The role of sampling time in measurements of the CO2 kinetics of the bistable reaction CO+O2→CO2 on Ir(1 1 1) surfaces

Admission of a mixed CO (fraction Y) and oxygen (fraction 1−Y) reactant gas flux to Ir(1 1 1) surfaces at substrate temperatures between 420 and 560 K leads to CO2(Y,T) rates which can be very well described by reaction-diffusion differential equations. The solutions of these differential equations reproduce the experimentally observed bistability of the CO2 kinetics as a function of Y. We study by experiments as well as by modeling the role of the sampling time while scanning Y up and down across the rate hysteresis of the CO2 kinetics. In order to measure the true Y locations of the high/low and low/high CO2 rate transition at the boundaries of the hysteresis, the time scale for sampling, τsamp, needs to be adjusted to the time scales of CO desorption, τdeso, diffusion, τdiff, and CO+O reaction, τrea. All essential features observed experimentally are in qualitative and, frequently, in quantitative agreement with the results obtained from the underlying reaction-diffusion model. The remaining differences can be traced back to intrinsic limitations of the experimental set-up and to the dependence of the results on the parameters involved such as energies and frequency factors of reaction and CO desorption. To check the sensitivity of our results to variations of the parameters involved, we present numerical results for the dependence on the value of the desorption energy of CO while keeping all other parameters fixed.