Tin Oxide Supported on Nanostructured MnO₂ as Efficient Catalyst for Nitrophenol Reduction: Kinetic Analysis and Their Application as Heterogeneous Catalyst

Ata Ullah ¹ , Lutfur Rahman ¹ , Syed Zajif Hussain ² , Muhammad Bilal Yazdani ¹ , Asim Jilani ³ , Dayum Iqbal Khan ¹ , Musadiq Zeshan Nasir ¹ , Waheed S Khan ¹ , Irshad Hussain ² , Asma Rehman?? ¹

1 National Institute for Biotechnology and Genetic Engineering (NIBGE), Faisalabad (38000), Pakistan

2 Constituent College Pakistan Institute of Engineering and Applied Sciences, Faisalabad, Pakistan

3 Department of Chemistry and Chemical Engineering, SBA School of Science & Engineering (SBASSE), Lahore University of Management Sciences (LUMS), DHA, Pakistan

4 Center of Nanotechnology, King Abdul Aziz University, Jeddah, 21589, Saudi Arabia

Abstract

Recently, metal oxides-based nanomaterials have been widely used for catalytic reduction of nitro-aromatic compounds, which are notorious for their carcinogenic nature. The current study reports Sn-doped MnO₂ as an efficient catalyst for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The FE-SEM characterization of SnO₂-doped MnO₂ revealed the diffused flower-like morphology. Further, the XPS survey scans were performed to investigate the binding energies, oxidation states, and elemental compositions of both MnO₂ and Sn-doped MnO₂. Kinetics analysis revealed that the catalytic reduction (> 98.8%) of 4-NP into 4-AP by Sn-doped MnO₂ in the presence of NaBH₄ occurs within four min, following pseudo-first order kinetics. Importantly, no observable deactivation of catalytic efficiency was noticed even after five cycles. Our strategy of loading SnO₂ on the surface of semiconductor nanostructures offers a versatile approach to enhance the catalytic performance, stability, and recyclability which may further promote their practical industrial application.

Keywords

Industrial effluent, Carcinogenic, Catalytic reduction, Sn doped MnO2, 4-­Nitrophenol

INTRODUCTION

Advancement in metal oxides-based nanomaterials have garnered their application in several fields, including catalysis.¹ Metal oxides are used for the development of various catalysts due to their unique physicochemical properties such as high stability, multiple active-sites, and large surface-area. Such nano-catalysts have sufficient bandgap to catalyze several chemicalreactions.² Among these metal oxides, manganese dioxide (MnO₂) is considered as a promising material for designing the catalysts for efficient reduction of aro?matic compounds, such as 4-Nitrophenol (4-NP). It is because of their relative abundance, environmental compatibility, high surface-reactivity and chemical? stability.³ Morphology of such materials can be tuned into various shapes, such as nanosphere⁴, nanowires⁵, and nanotubes⁶, etc.

Figure 1: Field emission scanning electron micrograph of the designed catalyst (a) MnO₂ NPs. (b, c, d) Sn-doped MnO₂ morphologies at different resolutions.

However, their catalytic activity has been restricted due to the fast recombination of electron-hole pairs (ehp).⁷ Further, metal oxide nanoparticles (NPs) can agglomerate with each other and remain unstable due to their high-surface energy. In addition, from the reaction mixture, these NPs cannot be easily separated and thus difficult to recycle. Therefore, numerous efforts have been made to stabilize metal oxide NPs to prevent their agglomeration and enhance their stability/recyclability.⁸ Surface-capping of metal oxides NPs effectively prevent their agglomeration and maintain their stability. Similarly, these NPs may be loaded on supporting materials to design heterogeneous catalysts that exhibit high catalytic activity and are easier to recycle and reuse.⁹ Another approach is doping, which is the most promising, and give multiple properties of two or more NPs in a single composite resulting in extraordinary catalytic activity, stability, and recyclability.¹⁰

The current study reports a convenient and facile approach for the synthesis of Sn-doped MnO₂ based heterogeneous catalysts with unique morphology and proficient catalytic activity. Tin-oxide (SnO₂) is an n-type metal oxide semiconductor that exhibits high carrier density, oxygen vacancy, and chemical stability. Due to these properties, it can potentially catalyze several reactions in an environmental interest.¹¹ Therefore, Sn-doped MnO₂-based composite may have an excellent interface to prevent ehp recombination. The possible reason behind it could be the creation of surface defects and oxygen vacancies on the surface of SnO₂. Thus, increasing in the catalytic performance, stability and recyclability of the designed hybrid catalyst.^11,12

On the other hand, rapid industrialization has led to the contamination of water which is not only vital for human survival but also necessary for sustainable agriculture.¹³ Effluents from several, chemical, pharmaceutical, pulp/paper and tannery industries are frequently being discharged into aquatic bodies without prior treatment, which causes severe environmental concerns.¹⁴ These effluents contain heavy metals, recalcitrant dyes and other aromatic compounds.¹⁵ Among them, 4-NP is an essential precursor in the synthesis of synthetic dyes, paint, pesticides, herbicides explosive, drugs, wood preservatives, etc.¹⁶ Therefore, effluents discharged from these chemical industries are contaminated with 4-NP, which is highly carcinogenic for animals and induces toxicity in living tissues via the redox process.¹⁷ Therefore, the detoxification of 4-NP in aqueous bodies via reduction into their lesser toxic counterpart has significantly attracted the global interests.¹⁷

To address the above-mentioned environmental challenges, we report a facile hydrothermal approach for the synthesis of Sn-doped MnO₂ hybrid composite, as a promising catalyst for 4-NP reduction. The as-synthesized catalyst has been characterized through Field-Emission Scanning Electron Microscope (FE-SEM) and X-ray Photoelectron Spectroscopy (XPS). Their catalytic efficiency has been measured spectrophotometrically at different concentrations of 4-NP and sodium borohydride (NaBH₄). Additionally, the recyclability of Sn-doped MnO₂ catalyst has also been checked for 5-cycles that demonstrate its excellent potential for recyclability and reusability.

EXPERIMENTAL SETUP

All analytically grade chemicals i.e., potassium permanganate (KMnO₄) 98+ % from ACROS, anhydrous Tin (IV) chloride (SnCl₂.4H₂O) 98 % from Alfa Aesar, ethanol (absolute), hydrochloric acid (37 %) from Sigma-Aldrich, hydrogen peroxide (H₂O₂), and poly (vinyl)alcohol (PVA) were purchased from ACROS. All the solutions were prepared in deionized water (ρ = 18 MΩ cm from the Millipore Milli-Q system.

Synthesis of Manganese dioxide nanostructures

Manganese dioxide (MnO₂) nanostructures were synthesized by modifying the previously reported method.¹⁸ In a typical experiment, 0.048 g of KMnO₄ was added into 50 mL of HNO₃ (1.3M). After 30 min magnetic stirring, the solution was transferred to a Teflon-lined autoclave and hydrothermally treated at 80 °C for 5 hours. The product was recovered through centrifugation (3000 rpm), and washed several times with ethanol, and deionized water sequentially. Then, the prepared catalysts were vacuum dried at 60 ?C overnight and stored in a desiccator for further use.

Synthesis of Sn-doped MnO₂ nanocomposites

For the synthesis of Sn-doped MnO₂, as-synthesized MnO₂ nanostructures were allowed to a second low-hydrothermal treatment in the presence of SnCl₂.4H₂O ranging from 1:1 to 1:9 Sn/Mn atomic weight ratios.¹⁹ Briefly, a specific amount of thoroughly sonicated MnO₂ nanostructures were added to 20 mL acidic solution (0.7 mL of 38%) of SnCl₂.4H₂O (0.15 g). The mixture was magnetically stirred for 30 min. Then, transferred it to 100 mL Teflon-lined stainless-steel autoclave followed by hydrothermal treatment at 90 ?C for 5 hours. The as-synthesized Sn-doped MnO₂ were recovered through centrifugation, vacuum dried at 70 ?C and stored in a desiccator for further studies.

The morphology of the synthesize nanomaterials were explored through Field Emission-Scanning Electron Microscopy (FE-SEM). Further, the compositional information and binding energy of Sn-doped MnO₂ were determined through X-ray Photoelectron spectroscopy (XPS).

Catalytic study

The catalytic activities of both MnO₂ nanostructure and Sn-doped MnO₂ were investigated using 4-NP as a model pollutant. In a typical reaction, 2.7 mL of 4-NP (0.1 mM) was mixed with 0.3 mL of freshly prepared NaBH₄ (0.1 M), followed by the addition of catalyst in the range of 1-5 mg mL^-1 under optimized reaction conditions at room temperature. The catalytic reduction of the 4-NP was spectrophotometrically monitored in the range of 200-700 nm with respect to time (t). The recyclability of as-synthesized catalyst was also checked as follows: once the reaction was completed, the catalyst was recovered through centrifugation, washed with 10 % ethanol solution, and vacuum dried at 70 ?C for 5-6 h. The dried catalysts were then again used for the subsequent reactions to check their reusability and stability.

RESULTS AND DISCUSSION

Figure 2: (a-c) Survey scan and compositional analysis of MnO₂ and Sn-doped MnO₂, while (d) Sn3d5 analysis of Sn-doped MnO_2.

Figure 3: Mn2p analysis for (a) MnO₂ nanostructures (b) Sn-doped MnO₂ (c-d) O1s functional groups analysis for (c) MnO₂ nanostructures and (d) Sn-doped MnO₂.

Figure 4: UV-Vis absorption spectra of the catalytic reduction of 4-NP in the presence of Sn-doped MnO₂, using NaBH₄ as electrons donor.

The reduction kinetics of 4-NP were calculated from the UV-Vis absorption spectra of the reaction solution, which reveal that the reduction kinetics follow pseudo-first-order. Pseudo-first-order kinetics can be described as ln (Ct/Co) where Co and Ct are the concentrations of 4-AP at time zero and t, respectively. The ln (Ct/Co) was plotted against reaction time (t), which exhibited the best linear correlation (R² > 0.996) as shown in Figure 5(a). The rate constant (K_app) was calculated from the slope of _ln(Ct/Co)-t , which revealed that the Sn-doped MnO₂ had efficient catalytic reduction activity with K_app > 4.7884 min^-1 as compared to MnO₂ alone (K_app = 0).

Figure 5: (a) Plot of ln (Ct/Co) against the reaction time ‘t’ for MnO₂, Sn-doped MnO₂ under 1M NaBH₄, 0.1mM 4-NP, respectively at room temperature, (b) Plot of K_app against amount of Sn-doped MnO₂ under optimized conditions, (c, d) are the effect of 4-NP and NaBH₄ concentrations on the rate constant K_app.

Figure 6: (a) Plot of 4-NP % reduction against reaction time; (b) recyclability and reusability properties of the Sn-doped MnO₂ catalyst under optimized reaction condition.

The FE-SEM morphology of MnO₂ showed their diffused flower-like morphology as shown in Figure 1(a). Such flower-like morphology for MnO₂ has also been reported by Rivera-Lugo and co-workers.¹⁸ Our study shows that at low hydrothermal temperature and lesser treatment time (5-6 h), MnO₂ exhibits flower-like morphology. However, at high temperature and longer treatment time, the flower-like morphology transforms into rod-like structures. The FE-SEM micrograph of Sn-doped MnO₂ shows flower-like morphology, which is comprised of large, thick petal-like layers (Figure 1 (b-d)). This may indicate that second-low hydrothermal treatment of MnO₂ in the presence of Tin (IV) chloride (acidic condition) has improved their diffused flower-like morphology with well-defined, large and thick layers, like petals. Such transformation in morphology has also been demonstrated by Lan and co-workers.²⁰ Zhao et al have further explained that the flower-like morphology provides large surface area and enhance catalyst’s activities. Further reported that catalysts with flower-like morphology possesses more atoms at the edges and corners, thus provides more active sites for the interactions with reactants.²¹

The surface compositions of as-synthesized MnO₂ nanostructures and Sn-doped MnO₂ were investigated using XPS. The survey scan of MnO₂ nanostructure revealed the presence of Mn2p3 (18.5 %) and O1s (81.5%) without any residual impurities (Figure 2(a)). Further, the survey scan of Sn-doped MnO₂ (Figure 2(b)) confirms the presence of Sn3d5 (36.9%) along with Mn2p3 (7.6%) and O1s (55.5%). The graphical representation of each detected element of MnO₂ nanostructure and Sn-doped MnO₂ is shown in Figure 2(c).

Moreover, the chemical state analysis of Sn3d5, Mn2p3 and O1s were performed to determine the functional groups. The Sn3d5 analysis of Sn-doped MnO₂ (Figure 2(d)), Sn3d5 has two peaks around 486.97 eV and 495.47 eV that are attributed to the 3d5/2 and 3d/3/2, respectively. Further, the difference between these two peaks were noticed around 8.5 eV. Moreover, the peak fitting of 3d5/2 disclosed the manifestation of two peaks around 484.74 eV and 486.94 eV, which is recognized as SnO₂ and Sn in metallic form.²² Moreover, the contribution of SnO₂ was around 81.34 %, while 18.66% for Sn metallic.  

Mn2p analysis for MnO₂ nanostructures show (Figure 3(a)) the appearance of two main peaks around 640.50 and 651.41 eV which are attributed to the 2p3/2 and 2p1/2, respectively. Further, the peak fitting of 2p3/2 and 2p1/2 revealed four peaks around 640.49 eV, 643.95, 651.41 and 653.50 eV respectively for Mn3+, Mn4+ (2p3/2 ) and Mn3+, Mn4+ (2p1/2 ).22 Moreover, a change in Mn³⁺ and Mn⁴⁺ was observed after the addition of SnO₂ (Figure 3(b)).

O1s spectra of MnO₂ nanostructures (Figure 3(c)) show the presence of two peaks around 530.90 eV and 532.3 eV, which are attributed to Mn-OH and OH^-, respectively. The contribution of Mn-OH changed from 76.80 % to 60.70 % with the addition of Sn to MnO₂ (Figure 3(d)).^23-24 However, an increase in the OH^- was noticed from 23.20 % to 39.30 % is favorable for the enhanced catalytic activity of these materials.25 

The catalytic activity of the Sn-doped MnO₂ was explored for the reduction of 4-NP into 4-aminophenol (4-AP), using sodium borohydride (NaBH₄) as a source of electron donor. Catalytic reduction of 4-NP was monitored by UV-Vis absorption spectra of the solution (Figure 4). A maximum absorption spectrum of 4-NP ions in solution was recorded at 400 nm, when NaBH₄ was added to the 4-NP solution. Once, the Sn-doped MnO₂ catalyst was added to 4-NP solution, a significant decrease was observed in the intensity of absorption peak at 400 nm, while an increase in absorption peak at 300 nm was recorded that indicates the conversion of 4-NP to 4-AP. Moreover, the catalytic reduction was also observed visually by observing the fading of the greenish color of 4-NP solution to transparent.²⁶ As-synthesized Sn-doped MnO₂ catalysts have shown excellent reduction activity and completed the reduction within 4 minutes, however MnO₂ nanostructures haven’t shown significant catalytic activity. Thus, the best catalytic activity of Sn-doped MnO₂ is due to the synergistic effect of both metal oxides, namely SnO₂ and MnO₂. The reported literature has mentioned that Sn-doping on MnO₂ heterostructure may decreases their bandgap. Due to which the electrons are easily ejected for MnO₂ to produced electron-hole pairs that impart proficient catalytic activity to Sn-doped MnO₂ catalyst.²⁷

Further, the 4-NP reduction kinetics were explored at different reaction conditions. The amount of Sn-doped MnO₂ was changed in the range of 1-5 mg, while keeping NaBH₄ (1 M) and 4-NP (0.1 mM) concentrations constant. Under these optimized conditions, the K_app increased linearly with increasing the amount of catalyst (Figure 5(b)). It is obvious, that increase in catalyst amount provide more active sites for the reduction process. In the case of increasing NaBH₄ concentration from 0.025 to 0.1 M, an increase in K_ap p occurred; though no change in K_app was observed when the concentration increased from 0.15 M to 0.2 M (Figure 5(c)). Similarly, the effect of 4-NP concentration (0.05-0.2 mM) was investigated, and the results are shown in Figure 5(d). The K_app increases as the 4-NP concentration increases from 0.05 to 0.15 mM, but beyond 0.15 mM concentration K_app decreases. However, these nonlinear relationships between K_app with NaBH₄, along with a fall in K_app by increasing the 4-NP concentration shows that both reactants compete with each other on the surface of Sn-doped MnO₂ catalyst. Similar relation between rate constant and nitrophenol concentration was documented by Mona and Reza Gholami, using CeO₂ nanorods supported on CuNi nanoparticles.²⁸

The percent (%) reduction of 4-NP to 4-AP by the as-synthesized catalysts in the presence of NaBH₄ were calculated using the equation; η=1-Ct/Co×100 (Figure 6(a)),²⁹ which showed > 98% reduction of 4-NP into 4-AP within 4 min. The recyclability and reusability of Sn-doped MnO₂ were checked for five consecutive cycles under optimized conditions (4-NP (0.1 mM), 0.1 M NaBH₄ at RT) (Figure 4(b)). The results showed that in each consecutive cycle the reduction time increases slightly i.e., up to 15-30 sec, that might be due to the minute loss of catalyst during recovery processing (centrifugation, washing and drying).³⁰

CONCLUSIONS

A facile hydrothermal approach was used for the synthesis of hybrid Sn-doped MnO₂ catalyst. The flower-like morphology of Sn-doped MnO₂ with diffused petals showed that the deposition of specific amount of Sn on MnO₂ nanostructure has improved the petal-like layers of MnO₂. XPS study confirmed that the Mn-OH reduced from 76.80 % to 60.70 % due to the addition of Sn on MnO₂, which depict the successful deposition of Sn on MnO₂ nanostructure. The pure MnO₂ nanostructures were found catalytically inactive but the designed Sn-doped MnO₂ have reduced 4-NP into 4-AP (>98.8%) only in four minutes with high catalytic activity even after five cycles, thus demonstrating their effective reusability.

ACKNOWLEDGMENTS

The authors declare not conflict of interest.This study was funded by the Higher Education Commission (HEC) of Pakistan under the head of NRPU Project # 6118). The Principal investigator (PI) of this project was; Dr. Asma Rahman, Senior scientist at National Institute of Biotechnology and Genetic-engineering (NIBGE), Faisalabad. The authors highly acknowledge the financial support of the HEC, Pakistan.1

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