Analysis Of Energetic Heterogeneity Of Hgba2ca2cu3o8+ Based On Programmed Thermodesorption Under The Quasi-isothermal Conditions From The Q-tg And Q-dtg Data
ANALYSIS OF ENERGETIC HETEROGENEITY OF HgBa2Ca2Cu3O8+δ BASED ON PROGRAMMED THERMODESORPTION UNDER THE QUASI-ISOTHERMAL CONDITIONS FROM THE Q-TG AND Q-DTG DATA
D. Sternik1, P. Staszczuk1, V.V. Kutarov2
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 I.I. Mechnikov National University
Pastera st. 27, 65026 Odessa, Ukraine
In relation to energies, surfaces can be divided into two types: homogeneous and heterogeneous. These surface areas possess different energetic minimum depending on the type, thus different adsorption energy ΔE° and different potential barrier for the surface diffusion ΔV°. For ideally homogeneous surfaces ΔE° = constant but in practice there are two cases:
– when kT > ΔV° – the example of mobile adsorption and molecules can move along the surfaces;
– when kT
For heterogeneous surfaces: in this case it is characteristic that the minima have different depth on one surface, so there exists diffrent energetic barriers for diffusion.
Heterogeneous surfaces are divided, in relation to the minima, into:
– periodical heterogeneous surface when the energetic minima exhibit some reproducibility;
– completely random, when the minima do not exibit any reproducibility;
– patchwize, when some reproducibility of energetic minimum occurs, they are grouped in large.
Energetic heterogeneity of sorbents is conditioned by the differences in topology of adsorption centres, dispersion of pore sizes and other factors. The paper describes the method applied for estimation of sorbents energetic heterogeneity making use of the results of preadsorped liquid on porous solid
surface from single thermogravimetry Q-DTG curve recorded under quasi-equilibrium conditions. It is based on the application of condensation approximation to treat the desorption kinetics under non-isothermal conditions. General form of desorption kinetics is given in equation:
where: , q – the degree of surface coverage, n – the entropy factor, E– the desorption energy, To i T – the initial and given temperatures of desorption, b – the sample heating rate, t – the time. The final expression for determination of desorption energy distribution function has the form:
where .
After calculating the normalized density function jn(E) according to equation all size constants reduce reciprocally which makes calculation of the quantity jn(E) much easier:
The paper presents the way of calculating energetic heterogeneity for n-octane desorption from the surface of material being a high-temperature superconductor HgBa2Ca2Cu3O8+δ (Hg-1223) from the data Q-TG and Q-DTG as well as from the function of desorption energy distribution.
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