Sulphate Emissions from Automobile Exhaust

The installation of platinum group metal catalysts on 1975 model year U.S. automobiles for control of CO/HC emissions has led to concern over the tendency of such catalysts to oxidise SO₂ to sulphuric acid aerosol. Examination of sulphate emissions from Johnson Matthey and Volkswagen test vehicles suggests that although there is some tendency to form sulphate, especially under highly oxidising conditions, the levels of sulphate are in general low. Comparison of measured sulphate conversion with thermodynamic equilibrium values shows that only a small fraction of the available exhaust SO₂ is converted to sulphate, and that the catalyst’s ability to convert is shortlived. Future moves toward a three-way catalyst capable of simultaneously converting CO/HC and NO_x pollutants should eradicate any tendency towards sulphuric acid formation in automobile exhaust.