An atomic level understanding of the structure and stability of model catalysts is essential for surface science studies in heterogeneous catalysis. Scanning tunneling microscopy (STM) can operate both in UHV and under realistic pressure conditions with a wide temperature span while providing atomic resolution images. Taking advantage of the ability of STM, our research focuses on 1) investigating the structure and stability of supported Au catalysts, especially under CO oxidation conditions, and 2) synthesizing and characterizing a series of alloy model catalysts for future model catalytic studies. In our study, Au clusters supported on TiO2(110) have been used to model supported Au catalysts. Our STM studies in UHV reveal surface structures of TiO2(110) and show undercoordinated Ti cations play a critical role in the nucleation and stabilization of Au clusters on TiO2(110). Exposing the TiO2(110) surface to water vapor causes the formation of surface hydroxyl groups and subsequently alters the growth kinetics of Au clusters on TiO2(110). STM studies on Au/TiO2(110) during CO oxidation demonstrate the real surface of a working catalyst. Au clusters supported on TiO2(110) sinter rapidly during CO oxidation, but are mostly stable in the single component reactant gas, either CO or O2. The sintering kinetics of supported Au clusters has been measured during CO oxidation and gives an activation energy, which supports the mechanism of CO oxidation induced sintering. CO oxidation was also found to accelerate the surface diffusion of Rh(110). Our results show a direct correlation between the reaction rate of CO oxidation and the diffusion rate of surface metal atoms. Synthesis of alloy model catalysts have also been attempted in our study with their structures successfully characterized. Planar Au-Pd alloy films has been prepared on a Rh(100) surface with surface Au and Pd atoms distinguished by STM. The growth of Au-Ag alloy clusters have been studied by in-situ STM on a cluster-to-cluster basis. Moreover, the atomic structure of a solution-prepared Ru3Sn3 cluster has been resolved on an ultra-thin silica film surface. The atomic structure and adsorption sites of the ultrathin silica film have also been well characterized in our study.

In dieser Arbeit wurde ein Modell-Katalysator-Ansatz angewandt um Mischoxide mit Molybdän und Titan zu untersuchen. Die Schichten wurden auf TiO2(110) Einkristallen durch Verdampfen der Metalle in einer O2 Atmosphäre und in UHV und Nachbehandlung der Filme vorbereitet. Verschiedene Präparationen wurden in der Studie untersucht und diese werden in sechs Kategorien dargestellt. Wenn Mo und Ti in O2 gemeinsam aufgedampft wurden, wurde das meiste Mo an der Oberfläche abgeschieden mit einer nur geringen Mo-Konzentration in tieferen Schichten. Eine Mischung von Mo in TiO2 war sehr begrenzt, und die stimmt mit dem Phasendiagramm MoO2 und TiO2 überein. Mo6+ und Mo4+ sind die dominierenden Oxidationsstufen in den meisten der Schichten, wobei Mo6+ stets näher an der Oberfläche war als Mo4+. Schichten, in denen Mo vollständig in TiO2 gelöst ist, konnten durch Abscheidung von Metallen in UHV und Post-Oxidation der Filme erstellt werden. Im Inneren des TiO2 Gitters hat Mo die Oxidationsstufe 4+. Aufdampfen von Mo in O2 bei Raumtemperatur und anschließendes Tempern in UHV führte zur Bildung zweier Arten von Merkmalen in den STM-Bildern. Diese waren im UHV stabil bis mindestens 1000 K. Die Schichten mit hoher Mo-Konzentrationen erschienen uneinheitlich in den STM-Bildern aber sie zeigte das TiO2(110)-(1x1) LEED-Muster. Der Anstieg in der Mo-Konzentration führte zur Blockierung der Überbrückung Sauerstoffleerstellen (BOVs), was durch STM und Wasser-TPD nachgewiesen wurde. Die Reaktivitäten der Schichten wurden mit Methanol- und Ethanol-TPD getestet. Eine unerwartete Formaldehyd/Methanbildung bei hohen Temperaturen (~650 K) wurde bei der Methanol-TPD von reinem TiO2(110) beobachtet und mit BOVs in Verbindung gebracht. Der Anstieg der Mo-Konzentration unterdrückte diesen Effekt sowie die Ethylenbildung (~600 K) beim Durchführen von Ethanol-TPD. Sowohl in Ethanol als auch Methanol-TPD wurden neue Reaktionswege zu Ethylen und Methan-Bildung bei ~500 K beobachtet. / In this work, a model-catalyst approach has been taken to study oxide mixtures containing molybdenum and titanium. The layers were prepared on TiO2(110) single crystals by evaporating the metals in an O2 atmosphere or in UHV and post treating the deposited material. Different preparation procedures were employed in the study and these are presented in six categories. When Mo and Ti were co-deposited in O2, most of the molybdenum stayed at the surface with only a small Mo concentration in deeper layers. Mixing of Mo into TiO2 was very limited, consistent with the phase diagram of MoO2 and TiO2. Mo6+ and Mo4+ were the dominant oxidation states in most of the layers and Mo6+ was always nearer to the surface than Mo4+. Layers where Mo was completely mixed into TiO2 could be prepared by depositing metals in UHV and post-oxidizing the deposited material. Inside the TiO2 lattice, Mo had an oxidation state of 4+. Depositing Mo in O2 at room temperature and post annealing in UHV led to the formation of two types of features in the STM images. These features were stable in UHV up to at least 1000 K. The layers with high Mo concentrations appeared patchy in the STM images but they still exhibited the TiO2(110)-(1x1) LEED pattern. The increase in Mo concentration led to the blocking of the bridging oxygen vacancies (BOVs) as evidenced by STM and water TPD. The reactivities of the layers were tested by methanol and ethanol TPD. An unprecedented high temperature (~650 K) formaldehyde/methane formation channel was observed in the methanol TPD of clean TiO2(110) and was associated with BOVs. The increase in the Mo concentration led to the vanishing of this channel as well as the ethylene formation channel (~600 K) in the case of ethanol TPD. In both ethanol and methanol TPD, new reaction channels towards ethylene and methane formation at ~500 K appeared.

Nous avons étudié la synthèse, la structure et l'évolution sous gaz de nanoparticules (NPs) bimétalliques Au-Cu sur la surface (110) du rutile TiO2. Les NPs ont été obtenues par évaporation sous UHV. Pendant la croissance, la nature des sites de nucléation et l'évolution des densités et distributions de taille des NPs ont été suivies par microscopie à effet tunnel (STM), tandis que la structure et les relations d'épitaxies avec le substrat ont été suivies par diffraction de rayons X en incidence rasante (GIXD). Ces caractéristiques ont été mesurées sous oxygène, sous monoxyde de carbone ou sous mélange CO+O2 jusqu'à des pressions de 10-5 mbar.Nous montrons par STM que les défauts de la surface de type cluster de TiOx sont des sites de nucléation préférentiels pour les NPs. Par ailleurs, des NPs Au-Cu sont obtenues lors de l'évaporation séquentielle d'Au suivi de Cu. Les résultats de GIXD montrent que le Cu diffuse dans le volume des NPs d'Au initiales et forme une solution solide cfc. Les relations d'épitaxies entre les NPs alliées et le substrat indiquent que l'axe <110> des NPs est parallèle à l'axe [001] du substrat, mais que différentes orientations du plan interfacial sont possibles.En fonction de leur composition, la morphologie et à la structure des NPs sont modifiées sous faible pression d'oxygène. Tandis que les NPs de Cu pur disparaissent progressivement sous gaz, une faible proportion d'Au (de l'ordre de 5%) permet de les stabiliser. Cependant, les mesures de diffraction montrent que le Cu migre à la surface des NPs. Un recuit des NPs sous UHV permet de retrouver leur structure initiale. / We have studied the synthesis, the structure and the evolution in reactive environment of Au-Cu bimetallic nanoparticles (NPs) deposited under UHV on a (110) surface of rutile TiO2. During the growth, the type of the nucleation sites and the evolution of both density and size distribution of the NPs were followed with Scanning Tunneling Microscopy (STM), whereas the structure and the epitaxial relations with the substrate were determined using Grazing Incidence X-ray Diffraction (GIXD). These features were measured under oxygen, carbon monoxide and a mix of CO+O2 for pressures bellow 10-5 mbar.We show trough STM imaging that TiOx type of surface defects are a preferential nucleation site for NPs. Moreover GIXD results show that the Cu is able to diffuse inside the initial Au NPs to form a solid solution of fcc structure. The epitaxial relations between alloyed NPs and substrate indicate that the <110> axis of the NPs is parallel to the [001] axis of the substrate, but several orientations for the interfacial plan are possible.According to their composition, the structure and the morphology of the NPs can be modified in the presence of a low pressure of oxygen. Whereas Cu NPs progressively disappear in reactive environment, a small proportion of Au (around 5%) is enough to stabilize the morphology of the NPs. However, diffraction measurements show that in these conditions, the Cu segregates to the surface of the NP. A thermal annealing of the NPs under UHV allow to recover their initial structure.

A commercial non-contact atomic force microscope was improved to achieve utmost resolution on a routine basis. This system was used to study the (110) surface of rutile titanium dioxide. The focus was on understanding contrast formation in terms of tip-sample interaction mechanisms. Moreover, chromium and antimony-doped titanium dioxide was investigated. The implications of transition-metal doping on the surface structure of this highly interesting photocatalyst was studied at the atomic scale.

Recent advances in nanoscience and technology have provided the scientific community with new exciting opportunities to rationally design and fabricate materials at the nanometer scale with drastically different properties as compared to their bulk counterparts. A variety of challenges related to nanoparticle (NP) synthesis and materials characterization have been tackled , allowing us to make more homogenous, well defined, size- and shape-selected NPs, and to probe deeper and more comprehensively into their distinct properties. In this dissertation, a variety of phenomena relevant to nanosized materials are investigated, including the thermal stability of NPs and coarsening phenomena in different environments, the experimental determination of NP shapes, gaining insight into NP-support interactions, epitaxial relationships, and unusual thermodynamic and electronic properties of NPs, including the effect of adsorbates on the electron density of states of small clusters, and the chemical, and structural evolution of NPs under reaction conditions. In chapter 2, a general description of different characterization tools that are used in this dissertation is provided. In chapter 3, the details of two different methods used for NP synthesis, namely inverse micelle encapsulation and physical vapor deposition (PVD) are described. Chapter 4 describes the thermal stability and coarsening behavior of Pt NPs supported on TiO2(110) and γ-Al2O3 as a function of the synthesis method, support pretreatment, and annealing environment. For the Pt/TiO2(110) system, micellesynthesized NPs showed remarkable stability against coarsening for annealing temperatures up to 1060°C in vacuum, in contrast to PVD-grown NPs. When comparing v different annealing environments (H2, O2, H2O), Pt NPs on γ-Al2O3 annealed in O2 were found to be the least affected by coarsening, followed by those heated in H2O vapor. The largest NP growth was observed for the sample annealed in H2. The role of the PtOx species formed under oxidizing conditions will be discussed. In chapter 5, the shape of Pt and Au NPs and their epitaxial relationship with the TiO2(110) support was extracted from scanning tunneling microscopy (STM) measurements. Three main categories of NP shapes were identified, and through shape modeling, the contribution of facets with different orientations was obtained as a function of the number of atoms in each NP. It was also shown that the micellesynthesized Pt and Au NPs have an epitaxial relationship with the support, which is evident from the fact that they always have one symmetry axis parallel to TiO2(110) atomic rows in [001] directions. Chapter 6 describes how the presence of NPs on TiO2(110) surface affects its reconstruction upon high temperature annealing in vacuum. In contrast to NP-free TiO2(110) substrates, long and narrow TiO2 stripes are observed for Pt NP-decorated surfaces. This phenomenon is explained based on the stabilization of TiO2, induced by Pt NPs, which hinders the desorption of oxygen atoms in TiO2 to vacuum. In chapter 7, a systematic investigation of the thermodynamic properties of γ- Al2O3-supported Pt NPs and their evolution with decreasing NP size is presented. A combination of in situ extended x-ray absorption fine structure spectroscopy (EXAFS), ex situ transmission electron microscopy (TEM) measurements, and NP shape modeling is used to obtain the NPs shape, thermal expansion coefficient, and Debye vi temperature. The unusual thermodynamic behavior of these NPs such as their negative thermal expansion and enhanced Debye temperature are discussed in detail. Chapter 8 presents an investigation of the electronic properties of size-controlled γ-Al2O3-supported Pt NPs and their evolution with decreasing NP size and adsorbate (H2) coverage. The hydrogen coverage of Pt NPs at different temperatures was estimated based on XANES data and was found to be influenced by the NP size, and shape. In addition, correlations between the shift in the center of the unoccupied d-band density of states (theory) and energy shifts of the XANES spectra (experiment) upon hydrogen chemisorption as well as upon modification of the NP structure were established. Chapter 9 is dedicated to an operando study, describing the evolution of the structure and oxidation state of ZrO2-supported Pd nanocatalysts during the in-situ selective reduction of NO in H2 via EXAFS and XANES measurements.