Preparation and Properties of Microspherical Alumina with a Magnetic

Dec 10, 2008 - cores to prepare magnetic Ni0.8Co0.2Fe oxide/SiO2/γ-Al2O3 particles by hydrolysis of ... To be effective as the magnetic core in a com...
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Ind. Eng. Chem. Res. 2009, 48, 692–697

Preparation and Properties of Microspherical Alumina with a Magnetic Core/ Shell Structure Jun-Ting Feng, Yan-Jun Lin, Feng Li, David G. Evans, Dian-Qing Li,* and Xue Duan State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, Beijing, People’s Republic of China, 100029

A series of Ni2+-Co2+-Fe2+-Fe3+-SO42- layered double hydroxide precursors with different Ni/Co ratios have been synthesized by a method involving separate nucleation and aging steps. After calcination at 900 °C, the corresponding Ni1-xCoxFe oxides were obtained. Vibrating sample magnetometry indicated that the Ni1-xCoxFe oxide samples not only had high specific saturation magnetization, but also low coercivity and remanence. Ni1-xCoxFe oxides showed the optimum combination of magnetic properties for x ) 0.2. Silica was coated onto the surface of Ni0.8Co0.2Fe oxide particles, and the coated particles were used as magnetic cores to prepare magnetic Ni0.8Co0.2Fe oxide/SiO2/γ-Al2O3 particles by hydrolysis of aluminum isopropoxide. After repeating the hydrolysis twice more, Ni0.8Co0.2Fe oxide/SiO2/γ-Al2O3 particles with about 20 wt % Ni0.8Co0.2Fe oxide were obtained and shown to be suitable for practical applications as a magnetic catalyst or catalyst support by virtue of their efficacious magnetic properties and pore structure. 1. Introduction Alumina particles have been widely used as catalysts or catalyst supports in fluidized beds or suspended beds because of their microporosity, high specific surface area, excellent physical strength and resistance against acid and alkali.1-4 In such cases, the alumina is generally obtained by the dehydration of boehmite and pseudoboehmite. The hydrolysis of aluminum alkoxides as an alternative way to prepare pseudoboehmite has received increasing attention, because the alumina synthesized by this method has small particle size, high purity, fine crystallinity, and appropriate pore structure.5 However, as is the case for other microparticle catalysts, catalysis by microspherical alumina particles requires reactant turbulence and vigorous stirring in a fluidized bed or a suspended bed. As a result, there are considerable problems with low mass transfer during the reaction as well as difficulties in separation and recovery of the catalyst.6 In recent years, the magnetically stabilized bed (MSB) technology7 has been widely applied in various areas, particularly petrochemical engineering, biochemical engineering and environmental engineering.8,9 In an MSB, a stable bed of small magnetic particles of a catalyst is formed by axially applying a spatially uniform and stable magnetic field. An MSB has many advantages, including low pressure drop and high mass-transfer efficiency, since the magnetic catalyst particles are dispersed in a uniform and stable arrangement by the magnetic field. Furthermore, since the catalyst particles are held in the magnetic field, abrasion phenomena are markedly reduced and loss of catalyst is minimized. As a result, catalyst separation at the outlet of the reactor is not required.10-12 If a suitable magnetic material can be used as the core of alumina particles, then it should be possible to use the alumina as catalyst or catalyst support in an MSB reactor. In addition, the magnetic core should be chemically compatible with the alumina, so that a metal oxide is preferable. Layered double hydroxides (LDHs) are a class of anionic clays. The general formula13,14 of an LDH is [M2+1-xM3+x(OH)2]x+(An-)x/n · mH2O, where M2+ and M3+ are di- and trivalent metal * To whom correspondence should be addressed. Tel: +8610 64436992. Fax: +8610 64425385. E-mail: [email protected].

cations, An- denotes an organic or inorganic anion with negative charge n, and x( ) M2+/[M2+ + M3+]) is the value of the stoichiometric coefficient. The M2+ and M3+ cations are uniformly dispersed within the layers without the formation of “lakes” of like cations. We have previously shown15 that single phase MFe2O4 (M ) Mg, Ni, and Co) ferrite spinel materials can be synthesized by calcination of M2+-Fe2+-Fe3+A2--LDHs. To be effective as the magnetic core in a composite catalyst particle in an MSB, a material should have high specific saturation magnetization but low values of coercivity and remanence.16 Of the three M2+ cations employed, Ni2+ gave a spinel with the best combination of magnetic properties. After coating with silica, the coated particles were used as magnetic cores of porous microspherical alumina particles prepared by the oil column method. The resulting NiFe2O4/SiO2/Al2O3 particles only just meet the requirements for use in an MSB as catalyst or catalyst support and their magnetic properties could not be improved by increasing the amount of magnetic cores as this resulted in the exposure of Fe3+ ions on the surface of alumina. Therefore, in order to improve the magnetic properties of the magnetic catalyst, both the properties of the magnetic cores and the method of preparation of the alumina shell need to be optimized. In this work, we investigate use of mixtures of Co2+ and Ni2+ to synthesize Ni2+-Co2+-Fe2+-Fe3+-SO42--LDH precursors which can be converted to Ni1-xCoxFe oxides by calcination. After coating with silica, the resulting Ni1-xCoxFe oxides were used as magnetic cores for magnetic microspherical alumina particles prepared by hydrolysis of aluminum isopropoxide. 2. Experimental Section 2.1. Experimental Materials. NiSO4 · 6H2O, CoSO4 · 7H2O, FeSO4 · 7H2O, Fe2(SO4)3 · 6H2O, NaOH, Na2SiO3 · 9H2O, hydrochloric acid, aluminum isopropoxide Al[OCH(CH3)2]3, nitric acid, and ethanol were all A.R. grade and were used without further purification. The deionized water used in all experiments had a conductivity of less than 10-6 S · cm-1.

10.1021/ie801098k CCC: $40.75  2009 American Chemical Society Published on Web 12/10/2008

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2.2. Preparation of Ni -Co -Fe -Fe LDHs Precursors. The LDHs precursors were prepared by means of a method involving separate nucleation and aging steps (SNAS).16 For the Ni2+-Fe2+-Fe3+-SO42--LDHs, NiSO4 · 6H2O, FeSO4 · 7H2O and Fe2(SO4)3 · 6H2O, with Ni2+/Fe2+/Fe3+ molar ratio of 3/5/2 were dissolved in deionized water to make a mixed salt solution ([Ni2+] + [Fe2+] + [Fe3+] ) 0.8 mol · L-1). NaOH was dissolved in deionized water to make an alkali solution. The two solutions were added simultaneously to a modified colloid mill with a rotation speed of about 4000 r · min-1 under N2 protection.17 The resulting slurry was transferred to a four-neck flask as quickly as possible and aged at 40 °C for 15 h with vigorous stirring in a flowing N2 stream. The slurry was then cooled in ice and the precipitate was centrifuged and thoroughly washed with cool deionized water and ethanol until the pH value of the filtrate reached 7. The final Ni2+-Fe2+-Fe3+-SO42--LDHs was obtained after drying at 70 °C for 12 h, and denoted as precursor a. Co2+-Fe2+-Fe3+-SO42--LDHs was prepared following the same procedure, using CoSO4 · 7H2O in place of NiSO4 · 6H2O, and denoted precursor b. The mixed Ni2+-Co2+-Fe2+-Fe3+-SO42--LDHs were prepared following the above procedure using mixtures of NiSO4 · 6H2O and CoSO4 · 7H2O in molar ratios of 95/5, 90/10, 85/15, 80/20, and 75/25 in place of NiSO4 · 6H2O. The materials are denoted as precursors c, d, e, f, and g, respectively. 2.3. Synthesis of Ni1-xCoxFe Oxides and Surface Modification. The LDH precursors a-g were heated in air with a ramping rate of 10 °C · min-1 to 900 °C and calcined at that temperature for 5 h, followed by slow cooling to room temperature to give NiFe oxide, CoFe oxide, Ni0.95Co0.05Fe oxide, Ni0.9Co0.1Fe oxide, Ni0.85Co0.15Fe oxide, Ni0.8Co0.2Fe oxide and Ni0.75Co0.25Fe oxide. The samples are denoted a1-g1, respectively. Particles with sizes of about 3-8 µm were obtained after milling the Ni1-xCoxFe oxides in an A11 Basic high speed mill. Ni1-xCoxFe oxides (2.00 g) were dispersed in a solution (80 mL) containing Na2SiO3 · 9H2O (9.46 g) using ultrasound (Ni1-xCoxFe oxide/SiO2 mass ratio of ∼1). The resulting suspension was transferred into a four-neck flask, followed by slow addition of 2 mol · L-1 hydrochloric acid (34 mL) with vigorous stirring at 85 °C in a flowing N2 stream. The final pH value was 6. The resulting surface coatedmagnetic core particles of Ni1-xCoxFe oxide/SiO2 containing about 50% SiO2 were then washed and dried. 2.4. Synthesis of Magnetic Alumina. Ni0.8Co0.2Fe oxide/SiO2/ γ-Al2O3 was synthesized by the hydrolysis of aluminum isopropoxide. Ni0.8Co0.2Fe oxide/SiO2 (2.00 g) was dispersed in 532 mL deionized water using ultrasound. The resulting suspension was transferred into a four-neck flask with 5.32 g aluminum isopropoxide. After vigorous stirring at 85 °C for 30 min, nitric acid was added into the four-neck flask until the pH value reached 1. The Al(OH)3 gel containing Ni0.8Co0.2Fe oxide/SiO2 particles obtained after aging at 95 °C for 24 h was subsequently washed with ethanol and dried in vacuum. After calcination at 600 °C for 4 h in air, Ni0.8Co0.2Fe oxide/SiO2/γ-Al2O3 particles containing about 40 wt% alumina were obtained, denoted sample A. The resulting magnetic alumina sample was then coated with a second layer of alumina using the above method, and the resulting Ni0.8Co0.2Fe oxide/SiO2/γ-Al2O3 with about 50 wt% alumina is denoted sample B. Ni0.8Co0.2Fe oxide/SiO2/γ-Al2O3 containing about 60 wt% and 70 wt% alumina were obtained further repeating the hydrolysis step, and were denoted samples C and D respectively. 2.5. Analysis and Characterization. Powder XRD patterns were recorded on Shimadzu XRD-600 X-ray powder diffractometer 2+

2+

2+

-SO42--

3+

Figure 1. XRD patterns of Ni2+-Fe2+-Fe3+-SO42--LDHs (a), Co2+-Fe2+-Fe3+-SO42--LDHs (b) and Ni2+-Co2+-Fe2+-Fe3+SO42--LDHs with Ni/Co atomic ratio of 95/5 (c), 90/10 (d), 85/15 (e), 80/20 (f), and 75/25 (g).

(Cu Ka radiation, λ ) 0.15406 nm) between 3° and 70°, with a scan speed of 10° · min-1. Elemental analysis was performed using a Shimadzu ICPS-75000 inductively coupled plasma emission spectrometer (ICP-ES) and an Elementar Vario EL elemental analyzer. The magnetic properties of the samples were measured at room temperature using a JDM-13 vibrating sample magnetometer (VSM). Morphology and particle size were examined using a Hitachi S-4700 scanning electron microscope (SEM). The low temperature N2 adsorption-desorption experiments were carried out using a Quantachrome Autosorb-1 system. Surface elemental analysis for samples was performed using an ESCALAB250 X-ray photoelectron spectrometer. 3. Results and Discussion 3.1. Composition and Structure of Ni2+-Co2+-Fe2+Fe3+-SO42--LDHs precursors and Ni1-xCoxFe oxides. Elemental analysis of the LDHs precursors showed that the Ni/ Co ratio in the Ni2+-Co2+-Fe2+-Fe3+-SO42--LDHs (95/5, 90/10, 85/15, 79/21, and 75/25 in precursors c, d, e, f, and g, respectively) corresponded very closely to that in the corresponding precursor mixture (95/5, 90/10, 85/15, 80/20, and 75/ 25 in precursors c, d, e, f, and g, respectively), and therefore, the values in the precursor mixture are used to label the materials. The powder XRD patterns for Ni2+-Fe2+Fe3+-SO42--LDHs (a), Co2+-Fe2+-Fe3+-SO42--LDHs (b), and Ni2+-Co2+-Fe2+-Fe3+-SO42--LDHs with Ni2+/Co2+ molar ratios of 95/5 (c), 90/10 (d), 85/15 (e), 80/20 (f), and 75/25 (g) are shown in Figure 1. The peaks can be indexed in a three-layer 3R polytype with rhombohedral symmetry, with those at low angle being characteristic of the basal (003) and higher order (006 and 009) reflections of a layered structure and the peak around 60° 2θ corresponding to the (110) reflection of an LDH.13 The structural parameters of LDHs are listed in Table 1. The lattice parameter a () 2 d110) represents the distance between metal ions in two adjacent hexagonal cells. Its value increases with the proportion of cobalt in the LDH; this is consistent with the larger size of the Co2+ ion (values of the Shannon ionic radii for Ni2+ and Co2+ are 0.083 and 0.089 nm, respectively). The values of the lattice parameter c () d003 + 2d006 + 3d009) are similar to those previously reported for an LDH with intercalated SO42- anions.13 The powder XRD patterns of Ni1-xCoxFe oxides obtained after calcination of the above-mentioned LDHs at 900 °C are shown in Figure 2. The characteristic (220), (311), (400), (511), and

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Figure 2. XRD patterns of Ni1-xCoxFe oxides (a1) x ) 0; (b1) x ) 1; (c1) x ) 0.05; (d1) x ) 0.10; (e1) x ) 0.15; (f1) ) 0.20; and (g1) x ) 0.25. Table 1. Indexing of XRD Patterns for (a) Ni2+-Fe2+-Fe3+-SO42--LDHs, (b) Co2+-Fe2+-Fe3+-SO42--LDHs and Ni2+-Co2+-Fe2+-Fe3+-SO42--LDHs with Ni/Co atomic ratios of (c) 95/5, (d) 90/10, (e) 85/15, (f) 80/20, and (g) 75/25 indexing

a

b

c

d

e

f

g

d003/nm d006/nm d009/nm d110/nm lattice parameter a /nm lattice parameter c /nm

1.1085 0.5521 0.3678 0.1562 0.3124

1.1182 0.5543 0.3682 0.1579 0.3158

1.0968 0.5504 0.3674 0.1562 0.3124

1.1274 0.5560 0.3695 0.1566 0.3132

1.1146 0.5528 0.3678 0.1567 0.3134

1.1317 0.5572 0.3700 0.1569 0.3138

1.1279 0.5564 0.3695 0.1568 0.3136

3.3255 3.3546 3.2904 3.3822 3.3438 3.3951 3.3937

Table 2. Indexing of XRD patterns for patterns of Ni1-xCoxFe oxides (a1) x)0; (b1) x)1; (c1) x)0.05; (d1) x)0.10; (e1) x)0.15; (f1))0.20; (g1) x)0.25 indexing

a1

b1

c1

d1

e1

f1

g1

d220/nm d311/nm d400/nm d511/nm d440/nm lattice parameter a /nm

0.2957 0.2520 0.2089 0.1606 0.1476 0.8364

0.2973 0.2533 0.2099 0.1615 0.1483 0.8409

0.2957 0.2520 0.2088 0.1606 0.1475 0.8364

0.2957 0.2521 0.2089 0.1607 0.1476 0.8364

0.2958 0.2522 0.2090 0.1608 0.1477 0.8367

0.2962 0.2524 0.2091 0.1609 0.1477 0.8378

0.2958 0.2522 0.2090 0.1608 0.1477 0.8367

(440) reflections of NiFe oxide (Figure 2a) and CoFe oxide (Figure 2b) can be observed, consistent with the positions and relative intensities reported for MFe2O4 spinel ferrite (M ) Ni, JCPDS (10-0325); M ) Co, JCPDS (22-1086). In addition, the characteristic reflections of Ni1-xCoxFe oxides (x ) 0.05-0.25) are similar to that of the pure MFe2O4 spinel ferrite. This indicates that the layer hydroxyl groups and interlayer sulfate anions have decomposed with loss of water and sulfur dioxide/ trioxide, while the Fe2+ cations have been oxidized to Fe3+. As shown in Table 2, the lattice parameter a of CoFe oxide is higher than that of NiFe oxide, and furthermore, the lattice parameter a of Ni1-xCoxFe oxide increases with x; this is consistent with the ionic radii for Ni2+ and Co2+ given above. 3.2. Magnetic Properties of Ni1-xCoxFe Oxides. The field dependence of the magnetization of Ni1-xCoxFe oxide samples were measured using a vibrating sample magnetometer at room temperature. The resulting magnetization hysteresis loops are shown in Figure 3 and the magnetic parameters are listed in Table 3. To be effective as the magnetic core in a composite catalyst particle in an MSB, a material should have high specific saturation magnetization (Ms) but low values of coercivity (Hc) and remanence

Figure 3. Magnetization hysteresis loops of Ni1-xCoxFe oxides (a1) x ) 0; (b1) x ) 1; (c1) x ) 0.05; (d1) x ) 0.10; (e1) x ) 0.15; (f1) ) 0.20; and (g1) x ) 0.25. Table 3. Magnetic Parameters of Ni1-xCoxFe Oxides magnetic parameters

Ms (emu · g-1)

Mr (emu · g-1)

Hc (Oe)

NiFe oxide CoFe oxide Ni0.95Co0.05Fe oxide Ni0.9Co0.1Fe oxide Ni0.85Co0.15Fe oxide Ni0.8Co0.2Fe oxide Ni0.75Co0.25Fe oxide

47.3 90.0 52.4 55.3 57.5 59.8 61.60

13.3 45.2 15.1 16.9 18.4 20.4 27.93

400 1825 457 657 563 549 479

(Mr).16 As listed in Table 3, although the value of the Ms of CoFe oxide is much higher than that of NiFe oxide, the values of Hc and Mr are also higher. Thus, neither NiFe oxide nor CoFe oxide is an optimum choice as the magnetic species in the preparation of magnetic alumina. When replacing Ni2+ with Co2+ in Ni1-xCoxFe oxides, the value of Ms gradually increased with increasing x. Moreover, in comparison with the pristine CoFe oxide, Ni1-xCoxFe oxide samples had much lower values of Hc and Mr up to x ) 0.2. However, when x exceeded 0.2, the value of the Mr of Ni1-xCoxFe oxides significantly increased. Therefore, subsequent work focused on preparation of composite catalysts using Ni0.8Co0.2Fe oxide as the optimized core material. 3.3. Structure and Magnetic Properties of Ni0.80Co0.20Fe Oxide and Ni0.80Co0.2Fe Oxide/SiO2. SEM micrographs of Ni0.80Co0.20Fe oxide and Ni0.80Co0.20Fe oxide/SiO2 are shown in Figure 4. The sizes of the primary particles of Ni0.80Co0.20Fe oxide range from 50 to 200 nm and a limited extent of aggregation of the Ni0.80Co0.20Fe oxide particles occurs. In order to avoid the possibility of migration of Fe3+ ions from the Ni0.80Co0.20Fe oxide into alumina during the preparation of the composite particles, we first coated the Ni0.80Co0.20Fe oxide with a protective layer of SiO2 before formation of the microspherical alumina particles. As shown in Figure 4, discrete Ni0.80Co0.20Fe oxide/SiO2 particles are formed with sizes in the from range 5 to 10 µm. Powder XRD patterns for Ni0.80Co0.20Fe oxide and Ni0.80Co0.20Fe oxide/SiO2 are shown in Figure 5. The characteristic (220), (311), (400), (511), and (440) reflections of the spinel phase18 can be observed in the XRD patterns of both Ni0.80Co0.20Fe oxide and Ni0.80Co0.20Fe oxide/SiO2. In addition, there is a weak broad diffraction peak in the range 15° to 30°, which is an indication of the presence of an almost amorphous vitreous silica phase.19 This demonstrates that SiO2 and Ni0.80Co0.20Fe oxide exist as separate phases. An XPS survey spectrum of Ni0.8Co0.2Fe oxide/SiO2 is presented in Figure 6. The characteristic peaks of Ni, Co, Fe cannot be observed, which confirms that the Ni0.8Co0.2Fe oxide particles were thoroughly coated with a thick protective SiO2 layer. The Si2p3/2 and O1s

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Figure 6. XPS survey spectrum of Ni0.8Co0.2Fe oxide/SiO2.

Figure 4. SEM micrographs of Ni0.8Co0.2Fe oxide (a) and Ni0.8Co0.2Fe oxide/ SiO2 (b).

Figure 5. XRD pattern of Ni0.8Co0.2Fe oxide/SiO2.

binding energies are found to be 103.2 and 533.4 eV respectively, which are typical values20,21 for SiO2. The carbon peak is attributed to adventitious hydrocarbon (i.e., from the XPS instrument itself). Elemental analysis indicated that the mass fraction of Ni0.80Co0.20Fe oxide in Ni0.80Co0.20Fe oxide/SiO2 is 51%, which is consistent with the value in the synthesis mixture. The magnetization hysteresis loops for Ni0.80Co0.20Fe oxide and Ni0.80Co0.20Fe oxide/SiO2 are shown in Figure 7. The values of Ms (27.5 emu · g-1) and Mr (9.7 emu · g-1) of Ni0.80Co0.20Fe oxide/ SiO2 are lower than the corresponding values for Ni0.80Co0.20Fe

Figure 7. Magnetization hysteresis loops of Ni0.8Co0.2Fe oxide (a) and Ni0.8Co0.2Fe oxide/SiO2 (b).

oxide (59.8 emu · g-1 and 20.4 emu · g-1 respectively), whereas the value of Hc shows only a slight change (549 Oe as compared with 559 Oe). The results demonstrate that the magnetic properties of Ni0.80Co0.20Fe oxide/SiO2 particles make the material suitable for use as a magnetic core in magnetic catalysts. 3.4. Structure, Magnetic Properties and Pore Structure of Ni0.80Co0.20Fe Oxide/SiO2/γ-Al2O3. Magnetic microspherical alumina A, B, C, and D particles were prepared by the hydrolysis of aluminum isopropoxide in the presence of Ni0.80Co0.20Fe oxide/ SiO2. Elemental analysis indicated that the mass fractions of Ni0.80Co0.20Fe oxide in samples A, B, C, and D (in which the hydrolysis steps were carried out one, two, three, and four times, respectively) were 30%, 24%, 19%, and 14%, which are similar to the corresponding values in the synthesis mixture. The magnetization hysteresis loops are given in Figure 8. The magnetic parameters and pore structure parameters are listed in Table 4 and Table 5, respectively. The values of Ms of samples A, B, C, and D were observed to decrease with increasing amount of alumina, while the specific surface areas gradually increases. In designing a viable magnetic catalyst, not only magnetic properties but also pore structure must be taken into consideration.22 Therefore, of samples A-D, sample C (obtained by applying three coatings of alumina) is the most suitable catalyst or catalyst support in an MSB because it has both a reasonably high value of Ms and a high surface area. The N2 adsorption-desorption isotherms and the pore size distribution of sample C are shown in Figures 9 and 10, respectively. The isotherm is of type II with an obvious

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Figure 8. Magnetization hysteresis loops of Ni0.8Co0.2Fe oxide/SiO2/γ-Al2O3 samples A, B, C, and D coated with different amounts of alumina.

Figure 10. Pore size distribution of Ni0.8Co0.2Fe oxide/SiO2/γ-Al2O3 sample C.

Table 4. Magnetic Parameters of Ni0.8Co0.2Fe Oxide/SiO2/γ-Al2O3 Samples A, B, C, and D Coated with Different Amounts of Alumina sample

A

B

C

D

specific saturation magnetization (emu · g-1) remanence (emu · g-1) coercivity (Oe)

18.9 5.1 225

13.9 4.7 236

10.8 3.5 249

7.9 2.3 270

Table 5. Pore Structure Parameters of Ni0.8Co0.2Fe Oxide/SiO2/ γ-Al2O3 Samples A, B, C, and D Coated with Different Amounts of Alumina sample -1

specific surface area(m · g ) pore volume(cm3 · g-1) APDa (nm) 2

a

A

B

C

D

100 0.29 11.8

111 0.22 7.9

148 0.25 6.7

152 0.23 6.3

APD ) Average pore diameter (determined using BJH analysis).

Figure 11. SEM micrograph of Ni0.8Co0.2Fe oxide/SiO2/γ-Al2O3 sample C.

Figure 9. N2 adsorption-desorption isotherms of Ni0.8Co0.2Fe oxide/SiO2/ γ-Al2O3 sample C.

hysteresis loop.23 The shape of the hysteresis loop is a superposition of types H1 and H3. This is taken to indicate that the sample has both tubular and parallel slit-shaped capillary pores which are caused by the escaping gas during calcination and the stacking of alumina microcrystallites. The pore size of the magnetic alumina sample is in the range 1-10 nm, with a small number of micropores and a greater number of mesopores. The SEM micrograph shown in Figure 11 indicates that the particles in sample C have essentially spherical morphology and no aggregation or agglomeration of particles was observed. The particle size distribution is in the range 100-200 µm. In order to observe the internal structure of the Ni0.80Co0.20Fe oxide/SiO2/

Figure 12. SEM micrograph of the cross section of Ni0.8Co0.2Fe oxide/ SiO2/γ-Al2O3 sample C embedded in an acrylate adhesive.

γ-Al2O3 microspheres, they were embedded in modified acrylate adhesive and small pieces were made by cutting the block obtained after solidification. An SEM micrograph of one such section is shown in Figure 12. As expected, several layers of alumina can be observed on the surface of sample C. In addition, a number of Ni0.80Co0.20Fe oxide/SiO2 particles can be seen dispersed in the central region of the magnetic alumina sphere. The XPS survey spectrum of sample C presented in Figure 13 indicates that only the elements Al, O, and Si are exposed

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Figure 13. XPS survey spectrum of Ni0.8Co0.2Fe oxide/SiO2/γ-Al2O3 sample C.

on the surface of sample C. The Al2p binding energy is found to be 74.3 eV, which is consistent with octahedral coordination of aluminum.24 The presence of some Si peaks suggests that some parts of the alumina layer are not sufficiently thick, so that some of underlying silica layer is exposed. However, no Ni, Co, and Fe signals are observed on the surface of sample C which indicates that the resulting magnetic alumina has the potential to be used as a catalyst or catalyst support. 4. Conclusions Ni2+-Co2+-Fe2+-Fe3+-SO42--LDHs precursors have been synthesized by a method involving separate nucleation and aging steps (SNAS). After calcination at 900 °C, Ni1-xCoxFe oxides with not only high values of Ms but also low values of Ms and Hc could be obtained. For the Ni1-xCoxFe oxides, a value of x of 0.2 afforded the optimum combination of magnetic properties. Silica was coated onto the surface of Ni0.8Co0.2Fe oxide, and the coated particles were used as magnetic cores to prepare magnetic Ni0.8Co0.2Fe oxide/ SiO2/γ-Al2O3 particles by the repeated hydrolysis of Al isopropoxide. Ni0.8Co0.2Fe oxide/SiO2/γ-Al2O3 particles with about 20 wt% Ni0.8Co0.2Fe oxide were found to possess the optimum combination magnetic and textural properties for practical applications as a catalyst or catalyst support. Acknowledgment This work was supported by the National Natural Science Foundation, the 111Project, and the Program for Changjiang Scholars and Innovative Research Team in University. Literature Cited (1) Granado, S.; Ragel, V.; Cabanas, V.; Romana, J. S.; Vallet-Reg, M. Influence of R-Al2O3 morphology and particle size on drug release from ceramic/polymer composites. J. Mater. Chem. 1997, 8, 1581–1585. (2) Bertarione, S.; Prestipino, C.; Groppo, E.; Scarano, D.; Spoto, G.; Zecchina, A.; Pellegrini, R.; Leofanti, G.; Lamberti, C. Direct IR observation of vibrational properties of carbonyl species formed on Pd nano-particles supported on amorphous carbon: comparison with Pd/SiO2-Al2O3. Phys. Chem. Chem. Phys. 2006, 8, 3676–3681. (3) Niesz, K.; Yang, P. D.; Somorjai, G. A. Sol-gel synthesis of ordered mesoporous alumina. Chem. Comm. 2005, 15, 1896–1897.

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ReceiVed for reView July 17, 2008 ReVised manuscript receiVed October 24, 2008 Accepted October 27, 2008 IE801098K