Polysugar-Stabilized Pd Nanoparticles Exhibiting High Catalytic

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Langmuir 2008, 24, 328-336

Polysugar-Stabilized Pd Nanoparticles Exhibiting High Catalytic Activities for Hydrodechlorination of Environmentally Deleterious Trichloroethylene Juncheng Liu,† Feng He,‡ Ed Durham,† Dongye Zhao,*,‡ and Christopher B. Roberts*,† Department of Chemical Engineering and Department of CiVil Engineering, Auburn UniVersity, Auburn, Alabama 36849 ReceiVed May 17, 2007. In Final Form: October 8, 2007 In this paper, we present a straightforward and environmentally friendly aqueous-phase synthesis of small Pd nanoparticles (approximately 2.4 nm under the best stabilization) by employing a “green”, inexpensive, and biodegradable/ biocompatible polysugar, sodium carboxymethylcellulose (CMC), as a capping agent. The Pd nanoparticles exhibited rather high catalytic activity (observed pseudo-first-order reaction kinetic rate constant, kobs, is up to 828 L g-1 min-1) for the hydrodechlorination of environmentally deleterious trichloroethene (TCE) in water. Fourier transform IR (FT-IR) spectra indicate that CMC molecules interact with the Pd nanoparticles via both carboxyl (-COO-) and hydroxyl (-OH) groups, thereby functioning to passivate the surface and suppress the growth of the Pd nanoparticles. Hydrodechlorination of TCE using differently sized CMC-capped Pd nanoparticles as catalyst was systematically investigated in this work. Both the catalytic activity (kobs) and the surface catalytic activity (turnover frequency, TOF) of these CMC-capped Pd nanoparticles for TCE degradation are highly size-dependent. This point was further verified by a comparison of the catalytic activities and surface catalytic activities of CMC-capped Pd nanoparticles with those of β-D-glucose-capped Pd and neat Pd nanoparticles for TCE degradation.

Introduction Catalytic hydrodechlorination is an innovative technology for the remediation of soil and groundwater contaminated with chlorinated hydrocarbon compounds.1-4 Trichloroethylene (TCE) is one of the most common chlorinated organic pollutants detected in soil and groundwater, especially at sites adjacent to dry cleaners, automobile manufacturers or shops, asphalt processing plants, and military bases.5 TCE is a potent carcinogen and is resistent to natural degradation, thus posing long-term environmental and health risks.6 TCE can undergo reductive dechlorination by H2 within the aqueous phase in the presence of Pd or Pd-on-Au catalysts3,7 and ultimately be transformed into biodegradable ethane.1 Since Pd catalytic hydrodechlorination of TCE is a surface-mediated process, an increase in surface area of the Pd catalyst can greatly enhance the reaction rate.8,9 Therefore, lowering the particle size to the nanoscale is of great practical significance. One key step to the synthesis of nanoscale particles is to employ a capping agent during synthesis to passivate the surface and suppress the growth of the particles that would otherwise aggregate due to the high surface energy of the nanoparticles. The * Corresponding author. E-mail: [email protected] (C.B.R.) or [email protected] (D.Z.). † Department of Chemical Engineering. ‡ Department of Civil Engineering. (1) Lowry, G. V.; Reinhard, M. EnViron. Sci. Technol. 1999, 33, 1905. (2) Mcnab, W. W., Jr.; Ruiz, R. EnViron. Sci. Technol. 2000, 34, 149. (3) Nutt, M. O.; Hughes, J. B.; Wong, M. S. EnViron. Sci. Technol. 2005, 39, 1346. (4) Schuth, C.; Kummer, N.; Weidenthaler, C.; Schad, H. Appl. Catal. B. EnViron. 2004, 52, 197. (5) He, F.; Zhao, D. Y. EnViron. Sci. Technol. 2005, 39, 3314. (6) Agency for Toxic Substance and Disease Registry. U.S. Department of Health and Human Service. ToxFAQ for Trichloroethylene. http://www.atsdr.cdc.gov/tfacts19.html (7) Nutt, M. O.; Heck, K. N.; Alvarez, P.; Wong, M. S. Appl. Catal. B. EnViron. 2006, 69, 115. (8) Wang, C. B.; Zhang, W. X. EnViron. Sci. Technol. 1997, 31, 2154. (9) He, F.; Zhao, D. Y.; Liu, J. C.; Roberts, C. B. Ind. Eng. Chem. Res. 2007, 46, 29.

interactions of the functional molecular groups of the capping molecules with the nanoparticles play an important role in governing the particle size and size distribution and, hence, the catalytic activity of the nanoparticles. Typically, a capping agent that can interact strongly with the surface of Pd particles may effectively stabilize the nanoparticles and result in the formation of small Pd nanoparticles in aqueous systems. On the other hand, for environmental applications, the capping agent must be lowcost, nontoxic, and biodegradable/biocompatible. Therefore, it is highly desirable and challenging as well to develop a “green”, water-based process for synthesizing Pd nanoparticles with the aid of a low-cost, environmentally friendly, and effective capping agent. Of course, Pd is expensive and, as such, the practical application of these Pd nanoparticles would require their recovery and reuse by immobilizing them on a catalyst support. While the use of immobilized nanoparticle catalysts is an area of ongoing research, the focus of the current study involves the controlled synthesis of polysugar-capped Pd nanoparticles and investigation of their inherent activity toward TCE degradation in water as an initial step toward this goal. Synthesis of metal nanoparticles in aqueous phase systems has elicited great research interest in green chemistry and materials science. Raveendran et al.10,11 reported the completely green synthesis of Ag, Au, and Au-Ag alloy nanoparticles using starch as a capping agent in aqueous solutions. More recently, Liu et al.12 developed another green method for the synthesis of Au nanoparticles by employing β-D-glucose as both the capping and reducing agent. Although, starch and glucose can stabilize metal nanoparticles in aqueous solution, their effectiveness is limited by the relatively weak association of these capping agents with the particle surface.10-12 Sodium carboxymethylcellulose (CMC)13 (10) Raveendran, P.; Fu, J.; Wallen, S. L. J. Am. Chem. Soc. 2003, 125, 13940. (11) Raveendran, P.; Fu, J.; Wallen, S. L. Green Chem. 2006, 8, 34. (12) Liu, J. C.; Qin. G. W.; Raveendran, P.; Ikushima, Y. Chem. Eur. J. 2006, 12, 2131. (13) Kennedy, J. F. Cellulose and Its DeriVatiVes: Chemistry, Biochemistry, and Applications; Halsted Press: New York, 1985.

10.1021/la702731h CCC: $40.75 © 2008 American Chemical Society Published on Web 11/29/2007

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is a water-soluble polysaccharide adopting a similar macromolecular skeleton to that of starch. However, it possesses carboxylate groups in addition to hydroxyl groups. As a result, CMC can exert much stronger interactions with particles than starch and glucose and, thus, serve as a more effective capping agent.14-16 To date, CMC has been used as an effective capping agent for the preparation of superparamagnetic iron oxide,17 Ag,18 Pt,16 and bimetallic Fe-Pd nanoparticles.9 This work investigates the effectiveness of CMC-capped Pd nanoparticles for the catalytic hydrodechlorination of the pollutant TCE in an aqueous phase system. Specifically, the effect of the CMC concentration on the size and size distribution of the Pd nanoparticles and their catalytic activity toward the hydrodechlorination of TCE was systematically investigated. So far, there is little predictive knowledge about the influence of particle size and capping performance on metal nanoparticle catalysis.19,20 To this end, both the catalytic activities and surface catalytic activities of neat Pd nanoparticles, β-D-glucose-capped Pd nanoparticles (β-D-glucose-Pd), and CMC-capped Pd (CMCPd) nanoparticles for the hydrodechlorination of TCE were compared. Experimental Section Materials. Na2PdCl4·3H2O was purchased from Strem Chemicals. Carboxymethylcellulose sodium salt (average Mw ∼ 90 000), β-Dglucose, sodium borohydride (Purity: 99.99%), and TCE with spectrophotometric grade (>99%) were obtained from SigmaAldrich. Deionized ultrafiltered water with specific conductance of 2.0 µΩ/cm and hexane were obtained from Fisher Scientific. Aqueous Phase Synthesis of Pd Nanoparticles. The Pd nanoparticles were synthesized by reducing Pd2+ ions in a CMC or β-D-glucose aqueous solution using sodium borohydride (NaBH4) as the reducing agent. For the synthesis of β-D-glucose-stabilized Pd nanoparticles, a 1000 µL aliquot of a 0.05 M Na2PdCl4·3H2O aqueous solution was added to 250 mL of a 0.05 M β-D-glucose aqueous solution. Then, a 0.05 M NaBH4 aqueous solution (around 3500 µL) was added to the system under constant stirring at room temperature (∼23 °C). For a typical synthesis of the 0.15 wt % CMC-stabilized Pd nanoparticles, a 1000 µL aliquot of a 0.05 M Na2PdCl4·3H2O aqueous solution was added to 250 mL of a 0.15 wt % CMC aqueous solution. Subsequently, a 0.05 M NaBH4 aqueous solution (around 3500 µL) was added to the system under constant stirring at room temperature. Twenty-four hours after the addition of NaBH4 to these systems (with CMC or β-D-glucose as capping agent), the resultant Pd nanoparticles were used for catalytic hydrodechlorination of TCE. Preparation of Fourier Transform Infrared (FT-IR) Samples. Both solid β-D-glucose-Pd and CMC-Pd nanoparticles were separated from the aqueous solution by centrifugation at a speed of 6000 rpm/4185g using ethanol as an antisolvent. The FT-IR samples were obtained by forming a thin pellet of KBr (95 mg) and Pd nanoparticles (5 mg). A pure 100 mg KBr pellet used as background was subtracted from the FT-IR spectra of the Pd nanoparticle samples. Transmission Electron Microscopy (TEM) Characterization. For TEM analyses, samples of air-dried Pd nanoparticles were prepared by placing three droplets of aqueous Pd nanoparticle suspension onto a copper grid and subsequently air-drying the samples overnight under ambient conditions. The morphology and size (14) Wu, N.; Fu, L.; Su, M.; Aslam, M.; Wong, K. C.; Dravid, V. P., Nano Lett. 2004, 4, 383. (15) Kataby, G.; Cojocaru, M.; Prozorov, R.; Gedanken, A. Langmuir 1999, 15, 1703. (16) Liu, J. C.; Sutton, J.; Roberts, C. B. J. Phys. Chem. C 2007, 111, 11566. (17) Si, S.; Kotal, A.; Mandal, T.; Giri, S.; Nakamura, H.; Kohara, T. Chem. Mater. 2004, 16, 3489. (18) Magdassi, S.; Bassa, A.; Vinetsky, Y.; Kamyshny, A. Chem. Mater. 2003, 15, 2208. (19) Stowell, C. A.; Korgel, B. A. Nano Lett. 2005, 5, 1203. (20) Li, Y.; Boone, E.; El-Sayed, M. A. Langmuir 2002, 18, 4921.

Langmuir, Vol. 24, No. 1, 2008 329 distribution of the Pd nanoparticles were determined by a Zeiss EM 10 TEM at an operating voltage of 60 kV. The TEM images were analyzed using the image J software package to create the histogram.21 Dynamic Light Scattering (DLS) Measurement. The mean hydrodynamic diameter of the Pd nanoparticles stabilized by 0.05 M β-D-glucose or 0.15 wt % CMC in aqueous solution was determined using a DLS instrument (Nicomp 380, PSS, Santa Barbara, CA) equipped with a He-Ne laser (wavelength 633 nm) at a measurement angle of 90°. The excess β-D-glucose or CMC molecules were separated from the Pd nanoparticle system before DLS measurements by first precipitating the Pd nanoparticles via centrifugation at a speed of 6000 rpm/4185g using ethanol as an antisolvent and then redispersing them in aqueous solution upon shaking (the pH value is around 5.5 upon the redispersion of these Pd nanoparticles in aqueous solution). The viscosities of the aqueous solutions containing CMC-Pd and β-D-glucose-Pd nanoparticles were measured using a Gilmont falling ball viscometer (Pole-Parmer, Vernon Hills, IL). Each DLS experiment was repeated at least three times. The DLS data were processed with the software package CW380 to yield the volume-weighted size distributions. Powder X-ray Diffraction (XRD) Characterization. XRD patterns were collected with a Rigaku Miniflex powder X-ray diffractometer using Cu KR (λ ) 1.540 56 Å) radiation. Both the solid XRD samples of the β-D-glucose-Pd and CMC-Pd nanoparticles were separated from the aqueous solution by centrifugation at a speed of 6000 rpm/4185g while using ethanol as an antisolvent. TCE Hydrodechlorination Experiments. Batch experiments were conducted using 127 mL serum bottles capped with Teflon Mininert valves. In a typical experiment, 4.5 mL of a Pd nanoparticlewater suspension obtained from the aqueous phase synthesis was initially mixed with 85 mL of deionized water with stirring. Then, the reactor was sparged with hydrogen gas for 20 min to displace dissolved oxygen (DO) and to fill the headspace with H2 (1 atm); i.e., the system was saturated with H2. Then, 25 µL of a TCE stock solution (179 g/L TCE in methanol) was spiked into the suspension, resulting in an initial total TCE concentration of 50 mg/L. The catalytic dechlorination reaction was conducted at room temperature under constant magnetic stirring and was monitored for about 10 min. At constant time intervals (2 min), 0.1 mL of each aqueous sample including the suspended Pd nanoparticles was withdrawn using a 100 µL gastight syringe. Then, the samples were transferred into 2 mL GC vials, where the organic compounds were extracted into 1 mL of hexane phase. Upon phase separation, the TCE, vinyl chloride (VC), and dichloroethenes (DCE) were analyzed using an HP 6890 GC equipped with an electron capture detector (ECD). Assuming an equilibrium state of TCE between the headspace and the liquid phase, the concentration of TCE in the headspace was obtained by following Henry’s law.22 To examine the completeness of TCE hydrodechlorination, coupled TCE degradation and chloride production were followed by a separate set of batch experiments conducted in 127 mL serum bottles containing 85 mL of deionized water and 4.5 mL of aqueous Pd particle suspension. At selected time intervals (2 min), the degradation reaction was ceased by rapidly sparging the suspension with N2 gas to remove the residual TCE and H2. Then, the solution was filtered through 0.22 µm membrane (Millipore, Billerica, MA), and chloride in the filtrate was analyzed using a Dionex ion chromatography system (DX-120). All batch experiments were duplicated to ensure data quality.

Results and Discussions Synthesis and Stability of Pd Nanoparticles in Aqueous Solution. Upon addition of NaBH4 into the Pd2+ solution, the solution containing either 0.15 wt % CMC or 0.05 M β-D-glucose turned to transparent dark brown, visually indicating the formation of Pd nanoparticles. It is worth noting that the Pd nanoparticles in the aqueous phase were not stable in the absence of a capping (21) Rasband. W. Image J-Image processing and Analysis in JaVa, http:// rsb.info.nih.gov/ij (22) Schwarzenbach, R. P.; Gschwend, P. M.; Imboden, D. M. EnVironmental Organic Chemistry; John Wiley & Sons: New York, 1993.

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Figure 1. Representative TEM images of Pd nanoparticles synthesized using (a) 0.05 M β-D-glucose and (c) 0.15 wt % CMC in aqueous phase system, along with the corresponding particle size distribution histograms (b and d). The high-magnification TEM images of 0.05 M β-D-glucose-Pd and 0.15 wt % CMC-Pd nanoparticles are also given in a and c, respectively.

agent and underwent aggregation and precipitation within 1 day. On the other hand, the Pd nanoparticles dispersed in the 0.15 wt % CMC aqueous solution were found to be stable for prolonged periods (more than 9 months) and did not show any signs of aggregation and precipitation when stored in a 250 mL glass flask. Also, it was noted that when 0.001 wt % CMC was used to stabilize Pd nanoparticles in aqueous solution the particles would only be stable for 2 weeks before complete precipitation, suggesting that the CMC concentration significantly influences the stability of the Pd nanoparticles. β-D-Glucose-Pd and CMC-Pd Nanoparticles. TEM images of the 0.05 M β-D-glucose-Pd and 0.15 wt % CMC-Pd nanoparticles along with their corresponding particle size distribution histograms are presented in Figure 1. The highmagnification TEM images of the 0.05 M β-D-glucose-Pd and 0.15 wt % CMC-Pd nanoparticles are also given in parts a and c of Figure 1 (inset), respectively. The Pd nanoparticles synthesized and stabilized using 0.05 M β-D-glucose (0.90 wt %) in aqueous phase are observed to have a mean diameter of 4.3 nm with a standard deviation (SD) of 1.0 nm, whereas the Pd nanoparticles stabilized using 0.15 wt % CMC exhibit a smaller size (mean diameter ) 2.4 nm) and narrower size distribution (SD ) 0.50 nm). Clearly, CMC molecules exhibited much more effective capping performance for the synthesis and stabilization of the small Pd nanoparticles, with narrower size distribution in aqueous phase compared with that of β-D-glucose, even though the weight-based concentration of CMC used herein was onesixth of that for β-D-glucose. The exact capping interaction mechanisms of the β-D-glucose and CMC molecules with the surface of Pd nanoparticles, which govern the Pd nanoparticles size and size distribution, will be discussed in detail below. When the concentration of β-D-glucose and CMC was further increased beyond 0.05 M β-D-glucose or 0.15 wt % CMC, respectively, no significant effect on the size of the Pd nanoparticles was observed. This suggests that the surface of Pd nanoparticles is saturated with either 0.05 M β-D-glucose or 0.15 wt % CMC in the aqueous phase synthesis systems.

The effects of CMC concentrations lower than 0.15 wt % on Pd nanoparticle size and size distribution were also examined. The TEM images of the Pd nanoparticles synthesized using concentrations of CMC ranging from 0.10 to 0.001 wt %, along with their size distribution histograms, are presented in Figure 2. The high-magnification TEM images were also provided in Figure 2 in each case (inset). The use of 0.10 wt % CMC resulted in Pd nanoparticles of 2.5 nm average diameter with a SD of 0.6 nm (Figure 2a,b), while the use of 0.05 wt % CMC also yielded Pd nanoparticles of 2.5 nm average diameter with a slightly large SD of 0.8 nm (Figure 2c,d). Further reducing the CMC concentration to 0.01 wt % resulted in 2.9 nm Pd nanoparticles with a SD of 1.2 nm (Figure 2e,f), while 0.005 wt % CMC resulted in 3.1 nm Pd nanoparticles with 1.4 nm SD (Figure 2 g,h). As the concentration of CMC was decreased from 0.15 to 0.005 wt %, a slight increase in both the particle size and size distribution was observed; however, the CMC molecules were still quite effective in stabilizing the Pd nanoparticles in the aqueous solution, as evidenced by the small 3.1 nm diameter at 0.005 wt %. Note that a further decrease in the CMC concentration to 0.001 wt % gives 4.7 nm Pd nanoparticles with a SD of 3.0 nm (Figure 2i,j), a considerable increase in both particle size and size distribution. This low concentration of CMC does not allow the surface of the Pd nanoparticles to be effectively capped, thereby resulting in a lesser ability of 0.001 wt % CMC to stabilize the Pd nanoparticles in aqueous solution. As a result, the Pd nanoparticles in the 0.001 wt % CMC solution underwent complete precipitation upon aging for 2 weeks, as discussed above. Figure 3 shows the dependence of mean size and SD of the Pd nanoparticles on CMC concentration. These quantitative studies demonstrate that CMC molecules play an important role in stabilizing Pd nanoparticles in aqueous phase by selfassembling themselves into a protective macromolecular/ polysugar monolayer on the surface of the Pd nanoparticles. However, a certain amount of CMC molecules are required to ensure the effectiveness of this “green” polysugar to stabilize Pd nanoparticles in aqueous phase.

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Figure 2. TEM images of Pd nanoparticles synthesized using (a) 0.10 wt % CMC, (c) 0.05 wt % CMC, (e) 0.01 wt % CMC, (g) 0.005 wt % CMC, (i) 0.001 wt % CMC in the system, along with their corresponding histograms (b, d, f, h, and j).

DLS measurements were used to determine the hydrodynamic diameter of the Pd nanoparticles. The mean hydrodynamic diameters of the 0.05 M β-D-glucose-Pd and 0.15 wt % CMC-

Pd nanoparticles are 5.9 nm with a SD of 0.7 nm (Figure 4a) and 12.7 nm with a SD of 1.9 nm (Figure 4b), respectively. The hydrodynamic diameters of the CMC-Pd nanoparticles obtained

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Figure 3. The effect of CMC concentration on the Pd nanoparticle size and size distribution.

Figure 4. The mean hydrodynamic diameter and size distribution of the (a) 0.05 M β-D-glucose-Pd and (b) 0.15 wt % CMC-Pd nanoparticles measured using DLS.

in this study are consistent in magnitude with those of the CMCFe nanoparticles reported by He and Zhao.23 The difference in the diameters as determined from the DLS and TEM measurements is due in part to the fact that TEM allows measurement of only the electron-dense metal core, while DLS measures both the metal core and the attached capping agent.23,24 Also, comparing the metal core diameters from the TEM measurements with the hydrodynamic diameters from the DLS observations, it is found that the CMC molecular monolayer is bulkier than the β-Dglucose monolayer on the surface of Pd nanoparticles. XRD characterization was used to determine both the size and crystalline structure of the Pd nanoparticles synthesized in this study. The observed XRD patterns of both the 0.05 M β-Dglucose-Pd (a) and 0.15 wt % CMC-Pd (b) nanoparticles are (23) He, F.; Zhao, D. Y. EnViron. Sci. Technol. 2007, 41, 6216. (24) Zheng, J.; Stevenson, M. S.; Hikida, R. S.; Van Patten, P. G. J. Phys. Chem. B. 2002, 106, 1252.

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Figure 5. XRD patterns of (a) 0.05 M β-D-glucose-Pd and (b) 0.15 wt % CMC-Pd nanoparticles.

presented in Figure 5. In both cases, several peaks are observed in each XRD pattern at around 40°, 46°, 68°, 82°, and 86°. These peaks correspond to the {111}, {200}, {220}, {311}, and {222} planes of a face-centered-cubic (fcc) lattice, respectively, indicating the fcc structure of both the 0.05 M β-D-glucose-Pd and the 0.15 wt % CMC-Pd nanoparticles.25,26 The weak XRD peaks in Figure 5b may be due to the small CMC-Pd nanoparticle size or a disturbance from the CMC macromolecules on the surface of the Pd nanoparticles. Per the Scherrer equation27 and the XRD data in Figure 5, the average diameter of the 0.05 M β-D-glucose-Pd and the 0.15 wt % CMC-Pd nanoparticles are estimated to be 5.2 and 1.9 nm, respectively, nearly consistent with the TEM observations. The lower XRD diffraction angles of the 0.15 wt % CMC-Pd nanoparticles compared to the XRD diffraction angles of the 0.05 M β-D-glucose-Pd nanoparticles suggest that the particle size of the 0.15 wt % CMC-Pd nanoparticles is smaller than that of the 0.05 M β-D-glucose-Pd nanoparticles.25 This may be due to the expansion in Pd-Pd interatomic distance with a decrease in the particle size.25 FT-IR characterization was used to obtain information on the interaction between the capping agent and metal nanoparticles. Previous FT-IR studies have demonstrated that β-D-glucose interacts with the surface of Au nanoparticles through hydrogen bonding via its -OH groups.12,28 In this work, the interactions of both β-D-glucose and CMC with Pd nanoparticles were investigated using FT-IR spectroscopy. Figure 6a shows the pure β-D-glucose FT-IR spectrum in which two bands at 3260 cm-1 and 3340 cm-1 are observed. These two peaks result from the -OH stretch of β-D-glucose molecules. Upon reduction, stabilization by β-D-glucose, and subsequent separation of the Pd nanoparticles from the aqueous solution, the absorption bands of the -OH stretching mode undergo a significant high-frequency shift and change into one strong absorption peak at 3380 cm-1 (Figure 6b), revealing a weak hydrogen-bonding association between β-D-glucose and the surface of Pd nanoparticles.12,28 Moreover, a band of β-D-glucose at 1370 cm-1 vanishes upon the interaction with Pd nanoparticles (Figure 6a,b), further indicating the presence of β-D-glucose as an essential component of the Pd nanoparticles. The carboxylate (-COO-) groups and the surface of Pd nanoparticles may occur in four modes: monodentate (I), (25) Teranishi. T.; Miyake, M. Chem. Mater. 1998, 10, 594. (26) McClune, W. F. Powder Diffraction File Alphabetical Index Inorganic Phase, JCPDS, Swarthmore, PA, 1980. (27) Nuffield, E. W. X-ray Diffraction Methods; Wiley: New York, 1966. (28) Liu, J. C.; Anand, M.; Roberts, C. B. Langmuir 2006, 22, 3964.

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Figure 6. FT-IR spectra of (a) neat β-D-glucose and (b) β-D-glucosePd nanoparticles. Scheme 1. Modes of Metal-Carboxylate Complexation: Monodentate Chelating (I), Bidentate Chelating (II), and Bidentate Bridging (III)a

a ∆ represents the wavenumber separation between the asymmetric νas(COO-) and symmetric νs(COO-) stretching vibration.

Figure 7. FT-IR spectra of (a) neat CMC and (b) CMC-Pd nanoparticles.

chelating bidentate (II), bidentate bridging (III), and ionic interaction.14,29 Scheme 1 illustrates the first three interaction modes. The wave number separation, ∆, between the asymmetric νas(COO-) and symmetric νs(COO-) stretches can be used to identify the type of the interactions between the -COO- groups and particle surfaces. Characteristically, the ∆ value falls in the range of 200-320 cm-1 for monodentate, 140-190 cm-1 for bidentate bridging, and