A consistent reaction scheme for the selective catalytic reduction of nitrogen oxides with ammonia

For the first time, the standard and fast selective catalytic reduction of NO by NH₃ are described in a complete catalytic cycle, that is able to produce the correct stoichiometry, while only allowing adsorption and desorption of stable molecules. The standard SCR reaction is a coupling of the activation of NO by O₂ with the fast SCR reaction, enabled by the release of NO₂. According to the scheme, the SCR reaction can be divided in an oxidation of the catalyst by NO + O₂ and a reduction by NO + NH₃; these steps together constitute a complete catalytic cycle. Furthermore both NO and NH₃ are required in the reduction, and, nally, oxidation by NO + O₂ or NO₂ leads to the same state of the catalyst. These points are shown experimentally for a Cu-CHA catalyst, by combining in situ X-ray absorption spectrosocpy (XAS), electron paramagnetic resonance (EPR), and Fourier transform infrared spectroscopy (FTIR). A consequence of the reaction scheme is that all intermediates in fast SCR are also part of the standard SCR cycle. The calculated activation energy by density functional theory (DFT) indicates that the oxidation of an NO molecule by O₂ to a bidentate nitrate ligand is rate determining for standard SCR. Finally, the role of a nitrate/nitrite equilibrium and the possible in uence of Cu dimers and Brønsted sites are discussed, and an explanation is offered as to how a catalyst can be effective for SCR, while being a poor catalyst for NO oxidation to NO₂.