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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.

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 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

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.

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.

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.