A thermally stable porous material prepared by heating partially trimethylsilylated halloysite

Halloysite was trimethylsilylated in a mixture of hexamethyldisiloxane, 2-propanol and hydrochloric acid to the extent that small amounts of raw halloysite remained. The resulting product (PTSH) was heated in nitrogen and air. Heating in air scarcely influenced thermal degradation. The PTSH was in an amorphous state at 673–1273 K and crystallized into mullite at a higher temperature. The N₂-BET specific surface area (250 m²/g) of the PTSH increased to 390 m²/g after heating to 673 K and decreased to 270 m²/g at 1073 K in nitrogen. The PTSH, regardless of heat-treatment temperatures, included extremely homogeneous pores of 1.7 nm. The number of acidic sites and their acidic strength were lower than those of commercially available catalysts.     

Upgrading of asphalt with and without partial oxidation in supercritical water

Supercritical water and supercritical water partial oxidation treatments were applied to the upgrading of asphalt. Asphalt was converted at 613-673 K, 0-0.5 g/cm³ water density under argon or air atmosphere. Under an argon atmosphere and 0.5 g/cm³ water density, both the asphaltene conversion and desulfurization increased with increasing temperature. At 673 K, the asphaltene conversion and the yield of CO₂ increased with an increasing water density. Water apparently participated in the reaction and its hydrogen was used for capping the free radicals generated during the upgrading of asphalt resulting in an increased yield of maltene. Under an air atmosphere at 673 K, asphaltene conversion was lower but desulfurization was higher than those obtained in an argon atmosphere.     

Understanding the correlation between calcination temperature and performance in low-temperature methanation over Ni-Zr/Al2O3 catalysts

Low-temperature methanation of CO in the continuous stirred tank reactor (CSTR) over Zr doped Ni/Al₂O₃ catalyst calcined at different temperatures (673, 723, and 823 K) was investigated. XRD, TPR, XPS, ICP, SEM, and S-TPR techniques were employed to characterize the fresh and spent catalysts. Based on the characterization results, it was found that low-temperature (673 K) calcination could effectively prohibit the formation of NiAl₂O₄ spinel, thereby resulting in more reducible NiO particles, which were the essential precursor of methanation active sites over the catalyst surface. Thus, the highest CO conversion of 93.6% was achieved over the 25N3ZA-673 catalyst. In addition, the deactivation rate of 25N3ZA-673 was relatively slow in comparison to 25N3ZA-823 due to the presence of more reducible NiO. The formed nickel carbonyl species (Ni[CO]_x), which quickly decomposed at a higher reaction temperature, was closely related to the catalyst deactivation. Therefore, 25N3ZA-673 possessed much better stability at 593 K than that at 553 K.     

Electrolysis of HBr using molten, alkali-bromide electrolytes

The electrolytic oxidation of HBr was investigated using molten alkali-bromide salts in a porous scaffold of yttria-stabilized zirconia (YSZ), with porous Pt electrodes. Despite a relatively thick (1.0 mm) and dense (35% porous) YSZ scaffold, a total cell impedance of less than 3 Ωcm² was achieved with NaBr at 1033 K. The open-circuit potentials also agreed with theoretical potentials for the reaction H₂ + Br₂ = 2HBr. Lower operating temperatures were made possible by using a eutectic mixture of alkali bromides, (Li_0.56K_0.19Cs_0.25)Br. The electrolyte losses for the (Li_0.56K_0.19Cs_0.25)Br electrolyte, determined from the ohmic component of the impedance spectra, were less than 1.5 Ωcm² at 673 and 773 K, similar to the value found for NaBr at 1033 K, indicating that the ionic conductivities of the alkali bromides are high so long as the salts are molten. However, the electrode losses were dependent on temperature, decreasing from 18.1 Ωcm² to 3.0 Ωcm² in going from 673 to 773 K. Implications of these results for recycling HBr in hydrocarbon bromination reactions are discussed.     

Titania aerogels prepared by low-temperature supercritical drying. Influence of extraction conditions

Titania aerogels with meso- to macroporosity and high specific surface area were prepared by varying the conditions of semicontinuous extraction of methanolic titania gels with CO₂. The conditions varied were extraction temperature, extraction duration, and CO₂ in liquid or supercritical state. The resulting titania aerogels were characterised by means of nitrogen physisorption, X-ray diffraction, thermal analysis and transmission electron microscopy. All uncalcined aerogels contained significant amounts of organic residues (12–14 wt% elemental carbon), and remained X-ray amorphous during calcination in air up to 673 K. Thermoanalytical studies showed that crystallization generally occurred in the range 730–745 K. The variation of the extraction temperature at either constant density or pressure of CO₂, the use of either liquid or supercritical CO₂, and the duration of extraction greatly influenced surface area, pore size distribution, and pore volume. The highest specific surface area (623 m² g^–1) and nitrogen pore volume (4.0 cm³ g^–1) were obtained, if the density of supercritical CO₂ corresponded to that of methanol at the lowest temperature applied (313 K). The studies indicate that textural properties can be varied over a wide range by choosing appropriate extraction conditions.     

Electrochemical behavior of antimony and electrodeposition of Mg–Li–Sb alloys from chloride melts

The electrochemical behavior of Sb(III) ions was investigated in LiCl–KCl molten salt at 673 K. The reaction mechanism and transport parameters of electroactive species were determined by transient electrochemical techniques (such as cyclic voltammetry, square wave voltammetry, chronopotentiometry and chronoamperometry) at a molybdenum electrode. The results showed that electrochemical reduction of Sb(III) in LiCl–KCl melts occurred in a reaction step with an exchange of three electrons. A voltammogram with a different scan rate in LiCl–KCl containing 1.45 × 10⁻⁴ mol cm⁻³ SbCl₃ showed that the deposition/dissolution reaction of Sb(III) ions was not completely reversible. The diffusion coefficient of Sb(III) ions was 1.65(±0.01) × 10⁻⁵ cm⁻² s⁻¹ at 673 K. The electroreduction of Sb(III) ions at an Al electrode was also studied by cyclic voltammetry and open circuit chronopotentiometry in the temperature range of 668–742 K. The redox potential of Sb(III)/Sb at an Al electrode was observed at the more positive potentials values than those at an inert electrode. This potential shift due to the formation of intermetallic compound with Al electrode. AlSb alloys were prepared in LiCl–KCl–SbCl₃ melts at 742 K by potentiostatic electrolysis at an Al electrode. The activity of Sb and the Gibbs energy of AlSb formation were also calculated. Mg–Li–Sb alloys were obtained by galvanostatic electrolysis at 673 K and the electrochemical codeposition of Mg, Li and Sb was investigated on a molybdenum electrode in LiCl–KCl containing MgCl₂ and SbCl₃ melts at 673 K by cyclic voltammetry, chronopotentiometry and chronoamperometry. Cyclic voltammograms, chronopotentiometry and chronoamperometry measurements indicated that the electrochemical codeposition of Mg, Li and Sb metal occurred at current densities lower than −0.466 A cm⁻² or at an applied potential more negative than −2.350 V vs. Ag/AgCl. X-ray diffraction (XRD) suggested that Mg₃Sb₂ and Li₃Sb were formed in Mg–Li–Sb alloys. The distribution of Sb element in Mg–Li–Sb alloys from the analysis of scan electron micrograph (SEM) and energy dispersive spectrometry (EDS) indicated that Sb metal showed a distribution which resembled an interlaced network.     

Structural Reversibility of a Ternary CuO-ZnO-Al2O3 ex Hydrotalcite-Containing Material During Wet Pd Impregnation

A Cu-Zn-Al precursor was synthesized by coprecipitation of the corresponding cations with sodium carbonate at constant pH and temperature. CuO-ZnO-Al₂O₃ composite oxide support was obtained by calcination (673 K) of the Cu-Zn-Al precursor. Two palladium-modified CuO-ZnO-Al₂O₃ samples were prepared by impregnation of the mixed-oxide support and further calcination (673 K). The presence of remaining CO₃ ^2- anions in the CuO-ZnO-Al₂O₃ mixed oxide, as a result of incomplete Cu-Zn hydrotalcite phase decomposition, and the hydrothermal-like treatment during the Pd impregnation step, allow the partial reconstruction of the Cu-Zn hydrotalcite-type structure (memory effect). In addition, an enhancement in the CuO crystallinity was obtained for the Pd-modified oxides. A detailed characterization revealed that the hydrotalcite restoration enhances the crystallinity of the copper oxide as a consequence of a crystalline rearrangement of this oxidic phase.     

A new class of high-entropy fluorite oxides with tunable expansion coefficients,low thermal conductivity and exceptional sintering resistance

High-temperature thermal barrier coating (TBC) materials are desired for the development of high-efficient gas turbines and diesel engines. Herein, to meet up with this requirement, a new class of high-entropy fluorite-type oxides (HEFOs) has been synthesized via a solid-state reaction method. Comparing to La₂Ce₂O₇, a promising TBC material, the HEFOs exhibit similar high thermal expansion coefficients (TECs) of 11.92×10⁻⁶∼12.11×10⁻⁶ K^-1 at temperatures above 673 K but a better TEC matching performance at the temperature range of 473–673 K. It is also found that through tuning the average A-site cation radius, the TEC of the HEFOs could be tailored efficiently. The HEFOs also possess low thermal conductivities of 1.52-1.55 W∙m^-1∙K^-1 at room temperature, which is much lower than that of La₂Ce₂O₇ and comparable to pyrochlores as Gd₂Zr₂O₇. Moreover, the HEFOs display good sintering resistance and phase stability even at temperatures as high as 1873 K. The combination of these fascinating properties makes the HEFOs good candidates for thermal barrier coating and thermal insulating materials.     

Ethanol Steam Reforming for Hydrogen Production over Bimetallic Pt–Ni/Al2O3

Ethanol steam reforming was studied at 673–823 K over Pt–Ni/δ-Al₂O₃. Results indicate that bimetallic catalyst is resistant to coke deposition at steam-to-carbon ratios as low as 1.5 and higher ratios are beneficial for both ethanol conversion and hydrogen formation. About 773 K is the optimum since high H₂ production rates are accompanied by low CO and CH₄ production rates. A power-function rate expression obtained on the basis of intrinsic rates at 673 K gives reaction orders of 1.25 (±0.05) and −0.215 (±0.015) for ethanol and steam, respectively; the apparent activation energy is calculated as 39.3 (±2) kJ mol⁻¹ between 673 and 723 K.     

Nanocomposites of CuO/SWCNT: Promising thermoelectric materials for mid-temperature thermoelectric generators

Nanocomposites of ultra-thin copper oxide nanosheets (CuO NSs) and single-wall carbon nanotubes (SWCNTs) were produced and studied for their thermoelectric (TE) properties. Incorporating a small amount of SWCNTs into the matrix of CuO NSs enhanced the TE properties. This might be due to good band alignment between the two phases with large contact areas. The nanocomposite [CuO]_99.9[SWCNT]_0.1 showed a rapid increase with the increasing temperature in both the Seebeck coefficient and power factor. At 673 K, they reached 882 μV/K and 2500 μW/mK², respectively. The phonon and charge transport properties were attributed to the CuO NS/SWCNT interfaces. A small module of 2 p-n pairs based on CuO/SWCNT nanocomposites (p-type) and SnO₂ nanoparticles (n-type) was constructed and worked in a temperature range of 573–673 K with good stability and reusability. The measured output power was ˜1200 μW, which can power small electronic devices.     

Temperature effect on the nitrogen insertion in carbon nitride films deposited by ECR

The impact of the temperature on the local structure of carbon nitride coating a-C_1-x N_x:H was investigated by spectroscopic analysis. A set of carbon nitride films were deposited at several substrate temperatures (77 K, 300 K, 673 K and 900 K) by electron cyclotron resonance (ECR) ion gun technique fed of CH₄/N₂ plasma.The films were in situ characterized by X-ray photoelectron spectroscopy (XPS). A drastic decrease of the nitrogen content was observed when increasing the deposition temperature from 77 K to 900 K. Qualitative structural and electronic changes were followed after air exposure by infrared (FTIR), near-edge X-ray absorption fine structure (NEXAFS) and Ultraviolet photoelectron (UPS) spectroscopy. Below 300 K, the films are hydrogenated with aliphatic structure and nitrogen is bonded to carbon in many kind of configuration. Between 300 K and 600 K, the nitrogen amount is reduced while both the aromatic and the aliphatic carbons increase. The local structure of the films radically changes at 900 K, whereas the nitrogen surrounding is the same at 673 K. In that case the hydrogen fraction into the films is reduced to zero. The increase of the sp³ carbon as well as the ratio π?/σ? on the nitrogen K edge can be observed. This behaviour may be explain by nitrogen substituted to sp² carbon which induces local changes in the distribution of the π? states.     

Preparation of TiO2-pillared montmorillonite as photocatalyst Part I. Microwave calcination, characterisation, and adsorption of a textile azo dye

Two photocatalysts based on TiO₂-pillared intercalated montmorillonite have been prepared by microwave for 10 min at 700 W or by furnace heating at 673 K. Montmorillonite pillaring with TiO₂ increased the basal spacing to 14.7 Å (conventional heating) and 17.6 Å (microwave heating). XRD patterns of both materials showed the presence of 100% anatase with a slightly higher rate of crystallinity obtained through microwave calcination than by conventional heating at 673 K. The BET specific surface area of the microwave prepared photocatalyst (151 m² g^− 1) was 3 fold higher than those of the Degussa TiO₂ P25. At pH = 5.8, the maximum adsorption capacity of Solophenyl red 3BL (a textile azo dye) on the TiO₂-pillared montmorillonite calcined by microwave was 185 mg g^− 1, whereas it was 1.4 and 3 fold lower on the TiO₂-pillared montmorillonite calcined at 673 K, and on the Degussa TiO₂ P25 respectively. The influence of pH on the adsorption of the dye depended on the pH_ZPC of the pillared montmorillonites.     

Effect of Zn migration on the thermoelectric properties of Zn4Sb3 material

β-Zn₄Sb₃ is interesting as thermoelectric material at moderate temperature due to the extreme low thermal conductivity. Recent success in energy band engineering or nano-engineering led to a significant improvement in the thermoelectric properties of β-Zn₄Sb₃. In this work, we utilize the direct current to drive the migration of Zn by designing of sintering mould. Obvious Zn migration under the direct current applied in the plasma activated sintering (PAS) process is found in Zn₄Sb₃ compounds, and Zn exhibits significantly heterogeneous gradient composition distribution. At the top of sample, the single-phase Zn₄Sb₃ decomposes into ZnSb phase because of the loss of Zn, while Zn originated from lattice and interstitial sites in Zn₄Sb₃ is abundant in the bottom. The temperature-dependent transport measurements are also carried out at 323–673 K. Zn migration has a huge influence on the thermoelectric properties because of the sensitivity of Zn₄Sb₃. The maximum power factor can reach ~ 1.44 mW m⁻¹ K⁻² at 673 K due to the high Seebeck coefficient and low resistivity, which is one of the highest values in the reported results. The resulting peak ZT value of ~ 1.2 at 673 K is obtained. To control the Zn distribution by tuning the current is a feasible approach to improve the thermoelectric properties of Zn₄Sb₃ material.     

Pt/CeO2 catalysts in selective hydrogenation of crotonaldehyde: high performance of chlorine‐free catalysts

The hydrogenation of crotonaldehyde was conducted in gaseous phase, at atmospheric pressure, on Pt/CeO₂ catalysts prepared from metal precursors containing or not chlorine. The activities and selectivities were studied, at 253 K, as a function of the reduction temperature of the catalyst (473–993 K). The Pt/CeO₂ catalyst, prepared from tetraammineplatinum nitrate, led to 5–20% crotyl alcohol selectivity when the catalyst was reduced at low temperature (473–673 K), while increasing the reduction temperature up to 973 K, the crotyl alcohol selectivity reached more than 80%. Repeating a series of experiments after a re‐calcination treatment at 673 K, the selectivity decreased to only 40% after 473 K reduction to reach again more than 80% after 673 K reduction temperature. A phase transformation of Pt to CePt₅ was observed by XRD analysis after 973 K reduction treatment. Differently on Pt/CeO₂ catalysts containing chlorine, prepared from either chloroplatinic acid or tetraammineplatinum chloride, the crotyl alcohol selectivity never exceeded 30% and did not form alloy up to 973 K reduction temperature. The main results are interpreted considering that the activity of CePt₅ for C=C hydrogenation is low compared to unmodified platinum catalyst and the activation of the carbonyl bond is induced by the presence of oxygen vacancies sites located at the interface between ceria and the metallic particles. The results are in good accordance with the information known at the present time on the metal–support interactions in Pt deposited on CeO₂. This revised version was published online in August 2006 with corrections to the Cover Date.     

(100) facets of γ-Al2O3: The Active Surfaces for Alcohol Dehydration Reactions

Abstract  

Temperature programmed desorption (TPD) of ethanol, as well as ethanol and methanol dehydration reactions were studied on γ-Al₂O₃ in order to identify the active catalytic sites for alcohol dehydration reactions. Two high temperature (>473 K) desorption features were observed following ethanol adsorption. Samples calcined at T ≤ 473 K displayed a desorption feature in the 523–533 K temperature range, while those calcined at T ≥ 673 K showed a single desorption feature at 498 K. These two high temperature desorption features correspond to the exclusive formation of ethylene on the Lewis (498 K) and Br?nsted acidic (~525 K) sites. The amount of ethylene formed under conditions where the competition between water and ethanol for adsorption sites is minimized is identical over the two surfaces. Furthermore, a nearly 1-to-1 correlation between the number of under-coordinated Al³⁺ ions on the (100) facets of γ-Al₂O₃ and the number of ethylene molecules formed in the ethanol TPD experiments on samples calcined at T ≥ 673 K was found. Titration of the penta-coordinate Al³⁺ sites on the (100) facets of γ-Al₂O₃ by BaO completely eliminated the methanol dehydration reaction activity. These results demonstrate that in alcohol dehydration reactions on γ-Al₂O₃, the (100) facets are the active catalytic surfaces. The observed activities can be linked to the same Al³⁺ ions on both hydrated and dehydrated surfaces: penta-coordinate Al³⁺ ions (Lewis acid sites), and their corresponding –OH groups (Br?nsted acid sites), depending on the calcination temperature.     

Preparation and Characterization of Porous SiO2-Al2O3-ZrO2 Prepared by the Sol-Gel Process

An experimental strategy was developed to obtain Si—Al—Zr transparent sols via the sol-gel process. The sol was prepared from Al(OBu^s)₃ (OBu^s: C₂H₅CH(CH₃)O), Zr(OPrⁿ)₄ (OPrⁿ: OCH₂CH₂CH₃) and Si(OEt)₄. The chelating agents acetylacetone (2, 4 pentanedione, acacH), and itaconic anhydride (2-methylenesuccinic anhydride, anhH) were employed separately to stabilize Al and Zr precursors in order to control their chemical reactivity, avoiding precipitation. In all cases a prehydrolyzed tetraethyl orthosilicate (TEOS) sol was the Si source. We use the Partial Charge Model as a theoretical indication of the stabilization of the Al and Zr species derived from the reaction with anhH and acacH. The sols were polymerized at room temperature (293 K) to obtain gels and these were dried and calcined at 673, 773 and 873 K in air. The characterization techniques were Small Angle X-ray Scattering (SAXS), Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), Thermal Gravimetric (TGA) and Differential Thermal Analyses (DTA). The porosity and surface area of solids calcined at 673, 773 and 873 K were determined by N₂ adsorption/desorption isotherms. The corresponding average pore diameter was evaluated using the methods BJH, HK and DA. These models were used because all together cover the full range of the pore size.     

Determination of the Kinetics of the Reaction of CCl4 with Prefluorided Cr2O3 and Evaluation of the Active Site Area

The reaction of CCl₄ with prefluorided Cr₂O₃ has been studied by temperature-programmed and isothermal methods. Temperature-programmed reaction showed that CCl₃F and CCl₂F₂ were produced simultaneously with activation energies of 64.5 and 62.5 kJmol^–1, respectively, for the surface exchange reaction. It also allowed evaluation of the surface fluoride radius, a value of 1.86 Å being obtained which is slightly higher than the literature value of 1.33 Å. Isothermal reaction at 523 and 673 K of CCl₄ produced simultaneous and instantaneous sharp peaks of CCl₃F and CCl₂F₂. The fluoride ion radius derived from the 523 K experiment was 2.8 Å, suggesting that integration of the peak was terminated prematurely while the value at 673 K was 2.6 Å, suggesting involvement of the subsurface fluorine ions.     

Low temperature CO oxidation over Au/TiO2 and Au/SiO2 catalysts

After a high-temperature reduction (HTR) at 773 K, TiO₂-supported Au became very active for CO oxidation at 313 K and was an order of magnitude more active than SiO₂-supported Au, whereas a low-temperature reduction (LTR) at 473 K produced a Au/TiO₂ catalyst with very low activity. A HTR step followed by calcination at 673 K and a LTR step gave the most active Au/TiO₂ catalyst of all, which was 100-fold more active at 313 K than a typical 2% Pd/Al₂O₃ catalyst and was stable above 400 K whereas a sharp decrease in activity occurred with the other Au/TiO₂ (HTR) sample. With a feed of 5% CO, 5% O₂ in He, almost 40% of the CO was converted at 313 K and essentially all the CO was oxidized at 413 K over the best Au/TiO₂ catalyst at a space velocity of 333 h^–1 based on CO + O₂. Half the chloride in the Au precursor was retained in the Au/TiO₂ (LTR) sample whereas only 16% was retained in the other three catalysts; this may be one reason for the low activity of the Au/TiO₂ (LTR) sample. The reaction order on O₂ was approximately 0.4 between 310 and 360 K, while that on CO varied from 0.2 to 0.6. The chemistry associated with this high activity is not yet known but is presently attributed to a synergistic interaction between gold and titania.     

GAS PERMEANCE BEHAVIOR AT ELEVATED TEMPERATURE IN MESOPOROUS ANODIC OXIDIZED ALUMINA SYNTHESIZED BY PULSE-SEQUENTIAL VOLTAGE METHOD

Mesoporous anodic oxidized alumina (MAOA) capillary tubes with and without a barrier layer have been synthesized by applying a pulse-sequential voltage. The single gas permeances at an elevated temperature and the thermal and hydrothermal stabilities of MAOA were investigated. A highly oriented radial mesopore channel with pore sizes from 40 to 4 nm was formed in the MAOA tubes. Micropores with sizes from 0.4 to 0.8 nm were formed in the barrier layer. The H₂ permeance of MAOA with a barrier layer (barrier type) was approximately 540 times lower than that of MAOA without a barrier layer (block type) at 773 K. The H₂/N₂ permselectivity of the barrier type in the temperature range from 333 to 673 K was 3.4; those of the barrier type at 773 and 823 K were 4.4 and 11, respectively. On the other hand, the H₂/N₂ permselectivities of the block type were from 3.1 to 3.6 in the temperature range from 333 to 773 K. The H₂ permeance and the H₂/N₂ permselectivity of the amorphous silica membrane on the block type were 1.1 × 10^?7 mol/m² · s · Pa and 40 at 773 K, respectively. MAOA synthesized by the pulse-sequential voltage method can be applied to the mesoporous support of the gas separation membrane at elevated temperatures.     

High temperature microwave absorbing properties of plasma sprayed La0.6Sr0.4FeO3-δ/MgAl2O4 composite ceramic coatings

Microwave absorbing materials (MAM) which can be used in high temperature and oxidation environments are strongly demanded in application, for conventional MAM fail due to various factors at high temperature. In this work, ceramic coatings composed of La_0.6Sr_0.4FeO_3-δ and MgAl₂O₄ have been successfully prepared via atmospheric plasma spraying and the microwave absorbing properties at high temperature are first reported. When the coatings of LSF/MAS matched with the metal substrate on thermal expansion, they can be treated repeatedly in the temperature range of 300 K–1173 K without peeling off. Meantime, the ceramic coatings with thickness of 1.5 mm showed considerable microwave absorption in the range of 8 GHz–18 GHz at high temperatures between 673 K–1173 K. The absorption bandwidth of LSF30 was 3.5 GHz for RL < −10 dB and 8 GHz for RL < −5 dB in the temperature range of 673 K–1173 K. LSF/MAS possesses the advantages of thin thickness and broad bandwidth at high temperatures, suggesting great potential as MAM in ultra-temperature environment.