Insight into the preparation of Fe3O4 nanoparticle pillared layered double hydroxides composite via thermal decomposition and reconstruction

Applied Clay Science 140 (2017) 88–95

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Research paper

Insight into the preparation of Fe3O4 nanoparticle pillared layered double hydroxides composite via thermal decomposition and reconstruction Lei Li a, Guangxia Qi b, Masami Fukushima c, Bangda Wang a, Hui Xu a, Yi Wang a,⁎ a b c

School of Environment, Tsinghua University, Beijing 100084, PR China School of Food and Chemical Engineering, Beijing Technology and Business University, Beijing 100084, PR China Laboratory of Chemical Resources, Division of Sustainable Resources Engineering, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan

a r t i c l e

i n f o

Article history: Received 14 December 2016 Received in revised form 26 January 2017 Accepted 27 January 2017 Available online 8 February 2017 Keywords: Magnetite Layered double hydroxides Pillar Heterogeneous catalyst Thermal decomposition Reconstruction

a b s t r a c t As one of layered double hydroxides (LDH) based heterogeneous catalysts, transition metal oxide pillared LDH composite has attracted considerable research interests as its excellent catalytic efficiency and thermal stability. However, during the preparation especially by reconstruction techniques, abundant interlayer regions of LDH are contaminated by some unexpected anion such as LDH affinitive CO2− 3 , which will result in inaccessible interlayer active site. In this paper, magnetic nanoparticle Fe3O4 pillared LDH composite was synthesized with exchangeable NO− 3 as the main interlayer anion by decomposition and reconstruction route. The anion exchange, decomposition and reconstruction processes were comprehensively investigated. XRD, TG-DTA-MS, SEM and other methods have been employed to characterize materials. The results show that anion exchange between ferric citrate anion and LDH follows Langmuir isotherm model and pseudo-second-order model. The decomposition and oxidation of anion exchange product (LDH-Fe(C6H4O7)−) can be complete at 550 °C for 20 min and crystal Fe3O4 with nanostructure is formed between 500 and 600 °C. When calcined product (LDO-Fe3O4) is reconstructed by rehydration in 0.015 M HNO3-30 wt% NaNO3 mixture aqueous solution, CO2− 3 contamination derived from CO2 in air can be averted and lamellar structure is successfully recovered with Fe3O4 and NO− 3 remaining in the interlayer gallery of LDH. © 2017 Elsevier B.V. All rights reserved.

1. Introduction With the aim of developing new classes of selective catalysts, sandwich-like structural heterogeneous catalysts with good catalytic efficiency and stability have attracted increasing attention (Sels et al., 1999; Sels et al., 2001; Liu et al., 2009; Li et al., 2014; Hunter et al., 2016), which are generally prepared by intercalation of functional molecules into interlayer gallery of lamellar structural inorganic solids such as layered double hydroxides (LDH) (Zhao et al., 2011; Mac Leod et al., 2012; Gunjakar et al., 2013). As a type of anion exchange lamellar materials, LDH are well studied and widely used in various domains due to its excellent performance on anion exchange and facile preparation (Wang and O'Hare, 2012; Nagendra et al., 2015; Yang et al., 2016). Owing to the unique supramolecular structure with the controlled chemical composition and size of interlayer gallery, LDH is an ideal inorganic matrix for the synthesis of functional molecules intercalated LDH nanocomposites with sandwich-like structure (Darder et al., 2005). After intercalation of functional molecules, usually macromolecules with relatively higher molecular diameter such as titanium tartrate (Shi et al., 2010), paratungstate (Guo et al., 2001; Liu et al., 2008) and porphyrins (Lang ⁎ Corresponding author. E-mail address: [email protected] (Y. Wang).

http://dx.doi.org/10.1016/j.clay.2017.01.028 0169-1317/© 2017 Elsevier B.V. All rights reserved.

et al., 2007), (Káfuňková et al., 2010), the interlayer gallery of LDH is generally expanded to corresponding size, and consequently become a microreactor which can provide enough space for the reaction between reagents and catalytic molecules. In the field of catalytic chemistry, transition metal oxide incorporated LDH composite has also abstracted extensive research interests and investigation (Basile et al., 2002; Zou et al., 2013; He et al., 2014), since transition metal oxide species possess good catalytic function on oxidation, polymerization, photocatalysis and so on (Greiner et al., 2012). Dvininov (Dvininov et al., 2010) reported that SnO2/MgAl-layered double hydroxide nanocomposites prepared by wet impregnation and calcination method show a good photocatalytic activity for methylene blue decolourization and the flat band potential of SnO2 has been shifted due to the interaction with LDH, which is energetically favourable to the formation of hydroxyl radicals. However, during the preparation and application of such LDH based transition metal oxide composite, LDH only act as a supporter and active metal oxides merely load on external surface or in some holes, leaving abundant internal surface unused. If transition metal oxide can be intercalated into the interlayer gallery of LDH in a similar way to the intercalation of catalytic anions, more transition metal oxide can be embedded both on the external and internal surface, giving abundant reactive space and active sites for catalysis. Moreover, the height of interlayer gallery can be

L. Li et al. / Applied Clay Science 140 (2017) 88–95

adjusted to specific size, which can enhance the selectivity of catalysis to some extent (Feng et al., 2015). Villegas (Villegas et al., 2003) successfully incorporated manganese oxide species (MnOx) into the interlayer of LDH host by in-situ reduction reaction. Firstly, manganese precursor MnO− 4 was introduced into the interlayer of LDH by anion exchange, and subsequently MnO− 4 was transformed into MnOx by the reduction of organic compounds (glucose, ethanol and ascorbic acid). Accompanied by intercalation of MnOx; however, the interlayer gallery region was contaminated by such organic reductants, as the organic compounds in addition to reducing MnO− 4 ions were also partly exchanging with some of the counter anions from the interlayer. Moreover, not all transition metal has its corresponding oxyacid root and can be transformed into metal oxide species by reduction. Thermal decomposition-reconstruction method, which is a common route for the intercalation of some anions with different molecular size into LDH, can be adopted and improved to synthesize transition metal oxide pillared LDH composite. This synthetic route includes three processes: (I) anion exchange of transition metal precursor, (II) decomposition of transition metal precursor intercalated LDH to form transition metal oxide and layered double oxides composite, and (III) reconstruction of lamellar structure to obtain transition metal oxide pillared LDH. Transition metal precursor can be settled on abundant metal organic complex anions or metal oxyacid roots. Although decomposition-reconstruction method is a reasonable route, there is little report about preparation of transition metal oxide pillared LDH by this method. Gérardin (Gérardin et al., 2008) synthesized Ni0 intercalated LDH nanocomposites by thermal reduction of Ni hydroxyl citrate species intercalated Mg2Al layered double hydroxides in H2 atmosphere and reconstruction of calcined oxide compounds in aqueous solution. However, the interlayer gallery of regenerated Ni0-LDH is occupied by stable CO2− that is 3 unable to exchange with other anions, which means the reactant ions cannot get access to the active Ni0 particles loaded in the interlayer space and consequently the catalytic efficiency is reduced. During the rehydration procedure, the contamination and occupation of interlayer gallery by stable CO23 − derived from CO2 in air has been always observed. Rocha (Rocha et al., 1999) reconstructed calcination products of Mg-Al-CO3 LDH in saturated NH4Cl solution. However, the d003-value after reconstruction is only 7.65 Å, indicating CO2− 3 is the main interlayer anion in reconstructed LDH. Lv (Lv et al., 2006a; Lv et al., 2006b) adopted calcined Mg-Al-CO3 LDH to remove fluoride and chloride ions from aqueous solution, expecting intercalation of such halogen elements. However, the height of interlayer gallery was the same as that of Mg-Al-CO3 LDH, meaning fluoride and chloride ion did not intercalated into the interlayer as expected. Only if the solute molecular contains some functional groups which have a strong affinity with LDH such as carboxylic and sulfonic groups, the interlayer of reconstructed LDH can be partly intercalated by such special molecular with partial contamination of CO2− (You et al., 2002; El Gaini et al., 2009). Therefore, 3 the methods for eliminating or abating the contamination of CO2− 3 during the reconstruction has been an important issue, since it is significant for decomposition-reconstruction route as well as the application of transition metal oxide pillared LDH, on account of giving an accessible interlayer gallery for catalysis reaction. However, previous studies concerning about decomposition and reconstruction of LDH species mainly focus on the recovery of lamellar structure and crystal growth or adsorption performance of LDH composite material. In this study, magnetic nanoparticle Fe3O4, an important catalyst in many important chemical reactions, was intercalated into the interlayer gallery of LDH with exchangeable NO− 3 as the main interlayer anion by decomposition-reconstruction route. As a typical example for preparation of transition metal oxide pillared LDH composite heterogeneous catalyst, anion exchange of ferric citrate anion (transition metal precursor), decomposition and reconstruction process were comprehensively studied, providing an insight into the synthesis process. Thermodynamics and kinetics of anion exchange process was discussed. Thereafter, the crystal structure variation of calcined LDH, as well as the gas

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generated during decomposition were both analysed. Most importantly, the variation of interlayer anion during reconstruction in different solutions, as well as the kinetics property of reconstruction were also investigated in detail. 2. Experimental section 2.1. Materials Reagents used in this study were analytical grade and purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. including zinc chloride (ZnCl2), aluminium chloride crystal (AlCl3·6H2O), sodium hydroxide (NaOH), nitric acid (HNO3), sodium nitrate (NaNO3) and catechol (C6H6O2) and glycine (C2H5NO2). Ammonium ferric citrate was purchased from Aladdin Industrial Corporation and its Fe content was 21.6% as determined by ICP-OES; therefore, its molecular formula was proposed as NH4Fe(C6H4O7). 2.2. Synthetic method LDH were prepared by a facile method which involves a rapid mixing and nucleation process, followed by a separate aging process (Zhao et al., 2002). Solution A: ZnCl2 and AlCl3·6H2O with Zn2+/Al3+ ratios of 2.0 were dissolved in deionized water (100 mL) to give solutions with an Zn2+ concentration of 0.6 M. Solution B: NaOH were dissolved in decarbonated water (100 mL) with an concentration of 1.8 M. The concentration of NaOH correlated with the concentration of metal and calculated according to the stoichiometric LDH molecular formula (Zn2Al(OH)6Cl·2H2O): CNaOH = (CZn + CAl) × 2. Two solutions were simultaneously added into a flat-bottomed flask which contained 300 mL decarbonated water in advance and mixed at 1000 rpm for 30 min under N2 atmosphere to prevent CO2 contamination. The solid-to-liquid ratios (S/L) defined as the proportion of stoichiometric mass of LDH solid (g) to the volume of the reaction liquid (mL) were set as 1/25, 1/ 50 and 1/100. The flask with resulting suspension was sealed and aged at 80 °C for 12 h. The final precipitate was separated by centrifugation and extensively washed with decarbonated water. The wet LDH were divided in two portions: one was vacuum freeze drying for analysis, and the another part was used in the followed synthesis. The decarbonated water was produced by boiling deionized water at 100 °C for 10 min and the LDH was denominated as Zn2Al-Cl. Iron oxide pillared LDH were prepared firstly by anion exchange between ammonium ferric citrate and LDH. The ammonium ferric citrate solutions of different concentrations were added into the LDH suspension to reach a final S/L ratio of 1/50 and the suspension was stirred at 500 rpm. The adsorption isotherms and kinetics were investigated based on the equilibrium concentrations of Fe in filtrate. After centrifugation and vacuum freeze drying, the exchange product designated as LDH-Fe(C6H4O7)− was then subjected to thermal analysis by TG-DTAMS and decomposition at different temperature in order to obtain its oxide forms. The oxide compound derived from decomposition of LDH is denominated as LDO. Finally, this oxide compound designated as LDO-Fe3O4 was rehydrated in HNO3-NaNO3 mixed solution with different concentrations of HNO3 and 30 wt% of NaNO3. The final yellow product (Fe3O4 pillared LDH with NO− 3 as the interlayer anion, designated as LDH-Fe3O4-NO3) was centrifuged, washed with decarbonated water, and vacuum freeze drying prior to characterization. 2.3. Characterization techniques The concentration of Fe was analysed by using ICP-OES (PE OPTIMA8000, PerkinElmer, Inc. America). Thermal analyses of the materials were performed in corundum crucible by thermogravimetric analysis (TG) using thermal analyses instruments (STA449F3, NETZSCHGerätebau GmbH, German), with air under a flow rate of 100 cm3 min−1 by heating up to 1000 °C at heating rate of 10 °C

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min−1. Thermogravimetric-Differential Thermal Analyses-Mass Spectrum (TG-DTA-MS) (STA449F3, NETZSCH-Gerätebau GmbH, German) was used to simultaneously analyse Mass loss, heat flow and the tail gas generated during the decomposition of LDH-Fe(C6H4O7)−. X-ray diffraction (XRD) data were collected from powder samples using Xray Powder Diffractometer (D8 Advance, Bruker Corporation, German) with Cu Kα radiation and λ = 1.5418 Å at scanning rate of 0.28° per second for 2θ in the range from 5° to 75°. Morphology studies were carried out with scanning electron microscopy (JSM-6460LV, Japan Electron Optics Laboratory Co. Ltd., Japan) operated at 10 kV after coating the samples with carbon. 3. Results and discussion 3.1. The thermodynamics and kinetics of anion exchange between LDH and Fe(C6H4O7)− LDH preparation was divided into two processes including nucleation and aging processes. When solution A and B were added into flask, white precipitate was generated almost simultaneously, indicating fast nucleation of LDH. After stirring for 30 min, the pH of the white suspension was maintained at approximately 8 which fell in the optimal pH range for Zn2Al-Cl LDH crystallization (Rives, 2001). During aging at 80 °C, the LDH crystal nucleus grew gradually and finally settles down at the bottom of flask. The solid/liquid ratios (S/L) can affect the mixing and subsequently affect nucleation process of Zn2Al-Cl. When S/L was 1/100, the suspension became dilute and the LDH productivity declined. While the suspension became dilute when S/L was 1/100, resulting lower materials productivity. Therefore, 1/50 of S/L ratio was adopted to synthesize Zn2Al-Cl LDH. The XRD patterns of Zn2Al-Cl and anion exchanged product are shown in Fig. 1. The strikingly clear XRD pattern indicates a good crystallinity of LDH product. As the spacing value of crystal face (003) d003-value is 7.77 Å, it can be concluded that Cl− is the main interlayer anion of synthetic Zn2Al-Cl LDH (Morel-Desrosiers et al., 2003). Elemental analysis results showed 0.26% of C content, indicating contamination with a small amount of CO2; therefore, the molecular formula of LDH product could be approximately proposed as Zn2Al(OH)6Cl0.86(CO3)0.07·2H2O. After anion exchange with ammonium ferric citrate, the spacing values of (003) and (006) were both increased, specifically d003-value arose to 12.16 Å. The gallery height calculated by subtracting the brucite sheet width (4.8 Å) from the spacing of crystal face (003) is 7.36 Å which is as much as the height of ferric

citrate anion (Huang et al., 2012), indicating the successful intercalation of ferric citrate. In anion exchange process, the thermodynamics and kinetics of anion exchange are crucial to determine the equilibrium exchange capacity and equilibrium time, which are key parameters to figure out the optimal anion exchange conditions as well as the anion exchange mechanism. Fig. 2 shows the adsorption isotherm of Fe(C6H4O7)− at different temperature and also presents the fitted curves by using Langmuir isotherm model. Langmuir isotherm model: Ce 1 1 ¼ Ce þ bqm qe qm Where Ce (mg Fe/L) is the equilibrium concentration of adsorbate in solution and qe (mg Fe/g LDH) is the amount of adsorbate adsorbed on LDH after equilibrium. qm (mg Fe/g LDH) means the maximum amount of adsorbate adsorbed on LDH and b (L/mg) is a constant related to the energy of adsorption. The concentration of Fe represents the concentration of anion Fe(C6H4O7)−. Table 1 shows the Langmuir isotherm model fitting parameters. As shown in Fig. 2, the adsorption capacity increased as the concentration of Fe increased and reached the equilibrium when the equilibrium concentration of Fe was above 2500 mg/L. This indicates that the maximum adsorption capacity of anion Fe(C6H4O7)− at different temperature can be reached respectively when initial concentration of Fe(C6H4O7)− exceeds 1.44 times of the value of AEC (Anion Exchange Capacity, mmol/g). The Langmuir isotherm model gave a good fit on the basis of the correlation coefficients R2 N 0.99 shown in Table 1. Therefore, anion exchange process between Fe(C6H4O7)− and LDH can be described as the monolayer adsorption. When the temperature goes up, the adsorption capacity shows an improvement, which means the exchange process between ammonium ferric citrate and LDH is an endothermic reaction. However, the maximum exchange capacity increased indistinctively with the increasing temperature; therefore, the suitable experimental temperature was set as 45 °C. The stoichiometric AEC of this LDH Zn2Al(OH)6Cl0.86(CO3)0.07·2H2O can be calculated and the value is approximately 2.6 mmol/g. The anion exchange rates at different temperature can be calculated as follow and are shown in Table 1. anion exchange rate ¼

qm =56  100% AEC

where 56 is molar mass of Fe. qm (mg Fe/g LDH) is the maximum adsorption capacity of Fe. Huang (Huang et al., 2012) intercalated ferric

Fig. 1. The XRD patterns of LDHs and anion exchange product.

Fig. 2. Adsorption isotherms and Langmuir isotherm model fitting curves at different temperature (Adsorption conditions: LDHs = 1 g; V = 50 mL; t = 5 h).

L. Li et al. / Applied Clay Science 140 (2017) 88–95 Table 1 The Langmuir isotherm model fitting parameters and adsorption data at different temperature. T (°C)

qm (mg Fe/g LDHs)

b

R2

Anion exchange rate

25 35 45 55

90.91 90.91 93.46 95.24

0.01 0.02 0.03 0.11

0.9989 0.9997 0.9998 0.9999

62.4% 62.4% 64.1% 65.4%

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time t (min) and at equilibrium (mg Fe/g LDH), respectively. k2 (g/ (mg·min)) is the pseudo-second-order rate constant. Based on the above data, it is optimal to effectively intercalate Fe(C6H4O7)− into LDH at 45 °C for an exchange time of 60 min with S/ L ratio of 1/50 and an initial ammonium ferric citrate concentration of 1.5 AEC. The rest of ammonium ferric citrate remaining in filtrate can be recycled and exchange with Cl− in LDH again. 3.2. Thermal analysis and calcination of LDH-Fe(C6H4O7)−

citrate anion into Mg2Al-NO3 LDH by anion exchange at 75 °C for 12 h in an argon atmosphere, with an exchange rate of 66.5%. In this study, anion exchange rates exhibited marginal increase as the temperature raised and reached 65.4% at 55 °C, which was consistent with the reported data. This phenomenon may be attributed to the deeply located Cl− in LDH which is hard to diffusion and be exchanged. In order to investigate the rate-controlling mechanism of the anion exchange process, adsorption kinetics of Fe(C6H4O7)− by LDH was studied. As apparent from Fig. 3, irrespective of the initial adsorbate concentration, anion Fe(C6H4O7)− exchange with LDH was very rapid during the first 5 min, followed by a slower process to reach a plateau with increasing contact time. The equilibrium time for Fe(C6H4O7)− exchange was about 60 min. The rapid uptake of Fe(C6H4O7)− during initial 5 min is due to the adequately exchangeable Cl− and high adsorbate concentration gradient. After that, the anion exchange becomes less efficient, because the majority of exterior Cl− of LDH have been exchanged by Fe(C6H4O7)−, while the internally located Cl− requires a comparatively longer time for exchange with Fe(C6H4O7)− through interlayer diffusion. The adsorption kinetic data was further analysed in terms of pseudo-first-order kinetic model and the fitting curves were displayed in inserted chart of Fig. 3. As the high correlation coefficients (R2 = 0.9999), anion exchange data of Fe(C6H4O7)− fitted well with the pseudo-second-order model. Pseudo-second-order model: t 1 1 ¼ tþ qt qe k2 qe 2 where qt and qe are the concentration of adsorbate adsorbed on LDH at

Fig. 3. Effect of contact time on anion exchange process and pseudo-second-order model fitting curves as a function of different initial ammonium ferric citrate concentration (1 AEC and 1.5 AEC; another adsorption conditions: LDHs = 1 g; V = 50 mL; t = 5 h).

3.2.1. Thermal analysis of LDH-Fe(C6H4O7)− Thermal decomposition of LDH-Fe(C6H4O7)− was investigated through simultaneous thermal analysis. The TG-DTG curves of LDHFe(C6H4O7)−, LDH and NH4Fe(C6H4O7) in air atmosphere are shown in Fig. 4. When temperature was below 300 °C, the TG and DTG curves of LDH-Fe(C6H4O7)− almost coincided with that of LDH, presenting two mass loss at 110 °C and 270 °C respectively. When the temperature increased, especially at 370 °C, the mass loss of LDH-Fe(C6H4O7)− was much more obvious as comparison with horizontal TG curve of LDH around 370 °C, which might be due to the decomposition of intercalated Fe(C6H4O7)− in LDH-Fe(C6H4O7)−. As temperature rose to 525 °C, another mass loss shown in TG and DTG curves of LDH-Fe(C6H4O7)− was more evident than that of LDH at 525 °C, which might result from decomposition of residual intercalated Fe(C6H4O7)−. As for NH4Fe(C6H4O7), the main mass loss took place at 210 °C, 270 °C and 340 °C, which were ascribed to the decomposition of NH4Fe(C6H4O7). To further investigate the decomposition behaviours of LDHFe(C6H4O7)−, TG-DTA-MS of LDH-Fe(C6H4O7)− was carried out and the composition of gas generated during decomposition was simultaneously analysed by mass spectrometer. As CO2 and H2O are main gas products of citrate decomposition in air atmosphere (Park, 2009; Li et al., 2013), the intensity variation of molecular ion peak of CO2 (m/z = 44) and H2O (m/z = 18) were detected during the whole decomposition process of LDH-Fe(C6H4O7)−. When temperature was 110 °C, as shown in the mass spectrum in Fig. 5, the gas product was only H2O, indicating that mass loss at 110 °C was ascribed to dehydration of physically absorbed water at the surface of LDH-Fe(C6H4O7)−. The reaction occurring at 270 °C presented an endothermic decomposition, testified by an endothermic peak shown in the DTA curve and the gas products at 270 °C were composed of H2O and a small amount of CO2. In Fig. 4, the TG-DTG curves of LDH and NH4Fe(C6H4O7) also presented a mass loss at

Fig. 4. The TG-DTG curves of LDHs, LDHs-Fe(C6H4O7)− and NH4Fe(C6H4O7) in air atmosphere.

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Fig. 5. The TG-DTG-DTA curves of LDHs-Fe(C6H4O7)− and synchronous mass spectrum of gas generated during decomposition.

270 °C. Therefore, it can be concluded that mass loss at 270 °C of LDHFe(C6H4O7)− is due to dehydration of LDH host and slight decomposition of Fe(C6H4O7)−. At 370 °C, an evident exothermic peak was observed in the DTA curve, accompanying with a lot of CO2 and H2O generated. This can be ascribed to the oxidation of both C and H from the citrate, confirming the decomposition and oxidation of intercalated Fe(C6H4O7)− in LDH-Fe(C6H4O7)− at 370 °C. Moreover, CO2 was still detected at 525 °C and DTA curve also showed an exothermic peak at 525 °C; therefore, it could be attributed to the decomposition of residual Fe(C6H4O7)−. As comparison with NH4Fe(C6H4O7) which can be completely decomposed and oxidized at 400 °C, the Fe(C6H4O7)− intercalated in the interlayer of LDH is much more thermodynamic stable. When temperature was above 550 °C, both mass loss and intensity of ion peak remained constant up to 800 °C, indicating that no obvious decomposition reaction occurred. 3.2.2. The calcination of LDH-Fe(C6H4O7)− To reveal the variation of solid decomposition products, the calcination experiments were carried out, in which the temperatures were selected corresponding to the thermal analysis, at 250 °C, 350 °C, 400 °C, 450 °C, 500 °C, 550 °C, 600 °C, 650 °C and 700 °C. The XRD patterns of calcination products at different temperature for 1 h are illustrated in Fig. 6. The decomposition process of LDH-Fe(C6H4O7)− in air can be roughly divided into four stages. In stage I, below 400 °C, LDH-Fe(C6H4O7)− experiences dehydration and decomposition of LDH host, as well as decomposition and oxidation of intercalated Fe(C6H4O7)−, with violently oxidation at 370 °C. As a result of that, the lamellar structure of LDHFe(C6H4O7)− is collapsed and crystal structures disappear simultaneously, leaving an amorphous calcined product. In stage II, such amorphous structure is maintained from 400 °C to 500 °C, which is consistent with the unchanged TG curve of LDH-Fe(C6H4O7)− during this temperature range. In stage III, from 500 °C to 600 °C, the rest of Fe(C6H4O7)− is oxidized and amorphous calcined products gradually transform into ZnO and Fe3O4 whose diffraction peak are much more obvious at 550 °C. The crystallite size of Fe3O4 formed at 550 °C is approximately 50 nm as calculated by Scherrer formula, indicating decomposition of restrained Fe(C6H4O7)− can generate Fe3O4 nanoparticle. As the temperature further increasing to 600 °C and above (stage IV), although TG-DTG of LDH-Fe(C6H4O7)− shows no obvious change, the XRD

Fig. 6. The XRD patterns of products calcined in air for 1 h.

patterns demonstrate spinel Zn(AlFe)O4 is generated. Given Fe3O4 is a good catalyst and the lamellar structure of LDH can be reconstructed except that spinel structure exists in LDO (Rocha et al., 1999), the optimal decomposition temperature is set as 550 °C and the corresponding calcined product is named as LDO-Fe3O4. The mass loss of products remained roughly at 38% since from 550 °C to 700 °C (Fig. S1), which coincided with the TG-DTA data. The mass loss remained almost constant since 20 min of calcination time at 450 °C, while there was no change since 5 min at 550 °C and 650 °C, meaning that decomposition process of LDH-Fe(C6H4O7)− was very fast (Fig. S2). However, to make sure complete decomposition, the calcination time was set as 1 h in the following experiments.

3.3. Reconstruction of lamellar structure in HNO3-NaNO3 mixture solution It is reported that the LDO can be hydrated in aqueous solution and recovered to LDH (Rocha et al., 1999; Millange et al., 2000). However, the variation of interlayer anion during the reconstruction process has not been investigated in detail. The species of interlayer anion influence the anion exchange capacity as well as the contact between intercalated Fe3O4 with anion reactant, and further affect the catalysis reaction in the interlayer area. In this study, LDO-Fe3O4 obtained by calcination at 550 °C was subject to hydration and reconstruction. Meanwhile, NO− 3 which can be easily exchanged by other anions was settled on as the interlayer anion in order to provide enough reaction space and anion exchange capacity. When LDO-Fe3O4 was added into pure water with magnetic stirring, the pH of the solution ascended to alkaline 10.22 after hydration for 2 h, which led to CO2 contamination and consequently CO2− occupation in the interlayer space. This is why the contamina3 tion of CO23 − was observed during reconstruction of calcined LDH even when rehydration in the saturated NH4Cl solution as the literatures reported. Therefore, the reconstruction solution is settled on HNO3-NaNO3 mixture solution, in which HNO3 can reduce the pH and NaNO3 provides enough NO− 3 as well as decrease the solubility of CO2 (Duan and Sun, 2003). Nobuo Iyi (Iyi et al., 2004) successfully deintercalated carbonate ions from CO23 − intercalated LDH and

L. Li et al. / Applied Clay Science 140 (2017) 88–95

exchanged with a large excess of Cl− by treatment with HCl-NaCl mixture solution. Fig. 7 shows XRD patterns of products recovered in HNO3-NaNO3 mixture solution containing different concentration of HNO3 and 30 wt% NaNO3 for 12 h. Apparently, LDO-Fe3O4 recovered well to lamellar structure in all cases, verified by the clear diffraction peak of crystal face (003). Unfortunately, when reconstruction only in 30 wt% NaNO3 solution, the spacing value of crystal face (003) was 7.62 Å, indicating a severe contamination of CO2. When concentration of HNO3 was in the range of 0.0005–0.01 M, two different diffraction peaks of (003) 2− marked as (003)− NO3 and (003)CO3 appeared in the XRD patterns, corre2− sponding to the d003-value of NO− 3 (d003-value = 8.93 Å) and CO3 (d003-value = 7.62 Å) intercalated LDH, respectively. The intensity of diffraction peak (003)2− CO3 declined as the HNO3 concentration increased, while intensity of diffraction peak (003)− NO3 increased, indicating HNO3 could reduce the CO2 contamination during reconstruction process. When concentration of HNO3 reached 0.015–0.02 M, only diffraction − peak of (003)− NO3 was observed, meaning that NO3 was the main interlayer anion existed in the recovered product. The FT-IR spectrum of LDH-Fe3O4-NO3 (shown in Fig. S5) also demonstrated that NO− 3 was the main interlayer anion. Particularly, Fe3O4 generated from calcination of LDH-Fe(C6H4O7)− at 550 °C was inert during the hydration process and remained in the crystalline phase of LDH, as illustrated in Fig. 7. It is noteworthy that an excess of HNO3 will attack and destroy LDH host (as shown in Fig. S3). The mass percentages which were the mass of products recovered with setting HNO3 concentrations account for the mass of product recovered without HNO3 decreased to 85% and below when HNO3 concentration beyond 0.03 M, which was ascribed to the dissolution effect of excess HNO3. Moreover, when LDO-Fe3O4 hydrated in HNO3 single solution, the mass percentages of recovered products were lower as comparison with that in HNO3-NaNO3 mixture solution, indicating NaNO3 could abate the dissolution of LDH host caused by excess HNO3. To further investigate the mechanism of reconstruction process, the pH variation during LDO-Fe3O4 hydrated in 0.015 M HNO3-30 wt% NaNO3 mixture solution at different temperature was recorded and the kinetics of hydration was also studied, as illustrated in Figs. 8 and 9 respectively.

Fig. 7. XRD patterns of products recovered in HNO3-30 wt% NaNO3 mixture solution containing different HNO3 concentrations for 12 h.

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Fig. 8. pH variation during LDO-Fe3O4 hydrated in 0.015 M HNO3-30 wt% NaNO3 mixture solution at different temperature.

As LDO-Fe3O4 was respectively added into solutions at 30 °C, 45 °C and 60 °C, pH of solutions ascended very quickly within 25 min from initial around 1.5 to equilibrium 6.25, 5.91 and 5.72 respectively, demonstrating the hydration reconstruction process was exothermic. LDO is a type of Lewis base which can provide duplet and accept H+; therefore, hydration reconstruction process is actually an exothermic neutralization reaction between HNO3 and Lewis base LDO. The consumption of H+ at setting temperature were analysed in terms of pseudo-secondorder kinetic model and perfectly fitted the pseudo-second-order kinetic model (R2 = 1), as shown in the insert chart in Fig. 9. Millange (Millange et al., 2000) reported that the reconstruction process of LDO calcined from Mg3Al(OH)8(CO3)0.5·3H2O was fast and could be accomplished 99% within 40 min at 60 °C in sodium carbonate solution. However, in order to make sure the reconstruction products with well lamellar crystal structure, it is necessary to hydrate in HNO3-NaNO3 mixture solution with enough time and higher temperature. Therefore, in this study, the optimal reconstruction conditions are set as hydration in 0.015 M HNO3-30 wt% NaNO3 mixture solution at 45 °C for 2 h. The final product can be described as Fe3O4 pillared LDH with NO− 3 as the interlayer anion, named as LDH-Fe3O4-NO3. The XRD patterns of products obtained during preparation of Fe3O4 pillared LDH are illustrated in Fig. S4, the crystal structures, especially the crystal face (003) vary prominently as comparison.

Fig. 9. Variation of H+ concentration during hydration and pseudo-second-order model fitting curves.

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Fig. 10. SEM images of products (a) LDHs; (b) LDHs-Fe(C6H4O7)−; (c) LDO-Fe3O4; (d) LDHs-Fe3O4-NO3.

Fig. 10 shows the SEM images of products, LDH presented sheet structure with a little of agglomeration. After anion exchange, LDHFe(C6H4O7)− remained such sheet structure whose diameter was around 1 μm. However, the sheet structure of LDH-Fe(C6H4O7)− was almost destroyed after decomposition, as shown in Fig. 10(c), LDO-Fe3O4 was fragmentized particle. When LDO-Fe3O4 was subject to reconstruction in 0.015 M HNO3-30 wt% NaNO3 mixture solution, the lamellar structure was recovered, but the particle size was less than that of original LDH (shown in Fig. 10(d)). The EDS data shown in Fig. S6 demonstrated that Fe existed in the reconstructed product LDH-Fe3O4-NO3. 4. Conclusions In this study, as a typical example for synthesis of transition metal oxide pillared LDH composite, nanoparticle Fe3O4 was embedded into the interlayer gallery of LDH with exchangeable NO− 3 as the main interlayer anion through thermal decomposition-reconstruction route. The anion exchange process between ammonium ferric citrate and LDH fits well with Langmuir isotherm model and Pseudo-Second-Order model. The decomposition and oxidation of LDH-Fe(C6H4O7)− can be complete at 550 °C and above; however, crystal Fe3O4 is transformed only in the temperature range of 500–600 °C. Hydration reconstruction process of LDO-Fe3O4 in 0.015 M HNO3-30 wt% NaNO3 mixture solution can avert CO2 contamination and the recovery product is Fe3O4 pillared LDH with NO− 3 as the interlayer anion. The final sandwich-like structural composites show smaller sheet structure as comparison with well crystalline hexagonal sheet structure of LDH. Acknowledgements The authors appreciate the financial support from National Natural Science Foundation of China (No. 21407089), Key Laboratory for Solid Waste Management and Environment Safety (Tsinghua University), Ministry of Education of China (No. 201605), China Postdoctoral Science Foundation (No. 2014M560983) and Beijing Technology and Business University (No. QNJJ2016-21).

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