Synthesis of Ca-doped spinel by Ultrasonic Spray Pyrolysis

doi:10.1016/j.matlet.2016.02.114

Materials Letters

Volume 171, 15 May 2016, Pages 232-235

https://doi.org/10.1016/j.matlet.2016.02.114 Get rights and content

Highlights

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Synthesis of magnesium aluminate nanopowders is possible by Ultrasonic Spray Pyrolysis.
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Spherical porous agglomerates is obtained with high specific surface area independently of additive concentration.
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The CaO doping decreases the crystallite size.

Abstract

MgAl₂O₄ is a stable catalyst support with potential for replacing gamma-alumina in several applications. However, synthesis of magnesium spinel requires elevated temperatures to avoid phase separation (in MgO and Al₂O₃) at low temperatures, leading to coarsening and reduction of active surface area. In this work, nano CaO-doped and undoped magnesium aluminate were successfully prepared by Ultrasonic Spray Pyrolysis (USP), using a simple adapted experimental set-up operating at 1100 °C. During the process, the particles stay at high temperatures for a short period of time, allowing phase stability and limited coarsening. The influence of calcium oxide on the particles morphology and structure was investigated via X-ray diffraction, N₂ adsorption, X-ray fluorescence, scanning electron microscopy and transmission electron microscopy. The spinel nanopowders were obtained as spherical porous agglomerates of ~1 µm. The resulting powder showed low crystallite sizes in the 5–10 nm range and high specific surface area from 110.0 to 76.6 m² g⁻¹.

Introduction

Magnesium aluminate (MgAl₂O₄, also known as spinel) is primarily used as a refractory material [1], [2], [3] and has great potential as a transparent lightweight armor [4]. MgAl₂O₄ possesses superior mechanical properties, such as high elastic modulus (273 GPa) and flexural strength (110 MPa), associated with low density (3.58 g/cm³), low reflection index (1.736), high optical transmission in visible and mid-wavelength infrared spectra (0.2–5.5 µm), and no optical anisotropy due to its cubic structure [5]. MgAl₂O₄ is isostructural to gamma-alumina, and so suitable for catalytic applications [6], [7], [8], [9] but with the advantage of being more stable at high temperatures, since transition to alpha phase is not a concern.

Spinel nanoparticles have been previously prepared by different methods [6] However, a continuous, scalable, and versatile process for the preparation of doped MgAl₂O₄ still remain as a challenge for expanding applications. Among the usual synthesis routes used to produce nano-oxides, Ultrasonic Spray Pyrolysis (USP) has been successfully employed to synthesize nanoparticles as solid and hollow spheres, nanowires, nanoribbons and nanorods [10], [11], [12], [13], [14]. In this work we demonstrate the potential of USP to produce doped spinel in a continuous setup. Porous micrometric spheres of CaO-doped MgAl₂O₄ with crystallite size in the range from 5 to 10 nm and specific surface areas from 76 to 101 m² g⁻¹ were produced as soft spherical agglomerates of ~1 µm.

Section snippets

Powder synthesis

Aqueous solution (0.1 m Mg(NO₃)₂·6H₂O; 0.2 m Al(NO₃)₃·9H₂O) was doped with appropriate quantities of hydrated Ca(NO₃)₂ (all used reagents from Synth Ltda.). A commercial ultrasonic nebulizer (Britânia – 5 L) was used to spray the solutions into a tubular furnace (Lindeberg Blue-1 m long) with a 0.2 m isothermal temperature zone at 1100 °C. The resulting powder was deposited in a glass tube at the end of the tubular furnace. The spinel powder was recovered from the glass tube wall with ethanol and

Results and discussion

The MgO, Al₂O₃ and CaO contents were determined by XRF (Table 1) on the samples produced from the USP reactor. The (MgO+CaO)/Al₂O₃ ratio is close to unit, suggesting the synthesis allowed effective incorporation of all elements.

Fig. 2 shows the XRD patterns of Ca-doped MgAl₂O₄ powders prepared by USP which could be indexed as spinel structure (JCPDS card 05-0672) for all calcium concentrations. CaO as a second phase was only detected for the sample doped with the highest CaO content (7.3 mol%).

Conclusions

CaO-doped MgAl₂O₄ nanopowders were successfully prepared from precursor nitrates solutions by Ultrasonic Spray Pyrolysis (USP) at 1100 °C. The spinel nanopowder was produced as spherical agglomerates with approximately 1 µm size, which is specially suitable for catalyst support applications. The size was essentially independent on the additive concentration. However, CaO reduces simultaneously the crystallite size and specific surface area, probably due to the initial sintering of nanoparticles

Acknowledgements

The authors thank CNPq – Proc. 407149/2013-9 and FAPESP – Proc. 2013/23209-2 for financial support and CNPEM-LNNano TEM – 17265. RHRC thanks U.S. Department of Energy, BES ER46795 Early Career Program Award.

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Synthesis of Ca-doped spinel by Ultrasonic Spray Pyrolysis

Highlights

Abstract

Introduction

Section snippets

Powder synthesis

Results and discussion

Conclusions

Acknowledgements

Mater. Lett.

Scr. Mater.

Mater. Chem. Phys.

J. Eur. Ceram. Soc.

Mater. Lett.

Mater. Lett.

Mater. Lett.

Appl. Surf. Sci.

J. Alloy. Compd.