Formation of alumina-silver-palladium systems

Diffuse-reflection electron spectroscopy was used to investigate the state of alumina-silver-palladium catalysts as a function of the conditions of synthesis and heat treatment. Mono- and bimetallic particles were found to be present on the surface of a catalyst manufactured by impregnation of the carrier from one impregnating solution. Premodification of the carrier with palladium is accompanied by formation of metal implantation structures in the carrier lattice, while silver modification does not produce such structures. In the Pd/Ag/Al₂O₃ catalyst, palladium occurs at several oxidation levels. Institute of Physico-Organic Chemistry, Academy of Sciences of Belarus, 13, Surganov St., Misnk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 2, pp. 169–172, March–April, 1997.     

The influence of Fe,Cu, SiO<Subscript>2</Subscript>, TiO<Subscript>2</Subscript>, and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles in aqueous solution on proton relaxation times

Measurements of proton nuclear spin-spin and spin-lattice relaxation times are applied for determining the concentration of solid-phase nanoparticles in nanofluids. This approach is tested for metal oxides SiO₂, TiO₂, Al₂O₃ and metal-carbon nanoparticles of 3d-metals Fe and Cu. It is shown that the sensitivity of the method for determining concentrations of 3d-metals is much higher than for oxides (by 2–4 orders of magnitude). It is revealed that measurement of the proton spin-spin relaxation time allows one to determine the concentration of Cu nanoparticles to 0.0001 mg/ml and that of Fe nanoparticles to 0.00001 mg/ml.     

Structural and catalytic properties of ZnO and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanostructures loaded with metal nanoparticles

Nanostructured zinc oxide (ZnO) nanobelts and aluminum oxide (Al₂O₃) nanoribbons have been grown successfully from the vapor phase. XRD results confirmed the purity and the high quality of the formed crystalline materials. TEM images showed that ZnO nanostructures grew in the commonly known tetrapod structure with nanobelts separated from the tetrapods with an average width range of 10–30 nm and a length of about 500 nm. Al₂O₃ nanostructures grew in the form of nanoribbons with an average width range of 20–30 nm and a length of up to 1 μm. The catalytic oxidation of CO gas into CO₂ gas over the synthesized nanostructures is also reported. Higher catalytic activity was observed for Pd nanoparticles loaded on the ZnO nanobelts (100% conversion at 270 °C) and Al₂O₃ nanoribbons (100% conversion at 250 °C). The catalytic activity increased in the order Cu < Co < Au < Pd for the metal-loaded nanostructures. The preparation methods could be applied for the synthesis of novel nanostructures of various materials with novel properties resulting from the different shapes and morphologies.     

Influence of the spacing between metal particles on the kinetics of reaction with spillover on the supported metal catalyst

The influence of the spacing between active metal particles placed on the supported metal catalyst on the kinetics of the catalytic reaction with spillover was investigated. The 2A+B₂→2AB reaction, modelling the CO oxidation on Pd/Al₂O₃ catalyst, was studied using Dynamic Monte Carlo simulations. It was shown that there exists an optimal spacing, that provides the maximum reaction rate. It was postulated that this optimum is a consequence of both competition and cooperative effects occurring between metal particles.     

Laser synthesis of nanopowders

YSZ, YSZ + Al₂O₃, Ce_0.78 Gd_0.22O_2−δ, and 5NdY₂O₃ nanopowders are obtained using target evaporation with a repetitively pulsed CO₂ laser and subsequent vapor condensation in the flow of a carrier gas. The design of the laser complex for producing the nanopowder and the block diagram and the characteristics of the repetitively pulsed CO₂ laser pumped by a combined discharge are presented. The size distribution of the nanoparticles is studied and the x-ray data are reported. It is demonstrated that a nanopowder output rate of 15–75 g/h linearly increases with the mean laser power. Under equal conditions, the size distribution of the particles is weakly affected by the type of the target material. The results obtained are interpreted. Original Text ? Astro, Ltd., 2006.     

Network structure and thermal property of a novel high temperature seal glass

In this study, novel glasses based on SrO–La₂O₃–Al₂O₃–B₂O₃–SiO₂ system are investigated for solid oxide fuel and electrolyzer cells. The network structure evolution of the glasses with increasing B₂O₃:SiO₂ ratio was studied using Raman spectroscopy. The thermal properties of the glasses, including glass transition temperature T _g and glass softening temperature T _d , were studied using dilatometry. The thermal stability of the glasses was investigated using X-ray diffraction. The study shows that as the B₂O₃:SiO₂ ratio increases, the SrO–La₂O₃–Al₂O₃–B₂O₃–SiO₂ glass micro-heterogeneity and the amount of non-bridging oxygen atoms increase. Correspondingly, the T _g of the SrO–La₂O₃–Al₂O₃–B₂O₃–SiO₂ glasses changes from 635 to 775°C, and the T _d changes from 670 to 815°C. Glass thermal stability decreases with B₂O₃:SiO₂ ratio increase. The glass without B₂O₃ is thermally stable after being kept at 850°C for 200 hrs.     

Spectrooptical investigation of the palladium state in Pd/TiO2 and Pd/TiO2(Nb2O5) systems

The state of Pd in Pd/TiO₂ and Pd/TiO₂(Nb₂O) systems is investigated by means of electron diffuse reflection spectroscopy, x-ray electron spectroscopy, and IR spectroscopy of adsorbed CO as a function of thermal and reduction treatments. Modification of titanium dioxide TiO₂ leads to intensification of metal-support interaction, which manifests itself as an increase in the content of surface complexes of Pd strongly bound to TiO₂. As a result of electron density transfer from TiO₂ with electron-excess Ti³⁺ ions to the metal deposited, the latter acquires an enhanced reduction ability. Institute of Physical and Organic Chemistry, National Academy of Sciences of Belarus, 13, Surganov St., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 1, pp. 84–88 January–February, 1999.     

Enhancement of ionic conductivity in the composite solid electrolyte system: TlI–Al2O3

This paper reports the heterogeneously doped alumina (Al₂O₃) on the ionic conductivity of thallium iodide. Composite materials of formula (1 − x) TlI–xAl₂O₃, x = 0–0.7 have been prepared and studied by X-ray diffraction, differential scanning calorimetry, and electrical conductivity. X-ray diffraction and differential scanning calorimetry proved the formation of composite in this binary system. The maximum enhancement observed is about three orders of magnitude with respect to the host material. The enhancement of electrical conductivity in comparison with pure thallium iodide can be interpreted with the space charge layer model. Moreover, the increased content of alumina in the system leads to the disappearance of phase transition β–α thallium iodide, which is usually observed in the pure compound. This behavior was explained by stabilizing effect of β-phase at high temperatures and suppression of α-phase at higher contents of alumina.     

Laser-assisted activation of dielectrics for electroless metal plating

2 O₃, SiC, diamond, ZrO₂, etc. are presented. The activation of the dielectric surface can be achieved in a wide range of laser wavelengths and is stable in time. This activation allows selective deposition of various metals (Cu, Ni, Pt, Pd, etc.) with lateral dimensions of several μm. The model of the activation process is discussed. This deals with the modification of the band gap of the dielectric, which involves the appearance of a non-zero density of electronic states in the vicinity of the potential of electroless metal reduction. These electronic states can arise either from the formation of point defects in the ablated surface (for example, F centers in Al₂O₃, CeO₂, or ZrO₂) or from the band bending of the dielectric caused by residual mechanical stresses left in the material after laser ablation (SiC or diamond). The data on the activation of dielectrics by mechanical indentation are qualitatively consistent with the model. Received: 5 January 1998/Accepted: 7 January 1998     

Adsorption of titanium,chromium, and copper atoms on thin aluminum and magnesium oxide film surfaces

Methods of Auger electron spectroscopy (AES), spectroscopy of characteristic electron energy losses (SCEEL), slow electron diffraction (SED), and contact potential difference (CPD) in ultrahigh vacuum are used to investigate the adsorption-emission properties and stability of two-component film systems formed by putting of Ti, Cr, and Cu atoms on MgO–Mo(011) and Al₂O₃–Mo(011) surfaces. All atoms have the properties of electronegative adsorbates. Continuous adatom monolayers are formed on the Al₂O₃–Mo(011) system surface, and three-dimensional islands are formed on the MgO–Mo(011) surface. The properties of monoatomic films on the oxide layer surface are close to those observed for bulk materials. No radical changes of the system properties are detected with increasing dielectric layer thickness. The thermal stability of the newly formed structures decreases in the order Ti, Cr, Cu, Al₂O₃(MgO), and Mo(011).     

Relaxation, self-trapping, and decay of electron excitations in wide-gap oxides

Emission spectra ofMgO, Al ₂O₃, Y₂O₃, andSc ₂O₃ crystals, excited at 8 K by photons with energies 5–40 eV, are measured. The luminescence of free excitons andGe ²⁺ centers forMgO, the self-trapped exciton emission (STEE) forAl ₂O₃ andY ₂O₃, and the tunnel self-luminescence forScO ₃ are studied. A distinction between hole polarons in oxides and self-trapped holes in halides leads to a sharp difference in the mechanisms of STEE creation on electronhole recombination for these two classes of solids. The electron-hole, hole-electron, and exciton mechanisms of excitation multiplication are identified. The high radioresistance of wide-gap oxides and the behavior of oxygen interstitials inMgO crystals are discussed. Institute of Physics of Estonia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 5–16, March, 2000.     

Controlling metal/oxide interactions in bifunctional nanostructured model catalysts: Pd and BaO on Al2O3/NiAl(1 1 0)

We have investigated the structure and growth of Pd and BaO containing nanoparticles sequentially co-deposited on an ordered Al₂O₃/NiAl(1 1 0) by scanning tunneling microscopy (STM), and their interaction with CO and NO₂ by infrared reflection absorption spectroscopy (IRAS).Ba deposition and subsequent oxidation result in Ba_xAl_2yO_x+3y nanoparticles being formed which are characterized by high particle densities and nucleation both on regular terraces and at oxide defects. In contrast, Pd interaction with the model support is weaker and preferential nucleation occurs mainly at rotational domain boundaries and to a lesser extent at anti-phase domain boundaries. For subsequent co-deposition of Pd on preformed Ba_xAl_2yO_x+3y/Al₂O₃/NiAl(1 1 0), Pd nucleates at the Ba_xAl_2yO_x+3y nanoparticles and covers them. The reverse deposition sequence, i.e. subsequent Ba co-deposition and oxidation on preformed Pd/Al₂O₃/NiAl(1 1 0), leads to formation of small isolated Ba_xAl_2yO_x+3y nanoparticles without contact to Pd, together with large Pd crystallites modified by Ba_xAl_2yO_x+3y. The present results provide control over the degree of interaction between metal nanoparticles and oxide nanoparticles on a well-defined model catalyst and thus allow us to study effects related effects on the reactivity and catalytic behavior.     

New advanced in situ carbon monoxide sensor for the process application

Mixed potential solid electrolyte CO sensors with sensing electrodes based on composite with various semiconducting oxides were extensively studied in the temperature range 500–650 °C for sensitivity, stability and cross-sensitivity besides CO to other combustion components like CO₂, H₂O, O₂, and SO₂. The highest CO sensitivity was found for the CO sensor with composite electrode based on Au/Ga₂O₃ showing also good reproducibility and stability in hazardous combustion environment. CO sensor response behavior in non equilibrated oxygen containing gas mixtures is mainly determined by the catalytic activity of the measuring electrode and depends strongly on preparation and measuring conditions. Mixed oxides based on doped chromites show only a little sensitivity to CO. CO sensor based on Au/Ga₂O₃ composite electrodes was showing good CO selectivity in the presence of other combustion gas species and finally was tested in combustion environment at power plant. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15, 2007.     

Angular dependence of surface acoustic wave characteristics in AlN thin films on a-plane sapphire substrates

AlN thin films have been grown on a-plane sapphire (Al₂O₃(112̄0)) substrates. X-ray diffraction measurements indicate the films are fully c-plane (0001) oriented with a full width at half maximum of the AlN(0002) rocking curves of 0.92. The epitaxial growth relationships have been determined by the reflection high energy electron diffraction analysis as AlN[11̄00]//Al₂O₃[0001] and AlN[112̄0]//Al₂O₃[11̄00]. Angular dependence of important surface acoustic wave (SAW) characteristics, such as the phase velocity and electromechanical coupling coefficient, has been investigated on the AlN(0001)/Al₂O₃(112̄0) structure. While the SAW is excited at all propagation angles with an angular dispersion of the phase velocity in the range of 5503–6045 m/s, a higher velocity shear-horizontal (SH) mode is observed only at 0°, 105° and 180° off the reference Al₂O₃[11̄00] over a 180° angular period. The phase velocity of the SH mode shows dispersion (6089–6132 m/s) as a function of the SAW wavelength. Temperature coefficients of frequency are also demonstrated for both modes. PACS 81.15.Hi; 77.84.-s; 77.65.Dq     

Shape of the Cu(2) NQR spectra in YBa2Cu3O7, TmBa2Cu3O7 and TmBa2Cu4O8

We present a study of shape of the Cu(2) NQR spectra in YBa₂Cu₃O₇, TmBa₂Cum₃O₇, and TmBa₂Cu₄O₈ compounds at temperatures of 4.2–300 K. The results of the quantitative analysis lead us to conclude that the shape of the Cu(2) NQR spectra in all the samples studied can be described in the framework of the “motional narrowing” model, which implies that the Cu(2) nucleus possesses two different NQR frequencies between which it can rapidly jump. The difference in frequencies seems to be related to the charge-stripe correlations in CuO₂ planes resulting in a dynamical modulation of the electric field gradients at the Cu(2) nuclei. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 9, 594–598 (10 November 1997)     

Electrochemical promotion of Pd for the hydrogenation of C2H2

The effect of Non-Faradaic Electrochemical Modification of Catalytic Activity, orin-situ controlled promotion, was investigated during Acetylene selective hydrogenation on Pd films deposited on β″ - Al₂O₃, a Na⁺ conductor, at temperatures from 60 to 100 °C and GHSVs from 10³ to 10⁴ h-⁻¹, i.e., under conditions similar to those used in industrial processes. It was found that both acetylene conversion and hydrogenation selectivity can be affected by means of externally applied potentials, i.e., by supplying or removing sodium ions to or from the catalyst surface. Electrochemical sodium supply to the Pd catalyst surfaċe was found to supress both the rate of acetylene hydrogenation and, to a larger extent the rate of ethylene hydrogenation to ethane. Consequently electrochemical sodium supply was found to enhance the selectivity to ethylene. Thus, β″ - Al₂O₃ can act as an active catalyst support causing enhanced performance of the Pd catalyst. Acetylene conversion and hydrogenation selectivity values up to 90 % and 95 %, respectively, were obtained. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Thermally stimulated luminescence of bismuth germanate ceramics with the benitoite,eulitine, and sillenite structures

Thermally stimulated luminescence (TSL) of Bi₂Ge₃O₉, Bi₄Ge₃O₁₂, and Bi₁₂GeO₂₀ and the primary components Bi₂O₃ and GeO₂ was studied under x-ray excitation. Thermal activation energies and frequency factors of trapping centers in the studied ceramics were determined. The relationships of TSL bands of the studied ceramics with maxima at 141–145 and 166–170 K and damage to the Ge sublattice and of TSL bands with maxima at 104–110 and 180–190 K and recombination processes in the Bi sublattice were demonstrated. Recombination processes causing luminescence upon nonequilibrium charge carrier release from trapping centers occur in structural complexes of similar configuration that contain the Bi ion in a nearest environment of O atoms. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 3, pp. 359–364, May–June, 2008.     

Ion synthesis and laser annealing of Cu nanoparticles in Al2O3

Al₂O₃ samples with Cu nanoparticles, synthesised by ion implantation at 40 keV with a dose of 1×10¹⁷ ion/cm² and a current density from 2.5 to 12.5 μA/cm², were annealed using ten pulses from a KrF excimer laser with a single pulse fluence of 0.3 J/cm². The copper depth distribution, formation and modification of metal nanoparticles under the ion implantation and laser treatment were studied by Rutherford backscattering (RBS), energy dispersive X-ray (EDX) analysis, atomic force microscopy (AFM) and optical spectroscopy. It was found that laser annealing leads to a reduction in the nanoparticle size without diffusion of metal atoms into the bulk. The change in particle size and the possibility for oxidation of the copper particles are examined in the framework of Mie theory. Calculations presented show that under excimer laser treatment, Cu nanoparticles are more likely to be reduced in size than to undergo oxidation. Received: 19 April 2001 / Accepted: 7 November 2001 / Published online: 23 January 2002     

Reaction mechanism of spinel CuFe2O4 with CO during chemical-looping combustion: An experimental and theoretical study

Spinel CuFe₂O₄ is a promising oxygen carrier due to its synergistic enhanced performance. A fundamental understanding of the reaction mechanism between oxygen carrier and fuels is important for a rational design of highly efficient oxygen carrier. The reaction mechanism of spinel CuFe₂O₄ with CO during chemical-looping combustion (CLC) was studied based on thermogravimetric analyses (TGA) and density functional theory (DFT) calculations. Two distinct reaction stages were clearly observed. CuFe₂O₄ was mainly transformed into Cu and Fe₃O₄ with a rapid reaction rate in the initial stage, and then product Fe₃O₄ was slowly reduced to FeO or even to Fe. The reactivity of CuFe₂O₄ is much higher than that of Fe₂O₃, which is ascribed to the existence of Cu. The enhanced oxygen evolution activity of CuFe₂O₄ at low temperature is validated by both the experimental and theoretical methods. Three types of surface oxygen coordinated with different metal atoms show different reactivity. Two kinds of reaction pathways are involved in CO oxidation over CuFe₂O₄. In the one-step reaction pathway, CO directly reacts with the oxygen bonding to two octahedral Cu and one octahedral Fe atoms to form a CO₂ molecule without an energy barrier, which corresponds to the surface oxygen consumption observed in TGA experiments. In the possible two-step reaction pathway, CO first adsorbs on the surface, and then reacts with the oxygen bound to one octahedral Cu and two octahedral Fe atoms to generate CO₂ by surmounting an energy barrier of 10.84 kJ/mol, which is the most kinetically and thermodynamically favorable pathway.     

UV-protection properties of electrospun polyacrylonitrile nanofibrous mats embedded with MgO and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles

This article describes the ultraviolet (UV) protection of MgO and Al₂O₃ nanoparticles embedded electrospun polyacrylonitrile (PAN) nanofibrous mats. UV radiation is a harmful part of sunlight and prolonged exposure to it can cause serious skin damages. In this research, nanofibrous mats consisting of nanofibers with different diameters containing different amounts of MgO, Al₂O₃, MgO Plus, and Al₂O₃ Plus nanoparticles were produced, and their UV-protection was measured. The specific surface area of MgO, MgO Plus, Al₂O₃, and Al₂O₃ Plus nanoparticles was 230, 600, 275, and 550 m²/g, respectively. The mean diameter of electrospun PAN nanofibers embedded with metal oxide nanoparticles was in the range of 665–337 nm. The results showed that the UV-protection (shielding) capability of the mats strongly depends on fiber diameter; in fact a thin mat of nanofibers has a much stronger UV-protection in comparison to a thicker mat composed of regular fibers. UV transmission is reduced as a result of embedding MgO and Al₂O₃ nanoparticles in the electrospun PAN nanofibrous mats. MgO Plus and Al₂O₃ Plus show higher UV-protection than MgO and Al₂O₃.