Characterization and Evaluation of Catalytic Dechlorination Activity of

Oct 15, 2008 - Batch experiments were conducted to evaluate the catalytic dechlorination activity of the prepared Pd/Fe NPs. Evidence observed ... The...
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Ind. Eng. Chem. Res. 2008, 47, 8645–8651

8645

MATERIALS AND INTERFACES Characterization and Evaluation of Catalytic Dechlorination Activity of Pd/Fe Bimetallic Nanoparticles Xiangyu Wang, Chao Chen, Huiling Liu,* and Jun Ma School of Municipal and EnVironmental Engineering, State Key Laboratory of Urban Water Resources and EnVironment, Harbin Institute of Technology, Harbin 150090, China

Two kinds of Pd/Fe nanoparticles (NPs) (k-Pd/Fe NPs and p-Pd/Fe NPs) were prepared by using an aqueous solution of potassium hexachloropalladate and an ethanol solution of palladium acetate as palladization solution, respectively, through the chemical precipitation method, followed by drying in an oven under vacuum, and characterized systematically in terms of morphology (SEM), Pd loading (EDS), surface composition (XRF), specific surface area (BET), particle size (TEM), and crystal structure (XRD). Batch experiments were conducted to evaluate the catalytic dechlorination activity of the prepared Pd/Fe NPs. Evidence observed suggests that the activities of various kinds of particles used for dechlorination of chlorinated methanes including carbon tetrachloride (CT), chloroform (CF), and dichloromethane (DCM) followed the order p-Pd/Fe NPs > Fe NPs > k-Pd/Fe NPs > Micro-Fe (commercial microscale iron powder). This order was found to be dependent on the size, surface area, degree of Pd dispersion, and purity extent of metal particles. These results may provide better understanding of the effects of preparing and drying methods on the catalytic dechlorination activity of Pd/Fe NPs. 1. Introduction Chlorinated organic compounds (COCs) are widespread environmental contaminants both in groundwater and in soils,1,2 many of which are carcinogenic, teratogenic, and mutagenic, and of public health concern. They are persistent in the environment over a long period of time, and are difficult to directly degrade under most natural biological or abiotic conditions. Therefore, the degradation of COCs in aqueous solution and soil has received particular attention in the environmental engineering community. In general, COCs can be removed by means of physical treatment, biological treatment, and chemical treatment. Physical treatment includes gasblowing desorption, activated carbon adsorption, volatilization, extraction, and so on. Physical treatment can merely separate COCs from gas or water in terms of their physical properties, such as volatility and solubility, but cannot destroy them.1-3 Furthermore, the transferred COCs must be treated at an additional cost. Biological treatment can decompose COCs by the metabolic process of microorganisms, and optimal conditions are necessarily required to sustain the degradation of COCs. However, biological treatment is inhibited by the toxicity of COCs to microorganisms, the selectivity of microorganisms to various pollutants, and the relatively slow degradation rate of COCs.4-8 Chemical treatment includes incineration, wet oxidation, ozone oxidation, Fenton’s process, photocatalytic oxidation, and so on.1,9,10 COCs are degraded into CO2, H2O, and HCl by oxidation processes. Recently, researchers have tended to degrade COCs by direct reduction with zerovalent iron (ZVI). Since ZVI is low-cost and can be obtained in bulk quantity, considerable interest has been aroused in groundwater treatment and site remediation with ZVI technology. Gillham and O’Hannesin employed ZVI as a reductant to dechlorinate COCs in 1994.11 The final products were nontoxic hydrocarbons and * To whom correspondence should be addressed. Tel.: +86 451 53625118. Fax: +86 451 53625118. E-mail: [email protected].

chloride ions.11-17 However, ZVI is not active enough to dechlorinate some COCs, and an extremely long reaction time is needed for complete dechlorination of these pollutants. In addition, the surface of ZVI tends to be covered by a metal (hydr)oxide layer with the increase of elapsed time, resulting in the deactivation of ZVI. Muftikian et al. deposited a thin and discontinuous layer of a second metal such as Ni, Cu, Pt, and Pd onto iron surface. Comparing with ZVI, the activity of bimetallic particles can be increased dramatically, and the production of chlorinated byproducts is curtailed. Thus, the

Figure 1. Schematic diagram showing the proposed pathways for dechlorination of COC with Pd/Fe bimetallic particles: (a) in acidic solution; (b) in basic solution.

10.1021/ie701762d CCC: $40.75  2008 American Chemical Society Published on Web 10/16/2008

8646 Ind. Eng. Chem. Res., Vol. 47, No. 22, 2008 Table 1. Comparison of Theoretical and Determined Pd Loadings of Pd/Fe NPs k-Pd/Fe NPs theoretical Pd loading (wt %)

actual Pd loading (wt %)

0.050 0.100 0.180 0.200

0.080 0.130 0.223 0.247

p-Pd/Fe NPs

deviation

actual Pd loading (wt %)

deviation

+0.030 +0.030 +0.043 +0.047

0.044 0.086 0.169 0.183

-0.006 -0.014 -0.011 -0.017

remediation technology with bimetallic particles is considered to be a promising method to enhance the transformation of COCs and expedite the dechlorination reaction rate.18-22 Cheng23 and Grittini19 postulated that metal Pd is an excellent catalyst to absorb H2, and on the surface of Pd/Fe bimetallic particles, a concentrated reactive surface forms. As one of the transition metals, noble metal Pd has a void orbit, and can combine with the chlorine atoms of COCs leading to the formation of a transitional complex compound. The dechlorination activation energy is decreased in this way. Catalytic dechlorination of COCs by Pd/Fe bimetallic particles is proposed to follow three steps: (i) H2 is produced by the corrosion of iron in water (eqs 1 and 2), (ii) a generalized chlorinated hydrocarbon, RCl, transports to catalyst Pd, accompanied by the formation of the complex Pd · · · Cl · · · R (eq 3), and (iii) RCl is consequently transformed to the corresponding hydrocarbon RH (eq 4). The plausible mechanisms for the catalytic dechlorination reaction are shown in Figure 1. Fe + 2H+ f Fe2+ + H2

(in

Fe + 2H2O f Fe2+ + H2 + 2OH-

acidic solution) (in

basic solution) (2)

Pd + RCl f Pd · · · Cl · · · R +

(1)

(3) -

H2 + Pd · · · Cl · · · R f RH + H + Cl + Pd

(4)

Since dechlorination takes place on the metal surface, the dechlorination efficiency may be expected to increase with increasing the surface area available for reaction. The utilization of Pd/Fe nanoparticles (NPs) is an excellent way to attain large surface area and has become one of the latest innovative technologies.24-27 Pd/Fe NPs are not yet commercially available and must be prepared in the laboratory. Currently, the preparation of Pd/Fe bimetallic NPs is achieved by depositing Pd on freshly prepared Fe NPs. At least three distinct reductive deposition routes are used to palladize Fe NPs: (1) the reaction of Fe NPs with an aqueous solution of potassium hexachloropalladate,19,20,26,28,29 (2) the reaction of Fe NPs with an ethanol solution of palladium acetate,21,25,27 and (3) the reaction of Fe NPs with an acidic solution of palladium chloride.30 Among the three routes, an aqueous solution of potassium hexachloropalladate and an ethanol solution of palladium acetate are widely adopted as palladization solutions for preparation of Pd/Fe NPs. In these cases, the prevention of iron oxidation is especially important during the preparation process in order to get favorable dechlorination efficiency, because iron is known to be thermodynamically unstable and can be easily oxidized. The larger the amount of iron that is oxidized, the less concentration of effective iron is left, and the lower the activity of Pd/Fe NPs. In previous studies, considerable attention has been devoted to the pathway and the kinetics of dechlorination of COCs by using zerovalent iron or bimetallic particle remediation technology. So far to the best of our knowledge, the wet prepared Pd/ Fe NPs were either dried before further experimental survey or

used in dechlorination reactions without drying.20,28 However, the use of wet prepared NPs represents a formidable technical hurdle when considering an in situ remediation of COCcontaminated groundwater and soils, and on account of this, drying NPs is significantly worthwhile and will become a major issue for the feasible and convenient application of this remediation technology. Compared with conventional wet NPs, dried NPs offer flexibility in deployment. In order to investigate some important parameters (e.g., surface area, dosage, size, morphology, noble metal dispersion, and structure of Pd/Fe NPs) that have been demonstrated to have an effect on dechlorination efficiency, drying processing of freshly prepared wet Pd/Fe NPs is essentially demanded. By far, the drying methods of wet NPs include (1) drying by nitrogen gas,27 (2) storing in the freezedrier,25 (3) drying under nitrogen at 100 °C for 30 min,31 (4) drying at 110 °C under a flow of nitrogen,23 (5) spreading the wet NPs in a thin layer and drying at room temperature in argon atmosphere,32 and (6) drying via vacuum freeze-drying technique at -56 °C for 24 h.33 Unfortunately, the aforementioned drying methods were shown to be unfavorable to commercial aspects. Even though Pd/Fe NPs were reported to be highly effective for catalytic dechlorination of COCs, little effort has been made to investigate the activity of Pd/Fe NPs after being dried for the purpose of prolonged use and application. Up to now, fundamental information about the understanding of the effect of the drying process on the physical and chemical properties and the activity of NPs has not been conclusively documented. Nevertheless, a more detailed understanding of such an effect is of obvious importance for the effective application of COC remediation technology by using bimetallic NPs. The purpose of this study is to contribute to such an understanding. In this study, the preparation, characterization, and catalytic activity of Pd/Fe NPs were investigated. Two kinds of Pd/Fe NPs were prepared by soaking Fe NPs in an aqueous solution of potassium hexachloropalladate and an ethanol solution of palladium acetate, respectively. In particular, under economical consideration, these two kinds of NPs were vaccuum-dried in an oven without the protection of nitrogen. The further objective of this study is to assess the feasibility of the herein provided relatively facile drying method. The size, morphology, Pd dispersion, and crystal structure of the prepared Pd/Fe NPs were extensively and systematically characterized. The catalytic activity of the two kinds of Pd/Fe NPs was quantitatively evaluated in terms of dechlorination efficiency of chlorinated methanes in the presence of Pd/Fe NPs in the dry state. Three chlorinated methanes, namely carbon tetrachloride (CT), chloroform (CF), and dichloromethane (DCM), which are among the most prevalent COCs, were chosen as target pollutants. Moreover, the reactivity of Pd/Fe NPs was compared with Fe NPs and commercial microscale iron powder, aiming at providing additional information about their relative activities. These results may provide essential information for implementing the in situ application of bimetallic NP remediation technology. 2. Experimental Section 2.1. Materials. Potassium borohydride (KBH4, 99%), ferric chloride hexahydrate (FeCl3 · 6H2O), potassium hexachloropalladate (K2PdCl6), palladium acetate ([Pd(C2H3O2)2]3, 47.4 wt % Pd), potassium hydroxide (KOH), methanol, ethanol, acetone, microscale iron powder (Micro-Fe, 98%, 200 mesh), CT (CCl4, 99.8%), CF (CHCl3, 99.8%), and DCM (CH2Cl2, 99.5%).

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Figure 2. XRD patterns of Pd/Fe NPs: (a) k-Pd/Fe NPs; (b) p-Pd/Fe NPs.

All the chemicals were reagent pure grade or above and used without further purification. Deionized water was used for preparation of reagent solutions. 2.2. Preparation of Fe NPs. Fe NPs was prepared by adding 179 mL of KOH solution containing 30 g of dissolved KBH4 dropwise to 179 mL of 1.0 M FeCl3 · 6H2O solution under vigorously magnetic stirring condition for complete mixing. The preparation was carried out in an anaerobic chamber while supplying N2 gas to the chamber to prevent intrusion of O2; otherwise the particles would be oxidized rapidly on the surface. The solution was stirred for an additional 5 min after all the KBH4 solution was added to FeCl3 solution, which ensured completion of the redox reaction. Fe NPs were made according to the following reaction: 4Fe3+ + 3BH4- + 9H2O f 4Fe0V + 3H2BO3- + 12H+ + 6H2v (5) Next, Fe NPs were rinsed successively three times or more with a large excess of deionized water until no chlorine ions could be detected in the filtrate, followed by harvesting via vacuum filtration through a 0.22-µm cellulose acetate membrane filter. 2.3. Preparation of Pd/Fe NPs. 2.3.1. Preparation of k-Pd/Fe NPs. The wet prepared Fe NPs were soaked in a dilute hydrochloric acid solution containing the required weight percentage of palladium by carefully weighing potassium hexachloropalladate, accompanied by continuous magnetic stirring for about 20 min. Taking into account that potassium hexachloropalladate is hard to dissolve in water and easy to dissolve in hydrochloric acid, a small amount of hydrochloric

acid was added to make the potassium hexachloropalladate soluble. This caused the reduction of PdCl62- and deposition of Pd on the surface of Fe NPs (eq 6). PdCl62- + 2Fe f 2Fe2+ + PdV + 6Cl-

(6)

The wet Pd/Fe NPs were vacuum-filtered, then dried in an oven under vacuum at 100 °C for 6 h, and restored in vials. Pd/Fe NPs prepared by using an aqueous solution of potassium hexachloropalladate as palladization solution were referred to as k-Pd/Fe NPs throughout the presentation. 2.3.2. Preparation of p-Pd/Fe NPs. Freshly prepared Fe NPs were charged into an ethanol solution of palladium acetate under magnetic stirring for 20 min. This caused the reduction of Pd2+ and deposition of Pd on Fe NPs surface. The reaction occurred according to the following equation: Pd2+ + Fe0 f Pd0V + Fe2+

(7)

The prepared Pd/Fe NPs were rinsed seriatim with ethanol and acetones until no chlorine ions were detected in the filtrate, and thereafter were vacuum-filtered through a piece of poly(vinylidene fluoride) (PVDF) membrane. The wet Pd/Fe NPs were dried in an oven under vacuum at 100 °C for 6 h, and then restored in vials. The Pd/Fe NPs prepared by using an ethanol solution of palladium acetate as palladization solution were termed p-Pd/Fe NPs throughout the presentation. 2.4. Characterization of Pd/Fe NPs. The specific surface areas of Pd/Fe NPs were measured from N2 physisorption using BJH (Barrett-Joyner-Halenda) and multipoint BET (BrunauerEmmett-Teller) methods with an Autosorb-1 surface area

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Figure 3. SEM images of Pd/Fe NPs: (a) k-Pd/Fe NPs; (b) p-Pd/Fe NPs. Table 2. Pd Dispersion on the Surface of Pd/Fe NPs in Various Regions k-Pd/Fe NPs region

Pd loading (wt %)

1 2 3 4 5

0.283 0.251 0.232 0.242 0.273

p-Pd/Fe NPs

deviation

Pd loading (wt %)

deviation

+0.083 +0.051 +0.032 +0.042 +0.073

0.192 0.184 0.186 0.187 0.208

-0.008 -0.016 -0.014 -0.013 +0.008

analyzer (Quantachrome Corp., Boynton Beach, FL, USA). The morphology of Pd/Fe NPs was examined using a MX2600FE scanning electron microscope (SEM, Camscan Ltd., U.K.). Localized Pd/Fe NP information from the chosen regions was obtained with an INCA X-ray energy-dispersive spectrometer (EDS, Oxford Instruments, U.K.) in conjunction with the SEM. The particle size and size distribution of Pd/Fe NPs were observed with a JEM 1200EX transmission electron microscope (TEM, JEOL Ltd., Japan) at an acceleration voltage of 120 kV. Pd/Fe NPs were ultrasonicated for 5 min in acetone. The resulting suspension was dropped and air-dried on a 300-mesh gold/copper lacey carbon grid before visualization under the TEM. The crystal structures of Pd/Fe NPs were characterized by X-ray diffraction (XRD) on a D/max-rB X-ray diffractometer

(Rigaku Corp., Japan) using Ni-filtered Cu KR radiation operating at an accelerating voltage of 45 kV and emission current of 40 mA (λ ) 0.15418 nm) in a step scanning mode, with a step size of 0.02° and a counting time of 1 s per step in the range 20-80°. All major species of iron and/or iron oxides were covered in this scan range. The crystal sizes of Pd/Fe NPs were calculated from X-ray line broadening analysis by Scherrer diffraction formula. The intermolecular distance d was calculated using Bragg’s first-order X-ray diffraction equation (d ) λ/(2 sin θ)). Pd loading of Pd/Fe NPs was determined with an AXIOSPW4400 X-ray fluorescence spectrophotometer (XRF, PANalytical Company, Holland). 2.5. Batch Experiments. A stock solution containing 25 g/L chlorinated methane was prepared by dissolving an exact amount of pure chlorinated methane in methanol and used within 24 h. The chlorinated methane solution at desired concentration was prepared by spiking a controlled volume of methanolic stock solution into deionized water. For each dechlorination reaction, 150 mL volume serum bottles filled with 50 mL of chlorinated methane with an initial concentration of 100 mg/L were loaded with 0.5 g of Pd/Fe NPs (0.20 wt % Pd loading) and sealed with Teflon-lined rubber septums immediately. These bottles were agitated at a constant shaking rate of 170 rpm in a temperature-controlled orbital shaker maintained at 22 ( 1 °C. Parallel experiments were conducted with 0.5 g of Fe NPs and 3 g of Micro-Fe under the same reaction conditions, respectively. Aliquots of samples were taken at times using a gas-tight syringe from the supernatant. The solution in the syringe was filtered for subsequent analyses. Duplicated experiments were performed at each sampling time. 2.6. Analytical Method. The dissolved chloride ion was quantified by using a 4500i ion chromatograph (IC, Dionex Corp., USA) equipped with a Dionex IonPac-AS 14 (4 mm × 250 mm) column and a conductivity detector at a flow rate of 1.0 mL min-1. The eluent solution was 1.7 mM NaHCO3/1.8 mM Na2CO3 (with chemical suppression), and the loop volume of each sample was 20 µL. Sodium chloride solution was used as a standard. The detection limit was 0.075 mg/L, and the average analytical error was less than 3.5%. Before injection, samples of the clear solution (supernatant) were filtered through a 0.22-µm Millipore syringe filter. Dechlorination efficiency was the ratio of the chloride detected to the chloride ion theoretically produced by the complete dechlorination of chlorinated methane (i.e., dechlorination efficiency of CT ) (CCl-/4C0) × 100%, where CCl- is the concentration of chloride ion and C0 is the initial concentration of carbon tetrachloride (CT)). 3. Results and Discussion 3.1. Pd Loading of Pd/Fe NPs. XRF analysis reveals the difference between theoretical Pd loading and actual Pd loading of Pd/Fe NPs. As can be seen in Table 1, metrical Pd loadings of p-Pd/Fe NPs were closer to theoretical Pd loading than that of k-Pd/Fe NPs. In regard to k-Pd/Fe NPs, the result suggests that the actual Pd loading is higher than the theoretically expected value. The discrepancy might be attributed to the loss of iron when Fe NPs were palladized in a dilute hydrochloric acid solution of potassium hexachloropalladate. It could be observed in the experimental process that the filtrate color of k-Pd/Fe NPs was light green; while ferrous, the evidence of the loss of iron could be detected in that filtrate. In Table 1, the actual Pd loadings of p-Pd/Fe NPs were a bit lower than the theoretical Pd loading, which might be attributed to the possibility that Pd2+ of palladium acetate was relatively active,

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and some could be reduced before palladization. The filtrate color of p-Pd/Fe NPs was colorless, and no ferrous ions could be detected, implying that no iron was oxidized. 3.2. Crystal Structure of Pd/Fe NPs. The XRD patterns of the two kinds of Pd/Fe NPs are shown in Figure 2. Characteristic peaks belonging to Pd were unobservable due to the small Pd loading of Pd/Fe NPs (0.2 wt %). In Figure 2a, the characteristic peaks at 2θ of 35.62°and 44.66° were assigned to the 311 diffraction peak of Fe2O3 and the 110 diffraction peak of R-Fe, respectively. The result illustrates that k-Pd/Fe NPs contained 76 wt % Fe2O3 and 23 wt % Fe, which unequivocally confirmed that a large amount of k-Pd/Fe NPs had been oxidized. For freshly prepared wet k-Pd/Fe NPs, the percentage of ZVI is supposed to be higher. Figure 2a indicates that the k-Pd/Fe NPs likely had a core/shell structure (R-Fe core and iron oxide shell).34 Macroscopically, the color of k-Pd/Fe NPs was dark reddish-brown, indicating that dry k-Pd/Fe NPs were oxidized. Above results showed that palladizing Fe NPs in an aqueous solution of potassium hexachloropalladate and drying k-Pd/Fe NPs in an oven under vacuum caused the oxidization of k-Pd/ Fe NPs. On the basis of XRD results, the mean crystal size of k-Pd/Fe NPs was determined by the Scherrer diffraction formula to be 46.35 nm. As shown in Figure 2b, the three strongest major peaks appeared at 44.66°, 64.50°, and 82.36°, respectively. The d spacings for the XRD patterns (0.202 nm (110), 0.144 nm (200), and 0.117 nm (211)) are consistent with d spacings for the three characteristic reflections of body-centered-cubic (bcc) Fe0 (0.203 nm (110), 0.143 nm (200), and 0.117 nm (211)), indicating that p-Pd/Fe NPs is present as R-Fe(0) crystal structure. No diffraction peak of Fe2O3 was found in Figure 2b, and no visually observable color change was noticed on the dry p-Pd/ Fe NPs, suggesting that p-Pd/Fe NPs were not oxidized during the palladizing and drying process. Comparatively, the palladizing process using an ethanol solution of palladium acetate led to a more regular phase structure and a higher purity extent of Pd/Fe NPs. The crystal size of p-Pd/Fe NPs was calculated by the Scherrer diffraction formula to be 38.44 nm. The most probable explanation for the above observation should assume that the different kinds of solutions were used for palladizing Fe NPs. During the process of preparing k-Pd/ Fe NPs, deionized water was used as the solvent, and therefore, a large amount of microgalvanic batteries were formed on the surface of k-Pd/Fe NPs due to the conductivity of water and the presence of dissolved oxygen in the reaction system. ZVI particles of k-Pd/Fe NPs could be easily oxidized. The superiority of p-Pd/Fe NPs can be attributed to the key factor that ethanol was the solvent of the palladization solution for the preparation of p-Pd/Fe NPs, and likewise, the washing agents of p-Pd/Fe NPs were organic compounds (ethanol and acetone). Comparing with k-Pd/Fe NPs, p-Pd/Fe NPs had little opportunities to be oxidized. 3.3. Morphology and Surface Area of Pd/Fe NPs. The specific surface area is expected to be an important factor of the reaction rate and has a significant impact on the dechlorination rate. According to the theory of Tratnyek et al.,35 the relationship between the dechlorination rate and the specific surface area is a direct proportion (eq 8). d[P] ) kSAaSFm[P] (8) dt where kSA denotes the surface-area-normalized rate coefficient, aS denotes the specific surface area, Fm denotes the mass concentration of metal, and P denotes the reacting halocarbon in the aqueous phase. As reactive sites on the metal surface are -

involved, NPs with larger surface area are recognized as having higher surface energy and thus higher activity.14 The BET surface areas of k-Pd/Fe NPs and p-Pd/Fe NPs were 33.6 m2/g and 51.4 m2/g, respectively. The catalytic activity and dechlorination rate of the latter would presumably be higher than that of the former. Figure 3a shows that k-Pd/Fe NPs aggregated severely: no single and regular particle could be easily recognized from a group of particles. In contrast, p-Pd/Fe NPs have a very different morphology. As shown in Figure 3b, the roughly spherical and evenly sized beads were aggregates of nanosized primary particles, and the long chains composed of spherical beads intertwined and formed the microstructure of p-Pd/Fe NPs. The stepped surface of p-Pd/Fe NPs would benefit the increase of catalytic dechlorination activity. Dechlorination of COCs is known to occur on the metal surface,14 and therefore, the well-proportioned distribution and dispersion of Pd on Fe NPs is beneficial to the catalytic dechlorination of target pollutants. EDS analyses give supplementary information on the Pd dispersion of five randomly chosen regions of Pd/Fe NPs. The theoretical Pd loading of the Pd/Fe sample was 0.2 wt %. Table 2 reveals that the Pd dispersion of p-Pd/Fe NPs is more homogeneous than that of k-Pd/Fe NPs. 3.4. Shape and Size of Pd/Fe NPs. The size and size distribution of k-Pd/Fe NPs and p-Pd/Fe NPs are presented in TEM images of Pd/Fe NPs (Figure 4). As the image reveals, k-Pd/Fe NPs with diameters in the range 30-80 nm agglomerated severely due to magnetic and electronic interactions between the particles (Figure 4a). The outline of their chains was stubbed and unclear, and such an observation might be ascribed to the oxidation of Fe. Figure 4b indicates that diameter of finer sized and distinct p-Pd/Fe NPs was in the range 20-50 nm. Evidently, the shape and outline of p-Pd/Fe NPs were clearer than those of k-Pd/Fe NPs. Not surprisingly, the morphology and particle shape of two kinds of Pd/Fe NPs shown in TEM images were consistent with those shown in SEM images. 3.5. Dechlorination of Chlorinated Methanes. Figures 5, 6, and 7 depict the dechlorination efficiencies of chlorinated methanes with four kinds of particles. From Figure 5, the dechlorination efficiencies of CT with p-Pd/Fe NPs, Fe NPs, k-Pd/Fe NPs, and Micro-Fe were 65%, 25.3%, 9.8%, and 8.5% within 180 min, respectively. Apparently, p-Pd/Fe NPs exhibited the highest activity among the four kinds of particles for the dechlorination of CT. As can be seen in Figure 6, the dechlorination efficiency of CF with p-Pd/Fe NPs was by far the highest among the four kinds of particles, and it increased steadily with time and reached 67.4% within 240 min. In comparison, the dechlorination efficiency of CF in the presence of k-Pd/Fe NPs was less than 10%, and CF was not measurably dechlorinated in the presence of Micro-Fe. No apparent dechlorination of DCM was observed in the presence of the four kinds of particles within 480 min (Figure 7). The dechlorination efficiency of DCM with p-Pd/Fe NPs was about 7%, the highest among the four particles. The result suggests that Fe NPs, k-Pd/Fe NPs, and Micro-Fe could not dechlorinate DCM effectively. In summary, the dechlorination reaction rate decreased with the number of chlorine substituents. Muftikian et al. rationalized the decrease of reaction rate by assuming that intermediate species with Pd-Cl bonds participate

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Figure 6. Dechlorination efficiency of CF with Micro-Fe, Fe NPs, k-Pd/Fe NPs, and p-Pd/Fe NPs.

Figure 7. Dechlorination efficiency of DCM with Micro-Fe, Fe NPs, k-Pd/ Fe NPs, and p-Pd/Fe NPs. Figure 4. TEM images of Pd/Fe NPs: (a) k-Pd/Fe NPs; (b) p-Pd/Fe NPs.

Comparing with p-Pd/Fe NPs, k-Pd/Fe NPs showed lower catalytic activity for the dechlorination of chlorinated methanes. The possible explanation for this phenomenon is the formation of a passivation layer of Fe oxides on the surface which inhibits the activity of k-Pd/Fe NPs. It can be further postulated that Pd, which serves as a hydrodechlorination catalyst, was also poisoned and deactivated by iron oxides. More interestingly, even though the mass concentration of Micro-Fe was 6 times higher than that of p-Pd/Fe NPs in this study, the dechlorination efficiency of chlorinated methanes with Micro-Fe was much lower than that with p-Pd/Fe NPs. Comparing with Micro-Fe (specific surface area of less than 0.9 m2/g), the surface area of p-Pd/Fe NPs was dramatically increased, resulting in an increase of the reactive sites that participated in surface reactions. The higher catalytic dechlorination activity of p-Pd/Fe NPs evidenced that the deposition of noble metal Pd on Fe NPs enormously prompted the ongoing process of the dechlorination reaction. 4. Conclusions

Figure 5. Dechlorination efficiency of CT with Micro-Fe, Fe NPs, k-Pd/Fe NPs, and p-Pd/Fe NPs.

in the dechlorination of chlorinated methane. The probability of forming such an intermediate changed from the greatest to the least.20

Compared with k-Pd/Fe NPs, Fe NPs, and Micro-Fe, p-Pd/ Fe NPs have remarkably higher activity for the catalytic dechlorination reaction of chlorinated methanes. The dechlorination efficiency of chlorinated methanes with the presence of p-Pd/Fe NPs was about 56.6%-4% higher than that with the presence of k-Pd/Fe NPs. Taken together, the overall order of the activity can be presented as p-Pd/Fe NPs > Fe NPs > k-Pd/

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ReceiVed for reView December 25, 2007 ReVised manuscript receiVed August 20, 2008 Accepted September 2, 2008 IE701762D