Insights into Antimony Adsorption on {001} TiO2: XAFS and DFT Study

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Insights into antimony adsorption on {001} TiO2: XAFS and DFT study Li Yan, Jiaying Song, Ting-Shan Chan, and Chuanyong Jing Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 17 May 2017 Downloaded from http://pubs.acs.org on May 17, 2017

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Environmental Science & Technology

Insights into antimony adsorption on {001} TiO2: XAFS and DFT study

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Li Yan†,‡, Jiaying Song†, Tingshan Chan§, Chuanyong Jing†,‡,*

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†

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for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China

State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center

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University of Chinese Academy of Sciences, Beijing 100049, China

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§

National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu 30076, Taiwan

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Tel: +86 10 6284 9523; Fax: +86 10 6284 9523

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E-mail: [email protected]

15 16

1 ACS Paragon Plus Environment


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Abstract

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Antimony (Sb) contamination poses an emerging environmental risk, whereas its

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removal remains a contemporary challenge due to the lack of knowledge in its surface

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chemistry and efficient adsorbent. In this study, self-assembly {001} TiO2 was examined

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for its effectiveness in Sb removal, and the molecular level surface chemistry was studied

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with X-ray absorption spectroscopy and density functional theory calculations. The

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kinetics results show that Sb adsorption followed the pseudo-second order reaction, and

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the Langmuir adsorption capacity was 200 mg/g for Sb(III) and 156 mg/g for Sb(V). The

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PZC of TiO2, which was 6.6 prior to the adsorption experiment, shifted to 4.8 and 5 days) of the Sb(III) oxidation to Sb(V),16, 19-21 since Sb(V) is more

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mobile and difficult to remove. Therefore, developing novel adsorbents with high

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adsorption capacity and chemical stability becomes an urgent requirement for Sb

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remediation. TiO2 is a promising material due to above advantages. Generally, the

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adsorption capacity of TiO2 is determined by its specific surface area.22 However, recent

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studies demonstrate that TiO2 adsorption performance largely depend on its exposed

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crystal facets and corresponding surface energy.23, 24 The surface energy of three low-

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index anatase TiO2 facets, which was widely studied for water and dye adsorption,

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follows the order {001} (0.90 J/m2) > {100} (0.53 J/m2) > {101} (0.44 J/m2).25-27 The

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equilibrium shape of an anatase crystal according to the Wulff construction and surface

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energy is a slightly truncated bipyramid enclosed by about 94% {101} and 6% {001}

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facets.25 However, water and dye molecules prefer to bind to high-energy {001} rather

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than {101} facets.26-28 Therefore, a reasonable surmise is that TiO2 material with large

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percentage of exposed {001} facet with high surface energy and large surface area should

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exhibit an excellent performance for Sb removal.

Although the sequestration of Sb(III) by iron (oxyhydr)oxides is

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The objective of this study was to investigate the molecular level mechanisms of

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Sb removal using a novel high-energy {001}-faceted TiO2 adsorbent. The surface

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reactions were explored using multiple complementary techniques including macroscopic

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wet chemistry, X-ray absorption fine structure (XAFS) spectroscopy, and density

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function theory (DFT) calculations. The insights gained from this study further our

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understanding of Sb chemistry at the mineral-water interface.



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Materials and methods

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Materials. KSbC4H4O7·1/2H2O, K2H2Sb2O7·4H2O, and titanium (IV) isopropoxide

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(TTIP) were purchased from Sinopharm Chemical Reagent Co., Ltd. Sb(III) and Sb(V)

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stock solutions of 500 mg/L were prepared by dissolving KSbC4H4O7·1/2H2O and

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K2H2Sb2O7·4H2O in deionized (DI) water, respectively. A Sb contaminated river water

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sample was collected from a Sb mining site at Xikuangshan area (27°47′ N latitude,

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111°29′E longitude, Hunan, China) and the detail water analysis is shown in the

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Supporting Information (SI). TiO2 was prepared with hydrothermal method by using

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TTIP as precursor, and the synthesis and characterization are detailed in the SI.

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Adsorption Experiments. Dosage experiments were performed to evaluate the

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effectiveness of TiO2 and widely-used goethite on Sb removal from a contaminated river

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water. To the river water sample, increasing adsorbent dosage from 0 to 10 g/L was

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added. After the samples were mixed on a rotator for 24 h, the suspensions were filtrated

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through a 0.22-µm membrane filter for analysis.

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Sb(III) and Sb(V) adsorption isotherms were conducted at pH 7.0 ± 0.2 by adding

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10 mg of TiO2 into 100 mL 0.04 M NaCl solution with initial Sb concentration ranging

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from 5 to 500 mg/L. The adsorption kinetics on 0.1 g/L TiO2 were studied at pH 7.0 ±

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0.2 in 150 mL of 0.04 M NaCl solution with 40 mg/L Sb(III/V). The adsorption pH

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envelope experiments with 5 mg/L Sb(III/V) and 0.1 g/L TiO2 were conducted in

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triplicate to determine the adsorption edge. To study the competitive adsorption of

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Sb(III/V) with anions including As(III/V), PO43-, SO42-, NO3-, and F-, similar pH edge

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experiments were conducted with Sb to anion molar ratio at 1:1 and 1:5. The suspension


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was adjusted to desired pH values in the range 2 to 11 with HCl and NaOH. After the

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samples were reacted for 24 h, the final pH were measured and the concentraitons were

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detected.

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Zeta (ζ) Potential Measurements. Zeta (ζ) potential measurements were conducted

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using a Zetasizer Nano ZS (Malvern Instrument Ltd., UK). All samples were purged with

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N2 to eliminate CO2 from the system. The pH of the suspension containing 0.1 g/L TiO2

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and 5 mg/L Sb(III/V) in 0.04 M NaCl was adjusted to desired values using HCl and

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NaOH. Suspension samples were placed on a rotator for 24 h and the final pH was

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measured. The reported ζ potential value was the average of three measurements.

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Analysis. The concentrations of dissolved Sb were measured using an inductively

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coupled plasma optical emission spectroscopy (ICP-OES, Optima 2000 DV, Perkin Elmer

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Co., USA) with a detection limit of 0.032 mg/L. The Sb speciation was determined using

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a high performance liquid chromatograph (HPLC) coupled with a hydride generation-

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atomic fluorescence spectrometer (HG-AFS, Jitian, P.R. China) with a detection limit of

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0.6 µg/L for Sb(III) and 1.2 µg/L for Sb(V).

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XAFS Study and DFT Calculations. EXAFS spectroscopy was employed to

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characterize the local coordination environment of adsorbed Sb/As on TiO2. The K-edge

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spectra of Sb (30,491 eV) and As (11,867 eV) were collected on beamline 01C1 at the

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National Synchrotron Radiation Research Center (NSRRC), Taiwan. The Sb L-edge

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(LIII=4,132 eV, LII=4,380 eV, and LI=4,698 eV) X-ray absorption near edge structure

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(XANES) spectra were collected on beamline 16A1 at the NSRRC. Details of sample

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preparation and data analysis are reported in the SI. The DFT calculation was performed

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using Castep package in Materials Studio 7.0 (Accelrys, San Diego, CA),29 with model



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building and parameter set up are detailed in the SI.

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Surface Complexation Modeling. The charge distribution multi-site complexation (CD-

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MUSIC) model with 1-pK TPM adsorption option was employed to describe the Sb

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adsorption behaviors on TiO2 under the constraint of XAFS and DFT results. The

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calculation was performed using the chemical equilibrium program MINTEQ to simulate

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the adsorption and the aqueous reactions with a fixed ionic strength at 0.04 M NaCl. The

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surface parameters and species used in the model are shown in the SI.

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Results and discussion

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Characterization of {001} TiO2. The field-emission scanning electron microscopy (FE-

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SEM) and high resolution transmission electron microscopy (HR-TEM) characterization

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demonstrate that the TiO2 spheres by self-assembly 5 nm thick nanosheets exposed nearly

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100% {001} facet with a lattice fringe of 1.9 Å (inset to Figure 1a).30 The average size of

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TiO2 spheres was 870 ± 25 nm with a porous structure (Figure 1a). Such a porous

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structure resulted in a high specific surface area of 205 m2/g (Figure S1a). The Barrett–

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Joyner–Halenda (BJH) pore size distribution was ranged from 35 to 60 nm with the peak

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centered at 45 nm (Figure S1b). Raman spectrum revealed the characteristic peaks of

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anatase TiO2 at 144 (Eg), 397 (B1g), 516 (A1g), and 639 cm-1 (Eg) (Figure S1c).31 The

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XRD pattern indicated that all diffraction peaks of TiO2 can be indexed to anatase TiO2

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(JCPDS: 21-1272) (Figure S1d).32

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TiO2 Performance in Sb-laden Water Treatment. The river sample from a Sb mining

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site with Sb(V) (5.7 mg/L) at pH 7.6 was collected (Table S2). The effectiveness of

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{001} TiO2 for Sb removal was evaluated and compared with that of goethite, a

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commonly used adsorbent for metals.11, 18, 33 As shown in Figure 1b, the residue Sb(V)


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concentration was 3.9 µg/L with a TiO2 dosage at 2 g/L, which is well below the MCL of

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Sb (6 µg/L). In contrast, goethite did not reduce Sb to its MCL even up to a 10 g/L

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dosage. The specific surface area (SBET) normalized Sb(V) adsorption capacity was 3.76

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molecules/nm2 for {001} TiO2, which was slightly lower than goethite (4.31), suggesting

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that the large surface area of {001} TiO2 contributed to its excellent adsorption

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performance in batch experiments. The promising results motivate our study on Sb

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chemistry on {001} TiO2 surfaces using integrated macroscopic, spectroscopic and DFT

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study.

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Macroscopic Characterization of Sb(III/V) Adsorption. The adsorption isotherms of

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Sb(III/V) on {001} TiO2 conformed to the Langmuir model (R2 > 0.98, Figure 1c). The

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maximum adsorption capacity was 200 mg/g for Sb(III) and 156 mg/g for Sb(V), which

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is considerably higher than that of previously used adsorbents (0.6-198 mg/g for Sb(III),

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0.2-139 mg/g for Sb(V), Table S1). The SBET normalized adsorption capacity was 4.82

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and 3.76 molecules/nm2 for Sb(III) and Sb(V), respectively, which was in the range of

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reported values (0.028-235.85 molecules/nm2 for Sb(III), 0.025-44.89 for Sb(V), Table

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S1). Notably, Sb adsorption on {001} TiO2 is higher than other two {100}- and {101}-

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faceted TiO2 samples (Figure S2-4, Table S3), indicating that both high surface area and

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surface energy of {001} TiO2 determine its high adsorption capacity for Sb, the detailed

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discussion is shown in the SI.

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The kinetics of Sb(III/V) adsorption on {001} TiO2 followed the pseudo-second

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order reaction (R2>0.99, Figure 1d), suggesting that chemical adsorption is the rate-

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controlling step.34 The adsorption was fast in the first 20 min, and then reached

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equilibrium after 40 min. The rate constant, k, for Sb(III) adsorption was 0.114 g/mgh,



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which is higher than that of Sb(V) (0.075 g/mgh, inset to Figure 1d). The faster kinetics

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of Sb(III) may be attributed to its smaller molecular size and stronger adsorption affinity

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with TiO2 {001} facet compared with Sb(V).

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pH Dependence of Sb Adsorption with Coexisting Anions. Figure 2 shows the effect of

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pH on Sb(III/V) adsorption on {001} TiO2. The results show that pH had a negligible

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effect on Sb(III) adsorption as evidenced by its consistent 87% removal in the pH range 2

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to 11. On the other hand, Sb(V) adsorption was favorable at acidic pH with 45-64%

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adsorption when pH PO43- > SO42- > F- > NO3-. Notably, Sb(V) adsorption was inhibited at high pH due

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to its anionic adsorption characteristics, and this pH effect was pronounced in the

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presence of competing ions.

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The surface chemistry of Sb and As was further compared due to their coexistence

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and carcinogenicity.1, 8 The results in Figure 2c showed that the Sb(III) adsorption was

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preferential to As(III). For instance, 89% of Sb(III), compared with 45% of As(III), were

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adsorbed with the same initial concentrations at 0.041 mM at pH 7. This observation was

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attributed to the stronger Lewis base property of Sb(III) than As(III),16 exhibiting a

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stronger binding affinity with Lewis acid of Ti5c atoms on TiO2 surface. Nevertheless,

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Sb(V) adsorption was slightly less than As(V), as evidenced by only 25% of Sb(V),

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compared with 30% of As(V), was adsorbed (Figure 2d). This is understandable due to a

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larger octahedral Sb(OH)6- structure than AsO43-, attributing to a greater steric hindrance.

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This surmise was justified by our DFT calculations as detailed in the SI. Based on the

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following XAFS and DFT results, the surface species including Ti2O2SbO-5/3 (Sb(III)-

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TiO2), Ti2O2Sb(OH)4-5/3 (Sb(V)-TiO2), Ti2O2AsO-5/3 (As(III)-TiO2), and Ti2O2AsO2-5/3

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(As(V)-TiO2) were used in the CD-MUSIC model to calculate the surface complexation

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reaction (Table S4). With the inclusion of these surface species, the modeling results

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agreed well with the experimental adsorption edge curves (Figure 2).

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XAFS Study. Figure 3a-c present the k2-weighted EXAFS spectra and corresponding



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Fourier transform (FT) for Sb(III) adsorption samples, and the fitting parameters are

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listed in Table S5. The results showed that the first Sb coordination shell consisted of

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three oxygen atoms at a distance of 1.98 Å, suggesting a trigonal pyramidal geometry of

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Sb(OH)3. The second FT peak was ascribed to a Sb-Ti single scattering (SS) path at the

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distance of 3.50 Å with a coordination number (CN) of 0.5-0.8. XAFS-derived bond

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lengths have a smaller relative error (~ 0.01-0.02 Å), compared to the coordination

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number (10-20% error).38 Therefore, the fact that the measured bond length of 3.50 Å, is

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consistent with a bidentate binuclear (BB) adsorption complex, which is also observed on

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iron and aluminum oxides.10, 12, 13 In fact, it is not the first case where the structural

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configuration derived from atomic distance does not match well with that from

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coordination number,39-41 considering the monodentate mononuclear (MM) complex with

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hydrogen bonding to an adjacent singly-coordinated hydroxyl group.40 In this study, the

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thermodynamically stable BB structure of Sb(III) on TiO2 {001} facet was confirmed by

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DFT calculations (Figure 4a), with a comparable Sb-O (2.01 Å) and Sb-Ti (3.47 Å)

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distances with EXAFS results.

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For Sb(V) adsorption samples, the k2-weighted EXAFS spectra are shown in

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Figure 3d-f. The first Sb coordination shell was resolved by six O atoms at 1.98 Å,10, 11, 13

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and the second shell resulted in a Sb-Ti SS path at 3.59-3.72 Å (Table S6). These

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distances were within the range of BB configuration with an average Sb-O distance at

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2.05 Å and Sb-Ti distance at 3.70 Å as elucidated by our DFT results (Figure 4b).

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Notably, a Sb-Sb shell at 4.00 Å with CN of 1.0-2.2 was also observed, indicative

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of surface precipitation.10, 13 The formation of surface precipitation was attributed to the

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high surface density over 60 mg/g with initial Sb concentration of 40 mg/L. However,


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precipitation was a slow dynamic process. For instance, only 2.7% and 2.2% loss in

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dissolved Sb(III) and Sb(V) concentrations were observed within 24 h, respectively, and

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less than 8.2% in four days with an initial concentration of 213 mg/L for Sb(III) and 182

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mg/L for Sb(V) at pH 7 (Figure S7). The formation of precipitates should be assisted by

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TiO2 surfaces, which provide “a seed” to promote the nucleation process of

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precipitation.39, 42

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The normalized Sb K-edge XANES spectra demonstrated that no oxidation

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occurred during adsorption and XAFS data collection (Figure S8). Meanwhile, Sb K-

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edge EXAFS spectra exhibited no significant surface structural difference between single

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Sb and Sb-As coexisting samples (Figure 3 and Table S5-6), suggesting that the existence

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of As does not affect the adsorption configuration of Sb on TiO2. In turn, As K-edge

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EXAFS spectra indicated that the presence of Sb have negligible influence on the As BB

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adsorption structure (Figure S9 and Table S7).39 In addition, the change of pH condition

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exhibited no effect on adsorption configurations as evidenced by EXAFS analysis (Figure

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3 and S9, Table S5-7).

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Sb L-edge spectra were examined to study the distribution of density of states

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(DOS) upon surface complexation formation, and the spectra and their corresponding

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first derivatives are shown in Figure S10. The LI edge spectrum exhibited no change

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upon adsorption due to the negligible DOS change of p states in the vicinity of Fermi

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level (Figure 5).43 In contrast, the LII/III edges resolved discrepancies upon adsorption

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(Figure S10a-b, bottom). Notably, the transition of 2p to pd hybridization states (peak 7

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in Figure S10b) move towards higher energy, suggesting charge redistribution upon

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adsorption as detailed in the SI. Our DFT calculations demonstrated that the adsorption



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imparts 0.17 and 0.12 electrons to Sb(OH)3 and Sb(OH)6- molecules, respectively (Figure

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4). The acquisition of electrons by Sb was coupled with the formation of new bonding

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and DOS redistribution. Some p character states of Sb were elevated to above Fermi level

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of 2.5 eV (Figure 5), which hybrid with d character states of Ti to form pd hybridization

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states. Therefore, the peak 7 in Figure S10 was shifted to high energy upon adsorption. In

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sum, the change in orbital energy derived from orbital hybridizing of adsorbed molecules

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on surface should be expected, and this is the driving force that underlines the surface

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chemistry for Sb adsorption.

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Surface Chemistry for Sb Adsorption. When molecules react with surfaces, the

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interactions can be generally classified into the following three types:44 (i) the highest

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occupied molecular orbital (HOMO) of molecules bond with the conduction band (CB)

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of surface; (ii) the lowest unoccupied molecular orbital (LUMO) of molecules bond with

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the valence band (VB) of surface; and (iii) the HOMO of molecules bond with the VB of

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surface (Figure 6). These interactions bind the molecule onto the surface, and electrons

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are generally transferred from bonding orbitals in one component to antibonding orbitals

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in the other. Thus, a new bond is formed between the adsorbed molecule and the surface,

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inducing a stabilization effect. Then, the fundamental question is where the electrons go

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and at what energy the bonding occurs during the Sb adsorption process? Based on the

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molecular orbital (MO) theory and projected density of states (PDOS) analysis, the MO

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energy level diagram for Sb(III) and Sb(V) adsorption on TiO2 was constructed as shown

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in Figure 6.

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Our DOS results shown in Figure 5 demonstrated that for both TiO2 surface and

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Sb(III) molecule, the antibonding exists above the Fermi level with an energy range 2.5


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to 4.8 eV because of the interaction (iii) that electrons occupied the antibonding orbitals

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between Sb and surface (Figure 6, red line). The formation of chemical bond during

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adsorption was further elucidated by the overlap of PDOS as shown in Figures S11-12.

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The 2p orbitals of O1 and O2 from Sb(OH)3 overlap, respectively, with 3d orbitals of Ti34

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and Ti14 (Figure S11), forming a Sb-O-Ti surface complex (Figure 4a). By comparing the

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PDOS of uncoordinated O3 with the other two coordinated O1 and O2 atoms for Sb(III)

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adsorption, the energy of coordinated O1 and O2 atoms is lowered, and their p orbital

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energy is located between -5.2 to -0.2 eV, in the same energy range as the Ti-d orbitals

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(Figure S11). Thus, the major contribution to the newly formed Ti-O bonds is due to the

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electron sharing between O-2p and Ti-3d orbitals,45 which corresponds to the interaction

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(i) (Figure 6, purple line). When Sb(III) forms a BB complex on TiO2 surface,

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deprotonation occurs with two H atoms fall off from Sb(OH)3 and bond with the O2c

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atoms on TiO2 surface. The bonding of surface O2c and H atoms can be justified by their

314

overlap in PDOS. The results indicated that compared with the undissipated H3 atom, the

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dissociated H1 and H2 atoms reduce their s orbital energy to the range of -6.0 to -4.8 eV

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to match the lowered 2p orbital energy of surface O29 and O69 (Figure S11). The

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dissociative adsorption and surface reconstruction upon adsorption significantly decrease

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the surface energy due to the interaction (ii) (Figure 6, green line), leading to a stable

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adsorption configuration.

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Sb(V) adsorption also exhibited the DOS redistribution (Figure 5), and PDOS

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overlap for newly formed Ti14-O2 and Ti34-O5 bonds (Figure S12). Similarly, dissociative

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adsorption and formation of surface hydroxyl O29-H2 and O69-H5 were observed for

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Sb(V) molecule (Figure 4b). However, the hybrid orbital energy for Ti-O bonds located at



324

high energy range of -5.3 to 0.1 eV, indicating that these bonds are less stable than those

325

at low energy (-5.2 to -0.2 eV) for Sb(III) adsorption. The bonding resulted in a lower

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adsorption energy of Sb(III) (-4.99 eV), compared with that of Sb(V) (-4.71 eV, Figure

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4), indicating a favorable adsorption of Sb(III), which is in line with our macroscopic

328

adsorption results and other independent studies.34, 46

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The understanding of Sb surface chemistry enables us to explain why TiO2

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exposed with high-energy {001} facet exhibited favorable Sb adsorption. Our DFT

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calculations indicated that the surface reconstruction exists on {001} facets in contact

332

with Sb molecules.23 With respect to the pristine {001} facet with 100% Ti5c and 100%

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O2c atoms, surface reconstruction reduces the amount of O2c atoms by the formation of

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O2c-H bond, where H is contributed by the dissociation of Sb(OH)3 or Sb(OH)6-. Both

335

surface reconstruction and dissociation adsorption of Sb molecules contribute to a

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favorable adsorption configuration with a low adsorption energy.28 Therefore, the {001}

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TiO2 is capable of immobilizing Sb, and this molecular mechanism may be generalizable

338

and applicable to other metal oxide surfaces.

339 340

Acknowledgements

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We acknowledge the financial support of the National Basic Research Program of

342

China (2015CB932003, 2016YFA0203102), the Strategic Priority Research Program of

343

the Chinese Academy of Sciences (XDB14020201), and the National Natural Science

344

Foundation of China (41373123, 41425016, and 21321004). The XAFS spectra were

345

acquired at the National Synchrotron Radiation Research Center (NSRRC) BL01C1,

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Taiwan.


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347 348

Supporting Information

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Details of TiO2 synthesis, BET, XRD, Raman characterization, CD-MUSIC modeling, Sb

350

adsorption comparison, Competitive adsorption, XAFS results, DFT calculations, PDOS

351

analysis, and additional figures and tables. This material is available free of charge via

352

the Internet at http:// pubs.acs.org.

353 354

Notes

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The authors declare no competing financial interest.



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References

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Figure 1. (a) FE-SEM image of {001} TiO2 with inset HR-TEM characterization. (b) Residue Sb(V) concentrations in river water as a function of dosage of TiO2 and goethite, initial Sb(V) concentration was 5.7 mg/L, pH = 7.6. (c) Isotherm and (d) kinetics for Sb(III) and Sb(V) adsorption on 0.1 g/L TiO2 in 0.04 M NaCl solution at pH 7. Symbols are experimental data, and solid lines represent the Langmuir model (c) and pseudo-2nd kinetics model simulation (d). Inset tables in c and d shows model parameters.


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Figure 2. (a) Experimental pH adsorption edges (symbols) and charge distribution multisite complexation (CD-MUSIC) models (lines) of Sb(III) and Sb(V) adsorption on TiO2. (b) Zeta potential of 0.1 g/L TiO2 as blank (green), 0.041 mM Sb(III) (red), and 0.041 mM Sb(V) (blue) adsorption samples as a function of pH in 0.04 M NaCl solution. Error bars represent the standard deviation (n=3). Adsorption pH edges of coexisting (c) Sb(III) and As(III), (d) Sb(V) and As(V) on TiO2. Both the initial Sb and As concentrations were 0.041 mM; adsorbent dosage was 0.1 g/L.



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Figure 3. Observed (red circles) and fitted (black lines) (a, d) k2-weighted Sb K-edge EXAFS spectra and their corresponding (b, e) FT magnitude and (c, f) real parts. The samples were prepared by reacting 0.33 mM Sb, with 0.33 mM As for co-adsorption, on 0.1 g/L TiO2 in 0.04 M NaCl at pH 4, 7, 10. The fitting parameters are shown in Table S5 and S6.

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Figure 4. Density functional theory (DFT) optimized bidentate surface configuration for (a) Sb(III) and (b) Sb(V) adsorption on TiO2 {001} facet.



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Figure 5. Density of states (DOS) analysis for Sb(III) and Sb(V) adsorption on TiO2. The left and right ones indicated the DOS of TiO2 and Sb molecule before adsorption, respectively. The middle columns suggested the DOS of TiO2 and Sb molecule after adsorption.

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Figure 6. Molecular orbital energy level diagram for Sb(III) and Sb(V) adsorption on TiO2. Three types of interactions between Sb and TiO2 surface are represented with dash lines in (i) purple, (ii) green, and (iii) red.

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Insights into Antimony Adsorption on {001} TiO2: XAFS and DFT Study

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