Already in 1896, Svante Arrhenius discussed in his work the impact of atmospheric carbon dioxide (CO₂) on the greenhouse effect. According to his calculations, Arrhenius stated even back then the correlation of CO₂ content in the atmosphere and increase of Earth’s temperature.^I Nowadays concerns regarding greenhouse gases, particularly CO₂, and global warming affect politics, economy and society. In comparison to other greenhouse gases such as methane (CH₄) and water vapor, CO₂ has the highest impact on global warming, as the atmospheric residence time is the highest and the content in the atmosphere is moreover on second place after water vapor.^II,III

CO₂ is generated from the combustion of fossil carbon (oil, gas, coal) and biomass in which energy is released. Due to the finite reserve of fossil-C, another issue is now rising: the convenience to recycle carbon, more than to release it to the atmosphere or dispose it underground. Based on these facts primarily utilization of CO₂ and substitution of fossil fuels as energy carriers have become some of the most discussed topics and have especially drawn attention in scientific community.^IV

Carbon Dioxide as Chemical Feedstock

To reduce atmospheric CO₂ generally two approaches comprising different techniques are considered. In the Carbon Capture and Sequestration (CCS) approach CO₂ is stored in deep rock cavities under sea and land.^V Differently, in the Carbon Capture and Utilization (CCU) approach CO₂ is regarded as a carbon feedstock and starting material for artificial fuels and chemicals. With this strategy both issues, depletion of fossil fuels and reduction of CO₂ in the atmosphere, are taken into account.

From the chemical point of view carbon dioxide is a highly stable molecule with the carbon atom. To produce energy rich chemicals from CO2, energy and hydrogen are necessary. Both of them must be generated from renewable energies such as sun, wind, hydropower, geothermal energy. This is a must to achieve CO2 neutrality of such synthetic fuel cycles.

^IS. Arrhenius, Phil. Mag. 1896, 41, 237-276.
^IIH. Craig, Tellus 1957, 9, 1-17.
^IIIB. Weinstock, Science 1969, 166, 224-225.
^IVC. D. Keeling, Geochim. Cosmochim. Acta, 1958, 13, 322-334.
^VR. S. Hazeldine, Science 2009, 325, 1647-1652.