Cu2+-Exchanged MFI-ferrisilicate catalysts for the reduction of nitric oxide with ammonia

Abstract

Catalytically active sites in Cu²⁺-exchanged MFI-ferrisilicate have been studied for the reduction of nitric oxide with ammonia in the presence of excess oxygen. Cu-ferrisilicate with a Cu²⁺-exchange level of 26–162% catalyzed the reduction of NO selectively at 550 K and below. At higher temperatures, oxidation of ammonia with oxygen occurred, lowering the selectivity for reduction of NO into N₂. The rate of N₂ formation was first-order with respect to the partial pressure of NO. The catalytic activity of H-ferrisilicate was greatly enhanced by Cu²⁺-exchange, i.e., the activity of Cu-ferrisilicate increased with increasing Cu²⁺-exchange level to be more than eight times that of H-ferrisilicate. The specific activity per copper ion did not change with the Cu²⁺-exchange level, indicating that a Cu²⁺ ion introduced in ferrisilicate created an active site with constant activity irrespective of the exchange level. This was totally different from Cu²⁺-exchanged ZSM-5 catalysts whose specific activity increased steeply with increasing copper concentration. The difference between Cu-ferrisilicate and Cu-ZSM-5 was explained by the role of framework Fe³⁺ ions in ferrisilicate as a part of an active site; Fe³⁺ adsorbs NO, probably as NO₂ or NO₃⁻ species, and Cu²⁺ exchanged into the site close to the Fe³⁺ ion adsorbs NH₃. The reduction of NO proceeds on Cu-ferrisilicate through the reaction between these adsorbed species on the pair of Fe³⁺ and Cu²⁺ ions.