Preparation and characterization of Keggin-type heteropolysalts , Co x PMo 12 O 40 ( x = 0 – 1 . 5 )

Polyoxometalates (POMs), based transition-metal oxide clusters, have received much attention in various fields such as catalysis, photochemistry, nonlinear optics, biology and medicine [1]. Their physico-chemical, acidic and oxidative properties can be adjusted according with the nature of constituent elements [2]. In the field of catalysis, the most studied POMs are those with the Keggin. They were tested in wide variety of reactions in homogeneous and heterogeneous phases. In the redox processes, the nature of countercation in the [PMo12O40]3− system can play a significant role. Thus, it has been shown that using Fe(III), vanadyl(VO2+), antimony (Sb3+) or cobalt (Co2+) counter-cation develop a more favourable distribution of both reduced Mo(V) and oxidized Mo(VI) sites [3]. In the present work we report the synthesis and characterization using several techniques of Keggin-type heteropolysalts of composition H3−2x Cox PMo12O40(x = 0 − 1.5) denoted as Cox PMo12.


INTRODUCTION
Polyoxometalates (POMs), based transition-metal oxide clusters, have received much attention in various fields such as catalysis, photochemistry, nonlinear optics, biology and medicine [1].Their physico-chemical, acidic and oxidative properties can be adjusted according with the nature of constituent elements [2].In the field of catalysis, the most studied POMs are those with the Keggin.They were tested in wide variety of reactions in homogeneous and heterogeneous phases.In the redox processes, the nature of counter-cation in the [PMo 12 O 40 ] 3− system can play a significant role.Thus, it has been shown that using Fe(III), vanadyl(VO 2+ ), antimony (Sb 3+ ) or cobalt (Co 2+ ) counter-cation develop a more favourable distribution of both reduced Mo(V) and oxidized Mo(VI) sites [3].In the present work we report the synthesis and characterization using several techniques of Keggin-type heteropolysalts of composition H 3−2x Co x PMo 12 O 40 (x = 0 − 1.5) denoted as Co x PMo 12 .

Materials
Co x PMo 12 was synthesized by a cation exchange carried out in two stages: in the first step Ba x PMo 12 O 40 was prepared from addition of Ba(OH) 2 to an aqueous solution of H 3 PMo 12 O 40 in stoichiometric ratios, and in the second step BaSO 4 precipitates after addition of cobalt sulphate.After filtration, the solution was drained to 50 • C. Co x PMo 12 salts were conserved with any washing.

Characterization
Infrared spectroscopy of the solids was recorded on an ATR Thermo scientific Nicolet apparatus.Solution 31 P NMR spectra were recorded on a Bruker AC300 apparatus at 121.5 MHz, temperature with a Siemens D5000 diffractometer using Cu-Kα radiation (λ = 1.5418 • A).Thermal analysis (TG) were carried out using a Perkin Elmer Stimiltanous Thermal analyzer STA 6000 and the studies of the morphology of salts using JEOL JSM-5800LV Scanning MicroscopIe (SEM).a e-mail: tassa2002dz@yahoo.frTable 1.IR vibration wave numbers (cm −1 ) of the polyoxometalate.

ATR spectroscopy
The IR vibration wave numbers solids are summarized in Table 1.In the low wave number region (1100-500cm −1 ), the characteristic bands of the Keggin structure were obtained.According to Rocchiccioli-Deltcheff et al [4], the bands at 1065, 962, 865, 793 and 561 cm −1 correspond to υas(P-Oa), υas(Mo-Od), υas(Mo-Ob-Mo), υas(Mo-Oc-Mo) and υ(P-O) vibrations, respectively.In Keggin type unit (Fig. 1), Oa refers to oxygen atom common to PO 4 tetrahedron and one trimetallic group Mo 3 O 13 ; Ob connects two trimetallic groups, Oc binds two octahedral MoO 6 inside a trimetallic group and Od is the terminal oxygen atom.The whole structure has Td symmetry and corresponds to α isomer.This suggests that primary structure remains intact after the partial and /or the total protons substitution.Chemical shifts are referenced to 85% H 3 PO 4 .X-ray diffraction (XRD) patterns were recorded at room.

XRD analysis
The X-ray diffractograms of Co x PMo12 are similar to that of the parent acid H 3 PMo 12 O 40 (Fig. 2), that crystallizes in the triclinic system.These results agree with those of IR spectroscopy showing that the ions occupy the counter-ion position in the polyoxometallate.

31 P NMR
Solution 31 P NMR spectrum of Co x PMo 12 exhibits only one peak at around −3.18, −3.27, −3.36, −3.4,−3.31, −3.31 and −3.50 ppm for x = 0-1.5 respectively.These results evidenced the purity of the heteropolysalts and also show that the chemical shift of 31 P is very sensitive to the value of x.

Thermal analysis (TG)
The TG curves of all salts show two steps of weight loss.Before 200 • C, the weight loss was attributed to crystallization water desorption and between 200-250 • C, water desorption.The latter weight loss allows to calculate the number of protons and therefore to deduce the value of x.Deduced formula for cobalt salts were in good agreement with the theoretical ones (Table 2).

UV-Visible
In the UV-Vis spectra of Co 1.5 PMo 12 salt, (Fig. 3), a large band in the domain of wavelengths 200-450 nm was observed associated to ligand-metal charge transfers from oxygen to Mo(VI) in the Keggin anion [2,5,6].
In addition to this broad absorption band, another large charge transfers band was observed above 500-600 nm that may be attributed to the d-d transition band of  Co II + Mo VI ↔ Co III + Mo V .

SEM analysis
From electron microscopy measurement, H 3 PMo 12 and Co 1.5 PMo 12 show different morphologies as a consequence of the substitution of protons by the Co 2+ ions (Fig. 4).

CONCLUSION
The results showed that the gradual cationic exchange method allows to obtain a series of compounds, H 3−2x Co x PMo 12 O 40 (x = 0-1.5),whose properties could be adjusted according to the value of x.