Oxygen, monolayer structure

Figure 6 The half monolayer structure of atomic oxygen adsorbed...

Fig. 44. Dynamic scaling in the growth of Bragg peaks for LEED scattering from oxygen monolayers at 0 = 1/2 and T — 297 K for adsorption in the p(2xl) structure on W(110). Different symbols denote different times t in seconds) after the adsorption has taken place. The halfwidth (used for normalization exactly as in fig. 43) is denoted by FWHM ( full width at half maximum ). From Wu et al. (1989).

In ethane ODH, the activity of VjOj catalysts increases with the loading. The rates per V atom reach a maximum on domains of intermediate size. There exists a balance between the aetivity of surface VO species and their accessibility to reaetants. The seleetivity to ethylene reaches its maximum at intermediate values of loading eorresponding to the formation of the monolayer structure. The ethylene formation rate per V atom depends on the vanadia surface density and reaches its maximum value at intermediate surfaee densities [36-37]. Surface oxygen, 0-H groups and especially oxygen vaeaneies are considered as the most prevalent reactive intermediates during ODH on active VO domains [38-39]. Klose et al. observed that the adsorbed O species [40] mainly contribute to formation. 

M. De Crescenzi. Phys. Rev. Letts. 30,1949,1987. Use of surface electron energy-loss fine structure (SEELFS) to determine oxygen-nickel bond length changes for oxygen absorbed on Ni (100) on a function of coverage from 0 to 1.0 monolayer. 

Before 1950, it was impossible to examine the true structure of a solid surface, because, even if a surface is cleaned by flash-heating, the atmospheric molecules which constantly bombard a solid surface very quickly re-form an adsorbed monolayer, which is likely to alter the underlying structure. Assuming that all incident molecules of oxygen or nitrogen stick to the surface, a monolayer will be formed in 3 x 10 second at 1 Torr (=1 mm of mercury), that is, at 10 atmosphere a monolayer forms in 3 s at 10 Torr, or 10 atmosphere but a complete monolayer takes about an hour to form at 10 Torr. The problem was that in 1950, a vacuum of 10" Torr was not achievable lO Torr was the limit, and that only provided a few minutes grace before an experimental surface became wholly contaminated. 

FIG. 26 Optimized structure of a water monolayer on mica obtained from molecular dynamic simulations by Odelius et al. The water molecules and the first layer of sihca tetrahedra of the mica substrate are shown in a side view in the top. K ions are the large dark balls. The bottom drawing shows a top view of the water. Oxygen atoms are dark, hydrogen atoms light. Notice the ordered icelike structure and the absence of free OH groups. All the H atoms in the water are involved in a hydrogen bond to another water molecule or to the mica substrate. (From Ref. 73.)... 

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... 

The interatomic spacing within the rows of the c(2 x 4) structure is 0.5 nm, which is close to the Cs-Cs spacing in the monolayer of Cs formed at a Cu(l 10) surface at 80 K. The presence of the oxygen adlayer apparently prevents reconstruction of the surface with the caesium locked in within the rows of... 

Studies of Ag on Au(lll)87 yield very similar results in terms of the structure of the deposited monolayer (i.e., the silver atoms are bonded to three surface gold atoms and are located at three-fold hollow sites forming a commensurate layer) with again strong interaction by oxygen from water or electrolyte (perchlorate). 

Although conventional electron-probe microanalysis appears to be unsuitable for analysis of the exposed surface layer of atoms in an alloy catalyst, recent developments have shown that X-ray emission analysis can still be used for this purpose (89, 90). By bombarding the surface with high energy electrons at grazing incidence, characteristic Ka radiation from monolayer quantities of both carbon and oxygen on an iron surface was observed. Simultaneously, information about the structure of the surface layer was obtained from the electron diffraction pattern. 

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