Introduction

Currently, cerium has been paid great attention to the researchers due to its appreciated applications in engineering and technology. Furthermore, Cerium is very reactive with atmospheric oxygen and readily form a metal oxide with varying composition and known as ceria. In bulk, Cerium oxide has two form having different oxidation states (CeO2 and Ce2O3) (Tsai et al. 2008). It is the only rare earth element which shows stable tetravalent oxidation state unlike other members of lanthanide series mainly exist in trivalent oxidation state (Dao et al. 2011). Numerous methods have been attempted for the synthesis of cerium oxide NPs including precipitation (Pelletier et al. 2010), hydrothermal (Rojas et al. 2012; Alhaji et al. 2019), solvothermal (Zhang et al. 2011,2018), ball milling (Yadav and Srivastava 2012), thermal decomposition (Wang et al. 2002), spray pyrolysis (Demokritou et al. 2013), thermal hydrolysis (Hirano et al. 2000), and sol–gel methods (He et al. 2012; Darroudi et al. 2014a,b). Amongst all co-precipitation is most satisfactory, economical, simple and widely used at laboratory scale level (Farahmandjou et al. 2016).Cerium oxide NPs are widely applied for the treatment of several organic pollutants from the environment and aqueous system also (Li et al. 2018). Additionally, it is used for oxygen sensing (Zhao et al. 2017), pharmacological agent (Celardo et al. 2011), dye degradation (Li et al. 2018), medicines (Shcherbakov et al. 2020), catalysis (Pato et al. 2019; Mahar et al. 2020), fuel oxidation catalysis (Jung et al. 2005), and automotive exhaust treatment (Campbell and Peden 2005). Currently, water pollution is caused by various nitroarenes compounds due to vast usage in different industries such as pesticides, explosives and dyes (Zhang et al. 2018). The conversion of nitroarenes into anilines received much significance because of wide applications in numerous pharmaceutical and dyestuff products (Budanov et al. 2010). Industrial and agricultural activities nitro-phenolic compounds found to be a major pollutant in the aquatic system (Aditya et al. (2015)). The US Environmental Protection Agency (EPA) has listed 4-NP as major toxicant due to hazardous effect as it may damage liver, kidneys and central nervous system in humans and animals (Guo et al. 2014; Feng et al. 2009). Many harmful chemicals are produced on daily basis due to rapid development of various industries i.e. Fertilizers, dyes, pharmaceuticals and textile etc. (Feng et al. 2009; Saravanakkumar et al. 2019; Vincent and Guibal 2004; Deka et al. 2014). The reduction of 4-NP yields 4-AP which is pharmaceutically important candidate and being utilized as intermediate for the synthesis of a vast number of antipyretic and analgesic drugs like paracetamol, phenacetin (Chinnappan et al. 2016). Unfortunately, 4-NP is organic refractory toxicant which severely affects the environment indulging the ecosystem including aquatic animals as well as humans and consequently results in several lethal diseases (Bordbar et al. 2018). Under mild environmental conditions, Sodium borohydride is applied as a reductant/initiator for the reduction of 4-NP to 4-AP in aqueous media, which is cheap, simple, and greener one. However, the conversion is very slow in the absence of a suitable catalyst by comparing all reported conversion methods (Kojima et al. 2002; Liu et al. 2009). In addition, different conventional methods have been reported i.e. sonolysis method and Fenton degradation process. In former degradation process, the products formed were NO2−, NO3−, and H+ and later degradation practice employment of “Fenton reagent”, which is a mixture of a ferrous salt and H2O2. In this process, advanced oxidation takes places involving the hydroxyl radical as the oxidation agent (Zhang et al. 2007; Shahwan et al. 2011), but these methods are not very much effective. Therefore, it is more important to find a suitable/convenient ecofriendly and potential catalyst for the direct hydrogenation of 4-NP applying NaBH4 as reductant. Nowadays, the use of nonmaterial as an efficient catalyst has been attracted by researchers because of remarkable electronic properties and high surface to volume ratio. The noble metals like Au (Suchomel et al. 2018), Ag (Mohamed and Al-Sharif 2013), Pt (Pandey and Mishra 2014), and non-noble metals like Cu (Deka et al. 2014), Ni (Yang et al. 2014), Pd (Harish et al. 2009), have widely been used with different morphologies and sizes as a competent catalyst to reduces the 4-NP into 4-AP, which is major contamination in aquatic system. In Aqueous media, 4-NP has a greater stability and low solubility, so it is difficult job to degrade, to resolve water pollution issue numerous semiconductor materials including, TiO2 (Ahn et al. 2007), ZnO (He et al. 2018), and Cu2O (Liu et al. 2015) have been reported. Furthermore, different methods have been developed typical chemical catalysis and photocatalysis (Zhao et al. 2017; Magdalane et al. 2019). The catalytic degradation of 4NP to 4-AP utilizing excess NaBH4 as reductant is an important one. Because 4-AP is less hazardous and has huge demand in the industrial application (Liu et al. 2018a). Most of the synthetic protocol for metal nanomaterials is chemical reduction which creates hazardous environmental problems and chemical toxicity. In Environmental remediation photocatalytic process is considered a greener one, which assists to decreases the unwanted generation of by-products at mild experimental conditions and is becoming a fascinating alternative for the reduction of nitroarenes. In this regard, several efforts have been made to find out the ideal conditions for photocatalytic reduction of nitroaromatic compounds into anilines (Hernández-Gordillo et al. 2013; Ramírez-Rave et al. 2015; Guerrero-Araque et al. 2017; Castañeda et al. 2019; Wang et al. 2009; Imamura et al. 2011; Cipagauta et al. 2014).

Scientifically Cerium dioxide has importance in photocatalysis due to excellent physicochemical properties which include, chemical stability, redox capacity, nontoxicity, corrosion protection and Ce3+ defects in CeO2–x. At nanoscale, ceria possesses high redox cycling between Ce4+/Ce3+ ions in the semiconductor surface which make them a super candidate for photocatalyst (Chang et al. 2016; Li et al. 2017; Channei et al. 2014). The pure cerium oxide (IV) nanomaterial has not been reported for the catalytic reduction of 4-NP to 4-AP so far. Ceria@Y2O3 Binary metal oxide NPs was successfully synthesized by hydrothermal approach and applied as a heterogeneous catalyst for selective reduction of 4-NP into 4-AP utilizing an excess of NaBH4 as a reductant. The reduction reaction completed in 4 min and follows pseudo-first-order kinetics (Magdalane et al. 2017). Another approach was made by researchers to fabricate CeO2@CdO binary metal oxide nanocomposites by simple precipitation and hydrothermal method and tested against gram-negative and gram-positive bacteria along with applied as a heterogeneous photocatalyst for the degradation of toxic Rh-B dye (Magdalane et al. 2016). Various authors have reported their work on ceria as photocatalyst for the degradation of organic species present in wastewater. Khan et al. (2011) have degraded toxic dyes namely amido black and acridine orange by 45.6% and 37.7% respectively with an irradiation time of 170 min using hollow spheres CeO2 as photocatalyst. Yang et al (2010) published their work using hollow spheres CeO2 as efficient adsorbent material for the removal of Congo red dye 84% from the wastewater photocatalytically. Enhance photocatalytic degradation of acid orange 7 was observed under visible light irradiation using mesoporous CeO2 material. The author compares CeO2 with TiO2 P25, ceria shows excellent photocatalytic behavior than bulk counterpart (Ji et al. 2008).

This study mainly focuses on the synthesis of cost-effective and eco-friendly nanocatalyst with exceptional photocatalytic properties and excellent reusability towards nitroarene compounds.

Experimental work

Glasswares and chemical reagents

In this experimental study, all the needed glasswares were soaked overnight in (10% v/v) HNO3 solution to prevent from any contamination and then rinsed three times with distilled water and dried in an oven at 100 °C before used for the experiment. All chemicals were extra pure and analytical grade used without further purification. The precursor salt cerium ammonium nitrate (NH4)2[Ce(NO3)6] was purchased from Dae-Jung China, NH4OH (33%) is used as reducing agent, 4-nitro phenol (4-NP), sodium borohydride (NaBH4), were obtained from Sigma Aldrich, extra pure Milli-Q water was used a solvent throughout the study.

Synthetic procedure for CeO2 nanocatalyst

The ceria (CeO2) nanostructures were synthesized by simple co-precipitation method at room temperature using 0.548 g (0.01 M) of (NH4)2[Ce(NO3)6] as a precursor salt in 250 mL two neck round bottom flask containing 100 mL of milli-Q water then solution was sonicated for 05 min at 28 ± 1 °C for complete homogenation. Then the entire solution was stirred magnetically for 10 min after that 10 mL of NH4OH (33%) solution as a reducing agent was taken in burette added dropwise ultimately pH of the solution was reached at 10 and resultant color of solution rapidly changes from orange to straw yellow which confirms the formation of particles. The constant vigorous magnetic stirring was carried out for 30 min until complete precipitation was achieved. The precipitate was washed several times with Milli-Q water up to neutral pH then lastly washed with ethanol. After washing, CeO2 nanostructures were dried in a vacuum oven at 80 °C overnight, then annealed at 400 °C for 3 h.

Heterogeneous catalytic application of CeO2

In this experimental study, CeO2 nanostructures have been used as an efficient heterogeneous catalyst for the reduction of 4-NP to 4-AP under low power microwave irradiation in aqueous media. The catalytic reduction of 4-NP was carried out under normal laboratory conditions, 10 mL of 15 µM 4-nitrophenol solution was taken in Pyrex glass tube. The solution was tightly sealed and irradiated with microwave in the presence and absence of catalyst material. During the reaction, the 4-NP solution was continuously monitored and irradiated with constant low power microwave radiation. To check the catalytic performance of CeO2 nanostructures for 4-NP reduction The UV–Visible spectrophotometer (Biochrom Libra S22) is employed to check the absorption spectrum after every 10 s of reaction interval by taking 3 mL (500 µL of 4NP, 2.5 mL of H2O and catalyst dose) in quartz crystal cuvette cell (1 cm path length) and the spectrum is recorded.

Conditions for reduction of 4-NP

Appropriate amount of catalyst, 4-NP and NaBH4 were taken in Quartz Crystal cell path length of 1 cm at the normal environmental condition. UV–Visible spectra were recorded following four suitable procedure which includes: (a) constant concentration of 4-NP with variable concentration of reductant (NaBH4) in the absence of catalyst, (b) fixed concentration of reducing agent and 4-nitrophenol utilizing constant microwave power time interval using suitable amount of catalyst, (c) at a constant concentration of 4-NP and NaBH4 with a variable quantity of catalyst, and (d) all the quantities catalyst dose, 4-NP concentration and reducing agent NaBH4 were optimized and the solution mixture was irradiated with low power microwave. For better results, it was found that CeO2 nanostructure (100 µg) and Reducing agent (0.0005 M) and low power microwave irradiation for 60 s were suitable for complete reduction of 4-NP to 4-AP. During the experimental study the % reduction was calculated from the change in absorption profile of 4-NP using the given equation:

$$\frac{{C_{{\text{i}}} \_C_{{\text{f}}} }}{{C_{{\text{f}}} }} \times 100.$$

Herein, Ci is the initial concentration of 4-nitrophenol and Cf is the final concentration respectively.

Characterization

Different analytical techniques were performed to characterize the cerium oxide nanoparticles. The UV–Visible study was carried out in the range of 200–800 nm with a scan rate of 2 nm (Biochrom Libra S22). FT-IR (Thermo Nicolet 5700) study was performed to investigate the functionality of material from 400 to 4000 cm−1. The crystalline phase and purity of the sample were checked employing X-ray diffraction (Bruker D8 Model) with CuKα radiation. The size distribution and surface potential of synthesize material were investigated by employing zeta sizer-potential technique while the size and surface morphology were disclosed by SEM (JSM 6380 of Joel, Japan).

Results and discussion

SEM analysis

To check the surface topography of engineered cerium oxide nanoparticle SEM analysis was carried out. This study reveals that the material has untwined combined form rhombohedron shaped morphology as shown in Fig. 1a, b. The low magnification image shows agglomerated rhombohedron shaped particles. In high-resolution SEM images reveals homogenized well separated untwined combined form rhombohedron shaped CeO2 nanoparticles with a particle size in the range of 18–20 nm. The sharp aged shape of particles proved as efficient photocatalyst in for organic contamination.

Fig. 1
figure 1

SEM images of CeO2 nanostructures a low resolution and b high resolution

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X-ray diffraction study

The freshly prepared powdered nanoceria sample was explored through XRD technique. The crystallinity and phase purity of the material was investigated using X-ray diffraction (Bruker D8 Model) with CuKα irradiation and scanning rate of 0.025ºs−1. It can be seen from the diffraction plane at 110, 200, 220, 311, 222, 400, 331, 420 which confirms the cubic phase nature of the material (Majeed et al. 2019). Nonappearance of impurity indicates that pure CeO2 is synthesized by the chemical precipitation method and shows the good agreement with JCPDS No 34-0394. Furthermore, the Debye Scherrer formula was used to calculate the average crystalline size of the engineered CeO2 NPs using the most intense plane in the diffractogram, a formula is given below (Truffault et al. 2010):

$$\tau = \frac{K \lambda }{{ \beta \cos \theta }}.$$

Here τ is the mean crystalline size K is the dimensionless form factor taken as a constant which has a value of 0.9, lambda (λ) is the wavelength of the incident X-rays radiation, β (FWHM) which represents the width at the mean peak height and θ is the diffraction angle (Fig. 2). Average size of pure nanoceria was coming in nanometric range and calculated theoretically using the above-given equation and found to be 18 ± 2 nm.

Fig. 2
figure 2

XRD pattern of the CeO2

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Zeta sizer and potential study

This study was exploited to know the size distribution of synthesized material using Zeta Sizer (Zetasizer Ver. 7.11 MALVAREN). Fresh powdered Ceria sample was prepared by taking 3 mg of sample into the water, thoroughly dispersed placed in vial then analyzed through zeta sizer different peaks were observed within the range of 100 nm the average size of CeO2 nanomaterial was found to be 50 ± 5 nm. After that the same sample was analyzed to check the surface charge of the material because zeta sizer has another mode known as zeta potential which is used to calculate the surface potential of particles and ceria has surface potential charge of + 6.37 mV which exhibit good catalytic applicability (Fig. 3).

Fig. 3
figure 3

Zeta sizer profile (a) and Zeta potential profile (b) of CeO2 nanocatalyst

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FT-IR study

To check the surface functionality of cerium oxide nanomaterial FT-IR study was performed in the range of 4000–400 cm−1 (Fig. 4). The broad absorption band at 3443 cm−1 confirms the OH stretching vibration which displays absorption of water on the surface of the material (Majeed et al. 2019; Anand et al. 2019), another band at 1600 cm−1 shows N–H stretching mode of vibration, the peak at 1400 cm−1 exhibit C–H bending along with the appearance of the small band at 1000 cm−1 is due C–O stretching it might be due to environmental CO2 and major absorption band around 540 cm−1 it confirms Ce–O stretching vibration (Rajendran et al. 2014; Santos et al. 2008).

Fig. 4
figure 4

(FT-IR) Fourier transform Infrared investigation of CeO2 nanostructures

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UV–Visible study

To check the synthesized material basic characterization techniques were performed. The synthesize material was confirmed through UV–Visible spectroscopy a very common technique was employed in the range of 200–600 nm in range. CeO2 nanoparticle shows a maximum absorption in the Ultraviolet range and maximum lambda max was observed at 322 nm (Calvache-Muñoz et al. 2017), which is shown in Fig. 5a. Along with the UV–Visible peak stability study was carried out to monitor the stability the synthesized cerium oxide NPs is highly stable up to 2 month as depicted in Fig. 5b.

Fig. 5
figure 5

a UV–Visible absorption profile. b Stability of nanocatalyst

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The catalytic applicability of CeO2 nanostructures

The catalytic performance of CeO2 nanostructures was monitored for the reduction of 4-NP using microwave irradiation. The UV–Visible absorption study was carried out from 200–800 nm range. The UV–Visible absorption spectrum of 4-nitrophenol (15 µM) solution shows major characteristics peaks at 318 nm shown in Fig. 6a. The peak at 318 nm corresponding to the 4-nitrophenol molecule, when a small quantity of reducing agent was added a small change in spectral profile of 4-NP was observed due to formation of 4-nitrophenolate ion and absorption is shifted to longer wavelength approximately 400 nm resultant changes in color was observed from light yellow to bright yellow which is depicted in Fig. 6b. Heterogeneous catalytic applicability of CeO2 NPs toward 4-NP was evaluated from a decrease in absorption of 4-nitrophenolate ion at 400 nm and ultimately the appearance of small peak around 300 nm confirms the formation of 4-AP (He et al. 2014), from reduction mechanism using greener source of energy microwave irradiation.

Fig. 6
figure 6

a UV–Visible spectra of 4-NP (15 µM) solution. b Absorption profile of 4-nitrophenolate ion solution

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Effect of reducing agent

The 4-NP with a concentration of 15 µM was used during experimental study. The UV–Visible profile of 4-NP shows a maximum absorption at 318 nm which confirms from the reported literature (Shah et al. 2017). When a 2 µL small quantity of reducing agent NaBH4 (0.0005 M) was added into the standard 4NP solution with the same concentration, red shift was observed optical density shifted from 318 nm (black line) to 400 nm (Red line) due to the formation of 4-nitrophenolate ion in the solution and there is no change in peak height was observed in both spectra were given in Fig. 7a. After that, the concentration of NaBH4 was increased but no further shifting to longer or shorter wavelength was observed but peak intensity is decreased slowly. Therefore the effect of reducing agent concentration towards concentration of 4-NP conducted. Herein the dose of NaBH4was optimized at a constant concentration of 4-NP in the absence of a catalyst. To improve % reduction variant amount of reducing agent was used from 5–20 µL with a concentration of 0.0005 M. Initially the 4-NP peak at 400 nm decreased gradually with increasing the amount of NaBH4 Fig. 7(b-c). After achieving 15% reduction in concentration no significant change was noticed therefore 20 µL of NaBH4 was selected as optimized concentration for further study.

Fig. 7
figure 7

a Spectral changes of 4-nitrophenol solution before (black line) and after (red line) addition of 0.0005 M NaBH4 solution. b UV–Visible absorption spectra of 4-nitrophenol (15 µM) solution without catalyst using NaBH4 solution 0.0005 M concentration. c % reduction of 4-NP at the different volume of NaBH4

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Effect of low power microwave radiation

After optimizing NaBH4 concentration, the effect of greener and instant source of energy microwave was exploited. The commercial microwave oven with constant power (110 W) was used in the absence of reducing agent and catalyst material during experimental study. The reduction in the concentration of 4-NP was observed with increasing the time from 10 to 60 s with a 10 s of the interval at constant power (110 W). Total 27.5% reduction in the concentration of 4-NP was attained within 60 s of at constant microwave radiation exposure (Fig. 8).

Fig. 8
figure 8

a Optical absorption profile of 15 µM 4-NP solution utilizing constant (110 W) microwave power. b % reduction of 4-NP at a different time interval

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Effect of catalyst dose

After successful optimization of NaBH4 and microwave radiation with 25% reduction of 4-NP concentration with respect to time the effect of CeO2 nanocatalyst was checked. After the addition of nanocatalyst a considerable drop in intensity of peak at 400 nm was observed which shows approximately 40% reduction in 4-NP concentration, results has been depict in Fig. 9a. Later on, the catalyst amount was increased from 20 to 120 µg at optimized condition and changes in the absorption profile of 4-nitrophenolate ion, % reduction were calculated. It was confirmed from the absorption profile as the amount of catalyst increase the gradual decrease in peak intensity is obtained. Total 87.74% reduction in 4-NP is obtained at 120 µg within 40 s. Simultaneously a small peak was also observed around 300 nm which is due to the formation of 4-AP. The reduction reaction rate faster with increasing the dose of catalysts, which was contributed to the directly increases in the number of catalytic active sites on the surface of nanoceria. It is confirmed from the spectra by increasing the amount of catalyst more active sites were available to accelerate the breakdown of reducing agent NaBH4 hence promoting the rate of reaction faster (Cai et al. 2017).

Fig. 9
figure 9

a UV–Visible spectrum of 4-nitrophenol reduction at a different dose of CeO2 using constant MW and NaBH4. b The inset graph shows the 4-nitrophenol % reduction using different amount of nanocatalyst

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Time study

Finally, time study was performed using a constant amount of nanocatalyst 100 µg to get the maximum % reduction at optimized condition taking NaBH4 20 µl, low Microwave radiation power 110 W from 10 to 60 s that is depicted in Fig. 10a. It was observed that as the reaction time is enhanced the reduction rate is increased and about 99% decreased in intensity of peak at 400 was achieved within 60 s. The reduction of 4-nitrophenol in the presence of CeO2 NPs is favorable due to fast hydrogen transfer from NaBH4 under microwave irradiation simultaneously decrease in absorption peak intensity at 400 nm. Appearance of small peak was also observed around 300 nm which is due to formation of 4-aminophenol.

Fig. 10
figure 10

a Optical absorption profile of 4-NP reduction under constant microwave irradiation, optimized NaBH4 and fixed dose of catalyst. b % reduction of 4-NP at a different time interval

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Calibration of 4-nitrophenol

To calculate the unknown amount of 4-NP the standard calibration curve of 4-NP with various concentration was plotted in the range of 1–15 µM at UV–Visible spectrophotometer. Change in absorption peak intensity with respect to the concentration of standard concentration of 4-NP was obtained and standard calibration graph was plotted as per Lambert Beer’s Law as shown in Fig. 11a, b. However, the linear regression equation (y = mx + c) used for a calibration plot. A straight curve was obtained with a R2 value of 0.992, y = 0.1067x + 0.1626.

Fig. 11
figure 11

a Standard calibration plot for 4-nitrophenol solutions in the concentration range from 01 to 15 µM. b inset showing the corresponding linear calibration plot

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Reusability of CeO2 nanostructure

To check the practical and commercial application of catalyst the reusability of catalyst was performed. The reusability of catalyst plays a great role in socio-economic benefits and environmentally friendly impact of method. Therefore, after the successful catalytic reduction of 4NP, the particles were separated out from the reaction mixture through centrifugation and then filtered with watt man filter paper. The collected material was washed many times with Milli-Q water and dried in an oven under inert condition. The reusability of catalyst was checked, the same particles were applied in the same concentration of the fresh 4-NP solution and the same protocol was followed. To monitor any change in the catalytic performance of ceria nanostructure, five times recycled particles were performed efficiently, and it was observed that catalytic efficiency decreased approximately 8% from 99.05 to 87% as shown in graph Fig. 12. The obtained data indicate the good stability of nanoparticles the decrease in efficiency could be due to the loss of catalyst material during the washing and separation process.

Fig. 12
figure 12

Reusability study of CeO2 nanocatalyst for 4-nitrophenol reduction

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Comparison of CeO2 nanocatalyst with previously reported literature

At last, we have compared our reported data with already reported methods (Table 1). The comparative study of 4-NP reduction using various heterogeneous nanocatalysts in terms of catalyst dose, reaction time, concentration was conducted. The comparing table shows the present study using CeO2 NPs as heterogeneous catalyst is cost effective, greener environmentally friendly and far superior then already reported nanocatalyst.

Table 1 Comparison of reduction efficiency of CeO2 nanocatalyst over reported candidates used for 4-nitrophenol reduction
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Conclusion

It is concluded that efficient Ceria nanocatalyst was synthesized using ammonium hydroxide as a reducing agent via chemical precipitation which is cheap, facile, lucrative and greener route. The XRD pattern shows that the synthesized material is highly crystalline in nature. The zeta potential data confirms the nanostructured ceria having a highly positive value of + 6.36 mV. The topographical information conducted via SEM analysis which revealed cubic shaped morphology. The enhanced catalytic performance of nanoceria was achieved by the reduction of 4-NP utilizing NaBH4 under microwave radiation. The nanoceria has excellent stability and nanometric size provides a high surface to volume ratio, increased number of active sites and catalytic performance, enabled the proposed nanostructures to get an edge over other materials reported till to date. The outstanding characteristics of cerium oxide make it a phenomenal aspirant for the conversion of 4-nitrophenol into 4-amino phenol.