Unveiling Tetrafluoromethane Decomposition over Alumina Catalysts

Tetrafluoromethane (CF₄), the simplest perfluorocompound known as a "forever chemical", presents substantial environmental challenges due to its health risks and contribution to the greenhouse effect. Designing efficient catalysts for CF₄ decomposition remains difficult, compounded by limited understanding of the mechanisms. Here, we use constrained ab initio molecular dynamics (cAIMD) simulations and in situ experiments to elucidate the mechanism of alumina-catalyzed CF₄ decomposition, highlighting the pivotal role of surface hydroxyl groups. The initial C-F bond breaking is the rate-determining step, with surface hydroxyl groups reducing the reaction free energy from 1.69 to 1.34 eV. These hydroxyl groups also facilitate the self-healing of oxygen vacancies generated during the decomposition. Contrary to the belief that CF₄ decomposes directly into CO₂, our cAIMD simulations, supported by synchrotron vacuum ultraviolet photoionization mass spectrometry data, reveal significant CF₂O and CO byproducts. Experimental data in an anhydrous environment indicate that water primarily replenishes surface hydroxyl groups rather than directly participating in decomposition. We conclude that the relatively high efficiency of Al₂O₃ catalysts stems from three key properties: (1) the presence of active sites with a specific affinity for CF₄ adsorption, ensuring efficient substrate interaction; (2) appropriate metal-oxygen bond strength, enabling the participation of lattice oxygen in the reaction; and (3) a high density of surface hydroxyl groups that facilitate the initial C-F bond cleavage and the self-healing of oxygen vacancies.