Inhibition of post-combustion catalysts by alkynes: A clue for understanding their behaviour under real exhaust conditions

The exhaust gas of spark ignited engines contains a hydrocarbon mixture of complex composition ranging from C 1 to C 9 molecules including alkanes, olefins, diolefins, alkynes and aromatics. It has been evidenced that among the various unsaturated, hydrocarbons, alkynes show the largest inhibiting effect on all oxido-reduction reactions occuring on a PtRh catalyst, i.e. non-alkyne hydrocarbons, CO and NO are not converted until alkynes are eliminated. A scheme based on competitive adsorption has been proposed which accounts for the effect of the nature of the hydrocarbons on their own oxidation and on CO oxidation. It has also been shown that the inhibition effect exists even, for very low C 2 H 2 concentrations, representative of real exhausts gases. Furthermore, the addition of 15 ppm SO 2 slows down C 2 H 2 oxidation, which, increases the C 2 H 2 , inhibiting effect. From these results a laboratory test has been defined, enabling the simulation of engine bench tests.