CLASSIFICATION OF GIBBS ADSORPTION-ISOTHERMS

The current IUPAC classification of gas-solid adsorption isotherms has two deficiencies: it is incomplete and it gives the incorrect impress ion that adsorption isotherms are always monotonically increasing func tions of pressure. However, there are many isotherms that are not mono tonic and show maxima. The reason for this deficiency is that the IUPA C classification is based conceptually on ideas about the 'absolute' a dsorption rather than on the Gibbs adsorption. It is shown here that s upercritical gases show adsorption isotherms which are fundamentally d ifferent from those in the IUPAC classification. There are experimenta l data for hydrocarbons, inert gases, nitrogen, carbon oxide, carbon d ioxide, nitrogen oxides, and others on microporous and macroporous ads orbents for supercritical temperatures showing nonmonotonic isotherms with maxima. It also is shown that the Ono-Kondo lattice model is able to predict all known types of adsorption behavior. By changing two en ergetic parameters (energies for adsorbate-adsorbate and adsorbate-ads orbent interactions), one can obtain smooth or stepped multilayer adso rption isotherms for macroporous adsorbents. It also is possible to de scribe adsorption on microporous adsorbents by imposing the appropriat e boundary conditions. The Ono-Kondo model also predicts steps in isot herms similar to those observed in high resolution measurements for ni trogen and oxigen on zeolites. The monolayer version of the Ono-Kondo theory gives all known types of monolayer adsorption isotherms. (C) 19 98 Elsevier Science B.V. All rights reserved.