The characterization of Fe/ZSM5 zeolite materials, the nature of Fe-sites active in N 2 O direct decomposition, as well as the rate limiting step are still a matter of debate. This theoretical result was compared to the experimentally observed steady state kinetics of the N 2 O decomposition and temperature-programmed desorption (TPD) experiments. A switch of the reaction order with respect to N 2 O pressure from zero to one occurs at around 800 K suggesting a change of the rate determining step from the a-oxygen recombination to a-oxygen formation. The TPD results on the molecular oxygen desorption confirmed the mechanism proposed.
A novel "sandwich" microreactor designed as a thin (∼ 300 lm) porous plate of sintered metal fibers (SMF), sandwiched between metallic plates is reported. The SMF surface was coated by Fe-ZSM-5 thin film (< 2 lm) rendering a catalyst highly active in the decomposition of N 2 O. The 3D open microstructure of SMF plates presents a low pressure drop during the passage of reacting gases. The high thermoconductivity of metallic SMF improves the heat transfer, avoiding hotspot formation during exothermic reactions. The temperature measured in the middle and at the outlet of the reactor confirmed the isothermal reactor operation. The sandwich microreactor showed high permeability and a narrow residence time distribution close to an ideal plug-flow reactor. The kinetics of the N 2 O decomposition was studied and the reaction was shown not to be limited by mass transfer when conducted in the microreactor.
The observed rate increase is assigned to a slow accumulation on the surface of NO x,ads species formed from N 2 O and participating as co-catalyst in the N 2 O decomposition. The NO x,ads species accelerates the atomic oxygen recombination/ desorption, which is the rate-determining step of N 2 O decomposition. The formation and accumulation of NO x,ads species during N 2 O interaction with the catalyst was confirmed by TAP studies. The amount of NO x,ads was found to depend on the number of N 2 O pulses injected into the TAP reactor. In the presence of adsorbed NO x on the catalyst surface (NO x,ads ) the desorption of dioxygen is facilitated. This results in a shift of the oxygen desorption temperature from 744 K to considerably lower temperatures of 580 K in TPD experiments. Pulses of gaseous NO had a similar effect leading to the formation NO x,ads , thus facilitating the oxygen recombination/desorption.
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