Nitrogen oxide storage and reduction studies on platinum/barium/alumina monolithic catalysts.

摘要

Stricter emission standards and the increased used of lean-burn engine technology has prompted the research and development of NOX storage and reduction (NSR) or "NOX trap" technology. During the NSR process, NOX is oxidatively adsorbed on an alkali earth oxide (barium oxide), forming a surface nitrate. The nitrate is then chemically reduced over a noble metal (Pt) by periodic "rich" operation, which increases the level of hydrocarbons in the exhaust, promoting the release and reduction of NOX.; This work systematically investigated the performance of a series of Pt/BaO/Al2O3 monolithic catalysts. The objective was to gain insight into the effect of various operating parameters and catalyst compositions on the productivity of the monolithic reactor. An experimental set-up was designed and built to monitor the transient reactor temperatures and effluent gas concentrations.; Experimental results showed significant enhancement in time-averaged NOX conversion was achieved through the intermittent addition of short, intense pulses of reductant into a stream containing NOX and excess oxygen. Short, rich pulses were found most effective, and the time-averaged NOX conversion was maximized at an intermediate cycle time and reductant pulse duty, with values dependent on the feed temperature and flow rate. The NOX conversion achieved a maximum at an intermediate temperature and the temperature profiles at higher space velocities gave superior thermal conditions for NSR.; An extensive study of the NOX storage cycle was performed for the series in order to gain insight into the NOX storage mechanism. It was determined that increases in the BaO loading led to increases in the NOX storage, but the extent depended upon the exposure time. The effect of increasing the Pt loading on the NOX storage was not as clear.; The impact of catalyst composition on NOX storage and time-averaged NOX conversion under cycling conditions was also evaluated. Increases in Pt loading led to monotonic increases in NOX conversion, whereas increases in BaO loading did not. Transient concentration profiles were analyzed in order to gain insight into the role of Pt in the overall NSR process. The relationship between the NOX trapping efficiency and time-averaged NOX conversion was also examined.