Role of promoting oxide morphology dictating the activity of Au/SiO2 catalyst in CO oxidation

The interfacial interaction of gold nanoparticles deposited on either model SiO₂/Si(100) or high surface area amorphous or mesoporous silica with minute amounts of promoter oxide like “active” FeO_x, TiO₂ and CeO₂ has been discussed. The role of the active oxide, its contribution to the perimeter along the gold nanoparticles has been interpreted. The oxide may invoke electronic interaction and simultaneously the defect structure of oxides likely has a key issue in the formation and stabilization of very small Au particles. The activity of the Au/oxide perimeter depends not only on the size of the Au particles, but also on the size and morphology of the oxide component (likely amorphous structure) regardless of whether it is supporting Au nanoparticles or decorating them. The activity in CO oxidation over Au catalysts is strongly affected by the length of the Au/“active” oxide perimeter which is regarded as the “active interface”. The longer length of the perimeter is evidenced by the enhanced CO oxidation activity.     

Synthesis and properties of composites synthesized by deposition of TiO2 doped with SnO2 or NiO2 onto A-300 nanosilica

This paper focuses on the investigation of nanooxide composites SiO₂/TiO₂/X (X = SnO₂, NiO) synthesized by controlled low-temperature hydrolysis of TiCl₄, SnCl₄ (followed by thermal treatment of some samples) on nanosilica surface (specific surface area 300 m²/g) and by thermolysis of Ni(NO₃)₂ · 6H₂O at 600°C. The effect of the nature and concentration of a doping agent on the phase composition of the composites and their physicochemical properties is determined. The properties of the synthesized systems are examined by means of XRD analysis, nitrogen adsorption, potentiometric titration, and photon correlation spectroscopy.     

Sensing behavior of silica-coated Au nanoparticles towards nitrobenzene

In the present work, we report silica-stabilized gold nanoparticles (SiO₂/Au NPs) as a wide-range sensitive sensing material towards nitrobenzene (NB). Surface hydroxyl groups of silica selectively form Meisenheimer complex with electron-deficient aromatic ring of NB and facilitate its immobilization and subsequent catalytic reduction by Au cores. Silica-coated Au NPs were synthesized and characterized for their chemical, morphological, structural, and optical properties. SiO₂/Au NPs-modified electrodes were characterized with impedometric and cyclic voltammetric electrochemical techniques. SiO₂/Au NPs are found to have a higher optical detection window of range, 0.1 M to 1 μM and a lower electrochemical detection window of range, 10⁻⁴ to 2.5 × 10⁻² mM with a detection limit of 12.3 ppb. A significant enhancement in cathodic peak current, C ₁, and sensitivity (102 μA/mM) was observed with modified electrode relative to bare and silica-modified electrodes. The I _P was found to be linearly co-related to NB concentration (R ² = 0.985). The interference of cationic and anionic species on sensor sensitivity was also studied. Selectivity in the present sensing system may be further improved by modifying silica with specific functional moieties.     

Gold complexes with

The reactions of methane with the gold complexes [Au(OH)]⁻, [Au(OCH₃)₄]⁻, [Au(O(CO)₂O₂]⁻ and [Au(O₂CH)₂]⁺, [Au^I(acac)], [Au^III(acac)₂]⁺ (acac-acetylacetonato) were studied using the DFT/PBE method with the SBK basis set. High activation barriers were obtained for the electrophilic substitution in [Au(OH)]⁻, [Au(OCH₃)⁴]⁻, [Au(O(CO)₂O)₂]-and [Au^III(acac)₂]⁺ complexes, which excludes the possibility that these reactions might proceed under mild conditions. The reactions of the [Au(HCO₂)₂]⁺ and [AuI(acac)] complexes with methane have rather low energy barriers and proceed through the formation of an intermediate complex. The alternative mechanism of methane oxidation with a gold complex in the presence of oxygen is simulated.     

TiO2/SiOx core-shell nanowires generated by heating the multilayered substrates

By heating Au/TiN/Si substrates, we fabricated TiO₂/SiO_x core-shell nanowires. By changing the thickness of predeposited Au layers, we demonstrated that the thickness of the Au layer needs to be optimized to obtain nanowires. High-resolution transmission electron microscopy image, X-ray diffraction spectrum, and selected area electron diffraction pattern coincidentally revealed that the resultant core nanowires had a tetragonal rutile structure of TiO₂, and the shell was comprised of amorphous SiO_x. The dominant growth mechanism was a base-growth mode, in which Au played a catalytic role, resulting in morphological changes with variation of the Au layer thickness. The TiO₂/SiO_x core-shell nanowires exhibited a broad photoluminescence emission band, which comprised four peaks centered at 1.54, 2.34, 2.67, and 2.99 eV, respectively. We expected that the 1.54 eV- and 2.34 eV-centered peaks arised from the TiO₂ core, whereas the 2.67 eV- and 2.34 eV-peaks were ascribed to both the TiO₂ core and the SiO_x shell.     

Spectral and adsorption characteristics of Au(III) and Au(0) composite materials

The adsorption of HAuCl₄ chloroauric acid on silica modified with γ-aminopropyltriethoxysilane (aminosilica) and on aluminum oxide is studied. Diffuse-scattering spectra of these Au(III) composite materials and their reduced Au(0) composites are recorded in the visible and infrared spectral ranges. Au(III) composites are selective with respect to the adsorption of phenylacetylene (PA) from octane due to the formation of π-complexes with Au(III). Au(III)-aluminum oxide composite material has a much larger capacity for PA adsorption than does aminosilica composite with the same gold content. The formation of coordination bonds between free aminopropyl groups of the silica carrier and gold atoms prevents PA adsorption. The formation of such bonds is manifested in a shift in λ_max of the spectral line from 408 to 522 and 546 nm with a decrease in [Au(III)] concentration from 400 to 120 and 60 μmol/g. The decrease in the intensity and the red shift of the absorption bands of NH₂ stretching vibrations in the infrared spectra of the specimen upon modification also confirms the supposition. There are absorption bands of free hydroxyl radicals, but no band of bound radicals in the infrared spectra of Au(0) composites. The electronic spectra (λ_max = 511, 504, and 512 nm) are close for all three specimens that differ in gold content, which means that the sizes of immobilized Au(0) nanoparticles are similar. Upon the sorption of HAuCl₄ on a Au(III)-aluminum oxide specimen, the absorption band of surface OH groups disappears, however, in Au(0) composite, it appears again near 3105 cm^?1.     

Adsorption of small molecules on gold single crystal surfaces

Much more work has been carried out on supported gold catalysts (also called “real” catalysts) than on gold single crystal surfaces. However, for fundamental understanding of catalysis by gold, well-defined gold surfaces and controlled conditions using the surface science approach may provide useful information concerning reaction mechanisms and the nature of active centers. This paper presents a brief overview on the work carried out regarding adsorption of several small molecules on gold surfaces from 2004, date of the last review on gold surface science [R. Meyer, C. Lemire, S.K. Shaikhutdinov and H. Freund,Gold Bull., 2004, 37, 72], until recently. Both experimental and theoretical results are discussed. A large difference between the flat Au(111), by far the most studied surface, and stepped/kinked surfaces, is found, demonstrating the importance of low-coordinated Au atoms for high catalytic activity.     

Kinetic Parameters and Mechanism of Gold Electrooxidation in Thiocarbamide Solutions

Kinetics and mechanism of gold electrooxidation in sulfuric acid solutions of thiocarbamide (TCA) are studied by measuring voltammograms (VA) of gold and glassy carbon (GC) electrodes. In a potential (E) range from –0.2 to 1.2 V (SCE), VA of a gold electrode demonstrate three peaks. The first and the second peaks correspond to gold electrooxidation to [AuTCA_ads] + and [Au(TCA_ads)₃]³⁺ ions at E of 0.152 and 0.554 V, while the third one corresponds to TCA electrooxidation. The electrooxidation of TCA on GC electrodes is characterized by two VA peaks at 0.983 and 1.437 V. The first peak corresponds to the formation of formamidine disulfide (FADS) (k _a1 = 6.40 × 10^–4 cm/s), while the second peak corresponds to TCA oxidation to sulfides and hydrosulfides (k _a2 = 7.78 × 10^–5 cm/s). The composition of adsorption layers formed at the Au electrode is analyzed by Auger spectroscopy. The introduction of sodium sulfide into TCA solutions eliminates the formation of adsorption layers and accelerates Au oxidation in such solutions. The rate constants of gold electrooxidation k _a = 6.31 × 10^–3 cm/s and Au(I) electroreduction k _c = 5.46 × 10^–4 cm/s in TCA solutions are estimated. Kinetic parameters (charge transfer coefficients, reactions orders in carbamide) are determined and the mechanisms of Au and TCA oxidation in sulfuric acid solutions are proposed.__________Translated from Zashchita Metallov, Vol. 41, No. 3, 2005, pp. 316–325.Original Russian Text Copyright © 2005 by Kozin, Kozina, Bogdanova.     

Diverse electrodeposits from modified acid sulphate (Watts nickel) baths

A diverse range of nickel-based coatings can be cathodically deposited from a classical Watts electroplating bath containing suitable additives by appropriate control over operational conditions. Modern nickel-based electrodeposits are concisely reviewed and can be seen to be longer-term developments from the publication of optimised conditions for rapid nickel plating from an acid sulphate bath by Watts 100 years ago, in 1916. Recent examples of tribological nickel electrodeposits are illustrated by examples from the authors’ laboratory: (i) low friction, hard, wear resistant surfaces (containing 2-D, layered, self-lubricating particles of Ni–P–MoS₂), (ii) superhydrophobic layers of Ni–WS₂, showing a lotus petal effect (for self-cleaning surfaces), (iii) light emitting, luminescent signal bearing surfaces of Ni–BAM (BaMgAl₁₁O₁₇:Eu²⁺) metal oxide, (providing an in-situ phosphor wear tracer) and (iv) nanocrystalline surfaces of Ni, Co, Ni–Co and Ni–Co–P (including high surface area electrocatalysts). Important trends in the deployment of the longstanding, versatile Watts nickel electroplating bath are seen to include the development of (a) surfaces having tailored structure, hence special properties, by choice of bath composition and operational conditions, (b) nanostructured composite layers containing mixed inclusions and (c) layers able to diagnose in-service wear and provide an output signal. Subject areas needing further research are suggested.     

Influence of Nickel Sulfate Additives to Electrolytes Subjected to Microarc Oxidation on the Structure,Composition, and Properties of Coatings Formed on Titanium

The influence of nickel sulfate additives to acid and basic electrolytes for microarc oxidation on the structure, composition, and properties of coatings formed on VT1-0 titanium were studied to produce poorly soluble titanium anodes. We established the possibility of incorporating nickel (nickel oxide) into the composition of coatings. X-ray diffraction analysis showed that the coatings contained nickel oxide β-NiO with a cubic lattice, TiO₂ in the form of rutile, and SiO₂ in the form of high-temperature β-cristobalite. The maximum thickness and nickel content in the surface layer and the minimum values of both the anodic dissolution currents and the electrical strength were obtained for the coatings formed in silicate-alkali (3 g/L KOH + 4.5 g/L Na₂SiO₃) electrolyte with the addition of 1 g/L of nickel sulfate NiSO₄.     

Chronovoltammetry of the anodic dissolution of Ag-Au alloys in nitrate environments

A theory of quasi-reversible anodic chronovoltammetry of the selective dissolution of a homogeneous A-B alloy controlled by the solid-phase diffusion is proposed. The effects of the equilibrium solid-phase adsorption of the components and the roughness of the electrode surface with either homogeneously or statistically normally distributed microscopic irregularities of diverse kind on the shape of chronovoltammogram (the I _A current dependence on the η(t) overpotential) are successively taken into account. An expression for the I _{A, m} current at the voltammogram maximum, which is a generalization of the Randles-Sevik formula, is derived. A technique of estimating the recording-time boundaries of voltammograms, within which the surface roughness effect may be taken into account by simply multiplying the current by the roughness factor f _r , is founded. Outside the time interval, the I _A,m − ν^1/2 dependence (ν is the potential scanning rate), which is criterion for the diffusion kinetics, is shown to be substantially nonlinear. The diffusion-zone parameters (δ thickness and D _Ag heterodiffusion coefficient) obtained by processing the experimental data for Ag-Au alloys with a gold content of 15 or 20 at.% with the use of three models discussed are compared. Original Russian Text ? O.A. Kozaderov, V.V. Lozovskii, A.V. Vvedenskii, 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 4, pp. 359–368.     

Obtaining ZrO<Subscript>2</Subscript> + CeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> + TiO<Subscript>2</Subscript>/Ti compositions by plasma-electrolytic oxidation of titanium and investigating their properties

Regularities of the effect produced by Ce₂(SO₄)₃ salt introduced in an aqueous electrolyte containing Zr(SO₄)₂ on the plasma-electrolytic formation of oxide coatings on titanium, their composition, and structure are studied. ZrO₂ + CeO _x + TiO₂ three-phase oxide coatings with a thickness about 10 μm are obtained. The coatings involve ZrO₂ cubic phase. The ZrO₂-to-TiO₂ phase ratio in the coatings can be controlled. The zirconium content in the coatings reaches 20 at %, while that of cerium is 3–5 at %. The surface layer (∼3-nm thick) contains Ce³⁺ (∼30%) and Ce⁴⁺ (∼70%). Pores in the surface part of coatings have diameters around or smaller than 1 μm and are regularly arranged. The obtained systems have a certain catalytic activity with respect to the oxidation of CO to CO₂ at temperatures above 400–450°C. The coatings are corrosion-resistant in chloride-containing environments. The thickness h of coatings depending on the charge Q supplied to the cell is described by the equation h = h ₀(Q/Q ₀) ⁿ , where n = 0.35 and h ₀ is the thickness of the coating formed at Q ₀ = 1 C/cm².     

Grain growth and thermal stability of nanocrystalline Ni–TiO2 composites

The grain growth and thermal stability of nanocrystalline Ni–TiO₂ composites were systematically investigated. The nanocrystalline Ni–TiO₂ composites with different contents of TiO₂ were prepared via electroplating method with the variation of TiO₂ nano-particles concentration. The effect of TiO₂ content on the grain size, phase structure and microhardness was investigated in detail. The corresponding grain growth and diffusion mechanisms during the heating process were also discussed. The optimal microhardness of HV₅₀ 270 was achieved for the composite with addition of 20 g/L TiO₂ nano-particles after annealing at 400 °C for 90 min. The calculation of the activation energy indicated that lattice diffusion dominated at high temperatures for the nanocrystalline Ni–TiO₂ composites. It was indicated that the increase of TiO₂ nano-particles content took effect on restricting the grain growth at high temperatures by increasing the grain growth activation energy.     

Di-(2-ethylhexyl) dithiophosphoric acid surface protected gold nanoparticles: micellar synthesis, stabilization, isolation, and properties

Gold nanoparticles (AuNPs) were synthesized in the organic solution by means of the reduction of HAuCl₄ by hydrazine in reverse micelles of oxyethylated surfactant Triton N-42, with decane as the dispersion medium. To isolate the powder of particles, the micelles were destroyed with chloroform in the presence of di-(2-ethylhexyl) dithiophosphoric acid as a surface protecting agent. According to the results of several experiments, the yield is within the limits of 90–98%, calculated for gold. The obtained preparations are dark blue hydrophobic powders containing aggregated but not agglomerated gold nanoparticles, as well as microcrystals (∼0.08–0.2 μm) of NaCl. The powders get re-dispersed in weakly polar organic solvents with the formation of colloidal solutions. The shape of the nanoparticles is spherical. Their nuclei are gold single crystals with a narrow size distribution; their diameter (d _Au) is about two times as large as the diameter of the aqueous nucleus (d _c) of initial micelles: d _Au = 7.7 ± 1.4 nm (d _c = 3.6 nm) and 8.8 ± 1.5 nm (4.6 nm). The preparations were studied by means of dynamic light scattering, atomic force microscopy, transmission electron microscopy, UV–vis spectroscopy, IR spectroscopy, X-ray powder diffraction, and thermogravimetric and elemental analyses. In the case of the particles with d _Au = 8.8 nm, the product is a mixture of AuNPs and the salt with the molar ratio Au/NaCl ≈ 1:4.54, while the gross composition of AuNPs per one gold atom is estimated as Au(C₁₆H₃₄O₂PS₂Na∙2N₂H₄)_0.16 with the number of gold atoms in one particle ∼21,000.     

Microstructural Analysis and Photocatalytic Activity of Plasma-Sprayed Titania-Hydroxyapatite Coatings

Hydroxyapatite (HAp Ca₁₀(PO₄)₆(OH)₂) is known to be a biomaterial and an adsorbent for chromatography. In this study, HAp was agglomerated with anatase TiO₂ to manufacture thermal-spray powders to improve the adsorption activity of TiO₂, and then to improve its photocatalytic activity. The microstructures, compositions and photocatalytic activity of plasma-sprayed TiO₂, TiO₂-10%HAp, TiO₂-30%HAp, and HAp coatings were investigated. Due to the low thermal conductivity of HAp compound, not all HAp particles fully melted even under the arc current of 800 A. The addition of HAp inhibited the phase transformation of anatase TiO₂ to rutile. Under the arc current of 600 A, the anatase content in the TiO₂, TiO₂-10%HAp and TiO₂-30%HAp coatings was 11, 20 and 42%, respectively. With the increasing of the spraying distance from 70 to 110 mm, the anatase content in the TiO₂-30%HAp coatings decreased from 34 to 17% under arc current of 700 A. Furthermore, a slight decomposition of HAp to α-Ca₃(PO₄)₂ was found in the TiO₂-30%HAp coatings, it did not decompose to CaO and P₂O₅ according to the XRD and EDAX analysis. The addition of the secondary gas of helium had no significant influence on the melting state of the TiO₂-HAp feedstock powders. Moreover, the HAp in the TiO₂-10%HAp and TiO₂-30%HAp coatings had adsorption characteristic to acetaldehyde. The photocatalytic activity of TiO₂-10%HAp coating was highest among TiO₂, TiO₂-10%HAp, and TiO₂-30%HAp coatings sprayed under the arc current of 600 A for the optimum adsorption property and anatase content. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Catalytic performance of gold catalysts in the total oxidation of VOCs

Catalysts for the oxidation of volatile organic compounds (VOCs) were prepared by supporting 1% gold on cerium and zirconium oxides (CeO2, Ce_0.5Zr_0.5O₂, ZrO₂) using a simple impregnation method followed by reduction of gold in the presence or absence of ammonia (N). The catalysts were tested in model reactions, namely the total oxidation of benzene, hexane and chlorobenzene, using a micro flow reactor at atmospheric pressure in the temperature range 100–500°C and their activity was compared to that of Au/TiO₂ and Au/Fe₂O₃ reference catalysts supplied by the World Gold Council (WGC). Benefits on the light-off temperatures were observed by adding gold to cerium-containing oxides: 100% conversion of hexane was obtained with Au/Ce_0.5Zr_0.5O₂ at the lowest temperature (300°C). Full conversion of benzene was reached at only 250°C with Au/CeO₂ (N), at 290°C with Au/Fe₂O₃ (WGC) and at 300°C with Au/Ce_0.5Zr_0.5O₂. In the case of chlorobenzene oxidation, the addition of gold was of even greater relevance because of a drop in the light-off temperature of over 100°C for Au/Ce_0.5Zr_0.5O₂ and Au/CeO₂, with respect to the gold-free oxide supports; but in this case rapid deactivation took place.     

Properties of TiO2 support and the performance of Au/TiO2 Catalyst for CO oxidation reaction

Gold catalysts were prepared on TiO₂ supports of different phase structures (i.e., anatase, rutile and biphasic), TiO₂ crystal size (i.e., 9–23 nm), surface and textural properties (i.e., hydration and surface area). The CO oxidation on the gold catalysts was carried out in an operando-DRIFTS set-up equipped with DRIFTS reactor cell connected on-line to CO gas analyser and gas chromatograph enabling real time monitoring of surface reaction and simultaneous reaction rate measurements. Gold catalysts supported on pure anatase TiO₂ were more resistant to sintering compared to catalysts supported on rutile and bi-phasic TiO₂. Besides catalyst sintering, deposition of surface carbonates is an important cause of catalyst deactivation. The best gold catalyst was prepared on 13 nm anatase TiO₂. It displays both increased activity and stability for CO oxidation reaction at room temperature. Surface and textural properties of TiO₂ also play a role on the performance of the Au/TiO₂ catalyst.     

Effect of α-Al2O3 coatings on the interface of Ni/SiC composites prepared by electrodeposition

This paper describes an investigation on the effect of α-Al₂O₃ coating on the interface between nickel and SiC particle. Uniform, dense and well-adhered α-Al₂O₃ coatings were obtained on the surface of SiC particles by sol–gel technology. The nickel-based composites reinforced with α-Al₂O₃-coated SiC particles (CS_p) or uncoated SiC particles (UCS_p) prepared by composite electrodeposition were heated at 600 °C to study the reactivity and the resulting interfaces. The results showed that the Ni/CS_p composites presented excellent thermal stability without interfacial reaction, while nickel silicide formed in the Ni/UCS_p composites. It indicated that high-temperature interfacial reaction between SiC particles and nickel matrix was efficiently inhibited by the α-Al₂O₃ coatings. Moreover, great differences of the local mechanical properties of interfaces were observed by the nanoindentation characterization.     

Visible light-induced photocatalytic activity of Ag-containing TiO2/carbon nanofibers composites

TiO₂/carbon nanofiber (CNF) composites containing Ag nanoparticles (Ag–TiO₂/CNF) were prepared and their photocatalytic degradation ability upon visible light irradiation assessed. The reduction of methylene blue (MB) adsorption in an aqueous solution by the photocatalysts occurred due to the combined effects of MB adsorption in the pores of CNFs and decomposition by the photocatalysts throughout the process. The Ag–TiO₂/CNF photocatalyst degraded MB 17 times faster than the TiO₂/CNF composites with no Ag nanoparticles after 3 h under visible light illuminations.     

Effects of defects generated in ALD TiO<Subscript>2</Subscript> films on electrical properties and interfacial reaction in TiO<Subscript>2</Subscript>/SiO<Subscript>2</Subscript>/Si system upon annealing in vacuum

Thin TiO₂ layers grown at 130°C on SiO₂-coated Si substrates by atomic layer deposition (ALD) using TTIP and H₂O as precursors were annealed, and the effects of the annealing temperature on the resulting electrical properties of TiO₂ and the interface properties between a Pt electrode and TiO₂ were examined using transmission line model (TLM) structures. The as-deposited TiO₂ thin film had an amorphous structure with OH groups and a high resistivity of 6×10³Ω-cm. Vacuum annealing at 700 °C transformed the amorphous film into an anatase structure and reduced its resistivity to 0.04Ω-cm. In addition, the vacuum-annealing of the TiO₂/SiO₂ structure at 700°C produced free silicon at the TiO₂-SiO₂ interface as a result of the reaction between the Ti interstitials and SiO₂. The SiO₂ formed on the TiO₂ surface caused a Schottky contact, which was characterized by the TLM method. The use of the TLM method enabled the accurate measurement of the resistivity of the vacuum-annealed TiO₂ films and the characterization of the Schottky contacts of the metal electrode to the TiO₂.