Increasing the (Re)Activity of MgO for Carbon Capture under Continuous Ball Milling

Carbon capture and storage from point emission sources is a crucial technology able to mitigate the progress of climate change by slowing down the emission of greenhouse gases. Current technologies, such as amine scrubbing and calcium looping, face challenges including high energy consumption and limited efficiency under humid conditions. Magnesium oxide (MgO) is a promising CO₂ sorbent due to its lower decarboxylation enthalpy compared to calcium oxide, but its practical use is hindered by slow CO₂ uptake kinetics and the formation of a passivating carbonate layer that inhibits further CO₂ diffusion. While synthetic methods have improved MgO surface area, scalability remains an issue. This study addresses the kinetic limitations of MgO by investigating how continuous ball milling can reduce the performance hindrance from the passivating-carbonate layer on MgO and enhance CO₂ uptake under industrially relevant conditions. Here we show that continuous ball milling of MgO under a CO₂ atmosphere significantly increases its CO₂ uptake capacity. Unlike conventional approaches that rely on presynthesized high-surface-area MgO, our results demonstrate that ball milling during the sorption step does not only increase the specific surface area of MgO but also actively enhances its CO₂ capture performance, with a 6-fold increase in adsorption capacity compared to static exposure. The addition of water further boosts performance, doubling uptake under simulated flue gas conditions (10% CO₂, 40 °C), likely due to the formation of bicarbonates and hydroxycarbonates. The performance stability of MgO under continuous milling and moisture conditions was evaluated in several sorption-regeneration cycles, showing consistent CO₂ uptake over ten cycles with an average sorption capacity of 17.4 wt %. Structural analyses reveal that carbonation proceeds via magnesium hydroxycarbonate species, which fully decompose upon regeneration, preserving sorbent stability over ten cycles. This study underscores the practical relevance of mechanochemical activation for enhancing the capture capacity of MgO, offering a promising approach for industrial point-source CO₂ capture applications.