Mechanisms of carbon formation on nickel-containing catalysts

Earlier studies of the mechanism of carbon formation on nickel are reviewed. A complex dependence of rate upon temperature is observed in many systems. Different explanations for the existence of a rate maximum and an apparent negative activation energy are discussed. The observations are shown to be best explained in terms of adsorption effects, although other effects may influence the magnitude of the observed activation energy. Another aspect of the mechanism is the method of transportation of metal particles with the surface of the growing carbon. Temperature driven dissolution-precipitation of carbon or surface migration of carbonaceous species appear less probable than a mechanism involving diffusion of carbon through nickel under a concentration gradient.