From greenhouse gas to green fuel
The expansion of renewable energies will increasingly lead to temporary surpluses of electricity. The ability to store these surpluses is therefore of great importance.
This can be done with battery systems, for instance, but also with the Power-to-X concept. This concept uses surplus energy to synthesize chemical substances such as hydrogen, methanol, and ammonia, which are then used as energy carriers or raw materials. Another useful storage medium would be methane, which could replace fossil natural gas, for example. It can be produced via a reaction of hydrogen and CO2, known as methanation. The use of carbon dioxide from industrial waste gases would also prevent CO2 emissions and keep the carbon in the industrial material cycle. However, methanation releases a lot of heat, causing the temperature in the reactors to rise sharply, which would inactivate the nickel used as a catalyst. This would cause the reaction to stop. For this reason, there are currently no large-scale plants for methanation. The team led by Kai Sundmacher, CDS member and Director of the Process Technology Department at the Max Planck Institute for Dynamics of Complex Technical Systems in Magdeburg, developed the idea of so-called core-shell catalyst pellets further.
"With this structure consisting of a catalytically active core and an inactive shell, it is possible to limit the reactor temperature and thus create the basis for large-scale methanation of carbon dioxide," explains Kai Sundmacher. According to Ronny Tobias Zimmermann, a chemical engineer in Sundmacher's team, the inert porous shell of the core-shell catalyst pellets ensures diffusion of the reaction partners through the shell, which slows down the conversion rate and thus also a potentially excessive rise in temperature. Computer simulations and subsequent experiments were used to determine the optimum properties and dimensions of the core-shell catalyst and the reactor.
The Max Planck researchers' approach should not only be suitable for the reaction of hydrogen with carbon dioxide. In general, the concept of tailor-made core-shell catalysts can be applied to all gas reactions with high heat generation.