Dynamic modelling of a bubble column for particle formation via a gas–liquid reaction

This paper presents a dynamic model of a bubble column reactor with particle formation, accomplished by adopting a hybrid CFD-reaction engineering approach. CFD is employed for estimating the hydrodynamics and is based on the two-phase Eulerian–Eulerian viewpoint. The reaction engineering model links the penetration theory to a population balance that includes particle formation and growth with the aim of predicting the average particle size. The model is then applied to the precipitation of CaCO₃ via CO₂ absorption into Ca(OH)₂aq in a draft tube bubble column and draws insight into the phenomena underlying the crystal size evolution.     

Synthesis of TiC–TiB2 composite powders via carbothermal reduction and its reaction mechanism

Titanium carbide–titanium diboride (TiC–TiB₂) composite powders were synthesised through a carbothermal reduction method by using titanium dioxide, boric acid, and different carbon sources (namely, carbon black, sucrose, and glucose) as starting materials. The thermal decomposition behaviour of the precursors was studied by thermogravimetry–differential thermal analyser. Phase compositions and morphologies of the synthesised products were characterised by X-ray diffractometer and scanning electron microscope. When n(Ti):n(B):n(C)?=?1.0:2.5:5.0, the blended stock mainly formed TiB₂ with sucrose or glucose as a carbon source, whereas the stock produced TiC when carbon black was the source. At an optimum reaction temperature, the particles of the powders synthesised from carbon black as a carbon source were the smallest at approximately 100?nm. With increasing amount of boric acid in the precursor, the morphologies of the samples changed into less spherical particles, and more flaky grains and small particles with irregular structures were observed.     

Low-temperature combustion synthesis of nanocrystalline HoFeO3 powders via a sol–gel method using glycin

Single phase nanocrystalline HoFeO₃ powders were successfully synthesized by the sol–gel self-propagation combustion method using glycin (C₂H₅NO₂) as the chelating reagent at a low combustion temperature. Four different mole ratios of glycin to metal nitrate (G/M) were used to prepare HoFeO₃ powders in the experiment. The XRD patterns indicate monophasic HoFeO₃ powders can be formed by further calcination, the SEM micrographs show that the nano-sized grains with distinguishable boundaries had been obtained. The M–H curves show HoFeO₃ powders had the characteristic of antiferromagnetism at 50 K, while the powders had the characteristic of paramagnetism as the ambient temperature reached 100 K or 300 K. The FC/ZFC magnetic measurement results demonstrate that there was a transition from antiferromagnetism to paramagnetism in HoFeO₃ nanopowders as the temperature was increased.     

Etching of SiC–SiC-composites by a laser-induced plasma in a reactive gas

The precise machining of silicon carbide composite (SiC–SiC) as a high-tech material with extraordinary characteristics is required for different applications in aerospace, light weight construction and car industry. Laser machining enable new approaches for fabrication processes but the regularly applied ablation processes can cause damage to the SiC–SiC material. Here we propose and demonstrate a new approach for gentle SiC–SiC machining making use of a laser-induced plasma for reactive species generation enabling chemical material removal processes. A fs-laser (775 nm, 150 fs, 1 kHz) was focussed to a CF₄/O₂ gas mixture igniting a laser-induced plasma (LIP) approximately 100 μm in front of a SiC–SiC sample. This LIP initiate material removal processes of the textured, multiphase SiC–SiC sample without a mechanical damage of the SiC–SiC composite structure. Different surface features such as etching of the cover SiC layer, etching of the SiC matrix and exposure, thinning and sharpen of the SiC fibres, underetching of the fibres has been observed. Across the whole etched area, no mechanical damage such as cracks, delamination's, broken fibres were observed so that a gentle machining process can be expected.     

Iron sulfate/sulfur-catalyzed liquefaction of Wandoan coal using syngas–water as a hydrogen source

Water-soluble iron sulfate/sulfur-catalyzed coal liquefaction using three kinds of hydrogen sources including syngas–water has been investigated. The liquefaction of Wandoan coal, an Australian subbituminous, with iron sulfate/sulfur as a catalyst precursor using syngas–water or carbon monoxide–water afforded higher coal conversions and oil yields than those using pressurized hydrogen gas. The pretreatment at relatively low temperature (200°C) was indispensable to achieve the high coal conversion. In the two-staged liquefaction (400°C, 60 min+425°C, 60 min), the use of syngas–water as a hydrogen source afforded higher coal conversion of 90.1% together with a high oil yield of 46.2% than those using pure hydrogen, and almost comparable to those using carbon monoxide–water, indicating the presence of synergistic effects of two hydrogen sources. At the early stage of the reaction, the contribution of carbon monoxide–water was predominant, whereas hydrogen gas significantly took effect at the latter stage. The XRD and XPS study revealed the formation of pyrrhotite, a possible active species, covered with a small amount of sulfate species.     

An efficient green synthesis of quinoxaline derivatives using carbon-doped MoO3–TiO2 as a heterogeneous catalyst

A series of carbon-doped MoO₃–TiO₂ (CMT) material were prepared by sol–gel method. The catalyst prepared has been characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and Fourier transform infrared spectroscopy (FT-IR). The natural wood plant Acacia arabica used as a source for carbon substrate. The effect of variation of wt.% of carbon doping was successfully evaluated for the preparation of quinoxalines by the condensation of benzil and 1,2-diamine under ultra-sonication in EtOH:water (3:1). The merits of the present investigation are extremely short reaction times, mild reaction conditions, excellent yield, simple workup and environmentally benign process.     

Oxidative dehydrogenation of n-butane: a comparative study of thermal and catalytic reaction using Fe–Zn mixed oxides

In order to investigate how the presence of surface hydroxyl groups on oxide surfaces affects the interaction with the supported metal, we have modified a well-ordered alumina film on NiAl(110) by Al deposition and subsequent exposure to water. This procedure yields a hydroxylated alumina surface as revealed by infrared and high-resolution electron energy loss spectroscopy. By means of scanning tunneling microscopy, we have studied the growth of rhodium on the modified film at 300 K. Clear differences in the particle distribution and density are observed in comparison to the clean substrate. While, in the latter case, decoration of domain boundaries as typical defects of the oxide film governs the growth mode, a more isotropic island distribution and a drastically increased particle density is found on the hydroxylated surface. From infrared data, it can be deduced that the growth is connected with the consumption of the hydroxyl groups due to the interaction between the metal deposit and the hydroxylated areas. This finding is in line with photoemission results published earlier. This revised version was published online in July 2006 with corrections to the Cover Date.     

Modeling of gas–liquid flow in a rotating packed bed using an Eulerian multi-fluid approach

Rotating packed beds (RPBs) are ideal candidates for CO₂ removal from offshore natural gas due to their good mass transfer performance and significant volume savings. This article proposes an Eulerian multi-fluid approach to simulate the gas–liquid flow in RPBs. Three new multiphase drag force models are constructed based on single-phase drag force models for wire mesh packings. Based on the Eulerian multi-fluid approach, a new RPB simulation framework is developed. The predicted results using the new simulation framework with the new drag force models are compared with the experimental data. When using the Kołodziej model and the modified Kołodziej model, the predicted overall liquid holdup shows good agreement with the experimental data with errors less than 20%. In addition, the pressure drop predicted by these three models are reasonable compared with the experimental data. This work lays a foundation for RPB simulation of gas–liquid flow using Eulerian multi-fluid approach.     

Reduction of nitrogen oxides from simulated exhaust gas by using plasma–catalytic process

Removal of nitrogen oxides (NO_x) using a nonthermal plasma reactor (dielectric-packed bed reactor) combined with monolith V₂O₅/TiO₂ catalyst was investigated. The effect of initial NO_x concentration, feed gas flow rate (space velocity), humidity, and reaction temperature on the removal of NO_x was examined. The plasma reactor used can be energized by either ac or pulse voltage. An attempt was made to utilize the electrical ignition system of an internal combustion engine as a high-voltage pulse generator for the plasma reactor. When the plasma reactor was energized by the electrical ignition system, NO was readily oxidized to NO₂. Performance was as good as with ac energization. Increasing the fraction of NO₂ in NO_x, which is the main role of the plasma reactor, largely enhanced the NO_x removal efficiency. In the plasma–catalytic reactor, the increases in initial NO_x concentration, space velocity (feed gas flow rate) and humidity lowered the NO_x removal efficiency. However, the reaction temperature in the range up to 473 K did not significantly affect the NO_x removal efficiency in the presence of plasma discharge.     

Microstructure and thermal expansion of Al2TiO5–MgTi2O5 solid solutions obtained by reaction sintering

Solid solutions Mg_0.1Al_1.8Ti_1.1O₅ and Mg_0.5AlTi_1.5O₅ were obtained by reaction sintering of mixtures of the binary oxides at 1350–1600 °C using different precursor powders. For the composition Mg_0.1Al_1.8Ti_1.1O₅, ceramics sintered at 1400–1500 °C have high relative density (⩾90%), reduced grain size (2–6 μm), low thermal expansion (−0.8 to 0.3×10⁻⁶ K⁻¹ in the range 200–1000 °C) and reproducible expansion behaviour. At higher temperature, grain size rapidly increases owing to anisotropic and exaggerated grain growth (EGG) resulting in severe microcracking. Microstructure evolution is affected by the nature of the starting oxides, in particular for what concerns the onset temperature of EGG, the size and the fraction of abnormal grains. For the composition Mg_0.5AlTi_1.5O₅, EGG already takes place at 1350 °C and materials with grain size < 5 μm are difficult to obtain by conventional reaction sintering. Large grained samples (>10 μm) of both compositions show a reduced hysteresis and complex thermal expansion behaviour. In particular, heating to 1000 °C results in a significant increase in specimen size on return to room temperature. Repeated thermal cycling leads to an increase of the hysteresis.     

Ligandless palladium supported on SiO2–TiO2 as effective catalyst for Suzuki reaction

TiO₂–SiO₂ mixed oxides were prepared by the homogeneous precipitation method, and the supports were characterized by XRD, IR and BET. Ligandless palladium supported on SiO₂–TiO₂ was prepared by the deposition–precipitation method and used as recoverable catalyst for Suzuki reactions. The results revealed that the supported catalyst Pd/SiO₂–TiO₂ exhibited excellent catalytic activity for the coupling of aryl bromides with arylboronic acid, and also exhibited moderate catalytic activity for the coupling of aryl chlorides. The catalyst was recycled by simple filtration and reused without further disposal, no significant Pd leaching and loss of catalytic activity was observed except the first reuse.     

Improved activity of a graphene–TiO2 hybrid electrode in an electrochemical supercapacitor

We present a simple and fast approach for the synthesis of a graphene–TiO₂ hybrid nanostructure using a microwave-assisted technique. The microstructure, composition, and morphology were characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, Raman microscopy, X-ray photoelectron spectroscopy, and field-emission scanning electron microscopy. The electrochemical properties were evaluated using cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge tests. Structural analysis revealed a homogeneous distribution of nanosized TiO₂ particles on graphene nanosheets. The material exhibited a high specific capacitance of 165 F g⁻¹ at a scan rate of 5 mV s⁻¹ in 1 M Na₂SO₄ electrolyte solution. Theenhanced supercapacitance property of these materials could be ascribed to the increased conductivity of TiO₂ and better utilization of graphene. Moreover, the material exhibited long-term cycle stability, retaining ∼90% specific capacitance after 5000 cycles, which suggests that it has potential as an electrode material for high-performance electrochemical supercapacitors.     

Enhancement of thermal shock and slag corrosion resistance of MgO–ZrO2 ceramics by doping CeO2

The purpose of this paper was to develop ceramics materials with high thermal shock resistance and corrosion resistance for preparing gas blowing components. In this paper, MgO-rich MgO–ZrO₂ ceramics were obtained by using MgO powder and ZrO₂ powder as starting materials and CeO₂ as an additive. Changes in the properties in terms of thermal shock resistance, mechanical properties, and slag corrosion-resistance with chemical compositions were examined correlated to microstructure and phase changes. Especially, the effect of doping CeO₂ on phase transition of zirconia in MgO-rich system was discussed. The results showed that doping amount of CeO₂ significantly improved properties of MgO–ZrO₂ ceramics. Especially when doping amount of CeO₂ was 2 wt%, residual strength ratio was enhanced over 100% after thermal shock testing. In samples doped with CeO₂, ZrO₂ was stable in cubic or tetragonal form due to complete solution of CeO₂, which was important reason for the improvement of various properties of MgO–ZrO₂ ceramics.     

Performance of yttria-stabilized zirconia fuel cell using H2–CO2 gas system and CO–O2 gas system

This paper reports the performance of an yttria-stabilized zirconia fuel cell (YSZ) using five kinds of gas systems. The final target of this research is to establish the combined fuel cell systems which can produce a H₂ fuel and circulate CO₂ gas in the production process of electric power. A large electric power was measured in the H₂–O₂ gas system and the CO–O₂ gas system at 1073 K. The formation process of O²⁻ ions in the endothermic cathodic reaction (1/2O₂+2e⁻→O²⁻) controlled the cell performance in both the gas systems. The electric power of the H₂–CO₂ gas system, which allowed to change CO₂ gas into a CO fuel (H₂+CO₂→H₂O+CO) in the cathode, was 1/31–1/11 of the maximum electric power for the H₂–O₂ gas system. This result is related to the larger endothermic energy for the formation of O²⁻ ions from CO₂ molecules at the cathode (CO₂+2e⁻→CO+O²⁻) than from O₂ molecules. The CO–H₂O gas system and the H₂–H₂O gas system was expected to produce a H₂ fuel in the cathode (CO+H₂O→H₂+CO₂, H₂+H₂O→H₂+H₂O). Although relatively high OCV values (open circuit voltage) were measured in these gas systems, no electric power was measured. At this moment, it was difficult to apply H₂O vapor as an oxidant to the cathodic reaction in a YSZ fuel cell.     

Synthesis and properties of acrylate functionalized alkyds via a Diels–Alder reaction

The α-eleosterate pendent fatty acid of a tung oil based alkyd was functionalized via a Diels–Alder reaction with three different acrylate groups: (1) 2,2,2-trifluoroethyl methacrylate, (2) 3-methacryloxypropyl trimethoxysilane, and (3) triallyl ether acrylate. The modified alkyds were characterized by using ¹H NMR, ¹³C NMR, and gel permeation chromatography (GPC). The drying time was measured at ambient temperature. The viscoelastic properties of the alkyd-modified films were measured using dynamic mechanical thermal analysis. The viscoelastic and drying time results show that the alkyd modified with siloxane and triallyl group affords a faster drying time, higher crosslink density, and higher glass transition temperature compared to the unmodified alkyd, whereas the fluorinated alkyd possesses surface active properties, but suffers in terms of drying and crosslinking density.     

FeVO4 nanorods supported TiO2 as a superior catalyst for NH3–SCR reaction in a broad temperature range

In this work, a catalyst with FeVO₄ nanorods supported on TiO₂ was prepared and applied for NH₃–SCR reaction. A significantly enhanced low temperature catalytic activity has been achieved in the presence of 10% H₂O with the active window shifting by 15 °C to lower temperatures, compared to the classical catalyst with FeVO₄ nanoparticles supported TiO₂. For the catalyst containing FeVO₄ nanorods, enhanced redox ability and enriched surface active oxygen species originated from its unique crystal structure and predominantly exposed reactive crystal facets (− 2 1 0) on the catalyst surface are responsible for its improved low temperature catalytic activity.     

Structure analysis of cBN films prepared by DC jet plasma CVD from an Ar–N2–BF3–H2 gas system

The structure of the cBN films deposited by DC jet plasma CVD from an Ar-N₂-BF₃-H₂ gas system was investigated by transmission electron microscopy and electron-energy-loss spectroscopy. A sequent layered-structure of Si/amorphous /hexagonal/cubic BN was revealed, which was also confirmed by the confocal Raman technique. For comparison, the phase composition, crystal size and crystallinity of cBN films deposited for different times at initial growth stage were studied by infrared spectroscopy, Raman spectroscopy and glancing-angle X-ray diffraction. A columnar growth of the cBN grains with the average column width of approximately 0.2 μm was observed. The columns were proved to be cBN single crystals elongated from the nucleation sites on the hexagonal BN to the film surface. High-density twins and stacking faults were observed on the {111} planes of the cBN crystals, which subdivided the crystals into many lamellae of several to about 20 nm in thickness.     

TiO2–ZrO2 mixed oxide as a support for hydrotreating catalyst

Pure TiO₂, ZrO₂ and TiO₂–ZrO₂ mixed oxides are prepared by urea hydrolysis. Hydrotreating catalysts containing 12 wt% molybdenum are prepared using these oxides and characterized by BET surface area, pore volume, XRD and oxygen chemisorption. It is observed that oxides produced by the method of urea hydrolysis have higher surface area as compared to those available commercially. With increasing zirconia content in the mixed oxide, the surface area increases and a maximum value is obtained for a mixed oxide having Ti and Zr molar ratio of 65/35. XRD results indicate that mixed oxides are poorly crystalline in nature. Thiophene hydrodesulfurization, cyclohexene hydrogenation and tetrahydrofuran hydrodeoxygenation are taken as model reactions for evaluating catalytic activities. It is found that both O₂ uptake and catalytic activities increase with increasing zirconia content in mixed oxide and reach maximum values for the 12 wt% Mo/TiO₂–ZrO₂ (65/35) catalyst. With further increases in zirconia content, O₂ uptake and catalytic activities decrease and the lowest values are observed for the pure ZrO₂ supported catalyst.     

Microstructure and properties of ZrB2–SiC composites prepared by spark plasma sintering using TaSi2 as sintering additive

ZrB₂–SiC composites were fabricated by spark plasma sintering (SPS) using TaSi₂ as sintering additive. The volume content of SiC was in a range of 10–30% and that of TaSi₂ was 10–20% in the initial compositions. The composites could be densified at 1600 °C and the core–shell structure with the core being ZrB₂ and the shell containing both Ta and Zr as (Zr,Ta)B₂ appeared in the samples. When the sintering temperature was increased up to 1800 °C, only (Zr,Ta)B₂ and SiC phases could be detected in the samples and the core–shell structure disappeared. Generally, the composites with core–shell structure and fine-grained microstructure showed the higher electrical conductivity and Vickers hardness. The completely solid soluted composites with coarse-grained microstructure had the higher thermal conductivity and Young's modulus.     

Synthesis of ultra-fine tantalum carbide powders by a combinational method of sol–gel and spark plasma sintering

Tantalum carbide (TaC) nanopowders were synthesized by a novel method combining the sol–gel and spark plasma sintering (SPS) processes using tantalum pentachloride (TaCl₅) and phenolic resin as the sources of tantalum (Ta) and carbon (C), respectively. Gels of Ta-containing chelate with good uniformity and high stability were prepared by solution-based processing. The products with the structure of carbon-coated tantalum pentoxide (Ta₂O₅) were obtained after pyrolysis at 800?°C. Further heat treatment by SPS resulted in the fast formation of TaC at a relatively low temperature. The effects of the C/Ta molar ratio in the raw materials and the heat treatment temperature on the prepared powders were investigated. With increase in the C/Ta molar ratio from 3.75 to 4.25, the synthesis temperature, oxygen content and average crystallite size of the TaC powders decreased. Furthermore, the oxygen content of the powders prepared at the C/Ta molar ratio of 4.25 could be reduce by increasing the heat treatment temperature from 1400° to 1600°C, which unfortunately also induced a mean crystallite size increase from 30 to 100?nm. The TaC powders obtained at a comparatively low C/Ta molar ratios of 4.25 at 1500?°C had an average particle size of about 50?nm and a low oxygen content of about 0.43?wt%.