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 Z 2 Cu 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+O 2 and NH 3, titration of NH 3 -saturated samples with NO+O 2, and finally steady-state standard NH 3 -SCR reaction. During dehydration, EPR-active hydrated ZCuOH loses H 2 O 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+O 2 treatment of dehydrated samples, ZCu(I) species are oxidized to Cu(II)–NO 3 – species, regaining EPR visibility. During NH 3 titration, EPR-silent dehydrated ZCuOH can also regain EPR visibility by coordinating with NH 3 ligands. During NO+O 2 titration of NH 3 -saturated samples, EPR-active Cu contents first decrease due to Cu(II) reduction to Cu(I) and then increase due to Cu(II)–NO 3 – species formation. However, the Cu(II)–NO 3 – 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.