Structural and CO₂ capture analyses of the Li₁₊ₓFeO₂ (0 ≤ x ≤ 0.3) system: effect of different physicochemical conditions

In this work, we studied α-LiFeO₂ compounds and the effect of lithium excess on CO₂ capture properties. The α-LiFeO₂ phase (cubic phase) was synthesized using the nitrate pyrolysis method at moderate temperature and a short calcination time (up to 670 °C and 3 h). Four compositions were synthesized with the nominal formula Li₁₊ₓFeO₂ for 0 ≤ x ≤ 0.3, obtaining α-LiFeO₂ phases with different lattice parameters. The specific surface area was calculated for all the compounds from the BET model, fitting the N₂ adsorption–desorption curves. The CO₂ capture was studied under two different sets of physicochemical conditions. Initially, the CO₂ capture was analysed at high temperature, where the Li₁.₃FeO₂ composition showed the best properties at T > 600 °C. It was found that the amount of CO₂ captured at T > 600 °C depended on the lithium excess in the sample. Isothermal studies were performed in the 400–700 °C range, and the curves were fitted to a double exponential model. Kinetic constants were used in the Eyring formalism to obtain the activation enthalpies (ΔH≠) for the surface and bulk reactions. These values were similar to those reported for other Li-based ceramics. The CO₂ capture process in the presence of steam was evaluated at low temperature (from 40 °C to 80 °C, varying the relative humidity from 0 to 80%). In these physicochemical conditions, Li₁.₃FeO₂ could capture up to 24 wt% CO₂. This characterization has not previously been performed on these compounds. After CO₂ and steam capture, the product obtained contained magnetite, which is an iron oxide with mixed Fe²⁺ and Fe³⁺ oxidation states. The origin of the Fe²⁺ is attributed to the excess lithium added to the crystalline system. Its presence was confirmed by XPS spectroscopy, and it was also possible to identify two different chemical environments for oxygen ions, indicating that the Li⁺ ions occupy two different crystal sites. We associated the high reactivity of compounds under a steam and CO₂ atmosphere with the presence of both Fe²⁺ and the interstitial Li⁺ species.