Bulk Carbide Formation in the FTS

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 

Early work at the Bureau of Mines on Co/Th02/kieselguhr catalysts showed that bulk carbide was not an intermediate in the FTS, nor was it catalytically active.82 Excessive amounts of carbides, produced by CO exposure prior to the 

Advances in Fischer-Tropsch Synthesis, Catalysts, and Catalysis 

Recent work done by Xiong et al.84 on Co/AC (activated carbon) catalysts showed that a Co2C species formed during the catalyst reduction in hydrogen at 500°C. Evidence for the carbide in the Co/AC catalysts was obtained by x-ray diffraction and XPS measurements, and the formation of this Co2C species reduced the FTS activity over the Co-based catalysts. The presence of bulk carbide also seems to enhance alcohol selectivity.85 

Several workers have reported that bulk carbide does not form readily during normal FTS conditions.76 82 Bureau of Mines work, using laboratory XRD measurements, showed that detectable amounts of bulk carbide were not formed under synthesis conditions.82 

---