Search

A fundamental question associated with chirality is how mixtures containing equal amounts of interconverting enantiomers can spontaneously convert to systems enriched in only one of them. Enantiomers typically have similar chemical properties, but can exhibit distinct reactivity under specific conditions, and these differences can be used to bias the system’s composition in favor of one enantiomer. Transport properties are also expected to differ for enantiomers in chiral solvents, but the role of such differences in chiral symmetry breaking has not been clarified yet. In this work, we develop a theoretical framework to show that asymmetry in diffusion properties can trigger a spontaneous and selective symmetry breaking in mixtures of enantiomers. We derive a generic evolution equation for the enantiomeric excess in a chiral solvent. This equation shows that the relative stability of homochiral domains is dictated by the difference of diffusion coefficients of the two enantiomers. Consequently, deracemization toward a specific enantiomeric excess can be achieved when this difference is large enough. These results hold significant implications for our understanding of chiral symmetry breaking., SCOPUS: ar.j, info:eu-repo/semantics/published

info:eu-repo/semantics/nonPublished

Ammonia (NH3) synthesis is crucial for the production of artificial fertilizer and is carried out through the Haber-Bosch process. With an energy consumption of 30 GJ/t-NH3 and the emission of 2 kg-CO2/kg-NH3, ammonia is the chemical with the largest environmental footprint. Haber-Bosch operates under high pressure and high temperature conditions. Plasma technology potentially allows greener ammonia production. Dielectric barrier discharges are a popular plasma source in which a catalyst is easily incorporated. The combination of plasma and catalyst can circumvent the harsh reaction conditions of the Haber-Bosch process.Plasma kinetics modelling is used to gain insight into the mechanisms of such plasma-catalytic systems. Special attention is given to the instantaneous power absorbed by the electrons, the relevant fraction of the microdischarges and the discharge volumes.The importance of vibrational excitation is investigated. Depending on the exact discharge conditions, it was found that both the strong microdischarges and vibrational excitation can be simultaneously important for the ammonia yield.The temporal behavior of filamentary dielectric barrier discharges was explicitly taken into account. Ammonia was found to decompose during the microdischarges due to electron impact dissociation. At the same time atomic nitrogen and other excited species are created. Those reactive species recombine to ammonia in the afterglow through various elementary Eley-Rideal and Langmuir-Hinshelwood surface reaction steps with a net ammonia gain.Finally, the concept of the fraction of microdischarges was generalized. It directly represents the efficiency with which the applied electric power is transferred to each individual particle in the plasma reactor. It is argued that any type of spatial or temporal non-uniformity of the plasma will cause unequal treatment of the gas molecules in the reactor, corresponding to a lower efficiency at which the power is transferred to the gas, Option Chimie du Doctorat en Sciences, info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/published

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/published

info:eu-repo/semantics/nonPublished

The behaviour of a Frank-like chemical network model featuring autocatalytic production of chiral enantiomers from achiral reactants is studied numerically in 1D and 2D systems using fluctuating initial conditions and accounting for diffusion processes. Our results reveal that the achiral substrate concentration can play an ambivalent role. It is shown that when the achiral reactant concentration is maintained constant and homogeneous in 1D systems, global homochirality is not systematically reached when the size of the system or the achiral reactant concentration are increased. However, with a fixed concentration gradient, coexisting homochiral domains of opposite handedness are no longer observed and homogeneous homochirality, i.e. the presence of a single stable homochiral domain, is recovered. In 2D systems, reaching global homochirality is just a matter of time. This time is dramatically increased when insufficient or excessive amount of achiral reactant is used. An optimal amount of achiral material is observed to maximise the enantiomer production rates., info:eu-repo/semantics/published

The main motivation for the research presented in this thesis is the idea that an optimised laser pulse can be used to selectively populate a molecular (ro)vibrational state. One of our goals is to populate a non-stationary dark vibrational state of acetylene (C2H2) using a single optimized laser pulse. This goal was pursued theoretically and efforts were made to perform simulations as close to the experiments as possible. The core of the control studies was undertaken using a spectroscopic effective Hamiltonian. Effects of collisions on the dynamics in the gas phase were also considered within the Lindblad master equation formalism in which the phenomenological parameters were extracted from spectra. In addition, ab initio quantum dynamics was considered for other molecular systems. Within that context, we developed a semi-automatic adaptive fitting methodology to obtain potential energy surfaces in sum-of-product form., La motivation principale de cette thèse de doctorat est l’idée qu’un champ laser optimisé peut être utilisé pour peupler sélectivement un état (ro)vibrationnel d’une molécule. Notre objectif est de peupler un état non-stationnaire dark de l’acétylène (C2H2) avec un champ laser optimisé. Cet objectif est poursuivi théoriquement et nos efforts vont dans l’établissement de simulations les plus proches que possible de la réalité. Le cœur de nos travaux sur le contrôle laser est effectué à l’aide d’un Hamiltonien effectif spectroscopique. Les travaux sur C2H2 présentés dans cette thèse évitent donc l’établissement de l’Hamiltonien moléculaire. De ce fait, nous avons pu nous concentrer sur d’autres difficultés rencontrées dans les simulations du contrôle laser quantique. Les effets de collisions dans la phase gazeuse ont été paramétrisées dans le formalisme de l’équation maîtresse de Lindblad à partir de coefficients d’élargissement de raies issues de la spectroscopie à haute résolution. Ces paramètres ont ensuite été utilisés pour lasimulation du contrôle laser quantique avec effets de collisions. En collaboration avec le groupe OPERA-photonique (ULB), nous nous sommes concentrés sur les contraintes expérimentales de la mise en forme d’impulsions laser, en identifiant les techniques les plus adéquates à nos objectifs de contrôle. Nous avons adapté nos méthodes d’optimisation de champ de contrôle pour que les contraintes expérimentales y soient incluses explicitement à l’aide d’une méthode d’optimisation par gradients.Nous nous sommes également tournés vers l’établissement de la simulation de dynamique quantique vibrationnelle ab initio en nous focalisant sur la molécule d’acide formique, et au moyen de la méthode Multi-Configuration Time-Dependent Hartree (MCTDH). L’Hamiltonien avait déjà été construit dans la forme adéquate pour qu’il soit utilisé dans MCTDH. La qualité de l’Hamiltonien a été établie par comparaison d’états propres vibrationnels calculés avec les mesure, Doctorat en Sciences, info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/published

Dans cette thèse, nous avons analysé les propriétés dynamiques de fronts chimiques, à savoir de structures chimiques spatialement localisées et formées par des processus réaction-diffusion (RD). Ces propriétés peuvent être influencées par des mouvements macroscopiques de fluide (convection) induits spontanément par les gradients de concentration et/ou de température qui accompagnent la formation de ces structures chimiques. La prise en compte des effets convectifs devient alors cruciale pour une description complète de la dynamique de ces fronts. Deux classes de fronts chimiques ont été étudiées :les fronts bimoléculaires et ceux de polymérisation thermique. La première classe est formée par la diffusion de masse de deux espèces réactives initialement séparées et qui réagissent selon une réaction bimoléculaire. La seconde survient par l’action combinée de la diffusion de la chaleur et de la cinétique de type Arrhenius d’une réaction de polymérisation exothermique. Dans les deux cas, le front est supposé se propager dans une couche à deux dimensions effectives et perpendiculairement au champ de gravité. D’une façon générale, la dynamique du système est étudiée en résolvant les équations RD couplées aux équations Navier-Stokes de conservation de masse et du moment. Dans un premier temps, nous avons considéré la propagation de fronts bimoleculaires en présence d’écoulements induits par des gradients de tension de surface (écoulements Marangoni). Cette étude nous a permis de mettre en évidence les phénomènes uniques qui résultent du couplage entre la dynamique des écoulements Marangoni et celle du front chimique. En particulier, nous avons établi un critère d’évolution du front en présence de tels écoulements et nous avons décrit les nouvelles propriétés dynamiques qui résultent de ce couplage telles que les oscillations spatio-temporelles du champ de vitesses Marangoni et des concentrations chimiques.Dans un deuxième temps, nous avons étudié la propagation de fronts de, Doctorat en Sciences, info:eu-repo/semantics/nonPublished

Au-based catalysts present excellent low temperature activity and selectivity for partial oxidation reactions, but the fundamental issue of atomic oxygen availability, the key parameter to obtain such reactivity, remains present. To enable the O2 dissociation reaction, Au nanoparticles must be smaller than 5 nm, which induces structural issues for upscaled applications. Alloying Au creates synergistic catalytic effects, and this option is investigated here: using residual amounts of Ag enables the O2 dissociation, regardless of the size of the Au-Ag catalyst. Questions remain about the precise interplays between the surface structure, the composition and the observed reactivity and selectivity. The aim of this thesis is to investigate the phenomena occurring during oxidation catalysis on Au-Ag surfaces, at a molecular scale, using surface science techniques.Nanoporous (np) Au structures with residual amounts of Ag (1-5 at.%) are increasingly used as catalyst for oxidation reactions. They are made of an interconnected array of three-dimensional ligaments (20-70 nm in diameter) presenting highly crystalline structures and exposing different crystallographic facets with different properties. These structural features allow for a bottom-up surface science investigation using Au-Ag samples with increasing structural complexity. The used experimental approach provides representative data on single crystals (using photoemission electron microscopy (PEEM) and quadrupole mass spectrometry (QMS)), on model nanoparticles (using field ion microscopy (FIM) and field electron microscopy (FEM)) and on np-Au samples (using temporary analysis of products (TAP) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)) and allows to corroborate results from the different techniques. During experiments with the respective techniques, these samples are exposed to O2, NO2, H2, CO, CH4, CH3OH as well as their analogous reactive gas mixtures to study the corresponding oxida, Doctorat en Sciences, info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished

Stochastic thermodynamics is an extension of classical nonequilibrium thermodynamics that aims at including the effect of fluctuations occurring at small scales. In many cases, the starting point of such approaches is a Fokker–Planck equation for the probability density associated to the underlying stochastic variables. It is known that for chemically reactive systems, the usual equation for entropy production derived in this framework is not consistent with its macroscopic counterpart. It moreover also differs from the expression obtained with the master equation, from which the aforementioned Fokker–Planck equation derives. In the present work, we obtain a Fokker–Planck based alternative formula for entropy production in reactive systems that converges to the correct expression in the thermodynamic limit and is moreover consistent with expressions obtained with the master-equation based approach., SCOPUS: ar.j, info:eu-repo/semantics/published

Cette thèse est consacrée à l’élaboration et l’étude théorique de modèles de transport décrivant les dynamiques cellulaires et la communication intercellulaire dans les tissus épithéliaux. Nous nous intéressons d’abord à l’influence du transport de microARNs (miRNAs) sur la dynamique spatiotemporelle de réseaux de régulation génétique. Ces courtes séquences d’ARN régulent la synthèse des protéines en bloquant l’activité des ARN messagers et leur sécrétion via des vesicules extracellulaires en font des agents de communication intercellulaire. Différents modèles faisant intervenir des miRNAs extracellulaires ont été construits et étudiés numériquement. Les premiers sont des modèles génériques destinés à mettre en évidence l'effet d'une cellule ayant une production de miRNAs anormale sur l'expression génétique dans les cellules voisines. Nous abordons ensuite des modèles plus complexes et réalistes dans lesquels des oscillations (liées à des rythmes biologiques) et de la bistabilité (liée à une différenciation cellulaire) sont observées. Ces modèles permettent d’étudier des dynamiques de communication complexes observées en biologie, comme la synchronisation de cellules couplées ou la propagation d'un changement de phénotype. Nous mettons également en évidence le rôle de défauts, tels que des mutations génétiques ou encore des variations de densité cellulaire dans les tissus, sur ces phénomènes de propagation. La deuxième partie de la thèse est dédiée à la construction de modèles de réaction-diffusion dans lesquels la dynamique des cellules dépend de leur état interne. Sur base d’études expérimentales montrant l’influence de protéines et de miRNAs sur la mobilité et la prolifération des cellules, nous établissons un modèle multi-échelle dans lequel la dynamique intracellulaire et le mouvement des cellules interagissent. En effet, certaines protéines sont responsables de l’adhésion cellulaire ou régulent la vitesse de prolifération. Dans notre modèle, chaque cellule syn, Doctorat en Sciences, info:eu-repo/semantics/nonPublished

Avec les progrès de la technologie, il est désormais devenu possible de manipuler des faibles quantités d’objets nanométriques, voire des objets uniques. Observer une réaction chimique de quelques centaines de molécules sur des catalyseurs, étudier le travail exercé lors du déploiement d’un brin d’ADN unique ou mesurer la chaleur émise par un unique électron dans un circuit électrique constituent aujourd’hui des actes expérimentaux courants. Cependant, à cette échelle, le caractère aléatoire des processus physiques étudiés se fait plus fortement ressentir. Développer une théorie thermodynamique à ces échelles nécessite d'y inclure de manière exhaustive ces fluctuations.Ces préoccupations et les résultats expérimentaux et théoriques associés ont mené à l’émergence de ce que l’on appelle aujourd’hui la thermodynamique stochastique. Cette thèse se propose de développer une approche originale à la thermodynamique stochastique, basée sur une extension de l'hypothèse d'équilibre local aux variables fluctuantes d'un système. Cette théorie offre de nouvelles définitions des grandeurs thermodynamiques stochastiques, dont l'évolution est donnée par des équations différentielles stochastiques (EDS).Nous avons choisi d'étudier cette théorie à travers des modèles simplifiés de phénomènes physiques variés; transport (diffusif) de chaleur ou de masse, transport couplé (comme la thermodiffusion), ainsi que des modèles de réactions chimiques linéaires et non-linéaires. A travers ces exemples, nous avons proposé des versions stochastiques de plusieurs grandeurs thermodynamiques d'intérêt. Une large part de cette thèse est dévolue à l'entropie et aux différents termes apparaissant dans son bilan (flux d'entropie, production d'entropie ou dissipation). D'autres exemples incluent l'énergie libre d'Helmholtz, la production d'entropie d'excès, ou encore les efficacités thermodynamiques dans le transport couplé.A l'aide de cette théorie, nous avons étudié les propriétés statistiques de ces, Over the last decades, nanotechnology has experienced great steps forwards, opening new ways to manipulate micro- and nanosystems. These advances motivated the development of a thermodynamic theory for such systems, taking fully into account the unavoidable fluctuations appearing at that scale. This ultimately leads to an ensemble of experimental and theoretical results forming the emergent field of stochastic thermodynamics. In this thesis, we propose an original theoretical approach to stochastic thermodynamics, based on the extension of the local equilibrium hypothesis (LEH) to fluctuating variables in small systems. The approach provides new definitions of stochastic thermodynamic quantities, whose evolution is given by stochastic differential equations (SDEs).We applied this new formalism to a diverse range of systems: heat or mass diffusive transport, coupled transport phenomena (thermodiffusion), and linear or non-linear chemical systems. In each model, we used our theory to define key stochastic thermodynamic quantities. A great emphasis has been put on entropy and the different contributions to its evolution (entropy flux and entropy production) throughout this thesis. Other examples include also the stochastic Helmholtz energy, stochastic excess entropy production and stochastic efficiencies in coupled transport. We investigated how the statistical properties of these quantities are affected by external thermodynamic constraints and by the kinetics of the system. We first studied how the thermodynamic state of the system (equilibrium \textit{vs.} non-equilibrium) strongly impacts the distribution of entropy production. We then extended those findings to other related quantities, such as the Helmholtz free energy and excess entropy production. We also analysed how some external control parameters could lead to bimodality in stochastic efficiencies distributions.In addition, non-linearities affect stochastic thermodynamics quantities in different ways. Using, Doctorat en Sciences, info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/published

In nonexcitable cells, global Ca2+ spikes emerge from the collective dynamics of clusters of Ca2+ channels that are coupled by diffusion. Current modeling approaches have opposed stochastic descriptions of these systems to purely deterministic models, while both paradoxically appear compatible with experimental data. Combining fully stochastic simulations and mean-field analyses, we demonstrate that these two approaches can be reconciled. Our fully stochastic model generates spike sequences that can be seen as noise-perturbed oscillations of deterministic origin, while displaying statistical properties in agreement with experimental data. These underlying deterministic oscillations arise from a phenomenological spike nucleation mechanism., SCOPUS: ar.j, info:eu-repo/semantics/published

The adhesion properties and the mobility of biological cells play key roles in the propagation of cancer. These properties are expected to depend on intracellular processes and on the concentrations of chemicals inside the cell. While most existing reaction-diffusion models for cell migration consider that cell mobility and proliferation rate are constant or depend on an external diffusing species, they do not include the gene expression dynamics taking place in moving cells that affect cellular transport. In this work, we propose a multiscale model where mobility and proliferation depend explicitly on the cell's internal state. We focus more specifically on the case of cellular mobility in epithelial tissues. Wound-healing experiments have demonstrated that the loss of a key protein, E-cadherin, results in a significant increase in both mobility and invasiveness of epithelial cells, with dramatic consequences on cancer progression. We can reproduce the results of these experiments under various genetic conditions with a single set of parameters., SCOPUS: ar.j, info:eu-repo/semantics/published

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished

Stochastic thermodynamics is an extension of classical nonequilibrium thermodynamics to small systems, where fluctuations are expected to play an important role. A central difficulty met when developing such an extension is how to define a nonequilibrium fluctuating entropy. Typically, the expression used is based on Gibbs' formula for entropy at equilibrium. In this work, we show that one can construct an alternative framework for stochastic thermodynamics based on an extension of Einstein's formula connecting the probability of fluctuations and entropy around equilibrium states. We compare the two approaches and discuss, in particular, how they lead to different interpretations of what a stochastic entropy and entropy production represent., SCOPUS: ar.j, info:eu-repo/semantics/published

info:eu-repo/semantics/published

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished

We studied the catalytic NO2(g) + H2(g)/Pt system on model platinum catalysts with nanoscale spatial resolution by means of field emission microscopy (FEM). While the surface of the catalyst is in a non-reactive state at low H2 partial pressure, bursts of activity are observed when increasing this parameter. These kinetic instabilities subsequently evolve towards self-sustained periodic oscillations for a wide range of pressures. Combining time series analyses and numerical simulations of a simple reaction model, we clarify how these observations fit in the traditional classification of dynamical systems. In particular, reconstructions of the probability density around oscillating trajectories show that the experimental system defines a crater-like structure in probability space. The experimental observations thus correspond to a noise-perturbed limit cycle emerging from a nanometric reactive system. This conclusion is further supported by comparison with stochastic simulations of the proposed chemical model. The obtained results and simulations pave the way towards a better understanding of reactive nanosystems., SCOPUS: ar.j, info:eu-repo/semantics/published

Hybrid trap experiments are set-ups that allow to study the interaction between ions and atoms in cold controlled environment. In such context, molecular anions present specific theoretical and experimental interests and challenges. In this work, we have used extensive \textit{ab initio} methods to investigate several collisional anionic systems: (1) M + OH$^{-}$ (where M are alkali or alkaline earth atoms), (2) Rb and H + OH(H$_{2}$O)$_{n}^{-}$ (with $n=0,1,2,3,4$) and (3) Rb and Li + C$_{2}^{-}$. Several molecular properties such as vertical detachment energies or electroaffinities, optimized structures, harmonic frequencies, potential energy curves or surfaces, etc have been calculated using high level quantum chemistry approaches. The results have been used to make predictions on the related reactivity in low energy regime. We emphasis on electronic detachment processes by carefully analysing the difference between the neutral and anionic potential energy surface. The Rb + OH$^{-}$ system is currently under experimental investigation. Therefore, a detailed study of its reactivity is carried out in the present work. We have analysed the different reactive channels arising from both collision involving the ground state and first electronic excited state of Rb. Using our calculated potentials and a capture model based dynamics, we have extracted cross sections and rate constants. Comparison with other alkali and earth alkaline atoms are made. Hydrated hydroxide cluster anions are planned by the experimental group as upcoming studied systems. We present here our preliminary results on the possible outcome when considering collisions with Rb and we discuss their implications for hybrid trap experiments. We make comparison with H as a colliding partner and consider our results in the context of astrochemistry. Finally we propose the C$_{2}^{-}$ molecular anion as an alternative to OH$^{-}$. Its interaction and reactivity with Rb and Li are investigated and the results, Doctorat en Sciences, info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/nonPublished