NDLI: Cyclic Lean Reduction of NO by CO in Excess H$_{2}$O on Pt–Rh/Ba/Al$_{2}$O$_{3}$: Elucidating Mechanistic Features and Catalyst Performance

Content Provider	SpringerLink
Author	Dasari, Prasanna Muncrief, Rachel Harold, Michael P.
Copyright Year	2013
Abstract	This study provides insight into the mechanistic and performance features of the cyclic reduction of NO$_{x}$ by CO in the presence and absence of excess water on a Pt–Rh/Ba/Al$_{2}$O$_{3}$ NO$_{x}$ storage and reduction catalyst. At low temperatures (150–200 °C), CO is ineffective in reducing NO$_{x}$ due to self-inhibition while at temperatures exceeding 200 °C, CO effectively reduces NO$_{x}$ to main product N$_{2}$ (selectivity >70 %) and byproduct N$_{2}$O. The addition of H$_{2}$O at these temperatures has a significant promoting effect on NO$_{x}$ conversion while leading to a slight drop in the CO conversion, indicating a more efficient and selective lean reduction process. The appearance of NH$_{3}$ as a product is attributed either to isocyanate (NCO) hydrolysis and/or reduction of NO$_{x}$ by H$_{2}$ formed by the water gas shift chemistry. After the switch from the rich to lean phase, second maxima are observed in the N$_{2}$O and CO$_{2}$ concentrations versus time, in addition to the maxima observed during the rich phase. These and other product evolution trends provide evidence for the involvement of NCOs as important intermediates, formed during the CO reduction of NO on the precious metal components, followed by their spillover to the storage component. The reversible storage of the NCOs on the Al$_{2}$O$_{3}$ and BaO and their reactivity appears to be an important pathway during cyclic operation on Pt–Rh/Ba/Al$_{2}$O$_{3}$ catalyst. In the absence of water the NCOs are not completely reacted away during the rich phase, which leads to their reaction with NO and O$_{2}$ upon switching to the subsequent lean phase, as evidenced by the evolution of N$_{2}$, N$_{2}$O and CO$_{2}$. In contrast, negligible product evolution is observed during the lean phase in the presence of water. This is consistent with a rapid hydrolysis of NCOs to NH$_{3}$, which results in a deeper regeneration of the catalyst due in part to the reaction of the NH$_{3}$ with stored NO$_{x}$. The data reveal more efficient utilization of CO for reducing NO$_{x}$ in the presence of water which further underscores the NCO mechanism. Phenomenological pathways based on the data are proposed that describes the cyclic reduction of NO$_{x}$ by CO under dry and wet conditions.
Starting Page	1922
Ending Page	1936
Page Count	15
File Format	PDF
ISSN	10225528
Journal	Topics in Catalysis
Volume Number	56
Issue Number	18-20
e-ISSN	15729028
Language	English
Publisher	Springer US
Publisher Date	2013-07-04
Publisher Place	New York
Access Restriction	Subscribed
Subject Keyword	NO$_{x}$ CO Platinum Barium Lean NO$_{x}$ trap Isocyanates Catalysis Physical Chemistry Pharmacy Industrial Chemistry/Chemical Engineering Characterization and Evaluation of Materials
Content Type	Text
Resource Type	Article
Subject	Catalysis Chemistry

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