Low-pressure plasma cleaning of Au and PtIr noble metal surfaces

The effect of low-pressure hydrogen and oxygen plasma cleaning of Au and PtIr was investigated by X-ray photoelectron spectroscopy. Hydrocarbon contamination was efficiently removed by hydrogen and oxygen plasma. Hydrogen plasma additionally reduces oxygen compounds, especially metal oxides, while oxygen plasma results in the formation of a surface layer of Au₂O₃ and PtO, respectively. Both noble metal oxides are unstable and decompose with time. The decomposition of metal oxides occurs in parallel with the recontamination of the surface. Metal oxides can be removed completely for Au and partially for PtIr by an additional cleaning with hydrogen plasma. Hydrogen plasma treatment is very promising for noble metal surface cleaning.     

A general and low-cost synthetic route to high-surface area metal oxides through a silica xerogel template

Porous metal oxides with a large surface area are synthesised by means of a procedure based on the templating approach. An inexpensive porous silica xerogel synthesised at moderate temperatures (∼100 °C) in order to preserve the silanol superficial groups was used as template. In a first step, the silica porosity was filled with a concentrated solution containing a metallic salt. Then, the impregnated sample was calcined in air at a temperature of 600 °C. Under these conditions, the metal oxides were synthesised within the confined space provided by the silica pores. Finally, the product was recovered after dissolution of the silica framework in 2 M NaOH solution. The materials obtained by this procedure are made up of aggregates of nanoparticles and/or 3D solid structures containing confined pores. In this work, the synthetic route proposed is illustrated by the preparation of various binary metal oxides (i.e. Fe₂O₃, Cr₂O₃, NiO, CeO₂, Mn₂O₃, Co₂O₃ and Al₂O₃). The BET surface areas measured for these materials are in the range of 100-270 m² g⁻¹. The proposed method is not restricted to the binary metal oxides. It can also be used in the preparation of other inorganic materials such as metal sulphides or mixed metal oxides.     

Effect of combination of noble metals and metal oxide supports on catalytic reduction of NO by H2

We investigated the effects of combination of noble metals M (Rh, Pd, Ir, Pt) and metal oxide supports S (Al₂O₃, SiO₂, ZrO₂, CeO₂) on the NO + H₂ reaction using planar catalysts with M/S two layered thin films on Si substrate. In this study, NO reduction ability per metal atom were evaluated with a specially designed apparatus employing pulse valves for the injection of reactant molecules onto catalysts and a time-of-flight mass spectrometer to measure multiple transient products: NH₃, N₂ and N₂O simultaneously as well as with an atomic force microscopy to observe the surface area of metal particles. The catalytic performances of Rh and Ir catalysts were hardly affected by a choice of a metal oxide support, while Pd and Pt catalysts showed different catalytic activity and selectivity depending on the metal oxide supports. This assortment is consistent with ability to dissociate NO depending on metals without the effect of any support materials. There, the metals to the left of Rh and Ir on the periodic table favor dissociation of NO and those to the right of Pd and Pt tend to show molecular adsorption of NO. Therefore, the catalytic property of noble metals could be assorted into two groups, i.e. Rh and Ir group whose own property would mainly dominate the catalytic performance, and Pd and Pt group whose interaction with metal oxides supports would clearly contribute to the reaction of NO with H₂. NO reduction activity of Pd and Pt was found to be promoted above that of Rh and Ir, provided that Pd and Pt were supported by CeO₂ and ZrO₂.     

Enhancement of the electrochemical performance in MgO stabilized ZrO2 oxygen sensors by co-doping trivalent metal oxides

The present work looked at the co-doping effect of the typical trivalent metal oxides on the electrochemical performance of 8 mol% of MgO stabilized ZrO₂ (Mg-PSZ) electrolyte ceramics used for oxygen sensors. An appreciable increase in the ionic conductivity occurred on substituting MgO with trivalent metal oxides except La₂O₃ in the Mg-PSZ electrolyte ceramics across the measured temperature range. The conductivity was related to the co-dopant ionic radius and the ionic conductivity increased with the co-dopant ionic radius of the trivalent metal oxides, reached a maximum at about 1.02 Å, and thereafter decreased. Through comparison and analysis, the Y₂O₃ exhibited excellent superiority than other trivalent metal oxides and the oxygen sensor assembled by 1Y7Mg-PSZ electrolyte possessed more excellent response speed and stability.     

Fabrication and application of highly ordered mesoporous Co3O4, NiO, and their metals

Highly ordered mesoporous Co₃O₄, NiO, and their metals were synthesized by nanocasting method using there corresponding mesoporous SBA-15 silica as a template. The obtained porous metal oxides have high surface areas, large pore volume, and a narrow pore size distribution. The N₂-adsorption data for mesoporous metal oxides have provided the BET area of 257.7 m² g⁻¹ and the total pore volume of 0.46 cm³ g⁻¹. The mesoporous metals were employed as a catalyst in the synthesis of (S)-3-pyrrolidinol from chiral (S)-4-chloro-3-hydroxybutyronitrile, and a high yield to (S)-3-pyrrolidinol-salt was obtained on the mesoporous Co metal catalyst.     

Study of the electronic structure of rhenium and tungsten oxides on the O K-edge

Oxygen K-edge x-ray absorption spectra were studied on the electrochromic amorphous thin film a-WO₃ in the comparison with crystalline oxides having variable electronic (d⁰, d¹, d²) and atomic structure: monoclinic m-WO₃ (insulator - d⁰), cubic Na_0.6WO₃ (metal - d¹), cubic ReO₃ (metal - d¹), layered-type hexagonal h-WO₃, WO₃H₂O and with intercalated H_xReO₃ (metal - d²), H_xWO₃ oxides having a metal-isulating transition. The changes in the XANES range 10–15 eV above the absorption edge are interpreted based on the known band-structure calculations. The high-energy features are related to the multiple-scattering processes (EXAFS) on the nearest atoms. The intensity of the feature at 550–560 eV is attributed for the first time to the value of the metal-oxygen-metal bond angle. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

2D siloxene supported NiO/Co3O4 electrocatalyst for the stable and efficient hydrogen evolution reaction

The activity of NiO/Co₃O₄ for the hydrogen evolution reaction (HER) during water splitting was increased by depositing these metal oxides on siloxene multi-sheets. The improvement in active sites due to siloxene was used to increase the catalytic activity. The hierarchical structure of the composite with the synergistic effect of metal oxides helped enhance the catalytic activity to show a low overpotential of 110 mV at 10 mA/cm² in 1 M KOH and stability at 10 mA/cm² over 20 h without an obvious change in voltage. The as-prepared catalyst can be a promising electrocatalyst for the HER owing to the low cost of transition metal oxides, the abundance of silicon on Earth, and the simplicity of the synthesis process.     

Thermodynamic properties of noble metal clusters:molecular dynamics simulation

The thermodynamics properties of noble metal clusters Au_N, Ag_N, Cu_N, and Pt_N (N = 80, 106, 140, 180, 216, 256, 312, 360, 408, 500, 628, 736, and 864) are simulated by micro-canonical molecular dynamics simulation technique. The potential energy and heat capacities change with temperature are obtained. The results reveal that the phase transition temperature of big noble metal clusters (N ⩾ 312 for Au, 180 for Ag and Cu, and 360 for Pt) increases linearly with the atom number slowly and approaches gently to bulk crystals. This phenomenon indicates that clusters are intermediate between single atoms and molecules and bulk crystals. But for the small noble clusters, the phase transition temperature changes irregularly with the atom number due to surface effect. All noble metal clusters have negative heat capacity around the solid-liquid phase transition temperature, and hysteresis in the melting/freezing circle is derived in noble metal clusters.     

Eco-friendly combustion-based synthesis of metal aluminates MAl<Subscript>2</Subscript>O<Subscript>4</Subscript> (M = Ni,Co)

Nanosized metal aluminates, MAl₂O₄ (M = Ni, Co), have been prepared following a nonpolluting, low temperature, and self-sustaining starch single-fuel combustion synthesis. The mixed fuel-coordinating actions of starch have given rise to an intermediary precursor which afforded monodisperse metal aluminate nanoparticles. The thermal analysis of the [M(II), Al(III)]-starch precursors indicates a similar thermochemical reactivity for the two compounds, displaying a sequence of well-defined decomposition stages associated with three endothermic effects and three/four (nickel/cobalt) exothermic ones. The XRD data confirm the formation of spinelic phase and a continuous growth of particle sizes with the increase of calcination temperatures. The mechanisms proposed for the formation of metal aluminates essentially consist in a combination of solid-state reactions of amorphous NiO/Co₃O₄ and Al₂O₃ simple oxides. The evaluation criterion of Ni(II) cations into the spinelic lattice is original and is based on the distinct occupancy degree of tetrahedral and octahedral sites in NiAl₂O₄ and γ-Al₂O₃. TEM/HRTEM investigations performed on the cobalt(II) and nickel(II) aluminate oxide powders resulted after calcination at 800 and 900 °C, respectively, for 1 h show the formation of irregular and isolated plate-like particles for Co(II)-based spinelic oxides (the average particle size is 16.6 nm) and submicron aggregates of small, bimodal, and almost uniform (as shape and size) of NiAl₂O₄ mixed oxide (the mean particle size is 33.6 nm). The NIR–UV–Vis spectra for the resulted MAl₂O₄ (M = Co, Ni) mixed oxides reveal a massive presence of tetrahedral divalent cations both for short- and long-time calcined samples. NiO impurities are detected using FTIR and electronic spectra for all NiAl₂O₄ samples.     

Predicting the metal growth mode and wetting of noble metals supported on c-ZrO2

Employing periodic density-functional calculations, we predict the metal growth mode for ideal and defective interfaces of noble metals (Pd and Pt) deposited on top of cubic-ZrO₂. We discuss various energetic contributions and surface textures. We propose that isolated noble metal atoms are supported on both steps and terraces on the surfaces. Pt, on the other hand, shows a higher probability to decorate steps and other low oxygen coordinated sites, while Pd forms larger metal aggregates on both high oxygen coordinated sites (terraces) as well as low oxygen coordinated sites (steps). Oxygen defects on the support act as nucleation sites for the metal cluster growth. We also investigate metal wetting of Pd and Pt on the {1 1 1} surface of cubic-zirconia. Our calculations predict that the wetting angles are ca. 110-130° for 1 and 2 mono-layers of Pd and Pt, respectively, suggesting that metal wetting decreases as the metal loading increases.     

Energy expression of the chemical bond between atoms in metal oxides

The chemical bond between atoms in metal oxides is expressed in an energy scale. Total energy is partitioned into the atomic energy densities of constituent elements in the metal oxide, using energy density analysis. The atomization energies, ΔE_M for metal atom and ΔE_O for O atom, are then evaluated by subtracting the atomic energy densities from the energy of the isolated neutral atom, M and O, respectively. In this study, a ΔE_O vs. ΔE_M diagram called atomization energy diagram is first proposed and used for the understanding of the nature of chemical bond in various metal oxides. Both ΔE_M and ΔE_O values reflect the average structure as well as the local structure. For example their values vary depending on the vertex, edge or face sharing of MO₆ octahedron, and also change with the overall density of binary metal oxides. For perovskite-type oxides it is shown that the ΔE_O value tends to increase by the phase transition from cubic to tetragonal phase, regardless of the tilting-type or the 〈1 0 0〉 displacement-type transition. The bond formation in spinel-type oxides is also understood with the aid of the atomization energies. The present approach based on the atomization energy concept will provide us a new clue to the design of metal oxides.     

Relation between 2p X-ray photoelectron and Kα X-ray emission spectra of manganese and iron oxides

The high resolution Mn and Fe Kα X-ray emission spectra (XES), and Mn and Fe 2p X-ray photoelectron spectra (XPS) for manganese and iron oxides were measured. The spectra were compared with those of [MnO₄]⁻, [Fe(CN)₆]⁴⁻ and [Fe(CN)₆]³⁻ ions. As the electronic structure of the latter compounds do not change with electron hole creation in the core levels, satellite peaks due to charge transfer are not observed in the 2p XPS spectra, and the peak profiles of metal 2p XPS and Kα XES are governed by the exchange splitting between 2p and valence electrons. The metal 2p XPS spectra of the oxides had satellite peaks, but the XES spectra had no satellites. FWHMs of the metal 2p_3/2 main peaks of the compounds being low spin states are smaller than those of metal Kα₁ XES spectra. However, FWHMs of Mn 2p_3/2 of the manganese oxide were nearly equal to those of Mn Kα₁ XES spectra, and those of Fe 2p_3/2 XPS spectra of the iron oxides are greater than those of Fe Kα₁ XES spectra.     

Investigation of IrO2-SnO2 thin film evolution from aqueous media

The thermal evolution process of IrO₂-SnO₂/Ti mixed oxide thin films of varying noble metal content has been investigated under in situ conditions by thermogravimetry-mass spectrometry, Fourier transform infrared emission spectroscopy and cyclic voltammetry. The gel-like films prepared from aqueous solutions of the precursor salts Sn(OH)₂(CH₃COO)_2−xCl_x and H₂IrCl₆ on titanium metal support were heated in an atmosphere containing 20% O₂ and 80% Ar up to 600 °C.The thermal decomposition reactions practically take place in two separate temperature ranges from ambient to about 250 °C and between 300 and 600 °C. In the low temperature range the liberation of solution components and - to a limited extent - an oxidative cracking reaction of the acetate ligand takes place catalyzed by the noble metal. In the high temperature range the evolution of chlorine as well as the decomposition of surface species formed (carbonyls, carboxylates, carbonates) can be observed. The acetate ligand shows extreme high stability and is decomposed in the 400-550 °C range, only.Since the formation and decomposition of the organic surface species can significantly influence the morphology (and thus the electrochemical properties) of the films, the complete understanding of the film evolution process is indispensable to optimize the experimental conditions of electrode preparation.     

On the use of scanning tunneling microscopy to investigate surface structure and interface formation in transition metal oxides: SrTiO3 and TiO2

Since transition metal oxides are wide bandgap, low conductivity materials compared to conventional semiconductors, surface analysis by scanning tunneling microscopy (STM) is expected to be problematic. This paper considers the factors that affect atomic scale imaging of transition metal oxides and demonstrates how STM can be exploited to examine the geometric and electronic structures of SrTiO₃ and TiO₂ surfaces, their variations with thermochemical history, and the mechanisms of metal/oxide interface formation. The development of periodic atomic scale surface structure with variations in surface compositions are documented for both oxides. Further, the interactions of these surfaces with metal are examined by characterizing the morphologies that develop upon deposition of Cu on SrTiO₃ and Al on TiO₂.     

Theory of sum frequency generation from metal surfaces

zzz (ω₁,ω₂). Four types of metal surfaces are considered: simple metals, alkali metal overlayers, noble metals, and charged metal surfaces. Differences and similarities with respect to second harmonic generation from these surfaces are pointed out. Received: 20 September 1998     

Determination of Mg and Ti in Ziegler–Natta catalysts by x‐ray fluorescence spectrometry

The metal content in Ziegler–Natta catalysts based on Ti and Mg derivatives could not be determined directly by x‐ray fluorescence (XRF) spectrometry owing to the instability of the catalyst. The samples were then calcined and the resultant Mg and Ti oxides were characterized by x‐ray diffraction (XRD). A series of synthetic standards were prepared with high‐purity MgO and TiO₂, since there were no available certified standards for the catalysts. Samples and standards were compressed with high‐purity H₃BO₃ flux. This protocol was shown to be reproducible, having a correlation coefficient of 0.9999. The metal contents determined by XRF were in accordance with those obtained by a complexometric titration (Mg) and a UV–visible spectrophotometric method (Ti). Copyright © 2004 John Wiley & Sons, Ltd.     

Controlled preparation of well-ordered transition metal oxide layers on a metallic surface

A method to create various well-ordered two dimensional transition metal oxide films on a metallic substrate has been exploited. The formation of an intermediate amorphous layer with controllable metal-oxygen stoichiometry serves as an important precursor condition for the final transformation into a mono-phase, crystalline oxide layer via mild annealing. As a key ingredient serves a Cu₃Au(1 0 0) substrate covered by oxygen. The flat Cu-O topmost layer stops completely intermixing of the substrate material with the subsequently evaporated transition metal film. Likewise the wetting of the surface is considerably enhanced and a homogeneous oxidation of the film is strongly promoted. The proposed technique appears to be widely efficient for preparation of various two dimensional oxide films covering the entire Cu₃Au(1 0 0) substrate. Its usefulness is demonstrated successfully for vanadium, niobium and molybdenum to produce a set of single-phase transition metal oxides of different stoichiometry and geometrical structure. All created oxides are found to be thermally stable at least up to a substrate temperature of 800 K.     

Hydrogen production from mixed cerium oxides via three-step water-splitting cycles

This study deals with the production of hydrogen from water-splitting thermochemical cycles based on mixed metal oxides. The solar synthesis of mixed oxides based on ceria was achieved by melting mixed powders of component oxides at high temperature. The reduced oxides (Ce₂Ti₂O₇, Ce₂Si₂O₇, CeFeO₃, CeVO₄, and CeNbO₄) produced hydrogen in a three-step thermochemical cycle except CeVO₄. The three-step cycle uses an alkali hydroxide (NaOH or KOH) in an activation reaction to improve oxidation and to facilitate water-splitting during the third step. Experimental investigations were performed to demonstrate the concept and to quantify the reactions performance for hydrogen production. The high temperature synthesis of reduced cerium-based mixed oxides was performed in a laboratory-scale solar reactor. The mixed oxides were analysed by X-ray diffraction and characterized by XPS, which identified Ce(III) species in the structure. The activation reaction with NaOH or KOH producing hydrogen was studied to determine the effect of temperature and particle diameter, and to quantify the chemical conversion for each cerium-based mixed oxide.     

铁氧化物中电子隧道现象的扫描隧道显微术研究

报道了利用扫描隧道显微术(STM)对金属表面氧化物层进行电子隧道谱研究的结果。在对两类铁晶体表面氧化层进行的隧道谱和势垒高度测量结果进行分析后表明,常温条件下形成的氧化层(Ⅰ类)应主要是Fe₃O₄;而在高温氧化条件下形成的表面层(Ⅱ类)的主要成分则应是Fe₂O₃。从而表明(STM)可用于研究铁表面氧化过程的不同阶段,并且由Ⅰ类氧化层的低势垒特性说明STM还可以用于观测此类氧化层的内部结构。类似研究方法还可应用到对一系列 关键词：     

Sonochemical water splitting in the presence of powdered metal oxides

Kinetics of hydrogen formation was explored as a new chemical dosimeter allowing probing the sonochemical activity of argon-saturated water in the presence of micro- and nano-sized metal oxide particles exhibiting catalytic properties (ThO₂, ZrO₂, and TiO₂). It was shown that the conventional sonochemical dosimeter based on H₂O₂ formation is hardly applicable in such systems due to catalytic degradation of H₂O₂ at oxide surface. The study of H₂ generation revealed that at low-frequency ultrasound (20 kHz) the sonochemical water splitting is greatly improved for all studied metal oxides. The highest efficiency is observed for relatively large micrometric particles of ThO₂ which is assigned to ultrasonically-driven particle fragmentation accompanied by mechanochemical water molecule splitting. The nanosized metal oxides do not exhibit particle size reduction under ultrasonic treatment but nevertheless yield higher quantities of H₂. The enhancement of sonochemical water splitting in this case is most probably resulting from better bubble nucleation in heterogeneous systems. At high-frequency ultrasound (362 kHz), the effect of metal oxide particles results in a combination of nucleation and ultrasound attenuation. In contrast to 20 kHz, micrometric particles slowdown the sonolysis of water at 362 kHz due to stronger attenuation of ultrasonic waves while smaller particles show a relatively weak and various directional effects.