Oxygen cleaning metal surfaces with

The adsorption of C02 on metal surfaces is rather weak, with the exception of Fe, and no molecular or dissociative adsorption takes place at room temperature on clean metal surfaces. At low temperatures, lower than 180 to 300 K, a chemisorbed COf" species has been observed by UPS6 on Fe(lll) and Ni(110) surfaces, which acts as a precursor for further dissociation to CO and adsorbed atomic oxygen. A further step of CO dissociation takes place on Fe(l 11) above 300 to 390 K. 

A clean, solid surface is actually an active center for adsorption from the surroundings (e.g., air or liquid). A perfectly cleaned metal surface, when exposed to air, will adsorb a single layer of oxygen or nitrogen (or water). Or, when a completely dry glass surface is exposed to air (with some moisture), the surface will adsorb a mono-layer of water. In other words, the solid surface is not as inert as it may seem to the naked eye. This has many consequences in industry, such as with corrosion control. Accordingly, solid surfaces should always be exposed to vacuum prior to any kind of adsorption studies. 

FIG. 9.16 Low-energy electron diffraction (LEED) patterns for clean metal surfaces and surfaces with adsorption (a) clean W(100) surface (b) W(100) with adsorbed oxygen (c) W(100) with adsorbed hydrogen and (d) clean Pt(lll). (Figs. 19.16a-19.16c reprinted with permission from P. J. Estrup, In Modem Diffraction and Imaging Techniques in Materials Science (S. Amelincks, R. Gevers, G. Remaut, and J. Van Landuyt, Eds.), North Holland, Amsterdam, Netherlands, 1970 Fig. 9.16d reprinted with permission of Somorjai 1981.)... 

Only a small amount of research has been published dealing with the reactions of / -diketones with clean metal surfaces.513,514 The interaction of acetylacetone with iron and nickel films under ultra high vacuum conditions has been investigated. X-Ray photoelectron spectroscopy is a particularly useful analytical probe as data on gas phase metal acetylacetonates are available for comparison.515 On iron, dissociative adsorption giving acetylacetonate occurs at 90 K. This decomposes at about 290 K to form surface oxide, chemisorbed oxygen and a species considered to contain Fe—C bonds. 

Mechanical activity at the surface such as load, speed, and variations in surface energetics, play a role in surface chemistry. For example, if a clean metal surface is exposed to materials such as oxygen, chlorine, and sulfur, an interaction goes on. There is no activation energy necessary to achieve the reaction of the species with the metal surface to form surface compounds. 

Lubrication. Lubricants separate the solid surfaces, either so completely that the projections on the two surfaces cannot touch, or they provide sufficient covering on the surfaces, so that if any projections do really touch the other surface the intensity of seizure is much less than if the metals were clean. We have already seen that adsorbed films of oxygen, or oxide films, reduce the friction far below that of chemically clean metal surfaces in a sense, therefore, such films might be considered lubricants. But the practical problem of lubrication consists in reducing the friction between ordinary metal surfaces, with their surface films, to the lowest practicable value. 

When a clean metallic surface is exposed to ordinary ambient air, it acquires a contaminating film. Every common metal, with the exception of gold, reacts with the oxygen of the air to form an epitaxial film of oxide. In addition, there is a physically adsorbed film, comprised of oxygen, carbon dioxide, water vapor and other atmospheric constituents, that lies on top of the oxide layer. Consequently the process of "cleaning" surfaces for friction studies by abrasion, polishing, water-washing... 

Another approach to this problem is to allow dissociation to occur, but to provide a mechanism for recombination. With hydrogen this can be accomplished by surrounding the emitter with a clean metal surface. A gauge of the type built by Klopfer (112) should avert such difficulties. This remedy is likely to be less successful with oxygen and other gases for which recombination is not as efficient as for hydrogen. In some circumstances the chemical effects of the thermionic emitter can be minimized by operating in a flow system. A rapid flow of gas... 

Atmospheric corrosion resulting from the reaction of water vapor with a metal surface is a serious problem. Most clean metal surfaces will permit the bonding of water in molecular form. The oxygen atom bonds to the metal surface and acts as a Lewis base (donating an electron pair), since the bonding is connected with a net charge transfer from the water molecule to the surface. The water adsorbs on electron-deficient adsorption sites. 

If an atomically clean metal surface is generated in a vacuum system whose pressure is kept at 1.00 x 10 torr, how many atoms of oxygen per second are colliding with each square centimeter of the surface Assume that any residual gas in the system has the same composition as air, and a temperature of 295 K. 

---