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.
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The data that support the findings of this study are presented within the text and the supporting information or are available from the corresponding author upon reasonable request.
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Supplementary Tables 1 and 2, Supplementary Figs. 1–4 and Supplementary References.
Time lapse of 18 STM images recorded during a period of 80 min on the same area of the sample. Conditions were a total pressure of 200 mbar syngas, an H2:CO = 2:1 mixture, and a sample temperature of 493 K. The first frame was taken 3.5 h after heating the sample to 493 K. Images 3 and 8 are displayed in Fig. 3(a) and (b) of the main text. The video was corrected for a small thermal drift. One can see that the step positions fluctuate, demonstrating the high mobility of cobalt atoms under the reaction conditions (200 × 200 nm2, Vt = −0.5 V, It = 0.7 nA).
Time lapse of 18 STM images recorded during a period of 25 min. The data were taken under 950 mbar syngas (H2:CO = 2:1) and at a sample temperature of 493 K. The first frame was taken 3 h after heating the sample to 493 K. The fringes of the steps are caused by cobalt atoms diffusing along the steps or exchanging between steps and terraces at a faster rate than the rate of the scan lines (7 Hz) (these effects are still slow compared to the 10−13 to 10−12 s time scale of elementary chemical processes, such as the dissociation of a CO molecule, which thus ‘see’ a static step) (18× 18 nm2, Vt = −0.5 V, It = 0.7 nA).
Time lapse of 19 STM images recorded during a period of 2.5 h. The data were recorded on the sputtered Co(0001) sample in 950 mbar syngas (H2:CO = 2:1) and at a sample temperature of 493 K. The first frame was taken 80 min after heating the sample to 493 K. The video shows that several of the topmost terraces shrink and at the same time that lower cobalt layers become partially or completely filled (90 × 90 nm2, Vt = 0.05 V, It = 0.7 nA).
Time lapse of the 19 STM images from Supplementary Video 3 showing the differences between successive images. Image no. 5 served as a reference image and was subtracted from all images to highlight the morphological changes (images no. 1 to no. 4 were not suitable as references because in this phase of the experiment the thermal drift was still too strong). By this subtraction, areas in images no. 6 to no. 19 where Co terraces grow by adding Co atoms to the steps appear bright, and areas where Co atoms are removed from the steps appear dark. Accordingly, image no. 5 itself appears homogeneously grey, and in images no. 1 to nos. 4 the bright/dark contrast of growing and shrinking terraces is inverted.