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.