Abstract
The nature of the carbon deposits formed on an alumina-supported nickel catalyst have been characterized by their reactivity with H2 and H2O during temperature-programmed surface reaction (TPSR). Carbon deposits formed by exposure to hydrocarbons and carbon monoxide exhibit, depending primarily on the temperature during deposition, seven reactive carbon states during TPSR with 1-atm H2, including two very reactive states of chemisorbed carbon, a carbon film, nickel carbide, and two types of filamentary carbon. Filamentary carbon was identified by transmission and scanning electron microscopy, and Ni3C was identified from x-ray diffraction measurements. These results suggest that the deactivation of nickel catalysts is due to the accumulation of a carbonaceous film at low temperature, the rapid formation of filamentary carbon at moderate temperatures, and the formation of encapsulating carbon layers at very high temperature.
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