Study on the splitting mechanism of H2O/CO2 on the surface of reduced CaFe2O4

Abstract

In the process of biomass chemical looping gasification using calcium ferrite as an oxygen carrier, H2O or CO2 can be injected into the oxidation reactor to produce H2 or CO with high purity while oxidizing the reduced oxygen carrier, which is a new technology that has attracted wide attention in recent years. However, the splitting reaction mechanism of H2O or CO2 is still unclear. Therefore, based on density functional theory (DFT) calculations, the adsorption behavior of H2O and CO2, the electronic structure, and the splitting reaction path of H2O and CO2 on the surface of the reduced CaFe2O4 were systematically studied to deeply understand the reaction mechanism. The results show that H2O and CO2 form stable complexes on the CaO(111)/Fe(110) surface by chemisorption. H2O is splitted to H2 and adsorbed O* through a two-step reaction. In this process, the step of H2O cracking to HO* and H* is the rate-determining step. CO2 is splitted into CO* and O* through a one-step reaction, in which the desorption of CO from the surface is a rate-determining step. The total charges of H2O and CO2 after adsorption are –0.313 |e| and –1.086 |e|, respectively, and the total charges of Fe atoms on the surface are 0.444 |e| and 0.265 |e|, respectively. H2O and CO2 are electron acceptors, and Fe atoms on the surface are electron donors. The splitting of H2O or CO2 and the oxidation of reduced oxygen carriers are generally exothermic reactions, which can realize the oxidation of the reduced oxygen carrier and provide heat for the system.