J. Mai

The CO+O2 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 the 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 y2 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 fo...

A general stochastic model for the description of surface reaction systems

In this paper we introduce a stochastic ansatz which can be used to model surface reaction systems. The systems may include mono- and bimolecular steps (i.e. adsorption, desorption, reaction and diffusion steps). We take advantage of the Markovian behaviour of these systems by using master equations for their description. The resulting infinite chain of equation is truncated at a certain level: In a small lattice region we solve the exact lattice equations and connect their solution to continuous functions which represent the behaviour of the system for large distances from a reference point. The stochastic ansatz is used to model different surface reaction systems such as the oxidation of CO on a Pt surface and on a Pt/Sn alloy. Also the formation of NH3 is discussed.

A model for the catalytic oxidation of CO on fractal lattices

Abstract A simple model for the catalytic oxidation of CO on fractal lattices is studied via Monte Carlo simulations. We observe a strong influence of the lattice structure on the system behaviour. In the case of extremely fast particle diffusion the structure of the lattice becomes unimportant. We are able to explain these effects qualitatively by the aspect of particle segregation. It is shown that this reaction model belongs to the universality class of the Reggeon field theory.

A Monte Carlo simulation of the CO oxidation on probabilistic fractals

We study a model of the CO oxidation on percolation clusters which represent the surface of our system in the Monte Carlo simulation. We observe two phase transitions where the surface is completely covered (poisoned) by one species (in our system CO or O). These phase transitions are described by their order and the values of the mole fraction yCO of CO in the gas phase at y1 (O poisoning) and at y2 (CO poisoning). The interval (y1,y2) represents the reactive regime. The influence of the occupation probability p for generating a spanning cluster on the underlying square lattice, of the diffusion of CO and of the lattice size on the value and the character of the kinetic phase transitions is studied. Increasing p leads to a shift of y2 to larger values of yCO for all ratios of adsorption to diffusion events but the value of y1 is increased to a larger value of yCO only if diffusion is not allowed. In the case of diffusion the value of y1 is maximally independent of p. A change in the character of the phas...

A theoretical stochastic model for the A+1/2B2→0 reaction

A surface reaction model for the A+1/2B2→0 reaction is studied by a theoretical approach. To this end, we introduce a stochastical ansatz which describes the system behavior by master equations. These equations are solved numerically in the superposition approximation. The results of the theoretical description are in good agreement with the corresponding computer simulations of the system. We focus our interest on the study of oscillations. The correlation functions and the parameter ranges in which oscillations are possible are studied in detail. This model as well as the computer simulations describe some aspects of the heterogeneously catalyzed oxidation of CO on a Pt surface.

The influence of physisorption and the Eley-Rideal mechanism on a surface reaction: CO + O2

Abstract 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 y 1 (at which the surface of the catalyst is completely covered by O) to y CO = y 1 = 0.28 from its value of y 1 absence of physisorption. The value of the first-order phase transition y 2 (complete covering with CO) is not affected by the physisorption and remains at Y CO = y 2 = 0.650. y CO represents the mole fraction of CO molecules in the gas phase. For this simu a mean-field ansatz is introduced which is able to describe the reaction system to a certain degree. This ansatz results in y 1 = 0 and y 2 = 0.57 indicating that the second-order phase transition at y 1 is dominated by long-range correlations and is not mean-field character. The influence of the Eley-Rideal mechanism as an additional step for the formation of CO 2 changes the behaviour of the reaction system dramatically: the second-order phase transition at y 1 (which is not observed experimentally) vanishes ( y 1 = 0) and the first-order phase transition is shifted to y 2 = 0.646. From this simulations we are able to give an alternati way to understand why O-poisoning is not experimentally observed.

A monte carlo simulation for the catalytic oxidation of CO on DLA clusters

Abstract A Monte Carlo simulation for the heterogeneously catalyzed oxidation of CO on diffusion limited aggregation clusters is introduced. Kinetic phase transition points which appear in this reaction system are studied as a function of the system parameters. Both the O poisoning transition at y 1 and the CO poisoning transition at y 2 are found at very low values and they are second-order transitions. This holds even in the presence of CO diffusion. The results will be compared with simulations which were performed on regular and other fractal surfaces. The strong influence of the lattice structure is discussed in detail.

Diffusion and reaction in multicomponent systems via cellular‐automaton modeling: A+B2

A cellular‐automaton model for the diffusion on a lattice is introduced. The spatial and temporal correlations that appear in the model are compared with the results of the correlated random walk, which has been obtained by Monte Carlo simulations. Very good agreement was found between these completely different simulations for the spatial correlations. The temporal correlations are quite different because of the nonequivalent temporal evolution of the two models. This new cellular automaton model is used to describe a reaction‐diffusion system that represents the catalytic oxidation of CO on a metal surface.

A cellular automaton model for the catalytic formation of NH3

Abstract A cellular automaton model for the NH 3 synthesis is introduced. The simulation accounts for the rules of adsorption, reaction via intermediate compounds and desorption of the product. A first-order kinetic phase transition is found if the N 2 adsorption is allowed on all sites of the lattice. The order of the phase transition is changed to second order if the N 2 adsorption can only occur on activated sites. We study the system behaviour with respect to the composition of the gas phase and the concentration of activated sites of the surface which permit the dissociative adsorption of N 2 .

A cellular automaton model with diffusion for a surface reaction system

Abstract A cellular automaton model for a reactive system including adsorption, diffusion, reaction and desorption of the product is introduced. The rules of the chemical reactions obey the stoichiometry and the diffusion rules obey the diffusion equation. The model is applied to the catalytic oxidation of CO. The results are compared with the obtained results of Monte Carlo simulations.