Chemical synthesis of gold nanowires in acidic solutions

High aspect ratio gold nanowires with single crystalline surface have long been a missing piece in the toolbox of plasmonics metal nanostructures. Such wires are now made with a room temperature, surfactant assisted chemical synthesis in acidic aqueous solution. The diameters and lengths of the multiply twinned gold nanowires can be tuned by varying the amount of seed particles and acid in the growth solution. Nanowires with diameters around 35 nm and lengths up to 10 micron were made with a low seed concentration in pH approximately 1 solution.     

Origin and activity of gold nanoparticles as aerobic oxidation catalysts in aqueous solution

Whether gold is catalytically active on its own has been hotly debated since the discovery of gold-based catalysis in the 1980s. One of the central controversies is on the O(2) activation mechanism. This work, by investigating aerobic phenylethanol oxidation on gold nanoparticles in aqueous solution, demonstrates that gold nanoparticles are capable to activate O(2) at the solid-liquid interface. Extensive density functional theory (DFT) calculations combined with the periodic continuum solvation model have been utilized to provide a complete reaction network of aerobic alcohol oxidation. We show that the adsorption of O(2) is very sensitive to the environment: the presence of water can double the O(2) adsorption energy to ~0.4 eV at commonly available edge sites of nanoparticles (~4 nm) because of its strongly polarized nature in adsorption. In alcohol oxidation, the hydroxyl bond of alcohol can break only with the help of an external base at ambient conditions, while the consequent α-C-H bond breaking occurs on pure Au, both on edges and terraces, with a reaction barrier of 0.7 eV, which is the rate-determining step. The surface H from the α-C-H bond cleavage can be easily removed by O(2) and OOH via a H(2)O(2) pathway without involving atomic O. We find that Au particles become negatively charged at the steady state because of a facile proton-shift equilibrium on surface, OOH + OH ? O(2) + H(2)O. The theoretical results are utilized to rationalize experimental findings and provide a firm basis for utilizing nanoparticle gold as aerobic oxidation catalysts in aqueous surroundings.     

Mutual Cooperation in the Formal Allyl Alcohol Nucleophilic Substitution and Hydration of Alkynes for the Construction of γ‐Substituted Ketones

Mutual cooperation in the formal allyl alcohol nucleophilic substitution reaction and hydration of an alkyne has been utilized in the presence of a gold catalyst to give a series of γ‐functionalized ketones with high to excellent yields. This reaction actually involved an intramolecular O?H insertion cyclization of an alkyne to form the dihydrofuran intermediate, which was followed by the nucleophilic addition ring‐opening of a dihydrofuran to give the target compound.     

CO2和CH3OH直接合成碳酸二甲酯Cu-Ni/V2O5-SiO2催化剂

采用表面反应改性法制备了Ｖ２Ｏ５－ＳｉＯ２（ＶＳｉＯ）表面复合物,用等体积浸渍法制备了ＶＳｉＯ担载的Ｃｕ－Ｎｉ双金属催化剂,用ＩＲ,ＴＰＤ,ＴＰＳＲ和微反技术研究了ＣＯ２和ＣＨ３ＯＨ在催化剂表面上的化学吸附与反应性能。结果表明,在Ｃｕ－Ｎｉ／ＶＳｉＯ催化剂上存在着金属位Ｃｕ－Ｎｉ合金,Ｌｅｗｉｓ酸位Ｖ＾ｎ＋和Ｌｅｗｉｓ碱位Ｖ＝Ｏ三类活性中心;ＣＯ２在金属位和Ｌｅｗｉｓ酸位协同作用下可生成ＣＯ２卧式     

CO2和CH3OH直接合成碳酸二甲酯Cu-Ni/V2O5-SiO2催化剂

采用表面反应改性法制备了V2O5 SiO2(VSiO)表面复合物 ,用等体积浸渍法制备了VSiO担载的Cu Ni双金属催化剂 ,用IR、TPD、TPSR和微反技术研究了CO2 和CH3OH在催化剂表面上的化学吸附与反应性能.结果表明,在Cu Ni/VSiO催化剂上存在着金属位Cu Ni合金、Lewis酸位Vn 和Lewis碱位V=O三类活性中心 ;CO2 在金属位和Lewis酸位协同作用下可生成CO2卧式吸附态M -(CO) -O→Vn ,此吸附态在138℃左右可解离成M -CO和V=O ;CH3OH在Lewis酸位和Lewis碱位协同作用下可形成解离吸附态V -OCH3和V -OH ;CO2 和CH3OH在Cu Ni/VSiO催化剂表面上的反应产物主要为碳酸二甲酯(DMC)、CH2O、CO和H2O ,其生成DMC的选择性在85%以上.     

Influence of Excitation Wavelength (UV or Visible Light) on the Photocatalytic Activity of Titania Containing Gold Nanoparticles for the Generation of Hydrogen or Oxygen from Water

Gold nanoparticles supported on P25 titania (Au/TiO(2)) exhibit photocatalytic activity for UV and visible light (532 nm laser or polychromatic light λ > 400 nm) water splitting. The efficiency and operating mechanism are different depending on whether excitation occurs on the titania semiconductor (gold acting as electron buffer and site for gas generation) or on the surface plasmon band of gold (photoinjection of electrons from gold onto the titania conduction band and less oxidizing electron hole potential of about -1.14 V). For the novel visible light photoactivity of Au/TiO(2), it has been determined that gold loading, particle size and calcination temperature play a role in the photocatalytic activity, the most active material (Φ(H2) = 7.5% and Φ(O2) = 5.0% at 560 nm) being the catalyst containing 0.2 wt % gold with 1.87 nm average particle size and calcined at 200 °C.     

Selective Amplification of CO Bond Hydrogenation on Pt/TiO2: Catalytic Reaction and Sum‐Frequency Generation Vibrational Spectroscopy Studies of Crotonaldehyde Hydrogenation

The hydrogenation of crotonaldehyde in the presence of supported platinum nanoparticles was used to determine how the interaction between the metal particles and their support can control catalytic performance. Using gas‐phase catalytic reaction studies and in situ sum‐frequency generation vibrational spectroscopy (SFG) to study Pt/TiO₂ and Pt/SiO₂ catalysts, a unique reaction pathway was identified for Pt/TiO₂, which selectively produces alcohol products. The catalytic and spectroscopic data obtained for the Pt/SiO₂ catalyst shows that SiO₂ has no active role in this reaction. SFG spectra obtained for the Pt/TiO₂ catalyst indicate the presence of a crotyl‐oxy surface intermediate. By adsorption through the aldehyde oxygen atom to an O‐vacancy site on the TiO₂ surface, the C?O bond of crotonaldehyde is activated, by charge transfer, for hydrogenation. This intermediate reacts with spillover H provided by the Pt to produce crotyl alcohol.     

Plasmonic monitoring of catalytic hydrogen generation by a single nanoparticle probe

Plasmonic nanostructures such as gold nanoparticles are very useful for monitoring chemical reactions because their optical properties are highly dependent upon the environment surrounding the particle surface. Here, we designed the catalytic structure composed of platinized cadmium sulfide with gold domains as a sensitive probe, and we monitored the photocatalytic decomposition of lactic acid to generate hydrogen gas in situ by single-particle dark-field spectroscopy. The plasmon band shift of the gold probe throughout the reaction exhibits significant particle-to-particle variation, and by simulating the reaction kinetics, the rate constant and structural information (including the diffusion coefficient through the shell and the relative arrangement of the active sites) can be estimated for individual catalyst particles. This approach is versatile for the monitoring of various heterogeneous reactions with distinct components at a single-particle level.     

Gold nanoparticles located at the interface of anatase/rutile TiO2 particles as active plasmonic photocatalysts for aerobic oxidation

Visible-light irradiation (λ > 450 nm) of gold nanoparticles loaded on a mixture of anatase/rutile TiO(2) particles (Degussa, P25) promotes efficient aerobic oxidation at room temperature. The photocatalytic activity critically depends on the catalyst architecture: Au particles with <5 nm diameter located at the interface of anatase/rutile TiO(2) particles behave as the active sites for reaction. This photocatalysis is promoted via plasmon activation of the Au particles by visible light followed by consecutive electron transfer in the Au/rutile/anatase contact site. The activated Au particles transfer their conduction electrons to rutile and then to adjacent anatase TiO(2). This catalyzes the oxidation of substrates by the positively charged Au particles along with reduction of O(2) by the conduction band electrons on the surface of anatase TiO(2). This plasmonic photocatalysis is successfully promoted by sunlight exposure and enables efficient and selective aerobic oxidation of alcohols at ambient temperature.     

低温液相合成甲醇及甲酸甲酯用Cu-Cr-M-O催化剂的制备与表征

采用共沉淀法和配合物沉淀法制备了分别添加Ｂａ, Ｍｎ,Ｖ,Ｂｉ,Ｎｉ 和Ｚｎ 的ＣｕＣｒＭＯ三组分催化剂,采用ＢＥＴ,ＸＲＤ和ＴＥＭ 对催化剂进行了表征,并考察了其对合成甲醇及甲酸甲酯反应的活性及选择性． 结果表明,添加第三种金属使催化剂的稳定性及甲酸甲酯选择性有所提高,但使催化活性有所下降． 配合物沉淀法制备的催化剂的比表面积较大,粒径较小． 热分解温度越高,催化剂的比表面积越小． 除ＣｕＣｒＶＯ 催化剂为纳米非晶态之外,其它催化剂均呈晶态． 催化剂的最小平均粒径为３８ ｎｍ , 最大平均粒径为６８ ｎｍ , 属纳米级催化剂．     

单晶Au表面和Au/TiO2（110）模型催化剂表面CO氧化反应机理和活性位

在分子尺度上介绍了Au/TiO2(110)模型催化剂表面和单晶Au表面CO氧化反应机理和活性位、以及H2O的作用.在低温(<320 K), H2O起着促进CO氧化的作用, CO氧化的活性位位于金纳米颗粒与TiO2载体界面(Auδ+–Oδ––Ti)的周边. O2和H2O在金纳米颗粒与TiO2载体界面边缘处反应形成OOH,而形成的OOH使O–O键活化,随后OOH与CO反应生成CO2.300 K时CO2的形成速率受限于O2压力与该反应机理相印证.相反,在高温(>320 K)下,因暴露于CO中而导致催化剂表面重组,在表面形成低配位金原子.低配位的金原子吸附O2,随后O2解离,并在金属金表面氧化CO.     

Selective hydrogenation of monosubstituted alkenes by Pd nanoparticles embedded in polyelectrolyte films

Pd nanoparticles embedded in multilayer polyelectrolyte films can be easily prepared through layer-by-layer adsorption of poly(acrylic acid) (PAA) and poly(ethyleneimine)-Pd2+ (PEI-Pd(II)) complexes followed by reduction of Pd(II) with NaBH4. Transmission electron microscopy confirms the formation of Pd particles with diameters of 1-3 nm. Remarkably, [PAA/PEI-Pd(0)]3PAA films catalyze the hydrogenation of monosubstituted alkenes with turnover frequencies that are as much as 100-fold higher than turnover frequencies for hydrogenation of multiply substituted double bonds. Selectivities in the hydrogenation of monosubstituted over multisubstituted double bonds are higher than those of Wilkinson's catalyst. Moreover, the turnover frequency for the hydrogenation of allyl alcohol did not change when the catalyst was recycled three times. Intramolecular selectivity for the hydrogenation of monosubstituted alkenes also occurs when substrate molecules contain both mono and multiply substituted double bonds. The combination of the encapsulating polyelectrolyte film and small nanoparticles apparently results in hindered access of multiply substituted double bonds to catalytic sites.     

Understanding the effect of cobalt particle size on Fischer-Tropsch synthesis: surface species and mechanistic studies by SSITKA and kinetic isotope effect

Co/γ-Al(2)O(3) catalysts with particle sizes in the range of 4-15 nm were investigated by isothermal hydrogenation (IH), temperature programmed hydrogenation (TPH), and steady-state isotopic transient kinetic analysis (SSITKA). Kinetic isotope effect experiments were used to probe possible mechanisms on Co/γ-Al(2)O(3) with different particle size. It was found that CO dissociated on Co/γ-Al(2)O(3) catalysts at 210 °C. The total amount of CO(2) formed following the dissociation depends on the cobalt crystal size. O-Co binding energy was found to be highly dependent on the Co metal particle size, whereas similar C-Co binding energy was found on catalysts with different Co particle size. Very strongly bonded carbon and oxygen surface species increased with decreasing particle size and acted as site blocking species in the methanation reaction. SSITKA experiments showed that the intrinsic activity (1/τ(CH(x))) remained constant as the particle size increased from 4 to 15 nm. The number of surface intermediates (N(CH(x))) increased with increasing particle size. The apparent activation energies were found similar for these catalysts, about 85 kJ/mol. D(2)-H(2) switches further confirmed that the particle size did not change the kinetically relevant steps in the reaction. The reactivity of the active sites on the 4 nm particles was the same as those on the 8, 11, and 15 nm particles, and only the number of total available surface active sites was less on the 4 nm particles than on the others.     

Mechanism of silver(I)-assisted growth of gold nanorods and bipyramids

The seed-mediated growth of gold nanostructures is shown to be strongly dependent on the gold seed nanocrystal structure. The gold seed solutions can be prepared such that the seeds are either single crystalline or multiply twinned. With added silver(I) in the cetyltrimethylammonium bromide (CTAB) aqueous growth solutions, the two types of seeds yield either nanorods or elongated bipyramidal nanoparticles, in good yields. The gold nanorods are single crystalline, with a structure similar to those synthesized electrochemically (Yu, Y. Y. et al. J. Phys. Chem. B 1997, 101, 6661). In contrast, the gold bipyramids are pentatwinned. These bipyramids are strikingly monodisperse in shape. This leads to the sharpest ensemble longitudinal plasmon resonance reported so far for metal colloid solutions, with an inhomogeneous width as narrow as 0.13 eV for a resonance at approximately 1.5 eV. Ag(I) plays an essential role in the growth mechanism. Ag(I) slows down the growth of the gold nanostructures. Ag(I) also leads to high-energy side facets that are {110} for the single crystalline gold nanorods and unusually highly stepped {11n} (n approximately 7) for the bipyramid. To rationalize these observations, it is proposed that it is the underpotential deposition of Ag(I) that leads to the dominance of the facets with the more open surface structures. This forms the basis for the one-dimensional growth mechanism of single crystal nanorods, while it affects the shape of the nanostructures growing along a single twinning axis.     

Gold nanoparticles incarcerated in nanoporous syndiotactic polystyrene matrices as new and efficient catalysts for alcohol oxidations

The controlled synthesis of gold nanoparticles (AuNPs), incarcerated in a semicrystalline nanoporous polymer matrix that consisted of a syndiotactic polystyrene-co-cis-1,4-polybutadiene multi-block copolymer is described. This catalyst was successfully tested in the oxidation of primary and secondary alcohols, in which we used dioxygen as the oxidant under mild conditions. Accordingly, (±)-1-phenylethanol was oxidised to acetophenone in high yields (96%) in 1 h, at 35 °C, whereas benzyl alcohol was quantitatively oxidised to benzaldehyde with a selectivity of 96% in 6 h. The specific rate constants calculated from the corresponding kinetic plots were among the highest found for polymer-incarcerated AuNPs. Similar values in terms of reactivity and selectivity were found in the oxidation of primary alcohols, such as cinnamyl alcohol and 2-thiophenemethanol, and secondary alcohols, such as indanol and α-tetralol. The remarkable catalytic properties of this system were attributed to the formation, under these reaction conditions, of the nanoporous ε crystalline form of syndiotactic polystyrene, which ensures facile and selective accessibility for the substrates to the gold catalyst incarcerated in the polymer matrix. Moreover, the polymeric crystalline domains produced reversible physical cross-links that resulted in reduced gold leaching and also allowed the recovery and reuse of the catalyst. A comparison of catalytic performance between AuNPs and annealed AuNPs suggested that multiple twinned defective nanoparticles of about 9 nm in diameter constituted the active catalyst in these oxidation reactions.     

The mystery of gold's chemical activity: local bonding, morphology and reactivity of atomic oxygen

Recently, gold has been intensely studied as a catalyst for key synthetic reactions. Gold is an attractive catalyst because, surprisingly, it is highly active and very selective for partial oxidation processes suggesting promise for energy-efficient "green" chemistry. The underlying origin of the high activity of Au is a controversial subject since metallic gold is commonly thought to be inert. Herein, we establish that one origin of the high activity for gold catalysis is the extremely reactive nature of atomic oxygen bound in 3-fold coordination sites on metallic gold. This is the predominant form of O at low concentrations on the surface, which is a strong indication that it is most relevant to catalytic conditions. Atomic oxygen bound to metallic Au in 3-fold sites has high activity for CO oxidation, oxidation of olefins, and oxidative transformations of alcohols and amines. Among the factors identified as important in Au-O interaction are the morphology of the surface, the local binding site of oxygen, and the degree of order of the oxygen overlayer. In this Perspective, we present an overview of both theory and experiments that identify the reactive forms of O and their associated charge density distributions and bond strengths. We also analyze and model the release of Au atoms induced by O binding to the surface. This rough surface also has the potential for O(2) dissociation, which is a critical step if Au is to be activated catalytically. We further show the strong parallels between product distributions and reactivity for O-covered Au at low pressure (ultrahigh vacuum) and for nanoporous Au catalysts operating at atmospheric pressure as evidence that atomic O is the active species under working catalytic conditions when metallic Au is present. We briefly discuss the possible contributions of oxidants that may contain intact O-O bonds and of the Au-metal oxide support interface in Au catalysis. Finally, the challenges and future directions for fully understanding the activity of gold are considered.     

Nanoclusters in catalysis: a comparison of CS(2) catalyst poisoning of polyoxoanion- and tetrabutylammonium-stabilized 40 +/- 6 A Rh(0) nanoclusters to 5 Rh/Al(2)O(3), including an analysis of the literature related to the CS(2) to metal stoichiometry issue

It is crucial in metal particle catalysis to know the true number of catalytically active surface sites; without this knowledge it is impossible (i) to know the true turnover frequency (TOF, i.e., the moles of product/(moles of active metal atoms x time)); (ii) to know for certain whether a (quantitatively) better catalyst has been made-on a per-active-metal-atom basis; (iii) to know the amount of active sites remaining in a deactivated catalyst; and (iv) to know how many active sites have been regenerated in a reactivated catalyst. For this reason, herein we report the first quantitative, more complete and fundamental study of nanocluster catalyst poisoning using the preferred CS(2) method with polyoxoanion- and tetrabutylammonium-stabilized Rh(0) nanoclusters; 5% Rh/Al(2)O(3) is also examined as a valuable comparison point. Both catalysts are examined under essentially identical conditions and while catalyzing a prototype reaction, cyclohexene hydrogenation. A number of control studies are also reported to be sure that the kinetic method used to follow the CS(2) poisoned hydrogenation reaction is reliable, to test for H(2) gas-to-solution mass-transfer limitations, to test for reversibility in the CS(2) poisoning, and to test for loss of the volatile CS(2). The results allow 10 previously unavailable insights and conclusions, including the first quantitative comparison of the active-site corrected TOF for a nanocluster catalyst (in this case Rh(0) nanoclusters) to its supported heterogeneous counterpart (the 5% Rh(0) on Al(2)O(3)). The results show that the nanocluster surface Rh(0) is between 2.3 and 23 times more active on a per-active-metal-atom basis. Overall, the results introduce to the transition-metal nanocluster area the catalyst poisoning methodology necessary for the determination of the number of active metal sites. The important literature of CS(2) catalyst poisoning studies is also cited and discussed with a focus on the previously neglected issue of the exact poison/metal stoichiometry ratio. Significantly, the single metal crystal plus CS(2) literature provides evidence that the CS(2)/metal ratio probably lies between 1/1.5 and 1/10 in most cases. The data presented herein suggest that the CS(2)/Rh ratio for the Rh(0) nanoclusters is very likely within this range and for certain is <1/17.     

Investigation of hydrothermal routes to mixed-metal cerium titanium oxides and metal oxidation state assignment using XANES

The reaction between TiF(3) or TiO(2) and Ce(3+) in sodium hydroxide solutions yields highly crystalline NaCeTi(2)O(6) at room temperature and under mild hydrothermal conditions (T < or = 240 degrees C). There is no evidence for the formation of ternary Ce-Ti-O materials by this method, and the use of bases other than NaOH always produces poorly crystalline materials. The material NaCeTi(2)O(6) has a distorted perovskite structure with sodium and cerium ions randomly occupying the A sites: Pnma, a = 5.4517(8) A, b = 7.7292(6) A, c =5.4573(3) A. XANES spectroscopy at the Ti K edge and Ce L(III) edge, with reference to crystalline model compounds, reveals that cerium is found solely as Ce(III) and titanium as Ti(IV) in NaCeTi(2)O(6). Isomorphous substitution of Ce(3+) by Nd(3+) or Ti(4+) by V(4+) is found to be very facile under hydrothermal conditions (at a temperature of 240 degrees C), by addition of appropriate amounts of metal salts to the hydrothermal reaction mixtures. The series NaCe(1-x)Nd(x)Ti2O6 (0 < or = x < or = 1) and NaCeTi(2-x)V(x)O6 (0 相似文献   

O2和CO表面催化反应的活性位分布模型

对于O_2和CO表面催化反应，建立了一个新的不可逆Monte Carlo模拟模型。在 二维格子中，引进了表面活性位和非活性位的要领。模型假设，一定浓度的活性位 随机分布在非活性位上，形成了活性位分布的二维格子模型反应器，并在ZGB模型 的基础上，考虑了氧原子和CO分子的表面扩散，特别是引进了吸附粒子的定向表面 扩散。其中，活性位和活性位最近邻是表面吸附物质反应的活性中心，而非活性位 的作用是通过表面扩散传质。当活性位浓度C_a = 1且考虑扩散时，模型还原为增 加了扩散的ZGB模型。当活性位浓度C_a = 1且只考虑氧的扩散时，模拟结果表明， 扩散几率达到某一数值（0.3）时，二级相变点完全消失。当活性位浓度C_a逐渐减 小时，单位活性位产生的CO_2的速率不断增大，这表明活性位的利用率提高了。     

Palladium and carbon synergistically catalyzed room-temperature hydrodeoxygenation(HDO) of vanillyl alcohol–A typical lignin model molecule

Vanillyl alcohol,which is made up of an aromatic ring,an alcoholic hydroxyl group,a phenolic hydroxyl group and a methoxy group,was selected as the model molecule of lignin.Various carbon materials supported Pd catalysts were chosen to catalyze the HDO of vanillyl alcohol.The catalysts were characterized via TEM,TPD,XRD,XPS and CO-chemisorption.It was found that different carbon materials could obviously influence the particle sizes,dispersion and distribution of Pd or Pd O particles.Palladium and carbon can synergistically catalyze the room-temperature HDO of vanillyl alcohol even at room temperature,and the carboxyl group was found to be the effective active acid site during the reaction.Possible reaction mechanism was also proposed.The existence of the effective active acid sites on the carbon supports could obviously lower the reaction temperature without decreasing the selectivity,as a result,making the production of renewable fuels by HDO much more economically feasible,which is of much importance.