Langmuir 1996, 12, 2961-2968
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Mechanically Activated MoO3. 5. Redox Behavior G. Mestl,† N. F. D. Verbruggen, E. Bosch, and H. Kno¨zinger* Institut fu¨ r Physikalische Chemie, Universita¨ t Mu¨ nchen, Sophienstrasse 11, 80333 Mu¨ nchen, Germany Received September 21, 1995. In Final Form: April 1, 1996X Mechanical activation leads to a variety of defects in MoO3. The behavior of paramagnetic defects in unmilled MoO3 and in MoO3 mechanically activated in a planetary mill for 600 min was investigated by ESR spectroscopy. MoO3 milled for 600 min, and unmilled MoO3 though in a smaller concentration, contain Mo5+ ions. N2 treatment at elevated temperatures increases the Mo5+ concentration in the samples due to oxygen loss. Both MoO3 samples contain octahedrally coordinated Mo5+ species prior to any treatment which interact with a proton in close proximity. An additional signal is observed in both samples that is characteristic of F centers. An inverse Curie-Weiss behavior between 90 and 300 K indicates the pairing of electrons at lower temperatures. Fivefold coordinated Mo5+ species in distorted, square-pyramidal symmetry are generated in the bulk of unmilled MoO3 during N2 flushing at 523 and 623 K. MoO3 milled for 600 min contains, besides 6-fold coordinated Mo5+ in C2v symmetry, a second Mo5+ species in almost C4v symmetry. The Mo-O distances in the equatorial plane are shortened and these Mo5+ centers are assigned to a precursor structure of crystallographic shear planes. Besides 6-fold coordinated Mo5+, tetrahedrally coordinated Mo5+ centers are also generated, if milled MoO3 is treated in N2 at 623 or 673 K which are assigned to the Mo sites on shear defects protruding from the surface. The total intensity of the Mo5+ signals follows the Curie-Weiss law between 90 and 200 K. Above 200 K a deviation from linearity is observed. Hence, a considerable part of the Mo5+ centers does not contribute to the signal intensity at higher temperatures. In situ Raman spectroscopy of mechanically activated MoO3 revealed the formation of some nonstoichiometric MoO3-x after thermal treatment at 673 K in N2 for 1 h. In addition, diffuse reflectance infrared spectroscopy (DRIFTS) was used to characterize the stability of OH groups on the surface of MoO3 and in the bulk of the milled sample. The OH species located in the bulk of MoO3 was found to be stable even at 673 K under flowing N2.
1. Introduction Tammann1
Already in 1929, observed that the mechanical energy transferred to a solid is not quantitatively converted into heat. About 15% of the transferred energy is converted into internal energy, thus, increasing the thermodynamic potential of the solid. The main reason for the enhanced reactivity of mechanically activated solids is their enhanced enthalpy due to structural changes.2 Particularly defect structures induced by mechanical activation determine the solid reactivity. Correlations between the energy stored in a solid and its reactivity have been reported frequently.2,3 Thus, a close correlation between the nature and concentration of lattice defects and the specific catalytic activity was found4-7 and explained by different theories.8-10 In general, point defects, dislocations, and grain boundaries enhance the mass transport in solid state reactions, e.g., in sintering. Hence, the spreading of a mobile oxide across the surface of a carrier oxide, which has been described, e.g., for MoO3 over alumina,11-15 may also be affected by the milling process. To elucidate the role of defects, created in MoO3 by mechanical stress, during the spreading process, we * To whom correspondence should be addressed. † Present address: Department of Chemistry, Texas A&M University, College Station, TX 77843-3255. X Abstract published in Advance ACS Abstracts, May 15, 1996. (1) Tammann, G. Z. Elektrochem. 1929, 35, 21. (2) Heinicke, G. Tribochemistry; Carl Hanser Verlag: Mu¨nchen, 1984. (3) Boldyrev, V. V.; Meyer, K. Festko¨rperchemie 1973. (4) Uhara, I.; Yanigamoto, S.; Tani, K.; Adachi, G.; Teratani, S. J. Phys. Chem. 1962, 66, 2691. (5) Schrader, R.; Sta¨tter, W.; Oettel, H. Chem. Tech. (Leipzig) 1971, 23, 363. (6) Kishimoto, S. J. Phys. Chem. 1962, 66, 2694. (7) Schrader, R.; Deren, J.; Fritsche, B.; Ziolkowski, J. Z. Anorg. Allg. Chem. 1970, 379, 25. (8) Schrader, R.; Vogelsberger, W. Z. Anorg. Allg. Chem. 1969, 368, 187. (9) Schrader, R.; Sta¨tter, W. Acta Chim. Acad. Sci. Hung. 1968, 55, 39. (10) Paudert, R. Chem. Tech. (Leipzig) 1965, 17, 449.
S0743-7463(95)00788-8 CCC: $12.00
have analyzed the nature of defects in unmilled MoO3 and in MoO3 that was milled for 600 min, when these materials were treated under conditions similar to those applied during the spreading process. In a series of previous papers, we have shown that MoO3 crystallites were disintegrated when treated in an agate planetary mill for 600 min.16 During this process, the BET surface area increased from about 1.3 to 32 m2/g. A considerable decrease in the particle size from about 1 µm to about 50 nm was confirmed by XRD and SEM. Ultrafine amorphous material was indicated by the difference between the BET and XRD particle size, by an X-ray scattering background, and it was also found in SEM micrographs. Variations in the X-ray pattern quality and the X-ray background suggested a complex process of particle size reduction involving the migration and clustering of defects. Changing diffraction profiles, anomalous X-ray diffraction intensities, and the disappearance of lines and the appearance of new ones pointed to the formation of shear defects.16 In DR-UV-vis spectra,17 the position of the band attributed to polaron conductance, its increasing intensity, and its linear dependence on the charge carrier concentration revealed that a substoichiometric MoO3-x is formed during mechanical activation. In addition, ESR spectroscopy17 corroborated the presence of Mo5+ centers in coordination spheres of different symmetries. Thus, (11) Leyrer, J.; Zaki, M. I.; Kno¨zinger, H. J. Phys. Chem. 1986, 90, 4775. (12) Margraf, M.; Leyrer, J.; Kno¨zinger, H.; Taglauer, E. Surf. Sci. 1987, 189/190, 842. (13) Kno¨zinger, H. Mater. Sci. Forum. 1988, 25/26, 223. (14) Kno¨zinger, H. In Fundamental Aspects of Heterogeneous Catalysis Studied by Particle Beams; Brongersma, H. H., van Santen, R. A., Eds.; Plenum Press: New York, 1991; p 7. (15) Kno¨zinger, H.; Taglauer, E. In Catalysis; Spivey, J. J., Ed.; The Royal Society of Chemistry: Cambridge, 1993; Vol. 10, p 1. (16) Mestl, G.; Herzog, B.; Schlo¨gl, R.; Kno¨zinger, H. Langmuir 1994, 11, 3027. (17) Mestl, G.; Verbruggen, N.; Kno¨zinger, H. Langmuir 1994, 11, 3035.
© 1996 American Chemical Society
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Mo5+ species were detected in microcrystalline MoO3 which were suggested to be the precursor of crystallographic shear structures. Another detected species was shown to interact with OH groups. O2 adsorption revealed that all of these Mo5+ centers are located within the bulk and not at the surface of the MoO3 crystallites. Vibrational spectroscopy (Raman, diffuse reflectance infrared Fourier transform (DRIFT))18 showed certain variations in MoO3 band intensity ratios and band half widths upon mechanical activation which were attributed to particle size reduction and the generation of defects. DRIFTS experiments18 indicated a drastic increase in the intensity of OH bands. This indicated the presence of water or OH groups in microcrystalline MoO3 which are interconnected by H bonds. Moreover, bands were detected which could be attributed to molybdate hydrates formed during the milling process. In situ experiments (ESR, DRIFTS, Raman) at elevated temperatures in N2 or O2 may yield information about changes in the concentration of defects, their mobility, and their reactivity toward reoxidation, and may permit the detection of structural changes. We therefore report here experimental results obtained by in situ ESR, DRIFT, and Raman spectroscopy. It is expected that a better understanding of properties of mechanically activated MoO3 will provide information on the microscopic processes occurring during the spreading of MoO3 across the surface of support oxides such as alumina, when the two components have been physically mixed by, e.g., milling in a planetary mill. 2. Experimental Section MoO3 (Merck, p.a., BET surface area 1.3 m2/g) was disintegrated in a planetary mill during 600 min as described previously.16 Samples were drawn at distinct times (10, 20, 60, 120, 180, 240, 420, and 600 min) and BET surface areas were measured. After reaching a constant BET surface area of 32 m2/g after 600 min, the milling process was discontinued. ESR spectra were recorded in the X-band (9.67 GHz) at room temperature and at 90 K on a Varian E-Line spectrometer (E9) equipped with a TE104-mode cavity. Since a linear relation between signal intensity and the square root of the applied power was found up to 20 mW, all spectra were recorded using 10 mW microwave power. Saturation effects should therefore be excluded. Mn2+ ions in a MgO matrix were measured in the second cavity and used for field calibration. To reduce paramagnetic interaction with adsorbed oxygen, the samples were flushed with dry N2 at room temperature for 18 h, prior to in situ treatments. The simulation of the ESR spectrum was achieved using the Bruker program “SimFonia”. DRIFT spectra were obtained in the range between 400 and 4000 cm-1 on a Bruker IFS 66 spectrometer using an MCT detector. An in situ DRIFTS cell (Spectra Tech, model 0030100) equipped with KBr windows was used. All spectra were recorded with 1000 scans and a frequency resolution of 2 cm-1. The measured reflectance was converted into Schuster-KubelkaMunk units. The Raman spectra were recorded on an OMARS 89 (Dilor, France) spectrometer equipped with an electrically cooled diode multichannel detector (Spectroscopy Instruments, Germany) using the conventional multichannel technique. The detector controller and the stepping motor controller were also from Spectroscopy Instruments, Germany. The samples were excited in situ in rotating ESR tubes to reduce laser heating with the line at 487.9 nm of an Ar+ ion laser (Spectra Physics, Model 2020) in backscattering geometry. The optical resolution was set to 5 cm-1 and the laser power was 50 mW at the sample. For the in situ ESR and Raman experiments the following treatment steps were chosen: Heating in flowing N2 (20 mL/min) at 423 K for 1 h to remove physically adsorbed H2O and CO2 from the surface. (18) Mestl, G.; Srinivasan, T. K. K.; Kno¨zinger, H. Langmuir 1995, 11, 3795.
Mestl et al. Table 1. Summary of Mo5+ Signals Observed after Different Treatments treatment
unmilled MoO3
600 min milled MoO3
untreated N2, RT N2, 423 K, 1 h N2, 523 K, 1 h N2, 623 K, 1 h N2, 673 K, 1 h 5% O2, RT 5% O2, 423 K, 1 h 5% O2, 523 K, 1 h 5% O2, 623 K, 1 h 5% O2, 723 K, 1 h
A1 (RT); D weak A1 (RT); D weak A1 (RT) A1; B weak A1; B A1; B A1; B A1; B; A2 weak B; A1 weak; A2 weak B; A1 weak; A2 weak B; A2; D weaka
A2; C weak; D weaka A2; C weak; D weaka A1; A2 weak A1; A2 weak A1; A2 weak; E weak A1; A2 weak; E weak A1; A2 weak; E weak A2; A1 weak; A2 A2; C weak A2; C weak; D weaka
a
Spectrum with hyperfine splitting due to
95Mo
and
97Mo.
Table 2. g Values of the Different ESR Signals of Mo5+ Species signal A1 signal A2 signal B signal C signal D signal E free electron
g⊥ ) 1.946 g1 ) 1.957, g2 ) 1.944 g1 ) 1.959, g2 ) 1.952 g⊥ ) 1.935 g⊥ ) 1.975 A⊥ ) 0.39 mT g⊥ ) 1.921 g ) 2.003
g| ) 1.871 g3 ) 1.871 g3 ) 1.867 g| ) 1.901 g| ) 1.889 (g| ) 1.77-1.75)
Heating in flowing N2 (20 mL/min) for 1 h at 523, 623, and 673 K to generate increasingly more and possibly different defects in MoO3. The enhanced internal energy of mechanically activated MoO3 should lead to differences in the kind, concentration, and behavior of defects as compared to unmilled MoO3. In addition, the high temperature stability of the OH species17,18 should be tested. The reactivity toward reoxidation of the defects generated by mechanical activation and by the high temperature treatment in N2 was investigated by treating the samples in a flow of N2 containing 5% O2 (20 mL/min) for 1 h at 290, 423, 523, 623, and 673 K, respectively.
3. Results and Discussion 3.1. ESR Spectroscopy. The notation of the ESR signals detected at the different stages of the experimental procedure is summarized in Table 1, and the corresponding g values are given in Table 2. Unmilled MoO3. A broad asymmetric signal denoted A1 at g ≈ 1.95 is observed at room temperature for crystalline MoO3 prior to any mechanical treatment (spectrum a in Figure 1). Thermal treatment in N2 at 423 K (spectrum c in Figure 1) leads to an increase of the overall intensity of this signal, which then decreases again after treatment at higher temperatures (spectra d-f in Figure 1). The intensity of the signal decreases further when the sample is exposed to an O2 containing atmosphere (spectra g-k in Figure 1) and finally reaches an intensity after O2 treatment at 723 K (spectrum k in Figure 1). That is comparable to the initial intensity of signal A1. When the spectra after the same thermal treatments were recorded at 90 K (not shown) two signals denoted A1 and B (see Tables 1 and 2) with axial and rhombic symmetry, respectively, were detected. Signal A1 reaches maximum intensity in N2 at 523 K. As the treatment temperature was further increased, the intensity of signal A1 decreased while that of signal B started increasing. This trend continued during treatments in an O2-containing atmosphere. At 423 K in O2/N2 a third weak signal became detectable, which will be denoted A2 for the milled sample. As shown in Figure 2, the reciprocal intensity of signal A1 (g⊥ ≈ 1.95) of samples that were treated in N2 at 623 K for 1 h increases proportional to the temperature below 200 K. The adsorption of O2 at 300 K on a sample that
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Figure 2. Temperature dependence of the reciprocal signal intensity (at signal maximum) after 1 h of N2 treatment at 623 K of unmilled MoO3: all contributions to the signal at g ≈ 1.95 (+) and signal at g ≈ 2.003 (0).
Figure 1. Low-temperature ESR spectra of unmilled MoO3 prior to any thermal treatment (a), treated in N2 at room temperature (b), N2 at 423 K (c), N2 at 523 K (d), N2 at 623 K (e), N2 at 673 K (f) and in 5% O2/N2 at RT (g), 5% O2/N2 at 423 K (h), 5% O2/N2 at 523 K (i), 5% O2/N2 at 623 K (j), and 5% O2/N2 at 723 K (k).
was pretreated in N2 at 423 K did not influence the signal intensity nor its line width when measured at 300 K. For samples that were flushed in N2 at 300 K and reoxidized at 723 K, a doublet signal (denoted D in Tables 1 and 2) was detected in low-temperature spectra near g⊥ ) 1.975. This signal may be due to an orthorhombic site with only slightly different gx and gy values or it may alternatively be attributed to an axial signal with hyperfine splitting by an I ) 1/2 nucleus. The parallel component is only poorly resolved. In addition to the signals described above, a sharp isotropic signal with g ) 2.003 is observed (see Figure 1) after all treatments. The intensity of this signal strongly increases after thermal treatment in N2 for 1 h at 423 and 523 K, and then decreases again at still higher temperatures. Thermal treatment in O2 leads to a further decrease of the signal intensity. After a final O2 treatment at 723 K, the signal assumes an intensity comparable to that in the original sample prior to any thermal treatment. As shown in Figure 2, the reciprocal intensity of this signal after thermal treatment in N2 at 623 K decreases almost linearly with temperature between 100 and 300 K. Adsorption of O2 at 300 K on a sample that was treated in N2 at 423 K leads to the complete disappearance of the signal, which can be restored by flushing in N2 at room temperature. This behavior suggests that the signal must be attributed to a surface species. Milled MoO3. The ESR spectrum of the milled MoO3 sample also showed the asymmetric signal with g ≈ 1.95;
its intensity, however, was significantly higher (about 20×) than that of the unmilled oxide. This signal did not exhibit any fine structure at room temperature. Its intensity increased during thermal treatment in N2 up to 423 K and decreased again at higher temperatures. Subsequent treatment in the presence of O2 (5% in N2) led to an intensity enhancement before the signal again decayed at temperatures higher than 523 K. Spectra recorded at 90 K after the same thermal treatments (Figure 3) were significantly more intense and better resolved. Spectrum a in Figure 3 of the milled sample prior to any thermal treatment exhibits the strongest, orthorhombic signal A2 with g1 ) 1.957, g2 ) 1.944, and g3 ) 1.871. The simulation of signal A2 is shown in Figure 4 and compared to the experimental spectrum a of Figure 3. The weak signal C with g⊥ ) 1.935 and g| ) 1.901 has not been taken care of in the simulation. When the treatment temperature was increased stepwise to 673 K in N2 (spectra b-f in Figure 3), signal A2 almost disappeared and the structureless signal A1 was built up. Simultaneously a very weak signal E (g⊥ ) 1.921, see Table 2) formed, which disappeared in the presence of O2 at higher temperature. Signal A1 re-formed in 5% O2/N2 at increasing temperature (spectra h in Figure 3) and was the only strong signal at 523 K (spectrum i in Figure 3). Its intensity decreased again at 623 K (spectrum j in Figure 3). Adsorption of O2 at room temperature on the sample that was thermally treated at 423 K in N2 affected neither the total intensity nor the line widths of the ESR signals. A weak signal D (g⊥ ) 1.975, g| ) 1.889) was only observed for the milled sample prior to thermal treatment and on the sample that was reoxidized in 5% O2/N2 at 623 K. The sharp isotropic signal at 2.003 that was seen in the spectra of the unmilled MoO3 (Figure 1), was also detected for the milled MoO3 and its intensity showed the same qualitative trends during the thermal treatments as with the unmilled oxide. The intensity of the signal was significantly reduced in spectra recorded at 90 K and also in room temperature spectra when O2 was present.
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Mestl et al.
Figure 4. Simulation of signal A2 and of the signal of a free electron: (a) experimental; (b) simulated spectrum.
Figure 3. Low-temperature (90 K) ESR spectra of milled MoO3, untreated (a) and treated in N2 at RT (b), in N2 at 423 K (c), in N2 at 523 K (d), in N2 at 623 K (e), in N2 at 673 K (f), in 5% O2/N2 at RT (g), in 5% O2/N2 at 423 K (h), in 5% O2/N2 at 523 K (i), and in 5% O2/N2 at 623 K (j).
Two weak signals which showed hyperfine splitting into four lines (g1 ) 2.424, g2 ) 2.08, g3 ) 2.05, and A1 ) 9,4 mT; g1′ ) 2.418, g2′ ) 2.08, g3′ ) 2.05, and A1′ ) 10.0 mT) suggested the presence of Cu2+ impurities of less than 4 ppm. Another isotropic signal (g ) 4.22) is probably due to the 0.0004% Fe impurity that was indicated by the producer of the MoO3. In the following we will now give an assignment of the various ESR signals that were seen in the spectra of unmilled and milled MoO3. Signals A through E can be attributed to Mo5+ in various coordinations and symmetries. Signals A1 and A2 were observed in practically all spectra of unmilled and milled MoO3. The g values of signal A2 are very similar to those reported for MoO3 single crystals, namely, gx ) 1.942, gy ) 1.953, and gz ) 1.878.19 They are characteristic for Mo5+ in 6-fold coordination with rhombic distortion, consistent with the simulation shown in Figure 4. Assuming C2v crystal field symmetry with dxy ground state as shown in Figure 5, the g values are given by
gz ) ge(1 - 4λ/∆)
(1a)
gx ) ge(1 - 4λ/δ)
(1b)
gy ) ge(1 - 4λ/η)
(1c)
with ge ) 2.0023 being the g factor of the free electron, λ (19) Ioffe, V. A.; Patrina, I. B.; Zelenetskaya, E. V.; Mikheeva, V. P. Phys. Status Solidi 1969, 35, 535.
Figure 5. Energy levels of Mo5+ in Oh, C4v, and C2v environments. Table 3. Crystal Field Parameters of Various Mo5+ Species signal A1 signal A2 signal B signal C
∆
δ
22 875 22 875 22 198 29 649
13 337 12 879 14 928 11 157
η
R
16 575 17 341
2.33 2.53 2.89 1.5
) 375 cm-1 the spin-orbit coupling constant for Mo5+,19 and ∆, δ, and η the energy splittings as defined in Figure 5. The parameter ∆ for Mo5+ (d1) depends only on the equatorial Mo-O distances of a distorted MoO6 octahedron. Parameter δ depends on all Mo-O distances in the octahedron and is a measure of tetragonal distortion. ∆, δ, and η parameters as calculated using eq 1a-c are summarized in Table 3. The data obtained for signal A2 match satisfactorily with values reported for Mo5+ in 6-fold coordination.20,21 Dyrek and Labanowska22 and Serwicka (20) Gray, H. B.; Hare, C. R. Inorg. Chem. 1962, 1, 363. (21) Che, M.; Fournier, M.; Launay, J. P. J. Chem. Phys. 1979, 71, 1954. (22) Dyrek, K.; Labanowska, M. J. Chem. Soc., Faraday Trans. 1991, 87, 10003.
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and Schindler23 observed the same signal and also assigned it to 6-fold coordinated Mo5+. Signal A1 is broader than signal A2 and less well resolved. Its g| value coincides with the g3 value of signal A2 (see Table 2) while the g1 and g2 values are not resolved. Since the Mo5+ concentration in the crystalline sample is low, this observation is probably related to higher symmetry (similar to C4v) rather than being caused by line broadening. The poor resolution of the signal may originate from an inhomogeneous distribution of Mo5+ centers or from slightly varying symmetries. The transformation of signal A2 into signal A1 during thermal treatment in N2 is attributed for the milled sample to the increased density of Mo5+ centers, which would lead to dipolar interactions and, hence, line broadening. A parameter R, namely
R)
ge - g| 4δ ) ge - g⊥ ∆
(2)
has been defined as a relative measure of the tetragonal distortion around the Mo5+ center.24,25 In eq 2, g⊥ ) (gx + gy)/2 and g| ) gz. In a perfect octahedral coordination, the R-value is equal to unity (R ) 1),24 while R values greater than unity are indicative of a distortion of the coordination polyhedron. The calculated R values for the various observed signals are also summarized in Table 3. An R value of 2.53 was found for signal A2 indicating a significant distortion of the MoO6 octahedron. The R value of signal A1 is lower, namely, 2.33, consistent with the higher proposed symmetry. Signals A1 and A2 are the dominant signals in both the unmilled and the milled MoO3 samples prior to thermal treatment and during thermal treatment in oxygen-free atmosphere. Both signals can be attributed to 6-fold coordinated Mo5+ centers in distorted octahedral environment. It is thus likely that the Mo5+ centers coincide with the crystallographic positions of Mo atoms in the MoO3 lattice. The stronger distortion in the milled sample may easily be explained by the mechanical stress induced by the milling process. The assignment of the A signals to Mo sites in the bulk structure is consistent with the fact that they remain unaffected by the presence of O2. Signal B with g1 ) 1.959, g2 ) 1.953, and g3 ) 1.867 was observed in the unmilled sample during thermal treatment in N2 at 523 K. Its ∆ parameter is smaller than that of signals A1 and A2, suggesting a weaker octahedral crystal field strength. This may be caused by the creation of an oxygen vacancy. The R parameter is greater than that of the A signals. Serwicka,26 Che and co-workers,27-30 and Dyrek and Labanowska22 observed the same crystal field parameters and attributed the signal to Mo5+ in 5-fold coordination with a distorted quadratic-pyramidal environment. Since signal B remains almost unaffected by treatments in oxidizing atmosphere, the corresponding Mo5+ center is most likely located in the bulk. Signal C has g values at g⊥ ) 1.935 and g| ) 1.901 and is exclusively found for the milled sample. Its ∆ parameter is very high, whereas the parameter δ is smaller than those of signals A1, A2, and B. This suggests a higher MoO6 symmetry with shorter equatorial Mo-O bonds. (23) Serwicka, E.; Schindler, R. N. Z. Phys. Chem. (Munich) 1982, 133, 175. (24) Dufaux, M.; Che, M.; Naccache, C. J. Chim. Phys. 1970, 67, 527. (25) Seshasdri, K. S.; Petrakis, L. J. Catal. 1973, 30, 195. (26) Serwicka, E. J. Solid State Chem. 1984, 51, 300. (27) Che, M.; McAteer, J. C.; Tench, A. J. J. Chem. Soc., Faraday Trans. 1 1978, 74, 2378. (28) Louis, C.; Che, M. J. Phys. Chem. 1987, 91, 2875. (29) Che, M.; Louis, C.; Tatiboue¨t, J. M. Polyhedron 1986, 5, 123. (30) Louis, C.; Che, M.; Anpo, A. J. Catal. 1993, 141, 453.
The R parameter has a value of 1.50 and supports this conclusion. A signal with the same parameters was reported by Dyrek and Labanowska22 and attributed to a 6-fold coordinated Mo5+ center in an axially distorted MoO6 octahedron. Similar g values were observed for the isopolyanion Mo6O193- in solution,21 in which the point symmetry of the Mo center is very close to C4v. Short Mo-O bond distances relative to the average distances in MoO3 also occur in Mo18O52. This phase is one of the wellknown shear structures31 which is formed from MoO3 at high degrees of reduction. The high intensity of the ESR signals of the milled MoO3 relative to the unmilled oxide is indicative of a high degree of reduction. The completely developed shear structure contains Mo5+ pairs with MoMo bond character in the {120} plane. Mo5+ ions that produced signal C were considered to be in a precursor state of the shear structure by Dyrek and Labanowska,22 since only clusters of edge-linked octahedra had formed in their mildly reduced sample. Signal D has g values of g⊥ ) 1.975 and g| ) 1.889. The perpendicular component is greater than that of the other ESR signals. Its intensity is typically weak. Dyrek and Labanowska22 attributed this signal to Mo3+ and considered it to be anisotropic with three g factors. A greater g factor relative to Mo5+ is the result of a smaller spinorbit coupling constant. The λ parameter for Mo3+ is 272 cm-1. However, the existence of Mo3+ ions after treatment at 623 K in an oxidizing atmosphere is rather unlikely. Alternatively, the g⊥ value at 1.975 may be caused by a hyperfine splitting with A⊥ ) 0.39 mT, suggesting that a 1H nucleus may be located close to a Mo5+ ion. It is known, that OH groups can be incorporated in MoO3 single crystals when they are grown in air.19 Sperlich et al.32 estimated the effective distance between Mo5+ and the proton as 0.26 nm. Signal E has g values of g⊥ ) 1.921 and g| ) 1.75-1.77 and can be attributed to 4-fold coordinated Mo5+.22,28,29,33,34 This Mo5+ species is very reactive and disappears at 423 K in the presence of O2. The centers should therefore be located at or close to the surface. The temperature dependence of the ESR spectra showed (Figure 2) that the total intensity of the Mo5+ signals was inversely proportional to the temperature in the range 90 e T e 200 K, thus fulfilling the Curie-Weiss law. This result indicates that all Mo5+ centers are detected within the mentioned temperature range. However, a significant deviation from the Curie-Weiss law was observed for temperatures higher than 200 K (see Figure 2). It has been reported that Mo5+ in 4-fold coordination can only be detected at low temperature.33,35 The spin-lattice relaxation time T1 of Mo5+ in 4-fold coordination is smaller, that of Mo5+ in 5-fold coordination is larger than that of 6-fold coordinated Mo5+.28 The sharp isotropic signal at g ) 2.003 lies close to the value of the free electron and suggests that the electron should have a small angular momentum. Carbon impurities and organic radicals can be excluded as a possible source of this signal since the signal would be expected to disappear after thermal treatment in an oxidizing atmosphere, which is not the case. Therefore, the signal is attributed to F centers. These are known to be created on the surface of MoO3 by radiation and by thermal treatment in vacuum or N2 and they are responsible for (31) Kihlborg, L. Ark. Kemi 1963, 21, 443. (32) Sperlich, G.; Frank, G.; Rhein, W. Phys. Status Solidi B 1972, 54, 241. (33) Shelimov, B. N.; Pershin, A. N.; Kazansky, V. B. J. Catal. 1980, 64, 462. (34) Latef, A.; Aissi, C. F.; Guelton, M. J. Catal. 1989, 119, 368. (35) Seyedmonir, S. R.; Howe, R. F. J. Catal. 1988, 110, 229.
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Figure 6. In situ Raman spectrum of MoO3 milled for 600 min after the series of N2 treatments at different temperatures (measured through the ESR tube).
the blue color of such materials.36-38 The interaction with gas phase O2 at room temperature induced a significant line broadening caused by dipolar interactions, suggesting that the F centers are located at the surface of the MoO3. In contrast to the Curie-Weiss behavior, the reciprocal intensity of the signal decreases with increasing temperature (see Figure 2) suggesting that pairing of the F centers occurs at low temperatures. In summary, the present ESR results demonstrate that the milling process induces the creation of Mo5+ centers. The majority of the Mo5+ ions is 6-fold coordinated and in symmetries close to C2v or C4v. N2 treatment at elevated temperatures increases the Mo5+ concentration; treatment in an oxidative atmosphere reduces it. Unmilled and milled MoO3 also contain octahedrally coordinated Mo5+ with an OH group in close proximity. A 5-fold coordinated Mo5+ species in distorted square pyramidal symmetry is created in unmilled MoO3 after thermal treatment in N2 at 523 and 623 K. These species are located in the bulk of the MoO3 crystallites. Besides Mo5+ centers in 6-fold coordination with symmetry similar to C2v, six-coordinated Mo5+ ions with symmetry close to C4v are also present in milled MoO3. These species are probably precursors in the formation of shear structures. In addition, 4-fold coordinated Mo5+ is formed when milled MoO3 is thermally treated in N2 at 623 and 673 K. These centers are found to be highly reactive. The total intensities of Mo5+ signals obey the Curie-Weiss law at temperatures below 200 K, while significant deviations occur at higher temperatures. Finally, F centers are present in both unmilled and milled MoO3. 3.2. In Situ Raman Spectroscopy. The Raman spectrum (Figure 6) of MoO3 milled for 600 min after flushing with N2 at 673 K for 1 h exhibits certain changes (36) Mann, R. S.; Khulbe, K. C. Bull. Chem. Soc. Jpn. 1975, 48, 1021. (37) Deb, S. K.; Chopoorian, J. A. J. Appl. Phys. 1966, 37, 4818. (38) Haber, J.; Serwicka, E. Polyhedron 1986, 5, 107.
Mestl et al.
as compared to regular MoO3.39 Thus, in the low-frequency regime a broad background, probably due to combination modes of optical and acoustic phonons, is indicating a relaxation of the k-selection rule. Moreover, the small band of the Ag, B1g deformation mode of Mo-O-Mo bridges is found at 466 cm-1 and has broadened considerably. This broadening may arise from the energetic separation of the originally coinciding modes. Probably, exclusively IR allowed modes (LO-TO modes of B2u, B3u Mo-O-Mo deformations) are also detected, again indicating a relaxation of the k-selection rule. After heat treatment at 673 K, a band at 606 cm-1 is detected. The position of this band is in agreement with an IR-active mode at 608 cm-1 observed in polycrystalline MoO3.18 Once again, the detection of this signal confirms the weakening of the spectroscopic exclusion rule. Furthermore, a very weak band is observed at 894 cm-1. A comparable band was detected at 888 cm-1 after evacuating MoO339 and mixtures of MoO3 with antimony oxides40 at 648 K for 10 h. 18O reoxidation revealed that this band arises from oxygendeficient MoO3-x. Unmilled MoO3 did not show any of these changes relative to the reported Raman spectrum of polycrystalline MoO3 after this temperature treatment in N2 (spectra not shown). After reoxidation, the Raman spectra did not give any further information, except that the above mentioned changes (high background, additional bands at 606, 894 cm-1) disappeared again. Therefore, these spectra are not shown. 3.3. In Situ Diffuse Reflectance Fourier Transform IR Spectroscopy. Unmilled as well as milled MoO3 (600 min) was treated in the in situ DRIFTS cell at 673 K in flowing N2 for 1 h and was characterized by IR spectroscopy before and after the treatment. No differences were observed in the DRIFT spectra of unmilled MoO3 before and after the temperature treatment, either in the fundamental or in the combination mode regime. In Figure 7, the spectral features in the OH stretching regime are shown. While in spectrum a of untreated MoO3, the broad OH stretching band at about 3220 cm-1 exhibits two shoulders at 3320 and 3470 cm-1 (see also ref 18), in spectrum b recorded after 1 h at 673 K in flowing N2, only the low-frequency maximum at about 3220 cm-1 can be recognized on top of a strongly increased background which cannot arise from particle size induced scattering and, therefore, is attributed to the IR polarization of free charge carriers. In Figure 8, the OH stretching regime of MoO3 milled for 600 min is reproduced. Spectrum a shows a very intense broad maximum between 3600 and 3000 cm-1. The signal maximum is about 3350 cm-1, having two shoulders at 3500 and 3240 cm-1 (see also ref 18). After 1 h at 673 K in flowing N2 (spectrum b) only a broad, weak band remains at about 3250 cm-1, again on top of a considerably increased background due to the polarization of free charge carriers in the infrared. Adsorption of H2O leads to H bonding of all surface OH groups and to a broad absorption between 3650 and 3200 cm-1. Thus, the broad OH band observed was assigned to H bonded OH groups and adsorbed water.18 Comparing the spectra before and after purging with N2 at 673 K, one recognizes that the low-frequency maximum (spectra a of (39) Py, M. A.; Schmid, Ph. E.; Vallin, J. T. Nuovo Cimento 1977, 38B, 271. (40) Mestl, G.; Ruiz, P.; Delmon, B.; Kno¨zinger, H. J. Phys. Chem. 1994, 98, 11269. (41) Mestl, G.; Ruiz, P.; Delmon, B.; Kno¨zinger, H. J. Phys. Chem. 1994, 98, 11283.
Mechanically Activated MoO3
Figure 7. In situ DRIFT spectra of the OH stretching region of unmilled MoO3: (a) before any treatment; (b) after 1 h at 673 K in flowing N2. Spectra are normalized to the fundamental mode at 995 cm-1. For better visualization spectra are vertically shifted. *: org. impurities of the in situ cell windows.
Figures 7 and 8) remains under such conditions (spectra b of Figures 7 and 8). The high-frequency bands or shoulders at 3320 and 3460 cm-1 or 3500 and 3350 cm-1 (spectra a of Figures 7 and 8, respectively) have disappeared. Therefore, the latter bands and shoulders are assigned to H-bonded H2O adsorbed on MoO3. It is known that H-bronzes, HxMoO3, and MoO3 hydrates, MoO3‚xH2O, contain extended H-bond systems.42-49 Sheik, Saleem and Aruldhas50 observed very intense IR bands at 3215, 3185, 3172, and 3155 cm-1 in molybdenum hydrates. The position of the high-frequency band observed by these authors is in good agreement with the absorption at 3220 cm-1 detected in this experiment. Therefore, the remaining band at about 3220 cm-1, after purging at 673 K, may be attributed to H bonded O‚‚‚HO-species within the MoO3 lattice. In Figure 9, the OH bending and ModO combination mode regime is reproduced. While the combination modes do not vary under N2 treatment at 673 K, the intensity of the OH-bending vibration at 1630 cm-1 (see also ref 18), assigned to adsorbed molecular water, decreases as expected. The band assigned to the O‚‚‚H-O bending at (42) Taylor, R. E.; Silva Crawford, M. M.; Gerstein, B. C. J. Catal. 1980, 62, 401. (43) Taylor, R. E.; Ryan, L. M.; Tindall, P.; Gerstein, B. C. J. Chem. Phys. 1980, 73, 5500. (44) Cirillo, A. C.; Ryan, L.; Gerstein, B. C.; Fripiat, J. J. J. Chem. Phys. 1980, 73, 3060. (45) Marinos, C.; Plesko, S.; Jonas, J.; Tinet, D.; Fripiat, J. J. Chem. Phys. Lett. 1983, 96, 357. (46) Maricic, S.; Smith, J. A. S.; Ritter, Cl.; Mu¨ller-Warmuth, W.; Scho¨llhorn, R. J. Chem. Phys. 1985, 83, 6130. (47) Ritter, Cl.; Mu¨ller-Warmuth, W.; Scho¨llhorn, R. Ber. Bunsenges. Phys. Chem. 1982, 86, 1101. (48) Slade, R. C. T.; Hirst, P. R.; Pressman, H. A. J. Mater. Chem. 1991, 1, 429. (49) Crouch-Baker, S.; Dickens, P. G. Acta Crystallogr. 1984, C40, 1121. (50) Sheik Salem, S.; Aruldhas, G. Pramana 1983, 21, 283.
Langmuir, Vol. 12, No. 12, 1996 2967
Figure 8. In situ DRIFT spectra of the OH stretching region of MoO3 milled for 600 min: (a) before any treatment; (b) after 1 h at 673 K in flowing N2. Spectra are normalized to the fundamental mode at 995 cm-1. For better visualization spectra are vertically shifted. *: org. impurities on the in situ cell windows.
1410 cm-1 decreases as well, and in spectrum b two small bands are observed at 1508 and 1410 cm-1. These observations are suggested to indicate changes in the H-bond network. In Figure 10, the Mo-O fundamental regime of milled MoO3 is reproduced. In contrast to the unmilled sample, certain changes are observed between the untreated sample (a) and the sample after purging with N2 at 673 K for 1 h (b). The band at 770 cm-1 in spectrum a, which was attributed to Mo-OH vibrations,50 is shifted to 759 cm-1 (spectrum b). The Mo-OH band at 955 cm-1 50 in spectrum a, which formed during mechanical activation,18 has shifted to 948 cm-1 in spectrum b. After dehydration, the shoulder at 980 cm-1 (LO mode of the B3u-stretching vibration), is again a resolved signal at 979 cm-1. Obviously, the loss of water from the MoO3-lattice affects the Mo-OH stretching vibrations. 4. Conclusions MoO3 milled for 600 min as well as unmilled MoO3 contains Mo5+ ions, the latter in lower concentration. Most of these ions are 6-fold coordinated in symmetries similar to C2v or C4v. N2 treatment at elevated temperatures increases the Mo5+ concentration in the samples; oxidation, on the other hand, leads to a decrease. In addition, both MoO3 samples, without any treatment, contain octahedrally coordinated Mo5+ species the ESR signal of which is split by the interaction with a proton in close proximity. Fivefold coordinated Mo5+ species in distorted, squarepyramidal symmetry are generated in unmilled MoO3 during N2 purging at 523 and 623 K. These species are located in the bulk MoO3, since they are stable in O2 atmosphere even at 623 K. MoO3 milled for 600 min contains, besides 6-fold coordinated Mo5+ in C2v similar
2968 Langmuir, Vol. 12, No. 12, 1996
Figure 9. In situ DRIFT spectra of the OH bending and ModO combination mode region of MoO3 milled for 600 min: (a) before any treatment; (b) after 1 h at 673 K in flowing N2. For better visualization, spectra are vertically shifted. Spectra were normalized to the combination mode at 1995 cm-1.
symmetry, a second Mo5+ species in almost C4v symmetry. The Mo-O distances in the equatorial plane are shortened and these Mo5+ centers are assigned to a precursor structure of crystallographic shear planes. Besides 6-fold coordinated Mo5+, tetrahedrally coordinated Mo5+ centers are also generated, if milled MoO3 is treated in N2 at 623 or 673 K. This Mo5+ species is related to shear defects protruding from the surface into the bulk of MoO3. The disappearance of this species under mild oxidative conditions indicates its high reactivity. The total intensity of the Mo5+ signals follows the Curie-Weiss law between 90 and 200 K. Above 200 K it deviates strongly from linearity. Hence, a considerable part of the Mo5+ centers does not contribute to the signal intensity at higher temperatures. An additional signal is observed which is characteristic of free electrons. N2 treatment at 423 K strongly enhances their concentration, while at higher temperatures and under O2 their concentration decreases again. An inverse Curie-Weiss behavior between 90 and 300 K indicates the pairing of electrons at lower temperatures. The enhanced scattering background in the Raman spectrum of milled MoO3 thermally treated in N2, as well
Mestl et al.
Figure 10. In situ DRIFT spectrum of the Mo-O fundamental mode regime of MoO3 milled for 600 min: (a) before any treatment; (b) after 1 h at 673 K in flowing N2. For better visualization, spectra are vertically shifted. Spectra are normalized to the band at 995 cm-1.
as the broadening and growing of the band at 488 cm-1, which is assigned to Mo-O-Mo bridges, is correlated with the increasing defect concentration detected by ESR. The two small bands in the Raman spectrum at 606 and 894 cm-1 observed after 1 h at 673 K in N2 are probably connected with the formation of shear defects which were detected by ESR spectroscopy. In situ DRIFT spectra before and after purging with N2 at 673 K reveal that a small amount of H bonded OH groups remains under these conditions in both unmilled and milled MoO3. In the case of milled MoO3, characteristic vibrations in the fundamental and O‚‚‚H-O bending regime indicate the formation of Mo-OH groups during the mechanical treatment. Acknowledgment. This work was financially supported by the Deutsche Forschungsgemeinschaft (SFB 338) and by the Fonds der Chemischen Industrie. The authors thank Professor Dr. J. Voitla¨nder for providing the possibility to measure the ESR spectra. G.M. acknowledges the award of a Feodor Lynen-Research Grant from the Alexander-von-Humboldt-Foundation. LA950788C
