Evaporated iron films

Fig. 16. Heats of adsorption of hydrogen on evaporated iron films at 23°C. as a...

Fig. 17. Heats of adsorption of hydrogen on evaporated iron film at — 183°C. (points) in comparison with the heats obtained at 23°C. (curve) as a function of surface covered.

It can be seen (Fig. 20) from the results of Brennan and Hayes 80) on evaporated iron films that the initial CO calorimetric heat of adsorption is low (about 40 kcal/mole) in comparison with the 100 kcal/mole measured on tungsten and its nearest neighbors. This value obtained for the initial heat of adsorption of CO on iron films is very close to the values measured by these authors on all the other group VIII elements studied (Ni, Co, Rh, Pd, and Pt). 

In sharp contradiction with the result of Balandin (6), of Long, Frazer and Ott (22) and of Emmett and Skau (24) that iron powders cannot hydrogenate benzene, Beeck and Bitchie (Faraday Society Discussion on Heter< eneous Catalysis, liver-pool, England, April 1950) have reported that evaporated iron films are very active in this reaction. The discrepancy if difficult to explain— it may be that the iron films have an abnormal crystal structure, as have those of tungsten— but must be regarded as important. 

The type of intermediate shown in structure (B) has also been supported by Muller and Gault (119) who showed that in the reaction of 1,1-dimethylcyclopropane with deuterium over a series of thick evaporated metal film catalysts, it was only on platinum that 1,1,3-da-neopen-tane (and 1,1,3,3-d4-neopentane) were dominant products. On palladium, iron, rhodium, nickel, and cobalt the major product was 1,3-d2-neopentane. 

In this section a method for the direct calorimetric determination of heats of adsorption on evaporated metal films is described and results for the heals of adsorption of hydrogen on nickel, iron, and tungsten are reported. In all cases the heats of adsorption decrease with the fraction of surface covered in a mode that can satisfactorily be explained by interaction of adsorbed atoms. A criterion for mobility of the adsorbed atoms is developed... 

The gas reactants and products of the reaction were studied with a mass spectrometer and the solid reaction of the oxidation of iron was studied with Mossbauer spectrometry with electron diffraction. The Mossbauer study of the oxidized iron powder was carried out in a constant acceleration equipment [121], The electron diffraction study of the oxidized iron film, evaporated over a carbon covered transmission electron microscopy sample holder and introduced into the 5L spherical Pyrex glass container, where the Fe evaporation takes place, was carried out with the help of an Hitachi 100C transmission electron microscope [119],... 

The oxidation of the evaporated iron thin films after the action of NO at 527 K was established with the help of electron diffraction. In Figure 9.13a and b, the electron diffraction patterns of the solid phase reactant, Fe, and the product of the action of NO over Fe are reported [119]. 

The heats of adsorption of nitrogen on evaporated metal films of nickel and iron have been reported to decrease from 10 to 5 kcal./mole as the surface coverage increased from about 0.1 to 1.0 monolayer. Beeck (150) states that nitrogen is unsuitable for the evaluation of surface areas of evaporated iron and nickel films by the Brunauer-Emmett-Teller method because of its high heat of adsorption at 78°K., which would yield a value for monolayer adsorption too high by 50%. The author feels that this objection does not apply to the nitrogen adsorption on reduced electropolished planar copper plates for the following reasons. 

Fig. 4. Abnormal shapes of the adsorption isobars of H2 on zinc oxide, P = 760 mm. Hg (Taylor and Strother) promoted iron, P = 760 mm. Hg (Emmett and Harkness) reduced nickel, P = 0.01 mm. Hg (Eucken) and evaporated nickel film presintered at 400°C., P = 0.1 mm. Hg. (The ordinates are arbitrarily), (Beeck).

Films of metal in a highly reactive condition can be prepared by flash evaporation in vacuum from a hot filament onto a receiving surface. The gaseous substance whose reaction is to be studied is then leaked into the system at low pressures. Morecroft [1], on admitting n-octadecane at 0.1 Pa (7.5 X 10 torr) to a film of evaporated iron at room temperature, recovered methane, hydrogen and carbon monoxide in the ratio... 

The similarities between the CO adsorption properties of cobalt and those of iron evaporated metal films have already been mentioned. The calorimetric heat of CO on cobalt films is very close to that on iron 80) as is the surface potential caused by this adsorption 123). The CO uptakes on catalysts of the three metals iron, cobalt, and nickel have also been reported to be very close in value 133), although the group Vlllb and c elements all crystallize normally in face center cubic lattices, unlike the group Villa metals. 

Infrared studies on evaporated cobalt films 91) show an extreme similarity to the spectra obtained on iron films evaporated under the same conditions. They show a maximum in absorption at 1990 cm with a broad tail extending below 1800 cm when CO is adsorbed at... 

The amounts of the various deutero-benzenes and deutero-cyclohex-anes formed during the course of reaction between benzene and deuterium on a number of evaporated metallic films have been followed by a mass-spectrometric technique. The most extensive results were obtained over platinum and palladium films because both the exchange reaction and the deuteration reaction were found to occur simultaneously on these metals, but some results were also obtained with nickel, tungsten, iron, and silver films. 

Kishi, K., Ikeda, S., and Hirota, K. (1967) Infrared and ultraviolet studies of the adsorption of acetylaeetone on evaporated iron and nickel films, J. Phys. Chem. 71, 4384 - 4389. 

Very recently, Abel and Clair described an elegant approach to address these issues, whereby they co-evaporated iron and 1,2,4,5-tetracyanobenzene (TCNB) 61 under UHV, not only onto Au(lll) and Ag(lll) surfaces but also onto an insulating NaCl film (Figure 28.27) [133]. While the polymerizations were found sensitive to the stoichiometric ratio of the two components, they could occur on the metal surfaces at room temperature, without the formation of byproducts. When TCNB was polymerized with iron in a 2 1 stoichiometric ratio on Au(lll) and Ag(lll), a 2-D polymeric Fe-phthalocyanine network was obtained. The resultant patchwork pattern composed of three differently oriented domains (ca. 10-30 nm wide) could be accounted for by the threefold symmetry of the substrates. On the other hand, when the polymerization was carried out on NaCl islands (50-100nm wide) deposited on Ag(lOO), the polymeric Fe-phthalocyanine was produced as a monodomain phase. In this way, a direct preparation of organic metal-on-insulator systems can be envisaged, as well as a smooth transferral of products onto various other substrates for device applications. 

Thurner, G. and Abermann, R. (1990), Internal stress and structure of ultrahigh vacuum evaporated chromium and iron films and their dependence on substrate temperature and oxygen partial pressure. Thin Solid Films 192, 277-285. 

As an initial exploratory experiment dodecafluoro n-pentane was chosen as a model compound for P.T.F.E. to simulate part of the perfluorinated molecular chain and allowed to adsorb at 0.13 pPa pressure and ambient temperature on a freshly prepared iron film produced by an evaporation process in situ from an iron wire wrapped around a tungsten filament. The events inside the vacuum chamber were followed with an MS 10 mass spectrometer and (where appropriate) a Pi rani gauge. These experiments suggest that the clean iron acts as a catalyst for the breakdown of the perf1uorinated alkane producing a variety of transiently reactive fragments, such as CF2, that in turn may react with the borosilicate (Pyrex) glass walls of the reaction chamber. 

Anderson and Kemball (35) examined the reaction between gaseous deuterium and benzene catalyzed by evaporated films of iron, nickel, palladium, silver, tungsten, and platinum. The order of reactivity (estimated from the temperature at which the addition reaction achieved an initial rate of 1% per minute for a 10 mg film at certain specified reactant... 

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