Revisiting the nature of Cu-sites in activated Cu-SSZ-13 catalyst for SCR reaction

Cu-SSZ-13 is a highly active NH3-SCR catalyst for the abatement of harmful nitrogen oxides (NOx, x = 1, 2) from the exhaust of lean-burn engines. The study of Cu-speciation occurring upon thermal dehydration is a key step for the understanding of the enhanced catalytic properties of this material and for identifying the SCR active sites and their redox capability. Herein, we combined FTIR, X-ray absorption (XAS) and emission (XES) spectroscopies with DFT computational analysis to elucidate the nature and location of the most abundant Cu-sites in the activated catalyst. Different Cu species have been found to be dominant as a function of the dehydration temperature and conditions. Data analysis revealed that the dehydration process of Cu cations is essentially completed at 250 °C, with the formation of dehydrated [CuOH]+ species hosted in a close proximity of 1 Al sites in both d6r and 8r units of the SSZ-13 matrix. These species persist at higher temperature only if a certain amount of O2 is present in the gas feed, while in inert conditions they undergo virtually total “self-reduction” as a consequence of an OH extra-ligand loss, resulting in bi-coordinated bare Cu+ cations. Synchrotron characterization supported by computational analysis allowed a unprecedented quantitative refinement of local environment and structural parameters of these Cu(II) and Cu(I) species.