EFFECTS OF SULFUR ON NOBLE-METAL AUTOMOTIVE CATALYSTS

The small amount of sulfur, ca. 300 ppm, in gasoline which is equivalent to ca. 20 ppm SO2 in exhaust, has multiple effects on the operation of automotive catalysts. These effects are not widely appreciated. Firstly, there are large differences in the capacity of different noble metals to chemisorb sulfur dioxide and secondly, the dispersion of the noble metal influences the activity in the oxidation of sulfur dioxide to sulfur trioxide. The ability of the catalyst to oxidize SO2 determines the resistance to poisoning by residual amounts of lead in the fuel and whether the surface of the catalyst support will store sulfur, as sulfate, under oxidizing conditions. The surface sulfate groups on the support enhance the oxidation of saturated hydrocarbons, which is very desirable for the attainment of the stringent regulatory limits on hydrocarbon emissions. On the other hand, the stored sulfur can be emitted as a transient spike of hydrogen sulfide if the engine operating conditions shift momentarily to the rich side of the stoichiometric air/fuel ratio, which is an undesirable phenomenon. The presence of sulfur species in the exhaust influences the partitioning of the products of nitric oxide reduction between dinitrogen and ammonia strongly suppressing the latter. The optimization of the composition of the three-way catalyst has to take into account the effect of sulfur compounds present in the exhaust.