Sol-Gel-Derived Ordered Mesoporous High Entropy Spinel Ferrites and Assessment of Their Photoelectrochemical and Electrocatalytic Water Splitting Performance.

The novel material class of high entropy oxides with their unique and unexpected physicochemical properties is a candidate for energy applications. Herein, it is reported for the first time about the physico- and (photo-) electrochemical properties of ordered mesoporous (CoNiCuZnMg)Fe2 O4 thin films synthesized by a soft-templating and dip-coating approach. The A-site high entropy ferrites (HEF) are composed of periodically ordered mesopores building a highly accessible inorganic nanoarchitecture with large specific surface areas. The mesoporous spinel HEF thin films are found to be phase-pure and crack-free on the meso- and macroscale. The formation of the spinel structure hosting six distinct cations is verified by X-ray-based characterization techniques. Photoelectron spectroscopy gives insight into the chemical state of the implemented transition metals supporting the structural characterization data. Applied as photoanode for photoelectrochemical water splitting, the HEFs are photostable over several hours but show only low photoconductivity owing to fast surface recombination, as evidenced by intensity-modulated photocurrent spectroscopy. When applied as oxygen evolution reaction electrocatalyst, the HEF thin films possess overpotentials of 420 mV at 10 mA cm-2 in 1 m KOH. The results imply that the increase of the compositional disorder enhances the electronic transport properties, which are beneficial for both energy applications.

Effects of Hydrothermal Treatment on Mesopore Structure and Connectivity in Doped Ceria-Zirconia Mixed Oxides.

Pore size and pore connectivity control diffusion-based transport in mesopores, a crucial property governing the performance of heterogeneous catalysts. In many cases, transition-metal oxide catalyst materials are prepared from molecular precursors involving hydrothermal treatment followed by heat treatment. Here, we investigate the effects of such a hydrothermal aftertreatment step, using an aqueous ammonia solution, on the disordered mesopore network of CexZr1-x-y-zYyLazO2-δ mixed oxides. This procedure is a common synthesis step in the preparation of such ceria-based oxygen storage materials applied in three-way catalysis, employed to improve the materials' thermal stability. We perform state-of-the-art Ar-physisorption analysis, especially advanced hysteresis scanning, to paint a detailed picture of the alterations in mesopore space caused by the hydrothermal aftertreatment and subsequent aging at 1050 °C. Furthermore, we investigate the network characteristics by electron tomography in combination with suitable statistical analysis, enabling a consistent interpretation of the desorption scans (physisorption). The results indicate that the hydrothermal aftertreatment enhances the mesopore connectivity of the continuous 3D network by widening pores and especially necks, hence facilitating accessibility to the particles' internal surface area and the ability to better withstand high temperatures.

Tuning Mesopore Accessibility of Ce0.18Zr0.64Y0.15La0.03O2-δ by Hydrothermal Post-treatment─A Case Study for Ceria-Based Oxidation Storage Materials.

The connectivity and thermal stability of pores in heterogeneous, mesoporous metal oxide catalysts are key properties controlling their (long-term) efficacy. In this study, we investigate the influence of pH and temperature during a common hydrothermal aftertreatment step in the synthesis of mesoporous CexZr1-x-y-zYyLazO2-δ oxides obtained from molecular precursors via hydrothermal synthesis. This study has a strong focus on the methodological approach, elucidating whether and how even the smallest changes in morphology and connectivity may be unraveled and related to the underlying chemical processes to uncover key parameters for the ongoing improvement of material properties. Deep insights into the mesopore space were obtained by state-of-the-art physisorption (including hysteresis scanning), electron tomography, and small-angle X-ray scattering (SAXS) analysis. We also provide a simple tool to simulate SAXS curves from electron tomography data that allow direct comparison to experimentally obtained SAXS curves. Furthermore, the impact on surface-bound nitrate groups and the development during calcination were studied in detail by thermogravimetric analysis coupled with mass spectrometry. The key observations indicate a significant increase in thermal stability at temperatures as high as 1050 °C and improved mesopore accessibility with an increase in pH of the aftertreatment solution. The combined observations from the employed methods suggest a pH-dependent removal of surface-bound nitrate groups as well as a dissolution and reprecipitation-based fusing of the primary particles that constitute the mesopore skeleton. This transformation yields a mechanically and thermally stronger mesopore space with the capability to endure high temperatures.

H2 S Dosimetry by CuO: Towards Stable Sensors by Unravelling the Underlying Solid-State Chemistry.

The precise detection of the toxic gas H2 S requires reliable sensitivity and specificity of sensors even at minute concentrations of as low as 10 ppm, the value corresponding to typical exposure limits. CuO can be used for H2 S dosimetry, based on the formation of conductive CuS and the concomitant significant increase in conductance. In theory, at elevated temperature the reaction is reversed and CuO is formed, ideally enabling repeated and long-term use of one sensor. Yet, the performance of CuO tends to drop upon cycling. Utilizing defined CuO nanorods we thoroughly elucidated the associated detrimental chemical changes directly on the sensors, by Raman and electron microscopy analysis of each step during sensing (CuO→CuS) and regeneration (CuS→CuO) cycles. We find the decrease in the sensing performance is mainly caused by the irreversible formation of CuSO4 during regeneration. The findings allowed us to develop strategies to reduce CuSO4 formation and thus to substantially maintain the sensing stability even for repeated cycles. We achieved CuO-based dosimeters possessing a response time of a few minutes only, even for 10 ppm H2 S, and prolonged life-time.

Development of Pore Morphology During Nitrate Group Removal by Calcination of Mesoporous CexZr1-x-y-zYyLazO2-δ Powders.

Here, we present a study of the development of the micro- and mesoporosity of a CexZr1-x-y-zYyLazO2-δ oxygen storage material upon treatment at temperatures up to 1050 °C. The investigated powder, obtained from nitrate-based metal oxide precursors in a specially developed hydrothermal synthesis, is highly crystalline, features a high surface area and does not show phase segregation at high temperatures. By employing an advanced methodology, consisting of state-of-the-art argon physisorption, thermogravimetric analysis coupled with mass spectrometry (TG-MS) and X-ray powder diffraction (XRD) along with Raman spectroscopy, we correlate the stability of the mesopore system to the presence of surface-bound nitrate groups introduced during synthesis, which prevent sintering up to a temperature of 600 °C. In addition, the connectivity of mesopores was further studied by hysteresis scanning within the argon physisorption measurements. These advanced physisorption experiments suggest a three-dimensionally interconnected pore system and, in turn, a 3D network of the material itself on the nanometer scale which appears to be beneficial to endow the mesopore space with enhanced stability against sintering and mesopore collapse once the removal of nitrate groups is completed.

Determining Mechanical Moduli of Disordered Materials with Hierarchical Porosity on Different Structural Levels.

The impact of synthesis parameters and structural properties, respectively, on mechanical properties of porous materials on different structural levels provides valuable information for designing materials for specific applications. Within this study, we apply two nonstandard approaches for determining the mechanical properties of the mesoporous backbone phase in a series of disordered SiO2-based monolithic materials possessing hierarchical meso-macroporosity, that is, deformation upon mercury porosimetry and in situ dilatometry during nitrogen adsorption analysis. By using ordered porous model materials, the latter method has been recently proven to provide reliable mechanical moduli. This concept was now applied to a SiO2 monolith developed for high-performance liquid chromatography exhibiting disordered hierarchical meso- and macroporosity, as well as a series of analogue phenyl-modified meso-macroporous SiO2 monoliths with up to 36.1 at% organic modification. The phenyl group was introduced by adding phenyltrimethoxysilane to the sol-gel mixture. The study aimed at investigating in detail the impact of the organic modification on the morphology of the porous solid and the resulting mechanical properties. The study shows that both Hg porosimetry and in situ dilatometry performed during N2 adsorption at 77 K provide similar and reasonable moduli of compression for the mesoporous backbone of the silica materials investigated. These data were compared with moduli of the macroscopic sample as determined from sound velocity measurements by describing the fully connected macroporous backbone with a foam model. The comparison reveals an otherwise overseen side effect of the organic modification of the silica framework: in contrast to the pure reference SiO2 meso-macroporous monoliths, the hybrid material is composed of a more particulate morphology on the mesoscale, that is, mesoporous particles and corresponding necks between them are formed, which results in significant softening of the porous solid on the macroscale.

Catalytic Stability Studies Employing Dedicated Model Catalysts.

Long-term stability of heterogeneous catalysts is an omnipresent and pressing concern in industrial processes. Catalysts with high activity and selectivity can be searched for by high-throughput screening methods based maybe on educated guesses provided by ab initio thermodynamics or scaling relations. However, high-throughput screening is not feasible and is hardly able to identify long-term stable catalyst so that a rational and knowledge-driven approach is called for to identify potentially stable and active catalysts. Unfortunately, our current microscopic understanding on stability issues is quite poor. We propose that this gap in knowledge can be at least partly closed by investigating dedicated model catalyst materials with well-defined morphology that allow for a tight link to theory and the application of standard characterization methods. This topic is highly interdisciplinary, combining sophisticated inorganic synthesis with catalysis research, surface chemistry, and powerful theoretical modeling. In this Account, we focus on the stability issues of Deacon catalysts (RuO2 and CeO2-based materials) for recovering Cl2 from HCl by aerobic oxidation and how to deepen our microscopic insight into the underlying processes. The main stability problems under harsh Deacon reaction conditions concomitant with a substantial loss in activity arise from deep chlorination of the catalyst, leaching of volatile chlorides and oxychlorides, and decrease in active surface area by particle sintering. In general, powder materials with undefined particle shape are not well suited for examining catalyst stability, because changes in the morphology are difficult to recognize, for instance, by electron microscopy. Rather, we focus here on model materials with well-defined starting morphologies, including electrospun nanofibers, shape-controlled nanoparticles, and well-defined ultrathin crystalline layers. CeO2 is able to stabilize shape-controlled particles, exposing a single facet orientation so that comparing activity and stability studies can reveal structure sensitive properties. We develop a quasi-steady-state kinetic approach that allows us to model the catalyst chlorination as a function of temperature and gas feed composition. For the case of pure CeO2 nanocubes, this simple approach predicts chlorination to be efficiently suppressed by addition of little amounts of water in the reaction feed or by keeping the catalyst at higher temperature. Both process parameters have great impact on the actual reactor design. Thermal stabilization of CeO2 by intermixing Zr has been known in automotive exhaust catalysis for decades, but this does not necessarily imply also chemical stabilization of CeO2 against bulk-chlorination since Zr can readily form volatile ZrCl4 and may quickly lose its stabilizing effect. Nevertheless, with model experiments the stabilizing effect of Zr in the Deacon process over mixed CexZr1-xO2 nanorods is clearly evidenced. Even higher stability can be accomplished with ultrathin CeO2 coatings on preformed ZrO2 particles, demonstrating the great promise of atomic layer deposition (ALD) in catalysis synthesis.

Comprehensive Characterization of a Mesoporous Cerium Oxide Nanomaterial with High Surface Area and High Thermal Stability.

In the present study, the pore space of a mesoporous cerium oxide material is investigated, which forms by the self-assembly of primary particles into a spherical secondary structure possessing a disordered mesopore space. The material under study exhibits quite stable mesoporosity upon aging at high temperatures (800 °C) and is, thus, of potential interest in high-temperature catalysis. Here, different characterization techniques were applied to elucidate the structural evolution taking place between heat treatment at 400 °C and aging at 800 °C, i.e., in a water-containing atmosphere, which is usually detrimental to nanoscaled porosity. The changes in the mesoporosity were monitored by advanced physisorption experiments, including hysteresis scanning, and electron tomography analysis coupled with a 3D reconstruction of the mesopore space. These methods indicate that the 3D spatial arrangement of the primary particles during the synthesis under hydrothermal conditions via thermal hydrolysis is related to the thermal stability of the hierarchical mesopore structure. The assembly of the primary CeO2 particles (∼4 nm in size) results in an interparticulate space constituting an open 3D mesopore network, as revealed by skeleton analysis of tomography data, being in conformity with hysteresis scanning. At elevated temperatures (800 °C), sinter processes occur resulting in the growth of the primary particles, but the 3D mesopore network and the spherical secondary structure are preserved.

Pore Size Gradient Effect in Monolithic Silica Mesopore Networks Revealed by In-Situ SAXS Physisorption.

In disordered mesopore networks, the size distribution and connection between adjacent pores control desorption. How network characteristics can be extracted from corresponding physisorption isotherms is still a matter of research. To elucidate this, we study krypton physisorption (117.8 K) in the mesopore networks of "Nakanishi"-type monolithic silica. Combining physisorption in scanning acquisition mode with synchrotron-based in-situ SAXS provides complementary information on pore-filling states. These data reveal a mean pore size gradient in which pores grow smaller towards the material's network center. This structural motif cannot be derived through conventional isotherm analysis, but it is clearly exposed through scanning desorption curves which do not quite converge but merge individually with the main desorption isotherm before the lower hysteresis closing point. Hence, our findings provide the basis to build advanced models for analyzing scanning isotherms and extracting network characteristics through new descriptors, such as pore size and connectivity distributions as a function of the distance from the network center.

Peering into the Formation of Cerium Oxide Colloidal Particles in Solution by In Situ Small-Angle X-ray Scattering.

The formation of CeO2 colloidal particles upon heating an aqueous solution of (NH4)2Ce(NO3)6 to 100 °C was investigated by time-resolved in situ SAXS analysis using synchrotron radiation, providing absolute intensity data. In particular, the experiments were performed by applying different temperatures between room temperature and 100 °C as well as under variation of the ionic strength and concentration. Using validated SAXS evaluation tools (SASfit and McSAS software), the analyses revealed the presence of two types of particle populations possessing average dimensions of ca. 2 nm and 5-15 nm, with the latter being agglomerates of the 2 nm particles rather than single crystallites. The analysis revealed not only the changes in the size, but also the relative volume fractions of these two CeO2 particle populations as a function of the aforementioned parameters. Increasing the temperature increases the number of the 5-15 nm agglomerates on one hand by the enhanced nucleation rate of the primary particles. On the other hand, especially at high temperatures (90 and 100 °C) the larger agglomerate particles precipitate, resulting in interesting trends in the fractions of the two populations as a function of time, temperature, ionic strength, and precursor concentration. The experimental studies are complemented by calculating colloidal interaction energies based on classical DLVO theory. Thereby, this study provides detailed insight into the nucleation, growth, and agglomeration of CeO2 nanoparticles. The primary objective of this study is to provide a better understanding of the nucleation and growth of particles by the hydrolysis of the tetravalent cerium ion in aqueous solutions.

TiCl4 Dissolved in Ionic Liquid Mixtures from ab Initio Molecular Dynamics Simulations.

To gain a deeper understanding of the TiCl4 solvation effects in multi-component ionic liquids, we performed ab initio molecular dynamics simulations of 1-butyl-3-methylimidazolium [C4C1Im]+, tetrafluoroborate [BF4]-, chloride [Cl]- both with and without water and titanium tetrachloride TiCl4. Complex interactions between cations and anions are observed in all investigated systems. By further addition of water and TiCl4 this complex interaction network is extended. Observations of the radial distribution functions and number integrals show that water and TiCl4 not only compete with each other to interact mainly with [Cl]-, which strongly influences the cation-[BF4]- interaction, but also interact with each other, which leads to the fact that in certain systems the cation-anion interaction is enhanced. Further investigations of the Voronoi polyhedra analysis have demonstrated that water has a greater impact on the nanosegregated system than TiCl4 which is also due to the fact of the shear amount of water relative to all other components and its higher mobility compared to TiCl4. Overall, the polar network of the IL mixture collapses by including water and TiCl4. In the case of [Cl]- chloride enters the water continuum, while [BF4]- remains largely unaffected, which deeply affects the interaction of the ionic liquid (IL) network.

Comparison of In-Plane Stress Development in Sol-Gel- and Nanoparticle-Derived Mesoporous Metal Oxide Thin Films.

By using an evaporation-induced self-assembly (EISA) process, mesoporous metal oxide thin films are prepared via molecular precursors undergoing a sol-gel transition or by using nanoparticle dispersions as the starting materials. Both methods are employed together with PIB50-b-PEO45 as the structure-directing agent to produce porous TiO2 and ZrO2 thin films with spherical mesopores of around 14 nm in diameter. These nanoparticle- and sol-gel-derived films were investigated in terms of the intrinsic in-plane stress development during the heat treatment up to 500 °C to evaluate the impact of solvent evaporation, template decomposition and crystallization on the mechanical state of the film. The investigation revealed the lowest intrinsic stress for the nanoparticle-derived mesoporous film, which is assigned to the combination of the relaxing effects of the utilized diblock copolymer and the interparticular gaps between the precrystalline nanoparticles. Furthermore, the residual in-plane stress was studied after annealing steps ranging from 300 to 1000 °C and cooling down to room temperature. Here, TiO2 nanoparticle-derived mesoporous films possess a lower residual stress than the sol-gel-derived mesoporous films, while in the case of ZrO2 films, sol-gel-derived coatings reveal the smallest residual stress. The latter is based on the lower thermal expansion coefficient of the dominant monoclinic crystal phase compared to that of the silicon substrate. Hence, the present crystal structure has a strong influence on the mechanical state. The observation in this study helps to further understand the stress-related mechanical properties and the formation of mesoporous metal oxides.

Independent Tailoring of Macropore and Mesopore Space in TiO2 Monoliths.

TiO2 monoliths were synthesized by a partially hindered sol-gel process. Various synthesis parameters like precursor concentrations and gelation temperature were varied to investigate changes in the macroporosity (being in the range of micrometers) and to determine influences on the macropore formation mechanism. Ionic liquids (ILs) were used as templates to vary the mesopore size independently from the macropore size. Depending on the synthesis parameters, TiO2 monoliths with exclusive mesoporosity or with hierarchical meso-/macropore structure were received, and the range of macropores can be shifted between 100 nm and 10 µm without influencing the mesopore diameter. Pore volumes up to 880 mm3/g were achieved, as determined by mercury intrusion porosimetry. The mesopores' diameter can be adjusted between 6 and 25 nm by adding different amounts of IL, and surface areas up to 260 m2/g and mesopore volumes of 0.5 cm3/g were obtained, based on N2-physisorption measurements. The monoliths were cladded by polymer, allowing for studying the flow-through properties depending on the macropore size. This precise control for tailored macropores enables the design of optimized TiO2 monoliths with respect to the desired application requirements.

On the underestimated impact of the gelation temperature on macro- and mesoporosity in monolithic silica.

The preparation of monolithic SiO2 with bimodal porosity using a special sol-gel procedure ("Nakanishi process") generally shows a pronounced sensitivity towards several physico-chemical parameters of the initial solution (concentrations, precursors, pH, temperature, etc.). Thus, temporal and spatial variations of these parameters during the sol-gel reactions can affect the final meso- and macropore space with respect to the pore size distributions and homogeneity. In this study we thoroughly examine the sol-gel reaction in terms of the impact of temperature accuracy and homogeneity during the gelation and their effect on meso- and macropore space. The in-depth characterization of the macroporosity in monolithic SiO2 rods, prepared by utilizing a highly homogeneous and accurate temperature profile, shows that a decrease of only 1.5 °C during the reaction doubles the mean size of the macropores in the analyzed temperature ranges (22.0-28.0 °C and 33.5-36.5 °C). Rheological measurements of the gelation points and the viscosity of the starting solutions prove that a higher reaction rate is the main reason for this marked temperature-sensitivity. Furthermore, the mesoporosity is affected to a surprising extent by the applied small temperature differences during the gelation reaction. This phenomenon is shown to be mainly caused by the temperature-dependent differences in macropore and skeleton dimensions and an inhomogeneous distribution of mesopore sizes within the skeleton. In essence, our study reveals that the impact of temperature on the formation of meso- and macroscale dimensions during the sol-gel process has been underestimated so far. The impact of a poor temperature homogeneity during monolith synthesis is exemplarily demonstrated by the application of monolithic silica capillary columns in HPLC.

Elucidating the Sorption Mechanism of Dibromomethane in Disordered Mesoporous Silica Adsorbents.

The mechanism of dibromomethane (DBM) sorption in mesoporous silica was investigated by in situ small-angle X-ray scattering (SAXS). Six different samples of commercial porous silica particles used for liquid chromatography were studied, featuring a disordered mesoporous structure, with some of the samples being functionalized with alkyl chains. SAXS curves were recorded at room temperature at various relative pressures P/P0 during adsorption of DBM. The in situ SAXS experiment is based on contrast matching between silica and condensed DBM with regard to X-ray scattering. One alkyl-modified silica sample was evaluated in detail by extraction of the chord-length distribution (CLD) from SAXS data obtained for several P/P0. On the basis of this analytical approach and by comparison with ex situ obtained data of nitrogen and DBM adsorption, the mechanism of DBM uptake was studied. Results of average mesopore sizes obtained with the CLD method were compared with pore size analysis using nitrogen physisorption (77 K) with advanced state-of-the-art nonlocal density functional theory (NLDFT) evaluation. The dual SAXS/physisorption study indicates that microporosity is negligible in all silica samples and that surface functionalization with a hydrophobic ligand has a major influence upon the process of DBM adsorption. Also, all of the mesopores are accessible as evidenced by in situ SAXS. The data suggest that no multilayer adsorption occurs on C18-(octadecyl-)modified silica surfaces using DBM as adsorptive, and it is possibly also negligible on bare silica surfaces.

Morphological Analysis of Physically Reconstructed Silica Monoliths with Submicrometer Macropores: Effect of Decreasing Domain Size on Structural Homogeneity.

Silica monoliths are increasingly used as fixed-bed supports in separation and catalysis because their bimodal pore space architecture combines excellent mass transport properties with a large surface area. To optimize their performance, a quantitative relationship between morphology and transport characteristics has to be established, and synthesis conditions that lead to a desired morphology optimized for a targeted application must be identified. However, the effects of specific synthesis parameters on the structural properties of silica monoliths are still poorly understood. An important question is how far the macropore and domain size can be reduced without compromising the structural homogeneity. We address this question with quantitative morphological data derived for a set of eight macroporous-mesoporous silica monoliths with an average macropore size (d(macro)) of between 3.7 and 0.1 µm, prepared following an established route involving the sol-gel transition and phase separation. The macropore space of the silica monolith samples is reconstructed using focused ion beam scanning electron microscopy followed by a quantitative assessment of geometrical and topological properties based on chord length distributions (CLDs) and branch-node analysis of the pore network, respectively. We observe a significant increase in structural heterogeneity, indicated by a decrease in the parameter k derived from fitting a k-gamma function to the CLDs, when d(macro) reaches the submicrometer range. The compromised structural homogeneity of silica monoliths with submicrometer macropores could possibly originate from early structural freezing during the competitive processes of sol-gel transition and phase separation. It is therefore questionable if the common approach of reducing the morphological features of silica monoliths into the submicrometer regime by changing the point of sol-gel transition can be successful. Alternative strategies and a better understanding of the involved competitive processes should be the focus of future research.

Ionic liquid- and surfactant-controlled crystallization of WO3 films.

WO3 films were obtained via evaporation-induced self-assembly (EISA) using ionic surfactants such as long-chain ionic liquids 1-hexadecyl-3-methyl imidazolium chloride and bromide (C16mimCl and C16mimBr, respectively) and cetyltrimethylammonium chloride and bromide (CTAC and CTAB, respectively) as additives. Owing to the presence of the ionic surfactants, WO3 films crystallize in a preferred orientation along the a-axis on different substrates, as evidenced by X-ray diffraction. WO3 films with this orientation show improved electrochromic properties when compared to films with a lower degree of crystallographic orientation, prepared in an analogue fashion.

An all low-temperature fabrication of macroporous, electrochemically addressable anatase thin films.

Macroporous TiO2 (anatase) thin films are fabricated by an all low-temperature process in which substrates are dip-coated in suspensions of mixed anatase nanoparticles and polystyrene beads, and the templating agents are removed by ultraviolet (UV) irradiation at a temperature below 50 °C. Scanning electron microscopy (SEM) and Raman spectroscopy show that the templating polymer beads are removed by UV irradiation combined with the photocatalytic activity of TiO2. X-Ray diffraction reveals that nanoparticle growth is negligible in UV irradiated films, while nanoparticle size increases by almost 10 times in calcined films that are prepared for comparison. The macroporous films are prepared on FTO-(fluorine-doped tin oxide) coated glass and ITO (indium tin oxide) coated flexible plastics and thereby used as working electrodes. In both cases, the films are electrochemically addressable, and cyclic voltammetry is consistent with the response of bulk TiO2 for calcined films and of nanoscale-TiO2 for UV-irradiated films.

Morphological analysis of disordered macroporous-mesoporous solids based on physical reconstruction by nanoscale tomography.

Solids with a hierarchically structured, disordered pore space, such as macroporous-mesoporous silica monoliths, are used as fixed beds in separation and catalysis. Targeted optimization of their functional properties requires a knowledge of the relation among their synthesis, morphology, and mass transport properties. However, an accurate and comprehensive morphological description has not been available for macroporous-mesoporous silica monoliths. Here we offer a solution to this problem based on the physical reconstruction of the hierarchically structured pore space by nanoscale tomography. Relying exclusively on image analysis, we deliver a concise, accurate, and model-free description of the void volume distribution and pore coordination inside the silica monolith. Structural features are connected to key transport properties (effective diffusion, hydrodynamic dispersion) of macropore and mesopore space. The presented approach is applicable to other fixed-bed formats of disordered macroporous-mesoporous solids, such as packings of mesoporous particles and organic-polymer monoliths.

Coherent analysis of disordered mesoporous adsorbents using small angle X-ray scattering and physisorption experiments.

Characterization of mesoporous adsorbents is traditionally performed in terms of the pore size distribution with bulk methods like physisorption and mercury intrusion. But their application relies on assumptions regarding the basic pore geometry. Although novel tools have enabled the quantitative interpretation of physisorption data for adsorbents having a well-defined pore structure the analysis of disordered mesoporosity still remains challenging. Here we show that small angle X-ray scattering (SAXS) combined with chord length distribution (CLD) analysis presents a precise and convenient approach to determine the structural properties of two-phase (solid-void) systems of mesopores. Characteristic wall (solid) and pore (void) sizes as well as surface areas are extracted without the need to assume a certain pore shape. The mesoporous structure of modern, commercially available fully porous and core-shell adsorbent particles is examined by SAXS/CLD analysis. Mean pore size and surface area are compared with results obtained from nitrogen physisorption data and show excellent agreement.