Î³-Al2O3 Modified with Praseodymium: An Application in the

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γ-Al2O3 Modified with Praseodymium: An Application in the Heterogeneous Catalytic Ozonation of Succinic Acid in Aqueous Solution Zhiqiao He,† Angliang Zhang,† Shuang Song,*,† Zhiwu Liu,† Jianmeng Chen,† Xinhua Xu,‡ and Weiping Liu‡ College of Biological and EnVironmental Engineering, Zhejiang UniVersity of Technology, Hangzhou 310032, People’s Republic of China, and College of EnVironmental and Resource Sciences, Zhejiang UniVersity, Hangzhou 310029, People’s Republic of China

In an attempt to utilize ozone effectively, a series of praseodymium-modified γ-Al2O3 (Pr/Al2O3) was prepared via incipient wetness impregnation using Pr(NO3)3 · 6H2O as the precursor. The structure and properties of the catalysts were studied by X-ray diffraction (XRD) and the Brunauer-Emmett-Teller (BET) method. Catalytic activity was evaluated by monitoring the degradation of succinic acid (SA) in the presence of ozone. The praseodymium modification can effectively enhance the ozonation activity of γ-Al2O3 upon SA removal. Increasing the calcination temperature of Pr/Al2O3 is disadvantageous for the catalytic process, whereas increasing the load of praseodymium is helpful. After three successive cycles, the Pr/Al2O3 catalyst remained stable in the catalytic ozonation of SA. Overall, the initial degradation rate of SA, as well as the saturated adsorption capacity of SA, were found to have a linear relation to changes of the surface hydroxyl groups of the catalyst. On this basis, we conclude that the significant enhancement of SA degradation using Pr/Al2O3 as a catalyst should be because praseodymium, in the form of Pr6O11, promoted the formation of surface hydroxyl groups. Hence, the adsorption was increased, and the degradation rate of SA was enhanced. 1. Introduction Ozone has attracted considerable attention due to its strong oxidizing, environmentally friendly nature and its potential applications in industry and agriculture as well as in other fields. However, single ozonation is difficult to completely oxidize some chemical species, such as saturated carboxylic acids, which are the main residual organic byproducts of the degradation of organic pollutants.1,2 In this respect, various methods, including treatment with hydrogen peroxide, UV radiation, or sonolysis, have been applied in combination with ozonation to increase the oxidizing capability via the generation of more hydroxyl radicals (HO · ).3–7 Unfortunately, the high energy consumption and low energy efficiency of these methods make them economically unattractive.8 Recently, catalytic ozonation, as an alternative technique of advanced oxidation processes (AOPs), has attracted much interest because the method offers more efficient consumption of ozone and shorter operating times as compared to ozonation alone.9 Catalytic ozonation includes both homogeneous and heterogeneous catalytic ozonation. The former improves the ozonation capability by the presence of metal ions in solution, whereas the latter is based on the surface catalysis on solid catalysts.10 Some metal ions, such as Cu(II), Mn(II), Fe(II), Ni(II), and Co(II), have been shown to be effective against refractory organic pollutants as catalysts of ozonation.11–15 However, most of them are extremely harmful to human health and the environment, even if their concentrations in water are very low. Thereby, the homogeneous catalytic ozonation requires additional separation of trace catalysts from water at the end of the catalytic reaction, limiting the broad application of this technology. The use of solid heterogeneous catalysts can * To whom correspondence should be addressed. Tel.: 86-57188320726. E-mail: [email protected]. † Zhejiang University of Technology. ‡ Zhejiang University.

overcome the drawbacks from the point of view of recovering the catalyst and the disposal of environmentally hazardous residues.16 Both the catalyst and the support play important roles in heterogeneous catalysis because of their surface characteristics that influence the properties of the surface active sites and the decomposition of organic pollutants.17,18 The most commonly used supports in catalytic ozonation are metal oxides (Al2O3, CeO2, TiO2), clay, ceramic honeycomb, and activated carbon, because of their large surface area, chemical stability, and the ability to promote catalytic effects.19–23 For further enhancing catalytic activity, metals or their oxides have been used as the active species to explore the capability of the catalyst supports. To date, numerous catalysts, including Ru, Cu, Pt, Fe, Mn, Ni, Zn, Ag, Cr, and Co under various forms, have been investigated;15,18,20,21,24–27 however, only a few studies on the loading of praseodymium oxide into catalyst supports have been reported. Praseodymium ions can exchange between the trivalent and tetravalent states, allowing the release and uptake of oxygen. Therefore, praseodymium oxide appears to be very promising as a catalyst with the help of high oxygen mobility.28 At present, the study of pure praseodymia as an effective catalyst is still limited, ranging from simple oxidation of carbon monoxide,29–31 nitric oxide,32,33 and hydrogen34 and oxidative coupling of methane35,36 to decomposition reactions of organic molecules such as 2-propanol37 and liquid-phase benzylation.38 Typically, praseodymia was considerably more active than other rare earth oxides for oxidizing hydrogen34 and NO.32,33 The primary objective of this study was to gain insight into the use of praseodymium oxide as a modifier of alumina (Pr/ Al2O3) in the degradation of succinic acid (SA) by catalytic ozonation. The influence of the preparation conditions of Pr/ Al2O3, such as calcination temperature and loading content, on the catalytic activity was investigated. Also, the stability and role of praseodymium in the catalytic ozonation of SA were explored.

10.1021/ie101233h  2010 American Chemical Society Published on Web 11/04/2010

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SA was used as the indicator in our study because (1) it is a key compound in the preparation of more than 30 commercially important products; and (2) it is a relatively stable product in the ozonation of natural water.39,40 2. Experimental Section 2.1. Materials and Reagents. SA and praseodymium nitrate (Pr(NO3)3 · 6H2O; purity 99.99%) were purchased as analytical grade reagents from Aladdin Reagent (China) Co., Ltd., and γ-Al2O3 (purity 99.99%) was obtained from Hangzhou Wanjing New Material Co., Ltd., China. Other materials used in the test were of analytical grade. Deionized doubly distilled water was used throughout the study. 2.2. Catalyst Preparation and Characterization. The Pr/ Al2O3 catalysts were prepared by impregnating γ-Al2O3 with an aqueous solution of Pr(NO3)3 · 6H2O to give 5, 15, and 30 mol % of Pr/Al2O3. The volume of solution used was just sufficient to wet the powder completely. The thoroughly moistened powder was dried in a vacuum oven (DZG-6050SA, Shanghai Sumsung Laboratory Instrument Co., Ltd., China) at a temperature of 110 °C and a vacuum pressure of 0.1 MPa. The dried powder was calcined in a furnace (CWF1100, CARBOLITE, England) from ambient temperature to 600, 800, or 1000 °C at a heating rate of 5 °C min-1 and kept at that temperature for 2 h before being allowed to cool to room temperature. The nomenclature used for the catalysts is Pr/ Al2O3-X%-Y °C, where X is to the molar percentage of praseodymium in the catalyst and Y is the calcination temperature. The preparation of Mn, Ni, Fe, and Zn reference catalysts was similar to the procedure used for Pr/Al2O3. The precursors were Mn(NO3)2 · 4H2O, Ni(NO3)2 · 6H2O, Fe(NO3)3 · 9H2O, and Zn(NO3)2 · 6H2O, the molar ratio of metal element to Al2O3 was 30:100, and the calcining temperature was 600 °C. The phase identification of the powders was examined by X-ray diffraction (XRD) with a Thermo ARL SCINTAG X’TRA diffractometer at room temperature using Cu KR irradiation at 45 kV and 40 mA. The Brunauer-Emmett-Teller (BET) surface area measurements were made with a Micromeritics ASAP 2010 Analyzer using nitrogen adsorption at 77 K. The pH at zero charge (pHPZC) of the prepared catalysts was measured by the drift method as described.41 In addition, the concentration of surface hydroxyl groups was determined by reference to the literature.42,43 Briefly, 0.3 g of catalyst powder was added to 50 mL of NaOH solutions in the concentration range 2-100 mM followed by shaking at 25 °C for >4 h. The solution was passed through a membrane with a 0.22 µm pore size, and the NaOH remaining in the supernatant was determined by titrating with a standard HNO3 solution. Because the acidic hydroxyl groups react with NaOH, they can be readily quantified via calculation of the NaOH consumption. According to the principle of charge balance, the acidic and basic hydroxyl groups should be quantitatively equal. Hence, the total amount of the surface hydroxyl groups is twice that of the acidic groups. 2.3. Catalytic Activity Measurement. Catalytic ozonation was done in a cylindrical Pyrex glass reactor (diameter 80 mm; height 260 mm). The reaction solution was stirred continuously by a magnetic stirring bar to guarantee good mixing. Ozone was generated from 100% extra-dry oxygen with an ozone generator (CHYF-3A, Hangzhou Rongxin Electronic Equipment Co., Ltd., China) and purged through a small glass gas dispersion tube with coarse porosity placed at the bottom of the reaction cylinder to ensure the formation of small bubbles for effective purging throughout experiments. The flow rate (300 mL min-1) was controlled by a mass flow controller (D07-12A/ZM, Beijing

Figure 1. XRD patterns of γ-Al2O3 and Pr/Al2O3 with 30 mol % of Prload calcined at 600, 800, or 1000 °C.

Qixing Electronic Equipment Co., Ltd., China), and the concentration of O3 was determined to be ∼55 mg L-1 using an iodimetric method.44 The heterogeneous ozonation was done with 800 mL of 1.00 mM SA aqueous solution containing 1.0 g L-1 of the solid catalyst. Samples were withdrawn at predetermined time-points, centrifuged, and filtered through a 0.22 µm pore size membrane filter to remove the catalyst particles. The initial pH of the solution was ∼3.4 due to the presence of SA. The reaction solution was not buffered because the pH did not vary by more than (0.1 after introduction of the catalysts into the aqueous solution, and the initial degradation rate was used to evaluate the activity of the catalysts to avoid the complicating effects of reaction products undergoing further reactions. SA and its degradation intermediates were analyzed with a Dionex model ICS 2000 ion chromatograph equipped with a dual-piston (in series) pump, a Dionex IonPac AS19 analytical column (4 mm × 250 mm), an IonPac AG19 guard column (4 mm × 250 mm), and a Dionex DS6 conductivity detector. Suppression of the eluent was achieved with a Dionex anion ASRS electrolytic suppressor (4 mm) in the autosuppression external water mode. Total organic carbon (TOC) was measured with a TOC analyzer (Shimadzu, Japan), and the concentration of the dissolved Pr was measured with a Perkin-Elmer ELAN DRC-e inductively coupled plasma mass spectrometer (ICP-MS). 3. Results and Discussion 3.1. Catalyst Characterization. The XRD patterns of Pr/ Al2O3-30% calcined at 600, 800, or 1000 °C are shown in Figure 1. For the pure γ-Al2O3, the XRD peaks were observed at 2θ values of approximately 33°, 37°, 39°, 45°, 60°, and 67°. The corresponding XRD peaks of Pr/Al2O3-30%-600 °C and Pr/ Al2O3-30%-800 °C were very similar to those of pure γ-Al2O3, indicating that the sample did not change its structure or properties in this calcination temperature range. Nonetheless, the diffraction patterns in the 2θ range 26-33° were markedly enlarged, which should be caused by the reflection typical of the (111) plane of the Pr6O11 (PrO1.833).45 In other words, the praseodymium on the surface of γ-Al2O3 was present in the form of Pr6O11 within the sintering temperature range of 600-800 °C. Phase transition occurred when the calcination temperature was 1000 °C, resulting in the disappearance of Pr6O11 and the generation of PrAlO3 (ref. code 00-029-0076). The surface area of the calcined catalysts was determined by the BET method. At the calcination temperature of 600 °C, with


Figure 2. Influence of calcination temperature (a) and the amount of praseodymium loading (b) on the catalytic ozonation of SA. Experimental conditions: initial concentration of SA, 1.00 mM; catalyst dose, 1.0 g L-1; O3/O2 flow rate, 300 mL min-1.

an increasing loading percentage of praseodymium (0, 5, 15, 30 mol %), the surface area decreased from 205.0 m2 g-1 of the pure γ-Al2O3 to 159.4, 97.9, and 69.2 m2 g-1 of Pr/Al2O3, respectively. In addition, at the calcination temperatures of 600, 800, and 1000 °C, the surface area of Pr/Al2O3-30% was 69.2, 61.3, and 37.5 m2 g-1, respectively. These results are in accord with expectation; the surface area decreased with increasing praseodymium loading at a given calcination temperature, and for the same loading, it decreased with increasing calcination temperature. 3.2. Activity and Stability of Pr/Al2O3 in the Catalytic Ozonation. Activity and stability are key factors in determining the applicability of catalysts in a commercial scale. In view of this, activity and stability of the Pr/Al2O3 catalysts in the ozonation of SA were investigated in this work. 3.2.1. Effect of Calcination Temperature and Praseodymium Loading. The effect of calcination at various temperatures on the activity of the Pr/Al2O3-30% was evaluated. It can be seen from Figure 2a and Table 1 that the catalytic activities of Pr/Al2O3-30% varied with the change of calcination temperature. An increase of calcination temperature from 600 to 800 °C slightly decreased the activity, and the initial degradation rates were ∼9.28 and ∼6.69 µM min-1, respectively. Nevertheless, the catalytic activity was decreased very sharply when the calcination temperature was 1000 °C. In this case, the initial degradation rate was only 1.30 µM min-1. This trend was due mainly to the loss of surface hydroxyl groups and surface area. The surface hydroxyl group as the surface-

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Figure 3. Evolution (a) and TOC removal (b) of SA by single ozonation, ozonation over γ-Al2O3 or Pr/Al2O3-30%-600 °C, ozonation over Pr/Al2O330%-600 °C with 100 mM t-BuOH or 10 mM t-BuOH, and adsorption onto γ-Al2O3 or Pr/Al2O3-30%-600 °C. Experimental conditions: initial concentration of SA, 1.00 mM; catalyst dose, 1.0 g L-1; O3/O2 flow rate, 300 mL min-1.

active site was generally reduced while elevating the calcination temperature, as confirmed below. Additionally, according to the BET analysis for the catalysts, raising the calcination temperature reduced the surface area correspondingly, thereby decreasing the number of active sites. Furthermore, we can conclude from Table 1 that the PrAlO3 obtained at a calcination temperature of 1000 °C was not propitious for the catalytic ozonation of SA. The influence of the amount of praseodymium loading on the activity of Pr/Al2O3-600 °C was investigated with respect to SA degradation. Figure 2b and Table 1 show that the catalytic activity of the Pr/Al2O3-600 °C increased gradually as the praseodymium loading increased, and the catalytic activity of the pure Al2O3 powder (without praseodymium load) had the lowest reaction rate. This suggests that the pure Al2O3 powder does not have enough active sites, which are an essential component of the reaction. The number of active sites for ozonation was increased with the increase of praseodymium content, as determined below. 3.2.2. Comparison of the Degradation of SA. We compared the catalytic activities of pure γ-Al2O3 and Pr/Al2O3-30%-600 °C by the ozonation of 1.00 mM SA in aqueous solution, and the results are shown in Figure 3a. To clarify the effects of

Table 1. Refined Physicochemical Parameters and the Initial Degradation Rate of SA for the Prepared Catalysts

sample

BET surface area (m2 g-1)

concentration of surface hydroxyl groups (mmol g-1)

density of surface hydroxyl groups (µmol m-2)

γ-Al2O3 Pr/Al2O3-5%-600 °C Pr/Al2O3-15%-600 °C Pr/Al2O3-30%-600 °C Pr/Al2O3-30%-800 °C Pr/Al2O3-30%-1000 °C

205 159.4 97.9 69.2 61.3 37.5

0.49 1.49 4.45 4.70 3.65 0.92

2.39 9.35 45.4 67.9 59.5 24.5

pHPZC

saturated adsorption capacity (mM)

initial degradation rate (µM min-1)

7.53 8.10 8.97 9.23 8.68 7.70

0.0061 ( 0.0026 0.067 ( 0.0049 0.15 ( 0.0079 0.19 ( 0.011 0.13 ( 0.0095 0.031 ( 0.0052

0.81 ( 0.55 3.57 ( 0.83 8.65 ( 0.24 9.28 ( 0.93 6.69 ( 0.20 1.30 ( 0.46

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adsorption and catalytic ozonation activity, a series of experiments were performed, including single ozonation, ozonation in the presence of γ-Al2O3 and ozonation in the presence of Pr/Al2O3-30%-600 °C, as well as adsorption on γ-Al2O3 and Pr/Al2O3-30%-600 °C. We used the initial degradation rate to compare the catalytic ozonation activity of different samples to eliminate any possible influence of the degradation products (Table 1). Because the initial degradation rate depends mainly on the initial pH, the pH of the solutions was not controlled in our experiments. During ozonation alone, the initial degradation rate was 0.17 ( 0.11 µM min-1, and the corresponding degradation efficiency of SA was no more than 3% after treatment for 4 h. This is reasonable, because the reaction rate of SA with molecular ozone is rather slow under the acidic conditions of our experiments.15 As far as the adsorption is concerned, the adsorption capacity of Pr/Al2O3-30%-600 °C was approximately 3 times that of γ-Al2O3, and the BET surface area of Pr/Al2O3-30%-600 °C was approximately one-third that of γ-Al2O3. At the end of the adsorption, nearly 5% and 15% of SA was adsorbed by γ-Al2O3 and Pr/Al2O3-30%-600 °C, respectively. This is probably because the praseodymium loading caused an increase of the surface hydroxyl groups, as discussed below. As for the SA degradation, the decrease of SA during ozonation with γ-Al2O3 as catalysts was only slight. Nevertheless, Ernst et al. achieved a TOC removal efficiency of 90% after 60 min reaction using γ-Al2O3 as the catalyst. The difference can be attributed to different reaction conditions. In the study by Ernst et al., the height to diameter ratio was 30:1, 10-fold greater than ours, and the catalyst dose was 20 g L-1, which is about 20-fold greater than ours. Furthermore, Ernst et al. adjusted the initial pH of the solution to 7.0 by the addition of NaOH,1 whereas we did not control the pH of our solution. In the case of the ozonation of SA in the presence of Pr/ Al2O3-30%-600 °C, the initial degradation rate is dramatically higher than that of adsorption on Pr/Al2O3-30%-600 °C. Also, the initial degradation rate is 246% higher for the catalytic ozonation system than that for the linear combination of ozonation alone and adsorption of Pr/Al2O3-30%-600 °C. All these results indicate that the presence of praseodymium has an important effect with regard to the significant improvement of γ-Al2O3 catalytic activity for the ozonation of SA. Destruction of the SA should be evaluated as an overall degradation process that ultimately involves mineralization of both the parent substance and its intermediates. The most practical means of estimating this overall process is to monitor the reduction of TOC. As shown in Figure 3b, the efficiency of catalysts for TOC removal showed the same trend as the elimination of SA, with the Pr/Al2O3-30%-600 °C catalyst being the most efficient, leading to 89% mineralization after 4 h of reaction. Moreover, the degree of mineralization achieved is less than that of the reduction of SA, indicating the formation of byproducts. It is generally accepted that the degradation of SA involves the generation of acetic acid, acrylic acid and fumaric acid.46 Acetic acid was determined quantitatively as the main intermediate compound (inset of Figure 3a), whereas acrylic acid and fumaric acid were present in only trace amounts that were difficult to quantify accurately. The concentration of acetic acid first increased and then decreased with time, indicating that the catalysts were also effective for the catalytic ozonation of acetic acid. To further confirm the superiority of Pr/Al2O3 in the removal of SA, commonly used metal oxides, including oxides of Mn,

Figure 4. Evolution of SA by ozonation over Al2O3 modified with different transition metals. Experimental conditions: initial concentration of SA, 1.00 mM; catalyst dose, 1.0 g L-1; O3/O2 flow rate, 300 mL min-1.

Ni, Fe, and Zn, were selected as reference catalysts. Figure 4 shows that all the transition metal catalysts enhanced the rate of removing SA by ozonation with catalytic capability in the order Mn > Ni > Fe > Zn. However, the catalytic activities of these metal oxides were all lower than that of Pr/Al2O3-30%600 °C. For example, ∼34% removal efficiency was observed for Mn after 4 h reaction, whereas ∼92% of the SA was removed with Pr/Al2O3-30%-600 °C in the presence of ozone under the analogous conditions. Obviously, praseodymium was more active than transition metal oxides in the ozonation of SA. 3.2.3. Leaching of Praseodymium from Pr/Al2O3 Catalysts. An important factor that needs to be taken into consideration when using a Pr/Al2O3 catalyst in heterogeneous catalytic ozonation reactions is the loss of catalyst by leaching. Metal leaching into the aqueous solution is an important deactivation factor and might become a source of pollution. With the Pr/ Al2O3-30%-600 °C catalyst dose at 1.0 g L-1, the O3/O2 flow rate at 300 mL min-1 and the initial concentration of SA at 1.00 mM, 1.78 mg/L of Pr in the liquid phase was detected by ICP-MS after 240 min of reaction, suggesting Pr solubilization remained at a low level (∼0.6%) as compared to the quantity of Pr. To check the reusability of the Pr/Al2O3 catalysts, catalytic ozonation of SA was done for three successive cycles using Pr/Al2O3-30%-600 °C as the catalyst. All of the catalyst recycling tests were done under identical reaction conditions. At the end of each run, systematic injections (each 240 min) of a precalculated SA solution were done to obtain a concentration close to the initial value (1.00 mM). As can be seen in Figure 5, the catalytic activity of Pr/Al2O3-30%-600 °C did not decrease significantly in the catalytic ozonation of SA after three successive cycles in the presence of ozone. In the three catalytic ozonation experiments, 92%, 85%, and 82% of removal of SA was observed at a reaction time of 240 min, indicating that the Pr/Al2O3 catalysts possess good stability against ozonation. 3.3. Relationship of Initial Degradation Rate, pHPZC, and Surface OH Groups. The surface hydroxyl groups of the catalyst, as a major factor of the surface characteristics, are believed to be crucial for the catalytic effect in heterogeneous catalytic ozonation. Therefore, experiments were done to investigate the relationship among initial degradation rate and surface hydroxyl groups. Figure 6a shows that the initial degradation rate of SA is linearly correlated with the concentration of surface hydroxyl groups, confirming that the degradation


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Figure 5. The removal of SA in the presence of recycled Pr/Al2O3-30%600 °C. Experimental conditions: initial concentration of SA, 1.00 mM; catalyst dose, 1.0 g L-1; O3/O2 flow rate, 300 mL min-1.

of SA is determined by the concentration of surface hydroxyl groups under the present experimental conditions. In addition, the data in Table 1 show that there is a significant change in the concentration of surface hydroxyl groups with different calcination temperatures and the amount of praseodymium loading. The greater is the amount of praseodymium loading or the lower the calcination temperature, the greater is the concentration of surface hydroxyl groups. It is widely accepted that the concentration of surface hydroxyl groups depends on the number of oxygen vacancies. As mentioned above, Pr6O11 (PrO1.833), which has a structure similar to that of PrO2 but with increased oxygen vacancy,47 is produced via the thermal decomposition of praseodymium nitrate. Praseodymium oxide, which forms fluorite-related oxide systems of variable composition, is ceria’s partner among the higher rare-earth oxides.48 The oxygen vacancies on CeOx were caused by Ce3+.49 Likewise, we infer that more oxygen vacancies in Pr/Al2O3 originated from Pr3+ in Pr6O11. The surface hydroxyl groups have different charge states, which are determined by the pHPZC of catalysts and the pH of the aqueous solution. This can be seen from eqs 1 and 2: MeOH + OH- h MeO- + H2O (pH > pHPZC)

(1)

MeOH + H+ h MeOH+ 2 (pH < pHPZC)

(2)

The fact that there is a pHPZC of the solid catalyst shows that the reaction responsible for the surface charge of the solid is that given as eq 1 below the pH of aqueous solution and in eq 2 above the pH of aqueous solution. The pHPZC of our catalysts changed in the range 7.53-9.23 (Table 1), which is higher than the initial pH of the reaction solution (∼3.4). Thus, in this case, eq 2 is more valid than eq 1, and the surface-OH2+ may be the primary surface group on the surface of all catalysts at beginning of the reaction. A very good positive correlation was observed between the concentration of surface hydroxyl groups and the pHPZC of the catalyst (Figure 6b). This correlation is good evidence that the concentration of surface hydroxyl groups is directly related to the pHPZC of the catalyst. 3.4. Role of Praseodymium on the Surface of γ-Al2O3. Usually, there are two roles of catalysts for the heterogeneous catalytic ozonation of organic acid. One is to increase the generation of HO · , which then reacts with organic compounds to form oxidized species or decomposed products. The other is formation of the surface complex between the carboxylic groups

Figure 6. Relationship between the initial degradation rate of SA and the concentration of surface hydroxyl groups on catalysts (a), the concentration of surface hydroxyl groups and pHPZC (b), as well as the saturated adsorption capacity of SA and the concentration of surface hydroxyl groups on catalysts (c).

of the pollutants and the active sites of the catalyst. The surface complex then undergoes degradation because the coordinated pollutants formed are more reactive toward oxidative species.50–53 To estimate whether the high conversion rate of SA achieved in the catalytic ozonation process was due to the action of HO · , excess tert-butyl alcohol (t-BuOH; 10 and 100 mM) was added to the reaction solution because it can react rapidly with HO · with a rate constant of 6.0 × 108 M-1 s-1 and does not compete with the organic acid in the adsorption process.27,54 As shown in Figure 3a, adding excess t-BuOH did not change the removal rate of SA, which implies that the significant enhancement of the degradation of SA (an increase of initial degradation rate of 1046%) by praseodymium is not attributable to the increase of HO · . Thus, we infer that the interaction of SA and active sites on the catalyst surface facilitated the process of degradation. Table 1 shows that a high adsorptive capacity was observed in relation to the high degradation efficiency of SA; therefore,

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adsorption is the crucial step of the catalytic ozonation process. Also, we found that the saturated adsorption capacity of SA (after 24 h adsorption) of the catalysts in the absence of ozone increased linearly with increasing concentration of surface OH groups on catalysts, as shown in Figure 6c. On the basis of these experimental results, it can be concluded that the degradation of SA has to undergo the carboxylic group-praseodymium interaction. Next, the precursor complex adsorbed onto the catalyst surface is oxidized by the adsorbed and/or dissolved ozone. Yao and Yeh have investigated the behavior of succinate adsorption on hydrous δ-Al2O3. They concluded that the adsorption mode between SA and δ-Al2O3 is not due to hydrogen-bonding interactions. Instead, the electrostatic attractive forces between SA and the surface OH groups on δ-Al2O3 are the main contributors to adsorption in this system.55 In brief, the role of praseodymium is to promote the generation of surface OH groups, leading to an increase of the effective adsorption sites to SA. Thus, the rate of heterogeneous catalytic ozonation of SA was significantly increased. 4. Conclusions The present work demonstrated that the Pr/Al2O3 catalyst exhibited more efficient catalytic ozonation activity than pure γ-Al2O3 in the degradation of SA. The superiority of the Pr/ Al2O3 catalyst was attributed to the fact that the praseodymium loading can effectively increase the surface hydroxyl groups, which can significantly influence adsorption and catalytic behavior. In the range investigated, increasing praseodymium loading and decreasing calcination temperature had a positive effect on the catalytic activity of Pr/Al2O3. Further, leaching of Pr was limited, and the catalyst maintained its catalytic activity for at least three successive cycles. Overall, Pr/Al2O3 appears to offer the possibility to remove other contaminants from water. Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant Nos. 20977086 and 21076196), the National Basic Research Program of China (Grant No. 2009CB421603), and the Zhejiang Provincial Natural Science Foundation of China (Grant Nos. Z5080207 and Y5100310). Literature Cited (1) Ernst, M.; Lurot, F.; Schrotter, J. C. Catalytic ozonation of refractory organic model compounds in aqueous solution by aluminum oxide. Appl. Catal., B 2004, 47, 15. (2) Karpel Vel Leitner, N.; Delanoe¨, F.; Acedo, B.; Legube, B. Reactivity of various catalysts during Ru/CeO2 ozonation of succinic acid aqueous solutions. New J. Chem. 2000, 4, 229. (3) Gogate, P. R.; Pandit, A. B. A review of imperative technologies for wastewater treatment II: hybrid methods. AdV. EnViron. Res. 2004, 8, 553. (4) Zhou, H.; Smith, D. W. Advanced technologies in water and wastewater treatment. J. EnViron. Eng. Sci. 2002, 1, 247. (5) Kang, J. W.; Hoffmann, M. R. Kinetics and mechanism of the sonolytic destruction of methyl tert-butyl ether by ultrasonic irradiation in the presence of ozone. EnViron. Sci. Technol. 1998, 32, 3194. (6) He, Z. Q.; Song, S.; Ying, H. P.; Xu, L. J.; Chen, J. M. p-Aminophenol degradation by ozonation combined with sonolysis: operating conditions influence and mechanism. Ultrason. Sonochem. 2007, 14, 568. (7) He, Z. Q.; Zhu, R. Y.; Xu, X.; Song, S.; Chen, J. M. Ozonation combined with sonolysis for degradation and detoxification of m-nitrotoluene in aqueous solution. Ind. Eng. Chem. Res. 2009, 48, 5578. (8) Ma, J.; Graham, N. J. D. Degradation of atrazine by manganesecatalysed ozonation: influence of humic substances. Water Res. 1999, 33, 785.

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ReceiVed for reView June 7, 2010 ReVised manuscript receiVed September 16, 2010 Accepted October 17, 2010 IE101233H

Î³-Al2O3 Modified with Praseodymium: An Application in the

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