Palladium Speciation in Zeolites for Passive NOx Adsorption
Trevor Lardinois, Purdue University
Trevor M. Lardinois1, Jason S. Bates1, Kinga A. Unocic2, Vitaly Y. Prikhodko2, Andrew “Bean” Getsoian3, Joseph Theis3, Christine Lambert3, Nicole LiBretto1, Rajamani Gounder1*
1Purdue University, West Lafayette, IN 47907 (USA)
2Oak Ridge National Laboratory, Oak Ridge, TN 34830 (USA)
3Ford Motor Company, Dearborn, MI 48121 (USA)
*rgounder@purdue.edu
Current automotive engine aftertreatment NOx (x = 1,2) abatement systems are challenged to meet NOx emission standards during cold-start and low-load operation, where exhaust gas temperatures (<150 oC) are below converter operating limits. Pd-exchanged zeolites have been reported to adsorb NOx at low temperature, and thermally desorb such compounds within the temperature operating window of downstream catalytic converters [1]. Ion-exchanged Pd species charge-compensated by framework Al sites are proposed NOx storage sites, whereas agglomerated PdO species do not store NOx [2]. Here, [Pd(NH3)4]2+ was deposited onto Beta and CHA zeolites with varying Al density (4.5 < Si/Al < 30) via either incipient wetness impregnation or aqueous ion-exchange (AIE) and exposed to various post-synthetic treatments to study the speciation and dynamic restructuring of Pd in zeolites. Temperature-programmed desorption experiments on as-made AIE Pd-zeolites resulted in a net +2 NH3 per exchanged Pd ion, suggesting the predominant ion-exchanged Pd species is [Pd(NH3)4]2+ at two framework Al sites. Although Pd was predominantly isolated after ion-exchange, high temperature (550 oC< T <750 oC) treatments in flowing air caused Pd to aggregate to PdO, as evidenced by scanning transmission electron microscopy (STEM). Prolonged exposure to high temperature (>550 oC) treatments in dry air transformed agglomerated PdO to ion-exchanged Pd, as evidenced by in-situ H2 temperature-programmed reduction and STEM. Additionally, treatments in flowing dry air (750 oC) resulted in predominantly ion-exchanged Pd in a high Al CHA zeolite (Si/Al = 4.5), while hydrothermal aging treatments in flowing ~3% H2O/air at 750 oC [3] led to dealumination and structural collapse of the CHA zeolite, suggesting high-temperature (750 oC) post-synthetic treatments in flowing dry air are sufficient to re-disperse PdO to ion-exchanged Pd.
References
[1] Chen et al., U.S. Patent 10,005,075 (2018)
[2] Lee et al., App. Catal. B: Env., 226 (2018) 71-82
[3] Ryou et al., App. Catal. B: Env., 212 (2017) 140-149