Home
Search
Collections
Journals
About
Contact us
My IOPscience
Synthesis and characterization of supported polysugar-stabilized palladium nanoparticle catalysts for enhanced hydrodechlorination of trichloroethylene
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2012 Nanotechnology 23 294004 (http://iopscience.iop.org/0957-4484/23/29/294004) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 131.204.15.107 This content was downloaded on 15/08/2017 at 15:48 Please note that terms and conditions apply.
You may also be interested in: Immobilization of mercury in field soil and sediment using carboxymethyl cellulose stabilized iron sulfide nanoparticles Yanyan Gong, Yuanyuan Liu, Zhong Xiong et al. Facile and rapid synthesis of spherical porous palladium nanostructures with high catalytic activity for formic acid electro-oxidation Shaochun Tang, Sascha Vongehr, Zhou Zheng et al. Phase stability and photocatalytic activity of Zr-doped anatase synthesized inminiemulsion Renate Schiller, Clemens K Weiss and Katharina Landfester Effects of surface activation on the structural and catalytic properties of ruthenium nanoparticles supported on mesoporous silica Xianfeng Ma, Rui Lin, Christopher Beuerle et al. Green synthesis of Pt and Ag nanoparticles and their use towards nitric oxide abatement Marcus V Castegnaro, Jéssica Alexandre, Ione M Baibich et al. Ultrafine sputter-deposited Pt nanoparticles for triiodide reduction in dye-sensitized solar cells: impact of nanoparticle size, crystallinity and surface coverage on catalytic activity Somik Mukherjee, Balavinayagam Ramalingam, Lauren Griggs et al. A ‘green’ chitosan–silver nanoparticle composite as a heterogeneous as well asmicro-heterogeneous catalyst A Murugadoss and Arun Chattopadhyay
IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 23 (2012) 294004 (13pp)
doi:10.1088/0957-4484/23/29/294004
Synthesis and characterization of supported polysugar-stabilized palladium nanoparticle catalysts for enhanced hydrodechlorination of trichloroethylene Deborah B Bacik1,5 , Man Zhang2,5 , Dongye Zhao2 , Christopher B Roberts1 , Mohinar S Seehra3 , Vivek Singh3 and Naresh Shah4 1
Department of Chemical Engineering, Auburn University, Auburn, AL 36849, USA Department of Civil Engineering, Auburn University, Auburn, AL 36849, USA 3 Department of Physics, West Virginia University, Morgantown, WV 26506, USA 4 Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY 40506, USA 2
E-mail:
[email protected]
Received 5 November 2011, in final form 10 January 2012 Published 28 June 2012 Online at stacks.iop.org/Nano/23/294004 Abstract Palladium (Pd) nanoparticle catalysts were successfully synthesized within an aqueous phase using sodium carboxymethyl cellulose (CMC) as a capping ligand which offers a green alternative to conventional nanoparticle synthesis techniques. The CMC-stabilized Pd nanoparticles were subsequently dispersed within support materials using the incipient wetness impregnation technique for utilization in heterogeneous catalyst systems. The unsupported and supported (both calcined and uncalcined) Pd nanoparticle catalysts were characterized using transmission electron microscopy, energy dispersive x-ray spectrometry, x-ray diffraction, and Brunauer–Emmett–Teller surface area measurement and their catalytic activity toward the hydrodechlorination of trichloroethylene (TCE) in aqueous media was examined using homogeneous and heterogeneous catalyst systems, respectively. The unsupported Pd nanoparticles showed considerable activity toward the degradation of TCE, as demonstrated by the reaction kinetics. Although the supported Pd nanoparticle catalysts had a lower catalytic activity than the unsupported particles that were homogeneously dispersed in the aqueous solutions, the supported catalysts retained sufficient activity toward the degradation of TCE. In addition, the use of the hydrophilic Al2 O3 support material induced a mass transfer resistance to TCE that affected the initial hydrodechlorination rate. This paper demonstrates that supported Pd catalysts can be applied to the heterogeneous catalytic hydrodechlorination of TCE. S Online supplementary data available from stacks.iop.org/Nano/23/294004/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
et al 2006, Moran et al 2007). It has primarily been used as a solvent for cleaning and degreasing in the dry cleaning and automotive industries. Unfortunately, TCE is a potent carcinogen and is resistant to the natural attenuation process. Therefore, the presence of TCE in the environment is a significant health and safety concern. Considering the
Trichloroethylene (TCE) is one of the most common organic pollutants detected in both soil and groundwater (Zogorski 5 Equal contributions.
0957-4484/12/294004+13$33.00
1
c 2012 IOP Publishing Ltd Printed in the UK & the USA
Nanotechnology 23 (2012) 294004
D B Bacik et al
(Liu et al 2006, He and Zhao 2007, Liu et al 2007, 2008). In addition, we investigate the effectiveness of these heterogeneous supported CMC–Pd nanoparticles as catalysts in the hydrodechlorination of TCE. Supporting the Pd nanoparticles provides an opportunity to efficiently retain and recover the active Pd catalyst material, which is essential for any practical environmental application. Heterogeneous catalysts offer certain advantages including ease of product and catalyst recovery and suitability for continuous processing in continuous flow-type reactors (Trewyn et al 2009, Sheldon 1991). For example, Macquarrie (2010) states that there are several reasons to heterogenize a catalyst: ‘the heterogenized catalyst is almost certainly more easily recovered, and therefore reuse becomes more realistic (although reactivation may be required). Therefore less waste is generated and expensive, difficult-to-obtain components such as scarce metals can be effectively recovered and reused’.
persistence of TCE in the environment and the long-term health effects associated with chlorinated contaminants, site cleanup is essential. The remediation of contaminated sites requires a site-specific approach, which is often very expensive and time-consuming. Fortunately, the implementation of more stringent regulatory controls for the use of chlorinated solvents in manufacturing has resulted in the replacement of chlorinated solvents with more environmentally friendly alternatives (Sherman et al 1998). There has been an overall reduction in the use of chlorinated solvents in commercial processes; however, years of heavy usage and poor disposal of chlorinated compounds have resulted in a significant environmental problem (US EPA 1994). Various technologies have been developed for the successful remediation of soil and groundwater contaminated with these harmful pollutants (US EPA 2010). However, site cleanup is often a cost-intensive process spanning several years. The estimated cost of remediation of contaminated waste sites across the United States is approximately $750 billion over the next 30 years (NRC 1994, Chiras 2010). Catalytic hydrodechlorination is an innovative technology developed specifically for the remediation of soil and groundwater contaminated with chlorinated hydrocarbons (Lowry and Reinhard 1999, Alonso et al 2002, He and Zhao 2008, Liu et al 2008). During this process, TCE is hydrogenated in the presence of a catalyst (i.e. Pd, Au) to yield biodegradable ethane. Wong et al (2008) have proposed a Langmuir–Hinshelwood-type mechanism for the hydrodechlorination of TCE. According to this mechanism, the chlorine atoms are removed in series and replaced by hydrogen atoms. Wong et al (2008) have also shown that less chlorinated compounds react faster than TCE or perchloroethylene (PCE), which explains why no chlorinated intermediates build up in the system. Since the catalytic hydrodechlorination of TCE is a surface-mediated process, increasing the surface area of the catalyst can improve the degradation rate (Nutt et al 2005, 2006, He and Zhao 2007, Liu et al 2008). Thus, reducing the size of the catalyst to nanometer-sized particles provides an opportunity to have a more effective catalyst. The aqueous phase synthesis of metal nanoparticles using ‘green’ capping agents such as glucose (Raveendran et al 2003, Liu et al 2005, 2006), starch (Raveendran et al 2003, 2006, Vigneshwaran et al 2006) and carboxymethyl cellulose (Magdassi et al 2003, He and Zhao 2007, He et al 2007) has been examined to determine the ability of each ligand to effectively cap and stabilize the particles within solution. The advantages associated with using these materials as capping agents is that they are all very inexpensive, nontoxic, and biodegradable. In this paper, we explore the aqueous phase synthesis of sodium carboxymethyl cellulose (CMC) stabilized Pd nanoparticles and the deposition of these particles onto supporting materials to create heterogeneous Pd nanoparticle catalysts. Utilizing sugar and polysugar capping agents in aqueous media offers ‘green’ alternatives to the conventional techniques that are employed in synthesizing nanoparticles
2. Experimental section 2.1. Materials Na2 PdCl4 ·3H2 O precursor salt (purity = 99%) was purchased from Strem Chemicals. CMC sodium salt (average Mw ∼ 90 000), sodium borohydride (purity = 99.99%), dodecanethiol (purity > 98%), n-hexane (purity = 97.0%), and TCE (purity > 99.5%) were all obtained from SigmaAldrich. Deionized ultra-filtered water, hydrochloric acid (37.4%), and methanol (ACS grade) were all obtained from Fisher Scientific. Hydrogen gas (ultra-high purity) and 5% hydrogen/nitrogen specialty gas mixture (certified standard) were obtained from Airgas South. Aluminum oxide supporting material was obtained from Alfa Aesar and titanium silicalite (TS-1) supporting material was obtained from Sud-Chemie. All materials were used as-received without further purification. 2.2. Aqueous phase synthesis of CMC-stabilized Pd nanoparticles For the typical aqueous phase synthesis of CMC-stabilized Pd nanoparticles, a 0.15 wt% CMC solution was prepared by diluting 0.150 g of the CMC capping ligand in 100 ml of deionized water at room temperature. The solution was stirred until the capping agent was completely dissolved. A 1000 µl aliquot of a 0.05 M Na2 PdCl4 ·3H2 0 solution was added to the 0.15 wt% CMC solution, while continuously stirring. After dissociating the Pd2+ ions in solution, a 3600 µl aliquot of a 0.05 M NaBH4 aqueous solution was added to the CMC–salt solution at a stir rate of 500 rpm in order to initiate nanoparticle synthesis, growth, and stabilization. The system was aged for 24 h under constant stirring. 2.3. Deposition of CMC-stabilized Pd nanoparticles onto supporting materials The incipient wetness impregnation technique was followed for the preparation of all supported catalysts. After 24 h 2
Nanotechnology 23 (2012) 294004
D B Bacik et al
of aging, the CMC-stabilized Pd nanoparticle dispersions were concentrated to a volume of ∼5 ml by using a rotary evaporator. The concentrated nanoparticle solution, containing the appropriate amount of Pd to achieve a desired metal loading, was added dropwise until the total pore volume for the support material was reached. The material was then dried in an oven overnight at 353 K. The steps were repeated until all of the nanoparticle solution (∼5 ml) was added to the support material. After drying, calcination of the catalyst was carried out at 773 K for 5 h in air to remove the polysugar capping ligand. Subsequent reduction of the supported Pd catalyst was performed at 673 K for 4 h in the presence of hydrogen.
2.5. High-resolution transmission electron microscopy (HRTEM) characterization HRTEM analysis was performed in order to determine the morphology and approximate particle size for both the unsupported and alumina-supported CMC-stabilized Pd nanoparticles. A JEOL 2010F microscope with a field emission electron source operating at 200 keV accelerating voltage was used for HRTEM analysis of the samples. To prepare HRTEM samples for the unsupported CMC–Pd nanoparticles, one droplet of the CMC-stabilized Pd R nanoparticle aqueous dispersion was placed on a Quantifoil TEM grid. The aqueous solvent was allowed to evaporate from the grid at ambient conditions. It is noted that the dispersions were not concentrated in this case so as to obtain TEM images of isolated particles. The alumina-supported catalyst powder (212–355 µm) was crushed to a smaller size using a miniature mortar and pestle, and approximately 1 mg of this powder was placed in 3–5 ml of acetone. The suspension was ultrasonicated for about 5 min using an Omni rupture (250 W) ultrasonic probe. A few drops R of this suspension were placed on a Quantifoil TEM grid for HRTEM analysis. Bright field images were captured using a Gatan 1024 pixel × 1024 pixel CCD camera with a typical acquisition time of 1 s. Image collection and post-processing were done using Gatan Digital Micrograph software. Comparison was made to images obtained from traditional TEM analysis at Auburn University using a Zeiss EM 10 TEM at an operating voltage of 60 kV. The energy dispersive x-ray spectrometry (EDS) spectra were collected using an Oxford EDS detector and Gatan Digital Micrograph software. EDS spectra were collected for approximately 1 min live time using a smaller condenser aperture and after condensing the electron beam down to the region of interest.
2.4. Transmission electron microscopy (TEM) characterization TEM analysis was used to obtain the average particle sizes and particle size distributions for the CMC-stabilized Pd nanoparticles. After aging for 24 h, the particles were extracted from the aqueous phase in which they were synthesized into an organic phase from which TEM samples were prepared for characterization. Specifically, Pd nanoparticles were extracted from the aqueous phase into a hexane phase by adding 10 ml of hexane to 20 ml of the aqueous phase CMC-capped Pd nanoparticle dispersion. This was followed by the addition of 100 µl of dodecanethiol and five drops of hydrochloric acid (HCl), respectively. HCl was added in order to weaken the strong interactions existing between the CMC capping ligand and the surface of the Pd nanoparticles by protonating the –COO− functional groups (Liu et al 2007). Once the interaction between the CMC capping ligand and the Pd nanoparticle surface was sufficiently weakened, dodecanethiol was able to form a chemical bond with the surface of the Pd nanoparticle and solvate the nanoparticles within the hexane phase. The two-phase system was vigorously shaken in order to provide good contact between dodecanethiol and the surface of the Pd nanoparticle. After phase separation, the Pd nanoparticle dispersion in hexane was removed from the aqueous phase and washed with deionized water. The organic phase was washed twice more with deionized water in order to remove excess acid from the system and to provide a good sample for evaluation by TEM. All extractions were performed at room temperature. To prepare TEM samples, the Pd nanoparticle dispersion in hexane was concentrated by evaporating two-thirds of the hexane solvent by flowing air over the system at room temperature and atmospheric pressure. The nanoparticle dispersion was concentrated in this case in order to obtain a sufficient number of particles per TEM image for particle size analysis. Two droplets of the concentrated dodecanethiolcapped Pd nanoparticle dispersion in hexane were placed on a 300-mesh nickel-Formvar/carbon TEM grid. The solvent was allowed to evaporate from the grid at ambient conditions. The particles were analyzed using a Zeiss EM 10 TEM at an operating voltage of 60 kV. Both Image J and Microsoft Excel software were used to determine the average particle sizes, and to create a particle size distribution histograms. In each case, images of more than 5000 particles were analyzed.
2.6. Powder x-ray diffraction (XRD) characterization Characterization by XRD was performed in order to determine the average crystallite size and crystalline structure for the alumina-supported Pd nanoparticle catalysts. The XRD patterns were measured using a Rigaku D/Max B system with ˚ radiation source. Each catalyst a Cu Kα1 (λ = 1.541 85 A) sample (about 1 cm2 area) was loaded onto a special silicon plate using ethanol/acetone for adhesion, and then mounted vertically into the x-ray diffractometer. All samples were run twice using two different sensitivities. First, samples were run with a 2θ scan from 5◦ to 100◦ using a step size of 0.06◦ and a counting time of 5 s. In the second higher sensitivity scan of 2θ from 75◦ to 90◦ the step size was reduced from 0.06◦ to 0.01◦ and the counting time at each step was increased from 5 to 10 s. 2.7. Physisorption BET analysis Physisorption measurements were performed and Brunauer– Emmett–Teller (BET) theory was applied in order to 3
Nanotechnology 23 (2012) 294004
D B Bacik et al
d[TCE] = kapp [TCE] = kobs [Pd][TCE] dt = kSA as [Pd][TCE]
determine the specific surface area for the aluminasupported Pd nanoparticle catalysts prepared using various CMC concentrations. The surface area measurements were collected using a Micromeritics AutoChem II 2920 automated catalyst characterization system. Each sample was outgassed in a helium gas flow at 50 ml min−1 while the temperature was ramped to 350 ◦ C at 50 ◦ C min−1 . After holding the temperature constant at 350 ◦ C for 30 min, the sample surroundings were then brought to ambient conditions and a mixture of 30% N2 /balance helium was passed through the sample. The sample was immersed in a liquid nitrogen (LN2 ) bath to measure the uptake of nitrogen. The N2 dewar was then immediately replaced by a dewar of water at ambient temperature, and the amount of desorbed N2 was measured. The BET equation was used to calculate the specific surface area. All samples were duplicated, and the variations in the surface area between different samples are at most about 20%.
−
(1)
kapp is the apparent reaction rate constant, which was obtained from experimentally determined data by plotting ln([TCE]t /[TCE]0 ) versus time and calculating the slope. [Pd] is the concentration of Pd nanoparticles (g l−1 ) in the reactor, and [TCE] is the concentration of TCE during the reaction (g l−1 ). The specific surface area, as , is the surface area of Pd per gram of Pd nanoparticles. as is inversely related to the particle radius as expressed in equation (2) (Liu et al 2008): r = 3(ρas )−1
(2)
where r is the particle radius and ρ is the density of the particles. The density (ρ) of the Pd nanoparticles was assumed to be that of bulk Pd where ρ is 12 023 kg m−3 . Using the mean particle radii obtained via TEM analysis, the as of the CMC–Pd particles were calculated. kapp is normalized by the catalyst concentration to yield the observed reaction kinetic rate constant, kobs . In addition, kobs is normalized on the basis of the specific surface area per gram of Pd nanoparticles to yield the surface-area-based reaction kinetic rate constant, kSA . Turn over frequency (TOF) is often used to compare the activity of different catalysts (Boudart 1995, Li et al 2002, Stowell and Korgel 2005). TOF is the number of molecules that react per active site per unit time and was calculated using equation (3): 1 d[TCE] . (3) TOFinit = − dt t=0 [Pd]DPd
2.8. TCE hydrodechlorination experiments For the batch hydrodechlorination studies using supported Pd nanoparticle catalysts, 0.1 g of catalyst (0.031 mM Pd) was added to a 127 ml serum bottle along with 100 ml of deionized water. A stir bar was included for proper agitation. The system was purged with hydrogen gas for 20 min to displace any dissolved oxygen and to saturate both the aqueous phase and headspace with H2 . The system was then sealed with a Teflon Mininert valve and spiked with 25 µl (179 g l−1 in methanol) of a TCE stock solution to yield an initial TCE concentration of 50 mg l−1 . The TCE solution was added under constant stirring and the system was sampled at regular time intervals. The degradation of TCE was monitored for 30 min by removing 100 µl of the aqueous phase at constant intervals using a gastight syringe. The sample was transferred to a 2 ml GC vial and extracted into 1 ml of hexane. A HP 6890 GC fitted with a micro-electron capture detector (ECD) and a RTX-624 capillary column (32 m × 0.32 mm) was used for analysis of all samples. The data were collected using a HP GC Chemstation. A chlorine mass balance was performed in order to confirm the complete conversion of TCE to biodegradable products. For this set of experiments, after the system was sampled at a specific time interval the reaction was stopped by purging with air to remove any unreacted TCE. A Dionex ion chromatography system was used to measure the chloride concentration in a 1 ml aliquot. All experiments were at least duplicated.
The dispersion (DPd ) of a Pd nanoparticle is defined as the ratio of surface Pd atoms to the total number of Pd atoms in a nanoparticle. The DPd was estimated using a core–shell model (Liu et al 2008). In addition, both the molar concentration of Pd atoms, [Pd], in the reaction system and the initial rate of TCE degradation were used in the calculation of TOFinit .
3. Results and discussion 3.1. Synthesis of CMC-stabilized Pd nanoparticles in aqueous solution The effect of CMC concentration on both the particle size and the size distribution of the unsupported Pd nanoparticles synthesized at room temperature has been examined. TEM images of Pd nanoparticles synthesized at room temperature and stabilized with CMC (concentrations ranging from 0.05 to 0.15 wt% CMC), along with the corresponding particle size distribution histograms, are presented in figure 1. For all concentrations studied, Pd nanoparticles of small size (
