Thorium formate, decomposition

The kinetics of the decomposition [17] of thorium tetraformate to ThOj can be described by the Prout-Tompkins equation with = 150 kJ mol" from 498 to 553 K. The autocatalytic process was ascribed to participation of the oxide in breakdown of the carboxyl groups at the reaction interface to yield ThOj, formaldehyde and carbon dioxide as the primary products of reaction. The volatile products could, however, react further on the surface of the active solid to yield a number of secondary products amongst which the following gases were identified Hj, CO, HjO, CHjOH, HCOOCHj, HCOOH and (CHj). Addition of nickel formate to the reactant not only accelerated decomposition but also influenced the composition of the gases evolved, yielding predominantly CO, COj and H2 (which are the main products of nickel formate decomposition). 

The decomposition products identified following reaction are not necessarily the primary compounds which result directly from the rate limiting step. Particularly reactive entities may rapidly rearrange before leaving the reaction interface and secondary processes may occur on the surfaces of the residual material which often possesses catalytic properties. The volatile products identified [144] from the decomposition of nickel formate were changed when these were rapidly removed from the site of reaction. The primary products of decomposition of thorium formate were identified [17] as formaldehyde and carbon dioxide, but secondary processes occurring on the residual thoria yielded several additional compounds. The oxide product similarly catalysed interactions between the primary products of decomposition of zinc acetate [145]. During the decomposition of rare earth oxalates, carbon monoxide disproportionates extensively to carbon dioxide and carbon [81,82]. 

A third method for the preparation of slurry oxide is the thermal decomposition of thorium formate [28]. In this procedure, thorium nitrate in solution is decomposed on adding it to concentrated formic acid at 95°C [29,30]. The precipitated thorium formate is washed free of excess acid and decomposed by calcination at 500 to 800 C. The oxide from the formate procedure is similar in its slurry behavior to that produced by thorium oxalate thermal decomposition however, less is known about its handling characteristics. Because of this, the oxalate preparation method is preferred at the present time. 

The old method of heating the calcium salts of formic and a second carboxylic acid for aldehyde formation has been modified by the use of a catalytic decomposition technique. By this scheme, the acid vapors are passed over thorium oxide, titanium oxide, or magnesium oxide at 300° or the acids are heated under pressure at 260° in the presence of titanium dioxide. In the latter procedure, non-volatile acids can be used. With aliphatic acids over titanium oxide, reaction occurs only when more than seven carbon atoms are present, the yields increasing with increase in the molecular weight (78-90%). Aromatic-acids having halo and phenolic groups are converted in high yields to aldehydes, e.g., salicylaldehyde (92%) and p-chlorobenzaldehyde (8S>%). Preparation of a thorium oxide catalyst has been described (cf. method 186). 

Pichot et al. (2001) conducted a preliminary study of irradiation effects on thorium phosphate-diphosphate. Powdered samples were irradiated with 1.5 Gy dose of gamma-rays. The formation of PO b and POO free radicals were detected using electron spin resonance (ESR) and thermoluminescence (TL) methods. These free radicals do not modify the macroscopic properties of the TPD and disappear when the sample is heated at 400°C. The implantation of 1.6 MeV He with a fluence of 10 ions/cm and 5 meV Au with a fluence 4 x 10 ions/cm causes some surface damage to sintered samples. Amorphization and chemical decomposition of the matrix were observed for the dose of 10 ions/cm and higher when irradiated with Pb (200 keV) and Au " (5 MeV). These effects were evidenced by means of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). 

Hayek et al. [1951HAY/REH] obtained products with compositions close to ThCl2 and ThCh from the reaction of thorium metal with gaseous chlorine in stoichiometric quantities while Jantsch and Homayr [1954JAN/HOM] reported the formation of ThCls as a result of the reduction of ThCh with aluminium and also, which is more surprising, from the thermal decomposition of ThCh at 673 K. As noted by Rand [1975RAN], contamination by oxygen and silica may have been serious in these experiments. 

The following enthalpies of formation for the two thorium compounds are given without any details of the experimental results or method of calculation (ThOS, cr, 298.15 K) = -(862 +26) kJ-mol and (ThS2, cr, 298.15 K) = - (816 + 24) kJ-mor. However, the value of Af/7° (ThS2, cr) is almost certainly too negative, since these values would indicate that the enthalpy of the decomposition reaction ... 

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