Single step solid-state fusion for MgAl2O4 spinel synthesis and its influence on the structural and textural properties

Accepted Manuscript Single step solid-state fusion for MgAl2O4 spinel synthesis and its influence on the structural and textural properties

Norhasyimi Rahmat, Zahira Yaakob, Manoj Pudukudy, Norazah Abdul Rahman, Seri Suriani Jahaya PII: DOI: Reference:

S0032-5910(18)30127-X https://doi.org/10.1016/j.powtec.2018.02.007 PTEC 13183

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Powder Technology

Received date: Revised date: Accepted date:

8 November 2017 25 January 2018 5 February 2018

Please cite this article as: Norhasyimi Rahmat, Zahira Yaakob, Manoj Pudukudy, Norazah Abdul Rahman, Seri Suriani Jahaya , Single step solid-state fusion for MgAl2O4 spinel synthesis and its influence on the structural and textural properties. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Ptec(2017), https://doi.org/10.1016/j.powtec.2018.02.007

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Single step solid-state fusion for MgAl2O4 spinel synthesis and its influence on the structural and textural properties

Norhasyimi Rahmata, b,*, Zahira Yaakoba, Manoj Pudukudya, c, Norazah Abdul Rahmanb,

Seri Suriani Jahayad Department of Chemical and Process Engineering, Faculty of Engineering and Built

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a

Environment, Universiti Kebangsaan Malaysia, Bangi, 43600, Selangor, Malaysia. b

Faculty of Chemical Engineering, Universiti Teknologi MARA, Shah Alam, 40450, Selangor,

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Research Centre for Sustainable Process Technology (CESPRO), Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, Bangi, 43600, Selangor, Malaysia Research and Development Centre, Sime Darby Research Sdn. Bhd., Carey Island, 42960,

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Selangor, Malaysia

*E-mail: [email protected], [email protected]

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Tel no: +601-115076501; Fax no: +603-55436300

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d

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c

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Malaysia.

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Abstract

Magnesium aluminate (MgAl2O4) spinel nanostructure has been synthesized using a single step solid-state fusion method, with citric acid as a surfactant. The samples prepared at different annealing conditions such as temperature and duration, were analyzed to study their effects on the structural, crystalline and textural properties

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of MgAl2O4 spinel, such as specific surface area, morphology, crystallite size and agglomeration. The prepared samples were characterized using X-ray diffraction

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(XRD), Brunauer-Emmett-Teller (BET) surface area analysis, Fourier transform

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infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Increasing the annealing duration has insignificant influence on the crystallinity and the

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formation of spinel phase. However, with increasing annealing temperature, significant effects were observed, namely, decreasing specific surface area, increasing pore size

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and crystallite size, and increasing degree of agglomeration. Samples prepared at different annealing temperatures were then tested for catalytic methane steam reforming to understand their catalytic properties. It was found that the MgAl2O4 spinel with high

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steam to carbon ratio of 2.

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surface area and small crystallite size showed good catalytic activity at 700°C, with a

Keywords: Solid-state citrate fusion; Spinel; Mixed oxides; Magnesium aluminate; Annealing

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effects

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1.

Introduction

Over the past few years, spinel materials have become widely applied in various fields, including rubber and plastic production [1], painting industries [2], and in catalysis [3]. Spinel materials are metal oxides with a general molecular formula of XY2O4, which could be prepared either in powder, or in other forms, such as frameworks [4], films [5], monoliths [6], and nanotubes [7]. Metal oxide spinels, such as CuCr2O4 [8], CoMn2O4

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[9], NiCo2O4 [10], NiAl2O4 [11], NiFe2O4 [12], NiMn2O4 [13], ZnAl2O4 [14], and CaFe2O4 [15] are not only applied in the transformation of raw materials into chemical

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feedstock and energy production, but also in pollutant decomposition in wastewater and

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air pollution.

Magnesium aluminate (MgAl2O4) is an excellent spinel material [16] that is currently utilized in refractory [17] and metallurgy [18] industries. Previous studies

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have reported that MgAl2O4 possesses special properties, such as high melting point, inert towards chemical reactions, resistant to thermal disturbances, low dielectric

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constant, and high mechanical strength [19]. As such, MgAl2O4 is also widely applied in chemical [20], optical transmission [21], electrochemical [22], and radio technical fields [23]. In addition, MgAl2O4 is also investigated for catalysis because of its

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hydrophobic behaviour, low acidity, and good catalytic properties. MgAl2O4 is being

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considered an excellent catalyst support because it displays metallic compatibility and it can disperse active metal sites on its surface to facilitate the chemical reactions [24]. Nobel metals [25], transition metals [26–28], and lanthanides [29] are examples of

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metallic species that are usually adopted to be loaded onto the surface of MgAl2O4 spinels for different types of catalytic reactions. In addition to acting as a catalyst

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support, MgAl2O4 itself has been directly used as a catalyst for fluid catalytic cracking for SOx abatement [30], the production of chemical feedstocks such as acetaldehyde [31], 2,4,6-triarylpyridines [32], and 2-ketomethylquinolines [33], and in photocatalytic reaction [19]. MgAl2O4 exhibits unique metal oxide characteristics, such as high chemisorption and redox properties, which are important in the oxidative adsorption of SOx abatement and the decarbonylation of lactic acid to acetaldehyde. Moreover, MgAl2O4 spinel displays ferromagnetic and optical characteristics, allowing it to act as a separable photocatalyst that can degrade 90.0 to 95.45% of Reactive Red Me 4BL dye under UV, or sunlight irradiation and 50% of methyl orange under UV light [34].

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MgAl2O4 can be synthesized through direct solid-state reaction [28], wet chemical solution techniques [37], sonochemical [33], sol-gel [27], and mechanical activation [38]. MgAl2O4 synthesized through solid-state reaction generally involves the mechanical mixing or grinding of Mg and Al precursors, in the presence of a surfactant or fuel, followed by annealing to produce fine powders [35, 39]. In the wet chemical solution technique, the Mg and Al precursors are dissolved in a chemical reagent

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(usually a solvent), through thermal heating at a low temperature, ranging between 80 to o

C, followed by annealing [27]. The annealing temperature required for

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synthesizing MgAl2O4 spinel as a ceramic material is higher than that for synthesizing

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MgAl2O4 spinel as a catalyst, or a catalyst support [19, 40]. A surfactant, or fuel is generally used in synthesis techniques, such as solid-state and wet chemical solution, to facilitate the Al3+ and Mg2+ homogenization [6], as well as to act as a capping [41] and

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templating agent [42]. Adding a surfactant in Al3+ and Mg2+ homogenization in a wet chemical solution route would create spaces between adjacent metal ions, thereby

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restricting the hydrolysis interaction. A surfactant with large molecular weight and molecular size corresponds to a wider distance between the adjacent metal ions, which ultimately diminishes the hydrolysis interaction in a wet synthesis, thus delivering finer

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particle size, higher surface area, and lesser agglomerated particles [43]. In a wet

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chemical solution technique, the amount of escaping gas during combustion is closely related to the molecular weight and molecular size of the surfactant, and it could influence the morphology and textural properties of the synthesized MgAl2O4 spinel

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[44]. Thus, the wet chemical solution route, with an appropriate surfactant, is considered as a superior method for achieving high surface area, small crystallite and

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particle size of MgAl2O4 spinel, with narrow particle size distribution at low annealing temperatures and durations [45]. Sucrose [46], urea [45], citric acid [47], carbohydrazide [43], and triethylenetetramine [44] are examples of the surfactants that have been used to synthesize MgAl2O4 spinel through the wet chemical solution technique. Conventional solid-state method requires double stages of annealing to form uniform and highly dense spinel phases [48]. Whereas, a wet chemical solution technique requires a liquid medium to assist the coordination of Al3+ and Mg2+. Both conventional solid-state and wet chemical solution methods require extended processing

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time and tedious synthesis steps to obtain spinel powder with specific characteristics [48, 49]. These methods also require expensive chemical reagents to assist in the spinel formation and densification [50]. In the present study, MgAl2O4 was synthesized via a solid-state fusion method with citric acid. This approach was completed in a single step, within a short processing time and dry medium using basic ingredients, namely, Mg and Al precursors and citric acid as the surfactant. Citric acid has a relatively high molecular

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weight, making it capable to act as a reductant and templating agent [44]. It possesses one hydroxyl and three carboxyl groups, making it stable to form complexes with

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multivalent ions. Saberi et al. successfully synthesized nanocrystalline MgAl2O4 using

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citric acid as a chelating agent that coordinated Al3+ and Mg2+, while preventing the precipitation that occurred during pH change. Citric acid also promotes the combustion of the solution at lower temperatures, thereby producing MgAl2O4 powder with small

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particles and crystallite size [47]. Therefore, citric acid was selected as the surfactant to prepare MgAl2O4 under different annealing conditions.

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MgAl2O4 spinel is generally employed as a support, or loaded with metals as a catalyst. Since a catalyst support could significantly influence catalytic activity and product selectivity [51], the synthesized MgAl2O4 spinel was investigated for methane

Materials and methods

2.1 Chemicals

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2.

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structure, and morphology.

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steam reforming to correlate its catalytic performance with surface area, crystallite size,

In this experiment, Mg precursor used was magnesium nitrate hexahydrate

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(Mg(NO3)2.6H2O, Bendosen) and aluminium nitrate nonahydrate (Al(NO3)3.9H2O, R&M Chemicals) was used as the Al precursor. Citric acid monohydrate (C6H8O7.H2O, R&M Chemicals) was used as the surfactant for fusing the metal precursors of Mg2+ and Al3+. For the catalytic activity, methane (purity 99.99%) and distilled water were used as feedstock, while N2 (purity 99.99%) and H2 (purity 99.99%) were used for purging the reactor and for catalyst reduction, respectively.

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2.2 MgAl2O4 synthesis The stoichiometric amounts of the metal precursor and citric acid were prepared. Briefly, 0.1 mol Mg(NO3)2.6H2O, 0.2 mol Al(NO3)3.9H2O, and 0.3 mol C6H8O7.H2O were mixed using a mortar and a pestle, and rigorously pulverized for at least 1 hour. The pulverized mixture was then transferred into a crucible and placed in an oven at 100 °C for 2 hours to facilitate the proper fusion of the Al3+ and Mg2+ precursors. Then, the

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mixture was annealed at 700 °C for 5 hours in a furnace under ambient air. This process was repeated under different annealing temperatures of 600, 800, 900, and 1000 °C. To

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obtain samples for different annealing durations, the samples were annealed at 700 °C

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for 4, 5, 6, and 7 hours. 2.3 Characterization of materials

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The specific surface area for all MgAl2O4 samples at different annealing temperatures was determined using the Brunauer-Emmett-Teller (BET) method. The pore size and

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pore volume were measured using the Barrett-Joyner-Halenda (BJH) method. The analysis was conducted using a micropore surface area analyzer (model 3 Flex Micromeritic), with N2 as the sorption gas. The grain equivalent diameter for

3000   DBET

Eq. (1)

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S

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and the following Eq. (1):

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agglomeration determination [52] was calculated using the BET specific surface area

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in which DBET is the grain equivalent diameter (nm), ρ is the theoretical density of MgAl2O4 (3.58 gm-3), and S is the specific surface area (m2g-1). The phase evolution of the MgAl2O4 spinel at different annealing temperatures and durations was identified using X-Ray diffraction (XRD), model Bruker D8 Focus Advance powder diffractometer, with Cu Kα radiation at 2θ degree, from 10° to 80° at a scanning step of 0.02° and scanning speed of 4°min-1. The crystallite size of the spinel samples was calculated using Scherrer’s equation, shown by Eq. (2):

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D XRD 

k  cos  o

Eq. (2)

where DXRD is the crystallite size (nm), k is 0.9, β is the full width at half maximum (FWHM) in radian for the intense peaks at the planes (3 1 1), (4 0 0), and (4 4 0), λ is the wavelength (1.5406 Å), and θ is the Bragg angle in degree.

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The surface functional groups of MgAl2O4 were identified using Fourier transform infrared spectroscopy (FTIR) at room temperature, ranging between 400 to

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4,000 cm-1 at a 4 cm-1 resolution.

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Thermogravimetric analysis (TGA) was conducted in a Mettler Toledo Gas STAR System, model TGA HT instrument, in atmospheric air at a heating rate of

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10 °min-1 and an air flow rate of 20 ml min-1 from room temperature to 900 °C. The morphologies of the MgAl2O4 spinel at different annealing temperatures were examined using the field emission scanning electron microscope (FESEM), model

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Zeiss SUPRA 55, and the transmission electron microscope (TEM), model Philips CM12. TEM samples were prepared by dispersing the powders in ethanol. Then, the samples were ultrasonicated for 10 min, dropped onto a carbon-coated copper grid,

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2.4 Catalytic activity

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dried, and analyzed.

The catalytic activities of the representative MgAl2O4 samples were evaluated for the

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catalytic steam reforming of methane. Approximately 1 g of MgAl2O4 was loaded into a stainless steel fixed bed reactor, with an internal diameter of 0.025 m and a length of 0.6

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m. N2 gas was purged at a flow rate of 150 ml min-1 and the reactor was warmed up from room temperature to 650 °C at a ramping rate of 20.83 °C min-1. The catalyst loaded in the center of the reactor was reduced with H2 gas for 1.5 hours at 650oC with flow rate of 150 ml min-1. Then, N2 gas was introduced at a flow rate of 150 ml min-1 and the reactor was prepared for reaction at 700 °C at a ramping rate of 1.67 °C min-1. Once the reaction temperature was reached, CH4 was introduced at a flow rate of 150 ml min-1 and water was pumped at a flow rate of 0.221 ml min-1. The steam to carbon ratio used for this reaction was 2. The reaction was conducted for 6 hours and the products at different time intervals were analyzed using a gas chromatograph (GC), model GC-

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2014C (Shimadzu). The GC was equipped with a TDX-01 column and a thermal conductivity detector (TCD), using helium as the carrier gas, to calculate the methane conversion. The steam reforming of methane is theoretically illustrated in Eq. (3), and this reaction is accompanied by the water-gas shift reaction, which is represented by Eq. (4). H o  206.0 kJ mol -1

Eq.

CO( g )  H 2O( g )  CO2 ( g )  H 2( g )

H o  41.2 kJ mol -1

Eq.

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CH 4( g )  H 2O( g )  CO( g )  3H 2( g )

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(3)

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(4)

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The coke formation of carbon is possible in the steam reforming of methane because of the undesired methane cracking, as shown in Eq. (5), and the Boudouard

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reaction in Eq. (6) due to the high reaction temperature. CH 4( g )  C  2H 2( g ) H o  75.0 kJ mol -1

Eq. (5)

2CO( g )  CO2  C

Eq. (6)

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H o  86.2 kJ mol -1

3.

Eq. (7)

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The conversion of methane was calculated as follows:

Results and discussion

3.1 Single step solid-state fusion reaction pathways The reaction pathways of the MgAl2O4 synthesis through citrate fusion method can be illustrated as follows:

(a) Pulverization: During the pulverizing step, citric acid was mixed with Al and Mg precursors so that metal chelating and binding processes could occur.

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(b) Fusion: This step involves the confirmation of the chelation and binding of Al and Mg with citrate groups. Knowing that the citrate groups would be coordinated with the metals in a monodentate manner [1], the following reaction might occur during the process: Mg NO3 2  6H 2 O  Al ( NO3 ) 3  9H 2 O  5C6 H 8O7  H 2 O  Mg (C6 H 7 O7 ) 2  Al (C6 H 7 O7 ) 3  20H 2 O  5HNO3

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(c) Annealing: During the annealing process, the fused citrate-metal ligands were first decomposed into individual metal ligands (Mg-citrate and Al-citrate ligands), and

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further increase in the annealing temperature would trigger the formation of MgAl2O4 spinel [37].

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Mg (C6 H 7 O7 ) 2  Al (C6 H 7 O7 ) 3  Mg (C6 H 7 O7 ) 2  Al (C6 H 7 O7 ) 3

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5Mg (C6 H 7 O7 ) 2  10 Al (C6 H 7 O7 ) 3  144HNO3  5MgAl 2 O4  240CO2  212H 2 O  72 N 2

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3.2 XRD characterization

The XRD diffraction patterns of the MgAl2O4 spinel synthesized via solid-state citrate fusion at different annealing durations are displayed in Fig. 1(a), and the samples

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prepared at different annealing temperatures, ranging between 600 to 1000 °C for 5 hours are shown in Fig. 1(b). As shown in Fig. 1(a), MgAl2O4 spinel samples that were

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annealed at 700 °C, with different durations, displayed the strongest peaks at the 2θ values of 36.9°, 44.73°, and 65.3°, which corresponded to the (3 1 1), (4 0 0), and (4 4

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0) diffraction planes. Noticeable weak peaks were also observed at 2θ values of 19.0°, 31.28°, 55.84°, 59.33°, and 77.40°, respectively for the (1 1 1), (2 2 0), (4 2 2), (5 1 1), and (5 3 3) diffraction planes of the face-centered cubic phase of the MgAl2O4 spinel,

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with Fd3m space group (JCPDS card #01-070-5187). These results indicated that the various annealing durations have insignificant effect on the crystalline properties of MgAl2O4 spinel.

As shown in Fig. 1(b), the sample annealed at 600 °C showed no obvious peak, suggesting that phase formation and crystal growth occurred at a temperature higher than 600 °C. As the annealing temperature was increased from 700 to 1000 °C, the peak intensities of the MgAl2O4 spinel gradually increased, particularly at the diffraction planes of (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0), and (5 3 3), which reflected 2θ values at 18.95°, 31.23°, 36.78°, 44.85°, 55.62°, 59.23°, 65.2°, and 77.4°,

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respectively. Noticeable peaks at 2θ values of 37.62°, 68.65°, and 74.09° at the respective diffraction planes of (2 2 2), (5 3 1), and (6 2 0), which were formed at higher than 800 °C, as shown in Fig. 1(b), were also identified as the face-centred cubic of the crystalline structure of MgAl2O4 spinel. Table 1 displays the approximate crystallite sizes and lattice parameters of the spinel powders annealed at different annealing durations and temperatures. Three strong

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and high intensity diffraction peaks at (3 1 1), (4 0 0), and (4 4 0) diffraction planes were selected for the crystallite size measurements, using Scherrer’s equation. The

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crystallite size of the MgAl2O4 spinel annealed at 700 °C at different durations in the

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range of 4 to 7 hours has shown insignificant differences (6 to 7 nm) for the (3 1 1) diffraction plane, indicating that this range of duration exerted an insignificant influence on the crystal growth of the spinel powders. However, the annealing temperatures had

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significantly affected the crystallite size, in which an increase of 100 °C in the range of 700 to 1000 °C had increased the crystallite size from 6 to 22 nm for the (3 1 1)

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diffraction plane, from 8 to 23 nm for the (4 0 0) diffraction plane, and from 8 to 21 nm for the (4 4 0) diffraction plane. The lattice constant of the synthesized MgAl2O4 at different annealing durations and temperatures had ranged from 8.07 Å to 8.09 Å,

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which is in agreement with the JCPDS results. The increased crystallite size due to

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increasing annealing temperature observed in this study is corroborated by the study of MgAl2O4 through a chemical solution method by Rufner et al. [52] and Ewais et al.

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[53].

3.3 FTIR characterization

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Figs. 2(a) and 2(b) show the FTIR spectra of the MgAl2O4 precursor and powders at different annealing temperatures and durations, respectively. According to a previous report, the stretching vibration for the free carboxyl group of citric acid could be observed between 1760 and 1700 cm-1, and it was split into two bands belonging to the middle and terminal carboxyl groups [1]. However, as shown by the MgAl2O4 precursor sample in Fig. 2(a), given that citric acid could form stable complexes with metal ions, the asymmetric and symmetric stretching vibrations for the carboxyl groups appeared at 1606.4 and 1398.3 cm-1, respectively [2]. The bands observed at 1075 and 666.8 cm-1 were attributed to the NO3 group, whereas the wide bands displayed between 3600 and

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3200 cm-1 in the precursor sample were assigned to the stretching vibrations of the hydroxyl OH group. In Fig. 2(a), the sample annealed at 600 °C shows a broad band in the range of 900 to 700 cm-1, which could be attributed to the coordination between Mg2+ and Al3+ [47]. This phenomenon is not displayed in the spectra for the sample annealed at 700 °C. This result is in agreement with the XRD results in Fig. 1(b), which exhibited no obvious peaks for the sample annealed at 600 °C but exhibited peaks for

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the samples annealed at 700 °C and higher. The intensity of the peaks observed in the range of 590 to 620 cm-1 for the sample was increased when the annealing temperature

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was increased from 700 to 1000 °C. This could be due to the metal oxide bonds that

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formed in MgAl2O4 spinel [54]. The disappearance of the NO3 groups and the appearance of the metal oxide bonds suggested the formation of MgAl2O4 spinel, which began at a temperature higher than 600 °C. This observation is confirmed by the XRD

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analysis of MgAl2O4 shown in Fig. 1(b). The band at 1410 cm-1 in Fig. 2(a) was associated with the C-H stretching vibrations of the citrate group. No nitrate groups, or

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organic bands were found in the samples annealed at high temperatures, indicating the high surface purity of the samples through a complete decomposition reaction. As shown in Fig. 2(b), the annealing duration has exerted no obvious effect on

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the MgAl2O4 phase formation. The formation of the metal oxide bonds in MgAl2O4

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spinel was clearly displayed by a band at 590 to 620 cm-1 in all samples. This finding is consistent with the XRD results in Fig. 1(b), in which all intense and noticeable peaks were present in the samples annealed at 700 °C for 4 to 7 hours. Thus, it can be

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concluded that the annealing duration was indistinctive in this study, and therefore, only

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the samples annealed at different temperatures were further investigated.

3.4 Thermogravimetric analysis The simultaneous thermal curves of the precursor and the samples annealed at 600 °C are shown in Figs. 3(a) and 3(b), respectively. Meanwhile, Fig. 3(c) compares the thermogravimetric (TG) curves of the mixed sample of Mg-Al precursor and sample powder annealed at 600 and 1000 °C. The endothermic peak at low temperature, ranging between 99 and 111 °C of the differential thermal analysis (DTA) curves could be attributed to the moisture weight loss of the spinel precursor [40, 41]. The second broad endothermic peak at the temperature range of 200 to 300 °C for the spinel

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precursor and sample powder annealed at 600 °C corresponded to the elimination of the surface hydroxyl group of the spinel [44, 45]. The sharp exothermic peak displayed at 160 °C indicated the decomposition of the nitrates and the citrate groups [14, 41], whereas the broad exothermic peak at the temperature range of 380 and 450 °C was assigned to the decarboxylation of the precursor to metal oxide [14, 46]. In Fig. 3(a), this exothermic peak due to decarboxylation was still observed in the spinel sample

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annealed at 600 °C, as shown in Fig. 3(b). Weight losses due to moisture, nitrate and citrate groups at the temperature range of 100 to 300 °C was 74% for the mixed sample

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of Mg-Al precursors and only 4.1% for the sample annealed at 600 °C. The TG results

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in Fig. 3(c) revealed that the organic compounds at the temperature range of 380 to 600 °C were reduced in weight as the annealing temperature was increased. A total weight loss of 62.5% and 7.8% was measured for the mixed sample of Mg-Al precursors and

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the sample annealed at 600 °C, respectively, in this temperature range. The negligible weight loss of the sample powder annealed at 1000 °C demonstrated the complete

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disappearance of organic compounds through decarboxylation. The TGA/DTA results in Figs. 3(a)-3(c) are corroborated by the FTIR results in Fig. 2(a), in which the organic compound of the citrate group was observed and the compound was completely

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decarboxylated at high temperatures. A small exothermic peak at 789 °C could be

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ascribed to the reaction between Al2O3 and MgO, which led to crystallization and spinel formation [45]. This exothermic peak at higher than 700 °C, in the absence of weight loss, was also observed by Wajler et al. at 840 °C [55] and Rufner et al. [52] in the

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temperature range of 725 to 850 °C.

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3.5 Textural properties

The textural properties of MgAl2O4 samples at different annealing conditions are presented in Table 2. Significant decreases in specific surface area and pore volume were observed after annealing at different temperatures for 5 hours. However, the pore width of the spinel showed a considerable increase from 5.6 to 11.7 nm when the temperature was increased from 700 to 1000 °C. This finding is in agreement with the results obtained by Rezaei et al. [27] and Alvar et al. [42]. They suggested that an increase in the annealing temperature for the same reaction period has resulted in the release of high amount of heat and a high decomposition rate of fused sample, which

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could deteriorate the pore walls of the material. Consequently, the destruction of the pore walls resulted in a high pore diameter, but with low specific surface area and pore volume. The relationship between the decrease in specific surface area and the increase in crystallite size, with increasing annealing temperature, has been illustrated by Prabhakaran et al. [45] and supported by the studies of Alinejad et al. [46] and Saberi et al. [47]. In the present study, the decrease in the specific surface area in Table 2

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corresponds with the increase in crystallite size as the annealing temperature is increased, as shown in Table 1.

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The analysis results of the BET surface area in Table 2 are consistent with the

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isotherm results depicted in Fig. 4. Samples annealed at 600 °C for 5 hours exhibited a type IV characteristic, with H3 hysteresis loop, suggesting the presence of non-rigid aggregated particles. This finding is highly consistent with the inset pore distribution

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curve and the BET surface area analysis, which revealed the indistinctive pore volume and the low specific surface area. The BET isotherms of the samples annealed at 700,

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800, and 900 °C can be classified as type IV of the International Union of Pure and Applied Chemistry (IUPAC) classification that indicated the presence of mesoporous texture in the samples. The presence of hysteresis loops at a low relative pressure

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indicated type H2(b), which is a typical characteristic for mesoporous structures, with

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features of desorption branch attributed to the evaporation at pore necks because of the removal of citric acid during the annealing process. By contrast, the sample annealed at 1000 °C displayed a type V isotherm, which indicated the presence of a weak

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adsorbent-adsorbate interaction. The hysteresis loops were slightly observed at a high relative pressure range, suggesting that the sample has a mesoporous structure, with a

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wider pore neck [56]. This finding is consistent with the inset curve of pore size distribution and the results shown in Table 2.

3.6 Morphology characterization The TEM micrographs of the samples at different annealing temperatures are presented in Fig. 5, which shows the samples as a sheet-like aggregate of nanocrystals. The particles observed in the TEM micrograph of the samples annealed at 700 °C were more loosely aggregated than those at 800, 900, and 1000 °C. The micrographs of the samples annealed at 900 and 1000 °C showed densely agglomerated particles, as suggested by

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the reduced spaces between particles [45]. This finding is in agreement with the agglomeration determination by Eq. (1). For the sample annealed at 700 °C, the calculated specific surface area (S) obtained was 111.73 m2g-1 for the crystallite size from the XRD analysis, which was 0.35% higher than the obtained BET specific surface area of 111.34 m2g-1. The small difference between the calculated and the analyzed results indicated that the powders were loosely agglomerated. The BET specific surface

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area of the sample annealed at 1000 °C was 9.49 m2g-1, which was significantly lower than the calculated result of 38.14 m2g-1 for the crystallite size measured using XRD.

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This result concurred with the particles displayed in the TEM micrograph in Fig. 5(d),

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in which 75% of the particles were tightly agglomerated, resulting in an unfavourable coverage of the adsorbing gas during BET analysis, which in turn resulted in the reduction of the surface area of the sample. Table 3 represents a comparison of the

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calculated and measured specific surface area [52].

Particle agglomeration was also observed in the FESEM images, as presented

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in Fig. 6. As shown in Figs. 6(a) and 6(b), the samples annealed at 700 °C for 5 hours displayed flakes of MgAl2O4. The microflakes were expected to contain a number of tightly aggregated particles. A coral-like, highly porous structure was observed for the

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sample annealed at 800 °C. At 900 °C, nanoflakes were formed on the surface of the

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microflakes, as shown in Figs. 6(e) and 6(f). Significant agglomeration was observed in the MgAl2O4 powder annealed at 1000 °C (see Figs. 6(g) and 6(h)), which is in agreement with the TEM micrograph shown in Fig. 5(d), and the agglomeration

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determination in Table 3. Apart from agglomeration, the MgAl2O4 powder annealed at

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1000 °C also showed high sintering where the dense particles were coagulated [37].

3.7 Catalytic activity of MgAl2O4 in the steam reforming of methane To evaluate the catalytic activity of MgAl2O4 spinel in methane steam reforming, the powder samples annealed at 700 and 1000 °C for 5 hours were chosen, as they presented with significant results on specific surface area, crystallite size, agglomeration, and sinterability. As depicted in Fig. 7, the sample annealed at 700 °C has achieved a high methane conversion of 57%. The high specific surface area, small particle size, and low agglomeration of this sample were able to maintain its catalytic activity. However, the catalytic activity was prominently decreased after 180 min of reaction, while the

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methane conversion was maintained at 19 to 21%. This could be attributed to the formation of carbonaceous materials deposited on the MgAl2O4 surface, which decreased the surface area and consequently, inhibited its catalytic activity [51]. For the sample annealed at 1000 °C, a maximum methane conversion of 25% was achieved in the initial period of reaction, after which the conversion started to decline and was maintained at 15 to 17% throughout the reaction.

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The formation of carbonaceous materials deposited on MgAl2O4 was further investigated by characterizing the spent MgAl2O4 sample using TGA analysis, to

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measure the weight loss due to the oxidation of carbon. The sample annealed at 700 °C,

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which was used in the steam reforming reaction, exhibited 34.05% of weight loss between 500 to 710 °C, as depicted in Fig. 8. A weight loss at this temperature range typically indicates the oxidative decomposition of graphitic carbon. This finding is also

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confirmed by the studies of Lee et al. [57] and L. Ann et al. [58]. The high reaction temperature in the methane steam reforming reaction was responsible for the formation

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of graphitic carbon, possibly due to the decomposition of methane, which decreased the catalytic activity of the catalyst. Negligible amount of carbon formed on the spent catalyst annealed at 1000 °C, which accounted for less than 1% of the weight loss,

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indicated the absence of carbon on the MgAl2O4. This could be due to the low catalytic

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activity of the MgAl2O4 spinel annealed at this temperature. This phenomenon probably corresponded to the low specific area, large crystallite size, and sintered particles, which limited its catalytic activity. The morphology of the spent MgAl2O4 samples was also

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investigated by using TEM to characterize the type of graphitic carbon produced. The TEM image in Fig. 9(a) shows both filamentous and layered sheets of graphitic carbon

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formed on the samples annealed at 700 °C. The highly magnified image in Fig. 9(b) shows the formation of multi-walled carbon nanotubes (MWCNTs) from filamentous carbon. However, no specific carbon formation was observed on the sample annealed at 1000 °C, which is consistent with the TEM image of the fresh catalysts, as shown in Figs. 5(c) and 5(d).

4.

Conclusions

MgAl2O4 spinel nanoparticles were successfully synthesized via a solid-state fusion with citric acid. The effects of annealing conditions, such as annealing temperature and

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duration were investigated on the structural, textural, and morphological properties of the resulting MgAl2O4 spinel. Various characterizations indicated the formation of high surface area spinel, with porous textures at different annealing temperatures. Annealing duration exerted an insignificant effect on spinel phase formation and crystallite size. However, the annealing temperature played a critical role in determining the crystalline, structural, textural, and morphological properties of the MgAl2O4 spinel. Increasing the

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annealing temperature has increased the crystallite size, but decreased the specific surface area of the MgAl2O4 spinel. Its catalytic activity in the steam reforming of

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methane has revealed that the MgAl2O4 spinel, with a high surface area, small crystallite

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size, and low aggregations could result in a high conversion of methane (57%) in the first 180 minutes of reaction. Meanwhile, sample annealed at 1000 °C exhibited low

5.

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catalytic activity due to the thermal sintering at high temperatures.

Acknowledgements

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This work has been financially supported by Yayasan Sime Darby, UKM [grant number PKT 6/2012] and Sime Darby Research [grant number KK-2014-014]. The team would

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also like to thank the Ministry of Higher Education, Malaysia, the Centre of Research Instrumentation and Management, Universiti Kebangsaan Malaysia, and the Faculty of

support.

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Figure captions Fig. 1. X-ray diffraction patterns of the samples at different annealing (a) duration and (b) temperatures Fig. 2. FTIR spectra of the synthesized samples at different annealing (a) temperatures and (b) durations

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Fig. 3. TGA/DTA curves of the samples (a) mixed sample, (b) annealed at 600°C and (c) a comparison Fig. 4. Nitrogen adsorption desorption isotherms of the spinel samples at different annealing temperatures

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Fig. 5. TEM images of the samples at different annealing temperatures (a) 700oC, (b) 800oC, (c) 900oC and (d) 1000oC Fig. 6. FESEM images of the samples annealed at temperatures (a, b) 700oC, (c, d) 800oC, (e, f) 900oC and (g, h) 1000oC at different magnifications (1 µm and 200 nm)

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Fig. 7. Catalytic performance of the selected spinel samples for the steam reforming of methane at a reaction temperature of 700°C and a steam to carbon ratio of 2 under atmospheric pressure

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Fig. 8. Thermogravimetric curves of the spent catalysts

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Fig. 9. TEM images of the spent MgAl2O4 annealed at temperature (a, b) 700oC and (c, d) 1000oC at different magnifications (200 nm and 100 nm)

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Tables Table 1. Crystalline properties of the synthesized spinel samples at different annealing conditions h k l (4 0 0)

a (Å)

V (Å )

2.4385 2.4385 2.4351 2.4349 2.4385 2.4379 2.4418 2.4418

8.0877 8.0876 8.0765 8.0757 8.0876 8.0856 8.0985 8.0985

529.0215 529.0085 526.8313 526.6755 529.0085 528.6116 531.1526 531.1526

h k l (4 4 0) 3

d

a (Å)

V (Å )

2.0215 2.0241 2.0216 2.0168 2.0241 2.0187 2.0237 2.0191

8.0860 8.0965 8.0863 8.0671 8.0965 8.0748 8.0947 8.0763

528.6980 530.7485 528.7451 524.9955 530.7485 526.4963 530.4024 526.7859

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d

D (nm) 5.84 6.15 6.67 6.52 6.15 6.73 11.95 22.04

D (nm) 7.67 8.16 8.99 8.98 8.16 8.85 12.31 22.64

d

a (Å)

V (Å3)

1.4296 1.4277 1.4297 1.4264 1.4277 1.4283 1.4296 1.4296

8.0873 8.0766 8.0875 8.0691 8.0766 8.0795 8.0868 8.0868

528.9494 526.8433 528.9938 525.3724 526.8433 527.4192 528.8495 528.8495

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3

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700oC-4h 700oC-5h 700oC-6h 700oC-7h 700oC-5h 800oC-5h 900oC-5h 1000oC5h

h k l (3 1 1)

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Parameter

D (nm) 8.21 8.21 9.27 9.06 8.21 9.57 12.08 21.23

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Table 2. BET/BJH textural properties of the spinel samples at different annealing temperatures Surface area (m²/g)

Pore size (Å)

Pore volume (cm³/g)

MgAl2O4-600 oC-5hr

1.4099

112.5872

0.003968

MgAl2O4-700 oC-5hr

111.3378

56.0639

0.156051

MgAl2O4-800oC-5hr

91.0176

56.5664

0.128194

MgAl2O4-900oC-5hr

58.1954

67.9741

MgAl2O4-1000oC-5hr

9.4941

117.1831

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Sample

0.098213 0.028248

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Table 3. Comparison of measured and calculated specific surface area

BET surface area (m²/g)

Calculated surface area (m²/g)

MgAl2O4-700 oC-5hr

111.3378

111.73

MgAl2O4-800oC-5hr

91.0176

99.99

MgAl2O4-900oC-5hr

58.1954

MgAl2O4-1000oC-5hr

9.4941

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Sample

69. 19

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Highlights 

A one-pot solid state citrate fusion method is reported for the synthesis of MgAl2O4.

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The effect of annealing conditions on the properties of the spinel is studied.

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A series of reaction pathways of solid-state fusion is proposed.

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Annealing temperature exerts significant effects on the structural and textural

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properties.

Graphics Abstract

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