Cobalt as an Efficient Promoter in Low-loading Pd/BEA Catalysts for CH4 Oxidation

Junjie  Chen, University at Buffalo

Cobalt as an Efficient Promoter in Low-loading Pd/BEA Catalysts for CH₄ Oxidation

Junjie Chen, 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

 

Natural gas vehicles (NGVs) have shown the potential for decreased greenhouse gas emissions, but they are limited by the EPA requirements for reducing methane (CH₄) emissions, the main component of natural gas.  Palladium (Pd)-based zeolite catalysts have been quite successful at reducing CH₄ emissions. The main drawback of these catalysts is the prohibitive cost of Pd.  Decreasing the Pd loading to lower than 1-2 wt.% is of current interest but it can lead to an undesirable increase in the ionic Pd/PdO ratio [1], since ionic Pd is less active than PdO nanoclusters for the CH₄ oxidation reaction.  Thus, there is a critical need to design catalysts for NGVs aftertreatment applications containing a decreased amount of Pd while compressing the formation of ionic Pd.

Herein, we report a facile method that decreases Pd usage in 1 wt.% Pd/BEA while maintaining the CH₄ oxidation performance.  These results are achieved by introducing 0 – 1.5 wt.% cobalt (Co) in 0.5 wt.% Pd/BEA (Pd(0.5)/BEA) zeolites (Si/Al = 12.5).  CH₄ conversion of Pd(0.5)/BEA at 500℃ was as low as 12% (Fig. 1a), attributed to the majority of Pd being present in its ionic form.  Co-impregnation of Co with Pd in BEA zeolites led to an enhanced CH₄ oxidation performance.  Specifically, an increase in Co loading from 0.1 to 1 wt.% led to a decrease in the CH₄ oxidation T₅₀ from 456 to 352℃.  Pd(0.5)Co(1.0)/BEA showed the best performance with T₅₀ and T₉₀ equal to 352 and 419℃, respectively.  Stability tests at 450℃ (Fig. 1b) showed that Pd(0.5)Co(1.0)/BEA had a similar CH₄ conversion (~94%) and was more water resistant than Pd(1.0)/BEA.  This work illustrates that Co incorporation in Pd/BEA zeolites can achieve the same CH₄ oxidation performance results with 50% reduction of precious metal usage. 

Fig. 1. (a) CH₄ conversion over Pd(0.5)Co(x)/BEA (x = 0, 0.1, 0.28, 0.5, 1.0, 1.5), 1 wt.% Co/BEA, and 1 wt.% Pd/BEA catalysts, (b) Long-term stability test for CH₄ oxidation over Pd(0.5)Co(x)/BEA (x = 0, 1.0) and 1 wt.% Pd/BEA catalysts under dry and wet (5% H₂O) conditions at 450℃.  Reaction conditions: 1500ppm CH₄, 5% O₂, Ar balance, GHSV = 99,900 mLg_cat^-1h^-1.

 

[1] K. Khivantsev, N.R. Jaegers, L. Kovarik, J.C. Hanson, F. Tao, Y. Tang, X. Zhang, I.Z. Koleva, H.A. Aleksandrov, G.N. Vayssilov, Angew. Chem., 57 (2018) 16672-16677.

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