Chapter 4: Lean NO_x Trap Performance Degradation – Reversible Sulfur Poisoning and Irreversible Thermally-induced Sintering

The principle behind the lean NO_x trap (LNT) mechanism is that NO_x is trapped on the catalyst and is periodically released/reduced thereby regenerating the surface for the next cycle. The trapping chemistry occurs via nitrite and nitrate formation. Unfortunately, similar chemistry occurs for SO_x species, which leads to sulfate formation and a loss in NO_x trapping capacity. Again, much like the regeneration phase of a LNT catalyst cycle to reduce the nitrites and nitrates ultimately to N₂, a reductant-rich phase can be designed to decompose the sulfate species and induce sulfur release. Unfortunately, these sulfate species are more thermally stable than their nitrate counterparts therefore requiring elevated temperatures. High temperature exposure leads to a second degradation mode, thermal degradation via metal and trapping component sintering. With regeneration, surface sulfur species decompose and sulfur is released – the sulfur poisoning mode is reversible. The thermal degradation mode is not. A key here is that thermal degradation occurs primarily because of the sulfur regeneration phase. So, if sulfur was not a poison, thermal degradation itself would be limited. However, with the NO_x and SO_x trapping chemistry being so similar, to date, a sulfur resistant catalyst has not been developed. In this chapter, the effects of thermal degradation on catalyst performance and how surface or structural changes lead to this loss will first be reviewed. This will be followed by sulfur poisoning, and how sulfur interacts with different LNT catalyst surface components. As this is reversible, a review of regeneration processes and their efficacies will follow. Finally, a review of improvements made in sulfur poisoning resistance through formulation changes will be presented.