Adsorption and Catalytic Debromination of 2,6-Dibromophenol on Ni(111): A First-Principles Mechanistic Study

Catalytic debromination is increasingly relevant for environmental remediation and the recycling of plastics containing brominated flame retardants, yet atomistic understanding of the underlying surface chemistry on transition-metal catalysts remains limited. Density functional theory calculations were used to investigate the adsorption and surface-mediated debromination of 2,6-dibromophenol on Ni(111), with 2-bromophenol and phenol included for systematic comparison. Bromine substitution was found to fundamentally alter the adsorption mode: while phenol preferentially chemisorbs at ∼2.2 Å from the surface, 2,6-dibromophenol is more stable in a physisorbed configuration at ∼3.5 Å, with 2-bromophenol at the crossover. This reversal originates from the enhanced polarizability of the C-Br bonds, which strengthens long-range dispersion sufficiently to outweigh the covalent stabilization available in close-coordination geometries. The effect is captured by BEEF-vdW and vdW-DF but missed by PBE, placing a methodological requirement on DFT studies of halogenated aromatic adsorption. Reaction pathway analysis identifies a stepwise debromination-hydrogenation sequence as the preferred route, with ring hydrogenation (0.56 eV barrier) rather than C-Br cleavage (0.16 eV) as the rate-limiting step. Direct hydrogenolysis (0.77 eV) and sequential debromination prior to hydrogenation are both disfavored. The dominant practical limitation is not the activation of the C-Br bond but the strong binding of the released bromine on the surface (-1.45 eV), which blocks active sites and rationalizes the experimental need for halogen scavengers or regeneration strategies. The findings provide an atomistic foundation for understanding the catalytic dehalogenation of polybrominated aromatics, including tetrabromobisphenol A, on earth-abundant transition-metal catalysts.