Two color up-conversion emissions of Er<Superscript>3+</Superscript>-doped Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanopowders prepared by non-aqueous sol-gel method

Er³⁺-doped Al₂O₃ nanopowders have been prepared by the non-aqueous sol-gel method using the aluminum isopropoxide as precursor, acetylacetone as a chelating agent, nitric acid as a catalyzer, and hydrated erbium nitrate as a dopant under isopropanol environment. The different phase structure, including three crystalline types of (Al, Er)₂O₃ phases, α, γ, θ, and an Er–Al–O stoichiometric compound phase, Al₁₀Er₆O₂₄, was observed for the 0.01–0.5 mol% Er³⁺-doped Al₂O₃ nanopowders at the sintering temperature of 1,000 °C. The green and red up-conversion emissions centered at about 523, 545 and 660 nm, corresponding respectively to the ²H_11/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions of Er³⁺, were detected by a 978 nm semiconductor laser diodes excitation. With increasing Er³⁺ doping concentration from 0.01 to 0.1 mol%, the intensity of the green and red emissions increased with a decrease of the intensity ratio of the green to red emission. When the Er³⁺ doping concentration rose to 5 mol%, the intensity of the green and red emissions decreased with an increase of their intensity ratio. The maximum intensity of both the green and red emissions with the minimum of intensity ratio was obtained, respectively, for the 0.1 mol% Er³⁺-doped Al₂O₃ nanopowders composed of a single α-(Al,Er)₂O₃ phase. The intensity ratio of the green emission at 523 and 545 nm increased monotonously for all Er³⁺ doping concentrations. The two-photon absorption up-conversion process was involved in the green and red up-conversion emissions of the Er³⁺-doped Al₂O₃ nanopowders.     

Strong visible up-conversion emissions and thermometric applications of Er<Superscript>3+</Superscript>–Yb<Superscript>3+</Superscript> codoped Al<Subscript>2</Subscript>O<Subscript>3</Subscript> prepared by the sol–gel method

The Er³⁺–Yb³⁺ codoped Al₂O₃ has been prepared by the sol–gel method using the aluminium isopropoxide [Al(OC₃H₇)₃]-derived Al₂O₃ sols with addition of the erbium nitrate [Er(NO₃)₃ · 5H₂O] and ytterbium nitrate [Yb(NO₃)₃ · 5H₂O]. The phase structure, including only two crystalline types of doped Al₂O₃ phases, θ and γ, was obtained for the 1 mol% Er³⁺ and 5 mol% Yb³⁺ codoped Al₂O₃ at the sintering temperature of 1,273 K. By a 978 nm semiconductor laser diodes excitation, the visible up-conversion emissions centered at about 523, 545, and 660 nm were obtained. The temperature dependence of the green up-conversion emissions was studied over a wide temperature range of 300–825 K, and the reasonable agreement between the calculated temperature by the fluorescence intensity ratio (FIR) theory and the measured temperature proved that Er³⁺–Yb³⁺ codoped Al₂O₃ plays an important role in the application of high temperature sensor.     

Yb<Superscript>3+</Superscript> → Er<Superscript>3+</Superscript> energy transfer in Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and temperature characteristic of near-infrared photoluminescence

For the Er³⁺–Yb³⁺ codoped Al₂O₃ powders, the strong near-infrared photoluminescence (PL) centered at 1.535 μm derived from the energy transfer (ET) from Yb³⁺ to Er³⁺ was detected by a 978 nm laser diode excitation. Compared with that of Er³⁺ doped Al₂O₃ powders, the PL intensity enhanced about 9 times, the full width at half maximum (FWHM) extended from 82 to 90 nm, and the lifetime increased from 3.22 to 4.17 ms for Er³⁺–Yb³⁺ codoped Al₂O₃ powders at room temperature. The ET coefficient of 2.18 × 10⁻¹⁸ cm³ s⁻¹ from Yb³⁺ to Er³⁺ was obtained based on the rate equations. The decrease of PL intensity with increasing temperature in the range of 298–733 K was observed, due to thermally enhanced nonradiative relaxation ⁴I_13/2 → ⁴I_15/2 dominated over thermally enhanced phonon-assisted ET in the Er³⁺–Yb³⁺ codoped Al₂O₃.     

Sequential coordination of orthosemiquinone radical ligands by surface centers on γ-Al2O3

The kinetics of sequential generation of mono-, bi-, and triradical centers in heterogeneous reactions of toluene solutions of orthoquinones, catechols, and their mixtures with γ-Al₂O₃ was studied. The localization of mono- and biradical centers on the solid oxide surface and the transfer of triradical Al³⁺ (Q⁻)₃ complexes (Q⁻ is the corresponding 3,6-di-tert-butylsemiquinone radical) to a solution (extraction of Al from the solid phase) were proved. The number of the triradical complexes extracted is considerably higher than that of active surface centers accessible for coordination with radical ligands. The conditions favoring the formation of the radical complexes on the Al ions were established. Possible mechanisms of these processes were proposed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2223–2227, November, 1998.     

The state of metals in the Pt/Al2O3 and (Pt-Cu)/Al2O3 catalysts as indicated by IR spectroscopy with isotope dilution of 12C16O with 13C18O molecules

Adsorption of ¹³C¹⁸O+¹²C¹⁶O mixtures on the Pt(2.9%)/γ-Al₂O₃, (Pt(2.6%)+Cu(2.7%))/γ-Al₂O₃, and (Pt(2.6%)+Cu(5.1%))/γ-Al₂O₃ catalysts was studied by FTIR spectroscopy. On the metallic Pt surface at coverages close to saturation, CO is adsorbed both strongly and weakly to form linear species for which the vibrational frequencies of the isolated ¹³C¹⁸O molecules adsorbed on Pt are ∼1940 and ∼1970 cm⁻¹, respectively. The redistribution of intensities of the high-and low-frequency absorption bands in the spectra of adsorbed ¹³C¹⁸O indicates that these linear forms are present on the adjacent metal sites. The weak adsorption is responsible for the fast isotope exchange between the gaseous CO and CO molecules adsorbed on metal. The Pt-Cu alloys, in which the electronic state of the surface Pt atoms characteristic of monometallic Pt remains unchanged, are formed on the surface of the reduced Pt-Cu bimetallic catalysts. The decrease in the vibrational frequencies of the isolated C=O bonds in the isolated Pt-CO complexes suggests that the CO molecules adsorbed on the Cu atoms affect the electronic properties of Pt. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 831–836, May, 2007.     

Application of FTIR spectra for evaluating interfacial reactions in metal matrix composites

Manufacturing of Saffil/MgLi metal matrix composites by the melt infiltration process is accompanied by extensive interfacial redox reaction between δ-Al₂O₃ fibers (Saffil) and lithium. The present paper deals with the Fourier transform infrared spectroscopy examination of Saffil fibers isolated from Mg–8 wt% Li alloy by the bromine/methylacetate agent focusing on the insertion of Li⁺ ions into δ-Al₂O₃ and their influence on water adsorption. Insertion of Li⁺ into δ-Al₂O₃ is monitored by gradual change of Al–O stretching bands (400–900 cm⁻¹) towards more simple patterns of a spinel-like product assigned as δ(Li) which transforms to LiAl₅O₈ during subsequent annealing. Rapid increase in the water adsorption with increase in Li content, indicated by the changes in H–O–H bending (about 1,650 cm⁻¹) and O–H stretching (about 3,500 cm⁻¹), is connected with the ionicity of the δ(Li) phase, which attracts polar water molecules.     

Structural and spectroscopic characterization of Al2−x Cr x O3 powders obtained by polymeric precursor method

Al₂O₃ and Al_2−x Cr _x O₃ (x = 0.01, 0.02 and 0.04) powders have been synthesized by the polymeric precursors method. A study of the structural evolution of crystalline phases corresponding to the obtained powders was accomplished through X-Ray Diffraction and UV-vis spectroscopy (reflectance spectra and CIEL^*a^*b^* color data). The obtained results allow to identify the γ-Al₂O₃ to α-Al₂O₃ phase transition. The single-phase α-Al₂O₃ powder was obtained after heat treatment at 1050 °C for 2 h. The results show that the green to red color transition and ruby luminescence lines observed for the powders of Al_2−x Cr _x O₃ are related to the γ to α-Al₂O₃ phase transition and the temperature and time range for such transition depends on the chromium content.     

A study of the influence of the conditions of preparation of γ-aluminum oxide as a carrier for reforming catalysts on its physicochemical properties

The formation of highly imperfect γ-Al₂O₃ oxide prepared by calcining pseuodoboehmite and plasticized by organic acids was studied. The nature of the organic acid-aluminum hydroxide plasticizer was found to substantially influence the degree of γ-Al₂O₃ structure imperfection estimated qualitatively as the difference between the X-ray structural density and effective density with respect to helium and aluminum oxide. A high degree of imperfection caused an increase in the intensity of the absorption band at 3775 cm⁻¹ corresponding to OH groups localized on five-coordinate Al³⁺ and the concentration of Lewis acid centers. The adsorption and catalytic properties of systems based on these carriers were studied.     

Catalytic properties of Pt/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts in the aqueous-phase reforming of ethylene glycol: Effect of the alumina support

The influence of the alumina support on the catalytic activity of Pt/Al₂O₃ catalysts in aqueous phase reforming of ethylene glycol to hydrogen was studied. The catalysts were prepared by impregnation of γ-, δ-, and α-alumina with H₂PtCl₆. The highest rate of hydrogen production (452 μmol min⁻¹ g⁻¹) obtained with the Pt/α-Al₂O₃ catalyst can be related to the highest extent of dispersion of Pt on α-Al₂O₃. XPS, TEM-EDX and TPR-H₂ measurements showed the absence of chloride-containing surface complexes in the Pt/α-Al₂O₃ catalyst. However, chloride-containing entities were found on the surface of Pt/γ-Al₂O₃ and Pr/δ-Al₂O₃ catalysts. When chloride ions are removed chlorinated Pt species facilitate the sintering of Pt crystallites and in this way affect the extent of Pt dispersion. Moreover, depending upon the particular crystalline form, alumina atoms have different coordination and alumina surfaces contain varying amounts of OH groups of different nature which affect the interaction between Pt and the support.     

Effect of phenyl group on the structure and formation of transitional alumina from Al (OPh)<Subscript>3</Subscript>

The structure of freshly prepared Al(OPh)₃, its decomposition product, the hydrolyzed products and their structural evolution were investigated employing ²⁷Al MAS NMR spectroscopy, PXRD, TGA/DTA/DSC/FTIR techniques. In the ²⁷Al MAS NMR spectrum of the aluminium phenoxide, three signals with the chemical shift at 3.78, 21 and 45 ppm were observed. The chemical shift at 3.78 and 45 ppm revealed the presence of four and sixfold coordinated aluminum. The signal at 21 ppm corresponded to fivefold coordinated aluminium. When the aluminium phenoxide was directly decomposed in air at 600 °C, it resulted in amorphous product as evidenced from the PXRD pattern. The observed signals with chemical shifts at 10.1, 42, 73.6 ppm in the ²⁷Al MAS NMR spectrum indicated the presence of 6, 5 and 4 coordination for the aluminium atoms suggesting disordered transitional γ-alumina to be the product. The hydrolysis studies of Al(OPh)₃ with excess of water at 70 °C yielded bohemite (γ-AlOOH). The alumina obtained after dehydration at 600 °C was X-ray amorphous. The dehydrated product at 600 °C showed the presence of four and six coordinated aluminium atoms in the ²⁷Al MAS NMR spectrum confirming it to be ordered γ-Al₂O₃. Crystalline γ-Al₂O₃ was obtained on further heating at 800 °C.     

Acid-base properties of alumina prepared from a hydrated product of centrifugal thermal activation of hydrargillite (cta-product)

Summary Acid-base properties of aluminas prepared by thermal treatment of a hydrated CTA-product at 600°C were studied. The CTA-oxides, representing γ-Al₂O₃, were shown to contain terminal and bridged OH-groups. The concentration of the terminal OH-groups in the CTA-oxides was found to exceed their concentration in γ-Al₂O₃ prepared by dehydration of the “precipitated” pseudoboehmite, whereas the concentration of the bridged OH-groups in the CTA-oxides was lower than that in γ-Al₂O₃ prepared from pseudoboehmite. The total concentration of the surface Lewis acid sites in CTA-oxides varies within the limits of 2.80-4.14 mmol/m² and is essentially above that in g-Al₂O₃ (2.25 mmol/m²). The distinctive feature of the CTA-oxides is that their surface contains strong Lewis acid sites with n_CO = 2220 and 2238 cm^-1. The total concentration of basic sites in the CTA-oxides is lower than that in g-Al₂O₃, however, in contrast to g-Al₂O₃,they contain strong basic sites with n_CDCl3 = 2200 cm^-1.     

Effect of heating rates on TG-DTA results of aluminum nanopowders prepared by laser heating evaporation

Aluminum (Al) nanopowders with mean diameter of about 50 nm and passivated by alumina (Al₂O₃) coatings were prepared by an evaporation route: laser heating evaporation. Thermal properties of the nanopowders were investigated by simultaneous thermogravimetric-differential thermal analysis (TG-DTA) in dry oxygen environment, using a series of heating rates (5, 10, 20, 30, 50 and 90°C min⁻¹) from room temperature to 1200°C. With the heating rates rise, the onset and peak temperatures of the oxidation rise, and the conversion degree of Al to Al₂O₃ varies. However, the specific heat release keeps relatively invariant and has an average value of 18.1 kJ g⁻¹. So the specific heat release is the intrinsic characteristic of Al nanopowders, which can represent the ability of energy release.     

On the molecular models of lewis acid sites on the surface of γ-Al2O3 and in zeolites: a density functional study of CO adsorption

Adsorption complexes of CO-Lewis acid sites with 3- and 5-coordinated Al³⁺ are modeled by the density functional method using the cluster approach. Cluster models of the site with 4-coordinated Al³⁺ on the surface of γ-Al₂O₃ and in zeolites are suggested. For these models of adsorption complexes, C-O vibration frequencies are calculated and the energetics of CO adsorption is evaluated. Translated fromZhurnal Struktumoi Khimii, Vol. 38, No. 5, pp. 834–839, September–October, 1997.     

Effect of gamma-irradiation on surface and catalytic properties of nanocrystalline CuO,NiO and Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> supported on alumina

The effect of γ-irradiation on surface and catalytic properties of CuO/Al₂O₃, NiO/Al₂O₃ and Fe₂O₃/Al₂O₃ was investigated. The techniques employed were XRD, nitrogen adsorption at −196 °C and catalytic conversion of ethanol and isopropanol at 250–400 °C using micropulse technique. The results showed that the supported solids being calcined at 400 °C consisted of well crystallized CuO, NiO, Fe₂O₃ and AlOOH phases. The AlOOH crystallized into a poorly crystalline γ-Al₂O₃ upon heating at 600 °C. All phases present in different solids calcined at 400 and 600 °C showed that these solids are of nanocrystalline nature measuring an average crystallite size between 6 and 85 nm. The crystallite size of crystalline phases present was found to be much affected by the dose of γ-rays and the nature of the metal oxide. This treatment resulted in a progressive increase in the specific surface area reaching to a maximum limit at a dose of 0.8 MGy. The dose of 1.6 MGy exerted a measurable decrease in the S _BET. A radiation dose of 0.2 to 0.8 MGy brought about a progressive significant decrease in the catalytic activity of all the catalytic systems investigated. All the catalytic systems retained their high activity upon exposure to a dose of 1.6 MGy. The rise in precalcination temperature of the systems investigated from 400 to 600 °C brought about a measurable increase in their catalytic activity in the conversion of alcohols.     

Regularities of photostimulated conversions in nanometer aluminum layers

The gravimetric and optical spectroscopic methods reveals that light irradiation with λ = 300–750 nm and intensity I = 6.9 × 10¹⁴–1.1 × 10¹⁶ quanta cm⁻² s⁻¹ for τ = 1–160 min in atmospheric conditions significantly changes the absorption and reflection spectra and mass of aluminum films (d = 2–200 nm). The kinetic curves of the degree of conversion versus aluminum film thickness are satisfactorily described in the inverse logarithmic and parabolic terms. The contact potential difference is measured for Al and Al₂O₃ films along with the photo-EMF of Al-Al₂O₃ systems. The suggested model includes the stages of generation and redistribution of nonequilibrium charge carriers in the contact field of Al-Al₂O₃ systems, oxygen adsorption, Al³⁺ diffusion, and Al₂O₃ formation.     

Kinetic studies on the hydrogenation of nitrate in water using Rh/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Rh-Cu/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts

Liquid-phase reduction NO ₃ ⁻ using monometallic and bimetallic catalysts (5% Rh/Al₂O₃, 5% Rh-0.5% Cu/Al₂O₃, 5% Rh-1.5% Cu/Al₂O₃, 5% Rh-5% Cu/Al₂O₃ and a physical mixture of 5% Rh/Al₂O₃ and 1.5% Cu/Al₂O₃) was studied in a slurry reactor operating at atmospheric pressure. Kinetic measurements were performed for a low concentration of nitrate (0.4 × 10⁻³−3.2 × 10⁻³ mol dm⁻³) and the temperature range 293–313 K. From the experimental data, it was found that the reduction of nitrate is first order with respect to nitrate. On the basis of the rate constants, the apparent activation energy was established using a graphic method. Published in Russian in Kinetika i Kataliz, 2007, Vol. 48, No. 6, pp. 881–886. This article was submitted by the authors in English.     

Pt LIII-edge XANES and EXAFS studies on γ-Al2O3 supported Pt catalysts

Pt L_III-edge XANES and EXAFS were employed to investigate the nature of Pt/γ-Al₂O₃, Pt−Sn/γ-Al₂O₃ and Pt−Fe/γ-Al₂O₃ catalysts. The results indicated that Pt species on these catalysts were all in the oxidized states before reduction, and in the metallic states after reduction. The dispersity of the Pt species on the catalysts was very high after reduction. The electronic properties of the highly dispersed Pt species were different from that of the bulk Pt in large crystallites. An interaction between Pt and the metal-oxide modified γ-Al₂O₃ support is proposed. The interaction improved the dispersity of the Pt species on the catalysts and is thought to be the reason for the enhanced activity and selectivity for dehydrogenation reactions over these catalysts.     

Characterisation of Ni carbonate-bearing minerals by UV–Vis–NIR spectroscopy

Four nickel carbonate-bearing minerals from Australia have been investigated to study the effect of Ni for Mg substitution. The spectra of nullaginite, zaratite, widgiemoolthalite and takovite show three main features in the range of 26,720–25,855 cm⁻¹ (ν₁-band), 15,230–14,740 cm⁻¹ (ν₂-band) and 9,200–9,145 cm⁻¹ (ν₃-band) which are characteristic of divalent nickel in six-fold coordination. The Crystal Field Stabilization Energy (CFSE) of Ni²⁺ in the four carbonates is calculated from the observed ³A_2g(³F) → ³T_2g(³F) transition. CFSE is dependent on mineralogy, crystallinity and chemical composition (Al/Mg-content). The splitting of the ν₁- and ν₃-bands and non-Gaussian shape of ν₃-band in the minerals are the effects of Ni-site distortion from regular octahedral. The effect of structural cation substitutions (Mg²⁺, Ni²⁺, Fe²⁺ and trivalent cations, Al³⁺, Fe³⁺) in the carbonate minerals is noticed on band shifts. Thus, electronic bands in the UV–Vis–NIR spectra and the overtones and combination bands of OH and carbonate ion in NIR show shifts to higher wavenumbers, particularly for widgiemoolthalite and takovite.     

Study of chemical processes in the modification of epoxide polymers by aluminum oxide

The processes occurring during the modification of epoxy polymers by various polymorphic aluminum oxide modifications (γ-AlO(OH), γ-Al₂O₃, α-Al₂O₃) with epoxy groups were studied by the methods of IR Fourier spectroscopy, chemical analysis, and differential scanning calorimetry (DSC) by an example of a model compound (phenyl glycidyl ether). Two types of interactions were revealed: a direct chemical reaction of phenyl glycidyl ether with the surface hydroxy groups of alyminum oxide, and phenyl glycidyl ether homopolymerization. By processing by graphical method the data of chemical analysis on the diminishing in amount of epoxy groups in the course of the polycondensation reaction the value of activation energy 106–110 kJ mol⁻¹ of the process of phenyl glycidyl ether interaction with aluminum γ-oxide was determined.     

State of active components on the surface of the PdCl<Subscript>2</Subscript>-CuCl<Subscript>2</Subscript>/γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst for the low-temperature oxidation of carbon monoxide

The state of the active constituents of the freshly prepared PdCl₂-CuCl₂/γ-Al₂O₃ catalyst for the low-temperature oxidation of the carbon monoxide by molecular oxygen was studied by X-ray absorption spectroscopy (XAS), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and diffuse reflectance IR Fourier transform spectroscopy (DRIFTS). It was shown that copper in the form of a crystalline phase of Cu₂Cl(OH)₃ with the structure of the mineral paratacamite and palladium chloride in an amorphous state occurred on the surface of γ-Al₂O₃. According to XAS data, the local environment of palladium consisted of four chlorine atoms, which formed a flat square with an increased distance between palladium and one of the chlorine atoms. The evolution of the local environments of copper and palladium upon a transition from the initial salts to the impregnating solutions and chlorides on the surface of γ-Al₂O₃ was considered. The role of γ-Al₂O₃ in the formation of the Cu₂Cl(OH)₃ phase was discussed. It was found by the DRIFTS method that linear (2114 cm⁻¹) and bridging (1990 and 1928 cm⁻¹) forms of coordinated carbon monoxide were formed upon the adsorption of CO on the catalyst surface. The formation of CO₂ upon the interaction of coordinated CO with atmospheric oxygen was detected. Active sites including copper and palladium were absent from the surface of the freshly prepared catalyst.