Investigating the role of Cu-oxo species in Cu-nitrate formation over Cu-CHA catalysts

Abstract

The speciation of framework-interacting Cu^II sites in Cu-chabazite zeolite catalysts active in the selective catalytic reduction of NO_x with NH₃ is studied, to investigate the influence of the Al content on the copper structure and their reactivity towards a NO/O₂ mixture. To this aim, three samples with similar Cu densities and different Si/Al ratios (5, 15 and 29) were studied using in situ X-ray absorption spectroscopy (XAS), FTIR and diffuse reflectance UV-Vis during pretreatment in O₂ followed by the reaction. XAS and UV-Vis data clearly show the main presence of Z₂Cu^II sites (with Z representing a framework negative charge) at a low Si/Al ratio, as predicted. EXAFS wavelet transform analysis showed a non-negligible fraction of proximal Z₂Cu^II monomers, possibly stabilized into two 6-membered rings within the same cage. These sites are not able to form Cu-nitrates by interaction with NO/O₂. By contrast, framework-anchored Z[Cu^II(NO₃)] complexes with a chelating bidentate structure are formed in samples with a higher Si/Al ratio, by reaction of NO/O₂ with Z[Cu^II(OH)] sites or structurally similar mono- or multi-copper Z_x[Cu^II_xO_y] sites. Linear combination fit (LCF) analysis of the XAS data showed good agreement between the fraction of Z[Cu^II(OH)]/Z_x[Cu^II_xO_y] sites formed during activation in O₂ and that of Z[Cu^II(NO₃)] complexes formed by reaction with NO/O₂, further confirming the chemical inertia of Z₂Cu^II towards these reactants in the absence of solvating NH₃ molecules.