γ-Radiation synthesis of ultrasmall noble metal (Pd,Au, Pt) nanoparticles embedded on boron nitride nanosheets for high-performance catalysis

The large-scale synthesis and the stabilization of noble metal nanocatalysts with ultrasmall sizes are crucial to their practical application. Herein, Pd, Au, and Pt nanoparticles (NPs) are in situ embedded on boron nitride nanosheets (BNNSs) via the reduction of precursor salts induced by ⁶⁰Co γ-irradiation, which is a mature technique widely used in industry. The reduction mechanism was elucidated by pulse radiolysis experiments. In comparison with other BN substrates like hexagonal BN (h-BN), BNNSs had not only large surface area of 263 m² g⁻¹, but also abundant hydroxyl groups produced during the sonicated exfoliation, resulting in ultrasmall sizes, large loading amounts, and uniform distribution of the anchored NPs. The particle sizes of the loaded Pd, Au, and Pt NPs were measured to be 2.9, 3.1, and 3.0 nm, respectively. As a consequence, in the model reaction of 2-nitroaniline reduction, the Pd-, Au- and Pt-BNNSs composite nanocatalysts showed much higher catalytic activities than those composites loading the corresponding NPs on other BN substrates. Particularly, most of the 2-nitroaniline could be reduced within merely 40 s when using Pd-BNNSs as catalysts, performing a rate constant of 1.1 × 10⁻¹ s⁻¹. The number of hydroxyl groups on BNNSs support as well as the catalytic performance of the corresponding NPs-loaded BNNSs composites could be improved by extending the sonicated exfoliation time, because more NPs were loaded and the hydroxyl groups were also proved to play an important role in enhancing the catalytic activity. Moreover, Pd-BNNSs catalysts still displayed prominent activity even after five cycles owing to the high stability of BNNSs and their strong affinity for PdNPs. This work not only demonstrates BNNSs as ideal supports to load noble metal NPs for catalysis, but also reveals the feasibility to synthesize and in situ embed ultrasmall noble metal NPs on BNNSs via γ-irradiation.     

Conducting polymer electrodes modified by metal tetrasulfonated phthalocyanines: Preparation and electrocatalytic behaviour towards dioxygen reduction in acid medium

Polypyrrole (Au/PPy) and polyaniline (Au/PAni) electrodes were prepared and their activities towards oxygen reduction in acid medium were examined. The insertion of iron or cobalt phthalocyanines into the conducting polymer during the electropolymerisation process was carried out and the modified electrodes were characterised by esr and uv-visible differential reflectance spectroscopies. The electrocatalytic behaviour of such electrodes towards oxygen reduction was examined.The influence of the central metal of the macrocycle and of the kind of polymer was investigated. It appears that the modified electrodes containing iron tetrasulfonated phthalocyanine are the most active ones but they are less stable than electrodes containing cobalt tetrasulfonated phthalocyanine. The comparison of the electrocatalytic behaviour of the Au/polymer-FeTsPc electrode with that of a bare platinum electrode towards oxygen reduction indicates that the reduction process is the same for both electrodes. The Au/polymer-FeTsPc electrode allows then to reduce the oxygen molecule mainly via the 4-electron process into water as main product.In the case of the Au/polymer-CoTsPc electrode, the role of the conducting polymer in the whole reduction process is demonstrated. The Au/PAni-CoTsPc electrode allows to reduce the oxygen molecule mainly via the 2-electron reaction into hydrogen peroxide, whereas the Au/PPy-CoTsPc electrode allows it to reduce into water via the hydrogen peroxide formation for potentials lower than 0.4 V rhe.     

Reaction of Au(111) with Sulfur and Oxygen: Scanning Tunneling Microscopic Study

The reaction of sulfur and oxygen with the gold surface is important in many technological applications, including heterogeneous catalysis, corrosion, and chemical sensors. We have studied reactions on Au(111) using scanning tunneling microscopy (STM) in order to better understand the surface structure and the origin of gold’s catalytic activity. We find that the Au(111) surface dynamically restructures during deposition of sulfur and oxygen and that these changes in structure promote the reactivity of Au with respect to SO₂ and O₂ dissociation. Specifically, the Au(111) herringbone reconstruction lifts when either S or O is deposited on the surface. We attribute this structural change to the reduction of tensile surface stress via charge redistribution by these electronegative adsorbates. This lifting of the reconstruction was accompanied by the release of gold atoms from the herringbone structure. At high coverage, clusters of gold sulfides or gold oxides form by abstraction of gold atoms from regular terrace sites of the surface. Concomitant with the restructuring is the release of gold atoms from the herringbone structure to produce a higher density of low-coordinated Au sites by forming serrated step edges or small gold islands. These undercoordinated Au atoms may play an essential role in the enhancement of catalytic activity of gold in reactions such as oxygen dissociation or SO₂ decomposition. Our results further elucidate the interaction between sulfur and oxygen and the Au(111) surface and indicate that the reactivity of Au nanoclusters on reducible metal oxides is probably related to the facile release of Au from the edges of these small islands. Our results provide insight into the sintering mechanism which leads to deactivation of Au nanoclusters and into the fundamental limitation in the edge definition in soft lithography using thiol-based self-assembled monolayers (SAMs) on Au. Furthermore, the enhanced reactivity of Au after release of undercoordinated atoms from the surface indicate a relatively insignificant role of an oxide support for high reactivity.     

Synthesis of metal–cadmium sulfide nanocomposites using jingle-bell-shaped core-shell photocatalyst particles

Photocatalytic deposition of gold (Au) and silver (Ag) nanoparticles was investigated using jingle-bell-shaped silica (SiO₂)-coated cadmium sulfide (CdS) nanoparticles (SiO₂/CdS), which each had a void space between the CdS core and SiO₂ shell, as a photocatalyst. A size-selective photoetching technique was used to prepare the jingle bell nanostructure of SiO₂/CdS. Nanoparticles of Au and Ag were deposited by irradiation of the photoetched SiO₂/CdS in the presence of the corresponding metal complexes under deaerated conditions. Chemical etching of Au-deposited particles enabled the selective removal of CdS without any influence on the surface-plasmon absorption of Au. TEM analyses of the resulting particles suggested that some particles were encapsulated in hollow SiO₂ particles, while other Au particles were deposited on the outer surface of the SiO₂ shell. Emission spectra of the photoetched SiO₂/CdS showed that the metal deposition developed a broad emission with a peak around 650 nm originating from surface defect sites, the degree being dependent on the kind of metal nanoparticles and their amount of deposition. This fact can be explained by the formation of metal–CdS binary nanoparticles having defect sites at the interface between metal and CdS.     

Studying the localized deposition of Ag nanoparticles on self-assembled monolayers by scanning electrochemical microscopy (SECM)

The local deposition of Ag nanoparticles (NPs) on ω-mercaptoalkanoic acid, HS(CH₂)_nCO₂H, (n = 2, 10) self-assembled monolayers (SAMs) by scanning electrochemical microscopy (SECM) is reported. We found that the presence of a SAM had a pronounced effect on Ag deposition. Experiments were conducted by applying different potentials to an Au(1 1 1) substrate either in the presence of a constant concentration of Ag⁺ ions in solution (bulk deposition) or by generating a flux of Ag⁺ from an Ag microelectrode that was positioned close to the Au(1 1 1) substrate (SECM deposition). SECM was used for generating a controlled flux of silver ions by anodic dissolution of an Ag microelectrode close to the SAMs modified Au(1 1 1). We found that the shape of the NPs was affected by the length of the carbon-chain of the SAM. Tetrahedral NPs were obtained on bare Au(1 1 1) surfaces while rod like and cubic Ag NPs were deposited onto 3-mercaptopropanoic acid (MPA) and 11-mercaptoundecanoic acid (MUA) SAMs, respectively. The size and shape of the deposited NPs were influenced by the deposition potential.We conclude that the shape and distribution of locally deposited Ag NPs on Au(1 1 1) can be controlled by modification of the substrate with a SAM and through controlling the Ag⁺ flux generated by SECM.     

Dynamic behavior of electroless nickel plating reaction on magnesium alloys

In order to obtain better control over the quality of electroless nickel–phosphorous (EN) coatings on magnesium AZ91D alloy, the effects of metal salt concentrations, reducing agent, pH, and temperature on deposition rate were studied. The reaction orders and activation energy of the deposition were determined. The results show that the apparent activation energy (E _a) in the EN plating reaction is approximately 38.03 kJ/mol. The deposition rate increased with increasing temperature, concentration of H₂PO₂ ⁻, and pH, and decreased with increasing concentration of complexing agents. For nickel ions, the deposition rate increased gradually with increasing concentration (x) when x < 4.69 g/L. However, it decreased when the concentration exceeded 4.69 g/L. Finally, various types of the deposition reaction were also discussed. The results indicate that nickel was first deposited by replacement deposition in the initial 0.5 min, then by both replacement deposition and autocatalytic deposition in plating, and finally by autocatalytic deposition after 5 min of plating.     

Nanodispersed Au/Al2O3 catalysts for low-temperature CO oxidation: Results of research activity at the Boreskov Institute of Catalysis

This paper presents some important results of the studies on preparation and catalytic properties of nanodispersed Au/Al₂O₃ catalysts for low-temperature CO oxidation, which are carried out at the Boreskov Institute of Catalysis (BIC) starting from 2001. The catalysts with a gold loading of 1–2 wt.% were prepared via deposition of Au complexes onto different aluminas by means of various techniques (“deposition-precipitation” (DP), incipient wetness, “chemical liquid-phase grafting” (CLPG), chemical vapor deposition (CVD)). These catalysts have been characterized comparatively by a number of physical methods (XRD, TEM, diffuse reflectance UV/vis and XPS) and catalytically tested for combustion of CO impurity (1%) in wet air stream at near-ambient temperature. Using the hydroxide or chloride gold complexes capable of chemical interaction with the surface groups of alumina as the catalyst precursors (DP and incipient wetness techniques, respectively) produces the catalysts that contain metallic Au particles mainly of 2–4 nm in diameter, uniformly distributed between the external and internal surfaces of the support granules together with the surface “ionic” Au oxide species. Application of organogold precursors gives the supported Au catalysts of egg shell type which are either close by mean Au particle size to what we obtain by DP and incipient wetness techniques (CVD of (CH₃)₂Au(acac) vapor on highly dehydrated Al₂O₃ in a rotating reactor under static conditions) or contain Au crystallites of no less than 7 nm in size (CLPG method). Regardless of deposition technique, only the Cl-free Au/Al₂O₃ catalysts containing the small Au particles (d_i ≤ 5 nm) reveal the high catalytic activity toward CO oxidation under near-ambient conditions, the catalyst stability being provided by adding the water vapor into the reaction feed. The results of testing of the nanodispersed Au/Al₂O₃ catalysts under conditions which simulate in part removal of CO from ambient air or diesel exhaust are discussed in comparison with the data obtained for the commercial Pd and Pt catalysts under the same conditions.     

Formation kinetics and stability of phosphonate self-assembled monolayers on indium–tin oxide

Phosphonate self-assembled monolayers (SAMs) have been widely used for the surface modification of indium–tin oxide (ITO) electrodes; however, their formation kinetics and stability are not well understood. In this paper, we describe our electrochemical studies of the formation kinetics and stability of a series of phosphonate SAMs on ITO electrodes. In particular electrochemical impedance spectroscopic (EIS) and cyclic voltammetric (CV) measurements have been carried out on three carboxy-terminated phosphonate SAMs by using Fe(CN)₆^3−/4− as redox indicators. The dependence of the charge-transfer resistance (obtained from the EIS plots) on the incubation time allows an estimation of the apparent fractional surface coverage of phosphonate SAMs. The apparent formation rate constant (k_obs) was determined by fitting the experimental data to a Langmuir adsorption model. For 3-phosphonopropanoic acid (PPA), the k_obs value increases when the PPA concentration increases in the deposition solution, and is smaller than those of thiolate SAMs on Au. The stability of phosphonate SAMs was investigated in three different media (pure water, phosphate-buffered saline (PBS) solution, and ambient air condition). It has been shown that the phosphonate SAMs are rather stable in either PBS solution or ambient air condition.     

Formic acid decomposition on Au catalysts: DFT,microkinetic modeling,and reaction kinetics experiments

A combined theoretical and experimental approach is presented that uses a comprehensive mean‐field microkinetic model, reaction kinetics experiments, and scanning transmission electron microscopy imaging to unravel the reaction mechanism and provide insights into the nature of active sites for formic acid (HCOOH) decomposition on Au/SiC catalysts. All input parameters for the microkinetic model are derived from periodic, self‐consistent, generalized gradient approximation (GGA‐PW91) density functional theory calculations on the Au(111), Au(100), and Au(211) surfaces and are subsequently adjusted to describe the experimental HCOOH decomposition rate and selectivity data. It is shown that the HCOOH decomposition follows the formate (HCOO) mediated path, with 100% selectivity toward the dehydrogenation products (CO₂ + H₂) under all reaction conditions. An analysis of the kinetic parameters suggests that an Au surface in which the coordination number of surface Au atoms is ≤4 may provide a better model for the active site of HCOOH decomposition on these specific supported Au catalysts. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1303–1319, 2014     

Complete oxidation of 2-propanol over gold-based catalysts supported on metal oxides

This paper concerns the preparation of metal oxide-supported gold catalysts and their application to 2-propanol abatement in order to lower the light off temperature. Catalytic oxidation of 2-propanol was investigated on Au/CeO₂, Au/Fe₂O₃, Au/TiO₂ and Au/Al₂O₃ catalysts prepared from the deposition–precipitation (DP) method. The catalysts are characterized by XRD (X-ray diffraction), BET (Brunner–Emmett–Teller), TEM (transmission electron microscopy), NH₃-TPD (NH₃-temperature programmed desorption), H₂-TPR (H₂-temperature programmed reduction), ICP-AES (inductively coupled plasma-atomic emission spectroscopy) and XPS (X-ray photoelectron spectroscopy) techniques. The catalytic activity of Au/metal oxide samples towards the deep oxidation of 2-propanol to CO₂ and water has been found to be strongly dependent on the kind of supports, the amount of gold loading, the calcination temperature and the moisture content in the feed.     

Deposition of Pt, Pd, Ru and Au on the surfaces of titanate nanotubes

Nanoparticles of different metals (Pt, Pd, Au) or a metal hydroxide (Ru) have been immobilized on the surface of mesoporous titanate nanotubes produced by alkali hydrothermal treatment of TiO₂, and have been characterized by HRTEM. Two different approaches have been utilised for the deposition of metal particles into the internal pores of titanate nanotubes: (i) deposition from solution confined inside the nanotubes and (ii) blocking the external surface of the nanotubes. A third method, ion-exchange of protons onto metal cations in titanate nanotubes followed by reduction or alkali treatment (in the case of Ru hydroxide), has been used for deposition of metal nano-particles on both the internal and external surfaces of the nanotubes. Nanoparticles of metal or metal hydroxide deposited by the ion-exchange method are characterised by an average size in the range of 1.2–5 nm, and deposits are uniformly distributed on the surface, resulting in a very high loading density. An increase in the amount of deposited metal resulted predominantly in a higher nanoparticle loading density, without growth in the particle size. This was correlated with the retention of high specific catalytic activity of ruthenium hydrated oxide deposited on titanate nanotubes in the reaction of selective oxidation of benzyl alcohol over a wide range (0.6–8.7 wt%) of ruthenium loading. The methods for metallization of titanate nanotubes are critically discussed.     

Kinetic analysis of electrosorption using fast Fourier transform electrochemical impedance spectroscopy: underpotential deposition of Bi in the presence of coadsorbing ClO4 on gold

We demonstrate a relatively simple method for studying various kinetic aspects of metal electrodeposition in the presence of coadsorbing anions on an electrode surface. This method combines cyclic voltammetry (CV) with fast Fourier transform electrochemical impedance spectroscopy (FFT-EIS), where both steady state and transient behaviors of the interface are probed simultaneously. As a model surface reaction, we use electrodeposition of Bi³⁺ in the presence of ClO₄⁻ adsorption from an aqueous solution onto a gold electrode. The voltage range for underpotential deposition (UPD) of Bi³⁺ is determined with potential step (PS) experiments. The voltage dependent UPD coverage of Bi on Au is determined by analyzing the CV data. The relevant kinetic parameters of both Bi³⁺ and ClO₄⁻ deposition reactions under the conditions of CV are measured with FFT-EIS, and analyzed using a complex nonlinear least square method. The differences and similarities between the electrosorption characteristics of Bi³⁺ and ClO₄⁻ on Au are discussed in terms of these kinetic parameters.     

Special Sites at Noble and Late Transition Metal Catalysts

An overview of recent advancements in density functional theory modeling of particularly reactive sites at noble and late transition metal surfaces is given. Such special sites include sites at the flat surfaces of thin metal films, sites at stepped surfaces, sites at the metal/oxide interface boundary for oxide-supported metal clusters, and sites at the perimeter of oxide islands grown on metal surfaces. The Newns–Anderson model of the electronic interaction underlying chemisorption is described. This provides the grounds for introducing the Hammer–N?rskov d-band model that correlates changes in the energy center of the valence d-band density of states at the surface sites with their ability to form chemisorption bonds. A reactivity change described by this model is characterized as an electronic structure effect. Br?nsted plots of energy barriers versus reaction energies are discussed from the surface reaction perspective and are used to analyze the trends in the calculated changes. Deviations in the relation between energy barriers and reaction energies in Br?nsted plots are identified as due to atomic structure effects. The reactivity change from pure Pd surfaces to Pd thin films supported on MgO can be assigned to an electronic effect. Likewise for the reactivity change from flat Au surfaces, over Au thin films to Au edges and the Au/MgO interface boundary. The reactivity enhancement at atomic step sites is of both electronic and atomic structure nature for NO dissociation at Ru, Rh and Pd surfaces. The enhancement of the CO oxidation reactivity when moving from a CO+O coadsorption structure on Pt(111) to the PtO₂ oxide island edges supported by Pt(111) is, however, identified as mainly an atomic structure effect. As such, it is linked to the occurrence of favorable pathways at the oxide island edges and is occurring despite of stronger adsorbate binding of the oxygen within the oxide edge, i.e. despite of an opposing electronic effect. As a final topic, a discussion is given of the accuracy of density functional theory in conjunction with surface reactions; adsorption, desorption, diffusion, and dissociation. Energy barriers are concluded to be more robust with respect to changes in the exchange-correlation functional than are molecular bond and adsorption energies.     

The nucleation, growth, and stability of oxide-supported metal clusters

The optimization of oxide-supported metal clusters as heterogeneous catalysts requires a detailed understanding of the metal cluster–oxide interface. Model catalysts, prepared by deposition of a catalytically active metal onto a thin film oxide support, closely mimic real-world catalysts, yet are amenable to study using surface sensitive techniques. Surface science methods applied to model catalysts, combined with the use of in situ high-pressure reaction studies, have provided a wealth of information about cluster structure and reactivity. STM capabilities for imaging individual particles under reaction temperatures and pressures offer a new approach for studying supported cluster catalysts on a particle-by-particle basis. This article describes recent work in our laboratories using variable temperature STM to investigate the role of the support and its defects in the nucleation and stabilization of metal clusters.     

Electrochemical characterization of the underpotential deposition of tellurium on Au electrode

Electrochemical characterization of the underpotential deposition (UPD) of tellurium on Au substrate has been performed in this paper. The mechanism of Te deposition and its voltammetry dependence on the Te ion concentration were studied, and it suggests that variations in the metal ion concentration may affect the UPD process kinetics. The effect of tellurium adsorbates on UPD behavior of Te has also been investigated. The results show that the tellurium adsorbates could be irreversibly adsorbed upon the Au substrate surface under the open-circuit conditions. Subsequent removal of the Te adsorbates was also proved to be very difficult within the Au double-layer region, and a standard electrochemical cleaning procedure is necessary to remove the Te adsorbates completely. When the potential was cycled into the Au oxidation region, a substantial loss of Te adsobates was observed, which occurs simultaneously with the Au oxidation features. Scan rate dependent cyclic voltammetry experiments reveal that the peak current in the Te UPD peak is not a linear function of the scan rate, ν, but of a 2/3 power of the scan rate, ν^2/3. It is in good consistent with a two-dimension nucleation and growth mechanism.     

Fabrication of patterned nanostructures with various metal species on Si wafer surfaces by maskless and electroless process

Maskless and electroless fabrication was demonstrated to form patterned nanostructures of various metal species, based upon the process previously developed by the authors. In this process, the metallic nanostructures were formed on the surface of clean, hydrogen terminated p-(1 0 0) Si wafer with pre-patterned nanoscopic defects, which were confirmed to possess higher activity for the reductive deposition reaction of the metal ion species. The deposition was achieved spontaneously and selectively at the defect sites on the wafer surface by immersing into dilute aqueous fluoride solution containing trace amount of metal ion species. By optimizing the formation condition of the patterned defects and composition of the solution, fabrication of patterned nanostructures of various metallic species such as Au, Ag, and Co, was achieved. Formation of the patterned nanostructures to 10 μm² in extent, as well as control of the feature size of the deposits by adjusting the formation condition of the patterned defects were also attempted.     

Deposition of selenium thin layers on gold surfaces from sulphuric acid media: Studies using electrochemical quartz crystal microbalance, cyclic voltammetry and AFM

In this paper we report here new considerations about the relationship between the mass and charge variations (m/z relationship) in underpotential deposition (UPD), bulk deposition and also in the H₂Se formation reaction. Nanogravimetric experiments were able to show the adsorption of H₂SeO₃ on the AuO surface prior to the voltammetric sweep and that, after the AuO reduction, 0.40 monolayer of H₂SeO₃ remains adsorbed on the newly reduced Au surface, which was enough to gives rise to the UPD layer. The UPD results indicate that the maximum coverage with Se_ads on polycrystalline gold surface corresponds to approximately 0.40 monolayer, in good agreement with charge density results. The cyclic voltammetry experiments demonstrated that the amount of bulk Se obtained during the potential scan to approximately 2 Se monolayers, which was further confirmed by electrochemical quartz crystal microbalance (EQCM) measurements that pointed out a mass variation corresponding of 3 monolayers of Se. In addition, the Se thin films were obtained by chronoamperometric experiments, where the Au electrode was polarized at +0.10 V during different times in 1.0 M H₂SO₄ + 1.0 mM SeO₂. The topologic aspects of the electrodeposits were observed in Atomic Force Microscope (AFM) measurements. Finally, in highly negative potential polarizations, the H₂Se formation was analyzed by voltammetric and nanogravimetric measurements. These finding brings a new light on the selenium electrodeposition and point up to a proposed electrochemical model for molecule controlled surface engineering.     

A branched pore kinetic model applied to the sorption of metal ions on bone char

A slightly modified form of the branched pore model of Peel, Benedek and Crowe was successfully applied to describe the batch sorption kinetics of three metal ions—cadmium, copper and zinc—on bone char. In comparison with an analytical film‐surface solution, the additional parameters of the branched pore model were observed to produce a significant improvement in correlating the experimental results. The ranges of the values of the model parameters derived were deemed reasonable and the branched pore sorption capacities of two of the three metal ions were comparable (ca 0.16 mmol g^?1). Given that the surface diffusivities of the metal ions were observed to vary with averaged surface loading, a number of correlations were examined for their accuracy in describing this behaviour. The exponential expression of Neretnieks resulted in the smallest total error when the data for all three metal ions were considered together. Copyright © 2005 Society of Chemical Industry     

Chalcogenide metal centers for oxygen reduction reaction: Activity and tolerance

This mini-review summarizes materials design methods, oxygen reduction kinetics, tolerance to small organic molecules and fuel cell performance of chalcogenide metal catalysts, particularly, ruthenium (Ru_xSe_y) and non-precious transition metals (M_xX_y: M = Co, Fe and Ni; X = Se and S). These non-platinum catalysts are potential alternatives to Pt-based catalysts because of their comparable catalytic activity (Ru_xSe_y), low cost, high abundance and, in particular, a high tolerance to small organic molecules. Developing trends of synthesis methods, mechanism of oxygen reduction reaction and applications in direct alcohol fuel cells as well as the substrate effect are highlighted.