Impacts of Morphology and Crystallite Phases of Titanium Oxide on

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Environ. Sci. Technol. 2010, 44, 3913–3918

Impacts of Morphology and Crystallite Phases of Titanium Oxide on the Catalytic Ozonation of Phenol SHUANG SONG,* ZHIWU LIU, ZHIQIAO HE, ANGLIANG ZHANG, AND JIANMENG CHEN* College of Biological and Environmental Engineering, Zhejiang University of Technology, Hangzhou 310032, People’s Republic of China YUEPING YANG AND XINHUA XU Department of Environmental Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China

Received February 10, 2010. Revised manuscript received April 8, 2010. Accepted April 9, 2010.

TiO2 nanomaterial is widely used for catalytic ozonation. In the present work, TiO2 nanostructures with various morphology and crystallite phases were synthesized by a hydrothermal method, followed by calcination using Degussa P25 as precursor. The nanotube, nanorod, and nanowire forms were obtained by varying the hydrothermal temperature, and the anatase/rutile ratios were adjusted by controlling the annealing temperature. The catalytic activity of the samples was evaluated by degradation of phenol in aqueous solution in the presence of ozone. We found that the initial degradation rates (IDR) of phenol were dominated primarily by the surface OH groups. Thus, with the help of transmission electron microscopy (TEM), X-ray diffraction (XRD), and Brunauer-Emmett-Teller (BET) analyses, the number of surface OH groups per unit area of TiO2 was correlated with the morphology and crystallite phases. Finally, we conclude that the vast surface area and higher rutile phase ratios are favorable for the catalytic ozonation of phenol and the morphology of TiO2 had negligible effect in our experiments.

Introduction Heterogeneous catalytic ozonation, a novel alternative to traditional advanced oxidation processes (AOPs), has been receiving a great deal of attention as a promising technology, owing to its potentially greater effectiveness in the mineralization of dissolved organics and its lower negative effect on water quality (1-3). The transition metal oxides, viz., MnO2, TiO2, Ni2O3, NiO, Fe2O3, CuO, ZnO, CoO, V2O5, Cr2O3, and MoO, are the most commonly used catalysts for the ozonation processes (4-9). Among them, titanium oxide (TiO2) is chemically stable during the ozonation process, is comparatively inexpensive to produce, and is nontoxic (10-12). The role of TiO2 in promoting ozonation reactions is well documented (9, 10, 13-15). It is well-known that oxygen vacancies with Ti3+ atoms in the lattice are present on both anatase and the rutile phase TiO2 (16). Ozone and/or water * Address correspondence to either author. Tel.: 86-571-88320726 (S.S.); 86-571-88320386 (J.C.). Fax: 86-571-88320276 (S.S.); 86-57188320884 (J.C.). E-mail: [email protected] (S.S.); [email protected]. 10.1021/es100456n

 2010 American Chemical Society

Published on Web 04/21/2010

molecules fill the oxygen vacancies on the TiO2 surface where they undergo distortion until they become unstable and then dissociate to form surface oxygen atoms and surface OH groups, which could react with the ozone molecules, leading to generation of unstable oxidizing species reacting with nonchemisorbed organic molecules either in solution or on the surface (4, 9, 17, 18). The influence of morphology on catalytic activity has been demonstrated in many fields (19-21). One-dimensional (1D) nanostructures (e.g., nanotubes, nanowires, and nanorods) have been studied extensively as a result of their unique electronic and optical properties and their potential applications in modern technology (22-24). Nanotubes have been proposed to facilitate the adsorption of reactants on the active surface sites, due to a high specific surface area and an open mesoporous morphology (25, 26). Nanowires are regarded as a superior structure for charge transport, which makes it an interesting factor in sensing applications (20, 27). Nanorods have higher reversibility and insertion ratios with respect to compact materials in lithium rechargeable batteries, which is ascribed to their large surface area and the numerous structure defects (28, 29). Hence, care is needed when assuming that morphology is the decisive factor of activity in a catalytic ozonation reaction system. In addition, the crystal phase has been suggested to be one of the major factors responsible for the catalytic activity of TiO2 (14, 30, 31). There are four main physical forms of TiO2: brookite (orthorhombic), anatase (tetragonal), rutile (tetragonal), and TiO2-B (probably monoclinic) (32). It is commonly believed that anatase is the active phase in the photocatalytic reaction, whereas pure rutile normally shows no activity in this case (33). TiO2-B has been shown to be an excellent candidate for hosting lithium because its structure is more open than that of the other forms (34). Moreover, it is worth noting that rutile is more beneficial in enhancing ozonation catalytic activity than anatase in the degradation of nitrobenzene (14). However, there is no report of a systematic investigation into the dependence of the crystal phase of TiO2 on catalytic ozonation. The effect of the morphology and crystal structure of TiO2 on the initial degradation rate (IDR) of the catalytic ozonation of phenol was investigated in this study. The surface OH groups were quantified to establish the relationship between the number of surface OH and the morphology as well as the number of surface OH groups and the crystal phase composition.

Experimental Section Materials and Preparation. All chemicals used in this study were of analytical grade and were used without further purification. Deionized water was used in all experiments. The TiO2 samples with different morphology were fabricated as described (35). Briefly, 3 g of TiO2 powder (P25, Degussa) was dispersed in 10 M NaOH and then charged into a 180 mL Teflon-lined autoclave container. The autoclave was maintained at various temperatures (110, 160, and 200 °C, respectively) under autogenous pressure for 24 h. After cooling to room temperature, the precipitated powder was filtered and washed with 0.1 M HCl and deionized water until the pH of the rinse solution was ∼7.0 and very little, if any, HCl or NaOH remained in the powder, ensuring that no additional acid or base was introduced into the reaction solution for heterogeneous catalytic ozonation accompanied with the addition of catalysts. The product was air-dried at 80 °C and then calcined in a chamber furnace (CWF 1100, CARBOLITE) at preset temperature (400, 600, 750, and 800 VOL. 44, NO. 10, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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°C) for 2 h with a heating rate of 5 °C min-1 and was then allowed to cool naturally to room temperature. The nomenclature used to represent the catalysts is as follows: X-Y-Z, where X refers to the abbreviation of the shape of the catalyst, including nanotube (NT), nanowire (NW), nanorod (NR) and nanopolyhedron (NP), Y denotes the hydrothermal temperature, and Z is the calcination temperature. Characterization of Catalysts. Transmission electron microscopy (TEM; Tecnai G2 F30 S-Twin microscope operated at an accelerating voltage of 300 kV with 0.20 nm point resolution) was used to investigate the morphology of the TiO2. X-ray powder diffraction (XRD; Thermal ARL X-ray diffractometer (Thermo, France) with Cu KR radiation, accelerating voltage 45 kV, and applied current 40 mA) measurements were done to evaluate the identity of any 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 Micromeritics ASAP 2010 apparatus, and the specific surface area was determined from the Brunauer-Emmett-Teller (BET) equation. The concentration of surface OH groups (expressed in mmol per unit gram) was measured by a saturated deprotonation method as described (36). Procedures and Analysis. The catalytic degradation of phenol in the presence of ozone was done in a semicontinuous flow mode, which consists of a CHYF-3A ozone generator (Hangzhou Rongxin Electronic Equipment Co., Ltd., China), a 1000 mL reactor (diameter of 80 mm, height of 260 mm), a thermostatically controlled bath (THD-2006, Ningbo Tianheng Instrument Factory, China), and a mass flow controller (D07-12A/ZM, Beijing Qixing Electronic Equipment Co., Ltd., China). In a typical catalytic degradation procedure, a volume of 800 mL of simulated wastewater (the initial concentration of phenol was 100 mg L-1) and 0.8 g of catalyst powder were placed in the reactor. The solution was stirred continuously by a magnetic follower, and a mixture of O2 and O3 was fed into the reactor at a flow rate of 50 mL min-1 through the porous plastic diffuser located at the bottom of the reactor to produce fine bubbles. The concentration of O3 in the O3/O2 mixture was ∼55 mg L-1 as measured by an iodimetric method (37). The temperature of the solution was kept constant at 20 ( 1 °C. The same procedures were used for the control experiments of single ozonation, without a catalyst. To absorb the residual ozone, the off-gas was introduced into a glass bottle containing 2% (w/v) KI solution. A sample of the suspension was withdrawn at regular time intervals to determine the residual chemical oxygen demand (COD). The COD values were obtained by oxidation with K2Cr2O7 under acidic conditions, and titration analysis was with aqueous (NH4)2Fe(SO4)2 (38). It is worth noting that the effect of adsorption on the catalytic ozonation process could be neglected in our experiments because the COD values were measured without removing TiO2. A similar experimental method was used by Stylidi and co-workers (39).

Results and Discussion Characterization of the Catalysts. The catalysts were characterized by XRD, TEM, and BET techniques, in an attempt to correlate the physicochemical properties of the catalysts with the heterogeneous catalytic degradation of phenol with ozone. Details of the characterization of the TiO2 catalysts are given in the Supporting Information. Briefly, the crystalline structure of the catalyst samples was assessed on the basis of the XRD profiles, as shown in Figure 1. The patterns show that the phase structure of TiO2 compounds depend on the hydrothermal temperature. The diffraction peaks of samples NT-110-400 and NR-160-400 were assigned to the well crystallized anatase phase, and the 3914

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FIGURE 1. XRD patterns of TiO2 hydrothermal treatment at (a) 200 °C, (b) 160 °C, and (c) 110 °C, followed by calcining at 400 °C and TiO2 hydrothermal treatment at 110 °C, followed by calcining at (d) 600 °C, (e) 750 °C, and (f) 800 °C. peaks of NW-200-400 corresponded well to TiO2-B. Additionally, NT-110-400, NP-110-600, NP-110-750, and NP110-800 had similar 2θ values of the diffraction peaks, suggesting that no additional phase was contained in the samples, except the anatase and rutile forms in the calcination temperature range of 400-800 °C. However, the calcination temperature has an important role in the composition of the catalyst crystals. The phase contents of TiO2 were calculated according to eqs 1 and 2 (40), and the results are given in Table 1. WR ) 1/(1 + 0.8IA /IR)

(1)

WA ) 1 - WR

(2)

where WR and WA are the rutile and anatase titania content, respectively, and IA and IR represent the diffraction intensities of anatase (101) and rutile (110). The structure and morphology of the sample synthesized were affected substantially by both the hydrothermal temperature and the calcination temperature. As reflected in Figure 2, the samples of NT-110-400, NR-160-400, and NW200-400 display clear-cut images of nanotube, nanorod, and nanowire morphology, respectively. In addition, when the calcination temperature reached 600 °C, the tube-like structure vanished completely and it was transformed into a nanopolyhedron-like structure. The BET surface area (SBET) of the prepared TiO2 is given in Table 1. We found that the largest SBET comes from the sample treated hydrothermally at 110 °C followed by sintering at 400 °C. Further, SBET decreased with increased hydrothermal temperature and annealing temperature. Comparison of the Catalytic Ozonation Activity of the Catalysts. The COD removal, an important parameter with which to evaluate the mineralization degree of organic matter, was used to affirm the efficiency of catalytic degradation of phenol with ozone. The COD removal after reaction for 180 min in the eight processes is presented in Figure 3a,b. When comparing the degradation rates of single ozonation (without catalyst) and heterogeneous catalytic ozonation, we found that the TiO2 catalysts exhibited excellent performance in the degradation of phenol and ozone itself is not strong enough to degrade phenol. Thus, the TiO2 catalysts can cause an increase in the ozone decomposition rate to provide highly oxidant species. As depicted in Table 1, different catalysts show distinct contributions to the COD removal of phenol. The catalytic

TABLE 1. Refined Physicochemical Parameters and the Initial Degradation Rate of Phenol for the Prepared Catalysts sample

SBET (m2 g-1)

crystallite phase (%)

concentration of surface OH (mmol g-1)

density of surface OH (µmol m-2)

initial degradation rate (IDR) (mg L-1 min-1)

IDR/S (mg L-1 min-1 m-2)

NT-110-400 NR-160-400 NW-200-400 P25 NP-110-600 NP-110-750 NP-110-800

287 110 38 51 62.4 23 6.2

A 100 A 100 TiO2-B 100 A 80 R 20 A96 R 4 A 50 R 50 A 10 R 90

0.551 ( 0.048 0.200 ( 0.015 0.063 ( 0.008 0.136 ( 0.014 0.138 ( 0.013 0.190 ( 0.016 0.097 ( 0.012

1.92 ( 0.17 1.82 ( 0.14 1.66 ( 0.21 2.67 ( 0.28 2.22 ( 0.21 8.24 ( 0.72 15.69 ( 2.21

6.14 ( 1.23 2.13 ( 0.29 0.79 ( 0.23 2.30 ( 0.71 1.50 ( 0.92 2.49 ( 1.11 1.04 ( 0.43

0.027 ( 0.005 0.024 ( 0.003 0.026 ( 0.008 0.056 ( 0.018 0.030 ( 0.019 0.135 ( 0.060 0.210 ( 0.086

activities, denoted as the IDR of phenol, follow the decreasing order NT-110-400 > NP-110-750 > P25 > NR-160-400 > NP110-600 > NP-110-800 > NW-200-400. For catalysts obtained by treatment at various hydrothermal temperatures, NT110-400 had the highest catalytic activity and the IDR reached ∼6.14 mg L-1 min-1, which exceeded that of NR-160-400 and NW-200-400 by a factor of about 2.88-fold and 7.77-fold, respectively. For catalysts obtained at different sintering temperature, the IDR decreased with increasing temperature of calcination. The surface OH groups on the heterogeneous catalytic surface are believed to have an important function for catalytic ozonation (3, 4, 8). The quantity of the surface OH groups was determined to acquire more detailed information on their relationship with the morphology and crystallite phases of catalysts, and the results are given in Table 1. Figure 4 shows that there is a good linear correlation between IDR and the relative concentration of surface OH in the process of phenol catalytic ozonation, indicating that the removal efficiency of COD is determined by the concentration of surface OH in this case. On the basis of this, the degradation trend described above could be interpreted, in part, in terms of the influence of the crystal phase composition and morphology of TiO2 on the ozone catalytic reactions of phenol, as discussed below. It should be noted that the pH of the solution was not controlled in our experiments. As mentioned above, the reduction of COD depends strongly on the concentration of surface OH during the catalytic ozonation of phenol. In other words, the efficient degradation of phenol with ozone is attributable mainly to catalysis by TiO2. Moreover, considering that the degradation rate can change during each run,

the IDR was used to evaluate the catalytic activity of TiO2. Consequently, we performed all catalytic ozonation experiments without attempting to control the solution pH. Effect of Crystal Phase on the Catalytic Ozonation of Phenol. Different crystal phases have different amounts of oxygen vacancy sites, which is considered to be a decisive parameter contributing to the performance of the catalytic ozonation process (41-44). We can, therefore, deduce that the different crystal phase component might affect the degradation of phenol during catalytic ozonation. To investigate whether the crystal phase of TiO2 is correlated with the degradation of phenol, samples with different crystal phase combinations were used to compare the degradation rate. To avoid the influence of surface area, values of the initial degradation rate per unit area of TiO2 (IDR/S) were calculated, and the results are given in Table 1. Figure 3c shows that there is a positive correlation between the amount of rutile phase contents of samples P25, NT-110-400, NP110-600, NP-110-750, and NP-110-800 and the corresponding data of IDR/S. Additionally, the yield of the number of surface OH per unit area (called density of surface OH here) increased linearly with the rutile phase content, as shown in Figure 5a. Consequently, the rutile phase exhibited superior catalytic activity in ozonation in comparison to the anatase phase. The following hypothesis is put forward to explain the reason for the high level of the catalytic activity of the rutile phase for ozonation reactions. As reported, the surface of anatase is less propitious for the formation of oxygen vacancies with respect to rutile, because the removal of a bridging oxygen results in the formation of a 5-fold coordinated Ti site on rutile, which is more stable than a 4-fold coordinated Ti site generated at oxygen vacancies on anatase

FIGURE 2. TEM images of TiO2 hydrothermal treatment at (a) 110 °C, (b) 160 °C, and (c) 200 °C followed by calcining at 400 °C and TiO2 hydrothermal treatment at 110 °C, followed by calcining at (d) 600 °C, (e) 750 °C, and (f) 800 °C. VOL. 44, NO. 10, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. COD removal of phenol by ozonation over TiO2 with hydrothermal treatment at 110 °C, followed by calcining at 400, 600, 750, and 800 °C and by ozonation over P25 (a, c), as well as by ozonation over TiO2 with hydrothermal treatment at 110, 160, and 200 °C, followed by calcining at 400 °C and by single ozonation (b, d). Experimental conditions: initial concentration of phenol, 100 mg L-1; catalyst dose, 1.0 g L-1; O3/O2 flow rate, 50 mL min-1.

FIGURE 4. Correlation between the IDR of phenol and the concentration of surface OH groups on catalysts. (45-47). Furthermore, the number of Ti3+ sites located in the rutile component is significantly greater than those in the anatase phase (44). The concentration of surface OH groups depends on the amount of oxygen vacancy sites, and several investigators have asserted that a water molecule would dissociate at an oxygen vacancy site, leading to the formation of two surface OH groups (16, 48); therefore, we 3916

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conclude that the crystal phase is responsible for catalytic ozonation and the rutile phase is more appropriate for the system. Effect of Morphology on the Catalytic Ozonation of Phenol. The catalytic ozonation might depend on the morphology of catalysts. To investigate the effect of the shape of TiO2 on the catalytic degradation of phenol in the presence of ozone, NT-110-400, NR-160-400, and NW-200-400 catalysts, representing nanotube-like, nanorod-like, and nanowire-like, respectively, were chosen to eliminate the action of the rutile phase. As can be seen from Table 1, a higher hydrothermal temperature led to a slower IDR, which was 6.14, 2.13, and 0.79 mg L-1 min-1 when the hydrothermal temperature was 110, 160, and 200 °C, respectively. Nevertheless, the IDR/S was almost the same, regardless of the different morphology of TiO2 involving nanotube, nanorod, and nanowire (Figure 3d). Surprisingly, no obvious difference of IDR/S could be observed for TiO2-B and anatase with respect to the catalytic ozonation of phenol in our experiments. It can be seen from Table 1 that the concentration of surface OH is positively correlated to surface area for samples NT-110-400 and NR-160-400. The anatase TiO2 catalysts with larger surface areas, corresponding to more surface OH groups, show more efficient reactivity toward ozone for the decomposition of phenol, implying that the surface area is another main factor in achieving high catalytic reactivity of ozone. However, if we subtract the effects of surface area, the difference in surface OH numbers per surface area unit

FIGURE 5. Relationship between (a) the density of surface OH groups on catalysts and the content of the rutile phase of TiO2 and (b) the density of surface OH groups and the morphology. of samples NT-110-400, NR-160-400, and NW-200-400 is not marked enough to correlate the catalytic activity with the type of morphology of the catalysts (Figure 5b). From the results presented here and discussed above, we conclude that the catalytic ozonation of phenol with TiO2 as catalyst is independent of morphology. In summary, the catalytic activities of TiO2 depend mainly on their specific surfaces and crystallite phases in the process of ozonation of phenol, whereas the morphology per se has very little influence on the process. A superior catalyst should maximize the surface area and the content of the rutile form of TiO2. Therefore, development of methods for the synthesis of TiO2 with a large surface area and a high content of rutile is worthy of further study.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 20977086), National Basic Research Program of China (Grant No. 2009CB421603), and Zhejiang Provincial Natural Science Foundation of China (Grant Nos. Z5080207 and R5090033).

Supporting Information Available A detailed discussion of the XRD and TEM characterization of the TiO2 catalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

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