Theoretical study on the mechanism of the reaction of CH4+MgO

The reactions of (1) CH4 + MgO --> MgOH. + CH3. and (2) CH4 + MgO --> Mg + CH3OH have been studied on the singlet spin state potential energy surface at the MP2/6-311+G(2d,2p) level. These two reaction channels, both involving intermediates and transition states, have been rationalized by the structures of the species involved, natural bond orbital (NBO), and vibrational frequency analysis. We have considered two initial interacting models between CH4 and MgO: a collinear C-H approach to the O end of the MgO forming the MgOCH4 complex with C-3nu symmetry and three hydrogen atoms of the methane point to the Mg end of the MgO forming the OMgCH4 complex with C-1 symmetry. The calculations predict that reactions 1 and 2 are exothermic by 39.8 and 86.5 kJ mol(-1), respectively. Also, the former reaction proceeds more easily than the latter, and the complex HOMgCH3 is energetically preferred in the reaction of MgO + CH4.

Theoretical study on the mechanism for the addition reaction of SiH3 with propylene and acetic acid

To explore the reactivities of alkene (-CH=CH2) and carboxy (-COOH) group with H-Si under UV irradiation, the addition mechanism for the reactions of SiH3 radical with propylene and acetic acid was studied by using the B3LYP/6-311++ G(d,p) method. Based on the surface energy profiles, the dominant reaction pathways can be established; i.e., SiH3 adds to the terminal carbon atom of the alkene (-CH=CH2) to form an anti-Markovnikov addition product, or adds to the oxygen atom of the carboxy group (-COOH) to form silyl acetate (CH3-COOSiH3). Because the barrier in the reaction of the carboxy group (39.9 kJ/ mol) is much larger than that of alkene (11.97 kJ/mol), we conclude that the reaction of bifunctional molecules (e.g., omega-alkenoic acid) with H-Si under irradiation condition is highly selective; i.e., the alkene group (-CH= CH2) reacts with SiH3 substantially faster than the carboxyl group (-COOH), which agrees well with the experimental results. This provides the possibility of preparing carboxy-terminated monolayers on silicon surface from omega-alkenoic acids via direct photochemical reaction.

Growth Mechanism And Corrosion Behavior Of Ceramic Coatings On Aluminum Produced By Autocontrol Ac Pulse Peo

Ceramic coatings are produced on aluminum alloy by autocontrol AC pulse Plasma Electrolytic Oxidation (PEO) with stabilized average current. Transient signal gathering system is used to study the current, voltage, and the transient wave during the PEO process. SEM, OM, XRD and EDS are used to study the coatings evolution of morphologies, composition and structure. TEM is used to study the micro profile of the outer looser layer and inner compact layer. Polarization test is used to study the corrosion property of PEO coatings in NaCl solution. According to the test results, AC pulse PEO process can be divided into four stages with different aspects of discharge phenomena, voltage and current. The growth mechanism of AC PEO coating is characterized as anodic reaction and discharge sintering effect. PEO coating can increase the corrosion resistance of aluminum alloy by one order or two; however, too long process time is not necessarily needed to increase the corrosion resistance. In condition of this paper, PEO coating at 60 min is the most protective coating for aluminum alloy substrate. (C) 2008 Elsevier B.V. All fights reserved.

THE FORMATION MECHANISM OF THE VITRIFIED SLAG IN A PLASMA ARC REACTOR FOR HAZARDOUS WASTE TREATMENT

Various hazardous wastes with additives have been vitrified to investigate the formation mechanism of the glassy slag by a 30 kW DC plasma-arc reactor developed by the Institute of Mechanics, Chinese Academy of Sciences. The average temperature in the reaction area is controlled at 1500°C. The chemical compositions of three sorts of fly ashes are analyzed by XRF (X-Ray Fluorescence). Fly ashes with vitrifying additives can be vitrified to form glassy slag, which show that the ratio of the whole oxygen ions to the whole network former ions in glass (R) is appropriate in the range of 2~3 to form durable vitrified slag. In this experiment, the arc power is controlled below 5 kW to inhibit waste evaporation. To enhance the effects of heat transfer to wastes, ferrous powder has been added into the graphite crucible, which aggregates as ingot below the molten silicate after vitrification. The slag fails to form glass if the quenching rate is less than 1 K/min. Therefore, the slag will break into small chips due to the sharp quenching rate, which is more than 100 K/sec.

Wear resistance of reaction sintered alumina/mullite composites

Alumina and alumina/mullite composites with mullite content of 0.96-8.72 vol.% were subjected to an abrasive wear test under loads of 0.1-2.0 N with a ball-on-disc apparatus. The wear rate and area fraction of pullout f(po) on the worn surfaces were measured. The wear resistances of the alumina/mullite composites were better by a factor of 1-2 than that of pure alumina. The main wear mechanism of alumina is fracture wear, and for alumina/mullite composites, fracture wear and plastic wear mechanisms work together. The influence of mechanical properties on wear resistance was estimated by Evans' method. It was found that the wear rate depends on f(po), and the primary reason for the better wear resistance of alumina/mullite composites is the reduction off, induced by fracture mode transition. (c) 2007 Elsevier B.V. All rights reserved.

Study of the mechanism for synthesizing superheavy nuclei based on dinuclear system

Based on the concept of dinuclear system, considering the strong competition between fusion and quasiffision processes, by solving the master equation numerically to calculate the fusion probability of superheavy nuclei, we have estimated the excitation functions for the reactions Ti-50, Fe-58 + Pb-208, Bi-209, and the experimental data are basically reproduced. For different incident energies and different angular momentum, the effects on fusion and survival probability and the contribution to evaporation residue cross section have been given. These results help to further understand the mechanism for, synthesizing superheavy nuclei.

The mechanism studies of ethanol oxidation on PdO catalysts by TPSR techniques

The paper studies the direct oxidation of ethanol and CO on PdO/Ce0.75Zr0.25O2 and Ce(0.75)Zr(0.2)5O(2) catalysts. Characterization of catalysts is carried out by temperature-programmed desorption (TPD), temperature-programmed surface reaction (TPSR) techniques to correlate with catalytic properties and the effect of supports on PdO. The simple Ce0.75Zr0.25O2 is in less active for ethanol and CO oxidation. After loaded with PdO, the catalytic activity enhances effectively. Combined the ethanol and CO oxidation activity with CO-TPD and ethanol-TPSR profiles, we can find the more intensive of CO2 desorption peaks, the higher it is for the oxidation of CO and ethanol. Conversion versus yield plot shows the acetaldehyde is the primary product, the secondary products are acetic acid, ethyl acetate and ethylene, and the final product is CO2. A simplified reaction scheme (not surface mechanism) is suggested that ethanol is first oxidized to form intermediate of acetaldehyde, then acetic acid, ethyl acetate and ethylene formed going with the formation of acetaldehyde, acetic acid, ethyl acetate; finally these byproducts are further oxidized to produce CO2. PdO/Ce0.75Zr0.25O2 catalyst has much higher catalytic activity not only for the oxidation of ethanol but also for CO oxidation. Thus the CO poison effect on PdO/Ce0.75Zr0.25O2 catalysts can be decreased and they have the feasibility for application in direct alcohol fuel cell (DAFC) with high efficiency.

Controllable synthesis of VSB-5 micirospheres and microrods: Growth mechanism and selective hydrogenation catalysis

Nanoporous VSB-5 nickel phosphate molecular sieves with relatively well controllable sizes and morphology of microspheres assembled from nanorods were synthesized at 140 degrees C over a short time in the presence of hexamethylenetetramine (HMT) by a facile hydrothermal method. The pH value, reaction time, and ratio of HMT to NaHPO2-H2O crucially influence the morphology and quality of the final products.

Vilsmeier Reaction of Enaminones: Efficient Synthesis of Halogenated Pyridin-2(1H)-ones

A facile and efficient one-pot synthesis of halogenated pyridin-2(1H)-ones from a series of readily available enaminones under Vilsmeier conditions is described, and a mechanism involving sequential halogenation, formylation, and intramolecular nucleophilic cyclization is proposed.

Growth mechanism. of synthetic imogolite nanotubes

By controlling the surface effects during droplet evaporation of imogolite solutions, imogolite nanotubes were dispersed individually and directly observed by transmission electron microscopy (TEM), and the structure evolution of imogolite nanotubes in the synthetic process was investigated. It was found that the number of imogolite nanotubes continuously increased with time in the whole reaction. The average length grew slowly over time after a remarkable increase in the initial 24 h, and the length distribution experienced a similar variation with the polydispersity index always below 2. No appreciable changes in tube diameters were detected under TEM observation.

Polyol-Mediated Synthesis of PbS Crystals: Shape Evolution and Growth Mechanism

An interesting shape evolution of. PbS crystals, that is, from cubes to (truncated) octahedra and finally to stable star-shaped multipods with six arms along the < 100 > directions is first realized via a facile polyol-mediated reaction between lead acetate and sulfur powder in the absence of surfactants. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and Fourier transform infrared (FT-IR) techniques were employed to characterize the samples. We elucidate the important parameters (including reaction temperature and sulfur sources) responsible for the shape-controlled synthesis of PbS crystals.