Thermogravimetric Analysis (TGA)
In thermogravimetric analysis the change in mass of samples is monitored with an increase in temperature at specified gas environment and heating rate.
Fig. 6. Schematic diagram for thermogravimetric analysis technique
In this analysis, the sample is loaded in a small platinum crucible suspended from the arm of a microbalance. The entire arrangement is placed within a small oven, temperature of which is controlled and monitored. Analysis is carried out by raising the temperature of the sample gradually in a flow of air or inert gas such as N2, Ar or He and the sample weight (or percentage) is plotted against temperature. The parameters affecting the TGA are: heating rate, amount of sample, carrier gas flow, nature of carrier gas etc.
Applications
In catalysts TGA characterization is used to determine the thermal stability, content of moisture and volatile material if any or decomposition of inorganic and organic material in the catalysts. This method can also be used to study reaction kinetics with reactive gases (e.g., oxidation, hydrogenation, chlorination, adsorption/desorption) as well as pyrolysis kinetics (e.g., carbonization, sintering).
Only thermal processes that are associated with change in mass can be detected by TGA process which makes its use rather limited. However for desorption, decomposition or oxidation reaction, TGA can be used to gain useful information. The thermal stability of materials in both inert and oxidizing atmosphere can be determined by this technique. For catalysts, information regarding moisture content, decomposition or any reactions involving mass change that may happen during calcination / reactions conditions can be determined using a similar gaseous environment. Any catalytic reaction that is accompanied by mass change such as combustion or pyrolysis can be studied using TGA. TGA can be also used to determine the extent of deactivation by deposition of coke. TGA of deactivated sample in oxidizing atmosphere will remove the deposited coke of the used catalyst and thereby the amount of deposited coke can be quantified.
Fig. 7. Different TGA curves (A) single step change (B) multistep change (C) increasing mass
Fig. 7 shows some typical curves observed during TGA analysis. Curve A represents change in a single step. This can be moisture removal or decomposition or reaction of the material. For example, TGA of carbon in air is an example of this type of curve, where carbon after reaching the specific temperature in air burn down oxidizes, producing CO2 and CO gases.
The curve B corresponds to any change in multiple steps. The curve C on the other hand, shows an increase in the mass of the sample. This is observed for oxidation of metals such as oxidation of silver in air which results in gain in weight by formation of AgO2. TGA does not give any information on processes which are not associated with mass change such as phase change, melting or crystallization etc.