Enhanced photocatalytic performance of coaxially electrospun titania nanofibers comprising yolk-shell particles

The present paper reports the fabrication of novel types of hybrid fibrous photocatalysts by combining block copolymer (BCP) templating, sol-gel processing, and coaxial electrospinning techniques. Coaxial electrospinning produces core-shell nanofibers (NFs), which are converted into hollow porous TiO2 NFs using an oxidative calcination step. Hybrid BCP micelles comprising a single plasmonic nanoparticle (NP) in their core and thereof derived silica-coated core-shell particles are utilized as precursors to generate yolk-shell type particulate inclusions in photocatalytically active NFs. The catalytic and photocatalytic activity of calcined NFs comprising different types of yolk-shell particles is systematically investigated and compared. Interestingly, calcined NFs comprising silica-coated yolk-shells demonstrate enhanced catalytic and photocatalytic performance despite the presence of silica shell separating plasmonic NP from the TiO2 matrix. Electromagnetic simulations indicate that this enhancement is caused by a localized surface plasmon resonance and a confinement effect in silica-coated yolk-shells embedded in porous TiO2 NFs. Utilization of the coaxially electrospun TiO2 NFs in combination with yolk-shells comprising plasmonic NPs reveals to be a potent method for the photocatalytic decomposition of numerous pollutants. It is worth noting that this study stands as the first occurrence of combining yolk-shells (Au@void@SiO2) with porous electrospun NFs (TiO2) for photocatalytic purposes and gaining an understanding of plasmon and confinement effects for photocatalytic performance. This approach represents a promising route for fabricating highly active and up-scalable fibrous photocatalytic systems.

Active metals decorated NiCo2O4 yolk-shell nanospheres as nanoreactors for catalytic reduction of nitroarenes and azo dyes

Transition-metal oxides (TMOs) have received a great deal of research attention and have been widely used in a variety of fields. However, conventional TMOs do not possess high specific surface area, sufficient active site on their surfaces, and limited their applications in catalysis. This study presents a two-step method for synthesizing active metal (M) decorated NiCo2O4 (M/NiCo2O4, M = Pd or Cu) nanospheres with yolk-shell nanostructures. Taking advantage of the unique morphology and the combination of dual active components (i.e., active NiCo2O4 substrate and decorated active metal), the as-prepared M/NiCo2O4 yolk-shell nanospheres can be employed as nanoreactors in the organic reactions. In catalyzing the reduction of a representative nitroarene (i.e., 4-NP) by NaBH4, the Pd/NiCo2O4 nanoreactors exhibit a superior catalytic efficiency to their counterparts (Cu/NiCo2O4 and NiCo2O4). The turnover frequency is much higher than that of various TMOs supported nanocatalysts have been reported over the past five years. Furthermore, the Pd/NiCo2O4 nanoreactors show excellent stability and common applicability of the reduction of various substituted nitrobenzenes and azo dyes. This work provides new rational design concept and preparation strategy for efficient nanoreactors with dual active components and sheds light on the practical application of chemical reactions.

Engineering core-shell mesoporous silica and Fe3O4@Au nanosystems for targeted cancer therapeutics: a review

The extensive utilization of nanoparticles in cancer therapies has inspired a new field of study called cancer nanomedicine. In contrast to traditional anticancer medications, nanomedicines offer a targeted strategy that eliminates side effects and has high efficacy. With its vast surface area, variable pore size, high pore volume, abundant surface chemistry and specific binding affinity, mesoporous silica nanoparticles (MPSNPs) are a potential candidate for cancer diagnosis and treatment. However, there are several bottlenecks associated with nanoparticles, including specific toxicity or affinity towards particular body fluid, which can cater by architecting core-shell nanosystems. The core-shell chemistries, synergistic effects, and interfacial heterojunctions in core-shell nanosystems enhance their stability, catalytic and physicochemical attributes, which possess high performance in cancer therapeutics. This review article summarizes research and development dedicated to engineering mesoporous core-shell nanosystems, especially silica nanoparticles and Fe3O4@Au nanoparticles, owing to their unique physicochemical characteristics. Moreover, it highlights state-of-the-art magnetic and optical attributes of Fe3O4@Au and MPSNP-based cancer therapy strategies. It details the designing of Fe3O4@Au and MPSN to bind with drugs, receptors, ligands, and destroy tumour cells and targeted drug delivery. This review serves as a fundamental comprehensive structure to guide future research towards prospects of core-shell nanosystems based on Fe3O4@Au and MPSNP for cancer theranostics.

Nano-architectural design of TiO2 for high performance photocatalytic degradation of organic pollutant: A review

In the past several decades, significant efforts have been paid toward photocatalytic degradation of organic pollutants in environmental research. During the past years, titanium dioxide nano-architectures (TiO₂ NAs) have been widely used in water purification applications with photocatalytic degradation processes under Uv/Vis light illumination. Photocatalysis process with nano-architectural design of TiO₂ is viewed as an efficient procedure for directly channeling solar energy into water treatment reactions. The considerable band-gap values and the subsequent short life time of photo-generated charge carriers are showed among the limitations of this approach. One of these effective efforts is the using of oxidation processes with advance semiconductor photocatalyst NAs for degradation the organic pollutants under UV/Vis irradiation. Among them, nano-architectural design of TiO₂ photocatalyst (such as Janus, yolk-shell (Y@S), hollow microspheres (HMSs) and nano-belt) is an effective way to improve oxidation processes for increasing photocatalytic activity in water treatment applications. In the light of the above issues, this study tends to provide a critical overview of the used strategies for preparing TiO₂ photocatalysts with desirable physicochemical properties like enhanced absorption of light, low density, high surface area, photo-stability, and charge-carrier behavior. Among the various nanoarchitectural design of TiO₂, the Y@S and HMSs have created a great appeal given their considerable large surface area, low density, homogeneous catalytic environment, favorable light harvesting properties, and enhanced molecular diffusion kinetics of the particles. In this review was summarized the developments that have been made for nano-architectural design of TiO2 photocatalyst. Additional focus is placed on the realization of interfacial charge and the possibility of achieving charge carriers separation for these NAs as electron migration is the extremely important factor for increasing the photocatalytic activity.

Synthetic Approaches to Colloidal Nanocrystal Heterostructures Based on Metal and Metal-Oxide Materials

Composite inorganic nanoarchitectures, based on combinations of distinct materials, represent advanced solid-state constructs, where coexistence and synergistic interactions among nonhomologous optical, magnetic, chemical, and catalytic properties lay a basis for the engineering of enhanced or even unconventional functionalities. Such systems thus hold relevance for both theoretical and applied nanotechnology-based research in diverse areas, spanning optics, electronics, energy management, (photo)catalysis, biomedicine, and environmental remediation. Wet-chemical colloidal synthetic techniques have now been refined to the point of allowing the fabrication of solution free-standing and easily processable multicomponent nanocrystals with sophisticated modular heterostructure, built upon a programmed spatial distribution of the crystal phase, composition, and anchored surface moieties. Such last-generation breeds of nanocrystals are thus composed of nanoscale domains of different materials, assembled controllably into core/shell or heteromer-type configurations through bonding epitaxial heterojunctions. This review offers a critical overview of achievements made in the design and synthetic elaboration of colloidal nanocrystal heterostructures based on diverse associations of transition metals (with emphasis on plasmonic metals) and transition-metal oxides. Synthetic strategies, all leveraging on the basic seed-mediated approach, are described and discussed with reference to the most credited mechanisms underpinning regioselective heteroepitaxial deposition. The unique properties and advanced applications allowed by such brand-new nanomaterials are also mentioned.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Support-Activity Relationship in Heterogeneous Catalysis for Biomass Valorization and Fine-Chemicals Production

Heterogeneous catalysts are progressively expanding their field of application, from high-throughput reactions for traditional industrial chemistry with production volumes reaching millions of tons per year, a sector in which they are key players, to more niche applications for the production of fine chemicals. These novel applications require a progressive utilization reduction of fossil feedstocks, in favor of renewable ones. Biomasses are the most accessible source of organic precursors, having as advantage their low cost and even distribution across the globe. Unfortunately, they are intrinsically inhomogeneous in nature and their efficient exploitation requires novel catalysts. In this process, an accurate design of the active phase performing the reaction is important; nevertheless, we are often neglecting the importance of the support in guaranteeing stable performances and improving catalytic activity. This review has the goal of gathering and highlighting the cases in which the supports (either derived or not from biomass wastes) share the worth of performing the catalysis with the active phase, for those reactions involving the synthesis of fine chemicals starting from biomasses as feedstocks.

Ni@ZrO2 yolk-shell catalyst for CO2 methane reforming: Effect of Ni@SiO2 size as the hard-template

This paper reports the usage ratio of TEOS and Zr(OBu)₄ on the formation of Ni@ZrO₂ yolk-shell for dry reforming of methane. From XPS analysis, the ZrO₂ hollow shell texture is demonstrated to be [TEOS]/[Zr(OBu)₄] dependent due to different sizes of SiO₂ produced. It found that an adequate ratio of [TEOS]/[Zr(OBu)₄] improves the catalytic conversion of dry reforming of methane. It (Ni@ZrO₂-SiZr-7.7) shows 90% conversion for CH₄ and 93% for CO₂ at a WHSV of 72,000 mLg_cat^-1h^-1 for 50 h at 800 °C with TOF_CH4 of 8.7 s^-1. It proposed that the changes in surface Si/Zr and gradual interconnecting pores contributed to its activity and stability. These finding's potential to be utilized in other high-temperature reactions.

Pomegranate-like Core-Shell Ni-NSs@MSNSs as a High Activity, Good Stability, Rapid Magnetic Separation, and Multiple Recyclability Nanocatalyst for DCPD Hydrogenation

A novel pomegranate-like Ni-NSs@MSNSs nanocatalyst was successfully synthesized via a modified Stöber method, and its application in the hydrogenation of dicyclopentadiene (DCPD) was firstly reported. The Ni-NSs@MSNSs possessed a high specific area (658 m²/g) and mesoporous structure (1.7-3.3 nm). The reaction of hydrogenation of DCPD to endo-tetrahydrodicyclopentadiene (endo-THDCPD) was used to evaluate the catalytic performance of the prepared materials. The distinctive pomegranate-like Ni-NSs@MSNSs core-shell nanocomposite exhibited superior catalytic activity (TOF = 106.0 h^-1 and STY = 112.7 g·L^-1·h^-1) and selectivity (98.9%) than conventional Ni-based catalysts (experimental conditions: Ni/DCPD/cyclohexane = 1/100/1000 (w/w), 150 °C, and 2.5 MPa). Moreover, the Ni-NSs@MSNSs nanocatalyst could be rapidly and conveniently recycled by magnetic separation without appreciable loss. The Ni-NSs@MSNSs also exhibited excellent thermal stability (≥750 °C) and good recycling performance (without an activity and selectivity decrease in four runs). The superior application performance of the Ni-NSs@MSNSs nanocatalyst was mainly owing to its unique pomegranate-like structure and core-shell synergistic confinement effect.

Engineering of Yolk/Core-Shell Structured Nanoreactors for Thermal Hydrogenations

Heterogeneous hydrogenation reactions are of great importance for chemical upgrading and synthesis, but still face the challenges of controlling selectivity and long-term stability. To improve the catalytic performance, many hydrogenation reactions utilize special yolk/core-shell nanoreactors (YCSNs) with unique architectures and advantageous properties. This work presents the developmental and technological challenges in the preparation of YCSNs that are potentially useful for hydrogenation reactions, and provides a summary of the properties of these materials. The work also addresses the scientific challenges in applications of these YCSNs in various gas and liquid-phase hydrogenation reactions. The catalyst structures, catalytic performance, structure-performance relationships, reaction mechanisms, and unsolved problems are discussed too. Also, a brief outlook and opportunities for future research in this field are presented. This work on the advancements in YCSNs might inspire the creation of new materials with desired structures for achieving maximal hydrogenation performances.

The effect of Ag atom doped Cu@CuO core-shell structure on its electronic properties and catalytic performance: a first principles study

Density functional theory was used to study the Ag-doped Cu@CuO core-shell structure, electronic properties and catalytic properties. Similar to the undoped Cu@CuO clusters, the Ag doped clusters also retain the core-shell structure. Ag doping increases the charge transfer between surrounding O atoms and Cu atoms and reduces the potential of the core-shell structure, thereby increasing its surface activity. The study of its orbital distribution found that the doping of Ag atoms caused the interaction between the inner Cu core and the outer CuO shell, which changed the electron orbital motion inside the shell. The internal chemical stability of the core-shell material is improved. In addition, Ag atom doping accelerates the decomposition of H₂O₂ on Cu@CuO structure and increases its adsorption of small molecules, which indicates that Ag atom doping improves the catalytic performance of Cu@CuO structure.

Smart Designs of Anti-Coking and Anti-Sintering Ni-Based Catalysts for Dry Reforming of Methane: A Recent Review

Dry reforming of methane (DRM) reaction has drawn much interest due to the reduction of greenhouse gases and production of syngas. Coking and sintering have hindered the large-scale operations of Ni-based catalysts in DRM reactions at high temperatures. Smart designs of Ni-based catalysts are comprehensively summarized in fourth aspects: surface regulation, oxygen defects, interfacial engineering, and structural optimization. In each part, details of the designs and anti-deactivation mechanisms are elucidated, followed by a summary of the main points and the recommended strategies to improve the catalytic performance, energy efficiency, and utilization rate.

Fabrication of Hierarchical Indium Vanadate Materials for Supercapacitor Application

Transition metal orthovanadates are emerging 2D materials for promising electrochemical energy storage applications. Facile hydrothermal method for nanocrystalline indium vanadate (InVO4) semiconducting materials' fabrication is economical because of its direct chemical synthesis. X-ray diffraction studies, field emission scanning electron microscope (SEM) images, transmission electron microscopy (TEM), and photoelectron X-ray spectrum are used to describe the semiconductor materials as synthesized. InVO4 microspheres have attracted a lot of attention in the energy and environmental sector. These microsphere-derived semiconductor materials are recognized to offer the advantages of their large surface area, tunable pore sizes, enhanced light absorption, efficient carrier (electron-hole) separation, superior electronic and optical behavior, and high durability. From the results of SEM and TEM, InVO4 shows a microsphere construction with a mixture of nanosized particles. Diffuse reflectance UV-visible measurements are used to determine the bandgap, and it is found to be 2.1 eV for InVO4. The electrochemical analysis reveals a superior performance of the pseudocapacitor with hydrothermally derived microspheres of InVO4. Alongside an improved pseudocapacity, developed after 4000 cycles, it has excellent cycling stability with a retention of ≈94% of its original specific capacitance efficiency.

Hollow Silica Particles: Recent Progress and Future Perspectives

Hollow silica particles (or mesoporous hollow silica particles) are sought after for applications across several fields, including drug delivery, battery anodes, catalysis, thermal insulation, and functional coatings. Significant progress has been made in hollow silica particle synthesis and several new methods are being explored to use these particles in real-world applications. This review article presents a brief and critical discussion of synthesis strategies, characterization techniques, and current and possible future applications of these particles.

One-pot synthesis of highly active and hydrothermally stable Pd@mHSiO2 yolk-shell-structured nanoparticles for high-temperature reactions in hydrothermal environments

The facile synthesis of yolk-shell-structured nanoparticles (YSNPs) with mobile active metal cores and mesoporous inorganic-organic hybrid silica shells (mHSiO2) is important for their applications. In this work, Pd@mHSiO2 YSNPs have been synthesized in aqueous solution at 95 °C by a one-pot method without the need for extensive purification and separation steps. The method is simple and facile, and ingeniously combines the controlled synthesis of Pd nanocubes, coating of mesoporous silica, and transition from core-shell-structured nanoparticles (CSNPs) to YSNPs. 29Si NMR spectroscopy, FTIR spectroscopy, and detailed control experiments have demonstrated that the incorporation of 1,2-bis(trimethoxysilyl)ethane (BTME) modifies the degree of condensation between the outer hybrid silica layer and the inner pure silica section, and that high temperature water is really responsible for dissolving the inner pure silica layer leading to a transition from the CSNPs to the YSNPs. The obtained Pd@mHSiO2 YSNPs have a controllable diameter, tunable shell thickness, a high specific surface area, and uniform mesoporosity. Thermal stability tests have indicated that the Pd@mHSiO2 YSNPs are remarkably stable at high temperatures up to 650 °C. Importantly, the Pd@mHSiO2 YSNPs exhibit a much higher catalytic activity and hydrothermal stability than Pd@mSiO2 CSNPs or Pd/mHSiO2 NSs in the conversion of levulinic acid (LA) into γ-valerolactone (GVL), because the hollow voids provide low mass-transfer resistance and improve the accessibility of the catalytic sites, and the incorporation of organic groups enhances the hydrothermal stability of the outer shell.

Al2O3-Coated Ni/CeO2 nanoparticles as coke-resistant catalyst for dry reforming of methane

To mitigate catalyst deactivation during the dry reforming of methane, Ni/CeO₂ catalysts composed of monodisperse Ni nanoparticles supported on CeO₂ nanorods are designed and coated with Al₂O₃ layers by atomic layer deposition.

Hollow Au@TiO2 porous electrospun nanofibers for catalytic applications

Catalytically active porous and hollow titania nanofibers encapsulating gold nanoparticles were fabricated using a combination of sol–gel chemistry and coaxial electrospinning technique. We report the fabrication of catalytically active porous and hollow titania nanofibers encapsulating gold nanoparticles (AuNPs) using a combination of sol–gel chemistry and coaxial electrospinning technique. The coaxial electrospinning involved the use of a mixture of poly(vinyl pyrrolidone) (PVP) and titania sol as the shell forming component, whereas a mixture of poly(4-vinyl pyridine) (P4VP) and pre-synthesized AuNPs constituted the core forming component. The core–shell nanofibers were calcined stepwise up to 600 °C which resulted in decomposition and removal of the organic constituents of the nanofibers. This led to the formation of porous and hollow titania nanofibers, where the catalytic AuNPs were embedded in the inner wall of the titania shell. The catalytic activity of the prepared Au@TiO₂ porous nanofibers was investigated using a model reaction of catalytic reduction of 4-nitrophenol and Congo red dye in the presence of NaBH₄. The Au@TiO₂ porous and hollow nanofibers exhibited excellent catalytic activity and recyclability, and the morphology of the nanofibers remained intact after repeated usage. The presented approach could be a promising route for immobilizing various nanosized catalysts in hollow titania supports for the design of stable catalytic systems where the added photocatalytic activity of titania could further be of significance.

Catalytically active porous and hollow titania nanofibers encapsulating gold nanoparticles were fabricated using a combination of sol–gel chemistry and coaxial electrospinning technique.

A new systematic synthesis of ultimate nickel nanocatalysts for compact hydrogen generation

A new systematic synthesis: an AIO reactor can materialize ultimate Ni nanocatalysts containing active Ni nanoparticles (4.5 nm) with high Ni loading (25 wt%), using a programmed sequence based on simple melt-infiltration and thermal treatment.

Synthesis of nano-sized tungsten oxide particles encapsulated in a hollow silica sphere and their photocatalytic properties for decomposition of acetic acid using Pt as a co-catalyst

Nano-sized tungsten oxide (WO₃) particles, each of which was encapsulated as a core in a hollow silica sphere (WO₃@SiO₂), were synthesized using calcium tungstate particles as the starting material. The calcium tungstate particles, each of which was covered with a silica shell, were converted to tungstic acid by nitric acid treatment and then to WO₃ by heat treatment to obtain WO₃@SiO₂. A hollow space was formed in WO₃@SiO₂ between the WO₃ core and the SiO₂ shell as a result of shrinkage of WO₃ during the heat treatment. The thus-obtained WO₃@SiO₂ was 40 nm in diameter, the WO₃ core was 10 nm in diameter, and the silica shell, which was permeable to gas and liquid, was 10 nm in thickness. WO₃@SiO₂ absorbed visible light to the wavelength of 454 nm, which enabled photocatalytic reaction under visible light; Pt was loaded on the WO₃ cores in the photocatalytic reactions. In contrast to Pt-loaded bulk WO₃ photocatalysts without an SiO₂ shell, Pt-loaded WO₃@SiO₂ showed continuous and complete decomposition of gaseous acetic acid in air under visible as well as UV irradiation.

We present a procedure for the synthesis of WO₃ nanoparticles encapsulated in a hollow silica sphere, and their unique photocatalytic properties.

Architecture and Preparation of Hollow Catalytic Devices

Since pioneering work done in the late 1990s, synthesis of functional hollow materials has experienced a rapid growth over the past two decades while their applications have been proven to be advantageous across many technological fields. In the field of heterogeneous catalysis, the development of micro- and nanoscale hollow materials as catalytic devices has also yielded promising results, because of their higher activity, stability, and selectivity. Herein, the architecture and preparation of these catalysts with tailorable composition and morphology are reviewed. First, synthesis of hollow materials is introduced according to the classification of template mediated, template free, and combined approaches. Second, different architectural designs of hollow catalytic devices, such as those without functionalization, with active components supported onto hollow materials, with active components incorporated within porous shells, and with active components confined within interior cavities, are evaluated respectively. The observed catalytic performances of this new class of catalysts are correlated to structural merits of individual configuration. Examples that demonstrate synthetic approaches and architected configurations are provided. Lastly, possible future directions are proposed to advance this type of hollow catalytic devices on the basis of our personal perspectives.

Synthesis and thermal stability of ZrO2@SiO2 core-shell submicron particles

ZrO2@SiO2 core-shell submicron particles are promising candidates for the development of advanced optical materials. Here, submicron zirconia particles were synthesized using a modified sol-gel method and pre-calcined at 400 °C. Silica shells were grown on these particles (average size: ∼270 nm) with well-defined thicknesses (26 to 61 nm) using a seeded-growth Stöber approach. To study the thermal stability of bare ZrO2 cores and ZrO2@SiO2 core-shell particles they were calcined at 450 to 1200 °C. After heat treatments, the particles were characterized by SEM, TEM, STEM, cross-sectional EDX mapping, and XRD. The non-encapsulated, bare ZrO2 particles predominantly transitioned to the tetragonal phase after pre-calcination at 400 °C. Increasing the temperature to 600 °C transformed them to monoclinic. Finally, grain coarsening destroyed the spheroidal particle shape after heating to 800 °C. In striking contrast, SiO2-encapsulation significantly inhibited grain growth and the t → m transition progressed considerably only after heating to 1000 °C, whereupon the particle shape, with a smooth silica shell, remained stable. Particle disintegration was observed after heating to 1200 °C. Thus, ZrO2@SiO2 core-shell particles are suited for high-temperature applications up to ∼1000 °C. Different mechanisms are considered to explain the markedly enhanced stability of ZrO2@SiO2 core-shell particles.

Thermally stable core-shell Ni/nanorod-CeO2@SiO2 catalyst for partial oxidation of methane at high temperatures

During partial oxidation of methane (POM), the greatest challenge is to maintain the thermal stability of the catalyst at high temperatures. One of the most effective ways to improve thermal stability is to construct core-shell structure. Herein, using a microemulsion method, we synthesized a core-shell Ni/nanorod-CeO2@SiO2 catalyst, in which the Ni nanoparticles were supported on the CeO2 nanorods and encapsulated by SiO2 shells. Based on a series of characterizations, we found that the Ni particles are of nanosize (2.2 nm) and the thickness of the SiO2 shell is about 8 nm in the core-shell catalyst. Moreover, the Ni/nanorod-CeO2@SiO2 catalyst can perfectly maintain rod-like structures of the CeO2 support and enhance interaction between the metal Ni and CeO2, significantly reducing the sintering of metal Ni particles at high temperatures. Therefore, the as-prepared Ni/nanorod-CeO2@SiO2 catalyst shows high catalytic activity and good thermal stability during the POM reaction.

Advanced Chemical Looping Materials for CO₂ Utilization: A Review

Combining chemical looping with a traditional fuel conversion process yields a promising technology for low-CO₂-emission energy production. Bridged by the cyclic transformation of a looping material (CO₂ carrier or oxygen carrier), a chemical looping process is divided into two spatially or temporally separated half-cycles. Firstly, the oxygen carrier material is reduced by fuel, producing power or chemicals. Then, the material is regenerated by an oxidizer. In chemical looping combustion, a separation-ready CO₂ stream is produced, which significantly improves the CO₂ capture efficiency. In chemical looping reforming, CO₂ can be used as an oxidizer, resulting in a novel approach for efficient CO₂ utilization through reduction to CO. Recently, the novel process of catalyst-assisted chemical looping was proposed, aiming at maximized CO₂ utilization via the achievement of deep reduction of the oxygen carrier in the first half-cycle. It makes use of a bifunctional looping material that combines both catalytic function for efficient fuel conversion and oxygen storage function for redox cycling. For all of these chemical looping technologies, the choice of looping materials is crucial for their industrial application. Therefore, current research is focused on the development of a suitable looping material, which is required to have high redox activity and stability, and good economic and environmental performance. In this review, a series of commonly used metal oxide-based materials are firstly compared as looping material from an industrial-application perspective. The recent advances in the enhancement of the activity and stability of looping materials are discussed. The focus then proceeds to new findings in the development of the bifunctional looping materials employed in the emerging catalyst-assisted chemical looping technology. Among these, the design of core-shell structured Ni-Fe bifunctional nanomaterials shows great potential for catalyst-assisted chemical looping.

Porous Silica-Coated Gold Sponges with High Thermal and Catalytic Stability

A method to fabricate porous silica-coated Au sponges that show high thermal and catalytic stability has been developed for the first time. The method involves dense surface functionalization of Au sponges (made by self-assembly of Au nanoparticles) with thiolated poly(ethylene glycol) (SH-PEG), which provides binding and condensation sites for silica precursors. The silica coating thickness can be controlled by using SH-PEG of different molecular weights. The silica-coated Au sponge prepared by using 5 kDa SH-PEG maintains its morphology at temperature as high as 700 °C. The calcination removes all organic molecules, resulting in porous silica-coated Au sponges, which contain hierarchically connected micro- and mesopores. The hierarchical pore structures provide an efficient pathway for reactant molecules to access the surface of Au sponges. The porous silica-coated Au sponges show an excellent catalytic recyclability, maintaining the catalytic conversion percentage of 4-nitrophenol by NaBH₄ to 4-aminophenol as high as 93% even after 10 catalytic cycles. The method may be applicable for other porous metals, which are of great interests for catalyst, fuel cell, and sensor applications.

Silica-Coated Plasmonic Metal Nanoparticles in Action

Hybrid colloids consisting of noble metal cores and metal oxide shells have been under intense investigation for over two decades and have driven progress in diverse research lines including sensing, medicine, catalysis, and photovoltaics. Consequently, plasmonic core-shell particles have come to play a vital role in a plethora of applications. Here, an overview is provided of recent developments in the design and utilization of the most successful class of such hybrid materials, silica-coated plasmonic metal nanoparticles. Besides summarizing common simple approaches to silica shell growth, special emphasis is put on advanced synthesis routes that either overcome typical limitations of classical methods, such as stability issues and undefined silica porosity, or grant access to particularly sophisticated nanostructures. Hereby, a description is given, how different types of silica can be used to provide noble metal particles with specific functionalities. Finally, applications of such nanocomposites in ultrasensitive analyte detection, theranostics, catalysts, and thin-film solar cells are reviewed.

Bringing attention to metal (un)availability in encapsulated catalysts

The encapsulation method significantly affects the shell porosity, the availability of active sites and the catalytic behavior of Pd@SiO₂ materials in methane combustion.

Geometric design of a Ni@silica nano-capsule catalyst with superb methane dry reforming stability: enhanced confinement effect over the nickel site anchoring inside a capsule shell with an appropriate inner cavity

Ni particles confined in sealed nano-capsule shells with anchoring effect demonstrate improved catalytic performance.

Synthesis and chemical transformation of Ni nanoparticles embedded in silica

Ni nanoparticles (NPs) catalyze many chemical reactions, in which they can become contaminated or agglomerate, resulting in poorer performance. We report deposition of silica (SiO₂) onto Ni NPs from tetraethyl orthysilicate (TEOS) through a reverse microemulsion approach, which is accompanied by an unexpected etching process. Ni NPs with an average initial diameter of 27 nm were embedded in composite SiO₂-overcoated Ni NPs (SiO₂-Ni NPs) with an average diameter of 30 nm. Each SiO₂-Ni NP contained a ∼7 nm oxidized Ni core and numerous smaller oxidized Ni NPs with diameters of ∼2 nm distributed throughout the SiO₂ shell. Etching of the Ni NPs is attributed to use of ammonium hydroxide as a catalyst for deposition of SiO₂. Aliquots acquired during the deposition and etching process reveal agglomeration of SiO₂ and Ni NPs, followed by dissociation into highly uniform SiO₂-Ni NPs. This etching and embedding process may also be extended to other core materials. The stability of SiO₂-Ni NPs was also investigated under high-temperature oxidizing and reducing environments. The structure of the SiO₂-Ni NPs remained significantly unchanged after both oxidation and reduction, which suggests structural durability when used for catalysis.

Porous yolk-shell microspheres as N-doped carbon matrix for motivating the oxygen reduction activity of oxygen evolution oriented materials

It is highly challenging to explore high-performance bi-functional oxygen electrode catalysts for their practical application in next-generation energy storage and conversion devices. In this work, we synthesize hierarchical N-doped carbon microspheres with porous yolk-shell structure (NCYS) as a metal-free electrocatalyst toward efficient oxygen reduction through a template-free route. The enhanced oxygen reduction performances in both alkaline and acid media profit well from the porous yolk-shell structure as well as abundant nitrogen functional groups. Furthermore, such yolk-shell microspheres can be used as precursor materials to motivate the oxygen reduction activity of oxygen evolution oriented materials to obtain a desirable bi-functional electrocatalyst. To verify its practical utility, Zn-air battery tests are conducted and exhibit satisfactory performance, indicating that this constructed concept for preparation of bi-functional catalyst will afford a promising strategy for exploring novel metal-air battery electrocatalysts.

Sc promoted and aerogel confined Ni catalysts for coking-resistant dry reforming of methane

Sc promoted and aerogel confined Ni catalysts were developed for coking-resistant dry reforming of methane.

In situ preparation of Ni nanoparticles in cerium-modified silica aerogels for coking- and sintering-resistant dry reforming of methane

Ni nanoparticles in nanochannels of cerium-modified silica aerogels were in situ prepared for coking-resistant dry reforming of methane.

Silica-supported Au@hollow-SiO2 particles with outstanding catalytic activity prepared via block copolymer template approach

Catalytically active Au@hollow-SiO₂ particles embedded in porous silica support (Au@hollow-SiO₂@PSS) were prepared by using spherical micelles from poly(styrene)-block-poly(4-vinyl pyridine) block copolymer as a sacrificial template. Drastic increase of the shell porosity was observed after pyrolytic removal of polymeric template because the stretched poly(4-vinyl pyridine) chains interpenetrating with silica shell acted as an effective porogen. The embedding of Au@hollow-SiO₂ particles in porous silica support prevented their fusion during pyrolysis. The catalytic activity of Au@hollow-SiO₂@PSS was investigated using a model reaction of catalytic reduction of 4-nitrophenol and reductive degradation of Congo red azo-dye. Significantly, to the best of our knowledge, Au@hollow-SiO₂@PSS catalyst shows the highest activity among analogous systems reported till now in literature. Such high activity was attributed to the presence of multiple pores within silica shell of Au@hollow-SiO₂ particles and easy accessibility of reagents to the catalytically active sites of the ligand-free gold surface through the porous silica support.

Noncontact Synergistic Effect between Au Nanoparticles and the Fe2O3 Spindle Inside a Mesoporous Silica Shell as Studied by the Fenton-like Reaction

An Au-Fe₂O₃@mesoporous SiO₂ nanoreactor with a multiyolks/shell structure was synthesized through a multistep method. In this nanoreactor, the spindle Fe₂O₃ and Au nanoparticles were inside the same mesoporous SiO₂ shell as the yolks but in a noncontact manner. The noncontact synergistic effect between Au nanoparticles and the Fe₂O₃ spindle was studied with a Fenton-like reaction. The catalytic activity of the Au-Fe₂O₃@mesoporous SiO₂ nanoreactor to the Fenton-like reaction for the degradation of organic dyes was dramatically enhanced by the noncontact synergistic effect.