On-Line Analysis of Individual Aromatic Hydrocarbons in Automotive Exhaust:Dealkylation of the Aromatic Hydrocarbons in the Catalytic Converter

The real-time concentrations of benzene, toluene, xylene, trimethyl-benzene and naphthalene in vehicle exhaust have been monitored during the FTP-cycle with a time-resolution of 20 ms and a sensitivity of 50 ppb. Using a laser mass spectrometer, the aromatic hydrocarbons in unconditioned exhaust gas at sampling positions behind the exhaust valve, before and behind the catalytic converter have been analyzed.

The comparison of the emissions sampled before and behind the catalytic converter reveals the effect of dealkylation of the aromatic hydrocarbons in the catalytic converter. Whereas most of the aromatic hydrocarbons are burned in the hot catalytic converter, however, bursts of aromatic hydrocarbons are released at transient motor operation. In these moments, which can be attributed to phases of closed throttle valve and very low engine load at gear changes, a significant part of the C₁-, C₂- and C₃- benzenes has been converted into benzene. Naphthalene as a double ring molecule without alkyl groups being itself the product of other dealkylation reactions is also emitted with slightly increasing concentration after catalyst over the FTP-cycle.