Electron paramagnetic resonance (EPR) study of solid solutions of MoO3 in SbVO5

Electron paramagnetic resonance

(EPR) study of solid solutions

of MoO in SbVO

₃

₅

1

2

1

1,3

J. Typek , E. Filipek , G. Zolnierkiewicz and N. Guskos

1

Institute of Physics, West Pomeranian University of Technology, Al. Piastw 48, 70-311 Szczecin, Poland.

2

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

Poland.

3

Solid 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. 5

4+ 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 2

5+ 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).

Acknowledgements

Publication of this paper was realised

with partial financial support from the

budget resources of the West Pomeranian

Voivodeship.

0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 M o_O 3_% m o_l Sb2 O%4 mo l V₂O₅ %mol V2O5 Sb₂O₄ MoO₃ SbVO5: MoO3 SbVO₅

Composition of initial mixtures [%mol] Formulae index x MoO3 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 x MoO3 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: Sb_1-6x _xV_1-6x_xMo_10xO₅ V5+O6 V5+O₆ Sb5+O₆ Sb5+O6 Mo6+ Mo6+ Mo6+ Mo6+ Mo6+ Mo6+ 250 300 350 400 450 -8 -6 -4 -2 0 2 4 6 8 A b s o rp ti o n d e ri v a ti v e [a rb . u n it s ] Magnetic field [mT] 11.8 K 14.6 K 18.7 K 22.9 K 29.3 K 37 K 43 K 50 K 60 K 72 K 0 20 40 60 80 100 0,0 0,2 0,4 0,6 0,8 1,0 (I n te g ra te d E P R in te n si ty ) -1 [a rb .u n it s] Temperature [K] 100 200 300 400 500 600 -2 -1 0 1 2 3 A b s o rp ti o n d e ri v a ti v e [a rb . u n it s ] Magnetic field [mT] 2000 3000 4000 -400 -200 0 200 400 2000 3000 4000 -400 -300 -200 -100 0 100 200 300 400 500 2000 3000 4000 -400 -300 -200 -100 0 100 200 300 400 500 2000 3000 4000 -400 -200 0 200 400 2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000 4200 4400 -400 -200 0 200 400 2000 2400 2800 3200 3600 4000 4400 -400 -200 0 200 400 1 2 3 4 E P R s ig n a l in te n s it y [a .u .] Magnetic field [G] 5 6 0 2 4 6 8 10 12 14 16 150 200 250 300 350 400 E P R a b s o rp ti o n [a .u .] Magnetic field [mT] 1 2 3 4 5 6 4.5 5.0 5.5 6.0 6.5 7.0 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 30 32 34 36 38 E P R in te g ra te d in te n s it y [a .u .] E P R lin e w id th [m T ] Concentration of Mo [%mole] 0.5 1.0 1.5 2.0 2.5 3.0 15 20 25 30 35 F ra c ti o n o f M o io n s c a u s in g v a le n c e re d u c ti o n [% ]

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=4 SbO 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 solid solution SbVO :MoO (15mol%) 5 3

EPR: paramagnetic centers:

Vanadium

5+ 6

V (3p ) nominal, nonmagnetic

4+ 1

V (3d ) defect, magnetic S=1/2

Antimony

5+ 10

Sb (4d ) nominal, nonmagnetic

4+ 1

Sb (5s ) defect, magnetic

Molybdenum

6+ 6

Mo (4p ) nominal, nonmagnetic

5+ 1

Mo (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.5 2+

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: p

4+

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.

Conclusions

6+

-- 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.