Prevention of Hydrogen Sulphide Formation on Three-Way Catalysts

The storage of sulphur and the formation of hydrogen sulphide on three-way automotive exhaust catalysts during the lean to rich operation sequence has been investigated. Fundamental thermodynamic studies were carried out in order to elucidate the interaction of the SO₂-containing exhaust gas with different metallic additives, as a function of both temperature and equivalence ratio.

Based upon these calculations several three -way catalysts containing transition metals were prepared and tested in the laboratory with flow reactor studies. A scavenger-free three-way catalyst was used as a comparison. The results confirmed that a sulphur storage/H2S-release mechanism prevails in the formation of odour compounds. The Cu-containing catalyst proved to be the best transition metal containing catalyst to eliminate the rapid reductive release of hydrogen sulphide. The metal additives (Ni, Zn, Fe and Co) gave essentially the same three-way activity as the base catalyst. The Cu-additive had a deactivating effect on catalyst three-way behaviour. A small COS formation was observed at high temperature (700°C) during very rich conditions.