Nickel foils, carbon deposition

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). 

The amount of carbon deposited on nickel foils following reaction in ethane at atmospheric pressure and temperatures from 870 to 1070 K is presented in Table 1. Examination of these data shows that titanium oxide provides an effective barrier towards carbon deposition at 870 K with or without hydrogen pretreatment however, at higher temperatures only titanium oxide reduced in hydrogen exhibits an inhibiting effect. 

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. 

Previous results from studies on the deposition from benzene (12) and other hydrocarbons (13) in the absence of hydrogen indicate that the reaction terminates after a fixed amount of carbon has been incorporated into the metal foil. Not surprisingly, the amount of carbon uptake by nickel foils corresponds to the solubility of carbon in nickel. 

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. 

Figure 2 shows the effect of temperature and benzene concentration on the kinetics of carbon deposition on a nickel foil. The temperature behavior follows a pattern that has been previously observed in the catalytic carbon formation from non-aromatic hydrocarbons (1 ,2). There are three regions in the Arrhenius plot. At low temperatures, the rate increases with increasing temperature and, thus, a negative slope-line is obtained in the Arrhenius plot. This low temperature region is denoted as Region I. 

Figure 2. Arrhenius plot for the rate of carbon deposition from benzene onto nickel foils (total pressure — 1 atm). Key to P c c e. O 0.132 atm and , 0.091 atm.

Figure 6. SEM photograph of carbon deposited on a nickel foil showing filament growth. Magnification is 9,350/..

Figure 8. X-Ray diffraction patterns of carbon deposited on a nickel foil. Key top, after deposition and bottom, before deposition (clean foil).

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.

Nickel was electrodeposited on polished brass foil at different temperatures (Ni deposit roughness with temperature). Also, Martis et a/. reported that compact nickel multiwalled carbon nanotube composites eould be electrodeposited on a copper substrate using the eutectic mixture ChCl/EG as solvent. Later, electrodeposition of Ni-P alloys (with tunable phosphorous eontent) at room temperature was reported by You and co-workers. ... 

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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