Rapid synthesis and characterization of mesoporous nanocrystalline MgAl2O4 via flash pyrolysis route

Journal of Asian Ceramic Societies 1 (2013) 328–332

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Rapid synthesis and characterization of mesoporous nanocrystalline MgAl2 O4 via flash pyrolysis route Shuvendu Tripathy ∗ , Debasis Bhattacharya Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India

a r t i c l e

i n f o

Article history: Received 7 July 2013 Received in revised form 24 August 2013 Accepted 31 August 2013 Available online 19 September 2013 Keywords: Mesoporous Nanocrystalline Rapid synthesis Cation disorder MgAl2 O4

a b s t r a c t In this work, we demonstrate an effective, fast and low-cost route for the synthesis of mesoporous MgAl2 O4 powder. This synthesis procedure successfully reduces the overall synthesis time for nanosized MgAl2 O4 powder from days to a few hours. The as-synthesized powder was found to be amorphous in nature. Upon calcination at 700 ◦ C (2 h) and 900 ◦ C (4 h) in presence of air, the as-synthesized MgAl2 O4 amorphous powder was transformed into a disorder and order nanocrystalline phase respectively. The structural information of evolvement phase was analyzed through Rietveld refinement technique and Fourier transform infrared spectroscopy. The specific surface area of MgAl2 O4 powder calcined at 700 ◦ C and 900 ◦ C was found to be 118.14 m2 /g and 74.67 m2 /g, respectively. © 2013 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved.

1. Introduction Magnesium aluminate (MgAl2 O4 ) is one of the most promising refractory oxide materials with useful electrical, mechanical thermal and optical properties. It is widely used as a catalyst support material for various chemical reactions such as dehydrogenation of n-butane [1], high temperature CO2 capture [2], CO oxidation [3] and water shift reactions [4]. In addition to its uses in chemical industries, MgAl2 O4 has major applications in refractory, optical and sensor industry [5,6]. Nanocrystalline MgAl2 O4 is a low density additive material for solid propellant [7]. In general, MgAl2 O4 is prepared by methods like coprecipitation method [8], wet-chemical process [9], template method [10], sol–gel citrate technique [11], silica xerogel template method [12] and self-generated organic template method [7]. Zhang et al. have reported hydrothermal route for the synthesis of mesoporous MgAl2 O4 [13]. This process includes hydrothermal treatment at 180 ◦ C for 24 h and additional 4 h for drying. Recently,

∗ Corresponding author. Tel.: +91 3222 283976; fax: +91 3222 255303. E-mail address: [email protected] (S. Tripathy). Peer review under responsibility of The Ceramic Society of Japan and the Korean Ceramic Society.

mesoporous MgAl2 O4 was reported to be synthesized via a surfactant assisted precipitation route within ∼48 h [14]. Furthermore, citric acid–ethylene glycol route was adopted for synthesis of MgAl2 O4 nano-powder with a synthesis time of 40 h [15]. Nevertheless, these aforementioned techniques require multiple steps and are incredibly time-consuming. From the perspective of synthesis, it is still an important challenge to synthesize mesoporous MgAl2 O4 via a single-step, rapid and economically viable approach. Moreover, there are few literature references available about the synthesis of mesoporous MgAl2 O4 within a short time frame [16]. Herein, we demonstrate a simple, reproducible and rapid sol–gel route to synthesize highly mesoporous MgAl2 O4 crystalline framework which reduces the synthesis time from several days to 2 h [17]. The present method has advantages of (1) templatefree growth process, (2) use of inexpensive chemicals such as metal nitrates, citric acid and glycine instead of metal alkoxides [18,19], and (3) air atmosphere reaction rather than nitrogen or argon [7,11]. The calcined mesoporous MgAl2 O4 powder was studied using transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and N2 adsorption–desorption. In particular, distribution of cations (Mg and Al) over tetrahedral and octahedral intestinal sites as a function of calcinations temperature is studied using FTIR and Rietveld refinement of XRD patterns. 2. Experimental works

2187-0764 © 2013 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jascer.2013.08.006

Stock solutions of precursor salts of Mg(NO3 )2 ·6H2 O and Al(NO3 )3 ·9H2 O were prepared by dissolving the salts in distilled water. A mix of aqueous solutions of the said nitrate salts of Al3+ and

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Fig. 2. TG and DTG plot of the MgAl2 O4 precursor materials.

Fig. 1. Experimental setup for flash pyrolysis method.

Mg2+ in the molar ratio of 2:1 was taken in a 2000 mL glass beaker. Citric acid was added to this solution so that the ratio of citric acid to the total mole of cations was 1.5. The pH of the resultant solution was adjusted to 6 by adding liquor ammonia in drops. Calculated amount of glycine, ammonium nitrate and ethylene glycol were added thereafter to the solution. Citric acid and glycine initially acted as chelating agents and at a later stage they served as fuel and pore former [7]. The beaker containing the metal ions was then heated in a pit furnace set at 350 ◦ C with concurrent heating from the top by an infrared lamp. The water slowly evaporated leading to the formation of a gel. This was followed by sudden combustion with rapid liberation of gases. We obtained a black and brown fine powder in the shape of a low density sponge. The as-synthesized powder was collected and calcined at different temperatures. Fig. 1 shows the experimental set-up for the flash pyrolysis method. Decomposition behavior of the black and brown colored mass was examined with the help of a thermal analyzer at a heating rate 10 ◦ C/min in air (NETZCH TG 209F). XRD measurements were carried out at room temperature using a powder diffractometer (PANalytical High Resolution XRD PW 3040). The particle morphology of the calcined powder was studied using transmission electron microscope (JEOL-JEM-2100). The surface area and pore size distributions of powder were estimated by using the N2 adsorption and desorption method (Quantachrome Autosorb-1). Infrared spectra were recorded on Bruker (TENSOR 27).

of mesoporous MgAl2 O4 precursor powder. Here, the presence of calculated amount of glycine initiates the combustion process as well as supplied the adequate amount of heat to the combustion reactions. Fig. 2 shows the TG and DTG thermal analysis of the assynthesized MgAl2 O4 precursor materials. The weight loss can be classified mainly into three zones: AB, CD and EF. The initial weight loss was primarily associated with the elimination of physically adsorbed water. The highest mass loss was observed in the region CD of the typical TG curve. It can be attributed to the decomposition of metal complexes, polymer charring, pyrolysis of organic compounds and loss of chemisorbed water. A region EF of the curve reveals a mass loss of 7.53%. It may be due to the decarboxylation of residual carbonaceous ingredients as reported by Yatsui et al. [15]. The whole chemical changes were due to pyrolysis of the organics with concurrent elimination of gases which was reflected in the total weight loss of 71.03% in the TG curve. The peak at 455 ◦ C in the DTG curve may be indicative of the decomposition of precursor materials. The XRD patterns of the as-synthesized sample and calcined at various temperatures are shown in Fig. 3. The XRD pattern of as-prepared sample has a broad peak at low 2, indicating amorphous phase. A sharp low intense peak is found at 72◦ , indicating the existence of carbon nanoflakes, which has also been reported by Pristavita et al. [23]. MgAl2 O4 stays amorphous at 500 ◦ C as evidenced from XRD. A crystalline phase of MgAl2 O4 is obtained at 700 ◦ C. However, with the increase in calcination temperature,

3. Results and discussion Fuel plays a significant role in controlling shape, size, morphology, distribution and surface area of the final powder. The most frequently used organic fuels are glycine, urea and citric acid and their heat of combustion are −3.24 kcal g−1 , −2.98 kcal g−1 and −2.76 kcal g−1 , respectively [20]. Glycine makes the reaction quick, highly exothermic and sometimes unsafe. However, a very quick and high reaction temperature would partially sinter the ultra fine particles resulting in bigger size particles with smaller surface area. However, citric acid is a better chelating agent, forming more stable complexes compared to glycine [21,22]. On the other hand, it makes the combustion slow because of its less negative combustion heat [18]. Hence, in present work, glycine was added to aqueous solution of citric acid–metal nitrate complexes and heated to produce quick and controlled reactions leading to formation

Fig. 3. XRD patterns of the as-synthesized powder and calcined at various temperature.

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Fig. 5. FTIR spectra of calcined MgAl2 O4 powder (a) calcined at 700 ◦ C (2 h) and (b) calcined at 900 ◦ C (4 h).

Fig. 4. Rietveld refinements XRD pattern of MgAl2 O4 powder (a) calcined at 700 ◦ C (2 h) and (b) calcined at 900 ◦ C (4 h).

the intensity of the peaks representing to spinel phase increased. The XRD peaks in Fig. 3(c)–(e) match the peak given in the JCPDS card no. 04-008-1061 [24]. Applying Scherrer’s formula the average crystallite sizes of (3 1 1) plane (Fig. 3c, e) calcined at 700 ◦ C (2 h) and 900 ◦ C (4 h) were found to be 14 nm and 24 nm, respectively. In order to get details of the structural and micro-structural parameters, the diffraction data of MgAl2 O4 calcined powder were examined by the Rietveld refinement (program FULLPROF) using pseudo-Voigt function for profiling [25]. Fig. 4 shows the Rietveld refinement of XRD patterns for MgAl2 O4 calcined powder. The parameters such as scale factor, zero correction, displacement, lattice constant, atomic coordinates and occupancy of the ions get updated in the course of refinement. No violation of space group symmetry was detected during the investigation even in metastable state. Based on the XRD refinement results (Table 1) of the powder calcined at 700 ◦ C for 2 h, one can clearly figure out that Al3+ ions present on both tetrahedral and octahedral interstitial sites. The refinement results estimate that the powder forms a disorder spinel structure. Combining the occupancies of tetrahedral and

octahedral sites, the chemical formula for powder calcined at 700 ◦ C (2 h) is found to be (Mg.57 Al.27 ) Al2 O4 ( represents a symbol for vacancies). This type of cation disorder and vacancy is usually observed in lower temperate calcined spinel powder. Our results are in accordance with the Rietveld refinements done by Reinen et al. [26]. On further increase of the calcination temperature to 900 ◦ C for 4 h, all the Al3+ ions move from tetrahedral to octahedral sites and the defect spinel is turned into an ordered spinel structure. The redistribution of cations is consistent with the change of lattice parameter. At higher calcination temperature, these cations gain a sufficient amount of energy and migrate towards the most stable equilibrium position. Among a few investigations that have been reported on disorder in the spinel samples, the most relevant seems to be Sreeja et al. [27]. According to their study the cation distribution was depending upon the particle size of the nanoparticles. They also observed that the inversion of Al3+ ions in the tetrahedral sites decreases with the increase in particle size. Bhattacharya et al. discussed that the order–disorder phase transformation in MgAl2 O4 is due to the joint effect of chemical composition and amount of heat involved in citrate nitrate auto ignition route [28]. Here we observed a disordered magnesium aluminate nanocrystalline phase (700 ◦ C). From above argument, it is evident that the distribution of Al3+ ions in the tetrahedral sites depends upon the particle size as well as the calcination temperature. The evaluated statistical parameters depicted a good correlation between the calculated and experimental data (Table 1). The co-ordinance state of Al and Mg cations in nanocrystalline spinel phase is ensured by FTIR analysis. Fig. 5 shows the FTIR spectra of the calcined powders gained from the flash pyrolysis route. It is conceived that the broad peak in the region 3500 cm−1 and peak at 1624 cm−1 corresponded to the OH stretching and H O H banding of adsorbed water, which reflects high surface area of the powder [15]. The spinel type structure is proved by the bands at

Table 1 Refined structural parameters of MgAl2 O4 using cubic structure (space group: F d 3¯ m). Sample composition Mg.57 Al2 .27 O4 Mg Al2 O4

Calcination temperature ◦

700 C (2 h) 900 ◦ C (4 h)

Lattice parameter (Å)

Volume (Å3 )

Rp (%)

Rwp (%)

2

8.06 8.08

523.60 527.51

11.4 7.77

12.6 8.43

1.95 1.65

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Fig. 6. TEM images of the calcined nanoparticles of MgAl2 O4 at two different temperatures. (a) Bright field image of the mesoporous nanoparticles and (b) correspond dark field at 700 ◦ C for 2 h; (c) bright field at 900 ◦ C for 4 h and (d) corresponding HRTEM image of a single grain.

(532 cm−1 and 699 cm−1 ), which are associated with lattice vibrations of tetra and octahedrally coordinated metal ions [29]. The IR spectral regions (700–500 cm−1 ) are assigned to the bending mode of Al3+ octahedrally occupied in normal spinel. Moreover the powder calcined at 700 ◦ C (Fig. 5a) the band at 699 cm−1 and a shoulder band detected at 802 cm−1 were observed, this implied that some of the Al3+ occupied the tetrahedral position; this is consistent with several theoretical and experimental results [30,31]. This kind of inversion diminishes with increase in calcinations temperature. Xray Rietveld refinement results and FTIR spectra confirm that some of the Al3+ ions occupy tetrahedral sites creating a disordered structure. FTIR studies indicate the presence of slight impurities. This might be absorption of CO2 and moisture from the air [32]. Fig. 6 shows the TEM images of self-agglomerated MgAl2 O4 nanoparticles. In general the morphology shows that a bunch of very fine nanocrystals self-organize to form a bigger cluster which was much smaller than our earlier reported value [17]. Addition of ethylene glycol in the present work played a crucial role in minimizing the particle size and imparting homogeneity. The hydrolysis reaction between citric acid and ethylene glycol led to the formation of a three-dimensional rigid polymer network. The polymer resin restricted the mobilization of metal ions. The hard rigid polymer network resulted in uniform dispersion of cations, reduced the segregation of metal ions and hence resulted in formation of particles with a uniform particle size in narrow range [33,34]. It is also observed that the calcination process does not show a significant impact on the particle shape. Both dark field and bright field images confirm the mesoporous nature of the powder. It is also found that the pores are worm-hole type. From the TEM images (Fig. 6), we can see the very small nanoparticles, which are

embedded in a carbon matrix [35]. In the case of powder calcined at 900 ◦ C (4 h) the aggregates are composed of uniform MgAl2 O4 nanocrystals with irregular shape and nearly about 25 nm in diameter (Fig. 6c). In the previous published work, various chemical routes have been employed to prepare single phase nanocrystalline MgAl2 O4 where the average particle size was 20–600 nm after being calcined from 600 ◦ C to 900 ◦ C [15,36–38]. HRTEM technique was utilized to examine crystal structure, which suggested that the ˚ powder was well crystalline .The 2D lattice fringe spacing was 4.5 A, corresponding to inter-planner distance of cubic MgAl2 O4 (with F d 3¯ m symmetry Fig. 6d). Fig. 7 represents the N2 adsorption/desorption isotherms and pore size distribution of the calcined powder. The isotherms obtained for powder calcined at 700 ◦ C (2 h) can be attributed to the type IV with typical H3 hysteresis loops (Fig. 7A (a)). On the other hand, powder calcined at 900 ◦ C (4 h) displays a type IV isotherm with a H1 hysteresis loop (Fig. 7A (b)) [33,39]. The surface area of the sample was 118.14 m2 /g at 700 ◦ C, when the temperature was increased to 900 ◦ C, this value decreased to 74.67 m2 /g. The BET surface area of MgAl2 O4 is further compared with earlier work. The nanocrystalline MgAl2 O4 obtained by different chemical routes (and calcined at 900 ◦ C) possesses a surface area of 12.14–40 m2 /g [7,40–42]. It is important to note that the nanocrystalline MgAl2 O4 particles (calcined at same 900 ◦ C) obtained in the present work possess a much higher surface area of 74.67 m2 /g. However, Saberi et al. reported citrate–nitrate route for synthesis of MgAl2 O4 precursor powder [11]. The surface area of nanocrystalline MgAl2 O4 was found to be 156.3 m2 /g after subsequent heat treatment to precursor powder in argon and air atmosphere. Nevertheless this present method has several benefits. Firstly, all the reactions and

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overcome the time consuming and complex nature of the existing processes as it takes only 2 h for completion. The structural data found after Rietveld refinement have provided adequate evidence of cation disorder at lower calcination temperature powder. The movement of the cations between the octahedral and tetrahedral sites was confirmed by particular absorption bands from FTIR spectra. The distribution of cations can be manipulated by varying the calcination temperature. References

Fig. 7. (A) N2 adsorption/desorption isotherms and (B) pore size distribution of the MgAl2 O4 prepared by flash pyrolysis method.

calcination were performed in air atmosphere using normal electric furnace. Secondly, synthesis process is quick and the powder is highly reproducible from experiment to experiment. The existence of mesopore size pore distribution (Fig. 7B) is of particular interest for catalytic, sensor and adsorption applications. Sample calcined at 700 ◦ C (2 h) exhibited an average pore diameter 7.03 nm while sample calcined at 900 ◦ C (4 h) exhibited an average pore diameter 14.08 nm. The pore size increase may be agglomeration/sintering as evidenced from the decrease surface area and increase in particle size by HRTEM. From XRD, HRTEM, FESEM and N2 adsorption/desorption results, the calcined powder possesses a crystalline mesoporous three-dimensional framework of MgAl2 O4 . It is also noticed that the distribution of cations and structural parameters are strongly dependent on calcination temperature. The Rietveld refinement results are in close agreement with cations co-ordination state acquired by FTIR spectra. 4. Conclusions Mesoporous nanocrystalline MgAl2 O4 powder was successfully synthesized by the flash pyrolysis route. Our approach helps to

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