Memory effect on preexponential factor of rate constant for dissociation of adsorbed molecules

1992 ◽  
Vol 96 (8) ◽  
pp. 3452-3454 ◽  
Author(s):  
R. Tsekov ◽  
G. Vayssilov
Keyword(s):  
Memory Effect ◽  
Rate Constant ◽  
Preexponential Factor ◽  
Adsorbed Molecules

The lifetime of hydrogen adsorbed on a slightly oxidized nickel surface

1957 ◽  
Vol 240 (1222) ◽  
pp. 423-436 ◽  
Keyword(s):  
Oxide Film ◽  
Mass Spectrometer ◽  
Rate Constant ◽  
Molecular Beam ◽  
Copper Surface ◽  
Solid Surfaces ◽  
Nickel Surface ◽  
Order Process ◽  
Adsorbed Molecules ◽  
First Order

A method for investigating the lifetimes of adsorbed molecules on solid surfaces is described. A molecular beam of hydrogen was projected on to the surface of a spinning nickel disk, and the hydrogen evaporating from the surface at different times after deposition was collected and measured with a mass spectrometer. On a slightly oxidized nickel surface all the hydrogen was adsorbed. The subsequent evaporation was a first-order process with a rate constant of 4·5 x 10 11 exp ( -11·5 kcal/ RT ) S -1 . With a mixed molecular beam of hydrogen and deuterium no exchange was produced by the adsorption, which is considered to be molecular. Attempts to remove the oxide film from the nickel surface gave an un­stable surface on which no clear results were obtained. The lifetime of hydrogen on a copper surface was too short to measure.

scholarly journals Determining Preexponential Factor in Model-Free Kinetic Methods: How and Why?

Molecules ◽  
2021 ◽  
Vol 26 (11) ◽  
pp. 3077
Author(s):  
Sergey Vyazovkin
Keyword(s):  
Activation Energy ◽  
Kinetic Analysis ◽  
Rate Constant ◽  
Condensed Phase ◽  
Compensation Effect ◽  
Preexponential Factor ◽  
Isoconversional Methods ◽  
Activation Entropy ◽  
Model Free ◽  
Kinetics Of

The kinetics of thermally stimulated processes in the condensed phase is commonly analyzed by model-free techniques such as isoconversional methods. Oftentimes, this type of analysis is unjustifiably limited to probing the activation energy alone, whereas the preexponential factor remains unexplored. This article calls attention to the importance of determining the preexponential factor as an integral part of model-free kinetic analysis. The use of the compensation effect provides an efficient way of evaluating the preexponential factor for both single- and multi-step kinetics. Many effects observed experimentally as the reaction temperature shifts usually involve changes in both activation energy and preexponential factor and, thus, are better understood by combining both parameters into the rate constant. A technique for establishing the temperature dependence of the rate constant by utilizing the isoconversional values of the activation energy and preexponential factor is explained. It is stressed that that the experimental effects that involve changes in the preexponential factor can be traced to the activation entropy changes that may help in obtaining deeper insights into the process kinetics. The arguments are illustrated by experimental examples.

scholarly journals Study on the Size-Dependent Oxidation Reaction Kinetics of Nanosized Zinc Sulfide

2014 ◽  
Vol 2014 ◽  
pp. 1-8 ◽  
Author(s):  
Qing-Shan Fu ◽  
Yong-Qiang Xue ◽  
Zi-Xiang Cui ◽  
Ming-Fang Wang
Keyword(s):  
Activation Energy ◽  
Particle Size ◽  
Surface Energy ◽  
Apparent Activation Energy ◽  
Zinc Sulfide ◽  
Rate Constant ◽  
Oxidation Reaction ◽  
Reaction Order ◽  
Preexponential Factor ◽  
Surface Entropy

Numerous oxidation problems of nanoparticles are often involved during the preparation and application of nanomaterials. The oxidation rate of nanomaterials is much faster than bulk materials due to nanoeffect. Nanosized zinc sulfide (nano-ZnS) and oxygen were chosen as a reaction system. The influence regularities were discussed and the influence essence was elucidated theoretically. The results indicate that the particle size can remarkably influence the oxidation reaction kinetics. The rate constant and the reaction order increase, while the apparent activation energy and the preexponential factor decrease with the decreasing particle size. Furthermore, the logarithm of rate constant, the apparent activation energy and the logarithm of preexponential factor are linearly related to the reciprocal of particle diameter, respectively. The essence is that the rate constant is influenced by the combined effect of molar surface energy and molar surface entropy, the reaction order by the molar surface area, the apparent activation energy, by the molar surface energy, and the preexponential factor by the molar surface entropy. The influence regularities and essence can provide theoretical guidance to solve the oxidation problems involved in the process of preparation and application of nanomaterials.

Rate Constant Approximation with Cubic Splines for Kinetic Analysis of Temperature-Programmed Reduction Data

2019 ◽  
Vol 806 ◽  
pp. 87-92
Author(s):  
Arseniy Portnyagin ◽  
Alexey Golikov ◽  
Evgenii K. Papynov ◽  
Valentin Avramenko
Keyword(s):  
Kinetic Analysis ◽  
Rate Constant ◽  
Functional Materials ◽  
Preexponential Factor ◽  
Cubic Splines ◽  
Temperature Programmed Reduction ◽  
Heating Rates ◽  
Novel Approach ◽  
Analysis Technique ◽  
Temperature Programmed

Temperature-programmed reduction (TPR) is a widely used method for characterization of oxide-based catalysts, sorbents, and functional materials, but its results lack quantitative assessment. Here, we present a novel approach to kinetic analysis of the TPR that can be applied to a large variety of systems involving multiple limiting stages. Implementation of cubic splines to approximate rate constant vs. conversion dependencies obtained from several TPR curves recorded at different heating rates yields in a set of kinetic parameters (activation energy and preexponential factors) for all reduction stages. Relationship between preexponential factor of the first reduction stage and the specific surface area of the sample has been shown. Reduction of hematite has been studied to prove the performance of the developed kinetic analysis technique.

The Effects of Particle Size on the Kinetic Parameters in the Reaction of Nano-Nio with Sodium Bisulfate Solution

2011 ◽  
Vol 36 (4) ◽  
pp. 329-341 ◽  
Author(s):  
Yongqiang Xue ◽  
Xiaopeng Wang ◽  
Zixiang Cui
Keyword(s):  
Activation Energy ◽  
Particle Size ◽  
Apparent Activation Energy ◽  
Kinetic Parameters ◽  
Rate Constant ◽  
Reaction Order ◽  
Particle Diameter ◽  
Preexponential Factor ◽  
Exponential Factor ◽  
Sodium Bisulfate

The kinetic parameters for the chemical reaction of nano-NiO of different particle sizes with aqueous sodium bisulfate solution were determined; additionally, the influence of particle size on the kinetic parameters were studied and were discussed. The results show that: there are clear effects of the particle size of nano-NiO on the rate constant, the reaction order, the apparent activation energy and the pre-exponential factor; thus the rate constant and the reaction order increase, and the apparent activation energy and the pre-exponential factor decrease, with decrease of the particle diameter; the logarithm of the preexponential factor, and the apparent activation energy, exhibit a linear relationship with the reciprocal of the particle diameter respectively. The bases of the role of the particle size are that the reaction order is influenced by the molar surface area of nano-NiO, the apparent activation energy by the molar surface energy, the preexponential factor by the molar surface entropy, and the rate constant by both the molar surface energy and the molar surface entropy.

scholarly journals CORRELATION BETWEEN PREEXPONENTIAL FACTOR AND ACTIVATION ENERGY OF ISOAMYLALCOHOL HYDROGENOLYSIS ON PLATINUM CATALYSTS

2010 ◽  
Vol 4 (1) ◽  
pp. 1-5 ◽  
Author(s):  
Triyono Triyono
Keyword(s):  
Activation Energy ◽  
Rate Constant ◽  
Reaction Rate ◽  
Arrhenius Equation ◽  
Preexponential Factor ◽  
Active Centers ◽  
Exponential Term ◽  
Lower Activation ◽  
Higher Temperature ◽  
Lower Activation Energy

Arrhenius equation stated that reaction will proceed faster at higher temperature and with lower activation energy (Ea). Many literatures reported that preexponential factor (A) is constant for certain reaction and there is no relation between A and Ea. Experiment on the reaction of isoamylalcohol hydrogenolysis showed that logarithm of A increased linearly with Ea. The result of this investigation suggests that the rate of a process is affected by the number of active centers on the surface of a catalysts, which influences the value of the pre-exponential term in the expression for the rate constant of a reaction. An increase in the number of active centers corresponds to a higher value of A, the active centers would be less effective and is attended by a growth in the value of Ea. Therefore, reaction with lower activation energy will not always has higher reaction rate due to decreasing of Ea.   Keywords: isoamylalcohol hydrogenolysis, preexponential factor, activation  energy.

Use of 2½ D imaging in analysis of NiTi martensite

1978 ◽  
Vol 36 (1) ◽  
pp. 610-611
Author(s):  
G. M. Michal
Keyword(s):  
Memory Effect ◽  
Monoclinic Phase ◽  
Reaction Path ◽  
Structural Characteristics ◽  
Salient Feature ◽  
Martensite Phase ◽  
Orientation Relationships ◽  
Low Symmetry ◽  
Hydride Phases ◽  
Unusual Shape

Several TEM investigations have attempted to correlate the structural characteristics to the unusual shape memory effect in NiTi, the consensus being the essence of the memory effect is ostensible manifest in the structure of NiTi transforming martensitic- ally from a B2 ordered lattice to a low temperature monoclinic phase. Commensurate with the low symmetry of the martensite phase, many variants may form from the B2 lattice explaining the very complex transformed microstructure. The microstructure may also be complicated by the enhanced formation of oxide or hydride phases and precipitation of intermetallic compounds by electron beam exposure. Variants are typically found in selfaccommodation groups with members of a group internally twinned and the twins themselves are often observed to be internally twinned. Often the most salient feature of a group of variants is their close clustering around a given orientation. Analysis of such orientation relationships may be a key to determining the nature of the reaction path that gives the transformation its apparently perfect reversibility.

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