Conversion of CO2 to hard carbon is an interesting technology for the removal of carbon dioxide from the atmosphere. Recently, it was shown that CeO2 can selectively catalyse this reaction but we still lack information regarding the reaction mechanism. Using density functional theory (DFT) modelling we explore possible reaction mechanisms that allow for the polymerization of CO2. According to our computations the reaction is initialized by the adsorption of CO2 in an oxygen vacancy. Owing to the rich defect chemistry of ceria a large number of suitable sites are available at the surface. C-C bond formation is achieved through an aldol condensation type mechanism which comprises the electrochemical elimination of water to form a carbene. This carbene then performs a nucleophilic attack on CO2 . The reaction mechanism possesses significant similarities to the corresponding reaction in synthetic organic chemistry. Since the mechanism is completely generic it allows for all relevant steps of the formation of hard carbon like chain growth, chain linkage and the formation of side chains or aromatic rings. Surprisingly, ceria mainly serves as an anchor for CO2 in an oxygen vacancy while all other subsequent reaction steps are almost completely independent from the catalyst. These insights are important for the development of novel catalysts for CO2 reduction and may also lead to new reactions for the electrosynthesis of organic molecules.
The SI includes a detailed summary of all considered mechanisms, a description of the computational methods, a rational for using GGA functionals for CeO2 and the PDOS.