Simultaneous Low-temperature Storage of NO and C3H6 over Pd/SSZ-13: Chemistry of Adsorption and Release

Huawang  Zhao, University of Michigan

Simultaneous Low-temperature Storage of NO and C3H6 over Pd/SSZ-13: Chemistry of Adsorption and Release

Huawang Zhao1,2, Xiaoyin Chen1, Yongdan, Li2,3, Johannes W. Schwank1*

  1. Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109, USA
  2. Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin Key Laboratory of Applied Catalysis Science and Technology, State Key Laboratory of Chemical Engineering (Tianjin University), School of Chemical Engineering, Tianjin University, Tianjin, 300072, China
  3. Department of Chemical and Metallurgical Engineering, School of Chemical Engineering, Aalto University, Espoo, 02150, Finland


The increasing stringent emission standard calls for a unique strategy to control emissions coming from vehicles during cold start, which contribute up to 80% harmful gases of the exhaust. It has been reported that ion-exchanged Pd2+ in the chabazite (SSZ-13) zeolite provides a promising method to store NOx at low temperatures, while the effective trap of hydrocarbons (HCs) needs different zeolites. Herein, we present the simultaneous trapping NO and C3H6 under the stimulated exhaust conditions at 100 oC over Pd/SSZ-13. The effects of the loading and state of Pd species, presence of intermediate CO, and oxidation reaction have been investigated and the results were discussed in highlight of chemistry of the adsorption of NO, C3H6, and the intermediate CO.

Pd/SSZ-13 catalysts with different Pd loading (1-3 wt.%) were prepared by IWI impregnation. The adsorption of NO and C3H6 was performed at 100 °C in the presence of CO2 and steam, followed by the release by the temperature-programmed procedures from 100 to 600 oC. The results of NH3 TPD, DRIFTs, and NO adsorption showed that most of ion sites was occupied by Pd2+ but the ion-exchange degree decreased with the increase of Pd loading. The non-exchanged Pd species present in the PdO nanoparticles (~ 1 nm), which was characterized by TEM and was confirmed by the results of TGA and the oxidation of both C3H6 and CO. It was found that Pd/SSZ-13 showed higher C3H6 adsorption capacity than NO under the same conditions. The presence of C3H6 can facilitate trapping NO and the storage capacity was increased by 18% (1% Pd), 27% (2% Pd), and 39% (3% Pd), respectively.

Characterization of adsorption species by the results of in-situ DRIFTs indicates that C3H6 and NO can be co-adsorbed on the Pd2+ sites to form a complex olefin-nitrosyl Pd2+(NO)(C3H6). The results of TPO under different conditions suggest the different functions of the Pd2+ ions and the PdO nanoparticles in SSZ-13. The Pd2+ ions provide the adsorption sites for both NO and C3H6 while the PdO nanoparticles are active for oxidation of C3H6, which is inhibited in the presence of NO. Both C3H6 and CO oxidation provided the evidence of the formation of CO as an intermediate over Pd/SSZ-13 with different Pd loadings.

The release of stored NO showed different behaviors with and without the presence of C3H6.  Without C3H6, one broad NOx desorption peak in the range of 200-450 oC. With a co-feeding of NO and C3H6, the NOx desorption with a small  peak in the range of 200-280 oC accompanied by a dominant peak in the range of 350-480 oC occurred and a strong NOx adsorption peak was observed in 280-350 oC The combined results of a series of DRIFTs and TPO revealed that the release of NOx resulted from Pd2+(NO)(C3H6) complex occurred at 200-280 oC, whereas a part of Pd2+(NO)(C3H6) can transform to more stable carbonyl-nitrosyl Pd2+(NO)(CO) complex via the partial C3H6 oxidation. At the meantime, the Pd2+ sites resulted from the NO desorption can re-adsorb both NO and CO at 280-350 oC. The quantified release amount of co-feeding NO+CO+C3H6 provides the association ability of Pd2+ with adsorbed gas species that follow a sequence of NO > CO > C3H6, which indicates the higher bonding between NO and Pd2+ in Pd2+(NO)(CO) than that in Pd2+(NO) and Pd2+(NO)(C3H6) complex.