Controlling the {111}/{110} Surface Ratio of Cuboidal Ceria Nanoparticles

The ability to control size and morphology is crucial in optimizing nanoceria catalytic activity as this is governed by the atomistic arrangement of species and structural features at the surfaces. Here, we show that cuboidal cerium oxide nanoparticles can be obtained via microwave-assisted hydrothermal synthesis in highly alkaline media. HRTEM revealed that the cube edges were truncated by CeO2{110} surfaces and the cube corners by CeO2{111} surfaces. When adjusting synthesis conditions by increasing NaOH concentration, the average particle size increased. Although this was accompanied by an increase of the cube faces, CeO2{100}, the cube edges, CeO2{110}, and cube corners, CeO2{111} remained of constant size. Molecular Dynamics (MD) was used to rationalise this behaviour and revealed that energetically, the corners and edges cannot be atomically sharp, rather they are truncated by {111} and {110} surfaces respectively to stabilise the nanocube; both experiment and simulation agreed a minimum size of ~1.6 nm associated with this truncation. Moreover, HRTEM and MD revealed {111}/{110} faceting of the {110} edges, which balances the surface energy associated with the exposed surfaces, which follows {111}>{110}>{100}, although only the {110} surface facets because of the ease of extracting oxygen from its surface, which follows {111}>{100}>{110}. Finally, MD revealed that the {100} surfaces are ‘liquid-like’ with a surface oxygen mobility 5 orders of magnitude higher than that on the {111} surfaces; this arises from the flexibility of the surface species network that can access many different surface arrangements due to very small energy differences. This finding has implications for understanding the surface chemistry of nanoceria and provides avenues to rationalize the design of catalytically active materials at the nanoscale

Influence of the morphology of cerium-based oxide on the relation (micro) structure/properties

Les oxydes à base de Cérium, ont fait l’objet de nombreuses études ces dernières décennies et se sont révélés des matériaux de choix, dans le domaine de la catalyse hétérogène. L’objectif à l’heure actuelle, est donc d’accroître la réactivité de ces oxydes, tout en élargissant leur gamme de températures optimales d’utilisation. Dans ce contexte particulier, il semble possible de moduler les propriétés des oxydes à base de cérium en contrôlant la morphologie des cristallites. Ce travail de thèse a donc été consacré à la détermination, l’élaboration et à la caractérisation de matériaux oxydes à base de cérium de morphologies contrôlées. Nous avons tout d’abord déterminé cristallographiquement et thermodynamiquement les morphologies accessibles au système étudié puis par traitement hydrothermale assistée par chauffage micro-ondes nous avons synthétisé les dites morphologies. Après caractérisation de la réactivité par ATG et thermographie Infrarouge nous avons optimisé ces matériaux par un dopage extrinsèque tout d’abord (dépôt de métaux précieux), puis par un dopage intrinsèque ensuite (Yttrium et Fer). Enfin, l’obtention de morphologies non accessibles cristallographiquement nous a amené à approfondir le(s) processus de germination croissance de ces particules et la forte réactivité des matériaux dopés fer nous a poussé à une caractérisation fine de la microstructure de ces matériaux. Au final nous avons pu corréler l’influence de la morphologie des cristallites sur la réactivité propre de l’ensemble des familles de matériaux étudiés.Recent decades, numerous studies on cerium-based oxides have been realized and have revealed that cerium-based oxides were materials of choice in the field of heterogeneous catalysis. The aim now is therefore to increase the reactivity of these oxides, while expanding their range of optimal temperatures of Use. In this particular context, it seems possible to modulate the properties of cerium-based oxides by controlling the morphology of the crystallites. This thesis has been devoted to the identification, development and characterization of materials based on cerium oxides of controlled morphology. We first determined possible morphologies for the studied system, by hydrothermal synthesis by micro-wave assisted heating we have synthesized these morphologies and we characterized reactivity of these materials by infrared thermography and TGA. We have optimized these materials first, by extrinsic doping (deposition of precious metals) and then by intrinsic doping (Yttrium and Iron). New morphologies have been synthesized so we have studied the processes of nucleation-growth set in. The high reactivity of iron-doped materials has led us to a detailed characterization of the microstructure of these materials. Finally correlation reactivity/morphologies of crystallites have been achieved

Influence de la morphologie d'oxydes à base de cérium sur les relations (micro)structures/propriétés

Les oxydes à base de Cérium, ont fait l objet de nombreuses études ces dernières décennies et se sont révélés des matériaux de choix, dans le domaine de la catalyse hétérogène. L objectif à l heure actuelle, est donc d accroître la réactivité de ces oxydes, tout en élargissant leur gamme de températures optimales d utilisation. Dans ce contexte particulier, il semble possible de moduler les propriétés des oxydes à base de cérium en contrôlant la morphologie des cristallites. Ce travail de thèse a donc été consacré à la détermination, l élaboration et à la caractérisation de matériaux oxydes à base de cérium de morphologies contrôlées. Nous avons tout d abord déterminé cristallographiquement et thermodynamiquement les morphologies accessibles au système étudié puis par traitement hydrothermale assistée par chauffage micro-ondes nous avons synthétisé les dites morphologies. Après caractérisation de la réactivité par ATG et thermographie Infrarouge nous avons optimisé ces matériaux par un dopage extrinsèque tout d abord (dépôt de métaux précieux), puis par un dopage intrinsèque ensuite (Yttrium et Fer). Enfin, l obtention de morphologies non accessibles cristallographiquement nous a amené à approfondir le(s) processus de germination croissance de ces particules et la forte réactivité des matériaux dopés fer nous a poussé à une caractérisation fine de la microstructure de ces matériaux. Au final nous avons pu corréler l influence de la morphologie des cristallites sur la réactivité propre de l ensemble des familles de matériaux étudiés.Recent decades, numerous studies on cerium-based oxides have been realized and have revealed that cerium-based oxides were materials of choice in the field of heterogeneous catalysis. The aim now is therefore to increase the reactivity of these oxides, while expanding their range of optimal temperatures of Use. In this particular context, it seems possible to modulate the properties of cerium-based oxides by controlling the morphology of the crystallites. This thesis has been devoted to the identification, development and characterization of materials based on cerium oxides of controlled morphology. We first determined possible morphologies for the studied system, by hydrothermal synthesis by micro-wave assisted heating we have synthesized these morphologies and we characterized reactivity of these materials by infrared thermography and TGA. We have optimized these materials first, by extrinsic doping (deposition of precious metals) and then by intrinsic doping (Yttrium and Iron). New morphologies have been synthesized so we have studied the processes of nucleation-growth set in. The high reactivity of iron-doped materials has led us to a detailed characterization of the microstructure of these materials. Finally correlation reactivity/morphologies of crystallites have been achieved.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Local organization of Fe3+ into nano-CeO2 with controlled morphologies and its impact on reducibility properties

Ce and Fe are the main metal additives which have been tested by many laboratories for their catalytic activity at low temperature for soot oxidation in diesel engines.The key role and impact of Fe on the reducibility properties of ceria have been investigated on the basis of composition and structural features considering various synthesis routes. Two different procedures were used to prepare iron-substituted cerium dioxide Ce1-xFexO2-x/2: a classical co-precipitation route followed by annealing at T = 600 [degree]C and an unusual microwave-assisted hydrothermal synthesis at T = 200 [degree]C. The highest surface areas around 100 m2 g-1, have been obtained for oxides containing the largest Fe amount and prepared by the microwave-assisted route. Solid solutions were obtained up to x = 0.15 and 0.20 through microwave-assisted and co-precipitation routes, respectively. The highest decrease of the lattice parameter is observed for compounds prepared by the microwave-assisted route. The TEM analysis reveals a nano-cubic shape (with mainly {001} planes) for compounds prepared by the microwave-assisted route and containing low Fe amount. For the other compounds prepared also by the co-precipitation process, a nano-polyhedron shape corresponding to a thermodynamically stable morphology is observed. EPR (T = 5 K, very low Fe rates), Mossbauer (room temperature, x = 0.10) and XANES-EXAFS (RT and T = 20 K) spectroscopies studies showed that Fe ions adopt the trivalent state and are located in isolated distorted (orthorhombic and axial distortions) octahedra or form Fe clusters. The local organization of Fe3+ plays a key role in the oxygen vacancy distribution and consequently the reducibility properties of the Ce1-xFexO2-x/2 solid solution. For the compounds prepared by the microwave-assisted route, a large proportion of Fe3+ is located in isolated distorted octahedral sites that contribute to affect a high number of Ce4+ nearest neighbors with a high mobility of oxygen vacancies. In this case, the oxygen vacancies are well distributed around Ce4+ and Fe3+. In the case of compounds obtained by the co-precipitation route, Fe clusters abundance is higher and consequently oxygen vacancies are mainly located around Fe clusters. Such a distribution should explain the lower solubility limit and the larger variation of the cell parameter versus x Fe content in the series prepared by the microwave-assisted route. It is shown that these last compounds exhibit better reducibility properties with a doubled reduction rate (comparison between x(Fe) = 0.05 and pure CeO2) equal to 60% at T = 550 [degree]C due to the iron local organization, a nano-cubic morphology and a higher surface area

Three-dimensional tomographic analyses of CeO2 nanoparticles

A detailed morphological and structural analysis of CeO2 nanoparticles has been performed using electron tomography in scanning transmission mode in high angle annular dark field. The nanoparticles have been prepared through a solvothermal synthesis assisted by microwave heating. An adequate choice of the synthesis parameters leads to particles with various well-defined morphologies: cubes, octahedrons, and nanorods. In the case of cubic CeO2 nanoparticles, the three-dimensional analysis allowed us to precisely calculate the type and the proportion of the minor facets exposed at the nanoparticle surface. For the CeO2 nanoparticles with an octahedron shape, it has been demonstrated that the ambiguous interpretation of the objects giving triangular views in classical transmission electron microscopy can be prevented; furthermore, precise assignments of their external shape, surface crystallography, and type of minor facets were realized. In the case of nanorods, it was shown that the external shape and the transversal symmetry are strongly dependent on the nanorod sizes. The presence of a well-defined porosity inside the rods was also evidenced thanks to the ability of the electron tomography to solve the internal structure of a nano-object