Article pubs.acs.org/IECR
Effective Enhancement of the Degradation of Oxalic Acid by Catalytic Ozonation with TiO2 by Exposure of {001} Facets and Surface Fluorination Zhiqiao He, Qiaolan Cai, Fangyue Hong, Zhe Jiang, Jianmeng Chen, and Shuang Song* College of Biological and Environmental Engineering, Zhejiang University of Technology, Hangzhou 310032, People’s Republic of China ABSTRACT: Heterogeneous catalytic ozonation is a promising advanced oxidation technology for water treatment. In the present work, the surface fluorination of TiO2 catalysts with a high percentage of exposed {001} facets (F-TiO2) were synthesized through a hydrothermal method using tetrabutyl titanate as the precursor and HF as the shape controlling agent. The structural properties of the catalysts were characterized by X-ray diffraction, the Brunauer−Emmett−Teller method, fieldemission scanning electron microscopy, and X-ray photoelectron spectroscopy. The sheetlike TiO2 is pure anatase, with ∼75% of highly reactive {001} facets. The surface fluoride of the F-TiO2 nanosheets can be removed by washing with dilute NaOH solution (abbreviated here as OH-TiO2), resulting in a decrease of Ti3+ content and an increase of the specific surface area. The catalytic activity of the samples was evaluated by degradation of oxalic acid in aqueous solution in the presence of ozone. It was found that F-TiO2 facilitated the catalytic ozonation process by comparison with OH-TiO2 and pure TiO2 nanoparticles prepared in pure water. We conclude that the high surface energies of {001} facets and the increased concentration of oxygen vacancies contributed to the enhancement of the ozonation activity of fluorinated TiO2 with dominant {001} facets. usually adopted as catalysts.2,8−13 Among these, TiO2 has been found to be a promising catalyst because of several advantages, including low cost, nontoxicity, and chemical stability during the ozonation process. Most available anatase TiO2 nanocrystals are mainly dominated by the thermodynamically stable {101} facets rather than the more reactive {001} facets, since the proportion of surface with high reactivity usually diminishes rapidly during the crystal growth process as a result of the minimization of surface energy.14 For anatase TiO2 nanocrystals, different facets exhibit distinct activities because their surface energies are different. The order of the average surface energies is γ{110} (1.09 J m−2) > γ{001} (0.90 J m−2) > γ{100} (0.53 J m−2) > γ{101} (0.44 J m−2).15−18 Water would dissociate into two hydroxyl groups on the (001) surface, whereas the reaction cannot occur on the {101} facets19 because the activity of the catalysts for heterogeneous ozonation is strongly influenced by the surface OH groups.8,9,11 TiO2 nanosheets with dominant {001} facets will offer a new opportunity for heterogeneous catalytic ozonation. TiO2 nanomaterials with exposed {001} facets have usually been synthesized using hydrofluoric acid as a morphology controlling agent. The role of fluorine in the photocatalytic process still remains disputed.20−24 Therefore, the dependence of surface fluorination on the catalytic ozonation process deserves special attention. In this work, TiO2 catalysts with various percentages of exposed {001} facets were fabricated via a hydrothermal method using tetrabutyl titanate (Ti(OC4H9)4) as the titanium
1. INTRODUCTION Ozone oxidation is an attractive alternative for the treatment of various organic and inorganic compounds in aqueous solution. Ozone either reacts directly as molecular ozone in a slow and selective fashion or reacts via activation to the hydroxyl radical that reacts faster and less selectively.1,2 However, several disadvantages limit the use of ozone in water and wastewater treatment, such as its relatively low solubility and stability in water. Also, in view of economic considerations, the application of ozonation might not be economically viable owing to the high cost of ozone production and only partial oxidation of organic substrates present in water. To enhance the efficiency of the ozonation process and optimize economic efficiency, advanced ozone-based oxidation processes have been under investigation, viz., O3/H2O2, UV/O3, ultrasound/O3, and catalytic ozonation.1,3 Recently, heterogeneous catalytic ozonation for water treatment has received much attention and developed rapidly, because of its potentially higher effectiveness in the degradation of refractory organic pollutants and its lower cost with respect to simple ozonation.1,4,5 Catalytic ozonation in heterogeneous systems offers several following possible reactions:6,7 (1) reaction between nonchemisorbed organic molecules and active species on the catalyst surface generated from the decomposition of chemisorbed ozone; (2) reaction between chemisorbed organic molecules in associative or dissociative form on the catalytic surface with gaseous or aqueous ozone; (3) reaction between chemisorbed ozone and organic molecules and the subsequent interaction between the chemisorbed species. Therefore, the physicochemical properties of the catalyst surface play an important role in the catalytic ozonation process. Transition metal oxides, such as MnO2, TiO2, Ni2O3, NiO, Fe2O3, CuO, ZnO, CoO, V2O5, Cr2O3, MoO, and Pr6O11, are © 2012 American Chemical Society
Received: Revised: Accepted: Published: 5662
October 14, 2011 March 31, 2012 March 31, 2012 April 1, 2012 dx.doi.org/10.1021/ie202357d | Ind. Eng. Chem. Res. 2012, 51, 5662−5668
Industrial & Engineering Chemistry Research
Article
Figure 1. FE-SEM images of (a) F-TiO2, (b) OH-TiO2, (c) pure TiO2, and (d) used F-TiO2.
distilled water several times, and then dried in an oven at 60 °C for 8 h. FE-SEM (Hitachi S-4800) was used to image the morphology of the TiO2 samples. XRD (Thermal ARL X-ray diffractometer (Thermo, France) with Cu Kα radiation, accelerating voltage 45 kV, and applied current 40 mA) was performed to determine the crystalline phase or structures in the TiO2 samples. The porous structure characteristics were inferred from the conventional analysis of nitrogen sorption− desorption isotherms measured at −196 °C with a Micromeritics ASAP 2010 apparatus, and the specific surface area was calculated based on the BET equation. XPS examination was carried out on an RBD upgraded PHI-5000C ESCA system (Perkin-Elmer) with Mg Kα radiation (1253.6 eV). In addition, the concentration of surface OH groups was determined according to a saturated deprotonation method.26,27 The basic principle of the method is that methane can be formed from the reaction between the surface OH groups and methyl magnesium iodide (CH3MgI). Thus, the amount of surface OH groups can be quantified by measuring the amount of generated methane. 2.3. Apparatus for the Ozonation System. As shown earlier,28 the experimental apparatus consisted of a cylindrical Pyrex glass reactor (diameter 70 mm, height 260 mm) with a porous plastic diffuser located at the bottom, an O3 supply system, and an exhaust treatment system. The solution in the glass cylinder was stirred continuously by a magnetic follower, and the temperature was controlled by a thermostatically controlled bath (THD-2015, Tianheng Instrument Factory, Ningbo, China). The O3 was electrically generated on-site from dry pure oxygen using an ozone generator (CHYF-3A, Hangzhou Rongxin Electronic Equipment Co. Ltd.). The oxygen flow rate to the generator was controlled by a mass flow controller (D07-12A/ZM, Beijing Qixing Electronic Equipment Co., Ltd., China). 2.4. Ozonation Procedures and Sample Analysis. The catalytic degradation of oxalic acid in the presence of ozone was done in a semicontinuous flow mode. In a typical catalytic degradation procedure, a volume of 800 mL of simulated
source and HF solution as the solvent. Systematic characterization by methods that include field-emission scanning electron microscopy (FE-SEM), X-ray powder diffraction (XRD) measurements, X-ray photoelectron spectroscopy (XPS), and Brunauer−Emmett−Teller (BET) measurement was used to elucidate the relationship between the physicochemical properties and catalytic ozonation activities of the catalysts. Further, the effect of the reactive {001} facets and surface fluorination on the catalytic ozonation of oxalic acid was investigated, based on the quantitative determination of the surface OH groups.
2. EXPERIMENTAL SECTION 2.1. Reagents. All major chemicals used were of reagent grade or higher. Ti(OC4H9)4 and HF solution (with a concentration 40 wt %) for the preparation of the TiO2 nanosheets with exposed {001} facets were purchased from Huadong Medicine Co., Ltd., China. Oxalic acid, as obtained from Hangzhou Linping Chemicals Co. Ltd. was over 99.5% pure, and was used without further purification. Doubly distilled water was used for all synthesis and treatment processes. 2.2. Preparation and Characterization of Anatase TiO2. Anatase TiO2 nanosheets were synthesized through a hydrothermal method similar to that described elsewhere.25 Typically, 50 mL of Ti(OC4H9)4 was hydrolyzed by adding 6 mL of hydrofluoric acid solution at room temperature. The solution was then transferred to a Teflon-lined autoclave with a capacity of 200 mL and kept at 180 °C for 24 h. Subsequently, the white precipitates were collected, washed with ethanol and distilled water several times, and then dried in an oven at 60 °C for 8 h. The resulting sample was denoted F-TiO2. As a comparison, control samples of pure TiO2 were synthesized by replacing 6 mL of hydrofluoric acid with 6 mL of H2O, keeping other conditions the same as for sample F-TiO2. To prepare samples in which the F− is replaced with hydroxyl groups (OH-TiO2), the white precipitates obtained after hydrothermal treatments were washed with 0.1 M NaOH and 5663
dx.doi.org/10.1021/ie202357d | Ind. Eng. Chem. Res. 2012, 51, 5662−5668
Industrial & Engineering Chemistry Research
Article
Table 1. Summary of Physicochemical Properties of F-TiO2, OH-TiO2, and Pure TiO2 catalyst
SBET (m2 g−1)
crystallite phase (%)
concn surf. OH (mmol g−1)
pore vol (cm3 g−1)
pore diam (nm)
percentage {001} (%)
COD removal efficienciesa,b (%)
F-TiO2 OH-TiO2 pure TiO2
105 149 182
A100 A100 A100
1.18 0.93 0.58
0.37 0.52 0.29
11.1 12.3 4.9
75 ± 4 72 ± 5
