Modification of CaCO₃ and CaCO₃ pin-coated cellulose paper under supercritical carbon dioxide–ethanol mixture for enhanced NO₂ capture

In this work, we examine two modifications of fine-ground calcium carbonate material (GCC) in order to enhanced sorption of NO₂ and subsequent reaction properties toward NO₂⁻/NO₃⁻ formation by firstly exposing the GCC to supercritical (sc) CO₂ in order to increase particle surface area, a choice specifically made to avoid altering the surface chemistry, and secondly considering the potential advantage of using a surface coupling agent toward NO₂. The modification by the coupling agent amino silane (AMEO silane) was applied in a supercritical CO₂–ethanol mixture. The samples were characterised before and after modification by field emission scanning electron microscopy (FESEM), specific surface area determination (BET nitrogen adsorption), ATR-FTIR spectroscopy and ion chromatography to reveal the effects of the surface modification(s) on the morphology, surface textural properties and sorption versus reaction properties with NO₂. The performance of the treated sorbents for NO₂ capture was evaluated at room temperature. Results show that reactivity of NO₂ with GCC was observed to increase as a function of increased surface area resulting from scCO₂ exposure, but that the presence of AMEO silane on the surface, while enhancing initial adsorption of NO₂ was seen subsequently to act to block reactivity. Thus, judicious use of coupling agent can provide desired rapid initial adsorption of the gas, but the goal of long-term CaCO₃-consuming reactivity, so as to prolong the uptake of NO₂ beyond surface saturation alone, is achieved by increasing surface area while retaining chemical-free exposed CaCO₃ surface.