Temperature Dependence of Oxidation and Reduction Kinetics during NOx Selective Catalytic Reduction with NH3 on Cu-Chabazite Zeolites
Siddarth Krishna, Davidson School of Chemical Engineering, Purdue University
Selective catalytic reduction (SCR) with NH3 using Cu-exchanged chabazite (CHA) zeolites is a commercial technology for NOx emissions control in diesel and lean-burn engines . The majority of emissions occur at low engine exhaust temperatures (<523 K), and operating temperatures change transiently in SCR aftertreatment systems, motivating a deeper understanding of active site requirements and kinetically relevant processes during SCR catalysis as a function of temperature and catalyst composition . SCR occurs via a redox mechanism in which NO and NH3 reduce Cu2+ and O2 oxidizes Cu+ . At low temperatures (<523 K), NH3 solvates and mobilizes Cu cations , enabling the reaction of two Cu+(NH3)2 complexes with O2 to form binuclear O2-bridged Cu2+ complexes in the SCR oxidation half-cycle . Apparent activation energy (Eapp) values measured at fixed gas conditions (e.g., 10 kPa O2) increase with Cu density, apparently correlating to a transition from Cu+ oxidation to Cu2+ reduction as the dominant kinetically relevant step [3, 5]. Such data are often interpreted as Cu+ oxidation having a lower Eapp than Cu2+ reduction. However, kinetic measurements at fixed gas conditions convolute the influences of kinetic regime and catalyst composition.
To decouple the influences of Cu density and kinetic regime on Eapp values, we measured steady-state SCR rates across widely varying O2 pressures to isolate rate constants for Cu+ oxidation and Cu2+ reduction on Cu-CHA of varying Cu density . Measurements at multiple temperatures (446–500 K) enable extraction of Eapp values for Cu+ oxidation and Cu2+ reduction, showing that Eapp values increase with Cu density under both oxidation and reduction-limited conditions. Consequently, the kinetic relevance of oxidation and reduction steps is only weakly sensitive to temperature, a conclusion that is not evident from prior literature data at fixed gas conditions. Increasing Cu density not only increases SCR rates (per Cu), but also increases the sensitivity of SCR rates to temperature. Mechanistically, such changes in Eapp values with active site density are inconsistent with mean-field kinetic descriptions, implying non-mean-field behavior that is sensitive to the mobility and proximity of Cu active sites. This work reveals the temperature- and composition-dependent kinetic behavior of Cu+ oxidation and Cu2+ reduction processes underlying SCR catalysis, which influence observed low-temperature NOx light-off behavior.
 Peden, C. H. F. J. Catal. 2019, 373, 384–389.
 Krishna, S. H., et al. J. Phys. Chem. Lett. 2020, 11 (13), 5029–5036.
 Paolucci, C., et al. Science 2017, 357 (6354), 898–903.
 Paolucci, C., et al. J. Am. Chem. Soc. 2016, 138 (18), 6028–6048.
 Gao, F., et al. J. Am. Chem. Soc. 2017, 139 (13), 4935–4942.
 Jones, C. B. , et al. J. Catal 2020, 389, 140–149.