energy distribution function (6). However, generally speak-In this paper the expression for adsorption isotherm expressed ing, there are no precise and simple models to characterize by an asymptotic development, the extended Freundlich's isothese adsorption systems.
therm, was applied to the study and interpretation of energetic
On the statistical mechanics approach, the energetic heterheterogeneity. This expression presents the relative monolayer covogeneity of adsorption surface has been analyzed by the erage of the entire surface as a dependence on adsorbate pressure fundamental equation for the overall adsorption isotherm (or concentration) and on parameters m and b 0 , which were identi- (7,8). fied with the Freundlich's parameters. The value of m and b 0 were obtained by adjusting the cited expression to the experimental data for hydrogen adsorption on tungsten. The m parameter shows a
quadratic dependence on temperature. Our analysis shows that N(Q), the exponential form for the continuous energy distribution, is very dependent on m. A representative parameter is the dispersion of N(Q), d Å RT/m. We interpret the dependence of N(Q) where Q T is the relative monolayer coverage of the entire on m as an indication of the energetic heterogeneity of this system. surface, Q(Q) is the relative coverage of the adsorption The parameter b 0 was adjusted by a van't Hoff expression, indicatsites with adsorption energy, Q, and N(Q) is the continuous ing a thermal process on the Langmuir adsorption sites. ᭧ 1998 energy distribution normalized to the unity. This expression Academic Press must be integrated in the limit D, that is, in all possible Key Words: extended Freundlich's isotherm; influence of the energy values. temperature; energetic heterogeneity.
Equation [1] formalism has been applied to the investigation of physical adsorption on heterogeneous solids by applying the analysis of the term N(Q) (9)(10)(11). In this case, N(Q) Å a.exp(0mQ/RT )
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