Record high CO2 emissions in the atmosphere and the need to find alternative energy sources to fossil fuels are major global challenges. Conversion of CO2 into useful fuels like methanol and methane can in principle tackle both these environment and energy concerns. One of the routes to convert CO2 into useful fuels is by using supported metal catalyst. Specifically, metal atoms or clusters (few atoms large in size) supported on oxide materials are promising catalysts. Experiments have successfully converted CO2 to products like methanol, using TiO2 supported Cu atoms or clusters. How this catalyst works and how CO2 conversion could be improved is an area of much research. We used a quantum mechanical tool called density functional theory (DFT) to obtain atomic and electronic level insights in the CO2 reduction processes on TiO2 supported metal atoms and clusters. We modeled small Cu clusters on TiO2 surface, which are experimentally synthesizable. Our results show that the interfacial sites in TiO2 supported Cu are able to activate CO2 into a bent configuration that can be further reduced. The Cu dimer was found to be the most reactive for CO2 activation but were unstable catalysts. Following Cu, we also identified other potential metal atoms that can activate CO2. Compared to expensive and rare elements like Pt, Au, and Ir, we found several early and mid transition metals to be potentially active catalysts for CO2 reduction. Because the supported metal atom or cluster is a reactive catalyst, under reaction conditions they tend to undergo aggregation and/or oxidation to form larger less active catalysts. We chose Co, Ni, and Cu group elements to study their catalyst stability under oxidizing reaction conditions. Based on the thermodynamics of Cu oxidation and kinetics of O2 dissociation, we found that TiO2 supported Cu atom or a larger Cu tetramer cluster were the likely species observed in experiments. Our work provides valuable atomic-level insights into improving the CO2 reduction activities and predicts potential catalysts for CO2 reduction to valuable fuels.

Les (sub)nanoclusters sont des agrégats d’atomes ou de molécules composés de quelques unités à quelques centaines d’unités. En raison de leur petite taille, ils peuvent avoir des propriétés électroniques, optiques, magnétiques et catalytiques très différentes par rapport au solide correspondant . D'un point de vue expérimental, il est encore très difficile de synthétiser des agrégats de taille calibrée. D'un point de vue théorique, le développement des puissances de calcul, des méthodes de calcul de structure électronique et des algorithmes de recherches globales de structures stables, permettent un calcul toujours plus précis de leurs propriétés physico-chimiques. L’étude théorique permet alors de déterminer de façon fiable les structures stables de ces systèmes qui président aux calculs de leurs propriétés . L’exemple qui illustre ce travail s’inspire du processus observé au sein des piles à combustible dans lequel le Platine (Pt) est couramment utilisé pour produire de l’énergie par oxydation du dihydrogène en favorisant notamment sa dissociation . L’objet de ce travail consiste à comparer la capacité des clusters de Platine de différentes tailles à adsorber la molécule de dihydrogène sous leur forme libre et adsorbée sur substrat. Le graphène , matériaux bidimensionnel cristallin formé de carbone est choisi dans ce travail en tant que substrat en raison de sa grande résistance mécanique et chimique. La première partie de ce travail est consacrée à la recherche d’éléments dopants qui vont permettent à la fois d’améliorer la capacité d’adsorption des clusters de Platine sur la surface et éviter leur migration. L’objectif est ici de proposer un substrat sur lequel peuvent être empêchés les phénomènes d’agglomération, de dissolution et de détachement du cluster qui ainsi limiteraient son efficacité catalytique . Des dopages de la surface, tel qu’ils sont réalisables expérimentalement , par l’Azote, le Bore et le Nitrure de Bore, par substitution atomique et avec ou sans considération préalable de lacunes, ont été étudiés. La seconde partie correspond à l’implémentation dans le code GSAM (Global Search Algorithm of Minima - algorithme de recherche globale de minima) développé au laboratoire , , des éléments qui permettent la recherche de structures de plus basse énergie de clusters moléculaires adsorbés sur substrat, tels que les systèmes [H2-Ptn-Graphène dopé] de cet exemple. La troisième partie concerne l’illustration de la fiabilité de la méthode de recherche globale employée et de la qualité de quelques méthodes de calcul de l’énergie moléculaire (DFT et GUPTA) vis-à-vis de résultats mentionnés dans la littérature sur les clusters de Platine. La dernière partie comporte l’investigation structurale des systèmes [H2-Ptn] et [H2-Ptn-Graphène dopé] pour différentes tailles de clusters allant de n=6 à n=20. La variation de l’énergie d’adsorption de H2 sur les clusters libres et supportés ainsi que celle du cluster moléculaire sur le substrat en fonction de la taille est reportée. / A sub-nanometer sized metal cluster consists of only several to tens of atoms. Due to its small size and quantum effects, it can have very specific electronic, optical, magnetic and catalytic properties as compared with their bulk behaviors . From an experimental point of view, it is still a big challenge to realize size-controlled synthesis for (sub) nanoclusters. From a theoretical point of view, benefiting from the development of faster high-performance computational sources, more efficient electronic structure modelling software and more reliable global search methods for the determination of the most stable structures, the chemical and physical properties of clusters can be determinate more accurately. As it is experimentally a big challenge to realize size-controlled synthesis for (sub) nanoclusters, theoretical studies can provide detailed information on their geometric structure, electronic structure, as well as adsorption and reaction properties . The example chosen to be treated in this study is inspired by the fuel cell, in which the Platinum (Pt) is a typical and most commonly used precious metal catalyst for the production of energy by the oxidation of dihydrogene . Graphene is a recently discovered 2D carbon net structure, has several special properties, such as: low weight, high strength, high surface area, high electrical conductivity, etc. With these properties and their novel combinations, graphene might prove a promising candidate to be used as catalyst supports. The first part of this study is devoted to the search of the doping elements which permit both enhance the adsorption capacity of Pt clusters on the surface and prevent their migration. The aim here is propose one substrate which can avoid the problems of cluster agglomeration, dissolution and detachment, which reduce the performance of the catalysts . The ways of doping of the surface, which have already been experimentally realized , such as Nitrogen, Boron, and N-B patches substitution of Carbon atoms with or without introducing the vacancy on the pristine graphene, are studied. The second part corresponds to the implementation of some new features into the code GSAM (Global Search Algorithm of Minima) developed in our laboratory , , , which permit the search of the most stable structures of the molecular clusters adsorbed on substrate, such as the complex systems of [H2-Ptn-doped Graphene]. The third part is to evaluate the reliabilities of the global search method used, as well as the DFT and the empirical (GUPTA) potential energy surface. Thus, the main discussion appears as a comparison with the results of the literature concerning the Pt clusters. The fourth part consists of the structural investigation of [H2-Ptn] and [H2-Ptn-doped Graphene] systems for different sizes of Pt clusters with n=6 to n=20. The variation of the adsorption energy of H2 on the free and supported Ptn clusters, and the adsorption energy of (H2+Ptn) system on the surface with respect to the size of the cluster is discussed.