Catalytic reduction in 4‐nitrophenol using Actinodaphne madraspatana Bedd leaves‐mediated palladium nanoparticles

Abstract

There is a growing demand for the development of non‐toxic, cost‐effective, and environmentally benign green synthetic strategy for the production of metal nanoparticles. Herein, the authors have reported Actinodaphne madraspatana Bedd (AMB) leaves as the bioreducing agent for the synthesis of palladium nanoparticles (PdNPs) and its catalytic activity was evaluated for the reduction of 4‐nitrophenol (4‐NP) to 4‐aminophenol with undisruptive effect on human health and environment. The broad and continuous absorbance spectrum obtained in the UV–visible region indicated the formation of PdNPs. The synthesized PdNPs were found to be crystalline, spherical, and quasi‐spherical in shape with an average particle size of 13 nm was confirmed by X‐ray diffractometer and transmission electron microscope. Fourier transform infrared spectra revealed the active photo constituents present in the aqueous extract of AMB involved in the bioreduction of palladium ions to PdNPs. The catalytic activity of biosynthesized PdNPs was demonstrated for the reduction of 4‐NP via electron‐relay process. Also, the influential parameters such as catalyst dosage, concentration of 4‐NP, and sodium borohydride were studied in detail. From the present study, PdNPs were found to be a potential nanocatalyst for nitro compound reduction and also for environmental remediation of wastewater effluents from industries.

1 Introduction

Palladium nanoparticles (PdNPs) are receiving a greater demand in recent years due to its diverse application in catalysis, sensors, fuel cells, hydrogen storage, and in the preparation of active membranes [1, 2, 3, 4]. Furthermore, PdNPs play a crucial role in catalysis, owing to their high surface–volume ratio, high surface energy, lower coordination of numbers, and increased surface atom [5, 6]. Till now, many conventional methods have been explored for synthesising PdNPs such as the wet chemical method, ultrasonic method, electrochemical method, and microwave‐polyol method [7, 8, 9, 10]. Unfortunately, most of these methodologies require operational conditions such as high temperature, pressure, energy, and utilisation of various organic solvents, stabilisers, toxic chemicals etc., which often have a major impact on human health and the environment [11, 12]. Therefore, to replace and minimise the hazards of conventional protocols, an eco‐friendly and non‐toxic methodology is required, at the earliest. Currently, the green chemistry method has received significant attention in the synthesis of nanomaterials as it is (i) environmentally safe, (ii) cost effective, and (iii) scalable for large production of nanoparticles [13, 14]. Owing to these advantages, the green technique is now focused by researchers on the biosynthesis of nanoparticles by exploiting various plants. In recent years, plants such as Eucommia ulmoides, Delonix regia, Colocasia esculenta, Gardenia jasminoides Ellis, and Gum olibanum biopolymer have been utilised for green synthesis of PdNPs, which were found to have a wide range of applications such as degradation of dyes, hydrogenation of aromatic nitro compounds, and coupling reactions [13, 15, 16, 17, 18]. However, very few reports are available on the catalytic activity of biosynthesised PdNPs towards reduction of 4‐nitrophenol (4‐NP) [6, 19, 20].

In the present investigation, Actinodaphne madraspatana Bedd (AMB) leaves were utilised for the synthesis of PdNP and catalytic potential of the nanoparticles was evaluated for the reduction in 4‐NP. AMB is commonly known as ‘Ray Laurel’. It is considered as one of the most important medicinal plants in the Indian medicine system. Actiodaphne madraspatana Bedd belongs to the family of medium‐sized evergreen trees: Lauraceae. Recent studies have demonstrated that AMB contains major phytoconstituents like flavonoids, tannins, saponins, terponoids, steroids, glycosides, alkaloids, and carbohydrates [21, 22, 23, 24]. It is widely used for wound and diabetic remediation and to cure fickle‐minded behaviour. Though the AMB is known for its various pharmacological activities, the richness in terms of phytoconstituents and its high abundance stimulated us to explore its efficacy towards the synthesis of PdNPs.

Hence, the present study is focused on the synthesis of AMB leaves‐mediated PdNPs. The catalytic activity of PdNPs was tested towards the reduction of 4‐NP in the presence of sodium borohydride, which is a universally accepted model reaction. The effect of influential parameters such as catalyst dosage, 4‐NP concentration, and NaBH₄ concentration was also investigated in detail.

2 Experimental

2.1 Materials

Palladium acetate Pd(OAC)_2, 4‐NP, and sodium hydroxide (NaOH) were purchased from Sisco Research Laboratory, India. Sodium borohydride (NaBH₄) and acetic acid were purchased from Merck, India. For all the experiments, solutions were prepared using deionised (DI) water. The leaves of AMB were picked from the Talakona Forest, Tirupathi, Andhra Pradesh, India.

2.2 Biosynthesis of PdNPs

The fresh AMB leaves were collected and washed thoroughly with DI water to remove dust and impurities deposited on the leaves. The cleaned leaves were shade dried for 5–7 days to remove moisture content from the leaves completely. Finally, the dried leaves were grounded into a fine powder and stored in an air‐tight container for further use. For the synthesis of PdNPs, different concentrations of AMB leaf extracts such as 1, 2, 3, 4, and 5% (w/v) were prepared separately by soaking 1, 2, 3, 4, and 5 g of the finely powdered AMB leaves in 100 ml of DI water for 10–20 min and further refluxed at 60°C for 30 min. The refluxed AMB leaf solution was cooled and then filtered through the Whatman filter paper. The filtered AMB aqueous extract of different concentrations was centrifuged further at 2000 rpm for 5–10 min to remove unwanted plant materials and was stored at 4°C.

To investigate the effect of AMB concentrations for the reduction of palladium ions, 1 ml of (1–5% (w/v)) concentration of AMB was mixed with 4 ml of 1 mM Pd(OAc)₂ solution separately. This reaction mixture was stirred at 400 rpm at 60°C for 10 min until the solution becomes dark brown, representing the formation of PdNPs. Also, the effect of Pd(OAc)₂ concentrations was studied by varying the concentration of Pd(OAc)₂ from 1 to 5 mM with 5% AMB separately. In the same way, the effect of pH was also studied by varying the pH (3, 6, 9, and 12) of 5% AMB by adjusting with HCl or NaOH and keeping the concentration of Pd(OAc)₂ as1 mM.

2.3 Characterisation techniques

The UV–visible absorption spectra of PdNPs and its catalytic activities were studied by UV–vis spectrophotometer (Jasco‐V670). The morphology of the PdNPs was recorded using high‐resolution transmission electron microscopy (HRTEM, FEI‐TECNAI G2‐20 TWIN) with an accelerating voltage of 200 kV. The selected area diffraction (SAED) patterns and lattice fringes of the PdNPs were obtained by the high‐resolution mode. X‐ray diffraction (XRD, BRUKER D8) analyser with monochromatic copper K _α radiation was used to determine the crystalline nature of the PdNPs. FTIR (SHIMADZU IR‐Affinity‐1) spectrophotometer was used to record the phytomoieties present in the AMB extract and also identify the role of phytomoieties in the synthesis of PdNPs. Zetasizer Nano ZS (Malvern) was used to measure the zeta potential of synthesised PdNPs.

2.4 Catalytic activity of PdNPs

The catalytic activity of synthesised PdNPs was monitored using UV–visible spectrophotometer for reduction of 4‐NP using sodium borohydride as a reducing agent in a quartz cuvette and scanned in the wavelength range of 200–500 nm. To evaluate the effect of various parameters such as palladium nanocatalyst dosage, concentration of 4‐NP, and concentration of NaBH₄ for reduction of 4‐NP to 4‐AP was studied as follows. Initially, the reaction was performed by varying the dosage of synthesised PdNPs (4–12 μl) as a catalyst for the reduction of 0.1 mM concentration of 4‐NP by keeping the concentration of NaBH₄ (5 mM) as constant and the change in absorbance values was observed. The obtained data were fit into pseudo‐first rate equation and the rate constant ‘k’ values were calculated. Similarly, the effect of concentration of 4‐NP (0.050–0.125 mM) in the reaction mixture was studied by keeping the concentration of NaBH₄ (5 mM) and palladium nanocatalyst (7 μl) as constant. In the same way, the effect of NaBH₄ concentration was examined by varying its concentration from 4 to 10 mM by keeping the concentration of 4‐NP (0.1 mM) and palladium nanocatalsyt (7 μl) as constant.

3 Results and discussion

3.1 Characterisation of the synthesised PdNPs

Fig. 1 a shows the absorption spectra of (i) Pd(OAc)₂, (ii) AMB extract, and (iii) synthesised PdNPs. From the figure, it is evident that the changes in absorbance intensities clearly indicated the formation of PdNPs. The peak at 280 nm of AMB extract originated from the polyphenols present in the extract [25, 26, 27]. The metal precursor Pd(OAc)₂ that showed a discrete absorption peak at around 400 nm was attributed mainly to the ligand–metal charge transfer of Pd²⁺ [13, 18, 28, 29, 30, 31]. Upon the reduction with AMB extract, the peak at 400 nm of palladium ions vanished and a broad continuous absorption band was obtained from 200 to 1400 nm in the UV–vis region (Fig. 1 a). Thus, the appearance of a strong and broad continuous absorption band indicated the formation of Pd⁰ nanoparticles during the bioreduction process [17]. Also the gradual increase in the intensity of PdNPs from the visible region to the UV region in the spectrum made more clear confirmation about the entire reduction of Pd²⁺ to Pd⁰ nanoparticles [13, 18, 19, 31]. In addition, there was no surface plasmon resonance band between 200 and 1400 nm which revealed that the produced PdNPs were in the range of around 10 nm in size [32]. These results are well matched with the earlier reports such as D. regia leaves and E. ulmoides bark extract‐mediated PdNPs [13, 17]. Furthermore, the role of AMB concentration for the reduction of 1 mM Pd(OAc)₂ was studied by varying the AMB concentration from 1 to 5% (Fig. 1 b). From Fig. 1 b, it was observed from the increase in absorption band intensity that the reduction in Pd²⁺ ions was enhanced by an increase in AMB concentrations. Hence, 5% AMB concentration was chosen to optimise the other parameters involved in the bioreduction process. Similarly, the bioreduction of Pd²⁺ ions with 5% AMB concentration was evaluated by varying (1–5 mM) the concentration of Pd(OAc)₂ as shown in Fig. 1 c. It was observed that the increase in intensity up to 3 mM concentration of metal precursor Pd(OAc)₂ was possibly due to the formation of a higher amount of nanoparticles. However, at higher concentrations [4 and 5 mM Pd(OAc)₂], the absorbance intensities slightly decreased due to insufficient availability of capping moieties on the surface of nanoparticles and the synthesised PdNPs tend to aggregate within a week. However, PdNPs synthesised at 2 and 3 mM concentrations of Pd(OAc)₂ showed maximum intensity of absorbance and also found to have a slight aggregation after a month (Fig. S1a, see Appendix). Therefore, PdNPs synthesised at 1 mM Pd(OAc)₂ concentration does not show any aggregation even after 1 month and this concentration was considered as ideal to study the pH effect in 5% AMB concentration. From Fig. 1 d, it was observed that the intensity of the absorption band increases slightly from pH 3 to pH 12 and was found more stable at an elevated pH (supported by zeta potential in Section 3.1). From the studies, it can be found that the optimum reaction conditions for the synthesise of PdNPs were 5% AMB with a 1 mM Pd(OAc)₂ solution at pH 12.

Fig. 1.

The absorption spectra of PdNPs

(a) absorption changes in (i) Pd (OAc)₂, (ii) AMB extract, and (iii) PdNPs after bioreduction, (b) by varying (1–5%) the concentrations of AMB with 1 mM Pd (OAc)₂, (c) by varying (1–5 mM) the concentrations of Pd (OAc)₂ with 5% AMB, (d) by varying pH (3–12) of 5% AMB with 1 mM Pd (OAc)₂

The particle size and morphology of the PdNPs synthesised using 5% AMB with 1 mM Pd(OAc)₂ at pH 12 was determined by TEM analysis. Fig. 2 apparently reveals that the PdNPs were spherical in shape and some of the nanoparticles were quasi‐spherical with nearly uniform particle size distribution. Also, the inter‐particle distance between the PdNPs was uniformly separated due to the AMB extract which also act as a stabiliser. The presence of a thin biolayer around the nanoparticles which can be observed from Fig. 2 apparently indicated that the phytomoieties of the AMB extract act as a capping layer and similar observation has been reported earlier [17, 33, 34]. From the TEM images, particle size histogram was plotted for >150 nanoparticles and demonstrated that the synthesised PdNPs were highly dispersed with the range of 6–22 nm in size and the average particle size was calculated to be 13 nm. The SAED pattern (Fig. 2 d) of the PdNPs exhibiting the concentric ring with intermittent bright spots, clearly substantiated the crystalline nature of synthesised nanoparticles which are consistent with the (111), (200), (220), and (311) lattice planes of fcc PdNPs. The d ‐spacing values obtained from the SAED pattern are 0.2249, 0.1976, 0.1403, and 0.1201 nm, respectively, revealed the fcc planes of PdNPs which are in good agreement with the earlier reported values [13, 31]. The twinned crystal structure was observed in most of the nanoparticles (Fig. 2) which is expected to improve the catalytic activity due to more active sites of the synthesised PdNPs [35, 36, 37].

Fig. 2.

TEM images of PdNPs

(a–c) different magnification with particle size distribution, (d) SAED pattern

The crystalline nature of the synthesised PdNPs was determined from the XRD pattern. From Fig. 3, the well defined as well as distinct diffraction peaks at 40.6°, 46.8°, and 68.1° were observed and indexed as (111), (200), and (220) reflection planes, respectively. From the diffraction peaks, it was evident that the synthesised PdNPs have a face‐centred cubic (fcc) crystal structure according to JCPDS file no: 46‐1043. The presence of some unidentified peaks in the X‐ray diffraction pattern in Fig. 3 suggests that the phytomoieties of the AMB extract crystallised as bio‐organic phase on the surface of the nanoparticles. Similar types of diffraction were inconsistent with previously reported plant‐mediated PdNPs [13, 18, 31, 38]. Furthermore, the intense and broadened diffraction peak at 2θ  = 40.6° indicated that the synthesised PdNPs are in the range of nanoregime [38]. Also, the average crystalline size was estimated from the full width at half maxima (β) of the diffraction peak (2θ  = 40.6) by using Debye–Scherrer's equation:

$$d=\frac{K\lambda }{\beta }\cos \theta$$

where ‘d’ is the crystalline size; λ is the X‐ray wavelength; β represents the full width at half maximum (FWHM) in radian; θ represents the diffraction angle; and K denotes the Scherrer constant (shape factor values from 0.9 to 1). The crystalline size of the synthesised nanoparticles was ∼13.8 nm which was in close agreement with TEM particle sizes.

Fig. 3.

XRD pattern of synthesised PdNPs

To identify the possible phytomoieties in the AMB extract and its essential role during the synthesis of PdNPs was revealed by FTIR spectroscopy. The significant major peaks at 3288, 1592, 1372, 1032, and 822 cm⁻¹ of AMB extract were observed (Fig. 4 a). The broader peak at 3288 cm⁻¹ represented O–H stretching vibration of phenolic compounds. The intense peak located at around 1592 cm⁻¹ belongs to C = O stretching of carbonyl/carboxylic groups present in flavonoids. The stretching vibrations present at 1372 and 1032 cm⁻¹ were associated with C‐OH band of carboxylic acid and ester bond in tannin, respectively. Also, the peak at 822 cm⁻¹ represented C–H vibration of aromatic groups in polyphenols. These major peaks indicated the presence of flavonoids, tannins, alkaloids, saponin, triterpenoids, steroids, glycosides, and carbohydrate compounds abundantly in the AMB extract and are evident by earlier reports [22, 23, 24]. During the bioreduction process, the appearance of a new peak at around 1700 cm⁻¹ (Fig. 4 b) might be ascribed as stretching vibration of carbonyl groups. Hence, it is speculated that the –OH functional group present in the phytomoieties of the AMB extract are oxidised into aldehydes/ketones during the formation of PdNPs and this exhibits a new peak at around 1700 cm⁻¹. Also, a much intense peak at 3288 cm⁻¹ for PdNPs was observed when compared with the AMB extract (Fig. S1b, see Appendix) and this could be due to the presence of water molecules during the bioreduction process [28, 39]. To confirm it, the biosynthesised PdNPs are kept in hot air oven for 48 h at 60°C to remove the moisture content and the FTIR spectrum was recorded. It was found that the intensity of the peak at 3288 cm⁻¹ was apparently reduced which indicates the removal of water molecules in PdNPs. Further, the FTIR spectrum of PdNPs (Fig. 4 b) showed significant changes in the position and intensity of the polyphenols/flavonoids functional peaks of AMB which apparently proved the involvement of its phytomoieties during the synthesis of PdNPs as a capping ligand and stabilising agent. This slight shift in the peaks indicated that the π ‐electrons present in the aromatic phytomoieties of AMB extract transferred to the free orbital of Pd⁺ ions by means of the red/ox process and gets reduced to Pd⁰ nanoparticles [25]. The obtained FTIR spectra results are in good concurrence with earlier reported plant‐mediated PdNPs [39, 40].

Fig. 4.

FTIR spectra of

(a) Dried powder of AMB leaves, (b) PdNPs

The stability nature of the synthesised PdNPs was measured from zeta potential values whose potential difference exists between the shear plane of the PdNP's surface and its dispersion medium. In this study, the zeta potential value of PdNPs synthesised at pH 12 was found to be −28.8 mV and a less negative zeta potential was observed for PdNPs synthesised at lower pH (Fig. S1c, see Appendix). Sathishkumar et al. [41] have reported a similar trend for the PdNPs synthesised by Cinnamomum zeylanicum bark extract. At elevated pH, more negative charges were available in the oxidised form of phytomoieties of AMB extract which adhered onto the surface of the PdNPs. This higher negative charge of the PdNPs at higher pH made the nanoparticles more stable in colloidal state and thereby reduces the agglomeration between nanoparticles [17, 42]. The synthesised PdNps was compared with earlier reported PdNPs [13, 17, 18, 19, 42] synthesised via green methods shown in Table 1 and found that the synthesised PdNPs are highly monodispersed, spherical in shape, with an improved zeta potential.

Table 1.

Comparison of zeta potential and average particle size of PdNPs synthesised using various bioreducing agents

Bioreducing agent	pH during synthesis	Zeta potential, mv	average particle size, nm	Reference
Delonix regia leaf	3	−23.8	2–4	Dauthal and Mukhopadhyay [17]
Eucommia ulmoides bark	6	−25.3	12.2 (spherical, quasi‐spherical)	Duan et al. [13]
Gum olibanum, bioploymer	—	$8.4\pm 1.6$	$6.6\pm 1.5$ (spherical)	Kora and Rastogi [18]
Piper betle L. leaf	—	−18.4	$3.8\pm 0.2$ (spherical, monodispersity)	Rajasekharreddy and Rani [42]
Cochlospermum gossypium, biopolymer	7	$-22\pm 3.8$	$6.5\pm 2.3$ (spherical)	Rastogi et al. [19]
Actinodaphne madraspatna Bedd leaf	12	−28.8	13 (spherical, quasi‐spherical, twinned crystals monodispersity)	present study

The possible mechanism for the formation of PdNPs using AMB extract as a bioreducing agent is illustrated in Fig. 5. On the basis of the bioreduction mechanism involved in the synthesis of PdNPs by various plant phenolic moieties reported by Yang et al. and Atarod et al. [28, 40], we proposed the mechanism that the hydroxyl groups of phenolic compounds present in the AMB extract has the capability to form the complex with palladium ion as an intermediate in the reduction process. Finally, the palladium ions are reduced to PdNPs (Pd⁰) with concomitant oxidation of phenolic compounds [17]. Here, we represented the schematic diagram (Fig. S1d, see Appendix) to provide a clear insight for the role of AMB leaves as a bioreducing agent for nanoparticle synthesis and PdNPs as catalyst by using NaBH₄ as a reducing agent for the conversion of 4‐NP to 4‐aminophenol (4‐AP) in the present study.

Fig. 5.

Possible mechanism for the reduction of palladium ions into PdNPs

3.2 Catalytic activity of PdNPs

The efficacy of biosynthesised PdNPs towards the catalytic reduction in 4‐NP to 4‐AP was investigated using UV–visible spectroscopy. The 4‐AP is a potent intermediate in the pharmaceutical drug synthesis, in the preparation of polymers, corrosion inhibiter, hair dyes, and rubber products. Hence, 4‐NP being a familiar precursor material for 4‐AP and in the reduction of 4‐NP is always in demand over the decades. In the present work, the cost‐effective method was carried out for reduction reaction of 4‐NP in the presence of PdNPs as a catalyst using NaBH₄ as the reducing agent. During the reduction process, the absorbance peak of 4‐NP at 317 nm was shifted to 400 nm due to 4‐nitrophenolate formation in the presence of excess alkaline NaBH₄ (Fig. 6). Nevertheless, the peak at 400 nm remaing unchanged even after a day implies that the reduction in 4‐NP did not proceed in the absence of a catalyst [43]. This is due to the fact that the 4‐NP reduction process is thermodynamically favourable but kinetically hindered. To overcome this, the introduction of synthesised PdNPs (redox metal catalysts) in the reaction system lowers the activation energy that exists between the NaBH₄ and 4‐NP and the reaction proceeds via successive decrease in the intensity of peak at 400 nm of 4‐NP with the appearance of a new peak at 295 nm indicating clearly the formation of 4‐AP successfully (Fig. 7).

Fig. 6.

UV–vis absorption spectrum of

(a) 4‐NP before, (b) After addition of NaBH₄

Fig. 7.

Successive UV–vis absorption spectra for the reduction of 0.1 mM 4‐NP with 5 mM NaBH₄ catalysed by PdNPs

The 4‐NP reduction reaction follows the pseudo‐first‐order kinetics and the rate equation is expressed as

$$\ln \left(\frac{A_{\mathrm{t}}}{A_0}\right)=-kt$$ (1)

Here, A_t is the absorbance of 4‐NP at time ‘t’, A ₀ the initial absorbance of 4‐NP, and ‘k’ is the rate constant. The observed rate constant ‘k’ also depends on the available surface area of the PdNPs and expressed as $k=(k_{1}S)$ according to the Langmiur–Hinshelwood mechanism where ‘k ₁’ is the rate constant normalised to ‘S’ and S is the surface area of PdNPs normalised to the unit volume of the system. The influential key parameters like catalyst dosage, 4‐NP, and NaBH₄ concentration were studied and the obtained kinetic data were evaluated in detail by plotting ln (A _t /A ₀) versus time ‘t’.

3.2.1 Effect of catalyst dosage

To investigate the effect of biosynthesised PdNPs as nanocatalysts towards the reduction in 4‐NP, the reaction was carried out with various dosages (4–12 µl) of PdNPs, while other parameters such as concentration of 4‐NP (0.1 mM) and NaBH₄ (5 mM) were kept constant. During the reduction in 4‐NP, the decrease in absorbance values of 4‐NP with time was noted. From the absorbance values, ln (A _t /A ₀) versus time ‘t’ was plotted for various dosages of PdNPs shown in Fig. S2 (see Appendix) and rate constant ‘k’ values were calculated from its concerned slope. In this case, ‘k’ values for the reduction of 4‐NP was found to increase with the dosage of PdNPs nanocatalyst is clearly picturised in Fig. 8 a and is also listed in Table 2. The higher dosage of PdNPs (12 µl) results in more number of nanoparticles present in the reaction system with surplus surface‐to‐volume ratio and availability of lot of active sites for the reactants (BH₄ ⁻ and 4‐NP) to be adsorbed easily onto the surface of the nanocatalyst followed by the faster relay of electrons between BH₄ ⁻ and 4‐NP. Therefore, the reduction of 4‐NP occurs fast with the higher rate constant ‘k’. In the case of lower dosage of PdNPs (4 µl), the rate constant ‘k’ is found to be lesser due to the lesser number of nanoparticles availability for the reactants to adsorb which lead to a competition between the reactants on the catalyst surface and such a hindrance would consequently result in slow reduction rate [44, 45].

Fig. 8.

Dependence of rate constant ‘k’ on the concentration of

(a) Catalysts, (b) 4‐NP, (c) Sodium borohydride

Table 2.

Effect of PdNPs for the reduction of 0.1 mM 4‐NP with 5 mM NaBH₄ and calculated rate constant ‘k’ values

PdNPs, µl	Calculated rate constant ‘k’, 10−3 s−1
4	3.59
6	9.48
8	11.27
10	30.84
12	33.38

3.2.2 Effect of 4‐NP concentration

We further want to explore the effect of 4‐NP concentration (0.050 to 0.125 mM) in the reduction reaction by keeping the concentration of NaBH₄ (5 mM) and PdNPs (7 µl) as constant. Fig. S3 (see Appendix) depicts the plot of ln (A_t /A ₀) versus time ‘t’ for the various concentrations of 4‐NP and the rate constant ‘k’ was calculated from its slope. The dependence of the rate constant ‘k’ with 4‐NP concentration in Fig. 8 b provides clear insight to the fact that the ‘k’ value decreases remarkably with higher concentrations of 4‐NP and is summarised in Table 3. It is found that higher concentrations of 4‐NP have a greater affinity to adsorb onto the surface of PdNPs compared with BH₄ ⁻ ions results in less occupancy sites of BH₄ ⁻ on the surface of PdNPs, which leads to competition between the two reactants (4‐NP and BH₄ ⁻) to occupy the surface of the PdNP nanocatalysts. As per the Langmiur–Hinshelwood model, efficient catalytic reduction through the electron relay process is only feasible if both the reactants are adsorbed onto the catalyst surface in nearly equal amounts. Therefore, poor electron relay between the reactants was observed with a retarded reduction rate at higher concentrations of 4‐NP. In the case of lesser concentration of 4‐NP, the competition between 4‐NP and BH₄ ⁻ would be less and accordingly a faster reaction is obtained with higher rate constant ‘k’ values. Kalekar et al. [46] have also obtained the same trend for the reduction of 4‐NP using chemically synthesised porous palladium nanostructures as catalyst.

Table 3.

Effect of 4‐NP concentration for the reduction reaction with (5 mM) NaBH₄ by PdNPs and the calculated rate constant ‘k’ values

4‐NP concentration, mM	Calculated rate constant ‘k’, 10−3 s−1
0.050	11.56
0.075	10.83
0.100	8.95
0.125	8.62

3.2.3 Effect of NaBH₄ concentration

Similarly, the effect of NaBH₄ concentration (4–10 mM) for the 4‐NP reduction was also studied by keeping the concentration of 4‐NP (0.1 mM) and PdNPs (7 µl) constant. From the absorbance values of 4‐NP during the reduction reaction, ln(A_t /A ₀) versus time ‘t’ was plotted for the various concentrations of NaBH₄ as shown in Fig. S4 (see Appendix) and its rate constant ‘k’ values were calculated from its slope. It was found that the rate constant ‘k’ values increased with an increase in the concentration of NaBH₄ (Fig. 8 c) and tabulated in Table 4. At lesser concentrations of NaBH₄, only a less amount of BH₄ ⁻ ions have been generated and it supplies lesser electrons to 4‐NP which is adsorbed onto the surface of PdNP nanocatalysts, resulting in a slower electron relay process. Hence, the reduction occurs in a slow manner with lower rate constant ‘k’ values. In the case of higher concentrations of NaBH₄, the rate constant ‘k’ values were found to increase, suggesting that more BH₄ ⁻ ions are produced which is able to supply electrons instantly to 4‐NP resulting in rapid reduction rate with faster electron relay process.

Table 4.

Effect of NaBH₄ concentration for the reduction of 0.1 mM 4‐NP with PdNPs and the calculated rate constant ‘k’ values

NaBH4 concentration, mM	Calculated rate constant ‘k’, 10−3 s−1
4	10.27
6	13.67
8	20.91
10	22.83

3.2.4 Mechanism for catalytic activity of PdNPs

The possible mechanism for the catalytic efficacy of PdNPs towards the reduction of 4‐NP is illustrated in Fig. 9. In this mechanistic study, it is apparently clear that the large kinetic barrier between donor (NaBH₄) to acceptor (4‐NP) reactant molecules results in reduction of 4‐NP which is kinetically hindered. Generally, the introduction of metal nanoparticles (having an intermediate redox potential compared with both the reactants) as catalyst in the reaction system have the capability to lower the kinetic barrier between the reactants and facilitate the electron relay process from NaBH₄ to 4‐NP by the adsorption of both the reactants onto the surface of PdNPs [1]. In the present work, the synthesised PdNPs with an average particle size of13 nm acts as a nanocatalyst in the reduction reaction of 4‐NP bringing down the kinetic barrier between the two reactants as shown in Fig. 9. Therefore, BH₄ ⁻ ions instigated to transfer the electron onto the surface of the PdNPs and further electrons were relayed to the 4‐NP molecules and finally gets reduced to 4‐AP by means of oxidation–reduction reaction [47]. This proposed mechanism suggested that the entire reduction process of 4‐NP mainly depends on the size and surface–volume ratio of PdNPs. Also, the catalytic activity of synthesised PdNPs was compared with the earlier reported biosynthesised PdNPs [17, 19, 20, 31] and found that the obtained rate constant ‘k’ value was higher than the other catalyst (Table 5). Hence, it is found that the catalytic efficacy of the biosynthesised PdNPs is directly related to the particle size which is considered as a critical factor towards the catalytic reduction of 4‐NP.

Fig. 9.

Schematic representation for catalytic performance of the synthesised PdNPs for the reduction in 4‐NP

Table 5.

Comparison of rate constant ‘k’ for the reduction of 4‐NP by the PdNPs synthesised using various bioreducing agents

Catalyst	Calculated rate constant ‘k’, min−1	Approximate time taken for completion of reaction, min	Reference
Actinodaphne madraspatana Bedd mediated PdNPs	2.00	2.5 min	present work
Delonix regia ‐mediated PdNPs	0.25	—	Dauthal and Mukhopadhyay [17]
Anogeissus latifolia ‐mediated PdNPs	0.11	—	Kora and Rastogi [31]
Xanthan gum‐mediated PdNPs	0.18	12 min	Venkatesham et al. [20]
Cochlospermum gossypium‐ mediated PdNPs	0.18	—	Rastogi et al. [19]

4 Conclusion

An environmentally benign protocol was demonstrated for the synthesis of PdNPs using AMB leaves as a reducing agent. The synthesised PdNPs are found to be spherical/quasi‐spherical in shape with twinned sites which helped to improve the catalytic activities. Also, PdNPs are more stable at elevated pH due to the presence of more negatively charged phytomoieties of AMB extract adhered onto the surface of the PdNPs as a capping agent as well as a stabiliser. The AMB‐capped PdNPs significantly reduces the agglomeration and clumping between the nanoparticles. Furthermore, the AMB‐stabilised PdNPs demonstrated remarkable catalytic activity towards reduction in 4‐NP via the electron relay process. The various influential parameter studies showed that the rate constant ‘k’ increases with PdNP dosage and NaBH₄ concentration and gets decreased with 4‐NP concentration which clearly presented the role of PdNPs as a nanocatalyst. Overall, this synthesised PdNPs has demonstrated to be an effective and alternative nanocatalyst for environmental remediation of toxic anthropogenic chemicals and organic pollutants present in industrial wastewater and also prompted use in the field of medicine and sensors.

Images of biosynthesised PdNPs, FTIR spectra of PdNPs before and after being dried at 60°C, zeta potential measurements, schematic representation of AMB‐capped PdNPs synthesis and its activity, and also the plots of ln(At /A 0) versus time by varying concentrations of PdNPs, 4‐NP, and NaBH4 are provided in this section.

Fig. S1a.

Freshly biosynthesised PdNPs and biosynthesised PdNPs after a month

Fig. S1b.

FTIR spectra of PdNPs and PdNPs dried after 60°C for 48 h

Fig. S1c.

Zeta potential of synthesised PdNPs at

(a) pH 12, (b) pH 9, (c) pH 6 and (d) pH 3

Fig. S1d.

Schematic representation for AMB capped PdNPs synthesis and its activity

Fig. S2.

Plot of ln(A_t /A₀) versus time ‘t’ for the reduction of 0.1 mM 4‐NP with varying amount of PdNPs

Fig. S3.

Plot of ln(A_t /A₀) versus time ‘t’ for the reduction of different concentration of 4‐NP by 5 mM of NaBH4with PdNPs

Fig. S4.

Plot of ln(A_t /A₀) versus time ‘t’ for the reduction of 0.1 mM 4‐NP with varying amount of concentration of NaBH4