The Estimation Of The Total Heterogeneity
THE ESTIMATION OF THE TOTAL HETEROGENEITY
OF THE ACTIVATED CARBON SURFACES USING THERMAL ANALYSIS AND SORPTOMATIC METHODS
D. Głażewski1, P. Staszczuk1, V.V. Kutarov2, B. Kats2
1Department of Physicochemistry of Solid Surface, Faculty of Chemistry Maria Curie-Sklodowska University,
Maria Curie-Sklodowska sq. 3, 20-031 Lublin, Poland
2Physical Research Institute, Odessa Mechnikov National University,
Pastera st. 27, 65026 Odessa, Ukraine
Many researchers investigated the preparation and properties of liquid films adsorbed on the surface of activated carbons. Such interest results from certain features accompanying liquid adsorption on activated carbons: its positive or negative influence on adsorption and chemisorption of other vapours or gases as well as its influence on catalytic activity and surface reactivity of these solids. Among various kinds of porous solids, active carbons have the widest application in industry, science and everyday life. These materials are strongly heterogeneous because they possess various surface groups, impurities and/or irregularities as well as fine pores of different sizes and shapes. A great number of approaches have been proposed to evaluate the adsorbed liquid films and heterogeneous properties of activated carbons using adsorption techniques. Although these approaches are extremely useful for characterisation of liquid/active carbon systems, they are time consuming and have become less and less popular. Thus, there is a need for developing of simpler methods to characterise the above systems.
The analytical procedure was developed in order to evaluate the desorption energy distribution functions of preadsorped liquid on mesoporous solid surface from single thermogravimetry Q-DTG curve recorded under quasi-equilibrium conditions. It is based on the application of condensation approximation to treate the desorption kinetics under non-isothermal conditions. In this approximation, the desorption energy at each temperature in the Q-DTG curve was calculated using the Redhead equation. The desorption
distribution was calculated from first derivative of the temperature on the desorption energy, Ed. The approximate desorption energy distribution from the volume pores Vpore for each temperature Ti in the Q-DTG curve is given by equation:
ρ(Ed) = - (Vpore/dTi) (dTi/dEd) (1)
The studies were carried out on pure active carbon samples (Merck Co., Germany, particle sizes 0.5 – 1 mm for gas chromatography). This granular activated carbon is characterised by relatively low (2.8 %) content of mineral impurities. Prior to making the thermogravimetry measurements the samples of pure activated carbon were heated at 200° C for 8 hrs and then wetted with polar and apolar liquid vapors in vacuum dessicator, where p/po = 1, for a day. The studies were made using simultaneous derivatograph Q-1500 D (MOM, Budapest, Hungary) using an analyzer equipped with an automatic ultra slow procedure and a special crucible, which is capable of maintaining quasi-isothermal heating procedure to increase the resolution of thermal analysis. The thermodesorption of liquid measurements were carried out in quasi-isothermal conditions in a temperature range of 20–200° C with a furnace heating rate of 6° C/min. The heating rate was regulated automatically by the instrument in order to maintain a constant temperature during monoenergetic thermodesorption stage. The mass loss Q-TG and differential mass loss Q-DTG curves in relation to temperature and time were registered digitally under the control of the program Derivat running on PC.
Specific surface area, pore size and total pore volume as a function of pore radius (i.e. geometrical heterogeneity) of the studied samples were calculated from the low-temperature nitrogen adsorption-desorption isotherms measured by means of the sorptomat apparatus type ASAP 2405 V1.01 (Micrometrics Inc., USA).
Using experimental Q-TG technique the adsorption and porosity parameters of the pure active carbons were studied and compared with analogous ones obtained by classical sorptometry and other techniques. It was stated that number, intensity and shape of Q-DTG curves reflect the desorption energy distribution functions of the activated carbon surfaces and are typical for those of the above materials measured by independent methods. Present study demonstrates the possibility to use the single Q-DTG curve for the quantitative characterization of the heterogeneity of the mesoporous surfaces.
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