CO Adsorption in Bare MXenes Ti_xC_x-1 (X=2,3), MAX Slabs and Their Surfaces Passivated with Oxygen

This work is focused on the carbon monoxide molecule (CO) adsorption on the MXenes Ti₂C, Ti₃C₂ in its MAX slabs phases Ti₂AlC, Ti₃AlC₂ and Ti₂CO₂, Ti₃C₂O₂, Ti₂AlCO₂, Ti₃AlC₂O₂.The calculation was performed using the density functional theory (DFT) as is implemented in the Vienna Ab initio Simulation Package (VASP). Since the structures are arranged in layers, Van der Waals dispersive force interactions were considered. A 3 x 3 x1 supercell was used to study the different structures. The consecutive slabs were separated by a vacuum of ∼15 Å to reduce self-interactions due to periodic conditions.<br/>The structure of the MXenes were built from the corresponding MAX phase, by removing the Aluminium atom. The slabs were obtained by cuts of the MAX phase with different surface terminations like Titanium, Aluminium or Carbon. The stable one was finished in Titanium, and we used it to construct slabs with stoichiometries: Ti₂AlC, Ti₂Al_0.5C, Ti₆Al₂C₃, Ti₃AlC₂, Ti₃Al_0.5C₂ Ti₉Al₂C₆, where every surface were passivated with oxygen atoms. In all of them we adsorbed the CO molecule on the high symmetry sites. Furthermore, the CO was placed in 3 different positions with respect to the surface: horizontal, vertical with the C pointing to the surface and vertical with the O pointing to the surface. Vibrational frequencies for the adsorbed CO were determined by freezing all MAX and Mxenes atoms from the support, that is, the normal vibrational modes of the CO have been uncoupled from the surface phonons and every atom displaced independently to its equilibrium position. The CO-surface interaction was neglected and only the adsorbate was calculated.<br/>We optimized the MAXs phases, their corresponding MXenes and different configurations for the MAXs phases, where we varied the proportion of Al:Ti:C. We organized them into 2 groups named: (I) MAX Ti₂AlC, Ti₂AlCO₂ and its derivatives and (II) MAX Ti₃AlC₂,Ti₃AlC₂O₂and its derivatives. All optimizations were carried out under the same calculation conditions. CO adsorbed perpendicular to the titanium surface is placed on both the C and O atom sides. The adsorption energy was calculated as:<br/>Where is the system energy with the adsorbate, adsorbate energy (CO) in vacuum, clean surface energy. The configurations with lower adsorption energy were selected to calculate the vibrational frequencies, where only the CO displacements perpendicular to the surface were active.<br/>In group (I) the value of the calculated frequency is slightly increased as the proportion of aluminum increases, the variation in intensity is negligible. The frequency and intensity values are very similar for group (II), except for MXene which has a lower value, indicating that Aluminum is not present inside the cell. Due to the differences in adsorption energy, as well as in frequencies and intensities, between these MXenes and slabs of their MAX phases, that are very close to each other, we considered that any of these stoichiometries can be used for the CO adsorption.<br/>Acknowledgments<br/>M. G. Moreno thanks to PASPA DGAPA UNAM for her sabbatical funding support and DGAPA-PAPIIT IN101523. Calculations were performed in the DGCTIC-UNAM Supercomputing Center, project LANCAD-UNAM-DGTIC-150.