460174 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  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 . 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 “standard” and “fast” 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.
 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) 12022–12027.
 C. Paolucci, A.A. Parekh, I. Khurana, J.R. Di Iorio, J. Albarracin, A. Shih, et al., Catalysis in a Cage : Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations In SSZ-13 Zeolites, J. Am. Chem. Soc. (2016) 1–63.
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