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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]

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]

In this work, V/M/TiO 2 catalysts (M = Li, Na, K, Rb and Cs) were investigated with Solid-state nuclear magnetic resonance (SSNMR), electron paramagnetic resonance (ESR) and first-principles calculations. The total vanadium content corresponds to half monolayer (4 V atoms·nm−2 of TiO 2 support surface), with V/M atomic ratio being 4/0.6. Static 51V and MAS 1H, 7Li, 51V, 23Na, 133Cs NMR spectra were acquired. Surface moieties of vanadium oxide on (001) anatase surface were modeled using density functional theory (DFT); their 51V NMR parameters were calculated with the Gauge-Including Projected Augmented Wave (GIPAW) method and compared to experimental data obtained with 51V SSNMR spectroscopy. It was found that mainly strongly bound vanadium sites (V 3) are formed on anatase samples, located on TiO 2 terminal oxygen atoms. Weakly bound vanadium sites (V 1 , V 2) are formed on bridged anatase oxygen atoms. Supported alkali metals (Li, Na, K, Rb and Cs) on TiO 2 were found to interact with protons located on the terminal and bridged TiO 2 oxygen atoms. The relative ratio of weakly bound vanadium sites (V 1 , V 2) increased on alkali-containing samples. Calculations performed showed lithium cation to prefer V-O-Ti or V-O-V bonds over V O, unsystematically deshielding vanadium nuclei. The presence of V3+ is likely to cause a loss of intensity in 51V NMR spectra. [Display omitted] • Mainly strongly bound vanadium sites (V3) form on supported VOx/TiO 2 catalysts. • The alkali metals increase the relative ratio of weakly bound vanadium sites (V1, V2). • The strongly bound vanadium sites (V3) form on terminal oxygen atoms. • The weakly bound vanadium sites (V1, V2) form on the bridged oxygen atoms. • The calculations show that lithium cation prefers V-O-Ti or V-O-V bonds over V O. [ABSTRACT FROM AUTHOR]

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]