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Three-way catalyst (TWC) technology introduced in the 1980s became an integral part of the spark-ignited engine. The TWC catalyst, operating on the principle of non-selective catalytic reduction of NOx by CO and HC, requires that the engine is operated at a nearly stoichiometric air-to-fuel (A/F) ratio . Modern catalyst systems for gasoline or natural gas engines include an oxygen sensor in front of the catalyst and a closed loop electronic control system. The electronic controller, based on a feedback system from the oxygen sensor, maintains the A/F ratio within a narrow range around the stoichiometric point, to assure maximum catalyst efficiency. In the presence of oxygen, the three-way catalyst becomes ineffective in reducing NOx.
For this reason, three-way catalysts cannot be employed for NOx control on diesel applications, which, being lean burn engines, contain high concentrations of oxygen in their exhaust gases at all operating conditions. Diesel emissions that can be controlled with high efficiency by today’s catalyst technologies are CO and HC, including such HC material as the organic fraction of diesel particulates (SOF) or polynuclear aromatic hydrocarbons (PAH). Catalyst systems for the reduction of NOx from diesel engines are either still under development or at early stages of commercialization, as summarized in Table 1.
The design target of early diesel oxidation catalyst (DOC) formulations was high removal efficiency of gas phase diesel pollutants, including carbon monoxide and hydrocarbons. At the time of their introduction, in the 1970s, carbon monoxide and hydrocarbon emissions from diesel engines were many times higher than they are in today’s engines. The first diesel catalysts were used in confined space applications, such as in underground mining, where air quality was extremely important.
Introduction of fuels with reduced sulfur content (< 500 ppm S) made it possible to achieve small to moderate reductions of PM emissions with the DOC. In highway engine applications of modern diesel catalysts, the PM reduction became an important—in some cases the only important—catalyst function. Optimization of the PM performance of the diesel oxidation catalyst was the main objective of catalyst research in the late 1980s and early 1990s, both in the USA and in EU. These diesel catalysts have been commercialized on some heavy-duty diesel engines in the US and on light-duty diesel engines in the EU.
Future expectations focus on the NOx reduction function of the diesel catalyst. Technologies for the reduction of NOx from lean burn engines, both diesel and gasoline, have been researched for many years by catalyst manufacturers and research institutes worldwide. That research has produced some promising catalyst concepts, some of them being on the onset of commercialization.
Future emission standards for both highway (US 2010, Euro V, light-duty US Tier 2) and nonroad engines (US Tier 4, EU Stage IV) legislate NOx emission levels that will require highly efficient NOx emission aftertreatment. For example, the US 2010 NOx limit of 0.2 g/bhp-hr represents an approximate 90% reduction relative to the 2004 standard. The regulators expect that NOx reduction catalysts will mature to provide this magnitude of NOx control on time. Indeed, the very future of the diesel engine may depend on technological developments in the catalytic reduction of NOx under lean conditions.
|Catalyst Technology||Reaction Type||Reduced Emissions||Commercial Status|
|Diesel oxidation catalyst||Oxidation||CO, HC (incl. PAH), PM (SOF), odor||Established commercial technology|
|SCR||Selective catalytic reduction by ammonia/urea||NOx||Commercial technology for stationary and marine engines, under development for truck and car engines|
|DeNOx (lean NOx) catalyst||Selective catalytic reduction by hydrocarbons||NOx, CO, HC, PM (SOF)||Some commercial oxidation catalysts incorporate small NOx reduction activity (passive DeNOx)|
|NOx adsorbers||Adsorption (trapping) of NOx from lean exhaust, followed by release and catalytic reduction under rich conditions||NOx, CO, HC||Technology under development, commercialized on lean burn gasoline engines|