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Author
Serra Cervera, Jose Maria
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Abstract
The Selective Catalytic Reduction (SCR) of nitrogen oxides (NOx=NO+NO2) with ammonia in the presence of oxygen is nowadays considered as the most prevailing method for the abatement of NOx pollutants present in the exhaust gases from Diesel engine combustion. Typical NH3-SCR catalysts consist of transition metal (e.g., Fe and Cu) ion-exchanged zeolites. In particular, due to its high deNOx activity, high selectivity towards N2 and hydrothermal stability, Cu-exchanged chabazite (Cu-CHA) is presently considered as the state of-the-art SCR catalyst.
However, the abatement of harmful NOx compounds emitted from mobile sources still remains a challenging task for the catalysis community. In fact, the typically used SCR catalysts are not active enough at low temperatures, which is problematic during the cold start period of the Diesel engines, when NOx gases emissions are abundant. Also, NH3, which is the reducing agent needed to reduce NOx, is used in the form of urea, which is injected and lately decomposed to NH3. However, this decomposition does not occur when the temperature is lower than 170 °C. In addition, in the whole temperature range nitrous oxide (N2O) can be produced as undesired by-product during the NH3-SCR process over conventional SCR catalysts, especially over Cu-CHA.
The high global warming potential of N2O (around 300 times that of CO2) led to its recent inclusion in the list of future regulated pollutants under the emerging greenhouse gas regulations. Thus, considering the requirements of the automotive industry, which are dictated by the present and future stringent emissions regulations, it is important to understand how to maximize NOx conversion and minimize N2O formation at the same time.
In this context, being the detailed N2O formation mechanism still unclear in the literature, the aim of the present thesis work is to investigate the N2O formation over a commercial Cu-CHA catalyst under different operating conditions, such as by varying temperature and/or gaseous feed concentrations. In view of the stringent emission regulations, the experimental results can be of fundamental and practical interest for the catalysis community and the automotive industry in the field of both NOx and N2O abatement from Diesel exhausts.
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