Browsing by Author "Aharoni, Chaim"

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Kinetics of Activated Chemisorption Pari 3.—Amount and Distribution of Adsorbate at Varying Temperatures and Pressures
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1977, 73 (12), 1977) Aharoni, Chaim; Ungarish, Moshe
The heterogeneous-surface model applied in Part 2 to the kinetics, is now applied to the effects of temperature and pressure on the amount adsorbed. It predicts isobars with a maximum that is displaced towards higher temperatures at higher pressures. The low-temperature, pseudo-equilibrium isotherms obey the Freundlich equation. Increase in temperature or decrease in pressure produce desorption followed by readsorption. Temperature-programmed desorption gives one peak when heating is slow and two peaks when heating is rapid. When isotopes are successively adsorbed, the isotope adsorbed last is desorbed at lower temperature. The distribution of the adsorbate, according to the regions of various energy on the adsorbent, depends on temperature, pressure and time and is given by a discontinuous function with a maximum.
Kinetics of activated chemisorption. Part 2.—Theoretical models
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1977, 73 (03), 1977) Aharoni, Chaim; Ungarish, Moshe
In activated chemisorption the plot of the reciprocal of the rate Z=(dq/dt)–1 against the time t is convex towards the Z axis at low t and concave at high t. This condition is satisfied if both the energy of activation, Et and the number of available sites, Nt decrease with the coverage q and if d2Et/dq2 < 0. Two models are considered:
Kinetics of activated chemisorption. Part 4.—Differential heat of adsorption
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (06), 1978) Aharoni, Chaim; Ungarish, Moshe
A model previously applied to the kinetics of chemisorption and to adsorption at true and apparent equilibrium, is applied to the differential heats of adsorption. The measurable function, heat-coverage, differs from the enthalpy against site distribution. When the differential heat of adsorption is measured continuously, at constant pressure and increasing time, it increases with coverage. When it is measured stagewise with stages at increasing pressures, it decreases with coverage. In both cases it increases with temperature.
Kinetics of Chemisorption Deducing Kinetic Laws from Experimental Data
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1981, 77 (05), 1981) Ungarish, Moshe; Aharoni, Chaim
The kinetics of chemisorption have been inadequately represented in the literature by second-order equations and by Elovich equations. The conditions at which non-valid equations become apparently applicable are discussed. Assumed isotherms are often congruent to the experimental one at a limited range of coverage and predict a correct value for the rate dq/dt and for the change of rate d2q/dt2 in that range; however, they have been applied beyond the range of validity by using plotting procedures that allow the location of the region of congruency to be adjusted and the effect of data outside this region to be attenuated. The second-order rate equation q∞/(q∞–q)=αt+ 1 where q∞ and α are constants, is congruent to the Elovich equation when q≈ 0.5q∞. Reciprocally, the Elovich equation q=c+(1/b)ln t where b and c are constant, is indistinguishable from a second-order equation when q≈ 2/b. Both equations can be fitted at certain conditions to data obeying the equation q=kt1/v where k and v are constants. Differential analysis shows that the experimental isotherms generally obey a complex function for which z=(dq/dt)–1 is sigmoid.