The standard SCR reaction, 4 NO + 4 NH3 + O2 → 4 N2 + 6 H2O, and fast SCR reaction, 2 NO + 2 NO2 + 4 NH3 → 4 N2 + 6 H2O, are key reactions in the current technology for emission control of diesel engines. Commonly applied catalysts are based on vanadium oxide or zeolites ion-exchanged with Cu or Fe. A complete catalytic reaction scheme describing the single steps for the standard and fast SCR reactions on Cu-zeolites has been proposed. The reaction mechanism of SCR is studied over a Cu-CHA catalyst, by a combination spectroscopic investigations (IR, EPR and EXAFS) and density functional theory (DFT). In this reaction scheme, only stable molecules can adsorb or desorb, and the mass and charge balances are maintained in each step, resulting in the correct stoichiometry for standard and fast SCR. The reaction scheme provides new insights in the SCR reaction, and some implications are discussed. A complete and consistent reaction scheme for SCR provides new insights in the function of Cu-zeolite based SCR catalysts on the atomic level. These insights help to understand the influence of structural properties of the zeolite, such as distribution of the Cu and Al atoms, on the SCR reaction. This both contributes to a fundamental understanding of the chemistry of the SCR reaction, and provides a basis for identification of more efficient SCR catalysts.

Complete catalytic cycle for fast and standard NH3-SCR reaction

253rd National Meeting (2017)

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