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

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

Authors 

Ambast, M. - Presenter, University of Houston
Harold, M., University of Houston

Experimental
and Modeling Study of Passive NOx 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 NOx (NO + NO2) with a
target-year implementation of 2024, reduction of NOx emissions has
become a great challenge for the vehicle industry. Advanced technologies like
selective catalytic reduction (SCR) and NOx storage and reduction
(NSR) have been implemented for NOx reduction in diesel and lean
gasoline emission control. However, their effectiveness at temperatures below
200ºC is limited.  Thus, NOx emissions during the vehicle cold-start
complicate the meeting of the aforementioned emission rules. The passive NOx
adsorber (PNA) has emerged as a solution to reduce NOx through capture
and conversion of NOx 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 NOx
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 NOx concentration was
minimized.

Results and discussions

Typical NOx uptake data
revealed the process to be kinetically limited and to not be washcoat diffusion
limited. Water was shown to significantly lower NOx 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 NOx while no increase in NOx uptake was
observed for dry feed. Fe/Pd-H-ZSM-5 showed increased NOx uptake
when compared to Pd-H-ZSM-5.

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

 

References

(1)  Zheng, Y.; Kovarik, L.; Engelhard, M.H.; Wang, Y.; Wang,
Y.; Gao, F.; Szanyi, J.; J. Phys. Chem. C 121, (2017) 15793−15803

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