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Emission control catalysts were first introduced on spark-ignited gasoline engines. Three-way catalyst (TWC) technology introduced in the 1980s became an integral part of the stoichiometric 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 rich burn natural gas) engines include an oxygen sensor and a closed-loop control system. An electronic controller, based on feedback 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.
While stoichiometric operation of diesel engines has been investigated to determine their suitability for three-way catalyst technology, the challenges to operating diesel with a stoichiometric air-to-fuel ratio are costly to overcome . Thus, commercial diesel engines operate with lean burn combustion and contain high concentrations of oxygen in their exhaust gases at all operating conditions. For this reason, three-way catalysts are not employed for NOx control on diesel applications. Diesel emissions that can be controlled with high efficiency by oxidation 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 that have been developed for the reduction of NOx from diesel engines can operate either through lean NOx storage followed by rich reduction (NOx adsorber catalysts) or through selective catalytic reduction using ammonia (SCR catalysts), as summarized in Table 1.
The design target of early diesel oxidation catalyst (DOC) formulations was a high removal efficiency of gas phase 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 critically important.
The 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 onroad applications of diesel catalysts in the 1990s, the PM reduction became an important—in some cases the only important—catalyst function. Optimization of PM performance of the diesel oxidation catalyst was the main objective of catalyst research in the late 1980s and early 1990s. These diesel catalysts have been commercialized on some heavy-duty diesel engines in the USA and on light-duty diesel engines in the EU.
A number of emission regulations that became effective in the late 2000s and early 2010s—including US 2010 and Euro V standards for heavy-duty onroad engines, US Tier 2 and Euro 5 for light-duty engines, and US Tier 4 and EU Stage IV for nonroad engines—legislated NOx emission levels that required highly efficient NOx reduction catalysts. For example, the US 2010 NOx limit of 0.2 g/bhp-hr represented an approximate 90% reduction relative to the 2004 standard. These emission standards were met using urea-SCR catalysts and, in some light-duty applications, NOx adsorber catalysts.
|Catalyst Technology||Reaction Type||Target Emissions||Commercial Status|
|Diesel oxidation catalyst*||Oxidation||CO, HC, PM (SOF), odor||Mature commercial technology.|
|Lean NOx catalyst||Selective catalytic reduction by hydrocarbons (HC-SCR)||NOx, CO, HC||Not a mature technology, commercial application very limited.|
|NOx adsorber catalyst||Adsorption (trapping) of NOx from lean exhaust, followed by release and catalytic reduction under rich conditions||NOx, CO, HC||Commercialized on lean burn gasoline engines and on some light-duty diesel engines.|
|SCR catalyst||Selective catalytic reduction by ammonia||NOx||Mature commercial technology for most categories of diesel engines.|
|* DOCs are also used to promote NO → NO2 shift in SCR and particulate filter systems.|