Plasma-Assisted Catalytic Reduction of NOx

Many studies suggest that lean-NO_x SCR proceeds via oxidation of NO to NO₂ by oxygen, followed by the reaction of the NO₂ with hydrocarbons. On catalysts that are not very effective in catalyzing the equilibration of NO+O₂ and NO₂, the rate of N₂ formation is substantially higher when the input NO_x is NO₂ instead of NO. The apparent bifunctional mechanism in the SCR of NO_x has prompted the use of mechanically mixed catalyst components, in which one component is used to accelerate the oxidation of NO to NO₂, and another component catalyzes the reaction between NO₂ and the hydrocarbon. Catalysts that previously were regarded as inactive for NO_x reduction could therefore become efficient when mixed with an oxidation catalyst. Preconverting NO to NO₂ opens the opportunity for a wider range of SCR catalysts and perhaps improves the durability of these catalysts. This paper describes the use of a non-thermal plasma as an efficient means for selective partial oxidation of NO to NO₂. When combined with some types of SCR catalyst, the plasma can greatly enhance the NO_x reduction and eliminate some of the deficiencies encountered in an entirely catalyst-based approach.