Tuning surface terminations and vacancy formation in Ti3C2Tx MXenes for formic acid dehydrogenation: a DFT study

Formic acid (HCOOH) is a promising liquid organic hydrogen carrier owing to its high hydrogen density, low toxicity, and ease of handling. In this work, density functional theory (DFT) calculations were employed to investigate the catalytic decomposition of HCOOH on Ti 3 C 2 T x MXenes and to evaluate how surface terminations, vacancy formation, and Ru incorporation influence catalytic performance. The bare Ti 3 C 2 surface was found to be excessively reactive, leading to spontaneous molecular dissociation and poor catalyst stability, whereas fully terminated Ti 3 C 2 F 2 and Ti 3 C 2 O 2 interact only weakly with adsorbates, with adsorption dominated by van der Waals interactions. Partial removal of surface termination atoms significantly enhances surface reactivity by exposing catalytically active sites while preserving structural stability. In particular, oxygen vacancies on Ti 3 C 2 O 2 promote spontaneous and strongly exothermic HCOOH dehydrogenation while maintaining weak adsorption of the reaction products, favouring catalyst turnover. Transition-state calculations revealed that the second H removal step constitutes the main kinetic bottleneck of the reaction pathway, with C–H cleavage barriers of 1.30 eV and 1.14 eV on the most favourable undoped and doped two-Ti 3 C 2 O 2 MXene surfaces, respectively. Ru incorporation does not substantially modify the intrinsic reactivity of fully terminated surfaces, but lowers the formation energy of oxygen vacancies by more than 1 eV, thereby facilitating the formation of catalytically favourable oxygen-deficient regions. Among the investigated systems, the Ru-doped Ti 3 C 2 O 2 surface containing two oxygen vacancies adjacent to the Ru site exhibits the most favourable balance between reaction exothermicity, activation barriers, product desorption, and structural stability. These findings indicate that tuning surface terminations and local surface composition constitutes an effective strategy for optimizing Ti 3 C 2 T x -based MXenes for catalytic hydrogen production from formic acid.