The 11th International Chemical Engineering Congress & Exhibition (IChEC 2020) Fouman, Iran, 15-17 April, 2020

Synthesis of Manganese Dioxide (MnO2) Nanoparticles using

Supercritical Water

M. Golmohammadi1*, A. Esmaeili2 1Department of Chemical Engineering, Birjand University of Technology, Birjand, Iran

2College of North Atlantic-Qatar golmohammadi@birjandut.ac.ir

Abstract Supercritical water, because of its unique properties, can be used as a appropriate environment for conducting a wide range of reactions including the synthesis of different nanoparticles. The main objective of the present work was to synthesize MnO2 nanoparticles via supercritical water technique as well as to investigate their properties. Structural and morphological properties of synthesized nanoparticles were determined using X-ray diffraction and scanning electron microscopy (SEM), respectively. The results demonstrated that the crystalline structure of MnO2 was well developed in supercritical water. Furthermore, it was found that the spherical fine particles with mean crystalline size of 35±4 were synthesized in supercritical medium. Keywords: Nanoparticles, Supercritical Water, Characterization, Manganese Dioxide.

Introduction In the last decades, nanoparticles has attracted much attention due to their extraordinary properties. Decreasing the particle size to nanometers leads to the change in magnetic, electrical, chemical behaviors as well as increasing the surface-to-volume ratio. There are various methods to manufacture the nanoparticles such as sol-gel [1], hydrothermal [2], chemical vapor deposition [3], ultrasonic irradiation [4] and hydrothermal synthesis in supercritical water [5]. Most of these methods consume large amounts of chemicals that can be harmful to the environment. In contrast, the production of nanoparticles in the supercritical water environment requires no additional chemicals and the nanoparticles are produced solely from the aqueous precursor solution. Therefore, this method can be considered as one of the green methods for the production of nanoparticles. Water above its critical point (374 °C and 22.1 MPa) possesses unique properties that distinguish it from water at room temperature. The dielectric constant (ε), as the most important factor controlling the solubility, drastically decreases around the critical point [6]. This property is about 78 at room temperature, making polar minerals soluble in water. Increasing the temperature at a given pressure reduces the dielectric constant of water to below 10. In this case, mineral salts are no longer soluble in water and consequently precipitate in the solution [7]. In the supercritical water medium, metallic hydrated ions are first hydrolyzed and then the metal oxide crystals are formed through a dehydration reaction. Accordingly supercritical water is an appropriate medium for

The 11th International Chemical Engineering Congress & Exhibition (IChEC 2020) Fouman, Iran, 15-17 April, 2020

the formation of nanoparticles. This method has many advantages such as short synthesis time, fine nanoparticle production with appropriate distribution, control of size and morphology only with temperature manipulating, and the use of minimum chemicals [8]. In the last two decades, the supercritical water has been employed to synthesize a wide range of metals, metal oxides, and mixed metal oxides. Synthesis of numerous nanoparticles in supercritical water have been extensively reviewed by Yoko et al. [9]. Manganese dioxide has a wide range of applications, such as high-density magnetic storage, catalysts, ion exchange, molecular adsorption, electrochemicals, varistors and solar energy conversion. MnOx-based catalysts have also been identified as active phases in several catalytic oxidation processes and hydrogenation reactions [10]. Accordingly, the main objective of the current work is the synthesis of MnO2, as an effective catalyst, by using supercritical water method. Experimental The MnO2 nanoparticles were synthesized in a batch-wise stainless steel (316L) reactor with a capacity of 100 ml. Firstly, 0.2 M solution of Manganese (II) nitrate tetrahydrate (Merck AG. Fur synthesis) was prepared by dissolving this salt in a given amount of double distilled water (see Figure 1). Then, 30 ml of obtained solution was poured into the reactor and its cap was tightly closed. The reactor was then placed in a furnace with temperature of 480 °C for 3 hours. After this time, the reactor was rapidly quenched by cold water and its contents were evacuated. The fabricated nanoparticles were washed using double distilled water as well as centrifuged three times by using a high speed centrifugation (10000 rpm, 10 min). The resulting precipitate was then poured into a petri dish to dry overnight at ambient temperature. The crystal structure and composition of the synthesized nanoparticles were analyzed by X-ray diffractometry (XRD, Philips PW 1800). Moreover, the morphology of nanoparticles was studied by a KYKY SEM-EM3200 apparatus.

Figure 1. (a): The salt of Mn(NO3)2.4H2O, (b) 0.2 M aqueous solution of the salt.

Results and discussion Figure 2 illustrates the XRD pattern of MnO2 nanoparticles. The highly resolved diffraction peaks implying that the crystalline structure of mixed oxide nanoparticles is well developed through supercritical water synthesis. The diffraction peaks, that appeared at 2θ= 28, 37, 41, 43, 46, 57, 59, 65, 67, and 72° appertain to tetragonal structure of MnO2 with miller indices (110), (101), (200), (111), (210), (211), (220), (002), (310), (301) , respectively and with lattice parameters a= 4.42 Å, b= 4.42 Å and c = 2.87 Å (JCPDS File No. 12-0716). In addition, the crystallite size was calculated about 25 nm by applying Debye-Scherer equation.

The 11th International Chemical Engineering Congress & Exhibition (IChEC 2020) Fouman, Iran, 15-17 April, 2020

Figure 2. The XRD pattern of MnO2 nanoparticles synthesized via supercritical water method.

Moreover, in order to study the morphology as well as particles size distribution, the SEM micrograph of produced nanoparticles was taken and presented in Figure 3. From this figure it obvious that the nanoparticles are spherical in shape, and are in the range of 20-60 nm.

Figure 3. SEM micrograph of MnO2 nanoparticles synthesized via supercritical water method.

Moreover, no significant nanoparticle agglomeration is observed indicating that supercritical water has been successful in manufacturing nanoparticle with uniform size distribution. To prove this claim, the mean size of the nanoparticles was determined using an image processing software and then the particle size distribution histogram was plotted based the mean size of 300 nanoparticles (Figure 4). As can be seen, the nanoparticles have a fairly normal distribution with an average particle size of 37±6 nm.

The 11th International Chemical Engineering Congress & Exhibition (IChEC 2020) Fouman, Iran, 15-17 April, 2020

Figure 4. Particle size distribution histogram of MnO2 nanoparticles synthesized via supercritical water

method. Conclusions In this study, MnO2 nanoparticles were synthesized in SCW medium and also were characterized by various analyses. The results of the XRD and SEM indicated that the obtained nanoparticles possessed satisfactory size and morphology with narrow particle size distribution. Acknowledgements We gratefully acknowledge the Birjand University of Technology for the support of this work.

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The 11th International Chemical Engineering Congress & Exhibition (IChEC 2020) Fouman, Iran, 15-17 April, 2020

[7] Marcus, Y., Supercritical Water, John Wiley & Sons, New Jersey, (2012). [8] Adschiri, T., Kanazawa, K., and Arai, K., "Rapid and Continuous Hydrothermal Crystallization of Metal Oxide Particles in Supercritical Water", J. Am. Ceram. Soc., 75, 1019–1022 (1992). [9] Yoko, A., Aida, T., Aoki, N., Hojo, D., Koshimizu, M., Ohara, S., Seong, G., Takami, S., Togashi, T., Tomai, T., Tsukada, T., Adschiri, T., and Naito, M. (Eds.), Nanoparticle Technol. Handb. Elsevier, pp. 683–689 (2018). [10] Ren, T.Z., Yuan, Z.Y., Du, G.H., and Su, B.L., "Facile preparation of nanostructured manganese oxides by hydrotreatment of commercial particles", 162, 425-432 (2006).