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

Abstract

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 H2/CO gas mixtures at pressures of up to 950 mbar and a temperature of ~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. Visualizing catalysts at work poses significant experimental challenges. Here, an operando scanning tunnelling microscopy study of the cobalt-catalysed Fischer–Tropsch synthesis is presented, which establishes steps as the active sites of the process by correlating activity under realistic conditions with step density.