Preparation,characterization and photocatalytic applications of amine-functionalized mesoporous silica impregnated with transition-metal-monosubstituted polyoxometalates

Amine-functionalized mesoporous silica materials impregnated with transition-metal-monosubstituted polyoxometalates, K₅[M(H₂O)PW₁₁O₃₉]–(EtO)₃SiCH₂CH₂CH₂NH₂–MCM-48 (M = Co/Ni), were prepared by coordination of nickel/cobalt centers in the clusters with the amine surface groups in amine-functionalized mesoporous silica supports. The materials obtained were characterized by powder X-ray diffraction (XRD), UV–vis diffuse reflectance spectra (UV–vis-DR), infrared (IR) spectra, magic-angle spinning ³¹P MAS NMR, transmission electron microscopy (TEM) and nitrogen adsorption measurements, indicating that the primary Keggin structures remained intact in as-prepared composites, and the composites possessed mesoporous structures. The composites exhibited UV-photocatalytic activity to degrade dye rhodamine B (RB), and the pesticides including hexachlorobenzene (HCB) and methylparathion (MPT). Leakage of K₅[M(H₂O)PW₁₁O₃₉] from the support was hardly observed during the photocatalytic tests, attributed to strong coordination interactions between the Keggin units and the amine-functionalized silica surface.     

Esterification of levulinic acid into hexyl levulinate over dodecatungstophosphoric acid anchored to Al-MCM-41

Aluminum-substituted MCM-41 (Al-MCM-41) were hydrothermally synthesised and used as supports to fabricate the mesostructured H₃PW₁₂O₄₀ (HPW) solid acid catalysts via an impregnation method. The influences of various HPW loadings on the structures of the catalysts were verified by X-ray diffraction (XRD), nitrogen physisorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet–visible (UV–Vis) spectroscopy and Fourier-transform infrared spectroscopy (FT-IR) measurements and their acidities were evaluated by infrared of pyridine adsorption (Py-IR) and temperature-programmed desorption of ammonia (NH₃-TPD) measurements. The optimisation of the reaction conditions for levulinic acid esterification with n-hexanol such as the reaction temperature and time, the reactant molar ratio, and the catalyst dosages was performed to maximise the conversion of levulinic acid. The effects of various HPW loadings on the conversion of levulinic acid were investigated under the optimised and harsh reaction conditions. The reusability of the catalysts was also investigated. The results showed that these catalysts retained the hexagonal mesoporous structure of Al-MCM-41 and the Keggin characteristic of HPW, although their textural parameters decreased with increasing loading of HPW. In particular, the catalysts were found to be efficient in the esterification of levulinic acid with n-hexanol, resulting in hexyl levulinate which could replace the petroleum-derived chemical feedstocks.     

Effect of ZrO2-doping of nanosized Fe2O3/MgO system on its structural,surface and catalytic properties

Fe₂O₃/MgO system was prepared by wet impregnation method followed by treatment with different amounts of Zr-dopant salt then heating at 500 and 700 °C. The dopant concentrations were 0.48, 0.95 and 1.4 mol% ZrO₂. Pure and variously doped solids were characterized using XRD, N₂-adsorption isotherms carried out at ?196 °C and catalytic decomposition of H₂O₂ in aqueous solution at 25–35 °C. The results revealed that the nanosized MgO phase was only detected in the diffractograms of pure and doped solids calcined at 500 °C. Heating pure and doped solids at 700 °C produced nanosized MgFe₂O₄ phase together with MgO phase. Pure and ZrO₂-doped solids calcined at 500 and 700 °C are mesoporous adsorbents. The doping process brought about a measurable decrease in the S_BET of Fe₂O₃/MgO system with subsequent increase in its catalytic activity. The catalytic activity of the investigated system toward H₂O₂ decomposition, expressed as reaction rate constant per unit surface area was found to increase as a function of dopant concentration. The maximum increase in the reaction rate constant per unit surface area measured for the reaction carried out at 30 °C attained 125% for the heavily doped samples. This significant increase was based on the catalytic activity of pure catalyst sample measured under the same conditions.     

Synthesis and characterization of novel magnetically separable NiFe2O4@AlMCM-41-Cu2O core-shell and its performance in removal of dye

The synthesis of magnetic NiFe₂O₄@AlMCM-41-Cu₂O core-shell as a new class of visible light driven photocatalyst was suggested. The magnetic NiFe₂O₄ core was prepared by solvothermal method. The intermediate AlMCM-41 shell was prepared by the method of liquid crystal templating mechanism and subsequently cuprous oxide (Cu₂O) nanoparticles (NPs) were synthesized in NiFe₂O₄@AlMCM-41core-shell via colloidal chemistry approach. The properties of prepared magnetic core-shell were characterized by scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), nitrogen adsorption–desorption measurement and vibration sample magnetometer (VSM). Based on EDX results, the weight percentage (wt%) of NiFe₂O₄ core, MCM-41 shell and Cu₂O NPs were calculated to be 68.89, 30.55 and 0.56%, respectively. It consisted of mesoporous structure with a surface area of 687.00 m² g^?1, an average pore size of 2.95 nm and possessed excellent magnetic properties of 4.74 emu g^?1. The TEM results indicated that the NiFe₂O₄ as core were regular spheres with diameter of 68 nm, and the average thickness of AlMCM-41 shells was ～35 nm. The particles size of Cu₂O incorporated in core-shell was less than 5 nm. The photocatalytic activity was evaluated under visible light irradiation using the removal of methylene blue (MB) dye as a model reaction. The removal rate of MB achieved up to 90% after 60 min under visible light irradiation, and the NiFe₂O₄@AlMCM-41-Cu₂O can be recycled and reused.     

The novel,one step and facile synthesis of ZnO nanoparticles using heteropolyoxometalates and their photoluminescence behavior

A novel and facile wet chemical method is presented to synthesize zinc oxide nanoparticles (NPs) under ambient atmosphere and temperature. Keggin type heteropolyoxometalate (H₃[PW₁₂O₄₀]) was used as stabilizer and the effect of stirring time and amount of H₃[PW₁₂O₄₀] (HPW) were studied. XRD and TEM techniques were applied for the morphological and structural characterizations of NPs. Size of nanoparticles were determined using TEM, Scherrer’s formula as well as effective mass approximation. The results of these three methods are in good agreements and revealed single hexagonal zincite type crystalline with average particle size in the range of 3–15 nm. Photoluminescence behavior of the prepared sample shows a strong orange to red emission centred at about 620–635 nm, a green emission at around 550 nm and broad UV emission at around 400 nm.     

A novel potentiometric hydrogen peroxide sensor based on pKa changes of vinylphenylboronic acid membranes

A potentiometric hydrogen peroxide (H₂O₂) sensing scheme was developed using arylboronic acid as the electrode modifier. It is well-known that both aliphatic and aryl boronic acid undergo electrophilic displacement reaction with H₂O₂. This reaction involves replacement of boronic acid by the hydroxyl group of peroxide resulting in a change in pK_a value that can be exploited for sensing of H₂O₂. Vinylphenylboronic acid (VPBA) ink was prepared using Nafion as the binder and it was drop cast on an electrode surface. Morphology of the modified electrode was analysed using scanning electron microscopy (SEM). The present modifier exhibited a linear relationship between the difference in electrode potential (?Ep) vs. [H₂O₂] with a Nernstian slope of 26 ± 2 mV in the concentration range of 10^? 1–10^? 5 M. Application of the VPBA modified electrode for hydrogen peroxide sensing was studied in an industrial dye-bleach effluent.     

Nanotubules-supported Ru nanoparticles for preferential CO oxidation in H2-rich stream

CO present in H₂ provided as fuel for polymer electrolyte membrane fuel cells (PEMFC) can degrade the electrochemical performance and needs efficient removal, which can be accomplished by CO preferential oxidation (PROX). PROX catalytic activities in a H₂-rich stream were tested and compared using ruthenium (Ru) nanoparticles supported on two different nanotubularly structured materials, carbon nanotubes (CNTs) and halloysite nanotubes (HNTs). In both of the support materials the morphology remains unchanged after Ru deposition and reduction, demonstrating that their tubular structure is thermally-stable. The catalytic results show that the Ru/HNTs perform better than Ru/CNTs at temperatures below about 110 °C, owing to the easier reducibility of Ru particles over the former than the latter. However, Ru/HNTs can only reach a maximum CO conversion of 55%, with an O₂ selectivity of around 27% applying an O₂/CO mole ratio = 1 at 123 °C, which is insufficient for PROX applications. CNTs, on the other hand, provide larger surface area and have functional groups with stronger interaction with Ru nanoparticles, presenting a better Ru dispersion, which accounts for the superior catalytic activity at higher reaction temperature. Ru/CNTs catalyst exhibits a CO conversion over 90% and O₂ selectivity of around 50% applying the same O₂/CO mole ratio at temperatures above 120 °C.     

H<Subscript>3</Subscript>PW<Subscript>12</Subscript>O<Subscript>40</Subscript> supported on mesoporous MCM-41 and Al–MCM-41 materials: preparation and characterisation

MCM-41 and Al–MCM-41 has been synthesized using cetyl-trimethylammonium bromide (CTAB) surfactant as template and adding the silica precursor to aqueous solutions containing CTAB. The obtained solids were calcined at 600 °C for 4 h. HPW heteropolyacid supported on the mesoporous were prepared using the incipient wetness method. The characterization of materials was performed by X-ray diffraction, Transmission Electron Microscopy, N₂ adsorption, ²⁹Si Cross Polarization–Magic Angle Spinning and ²⁷Al MAS NMR. Results showed that the hexagonal structure is obtained in both cases. The Aluminium species are located inside an extra-framework. The impregnation reduces the surface area of the mesoporous materials especially of the Al–MCM-41 suggesting a participation of aluminium during the impregnation. HPW is well dispersed in the mesoporous materials and is located inside the pores interacting with the silanol group of the pores wall. ²⁷Al MAS NMR measurements have showed that the impregnation causes the removal of the non-framework aluminium.     

Controlled synthesis and catalytic properties of mesoporous nickel–silica core–shell microspheres with tunable chamber structures

In 550 °C and H₂ current, the mesoporous nickel–silica composite core–shell microspheres with tunable chamber structures have been successfully prepared by reduction of Ni₃Si₂O₅(OH)₄ microspheres, which are synthesized by the reaction between Ni(Ac)₂·4H₂O and SiO₂ microspheres via a self-template approach. The chamber (SiO₂ core sizes) and shell thickness (40–150 nm) of the nickel–silica microspheres can be controlled by adjusting the synthetic parameters of Ni₃Si₂O₅(OH)₄, such as the reaction time. After reduction, these microspheres still have the same sizes, morphologies, and core–shell structures with porous shell as before. These mesoporous nickel–silica microspheres with large BET surface area, exhibit good catalytic activity in m-dinitrobenzene (m-DNB) and high selectivity of m-phenylenediamine (m-PDA) after 3.5 h, but different selectivity of m-PDA in the progress, showing good potential in the catalyst industry.     

Recovery of high surface area mesoporous silica from waste hexafluorosilicic acid (H2SiF6) of fertilizer industry

In this article we report recovery of mesoporous silica from the waste material (hexafluorosilicic acid) of phosphate fertilizer industry. The process involves the reaction of hexafluorosilicic acid (50 ml, 24 wt% H₂SiF₆) and 100 ml, 0.297 M Na₂CO₃ to generate the alkaline aqueous slurry. Silica was separated from the slurry by filtration and the sodium fluoride was extracted from the aqueous solution by evaporation method. The obtained mesoporous silica was characterized by N₂ absorption/desorption (BET), thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscope (SEM), and EDS. The results confirm that the separation of silica and NaF was successful and the final products have high purity. The silica product was found to have an average pore diameter of 4.14 nm and a high surface area (up to 800 m²/g). The process reported in this study may significantly reduce the release of hazardous materials into the environment and it might confer economic benefits to the responsible industries.     

Metal-organic framework derived hexagonal layered cobalt oxides with {1 1 2} facets and rich oxygen vacancies: High efficiency catalysts for total oxidation of propane

A cobalt-based metal–organic framework was used as a precursor to synthesize Co₃O₄ catalysts exhibiting a hexagonal layered morphology by calcination at varying temperatures. Various characterization techniques, such as XRD, SEM, Raman, H₂-TPR, O₂-TPD and N₂ adsorption–desorption, were used to study the effects of calcination temperature on the grain size, surface area, and pore volume of the catalysts. The Co₃O₄ catalyst obtained by calcination at 350 °C (Co₃O₄-350) exhibited the highest catalytic activity for the total oxidation of propane. Furthermore, the small grain size and layered structure of Co₃O₄-350 allowed it to possess a high specific surface area, a highly exposed {1 1 2} facets, and abundant oxygen defects that facilitated a favorable low-temperature reducibility and oxygen mobility, thereby improving catalytic activity. This research offers a simple strategy for synthesis of Co₃O₄ with layered structure, highly exposed {1 1 2} facets and rich oxygen defects.     

Stable controlled growth of 3D CuO/Cu nanoflowers by surfactant-free method for non-enzymatic hydrogen peroxide detection

Sensitive, convenient and rapid detection of hydrogen peroxide(H_2 O_2) is highly desirable in fields like fundamental biological research, food industries, and clinical environmental analysis. Herein, a hierarchical porous CuO/Cu flower-like active electrode material for non-enzymatic H_2 O_2 sensor was synthesized via a low-cost and one-step chemical oxidation of Cu powder in water bath without surfactants. In order to discuss the growth mechanism of the product, products with different growth time length were fabricated. The electro-catalysis of all products were first exhibited by cyclic-voltammetry,and the product under 6 h reaction shows the best result. The detailed electro-catalytic behaviors of the best product(under 6 h reaction) are characterized by cyclic-voltammetry and amperometry under alkaline conditions. The materials have high sensitivity of 103 μA mM~(-1) cm~(-2)(R~2= 0.9979), low detection limit of 2 μmol/L and wide concentration range(from 2 μmol/L to 19.4 mmol/L). Large specific surface area and stabled nanostructure enabled good features, such as stability and sensitivity for the H_2 O_2 determination.     

Synthesis of Ti-containing MCM41 using Ti(SO4)2 as Ti source

A simple and economic method was developed to synthesize Ti-containing MCM41 from titanium sulphate as Ti source, sodium metasilicate as Si source and cetyltrimethylammonium bromide (CTABr) as template. The resultant materials were characterized by using XRD, FT–IR, N₂ adsorption isotherms, HRTEM, ICP, and LRS techniques. It was revealed that the atoms of titanium were incorporated into the framework of MCM41 for the samples with Ti / Si mol ratios below 1 / 20. Small amount of titanium oxides that highly dispersed onto the wall of MCM41 were detected while the Ti / Si mol ratios up to 1 / 10. The prepared materials exhibited high catalytic activity in the oxidation of aromatic compounds.     

Low-temperature synthesis of mesoporous nanocrystalline magnesium aluminate (MgAl2O4) spinel with high surface area using a novel modified sol-gel method

A simple, template-free and scalable modified sol-gel route was developed for the synthesis of mesoporous flake-like magnesium aluminate spinel (MgAl₂O₄) at low temperature (700 °C) with high surface area (281 m² g^?1). The obtained spinel materials were characterized by means of physicochemical techniques including X-ray diffraction, thermogravimetric analysis, scanning electron microscopy with energy-dispersive X-ray spectroscopy, transmission electron microscopy, Fourier transform infrared spectroscopy and N₂ adsorption-desorption analysis. The propylene oxide was used as gelation and pore forming agent in the sol-gel process. Different morphologies and sizes of flake were generated by the varied synthesis conditions. The result materials reveal that the textural properties of the MgAl₂O₄ product are strongly associated with the nature and amount of addictive solvent and calcination temperatures. It shows that the BET surface area decrease as the increase of calcination temperature and the optimal temperature of 700 °C result in the pure phase of MgAl₂O₄ spinel. This synthesis strategy offers a feasible approach for scalable fabrication of mixed metal oxides for various catalytic reactions or catalyst supports due to the large surface area.     

Hierarchical hollow microsphere and flower-like indium oxide: Controllable synthesis and application as H2S cataluminescence sensing materials

In the present work, In₂O₃ hierarchical hollow microsphere and flower-like microstructure were achieved controllably by a hydrothermal process in the sodium dodecyl sulfate (SDS)-N,N-dimethyl-formamide (DMF) system. XRD, SEM, HRTEM and N₂ adsorption measurements were used to characterize the as-prepared indium oxide materials and the possible mechanism for the microstructures formation was briefly discussed. The cataluminescence gas sensor based on the as-prepared In₂O₃ was utilized to detect H₂S concentrations in flowing air. Comparative gas sensing results revealed that the sensor based on hierarchical hollow microsphere exhibited much higher sensing sensitivity in detecting H₂S gas than the sensor based on flower-like microstructure. The present gas sensor had a fast response time of 5 s and a recovery time of less than 25 s, furthermore, the cataluminescence intensity vs. H₂S concentration was linear in range of 2–20 μg mL^?1 with a detection limit of 0.5 μg mL^?1. The present highly sensitive, fast-responding, and low-cost In₂O₃-based gas sensor for H₂S would have many practical applications.     

Thermal stability of Si–MCM-41 in gaseous atmosphere

The thermal stability of Si–MCM-41 in different atmosphere (air, O₂, NH₃, N₂, and Ar) has been investigated in the present work; as-synthesized Si–MCM-41 was heat-treated at 800–1030 °C for 6–12 h in the selected atmosphere. Based on absorption–desorption isotherms and low-angle XRD measurement of the treated samples, it was found that the thermal stability varied greatly in different atmosphere. As-synthesized Si–MCM-41 retained mesoporous structure up to 1010 °C in NH₃, N₂, and Ar environment, but in air and O₂ environment, the highest thermal stable temperature of mesoporous structure in Si–MCM-41 was no more than 900 °C.     

Blending effect of sucrose and surfactant templating agents on silica mesostructure

We report the blending effect of surfactant and sucrose as a nonsurfactant templating agent on the silica mesostructure. The CTAB/sucrose-templated mesoporous silica (SCS) was compared with CTAB-templated MCM-41. The MCM-41 showed spherical morphology with a particle diameter of 1.1–1.5 μm, and gave a bimodal size distribution, centered at 2.1 nm and 8.9 nm, which is assigned to hexagonally-arrayed cylindrical pores and interparticle-pores between small MCM-41 clusters, respectively. SCS gave unique and extraordinary morphology in which two different mesostructures have grown with both of them facing each other. The ordered MCM-41 pore structure clung to silica nanosphere-framed wormlike mesostructure, resulting in a bimodal pore size distribution centered at 2.1 nm and 7.0 nm. It was revealed that both of CTAB and sucrose act independently as a surfactant and a nonsurfactant template.     

Synthesis and characterization of V,Mo and Nb incorporated micro–mesoporous MCM-41 materials

Highly microporous metal-MCM-41 ordered mesoporous structure catalysts having different metal/Si (V, Mo, Nb) atomic ratios and combinations of metal sources were hydrothermally synthesized. The structural properties estimated using different techniques were found to be in agreement with each other. Metals were successfully incorporated into MCM-41 without deteriorating the ordered hexagonal structure. The metal ions in the synthesis solutions probably settled on the hydrophilic end of the template hence the metal incorporation resulted improvements in the micropore structure. Low loading of metals caused an increase in the surface area and pore volume values of the catalysts. The highest total (1310 m² g^?1) and micropore surface area values (1083 m² g^?1) were obtained by Nb incorporation. The micro- and mesopore dimensions of MCM-41 increased from 0.5 to 1.1 nm and from 2.5 to 2.8 nm, respectively, with metal incorporation. Low V/Si ratios and presence of Nb in the starting solution enhanced narrow mesopore size distribution. The pore dimension and wall thickness values estimated from nitrogen adsorption and X-ray diffraction methods were consistent with the corresponding values obtained using transmission electron microscopy.     

Synthesis and catalytic performance of antimony trioxide nanoparticles by ultrasonic-assisted solid-liquid reaction ball milling

In this study, the cubic antimony trioxide (Sb₂O₃) nanoparticles were successfully synthesized by ultrasonic-assisted solid-liquid reaction ball milling technique. And the synthetic process, using Sb powder as raw material, was conducted with an atmosphere of acetic acid aqueous solution at low temperature (?100 °C). Some controlled trials, including without the assistance of ultrasonic wave, different reaction solutions and diverse ultrasonic frequencies, were performed. The products were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that the Sb₂O₃ nanoparticles with uniform and ultrafine particle size were obtained within short reaction time under the coaction of both ultrasonic wave and ball milling. Furthermore, the catalytic capacity of the Sb₂O₃ nanoparticles for decomposition of H₂O₂ was measured during the oxidative decomposition of methylene blue (MB). The test exhibited rapid and efficient color removal in the degradation of MB.     

A study on in situ growth of TaC whiskers in Al2O3 matrix powder for ceramic cutting tools

In situ growth of tantalum carbide (TaC) whiskers was synthesized in an α-Al₂O₃ matrix powder via a carbothermal reduction technique within a temperature range of 1350–1500 °C in an argon atmosphere. The starting materials consisted of Ta₂O₅, C, Ni and NaCl powders. Different mixing methods and various reaction temperatures were employed. Most of the prepared whiskers were 0.2–0.5 μm in diameter and 5–15 μm in length. The reaction temperature of 1400–1450 °C was suitable for the growth of TaC whiskers and a wet mixing method was beneficial to increase the whisker yield. Some of the whiskers exhibited the needle shape while others exhibited the screw shape. The growth mechanism of the whiskers was a complex mechanism involving a helical screw dislocation mechanism and a vapor–liquid–solid process. No obvious influences of the Al₂O₃ matrix powder on the growth of TaC whiskers were found and the major impurities in the obtained powder were TaC particles, nickel and unreacted carbon.