Quantitative XANES Studies on Metastable Equilibrium Adsorption of Arsenate on TiO2 Surfaces
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- 作者：Guangzhi He ; Gang Pan ; Meiyi Zhang ; Ziyu Wu
- 刊名：Journal of Physical Chemistry C
- 出版年：2009
- 出版时间：October 1, 2009
- 年：2009
- 卷：113
- 期：39
- 页码：17076-17081
- 全文大小：234K
- 年卷期：v.113,no.39(October 1, 2009)
- ISSN：1932-7455
文摘
Quantitative information on the electronic and geometric structures of adsorption complexes of arsenate on TiO2 surfaces was obtained on the basis of XANES analysis combined with DFT calculation. The density of states (DOS) showed that the As(V) K-absorption edge corresponded to the dipole transition from the As 1s electron into the As(4p)−O(2p) antibonding molecular orbital, suggesting that the bonding between arsenate and TiO2 was primarily due to the interaction of As 4p and O 2p orbitals. Previous EXAFS studies indicated that both bidentate binuclear (BB) and monodentate mononuclear (MM) surface complexes coexisted in the As(V)−TiO2 adsorption system. DFT calculation showed that the XANES transition energy of the BB complex was significantly higher than that of the MM complex. Thus, quantitative information on adsorption microstructures can be obtained by measuring the blue-shift of the XANES absorption edge. XANES results of adsorption samples at different kinetic stages indicated that there was a structural evolution from the MM complex to the BB complex as the reaction processed to adsorption equilibrium. Therefore, the final proportion between BB and MM complexes in the real equilibrium state was fundamentally affected by surface coverage and kinetic pathways. This fact implied that the real equilibrium constant, when defined by macroscopic parameters of concentration and adsorption density (mol/m2), are generally inconstant in nature and hence cannot describe the real equilibrium properties of adsorption, because different microscopic metastable equilibrium adsorption structures that construct the real adsorption equilibrium cannot be counted for both mass and energy by the macroscopic parameter of surface concentrations (mol/m2).