Excitations of the valence electrons in the high-temperature superconducting cuprate La2−xSrxCuO4 were measured by Resonant Inelastic X-ray Scattering (RIXS). Several types of electronic excitations resonant at the Cu 1s →4p transition were studied over a wide range of dopings 0<x<0.35. A 500 meV excitation was observed at a reduced momentum transfer q=(π 0) corresponding to the zone boundary, whose temperature and doping dependence was the same as the two-magnon Raman scattering mode. The momentum dependence of this 2-magnon excitation agrees with recent theoretical calculations. Momentum resolved measurements of the x=0 sample revealed a broad range of excitations above and below the main charge transfer peak, and their dispersions were measured across the Brillouin zone. These include a dispersionless ∼1.8 eV peak, which is either a local crystal field (d-d excitation) or dipole-forbidden charge transfer excitation, and a dispersive 2.2 eV peak identified as a Zhang-Ng type charge-transfer exciton. The 2.2 eV peak was less dispersive than predicted from the theoretical models, due to electron-phonon coupling, as illustrated by the temperature dependent shift in the peak position. With increased hole doping, the RIXS spectral weight transfers from higher to lower energies, analogous to earlier optical conductivity studies. At the finite momentum of q=(π 0), however, the changes are most systematic: an isosbestic point was observed at 2.2 eV where the spectra of all dopings cross, and spectral weight is transferred from high to low energies, with near-linear dependence on x, in a manner suggesting that the integrated RIXS intensity is preserved. The intensity and energy variations of the spectral peaks, as well as the isosbestic point and possible sum rule, could be explained qualitatively by a rigid three-band model which includes the non-bonding oxygen, upper Hubbard, and Zhang-Rice singlet bands. The estimated properties of the bands, such as width and energy separation, are in reasonably quantitative agreement with current theoretical models and angle-resolved photoemission measurements. Moreover, anomalies in the doping dependence are similar to those observed in other types of spectroscopies. These results underscore the relevance of the RIXS method in understanding the electronic behavior of the Lanthanum cuprates.

Excitations of the valence electrons in the high-temperature superconducting cuprate La2−xSrxCuO4 were measured by Resonant Inelastic X-ray Scattering (RIXS). Several types of electronic excitations resonant at the Cu 1s →4p transition were studied over a wide range of dopings 0<x<0.35. A 500 meV excitation was observed at a reduced momentum transfer q=(π 0) corresponding to the zone boundary, whose temperature and doping dependence was the same as the two-magnon Raman scattering mode. The momentum dependence of this 2-magnon excitation agrees with recent theoretical calculations. Momentum resolved measurements of the x=0 sample revealed a broad range of excitations above and below the main charge transfer peak, and their dispersions were measured across the Brillouin zone. These include a dispersionless ∼1.8 eV peak, which is either a local crystal field (d-d excitation) or dipole-forbidden charge transfer excitation, and a dispersive 2.2 eV peak identified as a Zhang-Ng type charge-transfer exciton. The 2.2 eV peak was less dispersive than predicted from the theoretical models, due to electron-phonon coupling, as illustrated by the temperature dependent shift in the peak position. With increased hole doping, the RIXS spectral weight transfers from higher to lower energies, analogous to earlier optical conductivity studies. At the finite momentum of q=(π 0), however, the changes are most systematic: an isosbestic point was observed at 2.2 eV where the spectra of all dopings cross, and spectral weight is transferred from high to low energies, with near-linear dependence on x, in a manner suggesting that the integrated RIXS intensity is preserved. The intensity and energy variations of the spectral peaks, as well as the isosbestic point and possible sum rule, could be explained qualitatively by a rigid three-band model which includes the non-bonding oxygen, upper Hubbard, and Zhang-Rice singlet bands. The estimated properties of the bands, such as width and energy separation, are in reasonably quantitative agreement with current theoretical models and angle-resolved photoemission measurements. Moreover, anomalies in the doping dependence are similar to those observed in other types of spectroscopies. These results underscore the relevance of the RIXS method in understanding the electronic behavior of the Lanthanum cuprates.

This thesis is based on experiments applying synchrotron based X-ray Absorption Spectroscopy(XAS) and Resonant Inelastic X-ray Scattering (RIXS) in the soft X-ray region to energy related systems. The main work of this thesis has been to develop a reaction cell that allowed for in situ XAS and RIXS investigations of the partial epoxidation of ethylene over a Ag catalyst at 1 atm and up to 250C. The developed in situ cell can be used in two sample modes: 1) the Ag catalyst is deposited directly onto the vacuum separating membrane with the reaction gases flowing beneath it or 2) a slightly compressed Ag powder sample is at a distance to the vacuum membrane with the reaction gases flowing between the Ag powder sample and the membrane.Both sample modes offers the total florescence yield, TFY, and the total electron yield, TEY, to be recorded simultaneously. By means of the developed in situ cell a number of oxygen species, residing in/on the Ag surface or in the Ag bulk, have been detected. We claim to detect adsorbed O2 under epoxidation conditions, as well as Ag–O–H groups. We are also able to monitor changes of the absorbed oxygen as we change the composition of the reaction gas feed. The first charging cycle of Li-ion batteries have been investigate by ex situ measurements on the cathode Li2-xMnSiO4 and the anode composite LixNi0:5TiOPO4/C . The initial crystalline material becomes amorphous due to lithiation during the first first charging. We find that the redox behaviors of these two states are significantly different. Sulfidation of natural copper oxides are is found to be strongly promoted when it is grown on the host metal by a disproportionation.

X-ray spectroscopic experiments, including X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS), were performed in a large energy range (60-9000 eV) on a variety of systems, probing different elements and orbitals. The systems can preferably be divided into three groups: Actinides, copper systems and amorphous materials. We studied oxidation states of Pu in PuO2 using a closed-source experimental setup, developed by us, to lower the safety restrictions associated with such experiments. The presence of Pu with higher oxidation state than Pu(IV) was found in polycrystalline PuO2. This is an important finding since Pu with higher oxidation states has higher solubility affecting the safety of a proposed undeground repository of spent nuclear fuel which can eventually be infiltrated by groundwater. In the proposed underground repository the molecular oxygen will vanish at some point due to microbial activity and mineral oxidation. At this stage sulfur is the most important candidate for causing copper corrosion. We studied Cu films and foils exposed to sulfide solutions at both Cu L2,3, Cu K and S L2,3 edges and found for example that S L2,3 XES is useful for distinguishing between different copper sulfides and Cu Kβ XES can be used to identify monovalent Cu compounds. Amorphous, zirconium containing alloys are potentially useful materials for future nuclear reactors. The electronic structure of such materials was also investigated.

Les phonons contribuent à la diffusion inélastique résonante des rayons X (RIXS) du fait du couplage entre les degrés de liberté électronique et ceux du réseau. Contrairement à d'autres techniques sensibles aux interactions électron-phonon, la technique RIXS peut donner accès aux constantes de couplage dépendantes du moment. Des informations sur la dispersion de l'interaction électron-phonon sont très précieuses dans le contexte de la supraconductivité anisotrope conventionnelle et non conventionnelle.Nous avons considéré la contribution des phonons sur la diffusion RIXS d’un point de vue théorique. Contrairement aux études précédentes nous soulignons le rôle du couplage du réseau avec les trous de cœur. Notre modèle, avec les paramètres obtenus ab-initio, montre que même dans le cas d'un trou de coeur profond, la technique RIXS sonde le couplage exciton-phonon plutôt qu’un couplage direct électron-phonon.Cette différence conduit à des écarts quantitatifs et qualitatifs pour le couplage électron-phonon implicite par rapport à l'interprétation standard dans la littérature. Ainsi, notre objectif est de développer une approche rigoureuse pour quantifier le couplage électron-phonon dans le contexte des mesures de diffusion RIXS. La possibilité de reproduire avec précision les résultats expérimentaux à partir des calculs ab-initio, sans recourir à des paramètres ajustés, doit être considérée comme le test ultime d'une compréhension correcte de la contribution des phonons sur la diffusion RIXS.Nous commençons notre travail en considérant uniquement l’interaction trou de coeur-phonon dans le contexte de la spectroscopie par photoémission de rayons X. Nous combinons un calcul ab-initio de la fonction de réponse en espace réel avec des techniques de fonctions de Green à plusieurs corps pour reproduire les bandes latérales vibrationnelles dans les molécules SiX4 (X = H, F). L'approche que nous avons développée peut être appliquée aux matériaux cristallins.Nous examinons ensuite la contribution des phonons aux spectres d'absorption des rayons X. Contrairement aux excitations chargées générées par la photoémission par rayons X, l'absorption des rayons X crée une excitation neutre que nous approchons en tant que trou de cœur et électron excité. Nous résolvons d’abord la partie électronique du problème au niveau de l’équation de Bethe-Salpeter, puis nous habillons la quasi-particule excitonique à 2 particules résultante avec les interactions exciton-phonon en utilisant l’Ansatz des cumulants. La viabilité de cette méthode a été testée en calculant le seuil K XAS de la molécule N2 et le seuil K d’Oxygène de l’acétone. Les spectres vibrationnels obtenus concordent avec les résultats expérimentaux.Enfin, nous construisons une formulation hybride de la section transversale RIXS qui préserve la sommation explicite sur un petit nombre d'états finals, mais remplace la sommation sur les états intermédiaires, ce qui pourrait être extrêmement coûteux, par une fonction de Green. Nous avons obtenu un développement de la fonction de Green et dérivé des solutions analytiques exactes (dans la limite de non-recul) et approximatives. Le formalisme a de nouveau été testé sur le seuil K de l'acétone et est bien en accord avec l'expérience. En perspectives des travaux futurs, nous discutons de l’applicabilité de notre formalisme aux matériaux cristallins. / Phonons contribute to resonant inelastic X-ray scattering (RIXS) as a consequence of the coupling between electronic and lattice degrees of freedom. Unlike other techniques that are sensitive to electron-phonon interactions, RIXS can give access to momentum dependent coupling constants. Information about the dispersion of the electron-phonon interaction is highly desirable in the context of understanding anisotropic conventional and unconventional superconductivity.We considered the phonon contribution to RIXS from the theoretical point of view. In contrast to previous studies, we emphasize the role of the core-hole lattice coupling. Our model, with parameters obtained from first principles, shows that even in the case of a deep core-hole, RIXS probes exciton-phonon coupling rather than a direct electron-phonon coupling.This difference leads to quantitative and qualitative deviations from the interpretation of the implied electron-phonon coupling from the standard view expressed in the literature. Thus, our objective is to develop a rigorous approach to quantify electron-phonon coupling within the context of RIXS measurements. The ability to accurately reproduce experimental results from first-principles calculations, without recourse to adjustable parameters, should be viewed as the ultimate test of a proper understanding of the phonon contribution to RIXS.We start by considering only the core-hole--phonon interaction within the context of X-ray photoemission spectroscopy. We combine an ab initio calculation of the real-space response function with many-body Green's functions techniques to reproduce the vibrational side-bands in SiX4 (X=H, F) molecules. The approach we developed is suitable for application to crystalline materials.We next consider the phonon contribution to X-ray absorption spectra. Unlike the charged excitations generated by X-ray photoemission, X-ray absorption creates a neutral excitation that we approximate as a core-hole and an excited electron. We first solved the electronic part of the problem on the level of the Bethe-Salpeter equation and then dressed the resulting 2-particle excitonic quasiparticle with the exciton-phonon interactions using the cumulant ansatz. The viability of this methodology was tested by calculating the N K-edge XAS of the N2 molecule and the O K-edge of acetone. The resulting vibronic spectra agreed favorably with experimental results.Finally, we construct a hybrid formulation of the RIXS cross section that preserves explicit summation over a small number of final states, but replaces the summation over intermediate states, which might be enormously expensive, with a Green's function. We develop an expansion of the Green's function and derive both analytically exact (in the no-recoil limit) and approximate solutions. The formalism was again tested on the O K-edge of acetone and agrees well with the experiment. To provide an outlook towards future work, we discuss application of the developed formalism to crystalline materials.

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No. of bitstreams: 2 Tese - Rafael Carvalho Couto - 2016.pdf: 25101346 bytes, checksum: 808fe5c57059f14ca58d1ef0dbe03f00 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Previous issue date: 2016-06-08 / Esta tese dedicada a estudos tericos e experimentais de espalhamento ressonante inelstico de raios-X (Resonant inelastic X-ray scattering - RIXS) de molculas de monxido de carbono e gua em fase gasosa. Usando estado da arte clculos ab initio de estrutura eletrnica e formalismo de pacotes de onda dependente do tempo, uma anlise completa dos espectros RIXS experimental dos dois sistemas moleculares foi realizada. Na anlise do CO RIXS, fomos capazes de reproduzir o experimento RIXS com excelente preciso, permitindo uma descrio completa dos espectros experimentais. Interferncia entre diferentes canais RIXS correspondentes disperso via orbitais moleculares ortogonais no estado excitado do CO descrito. Com a ajuda do espectro de alta resoluo e simulaes ab initio, mostramos a quebra da aproximao de Born-Oppenheimer na regio onde estados nais de Rydberg acoplam com o estado nal de valncia. Explicamos a formao de uma caracterstica espectral, que foi atribuda a um nico estado em estudos anteriores. Alm disso, atravs da combinao experimento-teoria, aprimoramos o mnimo do potencial do estado excitado de valncia E 1 , juntamente com o constante de acoplamento entre o estado de valncia e dois estados de Rydberg. A m de estudar a gua, desenvolvemos uma nova abordagem terica para descrever molculas triatmicas atravs do formalismo de propagao de pacote de ondas, que reproduz com grande preciso a estrutura vibracional os espectros experimentais RIXS de alta resoluo, permitindo obter importantes concluses. Ns demonstramos que, devido ao acoplamento dos modos vibracionais e anarmonicidade do potencial no estado fundamental e das superfcies de energia potencial do estados excitados, diferentes estados excitados de camada interna em RIXS podem ser usados como portas para sondar diferentes dinmicas de vibrao e para mapear o potencial do estado fundamental usando modos normais de vibrao molecular. O ajuste dos raios-X acima da ressonncia de absoro permite extrair informaes adicionais sobre o potencial do estado fundamental, devido alta excitao vibracional. Substituio isotpica investigada por meio de simulaes tericas e as importantes caractersticas da dinmica nuclear so discutidas, especialmente para o estado excitado de camada interna dissociativo, onde um chamado pico \atmico " formado. Este recurso crucial para explicar a dinmica nuclear em RIXS da gua. Mostramos o forte potencial de experimentos RIXS de alta resoluo combinados IV com simulaes tericas de alto nvel para estudos avanados de estados moleculares altamente excitados, bem como superfcies de energia potencial do estado fundamental, send utilizada como uma tcnica auxiliar para espectroscopia ptica e infra vermelho. / This Thesis is devoted to theoretical and experimental studies of resonant inelastic X-ray scattering (RIXS) of gas-phase carbon monoxide and water molecules. Using state-of-the-art ab initio electronic structure calculations and a time-dependent wave packet formalism, we make a complete analysis of the experimental RIXS spectra of the two molecular systems. In the CO RIXS analysis, we are able to reproduce the RIXS experiment with an excellent accuracy, allowing for a complete description of all experimental features. Interference between di erent RIXS channels corresponding to the scattering via orthogonal molecular orbitals in the core-excited state of CO is described. With the help of the high-resolution spectrum and extensive ab initio simulations we show the complete breakdown of the Born- Oppenheimer approximation in the region where forbidden nal Rydberg states are mixed with a valence allowed nal state. Here we explain the formation of a spectral feature which was attributed to a single state in previous studies. Moreover, through an experimentaltheoretical combination, we improve the minimum of the valence E 1 excited state, along with the coupling constant between the valence and two Rydberg states. In order to study the water system, we developed a new theoretical approach to describe triatomic molecules through the wave packet propagation formalism, which reproduces with high accuracy the vibrational structure of the high-resolution experimental quasi-elastic RIXS spectra, allowing to draw several important conclusions. We demonstrate that due to the vibrational mode coupling and anharmonicity of the ground and core-excited potential energy surfaces, di erent core-excited states in RIXS can be used as gates to probe di erent vibrational dynamics and to map the ground state potential using molecular vibrational normal modes. Tuning the X-rays above the absorption resonance allows to extract additional information about the ground state potential, due to high vibrational excitation. Isotopic substitution is investigated by theoretical simulations and important dynamical features are discussed, especially for the dissociative core-excited state, where a so-called \atomic" peak is formed. This feature is crucial to explain the nuclear dynamics in RIXS from water. We show the strong potential of high-resolution RIXS experiments combined with high-level theoretical simulations for advanced studies of highly excited molecular states, as well as of ground state potential energy surfaces, as an auxiliary technique to optical and IR spectroscopy.

Les composés intermétalliques à base de cérium présentent à basse température des propriétés magnétiques, électroniques et thermodynamiques tout à fait fascinantes. Deux interactions majeures entre la bande de conduction et les électrons localisés de la couche 4f entrent en compétition et régissent la physique de ces composés: l'effet Kondo tend à écranter les moments magnétiques 4f alors que les interactions RKKY favorisent plutôt des moments localisés et ordonnés. Un diagramme de phase complexe naît de cette compétition et le comprendre exige une caractérisation complète des niveaux 4f. La dégénérescence des multiplets liés au couplage spin-orbit est levée par les effets de champ cristallin, et les écarts entre les niveaux d'énergie qui en résultent ainsi que les symétries des états fondamentaux et excités doivent être déterminés expérimentalement. Les effets de champ cristallin dans les lanthanides peuvent être sondés par diverses techniques expérimentales dont la susceptibilité magnétique, la diffusion inélastique de neutrons, la spectroscopie par absorption de rayons X ou de photoémission. Cependant, chacune de ces techniques présente des limitations propres qui empêchent une caractérisation précise de ces effets. Nous présentons dans cette thèse les capacités de la technique de diffusion résonante inélastique des rayons X mous (soft-RIXS) à sonder les effets de champ cristallin dans les composés à base de cérium. Les dernières avancées en instrumentation ont permis de repousser les limites de la résolution en énergie jusqu'à 30 meV au seuil M5 du Cérium, suffisant pour séparer les transitions électroniques des plus bas des niveaux 4f. Le composé CeRh2Si2 a été étudié à titre d'exemple. Les énergies des excitations de couplage spin-orbit et de champ cristallin observées expérimentalement fournissent une mesure directe des écarts des niveaux 4f, tandis que la comparaison entre les spectres expérimentaux et des calculs basés sur le multiplet complet dans l'approximation à un ion permet d'identifier sans ambiguïté la symétrie des états de plus basse énergie du champ cristallin. L'orientation des fonctions d'ondes dans les plans ab du cristal peut aussi être déterminée de manière très simple en mesurant la polarisation des photons diffusés de manière inélastique. L'interaction avec les états itinérants peut introduire une variation de l'énergie des niveaux 4f en fonction du vecteur de diffusion q, qui se traduit par des signatures observables dans les spectres de diffusion inélastique résonante de rayons X. Un modèle simple est proposé pour expliquer la variation observée en fonction de q, mais des calculs théoriques plus poussés apparaissent nécessaires pour rendre compte de manière plus exacte de ces variations. L'étude des composés CeCu2Si2 et CeCo2Ge2 montre que la diffusion inélastique des rayons X peut sonder des effets de champ cristallin bien plus faibles ou bien plus importants que ceux observés dans CeRh2Si2. Dans le futur, une meilleure résolution en énergie et des modélisations plus poussées devraient encore améliorer la précision des études des effets de champ cristallin dans les composés de Cérium et autres lanthanides par diffusion inélastique de rayons X. / Cerium intermetallic compounds exhibit fascinating magnetic, electronic and thermodynamic properties at low temperatures. Their physics is governed by two competing interactions between the conduction band and localised 4f electrons: the Kondo effect tends to screen the 4f magnetic moments while RKKY exchange favours ordered local moments. From this competition, a complex phase diagram arises and its understanding requires a full characterisation the 4f levels. The crystal electric field (CEF) effects lift the degeneracy of the spin-orbit multiplets and the resulting energy splittings (tens of meV) as well as the symmetry of the ground and excited states must be determined experimentally. CEF effects in the lanthanides can be studied with a number of experimental probes including magnetic susceptibility, inelastic neutron scattering, X-ray absorption spectroscopy or photoemission spectroscopy. However, each of these techniques has limitations which can prevent an accurate characterisation. This thesis discusses the potential of Resonant Inelastic soft X-ray Scattering (soft-RIXS) to become a new probe of the CEF in cerium compounds. The latest advances in instrumentation have pushed the energy resolution in soft RIXS down to 30 meV at the Ce M5 edge, good enough to resolve the electronic transitions between the lowest lying 4f levels. This is demonstrated using the example of CeRh2Si2 spectra. The observed energies of the spin-orbit and CEF excitations provide a direct measurement of the 4f splittings and the comparison of the experimental spectra with full-multiplet single-ion calculations unambiguously identifies the symmetry of the lowest CEF levels. The orientation of the wave-functions in the crystal ab plane can also be determined in a straightforward manner by measuring the polarisation of the inelastically scattered photons. The interaction with itinerant states can lead to a momentum dependence of the 4f levels' energy and this leaves signatures in the excitation spectra that can be observed in RIXS. A simple model is proposed to explain the observed q dependence but further theoretical work is needed to properly account for the momentum dependence of the 4f excitations. Using the examples of CeCu2Si2 and CeCo2Ge2 it is demonstrated that RIXS can probe CEF splittings that are significantly smaller or much larger than those in CeRh2Si2. With better energy resolution and more advanced modelling RIXS studies of the CEF in cerium and other lanthanide ions will become even more accurate in the future.

L’objectif de cette thèse est d’appliquer en mode in situ les spectroscopies de diffusion inélastique résonante de rayons X (RIXS 1s2p) et d’absorption X hautement résolue en énergie par détection partielle de fluorescence (HERPFD-XAS) à l’étude de la sulfuration des catalyseurs HDS. En effet, seule une compréhension fine de la structure et de la nature de la phase active des catalyseurs à l’échelle moléculaire permet d’améliorer leurs performances dans les procédés catalytiques. Après avoir présenté un état de l’art sur la caractérisation de la phase active des catalyseurs d’hydrotraitement, nous avons montré le potentiel de ces méthodes spectroscopiques à travers des composés de référence à base de cobalt, où celui-ci se trouve dans des symétries différentes avec des degrés d’oxydation différents. Ces résultats révèlent que ces techniques spectroscopiques sont sensibles à la coordinence locale du cobalt ainsi qu’à sa structure électronique. Deux échantillons ont été étudiés pour la sulfuration in situ : cobalt supporté sur alumine et cobalt-molybdène supporté sur alumine. Les résultats par spectroscopie HERPFD-XAS au seuil K du cobalt montrent pour les deux échantillons la formation de CoS2 avant les phases CoMoS et Co9S8, ainsi qu’une contribution constante de CoAl2O4 (phase réfractaire à la sulfuration). L’analyse par la spectroscopie RIXS 1s2p a révélé que le cobalt, présent dans la phase active des catalyseurs, présente un caractère métallique, ce qui a permis de réinterpréter un certain nombre de données présentes dans la littérature. / The aim of this PhD dissertation is to apply resonant inelastic X-Ray scattering (RIXS 1s2p) and high energy resolution partial fluorescence detection X-ray absorption spectroscopy (HERPFD-XAS) to the study in situ sulfurization of HDS catalysts. Indeed, only a detailed understanding of the structure and nature of the active phase of the catalysts at the molecular level makes it possible to improve their catalytic performance. After presenting a state of the art on the characterization of the active phase of hydrotreating catalysts, we have shown the potential of these spectroscopic methods through the study of cobalt-based reference compounds where cobalt is present in different symmetries with different oxidation states. These results reveal that these spectroscopic techniques are sensitive to the local coordination of cobalt, as well as to its electronic structure. Two samples were studied for in situ sulfurization: cobalt supported on alumina and cobalt-molybdenum supported on alumina. The HERPFD-XAS spectroscopy results at cobalt K-edge show CoS2 formation before the CoMoS and Co9S8 phases for both samples, as well as the constant contribution of CoAl2O4 (phase that does not undergo sulfurization). Analysis by 1s2p RIXS spectroscopy revealed that the cobalt, present in the active phase of the both catalyst, has a metallic character, which allowed to interpret in a finer way data published in the literature.

Dehydrogenation is the catalytic process of removing hydrogen from a saturated hydrocarbon to produce an olefin. Olefins are important feedstocks for the petrochemical industry and can potentially be used to produce fuels through oligomerization. Alloys containing an active metal such as platinum and palladium and a non-catalytic metal offer improved selectivity towards the olefin. This body of work seeks to further the understanding of how heteroatomic bonds in alloys change the rate and selectivity of alloy catalysts used for dehydrogenation.In the first study, a series of Pt-V bimetallic catalysts are synthesized, which are highly selective propane dehydrogenation catalysts. The bimetallic nature of the nanoparticles was verified by in-situX-ray Absorption Spectroscopy(XAS)and the formation of the Pt3V alloy phase was shown by in-situ synchrotronX-ray Diffraction(XRD). A reduction-oxidation differenceXASmethod was used to examine the surface stoichiometry and found that a shell layer of the alloy phase forms when the particles are platinum rich. Electronic modification of Pt was studied by Pt L3edgeX-ray Absorption Near Edge Structure(XANES),X-ray Photoelectron Spectroscopy(XPS), Resonant Inelastic X-ray Scattering (RIXS)andDensity FunctionalTheory(DFT). The spectral changes observed were shown to be due to changes in the energy of the filled and unfilled 5d density of states, and not due to electron transfer. The electronic modifications cause a weakening of adsorbate binding and destabilization of deeply dehydrogenated hydrocarbons, which contributes to the dehydrogenation selectivity.In the second study, alloys between palladium and five different promoters were synthesized and tested as propane dehydrogenation catalysts.The structure ofthe alloy catalysts was characterized by in-situ XAS and in-situ synchrotron XRD.Zinc and indium form alloy structures with site isolated palladium, while gallium, iron and manganese do not. All of the alloys have improved propane dehydrogenation selectivity compared to monometallic palladium. The propylene production turnover rate of the alloys increased by almost an order of magnitude compared to monometallic Pd, but among the alloys the turnover ratesonly varied by a factor of two despite the different structures and electronic modifications inherent to each phase. The site isolated alloys had higher propylene selectivity than those that were not site isolated. The site isolated alloys showed strongerelectronic modification: both in binding strengths and in Pd projected Density of States (pDOS)by DFT than did the non-site isolated alloys. The commonly used computational selectivity descriptor for dehydrogenation, which is the difference in energy between alkene desorption and alkene C-H bond activation energy correctly predicts that the site isolated alloys will have high selectivity but shows weaker trends for alloys without site isolation. A modified selectivity descriptor, involving the C-C bond breaking barrier in the adsorbed alkyne more accurately reflects the high selectivity of the non-site isolated alloys.In a third study,RIXS and XPSare used to examine trends in the electronic modification of platinum alloys with transition metal and post transition metal promoters. All alloys show an increase in the energy transfer maximum, showing that alloying modifies energy the filled and unfilled density of states. The increase in the energy transfer maximum in platinum alloys with 3d metals islargerfor early transition metals, which by DFT show larger shifts in the d-band center. The post transition elements showsignificantlylarger shifts than to the transition elements, partially due to the lack of orbital overlap between the valence p orbitals and Pt 5d orbitals. Platinum has the same number of valence d electrons regardless of promoter or structure, and redistribution of the 5d electron energy brought about by heteroatomic bonds leads to the observed electronic modifications. The positive binding energy shifts measured by XPS reflect these energy changes, which occur due to changes in the Fermi energy of the alloy, initial state effects and intra and extra atomic relaxation (final state effects). The calculated initial state effect shift is correlated to descriptors of the valence d band, such as the d band center.

Cette thèse s'inscrit dans le cadre de l'étude des excitations électroniques dans des matériaux fortement corrélés, ici les monoxydes de métaux de transition CoO, NiO et CuO, le cuprate La2CuO4 et le nickelate La2NiO4. Nous avons utilisé la diffusion inélastique résonante des rayons X (RIXS) pour étudier la dynamique des excitations de basse énergie (quelques eV) dans ces isolants de transfert de charge. Cette étude a été possible grâce à un développement instrumental permettant une amélioration de la résolution expérimentale d'un facteur 3 pour atteindre 300 meV. <br /><br /> Dans les monoxydes, nous avons étudié les excitations localisées du champ cristallin. Dans La2CuO4, et La2NiO4, nous avons plus particulièrement étudié les excitations de transfert de charge. Un exciton (état lié électron-trou) a été observé dans les deux matériaux avec cependant une dynamique différente. Dans La2CuO4, l'exciton a un comportement dispersif alors que dans La2NiO4, il est localisé par les corrélations antiferromagnétiques. Un modèle invoquant la formation d'un singulet de Zhang et Rice dans le cuprate a été proposé pour expliquer cette dynamique excitonique dans le réseau antiferromagnétique sous-jacent.