Oxygen, adsorbed zinc oxide

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]. 

It should be noted that in some papers (53, 57) the specimens of zinc oxide were preliminarily calcined in an atmosphere of oxygen, i.e., the surface of the catalyst was enriched in the adsorbed oxygen. 

This work will attempt to demonstrate the importance of adsorbed oxygen on many properties of zinc oxide, namely the conductance, fluorescence, photoconductance, and the adsorption of hydrogen. The contribution, potential, and current of each of these studies to a more complete understanding of the adsorption process will be discussed. 

A possible intermediate case discussed in Section IV,3 applies to the adsorption of hydrogen on zinc oxide. The extra surface levels are considered to be previously adsorbed oxygen. 

Bevan and Anderson (32) deduced that adsorbed oxygen controlled the resistance of sintered zinc oxide at temperatures between 500°C and 1000°C. This conclusion was based on the observations that (1) the oxygen pressure had a reversible controlling effect on the resistance down to 500°C, too low a temperature for thermodynamic equilibrium to be... 

Figure 1 is an energy level diagram showing a proposed model for the band structure of zinc oxide. The valence band and conduction band are shown separated by a forbidden gap. Two levels which correspond to the trapping of two electrons by the interstitial zinc are indicated in the forbidden gap. Surface levels associated with adsorbed oxygen are shown. 

In this section, we have presented evidence to indicate that adsorbed oxygen on the surface may be a controlling factor in the electrical properties of zinc oxide. In the following section, the electron transfer theory of adsorption will be discussed. The properties then will be examined in more detail, agreement of the results with those predicted by the adsorption theory will be emphasized, and the contribution of those that have been studied to a more complete understanding of adsorption phenomena will be discussed. 

Evidence has been presented (31,32) that oxygen is chemisorbed on the surface of zinc oxide. The energy level for the first electron is denoted by Es. From conductivity investigations, a second surface energy level, associated with adsorption, has been indicated with an energy level at about 0.8 e.v. below the conduction band at the surface. The hypothesis has been presented that this level is associated with double ionization of the adsorbed oxygen. 

Photoconductivity in zinc oxide, on the other hand, appears to be influenced by the surface through a different effect. Absorption of light effectively excites the electrons trapped in surface levels into the conduction band. This chapter will be primarily devoted to a consideration of this concept, proposed by Melnick (11), that photoconducting electrons are produced through the ionization of surface levels, specifically the adsorbed oxygen levels on zinc oxide. The decay of the photoconductivity. 

A hypothesis has been presented to describe the adsorption of hydrogen on zinc oxide, wherein the hydrogen is assumed to react with surface energy levels which are associated with adsorbed oxygen. The predictions of the model are shown to be qualitatively consistent with experiment. 

Each of the various processes of adsorption may have desorptions of the reverse forms, for example, dissociative adsorption may have as its reverse, associative desorption. However, the process of chemisorption may not be reversible [ 1.2.2(c)]. Desorption may lead to species other than that adsorbed, for example, ethane dissociatively adsorbed on clean nickel gives little or no ethane upon desorption, 1-butene dissociatively adsorbed to methylallyl and H on zinc oxide gives mainly 2-butenes upon desorption, and some W03 may evaporate from tungsten covered with adsorbed oxygen. 

BET area (Table V) and the copper area from oxygen chemisorption. Table VII summarizes the copper and zinc oxide areas so determined for the whole compositional range. The oxygen chemisorption method suffers from the uncertainty that some oxygen may be adsorbed on the copper solute and on defects in the zinc oxide surface that are formed only in the presence of copper. There is indirect evidence from a comparative study of carbon monoxide and oxygen chemisorption, however, that this is not the case and that oxygen titrates only the copper metal surface. 

Quantitative and qualitative changes in chemisorption of the reactants in methanol synthesis occur as a consequence of the chemical and physical interactions of the components of the copper-zinc oxide binary catalysts. Parris and Klier (43) have found that irreversible chemisorption of carbon monoxide is induced in the copper-zinc oxide catalysts, while pure copper chemisorbs CO only reversibly and pure zinc oxide does not chemisorb this gas at all at ambient temperature. The CO chemisorption isotherms are shown in Fig. 12, and the variations of total CO adsorption at saturation and its irreversible portion with the Cu/ZnO ratio are displayed in Fig. 13. The irreversible portion was defined as one which could not be removed by 10 min pumping at 10"6 Torr at room temperature. The weakly adsorbed CO, given by the difference between the total and irreversible CO adsorption, correlated linearly with the amount of irreversibly chemisorbed oxygen, as demonstrated in Fig. 14. The most straightforward interpretation of this correlation is that both irreversible oxygen and reversible CO adsorb on the copper metal surface. The stoichiometry is approximately C0 0 = 1 2, a ratio obtained for pure copper, over the whole compositional range of the... 

To summarize the qualitative findings, the methanol synthesis activity in the binary Cu/ZnO catalysts appears to be linked to sites that also irreversibly chemisorb CO and not to sites that adsorb CO reversibly. Since irreversible adsorption of CO follows linearly the concentration of amorphous copper in zinc oxide, these sites are likely to be that part of the copper solute that is present on the zinc oxide surface. No correlation of the catalyst activity and the copper metal surface area, titrated by reversible form of CO or by oxygen, could be found in the binary Cu/ZnO catalysts (43). In contrast with this result, it has been claimed that the synthesis activity is proportional to copper metal area in copper-chromia (47), copper-zinc aluminate (27), and copper-zinc oxide-alumina (46) catalysts. In these latter communications (27,46,47), the amount of amorphous copper has not been determined, and obviously there is much room for further research to confirm one or another set of results and interpretations. However, in view of the lack of activity of pure copper metal quoted earlier, it is unlikely that the synthesis activity is simply proportional to the copper metal surface area in any of the low-temperature methanol-synthesis catalysts. 

A clear case of different forms of chemisorbed oxygen is provided by recent studies of zinc oxide (48, 49). The quantities of oxygen adsorbed by zinc oxide are very small, much less than 1% coverage, but the uptake can be conveniently studied at low pressures using a Pirani gauge. The adsorp-... 

---