Vanadium-based Catalysts Research Articles

Promoting C-Cl Bond Activation via a Preoccupied Anchoring Strategy on Vanadia-Based Catalysts for Multi-Pollutant Control of NOx and Chlorinated Aromatics.

Regulating vanadia-based oxides has been widely utilized for fabricating effective difunctional catalysts for the simultaneous elimination of NOx and chlorobenzene (CB). However, the notorious accumulation of polychlorinated species and excessively strong NH3 adsorption on the catalysts lead to the deterioration of multipollutant control (MPC) activity. Herein, protonated sulfate (-HSO4) supported on vanadium-titanium catalysts via a preoccupied anchoring strategy are designed to prevent polychlorinated species and alleviate NH3 adsorption for the multipollutant control. The obtained catalysts with -HSO4 modification achieve an excellent NOx and CB conversion with turnover frequency values of ∼ 3.63 and 17.7 times higher than those of the pristine, respectively. The protonated sulfate promotes the formation of polymeric vanadyl with a higher chemical state and d-band center of V. The modulated catalysts not only substantially alleviate the competitive adsorption of multipollutant via the "V 3d-O 2p-S 3p" network, but also distinctly strengthen the Brønsted acid sites. Besides, the introduced proton donor of the -HSO4 connecting polymeric structure could markedly reduce the reaction barrier of breaking the C-Cl bond. This work paves an advanced way for low-loading vanadium SCR catalysts to achieve highly efficient NOx and CB oxidation at a low temperature.

Main-Group Elements Enhance Electrochemical Nitrogen Reduction Reaction of Vanadium-Based Single Atom Catalysts Through d-p Orbital Hybridization.

Developing active sites with flexibility and diversity is crucial for single atom catalysts (SACs) towards sustainable nitrogen fixation at ambient conditions. Herein, the effects of doping main group metal elements (MGM) on the stability, catalytic activity, and selectivity of vanadium-based SACs is systematically investigated based on density functional theory calculations. It is found that the catalytic activity of V site can be significantly enhanced by the synergistic effect between MGM and vanadium atoms. More importantly, a volcano curve between the catalytic activity and the adsorption free energy of NNH* can be established, in which V-Pb dimer embedded on N-coordinated graphene (VPb-NG) exhibits optimal NRR activity due to its location at the top of volcano. Further analysis of electronic structures reveals that the unoccupancy ratio (eg/t2g) of V site is dramatically increased by the strong d-p orbital hybridization between V and Pb atoms, subsequently, N2 is activated to a larger extent. These interesting findings may provide a new path for designing active sites in SACs with excellent performance.

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.

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.

Deactivation effect of elemental mercury oxidation over a V2O5-WO3/TiO2 catalyst by Pb in simulated coal-fired flue gas

Lead (Pb) is a typical heavy metal in the flue gas of coal-fired power plants, which can cause the deactivation of SCR catalysts. In this study, Pb-doped vanadium-based catalysts were prepared and investigated for the deactivation effect and mechanism of elemental Hg oxidation caused by Pb. The results showed that doping Pb on vanadium-based catalysts led to severe deactivation, which was more pronounced with increased Pb content. Further investigation illustrated that the introduction of Pb not only weakened the BET surface properties of vanadium-based catalysts but also increased the aggregation of surface species on the catalysts. After Pb introduction, the adsorption capacity toward elemental Hg, proportion of highly oxidized V5+, and surface Oβ ratio significantly decreased severely weakening the activity of the catalyst. In addition, we demonstrated that the reaction between Pb and active VO/V−OH groups of the catalyst formed inactive V−O−Pb species, which was the direct cause of catalyst deactivation.

Atom Pairing Enhances Sulfur Resistance in Low-Temperature SCR via Upshifting the Lowest Unoccupied States of Cerium.

Environmentally benign cerium-based catalysts are promising alternatives to toxic vanadium-based catalysts for controlling NOx emissions via selective catalytic reduction (SCR), but conventional cerium-based catalysts unavoidably suffer from SO2 poisoning in low-temperature SCR. We develop a strongly sulfur-resistant Ce1+1/TiO2 catalyst by spatially confining Ce atom pairs to different anchoring sites of anatase TiO2(001) surfaces. Experimental results combined with theoretical calculations demonstrate that strong electronic interactions between the paired Ce atoms upshift the lowest unoccupied states to an energy level higher than the highest occupied molecular orbital (HOMO) of SO2 so as to be catalytically inert in SO2 oxidation but slightly lower than HOMO of NH3 so that Ce1+1/TiO2 has desired ability toward NH3 activation required for SCR. Hence, Ce1+1/TiO2 shows higher SCR activity and excellent stability in the presence of SO2 at low temperatures with respect to supported single Ce atoms. This work provides a general strategy to develop sulfur-resistant catalysts by tuning the electronic states of active sites for low-temperature SCR, which has implications for practical applications with energy-saving requirements.

Advances and future directions in vanadium-based catalysts for synergistic control of nitrogen oxides and chlorinated organic compounds

Advances and future directions in vanadium-based catalysts for synergistic control of nitrogen oxides and chlorinated organic compounds

Infrared Photodissociation Spectroscopy of Dinuclear Vanadium-Group Metal Carbonyl Complexes: Diatomic Synergistic Activation of Carbon Monoxide.

The geometric structure and bonding features of dinuclear vanadium-group transition metal carbonyl cation complexes in the form of VM(CO)n+ (n = 9-11, M = V, Nb, and Ta) are studied by infrared photodissociation spectroscopy in conjunction with density functional calculations. The homodinuclear V2(CO)9+ is characterized as a quartet structure with CS symmetry, featuring two side-on bridging carbonyls and an end-on semi-bridging carbonyl. In contrast, for the heterodinuclear VNb(CO)9+ and VTa(CO)9+, a C2V sextet isomer with a linear bridging carbonyl is determined to coexist with the lower-lying CS structure analogous to V2(CO)9+. Bonding analyses manifest that the detected VM(CO)9+ complexes featuring an (OC)6M-V(CO)3 pattern can be regarded as the reaction products of two stable metal carbonyl fragments, and indicate the presence of the M-V d-d covalent interaction in the CS structure of VM(CO)9+. In addition, it is demonstrated that the significant activation of the bridging carbonyls in the VM(CO)9+ complexes is due in large part to the diatomic cooperation of M-V, where the strong oxophilicity of vanadium is crucial to facilitate its binding to the oxygen end of the carbonyl groups. The results offer important insight into the structure and bonding of dinuclear vanadium-containing transition metal carbonyl cluster cations and provide inspiration for the design of active vanadium-based diatomic catalysts.

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.

Deeply revealing the mechanism of the synergistic effect of Cu and P on alleviating CaO poisoning performance of V-W-Ti catalysts in cement kilns

CaO was the inevitable cause of V2O5-WO3-TiO2 catalyst (VWTi) poisoning in cement kilns. The significant reduction in NO conversion was due to the combination of CaO and active species of the vanadium-based catalyst. In this work, the VWTi catalysts were modified with different contents of Cu and P to improve resistance to CaO, thereby extending the service life of catalysts in cement kilns. By doping Cu and P, the Ca-VWCu3P5Ti had better catalytic activity, reduced the poisoning effect of CaO to V5+, thus protecting the redox properties and increasing the surface acidic sites of the catalyst. Meanwhile, the in-situ DRIFTs results showed that the poisoning effect of CaO was weaker on the Ca-VWCu3P5Ti than that of the Ca-VWTi. In addition, the NH3 and NO species could be effectively adsorbed and activated on the surface of the Ca-VWCu3P5Ti by E-R mechanism. In this way, the harm of CaO to the vanadium-based catalysts was effectively reduced. This work provided a promising strategy to design vanadium-based catalysts with superior resistance to CaO in NH3-SCR reaction.

Systematic analysis of dual-functional catalysts for simultaneous CO-NOx reduction: Toward an effective catalyst design strategy

The development of a multifunctional catalyst suitable for application in existing facilities with limited space, with specific emphasis on its capability to concurrently remove CO and NOx is vital for meeting stringent environmental regulations. This is particularly pertinent to industries such as liquefied natural gas-based power plants and multipollutant-emitting industrial facilities. Another important consideration is that the catalyst should operate over a wide temperature range to facilitate its application across various industrial sites. In this study, we synthesized vanadium-based catalysts with a series of noble metals (Pt, Pd, and Au). The V–W–Pd/TiO2 catalyst achieved a simultaneous CO–NOx reduction efficiency greater than 90% in the widest temperature range, 228–321 °C. The excellent catalytic activities resulted from the Pd active sites, which oxidated CO and adsorbed NOx. This enables the catalyst to easily participate in the NH3-SCR reaction and facilitates NOx reduction. Moreover, we systematically analyzed the reaction properties of each catalyst using diffuse reflectance infrared Fourier transform and temperature programmed desorption methods. This enabled the elucidation of the reaction characteristics and competitive adsorption properties of the catalysts.

A detailed kinetic model for the methanol oxidative dehydrogenation on vanadia-based catalysts: Aggregation state role and active site requirements

A kinetic model was derived for the methanol oxidative dehydrogenation to formaldehyde, methyl formate and dimethoxymethane on sub-monolayer and multilayer V2O5/TiO2 catalysts. Considering a hemiacetal intermediate, the model combines Langmuir-Hinshelwood/Eley-Rideal/Mars-van Krevelen mechanisms, accounting for lattice oxygen redox sites and Brønsted acid sites. The fitted model depends on 4 thermodynamically consistent parameters, adequately simulating yield trends with temperature, residence time, and partial pressure of methanol, oxygen and added water. Oxidation steps follow a Mars-van Krevelen redox cycle, while the dimethoxymethane formation is reversible, as the water addition favored the hemiacetal intermediate. The catalyst with higher vanadia surface density was intrinsically more active (lower activation energy of the rate limiting step), attributed to its higher reducibility, while showing weaker methanol adsorption (lower enthalpy of chemisorption). The increase in reducibility correlates with the transition from monomers to polymers to crystalline vanadia when vanadium content increases, as observed by XRD, Raman spectroscopy and DRS-UV–vis.

Promotion effect of Ce and Ta co-doping on the NH3-SCR performance over V2O5/TiO2 catalyst

NH3-SCR (SCR: Selective catalytic reduction) is an effective technology for the de-NOx process from both mobile and stationary pollution sources, and the most commonly used catalysts are the vanadia-based catalysts. An innovative V2O5-CeO2/TaTiOx catalyst for NOx removal was prepared in this study. The influences of Ce and Ta in the V2O5-CeO2/TaTiOx catalyst on the SCR performance and physicochemical properties were investigated. The V2O5-CeO2/TaTiOx catalyst not only exhibited excellent SCR activity in a wide temperature window, but also presented strong resistance to H2O and SO2 at 275 ℃. A series of characterization methods was used to study the catalysts, including H2-temperature programmed reduction, X-ray photoelectron spectroscopy, NH3-temperature programmed desorption, etc. It was discovered that a synergistic effect existed between Ce and Ta species. The introduction of Ce and Ta enlarged the specific surface area, increased the amount of acid sites and the ratio of Ce3+, (V3++V4+) and Oα, and strengthened the redox capability which were related to synergistic effect between Ce and Ta species, significantly improving the NH3-SCR activity.

Insight studies on the deactivation of sulfuric acid regeneration catalyst

The SAR unit is used for regenerating spent sulfuric acid for alkylation reactions. In the SAR unit, the converter employs a potassium and vanadium-based catalyst for conversion of SO2 to SO3. The multiple bed fixed bed reactor is there to ensure that almost entire amount of SO2 is converted to SO3. On observing the generation of large amounts of fines in the reactor catalyst, the spent catalyst and fines are collected for insight studies. The current study enlightens the possible causes of catalyst deactivation and fines generation. Structural changes in potassium/vanadium based catalyst during the oxidation of sulfur dioxide identified as the major cause of catalyst deactivation. Such structural changes and their impact on catalyst property were determined using various physico-chemical characterization methods like crushing strength, Loss on drying (LOD), XRF, XRD, FTIR and XPS analysis. LOD analysis shows the higher moisture and volatile content in spent and fines compared to fresh catalyst. High moisture content in spent catalyst decreases hardness of catalyst resulting in leaching of vanadium salt and carrier degradation due to thermal cycling. Chemical analysis data found to show the active vanadium content in fines is higher compared to spent which indicates vanadium has leached out from fresh catalyst and is deposited on fines. These observations are further correlated with XRD and XPS results. XPS analysis shows the relative distribution of V4+ and V5+ species on the catalyst surface are altered and found very low concentration of V4+ sites on spent catalyst. The XRD analysis demonstrates the leaching of the active component, potassium vanadyl sulfate K(VO2) (SO4)·3H2O, which then accumulated in the fines fraction. FT-IR and XRD results further shows increase in sulfate accumulation on spent catalyst surface along with phase changes. FTIR analysis of simulated spent catalyst are well correlated with these findings.

Regulating Spin-Valence States of Vanadium-Based Single-Atom Oxygen Reduction Catalysts through Substrate and Axial Ligand Engineering

Regulating Spin-Valence States of Vanadium-Based Single-Atom Oxygen Reduction Catalysts through Substrate and Axial Ligand Engineering

Sustainable production of triazoles from lignin major motifs.

An efficiently catalyzed synthesis of pharmaceutically relevant 1,2,3-trazoles from renewable resources is highly desirable. However, due to incompatible catalysis conditions, this endeavor remained challenging so far. Herein, a practical access protocol to 1,2,3-triazoles, starting from lignin phenolic β-O-4 with γ-OH group utilizing a vanadium-based catalyst is presented. A broad substrate scope reaching up to 97 % yield of 1,2,3-triazoles are obtained. The reaction pathway includes selective cleavage of double C-O bonds, cycloaddition, and dehydrogenation. Mechanistic studies and density-functional theory (DFT) calculations suggest that the V-based complex acts as a bifunctional catalyst for both selective C-O bonds cleavage and dehydrogenation. This synthetic pathway has been applied for the synthesis of pharmacological and biological active carbohydrate derivatives starting from biomass components as feedstock, enabling a potential sustainable route to triazolyl carbohydrate derivatives, which paves the way for lignin-based heterocyclic aromatics in the pharmaceutical applications.

Catalyst development for O2-assisted oxidative dehydrogenation of propane to propylene.

O2-Assisted oxidative dehydrogenation of propane (O2-ODHP) could convert abundant shale gas into propylene as an important chemical raw material, meaning O2-ODHP has practical significance. Thermodynamically, high temperature is beneficial for O2-ODHP; however, high reaction temperature always causes the overoxidation of propylene, leading to a decline in its selectivity. In this regard, it is crucial to achieve low temperatures while maintaining high efficiency and selectivity during O2-ODHP. The use of catalytic technology provides more opportunities for achieving high-efficiency O2-ODHP under mild conditions. Up to now, many kinds of catalytic systems have been elaborately designed, including transition metal oxide catalysts (such as vanadium-based catalysts, molybdenum-based catalysts, etc.), transition metal-based catalysts (such as Pt nanoclusters), rare earth metal oxide catalysts (especially CeO2 related catalysts), and non-metallic catalysts (BN, other B-containing catalysts, and C-based catalysts). In this review, we have summarized the development progress of mainstream catalysts in O2-ODHP, aiming at providing a clear picture to the catalysis community and advancing this research field further.

Enhancing mechanisms of N-doped biomass carbon on the vanadium-based catalyst for furan degradation at low temperature

Enhancing mechanisms of N-doped biomass carbon on the vanadium-based catalyst for furan degradation at low temperature

Potential Risk of Significant N2O Emission without Changing NOx Conversion on Commercial V2O5/TiO2 Catalyst under Working Conditions.

Vanadium-based catalysts play a pivotal role in the emission control of industrial NOx via selective catalytic reduction (SCR) technology. However, little attention has been paid to the potential emission of greenhouse gas N2O under complex working conditions. This work reports that a commercial V2O5/TiO2 catalyst may lead to significant N2O emission without greatly changing the outlet NOx concentration after chromium (Cr) deposition. With a Cr loading of 2 wt %, N2O concentration increased from 27.8 to 199.2 ppm at 350 °C with the value of outlet N2O/(N2O+N2) from 2.5% to 19.4%. Experimental results combined with DFT+U calculations suggest that nonselective catalytic reduction (NSCR) is the main route for N2O formation in a wide temperature range of 250 ∼ 400 °C. It is stemmed from the fact that the covalent interaction between Cr and V species on the V2O5/TiO2 surface accelerates the conversion of V4+ + Cr6+ → V5+ + Cr3+, leading to a larger proportion of surface V5+. More importantly, surface V5+ is highly related to the redox property of the V2O5/TiO2 catalyst, which is beneficial to NSCR reaction rather than the standard SCR process. The work suggests that to better inhibit the emission of greenhouse gases during the NH3-SCR process, monitoring N2O emission should be included along with the NOx concentrations, especially in complex flue gases.

A tale of two pollutants: The mutual promotion mechanism of synergistic catalytic elimination of Hg0 and chlorobenzene on V2O5–WO3/TiO2 catalyst

The co-benefit of Hg0 or chlorinated volatile organic compounds (CVOCs) oxidation can be achieved by using vanadium-based catalysts in the selective catalytic reduction (SCR) unit. However, there is no research on the synergistic catalytic elimination of Hg0 and chlorobenzene (CB) in vanadium-based catalysts. Herein, the reaction mechanism of catalytic elimination of Hg0 and CB on vanadium-based catalysts was systematically analyzed and discussed for the first time. It is surprisingly observed that there is a synergistic effect between chlorobenzene catalytic oxidation (CBCO) and Hg0 oxidation. That is, CB could enhance Hg0 oxidation by providing active chlorine species, while the presence of Hg0 could also promote CBCO and reduce the formation of polychlorinated byproducts due to the consumption of chlorine species in the Hg0 oxidation reaction according to Le Chatelier’s principle. This synergistic effect was conducive to maintaining the physical structure and electronic properties of the catalyst and weakening the deposition of elements chlorine and mercury on the catalyst surface, promoting the re-exposure of active centers. This work provides unique insights for synergistic catalytic elimination of Hg0 and CB, and is expected to guide the prospective application on vanadium-based catalysts for multipollutant control.