Raman and Ultraviolet Spectroscopic Characterization of Molybdena

A laser power of. Materials and Catalyst Preparation. pH 6-0.84M MOM)+. pH11-0, ..... M a ~ s o t h ~ ~ assumed an epitaxial growth of the molybdena m...
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The Journal of Physical Chemistry, Vol. 83, No. 9, 1979

(30) The only case in which it IS possible to measure the difference of inner electric potential between two phases is the case when the two phases are of identical chemical composition, e.g., two pieces of pure copper metaL3' Therefore the correct diagram for an electrochemical cell shoukl always show the klentical metal terminal^.^' (31) R. Parsons in "Modern Aspects of Electrochemistry", Vol. 1, J. O'M. Bockris, Ed., Butterwotths, London, 1954, pp 103-179. (32) For details cf. ref 1, Appendix B. (33) For a particularly clear discussion of the significance of the surface of tension cf. R. Defay and I. Prigogine, "Tension Superficielle et Adsorption", Desoer, Liege, Belgium, 1951, pp 1-5. English language version: "Surface Tension and Adsorption", R. Defay, I. Prigogine, A. Bellemans, and D. H. Everett, Longmans, London, 1965.

H. Jeziorowski and H. Knozinger (34) Fredlein and Bockris' assumed that when the surface strain was zero the ebstic stress would be zero instead of being equal to the interfacial tension as we have shown. Couchman et ai.'' pointed out the difficulties involved in attempting to determine the state of surface strain for vanishing elastic stress and they indicated that the state of strain corresponding to the minimum on the interfacial tensionelastic strain curve is physically the most obvious to choose as the origin of surface strain if it is desired to deduce a Hookean type behavior. (35) E. W. Washburn. Ed.. "International Critical Tables of Numerical Data. Physics, Chemistry and Technology", Vol. I, McGraw-Hill, New York: N.Y., 1926, pp 102-103. (36) N. Pangarov and G. Kolarov, J. Nectroanal. Chem., 91, 281 (1978).

Raman and Ultraviolet Spectroscopic Characterization of Molybdena on Alumina Catalysts Helge Jezlorowski and Helmut Knozinger" Institut fur Physikalische Chemie. Universitat Munchen, 8000 Munchen 2, West Germany (Received November 9, 1978)

Molybdena catalysts supported on y- and 7-A1203have been prepared by impregnation (pore filling method) from aqueous molybdate solutions at pH 6 and pH 11, which contain respectively the M070246- and MOO^^ion. The development of the final oxidic state of the catalysts during impregnation, drying, and calcination was followed by Raman and ultraviolet spectroscopy. An assignment of the vibrational bands and of the ligand charge transfer bands of the molybdena surface species was made on the basis of the corresponding solution spectra of a variety of isopolymolybdates. It is concluded that even during impregnation from solutions at pH 6 the initial species which ion exchanges with surface OH groups is the tetrahedral MOO:- ion. The degradation of the M07024~-ion is brought about by an increased pH at the support surface. Depending on the total loading, a more or less extensive formation of Mo-0-Mo bridges occurs during the drying and calcination procedure; a polymeric surface molybdate species is formed. The Mo coordination cannot unambiguously be determined.

Introduction Raman spectroscopy has been shown during the past 2 years to be an additional experimental technique for the characterization of supported molybdena on alumina which provides extremely valuable structural information of the supported active oxide phases. Brown et a1.2 concluded from their Raman spectra of commercial catalysts that, depending on the amount of molybdena incorporated into the catalysts and the method of preparation, bulk Moo3, tetrahedral MOO>-, A12(MoOJ3,and specific interaction species were formed on individual samples. A very typical Raman band which is usually observed in the 930-970-~m-~ range could not be assigned unequivocally, though it was postulated that it was due to an octahedral molybdenum-oxygen structure on the surface. In a later paper Brown et a1.j followed the formation of the different compounds and interaction species as a function of the molybdena loading and support surface area (support pellets were impregnated using the technique of impregnation to incipient wetness). At the highest loadings (>25 wt % ) only bulk MooBwas detected on the outer layers of the catalyst pellet, while internal layers showed a very unique (non-Mo03) spectrum which was interpreted as being due to a non-monolayer interaction species. Medema et al.6 suggested MOO^^- tetrahedra and a two-dimensional polymeric form of distorted molybdate octahedra (which was assumed to give rise to the typical band in the 930-970-~m-~ region) as primary interaction species. This assignment was based on a comparison of the Raman spectra of the surface species with those of the paramolybdate anion. Analogously, Knozinger and Jeziorowskig suggested the preferential formation of a polymeric molybdenum-oxygen species with an octahedral 0022-3654/79/2083-1166$01 .OO/O

environment of the MoGfcation. According to Medema et aL6the polymeric form is growing at the expense of the isolated MOO:- tetrahedra as the loading is increased. At higher loadings bulk Moo3 was always f ~ r r n e d ,while ~?~a "subsurface" A ~ , ( M o O ~phase ) ~ was detected in some catalysts.6 Jannibello et aL8 and Jannibello and MitchelllO have shown that a degradation of polyanions occurs during impregnation in the wet state followed by the adsorption of monomeric MOO:- species. A Raman band at 940 cm-l observed on the calcined samples (823 K) was therefore assigned as the symmetric Mo-0 stretching vibration of Moot- with an increased double bond character: although this vibration gives rise to a band at 897 cm-l in the free MOO>- ion.ll As an additional argument for their assignment of the Raman band a t 940 cm-l, Jannibello et a1.8 refer to the fact that the observed electronic transitions between 250 and 300 nm are usually attributed to tetrahedral molybdenum-oxygen surface s p e c i e ~ . l ~ - ~ ~ This short literature survey clearly demonstrates the discrepancies which still exist in the interpretation and assignment of Raman spectra of supported molybdena catalysts and which lead to entirely different structural models of the surface phases. To get a better understanding of the nature of these systems, we have now prepared two series of molybdena on alumina by impregnation of the support from aqueous ( N H ~ ) ~ M o ~ O ~ ~ solution a t either pH 6 or 11. According to the equilibrium14-16 7MoO4'- + 8H' + M070246- 4H20 (1)

+

the original solutions either contain exclusively the paramolybdate anion at pH 6 with Mo in an octahedral environment or the tetrahedral MOO:- ion at pH 11. This 0 1979 American Chemical

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The Journal of Physical Chemistry, Vol. 83, No. 9, 1979

Raman and UV Study uf Molybdena-on-Alumina

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260 300 3 4 0 nm Figure 2. UV spectra of the Mo7O2:- (at pH 6) and of the Moo4’- (at pH 11) anions in aqueous solutions at two different concentrations. L A - 1. , I . 1 1000 800 600 LOO 200 A VR I crn-‘ 1

Figure 1. Raman spectra of the in aqueous solutions.

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(a) and M o o t - (b) anions

can easily be controlled by the Raman spectra of these solutions. The development of the supported catalysts was then controlled during the various stages (wet, dried at 393 K, and calcined at 773 K in air) by means of their Raman and ultraviolet spectra. The spectra will be interpreted on the basis of reference solution spectra of several isopolymolybdates.

Experimental Section Materials and Catalyst Preparation. 7-Al,03 was obtained from the hydroxide (Condea, Brunsbuttel, West Germany) by calcination in air at 1023 K for 18 h. Its BET surface area was 158 m2/g. Some catalysts were also prepared by using an 7-A1203support, which was obtained from aluminum isopropoxide by hydrolysis and subsequent calcination of the resulting hydroxide at 973 K in air for 16 h. This material had a BET surface area of 154 m2/g. The (NH4)6M070244H20 (puriss. p.A.) was from Fluka, Buchs, Switzerland. The paramolybdate was dissolved in distilled water to yield a pH 6 solution, while an aqueous NH40H solution was used for the preparation of the molybdate solution at pH 11. The Raman spectra of these two solutions allow an easy differentiation between the molybdenum-oxygen species present, as shown in Figure 1. At pH 6 four bands at 218, 359, 895, and 938 cm-l clearly indicate the presence of the paramolybdate while at pH 11 the characteristic bands at 318, 846, and 896 cm-’ of the M c 1 0 ~ion1’ ~ - are found. The ultraviolet spectra of the two solutions also show typical differences in so far as the MoOZ- (pH 11) species only absorbs at 230-240 nm (43480-41670 cm-‘), while an additional broad band near 290 nm (34480 cm-’) is found for the paramolybdate ion (Figure 2). The alumina supports were impregnated by the pore filling technique. Solutions containing the requisite amount of molybdate to yield catalysts of desired compositions were added to the alumina support in a volume to just fill the pores. The moist systems were left at room temperature overnight (16 h), they were then dried at 393 K for 24 h, and finally calcined a t 773 K in air for 2 h. Instrumentation. The Raman spectra were recorded on a Cary 82 spectrometer equipped with a triple monochromator. The 514.5-nm line of a Spectra Physics Model 165 Ar+ laser was used for excitation. A laser power of

approximately 60 mW (measured at the sample position) was applied. The spectral slit width was typically 4 cm-’ and wavenumbers obtained from the spectra are accurate to within k 2 cm-l. The oxide samples were mounted into a stainless steel frame which was rotated at a frequency of approximately 60 Hz to avoid any damage of the samples due to laser-induced heat effects as was described by Medema et ala7 The diffuse reflectance technique was applied to record the ultraviolet spectra on a Beckman spectro-reflectometer Type DK 2 A. The powder samples were mounted in quartz cells, which provided a sample thickness of 0.1 cm. In the ultraviolet region this sample thickness is sufficient to guarantee the measurement of R,, i.e., the reflectance at “infinite” sample thickness.17 The experimental spectra were replotted and are presented in form of the Kubelka-Munk function F(R,) vs. wavelength. F(R,) is given by17

K and S are phenomenological absorption and scattering coefficients, respectively. In the ultraviolet region S is independent of the wavelength at the given particle sizes. Pure 7-Al,03 was used as the reference sample. The spectral slit width at 300 nm was typically 0.3 nm. Results Raman Spectra. Spectra of molybdena on alumina samples recorded for the three stages, i.e., wet state, dried at 393 K, and calcined at 773 K, during their preparation are shown in Figures 3-6. Figures 3 and 4 represent the spectra of samples containing 8 wt % Moo3, which were impregnated at pH 6 and 11, respectively, while Figures 5 and 6 show the corresponding spectra for samples containing only 3 wt 9’0 MOO> As compared to the r-Al,O, support all samples produced a relatively weak fluorescence background, which usually decreased as the samples were heat treated. The dominant characteristic spectral features are very similar for all samples, in so far as main Raman bands are observed in three wavenumber ranges, namely, 200-250, 300-350, and 850-1000 cm-’. The detailed structure, position, form, width, and symmetry of these bands, however, reflect differences of the determining vibrational parameters of the individual samples. These parameters may be Mo-0 bond length and strength as well as bond angles and structure and symmetry of the respective molybdena species. The most prominent band in all spectra is the relatively

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Flgure 3. Raman spectra of 8 wt % molybdena on y-A1203 after impregnation of pH 6 in the wet state (a), after drying at 393 K (b), and after calcination at 773 K (c).

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Flgure 4. Rarnan spectra of 8 wt YO molybdena on y-AlzO3 after impregnation at pH 11 in the wet state (a), after drying at 393 K (b), and after calcination at 773 K (c).

Figure 6. Rarnan spectra of 3 wt % rnolybdena on y-AIz03 after impregnation at pH 11 in the wet state (a), after drying at 393 K (b) and after calcination at 773 K (c).

broad and asymmetric feature between 930 and 970 cm-l which was also reported by other^.^-^ This band usually shows some structure or splitting in the wet state of the samples which is lost during the drying procedure. The maximum of the resulting broad and asymmetric band shifts toward higher wavenumbers during the drying and calcination processes. On the 8% MOO, sample which was impregnated at pH 6 (Figure 3), two bands at 921 and 939 cm-’ can clearly be discerned in the wet state. After drying,

the maximum of a single broad band occurs at 943 cm-I which is further shifted to 950 cm-’ on calcination. After impregnation at pH 11to yield samples of the same Moos content (Figure 4), three bands a t 900,914, and 939 cm-’ are detected in the wet state, which again collapse on drying to form a single broad band at 932 cm-l which then shifts to 961 cm-l on calcination. The sample containing 3 wt 70 MooBwhich was impregnated at pH 6 (Figure 5) produced a band at 918 cm-’ with a shoulder at 894 cm-’

The Journal of Physical Chemistry, Vol. 83,No. 9, 1979

Raman and UV Study OS Molybdena-on-Alumina

Mo 1yAr-pH 6- 25 120 500

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Figure 7. UV spectra of 1 wt % molybdena on y-Al2O3.

in the wet state. This band increased in width and shifted to 930 and 938 cm-l after drying and calcination, respectively. A small shoulder at 900 cm-' can be recognized in these spectra and an additional band at 865 cm-l appears after calcination. Impregnation at pH 11produced very similar bands (Figure 6), although the shoulder at 900 cm-' cannot be observed on the very broad band at 938 cm-l, and the extra band a t 865 cm-l does not appear. The bands at lower Raman shifts in the ranges 200-250 and 300-350 cm-l undergo very complex band splittings and changes in form and position in all cases. At the lower MOO, contents the bands in the 200-250-cm-l range are very weak but can be detected a t least in the dried and calcined states. A very ill-defined weak and broad feature can be recognized between 500 and 600 cm-l in the spectra of both samples containing 8 wt % Moos in the wet state. After heat treatments this band cannot be detected with certainty. While no such band is found in the spectra of the sample containing 3 wt % MOO, after impregnation at pH 6, impregnation a t pH 11 produced a sharp band at 546 cm-l which is intensified on drying and vanishes after calcination. With the exception of the spectrum of this latter sample, those of all other samples after calcination are superimposed by a series of narrow lines, namely, a t 998 and 821 cm-' (together with a band at 667 cm-l and several others below 400 cm-' which can only be seen in the spectrum of Figure 3), which belong to bulk MOO,. The relative intensity of these bands and hence the relative concentration of bulk Moos in the samples decreases with decreasing molybdena content, of the samples (see Figures 3 and 5) and is lower a t the same total molybdena content for the sample which was impregnated at pH 11 (see Figures 3 and 4). Ultraviolet Spectra. Diffuse reflectance spectra between 220 and 360 nm of molybdena on alumina containing 1, 3, and 8 wt % MOO^, which were impregnated a t either pH 6 or 11, are shown in Figures 7-9. The samples containing 1and 3 w t % MOO, clearly develop two distinct UV bands, namely, between 235 and 250 nm and between 270 and 290 nm. Irrespective of the pH value of the impregnating solution a single band around 240 nm (41670 cm-') is observed for these samples in the wet state (Figures 7 and 8). On drying the samples a new additional band develops near 280 nm (35714 cm-l) which then shifts to 290 nm (34 483 cm-l) and grows in relative intensity at

Flgure 8. UV spectra of 3 wt % molybdena on y-Al,03.

Figure 9. UV spectra of 8 wt % molybdena on y-A120B.

the exDense of the initial band at 240 nm on the ilcined samples.ls On the samples which contain 8 w t % Moo3 (Figure 9) only one band can be discerned which is asymmetric toward lower wavelengths. This band, which appears a t 283 nm (35336 cm-l) on the wet sample impregnated at pH 6, does not shift after drying, but is observed at 297 nm (33670 cm-*)after calcination. On the samples which were impregnated at pH 11, the original band in the wet state has a position at 257 nm (38910 cm-') and shifts to 284 and 297 nm, respectively, after drying and calcination. The asymmetry in the low wavelength range may indicate contributions of the absorption band around 240 nm also in these samples at the higher Moo3 content. Finally, Figure 10 shows the UV spectra of molybdena on q-A1,03 (containing 8 wt % MOO,). The UV bands for the three treatments appear at 237, 272, and 286 nm,

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The Journal of Physical Chemistry, Vol. 83, No. 9, 1979 MoB?AI-pHll- 25 -c120 -0500 +--

Figure 10. UV spectra of 8 wt % molybdena on 7-A1,03.

respectively. Hence, these spectra very closely resemble those obtained for 7-A1203supported samples at the lower Moo3 contents (see Figures 7 and 8) but deviate from those of samples having the 8 wt % Moo3 content on a 7-A1203 support. Thus, the surface of 7-A1203obviously accommodates the molybdena even at higher Moo3 contents as species which are formed on the 7-Al,03 surface only at low Moo3 contents.

Discussion Assignment of Raman Bands. The assignment of the vibrational modes observed in the Raman spectra of the supported molybdenum-oxygen species is a difficult task since a detailed normal coordinate analysis of such surface species is practically impossible. An assignment by comparison with compounds of known structure also is not unambiguous since the vibrational frequencies of the stretching modes of terminal and bridging molybdenum-oxygen groups strongly depend on bond order and bond length as shown by Cotton and Wing.lg These authors demonstrated for a series of well-characterized Mo-0 oscillators that the frequency ranges for the stretching modes of terminal Mo-0 groups (1046-840 cm-l) and of bridging groups (946-820 cm-l) strongly overlap, so that an unambiguous assignment of vibrational bands in this frequency range is not straightforward. Brown et al.5 assigned the bands in their spectra of the surface species by comparison with the Raman spectrum of bulk Moo3. In particular, bands at 1006 and 822 cm-' were attributed, respectively, to stretching vibrations of terminal and bridging Mo-0 groups of a so-called "interaction species", while a relatively strong band at 380 cm-' was understood as the corresponding deformation vibration of terminal and bridging groups. This assignment appears to be incorrect since the spectra obtained by Brown et al.,5 which contain these three characteristic features, very closely resemble the Raman spectrum of A12(Mo04)3.689The formation of A12(Mo04)3at high loadings of up to over 50 wt % Moo3 applied by Brown et al.5 does not seem to be unreasonable and the corresponding spectra have most probably been misinterpreted. A band a t 896 cm-l has been assigned by Brown et ala5to a stretching mode in a Mo-0-A1 species although it might also be due to the characteristic high-frequency mode of an isolated MOO^^- tetrahedronall The broad and typical feature usually observed around 950 cm-I for supported molybdena on alumina was also detected by Brown et al.5

H. Jeziorowski and H. Knozinger

It was attributed to a Mo-0 stretch, although a conclusive assignment was not made. Jannibello et only detected this band in their relatively poor Raman spectra and they assigned this feature to a symmetric terminal Mo=O stretch in a surface bound M o o t - tetrahedron since the corresponding ultraviolet spectra seemed to indicate tetrahedral coordination of Mo. The high frequency was explained by a postulated increase of the Mo=O double bond character of the surface species as compared to the isolated MOO^^- ion.20 Medema et a1.6 attributed bands appearing a t low loadings a t 890 and 325 cm-' and a doublet at 840 and 800 cm-l (probably due to distortion splitting) to the vibrational modes of isolated Moottetrahedra." The bands arising from samples at higher loadings, namely, the typical band a t around 950 cm-l accompanied by a shoulder at 875 cm-l, were attributed to a two-dimensional polymeric surface molybdena species, since the spectra of this structure very closely resembled those of the well-characterized Mo70246-polyanion (see Figure 1). It was therefore also concluded that the twodimensional polymeric species should be built up by Moo6 octahedra. An analogous interpretation was given by Knozinger and Je~iorowski.~ The Raman spectra of a variety of molybdenum-oxygen isopolyanions are reported in the l i t e r a t ~ r e . ' ~ *These ~l-~~ isopolyanions occur in acidic solutions or in solids and are preferentially built up by Moo6 octahedra,15J6although structures such as the [Mo4014]n4n-chains are also known, in which Moo6 octahedra and Moo4 tetrahedra are int e r ~ o n n e c t e d .The ~ ~ Raman spectra of M o ~ O ~ ~of- , ~ ~ J ~ M o ~ O ~ and ~ ~of- M, 0~~ 0~ ~ ~2 -~ have ~ , been ~ ~ reported in detail. All these spectra exhibit five characteristic frequency ranges of vibrational modes at 200, 310-370, 500-650,700-850, and between 900 and 1000 cm-l, which are usually assigned respectively to Mo-0-Mo deformations, Mo=O bending vibrations, symmetric Mo-0-Mo stretches, antisymmetric Mo-0-Mo stretches, and symmetric and antisymmetric terminal Mo=O ~ t r e t c h e s . ~ ~ ? ~ ~ A detailed normal coordinate analysis has been performed for the Mo6O12- polyanion by Mattes et al.23 Many of these polyanions have highly symmetric structures and a large number of their normal modes are thus degenerate and belong to species E or F. As a result, relatively simple Raman spectra are observed. Slight distortions, however, may lead to band splitting and weak Raman bands may then escape detection. It is also worth mentioning that the frequency of the intense band of the Mo=O stretch around 950 cm-' seems to increase as the degree of polymerization increase^^^^^^,^^ (e.g., the band is found at 943 cm-' for M070246-and 965 cm-' for Mo8O22-in aqueous solution22)and it also seems to increase at constant degree of polymerization as the degree of hydration increases.24 As mentioned above, the spectra of the molybdena species supported on alumina very much resemble those of the molybdenum-oxygen polyanions as regards the observed frequency ranges. This may give a clue to the assignment of Raman bands of the surface species. A major difference between their spectra and those of the polyanions is the larger width and possible asymmetries in band shape, but these phenomena can readily be accounted for by the heterogeneous surface of the alumina carrier to which the molybdena species are bound and by unresolved band splittings due to distortions of the COordination polyhedra on the surface. The Raman spectra observed in the present study will now be interpreted on this basis, Le., the occurrence of bands (or shoulders) in the ranges between 310 and 370 cm-l and 900 and 1000 cm-l will be considered as being due to the normal modes 14122924

Raman and UV Study of Molybdena-on-Alumina

of terminal Mo=O groups, while bands in the ranges between 200 and 250 cm-l, 400-600 cm-l, and 700 and 850 cm-l, will be attributed to bridging Mo-O-Mo species. Especially the occurrence of bands near 200 cm-l (MoO-Mo deformation) is a conclusive indication for the existence of bridging groups, since no bands are expected in this range for structures which exclusively contain MFO terminal groups, as may be seen, e.g., in Figure 1, whereas in the other regions bands may overlap or escape detection. The sample containing 3 w t % Moo3 which was impregnated at pH 6 (Figure 5) only shows a vibrational band a t 918 cm-l with a shoulder at 894 cm-l and a broad band between 320 and 340 cm-l in the wet state. No Raman band near 200 cm-l is detectable, although the M0~024~polyanion in the solution exhibits this band very clearly (see Figure 1). I t must therefore be concluded that the surface species in this sample does not contain Mo-O-Mo bridges a t a detectahle level in the wet state. The bands observed can be assigmed to MFO stretching and bending modes, perhaps of distorted MOO:- species which are bound onto the heterogeneous support surface in varying local environments. On drying and even more pronounced on calcination the terminal Mo=O stretching band shifts to higher frequency (930 and 938 cm-l, respectively), broadens, and becomes asymmetric toward lower wavenumbers. The bending mode also changes shape to some extent, but most importantly weak bands can now be detected between 200 and 250 cm-l. These indicate that interaction of the initially isolated MOO:- species does now occur through some Mo-O-Mo bridges. The asymmetry of the bands at 930 and 938 cm-l toward lower wavenumbers indicates contributions to this band of the antisymmetric Mo-O-Mo stretching mode. The symmetric mode in the range between 500 and 650 cm-' is usually weak (compare Raman spectrum of Mo70246-in Figure 1) and is not detectable on this sample. The additional band a t 865 cm-l, which appears after calcination, can be assigned to either a Mo-O-Mo symmetric mode at relatively high wavenumber or to a terminal Mo=O stretch at relatively low frequency. Its exact assignment remains unsolved, however, and the band only occurs in this particular sample. Impregnation a t pH 11 leads to principally the same behavior of the samples containing 3 wt % MooB(Figure 6), namely, the Mo-O-Mo deformation band between 200 and 250 cm-l and the corresponding asymmetry on the low-wavenumber flank of the Mo=O stretching mode are developed only after drying. One major difference as compared to the sample impregnated a t pH 6 is the fact that the Mo=O bending modes produce sharper bands a t 325 cm-l. Moreover, in the wet and dried state a sharp band arises at 546 cm-l, Le., in the range of the antisymmetric Mo-O-Mo stretching modes. Since the usually more intense deformation band near 200 cm-' is not detected at least in the wet state, this assignment cannot apply and this band remains unassigned. The samples containing 8 wt % Moo3 in the wet state after impregnation a t pH 6 and 11 (Figures 3 and 4) give rise both to bands characteristic for terminal Mo=O and for bridging Mo-O-Mo groups. Particularly the MoO-Mo deformation band near 220 cm-l can be discerned, weak broad bands appear in the range of the antisymmetric Mo-0--Mo stretches between 500 and 600 cm-', and the asymmetry toward lower wavenumber of the strong bands above 900 cm-l is already observed. This latter band consists clearly of at least three distinguishable components with band maxima a t 939,914-921, and 900 cm-l,

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which indicates the presence of different species of probably different degree of interaction through Mo-O-Mo bridges. Namely, the band or shoulder at 900 cm-l can be identified with the Fl mode of a relatively free isolated M o o t - tetrahedron, while the band at 914 or 921 cm-' probably can be assigned to tetrahedra which already weakly interact with neighboring groups through bridges with relatively long Mo-0 bonds. The high-frequency component at 939 cm-l already falls into the range of the terminal Mo-0 stretch of polyanions. After drying and after calcination the structure of the band has vanished and the band shifts to a final position at 950 and 961 cm-', respectively, for samples impregnated at pH 6 and 11. This frequency shift most probably indicates an increased degree of condensation of the molybdenum-oxygen species after calcination with sharter Mo-0 bond length for the oxygen bridges. The loadings in all samples studied remain below the average monolayer capacity of a support with a surface area of 158 m2/g. The exact monolayer capacity, however, depends on the nature of the exposed crystallographic planes of the alumina support.37 Considering also the porous structure of the high-surface area support, local excess molybdate concentrations may well occur during the impregnation procedure, which will then lead to the formation of bulk Moo3 during the calcination procedure even at total loadings below the average monolayer capacity. As can be seen from the relative intensities of the typical sharp Moo3 bands in Figures 3 and 4, this situation occurs more easily if the impregnation is carried out in a paramolybdate solution (pH 6), while ion exchange with the MOO:- ion obviously leads to a better dispersion. Ultraviolet Spectra. The electronic transitions in Mo6+ (do)/oxygen systems are of the ligand charge transfer (LCT) type. The spectra of molybdena on alumina have hitherto exclusively been assigned by comparison with reference compounds of known stereochemistry (preferentially crystalline molybdates) to either Mo6+in tetrahedral or octahedral c o o r d i n a t i ~ n . l ~ Thus, J ~ ! ~ ~absorptions in the 260-280 nm (38460-35715 cm-l) region were attributed to tetrahedral Mo6+ and the 300-320 nm (33300-31250 cm-l) bands to octahedral Mo6+. Ashley and Mitchell12 concluded from their spectra that Mo6+ was tetrahedrally coordinated on the alumina support. Giordano et have demonstrated that the dominant band in the UV spectra of molybdena on alumina samples was centered a t 265 nm (37736 cm-l) for low Moo3 concentrations and heat treatment at low temperatures. For higher concentrations (e.g., above 10 wt 90 Moo3) and at higher calcination temperatures (773 K and above) a second band appeared at 300 nm (33 333 cm-'). These two bands were assigned to Mo6+ in a tetrahedral and octahedral environment. Very recently, Janibello and Mitchell'O again demonstrated UV spectra of molybdena on alumina prepared by the pore filling and equilibrium adsorption methods which they attributed to tetrahedrally coordinated Mo6+. Only under certain preparation conditions was an additional band at 312.5 nm (32 000 cm-l) observed which they attributed to octahedrally coordinated Mo6+. An attempt will be made in this study to assign the observed UV bands of molybdena on alumina by comparison with solution spectra of molybdenum-oxygen compounds. It is obvious from the spectra shown in Figures 7-10 that two typical bands between 230 and 250 nm and between 270 and 297 nm are detected for the various catalysts depending on the molybdena concentration and on the state of the system. Figure 2 clearly

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H. Jezlorawski and H. Knozinger

The Journal of Physical Chemistry, Vol. 83,No. 9, 1979

shows that a band between 230 and 240 nm is the only absorption in solutions containing the M o o t - species exclusively, while the paramolybdate ion Mo70246-gives rise to an additional absorption at 290 nm. The same spectral features have been reported by Bartecki and DembickaZ7for aqueous molybdate solutions. Since the Mo70246-ion is composed of only Moo6 octahedra,l5?l6 absorption bands below 300 nm (or above 33000 cm-l) obviously cannot be assigned unambiguously to Mo6+in a tetrahedral environment. However, the origins of the LCT bands of polyanions of molybdenum unfortunately are still not and Bartecki and DembickaZ7expressed the opinion that even in aqueous solutions the observed absorption bands can hardly be ascribed to well-defined species. The same authors suggested that an absorption band at 227 nm (43 900 cm-l) is characteristic of the Mo03- ion and is due to a transition of an oxygen 2p7r electron into an empty d orbital of the Mo atom. A band at around 260 nm (39000 cm-l) was considered as being characteristic of the bridging Mo-0-Mo system and it seemed to correspond to a transition involving the molecular orbitals of the entire group. This latter band seems to be typical for polymerized species (dimolybdates and polyanionic species)27and its exact position may be expected to depend on the Mo-0 bond length and the detailed geometry of the bridging system. Therefore, and because of the occurrence of the 290-nm band in the aqueous Mo7O2:- solution we believe that bands in the range between 250 and 290 nm can be ascribed to LCT transitions in the bridging group. The UV spectra of the samples containing 1or 3 w t 7'0 Moo3 (Figures 7 and 8) thus show the presence of tetrahedral Mo6+ irrespective of the nature of the species which exists in the impregnation solution. This conclusion agrees with recent adsorption studies by Jannibello et a1.8 and by Jannibello and Mitchell.lo The decrease in relative intensity of the corresponding band at 230-250 nm and the occurrence of a new band in the 270-295-nm range on drying and calcination suggests the formation Mo-0-Mo bridges. Thus, at the low Moo3 concentration the M070?46 ion seems to undergo degradation during the impregnation procedure, while heat treatments lead to a polymerization of the adsorbed isolated Moot- tetrahedra through oxygen bridges. The intensity ratios F(240)/F(280) decrease with increasing treatment temperature which indicates an increasing degree of polymerization. The samples containing 8 w t % Moo3 (Figure 9) show only one asymmetric band, the maximum of which shifts to longer wavelengths thus indicating that some bridging interactions already occur a t these high loadings on the surface of yAlzO3. These conclusions agree very well with those drawn from the Raman spectra. The similarity of vibrational spectra and of LCT spectra of samples calcined at 773 K and those of isopolyanions in solution suggests the formation of polymeric surface species the structure of which might be comparable for example to that of the paramolybdate ion with an octahedral coordination of Mo6+ ions.689 The present band assignment, however, does not allow for an unambiguous determination of the Mo6+ coordination number, since Mo-0-Mo may also occur in tetrahedrally and pentacoordinated structures. Conclusions (1)Even if the paramolybdate ion Mo7OZ4'-exists in the impregnating solution, adsorption occurs via the Moot-. The degradation of the polyanion must be brought about by an increased local pH close to the support surface, since the OH concentration there is increased by the ion exchange adsorption, which was formulated by Jannibello

-

and MitchelllO and by Jannibello et ale8as A1-OH

+ Mood2-

Al-OMo03-

+ OH-

(2)

Alternatively, one could also consider a bidentate surface species: AI-OH

A1-0 t MOO,'-

A1-OH

+

\ 1

MOO, t 2 0 H -

(3)

Al- 0

(2) The removal of OH- ions from the surface leads to a decrease of the sample fluorescence, which was previously shown to be due to a laser-induced electronic excitation of surface OH- ions.29 (3) Removal of water from the impregnated samples by drying at 393 K and by calcination at 773 K leads to a polymerization of the isolated tetrahedral species through Mo-0-Mo bridges to form a two-dimensional polymeric structure as was suggested p r e v i o ~ s l y . ~The ~ ~ J coordi~ nation number of Mo6+ in this structure cannot be determined unambiguously; it may be 4, 5, or 6. The degree of polymerization depends on the molybdena concentration and on the heat treatment and it may also depend on the rate of water removal during the drying process.30 Moreover, the degradation of an Mo7O22-ion near the surface will lead to the ion exchange adsorption of MOO:- species in close proximity, thus forming islands of tetrahedral species even at low loadings. This must facilitate polymerization and bridge formation as compared to samples at low loadings, which were impregnated from MOO?- solutions at pH 11. Evidence for this conclusion comes from the Raman spectra shown in Figures 5 and 6 and from the intensity ratios in the UV spectra of Figures 7 and 8. The bridge formation must also be determined by the structure of the crystallographic planes which are exposed on the support surface. This assumption may explain the differences in the UV spectra of samples supported on 17-A1203(Figure 10) as compared to those supported on yAlz03. The nature of the two-dimensional polymeric surface species may be schematically represented by structures suggested by Giordano et al.13 such as I or polymeric 0 0 0 0 \\ // \\ //

I

U

U

I11

species I1 and 111.

Slightly different structures were proposed by Medema et a1.6 which may be represented by IV. It was suggested for this structure that four oxygen

IV

atoms were a t shorter Mo-0 distances than the two bridging oxygen atoms. All these models do not take into account the structure of the alumina surface. Hall and

Raman and UV Study of Molybdena-on-Alumina

The Journal of Physical Chemistry, Vol. 83, No. 9, 1979

1173

M a ~ s o t hassumed ~~ an epitaxial growth of the molybdena Acknowledgment. Financial support of this research work by the Deutsche Forschungsgemeinschaft and by the monolayer on the (13.0) face of the 7-Al,03 support, which Fonds der Chemischen Industrie is gratefully acknowlmust result in tetrahedral coordination of Mo6+. Mamoth% edged. has recently described a model for the molybdena-alumina system, which suggests the occurrence of one-dimensional References and Notes chains of Mooz species, the third 0 associated with Mo G. T. Pott and W. H. J. Stork in “Preparation of Catalysts”, B. Delmon, being placed in vacancies of the alumina support. P. A. Jacobs, and G. Poncelet, Eds., Elsevier, Amsterdam, 1976, p Since the molybdena species are bound to the support 537. F. R. Brown, L. E. Makovsky, and K. H. Rhee, Appi. Spectrosc., 31, surface via an ion-exchange process in which alumina 563 (1977). hydroxyls are liberated, infrared spectroscopy in the OH F. R. Brown, L. E. Makovsky, and K. H. Rhee, J . Catai., 50, 162 stretching region should reveal important information (1977). regarding the type of alumina OH groups which are F. R. Brown, L. E. Makovsky, and K. H. Rhee, J . Catal., 50, 385 (1977). eliminated. In fact, it has been reported recently,35that F. R. Brown, R. Tischer, L. E. Makovsky, and K. H. Rhee, paper the alumina hydroxyl stretching band at 3730-3740 cm-l presented before the D on of Petroleum Chemistry, at the 175th is preferentially eliminated after supporting the molybdena National Meeting of the American Chemical Society, Anaheim, Callf., March 12-17, 1978. species. The corresponding OH groups are bridging groups J. Medema, C. van Stam, V. H. J. de Beer, A. J. A. Konings, and which are aligned in parallel rows on the (111)faces of the D. C. Koningsberger, J. Catal., 53, 386 (1978). alumina spinel lattice.% It had therefore been s u g g e ~ t e d ~ ~ ~ ~ J. Medema, R. Bosman, V. H. J. de Beer, A. J. A. Konings, and D. C. Koningsberger, to be published. that the supported molybdena species might indeed be A. Jannibello, S.Marengo, F. Trifiro, and P. L. Villa, paper A, presented aligned as one-dimensional chains on the support surface. at the second International Symposium on “Scientific Basis for the In any case, it seems plausible that the local density of Preparation of Heterogeneous Catalysts”, Louvain-!a-Neuve, Belgium, Sept 4-7, 1978. exchangeable hydroxyl groups and their geometric arH. Knozinger and H. Jeziorowski, J . Phys. Chem., 82, 2002 (1978). rangement must critically determine the structure of the A. Jannibello and P. C. H. Mitchell, paper E2 presented at the Second molybdena species, which are being formed at a given International Symposium on “Scientific Basis for the Preparation of loading.37 These factors are governed by the crystallogHeterogeneous Catalysts”, Lowain-la-Neuve, Belgium, Sept 4-7, 1978. A. Muller, N. Weinstock, W. Mohan,C. W. Schlpfer, and K. Nakamoto, raphy of the surface planes of the support, and the types Appl. Spectrosc., 27, 257 (1973). of hydroxyl groups which are exposed on them can be J. H. Ashley and P. C. H. Mitchell, J. Chem. Soc. A , 2730 (1969). identified by their infrared f r e q u e n c i e ~ .It ~ ~is therefore N. Giordano, J. C. J. Bart, A. Vaghi, A. Castellan, and G. Martinotti, J. Catal., 36, 81 (1975). hoped that an infrared study jointly carried out with J. Aveston, E. W. Anacker, and J. S.Johnson, Inorg. Chem., 3, 735 Raman and ultraviolet spectroscopic studies as described (1964). here will enable us to present a more detailed model of the D. L. Kepert, Compr. Inorg. Chem., 4, 607 (1973). K.-H. Tytko and 0. Glemser, Adv. Inorg. Chem. Radiochem., 19, supported molybdena catalysts which will be based on the 239 (1976). previously described36model of the alumina surface. G. Kortum, “Reflexionsspektroskopie”, Springer, Berlin, 1969. (4) The formation of A ~ , ( M o O ~ could ) ~ not be detected The absolute F(R,) values should not be compared quantitatively for different samples, since particle aggregation and packing density in the catalysts studied. in the UV cell are not strictly reproducible and moreover depend on (5) The formation of bulk Moo3 occurred to some extent the treatment of the samples. on all samples after calcination at 773 K except for the F. A. Cotton and R. M. Wing, Inorg. Chem., 4, 867 (1965). F. Trifirb, Ann. Chim. Roma, 64, 377 (1974). sample containing 3 wt % Moo3 which was impregnated J. Fuchs and K. F. Jahr, Z. Naturforsch. B , 23, 1380 (1968). from a solution a t p H 11. The relative amounts of bulk W. P. Griffith and P. J. B. Lesniak, J. Chem. SOC.A , 1066 (1969). Moo3 increased with the total loading and decreased at R. Mattes, H. Bierbusse, and J. Fuchs, Z. Anorg. A&. Chem., 385, constant loading if impregnation was made at pH 11. Thus 230 (1971). K.-H. Tytko and 8. Schonfeld, Z. Naturforsch. B,30, 471 (1975). the better distribution of the initial tetrahedral species J. Fuchs and J. Brudgam, Z. Naturforsch. B , 32, 853 (1977). reduced the formation of bulk Moo3. An increase in G. N. Asmolov and 0. V. Krylov, Kinet. Katal., 11, 1028 (1970). surface area has the same effect as reported previo~sly.~ A. Bartecki and D. Dembicka, J. Inorg. Nucl. Chem., 29, 2907 (1967). H. So and M. T. Pope, Inorg. Chem., 11, 1441 (1972). (6) It is interesting to mention that all good commercial H. Jeziorowski and H. Knozinger, Chem. Phys. Left., 51,519 (1977). hydrodesulfurization catalysts in the oxidic state give rise 0. Ochoa, R. Galiasso, and P. Andreu, paper E, presented at the to the characteristic broad Raman band near 950 cm-l Second International Symposium on “Scientific Basis for the Preparation of Heterogeneous Catalysts”, Louvain-la-Neuve, Belgium, suggesting that the two-dimensional polymeric interaction Sept 4-7, 1978. species is an essential catalyst precursor also in these G. C. A. Schuit and B. C. Gates, AIChEJ., 19, 417 (1973). catalysts which contain Co2+ and/or Ni2+as promoters. H. Tops&, B. S.Chusen, N. Burriesci, R. Candia, and S.MCrup, paper E, presented at the Second International Symposium on “Scientific One may therefore conclude that the monolayer model as Basis for the Preparation of Heterogeneous Catalysts”, Louvainproposed originally by Gates and Schuit31and recently also la-Neuve, Belgium, Sept 4-7, 1978. considered by Topsgle et al.32on the basis of Mossbauer (33) W. K. Hall and F. E. Massoth, J . Catal., 34, 41 (1974). experiments is a good structural description of this class (34) F. E. Massoth, J . Catal., 36, 164 (1975). (35) P. Ratnasamy and H. Knozinger, J . Catal., 54, 155 (1978). of catalysts. The effect of promoters such as Co2+,Ni2+, (36) H. Knozinger and P. Ratnasamy, Catal. Rev. Sci. Eng., 17, 31 (1978). and Po43on the catalyst structure is presently being (37) M. Lojacono, A. Cimino, and G. C. A. Schuit, Gar. Chim. Ita/., 103, studied in this laboratory. 1281 (1973).