Study by XPS and TPD of the interaction of n-pentane and n-butane with the surface of non-equilibrated' and equilibrated' V–P–O catalysts (original) (raw)
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Journal of Catalysis, 1996
Changes occurring on thermal treatment of the precursor of vanadium/phosphorus mixed oxide, the industrial catalyst for the oxidation of n-butane, were studied. The precursor was mixed with stearic acid, used as an organic binder for pelletization of the powder. The calcination of the precursor leads to a partially oxidized compound, constituted of an amorphous V IV -P mixed oxide and a crystalline hydrated V V -P-O phase. The calcined compound, when left in a 1% hydrocarbon/air stream for 100 h leads to a "nonequilibrated" catalyst, and after 1000 h to the "equilibrated" catalyst. The catalytic activity of the nonequilibrated and equilibrated catalysts in n-butane and n-pentane oxidation was studied and compared; the chemical-physical features of the two catalysts were studied by means of XRD, FT-IR, chemical analysis, TGA, XPS, and TPD. Only well crystallized (VO) 2 P 2 O 7 was detected in the equilibrated catalyst and a homogeneous distribution of surface centers seems to be present on its surface. In the case of nonequilibrated catalyst, a poorly crystallized (VO) 2 P 2 O 7 is present together with an amorphous V IV -P-O phase and γ-VOPO 4 ; these phases define a heterogeneous distribution of at least two kind of surface centers. This surface heterogeneity gives rise to a catalyst less selective in n-butane oxidation to maleic anhydride and less specific in the conversion of n-pentane to phthalic anhydride.
The Journal of Physical Chemistry B, 2003
In situ X-ray absorption spectroscopy (XAS) and in situ X-ray photoelectron spectroscopy (XPS) have been applied to study the active surface of vanadium phosphorus oxide (VPO) catalysts in the course of the oxidation of n-butane to maleic anhydride (MA). The V L 3 near edge X-ray absorption fine structure (NEXAFS) of VPO is related to the details of the bonding between the central vanadium atom and the surrounding oxygen atoms. Reversible changes of the NEXAFS were observed when going from room temperature to the reaction conditions. These changes are interpreted as dynamic rearrangements of the VPO surface, and the structural rearrangements are related to the catalytic activity of the material that was verified by proton-transfer reaction mass spectrometry (PTR-MS). The physical origin of the variation of the NEXAFS is discussed and a tentative assignment to specific V-O bonds in the VPO structure is given. In situ XPS investigations were used to elucidate the surface electronic conductivity and to probe the ground state of the NEXAFS spectra.
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.
Interaction of oxygen with the surface of vanadia catalysts
Journal of Molecular Catalysis A: Chemical, 2007
Kinetics of the re-oxidation of the H 2-reduced or vacuum treated VO x /TiO 2 catalyst was studied. It was found that oxygen does not adsorb in the form of ad-atoms on the fully oxidized surface of the catalyst but re-oxidizes oxygen vacancies. On the basis of the surface potential measurements it was concluded that nucleophillic oxygen O (s) 2− is almost exclusively present above 550 K on the catalyst surface. The quantumchemical calculations were supported by the experimental results. The implications of these results for the reaction mechanism of the oxidative dehydrogenation of propane are discussed.
Journal of Catalysis, 1997
The oxidation of n-butane to maleic anhydride was investigated over a series of model-supported vanadia catalysts where the vanadia phase was present as a two-dimensional metal oxide overlayer on the different oxide supports (TiO 2 , ZrO 2 , CeO 2 , Nb 2 O 5 , Al 2 O 3 , and SiO 2 ). No correlation was found between the properties of the terminal V= =O bond and the butane oxidation turnover frequency (TOF) during in situ Raman spectroscopy study. Furthermore, neither the n-butane oxidation TOF nor maleic anhydride selectivity was related to the extent of reduction of the surface vanadia species. The n-butane oxidation TOF was essentially independent of the surface vanadia coverage, suggesting that the n-butane activation requires only one surface vanadia site. The maleic anhydride TOF, however, increased by a factor of 2-3 as the surface vanadia coverage was increased to monolayer coverage. The higher maleic anhydride TOF at near monolayer coverages suggests that a pair of adjacent vanadia sites may efficiently oxidize n-butane to maleic anhydride, but other factors may also play a contributing role (increase in surface Brønsted acidity and decrease in the number of exposed support cation sites). Varying the specific oxide support changed the n-butane oxidation TOF by ca. 50 (Ti > Ce > Zr ∼ Nb > Al > Si) as well as the maleic anhydride selectivity. The maleic anhydride selectivity closely followed the Lewis acid strength of the oxide support cations, Al > Nb > Ti > Si > Zr > Ce. The addition of acidic surface metal oxides (W, Nb, and P) to the surface vanadia layer was found to have a beneficial effect on the n-butane oxidation TOF and the maleic anhydride selectivity. The creation of bridging V-O-P bonds had an especially positive effect on the maleic anhydride selectivity.
Journal of Catalysis, 2017
A set of vanadium phosphorous oxide (VPO) catalysts, mainly consisting of (VO)2P2O7, VO(PO3)2 or VOPO4•2H2O bulk crystalline phases, has been investigated for the oxidative dehydrogenation (ODH) of ethane to ethylene, a key potential reaction for a sustainable industrial and socioeconomic development. The catalytic performance on these VPO catalysts has been explained on the basis of the main crystalline phases and the corresponding surface features found by XPS and LEISS at 400 ºC, i.e. within the temperature range used for ODH reaction. The catalysts based on (VO)2P2O7 phase presented the highest catalytic activity and productivity to ethylene. Nevertheless, the catalysts consisting of VO(PO3)2 structure showed higher selectivity to ethylene, reaching 90% selectivity at ca. 10% ethane conversion. To the best of our knowledge, this is the highest selectivity reported on a vanadium phosphorous oxide at similar conversions for the ethane ODH. In general, catalysts consisting of crystalline phases with vanadium present as V 4+ , i.e. (VO)2P2O7 and VO(PO3)2, were found to be significantly more selective to ethylene than those containing V 5+ phases. The surface analysis by XPS showed an inverse correlation between the mean oxidation state of vanadium near surface and the selectivity to ethylene. The lower averaged oxidation states of vanadium appear to be favoured by the presence of V 3+ species near the surface, which was only found in the catalysts containing V 4+ phases. Among those catalysts the one based on VO(PO3)2 phase shows the highest selectivity, which could be related to the most isolated scenario of V species (the lowest V content relative to P) found at the outermost surface by low energy ion scattering spectroscopy (LEISS), a "true" surface technique only sensitive to the outermost atomic layer.
Catalysis Today, 2000
V/P/O-based catalysts were prepared by thermal treatments of VOHPO 4 0·5H 2 O precursors prepared with the organic procedure. Different methods for precursor dehydration led to compounds which were characterized by the prevailing presence of crystalline (VO) 2 P 2 O 7 , but which also contained either V 5+ species or V 3+ species. The catalytic performance of these compounds in n-butane oxidation under almost-equilibrated conditions was compared. It was found that the presence of either V 3+ or V 5+ enhances the specific activity in n-butane oxidation, while the selectivity to maleic anhydride at low n-butane conversion (30%) remains substantially unaffected. A fully equilibrated, well-crystallized (VO) 2 P 2 O 7 was reduced with H 2 . The reduced compound was more active than the fully equilibrated vanadyl pyrophosphate, while exhibiting comparable selectivity to maleic anhydride.
Recueil des Travaux Chimiques des Pays-Bas, 1996
Well-dispersed supported V-P-0 (Vanadium Phosphorus Oxyde) catalysts were prepared and tested in the selective oxidation of n-butane to maleic anhydride. Because of their high activity, it was possible to test titania-supported catalysts in a two-step oxidation reduction process at low temperatures. After n-butane adsorption and activation in the absence of molecular oxygen, desorption took place after subsequent introduction of molecular oxygen into the gas stream, leading to the selective formation of maleic anhydride. In-situ XANES spectroscopy was performed under reaction conditions in the two-step oxidation reduction process with a special titania-supported V-P-0 catalyst with a high loading. The results of these experiments showed that, after equilibration, no more changes in the valence state of vanadium were observed after reduction or subsequent re-oxidation. This indicates that the generally assumed Mars-Van-Kreuefen mechanism is not operative for the titania-supported V-P-0 catalyst.
Surface and Interface Analysis, 2004
Vanadyl phosphate catalysts supported on different oxides (g-Al 2 O 3 , TiO 2 , SiO 2) have been investigated by x-ray photoelectron spectroscopy (XPS). The surface chemical composition has been studied as a function of the thermal treatment under oxidizing (calcination) or reaction conditions in the oxidative dehydrogenation of ethane. Dispersion of vanadyl phosphates on g-Al 2 O 3 and TiO 2 results in the formation of vanadium species in different oxidation states, i.e. V 5+ phosphate, V 5+ and V 4+ oxide species, whose relative fraction depends both on the support and on the reaction temperature. A progressive reduction of vanadium species occurs in the samples as an effect of the temperature. Vanadyl phosphates exhibit a stronger interaction with the titania support, which undergoes surface modifications after impregnation, thermal treatments and catalysis runs. A noticeable interaction of the alumina support and a poor dispersion of the active phase on silica are also revealed by XPS.
Applied Catalysis A: General, 1995
The changes that take place in a vanadium antimonate catalyst during catalytic propane ammoxidation were studied by transient reactivity experiments and infrared spectroscopy. The investigation was focused on both the change in catalytic behaviour of a fresh sample and the change in reactivity with stepwise increases in the concentration of one or more reagents while maintaining the concentration of the other reactants constant. The results obtained show that two main types of modifications are present: (1) A change in the reactivity from the fresh to the activated samples due to the surface formation of a VSbO4 phase by reaction of vanadium and antimony oxides that have not reacted during calcination; this modification leads to formation of carbon oxides and a decrease in the rate of propane depletion as well as a considerable increase in the rate of acrylonitrile formation. (2) A change in the reactivity of the activated sample which is a function of the redox characteristics of the reagent mixture; this modification does not change the rate of propane depletion, but rather only results in differences in the relative selectivity to products. The results also show that (i) in the steady state, the activated catalyst is partially reduced in comparison with the same sample after treatment with oxygen and (ii) the rate of acrylonitrile formation is lower. Infrared results confirm the formation of oxygen vacancies in the catalyst upon interaction with ammonia and propene as well as the presence of these defects in the sample after the catalytic tests.