Nitrogen Oxide Removal in Lean-Bum Engines

Lean-bum engines are more fuel-efficient when there is an excess of oxygen in the combustion gas and emit less carbon dioxide per mile travelled. Lean conditions lead to lower levels of NOx in the exhaust gas, but the introduction of future legislation on the NOx content of the exhaust will require that further reduction will have to be considered. NOx removal in the presence of excess oxygen is more difficult than removal under stoichiometric conditions because normally the NOx is reduced by some of the unbumt carbon monoxide and hydrocarbons in the exhaust. 

A partial solution to the NOx removal problem with lean-bum engines may be the use of a platinum/rhodium catalyst combined with a barium oxide trap that can absorb nitrogen dioxide as nitrate. The mechanism of the reaction is  

The cycle then repeats during subsequent lean/rich operatiom 

The main problem with the barium oxide approach is that sulfur dioxide competes with NOx for the basic sites, and is converted irreversibly to barium sulphate, which is quite inert. Thus, the active barium sites are quickly saturated, and the removal of NOx from the emission is severely restricted. The sulfur content of the gasoline would need to be much lower for this approach to provide a long-term solution to the NOx problem. There may also be problems associated with the thermal stabihty of the barium oxide traps during the many redox cycles, which the catalyst would be expected to ejqrerience over the lifetime of an autocatalyst. 

The use of lean-bum engines will therefore be limited until a viable solution to the NOX removal problem has been developed. 

---