The Studies Of The Polar And Apolar Liquid Film Properties On Pure And Modified Activated Carbons By Means Of The Thermal Analysis
THE STUDIES OF THE POLAR AND APOLAR LIQUID FILM PROPERTIES ON PURE AND MODIFIED ACTIVATED CARBONS BY MEANS OF THE THERMAL ANALYSIS
P. Staszczuk1, R. Leboda2, D. Glazewski1
1 Department of Physicochemistry of Solid Surface
2 Department of Chemical Physics and Physicochemical Methods of Separation Chemistry Faculty Maria Curie-Sklodowska Uniervsity
M. Curie-Sklodowska Sq. 3, 20-031 Lublin, Poland
Many researchers investigated the 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 [1,2]. 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 simpler methods to characterise above systems [3,4,5].
For that reason, special technique of thermal analysis in quasi-isothermal conditions have been adopted to study the liquid/active carbon systems.The effects taking place in the processes opposite to adsorption, i.e. in the programmed thermodesorption of liquid from solid surfaces were registred. The pure activated carbons (Aldrich, Fluca and Meck Co.) and modified by HNO3 and H2O2 (Merck Co., Germany) were wetted with water, n-butanol, benzene and n-octane vapours in vacuum dessicator, where p/po = 1.
The thickness of adsorbed liquid layers on the surface can be controlled by the immersion mode of solid sample. The immersing of sample with liquid
vapours in a vacuum dessicator blocks all adsorption active centres, the surface and capillary forces of the studied active carbon samples. The studies were made using simultaneous derivatograph Q-1500 D (MOM, Budapest, Hungary). The thermodesorption of liquid measurements were carried out in quasi-isothermal conditions in a temperature range of 20–200 oC with a furnace-heating rate of 6 oC/min. The Q-TG and Q-DTG curves were registered digitally under the control of the program Derivat running on PC [6]. 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).
It can be stated that it is possible to obtain from one single and quick experiment the important information concerning surface capacity (thickness and volume of water films) and wetting phenomena, nature of active centres, discontinuous change of adsorption liquid layer properties (piks on Q-DTG curves), mechanism and surface film destruction, kinetic thermodesorption of liquid films and its stability. Moreover, it is possible to study the effect of surface heterogeneity by chemicals modification of active carbon surfaces on its adsorption capacity and formation of liquid films. Using experimental Q-TG technique the adsorption and porosity (e.g. pore volume) parameters of the tested materials were studied, compared with analogous ones obtained by classical sorptometry technique and good correlation have been obtained.
References
1. R.C. Bansal, J.B. Donnet, F. Steckli, Active Carbon, Marcel Dekker, New York, 1988.
2. R. Leboda, J. Skubiszewska-Zięba, W. Grzegorczyk, Carbon, 1998, 36, 417.
3. R. Leboda, J. Skubiszewska-Zięba, W. Tomaszewski, Carbon’97, 23rd Biennal Conference on Carbon, 18-23 July, 1997, Extended Abstracts and Program, Adsorption/Reactivity/Interaction, The American Carbon Society, Pennstate, University Park Campus, Pennsylvania, pp. 10-11.
4. P. Staszczuk, R. Dobrowolski, M. Jaroniec, Langmuir:, 1992, 8, 229.
5. P. Staszczuk, J. Thermal Anal., 1996, 46, 1821.
6. P. Staszczuk, J. Thermal Anal., 1998, 53, 597.
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