Nano-TiO2-Catalyzed Dehydrochlorination of 1,1,2,2

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Environmental Processes

Nano-TiO2-Catalyzed Dehydrochlorination of 1,1,2,2Tetrachloroethane: Roles of Crystalline Phase and Exposed Facets Xuguang Li, Tong Li, Tong Zhang, Cheng Gu, Shourong Zheng, Haijun Zhang, and Wei Chen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05479 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 19, 2018

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Environmental Science & Technology

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Nano-TiO2-Catalyzed Dehydrochlorination of

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1,1,2,2-Tetrachloroethane: Roles of Crystalline Phase and

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Exposed Facets

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Xuguang Li,† Tong Li,† Tong Zhang,† Cheng Gu,‡ Shourong Zheng,‡ Haijun Zhang,§ Wei Chen*,†

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†

College of Environmental Science and Engineering, Ministry of Education Key Laboratory of

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Pollution Processes and Environmental Criteria, Tianjin Key Laboratory of Environmental

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Remediation and Pollution Control, Nankai University, Tianjin 300350, China

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‡

State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment,

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Nanjing University, Nanjing, Jiangsu 210023, China §

School of Physics and Materials Science, Anhui University, Hefei, Anhui 230039, China

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Manuscript prepared for Environmental Science & Technology

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* Corresponding author: (Phone/fax) 86-22-85358169; (E-mail) [email protected].

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ACS Paragon Plus Environment


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TOC Art

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ABSTRACT Nano-scale titanium dioxide (nTiO2) is one of the most widely used metal oxide

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nanomaterials. Once released into the environment, nTiO2 may catalyze abiotic transformation of

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contaminants, and consequently affect their fate and effects. Here, we show that the overall

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catalytic efficiency of nTiO2 for the dehydrochlorination reaction of 1,1,2,2-tetrachloroethane, a

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commonly used solvent, depends on the crystalline phase and exposed facets of nTiO2, which

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significantly affect the adsorption capacity and surface catalytic activity of nTiO2. Specifically,

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under all three pH conditions tested (7.0, 7.5 and 8.0), the overall catalytic efficiency of eight

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nTiO2 materials (as indicated by the surface-area-normalized reaction kinetic constants) followed

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the order of rutile > anatase > TiO2(B). For anatase and TiO2(B) materials, the overall catalytic

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efficiency increased with the increasing percentage of exposed {001} and {010} facets,

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respectively. Crystalline phase and exposed facets significantly affected adsorption affinities of

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nTiO2, likely by modulating surface hydrophobicity of nTiO2. Crystalline phase and exposed

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facets also determined the activity of surface catalytic sites on nTiO2 by dictating the

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concentration and strength of surface unsaturated Ti atoms, as the deprotonated hydroxyl groups

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chemisorbed to these reactive Ti atoms served as bases to catalyze the base-promoted reaction.

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INTRODUCTION Nano-scale titanium dioxide (nTiO2) is one of the most widely used metal oxide

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nanomaterials.1 For instance, anatase and rutile TiO2 are essential ingredients of paints,

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cosmetics and food additives.1 TiO2(B) has growing applications in the fields of energy

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conversion, such as solar cells, lithium batteries, and environmental photocatalysts.2,3 It was

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estimated that the annual global production of TiO2 would reach approximately 2.5 million

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metric tons by 2025.4 With the rapidly increasing production and use of nTiO2, its environmental

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release and the associated implications (e.g., toxicity to aquatic organisms and effects on

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bioavailability of environmental contaminants) have received much attention.5-9 One of the

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potential environmental implications of nTiO2 is that once released into the environment, these

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materials may affect the fate and effects of environmental contaminants by catalyzing their

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transformation reactions, which consequently modifies the physicochemical properties and

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biological effects of the contaminants. While numerous studies have been conducted to

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understand the photocatalytic properties of nTiO2 (as photocatalysts for water and wastewater

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treatments),10-12 the catalytic effects of nTiO2 on abiotic reactions that are more relevant to the

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natural environments, such as hydrolysis, are not well understood.

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A limited number of studies have shown that TiO2, like many other metal oxides, can

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catalyze the hydrolysis reactions of certain organic contaminants (e.g., organophosphorus

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pesticides and carboxylate esters),13-17 one of the most important types of abiotic transformation

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reactions controlling the environmental fate of these contaminants.18,19 Even though

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nTiO2-catalyzed hydrolysis may occur relatively more slowly compared to photocatalysis, this

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process constantly occurs and is not limited by time (e.g., daytime) or location (e.g., photic zone)

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and thus may dominate the degradation of organic contaminants in natural aquatic systems. It has

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been proposed that the metal atoms on the surface of metal oxides are likely reactive sites that

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catalyze hydrolysis reactions.13,15,16,20 For instance, metal atoms can coordinate with organic

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compounds or leaving groups and enhance the hydrolysis reactions.15,16 Metal atoms can also

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induce deprotonation of metal-coordinated water molecules, generating metal-hydroxo species

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that can serve as nucleophiles to catalyze hydrolysis reactions.15,16 Considering that the surface

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of TiO2 contains both saturated six-coordinated Ti (Ti6c) and unsaturated five-coordinated Ti

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(Ti5c) atoms,21 it is probably reasonable to assume that the unsaturated (and thus more reactive)

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Ti5c atoms play the most important role in TiO2-mediated hydrolysis reactions of organic

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compounds.

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Naturally occurring crystalline phases of TiO2 include anatase, rutile, brookite, and

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TiO2(B),22 and each crystalline phase may possess different percentages of exposed facets.23 For

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example, anatase TiO2 generally possesses three fundamental low-index facets: {001}, {010}

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and {101}.24 It has been shown that nTiO2 materials of different crystalline phases and exposed

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facets differ significantly in photocatalytic reactivity. TiO2 nanomaterials of different crystalline

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phases possess different energy band structures (i.e., band gap, conduction and valence band

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positions), which in turn determine the light absorption and redox capacity of photogenerated

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charge carriers.25 Meanwhile, the exposed facets of TiO2 can also influence the photocatalytic

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performance, via differences in charge separation efficiency and adsorption of reactant

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molecules.25-27 Moreover, theoretical calculations also showed that the adsorption energies of

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organic compounds on nTiO2 materials vary with crystalline phases and exposed facets.28,29

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Hence, we hypothesize that crystalline phase and exposed facets can also significantly affect the

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efficiency of nTiO2 materials in catalyzing hydrolysis reactions, likely by influencing both the

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adsorption affinities of nTiO2 for organic molecules and activity of surface catalytic sites,

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particularly, Ti5c atoms. The objective of this study was to understand how crystalline phase and exposed facets of

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nTiO2 materials can affect their catalytic effects on environmentally relevant hydrolysis reactions.

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Eight nTiO2 materials—including three anatase TiO2 materials varying in exposed {001} facets

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(referred to as anatase_L001, anatase_M001 and anatase_H001), three rutile TiO2 materials

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varying mainly in particle size (rutile_1, rutile_2 and rutile_3), and two TiO2(B) materials

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varying in morphology and exposed {010} facets (TiO2(B) nanorods and TiO2(B) nanowires,

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referred to as TiO2(B)_rod and TiO2(B)_wire)—were selected to represent different variables of

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the surface structures of nTiO2. The dehydrochlorination reaction of 1,1,2,2-tetrachloroethane

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(TeCA) was selected as the model hydrolysis reaction,30,31 because TeCA is a widely used

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chlorinated solvent32 and dehydrohalogenation reaction is one of the most environmentally

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relevant pathways for the natural attenuation of chlorinated solvents.33 The overall catalytic

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efficiencies of different nTiO2 materials were examined under three pH conditions. The results

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were compared to understand how catalytic efficiency was influenced by crystalline phase and

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exposed facets. Batch adsorption experiments, two-phase kinetic modeling and cation-mediated

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reaction experiments were carried out to further understand the interactions between TeCA and

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different nTiO2 materials.

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MATERIALS AND METHODS

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Materials. Anatase_L001 and the three rutile materials were purchased from Aladdin

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Biological Technology (Shanghai, China). Anatase_M001, anatase_H001 and the two TiO2(B)

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materials were synthesized using the hydrothermal methods reported in the literature.34,35 The

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synthesis procedures are given in Supporting Information (SI).

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TeCA (> 99%) and trichloroethylene (TCE, > 99%) were purchased from Sigma–Aldrich

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(St. Louis, USA). Stock solutions of TeCA and TCE were prepared in methanol, and stored at

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-20 °C. The inorganic salts (KH2PO4, K2HPO4·3H2O, CuCl2, CdCl2 and PbCl2) were obtained

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from Tianjin Chemical Reagent Co. (Tianjin, China).

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Characterization of nTiO2 Materials. The shape and morphology of the nTiO2 materials

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were characterized with scanning electron microscopy (SEM, Nova Nano SEM 230, Hillsboro,

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USA). The Brunauer−Emmett−Teller (BET) specific surface areas of the nTiO2 materials were

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determined by multipoint N2 adsorption–desorption using a Micromeritics ASAP2010

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accelerated surface area and porosimetry system (Micromeritics Co., Norcross, USA). The

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crystal structures of the nTiO2 materials were determined with X-ray diffraction (XRD) using a

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D/Max-2500 diffractometer (Rigaku, Tokyo, Japan) with Cu Kα radiation. Raman spectra were

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recorded with a Renishaw inVia Raman spectrometer (RM2000, London, UK) with an exciting

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wavelength of 633 nm. Surface elemental compositions of the nTiO2 materials were determined

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with X-ray photoelectron spectroscopy (XPS) (PHI 5000 VersaProbe, Tokyo, Japan). Fourier

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transform infrared (FTIR) transmission spectra of the materials were obtained using a 110 Bruker

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TENSOR 27 apparatus (Bruker Optics Inc., Karlsruhe, Germany). The relative hydrophobicity of

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the nTiO2 materials was assessed by measuring n-dodecane–water partition coefficients (the

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detailed sample preparation methods are given in SI)36 and the static water contact angle of the

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materials. The contact angle measurements were carried out in the air using the sessile drop

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method on a contact angle system OCA 20 (DataPhysics Instruments GmbH, Stuttgart,

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Germany). The contact angles reported are mean values measured for 4 µL water droplets at five

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positions of each material.

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The types of nTiO2 surface acid sites were examined with FTIR spectra after pyridine

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adsorption37 using a Tensor 27 FTIR spectrometer (Bruker, Karlsruhe, Germany). The amount of

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acid sites was estimated from temperature-programmed desorption of NH3 (NH3-TPD)38 carried

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out using a Chemisorb 2720 (Micromeritics, Norcross, USA).

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Reaction of TeCA in Homogeneous Aqueous Solution and in nTiO2 Suspensions.

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Reaction kinetic experiments were conducted both in homogeneous aqueous solution (i.e.,

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electrolyte solution in the absence of nTiO2 materials) and in nTiO2 suspensions. The electrolyte

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solution that contained 0.05 M K2HPO4/KH2PO4 as a pH buffer and 0.01 M NaN3 as a

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bio-inhibitor was added to a series of 40-ml amber glass bottles. The pH of the solution was

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adjusted to 7.0, 7.5 or 8.0 with HNO3 or NaOH. The reaction matrices that contained nTiO2

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material and the electrolyte solution were sonicated in a water bath for 30 min and then

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equilibrated on an orbital shaker at 7 rpm for 24 h. Then, 20 µL of a TeCA stock solution (in

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methanol) was injected into the solution with a micro-syringe to give an initial TeCA

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concentration of 2.5 mg/L, and then the bottle was immediately filled with the phosphate buffer

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solution, sealed, and left on an orbital shaker operated at 7 rpm and a constant temperature of

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25 °C in the dark. At predetermined time intervals, 1.0 ml of the solution was withdrawn,

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transferred to a clean glass vial containing a pH-3 buffer solution (0.05 M KH2PO4; pH adjusted

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with HNO3) to terminate the reaction. The solution was filtrated through a 0.22-µm membrane

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filter to remove nTiO2 materials, and then extracted with hexane (3:1, v:v). The hexane extract

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was analyzed to determine the mass of TeCA and TCE (the reaction product) with an Agilent

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6890N gas chromatography with electron capture detector (Agilent Technologies, Santa Clara,

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USA) equipped with an HP-5 capillary column (30 m × 0.32 mm × 0.25 µm). The mass of TeCA

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adsorbed to nTiO2 materials was less than 2% of the total mass (see the next section on

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adsorption experiments) and thus was not quantified in all samples. Solution pH was also

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checked during sampling and was found to be essentially unchanged in all the experiments. Each

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kinetic experiment was run in duplicate. Mass balance was within the range of 92-98% for all the

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experiments (Figures S1-S4).

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In the experiments involving divalent cations (Cu2+, Pb2+ and Cd2+), a stock solution of

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CuCl2, PbCl2, and CdCl2 was added to a 3-morpholinopropanesulfonic acid (MOPS) buffer

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solution containing nTiO2, to reach a predetermined concentration of Cu2+, Pb2+ and Cd2+. Then,

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pH of the solution was adjusted to 7.0 with HNO3 or NaOH. All other procedures were identical

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as those stated above. The effect of cation addition on the aggregation status of nTiO2 was

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examined by dynamic light scattering (DLS) using a ZetaPALS (Brookhaven Instruments,

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Holtsville, USA).

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Adsorption Experiments of TeCA to nTiO2 Materials. Adsorption isotherms of TeCA to

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the nTiO2 materials were obtained using the procedures reported in the literature.39 Briefly, a 1

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g/L nTiO2 suspension (pH 5.0) was added to a series of 8-ml amber glass vials (the low pH was

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used to ensure that abiotic transformation of TeCA would not occur). Next, a stock solution of

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TeCA (in methanol) was added to the vials, and the vials were filled with the electrolyte solution,

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capped and placed on an orbital shaker at 7 rpm for 5 d. Then, the suspensions were filtrated

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through 0.22-µm membrane filters to remove nTiO2 materials, and extracted with hexane (3:1,

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v:v) to determine the mass of TeCA and TCE. The concentrations of TeCA adsorbed to nTiO2

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were calculated based on mass balance. All the adsorption experiments were run in triplicate.

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Data Analysis. The reaction kinetics data of TeCA were fitted with the pseudo-first-order kinetic model: C/C0 = exp (−kobs ⋅ t)

(1)

where C0 (mg/L) is the initial TeCA concentration; C (mg/L) is the TeCA concentration at a 9



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given time t (h); and kobs (h-1) is the observed pseudo-first-order rate constant for the overall

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TeCA degradation.

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A two-phase kinetic model developed in our previous study30 was used to estimate the reaction constant of TeCA adsorbed to the surface of nTiO2 materials: ‫ܥ({ = ܥ‬଴ଵି௡ +

௞౩ ∙௄ూ ⋅஼ొ౉ ௞౗

) ⋅ ݁ [௞౗ ⋅(௡ିଵ)⋅௧] −

௞౩ ∙௄ూ ⋅஼ొ౉ ௞౗

భ

}భష೙

(2)

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where ka (h−1) and ks (h−1) are the calculated pseudo-first-order rate constants for the dissolved

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and nTiO2-adsorbed TeCA, respectively; KF (mg1-nLn/g) is the affinity coefficient of the

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Freundlich isotherm; n (unitless) is the Freundlich linearity index; and CNM (g/L) is the

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concentration of nTiO2 materials in the reaction system. The values of ka were obtained by fitting

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the kinetics data in homogeneous aqueous solution (i.e., reaction in the absence of nTiO2

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materials) with Equation 1.

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RESULTS AND DISCUSSION Characteristics of Different nTiO2 Materials. The SEM images of the nTiO2 materials

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(Figure S5) show the morphology of the materials. Anatase_L001 consisted of primarily

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nanoparticles with 40-nm diameter, whereas anatase_M001 and anatase_H001 were nanoplates

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with an average edge length of ~50 nm. The three rutile nTiO2 materials (rutile_1, rutile_2 and

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rutile_3) were nanoparticles with diameters of 19.8 ± 6.9, 23.7 ± 7.9, and 30.2 ± 7.8 nm,

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respectively. The average length of TiO2(B)_rod was ~200 nm, whereas that of TiO2(B)_wire

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was ~2 µm.

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The XRD patterns and Raman spectra (Figure 1a and 1b) show that all the nTiO2 materials

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were in single crystal phase. According to the intensity ratios of the XRD peaks, the three

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anatase TiO2 materials all had the strongest (101) peak (peak at 25.3°) (Figure 1a), indicating 10


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preferential orientation growth in the {101} direction (JCPDS 21-1272). Nonetheless, the

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intensity ratios for the (004) and (101) diffractions of anatase_M001 and anatase_H001 were

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larger than that of anatase_L001 (Figure 1a, Table S1), indicating that these two materials

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possessed higher percentage of exposed {001} facets than did anatase_L001.40 The percentages

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of the {001} facets, estimated using the Raman spectra,41,42 were 7% for anatase_L001, 21% for

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anatase_M001 and 38% for anatase_H001 (Table S2, Figure 1b). The three rutile TiO2 materials

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all showed the strongest (110) peak (peak at 27.4°) (Figure 1a), indicating preferential

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orientation growth in the {110} direction (JCPDS 21-1276). Additionally, the intensities of the

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other peaks were similar for these three materials (Figure 1a, Table S1), indicating no significant

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differences in exposed facets. Both TiO2(B) materials showed the strongest (110) peak (peak at

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24.9°). However, the intensity ratios for the (010) and (110) diffractions were 0.82 for

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TiO2(B)_rod and 0.79 for TiO2(B)_wire, remarkably larger than the standard value (0.30) in the

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JCPDS card no. 74-1940 (Figure 1a, Table S1), suggesting that they both possessed a significant

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amount of exposed {010} facets and TiO2(B)_rod exposed more {010} facets than

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TiO2(B)_wire.

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All the nTiO2 materials had comparable surface compositions in the same chemical states as

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evidenced by the XPS spectra (Figure S6-8). The peaks with the binding energy of 458.9 and

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464.5 eV were assigned to the Ti 2p3/2 and Ti 2p1/2 spin orbital splitting photoelectrons in the Ti4+

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chemical state, and the O 1s signal revealed two peaks with binding energy of 530.6 and 532.2

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eV, attributable to the lattice oxygen and hydroxyl species, respectively.43,44 The presence of

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hydroxyl groups was also evident from the OH stretching vibrations between 3200 and 3600

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cm-1 in the FTIR spectra (Figure S9).45

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The concentrations of surface Ti5c atoms (C_Ti5c) of the nTiO2 materials were estimated by

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measuring the concentrations of surface Lewis acids (C_acid), since Lewis acid sites on TiO2

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surface are normally attributed to Ti5c atoms.38 The pyridine adsorbed FTIR spectra (Figure S10a)

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show that one strong peak at 1445 cm-1 was observed for the three rutile materials, whereas two

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strong peaks—at 1445 and 1605 cm-1—were observed for the other five materials. It was

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reported that the two peaks at 1445 cm-1 and 1605 cm-1 of pyridine-adsorbed FTIR spectra

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correspond to the coordinated pyridine adsorbed on Lewis acid sites on nTiO2 materials.46 The IR

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absorbance at 1545 cm-1, characteristic of pyridine chemisorbed at Brønsted acid sites, was not

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observed, indicating the absence of Brønsted acid sites on the surfaces of the nTiO2 materials.46

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Due to the absence of Brønsted acid sites on nTiO2 materials, the concentration of the Lewis acid

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sites could be estimated from NH3-TPD, which quantifies the total concentrations of Lewis and

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Brønsted acid sites.38 It is noteworthy that the peak areas of the NH3-TPD profiles of the nTiO2

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materials were markedly different (Figure S10b), indicating that the nTiO2 materials possessed

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different amounts of surface Ti5c. In general, the anatase materials contained higher

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concentrations of surface Ti5c than rutile and TiO2(B). Among the three anatase materials, the

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concentration of Ti5c increased with the percentage of exposed {001} facets, and TiO2(B)_rod

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contained larger content of surface Ti5c than TiO2(B)_wire, likely due to their differences in

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morphology and exposed facets (Table 1, Figure S11).

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Overall Catalytic Efficiency of nTiO2 in TeCA Hydrolysis is Dependent on Both

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Crystalline Phase and Exposed Facets. Under all three pH conditions (7.0, 7.5 and 8.0) the

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dehydrochlorination reaction of TeCA was enhanced in the presence of an nTiO2 material,

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regardless of the specific type of nTiO2 used (Figure S12). For example, in the absence of nTiO2

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(i.e., reaction in homogeneous aqueous solution), only 11% TeCA was transformed after 336 h at

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pH 7.0, whereas in the presence of 100 mg/L nTiO2, transformation of TeCA increased to as

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much as 33%. Catalytic effects of nTiO2 were also observed at pH 7.5 and 8.0 (Figure S12).

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Additionally, the reactivity of TeCA increased substantially with the increase of TiO2

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concentration (Figure S13). TeCA dehydrochlorination in homogeneous aqueous solution

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occurred through a β-elimination (E2) mechanism, in which hydroxide ion attacks the hydrogen

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atom attached to the β-carbon, resulting in the breaking of a C-Cl bond and the formation of a

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C=C bond.47 In the presence of TiO2, heterogeneous catalytic reaction of TeCA on the surface of

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TiO2 (i.e., reaction of TiO2-adsorbed TeCA) occurred simultaneously.

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The reaction kinetics followed the pseudo-first-order reaction kinetics reasonably, and the

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fitted kinetic constants (Table S3) were used hereafter to compare the overall catalytic

254

efficiencies between different nTiO2 materials. It is evident that the overall catalytic efficiencies

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of different nTiO2 materials differed from each other, especially at pH 7.0. Strikingly, the

256

BET-surface-area-normalized kobs values showed clear dependency on both crystalline phase and

257

exposed facets of nTiO2. Upon surface-area normalization, overall catalytic efficiencies of nTiO2

258

generally followed the order of rutile > anatase > TiO2(B), anatase_H001 > anatase_M001 >

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anatase_L001 among the three anatase TiO2, and TiO2(B)_rod > TiO2(B)_wire between the two

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TiO2(B) materials (Figure 2). Note that the three rutile materials differed mainly in particle size.

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According to previous research,48 particle size influences catalytic efficiency of nTiO2 primarily

262

by affecting surface area. However, the variation among the kobs values of the three rutile

263

materials remained after surface-area-normalization (Figure 2), indicating that other factors also

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affected the catalytic efficiency of the rutile materials. This might be attributable to the small

265

differences in exposed {101} and {211} facets among the three materials (Table S1).

266 267

Crystalline Phase and Exposed Facets Affect Adsorption Affinities of nTiO2. It is commonly accepted that the overall catalytic efficiency of a catalyst is largely affected by its

13



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adsorption affinity for target contaminants.49 Indeed, our study demonstrated the variable

269

adsorption affinities of the eight nTiO2 materials for TeCA (Figure S14), which likely have

270

contributed to the different reaction kinetics of TeCA catalyzed by these nanomaterials (Figure

271

S12). Strikingly, the surface-area-normalized adsorption coefficient, Kd (corresponding to an

272

equilibrium aqueous-phase TeCA concentration of approximately 2.5 mg/L), showed clear

273

dependence on both crystalline phase and exposed facets (Figure 3a). Specifically, the

274

normalized Kd values of the rutile TiO2 were significantly higher (3.2–5.3 times) than those of

275

the anatase TiO2 and TiO2(B), and among the three anatase TiO2 the normalized Kd increased

276

with the increasing percentage of exposed {001} facets (Figure 3a).

277

Adsorption of nonpolar, nonionic, hydrophobic organic contaminants (such as TeCA) to

278

metal oxides is driven primarily by the hydrophobic effect.50 Accordingly, a reasonable

279

explanation for the crystalline-phase- and facet-dependent adsorption affinity of nTiO2 for TeCA

280

is that the nTiO2 materials differed in crystalline phase and exposed facets possessed different

281

surface hydrophobicity. To verify this hypothesis, we compared the surface hydrophobicity of the

282

nTiO2 materials by measuring both their water contact angles51 and n-dodecane–water partition

283

coefficients (KDW).36 As expected, the relative adsorption affinities among the nTiO2 materials

284

correlated well with their surface hydrophobicity (Figures 3b and 3c). Overall, the relative

285

hydrophobicity of the rutile materials was higher than the anatase and TiO2(B) materials, as

286

shown by their larger water contact angles and KDW values (Table 1, Figure S15), leading to the

287

higher adsorption affinities of three rutile materials. Similarly, anatase materials with larger

288

percentage of exposed {001} facets had higher surface hydrophobicity and thus, higher

289

adsorption affinities. This demonstrated that crystalline phase and exposed facets determined the

290

adsorption of TeCA to nTiO2 by controlling the surface hydrophobicity of nTiO2.

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Crystalline Phase and Exposed Facets Determine Activity of Surface Catalytic Sites on

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nTiO2. Even though the trend in overall catalytic efficiencies of nTiO2 appeared to be consistent

293

with the trend in their adsorption affinities, the variations among the eight nanomaterials were

294

not quantitatively comparable between the adsorption and hydrolysis experiments (Figures 2 and

295

3a). In the case of anatase versus rutile, the difference in surface-area-normalized adsorption

296

coefficients appeared to be much greater than the difference in surface-area-normalized kinetic

297

constants (Figures 2 and 3a). Similarly, the TiO2(B) and anatase materials showed similar

298

adsorption affinity (Figure 3a), whereas anatase generally exhibited greater overall catalytic

299

efficiency (Figure 2). To discern the additional factors controlling the overall catalytic efficiency

300

of TiO2, we compared the apparent reaction kinetic constants of TiO2-adsorbed TeCA (ks) among

301

different materials. Remarkably, the ks values, which indicate the activity of surface catalytic

302

sites, were also dependent on both crystalline phase and exposed facets (Figure 4). Anatase TiO2

303

exhibited higher ks values than both rutile TiO2 and TiO2(B), consistent with the abovementioned

304

effect of crystalline phase. Additionally, anatase materials with larger percentage of exposed

305

{001} facets had greater ks values, and the TiO2(B) material containing more {010} facets

306

exhibited relatively larger ks (Table S1 and Figure 4).

307

As mentioned earlier, the unsaturated Ti5c atoms on the surface of nTiO2 are likely the most

308

important surface catalytic sites in TeCA dehydrochlorination. The amounts of surface reactive

309

Ti5c sites varied depending on crystalline phase and exposed facets (Figure S16). An interesting

310

observation was that the relative abundance of surface Ti5c among the nTiO2 materials (Figure

311

S11) appeared to follow the same trend as their ks values (Figure 4). Overall, reasonable

312

correlations were observed between the ks values and the estimated concentrations of Ti5c (C_Ti5c)

313

(Figure 5), indicating that Ti5c atoms likely served as the predominant active sites to catalyze the

15



314

dehydrochlorination of TeCA. In aqueous solution, the surface of nTiO2 surface is saturated with

315

water molecules, and the Ti5c atoms (that are unsaturated and thus more reactive) would tend to

316

be coordinated by hydroxyl groups, which after deprotonation, can catalyze the base-promoted

317

dehydrochlorination of TeCA.30,52

318

It is noteworthy that the correlation between ks and C_Ti5c is more significant under higher

319

pH values (Figure 5). This was consistent with the mechanism that the deprotonated hydroxyl

320

groups chemisorbed to Ti5c were the primary catalytic moiety, in that, increasing pH would

321

facilitate deprotonation of the hydroxyl groups, resulting in greater catalytic activity. Moreover,

322

the catalytic efficiencies of nTiO2 in TeCA dehydrochlorination were considerably weakened in

323

the presence of divalent metal cations (i.e., Cu2+, Pb2+ and Cd2+) that bound to and likely

324

inactivated the hydroxyl groups on nTiO2 surface (Figure S17). The relative inhibiting effects

325

followed the order of Cu2+ > Pb2+ > Cd2+, consistent with the binding strength of these cations to

326

hydroxyl groups.53 This result was also in line with the role of the surface hydroxyl groups in

327

nTiO2-catalyzed hydrolysis reactions, even though the potential effects of cation-induced

328

aggregation on the concentrations of exposed surface reactive sites, i.e., hydroxyl groups on Ti5c,

329

could not be ruled out (for instance, the hydrodynamic diameter of anatase_L001 was 303.6 ±

330

25.4 nm in MOPS buffer, but increased to 392.8 ± 18.4 nm when the buffer contained 0.5 mM

331

Cu2+).

332

An interesting observation was that the C_Ti5c-normalized ks appeared to positively

333

correlate with the percentage of the Ti5c sites with low acidic activity (Table S4 and Figure S18;

334

the Ti5c sites with low acidic activity are operationally defined as acid sites with NH3 desorption

335

temperature below 300 °C in Figure S10b), possibly because their conjugate bases (i.e.,

336

deprotonated hydroxyl groups coordinated with Ti5c) were relatively strong and thus can 16


Page 16 of 30

Page 17 of 30


337

efficiently catalyze the dechlorination process. This result not only further corroborated that the

338

surface hydroxyl groups chemisorbed to Ti5c were the active catalytic sites, but also underscored

339

that both the amount and strength of surface catalytic sites need to be considered in order to

340

accurately predict the catalytic efficiencies of nTiO2. For example, anatase_H001 showed large

341

ks value due to its high abundance of surface Ti5c sites (Figure 5), whereas the percentage of Ti5c

342

with low acidity on the surface of anatase_H001 was relatively small (Figure S10b and Table S4)

343

and as a result the ks per unit reactive sites (quantified as Ti5c concentrations) was low (Figure

344

S18).

345

Environmental Implications. Hydrolysis of contaminants is one of the most important

346

abiotic transformation processes controlling the environmental transport, fate and effects of

347

contaminants. The significant catalytic effects of nTiO2 materials observed in this study indicate

348

that when released into the environment, nTiO2 (and possibly other metal oxide nanomaterials)

349

may significantly affect hydrolysis reactions of organic contaminants. Evidently, overall catalytic

350

efficiencies of TiO2 nanomaterials are greatly affected by their intrinsic properties, including

351

crystalline phase and exposed facets that determine both the adsorption capacity of nTiO2 for

352

organic contaminants and the activity of surface catalytic sites. In particular, crystalline phase

353

and exposed facets control the abundance and activity of surface unsaturated Ti atoms and

354

therefore are essential parameters for predicting the kinetics of hydrolysis reactions occurring on

355

the surface of nTiO2. Findings of this study may have important implications for assessing the

356

environmental impacts of TiO2 nanomaterials, and may also shed light on the design of novel

357

catalytic nanomaterials for contaminant removal and environmental remediation via crystal

358

engineering.

359

17



360

Acknowledgments. This project was supported by the National Science Fund for Distinguished

361

Young Scholars (Grant 21425729), the Ministry of Science and Technology of China (Grant

362

2014CB932001), and the National Natural Science Foundation of China (Grants 21237002). We

363

thank three anonymous reviewers for their valuable comments and suggestions on mechanistic

364

interpretation.

365 366

Supporting Information Available: Material synthesis procedures; tables summarizing intensity

367

ratios of XRD, calculated percentage of {001} facets, experimental parameters and fitted kinetic

368

constants, and relative abundance of the weak and strong acid sites; figures showing mass

369

balance data of kinetic experiments, SEM images, XPS spectra, FTIR spectra, pyridine-adsorbed

370

FTIR spectra and NH3-TPD profiles of nTiO2 materials, Ti5c concentrations, dehydrochlorination

371

kinetics of TeCA, effects of TiO2 content and divalent cations on dehydrochlorination kinetics,

372

adsorption isotherms of TeCA, photographs of water contact angles on surface of nTiO2

373

materials, schematic illustration of faceted TiO2 surfaces, and effects of acid strength on catalytic

374

efficiencies of nTiO2 materials. This information is available free of charge via the Internet at

375

http://pubs.acs.org.

376 377

Notes—The authors declare no competing financial interest.

378 379

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380

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Table 1. Selected Physicochemical Properties of the nTiO2 Materials

type of nTiO2

dominant

SABET 2

a

exposed facets

(m /g)

Anatase_L001

{101}

Anatase_M001

surface O b surface Ti

(at%)

b

C_acid (C_Ti5c) c Olatt/Ti

(at%)

ratio

57.4

30.1

7.4

60.2

29

9.8

{110}

31

Rutile_2

{110}

Rutile_3

OH

Olatt

83

12.5

{101}

35

Anatase_H001

{101}

Rutile_1

(µmol/m2)

b

KDW d

water contact angle (°)

total

weak

strong

1.87

2.1

1.1

1.0

0.04

17

32.4

1.86

6.3

1.4

4.9

0.09

18

57.4

32.8

1.75

6.6

0.8

5.8

0.09

20

13.2

56.4

30.4

1.86

0.9

0.8

0.1

0.56

35

28

12.6

57.2

30.2

1.89

0.9

0.4

0.5

0.49

33

{110}

27

12.8

57.0

30.2

1.89

0.8

0.7

0.1

0.33

25

TiO2(B)_rod

{110} & {010}

106

15.4

54.2

30.4

1.78

2.4

0.8

1.6

0.04

20

TiO2(B)_wire

{110} & {010}

185

17.2

52.4

30.4

1.72

0.7

0.2

0.5

0.03

19

a

SABET = surface area measured using the Brunauer–Emmett–Teller (BET) method.

b

Analyzed by X-ray photoelectron spectroscopy (XPS). The values represent the average of duplicate samples.

c

C_acid represents acid concentration (measured with NH3-TPD) normalized by surface area, which is approximately the concentration of surface Ti5c of

nTiO2 materials (C_Ti5c). d

KDW represents n-dodecane–water partition coefficient of nTiO2.

25



(a)

Page 26 of 30

(b) A(101)

A(004)

I I II II

R(110)

R(101)

Intensity (a.u.)

Intensity (a.u.)

III R(211) IV

V

B(110)

10

20

30

B(010)

40

50

60

70

III IV V

VI

VI

VII

VII

VIII

VIII

100

80

200

300

400

500

600

700

Raman shift ( cm-1)

2θ ( Deg)

Figure 1. The XRD patterns (a) and Raman spectra (b) of the nTiO2 materials. The letters A, R and B refer to the anatase, rutile, and TiO2(B) phases, respectively. The numbers I, II, III, IV, V, VI, VII and VIII refer to anatase_L001, anatase_M001, anatase_H001, rutile_1, rutile_2, rutile_3, TiO2(B)_rod and TiO2(B)_wire, respectively.

26


Page 27 of 30


(b)

(a) pH 7.0

kobs/SABET (g h

kobs/SABET (g h

pH 7.5 6.8e-5

-1

2.8e-5

m-2)

8.5e-5

-1

m-2)

3.5e-5

2.1e-5 1.4e-5 7.0e-6

3.4e-5 1.7e-5 0.0

An a

An a

ta An se_ L0 at 0 a An se_ 1 at M as 00 e_ 1 H0 Ru 0 1 til e_ 1 R ut ile _2 R Ti util O e_ 2 3 Ti (B)_ O 2 ( rod B )_ w ire

ta An se_ L0 at 0 a An se_ 1 at M0 as 0 e_ 1 H 00 R ut 1 ile _1 R ut ile _2 R Ti uti le O _3 2( Ti B)_ O r o 2( d B )_ w ire

0.0

5.1e-5

(c) pH 8.0 1.6e-4

kobs/SABET (g h

-1

m-2)

2.0e-4

1.2e-4 8.0e-5 4.0e-5

An a

An a

ta s

e_

L0

ta 0 s An e_ 1 at M0 as 01 e_ H 0 R 01 ut ile _1 R ut ile _2 R Ti uti O le_ 2( 3 Ti B)_ O 2 ( rod B) _w i re

0.0

Figure 2. Surface-area-normalized apparent pseudo-first-order kinetic constants (kobs) of different nTiO2 materials at pH 7.0 (a), 7.5 (b) and 8.0 (c). The kobs values were obtained using Eqn. 1. Each color of the columns represents a crystalline phase of nTiO2.

27



Page 28 of 30

(a)

2 Kd/SABET (L/m )

8.0e-3 6.4e-3 4.8e-3 3.2e-3 1.6e-3

An at

a An se_ L at as 001 An e_ a t M0 as 0 e_ 1 H0 01 R ut ile _1 R ut ile _2 R ut Ti i l e O _3 2 Ti (B) O _r od 2( B )_ w ire

0.0

(c) 8.0e-3

6.4e-3

6.4e-3

2 Kd/SABET (L/m )

2 Kd/SABET (L/m )

(b) 8.0e-3

4.8e-3 3.2e-3 1.6e-3

2 R = 0.9132

0.0

4.8e-3 3.2e-3 1.6e-3

2 R = 0.9710

0.0 15.0

20.0

25.0

30.0

35.0

40.0

0.0

0.1

Water ontact angle (o) Anatase_L001 Anatase_M001

0.2

0.4

0.5

0.6

KDW Anatase_H001 Rutile_1

Rutile_2 Rutile_3

TiO2(B)_rod TiO2(B)_wire

Figure 3. Surface-area-normalized adsorption coefficients (Kd) of different nTiO2 materials (a), and correlations between surface-area-normalized Kd and water contact angle (b) and n-dodecane–water partition coefficients (KDW) (c) of the nTiO2 materials. The Kd values correlate to an equilibrium aqueous-phase 1,1,2,2-tetrachloroethane concentration of approximately 2.5 mg/L. Each color of the columns represents a crystalline phase of nTiO2.

28


Page 29 of 30


(b)

(a) 0.060

0.180

pH 7.0

pH 7.5

0.036

0.000

0.000 e_

se

ta s An a

An a

An a

ta

se

_L 00 1 An e_ M at 0 as 0 e_ 1 H 00 1 R ut ile R _1 ut ile _2 R u Ti O tile _3 2( B )_ Ti O ro 2( d B )_ w ire

0.012

L0 01 _M 0 ta se 01 _H 00 1 R ut ile R _1 ut ile _2 R u Ti t i l O e_ 2( 3 B ) Ti O _r o 2( B d )_ w ire

0.072

ta

0.024

0.108

An a

0.036

An a

-1 ks ((h )

0.144

ta s

-1 ks ((h )

0.048

(c) 0.240

pH 8.0

-1 ks ((h )

0.192 0.144 0.096 0.048

An a

An a

ta

se

_L 0 ta se 01 An _M at as 001 e_ H 00 1 R ut ile _ R ut 1 ile _2 R Ti uti le O _3 2( B )_ Ti O ro 2( d B )_ w ire

0.000

Figure 4. Reaction kinetic constants of 1,1,2,2-tetrachloroethane adsorbed to different nTiO2 materials (ks) at pH 7.0 (a), 7.5 (b) and 8.0 (c). The ks values were obtained using Eqn. 2. Each color of the columns represents a crystalline phase of nTiO2.

29



(a)

(b)

0.060

0.180

pH 7.5

pH 7.0 0.048

0.144

-1 ks (h )

-1 ks (h )

Page 30 of 30

0.036 0.024 0.012

0.108 0.072 0.036

2 R = 0.7726

0.000

2 R = 0.7923

0.000 0.0

2.0e-3

4.0e-3

6.0e-3

8.0e-3

0.0

2.0e-3

4.0e-3

6.0e-3

8.0e-3

2 C_Ti5c (mmol/m )

2 C_Ti5c (mmol/m ) (c) 0.240

Anatase_L001 Anatase_M001 Anatase_H001 Rutile_1

pH 8.0

-1 ks (h )

0.192 0.144

Rutile_2 Rutile_3 TiO2(B)_rod

0.096

TiO2(B)_wire

0.048

2 R = 0.8698

0.000 0.0

2.0e-3

4.0e-3

6.0e-3

8.0e-3

2 C_Ti5c (mmol/m )

Figure 5. Correlations between the reaction kinetic constants of 1,1,2,2-tetrachloroethane adsorbed to different nTiO2 materials (ks) and the Ti5c concentrations on nTiO2 surface at pH 7.0 (a), 7.5 (b) and 8.0 (c). The ks values were obtained using Eqn. 2. Each color of the symbols represents a crystalline phase of nTiO2.

30


Nano-TiO2-Catalyzed Dehydrochlorination of 1,1,2,2

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