Statistical Thermodynamic Calculations of Adsorption Data for the Adsorption of Simple Gases at Solid Surface — A Two Phase Approach

Abstract

Statistical thermodynamic calculations were carried out in the two-phase approach to describe the temperature dependence of adsorption data for simple gases on homogeneous and heterogeneous solid surface at low coverages. It was shown that the high temperature region is much more sensitive to the kind of model potential function representing the potential hypersurface of the absorbed phase than it is in the low temperature region. At low temperatures $\left(\frac{\varepsilon }{\text{kt}}\le 10\right)$ the adsorption data can be calculated from using a harmonic potential function with good realistic background.

For the description of a heterogeneous solid surface a potential function Φ(x,y,z) was proposed by Φ(x,y,z) = Φ(z) + Φ(x,y) or Φ(x,y,z) = Φ(z)-φ(x,y). At low temperatures and small heterogenities $\left(\frac{\Delta \varepsilon }{{\varepsilon }_0}<10\right)$ the adsorption data can be approximated by adding one part related to a Φ(z) potential and another one related to Φ(x,y). If an anharmonic potential function Φ(z) is used a maximum in Δ_aC (T) appears caused by the anharmonicity of this function. This high temperature maximum is overlayed by the heterogenity maximum in Δ_aC and gives additional peaks if Δε is not in the same order of magnitude than ε₀.