Influence of Cations on Direct CO₂ Capture and Mineral Film Formation: The Role of KCl and MgCl₂ at the Air/Electrolyte/Iron Interface

Abstract

Uncovering the mechanisms associated with CO 2 capture through mineralization is vital for addressing rising CO 2 levels. Iron in planetary soils, the mineral cycle, and atmospheric dust react with CO 2 through complex surface chemistry. Here, the effect of cations on the growth of carbonate films on iron surfaces was investigated. In situ polarized modulated infrared reflection absorption spectroscopy was used to measure CO 2 adsorption and oxidation of iron in MgCl 2 (aq) and KCl(aq), compared to FeCl 2 (aq) at the air/electrolyte/iron interface. The cation was found to influence the film composition and growth rates, as corroborated by infrared and photoelectron spectroscopy. In MgCl 2 (aq), a mixture of hydromagnesite, magnesite, and a Mg hydroxy carbonate film was grown on iron, while in KCl(aq), a potassium-rich bicarbonate film was grown. The cations were found to affect the rates of hydroxylation and carbonation, confirming a specific cation effect on carbonate film growth. In the submerged region, a heterogeneous mixture of lepidocrocite and iron hydroxy carbonate was produced, suggesting that Fe 2+ dominates the surface products. Surface roughness measurements from in situ atomic force microscopy indicate iron initially corrodes faster in MgCl 2 (aq) than KCl(aq), due to the Cl − ions that initiate pitting and corrosion. In this region, cations were not found to affect the morphologies. This study shows surface corrosion is necessary to provide nucleation sites for film growth and that the cations influence the carbonate film, relevant for CO 2 capture and planetary processes.