Synthesis of high-surface area Mn-doped barium hexa-aluminates by a combination of hydrothermal precipitation–calcination route

Materials Chemistry and Physics 82 (2003) 892–896

Synthesis of high-surface area Mn-doped barium hexa-aluminates by a combination of hydrothermal precipitation–calcination route D. Mishra a , S. Anand a,∗ , R.K. Panda b , R.P. Das a b

a Regional Research Laboratory, Bhubaneswar 751013, Orissa, India Materials Science Division, Department of Chemistry, Berhampur University, Berhampur 760007, Orissa, India

Received 13 January 2003; received in revised form 19 May 2003; accepted 3 August 2003

Abstract High-surface area (25 m2 g−1 ) Mn-doped barium hexa-aluminates with compositions BaMnAl11 O19 and BaMn2 Al10 O19 were prepared by a combination of hydrothermal precipitation–calcination route. The precursors of the above two compounds were obtained by hydrolyzing a mixture of aqueous solutions of Ba, Al, and Mn at 180 ◦ C for 1 h in the presence of urea. The analysis of the hydrothermally prepared samples showed the presence of carbonate and/or hydroxide phases of Ba and Mn and boehmite. Barium mono-aluminate (BaAl2 O4 ), Ba–␤II -Al2 O3 , ␥-Al2 O3 , and a small fraction of MnO2 were observed after calcination of these hydrothermally prepared precursors at 1200 ◦ C. A mixture of Ba–␤I -Al2 O3 and Ba–␤II -Al2 O3 phases were found after calcination at 1400 ◦ C for 2 h with no other oxide phases of Ba, Al or Mn. © 2003 Elsevier B.V. All rights reserved. Keywords: Hydrothermal route; Mn-doping; Barium hexa-aluminate; Calcination

1. Introduction The high thermal stability of barium hexa-aluminates (melting point 1900 ◦ C) makes them a suitable material for catalyst/catalyst supports used in high-temperature effective combustion of lean fuels such as in gas turbines with minimum CO and NOx emissions [1,2]. In this regard barium hexa-aluminate-based materials are superior to conventionally used corderite (2MgO·2Al2 O3 ·5SiO2 ) as the former maintains a high-surface area (>15 m2 g−1 ) above 1400 ◦ C due to its inherent layered structure which inhibits sintering and growth at high temperatures [3–5]. Transition/non-transition metal ions are often introduced into the barium hexa-aluminate structure in order to improve its catalytic behavior and develop properties suitable for colored phosphor applications [6,7]. Among various metal ion doped barium hexa-aluminates, the Mn-doped materials are most extensively studied as the presence of Mn does not affect the thermal stability, surface areas and phase compositions of the parent barium hexa-aluminates [8–10]. As the properties of the end-products are largely dependent on the synthetic routes followed and on the nature and history of the precursors, a careful choice of reagents and the preparatory route is essential to produce the de∗ Corresponding author. Tel./fax: +91-674-2581750. E-mail address: s p [email protected] (S. Anand).

0254-0584/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2003.08.003

sired end-materials. Several methods have been reported for the preparation of Mn-doped barium hexa-aluminates [11–18]. These methods include (i) preparation by a mechanical activation of a mixture of inorganic salt of Al, Ba, and Mn followed by calcination (the activation–calcination) route [11], (ii) by evaporation of a homogeneous solution mixture of Ba, Al, and Mn salts followed by calcination (the evaporation–calcination) route [12], impregnation of aqueous Mn2+ in Ba and Al hydroxides/oxo-hydroxides followed by calcination (impregnation–calcination) route [13,14], and by calcination of precursors obtained through sol–gel routes using inorganic salt solutions of Ba, Al and Mn (sol–gel–calcination) route [15–18]. We have recently reported the preparation of high-surface area barium mono-aluminates [19] and barium hexa-aluminates [20] by a combination of hydrothermal precipitation– calcination route using aqueous solutions of Ba and Al nitrates and urea. The main objective of the present study is to prepare high-surface area Mn-doped barium hexaaluminates from inorganic salt solutions following a similar hydrothermal precipitation–calcination route.

2. Experimental Reagent grade Ba(NO3 )2 and Al(NO3 )3 ·9H2 O (BDH India Ltd.) and guaranteed grade Mn(NO3 )2 ·4H2 O (Fluka,

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Table 1 Coding of Mn-doped barium hexa-aluminate samples Expected composition BaMnAl11 O19 BaMn2 Al10 O19 BaMnAl11 O19 BaMn2 Al10 O19 BaMnAl11 O19 BaMn2 Al10 O19

Calcination temperature (◦ C) – – 1200 1200 1400 1400

Sample code BM1 BM2 BM12 1 BM12 2 BM14 1 BM14 2

Germany) were dissolved in distilled water to obtain a 0.1, 1.0 and 0.01 M solutions, respectively. Desired amounts of these solutions were thoroughly mixed according to the compositions BaMnAl11 O19 and BaMn2 Al10 O19 and the volume was made up to 1000 ml by dilution with distilled water. To this solution mixture, a weighed amount of urea was added and the contents were transferred to a Parr Model-4542 autoclave of 2 l capacity for hydrothermal precipitation at 180 ◦ C. After 1 h at 180 ◦ C the autoclave was cooled immediately to room temperature, the products were filtered and washed thoroughly with distilled water to remove washable impurities and dried overnight (or until a constant weight was attained) inside an air-oven maintained at 95–100 ◦ C. Analyses of the filtrate and wash liquors suggest almost quantitative precipitation of all the three metals. These washed and air-oven dried powder samples were subjected to calcination at the desired temperatures inside a furnace for a desired period (i.e., 2 h). These samples were also used for thermal and other characterization studies. The washed and air-oven dried (but uncalcined) precursor powder samples were coded, for convenience as shown in Table 1. X-ray diffraction (XRD) patterns of the samples were obtained with a Philips Powder Diffractometer Model PW 1710 in the range of 2θ = 10–80◦ at a scanning rate of 2◦ min−1 , using a Ni-filtered Cu target. The Fourier transform infrared (FTIR) spectra of samples were recorded by a Perkin Elmer P-500 Spectrophotometer using the KBr pellet method. Surface area measurements were done by using a Monosorb Quantachrome instrument following the Brunauer, Emmett and Teller method. Prior to each measurement samples were degassed at 110 ◦ C for 2 h.

3. Results and discussion 3.1. XRD results The XRD pattern of the two powders BM1 and BM2 , obtained hydrothermally at 180 ◦ C are shown in Fig. 1a and b, respectively. The XRD pattern of BM1 (Fig. 1a) shows orthorhombic barium carbonate to be the major crystalline phase present with peaks at 2θ = 24.21◦ , 34.38◦ and 46.97◦ [21a], together with MnCO3 or rodhochrosite, peaks at 2θ = 31.39◦ , 37◦ , 42◦ , 45.14◦ and 47.86◦ [21b]

Fig. 1. The XRD patterns of hydrothermally prepared Mn-doped barium hexa-aluminate precursors (a) BM1 and (b) BM2 .

and Mn(OH)2 of which only two peaks at 2θ = 18.54◦ and 19.82◦ are visible [21c]. In contrast the XRD pattern of the BM2 sample shows MnCO3 (Fig. 1b) to be the major crystalline phase together with boehmite (␥-AlOOH) (peaks at 2θ = 14.74◦ , 28.36◦ and 38.54◦ [21d]), BaCO3 and Ba(OH)2 ·8H2 O (peaks at 2θ = 13.25◦ and 19.22◦ [21e]). It is interesting to note here that in sample BM1 no crystalline phase of Al is observed, suggesting that aluminum hydroxide(s)/oxo-hydroxide(s) might be present in an amorphous/non-crystalline form. However, when the Mn concentration increases broad peaks for boehmite appeared. 12 The XRD patterns of the samples BM12 1 and BM2 , obtained after calcination of BM1 and BM2 , respectively, at 1200 ◦ C for 2 h are shown in Fig. 2a and b. It is seen from their XRD patterns that both powders have similar phases present irrespective of their Mn content and initial phase compositions. Both samples show the characteristic lines of BaAl2 O4 [21f] at a d value of 3.16 and 4.54 Å, having relative intensities of 100 and 32%, respectively. Besides barium mono-aluminate, the other major phases present are ␥-Al2 O3 (major peaks at 2θ = 66.40◦ , 45.91◦ and 36.97◦ [21g]), Ba–␤II -Al2 O3 (major peaks at 2θ = 19.95◦ , 31.60◦ , 34.28◦ and 57.92◦ [21h]) and small amount of MnO2 (where the only peak corresponding to maximum intensity is apparent at 2θ = 22.15◦ , having an observed relative intensities of

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2HNO3 + 2NH4 OH → 2NH4 NO3 + H2 O

(3)

with the overall reaction of formation of BaCO3 as Ba(NO3 )2 + NH2 –CO–NH2 + 2H2 O → BaCO3 (c) + 2NH4 NO3

(4)

Similarly formation of MnCO3 will take place as given below: Mn(NO3 )2 + NH2 –CO–NH2 + H2 O → MnCO3 (c) + 2NH4 NO3

(5)

Manganese nitrate partly hydrolyses to give manganese hydroxide Mn(NO3 )2 + NH2 –CO–NH2 + 2H2 O → Mn(OH)2 + 2NH4 NO3 + CO2

(6)

(iii) The formation of boehmite in case of sample BM2 : 2Al(NO3 )3 + 7NH2 –CO–NH2 + (14 + 2x)H2 O → 2AlOOH · xH2 O (c) + 6NH4 NO3 + 4(NH4 )2 CO3 + 3CO2 (g)

(7)

In case of sample BM1 no crystalline phase of aluminum oxide/hydroxide was observed in the precursor obtained at 180 ◦ C. 3.1.2. Calcination in different temperature regions

12 14 Fig. 2. The XRD pattern of samples (a) BM12 1 , (b) BM2 and (c) BM2 .

7% [21i]). The XRD patterns of both the samples BM14 1 and ◦ C for 2 h were also identical; a typBM14 , calcined at 1400 2 ical XRD pattern of the former sample is shown in Fig. 2c. All the major reflections of this diffractogram correspond to either Ba–␤I -Al2 O3 [21j] or to Ba–␤II -Al2 O3 [21h]. This type of observation was made earlier in case of the parent (un-doped) barium hexa-aluminates [20].

(i) At 1200 ◦ C (from XRD), any crystalline or amorphous oxide/hydroxide of aluminum formed during hydrothermal treatment at 180 ◦ C would result in the formation of Al2 O3 . Small amounts of manganese hydroxide formed will dehydrate to form manganese oxide. Irrespective of the nature of precursors at 1200 ◦ C, the crystalline phases shown in XRD are of barium mono-aluminate, Ba–␤II -Al2 O3 and ␥-Al2 O3 . Al2 O3 , BaCO3 , MnCO3 and Mn(OH)2 → Mixture of BaAl2 O4 , Ba–␤II -Al2 O3 (c) and γ-Al2 O3

(8)

The incorporation of manganese could be in BaAl2 O4 or Ba–␤II -Al2 O3 . (ii) At 1400 ◦ C (XRD).

3.1.1. Chemical reactions taking place during hydrothermal precipitation (i) Hydrolysis of urea at 180 ◦ C NH2 –CO–NH2 (s) + H2 O → 2NH4 OH + CO2 (g) (1) (ii) Reaction with barium nitrate with the product CO2 of reaction (1) Ba(NO3 )2 + CO2 (g) + H2 O → BaCO3 (c) + 2HNO3 (2)

Mixture of Ba–␤I -Al2 O3 (c), Ba–␤II -Al2 O3 with doped manganese. The observations made in the XRD study of Mn-doped barium hexa-aluminate differ in two respects from that made in the case of un-doped barium hexa-aluminate [20]: (i) A lack of appearance of any crystalline phase of Al when the Mn concentration is low and (ii) the appearance of an intermediate barium mono-aluminate phase at 1200 ◦ C prior to the formation of Ba–␤I ␤II -Al2 O3 phase at 1400 ◦ C. The

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Fig. 3. The FTIR spectra of samples (a) BM1 and (b) BM14 1 .

BaMnx Al12−x O19 powder obtained via alkoxide hydrolysis [15] showed that mono-phasic samples were obtained up to x = 2, and segregation of BaAl2 O4 occurred upon further Mn addition. However, studies made from inorganic salts suggested the formation of BaAl2 O4 at a whole range of Mn substitution; the amount decreased with decrease in the Mn concentration in the range of x = 0.5–3 [16,17]. In the present study, we observe that under identical preparation conditions, the presence of Mn facilitates the formation of BaAl2 O4 at intermediate temperature of calcination. 3.2. FTIR results The FTIR spectra of the hydrothermally prepared samples BM1 and BM2 were almost identical and a typical spectra of BM1 is shown in Fig. 3a. The two broad peaks centered around 3300 and 3100 cm−1 are due to the stretching vibrations for –OH/H2 O. Another weak peak at 1650 cm−1 is due to the bending modes of vibrations of the –OH groups in hydroxides and/or oxo-hydroxides of Ba, Mn and Al [22,23]. Presence of carbonate species in these samples is confirmed by the characteristic peaks at 1445 and 1060 cm−1 [22]. A very broad and strong peak observed in the 900–400 cm−1 region can be due to several M–O stretching and bending vibrations which could not be resolved in the present case [22,23]. The FTIR spectrum of the corresponding sample ◦ BM14 1 calcined at 1400 C is shown in Fig. 3b. The spectrum shows only one sharp peak at 1040 cm−1 which is due to the Al–O vibrations in the spinel block of ␤-alumina structure

having symmetry A2u [24,25] and a broad band between 980 and 400 cm−1 , in this region most of the vibrations for Al–O bending modes appear with symmetries E1u and A2u [24,25]. 3.3. Surface area results It was observed that the surface area values of the calcined samples are independent of their Mn content but dependent on the calcination temperature. The surface areas of sam12 14 14 ples BM12 1 , BM2 , BM1 , and BM2 were found to be 96.5, 94, 23.8 and 24.9 m2 g−1 , respectively. The higher values of surface area for the samples calcined at 1200 ◦ C could be due to the presence of metastable aluminas (␥-Al2 O3 ) in these samples, and the decrease in surface area values with increasing the calcination temperature to 1400 ◦ C may be due to the formation of Ba–␤I ␤II -Al2 O3 phases as seen from their XRD patterns. In the present route the surface 14 areas measured for samples BM14 1 , and BM2 calcined at 1400 ◦ C are much higher than that of several earlier reported (12–15 m2 g−1 ) Mn-doped barium hexa-aluminates prepared from inorganic salt solutions [15,18].

4. Conclusions From the present study it is observed that hydrothermal precipitation–calcination technique is a convenient preparatory route for synthesis of high-surface area Mn-doped bar-

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ium hexa-aluminates from inexpensive aqueous inorganic salt solutions. Presence of Mn in the hydrothermally prepared precursors facilitates formation of BaAl2 O4 at intermediate temperature (1200 ◦ C) of calcination. After calcination at 1400 ◦ C for 2 h a mixture of barium ␤-aluminas were formed without the presence of any contaminant oxide phases of Mn, Ba, and Al. The high-surface area of the end-materials most probably stems from the fine and partly amorphous precursors, consisting of carbonates/hydroxides of Mn and Ba and oxo-hydroxides of Al produced by the present hydrothermal route.

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