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

*elenikyr@buffalo.edu

 

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 (450~550^oC) 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/H_0.17Na_0.83SSZ-13 catalyst was synthesized by ion-exchanging Na/SSZ-13 with 0.1M NH₄NO₃ (dried at 100^oC overnight and calcined at 500^oC/2h to obtain H_0.17Na_0.83SSZ-13) followed by an ionic exchanged of the synthesized support with Pd(NH₃)₄(NO₃)₂. The results showed that 1 wt. % Pd/H_0.17Na_0.83SSZ-13 (T₅₀ = 290^oC, T₉₀ = 391^oC) outperformed 1 wt. % Pd/NaSSZ-13 (T₅₀ = 346^oC, T₉₀ = 386^oC) (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/H_0.17Na_0.83SSZ-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.