Oxygen adsorbed tungsten

From experiments with the field emission microscope it is learned that for a system like oxygen on tungsten (a) the crystallographic plane of the tungsten has a marked influence on the adsorption properties (6) the heat of adsorption increases with the number of W atoms a particular 0 atom can contact (c) the heat of adsorption for the first layer, in which 0 atoms make first valence bonds with W atoms, is about 4 ev., for the second layer, in which 0 atoms make second valence bonds with W atoms, only about 2 ev. (d) at a constant pressure the rate of adsorption is constant until the first layer is complete, and for the second layer it is slower by a factor of 100 or more (e) beyond the second layer oxygen is adsorbed as admoles of O2, O4, Os. 

Langmuir put forward an extremely definite form of this idea. The adsorbed molecules are supposed to be held to the surface by ordinary valency forces , either primary valencies or secondary valencies . In the light of recent developments in the theory of atomic structure it would probably be sufficient to say that the adsorbed molecules are attached to the molecules constituting the surface by non-polar linkages. Thus the kind of union between tungsten and oxygen adsorbed on its surface, to... 

Oxygen and tungsten reacted in a totally different manner. A minute pressure of oxygen formed an adsorbed film over the whole surface of the filament. The presence of this film was shown in three different ways it was found to diminish the thermionic emission of electrons from the heated wire at 1,500° K. it was found to inhibit the dissociation of hydrogen at the surface of the wire and it was shown by quantitative studies that the rate of reaction of the filament covered by adsorbed oxygen, with further supplies of the gas, was so great as to require the presence of a monatomic film of adsorbed oxygen, already combined with the surface of the filament for, p. 1150). 

Also, since chemisorbed adsorbates like oxygen on tungsten are known to acquire a pronounced negative charge, the mechanism of adsorption for aqueous ions on a metal surface cannot be far different. Perhaps the existence of a diffuse double layer in the case of ions is the one distinguishing characteristic of these two systems. 

Adsorbates should have a marked influence on surface-atom vibrations, since they change the bonding environment with respect to that on the clean surface. The adsorption of oxygen on tungsten increases the surface Debye temperature with respect to the bulk value due to the stronger W—O bond as compared to the W—W bond. Studies of surface-atom vibrations in the presence of adsorbates provide information on the nature of the surface bond. 

Part (b) of Fig. 9.16 is the tungsten 100 face with adsorbed oxygen, while part (c) of the figure is the same surface with adsorbed hydrogen. 

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