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Johnson Matthey Technol. Rev., 2018, 62, (1), 60

doi:10.1595/205651317x696252

A Facile Green Tea Assisted Synthesis of Palladium Nanoparticles Using Recovered Palladium from Spent Palladium Impregnated Carbon

Biosynthesising palladium nanoparticles using green tea as a reducing agent

  • Ansari Palliyarayil, Kizhakoottu Kunjunny Jayakumar, Sanchita Sil* and Nallaperumal Shunmuga Kumar
  • Defence Bioengineering and Electromedical Laboratory (DEBEL), Defence Research and Development Organization (DRDO), C. V. Raman Nagar, Bangalore-560 093, India
  • *Email: sanchitasil@debel.drdo.in
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Article Synopsis

Palladium impregnated activated carbon (Pd/C) filters play a major role in air quality management by the removal of toxic carbon monoxide from confined environments. However, Pd is an expensive metal and therefore, recovery and reuse of Pd from spent filter cartridges is highly desirable. The objective of the present study was to biosynthesise Pd nanoparticles (NPs) using green tea as a reducing agent. The source of Pd for the NP synthesis was spent Pd/C. Three different acid based Pd extraction protocols constituting of hydrochloric acid-hydrogen peroxide (HCl-H2O2), 2 M HCl and aqua regia were systematically explored. The Pd impregnated carbon was characterised using scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), ultraviolet-visible (UV-vis) spectroscopy, X-ray powder diffraction (XRD) and atomic absorption spectrometry (AAS) before and after Pd extraction. It was found that the aqua regia based extraction protocol was the most efficient among the three chosen acid or acid mixtures with an average absolute yield of 96%. Finally, an attempt was made towards one pot biosynthesis of Pd NPs from the recovered extract by using green tea as a reducing agent. The synthesised NPs were characterised using UV-vis spectroscopy, SEM and XRD.

Introduction

Activated carbon is a versatile adsorbent possessing high surface area, microporous structure and a high degree of surface reactivity. As an adsorbent, activated carbon has found application in economic sectors as diverse as the food, pharmaceutical, chemical, petroleum, nuclear, automobile and vacuum industries as well as for the treatment of drinking water, industrial and urban wastewater and industrial flue gases. According to the European Council of Chemical Manufacturers’ Federation (European Chemical Industry Council or Cefic), it is defined as non-hazardous, processed, carbonaceous material having a porous structure and a large internal surface area (1). It is known as a ‘universal adsorbent’ due to its efficiency and has been employed for the adsorptive removal of impurities from exhaust gas and wastewater systems (27).

One of the ways to improve the efficiency of activated carbon is by impregnation of metals (8). Impregnated activated carbon has been extensively used for gas purification, catalysis, civil and military gas protection (915). It has also been successfully used in air cleaning filters in confined environments for the removal of toxic pollutants. These filters act as the ‘lungs’ of a closed system that ensure air purification, thereby maintaining a level of toxic pollutants under the permissible limits. Pd is one of the metals which is impregnated on carbon in air cleaning filters. It acts as a catalyst by oxidative removal of CO as carbon dioxide, as shown in Equation (i):

(i)

Pd as a catalyst is present in the range 4–8% by weight of total carbon (16, 17). Once the filter becomes saturated, it must be replaced. However, this is a cost intensive process and recovery of Pd is highly desirable so that it can be recycled and used for subsequent impregnation. Additionally, metal-containing wastes such as spent filters cannot be disposed of directly due to environmental concerns (18). According to a review by Butler, in the year 2011 alone, about 22% of the Pd market consisted of recovery from autocatalysts and jewellery scrap (19).

Many processes have been reported in the literature for extraction of Pd by using different solvents such as ortho-8-hydroxyquinoline, HCl, a HCl-H2O2 mixture and aqua regia (2023). The objective of this study was to identify a simple, cost effective and scalable technique for Pd recovery that allows reuse of the recovered metal for various applications and can be easily adopted by industry. In the present work, a systematic study for the recovery of Pd from Pd impregnated (6.5%) activated carbon was conducted using three different extraction routes. The first method comprised of digestion with 2 M HCl, the second consisted of digestion with a HCl-H2O2 mixture while the third used aqua regia for digestion. The details of the processes are provided schematically in the Supplementary Information (SI) accompanying this article online.

Additionally, it is important to reuse the extracted metal for various applications. As an extension of this study, biosynthesis of Pd NPs using a ‘green’ route was also conducted. It is known that metal NPs show unique properties different from their bulk counterparts and have therefore found applications in the areas of optoelectronics, catalysis, photothermal therapy, surface enhanced Raman spectroscopy (SERS) detection and biological labelling (24). Recently, the syntheses of metal NPs by following green chemistry principles have attracted a great deal of attention (2529). In the present work, green tea extract was used as a reducing agent for the biosynthesis of Pd NPs. Green tea, an infusion of Camellia sinensis leaves, is rich in polyphenols which are important biologically active components having antioxidative, antimutagenic and anticarcinogenic effects (3032). Chemically, green tea is a complex mixture of polyphenols including flavonoids, caffeine, amino acids, organic acids, proteins, volatiles (low molecular weight aldehydes and alcohols), minerals like sodium, potassium and calcium and trace elements (aluminium, manganese, selenium and iron) (3046). Flavanols and flavonols are the two major classes of polyphenols present in green tea (approximately 16–30% of the dry weight of the fresh leaf) (30, 33). The phenolic acid-type biomolecules which are present in the green tea infusion play a major role in the conversion of metal ions into metal NPs by reduction. During the NP formation, these biomolecules form complexes with metal ions present in the solution and reduce them into corresponding metals through an electron transfer mechanism. The catechins (flavan-3-ols), which are colourless, astringent and water soluble compounds, are characterised by the meta-5,7-dihydroxy substitution of the A-ring and di- or trihydroxy substitution of the B-ring as shown in Figure 1(a) and are the predominant species which are easily oxidisable. These are responsible for the biological activity of green tea. Green tea catechins are composed of eight catechin monomers (30, 34, 4549), the structures of which are given in Figure 1. The polyphenol profile of green tea along with their percentage composition is tabulated in the Supplementary Information (SI1).

The Pd NPs biosynthesised from the recovered extract using green tea as the reducing agent were characterised using UV-vis spectroscopy, SEM and XRD.

Fig. 1.

Two-dimensional (2D) and three-dimensional (3D) structure of various catechin monomers present in green tea

Two-dimensional (2D) and three-dimensional (3D) structure of various catechin monomers present in green tea

Materials and Methods

Chemicals

Spent Pd impregnated activated carbon (Pd/C) was received from Active Char Pvt Ltd, India. Analytical grade HCl, sodium hydroxide (NaOH), H2O2, sodium borohydride (NaBH4) and nitric acid (HNO3) were purchased from Merck Specialities Pvt Ltd, India. Ethanol was purchased from Changshu Yangyuan Chemical Co Ltd, China. All these chemicals were used as received without further purification. All aqueous solutions were prepared in distilled water.

Instrumental Details

The pH of the spent Pd/C was measured using a CyberScan pH 510, Eutech Instruments Pte Ltd, Singapore. SEM studies for carbon samples were carried out with a Quanta 200 ESEM, Icon Analytical Equipment Pvt Ltd, India, and EDS with a Genesis XM4, Icon Analytical. Field emission scanning electron microscope (FESEM) studies of the Pd NPs were performed using an Ultra-55, Carl Zeiss AG, Germany. The scanning was carried out using an in-lens secondary electron detector, the voltage was maintained at 2.0 kV and 5.0 kV, and the working distance was kept at ~10 mm and 5 mm respectively. The XRD analysis was carried out with an X’Pert PRO diffractometer, PANalytical, Spectris Plc, The Netherlands, using CuKα radiation of wavelength λ = 1.54 Å with 40 kV generator voltage and 30 mA tube current. The 2θ value for the analysis was selected between 5° and 90°. A BELSORP-mini II surface area analyser, MicrotracBEL Corp, Japan, was used to measure the surface area by a nitrogen adsorption method at –196°C. A LambdaTM 25 UV-vis spectrophotometer, PerkinElmer Inc, USA, was used to carry out the study of pH variation during palladium(II) chloride (PdCl2) precipitation. Scanning between the wavelengths 200–900 nm was performed to measure the absorbance values using the Lambda 25 software. Quantitative analysis was carried out with the help of an iCETM 3500 AAS, Thermo Fisher Scientific, USA, in air-acetylene mode using the SOLAARTM software.

Characterisation of Palladium Impregnated Activated Carbon

All the physical and chemical properties of the spent catalyst (Pd/C) were studied as per the Indian standards IS 877:1989 and IS 2752:1995 (reaffirmed in 2016). For each analysis, moisture free samples were used.

Palladium Recovery

All the extraction studies were carried out with moisture free Pd/C. The spent Pd/C was oven dried at 110°C for 4 h. To check the repeatability of the results, the extraction studies were carried out in duplicate.

In the first method, recovery of Pd from Pd/C was achieved using a HCl-H2O2 (10% HCl and 5% H2O2) mixture. The details of the process are provided in the Supplementary Information (SI2). The reaction occurs as shown in Equations (ii) and (iii) (23):

(ii)

(iii)

The obtained filtrate was made up to a definite volume and analysed for Pd content using AAS. Reduction of the Pd extract was performed by slow addition of 1% NaBH4 to the filtrate under magnetic stirring and the reaction temperature was maintained at 95–100°C (Equation (iii)) (23). The completion of the reduction process was monitored with the help of UV-vis spectroscopy. The precipitated Pd was filtered out, oven dried at 110°C and was analysed quantitatively by AAS.

In the second approach the extraction of Pd from Pd/C was achieved using 2 M HCl. The details of the process are provided in the Supplementary Information (SI2). The filtrate was analysed for Pd content using AAS after making up to a definite volume. The pH of the extract was raised by using NaOH solution to precipitate Pd as PdCl2. The precipitation of PdCl2 usually occurs at pH 9.5 (20). Initially the pH of the extract was raised to pH 6 to check for precipitation of any impurities such as chlorides of iron or other metals. No precipitation was observed in this range. The pH of the extract was then further raised up to pH 11 to ensure the completion of the precipitation of Pd as PdCl2. The precipitated PdCl2 was filtered by using a previously weighed G-3 crucible. The precipitate was then oven dried at 110°C and quantitatively analysed by AAS. Equations (iv), (v) and (vi) describe the chemical reactions:

(iv)

(v)

(vi)

In the final approach, Pd was extracted from Pd/C using aqua regia. The details of the process are provided in the Supplementary Information (SI2). The reaction proceeds according to Equations (vii) and (viii) respectively. The filtrate was made up to a definite volume and analysed for Pd using AAS. Part of the filtrate was used to remove nitrate ions and the residue was redissolved in acid (20% HCl) for precipitation of Pd as PdCl2 as described in Equation (vi).

(vii)

(viii)

The precipitated Pd was then quantitatively analysed by AAS.

Synthesis of Palladium Nanoparticles

Green tea extract used for the synthesis of Pd NPs was prepared by heating 0.25 g of tea leaves (Tetley green tea from TATA global beverages) with 50 ml of distilled water at 80–85°C for 2 min followed by filtration. Since the amount of Pd recovered using the HCl-H2O2 method was less compared to the other two methods (see Table I), only extracts from the 2 M HCl and aqua regia methods were used for the synthesis of NPs. In this case, 40 ml of the Pd extract (pH adjusted to pH 5.5–6) was heated with 2 ml of green tea extract at 90–95°C for 15–45 min with continuous stirring at 400 rpm. In the case of aqua regia extract, the extract was evaporated to dryness for the removal of nitrate ion. The obtained residue was dissolved in a minimum quantity of 20% HCl followed by addition of 40 ml distilled water and this solution was used for NP synthesis. The reaction was carried out in the dark to prevent the effect of light. The colour change of the solution from light to dark brown indicated the formation of Pd NPs. The formation of Pd NPs was further monitored by using UV-vis spectroscopy. The synthesised Pd NPs were then washed with deionised water by centrifugation at 10,000 rpm for 10 minutes. The process was repeated three times followed by dispersion in ethanol and the resulting dispersion was used for characterisation studies. The morphology of the biosynthesised Pd NPs was obtained by using FESEM. The Pd NPs were also studied using XRD.

Table I

Recovery of Palladium Estimated by Atomic Absorption Spectrometry Measurement using Different Extraction Media

MethodSample 1, %Sample 2, %Average recovery, %Standard deviation with respect to absolute recovery
HCl-H2O2 Extract 58.68 59.09 58.89 0.5939
Absolute recovery 55.96 56.80 56.38
2 M HCl Extract 75.01 73.56 74.29 0.6576
Absolute recovery 73.65 72.72 73.19
Aqua regia Extract 93.95 98.73 96.34 3.4577
Absolute recovery 93.84 98.73 96.29

Results and Discussion

Palladium on Carbon

The results of the characterisation of Pd/C are compiled in Table II.

Table II

Characterisation of Pd/C Used in the Present Study

TestResult
Moisture content 11.53%
Ash content 8.62%
pH 9.30
Decolourising power 365.47 mg
Iodine number 1463.4 mg g–1

The Pd/C was observed to have 11.53% moisture content. Although moisture content does not have any detrimental effect on the adsorption efficiency of carbon, it essentially dilutes the carbon content which necessitates additional weight compensation during the filtering process (50). One of the quality indicators for activated carbon is the ash content which is defined as the residue that remains when the carbonaceous portion is burnt off. Ash essentially consists of minerals such as silica, aluminium, iron, magnesium and calcium. This is generally not desirable and lower values are indicative of a good quality carbon. Sometimes ash may also interfere with the carbon adsorption by competitive adsorption and catalysis (51). The obtained ash content for the material used in the present study was 8.62% as it also contains Pd. Since the pH values of most commercial carbons are produced by their inorganic components, the ash content may affect the pH of the carbon. The acidic or basic nature of an activated carbon depends on its preparation and inorganic matter content along with the chemically active oxygen groups on its surface. In addition, the kind of chemical treatment to which the activated carbon is subjected would also determine the pH. The pH of the Pd/C used in this study was slightly alkaline suggesting that the carbon surface was essentially negatively charged (1).

The iodine number provides an idea of the micropore content of the activated carbon by adsorbing iodine from solution. Additionally, it also provides qualitative information about the surface area (52, 53). From the obtained iodine number in the present study (1463.4 mg g–1), it can be concluded that the surface area of the Pd/C carbon is ≥1100 m2 g–1 which is also confirmed by the BET surface area measurement value (1550.5 m2 g–1).

Adsorption capacity, an important property of an adsorbent material, was calculated with respect to decolourising power. The high value of decolourising power (365.47 mg) indicates that the Pd/C is a good adsorbent. Since methylene blue comes under the category of a cationic dye, a higher value for methylene blue adsorption indicates the presence of more negative charges on the Pd/C. This can be further corroborated from the pH value which is slightly in the alkaline range (54, 55).

SEM and EDS studies were carried out for the Pd/C before and after extraction via the described routes. Figure 2 depicts the micrographs of Pd/C and Figure 2(a) depicts the SEM microstructure of Pd/C before Pd removal. The Pd/C possesses pores of diameter in the micrometre range. On treatment with acids, HCl-H2O2, 2 M HCl and aqua regia as shown in Figures 2(b), 2(c) and 2(d) respectively, a visible change in the morphology of Pd/C treated with aqua regia was observed compared to the morphology of Pd/C treated with HCl-H2O2 and 2 M HCl. In the case of HCl-H2O2 and 2 M HCl treated Pd/C (Figures 2(b) and 2(c) respectively), there is no visible difference in surface morphology compared to the pure Pd/C. The HCl-H2O2 treated Pd/C appears to be flakier which may be due to grinding of Pd/C before the extraction. The aqua regia treated Pd/C (Figure 2(d)) shows a more porous structure with an increase in the pore densities. The EDS study of Pd/C reveals the presence of Pd on its surface. The disappearance of the Pd peak in the EDS spectrum of aqua regia treated Pd/C reveals that aqua regia treatment removes Pd effectively (Figure 2(d)) and is the most effective method for the extraction of Pd from Pd/C. This was also confirmed from AAS and XRD studies.

UV-vis spectrophotometric studies were carried out during the course of Pd recovery for all three acid digestion routes. The UV-vis spectra are provided in Figure 3. The trend of the decrease in absorption peaks during the progress of the reaction towards Pd recovery was monitored and is shown as an inset in the figure. In the case of Pd removal using HCl-H2O2 digestion as shown in Figure 3(a), absorption peaks centred at 231 nm and 277 nm respectively were obtained from the Pd containing solution, which are characteristics of Pd(II). The UV-vis spectra were acquired at different stages of reduction: during and after completion of the reaction. There was a steady decrease in the absorption band which finally disappeared at the end of the reduction. Similarly, in the case of recovery of Pd using 2 M HCl and aqua regia respectively, the UV-vis spectra at different pH were obtained which showed a steady decrease in the absorption band as the solution became depleted of Pd. At pH 11, the Pd(II) absorption peak disappeared which marked the completion of the Pd(II) precipitation.

Fig. 2.

SEM and EDS graphs of: (a) Pd impregnated carbon (Pd/C); (b) Pd/C treated with HCl-H2O2; (c) Pd/C treated with 2M HCl; (d) Pd/C treated with aqua regia. The SEM images were taken at magnifications 500× and 5000× respectively

SEM and EDS graphs of: (a) Pd impregnated carbon (Pd/C); (b) Pd/C treated with HCl-H2O2; (c) Pd/C treated with 2M HCl; (d) Pd/C treated with aqua regia. The SEM images were taken at magnifications 500× and 5000× respectively

Fig. 3.

UV-vis spectrophotometric studies of Pd impregnated carbon (Pd/C) at different stages of the reaction or pH variation extracted using: (a) HCl-H2O2; (b) 2M HCl; (c) aqua regia. The plots in the inset depict the trend in absorbance as the reaction progresses for Figure 3(a), and the PdCl2 precipitation occurring with the increase in pH in the case of Figures 3(b) and 3(c) respectively

UV-vis spectrophotometric studies of Pd impregnated carbon (Pd/C) at different stages of the reaction or pH variation extracted using: (a) HCl-H2O2; (b) 2M HCl; (c) aqua regia. The plots in the inset depict the trend in absorbance as the reaction progresses for Figure 3(a), and the PdCl2 precipitation occurring with the increase in pH in the case of Figures 3(b) and 3(c) respectively

The energy level ordering of the transition metal d-orbitals is greatly influenced by the ligand which is manifested in the spectroscopic transition. Additionally, the orbital interaction with ligands influences the magnitude of orbital splitting. In the case of complexes with π-donor ligands such as (PdCl4)2–, the energetic ordering is perturbed by π-donating ligands such as chloride (56). Based on the different acid treatments, the absorbance spectra show little shifts in the values. The absorbance values for the (PdCl4)2– complexes formed using various acids show bands around 231 nm and 277 nm in the case of HCl-H2O2 based extraction whereas there is an appearance of an additional band around 211 nm and 218 nm in the case of 2 M HCl and aqua regia based extraction routes respectively. All the observed absorbances can be attributed to a ligand-to-metal (Cl to Pd(II)) charge transfer transition for (PdCl4)2– (57, 58).

AAS was employed to quantitatively analyse the amount of Pd recovered using the three extraction routes. The initial estimation was carried out using the Pd(II) solution immediately after acid digestion. The AAS analysis results are provided in Table I. It is observed that aqua regia based digestion was most effective in recovering Pd, followed by the 2 M HCl and HCl-H2O2 routes with an average yield ~96%, 74% and 59% respectively for the three acids. Pd was then precipitated from the three extracts by using NaBH4 in the case of the extract from HCl-H2O2, and by increasing the pH of the solution in the case of the 2 M HCl and aqua regia extracts. Finally, AAS studies were performed on the final recovered Pd. The aqua regia extract gave the highest recovery of Pd at 96%, followed by the 2 M HCl (~73%) and HCl-H2O2 (~56%) extracts. All the extractions were carried out in duplicate and the results were reproducible within <5% variation as mentioned in Table I. The detailed calculations are included in the Supplementary Information.

An ash test was carried out on the Pd/C after aqua regia extraction. The amount of ash obtained from the Pd/C treated with aqua regia was 0.71%. The ash content value also reflects that the aqua regia treatment effectively removes Pd from the spent Pd/C.

XRD studies were carried out on Pd/C before and after treatment and the spectra are shown in Figure 4. The XRD pattern of Pd/C (Figure 4(a)) exhibited two broad peaks at 2θ corresponding to 25.56° and 43.89° respectively. The peaks are attributed to the (002) and (100) plane, respectively, characteristic of the hexagonal diffraction pattern of the graphitic structure. In addition to these two broad peaks, five major diffraction peaks centred at 2θ 40.51°, 47.00°, 68.47°, 82.38° and 86.87° were also present in the XRD pattern for Pd/C (Figure 4(a)). The appearance of these five peaks indicates the presence of Pd on the surface of Pd/C possessing (111), (200), (220), (311) and (222) planes (Bragg reflection) of the face centred cubic (fcc) crystalline structure. Two broad peaks characteristic of the hexagonal diffraction pattern of the graphitic structure were also observed in the XRD pattern for acid treated Pd/C (Figures 4(b), 4(c) and 4(d)). In the case of Pd/C treated with aqua regia, all five Pd peaks disappeared, indicating the complete leaching out of Pd from the Pd/C. The XRD pattern for 2 M HCl treated Pd/C shows peaks corresponding to Pd indicating the presence of Pd on the surface of Pd/C, implying that the extraction with 2 M HCl was not effective and the Pd/C contained some Pd even after treatment. The observation was in agreement with the EDS, XRD and AAS results. For the Pd/C treated with HCl-H2O2, all five peaks were absent and the pattern was similar to the spectrum obtained for Pd/C treated with aqua regia . However, the recovery percentage was lower for the HCl-H2O2 method compared to the aqua regia and 2 M HCl based extraction methods.

Fig. 4.

XRD diffraction pattern for: (a) Pd impregnated carbon (Pd/C); (b) Pd/C treated with HCl-H2O2; (c) Pd/C treated with 2 M HCl; (d) Pd/C treated with aqua regia

XRD diffraction pattern for: (a) Pd impregnated carbon (Pd/C); (b) Pd/C treated with HCl-H2O2; (c) Pd/C treated with 2 M HCl; (d) Pd/C treated with aqua regia

Nanoparticles

As mentioned in the earlier section, it is important to develop processes for the reuse of the recovered metal. In this direction, biosynthesis of Pd NPs was carried out with the obtained extracts using green tea as the reducing agent. The flavonoid (+)-catechin, an antioxidant present in green tea, is responsible for the formation of NPs. The reaction follows an electron transfer mechanism involving the release of both electrons and protons from the molecule (59). (+)-Catechin has two different pharmacophores, the catechol group in ring B and the resorcinol group in ring A (Figure 1(a)). Additionally, at position 3, it has a hydroxyl group in ring C (Figure 1(a)). Electrochemical studies on flavonoids show the trends of their electron donating ability. It has been shown that compared to resorcinol ring A, catechol ring B is more easily oxidisable, the ring with lower redox potential. The stability of various catechin radicals will follow the sequence: 4’-OH, 3’-OH, 7-OH, 5-OH (59, 60).

During the formation of the NPs, catechin forms a chelation complex with the Pd2+ ion, followed by an electron transfer that reduces Pd2+ to Pd0 and oxidises catechin to quinone (59, 61). The reduction mechanism for the conversion of Pd2+ to Pd0 by green tea is schematically expressed by using a representative catechin monomer in Figure 5. This can be stoichiometrically expressed as Equation (ix):

Fig. 5.

Mechanism for the formation of Pd NPs by using catechin as a representative reducing agent

Mechanism for the formation of Pd NPs by using catechin as a representative reducing agent

(ix)

where R and n represent the heterocyclic or alkyl groups and the number of the hydroxyl groups oxidised by Pd(II) species, respectively. Therefore, it is apparent that the formation of Pd NPs follows a 1:1 reaction and the amount of Pd NPs formed will be proportional to the amount of Pd(II) present in the solution.

The formation of NPs from the extracts (2 M HCl and aqua regia methods) was identified with the help of a UV-vis spectrophotometer. The UV-vis spectra are provided in Figures 6(a) and 6(b). The UV-vis spectrum of green tea shows an absorption peak centred at 273 nm. The absorption peaks centred around 211 nm, 234 nm and 277 nm corresponding to Pd(II) extracted from 2 M HCl and aqua regia start disappearing as the reaction proceeds, indicating the formation of Pd NPs. The complete reduction of Pd(II) to Pd(0) was confirmed by the appearance of a broad continuous absorption band (62). NP formation was further confirmed by FESEM images as shown in Figures 7(a) and 7(b). It is evident from the micrographs that fine NPs are formed which are well separated from each other. The approximate size of the synthesised NPs was calculated from the FESEM images. Although agglomerated particles are also visible in the image, the majority (~94%) of the formed NPs fall within the size range of 1–25 nm (Figures 8(a) and 8(b)).

Fig. 6.

UV-vis spectra for the Pd NPs synthesised from: (a) 2 M HCl extract; (b) aqua regia extract

UV-vis spectra for the Pd NPs synthesised from: (a) 2 M HCl extract; (b) aqua regia extract

Fig. 7.

FESEM images for Pd NPs synthesised from: (a) 2 M HCl extract; (b) aqua regia extract

FESEM images for Pd NPs synthesised from: (a) 2 M HCl extract; (b) aqua regia extract

Fig. 8.

Particle size distribution of Pd NPs synthesised from (a) 2 M HCl extract; (b) aqua regia extract. The inset shows the reaction mixture before and after the formation of Pd NPs

Particle size distribution of Pd NPs synthesised from (a) 2 M HCl extract; (b) aqua regia extract. The inset shows the reaction mixture before and after the formation of Pd NPs

The crystalline nature of the biosynthesised Pd NPs was confirmed by XRD analysis. The spectra are shown in Figure 9. The XRD pattern for Pd NPs prepared from the aqua regia extract shows five major diffraction peaks centred at 2θ 39.95°, 46.4°, 67.92°, 81.70° and 86.19°. The observation is similar for the Pd NPs prepared from the 2 M HCl extract with peaks centred at 2θ 40.07°, 46.65°, 68.03°, 82.02° and 86.00°. As mentioned in the previous section, the appearance of the five peaks represents the fcc structure of Pd. The results observed here are in agreement with previously published reports, thereby confirming a nanocrystalline form of Pd (6366). The crystallite size of the Pd NPs was calculated using the Scherrer equation (67). On the basis of the (111) peak, the average crystallite size of the Pd NPs prepared from aqua regia and 2 M HCl extract was calculated to be ~13 nm and ~23 nm respectively. The synthesised Pd NPs can be effectively used as a potential impregnant for the preparation of fresh Pd/C. Further studies on CO removal are being carried out with the Pd NPs impregnated Pd/C and shall be published later.

Fig. 9.

XRD diffraction pattern for Pd NPs synthesised from: (a) aqua regia extract; (b) 2 M HCl extract

XRD diffraction pattern for Pd NPs synthesised from: (a) aqua regia extract; (b) 2 M HCl extract

Economic Comparison

A tentative economic comparison was made for the three methods employed in this study towards the extraction of Pd from 1 kg of spent Pd/C. Three criteria to estimate economic viability were considered: materials, process and Pd recovery. The comparison method is summarised in the Supplementary Information (SI2c). Considering the cost of the chemicals involved in the extraction, the cost was slightly higher for the HCl-H2O2 and aqua regia methods compared to the 2 M HCl extraction route. However, the 2 M HCl extraction takes ~9 h for completion of the recovery process as compared to the other two methods which take ~4 h and ~5 h for the HCl-H2O2 and aqua regia methods respectively. It can thus be concluded from the material and process points of view that the three extraction routes are more or less similar, even though the HCl-H2O2 and aqua regia routes are slightly costly in terms of materials. However, the major cost incurred for Pd/C catalysts is the amount of Pd used for impregnation. As mentioned earlier, the percentage recovery of Pd was highest for the aqua regia based extraction method at ~96%, followed by the 2 M HCl (~73%) and HCl-H2O2 (56%) extraction methods. This suggests that the cost for the development of a fresh Pd/C with 6.5% Pd derived from extracted Pd could be greatly reduced by using the aqua regia route, with a ratio of ~1:7:11 corresponding to aqua regia, 2 M HCl and HCl-H2O2 extraction routes respectively. From a commercial perspective, the aqua regia based extraction procedure is an attractive method for the recovery of Pd as the method is economical, simple and highly effective.

Conclusion

Studies on Pd recovery from a spent activated carbon filter impregnated with Pd (Pd/C) were carried out. Recovery studies were performed using three acid or acid mixtures: 2 M HCl, HCl-H2O2 and aqua regia respectively. The absolute yield of the Pd obtained by different extraction methods showed a decreasing trend with aqua regia (~96%) > 2 M HCl (~73%) > HCl-H2O2 (~56%). Biosynthesis of Pd NPs from the recovered Pd was successfully carried out using green tea as the reducing agent. The synthesised Pd NPs had a crystallite size of ~13 nm (aqua regia extract) and ~23 nm (2 M HCl extract) and the particles were well separated. It is also clear from the XRD results that the synthesised Pd NPs had a fcc crystalline structure. The present study indicates that the recovered Pd could be reused successfully for the ‘green’ synthesis of Pd NPs which in turn can be effectively used for the development of fresh Pd/C for the removal of pollutants.

Further studies on impregnation and CO removal efficiency of the biosynthesised Pd NPs are being carried out in our laboratory. The authors are hopeful that with further optimisation of the experimental parameters, the synthesised NPs will have the potential to be reused for different applications.

Acknowledgement

The authors are grateful to Dr U. K. Singh, Director, DEBEL, for constant encouragement and support to carry out the work. The authors would also like to thank Dr Kadirvelu, Scientist, Centre for Life Sciences, Bharathiar University, India and D. R. Nayak, IISc, Bangalore for conducting the SEM-EDS.

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Supplementary Information

The Supplementary Information (PDF, 0.56 MB)  for this article can be downloaded here.

The Authors


Ansari Palliyarayil completed an MSc in Chemistry in 2010 at Kannur University, India. Presently he is a Senior Research Fellow at Defence Bioengineering and Electromedical Laboratory (DEBEL)-Defence Research and Development Organization (DRDO), India. His research interests include air quality management, catalysis, carbon based materials, adsorption, impregnation and nanoparticle synthesis and characterisation.


Kizhakoottu Kunjunny Jayakumar is a Technical Officer at DEBEL-DRDO. He has more than 10 years of experience in the field of air quality management, high energy materials, toxic emission studies and carbon based materials.


Dr Sanchita Sil is a Scientist at DEBEL-DRDO. She received her PhD in Chemistry in the area of Raman spectroscopy from the Indian Institute of Science, India. Prior to joining DEBEL, she was a Scientist at High Energy Materials Research Laboratory (HEMRL), India. She has more than 13 years of research experience. Her research interests include Raman spectroscopy, SERS, development of novel Raman based instrumentation towards detection of materials, air quality management, catalysis, carbon based materials, adsorption and nanoparticles.


Dr Nallaperumal Shunmuga Kumar is a Senior Scientist at DEBEL-DRDO, Bangalore. He received his PhD in Biomaterial Science and Technology in 1991 at Sree Chitra Tirunal Institute for Medical Sciences and Technology, India. He has more than 30 years of research experience in the field of air quality management, catalysis, adsorption, biomedical devices, polymers and development of biosensors.

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