Surface Adsorption with Infinite, or Finite, Residence Time, and the Resultant Pressure Decrease in a Reaction Chamber

Abstract

The pressure-response of a vessel (volume V) subjected to an injection of M molecules and containing a substrate capable of adsorbing said molecules depends on adsorption kinetics. If the adsorption sites hold an adsorbed molecule forever, i.e. if the binding energy of the molecule to a site is large compared to kT, the chamber’s pressure will drop continuously until all molecules are adsorbed. If the adsorption sites release adsorbands after some residence time tRes, i.e. if the binding energy is such that at temperature T there is a finite probability that the bond will be broken, the pressure in the chamber will decrease for a while, but then stay constant forever – when the rate of desorption equals the rate of adsorption. Hence observing the time dependence of the pressure drop reveals details of the adsorption kinetics.