Sticking coefficients, experimental investigation

Very recently, considerable effort has been devoted to the simulation of the oscillatory behavior which has been observed experimentally in various surface reactions. So far, the most studied reaction is the catalytic oxidation of carbon monoxide, where it is well known that oscillations are coupled to reversible reconstructions of the surface via structure-sensitive sticking coefficients of the reactants. A careful evaluation of the simulation results is necessary in order to ensure that oscillations remain in the thermodynamic limit. The roles of surface diffusion of the reactants versus direct adsorption from the gas phase, at the onset of selforganization and synchronized behavior, is a topic which merits further investigation. 

All activation energies and sticking coefficients in the model were taken from experimental investigations published in the literature. The pre-exponential factors, however, have not been measured for most surface reaction steps. Where reliable data was a ailable in the literature, those values were used. For the other reaction steps, we assumed a (pseudo-first order) kinetic constant of 10 s . which can be derived from transition state theory for a surface reaction step in which the transition state complex is not too different from the adsorbed state, i.e. if the ratio of the respective partition functions is close to unity (Zhdanov et al.). Starting from these initial values, we adjusted the pre-exponential factors until a satisfactory agreement between experimental data and model results was... 

Essentially exact values for V2 and have been reported by Wajnryb and Dahler [42] for both stick and slip boundary conditions. These values are recorded in Table V.4, along with estimates obtained by several previous investigators. In Figure 5.16, predictions based on the formula (5.297) are compared with the available experimental data. The solid curve is based on the stick boundary condition values V2 = 2.5 (Einstein) and = 5.9147 (Wajnryb and Dahler). To obtain the dashed curve, which agrees much better with (some of) the experimental data and for which virial coefficients with V2 = 5.0781. 

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