Low temperature methane combustion over palladium ion-exchanged zeolites

Chih-Han  Liu, University at Buffalo

Low temperature methane combustion over palladium ion-exchanged zeolites


Chih Han Liu, Yiran Chen, Eleni A. Kyriakidou*

Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, NY 14260, USA



Emissions of unburnt methane (CH4) from natural gas vehicles (NGVs) have an extensive impact on climate change since CH4 has a 28-36 times higher global warming potential than CO2 [1]. Catalytic combustion of unburnt CH4 is the main strategy for mitigating CH4 slip from the exhaust of NGVs. Pd based catalysts have been reported to be highly active for the CH4 combustion. However, Pd sintering due to high exhaust temperatures (450~550oC) in NGVs and weak Pd-Al2O3 interactions observed in commercial Pd/Al2O3 catalysts, can lead to rapid catalyst deactivation [2]. Incorporation of well dispersed Pd species within zeolite cages by ion-exchange can sufficiently prevent Pd sintering under CH4 combustion conditions. Herein, the CH4 combustion activity of Pd based ZSM-5 (Si/Al = 15) and SSZ-13 (Si/Al = 12.5) zeolites synthesized by incipient wetness (IW) and ion-exchange (IE) was evaluated.  The catalysts were pretreated at 500°C for 2 h (21% O2/Ar) followed by CH4 combustion (1500 ppm CH4, 5% O2/Ar) from 100 to 600°C with a ramp rate of 2°C/min (GHSV=78,900 h-1). The results indicated that 1 wt.% Pd/ZSM-5 synthesized via IE had a higher Pd dispersion (61%) and smaller Pd particle size (1.8 nm) compared to Pd/ZSM-5 synthesized via IW (53.4% dispersion, 2.1 nm Pd particle size). Moreover, the IE catalyst had a lower T50 by 76ºC compared to the IW Pd/ZSM-5 catalyst (Fig. 1A). Furthermore, 1 wt.% Pd/SSZ-13 showed a lower T50 by 45ºC compared to 1 wt.% Pd/ZSM-5 for the CH4 combustion reaction (Fig. 1B), that can be attributed to the increased number of CH4 collisions with the Pd sites in the smaller channels of SSZ-13(3.8 × 3.8 Å) compared to ZSM-5 (straight channels 5.6 × 5.3 Å and sinusoidal channels 5.5 × 5.1 Å) [3]. Further improvement in CH4 conversion was achieved by substituting Na+ for H+ in SSZ-13 zeolites to eliminate the sintering of Pd caused by Na [4]. 1 wt. % Pd/H0.17Na0.83SSZ-13 catalyst was synthesized by ion-exchanging Na/SSZ-13 with 0.1M NH4NO3 (dried at 100oC overnight and calcined at 500oC/2h to obtain H0.17Na0.83SSZ-13) followed by an ionic exchanged of the synthesized support with Pd(NH3)4(NO3)2. The results showed that 1 wt. % Pd/H0.17Na0.83SSZ-13 (T50 = 290oC, T90 = 391oC) outperformed 1 wt. % Pd/NaSSZ-13 (T50 = 346oC, T90 = 386oC) (Fig. 1C). This work illustrates the potential of developing Pd-based catalysts with enhanced low-temperature activity using H-promoted SSZ-13 support.


Figure 1. CH4 conversion as a function of temperature over (A) IW-1wt. % Pd/ZSM-5, IE-1wt. % Pd/ZSM-5, (B) IE-1wt.% Pd/ZSM-5, IE-1wt.% Pd/SSZ-13 and (C) IE-1wt.% Pd/SSZ-13, IE-1wt.% Pd/H0.17Na0.83SSZ-13 under 1500 ppm CH4, 5% O2/Ar (GHSV = 78,900 h-1).


[1] United State Environmental Protection Agency, Understanding Global Warming Potentials, Retrieved from https://www.epa.gov/ghgemissions/understanding-global-warming-potentials#Learn%20why

[2] A.W. Petrov, D. Ferri, F. Krumeich, M. Nachtegaal, J.A. van Bokhoven, O. Kröcher, Nat. Commun., 9 (2018) 2545.

[3] J.B. Lim, D. Jo, S.B. Hong, Appl. Catal., B, 219 (2017) 155-162.

[4] X. Auvray, A. Lindholm, M. Milh, L. Olsson, Catal. Today, 299 (2018) 212-218.