STRUCTURAL STABILITY OF KINETIC-MODELS - ANOMALIES DUE TO IRREVERSIBLE ADSORPTION

The assumption of irreversible adsorption of a reactant can lead to a structurally unstable kinetic model. The structural instability is cau sed by the fact that two steady-state solutions intersect in the bifur cation diagram of for example, the degree of surface coverage of one o f the adsorbates vs. the partial pressure of one of the reactants. One of the two solutions is a trivial solution, corresponding to a cataly st surface completely covered with the irreversibly adsorbing reactant . The intersection point is a transcritical bifurcation point A struct urally stable kinetic model is obtained if an arbitrarily small desorp tion rate coefficient is introduced. Two examples are discussed in whi ch a transcritical bifurcation point is encountered, that is, the tota l oxidation of ethene in three-way catalysis and the chemical vapor de position of refractory metals. The consequences of the existence of a transcritical bifurcation point for kinetic parameter estimation and r eactor simulation are emphasized.