THE CO+O-2 REACTION ON METAL-SURFACES - SIMULATION AND MEAN-FIELD THEORY - THE INFLUENCE OF DIFFUSION

A computer simulation for the heterogeneous catalyzed oxidation of CO is presented. The simulation includes adsorption, CO diffusion, reaction, and CO2 desorption. It is found that a first-order phase transition occurs at yco = y2 (yco is tne mole fraction of CO in the gas phase). In the interval [y2,1], the catalyst is almost completely covered with CO, i.e., the catalyst is poisoned. The value of y 2 is a function of the adsorption/diffusion ration. For no CO diffusion, Ziff, Gulary, and Barshad [Phys. Rev. Lett. 24, 2553 (1986)] found y2 = 0.525. In this paper, for A/D = 1/40, y2 = 0.650. In a mean-field ansatz with infinite diffusion rate, one obtains y2 = 0.666. With a linear stability analysis, the dependence of y2 on different initial coverage values can be explained. An initial coverage of oxygen does not influence the value of y2, but with increasing initial coverage of CO, the value of y2 decreases. It will be shown that oscillations are not possible in this simple reaction system. The adsorbed O atoms form large clusters that are found to be fractal in nature. The fractal dimension Df is equal to 1.88. A percolation transition at y co = 0.520 is observed, which corresponds to a critical oxygen coverage of ΘO,crit. = 0.561. At this oxygen coverage, an infinite cluster percolates through the lattice. The value of ΘO,crit. is nearly independent of the A/D ratio. In correlated percolation simulations, it is found that ΘO,crit. = 0.559, which is in good agreement with the value obtained from the reaction system. © 1990 American Institute of Physics.