TUngsten nitrogen adsorption

Not only the hydrogen adsorption but also the nitrogen adsorption as observed by Davis (17a), as well as by Beeck and coworkers fits well into this picture. It was observed by Beeck et al. that 60 % of the surface of evaporated tungsten films could be covered by nitrogen at room temperature, 50% being adsorbed instantaneously with a constant heat... 

The following data refer to the adsorption of /t-butane at 273 K by a sample of tungsten powder which has a specific surface area (as determined from nitrogen adsorption measurements at 77 K) of 6.5... 

Fia. 60. Surface diffusion of nitrogen emission centers, (a) View of (111) and surroundings on a clean tungsten surface, (b) After nitrogen adsorption at T = 300°K. Arrow indicates location of one of the new adatoms, (c) Identical tip after heating to T 420°K for 1 min. Emission centers have migrated away from the sites occupied in (b) without disturbing the lattice. 

It could still be argued that holes are created in the areas not accessible to direct observations such as the (110) and (211), and other low index planes for which the individual lattice atoms are not clearly defined. We can invoke thermochemical data on the energetics of nitrogen adsorption to eliminate this eventuality. The energy change xw f°r the transfer of tungsten atoms out of the (110) to an evaporation site, estimated on the assumption of pairwise additive lattice forces, amounts to 5.5 ev. It is still 3.16 ev for an atom abstracted from the (211). 

Fid. 65. Desorption of tungsten-nitrogen complex at high fields, (a) Clean tungsten. He image at T 20°K, 16.3 kv. (b) After adsorption of nitrogen at T 20°K. (c) Same surface after applied potential has been raised to 19 kv (in the absence of He). W atom in left comer of the plane is removed together with adatom. 

AlTUD-1, a new mesoporous, Bronsted acidic aluminosilicate with ideal characteristics for catalyst immobilization was used for the noncovalent anchoring of hydride tungsten complex [WH2(ti -OOCCH3) (Ph2PCH2CH2PPh2)2][BPh4]. The immobilization was carried out by an adsorption process in liquid phase. The new materials were characterized by several techniques spectroscopic methods (ICP-AES, FT-IR and UVA is), X-ray techniques (XPS and XRD), isothermal nitrogen adsorption and elemental analysis. 

The tungsten carbides on FSM-16 were prepared in carburizing of WO3/FSM-I6 with 20% CH4/H2 and characterized using nitrogen adsorption, XRD, TEM, and NMR. The relationship between the surface properties and the catalytic activity of tungsten carbides on FSM-16 in cyclohexene hydrogenolysis at 250°C was discussed. 

The other common factor is that molecular nitrogen adsorption is structure-sensitive, even on metals such as tungsten where the heat of N adsorption is substantial. Extensive studies have failed to link conclusively this sensitivity with any particular surface structure. It seems to require the presence of a very open surface which may be linked to high index planes or the presence of surface defects such as steps, kinks, etc. In any event this manifests itself in supported catalysts as an unusual dependence on the metal crystallite size, with the optimum activity occurring in the 2 to 6 nm range. 

Measuring the electron emission intensity from a particular atom as a function of V provides the work function for that atom its change in the presence of an adsorbate can also be measured. For example, the work function for the (100) plane of tungsten decreases from 4.71 to 4.21 V on adsorption of nitrogen. For more details, see Refs. 66 and 67 and Chapter XVII. Information about the surface tensions of various crystal planes can also be obtained by observing the development of facets in field ion microscopy [68]. 

The significance and impact of surface science were now becoming very apparent with studies of single crystals (Ehrlich and Gomer), field emission microscopy (Sachtler and Duell), calorimetric studies (Brennan and Wedler) and work function and photoemission studies (M.W.R.). Distinct adsorption states of nitrogen at tungsten surfaces (Ehrlich), the facile nature of surface reconstruction (Muller) and the defective nature of the chemisorbed oxygen overlayer at nickel surfaces (M.W.R.) were topics discussed. 

The adsorption of nitrogen on tungsten 51) was the first nitrogen system to be studied (Fig. 12) by XPS subsequently nitrogen interaction with iron was studied (52, 53), and two distinct N(ls) peaks were observed at 80 K, one at about 405 eV and the other at 400 eV. At room temperature with both single crystals and polycrystalline iron surfaces only a single peak is... 

The heat of adsorption of nitrogen on tungsten at room temperature was found to be 95,000 calories per mole and constant over the whole fraction of the surface which it will cover—that is, about 60%. The adsorption is instantaneous. In Fig. 21 is [shown the heat of adsorption of hydrogen on a tungsten surface covered 60% with nitrogen. 

IV. The Adsorption of Nitrogen on Tungsten as Deduced from Ion Gauge and... 

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