Unveiling the different physicochemical properties of M-doped β-NaFeO2 (where M = Ni or Cu) materials evaluated as CO2 sorbents: a combined experimental and theoretical analysis

Abstract

M-doped β-NaFeO₂ samples (where M = Cu or Ni) were synthesized through the nitrate-pyrolysis method, aiming to enhance the sodium ferrite's CO₂ capture properties. For the first time, these doped-ferrites were studied with a combined experimental and theoretical approach, to unveil the modifications in the structural, electronic, and optical properties. Several experimental techniques were employed to perform an in-depth characterization (XRD, XPS, SEM and N₂-ads–des, as well as Raman, FTIR-ATR and UV-vis spectroscopies), while the stable structure configurations of pristine β-NaFeO₂ and different doped ferrites were calculated through density functional theory (DFT) at the DFT + U level. Besides, ab initio molecular dynamics (AIMD) simulations were performed to analyze the temperature effect on the Na-ion diffusion, within the ferrite's crystal structure. Results showed that effective doping was achieved with less than 5 mol% of metal, while the efficiency was higher in the nickel-containing samples, as these systems showed the generation of oxygen vacancies with the insertion of divalent Ni²⁺ cations into the ferrite structure. These changes were linked to the improved CO₂ sorption of the low nickel-doped sodium ferrite (2.5 mol%). On the other hand, time-dependent DFT (TD-DFT) calculations showed significant changes in the structural, electronic and optical properties of both doped systems, which corroborate the characterization performed for the synthesized materials. In addition, AIMD simulations evidence a Na-ion mobility increment based on the obtained diffusion coefficients, due to thermal effects, which favors a better CO₂ capture. These theoretical results agree with performances shown experimentally.