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 analysis of both gas product distribution and the amount of solid carbon formed, it was possible to follow changes in the fraction of cobalt atoms present in the surface of the bimetallic catalyst as a function of time, temperature, and gas composition. It was found that, although pure cobalt exhibited a low activity for the decomposition of ethylene toward carbon deposition, the presence of as little as 2% copper in the particles produced a dramatic enhancement in the activity of the catalyst for this reaction. Using the total product data in conjunction with a previously developed model it has been possible to estimate the most active state of the bimetallic catalyst to be one where the surface contains approximately 80% cobalt atoms. The activity of the bimetallic catalyst was extremely sensitive to the temperature; at 725 degrees C or higher, the system underwent deactivation. When hydrogen was added 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 temperature to a previously active regime immediately restored the activity of the catalyst to its original level. This finding demonstrates that deactivation via the formation of a graphitic overlayer is not operative in this system. Examination of the solid deposits by transmission electron microscopy revealed that carbon filaments were the major component; however, significant differences were evident in the physical ad chemical characteristics of the structures formed from cobalt and copper-cobalt particles. The material produced from the metal catalyzed decomposition of ethylene was created by a whisker-like growth mode and was highly crystalline in nature. In contrast, the structures generated by the bimetallic system consisted of many filament limbs emanating from 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.