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

Abstract The effects of sulfur poisoning on Cu-SSZ-13 zeolites, used commercially for the selective catalytic reduction (SCR) of nitrogen oxides (NO X ) with ammonia, were studied by exposing model Cu-zeolite powder samples to dry SO 2 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 Cu 2+ 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 SO 2 -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 SO 2 -derived intermediates at Z 2 Cu and ZCuOH sites, revealing that bisulfates are particularly low in energy, and residual Bronsted protons are liberated at Z 2 Cu sites as bisulfates are formed. Molecular dynamics simulations also show that Cu sites bound to one HSO 4 − are immobile, but become liberated from the framework and more mobile when bound to two HSO 4 − . These findings indicate that Z 2 Cu sites are more resistant to SO 2 poisoning than ZCuOH sites, and are easier to regenerate once poisoned.