Hydrothermal Synthesis of Transition Metal Oxides

CHAPTER 3

SYNTHESIS AND CHARACTERIZATION of KV₃O₈

3.1 Introduction

Vanadium compounds are extensively studied because of their interesting redox, electrochemical, and catalytic or magnetic properties. For example, among the oxides, the layered lithium vanadates are used as electrode material in lithium batteries [46] [47], whereas the bismuth oxides are good candidates for solid electrolytes in fuel cells [48]. Some other vanadates show catalytic activities in the oxidative dehydrogenation of hydrocarbons [49, 50].

The vanadium oxide part of the hybrid materials has the V_xO_y^n- formula. In mineral chemistry, vanadium exhibits three oxidation states V(III), V(IV) and V(V). The coordination polyhedra observed for V(IV) are square pyramidal and distorted octahedral or square bipyramidal. The square pyramidal geometry may be described as 4 + 1. This square pyramidal geometry has one short vanadyl bond in the apical position and four longer equatorial bonds (1.80–2.12 Å). The six-coordinate geometry has been denoted as 4 + 1 + 1, that has four intermediate equatorial bonds (1.86–2.16 Å), one axial vanadyl bond, and a long axial bond (2.20–2.32 Å) (Figure 3.1) [51].

Pentavalent vanadium may exhibit tetrahedral, square pyramidal, distorted trigonal bipyramidal, and distorted octahedral or square bipyramidal geometries. The tetrahedral coordination exhibits bond lengths in the range 1.60 to 2.0 Å. These bond lengths are distributed into short (ca. 1.6 to 1.7 Å) and longer distance (ca. 1.8 to 2.0 Å) depending on the terminal or bridging nature of the V–O bond [51].

The five-coordinate geometry displayed by V(V) reflects the number of vanadyl groups. Thus, when there is a single V=O bond, the 4+1 square pyramidal geometry is observed, while the presence of two short V=O bonds results is distorted trigonal bipyramidal 3 + 2 geometry, with the short vanadyl bonds occupying two equatorial positions and the longer bonds occupying one equatorial and two axial positions [51].

The six coordinate V(V) polyhedra shows 4+1+1 or 2 + 2 + 2 bond distributions, depending on the presence of one or two short vanadyl bonds, respectively. The 4 + 1 + 1 geometry is similar to that described above for tetravalent vanadium. The 2 + 2 + 2 geometry is defined by two short vanadyl bonds in a cis orientation, two long bonds (2.1–2.3 Å) trans to the vanadyl groups and two intermediate bond lengths (1.85–2.05 Å) cis to the V=O bonds [51].

Figure 3.1 Coordination polyhedra adopted by V (V)- and V (IV)- oxo species:

(a) tetrahedral, (b) “4 + 1” square pyramidal, (c) “3 + 2” trigonal bipyramidal,

(d) “4 + 1 + 1” octahedral, and (e) “2 + 2 + 2” octahedral.
Vanadium oxide chemistry is characterized by several general families of compounds: binary oxides, bronzes, and molecular polyanions. The chemistry of polyanions is vast and has been reviewed recently [51, 52]. Representative examples of vanadate cluster chemistry include divanadate, (V₂O₇)⁴⁻ [53], tetravanadate (V₄O₁₂)⁴⁻ [54] and decavanadate (V₁₀O₂₈)⁶⁻ [55], shown in Figure 3.2. (V₄O₁₂)⁴⁻ structure is a ring of corner sharing V(V) tetrahedra, the decavanadate is constructed from edge-sharing V(V) octahedra. Vanadium polyhedra in both the cluster chemistry and the solid state may show edge-, corner-, and face-sharing.

Metavanadates are characteristic oxides of vanadium which have the empirical formula VO₃⁻ such as KVO₃ and β-NaVO₃ (Figure 3.3). KVO₃ consists of a chain of corner-sharing tetrahedras. In contrast, the structure of β-NaVO3 consists of a double chain of edge-sharing trigonal bipyramids (2 + 3 geometry), formed by the fusion of two corner-sharing tetrahedral chains [51].

Figure 3.2. Polyhedral representations of the structures of the (a) [V₂O₇]⁴⁻ and (b) [V₄O₁₂]⁴⁻ clusters; (c) ball and stick and (d) polyhedral views of the structure of [V₁₀O₂₈]⁶⁻.




Figure 3.3 Views of the one-dimensional vanadate chains of (a) KVO₃ and (b) β-NaVO₃.
The structure of V₂O₅ exhibits characteristic features of two-dimensional vanadium oxides. As shown in Figure 3.4, the structure is constructed from both edge- and corner-sharing of [VO5] square pyramids. The double chains of edge-sharing polyhedra consist of a chain with the apical oxo group directed above the plane of network while the second chain exhibits oxo groups directed below the plane. The layers pack such that there are weak interlayer V–O interactions at 2.79 Å [51].

The title compound has been synthesized by Oka et.al using hydrothermal method with different starting materials in the past. They have applied hydrothermal synthesis for alkali-metal vanadium oxides by using various vanadium and alkali-metal sources. In their work, starting materials for growing AV₃O₈ crytsals were V₂O₅ powders and alkali-metal nitrate A(NO)₃ solutions for A= K, Rb, Cs, NH₄. V₂O₅ powders were obtained by the thermal oxidation of VO(OH)₂ powders in air, where VO(OH)₂ powders were prepared in advance by the hydrothermal treatment of VOSO₄-NaOH slurries. A suspension of 0.5g V₂O₅ powders in 0.2 mol/L ANO₃ solution was sealed in a Pyrex ampoule and treated hydrothermally in an autoclave at 250^oC for 48 h. Then, orange transparent crystals were separated by filtration [56].

In contrast to this work, KV₃O₈ crystals were hydrothermally grown at 170^oC for 3 days in our laboratory. In this thesis, our goal was to synthesize metal oxide compounds by using hydrothermal method. Here an alkali-metal trivanadate, KV₃O₈, has been synthesized in aqueous solution, structurally characterized, and some of its properties explained. In this section we report the synthesis and structural characterization of KV₃O₈.



Figure 3.4 The network structure of V₂O₅.