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

Authors: 
Di Iorio, J. R., Purdue University
Gounder, R., Purdue University
Delgass, W. N., Purdue University
Khurana, I., Purdue University
Ribeiro, F. H., Purdue University
Miller, J. T., Purdue University

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.