Enhancement Mechanism of Heterogeneous Catalytic Ozonation by

Feb 9, 2009 - School of Municipal and Environmental Engineering, Harbin. Institute of Technology, 202 Haihe Road, Harbin 150090,. People's Republic of...
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Environ. Sci. Technol. 2009, 43, 2047–2053

Enhancement Mechanism of Heterogeneous Catalytic Ozonation by Cordierite-Supported Copper for the Degradation of Nitrobenzene in Aqueous Solution L E I Z H A O , * ,† Z H I Z H O N G S U N , ‡ J U N M A , * ,† A N D H U I L I N G L I U † School of Municipal and Environmental Engineering, Harbin Institute of Technology, 202 Haihe Road, Harbin 150090, People’s Republic of China, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, People’s Republic of China, National Engineering Research Center of Urban Water Resources, State Key Laboratory of Urban Water Resources and Environment, Harbin Institute of Technology, Harbin 150090, People’s Republic of China

Received November 5, 2008. Revised manuscript received December 20, 2008. Accepted January 9, 2009.

The use of cordierite or Cu-cordierite for heterogeneous catalytic ozonation enhances significantly the degradation efficiency and the TOC removal of nitrobenzene in aqueous solution relative to ozonation alone, owing to the synergistic effect between ozone and the catalysts, and the modification process with Cu can increase the catalytic activity of cordierite for the ozonation of nitrobenzene. The adsorption of nitrobenzene is too small to make a significant contribution to the degradation of nitrobenzene in either of the processes of catalytic ozonation. The initiation of hydroxyl radical (•OH) improves as the amount of Cu-cordierite catalyst is increased, meaning that the degradation of nitrobenzene is mainly attributed to •OH oxidation. The modification by loading Cu increases the density of surface hydroxyl groups and the pH at the point of zero charge (pHPZC) of raw cordierite. The investigation of the enhancement mechanism confirms that the modification by loading Cu causes changes of density of surface hydroxyl groups, pHPZC, and |pH - pHPZC|, resulting in the acceleration initiation of •OH which can promote the degradation of nitrobenzene. The result shows that the optimal critical loading percentage of Cu is 3% under the present experimental conditions.

Introduction Ozone is widely used in industrial and environmental processes such as semiconductor manufacturing, deodorization, disinfection and water treatment. Ozone is an environmentally friendly oxidant since it decomposes to O2 without producing self-derived byproducts in the oxidation reactions (1). Therefore, ozonation is an attractive and increasingly important method for the degradation of organic * Address correspondence to either author. Phone: +86-45182291644 (L. Z.); +86-451-86283010 (J. M.). Fax: +86-451-82368074 (L. Z. and J. M.). E-mail: [email protected] (L. Z.); majun@ hit.edu.cn (J. M.). † Harbin Institute of Technology. ‡ Heilongjiang University. 10.1021/es803125h CCC: $40.75

Published on Web 02/09/2009

 2009 American Chemical Society

pollutants in aqueous solution. However, the refractory organic compounds are usually not totally oxidized and only a small mineralization is achieved, e.g., nitrobenzene (see the Supporting Information). Moreover, the practical use of ozonation for wastewater treatment is limited by its highenergy demand. Several approaches have been taken to improve the oxidizing power of this technique leading to reduction of the required reaction time and hence decreasing its energy cost (2). Consequently, various advanced oxidation processes (AOPs) have been investigated as potential methods for degrading organic compounds. These are O3/H2O2, O3/ UV, UV/H2O2, Fenton and UV/Fenton reagents, UV/TiO2, electron beam and catalytic ozonation (3). Catalytic ozonation is a promising AOP because of its effective use of ozone and its improved treatability of organic compounds through radical reactions (4). AOPs are characterized by the generation of hydroxyl radical (•OH), species of high oxidizing power which reacts with the matter present in water in an unselective way (5). In recent years, heterogeneous catalytic ozonation, as an alternative technique of AOP, has received much attention in water treatment due to its high oxidation potential. Several studies have been reported on the degradation of nitrobenzene in aqueous solution by the heterogeneous catalytic ozonation processes. It is seen that the presence of heterogeneous catalysts such as nano-TiO2 (6), Mn-loaded granular activated carbon (7, 8), ceramic honeycomb (9, 10), Mn-ceramic honeycomb (11) and synthetic goethite (12), respectively, can significantly enhance the degradation efficiency of nitrobenzene compared with the case of ozonation alone, and the degradation of nitrobenzene mostly follows the •OH oxidation mechanism in the systems mentioned above. In order to develop a convenient vehicle for the copper catalyst (13) used in the previous study, the degradation efficiency of the organic micropollutant was investigated by the process of cordierite-supported Cu catalytic ozonation. Cu-cordierite was selected as the catalyst in the present study due to its excellent chemical and configurable properties, such as high mechanical strength, chemical stability, and higher catalytic activity. Nitrobenzene, as a special indicator of •OH, is chosen as the target organic compound due to its refractory nature to conventional chemical oxidation. Specifically, the experimental results indicate the novel relationship between the pH of the aqueous solution and the conversion of the pH at the point of zero charge (pHPZC), which is different from the process of ceramic honeycombsupported Mn catalytic ozonation used in a previous study (11). The primary objective of the present study was to reveal the relationships among the degradation efficiency of nitrobenzene, the initiation of •OH, the density of surface hydroxyl groups, the change of pHPZC, and the variation of |pH - pHPZC|, and then to illuminate the enhanced mechanism of Cu-cordierite catalytic ozonation for the degradation of nitrobenzene in aqueous solution.

Experimental Section Materials and Reagents. The model water was prepared by spiking 50 µg L-1 nitrobenzene (Beijing Chemical Factory, China, purified by distillation pretreatment, 99.80%) in Milli-Q water (Millipore Q Biocel system). Perchloric acid (Tianjin Dongli Chemical Factory of Tianjin University, China), and sodium hydroxide (Harbin Xinchun Chemical Factory, China) were added in aqueous solution to control the pH. Copper nitrate (Tianjin Nankai Chemical Factory, China), and all other chemicals used in the experiments were VOL. 43, NO. 6, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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analytical grade reagents. A diluted sodium thiosulphate solution was used in the experiments for quenching the reaction. Monoliths of cordierite (Shanghai Pengyinaihuo Material Factory, China) were used as the catalyst and the framework of the catalyst. These blocks have the following characteristics: cylindrical shape with a diameter of 50 mm and a length of 50 mm, wall thickness 0.4 mm, cell density 400 cells per square inch, and weight of a single block of cordierite was 34.6∼35.4 g. All the monolithic blocks were cleaned before the catalytic ozonation process and the modified procedure by immersing them into a vessel with redistilled water, which was placed in an ultrasonic bath for 10 min. Afterward, they were dried in an oven at 393 K overnight, and then stored in a dry vacuum oven for use. Analytical Method. The concentration of ozone in the gas was measured by the iodometric titration method (14). The concentration of residual ozone in aqueous solution was measured by spectrophotometer using the indigo method (15). The concentration of total applied ozone in this experiment was controlled at 1.0 mg L-1. The concentration of nitrobenzene was determined by a GC-14C gas chromatograph (Shimadzu, Japan). The pH of aqueous solution was measured by a PB-10 pH meter (Sartorious, Germany). The concentration of H2O2 formed in the oxidation system was determined by the photometric method (16). An electron paramagnetic resonance (EPR) experiment was conducted for the determination of •OH generated in the selected processes. Mineralization of nitrobenzene was monitored via total organic carbon (TOC) removal, which was determined with a TOC analyzer (Analytik Jena Multi N/C 3100). A gas chromatographic and mass spectrometric analyses system (GC/MS, 6890GC/5973MS, Agilent, U.S.) and ion chromatography (IC, CDD-6A, Shimadzu Co., Japan) were used to identify the byproducts of the degradation of nitrobenzene. X-ray power diffraction (XRD, Input Gokv Zokw Co. Ltd., Japan, model A-41 L-Cu) was used to analyze the crystal phase of the catalyst in monochromatized Cu Ka radiation using a curved graphite monochromator on the diffracted beam with an operating voltage of 45 kV and a current of 50 mA. The specific surface area of catalyst samples was measured according to the Brunauer-Emmet-Teller (BET) method with krypton adsorption at liquid nitrogen temperature on a Micromeritics ASAP 2020 system. To measure the BET specific surface area of catalyst monoliths, a particular homemade test tube was needed to host the sample. The density of surface hydroxyl groups was measured according to a saturated deprotonation method described by Laiti et al. (17) and Tamura et al. (18). pHPZC was measured with a mass titration method (19, 20). An inductively coupled plasma emission spectrometer (ICP, Optima 5300DV, Perkin-Elmer, U.S.) was used to determine the concentrations of metal ions in aqueous solution. The analytic conditions, catalyst preparation and ozonation procedure are provided in the Supporting Information.

Results and Discussion Comparison of the Degradation Efficiency of Nitrobenzene. The degradation of 50 µg L-1 nitrobenzene in aqueous solution has been carried out by the processes of ozonation alone, ozonation/cordierite, ozonation/Cu-cordierite, adsorption on cordierite, and adsorption on Cu-cordierite, respectively. Figure 1 shows the variation of normalized nitrobenzene concentration on reaction time under several experimental conditions. As shown in Figure 1, the lowest degradation efficiency of nitrobenzene appears with the use of the cordierite and Cu-cordierite materials in the absence of ozone, namely adsorption on cordierite and adsorption on Cu-cordierite, resulting in the removal of only 2.0 and 2.5% of the initial 2048

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FIGURE 1. Comparison of the degradation efficiency of nitrobenzene in the different processes (reaction conditions: temperature 293 K; initial pH 6.92; terminal pH 6.90∼6.91; initial nitrobenzene concentration 50 µg L-1; the concentration of total applied ozone 1.0 mg L-1; the number of cordierite and Cu-cordierite blocks used, respectively, in catalytic ozonation, five blocks; loading percentage of Cu (wt.%) 2%; CNitrot: the instantaneous concentration of nitrobenzene; CNitro0: the initial concentration of nitrobenzene). nitrobenzene after 20 min, respectively. Thus the modification of cordierite by loading Cu can slightly promote adsorption of nitrobenzene on the catalyst surface. However, compared with the experimental results of ozonation alone and catalytic ozonation processes, adsorption of nitrobenzene is too small to make a significant contribution to its degradation efficiency and can therefore be neglected. For all the other experimental situations, with the presence of ozone in aqueous solution, the concentration of nitrobenzene decreases with increasing reaction time. The presence of cordierite catalyst significantly enhances the degradation efficiency of nitrobenzene, and the best result is obtained when using ozone combined with Cu-cordierite. Simultaneously, comparing ozonation/cordierite with the cumulative effect of ozonation alone and adsorption of cordierite, an increment of approximately 19.5% of nitrobenzene degradation is observed. Under the same experimental conditions, the ozonation/Cu-cordierite system leads to about 77.9% nitrobenzene conversion, an increment of 41.6% compared to the cumulative effect of ozonation alone and adsorption on Cu-cordierite. The experimental results suggest that the presence of cordierite or Cu-cordierite catalyst, respectively, has a synergistic effect with ozone for the degradation of nitrobenzene, and the modification process can improve the catalytic activity of cordierite for the ozonation of nitrobenzene. Ozone reacts on various organic and inorganic compounds in aqueous solution, either by direct, selective reactions of molecular ozone or through a radical type reaction involving •OH induced by the decomposition of ozone in aqueous solution (3, 21). Since the oxidizing potential of •OH is much higher than that of molecular ozone, • OH is a less selective and more powerful oxidant. It is one of the most reactive free radicals and one of the strongest oxidants (22). The reaction rate constant of nitrobenzene with •OH is 2.2 × 108 M-1 s-1 (23), whereas the rate constant for reaction of nitrobenzene with ozone alone is only 0.09 ( 0.02 M-1 s-1 (24), meaning that nitrobenzene scarcely reacts with molecular ozone. This suggests that nitrobenzene is degraded mainly by oxidation of •OH in the processes of ozonation alone and catalytic ozonation. In addition, the previous research confirms that, under the present experimental conditions of reaction temperature 293 K and initial pH 6.92, nitrobenzene is oxidized primarily by •OH in the

processes of ozonation alone and catalytic ozonation (10, 11). During the course of the reaction, ozone alone reacts with nitrobenzene mostly through the oxidation of •OH initiated by the ozone self-decomposition in aqueous solution. As far as catalytic ozonation is concerned, it is revealed that the initiation of •OH is enhanced by the introduction of heterogeneous catalytic surface (10). For the process of ozonation/Cu-cordierite, it can be assumed that the initiation of •OH is accelerated further by the modification of cordierite by loading Cu. More attention was therefore focused on the investigation of heterogeneous catalytic ozonationbyCu-cordieriteforthedegradationofnitrobenzene. Referring to heterogeneous catalytic ozonation, it should be noticed that the system is composed of three phases (gas-liquid-solid). Therefore it is very important to investigate the effects of the various operating variables on the degradation efficiency of nitrobenzene and the characteristics of heterogeneous catalysts. The results in Supporting Information Figure S7 illustrate that the bulk crystalline phase of the both catalysts is 2MgO-2Al2O3-5SiO2, which is the standard structure of R-cordierite. In addition, comparing to the result of raw cordierite catalyst in Supporting Information Figure S7a, it can be observed that the CuO crystalline phase is present in Figure S7b, meaning that the modification process by loading Cu leads to the appearance of additional peaks of CuO. The results of BET measurements show that the specific surface area converts from 0.4 m2 g-1 of cordierite to 1.8 m2 g-1 of 2.0% Cu-cordierite, namely more than 4.5 times higher than the surface area of raw cordierite catalyst. However, the increase in loading percentage of Cu from 2.0 to 6% only results in a slight increase in specific surface area (0.2 m2 g-1). Furthermore, the introduction of Cu also changes the pHPZC from 6.60 of raw catalyst to 6.83 of 2.0% Cu-cordierite. However, it should be noticed that due to the loading percentage of Cu is selected in the experiment within a relatively lower concentration range of 2.0%, resulting in an unapparent conversion of the pHPZC. The determination results indicate that the density of surface hydroxyl groups increases from 0.91 × 10-5 mol m-2 of raw cordierite to 1.88 × 10-5 mol m-2 of 2.0% Cu-cordierite catalyst. It represents that the appearance of CuO crystalline phase is associated with the increases of the specific surface area, the pHPZC, and the density of surface hydroxyl groups, which probably influence the active sites on the catalyst surface resulting in the conversion of catalytic efficiency. Moreover, the measurement confirms that the degradation efficiency of nitrobenzene almost remain constantly after 40 times reuse of catalyst, namely the successive experiments with the same catalyst for 120 L model water. Meanwhile, the further increase in the leaching of Cu has never been determined with the increasing reuse times of catalyst, meaning that the catalyst is provided with a good chemical stability and a relative long lifetime under the experimental condition. It is reported that heterogeneous catalytic ozonation is a potential alternative AOP because the presence of heterogeneous surface appears to transfer ozone into aqueous solution more efficiently (25), as indicated by the experimental results shown in the Supporting Information. Moreover, the decrease in the concentration of residual ozone and the increase in the utilization efficiency of ozone imply that the presence of heterogeneous Cu-cordierite catalyst surfaces may also help to initiate the production of novel oxidative intermediate species from the decomposition of ozone (25). Production of Oxidative Intermediate Species. Ozone reacts with organic compounds in aqueous solution directly through molecular or selective reactions with specific functional groups (double bonds, nucleophilic positions) (26) and through free radicals generated from the decomposition

FIGURE 2. Comparison of relative intensity of DMPO-OH adduct signal in the process of catalytic ozonation (reaction conditions: temperature 293 K; initial pH 6.92; terminal pH 6.90∼6.91; initial nitrobenzene concentration 50 µg L-1; the concentration of total applied ozone 1.0 mg L-1; the number of Cu-cordierite blocks used, 0∼5 blocks; loading percentage of Cu (wt.%) 2%; initial DMPO concentration 100 mmol L-1). of ozone (27). Furthermore, ozone is very unstable in aqueous solution due to its highly active resonance structures (21), and the major secondary oxidant formed from the decomposition of ozone in aqueous solution is the •OH (28). The formation of •OH is found in the process of heterogeneous catalyst ozonation by the previous studies (4, 6, 29). An experiment was performed to determine the formation of • OH in the process of Cu-cordierite catalytic ozonation by means of the spin trapping/EPR technique, which can detect unstable radicals by measuring the intensity of DMPOOH adduct signal. The results are summarized in Figure 2. Like the profile in Figure 2a, the spectra of the DMPO-OH adduct signal at different amount of catalyst are all composed of quartet lines having a peak height ratio of 1:2:2:1, and the parameters are hyperfine constants RN ) 1.49 mT, RH ) 1.49 mT, and g-value ) 2.0055, which coincide with those of the DMPO-OH adduct as demonstrated previously (6, 30). From Figure 2b, the relative intensity of DMPO-OH adduct signal increases with increasing amount of catalyst, ranging from 0 to 5 blocks, suggesting that the higher amount of catalyst used the higher concentration of •OH achieved at the fixed total applied ozone of 1.0 mg L-1. The results of Figure 2 also confirm the expectation mentioned above that the degradation of nitrobenzene may be mainly attributed to •OH oxidation. In fact, the formation of •OH derives from a series complex matrix reactions including the initiation, the propagation, and the termination steps in aqueous solution containing molecular ozone, producing the intermediates HO2-, HO2•, O3•-, HO3•, O•-, O2•-, and HO4• (5, 28, 31). These novel species can react with ozone or each other, or decompose to form •OH, according to the reactions listed as follows (26, 31-33). O3+HO•2 T •OH + 2O2

(1)

O3+H2O f 2•OH + O2

(2)

• •O3+HO2 f OH + O2+O2

(3)

• HO•2+HO2 f OH + HO +O2

(4)

+ • O•3 +H f OH + O2

(5)

HO•3 f •OH + O2 •-

•

O +H2O f OH + HO

(6) -

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FIGURE 3. Comparison of removal efficiency of TOC in the different processes (reaction conditions: temperature 293 K; initial pH 6.92; terminal pH 6.90∼6.91; initial nitrobenzene concentration 50 µg L-1; the concentration of total applied ozone 1.0 mg L-1; the number of cordierite and Cu-cordierite blocks used, respectively, in catalytic ozonation, five blocks; loading percentage of Cu (wt.%) 2%; CTOCt : the instantaneous concentration of TOC; CTOC0: the initial concentration of TOC). In addition, another important oxidative intermediate species, H2O2, can be found during the course of oxidative reactions (5, 31, 34), as shown in Supporting Information Figure S9. The generation of •OH, an active oxidative species, is the important characteristic of AOPs. A common objective of heterogeneous catalytic ozonation is to produce •OH in sufficient quantity to improve the degree of mineralization of the target organic compound. Removal Efficiency of TOC in the Different Processes. Due to the significance of mineralization of organic compounds, the experiments were performed to detect the removal efficiency of TOC in the processes of ozonation alone, ozonation/cordierite, and ozonation/Cu-cordierite. The results are illustrated in Figure 3. Figure 3 indicates that the two catalytic ozonation processes are more effective than ozonation alone to remove TOC from aqueous solution containing nitrobenzene. Ozonation/cordierite degraded 36.4% of the initial TOC compared to 15.7% by ozone alone with the same total ozone applied 1.0 mg L-1. Simultaneously, the improvement of removal efficiency of TOC is even more pronounced in the presence of Cu-cordierite, approximately 62.3% of initial TOC was removed after 20 min treatment in the process of ozonation/ Cu-cordierite. Furthermore, comparing the removal efficiency of TOC in Figure 3 with the degradation efficiency of nitrobenzene in Figure 1 in the same process, respectively, it is found that the removal of TOC is lower than the disappearance of nitrobenzene, indicating that nitrobenzene has been mineralized partly into carbon dioxide and water, and the byproducts are formed via the degradation of initial compound in the every selected processes (see the Supporting Information). Initiation of •OH from the Heterogeneous Catalytic Surface. For the heterogeneous catalytic ozonation systems, many possible mechanisms have been proposed in the previous studies. For instance, the powdered TiO2 catalytic ozonation of oxalic acid indicates that free radicals are generated in the aqueous phase from the decompositiondesorption of ozone-adsorbed species (35). The literature reports that the final step of the mechanism is the surface reaction between oxalic acid adsorbed on titania active sites and ozone remaining nonadsorbed in solution in the presence of ozone and a TiO2/Al2O3 catalyst, and adsorption 2050

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of oxalic acid seems to be the rate-controlling step (36). In MnOx/GAC catalytic ozonation adsorption of nitrobenzene on the catalyst is an important step, which will have a direct influence on the catalytic effectiveness of the heterogeneous catalytic system, and it is also assumed that nitrobenzene can be further decomposed on the surface of catalyst (8). In conclusion, it is generally believed that there are three possible mechanisms of catalytic ozonation in heterogeneous systems: (1) chemisorption of ozone on the catalyst surface leading to the formation of active species which react with nonchemisorbed organic molecule; (2) chemisorption of organic molecule (associative or dissociative) on the catalytic surface and its further reaction with gaseous or aqueous ozone; (3) chemisorption of both ozone and organic molecules and the subsequent interaction between the chemisorbed species (21). The diversity is mainly due to the differences in the catalyst properties, the properties of target organic compounds, and the operating conditions used by the different research groups. In the present experiment, as shown in Figure 1, adsorption of nitrobenzene on the catalyst surface only leads to a slight removal, which may scarcely contributes to the increasing degradation level of nitrobenzene in the catalytic ozonation. Likewise, the formations of • OH and H2O2, respectively, corroborate that Cu-cordierite catalytic ozonation follows the first of the three possible mechanisms mentioned above. Based on the conversion of surface characteristics, the improvement brought about by modification with Cu is emphasized in the following discussion section. As shown in Figure 1, the wet impregnation of Cu, a modifier, enhances the catalytic activity of raw cordierite, yielding a remarkable increase in degradation efficiency of nitrobenzene. This phenomenon implies that, as a suitable active component, Cu has a synergistic effect with cordierite in catalytic ozonation for the degradation of nitrobenzene in aqueous solution. Under the same amount of catalyst, the variation of normalized nitrobenzene concentration with loading percentage of Cu was investigated and shown in Figure 4a. The results indicate that the degradation efficiency of nitrobenzene increases as the loading percentage of Cu is increased from 0 to 3%, while further increase in the loading percentage of Cu from 3 to 6% produces a negative effect on the degradation efficiency. Furthermore, Figure 4a also shows that the relative intensity of the DMPO-OH adduct signal, representing the concentration of •OH formation, reaches the maximum value at the loading percentage of Cu 3%, namely the degradation efficiency of nitrobenzene and the concentration of •OH formation follow the same conversion rule with loading percentages of Cu. From Figure 4b, it is seen that a good correlation between the degradation efficiency of nitrobenzene and the relative intensity of DMPO-OH adduct signal exists at the different loading percentages of Cu applied by the present experiment, further confirming that the degradation of nitrobenzene may be attributed to •OH oxidation. Therefore, a critical Cu loading percentage of 3% is found, above which the degradation efficiency of nitrobenzene decrease sharply. These results suggest that, as loading percentage of Cu is increased from 0 to 3%, the synergistic effect between cordierite and Cu is also promoted, resulting in an acceleration of •OH formation, which in turn enhances the degradation efficiency of nitrobenzene. With a further increase in the loading percentage of Cu from 3 to 6%, the synergistic effect is decreased because the superabundant Cu can cause the permanent blockage of the catalyst active surface sites and yield a decrease in their catalytic activity, which reduces the initiation of •OH leading to the decrease in degradation efficiency of nitrobenzene. Here, the catalytic efficiency of Cu-cordierite more and more represents the catalytic activity of Cu itself

FIGURE 4. Variations of degradation efficiency of nitrobenzene, relative intensity of the DMPO-OH adduct signal, density of surface hydroxyl groups, and pHPZC with the loading percentages of Cu (reaction conditions: temperature 293 K; initial pH 6.92; terminal pH 6.90∼6.91; initial nitrobenzene concentration 50 µg L-1; concentration of total applied ozone 1.0 mg L-1; number of Cu-cordierite blocks used in catalytic ozonation, five blocks; loading percentages of Cu (wt.%) 0, 1.0, 2.0, 3.0, 4.0, 5.0, and 6.0%; initial DMPO concentration 100 mmol L-1; reaction time 20 min; (a): The variations of degradation efficiency of nitrobenzene and relative intensity of DMPO–OH adduct signal with the loading percentages of Cu; (b): The relationship between degradation efficiency of nitrobenzene and relative intensity of DMPO–OH adduct signal at the different loading percentages of Cu; (c): The relationship between density of surface hydroxyl groups and relative intensity of DMPO-OH adduct signal at the different loading percentages of Cu; (d): The variations of density of surface hydroxyl groups and pHPZC with the loading percentages of Cu; (e): The relationship between |pH - pHPZC| and density of surface hydroxyl groups at the different loading percentages of Cu). rather than the practical synergistic effect between cordierite concentration exists between the modifier and the dominatand Cu. It can be deduced that a critical equilibrium ing catalyst, which is dependent on the properties of the VOL. 43, NO. 6, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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supporter and the modifier, the process of catalyst preparation and the practical operating conditions. This has to be taken into consideration when investigating the mechanism of catalytic ozonation. The initiation of •OH is the most significant characteristic of AOPs, and is mainly attributed to the introduction of a catalyst surface in the heterogeneous catalytic ozonation. Specifically, the surface hydroxyl groups on the heterogeneous catalytic surface are believed to be crucial for the initiation of •OH from the decomposition of ozone (29). The surfaces of metal oxides are generally covered with hydroxyl groups in aqueous solution due to chemisorption of dissociative water molecules on the surface ions. As described in the previous study, the surface hydroxyl groups also can be formed during cordierite, and Cu-cordierite is introduced into aqueous solution because of the metal oxide composition of 2MgO-2Al2O3-5SiO2, and CuO (11, 37). In the following step, ozone can react with the surface hydroxyl groups, initiating the production of •OH on the surface of the catalyst (29, 31). Figure 4c indicates a good correlation between density of surface hydroxyl groups and relative intensity of DMPO-OH adduct signal at the different loading percentages of Cu, meaning that the formation of •OH is determined by the density of surface hydroxyl groups according to the fixed linearity under the present experimental conditions. However, it should be noticed that the density of the surface hydroxyl groups can be significantly influenced by the variation of the surface characteristic of the catalyst, especially the pHPZC. The pHPZC is the pH of the zero charge point of the catalyst surface, at which the amounts of negative and positive surface charges developed by proton equilibrium are equivalent. Figure 4d elucidates the variation of the density of surface hydroxyl groups and the pHPZC, respectively, with the loading percentage of Cu. Coinciding with the conversion rule of the degradation efficiency and •OH formation, the density of surface hydroxyl groups exhibits a maximum at the loading percentage of Cu 3%. However, the pHPZC of the catalyst increases slowly with increased loading percentage of Cu from 0 to 6%, presenting a positive correlation. Moreover, density of surface hydroxyl groups and their charge properties have relationship with the activity of catalyst in enhancing the decomposition of ozone to generate •OH (12). On the one hand, the charge properties or acid/base properties of the surface hydroxyl groups can be changed with the conversion of pHPZC and pH of aqueous solution (12). This can be inferred from eqs 8 and 9. Therefore, the pH of aqueous solution is an important factor that determines the charge properties of surface hydroxyl groups at oxide/ water interface (38). MeOH + H+ S MeOH+ 2 (pH < pHPZC) –

–

MeOH + OH S MeO +H2O(pH > pHPZC)

(8) (9)

When the pH of aqueous solution is near the pHPZC of the oxide, most of the surface hydroxyl groups are in a neutral state, namely nearly no surface charge exists. Otherwise, the oxide surface becomes protonated or deprotonated when the pH of aqueous solution is below or above the pHPZC. However, only the surface hydroxyl groups at neutral state are believed to have a relative higher catalytic activity to accelerate the decomposition of ozone and the initiation of • OH (12). As far as the protonated surface hydroxyl group is concerned, its O is weaker in nucleophilicity than the O of a neutral state hydroxyl group. Therefore, the protonation of the surface hydroxyl group will be a disadvantage to the surface binding of ozone. Likewise, the deprotonated surface hydroxyl group can not provide the electrophilic H, which would also handicap the processes of the decomposition of ozone and the initiation of •OH (12). Based on the theory 2052

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mentioned above, the density of surface hydroxyl groups is dependent on the absolute value of the difference between the pH of aqueous solution and the pHPZC, namely |pH pHPZC|. In a departure from the previous work (10), in order to investigate the degradation efficiency of trace organic compounds, the initial nitrobenzene concentration was decreased from 10 mg L-1 to 50 µg L-1 (by 200 times), leading to only a slight variation of pH from the initial pH of 6.92 to terminal pH 6.90∼6.91 and can therefore be neglected. This is due to a reduction in the amount of oxidation products inducing the variation in pH under the present experimental conditions. In addition, compared to the scavenger effect of buffer solution (HCO3-, CO32-, H2PO4-, and HPO42-), the degradation of trace initial nitrobenzene concentration led to a slight conversion of initial pH which could not affect the experiment. Therefore, the experiments were carried out at initial pH 6.92 in this study without adding any buffer solution to maintain the pH at a constant value. Figure 4e shows the relationship between |pH - pHPZC| and density of surface hydroxyl groups at the different loading percentages of Cu, representing a good negative linear correlation. Thus the density of surface hydroxyl groups increases with the decrease of |pH - pHPZC| within the loading Cu percentage range selected for this experiment. Therefore, the enhancement mechanism of modification with Cu is obvious based on the results listed above. Enhancement Mechanism of Modification with Cu. From the experimental results and the theory mentioned above, the enhancement mechanism of modified cordierite with Cu is proposed and illustrated in Supporting Information Figure S11. At first, the presence of cordierite catalyst increases significantly the degradation efficiency of nitrobenzene compared to the case of ozonation alone (Figure 1) because the introduction of a heterogeneous catalytic surface accelerates the initiation of •OH (10). The modification of cordierite by loading Cu results in a variation of surface characteristics, such as the appearance of CuO crystalline phase (Supporting Information Figure S7) and an increase in specific surface area, specifically via the enhancement of pHPZC. The results of Figure 4d indicate that the pHPZC of the catalyst increases from 6.60 to 7.34 with the increase in loading percentage of Cu from 0 to 6%. Combined with the effect of change in the pH of aqueous solution, the enhancement of pHPZC produces the observed variation in |pH pHPZC|. In addition, based on eqs 8 and 9, it is deduced that the pH of aqueous solution and the pHPZC can determine the density of surface hydroxyl groups, which can be further confirmed by the good negative correlation observed between |pH - pHPZC| and the density of surface hydroxyl groups at the different loading percentages of Cu shown in Figure 4e. On the one hand, when pH > pHPZC, with the enhancement of pHPZC from 6.60 to 6.94 derived from the increasing loading percentage of Cu from 0 to 3%, the density of surface hydroxyl groups increases from 0.91 × 10-5 mol m-2 to 2.28 × 10-5 mol m-2 due to the decrease in the proportion of deprotonated surface hydroxyl groups. Contrarily, when pH < pHPZC, with the enhancement of pHPZC from 6.94 to 7.34 derived from the increase loading percentage of Cu from 3 to 6%, the density of surface hydroxyl groups decreases from 2.28 × 10-5 mol m-2 to 0.45 × 10-5 mol m-2 due to the increase in the proportion of protonated surface hydroxyl groups. Notice that the density of surface hydroxyl groups can reach a maximum value at the critical Cu loading percentage of 3%. At this point, |pH - pHPZC|effectively reaches a minimum, meaning that most of the surface hydroxyl groups on Cu-cordierite are nearly uncharged.

From the correlation between density of surface hydroxyl groups and relative intensity of the DMPO-OH adduct signal in Figure 4c and the correlation between relative intensity of DMPO-OH adduct signal and normalized nitrobenzene concentration in Figure 4b, it can be inferred that the density decrease of the surface hydroxyl groups directly casuses the weakening of the •OH initiation because the reduction of surface hydroxyl groups on the catalyst in aqueous solution provides fewer chances for the interaction between molecular ozone and the surface hydroxyl groups (12). Simultaneously, the weakening of •OH initiation causes a reduction in the degradation efficiency of nitrobenzene, which is mainly ascribed to oxidation by •OH achieved through the diffusion of generated •OH from the catalyst surface into the bulk aqueous solution. Therefore, the phenomenon, the increase in the proportion of protonated or deprotonated surface hydroxyl groups leads to the density decrease of surface hydroxyl groups at neutral state, should be taken into consideration when modifying the catalyst for catalytic ozonation.

Acknowledgments The support from the Scheme of 863 High Technology Research and Development Program of China (Grant No. 2006AA06Z306) and the China Postdoctoral Science Foundation (Grant No. 20080440130) are greatly appreciated.

Supporting Information Available Additional details of catalyst preparation, ozonation procedure, the analytic conditions, effect of operating variables, the leaching of effective components, the utilization efficiency of ozone, evolution of H2O2 formation, formation and evolution of byproducts in the different processes, and the figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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