In Situ/Operando STM of the Fischer–Tropsch Synthesis on a Co(101̅15) Surface─A Study to Bridge the Materials Gap between Single-Crystal Models and Supported Catalysts

Abstract

Co(101̅15), a vicinal surface of the Co(0001) surface, has a similar density of steps as the supported Co catalysts used in the industrial Fischer–Tropsch synthesis. We have investigated whether this surface can serve as a model system to bridge the materials gap between single crystals and supported catalysts. Experiments on the Co(101̅15) surface were performed by scanning tunneling microscopy (STM) under in situ/operando conditions and by gas chromatography (GC) with the same sample under the same conditions. X-ray photoelectron spectroscopy (XPS) was used for postreaction surface analysis. In the experiments, 2:1 mixtures of H2 and CO (syngas) were applied at a total pressure of 950 mbar and at a sample temperature of 503 K. The morphology resolved by STM under these conditions showed long, narrow terraces as expected for the Co(101̅15) surface but also step bunching. GC showed the formation of C1 to C4 hydrocarbons in considerably higher amounts compared to reference experiments with a Co(0001) sample. The activity of the Co(101̅15) sample was higher than that of the Co(0001) sample by a factor approximately proportional to the density of atomic steps. It is concluded that atomic steps are the active sites on this highly stepped surface. An analysis of literature data of supported Co catalysts shows that the activities reported in the majority of these studies are comparable to those of the Co(101̅15) sample.