Investigation of NO adsorption and desorption phenomena on a Pd/ZSM-5 passive NOx adsorber

[Display omitted] •Pd2+ ions exchanged into ZSM-5 exist in a hydrated state at low temperatures.•Hydration of the Pd2+ ions inhibits the rate of NO uptake.•CO displaces some of the H2O associated with Pd2+, mitigating the H2O inhibition.•Increasing temperature leads to Pd2+ dehydration, eliminating...

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Published in:Applied catalysis. B, Environmental Environmental, 2021-12, Vol.298 (X), p.120561, Article 120561
Main Authors: Gu, Yuntao, Sinha Majumdar, Sreshtha, Pihl, Josh A., Epling, William S.
Format: Article
Language:English
Subjects:
Adsorption
Adsorption mechanism
Carbon monoxide
Cations
Cold start
Cold starts
Composition effects
Dehydration
Diesel engines
Emissions
Gas composition
INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
Intermediates
Low temperature
Mitigation
NOx storage
Palladium zeolite catalyst
Passive NOx adsorber
Spectroscopy
Storage capacity
Surface chemistry
Temperature effects
Zeolites
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Summary:[Display omitted] •Pd2+ ions exchanged into ZSM-5 exist in a hydrated state at low temperatures.•Hydration of the Pd2+ ions inhibits the rate of NO uptake.•CO displaces some of the H2O associated with Pd2+, mitigating the H2O inhibition.•Increasing temperature leads to Pd2+ dehydration, eliminating CO and H2O effects.•A mechanism incorporating these features is proposed. Pd-based zeolite materials have gained significant attention as passive NOx adsorbers (PNAs) for diesel-engine cold-start NOx mitigation due to their ability to store NOx at low temperatures. Pd/ZSM-5 is a promising PNA candidate, however, the NO adsorption mechanism over this material is not well understood. This study combines flow reactor experiments and surface spectroscopy to investigate NO adsorption under a variety of conditions. The state of hydration of the Pd cations played an important role in determining the impact of H2O and CO concentration on the PNA performance. Below 150 °C, the inhibition effect of H2O on NO adsorption was mitigated by CO. Due to dehydration, neither H2O nor CO had an impact on NO uptake at 150 °C, where maximum NO storage capacity of the PNA was also observed. The observed gas composition and temperature effects on NO adsorption and formation of surface intermediates ultimately inform a proposed mechanism.
ISSN:0926-3373
1873-3883
DOI:10.1016/j.apcatb.2021.120561