Chimie verte sur catalyseurs perovskites

ABSTRACT. Development of active and stable noble metal-free catalysts is more and more required for environmental applications and valorization of renewable resources. Easy tuning of the reactivity of mixed oxide perovskites by cation and anion substitutions renders them a promising field of catalyst innovation. In the thesis, two classes of perovskite materials have been synthesized, characterized and tested in catalysis. Cation substitution of Co and Ga for Al in LaAlO3 has allowed to modify both redox and acid-base properties, whereas partial hydride substitutions for oxygen in LaSrCoO4 has provided a test-bed of hydrogenation activity. Tailoring the redox properties of the LaAlO3 mixed oxide by Co substitution has allowed to develop a multifunctional catalysts dealing with NOx and soot decontamination of diesel exhausts. NOx-assisted soot oxidation involves NO to NO2 conversion and subsequent soot oxidation by the formed NO2 at relatively low temperature. LaAlO3 materials with different degrees of Co substitution for Al were prepared by sol-gel method and their catalytic activity was correlated with crystallinity, redox properties and mobility of lattice oxygen. Among the studied samples LaAl1-xCoxO3 with x 0.75 exhibits superior catalytic activity for both NO to NO2 oxidation and NOx-assisted soot oxidation. The excellent performance is attributed to the synergetic interaction between Al-Co in B-site of the structure and may be linked to the high lattice surface oxygen supported by a well-crystallized perovskite structure. The production of bio-ethanol is one of the staples of the replacement of fossil fuels with renewable biomass-derived resources. Valorisation of bio-ethanol for the production of chemicals requires a fine control of the acid-base properties of catalysts. While most of the research on perovskite catalysts has focused on their redox properties, studies on their acid-base properties have been much less developed. Partial substitution of Co and Ga for Al in LaAlO3 has allowed to tune the acidity and basicity of the material and to control the relative activity of dehydration and dehydrogenation of ethanol, controlling the ratio between ethylene and acetaldehyde. The nature of the introduced cation allowed to form different Lewis acid-base pairs, orienting the reactivity of the acetaldehyde intermediates towards Tishchenko or aldol coupling, allowing to direct the reaction cascade towards the formation of butadiene or acetone and odd-C oxygenates. While most of the literature on perovskite reactivity has focused on their structure/properties modifications by cation substitutions, there is a limited number of works on substitutions in the anion sites. Oxyhydride perovskites, ABO3-xHx, where hydride anions Hcan substitute O2- in any sites of the BO6 octahedron, have been reported presenting important hydride mobility, suggesting potential applications in hydrogenation catalysis. Oxyhydrides LaSrCoO3H0.7 and BaTiO3-xHx have been prepared, characterized and tested in a model hydrogenation reaction. The reactivity of hydride species has been tested using chemical titration of toluene at temperatures at which hydrides are expected to be mobile. The results show that LaSrCoO4- xHy exhibited a much higher activity than BaTiO3-xHx, due to the metallic sites formed by partial reduction of LaSrCoO4-xHy, indicating a possible use of perovskites as support for hydrogenation catalysts.