Effects of Cu-Zeolite Topology on the Selective Catalytic Reduction of NOx with NH3

Casey  Jones, Purdue University

Casey B. Jones¹, Zhenghang Zhao², Claire T. Nimlos¹, Siddarth H. Krishna¹, Sichi Li², Subramanian Prasad³, Ahmad Moini³, William F. Schneider² and Rajamani Gounder^*,1

(1)Charles D. Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN, (2)Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN, (3)BASF Corporation, 25 Middlesex Essex Turnpike, Iselin, NJ

The selective catalytic reduction (SCR) of NO with NH₃ over Cu-zeolites proceeds via a redox cycle of Cu^I/Cu^II ions. At low temperatures (<573 K), Cu ions are solvated by NH₃, imparting local mobility to Cu within zeolite void spaces. As a result, the Cu^I oxidation and Cu^II reduction half-cycles have different Cu site and density requirements and the number of active Cu sites on Cu-CHA is a function both of reaction conditions and sample composition. Here, we measure SCR rates (473 K, per Cu) over a wide range of O₂ pressures (1-60 kPa) to quantify rate constants in kinetic regimes that are first-order and zero-order in O₂ pressure, which respectively describe rates of dual-site Cu^I oxidation and single-site Cu^II reduction events. These data reveal that the steady-state rates of both the oxidation and reduction half-cycles depend on the spatial density of Cu ions, and their mobility within void spaces defined by the pore topology. Measured first-order rate constants increase systematically with Cu density on a series of Cu-CHA samples, consistent with the proposed dual-site Cu^I oxidation mechanism. Zero-order rate constants on the same series of Cu-CHA samples shows a more gradual dependence on Cu-density, consistent with small changes in the fraction of O₂ oxidizable Cu measured during in operando transient oxidation experiments. Across zeolite topologies, apparent first-order rate constants are similar on FER (2D) compared to CHA (3D), while zero-order rate constants are lower on FER. These findings, consistent with metadynamics simulations, indicate that lower pore dimensionality reduces the effective volumetric footprint that mobile Cu^I(NH₃)₂ complexes can occupy during catalysis. We will also discuss data collected on zeolites of different pore size and dimensionality (1D, 2D, 3D) and topology (channel, cage-window), to understand how zeolite framework topology affects Cu ion mobility and the kinetics of low-temperature SCR.

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