catalysis bifunctional roles

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Complete degradation of perchlorate using Pd/Ndoped activated carbon with adsorption/catalysis bifunctional roles You-Na Kim a, Young-Chul Lee b, Minkee Choi

a,*

a Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea b Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Article history:

Although perchlorate (ClO4 ) has a detrimental effect on human health, there have been

Received 14 June 2013

only limited solutions for its complete degradation due to its extraordinary chemical stabil-

Accepted 18 August 2013

ity. Physical adsorption using activated carbons and ion-exchange resins can provide fast,

Available online 26 August 2013

economic methods for perchlorate removal, but energy-efficient and environmentally



benign ways for the disposal and regeneration of perchlorate-saturated adsorbents should be developed. Here we demonstrated an integrated process synergistically combining the strengths of physical adsorption and catalytic degradation by using 0.1 wt.% Pd on N-doped activated carbon (Pd/N-AC) as an adsorption/catalysis bifunctional material. During the perchlorate adsorption from polluted water, the N-doped carbon surface provides increased number of adsorption sites for perchlorate compared to pristine carbon surface. After the carbon is saturated with perchlorate, the perchlorate concentrated on the carbon surface can be fully decomposed into non-toxic chloride (Cl) by the catalytic function of supported Pd in H2 atmosphere. Notably, the N-doped carbon surface and adsorbed water synergistically enhance the catalytic decomposition rate of perchlorate. As a result, Pd/NAC showed complete perchlorate decomposition even at very mild condition (333 K). The perchlorate adsorption/catalytic decomposition cycle could be repeated up to five times without loss of perchlorate adsorption capacity.  2013 Published by Elsevier Ltd.

1.

Introduction 

Perchlorate (ClO4 ) is widely used in the manufacturing of explosives and solid fuel rocket propellants for the aerospace and defense industries, and also in the manufacturing of commercial products ranging from electronics to pharmaceuticals [1]. Due to its high solubility in water and kinetic inertness, perchlorate can be widely spread in surface and ground water systems. High loading of perchlorate is suspected to interfere with iodine uptake by the thyroid gland resulting

* Corresponding author. E-mail address: [email protected] (M. Choi). 0008-6223/$ - see front matter  2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.carbon.2013.08.031

in decreased production of thyroid hormones [1]. Thyroid is critical for growth, development and metabolism in the human body; the effects of perchlorate on the thyroid can be especially significant in cases of pregnant women and fetuses. Perchlorate is listed by the U.S. EPA on the drinking water contaminant candidate list [2]. The EPA has established an official reference dose of 0.0007 mg/kg day of perchlorate [3], which corresponds to a drinking water equivalent level (DWEL) of 24.5 ppb [4]. However, since this is not a federal maximum contamination level (MCL), various states are

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considering their own specific standards (e.g., Massachusetts: 2 ppb, New Jersey: 5 ppb) [1]. The various treatment technologies for perchlorate removal that have been developed so far can be grouped into three general categories: (1) bioremediation with microorganisms, (2) physical adsorption, and (3) chemical reduction [5]. Bioremediation uses enzymatic reduction function of microorganisms [6], which can reduce perchlorate to chloride (Cl) in the presence of various reducing agents (e.g., hydrogen, acetate, ethanol, or lactate) [7]. Biological treatment is cost-effective for poor-quality water containing high concentrations of perchlorate. However, biological processes can be costly for the treatment of water containing low-concentration perchlorate because a highly reducing environment is required [8]. This problem aside, the biggest issue with bioremediation is the lack of public acceptance for the introduction of pathogenic microorganisms into drinking water systems. Thus, additional treatment processes are required to remove the nutrients and potential pathogens used in the remediation process [5,9]. In the physical adsorption methods, activated carbon (AC) and anion-exchange resin have been most widely studied as adsorbents [8]. Although AC has large surface area and low cost, its adsorption capacity for perchlorate is relatively small due to its anionic surface nature in neutral pH condition. Various surface modification methods, such as pre-adsorption of cationic surfactants [10] and N-doping [11], have been reported to improve perchlorate adsorption capacity. Anionexchange resins generally show higher perchlorate adsorption capacity and ion-selectivity than AC. The great challenge with anion-exchange resins, however, is regeneration of the resins and subsequent treatment of a large amount of brine solution enriched with perchlorate. Gu et al. reported that concentrated FeCl3–HCl solution could effectively desorb perchlorate ions from ion-exchange resin [12], and the perchlorate in the regenerant solution could be reduced to chloride using Fe(II) as a reducing agent at 443 K [13]. In general, the physical adsorption techniques can provide fast, economic methods for perchlorate removal [14], but less energy-intensive and more environmentally benign disposal and regeneration processes for the perchlorate-saturated adsorbents should be developed. Similar to the enzymatic reduction of perchlorate by microorganisms, in principle, catalytic reduction can permanently decompose perchlorate into chloride in the presence of reducing agents. However, the catalytic and electrocatalytic reductions of perchlorate with noble metal catalysts have been reported to be extremely slow and infeasible under ambient conditions [15,16]. Huang et al. studied 78 heterogeneous catalysts and all showed negligible levels of perchlorate decomposition at ambient temperature [17]. The low catalytic activities of heterogeneous catalysts at ambient temperature seems inevitable due to both the high stability of perchlorate, and to the low concentration of perchlorate in polluted water. Abu-Omar and coworkers reported high activity for Re-based homogeneous catalysts in the presence of H3PO2 and organic sulfide as a reducing agent [18]. Despite reasonably high levels of activity, such a homogeneous system using soluble phosphorous or sulfur reducing agents is not compatible with use in a water purification system. More recently, supported

Fig. 1 – Schematic representation of the perchlorate adsorption-decomposition cycle using Pd/N-doped activated carbon (Pd/N-AC) as an adsorption/catalysis bifunctional material.

Pd–Re catalysts working with hydrogen as a reducing agent have been developed. The reaction rate was moderate but the optimal reduction efficiency of this catalyst was achieved only in acidic condition (pH  3), which is not really practical for the application [19]. In the present work, we demonstrated an integrated process synergistically combining the strengths of physical adsorption and catalytic decomposition by using 0.1 wt.% Pd on N-doped activated carbon (Pd/N-AC) as an adsorption/ catalysis bifunctional material (Fig. 1). During the perchlorate adsorption from polluted water, the N-doped carbon surface provides increased number of adsorption sites for perchlorate due to the presence of basic nitrogen functional groups. After the carbon surface is saturated with perchlorate, the perchlorate concentrated on carbon surface can be catalytically decomposed into non-toxic chloride (Cl) by supported Pd clusters in H2 atmosphere. It will be shown that the N-doped carbon surface and adsorbed water can synergistically enhance the catalytic decomposition rate of perchlorate. As a result, Pd/N-AC could show complete perchlorate decomposition even at very mild condition (333 K). The perchlorate adsorption/catalytic decomposition cycle could be repeated up to five cycles without loss of perchlorate adsorption capacity.

2.

Experimental methods

2.1.

Material synthesis

Activated carbon (Sigma–Aldrich, C3345) was treated with 1 M HNO3 (1 g carbon/50 mL solution) at 363 K for 2 h in order to remove impurities. This treatment can also enhance the acidity and concentration of oxygen functional groups of the asreceived activated carbon. After the treatment, the carbon

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was filtered, washed with deionized water until the pH of the filtrated solution reached 7, and then dried at 373 K. N-doped activated carbon was prepared by treating the acid-washed activated carbon with flowing NH3 (5%, He balance, flow rate = 50 mL/min g) at 873 K for 2 h, followed by He flushing at the same temperature for 1 h. Pd (0.1 wt.%) was supported on the activated carbon (AC) and N-doped activated carbon (N-AC) using a conventional wet impregnation method with an aqueous solution of Pd(NO3)2Æ2H2O (Sigma–Aldrich, 76070). After the impregnation, the sample was dried at 333 K for 5 h and reduced under H2 (99.999%, flow rate = 50 mL/min g) at 473 K for 2 h.

2.2.

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Characterization

N2 adsorption–desorption isotherms were measured with a Belsorp Max (BEL Japan) adsorption analyzer at 77 K. Prior to the adsorption measurements, all samples were degassed in a vacuum for 3 h at 573 K. The specific surface area was determined from the adsorption branch in a P/P0 range between 0.05 and 0.20, using Brunauer-Emmett-Teller (BET) equation. Pore diameter distributions were analyzed by the non-local density functional theory (NLDFT) method by assuming slit pore geometry [20]. Micropore volume was calculated by t-plot method and mesopore volume was calculated by subtracting the micropore volume from the total pore volume calculated at P/P0 of 0.95. The X-ray photoelectron spectroscopy (XPS) measurements of N-AC and Pd/N-AC samples were carried out in an ultrahigh vacuum (UHV) setup equipped with a Thermo Scientific Sigma–Probe (Thermo Scientific) analyzer. A monochromatic Al Ka X-ray source (1486.3 eV; anode operating at 15 kV and 7 mA) was used as incident radiation. The pass energy was fixed at 50 eV for all the measurements and overall energy resolution was better than 0.1 eV. A flood gun was used to compensate for the charging effects. The binding energies were calibrated based on the graphitic C 1s peak at 284.8 eV. The zeta potential of the AC, N-AC, Pd/AC and Pd/N-AC were measured with Zetasizer Nano ZS (Malvern Instruments Ltd.). 50 mL of 0.1 M NaCl solution was adjusted with 0.1 M HCl or NaOH solution to different pH values. The initial pH of the suspensions varied from 5 to 9. Then 0.01 g of AC, NAC, Pd/AC and Pd/N-AC were added to each of the solutions and stirred for 24 h. Approximately 1 mL of suspension was injected into the electrophoretic cell. The cell was placed under a microscope, where a laser beam is automatically illuminated. An electric field was applied to the cell by manually changing the voltage until the sample particles were stationary, as observed by the microscope. The average zeta potential was plotted against the pH at equilibrium. The metal dispersions (defined as the ratio of the number of surface metal atoms to the number of total metal atoms) of Pd on AC and N-AC were analyzed by volumetric chemical adsorption of O2 at 308 K by using a Micromeritics ASAP2020 adsorption analyzer (Micromeritics Instrument Corporation). Before the measurement, the samples were reduced in flowing 5% H2/Ar gas for 2 h at 473 K and evacuated for 1 h at the same temperature. Total gas uptake was determined by

extrapolation of the linear portion of the isotherm to zero pressure.

2.3.

Adsorption of perchlorate

In the batch adsorption experiments, two types of perchlorate  solution containing 10 mg/L ClO4 were used. The first solution was prepared by dissolving NaClO4 in deionized water. The second solution was prepared by dissolving NaClO4 in simulated ionic background solution containing 0.2 mM NaNO3, 0.2 mM Mg(NO3)2, 0.3 mM K2SO4, and 1 mM NaHCO3, following the composition given in the literature [21]. Hereafter, these solutions will be denoted as ‘pure solution’ and ‘simulated solution’, respectively. For the kinetic analysis of perchlorate adsorption, 0.12 g Pd/AC and 0.08 g Pd/N-AC samples were suspended in 20 mL of pure and simulated solutions. The amount of solution per gram sample was controlled so that the perchlorate adsorption capacities were roughly 50–70% of the maximum adsorption capacities (mg/g) of these materials, which were determined by Langmuir fitting of the equilibrium isotherms (i.e., qmax in Table 1). After magnetic stirring (400 rpm) for specified time intervals (5, 10, 20, 40, 60 and 120 min) at room temperature, the mixture was centrifuged (4000 rpm, 10 min) and the supernatant solution was collected. The collected solution was further purified with PVDF syringe filters (Whatman, 0.2 lm). The perchlorate concentration in the solution was analyzed using Dionex ion-exchange chromatography (IC) systems, equipped with a 25 lL sample loop, a set of 4 · 250 mm AS16 and AG16 columns, a 4 mm ASRS Ultra II suppressor, and an electrical conductivity detector. The suppressor current was 100 mA. The eluent was set to 35 mM KOH. The sample running time was 22 min. With this setup, the detection limit for perchlorate was 0.1 mg/L. The amount of adsorbed perchlorate q (mg/g) at any time was calculated as follows: q ¼ ðCi  Ct ÞV=m

ð1Þ

where Ci is the initial concentrations (mg/L) of perchlorate, Ct is the concentrations (mg/L) of perchlorate after adsorption, V is the volume of solution (L), and m is the mass of the dried adsorbent (g). Perchlorate adsorption isotherms were measured by equilibrating Pd/AC and Pd/N-AC in the pure and simulated solutions for 12 h. The aforementioned kinetic experiments revealed that 12 h equilibrium time was sufficient for all the present adsorption studies. In detail, 0.02–0.5 g of Pd/AC and Pd/N-AC samples were added to 20 mL solutions containing varied concentrations of perchlorate and stirred for 12 h. The supernatant solution was collected and analyzed by IC. All the isotherms were fitted using the Langmuir model.

2.4. Temperature experiments

programmed

reduction

(TPR)

In order to study the reduction behavior of perchlorate as adsorbed on the AC, N-AC, Pd/AC, and Pd/N-AC samples, H2 consumption during temperature rise was monitored by temperature programmed reduction (TPR) analysis. TPR profiles were collected using a Belcat (BEL Japan) instrument

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Table 1 – Langmuir isotherm parameters of Pd/AC and Pd/N-AC in pure and simulated solutions. Samples

R2

qmaxa (mg/g)

Kb (L/mg)

Pd/N-AC (purec) Pd/N-AC (simulatedd) Pd/AC (purec) Pd/AC (simulatedd)

0.95 0.99 0.92 0.93

3.67 2.28 2.66 1.50

3.21 1.84 2.31 1.97

a

Maximum adsorption capacity (mg/g). Langmuir coefficient related to adsorption strength (L/mg). c 10 mg/L ClO 4 solution prepared by dissolving NaClO4 in deionized water. d 10 mg/L ClO 4 solution prepared by dissolving NaClO4 in simulated ionic background solution containing 0.2 mM NaNO3, 0.2 mM Mg(NO3)2, 0.3 mM K2SO4, and 1 mM NaHCO3. b

equipped with a thermal conductivity detector (TCD). Due to the sensitivity limit of the TCD, the materials were fully saturated with an excessive amount of perchlorate before the TPR experiments. In a typical experiment, 0.2 g samples were stir red in 1000 mL of 100 mg/L ClO4 solution (NaClO4 dissolved in deionized water) for 12 h, filtered and dried at 373 K for 6 h. Dried samples (0.03 g) were placed in a quartz reactor and detector stabilization was performed for 2 h under 5% H2/Ar. Temperature was increased from room temperature to 623 K with a ramping rate of 10 K/min under 5% H2/Ar flow (30 mL/min). For comparison, TPR analyses of the fresh AC, N-AC, Pd/AC, and Pd/N-AC samples without perchlorate adsorption were also carried out in the same way.

2.5. Catalytic decomposition of adsorbed perchlorate in a batch reactor Pd/AC and Pd/N-AC were equilibrated in a pure perchlorate  solution (10 mg/L ClO4 in deionized water) for 12 h. The amount of solution per gram sample was controlled so that the perchlorate adsorption capacities were roughly 50–70% of the maximum adsorption capacities (mg/g) of these materials (qmax) in Table 1. The solid samples were collected by filtration. Approximately 1 g of samples before and after drying at 373 K for 6 h were placed in a stainless-steel reactor (inner volume: 20 mL). Prior to the reaction, the reactor was flushed with H2 for 10 min (50 mL/min) and then pressurized to 5 bar. The reaction temperature was measured using a K-type thermocouple inserted into the reactor through a thermocouple well. After maintaining the specified temperature for 10 h, the reactor was cooled to room temperature. The treated Pd/N-AC and Pd/AC samples were collected and stirred in 10 mM NaOH solution (200 mL/g sample) for 12 h. The perchlorate concentration in the supernatant solution was analyzed by IC. The amounts of perchlorate decomposed were determined by the difference between the initially adsorbed perchlorate amount and the amount released by the treatment with NaOH solution.

2.6. Cyclic experiments of perchlorate adsorption and catalytic decomposition To check whether the cycle of aqueous-phase perchlorate adsorption and catalytic perchlorate reduction could be repeated without loss of perchlorate adsorption capacity, the

perchlorate adsorption – decomposition cycles were repeated five times for Pd/AC and Pd/N-AC samples. Both pure and simulated perchlorate solutions were used for the cyclic experiments. Typically, 0.12 g Pd/AC or 0.08 g Pd/ N-AC sample was suspended in 20 mL of pure and simu lated solutions containing 10 mg/L ClO4 so that the perchlorate adsorption capacities were roughly 50–70% of the maximum adsorption capacities of these materials (qmax). The perchlorate adsorption capacities were determined by analyzing perchlorate concentration in a supernatant solution with IC. The perchlorate-adsorbed samples were collected by filtration and put into the stainless-steel batch reactor without drying. For the decomposition of adsorbed perchlorate, the samples were treated at 5 bar H2 at 333 K for 10 h (sub Section 2.5).

3.

Results and discussion

3.1.

Material characterization

N2 adsorption isotherms (Fig. 2a) reveal that Pd/N-AC has slightly decreased BET surface area of 842 m2/g, micropore volume of 0.21 mL/g, and mesopore volume of 0.52 mL/g, compared with AC (973 m2/g, 0.22 mL/g, and 0.71 mL/g, respectively). The results indicate that the presence of nitrogen doping at 873 K followed by impregnation with 0.1 wt.% Pd does not significantly change the porous structure of activated carbon. Both materials show dual porosities containing both micro- and mesopores of which diameters are respectively centered at 1.5 and 10 nm, based on pore diameter distributions calculated by NLDFT (Fig. 2b). Elemental analysis shows that nitrogen content is increased from 0.18% to 3.2% after nitrogen doping. The N 1s XPS analysis for N-AC before and after supporting Pd clusters was carried out to analyze the chemical nature of nitrogen functional groups on the activated carbon surface. As shown in Fig. 3, each XPS spectra could be deconvoluted into three peaks having different electron binding energies: pyridinic nitrogen (N1), pyrrolic nitrogen (N2) and pyridinic oxide (N3) [22]. The most dominant species is pyridinic nitrogen (N1) for both samples. The overall N 1s XPS spectra and the types of nitrogen species on both N-AC and Pd/N-AC samples are very similar. These results indicate that supporting Pd clusters does not change the chemical nature of surface nitrogen functional groups.

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Fig. 4 – Surface charge distribution of AC, N-AC, Pd/AC and Pd/N-AC determined by zeta potential analysis.

Fig. 2 – (a) N2 adsorption isotherms of Pd/AC and Pd/N-AC samples, and (b) the corresponding pore diameter distributions calculated by NLDFT.

fects the external particle mobility [11]. As shown in Fig. 4, the AC and Pd/AC samples show a more negative surface charge than N-AC and Pd/N-AC over the entire pH range studied. This result is consistent with previous reports showing that nitrogen doping can increase the positive charge on the carbon surface due to the incorporation of basic nitrogencontaining functional groups [11]. However, the overall surface charge at neutral pH is slightly negative even in the case of N-AC and Pd/N-AC, which indicates that the resultant nitrogen functional groups are only weakly basic. Both AC and N-AC samples showed very similar zeta potential behaviors before and after supporting Pd clusters. This means that supporting a trace amount of Pd (0.1 wt.%) does not significantly alter the overall surface charge of the carbon materials. The fractional dispersion (number of surface metal atoms/ number of bulk metal atoms) and the diameter of the supported Pd clusters were analyzed by volumetric chemical adsorption of O2 at 308 K. The results show that metal dispersions were 33% for Pd/AC and 16% for Pd/N-AC. These results correspond to the average cluster diameters of 3.4 and 7.1 nm (1.13/dispersion), respectively, if spherical geometry is assumed for the Pd clusters [23]. This implies that the Pd particles on average reside largely within the mesopores of these materials.

3.2.

Fig. 3 – N 1s XPS spectra for N-AC and Pd/N-AC samples.

The effect of nitrogen doping on the surface charge of the activated carbon was investigated by comparing the zeta potential of the AC, N-AC, Pd/AC and Pd/N-AC samples (Fig. 4). The zeta potential represents the external surface charge of a material as a function of pH; i.e., the surfaces charge that af-

Perchlorate adsorption

The kinetic results of perchlorate adsorption on Pd/AC and Pd/N-AC samples are presented in Fig. 5. Within 20 min, 95% of perchlorate adsorption was completed for Pd/AC and Pd/N-AC in both pure and simulated perchlorate solutions. Such rapid perchlorate adsorption onto activated carbons has previously been reported in the literature [8]. The fast approach to equilibrium can be attributed to the small ionic radius (0.34 nm) [24] of perchlorate and the co-presence of large mesopores in the activated carbons that provides a fast transport pathway for the perchlorate ions. Fig. 6 shows the adsorption isotherms of perchlorate for Pd/AC and Pd/N-AC samples in both pure and simulated solutions. All the isotherms were well fitted to the classical Langmuir model: qe ¼ qmax KCe =ð1 þ KCe Þ

ð2Þ

where qe is the amount of perchlorate adsorbed (mg/g), Ce is the equilibrium concentration (mg/L), qmax is the maximum

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Fig. 5 – Kinetics of perchlorate adsorption on Pd/AC and Pd/ N-AC samples in pure and simulated solutions. The pure solution was prepared by dissolving NaClO4 in deionized  water to obtain 10 mg/L ClO4 . The simulated solution was  prepared by dissolving NaClO4 (10 mg/L ClO4 ) in ionic background solution containing 0.2 mM NaNO3, 0.2 mM Mg(NO3)2, 0.3 mM K2SO4, and 1 mM NaHCO3, as reported in the literature [21].

gas [25]. Although the idea is smart, such regeneration condition is energy-intensive and difficult to use for practical application. In the present work, we supported 0.1 wt.% Pd on carbon adsorbents for catalytically decomposing the adsorbed perchlorate at much milder temperature by using H2 as a green reducing agent. In order to study the reduction behavior of the adsorbed perchlorate, H2 consumption during temperature rise (ramp: 10 K/min) was monitored by temperature programmed reduction (TPR) analysis after the perchlorate adsorption on AC, N-AC, Pd/AC and Pd/N-AC samples. We also measured the TPR profile without pre-adsorption of perchlorate, since H2 might be used to reduce the surface functional groups of carbon rather than the adsorbed perchlorate. As shown in the TPR profile (Fig. 7), the AC and N-AC samples did not show any visible H2 consumption before or after the perchlorate adsorption. The results indicate that H2 cannot be activated in the absence of 0.1 wt.% Pd catalysts and hence neither the adsorbed perchlorate nor functional groups on the carbon surface can be reduced at temperatures lower than 623 K. The fresh Pd/AC and Pd/N-AC samples without perchlorate adsorption showed much smaller H2 consumption peaks than those of the samples after perchlorate adsorption. The small H2 uptake by the fresh samples can be attributed to the reduction of the oxygen-containing functional groups on the carbon surfaces via hydrogen spillover. Hydrogen spillover is the surface migration of activated H atoms from a metal cluster to the surface of catalyst support where hydrogen dissociation is improbable [26]. Pd/AC showed a significantly larger H2 uptake peak than that of Pd/N-AC in the absence of pre-adsorbed perchlorate, indicating that AC has more oxygen func-

Fig. 6 – Perchlorate adsorption isotherms of Pd/AC and Pd/NAC in both pure and simulated solutions.

adsorption capacity (mg/g), and K is the Langmuir coefficient related to adsorption strength (L/mg). The fitting results are summarized in Table 1. The results show that Pd/N-AC exhibited 38% and 52% higher maximum adsorption capacities than Pd/AC in pure and simulated solutions, respectively. If the slightly lower BET surface area of Pd/N-AC than that of Pd/AC is considered, the increased perchlorate adsorption capacity of Pd/N-AC should be attributed to the more positively charged carbon surface (i.e., the presence of nitrogencontaining functional groups) as indicated by the zeta potential measurement (Fig. 4).

3.3. Catalytic decomposition of adsorbed perchlorate and regeneration of carbon adsorbents Physical adsorption techniques can provide fast, economic methods of perchlorate removal, but have significant limitations in the subsequent disposal and regeneration of adsorbent materials saturated with perchlorate. It was reported that perchlorate adsorption capacities of N-AC can be restored by the thermal treatment at 973 K under CO2 or NH3

Fig. 7 – Temperature programmed reduction (TPR) profiles of AC, N-AC, Pd/AC and Pd/N-AC before and after the  perchlorate adsorption (solid line: after ClO4 adsorption,  dashed line: before ClO4 adsorption, dotted line: peak  deconvolution of TPR profile measured after ClO4 adsorption). The y-axis units of TCD signal were plotted at the same scale for all curves.

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tional groups that can react with activated hydrogen. This is also consistent with earlier proposals, which asserted that nitrogen can also be incorporated into the carbon framework by the reaction between NH3 and the oxygen-containing functional groups on the carbon surface at elevated temperatures [11,27]. After the perchlorate adsorption, Pd/AC showed a major H2 uptake peak at 440 K while Pd/N-AC showed two peaks centered at 400 K and 480 K. These results indicate that Pd/AC has a uniform site for perchlorate adsorption while Pd/N-AC may possess two chemically different sites. By subtracting the peak areas of the fresh samples from those of the perchlorate-adsorbed samples (indicated by gray color in Fig. 7), the amount of H2 used for perchlorate reduction can be quantified. The results indicated that 0.12 and 0.31 mmol H2 were consumed per gram sample for the Pd/AC and Pd/N-AC samples. Since the pre-determined amounts of initially adsorbed perchlorate in these samples were 0.034 and 0.071 mmol/g respectively, the molar amounts of H2 consumed corresponded to roughly four times the pre-adsorbed amounts of perchlorate. This result strongly supports the idea that perchlorate is fully decomposed into chloride (Cl) as follows: 



ClO4 þ 4H2 ! Cl þ 4H2 O

ð3Þ

Effluent gas analysis with mass spectrometer during TPR analysis also confirmed that only H2O was produced during the reaction, which is consistent with Eq. (3). Although TPR is a versatile tool for studying perchlorate reduction behavior, it is carried out under transient condition (i.e., fast temperature ramping and short reaction time at a given temperature), not in a steady state. Moreover, the perchlorate reduction during the TPR experiment should be carried out only in dry H2 atmosphere. In a real experimental condition, water is condensed in the pore structure of activated carbons after the perchlorate adsorption from the polluted water. The pre-evaporation of water requires significant energy consumption due to the large heat of vaporization of water (40.7 kJ/mol) and thus should be avoided in a process design unless the presence of water significantly retards perchlorate decomposition. In view of these facts, we carried out the perchlorate reduction under both dry (i.e., with pre-evaporation of water) and wet (i.e., without pre-

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evaporation of water) H2 atmosphere using a batch reactor (see Experimental Section). The results (Fig. 8) shows that perchlorate adsorbed on Pd/ AC is fully (>90%) decomposed at 443 K, while perchlorate in Pd/N-AC is decomposed in a similar degree at 473 K under dry condition. The results are consistent with the reduction behavior observed during TPR analysis. Surprisingly, perchlorate decomposition in wet condition requires significantly lower reaction temperature than the decomposition in dry condition. Pd/N-AC showed >90% decomposition even at 333 K that is 140 K lower than the temperature required for dry condition (473 K). Pd/AC also showed lowered decomposition temperature (403 K for >90% decomposition) under wet condition compared with dry condition (443 K). We believe that the presence of water in the carbon pores can facilitate the surface diffusion of perchlorate ions to the dispersed Pd catalysts, which could result in an increase in the perchlorate decomposition rate. It should be noted, however, that the enhancement by water is much more pronounced in the case of Pd/N-AC than Pd/AC. Consequently, the Pd/N-AC sample, showing less decomposition than Pd/AC in dry condition, exhibited significantly greater perchlorate decomposition in wet condition. The results show that the surface nitrogen functional groups in Pd/N-AC and the adsorbed water may cooperate synergistically to drastically lower the activation barrier for perchlorate decomposition. If the catalytic reaction takes place only on the Pd surface, the presence of nitrogen functional groups on carbon surface could not significantly affect the catalytic activity. The present result indirectly indicates that the catalytic decomposition of perchlorate occurs not only on the Pd surface, but also on the carbon surface by using N-containing functional groups as a catalytic active site and by using spillover hydrogen as an active hydrogen source. Hydrogen spillover is known to readily occur on the conducting materials (e.g., carbon) because activated hydrogen can migrate as proton (H+) – electron pair [26]. It has been pointed out that the hydrogen spillover can be facilitated by the presence of water because water can enhance proton conductivity [26]. More detailed studies are required to comprehensively understand the reaction mechanism of perchlorate on the N-doped carbon surface.

3.4. Cyclic experiments of perchlorate adsorption and catalytic decomposition

Fig. 8 – Temperature dependence of perchlorate decomposition by Pd/AC and Pd/N-AC samples before and after the pre-evaporation of water.

The aforementioned perchlorate adsorption and decomposition experiments clearly reveal that Pd/N-AC has remarkable advantages in both the adsorption capacity and reduction behavior of perchlorate compared to Pd/AC. Now the question arises whether the cycle of perchlorate adsorption and catalytic decomposition in wet H2 can be repeated without loss of perchlorate adsorption capacity. To answer this question, the perchlorate adsorption – decomposition (at 333 K, 5 bar H2) cycle was repeated five times with Pd/AC and Pd/N-AC samples (Fig. 9). The perchlorate adsorption capacities of Pd/N-AC in both pure and simulated solutions were almost perfectly recovered up to five cycles (Fig. 9a). Also, the perchlorate adsorption capabilities of Pd/N-AC in both pure and simulated solutions did not significantly change up to five cycles. The present re-

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Fig. 9 – Effect of repeated regeneration of (a) Pd/N-AC and (b) Pd/AC on perchlorate adsorption capacities in pure and simulated solutions.

sults clearly indicate that Pd/N-AC can be reused without loss of perchlorate adsorption capacity. This also shows that Cl ions generated during the catalytic decomposition of ad  sorbed ClO4 can be easily replaced by ClO4 during adsorption in the next cycle. Previous studies with various ion-exchange  resins similarly showed that the adsorption sites for ClO4 can 2  generally adsorb SO4 and NO3 , while they only weak interact with Cl- and HCO 3 ions [21]. On the other hand, the initial adsorption capacity of Pd/AC was recovered only by 40–50% after cyclic experiments (Fig. 9b). This is because Pd/AC requires higher perchlorate reduction temperature than Pd/N-AC (Fig. 8).

4.

Conclusion

We successfully demonstrated that perchlorate can be removed by using Pd/N-AC as an adsorption/catalysis bifunctional material. During the treatment of polluted water, the porous structure of the carbon provides adsorption sites for perchlorate. After the carbon surface is saturated with perchlorate, the adsorbed perchlorate can be permanently decomposed by the catalytic function of supported Pd catalysts in H2. Due to the more positive surface charge of Ndoped carbon surface, Pd/N-AC exhibited significantly larger maximum adsorption capacities than Pd/AC in both the pres2  ence and absence of competing ions (NO 3 , SO4 and HCO3 ). Interestingly, the catalytic degradation of perchlorate is remarkably enhanced in the presence of water filling the carbon pores and the enhancement is much more pronounced in the case of Pd/N-AC than Pd/AC. Consequently, Pd/N-AC showed complete perchlorate decomposition even under very mild condition (333 K). It is confirmed that the perchlorate adsorption – decomposition cycle can be repeated up to five cycles without loss of perchlorate adsorption capacity. Although Pd is a precious noble metal, a trace amount (0.1 wt.%) of Pd is sufficient for the complete decomposition of adsorbed perchlorate. Also considering that the recovery yield of Pd catalysts in industry is over 99% [28], the use of Pd would not significantly damage the economic feasibility of entire process. Because the carbon adsorbents can be recycled using H2 as a green reducing agent (producing only H2O as a byproduct) at very mild temperature (333 K), significant cost savings and a favorable environmental impact can be

realized compared with conventional perchlorate removal strategies based on physical adsorption only. It is reasonable to expect that the economic feasibility of the process can be further improved by enhancing the perchlorate adsorption capacities of carbon materials. This might be achievable by the proper design of carbon pore structure and by adopting a more relevant surface modification technique.

Acknowledgments This work was supported by the Korean Ministry of Environment as a ‘Converging Technology Project’ and also by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20110011392).

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