Lead-free fuels

The picture we see now is that of new lead-free fuels having lower aromatics content but containing a variety of oxygenates. It is thus likely that the additives entering into the composition of gasoline will be also modified. 

The use of propane as a motor vehicle fuel has been highly developed in some countries, particularly in the USA, Holland and Italy. It is, of course, an entirely lead-free fuel. Very high efficiencies can be obtained using a gaseous fuel in spark-ignition engines since intimate mixing of the fuel and air is much more easily achieved than with a liquid fuel. This results in a much cleaner exhaust, with considerable reductions in CO and hydrocarbons. 

Their discoveries—white clothes, cheap soap and sugar, brightly colored washable fabric, clean water, fertilizer, powerful aviation and automotive fuel, safe refrigerants, synthetic textiles, pesticides, and lead-free fuel and food—were enthusiastically embraced by the buying public. Few of us today would want to do without them. 

Furthermore, lead compounds poison the platinum metal-based catalysts in catalytic converters, so that efforts to abate air pollution by automobiles through the use of catalytic converters (Section 8.4.2) are dependent on the use of lead-free fuels. Fuels of sufficiently high octane... 

Johnson and co-workers (62) have come to the conclusion that interaction of lead with Pt crystallites results in the formation of an inactive phase in which the Pt atoms are ionized and soluble in HC1. These data were derived from engine tests, in which the catalysts were exposed to fuels with 0.03-0.1 g Pb/gal. The amount of crystalline Pt in these catalysts was smaller than in catalysts run on lead-free fuels. The authors indicate that noncrystalline forms of Pt are present on A1203 supports under certain conditions, and that lead stabilizes such forms. The question whether the noncrystalline, ionic Pt is a surface or a bulk phase remains unanswered. Bulk mixed Pt-Pb oxides have been described (98, 99), but, again, the dispersed forms of noble metals supported on A1203, which lead (and other elements) may stabilize, are known to be associated with the surface only. Palladium can be expected to form such noncrystalline dispersed phases to a still greater extent since it is more easily oxidized than Pt. 

Lead and sulphur are derived from the fuel and there is a complex equilibrium dependent upon temperatures and gas composition controlling the absorption/desorption of these poisons. In the case of lead, extended trials have demonstrated the feasibility (ref. 20) of successful operation of oxidation catalysts on leaded fuel. However, it has been noted that in the decade since introduction of lead-free fuel in the USA, residual lead levels have fallen dramatically. In that market, where leaded and unleaded fuels are both available, incidents of poisoning reflect contamination of distribution equipment or deliberate misfuelling (refs. 21,22). Sulphur may also be derived from lube oil but its impact in the sense of poisoning is low on PGM catalysts. Interaction with catalyst components can, however, influence secondary/unregulated emissions of... 

This test programme has also illustrated that if lead levels do rise in future to 10 mg/1 or above, catalysts and HEGO sensor systems would be deactivated such that compliance with 83 US legal levels at 80K km would not be possible. This may arise in a territory that introduces lead free fuel with less control than has been exercised in Germany/Switzerland and Austria to date. 

Since 1975 catalysts have been fitted to vehicles in the USA to control emissions, initially of HC and CO (oxidation catalysts), and latterly also of NOx (three way catalysts). The mode of operation of these catalyst systems in the USA and Japan is now well characterised (1). The catalysts typically comprise the precious metals platinum, palladium and rhodium, either singly or in combination, together with base metal promoters or stabilisers, supported on alumina pellets or alumina coated ceramic monoliths. Catalysts for the US market are designed to withstand 50,000 miles of road use and must be operated in conjunction with lead free fuel since they are poisoned by lead. 

Another aspect related to fuel composition is the employment of catalytic converters to reduce automotive pollutant emission which requires the use of lead free fuels. This has the added advantage of reducing PAH emission. 

An everyday example of this is the poisoning of the previously mentioned exhaust catalyst in motor vehicles by the heavy metal lead (also compare Sect. 19.4). This is why lead-free fuels must always be used. 

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