Activité et stabilité de catalyseurs métalliques supportés sur oxydes testés en Oxydation Voie Humide du phénol

L'Oxydation Voie Humide Catalysée (OVHC) est un procédé de traitement des eaux prometteur, au cours duquel les substances polluantes sont totalement oxydées sous pression d'oxygène et à température élevée. L'objectif de cette thèse est d'étudier l'activité et la stabilité de différents catalyseurs métalliques testés en oxydation du phénol. Plusieurs catalyseurs en poudre, à base de platine ou de ruthénium supportés sur cérine ou cérine dopée ont été comparés en OVHC du phénol en réacteur fermé. À dispersion identique, Pt est toujours plus actif que Ru. Quatre principales causes de désactivation ont pu être identifiées : - le dépôt d'espèces organiques aromatiques, issues de réactions de couplage oxydant, est responsable d'une perte d'activité provisoire par blocage des sites catalytiques, - l'oxydation des particules de métal est un phénomène réversible causé par les fortes pressions d'oxygène imposées, - des conditions de réaction trop sévères peuvent provoquer une perte de métal et un frittage des particules, engendrant une perte d'activité définitive. Des catalyseurs sous forme de pellets ou de monolithes ont été préparés et testés en OVHC du phénol dans un Réacteur Continu Parfaitement Agité. Bien qu'actifs, ces catalyseurs sont inadaptés, en raison de leur résistance mécanique limitée. Des catalyseurs à base de cuivre échangé sur zéolithes 13X ont été testés dans un procédé impliquant à la fois l'oxygène et le peroxyde d'hydrogène. Un effet de synergie entre les deux oxydants, lié à une modification du mode de décomposition de H2O2 en présence de O2, a pu être constaté. Hélas, l'activité observée est principalement due aux espèces Cu passées en solution.Catalytic Wet Air Oxidation (CWAO) is a promising process for water treatment which consists in totally oxidizing pollutants under oxygen pressure and at elevated temperature. This thesis aims at studying the activity and the stability of different metallic catalysts tested in the oxidation of phenol. Several powder catalysts constituted by platinum or ruthenium supported on ceria or doped ceria were compared in CWAO of phenol in a batch reactor. At identical dispersion, platinum is always more active than ruthenium. Four main deactivation causes were identified : - the deposition of organic aromatic species, formed by oxidative coupling reactions, is responsible for a temporary loss of activity by blockage of catalytic sites, - the oxidation of metal particles is a reversible phenomenon which is caused by the use of important oxygen pressures, - too severe conditions of reaction can be responsible for a loss of metal and sintering of particles, which leads to a definitive loss of activity. Pelletized and monolithic catalysts were prepared and tested in CWAO of phenol in a Continuous-flow Stirred Tank Reactor. Although active, these catalysts are inappropriate since their mechanical strength is limited. Copper on 13X zeolite catalysts prepared by ion-exchange were tested in a process using oxygen and hydrogen peroxide. A synergistic effect, resulting from a modification of H2O2 decomposition pathway, in the presence of O2, was observed. Nevertheless, activity was mainly due to leached copper species.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

Pd Loading and Structure of Flame-Made Pd/YFeO3±δ

The interactions between a platinum-group metal (PGM) and a perovskite-type oxide are complex since the latter can accommodate the former in its structure, simply act as a support or, in specific cases, reversibly switch between these two behaviours, depending on the redox environment. Despite promising performances as oxidation catalysts, Y-based perovskite-type oxides are far less studied than their La-based counterparts and their interactions with PGM need to be better understood. The morphology, coordination and oxidation state of Pd species in Pd-doped YFeO3±δ catalysts prepared by flame spray synthesis were investigated in dependence on Pd loading in the range of 0-2.5wt%. Their thermal stability was assessed by calcination of the flame-made materials at 700°C. Fresh and calcined samples were thoroughly characterized by STEM, N2-physisorption, XRD, XPS, DRIFTS and OSCC. Pd species were predominantly in the form of metallic nano-particles supported on YFeO3±δ. The size of these nano-particles increased with increasing loading as evidenced by DRIFTS. XPS facilitated the identification of Pd2+ species in strong interaction with the hexagonal YFeO3 lattice, suggesting the partial incorporation of noble metal ions in the perovskite-type structure. After calcination at 700°C, this contribution vanished in the catalysts containing at least 2 wt% Pd. The catalysts were tested for methane oxidation under stoichiometric conditions up to 850°C. The catalyst with 2 wt% Pd exhibited the highest CH4 oxidation activity. Reduction of the Pd content to 0.5 wt% resulted in the shift of the 50% CH4 conversion by only ca. 40°C. Hence, flame-made Pd/YFeO3±δ demonstrated to be a suitable material to maintain CH4 conversion with reduced noble metal content

Wet Air Oxidation of phenol over Pt and Ru catalysts supported on cerium-based oxides: Resistance to fouling and kinetic modelling

International audienceCeria and doped ceria supported Pt and Ru catalysts were tested at 160 degrees C in the Catalytic Wet Air Oxidation (CWAO) of phenol. Catalysts were compared in terms of activity, selectivity and resistance towards fouling. The respective influences of metal phase and support were studied. Under the selected operating conditions, 100% phenol conversion could be reached. Contrary to what was expected, improved Oxygen Storage Capacities (OSC) accelerated the accumulation of adsorbed species on the catalyst surface, therefore limiting the catalytic performance. By contrast, high metal dispersions enhanced both the elimination of aqueous organic compounds and the degradation of heavy molecules involved in the catalyst fouling. The progressive decrease in activity induced by carbonaceous deposits could be kinetically modelled using a simple reaction schem

Chromium-induced deactivation of a commercial honeycomb noble metal-based CO oxidation catalyst

A commercially available honeycomb CO oxidation catalyst used to control the exhaust of a solid oxide fuel cell (SOFC) based power system has been characterized after prolonged use. X-ray fluorescence (XRF), X-ray diffraction (XRD), Raman spectroscopy, N2 physisorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were employed to determine the phase composition, the morphology and the chemical state of the various components. Besides sintering of the active phase, deactivation was found to occur mainly as the result of the deposition of chromium-containing species on the catalyst washcoat. These fouling species mainly appeared as highly crystalline Cr2O3 particles and could still maintain acceptable CO oxidation activity under dry atmosphere. However, the formation of surface chromium oxyhydroxide species was found to occur in the presence of water vapor, leading to significant catalyst deactivation