Zinc oxide activation energy

The low energy of activation of the change in electric conductivity of zinc oxide observed during adsorption of H-atoms ( 0.08 eV) [102] can correspond to the ionization energy of (0-H) -groups formed during direct interaction of H-atoms with O -ions of the lattice. 

The activation energy for evaporation of over-stechiometric zinc atoms Zn calculated from the tilt of this line is 32 kcal/gram/atom. This means that evaporation heat of superstechiometric zinc atoms (Zn ) from zinc oxide found in these experiments agrees well with the corresponding value found in [31] by mass-spectrometry. 

It follows from the formula that the experimentally evaluated value of / activation energy on the ZnO surface may be related to the activation energy of oxygen desorption from the zinc oxide surface. This value well agrees with the desorption activation energy measured with the aid of semiconductor detectors in work [109]. 

This conclusion is in agreement with experiments in which a smootb quartz and cellulose were used as substrates. For above materials the transfer of excitation energy of the dye into the substrate is low which is confirmed by intensive luminescence of adsorbed tripaflavine. Note, that the activation energy of emission of singlet oxygen is close for zinc oxide oxidized by oxygen atoms, quartz and cellulose and amounts to 5-10 kcal/mol [83]. 

Figure 24 shows the cfs-butene isomerization over zinc oxide as a function of time at room temperature (7/). On a per unit area basis the initial rate at room temperature is 4 X 1010 molecules/sec cm2, a rate roughly one third that reported for alumina (69). Since the activation energy for alumina is less than that found for zinc oxide, this means that zinc oxide is comparable (on a per unit area basis) to alumina as an isomerization catalyst at slightly higher temperatures. 

The complexity of the adsorption process, in particular its duality as illustrated above as well as in more recent data of Wicke (12), also shows in the irregular behavior of zinc oxide as a catalyst for the hydrogen-deuterium exchange (13). Thus this reaction proceeds at measurable rates at temperature as low as 14O K., indicating that at least part of the low-temperature adsorption is of the dissociating type. The apparent activation energy at low temperatures is low, but in the temperature... 

The existence of other deep surface levels, for example Tamm levels (discussed in the preceding section), on the surface of zinc oxide is placed in doubt by an experiment of Bevan and Anderson (32) on sintered zinc oxide. They observed that the activation energy of the conduction electrons (of the order of an electron volt when the sample is subjected to high oxygen pressure) decreases to a few hundredths of an electron volt if the measurements are taken at low pressure (less than 10 mm.) and high temperature (the order of 600°C). Surface traps other than those asso-... 

This evidently does not apply to sintered zinc oxide, since at low temperature its conductivity shows an activation energy of a few hundredths of an electron volt, the same as is shown by the Hall coefficient (26), independent of the previous treatment of the sample. This, then, indicates that the grains of sintered zinc oxide are actually fused, rather than merely touching. 

The latter reaction will occur (at temperatures sufficiently high to overcome the activation energy in Fig. 1) because adsorption of hydrogen (Type A) will upset the electronic equilibrium between the number of electrons leaving the O sites and those returning from the zinc oxide. There will be fewer empty O sites after the Type A adsorption, hence more electrons will leave the O sites than return. As more 0 sites are formed, more hydrogen will be adsorbed. 

As in the case of normal supported catalysts, we tried with this inverse supported catalyst system to switch over from the thin-layer catalyst structure to the more conventional powder mixture with a grain size smaller than the boundary layer thickness. The reactant in these studies (27) was methanol and the reaction its decomposition or oxidation the catalyst was zinc oxide and the support silver. The particle size of the catalyst was 3 x 10-3 cm hence, not the entire particle in contact with silver can be considered as part of the boundary layer. However, a part of the catalyst particle surface will be close to the zone of contact with the metal. Table VI gives the activation energies and the start temperatures for both methanol reactions, irrespective of the exact composition of the products. 

The reaction is of the acceptor type (28). Consistent with this and the views presented above, the data of Table VI suggest that contact with silver has bent the bands in zinc oxide and thus lowered the activation energy and the start temperature. However, a bifunctional catalytic action of the silver-zinc oxide system canot be excluded with certainty. 

Activation Energies and Starting Temperatures of Methanol Reactions over Silver, Zinc Oxide, and their Mixture (1 1 by wt)... 

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