Spectroscopic and kinetic responses of Cu-SSZ-13 to SO2 exposure and implications for NOx selective catalytic reduction

The effects of sulfur poisoning on Cu-SSZ-13 zeolites, used commercially for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ammonia, were studied by exposing model Cu-zeolite powder samples to dry SO2 and O-2 streams at 473 and 673 K, and then analyzing the surface intermediates formed using spectroscopic and kinetic assessments. Model Cu-SSZ-13 zeolites were synthesized to contain distinct Cu active site types, predominantly either divalent Cu2+ ions exchanged at proximal framework Al (Z(2)Cu), or monovalent CuOH+ complexes exchanged at isolated framework Al (ZCuOH). SCR turnover rates (473 K, per Cu) decreased linearly with increasing S content to undetectable values at equimolar S:Cu ratios, consistent with poisoning of each Cu site with one SO2-derived intermediate. Cu and S K-edge X-ray absorption spectroscopy and density functional theory calculations were used to identify the structures and binding energies of different SO2-derived intermediates at Z(2)Cu and ZCuOH sites, revealing that bisulfates are particularly low in energy, and residual Bronsted protons are liberated at Z2Cu sites as bisulfates are formed. Molecular dynamics simulations also show that Cu sites bound to one HSO4- are immobile, but become liberated from the framework and more mobile when bound to two HSO4-. These findings indicate that Z(2)Cu sites are more resistant to SO2 poisoning than ZCuOH sites, and are easier to regenerate once poisoned.