2461
J. Phys. Chem. 1991, 95, 2461-2465
Coverage Dependence of Molecular Adsorption Dynamics: Ethane on Pt( 111) Christopher R. Arumainayagam,t Mark C. McMaster,*and Robert J. Madix*qt** Departments of Chemistry and Chemical Engineering, Stanford University, Stanford, California 94305 (Received: May 29, 1990; In Final Form: October 5, 1990)
The dynamics of associative adsorption of ethane on Pt( 11 1) as a function of ethane coverage was probed with supersonic molecular beam techniques. Adsorption probabilities were directly measured at coverages ranging from zero to monolayer 0 and 60° at a surface saturation at incident translational energies between 10 and 40 kJ/mol and incident angles between ' temperature of 95 K. In contrast to the predictions of the original Kisliuk model, at all incident translational energies and incident angles the adsorption probabilities increases with ethane coverage up to near monolayer coverage. This behavior can be tit quite adequately by a model that incorporates enhanced adsorption onto the covered surface compared to the clean surface, utilizing the experimental value of the adsorption probability onto the saturated monolayer. The angular dependence of the adsorption probability shows progressive deviation from normal energy scaling with increasing surface coverage, suggesting that the effective corrugation of the gas-surface interaction potential increases with adsorbate coverage.
Introduction Studying the effects of adsorbates on the degree and nature of energy transfer during gassurface collisions is important since in most industrial applications, such as catalysis, the surface is partially covered by adsorbed reactants, products, and/or reaction intermediates. In traditional experiments designed to probe these effects a surface is immersed in a gas, and the uptake of the gas is monitored as a function of exposure. Such experiments, however, reveal only the average effects of gassurface encounters. Although supersonic molecular beams have been utilized extensively to probe the details of gassurface interactions, these studies have focused mainly on the adsorption dynamics at zero coverage. Study of the molecular adsorption dynamics as a function of surface coverage is a recent topic.' Langmuir Model. The simplest coverage dependence of the associative adsorption probability is given by the Langmuir site exclusion principle.* According to this principle, a molecule impinging on an occupied site is reflected while a molecule hitting an unoccupied site is adsorbed with a probability S,. Hence the adsorption probability at nonzero coverage, S(fl),is proportional to the fraction of unoccupied sites and hence decreases linearly with fractional coverage, 8:
s(e) = so(i- e)
(1)
Kisliuk Model. The inability of direct Langmuirian adsorption kinetics to explain the often observed insensitivity of the adsorption probability on adsorbate coverage up to near monolayer coverage prompted Kisliuk to derive kinetic expressions for adsorption as a function of coverage using a successive site statistical model and invoking a weakly bound mobile precursor statea3 Precursors are transient species and are generally classified as either intrinsic or extrinsic. Intrinsic precursor-mediated adsorption on the clean surface occurs via trapping into an equilibrated, weakly bound state existing above unoccupied surface sites. Extrinsic precursor-mediated adsorption occurs at nonzero adsorbate coverages via trapping into a thermalized, weakly bound, second-layer state existing above surface sites already occupied by adsorbed species. Both experimental and theoretical studies have invoked the role of precursors in the kinetics of molecular adsorption, dissociative chemisorption, and thermal de~orption.~ In the original model of K i ~ l i u kadsorption ,~ occurs via both an intrinsic precursor and an extrinsic precursor. The trapping probability, a,into both precursor states was assumed to be the same. No provision was made for direct Langmuirian adsorption. Kisliuk's model for associative adsorption may be represented by the following kinetic scheme: 'Department of Chemistry. *Department of Chemical Engineering. To whom correspondence should be addressed.
0022-3654/91/2095-2461$02.50/0
A'
where F is the incident flux in monolayers/second. The intrinsic and extrinsic precursor are denoted by an asterisk and a prime, respectively. By application of the steady-state approximation, the associative adsorption probability as a function of coverage was found to be
where Sois the associative adsorption probability at zero coverage SO = aka/(ka + kd) and K is a ratio of rate constants defined as follows:
(3)
If K
