Electron paramagnetic resonance (EPR) study of solid solutions

of MoO in SbVO3 51 2 1 1,3

J. Typek , E. Filipek , G. Zolnierkiewicz and N. Guskos 1Institute of Physics, West Pomeranian University of Technology, Al. Piastw 48, 70-311 Szczecin, Poland.

2Department of Inorganic and Analytical Chemistry, West Pomeranian University of Technology, Al. Piastow 42, 71-065 Szczecin, Poland.

3Solid State Section, Department of Physics, University of Athens, Panepistimiopolis, 15 784 Zografos, Athens, Greece.

The mixtures of V O , Sb O and MoO oxides as well as other phases formed in the Sb-V-Mo-O system are promising catalysts in the 2 5 2 4 3

selective oxidation of organic compounds, e.g. in the reaction of obtaining acrylonitrile by ammooxidation of propane. In the recent years intensive investigations have been performed aimed at establishing the thermal, electric and other physicochemical properties of these phases. In the present work monophase samples containing only solid solution of MoO in SbVO with the general formula Sb3 5 1-

¤ V ¤ Mo O (for x = 0.0051, 0.0077, 0.0104, 0.0132, 0.0159, and 0.018), (where ¤ designates vacancy) have been synthesized by 6x x 1-6x x 10x 5

using the solid-state reaction method and investigated by electron paramagnetic resonace (EPR) technique. Recent study has shown 6+that these materials are substitutional solid solutions, in which the Mo ions are incorporated into the crystal lattice of SbVO at both 5

5+ 5+Sb and V sites, and the compensation of an excessive positive charge occurs mostly through formation of cationic vacancies at the 5+ 5+ 4+ 5+Sb and V sites in 1:1 proportion. EPR study of these compounds aims at detecting V and Mo paramagnetic ions and calculating

5+their amount in order to discuss the possibilty of charge compensation by valence reduction of nominally V ions in the SbVO lattice. 54+ 5+As could be seen in Figs. 1 and 2 the EPR signal intensity, attributed to V and Mo ions and clusters increases with Mo ions

concentration what is a clear sign of the presence of valence reduction. Comparison with a reference sample of VOSO ·5H O allowed to 4 25+ 5+ 4+calculate the number of spin centers participating in the resonance and thus to estimate the ratio of Sb and V vacancies to V

5+ 4+paramagnetic centers. It has been calculataed that the percentage of Mo ions involved in V to V valence reduction decreases with the total content of Mo ions in Sb-V-Mo-O solid solution (see Fig. 3). This dependence might be roughly described by the decreasing exponential function (solid line in Fig. 3).

AcknowledgementsPublication of this paper was realised with partial financial support from the budget resources of the West Pomeranian Voivodeship.

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MoO

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Sb 2

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V2O

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V2O

5Sb

2O

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MoO3

SbVO5: MoO3

SbVO5

Composition of initial mixtures [%mol]

Formulae index

xMoO3 V2O5 Sb2O4

5.00 47.50 47.50 0.0051

7.50 46.25 46.25 0.0077

10.00 45.00 45.00 0.0104

12.50 43.75 43.75 0.0132

15.00 42.50 42.50 0.0159

17.50 41.25 41.25 0.0188

Composition of initial mixtures [%mol]

Formulae index

xMoO3 V2O5 Sb2O4

5.00 47.50 47.50 0.0051

7.50 46.25 46.25 0.0077

10.00 45.00 45.00 0.0104

12.50 43.75 43.75 0.0132

15.00 42.50 42.50 0.0159

17.50 41.25 41.25 0.0188

General formulae of the solid solutions:Sb1-6x �xV1-6x�xMo10xO5

V5+O6

V5+O6

Sb5+O6

Sb5+O6

Mo6+

Mo6+

Mo6+ Mo6+

Mo6+ Mo6+

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11.8 K14.6 K18.7 K22.9 K

29.3 K37 K43 K50 K60 K72 K

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Concentration of Mo [%mole]

0.5 1.0 1.5 2.0 2.5 3.0

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Concentration of Mo ions [% mole]

The SbVO matrix: crystal structure. 5

Monoclinic a=9.86 Å, b=4.93 Å, c=7.12 Å, â=109.79°, Z=4SbO octahedra, Vo deformed octahedra, Separate layers.6 6

SbVO :MoO - Charge compensation5 3

Preferred model of charge compensation, based on TG: 5+ 5+ 6+ 5+ 5+V and Sb vacancies; substitution of Mo at V and Sb sites.

Scanning Electron Microscope (SEM) picture of the matrix: SbVO5.

SEM picture of s SbVO :MoO (15mol%) 5 3olid solution

EPR: paramagnetic centers:

Vanadium

5+ 6V (3p ) nominal, nonmagnetic4+ 1V (3d ) defect, magnetic S=1/2

Antimony

5+ 10Sb (4d ) nominal, nonmagnetic4+ 1Sb (5s ) defect, magnetic

Molybdenum

6+ 6Mo (4p ) nominal, nonmagnetic5+ 1Mo (4d ) defect, magnetic S=1/2

S=1/2

6+. The intensity of EPR spectra increases with the Mo contents – only cation vacancy compensation model could not be used.

4+No hfs lines visible – all V ions strongly coupled to the magnetic spin system.

4+ 6+ No linear dependence of V content on amount of Mo ions.6+The EPR linewidth decreases with Mo content (exchange interaction narrowing).

6+ 5+ 4+ 6+The fraction of Mo ions involved in V ? V compensation decreases with Mo increase

EPR: solid solution SbVO :MoO .5 3

EPR: the SbVO matrix.5

4+Only 0.02% of all vanadium ions are EPR active (V ).4+ 4+ 5+There are separate V (showing 8 hfs narrow lines) and involved in a V –O–V bond with a mobile electron hopping (broad line).

4+ 2+Separate V in SbVO exist as VO ions in octahedral coordination with a tetragonal compression.52+There are also pairs of two interacting VO with a singlet S=0 (ground state) and a triplet S=1 state (excited state).

6+ 4+Solid solution: possible paramagnetic Mo -V 4+centres involving one V ion.

Solid solution: p4+more than one V ion (equatorial view).

ossible paramagnetic centres involving

Fig. 1: EPR absorption lines registered for samples having the same mass but with different concentration of Mo ions: 1 - 0.73% mole, 2 - 1.10% mole, 3 - 1.49% mole, 4 - 1.89% mole, 5 - 2.28% mole, 6 - 2.70% mole.

Conclusions6+-- At least one third of Mo ions are involved in charge

compensation through changing the oxidation state of cations

-- Compensation mechanism through cation vacancy is more 6+efficient for larger concentrations of Mo ions

4+-- V ions are strongly coupled to the rest of spin system – no distant charge compensation

Fig. 2: Dependence of EPR integrated intensity (upper panel) and linewidth of unit mass samples (bottom panel) on the molar concentration of Mo ions.

5+Fig. 3: Percentage of Mo ions involved in V to 4+V valence reduction as a function of total Mo

ions concentration in Sb-V-Mo-O solid solution.