Probing Active-Site Relocation in Cu/SSZ-13 SCR Catalysts during Hydrothermal Aging by In Situ EPR Spectroscopy, Kinetics Studies, and DFT Calculations

Cu/SSZ-13
selective catalytic reduction catalysts display activity
loss after mild hydrothermal aging without support degradation or
loss of isolated Cu cation content. By applying electron paramagnetic
resonance spectroscopy to compare hydrated, dehydrated, and NH3-saturated catalysts before and after hydrothermal aging,
as well as 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 structure with
two Cu­(II) cations positioned in a double six-membered ring prism.
These two Cu­(II) ions have a separation of ∼3.90 Å as
derived by EPR analysis, which agrees well with our DFT-based model.
The Cu­(II) cations 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, a 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.