2
J. Albadi et al. / Catalysis Communications 49 (2014) 1–5
transmission electron microscopy (HRTEM) techniques using a HITACHI
S-4160 instrument, and JEOL JEM-2100 (200 kV) microscope equipped
with an EDS analytical system.
2.4. General procedure
All reactions were performed in a glass flask slurry reactor connected
to an O2 tube for atmosphere control and a condenser for reflux condition.
A mixture of alcohol (1 mmol), Cs2CO3 (0.5 mmol) and 2Au/1CuO–ZnO
(0.05 g) in water was stirred under oxygen atmosphere in a slurry reactor
at total reflux condition. Then the catalyst was recovered by filtration,
washed two times with 5 ml hot EtOAc, and dried for consecutive
reaction runs. The filtrate was quenched with 2 M HCl aqueous solution,
extracted with EtOAc three times and dried over anhydrous MgSO4.
Evaporation of the solvent followed by column chromatography on silica
gel afforded the pure products (Table 3).
Scheme 1. Aerobic oxidation of alcohols catalyzed by the 2Au/1CuO–ZnO catalyst.
3. Results and discussion
3.1. X-ray diffraction
Fig. 1 exhibits the XRD analysis of pure ZnO and 2Au/1CuO–ZnO
samples. There are several peaks attributed to ZnO at a wide range of
2θ = 31.7, 34.3, 36.3, 47.3, 56.4, 62.8, 66.2, 67.8, 69.0, 72.5 and 76.9°
which are characteristics of (100), (002), (101), (102), (110), (103),
(200), (112), (201), (004) and (202) planes based on JCPDS card No.
89–1397 (Fig. 1b). The 2Au/1CuO–ZnO diffraction pattern shows only
the ZnO structure. This may suggests high dispersion of Au nanoparticles
on the catalyst surface. Also, the amount and the size of Au and CuO
species are too small to be detected by XRD [34]. The XRD peaks of
2Au/1CuO–ZnO have lower intensity and are slightly broadened after
addition of Au and CuO to the ZnO. This may indicate lower crystallinity
and smaller crystallite sizes of ZnO (Fig. 1b).The average crystallite size
of ZnO in 2Au/1CuO–ZnO catalyst, calculated by the Scherrer formula is
about 26 nm in comparison with 29 nm, for ZnO. The structural pro-
perties of prepared samples are presented in Table 1.
Fig. 1. XRD patterns of (a) ZnO, and (b) 2Au/1CuO–ZnO catalyst.
continuous mixing. The obtained suspension was aged at pH = 8.5 for
15 min at 65 °C, then filtered and washed with warm deionized
water. The precipitates were dried 12 h at 100 °C followed by calcina-
tion at 300 °C for 3 h. In addition, a batch of ZnO support was prepared
under this condition for the supplementary tests. For the synthesis of
2 wt.% Au on CuO–ZnO support, proper amount of gold was loaded
onto the support by a deposition–precipitation method. Simultaneous-
ly, an aqueous solution of HAuCl4 and 0.1 M Na2CO3.1H2O was added
drop-wise into the slurry of CuO–ZnO support at 50 °C and a constant
pH of 8 under vigorous stirring. The precipitate finally was aged, filtered,
washed and dried under air atmosphere at 100 °C. The as prepared
catalyst is entitled as 2Au/1CuO–ZnO, where 2 and 1 are the amount
of Au and CuO weight percent in the catalyst, respectively.
3.2. N2 adsorption/desorption analysis
The BET surface area of 1CuO–ZnO support is 63 m2/g, and as
2 wt.% Au is deposited on the support, the surface area slightly decreases
to 61 m2/g. The N2 adsorption/desorption profiles for 1CuO–ZnO and
2Au/1CuO–ZnO samples are shown in Fig. 2a. According to the IUPAC
classification, the nitrogen adsorption isotherms can be categorized as
a type IV at the borderline with type II, with a type H3 hysteresis loop.
This isotherm adsorption indicates the presence of large mesopores
with a pore size distribution continuing into the macropore domain
[2], (as shown in Fig. 2b). In addition, the type H3 hysteresis is usually
observed on solids containing aggregates or agglomerates of some par-
ticles leading to slit shaped pores, with nonuniform size and/or shape.
After deposition of 2 wt.% Au on 1CuO–ZnO support the pore volume
relatively increases.
2.3. Catalyst characterization
The Bruker AXS D8 advanced diffractometer carries out X-ray dif-
fraction (XRD) analysis to determine the structural properties of the cat-
alyst. The sample was scanned over a range of 2θ = 20–80° at a rate of
0.05°/s using Cu Kα radiation (λ = 1.5406 ˚A). The specific surface area
of samples was determined by nitrogen adsorption–desorption using
the BET method. BET tests were carried out using an automated gas
adsorption analyzer (Tristar 3020, Micromeritics). Prior to BET tests
the samples were degassed under vacuum at 150 °C for 2 h. The
morphology of the catalyst was investigated comprehensively by field
emission scanning electron microscopy (FESEM) and high resolution
3.3. FESEM and TEM
FESEM micrographs of 1CuO–ZnO and 2Au/1CuO–ZnO catalysts
have been illustrated in Fig. 3. The 1CuO–ZnO picture (Fig. 3a) shows ag-
gregation of nano-rod-like particles of about 35 nm in diameter. As Au is
deposited on to the 1CuO–ZnO with Na2CO3 in an aqueous media at
Table 1
Structural properties of the prepared samples.
Sample
DXRD crystallite size (nm)
SBET (m2g−1
)
Pore volume (cm3g−1
)
Pore size (nm)
1CuO–ZnO
2Au/1CuO–ZnO
29
26
63
61
0.20
0.25
21.5
21.9