Cobalt carbide, decomposition

The formation of bulk cobalt carbide is quite a slow process since it requires the diffusion of carbon into the cobalt bulk. It was reported that the full conversion of unsupported and reduced Co to Co2C only occurred after 500 h of exposure to pure CO at 230°C. Increasing the reaction temperature resulted in a faster rate of carburization.81 Bulk cobalt carbides are considered to be thermodynamically metastable species, and therefore Co2C will decompose to hep cobalt and graphite, while Co3C will decompose to fee cobalt and methane. Thermal decomposition of bulk carbides under an inert atmosphere is believed to occur at 400°C.81 Hydrogenation of the bulk carbides is believed to be a fast process and occurs around 200°C.82 83... 

The second study to be described has somewhat more complexity. This study which is also due to Hofer and his associates at the U. S. Bureaii of Mines, concerns the isothermal decomposition of cobalt carbide in a Fischer-Tropscl catalyst. The carbide, CojC, had previously been reported (Hofer and Peebles, 49, 55). At moderately elevated temperatures it undergoes the reaction... 

Hofer, L. J. E., Cohn, E. M., and Peebles, W. C. 1949. Isothermal decomposition of the carbide in a carburized cobalt Fischer-Tropsch catalyst. J. Phys. Coll. Chem. 53 661-69. 

Hofer et al. [29] used a magnetic method to measure the isothermal kinetics of the decompositions of cobalt and nickel carbides. The reaction of CojC was zero-order (0.20 < a< 0.75) with Z , = 226 kJ mol in the range 573 to 623 K, and became deceleratory thereafter. The behaviour of NijC differed in that there was a short induction period, but there was again a period of constant rate (0.3 < ar < 0.9) with , = 295 kJ mol between 593 and 628 K and the final period was deceleratory. There was no evidence for the intervention of a lower carbide. The mechanisms of these reactions were not discussed. 

Carbided cobalt reacts with hydrogen at synthesis conditions. However, the product of the conversion is methane only (113). Iron carbides are very stable against hydrogen at synthesis conditions. The decomposition of iron carbide with acids yields higher hydrocarbons in addition to methane (114). 

Cobalt molybdenum carbide was also found to be active as a catalyst for the methane decomposition for hydrogen production. 

Bimetallic Co(Ni)-Mo carbides can also be synthesized due to the decomposition of precursors (metal-hexamethylenetetramine complexes) in an inert atmosphere [9, 10], This is a simple one-stage method of the formation of double C03M03C and CogMogC carbides [11], In [12], the Co Moi j oxides prepared from aqueous solutions of cobalt nitrate and ammonium heptamolybdate were carbonized in a flow of pure methane and hydrogen (20% CH4/H2 mixture) to form double carbides. 

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