Activation energy carburization

An additional series of measurements were carried out under isothermal conditions at 1 atm (Figure 2). For these experiments, carburization of the hydrogen-reduced catalysts was limited to a period less than six hours. The data, analyzed in terms of the parabolic rate law, exhibit two distinct regions of carburization (Figure 3). The parabolic rate constants were calculated from the initial and final slopes (Table III). The parabolic rate constants yield the activation energies and preexponential factors summarized in Table IV. 

Table IV. Activation Energy and Preexponential Factor for Mass Gain during Isothermal Carburization of Catalysts B-2 and B-6...

The rates of these various steps will depend on carburization conditions and catalyst composition. That the activation energy for mass increase during carburization is higher for catalyst B-2 than for B-6 (Table IV) may be associated with the greater amount of Si02 in B-2. 

Iron is the industrial Fischer-Tropsch catalyst and is applied in practice. Reduced iron interacts strongly with carbon. Because the activation energy for carbon diffusion into the metallic iron lattice is low (40-65 kJ/mol), the metal converts to iron carbides during reaction. According to Niemantsverdriet et al, the initial rate of the Fischer-Tropsch reaction is low because the carburization process consumes most of the carbon. When the iron particles become saturated, carbon stays at the surface where it is available for the actual Fischer-Tropsch reaction. Molybdenum is also converted to a carbide or an oxide when exposed to synthesis gas in this state it is an active catalyst. However, the early transition metals form stable but unreactive compounds in synthesis gas and are inactive as Fischer-Tropsch catalysts. 

Reactions of carbon in alkali metals with carbide forming metallic elements are the driving processes of the carburization of stainless steels. The direction of the carbon exchange between the molten metals and the solid metallic materials depends on the carbon potential in the liquid metals and on the free energy of formation of the metal carbides and the chemical activity of the metallic element in the solid phase. 

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