Synthesis of Au, Au/Ag, Au/Pt and Au/Pd nanoparticles using the microwave-polyol method

Gold, Au/Ag, Au/Pt and Au/Pd bimetallic nanoparticles with varying mol fractions were synthesized in ethylene glycol and glycerol, using the microwave technique in the presence of a stabilizer poly(N-vinylpyrrolidone) (PVP). It was found that bimetallic colloids of Au/Ag, Au/Pd and Au/Pt form an alloy either on co-reduction of respective metal ions or on mixing individual sols.     

Formation of Pd/Au nanostructures from Pd nanowires via galvanic replacement reaction

Bimetallic nanostructures with non-random metal atoms distribution are very important for various applications. To synthesize such structures via benign wet chemistry approach remains challenging. This paper reports a synthesis of a Au/Pd alloy nanostructure through the galvanic replacement reaction between Pd ultrathin nanowires (2.4 +/- 0.2 nm in width, over 30 nm in length) and AuCl3 in toluene. Both morphological and structural changes were monitored during the reaction up to 10 h. Continuous changes of chemical composition and crystalline structure from Pd nanowires to Pd68Au32 and Pd45Au55 alloys, and to Au nanoparticles were observed. More interestingly, by using combined techniques such as high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), energy dispersive X-ray spectrometry (EDS), UV-vis absorption, and extended X-ray absorption fine structure (EXAFS) spectroscopy, we found the formation of Pd68Au32 non-random alloy with Au-rich core and Pd-rich shell, and random Pd45Au55 alloy with uniformly mixed Pd and Au atom inside the nanoparticles, respectively. Density functional theory (DFT) calculations indicated that alkylamine will strongly stabilize Pd to the surface, resulting in diffusion of Au atoms into the core region to form a non-random alloy. We believe such benign synthetic techniques can also enable the large scale preparation of various types of non-random alloys for several technically important catalysis applications.     

Adsorption and Dissociation of Methanol on Pd(111), Pd/Au(111) and Pd/Rh(111) Surfaces

The adsorption and dissociation behaviors of methanol on Pd(111), Pd/Au(111) and Pd/Rh(111) surfaces were studied using a periodical slab model and the PW91 generalized gradient approximation(GGA) within the framework of first-principles calculations based on density functional theory(DFT). The adsorption energy and geometric parameters for the three surfaces showed that methanol is preferentially adsorbed onto the top-Pd sites and that the adsorption energy of methanol on these surfaces decreases in the order Pd/Au(111) Pd/Rh(111) Pd(111). After adsorption, the C–O, C–H and O–H bonds in methanol adsorbed onto these surfaces are elongated and the vibrational stretching frequency of the O–H bond is obviously redshifted. Furthermore, the first step for the possible dissociation pathway for methanol on these surfaces was calculated. Our results indicate that the O–H bond in methanol decomposes producing methoxy and a hydrogen atom, with the Pd/Au(111) surface exhibiting the smallest dissociation barrier.     

Biodeposited Pd/Au bimetallic nanoparticles as novel Suzuki catalysts

The use of two nanoparticulate palladium based catalysts in the Suzuki reaction is described. One monometallic (Pd) and one bimetallic (Pd/Au) catalyst were prepared by the environmentally benign method of bioreductive precipitation by Shewanella oneidensis. Both catalysts successfully mediated the Suzuki coupling, however, the Au doped catalyst was shown to deliver more reproducible results with a broader reaction scope.     

Monitoring Stability and Sizes of Au/Pd Core/Shell Nanoparticles by SEC

This paper describes how size exclusion chromatography (SEC) can be used to rapidly characterize Au/Pd core/shell nanoparticles (NPs). We monitored the sizes of Au/Pd core/shell NPs by effecting SEC separation using a mobile phase of 10 mM sodium dodecyl sulfate (SDS); the plot of retention time with respect to the standard size of the Au NPs was linear (R ² = 0.991) for diameters falling in the range from 12.1 to 59.9 nm; for five consecutive runs, the relative standard deviations of these retention times were less than 0.4%. Under the optimized separation conditions, we found that the addition of the surfactant SDS stabilized the Au/Pd core/shell NP samples. In addition, SEC analysis revealed that the sizes of the Au/Pd core/shell NPs could be controlled via modification of the rate of addition of the reducing agent and the use of adequate volumes of the seed and shell precursor metal ion solutions. When using these conditions to analyze the Au/Pd core/shell NPs produced through seed-assisted synthesis, a good correlation existed between the sizes determined through SEC and transmission electron microscopy. Our results suggest that SEC is a useful technique for monitoring the sizes of NPs and nanomaterials in general.     

The binary alloy system Au/Pd; a SIMS study

Sputtering yields, positive secondary ion yields and relative degrees of ionization of sputtered atoms under bombardement of 10keV O₂⁺-ions have been determined for the single-phase binary alloy system Au/Pd. A weak minimum of the total sputtering yields was observed at a Pd-concentration around 75% atomic.Strong surface topography was found after high-dose ion bombardment. Sputtering yields, together with surface enrichment factors determined by Auger Electron Spectroscopy have been used to calculate surface binding energies for both alloy components.The apparent degree of ionization for positive ions shows a different behaviour for the two alloy components; whereas an almost constant value was found for Au, a distinct maximum at a Pd-concentration of about 40% was observed for Pd. The measurements indicate that, in the concentration range investigated, the concept of constant relative sensitivity factors would yield large analytical errors when applied to the quantization of secondary ion mass spectra of Au/Pd alloys.     

Reduced Pd density of states in Pd/SAM/Au junctions: the role of adsorbed hydrogen atoms

Experiments have shown that a Pd monolayer deposited electrochemically on a Au-supported self-assembled monolayer (SAM) of 4-mercaptopyridine (Mpy) exhibits a strongly reduced Pd local density of states (LDOS) at the Fermi energy (E(f)). Understanding the origin of this modified electronic structure is crucial for the use of the sandwich design as a platform for future nanoelectronics. Here we suggest that hydrogen adsorption might be the origin of the modified electronic properties. We performed periodic density functional theory calculation to explore the influence of hydrogen adsorption on the geometric and electronic structure of a Pd/Mpy/Au(111) complex. Dissociative adsorption of H(2) on a Pd monolayer on top of a Mpy SAM is a strongly exothermic process leading to atomic hydrogen atoms preferentially located at the hollow sites. Due to the formation of a strong Pd-H bond the Pd-SAM interaction realized via one-fold N-Pd bonds is substantially weakened. Upon hydrogen adsorption, the Pd LDOS becomes significantly modified exhibiting a drastic reduction of the density of states at E(f). The calculated spectra are in a good agreement with the experiment for a hydrogen coverage corresponding to two monolayers which is still thermodynamically allowed.     

Augmented formic acid electro-oxidation at a co-electrodeposited Pd/Au nanoparticle catalyst

In this study, the formic acid electro-oxidation reaction (FAEOR) was catalyzed on a Pd-Au co-electrodeposited binary catalyst. The kinetics of FAEOR were intensively impacted by changing the Pd²⁺:Au³⁺ molar ratio in the deposition medium. The Pd₁-Au₁ catalyst (for which the Pd²⁺:Au³⁺ molar ratio was 1:1) acquired the highest activity with a peak current density for the direct FAEOR (I_p) of 4.14 mA cm^?2 (ca. 13- times higher than that (ca. 0.33 mA cm^?2) of the pristine Pd₁-Au₀ catalyst). It also retained the highest stability that was denoted in fulfilling ca. 0.292 mA cm^?2 (ca. 19-times higher than 0.015 mA cm^?2 of the pristine Pd₁-Au₀ catalyst) after 3600 s of continuous electrolysis at 0.05 V. The CO stripping and impedance measurements confirmed, respectively, the geometrical and electronic enhancements in the proposed catalyst.     

纳米团簇Au19Pd和Au19Pt催化解离N2O

采用密度泛函理论研究Au-Pd和Au-Pt 纳米团簇催化解离N₂O. 首先根据计算得到Au₁₉Pd和Au₁₉Pt 团簇的最优构型(杂原子均位于团簇的表面). 以Au₁₉Pd催化解离N₂O为例研究催化解离的反应机理. 对此主要考虑两个反应机理, 分别是Eley-Rideal (ER)和Langmuir-Hinshelwood (LH). 第一个机理中N₂O解离的能垒是1.118 eV, 并且放热0.371 eV. N₂分子脱附后, 表面剩余的氧原子沿着ER路径消除需要克服的能垒是1.920eV, 这比反应沿着LH路径的能垒高0.251 eV. 此外根据LH机理, 氧原子在表面的吸附能是-3.203 eV, 而氧原子在表面转移所需的能垒是0.113 eV, 这表明氧原子十分容易在团簇表面转移, 从而促进氧气分子的生成. 因此, LH为最优反应路径. 为了比较Au₁₉Pd和Au₁₉Pt 对N₂O解离的活性, 根据最优的反应路径来研究Au₁₉Pt 催化解离N₂O, 得到作为铂族元素的铂和钯对N₂O的解离有催化活性, 尤其是钯. 同时, 将团簇与文献中的Au-Pd合金相比较, 得到这两种团簇对N₂O 解离有较高的活性, 尤其是Au₁₉Pd团簇. 再者, O₂的脱附不再是影响反应的主要原因, 这可以进一步提高团簇解离N₂O的活性.     

Using Size-Exclusion Chromatography to Evaluate Changes in the Sizes of Au and Au/Pd Core/Shell Nanoparticles Under Thermal Treatment

In this study, we used size-exclusion chromatography (SEC) to evaluate the sizes of Au and Au/Pd core/shell nanoparticles (NPs) that had been subjected to thermal treatment, with the eluted NPs monitored through diode array detection (DAD) of the surface plasmon (SP) absorption of the NPs. In the absence of an adequate stabilizer, thermal treatment resulted in longer retention times for the Au NPs and shorter retention times for the Au/Pd core/shell NPs in the SEC chromatograms. Thus, thermal treatment influenced the sizes of these Au and Au/Pd core/shell NPs, through digestive ripening and Ostwald-type growth, respectively. In addition, the trends in the SP absorption phenomena of the NPs in the eluted samples, as measured using DAD, were consistent with the trends of their size variations, as measured from their elution profiles. In the presence of 3A-amino-3A-deoxy-(2AS,3AS)-β-cyclodextrin (H₂N-β-CD) as a stabilizer, the retention times and SP absorptions of the eluted Au and Au/Pd NP samples remained constant. Thus, H₂N-β-CD is a good stabilizer against size variation duration the thermal treatment of Au and Au/Pd core/shell NPs. A good correlation existed between the sizes obtained using SEC and those provided by transmission electron microscopy. Therefore, this SEC strategy is an effective means of further searching for suitable stabilizers for NPs, especially those exposed to harsh reaction conditions (e.g., in catalytic reactions).

     

Using Size-Exclusion Chromatography to Evaluate Changes in the Sizes of Au and Au/Pd Core/Shell Nanoparticles Under Thermal Treatment

In this study, we used size-exclusion chromatography (SEC) to evaluate the sizes of Au and Au/Pd core/shell nanoparticles (NPs) that had been subjected to thermal treatment, with the eluted NPs monitored through diode array detection (DAD) of the surface plasmon (SP) absorption of the NPs. In the absence of an adequate stabilizer, thermal treatment resulted in longer retention times for the Au NPs and shorter retention times for the Au/Pd core/shell NPs in the SEC chromatograms. Thus, thermal treatment influenced the sizes of these Au and Au/Pd core/shell NPs, through digestive ripening and Ostwald-type growth, respectively. In addition, the trends in the SP absorption phenomena of the NPs in the eluted samples, as measured using DAD, were consistent with the trends of their size variations, as measured from their elution profiles. In the presence of 3A-amino-3A-deoxy-(2AS,3AS)-??-cyclodextrin (H₂N-??-CD) as a stabilizer, the retention times and SP absorptions of the eluted Au and Au/Pd NP samples remained constant. Thus, H₂N-??-CD is a good stabilizer against size variation duration the thermal treatment of Au and Au/Pd core/shell NPs. A good correlation existed between the sizes obtained using SEC and those provided by transmission electron microscopy. Therefore, this SEC strategy is an effective means of further searching for suitable stabilizers for NPs, especially those exposed to harsh reaction conditions (e.g., in catalytic reactions).     

Size-controlled synthesis of Au/Pd octopods with high refractive index sensitivity

Au/Pd octopods, nanostructures with eight branches and a primarily Au interior, have been synthesized as size-controlled samples through the manipulation of seed-mediated co-reduction. The position of their localized surface plasmon resonance can be controllably tuned throughout the visible and near-infrared regions, and this response is correlated with the structural features (branch length and tip width) of the octopods. These Au/Pd octopods were also found to be highly sensitive to changes in the local refractive index of the surrounding media and suitable substrates for surface enhanced Raman spectroscopy. These findings, coupled with their unique composition, highlight the multifunctional capabilities of the Au/Pd octopods and provide insight into the optical properties of architecturally controlled bimetallic nanostructures.     

Electrochemical oxidation of methanol using dppm-bridged Ru/Pd, Ru/Pt and Ru/Au catalysts

The electrochemical oxidation of methanol was carried out using a series of dppm-bridged Ru/Pd, Ru/Pt and Ru/Au heterobimetallic complexes as catalysts. The major oxidation products were formaldehyde dimethyl acetal (dimethoxymethane, DMM) and methyl formate (MF). The Ru/Pd and Ru/Pt bimetallic catalysts generally afforded lower product ratios of DMM/MF and higher current efficiencies than the Ru/Au catalysts. The Ru/Au bimetallics exhibited product ratios and current efficiencies similar to those obtained from the Ru mononuclear compound CpRu(PPh(3))(2)Cl. Increasing the methanol concentration afforded higher current efficiencies, while the addition of water to the samples shifted the product distribution toward the more highly oxidized product, MF.     

Atomically Dispersed Pt on Screw-like Pd/Au Core-shell Nanowires for Enhanced Electrocatalysis

Engineering noble metal nanostructures at the atomic level can significantly optimize their electrocatalytic performance and remarkably reduce their usage. We report the synthesis of atomically dispersed Pt on screw-like Pd/Au nanowires by using ultrafine Pd nanowires as seeds. Au can selectively grow on the surface of Pd nanowires by an island growth pattern to fabricate surface defect sites to load atomically dispersed Pt, which can be confirmed by X-ray absorption fine structure measurements and aberration corrected HRTEM images. The nanowires with 2.74 at % Pt exhibit superior HER properties in acidic solution with an overpotential of 20.6 mV at 10 mA cm⁻² and enhanced alkaline ORR performance with a mass activity over 15 times greater than the commercial platinum/carbon (Pt/C) catalysts.     

Pd/C promoted by Au for 2-propanol electrooxidation in alkaline media

Pd/C catalysts promoted by Au are investigated as electrocatalysts for the direct 2-propanol fuel cells in alkaline media. The results show that Pd is a good electrocatalyst for 2-propanol oxidation and the activity for 2-propanol electrooxidation is higher than that for methanol electrooxidation on the Pd/C electrocatalysts in alkaline media. Addition of Au can significantly increase the palladium catalytic activity and stability for the 2-propanol oxidation. PdAu4:1/C has higher electrocatalytic activity and better stability for the electrooxidation of 2-propanol than Pd/C and E-TEK Pt/C electrocatalysts. The present study shows the promising properties of Au promoted Pd/C as effective electrocatalysts for 2-propanol fuel based direct alcohol fuel cells.     

First principles study of oxygen adsorption and dissociation on the Pd/Au surface alloys

The formation of Pd/Au surfaces and their catalytic performance toward oxygen dissociation were investigated using periodic density functional methods. We show that Pd can readily incorporate into the second layer of Au(100) and Au(111) substrates with the assistance of Au vacancies. Pd/Au(100) exhibits better catalytic activity toward oxygen dissociation than Pd/Au(111). Specifically, the sub-layer Pd atoms of Pd/Au(100) can promote the oxygen dissociation and stabilize the surface structure after adsorbing oxygen atoms. On the contrary, the sub-layer Pd atoms of Pd/Au(111) slightly hinder the oxygen dissociation.     

Fabrication of monometallic (Co,Pd, Pt,Au) and bimetallic (Pt/Au,Au/Pt) thin films with hierarchical architectures as electrocatalysts

Co thin films with novel hierarchical structures were controllably fabricated by simple electrochemical deposition in the absence of hard and soft templates, which were used as sacrificial templates to further prepare noble metal (Pd, Pt, Au) hierarchical micro/nanostructures via metal exchange reactions. SEM characterization demonstrated that the resulting noble metal thin films displayed hierarchical architectures. The as-prepared noble metal thin films could be directly used as the anode catalysts for the electro-oxidation of formic acid. Moreover, bimetallic catalysts (Pt/Au, Au/Pt) fabricated based on the monometallic Au, Pt micro/nanostructures exhibited the higher catalytic activity compared to the previous monometallic catalysts.     

Au/Pd(111)双金属表面催化噻吩加氢脱硫的反应机理

采用密度泛函理论(DFT)计算了Pd(111)表面含有N(N=1-4)个Au原子数目时的表面形成能,选取最优构型进一步研究了噻吩在Au/Pd(111)双金属表面的吸附模式及加氢脱硫反应过程.结果表明:当Pd(111)表面含有1个Au原子时,其形成能最低.在Au/Pd(111)双金属表面噻吩初始吸附于Pd-Hcp-30°位时,其构型最稳定.在各加氢脱硫过程中,反应总体均放出热量.对于直接脱硫机理,其所需活化能较低,但脱硫产物较难控制;对于间接脱硫机理,反应最有可能按照顺式加氢方式进行,C―S键断裂开环时所需活化能最高,是反应的限速步骤.此外,与单一Au(111)面及Pd(111)面相比,Au/Pd(111)双金属表面限速步骤的反应能垒最低,表明AuPd双金属催化剂比Au、Pd单金属催化剂更有利于噻吩加氢脱硫反应的进行.     

Highly Ordered Macroporous Au and Pd by Colloidal Crystal Templating

The highly ordered macroporous Au and Pd with regular arrays of spherical pores have been synthesized by poly (styrene-co-acrylic) (PSA) colloidal crystal template. The pore size is tuneable in the range of 100-400 nm according to the size of PSA latex. The mechanism is based on the in-situ impregnating and reducing of metal ions in the interspaces of the PSA spheres then removing the template.     

Microporous nanocomposites of Pd and Au nanoparticles via hierarchical self-assembly

Microporous nanocomposites of Pd and Au nanoparticles were generated by utilizing electrostatic interaction between oppositely charged Au nanoparticles coated with carboxylate groups (Au-COO-) and spherical aggregates of Pd nanoparticles (Pd- NH3+) with a mean diameter of 80+/-20 nm stabilized and cross linked by octa(aminopropyl)silsesquioxane octahydrochloride (POSS-NH3+). Amide bonds were formed between the reactive ion couples that are well defined in the Pd-Au colloidal nanocomposites during a subsequent chemical reaction to generate more stable nanocomposites with improved chemical and physical properties.