To reduce NOx emissions from vehicles, catalytic exhaust systems have been developed and utilised for decades. Diesel engines are widely used, and are currently the most energy efficient combustion engines. Therefore, it is highly important that improvements in diesel engine technology can reach the goals for air quality and greenhouse gas emissions.<\/p>\n
Selective catalytic reduction can control NOx emissions
\n<\/strong><\/h3>\nThe fuel-efficiency of diesel engines is largely thanks to excess amounts of air in the combustion process, in contrast to gasoline-powered engines. Consequently, the exhaust gas of diesel engines still contains oxygen, implying that the reduction of NOx emission in diesel exhausts has to take place in the presence of oxygen.<\/p>\n
The selective catalytic reduction (SCR) of NOx by ammonia (NH3<\/sub>) (NH3<\/sub>-SCR) is an efficient reaction that converts NOx and NH3<\/sub> in the presence of oxygen to nitrogen (N2<\/sub>) and water (H2<\/sub>O), both occurring naturally in the Earth\u2019s atmosphere. Since NH3<\/sub> is not emitted from the engine, it must be added from an external source.<\/p>\nIn the technical implementation of SCR on diesel vehicles, ammonia is added as an aqueous solution of urea (AdBlue\u00ae<\/sup>), which releases NH3<\/sub> upon decomposition. The NH3<\/sub>-SCR technology for NOx reduction is applicable in all situations where oxygen is present, which is also often in the exhausts of power plants and ships.<\/p>\nFurthermore, the successful implementation of biofuels in diesel engines depends on controlling the emission of NOx. In all these applications, NH3<\/sub>-SCR is an important technology to mitigate unwanted NOx emissions.<\/p>\nThe potential of Cu-CHA catalysts
\n<\/strong><\/h3>\nThe common catalysts for NH3<\/sub>-SCR on vehicles are based on vanadium oxide supported on titania, Fe-zeolites, or Cu-zeolites. Zeolites are microporous crystalline aluminosilicate materials with a typical diameter of 0.3\u20130.8 nanometres, commonly used as commercial adsorbents and heterogeneous catalysts. The ordered porous three-dimensional framework is formed by linking of Al and Si atoms by bridging oxygen atoms. The negative charge induced on the framework by the presence of Al atoms requires a positive counterions, which usually infer to the solid its catalytic properties.<\/p>\nCu-exchanged zeolites, and Cu-CHA in particular (a small pore zeolite with the chabazite structure), performs excellently in the low temperature range (150-300\u00b0C). The focus for the development of engines is currently on improved fuel efficiency, requiring temperature in diesel exhausts systems to be lower. Consequently, the performance of SCR catalysts in the temperature range 150-300\u00b0C will be an important parameter for application in future diesel engines.<\/p>\n
Cu-CHA-type catalysts perform well up to 550\u00b0C, and can tolerate temperatures up to 800\u00b0C. Therefore, Cu-CHA catalysts are currently the material of choice for NH3<\/sub>-SCR applications particularly on freight transport.<\/p>\nDeactivation risks of Cu-CHA catalysts and mitigation
\n<\/strong><\/h3>\nDespite their superior stability, repeated exposure to high temperatures and the harsh environment of exhaust systems can still deactivate Cu-zeolite based catalysts \u2013 i.e. the performance deteriorates with time.<\/p>\n
In particular, the presence of water vapour in the exhaust gas at high temperatures can damage the zeolite structure by removing Al atoms from the framework (dealumination), in a process so-called hydrothermal ageing.<\/p>\n
Importantly, small amounts of SO2<\/sub>, a common component in diesel exhaust gas, can result in a severe deactivation of Cu-zeolites, most notably between 150 and 300\u00b0C, which is the range where Cu-zeolites otherwise show their superior performance.<\/p>\nAs deactivation may result in the malfunction of the exhaust system, the applicability of Cu-CHA catalysts<\/a> is limited to systems where poisoning by SO2<\/sub> is less probable. In practice, this means that ultra-low sulphur diesel is required and even then, exhaust systems must be designed to handle possible deactivation by SO2<\/sub>.<\/p>\nThe CHASS project and expert collaboration
\n<\/strong><\/h3>\n
In the technical implementation of SCR on diesel vehicles, ammonia is added as an aqueous solution of urea (AdBlue\u00ae<\/sup>), which releases NH3<\/sub> upon decomposition. The NH3<\/sub>-SCR technology for NOx reduction is applicable in all situations where oxygen is present, which is also often in the exhausts of power plants and ships.<\/p>\n Furthermore, the successful implementation of biofuels in diesel engines depends on controlling the emission of NOx. In all these applications, NH3<\/sub>-SCR is an important technology to mitigate unwanted NOx emissions.<\/p>\n The common catalysts for NH3<\/sub>-SCR on vehicles are based on vanadium oxide supported on titania, Fe-zeolites, or Cu-zeolites. Zeolites are microporous crystalline aluminosilicate materials with a typical diameter of 0.3\u20130.8 nanometres, commonly used as commercial adsorbents and heterogeneous catalysts. The ordered porous three-dimensional framework is formed by linking of Al and Si atoms by bridging oxygen atoms. The negative charge induced on the framework by the presence of Al atoms requires a positive counterions, which usually infer to the solid its catalytic properties.<\/p>\n Cu-exchanged zeolites, and Cu-CHA in particular (a small pore zeolite with the chabazite structure), performs excellently in the low temperature range (150-300\u00b0C). The focus for the development of engines is currently on improved fuel efficiency, requiring temperature in diesel exhausts systems to be lower. Consequently, the performance of SCR catalysts in the temperature range 150-300\u00b0C will be an important parameter for application in future diesel engines.<\/p>\n Cu-CHA-type catalysts perform well up to 550\u00b0C, and can tolerate temperatures up to 800\u00b0C. Therefore, Cu-CHA catalysts are currently the material of choice for NH3<\/sub>-SCR applications particularly on freight transport.<\/p>\n Despite their superior stability, repeated exposure to high temperatures and the harsh environment of exhaust systems can still deactivate Cu-zeolite based catalysts \u2013 i.e. the performance deteriorates with time.<\/p>\n In particular, the presence of water vapour in the exhaust gas at high temperatures can damage the zeolite structure by removing Al atoms from the framework (dealumination), in a process so-called hydrothermal ageing.<\/p>\n Importantly, small amounts of SO2<\/sub>, a common component in diesel exhaust gas, can result in a severe deactivation of Cu-zeolites, most notably between 150 and 300\u00b0C, which is the range where Cu-zeolites otherwise show their superior performance.<\/p>\n As deactivation may result in the malfunction of the exhaust system, the applicability of Cu-CHA catalysts<\/a> is limited to systems where poisoning by SO2<\/sub> is less probable. In practice, this means that ultra-low sulphur diesel is required and even then, exhaust systems must be designed to handle possible deactivation by SO2<\/sub>.<\/p>\nThe potential of Cu-CHA catalysts
\n<\/strong><\/h3>\nDeactivation risks of Cu-CHA catalysts and mitigation
\n<\/strong><\/h3>\nThe CHASS project and expert collaboration
\n<\/strong><\/h3>\n
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