Using in situ electron paramagnetic resonance (EPR) spectroscopy to probe reactivity and relocation of isolated Cu(II) active sites in Cu/SSZ-13 selective catalytic reduction (SCR) catalysts
Feng Gao, Pacific Northwest National Laboratory
Cu/SSZ-13 SCR catalysts contain two isolated Cu(II) active sites, i.e., Z2Cu and ZCuOH species. In order to better understand their reactivity difference under varying conditions relevant to SCR, in situ EPR studies are carried out, including 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. In 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.
We also apply in situ EPR to compare hydrated, dehydrated and NH3-saturated catalysts before and after hydrothermal aging. With the aid from density functional theory calculations, it is found that hydrothermal aging induces Cu relocation, leading to stronger Cu-support interactions. This is particularly evidenced by ZCuOH conversion to Z2Cu, which leads to the generation of a unique structure with two Cu(II) ions positioned in a double 6-membered ring prism. The Cu(II) ions that do not undergo such dramatic changes also appear to stay closer to framework windows as evidenced by the overall increased anisotropy during EPR measurements. In situ EPR measurements under SCR conditions demonstrate that, in comparison to the fresh catalyst, higher percentages of Cu species stay as Cu(II) in the aged catalyst. This suggests that the reduction half-cycle of the SCR redox, i.e., Cu(II) ® Cu(I), becomes slower for the hydrothermally aged catalyst.