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Mechanically Activated MoO3. 4. In Situ Characterization of Physical Mixtures with Al2O3 G. Mestl,†,‡ N. F. D. Verbruggen,† F. C. Lange,†,§ B. Tesche,| and H. Kno¨zinger*,† Institut fu¨ r Physikalische Chemie, Universita¨ t Mu¨ nchen, Sophienstrasse 11, 80333 Mu¨ nchen, Germany, and Max-Planck-Institut fu¨ r Kohleforschung, Postfach 1010353, 45466 Mu¨ lheim/Ruhr, Germany Received October 13, 1995. In Final Form: January 17, 1996X Mechanical activation of solids leads to an increased internal energy caused by the introduction of defects. This can result for example in a reduced surface melting temperature of activated particles, which in turn may affect their sintering behavior. Analogously the spreading behavior of MoO3 over Al2O3 may depend on the mechanical treatment during physically mixing the solids. Differences in the spreading over Al2O3 of unmilled MoO3 and MoO3 that was mechanically activated for 600 min were investigated by SEM/TEM, XPS, ESR, and in situ high-temperature Raman spectroscopy. SEM and EDX analyses of these physical mixtures make surface melting during the calcination very probable. In addition, analysis of XPS spectra also shows that spreading occurs under these conditions. However, spreading in the mixture with milled MoO3 is more effective. ESR spectroscopy shows that Mo5+ centers are reoxidized after calcining the mixtures with unmilled MoO3 in moist oxygen. For the mixture with milled MoO3 an additionally observed Mo5+ species in C2v distorted sixfold coordination is stable against oxygen for many hours, independent of the presence or absence of water. This higher stability of this defect species against reoxidation is attributed to an improved stabilizing effect of the Al2O3 support due to a pronounced spreading of mechanically activated MoO3. High-temperature Raman spectroscopy of pure, unmilled MoO3 reveals that a transformation into polymeric species occurs at temperatures above 948 K. At 1053 K, melting is observed and the Raman bands of crystalline MoO3 are lost. Calcination of an unmilled physical mixture of 9 wt % MoO3 and Al2O3 at the considerably lower temperature of 823 K in dry O2 leads to the observation of Raman bands of an amorphous polymeric Mo surface melt. Quenching this sample to room temperature results in a Raman spectrum which is attributed to a glassy surface MoO3 phase. Calcination of the physical mixture milled for 3 h at 823 K also leads to a Raman spectrum of the surface melt. Quenching to 298 K does not lead to a considerable change of the spectrum, this being explained by a more effective spreading of the Mo phase in the mechanically activated mixture. This surface Mo phase is highly reactive toward H2O during rehydration at 298 K, which leads to the formation of a polymeric surface species. A long time spreading experiment at 723 K reveals that this process is considerably slower and less effective at this lower temperature.

1. Introduction Recently a new route to supported molybdenum catalysts was described.1 In this preparation method, MoO3 is spreading over the carrier surface at elevated temperatures. The reduction of the free surface energy is considered as the driving force of this process; the microscopic mechanism of the spreading, however, is not yet understood. Surface melting of MoO3 at temperatures above the Tammann temperature and mass transport similar to plastic flow on interfaces during sintering2 may describe the spreading. Kno¨zinger and Taglauer3 proposed the so-called “unrolling carpet” mechanism for this process. The first step of this dry chemical synthesis is the intense mixing and grinding of MoO3 with the support oxide. Hence, the question arises whether the grinding affects the spreading process. Lattice defects induced by mechanical energy cause a change of the structural and physical properties of the †

Universita¨t Mu¨nchen. Present Address: Abteilung Oberfla¨chenchemie und Katalyse, Universita¨t Ulm, Albert.-Einstein-Str. 11, 89069 Ulm, Germany. § Present Address: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany. | Max-Planck-Institut fu ¨ r Kohleforschung. * To whom correspondence may be addressed. X Abstract published in Advance ACS Abstracts, March 15, 1996. ‡

(1) Leyrer, J.; Zaki, M. I.; Kno¨zinger, H. J. Phys. Chem. 1986, 90, 4775. (2) Doe, A.; Seidel, B. R.; Johnson, D. L. In Sintering and Related Phenomena; Kucynski, G. C., Ed.; Plenum Press: New York, 1973; Vol. 6, p 247. (3) Kno¨zinger, H.; Taglauer, E. In Catalysis; Spivey, J. J.; Agarwal, S. K., Eds.; The Royal Society of Chemistry: Cambridge, 1993; Vol. 10, p 1.

solid. As a consequence, tribochemical reactions are initiated or accelerated by the energy stored in the solid in the form of lattice defects. Heinicke,4 in his model of a multiple-step energy dissipation, distinguished three main groups of tribochemical reactions: (i) stochastic reactions induced by energetically highly excited states which are not in thermal equilibrium with their environment, (ii) reactions induced by high-energy states in the solid which have already adopted a local MaxwellBoltzmann distribution, and (iii) reactions which are initiated or accelerated by the energy stored in the solid in the form of lattice defects due to the dissipation of mechanical energy. These tribochemical reactions are extremely complex and depend on the kind and intensity of mechanical treatment, as well as on the specific mechanical properties of the solid. The enhancement of the solid state reactivity due to mechanical stress originates from four types of perturbations of the material:4 (i) creation of highly excited states at the fracture tip, (ii) rupture of chemical bonds, (iii) enlargement of the solid surface, and (iv) changes of the lattice structure in a layer close to the newly generated surface. It may be speculated that such mechanically induced perturbations will affect the spreading behavior of MoO3 on the surface of a support oxide. Already more than 200 years ago, it was observed that the reaction rate in heterogeneous systems is not proportional to the amount but to the surface area of the solid components.5 Thus, the surface area accessible to a second reactant significantly affects heterogeneous (4) Heinicke, G. Tribochemistry; Carl Hanser Verlag: Mu¨nchen, 1984.

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reactions. For example, diffusion in a solid state reaction is determined by the surface area, e.g., the oxidation rate of finely divided metals.6 Hence, the mass transport occurring during the spreading process of MoO3 across a support may also be affected by the BET surface area of the participating oxides. MoO3 has an open layer structure.7 The crystal structure of such oxides collapses under reducing conditions, and crystallographic shear planes can be formed.8 Sintering of such oxide materials results in a complex coherently proceeding rearrangement of the substructures, whereby complete crystal planes are regrouped in a concerted mechanism.9 One may also speculate whether spreading of MoO3 across the surface of Al2O3 is occurring via such complex rearrangements of larger arrays of the MoO3 lattice and is thus affected by the presence of defect clusters. In several papers of this series,10-13 the effects of mechanical stress exerted on MoO3 were reported. For this purpose, MoO3 was disintegrated in a planetary mill during 600 min, as described previously,10 until a surface area of 32 m2/g was reached, corresponding to a decrease in particle size from about 1 µm to about 50 nm, as confirmed by XRD and SEM.10 The presence of ultrafine amorphous material was indicated by the difference between the BET and XRD particle size and by an X-ray scattering background, and it was also found in SEM micrographs. Varying X-ray pattern quality and 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.10 The excellent agreement between the position of the DR-UV/vis 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.11 ESR spectroscopy11,13 corroborated the presence of Mo5+ centers in coordination spheres of different symmetries. Thus, Mo5+ species were detected in milled MoO3 which were suggested to be the precursor of a crystallographic shear structure. Another detected species was shown to interact with OH groups. Vibrational spectroscopy (Raman, diffuse reflectance infrared Fourier transform (DRIFT))12 showed certain variations in MoO3 band intensity ratios and band half widths upon mechanical activation attributed to particle size reduction and the generation of defects. DRIFTS experiments12 indicated a drastic increase in the intensity of OH bands, indicating the presence of water or interconnected OH groups in milled MoO3. Moreover, bands were detected which could be attributed to molybdate hydrates formed during the milling process. The concentrations and physical properties of defect structures induced by mechanical activation were investigated by ESR spectroscopy under high(5) Wenzel, C. F. Lehre von der chemischen Verwandtschaft; Dresden, 1777. (6) Heinicke, G.; Janova, L. P.; Chrustalev, Ju. A.; Krotowa, N. A. Z. Chem. 1979, 19, 118. (7) Kihlborg, L. Ark. Kemi 1963, 21, 357. (8) Kihlborg, L. Ark. Kemi 1963, 21, 471. (9) Tilley, R. J. D. Defect Crystal Chemistry and its Applications; Blackie & Sons: London, 1987. (10) Mestl, G.; Herzog, B.; Schlo¨gl, R.; Kno¨zinger, H. Langmuir 1995, 11, 3027. (11) Mestl, G.; Verbruggen, N. F. D.; Kno¨zinger, H. Langmuir 1995, 11, 3055. (12) Mestl, G.; Srinivasan, T. K. K.; Kno¨zinger, H. Langmuir 1995, 11, 3795. (13) Mestl, G.; Verbruggen, N. F. D.; Kno¨zinger, H. Submitted to Langmuir. (14) Verbruggen, N. Dissertation, Universita¨t Mu¨nchen, 1993.

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temperature conditions similar to those applied during the spreading of MoO3 on Al2O3.13 In the bulk of unmilled MoO3, fivefold coordinated Mo5+ species in distorted square-pyramidal symmetry are generated during N2 flushing at 523 and 623 K. If milled MoO3 is treated in N2 at 623 or 673 K, tetrahedrally coordinated Mo5+ centers are generated, which are assigned to Mo sites on shear defects protruding from the surface. SEM, EDX, and XPS characterization of two physical mixtures of Al2O3 with 10 wt % unmilled and milled MoO3 (600 min) is reported in the present paper, which aims at analyzing the spreading behavior of mechanically activated MoO3 as compared to unmilled MoO3. Defect characterization of the mixtures using ESR spectroscopy in comparison with pure MoO313 is expected to clarify the possible influence of defects on the spreading behavior of MoO3. The structures of molybdenum oxides in the mixtures are analyzed by in situ Raman spectroscopy under spreading conditions. 2. Experimental Section The alumina used in these experiments was prepared by calcining Al(OH)3 (Condea) at 1048 K for 24 h. The BET surface area of the resulting γ-Al2O3 was determined to be 132 m2/g. Unmilled MoO3 was directly used as delivered (Merck p.a.). Its BET surface area was determined to be about 1.3 m2/g. Milled MoO3 was disintegrated in a planetary mill during 600 min, as described previously10 until a surface area of 32 m2/g was reached. The physical mixtures of 10 wt % MoO3 with Al2O3 were produced by gently stirring unmilled and milled MoO3, respectively, with the alumina support for 30 min using a spatula. For in situ ESR and Raman characterization, the physical mixtures were filled in standard ESR tubes which were connected to a reactor for in situ treatments.14 In order to further characterize the powder mixtures before and after thermal treatment, SEM, EDX, and XPS were carried out. In order to further elucidate the behavior of these physical mixtures directly during the spreading, high-temperature in situ Raman experiments were carried out. For this purpose, physical mixtures of MoO3 and Al2O3 were thermally treated in the in situ Raman cell using either a flow of dry O2 or a static air atmosphere. In a second in situ Raman experiment, two additional physical mixtures were investigated: The first one was prepared by shaking a mixture of 9 wt % unmilled MoO3 (Merck p.a.) with Al2O3 in a glass beaker for 20 min. By this procedure any mechanical stress upon the oxide particles should be avoided. The second one was made by milling a mixture of 9 wt % MoO3 with Al2O3 in the above-mentioned planetary mill for 3 h. For these high-temperature in situ Raman experiments an especially designed in situ quartz cell was used, which can be heated up to 1073 K.15 Scanning electron microscopy (SEM) was carried out on a Hitachi S-4000. For SEM/EDX the samples were dispersed in ethanol by ultrasonic agitation and spread onto a Si carrier which was mounted onto an Al holder by an electrically conducting glue. The EDX spectra were recorded with the Kevex-Delta III System attached to the system. The detector contains a Si(Li) crystal with a BN window and has a resolution of 118 eV. The accelerating voltage was between 15 and 25 kV and is printed on the micrographs. The X-ray photoelectron spectra were recorded on a modified Vacuum Science Workshop (VSW) ESCA 100 spectrometer with a hemispherical analyzer HAC 100/285 mm. The system was equipped with a twin-anode X-ray gun for Al KR (1468.6 eV) or Mg KR (1253.6 eV) excitation, respectively. The gun was operated at 180 W (12 kV, 15 mA). The spectra were recorded in the fixed analyzer transmission (FAT) mode, the pass energy being 44 eV for survey scans and 22 eV for high-resolution scans. For survey scans, the step width was 0.4 eV with an integration time of 0.5 s; otherwise a step width of 0.06 eV and a total integration time after several scans of 1 s (O 1s, Al 2s, Al 2p) and 5 s (Mo 3d) was used. The maximum resolution of the system is 0.6 eV. The powders were pressed into stainless steel sample holders. There (15) Mestl, G. Dissertation, Universita¨t Mu¨nchen, 1994.

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Mechanically Activated MoO3 was no further in situ treatment of the samples except outgassing in the UHV at room temperature. The base pressure of the UHV chamber was 10-9 mbar. The spectra were referenced to the Al 2p emission at 74.3 eV. Sample charging, which can be very severe in the case of oxidic compounds, was controlled by monitoring the C 1s line arising from ubiquitous carbon contamination in the beginning and at the end of the measurements. Using the VSW software ECRUN version 7.02, the X-ray satellites were eliminated, the spectra were smoothed, and the background was subtracted to determine the peak areas. ESR spectra were recorded on a Varian E-Line spectrometer (E9) equipped with a TE104-mode cavity in the X-band at 300 and 90 K. The system was tested for saturation. Since there was a linear relation up to 20 mW between signal intensity and the square root of the applied power, all spectra were recorded using 10 mW microwave power. Mn2+ ions in a MgO matrix measured in the second cavity were used for field calibration. To reduce paramagnetic interaction with molecular oxygen, the samples were purged with dry N2 at room temperature for 18 h, prior to the in situ treatments. The Raman spectra were recorded on an OMARS 89 (Dilor) spectrometer equipped with an electrically cooled diode multichannel detector using the conventional multichannel technique. The detector and the stepping motor controller were from Spectroscopy Instruments. The spectra were excited in situ directly through the ESR tube with the line at 487.9 nm of an Ar+ ion laser (Spectra Physics, Model 2020) in backscattering geometry (180°). The optical resolution was set to 5 cm-1 and the laser power was set to 50 mW. As the Raman spectra were recorded in the conventional multichannel technique, the spectra are convoluted with the diode array characteristics and the spectrometer function. Real band shapes cannot be observed using this technique.16,17 Thus band shifts and changing band profiles between the different treatment steps in the spreading experiment are not discussed in the following except for drastic variations in band position, width, and the general spectral feature (e.g. background). The Raman spectra recorded during the high-temperature in situ experiments (except the ones recorded for pure unmilled MoO3), however, are recorded using the new scanning multichannel technique (SMT).16-18 This technique allows a continuous recording of the whole spectral range and results in Raman spectra with the detector characteristics being averaged out, and thus, the real band position and shape is obtained. In addition, determining the spectrometer function (plus detector sensitivity) allows one to correct the Raman spectra so that the physically real band intensities over the whole spectral range are obtained. During these hightemperature experiments, the laser (487.9 nm) power and the integration times had to be increased step wise, in order to compensate the decreasing scattering efficiency of MoO3 at higher temperatures (see also ref 19).

3. Results and Discussion 3.1. SEM and EDX Characterization of 10 wt % MoO3/Al2O3 Mixtures. Scanning electron micrographs were taken of the two physical mixtures of 10 wt % MoO3 with Al2O3 prior to any treatment and after the various temperature treatments, namely, (i) 423 K, N2, 8 h, in order to quantitatively remove adsorbed H2O, (ii) 723 K, O2, 3 h, to induce spreading across the Al2O3 surface, and (iii) 723 K, O2 + H2Ovap, 7 h, to induce the chemical transformation into surface polymolybdates. As a reference, pure Al2O3 was treated in the same way and characterized analogously. No differences between the treated and untreated Al2O3 samples (spherical particles of about 7 µm diameter) could be observed. Changes in the micrographs of the mixtures occurring after thermal treatments must therefore be due to an altered MoO3 morphology. (16) Knoll, P.; Singer, R.; Kiefer, W. Appl. Spectrosc. 1990, 44, 776. (17) Deckert, V.; Kiefer, W. Appl. Spectrosc. 1992, 46, 322. (18) Spielbauer, D. Appl. Spectrosc. 1995, 49, 650. (19) Mestl, G.; Ruiz, P.; Delmon, B.; Kno¨zinger, H. J. Phys. Chem. 1994, 98, 11283.

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Figure 1. Scanning electron micrograph of the physical mixture of Al2O3 and 10 wt % unmilled MoO3 after treatment in dry O2 (723 K, 3 h) followed by wet O2 (723 K, 7 h).

Micrographs of the untreated physical mixture with unmilled MoO3 (not shown) exhibit small platelet- or needle-like particles being mixed with the support. However, larger MoO3 crystals and Al2O3 particles can also be seen, indicating, as expected, a poor overall mixing between the two phases. The EDX spectra (not shown) taken from one spherical and one platelet-like particle confirm that the first one is pure Al2O3 while the latter is pure MoO3. A whole series of EDX spectra of this mixture (not shown) recorded at different spots exhibit a large variance in the Mo and Al intensities. Such behavior is characteristic of an inhomogeneous mixture of the two phases. These observations in electron microscopy are consistent with the low Mo 3d/Al 2p intensity ratio in the XP spectrum of this sample (see section 3.2). After the various calcination treatments at 723 K, the micrograph of the mixture with 10 wt % unmilled MoO3 (Figure 1) shows almost spherical particles which are larger in size relative to the particles before calcination. The surface of these particles looks smooth and, therefore, seems to have been molten during the temperature treatments. EDX analysis (spectra not shown) carried out with acceleration voltages of 20 and 5 kV only shows a small Mo signal besides a large Al peak. The decrease in the Mo intensity and the altered Al/Mo intensity ratio in the EDX spectra, compared to spectra prior to thermal treatment, can be explained by the assumption of a thin MoO3 film on the support particles, this film being formed by spreading of MoO3 over Al2O3. This spreading must result in a more uniform Al/Mo intensity ratio (there are no separated phases anymore) and in a reduced intensity of the Mo signal (larger dispersion of the MoO3 phase, small thickness of the MoO3 film). This EDX result is in line with an increased XPS ratio, since EDX only samples a small spot of the surface but its sampling depth is as large as 1 µm while XPS integrates over the whole powder surface, however, with high surface sensitivity (sampling depth: ca. 1 nm). In Figure 2, a micrograph of the physical mixture with milled MoO3 is reproduced. It shows agglomerates of about 80 µm in diameter. As compared to the mixture with unmilled MoO3, no separate phases (spherical Al2O3 and platelet-like MoO3 particles) are directly visible in the micrograph. EDX analysis (not shown) reveals that these agglomerates contain both Al2O3 and MoO3. However, a series of EDX spectra (not shown) showed varying Al/Mo intensity ratios, indicating the additional presence of less well mixed agglomerates.

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Figure 2. Scanning electron micrograph of the physical mixture of Al2O3 and 10 wt % MoO3 milled for 600 min in a planet mill.

Figure 4. XP spectra of physical mixtures containing 10 wt % MoO3 and alumina referenced to the Al 2p emission at 74.3 eV: (a) mixture with unmilled MoO3 prior to any treatment; (b) mixture with unmilled MoO3 after treatment in N2 (423 K, 8 h), dry O2 (723 K, 3 h), and wet O2 (723 K, 7 h); (c) mixture with milled MoO3 prior to any treatment; (d) mixture with milled MoO3 after treatment in N2 (423 K, 8 h), dry O2 (723 K, 3 h), and wet O2 (723 K, 7 h).

Figure 3. Scanning electron micrograph of the physical mixture of Al2O3 and 10 wt % MoO3 milled for 600 min after calcination in dry O2 (723 K, 3 h) followed by wet O2 (723 K, 7 h).

The micrograph (Figure 3) obtained after the temperature treatments shows spherical particles comparable to those prior to the calcination steps. As in the mixture with 10 wt % unmilled MoO3, their surface is smooth and, thus, seems to be molten during the temperature treatments. The EDX spectra (not shown) exhibit only a small Mo signal. As in the case of the mixture with unmilled MoO3, this observation can be explained by the spreading of the MoO3 phase across the Al2O3 support. 3.2. XPS Characterization of the 10 wt % MoO3/ Al2O3 Physical Mixtures. In Figure 4, the XP spectra of both physical mixtures (containing either 10 wt % unmilled, or milled MoO3) are shown which were recorded before and after the temperature treatments (423 K, N2, 8 h, plus 723 K, O2, 3 h, plus 723 K, O2 + H2Ovap, 7 h). Two main effects are observed in the two series of Mo 3d spectra: (i) After the high-temperature treatment the binding energies of the Mo 3d doublet are shifted to higher values (spectra b and d in Figure 4), and (ii) the Mo 3d doublet has lost resolution as compared to the untreated physical mixtures. In addition, a further effect is recognized between the two XP spectra of the untreated mixtures. The binding energies of the Mo 3d doublet of milled MoO3 (spectrum c), prior to any treatment, are shifted to higher energies by 0.4 eV as compared to the unmilled material (spectrum a). This shift is within the range of the spectral resolution. It is known that the binding energies in microcrystalline samples are shifted

toward higher values as compared to those in highly crystalline materials.20-25 A shift of the Mo 3d5/2 peak of about 1.4 eV to higher binding energies was observed by Polz after milling a physical mixture of 9 wt % MoO3 with Al2O3 for 1 h.26 Since in this experiment, the hard γ-Al2O3 has acted as an additional emery, smaller MoO3 particles were generated. Therefore, the smaller shift of 0.4 eV in the Mo 3d peak positions observed for the present mixtures with milled MoO3 may be attributed to the MoO3 particle size reduction observed previously during the milling of pure MoO3.10 Unfortunately, the independent determination of this binding energy shift with the pure MoO3 powders (unmilled and milled for 240 and 600 min) was not possible due to electrostatic charging of the samples. This makes an alternative explanation for the observed shift also very likely. The physical mixtures prior to calcination are a system with two separated phases in which different charging of Al2O3 and MoO3 particles may occur due to the poor electrical conductivity of the oxidic powders. Since the Mo 3d binding energy was referenced to the Al 2p signal, possible charging differences may lead to an artificial shift in the binding energy. After calcination the situation is different. Now a good contact has been established between the two phases present in the mixtures (see SEM/EDX). This good contact allows charge transfer between the oxides. The MoO3 phase carries the same charge as the Al2O3 support. Without ambiguity, shifts toward higher binding energy (0.5 eV for unmilled, 0.7 eV for milled MoO3) can now be attributed to particle size reduction. Therefore, the shifts toward higher binding energies observed after calcination are evidence for an increasing dispersion of MoO3 (i.e (20) Mason, M. G. Phys. Rev. 1983, 27, 748. (21) Jirka, I. Surf. Sci. 1990, 232, 307. (22) Koohiki, S. Appl. Surf. Sci. 1986, 25, 81. (23) Cheung, T. T. P. Surf. Sci. 1984, 140, 151. (24) Asakawa, T.; Tanaka, K.; Toyoshima, I. Langmuir 1988, 4, 521. (25) Fernandez, A.; Caballero, A.; Gonzales-Elipe, A. R. Surf. Interface Anal. 1992, 18, 392. (26) Polz, J. Dissertation, Universita¨t Mu¨nchen, 1992.

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spreading over alumina). The observed loss in resolution of the Mo 3d doublet can also be explained by this particle size effect. The observed spectrum is the sum of the spectra of all the differently sized crystallites in the sample. This must lead to inhomogeneous line broadening. While the MoO3 crystallites being in good contact with Al2O3 spread over the support surface during thermal treatment (particle size reduction), MoO3 crystallites being only in contact with other MoO3 particles will sinter together (particle growth). The resulting larger particle size distribution leads to the observed increase in the band width. The strongest evidence for a spreading process during the temperature treatments is provided by the Mo 3d/Al 2s signal intensity ratios. The larger this ratio, the more dispersed is the MoO3 phase in the physical mixture. For the sample with unmilled MoO3, this intensity ratio is 0.30 before any treatment, indicating a very poorly dispersed MoO3 phase. The intensity ratio increases to 0.88 upon calcination. Therefore, it is tempting to assume that spreading has occurred in this mixture, leading to a better dispersion (see section 3.1). Probably only the smallest MoO3 crystals in good contact with the support have spread. Raman data suggest that larger MoO3 crystals in poor contact with the support remain unchanged or even grow due to sintering (see section 3.4). For the mixture with milled MoO3, the signal intensity ratio prior to any treatment is 0.59 and it increases to 1.01 after the calcination steps. As expected from the smaller particle size of milled MoO3, the degree of dispersion in the physical mixture before starting the experiment is better as compared to that in the sample with unmilled molybdena (smaller particles can be better intermixed than larger ones). After calcination this ratio is larger as compared to that for the unmilled sample, indicating that the spreading process over alumina is more efficient with mechanically activated MoO3. Additional effects during these experiments were observed with the Al 2p and O 1s signals (not shown). The full width at half maximum (fwhm) of both signals decreased during calcination in both the mixture with unmilled and with milled MoO3. For the mixture with unmilled MoO3, the fwhm of the Al 2p emission was 2.64 eV and it decreased to 2.4 eV after heat treatment. The fwhm of the O 1s peak also decreased from 3.3 to 2.88 eV. In the mixture with milled MoO3, the width of the Al 2p signal decreased from 2.58 to 2.28 eV, while the O 1s peak width decreased from 3.24 to 2.76 eV. As expected, the differences in the fwhm between the two untreated physical mixtures are not significant. The decreasing fwhm of both signals during calcination, however, is significant. Four reasons may account for the observed fwhm decrease: (i) The Al2O3 surface loses physisorbed H2O and surface OH groups and (ii) ordering effects on the surface of the highly disperse alumina.26 (iii) The sharpening of the O 1s peak may be due to the spreading process. Prior to spreading, there are at least two different O species on the sample surface: Al-O and Mo-O groups. After spreading, the outermost Al2O3 surface is covered by Mo-oxygen species, thus leading to dominant contributions of the latter oxygen species to the O 1s emission. A narrowing of the O 1s peak should be the consequence. Furthermore, during the spreading the Mo-oxygen phase may have reacted with Al2O3 surface hydroxyls to form Al-O-Mo bonds. This loss of surface hydroxyl groups should also lead to O 1s peak narrowing. In addition, the latter process creates a more regular octahedral environment for the surface Al centers than on the unreacted surface. This should result in a decreasing fwhm of the Al 2p signal, which indeed is observed. (iv) Spreading

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Figure 5. Room-temperature ESR spectra of 10 wt % unmilled MoO3/Al2O3, treated in (a) N2 (423 K, 8 h) and subsequently in (b) O2 (723 K, 1 h), (c) O2 (723 K, 2 h), (d) O2 (723 K, 3 h), (e) O2/H2O (723 K, 1 h), and (f) O2/H2O (723 K, 7 h).

leads to a better electrical contact between the two oxide phases present in the mixture and therefore to a reduced differential charging. In conclusion, XP spectroscopy shows that the degree of dispersion of the MoO3 phase has increased under the conditions applied (423 K, N2, 8 h; 723 K, O2, 3 h; 723 K, O2 + H2O, 7 h), which may be considered as evidence for spreading of MoO3 across alumina. In the mixture with milled MoO3, this process is more efficient as compared to that for the mixture with unmilled MoO3, presumably due to the smaller particle size in the milled molybdena. These smaller MoO3 particles must be in better contact (amount and quality of contacts) with the support particles, which leads to more efficient spreading. In addition the milling process may be considered to increase the internal energy and to lower the surface melting point, which may also lead to more efficient spreading. 3.3. ESR Spectroscopy of 10 wt % MoO3/Al2O3 Physical Mixtures. Physical Mixture with Unmilled MoO3. After 8 h of N2 treatment at 423 K of a physical mixture of Al2O3 with 10 wt % unmilled MoO3, a Mo5+ signal near g ≈ 1.95 can hardly be detected (spectrum a in Figure 5). After 1 h of calcination at 723 K in dry oxygen, a broad and weak signal is observed near g ≈ 1.95 (Figure 5b). A detailed inspection of the spectrum reveals the presence of three different Mo5+ species. The main Mo5+ species is sixfold coordinated in a symmetry similar to C4v with g values of g⊥ ) 1.946 and g| ) 1.871.14 In addition, a second Mo5+ species is observed in fivefold coordination with rhombic distortion (g1 ) 1.959, g2 ) 1.952, g3 ) 1.867), and a third weak signal (g⊥ ) 1.975, A⊥ ) 0.39 mT, and

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g| ) 1.889) is assigned to Mo5+ species in distorted octahedral environments interacting with protons.13 The total signal intensity after calcination is comparable to the signal intensity of the unmilled sample after purging with N2 at 623 K.13 Since the physical mixture with Al2O3 contains only 10 wt % MoO3, it exhibits an amount of ESR active centers being about 10 times larger per unit weight MoO3 as compared to the unsupported sample. This observation corroborates that (i) a considerable amount of Mo5+ centers in pure unmilled MoO3 are ESR inactive,13 either due to the coupling between Mo5+ centers or due to coupling to free electrons thus forming Mo4+ centers, and (ii) some mechanism must be operative which leads to the formation of a larger number of Mo5+ centers in the physical mixture as compared to the pure oxide. Further oxidation at 723 K results in a small reduction of the signal intensity, and only sixfold coordinated Mo5+ species in C4v symmetry are detected in the sample (spectra c and d of Figure 5). Mo5+ centers having an oxygen vacancy are reoxidized. Thus, Mo5+ centers in six-fold coordination are obviously more stable toward oxygen than to Mo5+ centers having an oxygen vacancy. In pure unmilled MoO3, the Mo5+ species having an oxygen vacancy in the coordination sphere is formed under reoxidation,14 while the sixfold coordinated defects were reoxidized. This comparison of pure MoO3 and the mixture with Al2O3 indicates that the interaction of MoO3 with the support possibly results in an altered defect stability of the octahedral defect species as compared to the fivefold coordinated defects. This observation may be related to the incomplete spreading of MoO3 as detected by XPS, EDX, and Raman. The more efficient the spreading process, the more octahedral Mo5+ centers would be affected by the stabilizing effect of the Al2O3 phase. If this hypothesis is correct, this effect should be more pronounced in the mixture with milled MoO3, where the spreading is suggested to be more efficient. When the calcination was carried out at 723 K in H2Osaturated O2, 7 h were needed for complete disappearance of the signal near g ) 1.95 (spectrum f in Figure 5). Physical Mixture with 10 wt % Milled MoO3. After an N2 treatment at 423 K for 8 h, the room-temperature ESR spectrum (Figure 6) exhibits a broad, asymmetric signal at g ≈ 1.95. A spectrum recorded at 90 K identifies this signal as the Mo5+ species which is sixfold coordinated having a C4v similar symmetry (g⊥ ) 1.946 and g| ) 1.871). The general spectral apperance very much resembles that of the unsupported milled MoO3 when recorded after treatment for 1 h at 673 K in flowing N2.13 In addition, two weak signals assigned to sixfold-coordinated Mo5+ species in C2v symmetry (g1 ) 1.957, g2 ) 1.944, g3 ) 1.871) and to fourfold-coordinated Mo5+ species in tetrahedral symmetry (g⊥ ) 1.921, g| ) 1.77-1.75) are detected. The appearance of the tetrahedrally coordinated species is very important for two reasons: (i) It is known that tetrahedrally coordinated molybdenum atoms are located at the crystallographic shear planes in Mo suboxides.8 Therefore, these molybdenum sites may be connected with the presence of such shear defects. (ii) In pure milled MoO3, this species can be detected only after an N2 treatment at 673 K.13 In contact with Al2O3, they are detected even at 423 K. Of course, the longer duration of the N2 treatment at 423 K as compared to the experiments with pure MoO3 may play a role in the detection of this defect species. However, since the generation of defects must be mainly controlled by the activation energy of their formation, and therefore by the reaction temperature, their observation after an N2 treatment at considerably lower temperature is probably due to the known strong interaction between Al2O3 and MoO3. This observation after treatment at lower

Mestl et al.

Figure 6. Room-temperature ESR spectra of 10 wt % milled MoO3/Al2O3, treated in (a) N2 (423 K, 8 h) and subsequently in (b) O2 (723 K, 1 h), (c) O2 (723 K, 2 h), (d) O2 (723 K, 3 h), (e) O2/H2O (723 K, 1 h), and (f) O2/H2O (723 K, 7 h).

temperature may also indicate a different formation mechanism of the tetrahedral Mo5+ species in MoO3 in contact with the support as compared to pure MoO3. Under oxidative atmospheres, this sample shows a behavior analogous to that of the unsupported milled MoO3. After 1 h calcination in dry oxygen at 723 K, a drastic reduction of the signal intensity is observed and only the signal assigned to a Mo5+ species in distorted sixfold coordination (C2v) remains. Further calcination in dry oxygen at 723 K for 3 h in total does only result in a minor intensity reduction. Hence, this defect species is very stable toward reoxidation. After subsequent calcination for 1 h in O2 saturated with H2O (spectrum e of Figure 6), the signal of Mo5+ centers in C2v symmetry has hardly lost intensity. Here, the mixture with milled MoO3 behaves differently as compared to that with unmilled MoO3, where the sixfold defect species was stable against reoxidation but not against the presence of H2O. After calcination in humid oxygen also Mo5+ centers in interaction with protons are detected. Similar defect species were also observed in pure MoO3.11 The observation of Mo5+ ions interacting with protons in the physical mixture (dilution with Al2O3), however, suggests that their relative concentration is larger as compared to that in pure MoO3. Therefore, an additional interaction with protons due to the presence of water must have led to an increase in their concentration and thus to their detectability. It is known that Mo5+ defects may heal out via the incorporation of OH groups, thus leading to charge compensation.27 Therefore, it is (27) Joffe, V. A.; Patrina, I. B.; Zelenetskaya, E. V.; Mikheeva, V. P. Phys. Status Solidi 1969, 35, 535.

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Figure 7. Change in global ESR signal intensity with the duration of the temperature treatment at 723 K in dry and H2O-saturated oxygen: (a) physical mixture with 10 wt % unmilled MoO3; (b) physical mixture with 10 wt % milled MoO3.

suggested that the interaction with H2O leads to the formation of this defect species during calcination. After further calcination in wet O2 for a total of 7 h (spectrum f of Figure 6), again only a minor decrease in global intensity can be recognized. In contrast to the mixture with unmilled MoO3, a total oxidation time of 10 h was not sufficient to oxidize all the Mo5+ species. This behavior of the ESR intensities is shown in Figure 7, where the global ESR signal intensity of both MoO3/Al2O3 mixtures is plotted against the duration of the calcination treatment. The slope of the curve is a measure for the rate with which the defects are reoxidized. Curve a shows the intensity reduction for the mixture with unmilled MoO3, while curve b represents the changes in milled MoO3 mixed with alumina. The steeper slope of curve a in the range of exposure to humid O2 suggests a more pronounced sensitivity to water vapor of the defects in the physical mixture with unmilled MoO3. The behavior of intensity reduction in the mixture with milled MoO3 (Figure 7b) is completely different. The slopes are identical in the presence and absence of water vapor. One would expect that the microcrystalline, highly disperse MoO3 in this mixture should be reoxidized faster than the unmilled MoO3, in contrast to the experimental result. γ-Al2O3 possesses a spinel-like structure with cation vacancies.28 Al3+ is located statistically in octahedral as well as tetrahedral sites. At the γ-Al2O3 surface, there are open octahedra and tetrahedra. It is known that e.g. Cu2+ ions may occupy such empty distorted octahedral sites.29,30 Since the ionic radii of Cu2+ and Mo5+ are very similar (77 and 68 pm, respectively), it may be suggested that Mo5+ ions present on the Al2O3 surface (28) Hollemann; Wiberg. Lehrbuch der Anorg. Chem., 81-90 ed.; deGruyter: Berlin, 1976; p 649. (29) Berger, P. A.; Roth, J. F. J. Phys. Chem. 1967, 71, 4307. (30) Dufaux, M.; Che, M.; Naccache, C. J. Chim. Phys. 1970, 67, 527.

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Langmuir, Vol. 12, No. 7, 1996 1823

also occupy such cation sites. During the spreading process the molybdena phase is distributed over the support surface. Together with the spreading MoO3 phase, Mo5+ defects are also distributed over the support. For milled MoO3, the more intimate contact with alumina leads to a more pronounced spreading (see XPS). Therefore, a higher concentration of these Mo5+ centers may be located on the alumina surface (in analogy to the Cu2+ ions), which would result in their stabilization against oxidation. If spreading does not occur or if it is inefficient, like in the case with 10 wt % unmilled MoO3, these centers are not stabilized by the Al2O3 surface and can therefore be reoxidized. Alternatively, the higher Mo5+ defect intensity, observed for the mixture with milled MoO3, may be explained by their mutual paramagnetic interaction. It is reported for supported molybdena catalysts that only a small percentage of all Mo5+ centers are detectable by ESR.31,32 The mechanical activation of MoO3 leads to a more pronounced spreading of the molybdena phase. This must of course result in a better spatial distribution of the Mo5+ defects, which in turn would imply a decreasing paramagnetic interaction between the different centers. This lower paramagnetic interaction increases the relative percentage of detectable Mo5+ ions. The higher signal intensity as compared to that of the mixture with unmilled MoO3 would then reflect its better dispersion on alumina. 3.4. In Situ Raman Characterization of Physical Mixtures of 10 wt % MoO3 in Al2O3. Next the nature of the surface species which are formed under the experimental conditions during the ESR temperature treatments is to be analyzed. It is known1,33-36 that, in addition to the spreading process, MoO3 is transformed into polymeric surface species on alumina in humid oxygen. In situ Raman spectroscopy was applied to follow possible structural changes of the molybdenum oxide during thermal treatments. Physical Mixture with 10 wt % Unmilled MoO3. In Figure 8, in situ Raman spectra are shown of this physical mixture during the spreading experiment. The assignment of bands is following the work of Py et al.37,38 The two spectral windows needed to cover the entire MoO3 spectrum are normalized to the most prominent bands of the untreated sample at 821 and 285 cm-1, respectively. The spectra of Figure 8 show the signals of crystalline MoO3 powder samples. After purging with N2 at 423 K for 8 h and calcination at 723 K in dry O2 for 3 h and in H2O-saturated O2 for 7 h, in spectrum d the bands of crystalline MoO3 are still observed. A close inspection of the fwhm of the most prominent band at 820 cm-1 reveals that broadening of the band, which might indicate a decrease in particle size (see ref 12) and, hence, possible spreading, did not occur. Moreover, this band seems to be a little sharper after the 10 h calcination, thus perhaps indicating some recrystallization (sintering). No sign is observed in the spectra of an amorphous or molecular surface species. The discrepancy with the XPS results which show higher dispersion after the calcination steps (vide supra) may be explained by the assumption that only the smallest MoO3 crystallites in good contact with Al2O3 are spreading, whereas agglomerates of MoO3 (31) Latef, A.; Aissi, C. F.; Guelton, M. J. Catal. 1989, 119, 368. (32) Abdo, S.; Clarkson, R. B.; Hall, W. K. J. Phys. Chem. 1976, 80, 2431. (33) Margraf, R.; Leyrer, J.; Kno¨zinger, H.; Taglauer, E. Surf. Sci. 1987, 189/190, 842. (34) Kno¨zinger, H. Mater. Sci. Forum 1988, 25-26, 223. (35) Leyrer, J.; Mey, D.; Kno¨zinger, H. J. Catal. 1990, 124, 349. (36) Leyrer, J.; Margraf, R.; Taglauer, E.; Kno¨zinger, H. Surf. Sci. 1988, 201, 603. (37) Py, M. A.; Schmid, Ph. E.; Vallin, J. T. Il Nuovo Cimento 1977, 38B, 271. (38) Py, M. A.; Maschke, K. A. Physica 1981, 105B, 376.

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Figure 8. In situ room-temperature Raman spectra of the physical mixture with unmilled MoO3: (a) mixture prior to any treatment; (b) after 8 h in flowing N2 at 423 K; (c) after 3 h in flowing dry O2 at 723 K; (d) after an additional 7 h at 723 K in flowing O2 saturated with H2O. The spectra are normalized to the intensity of the untreated sample; for better visualization, spectra are vertically shifted.

crystallites would sinter to larger particles. Since the Raman scattering efficiency of the remaining crystalline phase is much larger as compared to that of a possible amorphous thin film or polymeric surface species, their Raman spectra would be overwhelmed by that of the crystalline phase. Physical Mixture with 10 wt % Milled MoO3. In Figure 9, Raman spectra of this mixture as recorded during the in situ experiments are shown. The differences between spectra a of Figures 8 and 9 were discussed in ref 12, where the general loss in intensity and the broadening of bands was attributed to the particle size reduction and the generation of lattice defects during the mechanical activation. Spectrum a of Figure 9 (untreated physical mixture) again shows the bands of crystalline MoO3. After purging with N2 at 423 K for 8 h, spectrum b of Figure 9 shows the same bands; however, the band at 470 cm-1 assigned to the Ag and B1g modes of the Mo-O-Mo inplane deformation has changed. It has gained intensity as compared to the band at 378 cm-1 and exhibits a shoulder at 446 cm-1 (see also ref 13). After oxidation at 723 K in dry O2 for 3 h, the spectrum of the untreated physical mixture is reproduced (spectrum c of Figure 9). ESR spectroscopy proved that during the N2 treatment at elevated temperatures the concentration of defects is considerably increased (see section 3.3). Hence, the width and intensity of the band at 470 cm-1 seem to be related to the generation of defects in MoO3 during the hightemperature nitrogen treatment. The Raman and ESR spectroscopic characterization of pure MoO313 also revealed that the spectral variation in the in-plane deformation vibration of the Mo-O-Mo bridges at 470 cm-1 is correlated with the generation of defects in MoO3. After calcination in dry oxygen, there are further differences between spectra c and a of Figure 9. Thus, the band of the stretching mode of the terminal oxygens along the a-axis at about 820 cm-1 is considerably sharper as compared to that of the original material. It was earlier shown that the width of this band is sensitively affected by the crystallinity of the sample.12 Therefore, this

Mestl et al.

Figure 9. In situ room-temperature Raman spectra of the physical mixture with milled MoO3: (a) mixture prior to any treatment; (b) after 8 h in flowing N2 at 423 K; (c) after 3 h in flowing dry O2 at 723 K; (d) after an additional 7 h at 723 K in flowing O2 saturated with H2O. The spectra are normalized to the intensity of the untreated sample; for better visualization, spectra are vertically shifted. The dip at about 1030 cm-1 marked by an asterisk is due to a defective detector diode.

observed narrowing and the variation in the shape of the band at 470 cm-1 indicate healing of defects during the calcination. After calcination at 723 K in O2 + H2O for 7 h, considerable changes in the Raman spectrum of the physical mixture with milled MoO3 (spectrum d of Figure 9) are observed as compared to the spectrum taken after the preceding step. The signal-to-noise ratio is decreased, indicating a loss in Raman intensity. A loss of spectral quality due to defect formation, as observed after the N2 treatment step, must be ruled out. It could be shown previously12 that the particle size reduction during the milling process is correlated with decreasing Raman intensity. Therefore, the decreased signal-to-noise ratio is attributed to a reduction in the mean particle size due to spreading. In addition, the band at 470 cm-1 is again broadened and increased in intensity. However, this spectral change cannot be explained in the same manner as that observed after the N2 treatment (vide supra), since oxygen is obviously not lost from the MoO3 lattice during this calcination. These spectral changes must therefore be ascribed to a particle size reduction during spreading. Furthermore, spectrum d exhibits a weak band at 58 cm-1 which is more intense as compared to that for the rigid chain mode (Ag) in the a-direction at 83 cm-1. Such bands, probably arising from the presence of superstructures, were of considerably lower intensity in pure unmilled MoO3.12 The increase in its intensity may thus be ascribed to a further reduction of the mean particle size. In addition, differences can be found in the bands of the terminal ModO groups after calcination in wet oxygen. Thus, the stretching band of terminal ModO groups along the a-axis, located at 817 cm-1 in spectrum a, is shifted to 826 cm-1 in spectrum d, and the stretching band of the terminal ModO groups along the c-axis, located at 991 cm-1 in spectrum a, is shifted to 1005 cm-1 (spectrum d),

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and a shoulder is observed at about 1014 cm-1. This most probably also indicates changes in the crystallinity of the MoO3 species in contact with Al2O3. For MoO3 single crystals, IR-active modes are reported at 1002 (B3u) and 1010 cm-1 (B2u).39,40 Hence, the observation of these bands may indicate weakening of the spectroscopic exclusion rule due to microcrystallinity leading to the appearance of new bands. An alternative assignment of the band at 1005 cm-1 to Al2(MoO4)239-43 does not seem to be plausible, since this compound is only formed at higher temperatures and should also exhibit bands at 1028 and 830 cm-1 as well as a very intense band at 381 cm-1. Besides the already discussed spectral variations, the increased background between the bands at 826 and 1005 cm-1 (note the dotted base line) may indicate the presence of a whole variety of Mo-oxygen clusters of different sizes. However, the main molybdenum species in this mixture after 7 h of calcination in humid oxygen is poorly crystalline MoO3. This confirms the observation of Polz26 that MoO3 once dispersed on alumina in dry oxygen cannot be completely converted into polymolybdates upon additional calcination in H2O-saturated O2. This experimental result together with the observed high stability of Mo5+ ions against reoxidation can only be explained by the strong interaction of Mo oxides with the alumina surface. During the spreading process via the proposed “unrolling carpet” model, strong bonds are formed between Al2O3 and the thin molybdena overlayer which prevents hydrolysis, formation of polymolybdates, and reoxidation of Mo5+ centers. If the spreading process is carried out in the presence of H2O, probably a different route is followed, in which the molybdenum phase is dispersed over the support via oxyhydroxides. So in the first step the molybdenum phase is hydrolyzed to oxyhydroxymolybdates, which in the second step spread over the surface. 3.5. In Situ High-Temperature Raman Characterization of the Spreading Process in Physical Mixtures with 9 wt % MoO3/Al2O3. High-Temperature Raman Spectra of Pure Unmilled MoO3. In Figure 10, a series of high-temperature Raman spectra of unmilled MoO3 are shown that were recorded in the temperature range between 298 and 1053 K in a static air atmosphere. As in the previous section, two spectral windows had to be recorded to cover the entire Raman spectrum of MoO3. The intensities of the respective spectra were normalized to the most intense signal in each window (band at 281 and 818 cm-1, respectively). The signals in the lower frequency range, therefore, are too intense with respect to the vibrations of the terminal ModO groups. Both spectral windows are overlapping at 550 cm-1. The room-temperature Raman spectrum (Figure 10) shows the bands of crystalline MoO337,38 at 80, 94, 112, 125, 152, 195, 215, 241, 281, 290 (sh), 334, 365, 375, 468, 664, 818, and 995 cm-1. Until a temperature of 873 K, no changes are observed in the Raman spectra, except a certain thermally induced line broadening. At 898 K a weak band at 940 cm-1 is detected for the first time. At temperatures g973 K, this signal gains intensity, while the background scattering in the spectra seems to drastically increase. At 1043 K an additional new band is observed at 880 cm-1, and simultaneously a shoulder develops at 600 cm-1. At this temperature the bands of (39) Spevack, P. A.; McIntyre, N. S. J. Chem. Phys. 1993, 97, 11020. (40) Bartlett, J. R.; Cooney, R. P. In Spectroscopy of Inorganic-based Materials; Clark, R. J. M., Hester, R. E., Eds.; J. Wiley & Sons: New York, 1987; p 187. (41) Stencel, J. M.; Makowsky, L. E.; Sarkus, T. A.; De Vries, J.; Thomas, R.; Mouljin, J. A. J. Catal. 1984, 90, 314. (42) Zingg, D. S.; Makowsky, L. E.; Tischer, R. E.; Brown, F. R.; Hercules, D. M. J. Phys. Chem. 1980, 84, 2898. (43) Cheng, C. P.; Schrader, G. J. Catal. 1979, 60, 276.

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Figure 10. In situ high-temperature Raman spectra of pure unmilled MoO3 powder in static air. The spectra are normalized to the band at 820 cm-1 of the room-temperature spectrum. For better visualization the spectra are shifted vertically. The dip at about 530 cm-1 marked with an asterisk is due to a defective detector diode.

MoO3 are detected at 77, 109, 122, 140, 196, 232, 277, 333, 371, 460, 659, 818, and 991 cm-1; thus, almost every band is shifted by 3-4 cm-1 to lower frequencies due to a general temperature effect caused by anharmonicity. While a few modes of MoO3 hardly show any temperature effect, especially the transverse mode (Ag, B1g; 152 cm-1) of the rigid chain in the b-direction is shifted by 11 cm-1 to 141 cm-1. According to the theoretical work of Py and Maschke,38 this shift can be explained by a decreasing interaction between the MoO3 layers. The most prominent change in the high-temperature Raman spectra is observed at 1053 K. The bands of crystalline MoO3 are completely lost. Thus, this Raman spectrum indicates the melting of MoO3 (reported melting point 1068 K). Bands can be identified at 352, ca. 820, 880, and 933 cm-1. For a better comparison, the observed Raman frequencies are summarized in Table 1. Only the band at 820 cm-1, the most prominent of crystalline MoO3, might still point toward the presence of a residual solid phase. In the gas phase above MoO3 melts, a whole series of MosO molecules have been detected44 and identified as (MoO3)3, (MoO3)4, and (MoO3)5.45 Hewitt et al.46 recorded the IR absorption of the gas phase above solid MoO3 at the nominal 1220 K and found bands at 815 and 965 cm-1. The latter band was assigned to a stretching vibration of ModO groups (44) Berkowitz, J.; Inghram, M. G.; Chupka, W. A. J. Chem. Phys. 1957, 32, 842. (45) Iorns, T. V.; Stafford, F. E. J. Am. Chem. Soc. 1966, 88, 4819. (46) Hewitt, W. D., Jr.; Newton, J. H.; Weltner, W., Jr. J. Phys. Chem. 1975, 79, 2640.

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Table 1. High-Temperature Raman Spectra of Unmilled MoO3 T ) 290 K

T ) 1043 K

80 cm-1 94 cm-1 112 cm-1 125 cm-1 152 cm-1 195 cm-1 215 cm-1 241 cm-1 281 cm-1 290 (sh) cm-1 334 cm-1 365 cm-1 375 cm-1 468 cm-1 664 cm-1 818 cm-1

77 cm-1

995 cm-1

T ) 1053 K

109 cm-1 122 cm-1 140 cm-1 196 cm-1 232 cm-1 277 cm-1 333 cm-1 352 cm-1 371 cm-1 460 cm-1 59 cm-1 818 cm-1 880 cm-1 940 cm-1 991 cm-1

820 cm-1 880 cm-1 933 cm-1

while the signal at 815 cm-1 was attributed to a ring vibration. The IR spectra of (MoO3)3 in Ar and Ne matrices were reported to show bands at 840 (s), 858 (vs), and 978 (m) cm-1.46 Other bands were attributed to MoO2 molecules (899 and 948 cm-1), MoO3 molecules (922 and 976 cm-1), (MoO3)2 molecules (350, 694, and 975 cm-1), and (MoO3)4 or (MoO3)5 molecules (514, 866, 884, 896, and 973 cm-1). An additional band observed at 987 cm-1 was ascribed to the existence of other polymeric species, since in crystalline MoO3 a comparable band is found at 989 cm-1.46 Mohan and Ravikumar47 recorded the Raman spectrum of the MoO3 molecule and found bands at 282 (s) (OsMosO symmetric bending), 335 (m) (OsMosO antisymmetric bending), 814 cm-1 (vs) (MosO symmetric stretch), and 993 (vs) cm-1 (antisymmetric stretch). The strong similarity of the bands reported by Mohan and Ravikumar and those of crystalline MoO3 and the missing bands at 282 and 993 cm-1 rule out a possible assignment of the present spectra to monomeric MoO3 molecules. Becher48 recorded the Raman spectra of molten K2Mo2O7 and observed only four bands in the range between 100 and 1000 cm-1, two bands in the stretching vibrational regime at 930 and 880 cm-1 and two bands assigned to deformation modes at 330 and 200 cm-1, which were attributed to Mo2O72- molecules. To our knowledge, a Raman spectroscopic investigation of molten MoO3 was not carried out up to date. Thus, an assignment of the observed bands is attempted using the mentioned literature data. The experimentally observed Raman spectrum has a certain similarity with the spectrum of molten K2Mo2O7. However, the strong bands of the latter at 200 and 330 cm-1 were not detected in the spectrum at 1053 K. Some coincidences are also found with the IR absorptions of polymeric (MoO3)x, and the spectrum also resembles to a certain extent the Raman spectrum of ammonium heptamolybdate in aqueous solution.49 Although a definite assignment of the observed bands is not possible, the Raman spectrum at 1053 K suggests the presence of oligomeric and polymeric molybdenum oxygen compounds in the melt. The band at 820 cm-1 most probably arises from residual solid MoO3. Calcination of the Unmilled Physical Mixture of 9 wt % MoO3 and Al2O3 in O2. In Figure 11, Raman spectra of the physical mixture of 9 wt % unmilled MoO3 with Al2O3 which were normalized to the laser plasma lines at 1057 (47) Mohan, S.; Ravikumar, K. G. Curr. Sci. 1984, 53(a), 471. (48) Becher, H. J. J. Chem. Res. 1980, 1053. (49) Tytko, K.-H.; Scho¨nfeld, B. Z. Naturforsch. 1975, 30b, 471.

Figure 11. In situ high-temperature SMT Raman spectra of the unmilled physical mixture containing 9 wt % MoO3: (a) prior to any treatment at 298 K; (b) at 723 K in dry O2 after heating at 423 K in dry O2 for 15 h; (c) at 823 K in dry O2; (d) after quenching to 298 K in dry O2. For better visualization the spectra are vertically shifted. The Raman intensity is normalized to the plasma lines (*) at 1057 cm-1.

cm-1 are shown. Clearly the decreasing scattering efficiency can be recognized between the SMT spectrum recorded at 298 K (Figure 11a) and the one recorded at 723 K (Figure 11b). The SMT Raman spectrum recorded at 823 K (Figure 11c) exhibits (besides the plasma lines which are marked by an asterisk) only a weak broad band at ca. 1000 cm-1. Quenching the thus treated physical mixture to room temperature within 1 min (Figure 11d) leads to the development of weak broad bands at 708, 820, and 1009 cm-1. In Figure 12, the SMT Raman spectra of the physical mixture which were recorded at 823 K (spectrum a) and after quenching to 298 K (spectrum b) are displayed at an expanded intensity scale, and the observed Raman bands are summarized in Table 2 for a better comparison. In the SMT spectrum (a) recorded at 823 K bands can be identified at 275, 472 (sh), 628 (sh), 839, and 998 cm-1. After quenching to 298 K, bands are recognized in the SMT spectrum (b) at 228 (sh), 288, 324 (sh), 487, 609 (sh), 706, 825, 869 (sh), and 1009 cm-1. A detailed assignment of these bands seems to be not possible; however, the structures in spectrum b are sharpened due to the quenching and a few changes can be noticed: (i) The broad band at 275 cm-1 in spectrum a seems to be split into four bands or shoulders at 228 (sh), 288, 324 (sh), and 375 (sh). (ii) The broad shoulder at 472 cm-1 (spectrum a) is found as a broad resolved band at 487 cm-1. (iii) In the frequency regime above 600 cm-1 a new signal is formed at 706 cm-1, while the broad band at 839 cm-1 seems to split into a signal at 825 cm-1 and a shoulder at 869 cm-1. Highly crystalline MoO3 has bands in the frequency range between 200 and 1000 cm-1 at 217, 244, 289, 335, 366 (sh), 376, 469, 663, 820, and 995 cm-1. Quenching to room temperature seems to increase the similarity of the Raman spectrum of this physical mixture to that of crystalline MoO3. Spevack and McIntyre39 recorded the Raman spectra of thin MoO3 films and observed for films with thicknesses smaller than 14 nm broad bands between 1000 and 600 cm-1 and at 450 and 200 cm-1 which were not

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assigned. The oxygen deficient Mo18O52 (MoO3 structure) shows bands in the IR spectrum at 650, 730, 830, 952, 975, 985, 992, and 1010 cm-1.50 MoO3 which was heated in vacuum at 648 K exhibits new bands at 724 (vw), 888, and 954 (vw) cm-1,19 which were assigned to defect structures on the basis of the results of reoxidation experiments. The ammonium heptamolybdate phase impreganted onto silica is not stable against thermal treatment and forms MoO3. In the Raman spectrum of such a sample bands were observed at 781 and 851 cm-1 which were generated due to laser heating.14 Py and Maschke,38 in their theoretical study, observed a decrease in the vibrational frequencies with decreasing interaction of the structure-forming layers and chains. Thus, the stretching vibration of the terminal ModO groups of an isolated chain should be found at ca. 760 cm-1. The similarity of spectrum b with that of crystalline MoO3 and the detected additional bands, therefore, suggests the detection of strongly disordered MoO3, which may be interpreted as a glassy surface film. Calcination of the Milled Physical Mixture of 9 wt % MoO3 and Al2O3. In Figure 13, the in situ SMT Raman spectra of the milled physical mixture are reproduced, and the observed Raman frequencies are summarized in Table 2 for a better comparison. After heating at 423 K in dry O2 for 15 h, the temperature was increased to 723 K and spectrum a, showing the bands of crystalline MoO3, was recorded. At 813 K (spectrum b) and 823 K (spectrum c) all bands are broadened considerably. The main difference between spectra b and c of Figure 13 is the splitting of the broad band at 282 cm-1 at lower temperature into three signals or shoulders at 228, 282, and 328 cm-1. These spectral changes may be interpreted as a beginning melting process, since only a small increase in temperature of 10 K leads to the observed loss in resolution.

Quenching this mixture to 298 K, however, does not lead to a Raman spectrum (spectrum d in Figure 13) comparable to spectrum b in Figure 12. Now the question arises why quenching in this case does not lead to major spectral changes. The only difference between the two physical mixtures is found in the milling process. It could be shown by XPS (section 3.2) that mechanically activated MoO3 is spreading more efficiently across the Al2O3 surface as compared to unmilled MoO3. Therefore, one may suggest that the differences in the Raman spectra of the quenched physical mixtures arise from the more efficient spreading process in the milled sample. Due to the more highly dispersed MoO3 phase in the milled physical mixture, only smaller agglomerates or thinner films of the amorphous glassy surface species can be formed. Thus, the Raman spectrum of the quenched sample has to resemble more the one of the suggested surface melt than the one of crystalline MoO3. The differences observed between SMT spectra d and e of Figure 13, furthermore, suggest that this surface species is highly reactive and most probably strongly disordered. Spectrum e of Figure 13 was recorded after the quenching step and after hydration at 298 K for 2 h in O2 saturated with H2O at room temperature. The spectrum shows bands at 218, 286 (w), 355, 566 (w), 820, and 952 cm-1. The appearance of the Raman spectrum in the stretching vibration regime is completely altered: (i) The stretching vibration of the ModO group with the shortest ModO distance is shifted from 1006 to 952 cm-1 and asymmetrically broadened to lower frequencies presumably by contributions from the band at 863 cm-1 seen in spectrum d. (ii) A relatively sharp band becomes visible at 820 cm-1. In addition, changes in the Raman spectra are visible in the range of the deformation modes: Thus, the broad band centered at 282 cm-1 is split into three bands at 218, 286, and 355 cm-1 after hydration. Wachs and co-workers reported the influence of the degree of hydration upon the Raman spectra of impregnated Mo catalysts.51,52 In the hydrated state, bands at 850 and 975 cm-1 were observed which were shifting upon the loss of water to 872 and 1012 cm-1, respectively. These shifts were attributed to additional coordination of H2O and/or the formation of H bonds to ModO groups. A similar effect must explain the observed shifts between Raman spectra d and e of Figure 13. In addition, the Raman spectrum obtained after hydration (Figure 13e) resembles very much those Raman spectra which were recorded after calcining both physical mixtures in humid O2 (spectra not shown), which also exhibited bands at 255, 350, and 961 cm-1. It is known36 that calcination in humid oxygen leads to the formation of polymeric surface molybdates. Therefore, Raman spectrum e of Figure 13 is attributed to such polymeric species. Surprising, however, is the short period of time and the low temperature which are needed to lead to the formation of these polymeric species. Leyrer needed 30 h of calcination at 720 K in humid O2 to observe complete transformation, while in this experiment this transition seems to have terminated already after 2 h at room temperature. The reasons for this behavior must be found in the higher temperature during the calcination and in the milling process. Both reasons lead to a more highly dispersed, amorphous Mo phase which is more reactive toward the reaction with water as compared to the spread but crystalline MoO3 surface phase after calcination at 720 K. Kno¨zinger and co-workers1,35,36 and Polz26 observed only the bands of crystalline MoO3 after calcination in dry O2 at 723 K. In the in situ Raman spectra (Figures 8 and 9)

(50) Cariati, F.; Bart, J. C. J.; Sgamelotti, A. Inorg. Chim. Acta 1981, 48, 97.

(51) Vuurmann, M. A.; Wachs, I. E. J. Phys. Chem. 1992, 96, 5008. (52) Hardcastle, F. D.; Wachs, I. E. J. Raman Spectrosc. 1990, 21, 683.

Figure 12. In situ high-temperature SMT Raman spectra of the unmilled physical mixture containing 9 wt % MoO3 after subtraction of the plasma lines and of the Rayleigh wing: (a) at 823 K in dry O2; (b) after quenching to 298 K in dry O2. For better visualization the spectra are vertically shifted. Solid lines, smoothed spectra; dots, experimentally recorded spectra.

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Mestl et al.

Table 2. In Situ Raman Bands of Physical Mixtures of Alumina and 10 wt % Molybdenum Oxide at Different Temperatures unmilled MoO3 T ) 823 K

milled MoO3

T ) 298 K cm-1

275 cm-1 472 cm-1 628 (sh) cm-1 839 cm-1 998 cm-1

228 (sh) 288 cm-1 324 (sh) cm-1 375 (sh) cm-1 487 cm-1 609 (sh) cm-1 706 cm-1 825 cm-1 869 (sh) cm-1 1009 cm-1

T ) 723 K cm-1

217 244 cm-1 289 cm-1 335 cm-1 366 (sh) cm-1 376 cm-1 469 cm-1 663 cm-1 820 cm-1 995 cm-1

T ) 813 K 228

T ) 298 K

cm-1

282 cm-1 328 cm-1

218 cm-1 ∼300 cm-1

∼820 cm-1 997 cm-1

T ) 298 K/H2O

863 cm-1 1006 cm-1

286 (sh) cm-1 355 cm-1

566 cm-1 820 cm-1 952 cm-1

Figure 13. In situ high-temperature SMT Raman spectra of the milled physical mixture (3 h) containing 9 wt % MoO3 after subtraction of the plasma lines and of the Rayleigh wing: (a) at 723 K in dry O2 after heating at 423 K in dry O2 for 15 h; (b) at 813 K in dry O2; (c) at 823 K in dry O2; (d) after quenching to 298 K in dry O2; (e) after rehydration in O2/H2O at 298 K for 2 h. For better visualization the spectra are vertically shifted. The Raman intensity is normalized to the line of crystalline MoO3 at 997 cm-1. Solid lines, smoothed spectra; dots, experimentally recorded spectra.

Figure 14. In situ high-temperature SMT Raman spectra of the milled physical mixture (3 h) containing 9 wt % MoO3 after subtraction of the plasma lines: (a) at 723 K in dry O2 after heating at 423 K in dry O2 for 4.5 h and at 723 K for 64 h; (b) at 298 K after heating at 423 K in dry O2 for 4.5 h and at 723 K for 64 h and slow cooling to 298 K; (c) after additional heating to 823 K in dry O2; (d) after quenching to 298 K in dry O2. For better visualization the spectra are vertically shifted. The Raman intensity is normalized to the band of crystalline MoO3 at 997 cm-1.

during the ESR experiments, these findings were reproduced. In the series of experiments reported in this paragraph (Figures 12 and 13), however, bands of an amorphous, polymeric surface species were detected in situ at elevated temperatures as well as after quenching to room temperature. The reason for these discrepancies may be found in the rate of the cooling process or in the efficiency of the spreading process at different temperatures. Slow cooling in the furnaces1,26,33-36 may probably lead to the recrystallization of MoO3, while quenching (1 min) may lead to the “freezing” of the surface melt to form an amorphous glassy layer. In order to prove this possible explanation, a spreading experiment for an extended time period under dry O2 was carried out under experimental conditions analogous to those used by Kno¨zinger and coworkers and by Polz. In Figure 14, the most important Raman spectra recorded during this long term experiment are shown. The Raman spectrum (not shown) recorded after heating

in dry O2 at 423 K for 4.5 h and calcining in dry O2 at 723 K for 64 h does exclusively exhibit the bands of crystalline MoO3. As in the experiments of Kno¨zinger and coworkers1,33-36 and of Polz,26 calcination at 723 K does not lead to detectable changes in the Raman spectra of the surface Mo phase. In the in situ high-temperature SMT Raman spectrum (Figure 14a), recorded at 723 K, additional bands are detected at 705, 903, and 952 cm-1. The positions of these bands very much resemble those observed for polymeric molybdates. Possibly these bands arise from the polymeric MosO species on the support surface which are formed during the spreading process at elevated temperatures. After quenching to room temperature, the SMT Raman (Figure 14b) spectrum only exhibits the bands of crystalline MoO3; the weak signals observed in spectrum a of Figure 14 are not detected any more. However, in spectrum b of Figure 14, an increased background between 820 and 995 cm-1 (note the dotted baseline) and a well resolved shoulder at 1011 cm-1 are

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recorded. This high background and the high-frequency shoulder most probably arise from a multitude of different ModO vibrations in this frequency regime and, therefore, from a multitude of different amorphous MosO species on the surface. Since the Raman spectra reported by Kno¨zinger and co-workers and by Polz were recorded ex situ at room temperature and after readsorption of water using the conventional multichannel technique, this background and the additional bands could never be detected. This in situ spreading experiment for extended time periods seems to indicate that the Mo phase is spreading over the support at 723 K; the Raman spectra, however, show that crystalline MoO3 is not transformed completely to an amorphous surface species at this temperature. If the temperature is increased to 823 K, again a Raman spectrum (Figure 14c, recorded at 823 K) is observed which is comparable to the high-temperature spectra of the preceeding series (Figures 12 and 13): a broad band at about 830 cm-1 and a sharp signal at about 1002 cm-1 are detected, which are, therefore, attributed to polymeric Mo-O surface species. Cooling the sample slowly to room temperature (spectrum d, Figure 14), however, does not lead to the detection of the bands of crystalline MoO3 and, therefore, proves that slow cooling to room temperature after calcination at 823 K does not lead to recrystallization of MoO3. This apparently contradictory observation can only be explained under the assumption that a molten MoO3 phase forms a thin film on the support but not small droplets. Only in the latter case would slow cooling result in recrystallization of MoO3. On the other hand, this observation proves that the spreading process at temperatures higher than 723 K is more efficient. One may argue now that at this high temperature a gas phase transport of MoO3 over the support surface is very probable. However, Leyrer et al.1 could show using a twobed reactor (the lower bed filled with MoO3, the upper one with Al2O3, upward gas stream) that no MoO3 could be detected in the alumina phase after calcination at 820 K in dry oxygen after 96 h. Gas phase transport thus does not occur under these conditions. The variations in the Raman spectra reported in this series of experiments which were observed after very short periods of time, therefore, can only be explained by the assumption of a melting surface MoO3 phase at temperatures above 800 K and by an increased efficiency of the spreading at this temperature. 4. Conclusions SEM, EDX, and TEM analyses show that the physical contact between Al2O3 and milled MoO3 is more intimate as compared to that of the mixture with unmilled MoO3. After calcination some particle growth was observed for both samples and the particles exhibited a different morphology, suggesting the possibility of surface melting during the heat treatment. The shift of the Mo 3d binding energies, the loss of resolution of the Mo 3d doublet, the decreasing fwhm of the Al 2s and O 1s peaks, and the increasing Mo 3d/Al 2s signal intensity ratio in the XP spectra of both samples are taken as evidence for spreading of MoO3 across the Al2O3 surface. However, spreading in the mixture with milled MoO3 is more efficient. After purging the physical mixtures with N2 at 423 K for 8 h, ESR spectroscopy shows that Mo5+ is present mainly in sixfold coordination in C4v symmetry. In addition, in the physical mixture unmilled MoO3 contains

Langmuir, Vol. 12, No. 7, 1996 1829

fivefold-coordinated Mo5+ defects. After 10 h of calcination (3 h dry and 7 h humid O2), all Mo5+ centers are reoxidized. After purging the physical mixture containing milled MoO3 with N2 at 423 K for 8 h, a small part of the Mo5+ centers are in C2v symmetry and in fourfold tetrahedral coordination. Only the Mo5+ species in C2v distorted sixfold coordination is stable against oxygen for several hours, independent of the presence or absence of water vapor. Here, the physical mixture with milled MoO3 behaves completely differently than that with unmilled MoO3. The high stability of this defect species against reoxidation is attributed to a stabilization by the Al2O3 support. Raman spectra recorded during these ESR experiments of the physical mixture with unmilled MoO3 do not give any indication for decreasing particle sizes or for the formation of new surface species during calcination. The Raman spectra of the mixture with milled MoO3 exhibit certain changes after each treatment step. Thus, after the treatment in N2, spectral changes are interpreted as being induced by the formation of defects in MoO3 due to the partial loss of oxygen. These spectral changes vanished during the calcination in O2. In addition, the narrowing of bands points to some gain in crystallinity (healing of defects). After treatment in wet oxygen, the spectrum is considerably changed. The loss in the spectral quality in this case was explained by spreading of the MoO3 phase, which is indicated by the upward shift of the most prominent bands and the detection of new Raman bands and shoulders. The presence of an enhanced background in the frequency range between 820 and 1100 cm-1 is interpreted as an indication of the presence of smaller clusters or amorphous Mo-oxygen surface species. In situ high-temperature Raman spectra of pure unmilled MoO3 reveal that this oxide is transformed into an oligomeric Mo-O species above temperatures of 948 K. At 1053 K melting seems to occur and the Raman bands of crystalline MoO3 are lost. Calcination of an unmilled physical mixture of 9 wt % MoO3 and Al2O3 at 823 K in dry O2 leads to the observation of Raman bands which are attributed to an amorphous polymeric Mo-O surface melt. Quenching this sample to room temperature results in a Raman spectrum which has a certain similarity with that of crystalline MoO3 and, therefore, is attributed to a glassy surface Mo phase. Calcination of the physical mixture milled for 3 h at 823 K again results in the Raman spectrum of the surface melt. Quenching to 298 K, however, does not lead to a considerable change of the spectrum. This observation is explained by the assumption of a more efficient spreading in mechanically activated mixtures. This results in a thin Mo-O film during the heat treatment which remains amorphous upon quenching to 298 K. The surface Mo phase generated during this treatment is highly reactive toward H2O at 298 K, which leads to the formation of a polymeric surface species comparable to heptamolybdateimpregnated catalysts. A long term spreading experiment at 723 K reveals that the spreading process is considerably slower and less efficient at this lower temperature. In addition, this series of spectra indicated that surface melting of the MoO3 phase is probably one step in the spreading across the support surface. Acknowledgment. This work was financially supported by the Deutsche Forschungsgemeinschaft (SFB 338) and by the Fonds der Chemischen Industrie. LA950864B

Mechanically Activated MoO3. 4. In Situ Characterization

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