NOx (x = 1,2) species are pollutants generated in oxygen rich combustion. In an increasingly more stringent emission regulations, ammonia Selective Catalytic Reduction (NH3-SCR) of NOx into N2 and H2O is the leading technology in diesel and stationary sources. However, the active sites and reaction mechanism of the commercially used Cu-exchanged zeolite SCR catalyst, i.e. Cu-SSZ-13, are still being investigated. Past research has shown that the presence of NO2 through catalytic oxidation of NO to NO2:
NO + ½ O2 → NO2
is known to affect NH3-SCR reaction mechanism and activity. Hence, we began our study by modeling the NO oxidation reaction on Cu-SSZ-13 using hybrid first-principles calculations to model two different active sites: single and dimeric Cu.
We first examine the position and redox capability of single and dimeric Cu species using first-principles thermodynamics. Cu prefers to be charge compensated by two Al in the zeolite near the 6-Ring as isolated Cu(II) and stays as Cu(II) in the presence of O2 and H2O. Cu pairs prefer to be in the 8-Ring and stay as Cu(II). On the other hand, Cu in the 6- or the 8-Ring can undergo redox when charge compensated by one Al. This analysis frame the discussion of site activities, variation of Si/Al ratio, Cu loadings, and reaction conditions in the zeolites.
NH3-SCR reactions occurs on isolated Cu charge compensated by two Al. During the redox cycle, Cu exists in both reduced Cu(I) and oxidized Cu(II) form, and oxidation of NO on a reduced Cu(I) is one of the key step in the mechanism. Our first-principles thermodynamic analysis on Cu(I) has identified NO3 as the most stable species under NO oxidation conditions (10% O2, 300 ppm NO, 150 ppm NO2 and 543 K). Further studies on the formation of NO3 is recommended for future work.
We identify the molecular adsorption and subsequent activation of oxygen as a kinetically relevant step during dry NO oxidation catalysis. This can be written as:
★ + O2 → O2★ + NO → O★ + NO2 + NO → ★ + 2 NO2
where ★ denotes the active sites for catalytic dry NO oxidation. On both isolated Cu sites, molecular adsorption of oxygen are not highly favored under dry NO oxidation conditions (10% O2, 300 ppm NO, 150 ppm NO2 and 543 K). We show CuxOy species in the 8 member ring of SSZ-13 as a more facile redox site for the dry oxidation of NO, consistent with experimental observations.
Dry NO oxidation, being selective to only CuxOy species in Cu-SSZ-13, can be used as a probe reaction to identify clustering of Cu ions. Further molecular investigation on paired Cu sites is recommended for future work. Finally, applying this model to other cations, e.g. Fe, and other catalysts materials are recommended for future work.