Three-way-catalyst induced benzene formation: A precursor study

The implementation of the three-way-catalyst technology was the major step for pollutant abatement of gasoline vehicles. There are, however, situations, where the engine has to be operated at sub-stoichiometric combustion. At such fuel-rich conditions, an intense formation of benzene was observed over a Pd/Rh-based three-way-catalyst (TWC), when operating the catalyst in a critical temperature window of 600–730 °C. At least four different reaction pathways can lead to benzene formation on the catalyst, viz. (i) dealkylation of alkylbenzenes under steam reforming conditions, (ii) hydrodealkylation, (iii) aromatization of cyclohexanes, and (iv) cyclotrimerization of ethyne. Based on the engine-out exhaust composition only routes (i) and (ii) seem to be reasonable. The pre-catalyst application of 12 different alkylbenzenes indeed revealed that benzene formation is possible from all these precursors. At most up to 60% of the spiked precursors were converted to benzene. For meta- and para-substituted alkylbenzenes, a multi-step mechanism is proposed because partial dealkylation products such as toluene were formed as well. But a different, one-step mechanism is assumed for ortho-substituted alkylbenzenes, since no intermediates could be detected. No C–C-bond cleavage was observed within alkyl side chains. It is concluded that dealkylation reactions of alkylbenzenes are the major pathways leading to benzene formation in the TWC. Because fuel-rich combustion conditions have to be applied for the regeneration of deNOX traps or certain particulate traps as well, this chemistry might also be of relevance for these exhaust gas treatment systems.