Could the inert greenhouse gas CO² be activated using lasers, LEDs or sunlight so that it can be used as a raw material for chemical products and synthetic fuels? Researchers all over the world are looking into this. Between 2010 and 2016, the German Federal Ministry of Education and Research (BMBF) provided EUR 100 million to subsidize research projects in this area. A second phase of the “CO²Plus” scheme with a EUR 17.5 million subsidy has been under way since 2016. In addition to efficient CO² separation and its use as a building block for chemical raw materials, it focuses on projects connected with the photocatalytic activation of the gas. The researchers carrying out the projects are looking for suitable combinations of light and catalysts in order to simulate natural photosynthesis in a technical manner.
The central question is whether the amount of energy used to activate the inert gas can be reduced to a point where the use of CO² as a material would have a positive effect on the climate. A project entitled “CarbonCat” is developing a microstructured reactor system in which high-performance LEDs and a carbon-based photocatalyst system can be used for photosynthesis. Researchers from the Microengineering and Microsystems department (ICT-IMM) at the Fraunhofer Institute for Chemical Technology (ICT), the University of Würzburg and Sahlmann Photochemical Solutions hope to achieve technical photosynthesis with sunlight. Instead of the chloroplasts in plant cells, the microreactor uses a diamond photocatalyst. In the reactor, CO² and water are continually mixed and exposed to light. The aim is to optimize the interaction of light and the catalytic process and modify the photocatalyst surfaces in order to maximize the chemical yield. This is the only way for technical photosynthesis to convert CO² into valuable C1 chemical building blocks such as methanol with minimal energy use.
A second project entitled “PROPHECY” is looking for ways of converting CO² into methane, methanol or syngas using sunlight. For researchers at the Leibniz Institute for Catalysis in Rostock, the University of Oldenburg, the Karlsruhe Institute of Technology and Siemens AG, one thing is crucial: The process must make ecological and economic sense. This is where the research carried out over the last three decades has failed. As an end product of combustion processes, CO² can only be activated to allow further chemical reactions if a great deal of energy is used. New material and process concepts are needed in order to establish a climate-neutral CO² recycling system. Researchers know that they need to increase the yield from photocatalytic CO² reduction by several orders of magnitude if they are to achieve this aim. In order to do this, they add additives such as bioethanol or regeneratively produced hydrogen to an inert mixture of CO² and water and try new photocatalysts other than titanium oxide (TiO²) or zinc oxide (ZnO).
Researchers from the Institute of Physical and Theoretical Chemistry at the University of Bonn recently publicized a promising observation in the specialist journal “Angewandte Chemie”. Using ultrashort UV laser pulses, the team headed by Prof. Peter Vöhringer has managed to make the inert greenhouse gas more reactive, albeit on a laboratory scale. An iron complex with positively charged iron atoms to which the CO2 components were bound in multiple fashion was at the heart of the process. The laser pulses break up these bonds, forming carbon dioxide radicals. These radicals have a single electron on the outer shell which has a high bonding affinity to other molecules or atoms. If this type of activation could be achieved on a large scale, it would be a huge step toward achieving a CO2 recycling system.
Incidentally, photonics plays a key role for the research team in Bonn not only in CO2 activation but also in proving the existence of the radicals. With the help of infrared spectrometry with high temporal resolution in the order of femtoseconds, the team was able to prove the formation of the carbon dioxide radical using the changed vibration spectra of this highly reactive molecule.