Bimetallic PdCo/BEA zeolites for passive NOx adsorption

Eleni  Kyriakidou, University of Buffalo

Bimetallic PdCo/BEA zeolites for passive NOx adsorption

Jungkuk Lee, Junjie Chen, Eleni A. Kyriakidou*

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



Passive NOx adsorbers (PNAs) using palladium (Pd) containing zeolites have attracted significant attention for the abatement of NOx emissions during the cold-start period [1].  Ionic Pd is considered to be the NOx adsorption site.  However, ionic Pd can be reduced to metallic Pd (Pd0) when CO is present in the exhaust stream resulting to a decrease in the PNA NOx adsorption capacity [2].  Herein, the effect of cobalt (Co) incorporation in Pd/BEA (Si/Al = 12.5) PNAs was investigated over five consecutive NOx adsorption/desorption cycles in the presence of CO.  The synthesized PNAs contained 1.0 wt.% Pd and Co loadings of 0, 0.14 and 0.28 wt.%.  NOx adsorption was examined for 3 min at 100oC using lean trapping conditions followed by temperature programmed desorption.  The results showed a decrease in the percentage of NOx desorption at temperatures >200oC from the 1st (34%) to the 5th (8%) cycle over Pd(1.0)/BEA (Fig. 1a).  This behavior can be attributed to the reduction of ionic Pd that led to a decrease in the amount of chemisorbed NOx.  Bimetallic PdCo/BEA showed a higher percentage of NOx release at temperatures >200oC.  Moreover, increase in Co loading led to an increase in the percentage of NOx desorbed above 200oC.  Specifically, only a small decrease in the percentage of NOx released above 200oC was observed from the 1st (60%) to the 5th (50%) cycle over Pd(1.0)Co(0.28)/BEA.  H2-TPR (Fig. 1b) showed that the Pd2+ to Pd0 reduction peak of Pd(1.0)/BEA (~62oC) is shifted to higher temperatures (~73oC) upon Co incorporation indicating the presence of more stable Pd2+ species over PdCo/BEA.  The results collectively suggest that Pd(1.0)Co(0.28)/BEA has more ionic Pd species compared to Pd(1.0)/BEA which results to desorption of NOx at temperatures >200oC.

Figure 1.  Comparison of (a) percentage of NOx desorption above 200oC and (b) H2-TPR over Pd/BEA with varying Co loadings from 0 to 0.28 wt.%.

[1] J. Lee, J. Theis, E.A. Kyriakidou, Appl.Catal. B 243 (2019) 397-414.

[2] Y. Gu, R.P. Zelinsky, Y.-R. Chen, W.S. Epling, Appl. Catal. B 258 (2019) 118032.

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