Heteropolyoxotungstates containing a wide catalytic target: reducibility and thermal stability

Materials Chemistry and Physics 87 (2004) 264–274

Heteropolyoxotungstates containing a wide catalytic target: reducibility and thermal stability Carmen I. Cabello a,∗,1 , Mar´ıa G. Egusquiza a , Irma L. Botto b , Giuliano Minelli c a

“Centro de Investigación y Desarrollo en Ciencias Aplicadas Dr. J. J. Ronco” CINDECA, CONICET- Facultad de Ciencias Exactas, Universidad Nacional de La Plata, calle 47 No. 257, (1900) La Plata, Buenos Aires, Argentina b Centro de Qu´ımica Inorgánica, CEQUINOR, CONICET—Facultad de Ciencias Exactas, Universidad Nacional de La Plata, calle 47 esq. 115, (1900) La Plata, Buenos Aires, Argentina c IMIP, CNR—Universidad “La Sapienza”, Pzz.le Aldo Moro 5, Roma, Italy Received 16 January 2004; received in revised form 16 January 2004; accepted 16 March 2004

Abstract The potassium salts of complex heteropolyoxotungstates K10 [(PW9 O34 )2 M4 (H2 O)2 ]·20H2 O containing tetranuclear clusters (MO6 )4 with M = Co, Zn and Mn were synthesized and characterized by means of X-ray Powder Diffraction Analysis (XRD), Vibrational Spectroscopy (FTIR and Raman), Diffuse Reflectance Spectroscopy (DRS) and Scanning Electron Microscopy (SEM and EDAX). The thermal decomposition of these species in air atmosphere (DTA-TGA) and in reducing conditions (H2 -N2 ) by Temperature Programmed Reduction (TPR) was analyzed and discussed on the basis of the characterization of intermediate and pyrolysis products. Additional thermal treatments were made by means of X-ray Powder Diffraction “in situ” technique. Besides, as the FTIR and Raman vibrational spectra of pure samples are poorly described in literature, the reassignment was included. Thermal products depend on geometrical factors of the heteropolyoxotungstate arrangement, according to the affinity between neighbouring atoms: terminal W/countercations, PO4 /WO6 internal groups of lacunary fragments and interaction between M cations of tetranuclear cluster/(PW9 O34 )9− lacunary groups. The reducibility of W(VI) strongly depends on the metal type of cluster. A decrease of ∼200 ◦ C for the W(VI)-Wo reduction temperature of the Co-phase respect to that of Mn-phase is observed by the comparative TPR study. The presence of bronze-like structures can be suggested in both oxidant and reducing atmospheres. On the other hand, PO4 species in low symmetry, associated to vitreous W–P–O phases, can be inferred for the TPR intermediate phases by means of FTIR spectroscopy. © 2004 Elsevier B.V. All rights reserved. Keywords: Heteropolyoxotungstates; Metal cluster; Temperature programmed reduction; Bronze-like structures

1. Introduction In the search of new multimetallic catalytic precursors, polyoxometalates (POMs) and non-conventional heteropolyoxometalates (HPOMs) have been studied due to the great interest of these species as bulk or supported materials in either heterogeneous or homogeneous catalysis fields [1–5]. In this sense the ability to produce metal clusters with specific sizes (1–100 nm) and compositions would mimic the control of homogeneous catalysts but may provide reactivity and separation advantages of heterogeneous catalysts. Al∗ Corresponding author. Tel.: +54-221-4220288; fax: +54-221-4220288. E-mail address: [email protected] (C.I. Cabello). 1 To whom proofs should be mailed. Member of the research staff of CICPBA Argentina.

0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.04.015

though the presence of clusters have been shown to possess interesting catalytic properties, much more work in this area needs to be done. The development of the subject has led to the preparation of complex phases that arise from the condensation of simple HPOMs [6–9]. In this context, complex heteropolyoxoanions [(PW9 O34 )2 M4 (H2 O)2 ]10− , where M = Co, Zn and Mn divalent cations, can structurally derive from the union of two fragments with [PW9 O34 ]9− composition. This union occurs by means of the interpolation of coplanar tetranuclear clusters of transition metallic ions, giving rise to a “sandwich-type” structure. Such heteropolyoxoanions linked to alkaline metallic ions such as Na+ , K+ or NH4 + generate highly symmetric structures. These inorganic salts show a good solubility and stability and constitute attractive multimetallic systems of potential catalytic interest [10–12].

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Recent works show that the preparation of heterogeneous catalysts from the impregnation of a given support with solutions of Anderson or Keggin type heteropolyanions, allowed to obtain surfaces with a uniform and ordered distribution of active sites [13–15]. On the other hand, different types of interactions between the complex arrangement with the surface support can promote the catalytic activity by means of two facts: the synergetic effect of the multimetallic system as well as the formation of more active intermediates by mild thermal conditions. In fact, recent reports concerning the thermal and structural stability of the Wells–Dawson-type heteropoly compound K6 P2 W18 O62 ·10H2 O, have demonstrated that one of its thermolysis stage involves a microstructural rearrangement with the formation of a mix of Keggin-type compound “K3 PW12 O40 ” and small amounts of an ill-defined phase which is a better isobutane oxidation catalyst than the precursor Wells–Dawson compound and than pure Keggin polyoxoanion [16]. The aim of this work is centred on the thermal behaviour of “sandwich type” isomorphous heteropolyoxometalates, which is analyzed by means of different thermal techniques in both oxidant (TGA-DTA) and reducing atmosphere (TPR or temperature programmed reduction) with the help of X-ray Powder Diffraction Analysis (XRD) including “in situ” thermal measurements, Scanning Electron Microscopy (SEM-EDAX) as well as vibrational (FTIR and Raman) and Diffuse Reflectance (DRS) spectroscopies.

equipment. “In situ” XRD measurements were carried out by using a high temperature and gas flow, Anton PAAR (HTK) 10 with built-in temperature programmer. Cu K␣ radiation (Ni filtered) was used and the register was carried out with a scanning angle between 5◦ and 60◦ at a rate of 2θ = 1◦ per minute.

2. Experimental

The pure phases were studied before and after thermal treatment in a Philips 505 microscope equipped with EDAX 9100 microprobe (for the electron probe microanalysis).

2.1. Synthesis The K10 [(PW9 O34 )2 M4 (H2 O)2 ]·20H2 O syntheses were carried out by reaction in aqueous solution from the Na8 H[PW9 O34 ]·19H2 O lacunary precursor (obtained by reaction of sodium tungstate and the corresponding phosphoric acid) and soluble salt of divalent metal (M = Co, Zn, Mn, as chlorides) in stoichiometric amounts and KCl excess. The obtained polycrystalline solid was separated by filtration and purified by recrystallization, according to the following reaction [8]: Na8 H[PW9 O34 ] · 19H2 O KCl excess 4MCl2

−−−−−−−−−→ K10 [PW9 M2 (H2 O)O34 ]2 · 20H2 O + KClaq + NaClaq

2.3. Diffuse reflectance spectroscopy (DRS) Spectra were recorded with UV–vis Super Scan 3 spectrophotometer with double beam, using BaSO4 as internal standard, in the 200–800 nm range and built-in recorder of programmable scanning. 2.4. Infrared spectroscopy (FTIR) The Fourier transformed infrared spectra were obtained by using a Bruker IFSS 66 FT-IR equipment (KBr pellet technique). 2.5. Raman spectroscopy It was carried out with a 1403 Spex-Ramalog spectrometer, double monochromator equipped with a SCAMP data processor (excitation line: 514.5 nm of an Ar-ion laser) and rotary sample holder. 2.6. Scanning electron microscopy (SEM)

2.7. Thermal decomposition in oxidant atmosphere It was carried out by means of differential thermal analysis (DTA) and thermo-gravimetric analysis (TGA) in air atmosphere with a 50 Shimadzu thermobalance. Oxidant thermal treatments were also carried out in air atmosphere in a controlled temperature electric furnace. 2.8. Temperature programmed reduction (TPR) The treatment was carried out in a Chembet 3000 equipment with TPR accessories attached. The reactor feed was 10% H2 reducing agent in N2 from 20 to 1000 ◦ C. The heating rate was 5 ◦ C min−1 .

where  means the mixture of A and B (PW9 O34 )9− isomers called “Thermalized (PW9 O34 )9− ”.

3. Results and discussion

2.2. X-ray diffraction

3.1. Structural and crystallographic properties

The X-ray diffraction diagrams obtained for both pure and treated samples were registered in a Philips-PW 1729

The heteropolyoxoanion [(PW9 O34 )2 M4 (H2 O)2 ]10− has C2h symmetry, deriving from the condensation of two

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Fig. 1. [(PW9 O34 )2 M4 (H2 O)2 ]10− structure.

[PW9 O34 ]9− units (obtained from removing triads of WO6 polyhedra of the (PW12 O40 )3− Keggin structure (tetrahedral symmetry) [6]. In this compound, two (PW9 O34 )9− truncated groups are joined to the divalent metal cluster. This is constituted by a tetranuclear arrangement of octahedra joined by edges and connected to (PW9 O34 )9− anionic unities through the corners. There are also two water molecules of constitution linked to the corners of both MO6 octahedra in opposite positions according to what can be observed in Fig. 1. XRD and FTIR techniques reveal that the phases with M = Co, Zn are isostructural to that of Mn (PDF 81-2062), also in agreement with literature [8]. Fig. 2 shows typical morphology of K10 [(PW9 O34 )2 M4 (H2 O)2 ]·20H2 O crystals by SEM Microscopy. EDAX results lead to a M/W (weight %) ∼ 0.07 in agreement with chemical analysis in aqueous solution and with theoretical values.

Fig. 3. Comparative FTIR spectra of (a) precursor (b) Zn phase (c) Mn phase (d) Co phase.

3.2. Spectroscopic properties 3.2.1. Vibrational spectroscopy The complex anionic structure of Fig. 1 reveals the existence of structural units more simple than the ones resulting in different kinds of links: (a) P–O (b) WOt (c) (W–O–W)A (d) (W–O–W)B (e) M–O–W. . . W–O–M. . . (f) M–O–M

Fig. 2. Micrograph of K10 [(PW9 O34 )2 Co4 (H2 O)2 ]·20H2 O, magnification 240×, scale bar 100 ␮m.

From PO4 tetrahedral group terminal bonds of WO6 . Bridge links resulting from the bond of corner-shared octahedra Bridge links resulting from the bond of edge-shared octahedra Bridges resulting from the bond between WO6 MO6 of clusters through corners From the MO6 edge-shared polyhedral (cluster)

Fig. 3 shows comparatively the FTIR spectra of the Co, Mn and Zn phases, including that of Na8 H[PW9 O34 ]·19H2 O for comparative purposes. Fig. 4 shows, in detail, the (FTIR and Raman) vibration spectrum of the precursor. While the PO4 groups in the structure of the complex heteropolyoxometalate present a unique band of P–O antisymmetric stretching (around 1030 cm−1 ) [17,18], the lacunary precursor shows a clear splitting of this mode. The differences are attributed to the decrease of Td local symmetry of PO4 group in the lacunary arrangement, due to the removal of a triad of corner-sharing WO6 groups. However, PO4 tetrahedral symmetry is restored from the condensation of (PW9 O34 )9− decapped structural units during the generation of the complex “sandwich” heteropolyanion. The de-

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Table 2 Raman spectrum of (a) [PW9 O34 ]8− and (b) [(PW9 O34 )2 Co4 (H2 O)2 ]10− heteropolyanions (a) Raman frequency (cm−1 )

(b) Raman frequency (cm−1 )

Assignment

1161 vw 1017 sh

1052 m 1052 m

νas P–O νs P–O

966 928 898 851 655 599

vs sh m w vw vw

571 vw 495 w

vs sh m w w w

νs W–Ot νas W–Ot νas W–O–WA νas W–O–WB ␯s W-O-W

523 w 429 w

δWOt W–O–M–O–

974 954 887 812 594 594

sh: shoulder; m: medium; s: strong; w: weak; vw: very weak.

Fig. 4. Vibrational FTIR and Raman spectra of the precursor Na8 H[PW9 O34 ]·19H2 O.

crease of PO4 symmetry in the lacunary precursor leads to the presence of very short P–O bonds (band in 1160 cm−1 ). Besides, the WO6 octahedral symmetry decreases sharply, presenting W–O bonds of different characteristics: the shortest W–O bond (992 cm−1 ), associated to the fragment of the anion that has undergone the octahedra removal, while the weakest W–O bond (d) is located at 818–746 cm−1 . The comparison of spectra belonging to the studied phases and the precursor, given in Table 1, shows that differences occur in the more sensible region of the heteropolyanion: the

W–O terminal bonds. Whereas a series of bands appear in the precursor spectrum (992–911 cm−1 ), only a broad band centred at ∼940 cm−1 is observed in spectra of condensed phases, in agreement with a low distortion of constitutive polyhedra. Below 900 cm−1 the bridged W–O–W bonds do not show appreciable differences, corroborating the stability of condensed WO6 in the Keggin derivative structure. On the other hand, the W–O–M bond would be associated to the bands situated in the lower energy region of spectra (approximately 500 cm−1 ) even though the assignment is difficult from this region due to the overlapping of M–O stretching bands with the angular deformations of vibrational modes depending on (c), (d) and (e) bonds. Table 2 shows the comparative assignment of the Raman bands for both the precursor and the typical Co-phase. In the former, it is interesting to observe that the most intense and sharp Raman line is located at 966 cm−1 whereas a series of thin lines of lower intensity are observed at lower values (928, 898 and 851 cm−1 ). The absence of strong Raman signals above 1000 cm−1 means that the P–O symmetric stretching can be related to an inactive Raman mode, as it is observed in some complex W–P oxides [19]. Likewise, the reinforcement and weakening of W–O bonds from the lowering of general symmetry lead to an overlapping of

Table 1 FTIR bands for K10 [(PW9 O34 )2 M4 (H2 O)2 ]·20H2 O (M = Mn, Co, Zn) and the Na8 H[PW9 O34 ]·19H2 O precursor species Precursor FTIR ν (cm−1 )

Co-phase FTIR ν (cm−1 )

Zn-phase FTIR ν (cm−1 )

Mn-phase FTIR ν (cm−1 )

Assignment

1160 w 1058 s 1015 m

1038 s

1032 s

1029 s

νP–O

m sh s sh

975 sh 944 s

978 sh 935 s

975 sh 938 s

νW–Ot

886 s 818 sh 746 s

881 s 829 sh 775 s

886 s 832 sh 741 s

872 s 835 sh 786 s

νW–O–Wa νW–O–Wb

992 972 938 911

sh: shoulder; m: medium; s: strong; w: weak; vw: very weak.

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electronic spectra of the d5 high spin Mn(II) is difficult to assign due to the number and low intensity of the bands. The spectrum of Co-phase (having Co(II) d7 ) presents a wide band centred in ∼560 nm typical of these species in an oxygen octahedral coordination. The band can be assigned to the third spin-allowed d–d transitions from the ground state: 4 T1g, to the 4 T2g, 4 A2g and 4 T1g(P) states, which generally fall within the ranges 1700–1000, 830–550 and 625–450 nm, respectively. The second transition is usually very weak and rarely unequivocally observed [20]. The band in ∼450 nm observed in the DRS of Mn-phase can be attributed to an overlapping of several d–d weak transitions as usually observed in Mn(II) compounds in octahedral symmetry [20]. DRS spectra of the precursor and that of species having Zn are used with comparative purposes. 3.4. Thermogravimetric (TG) and differential thermal analysis (DTA)

Fig. 5. Comparative DRS spectra of (a) precursor, (b) Zn phase, (c) Co phase, (d) Mn phase.

W–O and P–O Raman-active symmetric and antisymmetric (activated) modes, with the intense line at ∼966 cm−1 . The resolution is very difficult by the low P/W ratio. Remaining lines below 900 cm−1 can be attributed to the W–O–W symmetric bonds as well as the activation of corresponding antisymmetric modes by structural feature. Raman spectra of condensed heteropolyoxometalates seems to be more simple due to the increase of the general symmetry: a unique line of moderate intensity for the P–O symmetric stretching at ∼1050 cm−1 and the strongest and sharp line of W–Ot symmetric stretching mode at ∼970 cm−1 . The lines of intermediate intensity at 890 and 820 cm−1 can be assigned to the (W–O–W)A and (W–O–W)B vibrations, less affected by condensation effect. 3.3. Diffuse reflectance spectroscopy (between 200 and 800 nm) Fig. 5 shows DRS spectra of studied phases including that of the precursor as reference. Usually all the heteropolyoxometalate species show a common profile in the zone between 200 and 350 nm assigned to the TC O2− → M (mainly M = W) in an octahedral coordination. Bands of d–d electronic transitions are only observed for the Co(II) and Mn(II) octahedral species from 350 nm, although the

Thermal measurements of the studied phases indicate that the dehydration process starts at low temperature, about 50 ◦ C, and finishes at 250 ◦ C, without observing mass changes at subsequent temperatures. This thermal behaviour is typical for all studied phases, including the precursor and Fig. 6 shows its TG–DTA plot. The exothermic signal at 432 ◦ C is related to the crystallization of new and stable phases from the destruction and rearrangement of the original structure. So, the presence of Na2 WO4 and hexagonal WO3 as majority crystalline phases is clearly detected by XRD (PDF 74-2369 and 85-2459). The Na2 WO4 formation is also corroborated by the intense DTA endothermic signal of melting at 707 ◦ C. However, according to the original stoichiometry, the formation of another minority phases is also expected. In this sense, the presence of oxo-compounds generated by WO6 and PO4 condensation can be only suggested by FTIR spectroscopy. The W–P-oxides usually present a short range ordering behaviour and are difficult to be identified by XRD [21]. The decomposition process seems to be governed by geometrical factors, according to the affinity between neighbouring atoms: external WO6 with the counter-cations and PO4 groups with internal WO6 units. The comparative DTA diagrams for Co, Zn and Mn complex phases is shown in Table 3. The behaviour at low temperature is comparable for all phases, including the precursor. The Co-phase seems to be slightly less stable of all M phases. First endothermic signals can be attributed to the thermal dehydration process, while the exothermic signal in the mid zone (400–500 ◦ C) corresponds to the crystallization of new oxide phases by the structure breakage. Likewise, the endothermic signals above 600 ◦ C can be associated to the crystal-vitreous transformation of the minority phase containing phosphorous (process without mass-loss, according to TG data).

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Fig. 6. Comparative TGA and DTA patterns of the precursor.

The “in situ” XRD diagrams of the Co-, Mn-, and Zn-phases up to 800 ◦ C show a diminution of the resolution in the range of 80–250 ◦ C, in agreement with the dehydration process which does not imply changes of the primary structure of the heteropolyoxoanion. Fig. 7 shows the comparative patterns for the Co-phase. The samples are not crystalline between 200 and 400 ◦ C, which reveals a re-arrangement process, effect that is observed for all the studied species. A mixture of crystalline phases constituted by the hexagonal WO3 , potassium tungstates K2 W4 O13 and K2 WO4 (PDF 84-2126 and 74-0545) and the CoWO4 wolframite-type (PDF 72-0479) are observed at 600 ◦ C, although their incipient crystallization is observed from 550 ◦ C. The process follows a similar reaction scheme to that observed for the precursor. However, the structural

stability induced by the clusters and the competitiveness among the metal–W, W–P and W–K reactions determines the structure and energetic state of the products. The typical diffraction lines of bronze-like phases such as K0.33 W0.94 O3 (PDF 81-0005) [22,23] are observed as majority phases from 700 ◦ C. The resolution is lowered at this temperature due to the presence of vitreous phases. It is well known that bronzes are generally obtained by decomposing of potassium peroxo-polytungstate either in air or reducing conditions. The thermal treatment in air yielded pale yellow hexagonal K-tungstate (Kx WO(3+x/2) where x = 0.30) which corresponds to a fully oxidized state at the tempera-

Table 3 DTA signals for the precursor Na8 H[PW9 O34 ]·19H2 O and the K10 [(PW9 O34 )2 M4 (H2 O)2 ]·20H2 O (M = Co, Mn, Zn) Phase

T (◦ C) of exothermic signal

T (◦ C) of endothermic signal

Precursor

432

45 132 183 707

Co-phase

450 499 450 499

84 122 700 915

Mn-phase

490

87 704 923

Zn-phase

456

69 114 715 910

Fig. 7. XRD “in situ” pattern for Co phase thermally treated in air at different temperatures.

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Fig. 8. Comparative FTIR spectra of Zn phase thermally treated in air atmosphere (a) RT (b) 300 ◦ C (c) 550 ◦ C (d) 660 ◦ C, (e) 750 ◦ C.

ture range of 400–800 ◦ C [24]. On the other hand, although the stability field for wolframite-structure is adequate for the Mn, Zn and Co divalent species, Co(II) presents a slight difference adopting a derivative pseudo orthorhombic wolframite [25]. Fig. 8 shows comparatively the FTIR spectra of the pure Zn-phase and the thermally treated samples from 300 to 750 ◦ C. The similarity between FTIR spectra of the original and 300 ◦ C treated sample agrees with that previously suggested referred to the maintenance of the primary structure of the heteropolyoxometalate. The FTIR spectrum of sample heated at 550 ◦ C shows some differences: the shifting of the higher energy bands (1150 and ∼1050 cm−1 ) which can be related to the shortening of the P–O bonds in the new arrangement and the progressive resolution loss in the 900–500 cm−1 . The behaviour is closely related to that observed in W–P–O condensed glasses which usually produces a strongest absorption consisting of an imperfectly resolved doublet that lies in the same region [26]. The very broad band from ∼900 to 550 cm−1 , as increasing the temperature, includes the W–O stretching vibrations of wolframite-like compounds (800–870 cm−1 ) [27], the W–O modes of the tetrahedral group of potassium tungstates (∼800 cm−1 ), the W–O modes of hexagonal WO3 as well as bronze like structures (∼950 cm−1 ) [24]. The definition is highly lowered as energy decreases because of the overlapping of W and P deformation modes, the M–O stretching modes, coupling effects and the vitreous nature of the sample [21]. Finally, Fig. 9 shows the morphological changes that can be observed during the heating of the Zn-phase. Whereas only incipient changes are observed at 200 ◦ C by water loss, a mixture of products is evident at 660 ◦ C (Fig. 9(b)). The

Fig. 9. Micrographs of Zn phase in the typical air atmosphere heating sequence at (a) 200 ◦ C, magnification 500×, scale bar = 100 ␮m ; (b) 660 ◦ C, magnification 300×, scale bar = 100 ␮m, (c) 700 ◦ C magnification 1000×, scale bar = 10 ␮m.

magnification observed in Fig. 9(c) allows to observe with more detail the crystal morphology of majority phase at 700 ◦ C. K2 WO4 melts at ∼930 ◦ C. All results permit us to formulate the following decomposition scheme for the potassium salt of [(PW9 O34 )2 M4 -

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(H2 O)2 ]10− heteropolyoxoanions after dehydration (T > 400 ◦ C):

3.5. Thermal reduction behaviour As it is well known, the TPR technique provides valuable information about the reducibility of individual species, making it a useful tool to study catalytic materials [28]. Fig. 10 shows comparatively the TPR diagrams of the studied phases using the precursor as reference. The dia-

Fig. 10. Comparative TPR diagrams for (a) precursor; (b) Co phase; (c) Zn phase and (d) Mn phase, up to 1000 ◦ C.

271

grams are typical of W(VI) species with an intense band in the 800–950 ◦ C zone [27]. However, it is evident that the divalent metal cluster plays an important role in the W stability in reducing conditions, affecting the way of thermal reduction. Below ∼700 ◦ C some signals of low intensity can be associated to the more reducible divalent metals (EhMn(II)/Mn = −1.029 V, EhZn(II)/Zn = −0.76 V and EhCo(II)/Co = −0.28 V). In absence of divalent metal (as the precursor) the reducibility is associated to the oxygen availability of lacunary framework. In fact its structural arrangement ensures a fast W(VI) reduction which occurs through the formation of W(VI)–O–P → W(V)–O–P and “Na2 O·WO3 ” → Nax WO3 (PDF: 75-0458) bronze-like structures. It is interesting to remark that the thermal treatment without reducing atmosphere also presents the formation of some W(V) reduced species (but at higher temperature). Hence, the net TPR signal at 680 ◦ C for the precursor can be attributed to these processes. On the other hand, divalent metals promote the incipient reduction of W(VI) in octahedral positions, leading to a rearrangement of such polyhedra having W(V) and to the formation of non-stoichiometric oxides, structurally related to the Kx WO3 bronzes. This result can be verified by means of XRD analysis of the TPR samples at the peak temperatures in the 500–700 ◦ C range. Fig. 11 shows the comparative XRD patterns for the Co, Zn and Mn phases including that of precursor, after TPR treatments at 700 ◦ C. The sodium bronze-like phase (PDF 75-0458) is the unique intermediate product identified in the precursor. Likewise, in sandwich phases the formation of Kx WO3 bronzes is detected (such as PDF 83-1332 for the Co and Mn and 71-1474 for the Zn phases). The P–W–O phases are not clearly observed due to their short range ordering [24]. However, the presence of the crystalline WOPO4 (PDF 44-0349) could be suggested, although its proportion in the mixture of intermediate products is very low. It is interesting to remark that the low reducibility of Mn(II) and Zn(II) respect to Co(II) left a part of W(VI) unreduced in the Mn- and Zn-phases, by stabilizing wolframite-type structures (PDF 80-0152 and 88-0251, respectively). Contrarily, the CoWO4 is not observed because Co(II) in oxidic matrix reduces at ∼500 ◦ C [28]. Although the temperature of heteropolyoxometalate decomposition is lower than that at which the involved metals can be reduced, it is expected that the structure collapse occurs in a similar way in both oxidant and reducing atmospheres. Nevertheless, the dynamic reduction hinders the stability of some phases observed in air atmosphere such as potassium tungstates and CoWO4 , while another phases such as MWO4 (M = Mn, Zn) are not altered, but no distinct reduction signals of W(VI) are observed up to 500 ◦ C. So, the great part of segregated W(VI)oxide , interacting with an alkaline-rich matrix, is involved in the formation of bronze-like structures. Their reducibility is affected by the M(II) oxidant character, according to the order Co > Zn >

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ion involved in the bronze formation are reduced in the 800–1000 ◦ C range. But the reduction temperature in each case depends on the presence of free metal (even as traces or highly dispersed to be observed by XRD) which provides a higher concentration of H active atoms improving the reduction of intermediates to give metal W. Thus, the reduction pattern of W(VI) is affected by the promoting effect of active metal. The satellite signal at 680 ◦ C in the precursor and at 780 and 850 ◦ C, respectively, in the Zn and Mn phases can be only related to a W(VI) incipient reduction, which is associated to the bronze-like as well as WO6 -PO4 vitreous phases. These last ones can be only characterized by FTIR spectroscopy. However, the related WOPO4 crystalline compound can be suggested by XRD, according to the crystallinity of the samples. As the Co(II) cluster → Coo process occurs below 600 ◦ C, all the W(VI) reduction processes are included in the unique strong signal at 800 ◦ C. On the other hand, the comparison with the precursor behaviour reveals that the moderate reduction of this phase at 680 ◦ C is associated to the P–W–O and Na–W–O systems containing W(V). This fact can be supported on the basis that low energy is needed to reduce a lacunary framework. The characterization of intermediate and final products is carried out by FTIR spectroscopy. Fig. 12 shows

Fig. 11. Comparative XRD patterns for the samples treated by TPR at 700 ◦ C: precursor, (b) Co phase, (c) Zn phase and (d) Mn phase. Symbols: (*) NaWO3 (PDF: 75-0458) ; () WOPO4 (PDF: 44-0349) ; (␾) K0.32 WO3 (PDF: 83-1332); (+) K0.37 WO3 (PDF: 71-1474); (␻) ZnWO4 (PDF: 88-0251) and MnWO4 (PDF: 80-0152).

Mn. Hence, the temperature of the stronger reduction signal decreases close to 200 ◦ C for the phase containing the most reducible divalent species (Co) respect to that unreducible Mn. Likewise, as original stoichiometry is similar in all cases, the integrated area of the stronger TPR reduction signals to give metal W is practically comparable. It is well known that wolframite-like structures (MnWO4 and ZnWO4 ) are unreducible phases whereas pure K2 WO4 is reduced at 952 ◦ C in similar experimental conditions, as we have reported in a previous work [26]. So, the W(VI) ions of heteropolyan-

Fig. 12. FTIR spectra (a) Mn pure phase, (b) Mn phase after TPR 600 ◦ C, (c) Mn phase after TPR 865 ◦ C, (d) Mn phase after TPR 1000 ◦ C and (e) precursor after TPR 700 ◦ C.

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the spectra obtained for both pure Mn species and for their TPR-intermediates from 600 to 1000 ◦ C. In addition, Fig. 12(e) shows the spectrum of precursor after TPR at 700 ◦ C for comparative purposes. Between 600 and 900 ◦ C, the presence of different phases containing W permits only to corroborate the presence of PO4 groups in an environment of different symmetry (broad band centred at ∼1130 cm−1 ) [17–19,27]. However, the resolution increases at 1000 ◦ C, where the bronze is quasi totally reduced. Thus, the FTIR spectra is the result of overlapping bands of W(V)–P–O phase and MnWO4 [19,26]. As it was stated for the thermal behaviour in air atmosphere, it is possible to give the following reduction scheme:

The products belonging to the group (I) are reduced to metal W in the range of temperature between 800–1000 ◦ C, depending on the reducibility of divalent metal. On the other hand, the absence of free metal (observed in the precursor and in the Mn-phase) leads to the strong signal of W reduction at the highest temperature, close 1000 ◦ C.

4. Conclusions The comparison of spectroscopic, thermal and structural behaviour of the K10 [(PW9 O34 )2 M4 (H2 O)2 ]·20H2 O (M = Co, Zn, Mn) heteropolytungstates allows to analyze their structural stability in reducing and oxidant atmospheres and establish the effect of the metal cluster on the (PW9 O34 ) condensed units. The primary structure of the heteropolytungstates keeps its identity up to ∼300 ◦ C and the decomposition seems to be governed by geometrical factors, according to the interaction and affinity between neighbouring polyhedra. So, the collapse of the complex anion occurs by segregating three kinds of structural groups: the one associated to the divalent metal octahedra interacting with WO6 groups, the second one from the interaction between the PO4 tetrahedral group

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and the close WO6 polyhedra (from PW9 O34 decapped Keggin units) and the third type is related to the interaction between the external WO6 polyhedra and the K+ countercation. The decomposition scheme is valid in both oxidant and reducing atmospheres, although the products can be different, according to the stability in each case. The thermal behaviour in air atmosphere leads to the potassium tungstate (K2 WO4 and K2 W4 O13 ) and hexagonal WO3 as majority phases, whereas the formation of wolframite type structures MWO4 (M = Mn, Zn, Co) and the W–P–O short range ordering phases (melting at ∼700 ◦ C) are also observed from 500 ◦ C. The formation of Kx WO3 bronzes can be also suggested as temperature increases (∼800 ◦ C). The thermal behaviour in reducing atmosphere shows that the metal cluster affects the reducibility of W(VI) according to the order Co > Zn > Mn. So, whereas the Kx WO3 bronze-like structures are observed from 600–700 ◦ C, the W(VI) associated to Mn(II) and Zn(II) as wolframite-type structures remains unreduced up to 1000 ◦ C. Comparatively, the temperature of W(VI) reduction decreases ∼200 ◦ C as reducibility of divalent M cation increases. The structural stability induced by the metal cluster increases according to Co < Zn < Mn in both reducing and oxidant atmospheres. In relation to the thermal behaviour of the Na8 H[PW9 O34 ]·19H2 O lacunary precursor, its stability is related to its structural arrangement. Although the Na2 WO4 is the majority product by thermal treatment in air, the Nax WO3 bronze-like phase is the majority product in the TPR conditions at 700 ◦ C. The low stability in reducing conditions can be attributed to the lacunary structure, low symmetry and high availability of oxygen atoms for the reducing agent.

Acknowledgements Authors are grateful to Lic. Norberto Firpo, Mrs. Graciela Valle, Lic. Diego Peña, Ing. Claudio Garc´ıa and Mr. Néstor Bernava for their technical assistance. This work had the economic support of ANPCyT, of Consejo Nacional de Investigaciones Cient´ıficas y Técnicas (CONICET) and of Comisión de Investigaciones Cient´ıficas de la Provincia de Buenos Aires (CICPBA), Argentina.

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