AUTO EXHAUST CATALYSTS

Automobile exhaust is a well-known pollution source. It contains products of incomplete combustion—carbon monoxide and assorted light hydrocarbons—as well as combustion by-products such as oxides of nitrogen and sulfur. Catalytic converters that transform incomplete combustion products to carbon dioxide and water, and oxides of nitrogen back to nitrogen and oxygen, are required equipment on all automobiles sold today in most industrialized countries. Manufacturing these catalytic converters is a worldwide business worth several hundred million dollars per year. The oxidizing converters that finish off the incomplete combustion products were first commercialized in the United States in the fall of 1974, and "three-way" converters that also handle nitrogen oxides became available, again in the United States, in the fall of 1980. Recent tests of a new catalytic converter design show that substantial further reductions in hydrocarbon and carbon monoxide emissions may well be possible to meet the stringent requirements of the 1991 Clean Air Act.

Hydrocarbon and carbon monoxide emissions are at their highest levels during the first 10 minutes after the engine has been started. The cold engine burns fuel less efficiently, passing more incomplete combustion products on to the cold catalytic converter, which is itself less efficient. Heating the catalyst would make it more efficient faster, and the new design does just that. The converter is made of catalyst-coated stainless steel, which can be heated by passing an electric current through it (Figure 2.5). Tests of a prototype under the federal test procedure specified for catalytic converters have already shown substantial emission reductions in comparison to conventional technology. The electrically heated catalytic converter is being developed to meet the ultralow emission regulations enacted by California in late 1990 to help highly polluted areas such as the Los Angeles Basin.

NOx reduction), there is a need for the development of lower-cost NOx reduction catalysts or the development of practical catalysts for NOx decomposition. With cleaner gasolines, there may also be opportunities to develop lower-cost catalysts containing a smaller amount of the expensive noble metals. Finally, with the very high cost of rhodium, there is need for an improved technology for recovering it.

The control of power plant emissions is another major area of opportunity for catalysis. In particular, there is a need for removal of NOx emissions either via selective catalytic reduction (SCR) or, if achievable, via NOx dissociation. The abatement of NOx from power plants is important in efforts to control acid rain and photochemical smog, the latter being linked



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