To date, natural oil still serves as our primary source of fuel, even though a much cleaner alternative exists in the form of synthetic fuel. This contains much less sulfur and doesn't require oil as the starting material. At the moment, only five percent of the world production of diesel fuel makes use of this process, because it is cheaper for companies to use more polluting substances. If researchers have a better understanding of the production process of synthetic fuel, the balance could tip the other way.
Now, physicists from Leiden University have seen for the first time how this chemical process unfolds in the early stages. It was already known that the necessary chemical reaction between carbon monoxide and hydrogen takes place on the surface of small cobalt particles. These serve as the catalyst for the reaction. It is however very difficult to verify the exact working mechanism in experiments. Researchers have to deal with pressures of several atmospheres and temperatures of several hundred degrees Celsius. These are far from ideal circumstances to observe molecules. Group leader Joost Frenken and his team developed a special type of Scanning Tunneling Microscope -- a so-called Reactor-STM -- to bypass this problem.
To their surprise, they observed that in the first stages of the process, the surface covers itself up progressively in a single layer of hydrocarbon molecules with a highly ordered regular pattern. The molecules accumulate on the cobalt surface all with the same, surprisingly long length. The Leiden physicists were able to explain these findings with a simple theory, in which the catalyst constructs the molecules step-by-step at the atomic steps on the cobalt surface. Most molecules spend some time on the surface and then evaporate, but the longer ones stick more strongly and fill the surface. The most efficient way to do that is to fill the surface with a regular pattern, similar to cars in a parking lot.
Currently, catalysts are being developed mostly by trial and error. With the new discovery, first author Violeta Navarro and her colleagues pave the way for future generations of genuine 'designer' catalysts, with fully optimized efficiency and selectivity for the desired products. Frenken: 'The ultimate goal is that of true "designer catalysts." We're absolutely not there yet, but understanding the early stages of synthetic fuel production forms an essential component of unraveling the entire, complex reaction mechanism. We have introduced a new way of looking at an active catalyst with the ultimate resolution.'
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