(14a) Low Temperature NO and Hydrocarbon Trapping over Pd-Exchanged Zeolite Passive NOx Adsorbers

Sam Malamis, 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, as well as exploring the development of a kinetic model for 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). It was also found that as the adsorption temperature increased, the amount of NOx stored and desorbed decreased. 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. 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. 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.

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.