Catalysts: Reactions on the edge
Model of an atomic step on a catalyst surface. At such sites on a cobalt catalyst, industrial Fischer-Tropsch synthesis of diesel fuel is discontinued. (Picture: J. Wintterlin / LMU)
From chemical production to exhaust gas purification and the chemical storage of solar energy: Without catalysts, many technical processes would not be possible. In the chemical industry, the vast majority of products produced come into contact with at least one heterogeneous catalyst. It is a solid on the surface of which gaseous substances adsorb and react. The catalyst allows or accelerates their reaction to the product without changing themselves. In this process, there are still many unanswered questions, such as at what points of the catalyst, the events actually takes place. Chemists led by Professor Joost Joost Wintterlin from the Chemistry Department at LMU now show that steps on the catalyst surface play a crucial role. They report on their results in the journal Nature Catalysis.
For many heterogeneously catalyzed reactions, there are indirect indications that not all of the catalyst surface is active, but only sites with defects, such as the corners and edges of the catalyst particles, rather than the smooth surfaces in between. "Whether these sites really are the active centers, could not be shown directly, because under reaction conditions, that is, at gas pressures of several bar and elevated temperatures, it is very difficult to analyze the chemical processes on the surface," Wintterlin says.
With his team, Wintterlin has been working for some time on the development of a special scanning tunneling microscope, which can be used to investigate catalytic reactions on surfaces under near-industry conditions. Instead of the catalyst particles, which are often only a few nanometers in size, the scientists use crystals several millimeters in size. In the work that has now been published, scientists also determined the formation of catalytic reaction products on the same sample under the same conditions. "Only in this way we can detect correlations between the structural elements of the surface and the catalytic activity mapped in the microscope," Wintterlin says. "This combination makes the experiment especially difficult." A specially designed gas chromatograph, with which extremely low product concentrations can be detected, eventually led to success.
As an example of their investigations, the scientists chose the Fischer-Tropsch synthesis, a large-scale process in which synthesis gas is used to produce liquid hydrocarbons on a cobalt catalyst, such as synthetic diesel. For this system, the researchers were able to show that the catalytic activity of the sample increased the more atomic steps are located on the surface of the cobalt crystal used as a catalyst. The steps arise because the atomic layers of the crystal are incomplete at the surface. At the point where one layer ends, a step is created to the next layer. Such steps also exist on the surfaces of the small cobalt particles of the industrial catalyst, and its activity could be quantitatively predicted with the model catalyst data. "This is the first direct evidence that these atomic steps are the active centers of the catalyst," says Wintterlin. These results could help, the researchers hope to develop more effective catalysts. (Nature Catalysis 2019)