Introduction
Rhenium(IV) oxide (ReO2), or also known as rhenium dioxide, is a binary transition metal oxide with the lowest oxidation state (+4) of the different rhenium oxides. In normal conditions this specific oxide is in the form of an odorless gray-black solid [1][2], which is significantly different from the other colorful oxides that rhenium forms. ReO2 has very minor safety hazards[2] which has caused it to be mainly used as a catalyst or addition to superconducting ceramics[3].
Synthesis
There are a couple of ways to synthesize rhenium(IV) oxide.
The first way is through comproportionating, where the oxide is formed from two different rhenium compounds that have different oxidation states. The reaction can be seen in the following equation:
2 Re2O7 + 3 Re → 7 ReO2[4]
Single crystals can also be synthesized from chemical transport by utilizing iodine in the following reaction[5]:
ReO2 + I2 ⇌ ReO2I2
Another very common way of synthesis is using perrhenate compounds and acids. The solutions are bought to their boiling points and rhenium(IV) oxide or rhenium metal can be extracted through filtration as a result [1][5].
Structure
Rhenium(IV) oxide has three different structures that can be formed under different conditions. The first is a tetragonal phase with a rutile (TiO2)-type structure. The second is an orthorhombic phase with a PbO2-type structure. The last is a monoclinic phase with a MoO2-type structure. The monoclinic and orthorhombic structures have been known for a long time, while the rutile structure has been a more recent discovery. The monoclinic and orthorhombic structure have been known since around the 1960's, but the rutile structure was discovered in 2005.[3]
The monoclinic structure is metastable and transforms to the orthorhombic structure above 460ºC. [5][6]
The rutile structure formation requires annealing at 420ºC under an argon atmosphere to form[3]. Below are pictures of each of the different crystal structures.
Figure 1. Crystal structure of tetragonal ReO2 (Figure: Niklas Suominen. Data from ICSD (154021), visualized with Vesta).
Figure 2. Crystal structure of orthorhombic ReO2 (Figure: Niklas Suominen. Data from ICSD (24060), visualized with Vesta).
Figure 3. Crystal structure of monoclinic ReO2 (Figure: Niklas Suominen. Data from ICSD (151412), visualized with Vesta).
Properties
Historically the main application of rhenium(IV) oxide has been a catalyst in oxidation reactions [7]. In addition, it has been used as an additional component in superconducting ceramic fabrication[8]. Rhenium(IV) oxide can also be used to protect underlying dielectric and barrier layers or ferroelectric capacitors as some examples[3]. These applications have been known for a while now and new applications haven't really been found for this specific oxide.
Rhenium(IV) oxide isn't water soluble and it doesn't have hygroscopic properties, which is a reason why it has been used to protect dielectric layers as mentioned previously[3]. Below are the XRD patterns for all of the different crystal structures that rhenium(IV) oxide exhibits.
Figure 4. XRD patterns for the rutile crystal structure. (Data from ICSD (154021)).
Figure 5. XRD patterns for the orthorhombic crystal structure. (Data from ICSD (24060)).
Figure 6. XRD patterns for the monoclinic crystal structure. (Data from ICSD (151412)).
References
"Rhenium oxide" American Elements, 25.2.2023. https://www.americanelements.com/rhenium-oxide-12036-09-8 "Rhenium(IV) oxide" PubChem Identifier: CID 82847. https://pubchem.ncbi.nlm.nih.gov/compound/Rhenium_IV_-oxide A. L. Ivanovskii, T. I. Chupakhina, V. G. Zubkov, A.P. Tyutyunnik, V.N. Krasilnikov, G.V. Bazuev, S. V. Okatov, A. I. Lichenstein, Structure and electronic properties of new rutile-like rhenium (IV) dioxide ReO2, Physics Letters A, 2005, 348, 66-70 (https://doi.org/10.1016/j.physleta.2005.08.025) G. Glemser, Handbook of Preparative Inorganic Chemistry, Academic Press, New York, 1963. C. L. Rulfs, R. J. Meyer, Rhenium(IV) compounds: Synthesis and Properties. Analytical Chemistry. 1955, 77, 4505-4507 (https://doi.org/10.1021/ja01622a018) H. P. S. Corrêa, I. P. Cavalcante, L. G. Martinez, C. G. P. Orlando, M. T. D. Orlando, Refinement of monoclinic ReO2 structure from XRD by Rietveld method, Brazilian Journal of Physics, 2004, 34 (https://doi.org/10.1590/S0103-97332004000600013) Y. H. Yuan, Y. Iwasawa, Performance and Characterization of Supported Rhenium Oxide Catalysts for Selective Oxidation of Methanol to Methylal, The Journal of Physical Chemistry B, 2002, 106, 4441-4449 (https://doi.org/10.1021/jp013770l) S. Piñol, A. Sin, A. Calleja, J. Fontcuberta, X. Obradors, Synthesis of Hg1−XRexBa2Ca2Cu3O8+x Pure Phase at Normal Pressures, Journal of Superconductivity, 1998, 11, 125-126 (https://doi.org/10.1023/A:1022679206829)
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Table 1. Properties of ReO2
Properties of ReO2 | |
|---|---|
| Molar mass | 218.206 g/mol |
| Density | 11.4 g/cm3 |
| Melting point | 1000ºC (decomposition) |
| Magnetic susceptibility | +44.0·10−6 cm3/mol |
| Crystal structure | Crystal class | Space group | Lattice parameters (Å) | Formula units |
|---|---|---|---|---|
| Rutile | Tetragonal | P42/mnm | a = b = 4.79825 c = 2.80770 | Z = 2 |
Orthorhombic | Orthorhombic | Pbcn | a = 4.8094 b = 5.6433 c = 4.6007 | Z = 4 |
| Monoclinic | Monoclinic | P121/c1 | a = 5.615 b = 4.782 c = 5.574 | Z = 4 |