Operando STM of Fischer-Tropsch model catalysts
Final Report Abstract
Scanning tunneling microscopy (STM) is one of the few surface analysis techniques that is not restricted to vacuum but can be used at elevated pressures. It can therefore provide access to the atomic surface structure of operating catalysts. By combining such experiments with measurements of the catalytic activity one could identify active sites. However, such correlations have not been established previously, a result of considerable experimental difficulties. In this project, a complex, industrially applied reaction was investigated, the Fischer-Tropsch synthesis. The reaction is used to produce hydrocarbons from mixtures of H2 and CO, in the modern version by applying supported Co catalysts. In the project, experiments were performed with Co single crystals as model catalysts. The project required to solve several experimental problems. At the enhanced CO pressures, Ni(CO)4 was formed that decomposed on the samples, leading to a contamination by Ni. This problem could be solved by covering the entire reaction cell and most of the components in the cell with amorphous Si. The second major problem, the detection of the extremely low concentrations of Fischer-Tropsch products, was solved by means of a gas chromatograph (GC) for which a special gas sampling unit was developed. Experiments were performed with a Co(0001) sample, furthermore with a Co(0001) sample that was sputtered to enhance the density of steps, and with a Co(101 ̅ 15) sample that has a yet higher density of steps. In the STM eperiments atomic resolution was regularly achieved at syngas pressures of ~1 bar and at ~500 K. According to the GC measurements, the samples were Fischer-Tropsch active under these conditions. Turnover frequencies (TOFs) could be determined. Surprisingly, the STM images recorded under reaction conditions showed that the surface structures were unchanged with respect to the structure under ultra-high vacuum (UHV). It had been expected that the surfaces would become rough during the reaction, a wide-spread hypothesis that explained the formation of active sites. This hypothesis was disproven. However, the active sites could nevertheless be identified. The STM images allowed us to determine the densities of atomic steps on the various single crystal samples. It was found that the TOFs scaled linearly with the densities of steps. Steps were thus identified as active sites. This fact is consistent with a reaction mechanism of the Fischer-Tropsch synthesis according to which the dissociation of CO molecules is strongly favored at step sites. Moreover, the step density on the Co nano particles of the supported catalyst is almost identical to the step density of the Co(101 ̅ 15) sample. An analysis of a large number of literature studies of supported Co catalysts showed that the average TOFs agree quite well with the TOFs from the Co(101 ̅ 15) sample. The atomic steps on the Co particles are sufficient to explain the activity, and no roughening is required. Additional STM experiments were performed to obtain insight into the structure of the adsorption layer on the Co model catalysts. Because of the high mobility of the adsorption layer at the reaction temperature an indirect approach was chosen. The temperature was reduced to 300 K and the pressure was varied over 12 orders of magnitude. A series of ordered and disordered CO structures were observed and the resulting phase diagram was converted to the enhanced temperature of the reaction. It was concluded that under reaction conditions the surface is covered by a disordered layer of CO molecules.
Publications
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The active sites of a working Fischer–Tropsch catalyst revealed by operando scanning tunnelling microscopy, Nature Catal. 2, 1027 (2019)
B. Böller, K. M. Durner, J. Wintterlin
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High-pressure CO phases on Co(0001) and their possible role in the Fischer–Tropsch synthesis, ACS Catal. 10, 12156 (2020)
B. Böller, P. Zeller, S. Günther, J. Wintterlin
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A highly sensitive gas chromatograph for in situ and operando experiments on catalytic reactions, Rev. Sci. Instrum. 92, 124103 (2021)
K. M. Golder, B. Böller, G. Stienen, J. Sickerling, J. Wintterlin
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Method for the manual analysis of moiré structures in STM images, ChemPhysChem. 22, 870 (2021)
S. Günther, P. Zeller, B. Böller, J. Wintterlin
