Subsurface O atoms

Figure 7.6 Several of the structures considered by Li, Stampfl, and Scheffler1 in constructing their phase diagram for 02/ Ag( 111). The top left panel shows a top view of a structure with both surface and subsurface O atoms (large and small dark spheres, respectively). The top right panel shows a side view of the same structure. The DFT calculations do not predict this structure to be thermodynamically favored. The bottom panel shows a top view of the (4 x 4) surface oxide, which has a complex arrangement of Ag atoms (light spheres) and O atoms (dark spheres) on top of an Ag(lll) substrate (unfilled circles). DFT calculations predict this structure to be favored at certain temperatures and pressures. (Reprinted by permission from the source cited in Fig. 7.5.)...

Key Words Ethylene oxide, Ethylene, Epoxidation, Silver, Cl promotion, Cs promotion. Promotion, Selectivity, Oxametallacycle, Adsorption, Desorption, Chemisorption, Activation energy, Ag-O bond. Reaction mechanism, Oxidation, Cyclisation, Heterogeneous catalysis, Selective oxidation, Eletrophilic oxygen. Nucleophilic oxygen. Subsurface O atoms, Ag/a-A Oj catalyst. 2008 Elsevier B.V. 

Fig. 4. Left DFT structures of on-surface (top) and Of, subsurface (bottom) oxygen atoms (dark grey balls) on Ag 111 (light grey balls and sticks). Right STM topographic simulations for each overlayer. Black crosses and circles mark the locations of Ag and O atoms. STM conditions V = 100 mV, I = 1 nA. Corrugation Osurf = 0.2 A, Osub = 0.02 A.

Based on DFT calculations Scheftler and co-workers drew similar conclusions for atomic O adsorption at low coverages on Ag lll. Interestingly, however, their DFT calculations revealed that at higher coverages of atomic O (> 0.5 ML), O atoms would adsorb preferentially at subsurface sites [44,45],... 

Occupation of subsurface sites will commence at a certain total coverage Of, when a structure with a nonzero fraction of the O atoms below the sur-... 

The authors therefore concluded that subsurface oxygen was not essential for the selective oxidation of ethylene on an Ag/7.-AI2O3 catalyst. Its presence had no effect on the activity or selectivity of the surface O atoms [3]. 

Figure 3.62. The silver cluster studied (9) chemisorbed oxygen stoms. ( ) silver Atom in outer Uyer, (o) silver atom in inner layer, (x) subsurface oxygen atom (a) top view without ethylene (b) side view with ethylene.

Oxygen dissociates on all Group Vlll/Ib metals, although on Pt, Ag, and Au, oxygen can also exist in the molecularly bound form. Oxygen atoms almost always bind in sites of high symmetry with threefold or fourfold coordination to the substrate metal. In some metals, O atoms can also be accommodated in deeper layers of the metal. The existence of subsurface oxygen in silver, for example, is well documented. 

Structural relaxation not only determines the distribution of localized electrons, it is also directly related to the relative stabilities of top- and subsurface O vacancies. On the defective surface with a single top-surface vacancy, the three nearest O atoms around the vacancy are all subsurface bulk-like O4,. that are binding with four Ce ions. In contrast, the nearest three O arotmd a subsurface O vacancy are all threefold, which can be expected to be more mobile compared to the fourfold ones. Therefore, deeper relaxation occurs around the single subsurface O vacancy compared to the top-surface one, and gives lower vacancy formation energy (1.95 vs. 2.13 eV). 

Figure 8.5 shows the LEIS spectra of ZnAl204 and ZnO as a characteristic example of a multicomponent system analyzed by this technique [Brongersma and Jacobs, 1994]. Since only the surface peaks of A1 and O were detected for ZnAl204, the Zn atoms must be located in the subsurface layers. The onset of the tail agrees between the spectra, indicating that Zn is present in the second and deeper layers. This example illustrates the strength of the LEIS technique, in that characteristic peaks from different elements can be used to selectively analyze the atomic composition of the topmost surface. In addition, the shape of the tails could provide information on the in-depth distribution of the elements. 

---