Competitive chemisorption of reactants

The simple model just discussed shows multistability even when the system is clean but requires the involvement of a poison for oscillations. One reason for this is that the latter is needed to provide a second independent surface concentration, so we theij. have a two-variable system. It was mentioned in 12.3.1 that implicit in the rate law used above may be the adsorption of a second reactant which participates in the reaction step. The latter did not provide a second concentration variable there since its adsorption and desorption processes were assumed to be on a very much faster (instantaneous) timescale. 

In this section we turn to a model where the adsorption and desorption of two reactants occur on similar timescales. The adsorption is competitive, i.e. both reactants are adsorbed on to the same surface sites. Again, a number of vacant sites will be involved in the reaction step. The model is 

The rate equations for the surface coverages 0p and 0r can be written in the dimensionless form 

Although this set of equations has four parameters, it will be enough to demonstrate that multiple stationary states and sustained oscillatory responses are possible to consider variations in the dimensionless reactant partial pressures p and r, with fixed values for the desorption rate constants k1 = 0.001 and k2 = 0.002. 

Substituting these into eqn (12.58), the stationary-state condition then yields a quartic in Gp  

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