Controllable Synthesis of Mn₃O₄ Nanowires and Application in the Treatment of Phenol at Room Temperature

Abstract

Nanosized Mn₃O₄ nanowires are prepared with KMnO₄ and ethanol in mild conditions by facile hydrothermal method. Hydrothermal reaction temperature is optimized to get uniform nanowires. The prepared Mn₃O₄ nanowires exhibit high activity in the treatment of phenol at acid condition and room temperature. The 20 mg Mn₃O₄ nanowires can efficiently dispose of 50 mL phenol solution (0.2 g·L⁻¹) at pH 2 and 25 °C. The nanowires before and after phenol treatment are characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) and the reaction mechanism is discussed.

1. Introduction

Manganese is one of the most earth-abundant elements and manganese oxides are generally non-toxic. Manganese oxide nanomaterials also show great potential in sustainable nanotechnology. They are widely utilized in catalytic reactions, sensors and batteries because of their low cost and high activity [1,2]. Mn₃O₄, also known as hausmannite, is a mixed valence oxide and a promising candidate for catalysts, microwave absorption materials, sensors and anode materials [3]. It has been used in the catalytic oxidation of methane and reduction of nitrobenzene [4]. Lu et al. prepared the amorphous MnO₂ thin film first then hydrothermally transformed it into Mn₃O₄ nanowires under room temperature in a solution bath of 0.01 M manganese acetate and 0.01 M sodium sulfate mixture [5]. One-dimensional (1D) nanostructures, especially nanowires, are of great importance because of their specific shape and potential applications. Nanowires feature superior functional properties and mechanical strengths which have been used in micro/nanoelectromechanical systems and photovoltaic applications [6]. Veeramani et al. presented a room-temperature synthesis of Mn₃O₄ nanowires solely from water and manganese salt, catalyzed by iron oxide nanocrystals in the presence of piperazine-N,N^I-bis(2-ethanesulfonic acid) (PIPES) [7]. Sambasivam et al. synthesized single-crystalline Mn₃O₄ nanowires using solvothermal technique [8].

Tremendous discharge of toxic organic compounds has contributed to serious pollution to our eco-environment and human beings. Phenolic compounds play an important role in the production of pesticides, resins and antioxidants [9]. However, the phenolic waste waters are highly toxic and persistent, and are difficult to treat with traditional biodegradation. Strong oxidants such as H₂O₂ and O₃, without secondary pollution, have been utilized in the treatment of organic compound waste waters. However, the treatment efficiency is relatively low [10,11]. To achieve high mineralization, advanced oxidation processes (AOPs) were proposed to degrade organic pollutants through the generation of highly reactive hydroxyl radicals [12,13]. The Fenton process combines H₂O₂ and iron as a catalyst to generate hydroxyl radicals, which have strong oxidation capability on the phenol removal [14]. However, the homogeneous Fenton process is limited by its narrow pH range and iron sludge generation [15,16,17].

In this work, uniform Mn₃O₄ nanowires are fabricated with facile hydrothermal method and common raw materials in mild conditions. They are utilized to deal with phenol solution without extra H₂O₂ or O₃ at room temperature and air condition. The mechanism of the Mn₃O₄ nanowires on the treatment of phenol is also studied with the characterization of scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS).

2. Experimental

2.1. Preparation and Characterization of Mn₃O₄

The nanosized Mn₃O₄ was provided by US research Nanomaterials, Inc (Houston, TX, USA) for comparison. Analytically pure KMnO₄ and ethanol was used as raw materials. First, 0.2 g KMnO₄ and 20 mL ethanol/water solution were added in a 45 mL Teflon-lined steel autoclave. The ratio of ethanol and reaction temperature in the furnace are investigated to get uniform nanowire and improved performance in phenol treatment. All products were cleaned with deionized water several times and dried in air at 80 °C for 12 h. Then the Mn₃O₄ powder was ground before utilization. The 3Flex Surface Characterization Analyzer (Micromeritics Corporation, Norcross, GA, USA) is able to determine the adsorption–desorption isotherm of nitrogen at 77 K, then, as a function of the kind of isotherm, according to international union of pure and applied chemistry (IUPAC) classification, a mathematical model to find specific surface areas (e.g., Brunauer–Emmett–Teller (BET) adsorption). The prepared Mn₃O₄ and used nanoparticles were tested with SEM (FE-SEM, Hitachi S-4800, Tokyo, Japan), XPS (ThermoESCALAB250Xi, Waltham, MA, USA) and XRD (XRD-7000S, Shimadzu, Tokyo, Japan) to determine the morphology and chemical composition of the material.

2.2. Treatment of Phenol Waste Water at Room Temperature

The phenol was dissolved in de-ionized water with H₂SO₄ to adjust the pH. In a typical procedure, 20 mg Mn₃O₄ was added into a phenol solution (50 mL, 0.2 g·L⁻¹ and pH = 2) at room temperature in an air flow. The concentration of phenol is tested with UV-Vis spectrum at 270 nm every 15 min after the reaction solution was filtrated with a 0.1 μm poly tetra fluoroethylene (PTFE) syringe filter (Whatman^TM, Shanghai, China).

3. Results and Discussion

3.1. Composition and Morphologies of the Prepared Mn₃O₄

The effect of hydrothermal temperature on the diffraction patterns are shown in Figure 1. It is obvious that the prepared nanoparticles are all Mn₃O₄ which are similar with the commercial Mn₃O₄ nanoparticles. What is more, they are all in accordance with hausmannite type Mn₃O₄ PDF card (JCPDS no. 24-0734) even though the intensity of some peaks are different which may be induced by the formation of nanowires. When the reaction temperature decreases to 110 °C, the intensity of (211) plane is very high while some other planes such as (101), (220) and (400) planes almost disappeared.

The SEM images of Mn₃O₄ prepared at different temperatures are shown in Figure 2. When the hydrothermal temperature is 170 °C, uniform octahedron-like Mn₃O₄ particles with edge size about 60 nm are observed which are similar with the commercial Mn₃O₄ nanoparticles. With the decrease of temperature, the uniform Mn₃O₄ nanowires appear while the blocks disappear gradually. The nanowires with large specific surface area will improve the performance of Mn₃O₄. According to the data of BET surface area test, the BET surface area of commercial Mn₃O₄ is 14.48 m²·g⁻¹ while the BET surface area of Mn₃O₄ (110 °C) and Mn₃O₄ (130 °C) are 248.8 and 189.69 m²·g⁻¹, respectively, as shown in

Table 1. BET surface area of Mn₃O₄.

	Mn3O4 (Commercial)	Mn3O4 (130 °C)	Mn3O4 (110 °C)
BET surface area/(m2·g−1) Adsorption average pore diameter/nm	14.48 21.5	189.69 10.3	248.8 10.6

. Along with this, the adsorption average pore diameter of Mn₃O₄ (110 °C) is about 10.6 nm which is positive to the adsorption and reaction activity of the nanowires.

3.2. Treatment of Phenol with Mn₃O₄ at Room Temperature

Phenol is widely used in the manufacturing of plastics, pesticides and pharmaceuticals. Because of its high toxicity, a lot of method is recommended but still not efficient. In this work, nanosized Mn₃O₄ (20 mg) was used to deal with the phenol waste water at room temperature in the air condition. As shown in Figure 3, the phenol concentration decreases obviously in 60 min. Especially when the preparation temperature is 110 °C, the performance of the Mn₃O₄ is the best because the Mn₃O₄ prepared at 110 °C shows the uniform nanowires and highest specific surface area. As shown in

Table 2. Comparison of phenol removal with different reagents.

Reagent and Content	Phenol Concentration	Temperature /°C	Time /h	Conversion /%	References
2.5 g·L−1 Au/C + 5 mL H2O2	45 mL (5 g·L−1)	80	22	~100	[18]
20 mg CNT/PEG	50 mL(20 mg·L−1)	120	0.5	~98	[19]
20 mg Mn3O4 nanowires	50 mL (200 g·L−1)	25	1	93	This work

, the Mn₃O₄ nanowires can efficiently deal with phenol waste water with high concentration at low temperature.

Effect of pH on the performance of Mn₃O₄ is studied as shown in Figure 4. At neutral condition, almost no phenol will be consumed at all, which means that Mn₃O₄ will not react with phenol or adsorb it. With the decrease of pH, the reactivity of Mn₃O₄ increases obviously. When the pH is below 2, the Mn₃O₄ has good performance on treatment of phenol. However, lower pH will cause serious leaching of Mn₃O₄ because the nanosized Mn₃O₄ can be dissolved in concentrated sulfuric acid and hydrochloric acid.

3.3. Analyse of the Function of Mn₃O₄ in the Treatment of Phenol

Different reaction atmosphere is utilized to investigate the mechanism of phenol treatment. As shown in Figure 5, in all the reaction condition, the Mn₃O₄ has the similar performance which means oxygen is not necessary for the treatment of phenol waste water. In the product of the reaction, benzoquinone was detected which means Mn₃O₄ nanowires can oxidize phenol at room temperature.

H₂O₂ is an effective oxygen content with low cost and environmentally friendly nature. The excess H₂O₂ can decompose into safe H₂O and O₂ [20]. However, H₂O₂ (0.1 mol·L⁻¹) cannot oxidize phenol at the same pH in air condition. In contrast, Mn₃O₄ nanowires prepared in this work with large specific area has stronger oxidation reactivity compared with H₂O₂ because it can oxidize H₂O₂. After reaction with H₂O₂, oxygen is formed while Mn₃O₄ is dissolved in the solution. KMnO₄ which is the raw material of Mn₃O₄ nanowires also have strong oxidation ability. In order to completely oxidize phenol, excess dosage of KMnO₄ is needed which will induce secondary pollution to the waste water. Excess Mn₃O₄ nanowires can be easily filtrated and leaching Mn ions can also be removed by neutralization of the acidic waste water.

Benzoquinone and hydroquinone are the common oxidation products of phenol. In this work, pure benzoquinone and hydroquinone solutions are used as the waste water separately, while 20 mg Mn₃O₄ is added in the 50 mL solution (pH = 2, adjusted with H₂SO₄). After 5 min, about 60% hydroquinone is converted to benzoquinone as shown in Figure 6. It was also found that the solution is changed into yellow (the color of benzoquinone) while all Mn₃O₄ nanowires are dissolved which demonstrates that hydroquinone can react with the nanoparticles efficiently. For the benzoquinone solution, the absorbance does not change which means that the Mn₃O₄ nanowires cannot react with stable benzoquinone.

After reaction with phenol, the morphology of Mn₃O₄ changes obviously as shown in Figure 7. The uniform nanowires disappear while a lot of block-shaped particles are formed. After reaction, the reactivity of Mn₃O₄ also disappears because the reaction will not continue after adding extra phenol.

Although the morphology of the Mn₃O₄ particles changes obviously after reaction with phenol, the XRD patterns of Mn₃O₄ after reaction as shown in Figure 8 does not change very much and the characteristic peaks keep similar intensity which means only partial active surface area reacts with phenol. The reacted Mn₃O₄ still has the similar crystal structure with hausmannite.

The chemical composition and element valence of the materials before and after phenol removal are further examined by XPS (Figure 9). The Mn 2p3/2 peak of the prepared Mn₃O₄ and reacted Mn₃O₄ consisted of three separate peaks at 642.8, 641.6 and 640.6 eV (Figure 9b) which are in accordance with Mn⁴⁺, Mn³⁺ and Mn²⁺, respectively. Before reaction, the relative atomic percentages of Mn⁴⁺, Mn³⁺ and Mn²⁺ are 41.9%, 53.5% and 4.6%, respectively. After reaction with phenol, the relative percentages change into 68.1%, 25.5% and 6.4%. Although the average valence is improved according to the data, the possible reason is the dissolution of Mn with low valence in acid condition because the XPS spectrum of reacted Mn₃O₄ shows obvious noise which indicates pollution of the sample and low content of Mn.

4. Conclusions

Hydrothermal temperature and ethanol content have a strong effect on the morphology and structure of nanoparticles. Uniform Mn₃O₄ nanowires with high specific surface area (BET surface area, 248.8 m²·g⁻¹) are formed when the hydrothermal temperature is 110 °C and the ethanol content is 10 mL. The Mn₃O₄ nanowires show high efficiency in phenol removal at room temperature and air condition because it has strong oxidation ability compared with H₂O₂. The 20 mg Mn₃O₄ nanowires can efficiently dispose of 50 mL phenol solution (0.2 g·L⁻¹) at pH 2 and 25 °C. According to the SEM, XRD and XPS characterization, the Mn₃O₄ shows strong oxidation capability and reacts with phenol. After reaction, partial Mn with low valence is leaching into the acid solution. Therefore, it is very efficient in the quick removal of phenol in water treatment without special operation parameter. Excess Mn₃O₄ nanowires can be easily filtrated and leaching Mn ions also can be removed by neutralization of the acidic waste water.