Three-Way Catalyst Deactivation Associated With Oil-Derived Poisons

摘要

The results presented here indicate that oil-derived catalyst poisons such as P and heavy metals like Zn are most significantly deposited on three-way catalysts during low temperature engine operating modes. The design of accelerated catalyst aging protocols require modes with low temperature operation with high levels of potential poisons in order to impart the kinds of catalyst deactivation associated with poison accumulation on the catalyst surface. Combining low temperature modes that facilitate poison deposition on the catalyst surface with higher temperature modes appears to drive the poisons deeper into catalyst structure. The aging cycle developed in this study makes use of oil injection doped with high levels of ZDDP upstream of the converter in combination with both low temperature and high temperature operating modes. Adjustable aging parameters such as the level of ZDDP present in the injected oil, the relative duration of the low temperature mode, and the temperature associated with the low temperature mode can all be used to try and create poison profiles and catalyst performance observed after real world vehicle operation. The penetration of P and Zn into the washcoat of three-way catalysts observed by microprobe on the catalysts aged here, has been observed on real vehicle fleets that are known to have large relative consumption levels of engine oil and considerable operating time under low speed, low temperature exhaust condi-tions. Significantly higher catalyst light-off temperatures for regulated emissions such as hydrocarbons and NOx characterize the catalyst deactivation observed with this oil injection aging protocol and under real world conditions that favor high P and Zn levels on three-way catalysts. As discussed by Darr et al. (6), even modest amounts of poison deposition on a close-coupled catalyst's inlet face that lead to modest deactivation of a catalyst's light-off characteristics could be the difference between meeting a SULEV emission requirement or failing to meet these near-zero emission levels. In order to minimize these negative impacts on catalyst performance stemming from oil-derived poisons, it is critical that engine lubricant consumption be maintained at low levels throughout the catalyst regulated lifetime, or alternative lubricant additive packages that minimize the levels of potential poisons such as P and Zn be developed for emission critical applications.