Synthesis of higher surface area mayenite by hydrothermal method

Materials Research Bulletin 46 (2011) 1307–1310

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Synthesis of higher surface area mayenite by hydrothermal method Chunshan Li a,b,*, Daisuke Hirabayashi a, Kenzi Suzuki a a b

EcoTopia Science Institute, Nagoya University, Nagoya, 464-8603, Japan State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, CAS, Beijing, 100190, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 6 January 2010 Received in revised form 7 March 2011 Accepted 25 March 2011 Available online 6 April 2011

Mayenite (Ca12Al14O33) is an important material, being used as a transparent conductive oxide (TCO), catalysts for VOCs cleaning and ionic conductor. Unfortunately, the material has a very low BET surface area, generally below 10 m2/g and requires higher calcination temperature for the formation of mayenite phase. In this study, a new synthesis method, namely hydrothermal method, was employed for the synthesis of mayenite with higher BET surface area of about 70 m2/g, and also the calcination temperature decreased from over 1000 to just 400 8C. It was found that mayenite from different method shows different shapes. ß 2011 Elsevier Ltd. All rights reserved.

Keywords: Surface properties Chemical synthesis Inorganic compounds Thermogravimetric analysis

1. Introduction Mayenite (Ca12Al14O33) and its analogues (Sr12Al14O33, Ca12Al10Si2O35, etc.) described with a crystal structure C12A7, have stimulated research interest because of their oxygen mobility [1– 4], ionic conductivity [5–8], and catalytic properties [9,10]. An interesting feature of these mayenites is oxygen storage capacity in the structure, in which 32 of the 33 oxygen anions are tightly bound, containing large cages, 1/6 of them being filled randomly by the remaining ‘free’ oxygen. At ambient temperature excess oxygen is generally found, leading to a chemical composition of Ca12Al14O33.5, which has been attributed to the presence of hydroxide, peroxide and superoxide radicals in the cages. At high temperatures, the density of the ‘free’ oxygen is extremely spread out, with the expansion being related to the high ionic conductivity of this material. These oxygen ions are present in a very loosely bound state and are called ‘free’ oxygen ions; they can be replaced by monovalent anions such as OH, F, and Cl. These ‘free’ oxygen ions can be successfully replaced by active anion species unstable in the atmosphere such as O, O2, H, and electrons [11–13]. Owing to these exceptional characteristics, the C12A7 crystals have potential for exhibiting active functionalities. It has been reported that mayenite showed higher catalytic activity for the oxidation of volatile organic compounds (VOC) compared to either CaO or Al2O3. These crystal structures also possess higher anticarbon and anti-sulfur characteristics [14–17], due likely to the

* Corresponding author at: EcoTopia Science Institute, Nagoya University, Nagoya, 464-8603, Japan. Tel.: +81 52 7895845; fax: +81 52 7895845. E-mail addresses: [email protected], [email protected] (C. Li). 0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2011.03.023

presence of O2 and O22 radicals. In addition, the exchange of the ‘free’ oxygen between adjacent cages has also been predicted by theoretical calculations. Mayenite, in most of the previous reports, has been synthesized by ceramic route, by solid state reaction between CaCO3 and gA12O3 and by calcining the mixed powder at very high temperatures such as 1347 8C [18], or 1600 8C [19], or 1350 8C [20,21] and heating at these temperatures for longer time over 24 h [21]. Longer calcination times are needed because of poor homogeneity of the mixed starting powders. Also mayenite from these methods has a low surface area less than 10 m2/g [21] and low porosity, which limit its potentials for applications in catalytic reaction or material support. In this study, we report a new procedure for the synthesis of higher surface area mayenite by mixing Al(OH)3 and Ca(OH)2 and treating them hydrothermally followed by calcination at relatively lower temperatures. We also show that this preparation procedure offers homogeneous and higher surface area mayenite with a unique structure. 2. Experimental 2.1. Catalysts preparation In this study, mayenite materials were prepared by two different methods, namely hydrothermal synthesis, denoted as M-H, and the traditional ceramic route by solid state reaction without hydrothermal treatment, designed as M-C. In M-H method, a 1:1 stoichiometric mixture of Ca(OH)2 and Al(OH)3 was ground in a Planetary Ball Mill (Pulverisette-7, Fritsch, Japan) with distilled water for 4 h at a speed of 300 rpm, then the mixture

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was placed in a Teflon-lined stainless steel autoclave (25-ml volume). The autoclave was placed in a temperature controlled oven and the temperature was increased to 150 8C, and kept at 150 8C for about 5 h. The solid products were separated by filtration and dried at 120 8C for 8 h, followed by calcination at 200, 400, 600, 800, 1000 8C over 4 h in an air atmosphere. The calcined samples were crushed and sieved to obtain particle sizes between 212 and 425 mm. In the M-C method, mayenite was prepared by mixing powders of Ca(OH)2 and Al(OH)3 in stoichiometric ratio, grinding the mixture in Planetary Ball Mill apparatus (Pulverisette-7, Fritsch, Japan) with distilled water for 4 h at speed of 300 rpm, then dried at 120 8C, calcined at 400, 600, 800, 1000 8C over 4 h in an air atmosphere. 2.2. Catalysts characterization X-ray powder diffraction was taken with Mac Science M18 XHFSRA XRD apparatus with Ni-filtered Cu Ka radiation. The acceleration voltage was 30 kV and the acceleration current was 100 mA. BET specific surface areas were determined by nitrogen adsorption at the temperature of liquid nitrogen with a Belsorp II (BEL Japan, Inc.). Scanning electron microscopy (SEM) was measured by field emission-scanning electron microscope (JSM6330F, JEOL DATUM Ltd.). TG/DTA behavior was measured by Thermoplus TG 8120 (Rigaku).

formation of mayenite phase. It is likely that the formation of Ca3A12(OH)12 phase during hydrothermal treatment favors, or accelerates the formation of mayenite phase at a rapid rate even at a low calcination temperature of above 400 8C compared to the high calcination temperature of above 1000 8C required, if the precursors of Ca and Al have not been treated hydrothermally. The whole hydrothermal method process can be explained in detail from Eqs. (1)–(7). The starting materials Ca(OH)2 and Al(OH)3 treated by hydrothermal method, yielded the product Ca3Al2(OH)12, according to Eq. (1). 6CaðOHÞ2 þ 7AlðOHÞ3 ! 2Ca3 Al2 ðOHÞ12 þ 3AlðOHÞ3

(1)

At 300 8C, Ca3Al2(OH)12 and 3Al(OH)3 dehydrate according to Eqs. (2) and (3). At 300–500 8C, AlO(OH) also will dehydrate (Eq. (4)). Ca3 Al2 ðOHÞ12 ! Ca3 Al2 O6 þ 6H2 O

(2)

AlðOHÞ3 ! AlOðOHÞ þ H2 O

(3)

2AlOðOHÞ ! Al2 O3 þ H2 O

(4)

At 400 8C, the mayenite structure forms according to Eq. (5), or (6). 4Ca3 Al2 O6 þ 6AlOðOHÞ ! Ca12 Al14 O33 þ 3H2 O

(5)

4Ca3 Al2 O6 þ 3Al2 O3 ! Ca12 Al14 O33

(6)

3. Results and discussion

The Al(OH)3 dehydration process occurs according to Eq. (7).

3.1. Thermal and structural characteristics

 300  C

400500  C

AlðOHÞ3 ! AlOðOHÞ

The thermal and structural characteristics of the mayenite materials have been studied by TG/DTA and XRD. The XRD patterns of the materials prepared by the traditional ceramic route (M-C) without hydrothermal treatment and calcined at 400, 600, 800, 1000 8C over 4 h are shown in Fig. 1(a). It can be noted that no mayenite phase is formed for samples calcined at 400, 600, 800 8C. These calcination temperatures produce phases corresponding to CaO and Al2O3. However, the formation of mayenite phase is observed for the sample calcined at 1000 8C. The M-H samples, prepared by hydrothermal treatment followed by calcination at different temperatures as shown in Fig. 1(b), exhibit mayenite phase for samples calcined at 400 8C itself and the crystallinity of the phase increases with further increase in calcination temperature as seen by an increase in intensities of the peak. It is also interesting to note from XRD that the formation of a mixture of Ca3A12(OH)12 and Al(OH)3 phases occur for the samples calcined below 400 8C. The XRD results thus suggest that the hydrothermal treatment has an impact on the

!

 800  C

g-Al2 O3 ! a-Al2 O3 þ H2 O

The TG/DTA curves for M-C samples and M-H samples are shown in Fig. 2(a) and (b), respectively. The M-C sample exhibits two endothermic peaks, one between 200 and 400 8C and the other between 400 and 500 8C. Based on the XRD data shown in Fig. 1(a), the low temperature endothermic peak centering around 250 8C could be attributed to the partial dehydration of Al(OH)3 to Al2O3 while the second endothermic peak centering around 450 8C can be assigned to the complete decomposition of Ca(OH)2 and residual Al(OH)3 to form CaO and Al2O3. The weight loss is minimum above 500 8C, and exhibits an exotherm above 900 8C. This exotherm could be due to the formation of mayenite phase from CaO and Al2O3. In contract, the TG/DTA curves of M-H sample shown in Fig. 2(b) exhibit a single endotherm around 300 8C. Based on the XRD data discussed above, this endothermic peak is attributed to the formation of mayenite phase. These TG/DTA results further support the XRD data on the effect of hydrothermal treatment on the mayenite phase formation.

Intensity

Intensity

b

20

40

60

2theta (degree)

80

(7)

20

40

60

2theta (degree)

Fig. 1. XRD of mayenite without and with hydrothermal treatment at different calcination temperature.

80

120 110

0

-50

100 90

-100 80

DTA TG

70

-150 200

400

600

800 0

Temperature ( C)

1000

1200

b Weight left (%)

a Weight left (%)

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0

120 110

-30

100 -60 90

TG

DTA

80

-90

70 -120 200

400

600

800

1000

0

Temperature ( C)

Fig. 2. TG/DTA curve of mayenite without and with hydrothermal treatment.

Fig. 3. Scanning electron micrographs of (a) M-H-400, (b) M-H-600, (c) M-H-800, (d) M-H-1000, and (e) M-C-1000.

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Table 1 The physical properties of mayenite from different preparation methods.

2

as,BET (m /g) Pore volume (cm3/g) p/p0 = 0.990 The average diameter of hole (nm)

M-H-400

M-H-600

M-H-800

M-H-1000

M-C-1000

67.1 0.046 2.72

68.9 0.11 6.46

19.1 0.12 12.5

4.2 0.0088 8.34

5.1 0.011 8.73

3.2. Morphology The morphology of mayenite materials is studied by scanning electron microscopy (SEM). The SEM pictures of M-H 400, 600, 800, and 1000 are shown in Fig. 3(a)–(d), respectively. For the purpose of comparison, the SEM picture of the M-C 1000 sample also is included and shown in Fig. 3(e). It is interesting to note that the MH samples calcined at all temperatures show uniform size spherical morphology with pores while an irregular shaped particles are obtained for the M-C-1000 sample. 3.3. BET surface area and porosity The BET surface area and porosity of all the samples are summarized in Table 1. As can be seen, mayenite synthesized by hydrothermal treatment route and (M-H) calcined at low temperatures shows higher BET surface area of around 70 m2/g compared to the material synthesized by the routine ceramic method. It can be noted that the mayenite sample synthesized by M-H route and calcined at 600 8C exhibits high BET surface area and also high porosity of above 0.1 cm3/g. 4. Conclusions Mayenite material with high BET surface area of around 70 m2/ g can be synthesized by hydrothermal treatment followed by relatively low temperature calcination of around 600 8C compared to the routine ceramic method involving solid state reaction, which requires over 1000 8C to form the same phase, but very low BET surface area of below 10 m2/g. The material synthesized by hydrothermal treatment also produces regular

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