The selective catalytic reduction (SCR) of NOx with NH3 over Cu-CHA zeolites is used commercially to abate NOx emissions in lean-burn and diesel engines. NOx SCR in Cu-CHA proceeds via a redox cycle involving CuI and CuII ions. In the low temperature regime (&lt; 573 K), Cu cations are solvated by NH3 (CuII(NH3)4, CuII(OH)(NH3)3 CuI(NH3)2), and become ionically tethered to anionic charges in the framework introduced by Al substitution. NH3-solvated CuI(NH3)2 can diffuse through eight-membered ring windows (&lt;0.4 nm diam.) with reasonable barriers to occupy neighboring zeolite cages and form O2-bridged binuclear Cu intermediates necessary for O2 activation. Rate constants of the oxidation half-cycle are thus sensitive to the framework topology and pore structure, as they are a function of both Cu ion mobility and spatial density. In this work, SCR rates are measured from 1–60 kPa O2 in zeolites with varying three-dimensional cage size and shape (CHA, AEI, AFX) and different pore dimensionality (LEV, FER, MOR) and fit to an empirical Langmurian rate model to isolate kinetics of the reduction and oxidation half cycles. These data demonstrate that rates of the oxidation and reduction half cycles are governed by the Cu volumetric density and the framework topology, both of which influence Cu ion mobility. Apparent zero- and first-order rate constants (per Cu) are 2× lower on AFX than on CHA samples of similar Cu volumetric density, suggesting formation of the binuclear Cu complex may become unfavorable in the smaller AFX cages. Lower dimensionality frameworks (1D, 2D) than CHA (3D) cause the fraction of pairable Cu ions to decrease, consistent with theoretical simulations of Cu ion diffusion barriers and statistical predictions of pairable Cu sites in lower-dimensional pore topologies, as evidenced by apparent first-order rate constants that are ~10× lower on FER than CHA. Of all the zeolites studied, SCR zero- and first-order rate constants (per Cu) are the highest on Cu-CHA, reflecting the benefits conferred by 3D interconnected cage-window topologies to utilize the largest fraction of solvated and mobilized Cu ions during low-temperature NOx SCR.

Effect of zeolite framework otpology on Cu(I) oxidation and Cu(II) reduction kinetics during NOx selective catalytic reduction with NH3

ACS Spring 2022

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