Iron foils, carbon deposition

The macro-scale investigations showed that pretreatment of an iron surface with steam at 700°C itduces a dramatic increase in the catalytic activity for carbon deposition from hydrocarbons. Spectroscopic analysis (Auger and Mossbauer) combined with weight increase measurements prove that treatment of iron with steam at 700°C results in the conversion of the surface of the iron to FeO. At 800°C, this process is not just limited to the uppermost surface layers but penetrates to an appreciable depth of the material after a three hour treatment. Indeed Mossbauer spectroscopy data shows that nearly all of a 0.013 cm Fe foil is transformed to FeO in this time at 800°C. It should be mentioned that the reaction of steam with iron to produce FeO may be possible at temperatures above 570°C (3). The nonstoichiometric nature of FeO has been the subject of "a considerable number of papers. It is known, however, that the defects present in this material are vacant cation sites and trapped positive holes (26). 

The kinetics of carbon deposition from benzene over nickel and iron foils was studied at temperatures ranging from 500 to 650°C in atmospheres of hydrogen and mixtures of hydrogen and nitrogen, using a microbalance tubular flow reactor. Carbon deposition proceeds at a constant rate over both metals. The dependence of the rate on the partial pressure of hydrogen can be approximated by a power law kinetics. It is apparently third order over nickel and first order over iron. This behavior can be explained by a mechanism in which the benzene adsorbed on the surface is hydrogenated to produce intermediates. 

In this paper it is shown that the growth of carbon deposits can be maintained for long periods of time in the presence of hydrogen. The effect of hydrogen on the kinetics of carbon formation from benzene in hydrogen has been studied in experiments using nickel and iron foils. The results are presented below. 

Scanning Electron Microscope observations of the deposit structure showed similarities in the morphology both for nickel and iron foils. Figure 6 shows the structure on a nickel foil. Carbon has been formed in a filament-like structure, similar to what has been observed previously in deposits from other hydrocarbons (4y 5). The same type of structure is observed on the iron foils (see Figure 7). 

Figure 4. Effect of hydrogen on the rate of carbon deposition from benzene onto nickel (650°C) and iron (625°C). Nitrogen used as carrier (total pressure — 1 atm). Key 0, Ni foils and , Fe foils.

Figure 7. SEM photograph of carbon deposited on an iron foil showing filamentary growth. Magnification is 9,350x ...

Scanning Electron Microscope (S.E.M.) studies of the iron and nickel foils, after carbon deposition, showed the presence of filamentous growth of carbon on both metallic surfaces, as was expected. This is in agreement with the steady-state rate of carbon deposition observed for both iron and nickel foils. If filaments containing metal crystallites were not formed, the reaction would have to stop at a certain point because of deactivation and poisoning of the metal surface by the deposited carbon. 

Figure 10. Apparent reaction order with respect to hydrogen for carbon deposition rate at constant temperature, constant pressure (1 atm), constant benzene partial pressure on nickel foils (T = 650°C, P = 0.132 atm) and iron foils (T = 625°C, PB = 0.132 atm). Key , Ni foils and , Fe foils.

The core and shell type of particulates are similar to one of the deposit morphologies formed on an Fe-Ni alloy from CO at temperatures above 500°C where the core consisted of a metal particle in the size range 0.09 to 0.2 pm, with a shell thickness typically of 0.04 jjm(23). The structure of the particles, i.e. a carbon layer on metal, is comparable to the laminar film on the metal, suggesting that the carbon in the shell has been precipitated. Free metal particles have not been observed on the iron foils that could serve as active centres for growth directly from the gas phase. Therefore, it must be concluded that a solution-precipitation process plays a part in determining the final morphology of the core / shell particles, but further details of the mechanism of growth cannot be established at present. 

Sacco A Jr., Carbon deposition and filament initiation and growth mechanisms on iron particles and foils, Figueiredo JL, Bernardo CA, Baker RTK, Huttinger KJ eds.. Carbon Fibers Filaments and Composites, Kluwer, Dordrecht, 459-505, 1990. 

The sandwich technique has the advantage that it enables the analytical sample to react in a bath which is probably not yet saturated with carbon, and from which the escape of carbon monoxide is thus not yet impeded by needles of graphite or carbide deposited on the surface of the bath, as frequently occurs in the bath procedure. In addition, a further considerable effect on the release of gas, is obtained by the fact that titanium and zirconium form alloys with platinum or palladium, which results in a large evolution of heat. This causes a sudden and considerable increase in temperature (a flash of light ). The sandwich method is however limited to bath metals with very low oxygen contents, which is e.g. not the case with nickel, cobalt or iron. Therefore, the metal foil required is preferably a piece of platinum or palladium foil of about 50 Mm thick. The oxygen blank values of platinum and palladium in compact form are between 1 and 5 Mg/g. They amount to 5 to 15 Mg/g for the corresponding foils. 

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