both the surface vanadia units and the support. The calculated changes on vanadia structure are consistent with the changes evidenced by Raman spectroscopy. Reducibility is modeled by adsorption of one hydrogen atom, forming hydroxyl groups with the bridging V–O–Ti present in the model. The adsorption energy decreases in the series Li>Na>K, in agreement with experimental TPR results. [Copyright &y& Elsevier]

Vanadium and niobium species (together and separately) were loaded on gamma alumina, and the resulting catalysts were run in the methanol conversion. This reaction was studied by both GC analysis and FTIR study in the flow system. The catalytic properties are discussed based on the combined FTIR and 27Al, 51V and 1H MAS NMR studies. The NMR studies revealed a different mechanism of interaction between Nb and Al2O3 than that between V and Al2O3. This predetermines the structure of vanadium sites in bimetallic VNb/Al samples. The effect of coverage was considered for various metal loadings ranging from below to above monolayer. One of our most interesting findings is that the surface Nb oxide species exhibited a redox character below monolayer but were acidic above monolayer. 27Al MAS NMR revealed a strong alumina–Nb interaction that may account for its redox performance. Moreover, the role of sulfate from vanadium precursor is evidenced. [Copyright &y& Elsevier]

Abstract: Various catalysts containing niobium and vanadium oxides supported on alumina were prepared by wet impregnation via aqueous solution using several precursors. The total loading of V and Nb oxides were below their dispersion limit on alumina. Vanadyl sulfate, ammonium metavanadate and ammonium niobate(V) oxalate were the precursors for supported vanadia and niobia. The reduction/oxidation properties were studied by conventional TPR/TPO and TPR/TPO-Raman. Surface vanadium oxide species tend to increase their polymerization degree upon TPR/TPO cycles. A broad weak feature near 900cm−1 appears associated to V3+–O–Al3+ bond vibration in the reduced vanadia-alumina catalysts. Niobia appears to retard vanadia reduction. Regarding supported niobia, a fraction of surface niobia is significantly more reducible than surface vanadia and another fraction is significantly less reducible. The more reducible niobia appears associated to an incipient Nb–Al–O phase that may account for a fluorescence background observed in the Raman spectra. The less reducible niobia phases appears associated to dispersed niobium oxide species on alumina. Niobium has an effect on vanadia reduction profiles in VNb/Al2O3 system. [Copyright &y& Elsevier]

Abstract: Much attention has recently been devoted to the surface of bulk catalysts. It has been proved that the most critical phenomena occur at the surface, which differs significantly from the bulk. To evaluate the role of the bulk in VPO catalyst, vanadium and phosphorus were supported on γ-alumina by incipient wetness impregnation at a total V+P of two monolayers so that nanoscaled VPO phases (n-VPO) form on alumina, such size maximizes the surface-to-bulk ratio. Alumina support stabilizes V5+, which may be reduced, while V4+ predominates in bulk vanadyl pyrophosphate phase. In addition, the support modifies acid–base and textural properties. The presence of VPO nanocrystals supported on alumina (n-VPO) facilitates investigating the interaction between alumina-dispersed vanadia and VPO-lattice vanadium ions. Both, bulk and supported phases are active for propane ammoxidation affording acrylonitrile yields of 41% and 48%, respectively, at 500°C. The yield to acrylonitrile per mass unit of VPO increases by 150% at 500°C in the alumina-stabilized n-VPO system when compared to the bulk VPO. [Copyright &y& Elsevier]