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 NO_x emissions control in diesel and lean-burn engines [1]. 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 [2]. SCR occurs via a redox mechanism in which NO and NH₃ reduce Cu²⁺ and O₂ oxidizes Cu⁺ [3]. At low temperatures (<523 K), NH₃ solvates and mobilizes Cu cations [4], enabling the reaction of two Cu⁺(NH₃)₂ complexes with O₂ to form binuclear O₂-bridged Cu²⁺ complexes in the SCR oxidation half-cycle [3]. Apparent activation energy (E_app) 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 E_app 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 E_app 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 [6]. Measurements at multiple temperatures (446–500 K) enable extraction of E_app values for Cu⁺ oxidation and Cu²⁺ reduction, showing that E_app 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 E_app 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 NO_x light-off behavior.

References.

[1] Peden, C. H. F. J. Catal. 2019, 373, 384–389.

[2] Krishna, S. H., et al. J. Phys. Chem. Lett. 2020, 11 (13), 5029–5036.

[3] Paolucci, C., et al. Science 2017, 357 (6354), 898–903.

[4] Paolucci, C., et al. J. Am. Chem. Soc. 2016, 138 (18), 6028–6048.

[5] Gao, F., et al. J. Am. Chem. Soc. 2017, 139 (13), 4935–4942.

[6] Jones, C. B. , et al. J.  Catal 2020, 389, 140–149.