Protoxyde d'azote | EPFL Graph Search

Le protoxyde d'azote, ou monoxyde de diazote, oxyde nitreux, hémioxyde d'azote ou gaz hilarant, est un composé chimique de formule . Ce gaz incolore a une odeur et un goût légèrement sucrés. Il est utilisé en anesthésie, en chirurgie, en odontologie, en pédiatrie comme adjuvant (en mélange équimolaire avec de l'oxygène) pour ses propriétés anesthésiques et antalgiques. Il est dit « gaz hilarant » car euphorisant à l'inhalation, d'où son usage comme drogue récréative hallucinogène. Comme comburant, il accroît la puissance des moteurs en compétition automobile. Avec l'acétylène , il est utilisé dans certains appareils d'analyse (spectrométrie d'absorption atomique). Ses émissions sont d'origine naturelle et humaine (plus de 20 % d'augmentation dans l'air depuis l'époque pré-industrielle). Présent à l'état de traces dans l'air sec (), c'est un puissant gaz à effet de serre ( plus puissant que le ) et il est devenu le à la destruction de la couche d'ozone

Do synthetic Fe-zeolites mimic biological Fe-porphyrins in reactions with nitric oxide?

Bryan Bromley, Lioubov Kiwi

Iron containing ZSM-5 zeolites is extremely active in nitrous oxide decomposition. This reaction involves atomic oxygen species known as "alpha oxygen" on iron sites. Despite the multiple techniques devoted to its characterization, the active sites' structure remains nevertheless unknown. Herein, these centers are quantified via surface titration by nitrous oxide followed by temperature programmed desorption and characterized via nitric oxide probe molecule followed by infrared spectroscopy. In this latter case, two mono-nitrosyl species differing in the bonds' geometry are observed on the iron centers. Moreover, upon NO evacuation, the two corresponding IR bands suddenly transit to lower wavenumbers. A mirror trend is also reported in biology when NO interacts with iron porphyrins and explained in terms of linear to bent Fe-N-O modification. This structural change is reversible upon addition/evacuation of NO. In the present report, the same reversible nitrosyl transition is observed for Fe-ZSM-5. Based on the close analogies found, the active site's structure in Fe-ZSM-5 is built-up from the porphyrinatoiron(II) model. © 2010 Elsevier B.V. All rights reserved.

Elsevier2010

Nitrous Oxide Abatement over Fe-Zeolite

Bryan Bromley

The use of zeolites for catalytic reactions is a field in continuous development. Since it was demonstrated that metal-zeolites are efficient catalysts for NOx abatement, decomposition of nitrous oxide (N2O) over Fe-containing zeolites has attracted great interest by the scientific community mainly due: N2O is the third most important gas contributing to global warming, its concentration in atmosphere is still on the rise and direct N2O decomposition to N2 and O2 is a sustainable alternative. N2O interaction with Fe-zeolite leads to the formation of "special" adsorbed oxygen (often called α-oxygen) with great potential for selective oxidation. Although the focus of a number of studies, the nature of the actives sites for the formation of this adsorbed oxygen is still unclear. The main objectives of this work are: 1) study the decomposition mechanism of N2O over isomorpously substituted Fe-ZSM-5 zeolite aiming on catalyst optimization and 2) optimization and further use of structured Fe-zeolite in novel micro-structured reactor. A number of refined transient kinetics methods were intensively used such as transient response, Temperature Programmed Desorption (TPD) and Temporal Analysis of Product (TAP). Density Functional Theory (DFT) and in situ IR spectroscopy studies were conducted in collaboration and our results support the following: In chapter 3, quantitative measurements have been carrier out to analyze the effect of temperature on the activation (under helium flow) and the irreversible deactivations that can happen during the pretreatment. The results obtained indicate that high temperature treatment (1273 K) results in the formation of a high amount of α-sites. The addition of a very low fraction of oxygen (2%) in the feed demonstrates that the catalyst is sensitive to its presence. Certain sites can be irreversibly destroyed, a result ascribed to the oxidation of active Fe(II) to Fe2O3 clusters which contain inactive Fe(III). Catalyst treatment at high temperature (1273 K) in He for up to 3 hours, also reduced irreversibly the concentration of α-sites. TPD measurements have shown desorption of oxygen at 3 different temperatures. Desorption at the lowest temperature (650 K) was assign to the α-oxygen recombination. The oxygen which desorbs at 700 K was associated to the oxygen from the deactivated sites. Oxygen which desorbs at the highest temperature (730 K) is originated from the surface NOx that desorbed as NO and O2. After a comparison of the concentration of the molecules accumulated, we propose that NO2 is the adsorbed specie. The mechanism of N2O decomposition on the binuclear oxo-hydroxo bridged extraframework iron core site [FeII(μ-O)(μ-OH)FeII]+ inside the ZSM-5 zeolite has been studied by combining theoretical and experimental approaches (see chapter 4). Rate parameters computed using standard statistical mechanics and transition state theory reveal that elementary catalytic steps involved into N2O decomposition are strongly dependent on the temperature. This theoretical result was contrasted with the experimentally observed steady state kinetics of the N2O decomposition and TPD experiments. As predicted by the theoretical study, the switch of the reaction order with respect to N2O pressure (from zero to one) occurs at around 800 K which confirms a change of the rate determining step from the α-oxygen recombination to α-oxygen formation. The proposed mechanism was also confirmed by O2-TD which confirmed the viability of the binuclear complex. The α-oxygen recombination at low temperature (

EPFL2010

Nitrous Oxide Abatement over Fe-Zeolite

Bryan Bromley

EPFL2010