The dynamic nature of Cu sites in Cu-SSZ-13 and the origin of the seagull NOx conversion profile during NH3-SCR

Cu-Zeolites with chabazite structure show a peculiar dual-maxima NO conversion profile, also known as a seagull profile, during the selective catalytic reduction by ammonia. In order to understand the origin of this behavior, systematic catalytic tests and operando spectroscopy were applied to derive structure-performance relationships for Cu-SSZ-13 catalysts with low and high Cu loading. Operando X-ray absorption, X-ray emission and in situ electron paramagnetic resonance spectroscopy measurements, including novel photon-in/ photon-out techniques, demonstrated the interconversion of isolated Cu sites and dimeric bis.mu-oxo) Cu species, the former occurring via formation of ammonia Cu2+/Cu+ complexes and the latter in an oxidizing gas mixture. The formation of dimeric Cu+-O-2-Cu+ species by involving Cu sites in close vicinity was linked to the high activity at low temperatures of the highly loaded Cu-SSZ-13 sample. In contrast, the isolated Cu sites present at very low Cu loadings are strongly poisoned by adsorbed NH3. The activity decrease around 350 degrees C that gives rise to the seagull shaped NO conversion profile could be attributed to amore localized structure of mono(mu-oxo) dicopper complexes. Above this temperature, which corresponds to partial NH3 desorption from Cu sites, the isolated Cu sites migrate to form additional dimeric entities thus recovering the SCR activity.