Chemical oxidation of residual carbon from ZnAl2O4 powders prepared by combustion synthesis

The removal of carbon residue from ZnAl₂O₄ nanopowders by annealing at 500–800 °C leads to a decrease of specific surface area from 228.1 m²/g to 47.6 m²/g. At the same time, the average crystallite size increased from 5.1 nm to 14.9 nm. In order to overcome these drawbacks, a new solution for removing the carbon residue has been suggested: chemical oxidation using hydrogen peroxide. In terms of carbon removal, a H₂O₂ treatment for 8 h at 107 °C proved to be equivalent to a heat treatment of 1 h at 600 °C. The benefits of chemical oxidation over thermal oxidation were obvious. The specific surface area was much larger (188.1 m²/g) in the case of the powder treated with H₂O₂. The average crystallite size (5.8 nm) of ZnAl₂O₄ powder treated with H₂O₂ was smaller than the crystallite size (8.2 nm) of the ZnAl₂O₄ powder annealed at 600 °C.     

Particle Size Effect on CH4 Oxidation Over Noble Metals: Comparison of Pt and Pd Catalysts

Intrinsic catalytic activities (TOF values) in CH₄ complete oxidation under lean conditions were estimated as a function of Pt and Pd particle sizes (d_m) for two series of Pt/Al₂O₃ and Pd/Al₂O₃ catalysts. Comparison of TOF ~ f(d_m) relationships revealed significant difference between Pt and Pd catalysts. For Pt catalyst TOF showed tendency to increase by 2–3 times with increasing particle size from 1 to ca 3 nm and remained constant, when Pt particles became larger than 3 nm. On the other hand, for Pd catalyst TOF increased almost linearly when particle size grew from 1 to 20 nm. These different tendencies were attributed to the different mechanisms of CH₄ oxidation over Pt and Pd catalysts: Langmuir–Hinshelwood and Mars-Van Krevelen respectively.     

The effect of TiO2 particle size on the characteristics of Au–Pd/TiO2 catalysts

The nanocrystalline TiO₂ materials with average crystallite sizes of 9 and 15 nm were synthesized by the solvothermal method and employed as the supports for preparation of bimetallic Au/Pd/TiO₂ catalysts. The average size of Au–Pd alloy particles increased slightly from sub-nano (< 1 nm) to 2–3 nm with increasing TiO₂ crystallite size from 9 to 15 nm. The catalyst performances were evaluated in the liquid-phase selective hydrogenation of 1-heptyne under mild reaction conditions (H₂ 1 bar, 30 °C). The exertion of electronic modification of Pd by Au–Pd alloy formation depended on the TiO₂ crystallite size in which it was more pronounced for Au/Pd on the larger TiO₂ (15 nm) than on the smaller one (9 nm), resulting in higher hydrogenation activity and lower selectivity to 1-heptene on the former catalyst.     

Origin of Violet‐Blue Emission in Ti‐Doped Gahnite

ZnAl₂O₄ doped with Ti⁴⁺ (2%) was synthesized by the hydrothermal method at 220°C at pressure of 25 bars. An average grain size of the as‐prepared sample was 3 nm, the samples with biggest grain size were obtained after annealing at 300°C, 500°C, 600°C, 700°C, and 900°C, diameter of the latter was about 33 nm. IR spectroscopy indicated that ZnAl₂O₄ was partially inverted. The degree of the inversion decreases with increase in the annealing temperature but increases with increasing Ti⁴⁺ content. Absorption and emission spectra as well as emission decay profiles were recorded at 300 and 77 K. The observed spectra are due to charge‐transfer O^2??Ti⁴⁺ transitions. Color of the emission depends on the nanocrystal size and with increase in its diameter changes from violet to blue, accordingly the absorption bands exhibit redshift. The calculations based on Density Functional Theory confirmed the experimental results. 3d electrons of titanium ions form the bottom of the ZnAl₂O₄:Ti⁴⁺ conduction band, oxygen, aluminum or zinc vacancies create additional levels in the gahnite energy band gap. It was also found that in ZnAl₂O₄ aluminum or zinc vacancy induces magnetism with relatively high magnetic moment close to 1 μ_B per vacancy.     

Characterization of ZnAl2O4 Obtained by Different Methods and Used as Catalytic Support of Pt

In this work the synthesis of a ZnAl₂O₄ spinel to be used as a support of metals and its characterization were studied. The methods used for the ZnAl₂O₄ preparation were: ceramic method (CM), mechanochemical synthesis in humid medium (HMS) and coprecipitation (COPR). ZnAl₂O₄ CM and ZnAl₂O₄ HMS showed negligible acidity, but the ZnAl₂O₄ COPR displayed a low acidity. The spinels obtained by COPR and HMS showed higher specific surface area and pore volumes than that prepared by the ceramic method. In addition the catalytic performance of Pt supported on the prepared spinel was evaluated in the n-butane dehydrogenation reaction. The Pt catalysts prepared with ZnAl₂O₄ COPR presented better activity and selectivity to olefins than the ones prepared with ZnAl₂O₄ HMS and ZnAl₂O₄ CM, which could be correlated with a higher metallic dispersion and lower particle sizes, detected by TEM. The acidity of ZnAl₂O₄ COPR, observed by isopropanol dehydration and TPD of pyridine, and the sequence of specific surface areas of the different spinels (ZnAl₂O₄ COPR > ZnAl₂O₄ HMS > ZnAl₂O₄ CM) are other important factors to define the final dispersion of the catalysts.     

Syntheses and characterization of highly transparent ZnAl2O4 ceramic films using poly(acrylamide-co-acrylic acid) assisted microwave approach

Zinc aluminate nanopowders were synthesized via poly(acrylamide-co-acrylic acid) assisted microwave approach. The as-synthesized ZnAl₂O₄ nanopowders were characterized using X-ray diffraction (XRD), High resolution transmission electron microscopy (HRTEM) and selected area of electron diffraction (SAED). The prepared ZnAl₂O₄ nanopowders exhibited a spinel cubic polycrystalline structure. The increase of poly(acrylamide-co-acrylic acid) amounts decreased the particle size of the ZnAl₂O₄ nanopowders. The poly(acrylamide-co-acrylic acid) enhanced the densification rate of ZnAl₂O₄. The increasing of poly(acrylamide-co-acrylic acid) amount decreased the sintering temperature from 1300 °C to 950 °C. The hot-compressed ZnAl₂O₄ nanopowders in the existence of 2 wt% of poly(acrylamide-co-acrylic acid) exhibited full density at 950?C in just 20 min. The ZnAl₂O₄ ceramic films revealed a high transparency of 83 ± 1% at a wavelength range from 450?1200 nm.     

Electron Microscopy Study of γ-Al2O3 Supported Cobalt Fischer–Tropsch Synthesis Catalysts

Three supported catalysts containing 20 wt% cobalt and 0.5 wt% rhenium were subjected to electron microscopy studies in their calcined state. The catalysts were prepared by incipient wetness impregnation of γ-Al₂O₃ supports of different pore characteristics with aqueous solutions of cobalt nitrate hexahydrate and perrhenic acid. The influence of the support on the Co₃O₄ crystallite size and distribution was studied by X-ray diffraction and electron microscopy. There was a positive correlation between the pore diameter of the support and the post calcination Co₃O₄ crystallite size. On all three γ-Al₂O₃ supports, Co₃O₄ was present as aggregates of many crystallites (20–270 nm in size). Cobalt oxide did not crystallise as independent crystallites, but as an interconnected network, with a roughly common crystallographic orientation, within the matrix pore structure. The internal variations in crystallite size between the catalysts were maintained after reduction. Fischer–Tropsch synthesis was carried out in a fixed-bed reactor at industrial conditions (T = 483 K, P = 20 bar, H₂/CO = 2.1). Although the cobalt-time yields varied significantly (4.6–6.7 × 10^?3 mol CO/mol Co s), the site-time yields were constant (63–68 × 10^?3 s^?1) for the three samples. The C₅₊ selectivity could not be correlated to the cobalt oxide aggregate size and is more likely related to the cobalt particle size and chemical properties of the γ-Al₂O₃ support.     

Synthesis and Characterization of Catalytically Activity Fe3o4–3-Aminopropyl-triethoxysilane/Pd Nanocomposite

Herein we report the fabrication and characterization of Pd decorated Fe₃O₄ nanoparticles as highly effective catalysts for hydrogenation of 4-nitroaniline and 1,3-dinitrobenzene in liquid phase. The fabricated Fe₃O₄ nanoparticles exhibit an average size of 12 nm and super paramagnetic character with a high saturation magnetization (80 emu/g). The surface –NH₂ groups effectively binds the in situ formed Pd nanoparticles. Thus formed Fe₃O₄–APTES–Pd(0) catalyst showed a very high catalytic activity in reduction reactions of 4-nitroaniline and 1,3-dinitrobenzene in liquid phase. Electron donor –NH₂ groups supported Pd may be responsible for the increased catalytic activity. The superparamagnetic character of this system allows easy recovery and multiple uses without significant loss of its catalytic activity.     

Microwave Synthesis and Characterization of Spinel-type Zinc Aluminate Nanoparticles

In the present work, ZnAl₂O₄ nanoparticles have been synthesized with the aid of Zn(OAc)₂·2H₂O and Al(NO₃)₃·9H₂O as starting reagents in the presence of microwave irradiation. Besides, the effect of preparation parameters such as microwave power and irradiation time on the morphology and particle size of products was studied by SEM images. The as-prepared ZnAl₂O₄ nanoparticles were characterized extensively by techniques like XRD, TEM, SEM, FT-IR, PL, and EDS. Photoluminescence studies of the ZnAl₂O₄ nanoparticles displayed quantum confinement behavior with band gap of 3.2 eV. The XRD studies showed that pure orthorhombic ZnAl₂O₄ nanoparticles have been produced after calcination.     

Direct H2-to-H2O2 Oxidation in Aqueous Acidic Medium Containing Br Promoter Over Pd/Al2O3 and Pd/C Catalysts Thermally Pretreated Under Different Conditions

Influence of thermal pretreatments (under N₂, air or H₂ gas atmosphere at 500 °C or 700 °C) has been investigated for the Pd/Al₂O₃ and Pd/Carbon catalysts in terms of its effect on their Pd particle size and performance in the H₂-to-H₂O₂ oxidation and H₂O₂ destruction (by decomposition and/or hydrogenation) reactions in aqueous acidic medium containing Br promoter. The influence on the net H₂O₂ formation is found to depend strongly upon the catalyst support due to support–Pd cluster interactions. For both the catalysts, the thermal treatments (except in air) caused a large increase in their Pd particle size. The increase in Pd particle size caused an increase in the H₂O₂ formation activity of Pd/Al₂O₃ but a decrease in the H₂O₂ formation activity of Pd/C.     

Cumene Hydroperoxide Hydrogenation on a Pd/Al2O3 Catalyst in a Trickle Bed Reactor – Kinetics of Hydrogenation and Deactivation

Liquid‐phase hydrogenation using a Pd/Al₂O₃ catalyst provides a potential technique for the reduction of cumene hydroperoxide (CHP) to α‐cumyl alcohol (CA). In this paper, CHP hydrogenation was carried out in a cocurrent downflow trickle‐bed reactor over a wide range of reaction conditions to study the reaction and deactivation kinetics. The proposed intrinsic rate expression for CHP hydrogenation is based on an Eley‐Rideal mechanism that accounts for an irreversible surface reaction between the absorbed CHP with nonabsorbed hydrogen molecules. During CHP hydrogenation, an exponential decay in activity of the Pd/Al₂O₃ catalyst and the presence of residual activity were observed. A kinetic deactivation model with residual activity was developed. Based on reaction and deactivation kinetics, catalyst deactivation was attributed to oxidation of the catalyst surface by CHP. The presence of residual activity was due to the partial reduction of oxidized catalyst surface by hydrogen.     

Synthesis and Characterization of Sulfamic-Acid Functionalized Magnetic Fe3O4 Nanoparticles Coated by Poly(amidoamine) Dendrimer

Grafting of chlorosulfuric acid on the amino-functionalized Fe₃O₄ nanoparticles produced sulfamic acid-functionalized magnetic Fe₃O₄ nanoparticles as a novel organic–inorganic hybrid material, which was characterized with X-ray diffractometry (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, scanning electron spectroscopy, and magnetization measurements (VSM). The as-prepared nanocomposite with a narrow size distribution has the crystallite size from XRD (11 ± 4 nm) and particle size from TEM analysis (12.9 ± 0.4 nm) are consistent with each other. Magnetization measurements proved the superparamagnetic property of the product.     

Synthesis of high-surface-area micro/mesoporous ZnAl2O4 catalyst support and application in selective hydrogenation of o-chloronitrobenzene

High-surface-area ZnAl₂O₄ with micro/mesoporous frameworks was synthesized via a facile one-pot solvothermal approach without using any surfactants or post-treatment. The results revealed that the textural properties of micro/mesoporous ZnAl₂O₄ were tuned by changing alcohol/water mixture solvents used in solvothermal synthesis and Ag nanoparticles were dispersed uniformly on the surface of ZnAl₂O₄ supports. Especially, the supported Ag catalyst on the microporous ZnAl₂O₄ synthesized in methanol/water solvent exhibited good catalytic performance for liquid phase selective hydrogenation of o-chloronitrobenzene to o-chloroaniline, owing to the high dispersion of Ag nanoparticles and the strong metal-support interaction.     

Influence of acetylaceton on mesoporous structure and catalytic activity of ceria-based solid solution

Gd_0.1Ti_0.1Zr_0.1Ce_0.7O₂ solid solutions with a crystallite size of 3–8 nm have been prepared by the sonochemical method from inorganic salts with an acetylaceton additive. In all cases, ceria based materials exhibited a mesoporous structure with monomodal pore size distribution with diameter of 3–4 nm. It was demonstrated that such systems were more active in CO oxidation than samples, prepared without using acetylaceton (shift more than 150?°C of CO full oxidation).     

Acid Functionalized Multiwall Carbon Nanotube/Magnetite (MWCNT)-COOH/Fe3O4 Hybrid: Synthesis, Characterization and Conductivity Evaluation

A functionalized multiwall carbon nanotube (MWCNT)–COOH/Fe₃O₄ hybrid was fabricated by co-precipitation method. Fe₃O₄ nanoparticles were stably attached to the surface of carboxyl groups (COOH). The presence of Fe₃O₄ nanoparticles and their surface conjugation to MWCNT have been confirmed by XRD, TEM and FT-IR techniques. Magnetic evaluation revealed a superparamagnetic character of the hybrid and therefore the attached Fe₃O₄ nanoparticles. The crystallite size (9 ± 3 nm), particle size (9 ± 2 nm) and magnetic domain size estimated for Fe₃O₄ are consistent with each other, which reveal the single crystalline character of the nanoparticles. Electrical conductivity and dielectric behavior have also been characterized by utilizing impedance spectroscopy up to 3 MHz for an isotherm line varying from 293 to 393 K by 10 K steps. Electrical characteristics and its complex dielectric approaches might be elucidated with the existence of a conventional tunneling conduction mechanism of temperature-independency. The AC conductivity of MWCNT–COOH/Fe₃O₄ hybrid could also be a consequence of the estimations of the universal dynamic response.     

A novel combustion fuel for the synthesis of nanocrystalline ZnAl2O4 particles based on the thermodynamic correlations and their structural and optical studies

ZnAl₂O₄ nanocrystalline particles were prepared using the solution combustion method using a new combustion fuel, Leucine. The prepared samples' structural, microstructural–elemental composition, and optical characteristics were investigated using XRD, SEM-EDS, and UV–Visible spectroscopy. As-synthesized ZnAl₂O₄ nanoparticles are polycrystalline, with no secondary phases, and crystallized in a cubic - spinel structure. The polycrystalline nature of the prepared sample is due to the exothermicity of fuel and oxidizer, which demonstrate that the fuel utilized (Leucine) provided adequate energy for the production of nanoparticles in their as-synthesized form, as supported by adiabatic temperature through thermodynamic calculations. The thermodynamic calculations also include a universal method to estimate the specific heat capacity at constant pressure. Furthermore, even after 2 h of calcination at 600 °C, ZnAl₂O₄ exhibits a single phase with no secondary phases, indicating the material stability and single-phase nature. The crystallinity of ZnAl₂O₄ nanoparticles was observed to increase with increasing annealing temperature. SEM micrographs of as-synthesized samples exhibit the formation of dense particles, voids, and pores in the as-synthesized sample. In addition, tiny aggregates were detected on the surface of more prominent clusters, which reduced as the calcination progressed. In addition, calcined samples exhibit a greater optical reflectance than as-synthesized samples. Tauc's graphs were used to compute the optical energy bandgap. The calculated energy band gap is redshifted to that of the bulk material. The bandgap energy decreases upon calcination, suggesting that the prepared materials have a larger crystallite size or more crystallinity. Correlations were found between the T_ad, and the structural and optical properties of the prepared samples. The findings suggest that Leucine could be used as a novel combustion fuel to produce crystalline ZnAl₂O₄ nanoparticles in their as-synthesis form.     

Highly selective hydrogenation of phenol and derivatives over Pd catalysts supported on SiO2 and γ-Al2O3 in aqueous media

Pd/Al₂O₃ and Pd/SiO₂ catalysts containing Pd nanoparticles in the size range of 3–13 nm were prepared and investigated in direct selective hydrogenation of phenol to cyclohexanone. Catalysts with 3 nm Pd nanoparticles present highly active and promoted the selective formation of cyclohexanone under atmospheric pressure of hydrogen in aqueous media without additives. Conversion of 99% and a selectivity higher than 99% were achieved within 3 h at 333 K. The generality of Pd/Al₂O₃ catalyst with 3 nm Pd nanoparticles for this reaction was demonstrated by selective hydrogenation of other hydroxylated aromatic compounds with similar performance.     

Molten salt synthesis of zinc aluminate powder

ZnAl₂O₄ powder was synthesised by reacting equimolar ZnO and Al₂O₃ powders in alkaline chlorides (LiCl, NaCl or KCl). Formation of ZnAl₂O₄ started at about 700 °C in LiCl and 800 °C in NaCl and KCl. With increasing temperature, the amounts of ZnAl₂O₄ in the resultant powders increased with a concomitant decrease of ZnO and Al₂O₃. ZnAl₂O₄ powder was obtained by water-washing the samples heated for 3 h at 1000 °C (LiCl) or 1050 °C (NaCl and KCl). ZnAl₂O₄ formed in situ on Al₂O₃ grains from the surface inwards. The synthesised ZnAl₂O₄ grains retained the size and morphology of the original Al₂O₃ powders, indicating that a template formation mechanism dominated formation of ZnAl₂O₄ by molten salt synthesis.     

Precise recognition of catalyst deactivation during acetylene hydrogenation studied with the advanced TEMKIN reactor

C₆ hydrocarbons were identified as an important indicator for deactivation behaviour in acetylene hydrogenation on Pd–Ag/Al₂O₃ egg-shell catalysts. Thereby significant differences between highly developed catalysts can be identified, helping to forecast differences in deactivation behaviour over 100 h time-on-stream in a short and cheap catalytic test.     

Mathematical modeling of the partial hydrogenation of vegetable oil in a monolithic stirrer reactor

Experimental and theoretical studies on the partial hydrogenation of vegetable oil in a monolithic stirrer reactor are reported. A complete mathematical model of the reactor was developed, including hydrogenation and isomerization kinetics, catalyst deactivation, external gas–liquid and liquid–solid as well as internal mass transfer. The experimental studies were carried out in a Pd/Al₂O₃/Al monolithic stirrer reactor, at a wide range of temperatures (353–373 K), pressures (414–552 kPa), and catalyst loadings (0.00084–0.00527 kg_Pd,exp m^?3). Based on this model, simulated data can be used to evaluate the catalyst (Pd/Al₂O₃/Al) and the hydrogenation process in consecutive catalytic tests under different operating conditions. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3524–3533, 2014