The Studies Of Adsorption And Total Heterogeneity Properties Of Solid Surfaces By Means Of The Thermal Analysis
THE STUDIES OF ADSORPTION AND TOTAL HETEROGENEITY PROPERTIES OF SOLID SURFACES BY MEANS OF THE THERMAL ANALYSIS
P. Staszczuk
Department of Physicochemistry of Solid Surface, Faculty of Chemistry
Maria Curie-Sklodowska University
M. Curie-Sklodowska Sq. 3, 20-031 Lublin, Poland
Estimation of adsorption and porosity parameters is an important problem in the characterization of material sorption properties. This problem has been studied by means of classical adsorption, chromatography, spectroscopy, sorptometry and porosimetry. The equilibrium adsorption isotherms are usually used to obtain information about the surface (e.g. adsorption capacity, specific surface area, number of surface hydroxyl, pore radius and volume, adsorption energy distribution and pore-size distribution functions). Conventional sorption measurements are time-consuming and require a specially constructed apparatus. The use of commercially available equipment is often limited to simple gases and vapors. Thus, there is great interest in elaborating simple and quick methods for the characterization of porous solids.
Studies on the application quasi-isothermal technique of the thermal analysis (Q-TG) in investigations of physicochemical properties of liquid-solid systems (character of adsorbent-adsorbate interactions, properties of adsorption layers and heterogeneity of solid surface) were made [1,2,3]. The effects taking place during thermodesorption of liquids from solid samples were registered and used in the practical calculations of the physicochemical parameters. During the thermodesorption process of adsorbed liquid films from solid surfaces the physical bonds (e.g. hydrogen and Van der Waals bonds) are disrupted. When samples are previously saturated with liquid vapors in desiccator at p/po = 1 all capillary and surface forces are blocked. The Q-TG mass loss curves allow determination of the adsorption capacity of the studied sample, the total volume of pores present on the surface and the amount of liquid bound on the surface. In order to determine the inflection points in the Q-TG curves, the differential Q-DTG curves are used with
respect to temperature and time. The special thermal analysis techniques were adopted for adsorption and calorimetry measurements and calculation of the some thermodynamc adsorption functions (e.g. adsorption potential distribution functions), specific surface area and pore volume of adsorbents and different materials [4].
Recently, the numerical propcedure was developed in order to evaluate the pore-size distribution and desorption energy distribution functions (i.e. total heterogeneity) of preadsorped liquid on mesoporous solid surface from single Q-DTG curve recorded under quasi-equilibrium conditions [5,6]. It is based on the application of condensation approximation to treate the desorption kinetics under non-isothermal conditions. The approximate desorption distribution function in the condensation approximation, ρ(Ed), can be obtained as follows:
Ed (max)
Vpore = ∫ ρ(Ed) dEd (1)
Ed (min)
where Vm is relative volume of the mesopores.
In this approximation, the desorption energy at each temperature in the Q-DTG curve was calculated using Redhead’s equation:
ln [(βEd /RT2m) A θn-1]= - Ed/RTm (2)
where: Tm is the temperature of the peak maksimum (in K) and β is the coefficient (in (K/s).
The desorption distribution was calculated from first derivative of the temperature on the desorption energy. Because the relationship between the pore-size and the desorption energy is a function of vaporization heat of test liquid:
Ed = Qvp = Qovp + a/rk (3)
where: Qvp is the vaporization heat in the mesopores, Qovp is the vaporization heat of the pure liquid, a is a constant for a liquid and rk is the mesopore radius;
its molar volume and surface tension, the mesopore-size cumulative and differential distributions can be evaluated from the dependence of the desorption energy versus mesopore volume and above desorption energy distribution, respectively. The approximate desorption energy distribution from the pores for each temperature Ti in the Q-DTG curve is given by equation:
ρ(Ed) = - (Vpore/dTi) (dTi/dEd) (4)
By using the function Ed(rk), where rk is the radius of mesopores, we can evaluate the mesopore-size distribution, χ(rk), from the relationship:
χ (rk) = ρ(Ed) [dEd(rk)/drk] (5)
It was stated that number, intensity and shape of Q-DTG curves reflect the effects of the pore-size distribution and desorption energy distribution functions of the silica gel surface and are typical for those of the silica gels measured by independent methods [6]. Present study demonstrates the possibility to use the single Q-DTG curve for the quantitative characterization of the total (structural and energetic) heterogeneity of the mesoporous surfaces, for example typical adsorbents, and it is subject of our presentation.
References
1. P. Staszczuk, J. Thermal Anal. 29, 1984, 217; 53, 1998, 597; .
2. P. Staszczuk, Thermochimica Acta, 247, 1994, 169.
3. P. Staszczuk, Am. Lab., 28,1996, 21.
4. P. Staszczuk, Thermochimica Acta, 247, 1994, 169 (Review).
5. V.I. Bogillo, D. Głażewski and P. Staszczuk, Proceedings: Third International Symposium Effects of Surface heterogeneity in Adsorption and Catalysis on Solids, Toruń, Poland, 1998, p. 22.
6. V.I. Bogillo and P. Staszczuk, J. Thermal Anal. and Cal., 55,1999, 467.
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