In this work, the mesoporous SiO2 nanofibers from pyrolyzing precursor of electrospun nanofibers were employed as support to immobilize PtNi nanocatalyst (PtNi/SiO2 nanofibers). AFM, XRD, SEM, TEM, XPS, ICP-AES and N2 adsorption/desorption analysis were applied to systematically investigate the morphology and microstructure of as-prepared products. Results showed that PtNi alloy nanoparticles with average diameter of 18.7 nm were formed and could be homogeneously supported on the surface of porous SiO2 nanofiber, which further indicated that the SiO2 nanofibers with well-developed porous structure, large specific surface area, and roughened surface was a benefit for the support of PtNi alloy nanoparticles. The PtNi/SiO2 nanofibers catalyst exhibited an excellent catalytic activity towards the reduction of p-nitrophenol, and the catalyst’s kinetic parameter (kn = 434 × 10−3 mmol s−1 g−1) was much higher than those of Ni/SiO2 nanofibers (18 × 10−3 mmol s−1 g−1), Pt/SiO2 nanofibers (55 × 10−3 mmol s−1 g−1) and previous reported PtNi catalysts. The catalyst could be easily recycled from heterogeneous reaction system based on its good magnetic properties (the Ms value of 11.48 emu g−1). In addition, PtNi/SiO2 nanofibers also showed an excellent stability and the conversion rate of p-nitrophenol still could maintain 94.2% after the eighth using cycle.

An environmentally benign and economic reaction system with an effective catalyst for 4-nitrophenol reduction is highly desirable. Here, we synthesized reduced graphene oxide (RGO) supported Pt–Ni alloy catalysts with different atomic ratios of Pt and Ni, investigated their morphology, size, dispersity, structure and elemental valence, and studied their catalytic activity in order to tune their performance for 4-nitrophenol reduction. It is worth pointing out that the RGO support can efficiently avoid the aggregation of Pt–Ni alloy nanoparticles, and the most dispersed and smallest Pt–Ni particles on RGO can be obtained when the atomic ratio of Pt to Ni is 1:9. The Pt–Ni/RGO (1:9) nanocatalyst also shows a higher catalytic activity toward the conversion of 4-NP to 4-AP with a catalytic rate constant of 0.3700 min−1 than Pt–Ni/RGO (1:3) and Pt–Ni/RGO (1:25), and much higher than that of Pt/RGO, Ni/RGO and bare Pt–Ni owing to the well-defined composition, small particle size (10 nm), good dispersion, synergistic effect between Pt and Ni, and electron transfer between RGO and Pt–Ni alloy nanoparticles. In addition, the catalyst possesses good stability and recyclability for the catalytic reduction reaction. The Pt–Ni/RGO nanocatalyst, with well-defined composition, small particle size, uniform dispersity, high catalytic rate, and recyclability, should be an ideal catalyst for specific applications in liquid phase reactions.

Supported Pt-based alloy nanoparticles have attracted greater attention in catalysis due to their high activity, reduced cost, and easy recycling in chemical reactions. In this work, mesoporous SiO2 microspheres were employed as support to immobilize PtNi alloy nanocatalysts with different mass ratios of Pt and Ni (1:0, 3:1, 1:1, 1:3 and 0:1) by a facile in situ one-step reduction in the absence of any capping agent. SEM, EDS, TEM, FTIR, XRD, ICP-AES, XPS and nitrogen adsorption/desorption analysis were employed to systematically investigate the morphology and structure of the obtained SiO2 microspheres and SiO2/PtNi nanocatalysts. Results show that uniform PtNi nanoparticles can be homogeneously and firmly embedded into the surface of SiO2 microspheres. When the as-prepared SiO2/PtNi nanocatalysts were used in the reduction process of p-nitrophenol to p-aminophenol, the nanocatalyst with Pt and Ni mass ratio of 1:3 showed the highest catalytic activity (TOF of 5.35 × 1018 molecules⋅g−1⋅s−1) and could transform p-nitrophenol to p-aminophenol completely within 5 min. The SiO2/PtNi nanocatalyst can also maintain high catalytic activity in the fourth cycle, implying its excellent stability during catalysis.

Technological applications of heterogeneous nanocatalysts on supports have generally relied on the surface or interface properties of supports. Herein, we report a facile approach to fabricate roughened surfaces on halloysite nanotubes (HNTs) through etching the wall of HNTs in a molten-salt system. SEM, TEM, XRD, FTIR, AFM and N2 adsorption/desorption analysis are employed to systematically investigate the morphology, structure and surface properties. The results suggest that the roughness of HNTs surface has been significantly increased and defects are formed on the tube wall without structural damage. Subsequently, the roughened halloysite nanotubes (RHNTs) are used as supports to prepare heterogeneous nanocatalyst. Pt nanoparticles with a uniform size can be homogeneously deposited onto the RHNTs surfaces via a one-step hydrothermal reduction. The as-prepared Pt@RHNTs catalyst exhibits remarkably improved activity and selectivity for the hydrogenation of cinnamaldehyde towards cinnamyl alcohol compared with the pristine halloysite support. Furthermore, Pt@RHNTs catalyst shows rapid catalytic rates in hydrogenation reactions and excellent leaching resistance in cycle uses.

•Prepared m-BiVO4 with different morphologies using hydrothermal.•Morphologies of m-BiVO4 relied on pH value of solution.•The BiVO4 samples exhibit strong adsorption in visible range.•The peanut-like-shape BiVO4 shows an excellent catalytic activity.The monoclinic scheelite BiVO4 crystals with peanut-like, oval, twin-quadrangle and twin-four-pointed star morphologies were synthesized via a facile one step hydrothermal method by using sodium citrate as the chelating agent. The X-ray diffraction and scanning electron microscopy were employed to elucidate the structures and mophologies of the as-prepared BiVO4 samples. The results showed that the formation of m-BiVO4 with different morphologies relied on the pH value of the precursor solution. The band gaps values (Eg) of all the BiVO4 samples were around 2.37–2.45 eV according to the UV–vis diffuse reflectance spectrum, which indicated that samples could strongly absorb in the visible light region. The photocatalytic activities of BiVO4 crystals were evaluated by degradation of MB in aqueous solution under artificial solar-light. The BiVO4 samples obtained at different pH values showed different photocatalytic activities during the sunlight-driven photodegradation of methylene blue (MB). The sample with peanut-like-shape prepared at pH=1 exhibited the highest activity, and the photocatalytic conversion could reach above 90% after 3 h of irradiation. The result suggested that m-BiVO4 with peanut-like-shape could be used as an effective photocatalyst in practical application for organic pollutants degradation.

Inorganic nanostructures and their assemblies play important roles in immobilizing biomolecules. Herein, we developed a facile and green methodology to assemble natural halloysite nanotubes (1D building blocks) into nest-like porous microspheres (3D architecture). We further modified the microspheres with dopamine to form a biomimetic entity. The interconnected and hierarchical pores within the microspheres provide larger pore volume to entrap biomolecules, and the abundant functional groups on the pore surface bond covalently with enzyme to enhance the immobilization ability. The porous microspheres showed excellent loading capacity for laccase immobilization as high as 311.2 mg/g, around 30 times higher than the individual halloysite nanotubes (11.3 mg/g). The specific activity above 80% was retained for the immobilized laccase compared to the free laccase. In addition, the immobilized enzyme exhibited remarkable thermal and recycle use stability. The biomimetic microspheres are expected to be biologically safe and chemically stable microcapsules for immobilizing a variety of biomolecules because of their natural and biofriendly characteristics.Keywords: Dopamine; Enzyme immobilization; Halloysite; Natural nanotubes; Porous microspheres; Self-assembly

•Natural halloysite and chitosan was used as starting materials.•NaA zeolite/chitosan beads were prepared by in-situ hydrothermal synthesis.•NaA zeolite/chitosan hybrid beads showed a high adsorption capacity of ammonium.•NaA zeolite/chitosan hybrid beads were easy to regenerate and reuse.Inorganic/organic hybrid materials play important roles in removal of contaminants from wastewater. Herein, we used the natural materials of halloysite and chitosan to prepare a new adsorbent of NaA zeolite/chitosan porous hybrid beads by in-situ hydrothermal synthesis method. SEM indicated that the porous hybrid beads were composed of 6–8 μm sized cubic NaA zeolite particles congregated together with chitosan. The adsorption behavior of NH4+ from aqueous solution onto hybrid beads was investigated at different conditions. The Langmuir and Freundlich adsorption models were applied to describe the equilibrium isotherms. A maximum adsorption capacity of 47.62 mg/g at 298 K was achieved according to Langmuir model. The regenerated or reused experiments indicated that the adsorption capacity of the hybrid beads could maintain in 90% above after 10 successive adsorption–desorption cycles. The high adsorption and reusable ability implied potential application of the hybrid beads for removing NH4+ pollutants from wastewater.

Halloysite nanotubes (HNTs) have been proposed as a potential support to immobilize enzymes. Improving enzyme loading on HNTs is critical to their practical applications. Herein, we reported a simple method on the preparation of high-enzyme-loading support by modification with dopamine on the surface of HNTs. The modified HNTs were characterized by transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy analyses. The results showed that dopamine could self-polymerize to adhere to the surface of HNTs and form a thin active coating. While the prepared hybrid nanotubes were used to immobilize enzyme of laccase, they exhibited high loading ability of 168.8 mg/g support, which was greatly higher than that on the pristine HNTs (11.6 mg/g support). The immobilized laccase could retain more than 90% initial activity after 30 days of storage and the free laccase only 32%. The immobilized laccase could also maintain more than 90% initial activity after five repeated uses. In addition, the immobilized laccase exhibited a rapid degradation rate and high degradation efficiency for removal of phenol compounds. These advantages indicated that the new hybrid material can be used as a low-cost and effective support to immobilize enzymes.Keywords: dopamine; enzyme immobilization; halloysite nanotubes; laccase; modification;

Phase change materials (PCMs) have attracted extensively interests in solar storage. In the study, we prepared a new kind of composite PCM by impregnating paraffin (P) into halloysite nanotube. The as-prepared composite PCM was characterized by TEM, FT-IR and DSC analysis techniques. The composite can absorb paraffin as high as 65 wt.% and maintain its original shape perfectly without any paraffin leakage after subjected to 50 melt–freeze cycles. The melting temperature and latent heat of composite (P/HNT: 65/35 wt.%) were determined as 57.16 °C and 106.54 J/g by DSC. Graphite was added into the P/HNT composite to improve thermal storage performance, and the melting time and freezing time of the composite were reduced by 60.78% and 71.52% compared with the composite without graphite, respectively. Due to its high adsorption capacity, high heat storage capacity, good thermal stability and simple preparation method, the composite can be considered as cost-effective latent heat storage material for practical application.Highlights► The natural halloysite nanotube was used to prepare form-stable phase change material (PCM). ► The PCM maintains its original shape without any leakage after subjected to 50 melt–freeze cycles. ► The addition of graphite improved the thermal properties of the PCM. ► The PCM can be considered as cost-effective latent heat storage material for practical application.

Natural halloysite nanotubes (HNTs) were modified with a silane coupling agent, N-β-aminoethyl-γ-aminopropyl trimethoxysilane (KH-792), to form a new adsorbent for Cr(VI) removal. The as-prepared product was characterized by FTIR spectroscopy, TGA, TEM, and specific surface analysis. The results showed that KH-792 was successfully grafted onto the halloysite surface. Modified HNTs exhibited a rapid adsorption rate for Cr(VI) and approached 95% of the maximum adsorption capacity within 5 min. The effects of initial Cr(VI) concentration, temperature, pH, and ionic strength on the adsorption capacity were investigated in batch experiments. The results showed that low temperature was favorable to improve adsorption efficiency, and the adsorption capacity decreased significantly with the increase of pH and ionic strength. The optimum pH was found to be 3–5. The main adsorption mechanism was considered to be electrostatic interaction between protonated amino groups on the adsorbent surface and negatively charged Cr(VI). The results above confirmed that modified HNTs had the potential to be utilized as a low-cost and relatively effective adsorbent for Cr(VI) removal.

In this study, we use natural halloysite nanotubes as novel support materials to immobilize enzymes. Two typical industrial enzymes (α-amylase and urease) with different sizes were immobilized in channels of the nanotubes through simple physical adsorption. After 60 min heating, both immobilized enzymes retained more than 80% activity. Stored for 15 days, the immobilized enzymes still showed more than 90% activity. More than 55% initial activity of the enzyme was retained after 7 cycles. The immobilized enzymes exhibited thermal stability, good storage stability and reusability, which indicate that halloysite is a promising support material for enzyme immobilization.Download full-size imageResearch Highlights►Using natural halloysite nanotube as novel support materials for immobilization of enzymes. ►Two typical enzymes were immobilized on the nanotubes through simple physical adsorption. ►Immobilized enzymes retain a high fraction of their native activity. ►Low cost, the improved stability and reusability render halloysite potential support materials.

The magnetic composite of Fe3O4-halloysite nanotube (HNT) was prepared by chemical precipitation method. The prepared adsorbents were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), and multipoint Brunauer–Emmett–Teller (MBET). The results revealed that Fe3O4 particles with diameter of 3–5 nm dispersed on the nanotube surface and formed a composite with halloysite. The Fe3O4–HNTs composite exhibited fine magnetic property (Ms = 8.47 emu/g) and could be easily separated from aqueous solution by the application of an external magnetic field. Adsorption results showed that Fe3O4–HNTs composite could maintain a high adsorption capacity for methyl violet (MV) when the pH, concentration of metal ion and temperature varied. Adsorption kinetics was best described by the pseudo-second-order model. Equilibrium data fitted well with the Langmuir isotherm. The used Fe3O4–HNTs could be regenerated by simple calcinations. The recovered adsorbents could be used again for MV removal and magnetic separation. Because of the excellent adsorption capacity at different conditions, reproducibility and separability, Fe3O4–HNTs composite is a promising candidate for removing cationic dye from waste water.Download full-size imageHighlights► The magnetic composite of Fe3O4-halloysite was prepared by chemical precipitation method. ► The composite can be separated rapidly from solution by the application of external magnetic field. ► The composite shows excellent adsorption capacity and reproducibility for methyl violet dye.

The highly active and selective aerobic oxidation of aromatic alcohols over earth-abundant, inexpensive and recyclable catalysts is highly desirable. We fabricated herein MnO2/graphene oxide (GO) composites by a facile in-situ growth approach at room temperature and used them in selective aerobic oxidation of benzyl alcohol to benzaldehyde. TEM, XRD, FTIR, XPS and N2 adsorption/desorption analysis were employed to systematically investigate the morphology, particle size, structure and surface properties of the catalysts. The 96.8% benzyl alcohol conversion and 100% benzaldehyde selectivity over the MnO2/GO (10/100) catalyst with well dispersive ultrafine MnO2 nanoparticles (ca. 3 nm) can be obtained within 3 h under 383 K. Simultaneously, no appreciable loss of activity and selectivity occurred after recycling use up to six times. Due to their significant low cost, excellent catalytic performance, the MnO2/GO composites have huge application prospect in organic synthesis.Ultrafine MnO2 nanoparticles decorated on graphene oxide (GO) prepared by a facile in-situ growth method at room temperature showed excellent catalytic performance for aerobic oxidation of benzyl alcohol to benzaldehyde.