Preparation of hollow alumina microspheres by microwave-induced plasma pyrolysis of atomized precursor solution

Hollow alumina microspheres have been prepared by microwave-induced (MI) plasma pyrolysis of atomized aerosols of precursor solutions and subsequent calcination at 1300 °C for 2 h. When an aqueous solution of 0.5 mol dm⁻³ Al(NO₃)₃ without any additives was used as a precursor, hollow -Al₂O₃ microspheres with a thick shell wall were prepared after post-calcination at 1300 °C. The addition of a polypropylene (PO)–polyethylene(EO) blockcopolymer (molecular weight: 2900–6500) to the precursor solution was effective for increasing the yield of hollow microspheres, but resulted in the formation of many cracks and holes in the thinned shell wall. Hollow alumina microspheres with a thin, but strong, shell layer could be prepared by the simultaneous addition of tetraethylorthosilicate.     

Preparation of spherical yttrium oxide powders using emulsion evaporation

Water-in-oil type emulsions were used to prepare yttrium oxide powders by evaporation of the yttrium-containing aqueous phase in a hot oil bath. Emulsions were characterized with respect to emulsion type, droplet size distribution and stability. Y₂O₃ powders obtained by this method consisted of small (1–3 μm) spherical granules made up of 0·1 μm crystallites. The effects on the powder of emulsifier concentration, yttrium ion concentration, evaporation temperature and atmosphere were studied.     

In-situ growth of needlelike LaAl11O18 for reinforcement of alumina composites

In situ growth of needlelike LaAl₁₁O₁₈ grains reinforcing Al₂O₃ composites can be fabricated by a coprecipitation method using La(NO₃)₃√6H₂O and Al(NO₃)₃√9H₂O as starting materials. The new two-step process involved firstly preparing needlelike LaAl₁₁O₁₈ grains distributed homogeneously in Al₂O₃ powder and then pressureless sintering the composite powders. The Al₂O₃/25 vol.%LaAl₁₁O₁₈ samples pressureless sintered at 1550°C for 4 h achieve relative density up to 96.5% and exhibit a bending strength of 420±30 MPa and a fracture toughness of 4.3±0.4 MPa m^1/2.     

Water-induced bulk Ba(NO3)2 formation from NO2 exposed thermally aged BaO/Al2O3

Phase changes in high temperature treated (>900 °C) 8 or 20 wt% BaO supported on γ-Al₂O₃ model lean NO_x trap (LNT) catalysts, induced by NO₂ and/or H₂O adsorption, were investigated with powder X-ray diffraction (XRD), solid state ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, and NO₂ temperature programmed desorption (TPD) experiments. After calcination in dry air at 1000 °C, the XRD and solid state ²⁷Al MAS NMR results confirm that stable surface BaO and bulk BaAl₂O₄ phases are formed for 8 and 20 wt% BaO/Al₂O₃, respectively. Following NO₂ adsorption over these thermally treated samples, some evidence for nanosized Ba(NO₃)₂ particles are observed in the XRD results, although this may represent a minority phase. However, when water was added to the thermally aged samples after NO₂ exposure, the formation of bulk crystalline Ba(NO₃)₂ particles was observed in both samples. Solid state ²⁷Al MAS NMR is shown to be a good technique for identifying the various Al species present in the materials during the processes studied here. NO₂ TPD results demonstrate a significant loss of uptake for the 20 wt% model catalysts upon thermal treatment. However, the described phase transformations upon subsequent water treatment gave rise to the partial recovery of NO_x uptake, demonstrating that such a water treatment of thermally aged catalysts can provide a potential method to regenerate LNT materials.     

Al18B4O33 aluminium borate: A new efficient support for palladium in the high temperature catalytic combustion of methane

Well crystallised aluminium borate Al₁₈B₄O₃₃ has been synthesised from alumina and boric acid with a BET area of 18 m²/g after calcination at 1100 °C. Afterwards, 2 wt.% Pd/Al₁₈B₄O₃₃ was prepared by conventional impregnation of Pd(NO₃)₂ aqueous solution and calcination in air at 500 °C. The catalytic activity of Pd/Al₁₈B₄O₃₃ in the complete oxidation of methane was measured between 300 and 900 °C and compared with that of Pd/Al₂O₃. Pd/Al₁₈B₄O₃₃ exhibited a much lower activity than Pd/Al₂O₃ when treated in hydrogen at 500 °C or aged in O₂/H₂O (90:10) at 800 °C prior to catalytic testing. Surprisingly, a catalytic reaction run up to 900 °C in the reaction mixture induced a steep increase of the catalytic activity of Pd/Al₁₈B₄O₃₃ which became as active as Pd/Al₂O₃. Moreover, the decrease of the catalytic activity observed around 750 °C for Pd/Al₂O₃ and attributed to PdO decomposition into metallic Pd was significantly shifted to higher temperatures (820 °C) in the case of Pd/Al₁₈B₄O₃₃. The existence of two distinct types of PdO species formed on Al₁₈B₄O₃₃ and being, respectively, responsible for the improvement of the activity at low and high temperature was proposed on the basis of diffuse reflectance spectroscopy and temperature-programmed desorption of O₂.     

HYDROTHERMAL SYNTHESIS OF CRYSTALLINE HYDROXYAPATITE ULTRAFINE PARTICLES

Crystalline hydroxyapaiite ultrafine particles were synthesized by hydrothermal treatment of the same powders prepared by mining aqueous ratio of Ca to P (1·67) at room temperature. The relations of the particle size and shape to such operating conditions as hydrothermal temperature, Ca(NO₃) ₂ concentration, stirring speed and treatment time were investigated systematically. A mechanism of particle growth was proposed.     

The storage of nitrogen oxides on alumina-supported barium oxide

The storage and release of NO₂ on alumina-supported barium oxide has been studied with particular attention to the stoichiometry of the two processes. At 400 °C the storage process is characterised by a short period of complete uptake, possibly as nitrito or nitro species, followed by a slower partial uptake in which approximately one NO is released for every three NO₂ lost. The latter reaction appears to supply the oxygen necessary to store NO₂ as nitrate ions. Molecular O₂ has little direct involvement even if in large excess. The second storage reaction also occurs, but to a much lesser extent, with Al₂O₃ alone. During temperature programmed desorption, release of NO_x from Al₂O₃ peaks at 430 °C with evolution of NO₂ and some O₂. Release from BaO/Al₂O₃ exhibits an additional peak near 520 °C corresponding to formation of NO and a higher O₂ concentration. The NO may arise from NO₂ since BaO/Al₂O₃ has activity for NO₂ decomposition by 500 °C. Although CO₂ at low concentration is rapidly taken up by BaO/Al₂O₃ at 400 °C it is displaced by NO₂ and does not interfere with storage. Thermodynamic calculations show that the formation of Ba(NO₃)₂ by the reaction of NO₂ with bulk BaCO₃ under the conditions used here is more favourable above 380 °C if NO is evolved than if O₂ is consumed.     

Novel utilization of MCM-22 molecular sieves as supports of cobalt catalysts in the Fischer–Tropsch synthesis

Cobalt catalysts (2–10 wt% Co) supported on silica-rich MCM-22 zeolites have been prepared by impregnation with aqueous Co(NO₃)₂ solutions. The catalysts are characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), nitrogen adsorption, solid state nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The catalytic properties for the Fischer–Tropsch synthesis (FTS) at 280 °C, 12.5 bar and H₂/CO = 2 are evaluated. The catalysts supported on MCM-22 exhibit the highest selectivity to long-chain (C₅₊) hydrocarbons when MCM-22 supports are synthesized with the appropriate Si/Al ratio.     

Degradation of olive oil mill effluents by catalytic wet air oxidation: 2-Oxidation of p-hydroxyphenylacetic and p-hydroxybenzoic acids over Pt and Ru supported catalysts

Catalytic wet air oxidation of p-hydroxyphenylacetic acid and p-hydroxybenzoic acid, two important pollutants present in the olive oil mill wastewaters, was studied in a batch reactor using platinum and ruthenium catalysts supported on titanium and zirconium oxides at 140 °C and 50 bar of total air pressure. Reaction pathways for the oxidation of these two substrates were proposed, with formation of different aromatic compounds and short-chain organic acids through hydroxylation and decarboxylation reactions.

It was observed that the conversion and the mineralization of these two substrates were markedly affected by the nature of the ruthenium precursor (RuCl₃ or Ru(NO)(NO₃)₃), with the non-chlorine containing salt giving the best performances. Calcination of the catalyst precursor before reduction was detrimental. The nature of the metallic precursor (H₂PtCl₆ or Pt(NH₃)₄(NO₃)₂) had little influence on the catalytic properties of platinum catalysts, whereas the textural properties of the support were an important factor.     

A regenerable Fe/AC desulfurizer for SO2 adsorption at low temperatures

A novel regenerable Fe/activated coke (AC) desulfurizer prepared by impregnation of Fe(NO₃)₃ on an activated coke was investigated. Experiment results showed that at 200 °C the SO₂ adsorption capacity of the Fe/AC was higher than that of AC or Fe₂O₃. Temperature-programmed desorption (TPD) revealed that H₂SO₄ and Fe₂(SO₄)₃ were generated on the desulfurizer upon adsorption of SO₂. Effect of desulfurization temperature was also investigated which revealed that with increasing temperature from 150 to 250 °C, the SO₂ removal ability gradually increases. The used Fe/AC can be regenerated by NH₃ at 350 °C to directly form solid ammonium-sulfate salts.     

PRECIPITATION IN THE Mg+2-Na+-SO3-2-SO4-2 AQUEOUS SYSTEM

Spontaneous precipitation in the aqueous system Mg₊₂-Na₊-SO₃^-2-SO₄^-2 affected by mixing solutions of MgSO₄ and Na₂SO₃, together was studied for temperature varying from 40 to 80°C and for pH from 5,5 to 9. The initial composition of a precipitating system was 0·67 and 1·17 mol of MgSO₃ and Na₂SO₄ per liter and 0·82 mol of MgSO₃ and Na₂SO₄ together with 0·83 mol of MgSO₄ per liter, respectively. Depending on the prevailing reaction conditions, solid phase consisting of MgSO₃ 6H₂O, MgSO₃ 3H₂O, Mg₂NaOH(SO₃)₂ H₂O or their mixture is formed. Each solid phase forms crystals of typical size and shape. The precipitation diagrams drawn in the temperature and pH coordinates for three different initial composition of the studied system are presented.     

Hydration characteristics of monocalcium aluminate at a low water/solid ratio

The hydration characteristics of calcium monoaluminate were studied using an effective water/aluminate ratio of 0.15 at 20° or 80°C, from a few minutes of two months. The material hydrated at 80°C shows a large shrinkage while at the lower temperature a continuous expansion occurs. The product at 80°C shows a much higher strength than that hydrated at 20°C. The main initial hydration products are 2Ca0, Aℓ₂0₃, 8H₂0 and alumina gel. Microcracks are developed in the products hydrated at 20°C while at the higher temperature a very compact mass results. The data indicate that it is possible to obtain a durable high alumina cement by using a low water/cement ratio and hydrating at higher temperatures, and under these conditions C₃AH₃---C₃AH₆ bond is favoured.     

PREPARATION OF PEROVSKITE 0.75Pb(Ni1/3Nb2/3)O3-0.25PbTiO3 CERAMICS USING SEMICHEMICAL METHODS AND RELATED REACTION MECHANISMS

This article discusses a mechanism for preparing perovskite powders, 0.75Pb(Ni_1/3Nb_2/3)O₃-0.25PbTiO₃ (PNN-PT), using a semichemical method (SCM).Precursors were prepared by adding aqueous Ni(Ac)₂ solutions to an alcohol slurry of PbO, Nb₂O₅, and TiO₂. The TG-DTG and DSC analysis of the precursors and XRD analysis of the powders at different thermal treatment temperatures showed that the reaction mechanisms in this method differ from those in the conventional mixed-oxide method. The aqueous Ni(Ac)₂ solution reacted with PbO to form Pb(Ac)₂ · Pb(OH)₂ · H₂O and Ni(OH)₂, which decomposed to form nascent PbO and NiO, thereby improving the reactivity and distribution of PbO and NiO. Pb₃Nb₂O₈ and NiNb₂O₆ formed and were easily converted into the perovskite phase during the thermal treatment process. At a thermal treatment temperature of 850°C, the content of the perovskite phase reached 98%. Pyrochlore-free PNN-PT ceramic was obtained after 2 h of sintering at 1100°C, and its dielectric properties were found to be excellent at temperatures ranging between -55 and 120°C.     

Formation and stability of barium aluminate and cerate in NOx storage-reduction catalysts

The formation and stability of BaAl₂O₄ and BaCeO₃ in Pt-Ba/Al₂O₃ and Pt-Ba/CeO₂ based NO_x storage-reduction (NSR) catalysts has been investigated using kinetic measurements, X-ray diffraction, thermal analysis and X-ray absorption spectroscopy. In as-prepared state, the Ba-component in the NSR catalysts was made up of amorphous BaO and BaCO₃. The formation of BaAl₂O₄ started above 850 °C, whereas the formation of BaCeO₃ was already observed at 800 °C and was faster than that of BaAl₂O₄. The stability of BaAl₂O₄ and BaCeO₃ in various liquid and gaseous atmospheres was different. BaAl₂O₄ was rapidly hydrated at room temperature in the presence of water and transformed to Ba(NO₃)₂ and γ-alumina in the presence of HNO₃, whereas BaCeO₃ was decomposed to much lower extent under these conditions. Interestingly, BaCeO₃ was transformed to Ba(NO₃)₂/CeO₂ in the presence of NO₂/H₂O at 300–500 °C. Also, the presence of CO₂ led to decomposition of barium cerate, which has important consequences for the catalyst ageing under NO_x-storage conditions and can be exploited for regeneration of thermally aged NSR-catalysts.     

Hydrothermal transformation of the calcium aluminum oxide hydrates CaAl2O4·10H2O and Ca2Al2O5·8H2O to Ca3Al2(OH)12 investigated by in situ synchrotron X-ray powder diffraction

The hydrothermal transformation of calcium aluminate hydrates were investigated by in situ synchrotron X-ray powder diffraction in the temperature range 25 to 170 °C. This technique allowed the study of the detailed reaction mechanism and identification of intermediate phases. The material CaAl₂O₄·10H₂O converted to Ca₃Al₂(OH)₁₂ and amorphous aluminum hydroxide. Ca₂Al₂O₅·8H₂O transformed via the intermediate phase Ca₄Al₂O₇·13H₂O to Ca₃Al₂(OH)₁₂ and gibbsite, Al(OH)₃. The phase Ca₄Al₂O₇·19H₂O reacted via the same intermediate phase to Ca₃Al₂(OH)₁₂ and mainly amorphous aluminum hydroxide. The powder pattern of the intermediate phase is reported.     

Effect of Pd precursor salt on the activity and stability of Pd-doped hexaaluminate catalysts for the CH4 catalytic combustion

This study reports the influence of palladium salt precursor on the catalytic activity of palladium-doped hexaaluminate catalysts for the combustion of 1 vol% CH₄ in the presence of CO₂ and H₂O as inhibitors. Thermal stability of the catalysts is evaluated in long-term catalytic test at 700 °C. The hexaaluminate supports were synthesized using two different procedures: conventional coprecipitation and solid/solid diffusion procedure. Palladium impregnation was carried out by two different routes using Pd(NO₃)₂ in water or Pd(acac)₂ in toluene as impregnation solution. It was observed that using Pd(acac)₂ as precursor allows to attain higher dispersion of the active phase (Pd particles size <3 nm). Compared to the catalysts obtained by impregnation of Pd(NO₃)₂, higher catalytic activities are then obtained. Nevertheless, a deactivation of the samples obtained using Pd(acac)₂ is observed. At the end of the stability test, almost similar catalytic activity is obtained whatever the palladium precursor. Reduction–reoxidation experiment showed that this deactivation is irreversible, and TEM analysis suggest that this deactivation is related to the sintering of Pd particles under reaction over samples synthesized using Pd(acac)₂ as precursor.     

Pt-Ba/alumina NOx storage-reduction catalysts: Effect of Ba-loading on build-up, stability and reactivity of Ba-containing phases

A series of Pt-Ba/Al₂O₃ catalysts with Ba-loadings in the range 4.5–28 wt.% has been prepared by wet impregnation of Pt/Al₂O₃ with barium acetate (Ba(Ac)₂) as Ba precursor. The build-up and thermal stability of the deposited Ba-containing species was followed by means of XRD and thermogravimetry (TG) combined with mass spectroscopy (MS). Samples were characterized before and after thermal treatment (calcination). The study showed that the thermal stability of the Ba-containing phases depends on their interaction with the alumina support and the presence of dispersed platinum. In calcined catalysts, three different Ba-containing species could be distinguished based on their crystallinity and thermal stability. The relative concentration of these species varied with the Ba-loading. The first layer of Ba-containing species, corresponding to saturation of the alumina surface with Ba(Ac)₂, contained up to 12.5 wt.% of Ba in the form of amorphous BaO. Increasing the Ba-loading further resulted in 5–6 wt.% of Ba in the form of amorphous carbonates with relatively low thermal stability (LT-BaCO₃). At Ba-loadings higher than about 16 wt.%, crystalline barium carbonate became discernible which exhibited remarkably higher thermal stability (HT-BaCO₃). NO_x storage tests accomplished by exposing the catalysts to pulses of NO in oxygen containing carrier gas at 300 °C indicated that from all characterized Ba-containing phases, LT-BaCO₃ possesses the highest reactivity for NO_x storage, i.e. LT-BaCO₃ is transformed most rapidly to Ba(NO₃)₂.     

Morphologically controlled synthesis of mesoporous alumina

The morphologically controlled synthesis of mesoporous alumina using Al(NO₃)₃/NH₃/urea/surfactant reaction system is demonstrated. Both the mesostructures and morphologies of the resulting alumina can be effectively controlled via adjusting the synthesis parameters, including reactant compositions, surfactant kinds and hydrothermal conditions. With the aid of surfactants, mesoporous alumina with spheres, rods, fibers and three-dimensional dumbbell, flower-like hierarchical superstructures on the microscopic scale has been obtained. Through characterization by N₂ physisorption, X-ray diffraction, electron microscopy and thermogravimetry analysis, the effects of synthesis conditions on structures and shapes of products are explained and the possible formation mechanism of the controlled morphologies is also proposed.     

Hydrodesulfurization of model diesel using Pt/Al2O3 catalysts prepared by supercritical deposition

Pt/Al₂O₃ catalysts with Pt loadings ranging from 0.5 to 11 wt.% were synthesized by supercritical carbon dioxide (scCO₂) deposition method. Transmission electron microscopy (TEM) images showed that the synthesized catalysts contained small Pt nanoparticles (1–4 nm in diameter) with a narrow size distribution, no observable agglomeration, and uniformly dispersed on the alumina support. The catalysts were found to be active for hydrodesulfurization of dibenzothiophene (DBT) dissolved in n-hexadecane (n-HD) without sulfiding the metal phase. The reaction proceeded only via the direct hydrogenolysis route in the temperature range 310–400 °C and at atmospheric pressure. The activity increased with increasing the metal loading. Increasing [H₂]₀/[DBT]₀ by either increasing [H₂]₀ or decreasing [DBT]₀, increased the DBT conversion. At a fixed weight hourly space velocity and feed concentration, conversion did not increase with increasing temperature beyond 330 °C. The presence of toluene inhibited the catalyst activity presumably due to competitive adsorption between DBT and toluene. Under the operating conditions, the reaction was far from equilibrium.     

Sol-gel synthesis of BaTiO3 and Ba1−x−yCaySrx(ZryTi1−y)O3 perovskite powders

A BaTiO₃ powder has been prepared by the sol-gel process from the hydrolysis of a solution of barium acetate and titanium ethylate in the presence of acetic acid as a catalyst. Supplementary constituents in the form of Ca(CH₃COO)₂, Zr(OC₃H₇)₄, Sr(NO₃)₂ also can be used. Intermediate phases of barium acetate and barium carbonate have been identified by differential thermal analysis, X-ray diffraction, infra-red and scanning electron microscopy. BaTiO₃ with perovskite structure synthesizes in the temperature range from 600 to 1000°C.