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

*elenikyr@buffalo.edu

 

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.