Influence of tile synthesis parameters on the structural and textural properties of precipitated manganese oxides

International Journal of Inorganic Materials 3 (2001) 889–899

Influence of tile synthesis parameters on the structural and textural properties of precipitated manganese oxides a ´ ´ J. Boyero Macstre a , E. Fernandez Lopez , J.M. Gallardo-Amores b , *, R. Ruano Casero a , ´ ´ V. Sanchez Escribano a , E. Perez Bernal b a

´ ´ Inorganica , Universidad de Salamanca, Pl. de la Maced, s /n, E-37008, Salamanca, Spain Dpto. Quımica ´ ´ Universidad Complutense, Departmento de Quımica Inorganica , Laboratoria Complutense de Altas Presiones., Ciudad Universitaria, E-28040 Madrid, Spain

b

Accepted 24 July 2001

Abstract A systematic study has been conducted in order to compare the structural and morphological properties as well as the genesis course of manganese oxides prepared by a precipitation-calcination method under different conditions. In particular, the roles of the precursor salt, the pH, the precipitating agent and thermal treatment were investigated. It was found that calcination at 873 K of the starting materials gives rise in all cases to crystalline bixbyite. However, the phases formed at lower temperatures are quite different and depend largely on the synthesis conditions. The systems prepared from a Mn(III) ionic salt present maximum specific surface areas according to a greater development medium-sized pores.  2001 Elsevier Science Ltd. All rights reserved.

1. Introduction Manganese element gives rise to a rather complex oxides system, being the most usual stoichiometries MnO, Mn 3 O 4 , Mn 2 O 3 and MnO 2 [1]. These oxides are variously structured and each of them may exist in different modifications. Moreover, they can present disparate degrees of non-stoichiometry with Mn oxidation states ranging from II1 to IV1 [2]. These materials find increasing application in catalysis and electrochemistry fields. Polymorphs of MnO 2 [3–5], MnO 3 [6,7] and Mn 3 O 4 [8] have been proposed as cheap, environment-friendly catalysts for the combustion of volatile organic compounds (VOCs), the total oxidation of methane and CO [9] and the selective reduction of nitrobenzene [10–13]. Furthermore, the manganese dioxide MnO 2 has been found to be active in the synthesis of a-b unsaturated carbonylic compounds by the partial oxidation of alylic alcohols [14] and in the oxidation of aniline to hydroquinone [15], as well as it is regarded as an attractive component for reversible cathodes in non-aqueous lithium batteries [16–19] due to its high discharge voltage. However, the performance of the manganese oxides for *Corresponding author. E-mail address: [email protected] (J.M. Gallardo-Amores).

those applications is critically controlled by their phase composition and textural properties, which depend largely on the preparation method as well as on the precursor compound and the thermal pre-treatment. Therefore, the present investigation aims at determining the role of several parameters, namely pH, originating compound, precipitating agent and calcination temperature, on the synthesis by precipitation of MnOx systems. Due to that in many previous studies the attention has been restricted to the final products, in the present paper we intend to report also on the thermal genesis course of the manganese oxides as well as on its influence on the structural and morphological characteristics.

2. Experimental Four different manganese-oxide systems, denoted as M1, M2, M3 and M4, were prepared by a precipitation method and characterized from the points of view of their structural and morphological properties. The precursor salts, the precipitating agent and the pH of the solution during the aging of the precipitates were varied as reported in Table 1. The series M1, M2 and M3 were prepared from tetrahydrated Mn(II) acetate. For the series M1, the

1466-6049 / 01 / $ – see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S1466-6049( 01 )00091-5

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Table 1 Synthesis conditions of the manganese oxides System

Originating compound

Precipitating agent

pH

M1 M2 M3 M4

Tetrahydrated manganese (I) acetate Tetrahydrated manganese (II) acetate Tetrahydrated manganese (III) acetate Dihydrated manganese (III) acetate

Ammonium hydroxide Ammonium hydroxide Ammonium carbonate Ammonium hydroxide

9.5 8.5 9.5 9.5

precipitation was carried out at pH 8.5, which corresponds theoretically to a saturated aqueous solution of Mn(II) ions, whereas for the series M1 the pH was raised to 9.5 to ensure the quantitative precipitation of the Mn(II) species. The samples belonging to the series M3 were precipitated with ammonium carbonate (instead of ammonia) at pH 9.5 and the series M4 was prepared at pH 9.5 from dihydrated Mn(III) acetate. In all cases, the materials were obtained by the addition of the solid precursor salt to an aqueous solution of ammonia or ammonium carbonate (see Table 1) under continuous stirring and gentle heating. The precipitates were aged in contact with the solution for 1 day, dried overnight at room temperature and milled. Portions of each sample were taken and calcined in a Nabertherm furnace for 3 h at temperatures in the range 373-873 K with approximate heating rates of 10 K / min. Powder XRD analyses were carried out with a Siemens D-500 diffractometer (Cu K a radiation, Ni filter, 30 mA, 40 kV) equipped with the Diffract AT V3 software package. Rietveld refinement of the experimental X-ray diffiaction patterns was performed with Fullprof 99 software [20]. Particle sizes were evaluated with the Scherrer formula [21]. FT-IR spectra were recorded with a Perkin Elmer 1600 series spectrometer in the 4000–400 cm 21 range using the KBr dilution technique. RD–VIS–UV spectra were obtained with a Shimazdu 240 spectrometer in the 200–800 nm interval using MgO as a reference. Thermal analyses (DTA-TG) of the precipitated powders were performed in dynamic oxygen and nitrogen atmospheres from 300 to 1173 K with heating rates of 10 K / min on a Perkin Elmer DTA instrument and a Perkin Elmer TG analyzer. TPR profiles were recorded on a Micromeritics TPR / TPD 2900 instrument previously calibrated with CuO. Specific surface areas were measured by N 2 adsorption at the liquid nitrogen temperature (77 K) on a conventional volumetric apparatus according to the BET method. From the desorption branch of the isothermal representation, the pore size distribution was estimated by the BJH method [22]. The accumulated surface (SC ) and external surface (ST ) were calculated according to Cranston and Inkley’s method [23] and from the ‘t’ plot of de Boer [24], respectively.

3. Results

3.1. XRD analyses The crystallographic data of all materials synthesized in this study are summarized in Table 2. The XRD patterns of the precipitated powders are compared in Fig. 1. In general, the composition of the precipitated powders depends largely on the synthesis conditions. Thus, the precipitates both M1 and M2 (Fig. 1a,b) are composed by the low temperature form of hausmannite (g-Mn 3 O 4 , ICDD file no. 18-0803), although in the X-ray diffraction pattern for the former (Fig. 1a) this phase exhibits lower crystallinity and the background is slightly enhanced, which evidences the co-presence of an amorphous phase. On the other hand, in the precipitates M3 (Fig. 1c) the Mn 21 ions have been stabilized within the structure of rhodochrosite (MnCO 3 , ICDD file n8 44-1472) whereas the precipitates M4 (Fig. 1d) are essentially amorphous, though low amounts of crystalline hausmannite are also detected. The XRD patterns of all materials alter calcination at 673 K and M1 calcined at 873 K are compared in Fig. 2. In the systems M1 and M2, hausmannite undergoes a structural transformation from the g to the a form (aMn 3 O 4 , ICDD file n8 80-03 82) above 373 K with a parallel increase in the volume of the tetragonal unit cell. At 873 K (Fig. 2e), hausmannite oxidizes to bixbyite (ICDD file n8 31-0825). These facts point out that the thermal evolution of hausmannite is relatively independent from the preparation method. Moreover, in the X ray diffraction patterns of the materials M1 calcined at mild temperatures small amounts of other manganese oxides, sometimes difficult to identify, are also detected. Thus, at 373 K the presence of MnO is suggested whereas at 573–673 K very likely traces of Mn 5 O 8 and MnO 2 are also formed. Elemental chemical analyses performed on the materials M1 calcined at 773 and 873 K allow empirical formulae very close to MnO 1.5 to be calculated, according to the; major presence of bixhyite. Materials treated at lower temperatures display, in general, higher oxygen contents indicating the co-presence of over-oxidized manganese oxides. On the other hand, the rhodochrosite phase remains

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Table 2 Crystallographic data of the manganese oxides and the precursor materials System

T (K)

Phase

˚ D (A)

Cell parameters ˚ a (A)

M1

5.724 5.724

9.367 9.352

307 307

302

5.762

9.470

314

288

5.730

9.371

308

319 364 324

5.657 9.379 9.371

9.294 9.227 9.333

297 807 819

325 230 258

5.718 5.750 5.752

9.382 9.429 9.431

307 312 312

873

g-Mn 3 O 4 g-Mn 3 O 4 a-Mn 3 O 4 Mn 5 O 8 a-Mn 2 O 3

324

9.312

9.380

816

373 573 673 773 873

MnCO 3 MnCO 3 (a)1Mn 5 O 8 (t) a-Mn 2 O 3 a-Mn 2 O 3

15.531 17.699

303 311

as obtained 473 673 873

(a)1g-Mn 3 O 4 (a)1g-Mn 3 O 4 a-Mn 3 O 4 a-Mn 3 O 4

773 873

M4

˚ 3) V (A

331 354

673

M3

˚ c (A)

g-Mn 3 O 4 g-Mn 3 O 4 g-MnOOH (t) a-Mn 3 O 4 MnO 2 (t) Mn 5 O 8 (t) a-Mn 3 O 4 MnO 2 (t) Mn 5 O 8 (t) a-Mn 3 O 4 a-Mn 2 O 3 a-Mn 2 O 3

as obtained 373 573

M2

˚ b (A)

as obtained 473 673

9.325 9.368

9.338

4.749 4.782

184 270

9.400 9.321

9.387 9.335

9.416 9.322

830 811

5.716 9.387

9.302

9.716 9.366

308 818

a-Mn 2 O 3 (ICDD file n8 24-0508): a59.4161, b59.4237, c59.4051 (orthorhombic); g-Mn 3 O 4 (ICDD file n8 18-0803): a55.7800, c59.3300 (Tetragonal); a-Mn 3 O 4 (ICDD n8 80-0382): a55.7650, c59.4420 (Tetragonal). (a)5poorly crystallized material

Fig. 1. XRD patterns of the precipitated materials: (a) M1; (b) M2; (c) M3 and (d) M4.

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Fig. 2. XRD patterns of the materials M1 (a), M2 (b), M3 (c) and M4 (d) calcined at 673 K and M1 calcined at 873 K (e).

stable in the system M3 up to 573 K. Above this temperature, it decomposes giving rise to poorly crystallized manganese oxides as intermediates at 673 K (Fig. 2c) and to bixbyite at 773 K. Traces of Mn 5 O 8 are sporadically detected at 673 K. The system M4, prepared from a Mn(III) salt, remains primarily amorphous upon calcination up to 873 K, but weak reflections in the X-ray diffraction patterns (Fig. 2d) prove the presence of the hausmannite phase. At 873 K, bixhyite crystallizes as expected.

3.2. FT-IR spectroscopy The vibrational spectra of some materials of the series M1 and M4 are compared in Fig. 2. Those corresponding to the system M2 are not shown, since they are very similar to those of system M1. In general, weak bands at 1580, 1415 and 1345 cm 21 assigned to na C – O – O , ns C – O – O and dCH 3 modes, respectively, provide evidence for the permanence of traces of acetate compounds in all of the materials treated below 673 K The skeletal spectra of the precipitates M1 (Fig. 2a) and M2 display peaks at 630, 530 and 410 cm 21 , characteristic

of hausmannite [25–27]. In the spectrum of the precipitate M4 (Fig. 2b) two bends at about 595 and 510 cm 21 (the former also occurring as a shoulder in the spectrum of the precipitate M1) might be related to the presence of manganese hydroxides Like groutite or manganite. Additional components in the interval 1120–970 cm 21 (see inset in Fig. 2), attributed to Mn–O–H structural vibrations, support this assignment [28]. The features described above remain practically unchanged upon increasing the treatment temperature up to 673–773 K (Fig. 2c,d). At 873 K (Fig. 2e) all the spectra display bands at 665, 605, 580, 525 and 490 cm 21 , characteristic of highly crystalline byxbyite [29]. Eventually, some of them are already observed at 773 K. The spectrum of the precipitate M3 (not presented here) shows conversely the absorptions characteristic of rhodochrosite, namely peaks at 1440 cm 21 (nC5O ), 862 and 725 cm 21 (coupling of dMn – O – C and nMn – O modes) [30], which turn progressively into those of bixbyite upon heating above 673 K. At mild temperatures (573–673 K), a broad unresolved absorption over the skeletal range evidences the increasing formation of low crystalline manganese oxides as previous step to the stabilization of bixbyite.

J. Boyero Macstre et al. / International Journal of Inorganic Materials 3 (2001) 889 – 899

3.3. DR–UV–VIS–NIR spectroscopy DR–UV–VIS spectra of the materials of the M1 system are given in Fig. 3. The differences with respect to other systems will be opportunely pointed out. In general, electronic spectra for the precipitates M1, M2 and M4 are very similar and show a continuous absorption over the UV and visible regions with a maximum at about 380 nm, followed by an absorption tail which extends to the near infrared (NIR) range. The edge onset is located at about 620 nm, and several components are identified at 220, 260, 325, 500, 570 and 730 nm. This spectrum is typical of the hausmannite phase and the following assignments can be made according to literature: (i) the bands at 220, 260 and 325 nm are reasonably attributed to allowed O 22 →Mn 21 (the two former) and O 22 →Mn 31 (the latter) charge transfer transitions, respectively [31,32]; (ii) the components at higher wavelengths, in particular those at 570 and 730 nm, may be related to d–d crystal field transitions on octahedral Mi 31 species [33]. Subsequent calcination does not give rise to significant changes in the electronic spectra, except in those for the material M1. Hence, at 373 K the shoulders at 470 and 730 nm, associated to Mn 31 ions, increase notably in intensity

893

very likely due the formation of the g-MnOOH phase. At higher temperatures, the maximum of the main band shifts significantly to lower energies while the absorption in the visible region enhances systematically. These facts are attributed to: (i) The formation of the MnO 2 phase, which gives rise to a continuous absorption between 400 and 450 nm [34] and components at 575 and 705 rim characteristic of Mn 41 species [35]; (ii) The lower degree of crystallinity of the systems and the diminution in the symmetry, which entails the loss of degeneracy of the energy levels and the subsequent increase in the number of allowed d–d crystal field transitions; (iii) The presence of other manganese oxides such as Mn 5 O 8 . The spectrum of the precipitate M3 is consistent with the presence of Mn(II). The absorption in the visible region falls down, according to the non-allowed nature of d–d crystal field transitions on the Mn 21 ion. In all cases, the spectra change dramatically at 873 K due to the crystallization of bixbyite. Thus, they display a continuous absorption in the UV and visible regions with a maximum at 500 nm and shoulders at 220, 250, 390, 580 and 730 nm. Those appearing at higher energies (220 and 250 nm) prove the presence of small amounts of Mn 21 ions, whereas the others are definitely related to the presence of bixbyite and they are assigned, in increasing order of the wavelength, to O 22 →Mn 31 charge-transfer transitions (shoulders in the region below 400 nm), to superimposed 5 B 1g → 5 B 2g mid 5 B 1g → 5 E g crystal-field d→d transitions (main band), and to a 5 B 1g → 5 A 1g crystalfield d→d transition (shoulders in the visible region) [36,37].

3.4. Thermal analyses The DTA and TG analyses performed for the precipitates M1, M2 and M4 are, in general, quite similar (those for the precipitates M1 and M2 are compared in Fig. 4). In general, the thermal evolution of these materials can be qualitatively divided into three steps:

Fig. 3. Skeletal FT-IR spectra of the materials: (a) M1 as obtained; (b) M4 as obtained; (c) M1 calcined at 673 K; (d) M4 calcined at 673 K; (e) M1 calcined at 873 K. Inset: Mn–O–H stretching region for materials M1 (a) and M4 (b) as obtained.

1. The first one is characterized by two broad endothermic events occurring from room temperature up to 538 K, approximately. They are associated to regular weight losses in the TG curve (0.5% up to 373 K and 0.65% in the range 373–538 K for the system M1), being attributed the former to the removal of water molecules weakly adsorbed on the surface and the latter to the transformation of the poorly crystalline phases and the loss of structural water occluded in interplanar regions and lattice interstices. An additional exothermal peak identified near 450 K is due to the phase transition gMn 3 O 4 →a-Mn 3 O 4 , which proceeds in parallel to an increment in the particle size and the subsequent heat release [38].

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and TG curves show instead an continuous weight decrease (maximum for the system M4) according to the reduction of the Mn(III) species present, in varying amounts, in the precipitated materials. The TG curve in air for the precipitate M3 (not presented here) shows a continuous weight decrease from about 473 to 773 K subdivided in various steps according to the decomposition of rhodochrosite to bixbyite trough several intermediate compounds.

3.5. TPR analyses

Fig. 4. RD–VIS–UV spectra of the materials M1: (a) as obtained; (b) calcined at 373 K; (c) calcined at 573 K; (d) calcined at 673 K; (e) calcined at 773 K; (f) calcined at 873 K.

2. The second stage corresponds to a systematic weight increase in the interval 538–853 K. However, the profile of the TG curves is not homogeneous and accounts, at least, for two different processes. The former consists in a slow mass gain from 538 to 793 K (1.55% for the system M1) whereas the latter (absent for the system M4) is associated to a sudden weight increase at about 850 K (0.3% for the system M1) and to a sharp exothermic peak in the DTA plot. In general, both steps are related to transformations among the manganese oxides and hydroxides previously detected by other techniques in this thermal range, the former being due to the rapid oxidation of hausmannite to bixbyite. 3. The third stage is associated in all cases to low weight decreases near 870 K and to the slightly descending trend of the TG curve at higher temperatures. No significant thermal events are detected in DTA plots up to 1173 K. Therefore, the main weight loss is attributed to the decomposition of MnO 2 and Mn 5 O 8 to a-Mn 2 O 3 , whereas other decreases may be related to the volatilization of small amounts of the samples. In general, in the thermal analyses performed for the same samples under an inert nitrogen atmosphere, the steps corresponding to the overoxidation of manganese oxides and hydroxides in the range 473–773 K are suppressed,

The TPR profiles for several materials belonging to the systems M1 and M4 are compared in Fig. 5. Those corresponding to the system M1 (Fig. 5a,b) display always a main peak in the neighborhood of 673 K as well as an additional component (except for the sample treated at 773 K) which develops progressively at lower temperatures upon increasing the calcination temperature. The hydrogen consumption is lower for the materials M1 calcined below 373 K than for those calcined in the range 473–673 K, and becomes definitely higher in the system M4, according to the predominant presence of Mn(III) ions. It has been reported in literature that the reduction of bixbyite in an hydrogen atmosphere takes place in a two-step process, according to the reactions: 3Mn 2 O 3 1 H 2 → 2Mn 3 O 4 1 H 2 O Mn 3 O 4 1 H 2 → 3MnO 1 H 2 O, and consequently that of hausmannite in a one-step process corresponding to the second equation given above [39]. Thus, the curves for the materials M1 calcined at 773 (Fig. 1c) and 873 K (Fig. 1b) are fully consistent with the presence of hausmannite and bixbyite as main constituents, respectively. The low-temperature component identified for the same materials calcined in the range 473–673 K (see, for instance, Fig. 1a) can be now reasonably attributed to the reduction of manganese oxides with Mn ions in a overall oxidation state intermediate between 3 and 4 (like Mn 5 O 8 ), because it is well known that MnO 2 only gives rise to a unique reduction peak at approximately 773 K [38]. The non-symmetric shape of the peak is due to the simultaneous reduction of different compounds or to the non-uniform nature of the lattice O 22 . ions. In this way, Mn 5 O 8 is tentatively proposed to undergo a two-step reaction, being the former Mn 5 O 8 1 4H 2 → 5Mn 3 O 4 1 4H 2 O, and the latter that typical of the reduction of hausmannite. A small peak appearing at 503 K in its corresponding TPR profile is attributed to the reduction of Mn 31 ions located in tetrahedral holes in the hausmannite lattice [24]. Further differences among the curves in Fig. 5 must be attributed to differences in the specific surface areas or in the

J. Boyero Macstre et al. / International Journal of Inorganic Materials 3 (2001) 889 – 899

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Fig. 5. DTA (a) and TG (b) curves for the materials M1 (A) and M2 (B) as obtained.

crystallinity of the samples, rather than to the oxidation state of manganese ions [40]. The TPR curves fur the materials M4 (Fig. 1d) evidence, in all cases, for a two-step reduction analogous to that of bixbyite, but with an notable higher hydrogen consumption at the first stage (except for the sample calcined at 873 K), very likely due to the higher content of over-oxidized manganese ions in phases like manganite, with respect to the systems prepared from a Mn(II) salt.

3.6. Specific surface area trend and porosity The morphological characteristics of all materials are summarized in Table 3. The materials exhibit moderate specific surface areas and reach maximum values for the series M4. The isothermal plots for the materials calcined at 873 K are compared in Fig. 6A. That for the sample M2 has been excluded, because it is virtually identical to that for the

sample M1. The representations for the systems M1, M2 and M3 (Fig. 7A,a–b) correspond to the type II of the BDDT classification [41], characteristic of non porous or macroporous adsorbents. In general, the close agreement between the BET specific surface and the accumulated surface (ST ) rules out the existence of microporosity in these systems. This is confirmed by the negative values of the ordinate intercept on the extrapolated ‘t’ plot. The estimation of the pore size distribution according to the BJH method (Fig. 7B,a–b) reveals maximum population of the levels with diameters between 15 and 50 nm, according to the macroporous–mesoporous nature of these materials. The isothermal plot for the material M4 calcined at 873 K (Fig. 7A,c) is slightly different. It exhibits an incipient H1-hysteresis loop (according to the IUPAC classification) at relative pressures higher than 0.8, and the profiles are accordingly intermediate between the types II and IV of the BDDT classification. An H1-hysteresis loop is due to open

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Table 3 Morphological properties of the manganese oxides and the precursor materials System T (K)

Pore Vol. SBET (cm 3 / g) (m 2 / g)

SC (m 2 / g)

St (m 2 / g)

rm (nm)

M1

as obtained 373 573 673 773 873

0.04 0.05 0.03 0.02 0.02 0.01

17 27 11 9 10 5

19 21 9 7 9 4

20 25 11 9 11 6

10.4 8.0 12.3 8.4 9.6 9.6

M2

as obtained 0.11 673 0.06 873 0.04

30 34 25

28 33 21

33 37 24

14.6 20.1 17.9

M3

373 673 873

0.04 0.25 0.04

20 97 25

15 98 22

17 102 26

13.6 9.7 1.8

M4

as obtained 473 723 873

0.27 0.53 0.37 0.13

96 153 122 41

99 161 143 37

103 176 150 43

11.3 13.8 12.1 12.9

tubular pores with circular or polygonal sections. The pore size distributions are monomodal ones, with maximum frequencies for pores with radius between 10 and 30 nm. The absence of microporosity is evidenced by the extrapolated ‘t’ plot, as in the other systems. The specific surface areas decrease systematically upon heating, as expected.

4. Discussion The X-ray diffraction patterns corresponding to the precipitates of the series M1 and M2 prove that divalent manganese ions as such are not stable at room temperature in aqueous solution but they experience a partial oxidation to Mn(III), which results mainly in the crystallization of the low-temperature form of hausmannite (g-Mn 3 O 4 . When the pH of the solution raises during the synthesis above the theoretical value for a saturated aqueous solution of Mn(II) species (system MI), the formation of hausmannite is slightly hindered and an amorphous phase made up of Mn(II) oxy-hydroxides co-precipitates. g-Mn 3 O 4 undergoes a structural re-arrangement to aMn 3 O 4 at temperatures close to 473 K, with a parallel enlargement of its unit cell. The crystallographic parameters for the a-phase range slightly within the experimental error limits in all samples calcined in the range 473–773 K where present (see Table 2), which points out that aMn 3 O 4 is stable at intermediate temperatures independently from the synthesis method. Therefore, the manganite phase and manganese oxides with higher oxygen contents, such as Mn 5 O 8 and MnO 2 , detected in the system M1

Fig. 6. TPR runs for: (a) M1 calcined at 573 K; (b) M1 calcined at 873 K; (c) M1 calcined at 773 K; (d) M4 calcined at 773 K.

from 373 to 773 K, are believed to be formed preferentially from the amorphous Mn(III) oxy-hydroxide phase rather than from hausmannite. This statement is supported by the absence of any low-crystallinity phase in the diffraction patterns of all materials belonging to the system M1 as well as the lower hydrogen consumption for the materials M2 in TPR analyses. Taking into account these results, the following decomposition trend is proposed for Mn(II) hydroxides in our samples: 273 K

|373 K

MN(O,OH) → g-Mn(OOH) → Mn 5 O 8 , 773 K

MnO 2 → a-Mn 2 O 3 The partial pressure of oxygen has been reported to be a decisive factor on the final products obtained from the thermal decomposition of manganite [42] (and very likely of other manganese oxides and hydroxides). On basis of this, it is tentatively proposed that the oxidation of the suresh outer layer may give rise to different compounds

J. Boyero Macstre et al. / International Journal of Inorganic Materials 3 (2001) 889 – 899

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Fig. 7. Isothermal BET plots (A) and pore size distributions (B) for the materials M1 (a), M3 (b) and M4 (c) calcined at 873 K.

than the oxidation of the bulk, so contributing to the compositional heterogeneity of the systems calcined at mild temperatures. In fact, we have detected a significant amount of hausmannite on a solgel prepared pure manganese oxide calcined a 1073 K (within the thermal stability range for bixbyite), very likely due to the different behavior of the outer layer of the material upon cooling [43]. Moreover, the particle size is also believed to play a significant role, and different pathways for the oxidation to bixbyite of different-sized hausmannite particles have been proposed in literature [44]. According to this, the major product in our materials would be determined by the preferential particle size distribution in the material. On the other hand, Mn(II) ions can also be fully stabilized by the addition of appropriate species (like carbonate groups), as evidences the X-ray diffiaction pattern for the precipitate M3. In this case, rhodochrosite crystallizes and the formation of any amorphous phase, like Mn(II) hydroxides, is absolutely hindered. Rhodochrosite decomposes at higher temperatures (but significantly lower than in the non-stabilized systems) giving rise to traces of Mn 5 O 8 and other non-stoichiometric manganese oxides. When the synthesis is performed from a Mn(III) ionic salt (system M4), the main constituent of the precipitated materials is an amorphous phase different in nature from that observed in the system M1, as prove clearly the IR spectra compared in Fig. 2. It has been reported that

Mn(III) species are extremely unstable in aqueous solution and they rapidly undergo dismutation or reduction according to the reactions: 2Mn 31 1 2H 2 O → Mn 21 1 MnO 2 1 4H 1 Mn

31

1 H 2 O → 2Mn

21

1

1 1 / 2H 2 O 1 2H ,

respectively [45]. It should be noted that the traces of MnO 2 formed thereby catalyze the over-oxidation of all of the other manganese oxides or hydroxides coexisting in the material. Therefore, the amorphous phase is proposed to be composed of a mixture of g-Mn 3 O 4 , g-MnOOH and nsutite (hydrated manganese oxides with general formula Mn(O,OH) 2 ), which is consistent with the spectroscopic (FT-IR and RD–UV–VIS) studies. Anyway, TPR analyses prove that the materials M4 are notably more oxidized than those prepared from a Mn(II) precursor salt. Some authors have reported the decomposition of hausmannite to proceed under certain conditions via the intermediate g-MnO 3 phase, which presents the same tetragonal-distorted spinel structure and is therefore very difficult to distinguish in XRD patterns [46]. In general, g-Mn 2 O 3 is stable only under an inert atmosphere. It can be distinguished in skeletal infrared spectra, since the coupling of the vibration modes between the Mn 21 the Mn 31 cations in the hausmannite structure give rise to a lower number of components [47,48]. In this way, the presence of this phase is definitively excluded in our

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systems, being also supported by TPR analyses, which show a one-step reduction process for the sample M1 calcined at 723 K. All these materials are mesoporous adsorbents with moderate specific surface areas which become maximum for the system M4. The lower specific surfaces are related to the permanence of a crystalline phase (hausmannite or rhodochrosite) from room temperature which sinters progressively upon heating. The increase in the specific surface areas, in general, is associated to the formation of a low-crystallinity phase which gives rise to over-oxidized manganese oxides, such as Mn 5 O 8 or MnO 2 , upon calcining. The materials prepared from a Mn (III) precursor salt exhibit narrower pore sizes distributions with an increment of the population of the levels with lower diameters, which results in higher specific surface areas.

5. Conclusions The structural and morphological characteristics of manganese oxides studied prove to have a strong dependence on the synthesis conditions, mainly on the nature of the originating compound and the pH at which precipitation takes place. The materials prepared from a Mn(II) ionic salt and precipitated at a high pH are constituted at room temperature by a mixture of g-Mn 3 O 4 (as major phase) and amorphous non-stoichiometric Mn(lI) oxy-hydroxides. Upon calcining, the former undergoes a transformation at approximately 473 K to the a structure and remains stable up to 873 K, the latter, conversely, transforms successively to g-MnOOH, then to mixtures of MnO 2 , MnO and Mn 5 O 8 and finally to a-Mn 2 O 3 at 773 K. The precipitation at a pH value closer to 8.5 led to the crystallization of hausmannite directly. Its thermal behaviour is similar to the preceding system. On the other hand, Mn(II) ions can also be stabilized at low temperatures by the addition of suitable species (for instance, carbonate groups). The synthesis from a Mn(III) precursor gives rise to more oxidized materials, with the occurrence of new phases like nsutite or manganite, with manganese in a higher oxidation state. Independently from the preparation method, highly crystalline bixbyite is formed at 873 K or even lower temperatures. The decomposition of the precursor compounds (such as hausmannite, rhodocrosite, manganite, etc.) to a-Mn 2 O 3 always proceeds through intermediates (like Mn 5 O 8 , MnO 2 , etc.), whose presence is minimized in the case of hausmannite. The specific surface area of bixbyite depends mainly on the starting compound and it reaches maximum values when a Mn(III) precursor is used. This is associated to that the formation of bixbyite takes place primarily by the

crystallization from an amorphous compound rather than by the oxidation of hausmannite.

Acknowledgements This work has been supported by the Spanish Ministerio de Ciencia y Tecnologia (Ref. MAT2000-1 158) and Junta ´ Ref. SA37 / 98 (98-01). The authors de Castilla y Leon ´ acknowledge the collaboration of Leonardo Hernandez Delgado SALA-DIESEL. E.F.L. acknowledges Junta de ´ (Spain) for an FPI grant. Castilla y Leon

References [1] Bricker O. Am Miner 1965;50:1296. [2] Wells AF. In: 4th ed, Structural inorganic chemistry, Oxford: Clarendon Press, 1975, p. 458. [3] Boreskov GK, Popov BI, Bibin VN, Kozisimikova ES. Kinet Katal 1968;9:796. [4] Germain JE, Perez R. Bull Soc Chim France 1972:4683. [5] Lahousse C, Bernier A, Gaigneaux A, Ruiz P, Grange P, Delmon B, Grasselli RK et al., editors, Proceedings of the Third World Congress on Oxidation Catalysis, Amsterdam: Elsevier, 1997, p. 777. ´ [6] Baldi M, Sanchez Escribano V, Gallardo Amores JM, Milella F, Busca G. Appl Catal B, Environ 1998;17:L175. [7] Zang HM, Teraoka Y, Yamazoe N. Catal Today 1989;6:155. [8] Baldi M, Finocchio E, Milella F, Busca G. Appl Catal B, Environ 1998;16:43. [9] Van de Kleut D. Ph.D. Thesis. Utrech University; , 1994. [10] Grootendorst E, Verbeek T, Ponec V. J Catal 1995;157:706. [11] Weimin W, Yongniam Y, Jiayu Z. Appl Catal A 1995;133:81. [12] Sjoerd Kijistra W, Brands DS, Poels EK, Bielk A. Catal Today 1999;50:133. [13] Singoredjo R, Korver F, Kapteijn, JA, Moulijn. Appl Catal B, Environ 1992;1:297. [14] Haines AH. Methods for the oxidation of organic compounds, New York, 5: Academic Press, 1988. [15] Marhs J. Organic Chemistry, New Yok: Wiley and Sons, 1992. [16] Nardi JC. J Electrochem Soc 1982;129:861. ´ [17] Livage J, Henry M, Sanchez C. Prog Solid State Chem 1988;18:259. [18] Znaidi L, Baffler N, Huber M. Mater Res Bull 1989;12:1501. [19] Znaidi L, Baffler N, Pereira Ramos JP, Messina R. Solid State lonics 1988;28–30:886. ´ [20] Rodrıguez-Carvajal J. In: FULLPROF 2.4.2 December, ILL, 1993. [21] West AJ. Solid state chemistry and its applications, Cluichester: Wiley, 174. [22] Noh JA, Schwartz JA. J Colloid Interface Sci 1986;27:531. [23] Cranston RW, Inkley FA. Adv Catal 1957;9:143. [24] de Boer JH, Lippens BC, Linsen BG, Broekhoff A, Van der Heuvel T, Osinga J. J Coll Interface Sci 1966;21:405. [25] Nobman AKH, Zaki MI, Mansour SAA, Fahim RB, Kappestein C. Thermochim Acta 1992;210:103. [26] Pattanayak J, Sitakara V. Thermochim Acta 1989;153:193. ¨ [27] Lutz HD, Muller B, Steiner HJ. J Solid State Chem 1991;90:54. [28] McBride MB. Soil Sci Soc Am 1987;51:1466. [29] White WB, Keramidas VG. Spectrochim Acta 1971;28A:501. [30] Nakamoto K. Infrared and raman spectra of inorganic and coordination compounds, 4th ed, New York: Wiley, 1986. [31] Akrige JR, Kennedy JH. J Solid State Chem 1975;12:135.

J. Boyero Macstre et al. / International Journal of Inorganic Materials 3 (2001) 889 – 899 [32] Lenglet M, Bizi M, Jorgensen CK. J Solid State Chem 1990;86:82. [33] Lavalille F, Gourier D, Lejus AM, Vivien D. J Solid State Chem 1983;49:180. [34] Valigi M, Cimino C. J Solid State Chem 1975;12:135. [35] Milella F, Gallardo Amores JM, Baldi M, Busca G. J Mater Chem 1998;8:2525. [36] Dollimore D, Tonge KH. Proc Third ICTA, Davol 1971;2:91. [37] Dzhanastivilli BA, Bogoyavlenskii EN, Purlselazde KhG. Akad Nauk Gruz SSSR 1966;43:361. [38] Mendelovici E, Sagarzazu A. Thermochim Acta 1988;133:93. [39] Stobbe ER, de Boer BA, Geus JW. Catal Today 1999;47:161. [40] Stobbe ER, de Boer BA, Geus JW. Catal Today 1999;47:161.

899

[41] Gregg SJ, Sing KSW. Adsorption, surface area and porosity, London: Academic Press, 1991. [42] Morgan DJ, Milodowski AE, Warne S St J, Warrington SB. Thermochim Acta 1988;135:273. [43] To be published elsewhere. [44] Feitknecht W. Pure Appl Chem 1964;9:423. ´ ´ [45] Burriel Marti F, et al. Quımica Analıtica Cualitativa, etc. (238) [46] Bricker O. Am Miner 1965;50:1296. [47] Ishii M, Nakahira M, Yamanako T. Solid State Commum 1972;11:209. [48] Barbers VAM. Phys Status Solidi 1969;33:563.