La r eduction (\scaling") continue des dimensions des transistors MOS- FET nous a conduits a l' ere de la nano electronique. Le transistor a ef- fet de champ multi-grilles (MultiGate FET, MuGFET) avec l'architecture \nano l canal" est consid er e comme un candidat possible pour le scaling des MOSFET jusqu' a la n de la roadmap. Parall element au scaling des CMOS classiques ou scaling suivant la loi de Moore, de nombreuses propo- sitions de nouveaux dispositifs, exploitant des ph enom enes nanom etriques, ont et e faites. Ainsi, le transistor mono electronique (SET), utilisant le ph enom ene de \blocage de Coulomb", et le transistor a atome unique (SAT), en tant que transistors de dimensions ultimes, sont les premiers disposi- tifs nano electroniques visant de nouvelles applications comme la logique a valeurs multiples ou l'informatique quantique. Bien que le SET a et e ini- tialement propos e comme un substitut au CMOS (\Au-del a du dispositif CMOS"), il est maintenant largement consid er e comme un compl ement a la technologie CMOS permettant de nouveaux circuits fonctionnels. Toutefois, la faible temp erature de fonctionnement et la fabrication incompatible avec le proc ed e CMOS ont et e des contraintes majeures pour l'int egration SET avec la technologie FET industrielle. Cette th ese r epond a ce probl eme en combinant les technologies CMOS de dimensions r eduites, SET et SAT par le biais d'un sch ema d'int egration unique a n de fabriquer des transistors \Trigate" nano l. Dans ce travail, pour la premi ere fois, un SET fonction- nant a temp erature ambiante et fabriqu es a partir de technologies CMOS SOI a l' etat de l'art (incluant high-k/grille m etallique) est d emontr e. Le fonctionnement a temp erature ambiante du SET n ecessite une le (ou canal) de dimensions inf erieures a 5 nm. Ce r esultat est obtenu grce a la r eduction du canal nano l "trigate" a environ 5 nm de largeur. Une etude plus ap- profondie des m ecanismes de transport mis en jeu dans le dispositif est r ealis ee au moyen de mesures cryog eniques de conductance. Des simula- tions NEGF tridimensionnelles sont egalement utilis ees pour optimiser la conception du SET. De plus, la coint egration sur la m^eme puce de MOS- FET FDSOI et SET est r ealis ee. Des circuits hybrides SET-FET fonction- nant a temp erature ambiante et permettant l'ampli cation du courant SET jusque dans la gamme des milliamp eres (appel e \dispositif SETMOS" dans la litt erature) sont d emontr es de m^eme que de la r esistance di erentielle n egative (NDR) et de la logique a valeurs multiples. Parall element, sur la m^eme technologie, un transistor a atome unique fonc- tionnant a temp erature cryog enique est egalement d emontr e. Ceci est obtenu par la r eduction de la longueur de canal MOSFET a environ 10 nm, si bien qu'il ne comporte plus qu'un seul atome de dopant dans le canal (dif- fus ee a partir de la source ou de drain). A basse temp erature, le trans- port d' electrons a travers l' etat d' energie de ce dopant unique est etudi e. Ces dispositifs fonctionnent egalement comme MOSFET a temp erature am- biante. Par cons equent, une nouvelle m ethode d'analyse est d evelopp ee en corr elation avec des caract eristiques a 300K et des mesures cryog eniques pour comprendre l'impact du dopant unique sur les caracteristiques du MOSFET a temp erature ambiante.

Single atom catalysts are prospective class of materials, which holds promises to reach the ultimate limit of improvement in catalyst performance, selectivity, lifetime and cost reduction. The ability to efficiently capture the adsorbates at the active sites is the key prerequisite for catalytic transformation to the products. In this respect, our experimental study aims to describe the interaction of gas molecules (H2O, CO, O2 and NO) with single metal atoms (Rh, Ir, Cu) on the magnetite surface employing scanning tunneling microscopy.

Title: Role of surface defects in ceria-based catalysis Author: Andrii Tovt Department: Department of Surface and Plasma Science Supervisor of the doctoral thesis: doc. Mgr. Josef Mysliveček Ph.D., Department of Surface and Plasma Science Abstract: This work concentrates on the analysis of fundamental physicochemical properties of Pt-CeOx, single-atom Pt1 /CeOx, and inverse CeOx/Cu(111) catalysts. Preparation method for stabilized atomically-dispersed Pt2+ ions on ceria was developed and adsorption sites for Pt ions were thoroughly studied using advanced surface science techniques supported by theoretical methods. The mechanism of Pt2+ stabilization on ceria steps was revealed and the step capacity towards Pt2+ ions was estimated. Also, the preparation method for well-ordered cerium oxide ultrathin films with different stoichiometry and ordering of surface oxygen vacancies was developed, and the Ceria/Cu(111) interaction was investigated. Key words: heterogeneous catalysis, model systems, single-atom catalysis, platinum ions, cerium oxide.

<div>Thermochemical processing using fast-pyrolysis technology has been used to upgrade feedstocks like biomass and natural gas and more recently studied for plastic recycling. This work aims to improve the selectivity to desired products from a pyrolysis process through better catalysts and reactor design.</div><div>Fast-pyrolysis of biomass to fuels is considered a promising technology due to the higher yields to liquid fuel products. However, the process suffers from low carbon efficiency to hydrocarbon products due to carbon losses to biochar, accounting for 25-40 wt.% of the product stream depending on the biomass type. Using a combination of inorganic free-model compounds, biomass pretreatments and mass spectrometric analyses coupled with lab-scale reactor experiments, the char contribution from the lignocellulosic components (cellulose, hemicellulose, and lignin) and mineral content was investigated. The lignocellulosic components were found to follow the order: Lignin > Hemicellulose > Cellulose. Addition of inorganic salts (K, Na and Ca) to cellobiose, a model compound for cellulose, was found to catalyze additional dehydration reactions on primary pyrolysis products (e.g., levoglucosan) to yield secondary products (e.g., 5-HMF), and produce more char. This knowledge of char formation contributors can enable optimization of the bio-refining process sequencing using process system engineering tools and thus achieve higher carbon efficiency for biomass conversion.</div><div>While biomass has been viewed as a future energy source, there is a need for a transition fuel with the lowest possible greenhouse gas (GHG) footprint. Shale gas, consisting primarily of methane, is a potential candidate due to its large availability and high hydrogen to carbon ratio. Recently, single-atom catalysts have been studied as stable and non-coking catalysts for the non-oxidative coupling of methane (NOCM) to higher hydrocarbons (like ethylene). However, lack of post reaction catalyst characterization and rigorous kinetic testing have raised questions on the stability of these materials. This work combines homogenous (Chemkin simulations, gas phase kinetics) and heterogeneous reaction kinetic studies (reaction orders, steady state kinetics), coupled with microscopy (Scanning and Transmission Electron Microscopy (SEM, TEM)) and surface characterization tools (BET, TGA, Raman spectroscopy, CO-IR spectroscopy) to understand the role of the solid materials during NOCM. Post reaction catalyst characterization using transmission electron microscopy (TEM) analysis on the spent samples (CH4 treated at 975 deg C for 3 hours) reveals that the materials containing Pt single atoms (SA) and Pt nanoparticles (NP) are found to sinter to particles approximately 5-7 nm in size. Ethylene hydrogenation experiments, a kinetic probe for surface Pt, shows initial ethane formation rates that are four orders of magnitude lower on the isolated Pt+2 sites, found on Pt SAs, when compared to the rates obtained if all the surface Pt were assumed to be metallic. These results suggest that single atoms are not the active sites. However, under same reaction conditions (50 mL min-1 CH4 flow and 975 deg C), the ethylene formation rates (in mol h-1) on the solid materials are 2-7 times higher than the empty tube rates, indicating that the surface plays a role during NOCM. Addition of incremental amounts of the solid material increases methane conversion, extrapolating to the bare tube conversion at zero loading. This indicates that the solid materials improve the NOCM performance.</div><div>Experiments with pure methane feeds indicate that the solid materials are found to deactivate due to coking on the surface, evidenced by the coke buildup observed using thermogravimetric analysis (TGA) and the initial time-on-stream kinetic results showing rapid methane deactivation. Raman spectroscopy on the spent catalysts indicate at the development of a similar graphite-like surface intermediate under steady state conditions on all the materials. When compared under the same reaction conditions (975 deg C, 60 mL min-1 Pure CH4 with 10% UHP N2 feed, space velocity = 39.6 L h-1 gcat-1), these coked surfaces show a linear dependence for the ethylene formation rate (in mol h-1 gcat-1) with the spent surface area of the material (in m2 gcat-1). This observation is irrespective of the type of the material studied (alpha Al2O3, Davisil SiO2, 1 wt.% Pt/CeO2, Graphene, Graphite, etc.). In conclusion, these results prove that the spent surface area is critical for NOCM.</div><div>Similar experimental setup was used to study the dehydrogenation of methane, ethane, and propane mixture in the gas phase. Initial experiments at 1 bar pressure and reaction temperatures ranging from 650-850 deg C revealed that ethylene and hydrogen are the main gas phase products, with methane acting as a diluting agent under these reaction conditions. These results could enable direct processing of the shale gas without the use of a conventional ethane/propane separation step. These results were further studied by the system engineers using ANSYS ChemkinPro. For practical applications, these experiments were suggested to be performed at much higher operating pressures (~30 bar) and low residence time (~0.2 s), with a quick quenching step added after the reactor to prevent change in the exit stream compositions. A new reaction system was built to experimentally validate these recommendations.</div>

Dans cette thèse, nous mesurons la dynamique cohérente et les corrélations spatiales des excitations Rydberg dans des matrices 2D d’atomes uniques.Nous utilisons un modulateur spatial de lumière pour façonner la phase spatiale d'un faisceau laser de piégeage optique avant de le focaliser avec une lentille asphérique de grande ouverture numérique. En imprimant une phase appropriée sur le faisceau laser, nous pouvons créer des matrices 2D de pièges optiques, de forme arbitraire et facilement reconfigurables, avec jusqu'à 100 pièges séparées de quelques micromètres. Les pièges sont chargés à partir d'un nuage d'atomes froids de 87Rb, et due aux collisions assistées par la lumière, au plus un seul atome peut être présent dans chaque piège en même temps. Une caméra CCD sensible permet en temps réel l'imagerie de la fluorescence atomique émanant des pièges, ce qui nous permet de détecter individuellement la présence d'un atome dans chaque piège avec une précision presque parfaite.Pour créer des interactions importantes entre les atomes uniques, nous les excitons vers des états de Rydberg, qui sont des états électroniques avec un nombre quantique principal élevé.Un faisceau supplémentaire d'adressage permet la manipulation individuelle d'un atome sélectionné dans la matrice.La connaissance précise, de la fois de la matrice des atomes préparé et des positions des excitations Rydberg, nous a permis de mesurer l’augmentation collective de la couplage optique dans le régime de blocage Rydberg, où une seule excitation est partagée de façon symétrique entre tous les atomes de la matrice.Dans le régime où l'interaction ne s’étend que sur quelques sites, nous avons mesuré la dynamique et les corrélations spatiales des excitations Rydberg, dans des matrices d’atomes à une et deux dimensions. La comparaison à une simulation numérique d'un modèle d'Ising quantique d'un système de spin-1/2 montre un accord exceptionnel pour les matrices où l'effet de l'anisotropie de l’interaction Rydberg-Rydberg est faible. Les résultats obtenus démontrent que les atomes Rydberg uniques sont une plate-forme bien adaptée pour la simulation quantique des systèmes de spin. / In this thesis, we measure the coherent dynamics and the pair correlations of Rydberg excitations in two-dimensional arrays of single atoms.We use a spatial light modulator to shape the spatial phase of a single optical dipole trap beam before focusing it with a high numerical-aperture aspheric lens. By imprinting an appropriate phase pattern on the trap beam, we can create arbitrarily shaped and easily reconfigurable 2D arrays of high-quality single-atom traps, with trap-spacings of a few micrometers for up to 100 traps. The traps are loaded from a cloud of cold 87Rb atoms, and due to fast light-assisted collisions of atoms inside the traps, at most one atom can be present in each trap at the same time. A sensitive CCD camera allows the real-time, site-resolved imaging of the atomic fluorescence from the traps, enabling us to detect the presence of an atom in each individual trap with almost perfect accuracy.In order to induce strong, tunable interactions between the atoms in the array, we coherently laser-excite them to Rydberg states, which are electronic states with a high principal quantum number.An additional addressing beam allows the individual manipulation of an atom at a selected site in the array.The precise knowledge of both the prepared atom array and the positions of the Rydberg excitations allowed us to measure the collective enhancement of the optical coupling strength in the regime of full Rydberg blockade, where one single excitation is shared symmetrically among all atoms in the array.In the regime where the strong interaction only extends over a few sites, we measured the dynamics and the spatial pair-correlations of Rydberg excitations, in one- and two-dimensional atom arrays. The comparison to a numerical simulation of a quantum Ising model of a spin-1/2 system shows an exceptional agreement for trap geometries where the effect of the anisotropy of the Rydberg-Rydberg interaction is small. The obtained results demonstrate that single Rydberg atoms are a suitable platform for the quantum simulation of spin systems.

Ce travail de thèse démontre une technique de détection capable de mesurer, avec une sensibilité à l'atome unique, l'espace des impulsions d'un gaz ultrafroid chargé dans un réseau optique 3D. Nous avons développé un détecteur basé sur des galettes de micro-canaux, capable de sonder électroniquement des nuages d'Hélium-4 métastable. Le gaz est détecté après un temps de vol de 325ms, suffisamment long pour atteindre l'expansion de champ lointain, où la distribution spatiale du gaz coïncide avec la distribution d'impulsion asymptotique. En se plac{c}ant dans un régime proche du remplissage unitaire du réseau, les effets de collisions entre atomes aux premiers instants de l'expansion deviennent négligeables, et donc la distribution d'impulsion asymptotique est égale à la distribution d'impulsion in situ. Nous démontrons expérimentalement cette égalité en comparant nos mesures en champ lointain avec la distribution d'impulsion calculée à partir de l'Hamiltonien de Bose-Hubbard, gr^ace à des simulations Monte Carlo Quantique. Nous observons un bon accord avec la théorie sur plus de 3 ordres de grandeur en densité. Ces simulations sont calculées à partir de nos paramètres expérimentaux, la température étant la seule variable ajustable. Nous utilisons ensuite cette comparaison pour réaliser une thermométrie précise du gaz sur réseau, permettant une exploration de la transition superfluide-gaz normal à travers la mesure directe de différentes quantités, comme la fraction condensée ou la fonction de corrélation à deux particules. / In this thesis, we report the demonstration of a detection technique able to probe, with a single-atom sensitivity, the momentum distribution of an ultracold gas loaded inside a 3D optical lattice. We have developed a micro-channel plate detector, able to electronically probe clouds of metastable Helium-4. The gas is detected after a time-of-flight of 325ms, long enough to reach the far-field expansion, where the spatial distribution of the cloud can be mapped to the asymptotic momentum distribution. By putting ourselves in a regime where the lattice filling is close to unity, the atomic collisions in the first instant of the expansion become negligible, so that the asymptotic momentum distribution is equal to the in situ momentum distribution. We experimentally demonstrate this equality, by comparing our far-field measurements with the momentum distribution calculated from the Bose-Hubbard Hamiltonian, thanks to ab initio quantum Monte Carlo simulations. We show a good agreement with the theory over more than 3 orders of magnitude in density. Those simulations are calculated with our experimental parameters, the temperature being the only adjustable variable. We then use this comparison to perform a precise thermometry of the lattice gas, allowing us to explore the superfluid-normal gas transition through a direct measurement of different quantities, like the condensed fraction or the two-particles correlation function.

Single-atom and cluster catalysts have been receiving significant interest due to not only their capability to approach the limit of atom efficiency but also to explore fundamentally unique properties. Supported Pt-group single atoms and clusters catalysts in the subnanometer size regime maximize the metal utilization and were reported to have extraordinary activities and/or selectivities compared with nanoparticles for various reactions including hydrogenation reactions. However, the relationship between metal nuclearity, electronic and their unique catalytic properties are still unclear. Thus, it is crucial to establish their relations for better future catalyst design. Ethylene hydrogenation and acetylene hydrogenation are two important probe reactions with the simplest alkene and alkyne, and they have been broadly studied as the benchmark reactions on the various catalyst systems. However, the catalytic properties and reaction mechanism of those hydrogenation reactions for metal nuclearitiy in the subnanometer regime is still not well understood. In this study, we applied different characterization techniques including x-ray absorption fine structure (XAFS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy(XPS), diffuse reflectance infrared spectroscopy (DRIFTS), calorimetry and high-resolution scanning transmission electron microscopy (STEM) to investigate the structure of Pt/TiO2 and Pd/COF single-atom catalysts and tested their catalytic properties for hydrogenation reactions. In order to develop such relations, we varied the nuclearity of Pt supported on TiO2 from single atoms to subnanometer clusters to larger nanoparticles. For acetylene hydrogenation, Pt in the subnanometer size regime exhibits remarkably high selectivity to ethylene compared to its nanoparticle counterparts. The high selectivity is resulted from the decreased electron density on Pt and destabilization of C2H4, which were rationalized by X-ray photoelectron spectroscopy and calorimetry results. On the other hand, the activity of H2 activation and acetylene hydrogenation decreased as Pt nuclearity decreased. Therefore, our results show there's a trade-off between activity and selectivity for acetylene hydrogenation. Additionally, the kinetics measurements of ethylene hydrogenation and acetylene hydrogenation were performed on Pt/TiO2 catalysts, and they found to be structure sensitive for both reactions, which the reaction orders and activation energy changes as particles size change. The activity of ethylene hydrogenation decreases, and activation energy increase from 43 to 86 kJ/mol, as Pt nuclearity decreased from an average size of 2.1 nm to 0.7 nm and single atoms. The reaction orders in hydrocarbons (ethylene and acetylene) were less negative on subnanometer clusters and single atoms in contract to nanoparticles. The results imply that hydrocarbons, ethylene and acetylene species, do not poison the catalyst on Pt in the subnanometer size regime, and hydrogen activation turn to competitive adsorption path with surface hydrocarbons species. Moreover, single atom Pd supported on imine-linked covalent organic framework was synthesized, characterized by a various of techniques including X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed CO, and evaluated its catalytic properties for ethylene hydrogenation. The XAS results show that Pd atoms are isolated and stabilized by two covalent Pd–N and Pd-Cl bonds. DRIFTS of CO adsorption shows a sharp symmetrical peak at 2130 cm−1. The Pd single atoms are active for hydrogenation of ethylene to ethane at room temperature. The reaction orders in C2H4 and H2 were 0.0 and 0.5 suggesting that ethylene adsorption is not limiting while hydrogen forms on Pd through dissociative adsorption. / Doctor of Philosophy / More than 90% of chemicals come from petroleum and natural gas, and most of these chemicals are composed of alkene and alkyne, hydrocarbons containing at least one double bonds or triple bonds, such as ethylene, propylene, butenes, butadiene. These small hydrocarbon molecules with carbon-carbon bonds (double or triple) are in great interest of fundamental study and serve as probe units for understanding more complex reactions. Catalysts are materials that can be added to a chemical reaction to accelerate the specific rate of reactions. Most catalysts are supported noble metals thus increase the utilization of metal atoms are important. Decreasing the particle size to increase the metal dispersion is the simple approach to maximize the atom efficiency. However, it is not well understood how do the electronic property and catalytic performance change as particle size decrease. In this work, we focus on the structure sensitivity on catalysts in sub-nanometer region. Supported Pt and Pd catalysts, known to be highly active for hydrogenation reactions, are studied on hydrogenation reactions of acetylene and ethylene, the simplest alkene and alkyne. The Pd and Pt catalysts with particle sizes ranging from single atoms, sub-nanometer clusters and nanoparticles were prepared, characterized and tested for hydrogenation reactions mentioned above. The results show that significantly change in electronic property, catalytic performance (activity and/or selectivity) and reaction kinetics of the catalysts as the particle size changing from nanometer to sub-nanometer region. The fundamental understanding of structure sensitivity on catalysts and their relations between surface structure, electronic property and catalytic performance presented in this work can help the researchers design better catalysts for future work.

Supported noble metal catalysts make the best utilization of noble metal atoms. Recent advances in nanotechnology have brought many attentions into the rational design of catalysts in the nanometer and subnanometer region. Recent studies showed that catalysts in the subnanometer regime could have extraordinary activity and selectivity. However, the structural performance relationships behind their unique catalytic performances are still unclear. To understand the effect of particle size and shape of noble metals, it is essential to understand the fundamental reaction mechanism. Single atoms catalysts and subnanometer clusters provide a unique opportunity for designing heterogeneous catalysts because of their unique geometric and electronic properties. CO oxidation is one of the important probe reactions. However, the reaction mechanism of noble single atoms is still unclear. Additionally, there is no agreement on whether the activity of supported single atoms is higher or lower than supported nanoparticles. In this study, we applied different operando techniques including x-ray absorption fine structure (XAFS), diffuse reflectance infrared spectroscopy (DRIFTS), with other characterization techniques including calorimetry and high-resolution scanning transmission electron microscopy (STEM) to investigate the active and stable structure of Ir/MgAl2O4 and Pt/CeO2 single-atom catalysts during CO oxidation. With all these characterization techniques, we also performed a kinetic study and first principle calculations to understand the reaction mechanism of single atoms for CO oxidation. For Ir single atoms catalysts, our results indicate that instead of poisoning by CO on Ir nanoparticles, Ir single atoms could adsorb more than one ligand, and the Ir(CO)(O) structure was identified as the most stable structure under reaction condition. Though one CO was strongly adsorbed during the entire reaction cycle, another CO could react with the surface adsorbed O* through an Eley-Rideal reaction mechanism. Ir single atoms also provide an interfacial site for the facile O2 activation between Ir and Al with a low barrier, and therefore O2 activation step is feasible even at room temperature. For Pt single-atom catalysts, our results showed that Pt(O)3(CO) structure is stable in O2 and N2 at 150 °C. However, when dosing CO at 150 °C, one surface O* in Pt(O)3(CO) could react with CO to form CO2, and the reacted O* can be refilled when flowing O2 again at 150 °C. This suggests that an adsorbed CO is present in the entire reaction cycle as a ligand, and another gas phase CO could react with surface O* to form CO2 during low-temperature CO oxidation. Supported single atoms synthesized with conventional methods usually consist of a mixture of single atoms and nanoparticles. It is important to quantify the surface site fraction of single atoms and nanoparticles when studying catalytic performances. Because of the unique reaction mechanism of Ir single atoms and Ir nanoparticles, we showed that kinetic measurements could be applied as a simple and direct method of quantifying surface site fractions. Our kinetic methods could also potentially be applied to quantifying other surface species when their kinetic behaviors are significantly different. We also benchmarked other in-situ and ex-situ methods of quantifying surface site fraction of single atoms and nanoparticles. To bridge the gap between single atoms and nanoparticles and have a better understanding of the effect of nuclearity on CO oxidation, we also studied supported Ir subnanometer clusters with the average size less than 0.7 nm (< 13 atoms) prepared by both inorganic precursor and organometallic complex Ir4(CO)12. Low-temperature CO adsorption indicates that CO and O2/O could co-adsorb on Ir subnanometer clusters, however on larger nanoparticle the particle surface is covered by CO only. Additional co-adsorption of CO and O2 was studied by CO and O2 calorimetry at room temperature. CO oxidation results showed that Ir subnanometer clusters are more active than Ir single atoms and Ir nanoparticles at all conditions, and this could be explained by the competitive adsorption of CO and O2 on subnanometer clusters. / Doctor of Philosophy / CO oxidation is one of the important reactions in catalytic converters. Three-way catalysts, typically supported noble metals, are very efficient at high temperature but could be poisoned by CO at cold start. Better designed catalysts are required to improve the performance of the catalytic converter to lower the emissions of gasoline engines. To reach this goal, more efficient use of the noble metal is required. Single-atom catalysts consist of isolated noble metal atoms supported on different supports, which provide the best utilization of noble metal atoms and provides a new opportunity for a better design of heterogeneous catalysts. The unique electronic and geometric properties of metal single atoms catalysts could lead to a better activity and selectivity. Subnanometer clusters have also been shown to have unique electronic properties. With a better understanding of the structure of supported single atoms and subnanometer clusters, their catalytic performance can be optimized for better catalysts in the catalytic converter and other applications. In this work, we applied in-situ and operando characterization, kinetic studies and first principle calculations aiming to understand the active and stable structure of noble metal single atoms and vi subnanometer clusters under reaction condition, and their reaction mechanisms during CO oxidations. For MgAl₂O₄ supported Ir single atoms, our results suggest that CO could be co-adsorbed with O₂/O under reaction conditions. These multiple ligands adsorption leads to a unique reaction mechanism during CO oxidation. Though one CO was adsorbed during the whole reaction cycle, another gas phase CO could react with the O* species co-adsorbed with CO through an Eley-Rideal mechanism. This suggests that Ir single atoms are no longer poisoned by CO, and on the other hand the O₂ can be activated on an interfacial site with a low reaction barrier. Ir subnanometer clusters showed higher activities than Ir single atoms and nanoparticles. In-situ IR and high energy resolution fluorescence detected – X-ray absorption near edge spectroscopy (HERFD-XANES) showed that CO could co-adsorb with O₂ at room temperature, and this competitive adsorption could explain the high activity during CO oxidation. Supported Ir single atoms and subnanometer clusters are not poisoned by CO and O₂ could be co-adsorbed, this could be potentially applied to solve the poisoning of catalyst in the catalytic converter at cold start temperature. We also performed kinetic study on CeO₂ supported Pt single atoms. Similar behavior was observed, and we showed that the CO and O co-adsorbed complex is stable in O₂ and N₂, but could react in CO. With the understanding of the active structure of noble metal single atoms and the origin of activities, better-designed catalysts can be synthesized to improve the activity and selectivity of low-temperature oxidation reactions.

Decentralized electrochemical production of hydrogen peroxide (H₂O₂) is an attractive alternative to the industrial anthraquinone process, the application of which is hindered by the lack of high-performance electrocatalysts in acidic media. Herein, a novel catalyst design strategy is reported to optimize the Pd sites in pure metallic aerogels by tuning their geometric environments and electronic structures. By increasing the Hg content in the Pd–Hg aerogels, the Pd–Pd coordination is gradually diminished, resulting in isolated, single-atom-like Pd motifs in the Pd₂Hg₅ aerogel. Further heterometal doping leads to a series of M–Pd₂Hg₅ aerogels with an unalterable geometric environment, allowing for sole investigation of the electronic effects. Combining theoretical and experimental analyses, a volcano relationship is obtained for the M–Pd₂Hg₅ aerogels, demonstrating an effective tunability of the electronic structure of the Pd active sites. The optimized Au–Pd₂Hg₅ aerogel exhibits an outstanding H₂O₂ selectivity of 92.8% as well as transferred electron numbers of ≈2.1 in the potential range of 0.0–0.4 V/RHE. This work opens a door for designing metallic aerogel electrocatalysts for H₂O₂ production and highlights the importance of electronic effects in tuning electrocatalytic performances.

Cette thèse décrit l'observation expérimentale de l'effet Hong-Ou-Mandel avec une sourceatomique ultra-froide. L’expérience originale réalisée en 1987 par C. K. Hong, Z. Y. Ou et L. Mandel illustre de façon simple une interférence à deux particules explicable uniquement par la mécanique quantique : deux particules bosoniques et indiscernables, arrivant chacune sur une face d'entrée différente d'une lame semi-réfléchissante ressortent ensemble. Cet effet se traduit par une réduction du taux de détection en coïncidence entre les deux voies de sortie quand les particules arrivent simultanément sur la lame. Cette expérience fut originalement réalisée avec des photons et nous rapportons ici la première mise en oeuvre expérimentale avec des particules massives se propageant dans l’espace libre.Après présentation des différentes techniques nécessaires à sa réalisation, nous décrivons cette expérience et analysons les résultats obtenus. En particulier, la réduction du taux de coïncidence est suffisamment forte pour exclure toute interprétation classique ; l'observation de cet effet constitue une brique fondamentale dans le domaine de l’information quantique atomique. / In this thesis, we report the first realisation of the Hong–Ou–Mandel experiment with massive particles in momentum space. This milestone experiment was originally performed in quantum optics: two photons arriving simultaneously at the input ports of a 50:50 beam-splitter always emerge together in one of the output ports. The effect leads to a reduction of coincidence counts which translates into a dip when particles are indistinguishable. We performed the experiment with metastable helium atoms where the specificities of the Micro-Channel-Plate detector allows one to recover the momentum vector of each individual atom.After listing the necessary tools to perform this experiment with atoms, the experimental sequence is discussed and the results are presented. In particular we measured a coincidence count reduction that cannot be explained through any simple classical model. This corresponds to the signature of a two-particle interference, and confirms that our atomic pair source produces beams which have highly correlated populations and are well mode matched. This opens the prospect of testing Bell’s inequalities involving mechanical observables of massive particles, such as momentum, using methods inspired by quantum optics. It also demonstrates a new way to produce and benchmark twin-atom pairs that may be of interest for quantum information processing.