Experimental techniques for investigating the surface oxygen formation in the N2O decomposition on Fe-MFI zeolites

Abstract

Several experimental techniques were used to investigate the formation of surface oxygen in the N2O decomposition on an Fe-MFI zeolite. These techniques consisted of multipulse and step experiments as transient methods at ambient pressure and experiments in a closed set-up of Panov and co-workers at strongly reduced pressure. The total amount of surface oxygen determined through multipulse experiments was in the range of the amount obtained in the vacuum set-up at 523 K (40-50 mu mol O g(catalyst)(-1)). In contrast, the step technique revealed considerably higher values, up to 110 mu mol O g(catalyst)(-1), indicating an accumulation of oxygen in the zeolite. This phenomenon might also be responsible for the observation that higher cat reaction temperatures increase the total amount of surface oxygen, which can be deposited in the zeolite. In contrast to experiments in the vacuum set-up, the temperature-programmed desorption after multipulse and step experiments shows the presence of various oxygen species differing in thermal stability. The influence of temperature on the rate of surface oxygen formation was determined from step and vacuum experiments. The results of the step experiments lead to a lower activation energy, 14 kJ mol(-1), than the experiments in the vacuum set-up (49 kJ mol(-1)), probably due to sorption and mass transfer effects. Comparing the rate of surface oxygen formation and of total N2O decomposition reveals that the former is very fast and not rate determining for the latter. Furthermore, the rate of total N2O decomposition seems to be inhibited by adsorbed N2O, detected in the transient experiments. This observation is considered in an adequate kinetic model. (c) 2007 Elsevier B.V. All rights reserved.