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Research Article pubs.acs.org/journal/ascecg

Biomass Derived Sheet-like Carbon/Palladium Nanocomposite: An Excellent Opportunity for Reduction of Toxic Hexavalent Chromium Pitchaimani Veerakumar,*,†,‡ Pounraj Thanasekaran,§ King-Chuen Lin,*,†,‡ and Shang-Bin Liu† †

Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan § Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan ‡

S Supporting Information *

ABSTRACT: Palladium nanoparticles (Pd NPs) immobilized on a garlic skin-derived activated carbons (GACs) is reported. The morphology, structure, surface compositions, and textural properties of the GACs and Pd@GAC catalyst were investigated by a variety of physicochemical characterization techniques, which revealed a dispersion of Pd NPs with average particle size of ca. 21 nm on sheet-like graphitized GACs with surface areas and pore volumes as high as 1836 m2 g−1 and 0.89 cm3 g−1, respectively. As a result, the Pd@GAC with a Pd loading of ca. 1−2 wt% exhibited superior activity for catalytic reduction of toxic Cr(VI) to Cr(III) surpassing most metal-based catalysts reported in the literature. As evidenced by a comprehensive UV−vis spectrophotometric study, the eco-friendly Pd@GAC catalyst reported herein, which can be facilely prepared with biowaste feedstocks, also showed excellent catalytic performances for efficient reduction of Cr(VI) with extraordinary stability and recyclability over at least five repeated catalytic test cycles. KEYWORDS: Activated carbon, Palladium nanoparticles, Chromium, Formic acid, Catalytic reduction

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INTRODUCTION Hexavalent chromium, Cr(VI), has been recognized as one of the most strategic and critical pollutants commonly produced by industrial processes, such as electroplating, welding, leather tanning, and dyes, paints, inks, or papers manufacturing and so on.1 Being a toxic valence state (+6) of metallic element chromium, Cr(VI) is hazardous to human and animals, especially to respiratory system, kidneys, liver, skin, and eyes, leading to adverse effects on carcinogenic and potentially mutagenic damages even at trace concentrations.2 Unlike its nontoxic trivalent counterpart, Cr(III), which is an essential element for humans,3 Cr(VI) is a highly toxic element with excellent solubility and mobility, hence can easily be accumulated in human body through food chains.4,5 As such, the world health organization (WHO) has put a strain on Cr(VI) level to limit a maximum acceptable concentration (MAC) of 0.05 mg L−1 in drinking water.6 While scientists in relevant fields have also engaged their research efforts in development, fabrication, and implementation of novel environmentally benign catalytic materials for reduction of Cr(VI) to Cr(III).7−10 Garlic, Allium sativum L., is one of the potent vegetables worldwide which contains healthy constituents, such as carbohydrate, proteins, and thiamine.11 Garlic is abundant with elements, such as carbon, nitrogen, and sulfur, which are believed to be responsible for the antibacterial, antifungal, and antioxidant activities in human body.12−15 Nevertheless, garlic © 2017 American Chemical Society

skins, which are considered as biowastes, are being produced with increasing quantity, leading to growing problem in environmental pollution.16 As such, it would be desirable to identify some innovative approaches for practical utilization of garlic skins, for example, as an alternative precursor for carbon materials. Over the past decade, activated carbons (ACs) have been widely used as electrochemical sensors,17 catalyst supports,18,19 and for other energy-related applications20 owing to their high surface area, good electrochemical stability, desirable electrical conductivity, and so forth.21 Nowadays, in view of cost, accessibility, and renewability, large-scale production of ACs mostly invokes biomass precursors,22−24 rather than conventional raw materials, such as fossil fuels, petroleum coke, tar pitches, and coal. We report herein the fabrication of highly porous garlic-derived activated carbons (GACs) as catalyst support materials to incorporate palladium nanoparticles (Pd NPs) directly by a microwave-assisted method. The Pd@GAC nanocomposite materials were exploited as efficient catalysts for reduction of Cr(VI) to Cr(III) in the presence of formic acid (HCOOH), as illustrated in Scheme 1. Received: March 1, 2017 Revised: April 29, 2017 Published: May 15, 2017 5302

DOI: 10.1021/acssuschemeng.7b00645 ACS Sustainable Chem. Eng. 2017, 5, 5302−5312

Research Article

ACS Sustainable Chemistry & Engineering

Scheme 1. Schematic Illustrations of the Preparation, Characterization, and Application of the Pd@GAC Catalysts

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activation and carbonization treatments. Typically, chemical activation of the garlic-derived substrate was carried out by mixing ca. 1.0 g of dry powder with aqueous solution of ZnCl2 (5 M in 100 mL H2O) in a Teflon-coated flask. Then, the solution mixture was subjected to heating under microwave (MW; Discover series, CEM GmbH) irradiation at 100 °C with an output power of 300 W for 6 h. The obtained slurry-like material was then dried in air at 100 °C for 2 h. The chemically activated substrate was then subjected to a graphitization treatment in a tube furnace under N2 atmosphere (see Figure S1 of the Supporting Information; hereafter denoted as SI). Typically, starting from room temperature (RT), the graphitization treatment was carried out with a temperature ramp rate of 5 °C min−1 until reaching the targeted temperature (600−900 °C), then maintained at the final temperature for 2 h. Subsequently, the carbonaceous material was allowed to cool slowly back to RT, followed by thorough washing with 5 M HCl and deionized water until reaching a neutral pH value of 6−7, then dried in air at 100 °C overnight. The resultant garlic skin-derived AC samples so-prepared were denoted as GAC-x, where x represents the final carbonization temperature (in °C) used. To afford porosity enhancement in these powdered carbon substrate, the GAC substrates were further pyrolyzed in flowing CO2 (flow rate 30 mL min−1) at 400 °C for 30 min. For the preparation of Pd@GAC nanocomposite, Pd NPs were immobilized on the GAC-900 support by direct thermal reduction of PdNO3 under N2 atmosphere. In brief, 0.5 g of the as-prepared GAC-900 powdered sample and 10 mg PdNO3 were dispersed in an ethanol solution (10 mL) under ultrasonic vibration for at least 1 h, followed by heating via microwave irradiation (Milestone’s START; power 300 W) at 100 °C for 2 h. The sample was then heated in a furnace at 900 °C for 3 h under an inert gas (N2) atmosphere, and the resultant Pd-incorporated GAC nanocomposite was denoted as Pd@GAC (see Scheme 1). Catalytic Reduction of Cr(VI). Typically, an aqueous solution 3 mL of potassium dichromate (K2Cr2O7; 0.8 mM) and 0.3 mL of formic acid (FA, HCOOH; 0.45 M) were mixed under vigorous stirring at room temperature (RT, 25 °C). Upon introducing varied amounts (0.5, 1.0, and 2.0 mg mL−1) of Pd@GAC catalyst on to the mixture solution, the color of the yellow suspension solution tended to fade gradually with increasing reaction time, eventually a colorless solution was obtained. The reaction process was monitored by UV−vis spectroscopy. Upon adding excess amount of sodium hydroxide (NaOH) solution to the above colorless solution, a green mixture solution was observed, confirming the complete reduction of Cr(VI) to Cr(III) (vide inf ra).

EXPERIMENTAL SECTION

Materials. Palladium(II) nitrate dihydrate (PdNO3·2H2O, ca. 40% Pd basis), potassium dichromate, K2Cr2O7 (98%), formic acid, (HCOOH, 98%), and the palladium black (Pd/C, Pd loading 10 wt %) were supplied by Sigma-Aldrich. Fresh garlic (Allium sativum L.) skins were obtained from local market. All solutions were prepared using double distilled water and all analytical grade reagents were used as received. Characterization Methods. Powder X-ray diffraction (XRD) measurements were conducted on a PANalytical (X’Pert PRO) diffractometer using the Cu Kα radiation (λ = 0.1541 nm). Raman spectra were recorded on a Jobin Yvon T64000 spectrometer equipped with a charge coupled device (CCD) detector cooled under liquid nitrogen temperature (−196 °C). The backscattering signal was collected with a microscope using an Ar+ laser (λ = 488 nm) as the excitation source. FT-IR spectra were acquired using a Bruker IFS28 spectrometer in the region of 4000−400 cm−1 with a spectral resolution of 2 cm−1 using the KBr pellet method at room temperature (RT, 25 °C). Nitrogen physisorption studies were carried out with a Quantachrome Autosorb-1 volumetric analyzer at −196 °C. Prior to measurements, the sample was purged with N2 gas at 150 °C for at least 12 h. The pore size distribution (PSD) of various samples were derived from density functional theory (DFT) calculations. The dispersions of Pd NPs in various support catalysts were determined by hydrogen chemisorption using Micromeritics ASAP 2020 apparatus. Typically, each sample was dried at 120 °C under a He gas flow for 30 min, followed by a reduction treatment in 50% H2/He flow at a ramp rate of 5 °C min−1, then maintained at 250 °C for 2 h. All H2 absorption isotherms were measured at 323 K, and the corresponding Pd dispersion in each catalyst sample was derived assuming a complete Pd reduction with the stoichiometry of H:Pd = 1:1 atomic ratio. The morphology and structure of GACs were characterized by scanning electron microscopy (SEM; JEOL JSM-6500F) and field emission transmission electron microscopy (FE-TEM; JEOL JEM-2100F) operating at an acceleration voltage of 200 kV. The elemental compositions in each sample were determined by the energy dispersive X-ray analysis (EDAX) accessory equipped with the electron microscope. Surface constituent analyses by X-ray photoelectron spectroscopy (XPS) were also performed using a ULVAC PHI PHI 5000 Versa Probe apparatus. Moreover, thermogravimetric analyses (TGA) were carried out on a Netzsch TG-209 and ultraviolet−visible (UV−vis) spectra were recorded on a Thermo Scientific evolution 220 spectrophotometer. All the adsorption experiments were carried on using an incubator shaker (GENEI TMSLM-INC-OS-250). Preparation of GACs and Pd@GAC Composites. Garlic skins raw materials were first dried in sunlight for 2−3 days, followed by rinsing and washing with warm distilled water, then dried overnight in an air oven at around 45 °C. The processed substrates were crushed to powder form with an average particle size of ca. 3−5 mm, then, the dry powder was stored in an airtight desiccator for future chemical

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RESULTS AND DISCUSSION Physicochemical Properties. The structural properties and chemical compositions of the as-prepared GACs and Pd@ GAC samples were characterized by a variety of different physicochemical techniques. The GAC-x prepared at different carbonization temperatures, x (600−900 °C), exhibited similar

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Figure 1. (a) XRD patterns, (b) Raman spectra, (c) N2 adsorption/desorption isotherms, and (d) pore size distributions of various as-prepared GACs and the Pd@GAC sample.

Table 1. Physical Properties of the As-Prepared GACs and Pd/GAC Materials sample

STot (m2 g−1)a

GAC-600 GAC-700 GAC-800 GAC-900 Pd@GAC

616 868 1057 1836 1793

SMicro (m2 g−1)b 163 323 619 712 989

(26.5%) (37.2%) (58.6%) (38.8%) (50.1%)

SMeso (m2 g−1)b

VTot (cm3 g−1)a

453 545 438 1124 804

0.45 0.76 0.77 0.89 0.81

VMicro (cm3 g−1)b 0.07 0.14 0.26 0.32 0.31

(15.6%) (18.4%) (33.8%) (36.0%) (38.3%)

DP (nm)c 3.5 3.8 4.3 4.3 4.0

Dm (%)d

ID/IG

0.85

0.99 0.99 0.99 1.00 1.00

a

Total BET surface areas (STot) and pore volumes (VTot) derived at P/P0 = 0.99. bMicroporous surface areas (SMicro) and pore volumes (VMicro) obtained from t-plot analyses, values in parentheses indicate the portion relative to the total value. cAverage pore size (DP) derived by the nonlocal DFT method. dMetal dispersion measured by H2 chemisorption at 323 K.

such, the intensity ratio of the two bands, ID/IG, is commonly served as a criteria for evaluating the graphitic structure of the carbon.28 As depicted in Table 1, a typical ID/IG value close to 1.00 was observed for all GAC-x (x ≥ 600 °C), indicating the graphitized nature of the carbon materials carbonized at temperatures beyond 600 °C.29 Moreover, an identical ID/IG was observed for the GAC-900 and the Pd@GAC samples, indicating that the structure of the GAC support remained unchanged even after incorporation of Pd NPs. The textural properties of various samples were investigated by N2 adsorption/desorption isotherm measurements. As shown in Figure 1c, all GAC samples exhibit the typical typeIV isotherm (cf. IUPAC classification) with a weak H4 hysteresis loop at P/P0 of ca. 0.43, which is a signature for the presence of mesopores. Together with the notable increase in N2 uptake at very low partial pressures (i.e., in the Henry’s Law region), the isotherm curves clearly indicate the coexistence of micro- and meso-porosities in these carbon substrates. By analyzing the desorption curves of the isotherms obtained from various GACs using the Brunauer−Emmett− Teller (BET) method, their corresponding textural properties

XRD patterns with two broad diffraction peaks centering at a 2θ angle of ca. 20.8 and 43.5° (Figure 1a) due to (002) and (100) diffraction planes of the graphitized carbon, respectively.25 On the other hand, aside from the (002) diffraction peak, additional sharp diffraction peaks at 2θ = 40.1, 46.6, 67.9, and 82.0° were observed for the Pd@GAC nanocomposite (Figure 1a), which can be assigned to the (111), (200), (220), and (311) crystalline facets of the face-center-cubic (fcc) structure of Pd metal (JCPDS card no. 46−1043).10 On the basis of Scherrer equation, an average Pd NP particle size of ca. 22.1 nm was determined. Figure 1b displays the Raman spectra of corresponding samples. All samples showed Raman spectra with two intense absorption bands centered at 1350 and 1580 cm−1, which are associated with vibrational modes involving sp2-bonded carbon atoms in disordered and graphitic microcrystalline domains, respectively. The band at 1350 cm−1 (D band) is usually attributed to a lattice breathing mode with A1g symmetry, which prevails only in disordered carbonaceous materials.26,27 Whereas, the band at 1580 cm−1 (G band, E2g symmetry) is ascribed to bond stretching of pairs of sp2 carbons either in aromatic rings or chains of the graphitized carbons. As 5304


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Figure 2. SEM images of (a−d) GAC-900, and (e,f) Pd@GAC nanocomposite catalyst at different magnifications.

leading to a marginal decrease in the percentage of SMicro. Nonetheless, the GAC-900 was found to possess the highest STot (1836 m2 g−1) and VTot (0.32 cm3 g−1), hence, it is chosen as the catalyst support for Pd NPs. Upon incorporating Pd NPs onto the GAC-900 carbon support, notable decreases in STot, VTot, and Dp values compared to its parent support were observed for the Pd@GAC nanocomposite. A closer examination of the textural data listed in Table 1 revealed that the incorporated Pd NPs were mostly reside in mesopores, leading to a notable loss in mesoporous surface area and pore volume. The above results therefore clarify the successful dispersion of Pd NPs in the mesopores of GAC-900 support. The effects of activation and carbonization treatments on thermal properties of various GACs was further investigated by thermal analyses using the TGA-DTA technique. For comparison purpose, the TGA-DTA profiles of the parent dry garlic skin sample and its processed powder counterparts without and with chemical activation by ZnCl2 were recorded, as shown in Figure S2 (SI). The bulk garlic skin sample exhibited multistep weight-losses (Figures S2a; SI) corresponding to predominant DTA peak temperatures at ca. 100 and 275 °C (Figure S2c; SI), which may be attributed to desorption of water and volatile ingredients,23 respectively. Moreover, a somewhat shallower weight-loss was also observed in the temperature range of 400−900 °C. On the other hand, in

such as total surface areas and total pore volumes may be derived, as summarized in Table 1. Also listed in Table 1 are the surface area and pore volume accountable for micropores in each sample, which were derived by means of t-plot analysis. Moreover, their corresponding pore size distributions (Figure 1d) may also be predicted by DFT theoretical calculations, the results are also depicted in Table 1. It is clear that the total surface area (STot), total pore volume (VTot), and average pore size (DP) of GACs tended to increase with increasing temperature of carbonization. Moreover, a large portion of porosity was due to micropores. For example, upon increasing the carbonization temperature from 600 to 900 °C, the observed STot, VTot, and Dp values increased notably from 616 m2 g−1, 0.45 cm3 g−1, and 3.5 nm of the GAC-600 to 1836 m2 g−1, 0.89 cm3 g−1, and 4.3 nm of the GAC-900, respectively (see Table 1). While, the corresponding microporous SMicro and VMicro both gradually increased from 163 m2 g−1 and 0.07 cm3 g−1 of the GAC-600 to 619 m2 g−1 and 0.26 cm3 g−1 of the GAC-800, indicating that while majority of porosity in GACs were contributed by mesopores with an average pore size ranging from 3.5 to 4.3 nm (cf. Table 1), notable increases in microporosity (pore size ≤1.2 nm; see Figure 1d) of the substrate with increasing temperature of carbonization may also be inferred. Further increasing the carbonization temperature beyond 900 °C, partial shrinkage of micropores may occur, 5305


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Figure 3. FE-TEM images of (a) the as-prepared bare GAC-900, and (b−f) the Pd@GAC catalyst with different magnifications. Insets in (a) and (g) show the corresponding SAED pattern of GAC-900 and Pd@GAC, respectively. (h) Particle size distribution, and (i) EDAX profile of the Pd@GAC catalyst.

addition to the small weight-loss below 100 °C due to desorption of moisture, the processed, cleaned, and crushed powder samples before and after the chemical activation by ZnCl2 revealed sharp weight-loss peaks at ca. 450 and 550 °C, respectively, indicating that the latter samples were free of undesirable ingredients and/or impurities. Similar TGA-DTA profiles were observed for the GAC-x samples, except for the gradual increase in weight-loss temperature with increasing temperature of carbonization (x = 600−900 °C), revealing the enhanced structural stability with increasing x. It is also noteworthy that, none of the TGA profile observed for the GACs showed complete weight-loss, indicating the presences of trace amounts (