Ag ion-exchanged ZSM-5 zeolites for hydrocarbons trapping applications
Jungkuk Lee, University at buffalo
Ag ion-exchanged ZSM-5 zeolites for hydrocarbons trapping applications
Jungkuk Lee, Eleni A. Kyriakidou*
Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, NY 14260, USA
*elenikyr@buffalo.edu
Significant attention has been recently paid to the reduction of vehicle emissions during the cold-start that takes place the first 1-2 minutes of vehicle operation due to the stringent emission regulations [1]. A potential approach to regulate the cold-start emissions is to utilize hydrocarbon (HC) and nitrogen oxide (NOx) traps. Zeolites have been emerged as potential trapping materials due to their tunable acidity, high surface area, and thermal stability [2]. Herein, a series of Ag containing ZSM-5 (Si/Al = 11.5) zeolites with different Ag loadings (0.5 – 4.6 wt.%) were prepared via ion-exchange. Their trapping performance was evaluated using the CLEERS low temperature combustion of diesel (LTC-D) protocol (900 ppm C2H4, 900 ppm C7H8, 100 ppm NOx, 12% O2,6% CO2, 6% H2O, 2000 ppm CO, 400 ppm H2) [3]. The adsorption of single HCs and NOx was also investigated. As shown in Figs.1(a-b), Ag is able to act as an adsorption site for both ethylene (C2H4) and toluene (C7H8) in the presence of H2O. Moreover, increasing the Ag loading from 0.5 to 4.6 wt.% enhanced the HC adsorption capacity, attributed to an increase in the number of adsorption sites. However, NOx adsorption is completely inhibited by H2O. The oxidation state of Ag (e.g. Ag+, Agnδ+, Ag0) can also influence the HC adsorption (Fig.1(c)). Specifically, Ag+ showed higher C2H4 adsorption capacity (154 μmol/g) compared to Agnδ+ and Ag0 (135 and 96 μmol/g, respectively). This can be attributed to the decreased π-backbonding between C2H4 and Ag0. Moreover, C2H4 is desorbed at a higher temperature (314oC) over Ag+/ZSM-5 compared to Ag0/ZSM-5 (285oC), indicating a stronger interaction of Ag+ with C2H4 compared to Ag0. Finally, the durability of Ag/ZSM-5 was evaluated after hydrothermal aging at 800oC for 10 h under a 10% O2, 5% CO2, and 5% H2O flow. Our results showed that C2H4 and C7H8 adsorption capacity was maintained even after hydrothermal aging, indicating that Ag/ZSM-5 is thermally stable and suitable for HC trapping applications.
Figure 1. Adsorption profiles of (a) C2H4 and (b) C7H8 over HZSM-5, and 0.8 wt.% Ag/ZSM-5 in the absence and presence of 6% H2O; (c) summary of C2H4 adsorption/desorption capacity with different Ag oxidation state.