One‐Pot Synthesis of Carbon‐Supported Dendritic Pd‐Au Nanoalloys for Electrocatalytic Ethanol Oxidation

One‐pot synthesis of carbon‐supported Pd‐Au alloy nanoparticles with well‐defined dendritic shape (Pd‐Au_den/C) was achieved by co‐reduction of K₂PdCl₄/HAuCl₄ mixtures in a molar ratio of 1:1 with hydrazine in the presence of Vulcan XC‐72R. The prepared Pd‐Au_den/C exhibited significantly enhanced performance in the electrocatalytic oxidation of ethanol compared with dendritic Pd nanoparticles and a commercial Pd/C catalyst. Pd‐Au_den/C even showed higher durability in electro‐oxidation of ethanol than the supported catalyst prepared by the deposition of presynthesized dendritic Pd‐Au nanoparticles on the carbon support. The experimental results clearly indicate that enhanced interaction between nanoparticle catalysts and carbon support through the one‐pot synthesis protocol can improve the durability of the electrocatalysts.     

Porous organic polymer immobilized copper nanoparticles as heterogeneous catalyst for efficient benzylic C–H bond oxidation

A facile and practical heterogeneous copper nanoparticles catalyst (Cu@AEPOP) was prepared by the incorporation of Cu(OAc)₂ to amide and ether functionalized porous organic polymers (AEPOP) that were efficiently prepared by condensation of 4,4′-diaminodiphenyl ether with 1,3,5-benzenetricarbonyl chloride. The prepared AEPOP and Cu@AEPOP were characterized by SEM, BET, TEM, ICP, FT-IR and XRD. With TBHP (70 wt% in water) as oxidant and solvent, Cu@AEPOP showed excellent catalytic activity for the selective oxidation of benzylic C–H bonds. Among the oxidation of alkyl benzenes to ketones, secondary alcohols to ketones, and primary alcohols to acids, the copper catalyst exhibited remarkable selectivity and reactivity for a broad range of substrates in excellent yields. Furthermore, the experimental operation was very facile and the heterogeneous catalyst could be easily recovered by filtration separation.     

Au–Ag Nanoflower Catalysts with Clean Surfaces for Alcohol Oxidation

Shape‐controlled metal nanocrystals, such as nanowires and nanoflowers, are attractive owing to their potentially novel catalytic properties and bimetallic nanocrystals composed of two distinct metals are expected to act as highly active catalysts. However, their catalytic activities are limited because of the capping agents adsorbed on the metal surfaces, which are necessary for the preparation and dispersion of these nanocrystals in solvents. Therefore, the preparation of bimetallic shape‐controlled noble metal nanocrystals with clean surfaces, devoid of almost all capping agents, are expected to have high catalytic activity. Herein, we report the preparation of bimetallic Au–Ag nanoflowers using melamine as the capping agent. The bimetallic Au–Ag nanoflowers with a clean surface were subsequently obtained by a support and extraction method. The bimetallic nanoflowers with a clean surface were then used for the aerobic oxidation of 1‐phenylethyl alcohol and they exhibited high rates for the formation of acetophenone compared to Au nanoflowers and spherical nanoparticles with almost the same size and Au/Ag ratio. We also show that Au–Ag nanoflowers containing only 1 % Ag (Au₉₉–Ag₁NFs) exhibit the highest rate of acetophenone formation among Au–Ag nanoflowers with different Au/Ag ratios owing to an increase in the electron density of the Au atoms that act as active sites for the oxidation of 1‐phenylethyl alcohol.     

Chloride‐Induced Highly Active Au Catalyst for Methyl Esterification of Alcohols

Chloride is generally regarded as a harmful species for the heterogeneous catalysts, especially Au catalysts. In this work, a series of active Au/NiO_x catalysts were successfully prepared with co‐precipitation method by tracking the concentrations of chloride in the re‐dispersed aqueous solutions. For methyl esterification of alcohols, the highest active Au/NiO_x catalysts could be prepared from aqueous solutions containing 8‐13 ppm chloride, the yield of methyl benzoate of catalyst Au/NiO_x‐9 was 99%. The catalyst structures and the role of chloride in catalysts were explored by ICP, BET, XPS, TEM and EXAFS characterizations. It was found that the appropriate amount of residual chloride in Au catalysts was beneficial to their catalytic activities. Especially for Au/NiO_x‐9, the appropriate amount of residual chloride had positive effects on the physicochemical properties of Au/NiO_x catalyst, the position of Au nanoparticles (NPs) located on NiO_x crystallites and the ratio of Au^δ+/Au⁰ in catalyst, which together resulted in its high reactivity.     

Synthesis and characterization of <Emphasis Type="Italic">N</Emphasis>-hydroxyphthalimide immobilized on SiO<Subscript>2</Subscript>-coated Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles as magnetic catalyst for oxidation of benzyl alcohols and hydrocarbons

In this study, N-hydroxyphthalimide (NHPI) was successfully attached on functionalized SiO₂-coated Fe₃O₄ nanoparticles through amid bond. The sustained nanomagnetite-immobilized NHPI as a new magnetically recoverable catalyst was characterized by FT-IR, XRD, TGA, VSM, TEM and SEM techniques. The prepared catalyst exhibited high selectivity for oxidation of various benzyl alcohols and hydrocarbons in the presence of hydrogen peroxide as oxidant. The catalyst can be readily separated from the reaction mixture using an external magnet and reused several times without significant loss of its catalytic activity.     

Low‐Temperature Oxidation of Carbon Monoxide with Gold(III) Ions Supported on Titanium Oxide

Au/TiO₂ catalysts prepared by a deposition–precipitation process and used for CO oxidation without previous calcination exhibited high, largely temperature‐independent conversions at low temperatures, with apparent activation energies of about zero. Thermal treatments, such as He at 623 K, changed the conversion–temperature characteristics to the well‐known S‐shape, with activation energies slightly below 30 kJ mol^?1. Sample characterization by XAFS and electron microscopy and a low‐temperature IR study of CO adsorption and oxidation showed that CO can be oxidized by gas‐phase O₂ at 90 K already over the freeze‐dried catalyst in the initial state that contained Au exclusively in the +3 oxidation state. CO conversion after activation in the feed at 303 K is due to Au^III‐containing sites at low temperatures, while Au⁰ dominates conversion at higher temperatures. After thermal treatments, CO conversion in the whole investigated temperature range results from sites containing exclusively Au⁰.     

Thermal behavior of carbon nanotubes decorated with gold nanoparticles

Three different forms of carbon, i.e., multi-walled carbon nanotubes (CNTs), single-walled CNTs, and soot, were decorated with gold nanoparticles by a new method. In this method C₁₀H₈ ⁻ ions transfer electrons to the CNTs or soot. These electrons on the carbon surface can then reduce Au³⁺ species to form supported Au nanoparticles with a narrow particle size distribution. Thermogravimetric/differential thermal analyses (TG/DTA), XRD, Raman, and TEM show that naphthalene molecules remain trapped inside the Au nanoparticles and can only be removed by treatment at ca. 300 °C. Remarkable effect of the Au nanoparticles on the oxidation of carbon by O₂ is also observed by TG/DTA, i.e., on-set oxidation temperature and activation energy (E _a). It is shown that as the Au particle size decreases from 25 to 2 nm a linear decrease of the oxidation temperature is observed. Au particles larger than 25 nm do not produce any significant effect on carbon oxidation. These results are discussed in terms of spillover catalytic effect where Au nanoparticles activate O₂ molecules to produce active oxygen species which oxidize the different carbon supports.     

Preparation of Photocatalytic Au–Ag2Te Nanomaterials

A facile approach has been developed for the preparation of various morphologies of Au–Ag₂Te nanomaterials (NMs) that exhibit strong photocatalytic activity. Te NMs (nanowires, nanopencils, and nanorice) were prepared from TeO₂ in the presence of various concentrations (16, 8, and 4 M ) of a reducing agent (N₂H₄) at different temperatures (25 and 60 °C). These three Te NMs were then used to prepare Au–Ag₂Te NMs by spontaneous redox reactions with Au³⁺ and Ag⁺ ions sequentially. The Au–Ag₂Te nanopencils exhibit the highest activity toward degradation of methylene blue and formation of active hydroxyl radicals on solar irradiation, mainly because they absorb light in the visible region most strongly. All three differently shaped Au–Ag₂Te NMs (10 μg mL^?1) provide a death rate of Escherichia coli greater than 80 % within 60 min, which is higher than that of 51 % for commercial TiO₂ nanoparticles (100 μg mL^?1). Under light irradiation, the Au NPs in Au–Ag₂Te NMs enhance the overall photo‐oxidation ability of Ag₂Te NMs through faster charge separation because of good contact between Ag₂Te and Au segments. With high antibacterial activity and low toxicity toward normal cells, the Au–Ag₂Te NMs hold great potential for use as efficient antibacterial agents.     

Catalytic Oxidation of Ethyl Lactate to Ethyl Pyruvate over Au-Based Catalyst Using Authentic Air as Oxidant

Highly dispersed Au nanoparticles supported on Ni–Al mixed metal oxides (Au/NiAl-MMO) were prepared by a facile method, which is significantly efficient for the aerobic oxidation of ethyl lactate using authentic air as the oxidant, achieving 72.6% ethyl lactate conversion and 88.3% selectivity to ethyl pyruvate at 240 °C in a continuous fixed-bed reactor. The catalyst retained its catalytic performance during a long-term stability test. Characterization and experimental studies on the kinetic dependence sequence of the reactants and elementary reaction steps confirmed that the Au/NiAl-MMO catalyst followed the Mars–van Krevelen mechanism, with the activation of O₂ as the elementary step. The quasi in situ X-ray photoelectron spectroscopy spectra demonstrated that the active sites in the Au/NiAl-MMO catalyst were Au nanoparticles. This work may provide a novel technique for developing more efficient supported metal catalysts for the aerobic oxidation of ethyl lactate using authentic air as the oxidant.

Graphical abstract

Highly dispersed Au nanoparticle catalyst was facilely prepared for the efficient catalytic oxidation of ethyl lactate with authentic air.

     

Effect of oxygen mobility in the lattice of Au/TiO<Subscript>2</Subscript> on formaldehyde oxidation

Two Au catalysts supported on TiO₂ were prepared by impregnation method followed by sodium borohydride reduction or calcination in air (Au/TiO₂-R and Au/TiO₂-C, respectively). The 1 wt % Au/TiO₂-R sample was found to be highly efficient for the oxidation of low concentrated formaldehyde at room temperature. A HCHO conversion of 98.5% was achieved with this catalyst, whereas the Au/TiO₂-C sample showed almost no activity under the same conditions. Highly dispersed metallic Au nanoparticles with small size (∼3.5 nm) were identified in the 1 wt % Au/TiO₂-R catalyst. A significant negative shift of Au4f peak in XPS spectra with respect to bulk metallic Au was observed for the 1 wt % Au/TiO₂-R but no similar phenomena was found for the heat-treated catalyst. More Au nanoparticles and higher content of surface active oxygen were identified on the surface of the Au/TiO₂-R in comparison with the Au/TiO₂-C, suggesting that the Au/TiO₂-R catalyst can enhance the amount of active sites and species involved in for HCHO oxidation. The reduction treatment by sodium borohydride promotes the formation of dispersed metallic Au nanoparticles with small size because it facilitates the electron transfer and increases the content of surface Au nanoparticles and activated oxygen. All these factors are responsible for a high activity of this catalyst in the oxidation of HCHO.     

Gold nanoparticle decoration of insulating boron nitride nanosheet on inert gold electrode toward an efficient electrocatalyst for the reduction of oxygen to water

Overpotential for oxygen reduction reaction (ORR) at Au electrode is reported to be reduced by 0.27 V by the modification with boron nitride nanosheet (BNNS) but oxygen is reduced only to H₂O₂ by 2-electron process at Au electrode. Here we demonstrate that the decoration of BNNS with gold nanoparticles (AuNP) not only reduces the overpotential for ORR further by ca. 50 mV, but also opens a 4-electron reduction route to water. Both rotating disk electrode experiments with Koutecky–Levich analysis and rotating ring disk electrode measurements show that more than 50% of oxygen is reduced to water via 4-electron process at Au–BNNS/Au electrode while less than 20 and 10% of oxygen are reduced to water at the BNNS/Au and bare Au electrodes, respectively. Theoretical analysis of free energy profiles for ORR at the BN monolayer with and without Au₈ cluster placed on Au(111) shows significant stabilization of adsorbed oxygen atom by the Au₈ cluster, opening a 4-electron reduction pathway.     

Novel Nanoporous Binary Au?Ru Electrocatalysts for Glucose Oxidation

In this work, we study the fabrication, structural characterization, and electrochemical activity of titanium‐supported binary Au? Ru catalysts for glucose oxidation. The catalysts including Au₉₉Ru₁, Au₉₅Ru₅, Au₉₃Ru₇ and Au₈₈Ru₁₂ were prepared by a hydrothermal method using formaldehyde as a reduction agent. The morphologies of the prepared Au? Ru catalyst structures are characterized by porous dendritic particles with roughened surfaces with nano‐sized flakes. Electrochemical catalytic activity of the binary Au? Ru catalysts towards glucose oxidation in alkaline solutions was investigated using cyclic voltammetry and chronoamperometry. All binary Au? Ru catalysts facilitate glucose oxidation at the lower potentials and deliver higher anodic oxidation currents compared to pure Au catalyst. Among them, the binary Au₉₅Ru₅ catalyst presents the most negative onset potential of ?0.872 V (vs. Ag/AgCl, 3 M KCl) for glucose oxidation in 0.1 M NaOH solution. For the Au₉₅Ru₅ catalyst, chronoamperometric data at the potential step of ?0.65 V (vs. Ag/AgCl,3 M KCl) exhibit a well linear dependence of the anodic oxidation current density on glucose concentration in the range of 0 to 15 mM glucose.     

Enhancement of Fe3O4/Au Composite Nanoparticles Catalyst in Oxidative Degradation of Methyl Orange Based on Synergistic Effect

Enhancement of Fe₃O₄ /Au nanoparticles (Fe₃O₄ /Au NPs ) catalyst was observed in the oxidative degradation of methyl orange by employing H₂O₂ as oxidant. To evaluate the catalytic activity of Fe₃O₄ /Au nanoparticles, different degradation conditions were investigated such as the amounts of catalyst, H₂O₂ concentration and pH value. Based on our data, methyl orange was degraded completely in a short time. The enhanced catalytic activity and increased oxidation rate constant may be ascribed to synergistic catalyst‐activated decomposition of H₂O₂ to •OH radical, which was one of the strong oxidizing species. Besides, Fe₃O₄ /Au nanoparticles have exhibited satisfying recycle performance for potential industrial application.     

通过CeOx修饰提高金纳米颗粒在CO氧化反应中的催化活性和耐久性

CO催化氧化是一个重要的经典反应,与许多应用息息相关,包括痕量CO气体检测、汽车尾气净化和安全防护等,吸引了人们广泛的研究兴趣.负载型Au纳米颗粒在CO氧化等许多反应中有着与众不同的催化活性,具有广泛的应用前景,但依然存在着稳定性差、易团聚失活的问题.人们通过应用多孔载体隔离Au纳米颗粒,在Au纳米颗粒表面覆盖金属氧化物、二氧化硅或碳,以及对Au纳米粒子进行封装等方法解决这些问题.尤其是利用金属氧化物与Au纳米粒子间的强相互作用对其进行覆盖或封装,有效地提高了Au催化材料的稳定性.但以上策略操作流程复杂,不利于应用.本文发展了一种简单有效的方法,通过EDTA的络合作用引入CeOx对Au纳米粒子进行修饰,得到的CeOx@Au/SiO2催化剂活性和耐久性明显提升.采用X射线衍射(XRD)和高分辨透射电子显微镜(HRTEM)证明了CeOx成功地修饰在Au纳米颗粒上.且通过EDTA引入CeOx所制备的CeOx@Au/SiO2催化剂结构明显不同于直接加入纳米CeO2所得到的CeOx-Au/SiO2的结构.EDTA的络合作用能有效地连结Ce与Au物种,经焙烧消除EDTA后,加强了CeOx与Au间相互作用,最终在Au纳米粒子表面形成丰富的CeOx颗粒与原子级厚度的CeOx层.进一步应用X射线光电子能谱(XPS)和氢气程序升温还原(H2-TPR)等手段研究了CeOx修饰对Au纳米粒子的影响.XPS结果表明,CeOx@Au/SiO2催化剂带正电的Au+和Au3+的浓度明显高于一般的Au/SiO2和直接加入CeO2制备得到的CeOx-Au/SiO2催化剂.H2-TPR同样表明,CeOx修饰调变了Au纳米粒子的氧化还原性.这些均对其在CO催化氧化反应中的催化活性具有重要影响将CeOx@Au/SiO2催化剂用于CO催化氧化反应中,160°C时,CO转化率达98.8％,至180°C后实现了CO的完全转化.而一般的Au/SiO2催化剂在160°C时CO转化率仅为4.0％,CO的完全转化则需340°C.直接加入纳米CeO2所得到的CeOx-Au/SiO2催化剂,其催化活性略有提升,CO完全转化所需的温度为300°C.这充分证明了通过CeOx修饰Au纳米粒子,能有效提升其催化活性.原位漫反射红外光谱(DRIFT)结果表明,CeOx修饰促进了CO在Au表面的吸附,并能形成[Au(CO)2]δ+物种;同时还观察到大量的单齿CO32? 物种信号,反映了CeOx@Au/SiO2催化剂表面存在丰富的活性氧物种.通入O2后,观察到了大量CO32?物种信号和气相CO2,印证了催化剂表面发生的CO催化氧化过程,也表明其具有非常高的催化活性.考察了CeOx@Au/SiO2催化剂的耐久性,发现经50 h CO氧化反应,催化剂依然能有效保持活性.相比之下,Au/SiO2催化剂经10 h反应后,开始明显失活.由此可见,CeOx@Au/SiO2催化剂具有相当高的耐久性.在600°C将催化剂焙烧3 h,发现Au/SiO2催化剂中Au纳米粒子存在明显团聚现象,而CeOx@Au/SiO2催化剂的Au纳米粒子依然均匀分布在载体表面,且粒径未发生明显变化.     

A density functional theory study of gold clusters supported on layered double hydroxides

The geometrical structure and electronic properties of a series of Au _N (N = 1–8) clusters supported on a Mg²⁺, Al³⁺-containing layered double hydroxides (MgAl–LDH) are investigated using density functional theory. The Au clusters are supported on two typical crystal faces of the LDH platelet, the basal {0001} and the lateral $ \{ 10\,\bar{1}\,0\} $ crystal face, respectively, corresponding to the top and edge site of monolayer MgAl–LDH lamella for the sake of simplicity. It is revealed that an increase in the charge transfer from the LDH lamella to the Au _N clusters at the edge site rather than clusters on the top surface, demonstrating a preferential adsorption for Au _N clusters at the edge of LDH lamella. Moreover, the calculated adsorption energy of the Au _N clusters on the LDH lamella increases with the cluster size, irrespective of the adsorption site. The investigation on the interaction between O₂ and Au _N clusters on the LDH lamella is further carried out for understanding the catalytic oxidation properties of the LDH-supported Au catalyst. The formation of reactive O₂ ^? species, a necessary prerequisite in catalytic oxidation of CO, by O₂ bridging two Au atoms of Au _N clusters indicates that the LDH-supported Au catalyst has the required characteristics of a chemically active gold catalyst in CO oxidation.     

Au/α-Fe2O3 催化剂存贮失活环境因素及机理分析

 采用沉积沉淀法制备了 CO 低温氧化催化剂 Au/α-Fe₂O₃, 通过 X 射线衍射、X 射线光电子能谱、N₂ 吸附-脱附、傅里叶变换红外光谱、H₂ 程序升温还原和 CO₂ 程序升温脱附等手段对催化剂进行了表征, 探讨了在室温大气气氛下光线照射以及表面吸附等环境因素所导致的催化剂存贮失活及其作用机理. 结果表明, 经 110 ^oC 干燥的 Au/α-Fe₂O₃催化剂表面同时存在 Au³⁺和 Au^δ+ (0 ≤ δ ≤ 1) 物种, 且前者催化 CO 氧化的活性更高; 在室温大气气氛下, 紫外线照射会引起 Au³⁺的还原和 Au 颗粒的生长, 导致催化剂的不可逆失活. 此外, 空气中的 H₂O 和 CO₂ 可同时吸附在 α-Fe₂O₃的表面, 形成表面碳酸盐物种, 会引起催化剂的可逆失活.     

Oxoperoxo tungsten(VI) complex immobilized on Schiff base-modified Fe3O4 magnetic nanoparticles as a heterogeneous catalyst for oxidation of alcohols with hydrogen peroxide

Oxoperoxo tungsten(VI) complex immobilized on Schiff base-modified Fe₃O₄ super paramagnetic nanoparticles were synthesized and appropriately characterized using FT-IR, XRD, SEM, TEM, EDX, BET, and VSM analysis. The synthesized nanoparticles efficiently catalyzed oxidation of benzylic alcohols with H₂O₂ as oxidant in high yields, with high to excellent selectivity. The catalyst can be recovered using an external magnetic field and recycled for subsequent oxidation reactions without any appreciable loss of efficiency. The simple preparation, high activity, excellent selectivity, and simple recoverability of the catalyst are advantageous.     

Graphene nanoplatelets supported metal nanoparticles for electrochemical oxidation of hydrazine

Graphene nanoplatelets have been applied as the support to electrodeposit monometallic Au and Pd nanoparticles as well as bimetallic Au–Pd nanoparticles. These nanoparticles have been characterized with scanning electron microscope, energy dispersive X-ray spectroscopy, X-ray diffraction spectroscopy, and electrochemical techniques. They are further utilized as the catalysts for electrochemical oxidation of hydrazine. The oxidation peak potential is − 0.35 and 0.53 V (vs. SCE) when monometallic Pd and Au nanoparticle are used as the catalysts. When bimetallic nanoparticles are applied as the catalyst, their composition affects the peak potential and peak current for the oxidation of hydrazine. Higher oxidation current is achieved when bimetallic Au–Pd nanoparticles with an atomic ratio of 3:1 are deposited on graphene nanoplatelets. Metal nanoparticle-loaded graphene nanoplatelets are thus novel platforms for electrocatalytic, electroanalytical, environmental, and related applications.     

Anchoring of Cu(II)–vanillin Schiff base complex on MCM‐41: A highly efficient and recyclable catalyst for synthesis of sulfides and 5‐substituted 1H–tetrazoles and oxidation of sulfides to sulfoxides

A copper(II)–vanillin complex was immobilized onto MCM‐41 nanostructure and was used as an inexpensive, non‐toxic and heterogeneous catalyst in the synthesis of symmetric aryl sulfides by the cross‐coupling of aromatic halides with S₈ as an effective sulfur source, in the oxidation of sulfides to sulfoxides using 30% H₂O₂ as a green oxidant and in the synthesis of 5‐substituted 1H –tetrazoles from a smooth (3 + 2) cycloaddition of organic nitriles with sodium azide (NaN₃). The products were obtained in good to excellent yields. This catalyst could be reused several times without loss of activity. Characterization of the catalyst was performed using Fourier transform infrared, energy‐dispersive X‐ray and atomic absorption spectroscopies, X‐ray diffraction, thermogravimetric analysis, and scanning and transmission electron microscopies.     

Bimetallic Au/Pd catalyzed aerobic oxidation of alcohols in the poly(ethylene glycol)/CO2 system

Bimetallic Au/Pd nanoparticles were prepared and used to catalyze oxidation of alcohols in the poly(ethylene glycol) (PEG)/CO₂ biphasic system using O₂ as the oxidant without adding any base. The catalytic activity of Au/Pd bimetal with different mole ratios was studied using benzyl alcohol as the substrate. It was found that bimetallic Au/Pd nanoparticles with Au:Pd=1:3.5 had higher catalytic activity than monometallic Au, Pd and the bimetallic Au/Pd nanoparticles with other molar ratios. The effect of CO₂ pressure on the oxidation of benzyl alcohol and 1-phenylethanol in PEG/CO₂ was investigated. It was demonstrated that CO₂ pressure could be used to tune the conversion and selectivity of the reactions effectively. α,β,-Unsaturated alcohols were also studied and found to be more reactive than benzyl alcohol and 1-phenylethanol. Recycling experiments showed that the Au/Pd/PEG/CO₂ catalytic system could be recycled at least four times without reducing the activity. In addition, the catalytic system is clean and the products can be separated easily.