Variation of the number of metal atoms involved in active sites and of the true activation energy of hydrocarbon conversion and co hydrogenation over metals

This study is focused on the deformation mechanism and behavior of naturally aged 7010 aluminum alloy at elevated temperatures. The specimens were naturally aged for 60 days to reach a saturated hardness state. High-temperature tensile tests for the naturally aged sample were conducted at different temperatures of 573, 623, 673, and 723 K at various strain rates ranging from 5 × 10−5 to 10−2 s−1. The dependency of stress on the strain rate showed a stress exponent, n, of ~6.5 for the low two temperatures and ~4.5 for the high two temperatures. The apparent activation energies of 290 and 165 kJ/mol are observed at the low, and high-temperature range, respectively. These values of activation energies are greater than those of solute/solvent self-diffusion. The stress exponents, n, and activation energy observed are rather high and this indicates the presence of threshold stress. This behavior occurred as a result of the dislocation interaction with the second phase particles that are existed in the alloy at the testing temperatures. The threshold stress decreases in an exponential manner as temperature increases. The true activation energy was computed by incorporating the threshold stress in the power-law relation between the stress and the strain. The magnitude of the true activation energy, Qt dropped to 234 and 102 kJ/mol at the low and high-temperature range, respectively. These values are close to that of diffusion of Zinc in Aluminum and diffusion of Magnesium in Aluminum, respectively. The Zener–Hollomon parameter for the alloy was developed as a function of effective stress. The data in each region (low and high-temperature region) coalescence in a segment line in each region.

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Low-Temperature Diffusion of Transition Metals at the Presence of Radiation Defects in Silicon

Solid State Phenomena ◽

10.4028/www.scientific.net/ssp.178-179.421 ◽

2011 ◽

Vol 178-179 ◽

pp. 421-426

Author(s):

Jan Vobecký ◽

Volodymyr Komarnitskyy ◽

Vít Záhlava ◽

Pavel Hazdra

Keyword(s):

Low Temperature ◽

Active Sites ◽

Carrier Lifetime ◽

High Energy ◽

Deep Levels ◽

Radiation Defects ◽

Metal Atoms ◽

Temperature Diffusion ◽

Concentration Enhancement ◽

Application Potential

Low-temperature diffusion of Cr, Mo, Ni, Pd, Pt, and V in silicon diodes is compared in the range 450 - 800 oC. Before the diffusion, the diodes were implanted with high-energy He2+ to assess, if the radiation defects enhance the concentration of metal atoms at electrically active sites and what is the application potential for carrier lifetime control. The devices were characterized using AES, XPS, DLTS, OCVD carrier lifetime and diode electrical parameters. The metal atoms are divided into two groups. The Pt, Pd and V form deep levels in increased extent at the presence of radiation defects above 600 oC, which reduces the excess carrier lifetime. It is shown as a special case that the co-diffusion of Ni and V from a NiV surface layer results fully in the concentration enhancement of the V atoms. The enhancement of the acceptor level V-/0 (EC 0.203 eV) and donor level V0/+ (EC 0.442 eV) resembles the behavior of substitutional Pts. The second group is represented by the Mo and Cr. They easily form oxides, which can make their diffusion into a bulk more difficult or impossible. Only a slight enhancement of the Cr-related deep levels by the radiation defects has been found above 700 oC.

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On the catalytic dehydrogenation of naphthenes II. Competition experiments and energies of C-H bonds

Proceedings of the Royal Society of London Series A - Mathematical and Physical Sciences ◽

10.1098/rspa.1947.0078 ◽

1947 ◽

Vol 190 (1022) ◽

pp. 309-328 ◽

Cited By ~ 2

Keyword(s):

Activation Energy ◽

Active Sites ◽

Catalyst Surface ◽

Resonance Energy ◽

Activation Energies ◽

Theoretical Treatment ◽

Catalytic Dehydrogenation ◽

Simple Relationship ◽

Hydrogen Atoms ◽

Methyl Cyclohexane

The rates of dehydrogenation in competition experiments using mixtures of two naphthenes, or a naphthene and a cyclic mono-olefine or two cyclic mono-olefines, have been examined theoretically and experimentally for the stationary state conditions. Provided the two reactants can occupy the same sites on the catalyst surface, then the ratio of the rates should be directly proportional to the ratio of the partial pressures at any instant. Theory suggests that a constant which can be derived from these competition experiments should be independent of the overall pressures, or of the initial ratio of concentrations or of the overall extent of dehydrogenation. Further, the ratio of the rates in competition should bear no simple relationship to the ratio of the individual rates alone, but should be related to the slopes of the 1/rate against 1/pressure plot for the two components considered separately. Moreover, the constant should be a ratio of two functions each of which is characteristic of one of the naphthenes. The theoretical conclusions have been confirmed experimentally which proves either that the groups of active sites on the catalyst surface are widely separated or that any set of sites is available for the reaction of any molecular species, and no interference takes place between naphthene molecules adsorbed on adjacent sites. Proof that a naphthene and cyclohexene are dehydrogenated on the same sites is supplied by the observation that a constant is obtained when different mixtures of cyclohexene and trans -1:4-dimethyl cyclohexane are allowed to compete for the surface. The ratios for methyl, ethyl, the three dimethyl and the three trimethyl cyclohexanes in competition with cyclohexane have been accurately determined at temperatures of 400 and 450° C. From the constants so derived the activation energy differences for the removal of the first pair of hydrogen atoms has been obtained. These values are discussed in terms of the possible transition complexes, and it is shown that the reaction proceeds by the loss of a pair of hydrogen atoms simultaneously and not by a half-hydrogenated state mechanism. Using these activation energies and the experimentally found overall activation energy of 36 kcal./g. mol., the resonance energy per resonating structure was determined as 1-73 kcal. This is in good agreement with the energies of C-H bonds in alkyl radicals (2-2 kcal./g.mol./ resonating structure). The theoretical treatment suggests that the weakest C-H link in methyl cyclohexane should be in the three position to the methyl group. A study of the activation energies involved shows that the methyl cyclohexene produced from methyl cyclohexane is not 1-methyl-1-cyclohexene, thus confirming the theoretical deduction.

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Performance of Transition Metal Ions Exchanged Zeolite 13X in Greenhouse Gas Reduction

Volume 15: Sustainable Products and Processes ◽

10.1115/imece2007-41360 ◽

2007 ◽

Author(s):

K. S. Hui ◽

Christopher Y. H. Chao ◽

C. W. Kwong ◽

M. P. Wan

Keyword(s):

Activation Energy ◽

Catalytic Activity ◽

Transition Metal ◽

Apparent Activation Energy ◽

Surface Area ◽

Active Sites ◽

Methane Combustion ◽

Bet Surface Area ◽

Metal Loading ◽

Zeolite 13X

This study investigated the performance of multi-transition metal (Cu, Cr, Ni and Co) ions exchanged zeolite 13X catalysts on methane emission abatement, especially at methane level of the exhaust from natural gas fueled vehicles. Catalytic activity of methane combustion using multi-ions exchanged catalyst was studied under different parameters: mole % of metal loading, inlet velocity and inlet methane concentration at atmospheric pressure and 500 °C. Performance of the catalysts was investigated and explained in terms of the apparent activation energy, number of active sites and BET surface area of the catalyst. This study showed that the multi-ions exchanged catalyst outperformed the single-ions exchanged and the acidified 13X catalysts. Lengthening the residence time could also lead to higher methane conversion %. Catalytic activity of the catalysts was influenced by the mole % of metal loading which played important roles in affecting the apparent activation energy of methane combustion, active sites and also the BET surface area of the catalyst. Increasing mole % of metal loading in the catalyst decreased the apparent activation energy for methane combustion and also the BET surface area of the catalyst. In view of these, there existed an optimized mole % of metal loading where the highest catalytic activity was observed.

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Modified Cuo/ZnO/Al2O3 Catalysts for Methanol Synthesis from Co and CO2 Co-Hydrogenation

Advanced Materials Research ◽

10.4028/www.scientific.net/amr.690-693.1529 ◽

2013 ◽

Vol 690-693 ◽

pp. 1529-1534

Author(s):

Wen Gui Gao ◽

Hua Wang ◽

Wen Yan Liu ◽

Feng Jie Zhang

Keyword(s):

Methanol Synthesis ◽

Active Sites ◽

Co Hydrogenation ◽

Fixed Bed ◽

Fixed Bed Reactor ◽

Total Carbon ◽

X Ray Diffraction ◽

Wet Impregnation ◽

Temperature Programmed ◽

Co Precipitation

A series of CuO-ZnO-Al2O3catalysts modified by different promoter were prepared by co-precipitation or incipient wet impregnation and characterized by X-ray diffraction (XRD), N2physisorption, hydrogen temperature-programmed reduction (H2-TPR) and carbon dioxide temperature-programmed desorption (CO2-TPD). The modified catalysts were tested for methanol synthesis from CO/CO2co-hydrogenation in a fixed bed reactor with feed containing CO, CO2and H2(CO:CO2:H2=1.0:1.08:6.24, volume radio). It is revealed that the catalysts modified by Zr, Mg, Ca has higher activity of methanol synthesis by CO and CO2co-hydrogenation. Especially, the addition of Zr enhances the conversion of total carbon and the selectivity of methanol, which is due to the improved surface area, much more active sites, and the synergistically interaction between CuO and ZnO caused by the addition of Zr promoter.

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Calculation of the activation energy for surface self-diffusion of transition-metal atoms

Physics of the Solid State ◽

10.1134/1.1130717 ◽

1999 ◽

Vol 41 (1) ◽

pp. 8-10 ◽

Cited By ~ 16

Author(s):

S. Yu. Davydov

Keyword(s):

Activation Energy ◽

Transition Metal ◽

Self Diffusion ◽

Metal Atoms ◽

Transition Metal Atoms

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THE REACTION BETWEEN CYANATE AND HYPOCHLORITE

Canadian Journal of Chemistry ◽

10.1139/v56-070 ◽

1956 ◽

Vol 34 (4) ◽

pp. 489-501 ◽

Cited By ~ 9

Author(s):

M. W. Lister

Keyword(s):

Activation Energy ◽

Ionic Strength ◽

Rate Constant ◽

Hypochlorous Acid ◽

Effect Of Temperature ◽

Slow Step ◽

True Activation Energy ◽

Rate Of Reaction ◽

Different Temperatures ◽

Kinetics Of

The reaction between sodium hypochlorite and potassium cyanate in the presence of sodium hydroxide has been examined. The main products are chloride, and carbonate ions and nitrogen; but, especially if much hypochlorite is present, some nitrate is formed as well. The rate of reaction is proportional to the cyanate and hypochlorite concentrations, but inversely proportional to the hydroxide concentration: the rate constant is 5.45 × 10−4 min.−1 at 65 °C, at an ionic strength of 2.2. The rate constant increases somewhat as the ionic strength rises from 1.7 to 3.5. The effect of temperature makes the apparent activation energy 25 kcal./gm-molecule. The kinetics of the reaction suggest that the slow step is really a reaction of hypochlorous acid and cyanate ions, and possible intermediate products of this reaction are suggested. Allowing for the different extent of hydrolysis of hypochlorite at different temperatures, the true activation energy is found to be 15 kcal./gm-mol., which is consistent with the observed rate of reaction.

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Effect of Li2O Doping on the Surface and Catalytic Properties of the Fe2O3–NiO/Al2O3 System

Adsorption Science & Technology ◽

10.1177/026361749801600104 ◽

1998 ◽

Vol 16 (1) ◽

pp. 21-32 ◽

Cited By ~ 7

Author(s):

G.A. El-Shobaky ◽

A.M. Ghozza ◽

N.M. Deraz

Keyword(s):

Activation Energy ◽

Catalytic Activity ◽

Mixed Oxide ◽

Active Sites ◽

Catalytic Properties ◽

Surface Characteristics ◽

Appreciable Change ◽

Surface Areas ◽

Catalytically Active ◽

Specific Surface Areas

Ferric–nickel/aluminium mixed oxide solids have the formula Fe2O3–0.42NiO/Al2O3 were treated with Li2O (0.75–3 mol%) and heated in air for 4 h at 500°C and 800°C, respectively. The effects of this treatment on the surface characteristics of these solids and their catalytic properties in relation to CO oxidation by O2 have been investigated. The results reveal that Li2O doping at 0.75 mol% concentration resulted in an increase of 24% and 18%, respectively, in the value of the specific surface areas, SBET, of the solids precalcined at 500°C and 800°C, while the addition of 3 mol% Li2O led to a slight decrease of ca. 10% in the SBET value of the same solids. In contrast, irrespective of whether the doping process involved solids precalcined at 500°C or 800°C, a significant decrease of 37% and 78%, respectively, was observed in the catalytic activity of these materials. This decrease in catalytic activity was not accompanied by any appreciable change in the magnitude of the activation energy for the catalytic reaction, i.e. Li2O doping brings about a decrease in the concentration of catalytically active sites without changing their energetic nature.

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