Thermal decomposition studies on the exalate of uranium(IV), praseodymium and neodynium

Abstract

Thermogravimetric Analysis: The decahydrate on heating gives rise to a monohydrate, which loses its last mole of water on further heating to give the anhydrous oxalate that is quite stable in the range 350–400°C. The oxalate on further heating decomposes to give a basic carbonate 2Pr2O3CO3 2Pr2O3CO3 which decomposes slowly to give the oxide Pr6O11Pr6O11 as residue beyond 750°C.

In vacuum, the decomposition starts earlier. The decomposition thereafter is fairly slow compared to that in air, and the residue at 600°C corresponds to 2Pr2O3CO32Pr2

Differential Thermal Analysis: During the dehydration, stages of formation of the monohydrate and the anhydrous oxalate are clearly indicated by well-defined endothermic peaks. The decomposition of the oxalate is indicated by a slight shoulder and a small endothermic peak. However, a pronounced exothermic peak after the decomposition is observed, similar to the decompositions of other oxalates, which is attributed to the oxidation of the lower oxide of the metal to the higher oxide of the metal and to the burning off of carbon monoxide and elemental carbon produced during decomposition. There is another endothermic peak which is attributed to the decomposition of the basic carbonate of praseodymium.

Decomposition in Vacuum: The decomposition of praseodymium oxalate in vacuum commences at 300°C. At 300°C, in the initial stages, it gives almost equal volumes of carbon dioxide and carbon monoxide. The destruction of the oxalate is followed by the formation of a basic carbonate and evolution of carbon monoxide and carbon dioxide. At higher temperatures, there is a larger proportion of carbon dioxide compared to that of carbon monoxide evolved, and the residue is found to contain a basic carbonate and elemental carbon. This is explained as due to dismutation of carbon monoxide evolved into carbon dioxide and elemental carbon. This increases the proportion of carbon dioxide in the gaseous products.

The surface area of the crystalline hydrate is very low, and with the destruction of the oxalate, there is a large increase in its surface area.

Thermogravimetric Analysis (TGA) of Neodymium Oxalate: The decahydrate of neodymium oxalate, when heated, gives rise to a trihydrate, and with further loss of water produces the anhydrous oxalate, which appears to be stable between 300–350°C. On further heating, it decomposes to give a basic carbonate of the formula Nd2O3CO3Nd2

which decomposes beyond 600°C slowly to give the oxide Nd2O3Nd2

In an atmosphere of carbon dioxide, the dehydration of the decahydrate of the oxalate was similar to that in air. However, the decomposition of the oxalate is slower than in air, and a fairly stable basic carbonate Nd2O3CO3Nd2 is formed at 900°C.

The decomposition is found to be incomplete even in vacuum. Dehydration commences with an increase in temperature, and formation of intermediate hydrates is not indicated. Anhydrous oxalate is formed at 325°C, and decomposition of the oxalate commences earlier than in air. The residue at 600°C corresponds to the basic carbonate Nd2O3CO3Nd2

Differential Thermal Analysis (DTA): The different stages of dehydration of the decahydrate with the formation of trihydrate and anhydrous oxalate are clearly indicated by well-defined endothermic peaks. The decomposition of the oxalate is also shown as an endothermic peak but is soon followed by a prominent exothermic peak, similar to the decomposition of other oxalates. This exothermic peak is ascribed to the burning off of carbon monoxide and elemental carbon produced during decomposition of the oxalate. Another endothermic peak follows the exothermic peak and is attributed to the decomposition of the basic carbonate formed during decomposition of the oxalate.

Decomposition in Vacuum: The decomposition of neodymium oxalate in vacuum commences as early as 280°C. At 280°C, the volume of carbon monoxide is nearly 1½ times the volume of carbon dioxide evolved. With increase in temperature, the evolution of carbon dioxide is larger in proportion compared to that of carbon monoxide. At 450°C, the volume of carbon dioxide evolved is nearly three times that of carbon monoxide. The residue at 600°C is found to be a basic carbonate of neodymium mixed with a small quantity of elemental carbon. The formation of this elemental carbon is explained as due to the dismutation of carbon monoxide evolved into elemental carbon and carbon dioxide.

The surface area of the hydrated oxalate is very low, and with the destruction of the oxalate, there is a large increase in surface area.