G Model
CATTOD-10174; No. of Pages6
ARTICLE IN PRESS
S. Campisi et al. / Catalysis Today xxx (2016) xxx–xxx
Table 1
2
as the crystallinity of the support influenced both the immobiliza-
tion of PVA protected Au NPs and their selective in the oxidation of
glycerol [23].
Therefore the different distribution of anchoring sites for metal
nanoparticles on the selected supports is expected to differently
influence the catalytic activity and selectivity after the removal of
the protecting agent.
Statistical median and standard deviation of particle size analysis for Au based
catalysts.
Catalyst
Statistical median (nm)
Standard deviation
Au/AC unwashed
2.9
3.0
3.4
6.7
5.7
6.9
0.1
0.2
0.1
1.2
1.5
1.8
Au/AC washed at r.t.
Au/AC washed at 60 ◦
Au/Graph. Unwashed
C
Au/Graph. washed at r.t.
Au/Graph. washed at 60 ◦
C
2. Experimental
2.1. Materials
using a column (Alltech OA-10308, 300 mm 7.8 mm) with UV and
refractive index (RI) detectors. Aqueous H3PO4 solution (0.1 wt%)
was used as the eluent. Products were identified by comparison
with standard samples.
NaAuCl4·2H2O was from Aldrich (99.99% purity); activated car-
bon from Camel (X40S; SA = 900–1100 m2/g; PV = 1.5 cm3/g; Point
of zero charge (pzc) 9–10) and Graphite from Johnson Matthey
(SA = 14 m2/g) were used as supports. NaBH4 of purity >96% from
Fluka and poly(vinyl alcohol) (PVA) (Mw = 13 000–23 000, 87–89%
hydrolyzed) from Aldrich were used. Gaseous oxygen (99.99%)
from SIAD, NaOH from Aldrich and Glycerol (87 wt% solution) from
Riedel-de-Häen were used in the catalytic reactions.
3. Results and discussion
Gold nanoparticles stabilized with poly(vinyl alcohol) were sup-
ported on activated carbon and crystalline graphite. To disclose the
effect of the protecting agent and its relative amount, the water
lowing the procedure reported by Lopez-Sanchez et al. [20], slightly
modified for achieving a gradual removal of the PVA. Catalysts were
washed using deionized water at room temperature and at 60 ◦C.
Previous experiments performed on AuPVA/TiO2 [21] revealed that
the washing treatment at room temperature results in a partial
removal of PVA whereas the one at 60 ◦C is effective in completely
extracting residual PVA.
2.2. Catalyst preparation
Solid NaAuCl4·2H2O (0.043 mmol) and PVA (Au/PVA = 1:
1 wt/wt) solution were added to 130 mL of H2O. After 3 min, 0.1 M
NaBH4 (Au/NaBH4 = 1:4 mol mol−1) solution was added to the yel-
low solution under vigorous magnetic stirring. A ruby red Au(0) sol
was immediately formed. Within a few minutes from their gen-
eration, the colloids (acidified at pH 2, by sulphuric acid) were
immobilized by adding the support under vigorous stirring. The
amount of support was calculated in order to obtain a final metal
loading of 1 wt% (on the basis of quantitative loading of the metal
on the support). The catalysts were filtered, washed on the filter
(100 mL of distilled water for 1 g of catalyst) and dried at 80 ◦C
for 4 h. AuPVA/Support prepared using the Au/PVA ratio 1:1 were
poured into a large amount of water (100 mL g−1 of catalyst) at
room temperature or alternatively at 60 ◦C and stirred for 8 h. The
catalyst was then recovered by filtration and dried at 80 ◦C for 4 h.
In alternative, Au catalysts were further poured into a large amount
of water (100 × 5 mL/g of catalyst) at room temperature or in alter-
native at 60 ◦C and stirred for 8 h. The catalyst was then recovered
by filtration and dried at 80 ◦C for 4 h.
3.1. Catalyst characterization
Compared to thermal and oxidative treatments, the water
extraction procedure possesses the undeniable advantage of assur-
To explore the effect of PVA removal on the particle size and
distribution, aberration-corrected scanning transmission electron
treated samples (Figs. 1–2 and Table 1).
STEM data collected on activated carbon supported gold NPs
revealed that the gradual removal of PVA resulted in a slight
increase in particle size, 2.9–3.4 nm (Table 1) as a consequence of
the reduced stability and particle aggregation.
2.3. Catalyst characterization
The progressive removal of PVA was also confirmed by HRTEM,
where the layer of the protective agent surrounding the particle is
clearly observable in the as-prepared sample (Fig. 3a) whereas it is
even less evident in the washed samples (Fig. 3b and c). A similar
trend was previously observed for Au/TiO2 catalysts where IR spec-
tra also supported these findings [21]. Unfortunately a quantitative
trend of the mean particle size was observed for Au NPs supported
on graphite. Washing at room temperature leads to a decrease
of the mean particle size (from 6.7 nm to 5.7 nm) as shown in
Table 1 and Fig. 2. Moreover, examining the particle size histograms
(Fig. 2b, d and f), a higher amount of nanoparticles smaller than
2 nm was observed after washing at room temperature. Such an
unexpected evidence might be ascribed to the presence of small
gold clusters in the as-prepared catalyst. These clusters are too
small to be detected with STEM, but they coalesce after wash-
ing and PVA removal leading to the formation of few nanometer
gold particles and to the apparent decrease in mean particle size.
Increasing the temperature of washing, the nanoparticles tend to
further aggregate (from 5.7 nm to 6.9 nm at room temperature and
60 ◦C, respectively).
The specimens for Transmission Electron Microscopy (TEM)
were prepared by dispersing the catalyst powder on TEM grids
coated with holey carbon film. They were examined by means of a
FEI Titan 80–300 electron microscope, operating at 80 kV equipped
with CEOS image spherical aberration corrector, Fischione model
3000 high angle annular dark field (HAADF) scanning transmission
electron microscopy (STEM) detector.
Metal content in the solution was checked by ICP analysis on
a Jobin Yvon JY24 verifying the quantitative loading of Au on the
support.
2.4. Catalytic tests
Glycerol
(0.3 M)
and
the
catalyst
(substrate/total
metal = 1000 mol mol−1
)
were mixed in distilled water (total
volume 10 mL) and 4 equivalent of NaOH. The reactor was pres-
surized at 300 kPa of nitrogen and the temperature sets to 50 ◦C.
Once this temperature was reached, the gas supply was switched
to oxygen and the monitoring of the reaction started. The reaction
was initiated by stirring. Samples were removed periodically and
analyzed by high-performance liquid chromatography (HPLC)
Please cite this article in press as: S. Campisi, et al., Metal nanoparticles on carbon based supports: The effect of the protective agent