Crystal structure of cobalt molybdate hydrate CoMoO4·nH2O

doi:10.1016/j.jssc.2005.06.014

Journal of Solid State Chemistry

Volume 178, Issue 9, September 2005, Pages 2791-2797

https://doi.org/10.1016/j.jssc.2005.06.014 Get rights and content

Abstract

We have determined the crystal structure of the title compound, which has a triclinic cell with cell parameters of $a = 6.844 Å$ , $b = 6.933 Å$ , $c = 9.339 Å$ , α=76.617°, β=84.188°, γ=74.510° and space group $P \bar{1}$ . The crystal structure suggests the chemical formula CoMoO₄·3/4H₂O. The structure consists of MoO₄ tetrahedra and CoO₆ octahedra, confirming the earlier X-ray absorption near-edge spectroscopic (XANES) investigation on the hydrate. The comparison of the crystal structures of the hydrate and the α-,β-, and hp-phases shows that the hydrate exhibits metal cation coordinations similar to those of the β-phase, but had arrangements of CoO₆ and MoO_n polyhedra similar to those of the hp-phase.

Graphical abstract

Introduction

Cobalt molybdates are attractive compounds because of their structural, magnetic, and catalytic properties [1], [2], [3], [4], [5], [6], [7], [8], [9]. They are especially important components of industrial catalysts. Their catalytic properties are closely related to their structure [5], [7]. Four compounds with the CoMoO₄ stoichiometry are known: the low temperature α-phase (pale green, space group: C2/m; $a = 9.67$ , $b = 8.85$ , $c = 7.76 Å$ , β=113.49°) [10], the high temperature β-phase (pale violet, C2/m; $a = 10.21$ , $b = 9.31$ , $c = 7.01 Å$ , β=106.4°) [2], [11], the high-pressure (hp-) phase (black, P2/c, $a = 4.6598$ , $b = 5.6862$ , $c = 4.9159 Å$ , β=90.521°) [4], and the hydrate (violet, crystal structure: unknown up to now). The hydrate has been known to directly transform into the β-phase when it entirely loses its crystallization water at around 600 K. This transformation occurs at much lower temperature than the transition from α- to β-phase (ca. 750), and is probably facilitated by the structural similarities of the hydrate and β-phase [8]. Recently, we have found another direct transformation of the hydrate into the hp-phase under hydrothermal conditions, which transformation will be presented elsewhere. In order to understand not only these transformations but also their catalytic or physical properties, comparison of their structures is necessary. The crystal structures of three out of the four compounds (except the hydrate) are known as mentioned above. It is only suggested that the hydrate consists of CoO₆ and MoO₄ polyhedra according to X-ray absorption near-edge spectroscopic (XANES) investigation [8].

In order to reveal the structure of the hydrate, we succeeded in preparing a single crystal of it. The single crystal analysis confirmed that the hydrate consists of CoO₆ and MoO₄ polyhedra, and the polyhedra in the hydrate are connected similarly to that in the hp-phase rather than those in α- and β-phases. In the present paper we will present the structure of the hydrate and compare it with the structures of the CoMoO₄ phases.

Section snippets

Preparation of the hydrate

0.720 g of MoO₃ (5 mmol) and 0.375 g of CoO (5 mmol), were suspended in 30 ml of distilled water. 0.292 g of NaCl (5 mmol) was added to the solution in order to increase the ionic strength of the solution and promote crystal growth of the hydrate during the hydrothermal treatment. Then the solution was put into a 60 ml Teflon-lined autoclave and heated in a forced convection oven at 453 K under autogenous pressure (ca. 10 atm) for 1 day. The resulting product was filtered, washed with distilled water,

Crystal structures of the hydrate

In order to determine the crystal structure of the hydrate, some crystals obtained were used for collecting diffraction data. All crystals used exhibited non-merohedral twinning, which rotated the reciprocal axes 180° about the $a^{*} + c^{*}$ axis. Two distinct cell sets were separated using RLATT, identified using SMART (Bruker, 2001) and integrated using SAINT (Bruker, 2001). The combined data were refined with separate scale factors using SHELXTL. The resulting crystal data and refinement for the

Acknowledgments

The authors thank Dr. Masao Hashimoto for his valuable discussion. The work at Binghamton was supported by the National Science Foundation Grant 0313963.

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Crystal structure of cobalt molybdate hydrate CoMoO4·nH2O

Abstract

Graphical abstract

Introduction

Section snippets

Preparation of the hydrate

Crystal structures of the hydrate

Acknowledgments

J. Magn. Magn. Mater.

J. Catal.

J. Catal.

Science

Inorg. Chem.

J. Mater. Chem.

Catal. Lett.

Crystal structure of cobalt molybdate hydrate CoMoO₄·nH₂O