Controllable synthesis of multi-shelled NiCo2O4 hollow spheres catalytically for the thermal decomposition of ammonium perchlorate

The compatible catalytic structure of NiCo₂O₄ was modified into multi-shelled hollow spheres by one-pot synthesis, followed by heat treatment. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Brunauer–Emmet–Teller (BET) and N₂ adsorption–desorption approaches were used for the characterizations of nanoparticles and multi-shelled hollow porous structures and the morphologies and crystal structures of these hollow spheres, respectively. Differential scanning calorimetry (DSC) was adopted for comparing the thermal decomposition performances of ammonium perchlorate (AP) catalyzed by adding different contents of multi-shelled NiCo₂O₄ hollow spheres. Impressively, the experimental results showed that the NiCo₂O₄ hollow spheres exhibited more excellent catalytic activity than NiCo₂O₄ nanoparticles as a result of their large specific surface areas, good adsorption capacity and many active reduction sites. The decomposition temperature of AP with multi-shelled NiCo₂O₄ hollow spheres could be reduced up to 322.3 °C from 416.3 °C. Furthermore, a catalytic mechanism was proposed for the thermal decomposition of AP over multi-shelled NiCo₂O₄ hollow spheres based on electron transfer processes.

Double-shelled NiCo₂O₄ hollow spheres synthesized by a facile hydro-thermal method showed excellent catalytic properties for the thermal decomposition of AP.     

An efficient Au catalyst supported on hollow carbon spheres for acetylene hydrochlorination

Mesoporous hollow carbon spheres (HCSs) were prepared using SiO₂ spheres as a hard template, and Au nanoparticles were then synthesized using NaBH₄ as a reducing agent on the surface of the HCS support. Transmission electron microscopy characterization indicated that Au nanoparticles were much smaller on the HCS support than those on the active carbon (AC) support. HCl-TPD showed that the Au/HCS catalyst displayed a more active site than on Au/AC. The resulting Au/HCS catalyst showed excellent catalytic activity and stability for acetylene hydrochlorination. Acetylene conversion of Au/HCS can be maintained above 92% even after 500 h of lifetime. The excellent catalytic performance of Au/HCS was attributed to the presence of the HCS support, which limited the aggregation of Au nanoparticles.

Mesoporous hollow carbon spheres (HCS) were prepared and applied as the support of Au catalyst for acetylene hydrochlorination. Au/HCS exhibited excellent stability for acetylene hydrochlorination.     

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.     

Au nanoparticle-decorated TiO2 hollow fibers with enhanced visible-light photocatalytic activity toward dye degradation

TiO₂ hollow fibers (THF) were prepared by a template method using kapok as a biotemplate and subsequently decorated by plasmonic Au nanoparticles using a solution plasma process. The THF exhibited an anatase phase and a hollow structure with a mesoporous wall. Au nanoparticles with a diameter of about 5–10 nm were uniformly distributed on the THF surface. Au nanoparticles-decorated TiO₂ hollow fibers (Au/THF) have enhanced photocatalytic activity toward methylene blue degradation under visible light-emitting diode (Vis-LED) as compared to pristine THF and P25. This could be attributed to combined effects including effective light-harvesting by a hollow structure, large surface area due to a mesoporous wall of THF, and visible-light absorption and efficient charge separation induced by Au nanoparticles. The Au/THF also showed good recyclability and separation ability.

Plasmonic Au nanoparticles-decorated TiO₂ hollow fibers with enhanced visible-light photocatalytic activity have been successfully prepared by a two-step process: (i) template method using kapok and (ii) solution plasma process.     

Preparation of nanoporous alumina hollow spheres with a highly ordered hole arrangement

Nanoporous alumina spheres with an ordered hole arrangement were prepared through a two-step anodization of small Al particles. The hole periodicity in the ordered anodic porous alumina could be controlled by adjusting the anodizing conditions. Nanoporous hollow spheres were also obtained by removal of residual Al in an etchant. Additionally, nanoporous spheres loaded with Au nanoparticles on their surfaces were obtained through electrochemical or chemical deposition of Au nanoparticles. The obtained Au/alumina composite hollow spheres were used as a substrate for surface-enhanced Raman scattering measurements.

Nanoporous alumina hollow spheres with a highly ordered hole arrangement prepared by two-step anodization of small Al particles.     

A structured catalyst with high dispersity of Au species based on hollow SiC foam with porous walls for acetylene hydrochlorination

It is essential to improve the catalytic stability of Au-based catalysts for acetylene hydrochlorination. In this study, a novel hollow SiC foam with porous walls (HSF_p) is proposed to modify the Au catalyst distribution on the foam-based structured catalyst. The scanning electron microscopy and 3D X-ray tomography results showed that the hollow structure and the porous ceramic wall of HSF_p were successfully obtained. An Au/AC/HSF structured catalyst was prepared by the slurry-coating and impregnation method. Then the effects of the wall pore structure of HSF_p on the distribution of Au species and the stability of the catalyst were studied by comparison with a hollow SiC foam with compact walls (HSF_c). The HSF_p with 3D interconnected hollow channel structure was demonstrated to have a promotion effect on the reaction stability due to its ability to refine the catalyst particles during the impregnation and drying processes. The result indicates that the time on stream of acetylene conversion above 90% over the Au/AC/HSF_p structured catalyst reaches about 380 h at an acetylene gaseous hourly space velocity of 130 h⁻¹, which is much longer than that of the Au/AC/HSF_c. In addition, a bidirectional diffusion effect that can enhance the catalyst distribution by the porous wall of HSF was proposed in this paper. This may have positive significance in the field of preparation of structured catalysts.

A bidirectional drying to achieve high dispersity of structured catalysts using hollow SiC foam with porous walls.     

Improved hydrogen evolution performance by engineering bimetallic AuPd loaded on amino and nitrogen functionalized mesoporous hollow carbon spheres

A highly efficient heterogeneous catalyst was synthesized by delicate engineering of NH₂-functionalized and N-doped hollow mesoporous carbon spheres (NH₂–N-HMCS), which was used for supporting AuPd alloy nanoparticles with ultrafine size and good dispersion (denoted as AuPd/NH₂–N-HMCS). Without using any additives, the prepared AuPd/NH₂–N-HMCS catalytic formic acid dehydrogenation possesses superior catalytic activity with an initial turnover frequency value of 7747 mol H₂ per mol catalyst per h at 298 K. The excellent performance of AuPd/NH₂–N-HMCS derives from the unique hollow mesoporous structure, the small particle sizes and high dispersion of AuPd nanoparticles and the modified Pd electronic structure in the AuPd/NH₂–N-HMCS composite, as well as the synergistic effect of the modified support.

Anchoring ultrafine AuPd on NH₂-functionalized and N-doped hollow mesoporous carbon spheres for formic acid dehydrogenation.     

C-doped ZnO decorated with Au nanoparticles constructed from the metal–organic framework ZIF-8 for photodegradation of organic dyes

Recently, engineering metal–organic frameworks (MOFs) into metal oxides by solid state thermal decomposition has attracted wide attention for photocatalytic applications. Here, a series of C-doped ZnO materials decorated with Au nanoparticles (Au/C-ZnO) were constructed via controlled pyrolysis of ZIF-8 adsorbing different amounts of HAuCl₄·4H₂O. In this pyrolysis process, ZIF-8 was transformed into C-doped ZnO according to the EDX and XPS analysis. Meanwhile, HAuCl₄·4H₂O was transformed into Au nanoparticles that were uniformly dispersed on the surface of C-ZnO as seen in TEM images. The photocatalytic activity of as-prepared catalysts was evaluated by the degradation of methyl orange under UV-vis light irradiation. It was found that the photocatalytic activity of Au/C-ZnO was better than C-ZnO and pure ZnO. Furthermore, Au/C-ZnO exhibited high photocatalytic stability. After three consecutive cycles, there was no noticeable deactivation in the reaction. This unusual photocatalytic activity was attributed to the synergistic effect of C-doping and Au NPs.

C-doped ZnO decorated with Au nanoparticles (Au/C-ZnO) were prepared via one step pyrolysis of ZIF-8 adsorbing HAuCl₄.     

One-pot synthesis of hollow hydrangea Au nanoparticles as a dual catalyst with SERS activity for in situ monitoring of a reduction reaction

The controlled synthesis of metallic nanomaterials has attracted the interest of many researchers due to their shape-dependent physical and chemical properties. However, most of the synthesized nanocrystals cannot be combined with spectroscopy to measure the reaction kinetics, thus limiting their use in monitoring the catalytic reaction process to elucidate its mechanism. As a powerful analytical tool, surface-enhanced Raman spectroscopy (SERS) can be used to achieve in situ monitoring of catalytic reactions by developing bifunctional metal nanocrystals with both SERS and catalytic activities. Herein, we have developed a simple one-pot synthesis method for the large-scale and size-controllable preparation of highly rough hydrangea Au hollow nanoparticles. The growth mechanism of flower-like Au hollow nanostructures was also discussed. The hollow nanostructure with a 3D hierarchical flower shell combines the advantages of hollow nanostructure and hierarchical nanostructure, which possess high SERS activity and good catalytic activity simultaneously. Furthermore, the hydrangea Au hollow crystals were used as a bifunctional nanocatalyst for in situ monitoring of the reduction reaction of 4-nitrothiophenol to the 4-aminothiophenol.

We prepared hollow flower-shaped Au nanoparticles as a bifunctional material by a one-pot method for in situ monitoring of reduction reactions.     

Tuning the metal-support interaction in the thermal-resistant Au–CeO2 catalysts for CO oxidation: influence of a mild N2 pretreatment

Pretreatment is very important for altering the catalytic properties of the supported noble metal catalysts in many heterogeneous reactions. In this study, a simple and mild pretreatment with N₂ has been reported to re-activate the Au–CeO₂ catalysts that were prepared by a deposition–precipitation method followed by calcination at 600 °C. Upon N₂ pretreatment at 200 °C, the metal-support interaction between Au nanoparticles (NPs) and CeO₂ was observed with the evidence of particular coverage of Au nanoparticles by CeO₂, electronic interactions and changes in CO adsorption ability. As a result, the CO oxidation activity of the pretreated Au–CeO₂ catalysts largely improved compared with those without any pretreatment and even with those subjected to H₂ and O₂ pretreatments. N₂ pretreatment also makes the Au NPs more resistant to sintering at high temperature. Furthermore, this mild pretreatment strategy can provide a potential approach to improve the thermal stability of other supported noble metal catalysts.

The degree of encapsulation for Au–CeO₂ catalysts was identical to the catalysts exhibiting metal-support interaction, which improved the CO oxidation activity.     

In situ growth of CuS NPs on 3D porous cellulose macrospheres as recyclable biocatalysts for organic dye degradation

Aiming at recyclable catalyst carriers, porous cellulose macrospheres from wood pulp dissolved in an alkaline urea system were regenerated by simple injection regeneration. After solvent exchange, porous cellulose macrospheres (CMs) with a high specific surface area of 325.3 m² g⁻¹ were obtained by lyophilization, and CuS nanoparticles (CuS NPs) were coated on CMs by in situ growth in the liquid phase to achieve CuS-supported CM macrospheres (CuS@CM). The results indicated that the CuS@CM biocatalyst was successfully prepared with an average diameter of approximately 1.2 mm. In addition, CuS@CM was further used as a heterogeneous catalyst for the catalytic degradation of methylene blue (MB) and methyl orange (MO) model dyes during the oxidation of hydrogen peroxide (H₂O₂). In the presence of low doses of H₂O₂, the degradation rate of MB reached 94.8% within 10 min, showing high catalytic activity under neutral and alkaline conditions. After five cycles, 90.1% of the original catalytic activity was still retained, indicating that the prepared CuS@CM composite possessed excellent catalytic activity and reusability.

CuS nanoparticles were grown in situ on 3D porous cellulose macrospheres for an excellent rapid cycling removal of organic dyes.     

One-pot self-assembly preparation of thiol-functionalized poly(3,4-ethylenedioxythiophene) hollow nanosphere/Au composites,and their electrocatalytic properties

In this work, we developed a thiol-functionalized poly(3,4-ethylenedioxythiophene) hollow sphere (poly(EDOT-MeSH)/Au) polymer through a simple one-pot self-assembly method using polyvinylpyrrolidone (PVP) as a soft template. The monomer was used as both a reductant and a stabilizer to decorate gold nanoparticles (Au NPs). FTIR, XRD, EDX, SEM and TEM analyses were used to characterize the composite hollow spheres. The chemical bond between S and Au was confirmed by XPS. The electrochemical performance of the composite hollow spheres was determined by cyclic voltammetry (CV) and an ampere response timing current test. The results revealed that the poly(EDOT-MeSH)/Au hollow-sphere-based electrochemical sensor possesses excellent conductivity and high redox reversibility with detection limits (S/N = 3) of 0.2, 0.02, 0.08 and 0.05 μM in the linear ranges of 0.1–650 μM, 0.05–100 μM and 0.1–600 μM for the determination of ascorbic acid (AA), dopamine (DA), uric acid (UA) and nitrate ions (NO₂⁻), respectively. The preparation method for these composites will further the development of this type of conducting polymer/gold nano-composite material modified electrochemical sensor for biological species.

In this work, we developed a thiol-functionalized poly(3,4-ethylenedioxythiophene) hollow sphere (poly(EDOT-MeSH)/Au) polymer through a simple one-pot self-assembly method using polyvinylpyrrolidone (PVP) as a soft template.     

Efficient synthesis of highly dispersed ultrafine Pd nanoparticles on a porous organic polymer for hydrogenation of CO2 to formate

Precise design of catalytic supports is an encouraging technique for simultaneously improving the activity and stability of the catalyst. However, development of efficient heterogeneous catalysts for transforming CO₂ into formic acid (FA) is still a big challenge. Herein, we report that Pd nanoparticles (NPs) based on a porous organic polymeric support containing amide and pyridine functional groups (AP-POP) can be an efficient catalyst for selective hydrogenation of CO₂ to form formate with high efficiency even under mild reaction conditions (6.0 MPa, 80 °C). Electron density of the active Pd species modulated via the interaction between pyridine nitrogen and Pd play important roles in dramatic enhancement of catalytic activity and was indicated by X-ray photoelectron spectroscopy (XPS) along with CO chemisorption. This work provides an interesting and effective strategy for precise support design to improve the catalytic performance of nanoparticles.

Precise design of catalytic supports is an encouraging technique for simultaneously improving the activity and stability of the catalyst.     

Enhanced catalytic activity of monodispersed porous Al2O3 colloidal spheres with NiMo for simultaneous hydrodesulfurization and hydrogenation

Novel composites made from monodispersed porous Al-glycolate spheres (NiMo/Al-SP) were synthesized through alcoholysis or hydrolysis treatments. The obtained samples were characterized by a complementary combination of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), N₂ physisorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), hydrogen temperature-programmed reduction (H₂-TPR), and pyridine Fourier transform infrared spectroscopy (Py FT-IR). In addition, the catalytic performances of the resultant catalysts were evaluated in the simultaneous HDS of dibenzothiophene (DBT) and HYD of naphthalene (DBT and naphthalene represent the sulfur-containing compounds and polycyoalkanes, respectively). The experimental results showed that, 71.22% DBT and 88.28% naphthalene were converted by NiMo/Al-SP(H) under the conditions of 270 °C temperature, 5 MPa H₂ (initial pressure at room temperature) and 10 h reaction time. The design and preparation of NiMo/Al-SP provide an effective and novel pathway for the development of high-performance catalysts and production processes.

Fabrication of monodispersed porous Al₂O₃ spheres with controlled morphologies.     

Effect of nitrogen co-doping with ruthenium on the catalytic performance of Ba/Ru–N-MC catalysts for ammonia synthesis

Nitrogen co-doping with ruthenium mesoporous carbons (Ru–N-MC) was prepared by co-impregnation of sucrose and urea on a RuCl₃/SiO₂ template followed by a thermal carbonization process. The turnover frequency (TOF) of the Ba/Ru–N-MC catalyst in ammonia synthesis is 0.16 s⁻¹ under reaction conditions of 400 °C, pressure of 10 MPa and space velocity of 10 000 h⁻¹. The superior catalytic performance of the Ba/Ru–N-MC is proposed to originate from the strong metal-support interaction between Ru nanoparticles (NPs) and carbon support. In addition to the activity, the Ba/Ru–N-MC catalyst exhibits a long-term stability for 35 h without significant deactivation.

A highly active and stable mesoporous Ba/Ru–N-MC catalyst for ammonia synthesis was prepared by an in situ thermal carbonization method with nitrogen co-doping with ruthenium NPs.     

Synthesis of Pd-loaded mesoporous SnO2 hollow spheres for highly sensitive and stable methane gas sensors

High performance methane gas sensors have become more and more essential in different fields such as coal mining, kitchens and industrial production, which necessitates the design and synthesis of highly sensitive materials. Herein, mesoporous SnO₂ hollow spheres with high surface area (>90 m² g⁻¹) are prepared by a progressive inward crystallization routine, showing a high response of 1.31 to 250 ppm CH₄ at a working temperature of 400 °C. Furthermore, loading noble metal Pd onto the surface of SnO₂ hollow spheres by an adsorption–calcination process improves the response to 4.88 (250 ppm CH₄) at the optimal dosage of 1 wt% Pd. Meanwhile, the working temperature decreases to 300 °C, showing the prominent spillover effect of catalytic Pd and PdO–SnO₂ heterostructure sensitization as evidenced by the binding energy shift in the X-ray photoelectron spectroscopy (XPS) analysis. The response/recovery time is as short as 3/7 s and the sensitivity is stable for a test period as long as 15 weeks. All these performances show the promise of the highly porous Pd-loaded SnO₂ hollow spheres for high performance methane sensors.

This work reports a simple, rapid, effective and reliable CH₄ sensor based on Pd-loaded SnO₂ hollow spheres with high surface area and porosity, which is of great importance to gas sensing performance.     

Mussel-inspired immobilization of Au on bare and graphene-wrapped Ni nanoparticles toward highly efficient and easily recyclable catalysts

Bimetallic nanocatalysts have been gaining huge research attention in the heterogeneous catalysis community recently owing to their tunable properties and multifunctional characteristics. In this work, we fabricated a bimetallic core–shell nanocomposite catalyst by employing a mussel-inspired strategy for immobilizing gold nanoparticles (AuNP) on the surface of nickel nanoparticles (NiNP). NiNPs obtained from the reduction of Ni(ii) were first coated with polydopamine to provide the anchoring sites towards the robust immobilization of AuNPs. The as-synthesized nanocomposite (Ni–PD–Au) exhibited outstanding catalytic activity while reducing methylene blue (MB) and 4-nitrophenol (4-NP) yielding rate constants 13.11 min⁻¹ and 4.21 min⁻¹, respectively, outperforming the catalytic efficiency of its monometallic counterparts and other similar reported catalysts by large margins. The superior catalytic efficiency of the Ni–PD–Au was attributed to the well-known synergistic effect, which was experimentally investigated and compared with prior reports. Similar bio-inspired immobilization of AuNPs was also applied on graphene-wrapped NiNPs (Ni-G) instead of bare NiNPs to synthesize another composite catalyst (Ni-G–PD–Au), which yet again exhibited synergistic catalytic activity. A comparative study between the two nanocomposites suggested that Ni–PD–Au excelled in catalytic activity but Ni-G–PD–Au provided noteworthy stability showing ∼100% efficiency over 17 repeated cycles. However, along with excellent synergistic performance, both nanocomposites demonstrated high magnetization and thermal stability up to 350 °C ascertaining their easy separation and sustainability for high-temperature applications, respectively.

Employing a bio-inspired strategy we combine Ni and Au nanoparticles into a single scaffold to achieve excellent synergistic catalysis along with high recyclability.     

Facile morphology control of 3D porous CeO2 for CO oxidation

3D porous CeO₂ with various morphologies was successfully synthesized via a facile precipitation using glycine as the soft bio-template. During the synthesis, it was demonstrated that the morphology of CeO₂ depended on the molar ratio of reactants. Furthermore, the catalytic performance towards CO oxidation of the as-synthesized CeO₂ with different morphologies was investigated. CeO₂ with a bowknot shape showed excellent catalytic performance, giving complete CO conversion at 370 °C, due to its properties of much higher oxygen vacancies, loosely packed pore structure and larger specific surface area.

Different morphologies of CeO₂ were obtained via a green and facial method, which realized CO complete conversion at 370 °C.     

Zeolitic imidazolate framework derived ZnCo2O4 hollow tubular nanofibers for long-life supercapacitors

Uniform one-dimensional metal oxide hollow tubular nanofibers (HTNs) have been controllably prepared using a calcination strategy using electrospun polymer nanofibers as soft templates and zeolitic imidazolate framework nanoparticles as precursors. Utilizing the general synthesis method, the ZnO HTNs, Co₃O₄ HTNs and ZnCo₂O₄ HTNs have been successfully prepared. The optimal ZnCo₂O₄ HTNs, as a representative substance applied in supercapacitors as the positive electrode, delivers a high specific capacity of 181 C g⁻¹ at a current density of 0.5 A g⁻¹, an excellent rate performance of 75.14% and a superior capacity retention of 97.42% after 10 000 cycles. Furthermore, an asymmetric supercapacitor assembled from ZnCo₂O₄ HTNs and active carbon also shows a stable and ultrahigh cycling stability with 95.38% of its original capacity after 20 000 cycle tests.

Uniform metal oxides hollow tubular nanofibers have been controllably prepared by calcination strategy using electrospun polymer nanofibers as soft templates and zeolitic imidazolate framework nanoparticles as precursors for long-life supercapacitor.     

A porous Co–Ru@C shell as a bifunctional catalyst for lithium–oxygen batteries

We use SiO₂ as a template and dopamine as a carbon source to synthesize a hollow C shell, and we load Co and Ru nanoparticles onto it to obtain a Co–Ru@C shell composite. The diameter and thickness of the C shell are 100 nm and 5–10 nm, respectively, and numerous holes of different sizes exist on the C shell. Meanwhile, numerous C shells stack together to form macropores, thereby forming a hierarchical porous structure in the material. Brunauer–Emmett–Teller surface area analysis reveals that the specific surface area and pore volume of the Co–Ru@C shell are 631.57 m² g⁻¹ and 2.20 cc g⁻¹, respectively, which can result in many three-phase interfaces and provide more space for the deposition of discharge products. Compared with Co@C shell and C shell electrodes, the obtained Co–Ru@C shell-based electrodes exhibit the highest discharge capacity, the lowest oxygen reduction reaction/oxygen evolution reaction overpotential and the best cycle stability, indicating the excellent catalytic ability of the Co–Ru@C shell.

We use SiO₂ as a template and dopamine as a carbon source to synthesize a hollow C shell, and we load Co and Ru nanoparticles onto it to obtain a Co–Ru@C shell composite.