Activation for diffusion

Table IV. Surface basicity (0g), posTtional (kj 2) and geometric (kc t)isomerization constant of 1- and 2-Butene at 297 K and energy of activation for diffusion of 1-Butene adsorbed on SnSbO catalysts.

Figure 9 illustrates the results for kg. Neither the volume of activation for diffusion AVdiff (average —0.9 l.lcm mol ) nor the electrochemical cell reaction volumes A VAg/AgCl average (—22 2 cm mol i) for the C0W12 couple show significant dependence on electrolyte identity or concentration. For the PWi2 / ... 

Another drawback of the thermal diffusion model is the lack of a physically substantiated explanation of why the character of the reacting particle distribution over the time of "settled life at different points of the matrix (i.e. over the values of the energy and entropy of activation for diffusion)... 

Figure 8. Variation of the energy of activation for diffusion of Ne arid He with density of the diffusion media. In order of increasing densityy the media are silica glass, tridymite, and cristobalite...

Figure 9. Dependence of energy of activation for diffusion in several media upon the critical van der Waals dimension of the diffusing molecules...

The influence of moderators upon diffusion coefficients is shown in Figure 13 for mordenite moderated with ammonia for a number of diffusing species. Da may change by orders of magnitude for relatively small amounts of modifier. This can result in cut-offs in the amounts sorbed, on the time scale of normal experiments, which occur at different uptakes of NH3 for molecules of different dimensions. Different moderator molecules can, molecule for molecule, have different effects upon the diffusion coefiicients. These effects tend to follow the sequence of the molecular volumes of the moderator. As the amount of moderator increases, the energies of activation for diffusion increase (Table X), to parallel the decrease in Da. 

A number of NMR studies of the hydrides have appeared (307-310). Several have measured the relaxation times as a function of temperature and extracted the energy of activation for diffusion of the hydrogen. In one report, a value of 61.5 kJ mol" is reported (309), but another finds 45.6 kJ mol below 350 K and a mean value of 71.7 at 390 K (308). The data do not suggest much occupation of 5/" electrons near the Fermi level. Some measurements have been extended to 4.2 K (311). In addition, Knight shifts at the proton have been reported for ThH, a = 0 - 3.61 (312). 

In a related study, the volume of reaction, the volume of activation for diffusion (A F ff), and the volume of activation obtained from the standard electrode reaction rate constant at various pressures, have been determined for the dec-amethylferrocene (DmFc+/0) system, in several non-aqueous solvents.238 The deca-methylated ferrocene couple, rather than the unmethylated couple, was chosen. This... 

Figure 11. Expressions for the energy of activation for diffusion and the diffusion coefficient from the theory of Pace and Datyner (64).

From these figures he computed the energy of activation for diffusion normal to the 201 and 001 faces respectively to be 5400 and 9140 cal./mol. The temperature dependence of the diffusion constants he found did not depend appreciably upon the amount of water in the lattice, although we have seen that their absolute magnitudes do. 

The activation energies are somewhat smaller than for activation-limited reactions, often near the values for the energies of activation for diffusion processes. 

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