V2O5 catalysts supported on Al2O3–SiO2 mixed oxide: V, H MAS solid-state NMR, DRIFTS and methanol oxidation studies

Alumina–silica mixed oxide, synthesized by the sol–gel technique, was used as a support for dispersing and stabilizing the active vanadia phase. The catalysts were characterized employing ⁵¹V and ¹H solid-state MAS NMR, diffuse reflectance FT-IR, BET surface area measurements. The partial oxidation activities of the catalysts were tested using methanol oxidation as a model reaction. ⁵¹V solid-state NMR studies on the calcined catalysts showed the peaks corresponding to the presence of both tetrahedral and distorted octahedral vanadia species at low vanadia loadings and with an increase in V₂O₅ content, the ⁵¹V chemical shifts corresponding to amorphous V₂O₅ like phases were observed. DRIFTS studies of the catalysts indicated the vibrations corresponding tetrahedral vanadia species at low and medium loadings and at high V₂O₅ contents the vibrations corresponding V=O bonds of V₂O₅ agglomerates were observed. The V/Al–Si catalysts exhibited high selectivity for the dehydration product dimethyl ether in the methanol partial oxidation studies showing the predominance of the acidic nature of the alumina–silica support over the redox properties of the active vanadia phase.     

Ammoxidation of 3-picoline over V2O5/TiO2 (anatase) system I. Relationship between ammoxidation activity and oxidation state of vanadium

A series of titania (anatase)-supported vanadia catalysts ranging in V₂O₅ content from 0.4 to 9.9 mol% was prepared by wet impregnation technique, characterized by BET surface area measurement and X-ray diffraction, and evaluated for ammoxidation of 3-picoline. The average oxidation number of vanadium in the fresh and used catalysts was determined by titrimetric methods. The ammoxidation activity and the average oxidation number were observed to increase with vanadia loading up to 3.4 mol% in the catalyst which corresponds to a monolayer coverage. The phase transformation of anatase to rutile after the reaction was observed at a V₂O₅ loading of 5.9 mol%. The slow decrease of ammoxidation activity beyond 3.4 mol% V₂O₅ was attributed to the coverage of active monomeric VO_x species on the support by bulk vanadia and by other oxides, and also to compound formation with ammonia.     

Partial oxidation of toluene to benzaldehyde and benzoic acid over model vanadia/titania catalysts: role of vanadia species

Pure and K-doped vanadia/titania prepared by different methods have been studied in order to elucidate the role of vanadia species (monomeric, polymeric, bulk) in catalytic toluene partial oxidation. The ratio of different vanadia species was controlled by treating the catalysts in diluted HNO₃, which removes bulk vanadia and polymeric vanadia species, but not the monomeric ones, as was shown by FT-Raman spectroscopy and TPR in H₂. Monolayer vanadia species (monomeric and polymeric) are responsible for the catalytic activity and selectivity to benzaldehyde and benzoic acid independently on the catalyst preparation method. Bulk V₂O₅ and TiO₂ are considerably less active. Therefore, an increase of the vanadium concentration in the samples above the monolayer coverage results in a decrease of the specific rate in toluene oxidation due to the partial blockage of active monolayer species by bulk crystalline V₂O₅. Potassium diminishes the catalyst acidity resulting in a decrease of the total rate of toluene oxidation and suppression of deactivation. Deactivation due to coking is probably related to the Brønsted acid sites associated with the bridging oxygen in the polymeric species and bulk V₂O₅. Doping by K diminishes the amount of active monolayer vanadia leading to the formation of non-active K-doped monomeric vanadia species and KVO₃.     

Characterization and catalytic activity of Pd/V2O5/Al2O3 catalysts on benzene total oxidation

The role of vanadium oxide and palladium on the benzene oxidation reaction over Pd/V₂O₅/Al₂O₃ catalysts was investigated. The Pd/V₂O₅/Al₂O₃ catalysts were more active than V₂O₅/Al₂O₃ and Pd/Al₂O₃ catalysts. The increase of vanadium oxide content decreased the Pd dispersion and increased the benzene conversion. A strong Pd particle size effect on benzene oxidation reaction was observed. Although the catalysts containing high amount of V⁴⁺ species were more active, the Pd particle size effect was responsible for the higher activity.     

Molecular structure and reactivity of the group V metal oxides

The molecular structures and reactivity of the group V metal oxides (V₂O₅, Nb₂O₅ and Ta₂O₅) were compared. Their solid state structural chemistry, physical and electronic properties, number of active surface sites and their chemical reactivity properties were examined. For the bulk oxides, the solid state structural chemistry and the physical and electronic properties are well established. The number of active surface sites and the distribution of surface redox/acid sites were determined with methanol chemisorption and methanol oxidation, respectively. These studies revealed that the active surface sites present in pure V₂O₅ are primarily redox sites and the active surface sites in pure Nb₂O₅ are essentially acidic in nature. Furthermore, the surface redox sites present in pure V₂O₅ are orders of magnitude more active than the surface acid sites in pure Nb₂O₅. Consequently, the catalytic properties of bulk V₂O₅–Nb₂O₅ mixed oxides are dominated by the vanadia component. For the supported metal oxides, where the group V metal oxides are present as two-dimensional metal oxide overlayers, the structural and electronic properties are not well established in the literature. From a combination of molecular spectroscopic characterization methods (e.g., XANES, Raman, IR and UV–Vis DRS), it was possible to obtain this fundamental information. Methanol chemisorption studies demonstrated that a similar number of active surface sites are present in the supported vanadia and niobia catalyst systems. Similar to their bulk oxides, the surface vanadia species possess redox characteristics and the surface niobia species primarily possess acidic characteristics (Lewis acidity). The surface niobia species was a very sluggish redox site during oxidation reactions (e.g., methanol oxidation to formaldehyde and SO₂ oxidation to SO₃), but significantly promoted the surface vanadia redox sites for oxidation reactions that required dual surface redox and acid sites (e.g., butane oxidation to maleic anhydride and selective catalytic reduction of NO_x by NH₃ to produce N₂). These new fundamental insights are allowing for the molecular engineering of group V metal oxide catalysts (especially vanadia and niobia). In contrast, the molecular structure and reactivity properties of Ta₂O₅ catalysts are not yet established and will require significant research efforts.     

Atomic layer deposition and surface characterization of highly dispersed titania/silica-supported vanadia catalysts

The atomic layer deposition (ALD) method using surface-saturating gas–solid reactions was applied to modify a highly dispersed titania/silica support with submonolayer amounts of vanadia. The surface properties and acidity of the V₂O₅/TiO₂/SiO₂ materials were examined relatively to the corresponding silica and titania supported samples. The surface area, porosity and amorphous nature of the support were not affected by vanadia, which was present in the form of highly dispersed isolated species on titania/silica, as detected by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Microcalorimetry measurements showed that the vanadia species interacted with both the titania overlayer and the silica surface, confirming the observed V–O–Ti bonding Raman features. The surface reactivity towards ammonia was strongly enhanced by the modification, as probed by adsorption microcalorimetry and XPS. The superiority of the ALD method for the preparation of bilayered vanadia/titania/silica catalysts was demonstrated by examining for comparison a series of aqueous-phase impregnated catalysts.     

Chemical deactivation of V2O5/WO3–TiO2 SCR catalysts by additives and impurities from fuels, lubrication oils and urea solution: Part II. Characterization study of the effect of alkali and alkaline earth metals

A characterization study on a practice-oriented V₂O₅/WO₃–TiO₂ SCR catalyst deactivated by Ca and K, respectively, was carried out using NH₃-TPD, DRIFT spectroscopy, and XPS as well as theoretical DFT calculations. It was found from NH₃-TPD experiments that strongly basic elements like K or Ca drastically affect the acidity of the catalysts. Detailed DRIFT spectroscopy experiments revealed that these poisoning agents mostly interact with the Brønsted acid sites of the V₂O₅ active phase, thus affecting the NH₃ adsorption. Moreover, these experiments also indicated that the V⁵⁺ = O sites are much less reactive on the poisoned catalysts. XPS investigations of the O 1s binding energies showed that the oxygen atoms of the V⁵⁺ = O sites are affected by the presence of the poisoning agents. Based on these results and on DFT calculations with model clusters of the vanadia surface, the poisoning mechanism is explained by the stabilization of the non atomic holes of the (0 1 0) V₂O₅ phase as a result of the deactivation element. Consequently, V–OH Brønsted acid sites and V⁵⁺ = O sites are inhibited, which are both of crucial importance in the SCR process. The deactivation model also gives an explanation to the very low concentrations of potassium needed to deactivate the SCR catalyst, since one metal atom sitting on such a non-atomic hole site deactivates up to four active vanadium centers.     

TiO2-SiO2 and V2O5/TiO2-SiO2 catalyst: Physico-chemical characteristics and catalytic behavior in selective catalytic reduction of NO by NH3

TiO₂-SiO₂ with various compositions prepared by the coprecipitation method and vanadia loaded on TiO₂-SiO₂ were investigated with respect to their physico-chemical characteristics and catalytic behavior in SCR of NO by NH₃ and in the undesired oxidation of SO₂ to SO₃, using BET, XRD, XPS, NH₃-TPD, acidity measurement by the titration method and activity test. TiO₂-SiO₂, compared with pure TiO₂, exhibits a remarkably stronger acidity, a higher BET surface area, a lower crystallinity of anatase titania and results in allowing a good thermal stability and a higher vanadia dispersion on the support up to high loadings of 15 wt% V₂O₅. The SCR activity and N₂ selectivity are found to be more excellent over vanadia loaded on TiO₂-SiO₂ with 10–20 mol% of SiO₂ than over that on pure TiO₂, and this is considered to be associated with highly dispersed vanadia on the supports and large amounts of NH₃ adsorbed on the catalysts. With increasing SiO₂ content, the remarkable activity decrease in the oxidation of SO₂ to SO₃, favorable for industrial SCR catalysts, was also observed, strongly depending on the existence of vanadium species of the oxidation state close to V⁴⁺ on TiO₂-SiO₂, while V⁵⁺ exists on TiO₂, according to XPS. It is concluded that vanadia loaded on Ti-rich TiO₂-SiO₂ with low SiO₂ content is suitable as SCR catalysts for sulfur-containing exhaust gases due to showing not only the excellent de-NO_x activity but also the low SO₂ oxidation performance.     

Vanadia–silica aerogels. Structure and catalytic properties in selective reductionof NO by NH3

Vanadia-silica aerogels, containing 10 to 30 wt% V₂O₅, and a xerogel were prepared from vanadium(V) oxide triisopropoxide and vanadium (III) acetylacetonate (V(III)_acac) precursors using the solution-sol-gel method and different drying processes, including conventional evaporative and high-temperature and low-temperature supercritical drying. The behavior of these mixed oxides in the selective catalytic reduction of NO by NH₃ was tested and compared to that of other vanadia-silica and vanadia-titania catalysts. The structural and catalytic properties of the sol-gel derived vanadia-silica mixed oxides were found to be mainly influenced by the drying method, the vanadia content and the vanadia precursor used. For a particular vanadia content (10 wt%), low-temperature supercritical drying and evaporative drying resulted in significantly higher vanadia dispersion than high-temperature supercritical drying, which led to crystalline V₂O₅. Turnover frequencies for SCR at temperatures T < 475K were highest for low-temperature aerogels containing well-dispersed vanadium oxide species. Exposing these catalysts to higher temperatures under SCR conditions resulted in agglomeration/redispersion phenomena and at temperatures T > 550K best catalytic behavior was observed with vanadia-silica mixed oxides for which Raman spectroscopy indicated the presence of crystalline V₂O₅, as was the case for aerogels obtained by high-temperature supercritical drying and the low-temperature aerogel with the highest vanadia content (30 wt%).     

The role of acidity in the decomposition of 1,2-dichlorobenzene over TiO2-based V2O5/WO3 catalysts

The influence of Lewis and Brønsted acid sites on the performance of V₂O₅/TiO₂ and V₂O₅–WO₃/TiO₂ catalysts in the total oxidation of o-dichlorobenzene was investigated. Catalytic activity of these materials resulted strongly affected by their acidic properties. The presence of Brønsted acid sites significantly increases the o-DCB conversion but also leads to the uncompleted degradation of chlorinated compounds, promoting the formation of partial oxidation products, as dichloromaleic anhydride. On the contrary, Lewis acid sites, acting as absorbing sites, promote the further oxidation of intermediates to CO and CO₂, without any by-products desorption.

Furthermore, the presence of water in the feed-stream was proven to decrease o-DCB conversion but also to play a positive role on process selectivity, increasing COx production. Plausible reasons for this effect are the reduction of Brønsted acid sites and the hydrolysis of anhydride during wet tests.     

Selective oxidation of toluene on V2O5/TiO2/SiO2 catalysts modified with Te, Al, Mg, and K2SO4

The effect of the modification of vanadia catalysts supported on TiO₂/SiO₂ by the oxides of Al, Mg and Te, and K₂SO₄on the selective oxidation of toluene in the vapor phase has been studied. The catalysts were prepared by successive impregnation and characterized by BET surface measurements, XRD, XPS, and TPR. Addition of the second component decreased specific activity in all cases, except Al, mainly due to the decrease of surface area. Intrinsic activity was increased with addition of Te and Al, and decreased by that of Mg, while K₂SO₄ had little effect. These differences could be explained by the observed changes in either vanadium surface dispersion or reducibility. Selectivity to benzaldehyde increased markedly with addition of Te or K₂SO₄, that caused the formation of new oxide phases, V₃Ti₆O₁₇ and TiV₂O₆, in which vanadium is in a partially reduced state.     

Vanadium oxide supported on mesoporous Al2O3: Preparation, characterization and reactivity

The application of different techniques (diffuse reflectance-UV–vis, ⁵¹V NMR, FT-IR of adsorbed pyridine and TPR-H₂) in the characterization of vanadia supported on mesoporous Al₂O₃ catalysts shows that the nature of the vanadium species depends on the V-loading. At V-content lower than 15 wt.% of V-atoms (30% of the theoretical monolayer), vanadium is mainly in a tetrahedral environment. Higher V-contents in the catalyst leads to the formation of octahedral V⁵⁺ species and V₂O₅-like species. Both XRD and textural results indicate that the mesoporous structure of the support is mostly maintained after the vanadium incorporation, and therefore high surface areas were obtained on the final catalysts. Al₂O₃-suppported vanadia catalysts are active and selective in the oxidative dehydrogenation of ethane, although the catalytic behavior depends on the V-loading. High rates of formation of ethylene per unit mass of catalyst per unit time have also been observed as a consequence of the high dispersion of V-atoms on the surface of the support.     

Effects of microwaves on selective oxidation of toluene to benzoic acid over a V2O5/TiO2 system

Two series of catalysts, V₂O₅/TiO₂ and modified V₂O₅/TiO₂, were prepared with a conventional impregnation method. They were tested in the selective oxidation of toluene to benzoic acid under microwave irradiation. The reaction conditions were optimized over V₂O₅/TiO₂. It was found that in the microwave catalytic process the optimum reactor bed temperature of the titled reaction decreases to 500 K (600 K in the conventional process). The modification of V₂O₅/TiO₂ with MoO₃, WO₃, Nb₂O₅ or Ta₂O₅, which has no negative influence on the reaction in the conventional catalytic process, can greatly promote the catalytic activities in the microwave process, leading to a high yield of benzoic acid (41%). The effects of microwave electromagnetic field on the catalysts are discussed.     

Selective catalytic reduction of nitric oxide by propane over vanadia-titania aerogels

An investigation has been carried out of the effect of vanadia loading on the activity and selectivity ofV₂O₅TiO₂ aerogel catalysts, prepared by a two-step procedure, for the reduction of NO by propane. The structure of catalysts have been characterized by laser Raman spectroscopy and XRD measurements. At vanadia loading levels below ca. 4.4 wt%, the vanadia is present in the form of coordinated polymeric species, whereas crystallites of V₂O₅ are formed at higher vanadia contents. At this critical level of 4.4 wt% V₂O₅, the kinetic measurements showed also a maximum in the activity per mass of catalyst which very likely indicated that the coordinated polymeric surface species are more active than crystalline V₂O₅. The selectivity towards the formation of dinitrogen decreased as the loading increased, presumably because of the formation of larger polymeric species and V₂O₅ crystallites, below and above the critical loading level, respectively. For the reduction of NO by propane, titania supported vanadia aerogel catalysts are significantly more active, per mass of catalyst, and more selective towards N₂ formation than conventionalV₂O₅TiO₂ and V₂O₅Al₂O₃ aerogel catalysts, at vanadia loading levels below ca. 11 wt%.     

Reactivity of V2O5&z.sbnd;WO3TiO2 de-NOx catalysts by transient methods

The reactivity of ternary V₂O₅&z.sbnd;WO₃TiO₂ de-NO_xing catalysts with compositions similar to those of commercial catalysts (WO₃ ca. 9% w/w, V₂O₅ < 2% w/w) is investigated by transient techniques (temperature programmed desorption, TPD; temperature programmed surface reaction, TPSR; and temperature programmed reaction, TPR). The results indicate that the reactivity of the ternary catalysts in the SCR reaction increases on increasing the vanadia loading, and that the ternary catalysts are more active than the corresponding binary vanadia-titania samples with the same V₂O₅ loading. Indeed the SCR reaction is monitored at lower temperatures and high NO conversions are also preserved at high temperatures. TPSR and TPR data show that at low temperatures the SCR reaction occurs via a redox mechanism that involves at first the participation of the catalyst lattice oxygen and then the reoxidation of the reduced sites by gas-phase oxygen. Based on TPSR and TPR data, the higher reactivity of the ternary catalysts has been related to their superior redox properties, in line with previous chemico-physical characterisation studies. The catalyst redox properties thus appear as a key-factor in controlling the reactivity of V₂O₅&z.sbnd;WO₃TiO₂ de-NO_xing catalysts at low temperatures. The results also show that at high temperatures the surface acidity plays an important role in the adsorption and activation of ammonia.     

Selective oxidation of hydrogen sulfide containing excess water and ammonia over vanadia–titania aerogel catalysts

A series of V₂O₅–TiO₂ aerogel catalysts were prepared by sol–gel method with subsequent supercritical drying with CO₂. The aerogel catalysts showed much higher surface areas and total pore volumes than V₂O₅–TiO₂ xerogel and impregnated V₂O₅–TiO₂ catalysts. Two species of surface vanadium in the aerogel catalysts were identified by Raman measurements: monomeric vanadyl and polymeric vanadates. The selective oxidation of hydrogen sulfide in the presence of excess water and ammonia was studied over these catalysts. Aerogel catalysts showed very high conversion of H₂S without harmful emission of SO₂. Temperature programmed reduction (TPR), XRD and Raman analyses revealed that the high catalytic performance of the aerogel catalysts originated from their highly dispersed VO_x species and high reducibility.     

Reactivity of V2O5-WO3/TiO2 catalysts in the selective catalytic reduction of nitric oxide by ammonia

The physico-chemical characteristics and the reactivity of sub-monolayer V₂O₅-WO₃/TiO₂ deNO_x catalysts is investigated in this work by EPR, FT-IR and reactivity tests under transient conditions. EPR indicates that tetravalent vanadium ions both in magnetically isolated form and in clustered, magnetically interacting form are present over the TiO₂ surface. The presence of tungsten oxide stabilizes the surface V^IV and modifies the redox properties of V₂O₅/TiO₂ samples. Ammonia adsorbs on the catalysts surface in the form of molecularly coordinated species and of ammonium ions. Upon heating, activation of ammonia via an amide species is apparent. V₂O₅-WO₃/TiO₂ catalysts exhibits higher activity than the binary V₂O₅/TiO₂ and WO₃/TiO₂ reference sample. This is related to both higher redox properties and higher surface acidity of the ternary catalysts. Results suggest that the catalyst redox properties control the reactivity of the samples at low temperatures whereas the surface acidity plays an important role in the adsorption and activation of ammonia at high temperatures.     

Molecular structure and reactivity of the Group V metal oxides

The physical, electronic and reactivity properties of bulk and supported Group V metal oxides (V, Nb, Ta and Db) were compared at the molecular level. Dubnium is a very short-lived element, 60 s, whose properties have not been extensively studied, but can be predicted from knowledge of the other members of the Group V metal oxides. Bulk V₂O₅ possesses platelet morphology with the active surface sites only located at the edges: primarily surface redox sites and some surface acidic sites. Bulk Nb₂O₅ and Ta₂O₅, as well as to be expected for bulk Db₂O₅, possess isotropic morphologies and the active surface sites relatively homogeneously dispersed over their surfaces: only surface acidic sites. However, the bifunctional bulk V₂O₅ was found to exhibit a much higher specific acidic catalytic activity than the acidic bulk Nb₂O₅ and Ta₂O₅, the latter being almost identical in their specific acidic catalytic activity. The bulk properties of the Group V metal oxides were essentially transferred to the analogous supported Group V metal oxides, where the active Group V metal oxides were present as a two-dimensional monolayer on various oxide supports (e.g., Al₂O₃, TiO₂, ZrO₂ as well as Nb₂O₅ and Ta₂O₅). For supported vanadia catalysts, the active surface sites were essentially redox sites, with the exception of supported V₂O₅/Al₂O₃ that also contained strong acidic sites. For supported niobia and tantala catalysts, as well as to be expected for supported dubnia catalysts, the active surface sites were exclusively acidic sites. However, the TOF_redox for the supported vanadia catalysts and the TOF_acidic for the supported niobia and tantala catalysts varied over several orders of magnitude as a function of the specific oxide support with the electronegativity of the oxide support cation. However, the TOF_redox varied inversely to that of the TOF_acidic variation because of the opposite requirements of these active surface sites. Surface redox sites are enhanced by reduction and surface acidic sites are enhanced by stabilization (lack of reduction). The current fundamental understanding of the Group V metal oxides allows for the molecular engineering of their metal oxide applied catalytic materials.     

Gas-phase selective oxidation of toluene on TiO2–sepiolite supported vanadium oxides: Influence of vanadium loading on conversion and product selectivities

Gas-phase selective oxidation of toluene has been carried out on vanadium oxide systems (5–20 wt.% of V₂O₅, equivalent to 0.4–1.7 theoretical monolayers) supported on TiO₂–sepiolite (with titania loading around the theoretical monolayer, 12 wt.%) and on sepiolite. A study has been made on both the influence of vanadia loading and of the support on the catalytic behaviour of the supported vanadium systems. The reducibility by H₂ TPR was also studied as well as the acid and basic/redox sites from the results of conversion of the 2-propanol test reaction of the solids. Benzaldehyde, benzoic acid and several coupling products were the main ones detected, attaining over 50% selectivity towards the benzaldehyde and benzoic acid products at a total conversion around 10%. The activity and selectivity to the selective products exhibited by vanadium systems supported on mixed support were superior to those exhibited by the systems supported on sepiolite and increased notably in both series with the increase in vanadium loading. The best catalytic behaviour exhibited by the vanadium systems supported on mixed support, which also exhibited the highest density of sites capable of being reduced (as well as their reducibility) and of those responsible for propanone formation, could be attributed not only to the different balance of the vanadia species existing in the two supports (monomeric + oligomeric/polymeric), but also to such other factors as the nature of the support and, concretely, its chemical composition.     

The characterization of the V species and the identification of the promoting effect of dopants in V/Ti/O catalysts for o-xylene oxidation

The present work deals with the study of the role of promoters in TiO₂-supported vanadium oxide, catalyst for the oxidation of o-xylene to phthalic anhydride. Two different series of catalysts were prepared, the first one consisting of undoped samples having different vanadium oxide content, and the second one of samples having 7 wt.% V₂O₅ and variable amounts of Sb and Cs as promoters. All the samples were characterized by means of Raman spectroscopy, X-ray diffraction and thermal-programmed reduction and oxidation, in order to define a method for the quantification of the different V species (i.e., isolated vanadium, dispersed polyvanadate and bulk vanadium oxide) that develop on TiO₂ support in the presence of promoters. It was found that polyvanadate and bulk vanadium oxide spontaneously release molecular oxygen at 600–650 °C, whereas the isolated V is not susceptible of self-reduction. The latter species is predominant in samples having low vanadium oxide loading (≤2 wt.% V₂O₅, with TiO₂ surface area 22.5 m²/g), and possesses the highest intrinsic activity in o-xylene conversion. The presence of Sb, a promoter of activity, increases the dispersion of the most active species and also hinders its segregation in the reaction environment. These promoting effects are more pronounced when both Cs and Sb are present.