Apparent activation energies for reduction

Figure 6, Apparent activation energies for reductive dechlorination of CT at an iron electrode. Activation energies were determined from Arrhenius analysis of dechlorination rates at temperatures of2, 12, 22, 32 and 42...

Figure 51. Plots of apparent activation energies for (left) electron injection and (right) hole injection into cubic AgBrI microcrystals versus reduction and oxidation potentials of the sensitizing dyes, and the expected dependencies for AG based on Marcus theory, using 0.42 eV for A, —0.92 V versus Ag/AgCl for red and 1.41 V versus Ag/AgCl for E ox. Figure adapted from [158].

This is because the apparent activation energies for the interfacial processes are, in general, higher than those for oxygen ionic transport in solid electrolytes (Yamamoto, 2000). The reduction of the working temperature results in a lower oxygen vacancy concentration with concomitant increase of the role of ionic conductivity of electrode material. 

The most active Tc metal catalyst was obtained at the lowest temperature used for the reduction of TCO2 to Tc metal. Die apparent activation energy for the dehydrogenation process was 13.7 kcal/mole. At 220 C 30 % of isopropyl alcohol was converted to acetone. ITie activity of the technetium metal catalyst for the dehydrogenation of isopropyl alcohol is superior to that of manganese and close to that of metallic rhenium when, however, the content of rhenium in the catalyst was 30 wt% instead of around 0.2 wt% of technetium [11]. 

Since tanh

sufficiently large Thiele moduli, it follows from Eq. 29 that the effectiveness factor approaches the reciprocal value of the Thiele modulus. In the classical Thiele concept, this leads to the well-known reduction in the apparent activation energy for transport-controlled reactions, since the effective reactivity is now proportional to Vk rather than to fc, as in the absence of any transport limitation. 

For the reduction of NO with propene, the catalyst potential dependence of the apparent activation energies does not show a step change and is much less pronounced than it is for the CO+O2 and NO+CO systems. There is persuasive evidence [20] that the step change is associated with a surface phase transition - the formation or disruption of islands of CO. It is reasonable to assume that this phenomenon cannot occur in the NO+propene case, since there is no reason to expect that large amounts of chemisorbed CO can be present under any conditions. That there should be a difference in this respect between CO+O2/CO+NO on the one hand, and NO+propene on the other hand, is therefore understandable however, the chemical complexity of the adsorbed layer in the NO+-propene precludes any detailed analysis of the Ea(VwR> effect. 

For both series of metal oxides the apparent activation energies of the catalyzed (spillover) reduction reactions were found to be similar to those of the noncatalyzed reductions. Generally, the effect of the Pt was to increase the available reactive hydrogen and/or to increase the rate of the nucleation (pre-exponential factors). Thus, this catalysis increases the availability of H but does not ("classically ) decrease the activation energy. 

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