Hydrothermal growth of mesoporous SBA-15 silica in the presence of PVP-stabilized Pt nanoparticles: synthesis, characterization, and catalytic properties

A novel high surface area heterogeneous catalyst based on solution phase colloidal nanoparticle chemistry has been developed. Monodisperse platinum nanoparticles of 1.7-7.1 nm have been synthesized by alcohol reduction methods and incorporated into mesoporous SBA-15 silica during hydrothermal synthesis. Characterization of the Pt/SBA-15 catalysts suggests that Pt particles are located within the surfactant micelles during silica formation leading to their dispersion throughout the silica structure. After removal of the templating polymer from the nanoparticle surface, Pt particle sizes were determined from monolayer gas adsorption measurements. Infrared studies of CO adsorption revealed that CO exclusively adsorbs to atop sites and red-shifts as the particle size decreases suggesting surface roughness increases with decreasing particle size. Ethylene hydrogenation rates were invariant with particle size and consistent with a clean Pt surface. Ethane hydrogenolysis displayed significant structure sensitivity over the size range of 1-7 nm, while the apparent activation energy increased linearly up to a Pt particle size of approximately 4 nm and then remained constant. The observed rate dependence with particle size is attributed to a higher reactivity of coordinatively unsaturated surface atoms in small particles compared to low-index surface atoms prevalent in large particles. The most reactive of these unsaturated surface atoms are responsible for ethane decomposition to surface carbon. The ability to design catalytic structures with tunable properties by rational synthetic methods is a major advance in the field of catalyst synthesis and for the development of accurate structure-function relationships in heterogeneous reaction kinetics.     

Pt/glassy carbon model catalysts prepared from PS-b-P2VP micellar templates

Poly(styrene)-block-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymer was used as a micellar template to fabricate arrays of Pt nanoparticles on mica and glassy carbon (GC) supports. Polymer micellar deposition yields Pt nanoparticles with tunable particle size and surface number density on both mica and GC. After deposition of precursor-loaded micelles onto GC, oxygen plasma etching removes the polymer shell, followed by thermal treatment with H2 gas to reduce the Pt. Etching conditions were optimized to maximize removal of the polymer while minimizing damage to the GC. Arrays of Pt nanoparticles with controlled size and surface number density can be prepared on mica (for particle size characterization) and GC to make Pt/GC model catalysts. These model catalysts were characterized by tapping mode atomic force microscopy, X-ray photoelectron spectroscopy, and cyclic voltammetry to measure activity for oxidation of carbon monoxide or methanol. Cyclic voltammetry results demonstrate the existence of a correlation between Pt particle size and electrocatalytic properties including onset potential, tolerance of carbonaceous adsorbates, and intrinsic activity (based on active Pt area from CO stripping voltammetry). Results obtained with Pt/GC model catalysts duplicate prior results obtained with Pt/porous carbon catalysts therefore validating the synthesis approach and offering a new, tunable platform to study catalyst structure and other effects such as aging on proton exchange membrane fuel cell (PEMFC) reactions.     

不同介质对Pt/GC电极制备及其性能的影响

应用循环伏安法(CV),扫描电子显微镜(SEM)和电化学原位红外反射光谱(in situFTIRS)研究了不同介质对碳载铂纳米薄膜电极(Pt/GC)的表面结构以及该薄膜电极对甲酸电催化氧化性能的影响.结果表明,使用不同介质的镀铂溶液,均可电沉积出分布较为均匀的Pt粒子,但其尺寸与形貌却相差很大.当以H2SO4作介质,由循环伏安法于玻碳电极上电沉积Pt得到的(Pt/GC1)电极,其Pt粒子粒径约100~200 nm;而在HClO4介质得到的(Pt/GC2)电极,则含有两种Pt微晶:其一是立方体形,粒径约200 nm,其二为菜花状,粒径约400 nm.电化学循环伏安和原位红外反射光谱测试指明,不同介质制备的Pt/GC电极对甲酸的电催化氧化均表现出与本体铂电极(Pt)相类似的特性,即可通过活性中间体或毒性中间体将甲酸氧化至CO2,但不同结构的Pt/GC电极具有不同的电催化活性.进一步以Sb或Pb修饰Pt/GC电极,不仅可以有效地抑制毒性中间体CO的生成,而且还能显著提高其电催化活性.比较本文研究的7种电极,其电催化活性顺序依次为:Sb-Pt/GC2>Pb-Pt/GC2>Pb-Pt/GC1>Sb-Pt/GC1>Pt/GC2>Pt/GC1>Pt.     

Synthesis of Pt/Ru bimetallic nanoparticles in high-temperature and high-pressure fluids

A high-temperature and high-pressure flow-reactor system was applied to the synthesis of monometallic ruthenium (Ru) nanoparticles and platinum/ruthenium (Pt/Ru) bimetallic nanoparticles using the thermal reduction of ruthenium ion (Ru(III)) and the mixture of platinum (Pt(IV)) and ruthenium ions in water and ethanol mixture in the presence of poly(N-vinyl-2-pyrrolidone). Monometallic Ru nanoparticles with an average diameter of ca. 2 nm were synthesized above 200 degrees C at 30 MPa. The monometallic Ru nanoparticles tended to make large aggregates in colloidal dispersions. By the reduction of the mixture solution of Pt(IV) and Ru(III) in water and ethanol above 200 degrees C at 30 MPa, Pt/Ru bimetallic nanoparticles with an average diameter of ca. 2.5 nm were synthesized with relatively small size distribution. The EXAFS spectra for the Pt/Ru bimetallic particles indicated that the particle possesses metallic bonds between Pt and Ru atoms in contrast to the case of the nanoparticles produced by thermal reduction under ambient pressure at 100 degrees C [M. Harada, N. Toshima, K. Yoshida, S. Isoda, J. Colloid Interface Sci. 283 (2005) 64], and that the Pt/Ru bimetallic particle has a Pt-core/Ru-shell structure.     

Adsorption and co-adsorption of ethylene and carbon monoxide on silica-supported monodisperse Pt nanoparticles: volumetric adsorption and infrared spectroscopy studies

The adsorption of carbon monoxide and ethylene, and their sequential adsorption, was studied over a series of Pt/SBA-15 catalysts with monodisperse particle sizes ranging from 1.7 to 7.1 nm by diffuse-reflectance infrared spectroscopy and chemisorption. Gas adsorption was dependent on the Pt particle size, temperature, and sequence of gas exposure. Adsorption of CO at room temperature on Pt/SBA-15 gives rise to a spectroscopic feature assigned to the C-O stretch: nu(CO) = 2075 cm-1 (1.9 nm); 2079 cm-1 (2.9 nm); 2082 cm-1 (3.6 nm); and 2090 cm-1 (7.1 nm). The intensity of the signal decreased in a sigmoidal fashion with increasing temperature, thereby providing semiquantitative surface coverage information. Adsorption of ethylene on Pt/SBA-15 gave rise to spectroscopic features at approximately 1340, approximately 1420, and approximately 1500 cm-1 assigned to ethylidyne, di-sigma-bonded ethylene, and pi-bonded ethylene, respectively. The ratio of these surface species is highly dependent on the Pt particle size. At room temperature, Pt particles stabilize ethylidyne as well as di-sigma- and pi-bonded ethylene; however, ethylidyne predominated on the surfaces of larger particles. Ethylidyne was the only identifiable species at 403 K, with its formation being more facile on larger particles. Co-adsorption experiments reveal that the composition of the surface layer is dependent on the order of exposure to gases. Exposure of a C2H4-covered Pt surface to CO resulted in an approximately 50% decrease in chemisorbed CO compared to a fresh Pt surface. The nu(CO) appeared at 2050 cm-1 on Pt/SBA-15 pretreated with C2H4 at room temperature. The di-sigma-bonded and pi-bonded species are the most susceptible to displacement from the surface by CO. The formation of ethylidyne appeared to be less sensitive to the presence of adsorbed carbon monoxide, especially on larger particles. Upon exposure of C2H4 to a CO-covered Pt surface, little irreversible uptake occurred due to nearly 100% site blocking. These results demonstrate that carbon monoxide competes directly with ethylene for surface sites, which will have direct implications on the poisoning of the heterogeneously catalyzed conversion of hydrocarbons.     

电化学方法制备高分散Pt/CMK-3及其对甲醇电化学催化性能

采用直接电化学还原法在介孔碳(CMK-3)载体上直接电沉积高分散的铂纳米颗粒,制备CMK-3复合铂纳米颗粒电极(Pt/CMK-3)。 通过透射电子显微镜分析发现,铂纳米颗粒非常均匀的分布在CMK-3上,平均粒径约5 nm。 通过循环伏安测试,分析了催化剂不同负载铂含量时氯铂酸的利用率,在理论铂质量分数为20%时,这种方法制备的Pt/CMK-3所使用的氯铂酸的利用率最高,在1 mol/L CH3OH+0.5 mol/L H2SO4溶液中循环伏安测试电流密度达到382 A/g。 在相同实验条件下,Pt/CMK-3电极对甲醇电催化活性远高于Pt/XC-72(炭黑)电极和用常规电沉积方法制备的Pt/CMK-3电极。     

多壁碳纳米管负载铂立方体的制备及对乙醇电催化氧化性能

利用线性扫描电沉积的方法在玻碳电极或多壁碳纳米管表面制备出铂纳米立方体, 扫描电子显微镜(SEM)和透射电子显微镜(TEM)结果表明, 铂立方体的尺度约为38 nm, 由Pt(111)择优取向的小粒子围成. 运用电化学循环伏安和电位阶跃技术研究了所合成的2种催化剂和商用碳载铂对乙醇氧化的电催化活性, 发现在2种铂纳米立方体上乙醇氧化的电催化活性和稳定性均高于商用碳载铂, 其起峰电位较商业碳载铂降低168 mV. 采用电化学原位红外光谱对比研究了铂纳米立方体和商用碳载铂对乙醇氧化的电催化过程, 发现铂纳米立方体起始氧化电位提前, 催化活性增强. 乙醇在该催化剂上更易转化为乙酸, 且表现出较强的CO吸附能力.     

Electrocatalytic performance of fuel oxidation by Pt3Ti nanoparticles

A Pt-based electrocatalyst for direct fuel cells, Pt3Ti, has been prepared in the form of nanoparticles. Pt(1,5-cyclooctadiene)Cl2 and Ti(tetrahydrofuran)2Cl4 are reduced by sodium naphthalide in tetrahydrofuran to form atomically disordered Pt3Ti nanoparticles (FCC-type structure: Fm3m; a = 0.39 nm; particle size = 3 +/- 0.4 nm). These atomically disordered Pt3Ti nanoparticles are transformed to larger atomically ordered Pt3Ti nanoparticles (Cu3Au-type structure: Pm3m; a = 0.3898 nm; particle size = 37 +/- 23 nm) by annealing above 400 degrees C. Both atomically disordered and ordered Pt3Ti nanoparticles show lower onset potentials for the oxidation of formic acid and methanol than either pure Pt or Pt-Ru nanoparticles. Both atomically disordered and ordered Pt3Ti nanoparticles show a much lower affinity for CO adsorption than either pure Pt or Pt-Ru nanoparticles. Atomically ordered Pt3Ti nanoparticles show higher oxidation current densities for both formic acid and methanol than pure Pt, Pt-Ru, or atomically disordered Pt3Ti nanoparticles. Pt3Ti nanoparticles, in particular the atomically ordered materials, have promise as anode catalysts for direct fuel cells.     

Influence of substrate stiffness on cell-substrate interfacial adhesion and spreading: a mechano-chemical coupling model

Highly ordered three dimensionally macroporous carbon spheres (3DMPCS) were successfully prepared against removable colloidal silica crystal bead templates by carbonization of glucose. The unique structural characteristics of the well-developed three dimensionally interconnected macropores were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and nitrogen adsorption. The 3DMPCS have uniform large pore structures with size about 250 nm. Pt nanoparticles were supported on the macroporous carbon spheres by two aqueous impregnation methods, and it was found that the 3DMPCS supported Pt exhibited high electrocatalytic activity for methanol oxidation.     

Pt纳米空球粒径的可控制备及其甲醇电催化氧化

利用表面活性剂十二烷基磺酸钠（SDSN）的调控合成不同粒径的硒模板和铂纳米空球（Pthollow）,并将其修饰于玻碳（GC）基底即可制得Pthollow/GC电极;采用扫描电子显微镜（SEM）、透射电子显微镜（TEM）、高分辨透射电子显微镜（HR-TEM）和X射线光电子能谱等观察表征了Pthollow样品的形貌与组成;以甲醇为探针分子,研究Pthollow/GC和电沉积铂电极（Ptnano/GC）对甲醇氧化的电催化活性. 结果表明,由铂原子簇团构筑的多孔铂纳米空球粒径均匀,分散性好;用4 μmol·L^-1 SDSN控制合成的直径为130 nm的Pthollow制备的Pthollow/GC电极对甲醇氧化的电催化活性最佳.     

Electro-deposition of platinum nanoparticles on 4-mercaptobenzene-functionalized multi-walled carbon nanotubes

A new method to electro-deposit platinum nanoparticles on the surface of multi-walled carbon nanotubes (MWNTs) functionalized with 4-mercaptobenzene has been described. X-ray photoelectron spectroscopy results reveal that 4-mercaptobenzene was attached to the surface of MWNTs. Transmission electron microscope and X-ray diffraction analysis confirm that platinum nanoparticles were highly dispersed on the surface of MWNTs, and the average size of the platinum particle is 4.2 nm. The electrocatalytic properties of the Pt/MWNT composite electrode for methanol oxidation were investigated by cyclic voltammetry, and the results show that the fabricated composites exhibit high catalytic activity and good long-term stability. The study provides a feasible approach to fabricate Pt/MWNT composite electrode for direct methanol fuel cell.     

A facile synthesis and characterization of Ag, Au and Pt nanoparticles using a natural hydrocolloid gum kondagogu (Cochlospermum gossypium)

An environmentally benign method for the synthesis of noble metal nanoparticles has been reported using aqueous solution of gum kondagogu (Cochlospermum gossypium). Both the synthesis, as well as stabilization of colloidal Ag, Au and Pt nanoparticles has been accomplished in an aqueous medium containing gum kondagogu. The colloidal suspensions so obtained were found to be highly stable for prolonged period, without undergoing any oxidation. SEM-EDXA, UV-vis spectroscopy, XRD, FTIR and TEM techniques were used to characterize the Ag, Au and Pt nanoparticles. FTIR analysis indicates that -OH groups present in the gum matrix were responsible for the reduction of metal cations into nanoparticles. UV-vis studies showed a distinct surface plasmon resonance at 412 and 525 nm due to the formation of Au and Ag nanoparticles, respectively, within the gum network. XRD studies indicated that the nanoparticles were crystalline in nature with face centered cubic geometry. The noble metal nanoparticles prepared in the present study appears to be homogeneous with the particle size ranging between 2 and 10 nm, as evidenced by TEM analysis. The Ag and Au nanoparticles formed were in the average size range of 5.5±2.5 nm and 7.8±2.3 nm; while Pt nanoparticles were in the size range of 2.4±0.7 nm, which were considerably smaller than Ag and Au nanoparticles. The present approach exemplifies a totally green synthesis using the plant derived natural product (gum kondagogu) for the production of noble metal nanoparticles and the process can also be extended to the synthesis of other metal oxide nanoparticles.     

非碱性条件下不同粒径的碳载体负载Pt催化剂上甘油的选择性氧化(英文)

采用浸渍法制备了不同粒径的活性炭负载的Pt催化剂,并运用扫描电镜、N2吸附-脱附、透射电镜和X射线衍射对催化剂进行了表征.结果表明,当活性炭载体的粒径从253.2?m下降至9.3?m时,其表面积或孔体积变化不大,Pt颗粒高度分散于载体表面,平均粒径为2.8～5.5nm.这些高度分散的Pt催化剂在非碱性条件下的甘油氧化反应中表现出较高的活性,且随着载体粒径的减小而明显提升.其中粒径为9.3?m的活性碳负载的Pt催化剂上,游离的甘油酸收率达到47.6%,且催化剂可以重复使用.     

Continuous hot electron generation in Pt/TiO2, Pd/TiO2, and Pt/GaN catalytic nanodiodes from oxidation of carbon monoxide

Thin films (<10 nm) of platinum or palladium were deposited on TiO2 or GaN to form Schottky diodes. We detected and monitored the continuous electron flow across the metal-oxide interfaces of Pt/TiO2, Pd/TiO2, and Pt/GaN during the catalytic oxidation of carbon monoxide. The electron excitation and flow in the metals were due to conversion of energy released by the oxidation of carbon monoxide into the kinetic energy of free electrons in platinum and palladium. The best conversion of three electrons per four CO2 molecules was observed from 5 nm Pt/TiO2.     

Ultrafine Pt Nanoclusters for the Direct Methanol Fuel Cell Reactions

Controlling the size of Pt nanomaterials can be an effective route to improve their catalytic activity on a mass basis for the reactions of direct methanol fuel cell (DMFC). In this work we demonstrated the synthesis of ultrafine platinum nanoclusters based on the replacement reaction between Ag nanoparticles and Pt(IV) precursors. All the reactions were carried out in the presence of commercial carbon powders. The Pt/C thus formed could be directly used for catalyzing the DMFC reactions. By comparing with the commercial Pt catalysts from E-Tek, the Pt nanoclusters exhibit higher catalytic activity toward the methanol oxidation and oxygen reduction, the two key reactions in DMFC, due to their relative smaller sizes.     

“非保护型”金属胶体纳米簇形成机理研究

碱-乙二醇法制备的"非保护型"金属及合金纳米簇由表面吸附的溶剂分子和简单离子实现稳定化,它们被广泛用于制备高性能复相催化剂和研究复相催化剂中的尺寸、组成、载体表面基团以及修饰剂对催化性能的影响。关于此类非保护金属纳米簇的形成过程及机理的认识尚有待进一步深化。本文采用原位快速扫描X射线吸收精细结构谱(QXAFS)、原位紫外-可见(UV-Vis)吸收光谱、透射电子显微镜和动态光散射技术研究了碱-乙二醇法合成中非保护型金属胶体纳米簇的形成过程与机理。结果表明,在碱-乙二醇法合成非保护型Pt金属纳米簇的过程中,室温下即有部分Pt(IV)被还原至Pt(II)。随着反应温度的升高,OH-逐渐取代与Pt离子配位的Cl-,在Pt―Pt键形成之前,反应体系的UV-Vis吸收光谱中可观察到明显的纳米粒子的散射信号,原位QXAFS分析表明Pt纳米簇是由Pt氧化物纳米粒子还原所形成的;在Ru金属纳米簇的形成过程中,OH-首先取代了Ru Cl_3中的Cl~-,形成羟基配合物Ru(OH) _6~(3-),后者进一步缩合形成氧化钌纳米粒子,最终Ru金属纳米簇由乙二醇还原氧化钌纳米粒子形成。由于先形成了氧化物纳米粒子,后续的还原反应被限制在氧化物纳米粒子内,使最终得到的非保护型金属纳米簇具有尺寸小、分布窄的特点。本工作所获得的知识对发展高性能能源转化催化剂、精细化学合成催化剂、传感器等功能体系具有重要意义。     

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.     

Electrocatalytic properties of platinum anchored to the surface of highly oriented pyrolytic graphite

Nano-sized particles of platinum deposited on highly oriented pyrolytic graphite (HOPG) electrolessly (average sized = 3.4 nm) or electrochemically (d = 20 nm) and polycrystalline platinum are compared. The dispersed electrodes exhibit special features in the oxidation of ethylene glycol and adsorbed carbon monoxide. In particular, they reduce the overpotential of these processes. Possible reasons for the observed distinctions are discussed. The potential cycling leads to coalescence of nano-sized Pt particles on HOPG. As a result, their size distribution expands, and the distribution maximum shifts towards large sizes (d = 6.4 nm). This seriously complicates use of Pt/HOPG as a model electrode for investigating the size effect.     

Nanoparticle size effects on methanol electrochemical oxidation on carbon supported platinum catalysts

The particle size effect observed on the performance of Pt/C electrocatalysts toward the methanol oxidation reaction (MOR) has been investigated with differential electrochemical mass spectrometry (DEMS). The investigation has been conducted under both potentiodynamic and potentiostatic conditions as research on methanol electrochemical oxidation is closely related to interest in direct methanol fuel cells. The particle size effect observed on the MOR is commonly regarded as a reflection of different Pt-CO and Pt-OH bond strengths for different particle sizes. This work focuses mainly on the mechanism of methanol dehydrogenation on platinum which is central to the problem of the optimization of the efficiency of methanol electro-oxidation by favoring the CO(2) formation pathway. It was found that the partitioning of the methanol precursor among the end products on supported platinum nanoparticles is strongly dependent on particle size distribution. Also, it is postulated that the coupling among particles of different sizes via soluble products must be considered in order to understand the particle size effects on the observed trends of product formation. An optimum particle size range for efficiently electro-oxidizing methanol to CO(2) was found between 3 and 10 nm, and loss in efficiency is mostly related to the partial oxidation of methanol to formaldehyde on either too small or too large particles. The possible reasons for these observations are also discussed.     

聚乙烯吡咯烷酮稳定的钯纳米粒子催化加氢

Nanosized colloidal platinum was prepared by reduction of H2PtCl6 in methanol-water mixture by refluxing. The particle size and morphology were characterized by transmission electron microscopy and electron diffraction. The influence of polyvinylpyrrolidone (PVP) molecular mass (MM),PVP concentration,and reduction time on platinum particle size was investigated. Small (1-2 nm) Pt particles are formed in the case of PVP with MM=1.2×104. With increasing polymer MM and decreasing polymer concentration,large aggregates from small particles appear. High catalytic activity of the obtained colloidal platinum in hydrogenation of acetylene compounds is shown. The effect of Pt particle size on the catalytic activity was studied.