Oxalic Acid Adsorption on Rutile: Molecular Dynamics and ab Initio Calculations

Abstract

Detailed analysis of the adsorption of oxalic acid ions, that is, oxalate and hydrogenoxalate, on the rutile (110) surface was carried out using molecular dynamics augmented by free energy calculations and supported by ab initio calculations. The predicted adsorption on perfect nonhydroxylated and hydroxylated surfaces with surface charge density from neutral to +0.208 C/m 2 corresponding to pH values of about 6 and 3.7, respectively, agrees with experimental adsorption data and charge-distribution multisite ion complexation model predictions obtained using the most favorable surface complexes identified in our simulations. We found that outer-sphere complexes are the most favorable, owing to strong hydrogen binding of oxalic acid ions with surface hydroxyls and physisorbed water. The monodentate complex, the most stable among inner-sphere complexes, was about 15 kJ/mol higher in energy, but separated by a large energy barrier. Other inner-sphere complexes, including some previously suggested in the literature as likely adsorption structures such as bidentate and chelate complexes, were found to be unstable both by classical and by ab initio modeling. Both the surfaces and (hydrogen)oxalate ions were modeled using charges scaled to 75% of the nominal values in accord with the electronic continuum theory and our earlier parameterization of (hydrogen)oxalate ions, which showed that nominal charges exaggerate ion−water interactions.

Recent advances in theoretical investigation of titanium dioxide nanomaterials. A review

“…The predicted adsorption on perfect non-hydroxylated and hydroxylated surfaces (Fig. 3) was found in agreement with experimental adsorption data and predictions of the charge-distribution multisite ion complexation model [76]. The most favorable surface species were identified to be outer-sphere complexes because of the strong hydrogen bonding of oxalic acid ions with surface hydroxyls and adsorbed water molecules [76].…”

“…3) was found in agreement with experimental adsorption data and predictions of the charge-distribution multisite ion complexation model [76]. The most favorable surface species were identified to be outer-sphere complexes because of the strong hydrogen bonding of oxalic acid ions with surface hydroxyls and adsorbed water molecules [76]. Moreover, MD simulations were performed to identify the thermodynamically stable conformations of glycolate (CH 2 (OH)COO − ), lactate, and 2-hydroxybutyrate ions (Fig.…”

“…Recently, the use of the electronic continuum correction (ECC) theory, which suggests some adjustment of point charges at Ti and O atoms to reproduce the adsorption phenomena occurring at the TiO 2 −liquid interface, has received significant attention [32][33][71][72][73][74][75]. MD simulations, augmented by free energy calculations and supported by ab initio calculations, were used to study the adsorption of oxalic acid ions (oxalate and hydrogen oxalate) on the rutile (110) surface [76][77]. The predicted adsorption on perfect non-hydroxylated and hydroxylated surfaces (Fig.…”

Oxalic Acid Adsorption on Rutile: Experiments and Surface Complexation Modeling to 150 °C

“…These profiles indicated that monodentate inner-sphere adsorption was considerably more energetically favorable than bidentate inner-sphere adsorption. 42 Consequently, the PMF calculations presented here only compare monodentate inner-sphere adsorption with outersphere adsorption and were performed as follows. The reaction coordinate, the z-separation between a surface terminal Ti atom and one oxygen of an oxalate molecule, was divided into 37 windows of 0.1−0.3 Å width in the inner-sphere and outer-sphere adsorption regions and 0.5 Å width in the bulk region, thereby generating in all cases a sufficient sampling with overlapping distributions.…”

“…Further computational details of the studied systems and applied methods can be found in a paper focused on the detailed computational results of oxalate adsorption by rutile. 42…”