Some Phenomena Which Control Sulfuric Acid Emission from Automotive Catalysts

With the use of a simulated exhaust system, the sulfuric acid and sulfur dioxide emission from a monolith noble-metal oxidation catalyst (Engelhard IIB) is measured. It was found that the storage rate of sulfur onto an initially sulfur-free catalyst decreases to a few percent of the sulfur rejection rate within 3-4 h. The amount of sulfur on the catalyst when the catalyst is in equilibrium with 20 ppm sulfur in the gas phase varies between 0.3 weight percent of the catalyst at about 400°C to 0.1 weight percent at 600°C. The sulfur can readily desorb from the catalyst if the gas phase sulfur content is lowered or if the catalyst temperature is increased.

It was found that the conversion of sulfur dioxide to sulfuric acid reaches thermodynamic equilibrium at temperatures of 400-500°C and space velocities of 30,000 h^-1. These conditions correspond approximately to a small V8 engine at 20 mph cruise.

In the simulated exhaust system without a catalyst, propylene, propane and hydrogen, which are present in automobile exhaust, reduce sulfuric acid to sulfur dioxide. This reduction appears to be a homogeneous reaction.