Bimetallic Pd-Co/BEA zeolites for passive NOx adsorption

Jungkuk  Lee, University at Buffalo

Bimetallic Pd-Co/BEA zeolites for passive NOx adsorption

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

 

Passive NOx adsorbers using palladium (Pd) containing zeolites have attracted significant attention for the abatement of low temperature NOx emissions.  However, Pd containing zeolites can be deactivated after hydrothermal aging since Pd²⁺ can sinter forming PdO and Pd⁰ [1].  Incorporation of Co²⁺ as a second metal can facilitate the stabilization of Pd²⁺ species in zeolites [2].  Herein, the PNA performance of bimetallic Pd-Co/BEA (Si/Al = 12.5) traps synthesized via co-impregnation was evaluated for the first time.  The synthesized traps contained 0.5 wt.% Pd (or Pd(0.5)) and Co loadings varying from 0.5 to 2.0 wt.%.  H₂-TPR was carried out to investigate the reducibility of Pd species in the presence of Co.  The results indicated that Co incorporation led to an increase in the reduction temperatures of both PdO and Pd²⁺.  These results suggested that Pd²⁺ in bimetallic Pd-Co/BEA traps can interact stronger with BEA zeolites compared to monometallic Pd/BEA traps.  NOx adsorption  was examined for 3 min using lean trapping conditions (12% O₂, 6% CO₂, 6% H₂O, 100 ppm NOx) followed by temperature programmed desorption up to 600^oC (12% O₂, 6% CO₂, 6% H₂O).  The results showed that the NOx adsorption capacity slightly decreased with increasing Co loading from 0 to 2 wt.% (Fig. 1a).  Moreover, Pd(0.5)/BEA had two NOx desorption peaks at 150 and 400^oC.  Desorption at 150^oC is not desirable for PNA applications since the downstream SCR catalyst is not active at such low temperatures.  However, Co incorporation into Pd/BEA resulted to a decrease in the intensity of the low temperature desorption peak (at ~150^oC), while the desorption peak at 400^oC became more prominent, indicating that Co incorporation enhanced the NOx adsorption strength in Pd/BEA traps.  Furthermore, the low temperature desorption peak shifted to slightly higher temperatures upon incorporation of Co, whereas the position of the high temperature desorption peak (>300^oC) remainted unaltered (Fig. 1b).  More importantly, the NOx adsorption capacity of bimetallic Pd-Co/BEA improved after hydrothermal aging at 750^oC for 10h (10% O₂, 5% CO₂, and 5% H₂O) (Fig. 1(c)).  Specifically, the NOx capacity of Pd(0.5)/BEA trap decreased by 19% after hydrothermal aging, while the NOx adsorption capacity of Pd(0.5)-Co(0.5)/BEA trap increased by 26% after hydrothermal aging.  These results suggest that Co incorporation in Pd/BEA can enhance the stability of Pd²⁺ species, leading to an improved NOx adsorption capacity after hydrothermal aging.

Figure 1.  Comparison of (a) adsorption/desorption capacity, (b) temperature programmed desorption profiles of adsorbed NOx over Pd/BEA with varying Co loadings from 0.5 to 2.0 wt.%, and (c) NOx adsorption capacity of fresh and hydrothermally aged (750^oC/10h) Pd(0.5)/BEA and Pd(0.5)-Co(0.5)/BEA traps.

[1] Y. Ryou, J. Lee, H. Lee, C.H. Kim, D.H. Kim, Catal. Today 320 (2019) 175-180

[2] M. Ogura, S. Kage, T. Shimojo, J. Oba, M. Hayashi, M. Matsukata, E. Kikuchi, J. Catal. 211 (2002) 75-84.