The active sites of a working Fischer-Tropsch catalyst revealed by operando scanning tunnelling microscopy

Direct identification of the active sites of a working catalyst is still a major problem in heterogeneous catalysis. Here we present an operando scanning tunnelling microscopy study, in which insight into the nature of the active sites was obtained for the cobalt-catalysed Fischer-Tropsch synthesis. Experiments were performed on a Co(0001) sample under H-2/CO gas mixtures at pressures of up to 950 mbar and a temperature of similar to 500 K. On the same apparatus, turnover frequencies were measured with a customized gas chromatograph. The density of monoatomic steps of the sample was varied by sputtering. The Fischer-Tropsch activity scaled with step density, from which steps are identified as the active sites of this reaction. The long-standing idea that the activation of the Co catalyst is connected with a roughening of the surface is not confirmed. The known activity function can be explained by pre-existing steps without roughening.