Aging of Lean NOX Trap Catalysts and Hydrocarbon Trapping in Cold-Start Applications
It is well known that combustion processes worldwide give rise to emissions such as nitrogen oxides (NOX), carbon monoxide (CO), hydrocarbons (HC) and particulates. For vehicles, the method for handling these emissions has been through the development of the catalytic converter. For the diesel engine, the Lean NOX Trap (LNT) has been used since the 90s and is designed to function well in the high air/fuel ratio in which the diesel engine operates. The LNT often consist of a storage compound, e.g. barium, where NOX is stored in lean conditions (high air/fuel ratio). During short rich pulses (low air/fuel ratio) the NOX is released and reduced over the noble metal sites. Over time in use, these catalytic converters are exposed to the conditions that may reduce the catalytic properties of the catalyst and deactivation studies are therefore critical. Gasoline vehicles are often equipped with a hydrocarbon trap, to store HCs from the engine during the cold-start. The aim is that at higher temperature, when the catalyst is functioning, the HCs desorb and are oxidized.
Chemical poisoning of an LNT catalyst, i.e. Pt/Ba/Al2O3, was studied thoroughly. Both deactivation by phosphorus (P) as well as by zinc (Zn) was examined, because these elements can be found in the lubricant oil. The catalysts were exposed to P in two different ways. In the first study, P was introduced through gas-phase exposure by evaporating phosphoric acid. In the second study, both P and Zn were introduced to the Pt/Ba/Al2O3 catalyst through wet impregnation. The main findings were that gas-phase exposure results in an axial distribution where P can exist in different oxidation states at different positions over the washcoat. Moreover, it could be seen that Zn may help to suppress the deactivation caused by P on the LNT catalyst, and the suggested reason is due to formation of zinc phosphates. Furthermore, the main cause for deactivation of the LNT catalyst by P is attributed to the interaction between P and Ba.
The effect of catalyst composition was examined for HC Traps. The effect of promoting zeolites, by the addition of Pd, La and Fe, was studied. The addition of Fe did not show any significant effect on the toluene storage and release in comparison to zeolite beta. However, both Pd and La exhibited positive effects. The addition of La resulted in both an increase in the storage of toluene and an increase in the desorption temperature for toluene in wet conditions, which is beneficial. However, at higher La loadings, no beneficial effect of La could be seen. Lastly, during mixed-HC temperature programmed desorption experiments, an interaction mechanism between propane and toluene could be observed which resulted in higher adsorption capacity of propane on zeolite ZSM-5.