Have you ever realized that hazardous nitrogen oxides (NOX) are emitted when you are driving your car? NOx is primarily produced during combustion from reaction between nitrogen and oxygen at high temperatures. NOx can interact with water, oxygen and other compounds in the atmosphere to form acid rain. The nitrate species that result from NOx can also cause haze. As the legislations on NOx emission for the diesel engines are getting stricter, there is an urgent need to develop more efficient aftertreatment systems to reduce NOx emissions.
In order to remove NOx, a catalyst is needed. A catalyst speeds up a chemical reaction without being consumed such that it is available for multiple reaction cycles. Currently, NOx emitted in gasoline engine exhausts can be effectively converted to N2 and H2O through a series of reactions over a three-way catalyst. The function of the three-way catalyst is conditional on stoichiometric reaction conditions. However, for heavy duty diesel engines, the excess of oxygen prevents the use of the three-way catalyst technology. Instead, selective catalytic reduction (SCR) using NH3 as a reducing agent (NH3-SCR) is a promising approach for NOx abatement. Metal-exchanged zeolites, particularly Cu-exchanged small-pore zeolites, are presently the catalysts of choice for NH3-SCR, having a NOx conversion efficiency close to 100%. However, the catalysts are not perfect and need to be developed with respect to activity, selectivity and durability.
The developments of computational methods and computers have made it possible to simulate catalytic reactions at the atomic scale. In this kind of work, reaction paths can be explored, providing a detailed knowledge of kinetic bottlenecks.
In this thesis, a complete reaction mechanism for NH3-SCR over Cu-zeolites has been constructed from first principle calculations. The proposed reaction mechanism provides a solid basis for future investigations in the NH3-SCR area and development of catalysts with enhanced properties.