Competing reactions in three-way catalytic converters : modelling of the NOx conversion maximum in the light-off curves under net oxidising conditions

Automobile exhaust gas conversion was simulated with a reactor model based on first principles. The monolithic reactor was modelled as adiabatically operating with a uniform flow distribution over the channels and with constant heat and mass transfer coefficients. The kinetic rate equations in the model were constructed from elementary step kinetics of the individual global reactions. The model predictions for light-off curves compare quite well with experimental data from the literature. Light-off is in the sequence hydrogen, carbon monoxide, propene, but the light-off temperatures do not differ very much. The nitric oxide conversion as function of the reactor feed temperature passes through a maximum at high hydrocarbon conversion, but does not reach the level of 50%, defined as light-off. Hydrogen is the major oxygen consumer in the front part of the reactor, while a slow reaction proceeds between CO and NO. At sufficiently high feed temperatures thermal reactor ignition occurs when the degree of CO surface coverage drops considerably. The corresponding increase of the O adatoms coverage causes increased reaction rates and a sudden temperature rise. Beyond the ignition point oxygen is mainly consumed by propene and by unconverted carbon monoxide. NO is reduced by unconverted hydrogen, while NO reduction by the hydrocarbon is not significant. Catalysts capable of increasing the NO surface coverage or the NO dissociation, or leading to a lower oxygen sticking coefficient would show a higher NO conversion maximum. The results indicate that well-known reaction mechanisms are capable to describe the behaviour of automotive exhaust gas converters, if mutual interactions of gaseous components and surface species are taken into account via elementary step kinetics.