Adsorption and reactivity of nitrogen oxides (NO_2, NO, N_2O) on Fe-zeolites

摘要

Nitrous oxide decomposition and temperature programmed desorption tests on Fe-ZSM-5 and Fe-silicalite show that the catalytic conversion mechanism of N_2O into N_2 and O_2 over Fe-zeolites is more complex than expected. Nitrogen oxides are formed as byproducts of the catalytic process with the major part consisting in NO_2 species adsorbed on the iron sites. FTIR spectroscopy of adsorbed N_2O, NO, and NO_2 has been used to investigate the structure and environment of the iron active species of the Fe-MFI catalysts before and after atomic oxygen deposition. The interactions of NO and N_2O probes on activated Fe-ZSM-5 have evidenced two families of mononuclear Fe(II) centers (Fe_A and Fe_B) differing in the coordination state of Fe. N_2O also interacts with Br?nsted sites of Fe-ZSM-5 via hydrogen bonding. This type of interaction is nearly absent in Fe-silicalite. Polynuclear species (clusters) and iron oxide particles, whose concentrations are strongly influenced by the iron content and by the preparation methods are also present. When oxidized samples (by N_2O) are considered, the ability of Fe_A and Fe_B centers to adsorb N_2O and NO is strongly depressed. On the contrary, the surface chemistry of iron particles is not appreciably influenced. These results represent an indirect proof of the preferential presence of adsorbed oxygen on isolated Fe centers. NO titration of oxidized Fe-ZSM-5 results in the formation of a complex network of interplaying neutral (NO, NO_2, N_2O_4) and ionic species (NO~+, NO_2~-, NO_3~-). The cooperation of sites between Br?nsted and iron active sites is demonstrated. The last observation is fully confirmed by the experiments performed using NO_2 probe that titrates both Br?nsted and iron sites. On the basis of the comparison of catalytic results of N_2O decomposition and of spectroscopic results concerning the titration of surface sites with N_2O, NO, and NO_2 obtained on the same samples (which form the main scope of the paper), it clearly emerges that mononuclear sites characterized by lowest coordination are the most active in N_2O decomposition. Under the adopted conditions, low or negligible activity is shown by Fe_xO_y clusters and Fe_2O_3 particles.