Shape and Size Controlled Pt Nanocrystals as Novel Model Catalysts

A tremendous development of nanotechnology enabled us to prepare precisely structure-controlled nanocrystals (NCs). Recently, such structure controlled NCs have been applied as model catalysts with controlled active sites to evaluate reaction mechanisms. Here, we review our recent works on preparation of shape controlled Pt NCs, such as cube and wire, and on application as model catalysts for olefin hydrogenation to assess active Pt sites. A hydrogen reduction method of Pt(II) ion was adopted in an aqueous phase to prepare Pt cube in the presence of shape forming agent (NaI) and organic protective agent (sodium polyacrylate or sodium succinate). We succeed to prepare Pt cube with high size and shape selectivity, followed by very small Pt cube of less than 3 nm. Such simultaneous control of both size and shape of Pt NCs has been successfully achieved by the sophisticated tuning of multiple conditions for the growth kinetics of Pt nuclei during hydrogen reduction of PtCl₄ ²⁻ in aq. N-, N-dimethylformamide (DMF) solution. The key strategy is to produce small Pt nuclei and to avoid the excessive growth of Pt nuclei, in conjunction with face selective adsorption of anionic protective and shape-forming agents by the control of solvent system. For the preparation of Pt nanowires with high anisotropy, template-assisted methods have been usually used. We have developed facile liquid phase preparation method at room temperature in air without any template. The key factors of the method are: firstly, to use aqueous organic solvent system to control solvent polarity; secondly, to use co-solvent DMF as shape forming agent; thirdly, to use excess NaBH₄ as a stabilizer of Pt nuclei to prevent formation of conventional particles. Single-crystalline Pt nanowires of 2 nm diameter and more than micron length were easily obtained by the reduction of Pt(IV) with excess NaBH₄ in water–DMF–toluene solution (1:8:5 volume ratio) in a short reaction period of 3 h. Then, we have applied shape-controlled Pt NCs protected with PAA prepared by our original methods, such as cube, tetrahedron and wire, as model catalysts for olefin (cis- and trans-stilbenes) hydrogenation in ethanol to evaluate the active facet of Pt catalysts; e.g., Pt(111) or Pt(100). The estimated TOF values for the hydrogenation of cis- and trans-stilbene decreased in the order: cube > cuboctahedron > tetrahedron ~ nanowire. This tendency indicates that Pt(100) show high activity compared with Pt(111). The result is compatible with measurement of XPS and Raman spectra, suggesting strong adsorption of reactant on Pt(100). Hydrogenation of olefins (1-hexene and cyclohexene) has been carried out over Pt cubes with different sizes (8.2–10.1 nm) as catalysts to get information about active sites of Pt catalysts, e.g., flat facet or edge/corner atoms. The results of similar TOF values among Pt cubes with different sizes imply that the active sites are flat facets.     

PEGylated polyethyleneimine grafted silica nanoparticles: enhanced cellular uptake and efficient siRNA delivery

The present paper reports the utilization of hybrid nanocomposite particles consisting of PEI25k-PEG5k copolymer grafted silica nanoparticles (SiO₂NPs) for enhanced cellular uptake and siRNA delivery. High-resolution transmission electron microscopy and dynamic light scattering measurements ensured the average particle size of the final hybrid component as 45 nm (core SiO₂, 28–30 nm and shell PEI25k-PEG5k, 12–15 nm). Surface morphology from atomic force microscopy analysis showed the significant relationship between the particle size and shape. ²⁹Si and ¹³C cross-polarization–magic angle spinning solid state nuclear magnetic resonance (NMR), ¹H-NMR, and Fourier transform infrared spectroscopy were used to obtain the relevant structural information (such as Q3, silanol; Q4, siloxane functional groups of SiO₂NPs; resonance shifts and bending vibrations of PEI25k, –CH₂–CH₂–NH–; and PEG5k, –CH₂–CH₂–O–) from copolymer nanoparticle. Stable complexation of siRNA and nanocomposite particle (wt.%:wt.%) was achieved from 1:5 to 1:15 ratio. Nanocomposite particle (N/P) ratio and siRNA concentration determine the stability and knockdown efficiency of the PEI25k-PEG5k-graft-SiO₂NPs–siRNA complexes. It was shown that highly positively charged (zeta potential, +66 mV) PEI25k-PEG5k-graft-SiO₂NPs result in strong affinity with negatively charged siRNA. Confocal microscopy showed intensified cellular uptake of siRNA into cytoplasm of A549 cancer cell utilized for in vitro study. In conclusion, the coherence, graft density of copolymer-SiO₂NPs, and siRNA concentration were found to strongly influence the stability, and hence determine the knockdown efficiency, of PEI25k-PEG5k-graft-SiO₂NPs–siRNA complexes.     

Nano-architecture platinum catalyst layer prepared by electrophoresis deposition for PEM fuel cells

A Pt-loaded carbon black electrode was prepared by pulsed electrophoresis deposition in a Pt colloid solution as a plating bath to overcome the growth problem of a Pt catalyst during deposition in an electrochemical process. This method is a promising technique for preparing Pt catalyst layers at the polymer electrolyte/electrode interface. The particle size of the Pt catalyst loaded by electrophoresis deposition was the same as that of Pt nanoparticles (3–4 nm) in a colloid and the particle size was maintained even during deposition. The loading of the Pt catalyst was controlled by the pH of the Pt colloid and deposition time. The Pt nanoparticles were deposited on a carbon black electrode to a depth of 2.5 μm.     

Platinum nanoparticle size effect on specific catalytic activity in <Emphasis Type="Italic">n</Emphasis>-alkane deep oxidation: Dependence on the chain length of the paraffin

The specific activity of 0.8% Pt/Al₂O₃ catalysts in the deep oxidation of C₁–C₆ n-alkanes increases with an increase in the Pt particle size from 1 to 3–4 nm. Further coarsening of the particles insignificantly changes the specific activity. The size effect was studied for a series of catalysts containing platinum nanoparticles 1 to 11 nm in diameter. The specific catalytic activity variation range depends on the size of the reacting hydrocarbon molecules. As the platinum particle size increases, the specific catalytic activity increases 3–4 times for the oxidation of CH₄ and C₂H₆ and by a factor of 20–30 for the oxidation of n-C₄H₁₀ and n-C₆H₁₄.     

Tuning the High‐Temperature Wetting Behavior of Metals toward Ultrafine Nanoparticles

The interaction between metal nanoparticles (NPs) and their substrate plays a critical role in determining the particle morphology, distribution, and properties. The pronounced impact of a thin oxide coating on the dispersion of metal NPs on a carbon substrate is presented. Al₂O₃‐supported Pt NPs are compared to the direct synthesis of Pt NPs on bare carbon surfaces. Pt NPs with an average size of about 2 nm and a size distribution ranging between 0.5 nm and 4.0 nm are synthesized on the Al₂O₃ coated carbon nanofiber, a significant improvement compared to those directly synthesized on a bare carbon surface. First‐principles modeling verifies the stronger adsorption of Pt clusters on Al₂O₃ than on carbon, which attributes the formation of ultrafine Pt NPs. This strategy paves the way towards the rational design of NPs with enhanced dispersion and controlled particle size, which are promising in energy storage and electrocatalysis.     

General Strategy for Synthesis of Ordered Pt3M Intermetallics with Ultrasmall Particle Size

Controllable synthesis of atomically ordered intermetallic nanoparticles (NPs) is crucial to obtain superior electrocatalytic performance for fuel cell reactions, but still remains arduous. Herein, we demonstrate a novel and general hydrogel‐freeze drying strategy for the synthesis of reduced graphene oxide (rGO) supported Pt₃M (M=Mn, Cr, Fe, Co, etc.) intermetallic NPs (Pt₃M/rGO‐HF) with ultrasmall particle size (about 3 nm) and dramatic monodispersity. The formation of hydrogel prevents the aggregation of graphene oxide and significantly promotes their excellent dispersion, while a freeze‐drying can retain the hydrogel derived three‐dimensionally (3D) porous structure and immobilize the metal precursors with defined atomic ratio on GO support during solvent sublimation, which is not afforded by traditional oven drying. The subsequent annealing process produces rGO supported ultrasmall ordered Pt₃M intermetallic NPs (≈3 nm) due to confinement effect of 3D porous structure. Such Pt₃M intermetallic NPs exhibit the smallest particle size among the reported ordered Pt‐based intermetallic catalysts. A detailed study of the synthesis of ordered intermetallic Pt₃Mn/rGO catalyst is provided as an example of a generally applicable method. This study provides an economical and scalable route for the controlled synthesis of Pt‐based intermetallic catalysts, which can pave a way for the commercialization of fuel cell technologies.     

Synthesis of copper nanoparticles stabilized by polyoxyethylenesorbitan monooleate

We studied the influence of synthesis parameters and the composition of the reaction mixture on the size and morphology of copper nanoparticles (NPs). Use of a surfactant (polyoxyethylenesorbitan monooleate) is promising for confining NP growth and stabilizing NPs. Concentration ranges of existence were determined for copper NP dispersions stable to aggregation and sedimentation. Scanning electron microscopy and dynamic light scattering were used to show that: the NP size varied from 10 to 65 nm, the average diameter was 25–35 nm, and the shape was spherical. The sizes of copper NP aggregates were determined.     

Size sorting of Au and Pt nanoparticles from arbitrary particle size distributions

A K₂BSPP (dipotassium bis(p-sulfonatophenyl)phenylphosphane dihydrate)-based method to sort and size refine Au and Pt nanoparticles has been developed. It makes use of K₂BSPP to impart graduated stability to the nanoparticles in a number of NaCl solutions. The method offers a systematic approach to preparing metal nanoparticles of small diameters and a narrow size distribution from an arbitrary particle size distribution. TEM investigations confirmed the size refinement efficacy of this treatment method: in a typical experiment, the mean particle size and relative standard deviation of Au nanoparticles after three successive treatments were 5.18 and 0.055 nm, respectively, down from the corresponding values of 8.22 and 0.26 nm from the initial untreated nanoparticle solution. The K₂BSPP-based size refinement procedure is a simple alternative to more complex chemical procedures and stringent process control that are currently required for the preparation of mono-dispersed systems.     

Catalysts of ethanol anodic oxidation for ethanol-air fuel cell with a proton-conducting polymer electrolyte

Synthesis techniques for binary PtSn, PdM (M = Sn, V, Mo) and ternary PtSnNi, PtRuSn catalysts of ethanol electrooxidation on highly dispersed carbon materials are suggested. The highest activity in the 0.5 M H₂SO₄ solution containing 1 M C₂H₅OH corresponds to the system of PtSn (3: 1, 40 wt % Pt) with the particle size of 2–4 nm and tin content in the alloy with platinum of about 6%. It was shown that the catalyst efficiency as regards ethanol oxidation depth decreases in the series of Pt > PtRu ≈ PtSn, and the catalyst activity by current forms the series of PtSn > PtRu > Pt. The membrane-electrode assembly (MEA) with the anodes on the basis of the PtSn (3: 1, 40 wt % Pt) catalyst had stable characteristics for 220 h at the current density of ∼50 mA/cm².     

Chemical synthesis of tungsten-copper nanocomposite powder

In this study homogeneous powders of CuWO₄ and WO₃ was produced from ammonium para-tungstate (APT) and copper nitrate. Then, the product was used to prepare nano-sized W-Cu powder. Hence, a mixture of ammonium paratungstate and copper nitrate with predetermined weight proportion was made in distilled water, while the content of the beaker was being stirred at a certain speed to reach the desired composition of W-20 wt % Cu. Mixture was heated to 80–100°C for 6 h. Also, pH range was adjusted at about 3–4. The mixture was then evaporated and dried in the air. To reach W-Cu composite powder, the precursor powders burned out at 520°C for 2 h in the air to form W-Cu oxide powder and then were ball milled and reduced in H₂ atmosphere to convert it into W-Cu composite powder. The resulting powders were evaluated using scanning electron microscopy, X-ray diffraction, thermogravimetric analysis and differential thermal analysis techniques. The results showed that homogeneous powders of W-Cu with particle size of around 100 nm and a nearly spherical shape could be obtained by this process. Each particle include smaller parts with size of around 20–30 nm.     

DFT Studies of Palladium Model Catalysts: Structure and Size Effects

An important task for theory is the multi-scale modeling of catalytic properties of nanocrystallites with sizes ranging from clusters of few metal atoms to particles consisting of 10³–10⁴ atoms. To explore catalytic properties of nanosized metal catalysts, we developed an approach based on three-dimensional symmetric model clusters of 1–2 nm (~100 metal atoms) with fcc structure, terminated by low-index surfaces. With this modeling technique one is able to describe at an accurate DFT level various catalytic and adsorption properties of metal nanoparticles in quantitative agreement with experimental studies of model catalysts deposited on thin oxide films. Metal nanocrystallites exhibit properties that can significantly vary with their size and shape.     

Size effect of silica nanoparticles on thermal decomposition of PMMA

The size effect of silica nanoparticles (SiO₂) on thermal decomposition of poly(methylmethacrylate) (PMMA) was investigated by the controlled rate thermogravimetry. Thermal degradation temperature of PMMA–SiO₂ composites depended on both fraction and size of SiO₂, the thermal degradation temperature of 23 nm (diameter) SiO₂–PMMA (6.1 wt%) was 13.5 °C higher than that of PMMA. The thermal stabilities of 17 nm SiO₂–PMMA (3.2 wt%) and 13 nm SiO₂–PMMA (4.8 wt%) were 21 and 23 °C, respectively, higher than that of PMMA without SiO₂. The degree of degradation improvement was increased linearly with the surface area of SiO₂. The number of surface hydroxyl group in unit volume of SiO₂ particle increased with increasing the specific surface area of SiO₂, and the interaction between hydroxide group of SiO₂ and carbonyl group of PMMA had an important role to improve the thermal stability of PMMA.     

Size and composition control of Pt-In nanoparticles prepared by seed-mediated growth using bimetallic seeds

A two-step method has been developed for precise size and composition control of bimetallic Pt-In nanoparticles. Very small (1.62 nm) PtIn seed nanoparticles with 1:1 metal ratio were prepared in the absence of capping agents followed by growth of Pt on their surface in the presence of oleyl amine as reducing and stabilizing agent. Nanoparticles with bulk compositions of Pt(4)In, Pt(3)In, and Pt(2)In could be synthesized with average diameter smaller than 3 nm. TEM, EDX, and XPS provided evidence for homogeneous growth without separate nucleation of pure platinum nanoparticles in the reaction solution. Pt(3)In nanoparticles were deposited onto SiO(2) surface by incipient wetness impregnation. Temperature-induced changes in the particle surface were monitored by in situ IR spectroscopy and CO adsorption. It was found that surface alloy composition of the particles could be tuned by using oxidizing or reducing atmospheres.     

Bimetallic Cluster Provides a Higher Activity Electrocatalyst for Methanol Oxidation

The cluster complex Pt₂Ru₄(CO)₁₈ was used as a precursor to prepare a 60 wt% 1:2 Pt:Ru nanoparticles on carbon (PtRu/C) for use as an electrocatalyst for methanol oxidation. This bimetallic carbonyl cluster complex was found to provide smaller, more uniform bimetallic nanoparticle that exhibited higher electrocatalytic activity than a 60 wt% 1:1 Pt:Ru commercial catalyst from E-Tek. Using bimetallic cluster precursors simplifies the synthetic procedures by reducing the need for high temperature reduction and assures a more intimate mixing of the two different metals. Transmission electron microscopy (TEM) images of the catalyst obtained from the cluster precursor showed bimetallic nanoparticles having a narrow size range of 2–3 nm that were dispersed uniformly over the surface of the support. Images of the commercial catalyst showed particles 3–4 nm in diameter that tended to agglomerate near the edges of the carbon support particles. Cyclic voltammograms of methanol oxidation from the two catalysts showed significantly higher activity for the cluster-derived catalyst. The onset potential for methanol oxidation for the cluster-derived catalyst was approximately 170 mV lower than that of the commercial catalyst at 100 A/g Pt, and approximately 250 mV lower at 400 A/g Pt. ^* This report is dedicated to Prof. Günter Schmid on the occasion of his 70th birthday.     

Anticancer therapeutic efficacy of biogenic Am-ZnO nanoparticles on 2D and 3D tumor models

The present study fabricates biogenic zinc oxide nanoparticles (ZnO NPs) with the aqueous leaf extract of Annona muricata (Am) plant collected from semi-evergreen forests of Odisha, India. The synthesized Am-ZnO NPs were physicochemically characterized. The ultraviolet/visible spectrum showed the maximum optical absorbance of Am-ZnO NPs at 355 nm. High-resolution transmission electron microscopy analysis presented the nearly spherical shape of Am-ZnO NPs with an average particle size of 80 nm. The net surface charge and hydrodynamic size of Am-ZnO NPs were measured to be ～?2.59 mV and ～417 nm, respectively. Am-ZnO NPs were found to be biocompatible and hemocompatible nature. Furthermore, Am-ZnO NPs displayed strong anticancer effects on both 2D and 3D tumor models. We observed a dose-dependent toxicity on both A549 and MOLT4 cells and observed a size reduction in the A549 tumor spheroids. Subsequently, we observed a depolarization in mitochondrial membrane potential of Am-ZnO NP–treated cancer cells leading to the apoptosis induction in cancer cells.     

Sorption-luminescence determination of gold,silver, and platinum with the use of silica gel chemically modified with <Emphasis Type="Italic">N</Emphasis>-(1,3,4-thiodiazole-2-thiol)-<Emphasis Type="Italic">N</Emphasis>′-propylurea groups

Silica gel chemically modified with N-(1,3,4-thiodiazole-2-thiol)-N′-propylurea extracted gold(III) from solutions in the range of 6 M HCl-pH 8 and silver(I) from nitric acid solutions in the range of 6 M HNO₃-pH 8 and 1–2 M HCl at 20°C with 99.0–99.9% recovery and a sorption equilibration time of 5 min. Platinum(II) was quantitatively extracted at room temperature from solutions in the range of 4 M HCl-pH 8; the sorption equilibration time was 20 min. For the quantitative extraction of platinum(IV), it should be reduced to platinum(II). The intense yellowish orange luminescence (λ_max (Au) = 575 nm, λ_max (Ag) = 550 nm, and λ_max(Pt) = 620 nm) of surface complexes at 77 K under UV irradiation was used in the development of procedures for the low-temperature sorption-luminescence determination of gold, silver, and platinum. The detection limits were 0.15 (Au), 0.1 (Ag), and 0.05 μg (Pt) per 0.1 g sorbent. The calibration function was linear to 50 (Au, Ag) or 80 μg (Pt) per 0.1 g sorbent. The relative standard deviation in the determination of more than 5 μg of a metal was no higher than 6%. The sorption-luminescence determination procedures were tested in the determination of gold in gold-containing concentrates and their processing products and platinum in alumina-platinum catalysts.     

Synthesis of ZnS hollow nanospheres with holes using different amine templates

ZnS hollow nanospheres with holes were prepared by reacting ZnSO₄ with H₂S, the sulfide source formed in the reaction of CS₂ with ethylenediamine, 1,3-propylenediamine, butylamine or 2-(2-aminoethylamino) ethanol, which also acted as a template agent, at 50°C under agitation. The shape, particle size of about 100–850 nm and hole size of about 150–600 nm of ZnS hollow nanospheres with holes were shown by SEM and TEM images. These ZnS nanospheres with β cubic ZnS phase and composed of 2–5 nm nanocrystals were characterized by XRD and HRTEM. The blue shift of maximum absorption in UV-vis displayed the effect of quantum size. The two amino groups of amine templates reacted favorably with Zn²⁺ to form uniform and relatively smooth ZnS nanospheres with holes, while hydroxyethyl played a disadvantageous role. A reasonable mechanism of hole formation by H₂S rushing out is suggested. __________ Translated from Journal of Jinan University (Natural Science), 2007, 28(1): 92–95 [译自: 暨南大学学报(自然科学版)]     

Size and structure matter: enhanced CO2 photoreduction efficiency by size-resolved ultrafine Pt nanoparticles on TiO2 single crystals

A facile development of highly efficient Pt-TiO(2) nanostructured films via versatile gas-phase deposition methods is described. The films have a unique one-dimensional (1D) structure of TiO(2) single crystals coated with ultrafine Pt nanoparticles (NPs, 0.5-2 nm) and exhibit extremely high CO(2) photoreduction efficiency with selective formation of methane (the maximum CH(4) yield of 1361 μmol/g-cat/h). The fast electron-transfer rate in TiO(2) single crystals and the efficient electron-hole separation by the Pt NPs were the main reasons attributable for the enhancement, where the size of the Pt NPs and the unique 1D structure of TiO(2) single crystals played an important role.     

Chemoselective Transfer Hydrogenation of Cinnamaldehyde over Activated Charcoal Supported Pt/Fe3O4 Catalyst

A variety of spherical and structured activated charcoal supported Pt/Fe₃O₄ composites with an average particle size of ～100 nm have been synthesized by a self-assembly method using the difference of reduction potential between Pt (IV) and Fe (Ⅱ) precursors as driving force. The formed Fe₃O₄ nanoparticles (NPs) effectively prevent the aggregation of Pt nanocrystallites and promote the dispersion of Pt NPs on the surface of catalyst, which will be favorable for the exposure of Pt active sites for high-efficient adsorption and contact of substrate and hydrogen donor. The electron-enrichment state of Pt NPs donated by Fe₃O₄ nanocrystallites is corroborated by XPS measurement, which is responsible for promoting and activating the terminal C=O bond of adsorbed substrate via a vertical configuration. The experimental results show that the activated charcoal supported Pt/Fe₃O₄ catalyst exhibits 94.8% selectivity towards cinnamyl alcohol by the transfer hydrogenation of cinnamaldehyde with Pt loading of 2.46% under the optimum conditions of 120℃ for 6 h, and 2-propanol as a hydrogen donor. Additionally, the present study demonstrates that a high-efficient and recyclable catalyst can be rapidly separated from the mixture due to its natural magnetism upon the application of magnetic field.