Identification of the active Cu site in standard selective catalytic reduction with ammonia on Cu-SSZ-13

Abstract

Copper-exchanged SSZ-13 catalysts were used for the standard selective catalytic reduction (SCR) reaction at 473K with 320ppm NO, 320ppm NH3, 10% O2, 8% CO2, and 6% H2O. The copper to total aluminum atomic ratio (Cu:Altot) was varied from 0 to 0.35 (copper to framework Al ratio (Cu:Alf)=0–0.41) over seven samples with silicon to total aluminum atomic ratio (Si:Altot) ranging between 4.3 and 4.5 (silicon to framework Al (Si:Alf)=5.1–5.3). The standard SCR rate per gram was observed to increase linearly up to Cu:Altot=0.2 (Cu:Alf=0.23) with a maximum rate of 3.8×10−6molNOgcat−1s−1, which ruled out heat and mass transfer effects by the Koros–Nowak test. The rate per gram was observed to track with a hydrated Cu(II) species in ultraviolet–visible–near infrared spectroscopy (UV–Vis–NIR) at ambient conditions. This species was shown by operando X-ray absorption spectroscopy (XAS) to become the active, isolated Cu. Density functional theory calculations identified an exchanged isolated Cu(II) in the six-membered ring of SSZ-13 as the most stable position for isolated Cu(II). Statistical simulations showed that the number of isolated Cu in six-membered rings containing 2 framework Al reaches a maximum at Cu:Alf=0.22 for Si:Alf=5.3, which is consistent with the maximum observed rate per gram at Cu:Altot=0.2 (Cu:Alf=0.23) and supports the isolated Cu(II) in the six-membered ring as the active Cu species. Above Cu:Altot=0.2 (Cu:Alf=0.23), a new CuxOy (x,y⩾1) species was observed from ambient XAS measurements but did not contribute to standard SCR because the rate at Cu:Altot=0.35 (Cu:Alf=0.41) still tracked with the amount of hydrated Cu(II), the active site precursor. The standard SCR rate per gram was shown to be inversely related to the number of available Brønsted acid sites, suggesting the kinetically-relevant standard SCR steps were not occurring on those sites in the Cu-SSZ-13 samples, but not ruling them out as contributors to the catalysis. Thus, we suggest the kinetically-relevant steps for standard SCR occur on the isolated Cu(II) species located primarily in exchange positions of the six-membered ring of the SSZ-13 structure.