NDLI: The Dynamic Nature of Brønsted Acid Sites in Cu–Zeolites During NO$_{x}$ Selective Catalytic Reduction: Quantification by Gas-Phase Ammonia Titration

Content Provider	Springer Nature Link
Author	Di Iorio, John R. Bates, Shane A. Verma, Anuj A. Delgass, W. Nicholas Ribeiro, Fabio H. Miller, Jeffrey T. Gounder, Rajamani
Copyright Year	2015
Abstract	Brønsted acid sites on Cu-exchanged zeolites can be titrated selectively using gaseous ammonia when NH$_{3}$ saturation steps are followed by protocols that remove Lewis acid-bound and physisorbed NH$_{3}$, such as purging in flowing wet helium at 433 K. NH$_{3}$ titrates all H$^{+}$ sites on small-pore chabazite zeolites (SSZ-13) and leads to the complete disappearance of infrared stretches for Brønsted acidic OH groups after saturation (433 K), in contrast with larger n-propylamine titrants that access only a small fraction (<0.25) of H$^{+}$ sites on SSZ-13 under conditions sufficient to titrate all H$^{+}$ sites on medium-pore ZSM-5 zeolites (323 K, 2 h). NH$_{3}$ titration of the residual H$^{+}$ sites present in Cu-exchanged SSZ-13 samples (Si/Al = 4.5, Cu/Al = 0–0.20) after oxidative treatments detects two fewer H$^{+}$ sites per exchanged Cu$^{2+}$ ion, as expected to maintain framework charge neutrality. NH$_{3}$ titrants detect only one fewer H$^{+}$ site (per Cu) after Cu-SSZ-13 samples undergo a reductive treatment in flowing NO and NH$_{3}$ (473 K), however, indicating that each Cu$^{2+}$ cation reduces to form a Cu$^{+}$ and H$^{+}$ site pair. In the context of low temperature (473 K) selective catalytic reduction (SCR) on high aluminum Cu-SSZ-13, we discuss the different mechanistic roles of residual H$^{+}$ sites that remain after Cu$^{2+}$ exchange, whose primary function appears to be NH$_{3}$ storage, and of proximal H$^{+}$ sites that are generated in situ upon Cu$^{2+}$ reduction, whose role is to stabilize reactive NH$_{4}$ $^{+}$ intermediates involved in the standard SCR oxidation half-cycle. We highlight how gaseous NH$_{3}$ titrants can selectively count H$^{+}$ sites on small-pore, Cu-exchanged zeolites and, in doing so, enable probing the dynamic nature of active sites and catalytic surfaces during SCR redox cycles.
Starting Page	424
Ending Page	434
Page Count	11
File Format	PDF
ISSN	10225528
Journal	Topics in Catalysis
Volume Number	58
Issue Number	7-9
e-ISSN	15729028
Language	English
Publisher	Springer US
Publisher Date	2015-04-03
Publisher Place	New York
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Ammonia Brønsted acid site Copper-exchanged zeolites n-Propylamine Selective catalytic reduction Titration Catalysis Physical Chemistry Pharmacy Industrial Chemistry/Chemical Engineering Characterization and Evaluation of Materials
Content Type	Text
Resource Type	Article
Subject	Chemistry Catalysis

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