L’objectif de cette thèse de doctorat a consisté à déterminer l’influence des conditions réactionnelles du vaporeformage de l’éthanol (ESR), de la dispersion du catalyseur et de la promotion par le potassium sur l’état de la surface. Ce travail a aussi aidé à comprendre l’influence de ces facteurs sur les propriétés catalytiques. Nous avons utilisé les catalyseurs à base de cobalt (promus et non promus par le potassium) supportés à l’oxyde de cérium et à l’oxyde de zirconium à faible et à forte dispersion. Les changements de l’état de la surface des catalyseurs pendant la réaction d’ERS ont été étudiés à travers la spectrométrie photoélectronique X (XPS), alors que les changements des produits ont été analisés en utilisant la spectrométrie de masse et la chromatographie en phase gazeuse. Le catalyseur supporté sur oxyde de cérium à forte dispersion a été caractérisé sous une basse pression (0.2-20 mbar) avec le rapport molaire eau/éthanol de 3/1 (420ºC). Les autres tests ont été faits sur tous les catalyseurs sous une pression totale de 1 atm avec les rapports molaires de 3/1, 9/1, 12/1 (420ºC). Nous avons utilisé un mélange eau/éthanol dans un rapport molaire de 12/1 pour étudier les changements de l’état de la surface de tous les catalyseurs dans le temps. Il a été démontré que la sélectivité d’ESR des catalyseurs pour produire des gaz et pour déposer le carbone est réglée par la concentration des groupes hydroxyles sur la surface. Quant aux catalyseurs promus, elle dépend aussi de la concentration Kδ+–Osurfδ-. / The aim of the thesis was determination the influence of the ethanol steam reforming (ESR) reaction conditions, catalyst’s dispersion and potassium promotion on a surface’s composition and understanding the influence of these changes on catalysts’ performance. Cobalt-based catalysts (unpromoted and promoted with potassium) with low- and high-dispersed ceria and zirconia supports were used. The changes of the surface state of catalysts during the ESR were studied by means of X-ray photoelectron spectroscopy, whereas the reaction products evolution was followed by mass spectrometer or gas chromatograph. Highly-dispersed ceria-supported catalyst was characterized under low pressure conditions (0.2–20 mbar) with the water/ethanol molar ratio equal to 3/1 (at 420ºC). The other tests were carried out over all catalysts under total pressure of 1 atm with 3/1, 9/1 and 12/1 molar ratios (at 420ºC). The water/ethanol ratio of 12/1 was chosen for studies of the surface state of all catalysts with time-on-stream. It was found that the ESR selectivity to gaseous products and carbon deposition is governed mainly by surface hydroxyl species concentration; in the promoted catalysts together with Kδ+–Osurfδ- surface sites.

During this PhD thesis, cobalt (Co) catalysts have been prepared, characterized and studied in the carbon monoxide hydrogenation (CO+H2) reaction (also known as “Fischer-Tropsch” (FT) reaction). In industry, the FT synthesis aims at producing long chain hydrocarbons such as gasoline or diesel fuels. The interest is that the reactants (CO and H2) are obtained from other carbonaceous sources than crude oil: natural gas, coal, biomass or even petroleum residues. As it is well known that the worldwide crude oil reserves will be depleted in a few decades, the FT reaction represents an attractive alternative for the production of various fuels. Moreover, this reaction can also be used to produce high value specialty chemicals (long chain alcohols, light olefins…).<p>Two different types of catalysts have been investigated during this thesis: cobalt with magnesia used as support or dispersant (Co/MgO) and cobalt with silica used as support (Co/SiO2). Each catalyst from the first class is prepared by precipitation of a mixed Co/Mg oxalate in acetone. This coprecipitation is followed by a thermal decomposition under reductive atmosphere leading to a mixed Co/MgO catalyst. On the other hand, Co/SiO2 catalysts are prepared by impregnation of a commercial silica support with a chloroform solution containing Co nanoparticles. This impregnation is then followed by a thermal activation under reductive atmosphere.<p>The mixed Co/Mg oxalates and the resulting Co/MgO catalysts have been extensively characterized in order to gain a better understanding of the composition, the structure and the morphology of these materials: thermal treatments under reductive and inert atmospheres (followed by MS, DRIFTS, TGA and DTA), BET surface area measurements, XRD and electron microscopy studies have been performed. Moreover, an original in situ technique for measuring the H2 chemisorption surface area of catalysts has been developed and used over our catalysts.<p>The performances of the Co/MgO and Co/SiO2 catalysts have then been evaluated in the CO+H2 reaction at atmospheric pressure. Chemical Transient Kinetics (CTK) experiments have been carried out in order to obtain information about the reaction kinetics and mechanism and the nature of the catalyst active surface under reaction conditions. The influence of several experimental parameters (temperature, H2 and CO partial pressures, total volumetric flow rate) and the effect of passivation are also discussed with regard to the catalyst behavior.<p>Our results indicate that the FT active surface of Co/MgO 10/1 (molar ratio) is entirely covered by carbon, oxygen and hydrogen atoms, most probably associated as surface complexes (possibly formate species). Thus, this active surface does not present the properties of a metallic Co surface (this has been proved by performing original experiments consisting in switching from the CO+H2 reaction to the propane hydrogenolysis reaction (C3H8+H2) which is sensitive to the metallic nature of the catalyst). CTK experiments have also shown that gaseous CO is the monomer responsible for chain lengthening in the FT reaction (and not any CHx surface intermediates as commonly believed). Moreover, CO chemisorption has been found to be irreversible under reaction conditions.<p>The CTK results obtained over Co/SiO2 are quite different and do not permit to draw sharp conclusions concerning the FT reaction mechanism. More detailed studies would have to be carried out over these samples.<p>Finally, Co/MgO catalysts have also been studied on a combined DRIFTS/MS experimental set-up in Belfast. CTK and Steady-State Isotopic Transient Kinetic Analysis (SSITKA) experiments have been carried out. While formate and methylene (CH2) groups have been detected by DRIFTS during the FT reaction, the results indicate that these species play no role as active intermediates. These formates are most probably located on MgO or at the Co/MgO interface, while methylene groups stand for skeleton CH2 in either hydrocarbon or carboxylate. Unfortunately, formate/methylene species have not been detected by DRIFTS over pure Co catalyst without MgO, because of the full signal absorption.<p> / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished