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See detailSmall-Angle Scattering in Porous Solids, an overview highlighting data analysis challenges
Gommes, Cédric ULg

Conference (2015, June)

Countless technologies and chemical processes make use of nanoporous materials: heterogeneous catalysis, including electrochemical reactions in fuel-cell electrodes, adsorption separation processes ... [more ▼]

Countless technologies and chemical processes make use of nanoporous materials: heterogeneous catalysis, including electrochemical reactions in fuel-cell electrodes, adsorption separation processes, kinetically selective membrane processes, are but a few examples. Nanopores are also relevant to natural processes as diverse as the weathering of rocks and ion transport through biological membranes.[1] Small-angle scattering of x-rays (SAXS) or neutrons (SANS) is one of the few experimental methods currently available for the in situ analysis of phenomena in this type of materials at the mesoscopic scale.[e.g. 2,3] In this presentation, we briefly review some recent applications of small-angle scattering to the in situ analysis of phenomena inside mesoporous solids. A particular focus is put on the data analysis challenges, whereby the scattered intensity is converted to real-space structures with nanometer resolution. [less ▲]

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See detailNanometer-scale wetting transitions in mesopores: a SAXS analysis
Gommes, Cédric ULg

Conference (2015, May)

The equilibrium and metastable configurations of confined binary liquids has been a topic of research since the early work of Liu et al. in the nineties [1]. In particular, it has been proposed ... [more ▼]

The equilibrium and metastable configurations of confined binary liquids has been a topic of research since the early work of Liu et al. in the nineties [1]. In particular, it has been proposed theoretically that liquids may coexist inside nanopores in the form of layers covering uniformly the solid surface, of plugs filling locally the pore space, or of capsules floating in the middle of the pores. In the present contribution, we report in situ synchrotron small-angle scattering (SAXS) experiments on hexane/nitrobenzene solutions confined in mesoporous carbon xerogels [2]. The SAXS shows that these systems exhibit reversible temperature-induced transitions between the layer and the plug configurations. The scattering data is analyzed using a so-called plurigaussian model, which enables us to reconstruct the configurations of the confined liquids, and quantitatively analyze the wetting transitions at the nanometer-scale in terms of changing interface areas, contact angles, and triple-line lengths. [less ▲]

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See detailDerjaguin-Broekhoff-de Boer analysis of adsorption in very disordered mesopores using probabilistic models
Gommes, Cédric ULg

Poster (2015, May)

Our current ability to produce mesoporous materials with ordered morphology has raised fascinating questions about the impact of geometrical disorder on adsorption and desorption states [1]. Many recent ... [more ▼]

Our current ability to produce mesoporous materials with ordered morphology has raised fascinating questions about the impact of geometrical disorder on adsorption and desorption states [1]. Many recent works have investigated the role of mild elements of disorder, such as local constriction or corrugation superimposed to otherwise geometrically ideal cylindrical pores [2,3]. These works have notably shown that elements of disorder may act as nucleation sites and destabilize vapor-like metastable states. The relevance of these perturbation-like results to fundamentally disordered materials, such as gels, is unclear. In particular, do vapor-like metastable states exist at all in this type of very disordered material? In the present communication, we address this question using probabilistic models to investigate the role of disorder. We generalize the classical Gaussian field models of porous materials [4] and use them to analyze adsorption and desorption in the Derjaguin-Broekhof-de Boer approximation. Our approach differs from earlier contributions in that both the adsorbent and the adsorbate are described in terms of probabilities [5]. This enables us to analyze the adsorbate configuration in very disordered solids using a low-dimensional yet realistic configuration space. We notably show that vapor-like metastable states are unlikely in gel-like disordered materials. [1] D.Wallacher, N. Künzner, D. Kovalev, N. Knorr, K. Knorr, Capillary condensation in linear mesopores of different shape, Phys. Rev. Lett. 92 (2004) 195704; [2] B. Coasne, A. Galarneau, F. Di Renzo, R.M.J. Pellenq, Effect of morphological defects on gas adsorption in nanoporous silicas, J. Phys. Chem. C 111 (2007) 15759; [3] C.J. Gommes, Adsorption, capillary bridge formation, and cavitation in SBA-15 corrugated mesopores: A Derjaguin-Broekhoff-de Boer analysis, Langmuir 28 (2012) 5101-5115; [4] R.J. Pellenq, P. levitz, Capillary condensation in a disordered mesoporous medium: A grand canonical Monte Carlo study, Molecular Physics 100 (2002) 2059;[5] C.J. Gommes, A.P. Roberts, in preparation. [less ▲]

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See detailMesoscale Characterization of Nanoparticles Distribution Using X-Ray Scattering
Gommes, Cédric ULg; Prieto, Gonzalo; Zecevic, Jovana et al

in Angewandte Chemie International Edition (2015)

The properties of many functional materials depend critically on the spatial distribution of an active phase within a matrix or support material. In the case of solid catalysts, controlling the spatial ... [more ▼]

The properties of many functional materials depend critically on the spatial distribution of an active phase within a matrix or support material. In the case of solid catalysts, controlling the spatial distribution of metal (oxide) nanoparticles at the mesoscopic scale offers new strategies to tune their performance and enhance their lifetimes. However, such advanced control requires the development of suitable methods to characterize the spatial distribution of nanoparticles at the mesoscopic scale. Currently electron microscopy and more specifically electron tomography is close to being the only option. Here, we show how the background in x-ray scattering patterns can be analyzed to quantitatively access the distribution of metal nanoparticles within support materials displaying hierarchical porosity. Our approach is illustrated for copper catalysts supported on meso- and micro-porous silica, which display distinctly different metal spatial distributions. Results derived from the modeling of x-ray scattering patterns are in excellent agreement with electron tomography observations, while the amount of material being characterized at once is enhanced by twelve orders of magnitude. Our strategy opens unprecedented prospects to understand structure-property relationships and to guide the synthesis of advanced supported catalysts as well as a wide array of other functional nanomaterials. [less ▲]

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See detailSupport Functionalization To Retard Ostwald Ripening in Copper Methanol Synthesis Catalysts
Van den Berg, Roy; Parmentier, Tanja; Elkjaer, Christian et al

in ACS Catalysis (2015), 5

A main reason for catalyst deactivation in supported catalysts for methanol synthesis is copper particle growth. We have functionalized the support surface in order to suppress the formation and/or ... [more ▼]

A main reason for catalyst deactivation in supported catalysts for methanol synthesis is copper particle growth. We have functionalized the support surface in order to suppress the formation and/or transport of mobile copper species and thereby catalyst deactivation. A Stöber silica support was functionalized by treatment with aminopropyltriethoxysilane, which introduces aminopropyl groups on the surface. Copper was deposited on both unfunctionalized and functionalized Stöber silica via incipient wetness impregnation with aqueous copper nitrate solutions followed by drying and calcination. Similar particle size distributions (1−5 nm) were obtained for both supports by changing the flow of N2 to a flow of 2% NO/N2 during calcination of the unfunctionalized and amine functionalized silica, respectively. The effect of support functionalization with aminopropyl groups was an increased stability in the methanol synthesis reaction (40 bar, 260 °C, 23% CO/7% CO2/60% H2/10% Ar, 3% COx conversion) due to more limited copper particle growth as determined by transmission electron microscopy (TEM). Changing the interparticle distance did not have an influence on the deactivation rate, while the addition of few very large copper particles did, indicating that Ostwald ripening was most probably the dominant particle growth mechanism for these samples. In situ TEM images showed the contact angle between the reduced copper particles and the support. As shape and size was similar on silica as on amine-functionalized silica, the thermodynamic stability of the copper particles was unaltered. The driving force for copper particle growth was thus unchanged upon functionalization. We therefore suggest that Ostwald ripening in methanol synthesis catalysts was retarded by inhibiting the transport of copper species over the support surface. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) revealed a decrease in the number of surface groups (hydroxyl, methoxy, and aminopropyl) upon functionalization because aminopropyltriethoxysilane reacted with multiple hydroxyl groups. Because of that, the distance between neighboring functional groups was increased, suppressing the mobility of Ostwald ripening species from one copper particle to another. [less ▲]

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See detailThe range of validity of sorption kinetic models
Douven, Sigrid ULg; Pàez Martinez, Carlos ULg; Gommes, Cédric ULg

in Journal of Colloid & Interface Science (2015), 448

Several hundred papers are published yearly reporting liquid-phase adsorption kinetics data. In general the data is analyzed using a variety of standard models such as the pseudo first- and second-order ... [more ▼]

Several hundred papers are published yearly reporting liquid-phase adsorption kinetics data. In general the data is analyzed using a variety of standard models such as the pseudo first- and second-order models and the Intraparticle-Diffusion model. The validity of these models is often assessed empirically via their ability to fit the data, independently of their physicochemical soundness. The aim of the present paper is to rationalize the analysis of liquid-phase adsorption kinetics data, and to investigate experimental factors that influence the adsorption kinetics, in addition to the characteristics of the adsorbent material itself. For that purpose we use a simple Langmuir adsorption–diffusion model, which enables us to identify three dimensionless numbers that characterize the working regime of any batch adsorption experiment: an adsorption Thiele modulus, a saturation modulus, and a loading modulus. The standard models are found to be particular cases of the general adsorption–diffusion model for specific values of the dimensionless numbers. This provides sound physicochemical criteria for the validity of the models. Based on our modeling, we also propose a general yet simple data analysis procedure to practically estimate the diffusion coefficient in adsorbent pellets starting from adsorption halftimes. [less ▲]

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See detailSmall Angle X-ray Scattering Insights into the Architecture-Dependent Emulsifying Properties of Amphiphilic Copolymers in Supercritical Carbon Dioxide
Alaimo, David ULg; Hermida Merino, Daniel; Grignard, Bruno ULg et al

in Journal of Physical Chemistry B (2015), 119

The supramolecular assembly of a series of copolymers combining a PEO-rich hydrophilic and fluorinated CO2-philic sequences is analysed by synchrotron small-angle xray scattering (SAXS) in supercritical ... [more ▼]

The supramolecular assembly of a series of copolymers combining a PEO-rich hydrophilic and fluorinated CO2-philic sequences is analysed by synchrotron small-angle xray scattering (SAXS) in supercritical CO2, as well as in water/CO2 emulsions. These copolymers were designed to have the same molecular weight and composition, and to differ only by their macromolecular architecture. The investigated copolymers have random, block, and palm-tree architectures. Besides, thermo-responsive copolymer is also analysed, having a hydrophilic sequence becoming water-insoluble around 41 °C, i.e. just above the critical point of CO2. At the length scale investigated by SAXS, only the random copolymer appears to self-assemble in pure CO2, in the form of a disordered microgel-like network. The random, block and thermo-responsive copolymers are all able to stabilize water/CO2 emulsions but not the copolymer with the palm-tree architecture, pointing at the importance of macromolecular architecture for the emulsifying properties. A modelling of the SAXS data shows that the block and the thermo-responsive copolymers form spherical micelle-like structures containing about 70 % water and 30 % polymer. [less ▲]

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See detailModeling reaction-limited Ostwald ripening of supported Ni catalysts in presence of CO: the role of particle size distribution
Gommes, Cédric ULg; Munnik, Peter; de Jongh, Petra et al

Poster (2014, March)

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See detailThe Structure and Thermal Stability of Amylose−Lipid Complexes: A Case Study on Amylose−Glycerol Monostearate
Goderis, Bart; Putseys, Joke; Gommes, Cédric ULg et al

in Crystal Growth & Design (2014)

Three different crystalline amylose−glycerol monostearate (GMS) complexes with increasing thermal stability can be distinguished: type I, type IIa, and type IIb. All complexes consist of GMS-loaded ... [more ▼]

Three different crystalline amylose−glycerol monostearate (GMS) complexes with increasing thermal stability can be distinguished: type I, type IIa, and type IIb. All complexes consist of GMS-loaded amylose helices that pack hexagonally into lamellar habits. The complex melting points are proportional to the thickness of the lamellae and depend on the amount of water in the system. For type I complexes, SAXS experiments reveal folded amylose chains and a lamellar thickness governed by the presence of two stretched lipid molecules per amylose helix. In the conversion from type I to type IIa complexes, the short amylose chains unfold and assume a stretched conformation, which increases the number of aligned lipid molecules within the helices to four. In type IIb complexes, another pair of lipid molecules is added. The derived quantitative relation between crystal layer thickness, water content and melting point for amylose−GMS complexes also predicts the melting points of other amylose−monoacyl glycerol complexes. [less ▲]

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See detailA high pressure cell for supercritical CO2 on-line chemical reactions studied with x-ray techniques
Hermida-Merino, Daniel; Portale, Giuseppe; Fields, Peter et al

in Review of Scientific Instruments (2014), 85

A versatile high pressure X-ray sample cell has been developed for conducting in situ time-resolved X-ray scattering experiments in the pressure and temperature regime required (pressures up to 210 bars ... [more ▼]

A versatile high pressure X-ray sample cell has been developed for conducting in situ time-resolved X-ray scattering experiments in the pressure and temperature regime required (pressures up to 210 bars and temperatures up to 120 °C) for chemical reactions in supercritical fluids. The large exit opening angle of the cell allows simultaneous performance of SAXS-WAXS experiments. Diamond windows are used in order to benefit from the combination of maximum strength, minimal X-ray absorption and chemical inertia. The sample cell can also be utilised for X-ray spectroscopy experiments over a wide range of photon energies. Results of the online synthesis of a block copolymer, poly(methyl methacrylate-block-poly(benzyl methacrylate), by Reversible Addition-Fragmentation Chain Transfer (RAFT) in a supercritical CO2 dispersion polymerisation will be discussed. The contribution of the density fluctuations, as function of temperature, to the X-ray scattering signal has been quantified in order to allow appropriate background subtractions [less ▲]

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See detailRapid aqueous synthesis of ordered mesoporous carbons: Investigation of synthesis variables and application as anode materials for Li-ion batteries
Léonard, Alexandre ULg; Gommes, Cédric ULg; Piedboeuf, Marie-Laure ULg et al

in Microporousand Mesoporous Materials (2014), 195

Ordered mesoporous carbons (OMC) were synthesized via a direct templating pathway by a synthesis route that features short duration, moderate temperature and aqueous media. Resorcinol was used as carbon ... [more ▼]

Ordered mesoporous carbons (OMC) were synthesized via a direct templating pathway by a synthesis route that features short duration, moderate temperature and aqueous media. Resorcinol was used as carbon precursor and hexamethylenetetramine as a source of formaldehyde and ammonia to respectively cross-link the framework and regulate the pH. The temperature of the heat treatment leading to the formation of the solid polymer was shown to have a strong influence on the structural and textural parameters. In particular, moderate temperatures led to the coexistence of differently-sized entangled hexagonal mesostructures, whereas the higher temperatures led to a sharp decrease in the mesopore volume. The performance of these materials as anode materials for Li-ion batteries has been investigated in detail. Although these OMC show reversible capacities similar to those reported for hard carbons, their long-term cycling remains very stable for over 100 cycles of charge/discharge. The optimization of the reported short preparation pathway offers new possibilities regarding the application of ordered mesoporous carbons in various fields, such as energy storage, sorption and heterogeneous catalysis [less ▲]

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See detailNanoparticle Growth in Supported Nickel Catalysts during Methanation Reaction-Larger is Better
Munnik, Peter; Velthoen, Marjolein; de Jongh, Petra et al

in Angewandte Chemie International Edition (2014)

A major cause of supported metal catalyst deactivation is particle growth by Ostwald ripening. Nickel catalysts, used in the methanation reaction, may suffer greatly from this through the formation of [Ni ... [more ▼]

A major cause of supported metal catalyst deactivation is particle growth by Ostwald ripening. Nickel catalysts, used in the methanation reaction, may suffer greatly from this through the formation of [Ni(CO)4]. By analyzing catalysts with various particle sizes and spatial distributions, the interparticle distance was found to have little effect on the stability, because formation and decomposition of nickel carbonyl rather than diffusion was rate limiting. Small particles (3–4 nm) were found to grow very large (20–200 nm), involving local destruction of the support, which was detrimental to the catalyst stability. However, medium sized particles (8 nm) remained confined by the pores of the support displaying enhanced stability, and an activity 3 times higher than initially small particles after 150 h. Physical modeling suggests that the higher [Ni(CO)4] supersaturation in catalysts with smaller particles enabled them to overcome the mechanical resistance of the support. Understanding the interplay of particle size and support properties related to the stability of nanoparticles offers the prospect of novel strategies to develop more stable nanostructured materials, also for applications beyond catalysis. [less ▲]

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See detailThe number of structures compatible with any correlation function
Gommes, Cédric ULg

Conference (2013, July)

A two-point correlation function provides a crucial yet an incomplete characterization of a microstructure because distinctly different microstructures may have the same correlation function. We address ... [more ▼]

A two-point correlation function provides a crucial yet an incomplete characterization of a microstructure because distinctly different microstructures may have the same correlation function. We address here the microstructural degeneracy question: What is the number of microstructures compatible with a specified correlation function? We compute this degeneracy in the framework of reconstruction methods, which enables us to map the problem to the determination of ground-state degeneracies. Since the configuration space of a reconstruction problem is a hypercube on which a Hamming distance is defined, we can calculate analytically the energy profile of any reconstruction problem, corresponding to the average energy of all microstructures at a given Hamming distance from a ground state. The steepness of the energy profile is a measure of the roughness of the energy landscape associated with the reconstruction problem, which can be used as a proxy for the ground-state degeneracy. The relationship between this roughness metric and the ground-state degeneracy is calibrated using a Monte Carlo algorithm for determining the ground-state degeneracy of a variety of microstructures (e.g., realizations of hard disks and Poisson point processes at various densities), as well as with microstructures with a known degeneracy (e.g., single disks of various sizes and a particular crystalline microstructure). We show that our results can be expressed in terms of the information content of the two-point correlation functions. From this perspective, the a priori condition for a reconstruction to be accurate is that the information content, expressed in bits, should be comparable to the number of pixels in the unknown microstructure. We provide a formula to calculate the information content of any two-point correlation function, which makes our results directly applicable by other researchers in the field. [less ▲]

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See detailThree-dimensional reconstruction of liquid phases in disordered mesopores using in situ small-angle scattering
Gommes, Cédric ULg

in Journal of Applied Crystallography (2013), 46

Small-angle scattering of X-rays (SAXS) or neutrons is one of the few experimental methods currently available for the in situ analysis of phenomena in mesoporous materials at the mesoscopic scale. In the ... [more ▼]

Small-angle scattering of X-rays (SAXS) or neutrons is one of the few experimental methods currently available for the in situ analysis of phenomena in mesoporous materials at the mesoscopic scale. In the case of disordered mesoporous materials, however, the main difficulty of the method lies in the data analysis. A stochastic model is presented, which enables one to reconstruct the three-dimensional nanostructure of liquids confined in disordered mesopores starting from small-angle scattering data. This so-called plurigaussian model is a multi-phase generalization of clipped Gaussian random field models. Its potential is illustrated through the synchrotron SAXS analysis of a gel permeated with a critical nitrobenzene/hexane solution that is progressively cooled below its consolute temperature. The reconstruction brings to light a wetting transition whereby the nanostructure of the pore-filling liquids passes from wetting layers that uniformly cover the solid phase of the gel to plugs that locally occlude the pores. Using the plurigaussian model, the dewetting phenomenon is analyzed quantitatively at the nanometre scale in terms of changing specific interface areas, contact angle and specific length of the triple line. [less ▲]

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See detailTHREE-DIMENSIONAL RECONSTRUCTION OF LIQUID PHASES CONFINED IN DISORDERED POROUS MEDIA: AN IN SITU SYNCHROTRON SAS ANALYSIS
Gommes, Cédric ULg; Hermida Merino, Daniel

Conference (2012, December)

We use Synchrotron Small-Angle X-ray Scattering to investigate temperature-induced morphological changes in binary hexane/nitrobenzene solutions confined in the pores of disordered mesoporous solids. The ... [more ▼]

We use Synchrotron Small-Angle X-ray Scattering to investigate temperature-induced morphological changes in binary hexane/nitrobenzene solutions confined in the pores of disordered mesoporous solids. The scattering data is analyzed with a plurigaussian model, which enables us to reconstruct the 3D morphology of the phases with a nanometer resolution. The reconstructions bring to light wetting transitions whereby the morphology of the nitrobenzene passes from a nanometer-thin layer that uniformly covers the solid surface to plugs that locally occlude the pore space. Our analysis enables us to quantitatively analyze the SAS data in terms of changing interface areas and wetting angles. The present contribution offers unprecedented insight into nanometer-scale wetting transitions. It also presents a novel SAS data analysis methodology that is applicable to a host of experimental situations involving disordered mesoporous materials. [less ▲]

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