Nanostructured catalysts for the development of the hydrogen economy

La catalyse joue un rôle essentiel dans de nombreuses applications industrielles telles que les industries pétrochimique et biochimique, ainsi que dans la production de polymères et pour la protection de l’environnement. La conception et la fabrication de catalyseurs efficaces et rentables est une étape importante pour résoudre un certain nombre de problèmes des nouvelles technologies de conversion chimique et de stockage de l’énergie. L’objectif de cette thèse est le développement de voies de synthèse efficaces et simples pour fabriquer des catalyseurs performants à base de métaux non nobles et d’examiner les aspects fondamentaux concernant la relation entre structure/composition et performance catalytique, notamment dans des processus liés à la production et au stockage de l’hydrogène. Dans un premier temps, une série d’oxydes métalliques mixtes (Cu/CeO2, CuFe/CeO2, CuCo/CeO2, CuFe2O4, NiFe2O4) nanostructurés et poreux ont été synthétisés grâce à une méthode améliorée de nanocasting. Les matériaux Cu/CeO2 obtenus, dont la composition et la structure poreuse peuvent être contrôlées, ont ensuite été testés pour l’oxydation préférentielle du CO dans un flux d’hydrogène dans le but d’obtenir un combustible hydrogène de haute pureté. Les catalyseurs synthétisés présentent une activité et une sélectivité élevées lors de l’oxydation sélective du CO en CO2. Concernant la question du stockage d’hydrogène, une voie de synthèse a été trouvée pour le composét mixte CuO-NiO, démontrant une excellente performance catalytique comparable aux catalyseurs à base de métaux nobles pour la production d’hydrogène à partir de l’ammoniaborane (aussi appelé borazane). L’activité catalytique du catalyseur étudié dans cette réaction est fortement influencée par la nature des précurseurs métalliques, la composition et la température de traitement thermique utilisées pour la préparation du catalyseur. Enfin, des catalyseurs de Cu-Ni supportés sur silice colloïdale ou sur des particules de carbone, ayant une composition et une taille variable, ont été synthétisés par un simple procédé d’imprégnation. Les catalyseurs supportés sur carbone sont stables et très actifs à la fois dans l’hydrolyse du borazane et la décomposition de l’hydrazine aqueuse pour la production d’hydrogène. Il a été démontré qu’un catalyseur optimal peut être obtenu par le contrôle de l’effet bi-métallique, l’interaction métal-support, et la taille des particules de métal.

Developing an index of dietary estimation of fermentable carbohydrate to allow dietary analysis for epidemiological studies

It has been proposed that long-term consumption of diets rich in non-digestible carbohydrates (NDCs), such as cereals, fruit and vegetables might protect against several chronic diseases, however, it has been difficult to fully establish their impact on health in epidemiology studies. The wide range properties of the different NDCs may dilution their impact when they are combined in one category for statistical comparisons in correlations or multivariate analysis. Several mechanisms have been suggested to explain the protective effects of NDCs, including increased stool bulk, dilution of carcinogens in the colonic lumen, reduced transit time, lowering pH, and bacterial fermentation to short chain fatty acids (SCFA) in the colon. However, it is very difficult to measure SCFA in humans in vivo with any accuracy, so epidemiological studies on the impact of SCFA are not feasible. Most studies use dietary fibre (DF) or Non-Starch Polysaccharides (NSP) intake to estimate the levels, but not all fibres or NSP are equally fermentable. It has been proposed that long-term consumption of diets rich in non-digestible carbohydrates (NDCs), such as cereals, fruit and vegetables might protect against several chronic diseases, however, it has been difficult to fully establish their impact on health in epidemiology studies. The wide range properties of the different NDCs may dilution their impact when they are combined in one category for statistical comparisons in correlations or multivariate analysis. Several mechanisms have been suggested to explain the protective effects of NDCs, including increased stool bulk, dilution of carcinogens in the colonic lumen, reduced transit time, lowering pH, and bacterial fermentation to short chain fatty acids (SCFA) in the colon. However, it is very difficult to measure SCFA in humans in vivo with any accuracy, so epidemiological studies on the impact of SCFA are not feasible. Most studies use dietary fibre (DF) or Non-Starch Polysaccharides (NSP) intake to estimate the levels, but not all fibres or NSP are equally fermentable. The first aim of this thesis was the development of the equations used to estimate the amount of FC that reaches the human colon and is fermented fully to SCFA by the colonic bacteria. Therefore, several studies were examined for evidence to determine the different percentages of each type of NDCs that should be included in the final model, based on how much NDCs entered the colon intact and also to what extent they were fermented to SCFA in vivo. Our model equations are FC-DF or NSP$ 1: 100 % Soluble + 10 % insoluble + 100 % NDOs¥ + 5 % TS** FC-DF or NSP 2: 100 % Soluble + 50 % insoluble + 100 % NDOs + 5 % TS FC-DF* or NSP 3: 100 % Soluble + 10 % insoluble + 100 % NDOs + 10 % TS FC-DF or NSP 4: 100 % Soluble + 50 % insoluble + 100 % NDOs + 10 % TS *DF: Dietary fibre; **TS: Total starch; $NSP: non-starch polysaccharide; ¥NDOs: non-digestible oligosaccharide The second study of this thesis aimed to examine all four predicted FC-DF and FC-NSP equations developed, to estimate FC from dietary records against urinary colonic NDCs fermentation biomarkers. The main finding of a cross-sectional comparison of habitual diet with urinary excretion of SCFA products, showed weak but significant correlation between the 24 h urinary excretion of SCFA and acetate with the estimated FC-DF 4 and FC-NSP 4 when considering all of the study participants (n = 122). Similar correlations were observed with the data for valid participants (n = 78). It was also observed that FC-DF and FC-NSP had positive correlations with 24 h urinary acetate and SCFA compared with DF and NSP alone. Hence, it could be hypothesised that using the developed index to estimate FC in the diet form dietary records, might predict SCFA production in the colon in vivo in humans. The next study in this thesis aimed to validate the FC equations developed using in vitro models of small intestinal digestion and human colon fermentation. The main findings in these in vitro studies were that there were several strong agreements between the amounts of SCFA produced after actual in vitro fermentation of single fibre and different mixtures of NDCs, and those predicted by the estimated FC from our developed equation FC-DF 4. These results which demonstrated a strong relationship between SCFA production in vitro from a range of fermentations of single fibres and mixtures of NDCs and that from the predicted FC equation, support the use of the FC equation for estimation of FC from dietary records. Therefore, we can conclude that the newly developed predicted equations have been deemed a valid and practical tool to assess SCFA productions for in vitro fermentation.

Active oxygen by Ce–Pr mixed oxide nanoparticles outperform diesel soot combustion Pt catalysts

A Ce0.5Pr0.5O2 mixed oxide has been prepared with the highest surface area and smallest particle size ever reported (125 m2/g and 7 nm, respectively), also being the most active diesel soot combustion catalyst ever tested under realistic conditions if catalysts forming highly volatile species are ruled out. This Ce–Pr mixed oxide is even more active than a reference platinum-based commercial catalyst. This study provides an example of the efficient participation of oxygen species released by a ceria catalyst in a heterogeneous catalysis reaction where both the catalyst and one of the reactants (soot) are solids. It has been concluded that both the ceria-based catalyst composition (nature and amount of dopant) and the particle size play key roles in the combustion of soot through the active oxygen-based mechanism. The composition determines the production of active oxygen and the particle size the transfer of such active oxygen species from catalyst to soot.

Mesoporous mixed Nb2O5-ZrO2 catalysts for dehydration of glucose into 5-hydroxymethylfurfural

Biomass is the world’s most important renewable carbon source, whose major component, carbohydrates, can be valorized by transformation into biofuels and high value-added chemicals. Among the latter, 5-hydroxymethylfurfural (HMF), obtained by C6 carbohydrates dehydration, is a versatile and key intermediate for the production of a large spectrum of biobased chemicals. Different catalytic systems have been evaluated for HMF production, mostly based on heterogeneous catalysis as alternative to the use of conventional mineral acids [1]. Moreover, niobium oxide has shown interesting properties as acid catalyst for dehydration of sugars [2-3]. On the other hand, the high surface area and large pore size of mesoporous solids make them suitable for many catalytic processes. In the present work, the dehydration of glucose to HMF has been evaluated by using different mesoporous mixed Nb2O5-ZrO2 in a biphasic water–Methyl Isobutyl Ketone (MIBK) solvent system to avoid the HMF degradation. Different experimental parameters, such as reaction temperature and time, as well as the addition of CaCl2 have been studied in order to maximize the HMF yield.N2 adsorption-desorption isotherms have corroborated the mesostructured character of catalysts, being all isotherms of Type IV according to the IUPAC classification. BET surface area decreases for catalysts with higher Zr content (Table 1). Likewise, pore volume and average pore diameter values diminish after Zr incorporation. Concerning the acid properties, a clear correlation between Nb and acidity can be observed, in such a way that total acidity, as deduced from NH3-TPD, decreases when the Zr content rises, and consequently the amount of Nb is reduced.These mesoporous Nb-Zr catalysts have been tested in the dehydration of glucose to HMF at 175 ºC under batch operation in aqueous solution, using MIBK as co-solvent. It can be observed that both glucose conversion and HMF yield increase with the Nb content, being maximum (90% and 36%, respectively) after 90 minutes for Nb2O5. This trend changes when CaCl2 is added to the reaction medium, improving the catalytic performance of mixed oxides and ZrO2, but Nb2O5 maintains similar results than without salt addition. This could be justified by the interaction between CaCl2 and Lewis acid sites, since zirconium oxide possesses a higher amount of this acid sites type.

Study of the Electrochemical, Physical and Morphological Characteristics of Anode Catalysts for Perfluorosulfonic Acid (PFSA) Membrane Water Electrolysers

A number of supported and un-supported Oxygen Evolution Reaction (OER) iridium based electrocatalysts for Polymer Electrolyte Membrane Water Electrolysis (PEMWE) were synthesized using a polyol method. The electrocatalysts and the supports were characterized using a wide range of physical and electrochemical characterization methods. The effect of morphological characteristics of the OER electrocatalyst and the support on the OER activity was studied. The results of this thesis contribute to the existing research to reduce the cost of PEMWE by enhancing the utilization of precious metal for OER electrocatalysis. Iridium electrocatalysts supported on antimony tin oxide (Ir/ATO) were synthesized using the polyol method with two different heating techniques: conventional and microwave-irradiation. It was shown that the physical morphology and electrochemical properties of Ir/ATO synthesized with the two heating methods were comparable. However, the microwave irradiation method was extremely faster than the conventional heating method. Additionally, the effect of heat treatment (calcination temperature) on the morphology and OER activity of Ir/ATO synthesized electrocatalyst with the conventional polyol method. It was found that the iridium electrocatalyst synthesized with the polyol method, consisted of 1-5 nm particles, possessed an amorphous structure, and contained iridium with an average oxidation state of less than +4. Calcining the catalyst at temperatures more than 400 ºC and less than 700ºC: 1) increased the size of the iridium particles to 30 nm, 2) changed the structure of iridium particles from amorphous to crystalline, 3) increased the iridium oxidation state to +4 (IrO2), 4) reduced the electrochemically active surface area by approximately 50%, and 5) reduced the OER activity by approximately 25%; however, it had no significant effect on the physical and chemical morphology of the ATO support. Moreover, potential support metal carbides and oxides including: Tantalum Carbide (TaC), Niobium Oxide (Nb2O5), Niobium Carbide (NbC), Titanium Carbide (TiC), Tungsten Carbide (WC) and Antimony-doped Tin Oxide (ATO, Sb2O5-SnO2), were characterized, and used as support for the iridium OER electrocatalysts. TaC was found to be a promising support, and increasing its surface area by 4% improved the OER performance of the final supported catalyst by approximately 50%.