Cleaner Combustion by Frédérique Battin-Leclerc John M. Simmie & Edward Blurock

Author:Frédérique Battin-Leclerc, John M. Simmie & Edward Blurock
Language: eng
Format: epub
Publisher: Springer London, London

13.2.6 Thermogravimetry Analysis

Thermogravimetry Analysis (TGA) allows the determination of purity and thermal stability of a sample, the study of solid state reactions, the study of decomposition of inorganic and organic compounds, the determination of moisture, volatile and ash contents, the determination of kinetic parameters, the study of vaporizations, sorption, desorption and chemisorption of molecules on a sample, etc. (Brown 2001).

TGA evaluates the mass loss when a substance is heated at a given heating rate (dynamic TGA) or kept at constant temperature (static TGA). A plot of mass loss versus temperature (T) or heating time is defined thermogravimetric curve (TG plot). The sample mass losses are influenced by the set up parameters (heating rate, furnace atmosphere, geometry of sample holder and furnace, sample holder material, thermocouple location…), and by the material’s chemical-physical features (particle size, sample packing, sample thermal conductivity, heat capacity of the sample material, occurrence of trapped gases or volatile species in the sample, sample stability, etc.).

TGA in oxidative environment is commonly used to study the oxidation behavior of carbon materials and the determination of kinetic parameters (Stratakis and Stamatelos 2003). TGA analyses performed in inert atmosphere are useful to evaluate the amount of volatile species (unburned fuel, PAH, etc.) adsorbed on the sample and/or the presence on the surface of labile functional groups. TGA measurements are also used to analyze the chemical-physical modification occurring during the heating (volatilization, polymerization, polycondensation, cracking of side chains from aromatic rings and isomerization) (Luis 1987).

The TG plot of carbon materials typically exhibits different weight losses as function of temperature. The weight loss up to about 423 K is attributable to the water evaporation-desorption, a weight loss between 473 and 673 K is related to hydrocarbon desorption or decomposition of labile functional groups (hydroxyl, carboxylic, carbonylic, etc.). Between 673 and 1173 K, the weight loss is related to the oxidation of the carbonaceous core in the case of TG performed in oxidative environment (Paredes et al. 2009).

A correlation between nanostructure and reactivity toward oxidation of carbon-based materials was proposed (Vander Wal and Tomasek 2003). It was found that the oxidation reactivity is dependent on the accessibility of carbon in edge sites that are much more reactive than the basal plane carbon atoms, on the weakening of C–C bonds for effect of curvature due to five-membered rings (driving to an increase of sp3 character, like in fulleroid structures) (Dresselhaus et al. 1996) and on the hydrogen content at the edge sites (Alfè et al. 2009). It was pointed out that (Alfè et al. 2009; Vander Wal and Tomasek 2003), provided that the lengths of the graphitic layers are the same (i.e., the accessibility of the carbon edge sites), the more the curvature of the soot (i.e., the surface area), the faster the reactivity toward oxidation.

Printex-U sample exhibits a higher oxidation temperature at about 963 K (Fig. 13.6-inset) with respect to the combustion-formed carbon materials, 893–953 K (Alfè et al. 2009), indicating a higher graphitization degree, what is in agreement with the very low H content (with an H/C molar ratio <0.

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