Synthesis of manganese oxide microparticles using supercritical water

Accepted Manuscript Title: Synthesis of manganese oxide microparticles using supercritical water Author: Minsoo Kim Seung-Ah Hong Naechul Shin Keun Hwa Chae Hong-shik Lee Sun Choi Youhwan Shin PII: DOI: Reference:

S0896-8446(16)30041-9 http://dx.doi.org/doi:10.1016/j.supflu.2016.03.004 SUPFLU 3582

To appear in:

J. of Supercritical Fluids

Received date: Revised date: Accepted date:

4-2-2016 7-3-2016 7-3-2016

Please cite this article as: Minsoo Kim, Seung-Ah Hong, Naechul Shin, Keun Hwa Chae, Hong-shik Lee, Sun Choi, Youhwan Shin, Synthesis of manganese oxide microparticles using supercritical water, The Journal of Supercritical Fluids http://dx.doi.org/10.1016/j.supflu.2016.03.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis of manganese oxide microparticles using supercritical water

Minsoo Kima, Seung-Ah Hongb, Naechul Shinc, Keun Hwa Chaed, Hong-shik Leee, Sun Choia, Youhwan Shina*

a

Center for Urban Energy Research, Korea Institute of Science and Technology (KIST), Hwarangno

14-gil 5, Seongbuk-gu, Seoul 136-791, Korea b

Department of Chemical and Biological Engineering, Korea University, 5-1 Anam Dong, Seongbuk-

gu, Seoul 136-701, Korea c

Department of Chemical Engineering, Inha University, 100 Inha-ro, Nam-gu, Incheon 402-751,

Korea d

e

Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 136-791, Korea Green Material and Process Group, Korea Institute of Industrial Technology (KITECH), 89

Yangdaegiro-gil, Ipjang-myeon, Seobuk-gu, Cheonan-si, Chungcheongnam-do 331-822, Korea

Graphical abstract

Highlights -

Manganese oxides of different oxidation states were synthesized in SCW

-

MnCO3 was formed when glycerol was used with SCW

-

Calcination step was added to obtain various phases of manganese oxides

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MnO2, MnCO3, Mn2O3, and MnO+Mn3O4 were successfully synthesized

Abstract Manganese compounds of different oxidation states such as MnO2, MnCO3 Mn2O3, and a mixture of MnO + Mn3O4 were synthesized using supercritical water (SCW) and calcination process. The Xray Diffraction (XRD) patterns confirmed that the use of glycerol as a reducing agent in SCW process was successful in preventing oxidation of manganese products. Scanning electron microscopy (SEM) images of the manganese products showed micro-sized particles with different morphology depending on the product. The simple two step synthesis procedure described in this paper allows easy control of manganese oxidation states with direct applicability in large scale production on an industrial level.

Keywords: Manganese oxide; Supercritical water; Hydrothermal synthesis; Glycerol

1. Introduction Manganese oxide is a commonly used transition metal oxide that has a broad range of applications in catalysis, fertilizers, paints, ceramics, capacitors, sensors, and lithium ion batteries. Manganese oxides exist in different oxidations states such as MnO, MnO2, Mn2O3, and Mn3O4, all of which are used in different applications [1-4]. Manganese oxides can be synthesized by sol-gel [5], hydrothermal route [6], wet chemical method [7], and solid state method [8, 9]. Among the various methods of synthesis, use of supercritical water (SCW) is part of the hydrothermal route which is an environmentally friendly option [10]. Inorganic materials crystallization using SCW began in the early 1990s where SCW was used as a solvent for the synthesis of 6 different metal oxides [11]. Materials synthesis using SCW technology has many advantages; the process can be easily scaled up using a plug flow system, uses cheap and clean solvents, and the process is rapid where residence time can be in the order of seconds.

Manganese oxide synthesis using SCW has been attempted by Lee et al. and Nugroho et al [12, 13]. Lee et al. synthesized MnO2, Mn2O3 and LiMn2O4 continuously using SCW at varying temperatures where hydrogen peroxide (H2O2) and/or potassium hydroxide (KOH) were added to assist oxidation. Nugroho et al. synthesized MnO2 and Mn3O4 continuously using SCW at 400 °C where KOH was used as a reducing agent. Jankovsky et al. synthesized MnO, Mn2O3 and Mn3O4 by thermal decomposition of manganese glycerolate at different temperatures and atmospheres [14]. They prepared manganese glycerolate by reacting manganese precursor with glycerol under reflux followed by a subsequent calcination. Manganese has a low reduction potential [15] and therefore is easily oxidized upon hydrothermal reaction. In this work, however, manganese oxides of different oxidation states were synthesized in SCW where glycerol was utilized as a reducing agent to either reduce manganese oxides or prevent oxidation. The objective was to develop an energy efficient two step production process of controlling oxidation states of manganese oxides using SCW and calcination process stepwise. Calcination temperature was fixed at 500 °C to minimize the use of energy.

2. Materials and methods 2.1 Materials Manganese (II) nitrate hexahydrate [Mn(NO3)2·6H2O] was purchased from Junsei. Glycerol (99.0%) was obtained from Samchun Chemicals. Deionized water (DIW) was filtered using Milli-Q Ultrapure Water Purification System with a 0.22 μm filter (Millipore). Nitrogen gas was supplied by Shinyang Sanso Company.

2.2 Experimental methods Hydrothermal synthesis of manganese oxides was carried out using a stainless steel (SUS316) reactor with 30 ml inner volume. A salt bath (consisting of KNO3, NaNO3 and Ca(NO3)2) furnace controlled by a thermostat was used to heat up the reactor to the desired temperature. Manganese nitrate hexahydrate reagent is in solid form at room temperature which required warming up in an oven at 50 °C to obtain liquid form. This liquid form Mn(NO3)2·6H2O was used to make 1 M precursor solution in DIW. When glycerol was used, the mole ratio of glycerol to manganese nitrate was 5. A certain amount of the precursor solution was placed in the reactor so that it can reach 300 bar at the experimental temperature. The precursor solution input was calculated using the fluid’s density provided in NIST Chemistry Webbook [16]. The tightly sealed reactor was inserted into the molten salt bath with constant shaking for 12 min which includes 2 min of heat-up time. After 12 min, the

reaction was terminated by quenching the reactor in a water bath at room temperature. The obtained powders were collected and washed three times with DIW before drying in an oven at 80 °C for overnight. Calcination was conducted in a custom made furnace installed with a PID temperature controller and a gas flow system. When nitrogen gas was used, the flowrate was 200 cc/min. Heating rate was 5 °C/min and calcination was performed at 500 °C for 60 min. MnCO3 microparticles for calcination were hydrothermally synthesized at 400 °C with glycerol.

2.3 Physical characterization X-ray diffraction (XRD) patterns were obtained using Bruker AXS Diffraktometer D8 with Cu Kα radiation. FEI Inspect F was used to collect field emission scanning electron microscopy (SEM) images for visual inspection of particle size and morphology.

3. Results and discussion 3.1 Synthesis of various manganese oxides using sub and supercritical water Initially, manganese oxides were synthesized using water only and the results are summarized in Table 1. A temperature range of 250 to 400 °C was tested and the obtained particles were studied using XRD. Fig. 1 shows the XRD patterns of manganese oxides hydrothermally synthesized at 300 °C, 350 °C, and 400 °C. The XRD pattern for 250 °C reaction couldn’t be obtained because very few particles were synthesized at 250 °C, indicating insufficient activation energy for crystallization. Crystalline manganese (IV) oxides were synthesized at 300 to 400 °C where the XRD patterns matched MnO2 JCPDS card No. 24-0735. Fig. 1 shows the decrease of (110) peak intensity along with the increase in reaction temperature. This decrease in (110) peak intensity is due to the different metal oxide solubility of water in near-critical conditions. In the subcritical water region of 300 – 350 °C, water is in liquid state with high solubility of metal compounds [17] which allows steady crystal growth. In contrast, supercritical water has negligible metal compound solubility causing rapid crystallization which consequently retards crystal growth. The change in (110) peak intensity was also observed in the SEM images of MnO2 microparticles. Fig. 2 (a) and (b) shows the SEM images of MnO2 microparticles synthesized at 300 °C and 350 °C, respectively, which were found to be rod-shaped. The rod length was smaller for MnO2 synthesized at 350 °C. However MnO2 microparticles formed at 400 °C shown in Fig. 2(c) were pebble-shaped in the size range of 0.5 – 10 μm. The SEM images confirmed that the MnO2 rods synthesized at 300 °C and 350 °C were grown along the [001] direction.

Fig. 3 shows XRD patterns of manganese oxides which were synthesized in water with glycerol added as reducing agent. At 250 °C, very few particles were produced and therefore XRD data was not available. From 300 °C to 400 °C, manganese carbonate (JCPDS card No. 86-0173) was synthesized which showed increase in crystallinity along with increase in reaction temperature. The precursor manganese (II) nitrate and the product manganese (II) carbonate share the same electronic valence, Mn was neither oxidized nor reduced. Glycerol was an anti-oxidant in this reaction where it prevented the formation of manganese (IV) oxide, MnO2. Kim et al. [15] carried out a similar experiment where manganese (II) nitrate was reacted in SCW with glycerol at 400 °C and their reaction yielded a mixture of MnCO3 and MnO. In their XRD data, the peak intensity for MnO was much smaller than that of MnCO3, indicating MnO as a minor product. This research conducted tests from 250 °C to 400 °C and reaction temperatures 300 °C to 400 °C yielded pure MnCO3. In order to clarify the reproducibility of data, the experiment was repeated three times at 400 °C to check on the formation of MnO which was not identified in any of the XRD analyses, as shown in Fig. A1 in the Supplementary Material. The reason for the difference in the XRD results of Kim et al. and this work is yet unknown and is an area of further investigation. MnCO3 microparticles were analyzed by SEM as shown in Fig. 4. MnCO3 produced at 300 °C had a size range of 10 – 50 μm with cubic and diamond shape. Crystals growing on the surface of MnCO3 was observed indicating that the reaction was on going when the reactor was quenched to stop the reaction. In 350 °C, MnCO3 particles obtained had smaller sizes in the range of 1 – 30 μm with randomly aggregated looks containing small cubic particles on surfaces. Particle size was reduced to 1 – 10 μm at 400 ° C with relatively more defined cubic morphology aggregated into clusters, as shown in Fig. 4(c).

3.2 Effect of post heat treatment A simple post heat treatment was carried out on Mn(NO3)2·6H2O and hydrothermally synthesized MnCO3 to obtain pure manganese oxides, of which the results are summarized in Table 2. Calcination condition was fixed at 500 °C for 60 min, which was aimed minimizing the use of energy. Calcination of Mn(NO3)2·6H2O in air resulted in MnO2, as shown in Fig. 5(a). However, calcination of MnCO3 in air yielded manganese (III) oxide, Mn2O3, shown in Fig. 5(b). The MnCO3 used for calcination was synthesized in SCW at 400 °C with glycerol. Mn(NO3)2·6H2O and MnCO3 have the same electron valence but upon calcination, transformed into different manganese oxides. This difference in oxidation is due to the carbonate component having a reductive quality. Thermal degradation of MnCO3 can yield CO, CO2 and CO32-, all of which are known to be reducing agents [18]. The formation of Mn2O3 from MnCO3 calcination in air is in perfect agreement with work by [8, 9], of which they have analyzed the formation process using thermogravimetric analysis (TGA). From their TGA study, it was found that MnCO3 was first converted into MnO2, which is an intermediate that

loses oxygen at temperatures above 400 °C to finally become Mn2O3. [8]. A mixture of manganese (II) oxide (MnO) and manganese (II, III) oxide (Mn3O4) was formed when MnCO3 was calcined with N2 constantly flowing, as shown in Fig. 5(c). The removal of oxygen in the furnace during heat treatment resulted in partial reduction of the manganese oxides, however two phase mixture was formed. For this case, the adjustment of calcination conditions such as types of gas flowing, temperature, and reaction time is expected to have an effect on the final product formation such as phase purity and particle size [19, 20]. The SEM images of the calcined manganese oxides are shown in Fig. 6. Direct calcination of Mn(NO3)2·6H2O resulted in aggregated clusters of microparticles without a clear morphology, shown in Fig. 6(a). When MnCO3 was calcined in air, cubic Mn2O3 microparticles of 1 – 10 μm were aggregated in larger clusters where the morphology of MnCO3 was preserved even after the phase transition into Mn2O3. This is consistent with the results by Ashoka et al. [9] where they have heattreated hydrothermally derived MnCO3 and found that the MnCO3 frame structure was maintained. MnCO3 calcined under constant flow of N2 also yielded clusters of aggregated cubic microparticles where some particles were growing into larger lumps, indicating the growth of molten MnCO3 into pure manganese oxides. The formation pathways of manganese oxides with different oxidation states are summarize in Fig. 7. Hydrothermal synthesis using near-critical water in 300 - 400 °C resulted in MnO2. The use of glycerol as a reducing agent in the hydrothermal system yielded MnCO3. When this MnCO3 was heat treated at 500 °C for 60 min in air, pure Mn2O3 microparticles were obtained. Introducing N2 gas flow into the heat treatment yielded a mixture of reduced oxides, MnO and Mn3O4.

4. Conclusion Various phases of manganese oxides were synthesized using sub and supercritical water and a subsequent heat treatment. MnO2 and MnCO3 microparticles were formed from a single step hydrothermal route. Mn2O3 and a mixture of MnO + Mn3O4 were prepared by a two-step procedure composed of hydrothermal and calcination. Use of glycerol as a reducing agent in the hydrothermal system proved to be effective in cubic MnCO3 microparticle formation. Particle size and morphology were controllable by adjusting the process temperature in the hydrothermal system. Calcination of MnCO3 at 500 °C in air and N2 flow resulted in formation of Mn2O3 and MnO + Mn3O4, respectively, which are comparable to the formation of MnO2 from either hydrothermal synthesis or calcination. This research shows the versatility and efficiency of sub and supercritical water for producing manganese oxides of different oxidation states. The hydrothermal system can be easily scaled up using a continuous tubular plug flow reactor.

Acknowledgement This work was supported by the Green City Technology Flagship Program funded by Korea Institute of Science and Technology (Project No. 2E26300)

References [1] K. Zhong, X. Xia, B. Zhang, H. Li, Z. Wang, L. Chen, MnO powder as anode active materials for lithium ion batteries, Journal of Power Sources, 195 (2010) 3300-3308. [2] Y. Zhang, G.-y. Li, Y. Lv, L.-z. Wang, A.-q. Zhang, Y.-h. Song, B.-l. Huang, Electrochemical investigation of MnO2 electrode material for supercapacitors, International Journal of Hydrogen Energy, 36 (2011) 11760-11766. [3] B. Zhou, J. Yang, X. Jiang, Porous Mn2O3 nanorods synthesized from thermal decomposition of coordination polymer and used in hydrazine electrochemical sensing, Materials Letters, 159 (2015) 362-365. [4] M. Mansournia, F. Azizi, N. Rakhshan, A novel ammonia-assisted method for the direct synthesis of Mn3O4 nanoparticles at room temperature and their catalytic activity during the rapid degradation of azo dyes, Journal of Physics and Chemistry of Solids, 80 (2015) 91-97. [5] S. Ching, E.J. Welch, S.M. Hughes, A.B.F. Bahadoor, S.L. Suib, Nonaqueous Sol−Gel Syntheses of Microporous Manganese Oxides, Chemistry of Materials, 14 (2002) 1292-1299. [6] D. Yan, P.X. Yan, G.H. Yue, J.Z. Liu, J.B. Chang, Q. Yang, D.M. Qu, Z.R. Geng, J.T. Chen, G.A. Zhang, R.F. Zhuo, Self-assembled flower-like hierarchical spheres and nanobelts of manganese oxide by hydrothermal method and morphology control of them, Chemical Physics Letters, 440 (2007) 134138. [7] Y. Oaki, H. Imai, One-Pot Synthesis of Manganese Oxide Nanosheets in Aqueous Solution: Chelation-Mediated Parallel Control of Reaction and Morphology, Angewandte Chemie, 119 (2007) 5039-5043. [8] H.-z. Wang, H.-l. Zhao, B. Liu, X.-t. Zhang, Z.-l. Du, W.-s. Yang, Facile preparation of Mn2O3 nanowires by thermal decomposition of MnCO3, Chem. Res. Chines. Universities., 26(1) (2010) 5-7. [9] S. Ashoka, P. Chithaiah, C.N. Tharamani, G.T. Chandrappa, Synthesis and characterisation of microstructural α-Mn2O3 materials, Journal of Experimental Nanoscience, 5 (2010) 285-293. [10] T. Adschiri, Y.-W. Lee, M. Goto, S. Takami, Green materials synthesis with supercritical water, Green Chemistry, 13 (2011) 1380-1390. [11] T. Adschiri, K. Kanazawa, K. Arai, Rapid and Continuous Hydrothermal Crystallization of Metal Oxide Particles in Supercritical Water, Journal of the American Ceramic Society, 75 (1992) 1019-1022. [12] J.-H. Lee, J.-Y. Ham, Synthesis of manganese oxide particles in supercritical water, Korean J. Chem. Eng., 23 (2006) 714-719. [13] A. Nugroho, J. Kim, Effect of KOH on the continuous synthesis of cobalt oxide and manganese oxide nanoparticles in supercritical water, Journal of Industrial and Engineering Chemistry, 20 (2014) 4443-4446. [14] O. Jankovský, D. Sedmidubský, P. Šimek, Z. Sofer, P. Ulbrich, V. Bartůněk, Synthesis of MnO, Mn2O3 and Mn3O4 nanocrystal clusters by thermal decomposition of manganese glycerolate, Ceramics International, 41 (2015) 595-601. [15] M. Kim, W.-S. Son, K.H. Ahn, D.S. Kim, H.-s. Lee, Y.-W. Lee, Hydrothermal synthesis of metal nanoparticles using glycerol as a reducing agent, The Journal of Supercritical Fluids, 90 (2014) 53-

59. [16] http://webbook.nist.gov/chemistry/. [17] T. Adschiri, Y. Hakuta, K. Arai, Hydrothermal Synthesis of Metal Oxide Fine Particles at Supercritical Conditions, Industrial & Engineering Chemistry Research, 39 (2000) 4901-4907. [18] N.V. Osetrova, V.S. Bagotzky, S.F. Guizhevsky, Y.M. Serov, Electrochemical reduction of carbonate solutions at low temperatures, Journal of Electroanalytical Chemistry, 453 (1998) 239-241. [19] B. Sorensen, S. Gaal, E. Ringdalen, M. Tangstad, R. Kononov, O. Ostrovski, Phase compositions of manganese ores and their change in the process of calcination, International Journal of Mineral Processing, 94 (2010) 101-110. [20] A. Gil, L.M. Gandía, S.A. Korili, Effect of the temperature of calcination on the catalytic performance of manganese- and samarium-manganese-based oxides in the complete oxidation of acetone, Applied Catalysis A: General, 274 (2004) 229-235.

Fig. 1 XRD patterns of MnO2 synthesized in pure water at 300 – 400 °C

Fig. 2 SEM images of MnO2 synthesized in pure water at (a) 300 °C, (b) 350 °C, and (c) 400 °C.

Fig. 3 XRD patterns of MnCO3 synthesized from 300 to 400 °C (Mole ratio of glycerol to Mn(NO3)2·6H2O was 5).

Fig. 4 SEM images of MnCO3 synthesized in water with glycerol at (a) 300 °C, (b) 350 °C, and (c) 400 °C. (Mole ratio of glycerol to Mn(NO3)2·6H2O was 5)

Fig. 5 XRD patterns of (a) MnO2 from direct calcination of Mn(NO3)2·6H2O in air, (b) Mn2O3 from calcination of MnCO3 in air, and (c) MnO + Mn3O4 from calcination of MnCO3 under N2 flow. All calcination processes were conducted at 500 °C for 60 min and MnCO3 was made at 400 °C in SCW where glycerol to Mn(NO3)2·6H2O mol ratio was 5.

Fig. 6 SEM images of (a) MnO2 from direct calcination of Mn(NO3)2·6H2O in air, (b) Mn2O3 from calcination of MnCO3 in air, and (c) MnO + Mn3O4 from calcination of MnCO3 under N2 flow. All calcination processes were conducted at 500 °C for 60 min and MnCO3 was made at 400 °C in SCW where glycerol to Mn(NO3)2·6H2O mol ratio was 5.

Fig 7. Formation pathways of manganese oxides with different oxidation states.

Table 1. Hydrothermal conditions summarized. a mole ratio of glycerol and Mn(NO3)2·6H2O, N/A: not applicable. Temp

Pressure

Glycerol/

Particle Size

(°C)

(bar)

Mn(NO3)2·6H2O

(μm)

250

300

0

N/A

N/A

300

300

0

0.5 – 15

MnO2

350

300

0

0.1 – 10

MnO2

400

300

0

0.5 – 10

MnO2

250

300

5

N/A

N/A

300

300

5

10 – 50

MnCO3

350

300

5

1 – 30

MnCO3

400

300

5

1 – 10

MnCO3

a

Product

Table 2. A summary of calcination conditions and results. *Synthesized in SCW at 400 °C with glycerol Temp (°C)

Manganese

Calcination

precursor

time

Calcination

Particle size

Product

atmosphere

(μm)

In air

1 – 30

MnO2

1 – 10

Mn2O3

1 - 10

MnO +

(min) 500

Mn(NO3)2·6H2O

60

(no gas flow) 500

* MnCO3

60

In air (no gas flow)

500

* MnCO3

60

N2 flowing 200 cc/min

Mn3O4