ChemInform Abstract: Nature of Vanadium Species in SnO2-V2O5-Based Catalysts. Preparation, Characterization, Thermal Stability and Reactivity in Ethane Oxidative Dehydrogenation over V-Sn Mixed Oxides (original) (raw)
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Chemical and structural characterization of V2O5/TiO2 catalysts
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2001
A series of V 2 O 5 /TiO 2 samples was synthesized by sol-gel and impregnation methods with different contents of vanadia. These samples were characterized by x-ray diffraction ͑XRD͒, Raman spectroscopy, x-ray photoelectron spectroscopy ͑XPS͒, and electronic paramagnetic resonance ͑EPR͒. XRD detected rutile as the predominant phase for pure TiO 2 prepared by the sol-gel method. The structure changed to anatase when the vanadia loading was increased. Also, anatase was the predominant phase for samples obtained by the impregnation method. Raman measurements identified two species of surface vanadium: monomeric vanadyl (V 4ϩ) and polymeric vanadates (V 5ϩ). XPS results indicated that Ti ions were in octahedral position surrounded by oxygen ions. The V/Ti atomic ratios showed that V ions were highly dispersed on the vanadia/titania surface obtained by the sol-gel method. EPR analysis detected three V 4ϩ ion types: two of them were located in axially symmetric sites substituting for Ti 4ϩ ions in the rutile structure, and the third one was characterized by magnetically interacting V 4ϩ ions in the form of pairs or clusters. A partial oxidation of V 4ϩ to V 5ϩ was evident from EPR analysis for materials with higher concentrations of vanadium.
Preparation, characterization, and activity of Al 2 O 3-supported V 2 O 5 catalysts
A series of activated alumina-supported vanadium oxide catalysts with various V 2 O 5 loadings ranging from 5 to 25 wt% have been prepared by wet impregnation technique. A combination of various physicochemical techniques such as BET surface area, oxygen chemisorption, X-ray diffraction (XRD), temperature-programmed reduction (TPR), thermal gravimetric analysis (TGA), and Fourier transform infrared (FTIR) were used to characterize the chemical environment of vanadium on the alumina surface. Oxygen uptakes were measured at 370 • C with prereduction at the same temperature, which appears to yield better numerical values of dispersion and oxygen atom site densities. XRD and FTIR results suggest that vanadium oxide exists in a highly dispersed state below 15 wt% V 2 O 5 loading and in the microcrystalline phase above this loading level. TPR profiles of V 2 O 5 /Al 2 O 3 catalysts exhibit only a single peak at low temperature up to 15 wt% V 2 O 5 . It is suggested that the low-temperature reduction peak is due to the reduction of surface vanadia, which has been ascribed to the tetrahedral coordination geometry of the V ions. TPR of V 2 O 5 /Al 2 O 3 at higher vanadia loadings exhibited three peaks at reduction temperatures, indicating that bulk-like vanadia species are present for these catalysts only at higher vanadia loadings, with V ions in an octahedral coordination. The TPR profiles of V 2 O 5 /Al 2 O 3 catalysts indicate that at loadings lower than 15% vanadia forms isolated surface vanadia species, while two-dimensional structure and V 2 O 5 crystallites become prevalent in highly loaded (>15% V 2 O 5 ) systems. Liquid-phase oxidation of ethylbenzene to acetophenone has been employed as a chemical probe reaction to examine the catalytic activity. Ethylbenzene oxidation results reveal that 15%V 2 O 5 /Al 2 O 3 is more active than higher vanadia loading catalysts.
Design of stable and reactive vanadium oxide catalysts supported on binary oxides
Catalysis Today, 1999
A series of titania based mixed oxides viz., TiO 2 ±SiO 2 , TiO 2 ±Al 2 O 3 , TiO 2 ±ZrO 2 , and TiO 2 ±Ga 2 O 3 were prepared by a coprecipitation method. These mixed oxides were impregnated with V 2 O 5 ranging from 2 to 30 wt% by using ammonium metavanadate as source of vanadium oxide. The mixed oxide supports and the vanadia impregnated catalysts were then subjected to thermal treatments from 773 to 1073 K and were investigated by XRD, FTIR, O 2 uptake and BET surface area methods to establish the effects of vanadia loading and thermal treatments on the surface structure of dispersed vanadia species and thermal stability of the catalysts. Calcination of coprecipitated support hydroxides at 773 K resulted in the formation of an amorphous phase, and further heating to 1073 K resulted in the formation of titania anatase phase, except with TiO 2 ±ZrO 2 support where a ZrTiO 4 compound was observed. All these mixed oxides exhibited a high thermal stability. Oxygen uptake results suggested a high dispersion of vanadia on these mixed oxide supports when calcined at 773 K. The mixed oxide based V 2 O 5 catalysts studied are found to be very active and selective for the synthesis of isobutyraldehyde from methanol and ethanol, and for the selective oxidation of 4-methylanisole to anisaldehyde. # 0920-5861/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. P I I : S 0 9 2 0 -5 8 6 1 ( 9 8 ) 0 0 4 1 5 -5
n-Butane selective oxidation on vanadium-based oxides : Dependence on catalyst microstructure
1986
The oxidation of n-butane and of its intermediates to maleic anhydride has been studied on different vanadium-phosphorus oxides and on Ti02-and zeolites-supported vanadium oxides. On vanadium-phosphorus oxides the activity in n-butane selective oxidation depends strongly on the catalyst microstructure. On supported vanadium oxides n-butane is not selectively oxidized ; however, when the amount of vanadium deposed largely exceeds the monolayer amount, low yields of acetic acid are obtained. The analysis of the oxidation of some intermediates suggests that the mechanism of maleic anhydride formation from n-butane occurs through the successive formation of butadiene, 2,5-dihydrofuran and furan via successive cycles of oxygen insertion and allylic H-abstraction and that these properties are connected to the vanadium ions and not to a particular surface structure. On the contrary, the alkane activation requires a particular surface structure of vanadium deriving by straining of V-(OP) connections. A model of the possible mechanism for n-butane activation iS also given. INTRODUCTION Notwistanding the growing interest in the selective oxidation of n-butane to maleic anhydride (11, the nature of the active sites able to activate the paraffins is not clear and the only hypothesis made in the literature, involving the Presence Of D-species (21, is lacking the experimental support necessary in order to extraPol_ ate the results to real catalysts. Furthermore, due to the very complex and.multisteps reaction pattern, lacking knowledges are present in literature on the nature and structure of the active sites necessary for the successive steps from n-butane to maleic anhydride. Aim of this work was to analyze our data of the selective oxidation of n-butane and of its possible intermediates to maleic anhydride on different vanadium-based catalysts, for the Purpose of determining the reaction mechanism and the nature and ?roPerties of the active centers for the successive steps from n-butane to maleic 0166-9834/86/$03.50 0 1986 Elsevier Science Publishers B.V. anhydride. EXPERIMENTAL Vanadium-phosphorus oxides (VP): V205 was reduced in 37 % HCl, then o-H3P04 added to obtain a P:V atomic ratio of 1.0. The resulting solution was concentrated and then water added to obtain a blue precipitate which was dried at 150 C for 24 h (VP a*). ~205 was reduced in a mixture (3:2) of isobuthyllbenzyl alcohols, then o-H3P04 added to obtain a P:V atomic ratio of 1.0. The resulting slurry was filtered and dried at 150 C for 24 h (VP b*). Both precursor samples VP(a*) and VP(b*) were then activated in a flow of 1% n-butane/air at 400 C for 6 h, to give the VP(a) and VP(b) catalysts, respectively. Vanadium-supported oxides : VT1 18 and VTi 117 catalysts were prepared by impregnation with a NH4V03/oxalic acid/water solution, drying at 150 C for 24 h and calcination at 430 C for 3 h. Both Ti02 supports in the anatase form were Tioxide, CLDD 1587/ /2 (18.4 m2/g) and CLDD 1764/Z (117 m*/g) respectively. The amount of vanadium deposited in both cases was 10% wt of V205. Zeolites-supported vanadium oxides (VZ) were prepared by impregnation with a NH4V03 solution of Y-zeolite [ VZ(HY) 1, HZSM-5 or HZSM-11 [ VZ(ZSM5) and VZ(ZSM11) j and by ionic exchange with VOS04 solution of Y-zeolite [ V02+ Y ]. Amount of deposed vanadium is about 2 % wt of V205 and 3.5% wt in the case of V02+Y . Before catalytic tests, samples were activated in air at 430 C for 6 h. Further details on the preparation of all these catalysts and on their characterization have been reported previously (j-10). Catalytic tests Catalytic tests were performed in a flow reactor with analysis on-line of the reagent composition and products of reaction by means of two gas-chromatographs. Details of the reactor and method of analysis are reported elsewhere (8). One g of catalyst was used for each test. The reagent composition was hydrocarbon:oxygen:nitrogen 0.6:12.0:87. The total flow of the reactant was 70 cc/min. RESULTS Vanadium-phosphorus oxides The catalytic behaviors of VP(a) and VP(b) catalysts in n-butane and 1-butene selective oxidation are reported in Figure 1. Catalyst VP(b) is more active than catalyst VP(a) both in alkene and alkane oxidation, in agreement with the higher surface area (27 and 6 m2/g, respectively). However the difference in activity is much greater in the n-butane than in 1-butene oxidation. Furthermore, whereas the V -+(0-P) bond, the presence of medium-strong Lewis acidity due to Va atoms can be thus explained in the catalyst VP(a) surface. However the enhanced Lewis acidity of the catalyst VP(b) surface cannot be explained. The (020) planes are connected by pyrophosphate groups and thus disorder in the stacking-fold of the (020) planes induces the straining of the V~lr~~(O-Pl bonds, which can be schematically represented as follows : A SCHEME 2 The Lewis acidity of the Vb atoms is enhanced by this effect as compared to the Lewis acidity of the corresponding Va atoms. It has been shown on solid super acid (16) that the first step in the activation of n-butane is the extraction of an H-from the n-butane by very-strong Lewis sites. Similarly, it is possible to hypothesize that the very-strong Lewis sites observed on catalyst VP(b) and to a lesser extent on catalyst VP(a), are the sites responsible for the first step in alkane activation on vanadium-phosphorus oxides. It is thus possible to propose the mechanism of n-butane selective activation showed in Scheme 3. A coordinated attack of itrong Lewis sites ( Vb atoms ) and of a strong base ( OS-) activates the n-butane, giving the corresponding olefins which are further quickly oxidized due to their higher reactivity. Relationship between structure and mechanism of oxidation --In a previous work we have showed (9) that the general mechanism of maleic anhydride formation from n-butane can be written as follows : n-butane -Dbutenes --Obutadiene -4furan -+maleic anhydride 7 G.Centi, Z.Tvaruzkova, F.TrifirB, P.Jiru and L.Kubelkova, Appl. Catal., 13 (19841 69. 8 G.Centi, G.Fornasari and F.Trifir6, Ind. Eng. Chem. Prod. Res. Dev., 24 (1985) 32. 9 G.Centi, G.Fornasari and F.Trifird, J. Catal., 89 (1984) 44. 10 F.Cavani, G.Centi and F.Trifir6, Appl. Catal., 15 (1985) 151. 11 E.Bordes, P.Courtine and J.W.Johnson, J. Solid State Chem., 55 (1984) 270. 12 J.
Chemical and structural characterization of V[sub 2]O[sub 5]/TiO[sub 2] catalysts
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2001
A series of V 2 O 5 /TiO 2 samples was synthesized by sol-gel and impregnation methods with different contents of vanadia. These samples were characterized by x-ray diffraction ͑XRD͒, Raman spectroscopy, x-ray photoelectron spectroscopy ͑XPS͒, and electronic paramagnetic resonance ͑EPR͒. XRD detected rutile as the predominant phase for pure TiO 2 prepared by the sol-gel method. The structure changed to anatase when the vanadia loading was increased. Also, anatase was the predominant phase for samples obtained by the impregnation method. Raman measurements identified two species of surface vanadium: monomeric vanadyl (V 4ϩ ) and polymeric vanadates (V 5ϩ ). XPS results indicated that Ti ions were in octahedral position surrounded by oxygen ions. The V/Ti atomic ratios showed that V ions were highly dispersed on the vanadia/titania surface obtained by the sol-gel method. EPR analysis detected three V 4ϩ ion types: two of them were located in axially symmetric sites substituting for Ti 4ϩ ions in the rutile structure, and the third one was characterized by magnetically interacting V 4ϩ ions in the form of pairs or clusters. A partial oxidation of V 4ϩ to V 5ϩ was evident from EPR analysis for materials with higher concentrations of vanadium.
Catalysts, 2018
V-containing mixed oxide catalytic materials are well known as active for partial oxidation reactions. Oxidation reactions are used in industrial chemistry and for the abatement of pollutants. An analysis of the literature in this field during the past few years shows a clear increase in the use of vanadium-based materials as catalysts for environmental applications. The present contribution makes a brief revision of the main applications of vanadium containing mixed oxides in environmental catalysis, analyzing the properties that present the catalysts with a better behavior that, in most cases, is related with the stabilization of reduced vanadium species (as V4+/V3+) during reaction.
The Journal of Physical Chemistry B, 1998
A series of V 2 O 5 supported on the SnO 2 /SiO 2 and CeO 2 /SiO 2 mixed oxides were investigated during methanol oxidation by in situ Raman spectroscopy, and the catalytic properties of these catalysts were probed by methanol oxidation kinetic studies. The Raman studies revealed that tin oxide forms a surface SnO x overlayer on the silica surface owing to the absence of Raman features of the SnO 2 crystallite, but cerium oxide forms bulk CeO 2 particles on the silica surface. The impregnated vanadium oxide formed a surface vanadia overlayer on all the catalysts owing to the absence of V 2 O 5 crystallite Raman features. In situ Raman studies of the V 2 O 5 /SnO 2 /SiO 2 and V 2 O 5 /CeO 2 /SiO 2 catalysts during methanol oxidation indicate that the formation of the VO x-SnO x and VO x-CeO 2 interactions totally blocks the formation of surface V-OCH 3 groups, which are observed in the V 2 O 5 /SiO 2 catalysts. The interaction between the surface VO x and the surface SnO x overlayer on silica increases the methanol oxidation reactivity by 1-2 orders of magnitude relative to V 2 O 5 /SiO 2 , and partial interaction between the surface VO x and bulk CeO 2 particles increases the methanol oxidation reactivity by 0-1 order of magnitude relative to V 2 O 5 /SiO 2. Temperature programmed reduction (TPR) studies indicate that the reducibility of the surface vanadium oxide species is dependent on the reducibility of the specific oxide support and confirm the formation of the VO x-SnO x bonds for the V 2 O 5 /SnO 2 /SiO 2 catalyst and the formation of VO x-CeO 2 as well as VO x-SiO 2 bonds for the V 2 O 5 /CeO 2 /SiO 2 catalyst.
Catalysis Today, 2019
A XANES study under reaction conditions has been performed with two different V-based catalytic systems, Mo-V-Nb-Te-O and V-Sb-O. For this study, an alumina-supported nanoscaled bulk catalyst has been used. In all cases XANES determined the average vanadium oxidation state during reaction. XANES also demonstrated that the nanosized phases are more dynamic, and able to participate in the redox catalytic cycle without significant changes either in their structure or in the overall vanadium oxidation state. Such a stability is also apparent under oxidizing conditions.