THE INFLUENCE OF PHYSISORPTION AND THE ELEY-RIDEAL MECHANISM ON A SURFACE-REACTION - CO + O-2

In this paper the effects of physisorption and of the Eley-Rideal mechanism on a surface reaction model which describes the catalytic oxidation of CO on a metal catalyst are discussed. The physisorption causes a shift in the value of the second-order phase transition y1 (at which the surface of the catalyst is completely covered by O) to y(CO) = y1 = 0.28 from its value of y1 = 0.395 in the absence of physisorption. The value of the first-order phase transition y2 (complete covering with CO) is not affected by the physisorption and remains at y(CO) = y2 = 0.650. y(CO) represents the mole fraction of CO molecules in the gas phase. For this simulation a mean-field ansatz is introduced which is able to describe the reaction system to a certain degree. This ansatz results in y1 = 0 and y2 = 0.57 indicating that the second-order phase transition at y1 is dominated by long-range correlations and is not of mean-field character. The influence of the Eley-Rideal mechanism as an additional step for the formation of CO2 changes the behaviour of the reaction system dramatically: the second-order phase transition at y1 (which is not observed experimentally) vanishes (y1 = 0) and the first-order phase transition is shifted to y2 = 0.646. From this simulations we are able to give an alternative way to understand why O-poisoning is not experimentally observed.