Structural effects of Pd exchanged zeolites on ethylene trapping performance

Abstract

The three-way catalyst (TWC) was well-known effective for control the hydrocarbons (HCs), CO, nitrogen oxide form engine emission. However, the TWC has needed to working temperature (e.g., more than 250 ℃). To meet ever tightening vehicle emission standards, the vehicle emissions specifically for lean-burn combustions needs to be controlled even during cold-start period. Among the controlling of emission during cold-start period, the methods that can be practically applied is the adoption of trapping materials. The trap to control the large amount of hydrocarbon discharged during cold-start period can store the HC molecules at low temperature region below 150 ℃, subsequently converting stored HCs to CO2 by the oxidation over active sites. Zeolites as the microporous material possesses a specific channel size and structure, large surface and acid sites therefor. These characteristics can be applied trapping HCs selectively and strongly with respect to the pore size and cation exchanged. These properties can be stored at low temperature and released the adsorbed hydrocarbons at high temperature. In this study, we compared the trapping performances of ethylene (kinetic diameter: 3.9 Å) as a function of the pore size of zeolites. The H-BEA (12-membered ring, large pore: 6.7 Å), H-ZSM-5 (10-membered ring, medium pore: 5.6 Å) and H-SSZ-13 (8-membered ring, small pore: 3.8 Å) have been employed, while keeping the Si/Al ratio as 12.5, 16, and 15. The zeolites that induced palladium ion-exchanged have been prepared to identify the role of active cation sites in the adsorption capacity and their oxidation activities during the desorption period. The adsorption of ethylene was carried out at 80 ℃ for 30 min followed by purging a catalyst bed. We also observed the desorption/oxidation of ethylene from 80 ℃ to 500 ℃ with ramping of 10 ℃/min. The H-form bare zeolites were not effective in the ethylene trapping under both dry (w/o water vapor) and wet feed (w/ 5 % water vapor) conditions. However, the presence of Pd cations in zeolites clearly improved the adsorption capacity even under the wet feed condition. Particularly, the medium pore Pd/ZSM-5 showed a superior performance of the ethylene trapping and subsequent oxidation compared to the large pore Pd/BEA and the small pore Pd/SSZ-13 under both condition. A conversion reaction actively occurs over the Pd/BEA and Pd/SSZ-13 under dry condition, respectively, as confirmed by aldehyde-based HC band spectra from the gas-cell FTIR. To study the role of palladium, Pd/ZSM-5 with superior trapping performance were prepared by content. Pd/ZSM-5 were prepared from 0 wt. % to 3 wt. % and tested in the same way under wet condition. Even a low palladium loading on ZSM-5 showed satisfactory performance under wet condition. Contrary to expectation that adsorption performance would increase as palladium content increased, adsorption capacity of ethylene decreased at higher content. 1 wt. Pd/ZSM-5 still showed the best trapping performance. High content (2 and 3 wt. %) Pd/ZSM-5 occurred the HC products during adsorption period such as Pd/BEA and Pd/SSZ-13. Presence of palladium species in zeolite samples were investigated from XPS and CO-DRIFTS characterizations. The XPS analysis was measured the Pd species on external surface the zeolite according to the pore size. Pd/BEA and Pd/SSZ-13 were more relatively distributed PdO species than Pd/ZSM-5. The medium pore size of ZSM-5 has various palladium distribution on its external surface. The performance of CO-DRIFTS was confirmed the presence of distributed species of PdO species on SSZ-13 with small pore size. Through the adsorption of CO in Pd/Zeolite, the distribution and particle size of Pd species by structure could be indirectly determined. The difference in Pd species on ZSM-5 by content was performed through XPS and CO-DRIFTS. The high palladium content observed that PdO was produced as external surface on ZSM-5. As the palladium content increased, exchanged-palladium in ZSM-5 increased. But the adsorption efficiency decreased due to the formation of large particles. To investigate the acid properties, NH3-temperature programmed desorption and Pyridine Fourier transform infrared spectra (FTIR) was performed. And these studies revealed that Brønsted acid site and Lewis acid site different generation of zeolites. Presence of palladium was generated on Lewis acid site at all Pd/Zeolite samples. Pd/ZSM-5 and Pd/SSZ-13 provided adsorption site by forming the Brønsted acid sites than H-form. Pd/ZSM-5 and Pd/SSZ-13 were less affected by water inactivation. The influence of the Brønsted acid sites could also be measured in the amount of Pd/ZSM-5 catalysts. The increase in Lewis acid site and the decrease in Brønsted acid site observed in high content of palladium in ZSM-5 prove the adsorption efficiency not proportional to the palladium loading. Furthermore, microporous material and mesoporous material were compare to control the ethylene during cold-start period. Pd/ZSM-5 with superior adsorption performance can reduce cumulative emission of HC with transient ramp up tests, comparing the reduction performance of Pd/BEA and Pd/Al2O3 catalyst. The combination of trapping materials to existing commercial exhaust control catalysts suggests the possibility of reducing hydrocarbons emitted during the cold-start period.