Emission Control Catalysts

W. Addy Majewski

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Abstract: Emission control catalysts, introduced in the 1970s, are now used on all types of internal combustion engines, as well as in a number of stationary applications. Catalytic reactors for mobile applications, known as catalytic converters, utilize catalyst-coated monolithic substrates. Terms which are used to characterize the catalyst performance include conversion efficiency, light-off temperature, and space velocity.

Background

Environmental catalyst technologies take their roots in air pollution problems from gasoline fueled passenger cars. Gasoline car emissions were primary targets of early air pollution legislation, dating back to 1970s and 1980s. In the USA, federal emission regulations are based on four acts: the Clean Air Act (CAA) of 1967, the CAA of 1970, and its two amendments adopted in 1977 and in 1990 [406].

Following the adoption of the CAA and the resulting vehicle emission standards, oxidation catalysts have been commercialized in the USA in the mid-1970s (first commercial application in model year 1975 cars) to control emissions of CO and HC from spark ignited (SI) gasoline engine applications. Three-way catalyst technology, introduced in the 1980s, made it also possible to control NOx emissions from SI engines at levels that are significantly lower than NOx emissions from diesel engines. Finally, in the 1990s, oxidation catalysts have been introduced to diesel fueled cars in Europe, as well as to heavy-duty diesel bus and (to a limited scale) truck engines in the USA. Since as early as the 1970s, diesel catalysts have also been used in some occupational environments, such as underground mining. Most catalysts in all of the above applications use noble, platinum group metals as their active components.

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Figure 1. Catalytic converter on a gasoline car

Following the success in mobile engine applications, catalyst technologies were introduced for stationary applications, for the control of volatile organic compounds (VOC) and NOx emissions. The list of catalyst applications covers such emission sources as chemical plants, painting and coating processes, ovens, printing, dry cleaning, power generation, and, last but not least, stationary engines. Examples of catalyst technologies for stationary engines include non-selective catalytic reduction (NSCR) of NOx from rich burn natural gas engines and selective catalytic reduction (SCR) of NOx by ammonia from diesel engines.

[photo]
Figure 2. SCR catalyst system on a stationary diesel engine

The wide acceptance and popularity of catalysts for emission control from internal combustion engines can be attributed to a number of advantages, as follows:

Catalysts also have a number of disadvantages and potential issues, which need to be carefully observed when designing emission control systems. Some of the issues include:

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