Oxygen adsorbed silver

These effects are most striking on silver since it is, itself, a very unreactive surface. There is every reason to expect, however, that oxygen will behave similarly on other metals. More complex reaction behavior will, of course, be observed as the intrinsic reactivity of the metal increases. Oxygen adsorbed on platinum should show similar properties. In fact the formation of surface OH groups from HjO and 0(a) was recently reported 145). The ability of platinum itself to break C-H and C-C bonds complicates oxidation mechanisms, but future work should provide a greater understanding of the relative role of surface oxygen in oxidation catalysis. 

Cation radicals of several metallated TPP and OEP complexes were reported to have unique IR active ring modes (21. These absorptions were observed in the region 1250-1290 cm for TPP complexes and 1520-1570 cm for OEP complexes. IR and raman active modes have also been observed for several different reduced oxygen species adsorbed on various metal substrates. Observations of the adsorption of oxygen on silver by surface enhanced raman spectroscopy (SERS) 22), show superoxide molecules (O2) have a frequency of 1053 cm, and peroxide (0 ) of 697 cm. This same study reported additional peaks were observed at 815 cm and 1286... 

Several studies have been devoted to adsorption of oxygen on silver surfaces. It has become evident that both molecular and atomic species of adsorbed oxygen exist. It is generally considered that fast dissociative adsorption takes place first and leads to a strongly bonded species, which... 

Apart from kinetic measurements, the studies of oxygen adsorption on silver (63, 64), electron work function variations accompanying oxygen adsorption (65-67), heat effects of adsorption (65), reactivity of oxygen adsorbed on silver (67), and oxygen isotopic exchange on silver (56-55) were used for the elucidation of the mechanism of the reaction. The papers cited contain references to the works of other authors that were used for the formulation of the reaction mechanism on a level with our results. Here we shall mention, first of all, the work by Twigg (69), who has ascertained... 

The free molecules of metallic oxides can best be described as having covalent links, possessing dipoles (58). Similarly oxygen atoms adsorbed on metallic surfaces form covalent bonds, sharing two pairs of electrons with the metal (59) or with one specific atom or two atoms of the metal. Their dipoles point with the negative ends away from the metal. We may, to give an example, express the situation of the adsorption of oxygen on silver by... 

Oxygen and ethylene adsorb on a supported silver catalyst. A Rideal-Eley mechanism is favored for the adsorption. Chloride is a promoter/modifier for the catalyst and reduces the surface oxygen content. Silver on the surface tends to increase ethylene adsorption, which explains why small levels of chloride increase the rate of reaction. 

It is generally accepted that there are two types of adsorbed oxygen on silver, monoatomic and diatomic. 

A clean silver surface will adsorb oxygen in a dissociated form. LEED studies show that on the Ag(l 11) face oxygen adsorbs to form a stable (4 x 4) superstructure. This has been interpreted as a coincidence lattice between the Ag(lll) plane and the (111) plane of silver(i) oxide. There is evidence that oxygen adsorption on faces other than Ag(lll) results in the formation of Ag(l 11) facets. " Incorporation of oxygen into the subsurface appears to be rather slow in the presence of molecular oxygen, but alternate cycles of oxidation and reduction with CO results in the build up of a thin subsurface layer of oxidized silver. ... 

The rate of exchange of molecular oxygen on the silver film at 350° C is ten times less than that on the platinum film. The energy of activation is 19 kcal/mole. Oxygen adsorbed on the silver film is uniform. 

Kalish and Burstein (60) found that the amount of oxygen adsorbed by a platinum layer adjacent to the surface was about one hundred times that required for a monolayer. Temkin and Kul kova (61) have noted a similar phenomenon for oxygen adsorption on silver. The amount of... 

I>ensity functional theory (DFT) calculations were also used to confinn the structure of adsorbed enolic species on Ag/AFO using the (iaussian 98 suite of programs. The simulation molecular structure model and FTIR spectrum of the surface enolic species on Ag Al Oi are shown in Fig. 7. In model A. the enolic species (OI <.TI-() ) binided with a silver atom, and the optimized distance between the oxygen and silver atom w as 2.072 A.. Apparently, ihe calculated FTIR spectrum was reasonably similar to the correspimding experimental result. 

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