| Names | |
|---|---|
| IUPAC name Praseodymium(III,IV) oxide | |
| Identifiers | |
3D model (JSmol) | |
| ChemSpider | |
| ECHA InfoCard | 100.031.676 |
| EC Number |
|
| |
| |
| Properties | |
| Pr6O11 | |
| Molar mass | 1021.44 g/mol |
| Appearance | dark brown powder |
| Density | 6.5 g/mL |
| Melting point | 2,183 °C (3,961 °F; 2,456 K).[1] |
| Boiling point | 3,760 °C (6,800 °F; 4,030 K)[1] |
| Hazards | |
| GHS labelling: | |
 | |
| Warning | |
| H315,H319,H335 | |
| P261,P305+P351+P338 | |
| Lethal dose or concentration (LD, LC): | |
LD50 (median dose) | 5000 mg·kg−1 Rat oral |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Praseodymium(III,IV) oxide is the inorganic compound with the formulaPr6O11 that is insoluble in water.[2] It has acubicfluorite structure.[3] It is the most stable form of praseodymium oxide atambient temperature and pressure.[4]
Pr6O11 adopts a cubicfluorite crystal structure, measured byXRD,TEM andSEM methods.[3][5] It can be considered an oxygen deficient form ofpraseodymium(IV) oxide (PrO2), with the Pr ions being in a mixedvalency state Pr(III) and Pr(IV).[5] This characteristic is what gives the oxide its many useful properties for itscatalytic activity.
Praseodymium oxide nanoparticles are generally produced via solid-state methods such as thermolysis, molten salt method,calcination orprecipitation.[3][4][6] Practically all processes, however, contain a calcination step in order to obtain a crystallinePr6O11nanoparticles.
Typically, praseodymium nitratePr(NO3)3·6H2O[3][5] or praseodymium hydroxidePr(OH)3[7] is heated at high temperatures (usually above 500 °C) under air to give praseodymium(III,IV) oxide. While less common, synthesis from other organic precursors such as praseodymium acetate, oxalate[8] and malonate[9] have also been reported in chemical literature.
The physical properties of the prepared nanoparticles such as particle shape orlattice parameter depend strongly on the conditions of calcination, such as the temperature or duration, as well as the different preparation methods (calcination,sol-gel,precipitation, for example). As a result, many synthesis routes have been explored to obtain the precise morphology desired.[3][4][5]
Praseodymium(III,IV) oxide has a number of potential applications in chemicalcatalysis, and is often used in conjunction with a promoter such assodium orgold to improve its catalytic performance. It has a high-K dielectric constant of around 30 and very low leakage currents[10] which have also made it a promising material for many potential applications in nanodevices and microelectronics.[6]
Sodium orlithium promoted praseodymium(III,IV) oxide displays good conversion rate ofmethane with a good selectivity towardsethane andethene as opposed to unwanted byproducts such ascarbon dioxide.[11][12] While the precise mechanism for this reaction is still under debate, it has been proposed that typically, methane is activated to a methylradical by oxygen on the surface of the catalyst which combines to form ethane. Ethene is then formed byreduction of ethane either by the catalyst or spontaneously. The multipleoxidation states of Pr(III) and Pr(IV) allows rapid regeneration of the active catalyst species involving a peroxide anionO2−2.[11]
This reaction is of particular interest as it enables the conversion of abundant methane gas (composing up to 60% ofnatural gas)[11][12] into higher orderhydrocarbons, which provide more applications. As a result, the oxidative coupling of methane is an economically desirable process.
In the proposed mechanism forPr6O11–catalysedoxidation ofCO toCO2, CO first binds to the catalyst surface to create abidentate carbonate then converted to a monodentate carbonate species which can decompose asCO2, completing the catalyst cycle. The conversion of a bidentate carbonate to a monodentate species leaves an oxygen vacancy on the catalyst surface which can quickly be filled due to the high oxygen mobility deriving from the mixed oxidation states of Pr centres. This proposed mechanism is presented schematically below, adapted from Borchert, et al.[5]
Addition of gold promoters to the catalyst may significantly lower the reaction temperature from 550 °C to 140 °C, but the mechanism is yet to be discovered. It is believed that there is a certain synergistic effect between gold and praseodymium(III,IV) oxide species.[13]
The interest in CO oxidation lies in its ability to convert toxic CO gas to non-toxicCO2 and has applications in car exhaust, for example, which emits CO.[14]
Pr6O11 is also used in conjunction with other additives such assilica orzircon to produce pigments for use in ceramics and glass[15]