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

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 Cu^I ions, which are present as mobile Cu^I diamine complexes [Cu^I(NH₃)₂]⁺. To determine the structure and reactivity of the species formed by oxidation of these Cu^I 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 [Cu₂(NH₃)₄O₂]²⁺ mobile complex with a side-on μ-η²,η²-peroxo diamino dicopper(II) structure, accounting for 80-90% of the total Cu content. These [Cu₂(NH₃)₄O₂]²⁺ are completely reduced to [Cu^I(NH₃)₂]⁺ at 200 °C in a mixture of NO and NH₃. Some N₂ is formed as well, which suggests the role of the dimeric complexes in the low-temperature NH₃-SCR reaction. The reaction of [Cu₂(NH₃)₄O₂]²⁺ complexes with NH₃ leads to a partial reduction of the Cu without any formation of N₂. The reaction with NO results in an almost complete reduction to Cu^I, under the formation of N₂. This indicates that the low-temperature NH₃-SCR reaction proceeds via a reaction of these complexes with NO.