Coordination Structure of Adsorbed Zn(II) at Water−TiO₂Interfaces

Abstract

The local structure of aqueous metal ions on solid surfaces is central to understanding many chemical and biological processes in soil and aquatic environments. Here, the local coordination structure of hydrated Zn(II) at water-TiO 2 inter faces was identified by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectroscopy combined with density functional theory (DFT) calculations. A nonintegral coordination number of average ~4.5 O atoms around a central Zn atom was obtained by EXAFS analy sis. DFT calculations indicated that this coordination structure was consistent with the mixture of 4-coordinated bidentate bi nuclear (BB) and 5-coordinated bidentate mononuclear (BM) metastable equilibrium adsorption (MEA) states. The BB com plex has 4-coordinated Zn, while the monodentate mononuclear (MM) complex has 6-coordinated Zn, and a 5-coordinated adsor bed Zn was found in the BM adsorption mode. DFT calculated energies showed that the lower-coordinated BB and BM modes were thermodynamically more favorable than the higher-coordinated MM MEA state. The experimentally observed XANES fingerprinting provided additional direct spectral evidence of 4-and 5 coordinated Zn-O modes. The overall spectral and computational evidence indicated that Zn(II) can occur in 4-, 5-, and 6-oxygen coordinated sites in different MEA states due to steric hindrance effects, and the coexistence of different MEA states formed the multiple coordination environments.

Studies on the reaction pathway of arsenate adsorption at water–TiO2 interfaces using density functional theory

“…Experimentally measured equilibrium adsorption constants, therefore, bear the property of inconstancy in nature. Although it is now possible to measure the microscopic adsorption structures from samples using spectroscopic methods such as X-ray absorption fine structure (XAFS) [15,[18][19][20], it is extremely difficult to experimentally catch up the reaction pathways and activation barriers for the ion adsorption at mineralwater interfaces. Moreover, little is known about the interrelation of different MEA states.…”

Promotional effects and mechanism of second cations on activity and stability of Co-MOR for nitrous oxide decomposition: UV–Vis spectroscopy and EXAFS analysis

“…The data collected and analyzed processes are similar to Refs. [18,19] and the detailed descriptions are given in Supporting information.…”

Advances in Interfacial Adsorption Thermodynamics: Metastable-Equilibrium Adsorption (MEA) Theory

“…In dilute solution, activity   bb   ; b and b' are constant under fixed temperature and pressure, and are truly independent of the kinetics of the process. Equations [13] and [15] are called Langmuir-type metastable-equilibrium isotherm equations, since when K me =1, i.e., under the ideal equilibrium condition, they are reduced to the conventional Langmuir equation. Only under this ideal condition (K me = 1) can the isotherm be independent of the kinetic process.…”

“…3,11 Thus, the problem becomes rather confused based on empirical or experimental analysis only. Metastable-equilibrium adsorption (MEA) theory indicates that, [12][13][14] for a given adsorption reaction under fixed thermodynamic conditions, a polyhedral adsorbate molecule is generally ended in various MEA states with different energies and geometries rather than a unique equilibrium state when the reaction reaches to the apparent equilibrium. Unlike concentration in solutions, adsorption density (mol/m2) on solid surfaces no longer unambiguously corresponds to thermodynamic state variables, because adsorption density can only count for the mass but not the chemical potentials/energies of different microscopic MEA states that construct the real equilibrium adsorption state.…”