D. Dollimore

The bet method of analysis of gas adsorption data and its relevance to the calculation of surface areas

Abstract The BET method of calculating specific surface areas of solid powders from measurements of the adsorption isotherm is reviewed. The theoretical background to the BET theory is outlined and a critical examination made of the two parameters calculated from this theory — the BET C constant and the monolayer capacity. Experimental methods are also reviewed, with special emphasis given to the calculation of surface area from a single adsorption point. The use of the BET equation in some methods of comparing adsorption isotherms is stressed, as it is often overlooked that these methods depend on the location of the monolayer, which ultimately is performed by the use of the BET equation.

A critical study of the suitability of the freeman and carroll method for the kinetic analysis of reactions of thermal decomposition of solids

Abstract It is demonstrated that non-isothermal traces of thermal decomposition reactions of solids must fit the equation developed by Freeman and Carroll for performing the kinetic analysis of “n order” reactions even if they are following a quite different mechanism. Therefore, it is concluded that this method does not allow one to determine if a reaction is obeying an “n order” kinetic law or a different one.

The application of thermal analysis to the combustion of cellulose

Abstract It is shown that the use of differential thermal analysis (DTA) to follow the pyrolysis of cellulose in air products two and sometimes three exothermic peaks. The first peak is associated with the combustion of volatile material, released in the degradation process, the second is caused by the glowing combustion of the carbonaceous residue, and the final exotherm is probably due to the combustion of product gases. The thermogravimetric analysis (TG) data in air show a preliminary loss of water followed by a mass loss of about 85% due to the production of the combustible volatiles. This second step appears identical to the degradation process in nitrogen, but in air the degradation products ignite to produce the first exothermic peak on the DTA. The glowing combustion DTA peak is associated with a further mass loss of about 15% on the TG plot. The use of a thermomechanical analyser shows that a small shrinkage of 3% occurs between 45 and 110°C, with the major collapse taking place between 300 and 370°C. There is, however, an expansion of 10% between 370 and 405°C, believed to be due to a crosslinking reaction.

Mass spectrometric determination of kinetic parameters for solid state decomposition reactions. Part 1. Method; calcium oxalate decomposition

Abstract A description is given of an experiment in which a time-of-flight mass spectrometer is used to monitor the thermal decomposition of solid samples placed in the ion source region. Because the mass spectrometer can both identify and quantify the gaseous products, the technique yields direct insight into the decomposition mechanism as well as values for kinetic parameters. An assessment of the technique is given and its application illustrated by a study of anhydrous calcium oxalate decomposition “in vacuo”. The decomposition occurred in two stages, the kinetic parameters determined being The decomposition mechanism is found not to be simple and thus the selection of calcium oxalate as a “model” compound to show the applicability of a particular method of analysing kinetic data obtained in vacuum is questioned.

The use of the rising temperature technique to establish kinetic parameters for solid-state decompositions using a vacuum microbalance

Abstract There are certain experimental aspects concerning the effect of rising temperature on the apparent weight of a substance which have been extensively studied. The theoretical treatment of the kinetics of decomposition under rising temperature conditions rests largely on a combination of equations one of which is the Arrhenius equation. One thus has to combine the kinetic law; the law describing the temperature coefficient of the rate (usually the Arrhenius equation); and the equation describing the impose temperature ( T in degrees Kelvin) against time ( t ); where α is the fraction decomposed, k is the specific rate constant, A is a constant (the pre-exponential term in the Arrhenius equation), T o is the initial starting temperature in the rising temperature experiment, and β is the heating rate. The combination of these three equations carries certain implications. The Arrhenius equation is almost invariably assumed to hold over the entire temperature range. The assumption may not hold and the most common deviation is the occurrence of two or more linear plots when plotting log k against 1/ T . It is held that this would apply when there is a discontinuous alternation in “reaction site distribution” when a common compensation plot of log A against E (the activation energy) should result. Another matter which is essential to the calculation is the correct choice of the specific reaction rate incorporated in the Arrhenius expression. It is concluded that E and A values for solid-state decompositions are environmentally dependent and that values calculated from rising temperature experiments should not necessarily agree with those obtained from the more traditional isothermal experiments.

The appearance of a compensation effect in the thermal decomposition of manganese(II) carbonates, prepared in the presence of other metal ions

Abstract In this study, a compensation effect is observed for the thermal decomposition of manganese(II) carbonates, prepared in the presence of Al 3+ and Na + ions. This compensation effect is described by the equation log A = aE + b , and the parameters are showm to be a = 0.1 and b = −2.9. The mechanism of decomposition was found to follow first order kinetics. Both A , the pre-exponential function, and E , the energy of activation, depended on the concentration and type of metal ion present in the carbonate preparation, and on the experimental method used to obtain Arrhenius parameters. In the rising temperature experiments, more than one Arrhenius plot was obtained over different temperature ranges.

Mass spectrometric evolved gas analysis: an overview

Abstract The uses of mass spectrometric evolved gas analysis systems have been described and discussed in relation to solid state reactions giving rise to gaseous products. Emphasis has been placed on the use of such systems for kinetic studies. The theoretical considerations necessary to ensure quantitative and linear results have been described.

Mass spectrometric determination of kinetic parameters for solid-state decomposition reactions: Part 2. Calcium carbonate1

Abstract The thermal decomposition of calcium carbonate has been investigated using a non-isothermal mass spectrometric thermal analysis technique. A small sample size and rapid removal of evolved CO; minimised gas-residue interaction. Initial experiments showed that T max , at which the maximum rate of CO 2 evolution occurred at a given heating rate, fluctuated over a 20°C range. Experiments conducted 12 months later had an even greater fluctuation range of 90°C. Evidence is presented that the decomposition temperature of CaCO 3 , in vacuum, is controlled by two major factors, namely, ageing and the extent of chemisorbed water. A linear compensation plot of log A vs. E was obtained despite the wide fluctuation in the T max values used to evaluate A and E . The reasons for this are discussed.

The investigation of the decomposition kinetics of calcium carbonate alone and in the presence of some clays using the rising temperature technique

Abstract The kinetic parameters for the decomposition of calcium carbonate are established using a rising temperature programme experiment and from isothermal experiments. The effects on the decomposition of adding various clays are then established. Four methods of analysing the rising temperature data are then attempted.

The thermal decomposition of oxalates Part 14. Dehydration of magnesium oxalate dihydrate

Abstract The isothermal dehydration of magnesium oxalate dihydrate has been studied under various pressures of water vapour by use of a thermogravimetric balance. The rate of dehydration was found to be dependent upon the water vapour pressure. The reaction rate at temperatures below 124°C decreases sharply with an increase in water vapour pressure up to 0.5 mm Hg. With further increase in pressure an increase in rates is observed; this rises to a maximum and then falls again as the pressure is increased. The limitation of this phenomena to a limited temperature range is shown in the case of magnesium oxalate dihydrate. Above 124°C the initial fall in rate is not observed, the rate rises with increasing pressure from vacuum to a maximum and the falls. X-ray diffraction studies indicated that the anhydrous product prepared in the second region where a decrease of rate of dehydration occurred was crystalline but the sample dehydrated under vacuum or in the first region produced an amorphous anhydrous salt. A compensation effect is demonstrated with plots of the activation energy against the logarithm of the pre-exponential term in the Arrhenius equation.