(216g) Effects of Cu Density in Cu-SSZ-13 Zeolites on Low-Temperature NOx Selective Catalytic Reduction with NH3

Ishant Khurana¹, Atish A. Parekh¹, Jonatan D. Albarracin-Caballero¹, Christopher Paolucci², Arthur Shih¹, John R. Di Iorio¹, W. Nicholas Delgass¹, Jeffrey T. Miller¹, William F. Schneider², Rajamani Gounder¹, Fabio H. Ribeiro¹

¹School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, IN 47907, USA

²Department of Chemical and Biomolecular Engineering University of Notre Dame

182 Fitzpatrick Hall, Notre Dame, IN 46556, USA

NOx selective catalytic reduction (SCR) using NH₃ as a reductant over Cu-SSZ-13 zeolites is a commercial technology used to meet emissions targets in lean-burn and diesel exhaust. Low temperature (473 K) NOx SCR on Cu-SSZ-13 occurs via a Cu(II)/Cu(I) redox cycle [1] on two types of isolated Cu sites, both divalent Cu²⁺ cations compensating paired framework Al atoms and monovalent [CuOH]⁺ complexes at isolated framework Al atoms, which are solvated by NH₃ during reaction [2]. Both types of cationic Cu sites were identified by infrared, UV-Visible and operando X-ray absorption (XAS) spectroscopies and also from their exchange stoichiometry with surface protons, the residual number of which was quantified by NH₃ titration. Under standard SCR conditions, in which O₂ is used as a four-electron oxidant in the Cu(I)/Cu(II) redox cycle, we have identified two different kinetic regimes that are characterized by different apparent orders, activation energies, and steady-state Cu(I)/Cu(II) distributions. The prevalence of standard SCR kinetic regimes depends strongly on the Cu density (determined by the Si/Al and Cu/Al) of the Cu-SSZ-13 catalysts. One regime corresponds to SCR cycles limited by Cu(I)->Cu(II) oxidation with O₂, which we probe in operando by varying reaction parameters (temperatures, gas pressures) while collecting steady-state kinetic data and XAS spectra, and in transient oxidation experiments of reduced Cu(I)-SSZ-13 catalysts using either O₂ and NO₂. These experiments reveal differences in the kinetics and Cu site requirements between O₂ and NO₂-catalyzed Cu(I)->Cu(II) oxidation, which respectively are four-electron and one-electron oxidants that propagate SCR cycles under â??standardâ? and â??fastâ? conditions. We provide evidence that O₂-oxidation requires two Cu sites and two NO molecules to be oxidized, while NO₂-oxidation occurs at single, isolated Cu sites. As a result, standard SCR rates depend on Cu cation proximity and distribution in Cu-SSZ-13 zeolites when Cu(I) oxidation steps are kinetically-relevant, but are independent of Cu density otherwise.

[1] C. Paolucci, A.A. Verma, S.A. Bates, V.F. Kispersky, J.T. Miller, R. Gounder, et al., Isolation of the Copper Redox Steps in the Standard Selective Catalytic Reduction on Cu-SSZ-13, (2014) 12022â??12027.

[2] C. Paolucci, A.A. Parekh, I. Khurana, J.R. Di Iorio, J. Albarracin, A. Shih, et al., Catalysis in a Cageâ?¯: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations In SSZ-13 Zeolites, J. Am. Chem. Soc. (2016) 1â??63.