Aspects of asphaltene aggregation obtained from coarse-grained molecular modeling

We have performed a molecular-simulation-based study to explore some of the underlying mechanisms of asphaltene aggregation. The daunting complexity of the crude oil + asphaltene system precludes any type of meaningful molecular simulation unless some assumptions are made with respect to the key physical and chemical properties that must be explicitly described. In the present work, we focus on molecular simulations of a coarse-grained model of asphaltene molecules in pure solvents, which are based on the assumption that the general size asymmetry and asphaltene morphology play a key role in the aggregation process. We use simple single isotropic Lennard-Jones sites to represent paraffinic and aromatic C₆ segments, which are used as building blocks for the description of continental asphaltene models and solvent moieties. The energy and size parameters for the intermolecular models (ε and σ) for solute and solvent molecules are chosen to reproduce the experimental density of the liquid phase for different mixtures. An explicit pure solvent is considered, and the relationship between the aggregation mechanism and the solvent nature is investigated through direct simulation of the aggregation process. The results reproduce accurately expected trends observed for more-complex models as well as experiments, for example, strong aggregation of asphaltene molecules in <i>n-</i>heptane and high solubility in toluene. Different asphaltene models based on different geometries reveal that even at this level of simplification the topology of the molecules (number and position of aliphatic branches) does affect the way molecules aggregate

In Situ Detection of Active Edge Sites in Single-Layer MoS2_2 Catalysts

MoS2 nanoparticles are proven catalysts for processes such as hydrodesulphurization and hydrogen evolution, but unravelling their atomic-scale structure under catalytic working conditions has remained significantly challenging. Ambient pressure X-ray Photoelectron Spectroscopy (AP-XPS) allows us to follow in-situ the formation of the catalytically relevant MoS2 edge sites in their active state. The XPS fingerprint is described by independent contributions to the Mo3d core level spectrum whose relative intensity is sensitive to the thermodynamic conditions. Density Functional Theory (DFT) is used to model the triangular MoS2 particles on Au(111) and identify the particular sulphidation state of the edge sites. A consistent picture emerges in which the core level shifts for the edge Mo atoms evolve counter-intuitively towards higher binding energies when the active edges are reduced. The shift is explained by a surprising alteration in the metallic character of the edge sites, which is a distinct spectroscopic signature of the MoS2 edges under working conditions

Screening Methodology for the Efficient Pairing of Ionic Liquids and Carbonaceous Electrodes Applied to Electric Energy Storage

A model is presented that correlates the measured electric capacitance with the energy that comprises the desolvation, dissociation and adsorption energy of an ionic liquid into carbonaceous electrode (represented by single-wall carbon nanotubes). An original methodology is presented that allows for the calculation of the adsorption energy of ions in a host system that does not necessarily compensate the total charge of the adsorbed ions, leaving an overall net charge. To obtain overall negative (favorable) energies, adsorption energies need to overcome the energy cost for desolvation of the ion pair and its dissociation into individual ions. Smaller ions, such as BF4 −, generally show larger dissociation energies than anions such as PF6 − or TFSI−. Adsorption energies gradually increase with decreasing pore size of the CNT and show a maximum when the pore size is slightly greater than the dimensions of the adsorbed ion and the attractive van der Waals forces dominate the interaction. At smaller pore diameters, the adsorption energy sharply declines and becomes repulsive as a result of geometry deformations of the ion. Only for those diameters where the adsorption reaches maximum values is the adsorption energy sufficiently negative to balance the positive dissociation and desolvation energies. We present for each ion (and ionic liquid) what the most adequate electrode pore size should be for maximum capacitance

Leaching of a zirconolite ceramic waste-form under proton and HE2+ irradiation

In the hypothesis of a nuclear waste geological disposal, zirconolite is a candidate host material for minor tri- and tetra-valent actinides arising from enhanced nuclear spent fuel reprocessing and partitioning. Its chemical durability has been studied here under charged particle-induced radiolysis (He2+ and proton external beams) to identify possible effects on dissolution rates and mechanisms in pure water. Two experimental geometries have been used to evaluate the influence of the following parameters: solid irradiation and total deposited energy. Results on the evolution of the elemental releases due to the enhanced dissolution of the zirconolite surface during charged particle-induced irradiation of water are presented. Under radiolysis, elemental releases are first kinetically controlled. When the titanium and the zirconium releases reach (or exceed) their corresponding hydroxide solubility limits, the zirconolite dissolution becomes thermodynamically controlled

Enhancement of Zirconolite Dissolution Due to Water Radiolysis

International audienceZirconolite is a candidate host material for conditioning minor tri- and tetra-valent actinides arising from enhanced nuclear spent fuel reprocessing and partitioning, in the case of disposal of the nuclear waste. Its chemical durability has been studied here under charged particle-induced radiolysis (He2+ and proton external beams) to identify the possible effects of water radiolysis on the dissolution rates in pure water and to describe the alteration mechanisms. Two experimental geometries have been used in order to evaluate the influence of the following parameters: solid irradiation, water radiolysis. In the first geometry the beam gets through the sample before stopping at the surface/water interface. In the second one the beam stops before the surface/water interface. Results on the elemental releases due to the enhanced dissolution of the zirconolite surface during charged particle-induced irradiation of water are presented. Under radiolysis, an increase of one order of magnitude is observed in the Ti, Zr and Nd elemental releases. No difference in the total elemental releases can be noticed when the solid is also irradiated

Leaching of zirconolite ceramics under H+H^+ and He2+He^{2+} irradiation

Zirconolite is a candidate host material for conditioning minor tri- and tetra-valent actinides arising from enhanced nuclear spent fuel reprocessing and partitioning, which can be disposed in a geological repository for nuclear waste. Its chemical durability has been studied here under charged particle-induced radiolysis (He2+ and proton external beams) to identify possible effects on dissolution rates and mechanisms in pure water. Two geometries of experiments have been used to evaluate the influence of the following parameters: solid irradiation, Linear Energy Transfer (LET) at the interface and total deposited energy. Preliminary results on the elemental releases due to the enhanced dissolution of the zirconolite surface during charged particle-induced irradiation are first presented. Then, we focus on H2O2 production which is one of the major molecular species, created under water radiolysis, and likely to interact with the zirconolite surface. In presence of zirconolite, first results indicate an apparent consumption of the radiolytic hydrogen peroxide or its precursors compared to the production in pure water calculated from the primary yield GH2O2. The measured H2O2 concentration varies linearly with the total deposited energy in water over the irradiation duration (between 1 h and 6 h) and in the conditions of our experiments. Moreover, the H2O2 concentration decreases when the local density of the deposited energy close to the interface increases. Thus, we suggest that the mechanism(s) leading to the consumption of H2O2 or its precursors involve zirconolite surface reactions

Sorption of aqueous carbonic, acetic, and oxalic acids onto et945;-alumina

International audienceThe presence of organic complexing agents can modify the behaviour of a surface. This study aims to better understand the impact of carboxylic acids (acetic, oxalic, and carbonic acids) issued from cellulose degradation and equally naturally present in soils. First, evidence of two different kinds of sites for chloride adsorption onto et945;-alumina and another for sodium sorption was provided. Consequently, no competition between these cation and anion sorptions occurs on et945;-alumina. The associated exchange capacities and ionic exchange constants were measured. Second, the adsorption behavior of the carboxylic acids was studied as a function of aqueous -lg(H+) and 0.01 to 0.1 M ionic strength (NaCl), and modeled by using mass action law for ideal biphasic systems. The carboxylic acids were found to be adsorbed on the same sites as chloride ions. The competition between organic ligands and chloride ions was satisfactorily accounted for by the model assuming the deprotonated form of the ligands was sorbed on et945;-alumina. The model also allowed us to interpret the adsorption of all species under various conditions without any extra fitting parameters

Dft Studies of Fluid-Minerals Interactions At the Molecular Level: Examples and Perspectives

The scope of applications of first-principle theoretical chemistry methods has been vastly expanded over the past years due to the combination of improved methods and algorithms for solving the polyelectronic Schrödinger equation with exponential growth of computer power available at constant cost (the so-called Moore law). In particular, atomistic studies of solid-fluid interfaces are now routinely producing new qualitative and quantitative insights into adsorption, surface speciation as a function of the prevailing chemical potentials, and reactivity of surface species. This approach is currently widely exploited in the fields of heterogeneous catalysis and surface physics, and so far to a lesser extent for geochemical purposes, although the situation is rapidly evolving. Many fundamental issues of fluid-minerals interaction phenomena can indeed be addressed ab initio with atomistic 3D periodic models of fluid-solid interfaces involving up to 200-300 unequivalent atoms. We illustrate this proposal with recent IFP results, some of which are of primary interest with respect to the manufacture of catalysts supports, but which also show some relevance for inorganic geochemical issues in the context of the sequestration of acid gases in subsurface porous rocks: - reactive wetting of boehmite AlOOH and morphology prediction; - acido-basic surface properties of a transition alumina; - hydroxylation and sulfhydrylation of anatase TiO2 surfaces. Through these examples, the performances of DFT and a variety of up-to-date modeling techniques and strategies are discussed

Sorption of aqueous carbonic, acetic, and oxalic acids onto et945;-alumina

International audienceThe presence of organic complexing agents can modify the behaviour of a surface. This study aims to better understand the impact of carboxylic acids (acetic, oxalic, and carbonic acids) issued from cellulose degradation and equally naturally present in soils. First, evidence of two different kinds of sites for chloride adsorption onto et945;-alumina and another for sodium sorption was provided. Consequently, no competition between these cation and anion sorptions occurs on et945;-alumina. The associated exchange capacities and ionic exchange constants were measured. Second, the adsorption behavior of the carboxylic acids was studied as a function of aqueous -lg(H+) and 0.01 to 0.1 M ionic strength (NaCl), and modeled by using mass action law for ideal biphasic systems. The carboxylic acids were found to be adsorbed on the same sites as chloride ions. The competition between organic ligands and chloride ions was satisfactorily accounted for by the model assuming the deprotonated form of the ligands was sorbed on et945;-alumina. The model also allowed us to interpret the adsorption of all species under various conditions without any extra fitting parameters

Evolution au cours du temps d'un grain de catalyseur d'hydrodémétallisation : module et simulation numérique de résultats expérimentaux Evolution in Time of a Hydrodemetallization Catalyst Pellet: Modeling and Numerical Simulation of Experimental Results

On propose un modèle de désactivation lente d'un catalyseur de répartition poreuse bimodale par dépôt solide d'un des produits de réaction dans les pores. Le dépôt solide catalyse également la réaction, la désactivation s'effectuant par diminution de la surface spécifique et bouchage progressif des pores. Le modèle prend en compte les limitations diffusionnelles existant à l'intérieur du grain de catalyseur. Ce modèle utilise une représentation géométrique particulière des pores, et une loi de diffusion tenant compte de la variation des paramètres texturaux au cours du temps. Il suppose la connaissance expérimentale de la surface spécifique totale et de la distribution du volume poreux en fonction du diamètre des pores. Ce modèle a permis de retrouver avec un très bon accord quantitatif la courbe de captation des métaux (Ni + V) en fonction du temps de fonctionnement obtenue expérimentalement pour un catalyseur d'hydrodémétallisation (HDM) sur un pétrole brut de Boscan désasphalté. Il permet en outre de prédire la durée de vie et le taux d'occupation final du volume poreux d'un catalyseur d'HDM modèle. <br> A model is proposed for the slow deactivation of a catalyst with a bimodal pore distribution by a solid deposit of one of the reaction products inside the pores. The solid deposit is by itself a catalyst for the reaction, with deactivation resulting from the progressive decrease of specific area and blockage of the pores. The model takes Intragranular diffusional limitations into account. This model uses an original geometric representation of the pores together with a diffusion law incorporating the variation of textural parameters in time. It requires the experimental determination of the total specific area and of pore volume distribution as a function of pore diameter. Numerical simulations with this model provided a very good quantitative fit with the capture curve for metals (Ni + V) as a function of operating time determined experimentally for a hydrodemetallization (HDM) catalyst with a deasphalted Boscan crude oil. This model is able to forecast the lifetime and end storage-capacity for the pore volume of a model HDM catalyst