Adsorption of oxygen on tungsten

Treglia G, Desjonqueres MC, Spanjaard D, LassaiUy Y, Guillot C, Jugnet Y, Due TM, Lecante J. 1981. Study of the W (Ta) cote level shifts induced hy the adsorption of oxygen on tungsten (tantalum) (110). J Phys C 14 3463-3474. 

The dissociative adsorption of oxygen on tungsten described by the Langmuir isotherm with K = 0.35 KPa l. The fractional surface coverage at a pressure of 1 KPa. Calculated ... 

V. The Adsorption of Oxygen on Tungsten as Observed in the Field Emission... 

The adsorption of oxygen on tungsten surfaces has been investigated most extensively. Relevant data can be found elsewhere [3.1]. 

Menzel and Corner 147) observed desorption of gases from a tungsten field-emitter tip under slow-electron bombardment. They note that the process is in general inefficient, with a cross section about 10 of that for ionization or excitation in the gas phase. This desorption under bombardment with slow electrons was used by Zingerman and Ishchuk 148) as an analytical technique in a study of adsorption of oxygen on tungsten. Desorption by electron bombardment from copper and stainless steel was reported by Auslender and Minchenkov 148a). 

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. 

In one of the earliest reports of the use of clean evaporated alloy films in surface studies, Stephens described the preparation and characterization of Pd-Au films and presented some results for the adsorption of oxygen on them 46). Films of pure Pd and 60% Au were evaporated directly from wires, while films of 80% Au and pure Au were evaporated from a pre-outgassed tungsten support wire. The films were evaporated in a UHV system and the pressure was kept below PC8 Torr during evaporation. After evaporation, the films were stabilized by cycling between —195° and 30°C four times. They w ere characterized by X-ray diffraction and chemical analysis surface areas were measured by the BET method using krypton adsorption. 

In chemical adsorption(chemisorption) definite chemical bonds are produced betw the atoms molecules on the surface of solid absorbents and the molecules or atoms of "adsorbates . Chemisorption is usually accompained by an enormous evolution of heat(of the order of tens of thousands kcal per mole) and is very difficult to reverse. As an example of chemisorption, may be cited adsorption of oxygen on incandescent tungsten or of hydrogen nitrogen by tungsten even in the cold... 

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. 

Careful application of some new experimental techniques promises advances in the elucidation of the relation between detailed surface structure and reactivity in the chemisorption of a gas on a metal. Among these experimental techniques are the field-emission microscope (1-3), the inverted ionization gage (4), and modern high-vacuum technology (5). The use of the field-emission microscope technique for the study of the adsorption of oxygen (6) on tungsten has yielded recently data on the surface mobility... 

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. 

---