T. Ishida et al. / Applied Catalysis A: General 377 (2010) 42–46
43
abundant natural resource, chitosan by itself cannot be used as a
support due to its weak mechanical strength and poor diffusion
into solvent.
In this work, in order to establish a generic route to prepare
‘‘green’’ gold catalysts, we have attempted to deposit Au NPs onto
cellulose directly from gold complexes by the deposition–
reduction (DR) method [19,20] and by the solid grinding (SG)
method [14]. We have also studied their catalytic performance for
the aerobic oxidation of glucose in aqueous media.
60 8C. The reaction was kept at pH 9.5 by titrating 1 M aqueous
NaOH solution. The conversion of glucose was calculated from the
total amount of NaOH added. Metal-time-yield was calculated
from a straight line fitted to the conversion-time curve. Turnover
frequency (TOF) was calculated from MTY based on the number of
Au atoms exposed to the surfaces, which were calculated from the
mean particle diameters.
The rate dependency on glucose concentration was obtained
under the following conditions: glucose/Au = 32,000 mol/mol, O2
120 mL/min, 50 8C, and pH 9.0. The concentration of glucose was
changed over the range of 2.5–10 wt%. The rate of glucose
oxidation is expressed by Eq. (1) since the flow of O2 was in large
excess with respect to glucose concentration:
2. Experimental
2.1. Materials
a
0
a
0
b
b
Cellulose (<100
mm powder) was purchased from Aldrich and
r ¼ k½glucoseꢂ ½O2ꢂ ¼ k ½glucoseꢂ
ðk ¼ k½O2ꢂ Þ
(1)
used as received. Bis(ethylenediamine)gold(III) trichloride
(Au(en)2Cl3) was prepared from HAuCl4ꢁ4H2O according to the
literature [21]. Dimethyl gold(III)acetylacetonate, Me2Au(acac),
was purchased from Trichemical Laboratories Inc. and used as
received. Reagent grade glucose and NaOH were purchased and
used without further purification.
Arrhenius plots were obtained under the following conditions:
5 wt% aqueous glucose solution (glucose/Au = 32,000 mol/mol), O2
120 mL/min, pH 9.0. The reaction temperature was changed in the
range of 40–70 8C.
3. Results
2.2. Instruments
3.1. Direct deposition of gold nanoparticles and clusters onto cellulose
Zeta potentials were measured on a Microtec ZEECOM in 0.01 M
NaCl aqueous solution. The pH of the solution was adjusted by
adding HCl or NaOH solution. Transmission electron microscopic
(TEM) observations were carried out by using a JEOL JEM-3000F
operating at 300 kV and a JEM-2100F operating at 200 kV. High-
angle annular dark-field scanning TEM (HAADF-STEM) observation
was also carried out to observe Au clusters smaller than 2 nm by
using a JEOL JEM-3000F. The gold contents of the catalysts were
determined by atomic absorption spectrometry (AAS) by using a
SHIMADZU AA-6200. The products of glucose oxidation were
identified by using 1H NMR (270 MHz in D2O) and compared with
authentic samples.
The deposition–reduction (DR) method is based on the use of
weak electrostatic interactions of polymer surfaces with the
opposite charged Au(III) complex ions, leading to the reduction of
Au(III) exclusively on polymer surfaces [19,20]. Cationic Au(en)2Cl3
was chosen as a Au precursor because of the negative zeta
potentials of cellulose in the pH range of 3–10 (Fig. 1). We have
reported that the dropwise addition of a reducing agent into an
Au(III) aqueous solution was effective to obtain small Au NPs and
to prevent them from aggregation [19]. Gold NPs supported on
cellulose were also prepared by slow reduction in a similar
manner. Most of the loaded Au(III) precursors were reduced to
Au(0) exclusively on cellulose surfaces up to 0.5 wt% of Au loading.
As shown in Table 1 and Fig. 2, 0.5 wt% of Au loading yielded large
Au NPs around 10 nm, accompanied by the formation of aggregates
(entry 1, Fig. 2). Lower Au loading (0.05 wt%) gave smaller Au NPs
having a mean diameter of 7.7 nm with a standard deviation of
6.0 nm (entry 2).
We have recently reported that the solid grinding method was
effective to deposit Au as clusters smaller than 2 nm onto inert
supports such as polymers [14], carbons [8,22], and insulating metal
oxides [8]. This method was applicable to cellulose and gave highly
dispersed Au clusters and NPs with diameters of around 2 nm in the
range of 0.05–0.23 wt% of Au content (entries 3–5 in Table 1, Fig. 3).
In the case of cellulose, the addition of small amounts of H2O during
grinding was effective to deposit small Au NPs in particular for
2.3. Direct deposition of gold nanoparticles onto cellulose by the
deposition–reduction method
To a suspension of cellulose (0.5 g) dispersed in distilled water
(100 mL), a solution of Au(en)2Cl3 (5.4 mg, Au loading 0.5 wt%) in
H2O (5.0 mL) was added. The pH of the suspension was adjusted to
10 by adding aqueous NaOH solution. The mixture was stirred at
0 8C for 30 min and then 0.01 M NaBH4 (1.9 mL) was slowly added
over 4 min. After additional stirring at 0 8C for 1 h, the mixture was
filtered, washed, and dried under vacuum for 2 h to yield Au/
cellulose.
2.4. Direct deposition of gold clusters onto cellulose by the solid
grinding method
Cellulose (500 mg) and Me2Au(acac) (4.2 mg for 0.5 wt% Au
loading) were ground in an agate mortar in air at room
temperature for 40 min. A small amount of water (0.4 mL) was
added during grinding for 40 min. The mixture was treated in a
stream of 20 vol% H2 in N2 at a flow rate of 50 mL/min at 120 8C for
2 h to reduce Au(III) to Au(0).
2.5. Catalytic tests
Glucose oxidation was carried out at atmospheric pressure by
bubbling molecular oxygen (60 mL/min) through the glucose
(4.39 g) aqueous solution (74 mL). The catalyst dispersed in H2O
(10 mL) was added into the 5 wt% glucose solution (glucose/loaded
Au = 16,000 mol/mol), and the aqueous dispersion was stirred at
Fig. 1. Zeta potentials of cellulose in 0.01 M NaCl aqueous solution as a function of pH.