Analysis of Unregulated VOCs Downstream a Three-Way Catalyst in a Simulated Gasoline Engine Exhaust under Non-Optimum Conditions

Urban air pollution is partly due to exhaust emissions from road transport. Vehicle emissions have been regulated for more than 30 years in many countries around the world. Each motor type is equipped with a specific emission control system. In gasoline vehicles, a three-way catalytic converter (TWC) is implemented to remove at the same time hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). However, TWCs are only efficient above 200 degrees C and at a stoichiometric air-to-fuel ratio in the exhaust. However, deviations from stoichiometry occur during fast accelerations and decelerations. This study reports the analysis of unregulated VOCs commercial mini-TWC fed by model gasoline gas mixtures. A synthetic gas bench was used to control the model exhaust containing two model hydrocarbons (propene and propane) to identify the conditions at which VOCs are created under non-optimal conditions. Most of the pollutants such as N2O and VOCs were emitted between 220 and 500 degrees C with a peak at around 280 degrees C, temperature which corresponds to the tipping point of the TWC activity. The combination of different mass spectrometric analysis (online and offline) allowed to identify many different VOCs: carbonated (acetone, acetaldehyde, acroleine), nitrile (acetonitrile, propanenitrile, acrylonitrile, cyanopropene) and aromatic (benzene, toluene) compounds. Growth mechanisms from propene and to a lesser extend propane are responsible for the formation of these higher aromatic compounds that could lead to the formation of secondary organic aerosol in a near-field area.

Automated summary

Urban air pollution is caused by vehicle exhaust emissions, which have been regulated for over 30 years. Specific emission control systems are implemented in different motor types, such as the three-way catalytic converter (TWC) in gasoline vehicles. However, TWCs are only effective at certain temperatures and air-to-fuel ratios. This study analyzes the emission of unregulated VOCs from a mini-TWC under non-optimal conditions and identifies the formation of different VOCs, including carbonated, nitrile, and aromatic compounds.