Catalytic activity of bare and porous palladium nanostructures in the reduction of 4-nitrophenol

Abstract

The catalytic activity of bare and porous palladium nanostructures viz. palladium nanoballs (PdNBs) and palladium urchins (Pdurc) synthesized in surfactant based liquid crystalline mesophase have been investigated in the reduction of 4-nitrophenol. The loading of PdNBs and Pdurc nanocatalyst in the reaction are optimized to 1.57 × 10⁻⁴ mol% and 1.57 × 10⁻³ mol% respectively. The surface area normalized rate constant obtained at 303 K for PdNBs (0.74 ± 0.03) s⁻¹ m⁻² L and Pdurc (0.41 ± 0.05) s⁻¹ m⁻² L is the highest, considering the low palladium loading used in the reaction. The specific surface area determined for the PdNBs and Pdurc are 153 m² g⁻¹ and 27 m² g⁻¹ respectively. The high surface area of PdNBs nanostructure is consistent with the highest catalytic activity. The palladium nanostructure catalyzed reaction lacks induction time (t₀), indicating the influence of porosity in the Langmuir–Hinshelwood mechanism. Besides, PdNBs and Pdurc exhibited thermal stability and structural integrity with good conversion of the substrate (>97%) even up to five consecutive reactions. The mechanistic insight of the bare and porous palladium nanostructures presented in this study is significant for the development of stable and efficient palladium metal based nanocatalyst.

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1. Introduction

Studies on metal nanostructures and their composites in the field of catalysis are focused on developing a durable and efficient heterogeneous catalyst.¹ Research on morphology controlled supported metal nanostructures are gaining momentum in catalysis science owing to their morphological dependent properties.^2,3 In most of the cases, nanocatalyst or their composites contain noble metals such as platinum and palladium which are efficient but expensive. Amongst these noble metals, palladium (Pd) and its composites is one of the widely investigated nanomaterials for catalytic applications.^4–11 These literatures mainly concerns with the use of supported Pd nanocatalyst, which are easier to separate from reaction mixture. However, there is an inherent disadvantage of low efficiency, low selectivity and less reproducibility due to variation in the support used during the synthesis and stabilization materials used as well. Further, the support may also influence the catalytic reaction by altering the homogeneity and diffusibility of the substrates on the surface of the nanocatalyst.¹² Thus, research in nanocatalysis is focused to address and overcome these disadvantages by developing different route to synthesize metal nanostructures for better efficiency, selectivity, reproducibility and in addition rigidity.¹³ Most of the supported Pd nanostructured catalyst contains a Pd loading of >5 mol% in the catalytic activity measurement which is quite high as the catalytic activity is proportional to the catalyst concentration.^14,15 But for a cost effective and practical approach, higher catalytic activity with low catalyst loading is desirable for noble metal catalyst. The literatures available so far have focused on reducing the cost of the catalyst by lowering the Pd contents or replacement of the Pd metal itself.^16,17 Alternatively, these objectives can also be achieved by designing and synthesis of bare and porous Pd nanostructures with larger surface area which will lower the catalyst loading without compromising with the catalytic activity.

It is well documented that, supported Pd nanostructures with different morphologies can be used effectively in the catalytic reduction and electro catalytic applications.^15,18–21 The synthesis of porous nanostructures with significantly high surface area possesses a major challenge due to the stability issues. The stability of porous nanostructures is dependent on its surface energy which is inversely proportional to the size of the nanostructures leading to aggregation.²² The aggregation of the nanostructures significantly reduces the surface area resulting in lowering number of active sites and hence decrease in the catalytic activity of the nanostructures. Therefore, for genuine technological applications, the insertion and removal of templates (hard/soft) for stability of Pd nanostructures draws wide attention to the material chemists.^15,18–22

Of these, the soft templates offer a better alternative to its counterpart (hard templates) which is difficult to remove from the surface of the nanostructures. The surfactant based swollen liquid crystalline mesophases (SLCs) are powerful supramolecular self assemblies that provides soft template nanoreactors for synthesizing porous nanostructures into a specific phase having a well-defined microscopic orientation.^23–26 The metal doped SLCs have demonstrated to be a feasible nanoreactor for morphology controlled synthesis of nanostructures by radiolysis with robust reproducibility.²⁷ The bare and porous Pd nanostructures synthesized in SLCs have shown good electrocatalytic efficiency in ethanol electro-oxidation²⁴ and hydrogen storage property.²⁸ However, a systematic study of these bare and porous Pd nanostructures with respect to the catalysis, kinetics, thermal stability and structural rigidity as a catalyst is lacking.

4-Nitrophenol (4-NP) is highly toxic, non-biodegradable compound present in the industrial effluent that can be converted to 4-aminophenol (4-AP) by sodium borohydride (NaBH₄) in the presence of catalyst.²⁹ The mild reaction conditions and ease in monitoring the reaction by UV-Vis spectrophotometer in aqueous medium favors the 4-NP reduction reaction as a model reaction for the evaluation of the catalytic activity of nanostructures.^30–32 The kinetics of Pd nanoparticles supported in polymeric materials, spherical polyelectrolyte brushes and thermo sensitive microgels shows a consistent induction time (t₀) owing to the presence of dissolved oxygen in the system.³⁰ The mechanism and the possibility of mass transfer for these solid Pd nanostructures have been recently reported experimentally³³ and theoretically.³⁴ Similar mechanism has been also deduced for solid colloidal silver nanoparticles.³⁵ However, to the best knowledge of the authors, no literature report is available on the mechanistic details of bare and porous Pd nanostructures which is free of any stabilizers, as catalyst in the reduction of 4-NP by NaBH₄. The catalytic activity of bare and porous platinum nanostructures as catalyst has been reported by our group.²⁷

We report here the catalytic activity of bare and porous PdNBs and Pdurc using the model reaction of 4-NP reduction by sodium borohydride. The mechanism is significant as it provides information on the role of porosity in eliminating the induction time, enhanced catalytic activity and stability of the synthesized PdNBs and Pdurc nanostructures.

2. Experimental section

2.1 Chemicals

Tetraamminepalladium(II) chloride (98%) was purchased from Sigma Aldrich. Cetyltrimethylammonium bromide (CTAB) 99%, cetylpyridiniumchloride monohydrate (CPCl) 99% from SD-Fine Chemicals, 4-NP (99% purity) cyclohexane (99%) from SRL, 1-pentanol (99%) from Spectrochem, propan-2-ol (99.7%), NaBH₄ (97%) from Fisher scientific were purchased. Nitrogen gas with purity 99.995% was purchased from Inox air products. All the materials were used as received. Deionized water from Milli-Q system with resistance 18.2 MΩ M⁻¹ cm⁻¹ was used for preparation of all the solutions.

2.2 Synthesis of palladium nanostructures

The Pd doped SLCs were synthesized by modifying the method reported earlier using CTAB and Pd (NH₃)₄Cl₂ metal precursor.²⁴ Briefly, 1.03 g of CTAB was dissolved in 2 mL Milli Q water along with 0.1 M Pd precursor. Further 2.98 mL of cyclohexane was added to obtain a white microemulsion which was then vortex by adding 1-pentanol drop wise from the side of the glass tube. Finally, stable and perfectly transparent Pd doped SLCs were formed. To eliminate any dissolved oxygen present in the system, the prepared SLCs were kept under nitrogen bubbling for 30 minutes prior to irradiation. Similarly another nanostructures were synthesize by dissolving 0.1 M Pd(NH₃)₄Cl₂ in 1.03 g CPCl completely. In this micellar solution, 0.015 μL propan-2-ol (final concentration of solution 0.1 M) was added to scavenge the ˙OH radicals.²⁸ The oxygen interference was removed by bubbling nitrogen gas for 30 minutes before irradiation. The SLCs and micellar solution of Pd was irradiated in the same glass tubes sealed with silicon septum. The stable, perfectly transparent SLCs and micellar solution were exposed to γ radiation using a ⁶⁰Co γ source facility at Department of Chemistry, Savitribai Phule Pune University, Pune. The dose rate determined using Fricke dosimetry was found to be 3.6 kGy h⁻¹. After 80 kGy of absorbed dose, the Pd nanostructures formed were destabilized by propan-2-ol. The suspended Pd nanostructures were centrifuged and the supernatant was removed by decantation. The residues were washed several times with propan-2-ol and water to remove any CTAB or CPCl traces (nearly 2 liters of propan-2-ol taking small quantity at a time). The isolated Pd nanostructures were dried in oven at 60 °C for 12 hours prior to characterization and determination of the catalytic activity.

2.3 Characterization

The UV-Vis absorption spectra of Pd precursor, PdNBs, Pdurc were recorded using a Shimadzu UV1800 in quartz cuvettes. Transmission electron microscopy (TEM) analysis was performed using TECNAI-G2 20 ULTRA-TWIN FEI with an accelerating voltage up to 200 kV. A FEI TECNAI 3010 transmission electron microscope operating at 300 kV (resolution 1.7 Å) was used for high resolution-transmission electron microscopy (HR-TEM) measurements. The Pd nanostructures were sonicated using Oscar ultrasonicator prior to deposition on a carbon coated copper grid having 200 mesh size. X-ray diffraction (XRD) of the dried Pd nanostructures were recorded using PANalytical X'pert Pro diffractometer with a CuKα (1.5418 Å) line. The voltage and current applied to X-ray anode was 45 kV and 40 mA respectively. The nitrogen adsorption isotherms at 77 K were measured on a Quantachrome Autosorb iQ gas analyzer. The samples were rinsed with diethyl ether, heated in an oven at 358 K in order to remove the excess of diethyl ether and were then dried at 323 K under vacuum for 5 hours for the BET measurements. The BET surface area was calculated using the [0.05–0.30] relative pressure range, the correlation coefficient being 0.999 or better.

2.4 Catalytic reduction of 4-nitrophenol (4-NP)

The catalytic reduction of 4-NP by NaBH₄ in presence of PdNBs/Pdurc was performed as follows: 1 × 10⁻⁴ M of 4-NP (1.5 mL) and 5 × 10⁻² M NaBH₄ (1.0 mL, ice cold) were mixed and stirred for 1 min in a quartz cuvette. To this mixture, 1 mg L⁻¹ of PdNBs or 10 mg L⁻¹ of Pdurc (0.5 mL) were added and the reaction was monitored by observing the change in absorbance of 4-nitro phenolate anion absorbing at 400 nm. The Pd nanostructure concentration in the reaction mixture was calculated to be 0.166 mg L⁻¹ (1.57 × 10⁻⁴ mol%) and 1.66 mg L⁻¹ (1.57 × 10⁻³ mol%) respectively. The absorption spectra were recorded within the wavelength range of 200–600 nm. For uncatalyzed reaction, instead of PdNBs/Pdurc 0.5 mL of Milli Q water was added and spectra were recorded at the same wavelength.

2.5 Kinetic analysis

The kinetic analysis was performed using Langmuir–Hinshelwood mechanism assuming both the reactants adsorbed on the surface of PdNBs and Pdurc. Interaction of borohydride ions at these nanostructure surface is quite critical and involves multiple steps which ultimately leads to the liberation of hydrogen.³³ The steps involved during the reduction of 4-NP by NaBH₄ in the presence of Pd catalyst is represented in ESI Scheme S1.† Reactions 1 and 2 of Scheme S1† denote the adsorption of 4-NP and BH₄⁻ on the surface of the nanocatalyst. A pseudofirst-order condition with respect to BH₄⁻ is maintained throughout the experiment. The adsorbed BH₄⁻ then reacts with water on the surface of catalyst producing hydrogen as a reducing source (step 3). In step 4, subsequent addition of 4-NP^*_ad and H^*_ad takes place, forming 4-AP at the surface of the Pd nanocatalyst. Desorption of 4-AP (step 5) from the surface of the Pd nanocatalyst is the rate determining step in the reaction proposed for the catalytic mechanism of Pd nanocatalyst.

The general rate law for the 4-NP reduction by NaBH₄ is given by the eqn (1),

The apparent pseudofirst-order rate constants (k_app) were determined using a classical Langmuir–Hinshelwood equation which is deduced as reported earlier for platinum nanostructures²⁷ expressed by eqn (2).

where C₀ and C are the initial and final concentrations having equivalent in terms of absorbance (A₀ and A_t respectively) was monitored at a fixed wavelength (λ) at time t. k_app is the apparent pseudofirst-order rate constant assumed to be proportional to the accessible surface area (S) for the PdNBs and Pdurc. The surface area normalized rate constant k, is calculated by using k = k_app/S. Therefore, for constant catalyst concentration and uniform accessible surface area of Pd nanostructures, a plot of ln(A₀/A_t) with respect to time gives a straight line whose slope is k_app. The kinetic analysis was performed by varying one of the reactant concentration while keeping other reactant constant to check any deviation in the reaction mechanism. The k_app were also determined at temperature ranging from 293–333 K and activation energy (E_a) of the catalytic reduction by these Pd nanostructures was calculated employing eqn (3).where, k_app is the apparent pseudofirst-order rate constant at temperature T, A is the frequency factor, E_a is the activation energy and R is the universal gas constant (8.314 J K⁻¹ mol⁻¹).

The mol% of both Pd nanocatalyst was calculated based on the actual concentration of catalyst used in the reaction using eqn (4).

where, C is concentration of Pd nanocatalyst; M is molecular weight of Pd metal.

2.6 Recycling studies

The recycling study of Pd nanostructures were carried out by mixing 3 × 10⁻³ M of 4-NP (0.1 mL) and 2.5 mL milli Q water, followed by the addition of ice cold 1 × 10⁻¹ M NaBH₄ (0.1 mL). The mixture was stirred for 1 minute in a quartz cuvette. To this mixture, 1 mg L⁻¹ of PdNBs or 10 mg L⁻¹ of Pdurc (0.3 mL) equivalent to 0.1 mg L⁻¹ (9.4 × 10⁻⁵ mol%) and 1 mg L⁻¹ (9.4 × 10⁻⁴ mol%) were added and the reaction was monitored by observing the change in the absorbance of 4-nitro phenolate anion at 400 nm as a function of time. After the completion of the first cycle of the reaction, 0.1 mL each of 3 × 10⁻³ M of 4-NP and 1 × 10⁻¹ M ice cold solution of NaBH₄ was added to the reaction mixture in a sequential manner as in the first cycle without adding Pd nanostructures and monitored till the second reaction cycle completes. Similar procedure was followed for the remaining cycles.

3. Results and discussion

3.1 Morphology of palladium nanostructures

TEM images of the synthesized Pd nanostructures in SLCs are depicted in Fig. 1, panel (a and b). The images show a three-dimensionally (3D) interconnected Pd nanowires forming hexagonal shaped cells of 13–15 nm diameter which are interlinked into ball shaped nanostructures (PdNBs) of size ranging from 50–90 nm.

Fig. 1 TEM image of PdNBs synthesized in SLCs after 80 kGy of γ irradiation (panel (a and b)); panel (a), inset is the selected area electron diffraction (SAED) pattern. (c) HR-TEM image of PdNBs showing a continuous lattice fringes with d spacing 0.238 nm. Dose rate = 3.6 kGy h⁻¹.

These 3D nanowires have average diameter of about 3–4 nm, consistent with the water channel thickness between the two cyclohexane cylinders of hexagonal SLCs.²⁴ The HR-TEM image of PdNBs before the catalytic reaction (Fig. 1c) shows continuous lattice fringes with 0.238 nm which represents the d spacing. This d spacing of the crystalline domain corresponds to the d(111) plane of fcc Pd.

Similarly, the TEM images of Pd nanostructure synthesized in CPCl reveals urchin shaped nanostructure with arms growing away 3-dimensionally from the center (Fig. 2a and b). The diameter of these Pdurc ranges from 40–80 nm while the individual nanowires has diameter of about 3 nm. The HR-TEM image showing d spacing 0.238 nm corresponds to (111) plane of crystalline Pd.

Fig. 2 TEM images of Pdurc after 80 kGy of γ irradiation synthesized in CPCl (panel (a and b)); panel (a), inset is the selected area electron diffraction (SAED) pattern. (c) HR-TEM image showing continuous lattice fringes with d spacing of 0.238 nm. Dose rate = 3.6 kGy h⁻¹.

The UV-Vis absorption spectra for the Pd salts before irradiation, PdNBs and Pdurc after 80 kGy of γ irradiation are shown in Fig. 1S.† The spectral features suggest the reduction of the metal precursor upon radiolysis. The crystalline nature of PdNBs and Pdurc were further confirmed by the XRD analysis which complements the ridges observed in the HR-TEM images. The XRD pattern of the prepared PdNBs show 2θ peaks at 40.13°, 46.96° and 68.13° while for Pdurc show 2θ peaks at 39.93°, 46.81° and 68.01° respectively. These diffraction patterns were correlated to the diffractions from (111), (200) and (220) planes of crystalline Pd (Fig. 2S,† left panel) suggesting the crystallinity of PdNBs and Pdurc respectively.

In order to estimate the total surface area of the Pd nanostructures, BET measurements by nitrogen adsorption isotherms at 77 K were performed on the PdNBs (black color) and Pdurc (lime color). The adsorption isotherms at 77 K for PdNBs and Pdurc; before and after 4-NP reduction reaction are depicted in Fig. 3. Both, PdNBs and Pdurc represent type II isotherms, characteristic of macroporous (or non-porous) materials. The total surface area for PdNBs was calculated to be 86 m² g⁻¹ while that for the Pdurc was 15 m² g⁻¹. Comparing these Pd nanostructures with silica on a molar ratio basis (SiO₂, mol. wt = 60 g, and Pd, mol. wt = 106.42 g), the value would corresponds to specific surface area of 153 m² g⁻¹ and 27 m² g⁻¹, respectively. The surface area obtained for the PdNBs nanostructures are the highest of any bare Pd nanomaterials so far reported in the literature. It is worth to note that these nanostructures were washed several times with propan-2-ol and water before characterization.

Fig. 3 Nitrogen adsorption isotherms at 77 K for PdNBs before reaction (black line), PdNBs after 4-NP reduction reaction (red line), Pdurc (lime line), Pdurc after 4-NP reduction reaction (pink line).

3.2 Kinetics of catalytic reduction 4-nitrophenol (4-NP)

The general model reaction of 4-NP to 4-AP reduction by NaBH₄ is used for testing the catalytic activity of PdNBs and Pdurc in aqueous medium. The high concentration of NaBH₄ used in this reaction maintains a pseudofirst-order condition with respect to 4-nitrophenolate anion absorbing at 400 nm. The decrease in the absorbance of 4-nitrophenolate anion is followed by monitoring the absorbance at 400 nm. The change in absorbance with time after the addition of 0.5 mL of 0.166 mg L⁻¹ PdNBs and/or 1.66 mg L⁻¹ Pdurc is shown in Fig. 4a and b respectively. However, in the absence of PdNBs and/or Pdurc almost negligible change is observed at 400 nm (Fig. 4c) suggesting that reaction takes place only in presence of PdNBs and/or Pdurc nanostructures.

Fig. 4 The plots of absorbance as a function of wavelength at 303 K at regular time intervals indicates the disappearance of the peak of 4-nitrophenolate anion at 400 nm due to reduction of 4-NP to 4-AP by NaBH₄ (a) in presence of PdNBs; inset – linear plots of ln(A₀/A_t) versus time; (b) in presence of Pdurc; inset – linear plots of ln(A₀/A_t) versus time; (c) in absence of PdNBs/Pdurc; inset – left panel: zoomed around 400 nm, inset – right panel: linear plots of ln(A₀/A_t) versus time. Reaction conditions: [4-NP] = 1 × 10⁻⁴ M, [NaBH₄] = 5 × 10⁻² M, [PdNBs] = 0.166 mg L⁻¹, [Pdurc] = 1.660 mg L⁻¹.

As shown in Fig. 4 (panel (a and b)), the absorbance at 400 nm was found to be negligible within 15 minutes for both the Pd nanostructure catalyzed reaction. The apparent pseudofirst-order rate constants (k_app) were determined by plotting ln(A₀/A_t) as a function of time (Fig. 4, insets (a and b)). From the surface area of the Pd nanostructures, the obtained k_app values are normalized by eqn (2) and the surface area normalized rate constants (k) values are found to be (0.74 ± 0.03) s⁻¹ m⁻² L for PdNBs and (0.41 ± 0.05) s⁻¹ m⁻² L for the Pdurc at 303 K. It has to be taken into account that the concentration of Pdurc was 10 times higher than that of PdNBs resulting in the higher k values for Pdurc. Numerous reports available in the literature for the reduction 4-NP using metal nanostructures consistently shows t₀, which is explained by the oxygen adsorption on the surface of the catalyst and a dynamic substrate induced restructuring process of the catalyst surface slowing down the equilibrium between the reactants on the surface of the catalyst.^4,30,36 Interestingly, we have observed that t₀ is absent even if the reaction is carried out in air. The absence of t₀ is possibly due to the porosity of metal nanostructures which establishes fast equilibrium between the reactant leading to enhanced catalytic activity of PdNBs and Pdurc. Further, the interconnected 3D network of nanowires creates ball shaped morphology where both the reactants can easily diffuse on the surface of nanostructures thereby reducing the chance of mass transfer process. Mei et al. found the surface restructuring in thermosensitive microgel-Pd composites at higher temperature which subsequently leads to the decrease in the catalytic activity.³⁰ In order to investigate such morphological changes at high temperature, 4-NP reduction reaction was carried out at 60 °C by maintaining other reaction parameters constant. After completion of the reaction, the centrifuged PdNBs was analyzed by TEM and HR-TEM techniques. The images obtained for the PdNBs after the reaction are shown in Fig. 5. From the TEM images of PdNBs (Fig. 5, panels (a and b)), it can be noted that the morphological integrity of the PdNBs is retained even at 60 °C. This is further confirmed by the HR-TEM analysis depicted in Fig. 5c, which shows identical d spacing of 0.238 nm corresponding to the (111) plane of the crystalline fcc Pd.

Fig. 5 TEM images of (panel (a and b)) PdNBs after 4-NP reduction reaction, panel (c) the corresponding HR-TEM image showing continuous lattice fringes with d spacing of 0.238 nm.

Similarly, experiments at 60 °C were also performed using Pdurc as catalyst to confirm the structural rigidity and the crystallinity of Pdurc. The TEM and HR-TEM images of the Pdurc after the reaction are depicted in Fig. 6 (panel (a–c)) which showed marginal change in size after the reaction (Fig. 6, panel (a and b)), but no change in the shape was observed. The crystallinity of Pdurc after the reaction is confirmed by HR-TEM (Fig. 6, panel (c)) with 0.238 nm d spacing corresponding to (111) plane of crystalline fcc Pd.

Fig. 6 TEM images of (panel (a and b)) Pdurc after 4-NP reduction reaction, panel (c) corresponding HR-TEM image showing continuous lattice fringes with d spacing of 0.238 nm.

The crystalline nature of the Pd nanostructures were also confirmed by the XRD analysis after the 4-NP reduction reaction (Fig. 2S,† right panel). The XRD after the 4-NP reduction reaction show 2θ peaks at 40.15°, 46.90° and 68.09° while Pdurc show 2θ peaks at 39.96°, 46.72° and 68.06° respectively. These patterns correspond to the diffractions from (111), (200) and (220) planes of crystalline Pd. In order to study the structural rigidity, we performed the nitrogen adsorption isotherm analysis at 77 K for PdNBs and Pdurc after the 4-NP reduction reaction (Fig. 3). The accessible surface area for PdNBs after 4-NP reduction is 75 m² g⁻¹ while for Pdurc is 16 m² g⁻¹ as calculated from BET isotherm.

The dependence of k_app with concentration of 4-NP and NaBH₄ is studied (Fig. 7a and b). As shown in Fig. 7a, decreasing trend is observed and it can be attributed to the higher adsorption constant of 4-NP and the limited hydrogen produced by the NaBH₄ on the surface of PdNBs which is in agreement with the literature reported.³⁷ The higher affinity and increasing concentration of 4-NP favors their adsorption on the surface of PdNBs as compared to NaBH₄, decreasing the occupancy sites of NaBH₄ which limits the liberation of hydrogen at the surface of PdNBs as depicted in reaction 3 of ESI Scheme S1.† Therefore, at higher 4-NP concentration, a decreasing trend in k_app is observed.

Fig. 7 Dependence of k_app as a function of concentration of (a) variation of [4-NP] at constant [NaBH₄] of (5 × 10⁻² M) and (b) variation of [NaBH₄] at constant [4-NP] of 1 × 10⁻⁴ M; in the presence of 0.166 mg L⁻¹ PdNBs.

On the other hand, while keeping 4-NP concentration fixed and by varying the NaBH₄ concentration, the k_app increases linearly with increasing concentration of NaBH₄ (Fig. 7b). It is for the first time that such linear behavior of k_app with NaBH₄ concentration is reported. The literatures available on metallic nanocatalyst reports a Langmuir isotherm type behavior where k_app is linear at lower NaBH₄ concentration but plateau out at higher concentration of NaBH₄ due to monolayer formation of the adsorbed molecules on the surface of the catalyst.^37,38 Therefore, surface area of the nanocatalyst as shown in ESI Scheme S1,† reaction 3 will be a limiting factor at higher NaBH₄ concentration. In the current study, owing to the bare and porous nature of the Pd nanostructures with highest surface area, we propose that even at high NaBH₄ concentration, the Pd nanostructure capacity of hydrogen adsorption increases avoiding monolayer formation on the surface of the Pd nanocatalyst. Such conditions favor the efficiency of reactions 3 and 4, shown in ESI Scheme S1.†

To rule out a diffusion controlled reaction, the bimolecular rate constant (k_bm) from mass transport process is calculated by plotting the k_app versus PdNBs concentration (Fig. 8). The bimolecular rate constant is calculated to be 6.78 × 10² M⁻¹ s⁻¹. Since, the diffusion controlled mass transport process limit proposed by Smoluchowski is of the order of 10⁶ M⁻¹ s⁻¹ as reported by Bingwa et al.,³⁵ the mass transport limitation possibility can be ruled out as the experimental value is of the order of 10² M⁻¹ s⁻¹.

Fig. 8 Plot of apparent pseudo-first order rate constants (k_app) as a function of concentration of PdNBs. Reaction condition: [4-NP] = 1 × 10⁻⁴ M and [NaBH₄] = 5 × 10⁻² M.

In order to determine the activation energy, the reaction was carried out at different temperatures in presence of PdNBs and Pdurc. The plots of ln(A₀/A_t) against time at temperature range from 293–313 K for the PdNBs catalyzed reaction are shown in Fig. 3S,† panel (a). The corresponding Arrhenius plot of ln(k_app) versus 1000/T for the evaluation of the activation energy is depicted in Fig. 3S,† panel (b). Similarly, the temperature dependent linear plot and the corresponding Arrhenius plot for the Pdurc are depicted in Fig. 4S† panel (a and b). The k_app and E_a are calculated using eqn (2) and (3) and the values are depicted in Table 1.

Table 1 The surface area normalized rate constants (k), at different temperature, the activation energy (E_a) for the Pd nanostructure catalyzed reduction of 4-NP to 4-AP by NaBH₄

	Temperature T/(K)	Normalized rate constant k/s^−1 m^−2 L	Activation energy (Ea)/kJ mol^−1
PdNBs	293	0.32 ± 0.01	45.81
	303	0.74 ± 0.03
	313	1.06 ± 0.04
Pdurc	293	0.32 ± 0.05	7.75
	303	0.41 ± 0.05
	313	0.44 ± 0.07
	323	0.46 ± 0.08
	333	0.48 ± 0.08

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From the kinetic studies of the porous Pd nanostructures, the catalytic activity of PdNBs is found to be higher than that of Pdurc with minimum loading of catalyst. This observation is in agreement with the larger accessible surface area of PdNBs than Pdurc. Further, the absence of t₀ in this investigation is also in agreement with our previous studies on porous PtNBs.²⁷

The possibility of competitive equilibration of the reactants in this study is ruled out by performing experiments with PdNBs in two different reaction conditions, (1) addition of PdNBs (0.166 mg L⁻¹) to NaBH₄ (1 × 10⁻¹ M) solution followed by 4-NP (3 × 10⁻³ M) solution and (2) addition of PdNBs (0.166 mg L⁻¹) to 4-NP (3 × 10⁻³ M) solution followed by NaBH₄ (1 × 10⁻¹ M) solution. The k_app is found to be identical (0.17 min⁻¹), ruling out the possibility of a competing equilibrium of 4-NP and NaBH₄. Interestingly t₀ is absent in both the reaction conditions (Fig. 9). These result further rule out any competitive binding of the reactants on the surface of the PdNBs indicating the accessibility of the active sites of the PdNBs to the reactants by diffusing through the 3D porous morphology. For the porous Pd nanostructures, the diffusion of the reactants on the surface is independent of the reaction condition and impurities on the surface of the nanostructure.

Fig. 9 Linear plot of ln(A₀/A_t) as a function of time. Reaction conditions: (black line) 2.5 mL water + 0.3 mL PdNBs (1 mg L⁻¹) + 0.1 mL NaBH₄ (1 × 10⁻¹ M) + 0.1 mL 4-NP (3 × 10⁻³ M); (red line) 2.5 mL water + 0.3 mL PdNBs (1 mg L⁻¹) + 0.1 mL 4-NP (3 × 10⁻³ M) + 0.1 mL NaBH₄ (1 × 10⁻¹ M).

For the real time use of nanocatalyst in industries, recyclability and conversion of the substrate are important parameters for application of the nanocatalyst. Therefore, the recyclability and % conversion of substrate was investigated for the PdNBs and Pdurc. To avoid leaching of the nanocatalyst, the reactions were carried out by adding the substrates consecutively in the reaction mixture after the completion of first cycle, for the next cycle. The recycling of the PdNBs was carried out for 5 cycles while 3 cycles was performed on the same Pdurc. The k_app determined from these recycling reactions are shown in Fig. 10 panel (a and b) respectively. The k_app values for PdNBs marginally changes while for Pdurc it was found to be almost identical for all the cycles. These results indicate that PdNBs and Pdurc have good stability against catalytic poisoning.

Fig. 10 The observed k_app (min⁻¹) for the different cycles of catalytic reduction of 4-NP in the presence of NaBH₄ (a) with PdNBs (b) with Pdurc as catalysts. Reaction condition: 0.1 mL 4-NP (3 × 10⁻³ M) + 2.5 mL water + 0.1 mL NaBH₄ (1 × 10⁻¹ M) + 0.3 mL PdNBs (1 mg L⁻¹)/Pdurc (10 mg L⁻¹).

Further, the stability of the catalyst have been also studied for the PdNBs which remains stable even after 5 reaction cycles by comparing the % of conversion of 4-NP for each reaction cycle. As shown in Fig. 11, almost 97% conversion of the reactant is observed within 34 minutes at room temperature for every cycle.

Fig. 11 The % conversion with respect to reaction time (min) for different reaction cycles in the catalytic reduction of 4-NP with NaBH₄ in the presence of PdNBs as catalysts. Inset: conversion% versus number of cycles indicating the selectivity of the reaction. Reaction condition: 0.1 mL 4-NP (3 × 10⁻³ M) + 2.5 mL water + 0.1 mL NaBH₄ (1 × 10⁻¹ M) + 0.3 mL PdNBs (1 mg L⁻¹).

Another important constraint for a good catalyst concerns the loading of the catalyst which is directly proportional to the rate constant. The comparison of catalytic activities in terms of the surface area normalized rate constants k, size of nanostructure, turn over frequency (TOF), activation energy, loading in mol% for the Pd nanocatalyst with different stabilizing agents/supports are depicted in Table 2. The turn over frequency of the catalytic reaction is calculated based on consumption of 4-nitrophenolate anion per unit time (hours) with respect to moles of PdNBs or Pdurc used.

Table 2 The comparison of Pd nanoparticles with different stabilizing agents, Pd loading, size, k, E_a and TOF in the reduction of 4-NP to 4-AP by NaBH₄

Stabilizer^a/support	[Pd]/[Pd-support]^b	Size^c	k^d	Ea^e	TOF^f
a Stabilizing agent/support used in the study.b Catalyst loading in mol%.c Size of nanostructures in nm.d Normalized rate constant in (s^−1 m^−2 L).e Activation energy in (kJ mol^−1).f Turn over frequency is calculated from the moles of reduced 4-NP molecules/moles of Pd nanostructures per hour. NC: not calculated, *: this work.
PAMAM (ref. 4)	NC	1.7 ± 0.5	1.65	20.2	NC
PEDOT-PSS (ref. 5)	NC	1–9	0.0222	NC	13
Al2O3 (ref. 6)	0.09	6.0 ± 0.5	0.136	43	NC
SBA-15 (ref. 7)	NC	8.1	0.00167	NC	6
CNT/PiHP (ref. 8)	4	2.7	NC	NC	300
PPy/TiO2 (ref. 9)	2.6	2	0.000138	NC	326
SNT (ref. 10)	0.38	5	NC	NC	192
PEG (ref. 11)	0.2	1.6 ± 0.3	NC	NC	7500
SPB (ref. 29)	NC	2.4 ± 0.5	1.10	NC	NC
Microgel (ref. 29)		3.8 ± 0.6	0.101	44	NC
PAMAM (ref. 36)	NC	1.8 ± 0.42	0.00307	44	NC
PPI (ref. 36)		1.6 ± 0.36	0.776	NC	NC
Ag/Fe3O4 (ref. 37)	0.20	16.9 ± 1.3	0.485	NC	300
CeO2 (ref. 38)	0.56	3–5	1.45	NC	1068
Fe3O4 (ref. 39)	0.032	1.97 ± 0.4	1.08	NC	NC
SLC*	0.000157	50–90	0.7435	45.81	2452
CPCl*	0.00157	40–80	0.4086	7.75	544

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As, it can be noted from Table 2, the lowest catalyst loading of Pd is found for that of PdNBs (1.57 × 10⁻⁴ mol%) with higher k and TOF values. These findings are very significant as it suggests that, bare, porous and rigid Pd nanostructures can be used as efficient nanocatalyst.

4. Conclusion

In conclusion, we have systematically investigated the catalytic performance, thermal stability, structural integrity, catalyst loading, recyclability of bare and porous Pd nanostructures (PdNBs and Pdurc) in the catalytic reduction of 4-NP by NaBH₄. The TEM, HR-TEM and BET analysis showed structural rigidity of these nanostructures and similar accessible surface area before and after 4-NP reduction at higher temperature. The porosity with higher surface area favors adsorption of the hydrogen produced in the reaction without forming a monolayer which plays an important role in eliminating the induction time (t₀), generally reported in the case of supported Pd nanocatalyzed reactions and many other metal/supported metal nanocatalysts. The PdNBs are found to have significantly higher catalytic activity with lowest Pd loading in mol%. The activation energy is found to be lower in case of Pdurc. The Pd nanostructures have good recyclability, retaining high apparent pseudo-first order rate constant. The results could be useful to material scientist for designing an effective, efficient, porous, and thermodynamically stable noble metal based nanocatalyst.

Acknowledgements

The authors would like to thank Prof. B. S. M. Rao for his valuable suggestions, Prof. A. S. Kumbhar for UV-Vis Spectrophotometer. AMK acknowledge UGC for Meritorious Fellowship. KKKS acknowledges the DST-FTSYS (SR/FT/CS-077/2012) for the financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra23138h

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