Structure and Reactivity of Oxygen-Bridged Diamino Dicopper(II) Complexes in Cu-Ion-Exchanged Chabazite Catalyst for NH-Mediated Selective Catalytic Reduction.

The NH-mediated selective catalytic reduction (NH-SCR) of NOx over Cu-ion-exchanged chabazite (Cu-CHA) catalysts is the basis of the technology for abatement of NOx from diesel vehicles. A crucial step in this reaction is the activation of oxygen. Under conditions for low-temperature NH-SCR, oxygen only reacts with Cu ions, which are present as mobile Cu diamine complexes [Cu(NH)]. To determine the structure and reactivity of the species formed by oxidation of these Cu diamine complexes with oxygen at 200 °C, we have followed this reaction, using a Cu-CHA catalyst with a Si/Al ratio of 15 and 2.6 wt% Cu, by X-ray absorption spectroscopies (XANES and EXAFS) and diffuse reflectance UV-Vis spectroscopy, with the support of DFT calculations and advanced EXAFS wavelet transform analysis. The results provide unprecedented direct evidence for the formation of a [Cu(NH)O] mobile complex with a side-on μ-η,η-peroxo diamino dicopper(II) structure, accounting for 80-90% of the total Cu content. These [Cu(NH)O] are completely reduced to [Cu(NH)] at 200 °C in a mixture of NO and NH. Some N is formed as well, which suggests the role of the dimeric complexes in the low-temperature NH-SCR reaction. The reaction of [Cu(NH)O] complexes with NH leads to a partial reduction of the Cu without any formation of N. The reaction with NO results in an almost complete reduction to Cu, under the formation of N. This indicates that the low-temperature NH-SCR reaction proceeds via a reaction of these complexes with NO.