Solvent Cavitation during Ambient Pressure Drying of Silica Aerogels.

Ambient-pressure drying of silica gels stands out as an economical and accessible process for producing monolithic silica aerogels. Gels experience significant deformations during drying due to the capillary pressure generated at the liquid-vapor interface in submicron pores. Proper control of the gel properties and the drying rate is essential to enable reversible drying shrinkage without mechanical failure. Recent in operando microcomputed X-ray tomography (µCT) imaging revealed the kinetics of the phase composition during drying and spring-back. However, to fully explain the underlying mechanisms, spatial resolution is required. Here we show evidence of evaporation by hexane cavitation during the ambient-pressure drying of silylated silica gels by spatially resolved quantitative analysis of µCT data supported by wide-angle X-ray scattering measurements. Cavitation consists of the rupture of the pore liquid put under tension by capillary pressure, creating vapor bubbles within the gels. We found the presence of a homogeneously distributed vapor-air phase in the gels well ahead of the maximum shrinkage. The onset of this vapor/air phase corresponded to a pore volume shrinkage of ca. 50 vol % that was attributed to a critical stiffening of the silica skeleton enabling cavitation. Our results provide new aspects of the relation between the shape changes of silica gels during drying and the evaporation mechanisms. We conclude that stress release by cavitation may be at the origin of the resistance of the silica skeleton to drying stresses. This opens the path toward producing larger monolithic silica aerogels by fine-tuning the drying conditions to exploit cavitation.

Influence of Si3N4 fillers and pyrolysis profile on the microstructure of additively manufactured silicon carbonitride ceramics derived from polyvinylsilazane.

In this work, various methods were used to improve the printability of a photocurable polyvinylsilazane resin filled with silicon nitride particles for digital light processing. The developed resin was used as a preceramic polymer for polymer-to-ceramic conversion. The pyrolysis-induced structural changes of the additively manufactured objects were evaluated by comparing samples with different thicknesses, filler amounts and heating profiles. The printed green body retained its original geometry better and showed fewer cracks due to the addition of silicon nitride particles to the resin. Based on the thermally induced changes in a polyvinylsilazane resin system, a customized heating profile for the pyrolysis process was developed, which contributed to the reduction of pores and cracks while the average pyrolysis heating rate remained relatively high. This work provides insight into the pyrolysis of additively manufactured preceramic polymer green bodies and highlights various strategies for additive manufacturing of polymer-derived ceramics.

The presented work systematically demonstrates the microstructural optimization of additively manufactured polymer-derived ceramics through combination of high refractive index filler inclusion and pyrolysis procedure customization.

In situ/operando plug-flow fixed-bed cell for synchrotron PXRD and XAFS investigations at high temperature, pressure, controlled gas atmosphere and ultra-fast heating.

A plug-flow fixed-bed cell for synchrotron powder X-ray diffraction (PXRD) and X-ray absorption fine structure (XAFS) idoneous for the study of heterogeneous catalysts at high temperature, pressure and under gas flow is designed, constructed and demonstrated. The operating conditions up to 1000°C and 50 bar are ensured by a set of mass flow controllers, pressure regulators and two infra-red lamps that constitute a robust and ultra-fast heating and cooling method. The performance of the system and cell for carbon dioxide hydrogenation reactions under specified temperatures, gas flows and pressures is demonstrated both for PXRD and XAFS at the P02.1 (PXRD) and the P64 (XAFS) beamlines of the Deutsches Elektronen-Synchrotron (DESY).

Hydrogenation of Carbon Dioxide to Dimethyl Ether on CuO-ZnO/ZSM-5 Catalysts: Comparison of Powder and Electrospun Structures.

The promising direct dimethyl ether (DME) production through CO2 hydrogenation was systematically analyzed in this research by synthesizing, characterizing, and testing several catalytic structures. In doing so, various combinations of precipitation and impregnation of copper- and zinc-oxides (CuO-ZnO) over a ZSM-5 zeolite structure were applied to synthesize the hybrid catalysts capable of hydrogenating carbon dioxide to methanol and dehydrating it to DME. The resulting catalytic structures, including the co-precipitated, sequentially precipitated, and sequentially impregnated CuO-ZnO/ZSM-5 catalysts, were prepared in the form of particle and electrospun fibers with distinguished chemical and structural features. They were then characterized using XRD, BET, XPS, ICP, TGA, SEM, and FIB-SEM/EDS analyses. Their catalytic performances were also tested and analyzed in light of their observed characteristics. It was observed that it is crucial to establish relatively small-size and well-distributed zeolite crystals across a hybrid catalytic structure to secure a distinguished DME selectivity and yield. This approach, along with other observed behaviors and the involved phenomena like catalyst particles and fibers, clusters of catalyst particles, or the whole catalytic bed, were analyzed and explained. In particular, the desired characteristics of a CuO-ZnO/ZSM-5 hybrid catalyst, synthesized in a single-pot processing of the precursors of all involved catalytically active elements, were found to be promising in guiding the future efforts in tailoring an efficient catalyst for this system.

Springback effect of ambient-pressure-dried silica aerogels: nanoscopic effects of silylation revealed by in situ synchrotron X-ray scattering.

Ambient pressure drying (APD) allows for synthesizing aerogels without expensive and sophisticated equipment for achieving supercritical conditions. Since APD does not eliminate the capillary stress that is induced by the liquid/vapour phase boundary, the shrinkage during drying needs to be prevented or reversed. The re-expansion of the silylated silica gels during drying is commonly referred to as the springback effect (SBE). The SBE is not only important for producing aerogels via APD, but is also a fascinating phenomenon, since it is accompanied by a significant volume change unusual for rigid ceramics. Synchrotron X-ray scattering has proven to be especially effective for the investigation of the volume change of these fractal silica structures on different length scales. In this work, we follow the drying, shrinkage, and (partial) re-expansion of various monolithic samples in situ to explore the occurrence of the SBE. For this purpose, various silylation agents, i.e., hexamethyldisilazane, trimethylchlorosilane, and triethylchlorosilane were used to investigate different shrinkage and re-expansion behavior. A scattering model was used to extract additional information of the evolving primary particle size, correlation length, fractal dimension, and other intensity contributions of the silica network and the hexane. While the primary particles pointed towards a relaxation at near molecular size, they were likely not involved in the SBE. However, structures near the size of the correlation length could be essential for the occurrence of this phenomenon. These findings may lead to the origin of this interesting phenomenon, as well as a better understanding of the production of APD aerogels.

Origin of the Springback Effect in Ambient-Pressure-Dried Silica Aerogels: The Effect of Surface Silylation.

Ambient pressure drying (APD) can prospectively reduce the costs of aerogel fabrication and processing. APD relies solely on preventing shrinkage or making it reversible. The latter, i.e., the aerogel re-expansion after drying (so-called springback effect-SBE), needs to be controlled for reproducible aerogel fabrication by APD. This can be achieved by an appropriate surface functionalization of aerogel materials (e.g., SiO2). This work addresses the fabrication of monolithic SiO2 aerogels and xerogels by APD. The effect of several silylation agents, i.e., trimethylchlorosilane, triethylchlorosilane, and hexamethyldisilazane on the SBE is studied in detail, applying several complementary experimental techniques, allowing the evaluation of the macroscopic and microscopic morphology as well as the composition of SiO2 aerogels. Here, we show that some physical properties, e.g., the bulk density, the macroscopic structure, and pore sizes/volumes, were significantly affected by the re-expansion. However, silylation did not necessarily lead to full re-expansion. Therefore, similarities in the molecular composition could not be equated to similarities in the SBE. The influences of steric hindrance and reactivity are discussed. The impact of silylation is crucial in tailoring the SBE and, as a result, the APD of monolithic aerogels.

Core-Sheath Pt-CeO2/Mesoporous SiO2 Electrospun Nanofibers as Catalysts for the Reverse Water Gas Shift Reaction.

One-dimensional (1D) core-sheath nanofibers, platinum (Pt)-loaded ceria (CeO2) sheath on mesoporous silica (SiO2) core were fabricated, characterized, and used as catalysts for the reverse water gas shift reaction (RWGS). CeO2 nanofibers (NFs) were first prepared by electrospinning (ES), and then Pt nanoparticles were loaded on the CeO2 NFs using two different deposition methods: wet impregnation and solvothermal. A mesoporous SiO2 sheath layer was then deposited by sol-gel process. The phase composition, structural, and morphological properties of synthesized materials were investigated by scanning electron microscope (SEM), scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), nitrogen adsorption/desorption method, X-ray photoelectron spectroscopy (XPS), inductively coupled plasma-optical emission spectrometry (ICP-OES) analysis, and CO2 temperature programmed desorption (CO2-TPD). The results of these characterization techniques revealed that the core-sheath NFs with a core diameter between 100 and 300 nm and a sheath thickness of about 40-100 nm with a Pt loading of around 0.5 wt.% were successfully obtained. The impregnated catalyst, Pt-CeO2 NF@mesoporous SiO2, showed the best catalytic performance with a CO2 conversion of 8.9% at 350 °C, as compared to the sample prepared by the Solvothermal method. More than 99% selectivity of CO was achieved for all core-sheath NF-catalysts.

Revealing the Chemical State of Palladium in Operating In2 O3 Gas Sensors: Metallic Pd Enhances Sensing Response and Intermetallic Inx Pdy Compound Blocks It.

The sensing response of metal oxides activated with noble metal nanoparticles is significantly influenced by changes to the chemical state of corresponding elements under operating conditions. Here, a PdO/rh-In2 O3 consisting of PdO nanoparticles loaded onto rhombohedral In2 O3 was studied as a gas sensor for H2 gas (100-40000 ppm in an oxygen-free atmosphere) in the temperature range of 25-450 °C. The phase composition and chemical state of elements were examined by resistance measurements combined with synchrotron-based in situ X-ray diffraction and ex situ X-ray photoelectron spectroscopy. As found, PdO/rh-In2 O3 undergoes a series of structural and chemical transformations during operation: from PdO to Pd/PdHx and finally to the intermetallic Inx Pdy phase. The maximal sensing response (RN2 /RH2 ) of ∼5 ⋅ 107 towards 40000 ppm (4 vol %) H2 at 70 °C is correlated with the formation of PdH0.706 /Pd. The Inx Pdy intermetallic compounds formed around 250 °C significantly decrease the sensing response.

Additive manufacturing and performance of bioceramic scaffolds with different hollow strut geometries.

Additively manufactured hollow-strut bioceramic scaffolds present a promising strategy towards enhanced performance in patient-tailored bone tissue engineering. The channels in such scaffolds offer pathways for nutrient and cell transport and facilitate effective osseointegration and vascularization. In this study, we report an approach for the slurry based additive manufacturing of modified diopside bioceramics that enables the production of hollow-strut scaffolds with diverse cross-sectional forms, distinguished by different configurations of channel and strut geometries. The prepared scaffolds exhibit levels of porosity and mechanical strength that are well suited for osteoporotic bone repair. Mechanical characterization in orthogonal orientations revealed that a square outer cross-section for hollow struts in woodpile scaffolds gives rise to levels of compressive strength that are higher than those of conventional solid cylindrical strut scaffolds despite a significantly lower density. Finite element analysis confirms that this improved strength arises from lower stress concentration in such geometries. It was shown that hollow struts in bioceramic scaffolds dramatically increase cell attachment and proliferation, potentially promoting new bone tissue formation within the scaffold channel. This work provides an easily controlled method for the extrusion-based 3D printing of hollow strut scaffolds. We show here how the production of hollow struts with controllable geometry can serve to enhance both the functional and mechanical performance of porous structures, with particular relevance for bone tissue engineering scaffolds.

Zirconium Carbide Mediates Coke-Resistant Methane Dry Reforming on Nickel-Zirconium Catalysts.

Graphitic deposits anti-segregate into Ni0 nanoparticles to provide restored CH4 adsorption sites and near-surface/dissolved C atoms, which migrate to the Ni0 /ZrO2 interface and induce local Zrx Cy formation. The resulting oxygen-deficient carbidic phase boundary sites assist in the kinetically enhanced CO2 activation toward CO(g). This interface carbide mechanism allows for enhanced spillover of carbon to the ZrO2 support, and represents an alternative catalyst regeneration pathway with respect to the reverse oxygen spillover on Ni-CeZrx Oy catalysts. It is therefore rather likely on supports with limited oxygen storage/exchange kinetics but significant carbothermal reducibility.

Porous Silicon Oxycarbonitride Ceramics with Palladium and Pd2Si Nanoparticles for Dry Reforming of Methane.

Pd-containing precursor has been synthesized from palladium acetate and poly(vinly)silazane (Durazane 1800) in an ice bath under an argon atmosphere. The results of ATR-FTIR and NMR characterizations reveal the chemical reaction between palladium acetate and vinyl groups in poly(vinyl)silazane and the hydrolyzation reaction between -Si-H and -Si-CH=CH2 groups in poly(vinyl)silazane. The palladium nanoparticles are in situ formed in the synthesized precursors as confirmed by XRD, XPS, and TEM. Pd- and Pd2Si-containing SiOCN ceramic nanocomposites are obtained by pyrolysis of the synthesized precursors at 700 °C, 900 °C-1100 °C in an argon atmosphere. The pyrolyzed nanocomposites display good catalytic activity towards the dry reforming of methane. The sample pyrolyzed at 700 °C possesses the best catalytic performance, which can be attributed to the in situ formed palladium nanoparticles and high BET surface area of about 233 m2 g-1.

Revealing Nanodomain Structures of Bottom-Up-Fabricated Graphene-Embedded Silicon Oxycarbide Ceramics.

Dispersing graphene nanosheets in polymer-derived ceramics (PDCs) has become a promising route to produce exceptional mechanical and functional properties. To reveal the complex nanodomain structures of graphene-PDC composites, a novel reduced graphene oxide aerogel embedded silicon oxycarbide (RGOA-SiOC) nanocomposite was fabricated bottom-up using a 3D reduced graphene oxide aerogel as a skeleton followed by infiltration of a ceramic precursor and high-temperature pyrolysis. The reduced graphene oxide played a critical role in not only the form of the free carbon phase but also the distribution of SiOxC4-x structural units in SiOC. Long-ordered and continuous graphene layers were then embedded into the amorphous SiOC phase. The oxygen-rich SiOxC4-x units were more prone to forming than carbon-rich SiOxC4-x units in SiOC after the introduction of reduced graphene oxide, which we attributed to the bonding of Si atoms in SiOC with O atoms in reduced graphene oxide during the pyrolysis process.

Hierarchically porous and mechanically stable monoliths from ordered mesoporous silica and their water filtration potential.

Mechanically stable structures with interconnected hierarchical porosity combine the benefits of both small and large pores, such as high surface area, pore volume, and good mass transport capabilities. Hence, lightweight micro-/meso-/macroporous monoliths are prepared from ordered mesoporous silica COK-12 by means of spark plasma sintering (SPS, S-sintering) and compared to conventionally (C-) sintered monoliths. A multi-scale model is developed to fit the small angle X-ray scattering data and obtain information on the hexagonal lattice parameters, pore sizes from the macro to the micro range, as well as the dimensions of the silica population. For both sintering techniques, the overall mesoporosity, hexagonal pore ordering, and amorphous character are preserved. The monoliths' porosity (77-49%), mesopore size (6.2-5.2 nm), pore volume (0.50-0.22 g cm-3), and specific surface area (451-180 m2 g-1) decrease with increasing processing temperature and pressure. While the difference in porosity is enhanced, the structural parameters between the C-and S-sintered monoliths are largely converging at 900 °C, except for the mesopore size and lattice parameter, whose dimensions are more extensively preserved in the S-sintered monoliths, however, coming along with larger deviations from the theoretical lattice. Their higher mechanical properties (biaxial strength up to 49 MPa, 724 MPa HV 9.807 N) at comparable porosities and ability to withstand ultrasonic treatment and dead-end filtration up to 7 bar allow S-sintered monoliths to reach a high permeance (2634 L m-2 h-1 bar-1), permeability (1.25 × 10-14 m2), and ability to reduce the chemical oxygen demand by 90% during filtration of a surfactant-stabilized oil in water emulsion, while indicating reasonable resistance towards fouling.

Springback effect and structural features during the drying of silica aerogels tracked by in-situ synchrotron X-ray scattering.

The springback effect during ambient pressure drying of aerogels is an interesting structural phenomenon, consisting of a severe shrinkage followed by almost complete re-expansion. The drying of gels causes shrinkage, whereas re-expansion is believed to be linked to repelling forces on the nanoscale. A multi-scale structural characterization of this significant volume change is key in controlling aerogel processing and properties. In this work, hydrophobic, monolithic silica aerogels with high specific surface areas were synthesized by modification with trimethylchlorosilane and ambient pressure drying. Here, we report a multi-method approach focusing on in-situ X-ray scattering to observe alterations of the nanostructured material during the drying of surface-modified and unmodified silica gels. Both show a porous fractal nanostructure, which partially collapses during drying and only recovers in surface-modified samples during the springback effect. Distinct changes of the X-ray scattering data were reproducibly associated with the shrinkage, re-expansion and drying of the gel network. Our findings may contribute to tailor aerogels with specific functionality, as the springback effect has a direct influence on properties (e.g., porosity, pore size distribution), which is directly affected by the degree of re-expansion.

Elucidating the role of earth alkaline doping in perovskite-based methane dry reforming catalysts.

To elucidate the role of earth alkaline doping in perovskite-based dry reforming of methane (DRM) catalysts, we embarked on a comparative and exemplary study of a Ni-based Sm perovskite with and without Sr doping. While the Sr-doped material appears as a structure-pure Sm1.5Sr0.5NiO4 Ruddlesden Popper structure, the undoped material is a NiO/monoclinic Sm2O3 composite. Hydrogen pre-reduction or direct activation in the DRM mixture in all cases yields either active Ni/Sm2O3 or Ni/Sm2O3/SrCO3 materials, with albeit different short-term stability and deactivation behavior. The much smaller Ni particle size after hydrogen reduction of Sm1.5Sr0.5NiO4, and of generally all undoped materials stabilizes the short and long-term DRM activity. Carbon dioxide reactivity manifests itself in the direct formation of SrCO3 in the case of Sm1.5Sr0.5NiO4, which is dominant at high temperatures. For Sm1.5Sr0.5NiO4, the CO : H2 ratio exceeds 1 at these temperatures, which is attributed to faster direct carbon dioxide conversion to SrCO3 without catalytic DRM reactivity. As no Sm2O2CO3 surface or bulk phase as a result of carbon dioxide activation was observed for any material - in contrast to La2O2CO3 - we suggest that oxy-carbonate formation plays only a minor role for DRM reactivity. Rather, we identify surface graphitic carbon as the potentially reactive intermediate. Graphitic carbon has already been shown as a crucial reaction intermediate in metal-oxide DRM catalysts and appears both for Sm1.5Sr0.5NiO4 and NiO/monoclinic Sm2O3 after reaction as crystalline structure. It is significantly more pronounced for the latter due to the higher amount of oxygen-deficient monoclinic Sm2O3 facilitating carbon dioxide activation. Despite the often reported beneficial role of earth alkaline dopants in DRM catalysis, we show that the situation is more complex. In our studies, the detrimental role of earth alkaline doping manifests itself in the exclusive formation of the sole stable carbonated species and a general destabilization of the Ni/monoclinic Sm2O3 interface by favoring Ni particle sintering.

Ceramic Stereolithography of Bioactive Glasses: Influence of Resin Composition on Curing Behavior and Green Body Properties.

Herein we report on the preparation of a bioactive glass (BAG)-based photocurable resin for the additive manufacturing of BAG scaffolds with high filler loadings. The preparation of glass/ceramics resins for stereolithography with high filler loading is always a challenge, especially for fillers with a high refractive index variance. Various photocurable resin compositions with and without bioactive glass fillers have been investigated to see the influence of bioactive glass on physical properties of the resin and resulting green body. The effect of concentration of monomers, reactive diluent, light absorber (Sudan orange G dye), photoinitiator (PI), non-reactive diluent, and fillers (BAG) on rheology and photocuring behavior of the resin and tomography of the resulting 3D structures have been investigated. The BAG contents affect the rheology of resin and influence the rate of the polymerization reaction. The resin compositions with 55-60% BAG, 10% PEG-200 (diluent), 1% of PI and 0.015% of the dye were found to be suitable compositions for the stereolithographic fabrication. A higher percentage of PI caused over-curing, while a higher amount of dye decreased the cure depth of the resin. The micro-computed tomography (µ-CT) and scanning electron microscopic (SEM) images of the resulting green bodies display a relatively dense glass scaffold without any visible cracks and good interlayer connection and surface finishing. These properties play an important role in the mechanical behavior of 3D scaffolds. This study will be helpful to prepare high density glass/ceramic slurries and optimize their printing properties.

Effect of Fomes fomentarius Cultivation Conditions on Its Adsorption Performance for Anionic and Cationic Dyes.

Lab-cultivated mycelia of Fomes fomentarius (FF), grown on a solid lignocellulose medium (FF-SM) and a liquid glucose medium (FF-LM), and naturally grown fruiting bodies (FF-FB) were studied as biosorbents for the removal of organic dyes methylene blue and Congo red (CR). Both the chemical and microstructural differences were revealed using X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, zeta potential analysis, and scanning electron microscopy, illuminating the superiority of FF-LM and FF-SM over FF-FB in dye adsorption. The adsorption process of CR on FF-LM and FF-SM is best described by the Redlich-Peterson model with ß constants close to 1, that is, approaching the monolayer Langmuir model, which reach maximum adsorption capacities of 48.8 and 13.4 mg g-1, respectively, in neutral solutions. Adsorption kinetics follow the pseudo-second-order model where chemisorption is the rate-controlling step. While the desorption efficiencies were low, adsorption performances were preserved and even enhanced under simulated dye effluent conditions. The results suggest that F. fomentarius can be considered an attractive biosorbent in industrial wastewater treatment and that its cultivation conditions can be specifically tailored to tune its cell wall composition and adsorption performance.

Atomic-Scale Insights into Nickel Exsolution on LaNiO3 Catalysts via In Situ Electron Microscopy.

Using a combination of in situ bulk and surface characterization techniques, we provide atomic-scale insight into the complex surface and bulk dynamics of a LaNiO3 perovskite material during heating in vacuo. Driven by the outstanding activity LaNiO3 in the methane dry reforming reaction (DRM), attributable to the decomposition of LaNiO3 during DRM operation into a Ni//La2O3 composite, we reveal the Ni exsolution dynamics both on a local and global scale by in situ electron microscopy, in situ X-ray diffraction and in situ X-ray photoelectron spectroscopy. To reduce the complexity and disentangle thermal from self-activation and reaction-induced effects, we embarked on a heating experiment in vacuo under comparable experimental conditions in all methods. Associated with the Ni exsolution, the remaining perovskite grains suffer a drastic shrinkage of the grain volume and compression of the structure. Ni particles mainly evolve at grain boundaries and stacking faults. Sophisticated structure analysis of the elemental composition by electron-energy loss mapping allows us to disentangle the distribution of the different structures resulting from LaNiO3 decomposition on a local scale. Important for explaining the DRM activity, our results indicate that most of the Ni moieties are oxidized and that the formation of NiO occurs preferentially at grain edges, resulting from the reaction of the exsolved Ni particles with oxygen released from the perovskite lattice during decomposition via a spillover process from the perovskite to the Ni particles. Correlating electron microscopy and X-ray diffraction data allows us to establish a sequential two-step process in the decomposition of LaNiO3 via a Ruddlesden-Popper La2NiO4 intermediate structure. Exemplified for the archetypical LaNiO3 perovskite material, our results underscore the importance of focusing on both surface and bulk characterization for a thorough understanding of the catalyst dynamics and set the stage for a generalized concept in the understanding of state-of-the art catalyst materials on an atomic level.