Fast synthesis of the polycrystalline materials on the base of Zn 3 V 2 MoO 11 and Zn 2

In our study we applied two different techniques for the preparation of Zn3V2MoO11 and Zn2.5VMoO8 polycrystalline materials melt quenching method (up-bottom) and mechanochemical synthesis (bottomup). These compounds belong to the family of materials with general formula M2.5VMoO8 (M=Zn, Mg, Mn, Co). They are potential candidates as catalysts in processes of selective oxidation of hydrocarbons. Until now, these two compounds were obtained by conventional solid state reaction. Using infrared spectroscopy and X-ray diffraction we proved that the melt quenching technique is a quite appropriate method for the synthesis of bulk Zn3V2MoO11 phase. Mechanochemical activation is more appropriated for the preparation of nonosized Zn2.5VMoO8 powder. It was established that the melt quenching technique and mechanochemical activation are faster in comparison with conventional ceramic methods for the given synthesis.


I. Introduction
During the past decades, numerous investigations have been published about the compounds found in the ternary system MO-MoO 3 -V 2 O 5 (M = Zn, Mg, Mn, Co) [1][2][3][4][5][6][7][8].For example, many compounds of the M 2.5 VMoO 8 family were synthesized.This kind of materials adopt structures similar to molybdates such as NaCo 2.31 (MoO 4 ) 3 [9] and Cu 3.85 (MoO 4 ) 3 [10].They are featured with one-dimensional columns built from face-sharing octahedra and isolated VO 4 and MoO 4 tetrahedra, which determine their promising catalytic properties (high selectivity and conversion) in the dehydrogenation of alkanes [11,12].Because of the presence of two transition oxides in the composition, these compounds are also potential candidates for semiconductor applications.
The objects of our investigation are two compounds, which exist in the ZnO-MoO 3 -V 2 O 5 system -Zn 2.5 VMoO 8 and Zn 3 V 2 MoO 11 .Up to now they have been obtained by solid state reactions during long time heat treatment (20-100 hours) at different temperatures in the range 480-700°C [13].
The aim of this study is to check the possibility to prepare Zn 2.5 VMoO 8 and Zn 3 V 2 MoO 11 for shorter time, using melt quenching and mechanochemical method of synthesis.The melt quenching method is well known for the preparation of glassy or metastable phases and also for the design of the so-called glass ceramics.Mechanical treatment in order to increase the reactivity of solids has been known in the ceramics industry for a long time, as a method for generating new surfaces and various defects.The advantages of mechanochemical activation are that the use of voluminous solutions and complicated operations as well as the sintering of the particles can be avoided.

Sample Characterization
Powder XRD patterns were obtained at room temperature with a Bruker D8 Advance diffractometer using Cu Kα radiation in the θ range 10< θ <60° in a step scan mode.Infrared spectra were performed on a Nicolet -320 FTIR spectrometer using the KBr pellet technique in the range 400-1200 cm -1 .The morphology of the obtained samples was derived using scanning electron microscopy (JEOL JEM-200, CX).

III. Results
Fig  Additional information on the phase formation and structural features of Zn 2.5 VMoO 8 and Zn 3 V 2 MoO 11 phases could be obtained by IR spectroscopy (Figs. 3  and 4).As one can see from the Fig. 3, the vibrational spectra of the samples prepared by melt quenching method are very similar, corresponding to the phase Zn 3 V 2 MoO 11 (XRD diffraction Fig. 1).The main absorption band is situated at 750-770 cm -1 .Three other bands -870-970 cm -1 are also recognized.All bands in the range 750-970 cm -1 can be assigned to the vibrations of isolated strongly deformed MO 4 polyhedra.The absorption between 550-650 cm -1 is probably due to the formation of edge-shared polyhedra (Fig. 3b -slow cooled sample) [16,17].The spectra of fast quenched samples (Fig. 3a,c) are not well resolved because of their partial amorphization.The bands in all spectra below 500 cm -1 should be connected with vibrations of ZnO 6 units [18,19].
Fig. 4a demonstrates infrared spectra of the sample 62.5ZnO•25MoO 3 •12.5V 2 O 5 mol% after 2 hours of mechanochemical treatment.On Fig. 4b,c are presented vibrational spectra of the mechanically activated sample with initial composition 60ZnO•20MoO 3 •20V 2 O 5 mol% after 3 and 5 hours milling time.The spectra of mechanochemical treated samples are characterized with very strong absorption near 790 cm -1 and not well resolved shoulders in the range 870-970 cm -1 .The band at 790 cm -1 is due to asymmetrical stretching vibrations of isolated MO 4 tetrahedra.
Fig. 5 represents SEM image of Zn 3 V 2 MoO 11 phase, obtained by melt quenching method of fast cooling sample with composition 60ZnO•20MoO 3 •20V 2 O 5 mol%.It is seen the formation of small crystals (5-10 μm) immersed into the amorphous matrix.The SEM micrograph of Zn 2.5 VMoO 8 phase, obtained by mechanochemicaly activated precursors shows the formation of spherical-like aggregates with size dimension of the crystallites below 1 μm (Fig. 6).

IV. Discussion
In order to understand the features of the applied techniques we take into account the crystallographic structure of Zn 2.5 VMoO 8 and the connectivity of the different polyhedra.Zn 3 V 2 MoO 11 phase crystallizes in the monoclinic system, but there is no data for the structural polyhedra in the unit cell [13,14].This compound melts incongruently at 800±5°C with a deposition of Zn 2.5 VMoO 8 crystals.Crystallographic data of the other compound -Zn 2.5 VMoO 8 are summarized by Wang et al. [12].According to them, this compound crystallizes in the orthorhombic system.In the structure, Zn(3)O 6 At the same conditions, from the melt with nominal composition 62.5ZnO•25MoO 3 •12.5V 2 O 5 mol%, mixture of Zn 2.5 VMoO 8 , Zn 2 V 2 O 7 , MoO 3 and one unknown phase was obtained.More over, part of the melts is possible to be supercooled in solid amorphous state.This is seen from the presence of amorphous halo together with diffraction peaks.
Applying mechanochemical reaction (bottom-up), irrespective to the initial compositions, the only crystalline phase is Zn 2.5 VMoO 8 .As it is shown above, its crystal structure contains different types of associated ZnO 6 units connected with isolated V/MoO 4 tetrahedra.According to the presented IR spectra, the isolated tetrahedra of this compound are more symmetrical, because the bands in the range 870-950 cm -1 are not well resolved.In the spectrum of compound Zn 3 V 2 MoO 11 these bands are more intensive, which resulted from the removal of the degeneracy and the activation of the symmetrical vibrations.This kind of spectra is typical for the vibration of more deformed polyhedra.In the spectrum of Zn 3 V 2 MoO 11 , bands in the range 620-590 cm -1 also appear which may be connected with the formation of edge-sharing units Mo 2 O 2 [16].This is why, from kinetic point of view, during mechanochemical treatment we suggest that it is easier to obtain Zn 2.5 VMoO 8 in which weak bonds of ZnO 6 units with corner-shared tetrahedra are realized.

IV. Conclusions
In this study we proved that the melt quenching is a very appropriate method for the synthesis of bulk Zn 3 V 2 MoO 11 phase for a short time.By mechanochemical reaction, Zn 2.5 VMoO 8 phase was obtained at room temperature after two hours, which is several time faster than classical solid state reaction.

Figure 5 .
Figure 5. SEM micrograph of fast cooling sample Z3V2M, containing Zn 3 V 2 MoO 11 crystals, according to X-ray data