Oxygen adlayer

Figure 2.10, Increase in work function (AO) with increasing oxygen concentration up to 3.8xl014 O atoms cm 2 (circles) at room temperature. The squares show the change in work function (AO) with increasing total (oxygen plus chlorine) concentration, when chlorine is dosed on the saturated oxygen adlayer at room temperature.37 Reprinted with permission from Elsevier Science.

Figure 4.3 Atomically resolved STM image (1.5 x 1.5 nm) of a clean Cu(110) surface (a) before and (b) after the formation of a fully developed (2 x 1) oxygen adlayer at room temperature. (Reproduced from Ref. 10).

Figure 5.2 Oxygen states present at the ends of -Cu-O-Cu-O- chains are established as the active sites in ammonia oxidation at Cu(110) from a Monte Carlo simulation of the growth of the oxygen adlayer. The reactivity (the experimental curve) is best fitted to the atoms present at chain ends. (Reproduced from Ref. 7).

At Ag(110), sulfur dioxide reacts with a p(2 x 1)0 oxygen adlayer to give a c(6 x 2) sulfite structure with six sulfite (S03) species associated with each unit... 

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... 

Fig. 34. Rate of C02 formation and variation of 0CQ and 0O with time for a CO beam impinging on an oxygen adlayer on Pd(l 11) at T = 374 K (176).

Table 11.1 The reconstruction of chemisorbed oxygen on selected catalytic metal surfaces, and the corresponding coverages of oxygen and metal adatoms in the oxygen adlayer...

The incorporation of metal atoms into the oxygen adlayer is a general phenomenon on many metal surfaces. The surface restructuring upon oxygen adsorption provides general information about the dynamic nature of metal surfaces, such as ... 

A significant research effort has been devoted to the characterization of Pd surface oxides [17, 63]. In situ studies under non-UHV conditions are most relevant, because surface oxides may only be present under reaction conditions (and not accessible by post-reaction (ex situ) characterization). A stepwise oxidation of Pd(lll) was reported [64-66], starting from the well-known (2x2) chemisorbed oxygen adlayer, followed by the formation of a two-dimensional Pd O surface oxide, which eventually transforms to a PdO bulk oxide. At high temperature, PdO decomposes with the concurrent dissolution of oxygen atoms into the bulk. In situ synchrotron HP-XPS [64, 65] indicated that a two-dimensional Pd O surface oxide was formed on Pd(lll) at 0.4 mbar O and >470 K, which transformed to PdO at temperatures >660 K. PdO was found to decompose at temperatures >720 K. 

Many conjectures on the chemical state and role of potassium in ammonia synthesis can be made from the surface-science results. The potassium TPD results show clearly that elemental potassium will not be stable at the temperatures necessary to perform the synthesis of ammonia. The industrial reaction is usually run between 673 K and 748 K and the TPD results show that elemental potassium would rapidly desorb at these temperatures. However, with the coadsorption of oxygen, potassium can be thermally stabilized, and it desorbs at temperatures greater than 1000 K. Bulk potassium compounds such as K2O or KOH would not be stable under ammonia synthesis conditions. This suggests that the formation of potassium ferrites in the industrial catalyst results in a chemisorbed potassium and oxygen adlayer, stable under industrial ammonia synthesis conditions. 

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