TOPICAL REVIEW: Nanoparticle catalysts

Nanocomposite catalysts (MWNT-TiO(2)) were prepared hydrothermally from multi-walled carbon nanotubes (MWNTs) and titanium sulfate as the titanium source, and then systematically analyzed using electron microscopy, Raman, FT-IR, and UV-vis spectroscopy. Pt-loaded nanocomposites, pristine TiO(2) and MWNTs were examined for their photocatalytic activity on splitting water with triethanolamine as an electron donor. Under visible light irradiation (lambda>420 nm), hydrogen was successfully produced over the Pt/MWNT-TiO(2), while no capacity to split water showed on the Pt-loaded pristine TiO(2) and MWNTs. Under full spectral irradiation of a Xe-lamp, a hydrogen generation rate of up to 8 mmol g(-1) h(-1) or more was achieved. The significant photocatalytic activity of the nanocomposites was attributed to the synergetic effect of the intrinsic properties of its components such as an excellent light absorption and charge separation on the interfaces between the modified MWNTs and TiO(2), resulting from direct growth of TiO(2) nanoparticles on the surface of the MWNTs during the hydrothermal process.

https://iopscience.iop.org/article/10.1088/0957-4484/20/12/125603/pdf

Photocatalytic hydrogen generation using a nanocomposite of multi-walled carbon nanotubes and TiO2 nanoparticles under visible light irradiation

Advanced carbon nanomaterials for state-of-the-art flexible supercapacitors

Selective oxidation of glycerol with molecular oxygen was studied over different functionalized MWNTs supported Pt catalysts in base-free aqueous solution. N2 adsorption, IR, Raman and SEM–EDS analysis confirmed that the tubular structure of MWNTs is stable during HNO3 and H2O2 pretreatment and the surface of MWNTs was successfully functionalized by HNO3, H2O2 and NH2–CH2–CH2–SH. XRD, SEM and TEM analysis demonstrated that unique sized Pt particles formed on S-pretreated MWNTs. Pt/S-MWNTs catalyst with small Pt particles was more active than Pt/MWNTs, Pt/HNO3-MWNTs and Pt/H2O2-MWNTs for glycerol oxidation in base-free aqueous solution. Free glyceric acid formed (with 68.3% selectivity and 90.4% conversion of glycerol) on Pt/S-MWNTs in base-free solution. While dihydroxyacetone formed firstly in an aqueous solution of NaOH/glycerol = 2:1 and the cleavage C–C bonds (catalyzed by alkali) arise severely.

Selective oxidation of glycerol with oxygen in a base-free aqueous solution over MWNTs supported Pt catalysts

Graphitic carbon nitride (g-C3N4) hybridized with a small number of multi-walled carbon nanotubes (CNT) was synthesized using cyanamide as precursor. The optimal CNT content is found to be ∼0.2 wt% in the composite, which displays a 2.4-fold enhancement in photocatalytic water splitting over pure g-C3N4. Characterizations by a series of joint techniques including Raman spectra, UV/vis diffuse reflectance spectra, steady and time-resolved fluorescence emission spectra, and photocurrent responses were carried out, aiming to reveal the determinative factor for the improved visible-light response. Our results indicate that the increased photoactivity originates from the enhanced charge-transfer effect due to the intimate interactions between g-C3N4 and conjugated CNT. The presence of CNT in the hybrids is beneficial for improving electron–hole separation on the excited g-C3N4 by prolonging the lifetimes of charge carriers and improving the population distribution of short-lived and long-lived charge carriers.

Origin of the enhanced visible-light photocatalytic activity of CNT modified g-C3N4 for H2 production

Intermetallic catalyst for carbon nanotubes (CNTs) growth by thermal chemical vapor deposition method

AlPO-5 molecular sieves were prepared by the hydrothermal reaction of a gel mixture with the following compositions: Al2O3 : P2O5 : Et3N : H2O = 1:1:1.5:x, where x is between 100 and 750 H2O molar ratio. The structure and morphology of the AlPO-5 molecular sieves depend on the gel mixture's composition, hydrothermal temperature, hydrothermal reaction time, and pH. Without pH control, the AlPO-5 structure changed from a spherical shape at H2O = 100 to a hexagonal pillar shape at H2O = 450. With pH control in the range of about 2.5-3.5, the hexagonal pillar crystals began to form at H2O = 100 and an island of hexagonal pillars with radiation form appeared at H2O = 300-450 due to the formation of a tridymite type of dense AlPO4 phase. It appears that the formation rate of hexagonal pillar crystals to form a dense AlPO4 phase is favorable under acidic conditions, and an amorphous AlPO-5 structure forms under basic conditions. Thus, the H2O concentration and pH value have a dramatic effect on the AlPO-5 structure.

Effect of synthesis pH and H2O molar ratio on the structure and morphology of aluminum phosphate (AlPO-5) molecular sieves

Surface nickel (NiO
 x
 ) species, surface NiAl
 x
 O
 y
 compound, and NiO crystallites are present on the Ni/Al2O3 catalysts, and the ratio of these nickel species is dependent on the nickel loading. Surface nickel interacts with the TiO2 support to form a surface nickel titanate compound (NiTiO
 x
 ) which has a lower reducibility. The weak interaction between the surface nickel and the silica support results in the formation of NiO crystallites on the SiO2 surface. The Ni/Al2O3 and Ni/TiO2 catalysts contain new surface Lewis acid sites and the amount of surface Lewis acid sites increases with increasing nickel concentration. The Ni/SiO2 catalysts have no sign of the presence of the surface Lewis acid sites. Only the Ni/Al2O3 catalysts have shown the ammonia adsorption at temperature of 200°C. Supported nickel on alumina catalysts possess the highest amination conversion, and the amine yield increases with increasing nickel loading up to ∼15% and starts to level off. By comparing amination catalysis with quantitatively TPR studies of the H2 consumed of the Ni/Al2O3 catalysts, it appears that the dispersed nickel species are the active sites for amination. In addition, the amination product is mainly the secondary amine due to the presence of water.

Amination of Polyethylene Glycol to Polyetheramine over the Supported Nickel Catalysts

Some nano-intermetallic alloys such as Mg–Ni were prepared by a chemical reduction method (named polyol process) using ethylene glycol, polyvinylpyrrolidone (PVP 10000) and PdCl 2 as reduction reagent, protect reagent, and nuclear reagent, respectively, to reduce the metal salts. The particle size of the synthetic Mg–Ni alloy was affected by the amount of the protect agent and nuclear agent and by the reduction temperature. All synthetic Mg–Ni alloys possess higher surface area about 113–315 m 2 /g and four kinds of components: Mg 2 Ni, Ni, Mg and MgO. The contents of Mg 2 Ni, Ni, Mg and MgO are affected by the Mg/Ni molar ratio. High molar ratio of Ni makes the single phase of Ni easy to reduce and results in the increase of Ni crystalline degree. The results of XRD and TEM and electron diffraction pattern indicate that PVP concentration only changes the particle size, but not the structure and composition of synthetic Mg–Ni alloys. The catalytic studies of PEG amination show that the total amine yield reaches to 58.4% over 1 g of Mg–Ni alloy with Mg:Ni=1:2 due to the formation of more single phase of Ni. The formation of Mg 2 Ni alloy and MgO prevent the aggregation of Ni, and possesses a higher amination activity.

Amination application over nano-Mg–Ni hydrogen storage alloy catalysts

The structure and morphology of both pure AlPO-5s and nickel containing AlPO-5s were affected by the preparation conditions such as pH value, nickel incorporated method (hydrothermal method or traditional incipient wetness impregnation). The dense tridymite phase of AlPO-5 can be formed by two controlling factors. One is controlling the pH at lower value to the AlO4 and PO4 moleculars to form AlPO4 with dense phase, and the other is used the excess nickel adding to increase the crystallinity of AlPO-5 to form dense tridymite phase. The reactivity of PEG-400 amination was also affected preparation methods and conditions. At higher nickel loading, the nickel supported on AlPO-5 prepared by traditional incipient wetness impregnation forms the surface nickel species, and both the surface nickel species and nickel incorporated species form by the hydrothermal method. The amination catalysis indicates that the reactivity of PEG-400 prepared by incipient wetness impregnation method was higher than that prepared by hydrothermal method. The formation of the dense tridymite phase of AlPO-5 can be further determined by the X-ray powder diffraction (XRD). The nickel containing AlPO-5s prepared with lower pH value possesses only the tridymite phase and exhibit higher catalytic reactivity. It appears that the trydimite structure can improve the catalytic activity for PEG-400 amination due to the grain size of the formed crystal of NiAlPO-5 is smaller than the catalyst without the formation of the trydimite phase.

Chia-Ming Chen

Papers

Intermetallic catalyst for carbon nanotubes (CNTs) growth by thermal chemical vapor deposition method

Effect of synthesis pH and H2O molar ratio on the structure and morphology of aluminum phosphate (AlPO-5) molecular sieves

Amination of Polyethylene Glycol to Polyetheramine over the Supported Nickel Catalysts

Amination application over nano-Mg–Ni hydrogen storage alloy catalysts

Structure control of metal aluminum phosphate (MeAlPO-5) molecular sieves and applications in polyethylene glycol amination