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Urea-SCR(selective catalytic reduction) system is widely used as a technology of NOx(Nitrogen Oxides) reduction from diesel engine exhaust gases. Emission regulations have becoming stricter all over the world, and high NOx reduction performance is necessary to meet the emission regulations. To get higher NOx reduction performance of the Urea-SCR system, it is important to understand detailed chemical reaction mechanisms of Urea-SCR catalysts. In this study, we focused on elucidation of the reaction mechanism of the Urea-SCR catalyst by numerical simulation approach. The chemical reaction models with detail chemical reactions were built for both Fe-catalyst and Cu-catalyst. Both of the catalytic reaction models can predict difference of the catalytic reaction performance between the Fe-catalyst and the Cu-catalyst. In addition, rate-determining reaction step of the Cu-catalyst was successfully identified by the numerical simulation results.

To reduce NOx and PM emissions in exhaust gas, after-treatment systems for low NOx emissions are being developed in combination with improvements of engine combustion. In recent years, the exhaust gas temperature has been dropping because of enhanced low-fuel consumption of the engine. Therefore, it is urgent to develop NOx reduction technologies that work at a low temperature under 200degC. Since NOx is reduced by reacting with ammonia supplied to the SCR catalyst, it is necessary to make the urea solution decompose into ammonia using the heat of the exhaust gas to supply sufficient ammonia to the SCR catalyst. However, both the decomposition reaction and hydrolysis reaction of the urea have insufficient exhaust heat, thus making it difficult for urea to decompose and hydrolyze to ammonia at a low temperature. To solve this problem, it focuses on forcibly decomposing the urea solution without depending on the exhaust gas temperature.

1 To meet the Japan Post New-Long-Term (Japan 2009) emissions regulation introduced in 2009, The Hydrocarbon Selective Catalytic Reduction (HC-SCR) system for the NOx emission with a diesel fuel was chosen among various deNOx after-treatment systems (the Urea-SCR, the NOx storage-Reduction Catalyst and so on). The HC-SCR was adopted, in addition to combustion modification of diesel engine (mainly cooled EGR) as the New DPR system. The New DPR system for medium and light duty vehicles was developed as a world's first technology by Hino Motors. Advantages of the New DPR are compact to easy-to-install catalyst converter and no urea solution (DEF) injection (regardless urea infrastructure) as compared the Urea-SCR system.