Coalescence, kinetic constant

The coalescence effects have been taken into account. This was accomplished by determining the coalescence kinetic constant in an internal mixer and incorporating its numerical value into the model computations. 

Ray, Jain and Salovey (14) modeled these phenomena using the kinetic constant of coalescence as their major parameter. 

It can be seen that the reduction of Pd2 + proceeded slowly and was complete in 20 min. from the start of the introduction of H2 in the first exposure. Unlike in the first H2 exposure, the reduction of Pd in the second, third, and fourth exposures was quickly completed, in less than 3 min. In the latter cases, 10%-20% of the Pd content was oxidized after the introduction of 02 and before the admission of H2. These facts are consistent with the change in the Pd—O and Pd—Pd bonds, as observed in the Fourier transforms of Figure 22.9. The oxidation state of Pd in the first H2 exposure was kinetically analyzed using the data of Figure 22.11 the first-order rate constant k was determined to be 0.28 min-1. In addition, the first-order rate constant of the CN of the Pd—Pd bond was determined independently, based on the data of Figure 22.10. The obtained k value was 0.35 min-1, which is close to that of the kinetic constant k for the reduction of Pd2 +, suggesting the reduction of Pd2 + and the coalescence of Pd clusters progressed simultaneously in H-USY. 

On the other hand, if xq is unchanged and sq different, as in Figure 6 compared with Figure 4, the critical time is the same (-40 ms) because it corresponds to the time required to reach the critical nuclearity fixed by the donor potential, and the growth kinetics depend only on the initial monomer concentration xq and on the coalescence rate constant fed ... 

For clusters of higher nuclearity too, the kinetic method for determining the redox potential (M /Mn) is based on electron transfer, for example, from mild reductants of known potential which are used as reference systems, towards charged clusters MjJ. [31] Note that the redox potential differs from the microelectrode potential °(M, M /M ) by the adsorption energy of Mon M (except for = 1). The principle [31 ] is to observe at which step n of the cascade of coalescence reactions, a reaction of electron transfer occurring between a donor S and the cluster could compete with the coalescence. Indeed, n is known from the coalescence rate constant value, measured in the absence of S, and from the time elapsed from the atom appearance to the start of coalescence. The donor S is produced by the same pulse as the atoms M°, the radiolytic radicals being shared between M and S. One form at least in the couple S/S should possess intense optical absorption properties to permit a detailed kinetics study. 

The pseudo first-order rate constants, obs, are obtained by least-squares fits of the measured peak height increase of the relevant coalesced oxygen-17 signal as a function of time to the modified McKay equation (7, 67, 68). The kinetics can be studied by manipulation of... 

A number of rate constants for reactions of transients derived from the reduction of metal ions and metal complexes were determined by pulse radiolysis [58]. Because of the shortlived character of atoms and oligomers, the determination of their redox potential is possible only by kinetic methods using pulse radiolysis. In the couple Mj/M , the reducing properties of M as electron donor as well as oxidizing properties of as electron acceptor are deduced from the occurrence of an electron transfer reaction with a reference reactant of known potential. These reactions obviously occur in competition with the cascade of coalescence processes. The unknown potential °(M /M ) is derived by comparing the action of several reference systems of different potentials. 

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