An STM project to overcome the remaining pressure gap for the Fischer-Tropsch synthesis

• Applicant: Professor Dr. Joost Wintterlin
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 552794718

Despite considerable progress with surface analytical methods that aim at bridging the pressure gap of heterogeneous catalysis, most investigations are still performed under restrictions, in particular when single crystals model catalysts are investigated. In most cases, pressures do not exceed 1 bar, often relatively simple, model-like reactions are investigated, and there are not many cases where catalytic turnover numbers are determined in the sense of "operando" experiments. Here we propose a project that aims at overcoming the remaining restrictions for the Co-based Fischer-Tropsch synthesis, an industrial process that forms hydrocarbons from H2/CO mixtures (syngas). The proposal is based on a previous project in which scanning tunneling microscopy (STM) has been used to record atomically resolved images of Co single crystal surfaces at a syngas pressure up to ~1 bar and at temperatures of ~500 K. With a special gas chromatograph (GC) Fischer-Tropsch products were detected under these conditions. The previous project aimed at resolving a predicted pressure gap effect, a surface roughening caused by the formation of Co-islands, which could explain the activity of supported Co catalysts. However, our experiments showed that such an effect does not exist under the applied conditions. This opens the question whether the interpretation of previous data was incorrect, or whether the maximum syngas pressure of ~1 bar applied in our experiments was still too low. There is some evidence that the state of the surface may, in fact, change between ~1 bar and 4 - 5 bar. We propose to solve this question by an operando STM project at higher pressures. For this purpose, the existing STM setup will be partially redesigned by replacing components of the current setup that do not withstand an internal overpressure. Gas mixing will be achieved by recirculation. A residual source of Ni tetracarbonyl, which may lead to an increased Ni contamination during reactions, has been identified and will be removed. With these modifications, STM experiments will be performed at syngas pressures of 4 - 5 bar and reaction temperatures of ~500 K. When the mentioned evidence is valid, it may not be necessary to increase the pressure to the industrial value of 20 - 40 bar to detect pressure gap effects. We expect that any pressure gap effects occurring under the chosen conditions, e.g., the mentioned roughening or the formation of a Co carbide layer, can be imaged with atomic resolution. Reaction products will be analyzed with the existing GC. In this way, all parameters that characterize Fischer-Tropsch catalysts, namely the overall activity, the chain growth probability, and the olefin-to-paraffin ratio, will be correlated with the state of the surface. X-ray photoelectron will be used for post-reaction surface analysis. The project offers the chance to overcome the remaining pressure gap for an actual industrial process.

• DFG Programme: Research Grants