Formation and Catalysis of Gold Nanoparticles Generated through the Physical Mixing of AuCl(PPh3) and Oxides

Article abstract of DOI:10.1055/s-0035-1561200

AuCl(PPh₃) was mixed with various kinds of inorganic supports with a mortar. Thermal treatment of the mixture resulted in the formation of gold nanoparticles, particularly when NaY zeolite and CeO₂ were employed as the supports for g

Full text of DOI:10.1055/s-0035-1561200

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Formation and Catalysis of Gold Nanoparticles Generated through
the Physical Mixing of AuCl(PPh3) and Oxides
Kazu Okumura*
Department of Applied Chemistry, Graduate School of
Engineering, Kogakuin University, 2665-1, Nakano-machi,
Hachioji, Tokyo, 192-0015, Japan
okmr@cc.kogakuin.ac.jp
Received: 30.10.2015
Accepted after revision: 24.12.2015
Published online: 18.01.2016
tage of the AuCl(PPh3) is that the complex is stable in air,
readily available; therefore widely utilized as the precursor
for the synthesis of gold(I) complexes5 and Au NPs.6–8
Among employed supports, we focused on NaY zeolite and
CeO2 as the support for gold in this study.
NaY (Tosoh, HSZ-320NAA), CeO2 (JRC-CEO-2), SiO2
(fumed silica, Sigma), Al2O3 (JRC-ALO-3), ZrO2 (JRC-ZRO-6),
TiO2 (JRC-TIO-11), and activated carbon (AC, Wako Co.)
were employed as the support of gold. The CeO2, Al2O3,
ZrO2, and TiO2 supports were reference catalysts supplied
from the Catalysis Society of Japan.9 Under typical condi-
tions, AuCl(PPh3) (Aldrich) and supports were ground in a
mortar for 0.5 hours. The loading of gold was 3 wt%. The
mixture was thermally treated in the stream of Ar, 6% H2 di-
luted with Ar or O2 for 0.5 hours. The flowing rate of gas
was fixed at 30 mL min–1. The obtained solid was subjected
to the characterization and catalytic reaction.
Figure 1 (a) shows X-ray diffraction (XRD) patterns of
gold loaded on different kinds of supports other than zeo-
lites thermally treated at 773 K in 6% H2. Reflections assign-
able to the Au(111) and Au(200) appeared at 2θ = 37.8 ° and
44.0 °, respectively. It can be seen in the figure that the in-
tensity and broadness of the reflections changes depending
on the kind of supports. Namely, sharp reflections appeared
on gold loaded on AC and SiO2. On the other hand, the in-
tensity of these peaks were much smaller over gold loaded
on Al2O3, TiO2, ZrO2, and CeO2, which had basic character. In
particular, the intensity of the diffraction was the smallest
and broader in Au/CeO2, suggesting gold particles were well
dispersed on the CeO2 surface because the broadness of the
diffraction could be correlated with the particle size.10 The
possible effect of CeO2 for the dispersion of Au NPs might be
that the vacant sites of CeO2 played a crucial role for the dis-
persion of gold as has been evidenced by the DFT analysis.11
DOI: 10.1055/s-0035-1561200; Art ID: st-2015-u0854-c
Abstract AuCl(PPh3) was mixed with various kinds of inorganic sup-
ports with a mortar. Thermal treatment of the mixture resulted in the
formation of gold nanoparticles, particularly when NaY zeolite and
CeO2 were employed as the supports for gold. Catalytic performance in
the aerobic oxidation of benzyl alcohol was in good correlation with the
degree of the dispersion of gold.
Key words gold, nanoparticle, zeolite, alcohol, oxidation
Recently, much attention has been paid to the gold-
loaded catalysts because gold nanoparticles have been
found to exhibit high catalytic activity in various reaction
including oxidation of CO and alcohols.1 Several preparation
methods of supported gold catalysts have been developed
such as deposition–precipitation,2 ion exchange,3 and
chemical vapor deposition (CVD).4 Among these methods,
CVD seems to be one of the most attractive ones because
highly dispersed gold nanoparticles (NPs) are feasibly ob-
tained on various supports. As a matter of fact, Au NPs are
readily obtained through solid grinding of Me2Au(acac) and
active carbon or metal organic framework.4b The catalysts
were applied to the oxidation of alcohols and glucose. De-
spite the feasibleness of the solid gridding, kinds of gold
complexes suitable for CVD are limited. In addition, the
gold complexes are unstable in air in many cases. From in-
dustrial and environmental points of view, finding of a
readily available and stable gold complexe as the precursor
of Au NPs is highly desirable. Here, we report that Au NPs
are obtained just by mixing of the chloro(triphenylphos-
phine)gold(I) [AuCl(PPh3)] and supports, followed by ther-
mal treatment in the atmosphere of hydrogen. The advan-
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heating the sample at 773 K, suggesting the gold(I) was re-
duced to give gold(0) in agreement with the EXAFS data.
Figure 1 XRD patterns of (a) Au loaded on different kinds of supports
other than zeolites. The samples were treated at 773 K for 30 min in a
6% H2 flow prior to the measurement. (b) Au loaded on zeolites (ZSM-5
and Y zeolite) thermally treated at 773 K under different atmosphere.
Figure 2 (a) Au L3-edge EXAFS Fourier transformed and (b) FTIR spec-
tra of 3 wt% Au loaded on NaY treated at different temperatures in the
atmosphere of 6% H2.
Figure 1 (b) shows the XRD patterns of gold mixed with
various kinds of zeolites. Intensity of the reflections assign-
able to the Au(111) and Au(200) planes were the smallest
when NaY was employed as the support for gold, although
the reflection from Y-type zeolite partially overlapped the
region. The XRD patterns of Au/NaY treated in the atmo-
sphere of Ar and O2 were included in Figure 1 (b). Intensity
of the reflections arising from Au(111) and Au(200) planes
were smallest when the Au/NaY was thermally treated in
the atmosphere of 6% H2. The fact agrees with the previous
studies on Au/USY zeolites prepared in the liquid phase
where the Au NPs with diameters of ca. 2 nm were obtained
through the thermal treatment in the atmosphere of H2.12
In order to monitor the formation process of Au NPs on
NaY zeolite, Au L3-edge EXAFS and FTIR spectra of the
Au/NaY were measured (Figure 2). In the EXAFS of the 473
K treated sample the small peak appeared at 0.20 nm to-
gether with the large Au–Au bond at 0.26 nm (Figure 2, a).
The curve fitting analysis revealed that the small peak
could be assigned to the Au–P and Au–Au bonds, respec-
tively, while the Au–Cl bond was not detected (Table S1). On
further increase in the treatment temperature, intensity of
the peak assignable to the nearest neighboring Au–Au bond
increased (0.26 nm, phase shift uncorrected). Intensity of
the Au–Au bond of the 673–873 K treated samples almost
agreed with that of gold foil implying the formation of
gold(0) was almost complete at 673 K. On the other hand,
the two peaks assignable to the stretching vibration modes
of the benzene ring were observed between 1420 and 1480
cm–1 in the IR spectrum of AuCl(PPh3) (Figure 2, b). A simi-
lar spectrum was observed in the as-prepared sample with
physical mixing, although the peak position slightly shifted
to the higher wavenumber. Intensity of the two peaks grad-
ually decreased with increase in the temperature and van-
ished at 773 K. The change suggested that the Ph3P was
completely removed from Au/NaY at 773 K. At the same
time, the color of the solid changed from white to red on
Figure 3 (a) shows the transmission electron micro-
scope (TEM) image of Au/NaY zeolite which was treated in
6% H2 at 773 K. Formation of Au NPs were seen in the sam-
ple with an average diameter of 3.8 nm with narrow distri-
bution of the diameter (Figure 3, c). The TEM image directly
shows that the formation of Au NPs is achieved through the
use of AuCl(PPh3) as the gold source. Figure 3 (b) shows the
enlarged TEM image of Figure 3 (a). The Au NPs were pref-
erentially deposited at the surface (edge), suggesting the
Au NPs were preferentially located on the external surface
of NaY zeolite. This may be due to the bulkiness of the
AuCl(PPh3); the Au complex could not enter into the micro-
pore of NaY zeolite when it was vaporized in the course of
thermal treatment. In order to obtain insight into the
mechanism for the formation of Au NPs, the 473 K treated
AuCl(PPh3)/NaY in 6% H2 was rinsed with water, followed by
filtration. The analysis of the filtrate by inductively coupled
plasma revealed the dissolution of Na+. The amount of Na+
detected in the filtrate was equimolar to that of gold pres-
ent in Au/NaY, suggesting that the reaction between Cl– in
AuCl(PPh3) and Na+ of NaY took place at 473 K. Therefore, it
could be assumed that the formation of Au NPs proceeded
via the steps given in Equation 1.
Probably, the first step (1) took place at 473 K consider-
ing that the temperature was close to the melting point of
AuCl(PPh3) (493 K). In agreement with the assumption, the
curve-fitting analysis revealed that the 473 K treated sam-
ple could be fitted by the combination of Au–P and Au–Au
bond, while the Au–Cl was not detected as described earlier
(Table S1). The absence of the Au–Cl bond suggested the
formation of Au NPs proceeded after the reaction between
AuCl(PPh3) + NaY
Au(PPh3)Y + 1/2H2
Au(PPh3)Y + NaCl
Au0-HY + PPh3
(1)
(2)
Equation 1
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of the concentration of H2 in the range of 6–100%. The effect
of H2 for the formation of Au NPs was not clearly under-
stood at this stage. But one hypothesis was that H2 promot-
ed the reduction of gold(I) to gold(0) at lower temperature
compared to those treated in Ar and O2, so that the coales-
cence of gold(0) was suppressed in the atmosphere of H2.
Catalytic performance of Au/CeO2 is displayed in Figure 4
(b). The highest conversion of BzOH was obtained when
Au/CeO2 was treated in 6% H2 at 773 K in a similar manner
to the Au/NaY. Moreover, the activity was higher than those
treated in O2 and Ar, similarly to the case of Au/NaY zeolite.
Figure 4 Dependence of the conversion of benzyl alcohol on the tem-
perature for thermal treatment of (a) Au/NaY and (b) Au/CeO2 treated
in different atmosphere. Reagents and conditions: BzOH (1 mmol),
K2CO3 (1 mmol), toluene (5 mL), catalyst [1.5×10–3 mmol (Au)], 323 K,
30 min, air.
Figure 3 (a, b) TEM images of Au/NaY thermally treated in 6% H2 at
773 K; (c) distribution of the diameter of Au NPs.
Cl– of the gold complex and the Na+ present on the external
surface of zeolite.
In conclusion, we found that the Au NPs were readily
obtained by mixing the AuCl(PPh3) and various kinds of
supports, followed by thermal treatment. Among employed
supports, the NaY and CeO2 seemed to be most promising
because high catalytic activity could be obtained in the aer-
obic oxidation of benzyl alcohol.
Finally, oxidation of benzyl alcohol (BzOH) was carried
out over Au/NaY and Au/CeO2. The reaction was performed
at 323 K in the atmosphere of air. Benzaldehyde was ob-
tained as the primary product (selectivity: ca. 90%) togeth-
er with a small amount of benzyl benzoate and dibenzyl-
ether. The possible role of K2CO3 might be the suppression
of the inhibition caused by the formation of benzoic acid,13
because benzoic acid was not detected in the analysis with
gas chromatography.
Figure 4 (a) shows the dependence of the conversion of
BzOH on the thermal treatment temperature of Au/NaY.
The conversion of BzOH reached maximum when the sam-
ple was treated at 773 K in 6% H2. Probably, the highest ac-
tivity was attained as a result of the completion of the re-
moval of the PPh3 ligand and the generation of the gold(0)
as proved by EXAFS and FTIR spectroscopy. Data on the con-
version of BzOH over the Au/NaY treated in the stream of Ar
and O2 are included in the figure. The highest conversion of
BzOH over Au/NaY treated in Ar and O2 were much lower
than that of Au/NaY treated in 6% H2. The fact coincided
with the data of XRD patterns in that the intensity of the
reflections from gold(0) were the smallest when Au/NaY
was treated in 6% H2. We have carried out the reaction over
the Au/NaY by varying the concentration of H2 for the ther-
mal treatment. But the conversion was almost independent
Acknowledgment
Au L3-edge XAFS data were obtained from synchrotron radiation ex-
periments performed at the BL01B1 station by the approval of the Ja-
pan Synchrotron Radiation Research Institute (JASRI, SPring-8)
(Proposal No. 2014B1064, 2015B1068).
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0035-1561200.
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