Langmuir 1995,11, 3035-3041
3035
Mechanically Activated MOOS. 2. Characterization of Defect Structures G . Mestl,+ N. F. D. Verbruggen, and H. Knozinger* Institut fur Physikalische Chemie, Universitat Munchen, Sophienstrasse 11, 8000 Munchen 2, Germany Received February 27, 1995. In Final Form: May 17, 1995@ The influence of mechanical activation in a planetary mill upon the nature and concentrationof defects in Moo3 powders was investigated by means of diffuse reflectance spectroscopy in the Wlvis regime (DR-UV-vis) and by electron spin resonance (ESR). Defects located at the crystallite surfaces were characterized by infrared spectroscopy of adsorbed CO as a probe molecule. For this purpose, Moo3 was ball-milled in a planetary mill during 600 min. In the DR-UV-vis spectra, the valence-to-conductionband transition exhibits a considerable blue shifi with decreasingparticle size. Furthermore, excitonic absorptions observed in these spectra are also drastically affected by the smaller particle size and probably by the altered crystallite surfaces. A n increasing intensity of the polaron bands was observed. In addition, a linear dependence was obtained between the position of the band attributed to polaron conductance and the logarithm of the carrier concentrationper Mo atom. Both the increasing intensity and the shift of the polaron band revealed that a substoichiometric Moos-, was formed upon mechanical treatment. ESR spectroscopy showed that Moo3 milled for 600 min, and unmilled Moo3 although in much smaller concentration,contained Mo5+centers. The main part of these Mo5+ions had CzUor C4usymmetry. Both samples also contained Mo5+centers interacting with protons in close vicinity. Adsorption of 0 2 did not lead to paramagnetic broadening; hence these Mo5+centers are located within the bulk Moos. In addition, a signal in the ESR spectra of both samples is assignable to free electrons at the crystallite surfaces as revealed by paramagnetic broadening upon 0 2 adsorption. One Mo5+defect species, however, was only detected in milled MOO3 and attributed to the precursor structure of shear defects, thus corroborating the reported XRD and HRTEM results. The high surface sensitivity of the IR technique using adsorbed probe molecules revealed the formation of coordinatively unsaturated (cus)Mo4+surface states in Moo3 samples which were mechanically activated. 1. Introduction In the first contribution of this series of papers we communicated on the changes in particle size, morphology, and crystallinity of Moo3 upon mechanical activation in a planetary mill during 600 min.l The BET surface area increased from about 1.3m2/gfor the educt to 32 m21gfor the final product and the mean particle size calculated using these BET data decreased from about 1pm to about 50 nm. The calculation of the dimensions of X-ray coherent regions also showed a decreasing crystallite size from about 180 nm to about 70 nm. The particle size reduction was confirmed by SEM and HRTEM.2 The formation of ultrafine amorphous material was indicated by the difference between the particle size as calculated from BET and XRD data and confirmed by an amorphous scattering background in the X-ray patterns and by SEM and HRTEM images. The variations in the X-ray profile quality and in the amorphous scattering background during mechanical activation were considered to point toward a complex process of particle size reduction. Anomalous X-ray reflection intensities, asymmetric reflection profiles, as well as disappearance or appearance of small diffraction peaks indicated the formation of shear defects which were also detected by HRTEM. Internal strain, therefore, was only marginally enhanced by mechanical activation. One effect of this mechanical treatment was the change in color from pale yellow for unmilled Moo3 to a deep grayish blue for the final product. This observation
* To whom correspondence should be addressed. Present address: Abt. Obertlachenchemieund Katalyse, Universitat Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany. Abstract published in Advance ACS Abstracts, July 1, 1995. (1)Mestl, G.; Herzog, B.; Schlogl,R.; Knozinger, H. 1996,11,3027. ( 2 ) Uzhida, Y.; Pfdnder, N.; Weinberg, G.; Herein, D.; Schlogl, R.; Mestl, G.; Knozinger, H. To be published. +
@
0743-7463195f2411-3035$09.00/0
pointed to the formation of certain defects in Moo3. These defect structures may also affect the electronic properties ofMo03. This should be reflected in the electronic as well as in the electron spin resonance (ESR) spectra of mechanically activated MoO3. Unsaturated chemical bonds at grain boundaries may also lead to effects in the ESR spectra. Such centers, if located at the surface, may also be detected by the adsorption of probe molecules like CO. The electronic properties of a solid are determined by its valence electrons which are described by delocalized wave functions forming bands extending throughout the crystal. The most exact values for band gap energies are determined by optical absorptiodreflection measurements. The threshold for a continuous optical absorption at w g is determined by the band gap energy E, = Additional absorption bands in the spectra arise from exciton (bound electron-hole pair) formation. The intensity of defect induced optical transitions is proportional to the concentration of these defect^.^ Thus, studying the optical spectra of milled Moo3 should yield information on defect formation, concentration, and stability. In general, mechanical stress results in the breakage of chemical bonds and may thus lead to highly reactive radical centers. ESR spectroscopy is used for the investigation of paramagnetic materials and to characterize radicals. ESR spectroscopy can yield information about the oxidation state, symmetry around the active center, and the kind of surrounding atoms. The interpretation of ESR spectra, however, of these radical centers in solids is very complex, and an unequivocal assignment of single signals to distinct structures is often impossible. ( 3 ) Kittel, Ch. Introduction to Solid State Physics, 6th ed.; Wiley: New York, 1986. (4) Greenwood, N. N. Ionic Crystals, Lattice Defects and Nonstoichiometry; Buttenvorth: London, 1967.
0 1995 American Chemical Society
3036 Langmuir, Vol. 11, No. 8, 1995
Metal oxides show some degree of selectivity in chemisorption of probe molecules. The extent of adsorption is depending on the chemical nature of the oxide and the temperature of adsorption. The electronic configuration of the adsorbate and of the surface center as well as the coordination of the latter are the most important parameters which control the selectivity of a given adsorption site.5 IR spectroscopy allows the observation of internal modes of surface complexes between probe molecules and specific surface sites. Especially CO is used as a probe molecule to characterize surface sites, since it is almost chemically inert.6 Using the above described techniques, one can gain information about the nature and concentration of different types of defects which may be formed during the mechanical activation ofMo03. This knowledge is ofbasic interest for the understanding of the electronic and ionic conductivity of this oxide, both of which in turn may influence the catalytic properties of Moo3, e.g., in selective catalytic oxidation processes.
Mestl et al.
n
8
? c
-IJI
U
2. Experimental Section MOOS(Merck, p.a.) was ground in a planetary mill with 145 rpm over an extended period of time (for a detailed description of the planetary mill see ref 1). During the milling process, samples were drawn at distinct times (10,20,60,120,180,240, 420, and 600 min; leading to the notation MoOO, MolO, ...,Mo600) and the BET surface area was measured by Nz adsorption in a Quantasorb jr. apparatus. After a constant BET surface area was reached, the milling process was discontinued. Room temperature diffuse reflectance spectra in the ultraviolet and visible regime (DR-UV-vis) were recorded on a Lambda 15 diffuse reflectance spectrometer (Perkin-Elmer) using 10mm cells directly after mechanical activation with a wavelength reproducibility of 0.1 nm and a spectral resolution of 0.25 nm. The base line corrected UV-vis diffuse reflectance spectra (Bas04 was used as white standard)were recorded in the range between 250 nm (4.96 eV) and 800 nm (1.55 eV) with a scan speed of 30 ndmin. ESR spectra were recorded on a Varian E-Line spectrometer (E9)equipped with a TElod-mode cavity in the X-band a t 90 and at 300 K. The system was tested for saturation. There was a linear relation between signal intensity and the square root of the power applied up to 20 mW, and thus all spectra were recorded using 10 mW of microwave power. Mn2+ions in a MgO matrix measured simultaneously in a second cavity were used for field calibration. To reduce paramagnetic interaction with adsorbed oxygen, the samples were purged with dry NZa t room temperature for 18 h prior to the measurements. Infrared transmission spectra were recorded in the wavenumber range 2300-1900 cm-' using a Perkin-Elmer Model 580 B spectrophotometer. The spectral resolution was 6.8 cm-l, and the wavenumber accuracy was k l cm-l in this frequencyregime. Self-supporting wafers, pressed between two sheets ofmica with a pressure of @bout20 MPa, were mounted in an in situ cell,' which was evacuated, purged with Nz, evacuated again, and cooled down to 90 K. Then the IR transmission of the empty in situ cell and of the sample wafer was measured. After the cell was refilled with 75 hPa CO, the IR spectrum of CO adsorbed on the wafer was recorded. Subsequently, the wafer was removed from the IR beam and the IR transmittance of the CO atmosphere in the in situ cell was recorded. The spectra shown are obtained as difference spectra of the sample with CO adsorbed minus the sample without CO, minus the gas phase in the in situ cell. The Moos fractions with the lowest BET surfaces, Le., those having ( 5 ) Fierro, J. L. G.; Garcia del la Banda, J. F. Catal. Rev.-Sci. Eng. 1986. 28. 265.
(6jKnozinger, H. Fundamental Aspects of Heterogeneous Catalysts Studied by Particle Beams; Brongersma, H . H., van Santen,R. A., Eds.; Plenum Press: New York, 1991; p 167. (7) Kunzmann, G. W.D. Thesis, University of Munich, 1987. (8) Goodenough, J. B. Proceedings of the Climax 4th International Conference on the Chemistry and Uses of Molybdenum; Barry, H . F., Mitchell, P. C. H., Eds.; Climax Molybdenum Corp.: Ann Arbor, MI, 1982; p 1.
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Energy [eV]
Figure 1. UV-vis diffise reflectance spectra of mechanically activated MOOS. Spectra a-i correspond to increasing BET surface area from 1.3 to 32 m2/g. the largest crystallites (unmilled Moos to 60 min milling), did not give stable wafers.
3. Results and Discussion 3.1. W-vis Diffuse Reflectance Spectroscopy. In Figure 1 the UV-vis diffuse reflectance spectra of differently mechanically activated MOOSsamples are reproduced (for better visualization, spectra are shifted vertically). The spectra are represented as log(llR,) vs energy, R, being the reflectance at infinite sample thickness. The spectra a to i were recorded after 0, 10, 20, 60, 120, 180, 240, 420, and 600 min milling time, respectively. The usually broad bands observed in UV-vis spectra contain information which can be extracted by calculating or measuring the first or second derivatives of the spectra.s Changes in the spectra (Figure 1) observed due to the milling process are more pronounced in the first derivatives (Figure 2): (1) The general position, intensity, and shape of the absorption spectrum changes during the milling process (Figure la-i). The absorption edge, however, does not shift, which is confirmed by the derivative spectra (Figure 2). In Table 1the main critical points of these derivative spectra are summarized. The absorption edge (marked in Figure 2 by an A) only shifts from 3.22 to 3.28 eV. (2) The maximum in the direct spectra located at 3.76 eV for MoOO (Figure l a ) broadens and loses intensity. It remains as a shoulder in the DR-UV-vis spectrum of Mo600 (Figure li). This is also confirmed in the first derivative spectra (Figure 2) where the structure marked B stays almost constant and the absorption energies are also summarized in Table 1. (3) This band seems to be overwhelmed in the spectra ofthe final stages (Figure le-i) by bands located at about 4.1 eV and about 4.96 eV. Again the first derivatives (Figure 2) show these changes more drastically. The additional band at about 4.2 (marked by C ) starts to grow in spectrum d of Figure 2 (after 60 min of milling) and, in addition, spectral changes above 5 eV, outside the range
Langmuir, Vol. 11, No. 8, 1995 3037
Mechanically Activated Moo3
beyond 3.1 eV containing three peaks at 3.64, 4.86, and 5.39 eV. The first two correlate with the values reported by Deb and Chop~orian.'~The broad maximum above the absorption edge at 3.76 eV and the shoulder at 3.94 eV (spectra a in Figures 1 and 2) are thus assigned to exciton formation in Moo3 (Figure 3). The two small bands at about 2.0 and below 1.5 eV are attributed to Mo5+ Mo6+intervalence t r a n s i t i o n ~ l ~ - ~ ~ and are discussed in detail in the next section. MOOSMilled for 600 min (M0600). In the classical local approximation the interaction of electromagnetic waves with matter is described by Maxwell's equations. However, the dielectric constant is a function not only of the frequency but also of the wave vector of light, thus leading to nonlocal effects contributing to the overall signal (reflectivity, absorption). Nonlocal effects generally are large when surface contributions are important.21 The maximum reflectivity of Moo3 single crystals at 2.6 eV is reported to shift to 2.8 eV for thin films.22 While the minimum absorption to MoOO is observed at 2.58 eV, it is located at 2.79 eV in Mo600 (Table 1). If an electromagnetic wave is acting on a solid, the interaction of the free electrons in the solid with the external field can be understood in terms of an acceleration into the direction ofthe field. These accelerated electrons are specularly reflected at surfaces and interface^.^^ Excitons are also considered to move under the influence of an electrical field. Since excitons are considered to have sizes of several nanometers, the reflection at interfaces leads to an exciton depletion layer 3 times thicker than the exciton size.14 Therefore, optical spectra of excitonic crystals are extremely sensitive to the detailed structure of the surfaces, even for thicknesses much larger than the characteristic width of the exciton depletion layer. To date, it is not fully understood how the exciton behavior is changing as the physical space available is reduced.14 Since the Moo3 particles with the highest BET surface area are about 50 nm in average diameter,1,2a considerable influence of the reduced space and of the particle surface can be expected. Spectral changes above 3.0 eV, assigned to excitons, between MoOO and Mo600 (Figures 1 and 2, parts a-i, and Table 11, thus, may be attributed to changes in size and morphology of the crystallites. Furthermore, exciton states are also changed by defect cluster formation (e.g., the formation of shear defects1r2)or by interaction with protons in the lattice. Protons may be introduced by adsorption of water at coordinatively unsaturated sites (cus) created during mechanical activation of Moo3 (see section 3.2 and refs 23 and 24). Thus, the band at about 3.5 eV (Figure 1 and 2) may be attributed to excitons arising from Mo5+ center^^^-^^ which probably are in a
-
1.5 2.0
2.5
3.0
3.5 4.0
4.5
5.0
Energy [eV]
Figure 2. First derivatives with respect to energy of UV-visdiffuse reflectance spectra of MOOS: Spectra a-i correspond to increasing BET surface area from 1.3 m2/gt o 32 mVg. of the spectrometer, are also reflected by the changing shapes of the first derivatives. (4) The minimum located at 2.58 eV in the spectrum of MoOO (Figure la), indicated in the derivative spectra (Figure 2) by the first zero value, is shifted to 2.79 eV in the spectrum of Mo600 (spectrum i of Figures 1 and 2). The value of the minimum reflectivity is also summarized in Table 1. (5) The shoulder at 2.0 eV and the band located below 1.55 eV, which are of low intensity in the UV-vis diffuse reflectance spectrum of MoOO (Figure la, see also first derivative spectra of Figure 2), gain intensity during the milling process. These bands significantly increase in intensity from spectrum f to spectrum i in Figure 1. Umilled Moo3 (MoOO). Moo3is a photosensitive n-type s e m i c o n d u ~ t o rwith ~ ~ ~ indirect band gaps that have reported widths between 2.9 eVIO and 3.15 eV1' for MOOS single crystals (Figure 3). The valence band is generated by oxygen 2p z-orbitals,l2 while the conduction band is formed by overlapping metal 4d and 5s 0rbita1s.l~The position of the presently observed absorption edge of unmilled Moo3 (Figure l a and Figure 2a) at 3.2 eV coincides well with the reported values. If light is absorbed in matter, electrons and holes are formed. The electron-hole pair may form a bound state, the exciton, which bears similarity to the hydrogen atom.14 Stoichiometric Moo3 is reported to have three bands at 3.7, 4.3, and 4.5 eV which were attributed to exciton f0rmati0n.l~ Tinet et a1.16 also recorded a broad band (9) Pichat,P.;Mozzanega,M.-N.;Hoang-Van,C . J .Phys. Chem. 1988,
92, 467.
(10)Krylov, 0.Catalysis by Non-Metals; Academic: New York,1980. (11) Erre, R.; Legay, M. H.; Fripiat, J. J. Surf. Sci. 1983, 127, 69. (12)Deb, S. K. h o c . R. SOC.London, A 1968,304, 211. (13) Goodenough, J. B. Progress in solid state chemistry; Reiss, H., Ed.; Pergamon: London, 1971; Vol. 5, p 145. (14) Halevi, P. Electromagnetic Waves, Vol. 1, Spatial Dispersion in Solids and Plasmas; Halevi, P., Ed.; Elsevier: Amsterdam, 1992; p 339. (15) Deb, S.K.; Chopoorian, J. A. J.Appl. Phys., 1966, 37, 4818.
(16) Tinet, D.;Canesson,P.;Estrade, H.;Fripiat,J. J. J.Phys. Chem. Solids 1979, 41, 583. (17)Faughnan, B. W.; Crandall, R. S.;Heyman, P. M. RCA Rev. 1968,36, 177. (18) Hollinger, G.; Duc, T. M.; Deneuville, A. Phys. Rev. Lett. 1976, 37, 1564. (19) Crandall, R. S.;Faughnan, B.W. Appl. Phys. Lett. 1976,28,95. (20)Louis, C.; Che, M.; Bozon Verduraz, F. J. Chim. Phys. Phys.Chim. Biol. 1982, 79, 803. (21) Lopez-Rios,T. Electromagnetic Waves, Vol. 1, Spatial Dispersion in Solids and Plasma; Halevi, P., Ed.; Elsevier: Amsterdam, 1992; p 215. (22) Fuchs, R. In Electromagnetic Waves, Vol. 1, Spatial Dispersion in Solids and Plasma; Halevi, P., Ed.; Elsevier: Amsterdam, 1992; p 289. (23) Mestl,. G.:. Srinivasan. T. K. K.: Knijzinger, - . H. Submitted to Langmuir. (24) Mestl, G.; Verbruggen, N.; Knijzinger, H. To be submitted to Langmuir. (25) Che, M.;Figueras, F.;Forissier, M.; McAteer, J. C.; Perrin, M.; Portefaix, J. L.; Praliaud, H. Proc. Int. Congr. Catal., 6th 1977,1,261. (26) Giordano, N.; Bart, J. C. J.;Vaghi, A,; Castellan, A.;Martinotti, G. J . Catal. 1976, 36, 81.
3038 Langmuir, Vol. 11, No. 8, 1995
Mestl et al.
Table 1. CharacteristicFeatures in the First Derivatives (Figure 2a-i) of the DRW-vis Spectra (ev) of Mechanically Activated Moos (Figure la-i) 1. zero value 2. zero valudor shoulder maximumA 1. shoulder B 2. shoulder C sample
minimum absorption
maximum absorption
absorption edge
exciton bands
exciton bands
2.58 2.65 2.68 2.72 2.71 2.72 2.76 2.78 2.79
3.76 3.86 3.90 4.21 sh 4.36 sh 4.40 sh 4.63 sh 4.57 sh 4.62 sh
3.22 3.25 3.27 3.27 3.30 3.30 3.28 3.28 3.28
3.94 3.67 3.69 4.0 br 3.67 3.67 3.65 3.65 3.70
4.08 4.09 4.09 4.06 4.10
MoOO MoOlO Moo20 Moo60 Mol20 Mol80 Mo240 Mo420 Mo600
Exciton
2
,,
,
,
i3.1 eV 1 13.7eV
Mo5+-
E
c
0
I
8
0.5
\ K
Figure 3. Schematic diagram of the energetic levels of electron states in MoOQ(-~).
2
+ u
B
tetragonally distorted 6-fold environment (C4" or D w ) ~ ~ 0.2 P'Mo,O,, and in close neighborhood to protons29(see section 3.2). ,,@Moo,: 3 2 m2/g In addition, for Mo03*H20two bands are reported at 4.42 ,, and 3.64eV0v31which are close to the absorptionsobserved , for the present sample at about 4.2eV and 3.5 eV (Figure li). 0.0 If Mo6+ions (Moos) are reduced to Mo5+(Mosuboxides), 2.0 2.1 2.2 2.3 2.4 the Mo5+ centers, through Mo5+-O-Mo6+ interactions, form an additional conduction band above the valence Energy [eV] band (see Figure 3). The additional charge carriers, injected into these conduction bands by reduction, distort Figure 4. Linear relationship between the band position due to polaron conductance at about 2 eV and the carrier conthe lattice in their surroundings and the coupled system centration: 0,data from ref 35; 0,data from ref 17; 0, this electron-lattice distortion is called polaron.32 Porter et study. al.33observed these polaron or intervalence bands at 2.48 eV for MoO2, at 2.13,2.42,and 1.3 (sh) eV for Mo4011, and ing their band position with the data reported by Porter at 2.11 and 1.3 (sh) eV for M09026. The systematic shift et al.= Analogously, A r r i s et al.35reported the band, which of the absorption in their spectra from 2.48 to 2.11 eV they assigned to Mo5+, to shift toward higher energies with decreasing metal-oxygen ratio was correlated by with increasing electron donation. these authors to the number of carriersby applying Meyers In Figure 4,the band positions observed by Porter et rule34 al.33and by Tinet et a1.16 are plotted against the number of electrons per Mo ion on a half-logarithmic scale. From nd*n,=AeKv . (1) this linear relationship between carrier concentration and band position, the number of electrons per Mo atom can where nd is the number of d-electrons, n, is the number be estimated for the present samples. The band position of carriers, Y is the frequency, and A and K are proporof 2.1 eV observed for Mo600 gives a degree of reduction tionality constants. Plotting the measured band positions of about 0.2 electron per Mo atom (see also section 3.2), Y against the logarithm of the number of electrons, Porter ascompared to MoOO (2.0eV)having less than 0.02electron et al.33obtained a linear correlation between the Mo5+ per Mo atom. band position and the statistical number of electrons per The band at about 1.5 eV is also attributed to interMo center. valence electron transfer or polaron conductance along Tinet et al.,16 who observed a polaron band a t 2.26 eV -Mo5+-02--Mo6+ ~hains.l'-~l Photoelectron data for in H,MoO3 estimated a carrier concentration of 0.43 4d Moo3 films support the intervalence or small polaron electrons per Mo atom in the hydrogen bronze by comparabsorption On the other hand, the blue color Moo3 thin films3' was attributed to an observed for (27) Castellan, A.; Bart, J. C. J.; Vaghi, A.; Giordano, N. J. Catal. 1976,42, 162. (28) Praliaud, H. J . Less Common Met. 1977,54,387. (29) Gmelin, Mo-Erg. Bd., Bl; Chapter 3.5.3.4.1. and Chapter 3.5.3.4.3. (30) Vedrine, J. C.; Praliaud, H.; Meriaudeau, P.; Che, M. Surf. Sci. 1979,80, 1010. (31)Asbrink,S.; Brandt, B. G. Chem. Ser. 1971,1, 169. (32) Tilley, R. J. D. Defect Crystal Chemistry and its Applications; Blackie & Son: London, 1987.
(33) Porter, V. R.; White, W. B.; Roy, R. J. Solid State Chem. 1972, 4, 250. (34) Meyer, W.; Neddel, H. Phys. Z.1937,38,1014. (35)Arris, J.; D u e , J. A. J . Chem. Soc., 1964, 1116. (36) Schirmer,0.F.; Wittever,V.; Baur, G.;Brandt, G. J.Electmhem. SOC. 1977,124,749. (37) Sanchez, C.; Livage, J.; Launay, J. P.; Fournier, M.; Jeannin, Y. J. Am. Chem. SOC.1982,104,3194.
Langmuir, Vol. 11, No. 8, 1995 3039
Mechanically Activated MOOS g
= 2.003 1.95
-87
r
i 320
340
360
380
Field Strength [mT]
Figure 6. ESR spectra O f MOO3 after purging with Nz at room temperature for 18h: (a)unmilled MOO3 (300 K); 50-nmMoo3 crystallites at (b) 300 K and (c) 90 K. electron transfer from an oxygen 2p-orbital to a molybdenum 4d-orbital which creates an isolated Mo5+oxidation state. Thus, the literature not being coherent, the broad absorption around 1.5 eV can be assigned either to polaron conductance (Mo5+-c Mo6+or Mo5+-O-Mo6+ intervalence transition) or to an 02- Mo6+charge transfer (see also section 3.2). 3.2. ESR Spectroscopyof MechanicallyActivated MOOS. Unmilled MOOS(MoOO). The room-temperature ESR spectrum of unmilled Moo3 (Figure 5a), recorded after purging with Nz at 300 K for 18 h, exhibits a very broad asymmetric signal characterized by axial symmetry withg, = 1.946 andg,,= 1.871. It can be assigned to Mo5+ ions in distorted octahedral coordination. The symmetry of this center is most probably higher than C2", near to C4". Since the site symmetry of Mo6+ in Moo3 is C,, the additional negative charge on the Mo center seems to lead to a distortion of the lattice. A calculation of the crystal field parameters gave splittings between the xy and the (x2 - y2), or the xz levels, of 2.84 and 1.65 eV, respectively. A comparison with the UV-vis spectrum (a) of Figure 1 shows an increasing absorption at about 1.5 eV. An absorption at 2.8 eV, however, is not detectable due to the overwhelming band gap transition at about 3 eV. A second ESR signal can be attributed to a site in orthorhombic symmetry withg, = 1.889 and with hardly differing g, and g, values. Alternatively, it can also be assigned to a site having axial symmetry withgL = 1.975 and being split by hyperfine interactions with a nuclear spin I = l/Z (e.g., a proton). Since the signal position relative to the first Mo5+ signal is at lower fields, the assignment to orthorhombic symmetry would imply that the oxidation state of this center is 3+. This is indeed very unlikely. Hence, the alternative description as a Mo5+ interacting with a proton seems more reasonable. Protons due to OH groups are reported for Moo3 single crystals grown in air.45 In addition, UV-vis spectroscopy corroborate the (see section 3.1) and IR spectroscopy23~24
-
presence of protons (HzO or OH groups) even after heat treatment at 670 K in NZfor 1h. In addition, a sharp isotropic signal can be found at g = 2.003 close to the value of free electrons. Carbon contamination or organic radicals could be the reason for this signal, but high temperature in situ experiments under NZor Oz38exclude this possibility. Hence, the signal must be assigned to free electrons present in Moo3. Exposure of the sample to 0 2 at room temperature leads to dipolar line broadening, suggesting that the free electrons are located at the surface. Moo3 Milled for 600 min (Mo600). An asymmetric signal a t g = 1.95 is visible in the room-temperature ESR spectrum ofMoGOO, recorded after purging with NZat 300 Kfor 18h (Figure 5b). Its intensity, however, is about 20 times greater as compared to the unmilled sample. The room-temperaturespectra do not show any additional fine structure in the resonance signals (Figure 5b). Spectra recorded at 90 K (Figure 5c) exhibit a better resolution and increased signal intensity. The most intense signal is due to a site in orthorhombic symmetrywithgl = 1.957,gz = 1.944, andg3 = 1.871. The g-values are comparable to those reported for Moo3single crystals39and are characteristic for Mo5+ ions in orthorhombic distorted, 6-fold coordination. Dyrek and Labanowska40 and Serwicka and Schindlefll also detected this signal and assigned it to 6-fold coordinated Mo5+.The symmetry around this Mo5+ center is slightly more distorted as compared to the'Mo5+species observed for unmilled MOOS. This may be explained by the destruction of the Mo03-lattice due to mechanical activation.1,2 Furthermore, it is known that defects in Moo3 form ordered shear structures with largely varying Mo positions ranging from octahedral to tetrahedral coordination (see also ref 1). Exposing the sample to 0 2 at 300 K did not cause line broadening due to dipole-dipole interaction. Hence, the Mo5+ centers are not located at the sample surface but in the bulk. A calculation of the crystal field parameters gave splittings between the xy and the (x2 y 2 )level of 2.84 eV, similar to the major signal observed in MoOO. The split of 1.59 eV between the xy and the xz levels is smallar than that measured for MoOO, while the split of about 2.0 eV between the xy and the y z levels was unresolved for MoOO. A comparison with the UV-vis spectrum (i) of Figure 1again shows that an absorption at 2.85 eV cannot be detected due to the overwhelming band gap transition. On the other hand, bands are observed at about 1.5 and 2.0 eV that can be assigned to polaron conductance (Mo5+-O-Mo6+intervalence transfer) known for molybdenum suboxides. As in MoOO, a second weak signal is detected with g, = 1.975, gll = 1.889 exhibiting hyperfine coupling (Al= 0.39 mT) with a nuclear spin1= l/z (vide supra). Shelimov et al.42ascribed this signal to Mo6+ ions in distorted octahedral coordination. Sperlich et al.43estimated an effective distance of about 0.26 nm between the Mo site and the proton. Additionally, a whole series of nonsto(38)Mestl, G.; Verbruggen,N.; Lange, F.; Tesche,B.; Knazinger,H. To be submitted to LanPmuir. (39)Ioffe, V. A.; Patrilna, I. B.; Zelenetskaya, E. V.; Mikheeva, V. P. Phys. Status Solidi 1969,35, 535. (40)Dyrek,K.; Labanowska, M. J.Chem. SOC.,Faraday Trans. 1991, 87. 10003. ~ ~ - - (41)Serwicka, E.;Schindler, R. N. 2.Phys. Chem. (Munich) 1982, 133, 175. (42)Shelimov,B.N.;Pershin, A. N.; Kazansky,V. B. J.Catal. 1980, 64, 426. (43)Sperlich, G.; Frank, G.; Rein, W. Phys. Status Solidi b 1971,54, 241. (44)Schollhorn, R.Angew. Chem. 1980,92,1015. >
3040 Langmuir, Vol. 11, No. 8, 1995 ichiometricH,Mo03 bronzes is knownin the literature.44-46 Protons usually adsorb on the basal planes OfMOO3 slabs,46 formed by Mo-0 groups along the b axis. But protons are mobile and also diffuse into the host l a t t i ~ e . Thus, ~~,~~ due to the short proton distance and the fact that oxygen does not lead to dipolar broadening, the Mo5+ions must be located in the bulk. This ESR signal also shows hyperfine splitting due to 95Mo or 97Mo nuclei, in the unmilled as well as in the mechanically activated sample. Again a sharp isotropic signal is observed at g = 2.003 assigned to free electrons in the sample. As for MoOO, the free electrons are affected by oxygen exposure indicating their presence at the surface. A weak signal, with g-values of g, = 1.935 and gll = 1.901, is observed only in the mechanically activated sample. A calculation of the crystal field parameters revealed a large splitting of 3.6 eV between the xy and the (x2- y2)levels, and a split of 1.4 eV between the xy and xz levels. In comparison to unmilled MOOSwhere only distorted Mo5+species are detected, this Mo5+center has a higher symmetric coordination (symmetry almost C4J with shorter Mo-0 bonds in the equatorial plane. These short Mo-0 distances (as compared to crystalline Moos) are also formed in crystallographic shear structures, like Mo18052.47~48 Dyrek et al.40ascribed this ESR signal to the precursor structure of crystallographic shear planes. The 20 times higher ESR signal intensity of Mo600 as compared to that of MoOO confirms a considerable degree of reduction, and thus, shear defects may be formed.1,2 A quantitative measurement of the different Mo5+ centers in both samples was not possible, since the signals are not resolved at 300 K. Even the determination of the total concentration of reduced centers is impossible, since it is not guaranteed that all Mo5+centers are unpaired. Abdo et al.49and Latef et al.50claim that only 10% of the Mo5+centers in supported Mo catalysts are ESR active. Even, the temperature dependence of the different Mo5+ signals could not be obtained, since the signals were only resolved at 90 K. Furthermore, ESR spectroscopy of both samples reveals that the Moos batch (Merck) used for these experiments is contaminated with Cu and Fe. The detection of the ESR signals in the unmilled sample proves that these contaminations do not arise from agate abrasion during the milling process. XPS spectroscopic characterization of these samples, in addition, did not show any trace of these impurities. A possible seggregation of these impurities to the particle surfaces must be ruled out. 3.4. IR Spectroscopyof CO Adsorbedon Activated MOOS. A real surface-sensitivemethod to explore reduced centers is IR spectroscopy of adsorbed probe molecules, e.g., CO. In Figure 6 the IR transmission spectra of CO adsorbed on mechanically activated Moo3having W e r e n t BET surface areas are reproduced. The spectra of Figure 6 were normalized since the absolute intensity is not comparable due to the unknown (and uncontrolable) waver thicknesses (concentration of absorbing centers). The spectra show a broad band at about 2170 cm-l with an asymmetry to lower wavenumbers. Since CO does not (45) Ritter, Cl.; Muller-Warmuth, W.; Schollhorn,R. J.Chem. Phys. 1985,83, 6130. (46) Mehandru, S.P.; Anderson, A. B. J.A m . Chem. SOC.1988,110, 2061. (47) Kihlborg, L. Ark. Kemi 1963,21, 357. (48) Kihlborg, L. Ark. Kemi 1963,21, 443. (49)Abdo, S.; Clarkson, R. B.; Hall, W. K. J.Phys. Chem. 1976,80, 2431. (50) Latef, A.; Aissi, C. F.; Guelton, M. J. Catal. 1989, 119, 368. (51)Diaz, A. L.; Bussell, M. E. J. Phys. Chem. 1993, 97, 470.
Mestl et al.
lo'
O5I
2230
2180
2130
2080
Wavenumbers [cm-'1
Figure 6. Transmission IR spectra of CO (75 mbar CO pressure, 80 K)adsorbed on mechanically activated MOOSwith increasing BET surface areas (normalized band intensities): (a) 18 mVg; (b) 21 mVg; (c) 30 m2/g; (d) 32 m2/g. coordinate to oxidic M o ~ +bands , ~ ~ in the range between 2195 and 2170 cm-l were attributed to CO terminally coordinated to unsaturated Mo4+centers in H2-reduced supported Mo catalyst^.^^ The asymmetry extending to 2150 cm-' can be attributed to physisorbed C053 by analogy to the band observed for physisorbed CO onAl203. Contributions to the asymmetry may also arise from CO adsorbed to a second Mo4+ species having a different coordination. XPS measurements on reduced Mo oxidess4 detected two different Mo4+species, isolated centers and Mo4+ coupled by metal-metal bonds across an oxygen vacancy. Nonvanishing components of OH stretching modes (not shown) at 3511,3338,3211, and about 3054 cm-' and of HOH deformation modes at about 1630 to 1400 cm-l in the difference spectra (see also ref 23) corroborate an interaction of CO with OH groups in Mo600, since undisturbed bands should cancel out in the difference spectrum. The OH stretching frequencies observed below the OH' gas phase value of 3735.2 cm-l 55 can be assigned to H bonded OH-groups (H bridges between surface OH groups or between adsorbed H20 and surface OH groups). The four different OH stretching frequencies observed may point to different OH or H2O species on Mo600. In addition, Mo-oxygen combination and overtone modes are observed at about 1930, 1960, and 1980 cm-l in the difference spectra (not shown). Their observation reflects an interaction of CO molecules with MoA+centers since unperturbed signals should cancel out in the difference spectrum. The interaction of CO with MoA+centers on (52)Delgado, E.; Fuentes, G. A,; Hermann, C.; Kunzmann, G.; Knozinger, H. Bull. Soc. Chim. Belg. l9f34, 93, 735. (53) Zaki, M. I.; Vielhaber, B.; Knozinger, H. J. Phys. Chem. 1986, 90, 3176. (54)Griinert, W.; Yu Stakheev,A.; Feldhaus, R.; Anders, K.; Shpiro, E. S.;Minachev, Kh. M. Proceedings ofthe 10th International Congress on Catalysis, Budapest, 1992; Guczi, L., Solymosi,F., TBtBnyi,P.,Eds.; Akadgmiai Kiad6: Budapest, 1993; p 1053. (55)Jones, L. H. J. C h e n . Phys. 1954,22,217.
Langmuir, Vol. 11, No. 8, 1995 3041
Mechanically Activated Moo3 Mo600 is corroborated by additional absorptions in the fundamental mode regime of the difference spectrum (not shown). 4. Conclusions The DR-UV-vis spectra exhibit considerable changes with decreasing particle size. Excitonic absorptions are also affected by the smaller particle size as well as the altered crystallite surfaces. The excellent agreement between the position of the band attributed to polaron conductance, which was observed for microcrystalline Moo3, and the linear dependence on the carrier concentration, as well as the increasing intensity of the polaron bands, reveals that a substoichiometric MoOa-, is formed upon mechanical activation. Mo600 and MoOO, although in much smaller concentration, contain Mo5+centers. The main part of these Mo5+ ions has 6-fold coordination with Czu or C4v symmetry. Both samples also contain 6-fold coordinated Mo5+centers interacting with OH groups in close vicinity. In addition, Mo5+centers are detected in milled Moo3 having almost Clu symmetry. These Mo5+ ions are suggested to be the
precursors of a crystallographic shear structure. Exposure to 0 2 reveals that all these Mo5+centers are located in the bulk and not on the surface. A further signal is detected that is assigned to free electrons. Oxygen exposure affects the corresponding ESR-signal revealing the presence of free electrons at the surface. The higher surface sensitivity of the IR technique suggests the formation of cus Mo4+ surface states in mechanically activated Moo3 samples. In addition, nonzero absorptions in the Mo-oxygen stretching regime (combination modes and fundamental modes) indicate that CO adsorption affects the Mo=O and Mo-0 bonds. Furthermore, CO adsorption reveals an interaction of CO with surface OH groups.
Acknowledgment. This work was financially supported by the Deutsche Forschungsgemeinschaft (SFB 338) and by the Fonds der Chemischen Industrie. The authors also like to thank Dr. B. Muller for her assistance with the of IR spectroscopy. LA950146P
