Active Sites for Heterogeneous Catalysis

The catalytic decomposition of carbon monoxide on (100), (110), and (111) monocrystalline nickel thin films has been studied. Reacted nickel films and extraction replicas were examined by transmission electron microscopy to determine the influence of orientation, lattice defects, and topographical imperfections on the catalytic activity of these films. The order of decreasing catalytic activity, determined by the rate of graphite deposition, was (111) > (110) > (100). The results on (100) films indicated that a Ni–C phase was formed initially and subsequently decomposed to give polycrystalline graphite in nickel‐depleted regions, generally associated with lattice defects or strain paths along $\text{〈001〉}$ directions. On (110) and (111) films, oriented graphite deposits were found on the nickel surfaces, and the results indicated that nucleation occurred at active sites for the catalytic reaction with no intermediate Ni–C phase detectable. No correlation was found between lattice defects and graphite nucleation sites. These nucleation sites in general corresponded to specific steps and kink sites on the nickel surface. It was proposed that the important factors governing the activity of steps for a given catalytic reaction included the step height and the nature of the step base and step face.