J. CHEM. SOC. FARADAY TRANS., 1994, 90(8), 1161-1170 Spectroscopic Characterization of Magnesium Vanadate Catalysts Part 1.-Vibrational Characterization of Mg,(VOJ2 Mg2V20, and MgV20, Powders Guido Busca* and Gabriele Ricchiardi lstituto di Chimica, Facolta di lngegneria, Universita P.le Kennedy, 1-16129 Genova, Italy D. Siew Hew Sam Elf Atochem, Centre de Recherche de l'Est BP 1005, F-57501 Saint-Avold Cedex, France Jean-Claude Volta lnstitut de Recherches sur la Catalyse, CNRS,Avenue A. Einstein, F49626 Villeurbanne Cedex, France The IR and Raman spectra of Mg orthovanadate Mg,(VO,), , Mg pyrovanadate Mg,V,O, and Mg metavanadate MgV,O, powders are reported, described and discussed on the basis of the crystal structures of these phases and of the optical activity of the fundamental vibrational modes predicted with the use of the correlation method.The IR-active combinations are also discussed. The structural and vibrational features of these compounds are discussed in relation to those of other V-based oxidic catalytic materials. The magnesium oxide-vanadium oxide system has been the object of attention recently because it presents promising catalytic activity in the oxidative dehydrogenation of hydro- carbons, such as alkanes (propane and butanel-') and ethyl- benzene.6 Processes allowing the production of light alkenes (ethene, propene and butenes) and butadiene by oxy-dehydrogenation of the corresponding alkanes are becoming very attractive because of the availability of very cheap, light alkanes from natural gas.Our research groups have been previously involved in investigations concerning other vanadia-based catalysts extensively used in industry in partial oxidation processes such as the V,O,-P,O, systems for maleic anhydride syn- thesis from b~tane~'~ and V,O,-TiO, systems for phthalic anhydride synthesis from o-~ylene.~.' A fundamental investi- gation of the structure and chemistry of these systems can allow an interpretation of the role of the components addi- tional to V205 (phosphorus, titania and magnesium), in gov- erning activity and selectivity. Attempts in this direction have already been proposed.' ',12 Several authors have investigated the solid-state chemistry of the MgO-V205 ~ystem.'~-'~Mo st of them'3.14*'7 con- sider that only three stable compounds appear in this phase diagram : the orthovanadate, Mg,(VO,), , the pyrovanadate, Mg,V,O, ,and the metavanadate, MgV206 .Moreover, both Mg,V,O, and MgV,06 exhibit the phenomenon of poly- m~rphism.'~.''.'~ In the present paper a bulk vibrational characterization of V-Mg-0 catalysts is reported. The materials investigated here appear to X-ray diffraction (XRD) analysis to be almost pure Mg orthovanadate [Mg,(VO,),], Mg pyrovanadate (Mg,V,O,) and Mg metavanadate (MgV206).3 The catalytic activity of these materials has also already been described., Experimental Catalyst Preparation The three pure magnesium vanadates were obtained by VMgO precursors generated by adding an appropriate amount of Mg(OH), to a basic aqueous solution (1% NH,OH) containing NH,VO,.The solid was then dried under vacuum at 373 K and immediately calcined at 823 K for 6 h to avoid any carbonation. Mg orthovanadate, Mg3(V0,), , was then prepared from the 60VMgO precursor (58.5% V205 w/w) by calcination in air for 49 h at 898 K, 60 h and 913 K, 15 h at 1023 K and 15 h at 1073 K. Mg pyrovanadate, Mg2V207, was prepared from the 69VMg0 precursor (66.4% V205 w/w) by calcina- tion in air for 6 h at 923 K and 6 h at 973 K. The metavana- date, MgV206, was prepared from the 82VMg0 precursor (79.8% V,O, w/w) by calcination in air for 6 h at 873 K and 24 h at 973 K. The XRD analyses (Siemens goniometer equipped with a quartz front monocromator, Cu-Ka radiation) showed quite pure phases except for Mg metavanadate, which presented traces of Mg pyr~vanadate.~ Spectroscopic Measurements The IR spectra were recorded using a Nicolet Magna 750 Fourier-transform instrument.The skeletal spectra in the region above 400 cm- 'were recorded with KBr pressed discs and with a KBr beam splitter, while those in the far-infrared (FIR) region (400-50 cm-') were recorded using the powder deposited on polyethylene discs, and with a 'solid substrate' beam splitter. The spectra of the overtone region (above lo00 cm-') were recorded using pressed discs of the pure powders, outgassed at 673 K in a heatablebiquid-nitrogen-cooledcell connected to a conventional gas-handling system. The laser Raman spectra were recorded on a Dilor Omars 89 spectrophotometer equipped with an intensified photo- diode array detector.The emission line at 514.5 nm from an Ar+ ion laser (Spectra Physics, mod. 124) was used for excita- tion. The power of the incident beam on the samples were 36 mW. The aquisition time was adjusted according to the intensity of the Raman scattering. 100 spectra were accumu- lated in order to improve the signal-to-noise ratio. The wave- number values obtained from the spectra were accurate to within about 2 cm-'.To reduce both thermal and photo- degradation of samples, the laser beam was scanned on the sample surface by means of a rotatory lens. The scattered light was collected in back-scattering geometry. Results The structures of the three Mg vanadates, deduced from liter- ature are shown schematically in Fig.1,2 and 3. In the case of the pyrovanadate (Mg2V,07), according to the XRD pattern and to ref. 17, the structure has been assumed Fig. 1 Scheme of the structure of Mg,(V04),, from ref. 18, On the right: the coordination of the vanadate ion. Symbols :black spheres, V; white spheres, Mg; grey spheres, oxygen. b Fig. 2 Scheme of the structure of monoclinic Co,V,O, from ref. 20, assumed to be isostructural with monoclinic Mg,V,O,. On the right: pairs of V,O:- ions. Symbols: black spheres, V; white spheres, Mg; grey spheres, oxygen. [010] t Fig. 3 Scheme of the structure of MgV,O,, from ref. 21. On the right: structure of the [(V,0,)2-], sheets.Symbols: black spheres, V; white spheres, Mg; grey spheres, oxygen. to be the same as that of Co,V,O, ,2o both being monoclinic (space group P2Jc =Cz,). In Table 1 the V-0 distances present in the relevant structures are summarized, also including those of triclinic Mg,V,O, l9 and of V,O,, 22 for the sake of completeness. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 V-0 distances (A)in metal vanadates (from ref. 18-22) compound ref. I I1 I11 IV V VI Mg3(V0,), Mg,V,O,triclinic Co,V,O, monoclinic 18 19 I I1 20 I I1 1.70" 1.63' 1.68" 1.63' 1.69" 1.70" 1.70" 1.71" 1.71" 1.70" 1.72" 1.71d 1.74d 1.72" 1.70" 1.81b 1.82' 1.78' 1.85' 1.84' 2.87/ 2.44/ MgV,O, v205 21 22 1.66" 1.58' 1.67" 1.77' 1.85h 1.88' 1.85' 1.88' 2.11' 2.02' 2.57g 2.83' "V-0-Mg, terminal oxygen coordinating two bivalent cations.V-O-Mg, terminal oxygen coordinating three bivalent cations. V-0-Mg terminal oxygen coordinating one bivalent cation. Asymmetric V,O-Mg,: like a but acting as weak fifth ligand to another V atom. 'Nearly symmetric V,O-Mg. V-0-V nearly symmetric bridge. Asymmetric V,O-Mg: like c but acting as weak sixth ligand to another V atom. Asymmetric V,O-V. V--. ..V terminal oxygen acting as weak sixth ligand to another V atom. In pyrovanadates the two V atoms (I and 11) are not equiva- lent. The FTIR, FT-FIR and laser-Raman spectra (1200-50 cm-') are summarized in Fig. 4, 5, 6 and 7. The positions of the observed bands are summarized in Table 2. Interpreta- tion of the spectra was achieved using the correlation method, as reported in ref.23 and 24. The Orthovanadate, MgJVO,,), The orthorhombic magnesium orthovanadate (Fig. 1) belongs to the Di8 =Cmca space group,18 with a =6.053 A, b =11.442 1,c =8.330 8, and four molecular units per unit cell. The crystallographic unit cell contains two Bravais cells, 1200 1000 800 600 400 200 waven umberjcm- ' Fig. 4 FTIR (a), FT-FIR (b) and laser Raman (c) spectra of Mg3(V04)2 powder -0.9 0.8-0.7-0.6-0.5-0.4-0.3: 4 ............................................. 1200 1000 800 600 400 200 ' wavenumber/cm - Fig. 5 FTIR (a), FT-FIR (b) and laser Raman (c) spectra of Mg,V,O, powder J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 0.60 0.55 0.50 0 0.45 $% 0.40 0.35 0.30 1200 1000 800 600 400 200 wavenumber/cm-' Fig. 6 FTIR (a), FT-FIR (b) and laser Raman (c) spectra of MgV,O, powder. The peaks marked with stars belong to Mg,V,O, impun ties. I I al Cm e 0 2400 2200 2000 1800 1600 1400 1200 wavenumber/cm-' Fig. 7 FTIR spectra in the overtone region: Mg3(V0,), (a), Mg,V,O, (b),MgV,O, (c)and V,O, (d)powders each one containing two molecular units, i.e. 26 atoms. Accordingly, 78 total modes are expected, three of which cor- respond to the acoustical modes and 75 are vibrational models. Two different types of octahedrally coordinated Mg ions are present while the tetrahedrally coordinated V atoms are all equivalent. The VOi- ions have C, symmetry, although approaching a symmetric tetrahedral configuration.According to factor group analysis we can obtain the fol- lowing irreducible representation for the optical modes of Mg,(VO,),: + 8AJinactive) + 12B,,(IR) + 11B2,(1R)+ 8B,,(IR) to which the acoustic modes (Blu + B,, + B,,) should be added. Consequently, we expect 31 IR-active modes and 36 Raman-active modes. The vibrational structure of Mg,(VO,), can be discussed by considering covalently bonded VO: -vanadate ions as molecular units placed in C, symmetry sites, coordinated through ionic bonds to Mg2+ cations. For each VOZ-molecular ion, nine internal vibrational modes are expected, which corresponds to 36 modes for the entire Bravais cell.The assignments of the remaining 39 optical modes are sum- marized in Table 3. The free V0:- ion is tetrahedral (T', point group) and cor- respondingly the following irreducible representation is valid : The A, mode is the symmetric stretching vl, while the E mode is the symmetric bending v2 . The two F, modes corre- spond to the asymmetric stretching (v,) and the asymmetric deformation (v,). In our case, one oxygen atom is bonded to V with a longer bond than the other three, so the symmetry is near C3",the two F, modes being split into A, + E. When the symmetry is lowered to C,, the site symmetry in our case, the degeneracies are completely broken and the irreducible representation changes to the following: rCsVOa= 6A' + 3A" where all modes are both IR- and Raman-active.Under the factor group D,, each A' mode gives rise to A,(R) + B3,(R)+ B,,,(IR) + B,,(IR) modes, while each A" mode gives rise to B,,(R) + B,,(R) + A,(inactive) + B,,(IR). According to these correlations, the symmetric stretching of the V0:-ions (v,, A, in Td point group) gives rise in the Mg orthovanadate to four modes [A,(R) + B3,(R) + B,,,(IR)+ B2,(1R)], while the asymmetric stretching (v3, F, in & point group) gives rise to 12 modes [2A,(R) + 2B,,(R)+ 2B1,(IR) + 2B,,(IR) + B2,(R) + B,,(R) + AJinactive) + B3J1R)]. Accordingly, also the number and activities of the vibrational modes arising from the asymmetric and sym- metric deformation modes of the VOZ-entity, as well as from the other lattice modes, can be predicted.The distribu- tion of the fundamental modes of Mg,(VO,), is summarized in Table 3. According to the above discussion, we can attempt some assignments of the observed IR and Raman spectra of Mg orthovanadate. In the region above 500 cm-' we expect the presence of bands arising from the stretchings of VO, tetra-hedra. The symmetric stretching (v,) of the isolated orthova- nadate ion is expected and is reported to correspond to the strongest Raman peak, quoted at 827 cm- ' for the free ion in aqueous solution.25 For the free ion this band is IR-inactive. According to the above discussion, under coupling of the four VO, unities in the Bravais cell, this mode is expected to split into two strong Raman peaks and two weak IR bands.So, we assign the two strongest Raman peaks in the spectrum of Mg orthovanadate, at 862 and 827 cm-', to the two com- ponents of the symmetric stretching mode. The strongest peak at higher frequency is assigned to the totally symmetric A, mode, the other being assigned to the B,, mode, i.e. to a mode that is symmetric with respect to the x, z symmetry plane and antisymmetric with respect to the x, y and y, z planes. It seems reasonable to assign the weak but sharp IR peak at 833 cm- to the two corresponding IR active modes with B,, and B,, symmetries, that are symmetric with respect to the x, y and x, z symmetry planes, respectively, and anti- symmetric with respect to the others. These modes could be superimposed upon each other, and are almost coincident with the Raman-active B,, mode, assuming that the three symmetry planes are equivalent, which is, obviously, not true.The frequency of these three modes, near 830 cm-', and also the 'centre of gravity' of the four components assigned to modes arising from v1 (837 cm- ') are very near to the wave- number of v1 of the unperturbed vanadate ion (827 cm- '). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Position of the observed vibrational bands for the Mg vanadate powders Mg,(V04)2 Mg2V20, MgV20, IR Raman assignment IR Raman assignment IR Raman assignment fundamentals 975 sh 968 VO str. 915 897 sh 917 923i881 sh 880 sh 888 861 840 VO, str. 840 sh asVOV str." VO, str. 833 827 818 730 sh 724 w 770 sh 690 vw 690 695 shI687 862 610 br sh 668 VOV str.655 575 570 620 575 sh 552 485 462 473 473 439 440 430 440 448 410 415 41 1 402 403 394 39 1 379 377 383 370 35 1 362 354 350 br 332 336 344 325 sh 335 320 330 3 14 316 308 vw 302 sh 305 302 309 291 290 285 282 286 275 V04 def. 268 VO, def. 268 248 245 br + lattice 242 243 + lattice 235 vw 228 205 200 220 216 212 204 198 198 198 174 190 181 171 165 156 145 155 150 149 145 136 137 130 131 122 113 IR combination 1933 975 + 948 1910 968 + 948 1867 655 + 655 + 552 732 + 695 + 440 1790 915 + 881 1790 917 + 873 1780 923 + 888 923 + 840 1720 862 + 861 1694 873 + 818 1672 861 + 827 845 + 840 1615 845 + 770 1347 862 + 485 1430 818 + 620 1408 731 + 695 1210 630 + 575 1208 695 + 523 1116 569 + 575 1117 731 + 383 str., stretch; def., deformation; sh, sharp; br, broad; as, asymmetric; vw, very weak.Very asymmetric VOV bridges. Table 3 Distribution and assignments of the fundamental modes of orthorhombic Mg3(V04), vo4 origin symmetry activity total acoustical optical lattice librational internal v1 v2 v3 v, 10 10 3 1 6 2 2 8 8 3 2 3 1 1 7 7 2 2 3 1 1 11 11 4 1 6 2 2 8 8 3 2 3 1 1 13 12 5 1 6 2 2 12 11 4 1 6 2 2 9 8 3 2 3 1 1 78 75 27 12 36 12 12 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 The asymmetric stretching mode (v,) corresponds to the strongest IR band for the isolated V0:-ion, and is also Raman-active, although weak, at 780 ern-'.25 This mode is expected to give rise to five strong IR bands and six weak Raman peaks in our case. We must consequently assign the two very strong IR bands, both showing multiplicity, centred at 861 and 687 cm-', with components at 915, 740 and 610 em-' to the IR-active modes arising from v3. The weak Raman peaks at 897, 881 and 724 em-' should be assigned to three of the six expected weak Raman-active modes arising from the splitting of the asymmetric stretching mode, v3. To attempt an identification of these modes we recall that among the four oxygen atoms that are bonded to vanadium, only two are equivalent, according to the C, site symmetry. One of the two in-plane V-0 bonds is definitely longer than the others. So, we propose that the main cause of splitting of the asymmetric stretching mode is internal to the vanadate ion, due to the non-equivalence of the V-0 bonds.This allows us to explain why the splitting of v3 is much stronger than that of v,, which is only due to the crystal structure effect, i.e. to the copresence of four equivalent units in the Bravais cell, whose vibrations are coupled. As shown in Table 4, if one V-0 bond is longer than the others (C," symmetry) the triply degenerate v3 mode (F, in & point group) splits into two components (A, + E). If only two oxygens are equivalent, as in our case (C, site symmetry), the E mode further splits into A' + A", and the A, mode becomes A'.The last mode is expected to fall at frequencies even higher than the symmetric stretching mode. The A" modes arising from the E mode in C,, can be expected at lower frequency. Each A' mode under D,, point group produces four components [A,(R) + B,,(R) + B,,(IR) + B3,(1R)]. So, we assign the IR bands at 915 and 861 em-' to the B,,+ B,, modes and the Raman shoulders at 897 and 881 em-' to the A, + B,, modes, all arising from the high-frequency A' component of v,. The 'centre of gravity' of both pairs of bands is near 888 cm-l, which can be assigned to this com- ponent of the asymmetric stretching mode, modified by lattice effects. As a consequence, we can assign the Raman peak at 724 em-' and an extremely weak feature that can be envisaged near 680 em-' to the remaining four Raman-active modes arising from the asymmetric stretching of the shorter V-0 bonds, probably superimposed in pairs, although it is pos- sible that some of them are too weak to be detected, and lie at lower frequency.The IR bands at 740, 687 em-' are assigned to the corresponding IR-active components of the asymmetric stretching mode, a third one probably being superimposed on them and undetectable. The centre of gravity of these modes is near 720-700 cm-', which can be taken as the value of the asymmetric stretching mode under C,, symmetry, modified by crystal effects. A summary of our assignments for the stretching modes is given in Table 4. Even more complex would be the assignment of the many peaks observed in both IR and Raman spectra below 500 em-',to the deformation modes of vanadate ions and to the rotational and translational lattice modes.We will limit our- selves to the observation that we detect at least 15 Raman and 15 IR bands in the region 500-100 cm-', in contrast to the expected 28 Raman-active and 24 IR-active modes. This may be due to the superimposition of different components. The IR spectrum is apparently composed of a weakly split medium-intensity band at 485,473 ern-and of a very broad absorption centred near 400 em-',on which at least 13 com-ponents are superimposed. The former split absorption is assigned to a component arising from the v4 asymmetric deformation mode, also IR-active for the symmetric V0:- anion, while the latter broad adsorption agrees, because of its position and broadness, with the assignment to lattice modes mainly involving stretching modes of MgO, octahedra.,, From our tentative assignments, we can attempt some con- clusions.Comparison of the spectra we report here with those reported by other authors for the same ~ompo~nd~-~~~~~~~shows good agreement, but with non-negligible differences. For example, we do not detect at all of the weak IR component reported both by Hanuza et d6and by Baran and AymoninoZ7 at 962 cm-', which is likely to be due to an impurity arising from Mg2V,07 (see below). Simi- larly, the Raman spectrum of Mg3(V04), reported by Owen and Kung28 clearly shows additional peaks at 948 and 900 em-', also likely to be due to Mg,V,O, impurities.More- over, the interpretation of the spectra of Mg3(V04), we propose here shows some marked differences from the pre- vious based on an imperfect knowledge of the struc- ture. Comparison of our spectra with those reported for the compounds Ca,(VO,),, Sr,(VO,), and Ba,(V04),,2i27929*30 Table 4 Scheme for assignment of the dundamental stretching bands of orthorhombicMg3(VOJ2 (an-I) and their IR-active combinations site symmetry factor group 915 + 881 1790 IR 862 + 861 1720 IR 861 + 827 1672 IR 862 + 485 1347 IR 915 IR 897 R 881 R 862 R 861 IR 833, IR 827 R 740 IR 724 R 687 IR (690) R 1166 which are not isostructural with Mg3(V04), , clearly points to the effect of the crystal structure and symmetry on the vibrational spectra.This makes unreliable simple correlations on the position of bands with, e.g. catalytic activity, for non- isostructural compounds, such as those proposed in ref. 2 and 31. The Pyrovanadate, Mg,V,O, Mg pyrovanadate is polymorphic, showing at least three dif- ferent structures at ordinary pressure. 14*' According to Clark and Morley,', the form stable at room temperature is monoclinic (Fig. 3), belonging to the P2,/c = c;h space group, with a = 6.605 8,, b = 8.415 8, and c = 9.487 8, and /3 = 100.61", and with four molecules per unit cell. Although the structure of this form has not been refined, it is thought to be isostructural with Ni and Co divanadates, whose struc- tures have been studied in detail.20 Mg,V,O, transforms near 1000 K to a triclinic form, whose structure has been refined by Gopal and Ca1v0.l~ A further phase transform- ation occurs at even higher temperatures.14v1 , Triclinic magnesium pyrovanadate belongs to the Pi = Ci space group with a = 13.767 A, b = 5.414 8, and c = 4.912 A, a = 81.42", /3 = 100.61", y = 130.33", and with two molecular units per unit cell. The triclinic structure of Mg,V,O,, as well as that of several other bivalent divanadates, can be termed thortveitite-like, being closely related to that of the mineral thortveitite, Sc,Si,O, (C2/m = cih space group), because in both cases sheets of M,O, plyanions are present. However, the structure of triclinic Mg,V,O, differs in many respects from that of thortveitite owing to the geometry of the M-0-M bridge of the M,O, anions (linear in thortvei- tite and bent in Mg pyrovanadate) and because of the coordi- nation of the vanadium ions, which is approaching five-fold owing to weak interactions with the oxygens of the nearest pyrovanadate ions. The structure of monoclinic Mg,V,O, , assumed to be the same as that of monoclinic C0,V207, as proposed by Clark and Morley,17 is not thortveitite-like because the M207 entities do not form sheets.The unit cell of the monoclinic structure contains 44 atoms, and consequently, 129 optical modes are expected. The V,074-ions are bent, with a nearly eclipsed cis orienta-tion of the oxygens of the VO, tetrahedra.So, the ion sym- metry is near C,,.Factor group analysis for the unit cell of monoclinic Mg,V,07 leads to the following irreducible rep- resentation : rapt = 33Ag(R)+ 33Bg(R)+ 32A,(IR) + 31B,(IR) to which the acoustic modes A, + 2B, should be added. The unit cell of the triclinic structure contains 22 atoms, J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 and consequently, 63 optical modes are expected, which are either IR- (A,) or Raman-active (BJ. The V20,4-ions are bent, with a nearly staggered trans orientation of the oxygens of the VO, units. So, the ion symmetry is near C,. Factor group analysis for the unit cell of triclinic Mg,V,O, leads to the following irreducible representation: rap, = 33Ag(R)+ 30A,(IR) to which the acoustic modes 3A, should be added.Since the volume of the monoclinic unit cell is nearly double that of the triclinic unit cell (both primitive), the two structures could, in principle, be distinguishable by vibra- tional spectroscopies, because for the former structure the number of bands should be nearly double that for the latter. However, if the A,B coupling for the monoclinic compound is sufficiently small, only half of the bands appear because they are superimposed in pairs. So, the two structures are indistin- guishable by vibrational spctroscopy. This is probably what occurs for our monoclinic Mg,V,O, , where nearly 30 IR and Raman components, instead of 66 Raman and 63 IR, are observed. In Table 5 the assignments of the vibrational modes to lattice modes and internal vibrations of the pyrovanadate groups are also reported for both structures.These groups approach C,, symmetry in the monoclinic structure, for which the following irreducible representation is obtained : rc.vv207 = 7A,+ 4A2 + 6B1+ 4~, Among these 21 vibrational modes of isolated V,07 ions, 17 are IR-active (A,, B, and B, symmetry modes) and 14 are Raman-active (A,, B, and B,). Under site symmetry C,, all modes become A and all split into Ag(R) + B,(R) + A,(IR)+ BJIR) under the factor grup Ci. Of these 21 modes, six are terminal V-0 stretchings, 12 terminal bendings, two bridge stretchings and one bridge bending. Note that of the six terminal V-0 bonds, one each side is particularly short, while the other four are equivalent to each other and to the shorter three bonds of the orthovandate ion in Mg3(V04), (see Table 1).To attempt an assignment of the observed bands we can again divide the spectrum at 500 cm-'. Above this frequency we expect only V-0 stretching modes. Above this frequency we detect eight well resolved IR bands and seven well resolv- ed Raman peaks. It seems reasonable to assign the IR bands at 668 cm- and at 575 cm-',as well as the Raman bands at 620 and 570 cm- to the asymmetric and symmetric stretch- ing modes of the V-0-V bridge. This assignment agrees with that of Pedregosa et dJ2The higher-frequency com- ponents of these bands in both the IR and Raman spectra are clearly split (690, 668 cm-' in IR, 630, 620 cm-' in Raman) Table 5 Distribution and assignments for the fundamental modes of monoclinic and triclinic Mg,V,O, v20, terminal V-0 v-0-v symmetry activity total acoustical optical lattice librational internal stretch bend stretch bend monoclinic, S.G.P2Jc A, B,A, Bu R R IR IR 33 33 33 33 33 33 32 31 9 9 8 7 21 21 21 21 6 6 6 6 12 12 12 12 total 132 129 33 84 24 48 triclinic, S.G. Pi A,A, R IR 33 33 33 30 9 6 21 21 6 6 12 12 total 66 63 15 42 12 24 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 and this can be taken as evidence for the A,-B, and A,-B, splittings, respectively. The bands in the region 1000-700 cm-’ should be due to terminal V-0 stretching modes. In both the IR and Raman spectra we observe two sharp bands just above 900 cm-’, but their intensity ratio is inverted in the two spectra.More- over, this pair of bands is stronger than the bands in the region 900-700 cm-’ in the Raman spectrum, while the reverse is found for the IR spectrum. It seems reasonable to assign the strongest Raman mode (902 cm-’) to the totally symmetric stretch, which has the character of a symmetric stretch of the two shortest V-0 bonds. Consequently, the weaker mode at higher frequency (948 cm-’) can be assigned to the asymmetric stretching of the two shorter V-0 bonds. The same modes origmate the IR bands at 917 and 968 cm-’,respectively, whose intensity ratio is, accordingly, the inverse of that in the Raman case. The weak splitting of the band at 968 cm-’ can be further evidence for the weak A,-B, splitting. The remaining bands, located in the 930-700 cm-region, are assigned to stretching modes of the longer terminal V-0 bonds.Five components can be distinguished in the IR spec- trum, as expected assuming no A,-B, splitting, while only two are found in the Raman spectrum, perhaps owing to the very weak strength of some of them. The region below 500 cm-’ shows a split IR band at 462, 439 cm-’, similar to that observed at 485 and 473 cm-’ in the case of the orthovanadate, assigned to a deformation mode of the VO, entities. The components in the region 450- 100 cm- ’ are assigned to lattice modes and to deformations of the pyrovanadate ions. According to this interpretation of the spectrum in the V-0 stretching region, we have very weak A,-B, and A,-B, splittings, while A,A, and B,-B, splittings are sig- nificant (up to 40 cm-’), as evidenced by the separation of the IR and Raman modes.This might be due to the particu- lar association in centrosymmetric pairs of the pyrovanadate ions in the structure of monoclinic Mg2V20, (see Fig. 3”), whose vibrations are consequently strongly coupled. Note that the IR spectrum we report here for Mg,V,O, is consistent, although not entirely correspondent, with that dis- cussed by Pedregosa et aL3’ and with that reported by Pate1 et but both the IR and Raman spectra are definitely different from those reported by Hanuza et a/.,, which corre- spond to a mixture of phases. On the other hand, the Raman spectra reported by Stencel” for a-Mg,V,O, (monoclinic polymorph) and by Hardcastle and Wachs3’ for #?-Mg2V20, (triclinic polymorph) are instead due to mixtures of a-Mg,V,O, and MgV,O,.The IR spectrum we report here does not compare well with those of the other alkaline-earth- metal pyrovanadates” which, in fact, are not isostructural with monoclinic MgV,O, . The Metavanadate MgV,O, Monoclinic magnesium metavanadate (Fig. 5) is isomorphous with the brannerite mineral having the formula (Th,U)Ti,O, : both belong to the C2/m = Czh space group,2’ with two molecular units per crystallographic cell. The unit cell dimen- sions are a = 9.279 A,b = 3.502 A,c = 6.731 A,#? = 111.77”. This phase transforms near 535°C into a ‘pseudo-brannerite’-type form, and at high pressure into a ‘cou1ombite’-type form.The smallest Bravais cell of the bran- nerite structure contains one molecule only, i.e. nine atoms. Accordingly, 27 total modes are expected of which three are acoustical modes and 24 optical modes. The structure con- sists of both Mg2+ ions and V5+ ions in octahedral coordi- nation, and of three different types of oxygen atom, each tricoordinated. VO, octahedra are linked by three edges, forming infinite anionic [(V,0,)2 -1, layers parallel to the (001) face. Inside the layers, zig-zag chains of edge-shared VO, octahedra may be distinguished along the [OlO] direc-tion. Factor group analysis allows us to obtain the following irreducible representation for the optical modes of Mg meta- vanadate: rapt = 8A,(R) + 4B,(R) + 4A,(IR) + 8B,(IR) to which the acoustic modes (A, + 2B,) should be added.To divide the optical modes into lattice and ‘internal’ V-0 vibrations with known physical meaning, we must divide the vibrations associated with motions of Mg ions (A, + 2B,) from those associated with internal vibrations of the [(v206)”], polymeric layered molecular anion lying in the (020) plane of the unit cell (with Mg ions assumed to be at the corners). For this layer, 21 internal vibrations are expected, whose distribution among symmetry species, obtained by the difference spectra, is reported in Table 6. The three oxygen atoms of each VO, unit, two of which are present in the smallest Bravais cell, are of three different types.O(1) is bonded to a single V with a short V-0 bond (1.666 A),but also bridges two Mg ions; O(I1) bridges two V atoms very asymmetrically with one very short (1.671 A) and one very long (2.671 A)V-0 bond, but also coordinates one Mg ions; O(II1) triply bridges V atoms with bonds of inter- mediate length. To simplify the structure, we can neglect the sixth weakest coordination at the vanadium ions, so that the O(I1) atoms become terminally bonded to V atoms like O(I), and the zig-zag chains of edge-shared V06 octahedra along the [OlO] direction are separated from each other. The structure of one chain is reported in Fig. 5. In this view, the 24 optical modes of the structure are divided into 20 internal vibrations, one rotational mode along the b axis of the [v,06]z-chains and three lattice translational modes.The vibrations associated with the weak bonds we have artifically ‘broken’ become rotations of the chains. Each V ion is now pentacoordinated, with two short ter- minal V-0 bonds located in the symmetry plane of the unit cell, parallel to the (010) plane, and three longer V-OV, bonds with O(II1) atoms in a plane parallel to the [OlO] direction. We can now divide the vibrations associated with VO, units with short V-0 bonds from those associated with the motions of the O(II1) atoms in the (V,O,), layers. Our 24 total modes are now constituted by three lattice vibrations, one rotation of the V,O, chains, eight modes of the V,O, ‘network’ involving O(II1) forming a chain along the b axis, and finally, six out-of-plane deformations, two in-plane defor- mations and four stretchings of VO, short-bond units.The symmetry species and activity of these modes can be found in Table 6. The observed spectra of the Mg metavanadate are compa- ratively simpler than those discussed above for the ortho- and meta-vanadate, owing to the smaller size of the unit cell. The Raman spectrum shows 12 very well resolved bands, just as expected. The IR spectrum is much less well resolved, but also shows almost 12 components, as expected. However, we also observe in the IR spectrum weak components arising from Mg,V,O, impurities. Above 800 cm-’ we observe two Raman bands and two IR bands, which are assigned to the stretching modes of the VO, units.The strongest Raman mode at 923 cm- ’ certainly arises from the symmetric stretching mode of V02 ,while that at 836 cm-’ arises from the asymmetric mode. By analogy, the IR band at 888 cm- ’ is due to the IR-active component arising from the symmetric stretching mode, while that at 840 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 6 Assignments for the fundamental optical modes of monoclinic MgV'O, (VO,)+ a cv202Ia symmetry activity total acoustical optical Mg [V,06]'- (internal) stretch ip op int. rot. 4% R 8 0 8 0 8 2 1 1 4 0 B* A,B" R IR IR 4 5 10 0 1 2 4 4 8 0 1 2 4 3 6 0 0 2 0 0 1 2 2 1 1 1 2 1 0 0 total 27 3 24 3 21 4 2 6 8 1 The vibrations of the planar polymeric macroanion [v,06]'-have been divided into those of the bent (VO,)' units and those of the [V,O,] polymeric zig-zag chains.ip, in-plane deformations of V02 units (short bonds); op, out-of-plane deformations of VO, units (short bonds). cm-' is assigned to the IR-active mode arising from the asymmetric stretching. This agrees with the stronger intensity of the former band. We can recall that only one of the V-0 bonds is a true terminal bond (although the oxygen is coordi- nated to two Mg ions), the other being a very asymmetric V-0-V. An alternative, more rigorous assignment for the peaks at 923 cm-' (Raman) and at 888 cm-' (IR) is to the asymmetric and symmetric stretchings of the two true termin- al V-0 bonds, one per VO, unit. The IR and Raman peaks, almost coincident near 840 cm-',are consequently assigned to the stretching of the very asymmetric V-0-V bridging system, involving O(I1).In the region 800-500 cm-' we observe two peaks in the Raman spectrum at 731 and 523 cm-', and a complex IR absorption with the most intense bands at 655 and 552 cm- ', and probably two other components at 695 and 620 cm-'. These bands are assigned to the stretching motions of the V20, network with triply bridging O(II1) atoms. These assignments find confirmation from the assignments of the IR and Raman spectra of V205.34*35In total, six modes are observed, as expected. At lower frequencies, a band almost coincident in the IR and Raman spectra is observed (430 cm-' in IR, 440 cm-' in Raman).This band can be assigned to the two components of the in-plane deformation mode of the VO, unit. Note that the IR spectrum of the Mg metavanadate differs significatly from those of the orthovanadate and of the pyro- vanadate (as well as those of many Mg2+ compounds) because of the absence of a strong, broad absorption in the 450-350 cm-' region. This can be related to the observation, reported by Mocala and Ziolk~wski,~~ that in this compound Mg ions occupy more space than usual, probably because of the particular lack of elasticity of the layered macroanion. So, the MgO, octahedra are unusually expanded, and the Mg-0 bond order is unusually low.We found less well defined absorptions at lower frequencies than usual, as expected. The IR spectrum we report here does not compare well The spectrum of Mg3(V0,), is composed of a triplet at 1790, 1720 and 1697 cm-' and of a further strong band at 1347 cm-'. The triplet clearly corresponds to the com-bination modes of the bands arising from the splitting of v1 (symmetric stretching of the V0:- anion) as well as of the highest-frequency components of v3 (asymmetric stretching of VO:-), while the band at 1347 cm-' could be due to a com- bination of the Raman-active mode at 862 cm- ',arising from the symmetric stretching vl, which the IR mode at 485 cm- ', probably arising from the deformation mode v4. Note also an additional very weak, sharp band at 1964 cm- ',which is due to a surface vibration (see Part 2 of this series).37 The spectrum of the pyrovanadate shows a very character- istic sharp doublet at 1933, 1910 cm-'.These two com-ponents necessarily arise from the two crossed combinations of the two superimposed Raman-active and two weakly resolved IR-active modes arising from the stretchings of the shorter V-0 bonds. The theoretical values of these modes are 1923 and 1916 cm- '. The strong band in the intermediate region, composed of at least three bands, is due to com- binations of VO, terminal stretchings. The broad, weak band near 1430 cm-' is assigned to a combination involving a V-0-V asymmetric stretching and a VO, stretching. Finally, the two strong harmonics at 1210 and 1116 cm-' can be assigned to crossed combinations of the Raman- and IR-active V-0-V stretching modes.The overtone region of the spectrum of Mg metavanadate shows, besides weak components arising from Mg2V,0, impurities, five main bands. The low-frequency pair of bands looks similar to that already discussed for Mg,V,O, and also observed for V205, although at higher frequencies in this case (1276, 1200 cm-'). These bands, absent for the orthova- nadate, should arise from modes involving bridging oxygens. Also, the band near 1400 cm-' certainly involves VO, stretchings, while the band at 1780 cm-' is assigned to com- binations of terminal V-0 stretchings. The highest-frequency component, instead, cannot be assigned to a binary combination.The most reasonable assignment is to a ternary combination of IR-active modes, or of two Raman modes with those reported previously for the same c~mpound~?~~ and with those of other alkaline-earth-metal metavana-dates,29 while the Raman spectrum corresponds entirely with that reported by Sten~el.'~ IR Spectra in the 'First Overtone' Region The IR spectra of pressed discs of the pure Mg vanadate powders (and of V205 for comparison) in the region of the first overtones of the skeletal vibrations are reported in Fig. 7. Since all of the structures we are dealing with are centro- symmetric, all IR-active first harmonic bands should be assigned to binary combinations of one Raman-active (or inactive) mode and one IR-active (or inactive) mode.The positions of the observed harmonic bands and tentative assignments are reported in Table 2. plus one IR mode. In Fig. 7 the spectrum in the overtone region of V205 is also reported. The two strong bands at 1276 and 1200 cm- ' and the shoulder near 1350 cm-' can be assigned to com- binations of the fundamental mode~~~,~ arising from stretch- ings of the V-0-V and VO, entities at 820 cm- ' (IR), 703 cm-' (R), 600 cm-' (IR) and 528 cm-' (R) in the following manner: 703 cm-' (R) + 520 cm-' (IR) = 1123 cm-'; 703 cm-' (R)+600 cm-' (IR)= 1303 cm-'; 528 cm-' (R) + 820 cm-' (IR) = 1348 cm-'. At higher frequencies two strong bands are observed at 2020 and 1975 cm-'. In pre- vious publications by Busca et u1.38,39 a weak shoulder in the middle was envisaged, but is probably non-existent.This doublet, considered to be a triplet, was erroneously attributed to the summation and combination modes of two V-0 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 stretchings. In reality, four short V=O bonds are present in the Bravais cell of V,O, (Prnrnn = Dii space group, 2 = 2). Consequently, four V=O fundamental stretching modes are present, two of which are Raman-active (Ag, Blg, almost coincident near 994 cm-’ 34) and two IR-active (Bz,, 1035 cm-’, B3,, 995 cm-’). The IR spectrum in the overtone region is consequently constituted by four combinations, superimposed in pairs, expected near 2027 and at 1990 cm-’, both B,, + B3,, and just found at 2020 and 1975 cm-’.In conclusion, the IR spectra in the ‘first overtone’ region are characteristic of the single phases as well as of the struc- tural units they contain, and can be used to detect the state and the phase purity of the active catalyst phases during IR adsorption experiments using pure powder pressed discs, as already proposed.38 The observed harmonic bands can be assigned according to the assignments of the fundamental modes, and allow us to propose some correlations. Note that the stretching modes of short terminal V-0 bonds give rise to sharp strong combination modes, in the region 2050-1800 cm-’. Broader and multiple combination bands are found in the region 1900-1500 cm-’ arising from the different asym- metric and symmetric stretching modes of terminal VO, entities.Single and triple bridges, V-0-V and V30, give rise to strong and relatively sharp bands in the region 1500-1OOO cm-’. The strength of these combination modes is related to the covalency of the V-0 bonds. When only low- oxidation-state cations are involved in similar structures, these modes are very weak or even absent. Discussion The analyses of the structures of the stable compounds in the MgO-V,O, system show a progressive modification of the coordination sphere of vanadium. While the coordination at vanadium is an asymmetrically distorted octahedron with one very short V-0 bond and one very long bond in V’O,, in the case of magnesium metavanadate the structure is similar but with the shortest bond being longer (two nearly equivalent V-0 bonds) and the longest bond shorter (Table 1).In the triclinic magnesium pyrovanadate, vanadium is nearly pentacoordinated, with a fairly symmetric tetrahedron entertaining a fifth weak coordination. The situation is similar in the monoclinic magnesium pyrovanadate (assumed to be isostructural with monoclinic C0,V207) where the exis- tence of a fifth coordination at vanadium is also suspected.22 In the case of the magnesium orthovanadate the structure is definitely tetrahedral, which implies a further expansion of the shorter V-0 bonds. This confirms the existence of vanadylic nature in V,O,, of a VO, unit in MgV,O, (although one of the terminal oxygens actually bridges to another V ion), and the presence of four almost equivalent bonds in Mg3(V04), . In Mg,V,07 the ‘tetrahedra’ are more asymmetric.This situation looks similar to that observed in aqueous solution, where at low pH the VO; ion and polyoxovana- dates with nearly octahedrally coordinated vanadium are observed, which convert to isolated tetrahedral pyrovana-dates and later to orthovanadate ions with increasing PH.~’ Therefore, this progressive evolution observed in the solid state for the VMgO (MgO-V,O,) system can be attributed to the increasing basic character of the compound caused by increasing the nominal MgO content. The extension to the solid state of this conclusion valid for oxovanadium species in solution, provides a potentially useful concept when real catalysts and materials are taken into consideration.Accord- ingly, we note that vanadia compounds mixed with acidic components (as in the cases of V,O,-MOO, and V,O,-P,O, catalysts) have vanadium in a nearly octa-hedral vanadylic coordination, in contrast with Mg orthova- nadate where it is definitely tetrahedral. The same concept is useful when the surface species on vanadia catalysts sup- ported on metal oxides are considered. One can expect that the acid-base character of the metal oxide strongly influences the nature of the surface species in this sense. This concept agrees with the observation that basic dopants on V,O,-TiO, decrease the vanadylic character of the surface vanadium oxide species, with a decreasing V=O bond order4’ and a significant effect on the catalytic behaviour.The IR and Raman spectra presented and discussed here represent reference data which allow a better structural char- acterization of real catalysts, generally polyphasic, as well as of materials where other techniques (e.g. XRD) cannot give definite information, such as highly amorphous catalysts, MgO-V,O, glasses:’ and ‘monolayer’-type supported catalysts. In particular, comparison with the present data was useful for the identification of surface species observed on vanadia-titania catalysts.43 Laser Raman experiments were performed at Ecole Centrale de Lyon. We thank Dr. R. Olier for his kind assistance. Part of this work was supported by MURST (Rome, Italy). References 1 M.A. Chaar, D. Patel, M. C. Kung and H. H. Kung, J. Catal., 1987,105,483. 2 K. Seshan, H. M. Swaan, R. H. H. Smits, J. G. VanOmmen and J. R. H. Ross, in New Developments in Selective Oxidation, ed. G. Centi and F. Trifiro, Elsevier, Amsterdam, 1990, p. 505. 3 D. Siew Hew Sam, V. Soenen and J. C. Volta, J. Catal., 1990, 123,417. 4 D. Bhattacharyya, S. K. Bej and M. S. Rao, Appl. Catal. A, General, 1992,87,29. 5 R. Burch and E. M. Crabb, Appl. Catal. A, General, 1993, 100, 111. 6 J. Hanuza, B. Jezowska-Trzebiatowska and W. Oganowski, J. Mol. Catal., 1985,29, 109. 7 G. Busca, F. Cavani, G. Centi and F. Trifiro, J. Catal., 1986, 99, 400. 8 J. C. Volta, K. Bere, Y. J. Zhang and R. Olier, in Catalytic Selec- tiue Oxidation, ed. T. Oyama and J.Hightower, American Chemical Society, Washington, DC, 1993, pp. 217-230. 9 G. Busca, in ref. 8, pp. 168-182. 10 V,0,-Ti02 Eurocat standard catalyst, ed. G. C. Bond and J. C. Vedrine, Catal. Today, in the press. 11 P. M. Michalakos, M. C. Kung, I. Jahan and H. H. Kung, J. Catal., 1993, 140, 226. 12 G. Busca, G. Ramis and V. Lorenzelli, in New Deuelopments in Selective Oxidation, ed. V. Cortes Corberan and J. L. G. Fierro, Elsevier, Amsterdam, in the press. 13 R. Kohlmuller and J. Perraud, Bull. SOC. Chim. Fr., 1964,3, 645. 14 R. Wollast and A. Tazairt, Silicates Ind., 1969,34,42. 15 E. I. Speranskaya, Inorg. Muter., Engl. Trans., 1971,7, 1611. 16 R. C. Kerby and J. R. Wilson, Can. J. Chem., 1973,51, 1032. 17 G. M. Clark and R.Morley, J. Solid State Chem., 1976, 16,429. 18 N. Krishnamakhari and C. Calvo, Can. J. Chem., 1971,49,1630. 19 R. Gopal and C. Calvo, Acta Crystallogr., Sect. B, 1974, 30, 249 1. 20 E. E. Sauerbrei, R. Faggiani and C. Calvo, Acta Crystallogr., Sect. B,1974,30, 2907. 21 H. N. Ng and C. Calvo, Can. J. Chem., 1972,50,3619. 22 H. G. Bechman, F. R. Ahmed and W. H. Z. Barnes, 2. Kristal-logr., 1961, 115, 110. 23 W. G. Fateley, F. R. Dollish, N. T. McDevitt and F. F. Bentley, Infrared and Raman Selection Rules for Molecular and Lattice Vibration: The Correlation Method, Wiley, New York, 1972. 24 J. C. Decius and R. M. Hexter, Molecular Vibrations in Crystals, McGraw-Hill, New York, 1977. 25 J. M. Stencel, Raman Spectroscopy for Catalysis, Van Nostrand, New York, 1990. 26 P. Tarte, Spectrochim. Acta, 1962, 18,467. 27 E. J. Baran and P. J. Aymonino, 2. Anorg. Allg. Chem., 1969, 365, 211. 1170 28 0.S. Owen and H. H. Kung, J. Mol. Catal., 1993,79,265. 29 T. Dupuis and V. Lorenzelli, J. Therm.Anal., 1969, 1, 15. 30 P. Tarte and J. Thelen, Spectrochim. Acta, Part A, 1972, 28, 5. 31 F. D. Hardcastle and I. E. Wachs, J. Phys. Chem., 1991,%, 5031. 32 J. C. Pedregosa, E. J. Baran and P. J. Aymonino, 2. Anorg. Allg. Chem., 1974,404,308. 33 D. Patel, M. Kung and H.H. Kung, in Proc. 8th Int. Congr. Catal., Chemical Institute of Canada, Calgary, 1988, p. 1554. 34 T. R. Gilson, 0.F. Bizri and N. Cheetham, J. Chem. SOC., 1973, 291. 35 C. Sanchez, J. Livage and G. Lucazeau, J. Raman Spectrosc., 1982, 12, 68. 36 K. Mocala and S. Ziolkowski, J. Solid State Chem., 1987, 69, 299. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 37 G. Ramis, G. Busca and V. Lorenzelli, J. Chem. SOC., Farday Trans., 1994,!40, in the press. 38 G. Busca and J. C. Lavalley, Spectrochim. Acta, Part A, 1986,42, 443. 39 G. Busca, G. Ramis and V. Lorenzelli, J. Mol. Catal., 1989, 50, 231. 40 W. P. Griffth and P. J. B. Lesniak, J. Chem. SOC.A, 1969, 1066. 41 G. Ramis, G. Busca and F. Bregani, Catal. Lett., 1993, 18,299. 42 A. Tsuzuki, K. Kani, K. Watari and Y. Torii, J. Muter. Sci., 1993,28,4063. 43 L. Lietti, P. Forzatti, G. Ramis and G. Busca, Appl. Catal. B, Environmental, 1993,3, 13. Paper 3/06456E; Received 28th October, 1993