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

The hydration behavior of rutile(110) and anatase(101) TiO2 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 H2O to a neighboring deprotonated water molecule. In contrast, anatase(101) exhibits a single mechanism involving proton transfer from an associatively adsorbed H2O 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 TiO2 surfaces, offering valuable guidance for designing TiO2-based catalysts and proton-conductive materials for energy conversion and storage applications.