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Structural and kinetic changes to small-pore Cu-zeolites after hydrothermal aging treatments and selective catalytic reduction of NO_x with ammonia

Albarracin-Caballero, Jonatan D. and Khurana, Ishant and Di Iorio, John R. and Shih, Arthur J. and Schmidt, Joel E. and Dusselier, Michiel and Davis, Mark E. and Yezerets, Aleksey and Miller, Jeffrey T. and Ribeiro, Fabio H. and Gounder, Rajamani (2017) Structural and kinetic changes to small-pore Cu-zeolites after hydrothermal aging treatments and selective catalytic reduction of NO_x with ammonia. Reaction Chemistry and Engineering, 2 (2). pp. 168-179. ISSN 2058-9883. doi:10.1039/c6re00198j.

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Three small-pore, eight-membered ring (8-MR) zeolites of different cage-based topology (CHA, AEI, RTH), in their proton- and copper-exchanged forms, were first exposed to high temperature hydrothermal aging treatments (1073 K, 16 h, 10% (v/v) H_2O) and then to reaction conditions for low temperature (473 K) standard selective catalytic reduction (SCR) of NO_x with ammonia, in order to study the effect of zeolite topology on the structural and kinetic changes that occur to Cu-zeolites used in NO_x abatement. UV-visible spectra were collected to monitor changes to Cu structure and showed that band intensities for isolated, hydrated Cu^(2+) cations (∼12 500 cm^(−1)) remain constant after hydrothermal aging, but decrease in intensity upon subsequent exposure to low temperature SCR reaction conditions. Standard SCR rates (per Cu, 473 K), activation energies, and reaction orders are similar between Cu-AEI and Cu-CHA zeolites before and after hydrothermal aging, although rates are lower after hydrothermal aging as expected from the decreases in intensity of UV-visible bands for Cu^(2+) active sites. For Cu-RTH, rates are lower (by 2–3×) and apparent activation energies are lower (by ∼2×) than for Cu-AEI or Cu-CHA. These findings suggest that the RTH framework imposes internal transport restrictions, effectively functioning as a one-dimensional framework during SCR catalysis. Hydrothermal aging of Cu-RTH results in complete deactivation and undetectable SCR rates, despite X-ray diffraction patterns and Ar micropore volumes (87 K) that remain unchanged after hydrothermal aging treatments and subsequent SCR exposure. These findings highlight some of the differences in low temperature SCR behavior among small-pore Cu-zeolites of different topology, and the beneficial properties conferred by double six-membered ring (D6R) composite building units. They demonstrate that deleterious structural changes to Cu sites occur after exposure to hydrothermal aging conditions and SCR reactants at low temperatures, likely reflecting the formation of inactive copper-aluminate domains. Therefore, the viability of Cu-zeolites for practical low temperature NO_x SCR catalysis cannot be inferred solely from assessments of framework structural integrity after hydrothermal aging treatments, but also require Cu active site and kinetic characterization after hydrothermally aged zeolites are exposed to low temperature SCR reaction conditions.

Item Type:Article
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URLURL TypeDescription Information
Dusselier, Michiel0000-0002-3074-2318
Davis, Mark E.0000-0001-8294-1477
Gounder, Rajamani0000-0003-1347-534X
Additional Information:© 2016 The Royal Society of Chemistry. Received 01 Nov 2016, Accepted 07 Dec 2016, First published online 07 Dec 2016. We acknowledge the financial support provided by the National Science Foundation GOALI program under award number 1258715-CBET. RG acknowledges the financial support from a Ralph E. Powe Junior Faculty Enhancement Award from the Oak Ridge Associated Universities (ORAU). MD acknowledges Research Foundation Flanders (FWO) for postdoctoral funding. We thank Dr. John Harwood (Purdue Interdepartmental NMR Facility) for assistance collecting the NMR spectra, and Dr. Atish A. Parekh for helpful technical discussions. We also thank Daniel Gonzalez (Universidad Nacional de Colombia), through the Undergraduate Research Experience Purdue-Colombia (UREP-C) program, for experimental assistance constructing the apparatus to perform hydrothermal aging treatments and performing some aging experiments on CHA zeolites. We also thank Sachem, Inc. for providing the organic structure-directing agent used to synthesize SSZ-13.
Funding AgencyGrant Number
Oak Ridge Associated UniversitiesUNSPECIFIED
Fonds Wetenschappelijk Onderzoek - Vlaanderen (FWO)UNSPECIFIED
Issue or Number:2
Record Number:CaltechAUTHORS:20161214-094030853
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:72805
Deposited By: Tony Diaz
Deposited On:14 Dec 2016 18:36
Last Modified:11 Nov 2021 05:07

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