Langmuir 1996, 12, 51-56
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Experimental Hyperfine Characterization of Adsorbed Molybdates on Oxide Surfaces† F. G. Requejo*,‡ and A. G. Bibiloni§ TENAES, Departamento de Fisica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, CC/67, 1900 La Plata, Argentina Received September 1, 1994. In Final Form: January 10, 1995X The dispersion of metal oxides on supports is a problem currently being investigated in supported catalysts. On the other hand, a nuclear spectroscopy technique, namely time differential perturbed angular correlation (TDPAC), has been used in extensive investigations on Mo-based catalysts. This technique, through the measurement of the local electric field gradient (EFG) at radioactive probes sites, can give information about the characteristics (coordination, symmetry, distortions, etc.) of the different environments of the probes, their concentration and modifications related to in situ conditions (temperature, atmosphere, pressure, etc.) TDPAC technique was used in the present work to investigate the dispersion of molybdenum oxides on Al2O3, TiO2/Al2O3, and SiO2. The 99Mo obtained by neutron irradiation of natural molybdenum present in the samples was used as TDPAC probe. Measurements at room temperature and higher ones as well as with different Mo concentrations in the samples were performed. The corresponding results allowed us through the fitted hyperfine parameters to find out the dispersion, type of adsorbed Mo-O species, and their relative abundances. For Mo concentrations below the theoretical monolayer we were able to find bulklike MoO3 and two-dimensional-like Mo oxides. Previous results obtained with TDPAC and calculations based on a point charge model suggest different MoO3 crystallites sites for each support. The structures of the monolayer type molybdena are probably different in each catalyst.
1. Introduction In heterogeneous catalysis many catalysts consist of oxide species adsorbed on supports with adequate surfaces. The surfaces must have high effective areas and certain reactivity to adsorb the catalytic precursors. These properties of the supports can affect the structure of the supported species. Once adsorbed the reagents can interact. To this purpose the dispersion of the oxides is important. The dispersion or spreading of metal oxides on supports is a problem currently investigated in the supported catalysts. It is believed that the surface morphology of the support is the first cause for the dispersion and that dispersion has influence on the activity and selectivity of heterogeneous catalysts, especially for multicomponent ones like those used for partial oxidation reactions. In summary, the characteristics of the supports are related with the structure and the dispersion of the active species, i.e., with the catalytic activity. For supported molybdena, a variety of supports like Al2O3, SiO2, TiO2, CeO2, SnO2, MgO, and SiO2/Al2O3 have been studied. In those cases the influence of surface morphology on Mo dispersion was determined.1-7 In particular, significant differences in the effects produced on MoO3 were observed when Al2O3 or TiO2 were employed † Presented at the symposium on Advances in the Measurement and Modeling of Surface Phenomena, San Luis, Argentina, August 24-30, 1994. ‡ Fellow of Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas, Argentina. § Member of Consejo Nacional de Investigaciones Cientificas y Te´cnicas, Argentina. X Abstract published in Advance ACS Abstracts, January 1, 1996.
(1) Matsuoka, Y.; Niwa, M.; Murakami, Y. J. Phys. Chem. 1990, 94, 1477. (2) Ca´ceres, C. V.; Garcı´a Fierro, J. L.; La´zaro, J.; Lo´pez Agudo, A.; Soria, J. J. Catal. 1990, 122, 113. (3) Masuyama, Y.; Tomatsu, Y.; Ishida, K.; Kurusu, Y.; Segawa, K. J. Catal. 1988, 114, 347. (4) Stampfl, S. R.; Chen, Y.; Dumesic, J. A.; Niu, C.; Hill, C. G., Jr. J. Catal. 1987, 105, 445. (5) Martı´n, C.; Martı´n, M. J.; Rives, V. Proceedings of the 3rd Europ. Work. Mett. Select. Oxid. Heter. Catal., Louvain-la-Neuve; Elsevier Science Publ. Co.: Amsterdam, 1991.
0743-7463/96/2412-0051$12.00/0
as support.2 While in both cases molybdenum dispersion was high, the samples showed different reducibility due to the different type of interactions with the supports. The catalytic activity of the supported catalysts is usually determined by the amount and dispersion of the active phase. Bulklike molybdena species are generally present limiting the activity in those cases where they do not constitute the active phase. Therefore, the determination of its relative abundance became important. This determination is even more important in cases like Mo/ SiO2 catalysts, where bulk MoO3 appears as a promoter of the activity.8 The detection limits of the conventional techniques makes difficult to established its existence for sizes smaller than 4 nm as in ref 2 using X-ray difraction or for Mo loadings below 5 wt % as in ref 9 using infrared spectroscopy. A nuclear spectroscopy technique namely time differential perturbed angular correlation (TDPAC) has been used in many investigations of Mo-based catalysts.10-18 This technique, through the measurement of the local electric field gradients (EFG) at the radioactive probe site, (6) Requejo, F. G.; Quaranta, N.; Thomas, H.; Coronado, J. M.; Soria, J. In Stud. in Surf. Sciences And Catalysis; Corte´s Corbera´n, V., Delon, V., Eds.; Elsevier Pub. Co.: Amsterdam, 1994; Vol. 82. (7) Irusta, S.; Requejo, F. G.; Lombardo, E. A.; Miro´, E. E. Lat. Am. App. Res., in press. (8) Spencer, N. D. J. Catal. 1988, 109, 187. (9) Ng, K. Y. S.; Gulari, E. J. Catal. 1985, 92, 340. (10) Lerf, A.; Vogdt, C.; Butz, T.; Eid, A. M. M.; Kno¨zinger, H. Hyp. Int. 1983, 15/16, 921. (11) Butz, T.; Vogdt, C.; Left, A.; Kno¨zinger, H. J. Catal. 1989, 116, 31, and references therein. (12) Requejo, F. G.; Bibiloni, A. G.; Massolo, C. P.; Desimoni, J.; Renteria, M. Phys. Status Solidi A 1989, 116, 503. (13) Requejo, F. G.; Bibiloni, A. G.; Saitovitch, H.; Silva, P. R. J. Phys. Status Solidi A 1990, 120, 105. (14) Xinbo, N.; Butz, T. Hyp. Int. 1989, 52, 131. (15) Requejo, F. G. Unpublished (1988). (16) Guida, S.; Tingyun, Y.; Fushan, Y.; Liguo, R.; Xinbo, N. Proceedings of the 10th Int. Cong. Catal., Budapest, 1992, Guczi, L., et al., Eds.; New Frontier in Catalysis, Part B; Elsevier Science Pub. Co.: Amsterdam: 1992; p 1895. (17) Lerf, A.; Butz, T. Angew. Chem., Int. Ed. Engl. 1987, 26, 110. (18) Schatz, G.; Klas, T.; Platzer, R.; Voigt, J.; Wesche, R. Hyp. Int. 1987, 34, 555.
© 1996 American Chemical Society
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can give information about the characteristics (coordination, symmetry, distortions, etc.) of the different environments of the probes, their concentration, and modifications related to in-situ conditions (temperature, atmosphere, pressure, etc.). Compounds containing molybdenum have the advantage that after neutron irradiation the TDPAC probe 99Mo is formed from natural 98Mo. Thus, the extracted information is reliable since the TDPAC probe does not introduce an impurity. After the pioneer work of the group of Mu¨nchen,10 several papers devoted to bulk MoO312-14 and Mo-O supported species on alumina.11,15,16 Butz et al. determined the Mo species related to Mo loading, pH values of alumina, and different treatments.11 More recently, Guida et al. also characterized the different Mo-O adsorbed species in the same system and found out the active ones.16 It seems very interesting to continue the studies of Mo-O supported species on Al2O3 and to extend them to TiO2/Al2O3 and SiO2 supports using the TDPAC technique to contribute to the knowledge of the Mo oxide dispersion on different supports. In effect, the EFG at an ion site is defined by a few atomic shells due to its 〈r-3〉 dependence, allowing the technique to detect, for example, the presence of very small molibdena cristallites. With this purpose we irradiated with thermal neutrons Mo-Al-O, Mo-Ti-Al-O, and Mo-Si-O samples with different Mo loading to obtain the TDPAC probe 99Mo, which decyas to 99Tc incorporated to the catalyst. Determinations of different Mo environments in Mo-supported species are discussed related to previous characterizations with other techniques on the same samples.6,7 We established the possible Mo species and relative abundance for different concentrations of molybdena and temperature of measurement. 2. Experimental Section 2.1. Sample preparation. (a) Mo-Al-O Samples. Small spheres of γ-Al2O3 with high specific surface (BET surface area 230 m2 g-1) were impregnated with aqueous solutions of ammonium paramolybdate (APM) so to obtain samples with 5 and 21 wt% Mo. The first amount of molybdenum is less than that necessary to obtain theoretical monolayer coverage. The samples were dried in air 24 h at room temperature (RT). Then, the samples were calcined in air at 200 and 400 °C at a rate of 10 °C/min. (b) Mo-Ti-Al-O Samples. The starting material Al2O3 (Condea, BET surface area 214 m2 g-1 was added to a solution of titania isopropoxide in hexane at RT (6 mL of solution per gram of alumina). Acetyl acetonate, in a molar ratio 1:1 to titania isopropoxide, was also added to the solution. After being stirred for 1 h under N2, the mixture was heated up to 400 °C in N2 flux and subsequently calcined at 600 °C in air for 3 h. The final product (BET surface area 193 m2 g-1) of this process was a sample of Al2O3 with a titania (rutile) content of 12 wt %. This concentration corresponds to the theoretical monolayer covering of titania (rutile) on alumina. Molybdena-supported catalysts were prepared using the excess water impregnation method from an APM solution. Solutions contained the required amounts of molybdate to yield catalysts with 16 wt % Mo (sample 1) and 8 wt % Mo (sample 2) on TiO2/ Al2O3. The last concentration is used when a monolayer species of Mo oxide on alumina is needed. The APM solutions were maintained at pH ) 6 by addition of NH4(OH). Excess of water was removed using a rotary evaporator while heating at 100 °C. Moist materials were left overnight (15 h) at RT and then dried at 110 °C for 8 h. Finally, the samples were air-calcined in two steps: 250 °C for 2 h and 500 °C for 4 h. (c) Mo-Si-O Samples. Commercial silica Cabot Aerosil, particle size 5-30 nm; BET surface area 200 m2 g-1 and composition SiO2 > 99.8%; Al < 5 ppm; Fe < 2 ppm and Na < 0.5 ppm was used as carrier. This was impregnated during 12 h with aqueous solutions of APM in appropriate amounts to yield 5 wt % metal concentration. The impregnates were dried at 120
Requejo and Bibiloni °C and calcined 2 h at 200 °C and 12 h at 550 °C. The metal salt used for the impregnations was Merck pro-analysis ammonium heptamolybdate. All the samples, once encapsulated in a quartz tube in air, were irradiated in the reactor RA-3 of the Comisio´n Nacional de Energı´a Ato´mica (Argentina) with a flux of 1013 neutrons cm-2 s-1. In this way, the necessary activity 99Mo was obtained from 98Mo isotope present in natural molybdenum. The amount of metal loading is the ratio of the weight of metal divided by the total weight of the catalyst. 2.2. TDPAC Technique. 2.2.1. Theoretical Background. TDPAC technique is a well-establish method for solid-state physics.17,18 The technique requires a nucleus in an excited state that decays via the successive emission of two γ-rays. When a radioactive nucleus undergoes a γ decay, the probability of emission of the γ-photon in a certain direction depends on the angle between the nuclear spin and that direction according to the rules of angular momentum conservation. Then, in a radioactive source with nuclei randomly oriented, it is possible to select a set of nuclei with a particular spin orientation. This is achieved by detecting the first radiation γ1 of the γ-γ cascade in a fixed direction k1. The detection of the second γ ray will therefore show a certain angular distribution pattern with respect to the direction k1. The probability density of the second emission in the direction k2 detected at an angle θ with respect to k1 is given by the angular distribution function W(θ,t)17
W(θ) ∼ 1 + A2P2(θ)
(1)
where higher order terms have been neglected. A2 is a known function of the nuclear spin level and the multipolarities of both γ-rays and P2 is the second-order Legendre polynomial. If during the time t that nuclei live in the intermediate level, it interacts with extranuclear fields, the direction of the nuclear spin will be modified and the correlation function will become19
W(θ,t) ∼ 1 + A2G2(t)P2(cos θ)
(2)
where the interaction is described by the attenuation factor G2(t). For static interactions the factor G2(t) is a periodic function of time. Magnetic interactions are characterized by the Larmor precession frequency ωB while electric quadrupole ones are characterized by the so-called quadrupole interaction frequency ωQ. The aim of the TDPAC technique is the determination of G2(t) through the measurement of W(θ,t). Since the product A2G2(t) depends on both nuclear and solid-state parameters, it is possible to find nuclear parameters (spins, magnetic dipole moments, electric quadrupole moments, or half-lives of intermediate states) or solid state parameters (magnetic fields or electric field gradients) at the radioactive nuclei sites. In the latter case, under favorable conditions, it is possible to find out different interactions corresponding to nonequivalent lattice sites. For the cascade 740-181 keV in 99Tc, A2th ) 0.10 is the theoretical angular correlation coefficient, which depends only on the spin of the states and nature of the transitions involved in the nuclear decay. The form of the perturbation factor G2(t) depends on the nature of the fields acting on the nucleus. For a polycrystalline sample G2(t) has the form: 3
G2(t) )
∑ ∑S fi
i
2ni
cos(ωnit)e-δiωnit
(3)
n)0
where fi are the relative fractions of nuclei that experience a given perturbation. The ωn frequencies are related through ωn ) gn(η)ωQ to the quadrupole frequency ωQ ) eQVzz/40p. The gn and S2,n coefficients are known functions of the axial symmetry parameter η ) |Vxx - Vyy|/Vzz, where Vii are the principal components of the EFG tensor. The exponential functions account for Lorentzian frequency distributions of relative width δ around ωn. (19) Frauenfelder, H.; Steffen, S. M. In Alpha-, Beta-, Gamma-RaySpectroscopy; Siegbahn, K. Ed.; North-Holland Publ. Co.: Amsterdam, 1965, Vol. 2, p 917.
Characterization of Adsorbed Molybdates
Langmuir, Vol. 12, No. 1, 1996 53 Table 1. Hyperfine Parameters and Relative Abundance of the MoO3 Bulklike Supported Species Corresponding to the Different Studied Supportsa support Al2O3 Al2O3/TiO2 SiO2 unsupported
Mo concn (wt %)
rel abundance (%)
ωQ (Mrad/s)
η
21b
90 70c 22 17 80
41 ( 5 47 ( 1 46 ( 4 46 ( 4 28 ( 3 29 ( 1
0 0 0.15 0 0.45 0.45
5 16 8 5
a The results obtained for a pure powder MoO sample are 3 included for comparisons. b After calcination at 500 °C. c The rest corresponds to monomeric species.
Table 2. Hyperfine Parameters and Relative Abundance of the Two-Dimensional Mo Oxides on the Different Oxide Supports support Al2O3 Al2O3/TiO2 SiO2
Figure 1. Hyperfine parameters ωQ (a) and η (b) calculated by the point charge model at molybdenum sites in MoO3 structure. r indicates the radius of a sphere around the considered molybdenum, and the calculus takes account of all the electric charges included in the sphere. 2.2.2. Apparatus and Data Reduction. A three-detector CsF fast-slow coincidence system in a coplanar arrangement, with a time resolution of 0.8 ns, was used. Four-coincidence spectra Cij(θ,t) of all possible start-stop combinations of the three detectors were recorded simultaneously at each detector position in a multichannel analyzer. The spectra, corrected for accidental counts, were combined in the following form to obtain the asymmetry ratio R(t):
R(t) )
2 3
x
C13(π,t)C24(π,t) - 1 ∼ A2G2(t) π π C14 ,t C23 ,t 2 2
( ) ( )
(4)
where A2exp is the measured anisotropy of the γ-γ cascade and G2exp is the appropriate perturbation factor. TDPAC measurements were carried out in air at standard pressure. 2.2.3. Point Charge Model Calculations. Usually, comparison of the experimental results for the EFG (i.e. ωQ and η) with theoretical calculations helps in its physical interpretation. When the charge distribution of the solid is known, by neutron diffraction for example, an exact theoretical calculation of the components Vii of the EFG tensor is possible. Sophisticated methods like the Huckel extended one20 are also helpful. But in many cases a simple approximation such as the point charge model gives useful results. In fact, we applied successfully this model where the nominal electric charges are concentrated in the crystallographic site to binary oxides.21 In particular, we made use of this calculations to investigate the origin and structure of defects on MoO3.12,13 In heterogeneous catalysis the determination of the dispersion of the adsorbed species is very important. Calculations with PCM may help in this concern. Let us consider a sphere centered at the site of a molybdenum ion in the structure of MoO3. Figure 1 shows the values of Vzz and η calculated using the PCM and taking into account all the electric charges included in a sphere with radius r. It can be seen that for r g 2 nm the convergence of Vzz and (20) Weht, R.; Fabricius, G.; Weissmann, M.; Renterı´a, M.; Massolo, C. P.; Bibiloni, A. G. Phys. Rev. B 1994, 49, 14939. (21) Renterı´a, M.; Massolo, C. P.; Bibiloni, A. G. Mod. Phys. Lett. B 1992, 6 (28), 1819.
Mo concn (wt %)
rel abundance (%)
ωQ (Mrad/s)
η
21a 16 8 5
10 78 83 20
105 ( 10 159 ( 7 153 ( 6b 145 ( 7
0.3 0.44 0.3 0.76
a After calcination at 500 °C. b An additional fitted interaction with ωQ ) 335 ( 10 Mrad/s was assigned to Mo nuclei at the boundary of the two-dimensional species.
η is better than 10-3, far beyond the accuracy limit of the TDPAC techniuqe. This means that we expect unique Vzz (i.e., ωQ) and η values for MoO3 cristallites with radii larger than 2 nm, while for 0.6 nm < r < 2 nm both parameters have dispersion of nearly 20%. This dispersion will be reflected in the distribution parameter δ, while the value of ωQ will represent the centroid of the distribution. If no inhomogeneities in the environment of a molybdenum ion in the crystallite to whom it belongs affect the measurement, and we consider that the distances from the oxygen ions to the molybdenum one are in the order of 0.25 nm, all this means that crystallites with four Mo-O layers or more (r g 1 nm) will be indistinguisheable since Vzz and η will be the same. Then, hyperfine parameters different from those that characterize massive MoO3 would indicate the presence of monomers (ωQ ) 0), monolayer species, bulk structured crystals with defects, or very small crystals (r < 1 nm) of molybdena.
3. Results and Discussion The TDPAC spectra taken on the samples described in the previous section are shown in Figures 2-5. The thermal treatments performed on the samples before the TDPAC measurements are indicated in the captions. The temperatures at which the measurements were done are also showed. Solid lines represent least-squares fits to eq 3 to the data. The results of the fits are displayed in Tables 1 and 2. Unfortunately, due to the short mean life τ of the intermediate level of the cascade and the short time calibration needed (0.29 ns/channel), we have large errors in the R(t) values for t > 15 ns. So we fit the A2G2(t) function to the data with no more than two unknown hyperfine interactions. Fits with more unknown interactions would introduce considerable relative errors in the fitted parameters. For simplicity we will present and discuss separately the results obtained for each system. The comparison of the results and the conclusions that can be extracted will be done in the next section. (a) Mo-Al-O Samples. In standard conditions the surface of alumina should have basic groups. With these groups react the tetrahedric monomers from the APM solution. The binding is very strong in these cases and we can imagine the monomers fixed to the surface. If the interaction with the surface does not destroy the symmetry of the electric charges of the monomers, as showed by Lerf
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et al.,10 the Mo nuclei should experiment a null electric field gradient and so the quadrupole frequency ωQ will be also null. The same would happen for undistorted octahedral symmetry. As the occupation of the basic sites progresses, the molybdate ions will be adsorbed on less basic or even neutral sites at the surface of the alumina. This will be the case of the sample with 21 wt % Mo. The increase of the concentration of the metal ions combined with the migration of those weakly bound causes the build up of small Mo-O islands. The coordination of Mo in these polymeric species is octahedral22 and the octahedra is distorted due to the piling up of the ions.10,11 Here electric field gradients and, so, quadrupole frequencies different from zero are expected. If an island is large enough, the inner Mo ions should have the charge symmetry of bulk MoO3. Here the hyperfine parameters should be those found out in ref 10, if the influence of adsorption on the symmetry of the electric charges should be negligible. On the other hand, after the migration of the weakest bound ions the relative concentration of monomers at the surface should increase. In effect, those ions tightly bound to the surface at the basic sites should recuperate their undistorted symmetry once left alone. The TDPAC results displayed in Figure 2 and the fits of eq 3 to the data agree quite well with the above described situation. First, always a certain amount of nuclei is found to experience a null EFG, i.e., has the characteristic of the
monomeric species. The relative concentration is 30% for the sample with 5 wt % Mo and diminishes to 10% for the one with 21 wt % as it should. After the migration and rearrangement of the ions that promoted the thermal treatment at 500 °C in air, this concentration rises up to 35%. Second, all the spectra exhibit the interaction whose hyperfine parameters are 40 e ωQ e 50 Mrad/s and η ≈ 0. The quite small frequency distribution δ of this interaction accounts for a very well-defined structure of this species. All Mo ions in this species experience the same EFG; i.e., ions are surrounded by almost the same electric charge distribution. This interaction amounts to 70% for the sample with 5 wt % Mo and 90% for that with 21 wt %. Finally, after the thermal treatment and the corresponding rearrangement of Mo ions, it amounts to 55%. By use of the results obtained by PCM already presented in section 2.2.3, some enlightment on the nature of the adsorbed species is then possible. The centroid of this interaction is about 30% higher to that which characterize the massive MoO3 free of defects (ωQ ) 29 Mrad/s and η ) 0.45).11,13 Combining this results with the value of the frequency distribution δ (e5%), from PCM predictions shown in Figure 1 we could conclude that we are in the presence of small crystallites with 2-3 molybdena-like layers. In the sample with 21 wt % Mo after the thermal treatment at 500 °C (Figure 2c), the presence of a new hyperfine interaction is apparent. This interaction is characterized by ωQ ) 10510 Mrad/s and η ) 0.32. Its relative intensity was 10%. Being not bulk, not monomers, it should be assigned to the presence of monolayers of Mo-O or to MoO3 bulklike species with defects. In studies on massive MoO3 in the presence of NH34(OH), also present in our sample due to the impregnation with APM, we found a similar interaction.12,13 PCM calculations allowed us to assign then the interaction to the presence of an oxygen vacancy in the site of the weakest bound oxygen. This assignment for the third interaction would be consistent with the bluish coloration taken by the present sample after the treatment. We may now analyze the other possibility, the monolayer of Mo-O. Giordano et al.,23 found evidence of a monolayer deposition for Mo-Al-O samples up to 15-20 wt % Mo activated at 500 °C. We assume that this species is formed by two layers of oxygens that include an intermediate one of molybdenum. The total thickness being, therefore, about 0.5 nm. From Figure 1 we can see that, in the frame of this approximation, we find electric field gradients 3 times larger than that corresponding to bulk MoO3 for thicknesses shorter than 0.7 nm (r ) 0.35 nm in Figure 1). This ratio is approximately that which exists between ωQ ) 105 Mrad/s and ωQ ) 29 Mrad/s. Anyhow, it is not possible to model reliably a twodimensional species of Mo-O adsorbed on Al2O3, that allows a PCM calculation. Probably an extended study of adsorbed species of Mo-O on different supports will establish a correlation that permits the determination of its precise nature. (b) Mo-Ti-Al-O Samples. Two samples were prepared. Sample 1 with 16 wt % Mo and sample 2 with 8 wt % Mo. Both samples were warmed to 300 °C in air for 2 h approximately after neutron irradiation to eliminate the oxygen vacancies produced during irradiation. Figure 3 shows the TDPAC spectra at RT (Figure 3a) and 280 °C (Figure 3b) for sample 1. Figure 4 shows the
(22) Spanos, N.; Lycourghiotis, A. J. Catal. 1994, 147, 57, and references therein.
(23) Giordano, N.; Bart, J. C. J.; Vaghi, A.; Castellan, A.; Martinotti, G. J. Catal. 1975, 36, 81.
Figure 2. PAC spectra of 99Mo in Mo-Al-O samples taken at room temperature (RT): (a) 5 wt % Mo as irradiated; (b) 21 wt % Mo as irradiated; (c) 21 wt % Mo after a thermal treatment in air at 500 °C for 5 h. The arrow shows the baseline related to the presence of monomeric species of Mo-O.
Characterization of Adsorbed Molybdates
Figure 3. PAC spectra of 99Mo in the Mo-Ti-Al-O with 16 wt % Mo sample taken at (a) room temperature and (b) 280 °C.
Figure 4. PAC spectra of 99Mo in the Mo-Ti-Al-O with 8 wt % Mo sample taken at (a) room temperature and (b) 200 °C.
spectra at RT (Figure 4a) and 220 °C (Figure 4b) for sample 2. The high temperature measurements (280 and 220 °C) were done in the temperature range of the catalyst for methanol oxidation reaction (180 to 280 °C). All the TDPAC patterns show a strong nuclear quadrupole interaction (i.e. ωQ > 100 Mrad/s) shown by oscillation peaks below 10 ns and the rapid decrease of R(t). The results of ref 6 show that in samples with 16 wt % Mo there is a small concentration of bulk MoO3 and another highly dispersed species with Mo having octahedral coordination. In samples with 8 wt % Mo the bulk MoO3 concentration diminishes and the rest of the Mo atoms must remain in a monolayer species. Since this implies different coordination number and/or position of the first
Langmuir, Vol. 12, No. 1, 1996 55
neighbors atoms, different electric quadrupole interactions on 99Mo are expected. For sample 1 we fitted two hyperfine interactions (HI). One of them, the weaker one, is characterized by ωQ ) 345 Mrad/s assuming η ) 1 or ωQ ) 4510 Mrad/s assuming η ) 0. These values are the extreme assumptions for the asymmetry parameter η since it is not possible to obtain a reliable η-value. Similar hyperfine parameters were reported in refs 11 and 13 as characteristic of 99Mo in bulk MoO3 free of defects. We think that this interaction does not correspond to 99Mo in Mo-O species adsorbed on Al2O3 because in our case the alumina surface should be completely covered by titania. Actually, infrared measurements of the same samples revealed that the O-H groups of the alumina disappear after impregnation with titania isopropoxide.6 The fact that, in addition, we expect the presence of bulk MoO3 in this sample allows us to assign this HI to bulk MoO3 free of defects. Therefore, we fixed the parameters to ωQ ) 45 Mrad/s and η ) 0. The relative abundance of this interaction became 37%, and its frequency distribution δ became no more than 10%. In the future we will call this HI1. The other one, HI2, is characterized by ωQ ) 1597 Mrad/s and η ) 0.448. As in the case of the Mo-Al-O samples, PCM results suggest for HI1 its correspondence to thin MoO3 crystallites with thickness between two and three molybdena layers. When we fitted two unknowns HI plus HI1 to the spectra of sample 2, the agreement of the A2G2(t) function to the experimental data is quite good. This fit showed the same two interactions found in sample 1 plus another one, HI3, characterized by ωQ ) 33510 Mrad/s and η ) 0. Relative fractions of nuclei that experience HI1, HI2, and HI3 are 17%, 52%, and 31%, respectively. From the results obtained with complementary experiments performed on the same samples,6 we expect the presence of bulk MoO3 in both samples and a monolayer species of Mo oxide, especially for that with lower Mo concentration (sample 2). Moreover, the TiO2/Al2O3 surface in sample 1 should be completely covered while in sample 2 the surface should be partially covered. In this way, a simple interpretation of our results is possible. Since we assigned HI1 to 99Mo in bulk MoO3, HI2 and HI3 can be attributed to 99Mo in Mo-O monolayer species. Moreover, we may assign HI3 to 99Mo in the boundary region of monolayer crystallites. This boundary region should not exist in sample 1, where the surface should be completely covered. Finally, HI2, present in both samples, should be assigned to 99Mo in the monolayer species. Considering the relatively high frequency distribution of HI2 (15 e δ e 25), the PCM calculations are consistent with the presence of species whose thicknesses are less than two layers. For both samples the results show that the hyperfine parameters do not change when the samples are measured in the operating range of the catalysts. The main changes were observed in fractions, fi, and frequency distributions, δi. All the fitted parameters agree within the errors. Even when these changes have a real physical meaning, this will have no influence in our assignment of the species of each hyperfine interaction. (c) Mo-Si-O Samples. The surface of SiO2 support is considered relatively inert. Therefore we expect fewer monomers for a given concentration here than in the case of alumina.24 Figure 5 shows our TDPAC results for this sample. The spectra were taken after thermal treatments at 400 °C for 4 h and 700 °C for 7 h, respectively. As in the previous cases the presence of a hyperfine interaction (24) Smith, M. R.; Ozkan, U. S. J. Catal. 1993, 141, 124.
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extensively studied by Smith and Ozkan.24 According to that reference, the catalysts which preferentially exposed the (010) plane produced more complete oxidation products. This suggests that the Mo-O species adsorbed on SiO2 characterized by the hyperfine parameters ωQ ) 145 Mrad/s and η ) 0.76 may be associated to two-dimensional species that exposed the (010) plane.
Figure 5. PAC spectra of 99Mo in the Mo-Si-O sample with 5 wt % Mo taken at room temperature (RT) (a) after a thermal treatment at 400 °C for 4 h and (b) after a thermal treatment at 700 °C for 7 h.
characterized by ωQ ) 283 Mrad/s and η ) 0.45 is apparent. This interaction can be attributed to the presence of bulk MoO3. A second interaction is also present. Their parameters are ωQ ) 1457 Mrad/s and η ) 0.76. For the first interaction (ωQ ) 283 Mrad/s, η ) 0.45, and δ ≈ 12%) PCM calculations are consistent, as previously discussed, with the presence of MoO3 crystallites large enough to have the same EFG of the bulk MoO3. This result is also consistent with the known fact that 5 wt % of Mo on SiO2 supports is enough to form bulk molybdena.8 In effect, for 5.16 wt % Mo Spencer et al. found crystallites of MoO3 that are ∼5 nm or more in diameter (r ) 2.5 in Figure 1). Concerning the interaction characterized for ωQ ) 1457 Mrad/s and η ) 0.76 with δ e 5%, as in the Mo-Ti-Al-O samples, we may attribute it to two-dimensional species of Mo-O. Now, the same sample we used in our experiments showed catalytic activity for the selective oxidation of methane to formaldehyde.7 The role of different crystal planes of MoO3 in this reaction has been
4. Conclusions The TDPAC technique has been applied to the study of the Mo-O species which are formed when different supports like alumina, alumina modified by titania and silica are impregnated with molybdenum salts. Taking advantage of its capability to distinguish different environments in the surrounding of the probe, we characterize at least two different Mo-O adsorbed species on each support. One of them may be associated to the presence of bulk MoO3 like species. For Al2O3 and TiO2/ Al2O3 supports, the crystallites could have between two and three layers. SiO2 as support exhibits the presence of larger crystallites of MoO3 than in the other cases. The high hyperfine interaction fitted in the TDPAC spectra may be associated with the presence of a two-dimensionallike species of Mo-O. In the case of Al2O3 this interaction could also correspond to the presence of bulk MoO3 with single oxygen vacancies. Now evidence is not enough to make a definitive assignment. In all cases (i.e., Al2O3, TiO2/Al2O3, and SiO2) these adsorbed species are different. Although the PCM calculation is a very suitable tool to the knowledge of the origin of the characterized species, its results are reliable mainly for large and regular structures. Two-dimensional species are very difficult for modeling for PCM calculations. In the case of Mo-Ti-Al-O, measurements at temperatures higher than RT, in the catalytic operating range, show that the configuration of the Mo-adsorbed species does not change in that condition. Measurements on different oxides as support are now in progress to obtain a wide set of TDPAC results on Mosupported systems. In particular, studies of the Mo/TiO2 catalyst would be useful to clarify the origin of the presence of the high values of EFG in the site of 99Mo when TiO2/ Al2O3 is used as support. Acknowledgment. This work was partially supported by CONICET, Argentina. The authors acknowledge Dr. H. Thomas, Dr. R. Candia, and Dr. C. Ca´ceres for fruitful discussions. LA940695F
