Mathematical modeling of kinetic oscillations in the catalytic CO oxidation on Pd(110): The subsurface oxygen model

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Abstract
Experimental investigations of the catalytic CO oxidation on a Pd(110) surface revealed that the temporal oscillations in the reaction rate (measured in a range 10−3 Torr<p<1 Torr) can be traced back to a periodic formation and depletion of oxygen in the subsurface region. Oscillations in the reaction rate arise because the variation in the subsurface oxygen concentration modulates the oxygen sticking coefficient and hence the catalytic activity. Based on the proposed reaction mechanism, which has been well supported by experimental data, a set of three coupled differential equations was established describing the variations in the absorbate coverages ΘO and ΘCO and in the subsurface oxygen concentration for the system Pd(110)/CO+O2. Numerical solutions from the mathematical model reproduce the essential qualitative and quantitative features of the experiment. Characteristic features in the experiment that indicate the presence of subsurface oxygen, such as a reversal of the usual clockwise hysteresis in the reaction rate, are also found in the simulation. The model reproduces the existence region for kinetic oscillations in good agreement with the experimental data and it exhibits similar bifurcation behavior to that observed in the experiment. The remaining, mainly quantitative, differences can be traced back to simplifications made in the formulation of the kinetics.