Experimental and Computational Elucidation of the Li₂MnO₃-Mediated Mechanism for CO Oxidation-Capture

Using a combination of thermogravimetric and sorption/desorption experiments along with theoretical calculations, this study presents a comprehensive investigation of the CO oxidation-capture mechanism catalyzed by lithium manganate (Li2MnO3). Experimental results demonstrate efficient CO oxidation-capture by Li2MnO3 across a broad temperature range. However, the introduction of CO2 into the reaction flow induces significant changes in the CO oxidation-capture process. Specifically, CO2 alters the sorption–desorption equilibrium toward carbonate formation, leading to decreased CO capture efficiency and increased CO2 partial pressure owing to elevated activation energy. Theoretical calculations based on a cluster model explain the experimental observations of the primary CO oxidation pathway on the Li2MnO3 surface. This pathway involves the formation of intermediates for CO capture, CO2 generation, and subsequent release, leading to activation of the surface from Li2MnO3 to Li2MnO3−δ. This process resulted in the reduction of Mn4+ to Mn3+ and Mn2+. The competitive processes for CO2 and CO capture explain the inhibition of CO sorption. Additionally, carbonate moieties form on the ceramic surface, with Gibbs-free activation energies corresponding to the experimental values for the CO oxidation-capture process. Calculations revealed stability differences between CO and CO2 intermediates for Li2MnO3 and Li2MnO3−δ, which agreed with the experimental observations. To validate this theoretical model, a second comparable system was analyzed. Experimental reports have demonstrated that lithium zirconate (Li2ZrO3) captures CO2, unlike Li2MnO3. Computational analysis supports this, showing that the interaction between the highest occupied molecular orbital (HOMO) of CO2 and lowest unoccupied molecular orbital (LUMO) of the Zr atom enables CO2 capture by lithium zirconate, in contrast to that determined for Mn in lithium manganate. This disparity in reactivity elucidates the differing responses of Li2MnO3 and Li2ZrO3 toward CO and CO2. The correspondence between these insights and experimental data highlights the indispensable role of computational models in clarifying the reaction mechanisms and providing theoretical explanations for chemically significant processes.