Catalytic control of emissions from cars

Air quality problems in America that were caused by pollutants from car exhaust and their photochemical reactions producing secondary pollutants in the urban environment had become of such a concern by the late 1960s that forcing environmental legislation was introduced in 1970, which became effective in 1975. Only catalysts containing platinum group metals were sufficiently effective, and their fitment in the exhaust line of gasoline cars coupled with other technical advances led to reduced pollutant emissions and significant improvements in air quality. Oxidation catalysts (typically Pt/Pd and Pt/Rh) were introduced first to control hydrocarbons (HCs) and CO emissions. Then these were combined with an upstream Pt/Rh catalyst to control NOx emissions as well. By the early 1980s Pt/Rh three-way catalysts (TWCs) were used in combination with electronic fuel injection, oxygen sensors and a microprocessor to provide closed loop control of the engine around the stoichiometric point. Since their introduction TWC performance has been hugely improved and adopted increasingly around the world. Legislation made catalyst fitment mandatory in Europe in 1993, and as a consequence many millions of tons of pollutants have not been released into the atmosphere with tremendous environmental benefits. More recently in Europe there has been a move towards diesel cars, and they presented technical challenges associated with low temperature exhaust and the presence of excess free oxygen that prevents fitment of TWCs. First Pt oxidation catalysts were used to control HC and CO emissions, and more recently catalysed (Pt/Pd) filters have very effectively controlled particulate matter emissions (soot) that are associated with direct health concerns. Now diesel NOx emissions are beginning to be controlled by Pt/Rh NOx-trapping catalysts that are regenerated by periodic enrichment of the exhaust, and by base metal selective catalytic reduction (SCR) catalysts using ammonia derived from aqueous urea. In the future it may be expected that multi component diesel emissions control systems will be combined into sophisticated four-way single units under computer control in much the same way TWCs are used on gasoline cars. (C) 2011 Elsevier B. V. All rights reserved.