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The classical approach for the synthesis of catalysts based on biogenic silica from rice husk or rice straw involves a high temperature (600 °C) calcination step to remove the organic matrix from the silica backbone. The remaining silica can then be impregnated with a variety of active components in the form of Mn-, Fe- and Ce-precursors, followed by subsequent thermal treatment to form the corresponding metal oxides. In this work, we introduce a novel energy optimized route for the preparation of biogenic silica supported metal oxides. The proposed synthesis procedure aims towards the reduction of pyrolysis steps by infiltrating metal salts into the biomass lignocellulosic matrix. The modified rice husk or rice straw can then be calcined to directly yield the desired metal oxide supported on biogenic silica. To evaluate the described method, as model system manganese oxide modified rice husk silica was compared to a set of materials synthesized via classical calcination-impregnation by means of textural (N 2 -Sorption, SEM/EDX), structural (XRD), thermal (TG/DTA) and elemental (ICP-OES; XRF) analysis. Results indicate that the introduction of transition metals into biomass powder promotes the combustion of the organic matrix in terms of temperature requirements for its complete removal. In addition, catalytic tests of the materials obtained by the novel synthesis route clearly show similar or even improved performance as compared to catalysts prepared via impregnation method. [Display omitted] • Rice husk combustion requires temperatures up to 600 °C. • Introduction of transition metals allows for a reduction of the required temperature. • Manganese, iron and cerium act differently during the catalysed biomass combustion. • Metal modified rice husk ash prepared at 450 °C shows good activity towards carbon monoxide oxidation. [ABSTRACT FROM AUTHOR]

Surface modified silica nanoparticles are well-established composite materials. To improve their embedding and application behavior, surface energy analysis has been performed before and after silanization with mercaptopropyltrimethoxysilane (MPTMS) using inverse gas chromatography technique (IGC). Experiments with two silica materials have been conducted at infinite dilution to determine the dispersive component of the surface energy (γ s d) as well as the specific component (γ s sp) using the van Oss theory and a least-squares procedure evaluating the IGC data of 8 polar probe molecules collectively (instead of evaluating only the data of a pair of monopolar probes as is often the case in IGC studies). After surface silylation, the total surface energy (γ s t) of pyrogenic silica nanoparticles decreased from 225 mJ/m2 to 149 mJ/m2 referring to both a reduced wettability and an increased hydrophobicity of the MPTMS-modified sample. Moreover, the acidity/basicity parameters according to the van Oss and the Gutmann approach indicated that the acidity of the silica surface decreases by MPTMS grafting. In addition, IGC at finite concentration (using isopropanol as probe molecule) was applied to obtain the energetic heterogeneity of the silica surface. The results showed a bimodal energy distribution with maxima at about 18 and 25 kJ/mol and a reduction of the higher energetic sites from 65% to only 35% after MPTMS treatment. Overall, it can be seen that the dominance of specific interactions on silica surfaces is maintained even after extensive silanization. These findings are in agreement with the results of 29Si CP MAS NMR measurements which confirm the assignment of higher energetic sites to silanol groups. For the interpretation of the IGC results, quantum chemical calculations proved the preferred interaction of isopropanol onto silica surfaces via hydrogen bridging. The effect of silanization on the energetic distribution of adsorption sites of silica materials has been studied by inverse gas chromatography. For surface modification by 3-mercaptopropyltrimethoxysilane, the proportion of high energetic surface sites has only been reduced from 65% to 35%, bringing into question the often assumed complete coverage of surface silanol (Si‒OH) groups by organosilane grafting. [Display omitted] • The total surface energy of silica nanoparticles has been reduced after silanization. • Dispersive surface energy is significantly lower than the specific (polar) component. • Specific surface energy has been determined by evaluating several polar probe molecules collectively via van Oss approach. • Acid-base parameters using the Gutmann approach showed a more acidic character of pristine nanoparticles. • Quantum chemical calculations proved preferred polar interactions of isopropanol via hydrogen bridging. [ABSTRACT FROM AUTHOR]

Thermoporometry, namely revealing pore sizes based on the shifts of the freezing and melting transitions of liquid in the pores, has proven a powerful approach for mesostructure characterization of porous solids. Complementary to gas adsorption, it has several advantages, such as applicability to wet samples. At the same time, while gas sorption characterization has largely profited from the recent advances in the understanding of the gas-liquid phase equilibria in confined spaces, the solid-liquid transitions have remained poorly covered. In this work, in an analogy to the advanced gas sorption analysis, we implement a novel approach in thermoporometry by introducing the transition kernels, i.e. the families of solid-liquid transition curves describing different transition scenarios in single pores. In this way, a variation of the non-frozen layer thickness with temperature and the effects of thermodynamic fluctuations on phase transitions become automatically encoded in the transition kernels. As a next step, we incorporate these kernels into a recently developed theoretical framework, the serially-connected pore model, allowing for the analysis of pore networks beyond collections of independent pores conventionally assumed in the literature. This combination yields a substantial improvement of the accuracy of the thermoporometry method which is further demonstrated by applying NMR cryoporometry to MCM-41 and SBA-15 materials. Image 1 • Transition kernels fully describing solid-liquid equilibria for water in cylindrical channels are introduced. • Serially-connected pore model combined with the transition kernels describe phase equilibria in pore networks. • Serially-connected pore model reproduces entire phase behavior for water in MCM-41 and SBA-15. [ABSTRACT FROM AUTHOR]

This work addresses the advanced textural characterization of biogenic silica obtained from different Si-accumulating biomasses and agricultural waste products like rice, oat and spelt husk as well as horsetail. These materials exhibit a non-uniform, disordered pore structure in a broad size range deviating from well-ordered "artificial" species of porous materials. For the first time in literature, this contribution determines and validates mainly the mesoporosity by means of nitrogen physisorption, positron annihilation lifetime spectroscopy (PALS) and hyperpolarized 129Xenon NMR (HP-129Xe NMR). Nitrogen physisorption and HP 129Xe NMR both were able to describe the pore structure in the mesopore range. In addition, the specific surface area and the mesopore volume were determined by nitrogen physisorption. PALS complemented the results by an assessment of microporosity in biogenic silica. It was shown that biogenic silica contains micropores (1.5 nm–1.9 nm), mesopores in a broad range and indications of macroporosity. In addition, the results led to the assumption that different plant origins and plant species evoke different textural properties and inter-connections between the pores in biogenic silica. This contribution demonstrates that only a combination of all techniques is viable to characterize the non-uniform and complex pore structure of biogenic silica of different origin. Image 1 • The environmental and economical impact of extracting silica from biomasses wastes. • Three different techniques (positron annihilation lifetime spectroscopy, 129Xe-NMR, and N 2 adsorption) have been utilized. • The study helped to get deeper insights on the inner porosity of the naturally-build heterogeneous porosity of biomasses. • Closed micropores and broad mesopores with interconnectivity have been detected. [ABSTRACT FROM AUTHOR]

• Addition of Al 2 O 3 in lithium borosilicate glass changes the microstructure from isolated into bimodal and monomodal interconnected phases. • Lithium borosilicate glass accommodate higher Al 2 O 3 content with phase separation ability compared to sodium borosilicate glass system. • Al 2 O 3 decrease crystallization and phase separation in sodium borosilicate and lithium borosilicate glass systems. • The glass transition and liquidus temperature decrease with increase in Al 2 O 3 content in sodium borosilicate and lithium borosilicate glass systems. The investigation of selected properties in non-porous and porous R 2 O-B 2 O 3 -Al 2 O 3 -SiO 2 (R = Li, Na) glasses indicates different effects of Al 2 O 3 on phase separation and microstructure. Decrease of [BO 4 ]− units in Na 2 O-B 2 O 3 -SiO 2 glass system reveals a constant inhibition of phase separation with addition of Al 2 O 3. In Li 2 O-B 2 O 3 -SiO 2 glass system, the addition of Al 2 O 3 leads to a non-linear behaviour. XRD pattern and MAS-NMR studies display a microstructure-dependence on partial crystallization and phase separation. An addition of up to 4 mol.-% Al 2 O 3 changes the microstructure from isolated into bimodal and monomodal interconnected phases. Based on the analysed properties a simplified model is set up which explains the influence of Al 2 O 3 on phase separation, crystallization and microstructure in Li 2 O-B 2 O 3 -Al 2 O 3 -SiO 2 glasses. [ABSTRACT FROM AUTHOR]

The present study investigated the adsorption of diclofenac sodium (DCF) and carbamazepine (CBZ) on carbon-silica composites (CSC), activated carbon (RH-AC) and biogenic silica (RH-BS) based on rice husks from aqueous solutions. The materials were characterised using scanning electron microscopy, infrared spectroscopy, inductively coupled plasma optical emission spectroscopy, nitrogen sorption and elemental analysis. These methods provided essential information on the morphology, chemical composition, textural properties and surface characteristics of porous materials. The results of the adsorption studies demonstrate that the investigated materials exhibit varying adsorption capacities for DCF and CBZ. The maximum adsorption capacity was achieved by CSCs, with 1111 mg g -1 for DCF and 455 mg g -1 for CBZ and indicates additive effects on the adsorption capacity of CSCs compared to RH-AC and RH-BS. In addition to the hydrogen bonds and the π-π electron donor-acceptor interactions of the carbon component, further hydrogen bonds are formed by the silanol groups of the silica component. The CSCs derived from rice husks represent an innovative approach to the more efficient removal of pharmaceutical residues from wastewater. This is accomplished by utilizing a single starting material for both components, thereby yielding a unique structural combination., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

The development of non-precious metal-based electrodes that actively and stably support the oxygen evolution reaction (OER) in water electrolysis systems remains a challenge, especially at low pH levels. The recently published study has conclusively shown that the addition of haematite to H 2 SO 4 is a highly effective method of significantly reducing oxygen evolution overpotential and extending anode life. The far superior result is achieved by concentrating oxygen evolution centres on the oxide particles rather than on the electrode. However, unsatisfactory Faradaic efficiencies of the OER and hydrogen evolution reaction (HER) parts as well as the required high haematite load impede applicability and upscaling of this process. Here it is shown that the same performance is achieved with three times less metal oxide powder if NiO/H 2 SO 4 suspensions are used along with stainless steel anodes. The reason for the enormous improvement in OER performance by adding NiO to the electrolyte is the weakening of the intramolecular O─H bond in the water molecules, which is under the direct influence of the nickel oxide suspended in the electrolyte. The manipulation of bonds in water molecules to increase the tendency of the water to split is a ground-breaking development, as shown in this first example., (© 2024 The Authors. Small published by Wiley‐VCH GmbH.)

Surface-modified porous silica is a well-established composite material. To improve its embedding and application behavior, adsorption studies of various probe molecules have been performed using the technique of inverse gas chromatography (IGC). For this purpose, IGC experiments were carried out in the infinite dilution mode on macro-porous micro glass spheres before and after surface modification with (3-mercaptopropyl)trimethoxysilane. To provide information about the polar interactions between probe molecules and the silica surface, in particular, eleven polar molecules have been injected. In summary, the free surface energy for pristine silica ( γ S t o t a l = 229 mJ/m 2 ) and for (3-mercaptopropyl)trimethoxysilane-modified silica ( γ S t o t a l = 135 mJ/m 2 ) indicates a reduced wettability after surface modification. This is due to the reduction of the polar component of the free surface energy ( γ S S P ) from 191 mJ/m 2 to 105 mJ/m 2 . Simultaneously, with the reduction of surface silanol groups caused by surface modification of silica and, therefore, the decrease in polar interactions, a substantial loss of Lewis acidity was observed by various IGC approaches. Experiments with all silica materials have been conducted at temperatures in the range from 90°C to 120°C to determine the thermodynamic parameters, such as adsorption enthalpy ( Δ H a d s ) and adsorption entropy ( Δ S a d s ), using the Arrhenius regression procedure evaluating the IGC data. With the help of the enthalpy-entropy compensation, two types of adsorption complexes are assumed between polar probe molecules and the silica surface because of different isokinetic temperatures. Identical adsorption complexes with an isokinetic temperature of 370°C have been assigned to alkanes and weakly interacting polar probes such as benzene, toluene, dichloromethane, and chloroform. Polar probe molecules with typical functional groups such as OH, CO, and CN, having the ability to form hydrogen bonds to the silica surface, exhibit a lower isokinetic temperature of 60°C. Quantum chemical calculations of the probe molecules on a non-hydroxylated and hydroxylated silica cluster supported the formation of hydrogen bonds in the case of a strong polar adsorption complex with a bonding distance of 1.7 nm-1.9 nm to the silica surface., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Meyer, Mueller, Naumov, Bauer and Enke.)

High-temperature treatment of functional nanomaterials, through postsynthesis calcination, often represents an important step to unlock their full potential. However, such calcination steps usually severely limit the preparation of colloidal solutions of the nanoparticles due to the formation of sintered agglomerates. Herein, a simple route is reported to obtain colloidal solutions of calcined n-conductive antimony doped tin oxide (ATO) as well as titanium dioxide (TiO 2 ) nanoparticles without the need for additional sacrificial materials. This is achieved by making use of the reduced contact between individual nanoparticles when they are assembled into aerogels. Following the calcination of the aerogels at 500 °C, redispersion of the nanoparticles into stable colloidal solutions with various solvents can be achieved. Although a slight degree of sintering is inevitable, the size of the resulting aggregates in solution is still remarkably small with values below 30 nm., (© 2023 The Authors. Small published by Wiley-VCH GmbH.)

In this study, we present a detailed comparison between a conventional supercritical drying process and an evaporative drying technique for hierarchically organized porous silica gel monoliths. These gels are based on a model system synthesized by the aqueous sol-gel processing of an ethylene-glycol-modified silane, resulting in a cellular, macroporous, strut-based network comprising anisotropic, periodically arranged mesopores formed by microporous amorphous silica. The effect of the two drying procedures on the pore properties (specific surface area, pore volume, and pore widths) and on the shrinkage of the monolith is evaluated through a comprehensive characterization by using nitrogen physisorption, electron microscopy, and small-angle X-ray scattering. It can clearly be demonstrated that for the hierarchically organized porous solids, the evaporative drying procedure can compete without the need for surface modification with the commonly applied supercritical drying in terms of the material and textural properties, such as specific surface area and pore volume. The thus obtained materials deliver a high specific surface area and exhibit overall comparable or even improved pore characteristics to monoliths prepared by supercritical drying. Additionally, the pore properties can be tailored to some extent by adjusting the drying conditions, such as temperature.

Integration of solvothermal reaction products into complex thin-layer architectures is frequently achieved by combinations of layer transfer and subtractive lithography, whereas direct additive substrate patterning with solvothermal reaction products has remained challenging. We report reactive additive capillary stamping under solvothermal conditions as a parallel contact-lithographic access to patterns of solvothermal reaction products in thin-layer configurations. To this end, corresponding precursor inks are infiltrated into mechanically robust mesoporous aerogel stamps derived from double-network hydrogels. The stamp is then brought into contact with a substrate to be patterned under solvothermal reaction conditions inside an autoclave. The precursor ink forms liquid bridges between the topographic surface pattern of the stamp and the substrate. Evaporation-driven enrichment of the precursors in these liquid bridges, along with their liquid-bridge-guided conversion into the solvothermal reaction products, yields large-area submicron patterns of the solvothermal reaction products replicating the stamp topography. For example, we prepared thin hybrid films, which contained ordered monolayers of superparamagnetic submicron nickel ferrite dots prepared by solvothermal capillary stamping surrounded by nickel electrodeposited in a second orthogonal substrate functionalization step. The submicron nickel ferrite dots acted as a magnetic hardener, halving the remanence of the ferromagnetic nickel layer. In this way, thin-layer electromechanical systems, transformers, and positioning systems may be customized.

The deliquescence behavior of salt nanocrystals is different from that of bulk crystals. Here, we report the first systematic measurements of the deliquescence relative humidity (DRH) of sodium chloride crystals confined in various nanoporous silica materials with pore diameters ranging from 8 to 89 nm. Deliquescence humidities were determined by water vapor sorption measurements. In comparison to the DRH of bulk NaCl crystals (75.3% RH), the DRH decreases from 73 to 58% as the pore size decreases from 89 to 8 nm. In contrast, according to literature data, the DRH of NaCl aerosol nanoparticles increases with decreasing crystal size. A thermodynamic model approach, based on the combined use of an ion-interaction model, the Laplace equation, and the Kelvin equation, is used to calculate the influence of the confinement in nanopores on the solid-liquid and liquid-vapor phase equilibria. These calculations reveal that the main reason for the decrease in the DRH in nanopores is the concave curvature of the liquid-vapor interface that is formed during deliquescence. The same model approach shows that the increase in DRH of nanosized aerosol particles is due to the convex curvature of the liquid-vapor interface, whereas the effect of increases in solubility with decreasing crystal size is small.

Since the recent discovery of the template-free synthesis of porous boron nitride, research on the synthesis and application of the material has steadily increased. Nevertheless, the formation mechanism of boron nitride is not yet fully understood. Especially for the complex precursor decomposition of urea-based turbostratic boron nitride (t-BN), a profound understanding is still lacking. Therefore, in this publication, we investigate the influence of different common pre-heating temperatures of 100, 200, 300, and 400 °C on the subsequent properties of t-BN. We show that the structure and porosity of t-BN can be changed by preheating, where a predominantly mesoporous material can be obtained. Within these investigations, the sample BN-300/2 depicts the highest mesopore surface area of 242 m 2 g -1 with a low amount of micropores compared to other BNs. By thermal gravimetric analysis, X-ray photoelectron spectroscopy, and Raman spectroscopy, valid details about the formation of intermediates, types of chemical bonds, and the generation of t-BN are delivered. Hence, we conclude that the formation of a mesoporous material arises due to a more complete decomposition of the urea precursor by pre-heating., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

Applications in catalysis, adsorption and separation require high surface areas as provided by mesoporous materials. Particularly attractive is the class of silica-based mesoporous glasses, which are mechanically and chemically very stable and post-synthetically modifiable allowing specific surface properties to be introduced. One of the catalytically relevant moieties is the sulfonic acid group. To optimize the performance of mesoporous glass systems, analytical methods are required to determine the state of surface modification and its effect on the porosity. To this end, we here propose a specific combination of spectroscopic methods: The porosity during the introduction of thiol functionalities and subsequent oxidation into sulfonic acid groups on the surface of porous micro glass beads is investigated using hyperpolarized 129 Xe NMR, revealing that during the two modification steps the textural properties are preserved. The grafting mode as well as the surface coverage are determined using 29 Si MAS NMR. The oxidation step is demonstrated to be complete as probed by Raman spectroscopy and 13 C MAS NMR. Our combined analysis demonstrates the successful and complete surface modification as well as the maintenance of the free accessibility of the mesopore system.

Alterations of fluid phase transitions in porous materials are conventionally employed for the characterization of mesoporous solids. In the first approximation, this may be based on the application of the Kelvin equation for gas-liquid and the Gibbs-Thomson equation for solid-liquid phase equilibria for obtaining pore size distributions. Herein, we provide a comparative analysis of different phase coexistences measured in mesoporous silica solids with different pore sizes and morphology. Instead of comparing the resulting pore size distributions, we rather compare the transitions directly by using a common coordinate for varying the experiment's thermodynamic parameters based on the two equations mentioned. Both phase transitions in these coordinates produce comparable results for mesoporous solids of relatively large pore sizes. In contrast, marked differences are found for materials with smaller pore sizes. This illuminates the fact that, with reducing confinement sizes, thermodynamic fluctuations become increasingly important and different for different equilibria considered. In addition, we show that in the coordinate used for analysis, mercury intrusion matches perfectly with desorption and freezing transitions.

Shape-stabilized phase change materials (ss-PCMs) based on silica and butyl stearate were thermally enhanced via the addition of different hexagonal boron nitride particles (BN) to the in situ sol-gel synthesis. The dataset is used in conjunction with the experimental data of the influence of the particle size and surface area of BN on the thermal and mechanical properties of ss-PCMs discussed in Marske et al. (2021). To study the effect of the different BN particles on the hydrolysis degree of the silica network and on the chemical nature of the porogens sodium dodecyl sulfate and poly(vinyl alcohol) used for the ss-PCM synthesis, the ss-PCM samples are measured via High Power Decoupling (HPDEC) Magic Angle Spinning (MAS) 29 Si NMR and attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, respectively. Additionally, data of the influence of BN on the thermal properties is presented as thermogravimetric analysis (TG). The 29 Si MAS NMR spectra are referenced to tetramethylsilane and show the different silica species in ppm. The different value of wavenumber and intensity of each reference and ss-PCM sample is listed in the IR spectra. The decomposition points of the ss-PCMs are calculated from the TG data via OriginLab. The spectra and data can be used as a reference for other researchers and engineers to use in synthesizing ss-PCMs based on silica and other polymeric materials or as reference for pure BN, SDS, stabilized silica sol and PVA., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships which have or could be perceived to have influenced the work reported in this article., (© 2021 The Authors.)

This article combines a systematic literature review on the fabrication of macroporous α-Al 2 O 3 with increased specific surface area with recent results from our group. Publications claiming the fabrication of α-Al 2 O 3 with high specific surface areas (HSSA) are comprehensively assessed and critically reviewed. An account of all major routes towards HSSA α-Al 2 O 3 is given, including hydrothermal methods, pore protection approaches, dopants, anodically oxidized alumina membranes, and sol-gel syntheses. Furthermore, limitations of these routes are disclosed, as thermodynamic calculations suggest that γ-Al 2 O 3 may be the more stable alumina modification for A BET > 175 m 2 /g. In fact, the highest specific surface area unobjectionably reported to date for α-Al 2 O 3 amounts to 16-24 m 2 /g and was attained via a sol-gel process. In a second part, we report on some of our own results, including a novel sol-gel synthesis, designated as mutual cross-hydrolysis . Besides, the Mn-assisted α-transition appears to be a promising approach for some alumina materials, whereas pore protection by carbon filling kinetically inhibits the formation of α-Al 2 O 3 seeds. These experimental results are substantiated by attempts to theoretically calculate and predict the specific surface areas of both porous materials and nanopowders.

The results of investigations of porous glasses (PG) and porous glasses-ammonium hydrogen sulfate ferroelectric nanocomposites (AHS-PG) are presented. On the basis of dielectric and calorimetric measurements it was shown that in the AHS-PG nanocomposites with average pore size of 44, 68, 95, and 320 nm the anomalies of dielectric permittivity and specific heat similar to those in bulk crystals AHS are observed. An influence of the mean value of pores sizes on the ferroelectric phase transition temperatures of AHS nanocrystals embedded into the porous matrices was determined. It was shown that in AHS-PG dispersion of the dielectric permittivity is observed in both para- and ferro-electric phases and above room temperature AHS-PG nanocomposites exhibit the ionic conductivity.