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1.
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. 相似文献
2.
K. V. Narayana A. Venugopal K. S. Rama Rao V. Venkat Rao S. Khaja Masthan P. Kanta Rao 《Applied Catalysis A: General》1997,150(2):269-278
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 VOx species on the support by bulk vanadia and by other oxides, and also to compound formation with ammonia. 相似文献
3.
Partial oxidation of toluene to benzaldehyde and benzoic acid over model vanadia/titania catalysts: role of vanadia species 总被引:1,自引:0,他引:1
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. 相似文献
4.
R. S. G. Ferreira P. G. P. de Oliveira F. B. Noronha 《Applied catalysis. B, Environmental》2004,50(4):243-249
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. 相似文献
5.
Israel E. Wachs Laura E. Briand Jih-Mirn Jehng Loyd Burcham Xingtao Gao 《Catalysis Today》2000,57(3-4):323-330
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 NOx 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. 相似文献
6.
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. 相似文献
7.
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. 相似文献
8.
Motonobu Kobayashi Ryoji Kuma Sinyuki Masaki Noboru Sugishima 《Applied catalysis. B, Environmental》2005,60(3-4):173-179
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-NOx activity but also the low SO2 oxidation performance. 相似文献
9.
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%). 相似文献
10.
Stefania Albonetti Sonia Blasioli Rosa Bonelli Joseph Epoupa Mengou Salvatore Scir Ferruccio Trifir 《Applied Catalysis A: General》2008,341(1-2):18-25
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. 相似文献
11.
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. 相似文献
12.
P. Concepcin M.T. Navarro T. Blasco J.M. Lpez Nieto B. Panzacchi F. Rey 《Catalysis Today》2004,96(4):179-186
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. 相似文献
13.
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. 相似文献
14.
Selective catalytic reduction of nitric oxide by propane over vanadia-titania aerogels 总被引:1,自引:0,他引:1
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%. 相似文献
15.
Luca Lietti 《Applied catalysis. B, Environmental》1996,10(4):281-297
The reactivity of ternary V2O5&z.sbnd;WO3TiO2 de-NOxing 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-NOxing 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. 相似文献
16.
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 VOx species and high reducibility. 相似文献
17.
Reactivity of V2O5-WO3/TiO2 catalysts in the selective catalytic reduction of nitric oxide by ammonia 总被引:1,自引:0,他引:1
L. Lietti J. L. Alemany P. Forzatti G. Busca G. Ramis E. Giamello F. Bregani 《Catalysis Today》1996,29(1-4):143-148
The physico-chemical characteristics and the reactivity of sub-monolayer V2O5-WO3/TiO2 deNOx 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 VIV 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. 相似文献
18.
Molecular structure and reactivity of the Group V metal oxides 总被引:2,自引:0,他引:2
Israel E. Wachs Yongsheng Chen Jih-Mirn Jehng Laura E. Briand Tsunehiro Tanaka 《Catalysis Today》2003,78(1-4):13-24
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 TOFredox for the supported vanadia catalysts and the TOFacidic 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 TOFredox varied inversely to that of the TOFacidic 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. 相似文献
19.
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. 相似文献
20.
F. Cavani C.Cortelli A. Frattini B. Panzacchi V. Ravaglia F. Trifir C. Fumagalli R. Leanza G. Mazzoni 《Catalysis Today》2006,118(3-4):298-306
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. 相似文献