Synthesis of highly dispersed Pt/C electrocatalysts in ethylene glycol using acetate stabilizer for methanol electrooxidation

Pt/C electrocatalysts were prepared from a solution of H₂PtCl₆ in ethylene glycol in the presence of XC-72 carbon by adding a small amount of sodium acetate as stabilizer. Repeated TEM images showed that the platinum nanoparticles were small and uniform in size and highly dispersed on XC-72 carbon supports when a small amount of sodium acetate solution was added to the synthesis solution. The Pt/C electrocatalysts exhibited very high electrocatalytic activity for liquid methanol oxidation. The effects of adding acetate on Pt particle size and size distribution are discussed. It is demonstrated that acetate can be used as a good stabilizer for preparing Pt/C catalyst with fine and uniform Pt particles.     

Formation of carbon nanotubes through ethylene decomposition over supported Pt catalysts and silica-coated Pt catalysts

Ethylene decomposition was performed over supported Pt catalysts to fabricate composites of Pt metal nanoparticles and carbon nanotubes (CNTs). All supported Pt catalysts (Pt/carbon black, Pt/CNT, Pt/MgO, Pt/Al₂O₃ and Pt/SiO₂) showed catalytic activity for ethylene decomposition at 973 K to form CNTs. Pt metal particles were found at tips of CNTs. These results indicate that Pt metal particles have catalytic activity for growth of CNTs through hydrocarbon decomposition. A broad range (5-50 nm) of CNT diameters were formed from the use of supported Pt metal catalysts although Pt metal particles in the catalysts before ethylene decomposition were relatively uniform in size (2-5 nm). These results imply that Pt metal particles in the catalysts aggregated during ethylene decomposition at 973 K. Aggregation of Pt metal particles in catalysts during ethylene decomposition could be suppressed by covering catalysts with silica layers that were a few nanometers thick. Silica-coated Pt catalysts showed high activity for ethylene decomposition to form CNTs with uniform diameters (8-10 nm) despite the uniform coverage of Pt metal particles with silica layers.     

Highly dispersed Pt nanoparticles by pentagon defects introduced in bamboo-shaped carbon nanotube support and their enhanced catalytic activity on methanol oxidation

Bamboo-shaped carbon nanotubes (BCNTs), which were synthesized through chemical vapor deposition by using cresol as the carbon source, were explored as Pt catalyst support in comparison with conventional carbon nanotubes (CNTs) and Vulcan XC carbon blacks. The pyrolysis of cresol produced a large amount of pentagon defects introduced in the walls of BCNTs, which could possess higher chemical activity and stronger interaction with metal particles. After a mild purification, the BCNTs exhibited more oxygen-containing functional groups than CNTs, as shown by Fourier transform infrared spectra and cyclic voltammetry. The formed oxygen-containing functional groups as well as the pentagon defects could act as uniform active sites for metal particle loading. By ethylene glycol reduction, highly dispersed Pt nanoparticles with a narrow size distribution of 2-3 nm were easily supported on BCNTs, as shown by transmission electron microscope. The Pt/BCNT catalyst showed higher electro-catalytic activity on the methanol oxidation than the Pt/CNT and Pt/Vulcan XC catalyst, which could be largely ascribed to the highly dispersed Pt nanoparticles due to the introduced pentagon defects in the tube-walls (comparing with Pt/CNT) and the graphitic nanotube network that could provide good electron conduction (comparing with Pt/Vulcan XC).     

Particle Size Control and Dependence on Precursor pH: Synthesis Uniform Submicrometer Zinc Aluminate Particles

Zinc aluminate (ZnAl₂O₄) particles have been synthesized by the hydrothermal method using NH₃·H₂O as a pH adjustment mineralizer. Experimental results showed that ZnAl₂O₄ particle size was dependent on the precursor pH, and could be controlled through pH adjustment. It was 5.5, 11.5, and 27 nm when the precursor pH was 8.2, 9.3, and 10.5, respectively. On the other hand, the particle size distribution changed broader with increase in pH. These differences were attributable to the different NH₃·H₂O function. NH₃·H₂O was mainly used as a base at lower pH (<9.0), while its complex function predominated at higher one (>9.5). From thermodynamic viewpoint, the rate-limiting steps were dissolution of Al(OH)₃ and γ-AlO(OH) to Al(OH)₄⁻ at lower and higher pH, respectively. The newly formed γ-AlO(OH) with high reactivity was the critical factor in the synthesis of bimodal particles. Higher temperature treatment of γ-AlO(OH) could decrease the reactivity, and could be used as an aluminum source for synthesis uniform ZnAl₂O₄ particles.     

Influence of catalytic particle size on the performance of fluidized-bed chemical vapor deposition synthesis of carbon nanotubes

In this paper, the impacts of catalytic particle size on the overall reactor performance for carbon nanotubes (CNTs) production using a fluidized-bed chemical vapor deposition (FBCVD) process have been studied. Six different particle size fractions (10-20 μm, 20-53 μm, 53-75 μm, 75-100 μm, 100-200 μm, and 200-300 μm) were selected. It was observed that the smaller the catalytic particle diameter, the greater the carbon deposition efficiency and the greater CNT synthesis selectivity. The 10-20 μm catalytic particles exhibited 30% higher carbon deposition efficiency than the 200-300 μm catalytic particles. The selectivity toward CNTs formation was also approximately 100%. These observations could be explained by the fact that when the diameter of the catalytic particle gets smaller, the breakthrough capacities during frontal diffusion will be bigger due to a shorter diffusion path length within the particle. Moreover, the fine particles ensured high interstitial velocity which subsequently enhances the heat and mass transfer, and consequently improves the CVD reaction.     

The effect of the Pt deposition method and the support on Pt dispersion on carbon nanotubes

Carbon nanotube supported platinum (Pt/CNTs) catalysts prepared by different Pt deposition methods and on different CNT supports were studied. Colloidal based methods were demonstrated to be more effective than other wet chemistry deposition methods (e.g., impregnation and precipitation) for the preparation of highly dispersed Pt/CNTs. Pt catalyst supported on CNTs with a dispersion uniformity comparable to that supported on carbon powder was achieved using a zwitterionic surfactant 3-(N,N-dimethyldodecylammonio) propanesulfonate (SB12) as stabilizer in a monitored pH environment. It was experimentally observed that oxygen-containing surface functionalities on CNTs can greatly affect the catalyst particle dispersion by manipulating Pt anchoring and/or nucleating sites. Furthermore, it was revealed that the performance of Pt/CNTs based fuel cell is strongly dependent on the electrode fabrication method.     

Hydrothermal synthesis of size-dependent Pt in Pt/MWCNTs nanocomposites for methanol electro-oxidation

A hydrothermal method has been developed to prepare size-controlled Pt nanoparticles dispersed highly on multiwalled carbon nanotubes (Pt/MWCNTs). It was found that the size of Pt nanoparticles was strongly dependent on the solution pH in synthesis. The Pt nanoparticles with mean size of 3.0, 4.2 and 9.1 nm were obtained at pHs 13, 12 and 10 separately. After Pt/MWCNTs composites were fabricated, the different properties of cyclic voltammetry and chronoamperometry in electro-oxidation of methanol were found. The results showed that the smaller diameter Pt deposited Pt/MWCNTs nanocomposites exhibited higher electrocatalytic activity for methanol oxidation. By characterization of X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), size-dependent activities were identified.     

合成4，6-二氨基间苯二酚用钯炭催化剂的制备及失活

用KCl预处理载体,采用均匀设计考察了浸渍过程中浸渍液体积、pH、浸渍温度和浸渍时间对催化合成4,6-二氨基间苯二酚的影响,结果表明,KCl浓度0.5 mol·L^-1时,催化剂上钯粒径最小;pH是浸渍过程影响活性的主要因素,催化剂优化后,产物收率较高。堵孔和钯价态的升高是失活的主要因素,通过洗涤和还原可部分恢复催化剂活性。     

Production of Carbon Nanotube by Ethylene Decomposition over Silica-Coated Metal Catalysts

Formation of carbon nanofibers (CNFs) and carbon nanotubes (CNTs) through the decomposition of ethylene at 973 K was achieved using various metal catalysts covered with silica layers. CNFs of various diameters were formed by ethylene decomposition over a Co metal catalyst supported on the outer surface of the silica. In contrast, silica-coated Co catalysts formed CNTs with uniform diameters by ethylene decomposition. Silica-coated Ni/SiO₂ and Pt/carbon black also formed CNTs with uniform diameters, while CNFs and CNTs with various diameters were formed over Ni/SiO₂ and Pt/carbon black without a silica coating. These results indicate that silica layers that envelop metal particles prevent sintering of the metal particles during ethylene decomposition. This results in the preferential formation of CNTs with a uniform diameter.     

High Methanol Oxidation Activity of Well-Dispersed Pt Nanoparticles on Carbon Nanotubes Using Nitrogen Doping

Pt nanoparticles (NPs) with the average size of 3.14 nm well dispersed on N-doped carbon nanotubes (CNTs) without any pretreatment have been demonstrated. Structural properties show the characteristic N bonding within CNTs, which provide the good support for uniform distribution of Pt NPs. In electrochemical characteristics, N-doped CNTs covered with Pt NPs show superior current density due to the fact that the so-called N incorporation could give rise to the formation of preferential sites within CNTs accompanied by the low interfacial energy for immobilizing Pt NPs. Therefore, the substantially enhanced methanol oxidation activity performed by N-incorporation technique is highly promising in energy-generation applications.     

载5-氟尿嘧啶的pH敏感自组装纳米粒制备及体外释放

In order to improve the cancer-targeting and selective activity of antineoplastic agent [5-fluorouracil （5-FU）], a novel pH-responsive drug delivery system [pullulan acetate/sulfonamide （PA/SDM） conjugate] was synthesized by a diafiltration method. Sulfonamide was grafted to the hydrophobicaUy modified pullulan acetate to enhance the pH sensitivity for better cancer-targeting delivery. 5-FU was loaded into the self-assembled nanoparticles by the same method. The drug-loaded self-assembled nanoparticles were successfully obtained and characterized in terms of particle size, morphology and drug loading and release profile at various pHs. The results showed that the mean diameter of the self-assembled particles was approximately 100nm, with uniform size and good spherical morphology. The nanoparticles showed good stability at pH 7.4, which is equal to that of the normal body fluid, but shrank and aggregated below pH 6.8, which is close to the pH with tumors. The loading efficiency and concentration of released 5-FU was monitored at 269 nm on the UVNis spectrophotometer. The release profile was heavily pH-dependent around phvsiological pH, and the release rate was significantly enhanced under pH of 6.8.     

Electrochemical deposition of Pt nanoparticles on CNTs for fuel cell electrode

In order to increase the performance of fuel cell electrode, carbon nanotubes (CNTs) were used as support instead of conventional carbon black, and the Pt catalyst was synthesized by using electrochemical deposition (ECD) method which has recently been adopted as a synthetic tool of metal nanoparticles. CNTs used in this paper were grown directly on carbon paper by chemical vapor deposition (CVD) of acetylene. Highly dispersed and nano-sized Pt particles were electrochemically deposited on CNTs surface, which would simplify the manufacturing process of membrane-electrode-assembly (MEA). Pt particles on CNTs were investigated by SEM and TEM. The particle size of Pt is less than 2 nm, which is relatively small compared to that of conventional wet impregnated catalyst (2–8 nm). CO chemisorption results show that the amounts of catalytic sites are about three times larger in Pt/CNT prepared by ECD than those in conventional wet-impregnated one. The mass activity of the former catalyst for oxygen reduction is more than three times higher compared to that of the latter one. This paper was presented at the 11th Korea-Japan Symposium on Catatysis held at Seoul, Korea, May 21–24, 2007.     

不同聚苯乙烯微球的磺化

胶体颗粒是研究凝聚态物理性质、生命物质组装的理想模型体系,对胶体颗粒的研究具有十分重要的理论价值。聚苯乙烯(PS)微球是目前应用最为广泛的胶体粒子,由于其在合成过程中粒径可控且形貌均匀,因此,受到了越来越多的关注。文章制备了未交联的和交联的2种PS微球,研究了磺化时间对这2种PS微球的Zeta电势和粒径的影响。结果表明,在一定的反应时间内,未交联PS微球的Zeta电势和粒径,随磺化时间增加至原粒径的2.5倍,最后溶解。交联PS微球的Zeta电势和粒径随磺化时间增加至原粒径的5.5倍,但不溶解,而是形成C型胶体粒子。掌握了聚苯乙烯微球的磺化规律,可以得到尺寸及带电量不同的磺化聚苯乙烯,对凝聚态的物理性质的等研究具有重要意义。     

The effect of pH and ionic strength on the dispersion of carbon nanotubes in poly(acrylic acid) solutions

The dispersion of three kinds of acid‐treated carbon nanotubes (CNTs) in poly(acrylic acid) (PAA) aqueous solution of different pH and ionic strengths (varied by NaCl, KCl and ZnCl₂) was investigated by visual observation, zeta potential, particle size analysis, transmission electron microscopy and scanning electron microscopy. Visual observation revealed that the dispersion of CNTs acid treated at 60 °C for 3 h and at 80 °C for 2 h was poor in aqueous solutions with pH < 2 or pH > 12. The poor dispersion of acid‐treated CNTs may be improved by adding PAA. In particular, PAA improved the dispersion of CNTs with greater COOH content. In a low pH solution (pH 1.5), a higher PAA content resulted in poorer CNT dispersion while in a high pH solution (pH 12.5), a higher PAA content led to better CNT dispersion. For superior dispersion in a basic aqueous solution (pH 12.5), experimental data showed that a greater atomic radius or a higher cationic charge of the added salt may result in faster aggregation and thus precipitation of CNTs. Copyright © 2011 Society of Chemical Industry     

On the influence of Pt particle size on the PEMFC cathode performance

Colloidal suspensions of almost spherical and crystalline Pt nanoparticles between 1.6 and 2.6 nm in diameter and with narrow size distribution were synthesized using the phase transfer method (PTM) with alkylamines, C_nNH₂, as stabilizing agents. Batches of such homogenous Pt-C_nNH₂ (n = 8, 12) nanocrystals were deposited onto Vulcan XC-72 carbon powder, and the activity for the oxygen reduction reaction (ORR) of this series of Pt/C materials was evaluated under PEMFC conditions. The aim was to elucidate whether this type of stabilized Pt nanoparticles were as active for the ORR as a corresponding commercial Pt/C material, and if any difference in mass activity could be observed between catalysts with different Pt particle size. In the PEMFC experiments, i.e. voltammetry in oxygen and nitrogen, it was found that, after an initial electrode activation, the ORR activity of the catalysts prepared from the alkylamine-stabilized Pt nanoparticles deposited on carbon was as high as that of the employed commercial reference catalyst. In fact, all samples in the Pt/C series showed high and very similar ORR activity normalized to Pt-loading, without significant dependence on the initial Pt particle size. However, pre- and post-electrochemical characterization of the Pt/C material series with TEM showed that structural changes of the Pt nanoparticles occurred during electrochemical evaluation. In all samples studied the mean Pt particle size increased during the electrochemical evaluation resulting in decreased differences between the samples explaining the observed similar ORR performance of the different materials. These results emphasize the necessity of post-operation characterization of fuel cell catalysts when discussing electrocatalytic activity. In addition, employing complex preparation efforts for lowering the Pt particle size below 3 nm may have limited practical value unless the particles are stabilized from electrochemical sintering.     

Controlled platinum nanoparticles uniformly dispersed on nitrogen-doped carbon nanotubes for methanol oxidation

This work presents the synthesis of platinum nanoparticles (Pt NPs) and their subsequent deposition on the nitrogen-doped carbon nanotubes, which have been directly grown on a carbon cloth (CNT-CC electrode). The CNT-CC electrode provides a fast electron-transfer path to the carbon cloth, resulting in energy-loss reduction and enhancing catalytic activity of Pt NPs. The N-dopants in CNT serve as the defect sites to enhance nucleation of Pt particles. The reduction of the Pt precursor salt was carried out in the ethylene glycol solution at an elevated temperature. In order to control the Pt NP size, the pH of the reaction solution was controlled by the addition of NaOH. Zeta potential measurements of the as-prepared sample indicate that a higher zeta potential results in a smaller particle size, due to a stronger electrostatic repulsion between NPs. This serves a powerful tool for size control of the Pt nanoparticle. The Pt NPs dispersed on the CNT-CC have an average size of 2.81 nm (Pt/CNT-CC) prepared using 15 mM NaOH, with high uniformity under electron microscopy. Cyclic voltammetry measurements of the electrocatalytic activity of the Pt/CNT-CC for methanol oxidation indicate that it exhibits excellent electrocatalytic activity and are ideal for direct methanol fuel cell applications.     

酸化絮凝包覆法制备硫调节型粉末氯丁橡胶

采用酸化絮凝包覆法制备硫调节型粉末氯丁橡胶(PCR),采用均匀试验设计的方法研究了胶乳酸化后pH值、NaCl用量、羧酸钠用量和包覆剂用量对PCR粒子的析出时间、粒径分布及其硫化胶物理机械性能的影响,建立了酸化絮凝包覆法制备PCRS工艺的粉末化模型,发现DH为8．4时,可以获得最为均匀的粒径介于1～3mm之间的产物粒子,并通过逐步回归建立了制备工艺与产物最终性能之间的关系模型。SEM分析表明,PCR粒子具有多孔结构,这种结构为PCR粒子的快速干燥提供了良好的条件。能谱分析表明,酸化絮凝法制备的PCR表面的Ca含量较粒子内部要高,说明粒子的表面钙化是形成包覆和隔离的主要原因。     

Influence of support porosity and Pt content of Pt/carbon aerogel catalysts on metal dispersion and formation of self-assembled Pt-carbon hybrid nanostructures

Three carbon aerogels with different meso-macropore networks were used as supports for Pt catalysts using [Pt(NH₃)₄]Cl₂ as precursor salt. Results obtained showed mesopore volume and mean mesopore size to be important parameters that control Pt particle size and dispersion in catalysts containing 2 wt.% Pt. Once the most appropriate porosity to obtain the highest dispersion was determined, the metal content was increased to 20 wt.% Pt. However, the mean Pt particle size only increased from 1 to 2 nm, indicating the importance of an appropriate mesoporosity for obtaining a high dispersion at high metal loading. Mean Pt particle size was always slightly smaller by transmission electron microscopy than by H₂ chemisorption, because some Pt particles were not reduced during pre-treatment, as confirmed by X-ray photoelectron spectroscopy. Finally, transmission electron microscopy observations of catalysts with metal loading of 8-20 wt.% before pre-treatment showed the formation of self-assembled Pt-carbon hybrid nanorods and nanowires. To the best of our knowledge, this is the first observation of this phenomenon in Pt/C catalysts.     

调酸介质对二甲戊灵微胶囊囊形及粒径的影响

[目的]优选出原位聚合法制备二甲戊灵微胶囊的较佳调酸介质,为二甲戊灵的微胶囊化提供一定的理论指导.[方法]以脲醛树脂为壁材采用原位聚合法制备了二甲戊灵微胶囊,研究了不同调酸介质对二甲戊灵微胶囊囊形及粒径的影响.[结果]5％盐酸溶液作为调酸介质形成的微胶囊囊形粗糙,产生粘连,粒径分布较宽;5％硝酸铵溶液、5％硫酸氢铵溶液作为调酸介质形成的微胶囊囊形松散,容易破裂,产生粘连,粒径分布较窄;5％氯化铵溶液作为调酸介质形的微胶囊囊形光滑致密且坚固,粒径分布较窄.[结论]调酸介质对原位聚合法制备的微胶囊囊形及粒径影响很大,选择合适的调酸介质能获得囊形致密、粒径分布窄的微胶囊.     

Effect of Fouling Conditions and Cake Layer Structure on the Ultrasonic Cleaning of Ceramic Membranes

Abstract

Homogeneous alumina membranes fouled by polystyrene latex particles at different pH values and ionic strengths were subjected to ultrasonic cleaning. Cleaning was more effective at high and low pH than at neutral pH. At low pH values, less repulsive particle‐particle interactions resulted in the removal of millimeter‐scale aggregates and highly effective cleaning. At near‐neutral pH, stronger repulsive particle‐particle interactions caused detachment to occur as individual particles from the cake layer rather than as flocs, which was a slightly less effective cleaning mechanism. Ultrasonic cleaning of cake layers formed at high ionic strength (>0.3 M KCl) was less effective than cleaning at lower ionic strength (<0.3 M KCl). High ionic strength caused particles to coagulate in solution and deposit as flocs on the membrane surface forming a highly permeable fouling layer. This fouling layer was resistant to ultrasound at the sub‐optimal cleaning conditions used in this study, perhaps due to particle attachment occurring within a primary energy minimum. Membrane cleaning experiments performed with particles of varying size showed that particle size was less important than the surface potential of the particles. For a given mass, particles that possessed the largest surface potential formed the thickest fouling layer, irrespective of particle size, and showed the greatest improvement in flux with ultrasonic cleaning. These results demonstrate that solution conditions influence ultrasonic cleaning of membranes primarily by modifying particle‐particle and particle‐membrane interactions as well as cake layer structure, rather than by impacting the extent or magnitude of cavitation events.