Redox intercalation of alkali metals into vanadyl phosphate dihydrate

MATERIALS CHEM;~TWND ELSEVIER

Materials

Redox

intercalation

Chemistry

and Physics

40 (1995) 207-211

of alkali metals dihydrate

into vanadyl

phosphate

A. Chauvel a, M.E. de Roy a, J.P. Besse a, A. Benarbia b, A. Legrouri b, A. Barroug b aLaboratoire de Physico-Chink b Laboratoire

des Mat&xx, V.R.A. 444 du CNRS, Vnivemite’ Blake Pascal, 63177 Aubike Ceder, France de Chimie Physique, FacultP des Sciences, Univemite Cadi Ayyad, B.P. S 15, 40001 Marrakech, Morocco

Received 5 December

1994; accepted 6 January 1995

Abstract Alkali-metal-containing intercalates, of the general formula MxVOP04.2Hz0, where M=Li, Na, K, Rb and Cs, have been prepared by reduction of VOP04.2H20 in iodide solution. The results of both structural and morphological studies carried out upon these intercalation compounds are reported. Keywords: Intercalation

compounds;

Phosphates;

Vanadyl phosphates;

1. Introduction

The vanadyl phosphate dihydrate, VOPO,.2H,O, and its intercalation compounds are the subject of considerable interest for their applications as catalysts or cathode materials [1,2]. The structure of this layered compound, which belongs to the isomorphous series MOXO,.nH,O, with M = V, Nb, Ta and X= P, As, is built up of distorted VO, octahedra and PO, tetrahedra linked together by shared oxygen atoms to form infinite sheets. These sheets are linked together through shared water molecules [3-51. VOPO, .2H,O was reported to undergo intercalation reactions either by neutral molecules, such as alcohols [6,7] or amines [8,9], or by organic [lo] or inorganic [5] cations, whose charge is compensated for by vanadium reduction. This study investigates the intercalation of VOPO,. 2H,O by different metallic cations. It forms part of a wider study encompassing the preparation, characterisation and catalytic tests of compounds of the formula MOPO,.nH,O, where M =V, Nb. We report here on a series of alkali-metal-intercalated vanadyl phosphate dihydrates whose general formula is M,VOPO,.2H,O with M= Li, Na, K, Rb and Cs, and its characterisation by X-ray diffraction (XRD), X-ray absorption spectroscopy (XANES), thermal analyses (TG and DTA), infrared spectroscopy (IR) and both scanning (SEM) and transmission electron microscopy (TEM). 0254-0584/95/$09.50 0 1995 Elsevier Science S.A. AI1 rights reserved SSDI 0254-0584(95)01482-A

Alkaline metals

2. Experimental

2.1. Materials

VOPO, .2H,O was prepared as described by Ladwig [ll]; it was greenish yellow in colour. The intercalation compounds were prepared by the iodide method [12]. VOPO,.2H,O was reduced in a saturated MI (M =Li, Na, K, Rb or Cs) solution for one week at room temperature. The iodine that evolved during reduction was titrated in order to determine the amount of metal intercalated. The water content of the compounds was determined by thermogravimetry. The final products were green in colour, indicating that vanadium reduction had occurred. The products prepared in the course of this study are listed in Table 1. Table 1 Amount of metal intercalated M,VOPOI.nH20 compounds

(x) and water content

(n) for

Metal

x

n

Li Na

0.96 0.20-0.30 0.45 0.75

2.0 2.0 2.0

K Rb

0.45

0.10

cs

0.50 0.09

2.0 2.0 2.0 2.0

1.0

A. Chauvel et al. / Materials

208

Chemistry and Physics 40 (1995) 207-211

2.2. Methods XRD measurements were carried out on powdered samples with a Siemens D 501 diffractometer using graphite monochromatized Cu Ka radiation. Vanadium K-edge X-ray absorption data were recorded at the Laboratoire pour l’utilisation du Rayonnement Electromagnetique (LURE) (Orsay, France) on the four-line equipment. The IR absorption spectra were registered on a Perkin-Elmer 580-B spectrophotometer. Samples were examined in the form of thin discs containing approximately 2 mg of sample and 300 mg of spectroscopic potassium bromide. Spectra were recorded in the range 4000-400 cm-‘. TG and DTA studies were performed with a Setaram 92 unit. Curves were recorded over a range of temperatures up to 900 “C at a heating rate of 5 “C min-l. Samples for TEM examination were mounted on standard 3 mm copper grids covered with a carbon film. Observations were made with an Akashi EM200B electron microscope fitted with a Link X-ray microanalysis system. SEM was undertaken with a Philips 500 instrument. Samples were mounted on aluminium stubs and precoated with gold in a sputter coater to minimise the severe charging effects that proved to be characteristic of these materials.

I

20

20 (‘1 Fig. 1. XRD K-intercalated

patterns of (a) VOP04.2H20, compounds.

Table 2 Values of the interlayer

and discussion

3.1. X-ray diffraction and X-ray absorption spectroscopy

The XRD pattern perfectly indexed to by Tietze [3] for this rre determined to

of our VOPO,.2H,O sample was the tetragonal structure described compound. The lattice parameters be a = 6.20(6) 8, and c=7.38(9)

The XRD patterns of the intercalated compounds (Fig. 1) show that, upon intercalation, the intensity of XRD lines decreases, indicating a loss of crystallinity and a net reduction in the interlayer long-range order. In addition, intercalation is observed to cause a contraction in the interfoliar space owing to an electrostatic attraction between the vanadyl phosphate layers, which possess a negative charge due to vanadium(V) reduction, and the intercalating cation (Table 2). This is in line with XANES analyses, which indicate that vanadium(V) is reduced to its +4 oxidation state. In fact, it was observed that the spectrum of Na,,q,VOPO,. 2H,O is situated between those of the reference compounds VOSO, . 5Hz0 and VOPO, . 2H20, where the vanadium atoms possess the oxidation states five and four, respectively (Fig. 2). Similar results were observed in the case of other inorganic cations [1,2].

spacing

(b) Li-, (c) Na- and (d)

d in M,VOPO,.nHzO

compounds

x

Metal

’

d (A)

0.96 0.20-0.30 0.45 0.75 0.45 0.10 0.50 0.09

Li Na

K Rb

3. Results

I

60

40

cs 1 The value

for VOPO,.2H,O

I

I] 5440

6.38 6.73-6.67 6.42 5.60 6.42 6.48 7.00 7.03

is 7.38 A.

ri 5460

I

5480

,

5500

5 (ev) Fig. 2. XANES spectrum of (a) Na-intercalated compound, that its threshold is between those of VOP04.2Hz0 VOSO,.SH,O (c), in agreement with the mixed V(V)-V(W) in these materials.

showing (b) and valency

Indexing of the spectra of intercalated compounds (Fig. 1) to tetragonal structures, as made by Jacobson et al. [5], or even by assuming an orthorhombic distortion, as suggested by Casafi et al. [13] for analogous compounds, was not possible. The intercalation of ions into a lamellar structure can lead to the following situations: - The ions totally occupy one specific type of site in the structure, in which case the symmetry is conserved.

A. Chauvel et al. I Materials

Chemistry

- Cations induce a superstructure by occupying, partially but regularly, one specific type of site in the structure. - Cations occupy some sites partially but not regularly. - The cations are arranged in a regular manner in the interfoliar space, independently of the original structure. - The cations distort the original structure. Since indexing of the spectra obtained in the course of this study was not possible by considering a reasonable superstructure along the c axis or in the ab plane, the first three possibilities can be ruled out. The fourth possibility suggests weak interactions between the intercalated cations and the host structure, but this contradicts the observed electrostatic interactions. It is therefore likely that intercalation has created a deformation of the guest structure which seems to become more important as the radius of the intercalated cation increases. The less energy-demanding deformation consists of the rupture of elementary sheets.

and Physics 40 (1995) 207-211

209

can be attributed to the formation of the pyrophosphate (VO),P,O,. The intercalated compounds exhibit different behaviour during heating (Fig. 4). In the case of the Naintercalated compound, Na ions seem to be replacing water molecules which were localised truns to the V=O bond of reduced vanadyl groups (V02+). The charge deficiency is thereby neutralized locally. A similar effect has been observed for the intercalation of Na [13] and for some di- and trivalent cations [l] in VOPO, .2H,O. However, the other alkaline-metal-intercalated products present two indistinguishable water molecules. 3.3. Infrared spectroscopy The IR spectrum of VOPO,.2H,O (Fig. 5) presents well-resolved bands which can be attributed properly, based on previous studies of MOXO, compounds [1,5,8-10, 14-191. The vibration bands associated with water molecules and orthophosphate groups, present in the VP0 spectrum, and their attributions are reported in Table 3.

3.2. Thermal analyses According to DTA and TG curves (Fig. 3) VOPO, :2H,O contains three types of water. The two water molecules are lost during heating at about 100 and 150 “C; they seem to correspond to molecules which are weakly bound to the layers and those which complete the coordination of vanadyl groups, respectively. Complete dehydration of the product is obtained only at around 600 “C, indicating the presence of small amounts of water molecules strongly retained by the product. The endothermic peak present in the DTA curve at 320 “C corresponds to the first loss of water molecules strongly retained in the solid. The peaks present at 510 and 600 “C might be due to the topotactic transformation of the cr, form of the anhydrous vanadyl phosphate to the a,, form [14]. The DTA diagram also shows a phase transition at 750 “C, accompanied by a loss of small amounts of oxygen detected by TG, which

0-

T(‘C) Fig. 3. TG and DTA curves for VOPOI.2Hz0.

b

*OOW

800 T (“C)

Fig. 4. TG curves for (a) LiVOPO,.ZH,O, and (c) Na,,.,VOPO,-2H,O.

(b) Na,,.,,VOP04

.2H>O

Fig. 5. IR spectra of (a) VOPO,.2H,O, (b) Li-, (c) Na- and (d) Kintercalated compounds. See text and Table 3 for explanation of bands a-l.

210

A. Chauvel et al. I Materials

Table 3 band attributions for IR Li0.96VOP04. 2H,O (LiVPO)

VOP0,.2Hz0

Band

a b c d e f g, h i j k

1

Wavenumber

(cm-‘)

VP0

LiVPO

3568 3374 3136 1608 1182 1086 1033, 995 950 906 680 570

3524 3356 3255 1652 1134 1042 1026 974 762 682 550

Fig. 6. SEM micrographs intercalated compound.

(VPO)

Chemisfty

and

Attribution

Y

Hz0

VI

Hz0

v, H,O=V Y H,O Y PO. Y PO, ” v=o “, Pod+6 o-v-o v, PO,, or prH20 lattice Y w&O

of (a) VOP0,.2H20

and (b) the K-

l

v*

and Physics 40 (1995) 207-211

The IR spectra of the intercalated compounds (Fig. 5) are nearly identical to that for VOPO, .2H,O. Table 3 reports the absorption bands observed in the spectrum of Li,,,VOPO, - 2H,O, which corresponds to the reduction of the majority of vanadium(V) atoms to vanadium(IV). Band d, well defined and corresponding to the angular deformation H-O-H, is displaced towards higher energies, while the elongation vibrations v1 and v, of water molecules absorb at lower energy. This result indicates a more important hydrogen bond network or water molecules strongly bonded in the structure. The absorption bands of POa3- groups are sharper, and the interactions between the vibrations of water molecules and those of the sheets seem to be lower. In addition, the ionic@ of the bonds inside the sheet must be increased owing to vanadium atom reduction. This can explain the shift towards lower energy for the P-O angular vibrations, in particular band j, which is attributed to the angular vibration O-P-O -I-60-V-0, although nothing allows confirmation that this band corresponds to the same phenomenon in the parent material VOPO, .2H,O. It is not possible, as in VOPO,- 2H,O, to make a distinction between the two types of water. These are not coordinated to vanadium, but must be surrounding the Li’ cation. This result confirms the observations made as a result of thermal analyses. The P-O-V lattice vibrations are not very much displaced (band k). The IR spectrum of NaVPO indicates that the P-O-V lattice vibrations appear at the same value of the energy. This shows that the VOPO, sheets retain the same morphology. The region below 1400 cm-’ is very difficult to interpret, owing to the mixing of P-O and V-O modes [20]; in particular, it was not possible to observe the splitting of the vanadyl (V=O) absorption, which was observed to correspond to V5 + =0 and V4+ =0 in Na,,VOAsO, .2H,O [ 121. It is noteworthy that the IR bands were not notably modified upon intercalation. The evident slight band ratio changes suggest that the perturbations of the parent structure, noticed in the XRD and thermal analysis studies, may have affected only the long-range order.

v

3.4. Scanning and transmission u 0

v 10

(b) 20

keV

Fig. 7. X-ray microanalysis spectra for (a) VOPO,.2H,O K-VOP0,.2Hz0.

and (b)

Band g of VP0 which overlaps with the phosphorus bands can be reasonably attributed to vanadyl groups.

electron microscopy

The crystals appear in the SEM micrographs (Fig. 6) to be of laminar nature with different particle sizes. The laminar nature is more pronounced in the case of the parent compound VOP04.2H20. Furthermore, SEM observations show that the morphology of the particles underwent sensitive changes during intercalation. The particles were delaminated and the layers seem to have been sliced into smaller parts.

A. Chauvel

et al. I Materials

Chemistry

The TEM study was more difficult to carry out, owing to the fact that the crystals were beam sensitive. Nevertheless, in order to check the homogeneity of the crystal composition, an analysis by electronic microprobe was carried out. In the case of the K-intercalated compound (Fig. 7), the intensities of the characteristic emissions for vanadium, phosphorus and potassium were acceptably constant for the different crystals and for separate points in the same crystal. This suggests that the sample was composed of only one type of crystal and that these crystals present a homogeneous composition.

and Physics 40 (1995) 207-211

the parent compound VOPO,.2H,O. The morphology of the particle underwent sensitive changes during intercalation. The particles were delaminated and the layers seem to have been sliced into smaller parts. X-ray microanalysis confirms potassium intercalation and indicates that the crystal composition is homogeneous. References 111 M.R. Antonio, [21

4. Conclusions

131 [41 I51

Alkaline-metal-intercalated VOPO,.2H,O compounds of the general formula M,VOP04. 2H20, where M = Li, Na, K, Rb and Cs, were prepared by the iodide method. XRD indicates a certain loss of crystallinity and a net reduction in the interlayer long-range order upon intercalation. In addition, intercalation is observed to cause a contraction in the interfoliar space. This contraction might be caused by an electrostatic attraction between the intercalating cations and the VOPO, layers, which bear negative charge due to partial vanadium reduction. This is in line with the XANES analyses, which indicate that vanadium(V) is reduced to its +4 oxidation state. Examination of the bands observed in the spectra of VOPO, . 2Hz0 and M,VOPO, .2H,O shows that these spectra are nearly identical. Water absorption bands are better resolved for the intercalated compounds; this could indicate that water molecules are more localised in these compounds. The slight band ratio changes observed suggest that the perturbations of the parent structure noticed in the XRD and thermal analysis studies may have affected only the long-range order. The laminar nature of the crystals, observed in the SEM micrographs, is more pronounced in the case of

211

161 [71 [81 [91

[lOI

Pll [I21 [I31

P41 1151 [I61 [I71 1181

[I91 1201

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