4.5.3 The effect of poisons
The use of catalytic converters was one of the major contributors to the phasing-in of unleaded petrol. Lead in petrol is a severe poison for the catalyst, and there have been many stories, particularly in the early days of the converter, of people disabling the catalyst by misfuelling. Figure 25 shows how the activity of a typical three-way catalyst is impaired during, and following, intermittent operation with leaded fuel (0.26 g l−1) during 15,000 miles of vehicle operation. The efficiency of a control vehicle (unleaded petrol only) was virtually unchanged at 94% for hydrocarbons, 95% for CO and 66% for NOx. Following the misfuelling, the CO-conversion efficiency (Figure 25a) decreased, but subsequently recovered to an acceptable level. By contrast, the conversion efficiencies for the hydrocarbons (Figure 25b), and especially NOx (Figure 25c), did not recover to passable values, and hence did not meet the emission regulations current at the time (1986).
Of the various noble metal components, Pd is the most sensitive to lead poisoning. Its activity decreases when there are just trace amounts in the fuel. Rh is slightly less susceptible, and Pt is by far the most resistant. Clues to the mechanism of lead poisoning have come from model systems, which are amenable to detailed surface analysis.
Figure 26 shows electron probe elemental maps of Pt/γ-Al2O3 after exposure to a simulated exhaust gas mixture containing 0.33 g l−1 of Pb. With which element, Pt or Al, is Pb associated?
Pb is associated with Pt, because the Pt and Pb maps are exactly superimposable. Similar results are obtained whether the metal is Pt, Pd or Rh.
This deposition of lead specifically onto the noble metal is believed to occur because the molecules that ‘carry’ the lead out of the engine, probably halides or oxyhalides, decompose on the noble metal, leaving the lead on the surface.
The fact that Pt is more resistant to lead poisoning than Rh or Pd is largely due to an indirect effect. The small amount of sulfur also present in fuel can act as a scavenger for lead. Provided that the sulfur is in its hexavalent oxidation state (SVI), in the form of SO3, it can combine with lead oxide to form a stable lead sulfate, which, although a poison itself, is not site-specific. Only Pt, however, is a good catalyst for the oxidation of SO2 (produced from sulfur in the combustion reaction, and present in the exhaust mixture) to SO3: indeed, it is used for this purpose in the industrial production of sulfuric acid.