Since automobile catalysts were first introducedin the U.S.A. and Japan in 1975, the negative effects from impurities in the fuel on catalyst performance have been recognised. Lead, added as an octane booster, or sulfur (S), a constituent of crude oil, are the major impurities.

Lead poisons the catalyst, reacting with the active noble metal sites. This reduces catalyst (and O₂ sensor) performance. Unleaded fuel was there-fore made available when catalysts were introduced.

Sulfur also poisons the catalyst, and severe worldwide vehicle emissions legislation is forcing lower S limits in gasoline and diesel fuel (see Table).

Gasoline: When gasoline is combusted SO₂ is formed in the exhaust. In vehicles with some early Pt-containing catalysts the S0₂ was oxidised to SO₃ which was converted to sulfuric acid on contact with water. Sulfur became a bigger issue when Pt-Rh three-way catalysts (TWCs) were introduced. On running slightly lean (for improved fueleconomy) the S oxides (SOx) formed were stored as sulfate on the TWCs. On rich operation (more fuel to accelerate) this sulfate was converted to H₂S and emitted in a pungent burst from the tailpipe. The H₂S/sulfate problem has been reduced by improved engine calibration, catalyst design and lowered S levels in the fuel. Indeed, tests on conventional catalyst-equipped gasoline engines haveshown that if S levels are lowered from 600 to 300 ppm, emissions of hydrocarbon, carbon monoxideand nitrogen oxides (NOx) are reduced by10-50%, depending on the vehicle.

Gasoline engines (such as direct injection) are in development for improved fuel economy and lower CO₂ emissions. Such engines will operate at much leaner air:fuel ratios than at present. But under lean operation the O₂ does not allow NOx to be reduced over the TWC, so special catalyst technology, a ‘NOx-trap’, will be needed. The trap will store NOx as a nitrate salt on a chemical trap (a Ba salt added to the TWC formulation). When the trap is full the engine will briefly switch to rich operation, releasing NOx which will be reduced to N₂ by the catalyst (1). Any SOx present will compete for NOx-trap storage sites, lowering the storage capacity, so, longer, rich operation will be needed to remove the SOx, compromising fuel economy. S-free fuel is therefore needed by these advanced engines.

Diesel: If a diesel vehicle carries a catalyst (Pt) lowering the S content in the fuel will improve catalyst efficiency as SOx adsorbs onto active siteslowering oxidation performance.

The main diesel emission problem is particulate matter (PM) (legislated). Sulfate (SO₂ oxidised on Pt) coalesces with water on the PM, increasing its measured mass. PM is removed from the exhaust by a particulate filter, which needs frequent cleaningby fast, high-temperature combustion with the O₂ present in the lean mixture. In this reaction S is oxidised to sulfate, again coalescing on the PM.

If the Pt catalyst and filter are part of a CRT^™ then trapped PM continuously reacts with NO₂(from NO oxidised over the Pt catalyst before the filter). SO₂ competes with NO for the catalyst sites, so lowering the amount of NO₂ formed.

NOx in diesel engine exhaust is controlled at present by the engine. NOx-traps may find use in the future. Again S would be an issue.

Clearly, fuel S reduction with advanced catalystsand engines, will result in cleaner exhaust gas.