Adsorption activation energy

Indeed, assuming that the surface of adsorbent is characterized by homogeneous distribution in adsorption activation energy we arrive to the Zeldovich - Roginsky - Elovich kinetic isotherm of In (f + + providing the validity of function (2.50) in case of direct in-... 

Kds are the constants of rates of chemical reactions of oxygen adsorption and desorbtion from ZnO film and Aq are electron work function from ZnO before oxygen gets adsorbed and its variation caused by dipole moment of adsorbed complexes being formed U is the adsorption activation energy of non-electrostatic nature [ M] is the concentration of solvent molecules. Apparently we can write down the following expression for the stationary system ... 

Norskov and co-workers have calculated the adsorption activation energy and the heat of adsorption of oxygen on Ag(lll) using density functional theory (DFT) [15]. They obtained a value of 145 kJ/mol for the adsorption activation energy and a value of 28 kJ/mol for the heat of adsorption, corresponding to a desorption activation energy of 173 kJ/mol. These values do not accord with those foimd experimentally, suggesting that some refinement of the method is required. 

Key words Zeolites, Heats of adsorption. Activation Energies... 

These two objections do not leave us with much confidence in the absolute numerical values quoted for the adsorption activation energy. When care was taken in low pressure experiments to preheat the gas, Blyholder and Eyring (30) reported differing values in the temperature range 600°-800°C (at 15 fini Hg) the activation energy was 80 kcal/mole... 

Figure 2. Mean square error for b = 0.000126 and adsorption activation energy E1 = 5,6,7,9 kcal vs. desorption activation energy E2. Note that for b =...

The change in chemisorptive bond strengths with changing catalyst potential and work function, described in (6), (7), and (8) above is the cause of NEMCA, or EP in catalysis, and leads to the observed dramatic non-Faradaic variations in catalytic rates with A(e). Due to the linear variation of heats of adsorption, activation energies and logarithms of preexponential factors with A(e), catalytic rates, r, are often found to depend exponentially on e ranges ... 

In conclusion, the lifetime analysis of the experimental data of Drazer and Zanette provided important clues regarding the kinetics and mechanism of the desorption process. In particular, the knowledge of the fractal exponents of the adsorption isotherm and of the tail of the lifetime distribution was enough to elucidate the shape and structure of the activation energy barrier. We have also shown that the experimental data indicate that the adsorption rate and the adsorption activation energies are constant and only the desorption rate and desorption activation energy are random. The application of the method may involve detailed theoretical developments for different systems nevertheless we expect that the results are worth the effort. 

In the laminated membranes, thin palladium (or palladium alloys) layer avoids the formation of oxides on the metallic surfaces resulting in a reduction of the hydrogen adsorption activation energy and, as a consequence, in an increase of the hydrogen permeation flux [55]. 

Supposing that the adsorption activation energy Ea) increases and the desorption activation energy Ed) decreases linearly with coverage 0, then we have. 

Studies on the adsorption kinetics have shown that the adsorption activation energy also increases with the increase of the surface coverage. As a result, the concept of adsorption activation energy as adsorption heat is referred to the given surface coverage, which is represented by the Eq. (7.81)... 

There exist a relation between the adsorption activation energy Ea, desorption activation energy Ed and adsorption heat Qo, that is, Ed — Ea = Qo- Therefore, study on the process of desorption also provides some message for the process of adsorption, that is, the TPD technology can be used to study desorption rate and obtain desorption activation energy. 

Under these circumstances the use of calculated preexponentials in kinetic model development is likely to lead to significant errors, in addition to those inherent in the use of calculated heats of adsorption. These problems are reflected in the attempt to model magnetite kinetics using observed heats of adsorption and estimated frequency factors, which gave rise to a calculated reaction rate a factor of 10 too low. The development of mechanistically sound, kinetic models will therefore remain dependent on the direct determination of the heats of adsorption, activation energies, and frequency factors for the forseeable future. 

Activation energy of adsorption Activation energy of desorption Enthalpy of adsorption X 100(%) Degree of reduction... 

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