Polymeric Vanadia Research Articles

Effective abatement of naphthalene from flue gas by V-W/Ti catalyst: Study of vanadium species and reaction mechanism

The removal of naphthalene is important but deficient, so that the available V-W/Ti catalysts (1.0 wt% V2O5/TiO2) were prepared for its efficient abatement in this work. Especially, influences of different existence status of vanadium species on naphthalene elimination were investigated via changing the pH value of precursor solution. With the increase of precursor solution acidity, more highly polymeric vanadium species were formed on the catalyst surface. The redox capability was enhanced markedly as a consequence of the stronger electron transfer between V/W and Ti that induced more surface active oxygen and V5+, whereas the catalytic activity was obviously boosted and almost 90 % conversion of naphthalene was achieved at 293 °C. Moreover, the weaker chemisorption between naphthalene and V-W/Ti catalyst allowed the deeper conversion to COx. GC–MS manifested that naphthalene seemed to be an organic component that was difficult to be totally degraded. Naphthalene oxidation was in the following pathway of naphthalene → 1,4-naphthoquinone → phthalic anhydride → phthalates → benzene → benzoquinone → maleic anhydride → COx/H2O. The opening of aromatic ring in phthalic anhydride and benzene ring in benzene were proven to be the rate-controlling step based on in-situ DRIFTS results. The key of naphthalene oxidation was the complete degradation of various intermediates.

Biohybrid microcapsules based on electrosprayed CS-immobilized nanoZrV for self-healing epoxy coating development.

In this work, zirconium vanadate nanoparticles were immobilized into chitosan using a facile electrospraying technique to produce CS-ZrV hybrid microcapsules for the development of a self-healing coating. Upon assessment, hybrid microcapsules possessed desirable properties with a mean particle size of 319 μm, maintaining good thermal stability of ∼55% at 700 °C, and were subsequently incorporated into an epoxy resin to develop a biocompatible self-healing coating, CZVEx, for carbon steel corrosion protection. Scratched samples of self-healing and control coatings were analyzed in a corrosion medium of 3.5 wt% aqueous NaCl. SEM images of the scratched coating sample, after days of immersion, revealed healing of defects through the appearance of an epoxide gel-like substance due to the release of polymeric vanadate that reacted with corrosion agents, resulting in polymerization of vanadium hydrates and subsequent self-healing, validated by the proposed mechanism of self-healing. Electrochemical impedance spectroscopy analysis further confirmed CZVEx coating possessed excellent self-healing capabilities through a significant impedance rise from 4.48 × 105 to 5.52 × 105 (ohm cm2) between the 7th and 14th day of immersion. Furthermore, comparative polarization assessment of coating samples with/without defects indicated the accuracy of EIS for self-healing analysis, and showed the sample with no defect was only 2.6 times more corrosion resistant than the scratched coating, as against bare steel substrate that was 22 times less resistant, revealing superior self-healing anticorrosion properties of the coating.

Activity and in situ DRIFT studies on vanadia catalysts during oxidative dehydrogenation of sulfur-contaminated methanol

Silica-titania (70/30) supported vanadium catalysts were prepared, characterized, and studied in oxidative dehydrogenation of sulfur-contaminated methanol. The quality of vanadia species is dependent on temperature and gas conditions during preparation, support type, support specific surface area and VOx surface density. For example, upon heating the amount of V2O5 decrease along with formation of polymeric species. Such changes may occur also during the catalytic reaction. The reaction experiments and characterization results showed that the stability of polymeric vanadia species and total acidity has a connection with better formaldehyde production performance. The best performance was observed for N2-calcined silica-titania catalyst. Easy reducibility of the catalyst, as in the case of reference catalysts, leads to further oxidation of formaldehyde.

Promotional effect of phosphorus addition on improving the SO2 resistance of V2O5-MoO3/TiO2 catalyst for NH3-SCR of NO

The oxidation of SO2 into SO3 promote the formation of NH4HSO4 (ABS), which can cover surface active sites catalyst, and then leading to the decrease of NH3-SCR De-NOx activity. In this work, phosphorus species were loaded on the V2O5-MoO3/TiO2 De-NOx catalyst to enhance its SO2 resistance via inhibiting the SO2/SO3 conversion. XRD and N2-adsorption results showed that phosphorus has little impact on the structure and pore property of the catalyst. The results of XPS, H2-TPR, and NH3-TPD analyses illustrated that suitable amount of phosphorus (1%) increases the content of polymeric vanadate and chemisorbed oxygen (Oα) of the catalyst, which is benefit to the SCR process. Phosphorus species decreases the SO2 absorbability and SO2 oxidation activity of V-Mo-P/Ti catalysts, thus it exhibits higher NH3-SCR activity than that of the P-free catalyst at presence of SO2 due to the lower SO2/SO3 conversion and fewer amount of NH4HSO4 deposition.

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.

Vanadia–Zirconia and Vanadia–Hafnia Catalysts for Utilization of Volatile Organic Compound Emissions

Utilization is a sustainable and interesting alternative for the destructive treatment of volatile organic compounds due to avoided CO2 emission. This work concentrates on the development of active and sulfur-tolerant catalysts for the utilization of contaminated methanol. Impregnated and sol–gel prepared vanadia–zirconia and vanadia–hafnia catalysts were thoroughly characterized by N2 sorption, analytical (S)TEM, elemental analysis, XRD and Raman spectroscopy, and their performances were evaluated in formaldehyde production from methanol and methanethiol mixture. The results showed higher activity of the sol–gel prepared catalysts due to formation of mono- and polymeric vanadia species. Unfortunately, the most active vanadia sites were deactivated more easily than the metal-mixed oxide HfV2O7 and ZrV2O7 phases, as well as crystalline V2O5 observed in the impregnated catalysts. Metal-mixed oxide phases were formed in impregnated catalysts through formation of defects in HfO2 and ZrO2 structure during calcination at 600 °C, which was evidenced by Raman spectroscopy. The sol–gel prepared vanadia–zirconia and vanadia–hafnia catalysts were able to produce formaldehyde from contaminated methanol with high selectivity at temperature around 400 °C, while impregnated catalysts required 50–100 °C higher temperatures.

Elucidation of active species and reaction mechanism of sulfide V-K/Al2O3 catalyst for isobutane dehydrogenation

Supported sulfide vanadium-based catalyst in dehydrogenation of light alkanes is becoming a promise to replace the commercial used Pt- and Cr-based catalysts, due to the lower cost and toxicity. Nonetheless, the nature of active sites and reaction mechanism for dehydrogenation of isobutane over sulfide vanadium-based catalysts are still ambiguous. In this study, a series of sulfide V-K/γ-Al2O3 catalysts were prepared, presenting diverse distribution of isolated, oligomeric and polymeric vanadium species on catalyst surface. The oligomeric species were demonstrated to be the active dehydrogenation sites, and the TOFoligo showed a closely correlation with the isobutane conversion. Besides, the dehydrogenation reaction mechanism was studied by in-situ DRIFTS. The results showed that isobutane would first dissociative adsorbed on catalyst surface, and the stable intermediates of vanadium-isobutenyl were detected. Furthermore, the deposited coke rather than sulfur loss was found to play the crucial role in deactivation. The vast majority of deposited coke existed as polyaromatic species and the other slight ones presented as aliphatic form. This study enriches the fundamental explorations in active sites and reaction mechanism of sulfide V-K/γ-Al2O3 catalysts during isobutane dehydrogenation, and provides an in-depth strategy to improve the activity and stability of vanadium-based dehydrogenation catalysts.

Ni-Al polyvanadate layered double hydroxide with nanoceria decoration for enhanced corrosion protection of aluminium alloy

The present study introduces a modified layered double hydroxide (LDH) based system for corrosion inhibition on dual actions by both active and passive mechanisms. Nickel aluminium LDH (NiAl LDH) intercalated with vanadate ion was further decorated with ceria nanoparticles. The modifications of LDH were confirmed by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, infrared and Raman spectroscopy. We have investigated the effect of nanoceria decoration on NiAl LDH for the corrosion inhibition efficiency with and without intercalation of vanadate species on A356 aluminium alloy by electrochemical analysis and immersion test. The bare metallic substrates in 3.5 wt% sodium chloride solution containing inhibitor loaded LDH were tested to understand the inhibition mechanism. The presence of stable ceria nanoparticles layer can be an effective cathode, which passively protects the metal from corrosive anions and water molecules. The release of vanadate species during anion exchange of LDH with chloride ion in the corrosive environment and formation of polymeric vanadate on the surface actively protects the metal from further corrosion. The modified LDH loaded epoxy composite coatings were also made on A356 alloy to study the corrosion resistance in the real applications.

Influence of Vanadate Structure on Electrochemical Surface Reconstruction and OER Performance of CoV2O6 and Co3V2O8

The increase in global population and modernization of the world created a surge in the energy demand. Currently, carbon-based fossil fuels like coal, petroleum, natural gases etc., are the most widely used energy sources. However, they are limited in resources, and the burning of these fuels produces greenhouse gases which create a harmful impact on the environment. Wind, solar, geothermal energies are considered efficient, clean energy sources, but their intermittent nature restricts their real-life applications [1]. In this regard, Hydrogen is increasingly envisaged as an alternative fuel due to its highest gravimetric energy density (142 MJ/kg) and environmentally benign nature. Electro-catalytic water splitting is a promising method to produce hydrogen in a sustainable manner. For efficient water splitting, the oxygen evolution reaction (OER) at the anode acts as a bottleneck due to its four-electron/proton-coupled mechanism. To facilitate OER, catalysts are generally employed, but most are based on expensive metals, such as Ir, Rh etc. This has enthused researchers to develop cheaper transition metal based catalysts. Although many transition metal based catalysts have been developed, cobalt vanadate systems as OER catalysts are less explored [2]. This enthused us to synthesize pure phase Co3V2O8 and CoV2O6 by thermal co-precipitation followed by calcination. We could able to get these two different phases of cobalt vanadate by altering the reaction conditions slightly. Among these materials, CoV2O6 showed higher geometric electrocatalyst activity towards OER. The overpotential requirement to attain the benchmark current density of 10 mA/cm2geo was 360 mV, whereas Co3V2O8 required 510 mV overpotential to reach 10 mA/cm2geo current density. In addition, we have investigated the factors responsible for the superior phase-dependent electrocatalytic OER activity of two different crystalline cobalt vanadate phases. Vanadates as a class of oxometallates can exist as discrete as well as polymeric ions. For example, while orthovanadate is a discrete [VO4]3− ion, metavanadate ([VO3]n)n− can be visualized as a polymeric network of VO3 units. Earlier reports on cobalt vanadate as OER catalyst have ascribed activity difference to optimal metal-oxygen bond strength. In contrast, we found out that the etching of vanadate moieties from the precatalyst during the electrochemical activation plays a pivotal role [3]. Further, we observed that the polymerization of vanadate ions significantly affects the etching process. CoV2O6 has a polymeric structure consisting of chains of [VO3]−, formed via corner-shared VO4 tetrahedra, corner-shared VO6 octahedra, and edge-shared VO6 octahedra, whereas in Co3V2O8 vanadate ions only exist as discrete [VO4]3- tetrahedral. Due to the polymeric structure of metavanadate, its etching from the material would tear off a long chain of vanadate, thereby aiding the transformation of the material to its active phase. In contrast, the etching of orthovanadate is structurally constrained and, thus, much slower in nature. The resulting higher surface reconstruction exposes a higher number of catalytically active cobalt sites that otherwise lay inactive due to their position in bulk. Cobalt vanadates with polymeric vanadates (CoV2O6) showed better OER over monomeric orthovanadates (Co3V2O8) due to higher vanadate etching. Further, this work validates that etching can happen even from lattice positions, where bonding between constituents are strong.

Deep and efficient removal of vanadium from molybdate solution using magnetic γ-Fe2O3 nanoparticles

It is urgently desired to develop a promising method for deep removal of vanadium from molybdate solution for the efficient recycle of spent hydrodesulfurization (HDS) catalyst. In this paper, a novel method was investigated to deeply remove vanadium using γ-Fe2O3 particles as adsorbent. The properties of synthesized adsorbent were analyzed by a series of characterization methods. The adsorbent composed of γ-Fe2O3 nanoparticles with a diameter of 10–15 nm shows superparamagnetism and the saturation magnetization is about 56 emu·g−1. The performance of this adsorbent including the vanadium removal efficiency and adsorbent stability was evaluated. The vanadium removal rate is up to 97.6% and the co-adsorbed Mo is lower than 5% at a pH of 10 within only 30 min. Furthermore, both magnetism and adsorption capacity of the γ-Fe2O3 adsorbent are nearly unchanged after 30 days storage. The adsorption mechanism was revealed that polymeric vanadium ions exhibit higher affinity with γ-Fe2O3 than MoO42−, and the adsorption follows ion exchange mechanism between hydroxyls covered on the adsorbent. The γ-Fe2O3 adsorbent presents a series of advantages of excellent V-removal performance, good recyclability, excellent stability, which may have significant potential for industrial-scale applications.

Crystal orientation-dependent activity of tungsten-based catalysts for selective catalytic reduction of NOx with NH3

Hexagonal WO3 crystals with [1 1 0] and [0 0 1] orientations (denoted as H-110 & H-001) were synthesized and impregnated with 5 wt% V2O5 for NH3-SCR catalysis. H-110 with dominant (0 0 1) exposed facet possesses 72% NOx conversion at 400 °C while H-001 with dominant (1 0 0) exposed facet only achieves 17%. The maximal NOx conversion over V/H-110 and V/H-001 catalysts was enhanced to 96% and 47%, respectively. The catalysts were characterized by H2 temperature-programmed reduction (TPR), temperature-programmed desorption (TPD) of NH3, X-ray photoelectron spectroscopy (XPS), Raman, ultraviolet-visible (UV-vis) and diffuse reflection infrared Fourier transform (DRIFT) spectroscopy. The more coordinative unsaturated W6+ atoms on (0 0 1) facet generated more acid sites and NH3 was more strongly adsorbed on H-110 surface. The better match relationship between vanadia and WO3 (0 0 1) exposed facets led to the formation of more polymeric vanadates on V/H-110 catalyst. The higher proportioned V4+ and chemisorbed oxygen on V/H-110 catalyst also facilitated NH3-SCR reaction.

K+ deactivation of V2O5-WO3/TiO2 catalyst during selective catalytic reduction of NO with NH3: Effect of vanadium content

The effect of vanadium content on the resistance to K+ deactivation of V2O5-WO3/TiO2 SCR catalyst in biomass-fired flue gas was investigated. Catalytic activity and ammonia oxidation were measured, and the properties of fresh and K+-deactivated catalysts were characterized by XRD, N2 physisorption, H2-TPR, NH3-TPD and NH3-DRIFT. The BET surface area decreases with increased vanadium content for both fresh and K+-poisoned samples, but it is not responsible for catalyst deactivation. Ammonia oxidation starts from 300 °C and becomes more important with increasing vanadium content and at higher temperature. K+ can inhibit ammonia oxidation, but inactivates the newly generated NO2 to be reduced. The increase of vanadium content reduces mainly the Lewis acid sites, while the amount of BrØnsted acid sites increases. Monomeric and polymeric vanadium are the dominant species on the TiO2 support, and the amount of isolated vanadyl (V = O) species decreases with V2O5 content while the amount of V-OH species in polymeric vanadia increases. Isolated vanadyl species are advantageous to high-temperature catalytic activity while polymeric vanadia species increase ammonia and K+ adsorption. The (3 wt% V2O5)-WO3/TiO2 catalyst shows the best performance for both NO reduction and K+ resistance due to it containing both monomeric and polymeric vanadia species (or V = O and V-OH). Catalysts with 3 wt% V2O5 are preferable for flue gases with high alkali metal contents. Finally, the mechanism of reaction for different vanadium contents and corresponding K+-poisoning are also discussed.

SO2 promoted V2O5-MoO3/TiO2 catalyst for NH3-SCR of NOx at low temperatures

V2O5-MoO3/TiO2 and V2O5-WO3/TiO2 monolithic catalysts were evaluated for selective catalytic reduction (SCR) of NOx with NH3 in the presence of SO2 and H2O. After a durability test at 220 °C for 30 h, the NOx conversion of V2O5-MoO3/TiO2 increased from 70% to 88% while it decreased slightly over V2O5-WO3/TiO2. The catalysts were characterized by H2 temperature-programmed reduction (TPR), temperature-programmed desorption (TPD) of NH3, thermo-gravimetric (TG) analysis, temperature-programmed decomposition (TPD) of deposits, temperature-programmed surface reaction (TPSR) of ammonium bisulfates with NO, surface-enhanced Raman spectroscopy (SERS), ultraviolet-visible (UV–vis) and diffuse reflection infrared Fourier transform (DRIFT) spectroscopy. There were fewer sulfates deposits on VMoTi-S catalyst due to lower SO2 oxidation which was related to less VOV bonds on the Mo-modified catalyst. The as-formed high proportioned stable tridentate sulfates on VMoTi-S catalyst acted as strong Brønsted acid sites, and more active polymeric vanadates remained on this catalyst to activate the absorbed NH4+. The separation of acid sites and active sites which facilitates NH2 formation was suggested to be responsible for the promoted SCR activity of V2O5-MoO3/TiO2 during NH3-SCR reaction.

Effect of hydrothermal aging on NOx reduction performance for Sb–V–CeO2/TiO2 catalyst

Herein, we investigated the NOx reduction performance of Sb–V–CeO2/TiO2 (SbVCT) catalyst subjected to hydrothermal aging, where 6 vol% of H2O was fed to the SbVCT for 16 h with variable temperatures of 550–750 °C. The structural, morphological, redox, and acid properties of fresh and aged catalysts were comprehensively characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman, N2-physisorption, temperature programmed reduction/desorption (H2–TPR/NH3/NO–TPD) and in situ DRIFTS. The XRD, Raman and TEM results of the fresh and the one hydrothermally aged at 550, 600 °C catalysts indicated the presence of finely dispersed polymeric vanadia, antimony, and nano-crystalline ceria species on TiO2 support. The SbVCT catalysts hydrothermally aged at the temperatures ≤ 600 °C showed better NOx conversions than the others aged > 600 °C. This was because aging at the temperatures ≤ 600 °C could help to minimize the loss of surface redox/acid properties that were originally inherent to surface vanadyl and CeO2 species that went into CeVO4 and large CeO2 aggregates. This in turn led to the loss of surface redox and acidic characters indigenous to the SbVCT and reduced its NOx reduction performance. Moreover, the NOx reduction performance of hydrothermally aged V–WO3–TiO2 catalyst, when compared with that of SbVCT catalyst, showed relatively low NOx conversions at the temperatures below 300 °C.

The Role of Fe2O3 Species in Depressing the Formation of N2O in the Selective Reduction of NO by NH3 over V2O5/TiO2-Based Catalysts

Promotion of 2.73% Fe2O3 in an in-house-made V2O5-WO3/TiO2 (VWT) and a commercial V2O5-WO3/TiO2 (c-VWT) has been investigated as a cost effective approach to the suppression of N2O formation in the selective catalytic reduction of NO by NH3 (NH3-SCR). The promoted VWT and c-VWT catalysts all gave a significantly decreased N2O production at temperatures >400 °C compared to the unpromoted samples. However, such a promotion led to the loss in high temperature NO conversion, mainly due to the oxidation of NH3 to N-containing gases, particularly NO. Characterization of the unpromoted and promoted catalysts using X-ray diffraction (XRD), NH3 adsorption-desorption, and Raman spectroscopy techniques could explain the reason why the promotion showed much lower N2O formation levels at high temperatures. The addition of Fe2O3 to c-VWT resulted in redispersion of the V2O5 species, although this was not visible for 2.73% Fe2O3/VWT. The iron oxides exist as a highly-dispersed noncrystalline α-Fe2O3 in the promoted catalysts. These Raman spectra had a new Raman signal that could be tentatively assigned to Fe2O3-induced tetrahedrally coordinated polymeric vanadates and/or surface V-O-Fe species with significant electronic interactions between the both metal oxides. Calculations of the monolayer coverage of each metal oxide and the surface total coverage are reasonably consistent with Raman measurements. The proposed vanadia-based surface polymeric entities may play a key role for the substantial reduction of N2O formed at high temperatures by NH3 species adsorbed strongly on the promoted catalysts. This reaction is a main pathway to greatly suppress the extent of N2O formation in NH3-SCR reaction over the promoted catalysts.

Influence of calcination temperature on the plate-type V2O5–MoO3/TiO2 catalyst for selective catalytic reduction of NO

A series of plate-type V2O5–MoO3/TiO2 catalysts for selective catalytic reduction (SCR) of NO from flue gas were prepared and calcined at different temperatures. XRD, XRF, N2-adsorption, Raman, H2-TPR, NH3-TPD and XPS were used to characterize the catalysts. From the experimental results, plate-type V2O5–MoO3/TiO2 catalyst calcined at 500 °C showed the best performance in the SCR of NO. Compared with the catalyst calcined at 440 °C, the calcination in the range of 500–620 °C resulted in an increase of polymeric vanadate and the augmentation of catalytic acidity. Furthermore, the V4/V5+, (V4+ + V3+)/V5+ and Oα/(Oα + Oβ) ratio of the catalysts also increased with the increase of calcination temperature, which resulted in the high catalytic efficiency in the SCR reaction. However, the higher calcination temperature would lead to the inevitable formation of N2O at high reaction temperatures (> 370 °C). Meanwhile, high calcination temperature resulted in decreased mechanical strength of the catalyst.

Synthesis of V‐MCM‐41 Catalysts and Their Application in CO2‐Assisted Isobutane Dehydrogenation

AbstractA series of V‐MCM‐41 were in situ synthesized. The textual properties and vanadium species were systematically characterized. The results showed that the catalysts could still maintain a mesoporous structure and high specific surface area. The vanadium species were highly dispersed on the surface, with the coexistence of polymeric vanadium species. Isobutane dehydrogenation was performed by employing CO2 as a soft oxidant. The results revealed an almost linear relationship between the activity and surface vanadium sites. Isolated vanadium species were more active for isobutane dehydrogenation. The isobutane dehydrogenation reaction in the presence of CO2 could proceed simultaneously through oxidative dehydrogenation and non‐oxidative dehydrogenation followed by the reverse water gas shift reaction.

One-step selective synthesis of 2-chlorobenzonitrile from 2-chlorotoluene via ammoxidation

Isolated and polymeric vanadia formulate V2O5/Al2O3 catalysts selective for the synthesis of 2-chlorobenzonitrile from 2-chlorotoluene in a single step through ammoxidation.

NOxRemoval over V2O5/WO3–TiO2Prepared by a Grinding Method: Influence of the Precursor on Vanadium Dispersion

V2O5/WO3–TiO2 (VWT) catalysts for selective catalytic reduction (SCR) of NOx removal were prepared by a mechanical grinding method using different vanadium precursors. The SCR performances were evaluated in simulated flue gas and explained through characterizations by X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, H2 temperature-programmed reduction, and in situ diffuse reflectance Fourier transform spectroscopy. VO(acac)2–VWT catalyst prepared using a vanadyl acetylacetonate (VO(acac)2) precursor exhibited the highest catalytic activity and excellent resistance to SO2 and H2O poisoning in 200–400 °C compared to the catalysts obtained using other precursors. The VO(acac)2 precursor could enrich V on the surface remarkably and promote the formation and dispersion of polymeric vanadia species. Furthermore, a relatively high percentage of low-valent vanadium atoms were found on the VO(acac)2–VWT surface, facilitating electron transfer between V4+ and V5+. Surface-adsorbed NH3 species on VO(acac)2–VWT were much more reactive. The initial geometry of vanadium precursors determined the tendency of V to accumulate and further influenced the dispersion and final form of V species.

Selective oxidation of ethanol over vanadia-based catalysts: The influence of support material and reaction mechanism

The catalytic performance of vanadia supported on silica, alumina, zirconia, and titania was investigated in the selective oxidation of ethanol. It was shown that the activity and product distribution strongly depend on the support material, which determines the structure of supported vanadia species. On silica and alumina, low-active V2O5 crystallites were mainly formed regardless of the vanadium content. These catalysts demonstrated high selectivity toward only acetaldehyde. In contrast, monomeric surface vanadia species and polymeric surface vanadia species were mainly formed over TiO2 when the vanadium content did not exceed what is necessary for the ideal monolayer. Over zirconia, both the surface vanadia species and the V2O5 crystallites existed regardless of the vanadium content. It was found that the surface vanadia species are more active in the selective oxidation of ethanol than the V2O5 crystallites. The highest activity was observed for the polymeric vanadia species and, correspondingly, the best catalytic performance was achieved on the monolayer V2O5/TiO2 catalyst. At low temperatures between 110 and 150°C, this catalyst demonstrated high activity in the oxidation of ethanol to acetaldehyde with the selectivity ranging between 80% and 100%. At temperature near 200°C, the same catalyst was active in the oxidation of ethanol to acetic acid with the selectivity of approximately 65%. The surface intermediates and the catalyst state were also studied in situ by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. It was shown that under reaction conditions near 100°C, non-dissociatively adsorbed molecules of ethanol, ethoxide species, and adsorbed acetaldehyde exist on the catalyst surface, while at higher temperatures, V2O5/TiO2 is mainly covered with acetate species. Titanium cations remained in the Ti4+ state, whereas V5+ cations underwent a reversible reduction under reaction conditions. On the basis of the in situ data complemented by the results of kinetic measurements, a reaction mechanism for the selective oxidation of ethanol to acetaldehyde and acetic acid over the monolayer catalysts was proposed.