(484a) NO Uptake and Desorption on Fe-Modified Pd/ZSM-5: Impact of Pd and Fe Loading

NO
Uptake and Desorption on Fe-Modified Pd/ZSM-5: 

Impact
of Pd and Fe Loading

Kyle Karinshak^*,
Michael P. Harold^*, and Kiran Premchand⁺

^*Dept. of Chemical & Biomolecular Engineering,
University of Houston, Houston, Texas

⁺FCA US LLC Auburn Hills, MI  48236

*corresponding author:
M.P. Harold   (mpharold@central.uh.edu)

Reducing
cold-start emissions, emissions under 200 °C, will play a significant role in
meeting future EPA and EU emissions standards. By adsorbing NO and NO₂
at low temperatures and releasing these gases in temperatures regimes where
traditional emission control catalysts can operate, Passive NOx Adsorbers
(PNAs) augment existing emission control systems and are a promising new field
of catalysis. The scant published literature has focused on Pd supported on
various materials, including   ceria or alumina. In contrast, the patent
literature suggests the efficacy of ZSM-5 as a support as well as the addition
of Fe to PNA compositions [1-6]. Our preliminary investigations into Palladium-exchanged
MFI zeolites show significant NOx uptake capabilities and complex desorption
behavior.

            Catalyst
samples were prepared through a wet impregnation protocol before being deposited
onto monolith cores. Palladium(II) nitrate was used as the precursor and
dissolved into water equal to the pore volume of the ZSM-5 zeolite (SAR 30).
Alumina solution was added to the washcoating solution to improve binding. Four
catalysts – 0 wt% Pd, 1wt% Pd, 1 wt% Pd/1 wt% Fe, and 2 wt% Pd, were washcoated
onto a 0.25in diameter, 2in length, 600 cspi monolith cores at a fixed loading
of 1.5 g/in³. A bench-scale quartz reactor  was utilized to simulate
emissions and study uptake and desorption characteristics. DRIFTS (diffuse
reflectance infrared Fourier transform spectroscopy) experiments of catalysts
samples were performed to examine surface species formation and the mechanism through
which NOx uptake occurs on Pd/ZSM catalysts.

            Figure
1 shows uptake characteristics of NOx uptake at 50 ^oC over a five
minute duration for the four catalyst. 400 ppm of NO was flowed through the
catalyst accompanied by 2% O2 and balance Ar. Similar trends were observed at
uptake temperatures of 80 ^oC and 150 ^oC with decreasing
uptake quantities and breakthrough periods.  NO adsorption was accompanied by
noticeable NO₂ slip from the catalyst.  The addition of Fe to the
catalyst composition significantly improved the performance of NOx uptake.

            Figure
2 shows the NOx desorption peaks from the same experiments. Over half of
desorbed NOx occurred in the form of NO₂. As NO adsorbs as a mono nitrosyl
palladium complex, this shows that NO is being converted into NO₂ through
a zeolite-surface reaction and likely storing on the Pd-less acid sites of the
zeolite and binder. Unlike Murata et al, who reported a single NOx desorption
peak during TPD experiments [7], our results reveal multiple desorption peaks. Three
distinct peaks are identified in Pd-containing samples whereas only two exist
in the Pd-less sample. A preliminary assignment of these peaks include acid
sites of the binder, the acid sites of the zeolite, and the Pd nitrosyl complexes.

            Figure
3 shows DRIFTS measurements of three samples: 0 wt% Pd, 1 wt% Pd/1 wt% Fe, and
2 wt% Pd. Experiments were conducted in the presence of oxygen. Peak analysis
matches that reported by Descorme and Gelin, confirmed the NO adsorption
mechanism [8]. The Pd/Fe sample did not show the peaks associated with NO
adsorption onto Fe complexes, confirming that the role of Fe in the catalyst
composition is ensure a high dispersion of Pd [9]. Experiments conducted
without oxygen (not shown) display peaks associated with NO adsorption onto Pd
but do not show peaks associated with NO₂ formation and storage.

            Additional
TPD experiments (not shown) have shown that the presence of water significantly
suppresses NOx uptake and desorption. The introduction of water inhibits the
surface-reaction formation of NO₂, likely through the suppression of
zeolitic acid sites. Ongoing experiments include powdered catalyst TPD
experiments as well as additional DRIFTS experiments to better understand and
quantify the effects the addition of binder.  

Ongoing work will include modifications to the
catalyst compositions as well as the addition of a modelling component for both
uptake and desorption.  Additionally, concurrent experiments are
being carried out to understand the effects of adding hydrocarbons
to the gas environment.

Figure
3: Evolution
of DRIFTS data over time from three samples.