Thermal decomposition studies on the oxalates of lanthanum cerium and thorium

Abstract

The thermal decomposition of the hydrated oxalates of lanthanum, cerium and thorium have been studied by (i) thermogravimetry, (ii) differential thermal analysis, (iii) vapour pressure measurements and (iv) vacuum decomposition measurements and X?ray diffraction of the products have been used to supplement the results wherever they could be of any diagnostic value. The main aspects of the study are summarized below.

Hydrated oxalate samples were prepared by precipitation with oxalic acid from aqueous solutions of the salts of lanthanum, cerium and thorium. The normal compositions of the rare?earth oxalates were Ln?(C?O?)?·9H?O (Ln = La and Ce) and in the case of thorium the oxalate was a hexahydrate Th(C?O?)?·6H?O. The lower hydrates of lanthanum and thorium oxalates were obtained by suitable thermal treatment such as drying at 120°C in an air oven or dehydration in vacuum over phosphorus?pentoxide at temperatures higher than the room temperature. In general, a dihydrate of lanthanum and cerium oxalate was obtained by dehydration of the other hydrate in vacuum at 67°C (boiling point of water at this pressure).

Thermogravimetric results of hydrated oxalates of lanthanum, cerium and thorium in air indicate three steps of dehydration between 60°–180°C for the lower hydrate (nearly dihydrate). At about this temperature further dehydration slows down in the case of cerous oxalate, whereas lanthanum and thorium have another interhydrate stage around 260°C. Further loss of water and the decomposition of the oxalate appear to take place simultaneously. A complete decomposition of cerous oxalate occurred at 240°C giving a horizontal at 340°C; this residue corresponds to CeO?. In the case of thorium oxalate the decomposition starts at a temperature above 300°C and the horizontal level of ThO? is obtained around 520°C. But in the case of lanthanum oxalate an intermediate stage lanthanum basic carbonate, La?O(CO?)? is obtained at 400°C and the residue at 600°C corresponds to the lanthanum basic carbonate, La?O?CO?. The lower hydrates are found to behave in a similar way (except for the absence of initial dehydration steps).

In carbon dioxide atmosphere the dehydration of these oxalates are identical with those in air. However, the decomposition of these oxalates is very much slowed down probably because of characteristic inhibition of the decomposition of carbonates (or basic carbonates) formed during the decomposition.

Results obtained in differential thermal analysis of these oxalates show that dehydration occurs in stages before the decomposition of the oxalate. The endothermic peaks corresponding to the dehydration and decomposition of the oxalates are observed in each case. However, after the decomposition of the oxalate an exothermic effect is observed in all the three cases, the effect being well pronounced in the case of cerium and less pronounced in cases of thorium and lanthanum. The exothermic peak can be due to the burning off of elemental carbon produced during the decomposition of the oxalate.

The lower hydrates show very similar behaviour except for the absence of the earlier endothermic dehydration peaks. Activation energies corresponding to different stages of decomposition exhibited by various peaks have been calculated on the assumption that the dehydration and decomposition reactions are of first order. The activation energies of the first dehydration peaks of the higher hydrates are found to be 26.2, 26.7, 31.3 kcal/mole for La?(C?O?)?·9H?O, Ce?(C?O?)?·9H?O and Th(C?O?)?·6H?O respectively. The next dehydration peak gives a value of activation energy for lanthanum, cerium and thorium as 43.5, 64.4 kcal/mole respectively. The activation energies of the first peak of decomposition of the oxalates required higher activation energies and the corresponding activation values are obtained from the DTA curves of the lower hydrate.

Heating the samples at various temperatures for several hours in vacuum indicates that the decomposition starts at a lower temperature than that observed in thermogravimetric analysis, except for cerium which decomposes at a higher temperature. In the initial stages of decomposition the gaseous products liberated contain carbon monoxide and carbon dioxide in nearly equal proportions. With progressive increase in temperature the carbon dioxide content increases until the ratio CO?/CO reaches a value of ~20 at 400°–450°C. Beyond this temperature carbon monoxide increases again in the gaseous product until the ratio of CO?/CO becomes about 5.

In the case of cerium oxalate the ratio CO?/CO decreases further and around 600°C it is ~3. Thoria again shows different behaviour; the ratio CO?/CO is obtained as ~60 and no decrease is observed. The residue obtained after heating the samples in vacuum at a definite temperature until the decomposition became very slow was analysed for the oxalate and carbonate contents. It was found that the oxalate content decreased progressively with increase in the decomposition temperature whereas carbonate content increased up to a temperature of about 500°C and decreased thereafter. In the case of lanthanum, there was a considerable carbonate residue even at 600°C but cerium had very little carbonate content at 450°C and beyond (?0.17 mole Ce?O?/CO?). Thorium had very little carbonate even at 350°C. It has to be pointed out that in almost every case where the gaseous products were analysed, the total quantity of carbon monoxide found could not be accounted for 100% in terms of the decomposition products. This necessitates a reaction of carbon monoxide with oxygen giving rise to carbon dioxide and elemental carbon.

Isothermal vapour pressure characteristics of the higher hydrated oxalates, vapour pressure measurements of dehydrations and lower oxalates of lanthanum and cerium oxalates as well as isothermal dehydration?heats of thorium oxalates are obtained. The hydrated oxalates of lanthanum and cerium have very low vapour pressures at room temperature which gradually increase with temperature. They attained an equilibrium vapour pressure of water (at room temperature) at ~50°C and 54°C respectively. Thorium oxalate tetrahydrate attained an appreciable vapour pressure of 6 cm at room temperature. In this case also the vapour pressure increases with temperature attaining the saturation vapour pressure of water at ~57°C. The Clausius–Clapeyron relationship was found to be valid for hydration vapour pressure measurements and heats of dehydration were found to be 20.3, 15.2 kcal/mole for lanthanum and cerium respectively.

In order to find out whether any stable intermediate hydrate was formed during dehydration, isothermal dehydration was carried out with the higher hydrates. It was found that the vapour pressure decreased continuously with decrease in the amount of water in the oxalate without showing a constant vapour pressure stage. It could therefore be concluded that no intermediate hydrates of definite composition exist. This behaviour could be taken as characteristic of “interstitial hydrates” which in turn could be considered as solid solutions of water with oxalate.

The magnetic measurements of hydrates, cerous oxalate and the products of decomposition in vacuum show that the magnetic values increase with increase in the temperature treatment up to 400°C. Beyond 400°C the magnetic susceptibility increases indicating the oxidation of cerous to ceric state.

In conclusion, it may be said that the decomposition of hydrated oxalates shows loss of water initially in stages followed by decomposition. The decomposition of the oxalates gives rise to carbon monoxide and carbon dioxide in the gaseous phase, carbonate and oxide in the residue. The residue obtained after heating in vacuum usually contained some free carbon. The presence of free carbon is well understood through reactions which follow:

(i) Oxidation of carbon monoxide giving rise to carbon dioxide and elemental carbon, (ii) Formation of free carbon monoxide and oxygen (during the decomposition) which combine afterwards to give carbon dioxide and carbon, and (iii) Carbon may decompose leaving elemental carbon on the surface.