Researchers at the University of Bonn and University of Montreal have developed a new type of catalyst and used it in their study to produce methane out of carbon dioxide and water in a highly efficient way using electricity. Methane can be used, for example, to heat apartments or as a starting material in the chemical industry. It is also the main component of natural gas. If it is produced using green electricity, however, it is largely climate neutral. The insights gained from the model system studied by the researchers can be transferred to large-scale technical catalysts. The system could also be used to produce other important chemical compounds. The study was recently published in the prestigious journal "Nature Chemistry."
Many chemical reactions require energy to get started and this energy can be added by, for example, heating the reaction partners or subjecting them to high pressure. "We used electricity as the driving force instead," explains Prof. Dr. Nikolay Kornienko. "By using climate friendly electricity, we can produce, for example, methane that doesn't contribute to global warming."
The researcher recently moved from the University of Montreal to the Institute of Inorganic Chemistry at the University of Bonn. He started his latest study while still in Canada and concluded it in his new home. "The production of methane – which has the chemical formula CH4 – is challenging because it is necessary to carry out a reaction between a gas and a liquid," says Kornienko.
In this case, we are talking about carbon dioxide (CO2) and water (H2O). The researchers used a gas diffusion electrode to bring these two partners together. In the reaction, it is necessary to separate the two oxygen atoms (chemical symbol: O) from the carbon atom (C) and replace them with four hydrogen atoms (H). The hydrogen is sourced from the water.
Preventing side reactions
The problem with this process is that water would much rather undergo another reaction and will split into hydrogen and oxygen as soon as it is exposed to an electric current. "This is a competing reaction that we have to avoid," emphasizes Kornienko's assistant Morgan McKee, who carried out a large proportion of the experiments. "Otherwise, it would stop us producing any methane. Therefore, we have to prevent the water coming into contact with the electrode. At the same time, we still need the water as a reaction partner."
This is where the newly developed catalyst – which is deposited onto the electrode – comes into play. It ensures above all that the carbon dioxide reacts more readily and quickly to produce methane. It achieves this with its so-called "active center" that holds the carbon dioxide and – in simple terms – also weakens the bonds between the carbon atom and the two oxygen atoms.
These oxygen atoms are then gradually replaced by four hydrogen atoms in the next step. The catalyst needs water at this stage of the process. However, it also has to keep it at a distance to avoid any undesired side reactions. "In order to achieve this, we bound long molecular side chains to the active center," explains Prof. Kornienko, who is also a member of the Transdisciplinary Research Area "Matter" at the University of Bonn. "Their chemical structure repels water or, in other words, they are hydrophobic."
Water-fearing molecular chains
This specialist term comes from Greek and literally means "having a fear of water." The side chains not only keep the H2O molecules away from the active center and the electrode but they also act as a sort of conveyor belt. Figuratively speaking, they snatch hydrogen atoms from the water molecules and transport them to the active center, where they react with the carbon atom. In this way, the CO2 is converted into CH4 in several steps.
This process has an efficiency of more than 80 percent and the reaction produces hardly any undesired side products as a result. Nevertheless, the catalyst is not really suitable for the large-scale production of methane. "The reaction principles we have achieved with this catalyst could, however, be realized in other catalyst materials for use in large-scale technical applications," says Kornienko.
The researcher believes that methane production is not the only area of application for this method. In his opinion, it could prove more lucrative in the production of other chemical compounds such as ethylene, which is used as the starting material for many plastics. In the medium term, the new catalyst method could thus be used where possible to make plastic production more environmentally friendly.
Participating institutes and funding:
The Universities of Bonn, Montreal (Canada), Swansea (Wales), Bayreuth, Oulu (Finland), Hohenheim, FU Berlin and the Synchroton SOLEIL in Saint-Aubin (France) participated in the study. The study was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Engineering and Physical Sciences Research Council (EPSRC), the Higher Education Funding Council for Wales (HEFCW) and the Erasmus+ programme from the EU.
Publication: Morgan McKee et. al.: Hydrophobic molecular assembly at the gas-liquid-solid interface drives highly selective CO2 electromethanation; Nature Chemistry; DOI: 10.1038/s41557-024-01650-6, URL: https://www.nature.com/articles/s41557-024-01650-6
