7 junio, 2024

Cobalt hydroxide: what it is, chemical structure, properties and uses

What is cobalt hydroxide?

He cobalt hydroxide is the generic name for all compounds involving cobalt cations and the OH– anion. All are inorganic in nature, and have the chemical formula Co(OH)n, where n is equal to the valence or positive charge of the cobalt metal center.

Since cobalt is a transition metal with half-filled atomic orbitals, through some electronic mechanism its hydroxides reflect intense colors due to Co-O interactions, which can be deep pink or bluish-green.

These colors, as well as the structures, depend highly on their charge and on the anionic species that compete with the OH–.

The colors and structures are not the same for Co(OH)2, Co(OH)3 or CoO(OH). The chemistry behind all these compounds is intended for the synthesis of materials applied to catalysis.

On the other hand, although they can be complex, the formation of a large part of them starts from a basic medium, such as that supplied by the strong base NaOH. Hence, different chemical conditions can oxidize cobalt or oxygen.

Chemical structure of cobalt hydroxide

Its general formula Co(OH)n is ionically interpreted as follows: in a crystal lattice occupied by a number of Con+, there will be n times that amount of OH– anions interacting with them electrostatically. Thus, for Co(OH)2 there will be two OH– for each Co2+ cation.

But this is not enough to predict which is the crystalline system that these ions will adopt. By reasoning of coulombic forces, Co3+ attracts OH– more intensely than Co2+.

This fact causes the distances or the Co–OH bond (even with its high ionic character) to shorten. Also, because the interactions are stronger, the electrons in the outer layers of the Co3+ undergo an energetic change that forces them to absorb photons with different wavelengths (the solid darkens).

However, this approach is insufficient to clarify the phenomenon of changing their colors depending on the structure.

The same is true for cobalt oxyhydroxide. Its formula CoO OH is interpreted as a Co3+ cation interacting with an oxide anion, O2–, and an OH–. This compound represents the basis for synthesizing a mixed cobalt oxide: Co3O4 [CoO·Co2O3].

Covalent

Cobalt hydroxides can also be visualized, although less precisely, as individual molecules. Co(OH)2 can then be drawn as a linear OH–Co–OH molecule, and Co(OH)3 as a flat triangle.

Regarding CoO(OH), its molecule from this approach would be drawn as O=Co–OH. The O2– anion forms a double bond with the cobalt atom, and another single bond with OH–.

However, the interactions between these molecules are not strong enough to “assemble” the complex structures of these hydroxides. For example, Co(OH)2 can form two polymer structures: alpha and beta.

Both are laminar but with different arrangements of the units, and are also capable of inserting small anions, such as CO32–, between their layers; which is of great interest for the design of new materials from cobalt hydroxides.

Coordination units

The polymeric structures can be better explained by considering a coordination octahedron around the cobalt centers. For Co(OH)2, since it has two OH– anions interacting with Co2+, you need four water molecules (if aqueous NaOH was used) to complete the octahedron.

Thus, Co(OH)2 is actually Co(H2O)4(OH)2. For this octahedron to form polymers, it needs to be linked by oxygen bridges: (OH)(H2O)4Co–O–Co(H2O)4(OH). The structural complexity increases for the case of CoO(OH), and even more for Co(OH)3.

Cobalt hydroxide properties

Cobalt(II) hydroxide

-Formula: Co(OH)2.

-Molar mass: 92.948 g/mol.

-Appearance: pinkish-red powder or red powder. There is an unstable blue form with the formula α-Co(OH)2

-Density: 3.597 g/cm3.

-Solubility in water: 3.2 mg/l (slightly soluble).

-Soluble in acids and ammonium. Insoluble in dilute alkali.

-Melting point: 168° C.

-Sensitivity: sensitive to air.

-Stability: it is stable.

Cobalt(III) hydroxide

-Formula: Co(OH)3

-Molecular mass: 112.98 g/mol.

-Appearance: two shapes. A stable black-brown form and an unstable dark green form with a tendency to darken.

Cobalt hydroxide production

The addition of potassium hydroxide to a cobalt(II) nitrate solution results in the appearance of a blue-violet precipitate that, when heated, becomes Co(OH)2, that is, cobalt hydroxide ( II).

Co(OH)2 precipitates when an alkali metal hydroxide is added to an aqueous solution of a Co2+ salt.

Co2+ + 2 NaOH => Co(OH)2 + 2 Na+

Cobalt Hydroxide Uses

-It is used in the preparation of catalysts for use in oil refining and in the petrochemical industry. In addition, Co(OH)2 is used in the preparation of cobalt salts.

-Cobalt (II) hydroxide is used in the manufacture of paint dryers and in the manufacture of battery electrodes.

Synthesis of nanomaterials

-Cobalt hydroxides are the raw material for the synthesis of nanomaterials with novel structures. For example, nanoflakes of this compound have been designed from Co(OH)2, with a large surface area to participate as a catalyst in oxidative reactions.

These nanoflakes are impregnated on porous crystalline carbon or nickel electrodes.

-It has been sought to implement nanobars of carbonate hydroxides with carbonate intercalated in their layers. They take advantage of the oxidative reaction of Co2+ to Co3+, proving to be a material with potential electrochemical applications.

-Studies have synthesized and characterized, using microscopy techniques, nanodiscs of mixed cobalt oxide and oxyhydroxide, from the oxidation of the corresponding hydroxides at low temperatures.

Bars, disks and cobalt hydroxide flakes with structures at nanometric scales open the doors to improvements in the world of catalysis and also in all applications concerning electrochemistry and the maximum use of electrical energy in modern devices.

References

Clark J. (2015). Cobalt. Taken from: chemguide.co.uk
Wikipedia (2018). Cobalt(II) hydroxide. Taken from: en.wikipedia.org
PubChem (2018). Cobaltic. Hydroxide. Taken from: pubchem.ncbi.nlm.nih.gov
Rovetta AAS et al. (2017). Cobalt hydroxide nanoflakes and their application as supercapacitors and oxygen evolution catalysts. Retrieved from: ncbi.nlm.nih.gov
D. Wu, S. Liu, SM Yao, and XP Gao. (2008). Electrochemical Performance of Cobalt Hydroxide Carbonate Nanorods. Electrochemical and Solid-State Letters, 11 12 A215-A218.
Jing Yang, Hongwei Liu, Wayde N. Martens, and Ray L. Frost. (2010). Synthesis and Characterization of Cobalt Hydroxide, Cobalt Oxyhydroxide, and Cobalt Oxide Nanodiscs. Retrieved from: pubs.acs.org

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