Structure and Reactivity of Oxygen-Bridged Diamino Dicopper(II) Complexes in Cu-Ion-Exchanged Chabazite Catalyst for NH3-Mediated Selective Catalytic Reduction

The NH3-mediated selective catalytic reduction (NH3-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 NH3-SCR, oxygen only reacts with CuI ions, which are present as mobile CuI diamine complexes [CuI(NH3)(2)](+). To determine the structure and reactivity of the species formed by oxidation of these CuI diamine complexes with oxygen at 200 degrees C, we have followed this reaction, using a Cu-CHA catalyst with a Si/Al ratio of 15 and 2.6 wt% Cu, by Xray 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-2(NH3)(4)O-2](2+) mobile complex with a side-on mu-eta(2), eta(2)-peroxo diamino dicopper(II) structure, accounting for 80-90% of the total Cu content. These [Cu-2(NH3)(4)O-2](2+) are completely reduced to [CuI(NH3)(2)](+) at 200 degrees C in a mixture of NO and NH3. Some N-2 is formed as well, which suggests the role of the dimeric complexes in the low-temperature NH3-SCR reaction. The reaction of [Cu-2(NH3)(4)O-2](2+) complexes with NH3 leads to a partial reduction of the Cu without any formation of N-2. The reaction with NO results in an almost complete reduction to CuI, under the formation of N-2. This indicates that the low-temperature NH3-SCR reaction proceeds via a reaction of these complexes with NO.