Adsorption of Trifluoromethanesulfonic Acid and Trimethyl Ammonium on Hydrated Titania Surfaces: A DFT Study

The hydration behavior of rutile(110) and anatase(101) TiO 2 surfaces and their interactions with trifluoromethanesulfonic acid (TFSA) and trimethylamine (TMA(OH – )) under varying hydration conditions (up to 2.0MonoLayer, ML) were systematically examined using density functional theory based electronic structure calculations. Our results demonstrate distinct water adsorption mechanisms: on rutile(110), water undergoes dissociative adsorption at low hydration (≤0.5 ML), shifting to mixed dissociative/associative adsorption at higher coverages (0.75–2.0 ML), whereas anatase(101) predominantly maintains associative adsorption across all hydration levels. TFSA adsorption transitions from dissociative to hydrogen-bonded configurations with increasing hydration, while TMA(OH – ) adsorbs strongly exothermically via hydroxide ion coordination to undercoordinated Ti sites. Structural optimizations were followed by climbing-image nudged elastic band (CI-NEB) calculations to determine a minimum energy pathway (MEP) and transition-state barriers for the transfer of a proton. Two proton transfer mechanisms were identified on rutile(110). The first occurs from a surface oxygen to a deprotonated water molecule and the second from associatively adsorbed H 2 O to a neighboring deprotonated water molecule. In contrast, anatase(101) exhibits a single mechanism involving proton transfer from an associatively adsorbed H 2 O to a surface oxygen atom. The energy barriers for proton transfer are surface-dependent, with rutile(110) exhibiting lower barriers (1.0–1.25 eV for the water/TFSA systems; 0.8–2.0 eV for the TMA-modified surfaces) compared to anatase(101) (1.8–2.8 eV for water/TFSA; 1.5–2.3 eV for the TMA systems) involving surface oxygen atoms. The reduced barriers in TMA-containing systems underscore the role of hydrogen-bonding networks in facilitating proton shuttling. These findings provide molecular-level insight into the interfacial proton dynamics on TiO 2 surfaces, offering valuable guidance for designing TiO 2 -based catalysts and proton-conductive materials for energy conversion and storage applications.