Low Temperature SCR Catalysts Optimized for Cold-Start and Low-Load Engine Exhaust Conditions

摘要

The main objective of this work is to develop a low-temperature SCR catalyst for the reduction of nitrogen oxides at cold start, low-idle and low-load conditions. A series of metal oxide- incorporated beta zeolite catalysts were prepared by adopting incipient wetness technique, cation-exchange, deposition-precipitation and other synthesis techniques. The resulting catalysts were characterized and tested for reduction of NO_x in a fixed bed continuous flow quartz micro-reactor using ammonia as the reductant gas. Initial catalyst formulations have been exhibited good NO_x reduction activity at low-temperatures. These catalyst formulations showed a maximum NO_x conversion in the temperature range of 100 - 350°C. Besides, more experiments were performed with the aim of optimizing these formulations with respect to the metal atomic ratio, preparation method, active components and supported metal type. Further, these catalyst formulations were screened and investigated for reduction of NO_x in the broad temperature range between 100 and 600C. Both the preparation method and the relative atomic ratio of metals were found to be critical variables for successful catalyst promotion. The beta zeolite supported Cu-Ce catalyst prepared by cation-exchange followed by deposition precipitation technique revealed the most promising performance and broadening of the temperature window from 100 to 600 °C. In order to gain fundamental insights which may acquaint further improvements to these catalysts, NH_3-TPD was performed to investigate the ammonia adsorption/desorption properties at realistic conditions. Finally, the most promising catalyst was tested for sulfur tolerance and hydrothermal aging studies. The powder micro-reactor time on stream test revealed that the beta zeolite supported Cu-Ce catalyst demonstrated high sulfur tolerance between 100 and 300 °C in the presence of 100 ppm SO_2, 5vol.% H_2O and 10 vol.% oxygen in the reaction feed while maintaining stable low temperature NO_x activity. It can be ascribed to the preferentially located active component metal ions in the beta zeolite framework.