Cette thèse est consacrée à l'étude de deux systèmes, à la fois proches et différents par leur comportement : Co-Pt, système emblématique de l'ordre chimique, et Pt-Ag, système hybride présentant à la fois un ordre chimique et une tendance à la démixtion, ainsi qu'une forte tendance à la ségrégation. Afin de répondre à ces diverses questions, nous adoptons une approche semi-empirique à travers un potentiel à $N$-corps, permettant les relaxations atomiques, dans l'approximation du second moment de la densité d'états (SMA), couplé à des simulations Monte Carlo dans différents ensembles. Des agrégats de différentes tailles (allant de 1000 à 10000 atomes) et de différentes morphologies (octaèdre tronqué, décaèdre, ou icosaèdre) sont analysés en terme de composition chimique sur les différents sites inéquivalents (sommet, arête, facettes (100) et (111) et coeur) puis comparés aux systèmes de référence (surfaces, volume) sur toute la gamme de concentration. Pour le système Co-Pt, nous observons des structures ordonnées similaires à celles du volume pour le coeur et similaires à celles des surfaces pour les facettes. L'impact de la phase bidimensionnelle (√3×√3)R30◦ propre à la surface, est d'autant plus important sur l'ordre chimique au coeur que la nanoparticule est de petite taille. Pour le système Pt-Ag, nous observons une importante ségrégation de l'Ag en surface, ainsi qu'un enrichissement de Pt en sous-surface, et la stabilisation de la phase ordonnée L1$_{1}$ au coeur. Cette structure peut apparaître en un seul variant ou bien en adoptant tous les variants possibles, conduisant ainsi à une structure en pelures d'oignon. / Due to the correlation between atomic arrangement and physical properties, ordered nanoalloys are particularly interesting in the field of catalysis, magnetism, or optics. By reducing the system size, from alloy to nanoalloy, a lot of questions arise: Is chemical ordering conserved? What is the morphology of nanoalloys? What is the properties evolution as a size function? Is there a coupling between segregation and core ordering? This thesis is dedicated to two systems: Co-Pt, a typical example of ordering and Pt-Ag, hybrid system between ordering and demixion. To answer these questions, we performed Monte Carlo simulations in different ensembles with semi-empirical many-body potential within the Second Moment Approximation (SMA) of the density of states which allows atomic relaxations. Nanoparticles of different sizes (from 1000 to 10000 atoms) and shapes (truncated octahedra, decahedra, or icosahedra) are analyzed considering chemical composition on each site (vertex, edge, (111) and (100) facets and core) and compared to reference systems (surfaces and bulk) on the whole range of composition. For Co-Pt, we get ordered structures similar to the bulk ones and similar to surfaces for facets. The bidimensional phase (√3×√3)R30◦, purely due to surface effect, impacts core ordering, even more for small clusters. For Pt-Ag, we get a strong Ag segregation on surface coupled with a Pt sub-surface enrichment, and a stable L1$_{1}$ phase in the core. This ordered structure may appear with a single variant or with multiple variants, leading to an onion-like structure.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this thesis we performed a theoretical study of the structural, electronic, and magnetic properties of transition metal (TM) particles using two models, with 13 and 55 atoms to describe clusters, nanoparticles (NPs), nanoalloys, protected NPs, and adsorption on clusters by Density Functional Theory. Firstly, we performed a systematic study for 3d, 4d, and 5d TMs of the Periodic Table using clusters with 13 atoms. This study gives the trends of the properties as function of the d occupation. We implemented a strategy to obtain the clusters structures, which is based on high-temperature molecular dynamic calculations and simulated annealing. New lower energy configurations were identified for some 13 atom clusters and previous known structures were confirmed. The following conclusions were identified: (i) The analysis of the binding energies and average bond lengths show a parabolic-like shape as a function of the occupation of the d states and hence, most of the properties can be explained by the chemistry picture of occupation of the bonding and antibonding states. (ii) Ground state structures are seen to depend on the d band occupation, with compact icosahedral-like (ICO) forms at the beginning of each metal series, more opened structures such as hexagonal bilayer-like (HBL) and double simple-cubic (DSC) layers at the middle of each metal series, and structures with an increasing effective coordination number occur for large d states occupation. (iii) For Au13, we found that spin-orbit coupling favors 3D structures, i.e., a 3D structure is about 0.10 eV lower in energy than the previously assumed lowest energy 2D configuration. (iv) The magnetic exchange interactions play an important role for particular systems such as Fe, Cr, and Mn. Several trends are similar for clusters and bulk, however, the atomic structures for Ru13, Rh13, Os13, and Ir13 are considered unexpected, since the respective elemental crystals crystallize in compact structures. In this context, we employed different local, semilocal, and non-local exchange and correlation energy functional, to understand the performance of different exchange and correlation schemes in the prediction of the physical and chemical properties of TM clusters. The local and semilocal functionals confirm the DSC configuration as the lowest energy structure for the studied TM13 clusters. A good agreement in the relative total energies is obtained even for structures with small energy differences, i.e., the PBE (Perdew, Burke, and Ernzerhof) results are confirmed. With the study employing PBE+U and hybrid functionals we found that a partial correction of the self-interaction problem decreases the relative stability of opened structures such as the DSC, and hence, compact structures became the lowest energy ones. The sd hybridization helps to explain the dependence of the structural stabilities with the self-interaction correction. We found that, for Co13 and Rh13, the sd hybridization decreases for DSC and increases for ICO. The study of NO adsorption on TM13 clusters, such as: Rh13, Pd13, Ir13 and Pt13, and the comparison with the results obtained for the respective TM(111) surfaces, allowed the finding that the adsorption on clusters changes significantly, with a strong dependence of the chemical environment close to the adsorption sites, whereas the trend obtained for the characteristic geometric parameters are similar to those observed for NO/TM (111). For the TM55 we get that Co55 and Rh55 NPs have ICO lowest energy structures, contrarily to the respective 13 atoms clusters. For Pt55 and Au55 NPs we found a non-icosahedral structure, with lower symmetry and the reduced core size, 7 - 9 atoms, which is very important for catalysis due to the larger number of atoms at the surface. After the TM55 study, we performed the study for PtnTM55-n (TM = Co, Rh, Au) nanoalloys as a function of the composition (n). It is confirmed that PtTM NPs prefer a composition pattern where the Co and Rh (Pt) atoms are in the core region and Pt (Au) atoms are at the surface region. Furthermore, we get that PtnRh55-n and, especially, PtnCo55-n tend to form alloys, mainly between n = 28 42 and n = 20 42, where the core-shell ICO configurations (Pt42Co13 and Pt42Rh13) are stable for both systems, due to the different atomic sizes that cause a release of stress in the NPs. For PtnAu55-n nanoalloys only n = 13 is energetically favorable, forming a core-shell structure. For the other compositions of PtAu we have the same trend as for the crystalline alloys reported experimentally, i.e., non-alloy formation. The effects on the catalytic properties of mixing two-TMs can be understood through the shift of the gravity center of the d occupied states. In this analysis, we observed that it is possible to obtain PtTM nanoalloys that can be more affordable and have better catalytic properties than pure Pt NPs. In terms of magnetic properties, we found that Pt55 and Co55 have smaller and larger values of magnetic moments, respectively, so PtCo follows the tendency where the Co atoms dominate the magnetic properties. For PtRh, the magnetic moment values are higher than for pure NPs. In the case of PtAu we observed the same trend, although with a lower magnitude. The lowest energy structures for Pt55 and Au55 are non-icosahedral, with an unexpectedly small core. Thus, we study these systems adding ligands, and verifying the changes in the stability. We studied the interaction of TM NPs with ligands such as: PH3, PH2, and SH2, in order to verify the changes in stability, structural, and electronic properties. We obtained that the relative stability differences between ICO and LOW (lowest energy configuration) structures decreases with the use of ligands. The LOW structures are not the most stable (Au) or very similar in energy than ICO structures (Pt) when 18 ligands are added to NPs. / Nesta tese de doutorado realizou-se o estudo teórico das propriedades estruturais, eletrônicas e magnéticas de partículas de metais de transição (TMs) utilizando modelos com 13 e 55 átomos para descrever clusters, nanopartículas (NPs), nanoligas, NPs protegidas por ligantes e adsorção sobre clusters, via Teoria do Funcional da Densidade. Primeiramente, realizou-se um estudo sistemático dos TMs 3d, 4d e 5d da Tabela Periódica usando o modelo de clusters com 13 átomos. Este estudo possibilitou a obtenção de tendências nas propriedades dos clusters em relação à ocupação dos estados d. Fazendo-se uso de uma estratégia de obtenção de estruturas de mais baixa energia baseada em simulações de dinâmica molecular e simulated annealing, foi possível não só obter as estruturas mais estáveis reportadas na literatura, mas também novas configurações de mais baixa energia ainda não reportadas. As seguintes conclusões foram obtidas: (i) A energia de ligação e o comprimento médio de ligação possuem uma curvatura parabólica em função da ocupação dos estados d e, assim, muitas das propriedades podem ser explicadas pelo modelo de níveis ligante e antiligante. (ii) Estruturas compactas do tipo icosaédricas (ICO) são energeticamente favoráveis no início de cada série; estruturas mais abertas, tais como bicamada hexagonal (HBL) e cúbica simples dupla (DSC) são energeticamente favoráveis no meio de cada série e estruturas com um alto número de coordenação ocorrem para grandes ocupações dos estados d. (iii) Para o caso específico de Au13, verificou-se que o acoplamento spin-órbita favorece estruturas 3D, ou seja, a estrutura 3D é 0,10 eV mais estável que a configuração de mais baixa energia 2D, a qual era tida como a mais estável na literatura. (iv) As interações de troca magnética possuem um importante papel para sistemas tais como Fe, Cr e Mn. Verificou-se que muitas tendências são compartilhadas por clusters e os respectivos cristais. Estruturas inesperadas (DSC), abertas e com baixa coordenação, foram obtidas para Ru13, Rh13, Os13 e Ir13, contrastando com os cristais, que possuem estruturas fechadas. A excepcionalidade destas estruturas abertas levou-nos a investigar a influência de diferentes aproximações para o termo de troca e correlação (locais, semilocais e não locais) na determinação das estruturas de equilíbrio destes clusters. O emprego de funcionais locais e semilocais confirmou a estrutura DSC como sendo a mais estável e apresentou boa concordância nas energias relativas obtidas, mesmo para estruturas com pequenas diferenças de energia, ou seja, confirmou-se os resultados obtidos com o funcional PBE (Perdew, Burke e Ernzerhof). No entanto, o emprego de abordagens utilizando as aproximações PBE+U e funcional híbrido aplicados para alguns sistemas, mostrou que o aumento da localização eletrônica influencia diretamente a estabilidade dos clusters. A correção parcial do problema de auto-interação aumenta a estabilidade das estruturas fechadas. A hibridização sd auxilia na explicação da estabilidade estrutural, já que esta decresce para as configurações DSC e aumenta para as ICO. O estudo da adsorção da molécula de NO sobre alguns dos TM13 de maior interesse na literatura: Rh13, Pd13, Ir13 e Pt13 e a comparação com os resultados obtidos para as respectivas superfícies de TM(111) possibilitou a constatação de que a adsorção sobre os clusters varia significativamente, com forte dependência do ambiente químico próximo aos sítios de adsorção; enquanto que as tendências obtidas para os parâmetros geométricos característicos são similares aos observados para NO/TM(111). Do estudo de TM55 obteve-se que as NPs de Co55 e Rh55 possuem a estrutura ICO como sendo a mais estável, enquanto que estruturas HBL e DSC foram obtidas respectivamente para Co13 e Rh13. Já para Pt55 e Au55 uma estrutura não-icosaédrica é obtida como sendo a mais estável, com baixa simetria e com o tamanho de caroço reduzido, 7 9 átomos, fato que tem conseqüências diretas para aplicações em catálise, devido a maior quantidade de átomos na superfície. Após o estudo de NPs de TM55 estudou-se nanoligas de PtnTM55-n (TM = Co, Rh, Au) em função da composição (n), verificou-se que as NPs de PtTM preferem um padrão de composição com os átomos de Co ou Rh (Pt) na região do caroço e os átomos de Pt (Au) na região de superfície. Obteve-se que PtnRh55-n e, especialmente, PtnCo55-n tendem a formar ligas, principalmente entre n = 28 42 e n = 20 42, sendo que as configurações core-shell ICO (Pt42Co13 e Pt42Rh13) são estáveis para esses sistemas, devido a diminuição do stress causada pelas diferenças de tamanhos atômicos. Já para PtnAu55-n apenas a composição n = 13 é favorável (estrutura core-shell), as demais composições não são favorecidas energeticamente, da mesma forma como ocorre para as fases cristalinas. Mostrou-se que os efeitos da mistura de dois TMs nas propriedades catalíticas podem ser entendidos por meio do deslocamento do centro de gravidade dos estados d ocupados. Desta análise, observou-se a possibilidade de obtenção de nanoligas PtTM que podem ser mais acessíveis economicamente e ter melhores propriedades catalíticas que NPs puras de Pt. Obteve-se que Pt55 e Co55 possuem baixos e altos valores de momento magnético, respectivamente, logo PtnCo55-n segue uma tendência onde os átomos de Co dominam o comportamento magnético. Para PtRh observou-se valores de momento magnético algumas vezes maiores que para as NPs puras e para o caso de PtAu temos a mesma tendência, porém em menor magnitude. Devido as estruturas inesperadas obtidas para as NPs Pt55 e Au55 estudou-se esses sistemas, acrescentando ligantes (PH3, SH2 e PH2), verificando assim, as alterações na estabilidade. Obteve-se que a diferença de estabilidade relativa entre as estruturas ICO e LOW (configuração de mais baixa energia) diminui com o emprego de ligantes. As estruturas LOW deixam de ser as mais estáveis (Au) ou ficam muito próximas em energia da estrutura ICO (Pt) quando 18 ligantes são adicionados às NPs.

L'objectif de ce travail était double : obtenir des informations sur le lien structure/réactivité dans les nanoalliages et évaluer l'impact de l'addition d'or sur les propriétés catalytiques de métaux actifs en hydrogénation. Des nanoparticules bimétalliques Au-Pd et Au-Rh ont été synthétisées et supportées principalement sur des nanobâtonnets TiO2 rutile, caractérisées par différentes techniques d'analyse (DLS, UV-Vis, HR-(S)TEM, XRD, XPS et CO-FTIR), et évaluées dans deux réactions d'hydrogénation. Les catalyseurs modèles ont été principalement préparés par voie colloïdale suivie d'une immobilisation sur le support et d'un post-traitement consistant à éliminer le surfactant sans détruire la structure des nanoparticules. Les particules Au-Pd (3-5 nm) possèdent une structure alliée de type solution solide. La structure des particules Au-Rh (3-5 nm), système immiscible en volume, est plus hétérogène, avec différentes configurations en fonction du post-traitement effectué : coeur-coquille, ségrégation de phases de type « Janus » et alliage à l'échelle atomique en faible proportion. Les catalyseurs bimétalliques ont été testés en hydrogénation de la tétraline en présence de soufre et en hydrogénation sélective du cinnamaldéhyde, dans les deux cas sous haute pression d'hydrogène, et comparés à leurs homologues monométalliques. En hydrogénation du cinnamaldéhyde, un effet considérable du traitement post-synthèse sur l'activité et sélectivité est mis en évidence. Les catalyseurs fraichement synthétisés montrent une sélectivité élevée en hydrocinnamaldéhyde alors que les traitements de réduction et de calcination-réduction diminuent l'activité pour les échantillons AuRh les plus riches en Rh. En hydrogénation de la tétraline, l'alliage avec l'or a pour effet de diminuer l'activité mais d'améliorer la stabilité des systèmes à base de Pd et Rh en présence de soufre, en augmentant, par effet électronique, la barrière de chimisorption du soufre ou de sulfuration / The objectives of this work were to gain an insight into the structure-selectivity relationships in nanoalloys and to evaluate the impact of gold addition on the catalytic properties of active metals in hydrogenation reactions. For this purpose, bimetallic Au-Pd and Au-Rh nanoparticles were synthesized and supported (mostly) on rutile TiO2 nanorods before being structurally characterized by various techniques (DLS, UV-Vis, HR-(S)TEM, XRD, XPS, and CO-FTIR) and evaluated in two hydrogenation reactions. The model catalysts were mainly prepared using a colloidal method and immobilized on the support. Post-treatments were carried out in order to eliminate the surfactant used during the synthesis, with minimal impact on the nanoparticle structure. The influence of the synthesis parameters on the nanoparticle structure and catalytic properties was evaluated. The Au-Pd particles (3-5 nm) exhibit an alloyed solid solution structure. The structure of the bulk-immiscible Au-Rh particles (3-5 nm) is more heterogeneous, with several structural configurations depending on the post-treatment: core-shell, Janus-type phase segregation, and atomic-scale alloyed structure to a small extent. The catalysts were tested for tetralin hydrogenation in the presence of sulfur (0-100 ppm H2S) and for the selective hydrogenation of cinnamaldehyde, both under high hydrogen pressure. The bimetallic systems were compared to their monometallic counterparts. The post-synthesis treatments have a dramatic impact on activity and selectivity in cinnamaldehyde hydrogenation. The fresh catalysts exhibit a high selectivity toward hydrocinnamaldehyde, whereas reduction and calcination-reduction mainly decrease the activity of Rh-rich Au-Rh samples. For tetralin hydrogenation, gold decreases the activity but improves the stability of Pd and Rh-based systems in the presence of sulfur through electronic effects increasing sulfur chemisorption or sulfidation barriers

Cette thèse présente une étude théorique de la structure électronique et de la tendance à l'ordre dans les nanoalliages de métaux de transition en se référant au cas des systèmes à base de cobalt-platine (CoPt) qui présentent un intérêt particulier dans les domaines du magnétisme et de la catalyse. Il est ainsi important de décrire au mieux l'évolution de la structure électronique en lien avec la structure atomique ou l'arrangement chimique en fonction de la taille de tels nanoalliages en vue de prédire des propriétés potentielles pouvant différer fortement de celles du matériau massif correspondant. Dans ce contexte, des calculs systématiques de DFT ont été mis en œuvre sur des systèmes modèles simples, alliages massifs, surfaces et nanoparticules, qui ont permis de montrer qu'une règle de neutralité de charge locale, par site espèce et orbitale, s'applique au système CoPt massif et s'étend aux nanoalliages. Sur cette base, des calculs auto cohérents de liaisons fortes ont été développés et ont permis de proposer un moyen précis de prédire les caractéristiques de nanoalliages réalistes, en termes de redistribution des états électroniques et de tendance à l'ordre. Les grandeurs caractéristiques déduites de ces calculs, telles que le désordre diagonal et non diagonal, peuvent être en effet être déterminées à partir de lois simples linéaires de variations des centres et des largeurs de bandes sur les sites Co et Pt. Les valeurs issues de ces lois peuvent être placées sur des cartographies de domaines de tendance à l'ordre et l'ensemble de la méthodologie devrait être étendue facilement à d'autres systèmes binaires. / The interest in alloys of late transition metals arises from their potential applications in high-density magnetic storage devices where they can be used as supported magnetic nanoparticle arrays and as stable, efficient and selective catalysts. The preparation of materials with optimal properties faces a number of technological and physical restrictions and requires an in-depth knowledge of the interplay between structural features on the atomic level and the desired macroscopic properties. In the thesis, after extensive discussion of Density Functional Theory and Tight Binding approaches the work focused on DFT calculations of bulk systems, surfaces and small clusters. The results allow to conclude on general validity of the method and especially to justify the local neutrality assumption in the case of low-coordinated sites in nanoparticles. Basic structural, magnetic and energetic properties were also studied and compared with the experimental data. Subsequently TB calculations were performed and verified with DFT results. The scope of the calculations was then extended for the case of nanoclusters of realistic sizes, unavailable in DFT. Local Densities of State on sites with different chemical environment and coordination numbers were analyzed. The observations prove that basic features of LDOS (d-band centre and d-band width) can be predicted by simple laws on the basis of two terms: a structural term represented by the linear function of the site coordination and a chemical term as a rigid shift which opens a new way to predict the ordering tendency (mapping of the ordering domains) for any transition metal nanoalloy as a function of its size.

Philosophiae Doctor - PhD / Ammonia has attracted attention as a possible fuel for direct fuel cells since it is easy to handle under low pressure, costs only slightly higher than methanol and can easily be cracked down into hydrogen and nitrogen. At low temperature, ammonia oxidation on noble metal electrodes is a sluggish reaction and efficient catalysts are required to convert ammonia to nitrogen and hydrogen at reasonable reaction rates. In this thesis, I present polycrystalline and oriented nanoalloys synthesised at room temperature in aqueous media and their catalytic effects on the oxidation of ammonia. The electro-oxidation of ammonia on palladium-goldsilver (PdAuAgNPs) ternary nanoalloys was systematically studied in alkaline solution of potassium hydroxide (KOH) by cyclic voltammetry (CV). The PdAuAg nanoalloys were prepared through a facile synthesis with ascorbic acid as a reductant and polyvinylpyrrolidone (PVP) as a stabilising agent from aqueous solutions of PdCh/HAuCI4.3H20/AgN03 mixtures. UV-visible spectroscopy was used to confirm the complete reduction of the metal ions; absorption peaks observed at 260 nm, 285 nm and 420 nm for Ag", Au3+ and Pd2+ ions respectively, disappeared after reduction indicating a complete reduction of the metal ions to zero-valent nanoparticles. High resolution transmission electron microscopy (HR TEM) revealed the formation of crystalline nonaggregated 25-35 nm sized nanoalloys. The elemental composition of the nanoalloys measured using energy dispersive X-ray spectroscopy (EDX) showed the presence of the three elements; Pd, Au and Ag. The well-dispersed non-agglomerated PdAuAg nanoalloys exhibited a reduced overpotential and a 33%, 400%,82% and 54% increase in current density for ammonia electro-oxidation compared to Pd, PdAg, PdAu nanoparticles and bare Pt electrode, respectively. The much improved current density of the well-dispersed PdAuAg nanoalloys is attributed to the increased electrochemically active surface area of the nanoalloys. This electro catalytic behaviour of the PdAuAg nanoalloys for ammonia oxidation in KOH solutions provides a promising route for development of low-cost and high performance electro catalyst for electro-oxidation of ammoniaMoreover, ammonia oxidation on platinum surfaces has been found to be a very structure sensitive reaction which takes place almost exclusively on Pt(100) surfaces. I report for the first time the preparation of sodium polyacrylate-capped Pt(100)Pd, pte 1OO)Au, pte 1OO)Ir, Pt(IOO)Rh, Pt(100)PdAu, Pt(100)IrAu, Pt(IOO)PdIr and Pt(IOO)RhAu nanoalloys. The reduction of the metal ions to nanoparticles was confirmed by UV-visible spectroscopy while the shapes and the structures of the nanoparticles were studied using HRTEM and CV. HRTEM analysis showed well distributed non-agglomerated 5-20 nm semi-spherical and cubic nanoalloys with lattice fridges on their surfaces indicating the crystalline nature of the nanoalloys. Pt(100) nanoalloy systems showed particles with triangular and cubic shapes. The existence of the preferentially cubic shaped nanoparticles in the samples indicated that the nanoalloys had some (100) sites orientation/a significant amount of (100) sites at their surfaces. The CV of the nanoparticles in the hydrogen adsorption/desorption region (-200 mV to 100 mV vs. Ag! AgCl) was used to obtain qualitative information about the surface structure of the nanoparticles. The voltammogram of oriented Pt(100) nanoparticles showed very clearly the presence of adsorption states associated with (110) sites, (100) domains and (l00) sites at -131 mV, -34 mV and 29 mV, respectively. The companson of this voltammetric profile with that obtained for a Pt(100) single crystal electrode clearly points out that the synthesised Pt nanoparticles have a high density of (100) sites. However, the peak that was observed at 29 mV in the CV of Pt(100) nanoparticles was not present in the vo ltammo grams of the Pt(100) nanoalloy systems confirming the formation of the nanoalloys. The results reported in this work demonstrate the importance of controlling the intrinsic structural properties of Pt nanoparticles; in terms of nature of the active sites and the effect of adding adatoms (such as Au, Pd, Rh, Ir) in order to understand their catalytic properties. The electrochemical activities of these nanoparticles for ammonia oxidation in basic medium showed an increase of over 100% current density compared to Pt electrode. Pt(lOO)RhAu nanoalloys showed the highest catalytic properties while Pt(lOO)PdAu had the lowest as shown in the trend: Pt(lOO)RhAu > Pt(lOO)PdIr > Pt(lOO) > Pt(lOO)IrAu > Pt(lOO)Pd> Pt(lOO)Rh > Pt(lOO)Au > Pt(lOO)Ir > Pt(lOO)PdAu. The synthesised oriented nanoalloys were further interrogated towards the oxidation of hydrazine as a fuel for hydrazine fuel cells. The oriented Pt(lOO) nanoparticles and Pt(lOO) nanoalloy systems exhibited over 1000% increase in current density and reduced oxidation overpotential compared to bare glassy carbon electrode. These excellent catalytic properties are attributed to the increased surface area and the presence of (100) sites which favour the oxidation of hydrazine.

In der vorliegenden Arbeit wurden facettierte Nanolegierungen aus Gold und Palladium in der Größenordnung von 1 – 3 nm synthetisiert und charakterisiert. Die Synthese erfolgte in sphärischen Polyelektrolytbürsten (SPB), welche als Stabilisatorsystem fungierten. Die Strukturaufklärung erfolgte mittels hochauflösender Transmissionselektronenmikroskopie (HR-TEM) und Extended X-ray Absorption Fine Structure (EXAFS) Spektroskopie. Die Nanolegierungen wurden als Katalysatoren zur Reduktion von 4-Nitrophenol verwendet, wobei eine gesteigerte katalytische Aktivität, im Vergleich zu monometallischen Nanopartikeln, beobachtet wurde. Im HR-TEM wurden auf den Kanten der Metallkristalle Oberflächendefekte in Form von Stufen sichtbar, wobei eine Korrelation der Oberflächendefekte zur katalytischen Aktivität hergestellt wurde. Durch EXAFS-Analysen konnte nachgewiesen werden, dass eine Zusammensetzung von 75 mol-% Gold und 25 mol-% Palladium annähernd eine statistische Mischung aufweist. Mit steigendem Palladiumgehalt wurde eine leichte Anreicherung von Palladium an der Oberfläche der Partikel, sowie nichtmetallisches Palladium der Oxidationsstufe 2+ gefunden. Diese unterschiedliche Anordnung der Elemente für verschiedene Zusammensetzungen korreliert ebenfalls mit der katalytischen Aktivität. Ferner konnte eine Kontraktion des Netzebenenabstands für kleine Partikel mittels EXAFS nachgewiesen werden. Bei der Verwendung als Katalysatoren wurde eine zeitliche Reaktionsverzögerung festgestellt, welche auf die Oberflächenrestrukturierung der Metallnanopartikel hinweist. Zusätzlich konnte durch den Vergleich von SPB stabilisierten und stabilisatorfreien Goldnanopartikeln festgestellt werden, dass das SPB System die katalytische Aktivität nicht signifikant beeinträchtigt. / In this work facetted gold-palladium nanoalloys have been synthesized and characterized in the range of 1 – 3 nm. The spherical polyelectrolyte brushes (SPB) have been used during synthesis and for stabilization of the nanoparticles. Structural analyses have been done by high resolution transmission electron microscopy (HR-TEM) and extended X-ray absorption fine structure (EXAFS) spectroscopy. The nanoalloys have been used as catalysts for the reaction of 4-nitrophenol, where an enhanced catalytic activity has been found compared to monometallic nanoparticles. Surface defects are visible by steps at the edge of the metallic crystal where a correlation of these surface defects and the catalytic activity has been found. By EXAFS spectroscopy an almost statistic mixture for a composition of 75 mol-% gold and 25 mol-% palladium, slight enrichment of palladium at the surface of the particle, and the presence of nonmetallic palladium with an oxidation number of 2+ has been observed. This different atomistic arrangement correlates with the catalytic activity as well. In addition, a contraction of the lattice parameter has been observed for small particles by EXAFS measurements. Using the nanoalloys as catalyst, a delay time for the reaction has been found which indicates a restructuring of the surface of the metal nanoparticles. By comparison of SPB stabilized and support-free gold nanoparticles no significant influence of the SPB system on the catalytic activity has been observed.

Clusters e nanoclusters têm recebido grande atenção devido à suas propriedades físicas e químicas, as quais divergem bastante dos materiais na fase bulk. Essas propriedades podem variar de acordo com a composição e tamanho do cluster. Uma compreensão da evolução das propriedades em relação a estes parâmetros é de grande importância para potencializar diversas aplicações, entretanto, esse entendimento permanece insatisfatório. Este trabalho foi dividido em duas etapas, em que a primeira busca investigar parâmetros energéticos, por meio do cálculo da energia de excesso, e estruturais, analisando parâmetro de ordem química, função de distribuição radial central, comprimento médio de ligação e número de coordenação efetiva, dos sistemas ZrnCum-n, ZrnAlm-n, CunAlm-n e ZrnCunAlm-2n para n = 55 e 561 átomos com o incremento n tomado de 1 em 1 para o sistema de 55 átomos e de 20 em 20 para os sistemas de 561 átomos. A segunda etapa consiste de investigar como variam as propriedades energéticas e estruturais do sistema ZrCu em função da evolução do tamanho do sistema. Para alcançar os objetivos propostos, neste trabalho foi usado o algoritmo de otimização global de clusters e nanopartículas basin-hopping Monte Carlo revisado. O potencial de interação atômica utilizado é o método do átomo imerso, que é bastante utilizado na descrição de sistemas metálicos. Os resultados obtidos sugerem que: (i) os sistemas puros apresentaram energia de coesão mais alta que seu análogo material na fase bulk, sugerindo que estes tendem a aglomerar-se formando estruturas bulk. Para os sistemas binários e ternários, foi identificado que todas as composições são energeticamente estáveis devido aos valores negativos obtidos pelo excesso de energia e, para o sistema ZrCu verificou-se a presença de efeitos de tamanho. (ii) Com relação à estrutura, as composições puras estudadas apresentaram simetria icosaédrica. Para o estudo da evolução do tamanho do sistema, Zr e Cu apresentaram estrutura com simetria icosaédrica até a composição de 561 átomos, além deste tamanho a simetria icosaédrica é quebrada. Para os sistemas binários e ternários foi obtido que os átomos tendem a distribuir-se dentro do nanocluster além de apresentarem quebra da simetria icosaédrica apresentando ausência de camadas atômicas ordenadas acompanhada de redução da coordenação efetiva. Os sistemas ZrCu e ZrAl demonstraram seguir a lei de Vegard, enquanto que os sistemas CuAl e ZrCuAl apresentaram desvio da lei de Vegard providos por efeitos eletrônicos, além de apresentarem a presença de efeitos de tamanho. / Clusters and nanocluster have attracted great attention due to their physical and chemical properties, very different from their analogous bulk. These properties can vary with composition and size cluster. An understanding of the properties evolution with respect these parameters is essential to improve several applications. However, this understanding is not complete. This study was piecemeal in two stage, being the first the investigation of energetic properties, by excess energy analisys, and structural properties, by chemical order parameter, radial distribution function, effective coordination number and average bond length, from ZrnCum-n, ZrnAlm-n, CunAlm-n and ZrnCunAlm-2n systems, where n = 55, 561 atoms and the increment n vary in one unit for 55-atoms system and twenty unit for 561-atoms system. The second stage is the investigation of how vary the energetic and structural properties from the size evolution ZrCu system. To do this study, was employed the global optimization algorith for cluster and nanoparticle Revised basin-hopping Monte Carlo, were this method use the classical calculation to determine the total energy of the system. The interatomic potential used was the embedded atom method, that was very usefull to describe metallic systems. Our results suggest: (i) the unary systems present cohesive energy higher than their analogous bulk, that indicate the trend of clusters to form bulk. To the binary and ternary systems, we had that all systems are favorable to form nanoalloys by negative value of excess energy. From ZrCu system, the stability decrease when increase the size of system. With respect the structure, the unary compounds present icosahedral symmetry. From the size-evolution study, the unary compounds present icosahedral symmetry until 561-atoms composition, after this size the icosahedral symmetry is broken. To binary and ternary systems, the atoms trend form mixture into the nanocluster, the icosahedral symmetry is broken with respect the unary compounds and presenting absence of ordered layers followed by effective coordination reduction. The ZrCu and ZrAl systems follow the Vegard law, while the CuAl and ZrCuAl systems present deviation from Vegard law, because electronic effects.

Ce travail de thèse propose de déterminer, en utilisant le formalisme des liaisons fortes, la relation entre les structures atomique, chimique et électronique des nanoalliages en se focalisant sur deux systèmes, archétypes d'une forte tendance à l'ordre (Cobalt-Platine - CoPt) d'un côté et d'une forte tendance à la démixtion (Iridium-Palladium - IrPd) de l'autre. Concernant les alliages CoPt et IrPd, l'évolution des caractéristiques des densités d'états locales (DEL) en fonction de la coordination de site (effet structural), de l’environnement chimique (effet d’alliage) et de la taille des systèmes a été analysée en détail. CoPt et IrPd ont un comportement tout à fait similaire en ce qui concerne les décalages de bandes d, ce qui s'explique par une règle de conservation de charge par espèce, par site et par orbitale entre systèmes mixtes et systèmes purs. Dans les nanoparticules pures d’Ir et de Pd, les centres de bandes d varient linéairement avec la coordination indépendamment de la taille. Le même comportement est observé pour les nanoalliages d’IrPd, la droite correspondant aux nanoalliages étant seulement décalée rigidement par rapport aux nanoparticules pures. Ce découplage entre effet structural et chimique, déjà observé dans les nanoalliages de CoPt, est ici généralisé car il s'applique quelle que soit la tendance chimique du système à l'ordre ou à la démixtion. Concernant la tendance chimique, le CoPt reste un système avec une tendance à l'ordre quelle que soit sa configuration, de même pour l'IrPd qui reste à la démixtion quelle que soit la configuration et quelle que soit la taille dans le cas des nanoalliages. Nous avons exploré plus finement le cas des alliages dilués, dans le cas de systèmes à base d'AuNi. On trouve, dans ce cas, un changement de tendance, en passant de la séparation de phase pour les systèmes concentrés à une tendance à l’ordre pour les systèmes dilués, incluant les systèmes de couches minces en surface. Des études complémentaires de Monte Carlo, en réseau rigide puis incluant les déplacements atomiques, montrent que les nanoparticules d'IrPd adoptent une structure cœur-coquille avec un cœur excentré malgré le faible effet de taille atomique entre les atomes de Pd et d'Ir. / The purpose of this thesis work is to determine, by using the tight-binding formalism, the link between atomic, chemical and electronic structures of nanoalloys focusing on two systems, characteristic on one hand of a strong order tendency (cobalt-platinum, CoPt) and, on the other hand, of a strong tendency to phase separation (iridium-palladium, IrPd). For both CoPt and IrPd, the evolution of the local densities of states (LDOS) as a function of the site coordination (structural effect), the chemical environment (alloy effect) and the size of the systems has been analyzed in detail. CoPt and IrPd have a same behavior concerning the d band shifts which is explained by a rule of charge preserving per species, per site and per orbital between mixed systems and corresponding pure systems. In pure Ir and Pd nanoalloys, the d band centers are found to vary linearly with the site coordination. In IrPd, a linear behavior is also observed, the corresponding line being only rigidly shifted with regards to the pure materials. This decoupling between structural and chemical effects, already observed for CoPt nanoalloys, is here generalized since it applies regardless the tendency of the system to order or to phase separate. Concerning the chemical tendency, CoPt remains a system with order tendency whatever the atomic configuration. In the same way, IrPd remains a system with a tendency to phase separation on the whole range of studied configurations although not so clearly defined in the dilute alloys. We have therefore investigated another dilute systems based on AuNi. In this case, a change of trend is observed going from phase separation for concentrated systems to order tendency for dilute systems, including thin layers at surfaces. Complementary structural studies have been performed by using Monte Carlo simulations, first on a rigid lattice and then including atomic displacements. The results show that nanoparticles of IrPd are core-shell with a strong Pd segregation at the surface. The core of nanoparticle is generally off-centered despite the very small atomic size effect between Pd and Ir atoms.

Les propriétés catalytiques des nanoparticules métalliques peuvent être améliorées par effet d’alliages. La synthèse en solution par voie colloïdale permet de préparer des nanoalliages homogènes en taille, en forme et en composition chimique, de structure ordonnée, désordonnée ou cœur-coquille. La nucléation et la croissance des nanoalliages de Pt-Pd sont étudiées ici par microscopie électronique en transmission, en condition standard, puis in situ dans une cellule liquide formée par des feuilles d’oxyde de graphène. La cinétique de croissance des nanoalliages de Pt-Pd correspond à l’incorporation directe des monomères en solution, compatible avec un processus limité par la réaction de surface, sans phénomène de coalescence, contrairement à la croissance du Pt pur. La structure théorique à l’équilibre des nanoalliages de Pt-Pd est déterminée par des simulations Monte Carlo. La structure la plus probable correspond à une surface riche en Pd et à une sous couche atomique riche en Pt, stable à des températures élevées. L’effet de l’adsorption de gaz oxydants ou réducteurs sur la forme des nanoparticules, est étudié in situ par microscopie environnementale sous pression de quelques mbar, dans un porte objet environnemental. On observe des changements de formes sous oxygène, dus au développement de facettes d’indices plus élevés. La réactivité des nanocubes de Pd@Pt est étudiée pour l’oxydation du CO en fonction du recouvrement de Pt à la surface. La réactivité maximale pour un faible recouvrement est interprétée par une baisse de l’énergie d’adsorption du CO liée au désaccord paramétrique entre le Pt et le Pd et à la modification de la structure électronique du Pt lié au Pd. / The catalytic properties of metal nanoparticles can be improved by the alloying effect. Nanoalloys homogeneous in size, shape and chemical composition can be prepared with the colloidal synthesis method, with an ordered, random or core-shell chemical structure. Nucleation and growth of colloidal Pt-Pd nanoalloys were studied by transmission electron microscopy (TEM), in standard conditions and in situ with the aid of a graphene oxide liquid cell. The growth kinetics of homogeneous Pt-Pd nanoalloys corresponds to the direct incorporation of the monomers in solution. It was compatible with a process limited by the surface reaction, without coalescence (Lifshitz-Slyozov-Wagner mechanism). On the contrary, coalescence occurs during the growth of pure Pt nanoparticles. The theoretical structure of Pt-Pd nanoalloys is determined by Monte Carlo simulations. The most stable structure corresponds to a Pd surface and Pt subsurface layer, which is stable up to high temperatures. The effect of adsorption of oxidizing or reducing gasses on the shape of pure Pd nanocubes and core-shell Pd@Pt nanocubes is studied in situ by TEM with an environmental cell. The observed changes in a few mbar of oxygen are due to the development of higher index facets. The CO oxidation reaction is used to compare the reactivity of homogeneous Pt-Pd nanoalloys and core-shell Pd@Pt nanocubes with increasing coverage of Pt at the surface. A maximal reactivity is attained for a low coverage. The effect is interpreted by a decrease in adsorption energy of CO, due to electronic effects originating from the lattice mismatch between Pt and Pd and the mixed Pt-Pd bonds.

On appelle nanoalliages les nanoparticules d'alliages, leur composition chimique et leur taille en modifient les propriétés physico-chimiques. La relation entre structure et propriétés est intéressante pour la différence existant entre les propriétés du volume et celles du nanoalliage. Comme les alliages volumiques, caractérisés par un diagramme de phase en température et concentration, les nanoalliages sont étudiés pour déterminer un diagramme de phase en fonction de la concentration, la température, la taille et la forme. Les nanoparticules de CoPt ont un intérêt pour le développement du stockage magnétique à haute densité nécessitant une forte anisotropie magnétique telle qu'existant dans la phase volumique L10. L'expérience a montré que pour des nanoparticules de CoPt de 2-3nm la phase L10 existe. Des études théoriques, tenant compte des relaxations du réseau, ont confirmé l'existence d'agrégat de type polyèdre de Wulff présentant un tel ordre sur un réseau cfc.Nous proposons une étude théorique de l'alliage CoPt, utilisant un modèle énergétique simple, le modèle d'Ising en liaisons fortes, reproduisant l'ordre en volume et les trois moteurs de la ségrégation de surface (effets d'alliage, de surface et de relaxation atomique). Les analyses statistiques conduites à partir de simulations Monte Carlo dans les ensembles canonique et semi-grand canonique caractérisent un diagramme de phase de volume et de nanoalliage. Dans l'ensemble semi-grand canonique on s'intéresse à la synergie entre l'ordre au coeur et la ségrégation de surface du système CoPt. L'ordre à courte et longue distance a été caractérisé au cœur de l'agrégat et nous en avons étudié la frustration. / Nano-alloys are commonly the name given to nano-particles of alloys. The size and chemical composition of nano-alloys affect physical and chemical properties. Relation between structure and properties is interesting for the difference of bulk and nano-alloys properties. As bulk alloys which are characterised by phase diagram (in concentration and temperature), nano-alloys can be studie to determine a phase diagram in function of concentration, temperature, size and shape. CoPt nano-particles are interesting for the development of ultra-high density magnetic memories which require strong magnetic anisotropy as it exists in the L10 bulk phase. It has been shown experimentally the CoPt nano-alloys order with the L10 structure exist for the small size (2-3nm). Theoretical studies, using model taking into account the lattice's relaxations have confirmed the ordering of the fcc nano-particles with a Wulff polyhedron (WP) shape. We propose a theoretical study focus on CoPt WP, use a simple energetic model the Tight Binding Ising Model which reproduce the bulk ordering and the three driving surface segregation force (alloying, surface and atomic relaxation effect). Statistical analysis are conducted using a Monte Carlo simulation in the canonical and semi-grand canonical ensemble to characterize a phase diagram for bulk and nano-particles. In the semi-grand canonical ensemble we are interested on the synergy or competition between cluster core ordering and surface segregation of CoPt system. We characterised the order (short and long distance) at the core of the cluster and we studied frustration of the ordering.