MODIFICATION OF THE CATALYTIC BEHAVIOR OF COBALT BY THE ADDITION OF COPPER

The interaction of ethylene and ethylene/hydrogen mixtures with cobalt and copper-doped cobalt has been studied using a combination of flow reactor and transmission electron microscopy techniques. From the anal ysis of both gas product distribution and the amount of solid carbon f ormed, it was possible to follow changes in the fraction of cobalt ato ms present in the surface of the bimetallic catalyst as a function of time, temperature, and gas composition. It was found that, although pu re cobalt exhibited a low activity for the decomposition of ethylene t oward carbon deposition, the presence of as little as 2% copper in the particles produced a dramatic enhancement in the activity of the cata lyst for this reaction. Using the total product data in conjunction wi th a previously developed model it has been possible to estimate the m ost active state of the bimetallic catalyst to be one where the surfac e contains approximately 80% cobalt atoms. The activity of the bimetal lic catalyst was extremely sensitive to the temperature; at 725 degree s C or higher, the system underwent deactivation. When hydrogen was ad ded to the reactant gas, the lifetime of the catalyst was extended up to 800 degrees C before the deactivation step ensued. This phenomenon was shown to be reversible in nature, since a lowering of the temperat ure to a previously active regime immediately restored the activity of the catalyst to its original level. This finding demonstrates that de activation via the formation of a graphitic overlayer is not operative in this system. Examination of the solid deposits by transmission ele ctron microscopy revealed that carbon filaments were the major compone nt; however, significant differences were evident in the physical ad c hemical characteristics of the structures formed from cobalt and coppe r-cobalt particles. The material produced from the metal catalyzed dec omposition of ethylene was created by a whisker-like growth mode and w as highly crystalline in nature. In contrast, the structures generated by the bimetallic system consisted of many filament limbs emanating f rom a single multifaceted catalyst particle. It was also apparent that the introduction of copper into the cobalt particles gave rise to an more disordered carbon structure.