Competing Kinetic Consequences\nof CO<sub>2</sub> on\nthe Oxidative Degradation of Branched Poly(ethylenimine)

Amine-functionalized porous solid materials are effective\nsorbents\nfor direct air capture (DAC) of CO<sub>2</sub>. However, they are\nprone to oxidative degradation in service, increasing the materials\ncost for widespread implementation. While the identification of oxidation\nproducts has given insights into degradation pathways, the roles of\nsome species, like CO<sub>2</sub> itself, remain unresolved, with\nconflicting information in the literature. Here, we investigate the\nimpact of CO<sub>2</sub> on the oxidative degradation of poly(ethylenimine)–alumina\n(PEI/Al<sub>2</sub>O<sub>3</sub>) sorbents under conditions encompassing\na wide range of CO<sub>2</sub>-air mixture compositions and temperatures\nrelevant to DAC conditions, thereby reconciling the conflicting data\nin the literature. Degradation profiles characterized by thermogravimetric\nanalysis, <i>in situ</i> ATR-FTIR, and CO<sub>2</sub> capacity\nmeasurements reveal nonmonotonic effects of CO<sub>2</sub> concentrations\nand temperatures on oxidation kinetics. Specifically, 0.04% CO<sub>2</sub> accelerates PEI/Al<sub>2</sub>O<sub>3</sub> oxidation more\nat low temperatures (<90 °C) compared to 1% and 5% CO<sub>2</sub>, but this trend reverses at high temperatures (>90 °C).\nFirst-principles metadynamics, machine learning accelerated molecular\ndynamics simulations, and <sup>1</sup>H relaxometry experiments show\nthat chemisorbed CO<sub>2</sub> acid-catalyzes critical oxidation\nreactions, while extensive CO<sub>2</sub> uptake reduces PEI branch\nmobility, slowing radical propagation. These contrasting kinetic effects\nof CO<sub>2</sub> explain the complex degradation profiles observed\nin this work and in prior literature. Collectively, this work highlights\nthe importance of considering atmospheric components in the design\nof DAC sorbents and processes. Additionally, it identifies the unconstrained\nbranch mobility and local acid environment as two of the major culprits\nin the oxidation of amine-based sorbents, suggesting potential strategies\nto mitigate sorbent degradation.