Infoscience

Thesis

Nitrous Oxide Abatement over Fe-Zeolite: Mechanistic Study of Decomposition and Reactor Development

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 (<723 K) is still under discussion due to its complexity. It was demonstrated that NOx enhances this step and proposed that NOx,ads species are formed and accumulated during N2O interaction with the catalyst. In the present study, the mechanism of oxygen desorption below 723 K was elucidated. TAP studies confirmed that NOx,ads is accumulated during the catalytic decomposition of N2O (chapter 5). The amount of NOx,ads was found to depend on the number of N2O pulses injected into the TAP reactor. The NO2 adsorption/accumulation which was suggested in chapter 3, was further confirmed by TPD experiments (see chapter 6). These experiments demonstrate that NO2 is formed during the N2O decomposition in consecutive steps. First, N2O dissociates forming surface atomic α-oxygen, which reacts with another N2O molecule rendering surface NO. The NO, in a further step, is oxidized by α-oxygen resulting in the formation of surface NO2. We prove that this molecule is suggested to be involved in a catalytic cycle of N2O decomposition playing the role of oxygen storage and carrier. Two different sites of NO2 adsorption have been identified (chapter 7). One fraction of these sites are in the vicinity of each α-sites which are active for N2O decomposition while, another fraction is spread on the catalyst. Both kind of sites were only observed after high temperature treatment (1273 K). It was then suggested that the NO2-sites in vicinity of α-sites were formed during the dehydroxylation of active Fe2+ (α-sites) while the NO2-sites dispersed through the catalyst were formed during the dehydroxylation of inactive Fe3+. These surface NO2 species facilitate molecular oxygen formation but reduce the activity of this adsorbed oxygen for oxydation. It was also observed that the incorporation of water in the feed results in fast NO2 desorption from the sites in the vicinity of α-sites but has no influence on the NO2 adsorbed in the vicinity of inactive Fe3+. The structure of the active sites is quantified via surface titration with nitrous oxide followed by TPD and characterized using nitric oxide probe molecule followed by infrared spectroscopy. In this latter case, two mononitrosyl 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 work presented in chapter 8, 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 was built-up from the porphyrinatoiron(II) model. Chapter 9 is devoted to a new reactor design (Sandwich Reactor) using Fe-zeolite which grows in thin (∼300 µm) porous plate of sintered metal fibers (SMF), sandwiched between metallic plates. This novel design involving a structured catalytic bed showed high permeability and a narrow residence time distribution close to an ideal plug-flow reactor. The use of SMF as a support decreases drastically the surface necessary to get very thin crystal deposition on the reactor and its high thermoconductivity improves the heat transfer, avoiding hot-spot formation. In order to reduce the pressure drop up to a factor of 50, a second reactor design (Structured catalystic wall microreactor, SCWMR) has been developped and is presented in annexe 1.

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