Microporous Silica Shells Research Articles

A hollow in hollow nanoreactor of H-PtCu@SiO2 for the selective transfer hydrogenation

Hollow metal nanoparticles have attracted great interest in heterogeneous catalysis research due to their unique structures as nanoreactors. Here, we report hollow PtCu nanoparticles encapsulated in microporous silica shells (H-PtCu@SiO2), which can serve as the nanoreactor with high catalytic activity and high stability. The H-PtCu@SiO2 nanoreactor shows the state-of-the-art catalytic performance for the transfer hydrogenation of benzonitrile to dibenzylamine, which achieves up to 100% conversion and 99% selectivity in only 0.5 h. The void-confinement effect of H-PtCu@SiO2 is crucial for the outstanding catalytic performance by encaging the reactants in the nanoreactor. The computation demonstrates the limiting diffusion effect of the silica shell toward the ammonia borane, thus reduces the concentration of reductant species and prolongs the lifetime of the intermediate imine to improve the selectivity of secondary amine. Moreover, the silica shells can effectively enhance the catalytic stability of the H-PtCu@SiO2 nanoreactor.

Effect of different silica coatings on the toxicity of upconversion nanoparticles on RAW 264.7 macrophage cells.

Upconversion nanoparticles (UCNPs), consisting of NaYF4 doped with 18% Yb and 2% Er, were coated with microporous silica shells with thickness values of 7 ± 2 and 21 ± 3 nm. Subsequently, the negatively charged particles were functionalized with N-(6-aminohexyl)-3-aminopropyltrimethoxysilane (AHAPS), which provide a positive charge to the nanoparticle surface. Inductively coupled plasma optical emission spectrometry (ICP-OES) measurements revealed that, over the course of 24h, particles with thicker shells release fewer lanthanide ions than particles with thinner shells. However, even a 21 ± 3 nm thick silica layer does not entirely block the disintegration process of the UCNPs. 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays and cell cytometry measurements performed on macrophages (RAW 264.7 cells) indicate that cells treated with amino-functionalized particles with a thicker silica shell have a higher viability than those incubated with UCNPs with a thinner silica shell, even if more particles with a thicker shell are taken up. This effect is less significant for negatively charged particles. Cell cycle analyses with amino-functionalized particles also confirm that thicker silica shells reduce cytotoxicity. Thus, growing silica shells to a sufficient thickness is a simple approach to minimize the cytotoxicity of UCNPs.

Assessing the protective effects of different surface coatings on NaYF4:Yb3+, Er3+ upconverting nanoparticles in buffer and DMEM

We studied the dissolution behavior of β NaYF4:Yb(20%), Er(2%) UCNP of two different sizes in biologically relevant media i.e., water (neutral pH), phosphate buffered saline (PBS), and Dulbecco’s modified Eagle medium (DMEM) at different temperatures and particle concentrations. Special emphasis was dedicated to assess the influence of different surface functionalizations, particularly the potential of mesoporous and microporous silica shells of different thicknesses for UCNP stabilization and protection. Dissolution was quantified electrochemically using a fluoride ion selective electrode (ISE) and by inductively coupled plasma optical emission spectrometry (ICP OES). In addition, dissolution was monitored fluorometrically. These experiments revealed that a thick microporous silica shell drastically decreased dissolution. Our results also underline the critical influence of the chemical composition of the aqueous environment on UCNP dissolution. In DMEM, we observed the formation of a layer of adsorbed molecules on the UCNP surface that protected the UCNP from dissolution and enhanced their fluorescence. Examination of this layer by X-ray photoelectron spectroscopy (XPS) and mass spectrometry (MS) suggested that mainly phenylalanine, lysine, and glucose are adsorbed from DMEM. These findings should be considered in the future for cellular toxicity studies with UCNP and other nanoparticles and the design of new biocompatible surface coatings.

Nickel-Ceria Nanowires Embedded in Microporous Silica: Controllable Synthesis, Formation Mechanism, and Catalytic Applications.

Designing highly efficient catalysts for use in fuel production is a highly attractive research area but still remains challenging. Herein, for the first time, ultrafine Ni nanoparticles (NPs) self-assembled on ceria nanowires (NWs) and then embedded in a microporous silica shell (denoted as Ni-CeO2@SiO2) are successfully designed and synthesized via a one-pot facile strategy. The average diameter of Ni-CeO2 NWs is just 2.9 nm, and the length is up to 102.7 nm. The resulting Ni-CeO2@SiO2 exhibits high performance and 100% hydrogen selectivity for H2 production from N2H4 and N2H4BH3 in aqueous solution. Unexpectedly, Ni-CeO2@SiO2 also has good catalytic performance and thermal stability for CO2 methanation. The high catalytic performance of Ni-CeO2@SiO2 can be attributed to the synergistic electronic effect and strong interaction between Ni NPs and CeO2 NWs with plenty of oxygen vacancies, as well as the unique structure effect. As an effective strategy, the present work provides an opportunity to embed ultrafine metal NPs-CeO2 NWs into a microporous silica shell, which has broad application prospects in various catalytic fields.

Product Distributions of Fischer‐Tropsch Synthesis over Core‐Shell Catalysts: The Effects of Diverse Shell Thickness

AbstractA series of core‐shell catalysts with different shell thickness were synthesized and the effect of microporous silica shell on product distribution of Fischer‐Tropsch synthesis was analyzed. Fe3O4 core was prepared by hydrothermal method and the amorphous SiO2 shell was coated on the Fe3O4 core by sol‐gel method. N2 adsorption‐desorption analysis indicated an irregular and microporous silica shell and the pore size distribution moved to smaller diameter with the shell thickness increased. In Fischer‐Tropsch synthesis, inactive silica shell increased catalytic activity (FTY) and stability by restricting catalyst cracking and sintering. Product distribution shifted to C1‐C4 gaseous hydrocarbons with the increasing shell thickness probably due to the diffusion limitations on reactants/products and structure confinement and separation effect of microporous silica shell on restricting the growth of catalytic active particles. Selectivities of C5+ were restricted within 10% for core‐shell catalysts. WGS reaction activity is reduced as well, which constrains side reaction and amplifies FTS catalytic productivity to hydrocarbons.

Design of Stable Ultrasmall Pt−Ni(O) Nanoparticles with Enhanced Catalytic Performance: Insights into the Effects of Pt−Ni−NiO Dual Interfaces

AbstractNoble metal/transition metal oxide interfaces are typically considered as highly active sites for most heterogeneous catalytic reactions and can effectively increase the utilization of noble metals. One of the challenges in such catalytic systems, particularly under high‐temperature reactions, is the lack of an effective strategy to stabilize the catalysts against sintering over time and maintain the metal/oxide interfaces. We developed a one‐pot facile strategy to embed the ultrafine Pt−Ni(O) nanoparticles (NPs) with dual interfaces (Pt−Ni−NiO) in a microporous silica shell to fabricate a high‐performance heterogeneous oxidation catalyst (assigned as Pt−Ni(O)@SiO2) for the first time. Owing to the ultra‐small size of the Pt−Ni(O) NPs and the confinement effect of the silica shell, the resulting Pt−Ni(O)@SiO2 catalyst exhibited enhanced performance for CO oxidation and toluene [one of the main volatile organic compounds, (VOCs)] total combustion. The effects of Pt−NiO, Pt−Ni−NiO, and Pt−Ni interfaces were also studied in detail. The results indicated that the Pt−NiO and Pt−Ni alloy dual interface have the improving effect for catalytic oxidation and reducing utilization of noble metals. The strategy developed in this work may represent a general approach in the rational design of other high‐performance catalysts combining the interface catalysis and physical confinement concept to improve the usage of the noble metals.

Reverse Microemulsion-mediated Synthesis of Monometallic and Bimetallic Early Transition Metal Carbide and Nitride Nanoparticles.

A reverse microemulsion is used to encapsulate monometallic or bimetallic early transition metal oxide nanoparticles in microporous silica shells. The silica-encapsulated metal oxide nanoparticles are then carburized in a methane/hydrogen atmosphere at temperatures over 800 °C to form silica-encapsulated early transition metal carbide nanoparticles. During the carburization process, the silica shells prevent the sintering of adjacent carbide nanoparticles while also preventing the deposition of excess surface carbon. Alternatively, the silica-encapsulated metal oxide nanoparticles can be nitridized in an ammonia atmosphere at temperatures over 800 °C to form silica-encapsulated early transition metal nitride nanoparticles. By adjusting the reverse microemulsion parameters, the thickness of the silica shells, and the carburization/nitridation conditions, the transition metal carbide or nitride nanoparticles can be tuned to various sizes, compositions, and crystal phases. After carburization or nitridation, the silica shells are then removed using either a room-temperature aqueous ammonium bifluoride solution or a 0.1 to 0.5 M NaOH solution at 40-60 °C. While the silica shells are dissolving, a high surface area support, such as carbon black, can be added to these solutions to obtain supported early transition metal carbide or nitride nanoparticles. If no high surface area support is added, then the nanoparticles can be stored as a nanodispersion or centrifuged to obtain a nanopowder.

Nanoconfined Amine-Functionalized Silicone Oil Sorbents for Rapid CO2-Capture

A novel CO2 sorbent system involving the confinement of aminated silicone oil-based liquid sorbents in hollow, microporous silica shells was investigated. In TGA experiments with three silicone oil sorbents (GAP-0, M′D′M′, and M3′T′), the nanoconfined materials exhibited initial CO2 sorption rates between 44 and 90 times faster than the corresponding unconfined, bulk liquid silicone oils. However, the liquids proved to be volatile at temperatures as low as 45 °C in the gaseous flow environment, resulting in a continuous decrease in CO2 sorption capacities over multiple sorption/desorption cycles. In contrast to their volatility in their CO2-free state, the nanoconfined sorbents exhibited excellent stability upon sorption of CO2, suggesting that the unique CO2 sorption-induced liquid-to-solid phase transition that characterizes these sorbents in their unconfined bulk form is not significantly altered through nanoconfinement. Overall, while the stability of these nanoconfined amine-functionalized silicone oils suffers from their high volatility at flue gas temperatures, the observed drastic acceleration of the CO2 sorption kinetics with very little energy penalty for the overall composite sorbent suggests that nanoconfinement is a flexible way to accelerate the sorption kinetics of liquid CO2 sorbents.

Lab-in-a-shell: encapsulating metal clusters for size sieving catalysis.

Here we describe a lab-in-a-shell strategy for the preparation of multifunctional core-shell nanospheres consisting of a core of metal clusters and an outer microporous silica shell. Various metal clusters (e.g., Pd and Pt) were encapsulated and confined in the void space mediated by the entrapped polymer dots inside hollow silica nanospheres acting first as complexing agent for metal ions and additionally as encapsulator for clusters, limiting growth and suppressing the sintering. The Pd clusters encapsulated in hybrid core-shell structures exhibit exceptional size-selective catalysis in allylic oxidations of substrates with the same reactive site but different molecular size (cyclohexene ∼0.5 nm, cholesteryl acetate ∼1.91 nm). The solvent-free aerobic oxidation of diverse hydrocarbons and alcohols was further carried out to illustrate the benefits of such an architecture in catalysis. High activity, outstanding thermal stability and good recyclability were observed over the core-shell nanocatalyst.

High payload Gd(iii) encapsulated in hollow silica nanospheres for high resolution magnetic resonance imaging

For clear MR imaging of blood vessels, a long blood circulation time of effective T1 contrast agents is necessary. Nanoparticulate MR contrast agents are much more effective owing to their enhanced relaxivity, a result of reduced tumbling rates, and large payloads of active magnetic species. PEGylated yolk-shell silica nanospheres containing high payloads of Gd(iii) with cross-linking ligands are synthesized and evaluated as a blood-pool magnetic resonance contrast agent. The hydrophilic PEG coating and the microporous silica shell allow water exchange while keeping the multi-nuclear Gd species from leaching out. These Gd(iii)-containing yolk-shell silica nanoparticles with PEGylated surfaces give excellent resolution and contrast in magnetic resonance angiography images of vasculature in rat brains.

Stabilization of CdS semiconductor nanoparticles against photodegradation by a silica coating procedure

Nanometre-sized CdS semiconductor particles were synthesized in the presence of sodium citrate, and subsequently surrounded by a homogeneous silica shell. The coating procedure makes use of 3-(mercaptopropyl) trimethoxy silane (MPS) as a surface primer to deposit a thin silica shell in water. The dispersion is then transferred into ethanol, where thicker shells can be grown. The citrate-stabilized particles are slowly degraded through photochemical oxidation in the presence of dissolved oxygen. This destabilizing process is suppressed when a homogeneous, microporous silica shell is built up around the particles, through a limited access of O 2 molecules to the CdS surface.