Dynamics of N2O decomposition over HZSM-5 with low Fe content

The dynamics of N2O decomposition to gaseous nitrogen and oxygen over HZSM-5 catalysts with a low iron content (200 and 1000 ppm) was studied by the transient response method in the temperature range 523-653 K. The active catalysts were prepared from HZSM-5 with Fe in the framework on its steaming at 823 K followed by thermal activation in He at 1323 K. Two main steps were distinguished in the dynamics of N2O decomposition. The first step represents N2O decomposition forming gaseous nitrogen and surface atomic oxygen. The second step is associated with surface oxygen recombination and desorption. At 523-553 K only the first step is observed. Above 573 K the decomposition of N2O to O-2 and N-2 in stoichiometric amounts starts at a rate increasing with time until a steady-state value is reached. This increase was assigned to the catalysis by adsorbed NO formed slowly on the catalyst surface from N2O, as indicated by temperature-programmed desorption. The catalytic effect of the adsorbed NO was also confirmed by transient experiments with forced addition of NO in the stream of N2O during its decomposition. A simplified kinetic model is proposed to explain the autocatalytic reaction. Catalyst pretreatment in O-2 did not affect N2O decomposition, but irreversible water vapor adsorption at 603 K resulted in a twofold decrease in surface oxygen loading from N2O and complete inhibition of the oxygen desorption. (C) 2003 Elsevier Inc. All rights reserved.