CARBON DEPOSITION ON IRON-NICKEL DURING INTERACTION WITH CARBON-MONOXIDE HYDROGEN MIXTURES

We have found that the composition of Fe-Ni catalysts can have a profo und effect on the activity for the decomposition of CO/H-2 mixtures at 600 degrees C. As the fraction of nickel in the bimetallic is increas ed above 70% there is a significant decline in the percentage conversi on of CO. The amount of solid carbon deposited on a given bimetallic c atalyst was shown to increase as the hydrogen content in the reactant mixture was raised to between 20 to 30% and then exhibited a steady de cline with further addition of hydrogen. It was also evident that the structure and crystalline perfection of the carbon filaments that are produced during the reaction were very sensitive to the ratio of the t wo components in the catalyst. Temperature programmed oxidation and TE M studies of carbon filaments grown from various Fe-Ni bimetallic part icles demonstrates that as the nickel content of the catalyst is incre ased there is a concomitant decrease in the crystalline perfection of the deposited carbon structures. These features are rationalized accor ding to the notion that the addition of nickel to iron results in a re construction of the particle surfaces that leads to a diminution in th e likelihood of iron atoms being nearest neighbors and the generation of atomic arrangements at carbon precipitating faces that do not favor the formation of graphite platelets in the deposited filamentous stru ctures. A reversible deactivation process was shown to exist for all t hese Fe-Ni catalysts. At temperatures of about 725 degrees C, the grow th of carbon filaments suddenly ceased; however, restoration of cataly tic activity could be readily achieved by lowering of the temperature to a previously active regime. Possible reasons for this unusual behav ior are presented. (C) 1997 Academic Press.