Reduction of single-crystal molybdenum trioxide: an electron spin

Reduction of single-crystal molybdenum trioxide: an electron spin resonance investigation after reaction with methanol. D. V. Mesaros, and Cecil Dybow...
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Langmuir 1988,

4, 983-988

983

Reduction of Single-Crystal Molybdenum Trioxide: An Electron Spin Resonance Investigation after Reaction with Methanol D. V. Mesaros1 and Cecil Dybowski* Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716

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Received February 1, 1988. In Final Form:

April

13, 1988

ESR spectra of Mo03 single crystals heated to 570 K in the presence of methanol reveal paramagnetic reduction sites with the stoichiometry, H1Mo2XI, similar to those observed after reaction with H2 at elevated temperatures. The principal components of the g-tensor for this species are 1.944 (1), 1.878 (1), and 1.954 (1). The symmetry, stoichiometry, and relative magnitudes of the tensor components along the crystal axes indicate that the reduction site contains a pair of molybdenum atoms in the [010] crystal plane, isolated from the bulk lattice. The production of such sites upon methanol reduction is consistent with a crystallographic shear mechanism for oxide defect compensation. The proton at the paramagnetic center originates from the hydroxyl group of methanol. ESR studies of Mo03 after reaction with mixtures of methanol and oxygen indicate very little oxide reduction under steady-state selective oxidation conditions.

Introduction

bulk reduction have been characterized in Mo03 under partial-oxidation conditions (i.e., with both 02 and CH3OH present) with IR spectroscopy.18 The relative degree of reduction on the oxide surface versus the bulk of the

Molybdenum trioxide (Mo03) and other more complex oxides containing molybdenum have been investigated as catalysts for the selective oxidation of methanol to form-

crystal was found to depend on the reaction temperature and the oxygen concentration. ESR spectroscopy, when applied to the characterization

aldehyde.1"13

ch3oh

+

catalyst

y2o2-

ch2o

+ H20

(1)

To provide details of methanol adsorption and reaction on Mo03, polycrystalline Mo03 has been studied by many techniques,3,11"15 including electron spin resonance (ESR) spectroscopy.16,17 It is generally agreed that the mechanism of this partial oxidation reaction involves dissociative adsorption of methanol to form surface-bound methoxyl

(1) Adkins, H.; Peterson, W. R. J Am. Chem. Soc. 1931,53,1512-1520. (2) Novakova, J.; Jiru, P.; Zavadil, V. J. Catal. 1970, 17, 93-97. (3) Novakova, J.; Jiru, P.; Zavadil, V. J. Catal. 1971, 21, 143-148. (4) Ai, M. J. Catal. 1978, 54, 426-435. (5) Tatibouet, J. M.; Germain, J. E. C. R. Seances Acad. Sci., Ser. C 1980, 290, 321-324. (6) Niwa, M.; Mizutani, M.; Takahashi, M.; Murakami, Y. J. Catal. 1981, 70, 14-23. (7) Tatibouet, J. M.; Germain, J. E. J. Catal. 1981, 72, 375-378. (8) Machiels, C. J.; Sleight, A. W. J. Catal. 1982, 76, 238-239. (9) Selenina, V. M.; Fricke, R.; Hanke, W.; Schnabel, K. H.; Ohlmann, G. Z. Anorg. Allg. Chem. 1983, 505, 67-78. (10) Tatibouet, J. M.; Germain, J. E.; Volta, J. C. J. Catal. 1983, 82,

groups.9,11"15 CH3OH

+

—-

77777"

CH3O

+

H.

(or H+)

(2)

7

Methyl group C-H bond cleavage is firmly established as the rate-determining step for the reaction.8,11,12 The local structure and electronic state of Mo03 during reaction with methanol have not been characterized as convincingly as the overall adsorption and kinetics. Structural transformation occurring in Mo03 upon reduction with H2 or CH3OH have been examined by a host of spectroscopic techniques,9,11,18"25 as well as electron microscopy7,10,26"28 and powder X-ray diffraction.29,30 Several investigators conclude that the (010) face of orthorhombic Mo03 is active for formaldehyde formation,7,9,10,28,31 whereas others maintain that this face is inactive since it contains only coordinatively saturated molybdenum centers11,13 that would not readily chemisorb methanol.11 A variety of active sites have been proposed, including Mo=0,7,10 doubly unsaturated Mo centers (0=Mo=0),31 Mo6+,3,13 Mo5+,9 and coordinatively un-

240-244. (11) Ghowdhry, U.; Ferritti, A.; Firment, L. E.; Machiels, C. J.; Ohuchi, F.; Sleight, A. W.; Staley, R. H. Appl. Surf. Sci. 1984, 19, 360-372. (12) Farneth, W. E.; Ohuchi, F.; Staley, R. H.; Ghowdhry, U.; Sleight, A. W. J. Phys. Chem. 1985, 89, 2493-2497. (13) Machiels, C. J.; Cheng, W. H.; Ghowdhry, U.; Farneth, W. E.; Hong, F.; McCarron, E. M.; Sleight, A. W. Appl. Catal. 1986,25, 249-256. (14) I to, M. Vib. Surf. [Proc. Int. Conf.], 2nd 1980 1982, 71-78. (15) Groff, R. P. J. Catal. 1984, 86, 215-218. (16) Hemidy, J. F.; Tench, A. J. J. Catal. 1981, 68, 17-21. (17) Narayana, M.; Zhan, R. Y.; Kevan, L. J. Phys. Chem. 1985, 89, 636-641. (18) Chung, J. S.; Bennet, C. O. J. Chem. Soc., Faraday Trans. 1 1986, 82, 2155-2167. (19) Abdo, S.; LoJacono, M.; Clarkson, R. B.; Hall, W. K. J. Catal. 1975, 36, 330-332. (20) Shelimov, B. N.; Pershin, A. N.; Kazansky, V. B. J. Catal. 1980, 64, 426-436. (21) Fricke, R.; Hanke, W.; Ohlmann, G. J. Catal. 1983, 79, 1-12. (22) Seyedmonir, S. R.; Howe, R. F. J. Chem. Soc., Faraday Trans. 1, 1984, 80, 87-97. (23) Serwicka, E.; Schindler, R. N. Z. Phys. Chem. (Wiesbaden) 1982, 133, 175-183. (24) Tinet, D.; Canesson, P.; Estrade, H.; Fripiat, J. J. J. Phys. Chem. Solids 1979, 41, 583-589. (25) Slade, R. C.; Halstead, T. K.; Dickens, P. G. J. Solid State Chem.

saturated molybdenum centers.11,13 Several techniques have provided information on the structure of the reduction sites in Mo03 relative to that of the parent crystal.18,26"30 Using electron microscopy, Gai and co-workers observed the growth and propagation of crystal defects during the reduction of Mo03 in both H2 and methanol.28 X-ray diffraction analysis has confirmed that Mo03 is transformed into the bronzes, Hg gMoOg29 and H0 3MoO330 by reduction in methanol. Both surface and

1980 34 183—192 (26) Thoni, W.; Kirsch, P. B. Philos. Mag. 1976, 33, 639-662. (27) Thoni, W.; Gai, P. L.; Hirsch, P. B. Philos. Mag. 1977, 35,

781-786. (28) Gai, P.

L; Labun, P. A. J. Catal. 1985, 94, 79-96. (29) Guidot, J.; Germain, J. E. React. Kinet. Catal. Lett. 1980, 15,

389-393. (30) Vergnon, P.; Tatibouet, J. M. Bull. Soc. Chim, Fr. 1980, 11-12, 455-458. (31) Allison, J. N.; Goddard, W. A. J. Catal. 1985, 92, 127-135.

Present address: E. I. du Pont de Nemours & Company, Agricultural Products Department, Experimental Station, Wilmington, DE 19898. 1

0743-7463/88/2404-0983$01.50/0

©

1988 American Chemical Society

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Mesaros and Dybowski

of paramagnetic centers in single crystals, is a powerful tool for the examination of local structure,32,33 often yielding information on local coordination geometry, site stoichiometry, metal oxidation state, extent of electron delocalization, and relation of reduced-site geometry to that of the parent crystal lattice. Despite such specific features, to our knowledge ESR spectroscopy of single-crystal systems has not been exploited as a tool for heterogeneous catalyst characterization. In this investigation we have applied ESR spectroscopy to study the structure of paramagnetic reduction sites in Mo03 single crystals after reaction with methanol. The use of 95Mo-enriched Mo03, in addition to samples of natural isotopic abundance, significantly increases the structural information available from the application of ESR to this oxide system.

Experimental Section Natural-abundance Mo03 crystals were grown from MoOs powder (Aesar, 99.998%) by a carbonate-flux technique in a porcelain crucible at 800-950 K in air.34 Enriched molybdenum powder (U.S. Services, 96.47% 95Mo) was converted to Mo03 under 1 atm of oxygen in a quartz reaction tube at 620 K. After the powder was outgassed to 2 X 1CT5 Torr and dosed in 760 Torr of oxygen, single crystals were formed by slowly cooling the molten oxide from 1090 K through the melting point. Typical crystals used for ESR measurements were 1 mm [100] X 0.1 mm [010] X 3 mm [001] platelets. Methanol (Fisher, 99.9%), methanol-d4 (Chemical Dynamics, 99.5% isotopic purity), and I3C-labeled methanol (Cambridge Isotope Laboratories, 99% isotopic purity) were dried and stored over 3-Á molecular sieves (Linde). The sieves were dried at 620 K for 24 h prior to use, and those used to store methanol-d4 were equilibrated with D20 for 24 h prior to drying to exchange labile protons. Methanol-d3 was prepared by distillation of methanol-d4 from an excess of water. After the collected fraction was redistilled from H20, the product was stored over molecular sieves to remove residual water. Proton NMR spectroscopy indicated an exchange of >99 atom % at the hydroxyl position. All reagents were carried through no fewer than three freeze—pump-thaw cycles to remove dissolved gases prior to use. Crystals were placed on a short length of 3-mm-diameter quartz rod in an ESR adsorption reactor.35 Sample alignment was maintained with quartz wool packed around the crystal. The samples were outgassed to 6 X 5 Torr, equilibrated with 80-110 Torr of methanol at room temperature, and sealed. The reactor was then maintained at 570 K for 1 h, after which an ESR spectrum was recorded. This cycle was repeated once or twice until an ESR spectrum with adequate signal-to-noise ratio was obtained. ESR spectra were measured with a Varían E-109 spectrometer at 9.1 GHz with 100-kHz modulation. An E-229 goniometer, modified for use with the adsorption reactor and a liquid-nitrogen dewar, was employed to align the sample in the cavity with an accuracy of ±5°. Where possible, site splittings of the resonances were used to correct the alignment to an accuracy of ±1° in the plane of rotation, while the out-of-plane alignment error remained approximately ±5°. All spectra were recorded at 77 K. Throughout this investigation ESR signals were not observable at room temperature. DPPH was used as an external standard for g value calibration (g = 2.0037).

Results An ESR spectrum of a methanol-reduced Mo03 crystal with its [001] axis lying along the magnetic field is shown in Figure 1. The complex hyperfine structure in this (32) Juryska, R.; Bill, H. Nuovo Cimento Soc. Ital. Fis., B 1977, 38B, 369-377. (33) Grunin, V. S.; Patrina, I. B. Phys. Status Solidi B. 1984, 123, 353-363. (34) Wanklyn, B. M.; Garrard, B. J. J. Mater. Sci. Lett. 1983, 2, 285-290. (35) Mesaros, D. V.; Dybowski, C. Appl. Spectrosc. 1987, 41, 610-612.

Figure 1. ESR spectrum of natural-abundance Mo03 reduced with methanol at 570 K. The spectrum was acquired at 77 K with the magnetic field along the [001] axis.

2. Experimental and simulated ESR spectra of ^Moenriched Mo03 reduced in methanol at 570 K. The spectra were acquired at 77 K with the magnetic field along the [100] (A), [010] (B), and [001] (C) axes. Simulations were performed with the coupling constants from ref 32 (see Table I). A 0.6:0.4 Gaussian:Lorenzian line shape was used with the following line widths: 6.5 G (D), 15 G (E), and 6.5 g (F).

Figure

spectrum arises, in part, because of the presence of two molybdenum isotopes with spin, 9SMo (spin 5/2,15.7%) and 97Mo (spin 5/2, 9.5%), as well as a number of isotopes with zero spin (98Mo, 23.8%; 96Mo, 16.5%; 92Mo, 15.8%; 100Mo, 9.6%; 94Mo, 9.0%). Despite the extensive overlap of patterns produced by the random substitution of the various isotopes at the paramagnetic center, it is clear that hyperfine coupling to at least two unique molybdenum nuclei is present. Further splitting of these lines into overlapping doublets also indicates coupling of the electron to a proton. A chemically identical paramagnetic center interacting with a proton and a spin-zero molybdenum atom yields the intense central doublet. A second signal, having the same g value but with a smaller coupling to molybdenum, overlaps the inner resonances of the first center. Coupling to a proton is not clearly discernible in

Reduction of Molybdenum Trioxide

3. Dependence of the observed g value on orientation of an Mo03 single crystal with respect to the magnetic field direction for rotation about the [100] (·), [010] ( ), and [001] ( ) axes. In each case 0° represents the [001], [001], or [100] axis, respectively, while 90° represents the [010], [100], or [010] axis,

Figure

respectively.

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Figure 4. Dependence of the observed proton hyperfine coupling on orientation of an Mo03 single crystal with respect to the magnetic field direction for rotation about the [100] (·), [010] ( ), and [001] ( ) axes. In each case 0° represents the [001], [001], or or

[100] axis, respectively, while 90° represents the [010], [100], [010] axis, respectively.

this pattern, due to its low relative intensity and the complexity of the spectrum as a whole. Similar treatment of a 95Mo-enriched Mo03 single crystal with methanol vapor results in the spectra shown in Figure 2, acquired with the magnetic field along each of the three principal crystal axes. Twenty-one-line spectra indicate coupling of the electron to either four nearly equivalent molybdenum atoms (I = 5/2) or to two inequivalent molybdenum atoms. In the latter case the six-line pattern of the major coupling to one molybdenum atom would be further split by a minor coupling to the second molybdenum atom, such that the resonances overlap to yield a nearly uniform 21-line pattern. Coupling to a proton, as observed in spectra of Mo03 with the natural isotopic abundance, is not seen in the case of the isotopically enriched samples because of line broadening caused by the increased concentration of nuclei with nonzero

spin. Visual examination of the crystals after reduction reveals blue coloration initially on all faces, except the basal (010) face. However, surface defects in the (010) face show similar color. Continued reduction produces a uniform deep-blue color throughout the crystal. No change in the ESR signal, except for an increase in intensity, is observed with increasing time of reduction at 570 K. At 670 K, reduction in methanol destroys the single crystal, yielding a deep-blue powder. In all samples, the signal intensity is found to decrease after prolonged outgassing or reoxidation at 570 K. Although the ESR instrumentation employed in this study does not provide means for accurate spin counting, we can estimate the number of spins observed. We can reasonably assume a detection limit of approximately 1 X 1015 spins. On our instrument residual signals present in fully oxidized samples were barely observable, i.e., at or near out limit of detection. Signals from reduced samples ranged from 100 to 400 times the residual intensity, indicating detection of 1 X 1016 to 4 X 1017 spins. The size of the many single crystals used in the study did vary, but one sample was carefully measured by using an optical microscope and a calibrated scale. Using the dimensions of that crystal, 8.5 X 1019 molybdenum atoms were calcualted for the bulk of the crystal and 9 X 1013

5. Dependence of the observed 95Mo hyperfine coupling orientation of an Mo03 single crystal with respect to the magnetic field direction for rotation about the [100] (·), [010] ( ), and [001] ( ) axes. In each case 0° represents the [001], [001], or [100] axis, respectively, while 90° represents the [010], [100],

Figure on

or

[010] axis, respectively.

atoms for the total of all exposed surfaces, assuming perfect surfaces. It is immediately obvious that reduction at only surface sites is well beyond our limit of detection for the size crystals we were able to grow. Also, the ratio of the estimated spin count to the number of molybdenum atoms in the sample indicates 0.1-5% of the Mo atoms in the crystal were observed as Mo(V) by ESR spectroscopy.

The anisotropies of the g-tensor and proton hyperfine coupling were determined from the variation in the ESR spectra of natural-abundance Mo03 with orientation of the crystal in the magnetic field. The g-tensor rotation diagrams are shown in Figure 3. Each curve represents the variation of the observed g value for rotation of the sample in the magnetic field about the three principal crystal axes. Similar plots for the proton hyperfine coupling and the 95Mo couplings are presented in Figures 4 and 5, respectively. In the latter case each data point shown results from a second-order nonlinear least-squares refinement of

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Table I. ESR Parameters for Methanol-Reduced MoOa Single Crystals and MoOa Single Crystals Grown in Hydrogen” reduced in methanol orientation vs MoOa, growth in hydrogen (from ref 32) deg axis orientation value, G [001] value, G [100] [010] g-Tensor a

b c

1.944 (1) 1.879 (1) 1.954 (1)

82 (1) 90 (1)

5.1 (1) 2.4 (1) 2.7 (1)

40 (1) 51 (1) 85 (1)

8 (1)

82 (1) 8 (1) 90 (1)

90 (1) 90 (1) 0 (1)

1.943 (1) 1.878 (1) 1.953 (1)

rotated 8.5° (1.5°) in (001) plane

Hyperfine Coupling Tensor a'

b' c'

a"

9.9 (1) 22.1 (1) 9.4 (1)

b" c"

51 (1) 41 (1) 73 (7)

89 (1) 83 (5) 19 (7)

5.2 (8) 2.6 (8) 2.8 (8)

rotated 50° (3°) in (001) plane

95Mo Hyperfine Coupling Tensor6 1 (2) 90 (2) 89 (2) 90 (2) 1 (2) 89 (2) 3 (2) 88 (2) 89 (2) 96Mo

Figure 6. ESR spectra of Mo03 single crystals reduced in CH3OH (A), CD3OD (B), and CD3OH (C). The spectra were acquired at 77K with the [001] axis aligned along the magnetic field direction.

Hyperfine Coupling Tensor”

a'"

29.4 (1.5) 65.3 (1.0) 28.5 (1.5)

b'" c"'

rotated -2.5° (2°) in (001) plane

Mo Hyperfine Coupling Tensord

a"" b"" c""

9.2 (1.5) 21.5 (1.5) 8.8 (1.5)

rotated 4° (5°) in (001) plane

Estimates of error are enclosed in parentheses. Error values listed for last significant digit, except where a decimal place is shown in b Enriched sample. ” Natural abundance sample. parentheses. d Natural abundance sample, minor coupling, isotopes unresolved. “

are

a plot of resonant field versus nuclear-spin quantum number for the 21 resonance lines measured at each orientation. The solid lines in Figures 3-5 are the nonlinear least-squares fits to eq 3a and 3b. g2obsd

A2obsd

§2u sin2

=

=

A2¡¿

sin2

± 2g2ij sin ± 2A2ij sin

cos cos

+ g2jj cos2 + A2j¡ cos2

(3a)

(3b)

and Aobed are the observed g value and hyperfine coupling constants, respectively. Values of the tensor elements (gi;· or Ay) are obtained by a Simplex nonlinear least-squares fitting procedure. Diagonalization of the g2-tensor derived from fitting the rotation patterns gives the principal values and orientations of the principal axes (Table I). Like the host Mo03 crystal, the g-tensor possesses orthorhombic symmetry with the c axis of the g-tensor along the [001] direction, but the a and b axes are rotated 8 ± Io from the [100] and [010] axes, respectively. The g values are the same (to within 0.002) as those reported for defects in Mo03 grown in hydrogen.32 The similar symmetry and relation of the principal axes to the parent crystal (a rotated by 8.5 ± 1.5° from the [100] axis) make it likely that the same center is formed in both cases. Taking into account the uncertainty of the measurements of the proton hyperfine couplings, the c'axis of the hyperfine coupling tensor is nearly parallel to the [001] crystal axis, but the o' and b' axes are rotated approximately 50° from the [100] and [010] axes, respectively. The uncertainty in the principal hyperfine couplings is due to overlapping site splittings. The proton coupling tensor compares well with the work of Juryska and Bill32 on hydrogen-reduced Mo03, again indicating the similarity of the two systems. Within experimental error, the principal axes of the ^Mo hyperfine coupling tensor are coincident with the unit-cell axes of the Mo03 crystal (Table I). The errors arise primarily from the lack of resolvable site splittings, which would have been used to compensate for uncertainties in gobsd

initial crystal positioning. The 95Mo coupling constants approximately one-third of the values reported by Juryska and Bill32 for the major coupling to molybdenum of the ESR species formed by reduction in hydrogen and are equal to those of the minor constituent to within 0.6 G (see Table I). If the 21-line patterns result from coupling to two inequivalent molybdenum nuclei, the forgoing analysis is obviously not a valid determination of the 95Mo hyperfine coupling tensor. However, after comparison with the previous study,32 this analysis does provide a close approximation to the minor coupling tensor. In the 21-line patterns every third resonance is significantly more intense (Figure 2). Our attempt to simulate these patterns using the coupling constant values from the literature for H2-reduced Mo0332 is shown in Figure 2D-F. For the latter two patterns (B and C) we were able to produce convincing simulations (E and F), but the more pronounced anisotropy of spectrum A could not be reproduced without the introduction of significant anisotropic effects. The proton incorporated into the structure of the reduced Mo03 during reaction with methanol vapor can originate from one of two sources, either the hydroxyl group or the methyl group of methanol. Deuterium labeling was used to determine the origin of the proton at the paramagnetic center. Figure 6 shows the central portion of the ESR pattern for three samples at the same orientation of the crystals in the magnetic field. Spectra A, B, and c arise from Mo03 reduced in CH3OH, CD3OD, and CD3OH, respectively. The CH3OH-reduced crystal shows a doublet due to coupling to one proton, while the CD3OD sample, treated under the same conditions, gives an unresolved triplet resulting from coupling to one deuteron (I = 1). Shoulders are observed that are attributable to coupling to residual protons present in the crystal prior to reduction with deuteriated methanol and possibly from exchange with hydroxyl groups on the reactor walls. Reduction of M0O3 in CD3OH yields an ESR spectrum nearly identical with that of the CH3OH-reduced sample, indicating transfer only from the hydroxyl position to the paramagnetic center. Exposure of a CH3OH-reduced crystal to D20 produced no detectable isotope exchange after 16 h at room temperature and only partial exchange or further reduction after heating at 570 K. The role of protonic species in the reduction of Mo03 was addressed by experiments in which water vapor, carbon monoxide, and mixtures of methanol and oxygen were used as reagents. Reaction of Mo03 single crystals in a mixture of oxygen and methanol yields negligible are

Reduction of Molybdenum Trioxide sample reduction, as evidenced by the lack of color change in the crystal and weak ESR signal intensity. Treatment of samples at 570 K under 18 Torr of H20 results in ESR spectra comparable in both morphology and intensity to spectra acquired after methanol reduction. Reduction with 700 Torr of CO at 570 K yielded only a slight increase in the ESR signal intensity, relative to the background resonance observed for the fully oxidized sample. At this level of intensity residual protons on the sample and the quartzware (present as bound water or hydroxyl groups) would have been a sufficient proton source. Subsequent exposure of the same sample to D20 at room temperature for up to 18 h gave no change in the ESR signal. When 13C-labeled methanol is used as the reductant, the ESR spectrum is identical with that observed after reduction with unlabeled methanol, indicating no significant incorporation of the methoxyl group at the paramagnetic

center.

Discussion The data obtained on natural-abundance Mo03 reduced in methanol are virtually identical with the results reported by Juryska and Bill on hydrogen-induced defects in Mo03 single crystals.32 From this comparison, and our results for reduction in the presence of water vapor, it appears that a variety of protonic reagents yield the same paramagnetic

Mo(V)-containing sites. The ESR data from crystals grown under hydrogen32 were interpreted as evidence for a reduction center containing two unique molybdenum atoms and one proton. The high probability that only one of the two molybdenum sites is occupied by a spin-5/2 isotope yields two sets of resonances, one showing strong coupling to molybdenum and to a proton and a second center characterized by a weaker coupling to molybdenum.32 We obtain similar spectra from crystals of natural isotopic distribution reduced under methanol or water vapor (Figure 1). Our resutls for 95Mo-enriched Mo03 also indicate two nonequivalent molybdenum atoms at the reduction site, as supported by the spectral simulations calculated by using the published coupling constants for H2-reduced crystals32 (Figure 2). Reduction with 13C-labeled methanol shows that the observed paramagnetic center does not contain an organic species, although surface-bound organic radicals have been observed on Mo03-containing oxides by using ESR spectroscopy.16,36 The proton at the reduction site is clearly bound directly to the oxide structure. The deuterium-labeling studies indicate that the proton at the reduction site is transferred exclusively, or nearly so, from the hydroxyl group of methanol (Figure 6). Such a transfer is the consequence of either dissociative methanol adsorption or dehydration to form dimethyl ether. Incorporation of methyl group deuterons would have indicated the prevalence of methanol oxidation (dehydrogenation) in the formation of these sites. Several studies have shown that, in the absence of gas-phase oxygen, the activity of Mo03 for formaldehyde formation is low and

quickly lost, while the dehydration reaction over the reduced oxide persists for much longer periods.10,30 It is conceivable that reduction with a methyl proton occurs initially, but isotopic exchange with labile hydroxyl species eliminates these species from the oxide prior to observation with ESR spectroscopy. We consider this scenerio unlikely, since our attempts to exchange protons for deuterons in the reduced oxide failed at room temperature and (36) Anpo, M.; Mihara, K.; Kubokawa, Y. J. Catal. 1986, 97, 272-276.

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apparently proceeded only slowly at elevated temperatures. (Isotope exchange at the higher temperatures is not

distinguishable from further reduction by our experiments.) The extent of reduction of Mo03 in the presence of a mixture of methanol and oxygen is quite low, since the strong central doublet, as seen in Figure 1, is barely observable. Similar signal intensity is observed for “fully” oxidized samples, attributable to the presence of residual protons trapped in the crystal lattice at defect sites or as bound water. Some structural details of the reduction centers in Mo03 can be specified from the symmetries of the g-tensor and the two hyperfine coupling tensors in Table I. The g-tensor approximates the orthorhombic symmetry of the parent crystal, but the principal axes are slightly rotated away from the unit-cell axes in the (001) plane. The principal g value along the b axis is significantly lower than the others, indicating much stronger contributions from spin-orbit coupling for this component. Similarly, the molybdenum hyperfine tensor is nearly orthorhombic, with its principal axes closely aligned with the unit-cell axes. The b" axis appears to be “unique”, having a much larger principal value. The physical structure of the reduced site must reflect the significant charge circulation and Mo hyperfine coupling in the a-c plane that would give rise to these magnetic anisotropies. The conclusions from ESR spectroscopy on the geometry and stoichiometry of the reduction site are consistent with structural evidence from other studies, including electron microscopy investigations of H2- and CH3OH-reduced Mo03 crystals.26-28 Low-tempeature reduction produces local defects that grow with increasing severity of reaction, ultimately producing crystallographic shear at 650 K (80 K higher than the temperature of our study). This electron microscopic evidence28 is said to confirm the AndersonHyde model37 for shear formation along jl20jR38 to compensate for lattice oxygen displacement. Figure 2 of ref 37 schematically shows the effect of such a displacement with the formation of one edge-shairing Mo4On unit for each lattice oxygen atom lost! The overall geometry of the four molybdenum atoms is planar in the (101) plane. These edge-sharing species are isolated from the bulk crystal by the mismatch in coordination. Since crystallographic shear formation was not observed below 650 K with electron microscopy,28 our ESR observations after reaction at 570 K reflect the structure of the local domain defects reported at reduction temperatures below 650 K28 and not the presense of macroscopic shear. However, the ESR data establish a structural similarity between the local defects and those produced by bulk crystallographic shear. The isolated two-molybdenum centers in the (101) crystal plane observed with ESR spectroscopy support the concept of propagation of local defects to form the more ordered crystallographic shear structure.28,37

At room temperature no ESR signal is observed for partially reduced Mo03, while at 77 K the ESR spectrum shows that the unpaired electron is confined to a twomolybdenum site. This limited delocalization at the reduction site is further supported by the known distortions in coordination in Mo03 and the electronic structure for an edge-sharing molybdenum oxide cluster proposed by Broclawik and Haber.39 Their calculations for a hypo(37) Anderson, J. S.; Hyde, B. G. J. Phys. Chem. Solids 1967, 28, 1392-1408. (38) Subscript R indicates the ReOa structure, related to MoOa, by an alternate designation of unit-cell parameters.

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thetical spin-paired Mo2O1010~ cluster place the valence electrons (d2) in orbitals of predominantly dI2_y2 character. On the basis of their model, the unpaired electron in the lattice after reduction (d1) would be expected to reside in an orbital of similar character. From crystal field theory a d3.2_y2 orbital would have higher energy for octahedral than for tetrahedral coordination. A significant difference in orbital energies between oxidized (more nearly octahedral) and reduced (more nearly tetrahedral) domains may explain the observation of an ESR signal at 77 K and the lack of one at room temperature. On the basis of this premise, the ESR data indicate that the coordination environments of fully oxidized and partially reduced domains in Mo03 differ in the extent of tetrahedral character, as evidenced by the localization of the unpaired electron over two isolated molybdenum atoms at 77 K. The structure of Mo03 can be expressed in terms of two models for the ligand environment about each molybdenum atom.40 In one view each molybdenum atom is surounded by six oxygen atoms, producing a highly distorted octahedral environment. The crystal structure is viewed as double layers of corner sharing octahedra in the (101) plane, stacked along the [010] axis to form sheets. Edge-sharing octahedra between the two sheets form the double layer. In the other view the two longest Mo-0 distances at each molybdenum center are ignored, producing chains of corner-sharing distorted tetrahedra along the [001] axis. Thus, even in its fully oxidized state, Mo03 can be viewed to exist in a distorted tetrahedral environment. Evidence for

a transition from octahedral to tetrahedral coordination in molybdenum oxides is found in other studies. One oxide of molybdenum that formally contains both Mo(VI) and Mo(V) is Mo4Ou.41'42 The crystal structure of this oxide shows coordination around molybdenum that is purely tetrahedral. At reduction temperatures above 850 K in the presence of methanol “disintegration” of single-crystal Mo03 is accompanied by the onset of an electron diffraction pattern characteristic of bulk Mo4On.28 It is perhaps not coincidental that the stoichiometry of this mixed-valence oxide corresponds to that of the reduction site proposed in the Anderson-Hyde model for shear plane formation.37 In summary, the coordination environment of molybdenum in fully oxidized domains of Mo03 is expected to be more nearly octahedral than that of the reduction sites observed with ESR spectroscopy. The relatively lower energy of the d*2_y2-like orbitals at the reduction site effectively isolates the unpaired electron and limits delocalization over only the two molybdenum atoms at the site. At temperatures above 77 K the energy barrier is easily overcome, and exchange produces a loss of ESR signal intensity. Further support for this model is found in many studies of hydrogen-molybdenum bronzes.24’26,29,30’43,44 The UVvis reflectance spectra of such bronzes24 reveal two absorption bands similar to those of bulk Mo4On.46 Although bronzes are usually prepared by treatment of Mo03 with hydrogen at relatively low temperatures, their for(39) Broclawik, E.; Haber, J. J. Catal. 1981, 72, 379-382. (40) Kihlborg, L. Ark. Kemi 1963, 21, 357-364. (41) Kihlborg, L. Ark. Kemi 1963, 21, 365-377. (42) Magneli, A. Acta Chem. Scand. 1948, 2, 861-871. (43) Dickens, P. G.; Birtill, J. J.; Wright, C. J. J. Solid State Chem. 1979, 28, 185-193. (44) Kilborg, L.; Hagerstrom, G.; Ronnquist, A. Acta Chem. Scand. 1961, 15, 1187-1188. (45) Mitchell, P. C.; Williams, R. J. J. Chem. Soc. 1962, 4570-4578.

Mesaros and Dybowski

mation by exposure of the oxide to methanol at higher temperatures has been indicated by powder diffraction studies.29,30

The unique feature of this ESR study is the definitive placement of a proton at the reduction site. With the exception of NMR spectroscopy26 and neutron diffraction,43 the techniques applied to characterize molybdenum oxides are not sensitive to the presence of protons. ESR spectroscopy shows that the slightest reduction of Mo03 at elevated temperatures in the presence of CH3OH, H2, or H20 results in incorporation of a proton at the reduction site.32 A precise placement of the proton relative to the Mo03 lattice is not possible due to uncertainties in the proton hyperfine coupling tensor. Theoretical modeling of the H1Mo2Oi reduction site would provide a better bonding picture for the proton position. It seems odd that treatment of Mo03 under vacuum with H2, H20, or CH3OH yields the same ESR-observable species. Although our experiments provide no specific mechanistic information, the following is offered to rationalize these observations. Clearly, both H20 and CH3OH can function as proton donors, and both H2 and CH3OH can be considered reducing agents. However, since the oxidation of both H2 and CH3OH would liberate H20, all three reagents can directly, or in the case of H2 indirectly, serve as a proton source. H20 also stands apart from the others as an unlikely reducing agent. But given that the treatments were carried out under vacuum at elevated temperature, the following transformation is possible. Mo03

03_*

+ 7202

(4)

Therefore, all three systems provide some extent of proton transfer and reducing conditions. Postulation of specific mechanisms for formation of ESR-observable species is beyond the scope of the data available from our experiments. It is noted that a sequential mechanism involving oxide reduction followed by proton transfer seems unlikely given the results we obtained by first reducing the oxide in CO at high temperature followed by room temperature exposure to D20. A modest gain in intensity of the proton-coupled signal was observed after CO treatment, but no exchange or intensity changes were observed after a deuteron source was presented to the “reduced” sample. Clearly, a source of protons, or deuterons, must be present during the reduction process.

Conclusions ESR spectroscopy of methanol-reduced Mo03 single crystals reveals paramagnetic reduction sites with the stoichiometry ^Mo^. The ESR data on these systems correlate well with other experimental and theoretical data, suggesting crystallographic shear as a significant process in the reduction of Mo03. Deuterium labeling of the methanol shows that the proton at the reduction site originates from the hydroxyl group for reaction at 570 K in the absence of gas-phase oxygen. This result is consistent with the predominance of a dehydration reaction over the oxide when oxygen is not present.

Acknowledgment. D.V.M. thanks the Department of Chemistry of the University of Delaware for fellowship support during the course of this study. We acknowledge Hercules, Inc., for partial support of this work under a grant-in-aid. Registry No. Mo03, 1313-27-5; MeOH, 67-56-1.