Effect of water on the reduction of NOx with propane on Fe-ZSM-5. An FTIR mechanistic study

Adsorption of NO on Fe-ZSM-5 leads to formation of Fen+-NO (n = 2 or 3) spe cies (1880 cm(-1)), Fe2+(NO)(2) complexes (1920 and 1835 cm(-1)) and NO+ (2 133 cm(-1)). Water strongly suppresses the formation of NO+ and Fen+(NO)(2) and more slightly the formation of Fen+-NO. Introduction of oxygen to NO c onverts the nitrosyls into surface nitrates (1620 and 1575 cm(-1)) and this process is almost unaffected by water. The nitrates are thermally stable u p to ca. 300 degrees C, but readily interact with propane at 200 degrees C, thus forming surface C-H-N-O deposit (bands in the 1700-1300 cm(-1) region ). Here again, water does not hinder the process. The C-H-N-O deposit is re latively inert (it does not interact with NO or NO +O-2 at ambient temperat ure) but, at temperatures higher than 250 degrees C, it is decomposed to NC O- species (bands at 2215 (Fe-NCO) and 2256 cm(-1) (Al-NCO)). In the presen ce of water, however, the Fe-NCO species only are formed. At ambient temper ature the NCO- species are inert towards NO and O-2, but easily react with a NO +O-2 mixture. The mechanism of the selective catalytic reduction of ni trogen oxides on Fe-ZSM-5 and the effect of water on the process are discus sed.