Characterization of Cu-SSZ-13 NH3 SCR catalysts: an in situFTIR study

Abstract

The adsorption of CO and NO over Cu-SSZ-13 zeolite catalysts, highly active in the selective catalytic reduction of NO_x with NH₃, was investigated by FTIR spectroscopy, and the results obtained were compared to those collected from other Cu-ion exchanged zeolites (Y,FAU and ZSM-5). Under low CO pressures and at room temperature (295 K), CO forms monocarbonyls exclusively on the Cu⁺ ions, while in the presence of gas phase CO dicarbonyls on Cu⁺ and adsorbed CO on Cu²⁺ centers form, as well. At low (cryogenic) sample temperatures, tricarbonyl formation on Cu⁺ sites was also observed. The adsorption of NO produces IR bands that can be assigned to nitrosyls bound to both Cu⁺ and Cu²⁺ centers, and NO⁺ species located in charge compensating cationic positions of the chabasite framework. On the reduced Cu-SSZ-13 samples the formation of N₂O was also detected. The assignment of the adsorbed NO_x species was aided by adsorption experiments with isotopically labeled ¹⁵NO. The movement of Cu ions from the sterically hindered six member ring position to the more accessible cavity positions as a result of their interaction with adsorbates (NO and H₂O) was clearly evidenced. Comparisons of the spectroscopy data obtained in the static transmission IR system to those collected in the flow-through diffuse reflectance cell points out that care must be taken when general conclusions are drawn about the adsorptive and reactive properties of metal cation centers based on a set of data collected under well defined, specific experimental conditions.