(121a) Experimental Study of Hydrocarbon Trapping over Passive NOx Adsorber

Experimental
Study of Hydrocarbon Trapping over Passive NOx Adsorbers

Sam Malamis, Kyle Karinshak, Michael P. Harold*

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

*corresponding author
e-mail: mpharold@central.uh.edu

Introduction

Automotive
catalysts including TWC, LNT, and SCR catalysts are all highly effective in
reducing NO_x and HC emissions above 200°C [1]. Most engines,
however, require a warm-up period during which the catalyst undergoes a “cold
start” environment. During this period of up to 200s, large amounts of
pollutants such as NOx, N₂O, CO and Hydrocarbons slip through the
catalyst into the atmosphere [2]. To mitigate this occurrence, it is beneficial
to develop a passive NOx adsorber, or PNA, that can store NOx and hydrocarbons
at these low temperatures, and then to release them at higher temperatures to
improve the effectiveness of the aftertreatment system [3].

Few
comprehensive studies have been performed on this topic, and only recently has
there been a movement toward understanding the chemistry behind these low
temperature traps. To date, a wide variety of materials has been explored,
including palladium on ceria-zirconia (Pd-CZO) [4], CeO₂-M₂O₃
with supported Pt or Pd [5], and Palladium on various zeolites including BEA,
MFI, CHA [3]. While it has been reported that Pd-CZO is a potentially effective
NO_x trap at low temperatures, it also strongly absorbs SO₂,
which inhibits NO_x storage [3,4]. This study focuses on the use of
zeolites as PNA materials in order to trap both NOx and HC during the cold
start period, with the potential to act as a partial oxidation catalyst
upstream of the SCR or LNT. The study mainly explores the effect of temperature
and various feed gases on NOx and HC trapping.

Materials and methods

Experiments
were performed using 2wt% Pd-BEA zeolite supported on a cordierite monolith. A
feed gas was flowed over the catalyst in a vertical quartz-tube reactor, and
the effluent species concentrations were measured using FT-IR. The catalyst was
first pre-treated in 5% O₂ (bal. Ar) for 30 min at 550ºC. To
determine the amount of NOx stored on the catalyst during an experiment, a feed
gas of 2% O₂ and 400ppm NO was flowed over the catalyst for 5 min at
a fixed feed temperature, either 50ºC, 80ºC or 150ºC. After 5 min., the NO was
turned off and the reactor was maintained at the adsorption temperature until
the concentration of NO dropped to less than 2ppm. To measure the species
desorbing after adsorbing on the catalyst surface, a temperature programmed
desorption (TPD) was performed at 20ºC/min from the adsorption temperature to
550ºC. The surface was then pretreated to re-oxidize the Pd to PdO to ensure
proper behavior during subsequent experiments [6]. To measure the effect of
hydrocarbons on catalyst performance, experiments similar to those described
above were performed, using a feed with either 800ppm C₃H₆
and 2% O₂ or a mixture of 400ppm NO, 800ppm C₃H₆
and 2% O­₂.

Results

It was found that using
a feed of only NO and O₂, Pd-BEA stored between0.4 and
1.0 mol NOx/gcat*10⁴, but released it too early during the TPD (i.e.
at a temperature <200ºC) (Figure 1). It was also found that as the
adsorption temperature increased, the amount of NOx stored and desorbed
decreased (Figure 2). Experiments using only propylene in the feed indicate a
very strong propensity for the catalyst to trap hydrocarbons at low
temperatures (1.5-6.0 mol C₃H₆/gcat*10⁴), more
so than in trapping NOx (Figures 3). Adding C₃H₆ to the
NO feed revealed a competitive adsorption effect of both NOx and C₃H₆,
as the stored quantities of these species was significantly lower than the pure
feeds, as shown in Figures 2 and 3. While the total amount of stored NOx
decreased with the addition of C₃H₆, our studies show
that the majority of NOx was released above 200ºC, which is a promising result
(Figure 1).

Figure 1. Percentage of NOx that desorbs at a catalyst temperature
higher than 200ºC for feeds with only NO and with added C₃H₆
on 2wt% Pd-BEA. Values are reported for each adsorption temperature. Note that
the addition of propylene in the feed significantly delays NOx desorption.
Feed: 400ppm NO with or without 800ppm C₃H₆, 2% O₂,
bal. Ar.

Figure 2. NOx adsorption and desorption quantities at various
adsorption temperatures and for feeds with or without C₃H₆.
Values are reported as moles of NOx/g cat*10⁴. Note that as the
adsorption temperature increases, adsorption/desorption of NOx decreases. Feed: 400ppm NO with or without 800ppm C₃H₆,
2% O₂, bal. Ar.

Figure 3. C₃H₆ adsorption and desorption
quantities at various adsorption temperatures and for feeds with or without NO.
Adsorption values are reported as moles of C₃H₆/g cat*10⁴,
while desorption values are reported as moles of C/g cat*10⁴. Note
that as the adsorption temperature increases, adsorption/desorption of C₃H₆
decreases. Feed: 800ppm C₃H₆ with or
without 400ppm NO, 2% O₂, bal. Ar.

References

1.      Twigg M: Progress
and future challenges in controlling automotive exhaust gas emissions. Applied
Catalyst B 2007, 70:2-15.

2.      Wilenmann M,
Favez J, Alvarez R: Cold-start emissions of modern passenger cars at
different low ambient temperatures and their evolution over vehicle legislation
categories. Atmospheric environment 2009, 43:241-2429.

3.      Chen HY et al.:
Low temperature NO storage of zeolite supported Pd or low temperature diesel
engine emission control. Catal. Lett. 2016.

4.      Theis JR,
Lambert C: The effects of CO, C₂H₄, and H₂O
on the NO_x storage performance of low temperature NO_x
adsorbers for diesel applications. SAE International 2017, 10.

5.      Jones S et
al.: CeO_2­-M₂O₃ passive NO_x adsorbers
for cold start applications. Emss. Contrl Sci. Technol. 2016 CLEERS
Issue.