Quantitative Cu counting methodologies for Cu/SSZ-13 selective catalytic reduction catalysts by electron paramagnetic resonance spectroscopy

Two Cu/SSZ-13 selective catalytic reduction (SCR) catalysts with distinct Si/Al ratios and isolated Z₂Cu and ZCuOH distributions are prepared for in situ electron paramagnetic resonance (EPR) spectroscopic studies. These in situ studies include dehydration, titration of dehydrated samples with NO+O₂ and NH₃, titration of NH₃-saturated samples with NO+O₂, and finally steady-state standard NH₃-SCR reaction. During dehydration, EPR-active hydrated ZCuOH loses H₂O ligands and becomes EPR-silent due to a pseudo Jahn−Teller effect; a portion of ZCuOH also undergoes autoreduction to ZCu(I) species, a process that also induces EPR invisibility. During NO+O₂ treatment of dehydrated samples, ZCu(I) species are oxidized to Cu(II)−NO₃⁻ species, regaining EPR visibility. During NH₃ titration, EPR-silent dehydrated ZCuOH can also regain EPR visibility by coordinating with NH₃ ligands. During NO+O₂ titration of NH₃-saturated samples, EPR-active Cu contents first decrease due to Cu(II) reduction to Cu(I) and then increase due to Cu(II)− NO₃⁻ species formation. However, the Cu(II)−NO₃⁻ formation chemistry is substantially slower for the Si/Al = 36 catalyst. In steady-state SCR studies, the EPR-active content decreases with increasing temperature in the kinetically controlled low-temperature regime and becomes largely invariant in the mass transfer-limited regime. Importantly, Cu sites in the SCR more active Si/Al = 6 catalyst display substantially higher EPR visibility than the SCR less active Si/Al = 36 catalyst at any reaction temperatures tested. The higher Cu loading for the former catalyst is believed to be the key for this difference.