Addition Of Au Research Articles

Gold modified cobalt-based Fischer-Tropsch catalysts for conversion of synthesis gas to liquid fuels

The addition of Au as a promoter/modifier for alumina supported Co catalyst has been studied by combined in-situ high temperature, high pressure Fourier transform infrared (FTIR) and on-line gas chromatography. The combination of these tools permitted the state of the active catalyst surface to be monitored while following the elution of reaction products during the first 5–7 h on stream of the catalyst. The catalysts under study were a 10%Co/Al2O3 and a 2.5%Au/10%Co/Al2O3. Samples were characterised before use using Raman and temperature programmed reduction (TPR). During the initial stages of reaction, hydrocarbons were built up on the surface of the catalyst as monitored by FTIR and the nature and amount of these species were assessed in terms of CH2/CH3 ratio and the density of these alkyl fragments by employing absorption coefficients for the individual components. The nature and reducibility of the Co particles were modified by the presence of Au while the later also shifted the CO/H2 balance by acting as an effective water-gas shift catalyst during the early stages of reaction. This characteristic was lost during reaction as a consequence of redistribution of the two metallic phases.

Densification of tantalum carbide ceramics with 5 mol.% Al, Cu, Ag and Au

The effects of the addition of 5 mol.% aluminum, copper, silver or gold on the densification, microstructures, mechanical properties and oxidation resistance in thin air of TaC ceramics were investigated. Full density and relatively high flexural strength (up to 738 MPa) were observed in the case of the Au addition after spark plasma sintering at 1700 °C for 5 min under 30 MPa pressure. Due to wetting of the TaC grains by a thin metallic film, the consolidated materials showed enhanced oxidation resistance against thin air.

Influence of Au addition on magnetic properties of iron oxide in a silica–phosphate glass matrix

The influence of gold particle addition on structural and magnetic ordering of iron oxide in a glass matrix was studied. The silica–phosphate glasses containing iron oxide and Au were prepared by the melt quench technique. Evolution of crystalline phases was studied by X-ray diffraction (XRD). Magnetic properties were investigated by means of a Superconducting Quantum Interference Device (SQUID) magnetometer and room temperature Mössbauer spectroscopy. The microstructure exhibited the formation of 30–40nm size particles. The samples showed the formation of magnetite and hematite as major crystalline phases. Magnetic studies revealed the relaxation of magnetic particles. Blocking temperature of investigated sample increased with an increase of Au content implying an increase in the strength of magnetic interactions. Mössbauer spectroscopy has shown the presence of both doublets and magnetic sextets. The magnetization value increased as Au content was increased, which is attributed to the increase in magnetic and structural ordering.

TiO2–Au composite for efficient UV photocatalytic reduction of Cr(VI)

TiO2 at the nanoscale perform well in the UV photocatalytic application depending on the particle size; a combination of TiO2 and Au may improve the situation. TiO2–Au composites are thus synthesized and examined in the UV photocatalytic reduction of Cr(VI). Results show that the photocatalytic efficiency of TiO2–Au reaches 91% in Cr(VI) reduction compared with the pure TiO2 (83%) under UV light irradiation. The increased light absorption intensity in a broader spectral range as well as the rate of electron-hole pair recombination in TiO2 with the addition of Au shall be responsible for the enhanced photocatalytic ability of the composite.

Enhancement of electrochemical properties of Pd/C catalysts toward ethanol oxidation reaction in alkaline solution through Ni and Au alloying

The effect of Au and/or Ni addition on the ethanol oxidation reaction (EOR) performance in alkaline media of Pd-based binary and ternary catalysts (Pd3Au/C, Pd3Ni/C, and Pd3AuNi/C) is systematically elucidated. The EOR activities, structures, morphologies, surface compositions and surface species of the prepared catalysts are analyzed by cyclic voltammetry, X-ray diffraction and X-ray absorption spectroscopy, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and temperature-programmed reduction, respectively. It is observed that the surface Ni with the chemical state of NiOOH can promote the EOR through bi-functional mechanism and spillover while surface Au can modify the Pd lattice and electron configuration which is helpful for the absorption of ethanol molecular. Chronoamperometric (CA) results obtained at room temperature demonstrate that the mass current density of ternary Pd3AuNi/C catalysts after the long-term EOR test for 4 h is about 1.39 and 1.10 times higher than that of the monometallic Pd/C and binary Pd3Au/C catalysts, respectively. It is proposed that the EOR stability enhancement of Pd3AuNi can be attributed to the synergistic effect of Ni and Au alloying.

Highly active PdAu alloy catalysts for ethanol electro-oxidation

Alloyed PdmAu nanoparticles (m refers to the atomic Pd/Au ratio), prepared through the co-reduction of K2PdCl4 and HAuCl4 precursors in aqueous solutions, are employed as the catalysts for ethanol electro-oxidation reaction (EOR) in alkaline electrolyte. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) are used to characterize the morphology and confirm the alloy structure of the samples. Cyclic voltammetry (CV) and chronoamperometry (CA) results indicate that the addition of Au to Pd can significantly improve the catalytic activity of Pd toward EOR and the improvement is strongly dependent on the composition of the catalysts. Pd1.0Au/C is identified as the most efficient catalyst since it produces the highest catalytic activity and long-term stability. The mass-specific activity (MSA) and intrinsic activity (IA) data of Pd1.0Au/C is 1.45 mA μg−1Pd+Au and 80.2 A m−2Pd, which is found to be 2.3 and 11.3 times that of the Pd/C catalyst, respectively. These findings clearly suggest that the proximity and relative amount of Au and Pd on the surface of the nanoparticles (NPs) are crucial for the improvement of Pd catalysis, which would have important implications for designed preparation of bimetallic catalysts for the direct alcohol fuel cells.

Methane dehydrogenation on Au/Ni surface alloys – a first-principles study

We present density-functional theory calculations of the dehydrogenation of CHx (x = 1–4) on Au-alloyed Ni(211) surfaces, where the Au atoms are substituted on the Ni surface with the ratio of Au atoms to the total stepped Ni atoms being 1:4, 1:2 and 3:4, respectively. To evaluate the role of Au at the step-edge on the process of methane dehydrogenation, CHx adsorption and dissociation on a pure Ni(211) surface is also conducted. Our results show that Au addition weakens the adsorbate–substrate interaction. With the increase of the Au concentration, the binding energies of CHx gradually decrease and correlate well with the number of Au atoms on each model. On the Ni(211) surface, methane experiences a successive dehydrogenation process at the step-edge site in which carbon is eventually formed. As Au is introduced, the relative formation rate of carbon is greatly hampered even with a small amount of Au addition, while an appropriate amount of Au modification on the Ni catalyst has little effect on the activity of the CHx dissociation. Finally, we also demonstrate that the active center for CHx dissociation is dynamic with the variation of the Au concentration.

Au microparticles mediated construction of a logic based dual channel molecular keypad lock

New melamine based ligand with pyrene as a fluorophore is synthesized and characterized. The sequential addition of Au(3+) ions and ascorbic acid to the solution of in EtOH-H2O (9.5 : 0.5 v/v) generates gold microparticles (AuMPs). However, the particle size distribution and fluorescence behaviour on sequential additions are different and contribute to the construction of a new logic based molecular dual channel keypad lock system.

Carbon-supported AuPd bimetallic nanoparticles synthesized by high-energy electron beam irradiation for direct formic acid fuel cell

Nanoparticle catalysts of carbon-supported Pd (Pd/C) and carbon-supported AuPd (AuPd/C) for the direct formic acid fuel cell (DFAFC) anode were synthesized by the reduction of precursor ions in an aqueous solution irradiated with a high-energy electron beam. We obtained three kinds of nanoparticle catalysts: (1) Pd/C, (2) AuPd/C of the core–shell structure, and (3) AuPd/C of the alloy structure. The structures of AuPd nanoparticles were controlled by the addition of citric acid as a chelate agent, and sodium hydroxide as a pH controller. The structures of nanoparticle catalysts were characterized using transmission electron microscopy, inductively coupled plasma atomic emission spectrometry, the techniques of X-ray diffraction and X-ray absorption fine structure. The catalytic activity of the formic acid oxidation was evaluated using linear sweep voltammetry. The oxidation current value of AuPd/C was higher than that of Pd/C. This indicated that the addition of Au to Pd/C improved the oxidation activity of the DFAFC anode. In addition, the AuPd/C of the alloy structure had higher oxidation activity than the AuPd/C of the core–shell structure. The control of the AuPd mixing state was effective in enhancing the formic acid oxidation activity.

Oxidative halogenation of dinuclear N-heterocyclic dicarbene gold(I) complexes

Cl2, Br2, and I2 are effective agents for the oxidative addition of carbene-coordinated Au(I) centres in [Au2(MeIm-xylylene-ImMe)2](PF6)2. The resulting complexes depend on the type of xylylene bridge between the two carbene units. With m- and p-xylylene, the obtained products are always the Au(III)/Au(III) complexes [Au2X4(MeIm-m,p-xylylene-ImMe)2](PF6)2 (X = Cl, Br, I). By contrast, with the o-xylylene bridge only in the reaction with iodine the Au(III)/Au(III) complex is isolated as the unique compound, while with chlorine or bromine a mixture of two complexes, the usual Au(III)/Au(III) and the Au(II)/Au(II) [Au2X2(MeIm-o-xylylene-ImMe)2](PF6)2 (X = Cl, Br), is obtained. This different behaviour is attributable to a greater tendency of o-xylylene bridge to favour an approach of the gold centres compared to the m- and p- analogues.

First‐Principles Study toward CO Adsorption on Au/Ni Surface Alloys

The introduction of a second metal, gold, into a nickel matrix can effectively improve the catalytic performance and thermal stability of the catalysts toward steam reforming of methane. To investigate the effect of Au on the adsorption properties and electronic structure of the Ni(111) surface, we chose CO as a probe molecule and examined CO adsorption on various Au/Ni surfaces. It was revealed that Au addition weakened the absorbate-substrate interactions on the Ni(111) surface. With increasing gold concentration, the binding energy declines further. The variation of the binding energies has been interpreted by exploring the electronic structure of surface nickel atoms. The effect of gold can be quantitatively characterized by the slopes of the fitting equations between the binding energy and the number of gold atoms surrounding the adsorption site. Our results show that the binding energy at top sites can be approximately estimated by counting the number of surrounding gold atoms. On one specific surface, the relative magnitude of the binding energy can be simply judged by the distance between gold and the geometrical center of the adsorption site. This empirical rule holds true for C, H, and O adsorption on the Au/Ni surface. It may be applicable to a system in which a doped atom of larger atomic size is incorporated into the host metal surface by forming a surface alloy.

Highly Fluorescent Graphene Oxide-Poly(vinyl alcohol) Hybrid: An Effective Material for Specific Au3+Ion Sensors

We have developed a new highly fluorescent graphene oxide (GO)/poly(vinyl alcohol) (PVA) hybrid (GO-PVA) in an acidic medium (pH 4). Fourier transform infrared (FTIR) spectra indicate the formation of hydrogen bonds between the hydroxy group of PVA and the hydroxy groups of GO. The hybrid is highly fluorescent, because of passivation by hydrogen bonding, as evident from Raman spectra. The quantum yields of GO-PVA hybrids are higher than that of GO. The fluorescent microscopic images of the hybrids exhibit a fibrillar morphology, and all of them emit highly intense green light. Field-emission scanning electron microscopy (FESEM) micrographs also show a fibrillar morphology, which is produced due to the supramolecular organization of GO-PVA complex. The highly fluorescent GO-PVA1 hybrid has been used as a fascinating tool for selective sensing of Au(3+) ions in aqueous media with a detectable limit of ~275 ppb. The sensitivity of the Au(3+) ion (300 μM) in the presence of 600 μM concentrations of each ion (Cu(2+), Ag(+), Mg(2+), Ca(2+), Zn(2+), K(+), Pb(2+), Co(2+), Ni(2+), Pd(2+), Fe(2+), Fe(3+), and Cr(3+)), taken together, is unique, exhibiting a quenching efficiency of 76%. The quenching efficiency in the presence of a biologically analogous mixture (d-glucose, d-lysine, BSA, Na(+), K(+), Ca(2+), Mg(2+), Zn(2+)) (600 μM each) is 73%, which suggests that the GO-PVA1 hybrid is an efficient sensor of Au(3+) ions. The average lifetime of GO at pH 4 increases in the GO-PVA1 hybrid, indicating the formation of a more stable excited state but the increase in lifetime value after addition of Au(3+) salt solution to the hybrid solution indicates dynamic quenching. The selectivity of sensing of Au(3+) is attributed to its reduction potential being higher than that of other metal ions and XPS data of GO-PVA1 hybrid with 300 μM Au(3+) substantiate the reduction of Au(3+) to Au(0), because of the transfer of excitons from the hybrid facilitating the selective photoluminescence (PL) quenching.

Enhancing the photocatalytic properties of TiO2 by coupling with carbon nanotubes and supporting gold

The photodegradation of methylene blue in aqueous solutions is studied using various photocatalysts, including neat TiO2, CNT–TiO2, Au–TiO2, and Au–CNT–TiO2 composites MB. Materials were synthesized and extensively characterized by XRD, TEM, DRFIT spectroscopy, N2 adsorption–desorption isotherms, as well as diffuse reflectance UV–vis spectroscopy. By using CNT–TiO2 composite as catalysts, it was found that CNT act as adsorbent and photosensitizer to improve the photoactivity of neat TiO2. Among the CNT–TiO2 composites with different CNT weight ratio (0.2–20%), the 2%CNT–TiO2 shows the best photoactivity. When CNT content is larger than 2%, the surplus CNT may absorb and scatter light photons. Combined with the decrease of TiO2 amount in composite, the photoactivity is reduced. To further improve the photoactivity of 2%CNT–TiO2, different Au loads varying from 0.25% to 1% were introduced by the deposition–precipitation method. The 0.25%Au–2%CNT–TiO2 composite had the highest photoactivity. The increase in activity was explained by the surface plasmon resonance of Au that makes the composite to absorb more photons than the 2%CNT–TiO2, thus overcoming the disadvantages of surplus CNT addition. On the other hand, 0.25%Au–2%CNT–TiO2 composite also presents higher activity than 0.25%Au–TiO2 due to higher adsorption capacity provided by CNT introduction. The addition of CNT and Au simultaneously has a much stronger synergic role than when each of them is introduced individually.

Effects of Au and Pd Additions on Joint Strength, Electrical Resistivity, and Ion-Migration Tolerance in Low-Temperature Sintering Bonding Using Ag2O Paste

A new bonding process using an Ag2O paste consisting of Ag2O particles mixed with a triethylene glycol reducing agent has been proposed as an alternative joining approach for microsoldering in electronics assembly, which currently uses Pb-rich, high-temperature solders. Ag nanoparticles were formed at approximately 130°C to 160°C through a reduction process, sintered to one another immediately, and bonded to a metal substrate. An Au-coated Cu specimen was successfully bonded using the Ag2O paste. The resulting joint exhibited superior strength compared with joints fabricated using conventional Pb-rich solders. To improve ion-migration tolerance, the Ag2O paste was mixed with Au and Pd microparticles to form sintered Ag-Au and Ag-Pd layers, respectively. The additions of Au and Pd improved the ion-migration tolerance of the joint. Regarding the mechanical properties of the joints, addition of secondary Au and Pd both resulted in decreased joint strength. To match the joint strength of conventional Pb-10Sn solder, the mixing ratios of Au and Pd were estimated to be limited to 16 vol.% and 7 vol.%, respectively. The electrical resistivities of the sintered layers consisting of 16 vol.% Au and 7 vol.% Pd were lower than that of Pb-10Sn solder. Thus, the additive fractions of Au and Pd to the Ag2O paste should be less than 16 vol.% and 7 vol.%, respectively, to avoid compromising the mechanical and electrical properties of the sintered layer relative to those of contemporary Pb-10Sn solder. Following the addition of Au and Pd to the paste, the ion-migration tolerances of the sintered layers were approximately 3 and 2 times higher than that of pure Ag, respectively. Thus, the addition of Au was found to improve the ion-migration tolerance of the sintered Ag layer more effectively and with less sacrifice of the mechanical and electrical properties of the sintered layer than the addition of Pd.

Effects of noble metal addition on response of ceria thick film CO sensors

Abstract Cerium oxide (ceria) CO sensors exhibit fast, but smaller response than tin dioxide CO sensors; therefore, improvement in the response of ceria CO sensors is required. The effects of noble metal addition (Pt, Ag, Pd, or Au nanoparticles) on the response of ceria thick film CO sensors were investigated. The ceria thick film sensors with Au nanoparticles exhibited good response to CO; the addition of 5 wt% Au caused a 40 times increase in the sensor response. In contrast, no change in response was observed for the addition of Au to tin dioxide sensors. The response of the ceria sensors with Au decreased in the presence of propane or ethane compared to that without the noble metal. The sensors with Ag nanoparticles showed an increase response in the first measurement, but this response decreased with each measurement. Pd addition to the ceria sensors resulted in uneven responses and no increase in resistance was observed, in contrast to the tin dioxide sensors of which the resistance increases by Pd addition. In the case of the ceria sensors with Pt nanoparticles, the response was approximately the same as the sensors without noble metal nanoparticles.

Carbon nanofiber supported bimetallic PdAu nanoparticles for formic acid electrooxidation

Carbon nanofiber (CNF) supported PdAu nanoparticles are synthesized with sodium citrate as the stabilizing agent and sodium borohydride as the reducing agent. High resolution transmission electron microscopy (HRTEM) characterization indicates that the synthesized PdAu particles are well dispersed on the CNF surface and X-ray diffraction (XRD) characterization indicates that the alloying degree of the synthesized PdAu nanoparticles can be improved by adding tetrahydrofuran to the synthesis solution. The results of electrochemical characterization indicate that the addition of Au can promote the electrocatalytic activity of Pd/C catalyst for formic acid oxidation and the CNF supported high-alloying PdAu catalyst possesses better electrocatalytic activity and stability for formic acid oxidation than either the CNF supported low-alloying PdAu catalyst or the CNF supported Pd catalyst.

Dehydrogenation of Propane to Propylene over Supported Model Ni–Au Catalysts

Hydrogenolysis and dehydrogenation of propane were studied over model nickel–gold catalysts. The supported model Ni–Au catalysts were prepared by depositing Ni and Au onto a planar silica film. Infrared reflection absorption spectroscopic data showed that isolated Ni sites appeared and became dominant on the surface with the addition of Au to Ni. For the conversion of propane in the presence of hydrogen, the dehydrogenation of propane to propylene was observed on the Ni–Au bimetallic catalysts, whereas only hydrogenolysis products were observed on the monometallic Ni catalyst. A correlation was found between the concentration of isolated Ni sites and the catalytic activity for propane dehydrogenation.

Enhanced electrochemical stability of PtRuAu/C catalyst synthesized by radiolytic process

Abstract

Enhanced Photodegradation of Toxic Pollutants on Plasmonic Au–Ag–AgI/Al2O3 Under Visible Irradiation

Au–Ag–AgI nanoparticles (NPs) were uniformly dispersed onto mesoporous alumina by deposition–precipitation and photoreduction at low temperature. Compared with Ag–AgI/Al2O3, the catalyst showed higher photocatalytic activity under visible irradiation. In particular, the release of metal ions from the catalyst was significantly inhibited during photodegradation of pollutants. Electron spin resonance and cyclic voltammograms studies under a variety of experimental conditions verified that the coupling of Au and Ag NPs increased the efficiency of light energy conversion and accelerated interfacial electron transfer processes. The main active species involved in the photoreaction of Au–Ag–AgI/Al2O3 were O2 •− and excited h+ on the metal NPs. The presence of several ubiquitous anions, including HCO3 −, SO4 2−, and NO3 − could act as electron donors trapping excited h+ on the metal NPs to facilitate electron circulation. The results obtained herein indicated that coupled, noble, bimetal NPs exhibited high photosensitivity and photostability due to enhanced surface plasmon resonance and interfacial electron transfer. Addition of Au made Au–Ag–AgI nanoparticles (NPs) disperse more uniformly onto mesoporous Al2O3 and showed higher photocatalytic activity under visible irradiation.

Microbial synthesis of core/shell gold/palladium nanoparticles for applications in green chemistry

We report a novel biochemical method based on the sacrificial hydrogen strategy to synthesize bimetallic gold (Au)–palladium (Pd) nanoparticles (NPs) with a core/shell configuration. The ability of Escherichia coli cells supplied with H2 as electron donor to rapidly precipitate Pd(II) ions from solution is used to promote the reduction of soluble Au(III). Pre-coating cells with Pd(0) (bioPd) dramatically accelerated Au(III) reduction, with the Au(III) reduction rate being dependent upon the initial Pd loading by mass on the cells. Following Au(III) addition, the bioPd–Au(III) mixture rapidly turned purple, indicating the formation of colloidal gold. Mapping of bio-NPs by energy dispersive X-ray microanalysis suggested Au-dense core regions and peripheral Pd but only Au was detected by X-ray diffraction (XRD) analysis. However, surface analysis of cleaned NPs by cyclic voltammetry revealed large Pd surface sites, suggesting, since XRD shows no crystalline Pd component, that layers of Pd atoms surround Au NPs. Characterization of the bimetallic particles using X-ray absorption spectroscopy confirmed the existence of Au-rich core and Pd-rich shell type bimetallic biogenic NPs. These showed comparable catalytic activity to chemical counterparts with respect to the oxidation of benzyl alcohol, in air, and at a low temperature (90°C).