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Kinetic analysis of thermogravimetric data

XXX. Thermal decomposition of some Reinecke salt like complexes

Journal of Thermal Analysis and Calorimetry
Authors: J. Zsakó, I. Ganescu, Cs. Várhelyi, and L. Chirigiu

Thermal decomposition of 6 complexes of the type AH[Cr(NCS)4 (am)2]· nH2O is studied with derivatograph. The formation of Cr(NCS)3 as a labile intermediate is presumed. For some decomposition stages kinetic parameters are derived. The kinetic compensation effect is discussed.

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Four many curves methods, viz. calculation techniques based on Eqs (30), (31), (34) and (36), respectively, for deriving kinetic parameters from several TG curves recorded with different heating rates are tested on two sets of theoretical TG curves. The maximum reaction rate temperature and conversion, as well as the approximate formulae used for their calculation are discussed. Some aspects of the kinetic compensation effect are analysed. The final conclusion is that the use of the many curves methods is not reasonable.

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The kinetic compensation effect observed in heterogeneous non-isothermal kinetics is only an apparent effect. In general, the correlation derived between the kinetic parameters E and log A from TG curves can be described by means of a non-linear compensation law, expressed by Eq. (14). This equation may become approximately linear in certain particular cases, i.e. it may change into an isokinetic relation. The validity of the non-linear compensation law has been tested by using over 1000 sets of kinetic parameters reported earlier.

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A number of 1145 sets of kinetic parameters derived in our earlier papers from TG curves have been worked up. The apparent activation energy and pre-exponential factor values have been found to obey a linear compensation law (isokinetic relation) if the thermal decomposition begins in the same temperature interval, irrespective of the nature of the chemical reaction. The isokinetic temperatureT i has been found to be very close to the mean value of the temperaturesT 0.1 at which the conversion becomes equal to 0.1 and atT i the rate constant has been found to be approximately equal to 10−3s−1 in allT 0.1 intervals investigated. It is concluded that the kinetic compensation effect observed in heterogeneous non isothermal TG kinetics is not a true one.

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