The following thesis applies an experimental design framework to investigate properties of electron transfer kinetics and homogeneous catalytic reactions. The approach is model-based and the classical Butler-Volmer description is chosen to describe the fundamental electrochemical reaction at a conductive interface. The methodology focuses on two significant design variables: the applied potential at the electrode and mass transport mode induced by physical arrangement. An important problem in electrochemistry is the recovery of model parameters from output current measurements. In this work, the identifiability function is proposed as a measure of correspondence between the parameters and output variable. Under diffusion-limit conditions, plain Monte Carlo optimization shows that the function is globally non-identifiable, or equivalently the correspondence is generally non-unique. However by selecting linear voltammetry as the applied potential, the primary parameters in the Butler-Volmer description are theoretically recovered from a single set of data. The result is accomplished via applications of Sobol ranking to reduce the parameter set and a sensitivity equation to inverse these parameters. The use of hydrodynamic tools for investigating electron transfer reactions is next considered. The work initially focuses on the rotating disk and its generalization - the rocking disk mechanism. A numerical framework is developed to analyze the latter, most notably the derivation of a Levich-like expression for the limiting current. The results are then used to compute corresponding identifiability functions for each of the above configurations. Potential effectiveness of each device in recovering kinetic parameters are straightforwardly evaluated by comparing the functional values. Furthermore, another hydrodynamic device - the rotating drum, which is highly suitable for viscous and resistive solvents, is theoretically analyzed. Combined with previous results, this rotating drum configuration shows promising potential as an alternative tool to traditional electrode arrangement. The final chapter illustrates the combination of modulated input signal and appro- priate mass transport regimes to express electro-catalytic effects. An AC voltammetry technique plays an important role in this approach and is discussed step-by-step from simple redox reaction to the complete EC′ catalytic mechanism. A general algorithm based on forward and inverse Fourier transform functions for extracting harmonic currents from the total current is presented. The catalytic effect is evaluated and compared for three cases: macro, micro electrodes under diffusion control condition and in micro fluidic environments. Experimental data are also included to support the simulated design results.

Le présent travail de thèse porte sur la synthèse et la caractérisation de nouvelles molécules et matériaux à base de polyoxométallates (POMs) pour des applications dans les domaines de l’énergie et l’environnement. Dans le domaine de l’environnement, notre choix s’est porté sur la transformation électro-catalytique et photo-catalytique des espèces polluantes comme les oxydes d’azote et les colorants azoïques toxiques comme l’Acide Orange 7. Pour ce qui concerne le domaine de l’énergie, nous nous sommes focalisés sur des systèmes électro- catalytiques pour la production de l’hydrogène (combustible à fort pouvoir calorifique) et pour la réduction du di-oxygène (intérêt dans le fonctionnement des piles à combustibles). Ces réactions nécessitent généralement plusieurs électrons pour se produire, c’est pour cette raison que notre choix s’est porté sur les catalyseurs à base de POMs. En effet, les POMs sont capables de stocker, puis de restituer, un grand nombre d’électrons sans changer de structure.Dans la première partie, divers POMs qui prennent en sandwich plusieurs métaux de transition sont synthétisés et caractérisés par des méthodes expérimentales (voltamétrie cyclique, coulométrie, microbalance) et théoriques (calculs DFT). Ces composés montrent une très bonne activité électro-catalytique pour la réduction des substrats tels que les oxydes d’azote, le di-oxygène et le peroxyde d’hydrogène.Dans la seconde partie, des matériaux aux propriétés améliorées sont synthétisés par incorporation de POMs dans des matrices tels que les réseaux moléculaires de type MOF (Metal Organic Framework) et dans les polymères liquides ioniques. L’association avec un semi-conducteur comme le TiO₂ est aussi considérée. Les matériaux obtenus sont caractérisés par diverses techniques : infrarouge à transformée de fourrier, électrochimie, spectroscopie en réflectance diffuse, analyse thermogravimétrique, diffraction de rayons X, spectroscopie de photo-électrons X, microscopie électronique à transmission. Les matériaux à base de POMs et de MOFs sont très performants pour la réduction électro-catalytique des protons, avec des potentiels de début effectif de la réaction, meilleurs que ceux des électrodes de platine. Les hybrides à base de POMs, de polymères liquides ioniques et du TiO₂ sont photo-sensibles sous lumière visible, contrairement à leurs composants. Ils démontrent une bonne activité vis-à-vis de la dégradation de l’Acide Orange 7. De plus, le composite obtenu par photo-déposition de nanoparticules d’argent sur ces matériaux présente de bonnes performances électro-catalytiques comme cathode pour la réduction du O₂ et du NO‾₃. / The present work concerns the synthesis and characterisation of new polyoxometalate (POMs) based molecules and materials for applications in energy and environment matters. As far as the environment is concerned, our efforts involved the electro-catalytic and photo-catalytic transformation of pollutants such as nitrogen oxides and recalcitrant azo dyes like Acid Orange 7. As for the domain of energy, we focused on electro-catalytic systems aimed at producing dihydrogen (a high calorific power fuel) and at reducing dioxygen (an important reagent in fuel cells). These reactions usually require several electrons in order to take place, which led us to choose POM-based catalysts. In fact, POMs are capable of stocking and returning an important number of electrons without changing their structure.In the first part, several POMs in which different transition metals are sandwiched in the equatorial plane of the molecular scaffold are synthesised and characterised by experimental (cyclic voltammetry, coulometry, microbalance) et theoretical (DFT calculations) methods. These compounds have shown a very good electro-catalytic activity towards the reduction of substrates such as nitrogen oxides, dioxygen and hydrogen peroxide.In the second part, some materials exhibiting improved properties are synthesised through the incorporation of POMs in matrices like Metal Organic Framework (MOF)-type molecular networks and ionic liquid polymers. The association with a semi-conductor such as TiO₂ has also been considered. The materials obtained were characterised by several techniques: Fourier transform infrared spectroscopy, electrochemistry, diffuse reflectance spectroscopy, thermogravimetric analysis, X ray diffraction, X ray photoelectron spectroscopy, transmission electron microscopy. The POM and MOFs based materials are very performing for the electro-catalytic reduction of protons, having onset potentials better than those exhibited by platinum electrodes. The hybrids consisting of POMs, ionic liquid polymers and TiO₂ are photo-sensitive under visible light, unlike each component taken individually. They have shown a good activity towards the degradation of Acid Orange 7. In addition, the composite obtained by photo-deposition of silver nanoparticles on these materials exhibits a good electro-catalytic performance as a cathode for the reduction of O₂ and NO‾₃.

Les polyoxométallates (POMs) peuvent être considérés comme des oxydes moléculaires de métaux de transition. Ils forment une famille de clusters moléculaires inorganiques dont les propriétés et les applications sont diverses et nombreuses. Ils sont notamment utilisés dans des domaines aussi divers que la catalyse, la biologie, ou encore la médecine. Les nanomatériaux de carbone sont une nouvelle classe de matériaux connus pour leur surface spécifique élevée et leur conductivité électrique accrue. Ce mémoire de thèse porte sur la synthèse et l’étude de différents systèmes moléculaires électro-catalytiques à base de POMs et de matériaux de carbone comprenant les nanotubes de carbone et le graphène, pour l’oxydation de l’eau ou la réduction des oxydes d’azote. La première partie de ce mémoire est consacrée à la synthèse, la caractérisation physico-chimique (spectroscopies infrarouge et UV-visible, analyse thermogravimétrique) et l’étude électrochimique (voltammétrie cyclique, coulométrie, microbalance couplé à l’électrochimie) d’un certain nombre de complexes poly-tungstiques prenant en sandwich des métaux de transition tels que le cobalt et/ou le manganèse. Deux d’entre eux, choisis pour l’étude de leurs propriétés électro-catalytiques, montrent une bonne activité électro-catalytique pour la réduction des oxydes d’azote et l’oxydation de l’eau. La seconde partie de ce mémoire est consacrée à l’optimisation des propriétés électrochimiques des POMs. Pour cela, les POMs sont immobilisés sur des matériaux de carbone structurés (graphène ou nanotubes de carbone simple paroi), et les matériaux hybrides obtenus sont caractérisés par différentes techniques analytiques : spectrométrie de photoélectrons X, spectroscopie infrarouge, voltammétrie cyclique et coulométrie. / Polyoxometalates (POMs) may be considered as molecular oxides containing transition metals. They form a family of inorganic molecular clusters with a variety of properties and a myriad of applications. They are used in different fields such as catalysis, biology or medicine. Carbon nanomaterials are a new class of materials known for their high surface area and enhanced electrical conductivity. This thesis focuses on the synthesis and study of different electro-catalytic molecular systems based on polyoxometalates and carbon materials (such as carbon nanotubes and graphene) for water oxidation or nitrogen oxides reduction. The first part of this thesis is devoted to the synthesis, physicochemical characterisation (infrared and UV-Vis spectroscopies, TGA) and the electrochemical study (cyclic voltammetry, coulometry, electrochemistry coupled with microbalance) of several poly-tungstic complexes sandwiching transition metals such as cobalt and/or manganese. Two of them, selected for the study of their electro-catalytic properties, show good electro-catalytic activity for nitrogen oxides reduction and water oxidation. The second part of this thesis is devoted to the optimisation of the electrochemical properties of POMs. For this, POMs are immobilised on structured carbon materials (graphene or single wall carbon nanotubes), and the composite materials obtained are characterised by different analytical techniques: X-ray photoelectron spectroscopy, infrared spectroscopy, cyclic voltammetry and coulometry.

The main objective of this work is to gain basis for rational design of catalysts used in fuel cells for conversion of chemical energy stored in hydrogen molecules into electric energy, as well as photo-catalysts used for hydrogen production from water under solar irradiation. This objective is achieved by applying the first principles computational approach to reveal relationship among compositions of materials under consideration, their electronic structure and catalytic activity. A major part of the work is focused on electro-catalysts for hydrogen fuel cells. Platinum (Pt) is widely used in the electrodes of fuel cells due to its good catalytic properties. However, Pt is an expensive and scarce element, its catalytic activity is not optimal and also it suffers from CO poisoning at anode. Therefore the search for new catalytic materials is needed for large scale implementation of fuel cells. The main direction of search of more efficient electro-catalysts is based in the design in which an active element monoatomic layer (AE) is deposited on a metal substrate (MS) made of a cost-effective material. Two goals are achieved by doing this: on the one hand, the cost of the catalytic system is reduced by reducing the amount of the AE in the system and on the other hand the catalytic properties of the AE can be tuned through its interactions with the MS. In the first part of this work the Pd-based alloys and layered structures have been studied as promising electro-catalysts for the ORR on the fuel cell cathodes, more precisely Pd-Co alloys and Pd/M/Pd (M=Co,Fe). There exists a robust model linking the activity of a surface toward ORR to computable thermodynamic properties of the system and further to the binding energies iv of the ORR intermediates on the catalyst surface. A more challenging task is to find how to tune these binding energies through modification of the surface electronic structure that can be achieved by varying the surface composition and/or morphology. To resolve this challenge, the electronic structure, binding energies of intermediates and the ORR free energies have been calculated within the density functional theory (DFT) approximation. The results presented in this work show that in contrast to the widely accepted notion, the strain exerted by a substrate on AE hardly affects the surface activity toward ORR, while the hybridization of the electronic states of the AE-and MS-electronic states is the key factor controlling the catalytic properties of these systems. Next it is shown that the catalytic activity of the promising anode electrocatalysts, such as Pt/M, M=Au, Ru and Pd, is also determined by the AE-MS hybridization with a minor effect of the strain. Furthermore, we have shown that, if AE is weakly bound to the substrate (as it is for Pt/Au), surface reconstruction occurs. This leads to the breaking of the relation between the electronic structure of the clean surface and the reactivity of the sytem. Other kind of promising ORR catalysts is designed in the form of Ru nanoparticles modified by chalcogens. In this work, I present the results obtained for small Ru clusters and flat Ru facets modified with chalcogens (S, Se and Te). The O and OH binding energies are chosen as descriptors of the ORR. The results on the two systems are compared, concluding that large clusters with relative large flat facets have higher catalytic activity due to the absence of low coordinated and thus high reactive Ru atoms. Regarding the problem of the hydrogen production via photo-catalytic splitting of water, one of the challenges is tuning the band gap of the photo-anodes to optimal levels. Graphitic carbon nitride (g-C3N4) is a promising material to be used as a photo-anode, however, a v reduction of the band gap width by rational doping of the material would improve the efficiency significantly. This issue is addressed in the last chapter of this work. Two problems are considered: a) the stability of the doped system and b) the band gap width. To address the first problem the ab-initio thermodynamics approach has been used, finding that the substitution of C and N with the doping agent (B, C, N, O, Si and P) is thermodynamically preferred over the interstitial addition of dopant to the g-C3N4 structure. However, due to high kinetic energy barriers for the detachment of C and N atoms, involved in the substitution doping, the interstitial addition found to be kinetically more favorable. Since the density functional theory fails to reproduce the band gap of semiconductors correctly, the GW approximation was used to study the band gap of the system. The results indicate that the g-C3N4 system maintain its semiconductor character if doped with B, O and P under certain conditions, while reducing the band gap.

A fundamental understanding of the kinetics and mechanisms of the catalytic reaction steps involved in the process of converting a fuel into hydrogen rich stream suitable for a fuel cell, as well as the electro-catalytic reactions within a fuel cell, is not only conceptually appealing, but could provide a sound basis for the design and development of efficient fuel processor/fuel cell systems. With the quantum chemical calculations on kinetics of elementary catalytic reaction steps becoming rather commonplace, and with increasing information now available in terms of electronic structures, vibration spectra, and kinetic data (activation energy and pre-exponential factors), the stage is set for development of a comprehensive approach. Toward this end, we have developed a framework that can utilize this basic information to develop a comprehensive understanding of catalytic and electrocatalytic reaction networks. The approach is based on the development of Reaction Route (RR) Graphs, which not only represent the reaction pathways pictorially, but are quantitative networks consistent with the Kirchhoff's laws of flow networks, allowing a detailed quantitative analysis by exploiting the analogy with electrical circuits. The result is an unambiguous portrayal of the reaction scheme that lays bare the dominant pathways. Further, the rate-limiting steps are identified rationally with ease, based on comparison of step resistances, as are the dominant pathways via flux analysis. In fact, explicit steady-state overall reaction (OR) rate expression can also be derived in an Ohm's law form, i.e. OR rate = OR motive force/OR resistance of an equivalent electric circuit, which derives directly from the RR graph of its mechanism. This approach is utilized for a detailed analysis of the catalytic and electro-catalytic reaction systems involved in reformer/fuel cell systems. The catalytic reaction systems considered include methanol decomposition, water gas shift, ammonia decomposition, and methane steam reforming, which have been studied mechanistically and kinetically. A detailed analysis of the electro-catalytic reactions in connection to the anode and cathode of fuel cells, i.e. hydrogen electrode reaction and the oxygen reduction reaction, has also been accomplished. These reaction systems have not so far been investigated at this level of detail. The basic underlying principles of the RR graphs and the topological analysis for these reaction systems are discussed.