Dispersion Of Vanadium Species Research Articles

Surface anchoring requirements for vanadia clusters on titanium oxide surfaces and their impact on activity for oxidative dehydrogenation of ethanol

Titanium oxide site requirements for the anchoring of catalytically active forms of dispersed vanadia species are investigated. Using a series of oxygen-modified titanium nitride materials, we systematically modify the available vanadia-anchoring moieties on the titanium-bearing support surface. This strategy in turn results in a very narrow size distribution of well-dispersed vanadia clusters anchored on the support. This synthesis technique ensures that only highly dispersed vanadia is present on the catalyst surface even at nominal monolayer coverages. A deeper insight into the catalytic behavior for ethanol partial oxidation of the active vanadia species is obtained by comparing intrinsic kinetic and thermodynamic parameters of kinetically relevant reaction steps (heat of ethanol adsorption, hydrogen abstraction activation energy, and the energetics of catalyst reoxidation). It is found that these parameters are dependent on the relative amount of oxygen in the titanium-bearing support and the resulting distribution of vanadia species, which validates the premise of a potential participation of lattice oxygen atoms of the titania support into the catalytic activity of active vanadia species.

In-situ DRIFTs study on different exposed facets of VOx-MnOx/CeO2 catalysts for low-temperature NH3-SCR

In order to achieve the low-temperature (200–250°C) NH3-SCR, highly dispersed VOx-MnOx/CeO2 catalysts with different exposed facets were produced. The NH3-SCR performance results indicate that the NO conversion over VOx-MnOx/CeO2-R with preferentially exposed {110} facets can achieve NO conversion >95% over a wide temperature span of 220–330°C. From the in-situ DRIFTs, gaseous NH3 and NO are favorable to be absorbed on the surface of the VOx-MnOx/CeO2-R catalyst with preferentially exposed {110} facets, which improves the efficiency of NO conversion. The mechanism study via in-situ DRIFTs demonstrates that the well dispersed vanadium species on CeO2{110} absorb NH3 to generate NH3(L) and NH+ 4(B) species, which in turn become highly reactive toward bridging nitrate and bidentate nitrate species to form N2 and H2O according to the Langmuir-Hinshelwood mechanism.

Titanium dioxide doped with vanadium as effective catalyst for selective oxidation of diphenyl sulfide to diphenyl sulfonate

Diphenyl sulfide was oxidized to sulfoxide and sulfone over V-doped TiO2 using a 30% solution of H2O2. The TiO2 samples with different intended content of vanadium (0.02, 0.05, 0.1 and 0.18 mass%) were prepared by incipient wetness impregnation. Physicochemical properties of the V-doped TiO2 were characterized by chemical analysis (ICP-OES), X-ray diffraction (XRD/in situ HT-XRD), UV–Vis diffuse reflectance spectrometry (UV–Vis DRS), N2-sorption measurements, electron paramagnetic resonance and cyclic voltammetry. Both vanadium oxide loading and calcination temperature influenced the structure of the V-TiO2 samples. Vanadium species deposited on TiO2 decreased temperatures required for anatase to rutile phase transformation. The V-TiO2 samples were found to be efficient catalysts for oxidation of sulfides to sulfones. The sample with the lowest vanadium content (0.02VTiO2) presented among the studied catalysts the best catalytic properties with respect to high conversion of diphenyl sulfide to diphenyl sulfonate. An increase in vanadium loading resulted in decrease in catalytic activity of the samples. Also non-modified TiO2 presented significantly lower catalytic activity in comparison with 0.02VTiO2. This interesting effect was related to the formation of highly dispersed vanadium species catalytically active in Ph2S oxidation in the case of the samples with lower V-content. An increase in vanadium loading results in the formation of more aggregated V-species inactive, or less active, in the process of diphenyl sulfide oxidation.

Transient operando study on the NH3/NH4+ interplay in V-SCR monolithic catalysts

The assessment of an integral catalytic reactor facilitating operando spectroscopic measurements on a monolith has been carried out using NH3-SCR on a vanadia-based catalyst as a probe reaction. The NH3-SCR mechanism is revisited by studying the adsorbed ammonia and ionically bound ammonium ions and their relations to Lewis and Brønsted acid sites during reaction. The simultaneous presence of molecular water and ammonia adsorbed on the surface is intrinsic to low temperature NH3-SCR, and their IR absorption bands overlap in the bending region around 1600cm−1. This has to be tackled in order to genuinely reproduce real reaction conditions and simultaneously extract relevant spectroscopic data of a working industrial monolithic catalyst. This operando study on a V2O5-WO3/TiO2–sepiolite monolith is performed without added water in the gas phase, but water formed upstream is adsorbed further down, and thus detected by FTIR spectroscopy. We observed that Multivariate Curve Resolution Alternating Least Squares (MCR-ALS) analysis of the 1620cm−1 envelope of peaks can successfully resolve the bands of adsorbed H2O and NH3 in the monolith during transients between NH3 and NO+O2 flow. Their evolution along with that of surface ammonium ions indicates that generated water hydrolyzes molecularly dispersed vanadia species of the catalyst creating new Brønsted acid sites, and that adsorbed ammonia is either used for the SCR reaction, or converted into the ammonium ion, which then participates in the SCR reaction.

Selective oxidation of benzyl alcohol through eco-friendly processes using mesoporous V-MCM-41, Fe-MCM-41 and Co-MCM-41 materials

MCM-41 nanostructured materials modified with Vanadium, Iron and Cobalt were synthesized by a hydrothermal method. The catalysts were characterized by XRD, UV–Vis-DR, ICP-OES and N2 adsorption. All the catalysts showed good structural order and high specific areas; however the lowest value in these parameters, corresponding to the Co-M(60) sample, could be due to the higher presence of oxide species determined by UV–Vis-DR. These mesoporous metalosilicates were evaluated in the liquid phase oxidation of benzyl alcohol (BzOH) to benzaldehyde (BzH) using H2O2 as oxidant. Results showed that V-M(60) had better catalytic performance than Fe-M(60) and Co-M(60) exhibiting high TON (1100mol/molV), selectivity to BzH (95%) and 31.7% yield at 7h under optimized reaction conditions. The main reason for the enhanced catalytic performance was attributed to the well dispersion of vanadium species in the framework which could be considered as the active sites for the benzyl alcohol oxidation reaction. At the same time, the catalyst could be recovered and effectively reused during three cycles without a significant loss in its activity and selectivity.

Selective cyclohexane oxidation over vanadium incorporated silica pillared clay catalysts: The effect of VOx content and dispersion

The design of efficient and stable clay based materials for catalytic technologies is a great challenge. We demonstrated a new approach of fabricating vanadium incorporated silica-pillared clay by interlayered self-assembly synthetic route. And such synthetic route utilized tartaric acid as the complexing reagent, both dodecyl dimethyl benzyl ammonium bromide and octadecylamine as the mixed structure-directing agent. The obtained vanadium-containned silica-pillared clay samples were characterized by X-ray fluorescence, X-ray diffraction, transmission electron microscopy, N2 adsorption-desorption isotherms, Fourier-transform infrared spectroscopy, diffuse reflectance ultraviolet–visible spectroscopy and X-ray photoelectron spectroscopy techniques. Characterization results indicate that there is an important correlation between the concentration of octadecylamine and uniform structure of the obtained vanadium-containned sample. The octadecylamine is not only used as swelling agency but also decreases the polarity to confirm the uniform mesoporous structure form in the interlayered regions. It was generally considered that hydrothermal synthesis bears the advantage of incorporating active metal ions into the framework in the tetrahedral coordination with V4+ state under the combined action of complexing reagent and mixed structure-directing agent. And the V2O5 phase was gradually formed via aggregation and agglomeration of increasing highly dispersed vanadium species. The catalytic behaviour and performance was examined using the selective oxidation of cyclohexane. The obtained catalyst possesses larger surface area and mesoporous volume and uniform slit-shaped mesopores, which are beneficial to the diffusion of the vanadium atom and responsible for higher yield of the KA oil. And the mechanism associated with the porous structure change has also been discussed.

Dehydrogenation of Isobutane with Carbon Dioxide over SBA-15-Supported Vanadium Oxide Catalysts

A series of vanadia catalysts supported on SBA-15 (V/SBA) with a vanadia (V) content ranging from 1% to 11% were prepared by an incipient wetness method. Their catalytic behavior in the dehydrogenation of isobutane to isobutene with CO2 was examined. The catalysts were characterized by N2 adsorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and temperature-programmed reduction (TPR). It was found that these catalysts were effective for the dehydrogenation reaction, and the catalytic activity is correlated with the amount of dispersed vanadium species on the SBA-15 support. The 7% V/SBA catalyst shows the highest activity, which gives 40.8% isobutane conversion and 84.8% isobutene selectivity. The SBA-15-supported vanadia exhibits higher isobutane conversion and isobutene selectivity than the MCM-41-supported one.

Oligomerization of Supported Vanadia: Structural Insight Using Surface-Science Models with Chemical Complexity

In light of the ongoing debate on the structure of supported vanadia, we report on a spectral marker enabling the direct identification of oligomeric surface structures. A series of VOx/SiO2/Si(100) planar samples with chemical complexity was synthesized by spin-coating and investigated in detail by UV resonance Raman spectroscopy at 256.7 nm excitation as well as by X-ray photoelectron spectroscopy. The enhanced Raman sensitivity allows vibrational spectra to be recorded as a function of vanadium loading (0 ≤ LV ≤ 20.2 V nm–2) despite the small surface area of the planar model samples. At low loadings (LV < 7.3 V nm–2) the spectra are dominated by dispersed vanadia species, whereas at higher loadings the presence of crystalline V2O5 is also observed. We identify new spectral features at 492, 562, and 676 cm–1, which are attributed to V–O–V-related modes of oligomeric vanadia surface species. The vanadia surface species show a linear increase with vanadium loading, saturating at a loading of LV = 7.3 V nm...

In-situ synthesis and characterization of V-MCM-41 for oxidative dehydrogenation of n-butane

A series of MCM-41 catalysts with different vanadium content were prepared via prehydrolysis hydrothermal method with ammonia metavanadate as precursors. The prepared samples were characterized by Powder X-ray diffraction (XRD), N2 adsorption–desorption, X-ray photoelectron spectra (XPS), H2 temperature-programmed reduction (TPR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results showed that V-MCM-41 could still maintain hexagonal mesoporous structure and large specific surface area when the vanadium content was low, whereas the mesoporous structure collapsed in the high V-containing samples. Two vanadium species (V5+/VSiOx and V5+/V2O5) were identified in the catalysts and the highly dispersed vanadium species decreased with the increasing vanadium content. The activity of the catalysts were tested for the oxidative dehydrogenation (ODH) of n-butane with O2 as oxidant, the results showed that V5+/VSiOx was more active than V5+/V2O5.

Active Sites Evaluation of Vanadia Based Powdered and Extruded SCR Catalysts Prepared on Commercial Titania

The active sites of powder and monolith V2O5–WO3/TiO2 catalysts prepared on commercial titania supports were studied for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) by ammonia (NH3) while varying vanadium oxide (V2O5) coverage. Four different types of commercial titania (DT-51, DT-56, G-5, and S-10) were used in this study. It was observed that the SCR activity of powder V2O5–WO3/TiO2 is highly dependent on the vanadium coverage. The catalytic activity increased with increasing V2O5 coverage and reached a maximum between one-half and one monolayer of vanadia. However, NOx conversion decreased at high V2O5 coverage. The catalysts were characterized by temperature-programmed desorption of NH3 (NH3 TPD), temperature-programmed reduction by hydrogen (H2 TPR), oxygen chemisorption, and N2 adsorption to evaluate the active sites responsible for high SCR activity. The spectroscopic studies revealed that the rapid decrease in SCR activity at higher vanadia coverage is due to the decrease in the reducibility and dispersion of vanadium species. This synthetic learning was extended to investigate the active sites in extruded monolith V2O5–WO3/TiO2 catalysts. A good correlation between redox sites and activity for powder and monolith V2O5–WO3/TiO2 catalysts was found.

Dispersion of tungsten oxide on SCR performance of V2O5[sbnd]WO3/TiO2: Acidity, surface species and catalytic activity

Effects of tungsten on vanadium-based catalysts were studied under differently dispersed conditions of tungsten oxide on titania surface. Acidity, surface species and NH3-SCR performance over commercial WO3/TiO2 and V2O5WO3/TiO2 have been investigated. The incomplete dispersion of tungsten oxide gives rise to the formation of crystalline WO3 on titania surface, and the mounts of surface Brønsted acid sites decrease consequently. From the TPR and Raman spectra, it is indicated that two-dimensional vanadium species exhibits a tendency to moderately anchor onto titania surface in the immediate vicinity of tungsten species. It is also found that amorphous tungsten oxide has different impacts on the dispersion of vanadium species over titania surface at variable loadings of vanadia. At low vanadium loadings, amorphously distorted tungsten species show promoting effects on the formation and dispersion of two-dimensional vanadium species. With vanadium loadings increasing, amorphous tungsten species exert compressing effects on surface vanadium species and lead to the aggregation of vanadium oxide.

Influence of temperature on hydroxylation of benzene to phenol using molecular oxygen catalyzed by V/SiO2

Abstract Effect of temperature on hydroxylation of benzene to phenol with molecular oxygen as an oxidant was studied over V/SiO2 using different reductants. The V/SiO2 with highly dispersed vanadium species was prepared by a sol–gel process and characterized by diffuse reflectance UV–Vis and ESR. This work shows that the onset temperature range of benzene hydroxylation and the temperature reaching the maximum phenol yield differ corresponding to each reductant. The reducing capacity of reductant can be shown by changing temperature and affects, in turn, benzene conversion, product selectivity, and the amount of leaching of V species.

Selective oxidation of methanol to dimethoxymethane over acid-modified V2O5/TiO2 catalysts

Selective oxidation of methanol to dimethoxymethane (DMM) was conducted in a fixed-bed reactor over an acid-modified V2O5/TiO2 catalyst. The influence of the acid modification on its structure, redox and acidic properties, and catalytic performance for methanol oxidation were investigated. The results indicated that the content of vanadia in the catalyst exhibits a vital influence on the dispersion of vanadium species, while the acid modification can enhance its surface acidity. Proper amounts of the acid (W(SO42-)=15%) and V2O5 (W(V2O5)=15%) components loaded in the acid-modified V2O5/TiO2 catalyst are able to build a bi-functional circumstance that is favorable for the formation of DMM with high activity and selectivity. As a result, for the selective oxidation of methanol, the H2SO4-modified V2O5/TiO2 catalyst gives a much higher DMM yield at 150°C than the unmodified one.

V2O5/Ce0.6Zr0.4O2‐Al2O3 as an Efficient Catalyst for the Oxidative Dehydrogenation of Ethylbenzene with Carbon Dioxide

V2O5 impregnated onto Ce0.6Zr0.4O2-Al2O3 is a highly active and stable catalyst for the oxidative dehydrogenation of ethylbenzene with CO2. For the first time, highly dispersed vanadium species stabilized with CeO2 and/or CeZrO2 solid solution and deposited carbonaceous materials are revealed to be main factors contributing to its high stability.

Effect of the support on the catalytic properties of vanadyl phosphate in the oxidative dehydrogenation of propane

Propane oxidative dehydrogenation has been investigated on vanadyl orthophosphate dispersed on different supports (Al 2O 3, TiO 2, SiO 2, ZrO 2, MgO and SnO) in a fixed bed reactor in the temperature range 300–550 °C. Catalysts have been characterized by bulk and surface sensitive techniques, such as X-ray diffraction (XRD), BET analysis, temperature programmed reduction with H 2 (H 2-TPR) and X-ray photoelectron spectroscopy (XPS). A different dispersion of vanadium species has been found depending on the different nature of the supports. Al 2O 3, ZrO 2 and TiO 2 promote the best dispersion of vanadium and provide the highest catalytic performances, due to the strong interaction of the support with the phosphate phase and the enhanced vanadium reducibility. A different reaction path is activated by catalysts supported on SiO 2, MgO and SnO due to the formation of vanadyl phosphate aggregates and in some cases also to the reducibility of the support which favour propane overoxidation leading to a larger CO 2 production.

The poisoning effect of Na+ and Ca2+ ions doped on the V2O5/TiO2 catalysts for selective catalytic reduction of NO by NH3

Deactivation of the supported V2O5/TiO2 catalysts due to deposition of alkali and alkaline earth metal salts in fly ashes is one of the most important problems in the selective catalytic reduction (SCR) of NOx by NH3. In this work, the poisoning effect of Na+ and Ca2+ ions doped on the V2O5/TiO2 catalysts for selective catalytic reduction of NO by NH3 has been studied. It has been shown that the Na+ and Ca2+ ions doped can poison the V2O5/TiO2 catalyst and the Na+ ions exhibit greater poisoning effect than Ca2+ ions. The NO rate constant k on the Na+-V2O5/TiO2 catalyst decreased very sharply from 97.1 to 35.0cm3g−1s−1 with increasing Na/V molar ratio from 0.00 to 0.20. In comparison, k on the Ca2+-V2O5/TiO2 catalyst remained almost unchanged when Ca/V molar ratio was below 0.05. Further increasing Ca/V molar ratio to 0.20 resulted in an obvious decrease of k from 101.8 to 46.3cm3g−1s−1. It was suggested that Na+ ions combined strongly with dispersed vanadia species, neutralized the Brønsted acid sites and reduced their reducibility. In comparison, Ca2+ ions slightly affected the Brønsted acid sites of catalyst. Besides, the dispersed vanadia species on the Ca2+-V2O5/TiO2 catalyst had higher reducibility than Na+-V2O5/TiO2 catalyst. The different poisoning effects of Na+ and Ca2+ ions doped on the V2O5/TiO2 catalyst were correlated not only to the surface acidity but also to the reducibility of catalyst.

The Oxidative Dehydrogenation of Propane Using Vanadium Oxide Supported on Nanocrystalline Ceria

Nanocrystalline ceria was prepared as a support for vanadium oxide catalysts and tested for the oxidative dehydrogenation of propane. Nanocrystalline ceria is very active for the total oxidation of propane under conditions used for oxidative dehydrogenation. The addition of vanadium results in a switch of activity to produce propene with appreciable selectivity. The catalyst performance depends on the vanadium loading. Lower vanadium loadings resulted in catalysts with highly dispersed vanadia species, which were selective towards propene production. Higher vanadium loadings resulted in the formation of a mixed cerium–vanadium phase, which was also active for propane selective oxidation. A catalyst with an intermediate loading was far less selective. Catalysts were characterised by a range of techniques (including XRD, laser Raman, TPR, SEM/EDX and XPS), and the activity of the catalysts can be related to their structure and chemistry.

A Non-sodium Synthesis of Highly Ordered V-MCM-41 and Its Catalytic Application in Isomerization

A rapid non-sodium process was developed to synthesize highly ordered V-MCM-41 mesoporous materials with atomic level dispersed vanadium species. NH3·H2O instead of NaOH was used to maintain the moderately basic synthesis condition. The characterization results of N2 physisorption and XRD showed that the synthesized samples had highly ordered hexagonal structure. UV–Vis, UV-Raman, and EPR spectra provided evidence that most vanadium ions were tetrahedrally incorporated into the framework of siliceous MCM-41. The NH3-TPD proved the mild acidity of V-MCM-41 due to the incorporation of vanadium cations. The most important variables were structural order and vanadium loading which can be controlled easily by changing synthesis variables in such a way that collapse of the V-MCM-41structure did not occur. The synthesized V-MCM-41 catalysts successfully catalyzed the isomerization of heptene. High conversion and isomer selectivity were achieved with good catalytic stability. The structure of V-MCM-41 can be well preserved after reaction.

Vanadium-modified molecular sieves: preparation, characterization and catalysis

Vanadium-containing molecular sieves are redox catalysts and are good candidates as substitutes for oxide-supported V2O5 in a number of reactions. These materials have the advantage of presenting better dispersion of vanadium species, as well as shape-selective properties and controllable acidities. They may be prepared by one-pot synthesis or by post-synthesis methods and a number of techniques such as diffuse reflectance UV-visible spectroscopy, 51V nuclear magnetic resonance and electron paramagnetic resonance, to name but a few, have been used to characterize these materials. In this review, methods of preparation of vanadium-modified molecular sieves, their characterization and applications in catalysis are discussed.

Nanoparticles of vanadia–zirconia catalysts synthesized by polyol-mediated route: Enhanced selectivity for the oxidative dehydrogenation of propane to propene

A polyol-mediated route was employed to obtain nanoparticles of vanadia–zirconia (∼10 nm) and Ti4+-modified zirconia catalyst for the selective oxidative dehydrogenation reaction of propane to propene. The catalytic activity and selectivity of samples thus prepared were compared with the values for the sample synthesized by the conventional impregnation method. More dispersed amorphous vanadia species on zirconia support could be obtained by polyol method compared to those obtained by conventional impregnation route, as discerned from transmission electron microscopy (TEM) and Raman studies. Raman spectra of samples prepared by polyol method indicated the presence of monovanadate and polyvanadate species on the zirconia support surface, whereas the impregnated sample showed the existence of aggregated vanadia besides mono and polyvanadate species though the vanadia loading was the same on all samples. XPS studies revealed that vanadia existed as both V5+ and V4+ in the samples prepared by the polyol method, whereas only V5+ state was seen in the impregnated sample. The catalysts prepared by polyol method exhibited enhanced selectivity for propene formation compared to the sample prepared by impregnation method. The enhanced selectivity is attributed to the presence of dispersed vanadia species with lower valence state of vanadium. The present results demonstrate that the polyol-mediated synthesis is an efficient method for the preparation of supported vanadia catalysts containing such active species.