Neptunium(IV) oxide or neptunium dioxide is a chemical compound with the chemical formula NpO2, composed of neptunium and oxygen. Solid neptunium(IV) oxide is one of two solid neptunium oxides, the other one being neptunium(V) oxide. It is synthesized in a variety of ways, most commonly by the chemical decomposition of other neptunium compounds (usually neptunium(IV) oxalate, Np(C2O4)2), but it is also produced by a process called modified direct denitration (MDD), which involves the calcination of a neptunium-containing aqueous solution. Environmentally, it can be formed by the hydrolysis of neptunium when it is in the +4 oxidation state, making it a relevant form of neptunium in the environment. In an oxidizing environment, it dissolves in water, being first converted into a mixed oxide-hydroxide phase which is then oxidized, releasing the neptunium as neptunyl(V) ions (NpO+2). During dissolution, parts of the neptunium(IV) oxide solid are also broken off. Neptunium(IV) oxide shows both a hypostoichiometric (which has less oxygen than in the chemical formula, represented as NpO2−x) and a hyperstoichiometric (which has more oxygen than in the chemical formula, represented as NpO2+x but more accurately NpO2+x−y(OH)y·zH2O) phase. Stoichiometric neptunium(IV) oxide (exactly NpO2) shows a fluorite structure, and has Np4+ and O2− ions. Compared to this, the hypostoichiometric phase shows oxygen vacancies and existence of Np3+ ions, and the hyperstoichiometric phase shows oxidation of neptunium to the +5 oxidation state and creation of oxo and hydroxide groups. At low temperature, it transforms into a complex phase whose existence is explained by the formation of magnetic octupoles. Neptunium(IV) oxide reacts with a variety of chemical compounds to produce neptunium halides. Two of these compounds, neptunium(IV) chloride (NpCl4) and bis(dimethoxyethane)neptunium tetrachloride (NpCl4(DME)2), are used as starting materials in neptunium chemistry. Another use of neptunium(IV) oxide is for the production of plutonium-238 as a heat source for radioisotope thermoelectric generators, used for deep space exploration. Neptunium(IV) oxide can also be incorporated into nuclear fuel like mixed oxide (MOX) fuel, or be used as a stable form of neptunium in nuclear waste or storage. If it contains neptunium-237, the most stable and chemically important form of neptunium, it is radioactive by emitting alpha particles and gamma rays.
Synthesis
From oxalate
The oxalate route is the main method of NpO2 production, originating in the 1960s. In it, neptunium(IV) oxalate, Np(C2O4)2, is prepared through a two-stage precipitation method. Production of the oxalate starts with a nitric acid (HNO3) solution, which can be prepared via ion exchange. Either hydrazine (N2H4) or hydrazinium nitrate (N2H5NO3) is added to stabilize neptunium's +4 oxidation state, and either ascorbic acid (C6H6O6) or iron(II) sulfamate (Fe(NH2SO3)2) is added to reduce any neptunium to the +4 state. This is necessary, as presence of higher oxidation states, namely the +5 oxidation state, reduces the amount of neptunium filtered out. At room temperature and low nitric acid concentrations, ascorbic acid is a slow reducing agent, so the reduction is done at either elevated temperatures (~50 °C) or high concentrations of nitric acid (>4 M). Addition of oxalic acid to the nitric acid solution precipitates neptunium(IV) oxalate (specifically the hexahydrate, Np(C2O4)2·6H2O), which is then dried in air.
2 NpO+2 + 6 H+ + C6H8O6 → 2 Np4+ + C6H6O6 + 4 H2O (ascorbic acid reduction) NpO+2 + Fe2+ + 4 H+ → Np4+ + Fe3+ + 2 H2O (iron(II) sulfamate reduction) Np4+ + 2 H2C2O4 + 6 H2O → Np(C2O4)2·6H2O + 4 H+ Neptunium(IV) oxide is prepared from the oxalate through thermal decomposition. First, the neptunium(IV) oxalate is heated in a stream of nitrogen or air from room temperature to 150 °C over a 1 hour period. Afterwards, the temperature is increased. Heating to 500–550 °C provides neptunium(IV) oxide of satisfactory quality, but temperatures between 400 °C and 900 °C will yield pure neptunium(IV) oxide as well. During decomposition, neptunium(IV) oxalate hexahydrate first loses water between 80 and 200 °C to produce the anhydrous form (Np(C2O4)2). Np(C2O4)2 decomposes further at higher temperatures, first mainly to neptunyl(V) oxalate at 270 °C and eventually to the oxide at even higher temperatures.
Np(C2O4)2·6H2O → Np(C2O4)2·2H2O + 4 H2O Np(C2O4)2·2H2O → Np(C2O4)2·H2O + H2O Np(C2O4)2·H2O → Np(C2O4)2 + H2O 2 Np(C2O4)2 + 2 O2 → (NpO2)2C2O4 + 6 CO2 (NpO2)2C2O4 → 2 NpO2 + 2 CO2
In modified direct denitration Oak Ridge National Laboratory produces neptunium(IV) oxide using a process called modified direct denitration (MDD). This process starts from a solution of neptunium nitrate, where neptunium is in the +5 oxidation state. Ammonium nitrate (NH4NO3) is added such that there is a 2.5:1 ratio of ammonium to neptunium. The resulting solution is fed into a rotary kiln and heated to 675 °C. This produces a mixture of neptunium(IV) oxide and neptunium(V) oxide (Np2O5), which is then heated further to 1185 °C to ensure complete conversion to neptunium(IV) oxide.
Other methods Neptunium(IV) oxide can be produced through the direct denitration (DD) process. In this process, a solution containing neptunium, purified by ion exchange, is partially dried and calcined in a furnace to produce the NpO2 product. In addition, heating various different neptunium(IV), neptunium(V), or neptunium(VI) compounds, like hydroxides, nitrates, or oxalates, at 600–1000 °C causes them to decompose to produce neptunium(IV) oxide. One such compound is neptunyl ammonium nitrate (NH4NpO2(NO3)3), prepared by the evaporation of a nitric acid solution containing neptunium and ammonium ions. A preparation method of neptunium(IV) oxide has been reported through synthesis of neptunium(IV) peroxide. After the nitric acid solution is prepared through ion exchange, and hydrazine is added, as in the oxalate route, hydrogen peroxide (H2O2) precipitates the peroxide. Hydrogen peroxide rapidly reduces neptunium to the +4 oxidation state, so there is no need to use reducing agents like ascorbic acid. This method offers less purification from impurities and is more sensitive than the oxalate method, so the oxalate method is preferred to it.
… excerpt ends here. Continue reading the full article.
