Emission Control Catalysts

W. Addy Majewski

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
Catalytic Reactors
Conversion of Pollutants: Basic Terms

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—with the 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. The first diesel oxidation catalysts have been introduced to diesel fueled cars in Europe and to some heavy-duty diesel engines in the USA in the 1990s. Within the following decade, diesel emission aftertreatment systems became more complex—additional catalysts have been incorporated, including NOx reduction catalysts such as selective catalytic reduction (SCR) catalysts and NOx adsorber catalysts, as well as catalytic particulate filters. Since as early as the 1970s, diesel catalysts have also been used in some occupational environments, such as underground mining. Most catalysts in the above applications (except for SCR catalysts) use noble, platinum group metals as their active components [5421].

[photo] — **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 using urea or ammonia from diesel engines.

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

simplicity and passive character of many catalyst systems,
negligible adverse effect on the engine or fuel economy (low pressure drop),
maintenance-free or low maintenance operation,
proven durability, and
relatively low cost.

Catalysts also have certain disadvantages and potential issues, which need to be carefully observed when designing emission control systems. These issues include:

Dependence of performance on temperature. In the case of internal combustion engines, this translates to the dependence of performance on engine operating conditions and duty cycle.
Loss of activity through poisoning and thermal deactivation. Emission control catalysts require a very significant development effort to ensure the emission durability required in a given application.
Possibility of undesired reactions. In some cases, secondary emissions may be formed over the catalyst.
Platinum group metals—mostly platinum, palladium and rhodium—released from catalytic converters due to attrition can accumulate in soil and plants along highways [1087] and have been found in road dust and urban waste such as incinerated sewage ash [2468], as well as in ambient particulate matter in urban environments [1343], the environmental consequences of which remain unclear.
Some base metals may be a potential source of adverse health and environmental effects when released from the catalytic converter through attrition or other processes. The use of such metals may be limited by government regulations and/or guidelines (for instance, Japanese government policies dating back to the 1970s restricted the use of Cu, Ni, V, Co, Cr and Mn in emission control catalysts).
Insufficient availability of noble metals and other catalytic materials may cause unpredictable price fluctuations and supply shortages.

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