Chemisorption of oxygen

CO oxidation catalysis is understood in depth because potential surface contaminants such as carbon or sulfur are burned off under reaction conditions and because the rate of CO oxidation is almost independent of pressure over a wide range. Thus ultrahigh vacuum surface science experiments could be done in conjunction with measurements of reaction kinetics (71). The results show that at very low surface coverages, both reactants are adsorbed randomly on the surface CO is adsorbed intact and O2 is dissociated and adsorbed atomically. When the coverage by CO is more than 1/3 of a monolayer, chemisorption of oxygen is blocked. When CO is adsorbed at somewhat less than a monolayer, oxygen is adsorbed, and the two are present in separate domains. The reaction that forms CO2 on the surface then takes place at the domain boundaries. 

This linear variation in catalytic activation energy with potential and work function is quite noteworthy and, as we will see in the next sections and in Chapters 5 and 6, is intimately linked to the corresponding linear variation of heats of chemisorption with potential and work function. More specifically we will see that the linear decrease in the activation energies of ethylene and methane oxidation is due to the concomitant linear decrease in the heat of chemisorption of oxygen with increasing catalyst potential and work function. 

Figure 4.5 STM images (6.2 x 6.5 nm) observed in the chemisorption of oxygen at Ni(110) at room temperature (a) the (3 x 1)0 state at 0 = 0.33 (b) the (2 x 1)0 state at 0 = 0.5 (c) the (3 x 1)0 state at 0 = 0.66. Corresponding ball models of these are shown in (d), (e) and (f) and are typical of oxygen-induced reconstructions at metal surfaces. The small black balls represent the O adatoms. (Reproduced from Ref. 12).

Chemisorption of oxygen at Pt(lll) has been studied in detail by Ertl s group25 and the STM evidence is for complex structural features present in the temperature range 54M60K (Figure 4.14). The limitations of the Langmuir model, frequently invoked for reactions at platinum surfaces, is obvious from... 

Chemisorption of oxygen at Ag(110) at 300 K forms added rows of-Ag-O-extending along the [001] direction much like those observed with Cu(110). At saturation the monolayer, as with Cu(110), has a (2 x 1)0 structure.16 On exposure to ammonia at 300 K, Guo and Madix established17 that this oxide structure undergoes extensive restructuring where the added silver atoms in the monolayer are released to form nanoscale islands with the formation of... 

Fig. 11. Relative contents of various forms of chemisorption of oxygen in dependence on the position of the Fermi level.

From thermodynamic considerations it is evident that bulk nickel cannot be oxidized by CO2. However, it is not justified to conclude from this that dissociative chemisorption of CO2 will not occur. Consider, for example, the chemisorption of oxygen or hydrogen which on several metals takes place under conditions where bulk oxides or hydrides are not at all thermodynamically stable. Dissociative adsorption of CO2 has indeed been observed by Eischens and Pliskin (35). 

Gold forms a continuous series of solid solutions with palladium, and there is no evidence for the existence of a miscibility gap. Also, the catalytic properties of the component metals are very different, and for these reasons the Pd-Au alloys have been popular in studies of the electronic factor in catalysis. The well-known paper by Couper and Eley (127) remains the most clearly defined example of a correlation between catalytic activity and the filling of d-band vacancies. The apparent activation energy for the ortho-parahydrogen conversion over Pd-Au wires wras constant on Pd and the Pd-rich alloys, but increased abruptly at 60% Au, at which composition d-band vacancies were considered to be just filled. Subsequently, Eley, with various collaborators, has studied a number of other reactions over the same alloy wires, e.g., formic acid decomposition 128), CO oxidation 129), and N20 decomposition ISO). These results, and the extent to which they support the d-band theory, have been reviewed by Eley (1). We shall confine our attention here to the chemisorption of oxygen and the decomposition of formic acid, winch have been studied on Pd-Au alloy films. 

Chemisorption of oxygen on char has often been discussed previously in terms of free radical concentration in the char (1.5,6). For cellulose chars Bradbury and Shafizadeh (1) found that free spin concentration reached a sharp maximum at HTT 550°C, coinciding with maximum CSA and drew the obvious conclusion that the extent of CSA was at least partly related to free radical content of the char. However, in subsequent work on cellulose char, DeGroot and Shafizadeh (3) have found that unpaired spin concentration continues to increase up to HTT 700"C. Ihe CSA of the char must therefore depend on factors other than free radical concentration. 

Table I. Effect of Heat Treatment Temperature (HTT) on Chemisorption of Oxygen... 

Figure 1 has shown that the maximum chemisorption of oxygen on chars from untreated wood occurs at HTT 450°-500°C. However, in order to understand better the effect of metal ions on the total process consisting of pyrolysis and subsequent chemisorption and oxidation of wood char, it was necessary to carry out pyrolysis, isothermal chemisorption and oxidation reactions in a single experiment. A typical overall pyrolysis, isothermal chemisorption (140°C) and oxidation curve is shown in Figure 2. The temperature program is (1) heat from 25° to 500°C at 5°C/min, (2) cool at...

Free radicals in the surface were found after irradiation of silica gel with ionizing radiation. Observations of hydrogen evolution during heating to high temperatures were not confirmed. Chemisorption of oxygen at high temperatures has also been reported. 

As an example, XPS has been used to analyze modifications induced by 2 to 20 eV electrons incident on a hydrogen-passivated and sputtered Si(lll) surface, onto which had been physisorbed thin films of H2O [293,294] and CF4 [295]. In both cases, following the electron-induced dissociation of the molecular adsorbate, a new XPS signal associated with the chemisorption of either O or F onto the Si surface was observed and an effective cross section for chemisorption was then calculated. This cross section for electron-induced chemisorption of oxygen from an H2O bilayer onto a hydrogen-passivated Si(l 11) surface is shown in Fig. 24 as a function of Ei [293,294]. The low energetic threshold for the chemisorption process (i.e., 5.2 eV) has been interpreted as due to the formation of OH via the DEA process... 

Chuvylkin et al. (54) have used this approach to discuss EPR signals arising from weak R02 surface complexes in a number of systems where the g tensor does not fit the pattern expected [Eq. (6) and Fig. 3] from the ionic model. This is not discussed quantitatively, but they conclude that the appearance of covalently bonded oxygen is impossible without a favorable orientation of appropriate electronic orbitals. A similar covalent bonding approach has been considered theoretically for the chemisorption of oxygen on silicon surfaces (55). Examples of weakly bonded oxygen are given in Section IV,E. 

Gopel et al. (135a) have reported ultraviolet photoemission spectra (UPS) of the interaction of 02 with the (lOlO) face of a single crystal of ZnO. Between 300 and 600 K, chemisorption of oxygen is observed on stoichiometric ZnO (lOlO) surfaces and UPS difference spectra indicate peaks at... 

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