STATISTICAL RATE THEORY DESCRIPTION OF BEAM-DOSING ADSORPTION-KINETICS

Absolute rate theory and the sticking probability approach have been p reviously examined as possible means of predicting the rate of adsorpt ion. However, when applied to examine adsorption kinetics, they have b een found not to contain the coverage and pressure dependence required for several important systems including CO-Ni(111). Statistical rate theory (SRT) is being developed with the objective of predicting the r ate of molecular (or atomic) transport across the interface between ma croscopic phases in terms of experimentally controllable variables and material properties of the two phases. Previous applications of SRT t o adsorption have been limited to systems for which both the gas phase pressure and the temperature could be assumed to be constant. Herein, the SRT approach is extended to systems in which the number of molecu les in the system (and hence the gas phase pressure) is not constant. To examine this extension, SRT is used to formulate the equations gove rning the rate of adsorption in isothermal, beam-dosing experiments. T hese equations are then combined with the values of certain material p roperties that have previously been established and a hypothesis that the value of the equilibrium adsorption cross section is given by the area of an adsorption site. The kinetic data for CO adsorbing on Ni(ll l) data reported by three different laboratories are then examined. Fo r each set of experimental data, constants had to be inferred that wer e related to the experimental apparatus used and as such they were not expected to have any coverage or pressure dependence. The good agreem ent found between the predicted and measured adsorption kinetics indic ates that all of the necessary coverage and pressure dependence was ex plicitly predicted from the SRT approach. (C) 1997 American Institute of Physics.