Catalytic conversion of N2O over FeZSM-5 zeolite in the presence of CO and NO

摘要

The conversion of N2O in the presence of NO and CO has been investigated over steam-activated FeZSM-5 in the temperature range of 473-873 K. Individually, both CO and NO have a positive effect on the N2O conversion, leading to a lower operation temperature with respect to direct N2O decomposition. At low partial pressures, the catalytic effect of NO as O-2 desorption-accelerator is more effective than the reducing effect of CO. At high partial pressures, the N2O removal is faster with CO than with NO at low temperature. Small amounts of NO in the feed are sufficient to severely inhibit the reduction of N2O with CO, approaching the activity of the binary N2O + NO system. The CO conversion is more negatively affected by NO than the N2O conversion, decreasing progressively with an increased partial NO pressure. The catalyst showed a very low activity in the reduction of NO with CO, evidencing the inability of steam-activated FeZSM-5 to dissociate the NO molecule. The N2O conversion is decreased in the presence of small NO amounts, but no longer inhibited at higher partial NO pressures. The dual role of NO, acting as a promoter in N2O decomposition and as an inhibitor in N2O reduction with CO, can be explained attending to the iron species involved in the various reactions. A recent in situ spectroscopic study [J. Catal. 223 (2004) 13] has demonstrated that both isolated iron ions and oligonuclear iron clusters in FeZSM-5 participate in the reaction of N2O + CO. The present results suggest that NO selectively inhibits the reduction of N2O of isolated iron sites by strong NO adsorption, particularly in the low-temperature range, blocking CO and N2O activation. As a consequence the reaction mechanism shifts to that of NO-assisted N2O decomposition, mainly involving oligonuclear iron species. (C) 2004 Elsevier B.V. All rights reserved.