Computing Surface Acidity Constants of Proton Hopping Groups from Density Functional Theory-Based Molecular Dynamics: Application to the SnO₂(110)/H₂O Interface

Abstract

Proton transfer at metal oxide/water interfaces plays an important role in electrochemistry, geochemistry and environmental science. The key thermodynamic quantity to characterize this process is the surface acidity constant. An ab initio method that combines density functional theory based molecular dynamics (DFTMD) and free energy perturbation theory has been established for computing surface acidity constants. However, it involves a reversible proton insertion procedure in which frequent proton hopping, e.g. for strong bases and some oxide surfaces (e.g. SnO2), can cause instability issues in electronic structure calculation. In the original implementation, harmonic restraining potentials are imposed on all O-H bonds (denoted by the VrH scheme) to prevent proton hopping, and thus may not be applicable for systems involving spontaneous proton hopping. In this work, we introduce an improved restraining scheme with a repulsive potential Vrep to compute the surface acidities of systems in which proton hopping is spontaneous and fast. In this Vrep scheme, a Buckingham-type repulsive potential Vrep is applied between the deprotonation site and all other protons in DFTMD simulations. We first verify the Vrep scheme by calculating the pKa's of H2O and aqueous HSsolution (i.e. strong conjugate bases), and then apply it to the SnO2(110)/H2O interface. It is found that the Vrep scheme leads to a prediction of the point of zero charge (PZC) of 4.6, which agrees well with experiment. The intrinsic individual pKa of terminal five-coordinated Sn site (Sn5cOH2) and bridge oxygen site (Sn2ObrH + ) are 4.4 and 4.7, respectively, both being almost the same as the PZC. The similarity of the two pKa values indicate that dissociation of terminal water has almost zero free energy at this proton hopping interface (i.e. partial water dissociation), as expected from the acid-base equilibrium on SnO2.

Investigations of water/oxide interfaces by molecular dynamics simulations

“…Interestingly, hematite (0001) surface is similar to alumina (0001) and the predicted pK a of Fe 2 OH by AIMD is 21.7, comparable with Al 2 OH, but much higher than the value predicted by the valence bond model (about 12) 142 . The pK a of SnO 2 is consistent with experimental PZC measurements 143 . Understanding disagreements between simulations and experimental measurements is crucial to model the interface in acidic or basic solutions because the surface protonation state, surface charge density, and interfacial water orientation are mainly determined by the surface pK a .…”

“…142 The pK a of SnO 2 is consistent with experimental PZC measurements. 143 Understanding disagreements between simulations and experimental measurements is crucial to model the interface in acidic or basic solutions because the surface protonation state, surface charge density, and interfacial water orientation are mainly determined by the surface pK a .…”

Origin of Asymmetric Electric Double Layers at Electrified Oxide/Electrolyte Interfaces

“…In the first work of applying finite-field DFTMD for modeling EDLs, 23 it was found that the Helmholtz capacitance at electrified rutile TiO 2 (110)/NaCl interfaces is much higher at high pH than that at low pH for the given surface charge density, and the interfacial proton transfer at low pH increases significantly the capacitance value. Compared with rutile TiO 2 , the isostructural cassiterite SnO 2 has a characteristic dissociative water adsorption, 26 , 27 which involves terminal adsorbed water Sn 5c O w H 2 to bridge oxygen site Sn 2 O br as …”

Electric Double Layer at the Metal-Oxide/Electrolyte Interface

“…Subsequently, finite-field DFTMD has also been applied to study SnO 2 , which is an isostructure of TiO 2 and has a characteristic dissociative water adsorption 57 . Jia et al 53 computed the Helmholtz capacitance of the SnO 2 (110)/NaCl electrolyte interface and developed a differential Helmholtz capacitance model accordingly.…”