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International audience; Despite the broad relevance of copper nanoparticles in industrial applications, the fundamental understanding of oxidation and reduction of copper at the nanoscale is still a matter of debate and remains within the realm of bulk or thin film-based systems. Moreover, the reported studies on nanoparticles vary widely in terms of experimental parameters and are predominantly carried out using either ex situ observation or environmental transmission electron microscopy in a gaseous atmosphere at low pressure. Hence, dedicated studies in regards to the morphological transformations and structural transitions of copper-based nanoparticles at a wider range of temperatures and under industrially relevant pressure would provide valuable insights to improve the application-specific material design. In this paper, copper nanoparticles are studied using in situ Scanning Transmission Electron Microscopy to discern the transformation of the nanoparticles induced by oxidative and reductive environments at high temperatures. The nanoparticles were subjected to a temperature of 150 °C to 900 °C at 0.5 atm partial pressure of the reactive gas, which resulted in different modes of copper mobility both within the individual nanoparticles and on the surface of the support. Oxidation at an incremental temperature revealed the dependency of the nanoparticles’ morphological evolution on their initial size as well as reaction temperature. After the formation of an initial thin layer of oxide, the nanoparticles evolved to form hollow oxide shells. The kinetics of formation of hollow particles were simulated using a reaction-diffusion model to determine the activation energy of diffusion and temperature-dependent diffusion coefficient of copper in copper oxide. Upon further temperature increase, the hollow shell collapsed to form compact and facetted nanoparticles. Reduction of copper oxide was carried out at different temperatures starting from various oxide phase morphologies. A reduction mechanism is proposed based on the dynamic of the reduction-induced fragmentation of the oxide phase. In a broader perspective, this study offers insights into the mobility of the copper phase during its oxidation–reduction process in terms of microstructural evolution as a function of nanoparticle size, reaction gas, and temperature.

International audience; Recent technologies such as those using coal, natural gas or biomass as fuel are often facing the challenge of removing H2S impurities. Among the various existing routes for sulfur removal, the conversion of transition metal oxides into sulfides is often considered for deep gas purification. The ideal regenerative system, preventing waste generation, should combine a high affinity material towards H2S and an easy way for its regeneration into the initial oxide form. The present paper describes the reactivity of the ZnMoO4 mixed oxide material and ZnO–MoO3 oxides mixture as potential candidates for the regenerative H2S sorption process. The use of the QXAS technique allowed us to get time resolved information about both sulfidation and oxidative regeneration processes at Mo and Zn K-edges. Faced with the complexity of gas–solid reactions involving several phases, QXAS in combination with multivariate data analysis enabled us to follow the sulfidation and oxidative regeneration kinetics of both materials, with a description of the evolution of several intermediate phases. Both Mo and Zn K-edge spectroscopic data were analyzed and comparison of the evolution of ternary oxides containing the two elements proved to be an effective way for validating the results.Graphical abstract: In situ QXAS study of sulfidation/oxidative regeneration reactions of zinc molybdate (ZnMoO4) and ZnO–MoO3 materials

Distinct and simultaneous COS and HCN hydrolysis reactions over an industrial TiO 2 based catalyst were extensively studied in this work in the scope of synthesis gas purification applications. 144 experiments were carried out, including 92 experiments that allowed to achieve partial conversion rates and showed reaction kinetics sensitivity to operating parameters. Significant crossed influences were evidenced between both COS and HCN hydrolysis reactions. The concomitant occurrence of both reactions showed to detrimentally affect each other upon COS and HCN conversion rates, and therefore upon kinetic rates. This was explained through a competitive adsorption of HCN and COS reactants upon catalyst surface active sites. Inhibition of catalytic activity by the presence of NH 3 and H 2 O was also evidenced and explained through competitive adsorption phenomena. For the operating conditions ranges explored, H 2 S and CO 2 had no sensitive impact on the kinetics of the COS and HCN hydrolysis reactions. However, the moderate impact of CO 2 upon COS and HCN conversion rates might be explained by the large CO 2 excess compared to COS and HCN levels. A reaction model has been fully developed considering hydrodynamic, external mass transfer and intra particle diffusion limitations, and Langmuir-Hinshelwood reaction mechanisms for both COS and HCN hydrolysis reactions. Langmuir-Hinshelwood kinetic rate laws were indeed considered to account for the detrimental effect of gaseous species upon COS and HCN conversion kinetic rates, through competitive adsorption upon catalyst active sites of COS, HCN, H 2 O, and NH 3 . Collected kinetic data as a function of reactor size, gas residence time, temperature and reactants partial pressures were used to validate and fit kinetic and adsorption constants. Very good agreement was achieved between experimental and calculated COS and HCN conversion rates from the model developed, that allowed complete validation of the Langmuir-Hinshelwood based modeling. The coupled hydrodynamic-reaction model also constitutes a complete industrial reactor model taking into account all the potential limitations, and can be used as a powerful predicting tool for industrial process design, i.e. fully usable for industrial process scale-up and optimization purposes.

Recent technologies such as those using coal, natural gas or biomass as fuel are often facing the challenge of removing H2S impurities. Among the various existing routes for sulfur removal, the conversion of transition metal oxides into sulfides is often considered for deep gas purification. The ideal regenerative system, preventing waste generation, should combine a high affinity material towards H2S and an easy way for its regeneration into the initial oxide form. The present paper describes the reactivity of the ZnMoO4 mixed oxide material and ZnO-MoO3 oxides mixture as potential candidates for the regenerative H2S sorption process. The use of the QXAS technique allowed us to get time resolved information about both sulfidation and oxidative regeneration processes at Mo and Zn K-edges. Faced with the complexity of gas-solid reactions involving several phases, QXAS in combination with multivariate data analysis enabled us to follow the sulfidation and oxidative regeneration kinetics of both materials, with a description of the evolution of several intermediate phases. Both Mo and Zn K-edge spectroscopic data were analyzed and comparison of the evolution of ternary oxides containing the two elements proved to be an effective way for validating the results.

Sulfidation of undoped and aluminum doped zinc oxide materials has been performed by TGA under a H2S atmosphere in order to evaluate the impact of the doping element on sulfidation reaction kinetics and mechanism. The presence of aluminum seems to slow-down the reaction kinetics. This phenomenon might be explained by a modification of the solid state diffusion processes involved in ZnO sulfidation reaction and the related ZnS outward growth, assuming the presence of aluminum atoms inside ZnO and ZnS phases. In order to determine solid state diffusion mechanisms controlling the reaction kinetics, molecular dynamics simulations were performed using a Coulomb-Buckingham potential. Firstly, the diffusion of the different elements (Zn, O, S) was simulated for both the oxide and sulfide phases considering a vacancy mechanism. Secondly, simulations of the oxide phase doped by a trivalent cation were also performed. The results obtained in this preliminary work are presented and compared to the literature.

International audience; This work aims to study the evolution of NiO/NiAl2O4 redox performance with CO as fuel for Chemical Looping Combustion (CLC) applications. The oxygen carrier (OC) was investigated under alternating oxidizing and reducing conditions in a fixed bed reactor simulating the cyclic conditions of CLC. The study of the operating temperature influence reveals that total reduction capacity increases with temperature, due to the reaction of the binder NiAl2O4. Regeneration step performed under low oxygen concentration (5 vol.%) shows that the decrease in total reduction capacity during multiple cycles is minimized and the purity of produced CO2 is higher. Therefore, carrying out the regeneration step at lower oxygen concentrations can increase oxygen carrier lifetime. Characterization studies revealed the formation of a nickel rich layer at the surface (up to 20 μm thick at 900 °C) and a partial sintering of particles regenerated under 20 vol.% of oxygen. However, these phenomena are not observed on the particles regenerated under low oxygen concentration. The formation of this layer can be explained by the fact that nickel oxidation is carried out by migration of Ni2+ cations through the layer of NiO initially oxidized at the surface of the particle, the layer becomes thicker during cycles, because NiO does not return to its initial position. At higher temperature, the diffusion velocity increases and therefore the layer formed is thicker.Specific surface area of the oxygen carrier decreases as cycles number increases from 9.4 m2/g for the fresh material to 4.4 and 2.3 m2/g after thirty cycles at 750 °C and 900 °C respectively. The same trend is observed for porous volume. However, the decrease is lower when particles are regenerated under low oxygen concentration (6 m2/g).

International audience; Fischer-Tropsch (FT) based B-XTL processes are attractive alternatives for future energy production. These processes aim at converting lignocellulosic biomass possibly in co-processing with petcoke, coal, or vacuum residues into synthetic biofuels. A gasification step converts the feed into a synthesis gas (CO and H2 mixture) , which undergoes the Fischer-Tropsch reaction after H2/CO ratio adjustment and CO2 removal. However synthesis gas also contains various impurities that must be removed in order to prevent Fischer-Tropsch catalyst poisoning. Due to the large feedstocks variety that can be processed, significant variations of the composition of the synthesis gas are expected. Especially, this affects the nature of the impurities that are present (element, speciation), as well as their relative contents. Moreover, due to high FT catalyst sensitivity, severe syngas specifications regarding its purity are required. For these reasons, synthesis gas purification constitutes a major challenge for the development of B-XTL processes. In this article, we focus on these major hurdles that have to be overcome. The different kinds of syngas impurities are presented. The influence of the nature of feedstocks, gasification technology and operating conditions on the type and content of impurities is discussed. Highlight is given on the fate of sulfur compounds, nitrogen compounds, halides, transition and heavy metals. Main synthesis gas purification technologies (based on adsorption, absorption, catalytic reactions, etc.) are finally described, as well as the related challenges.

The control of oxide nanoparticle size and shape is of great importance to tune their physical and chemical properties. As a consequence, routine methods are required to determine the nanoparticle morphology. Since diffraction patterns strongly depend on both size and shape, X-ray powder diffraction (XRD) is widely used to determine the particle morphology. Herein, an accurate approach is proposed based on the simulation of morphology-dependent diffraction patterns using the Debye formula and on the comparison of these calculated patterns to the experimental ones. The most representative particle morphology is also obtained. This methodology has been first validated on a simple model system of MgO cubic and octahedral nanoparticles. It has been then applied to three different morphologies of boehmite AlOOH samples, obtained by varying the synthesis pH value. The method is characterized in terms of sensitivity, and extensive comparisons with transmission electron microscopy (TEM) results are also performed...

The elaboration of nanoparticles designed for technological applications in various fields such as catalysis, optics, magnetism, electronics… needs the strict control of their characteristics, especially chemical composition, crystalline structure, size, and shape. These characteristics bring the physical properties (color, magnetism, band gap…) of the material, and also the surface to volume ratio of particles which is of high importance when they are used as a chemically active or reactive support, in catalysis for instance. The nanoparticles may have also to be surface functionalized by various species, and/or dispersed in aqueous or non aqueous media. We will show that the aqueous chemistry of metal cations is a very versatile and attractive way for the design of oxide nanomaterials, allowing the control of size, shape, and crystalline structure for polymorphic materials. Aqueous surface chemistry, including adsorption of various species, may be used to modify the morphology of nanoparticles. In some cases, redox processes can be involved to control the morphology of nanoparticles. Technologically important nanomaterials such as titania, alumina, and iron oxides are studied.

International audience; The aim of this work is to investigate the contribution of the binder (NiAl2O4) on the performances of the oxygen carrier NiO/NiAl2O4. To this purpose, oxidation/reduction cycles have been performed in a fixed bed reactor using CO as a fuel. The results reveal that the binder can react with the fuel to form CO2, and that its total reduction capacity increases with temperature. XRD characterizations performed on the binder (on the fresh and after several cycles) show a shift of the diffraction peaks of NiAl2O4 toward the ones of γ-alumina, which can be attributed to a progressive decomposition of NiAl2O4 to alumina and NiO.

Zinc oxide-based materials are commonly used for the final desulfurization of synthesis gas in IGCC and Fischer–Tropsch based XTL processes. The formation of large amount of solid waste is the major issue of this process. In-situ oxidative regeneration is a promising way to reduce this waste formation and enhance desulfurization process efficiency and economics. However, previous studies showed that one of the major drawbacks of oxidative regeneration of sulfided oxides relies in the high operating temperature range required to overcome the formation of inhibitory sulfate phases. A preliminary work of the authors focused on single oxides identified that regeneration temperature of zinc oxide-based sorbent could be reduced through the addition of molybdenum oxide. Two composites oxides – a single oxides mixture (ZnO and MoO3) and a mixed oxide (ZnMoO4) – were synthesized and characterized. Their sulfidation and oxidative regeneration properties were investigated through thermogravimetry and in-situ characterizations. Sulfidation of the single oxides mixture was shown to be similar to the combination of the sulfidation of both independent single oxides. Mixed oxide sulfidation leads to ZnMoO4 phase demixing into ZnS, MoS2 and ZnMoO3. The oxidative regenerations of the sulfided single oxide mixtures and mixed oxide are initiated, respectively, at 350 °C and 300 °C. These temperatures are 250 °C and 300 °C lower than the regeneration temperature of a pure ZnS. For the sulfided mixed oxide regeneration is even complete at 500 °C under isothermal conditions. Regeneration of sulfided oxides mixture and mixed oxide was thus shown to exhibit synergetic effects, resulting from exothermic oxidative reactions of molybdenum phases. Heat energy released during these reactions is assumed to enhance ZnS oxidation kinetics at a temperature lower than the previously measured one.

International audience; This work was devoted to study experimentally and numerically the oxygen carrier (NiO/NiAl2O4) performances for Chemical-Looping Combustion applications. Various kinetic models including Shrinking Core, Nucleation Growth and Modified Volumetric models were investigated in a one-dimensional approach to simulate the reactive mass transfer in a fixed bed reactor. The preliminary numerical results indicated that these models are unable to fit well the fuel breakthrough curves. Therefore, the oxygen carrier was characterized after several operations using Scanning Electronic Microscopy (SEM) coupled with equipped with an energy dispersive X-ray spectrometer (EDX). These analyses showed a layer rich in nickel on particle surface. Below this layer, to a depth of about 10 µm, the material was low in nickel, being the consequence of nickel migration. From these observations, two reactive sites were proposed relative to the layer rich in nickel (particle surface) and the bulk material, respectively. Then, a numerical model, taking into account of both reactive sites, was able to fit well fuel breakthrough curves for all the studied operating conditions. The extracted kinetic parameters showed that the fuel oxidation was fully controlled by the reaction and the effect of temperature was not significant in the tested operating conditions range

RAFFINAGE+CGE; Metal oxide based materials are commonly used for the final non-regenerative desulfurization of synthesis gas in IGCC and Fischer-Tropsch based XTL processes. In situ oxidative regeneration is a promising way to prevent massive waste formation resulting from usual H2S scavenger processes, and leads to a global process efficiency and economics improvement. However, it is shown in the literature that refractory sulfate formation of sulfided oxides in oxidative conditions constitutes one of the major limitations for the development of an economic low temperature regeneration process. In the present paper, the authors propose a rational methodology to compare and identify single oxides most-suited for the regenerative desulfurization of syngas. This work brings novel insight through a systematic comparison of thermodynamic gas-solid equilibrium data and experimental study of sulfidation and oxidative regeneration of single oxides of interest. Based on equilibrium calculations, Ce, Cu, Fe, Mn, Mo and Zn oxides were selected for further experimental investigations, as they proved to offer the best compromise between the stabilities of their related sulfide and sulfate phases, respectively in reductive sulfiding and oxidative conditions. Sulfidation tests showed that Ce, Cu, Fe, Mn and Zn oxides were quantitatively transformed in their relative sulfide phases whereas Mo oxides could hardly be sulfided in the studied operating conditions. The oxidative regeneration of Ce, Cu, Fe and Mn-based sulfided materials led to a massive formation of sulfate phases which could only be decomposed at very high temperatures (>600 degrees C). At the opposite, zinc sulfatation was shown to be limited. The regeneration of MoO2/MoS2 mixture resulting from Mo oxide sulfidation was observed at reduced temperature (starting at 400 degrees C) without sulfate formation and proved to be very exothermic. All those results led the authors to state that no single oxide could offer optimal concomitant sulfidation and regeneration abilities required for an efficient regenerative syngas desulfurization process. However, this target could be achieved via a new composite materials that may combine several single oxides properties, exhibiting respectively high sulfur capacity, such as ZnO, and low regeneration temperature brought by the exothermic re-oxidation of sulfo-reduced MoO3. (C) 2014 Elsevier Ltd. All rights reserved.

International audience; Zinc oxide based materials are commonly used for the final desulfurization of synthesis gas in Fischer-Tropsch based XTL processes. Although the ZnO sulfidation reaction has been widely studied, little is known about the transformation at the crystal scale, its detailed mechanism and kinetics. A model ZnO material with well-determined characteristics (particle size and shape) has been synthesized to perform this study. Characterizations of sulfided samples (using XRD, TEM and electron diffraction) have shown the formation of oriented polycrystalline ZnS nanoparticles with a predominant hexagonal form (wurtzite phase). TEM observations also have evidenced an outward development of the ZnS phase, showing zinc and oxygen diffusion from the ZnO-ZnS internal interface to the surface of the ZnS particle. The kinetics of ZnO sulfidation by H2S has been investigated using isothermal and isobaric thermogravimetry. Kinetic tests have been performed that show that nucleation of ZnS is instantaneous compared to the growth process. A reaction mechanism composed of eight elementary steps has been proposed to account for these results, and various possible rate laws have been determined upon approximation of the rate-determining step. Thermogravimetry experiments performed in a wide range of H2S and H2O partial pressures have shown that the ZnO sulfidation reaction rate has a nonlinear variation with H2S partial pressure at the same time no significant influence of water vapor on reaction kinetics has been observed. From these observations, a mixed kinetics of external interface reaction with water desorption and oxygen diffusion has been determined to control the reaction kinetics and the proposed mechanism has been validated. However, the formation of voids at the ZnO-ZnS internal interface, characterized by TEM and electron tomography, strongly slows down the reaction rate. Therefore, the impact of the decreasing ZnO-ZnS internal interface on reaction kinetics has been taken into account in the reaction rate expression. In this way the void formation at the interface has been modeled considering a random nucleation followed by an isotropic growth of cavities. Very good agreement has been observed between both experimental and calculated rates after taking into account the decrease in the ZnO-ZnS internal interface.

This overview features the chemical background on condensation phenomena of the aluminum cation in aqueous solution. The formation of polycationic molecular clusters and nanosized solid phases of aluminum oxy(hydroxi)des is interpreted with illustrative mechanisms, building a bridge between solution chemistry and solid-state chemistry. The formation of the main structural types of aluminosilicates (zeolites, clays, imogolite) is also illustrated through the aqueous chemistry of aluminum and silicates.

International audience; Zinc oxide based materials are commonly used for the final desulfurization of synthesis gas in Fisher-Tropsch based XTL processes. Although the ZnO sulfidation reaction has been widely studied, little is known about the different steps involved in the solid transformation. Various ZnO materials with well-determined characteristics have been synthesized to perform this study. The kinetics of ZnO sulfidation by H2S has been investigated using isothermal and isobaric thermogravimetry. In view of the kinetic curves appearance and in agreement with literature, the rate-determining step is more likely to be a diffusion step. However a kinetic blocking has been noticed for some ZnO particles. As expected, characterizations of the reaction products using electronic microscopies indicate that a polycrystalline ZnS layer is formed around the monocrystalline ZnO particles, but surprisingly, cavities in the bulk of ZnO have been noticed at partial conversion. At total conversion, a void is observed in the core of the sulfided particles, which proves that ZnS is formed through an outward growth mechanism. Regarding the ZnO sulfidation reaction, the relevance of the widely used shrinking core based models has thus to be reconsidered. The formation of two ZnS polymorphs (sphalerite Zn1−xS and wurtzite ZnS1−x) has also been evidenced by XRD characterizations of sulfided solids, which may be explained in the operating conditions according to the literature by considering the nanometric size of ZnS crystals formed. As sphalerite is a zinc deficient phase, whereas wurtzite is sulfur deficient, different scenarios can be postulated for the mechanisms of zinc and oxygen diffusion occurring through the ZnS layer and involving the various related ZnS point defects. Beyond a critical value of fractional conversion, cavities that are formed at the internal ZnO/ZnS interface may cause the kinetic blocking of the sulfidation reaction by preventing the zinc atoms from diffusing into the ZnS layer.

International audience; This overview features the chemical background on condensation phenomena of the aluminum cation in aqueous solution. The formation of polycationic molecular clusters and nanosized solid phases of aluminum oxy(hydroxi)des is interpreted with illustrative mechanisms, building a bridge between solution chemistry and solid-state chemistry. The formation of the main structural types of aluminosilicates (zeolites, clays, imogolite) is also illustrated through the aqueous chemistry of aluminum and silicates.

The aim of this paper is to show that a very simple but well controlled chemistry in an aqueous medium allows one to efficiently control the main characteristics of oxide nanoparticles. Examples concerning titania, alumina, iron and manganese oxides are discussed to illustrate various effects on the control of size, shape and structure of nanoparticles. Some examples of functionalization of these particles are also illustrated. Experimental data, procedures and detailed references can be found in the cited literature.

Alumina γ-Al2O3 is widely used as catalyst support in refining and petrochemistry and is often obtained by calcination of γ-AlOOH boehmite. Several crucial properties of γ-Al2O3, including particles size and shape, are fixed at the boehmite stage. For this reason, the control and the understanding of boehmite particles properties is a fundamental task to tune the catalyst support. Several complementary techniques have been developed to characterize the boehmite particles morphology and the γ-Al2O3 formation, taking into account the specificity of these materials. These methods have been applied to several samples, aiming at rationalizing the morphology observations and preparation conditions.

TiO2 rutile nanorods of average length L = 160 +/- 40 nm and average diameter D = 15 +/- 5 nm have been synthesized through a seed-mediated growth process by TiCl4 hydrolysis in concentrated acidic solution. These nanorods were dispersed in water to yield stable (aggregation-free) colloidal aqueous suspensions. At volume fractions phi3%, the suspensions spontaneously display a phase separation into an isotropic liquid phase and a liquid-crystalline phase identified as nematic by X-ray scattering. At phi12%, the suspensions form a nematic single phase, with large order parameter, S = 0.75 +/- 0.05. Very well aligned rutile films on glass substrate were produced by spin-coating, and their photocatalytic properties were examined by monitoring the decomposition of methylene blue under UV light. We found that UV-light polarized along the quadratic axis of the rutile nanorods was most efficient for this photocatalytic reaction.

This preliminary study focuses on the ability of organic compounds such as polyols to tune size and morphology of boehmite nanoparticles by aluminium precipitation in aqueous medium. Results show a notable effect of both the synthesis pH and the carbon chain length of polyols, ie C4 and C5, on the size of particles and their thickness to width ratio. The specific surface area can be increased up to 382 m2.g−1. In addition, the morphology of particles is modified by C5 polyol.

Polyols were successfully used as size and shape controllers of oxide nanoparticles synthesized by soft chemistry in aqueous solution. The efficiency of acyclic polyols as a complexing agent depends obviously on the number of OH groups bonded to the carbon chain (and thus on the carbon chain length), but also on their stereochemistry. This innovating way to control morphology has been experienced for the synthesis of boehmite nanoparticles, whose morphology variations related to xylitol adsorption (C5 alditol) have been previously reported. The use of polyols during synthesis causes a modification of the usual morphologies observed, specifically resulting in an increase of (101) faces area. It is evidenced here that the variations of the nanoparticle aspect ratio are related to polyol complexing ability, and more specifically to molecule topology and configuration. Indeed, the morphology variations increase as a function of polyol carbon chain length and number of hydroxyl groups, and is much pronounced for stereoisomers exhibiting hydroxyl groups all oriented on the same side of the molecule (threo-threo sequences). Thanks to these various polyols used, we showed how the progressive levels of complexing ability allow us to tune boehmite particle size and shape. Material characterizations were performed using relevant methods such as X-ray diffraction powder pattern simulation and zetametry in addition to transmission electron microscopy. Since γ-alumina is obtained from boehmite by a topotactic transformation, we expect that this method will provide a promising way to control surface properties of γ-alumina, an important industrial catalyst support.

The ability to design nanoparticles size and shape through the addition of simple and commercially available organic molecules is of particular interest in the catalytic domain because huge amounts of very fine powders are needed. The origin of this effect is all the more difficult to elucidate because the involved interactions are weak. In this paper, we have investigated the shaping of boehmite AlO(OH) nanoparticles in the presence of polyols like xylitol (C(5) alditol) by a combined experimental and theoretical approach. Experimental techniques such as XRD, TEM, IEP measurements, adsorption isotherms measurements, and (13)C MAS NMR experiments demonstrate that the effect of xylitol has a thermodynamic origin and suggest weak interactions between xylitol and the surface. Furthermore, the strongest proportion of lateral faces ((100), (001), and (101)) that of basal face would be in agreement with a preferential adsorption upon lateral surfaces. These results were refined by a computational approach. DFT calculations of surface energies (taking into account temperature and solvation effects) and of NMR shielding constants corroborate that molecular adsorption mode is preferred over all adsorption modes involving exchanges with surface OH groups. The preferred adsorption on lateral surfaces is attributed to the nest effect induced by hydroxyl groups localized on the concavities of the (001) and (101) surfaces, able to stabilize the xylitol molecule by hydrogen-bonding, whereas the basal (010) surface is almost flat. This combined experimental and computational approach thus provides interesting rationalization for the morphology effects observed.

Phase transition phenomena involving the mobility of the reaction interface are involved in a wide variety of chemical reactions and applications. A good example is the sulfidation reaction experienced by the metal oxide-based materials used in the framework of gas purification or catalysts preparation applications. These reactions involve solid-state diffusion phenomena of the reactive species (atomic or ionic form) through the layer of product formed during the reaction (oxide, sulfide, or metal phase). In many cases, solid-state diffusion has a direct impact on the reaction mechanisms while determining the growth direction of the formed phases, as well as the overall kinetics of the reactions. This PhD-thesis work aims at providing a better understanding of the solid-state diffusion phenomena of reactive species involved in gas-solid heterogeneous reactions. In particular, the study is focused on zinc oxide sulfidation reaction with H2S, in which the influence of the crystal structure of solids, the presence of impurities and / or native or extrinsic point defects, and the impact of diffusion phenomena on the overall reaction kinetics were evaluated. The research strategy relies on a first experimental approach via the synthesis and characterizations of doped materials, followed by the determination of their sulfidation reaction kinetics by thermogravimetry under reactive atmosphere. The experimental work was combined to a theoretical approach based on Molecular Dynamics, which allows the determination of diffusion coefficients in different systems (ZnO and ZnS), mono/polycrystalline, and with/without presence of doping elements. Knowledge of the diffusion processes and of key parameters involved leads to a better understanding of solid-gas heterogeneous reactions.; Les phénomènes de transformation de phases avec interface réactionnelle mobile interviennent dans une grande variété de réactions chimiques et d’applications. Les réactions de sulfuration d'oxydes métalliques mènent par exemple à la formation de phases sulfures dans des applications concernant la purification de gaz ou la préparation de catalyseurs. Ces réactions impliquent entre autres des phénomènes de diffusion des espèces réactives à l’état solide (sous forme atomique ou ionique) à travers la couche de produit formée lors de la réaction (phase oxyde, sulfure ou métal). Dans de nombreux cas, les phénomènes de diffusion à l’état solide ont un impact direct sur les mécanismes ainsi que sur la cinétique globale des réactions, et déterminent le sens de croissance des phases formées. Cette thèse a pour objectif d’apporter une meilleure compréhension des phénomènes de diffusion d’espèces réactives à l’état solide impliqués dans les réactions hétérogènes gaz-solide. En particulier, cette étude porte sur le cas de laréaction de sulfuration de l’oxyde de zinc par H2S menant à la formation de ZnS. L’influence de la structure cristalline, de la présence d’impuretés et/ou de défauts ponctuels natifs ou extrinsèques et l’impact des phénomènes de diffusion sur la cinétique réactionnelle globale ont été étudiés. La stratégie de recherche proposée comporte un volet expérimental via la synthèse et la caractérisation de matériaux dopés, et l’étude de la cinétique de la réaction de sulfuration par thermogravimétrie sous atmosphère réactive. Le travail expérimental a été complété par une approche théorique par dynamique moléculaire permettant la détermination de coefficients de diffusion dans différents systèmes (ZnO et ZnS), mono/polycristallins, et avec/sans présence de dopants. La détermination des processus de diffusion et des paramètres qui la gouvernent permet d’aboutir, in fine, à une meilleure compréhension des réactions hétérogènes solide-gaz.