In situ generated nickel on cerium oxide nanoparticle for efficient catalytic reduction of 4-nitrophenol

Abstract

Efficient and economic catalysts are required for the large scale degradation of hazardous pollutants. In the present work, two nickel (5 wt%) based compounds, Ni(NO₃)₂ and NiO, immobilized over a CeO₂ surface were tested for the reduction of 4-nitrophenol. Size, structural and surface properties of the catalyst were characterized by XRD, SEM & TEM – EDX, FTIR and Raman spectroscopy. UV-visible spectroscopic results indicated the better catalytic performance of the Ni(NO₃)₂ support than that of NiO supported CeO₂. The reduction rate of 4-nitrophenol in the presence of the Ni(NO₃)₂ support was found to be 12 times faster than that of NiO supported CeO₂. The time-dependent Raman spectroscopic investigation demonstrated that the performance of Ni(NO₃)₂ supported CeO₂ arises from the in situ generation of nickel in the presence of an excess of sodium borohydride in the reduction of 4-nitrophenol. Further, the reversible conversion of nickel to nickel nitrate enabled the recyclability of the Ni(NO₃)₂ supported CeO₂. The formation of nickel was found to be important for the reduction of 4-nitrophenol as NiO supported CeO₂ did not form nickel thereby exhibiting poor catalytic activity. Thus, the present work showcases the in situ generation of nickel as a novel strategy for the catalytic reduction of 4-nitrophenol.

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1. Introduction

Various industries such as pharmaceutical, textile, fertilizers, etc., have been utilizing/producing various hazardous dyes which are harmful to humans and the ecosystem.^1–3 Exposure of these hazardous dyes from the aqueous system, soil and environment leads to pollution in the atmosphere and causes serious health problems to living organisms. 4-Nitrophenol (NP) is one of the major organic pollutants generated from the above industries and also widely used as a starting material for the synthesis of aminophenol. Reduction of NP using reducing agents such as sodium borohydride (NaBH₄) in the presence of a metal catalyst is the generally adopted strategy for the removal of NP.^4,5 Noble metal nanoparticles such as Pd, Pt, Ag, Au, etc., have been studied extensively with different support materials as a heterogeneous catalyst due to their high catalytic performance in the reduction of NP.^6–9 But the use of noble metals as catalysts increases the overall cost thereby requiring an economic alternative for efficient catalytic performance.

A non-noble metal catalyst such as nickel has the advantages of higher catalytic activity, low cost and ease of availability.^10,11 A few reports are available on the effect of nickel on the catalytic reduction of NP, but nickel suffers from aggregation which significantly reduces the surface area leading to the closure of catalytically active sites.¹² As a result, lower surface area decreases the overall catalytic activity and reduces the recyclability of the catalyst. In order to overcome this aggregation problem, the catalyst is immobilized over the support material such as carbon, zirconia, ceria, alumina, titania, etc.^13–16 Nickel supported catalysts have been extensively investigated for the reduction of NP in the presence of sodium borohydride.^17–20

CeO₂ is reported to be one the effective support for Ni and widely used as a heterogeneous catalyst due to its strong metal support interaction nature with Ni.^21,22 This property of CeO₂ in turn improves the dispersion and induces the transition in the oxidation state of the support metal, which increases the active phase and performance of the catalyst.²³ Conventionally Ni supported ceria have been prepared by the wet impregnation of Ni over support followed by the reduction under an inert atmosphere which adds up more cost and time for the bulk production.²⁴ However, no reports are available in studying the catalytic performance of the in situ generated Ni for the reduction of NP which is likely to lead a simple, economic and scalable process. In the present work, we report the performance of two compounds, namely, nickel nitrate and nickel oxide on their relative ability to reduce nitrophenol with respect to catalytic activity as well as recyclability. Thus, the reported procedure likely to eliminate the post reduction process for the formation Ni and overcome the agglomeration related issues.

2. Experimental details

2.1. Chemicals

Cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O, Himedia, 99%), nickel(II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O, Himedia) were used as precursors. Ammonium hydroxide (28%, Merck) and sodium borohydride (NaBH₄, Sigma-Aldrich) were used as the oxidizing and reducing agent, respectively.

2.2. Synthesis

2.2.1. Support material. Cerium nitrate was dissolved in 100 ml of double distilled water (0.05 M) and ammonia was slowly added as a precipitating agent to maintain the pH at ∼9 under continuous stirring for 4 hours at room temperature. The resultant pale yellow colour precipitate was washed thoroughly using water for three times and then dried in an oven at 80 °C overnight to obtain CeO₂ powder. The as synthesised CeO₂ powder was used as support material for the catalyst loading.

2.2.2. Preparation of Ni (NO₃)₂ and NiO immobilized CeO₂. Two different nickel compounds namely Ni (NO₃)₂ and NiO were immobilized on the ceria surface to understand the dynamics of dissociation in the medium. In both the compounds, Ni was present in the +2 oxidation state. The calculated quantity of support material (CeO₂) was dispersed in 50 ml of water using an ultrasonicator for 30 minutes in order to break the weakly agglomerated particles. Ni(NO₃)₂ impregnated CeO₂ was prepared by the dropwise addition of 5 wt% of 50 ml aqueous Ni(NO₃)₂ to the ultrasonicated support under constant stirring. Subsequently the solution was continuously stirred while heating at 100 °C to obtain a dry powder of Ni(NO₃⁻)₂ impregnated CeO₂ (CN). The sample CN was annealed at 400 °C for 1 hour to form NiO impregnated CeO₂ (CNA). The colour of the synthesized CN powder changed from pale yellow to light brown colour upon heating (CNA).

2.3. Characterization

The crystallographic phase identification of the material was carried out using Rigaku Ultima IV X-ray diffractometer at 40 keV using monochromatic Cu Kα radiation (λ = 1.5406 Å) with a scan rate of 2° per minute with a step size of 0.02°. Raman spectra were recorded using confocal Raman spectrometer (Renishaw, RM 2000) with a laser excitation wavelength of 514 nm and the acquisition time of 30 s in the range of 200 to 1600 cm⁻¹ at room temperature. Infrared spectra for the samples were recorded after making a pellet with KBr at room temperature using Thermo Nicolet – 6700 FT-IR spectrometer in the range of 300 to 4000 cm⁻¹. A Hitachi, S-3400N scanning electron microscope (SEM) was used to analyse the surface topography of the catalyst. Energy dispersive X-ray spectroscopy (EDX) was used for the analysis of elemental composition and elemental mapping using Thermo Dry II spectrometer attached with SEM. High resolution transmission electron microscopic (HRTEM) images were obtained using FEI-Tecnai G² 20 S-TWIN operated at 200 keV attached with Bruker XFlash 6T130 EDX Detector.

2.4. Evaluation of catalyst performance in the reduction process of 4-nitrophenol

The reduction of 4-nitrophenol to 4-aminophenol for samples CN and CNA was monitored using UV-visible spectrometer (Perkin Elmer, Lambda 650) at 1 nm resolution. The 4-nitrophenol solution (0.01 mM, Spectrochem) was added to the freshly prepared NaBH₄ (0.1 mM) of 2.5 ml. To the above solution, 2 mg of nickel based catalyst was added to the reaction mixture in order to initiate the reduction process of 4-nitrophenol to 4-aminophenol. UV-visible spectroscopy was employed to monitor the real-time changes in absorption of 4-nitrophenol (404 nm) and 4-aminophenol (308 nm) with respect to a time interval of 5 minutes.

3. Results and discussion

3.1. Structural and surface characterization

X-ray diffraction (XRD) studies were carried out to understand the structural aspects of the as-prepared ceria support and impregnated catalyst. Fig. 1, shows the XRD pattern of CeO₂ support as well as nickel impregnated samples such as CN and CNA. The support and nickel loaded samples exhibited the characteristic peaks at 28.5°, 33.2°, 47.4° and 56.4° of cubic fluorite structured CeO₂ (ICDD no. 98-001-0948) corresponds to (111), (200), (220) and (311) reflection planes, respectively. Ni(NO₃)₂ and NiO peaks were not observed in CN and CNA, respectively, which indicates the impregnated catalyst were well distributed over the surface of the CeO₂ support.²⁵ The homogeneous dispersion of Ni (NO₃)₂ or NiO based catalyst over CeO₂ support limits the detectable range of XRD.^26,27 The mean crystallite size of CeO₂ was calculated from the XRD reflection planes by (111), (200), (220) and (311) using Debye–Scherrer equation and was found to be around 12 nm. The crystallite size of the support shows negligible variation even after heating to form CNA due to the slower growth kinetics at a lower temperature (400 °C for 1 hour). No significant shift in XRD peak position was observed between the samples and the variation in lattice parameter was found to be minimal. The above results indicate, that the property of the support did not vary with respect to size and lattice parameter for Ni(NO₃)₂ and NiO catalyst. Further, the absence of change in lattice parameter of CeO₂ indicates the presence of nickel compounds immobilized over the surface.

Fig. 1 XRD pattern of ceria and nickel based catalyst supported over CeO₂.

The Raman spectra of the support material and after nickel impregnation are shown in Fig. 2. The Raman spectra for all the samples exhibited an intense band at ∼465 cm⁻¹, which corresponds to the F_2g symmetric vibrational mode for CeO₂.²⁸ Additionally, a weak peak at 600 cm⁻¹ was observed in all the samples that can be attributed to the oxygen vacancy induced by the Ce³⁺ concentration.²⁹ In CN along with the F_2g band, a symmetric peak at ∼1050 cm⁻¹ was observed due to the vibrational mode of the nitrate group (ν₁NO₃⁻) which was absent in the CeO₂ support. This confirms the presence of nickel nitrate impregnated over the support.³⁰ Bulk NiO exhibits first order transverse optical mode (1TO) mode (∼440 cm⁻¹) and longitudinal optical (1LO) mode (∼560 cm⁻¹) in Raman spectra.^31–33 The broad spectra for CNA around the range of 575 cm⁻¹ can be attributed to the overlapping of 1TO and 1LO mode of NiO along with 465 and 600 cm⁻¹ peaks of CeO₂. This broad spectrum (575 cm⁻¹) in CNA along with F_2g band confirms the presence of NiO over CeO₂ support. The absence of a peak at 1050 cm⁻¹ in CNA further confirms the complete conversion of nickel nitrate into nickel oxide.

Fig. 2 Raman spectra for ceria and nickel based catalyst supported over CeO₂.

To understand the functional group modification, Fourier transform infrared spectra (FTIR) were recorded at room temperature for all the samples. FTIR spectra of CeO₂ and CN shown in Fig. 3, exhibits two bands around ∼3400 and ∼1648 cm⁻¹ which corresponds to the asymmetric stretching mode of ν (O–H) and δ (O–H) from the surface adsorbed water. All the samples exhibited a sharp absorption band around ∼1380 cm⁻¹ corresponds to the vibrational mode of ν(Ce–O–Ce).³⁴ FTIR spectra of the as-prepared CeO₂ exhibited weak absorption around 1058 and 834 cm⁻¹ due to the presence of residual nitrate moiety (ν(NO₃⁻)) over CeO₂ surface. In CN the observed bands at 1383 and 1058, 834 cm⁻¹ corresponds to the asymmetric stretching mode of the nitrate group, ν(NO₃⁻). The presence of nitrate vibrational band along with CeO₂ in CN confirms the impregnation of Ni(NO₃)₂ over CeO₂ support. Subsequent heating of CN at 400 °C for 1 hour, oxidizes the Ni(NO₃)₂ to NiO leading to the formation of NiO impregnated over CeO₂ support (CNA). The removal of water during the heating process lead to the disappearance of peaks at 3400 and 1648 cm⁻¹ in CNA. The broad peak around ∼430 cm⁻¹ in CNA can be attributed to the Ni–O vibration mode, which confirms the impregnation of NiO over CeO₂ surface.³⁵ FTIR, XRD and Raman studies confirm the presence of Ni(NO₃)₂ and NiO over CeO₂ support.

Fig. 3 FTIR pattern for ceria and nickel based catalyst supported over CeO₂.

Fig. 4, shows the SEM images of the as synthesized CN and CNA along with the corresponding elemental mapping and EDX spectra. Synthesis of ceria by precipitation method resulted in a weaker agglomerated support material (CeO₂) with spherical morphology for coated samples of CN and CNA as shown in Fig. 4(a and b). EDX spectra of the samples showed the presence of Ce and Ni, and the elemental analysis confirmed the presence of ∼5 wt% of nickel in both CN and CNA. Further elemental image mapping shows the uniform distribution of Ni over CeO₂ in both CN and CNA. In CNA, other elements such as Al and Si were found which could have emerged from the aluminum stub and silicon glue from the carbon tape used to hold the sample.

Fig. 4 SEM images of CN (a) and CNA (b) along with the corresponding elemental mapping and EDX spectra.

The microstructure of the as-synthesized CN and CNA were characterized through TEM and HRTEM analysis (Fig. 5). The low and high magnification TEM images of CN and CNA clearly indicates the presence of CeO₂ with no significant change in morphology and crystallite size (as support retains the same size). The nickel based catalysts got infiltrated between the CeO₂ crystallites as represented by the TEM images. Corresponding selected area electron diffraction (SAED) pattern in Fig. 5 shows a series of rings that can be indexed to cubic fluorite structured CeO₂. Though EDX data (ESI Fig. 1†) confirms the presence of nickel, no diffraction pattern corresponding to nickel compounds were observed suggesting the presence of later in the amorphous form in CN and CNA, supporting our XRD results. The in-detailed interfacial microstructure between CeO₂ and nickel based material were observed from HRTEM image (Fig. 5(c) and (f)) clearly indicates the strong infiltration of catalyst over CeO₂ support. Further, HRTEM images clearly indicate the presence of regular lattice fringes with an interplanar spacing of 0.313 and 0.318 nm for CN and CNA, respectively, corresponding to the (111) plane of CeO₂. However, no lattice fringes were observed in nickel based material due to the existence of catalyst in amorphous nature. The corresponding EDX spectrum of low magnification TEM images (ESI Fig. 1†) demonstrates the uniform distribution of ∼5 wt% of nickel based catalyst supported over CeO₂ crystallites in both CN and CNA. Thus, TEM studies exhibit the microstructural and interfacial interaction between the CeO₂ and nickel based catalyst at nano level.

Fig. 5 HRTEM images of as CN (a–c) and CNA (d–f); inset in (a) and (d) represents the selected area diffraction pattern of the corresponding samples.

3.2. Catalytic properties

To compare the synthesized material performance on catalytic activity, 4-nitrophenol was used as a model system for the present study. In CN and CNA samples, the weight percentage of the Ni atom remains the same. Since the variation in concentration of Ni, which is the catalytically active species, may result in difference in catalytic performance. In order to keep the concentration constant for comparison, the sample CNA was prepared from CN by annealing. Hence the concentration of nickel remains the same in both CN and CNA. The reduction of 4-nitrophenol (NP) to 4-aminophenol (AP) in the presence of NaBH₄ as reducing agent was monitored in UV-visible spectra with respect to time. The addition of NaBH₄ in NP solution leads to a strong red shift in the absorption peak from 326 to 404 nm as shown in Fig. 6(a) can be attributed to the formation of nitrophenolate ion with a corresponding change in colour from pale yellow to strong yellow.^36,37 The colour and the absorption peak position in UV-visible spectra upon addition of NaBH₄ in NP remains the same even after two days in the absence of a catalyst. Fig. 6(b)–(d) shows the time-dependent UV-visible spectra in the presence of NP and NaBH₄ along with CN, CNA and CeO₂ as catalyst respectively. It has been reported that the reduction of 4-nitrophenol to 4-aminophenol results in a colour degradation with a strong absorption around 308 nm attributes to the reduced absorbance due to the nitrophenolate ion.

Fig. 6 UV-visible spectroscopy for (a) addition of NaBH₄ converts NP (326) to NP ions (404 nm) (b) CN (c) CNA and (d) CeO₂ (e) reaction rate.

In the presence of CN catalyst, the degradation of the yellowish colour of NP was complete (Fig. 6(b)), in contrast degradation was incomplete in the presence of CNA (Fig. 6(c)) at the equal time interval of 50 minutes. Hence, it can be observed that the CN reduces the NP faster than that of CNA. In order to check the catalytic activity of support material alone, the synthesised CeO₂ alone without any supported catalyst is added to the catalytic solution is shown in the Fig. 6(d). The presence of CeO₂ did not alter the absorption of NP ion indicates the inert nature of support material during the catalytic process for the reduction of NP to AP.

The changes in the absorbance with respect to time corresponds to the reaction rate during the reduction process of NP to AP. The concentration of NP, NaBH₄ and supported catalyst in the solution was maintained constant in order to compare the relative catalytic performance. Due to the presence of high concentration of NaBH₄, the catalytic reaction is considered as pseudo-first-order reaction, thereby the reaction rate depends only on the concentration of NP. Since the reaction is pseudo-first-order, the linear correlation between initial absorption (C_o) and absorption at the different interval (C_t) with respect to time (t) apparently yields the reaction rate constant (k_p).

This pseudo-first-order reaction rate (k_p) was calculated from the linear slope value obtained from ln(C_t/C_o) with respect to time (t) using the eqn (1) as shown in the Fig. 6(e).

ln(C_t/C_o) = k_pt (1)

The reaction rate constant values were obtained from the logarithmic slope of the absorbance yield at 404 nm obtained for NP with time for the various catalysts. The reaction rate was found to be 0.0673, 0.0056 and 0.0010 min⁻¹ for CN, CNA and CeO₂, respectively. Thus, the nickel nitrate impregnation over CeO₂ (CN) shows 12 times improvement in catalytic activity over nickel oxide supported CeO₂ (CNA).

To elucidate the role of nickel nitrate and nickel oxide in the absence of support, the same materials were tested separately for the catalytic activity. Fig. 7(a and b) shows the UV-visible spectra for the reduction of NP to AP in the presence of an unsupported catalyst. In the case of Ni(NO₃)₂, the reduction was completed within 5 minutes as shown in the UV-visible spectra (Fig. 7(a)). However, recovering back of Ni(NO₃)₂ after the completion of the reaction is less possible, as the unsupported Ni(NO₃)₂ were difficult in separating from the reaction mixture through centrifugation, which limits the practical application during the recyclability process. Thus, the presence of support is required for the effective recovery of the catalyst during the recycling process. But NiO showed negligible activity than Ni(NO₃)₂ as indicated by the negligible absorbance changes in UV-visible spectra at 404 nm of NP ions (Fig. 7(b)).

Fig. 7 Reduction of NP to AP in unsupported catalyst (a) Ni(NO₃)₂ (b) NiO.

3.3. In situ Raman spectroscopic investigation

Raman spectra recorded for Ni(NO₃)₂ and NiO in the absence of the support are shown in the Fig. 8. In the case of Ni(NO₃)₂, an intense absorption band was observed at 1050 cm⁻¹ corresponding to the nitrate group ν₃(NO₃⁻). Weak bands present around 720 and 400 cm⁻¹ corresponds to the stretching modes of ν₄(NO₃⁻) and ν (Ni–N), respectively. Raman spectra of NiO exhibited a broad peak in between the range of 400–600 cm⁻¹ attributes to the two overlapping peaks at 440, 560 cm⁻¹ corresponds to 1TO and 1LO optical modes of Ni–O, respectively. While the weak bands at 740 and 1020 cm⁻¹ arise from 2TO and LO + TO stretching modes of NiO, respectively.

Fig. 8 Raman spectra of unsupported nickel nitrate and nickel oxide.

In order to understand the mechanism of catalytic reaction in real time, in situ Raman spectroscopic studies were carried out. The changes in CN and CNA on the addition of NaBH₄ was monitored through in situ Raman Spectroscopy. The Ni–N or Ni–O stretching modes provide an important insight into the mechanism of catalytic activity over the reduction of NP to AP. Raman spectrum recorded (ESI Fig. 2†) in the presence of CN dispersed in water along with the addition of NaBH₄ in the solution. The mole ratio between NaBH₄ to the catalyst was maintained at a ratio of 10 : 1. In the case of CN, strong bands were observed at 465 and 1050 cm⁻¹ which arise due to stretching of mode Ce–O and NO₃⁻, respectively. However, the weak band at 400 cm⁻¹ corresponding to Ni–N could not be observed in CN due to the lower weight (5 wt%) percentage of catalyst loading on the support material which limits the detection level of the weak band in Raman spectroscopy. In order to understand the catalytic mechanism, pure Ni(NO₃)₂ in the solution along with NaBH₄ was directly employed instead of CN for in situ Raman investigation as it clearly shows all the vibration mode of Ni(NO₃)₂ other than CeO₂. Since on understanding, the weak band Ni–N stretching modification is important to elucidate the mechanism of the catalyst during the reduction process of NP to AP.

After adding NaBH₄ (0.2 M) to Ni(NO₃)₂ solution, Raman spectra were recorded at a time interval of every 15 minutes as shown in Fig. 9. Before the addition of NaBH₄, Ni(NO₃)₂ solution exhibited three peaks at 400, 720 and 1050 cm⁻¹ similar to that of Fig. 8. Immediately upon adding NaBH₄ to the nitrate solution (0 minutes), the peak at 400 cm⁻¹ disappeared while the peak at 720 and 1050 cm⁻¹ remains unaffected. Raman spectroscopic result at 0 minute shows the presence of nitrate stretching alone with the absence of Ni–N stretching mode. This modification can be attributed to the formation of metallic nickel. As pure Ni metal is Raman inactive, which support the in situ generation of Ni from Ni(NO₃)₂ under reducing environment at 0 minutes. The spectra (Fig. 9) taken after 15 minutes shows the minor intensity at 400 cm⁻¹ due to the regeneration of Ni–N stretching band. With the increase in time (30 and 45 minutes), the intensity of the vibrational band at 400 cm⁻¹ increases indicates the gradual regeneration of Ni–N from metallic Ni. Thus, the addition of reducing agent resulted in the formation of nickel and subsequent regeneration of nickel nitrate with respect to time was supported by the strong metal-nitrate stretching band present in the Raman spectra. Fig. 10, shows the optical image of color changes associated with the addition of NaBH₄ in Ni (NO₃)₂ solution. The transparent green solution of Ni(NO₃)₂ converted to blackish opaque solution upon addition of NaBH₄ indicates the formation of nickel. The black color Ni metal solution slowly regenerates back into green color Ni(NO₃)₂ as evident from the gradual change from black to green color with increase in time.

Fig. 9 In situ Raman spectroscopy analysis for generated nickel metal from nitrate solution.

Fig. 10 Image of the cyclic process of Ni (NO₃) over addition NaBH₄.

In the case of CNA dispersed in water the broad spectra between the ranges of 400 to 600 cm⁻¹ can be attributed to the overlapping of NiO and CeO₂ vibrational mode. Thus pure NiO was tested under Raman spectroscopy in order to monitor the reduction process of NiO to Ni, upon addition of NaBH₄ (ESI Fig. 3†). But the addition of NaBH₄ did not alter the peak positions of NiO indicates the difficulties in reduction of NiO to Ni metal, supporting the slow reduction rate of NP to AP. Thus, in situ Raman spectroscopic results indicate that the catalytic performance depends on the ease of converting a nickel compound to metal which is involved in the reduction of NP to AP and subsequent regeneration of nickel ion for potential recyclability.

3.4. Catalytic reaction mechanism

The mechanism of the reduction process over a different form of Ni supported catalyst can be explained as given below. The catalytic model of pseudo-first-order reaction follows up the classic Langmuir–Hinshelwood model (LH), where both the 4-nitrophenolate ion and NaBH₄ absorbs over the metal.³⁸ According to LH model the active sites of metallic surface considered to have similar absorption and desorption rate, which makes the reaction rate depend on the concentration of NP ion alone leading to pseudo-first-order reaction. Many studies have been carried out on the reduction of NP to AP, however, the intermediate complex reactions and its mechanism during the reduction process are yet to be elucidated clearly.^33,39 In the present work, the tentative mechanism depending on the experimental observation of in situ Raman spectroscopy is illustrated in the Fig. 11. In the absence of a catalyst, the excess of NaBH₄ reduces the NP to AP to a lesser extent due to the electrostatic repulsion between the negatively charged 4-nitrophenolate and BH₄⁻ ions.⁴⁰ The metal catalyst is generally used to overcome this electrostatic repulsion for the reduction of NP to AP.⁴¹ The proposed model for the metal catalyst is to adsorb the NP ion over the surface of the particle and at the same time, the BH₄⁻ ion from NaBH₄ transfers the hydrogen atom to the metal surface. Accordingly the adsorbed surface hydrogen transfers the electron to the nitro group in order to reduce the NP to AP.⁴²

Fig. 11 Mechanism proposed for the reduction of NP to AP.

In the present work the metal catalyst (nickel) was in the form of nitrate and oxide supported over CeO₂. The reaction products formed during the reduction process and its available Gibbs free energy are summarized in eqn (2) and (3).⁴³

Ni(NO₃)₂ + NaBH₄ + 2H₂O → Ni + 2NO₃ + NaBO₂ + 4H₂, ΔG° = −377.39 kJ (2)

NiO + NaBH₄ + H₂O → Ni + NaBO₂ + 3H₂, ΔG° = −347.27 kJ (3)

The ease of formation of nickel by NaBH₄ mediated reduction differs with respect to nitrate (NO₃⁻) or oxide (O²⁻). In the case of CN, the nitrate molecules easily get reduced to form nickel metal over CeO₂ surface as observed from Raman spectroscopy analysis (Fig. 9).⁴⁴ Thus, the catalytic performance in CN arises due to the in situ conversion of Ni(NO₃)₂ to Ni supported over CeO₂ in the presence of NaBH₄.⁴ The reduction process of Ni(NO₃)₂ to Ni is more favourable in the presence of NaBH₄, as the bonding between nickel to nitrate is weak van der Walls force which resulted in higher negative value of Gibbs free energy (ΔG°, eqn (1)). The formation of metallic nickel supported CeO₂ surface act as an adsorption site for nitrophenolate ions and hydrogen, thereby efficiently reducing NP to AP. However in the open atmosphere the reducing agent of NaBH₄ is not strong enough to retain the reduced Ni metal without an external stabilizing agent, hence with the time the reduced nickel converts back to its original form by reacting with the nitrate molecule present in the solution (eqn (4)) is evident from the in situ Raman spectroscopy.⁴⁵

Ni + 2(NO₃) → Ni(NO₃)₂ (4)

However, in the solution due to the absence of any stabilizing agent nickel converts back to Ni(NO₃)₂ over CeO₂ once the evolution of hydrogen from the BH₄⁻ completes.⁴¹ The high concentration of NaBH₄ extends the duration of stability in the form of metallic nickel was extended up to ∼100 minutes was sufficient enough to complete the reduction of 0.01 mM of NP to AP as evident from the rate constant value calculated from the UV-visible spectrum (Fig. 5). The reduction of NP to AP got completed within 45 minutes over 5 mg of CN.

In CNA, the reduction process of NiO to Ni in the presence of NaBH₄ requires higher activation energy due to the presence of strong Ni–O bond which makes the generation of metallic nickel difficult for the catalysis as denoted by lower negative value of ΔG° value. Hence, the probability of reduction process of NiO into Ni over CeO₂ support is low in the presence of NaBH₄, which in turn slows down the reduction process of NP to AP as evident from the reaction rate constant calculated from the UV-visible spectrum (Fig. 6). The catalytic activity of 5 mg of CNA during the reduction process was incomplete even after 120 minutes which denotes the insufficient active sites of metal for catalytic reduction of NP to AP.

3.5. Recyclability of catalyst

The best catalytic performance obtained from CN was due to the easy reduction of Ni(NO₃)₂ ion over CeO₂ support in the presence of NaBH₄. In order to check the reusability of the catalyst, the CN was recovered from the reaction mixture by centrifugation and tested for the catalytic activity for various cycles (ESI Fig. 4†). Table 1 shows the reusability performance of the catalyst for 4 consecutive cycles with respect to the rate constant value. The CN exhibited good catalytic activity up to 4 consecutive cycles in reducing NP to AP. For the 4^th cycle, CN required nearly 95 minutes to complete the conversion of NP to AP. The increase in time for the complete conversion of NP to AP may also be attributed to the loss of catalyst during centrifugation and washing processes.⁴⁶

Table 1 Recyclability test for CN

Cycle number	Rate constant (min^−1)	Time take for complete conversion (∼minutes)
1	0.0633	55
2	0.0597	63
3	0.0415	70
4	0.0384	81

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4. Conclusion

Ni(NO₃)₂ and NiO supported CeO₂ nanoparticles were synthesized through wet impregnation method over CeO₂ support and subsequent thermal oxidation. The FTIR and Raman spectroscopic investigation confirmed the presence of nitrate in Ni(NO₃)₂ supported CeO₂ while SEM & TEM with EDX analysis confirmed the presence of nickel in a homogenous manner over CeO₂. The reduction of 4-nitrophenol by Ni(NO₃)₂ supported CeO₂ was found to be 12 times faster than that of NiO support. In situ Raman spectroscopic results indicated the formation nickel from Ni(NO₃)₂ supported CeO₂ which is important in the reduction process. Due to the difficulty to form nickel, NiO supported CeO₂ showed poor catalytic activity. The recyclability of Ni(NO₃)₂ supported CeO₂ was tested for four cycles and was found to be good. The current results assume significance in designing in situ generated catalyst for degrading hazardous pollutants. Nevertheless, a further investigation is necessary to disseminate the role of support in catalytic action to identify the economic support material.

Acknowledgements

Authors are grateful for the financial support from Start-up grant (PU/PC/Start-up Grant/2011-12/305) of Pondicherry University. Authors acknowledge the Central Instrumentation Facility (CIF), Pondicherry University for the characterization. Also the authors thank Mr S. Ramasamy (Technical Officer - I) and Mr R. Elumalai (Technician - II) in CIF for helping in carrying out the in situ Raman Spectroscopy.

References

1. M. Nemanashi and R. Meijboom, J. Colloid Interface Sci., 2013, 389, 260–267.
2. T. Vincent and E. Guibal, Environ. Sci. Technol., 2004, 38, 4233–4240.
3. T. Vincent and E. Guibal, Langmuir, 2003, 19, 8475–8483.
4. P. Deka, R. C. Deka and P. Bharali, New J. Chem., 2014, 38, 1789–1793.
5. Y. Tian, Y. Cao, F. Pang, G. Chen and X. Zhang, RSC Adv., 2014, 4, 43204–43211.
6. S. Saha, A. Pal, S. Kundu, S. Basu and T. Pal, Langmuir, 2010, 26, 2885–2893.
7. S. Panigrahi, S. Basu, S. Praharaj, S. Pande, S. Jana, A. Pal, S. K. Ghosh and T. Pal, J. Phys. Chem. C, 2007, 111, 4596–4605.
8. A. Wang, H. Yin, H. Lu, J. Xue, M. Ren and T. Jiang, Langmuir, 2009, 25, 12736–12741.
9. A. Chang and D. H. Chen, J. Hard Mater., 2009, 165, 664–669.
10. A. Wang, H. Yin, H. Lu, J. Xue, M. Ren and T. Jiang, Langmuir, 2009, 25, 12736–12741.
11. H. Lu, H. Yin, Y. Liu, T. Jiang and L. Yu, Catal. Commun., 2008, 10, 313–316.
12. J. Basu and R. Divakar, Ceram. Int., 2015, 41, 12658–12667.
13. M. Banu, S. Sivasanker, T. M. Sankaranarayanan and P. Venuvanalingam, Catal. Commun., 2011, 12, 673.
14. S. Schimpf, C. Louis and P. Claus, Appl. Catal., A, 2007, 318, 45–53.
15. X. C. Wang, L. Q. Meng, F. Wu, Y. J. Jiang, L. Wang and X. D. Mu, Green Chem., 2012, 14, 758.
16. S. Wang and M. Lu, Appl. Catal., B, 1998, 19, 267–277.
17. Y. Yang, Y. Ren, C. Sun and S. Hao, Green Chem., 2014, 16, 2273–2280.
18. O. A. de, P. E. Kagohara, H. Leandro, A. V. Joao, C. H. S. Iracema, R. Crusius, C. R. Claudete Paula and L. M. A. Porto, Enzyme Microb. Technol., 2007, 42, 65.
19. S. Z. S. Gai, F. He, Y. Dai, P. Gao, L. Li, Y. Chen and P. Yang, Nanoscale, 2014, 6, 7025–7032.
20. N. Sahiner, H. Ozay, O. Ozay and N. Aktas, Appl. Catal., A, 2010, 385, 201–207.
21. A. Maubert, G. A. Martin, H. Praliaud and P. Turlier, React. Kinet. Catal. Lett., 1984, 24, 183–186.
22. H. C. Yao and Y. F. Yu Yao, Ceria in Automotive Exhaust Catalysts, J. Catal., 1984, 86, 254–265.
23. C. Morterra, V. Bolis and G. Magnacca, J. Chem. Soc., Faraday Trans., 1996, 92, 1991–1999.
24. P. F. Fulvio, C. D. Liang, S. Dai and M. Jaroniec, Eur. J. Inorg. Chem., 2009, 2009, 605–612.
25. Y. H. Taufiq-Yap, U. Rashid and Z. Zainal, Appl. Catal., A, 2013, 468, 359–369.
26. T. S. Zhang, J. Ma, Y. J. Leng, S. H. Chan, P. Hing and J. A. Kilner, Solid State Ionics, 2004, 168, 187–195.
27. P. Arunkumar, S. Preethi and K. Suresh Babu, RSC Adv., 2014, 4, 33338–33346.
28. V. P. Vladimir, I. K. Vladimir and L. Julie, J. Phys. Chem. B, 2004, 108, 5341–5348.
29. M. Guo, J. Lu, Y. Wu, Y. Wang and M. Luo, Langmuir, 2011, 27, 3872–3877.
30. T. J. Taylor, D. Dollimore, G. A. Gamlen, A. J. Barnes and M. A. Stuckey, Thermochim. Acta, 1986, 101, 291–304.
31. W. J. Duan, S. H. Lu, Z. L. Wu and Y. S. Wang, J. Phys. Chem. C, 2012, 116, 26043–26051.
32. M. Marciuš, M. Ristić, M. Ivanda and S. Musić, J. Alloys Compd., 2012, 541, 238–243.
33. N. M. Ulmane, A. Kuzmin, I. Steins, J. Grabis, I. Sildos and M. Pärs, J. Phys.: Conf. Ser., 2007, 93, 012039–012044.
34. B. Yan and H. Zhu, J. Nanopart. Res., 2008, 10, 1279–1285.
35. A. V. Rao, R. R. Kalesh and G. M. Pajonk, J. Mater. Sci., 2003, 38, 4407–4413.
36. A. K. Patra, A. Dutta and A. Bhaumik, Catal. Commun., 2010, 11, 651–655.
37. A. Leelavathi, T. U. B. Rao and T. Pradeep, Nanoscale Res. Lett., 2011, 6, 1–9.
38. S. Wunder, F. Polzer, Y. Lu, Y. Mei and M. Ballauff, J. Phys. Chem. C, 2010, 114, 8814–8820.
39. Z. D. Pozun, S. E. Rodenbusch, E. Keller, K. Tran, W. Tang, K. J. Stevenson and G. Henkelman, J. Phys. Chem. C, 2013, 117, 7598–7604.
40. Z. Zhang, C. Shao, Y. Sun, J. Mu, M. Zhang, P. Zhang, Z. Guo, P. Liang, C. Wang and Y. Liu, J. Mater. Chem., 2012, 22, 1387–1395.
41. K. Kuroda, T. Ishida and M. Haruta, J. Mol. Catal. A: Chem., 2009, 298, 7–11.
42. X. Kong, Z. Sun, M. Chen, C. Chen and Q. Chen, Energy Environ. Sci., 2013, 6, 3260–3266.
43. K. S. Jang and J. D. Kim, J. Nanosci. Nanotechnol., 2011, 11, 4496–4500.
44. B. Liu, X. Y. Wang, H. T. Yuan, Y. S. Zhang, D. Y. Song and Z. X. Zhou, J. Appl. Electrochem., 1999, 29, 853–858.
45. K. J. A. Raj and B. Viswanathan, Indian J. Chem., 2011, 50, 176–179.
46. M. Meyer, A. Bée, D. Talbot, V. Cabuil, J. M. Boyer, B. Répetti and R. Garrigos, J. Colloid Interface Sci., 2004, 277, 309–315.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04120e

‡ Equally contributed to the work.

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