J. Phys. Chem. 1994, 98, 11269-11275
11269
Oxygen-Exchange Properties of Mo03: An in Situ Raman Spectroscopy Study G . Mestl: P. Ruiz,' B. Delmon,' and H. Kniizinger"3t Institut fur Physihlische Chemie, Universitat Miinchen, Sophienstrasse 1I , 80333 Miinchen, Germany, and Unit6 de Catalyse et Chimie des Materieux Divists, Universiti Catholique de Louvain, 1 Place Croix du Sud, 1348 Louvain-la-Neuve, Belgium Received: January 21, 1994; In Final Form: June 13, 1994@
In situ Raman spectroscopy was used to investigate the oxygen-exchange properties of Moo3 and the role of bulk defects during redox processes. A change of the absorption coefficients between Moo3 and molybdenum suboxides as evidenced by a change in color from pale yellow to gray blue may contribute to a general decrease in Raman intensities. An alteration of the Raman scattering tensor of MOOS-, is indicated by additional Raman bands and by changes in signal intensity ratios compared to stoichiometric Mo03. This different scattering tensor may also contribute to the lower Raman efficiency of Mo suboxides. Furthermore, besides temperature-induced broadening and signal shifts, Raman spectra of Moo3 seem to be sensitive to the crystallite size due to sintering and crystal growth. Calcination in 1 8 0 2 after evacuation at 393 K (sufficient to remove physically adsorbed H 2 0 and C 0 2 from the surface) does not result in any detectable spectral changes due to l80exchange. On the other hand, after evacuation at 648 K leading to a higher dzgree of reduction, calcination in 1 8 0 2 gives rise to bands due to an l80incorporation into Moos. Therefore, the intensity ratio of the l80-related bands relative to those of the l80-free stoichiometric oxide may correlate with the initial bulk defect concentration in oxygen-deficient Moos. The long time required for the l60reexchange of Moo3 when fully reoxidized in 1802, constitutes a proof of the essential role played by defects in the anion vacancy conductor Moo3 during the exchange process. H20 present in the gas phase has no detectable influence on the l60reexchange in Moo3.
Introduction Moo3 and polymolybdates are technically highly important compounds in catalysis, both as catalysts or as catalyst precursors. Supported molybdenum sulfides are the major components in most catalysts in the hydrotreating process of crude oil (hydrodesulfurization', hydrodenitrogenation,2 and hydrodemetalation3). Moo3 supported on Ti02 is a selective catalyst in photooxidation4and additionally active for selective catalytic reduction (SCR) of NO, with NH3.5 Furthermore, molybdenum oxides are active in oxidative coupling reactions,6 and together with group VIa oxides they are used in selective oxidation c a t a l y s i ~ . ~ - lEspecially ~ in the case of selective oxidation catalysis, synergistic effects occur between individual phases present in the catalyst. It has been postulated that their enhanced activity and/or selectivity could be explained by the occurrence of oxygen spillover proce~ses.'~-'~ It is well established by isotopic labeling that in the case of selective oxidation, "lattice oxygen" may take part in the reacti~n.'~-*~ In all the abovementioned catalytic processes, lattice defects play an important role either during the activation of the catalyst or in elementary catalytic steps. In situ laser Raman spectroscopy in conjunction with isotopic labeling has recently been used successfully to investigate selective catalytic oxidation systems. For example, Glaeser et a1.21and Heufs et aLZ2have used labeled hydrocarbon molecules and oxygen, respectively, as probes for reaction sites on bismuth molybdate catalysts. Matsuura et al. identified the sites which take part in lattice oxygen insertion over bismuth molybdate.23 Schrader et aLZ4and Lashier et aLZ5investigated a V-P-0 t
Universiat Miinchen.
* Universitk Catholique de Louvain.
* To whom correspondence should be addressed. @Abstractpublished in Advance ACS Abstracts, October 1, 1994.
catalyst using isotopically labeled oxygen, and Ozkan et a1.26 reported an isotopic labeling study of a two-phase MnMoOJ Moo3 catalyst. Phase characterizations mainly by X-ray diffraction techniques have been reported frequently for the molybdenumoxygen ~ y s t e m . ~Removal ~ - ~ ~ of oxygen from MOO3 initially results in Mo-Mo bonding across the resulting oxygen vacancy,# but on further reduction these vacancies order into extended shear defects.45 These defects are stabilized because of extended relaxations that are possible by cation displacement from the center of symmetry to an i n t e r ~ t i c e . They ~ ~ form readily at modest temperatures (ca. 350 K) because oxygen vacancies are mobile.45 As the density of extended defects increases, they interact to form ordered crystallographic shear structures of the M003-type.~~-~l Still more deeply reduced suboxides are stabilized in columnar or block structures of Re03type,32-43and finally Moo2 of the rutile type is formed. Solid-state Raman spectroscopy is sensitive to the metalligand distances, the coordination number and the symmetry around metal centers. It additionally supplies information on the symmetry at the Brillouin zone center. Hence, it constitutes in principle an important tool to unravel details about local and cooperative changes during the redox processes involving defect structures in MoO3-,. This ability of Raman spectroscopy together with the known role of lattice o ~ y g e n , ' ~probably -~~ located at those defect structures, should make it particularly suited to study the oxygen exchange properties of molybdena. Provided the experimental procedure is properly designed, one may gain information on whether and how oxygen defects in this compound affect the oxygen-exchange behavior. In selective oxidation, water is always formed. In industrial processes, water is added to the feed in many cases. Thanks to Raman spectroscopy the role of gas-phase H20 in these redox reactions, still not being well investigated, may be clarified.
0022-365419412098-11269$04.50/0 0 1994 American Chemical Society
Mestl et al.
11270 J. Phys. Chem., Vol. 98, No. 44, 1994
This contribution, with the objective described above, is the first of a series of three papers in which we ultimately address the question of synergy and oxygen spillover in mixtures of Moo3 with antimony oxides.
Experimental Section Moo3 was obtained by thermal decomposition of (NH4)6Mo7024-4H20 (Merck, pure grade with Fe < 0.0005%, Pb < 0.001%, and Cu < 0.001%)in air at 773 K for 20 h. 1 8 0 2 used in the exchange experiments after evacuation at 393 K was supplied by Yeda, Research and Development Co., Ltd., Rehovot, Israel, in a 20 mL sealed glass vial with an isotopic purity of 99.91 at %. The 1 8 0 2 used in the exchange experiments after evacuation at 648 K was supplied by Medgenix GmbH, Ratingen, Germany, in a stainless steel cylinder at 13 bar and with an isotopic purity of 99.5 at. %. For the in situ treatment an in situ Raman cell described elsewhere47 was used which allows to cover the entire temperature range between 90 and 773 K. The cell can also be evacuated to Pa, and all kinds of gas atmospheres can be applied. Six samples can be simultaneously mounted in the cell. All Raman spectra were recorded with a triple-monochromator spectrometer (OMARS 89, Dilor) operated in the subtractive mode for stray light reduction and to cut off the exciting laser line in the very low wavenumber regime. The entrance slit widths were set at 150 pm and occasionally 50 p m (lowtemperature spectra) which correspond to a spectral resolution of 5 and 2.5 cm-', respectively. The spectrometer is equipped with an optical multichannel diode array of 512 diodes (Spectroscopy Instruments) which is cooled to 250 K to reduce thermal noise. The stepping motor controller, the detector controller, and the software are from Spectroscopy Instruments. All Raman spectra were recorded in the backscattering geometry, since this arrangement reduces a possible thermal deadjustment to a single focal plane as compared to the conventional 90" scattering geometry, where two focal planes must be controlled. In reference experiments in 1 6 0 2 and for the l80exchange after evacuation at 393 K, the 514.5 nm line of an argon ion laser (Spectra Physics, Type 2020) with a laser power of 10 mW at the sample position was used to excite Raman scattering. The Raman spectra during the reference experiments in 1 6 0 2 were recorded with the conventional multichannel technique where the different spectral windows which cover the entire Raman spectrum are recorded sequentially. Raman spectra recorded with this technique are convoluted by the diode array characteristics, the spectral sensitivity of the detector and the spectrometer function. The characterization of the starting material as well as the Raman spectra recorded during the in situ oxygen exchange experiments after evacuation at 648 K (reoxidation in l8Oz, reexchange in dry and wet l602) were carried out using the 487.98 nm line of the Ar+-ion laser with a laser power of about 50 mW at the sample position. These spectra were recorded with the new scanning multichannel technique (SMT) where the monochromators are moved stepwise while recording the This allows continuous recording over the entire spectral range and leads to Raman spectra averaged for the diode array characteristics. A determination of the spectrometer function and the detector sensitivity leads to the possibility to correct the Raman spectra for these parameters. Thus, the physically true band positions, line shapes, and signal intensities are obtained. In some cases, when a fluorescence background was convoluting the Raman spectra, the spectra were corrected by subtracting a polynomial fitted to the background.
All in situ experiments were carried out under exclusion of COz and water except for those specially designed to investigate the role of water. This procedure should enable one to cope with the objection of Patterson?l who stressed the possible role of HzO and COS as gas phase transfer agents in the spillover mechanism. Two series of l80exchange experiments were carried out. In both cases the sample was treated identically to reference experiments in l 6 0 2 , in order to show which Raman bands of the compound under investigation would respond to the exchange. In the first series, the sample was evacuated at 393 K to 10-1 Pa for 14 h using a rotary pump, in order to just remove physically adsorbed H2O and COz, thus leading to coordinatively unsaturated (cus) sites only at the crystallite surfaces but not producing a reduced sample with a high density of bulk oxygen vacancies. Strongly adsorbed carbonates were not detectable after this treatment. In this series, half of the theoretical amount of 1 8 0 2 for a total oxygen exchange was provided in the in situ cell. In the second series of experiments, evacuation at 648 K to IO-' Pa for 10 h using a rotary pump led to a more pronounced oxygen deficiency in the sample under investigation, thus resulting in a specimen with a higher density of bulk anion vacancies. This series should reveal a possible role played by bulk defect sites in the l80exchange and reoxidation. Following the l8O treatments, reexchange with 1 6 0 2 was studied so as to obtain information on the time scale of oxygenexchange in the now fully oxidized material as compared to the oxidation in l 8 0 z in the previous step. Subsequently, an exchange experiment was carried out in 1 6 0 2 with a water partial pressure of 26 hPa using a saturator at 300 K to elucidate the possible role of H20 for the oxygen exchange in this compound. In an additional series of experiments, the behavior of Moo3 was tested in 1 6 0 2 under the conditions of the l 8 0 exchange experiments with the aim to detect any possible effects whose origins might be other than l 8 0 exchange, such as line broadening due to high temperature, shifting and splitting of bands due to changes in lattice symmetry, or appearance of new peaks due to a possible formation of compounds, etc. The effect of 1 6 0 2 on the samples was tested at different conditions, namely, at 573 and 737 K, and after evacuation at 373 and 648 K.
Results and Discussion
Raman Spectrum of Polycrystalline MOOS.Earlier workers interpreted the complicated layer structure of Moos as being built up by rather distorted Moo6 octahedra which are interconnected in one direction (c axis) by common edges and corners so as to form zigzag rows which are linked to each other via common comers in a perpendicular direction (a An alternative description is based on molybdenum atoms in that structure having a tendency toward 5-fold c ~ o r d i n a t i o n . ~ ~ Kihlborg's more recent results based on a more accurate X-ray analysis55indicate a rather different picture. Moo3 crystallizes in the space group Pbnm-D:; with four formula units in the unit cell of dimensions 396.28 pm (a axis), 1385.5 pm ( b axis), and 369.64 pm (c axis). The orthorhombic unit cell extends over one layer and two half-layers above and below the latter. The Moo3 layers are separated by a van der Waals gap of about 700 pm. All the atoms in the unit cell are situated on mirror planes of that space group and accordingly occupy the 4-fold point positions. The molybdenum atoms in Moo3 have four close neighbor oxygen atoms at distances ranging from 167 to 195 pm, while the distances of the remaining two oxygen atoms which would complete the distorted octahedron are as long as
Oxygen-Exchange Properties of Moo3
J. Phys. Chem., Vol. 98, No. 44, 1994 11271
167 pm
'7 225 pm
'L
195 pm
Figure 1. Coordination around the Mo centers in MOO^^^ and section of the layered Moos structure: chains of Moo4 tetrahedra along the c axis forming half-layers in the ac plane. Two half-layers build up one Moos layer which are stapled along the b axis. N 100
,
,
,
,
300
,
,
,
,
500
,
,
700
900
AvR [cm-'1
Figure 3. In situ high-temperature Raman spectrum of Mo03-, after evacuation to lo-' Pa at 648 K for 10 h (spectrum recorded at 648 K).
0)
N
(0
ir
,
100
,
.
, 300
,
,
,
, 500
I 700
900
AvR [cm-']
Figure 2. Room-temperatureRaman spectrum of fresh Moos prior to
thermal treatment. 225 and 233 pm. The four close neighbor oxygens tend to be tetrahedrally arranged around the metal atom (see Figure l), since all 0-Mo-0 angles have changed from the values 90" or 180", which characterize the regular octahedron, toward the tetrahedral value of 109.5", namely, to 143", 104", and 98". Thus, the structure of Moo3 evidently represents a transitional stage between octahedral and tetrahedral coordination and may be considered as built up by Moo4 tetrahedra connected by sharing two oxygen comers with two neighboring tetrahedra to form chains running in the direction of the c axis. Crystals of Moo3 usually grow in the form of needles or platelets with this direction as the principal axis. The room-temperature SMT-Raman spectrum (Figure 2) of pure Moo3 exhibits bands characteristic for this compound at 94 (Big), 114 (BZg), 125 (B3g), 157 (Ag, Big), 195 (Bzg), 217 (Ag), 244 (B3g)3 289 (BZg, B3g), 335 (Big, Ag), 366sh (Ag), 376 (Big), 469 (Ag? Big), 663 (BZg, B3gL 816 (Ag, Big), and 993 (Ag, B1,) cm-l. The assignment of the bands observed follows the single-crystal study and valence force field (VFF) calculations of Py et al.56*57 who used Kihlborg's structural picture of Moo3 to obtain new insights into its vibrational behavior. Earlier band assignments by Beattie and Gilson5*deviate from those given by Py et al. due to the fact that they were based on
a different structural mode152*53 of MoO3. As compared to the band positions reported by Py et al.:6 the spectrum of our polycrystalline Moo3 (Figure 2) indicates certain shifts, namely, (i) the translational (T,) rigid-chain mode (Big) has shifted downward to 94 cm-' as compared to 100 cm-' in the singlecrystal spectrum, (ii) the translational (T,) rigid-chain mode (B3g) has shifted down to 125 cm-' from 131 cm-' in the singlecrystal spectrum, and (iii) the pair of unresolved wagging modes (Bzg, B3g) of the terminal oxygens appears at 289 cm-' and is shifted by 6 cm-' to higher frequency relative to the singlecrystal spectrum (283 cm-'). Also significant intensity changes are observed as compared to the single-crystal spectrum:56(i) the translational (TJ rigidchain modes at 83 cm-' (Ag) and at 94 cm-' (B1,) are almost invisible, (ii) the translational (Tc)rigid-chain modes at 114 cm-' (Bzg) and at 125 cm-' (B3& show a reversed intensity ratio, (iii) the translational (TI,) rigid-chain mode at 157 cm-' (Ag, B1,) has an increased relative intensity, and (iv) in the powder spectrum, the twisting mode of the terminal oxygens at 244 cm-' (B3& is the most intense band in the frequency window between 200 and 250 cm-', while it has the lowest intensity of the triplet in the single crystal spectrum.56 The differences in the spectrum observed may be due to the microcrystalline nature of the Moo3 used in this study, since VFF calculation^^^ show that a decrease in frequency, e.g., of the rigid-chain modes, points to decreasing interactions between the half-layers and the constituting chains, and thus, to a perturbation of the layered structure. Comparable changing intensity ratios in the Raman spectra were observed when crystalline Moo3 was disintegrated in a ball mill.59 Raman Spectra after Evacuation at 648 K. After 10 h evacuation to lo-' Pa at 648 K, the high-temperature SMTRaman spectrum (Figure 3) shows remarkable differences relative to that of untreated Moo3 (Figure 2). It shows signals at 75,94, 110, 120, 142, 156sh, 193,234,279, 334, 370,472br, 660,724w, 818,888,954vw, and 990 cm-'. The blue color of Moos-, resulting from the polaron conductance (intervalence transition) exhibited by these suboxides60s61and thus, a higher absorption coefficient, and/or a decreasing Raman scattering cross section of molybdena shear structures, may be responsible for the poorer quality of the Raman spectrum. The modification
11272 J. Phys. Chem., Vol. 98, No. 44, 1994
Mestl et al.
of the Raman scattering tensor is indicated by signal intensity 773 K led to a heterogeneous sample which showed signals at ratios of all the bands being dramatically changed as compared 793, 836, 846, and 909 cm-' in the terminal stretching to the starting material. For example, (i) the pair of wagging vibrational region superimposed on a background (ca. 650925 cm-l). These signals were attributed to a not-identified, modes at 279 cm-' is more intense than the usually most intense band at 820 cm-', while (ii) the band of the rotational (RJ rigidmetastable intermediate phase. The authors noted that the small chain mode (Ag) at 217 cm-' in the spectrum of the starting number of peaks assignable to this species suggests a high material cannot be observed any more. symmetry or, alternatively, that several intermediate oxides be present but that these oxides are poor Raman scatterers, perhaps In addition, bands are shifted as compared to the starting even Raman inactive, so that their signals are overwhelmed by material: (i) the bands of the translational (T,) rigid-chain modes those of Mo03. Ozkan et aLZ6compared the Raman spectra at 114 cm-l (Bzg) and at 125 cm-I (B3g) have shifted down to obtained for fresh Moo3 and of Moo3 reduced in butadiene at 110 and 120 cm-', respectively, (ii) the translational (Tb) rigid723 K. They concluded to a high degree of reduction, this chain modes (Ag, B1,) at 157 cm-' shifted down to 142 cm-', interpretation being supportzd by the intensity loss of all major and (iii) the pair of wagging modes of the terminal oxygen Raman bands of this compound. The Raman spectrum observed atoms, being located at 289 cm-l (Bzg, BQ) in the oxidized by Ozkan et a1.26after reduction does not show traces due to state, has shifted down to 279 cm-l. MoOz and is completely different from that reported by Spevack Following the valence force field calculations of Py et al.,57 et a1.62 This indicates a more drastic reduction of MOO3 under the observed shifts to lower frequencies point to a perturbation their reducing conditions as compared to the present results and of the condensed layered structure towards vibrationally less a less severe reduction as compared to the sample studied by interacting chains. Actually, in suboxides29the distance between Spevack et a1.62 metal atoms connected through oxygen atoms (chain of tetraReoxidation in lSOz at 733 K after Evacuation at 648 K. hedra along the c axis) increases from undisturbed parts toward Moo3 previously evacuated at 648 K as described in the the shear plane with increasing distortion of the linkage. This previous section was calcined in l 8 0 z at 733 K for 14 h. The effect should affect all modes polarized parallel to the c axis, SMT spectrum of Figure 4A was recorded at the reaction while modes polarized perpendicular to the c axis, like scistemperature and shows that regular Moo3 is re-formed with soring, chain bending, and rotational rigid-chain modes, should bands appearing at 80,94, 114, 125, 148, 197, 216, 238, 284, be considerably influenced by the changes of the Mo=O 338, 379, 472, 664, 821, and 995 cm-l. In addition, three distances. signals (or shoulders) are observed which must be due to MoThe removal of oxygen from Moo3 leads to ordered crystall80vibrations, namely at 632sh, 794sh, and 945 cm-l. Either lographic shear s t r u ~ t u r e s ~via ~ -extended ~~ shear thus the changes observed in the spectrum relative to the Raman destroying the translational symmetry. This suggests that the spectrum of the starting material (Figure 2) may be due to highbroad background observed in the spectrum of Figure 3 probably temperature effects on the vibrational states of Moo3 or they arises from a multitude of combination modes of acoustic and may arise from crystal growth during the high-temperature acoustic plus optical modes due to a relaxation of the k-selection treatment. Note that the temperature applied is above the rule, indicative of a change in the morphology and crystallinity Tammann temperature of Moo3 (670 K) and, therefore, surface of the Moos-, crystallites. diffusion should occur, leading to sintering and crystal growth. Three additional signals (Figure 3) in the Mo-0 stretching Cooling the sample to 113 K within 30 min in oxygen results region, namely, at 724vw, 888, and 954vw cm-', are pointing in a low-temperature SMT-Raman spectrum (Figure 4B) toward a considerable change in coordination spheres around exhibiting the features of Mo-180 modes more clearly. HardMo centers. MoOz, having a rutile structure, exhibits signals castle and W a ~ h s could ~ ~ scorrelate ~ the high-frequency (400in the Raman spectrum at 203, 228, 345, 363, 461, 495, 571, 1000 cm-l) Raman bands of metal-oxygen compounds to the 589, and 744 cm-1.62 The four characteristic high-frequency M-0 bond distances and bond orders using a diatomic modes of that phase are not observed in the recorded spectrum approximation where each bond was assumed to vibrate (Figure 3). This shows that the reduction of Moo3 in this independently of all other oscillators in the molecule or solid. experiment did not proceed to MoOz. It is known that oxygen deficient molybdena contains tetrahedra along the shear planes;29 Indeed, a harmonic oscillator, although being a very rough furthermore the distortion of the coordination polyhedra is model for solid vibrations, may give an estimate of the decreasing from undisturbed parts of the Moo3 lattice toward maximum possible band shifts that can be expected after isotopic the shear planes.2g Thus, the appearance of the above-mentioned exchange. The band positions of modes due to the l8O bands in the spectrum of Figure 3 may suggest the presence of reoxidation at 648sh, 793sh, and 948 cm-' are in close octahedra (954 cm-') being less distorted than in Moo3 and of agreement with results of this type of calculation which give tetrahedral (888 cm-l) coordination polyhedra. A comparison signal positions for Mo-I80 vibrations at 631, 780, and 946 with differently distorted tetrahedral molybdates (about 800 cm-'. In addition, one may also apply the Teller-Redlich rule65 cm-' for Td symmetry in Ba2Mo04 to 900 cm-' for Dw in Kzfor polyatomic molecules, despite the fact that it is not entirely Mo04) shows that the signal at 888 cm-l is in the range usually valid for polymeric solids. This gives a correlation of the observed for tetrahedrally coordinated Mo centers. Still more measured frequencies of the AI, B1, and Bz modes with the severely reduced suboxides have block or columnar structures calculated ones (assumption of a CzVMoo4 molecule as the of the ReO3-type and even contain pentagonal b i p y r a m i d ~ ~ * - ~ ~building unit for Moos) with an error less than 5%. For a which must change the Raman spectra more significantly. It deeper discussion of the band intensity ratios of the 180-related should be mentioned that a correlated appearance of a band near bands relative to the unexchanged peaks, see Mestl et a1.66since 880-890 cm-' in the Raman spectrum of oxygen-deficient the effects are more pronounced in the experiments described Moos-, and the detection of Mo5+ by ESR was observed,59 there. The low intensity of the bands due to Mo-lsO vibrations this being consistent with the attribution of the Raman band to relative to the signal intensities of the unexchanged bands shows molybdenum suboxides. clearly that atoms are at least filling oxygen vacancy sites in oxygen-deficient molybdena; a more extensive l80exchange Moreover, Spevack et a1.62reported that reduction of Moos does not seem to occur. Thus, the signal intensity of Mo-180 to Moo2 under N2 in the temperature range between 723 and
J. Phys. Chem., Vol. 98, No. 44, 1994 11273
Oxygen-Exchange Properties of Moo3 A
00
300
500
700
900
AvR [cm.'] N 0
B
100
m I
300
500
700
900
AvR [cm.']
Figure 4. In situ Raman spectrum of outgassed Moo3 after calcination in 180z at 733 K for 14 h. (A) Spectrum recorded at 733 K; (B) spectrum recorded at 113 K.
modes seems to reflect the low defect density in Moo3 after vacuum treatment. As compared to the high-temperature SMT-spectrum (Figure 4A), all bands are narrower and shifted to higher wavenumbers by 4-8 cm-'. Additionally, the shoulder at 369 cm-' (Ag, scissoring of the terminal oxygens) is well resolved in the lowtemperature spectrum. These observations again indicate that there is a temperature effect modifying the dynamic properties of crystalline M003. This spectral broadening is generally observed at high temperatures. It is attributed to a decreased lifetime of the excited phonon due to a more efficient decay into-or scattering on-thermally populated acoustic and optical phonons throughout the Brillouin zone.67 The usual band shift to lower frequencies at higher temperatures is explained by the
anharmonic coupling to other phonons and by the thermal expansion of the whole crystal.67 Similar observations were made by Ozkan et al.,26who observed some broadening of all bands due to "bulk oscillations" and a shift of 3-4 cm-' to lower frequencies at high temperature compared to spectra obtained at room temperature. Possible crystallite size effects due to sintering may be indicated by the unexpectedly large shifts of the translational (Tb) rigid-chain mode at 166 cm-', which has shifted by 18 cm-', of the terminal oxygen twisting (B3g) at 251 cm-' which has shifted by 13 cm-', as well as of the terminal oxygen wagging (B3g) at 297 cm-', which has shifted by 13 cm-' and is now showing a clear splitting from its shoulder at 290 cm-' (B2g). Evacuation at Low Temperatures and Subsequent Reoxidation in l 6 0 2 . Evacuations to lo-' Pa of a fresh MOO3 sample at 290 K for 21 h and at 393 K for 18 h sufficient to remove physically adsorbed H20 and C02 led to the formation of point defects in Moos, as indicated by the slight blue color. This again had a negative influence on the quality of the Moo3 Raman spectra (not shown). A similar effect was observed when Moo3 was heated in a He-stream at 393 K for 90 h. Again, this must be explained by an increased absorption coefficient due to the coloration of the sample. No additional bands like in the spectrum of Figure 3 could be observed, which would indicate the presence of more deeply reduced MOO^-^. Subsequent calcination of the evacuated Moo3 sample at temperatures above 473 K again results in a pale yellow sample and higher quality spectra (not shown). The final spectrum after 110 h calcination at 723 K in 1 6 0 2 exhibits certain differences in the band widths and positions relative to the spectrum before the in situ treatments which can be attributed, by comparison with XRD, to crystal l 8 0 Exchange after Evacuation at 393 K. Evacuation of Moo3 to lo-' Pa at 393 K is sufficient to remove physically adsorbed H20 and CO2 and should only lead to cus-sites at the crystallite surfaces (see the previous subsection). Again the slight blue color of the sample, due to the formation of point defects, indicates some loss of oxygen from the Moo3 lattice. As described above, no additional bands, comparable to those detected in the Raman spectrum of Figure 3, were observed in the Raman spectrum (not shown). If l80 exchange occurs independent of the presence of bulk defects, Raman signals due to an l80incorporation should be observed. The room-temperature Raman spectrum (Figure 5) recorded after 14 h evacuation to lo-' Pa at 393 K, calcination in 1 8 0 2 at 573 K for 15 h, followed by a calcination in 1 8 0 2 at 733 K for 1 h does not exhibit any of the bands attributed previously to l 8 0 . The spectrum only shows bands at 82, 98, 116, 129, l59,198,217,245,284,290sh, 336,364sh, 379,413,666,819, and 997 cm-'. As in the previous paragraphs, differences in the signal intensity ratios, band positions and band widths between this spectrum and that of the starting material (Figure 2) may be due to crystal growth and sintering during the calcination. Since the spectrum does not give any indication for an l 8 0 incorporation, two conclusions can be drawn. First, the amount of oxygen vacancies generated in the near surface domains during the evacuation at 393 K, as indicated by the blue color observed after evacuation, must be far below the detection limit of Raman signals. Second, reoxidation of these defects in 1 8 0 2 does not lead to detectable Raman bands. This implies, in addition, that any oxygen exchange, independently occumng from the presence of defects, does not take place to an extent detectable by our measurements. Furthermore, this experiment
Mestl et al.
11274 J. Phys. Chem., Vol. 98, No. 44, I994 A
t N
h N
100
300
500
700
900
AvR [cm '1
00
300
Figure 5. In situ room-temperature Raman spectrum of Moos after 14 h evacuation to lo-' Pa at 393 K, calcination in lSOzat 573 K for 15 h and at 733 K for 1 h.
500
700
900
AvR [cm-'1
f
B
together with that after evacuation at 648 K clearly shows that anion vacancies in partly reduced molybdena are required for a measurable l80uptake to occur under the experimental conditions applied. A further l80exchange does not take place after reoxidation of the defects. l60Reexchange after Evacuation at 648 K and Calcination in l*Oz at 733 K. In the low-temperature SMT-Raman spectrum (Figure 6A) recorded at 133 K after a reexchange experiment at 733 K in 1 6 0 2 for 18 h, the bands assignable to Mo1603 in the lower wavenumber regime have sharpened and shifted to lower frequencies due to the temperature effect as compared to the high-temperature SMT spectrum (not shown). But unexpected shifts of the antisymmetric stretching modes (Bzg, B3g) of the Mo-0-Mo bridges along the c axis at 663 (670) cm-l, of the symmetric stretch (Ag, B1,) of the terminal oxygens at 815 (820) cm-' and of the antisymmetric stretch (Ag, B1,) of the terminal oxygens at 991 (999) cm-', and additionally the very weak signal around 899 cm-' may indicate some changes in this material. As compared to the spectrum of the starting Moo3 (Figure 2), the symmetric stretch at 815 cm-l seems to have gained intensity as compared to the antisymmetric stretch at 991 cm-'. Ozkan et a1.26 observed a similar effect arising from differently oriented Moo3 crystallites. In their Raman spectra of Moo3 samples with a higher ratio of basal-to-side plane area (Le., (010)/(100)), the band at 815 cm-l was more intense compared to the band at 99 1 cm-' . Compared to the spectrum after oxidation in l8Oz (Figure 4B),the pair of wagging modes at about 290 cm-l shows a more pronounced splitting, which probably also points to crystal growth, since in the single-crystal study56these two modes are well resolved. Thus, again the observed differences in the spectra indicate variations of particle size and morphology possibly due to crystal growth and sintering during the calcination. Since the spectrum after this very extensive calcination in l 6 0 2 (Figure 6A) still shows signals due to lsO-Mo vibrations at 637sh, 793sh, and 944 cm-', reexchange has not taken place. This again clearly demonstrates that anion vacancies in the crystal framework are required for oxygen exchange to occur in Moo3 under our experimental conditions. This is confirmed by the fact that the presence of reducing agents (e.g., propene)
Q
I
c) 0) b
I
h
m
N
I
100
300
500
700
900
AvR [cm-'1
Figure 6. (A) In situ low-temperature (133 K) Raman spectrum of 180-reoxidizedMoos after reexchange at 733 K in l602 for 18 h. (B) In situ low-temperature (133 K) Raman spectrum of I8O-reoxidized Moos after additional 14 h of calcination at 733 K in l 6 0 2 saturated
with HzO (26 P a ) . accelerates the rate of exchange. Extremely low exchange rates consistent with the present results have been reported by B o r e s k ~ v and , ~ ~Muzykantov et aL70 Thermal Treatment in HzO-Saturated l602. Further calcination of the same sample for 17 h at 733 K in flowing 1 6 0 2 saturated with HzO vapor (26 m a ) gives a low-temperature SMT-Raman spectrum (Figure 6B) (recorded at 133 K) which is in good agreement with the spectrum in Figure 6A, but the very weak signal observed at 899 cm-l probably again indicates changes in the structure or morphology of the crystallites. As observed in the previous subsection, the pair of wagging modes at about 290 cm-l exhibits now an even more pronounced splitting. Additionally, the intensity of the Bzg mode has
Oxygen-Exchange Properties of Moo3 increased compared to the B3g mode. In the single-crystal spectrum,56the BQ wagging mode is about 3 times as intense as the B3g mode. Thus, the changes observed in the Raman spectra show some crystal growth during calcination. Even after this second extended calcination, the spectrum still exhibits signals due to 180-Mo vibrations at 645sh, 793sh, and 944 cm-’. This demonstrates that HzO present in the gas phase does not play an important role for the oxygen exchange process in fully oxidized MoO3. However, a possible exchange between H2I60 and gaseous l 8 0 2 on defects of Moo3 (possibly generated by a reducing agent) is not excluded by the results described above.
Conclusions Besides thermal broadening and shifting, the Raman scattering tensor of Moo3 seems to change with temperature, as indicated by variations of signal intensity ratios. In addition, changes of intensity ratios (doublets and quartets characteristic for interacting chains and half-layers) and band shifts in the Raman spectra of MoO3-, as compared to Moo3 reveal that the scattering tensor of Moo3 is changing under oxygen-deficient conditions. Molybdenum suboxides are further characterized by additional bands in the Mo=O stretching vibrational regime at 954, 888, and 724 cm-’. This changed Raman tensor and the deeper color of Moos-, may be responsible for the reduced scattering efficiency of Mo suboxides. In addition, the Raman spectrum of Moo3 shows effects of the crystallite size due to sintering and crystal growth. Evacuation at 393 K, sufficient to remove physically adsorbed HzO and COZand leading only to defects in low concentration, does not result in any detectable spectral changes. Reoxidation in l8Oz also does not result in 180-related bands. Reoxidation in 1 8 0 carried ~ out after evacuation at 648 K leading to a higher degree of reduction, gives signals due to an l 8 0 incorporation into Mo03. Therefore, the l8O signal intensities, proportional to the amount of l80atoms incorporated into Moos, seem to correlate with the initial degree of reduction of Moos. An additional oxygen exchange, independent from the presence of defects, does not occur. The extremely slow l60reexchange observed for the fully oxidized material is consistent with the fact that Moo3 is an anion vacancy conductor and demonstrates therefore, the essential role played by the defects in Moo3 for the exchange process. The presence of H20 has no detectable influence on the oxygen exchange.
Acknowledgment. This work was financially supported by the Deutsche Forschungsgemeinschaft (SFB 338) and by the Fonds der Chemischen Ind. The support of the Region Wallonne (Belgium) in the frame of an “Action Concert6e” is gratefully acknowledged. References and Notes (1) Grange, P. Catal. Rev. Sci. Eng. 1980, 21, 135. (2) Katzer, J. R.; Sivasubramanian, R. Catal. Rev. Sci. Eng. 1979,20, 155. (3) Vielhaber, B.; Knozinger, H. Appl. Catal. 1886, 26, 375. (4) Liu, Y. C.; Griffin, G . L.; Chan, S. S.; Wachs, I. E. J . Catal. 1985, 94, 108 and references therein. ( 5 ) Bosch, H.; Jansen, F. Coral. Today 1988, 2, 369. (6) Tong, Y.; Lunsford, J. H. J . Am. Chem. SOC.1991, 113, 4741. (7) Trifiro, F.; Caputo, G.; Villa, P. L. J . Less-Common Met. 1974, 36, 305. (8) Ozkan, U. S.; Schrader, G. L. J . Catal. 1985, 95, 120. (9) Grasselli, R. K.; Bunington, J. D. Adv. Catal. 1981, 30, 133.
J. Phys. Chem., Vol. 98, No. 44, 1994 11275 (10) Ruiz, P.; Zhou, B.; Remy, M.; Machej, T.; Aoun, F.; Doumain, B.; Delmon, B. Catal. Today 1987, I , 181. (11) Centi, G.; Trifio, F. Catal. Rev. Sci. Eng. 1986, 28, 165. (12) Kung, H. H. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 171. (13) Berry, F. J. Adv. Catal. 1981, 30, 97. (14) Becker, K.; Steinberg, K. H.; Spindler, H. In 2nd Conf. on Spillover, Leipzig; Steinberg, K. H., Ed.; 1989, p 204. (15) Ruiz, P.; Delmon, B. Catal. Today 1988, 3, 199. (16) Delmon, B.; Ruiz, P. React Kinet. Catal. Lett. 1987, 35, 303. (17) Keulks, G. W. J . Catal. 1970, 19, 232. (18) Wragg, R. V.; A s h o r e , P. G.; Hockey, J. A. J . Catal. 1971, 22, 49. (19) Weng, L. T.; Ruiz, P.; Delmon, B.; Duprez, D. J. Mol. Catal. 1989, 52, 349. (20) Ozkan, U. S.; Driscoll, S. A.; Zhang, L.; Auk, K. L. J . Catal. 1990, 124, 183. (21) Glaeser, L.; Brazdil, J.; Hazle, M.; Mehicic, M.; Grasselli, R. J . Chem. Soc., Faraday Trans. I 1985, 79, 2903. (22) Heufs, E. V.; Monnier, J. R.; Keulks, G. W. J . Catal. 1979, 57, 331. (23) Matsuura, I.; Hashiba, H.; Kanesaka, I. Chem. Left. 1986, 533. (24) Schrader, G. L.; Moser, T. P.; Lashier, M. E. Proc. 9th In?. Congr. Catal., Calgary; Phillips, M. J., Teman, M., Eds.; Chem. Institute Canada: Ottawa, Canada, 1988; Vol. 4, p 1624. (25) Lashier, M. E.; Schrader, G. L. J . Catal. 1991, 128, 113. (26) Ozkan, U. S.; Smith, M. R.; Driscoll, S. A. J . Catal. 1992, 134, 24. (27) Kihlborg, L. Ark Kemi 1963, 21, 443. (28) Kihlborg, L. Ark Kemi 1963, 21, 461. (29) Kihlborg, L. Ark Kemi 1963, 21, 471. (30) Comaz, P. F.; van Hooff, J. H. C.; Pluijm, F. J.; Schuit, G. C. A. Discuss. Faraday SOC.1966, 41, 290. (31) Guyot, H.; A1 Khoury, E.; Marcus, J.; Schlenker, C.; Banville, M.; Jandl, S. Solid State Commun. 1991, 79, 307. (32) Bursill, L. A. Acta Crystallogr. 1972, A28, 187. (33) Kihlborg, L. Acta Chem. Scand. 1959, 13, 954. (34) Kihlborg, L. Acta Chem. Scand. 1960, 14, 1612. (35) Fhlborg, L. Acta Chem. Scand. 1963, 17, 1485. (36) Asbrink, S.; Kihlborg, L. Acta Chem. Scand. 1964, 18, 1571. (37) MagnCli, A. Acta Crysfallogr. 1953, 6, 495. (38) Hagg, G.; MagnCli, A. Ark. Kemi 1944, 19, 1. (39) Kihlborg, L. Ark. Kemi 1963, 21, 365. (40) Kihlborg, L. Ark. Kemi 1963, 21, 427. (41) MagnCli, A. Acta Chem. Scand. 1948, 2, 861. (42) MagnCli, A. Acta Chem. Scand. 1948, 2, 501. (43) Kihlborg, L.; MagnCli, A. Acta Chem. Scand. 1955, 9, 471. (44) Nakamura, 0.;Kodama, T.; Ogino, I.; Miyake, Y. Chem. Lett. 1979, 17. (45) Gay-Boyes, P. L. J . Solid State Chem. 1993, 104, 119. (46) Catlow, C. A. In Nonstoichiometric Oxides; Sorenson, 0.T., Ed.; Academic Press: New York, 1981; Chapter 2. (47) Zeilinger, H. Ph.D. Thesis, Universitat Munchen, 1992. (48) Knoll, P.; Singer, R.; Kiefer, W. Appl. Spectrosc. 1990, 4, 776. (49) Deckert, V.; Kiefer, W. Appl. Spectrosc. 1992, 46, 322. (50) Home-developed software. (51) Patterson, W. R. J . Mol. Caral. 1991, 65, L41. (52) B r U e n , H. Z. Kristollagr. 1931, 78, 484. (53) Wooster, N. Z. Kristallogr. 1931, 80, 504. (54) Anderson, G.; MagnCli, A. Acta Chem. Scand. 1950, 4, 793. (55) Kihlborg, L. Ark. Kemi 1963, 21, 357. (56) Py, M. A.; Schmid, Ph. E.; Vallin, J. T. Nouvo Cim. 1977, 38B, 271. (57) Py, M. A,; Maschke, K. Physica B 1981, 370. (58) Beattie, I. R.; Gilson, T. R. J . Chem. SOC.A 1969, 2322. (59) Mestl, G. Ph.D. Thesis, Universitat Munchen, 1994. (60) Porter, V. R.; White, W. B.; Roy, R. J . Solid State Chem. 1972,4, 250. (61) Louis, C.; Che, M.; Bozon-Verduraz, F. J . Chim. Phys. Phys.-Chim. Biol. 1982, 79, 803. (62) Spevack, P. A.; McIntyre, N. S. J. Phys. Chem. 1992, 96, 9029. (63) Hardcastle, F. D.; Wachs, I. E. J . Raman Spectrosc. 1990,21,683. (64)Hardcastle, F. D.; Wachs, I. E. J . Phys. Chem. 1991, 95, 5031. (65) Herzberg, G. Molecular Spectra and Molecular Structure; van Nostrand Reinhold: New York, 1945; p 231. (66) Mestl, G.; Ruiz, P.; Delmon, B.; Knozinger, H. J. Phys. Chem., this issue. (67) Sherman, W. F.; Wilkinson, G. R. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Heyden & Son: London, 1980; Vol. 6. (68) Winter, E. R. S. J . Chem. SOC.,A 1968, 2889. (69) Boreskov, G. K. Adv. Caral. 1964, 15, 286. (70) Muzykantov, V. S.; Cheshkova, K. T.; Boreskov, G. K. Kinet. Catal. 1973, 14, 365.
