Experimental\nand Computational Elucidation of the\nLi<sub>2</sub>MnO<sub>3</sub>‑Mediated Mechanism for CO Oxidation-Capture

Using a combination of thermogravimetric and sorption/desorption\nexperiments along with theoretical calculations, this study presents\na comprehensive investigation of the CO oxidation-capture mechanism\ncatalyzed by lithium manganate (Li<sub>2</sub>MnO<sub>3</sub>). Experimental\nresults demonstrate efficient CO oxidation-capture by Li<sub>2</sub>MnO<sub>3</sub> across a broad temperature range. However, the introduction\nof CO<sub>2</sub> into the reaction flow induces significant changes\nin the CO oxidation-capture process. Specifically, CO<sub>2</sub> alters\nthe sorption–desorption equilibrium toward carbonate formation,\nleading to decreased CO capture efficiency and increased CO<sub>2</sub> partial pressure owing to elevated activation energy. Theoretical\ncalculations based on a cluster model explain the experimental observations\nof the primary CO oxidation pathway on the Li<sub>2</sub>MnO<sub>3</sub> surface. This pathway involves the formation of intermediates for\nCO capture, CO<sub>2</sub> generation, and subsequent release, leading\nto activation of the surface from Li<sub>2</sub>MnO<sub>3</sub> to\nLi<sub>2</sub>MnO<sub>3−δ</sub>. This process resulted\nin the reduction of Mn<sup>4+</sup> to Mn<sup>3+</sup> and Mn<sup>2+</sup>. The competitive processes for CO<sub>2</sub> and CO capture\nexplain the inhibition of CO sorption. Additionally, carbonate moieties\nform on the ceramic surface, with Gibbs-free activation energies corresponding\nto the experimental values for the CO oxidation-capture process. Calculations\nrevealed stability differences between CO and CO<sub>2</sub> intermediates\nfor Li<sub>2</sub>MnO<sub>3</sub> and Li<sub>2</sub>MnO<sub>3−δ</sub>, which agreed with the experimental observations. To validate this\ntheoretical model, a second comparable system was analyzed. Experimental\nreports have demonstrated that lithium zirconate (Li<sub>2</sub>ZrO<sub>3</sub>) captures CO<sub>2</sub>, unlike Li<sub>2</sub>MnO<sub>3</sub>. Computational analysis supports this, showing that the interaction\nbetween the highest occupied molecular orbital (HOMO) of CO<sub>2</sub> and lowest unoccupied molecular orbital (LUMO) of the Zr atom enables\nCO<sub>2</sub> capture by lithium zirconate, in contrast to that determined\nfor Mn in lithium manganate. This disparity in reactivity elucidates\nthe differing responses of Li<sub>2</sub>MnO<sub>3</sub> and Li<sub>2</sub>ZrO<sub>3</sub> toward CO and CO<sub>2</sub>. The correspondence\nbetween these insights and experimental data highlights the indispensable\nrole of computational models in clarifying the reaction mechanisms\nand providing theoretical explanations for chemically significant\nprocesses.