(762f) Spectroscopic and Kinetic Responses of Cu-SSZ-13 to SO2 Exposure and Implications for NOx Selective Catalytic Reduction
Two Cu-SSZ-13 catalysts, one with only Cu2+ active sites (3.8 Cu wt%, 100% Cu2+) and another with predominantly [CuOH]1+ active sites (1.5 Cu wt%, 80% CuOH) were synthesized and characterized [3,4]. Our model Cu2+ and [CuOH]1+ catalysts were sulfated with dry SO2 at 200°C using a flow treatment that exposed samples to 5 times the number of SO2 (per Cu), resulting in molar S:Cu ratios of 0.3 and 0.7, respectively. Consistent with the experimental observation of the stronger preference for adsorption of SO2-derived species to [CuOH]1+, density functional theory (DFT) calculations indicate that the binding energy of SO2 on Cu2+ and [CuOH]1+ are -30 and -90 kJ mol-1, respectively. Sulfation decreased the SCR rate (300 ppm NO, 300 ppm NH3, 60% O2, 2% H2O, 8% CO2, balance N2, 200Â°C, per total Cu) by 26% for the Cu2+ catalyst and 64% for the [CuOH]1+ catalyst. Reaction rates are constant when normalized to the number of non-poisoned Cu sites (moles Cu â moles S) for all samples when rates are measured under in the reduction-limited regime (Eapp = 65 Â± 5 kJ mol-1), which indicates that sulfur deactivates Cu sites at a 1:1 S:Cu molar ratio on both Cu2+ and [CuOH]1+ active sites. The coordination environment probed by UV-Visible indicate that S-Cu interactions are observed on [CuOH]1+ but not on Cu2+. Thus, even though sulfur poisons both Cu2+ and [CuOH]1+ sites at a 1:1 molar ratio, spectroscopic observations reveal different sulfur interactions with [CuOH]1+ and Cu2+.
Cu2+ sites are preferred over [CuOH]1+ sites as more SO2-resistant sites based on theoretical DFT calculations and experimental elemental analysis. Reaction kinetics indicate that each SO2 poison binds to only one Cu site, while other non-poisoned Cu sites continue to turn over the SCR cycle. These results predict that synthesizing catalysts with higher fractions of Cu2+ sites will lead to improved emission control catalysts for commercial applications.
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