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

Abstract

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.