Hydrogenolysis of Dimethyl Disulfide to Methanethiol in the Presence of Cobalt Sulfide Catalysts

The hydrogenolysis of dimethyl disulfide to methanethiol at T = 180–260°C and atmospheric pressure in the presence of supported cobalt sulfide catalysts has been studied. Cobalt sulfide on aluminum oxide exhibits a higher activity than that on a carbon support or silicon dioxide. The maximum reaction rate per gram of a catalyst is observed on an 8% Co/Al₂O₃ catalyst. At temperatures of up to 200°C and conversions up to 90%, methanethiol is formed with nearly 100% selectivity regardless of the cobalt content, whereas the selectivity for methanethiol under more severe conditions decreases because of its condensation to dimethyl sulfide.     

Oxidation Behaviour of A Boron Carbide Based Material in Dry and Wet Oxygen

The oxidation behaviour of a B₄C based material was investigated in a dry atmosphere O₂(20 vol.%)-CO₂(5 vol.%)-He and also in the presence of moisture H₂O (2.3 vol%) as boron oxide is very sensitive to water vapour. The mass changes of samples consisting of a chemical vapour deposit of B₄C on silicon nitride substrates were continuously monitored in the range 500–1000°C during isothermal experiments of 20 h. The stability of boron oxide formed by oxidation of B₄C was also studied in dry and wet atmospheres to explain the kinetic curves. In both atmospheres, oxidation is diffusion controlled at 700 and 800°C and enhanced by water vapour. At 900°C and higher temperatures, boron oxide volatilisation and consumption by reaction with water vapour modifies the properties of the oxide film and the material is no more protected. At 600°C, B₄C oxidation is weak but the process remains diffusion controlled in dry conditions as boron oxide volatilisation is negligible. However, in the presence of water vapour, B₂O₃ consumption rate is significant and mass losses corresponding to this consumption and to the combustion of the excess carbon are observed. This revised version was published online in August 2006 with corrections to the Cover Date.     

A Study of the Activation of Carbon Using Sample Controlled Thermal Analysis

A constant rate method involving the control of the concentration of evolved CO₂ at a constant level was used to study the air activation of pure and copper-doped carbon prepared from sodium carboxymethylcellulose. Whereas under a linear heating regime, both types of carbon reacted suddenly and quickly with O₂, under constant rate conditions this violent reaction was avoided and oxidation proceeded steadily at a lower temperature until complete burn off of the carbon was achieved. The catalytic effect of the copper on carbon gasification was noted with lower reaction temperatures for both linear heating (380°C compared to 500°C) and for the constant rate experiments (320°C compared to 400°C). This revised version was published online in July 2006 with corrections to the Cover Date.     

Nanocomposites based on nickel ferrites dispersed in sol–gel silica matrices

In this work, we describe the effects of thermal treatments on the structural, morphological, and textural properties of nanocomposites formed by nickel ferrite dispersed in xerogel and aerogel silica matrices. The catalytic properties for the total oxidation of an organochloro model contaminant, the chlorobenzene, are also evaluated. Wet samples with different amounts of NiFe₂O₄ in matrix were prepared by sol–gel process. Xerogels and aerogels obtained in monolithic form were prepared by controlled and hypercritical drying, respectively, and heated at temperatures between 300 and 1,100°C. The specific surface area and total pore volume of the samples change with heating mainly due to the variation on their texture. The xerogel treated at 500°C and the aerogel treated at 700°C showed the most catalytic activity, converting chlorobenzene at temperatures as low as 150°C, while the other catalysts were active only at temperatures higher than 300°C. No organic by-products were observed in the oxidation of chlorobenzene, suggesting that total oxidation takes place under the reaction conditions. A strong decrease in catalytic activity was observed for nanocomposites treated at 1,100°C, due to matrix densification, which led to the encapsulation of the ferrite particles and hindered the access of the gas to the ferrite surface.     

Promotion effect of adsorbed water/OH on the catalytic performance of Ag/activated carbon catalysts for CO preferential oxidation in excess H2

Activated carbon (AC) supported silver catalysts were prepared by incipient wetness impregnation method and their catalytic performance for CO preferential oxidation (PROX) in excess H₂ was evaluated. Ag/AC catalysts, after reduction in H₂ at low temperatures (≤200 °C) following heat treatment in He at 200 °C (He200H200), exhibited the best catalytic properties. Temperature-programmed desorption (TPD), X-ray diffraction (XRD) and temperature-programmed reduction (TPR) results indicated that silver oxides were produced during heat treatment in He at 200 °C which were reduced to metal silver nanoparticles in H₂ at low temperatures (≤200 °C), simultaneously generating the adsorbed water/OH. CO conversion was enhanced 40% after water treatment following heat treatment in He at 600 °C. These results imply that the metal silver nanoparticles are the active species and the adsorbed water/OH has noticeable promotion effects on CO oxidation. However, the promotion effect is still limited compared to gold catalysts under the similar conditions, which may be the reason of low selectivity to CO oxidation in PROX over silver catalysts. The reported Ag/AC-S-He catalyst after He200H₂00 treatment displayed similar PROX of CO reaction properties to Ag/SiO₂. This means that Ag/AC catalyst is also an efficient low-temperature CO oxidation catalyst.     

Struktur und katalytische Eigenschaften von Molybdänoxid-Trägerkatalysatoren bei einigen Oxydationsreaktionen

Structure and Catalytic Properties of Molybdenum Oxide Supported Catalysts in Some Oxidation Reactions Molybdenum supported catalysts were prepared by using different precursor compounds such as Mo(π-C₃H₅)₄, [Mo(OC₂H₅)₅]₂, MoCl₅, (NH₄)₆Mo₇O₂₄, and their catalytic behaviour in some oxidation reactions was studied. During the preparation process, as a result of interaction between the molybdenum compound used and the support, different surface compounds with strongly differing catalytic properties have been formed. MoO₃ and supported catalysts with MoO₃ crystallites on the surface, catalyse the H₂ oxidation at temperatures above 400°C and the CO oxidation at temperatures of about 500°C. The reaction proceeds according to a redox mechanism. On surface compounds of molybdenum which exist on the surface if organic complexes are used as precursors, the catalytic H₂ oxidation occurs even at 100°C with a high reaction rate. The catalytic CO oxidation on these catalysts occurs at temperatures of about 300°C. An associative mechanism on coordinative unsaturated Mo^VI sites is discussed.     

Dimethyl disulfide conversion into dimethyl sulfide in the presence of sulfidized catalysts

The conversion of dimethyl disulfide in the presence of various supported sulfidized metal-containing catalysts at atmospheric pressure and T = 150−350°C was studied. Sulfidized transition metals supported onto aluminum oxide were more active than catalysts based on a carbon support, silicon dioxide, amorphous aluminosilicate, and zeolite ZSM-5. The most active catalyst was 10% Co/Al₂O₃ prepared with the use of cobalt acetate as an active component precursor and treated with a mixture of hydrogen sulfide with hydrogen at T = 400°C. From kinetic data, it follows that all of the reaction products were formed simultaneously at a temperature of <200°C, whereas a consecutive reaction scheme took place at higher temperatures. In the presence of a sulfidized alumina-cobalt catalyst, the output of dimethyl sulfide was higher than that reached with the use of other well-known catalysts.     

The oxidation kinetics study of ultrafine iron powders by thermogravimetric analysis

The oxidation kinetics of ultrafine metallic iron powder to hematite (α-Fe₂O₃) up to temperatures 800 °C were studied in air using non-isothermal and isothermal thermogravimetric (TG) analysis. The powders with average particles size of 90, 200, and 350 nm were made by the electric explosion of wire. It was observed that the reactivity of the iron powder is increased with the decreasing particle size of powder. The experimental TG curves clearly suggest a multi-step process for the oxidation, and therefore a model-fitting kinetic analysis based on multivariate non-linear regressions was conducted. The complex reaction can be best described with a three-step reaction scheme consisting of two concurrent and one parallel reaction step. In one reaction pathway Fe is oxidized to α-Fe₂O₃. The other pathway is described by the oxidation of Fe to magnetite (Fe₃O₄). At higher temperatures the formed Fe₃O₄ is further oxidized in a α-Fe₂O₃. It is established that the best fitting three-step mechanism employed a branching set of n-order equations for each step.     

Factors Affecting the Electrodeposition of Aluminum Metal in an Aluminum Chloride–Urea Electrolyte Solution

Electrodeposition of aluminum metal using an AlCl₃–urea ionic liquid electrolyte at room temperature is studied. The molar ratio of AlCl₃/urea, addition of toluene, stirring speed, deposition duration, and temperature are the major factors that affect the deposition of aluminum. The electroplating is carried out at temperatures in the range 20–60°C at a stirring speed 0–80 rpm using bias of 1 V applied for 2 h. The aluminum electrodeposition is enhanced at a high molar ratio of AlCl₃/urea using 20% diluted toluene electrolyte. The microstructure of the deposited aluminum layer is examined using X‐ray diffraction (XRD), energy dispersive spectroscopy (EDS), and scanning electron microscopy (SEM). The current density is found to decrease with the duration and at lower temperatures. In this study, a current efficiency as high as ~89.98% could be obtained at 60°C.     

Kinetics and mechanism of the oxidation of carbon by NO2 in the presence of water vapor

The kinetics and mechanism of the oxidation of carbon by NO₂ in absence and presence of water vapor were studied in a fixed bed reactor. The rate of carbon oxidation by NO₂ is enhanced in the presence of water vapor in the range of temperature 300–400°C. The benefit effect of water is attributed to the intermediate formation of traces of nitric and nitrous acids, which enhance the rate of the carbon oxidation without modifying the global mechanism reaction. Therefore, water acts as a catalyst for the carbon oxidation by NO₂. A kinetic mechanism derived from this parametric study shows a decrease in the activation energy of carbon oxidation by NO₂ in the presence of water vapor. This result is in agreement with the experimental observation. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 236–244, 2009     

Avoiding Self‐Poisoning: A Key Feature for the High Activity of Au/Mg(OH)2 Catalysts in Continuous Low‐Temperature CO Oxidation

Au/Mg(OH)₂ catalysts have been reported to be far more active in the catalytic low‐temperature CO oxidation (below 0 °C) than the thoroughly investigated Au/TiO₂ catalysts. Based on kinetic and in situ infrared spectroscopy (DRIFTS) measurements, we demonstrate that the comparatively weak interaction of Au/Mg(OH)₂ with CO₂ formed during the low‐temperature reaction is the main reason for the superior catalyst performance. This feature enables rapid product desorption and hence continuous CO oxidation at temperatures well below 0 °C. At these temperatures, Au/TiO₂ also catalyzes CO₂ formation, but does not allow for CO₂ desorption, which results in self‐poisoning. At higher temperatures (above 0 °C), however, CO₂ formation is rate‐limiting, which results in a much higher activity for Au/TiO₂ under these reaction conditions.     

Kinetic Analysis of Calorimetric Measurements of Niobium–Aluminum Nb/Al Multifilament Strands

The formation of different phases for the reaction of Nb and Al in M/s Hitachi Ltd made wires using differential scanning calorimetry (DSC) has been investigated. The interfacial diffusion reaction has been studied in the temperature range 100–990 °C. Niobium (Nb) and aluminum (Al) react to form the phase NbAl₃, subsequently NbAl₃ reacts with the remaining Nb to form the A15 phase. The interfacial reactions play an important role. At higher heating rates, NbAl₃ and bcc‐structured NbAl are formed prior to the formation of Nb₃Al, but at lower heating rates the phase observed indicates σ phase Nb₂Al. DSC has been performed at different heating rates under nonisothermal conditions aimed to measure activation energy of crystallization for different phases: employing Kissinger's equation, Matusita–Sakka theory, and the Augis‐Bennett method. Activation values calculated from these three different methods are found to be in good agreement with each other. The activation energy for the first phase (NbAl₃) is lower than the other two phases (Nb₃Al) A15 phase and Nb₂Al, suggesting that NbAl₃ formation occurs easily. The value of n relating to the nucleation and growth mechanisms has also been calculated using the modified Kissinger method at different temperatures.     

Carbon molecular sieve membrane formed by oxidative carbonization of a copolyimide film coated on a porous support tube

A copolyamic acid was synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) as the dianhydride and 4,4′-oxydianiline (4,4′-ODA) and 2,4-diaminotoluene (2,4-DAT) as the diamine and was coated on the outer surface of a porous alumina support tube. The film was imidized and then carbonized by varying reaction period, temperature and atmosphere. Permeances of the BPDA-ODA/DAT carbon membrane were much lower than those of BPDA-ODA carbon membranes. However, the performance of the BPDA-ODA/DAT copolyimide-based membrane was greatly improved by treating in air at temperatures up to 500°C for 1 h, followed by carbonizing in nitrogen at temperatures up to 700°C. Permeance to CO₂ for the BPDA-ODA/DAT carbon membrane prepared under optimum conditions was 3 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹, and the separation coefficient of CO₂ to CH₄ was 60 at a permeation temperature of 35°C. These were comparable to the results of carbon membranes prepared from BPDA-ODA polyimide. The micropore structure of the BPDA-ODA/DAT carbon membrane was thus successfully controlled by an optimized combination of oxidation and carbonization after imidization.     

Solid State Synthesis of Benzils at Low-Heating Temperatures

A simple and efficient method for the preparation of benzils through the solid state oxidation reaction of benzoins by FeCl₃ · 6H₂O at low-heating temperatures(80 ~ 90°C) is described. The oxidation conditions are mild. The products are easily separated with no pollutions produced.     

Reactions in aminosilane-epoxy prepolymer systems. II. Reactions of alkoxysilane groups with or without the presence of water

The hydrolysis and reactions of alkoxy silane groups have been studied on a model compound (TA) prepared from 2 mol of phenyl glycidyl ether and 1 mol of aminopropyl triethoxy silane. At low (40°C) and high (140°C) temperatures, the monomer conversion and the evolution of the molecular mass are followed by size exclusion chromatography (SEC). During the same reaction time, the evolution of the functional groups, hydroxyl CH? OH, ethoxy ? O? C₂H₅, and siloxane Si? O? Si, is observed by FTIR spectroscopy. Without the presence of water, reactions between hydroxyl and ethoxy silane lead to gelation at the end of the reaction. A by-product, probably a cyclic tetramer is also formed. After the hydrolysis, the reaction of the model compound is quite different. The product of reaction is always soluble, even after a treatment at high temperatures, and the evolution of the molecular mass versus the reaction time seems to correspond to the condensation giving a dead cyclic tetramer. From this study it is evident that the curing cycle has a great influence on the properties of the interface of a composite based on a epoxy matrix.     

Synthesis of ultrafine yttrium aluminum garnet using sol-gel technology

Mesoporous yttrium aluminum garnet Y₃Al₅O₁₂ powders were prepared using sol-gel technology proceeding from solutions of metal alkoxoacetylacetonates. Xerogel microstructure was studied by SEM, and the fact of mesopores being formed was established. The temperature range within which Y₃Al₅O₁₂ crystallizes in a dynamic mode from the xerogel was determined to be 850?C950°C using an SDT Q600 TGA/DTA/DSC analyzer. A 1-h isothermal treatment of the xerogel was shown to reduce the garnet phase formation temperature to 800°C. At lower temperatures (400, 450 or 500°C), even long-term (6-h) calcination yielded X-ray amorphous powders with developed surfaces (specific surface areas were 230?C350 m²/g). Powder particle coarsening was studied upon sintering for 2 and 4 h at 1000, 1200, and 1400°C.     

CO oxidation by oxygen of the catalyst and by gas-phase oxygen over (0.5–15)%CoO/ZrO<Subscript>2</Subscript>

CO adsorption on (0.5–15)%CoO/ZrО₂ catalysts has been investigated by temperature-programmed desorption and IR spectroscopy. At 20°С, carbon monoxide forms carbonyl and monodentate carbonate complexes on Co _m ²⁺ O _n ^2- clusters located on the surface of crystallites of tetragonal ZrO₂. With an increasing CoO content of the clusters, the amount of these complexes increases and the temperature of carbonate decomposition, accompanied by CO₂ desorption, decreases from 400 to 304°С. On the 5%CoO/ZrО₂ sample, the carbonyls formed on the Со²⁺ and Со⁺ cations and Со⁰ atoms decompose at 20, 90, and 200–220°С, respectively, releasing CO. At 20°С, they are oxidized by oxygen to monodentate carbonates, which decompose at 180°С. Adsorbed oxygen decreases the temperature of their decomposition on oxidation sites by ~40°C, and the sample remains in an oxidized state ensuring the possibility of subsequent CO adsorption and oxidation. The rate of the oxidation of 5%CoO/ZrО₂ containing adsorbed CO by oxygen is higher than the rate of the oxidation of the same sample reduced by carbon monoxide, because the latter reaction is an activated one. In view of the properties of the complexes, it can be concluded that the carbonates decomposing at 180°С are involved in CO oxidation by oxygen from the gas phase in the presence of hydrogen, a process occurring at 50–200°С. The rate-limiting step of this process the decomposition of the carbonates, which is characterized by an activation energy of 77–94 kJ/mol.     

Physicochemical Principles of the Synthesis of Porous Composite Materials through the Hydrothermal Oxidation of Aluminum Powder

The main versions of the synthesis of a new class of porous cermet materials such as Al₂O₃/Al, MO_x/Al₂O₃/Al, and M¹/MO_x/Al₂O₃/Al and ceramic composites on their basis were analyzed. These ceramic composites were prepared through the stage of the hydrothermal oxidation of aluminum powder and were designed for catalytic and adsorption processes. Equations that express the dependence of the apparent density of the resulting composite on the density of the initial powder mixture, on the concentration of the powdered active component, and on the conversion of aluminum are given. It was found that the formal kinetics of aluminum oxidation with water at 100°C can be described by the Kolmogorov-Erofeev equation. The results were compared with data obtained in an autoclave at higher temperatures and steam pressures. The synthesis parameters that affect the total pore volume and the specific surface area of aluminum oxide obtained from aluminum powder were determined. For the case of the transfer of soluble components from an autoclave to a press mold, the molar coefficients of this process were calculated. The texture peculiarities of composites were analyzed. The texture exhibited a polymodal character with developed micropore, mesopore, and ultramacropore structures, which are responsible for the high permeability of granulated composites. Factors affecting the mechanical properties of metal ceramics were studied. The catalysts and products of composite materials were exemplified.     

Kinetics of low temperature CO oxidation over Pt/SnO2 catalyst

In a CO−O₂ stoichiometric mixture, the kinetic parameters, reaction order, rate constant and activation energy of CO oxidation over a Pt/SnO₂ catalyst have been measured using a fixed bed flow reactor near 0°C. The results show that it is a first-order reaction. The activation energy of CO oxidation over Pt/SnO₂ prepared with SnO₂ calcined at 300°C was approximately 21 kJ/mol. The activation energy of CO oxidation over Pt/SnO₂ changed slowly with SnO₂ calcination temperature above 400°C, and reached approximately 45 kJ/mol.     

Aluminum oxide and systems based on it: Properties and applications

The metastable forms of aluminum oxide that exist in the range of 300–800°C are characterized; differences in the microstructures of homogeneous γ-, η-, and χ-Al₂O₃ are demonstrated; and the acid-base properties of the above modifications are compared. The catalytic properties of aluminum oxide in ethanol dehydration and propionitrile ammonolysis were studied. It was found that an increased surface concentration of Lewis acid sites, including strong acid sites (ν(CO) = 2237 cm^?1), is required for preparing an effective catalyst for the dehydration of ethanol, whereas the rate of propionitrile conversion increased proportionally to the surface concentration of Brønsted acid sites. γ-Aluminum oxide was used to prepare catalysts for carbon monoxide oxidation. It was found that the supporting of Pd on γ-Al₂O₃ did not change the support structure. Palladium on the surface of γ-Al₂O₃-550 (T _calcin = 550°C, S _BET = 300 m²/g) occurred as single particles (2–3 nm) and aggregates (～100 nm). The single particles were almost completely covered with a layer of aluminum oxide to form core-shell structures. According to XPS data, they were in atypical states (BE(Pd 3d _5/2) = 336.0 and 338.0 eV), which were not reduced by hydrogen in the range of 15–450°C and were resistant to the action of the reaction mixture. Palladium on the surface of γ-Al₂O₃-800 (S _BET = 160 m²/g) was in the states Pd⁰ and PdO, which are typical of Pd/Al₂O₃, and the proportions of these states can change under the action of the reaction mixture. An increase in the T _calcin of the Pd/Al₂O₃(800)-450 catalyst from 450 to 800 → 1000 → 1200°C led to the agglomeration of palladium particles and to an increase in the temperature of 50% CO conversion from 145 to 152 → 169 → 189°C, respectively. α-Aluminum oxide was used in the preparation of an effective Mn-Bi-O/α-Al₂O₃ supported catalyst for the synthesis of nitrous oxide by the oxidation of ammonia with oxygen: the NH₃ conversion was 95–97% at 84.4% N₂O selectivity.