2.5 Thermogravimetric Analysis and Differential Scanning Calorimetry
Thermal methods are useful in a variety of fields in materials science. Thermal analyses are carried out to evaluate the thermal stability of samples, to determine the amount of moisture or volatile compounds in samples and to quantify the percentage of any phase present in a composite. Also, thermogravimetric analysis gives useful information about the different compositions forming a composite if one of the substances present can be evaporated at certain temperature levels. The species amount adsorbed in the micropores of the adsorbent can influence the behaviour in applications such as adsorption and ion exchange (Alejandro, 2015).
According to Yang (2014) thermogravimetric analysis (TGA) measures the mass change in a sample as a function of temperature, under a controlled atmosphere. It provides a quantitative measurement of the mass changes in a material associated with both material transitions and thermal degradation and thus can be used in the determination of the thermal stability and decomposition products of a material.
Thermogravimetric analysis has other desirable features as well. First, only very small samples, usually about 30 mg, are required for each TG scan. Second, by suitable adjustments in the work-up procedures and the operating parameters, thermogravimetric analysis reveals important information concerning the reaction pathways (Dean et al., 1988).
Three different regions are considered when analyzing adsorbent under the action of temperature. These regions are not completely fixed, and they change depending on the type of material or composition. The first section ranges from ambient temperature to 220 ◦C, in which the desorption of interstitial water molecules contained within the adsorbent pores occurs, the second region ranges from 620◦C to 1200◦C, where adsorbent, depending on the type of framework, collapses and the third one, that occurs at temperatures higher than 1200◦C where recrystallization or phase changes take place (Ribeiro, 1984).
Figure 2.13 Simple scheme of Thermogravimetric Equipment
According to Bingzheng (2012) the heat of adsorption of a given adsorption process is one of the most useful parameters in surface of adsorbent studies. It is usually determined by the differential scanning calorimetry analysers.
However, there are essentially three methods for experimental determination of the heat of adsorption:
1. calculation from adsorption equilibrium data;
2. calculation from desorption kinetics data; and
3. calorimetric measurement which is the most reliable method of determining the heat of adsorption.
The calorimetric determination of heat of adsorption does not require any model of the surface process and measure the heat directly. This method of heat adsorption determination is capable of handling both reversible and irreversible processes (Dean et al., 1988).
Figure 2.14 Water adsorption on adsorbents
Figure 2.15 TGA-DSC curve
Figure 2.16 Heat of Adsorption
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