Facile synthesis, characterization and enhanced catalytic reduction of 4-nitrophenol using NaBH4 by undoped and Sm3+, Gd3+, Hf3+ doped La2O3 nanoparticles
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This work focuses on the synthesis of undoped and doped lanthanum oxide nanoparticles (La2O3 NPs) by a simple co-precipitation method for the catalytic reduction of 4-nitrophenol (4-NP) using NaBH4 as a reducing agent. Their optical properties, morphologies, structure, chemical compositions and electronic properties were carefully characterized by XRD, FTIR, SEM, TEM, PL and UV–visible absorption spectroscopy. The SEM and TEM images showed various shape morphologies and sizes of the particles. The XRD pattern revealed a polycrystalline nature with the hexagonal structure of the La2O3 NPs. The synthesized undoped and doped La2O3 NPs were also employed as catalysts for the reduction of 4-nitrophenol, it shows that the doped (Sm3+, Gd3+ and Hf3+) La2O3 NPs provided better catalytic activity than the undoped La2O3 NPs. Moreover, Hf3+ doped La2O3 NPs exhibited an enhanced catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol in 90 min. The catalytic conversion was studied by UV–vis spectroscopy with high reduction rate (k = 2.048 min−1). The applications of the present study may utilize in the removal of toxic pollutants in a cleaning of environmental pollution as well as in industrial applications.
KeywordsFacile synthesis La2O3 NPs Dopants Catalytic activity 4-Nitrophenol reduction Kinetics
In the past few years, due to the concerns about environmental pollution and potential exhaustion possibility of fossil fuels, more and more attention has been paid to the development of green and renewed energy resources. Therefore, the research on materials is necessary for the prevention of environmental pollution by the degradation or conversion of toxic compounds (dyes, pesticides and chlorinated phenols such as 4-nitrophenol) into the non-polluting compounds. 4-Nitrophenol (4-NP) has been wildly used in several applications such as pharmaceutical, dyeing agent, plastics, pesticides and anti-corrosion lubricant [1, 2, 3]. This compound has been identified as one of the most hazardous and toxic pollutants generated mainly from agricultural and industrial sources [4, 5]. Therefore, several methods have been introduced to remove 4-NP from wastewater with various advantages and limitations such as adsorption , microbial degradation , electrocoagulation  and reduction . Moreover, apart from the industrial and environmental viewpoints, reduction of 4-nitrophenol is considered as a model reaction for catalytic study [2, 10]. It is well-known that without a catalyst, the reduction of- 4-NP is extremely slow. Therefore, many investigators have paid attention to the development of catalysts for the reduction reaction of 4-NP.
The synthesis, production and manipulation of materials on the nanoscale are currently one of the favorable areas of research which also attracts the industrialists for designing and fabricating new functional materials with novel special properties [11, 12]. Rare earth elements are attractive materials for industry and play an important role in a number of current technologies as active components. La2O3 is a rare earth metal oxide, which has a band gap of 4.3 eV and the lowest lattice energy with the high electric constant. La2O3 ultrafine powders have a lot of attractive properties for industrial and technological applications. The electronic and magnetic properties of La2O3 differ considerably from those of the other oxides in the series because La3+ is the only trivalent rare earth cation that lacks 4f electrons and has the simple Xenon electronic structure. Because of their unique electronic configuration [4f electrons] lanthanides have been applied in various fields; also these lanthanide-based materials have attractive and interesting magnetic , optical [14, 15], electrical and therapeutic  properties.
Among the lanthanides, lanthanum has been extensively examined for its unique properties . The lanthanum based materials have been synthesized in various compositions such as La(OH)3 , LaF3 , La2(CO3)3 , LaPO4 , LaBO3 , LaOF , La2Sn2O7 , La2O3  nanoparticles. Although many methods have been developed for the synthesis of lanthanum nanostructures including hydrothermal , solvothermal , microemulsion or reverse micelles , sol–gel , laser deposition  and other chemical and physical methods; but some of these methods are affected by long reaction time, high temperature, high pressure, expensive surface materials and so on. Based on electronic, optical, and chemical characteristics arising from lanthanides 4f electrons, lanthanide compounds have been widely used as high-performance luminescent devices, upconversion materials, catalysts, and time-resolved fluorescence (TRF) labels for biological detection [30, 31]. In particular, recently La2O3 NPs were used in catalytic applications as a promising catalyst for the catalytic oxidative cracking of n-propane , ethanol oxidation , and degradation of rhodamine B under visible light irradiation . From the inspiration of these studies, we have investigated the reduction of 4-NP to 4-AP using La2O3 NPs and NaBH4 as a reducing agent.
In this work, we report the undoped and doped La2O3 NPs were successfully prepared from the reaction of lanthanum nitrate and urea by a simple co-precipitation method. The prepared products were characterized by XRD, SEM, TEM, UV–visible absorption spectroscopy, PL and FT-IR spectroscopy. The effect of La2O3 NPs in the presence and absence of dopants has been investigated in the catalytic reduction of 4-NP to 4-AP using NaBH4 as a reducing agent. It was found that the presence of dopants significantly improved the catalytic performance of La2O3 NPs in the 4-NP conversion catalyzed by sodium borohydride (NaBH4) as a strong reducing agent than the undoped La2O3 NPs. The application of the present work may utilize in the removal of industrial pollutants for the prevention of environmental pollution.
2.1 Materials and methods
All the materials were obtained from commercial suppliers and were used without further purification. Lanthanum nitrate hexahydrate (La(NO3)3·6H2O), carbamide (CH4N2O) and NaBH4 were purchased from Merck Chemicals, India. Samarium nitrate, gadolinium nitrate hexahydrate and hafnium nitrate were purchased from S D Fine Chemicals, India. The double distilled water used as a solvent for the preparation of stock solutions.
2.2 Synthesis of undoped La2O3 nanoparticles
The undoped La2O3 NPs has been synthesized by a simple co-precipitation method  using lanthanum nitrate and urea as starting materials. In a typical synthesis, 0.05 M of lanthanum nitrate and 0.05 M of urea were dissolved in 100 ml of double distilled water. The precursor solution was transferred into a round bottom flask and maintained at a constant temperature of 60 °C for 12 h. Then, the mixture was stirred for 30 min under the magnetic stirring for uniform distribution and formation of nanoparticles. The final products were collected by ultra-centrifugation and washed the obtained precipitate several times with ethanol and double distilled water for removal of unreacted precursors. Finally, the prepared La2O3 NPs were calcinated at 500 °C for 1 h and purified samples were further used in applications.
2.3 Synthesis of Sm3+, Gd3+ and Hf3+ doped La2O3 nanoparticles
The Sm3+, Gd3+ and Hf3+ doped La2O3 NPs have been synthesized by a simple co-precipitation method . In a typical synthesis, 0.05 M of lanthanum nitrate hexahydrate, 0.001 M metal salts (samarium nitrate for Sm3+; gadolinium nitrate hexahydrate for Gd3+ and hafnium nitrate for Hf3+) and appropriate concentration of urea were dissolved in 100 ml double distilled water. The precursor solution was transferred into a round bottom flask and maintained at a constant temperature of 60 °C for 12 h. Then, the mixture was stirred for 30 min under the magnetic stirring for uniform distribution and formation of nanoparticles. The final products were collected by ultra-centrifugation and washed the obtained precipitate several times with ethanol and double distilled water for removal of unreacted precursors. Finally, the as-prepared Sm3+, Gd3+ and Hf3+ doped La2O3 NPs were calcinated at 500 °C for 1 h. The collected samples were characterized by various physicochemical techniques for the confirmation of La2O3 NPs.
The synthesized undoped and doped (Sm3+, Gd3+ and Hf3+) La2O3 NPs, were analyzed by various physicochemical techniques. The UV–vis spectroscopic measurements were made at room temperature using a Shimadzu UV-3600 model double beam UV–vis spectrophotometer in the range of wavelength 200–800 nm. Fourier transform infrared (FTIR) spectra of La2O3 NPs were recorded in KBr pellets using an FTIR spectrophotometer (Bruker Optics, Germany, Model no. Tensor 27) in the range of wavenumber 400–4000 cm−1. X-ray diffraction (XRD) measurement was carried out on X’pert Pro X-ray diffractometer (Panalytical B.V., The Netherlands) operating at 40 kV and a current of 30 mA at a scan rate of 0.388 min−1. The morphology of the doped and undoped La2O3 NPs was characterized by scanning electron microscopy (SEM, ZEISS EVO18, 15 kV). The size distribution and crystallinity of the synthesized samples were obtained by transmission electron microscopy (TEM) measurement, casting NPs dispersion on carbon-coated copper grids and allowing drying at room temperature. TEM measurements were done on Tecnai G2 FEI F12 operated at an accelerating voltage of 200 kV.
2.5 Catalytic reduction of 4-nitrophenol
The catalytic reduction process of 4-NP was monitored by UV–vis absorption spectra. In a typical process, 0.5 ml of 0.15 M freshly prepared NaBH4 solution was added to a solution containing 0.05 ml of 0.005 M 4-NP and 2.25 ml of deionized water. At this stage, the 4-NP was converted into 4-nitrophenolate anion. After that, the 10 mg of catalyst (undoped and doped La2O3 NPs) was added and the reaction was spectrophotometrically monitored at different time intervals. A gradual change of the solution color from bright yellow to colorless was observed during the reaction. After reaction, the catalyst was recovered by precipitation/centrifugation. The absorption spectra were recorded within the wavelength range of 250–500 nm. The rate constants of the reduction reaction were calculated by measuring the peak intensity evolution every minute at wavelengths of 400 nm for 4-NP. The investigation of catalytic conversion was also studied using undoped, Sm3+, Gd3+ and Hf3+ doped La2O3 NPs against the reaction time.
3 Results and discussion
3.1 UV–vis absorption analysis
3.2 Photoluminescence study
3.3 FTIR analysis
3.4 XRD analysis
3.5 Morphology analysis
3.6 Catalytic activity
3.6.1 Catalytic reduction of 4-nitrophenol
The conversion percentage of 4-NP to 4-AP was calculated from the Fig. 7. It has been found that the reduction of 4-NP to 4-AP by NaBH4 in the presence of Hf3+ doped La2O3 NPs as a catalyst than the other samples.
3.6.2 The proposed mechanism of 4-nitrophenol reduction
A mechanism was proposed for the reduction of 4-NP to 4-AP using undoped and doped La2O3 NPs, which is exhibit excellent catalytic properties owing to the high rate of surface adsorption and high surface area to volume ratio. This hypothesis was subjected to test in two parallel studies. In the first approach, 4-NP was reduced to 4-AP in the presence of La2O3 NPs. Addition of NaBH4 to the reaction medium causes deprotonation of 4-NP resulting in the formation of intermediate nitrophenolate ion [2, 40]. Subsequently added NaBH4 reduces nitrophenolate ion to form 4-AP. The presence of NaBH4 favors the formation of 4-AP on account of the decrease in free energy (E0 for 4-NP/4-AP = − 0.76 V). After the addition of NaBH4, the La2O3 NPs start the catalytic reduction by relaying electrons from the donor NaBH4 to the acceptor 4-NP right after the adsorption of both onto the catalyst surface. The excess NaBH4 used, increases the pH of the reaction medium and thus retard the degradation of borohydride ions. The reduction of oxygen proceeds much faster than the nitrophenols present in the system. The reduction reaction of 4-NP only starts after all the oxygen in the system has reacted. The evolution of small bubbles of the hydrogen gas surrounding the catalyst particles remain well distributed in the reaction mixture during the course of the reaction and offer a favorable condition for a smooth reaction to occur. As NaBH4 is present in large excess, its consumption for the reduction of oxygen did not alter its concentration notably. However, the viability of reaction decreases as a result of the large potential difference between donor (NaBH4) and acceptor molecules (nitrophenolate ion) accounting for high kinetic energy barrier.
3.6.3 Evaluation of rate constants
3.6.4 Catalytic conversion of synthesized undoped and doped La2O3 NPs
In this study, we have successfully demonstrated the synthesis of undoped and doped La2O3 NPs under a facile co-precipitation method. The morphological, structural and optical properties of as-synthesized La2O3 NPs were characterized using TEM, SEM, XRD, FTIR, UV–vis, and PL spectroscopy. The XRD patterns indicate that the well-crystallized and hexagonal phase La2O3 nanocrystals can be easily obtained under the current synthetic conditions. The SEM and TEM analysis revealed the size of the NPs are in the range of 30 nm with different shapes due to some agglomeration. It also demonstrated that the synthesized undoped and doped La2O3 NPs as nanocatalysts strongly influent catalytic performance for 4-NP reduction using NaBH4 as a reducing agent. The catalytic activities of Hf3+ doped La2O3 NPs are higher than the Sm3+, Gd3+ doped and undoped La2O3 NPs due to the smaller size and high surface area. The well dispersed doped La2O3 NPs having the size 10–30 nm were exhibited the best catalytic activity for reduction of 4-NP. Our results demonstrate that the doped La2O3 NPs can be a promising material of choice for various practical applications in catalysis and industrial applications.
All authors have contributed to the writing of the manuscript. All authors read and approved the final manuscript.
The authors would like to thank DST-FIST, New Delhi, India for providing necessary analytical facilities and express sincere thanks to the Head, Department of Chemistry, Osmania University for providing infrastructure and other necessary facilities.
The authors declare that they have no competing interests.
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