AFM investigation of solid product layers of MgSO4 generated on MgO surfaces for the reaction of MgO with SO2 and O2

The nucleation, growth and arrangement of MgSO₄ product layers during the reaction of single-crystal MgO with SO₂ and O₂ were investigated via atomic force microscopy (AFM). The morphology of the single-crystal MgO surface after reacting with SO₂ and O₂ consisted of three-dimensional cone-shaped MgSO₄ product islands. Most of the islands appeared at locations with terraces, steps and kinks. With increasing reaction time, small product islands grew to large islands, finally coalescing into continuous islands. The nucleation, growth and arrangement of the solid products on the reactant surface are controlled by thermodynamics and kinetics. The morphology of the MgSO₄ product islands is governed by the competition between the intrinsic chemical reaction rate and the product molecular diffusion rate, which are both significantly affected by the reaction temperature. As the reaction temperature increases, the mean size of the MgSO₄ islands increases while the island density decreases. The nucleation, growth and arrangement of the solid product layers and the product layer morphology on the solid reactant surface directly affect the mechanism of the gas–solid reaction.     

Change in the optical properties of amorphous films of transition metal oxides upon formation of the nanocrystalline phase

Weakly absorbing films of tantalum oxide Ta₂O₅ with the crystallization kinetics typical of amorphous films have been investigated. The films have been prepared by reactive magnetron sputtering on substrates of different natures (titanium and optical silica glass). The crystallization heat treatment of the films has been performed at temperatures ranging from 500 to 700 °C. Changes in the optical properties of the films in the visible range have been studied at the initial stage of the formation of nanosized nuclei. Investigations have been carried out using X-ray powder diffraction, atomic-force microscopy, and optical measurements. It has been demonstrated that a new reflecting layer with the refractive index smaller than that of the film is formed on polycrystalline substrates due to the heterogeneous nucleation of the crystalline phase at the film-substrate interface. The crystallization of the films on amorphous substrates occurs through the homogeneous mechanism and leads to a change in the transmission coefficient of the film as a result of light scattering from the nanocrystalline phase.     

Electrocatalysis and the oxygen evolution reaction

The oxygen evolution reaction has been investigated on a number of electrodes which are electrocatalysts for the chlorine evolution reaction, by making measurements in NaClO₄ solution. Steady state current—potential and impedance—potential measurements, obtained using automated equipment, have been used as the preferred experimental method. Analysis of the impedance has been undertaken by means of curve fitting, and the resulting parameter curves of the double-layer capacity, the charge transfer resistance, and the electrolyte ohmic resistance displayed as a function of potential. Some speculation is made about the interpretation of the parameter curves. The ohmic loss parameter, R_w, is used to correct the potentials to the true values. In comparison to some other gas evolution reactions, such as hydrogen and chlorine, R_w does not depend strongly on potential. The corrected log i—E curve is also considered. It is suggested that a more complete picture of the electrical performance of these electrodes can be obtained by using these electrochemical methods.     

Catalytic activity of supported metal particles for sulfuric acid decomposition reaction

Production of hydrogen by splitting of water in the thermochemical sulfur-based cycles that employs the catalytic decomposition of sulfuric acid into SO₂ and O₂ is of considerable interest. However, all of the known catalytic systems studied to date that consist of metal particles on oxide substrates deactivate with time on stream. To develop an understanding of the factors that are responsible for catalyst activity, we investigate the fresh activity of several platinum group metals (PGM) catalysts, including Pd, Pt, Rh, Ir, and Ru supported on titania at 850 °C and perform an extensive theoretical study (density-functional-theory-based first-principles calculations and computer simulations) of the activity of the PGM nanoparticles of different size and shape positioned on TiO₂ (rutile and anatase) and Al₂O₃ (γ- and η-alumina) surfaces. The activity and deactivation of the catalytic systems are defined by (i) the energy barrier for the detachment of O atoms from the SO_n (n = 1, 2, 3) species, and (ii) the removal rate of the products of the sulfuric acid decomposition (atomic O, S, and the SO_n species) from metal nanoparticles. We show that these two nanoscale features collectively result in the observed experimental behavior. The removal rate of the reaction products is always lower than the SO_n decomposition rates. The relation between these two rates explains why the “softer” PGM nanoparticles (Pd and Pt) exhibit the highest initial catalytic activity.     

Morphological and structural study of plasma deposited fluorocarbon films at different thicknesses

The growth of fluorocarbon thin films has been obtained by continuous and modulated r.f. plasmas fed with C₂F₄. The kinetics surface morphology of the coatings has been investigated and analyzed on films of various thicknesses. Such a study has allowed the evolution of structures in size and shape to be followed. Specifically, it has been observed that modulated plasmas with the proper duty cycles (⩽7%) lead to the formation of micrometer-long ribbon-shaped nanostructures. The precursors of the ribbons are nuclei that align into a spiral-mimic. Highly nanostructured films with an optimal thickness and ribbon density exhibit a super-water repellent surface with contact angles up to 170°. The surface roughness and smoothing of fluorocarbon films are strongly affected by the modulated and continuous plasmas, respectively.     

High temperature reaction of kaolin with inorganic acids

Abstract

A new method is described in which raw materials prepared by mixing kaolin and concentrated acid are heated in a furnace at temperatures between 100 and 700°C. Experiments with HCl, HNO₃, H₃PO₄, and H₂SO₄ showed that only H₂SO₄ gave high reaction yields, which grew steadily as reaction temperature was raised. Reaction products were mostly Al₂(SO₄)₃. Leached kaolin samples showed BET specific surface areas of over 100 m² g^-1. Furthermore, reaction between kaolin and H₂SO₄ required no previous calcination of kaolin.     

Understanding the morphological variation in the formation of B4C via carbothermal reduction reaction

Despite the relatively successful effort for the synthesis of fine boron carbide (B₄C) powders, very little is known about the underlying interrelationships between processing, compositions and the resulting product morphology for B₄C formation via the carbothermal reduction reaction (CTR) process. In this paper, B₄C, including uniform submicron powders, were synthesized using CTR of finely mixed boron trioxide and carbon obtained from low-cost water-soluble precursors. To understand the aforementioned interrelationships and to better control product morphology, the effects of factors such as CTR thermal profile, CTR atmosphere and precursors on product phase purity and morphology were systematically studied. Among all the factors, it was found that CTR temperature, along with total reaction time, plays a dominant role in determining the morphology of B₄C products. Such observation was understood by analysis of CTR reaction kinetics and comparing it with microstructural evolution: significant non-uniformity in B₄C product morphology arises from the competition between nucleation and growth at low to intermediate CTR temperatures (e.g., ~1150–1450 °C) while highly uniform submicron or nano B₄C products result from very fast nucleation at high CTR temperatures (e.g., ≥~1750 °C) within a few minutes or even seconds.     

Preparation of a CeO2–ZrO2 based nano-composite with enhanced thermal stability by a novel chelating precipitation method

In this study, a chelating agent is introduced to prepare CeO₂–ZrO₂ nano-composite through a precipitation process. The physicochemical properties of the oxide precursors, nano composite materials are strongly dependent on the preparation method and whether a chelating agent is used. Adding an appropriate quantity of chelating agent SO₄²⁻ can facilitate thermal stability and phase structure uniformity of CeO₂–ZrO₂ mixed oxides. The calculation results showed that the Gibbs free energy of chelating complex of [ZrSO₄]²⁺ (ΔG = −127.2469 kJ/mol) is higher than the [Ce(III)SO₄]⁺ (ΔG = -29.8279 kJ/mol). The precipitation chemical potential of Zr⁴⁺ moves close to the precipitation chemical potential of Ce³⁺. The novel and low-cost chelating precipitation method can modify the homogeneity of the compounds at the atomic scale, which can offer a powerful opportunity for, and provide direction in, the design of materials with exceptional properties.     

Chloride-sulphate exchange on anion resins,kinetic investigations. VI. Differential exchange rates in diluted systems

The kinetics of the SO₄ uptake by 6 × 10^-3N Na₂SO₄ solution of some typical anion resins in Cl form have been followed differentially, i.e. with complete conversion progressively occurring in about five steps. Like for the integral exchange the kinetics for the differential conversions agree with a rate equation previously developed for systems with reactions accompanying exchange. The accordingly determined differential rate parameters increase regularly with the conversion degree, and the integral value seems to be determined by the final rate. The sulfate selectivity also arises during the conversion, while no appreciable differences in water fluxes have been associated with ionic modification of the resins.     

The reaction of 1,2-dichloroethane with copper

The decomposition of 1,2-dichloroethane on polycrystalline copper has been studied using a microreactor. The reaction is found to have an activation energy of 81±5 kJ mol^–1 generating gaseous ethene and chemisorbed chlorine. The reaction terminates on completion of a monolayer of chemisorbed chlorine and is followed by a much slower reaction. The rate limiting step is thought to be C₂H₄Cl_2(phys)C2H₄Cl(_ads)+Cl_ads The reaction is compared with a UHV study of the same molecule on Cu(l 11) and the possibility of a negative ion transition state is discussed.     

Influence of the boron doping level on the electrochemical oxidation of the azo dyes at Si/BDD thin film electrodes

In this study the efficiency of electrochemical oxidation of aromatic pollutants, such as reactive dyes, at boron-doped diamond on silicon (Si/BDD) electrodes was investigated. The level of [B]/[C] ratio which is effective for the degradation and mineralization of selected aromatic pollutants, and the impact of [B]/[C] ratio on the crystalline structure, layer conductivity and relative sp³/sp² coefficient of a BDD electrode were also studied. The thin film microcrystalline electrodes have been deposited on highly doped silicon substrates via MW PE CVD. Si/BDD electrodes were synthesized for different [B]/[C] ratios of the gas phase. Mechanical and chemical stability of the electrodes was achieved for the microcrystalline layer with relatively high sp³/sp² band ratio. Layer morphology and crystallite size distribution were analyzed by SEM. The resistivity of BDD electrodes was studied using four-point probe measurements. The relative sp³/sp² band ratios were determined by deconvolution of Raman and X-ray photoelectron spectra. The efficiency of degradation and mineralization of the reactive azo dye rubin F-2B was estimated based on the absorbance measurements at 545 nm. The influence of commonly used electrolytes NaCl and Na₂SO₄ on the dye removal efficiency was also investigated. The results suggest that, in general, the oxidation occurs indirectly at the anode through generation of hydroxyl radicals •OH, which react with the dye in a very fast and non-selective manner. In NaCl electrolyte the dye was also decomposed by more selective, active chlorine species (Cl₂, HOCl). However the efficiency of this process in BDD depended on the electrode's doping level. Higher amounts of dopant on the surface of BDD resulted in the higher efficiency of dye removal in both electrolytes.     

Effect of Cl/S molar ratio on theoretical partitioning of heavy metals under waste combustion conditions

A simulation calculation was carried out to predict the behavior of heavy metals during waste incineration according to the variation of chlorine content by using a thermodynamic equilibrium model. To predict the behavior of heavy metals in incineration of wastes, chlorine content in wastes was changed up to 3.00 on a Cl/S molar ratio basis. Then the partitioning characteristics of heavy metals with Cl/Metal molar ratio were investigated as solid, liquid, and gas phases of metals. For analysis of emission characteristics, incineration temperature, chlorine content and air-fuel ratio (λ_r) were chosen as major operating parameters. It was found that the distribution characteristics of heavy metals were not significantly affected by air-fuel ratio, but evaporation rate of heavy metals increased with operating temperature. Most of the heavy metals remained in solid phase of metal oxides, such as CdO, CrO₃, CrO₂, CuO and ZnO, except for lead which existed as PbSO₄ in the given operating conditions (i.e., T_b=800 °C, λ_r=1.3, and Cl/S=0). It was found that most of the heavy metals in solid phase changed to gaseous Cl compounds, which have a high volatility with regard to increasing the Cl/S molar ratio under the same conditions. However, Cr compounds were almost not affected as chlorine level increased. This paper is dedicated to Professor Dong Sup Doh on the occasion of his retirement from Korea University.     

Stability of iodine on ruthenium during copper electrodeposition and its effects on the nucleation behavior of electrodeposited copper

Cyclic voltammetry, current-time-transient measurements, and X-ray photoelectron spectroscopy (XPS) have been used to study the nucleation behavior of electrochemically deposited Cu films on Ru substrates as a function of Ru pre-treatment. Pre-treatment consisted of cathodic polarization in either 1 M H₂SO₄ or in 1 M H₂SO₄ + 1 mM KI, followed by sample emersion and placement in a 1 M H₂SO₄ + 50 mM CuSO₄ plating bath. XPS measurements confirmed the presence of adsorbed I on the Ru surface following pre-treatment in the KI/H₂SO₄ solution. Cyclic voltammogram (CV) data for electrodes either as-received or pre-reduced in H₂SO₄ and then immersed in the plating solution exhibited a broad peak in the overpotential region consistent with oxide reduction followed by Cu deposition. No underpotential deposition (UPD) feature was observed for these electrodes. In contrast, the sample pre-reduced in I-containing electrolyte exhibited a narrow Cu deposition peak in the overpotential region and a UPD Cu feature centered at 80 mV vs. Ag/AgCl. Current-time-transient (CTT) measurements of Cu deposition on as-received electrodes or electrodes pre-reduced in I-free solution exhibited potential-independent kinetics that are not well described by either progressive or instantaneous nucleation models and which at long times indicate a combination of diffusion and kinetic control. In contrast, CTT measurements of deposition kinetics for samples reduced in I-containing electrolyte exhibited complex, potential-dependent behavior and that at long times indicates diffusion control. XPS results also indicated that the iodine adlayer on Ru reduced in I-containing electrolyte is stable upon polarization to at least −200 mV vs. Ag/AgCl. These data indicate that a protective I adlayer may be deposited on an air-exposed Ru electrode as the oxide surface is electrochemically reduced, and that this layer will inhibit reformation of an oxide during the Cu electroplating process. Therefore, electrochemical pre-treatment in I-containing electrolyte may be of practical utility under industrial conditions for Cu electroplating.     

Effect of chlorine ion concentration on morphology and optical properties of Cl-doped ZnO nanostructures

Effect of Cl^?1 concentration on morphology and optical properties of Cl-doped ZnO nanostructures was studied. The Cl-doped ZnO nanostructures and undoped ZnO microstructures were grown on Si(1 1 1) substrates using a physical vapor deposition method. The ZnO nanostructures have been doped with different concentrations of chlorine. The Cl-doped ZnO nanostructures with 6% atom Cl, showed a nanodisk morphology with a hexagonal shape, while the Cl-doped ZnO nanostructures with 9% atom Cl, exhibited a stacked nanoplate morphology with smaller thickness in comparison to the Cl-doped ZnO nanodisks. In addition, with increasing Cl content to 13%, morphology of the products changed to more stacked nanoplates with nanoflakes morphology. X-ray diffraction results clearly showed a hexagonal structure for the all samples. Raman spectroscopy results showed a strong crystalline quality for the undoped ZnO microdisks and Cl-doped ZnO nanodisks; while these results indicated a weak crystalline quality for the Cl-doped ZnO nanoplates and nanoflakes. Photoluminescence (PL) studies also confirmed the Raman results and it exhibited a lower optical property for the Cl-doped ZnO nanoplates and nanoflakes in comparison to the undoped ZnO microdisks and Cl-doped ZnO nanodisks. Furthermore, the UV peak of the Cl-doped ZnO nanostructures was blue-shifted with respect to that of the undoped ZnO.     

Lignin–silica hybrids as precursors for silicon carbide

Lignin, prepared by digesting cedar with acetic acid, and tetraethoxysilane have been allowed for a sol–gel reaction in tetrahydrofuran using H₂SO₄ as catalyst to yield lignin–SiO₂ hybrids in the bulk gel form. The solid-state ¹³C nuclear magnetic resonance spectra of the hybrids and products formed solely from the lignin under the acidic conditions revealed that the lignin underwent crosslinking during the sol–gel reaction. The degree of crosslinking increased with an increasing amount of H₂SO₄. The powdered hybrids have been heated at 1500°C in Ar for carbothermal reduction, resulting in the formation of SiC powders. The lignin-to-tetraethoxysilane mixing ratio of the starting solutions varied free carbon content in the SiC powders. In addition, the amount of carbonaceous residue formed from the lignin upon heating depended on the degree of crosslinking of the lignin. Thus, to adjust the amounts of both the lignin and H₂SO₄ was necessary for producing the hybrids suitable for precursors for SiC powders with high purity. Critical adjustment of the amounts led to the formation of SiC powders with a free carbon content of 0.57 wt %, implying that the lignin is a beneficial carbon source for the production of SiC powders by the hybrid route. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1321–1328, 1999     

Hydrothermally prepared nanocrystalline Mn–Zn ferrites: Synthesis and characterization

Nanocrystalline particles of Mn_xZn_1−xFe₂O₄ were prepared by chemical precipitation of hydroxides, followed by hydrothermal processing and freeze–drying. The synthesis involves the hydrolysis of aqueous metal precursors by using ammonia as precipitating agent. The chlorine ion concentration in the solution and the pH of the precipitation, are shown to play a crucial role in retaining the initial stoichiometry of the solution to the nanoparticles. The obtained products exhibited some interesting and unique features: they consisted of nanoparticles with sizes ranging from 5 to 25 nm, they had surface areas between 60 and 110 m² g⁻¹ and pore sizes in the mesopore region (i.e. 8–20 nm). The produced materials were examined by powder X-ray diffraction for crystalline phase identification, scanning electron microscopy for grain morphology, high resolution transmission electron microscopy for particle size distribution and nitrogen sorption for surface area, pore volume and pore size distribution determination. The sintering of the ferrite powders was also studied by thermogravimetric analysis and dilatometry of the powders mixed with an organic binder to improve their compaction properties.     

Peculiarities of the Thermal Degradation of Chlorosulfonated Polyethylenes

Chlorosulfonated polyethylene (CSPE) is fairly widely employed in production of artificial leathers, the building industry, and others. Like all the halogen-containing polymers, CSPE is conspicuous for its low stability. There are two types of framing groups within the macromole- cules—Cl and SO₂Cl, and consequently under the thermal and other types of physical effects CSPE is decomposed with elimination of two low-molecular products: HCl and SO₂. It is clear that the thermal stability of the macromolecules, as many other service characteristics of CSPE, depends on the degree of the polymer chlorosulfonation, the character of distribution of Cl and SO₂Cl in the chain, and also on the presence of different labile structures in the macromolecules, such as vicinal chlorines and SO₂Cl groups, P-chloroallyl groups, Cl and S0₂Cl groups at tertiary carbons, etc.     

Synthesis of nanocrystalline ZrO2 powders by mechanochemical reaction of ZrCl4 with LiOH

Mechanochemical reaction of anhydrous ZrCl₄ with LiOH has been used to synthesize nanocrystalline powders of ZrO₂. The mechanism of chemical reaction was found to depend on the presence of LiCl diluent in the initial reactant mixture. In the absence of LiCl diluent, reaction of the precursors occurred in a combustive manner and resulted in the formation of agglomerated nanoparticles of tetragonal ZrO₂ embedded within a matrix of LiCl and LiCl·H₂O. Continued milling after the combustion event resulted in a progressive loss of specific surface area and transformation of the tetragonal ZrO₂ to the equilibrium monoclinic structure. Dilution of the initial reactant mixture with 4LiCl suppressed combustive reaction. Reaction of the precursors occurred gradually and resulted in the formation of an amorphous ZrO₂ gel that crystallized with the tetragonal structure on calcination at 500 °C for 1 h.     

Curing reaction and product properties of polysulfones terminated with active functional groups

The curing processes of maleimide-terminated polyether sulfone oligomers (M_n from 1100 to 3000) and ethynyl-terminated polysulfone prepolymers (M_n from 2600 to 12000) were investigated by means of DSC. The initial temperature of cure reaction is increased, and the apparent activation energy E_a of cure reaction is decreased as the M_n of the uncured prepolymer increases. On the basis of the T_g changes during the curing process, the kinetics of cure is proposed to be similar to a stepwise reaction, although the curing reaction is believed to proceed via a free radical chain mechanism. In this proposed reaction some initial reaction products with many branches and one remaining active functional group are formed first. The more these products occur, the more likely they can interreact to form a network. During the curing process the mobility of the active end group is restricted by the rigid polysulfone chain. Therefore, the reaction is mainly controlled by the movement of polymer, i.e., by the kinetics of diffusion. Films of these cured polymers were made by casting from solution. Their β relaxation peaks at low temperatures were examined by dielectric and dynamic mechanical measurements. The films made of the prepolymer with M_n > 3000 are flexible and foldable.     

Characterisation of monophasic and diphasic mullite precursors by solid state reaction study

Abstract

The characterisation by various spectroscopic techniques of amorphous mullite precursors formed from monophasic (SH) and diphasic (DG) gels is reviewed. In a new approach, both types of precursor have been heated with CaCO₃ and the crystalline phases formed during solid state reaction monitored using an X-ray powder diffraction technique. The formation of a calcium aluminosilicate phase (gehlenite) rather than calcium aluminate suggests that both precursors consist of an amorphous aluminosilicate phase of composition close to that of 3:2 mullite.