J. Phys. Chem. B 1999, 103, 7599-7606
7599
Characterization of V2O5-TiO2 Catalysts Prepared by Milling by ESR and Solid State 1H and 51V NMR Olga B. Lapina, Alexander A. Shubin, and Andrei V. Nosov BoreskoV Institute of Catalysis, Pr. Akad. LaVrentieVa 5, NoVosibirsk 630090, Russia
Eric Bosch, Jo1 rg Spengler,† and Helmut Kno1 zinger* Institut for Physikalische Chemie, UniVersita¨ t Mu¨ nchen, Butenandtstrasse 5-13 (Haus E), 81377 Munich, Germany ReceiVed: April 28, 1999; In Final Form: July 2, 1999
The interaction between V2O5 and TiO2 under milling in a ball mill (an alternative procedure of catalyst preparation) was characterized by 1H, 51V MAS, and 51V static (wide line) NMR spectroscopy supplemented by simulations of 51V NMR spectra. Additionally, ESR and magnetic susceptibility measurements were carried out for the characterization of the paramagnetic V4+ and V3+ sites. It has been shown that after milling, two different types of octahedrally coordinated vanadium (V5+) species (V5+(I) and V5+(II)) strongly bonded to TiO2 are formed. At the same time, the appearance of V3+ ions and an increase of their concentration is observed during milling-calcination processes, along with the formation of at least three different types of paramagnetic V4+ species corresponding to (i) V4+ centers in O-deficient V2O5-x; (ii) VO2+ vanadyl species (V4+(I)) with the vanadium centers in octahedral symmetry with axial distortion; (iii) V4+(II) species with vanadium centers also in octahedral symmetry, but with different bond lengths and strengths as compared to V4+(I). Relative amounts of different V4+ and V5+ species depend on the milling time, the presence of H2O in the system, and the subsequent calcination procedure (temperature and calcination time). Thus, V5+(I) species formed predominantly during milling, whereas V5+(II) species formed after thermal treatment. For the structural characterization of these species, complete sets of the quadrupole and chemical shielding tensor parameters, including relative tensor orientations, have been estimated. This allows us to conclude that the octahedral environment of vanadium in V5+(II) species is less distorted than in V5+(I) and in both cases the distortion is less axial than in V2O5. Combined NMR, ESR, and magnetic susceptibility measurements indicate that all vanadium species (V3+, V4+, and V5+) are isolated from each other on the TiO2 support.
Introduction V2O5 on TiO2 is widely used as a catalyst for partial oxidation of hydrocarbons. An important example is the oxidation of o-xylene or naphthalene to phthalic anhydride PSA in the gas phase.1,2 Other applications of V2O5/TiO2 catalysts are ammoxidation of alkyl aromatics3,4 and selective catalytic reduction of nitrogen oxides by ammonia (SCR).5,6 The catalysts used in industry are typically prepared by wet impregnation. Recently, it was shown that supported oxide catalysts can also be prepared by milling the different powder components in a ball mill (so called UHIG (ultra high intensity grinding)).7-11 In this paper, the influence of milling and subsequent calcination of pure V2O5 and TiO2, as well as of V2O5-TiO2 mixtures, was investigated. The major method chosen for this investigation was solid state 51V and 1H MAS NMR.12-15 51V NMR seems to be the most suitable technique for the characterization of V5+ sites since this method gives valuable information on the local environment of the vanadium nucleus. Some recently developed NMR techniques16-20 permit to extract all information on chemical shielding tensor and quadrupole tensor parameters from solid state NMR measurements that * To whom correspondence should be addressed. † Present address: Consortium fu ¨ r die elektrochemische Industrie, Zielstattstr. 20, 81379 Munich, Germany.
might be very important for the characterization of the local environment and its distortions of the nuclei under NMR study. However, these techniques are limited in application for real systems with a large magnitude of quadrupole constants and chemical shielding anisotropies. In the present work, a particular approach for the detailed analysis of NMR parameters described previously21 has been used. This approach permits the determination of quadrupole and chemical shielding tensor parameters with reasonable accuracy in cases when the line shape of the central transition is very complex because of comparable effects of second-order quadrupole interaction and chemical shielding interaction. Additionally, ESR and magnetic susceptibility measurements were carried out for the characterization of the paramagnetic V4+, V3+ sites that may be formed during milling. Experimental Section Sample Preparation. For the catalyst preparation V2O5 (J. T. Baker Chemicals B. V., 99% purity) and TiO2 (TiO2 suspension, dried and calcined at 600 °C for 10 h) were used. The TiO2 (provided by Consortium fu¨r die elektrochemische Industrie, Munich, 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 for some minor sulfur contamination, the concentration of which was close to the detection limit of XPS. The theoretical monolayer capacity of
10.1021/jp991405c CCC: $18.00 © 1999 American Chemical Society Published on Web 08/20/1999
7600 J. Phys. Chem. B, Vol. 103, No. 36, 1999 TiO2 for V2O5 was estimated according to the procedure described by Roozeboom et al.22 to be 6 wt % V2O5. 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 oxide 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 50 g of the oxide mixture. For discrimination between different VOx species, soluble components were extracted at ambient temperatures in 100 mL of NH3 solution using 1 g of catalyst. The suspension was filtered and the filtrate dried in air at 100 °C. Samples were measured in their hydrated state except for 1H NMR for which water of hydration was removed by evacuation and mild heating (see below). NMR and ESR 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 an NMR probe constructed by NMR Rotor Consulting ApS, Denmark.23 Quadrupolar 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 the rf pulses with 1 µs duration were used in the experiments. All chemical shielding values are referenced 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.24 The NMR1 program is based on an effective average Hamiltonian obtained in a manner similar to that presented in previous literature.25 A particular variant of this program (NMR2) especially adopted for the fast computation of spinning side band (ssb) intensities in MAS spectra of quadrupolar nuclei (Herzfeld and Berger approach26) was used for simulations of MAS and static spectra and for least-squares parameter fitting. Computations were performed on a dual Pentium II 300 MHz IBM PC compatible computer running Linux OS. 1H MAS NMR spectra were measured using a home-built MAS probe with low background signal. Prior to the measurements, the samples were placed into 7 mm o.d. glass ampoules, evacuated at 250 °C for 6 h and sealed. The samples were rotated in a quartz Andrey-type rotor at 3 kHz. Proton chemical shifts were referenced to TMS as an external standard. The ESR spectra were recorded on a Varian E-Line (E9) spectrometer with a microwave frequency of 9.2 GHz (X-band). The samples were placed into a TE104 double cavity which permitted the measurement of a sample and a reference under identical conditions, particularly at the same 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) was used as reference material. For quantitative calibration of the spectra a 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 simulations, the computer program Simfonia (Bruker) was used.
Lapina et al. TABLE 1: ESR Parameters of V4+ Species in V2O5/TiO2 after Milling at 1 and 20 h milling time (h) 1 20 20 + extraction a
species
g|
g⊥
A|
A⊥
∆g⊥a
1 1 2 2
1.93 1.93 1.94 1.925
1.978 1.976 1.963 1.958
199 196 170 179
78 76 57 52.5
0.024 0.026 0.038 0.044
∆g⊥ ) |g⊥ - 2.0023|.
Magnetic susceptibility measurements required for the estimation of the V3+ (not detectable by ESR) concentrations were carried out using a home-made magnetic balance (Gouymethod). The experimental accuracy of the reported V3+ concentrations is approximately (2 × 1018 sites/g of catalyst. Results and Discussion The pure TiO2 (anatase) support had a BET surface area of 45 m2/g which increased to a constant value of 52 m2/g after a milling time of 3 h. When the mixtures of TiO2 and V2O5 were milled, the resulting surface area reached a constant value of 52 m2/g within the limits of accuracy of the BET method in the uncalcined state irrespective of the presence or absence of water during the mechanical treatment. Subsequent calcination at 450 °C led to slight reduction of the surface area to 50 m2/g after 5 h and 47 m2/g after 20 h. Laser Raman spectroscopy clearly showed that the anatase modification was the only detectable form of TiO2 in all samples. Electron Spin Resonance and Magnetic Susceptibilv Measurements of Milled Oxides. The ESR parameters determined for V4+ species in variously treated binary V2O5/TiO2 samples are summarized in Table 1. Dry Milling. Milling of the V2O5/TiO2 mixture for 1 h produced an ESR signal showing well-defined hyperfine structure, thus indicating highly dispersed paramagnetic centers. The corresponding ESR parameters, namely g⊥ ) 1.978 and A⊥ ) 78 G, g| ) 1.93 and A| ) 199 G (line width ca. 20 G), can be attributed to VO2+ vanadyl species with the vanadium centers in octahedral symmetry with axial distortion.27-31 Characteristic for this species is a larger g⊥ value as compared to g|. This species will be denoted as species V4+(I) throughout this paper. A second contribution to the ESR spectrum of this sample was an isotropic line with Gaussian shape and ESR parameters of g ) 1.97 and a line width of 300 G. This signal with larger line width was also detected in pure V2O5 and is thus attributed to V4+ centers in O-deficient V2O5-x (presumably in regions exhibiting shear structures). The presence of V2O5 in the milled samples is consistent with the NMR results (see below). When the sample was milled for 20 h under dry conditions, a second species (V4+(II)) was detected in addition to species V4+(I) (see Table 1). The ESR parameters of species V4+(II) were g⊥ ) 1.963 and A⊥ ) 57 G, and g| ) 1.94 and A| ) 170 G which can again be attributed to an octahedral (perhaps quadratic pyramidal) symmetry, although with different bond lengths and strengths as compared to species V4+(I). A tetrahedral species as reported by Centi et al.29 for a differently prepared V2O5/TiO2 catalyst was not detected for the present preparations even at 77 K. The broad isotropic signal characteristic for paramagnetic centers in pure V2O5 was also present in the spectrum of the binary material after 20 h milling although with significantly reduced intensity (ca. 60% of the signal observed after 1 h milling). Additional calcination of the milled samples did not change the ESR spectra qualitatively, both species V4+(I) and V4+(II)
Characterization of V2O5-TiO2 Catalysts
J. Phys. Chem. B, Vol. 103, No. 36, 1999 7601
TABLE 2: Concentration of V3+, V4+, and Different Types of V5+ in Supported Vanadia on Titania prep cond dry
wet
milling time (h) 1 20 1 20 1 20 1 20 1 20 1 20
calcination time (h); temp (°C)
5; 450 5; 450 20; 450 20; 450 5; 450 5; 450 20; 450 20; 450
10-19[V3+] (g-1)
10-18[V4+] (g-1)
10-20[V5+] (g-1)
[V3+ + V4+]/[V5+] (%)
[V5+(I) + V5+(II)]/[V5+] (%)
2.6 3.7 3.6 5.2 3.6 5.0 1.3 4.9 3.3 7.8 4.2 8.2
7.0 5.7 8.3 8.0 2.3 6.3 6.3 5.0 5.3 2.7 1.7 2.0
3.7 3.6 3.6 3.4 3.7 3.5 3.8 3.5 3.7 3.2 3.6 3.2
8.9 11.8 12.3 17.4 10.5 16.2 5.0 15.4 10.5 25.0 12.0 26.0
0 23 10 58 22 100 10
being detectable. However, enhanced concentrations of V3+ as estimated from magnetic susceptibility measurements were induced during calcination. When the calcination temperature was increased to 450 °C, the concentration of species V4+(II) increased relative to species V4+(I). The quantitative estimates of the concentrations of paramagnetic centers as they develop with milling and subsequent calcination are summarized in Table 2. The ESR spectra were very different after the samples were extracted in ammoniacal solutions. Typically, signal V4+(I) and that characteristic for a V2O5-x phase disappeared after extraction, while signal V4+(II) remained almost unchanged in intensity with slightly modified ESR parameters: g⊥ ) 1.958 and A⊥ ) 52.5 G; g| ) 1.925 and A| ) 179 G (see Table 1). Analogous observations were reported by Centi et al.1 and by Busca et al.32 for differently prepared materials. Milling in the Presence of Water. The ESR spectrum of samples milled for 1 h in the presence of water was very different form that of dry-milled oxide mixtures. A signal with isotropic g-factor exhibiting eight hyperfine lines was observed. This signal is attributed to hydrated V4+ complexes having rotational freedom. The rotational freedom was frozen in at temperatures of -50 °C and below. The width W of individual lines of the hyperfine structure with magnetic quantum number mi is given by
W ) a + bmi + cmi2 The spectral simulation gave the following ESR parameters: g ) 1.965, A ) 115 G, a ) 26.3 G, b ) -3.06 G, and c ) 1.34 G. After evacuation, the rotational freedom of the V4+ complex disappeared and the resulting ESR spectrum was identical to that of species V4+(I) detected for the dry-milled samples. The concentration of V4+ centers was almost identical for samples milled under dry conditions and in the presence of water. Analogous spectra were obtained after milling for 20 h. The results suggest that the presence of water prevents the formation of species V4+(II) strongly bonded to the TiO2 support. Subsequent calcination, however, resulted in the typical ESR spectrum of species V4+(II). Also the concentration of V3+ centers increased for the samples milled in the presence of water after calcination at 450 °C (see Table 2). Nuclear Magnetic Resonance of Milled Oxides. V2O5. A 51V NMR characterization of V O after milling in a planetary 2 5 mill was reported earlier.33 The major conclusions that are important for the studies of the binary V2O5-TiO2 system are as follows: According to static (wide line) 51V NMR most of the vanadium atoms in the milled V2O5 are in the V5+ oxidation
18 67 31 100
Figure 1. 1H MAS NMR spectra of TiO2: (1) unmilled (0 h); (2-4) milled for 1, 10, and 20 h, respectively; (5) NMR spectrum of V2O5/ TiO2 dry milled for 20 h (asterisks denote spinning side bands).
state, the concentrations of V4+ and V3+ being small and increasing with the milling time. The distribution of V4+ and V3+ ions is most probably inhomogeneous. There is no significant influence of paramagnetic V4+ on the local environment of V5+ in large agglomerates of V2O5, presumably because the V4+ is located in small patches of V6O13-like shear structures, whereas paramagnetic V3+ ions are responsible for the loss (up to ca. 70%) of intensity in 51V NMR spectra. The local environment of vanadium sites of V2O5 milled for 20 h is more distorted as compared with the polycrystalline V2O5. No V5+ ions in tetrahedral coordination were apparent in detectable quantities. TiO2. 1H MAS NMR spectra of TiO2 are shown in Figure 1. The spectrum of the unmilled sample (Figure 1, spectrum 1) shows signals at 5.7, 2.4, and 0.1 ppm. The latter line in this and other spectra is caused by moisture on the outer surface of the rotor. The lines at 5.7 and 2.4 ppm are typical for the terminal and bridging surface hydroxyls of anatase.34 Milling for 1 h (Figure 1, spectrum 2) leads to the disappearance of the line at low field and to the appearance of a new signal at 7.2 ppm. Its intensity increases in the spectrum of the sample milled for 10 h, while longer milling times lead to a decrease of its
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Figure 3. 105.25 MHz 51V static (A, top) and MAS (B, bottom) (spinning frequency νr ) 14 kHz) NMR spectra of V2O5-TiO2 mixture after different milling times and calcination procedures with subsequent extraction of excess V2O5 by NH3 water solution: (1) 1 h, (2) 1 and 20 h at 450 °C, (3) 20 and 5 h at 450 °C, (4) 20 and 20 h at 450 °C.
Figure 2. 105.25 MHz 51V static (A, top) and MAS (B, bottom) (spinning frequency νr ) 10 kHz) NMR spectra of V2O5-TiO2 mixtures after different milling times and calcination procedures: (1) 1 h, (2) 20 h, (3) 20 and 5 h at 400 °C, (4) 1 and 5 h at 450 °C, (5) 20 and 5 h at 450 °C, (6) 1 and 20 h at 450 °C, (7) 20 and 20 h at 450 °C.
intensity (Figure 1, spectra 3 and 4). The intensity of the line at 2.4 ppm slightly decreases with increasing milling time. The appearance of the new line at 7.2 ppm may be due to the TiOH groups most probably located on the titania surface, because their concentration passes through a maximum as does the surface area. These new groups may originate from a dissociation of water molecules at coordinatively unsaturated titanium ions formed during milling. V2O5-TiO2. Dry Milling. The shape of the 1H MAS spectra of the dry-milled V2O5/TiO2 sample prior to calcination is substantially different from that of pure TiO2. The highfrequency line vanishes after 1 h milling and a broad shoulder
centered at ca. 5 ppm appears in the spectrum. It is interesting to note that the line at 7.2 ppm is not observed in the spectra either after 1 h milling or after 20 h (Figure 1, spectrum 5). The reason might be its consumption in the reaction with vanadia. Wide line and MAS 51V spectra of dry-milled V2O5-TiO2 samples as a function of milling time, calcination temperature, and duration are shown in Figure 2. The V2O5-TiO2 sample after 1 h dry milling (dry ML) shows the signal which is typical for polycrystalline V2O5 with δiso ) -612 ppm (Figure 2B, spectrum 1) and reveals quadrupole transitions of the first order in the static spectrum (Figure 2A, spectrum 1). After 20 h milling the distortion of the vanadium structure as judged from the blurring of the static spectrum and the appearance of the broad line in the bottom of the MAS spectrum (Figure 2, spectrum 2) is small. After calcination at 400 and 450 °C the relative intensity of the broad line in MAS spectra increases and this line becomes dominant for samples milled for 20 h and calcined at 450 °C for 5 and 20 h, whereas a calcination at
Characterization of V2O5-TiO2 Catalysts
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TABLE 3: 51V Quadrupole Tensor Parameters (CQ, νQ),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 for Strongly Bonded V5+ Species in VOx/TiO2 Catalyst Obtained from 105.25 MHz 51V MAS and Static NMR along with Literature Data for Bulk V2O5 signal
CQ (kHz)
ηQ
δσ (ppm)
ησ
σiso (ppm)
σ1c (ppm)
σ2c (ppm)
σ3c (ppm)
R (deg)
β (deg)
γ (deg)
Id IId strongly bonded V sites21 bulk V2O535
15900 11600 14700 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 (V1, V2, and V3 ) eq) are connected with CQ and ηQ by the relations: CQ ) e2qQ/h; V1 ) 1/2(-1 - ηQ) V3; V2 ) 1/2(-1 + ηQ)V3. b The eigenvalues of chemical shielding tensor are expressed by δσ, ησ, and σiso in the following manner: σ1 ) 1/2δσ (-1 - ησ) + σiso; σ2 ) 1/2δσ(-1 + ησ) + σiso; σ3 ) δσ + σiso. c σ1, σ2, and σ3, computed from the values of δσ, ησ, and σiso, using the relations above. d NMR parameters derived from simultaneous fitting to experimental static and MAS NMR spectra.
450 °C only slightly influences the spectrum of the sample milled for 1 h. Relative intensities of the broad line and of the lines characterizing V2O5 as estimated from MAS spectra are presented in Table 2. Rotation frequencies of 10 kHz as well as 14 kHz are not sufficient to obtain well-resolved MAS spectra of 51V nuclei corresponding to the broad line. The following features could be the reason for the ssb broadening in MAS spectra: large magnitude of quadrupolar constant, distribution of chemical shielding components, and superposition of several lines with similar ssb. As the vanadium content in these samples is small, the signal to noise ratio is low as compared with pure V2O5. This leads to an error of approximately 20% in the relative intensity estimation. Within this error limit, the intensity for the 1 h milled sample is in the same range as that expected for the same amounts of pure V2O5. After calcination at 400 °C for 5 h the value decreases to 0.75. With increasing calcination temperature and time, the total intensity remains constant within the error limits. Weakly bonded VOx species and excess V2O5 can be extracted by ammoniacal solutions, thus facilitating the detection of strongly bonded species. Static and MAS spectra (Figure 3) clearly demonstrate that the extraction process removes excess V2O5. The remaining vanadium most probably corresponds to strongly bound surface vanadium sites and/or to vanadium encapsulated in the bulk of titania. According to static and MAS spectra after the extraction, only a small portion of vanadium exists as regular tetrahedral species. Water remaining in the system may be responsible for the formation of mobile hydrated species with the spectra characteristic for vanadium in regular tetrahedral coordination. The majority of vanadium which is strongly bound to titanium oxide gives very complex, but wellresolved 51V MAS spectra (Figure 3B). A detailed analysis of the MAS spectra reveals two individual signals. The first signal V5+(I) predominates in samples obtained by milling only, whereas the intensity of the second signal V5+(II) increases with an increase of thermal treatment conditions (temperature and time). Thus, for the sample obtained after 1 h milling the main signal in the spectrum is signal V5+(I), while for the sample obtained after calcination at 450 °C for 20 h, signal V5+(II) prevails. MAS spectra of these two signals are shown in Figure 4. Despite the complexity of these spectra, simultaneous approximation of static and MAS spectra permits determination of NMR parameters for these two signals (set I and set II in Table 3). Comparison with the values of chemical shielding components for V2O5 (Table 3) shows that vanadium in V5+(I) and V5+(II) species is in distorted octahedral coordination. A large value of the quadrupolar coupling constant CQ for these vanadium sites (see Table 3) indicates significant changes of the electric field gradient at 51V nuclei positions for strongly bound vanadium species as compared to bulk V2O5. The
Figure 4. 105.25 MHz 51V MAS (spinning frequency νr ) 13.6 kHz) experimental (obtained by subtraction with appropriate weight of different spectra in Figure 3) NMR spectra of two signals (V5+(I) and V5+(II)), the superposition of which describes V2O5-TiO2 spectra presented in Figure 3. The narrow signal at ∼540 ppm belongs to hydrated mobile vanadium species located in TiO2 pore system. For comparison, simulated 105.25 MHz 51V MAS NMR spectra (νr ) 13.6 kHz ) with parameters from Table 3 (data sets 1 and 2) are shown.
asymmetry parameters ηQ of the quadrupole tensors for 51V in both V5+(I) and V5+(II) species are considerably larger than those in bulk V2O5. A similar dependence is observed, for the asymmetry parameters ησ of the chemical shielding tensors. These observations allow us to conclude that, in contrast to bulk V2O5 where vanadium nuclei are in axially distorted octahedral environment, the vanadium nuclei environment in both V5+(I) and V5+(II) species is characterized by an octahedral symmetry deviating significantly more from an axial distortion. It should be noted that the 51V quadrupole coupling constant for V5+(I) species is almost 1.5 times as large as that for V5+(II) species. This indicates that the 51V environment in V5+(I) is more distorted than in V5+(II). Wet Milling. When the samples were milled in the presence of water (wet ML) the spectra are slightly different (Figure 5). After 3 h milling, a very sharp and isotropic line at -543 ppm appears which can be assigned to hydrated mobile vanadium species. This species is likely located in the material’s pore system. The signal disappeared after evacuation at ambient temperature for 24 h. MAS spectra of the samples after milling and calcination (Figure 5B) reveal that the broad line discussed above appears after 1 h milling and becomes prevalent after 20 h milling and calcination at 450 °C for 20 h. The broad line observed in these
7604 J. Phys. Chem. B, Vol. 103, No. 36, 1999
Figure 5. 105.25 MHz 51V static (A, top) and MAS (B, bottom) (spinning frequency νr ) 10 kHz) NMR spectra of V2O5-TiO2 mixture after different milling times (with 10 wt % H2O) and calcination procedures: (1) 3 h, (2) 1 and 5 h 450 °C, (3) 10 and 5 h 450 °C, (4) 20 and 5 h 450 °C, (5) 1 and 20 h 450 °C, (6) 20 and 20 h 450 °C.
samples is much more pronounced than for the samples milled for the same time in the absence of water (Figure 2B) (the relative concentrations of the broad line species as it develops with milling and subsequent calcination are summarized in Table 2). Different rotation frequency MAS spectra for the latter sample are rather complex. Static and MAS NMR spectra more suitable for the analysis were obtained after extraction, as shown in Figure 6. As in the case of dry milling all the spectra exhibit the superposition of two lines: the first one (V5+(I)) is prevailing in the samples obtained by milling, the second line (V5+(II)) appears only after calcination treatment and becomes prevalent for the sample calcined at 450 °C for 20 h. The same NMR parameters as for dry samples were determined for these two lines (Table 3, sets 1 and 2). The very similar values of NMR parameters for vanadium(V) sites in samples after dry and wet milling can be explained
Lapina et al.
Figure 6. 105.25 MHz 51V static (A, top) and MAS (B, bottom) (spinning frequency νr ) 14 kHz) NMR spectra of V2O5-TiO2 mixture after different milling times (with 10 wt % H2O) and calcination procedures with subsequent extraction of the excess V2O5 by NH3 water solution: (1) 20 h, (2) 1 and 5 h at 450 °C, (3) 10 and 5 h at 450 °C, (4) 20 and 20 h at 450 °C.
by similar structures of vanadium sites formed in both cases, whereas the relative amounts of strongly bound species for samples prepared by wet milling are higher than for samples prepared by dry milling (at the same milling and/or calcination conditions, see Table 2). The formation of additional OH groups on the anatase surface during wet milling may be the reason for the higher density of vanadium species strongly interacting with the TiO2 support in comparison with samples after dry milling. Comparison with Catalysts Prepared by Different Procedures. It is interesting to compare NMR parameters of strongly bound vanadium(V) species formed in milled samples with NMR parameters of strongly bound vanadia sites in V2O5/ TiO2 catalysts. According to data presented by Shubin et al.,21 only one type of strongly bound V5+ species (Table 3, set 3) is
Characterization of V2O5-TiO2 Catalysts formed in catalysts prepared by the mixing of vanadyl oxalate solution and TiO2 with subsequent drying, calcination, and treating under catalytic reaction conditions. As follows from Table 3, vanadium species V5+(I) and V5+(II) are different from those observed in ref 21, namely, the 51V quadrupole coupling constant reported in ref 21 has an intermediate value between the smaller 51V quadrupole coupling constant in V5+(II) species and the larger constant in V5+(I) species. The value of the quadrupole coupling constant reflects the degree of distortion of the 51V local environment. Therefore, V5+(I) species formed during milling conditions are the most distorted while V5+(II) species formed after thermal treatment are the least distorted. At present, it is impossible to decide whether V5+(I) and V5+(II) species are catalytically active and are stable under catalytic conditions or whether they transform into strongly bonded vanadium species as observed in ref 21 which were shown to have high catalytic activity.36 This may be the subject for further investigations. V3+ and V4+ Distribution in Milled V2O5/TiO2 Catalysts. Analysis of the data summarized in Table 2 shows a parallel enhancement of the V3+ and the total concentrations of strongly bonded V5+(I) and V5+(II) species induced during milling and calcination. The relative concentration of V3+ may reach up to approximately 20% of the total vanadium content as may be seen for 20 h wet milled samples after subsequent calcination for 20 h at 450 °C. At the same time, all V5+ is observed in 51V NMR spectra. This is in contrast to milled bulk V O ,33 2 5 where a significantly smaller (2%) concentration of V3+ induced by the milling procedure caused a significant decrease (up to 70%) of the 51V NMR signal intensity. The most probable reason for the absence of V3+ influence on 51V NMR spectra intensity for V2O5/TiO2 catalysts is the absence of direct bonds V3+O-V5+ and the presence of only isolated V3+ ions or small V3+-O-V3+ clusters on the TiO2 support. As was noted earlier, the well-resolved hyperfine structure in ESR spectra of paramagnetic V4+ ions in these systems also points toward the presence of highly dispersed V4+ centers. Taking into account the high total relative concentration of V3+ and V4+, we may suppose that V5+(I) and V5+(II) species are also highly dispersed species consisting of one or a few vanadium sites on the TiO2 support. Conclusion V2O5-TiO2 (vanadium supported on titania) samples play an important role as basic materials for catalysts for selective oxidations1-4 and for the SCR process.5,6 These catalysts are typically prepared by impregnation techniques from aqueous solutions. Successful synthesis of supported vanadia catalysts by thermal spreading9-11 and by milling procedures7,8,37 of V2O5 and TiO2 mixtures has also been reported. Activity and selectivity of these supported catalysts are sensitively determined by the structures and dispersion of vanadia surface species and by the local environment of the vanadium centers. Therefore, V2O5-TiO2 catalysts prepared by ball-milling and calcination have been characterized in the present work by combined 1H MAS NMR, 51V MAS, and 51V static (wide line) NMR, spectra simulations, ESR, and magnetic susceptibility measurements. This combination of techniques is very powerful since the NMR results provide information on the local environment of V5+ centers, while ESR and magnetic susceptibility permit the detection of paramagnetic V4+ and V3+ centers and the determination of their spatial distribution. The major results and conclusions from these investigations can be summarized as follows: The milling of V2O5/TiO2
J. Phys. Chem. B, Vol. 103, No. 36, 1999 7605 mixtures induces spreading of the vanadia across the titania surface, whereby two types (V5+(I) and V5+(II)) of V5+ species strongly bonded to the titania are formed. V3+ centers are also present, the density of which increases, with milling time and additional calcination. In addition, at least three structurally different V4+ centers are detected by ESR, namely (i) V4+ in an oxygen-deficient V2O5-x matrix; (ii) VO2+ vanadyl species with V4+(I) centers in octahedral symmetry and axial distortion; and (iii) V4+(II) species with octahedral symmetry but different bond lengths and strengths as compared to species V4+(I). The relative concentrations of V4+ and V5+ centers are strongly dependent on the preparation parameters, namely milling time, presence or absence of water during milling, and calcination temperature and duration. V5+(I) species are predominantly formed during milling and then transformed into V5+(II) species during additional thermal treatments. A detailed analysis of quadrupole and chemical shielding tensor parameters permitted the structural analysis of these species, suggesting that the octahedral environment in species II is less distorted than in species I. Both structures deviate from the axial symmetry found in V2O5. There was very little effect of the paramagnetic V3+ and V4+ species on the 51V NMR spectra, suggesting that they were spatially separated from V5+ centers. The observed wellresolved hyperfine structure in the ESR signals of V4+ species indicates that these were isolated from each other as well. The present results only apply to materials in their hydrated state. Structural conclusions are expected to be different for dehydrated samples as suggested by Raman spectroscopy.38 However, under catalytic conditions of oxidation reactions water is present as a reaction product. It is therefore inferred that the results are relevant for catalysis. It must nevertheless be admitted that the materials characterized in this work can only be considered as catalyst precursors since the “real” catalyst is formed in a rather extended induction period, e.g. for o-xylene oxidation.39 Investigations into the nature of spent catalysts are in progress. Acknowledgment. This work has been supported by RFBR grant No. 98-03-32323a, and INTAS grant No. IR-97-0059. The work done in Munich was financially supported by the Bayerische Forschungsstiftung (FORKAT) and the Fonds der Chemischen Industrie. The international cooperation was made possible by the Russian Academy of Sciences and by the Deutsche Forschungsgemeinschaft. References and Notes (1) Centi, G.; Giamello, E.; Pinelli, D.; Trifiro, F. J. Catal. 1991, 130, 220. (2) Centi, G.; Pinelli, D.; Trifiro, F.; Goussoub, D.; Guelton M.; Gengembre, L. J.Catal. 1991, 130, 238. (3) Cavalli, F.; Cavani, F.; Manenti I.; Trifiro´, F. Catal. Today 1987, 1, 245. (4) Sanati, M; Anderson, A. J. Mol. Catal. 1990, 59, 233. (5) Blanco, J.; Avila, P.; Bahamonde, A.; Yates, M.; Belincho´n, J. L.; Medina E.; Cuevas, A. Catal. Today 1996, 27, 9. (6) Prins W. L.; Nuninga, Z. L. Catal. Today 1993, 16, 187. (7) Sobalik, Z.; Lapina, O. B.; Mastikhin, V. M. In Preparation of Catalysts V; Poncelet, G., Jacobs, P. A., Grange, P., Delmon, B., Eds.; Elsevier: Amsterdam, 1991; p 507. (8) Sobalik, Z.; Lapina, O. B.; Novgorodova, O. N.; Mastikhin, V. M. Appl. Catal. 1990, 63, 191. (9) Kno¨zinger, H.; Taglauer, E. Catalysis; The Royal Society of Chemistry: London, 1993; Vol. 10, p 1. (10) Ziolkowski, J.; Kozlowski, R.; Mocala, K.; Haber, J. J.Solid State Chem. 1980, 35, 297. (11) Kno¨zinger, H.; Taglauer, E. In Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨zinger, H., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, Germany, 1997; Vol. 1, p 216.
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