Kinetics of the Fischer–Tropsch Synthesis on Cobalt Single Crystals under Scanning Tunneling Microscopy Control

Previous studies of the Fischer–Tropsch synthesis on single crystal Co samples revealed a correlation between the catalytic activity and the density of atomic steps. This had been taken as evidence that the steps are the active sites for this reaction, most likely by facilitating the dissociation of the CO molecules. In this study, we have tested this conclusion by investigations of the kinetics. Apparent activation energies and hydrocarbon selectivities have been measured on Co(0001) and Co(101̅15) samples over a temperature range between 473 and 513 K, at a total pressure of 950 mbar and at a H2:CO ratio of 2:1. The Co(101̅15) sample has a higher density of steps than the Co(0001) sample, and the turnover frequency is higher, but the apparent activation energies for CO consumption are almost identical, approximately 100 kJ mol–1, on the two surfaces. This finding is strong evidence that the chemical processes are identical and that the different activities only result from the different densities of atomic steps. Scanning tunneling microscopy data taken under the same reaction conditions show metallic surfaces with the same morphologies as in ultrahigh vacuum over the entire temperature range. The kinetic quantities therefore refer to systems that are defined to a high degree. X-ray photoelectron spectra taken after the reactions confirm this conclusion. The apparent activation energies agree with simulations according to which atomic steps represent dissociation sites for the CO molecules. They also agree with many studies on supported Co Fischer–Tropsch catalysts that additionally display similar peculiarities of the selectivities.