(351a) Effects of Framework Aluminum Distribution in SSZ-13 on Extraframework Copper Speciation and Selective Catalytic Reduction of NOx with NH3

We discuss straightforward methods to differentiate between and quantify isolated Cu²⁺ and [CuOH]⁺ species exchanged onto SSZ-13 zeolites, which are used commercially for the selective catalytic reduction (SCR) of NOx with NH₃, and discuss the influence of framework aluminum density and distribution on Cu cation speciation. Under ambient conditions, both isolated Cu²⁺ and [CuOH]⁺ species are hydrated as hexa-aquo complexes and cannot be distinguished in X-ray absorption (XAS) or UV-Visible (UV-Vis) spectra. The saturation of Cu-SSZ-13 zeolites with gaseous NH₃ (433 K) followed by purging in wet helium (0.5-3% H₂O, 433 K) leads to the selective retention of NH₄⁺ at residual Brønsted sites but not at Lewis acidic Cu cations [1, 2]. The Brønsted sites present on metal-exchanged zeolites can then be quantified in a subsequent NH₃ temperature programmed desorption (TPD) experiment. These methods show that on Cu-SSZ-13 samples oxidized in air (773 K), Cu²⁺ and [CuOH]⁺ species exchange for two and one H⁺ site, respectively. Operando XAS experiments and DFT calculations indicate that both NO and NH₃ (473 K) are required to reduce isolated Cu²⁺ species to Cu⁺. Reduction of Cu-SSZ-13 samples in flowing NO and NH₃ (473 K) prior to titration of residual H⁺ sites shows that Cu²⁺ sites balancing two Al atoms reduce to form a Cu⁺ cation and a proximal H⁺ site, while [CuOH]⁺ sites reduce to form a Cu⁺ site and H₂O without generating an additional H⁺ site. We provide experimental evidence that Cu²⁺ cations preferentially exchange in SSZ-13 at paired framework Al atoms, which can be titrated and quantified using Co²⁺ cations, before [CuOH]⁺ species exchange at isolated Al atoms, consistent with DFT predictions of the strong energetic preference for Cu­²⁺ exchange at the double six-membered ring (D6R) of chabazite containing two Al atoms. This study illustrates how gaseous NH₃ can titrate residual H⁺ sites in metal-exchanged small-pore zeolites and be quantified in straightforward TPD experiments, how such titrations can be used in situ after oxidation and reduction treatments that resemble the Cu²⁺/Cu⁺ redox half-cycles prevalent during SCR, and how they can distinguish the reduction behavior of Cu²⁺ and [CuOH]⁺ sites. We will also provide evidence that the methods used to synthesize zeolites at a fixed Si/Al ratio can influence the density of aluminum pairs, which in turn influences the Cu speciation and catalytic behavior for NOx SCR with NH₃.

[1] Bates, S. A., Delgass, W. N., Ribeiro, F. H., Miller, J. T., Gounder, R., Journal of Catalysis, 312 (2014) 26-36.

[2] Di Iorio, J. R., Bates, S. A., Verma, A. A., Delgass, W. N., Ribeiro, F. H., Miller, J. T., Gounder, R., Topics in Catalysis, 58 (2015) 424-434.