STRUCTURE OF MOBILE DICOPPER COMPLEXES IN Cu-ZEOLITE CATALYSTS FOR NH3-SCR CAPTURED BY XAS AND UV-VIS SPECTROSCOPY
Cu-chabazite (Cu-CHA) is the catalyst of choice for the ammonia-mediated Selective Catalytic Reduction (NH3-SCR) for NOx abatement in diesel vehicle after-treatment systems. In-situ XAS and UV-Vis coupled to Wavelet Transform EXAFS reveal the structure of oxygen-bridged diamino dicopper (II) complexes responsible for the low- temperature activity of the catalyst.
The NH3-SCR technology is based on the reaction of NO and NH3 in presence of O2, to give N2 and H2O as clean products. In Cu ion- exchanged zeolites, the reaction proceeds by reduction of CuII to CuI with release of the products. The catalytic cycle is closed by reoxidation of CuI to CuII, which requires the activation of O2. In a catalyst pre-treated with O2, CuII ions compensate the negative charges of the framework generated by the substitution of AlIII in the otherwise neutral silicate network. At high Si/Al ratio, an extra ligand is needed for charge compensation, resulting in [CuIIOH]+ or oxygen-bridged multi-copper moieties (Figure 13d, grey panel) . These framework- interacting sites feature a well-defined second- shell peak in the extended X-ray absorption fine structure (EXAFS) spectra (due to the scattering of Si/Al framework atoms) and a very intense
and structured absorption in the d-d region of the ultraviolet-visible (UV-Vis) spectrum, the so-called quadruplet (dark grey dashed curves in Figure 13b and Figure 13c, respectively).
After reduction with NO/NH3, mobile [CuI(NH3)2]+ complexes are formed, characterised by a prominent rising-edge peak in the Cu K-edge X-ray absorption near-edge structure (XANES) spectrum . This corresponds to a decrease in both EXAFS first and second shell peaks and by an important change in the UV-Vis spectrum, in line with formation of closed-shell CuI ions (blue curves in Figure 13a-c).
Upon exposure of [CuI(NH3)2]+ to O2 at 200°C, CuI is reoxidised to CuII; this is a crucial step in the NH3-SCR reaction cycle, involving the interaction of an O2 molecule with a pair of
Fig. 13: a) XANES, (b) EXAFS and (c) UV-Vis spectra of Cu-CHA (Si/Al = 15, Cu/Al = 0.5) during pre- treatment in O2 (grey dashed curve), reduction in NO/NH3 (blue) and subsequent reaction with O2 (red). d) Pictorial representation of the corresponding Cu structures (same colour code). Atom colour: Cu, green; H, white; O, red; N, blue, Al, yellow; Si, grey.