The synergistic mechanism of NOx and chlorobenzene degradation in municipal solid waste incinerators

Abstract

The development of a selective catalytic reduction (SCR) catalyst that destroys both NO_x and chlorobenzene (CB) has drawn considerable interest for controlling emissions from municipal solid waste incinerators. SCR catalyst deactivation that is caused by Cl-containing species is one of the most serious problems in actual operation. We developed a MnO_x–CeO₂ catalyst for simultaneous control of NO_x and chlorobenzene and proposed a mechanism by which this occurs. The primary problem is that Cl can easily dissociate from CB and deposit on the catalyst surface via nucleophilic or electrophilic exchange effects between 200 and 300 °C. This leads to a considerable decrease in both redox properties and SCR performance. This passivation effect decreased the formation of undesired by-products, N₂O or NO₂, at high temperatures. CB adsorption was inhibited by competitive adsorption with excess NH₃; however, CB catalytic destruction was improved by NO and O₂ at 300 °C, whether NH₃ was present or not. The intermediate species, NH₄NO_x, or the product, NO₂, during the SCR process accelerated the dechlorination process on the catalyst surface and the formation of HCl or chlorinated hydrocarbons (e.g. CCl₄) instead of Cl₂. The long-term SCR activity depended mainly on the accumulation of Cl on the catalyst rather than competitive adsorption.