Les polyoxométallates (POMs) sont aujourd'hui reconnus pour leurs diverses architectures et applications. Nous nous en sommes ici servis afin de synthétiser des nanoparticules de palladium puisque le POM va jouer à la fois le rôle de réducteur du cation métallique mais aussi de surfactant des nanoparticules.Après avoir fait, dans un premier temps, l'étude électrochimique d'une série de POMs issus de la même famille, deux d'entre-eux ont été utilisés pour la synthèse de nanoparticules de palladium. D'une taille moyenne comprise entre 15 et 20 nm, ces nanoparticules ont été entièrement caractérisées et se sont avérées stables un intervalle de temps d'au moins un mois.Enfin, divers matériaux hybrides à base de palladium et/ou de cuivre ont été caractérisés par électrochimie à l'état solide et leur pouvoir catalytique vis-à-vis de la réduction des ions nitrate et de l'oxygène a été évalué. / Polyoxometalates (POMs) are known for their high diversification in terms of architectures and applications. POMs are used in this work for the synthesis of palladium nanoparticles since they act both as a reducer of metallic cation and as surfactant of nanoparticles.At first, we studied the electrochemical properties of several POMs belonging in the same family, then among this family, we chose to use two particular POMs to synthesize palladium nanoparticles. From an average size between 15 and 20 nm, these nanoparticles have been fully characterized and are stable over a month.Finally, various hybrid materials based on palladium and/or copper have been characterized by electrochemistry in solid state and their catalytic capacity towards the reduction of nitrate ions and dioxygene has been assessed.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20708号 / 工博第4405号 / 新制||工||1684(附属図書館) / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 江口 浩一, 教授 安部 武志, 教授 阿部 竜 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

Nanostructured tin oxide materials with a high specific surface area and porosity are promising for applications such as electrocatalysis, lithium ion batteries or sensors. Here, we present a facile strategy for the synthesis of tin oxide aerogels using inexpensive hexahydroxostannate as tin precursor. This easy and scalable method yields tin oxide aerogels with a high specific surface area and wide pore size distribution. The method can be modified by adding hexahydroxoantimonate to obtain antimony doped tin oxide aerogels that show an electrical conductivity after annealing. A cogelation with other preformed nanoparticles (e.g. Au, Pt) leads to mixed gels. Both modifications do not have a large impact on the porous properties of the obtained aerogels. Tin oxide materials prepared via this route can be tailored to a specific application by versatile modification possibilities.

This thesis documents efforts to synthesize Metal-Organic Frameworks (MOFs) and study their charge transport, electrocatalytic, and photoredox catalytic properties. Chapter 1 introduces concepts of pre-synthetic and post-synthetic metalation of MOFs. A series of four chemically identical but structurally different hydrolytically robust ZrIV-MOFs constructed from tetrakis(4-carboxyphenyl) porphyrinato iron (III) are examined to understand the influence of topological construction on redox hopping conductivity. The structural variation fixes center-to-center distances in the four MOFs and defines the hopping rate. The spin-state variation of the central metal in the porphyrin unit helps in further tuning the TCPP(FeIII/II) reorganization energy of the self-exchange process. The hopping rate significantly increased upon axial coordination of 1-methyl imidazole to the iron center, which converts a weakly halide bound five-coordinated high-spin (HS) TCPP(FeIII/II) to the six-coordinated low-spin (LS) complex. The population of LS vs HS species is shown to be a function of topology in the presence of an excess ligand. Chapter 2 investigates this idea further by using MOFs for electrocatalytic oxygen reduction reaction (ORR). Two cobalt-centered porphyrin-based MOFs are synthesized and deposited on various substrates to afford working electrodes that can be used in an electrochemical cell to catalyze the ORR. Chapter 3 investigates the linker-dependent photoredox catalytic activity of MOFs that possess the same topology. This is the first MOF-based study wherein a heavy metal like ruthenium is not employed to carry out the visible light-dependent photoredox catalysis.

As our world continues to develop and contend with the impacts of climate change, the scale up renewable energy technologies has never been more urgent. Alternative fuels derived from biomass-derived oxygenates and water splitting offer promising solutions for the transition towards sustainable chemical feedstocks and integration of clean renewable energy sources. However, this technology continues to be hampered by the need for scarce and costly precious metal catalysts. The work done in this thesis explored the facet-dependence of glycerol electrooxidation and studied the application of earth-abundant transition metal carbides (TMCs) and nitrides (TMNs) for reducing precious metal catalyst loadings in water electrolysis and electrooxidation of methanol and glycerol. Glycerol valorization has drawn significant interest in recent years due to the growth in biodiesel production leading to the market saturation of glycerol. While this molecule can be converted into a variety of value-added products, the possibilities have been limited by poor selectivity for C-C bond scission. The breaking of the C-C bonds in glycerol allows for complete extraction of energy from the molecule via complete glycerol oxidation, thereby opening the door for utilizing glycerol as an electrochemical fuel. While platinum (Pt) has been among the most popular catalysts, its tendency for poisoning due to adsorbed CO has hindered its activity. Previously demonstrated to enhance the catalytic activity of platinum (Pt) by reducing CO binding energy and increasing C-C bond scission selectivity in ethanol electrooxidation, TMCs were employed as catalyst supports for the glycerol electrooxidation reaction. This work used electrochemical techniques and in-situ IRRAS to study various loadings of Pt/TaC and Pt/WC to find enhanced C-C bond scission activity at reduced Pt loading because of the synergistic effects between Pt and TMCs. While Pt has remained the benchmark catalyst for glycerol electrooxidation due to its high C-C scission activity, gold (Au) has also found popularity with its high catalytic activity attributed to greater resistance to CO poisoning, despite its favorability for partial glycerol oxidation. Previous studies have hinted at the significance of Au surface facets on glycerol oxidation activity and product selectivity, but none had used nanoparticles with controlled surface facets. This thesis sought to bridge the knowledge gap using precisely-synthesized Au nanocrystals with well-characterized {100}, {110}, and {111} surface facets to provide insight into glycerol electrooxidation on Au. Electrochemical techniques were used in parallel with in-situ IRRAS analysis to uncover the differences in product selectivity and oxidation activity between the three Au surfaces, with Au {111} exhibiting the greatest activity for C-C bond scission, while Au {110} showed the lowest onset potential due to facile AuOH- formation. Hydrogen (H₂) fulfills a critical role in modern society, not only as a renewable fuel, but also as a key chemical feedstock. Production of H₂ from water electrolysis creates opportunities for storing excess energy from renewable sources as an energy-dense fuel and reducing the environmental footprint of chemical processes requiring H₂. However, efforts have been hampered by the dependence on scarce Pt-group catalyst materials. This thesis explores the application of TMNs as an earth-abundant material for enhancing the activity of Pt in the hydrogen evolution reaction (HER). Combined with DFT calculations, the HER activity of monolayer Pt- and Au-modified TMN thin films was correlated with the ΔGH* values in a volcano-type relationship. Electrocatalytic experiments in acidic electrolyte showed that TMN-supported monolayer Pt exhibited similar HER activity to the Pt foil, correlating with intermediate hydrogen adsorption strength. TiN-supported Pt and Au powders were studied to extend the correlations from thin films. Furthermore, the electrochemical stability of TMNs was studied across a wide range of potentials and pH values to generate pseudo-Pourbaix diagrams and identify TMN candidates for HER, alcohol oxidation, ORR and OER applications. Using the pseudo-Pourbaix findings, Pt/TMN catalysts were selected for studying methanol electrooxidation activity. Methanol electrooxidation has drawn significant attention particularly due to interest in direct alcohol fuel cells. Much like the case for glycerol oxidation, while Pt has been the benchmark catalyst, it has been hindered by strong adsorption of CO. As the modification of Pt with other materials, such as ruthenium, has shown promising enhancements to methanol electrooxidation activity, the synergistic effects of Pt modification with TMNs were studied in this work. In the resulting electrochemical experiments, Pt/Mo₂N was found to exhibit negligible activity likely because of its oxidative instability. In contrast, Pt/TiN showed enhanced activity, and in-situ IRRAS experiments suggest that Pt/TiN enhanced the COads-free pathway leading to increased formic acid selectivity. This thesis demonstrated avenues for developing more optimized catalysts with reduced loadings of Pt and other precious metals for applications in alternative fuel production and utilization. The influence of Au surface facets on glycerol oxidation was examined and the synergistic effects between Pt and earth-abundant TMC and TMN materials were used to enhance the electrooxidation of biomass-derived oxygenates and H₂ production from water electrolysis. These electrochemical stability and activity trends can guide future catalyst design for other critical reactions such as oxygen evolution and challenging applications like glycerol electroreduction.

Environmental pollution and health problems created by fossil fuels have led to the development of alternative energies such as solar and wind energies, hydroelectric power, and green hydrogen. The use of biohybrid materials in the development of this type of alternative energies is recent. Biohybrid materials are a unique type of advanced materials that have a biological component that can be a biomolecule or a whole cell and an abiotic or non-biological component that can be a ceramic, a synthetic polymer, or a metal, among others. They have applications in different fields that range from construction (such as bioconcrete) to catalysis (such as artificial enzymes). There are examples in the literature in which bacteria are hybridized with reduced graphene oxide or manganese oxide to catalyze the oxygen evolution of the electrochemical water splitting reaction that produces green hydrogen. The focus of this dissertation is to synthesize efficient biohybrid catalysts following a whole cell approach using electroactive bacteria as the biological component and metallic precursors that form particles ranging from single atoms, nanoclusters, and nanoparticles as the abiotic component. The Fe molecule that is part of the heme group of C-type cytochromes in the outer membrane of Geobacter sulfurreducens acted as the reduction center that allowed the synthesis and hybridization of the metals with the bacteria. Single atom metal catalyst of Ir, Pt, Ru, Cu, and Pd were synthesized and demonstrated a bifunctional catalytic activity towards the hydrogen evolution reaction and the oxygen evolution reaction. Ni single atoms were also synthesized with excellent activity in the water splitting reactions making this biohybrid catalyst very efficient but also green, as Ni is an abundant and cheap metal. Pd nanoclusters with size-control were synthesized by controlling the metal concentration, dosing, and incubation times and were tested in the electrochemical water splitting. Overall, the findings of these studies provide new knowledge on the field of biohybrid materials by contributing with novel methodologies for the synthesis of these materials and the application in the green hydrogen production with high efficiencies.

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Anthropogenic release of the greenhouse gas carbon dioxide is believed to be a leading cause in the global rise in temperature. The main source of the carbon dioxide released is from combustion of fossil fuels. Thus, its necessary to mitigate the release of CO2, look for alternatives for fossil fuels and capture and sequester or capture and convert CO2 to other useful fuels and chemicals hence creating carbon neutral or carbon negative energy cycles. This thesis work was primarily focused on design, adapt and understand the chemistry of two-dimensional (2D) layered materials, particularly transition metal dichalcogenide (TMD) molybdenum disulfide and transition metal carbides (MXenes) as catalytic materials for the conversion of renewable energy into fuels and chemicals as an alternative for fossil fuels. This investigation was accomplished by combining electrochemistry, state of the art characterization and density functional theory (DFT) calculations. We hypothesized that it would be possible to improve the electrocatalytic hydrogen evolution reaction (HER) on MoS2 by engineering catalytically active sites on the plane, their edges and their interlayer regions. We also hypothesized 2D MXene sheets would serve as good carbon dioxide reduction reaction (CO2RR) catalysts under aprotic conditions. Conceivably the broad impact of this thesis work utilizing experimental and theoretical studies is the realization of transition metal doped metallic MoS2 as a potential candidate towards HER in alkaline conditions. Initially the interlayer region of MoS2 were investigated for the HER by introducing Na+, Ca2+, Ni2+ and Co2+ cations in the interlayers of metallic phase MoS2. Experimental results show that intercalation of cations (Na+, Ca2+, Ni2+, and Co2+) into the interlayer region of 1T-MoS2 to lower the overpotential for the HER. In acidic media the overpotential to reach 10 mAcm-2 for 1T-MoS2 with intercalated ions is lowered by ~60 mV relative to pristine 1T-MoS2 (~230 mV). DFT calculations suggest that the introduction of states from the intercalated metals whether sp or d, to lower the Gibbs free energy for H-adsorption (ΔGH) relative to intercalant-free 1T-MoS2. The DFT calculations suggest that Na+ intercalation results in ΔGH closest to zero, which is consistent with our experiments where the lowest overpotential for the HER is observed with Na+ intercalation. In order to explore the activity of the edge sites of MoS2 and the effect of a conductive support we used a microwave-assisted growth technique to synthesize interlayer expanded MoS2 with a vertically orientation on conductive two-dimensional Ti3C2 MXene nanosheets (MoS2⊥Ti3C2). Judicious choice of reaction temperature allows a control over the density of the edges obtained. Compared to pure MoS2 this unique inorganic hybrid structure allows an increased exposure of catalytically active edge sites of MoS2. The produced materials were investigated as electrocatalysts for the hydrogen evolution reaction (HER) in acidic conditions. The MoS2⊥Ti3C2 catalyst synthesized at 240 0C exhibited a low onset potential (-95 mV vs RHE) for the HER and a low Tafel slope (~40 mV dec-1). The decrease in the overpotential is linked to decrease in the charge transfer resistance of the materials with the electrode and the increased edge site density. In a third study the basal plane of metallic MoS2 was engineered by doping with transition metals Co and Ni to be evaluated as a catalyst for the alkaline HER. Due to a lack of oxygen evolution catalysts that can oxidize water at the anode under acidic conditions, there is an urgency to realize HER catalysts that can efficiently reduce water to hydrogen gas under alkaline conditions. Though metallic MoS2 has an optimum H binding free energy for the HER, the sluggish water dissociation step under alkaline conditions has made the implementation of MoS2 as a catalyst at higher pHs harder. We hypothesized that doping transition metals in the basal plane of metallic MoS2 that can efficiently catalyze the water dissociation step in alkaline conditions would help to reduce the overpotential required for the HER under alkaline conditions. Ni and Co were doped in orthorhombic MoO3 which was then converted metallic MoS2 under hydrothermal conditions. The polarization plots obtained in 1.0 M KOH solution shows a low onset overpotential of -75 mV vs RHE for the 10% Ni doped metallic MoS2 with an overpotential of -145 mV to reach a current density of 10 mA/cm2. Pure metallic MoS2 reaches the same current density at an overpotential of -238 mV vs RHE while samples doped with 10% Co atoms reached 10 mA/cm2 at -165 mV. This improvement in the doped samples is attributed to the improved kinetics of the water dissociation step under the alkaline reaction conditions. DFT calculations suggests that an optimal binding of water for the water dissociation step, H binding free and low free energy of binding for OH intermediates. Rigorous cycling of the catalysts shows extremely high stability with the doped samples while the pure metallic MoS2 loses its activity with continuous cycling. DFT calculations show that the doped samples provide extra stability to the metastable metallic MoS2 thus improving their long-term stability. Photo/electrochemical conversion of CO2 is an important step in the path to renewable production of carbon-based fuels and chemicals. Activity and selectivity have been major concerns on the CO2RR catalysts. The activity of known materials are hindered by the scaling relationship in the binding energies of the many intermediates involved in the CO2RR. Thus, the simplest of CO2RR products CO and HCOOH are of great value. Nano structured precious metals like silver and gold have shown promise as cathode materials for the conversion of CO2 to CO. In this thesis work we evaluate the electrocatalytic properties of Mo2C and Ti3C2 MXenes towards the electrochemical CO2 reduction reaction (CO2RR) as cheaper alternatives for precious metals. Though there have been theoretical predictions of the ability of MXenes with certain composition to have the ability to reduce CO2 to hydrocarbons, there are no experimental findings to support these calculations. In this study we observe very high faradaic efficiencies, ~90% for the CO2 reduction to CO at low overpotentials ~250 mV in acetonitrile/ionic liquid electrolytes on Mo2C MXene while Ti3C2 shows ~65% FE at an overpotential of ~600 mV for the cathodic half reaction. Density functional theory calculations suggests that the enhanced activity of Mo2C relative to Ti3C2 is due to relative lowering of the energy barrier for the initial proton couple electron transfer step of CO2 and the spontaneous dissociation of the absorbed *COOH species to *CO and H2O on the Mo2C surface. The calculations also predict the most probable active sites for the CO2 conversion to be vacant oxygen sites. High selectivity and high FE of CO2 reduction to CO makes these earth abundant materials an attractive electrocatalyst for the CO2RR. / Chemistry