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Ind. Eng. Chem. Res. 2009, 48, 1490–1494
Effects of Processing Parameters on the Morphology of Precipitated Manganese Oxide Powders Qiang Zhao,§,† Wan Y. Shih,‡ and Wei-Heng Shih*,† Department of Materials Science and Engineering and School of Biomedical Engineering, Science, and Health Systems, Drexel UniVersity, Philadelphia, PennsylVania 19104
It is desirable that the surface area of catalytic materials can be controlled by adjusting the processing parameters. In this paper we report that the morphology of precipitated manganese oxide powders after 500 °C calcination is affected by the volume ratio between the manganese nitrate precursor and ammonium hydroxide and also by the concentration of the precursor solution. When a large amount of ammonium hydroxide (nominal concentration of NH4OH in the final suspension greater than 4 N) was used, the synthesized powder had plate-like particles with higher specific surface area compared to spherical particles made by a small amount of ammonium hydroxide (nominal concentration of NH4OH less than 1 N). Thinner plates were synthesized from the system with lower nominal concentration of Mn(NO3)2, resulting in even higher specific surface area. The surface area differences between these powders synthesized by different conditions decreased as the calcination temperatures increased from 500 to 900 °C because the high calcination temperatures eliminated the morphology difference among them. 1. Introduction Manganese-oxide-based materials are inexpensive, environmental friendly, and active catalysts for reduction-oxidation related reactions, such as the (photo)oxidation and combustion of hydrocarbon1-5 and volatile organic compounds (VOC),6,7 ozone decomposition,8 NO/NO2 reduction,9 and decomposition,10 etc. The catalytic activity is related to the valence change of Mn ions in response to the changing oxygen environment. For catalytic applications, it is desirable that the textural properties of oxide powders can be tailored by controlling the powder processing conditions. For example, it was reported that the surface area and crystallite size of precipitated zirconia powders were highly dependent on the concentration of zirconium precursor solutions.11 In a manganese oxide system, previous studies showed that using permanganate as the precursor can produce high surface area manganese oxide powders with superior oxygen storage capability compared to the powder obtained from nitrate precursor.12 However, it is still unclear why the change of precipitation parameters affects the surface area of oxide powders after high temperature calcination. We had reported that the surface area of synthesized manganese oxide powders can be tailored by simply controlling the concentration of Mn(NO3)2 precursor solution and the volume ratio between Mn(NO3)2 solution and ammonium hydroxide used in the precipitation method.13 However, no physical explanation for the effects of processing parameters was given. In this paper, we provide an explanation for the relationship between the surface area change of MnOx powders and processing parameters by investigating the microstructure of the synthesized powders through detailed SEM studies. We found that the phase and particle morphology of precipitated powders depended strongly on the precipitation conditions (mainly the nominal concentrations of Mn(NO3)2 and * To whom correspondence should be addressed. E-mail: shihwh@ drexel.edu. Tel.: 215-8956636. Fax: 215-8956760. † Department of Materials Science and Engineering. ‡ School of Biomedical Engineering, Science, and Health Systems. § Present address: Saint-Gobain Ceramics and Plastics, 9 Goddard Road, Northborough, MA 01532.
NH4OH), and this morphology difference resulted in different surface area values of the powders. 2. Experimental Methods Manganese oxide powders were synthesized by the precipitation method using manganese nitrate as the precursor (Mn(NO3)2 · xH2O, 98%, Aldrich) and ammonium hydroxide (NH4OH, 5.02 N standard solution, Alfa Aesar) as the base agent. All chemicals were used as received without further purification. A typical synthesis procedure is as follows. Aqueous solutions of Mn(NO3)2 were prepared at concentrations varying from 0.025 to 0.5 M. After stirring for 20 min, they were added to a 5.02 N ammonium hydroxide aqueous solution drop by drop through a funnel with the stopcock while continuously stirring. The volume ratio between Mn precursor solution and ammonium hydroxide solution was varied between 1:8 and 5:1. Precipitates were formed when the Mn precursor solution was dropped into ammonium hydroxide. After finishing the addition of the Mn precursor solution, the suspension was stirred for 20 h followed by centrifugation and washing with
Figure 1. Surface area of manganese oxide powders prepared by mixing Mn(NO3)2 precursor solution (0.1 M) with ammonium hydroxide (5.02 N) at different volume ratios (shown by the side of symbols). All samples were calcined at the indicated temperature for 4 h.
10.1021/ie801173e CCC: $40.75 2009 American Chemical Society Published on Web 01/05/2009
Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1491 Table 1. Crystalline Phases of Manganese Oxides Made from Different Volume Ratios between the Mn(NO3)2 Solution (0.1 M) and Ammonium Hydroxide (5.02 N)a precursor solution/ammonium hydroxide (vol. ratio)
1/8
1/4
1/1
5/1
before heat treatment heat treatment at 500 °C heat treatment at 700 °C heat treatment at 900 °C
β-MnOOH + Mn3O4 R-Mn2O3 R-Mn2O3 R-Mn2O3
β-MnOOH + Mn3O4 R-Mn2O3 R-Mn2O3 R-Mn2O3
β-MnOOH + Mn3O4 R-Mn2O3 R-Mn2O3 R-Mn2O3
Mn3O4 R-Mn2O3+ MnO2 R-Mn2O3 R-Mn2O3
a Note: If the sample contains more than one phase, the phases are listed in the sequence according to their relative intensity from high to low in the XRD pattern.
Figure 2. SEM images of manganese oxide powders made from different Mn(NO3)2 solution/ammonium hydroxide volume ratios: (a) 1/8, (b) 1/1, (c) 5/1, before the heat treatment.
distilled water for six times. The as-synthesized precipitates were dried at 70 °C overnight and ground to fine powders before heat treatment in air at 500, 700, and 900 °C for 4 h, respectively. The particle morphology was studied in the Philips XL30 environmental scanning electron microscope. The specific surface area (SSA) was measured by nitrogen adsorption with the five-point BET method using Quantachrome Nova 2200 analyzer. The powder X-ray diffraction (XRD) patterns of the samples were carried out on a Siemens D500 diffractometer using Cu KR radiation (λ ) 0.154 nm). 3. Results Figure 1 shows the specific surface area of manganese oxide powders prepared by adding 0.1 M Mn(NO3)2 precursor solution
Figure 3. SEM images of manganese oxide powders made from different Mn(NO3)2 solution/ammonium hydroxide volume ratios: (a) 1/8, (b) 1/1, and (c) 5/1 after 500 °C-4 h heat treatment.
dropwise into 5.02 N ammonium hydroxide solution at different volume ratios. When a lager amount of ammonium hydroxide was used, the specific surface area of the powder produced was higher after 500 and 700 °C heat treatment. The surface area difference among these powders became less significant after 900 °C heat treatment. XRD results of samples made from different reactant ratios are shown in Table 1. Before heat treatment the powders made from larger amounts of ammonium hydroxide were mixtures of Feitknechtite (β-MnOOH) and Hausmannite (Mn3O4) phases, while the powder made using the least amount of ammonium hydroxide was Hausmannite phase only. After 500 °C-4 h heat treatment nearly all of the samples transformed to R-Mn2O3 except the one made using the least amount of ammonium hydroxide, which became mixtures of R-Mn2O3 and MnO2. There is no phase difference
1492 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009
Figure 4. SEM image of manganese oxide powder made from different Mn(NO3)2 solution/ammonium hydroxide volume ratios: (a) 1/8, (b) 1/1, and (c) 5/1 after 900 °C-4 h heat treatment.
among the powders made from different reactant volume ratios after 700 and 900 °C heat treatment. The morphology of the powders made from different amounts of ammonium hydroxide before heat treatment is shown in Figure 2. The shape of the particles changed from plate-like to spherical as the amount of ammonium hydroxide used for the precipitation decreased (Figure 2). This phenomenon is consistent with the crystalline phase evolution shown in Table 1, since an isotropic morphology for Mn3O4 and a platy hexagonal morphology for β-MnOOH had been reported by Bricker et al.14 Heat treatment at 500 °C did not alter the particle morphology rather the particles inherited the features of those unheat-treated samples as shown in Figure 3, that is, the particle morphology changed from plates to spheres as the amount of ammonium hydroxide used for powder synthesis decreased. However, the particle morphology after 900 °C-4 h heat treatment changed significantly compared to those calcined at 500 °C and all the particles became highly sintered spheres as shown in Figure 4. When the volume ratio between Mn(NO3)2 solution and ammonium hydroxide was fixed at 1/4 during precipitation, the surface area of the synthesized powders varied with the concentration of Mn(NO3)2 solution. A detailed information regarding the surface area and phase evolution with the Mn(NO3)2 concentration can be found in our previous paper,13 so only a brief summary is included here in Table 2. Generally
Figure 5. SEM images of manganese oxide powders made from (a) 0.025, (b) 0.1, and (c) 0.5 M Mn(NO3)2 solutions before heat treatment.
speaking, it was found that powders produced from lower concentration precursors had higher SSA after 500 °C heat treatment, which was similar to the phenomenon observed in the zirconia system.11 As the heat treatment temperature increased, the SSA decreased, and the SSA difference among powders prepared from precursors with different concentrations became less significant. Before heat treatment, the major crystalline phases for all the powders were β-MnOOH and Mn3O4. After the 500 °C heat treatment, except for the powder prepared from 0.5 M precursor solution, which became mixtures of MnO2 and MnO1.88, all other samples transformed to R-Mn2O3. At 700 and 900 °C, all samples had the R-Mn2O3 phase. SEM images of powders made from 0.025 M, 0.1 M, and 0.5 M precursors before heat treatment are shown in Figure 5, showing plate-like grains, which were consistent with the β-MnOOH phase. The 500 °C-4 h calcined powders inherited the morphology of those before heat treatment as shown in Figure 6 although they had transformed to oxides from hydroxides (Table 2). The powders made from lower concentration precursor had thinner plates compared to the powder made from higher concentration precursor. The average plate thickness measured by visual inspection of SEM pictures was 32 ( 7, 35 ( 6, and 40 ( 9 nm for the powders made from 0.025, 0.1, and 0.5 M precursors, respectively. After 900 °C-4 h heat treatment, all powders became spherical and these spheres were
Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1493 Table 2. Major Crystalline Phases and Specific Surface Area of Manganese Oxides Made From Mn(NO3)2 Solutions with Different Concentrationsa precursor concentration (mol/L)
0.025
0.05
0.10
0.50
before heat treatment
phase
β-MnOOH + Mn3O4
β-MnOOH + Mn3O4
β-MnOOH + Mn3O4
β-MnOOH + Mn3O4
heat treatment at 500 °C
phase SSA (m2/g)
R-Mn2O3 42.0
R-Mn2O3 39.4
R-Mn2O3 34.1
MnO2 + MnO1.88 27.0
heat treatment at 700 °C
phase SSA (m2/g)
R-Mn2O3 22.0
R-Mn2O3 24.1
R-Mn2O3 20.0
R-Mn2O3 17.4
heat treatment at 900 °C
phase SSA (m2/g)
R-Mn2O3 7.5
R-Mn2O3 8.3
R-Mn2O3 5.9
R-Mn2O3 7.7
a
Note: all samples were calcined for 4 h at indicated temperatures. If the sample contains more than one phase, the phases are listed in the sequence according to their relative intensity from high to low in the XRD pattern.
Figure 7. SEM images of manganese oxide powders made from (a) 0.025, (b) 0.1, and (c) 0.5 M Mn(NO3)2 solutions after 900 °C-4 h heat treatment. Figure 6. SEM images of manganese oxide powders made from (a) 0.025, (b) 0.1, and (c) 0.5 M Mn(NO3)2 solutions after 500 °C-4 h heat treatment.
highly sintered to each other as shown in Figure 7. There was no apparent difference among the powders made from different concentration precursors at this temperature. 4. Discussion The surface areas of samples heat-treated at 500 °C from different reactant volume ratios and Mn precursor concentrations were combined together in Figure 8 and presented as a function of the nominal concentrations of Mn(NO3)2 and NH4OH in the final precipitated suspensions. The typical morphology of these
powders was also schematically drawn in the figure. Figure 8 clearly shows the relationship among processing condition, microstructure, and the specific surface area. When the concentration of ammonium hydroxide was high (>4 N), the dominant morphology of the powders was plate-like. Lower Mn(NO3)2 concentration solutions generated powders with thinner plates showing higher specific surface area. On the other hand, if the concentration of ammonium hydroxide decreased (
