Vanadyl Species Research Articles

Revealing the boosting roles of sulfate groups on NOx selective catalytic reduction over V2O5/CeO2 catalyst

Ceria-supported vanadia (VOx/CeO2) catalyst has been extensively investigated in the field of NOx selective catalytic reduction with NH3 (NH3-SCR) but is limited in practical application owing to its poor sulfur resistance and high-temperature activity. Herein, a series of SO42− modified V2O5/CeO2 catalysts were fabricated to improve the overall catalytic performance. The 2 %SO42−-V2O5/CeO2 catalyst exhibited above 95 % NOx conversion, 90 % N2 selectivity in the temperature range of 225–400 °C and excellent SO2 + H2O tolerance, which was much better than V2O5/CeO2 catalyst. Characterization results showed that the introduction of SO42− over V2O5/CeO2 increased the Ce3+/(Ce3++ Ce4+) ratio to promote the formation of oxygen vacancies and promoted the generation of more conducive polymeric vanadyl species. The synergistic effect of the redox sites and acid centers provided moderate redox property and enhanced surface acidity, thus improving the adsorption of NH3 and inhibiting the over-oxidation of NH3 that occurs on V2O5/CeO2. In-situ DRIFTS indicated that the NOx reduction over V2O5/CeO2 and 2 %SO42−-V2O5/CeO2 followed the Langmuir-Hinshelwood and Eley-Rideal routes concurrently. This study can expound the reasonable design of vanadium-based catalysts for efficient NOx reduction in industry.

Constructing highly active vanadyl species for efficient selective catalytic reduction of NOx

Vanadia-based catalysts have been commercially applied for selective catalytic reduction with NH3 of NOx (NH3-SCR). Considering the biotoxicity of V2O5, V-based catalysts are demanded to exhibit high catalytic activity with low V2O5 loadings. However, at low vanadium contents, surface VOx primarily present in the form of monomers, which show poor low-temperature activity. Here, efficient low-vanadium-loading catalysts were designed via constructing highly reactive polymeric vanadyl species by introducing disparate forms of niobia species. Niobia species were found to promote the polymerization of surface VOx, and the disparate forms of niobia species had a key influence on the formation of polymeric vanadyl species. Crystalline Nb2O5 promoted the formation of polymeric vanadyl species, induced by a structural effect. By contrast, uniformly dispersed niobium species on the surface of a Nb-Ti-O solid solution support served as new anchoring sites for VOx and promoted a greater extent of polymerization of VOx species via electronic interactions among V, Nb and Ti. Ultimately, Nb-Ti-O solid solution-supported V2O5 exhibited much higher low-temperature reaction rates than TiO2-supported Nb2O5 and V2O5 due to more polymeric vanadyl species. This study provides new methods to design low-vanadium-loading catalysts, offering new perspectives for the modification of transition metal oxides to design efficient catalysts.

In an attempt to add ligands to the sixth (axial) position of vanadyl bis-acetylacetonate: A unique tetranuclear vanadyl species

We have explored the interaction of [(acac)2V=O] (acac = acetylacetone) with a series of potential ligands which were chosen because of their expected ability to attach themselves onto its sixth (axial) position. Furthermore, some of the species chosen were expected to have the capability of linking pairs of [(acac)2V=O] molecules, thus creating magnetically coupled substances whose behavior would be interesting to document by magnetic as well as structural methods. Some of the synthetic results were surprising in that unexpected products were obtained which we had not envisioned; specifically, herein we describe a tetranuclear vanadyl cluster (Crystal data for C38H51N4O17ClV4: orthorhombic, space group Pca21 (no. 29), a = 26.4698(3) Å, b = 13.5167(2) Å, c = 12.7659(2) Å, V = 4567.44(11) Å3, Z = 4, μ(CuKα) = 7.842 mm-1, Dcalc = 1.53 g/cm3, 41277 reflections measured (6.538° ≤ 2Θ ≤ 137.892°), 7841 unique (Rint = 0.0428, Rsigma = 0.0421) which were used in all calculations; the final R1 was 0.0675 (I > 2σ(I)) and wR2 was 0.1641 (all data)), which is unusual in several aspects of its composition as well as its stereochemistry.

Expedient Azide-Alkyne Huisgen Cycloaddition Catalyzed by a Combination of VOSO4 with Cu(0) in Aqueous Media.

A series of vanadium(III), vanadyl(IV/V) species, inorganic metal oxides, and transition-metal oxides was examined as cocatalysts with Cu(0) powder for copper(I)-catalyzed azide-alkyne cycloaddition. Among them, vanadyl(IV) species bearing acetylacetonate, acetate, and sulfate, vanadyl(V) isopropoxide, and vanadate were suitable for the click reactions of per-acetyl and per-benzyl β-azido glycosides with three different terminal alkynes in CH3CN. Water-soluble vanadyl(IV) sulfate was further selected for efficient click reactions for unprotected β-glycosyl azides and even compatible with a thiol-containing substrate in aqueous media at ambient temperature.

Selective oxidation of methacrolein to methacrylic acid over CsH3PMo11VO40 with structural defects

A novel synthetic strategy using microwave-assisted precipitation was proposed to prepare defective CsH3PMo11VO40 for the oxidation of methacrolein (MAL) to methacrylic acid (MAA). A systematic comparison of catalysts synthesized using different heating methods and preparation conditions was conducted. Microwave treatment was found to accelerate the crystallization rate of the catalyst and increase vanadyl species (VO2+) content on its surface, yielding the formation of defective Keggin structures. A moderate degree of defective Keggin units improves the oxidation capacity of active Mo species and facilitates the activation of oxygen into lattice oxygen, enhancing the oxidation of MAL. Therefore, the catalyst obtained through microwave heating demonstrates significantly improved catalytic performance compared to the one prepared via conventional heating. The defective Keggin structure plays a key role in catalytic enhancing. Under optimal conditions, the 70MH-40 catalyst exhibits the highest MAL conversion of 90.3% and MAA selectivity of 87.2%.

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.

Pr-modified vanadia-based catalyst for simultaneous elimination of NOx and chlorobenzene

The V2O5/TiO2 catalyst is the most concerned NH3-SCR catalytic system for simultaneous removal of NOx and chlorinated volatile organic compounds (Cl-VOCs). There is an urgent need to develop a vanadia-based catalyst with low vanadium loading and excellent low-temperature catalytic performance. In this work, the Pr and W co-doped vanadia-based catalysts were prepared by a simple impregnation method. After the doping of Pr and W, the vanadia-based catalyst exhibited the excellent catalytic activity for NOx reduction and chlorobenzene (CB) oxidation. The more abundant polymeric vanadyl species and crystalline V2O5 were observed on the PrV5W/Ti catalyst, which are contributing to NH3-SCR and CB oxidation activity. The redox capability and the amount of surface-active adsorbed oxygen of vanadia-based catalyst were improved markedly, thus promoting NH3-SCR and CB oxidation reaction on PrV5W/Ti. This catalyst is promising to be used in the muti-pollutant control reaction of NOx and VOCs.

Polymerization State of Vanadyl Species Affects the Catalytic Activity and Arsenic Resistance of the V2O5-WO3/TiO2 Catalyst in Multipollutant Control of NOx and Chlorinated Aromatics.

The conventional V2O5-WO3/TiO2 catalyst suffers severely from arsenic poisoning, leading to a significant loss of catalytic activity. The doping of Al or Mo plays an important role in promoting the arsenic resistance on NH3 selective catalytic reduction (NH3-SCR), but their promotion mechanism remains in debate and has yet to be explored in multipollutant control (MPC) of NOx and chlorinated organics. Herein, our experimental characterizations and density functional theory (DFT) calculations confirmed that arsenic species preferentially adsorb on both Al and Mo to form arsenate, thereby avoiding bonding to the catalytically active V sites. More importantly, Al doping partially converted the polymeric vanadyl species into monomeric ones, thereby inhibiting the near-surface and bulk lattice oxygen mobility of the V2O5-WO3/TiO2 catalyst, while Mo doping resulted in vanadyl polymerization with an enriched V5+ chemical state and exhibited superior MPC activity and COx selectivity. Our work shows that antipoisoning catalysts can be designed with the combination of site protection and occurrence state modification of the active species.

SO2-induced dual active sites formation over VOx/Fe2O3 for accelerating NH3 selective catalytic reduction of NOx

The construction of plausible alternatives of V2O5-WO3/TiO2 for effectual NOx purification is becoming a research hotspot. Herein, a robust VOx/Fe2O3-S catalyst with SO2 tailored surface acid sites and active VOx sites was engineered for boosting the NOx reduction performance. The VOx/Fe2O3-S catalyst afforded above 90% NOx conversion and 90 % N2 selectivity at 275–425 °C, coupled with the exceptional long-term SO2 + H2O resistance. Py-IR and in situ DRIFTS disclosed that the formation of Fe2(SO4)3 endowed Fe2O3-S and VOx/Fe2O3-S with abundant medium-strong Lewis and Brønsted acid sites rather than covering the active sites, strengthening the NH3 adsorption and suppressing the overoxidation of NH3 on the reducible Fe2O3 support. Raman analysis revealed the generation of new species of polymeric vanadyl species on VOx/Fe2O3-S induced by the SO2. In-depth mechanistic understanding through DFT analyses indicated that the NOx reduction reaction over the Fe2O3-S and VOx/Fe2O3-S obeyed the Langmuir-Hinshelwood and Eley-Rideal mechanism simultaneously. Moreover, the presence of polymeric vanadyl species dramatically lower the overall energy barrier for NOx reduction, thus speeding up the SCR reaction of VOx/Fe2O3-S. This study can shed light on the more rational design of VOx-based catalysts for highly efficient NOx reduction in industry.

Water in the ball-milling process affects the dispersion of vanadia species on V2O5/TiO2 catalysts for NH3-SCR

Water in the ball-milling process increased polymeric vanadyl species and enhanced the redox capability, resulting in high NH3-SCR activity of V2O5/TiO2.

Elucidating the sensitivity of vanadyl species to water over V2O5/TiO2 catalysts for NOx abatement via operando Raman spectroscopy

Vanadia-based catalysts are widely used in the selective catalytic reduction of NOx with NH3 (NH3-SCR) for NOx abatement. However, the effect of water on specific vanadyl species is still unclear. Herein, the temperature-dependent sensitivity of vanadyl species to water has been originally elucidated via operando Raman spectroscopy. Interestingly, the stronger competitive adsorption of NH3 on polymeric vanadyl species in the presence of H2O dominants the NH3-SCR reaction above 250 °C despite the polymeric vanadyl show stronger adsorption to H2O than the monomeric ones. By contrast, the association of water molecules via hydrogen bonding dominants the activity degeneration to both monomeric and polymeric vanadyl species at temperatures below 250 °C. Furthermore, the stronger adsorption of water on polymeric vanadyl will give rise to even worse degeneration. This work elucidates the interaction of vanadyl species with water, which is essential for the development of high-performance catalysts for water-tolerant NOx abatement.

Hydrothermal Aging Treatment Activates V2O5/TiO2 Catalysts for NOx Abatement

Thermal stability is crucial for the practical application of deNOx catalysts. Vanadia-based catalysts are widely applied for the selective catalytic reduction of NOx with NH3 (NH3-SCR). Generally, hydrothermal aging at high temperatures induces the deactivation of deNOx catalysts. However, in this work, a remarkable increase in low- and medium-temperature NH3-SCR activity was observed for a V2O5/TiO2 catalyst after hydrothermal aging treatment, especially at 750 °C for 16 h. After the vanadia-based catalyst was hydrothermally treated at 750 °C, the specific surface area decreased and the surface VOx density and surface V ratio increased significantly. Therefore, the aged catalyst presented more abundant polymeric vanadyl species than the fresh one. Furthermore, the redox capability was improved markedly after hydrothermal treatment due to the strong interaction of vanadia and titania, contributing to the NH3-SCR reaction. 750 °C is the optimal temperature to activate the V2O5/TiO2 catalyst, improving the SCR performance significantly. This study provides an in-depth understanding of vanadia-based catalysts for practical applications.

Chromatographic and spectrophometric studies of vanadate (+V) reduction by 3–mercaptopropionic acid

The reduction of vanadate (+V) in the presence of 3-mercaptopropionic acid was studied using a chromatographic method for the determination of vanadate (+V) versus vanadyl (+IV) species. Ion chromatography was combined with spectrophotometric investigation of the absorption properties of the solution. The chromatographic method for the separation of vanadate (+V) and vanadyl (+IV) was carried out with an anion exchange column. In the initial reaction mixture containing vanadate (+V) and 3-mercaptopropionic acid, ethylenediaminetetraacetic acid – EDTA was added in an excessive amount relative to the concentration of reactants in the solution. After the ligand exchange reaction, the added EDTA terminates the reduction, allowing redox speciation in the solutions. A strong pH dependence of the reduction rates in the investigated solution was observed. The vanadate reduction seems to proceed in 2 steps: 1) formation of the intermediate vanadate (+V)-thioester; 2) reduction reaction and formation of the vanadyl (+IV)-thiol complex. The obtained results strongly suggest that the reaction of vanadate (+V)-thioester formation is proton catalyzed. It was observed that the overall reduction rates are pH dependent due to the complex vanadate (+V) solution speciation and changes in the ionic form of 3-mercaptopropionic acid.

Efficient NOx Abatement over Alkali-Resistant Catalysts via Constructing Durable Dimeric VOx Species.

The presence of alkali metals in flue gas is still an obstacle to the practical application of catalysts for selective catalytic reduction (SCR) of NOx by NH3. Polymeric vanadyl species play an essential role in ensuring the effective NOx abatement for NH3-SCR. However, polymeric vanadyl would be conventionally deactivated by the poison of alkali metals such as potassium, and it still remains a great challenge to construct robust and stable vanadyl species. Here, it was demonstrated that a more durable dimeric VOx active site could be constructed with the assistance of triethylamine, thereby achieving alkali-resistant NOx abatement. Due to the rational construction of polymerization structures, the obtained TiO2-supported cerium vanadate catalyst featured more stable dimeric VOx species and the active sites could survive even after the poisoning of alkali metal. Moreover, the depolymerization of VOx was suppressed endowing the catalysts with more Brønsted and Lewis acid sites after the poisoning of alkali metal, which ensured the efficient NOx reduction. This work unraveled the effects of alkali metal on the polymerization state of active species and opens up a way to develop low-temperature alkali-resistant catalysts for NOx abatement.

Phosphorous modified V-MCM-41 catalysts for propane dehydrogenation

The dehydrogenation performance of vanadyl catalysts was closely related to the form of surface vanadyl species. To enhance the vanadium dispersion, phosphorus was adopted to modify V-MCM-41 catalysts by using organic vanadium and phosphorus precursors. The influence of phosphorus introduction to the mesoporous structure and vanadyl species were investigated by various characterization techniques. The results showed that the catalysts could maintain ordered hexagonal mesoporous structures though the specific surface area slowly decreased along with the increase of phosphorus content. Both the reducibility and dispersion of the surface vanadyl species were improved. The proportion of polymerized vanadyl species obviously decreased due to the presence of phosphorus species. The propane dehydrogenation reaction results showed that both the catalytic performance and the catalyst stability were improved. Both the maximum surface vanadyl site density and optimum propane dehydrogenation performance were obtained over the sample with Si/P molar ratio of 30.

Synergistic Elimination of NOx and Chlorinated Organics over VOx/TiO2 Catalysts: A Combined Experimental and DFT Study for Exploring Vanadate Domain Effect.

Vanadium-based catalysts have been extensively applied for the synergistic control of NOx and chlorinated organics. However, how the vanadia species affect the reaction activity and products distribution, and what are the dominant reaction sites of these vanadia species are still unknown. Herein, we investigated the reaction characteristics of monomeric and polymeric vanadate domains for the catalytically synergistic elimination of NOx and chlorobenzene (CB). Density functional theory (DFT) calculations and experimental investigations have been combined to clarify the effects of different vanadyl species on the synergistic reaction. It was noted that the main adsorption site of CB on the monomeric domain was V-OH bond, and that on the polymeric one was V═O bond. The monomeric vanadyl was favorable for converting the Lewis V═O into Brønsted V-OH, which provided sufficient H protons for HCl formation, whereas the polymeric species could effectively retain the V4+/V5+ redox cycle, and yielded superior activity in CB catalytic oxidation (CBCO) reaction. However, the abundant oxygen vacancies and the inclined accumulation of Cl by forming the V-Cl bands led to significant polychlorinated byproducts on the polymeric vanadyl catalysts. Our work gives the first insight into different vanadate domain effects on the synergistic reaction, and is expected to provide theoretical basis for efficient design of the vanadium-based catalysts toward multipollutants elimination.

Adsorption-Induced Active Vanadium Species Facilitate Excellent Performance in Low-Temperature Catalytic NOx Abatement

Vanadia-based catalysts have been widely used for catalyzing various reactions, including their long-standing application in the deNOx process. It has been commonly considered that various vanadium species dispersed on supports with a large surface area act as the catalytically active sites. However, the role of crystalline V2O5 in selective catalytic reduction of NOx with NH3 (NH3-SCR) remains unclear. In this study, a catalyst with low vanadia loading was synthesized, in which crystalline V2O5 was deposited on a TiO2 support that had been pretreated at a high temperature. Surprisingly, the catalyst, which had a large amount of crystalline V2O5, showed excellent low-temperature NH3-SCR activity. For the first time, crystalline V2O5 on low-vanadium-loading catalysts was found to be transformed to polymeric vanadyl species by the adsorption of NH3. The generated active polymeric vanadyl species played a crucial role in NH3-SCR, leading to remarkably enhanced catalytic performance at low temperatures. This new finding provides a fundamental understanding of the metal oxide-catalyzed chemical reaction and has important implications for the development and commercial applications of NH3-SCR catalysts.

Significant promotion effect of the rutile phase on V2O5/TiO2 catalysts for NH3-SCR.

A large amount of polymeric vanadyl species owing to higher interaction energy between vanadia and anatase than rutile and the synergistic effect of vanadium oxides, anatase and rutile TiO2 contributes to an excellent NH3-SCR activity of the vanadia-based catalysts with high rutile content and low specific surface area.

Sn-doped rutile TiO2 for vanadyl catalysts: Improvements on activity and stability in SCR reaction

Sn-doped rutile TiO2 supports (SnxTi1−xO2, X = 0.1, 0.2, 0.5) were designed and V2O5/SnxTi1−xO2 catalysts were synthesized by incipient wetness impregnation. The V2O5/Sn0.2Ti0.8O2 exhibited higher low-temperature SCR activity and better H2O and SO2 resistance than the traditional V2O5/TiO2 (anatase) catalyst. In situ DRIFTs, XPS, H2-TPR and NH3/NO+O2-TPD results showed that Sn doping improved the redox property, NH3/NO adsorption and the amount of surface chemisorbed oxygen. The kinetic study indicated that Sn doping had little effect on SCR reaction pathways, but promoted SCR reaction, mainly through Eley-Rideal mechanism. More importantly, the V2O5/Sn0.2Ti0.8O2 showed better thermal stability than V2O5/TiO2. For V2O5/TiO2, the aggregation and sintering of the catalyst occurred, and the surface area decreased significantly after thermal aging. The monomeric vanadyl species partially transferred to crystalline V2O5. But for V2O5/Sn0.2Ti0.8O2, the surface area did not decrease as much as that for V2O5/TiO2, and the monomeric vanadyl species partially transferred to the polymeric vanadyl species.

Effect of treatment atmosphere on the vanadium species of V/TiO2 catalysts for the selective catalytic reduction of NOx with NH3

The decrease of polymeric vanadyl species due to treatment under different atmospheres results in the reduction of NH3-SCR activity.