The development of nanoscale heterostructure photocatalysts for the effective, direct utilization of visible light (400-750 nm, ~44% of solar spectrum) to drive important chemical conversions is a prime research area in the field of photocatalysis. Particles at nanoscale dimensions have a large surface area-to-volume ratio, expose a high number of active surface sites, and exhibit unique electronic properties (different from bulk) that are beneficial for improving the overall catalytic activity. However, the advantages of size reduction are often overshadowed by the low optical absorption (as absorption power  size3) and colloidal instability (extensive aggregation) of particles at the nanoscale. In this dissertation, we demonstrate a strategy to improve the colloidal stability and enhance the optical absorption of nano-sized semiconductor and metal nanoparticles (NPs) that exhibit weak visible light absorption. The colloidal, free-standing NPs are placed on transparent, dielectric silica nanospheres (SiOx NSs) that act as optical antenna supports, forming SiOx/NP composite nanostructures. The spherical morphology of SiOx enables scattering resonances (Fabry Perot or Whispering Gallery Modes) which enhances the local electric field on or near the surface of the NS. The NPs placed on the surface of SiOx NS interact with the locally enhanced electric field and exhibit improved optical absorption. By varying the size of the SiOx NS, the resonance wavelengths and the intensity of the local electric field enhancement can be tuned, offering the ability of such structures to effectively utilize a wide range of energies in the visible region. Composite nanostructures comprised of various classes of nanomaterials such as metal-doped semiconductor, plasmonic, and non-plasmonic metal NPs were investigated to perform the desirable solar-to-chemical transformations. First, we employed SiOx-loaded silver-doped silver chloride (SiOx/AgCl(Ag)) photocatalyst to investigate the role of metal-induced gap states in AgCl, a wide bandgap semiconductor. SiOx/AgCl(Ag) exhibit high catalytic performance and photostability after 10 cycles of the probe reaction, methylene blue (MB) degradation under visible light irradiation. The results indicate that the visible light absorption due to metal-induced gap states can be further improved by employing the SiOx NSs as supports that act as optical nanoantenna. We then studied the influence of NP size on the catalytic activity to understand the effect of size in promoting the generation and transfer of hot electrons to surface adsorbates. Our findings indicate that upon employing Ag NPs of different particle size (<10 nm and >10 nm) and normalizing for the optical absorption and moles of surface Ag atoms, the efficient generation and transfer of photoexcited hot electrons is favored in the small-sized Ag NPs (size <10 nm) than bigger Ag NPs. Next, we investigated the selective partial hydrogenation of nitroarene to N-aryl hydroxylamine using SiOx-loaded platinum (SiOx/Pt) photocatalysts. We found that change in the surface electronic structure of the small Pt NPs (size <5 nm) due to light illumination and surface modification (by adding suitable organic ligands), minimize the adsorption of the electron-rich hydroxylamine molecules and minimize their complete conversion to aniline, resulting in high N-hydroxylamine selectivity. Overall, our work shows that well-controlled composite nanostructures comprising of active catalyst loaded on dielectric SiOx NS supports that act as optical nanoantenna are a promising class of photocatalysts for driving photon-to-chemical transformations with high activity and product selectivity. / Chemistry

Le paracétamol est un analgésique parmi les plus consommés dans le monde. Les synthèses actuelles de cette molécule induisent la formation de quantités non-négligeables de sels ou de produits secondaires non valorisables. En plus d'induire de faibles économies d'atomes, la présence de ces déchets engendre des surcoûts importants pour la synthèse du paracétamol dus aux lourds traitements des réactions. Les objectifs de la thèse étaient à la fois de proposer une synthèse plus respectueuse de l'environnement mais également économiquement viable. En ce sens, deux synthèses du paracétamol ont été étudiées. La première synthèse étudiée concerne la réduction sélective du nitrobenzène en p-aminophénol, l'intermédiaire clé du paracétamol. Cette synthèse nécessite typiquement une quantité importante d'acide sulfurique qui est corrosif et engendre la formation de sels (sulfate d'ammonium) importante. Un catalyseur acide recyclable à base d'oxyde de niobium a été utilisé et associé à l'acide sulfurique. Ainsi les sélectivités en aminophénol de 74 % sans catalyseur de niobium ont été améliorées à 82 % en présence de ce catalyseur. En outre, la quantité d'acide sulfurique a été réduite au minimum sans pertes significatives de sélectivité. La deuxième synthèse est la substitution de l'hydroquinone par l'acétate d'ammonium en milieu acide acétique. Cette synthèse innovante s'est révélée être particulièrement performante car elle induit la formation du paracétamol en une étape en partant d'un produit disponible en grande quantité, avec de très bons rendements et sélectivités. De plus, un test à large échelle a permis de montrer que le paracétamol produit est facilement récupérable par précipitation et l'acide acétique récupérable par distillation. Enfin, la réaction a été testée avec succès à d'autres polyhydroxybenzènes et aux naphtols / Paracetamol is an analgesic among the most consumed in the world. Currents syntheses of paracetamol induce a quantity of salts and non-reusable by-products. These wastes lead to both a low atom economy and a high process cost due to the work-ups. The main objectives of this thesis were to propose eco-friendly and competitive synthesis of paracetamol. Two syntheses have been studied. The first one was the selective reduction of nitrobenzene to p-aminophenol, the key intermediate of paracetamol. This synthesis requires a large amount of sulphuric acid which is corrosive and induces salts formation. A reusable niobium oxide-based catalyst has been associated with sulphuric acid. This association gave better selectivities to aminophenol (82%) compare to sulphuric acid alone (74%). Moreover, the quantity of sulphuric acid has been minimized without significant loss of selectivities. The second synthesis study was the hydroquinone substitution to paracetamol with ammonium acetate in acid acetic. This new synthesis is very powerful due to the one-step synthesis of paracetamol from bulk quantity available products, with very good conversion and selectivity. Moreover, a large scale synthesis has been tested which demonstrates that paracetamol and acetic acid were easily recovered by precipitation and distillation, respectively. The reaction has been successfully extended to other polyhydroxybenzenes and naphtols