Vanadium Oxide Catalysts Research Articles

Unveiling the mechanistic insights into the potassium resistance of 3.5 W-V-1 %K NH3-SCR catalysts: The dual functionality of V-O-W structure as acid and redox sites

The poisoning effect of alkali metals over Vanadium-based catalysts still faced a significant challenge in the selective catalytic reduction of NOx with NH3 (NH3-SCR). In order to improve the alkali (mainly potassium resistance) resistance of the Vanadium-based catalysts, WO3 was introduced into the vanadium oxide catalysts. The obtained 3.5WO3-V2O5(3.5 W-V) catalyst exhibited the excellent low-temperature NH3-SCR performance owing to the formation of V-O-W structures. Noteworthily, when 1 wt% K was introduced into the 3.5 W-V catalyst, vanadium oxide (V-O, V=O) species preferentially bonded to alkali metal K as an acidic sacrifice site, prompting more active V-O-W species to remain. Thus, the 3.5 W-V catalyst also possessed excellent alkali resistance. The NH3-TPD results showed that the 3.5 W-V-1 %K catalyst had a considerable acid site to ensure the adsorption capacity of NH3, while the XPS results showed that the 3.5 W-V-1 %K catalyst had excellent redox performance to ensure the activation capacity of the reactant NH3. Moreover, in-situ DRIFTS transient experiments revealed that the 3.5 W-V and 3.5 W-V-1 %K catalysts were more capable of adsorbing and activating NH3 species to rapidly react with gaseous NO, obeying the Eley-Rideal (E-R) pathway in NH3-SCR reaction. This work provides new insights into poisoning resistant of vanadium-based catalysts.

O2 and CO2 assisted oxidative dehydrogenation of propane using ZrO2 supported vanadium and chromium oxide catalysts

Zirconia supported vanadium oxide (vanadia) and chromium oxide (chromia) catalysts of 1–3 wt% of V or Cr were prepared, characterized, and tested for the oxidative dehydrogenation (ODH) of propane (C3H8) with O2 and CO2. Characterization results reveal that 2.5% metal loading is in slight excess of monolayer loadings for this ZrO2, which has a surface area of 48 m2.g−1. The reaction results reveal that supported vanadia catalysts are better for O2-ODH and supported chromia catalyst are better for CO2-ODH at a reaction temperature of 550 °C. Furthermore, the C3H8 conversion and propene (C3H6) yield increases with loading and the highest conversion and yield are also achieved at 2.5% metal loading. With higher loadings the conversion and yield decreases, clearly indicating that the surface metal oxide species are more active than their crystalline counterparts. As the contact time increases, for inlet stoichiometric ratios of the reactants for the two reactions, C3H8/CO2 = 1 and C3H8/O2 = 2, C3H8 conversion and C3H6 yield both monotonically increase and appear to approach a constant value; however, the difference between C3H8 conversion and C3H6 yield also increases. The C3H6 selectivity during O2-ODH (∼30%) is much lower than C3H6 selectivity during CO2-ODH (86–94%). Furthermore, during CO2-ODH of propane the conversions of C3H8 and CO2 are similar, and it appears that under the operating conditions used, the dry reforming of propane does not occur. Overall, the CO2-ODH of propane reaction at 550 °C using stoichiometric ratios of C3H8/CO2 over ZrO2 supported chromia catalyst provides a better C3H6 yield and selectivity, with an additional advantage of converting CO2.

Catalytic Properties of Ethylene Reactions Using Silver and Reduced Vanadium Oxide Catalysts on MEMS Chips with Micro Hot Plate

Catalytic Properties of Ethylene Reactions Using Silver and Reduced Vanadium Oxide Catalysts on MEMS Chips with Micro Hot Plate

Mesoporous ceria-supported vanadia catalysts for selective aerobic oxidation of ethylbenzene

A series of mesoporous ceria-supported vanadium oxide catalysts were prepared and explored for aerobic oxidation of ethylbenzene to acetophenone without the involvement of any solvent or additive.

Uncovering the Dinuclear Mechanism of NO2-Involved NH3-SCR over Supported V2O5/TiO2 Catalysts.

Commercial vanadium oxide catalysts exhibit high efficiency for the selective catalytic reduction (SCR) of NO with NH3, especially in the presence of NO2 (i.e., occurrence of fast NH3-SCR). The high-activity sites and their working principle for the fast NH3-SCR reaction, however, remain elusive. Here, by combining in situ spectroscopy, isotopic labeling experiments, and density functional theory (DFT) calculations, we demonstrate that polymeric vanadyl species act as the main active sites in the fast SCR reaction because the coupling effect of the polymeric structure alters the elementary reaction step and effectively avoids the high energy barrier of the rate-determining step over monomeric vanadyl species. This study unveils the high-activity dinuclear mechanism of the NO2-involved SCR reaction over vanadia-based catalysts and provides a fundamental basis for developing high-efficiency and low V2O5-loading SCR catalysts.

A DFT study of the catalytic ODH of n-hexane over a cluster model of vanadium oxide

Supported vanadium oxides are one of the most widely reported catalysts for the selective oxidative dehydrogenation (ODH) of alkanes to alkenes. These vanadium oxide catalysts contain at least three distinct O species classified by their coordination to the vanadium cations. The involvement in ODH of the different oxygen species is thought to follow the coordination environment with singly coordinated vanadyl V(V)=O being the most reactive. A quantitative understanding of the relative activity of the different oxygen species in vanadium oxides could inform catalyst design to further enhance productivity and selectivity in the ODH reaction. Here we use computational models based on hybrid density functional theory to study the mechanistic pathway for n-hexane to 1- and 2-hexene. We compare the potential energy surfaces calculated for these reactions initiated by β-H abstraction with H transfer to the vanadyl oxygen, V(V)=O, with that involving H transfer to a two-coordinate bridging O atom. An isolated H4V2O7 cluster is used to represent the supported vanadium oxide catalyst so that the reactivity of oxygen species can be understood in the absence of support effects. Gibbs energy calculations were carried out at temperatures of 573, 673 and 773 K to mirror laboratory experimental conditions. We find that the rate-determining step (RDS) in the conversion of n-hexane to either alkene is associated with n-hexane interaction with H4V2O7 through a secondary CH bond. This leads to β-H abstraction with the calculated reaction barrier for abstraction by a V(V)=O (ΔE‡ = +32.7 kcal mol−1) significantly lower than that for the two-coordinate bridging O (ΔE‡ = +43.9 kcal mol−1). H-abstraction leads to reduction of one of the vanadium cations. Removal of the second H atom to form an alkene can follow several pathways. We have considered the second step leading to 2-hexene (γ-H abstraction) using either an adjacent V(V)=O on the reduced V(IV)-O-V(V) unit, a different active V(V)=O site on a fully oxidised cluster, or gas-phase molecular O2. Our results show that the ODH process is likely to proceed via a Mars-van Krevelen redox mechanism. In a practical catalyst the results imply that catalyst activity will depend on the surface coverage of V(V)=O active sites and the n-hexane to gas-phase molecular oxygen ratio. In addition to the pathways leading to alkene products we have noted the formation of CO bonds by the radical intermediate formed from the initial β-H abstraction. From this observation, we suggest that the low yields of 1- and 2-hexene (< 20%) obtained in our laboratory experiments with V2O5/MgO catalysts may be a result of the chemisorption properties of the radical intermediate (∙C6H13) on bridging or terminal O sites in the V(V)-O-V(V) units, leading to undesired products including oxygenates.

Active vanadium(iv) species synthesized at low-temperature facilitate excellent performance in low-temperature NOx-catalytic removal

V4+ species in a series of bulk vanadium oxide catalysts is more conducive to NH3 and O2 activation.

Supported vanadium oxide catalyst over HY-zeolite-alumina composite fabricated by extrusion for oxidative desulfurization of dibenzothiophene

Despite the advantages of zeolite catalysts/carriers, the small pore size of HY zeolite causes diffusion limitations especially for bulky refractory sulfur compounds in oxidative desulfurization (ODS) of fuels. To overcome this restriction, extruded HY-zeolite/Alumina composites containing both micro and meso‑pores were successfully synthesized and used as an attractive carrier for the vanadium dispersion. Insights about the interactions between zeolite and alumina and their effects on the vanadium impregnation were given implementing different analysis approaches; BET, SEM-EDX, TEM, NH3-TPD, and UV-vis. Alumina particle piling created meso‑pores in the composites providing a positive impact on the ODS of bulky dibenzothiophene (DBT). Comparing to physical mixing of the alumina and zeolite, the extrusion approach notably enhanced the weak acid sites due to the removal/distorting tetrahedrally coordinated Al from the zeolite framework. A less increase in the strong acid sites was also resulted from the reaction of Al-OH in the alumina with the free Si-OH over the zeolite. The proposed catalyst (V/HYZ-Al(alumina/zeolite ratio=50%) exhibited an excellent DBT removal so that 1000 ppmw of DBT was eliminated completely under the optimal investigated conditions (T = 50 °C, O/(S+N)=12 mol/mol, time=90 min, Mcatalyst/Vfuel=10 g/L). The catalyst recyclability was proved to be acceptable during four examined successive ODS cycles.

Kinetic Coupling of Redox and Acid Chemistry in Methanol Partial Oxidation on Vanadium Oxide Catalysts

Kinetic, isotopic, and spectroscopic studies establish the active site requirements for three kinetically coupled catalytic cycles catalyzed by redox, Brønsted, and Lewis acid–base sites, which occur during methanol and oxygen reactions on titania-supported vanadium oxide catalysts. The initial activation of methanol during its oxidative dehydrogenation to formaldehyde restricts the overall turnovers─this reaction proceeds via CH3OH dissociative adsorption followed by a kinetically relevant C–H bond scission of the CH3O intermediate on V–O redox site pairs found at the interface of VOx and TiO2. The Gibbs free energy change of these two steps (ΔGads and ΔG⧧, respectively) both decrease as V–O–V coordination decreases and V–O–Ti coordination increases, leading to higher turnovers per surface vanadium as VOx dispersion increases, except the extreme case of isolated VO4. Coupled kinetically with the oxidative dehydrogenation cycle is the Brønsted acid-catalyzed cycle that forms dimethoxymethane─this cycle proceeds via methanol- and formaldehyde-derived CH3OCH2OH adsorption to Brønsted sites at the VOx–TiO2 interface, followed by its kinetically relevant C–O bond scission. Its turnovers also increase with VOx dispersion following the same trend as the oxidative dehydrogenation cycle but at a much faster rate, so the reaction can readily approach chemical equilibrium. Alongside the other two catalytic cycles is the Tishchenko reaction that forms methyl formate from two formaldehyde molecules, produced by the methanol oxidative dehydrogenation cycle, on exposed Ti4+–O2– Lewis acid–base pairs uncovered by VOx─this cycle proceeds via kinetically relevant intermolecular C–O bond formation followed by a rapid 1,3-hydride shift. The interplay of coexisting redox, Brønsted, and Lewis sites, each with its unique catalytic roles, leads to different rates and yields of the three products. The active site structure and mechanistic knowledge established here allow us to optimize the product ratios required for downstream synthesis of larger oxygenates.

Determining the influence of microwave-induced thermal unevenness on vanadium oxide catalyst particles

• Thermal unevenness of V 2 O 5 catalyst due to microwaves was analyzed by thermography. • Local heat generation started from the contact point of spherical V 2 O 5 particles. • Thermal distribution dynamically changed depending on the oxidation state of V 2 O 5. • Raman spectra demonstrated the enhanced V 2 O 5 redox by microwaves. • Local hot spot and catalyst oxidation state synergistically accelerate catalytic reaction. Microwave (MW) heating accelerates various heterogeneous catalytic reactions at low temperatures, leading to energy-efficient catalytic processes. Herein we report the formation of MW–formed uneven thermal distribution at the interparticle of V 2 O 5 catalyst by using microscopic thermography and electromagnetic/thermal flow simulations. The local heat generation started from the contact point of the spherical catalyst particles. The uneven thermal distribution changed as the oxidation state of the catalyst dynamically changed during the dehydration of 2-propanol. In situ Raman spectroscopy suggested that dynamic change in the thermal gradient attributes to the oxidation state of the V 2 O 5 catalyst. As a result, the formation of the local hot spot at the contact point of the V 2 O 5 catalyst particles, as well as the oxidation state of V 2 O 5 due to MWs, synergistically affect the enhancement in the dehydration of 2-propanol. The present results provide important insights into a mechanistic understanding of the reaction enhancement of the fixed-bed flow reaction owing to the MW-induced uneven thermal distribution.

Effect of promoters on o-xylene oxidation pathways reveals nature of selective sites on TiO2 supported vanadia

The o-xylene oxidation pathways on TiO2 supported vanadium oxide catalysts are affected by Cs+ and Sb4+ promoters. Reaction kinetics change remarkably at 360 °C: while at lower temperatures the rate determining step is the C-H bond cleavage in xylene methyl groups, the catalyst reoxidation becomes kinetically limiting at higher temperatures. Promotion by Sb4+ is related to an increase in the availability of oxygen at high temperatures for the reoxidation of V sites. The presence of Cs+ enhances the reoxidation of V sites and increases the ratio of nucleophilic to electrophilic oxygen on the surface. Catalytic tests of benzene oxidation confirm the lack of electrophilic sites on Cs-promoted VOx/TiO2 catalyst. Combined evidence points to a key role of the bridging lattice oxygen in V-O-V species as active site for the activation of o-xylene and its selective transformation to phthalic anhydride.

Analogous Mechanistic Features of NH3-SCR over Vanadium Oxide and Copper Zeolite Catalysts

Analogous Mechanistic Features of NH₃-SCR over Vanadium Oxide and Copper Zeolite Catalysts

OXIDATIVE AMMONOLYSIS OF 4-METHYLPYRIDINEON OXIDE VANADIUM-TITANIUM-ZIRCONIUM CATALYST MODIFIED BY TIN AND TUNGSTEN OXIDES

Catalysts based on vanadium pentoxide modified by Ti, Sn, Zr and W oxides were tested in the oxidative ammonolysis of 4-methylpyridine. The role of the main process parameters such as temperature, the ratio of the initial components in the conversion of the methyl group to the nitrile one, and the optimal conditions for the oxidative ammonolysis of 4-methylpyridine were determined. It is determined that the V-Ti-Zr-O-catalyst and the sample containing 9% of tungsten oxide are superior in catalytic activity to the V-Ti-Zr-Sn-O contact. Conditions that ensure a high selectivity for the formation of 4-cyanopyridine were found. The highest yield of the target product (85-86%) was obtained on V-Ti-Zr-W-O at 270 °C, and the yield of 4-cyanopyridine was 87.5% at 310° C on the V-Ti-Zr-Sn-O catalyst. The phase composition and structural changes occurring in modified vanadium oxide catalysts have been studied. It is determined that mixed V-Ti-Zr-Sn-O and V-Ti-Zr-W-O catalysts contain ZrV2O7, the monoclinic modification of ZrO2 (baddeleyite), TiO2 (anatase), SnO2, WO3, and V2O5. In catalysts, it can exist in small amounts as a separate VO2 phase. The V-Ti-Zr-W-O catalyst showed the best catalytic properties. It has highactivity and selectivity towards 4-cyanopyridine.

Bulk tungsten-substituted vanadium oxide for low-temperature NOx removal in the presence of water

NH3-SCR (selective catalytic reduction) is important process for removal of NOx. However, water vapor included in exhaust gases critically inhibits the reaction in a low temperature range. Here, we report bulk W-substituted vanadium oxide catalysts for NH3-SCR at a low temperature (100–150 °C) and in the presence of water (~20 vol%). The 3.5 mol% W-substituted vanadium oxide shows >99% (dry) and ~93% (wet, 5–20 vol% water) NO conversion at 150 °C (250 ppm NO, 250 ppm NH3, 4% O2, SV = 40000 mL h−1 gcat−1). Lewis acid sites of W-substituted vanadium oxide are converted to Brønsted acid sites under a wet condition while the distribution of Brønsted and Lewis acid sites does not change without tungsten. NH4+ species adsorbed on Brønsted acid sites react with NO accompanied by the reduction of V5+ sites at 150 °C. The high redox ability and reactivity of Brønsted acid sites are observed for bulk W-substituted vanadium oxide at a low temperature in the presence of water, and thus the catalytic cycle is less affected by water vapor.

Effects of vanadium, vanadium carbide, and vanadium oxide catalysts on hydrogenation of Mg17Al12 (110) surface: A first principles study

Effects of V and V-based compounds (VC, VO, VO2) on hydrogenation performances of the Mg17Al12 (110) surfaces are studied by first principles calculations. Results indicate that V and V-based compounds are stably adsorbed on the (110) surfaces. Additionally, the V-doped Mg17Al12 (110) systems are simulated. Calculations indicate that compared with the pure (110) system, H2 adsorption and dissociation on the Mg17Al12 (110) surfaces with adding V, VC, VO, and VO2 are dramatically promoted. Particularly, H2 are spontaneously dissociated on the vanadium oxide-adsorbed (110) surfaces. Analyses of density of states for the VO-adsorbed and VO2-adsorbed systems exhibit stronger hybridizations between V d sates and H s states, leading to the improvement of hydrogenation for the Mg17Al12 (110) surface. The vanadium oxide displays a preferred catalytic effect in our investigations, and the relative catalytic mechanism is revealed.

Vanadium oxides anchored on nitrogen-incorporated carbon: An efficient heterogeneous catalyst for the selective oxidation of sulfide to sulfoxide

A highly efficient and durable metal catalyst stabilized by proper support is vital for organic catalytic transformations. In this study, a sequential pyrolysis–acid etching strategy was reported to prepare a nitrogen-rich nanocarbon inlaid with vanadium oxide catalyst (V@CN-750), which was evolved from the preassembly of a chitin−V precursor. V@CN-750 exhibited excellent activity and selectivity of 96% and 98%, respectively, for catalytic oxidation of thioanisole to sulfoxide at 40 °C for 8 h, which is considerably superior to that of V2O5 nanoparticles and homogeneous VO(acac)2. Moreover, V@CN-750 can be recycled at least eight times without affecting its catalytic performance.

Synthesis of bulk vanadium oxide with a large surface area using organic acids and its low-temperature NH3-SCR activity

Selective catalytic reduction using NH3 (NH3-SCR) is a chemical process that is used for the elimination of NOx (NO and NO2). Although current vanadia-based catalysts need a high reaction temperature, which leads to deactivation of the catalysts, bulk vanadium oxide showed the potential for NH3-SCR at a low temperature below 150 °C. We investigated a method for synthesis of vanadium oxide with a large surface area using an organic acid to enhance its NH3-SCR activity. Vanadium oxide catalysts were synthesized from ammonium metavanadate (NH4VO3) and organic acids (oxalic acid, succinic acid, malic acid and citric acid). When carboxylic acids (oxalic acid and succinic acid) were used as the organic acid source, the surface area of the catalysts increased up to 41 cm2 g−1, which was larger than that of vanadium oxide synthesized without an organic acid and with hydroxy acid. SEM and TEM measurements showed that vanadium oxide catalysts synthesized by the calcination of vanadyl oxalate at 300 °C (V2O5-OX_300) formed a pore structure that contributed to the increase in surface area. An increase in the number of acid sites and redox sites, which is important for NH3-SCR to proceed, was confirmed from NH3-TPD and H2-TPR measurements for V2O5-OX_300. The NO conversion of V2O5-OX_300 (47 % at 100 °C) was higher than that of the catalysts synthesized from only NH4VO3 (19 % at 100 °C), indicating that vanadium oxide catalysts synthesized using carboxylic acid have the low-temperature NH3-SCR activity due to the increase in their surface area.

Oxidative ammonolysis of 3,4-dimethylpyridine on the vanadium oxide catalysts

Oxidative ammonolysis of 3,4-dimethylpyridine on an individual vanadium oxide (V2O5) catalyst and binary vanadium oxide catalysts, modified by additions of SnO2 and ZrO2, has been studied. A connection between ??- acidity of the methyl groups of the substrate in the gaseous phase and in the chemosorbed state and the sequence of their transformation into a cyano group has been established. It has been shown that nucleophilicity of vanadyl oxygen, calculated by the Density Functional Theory method, increases with V2O5 modification by SnO2 and ZrO2 additions. Herewith, an increasing yield of 3- methyl-4-cyanopyridine and imide of pyridine-3,4-dicarboxylic acid was observed. A proposed mechanism of the imide of pyridine-3,4-dicarboxylic acid formation has been discussed

A bifunctional catalyst based on Nb and V oxides over alumina: oxidative cleavage of crude glycerol to green formic acid

A bimetallic vanadium and niobium oxide catalyst using alumina as support was developed for the conversion of crude glycerol from biodiesel production into formic acid.

Cover Feature: Understanding the Synthesis of Supported Vanadium Oxide Catalysts Using Chemical Grafting (Chem. Eur. J. 5/2020)

Cover Feature: Understanding the Synthesis of Supported Vanadium Oxide Catalysts Using Chemical Grafting (Chem. Eur. J. 5/2020)