(216c) Experimental and Modeling Study of Passive NOx Adsorbers: Pd-H-ZSM-5

Experimental
and Modeling Study of Passive NO_x Adsorbers: Pd-H-ZSM-5

Mugdha
Ambast and Michael P. Harold*

Dept.
of Chemical and Biomolecular Engineering, University of Houston,

 Houston,
TX 77204-4004, USA

mharold@uh.edu;
mugdhaambast@gmail.com

Introduction

With the announcement
by the Environmental Protection Agency (EPA) to impose more stringent
regulations for emissions of NO_x (NO + NO₂) with a
target-year implementation of 2024, reduction of NO_x emissions has
become a great challenge for the vehicle industry. Advanced technologies like
selective catalytic reduction (SCR) and NO_x storage and reduction
(NSR) have been implemented for NO_x reduction in diesel and lean
gasoline emission control. However, their effectiveness at temperatures below
200ºC is limited.  Thus, NO_x emissions during the vehicle cold-start
complicate the meeting of the aforementioned emission rules. The passive NO_x
adsorber (PNA) has emerged as a solution to reduce NO_x through capture
and conversion of NO_x emitted during a vehicle’s cold-start period. Pd
impregnated on zeolites (ZSM-5, BEA and SSZ-13) can adsorb NO at
temperatures below 100ºC and release NO+NO₂ at temperatures above 200ºC, enabling their
reduction by downstream SCR or NSR.
There is a need for mechanistic-based PNA kinetic and reactor models to
identify improved materials and to develop effective trapping strategies. A combined experimental and modeling study was conducted to
understand and predict the effects of various operating parameters on a model
PNA material: Pd-H-ZSM-5. Specifically, the effects of temperature, flowrate,
Pd-loading, Fe-loading, CO, O₂, and H₂O were investigated
and used to develop a predictive reactor model.

Catalyst preparation and experimental setup

A series
of Pd and Fe
impregnated on H-ZSM-5 samples were synthesized by incipient
wetness impregnation. The experiments were conducted in a quartz reactor
containing the washcoated monolith. Degreened samples were exposed to a feed
containing 400 ppm NO, O₂ (2%) and Ar with/without 7% H₂O
and with/without CO (800 ppm) over a range of uptake temperatures (50 – 170ºC) and flow rates (10 k
– 45 k h^-1). NO and NO₂ desorption were monitored with
FTIR under a temperature ramp to 550ºC at 20ºC /min.

Brief model description

A one-dimensional
two-phase transient monolith model is developed for a
single monolith channel. Model assumes laminar flow without axial dispersion.
Typical Conventional mass transport equations of monolithic reactor for fluid
phase, solid phase and species balance are used. The PDEs were discretized
using second order finite difference method. The differential equations
were then solved by MATLAB routine ODE23s. Initial activation energies
for most of the reactions were obtained from DFT calculations and subsequently
tuned to capture the spatio-temporal features of the passive NO_x
trap. Pre-exponential factors and the remaining activation energies were fitted
using MATLAB parameter estimation tool fmincon. The sum of the residue
squares of the measured and predicted NO_x concentration was
minimized.

Results and discussions

Typical NO_x uptake data
revealed the process to be kinetically limited and to not be washcoat diffusion
limited. Water was shown to significantly lower NO_x uptake, but the
extent of inhibition is a strong function of temperature. Typical NO uptake in
the presence of H₂O in the feed did not exceed ~0.4 NO/Pd indicating
that most of the Pd was not in cationic form. Wet
feed containing CO in the feed resulted in
higher uptake of NO_x while no increase in NO_x uptake was
observed for dry feed. Fe/Pd-H-ZSM-5 showed increased NO_x uptake
when compared to Pd-H-ZSM-5.

A transient monolith model for NO_x
uptake and TPD was developed for H-ZSM-5 and Pd-H-ZSM-5 without H₂O and
with H₂O in the feed as shown in fig. 1 (a), (b) and (c)
respectively. The model assumes that the dispersed Pd
cations are the active sites for NO_x adsorption together with the Bronsted
sites in Pd-H-ZSM-5, the latter of which are ineffective when water is present
in the feed. The model was validated for different uptake temperatures, feed
flow rates and Pd-loadings, affording its use to identify the optimal catalyst
formulation as well as operating strategy. Study to adapt the model to
different zeolites like SSZ-13 and to capture the effect of CO in the feed is
ongoing.

Significance

Passive NO_x adsorbers may
have the potential to significantly reduce NO_x emissions during
cold-start through trapping and release. The PNA model enables the
identification of optimal material composition and operating strategies to
maximize NO_x trapping and release for downstream conversion by SCR.

References

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(2)  Artioli,
N.; Lobo, R. F.; Iglesia, E.; J. Phys. Chem. C, 117, (2013)
20666−20674.