Activation energy of thermal decomposition of fully dehydrated NdC2O4Cl·3H2O to NdOCl

Abstract

An activation energy Ea = 195 kJ mol-1 has been determined for the thermal decomposition of NdC204C1 to NdOCI with simultaneous expulsion of CO and CO2. As compared with the value (127 kJ mo1-1) predicted by the Kahwa-Mulokozi empirical expression (i) + for Ln2(C204)3 (Ln = light lanthanide), the presence of C1-in the coordination polyhedron appreciably raises the activation energy, which is consistent with a corresponding increase in the covalent strength of the Ln-oxalate bond. The same conclusion regarding the strength of this bond is drawn from a study of the infrared spectrum.Mechanistically, the breaking of the Ln-oxalate bond is suggested to occur in the kinetic ratedetermining step.The thermal decomposition behaviour of the lanthanide oxalates and their derivatives LnC204X (X = C1 or Br) has been widely investigated [1-37]. The course of the reaction displays pronounced sensitivity to the nature of the bonding, not only with regard to the metal-oxalate interaction, but also as concerns the donor property of X. Those observations are explained in a neat theoretical framework. decomposition of oxalate can proceed via three alternativeThe thermal pathways:

Influence of sample particle size and heating rate on the thermal decomposition of K2C2O4

“…These calculations confirm our mechanism for the thermal decomposition of the oxalate, which, by means of (ii), links the activation energy to the interaction between the metal and oxalate ions [2,3,7].…”

“…Our mechanistic interpretation of the thermal decomposition of the oxalate is based on the expression [2,3,7] Ea = 127 + 1.4853.10-6(I~" -9800) 2 where the ionization potential term IT = ~(Y-d) V2 is defined by Pauling's covalent radius re, the ionic radius ri in hexacoordination and ~/, the sum of the ionization potentials of the metal atom. On the basis of (ii), theactivation energy of (1) is theoretically predictable.…”

“…We previously proposed a mechanism for oxalate thermal decomposition, in which breaking of the C-O bond is considered to be the rate-determining step [3,7]. Since the multiplicity of the C-O bond decreases with increasing interaction between the metal and oxalate ions, the activation energy will decrease with increasing interaction, to a minimum at 1~" = 9800.…”

“…For Nd2(C204)3, the activation energy is Ea = 127 kJ tool -1 (I~" ~ 9800) [3,7], and hence (I) was proposed as mechanism:…”

“…1. The dehydration of K2C204-H20 starts at 70 ~ and is complete at 126 ~ The onset temperature of reaction (1) where IT = 30.27(IO mol-i) ~ is the value of the ionization potential term [1][2][3]. The thermogravimetrie curves for fine powders of K2C204 at heating rates between 1 and 5 deg rain -1 are shown in Fig.…”