On the Nature of the Phenomenon of Oscillatory Adsorption

Abstract

The authors have recently advanced a hypothesis for the phenomenon of oscillatory adsorption suggesting that it is caused by a number of factors such as the competitive filling of the surface by A and B components, the ‘spotty’ nature of component adsorption, the dramatic change in the configuration of such ‘spots' at a certain step during surface filling with one of the components resulting in a transition of the adsorption layer from an ‘ A in B’ state to a ‘B in A’ state and vice versa, and the dependence of the values of the chemical potential of A and B on the configuration of the adsorption ‘spot’ causing a change in the direction of the sorption process. In the present communication, an attempt is made to examine the phenomenon from the viewpoint of non-equilibrium thermodynamics. By relying on the Gibbs equation, particularly with respect to the flux and affinity equations, and taking account of the equality of the reciprocal influence coefficients, it is possible to conclude that the chemical potentials of the competitive components are either equal or approximately equal at the periphery of the sorption ‘spots’. This indicates that the entropy of the system remains constant (i.e. dS = 0) leading to the possibility of sustained oscillations occurring in the adsorption process.

Oscillatory character of deactivation of zeolite catalysts in carbonium ion conversions of hydrocarbons

“…Then a new cycle begins (curves 5-6 and 5´-6), ending with the next inversion at point 6, i.e., the next merge of the spots under B. If component B (the coke) were chemically unchanged, then the oscillations would be undamped, as was theoretically justified for the "4-hydroxy TEMPO -hydrocarbon" system in [8]. However, the coke, in the initial stages of its appearance, is a rather indeterminate substance, the formation of which occurs through a number of stages: from chemisorption of olefins (the simplest precursors of coke) on acid sites of the catalyst, up to their oligomerization, cyclization, aromatization, and condensation.…”

“…Then the sections of the surface under component A become spotty. The values of the chemical potentials µ A a and µ B a for components A and B in the adsorbed state depend on the configuration of the adsorption layer they form [8]. For the same degree of coverage of the surface, in the case of a layer with a spotty configuration the values of µ A a and µ B a will be less than for the layer with a "holey" 42 0040-5760/05/4101-0042…”

“…The authors of [8] saw the characteristics of a bridging mechanism for the adsorption oscillations of 4-hydroxy TEMPO in the spotty adsorption of the latter on active sites of a surface initially coated with a hydrocarbon (component A). They hypothesized that as 4-hydroxy TEMPO (component B) is fed in portions into the "zeolite-hydrocarbon" suspension, the adsorption spots under the 4-hydroxy TEMPO increasingly expand, reaching their maximum dimensions at the instant they merge with each other (saturation of the zeolite with 4-hydroxy TEMPO), with inversion of the adsorption layer from the "B in A" state to the "A in B" state.…”

“…begins. Support for such a model is the fact that the geometric calculation of the minimum and maximum surface areas under the spots gives 21.5% and 78.5%, while the experimental values for different heterogeneous systems fluctuate within the range from 19.3%-25.5% up to 74.5%-75.4% [7][8][9].…”

“…We start from the fact that [8] the chemical potentials of components A and B in solution are equal at the instant of inversion. We may hypothesize that at this instant of time (Fig.…”