Search

In this paper, a heteropolyacid catalyst VPO-Cr-PEG/TiO2 was developed and the low temperature de-NOx performance of catalyst was studied. The experimental results showed that the de-NOx efficiency reached over 98% at the low temperature of 150–350 °C. The catalyst exhibited excellent sulfur and water resistance in the presence of 1000 ppm SO2 and 8 vol% H2O. In the presence of polyethylene glycol (PEG), the crystal surface energy of the catalyst decreased, and (VO)2P2O7 was the main active crystal phase of the catalyst. The selective catalytic reduction of NOx by NH3 over VP(1/5)O-Cr-PEG/TiO2 catalyst simultaneously followed the Langmuir-Hinshelwood and Eley-Rideal mechanisms.

Catalysts 11(11), 1325 (1-15) (2021). doi:10.3390/catal11111325, This study presents information about crystal imperfections in the main phase of industrial vanadium phosphorous oxide catalysts that are used to catalyze the oxidation of n-butane to maleic anhydride, being an important intermediate in the chemical industry. The mechanism of this reaction is still debated, and the catalytically active and selective surface centers have not yet been identified. The results presented are based on X-ray diffraction data obtained by both laboratory-scale and synchrotron powder diffraction experiments, as well as laboratory-scale single-crystal diffraction experiments. It has been proven that pronounced Bragg reflection broadening effects found in laboratory-scale powder diffraction patterns of industrial VPO catalysts are real and not due to an insufficient 2-�� resolution of the apparatus. In the framework of this work, a powder diffraction full profile fitting strategy was developed using the TOPAS software, which was applied to analyze the X-ray diffraction data of four differently activated industrial catalyst samples, originating from one batch after they had been catalytically tested. It was found that the reflection broadening is mainly caused by an anisotropic crystal size, which results in platelet-shaped crystallites of vanadyl pyrophosphate. A further contribution to the reflex broadening, especially for (111), was found to be a result of stacking faults perpendicular to the a direction in the crystal structure of vanadyl pyrophosphate. These results were used to elaborate on possible correlations between structural proxies and catalytic performance. A direct correlation between the extension of coherently scattering domains in the z direction and the catalyst���s selectivity could be proven, whereas the activity turned out to be dependent on the crystallite shape. Regarding the phase contents, it could be shown that sample catalysts containing a higher amount of ��-VO(PO$_3$)$_2$ showed increased catalytic activity., Published by MDPI, Basel

A catalyst 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10−4)/Ti for low-temperature SCR de-NOx was developed and the effects of SO2 and water vapor on its catalytic activity were investigated. Cr doping increases the molar ratio of V5+ to V4+ on the VPO catalyst, which promote oxidation of NO to NO2 and catalytic activity of the VPO catalyst. An appropriate amount of redox-coupled V5+/V4+ also promotes catalytic activity of the VPO catalyst. The denitration efficiency over 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10−4)/Ti was above 98% at 150–350°C. Cr doping also promotes the generation of Lewis acid around V5+ center and Brønsted acid (P-OH) on the VPO catalysts. The strong surface acidity of 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10−4)/Ti could restrain the adsorption of and oxidation of SO2. Characterization (FT-IR, TG) results indicate that nearly no sulfate was deposited on the surface of 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10−4)/Ti after activity test. The competitive adsorption of water molecules and nitric oxide on the catalyst surface decreases the catalytic activity of the VPO catalyst. The addition of Cr and PEG could increase the surface area and the exposure of active site of VPO catalyst, which also increase the unoccupied active sites of VPO catalysts in the presence of water vapor. The catalyst 0.1VP(0.2)O-Cr(0.01)-PEG(1 × 10−4)/Ti for low-temperature SCR de-NOx shows high catalytic activity and exhibits high resistance to SO2 and water vapor.

The present work addresses the issue of the promoter effect produced by group VIB elements added to the basic VPO formulation using different methods and loads in order to obtain information linking the catalytic behavior of the solids with their physicochemical properties. The unpromoted VPO precursor (VOHPO4·1/2H2O) was prepared from V 2O5 reduced with an alcoholic mixture and 100% H 3PO4. Cr, Mo or W were either impregnated on the VPO precursor or co-precipitated during the precursor synthesis. The precursors containing varying loads of the promoter were activated following a standard procedure in the same flow micro-reactor where the catalytic performance was evaluated. The precursors and the catalysts were characterized by X-ray diffraction (XRD) and laser Raman spectroscopy (LRS). Acetonitrile was used as a probe molecule to test the strength of the Lewis acid sites monitored by FTIR. The amount of chemisorbed acetonitrile was measured in a standard high vacuum system. The addition of the promoters invariably increased the catalytic activity but in every case there was an optimum load to achieve the best selectivity. The origin of this maximum could be ascribed to the right balance between the presence of very strong Lewis acid sites and the development of V5+ isolated sites in the matrix of the vanadyl pyrophosphate, by far the major crystalline phase present in the catalyst. The three group VIB promoters affect the balance between acid and redox properties but increase the overall activity in all cases. Fil: Pierini, Beatriz T.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Investigaciones en Catálisis y Petroquímica "Ing. José Miguel Parera". Universidad Nacional del Litoral. Instituto de Investigaciones en Catálisis y Petroquímica "Ing. José Miguel Parera"; Argentina Fil: Lombardo, Eduardo Agustin. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Investigaciones en Catálisis y Petroquímica "Ing. José Miguel Parera". Universidad Nacional del Litoral. Instituto de Investigaciones en Catálisis y Petroquímica "Ing. José Miguel Parera"; Argentina

International audience; The magnetic properties of [(VO)3(μ-PO4)2(2,2′-bpy)(μ-OH2)]1/3H2O (1) and (VO)2H4P2O9 (2), a tubular and a layered vanadium(IV) phosphates containing triply oxido bridged VIV dimers, are analyzed considering the Bleaney-Bowers S = 1/2 dimer model. In compound 1 the presence of an additional VIV connected with the VIV dimers through μ1,2-PO43− bridges is described with a Curie-Weiss type correction. This model reproduces the magnetic properties of compound 1 with g = 1.956, Jdim = −102.1 cm−1, θ = −0.4 cm−1 and Nα = 278 × 10−6 emu mol−1. In compound 2, the presence of a small percentage of paramagnetic impurity has to be considered to account for the divergence of χm at low temperatures. This simple model reproduces the magnetic data of compound 2 with g = 1.99, J = −62.4 cm−1 and a 1.2% of monomeric impurity. The moderate antiferromagnetic coupling found in the triply oxido bridges VIV dimers is justified from the structural parameters of the bridge. These studies confirm that the coupling through -O-P-O- bridges is antiferromagnetic and relatively weak, as well as previous magneto-structural correlations in this kind of oxido bridges. DFT calculations on a dinuclear fragment model for the two systems gave the following values of Jcalc = −72.4 cm−1 for 1 and Jcalc = −5.2 cm−1 for 2. These values reproduce the antiferromagnetic nature of the superexchange interactions between the VIV centers.

International audience; 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 V-V=O and V-IV=O 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 ((VO)-O-iv)(2)P2O7 phase is formed during propane ammoxidation at 753 K. Such transformation is less intense in inert atmosphere at 753 K. Ammonia adsorption on the VPO catalyst surface leads to the formation of VO2+ sites, probably bearing Bronsted and/or Lewis acidity. Propane ammoxidation appears to promote the formation of mixed valence compounds. (c) 2012 Elsevier B.V. All rights reserved.