Estudos sistemáticos utilizando a técnica de SAXS foram realizados no síncrotom do LURE (Orsay) com feixe intenso de geometria pontua:, para descrever a cinética e as estruturas obtidas durante as etapas de transformação sol -> gel úmido -> gel seco -> vidros de sílica . As análises foram feitas em termos de uma lei de potência I = q-? cujo expoente está relacionado a dimensão fractal de massa ou de superfície das estruturas. Devido a polidispersividade das unidades espalhadoras as dimensões fractais verificadas podem ser resultados de dimensões reais encobertas por uma distribuição de tamanhos. As curvas obtidas para a cinética de gelificação e envelhecimento dos géis úmidos mostram expoentes que indicam fractalidade de massa e, posteriormente, de superfície. Uma microestrutura composta, que pode resultar de uma agregação limitada por difusão (DLA) seguido de agregação de agregados (C-C) foi observado nas soluções básicas. Por outro lado, os agregados ácidos parecem surgir do crescimento contínuo de cadeias até atingir a interligação com outros agregados com uma cinética que pode ser descrita pelo modelo DLA. No estado de aerogeis outras técnicas como picnometria de mercúrio e hélio, microscopia eletrônica de transmissão e adsorção de gás de nitrogênio, foram também aproveitadas. Nestes materiais encontramos em uma faixa estreita de escala, um expoente indicando fractalidade de massa, embora com superfície lisa. Os resultados estão de acordo com um modelo de estrutura que tem uma matriz de SiO2 densa. A estrutura fractal desaparece durante o tratamento térmico de densificação devido ao rearranjo estrutural. / Systematic SAXS studies have been performed at the LURE synchrotron (Orsay) using an intense beam of point-like cross-section to describe the kinetic and the structure obtained during the sol-> humid gel -> dry gel ->silica glass. The analysis were done in terms of a power law, I = q-? , whose exponent is related to mass and surface fractal dimensions of the structures. Due to polidispersity of the scattering units the found fractal dimensions can be \"smeared\" dimensions that result from a size distribution. All the kinetic and aging humids curves showed evidenced of fractal structures. A composed microstructure that can be originated by clustering of clusters (C-C) of primary units grown by diffusion limited-aggregation (DLA), was observed for basic gels. By other hand, the acidic cluster seems to grow continuously reaching gelation through crosslinking of the clusters. The DLA model seems to describe this process. In the aerogels study others techniques like density measurements using Hg and He, TEM and adsorption of N2 gas were done along with SAXS measurements. Here we could find a narrow range in which the system is a mass fractal, although its surface has been already smoothened. The results agree with a dense SiO2 matrix model. During the densification no fractal structure was verified due to structural rearrangements at high temperature.

Cette thèse décrit le développement de procédés utilisables pour valoriser des extraits de bois afin de préparer : (1) des combustibles (hydrocarbures) liquides ; (2) des matériaux poreux avec des applications potentielles dans les domaines de l’énergie et l’environnement, notamment isolation thermique, catalyse, piégeage et séparation de micropolluants. Les extraits de bois en question sont des lignines, associées ou non à des tannins. Les deux types de matériaux sont actuellement peu valorisés, et l’on montre qu’ils peuvent être source de valeur ajoutée au travers des procédés rapportés dans ce mémoire / The present thesis describes the development of processes which can be used for valorizing wood extracts in the aim of preparing: (1) liquid (hydrocarbon) fuels; (2) porous materials with potential energy and environmental applications, namely thermal insulation, catalysis, abatement or separation of micropollutants. The wood extracts in question are lignins, associated or not with tannins. Both kinds of materials are presently poorly valorized, and it is shown here that they can lead to high added-value products through the processes reported in this PhD dissertation

Multi-metallic aerogels have recently emerged as a novel and promising class of unsupported electrocatalyst materials due to their high catalytic activity and improved durability for various electrochemical reactions. Aerogels can be prepared by a spontaneous one-step gelation process, where the chemical co-reduction of metal precursors and the prompt formation of nanochain-containing hydrogels, as a preliminary stage for the preparation of aerogels, take place. However, detailed knowledge about the homogeneity and chemical distribution of these three-dimensional Pd–Pt aerogels at the nano-scale as well as at the macro-scale is still unclear. Therefore, we used a combination of spectroscopic and microscopic techniques to obtain a better insight into the structure and elemental distribution of the various Pd-rich Pd–Pt aerogels prepared by the spontaneous one-step gelation process. Synchrotron-based extended X-ray absorption fine structure (EXAFS) spectroscopy and high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) in combination with energy-dispersive X-ray spectroscopy (EDX) were employed in this work to uncover the structural architecture and chemical composition of the various Pd-rich Pd–Pt aerogels over a broad length range. The Pd80Pt20, Pd60Pt40 and Pd50Pt50 aerogels showed heterogeneity in the chemical distribution of the Pt and Pd atoms inside the macroscopic nanochain-network. The features of mono-metallic clusters were not detected by EXAFS or STEM-EDX, indicating alloyed nanoparticles. However, the local chemical composition of the Pd–Pt alloys strongly varied along the nanochains and thus within a single aerogel. To determine the electrochemically active surface area (ECSA) of the Pd–Pt aerogels for application in electrocatalysis, we used the electrochemical CO stripping method. Due to their high porosity and extended network structure, the resulting values of the ECSA for the Pd–Pt aerogels were higher than that for a commercially available unsupported Pt black catalyst. We show that the Pd–Pt aerogels possess a high utilization of catalytically active centers for electrocatalytic applications based on the nanostructured bimetallic framework. Knowledge about the homogeneity and chemical distribution of the bimetallic aerogels can help to further optimize their preparation by the spontaneous one-step gelation process and to tune their electrocatalytic reactivity.

No description available.

A new class of multimetallic hierarchical aerogels composed entirely of interconnected Ni‐PdxPty nano‐building‐blocks with in situ engineered morphologies and compositions is demonstrated. The underlying mechanism of the galvanic shape‐engineering is elucidated in terms of nanowelding of intermediate nanoparticles. The hierarchical aerogels integrate two levels of porous structures, leading to improved electrocatalysis performance.

Multi-metallic aerogels have recently emerged as a novel and promising class of unsupported electrocatalyst materials due to their high catalytic activity and improved durability for various electrochemical reactions. Aerogels can be prepared by a spontaneous one-step gelation process, where the chemical co-reduction of metal precursors and the prompt formation of the nanochain-consisting hydrogels, as a preliminary stage for the preparation of aerogels take place. However, detailed knowledge about the homogeneity and chemical distribution of these three-dimensional Pd-Pt aerogels at the nano-scale as well as at the macro-scale is still unclear to date. Therefore, we used a combination of spectroscopic and microscopic techniques to obtain a better insight into the structure and elemental distribution of the various Pd-rich Pd-Pt aerogels prepared by the spontaneous one-step gelation process. Synchrotron-based extended X-ray absorption fine structure (EXAFS) spectroscopy and high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) in combination with energy-dispersive X-ray spectroscopy (EDX) were employed in this work to uncover the structural architecture and chemical composition of the various Pd-rich Pd-Pt aerogels over a broad length range. The Pd80Pt20, Pd60Pt40 and Pd50Pt50 aerogels showed heterogeneity in the chemical distribution of the Pt and Pd atoms inside the macroscopic nanochain-network. Features of monometallic clusters were not detected by EXAFS or STEM-EDX, indicating alloyed nanoparticles. However, the local chemical composition of the Pd-Pt alloys strongly varied along the nanochains and thus within a single aerogel. To determine the electrochemically active surface area (ECSA) of the Pd-Pt aerogels for applications in electrocatalysis, we used the electrochemical CO stripping method. Due to high porosity and extended network structure, the resulting values of the ECSA for the Pd-Pt aerogels were higher than that for a commercially available unsupported Pt black catalyst. We show that the Pd-Pt aerogels possess a high utilization of catalytically active centers for electrocatalytic applications based on the nanostructured bimetallic framework. Knowledge about the homogeneity and chemical distribution of the bimetallic aerogels can help to further optimize their preparation by the spontaneous one-step gelation process and to tune their electrocatalytic reactivity.

Multi-metallic aerogels have recently emerged as a novel and promising class of unsupported electrocatalyst materials due to their high catalytic activity and improved durability for various electrochemical reactions. Aerogels can be prepared by a spontaneous one-step gelation process, where the chemical co-reduction of metal precursors and the prompt formation of the nanochain-consisting hydrogels, as a preliminary stage for the preparation of aerogels take place. However, detailed knowledge about the homogeneity and chemical distribution of these three-dimensional Pd-Pt aerogels at the nano-scale as well as at the macro-scale is still unclear to date. Therefore, we used a combination of spectroscopic and microscopic techniques to obtain a better insight into the structure and elemental distribution of the various Pd-rich Pd-Pt aerogels prepared by the spontaneous one-step gelation process. Synchrotron-based extended X-ray absorption fine structure (EXAFS) spectroscopy and high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) in combination with energy-dispersive X-ray spectroscopy (EDX) were employed in this work to uncover the structural architecture and chemical composition of the various Pd-rich Pd-Pt aerogels over a broad length range. The Pd80Pt20, Pd60Pt40 and Pd50Pt50 aerogels showed heterogeneity in the chemical distribution of the Pt and Pd atoms inside the macroscopic nanochain-network. Features of monometallic clusters were not detected by EXAFS or STEM-EDX, indicating alloyed nanoparticles. However, the local chemical composition of the Pd-Pt alloys strongly varied along the nanochains and thus within a single aerogel. To determine the electrochemically active surface area (ECSA) of the Pd-Pt aerogels for applications in electrocatalysis, we used the electrochemical CO stripping method. Due to high porosity and extended network structure, the resulting values of the ECSA for the Pd-Pt aerogels were higher than that for a commercially available unsupported Pt black catalyst. We show that the Pd-Pt aerogels possess a high utilization of catalytically active centers for electrocatalytic applications based on the nanostructured bimetallic framework. Knowledge about the homogeneity and chemical distribution of the bimetallic aerogels can help to further optimize their preparation by the spontaneous one-step gelation process and to tune their electrocatalytic reactivity.