J. Yu et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 128–136
129
Table 1
Crystal spacing of HT-x.
2. Experimental
2.1. Preparation of HT
Entry
1
2
3
4
Catalysts
d value (nm)
HT-1
HT-2
HT-3
HT-4
0.772
0.783
0.787
0.796
The hydrotalcite supports (HT-x, x: Mg/Al mole rations, and x = 1,
2, 3 and 4) were prepared using a co-precipitation method. (1) A
certain ratio of MgCl2·6H2O and AlCl3·6H2O was dissolved in redis-
tilled water (1 L) under stirring. (2) NaOH and Na2CO3·10H2O were
was added drop-wise into the solution formed in (2) with vigorous
stirring over approximately 30 min. (4) The mixed suspension was
centrifuged and a white solid was obtained. Subsequently the solid
was washed six times with Na2CO3 (0.1 mol L−1). (5) The wet cake
was dried at 80 ◦C for 20 h in an oven [28].
2.5. Catalytic test: aerobic oxidation of benzyl alcohol
The reaction was carried out in a batch-type reaction with
stirring. Typically, the catalyst powder (50 mg), benzyl alcohol
(2 mmol) and solvent (10 mL) were placed in a 50 mL round-bottom
flask; a LED lamp (200 W, 400–800 nm) was used as a light source.
The system was filled with pure oxygen and sealed. A small amount
of reactant was separated at regular intervals and quantitatively
analyzed by GC. Afterward, the products were identified with a
GC–MS spectrometer. For comparison, the reaction was performed
simultaneously in the dark under the same conditions [30].
2.2. Preparation of Au/HT-x catalyst
The 2 wt% Au/HT-3 catalysts were prepared using a sequen-
tial deposition/reduction approach. HT-3 (2.5 g) was dispersed in
redistilled water (50 mL) and sonicated for 10 min. The pH of the
solution was adjusted to 10 using NaOH (0.5 mol L−1) with stir-
ring. Afterward, the HAuCl4 solution was added dropwise under
sonication. Subsequently, lysine (20 mL) was added to the mixture.
After stirring for 30 min, a fresh NaBH4 solution was added. This
slurry was filtered before being washed with deionized water and
ethanol, respectively. The resulting mixture was dried at 60 ◦C for
12 h. The other catalysts, including 2 wt% Au/HT-x (x = 1, 2 and 4),
y wt% Au/HT-3 (y: 0.5, 1, 2 and 3, respectively), 2 wt% Au/␥-Al2O3
and 2 wt% Au/Mg(OH)2 were prepared using the same method [28].
2.6. Hydrogen peroxide test
This kind of examination method is put forward by H. Bader
et al. in 1988 [31]. The particular way is shown as follows: the sam-
The buffer solution (pH = 6, 1 mL) was added to achieve pH 6 in
the final solution with stirring. Then DPD reagent (N,N-diethyl-
1,4-phenylenediammonium sulphate, 17 L) and POD reagent
(peroxidase product from horseradish, 17 L) were added in the
solution [31].
2.3. Recycling use of catalyst
3. Results and discussion
Method 1: Firstly, catalyst and reactant were separated via cen-
trifugation. Afterward, the catalyst was washed with deionized
water and ethanol. Finally, the catalyst was dried at 60 ◦C for 12 h.
The recycled catalyst was reused in the next run under the same
conditions [29].
Method 2: Firstly, catalyst and reactant were separated via
centrifugation. Afterward, the catalyst was washed with Na2CO3
(0.1 mol L−1) and ethanol. Finally, the catalyst was dried at 60 ◦C
for 12 h. The recycled catalyst was reused in the next run under the
same conditions.
3.1. Catalysts characterization
ports, including ␥-Al2O3, MgO and hydrotalcite with various Mg/Al
ratios (HT-x, x stands for Mg/Al mole rations, x = 1, 2, 3 and 4),
and revealing a series of characteristic patterns of inorganic struc-
tures. Fig. 1A (c–f) shows a series of characteristic patterns for the
tic reflections (0 0 3) that exhibit a high intensity and a narrow line
˚
width [28]. The lattice parameters of HT-3 are a = 3.058 0.004 A,
2.4. Catalyst characterization
◦
◦
◦
3
˚
c = 23.969 0.062 A, ˛ = 90 , ˇ = 90 , ꢁ = 120 , and V = 190.47 A .
Fig. 1B is the local amplification of Fig. 1A from 5◦ to 30◦, and blue
inter-planar crystal spacing is measured using the (0 0 3) diffraction
peak with the Bragg formula (2d sin ꢂ = nꢀ). The d value increases
with the molar proportions of magnesium (Table 1). This phe-
nomenon shows that a lattice expansion makes the XRD peaks
move toward a smaller angle. Fig. 1C shows the characteristic peaks
of supported samples (Au/␥-Al2O3, Au/Mg(OH)2 and Au/HT-x) with
the same gold content (2 wt%). An XRD pattern almost identical to
HT-x was obtained for Au/HT-x, indicating that the crystal structure
of the nano-crystalline support is conserved. Moreover, the char-
acteristic diffraction peaks of the Au nanoparticles can be observed
cite. (Calcined hydrotalcites can be reconstructed back to a layered
structure when it is in contact with water and appropriate anions.
This property is called retro-topotactical transformation. And it is
also known as the “memory effect” [32].). Fig. 1D shows the XRD
patterns of Au/HT-3 with different gold contents. The Au diffraction
The X-ray diffraction patterns (XRD) of the samples were
recorded on a RIGAKU D/MAX-2500 X-ray diffractometer using
CuK␣ radiation (ꢀ = 1.5405 A) at 40 kV and 100 mA. The diffrac-
˚
tion data were collected from 5◦ to 80◦. The gold contents of the
samples were quantitated using atomic absorption spectroscopy
(AAS) on a GBC AVANTA YX-05 instrument. The X-ray photoelec-
tron spectroscopy (XPS) measurements were performed on a Kratos
XSAM800 using AlK␣ radiation. The binding energy was calibrated
using the C1s photoelectron peak at 284.8 eV as a reference. The
particle sizes and morphologies of the samples were determined
using transmission electron micrographs (TEM) taken on a FEI Tec-
nai G2 F20 S-Twin apparatus with a 200 kV acceleration voltage.
UV–visible diffuse reflectance spectra (UV–vis DRS) were detected
on U-3900 made by Hitachi. Temperature-programmed desorption
of CO2 (CO2-TPD) was performed on a Micromeritics AutoChem
2920 II instrument. The product was quantitatively analyzed with a
GC-2014C gas chromatograph (GC). The identity of the product was
confirmed via gas chromatography–mass spectrometry (GC–MS)
on a Finnigan LCQ Advantage MAX instrument.