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J. Phys. Chem. B 2001, 105, 10772-10783
Antimony Oxide-Modified Vanadia-Based CatalystssPhysical Characterization and Catalytic Properties J. Spengler,†,⊥ F. Anderle,† E. Bosch,†,# R. K. Grasselli,*,† B. Pillep,‡,∇ P. Behrens,‡,X O. B. Lapina,§ A. A. Shubin,§ H.-J. Eberle,| and H. Kno1 zinger*,† Department Chemie, UniVersita¨ t Mu¨ nchen, Butenandtstrasse 5-13 (Haus E), 81377 Mu¨ nchen, Germany, Institut fu¨ r Anorganische Chemie, UniVersita¨ t Mu¨ nchen, Meiserstrasse 1, 80333 Mu¨ nchen, Germany, BoreskoV Institute of Catalysis, Pr. Akad. LaVrentieVa 5, NoVosibirsk 630090, Russia, and Consortium fu¨ r elektrochemische Industrie, Zielstattstrasse 20, 81379 Mu¨ nchen, Germany ReceiVed: June 13, 2001; In Final Form: August 21, 2001
Antimony-modified vanadia-on-titania catalysts were prepared for the selective oxidation of o-xylene to phthalic anhydride by ball milling of powder mixtures followed by calcination. A binary Sb2O3-V2O5 system was also prepared for comparison purposes. The resulting materials were physically characterized by surface area measurements, X-ray diffraction analysis (XRD), laser Raman spectroscopy, X-ray absorption fine structure (XAFS) spectroscopy, electron spin resonance (ESR), magnetic susceptibility determination, and 15V solidstate NMR. The catalytic performance of the TiO2-supported materials was tested for o-xylene oxidation. After calcination of the Sb2O3-V2O5 binary mixture at 673 K, Sb3+ is almost quantitatively oxidized to Sb5+, while both V3+ and V4+ are detected. V3+ and some V4+ are most likely located in a nonstoichiometric VSbO4-like structure, while the majority of V4+ preferentially concentrates within shear domains in oxygendeficient V2O5-x particles. In the titania-supported catalyst system, both Sb2O3 and V2O5 spread on the anatase surface. Sb3+ is oxidized to Sb5+, and V3+, V4+, and V5+ are detected. VSbO4-like structures are not observed. The presence of antimony leads to the formation of presumably V3+-O-V5+ redox couples. The paramagnetic centerssin contrast to the binary mixturesare largely isolated. Antimony preferentially migrates to the surface and appears to exhibit a dual function catalytically. It is inferred from the experimental data that the addition of antimony leads to site isolation and to a reduction of surface acidity. We suggest that V-O-V-O-V domains or clusters are interrupted by incorporation of Sb to form V-O-Sb-O-V species. As a consequence of this site isolation and a reduction of surface acidity, overoxidation of o-xylene is reduced. These two effects are therefore most probably responsible for the improved selectivity of the ternary catalyst system over the binary one toward phthalic anhydride.
Introduction Catalysts based on titania-supported vanadium oxide (V/TiO2) are technologically most important for the selective oxidation of o-xylene to phthalic anhydride (PA).1,2 Very detailed characterizations with the goal of providing a standard V/TiO2 catalyst (EUROCAT OXIDE) were reported and summarized by Vedrine3 based on investigations of 24 European research groups. Centi et al.4 reported on the nature of the active phase of V/TiO2 catalysts formed during o-xylene oxidation. They suggested that an anatase support is covered by a V4+-V5+ partially hydrated mono- or bilayer with an overlayer consisting of a hydrated V3O7-like phase as amorphous multilayer patches. Most industrial catalysts used for PA synthesis are V/TiO2 modified by antimony oxide,5-7 which is believed to enhance †
Department Chemie, Universita¨t Mu¨nchen. Present Address: Consortium fu¨r elektrochemische Industrie, Zielstattstrasse 20, 81379 Mu¨nchen, Germany. # Present Address: Maxlrainstrasse 2, 81541 Mu ¨ nchen, Germany. ‡ Institut fu ¨ r Anorganische Chemie, Universita¨t Mu¨nchen. 3 Present Address: Patent- und Rechtsanwaltskanzlei Kador und Partner, Corneliusstrasse 15, 80469 Mu¨nchen, Germany. X Present Address: Institut fu ¨ r Anorganische Chemie, Universita¨t Hannover, Callinstrasse 9, 30167 Hannover, Germany. § Boreskov Institute of Catalysis. | Consortium fu ¨ r elektrochemische Industrie. ⊥
the PA selectivity8 and vanadium antimonate-type catalysts are known as selective oxidation and ammoxidation catalysts.9-11 Despite the technical importance of the ternary Sb-V/TiO2 catalyst system, the structure and physical characteristics of this material have only sparely been investigated.12,13 Although the exact origin of the selectivity enhancement induced by the Sb2O3 modifier is still unknown, it might be speculated that the presence of the modifier oxide induces “site isolation”. The concept of site isolation was first formulated by Callahan and Grasselli14 and more recently reviewed in several articles.15,16 The concept states that reactive lattice oxygens must be structurally isolated in defined groupings on a catalyst surface to achieve selectivity. We have recently shown by X-ray photoelectron spectroscopy (XPS)13 that an Sb-V/ TiO2 material, which was prepared by ball milling of the individual oxides and subsequent calcination, contained partially reduced amorphous vanadium oxide overlayers and Sb3+ and Sb5+ oxides. The antimony oxide segregated into the outermost surface layers. It was therefore inferred that the presence of the antimony oxide modifier spatially separates V-O species and thereby leads in fact to site isolation. The latter is probably responsible for the observed positive effect of the modifier by enhancing the catalyst’s selectivity. In the present paper, we report results of extensive physical characterizations of Sb-V/TiO2 catalysts aiming at a better
10.1021/jp012228u CCC: $20.00 © 2001 American Chemical Society Published on Web 10/04/2001
Vanadia-Based Catalysts understanding of their structural characteristics and their redox properties. These catalysts are conventionally prepared by impregnation of the titania support with aqueous solutions containing suitable oxide precursors in the required concentrations. In continuation of a series of experimental studies on the preparation of supported oxide catalysts17-19 via a mechanochemical route, in the present work, we have prepared the SbV/TiO2 catalysts by gently mixing the TiO2 support in powder form with defined amounts of Sb2O3 and V2O5 powder to predetermine the final composition of the ternary catalyst system. The components of these mixtures of powders were then brought into intimate contact with each other by ball milling in a planetary mill followed by calcination, if desired. Therefore, the present paper also discusses the effect of ball milling and calcination on the dispersion and distribution of the active vanadium oxide and of the modifier antimony oxide. The effect of ball milling on some physical characteristics of the binary mixtures V/TiO217,18 and Sb/TiO219 was reported earlier and the possible genesis of active monolayer oxide catalysts by spreading and wetting in binary oxide mixtures was reviewed.20,21 Mechanochemical treatment of solids leads not only to alterations of their morphology and texture but also to the possible formation of surface solid solutions22, and it may induce modifications of their catalytic performance.23 For example, it was demonstrated24 that milling of V/TiO2-based PA catalysts led to an enhancement of PA yields accompanied by changes of the catalyst surface, which could be monitored by spectroscopic and microscopic techniques. The possibility of the synthesis of active and selective Sb-V/TiO2 catalysts by mechanochemical processes and the structural, redox, and catalytic properties of the resulting materials will be reported in the present paper. Experimental Section Materials and Catalyst Preparation. For the catalyst preparation, V2O5 (J. T. Baker Chemicals B. V., 99% purity), senarmontite Sb2O3 (Fluka, p. a.), and TiO2 (TiO2 suspension, dried and calcined at 873 K for 10 h) were used. The TiO2 (provided by Consortium fu¨r elektrochemische Industrie, Mu¨nchen, Germany) was pure anatase and had a surface area of 45 m2/g as determined by the BET method. No surface impurities were detected by XPS except some minor sulfur contamination, the concentration of which was close to the detection limit of XPS. The theoretical monolayer capacity of TiO2 for V2O5 and Sb2O3 (molar ratio V:Sb ) 13:5) was estimated according to the procedure described previously12 to be 7.7 wt %. The mechanical treatment was performed in a planetary mill in air with either the dry oxide mixture or with 10 wt % H2O added to the mixture. For this purpose, the corresponding amounts of the oxides were carefully mixed in an agate mortar and then milled at ca. 145 rpm in an agate vessel (250 mL) containing six agate balls (1.5 cm diameter). The milling times were chosen between 1 and 20 h. After 1, 2, 3, 5, 10, and 20 h the milling procedure was interrupted for sampling and for detaching the compressed powder from the walls of the vessel. All milling preparations were carried out with an initial loading of the agate vessel of 70 g of the oxide mixture. The resulting samples were calcined at 673 or 723 K for 5, 10, or 20 h. For comparison purposes, binary unsupported mixtures of Sb2O3 and V2O5 (molar ratio 1.5:1) were also prepared by ball milling for 1-20 h and subsequent calcination at 673 K for 5 and 20 h. It has been reported that VSbOx (rutile structure) can
J. Phys. Chem. B, Vol. 105, No. 44, 2001 10773 be prepared by a solid-state reaction of Sb2O3 with V2O5 at temperatures of 973-1073 K.25-27 Determination of Surface Areas. For the measurement of BET surface areas, samples were evacuated at 473 K and 4 × 10-2 mbar. N2-adsorption isotherms were measured on a Sorptomatic 1800 (Carlo Erba), which was controlled via the software Milestone 200. X-ray Diffraction. For X-ray diffraction analysis (XRD), a Siemens-Guinier diffractometer with Cu KR radiation was used. Laser Raman Spectroscopy. Raman spectra were recorded with a Dilor (OMARS 89 triple monochromator) spectrometer, equipped with a thermoelectrically cooled charge-coupled device camera (Princeton Instruments). An Ar+ ion laser (model series 2020, Spectra Physics) was used, and the samples in contact with air were rotated during measurements. Dehydration of the samples and the subsequent measurement of the spectra was carried out in flowing dry oxygen in a quartz flow reactor which was heated from room temperature to 723 K. The temperature was then held constant for 30 min. All spectra were recorded with the scanning multichanel technique28,29 with a laser power of 5-50 mW at the sample position and an exciting wavelength of 488 nm for the hydrated and 514 nm for the dehydrated samples. The slit width was 150 µm, and the scan time for a single spectrum was 10-90 s. The spectral resolution was 5 cm-1. X-ray Absorption Fine Structure (XAFS) Spectroscopy. For the X-ray absorption measurements, carefully ground powders of each sample were pressed to polyethylene pellets (spectroscopic grade polyethylene, Merck 107422). The amount of sample was adjusted to give edge jumps, ∆µd, from 0.3 to 0.8. Sb L edge XAFS spectra were recorded at HASYLAB/ DESY in Hamburg (Germany). The storage ring DORIS III was operating at 4.432 GeV with an injection current of 100 mA. All spectra were collected at room temperature in transmission mode by monitoring the X-ray intensities with ionization chambers. Recording of the spectra was performed at beamline E4, which was equipped with a focusing Au-coated mirror, a planar Ni-coated mirror, and a Si(111) double-crystal monochromator. The edge region was scanned with a step width of 0.2 eV, well below the estimated experimental resolution of 0.8 eV. For energy calibration of the Sb L edge spectra, the Ca K edge of a CaWO4 sample was recorded simultaneously with each sample. In the measurements, a detuning of the monochromator to 50% of the maximum intensity served to reduce contributions of higher harmonics to the X-ray beam. The reflectivity cutoff of the planar Ni-coated mirror at about 7 keV further improved this reduction. The spectra were processed with the program WinXAS.30 For energy calibration of the L edge spectra, the first inflection point (fip) at the Ca K edge of the CaWO4 sample was set to a value of 4.0491 keV. This corresponds to the value of 4.966 keV for the fip at the Ti K edge of a Ti foil. The background was modeled using Victoreentype functions and subtracted from the curves. The edge jump was then normalized to an absorption value of 1. Electron Spin Resonance (ESR) and Magnetic Susceptibility Measurements. The ESR spectra were recorded on a Varian E-Line (E 9) spectrometer with a microwave frequency of 9.2 GHz (X-band). The samples were placed into a TE104mode double cavity, which permitted the measurement of a sample and a reference under identical conditions, in particular, microwave power. The field modulation frequency was 100 kHz at a constant modulation amplitude of 4 G. A solid solution of Mn2+ in MgO (hyperfine structure with six lines) served as a reference material. For quantitative calibration of the spectra, a
10774 J. Phys. Chem. B, Vol. 105, No. 44, 2001 CuCl2 single-crystal standard was used. The accuracy of the quantitative determination of V4+ centers by ESR was approximately (2 × 1017 sites/g of catalyst. For spectra simulation, the computer program Simfonia (Bruker) was used. Magnetic susceptibility measurements required for the estimation of the V3+ (not detectable by ESR) concentrations were carried out using a self-made magnetic balance (Gouy method). The experimental accuracy of the reported V3+ concentrations is approximately (2 × 1018 sites/g of catalyst. NMR Measurements. 51V and 1H NMR measurements were performed using a Bruker MSL-400 spectrometer at 105.25 and 400.13 MHz, respectively, at room temperature. 51V MAS spectra of powder samples were recorded at rotation frequencies between 2 and 15 kHz using a 5 mm rotor and the NMR probe constructed by NMR Rotor Consulting ApS, Denmark.31 Quadropular echo experiments were performed with the two pulse sequence: {π/(12(X)) - t1 - π/(12(Y)) - t2}, where t1 and t2 were chosen empirically to be near 55 and 57 µs, respectively. Repetition times from 0.1 to 2 s and rf pulses with 1 µs duration were used in the experiments. All chemical shieldings are referred to VOCl3 as an external standard. Simulations of 51V static and MAS NMR spectra were performed taking into account the second-order quadrupole effects and using the general purposes NMR1 program.32 The NMR1 program is based on an effective average Hamiltonian obtained in a manner similar to that presented earlier.33 A particular variant of this program (NMR2) especially adopted for the fast computation of spinning sideband (ssb) intensities in MAS spectra of quadrupolar nuclei (Herzfeld and Berger approach)34 was used for simulations of MAS and static spectra and for least-squares parameter fitting. Oxidation of o-Xylene. The catalytic test reactions were carried out in a conventional tubular fixed-bed reactor working at atmospheric pressure. The standard reactant composition was 0.7 mol % o-xylene in air, which was obtained by passing an air flow, F, of 50 cm3/min through a thermostated saturator containing liquid o-xylene. The catalyst bed contained an amount (W) of 300 mg catalyst (particle size < 0.2 mm), which results in a W/F ratio of 0.36 g s cm-3. Reactant and product analyses were carried out with an on-line gas chromatograph using a HP-5 capillary column. Results and Discussion Unsupported Binary Sb2O3-V2O5 Mixture. When the binary oxide mixture was milled for 2 h, differential thermal analysis (DTA) and thermogravimetric analysis (TGA) indicated that the solid-state reaction forming VSbO4 started at about 773 K. A sample that was calcined at 1073 K did indeed show the X-ray diffraction signature of VSbO4, as reported in the literature.26,27 Because the calcination temperature of the ternary catalyst system was between 673 and 723 K, the nature of the binary oxides was studied as a function of the duration of milling prior to and after calcination at 673 K. The physical techniques applied were XAFS, NMR, magnetic susceptibility measurements, and ESR; XAFS provides information of the local environment and oxidation state of antimony,19 while 51VNMR, ESR, and magnetic susceptibility probe the chemical state of vanadium.17,35,36 X-ray Absorption Spectroscopy. We have performed Sb L1 edge and Sb L3 edge X-ray absorption near edge structure (XANES) measurements. As previously reported, 19 Sb L3 edge XANES spectra of oxidic Sb species show a characteristic preedge peak, A, at a photon energy of 4.132-4.136 keV (see, for example, Figure 1), which can be attributed to 2p3/2 f 5p
Spengler et al.
Figure 1. Sb L3 edge spectra of milled (mil.) and milled and calcinated (mil., calc.) unsupported Sb2O3-V2O5 mixtures. Calcination temperature was 673 K.
electronic transitions, the intensity and shape of which are related to the oxidation state of the absorbing Sb atoms. In the edge regime beginning at about 4.14 keV, the Sb L3 edge XANES spectra show a peak B (see also Figure 1), attributed to a 2p3/2 f 5d transition, and additional multiple scattering features at still higher photon energies. Peak B and, in particular, the multiple scattering features are indicative fingerprints of the local short to medium range order up to a maximum of about 4-5 Å around the absorbing Sb atoms. Sb L1 edge XANES spectra of oxidic Sb compounds show prominent absorption features, the so-called “white lines”, C, in the energy region between 4.70 and 4.71 keV assigned as 2s f 5p dipole-allowed transitions (see Figure 2). The energy position of these white lines allows for a clear distinction between Sb3+ and Sb5+ species for which the white line appears at ca. 4.702 and ca. 4.707 keV, respectively. Furthermore, in spectra of mixed valence Sb compounds, their relative intensities and their shapes also allow for a fair estimation of the Sb3+/ Sb5+ ratio present in the sample. Figure 1 shows the Sb L3 edge XANES spectra of the unsupported binary Sb2O3-V2O5 samples after different milling and calcination times. The spectrum of the sample that has been prepared by 1 h of milling without calcination is almost identical to that of pure crystalline senarmontite.19 This shows that neither the short to medium range order around the absorbing Sb atoms nor the oxidation state of the Sb atoms is significantly altered. The corresponding Sb L1 edge spectrum shown in Figure 2, which is also nearly identical to that of Sb2O3 senarmontite,19 confirms these findings. From the energy position (ca. 4.702 keV) and the shape (only one peak) of the white line C, it can be inferred that no oxidation of the Sb atoms has been induced by the milling process. It can thus be concluded that the milling of the Sb2O3-V2O5 mixture for short periods of time leaves at
Vanadia-Based Catalysts
Figure 2. Sb L1 edge spectra of milled (mil.) and milled and calcinated (mil., calc.) unsupported Sb2O3-V2O5 mixtures. Calcination temperature was 673 K.
least fragments of the Sb2O3 structure intact and does not affect the oxidation state of the Sb atoms. This behavior can well be understood by considering the structure of senarmontite. It consists of Sb4O6 subunits in which covalent bonding predominates, whereas between these subunits only weak van der Waals forces act in the solid state. Therefore, the Sb4O6 subunits of the senarmontite structure are preserved in the Sb2O3-V2O5 mixture. The same result has been obtained in the Sb2O3-TiO2 (anatase) system.19 The Sb L3 edge spectra of the sample that has been milled for 20 h and the sample that has been milled for 5 h in the presence of H2O (Figure 1) are very similar to each other and resemble that of Sb6O13.19 Both show a preedge peak A slightly modified with respect to the senarmontite spectrum, which indicates that at least part of the absorbing Sb atoms have changed their oxidation state from 3+ to 5+. This finding is confirmed by the corresponding Sb L1 edge spectra shown in Figure 2, in which the white line clearly shows two peaks at energy positions of 4.702 and 4.707 keV, indicative of the simultaneous presence of Sb3+ and Sb5+, respectively. The shape of the white line leads to a formulation of the antimony oxide species as Sb6O12+δ (with 0 < δ < 1). Furthermore, edge peak B in the Sb L3 edge spectrum and particularly the multiple scattering region at higher energies in Figure 2 show a drastic change as compared to the spectrum of senarmontite: peak B is significantly broadened out, and only one broad peak at an energy of 4.162 keV appears in the multiple scattering region, in contrast to the various structures observed in the senarmontite spectrum. From these observations, it can be concluded that dry milling of the Sb2O3-V2O5 samples for 20 h or wet milling for 5 h (almost) completely breaks down the Sb4O6 subunits, leads to a partial oxidation of the Sb atoms, and establishes a new short to medium range order around the absorbing Sb atoms. It is
J. Phys. Chem. B, Vol. 105, No. 44, 2001 10775 noteworthy that in contrast to these findings wet milling of the Sb2O3-TiO2 system, even after 20 h, does not lead to a breakdown of the Sb4O6 subunits and does not lead to a change in the Sb oxidation state.19 This different behavior may be attributed to the fact that in contrast to TiO2 V2O5 is easily reduced. Thus, the oxidation of Sb2O3 (at least at longer milling times or in the presence of H2O) is induced by the presence of V5+ oxide species via redox reaction between V5+ and Sb3+, which is not present in the Sb oxide/Ti oxide system (i.e., between Ti4+ and Sb3+). Furthermore, the Sb L3 edge spectra of the dry-milled samples that were calcined at a temperature of 673 K with calcination times of 5 and 20 h are shown in Figure 1. It can be observed that the spectra of the samples milled for 1 h and for 20 h are very similar to each other irrespective of the calcination time. The same observation is made when comparing the corresponding Sb L1 edge spectra in Figure 2. These observations indicate that a prolongation of the calcination time from 5 to 20 h affects neither the oxidation state nor the short to medium range order around the absorbing Sb atoms. Also, in all Sb L3 edge spectra, peaks A and B are observed, which show a higher intensity and a smaller width as compared to the spectra of the uncalcined samples. In the multiple scattering region above peak B, all spectra show one broad feature at an energy of ca. 4.164 keV, which according to its shape is constituted of at least two single peaks. The spectra of the calcined samples are thus similar to the spectra of the compound VSbO4.19 This compound has a rutile-type structure in which all Sb atoms have an oxidation state of 5+. In this structure, the Sb atoms are octahedrally surrounded by O atoms. The similarity with the spectrum of VSbO4 is still more pronounced for the spectra of the samples that have been milled for 20 h prior to calcination. These observations lead to the conclusion that most of the Sb atoms possess the oxidation state 5+ after calcination and that these atoms have an approximately octahedral O environment. These findings are confirmed and further specified by the corresponding Sb L1 edge spectra shown in Figure 2. The white lines in all spectra predominantly consist of one peak at an energy of ca. 4.707 keV, which clearly indicates that most of the Sb atoms after calcination have an oxidation state of 5+. However, small differences can be observed between samples that have been milled for 1 and for 20 h prior to calcination. After 1 h of milling, the white line shows a shoulder at energies of ca. 4.702 keV, indicating that there are still Sb3+ atoms present. The shape of the white line leads to a formulation of the antimony oxide species of Sb6O12-δ, with δ being approximately 0.2. In contrast, after 20 h of milling, the shoulder is lacking, thus showing that no or almost no Sb3+ atoms are remaining. From all these observations, it can be concluded that calcination of the Sb2O3-V2O5 mixture independently of the milling time prior to the calcination leads to a (complete) breakdown of the Sb4O6 subunits, which are still present at least after a milling time of 1 h. This break down is accompanied by an oxidation of the Sb3+ to Sb5+ atoms for which, however, the pretreatment of the samples plays a role. Whereas after milling for 20 h the oxidation to Sb5+ is complete after calcination, a small amount of Sb3+ is still present when milling has been applied for only 1 h. This difference in behavior may be attributed to the fact that after a milling time of 20 h already 1/ of the Sb atoms are oxidized to Sb5+ prior to the calcination, 2 whereas after milling for 1 h no oxidation of the Sb atoms is observed. In contrast, the duration of the calcination in excess of 1 h has no influence on the oxidation behavior of the Sb
10776 J. Phys. Chem. B, Vol. 105, No. 44, 2001
Spengler et al.
Figure 3. 105.25 MHz 51V static quad echo and MAS (spinning frequency, νr ) 2 and 10 kHz) NMR spectra of V2O5-Sb2O3 mixture after different milling times (1 h, 5 h, 20 h).
TABLE 1: Concentrations of V3+, V4+, and V5+ and Relative Intensities of Central NMR Transitions for Milled V2O5-Sb2O3 Samples milling time (h) 1 5 20 1 5 20
calcination time (h); temp (K)
[V3+] × 10-19 (g-1)
[V4+] × 10-19 (g-1)
[V5+] × 10-21 (g-1)
([V3+] + [V4+])/[V5+]
central NMR transition relative intensity
5; 673 5; 673 5; 673
11 12 18 58 100 97
1.2 11 54 0.7 0.7 0.7
4.0 3.8 3.4 3.5 3.1 3.1
3.05 6.05 21.2 18.6 34.5 33.5
1 0.47 0.07 0.26 0.09 0.13
atoms. In particular, the relatively short milling time of 1 h cannot be “compensated” by a longer calcination time. 51V NMR and ESR. 51V NMR, ESR, and RAMAN spectroscopy demonstrated36 that milling of pure V2O5 in a planetary mill resulted in an increase of its surface area and a partial reduction of V5+ to V4+ and V3+. The distribution of V4+ and V3+ ions is most probably inhomogeneous. V4+ is located in small patches of V6O13-like shear structures formed in V2O5 during milling, whereas (6%) paramagnetic V3+ ions are responsible for the loss of (up to ca. 70%) intensity in 51V NMR spectra. The local environment of vanadium sites in milled V2O5 is more distorted as compared with the polycrystalline V2O5. No V5+ ions in tetrahedral coordination were apparent in detectable quantities. After milling the Sb2O3-V2O5 mixture for 1 h, the wellknown 51V NMR spectrum reflecting an octahedral vanadium coordination sphere was observed (Figure 3). The vanadium nuclei in this mixture lead to the same intensity as for an identical number of nuclei in the absence of antimony oxide. Comparable to the spectra of pure V2O5, the static spectra demonstrate a further distortion in the long range order after milling for 5 and 20 h, which can be recognized by a broadening of the first-order quadrupolar transitions. MAS spectra at low rotation frequencies (νr ≈ 2 kHz) show a broad signal below the rotation structure representing a much stronger distortion of the close vicinity of V5+ than that for pure V2O5.36 After 20 h of milling, this distortion is sufficient to prevent the resolution of the rotational sidebands at 2 kHz and could be described by
a distribution of the isotropic shift of 5-10 ppm.36 On the other hand, a strongly distorted anisotropic signal appears at about -560 ppm, which is caused by a vanadium atom in a tetrahedral environment. After evacuation of this sample for 24 h at room temperature, the spectrum is not changed, showing that this new complex is not located at the surface and is not connected with the interaction with H2O. Probably, this tetrahedral complex is connected with the interaction with Sb2O3 in the bulk. MAS experiments do not reveal a new set of spinning sidebands in these spectra (even at high rotational frequencies of νr ≈ 10 kHz), thus demonstrating that the new tetrahedral complex has significant distribution of chemical shielding anisotropy parameters, i.e., some distortions of the local environment of vanadium sites must exist. The signal has now lost more than 90% of its intensity, which is not surprising since 21% of the V5+ ions in the oxide mixture are reduced after 20 h of milling (Table 1). As there is still no broadening visible in the spectra, the distribution of paramagnetic centers seems to be inhomogeneous in the samples. In addition, there is no hyperfine structure visible in the ESR spectra (not shown), and the line width is greater than that for pure V2O5. The relative number of V4+ ions formed is greater in the milled oxide mixture than that in pure V2O5, while that of V3+ ions is smaller. After calcination, only a very small amount of V4+ is left, the major part of the paramagnetic vanadium ions being in oxidation state 3+ (Table 1). The number of V3+ atoms remains essentially constant when the milling time is increased from 5 to 20 h (Table 1). Because of this high concentration of V3+ sites, the NMR spectra are now strongly broadened, which is
Vanadia-Based Catalysts
J. Phys. Chem. B, Vol. 105, No. 44, 2001 10777
Figure 4. 105.25 MHz 51V static quad echo and MAS (spinning frequency νr ) 2 and 10 kHz) NMR spectra of V2O5-Sb2O3 mixture after different milling times (1 h, 5 h, 20 h) and after calcination procedures at 673 K for 5 h.
visible in the static as well as in the MAS spectra (an envelope line) as shown in Figure 4. A very small broad line can be seen in the MAS spectra showing that there is only a small distortion of the vanadium-oxygen complexes after calcination. The signal of the tetrahedral vanadium complex has now totally vanished. Such behavior indicates that thermal treatment leads to a deep interaction between V2O5 and Sb2O3 via a redox process as reported by Centi and Perathoner.37 Ternary Sb-V/TiO2 System. Specific Surface Area. All TiO2-supported materials irrespective of the details of preparation had the same specific surface of 50 m2/g ((10%) within the limits of accuracy of the BET method. X-ray Diffraction. When the ternary oxide mixture was milled under dry conditions, the reflection intensities of V2O5 and Sb2O3 in the diffractograms (not shown) decreased significantly with increasing duration of milling. After 20 h of milling, the reflections of Sb2O3 are almost quantitatively eroded and those of V2O5 are strongly reduced in intensity and broadened. The reflections of the anatase support remain unchanged. These observations suggest that during milling the particle sizes of V2O5 and Sb2O3 are dramatically reduced and/or amorphization of these two oxide components occurs, which might be caused by spreading. When water was added to the oxide mixtures during milling, no reflections other than those of anatase could be detected. Hence, the presence of water during milling seems to accelerate the proposed spreading process. No crystalline phases other than anatase were detected for any sample after calcination, suggesting that the thermal treatment leads to an efficient spreading of both oxide components on the support surface. Raman Spectroscopy. Figure 5 shows Raman spectra of the ternary oxide mixture after milling for 1 or 20 h in the absence (spectra a and b) and presence (spectra c and d) of water. The spectra were recorded prior to calcination under ambient conditions. All spectra are dominated by bands at 147 and 198 cm-1 (not shown) and at 398, 515, and 640 cm-1. These bands characterize the anatase modification of the TiO2 support.38 A weak contribution near 800 cm-1 to the broad band around 810820 cm-1 must be attributed to an overtone of the B1g mode at
398 cm-1.38 After dry milling for 1 h, additional bands of V2O5 at 283, 304, 480, 701, and 995 cm-1 are observed. Bands of Sb2O3 are not detected, except for its strongest Raman band which appears at 253 cm-1 with very low intensity, suggesting that the senarmontite structure has been largely destroyed after 1 h of milling. This latter band is quantitatively eroded after 20 h of milling (spectrum b in Figure 5), and the intensity of the bands characterizing V2O5 are slightly reduced. These trends are consistent with the XRD results (vide supra). When water was present during the milling, neither Sb2O3 nor V2O5 could be detected (spectra c and d in Figure 5). Two very broad and weak features located at approximately 810 and 990 cm-1 have been assigned to surface polyvanadate stuctures.40,41 Figure 6 demonstrates the effect of calcination on the structure of the Sb-V/TiO2 samples. The spectra were also recorded at ambient conditions. Again, the spectra are dominated by the bands of the anatase support. After dry milling for 20 h followed by calcination at 673 K for 5 h (spectrum a of Figure 6), small amounts of V2O5 are still detected. The corresponding bands disappear when the calcination time and temperature are increased (spectrum b in Figure 6), suggesting that under these conditions both Sb2O3 and V2O5 have probably spread out on the surface of the TiO2. The only bands seen in the spectrum of this sample and the spectra of those calcined after milling in the presence of water (spectra c and d in Figure 6) are very broad and ill-defined features at approximately 815-830 cm-1 and 995-1000 cm-1, assigned in the literature to hydrated polymeric VOx species.40,41 These species have been considered to determine the catalytic properties of binary V/TiO2 catalysts.42,43 The Raman spectra of Sb-V/TiO2 recorded in their dehydrated state (Figure 7) show a band at 1034 cm-1 for all treatments as well as a broad ill-defined band near 935 cm-1. The latter band has been attributed to decavanadate species,44 while the former is most likely characterizing the VdO stretching mode of a four-coordinated vanadate species.44 This therefore suggests that the surface vanadate species undergo structural changes during dehydration in the present ternary oxide as was previously reported for the binary oxide V/TiO2
10778 J. Phys. Chem. B, Vol. 105, No. 44, 2001
Figure 5. Raman spectra of hydrated Sb-V/TiO2 after 1 and 20 h of milling in the absence and presence of water: (a) 1 h of dry milling; (b) 20 h of dry milling; (c) 1 h of milling in the presence of 10 wt % H2O; (d) 20 h of milling in the presence of 10 wt % H2O.
catalysts.44 In addition, small amounts of V2O5 are regenerated during dehydration of the ternary materials (band at 995 cm-1 in spectra a and c of Figure 7) when the samples were calcined at 673 K. The corresponding band disappears again on calcination at 723 K. X-ray Absorption Spectroscopy. In Figures 8 and 9, the Sb L3 edge and Sb L1 edge spectra, respectively, of Sb-V/TiO2 samples are shown. The Sb L3 edge spectra (Figure 8) show that even after milling for only 1 h the features of the senarmontite spectrum18 in the multiple scattering regime are no longer preserved but only a broad peak at an energy of 4.162 keV is obtained. In addition, peak A and also peak B show a sharpening and a gain in intensity. Small differences between the dry and the wet milled samples can be observed: In the spectrum of the sample milled for 20 h in the absence of water, the sharpening of peaks A and B and also of the peak at 4.162 keV is more pronounced than in the spectrum of the sample that was milled for 1 h. In contrast, both spectra of the samples
Spengler et al.
Figure 6. Raman spectra of hydrated Sb-V/TiO2: (a) 20 h of dry milling and subsequent calcination for 5 h at 673 K; (b) 20 h of dry milling and subsequent calcination for 5 h at 723 K; (c) 20 h of milling in the presence of water and subsequent calcination for 5 h at 673 K; (d) 20 h of milling in the presence of water and subsequent calcination for 5 h at 723 K.
milled in the presence of water are almost identical irrespective of the milling time and even show a slightly more pronounced sharpening of the features as compared to the spectrum of the sample milled for 20 h in the absence of water. Furthermore, the spectrum of the sample milled for 20 h in the absence of water and both spectra of the samples milled in the presence of water show a weak peak between features A and B at an energy slightly above 4.14 keV. These observations are supplemented by the corresponding Sb L1 edge spectra (Figure 9). Whereas in the spectrum of the sample milled for 20 h under dry conditions as well as in both spectra of the samples milled in the presence of water the white line occurs at an energy of 4.707 keV and has a one-peak shape, in the spectrum of the sample milled for 1 h in absence of water, the white line shows a twopeak shape with the second peak at an energy of about 4.702 keV. The shape of this white line leads to a formulation of the
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J. Phys. Chem. B, Vol. 105, No. 44, 2001 10779
Figure 7. Raman spectra of dehydrated Sb-V/TiO2: (a) 1 h of dry milling and subsequent calcination for 5 h at 673 K; (b) 1 h of dry milling and subsequent calcination for 20 h at 723 K; (c) 20 h of milling in the presence of water and subsequent calcination for 5 h at 673 K; (d) 20 h of milling in the presence of water and subsequent calcination for 20 h at 723 K.
Figure 9. Sb L1 edge spectra of milled (mil.) and milled and calcinated (mil., calc.) Sb-V/TiO2. Calcination temperature was 673 K.
Figure 8. Sb L3 edge spectra of milled (mil.) and milled and calcined (mil., calc.) Sb-V/TiO2. Calcination temperature was 673 K.
antimony oxide species of Sb2O5-δ with δ being approximately 0.6. These observations suggest the following: In the ternary SbV/TiO2 system, dry milling for only 1 h already leads to a pronounced breakdown of the Sb4O6 subunits of the senarmontite structure. In addition, as it is apparent from the Sb L1 edge spectra, this breakdown is accompanied by an oxidation of the major part of the Sb atoms from 3+ to 5+. Both processess structural breakdown and oxidationsare complete after dry
milling for 20 h or after wet milling (irrespective of the milling time). It can thus be inferred that the behavior of the ternary system differs from that of the binary systems in which either no oxidation (Sb/TiO219) or only oxidation of about half of the Sb atoms (unsupported Sb2O3-V2O5, see above) can be achieved by the milling process. Furthermore, the breakdown process of the Sb4O6 subunits also exhibits differences: In the Sb/TiO2 system, only dry milling for 20 h leads to a breakdown (of most) of the Sb4O6 subunits, whereas dry milling for only 1 h or wet milling even for 20 h does not.19 In the unsupported Sb2O3V2O5 system, dry milling for 1 h also leaves the Sb4O6 subunits unaffected as mentioned above, which, however, are destroyed by dry milling for 20 h or wet milling for 5 h. It is thus only in the ternary TiO2-supported V-Sb-O system that dry milling for short periods of time (1 h) already leads to a significant breakdown of the Sb4O6 subunits of the senarmontite structure. This shows a synergistic effect of all three components, which, when milled together, show a different behavior than would be expected from the results for the individual binary systems. Further, the occurrence of the weak peak at 4.14 keV, which is not observed in the spectra of the samples of the binary system, shows that the environment of the Sb atoms on the surface of the TiO2 support in the ternary system is different from that in the binary systems. This indicates that both V and Ti atoms are present in the short to medium distance range around the Sb atoms. The local structure arround the Sb atoms, however, appears to be rather irregular as shown by the broad structure in the Sb L1 edge spectra. It should also be emphasized that the Sb L3 edge spectra of the samples in the ternary system (particularly of the samples in which only Sb5+ species are present) do not resemble that of VSbO419 as is the case for the calcined binary Sb2O3-V2O5 (see Figure 1). This shows that the environment around the Sb
10780 J. Phys. Chem. B, Vol. 105, No. 44, 2001 atoms in the small and medium distance range in the ternary TiO2-supported system deviates from that in VSbO4. The Sb L3 edge spectra of the calcined ternary samples, which are also shown in Figure 8, are similar to those of the corresponding uncalcined samples, the main difference being that feature B is slightly less pronounced in the spectra of the calcined samples. The corresponding Sb L1 edge spectra (Figure 9) for all calcined samples show essentially one peak at an energy of 4.707 keV. Only the spectrum of the sample that was milled for 1 h under dry conditions prior to calcination shows an additional shoulder originating from a (small) peak at 4.702 keV. These observations indicate that calcination of the ternary Sb-V/TiO2 sample leaves the small to medium distance range order around the Sb atoms that was already attained by milling of the samples almost unaffected. Furthermore, neither oxidation nor reduction of the absorbing Sb atoms occurs by calcination with the exception of the sample that was milled for 1 h in the absence of water. In this case, the major part of the residual Sb3+ atoms are oxidized during calcination. Thus, it can be stated that binary and ternary systems behave differently during calcination. Whereas in the binary systems extensive changes in oxidation state and environment of the absorbing Sb atoms are induced, this is not the case in the ternary system. The main reason for this observation may be the fact that the milling of the ternary system (even at short milling times) leads to an extended breakdown and oxidation of the Sb atoms of the initial Sb2O3. Thus, calcination cannot induce additional significant alterations of these properties. This conclusion is arrived at by the fact that in the ternary system spreading of the Sb oxide on the support leads to an enhanced dispersion which cooperates with the redox interaction of V2O5 with Sb2O3. 51V NMR and ESR. Milling of binary V O -TiO mixtures 2 5 2 creates two different types of octahedrally coordinated V5+ species, which can be detected by 51V NMR and which are strongly bonded to TiO2.17 Simultaneously, the appearance of V3+ ions and an increase of their concentration is observed during milling and subsequent calcination, along with the formation of at least three different types of paramagnetic V4+ species. Relative amounts of these species depend on the milling time, the presence of water in the system, and the temperature and time of calcination. The total amounts of V4+ and V3+ do not exceed 25% of the total vanadium present. The structure of strongly bonded V5+ species depends on the preparation procedure. V5+(II) species formed after thermal treatment are less distorted than V5+(I) formed predominantly during milling only, and both types of species are less axially symmetric than V2O5. Combined NMR, ESR, and magnetic susceptibility measurements indicate that almost all vanadium species (V3+, V4+, V5+) are isolated from each other on the TiO2 support. Wide-line and MAS 51V NMR spectra of milled V2O5Sb2O3-TiO2 samples as a function of milling time, calcination temperature, and duration are shown in Figures 10 (milled in the absence of water) and 11 (milled in the presence of water). The spectra of the dry-milled ternary Sb-V/TiO2 sample are comparable to those of the binary V/TiO2 system.47 Magic angle spinning experiments at 15 kHz reveal a spectral superposition of several signals, the most characteristic of which is the signal of V2O5 (narrow spinning sidebands with the isotropic shift -612 ppm). The relative intensity of this signal permits a comparison of the strength of the interaction between oxides in dependence on the preparation conditions (milling time, calcination temperature and time, presence or absence of water). V2O5 disappeared completely only after milling for 20 h followed by
Spengler et al.
Figure 10. 105.25 MHz 51V static and MAS (spinning frequency νr ) 10-14 kHz) NMR spectra of V2O5-Sb2O3-TiO2 mixture after different milling times and calcination procedures: (1) 20 h of milling; (2) 20 h of milling and 5 h at 673 K; (3) 1 h of milling and 5 h at 723 K; (4) 20 h of milling and 5 h at 723 K; (5) 1 h of milling and 20 h at 723 K; (6) 20 h of milling and 20 h at 723 K.
calcination at 723 K for 20 h. Only traces of V2O5 could be detected in the sample after milling for 20 h followed by calcination at 723 K for 5 h. The same behavior was observed for the binary V/TiO2 system.17 Milling in the presence of 10 wt % H2O leads to a stronger interaction in the ternary system. Three hours of milling are sufficient for complete disappearance of V2O5 in the Sb-V/TiO2 system, while only a small part of V2O5 interacts at these conditions in the binary oxide system. The same tendency is observed for all other samples (compare spectra of the samples prepared by 1 h of milling and 5 h calcination at 723 K, by 20 h of milling and 5 h calcination at 723 K, and by 1 h of milling and 20 h calcination at 723 K in Figure 11 and in Figure 5 of this paper and in Fig. 5 in ref 16). A detailed analysis of MAS spectra shows that all spectra can be described by the superposition of four lines: the line of bulk V2O5, a line of strongly bonded species as observed by Shubin et al. in V/TiO2 catalysts,36 and two lines of strongly bonded species (V5+(I) and V5+(II)) as revealed by Lapina et al.17 in V/TiO2 catalysts prepared by milling.17 51V NMR parameters of these lines are summarized in Table 2. In all of these species, vanadium ions are in distorted octahedral coordination. A large value of the quadrupolar coupling constant, CQ, for strongly bonded vanadium species indicates a significantly larger electric field gradient at the position of 51V nuclei as compared to that of bulk V2O5. A significant deviation of octahedral symmetry of these species from an axial distortion follows from the large values of asymmetry parameters ηQ and ησ. Spectra of samples milled in the presence of water are more descriptive in this case. Thus, the spectrum of the sample after
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J. Phys. Chem. B, Vol. 105, No. 44, 2001 10781
TABLE 2: Literature Data for 51V Quadrupole Tensor Parameters (CQ, ηQ)a, Chemical Shielding Tensor Parameters (δσ, ησ, σiso)b, and Euler Angles (r, β, γ) Describing the Relative Orientation of Quadrupole Tensor with Respect to Chemical Shielding Tensor Obtained from 105.25 MHz 51V MAS and Static NMR Spectra for Strongly Bonded V5+ Species in VOx/TiO2 Catalysts Prepared by Different Techniques17,36 along with the Data for Bulk V2O545 c signal V5+(I)17 V5+(II)17 V5+ 36 bulk V2O545
CQ (kHz)
ηQ
δσ (ppm)
ησ
σiso (ppm)
σ1 (ppm)
σ2 (ppm)
σ3 (ppm)
R (deg)
β (deg)
γ (deg)
15 900 11 600 14 700 811
0.75 0.67 0.59 0.04
497 644 650 620
0.54 0.37 0.02 0.15
629 611 611 609
246 170 281 252
516 408 292 340
1127 1255 1261 1229
46 17 20 58
117 71 62 128
41 71 42
a Nuclear electric quadrupole moment, eQ; electric field gradient tensor eigenvalues (V , V , and V ) eq) are connected with C and η by the 1 2 3 Q Q relations CQ ) e2qQ/h; V1 ) 1/2(-1 - ηQ)V3; V2 ) 1/2(-1 + ηQ)V3. b The eigenvalues of the chemical shielding tensor are expressed by δσ, ησ, and 1 1 c σiso in the following manner: σ1 ) /2δσ(-1 - ησ) + σiso; σ2 ) /2δσ(-1 + ησ) + σiso; σ3 ) δσ + σiso. By the superposition of these species, all spectra in the V2O5-Sb2O3/TiO2 system can be described.
TABLE 3: Relative Intensities of Central NMR Transitions for Milled Oxide Samples calcination milling at 723 K for 20 h V2O5 V2O5-Sb2O3 V/TiO2 Sb-V/TiO2 time (h) 0 20 20
Figure 11. 105.25 MHz 51V static and MAS (spinning frequency νr ) 10-14 kHz) NMR spectra of V2O5-Sb2O3-TiO2 mixture after different milling times (with 10 wt % H2O) and calcination procedures: (1) milling for 1 h; (2) milling for 3 h; (3) milling for 20 h; (4) milling for 20 h and 5 h at 673 K; (5) milling for 1 h and 5 h at 723 K; (6) milling for 20 h and 5 h at 723 K; (7) milling for 1 h and 20 h at 723 K; (8) milling for 20 h and 20 h at 723 K.
3 h of milling (Figure 11) is almost identical to the spectrum measured for strongly bonded species in V/TiO2 catalysts prepared by the spray-drying technique.36 The spectrum of the sample after milling for 20 h in the presence of water (Figure 11) represents the superposition of the latter spectrum and the spectrum characterizing strongly bonded species as formed in V/TiO2 catalysts after high-temperature calcinations for long periods of time, which appears after calcination in all Sb-V/ TiO2 catalysts.
no no yes
100 27
100 7 13
100 75 75
100 60 50
The intensity of the NMR signal of the ternary system after dry milling starts with 80% (as compared to the amount of pure V2O5) after 1 h of milling, decreases to 60% after 20 h of milling, and reaches a value of about 50% after calcinations (Table 3). For all samples milled in the presence of water, the intensity has a value between 45% and 55%. For comparison, 20 h of milling decreases the intensity of pure V2O5 to 27%,35 the intensity of the binary mixture V2O5-Sb2O3 to 7%, and the intensity of the V2O5-TiO2 mixture to 75%.17 After calcination, the intensity of the NMR signal increases for the V2O5-Sb2O3 system to 13%, whereas it remains constant at 75% for V/TiO2. These data clearly indicate that the behavior of the ternary system is very similar to that of V/TiO2 samples. Catalytic Oxidation of o-Xylene. For the investigation of the effect of the antimony oxide modifier on the catalytic performance in the selective oxidation of o-xylene to phthalic anhydride, two catalysts were compared. The first was a SbV/TiO2 catalyst prepared by milling for 1 h in the presence of 10 wt % water and calcination for 5 h at 673 K, while the second was a V/TiO2 catalyst not containing antimony. Its vanadia loading was identical to that of the ternary catalyst and the preparation conditions were the same for the two materials except for a slightly higher calcination temperature for V/TiO2 (namely, 723 K). Figure 12 shows a phthalic anhydride selectivity vs total conversion plot of two V/TiO2 catalysts, one which contains antimony and one which does not. Intermediate products such as o-toluic aldehyde, o-toluic acid, and phthalide were also detected but are not taken into account in the present discussion, because their concentrations dropped to almost zero at increasing conversions. The variation in conversion was controlled by the reaction temperature between 523 and ca. 600 K. The data were always collected when the catalyst had reached the steady state. At temperatures higher than a critical temperature, namely, ca. 590 K for V/TiO2 and 600 K for Sb-V/TiO2, runaway conditions were reached in the microreactor and conversions abruptly jumped to 100%. Trends of selectivity with conversion analogous to those shown in Figure 12 have been reported in the literature and summarized by Grzybowska-Swierkosz.46 The phthalic anhydride selectivity increases with total conversion for both catalysts, the antimony-containing catalyst being more selective in the covered conversion range up to 40-50% as reported earlier.8 Thus, the Sb-V/TiO2 catalyst reaches a
10782 J. Phys. Chem. B, Vol. 105, No. 44, 2001
Figure 12. Selectivity vs conversion plot for o-xylene oxidation on an antimony-free (9) and an antimony oxide-modified (b) catalyst. Both catalysts contained a vanadium content corresponding to 60% of a theoretical monolayer; the molar ratio V:Sb was 13:5 for the Sb-V/TiO2 catalyst.
phthalic anhydride selectivity of 77% at 40% conversion as compared to only 38% selectivity with the V/TiO2 catalyst. It is interesting to note that the phthalic anhydride selectivity of V/TiO2 falls essentially to zero at 100% conversion (runaway conditions), the only products being COx, while the Sb-V/TiO2 catalyst still exhibits a selectivity of 44%. Although the absolute selectivity values at 100% conversion may be affected by several factors (runaway conditions), the observed trends appear to demonstrate that the antimony modifier reduces the tendency toward overreaction and, most importantly, that the presence of antimony oxide leads to a finite phthalic anhydride selectivity at 100% conversion although it is lower than that measured at lower conversion (i.e., lower temperatures). However, the selectivity of 44% at 100% conversion is still lower than that of an optimized industrial Sb-V/TiO2 catalyst, which gave a selectivity of 65% under the same experimental conditions. It is speculated that the present Sb-V/TiO2 catalyst requires additional modifiers or promoters for the enhancement of the selectivity at high conversion levels. Conclusions The combination of X-ray absorption spectroscopy (XAFS) with solid-state NMR, ESR, magnetic susceptibility measurements, and Raman spectroscopy turned out to be crucial for the physical characterization of V2O5-Sb2O3 and V2O5 + Sb2O3 on TiO2 mixed oxide catalysts. XAFS provided detailed information on oxidation states and local environment of antimony atoms, which is not accessible by any of the other techniques. The latter, however, are sensitive to vanadium atoms and permit oxidation states and local environment to be determined for vanadium. Binary V2O5-Sb2O3 Mixture. Milling of the binary V2O5Sb2O3 mixture (atomic ratio V:Sb ) 13:5) induced strong interactions between the two individual oxides. As shown by XANES, the Sb4O6 subunits of the senarmontite structure break down during extended periods of milling. Simultaneously, increasing amounts of Sb3+ are oxidized to Sb5+. It is interesting to note that the presence of the reducible V2O5 is essential for this oxidation process, which was not observed after milling of Sb2O3-TiO2 mixtures.19 Thus, the Sb3+ oxidation seems to be caused by a redox process in which V5+ is reduced to lower oxidation states.36 This is supported by the detection of V4+ and V3+ by combined ESR and magnetic susceptibility measurements (see Table 1) and by the observed reduction of the 51V NMR line intensities. A broadening of the first-order quadrupolar transitions indicates an increased distortion of the
Spengler et al. octahedral environment of the detectable V5+ nuclei. In addition, an anisotropic NMR signal characterizing tetrahedral V5+ centers was also observed. When the milled binary oxide mixture was additionally calcined at 673 K (calcination temperature of the ternary oxide mixture), Sb3+ was almost quantitatively oxidized to Sb5+, which is located in an octahedral environment, and the Sb4O6 building blocks of senarmontite were broken up completely. The tetrahedral V5+ species detected by 51V NMR prior to calcination was no longer detectable. The intensities of 51V NMR signals remained significantly reduced, and the absence of hyperfine splitting of the V4+ ESR signals indicated the existence of V4+ centers located in close proximity (line broadening by dipolar interactions). The data are consistent with the formation of a mixture of compounds as expected from the V/Sb atomic ratio. It is suggested that Sb5+ is preferentially existing in a VSbO4-like structure. Birchall and Sleight47 have shown that VSbO4 contains Sb only in the pentavalent state. This compound is usually nonstoichiometric and typically contains V in both the tri- and tetravalent states,27,47 the ratios depending on the preparation conditions. The remainder of the vanadium most likely forms oxygen-deficient V2O5-x containing shear structures in which V4+ cations are located in close proximity. Ternary Sb-V/TiO2 Catalyst. The surface area of the ternary catalyst was identical to that of the anatase support for all preparations and was close to 50 m2/g ((10%). The supported oxides were X-ray amorphous after milling with the exception of a small percentage of V2O5 at the shortest milling times only. This is consistent with the complete disappearance of Sb2O3 Raman lines and the very strongly reduced intensities of Raman lines characterizing V2O5. The reason for this observation is the spreading of both oxides across the support anatase surface as already demonstrated by XPS.13 The spreading process is more efficient in the presence of 10 wt % H2O during milling. XANES demonstrated that Sb4O6 building blocks of senarmontite were broken down completely after milling times greater than 1 h and Sb3+ was oxidized to Sb5+. A weak extra peak at 4.14 keV in the XANES spectra suggests that most likely vanadium and/or titanium atoms are located in the second coordination shell around an Sb absorber atom. Other than for the binary V2O5-Sb2O3 mixture, the formation of VSbO4 cannot be detected. Molecular structures of the VOx species in the milled samples can be envisaged from the Raman spectra and from solid-state 51V NMR spectra. Consistently, both spectroscopies prove the existence of small amounts of residual V2O5 after short milling times. The V5+ species in a strongly distorted octahedral environment detected by 51V NMR may well be related to the surface polymeric vanadia species inferred from their Raman spectra. The relative number of V5+ atoms detected by 51V NMR decreased to ca. 60% when the duration of milling was increased from 1 to 20 h (Table 3). In the presence of water during milling, this percentage was even lower. This clearly indicates the presence of paramagnetic centers created by reduction of V5+ during milling. In fact, ESR and magnetic suspectibility measurements clearly proved the formation V4+ and V3+. The V4+ centers should be largely isolated because the corresponding ESR signal shows hyperfine splitting. V3+ (and perhaps V4+) centers must at least partly be located in the vicinity of V5+ because their increasing number tends to reduce the 51V NMR signal intensity of V5+ species. As a possible structural
Vanadia-Based Catalysts arrangement, we infer the presence of -V3+-O-V5+- redox couples, which may have relevance for the catalytic performance of these materials. Additional calcination leads to more efficient spreading of the supported oxides and to the complete disappearance of residual V2O5. The relative percentage of reduced vanadium atoms is decreased in the calcined samples as compared to purely milled samples. It must be admitted that the characterization data were obtained for oxide materials prior to their use as catalysts. Possible modifications of their structural and morphological characteristics during the start-up period remain unknown at this time, because in situ experiments are yet to be done. Nevertheless, we believe that the catalytic tests demonstrate that active and highly selective catalysts for o-xylene oxidation to phthalic anhydride can be prepared by milling of oxide mixtures and subsequent calcination. The addition of Sb2O3 to V2O5/ TiO2 catalysts leads to the formation of vanadium-oxygen redox couples, which are most likely responsible for the good catalytic activity of these materials. Another important result possibly influencing the selectivity of the catalyst is the observation of an enhanced surface concentration of antimony oxide, probably caused by a segregation process.13 We infer that this antimony surface enrichment might lead to site isolation of VOx species.14-16 Site isolation of VOx species may occur via interaction on the surface with Sb moieties creating Sb-O-V species. Thereby, V-O-V-O-V domains or clusters might be interrupted by incorporation of Sb to form V-O-Sb-O-V, etc., species until (if too much antimony is present) Sb-O-Sb-O-Sb overlayers are formed. These overlayers become inactive for o-xylene oxidation. A direct interaction between the two elements is inferred by the observation of reduced vanadium and oxidized antimony sites. Consistent with literature,14-16 our present results also support the earlier supposition that V-O-V-O-V species lead to overoxidation, Sb-O-Sb-O-Sb species are inactive, and “isolated” V-O-Sb-O-V species are ideal for selective (o-xylene) oxidation. An additional catalytically beneficial effect of the antimony modifier may be the reduction of the surface acidity. This would lead to a weaker interaction of the reactant o-xylene, reaction intermediates, and products with the catalyst surface and, hence, to shorter residence times on the surface. Overoxidation will thus be less probable. In fact, Ivanovskaya and Sembaev50 reported that the least acidic VT catalysts gave the highest selectivity. We therefore infer that the selectivity enhancement toward phthalic anhydride induced by the antimony modifier might originate from a dual function, namely site isolation and reduction of surface acidity. Acknowledgment. This work was financially supported by the Bayerischer Forschungsverbund Katalyse (FORKAT) of the Bayerische Forschungsstiftung and the Fonds der chemischen Industrie. We thank the Hamburger Synchronstrahlungslabor HASYLAB at DESY for allocating beam time and the HASYLAB staff, especially M. Tischer, L. Tro¨ger, and J. Feldhaus, for their kind assistance during the measurements. We are also very grateful to HASYLAB for financial support concerning the travel expenses. The international cooperation between University of Munich and the Boreskov Insitute of Catalysis was made possible by the Russian Academy of Sciences and the Deutsche Forschungsgemeinschaft. The work done in Novosibirsk was financially supported by RFBR Grant No. 0103-32364 and INTAS Grant No. 97-0059. B.P. acknowledges a PhD scholarship from the Freistaat Bayern.
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