Tuning the reduction power of supported gold nanoparticle photocatalysts for selective reductions by manipulating the wavelength of visible light irradiation

Article abstract of DOI:10.1039/c2cc17977f

Gold nanoparticles supported on CeO₂ were found to be efficient photocatalysts for three selective reductions of organic compounds at ambient temperatures, under irradiation of visible light; their reduction ability can be tuned by manipulating the irradiation wavelength.

Full text of DOI:10.1039/c2cc17977f

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Tuning the reduction power of supported gold nanoparticle photocatalysts
for selective reductions by manipulating the wavelength of visible light
irradiationw
Xuebin Ke, Sarina Sarina, Jian Zhao, Xingguang Zhang, Jin Chang and Huaiyong Zhu*
Received 21st December 2011, Accepted 15th February 2012
DOI: 10.1039/c2cc17977f
Gold nanoparticles supported on CeO2 were found to be efficient
photocatalysts for three selective reductions of organic compounds
at ambient temperatures, under irradiation of visible light; their
reduction ability can be tuned by manipulating the irradiation
wavelength.
Au-NPs on the CeO2 support (Au-NPs/CeO2), under visible
light: deoxygenation of epoxides to alkenes, reduction of
ketones to alcohols, and hydrogenation of azobenzene to
hydroazobenzene. These reduction reactions are important
in organic synthesis and biological chemistry.6
The Au-NP photocatalyst exhibited excellent activity for
these syntheses under visible light irradiation as shown in
Table 1. 19.7% of styrene oxide was converted to styrene in
16 h with a high selectivity above 87.5%. 31% of acetophenone
was transformed into benzyl ethanol with a selectivity of 99%.
40% of azobenzene was reduced to hydrazobenzene in 6 h with
a selectivity of 78%. These reactions were also conducted in the
dark with other experimental conditions unchanged—very low
conversions of the reactants were observed. This indicates that
the reduction reactions can be driven by visible light irradiation.
The reductions of the substrates with various substituted
functional groups were also investigated (see Table 1). The
results indicate that the Au-NP photocatalyst can reduce a wide
range of epoxides, azo compounds, and ketones with high photo-
catalytic efficiency, yielding the corresponding alkenes, hydrazo
compounds and alcohols, respectively. Higher conversions were
generally achieved for reactants with an aromatic ring than those
without the ring, possibly due to the stronger affinity of aromatic
reactants to the Au-NP surface. The photon efficiency of the
reduction reactions is relatively high. For example, the photon
efficiency was 1.5% of the Au-NPs/CeO2 for azobenzene hydro-
genation, exceeding that of the well-known TiO2 photocatalyst
under UV irradiation (efficiency o0.2%).2,7 The high photon
efficiency and product selectivity are important advantages for
practical applications.
The application of sunlight-driven photocatalytic process to
synthesize fine organic chemicals achieved some promising
progress. Titania (TiO2) and silica-supported titania are active
for photocatalytic oxidation of light alkanes and alkenes
under ultraviolet (UV) irradiation at ambient temperature.1
Furthermore, various methods have been developed to produce
visible light photocatalysts.2 Given that reduction is a major
process for the synthesis of organic fine chemicals, it will be of
great interest if photocatalytic reduction processes can be applied
to synthesize fine chemicals under visible light. However, the
synthesis of organic compounds via visible light photocatalytic
reduction is even more challenging than that via visible-light
photocatalytic oxidation. Currently there are a few sparse
reports on this topic, which focus on the reduction of nitro-
aromatic compounds using cadmium sulfide or modified titania
as photocatalysts.3
Recently, we found that the supported Au-NPs exhibit superior
performance in the photocatalytic reduction of nitro-aromatic
compounds to produce corresponding azo compounds under both
visible and UV irradiation.4 It highlights that such a reduction
process can be driven by sunlight. Au-NPs absorb visible light
mainly due to the surface plasmon resonance (SPR) effect.5 High
product selectivity can be expected as the photocatalytic reductions
are conducted at ambient temperature; the low reaction
temperature reduces the formation of by-products. Besides,
products that are unstable intermediates in the reaction under
heating (e.g. thermal reaction) can be obtained in this way.
In the present study, we investigated three typical reduction
processes on supported Au-NP photocatalysts, 3 wt% of
The size and dispersion of the Au-NPs should influence their
catalytic performance, which can be readily tuned by adjusting
the ratios of sodium borohydride to HAuCl4 and HAuCl4 to
the CeO2 support. Fig. 1 shows the typical transmission electron
microscopy (TEM) images of the catalysts, the mean sizes of the
Au-NPs in the three samples were estimated to be 2 nm, 5 nm
and 8 nm, respectively. The X-ray photoelectron spectroscopy
(XPS) spectra of the photocatalysts (Fig. 1d) indicate that gold
in the photocatalysts exists in the metallic state.
School of Chemistry, Physics and Mechanic Engineering,
Queensland University of Technology, Brisbane, Qld 4001, Australia.
E-mail: hy.zhu@qut.edu.au; Fax: +61 7 3138 1804;
Tel: +61 7 3138 1581
w Electronic supplementary information (ESI) available: Preparation
of gold catalysts, influence of particle size, light intensity, and irradia-
tion wavelength. See DOI: 10.1039/c2cc17977f
An absorption band at about 530 nm is observed in the
UV-vis spectra of the photocatalysts; this is the characteristic SPR
absorption by Au-NPs as CeO2 exhibits little absorption of light
with wavelengths longer than 400 nm (Fig. 1e). Photocatalysts of
c
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Chem. Commun., 2012, 48, 3509–3511 3509

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Table 1 Performance of three photocatalytic reductions
Photon
efficiency TONb Conv. Sel.
Substrate
Product
0.4
0.7
1.4
41
19.7
34.5
67.4
87.5
88.2
85.5
72
140
CO2, H2O
—
Dec.c
13.1
31
—
0.3
0.4
0.6
0.2
27
90.9
>99
>99
70
Fig. 1 TEM images of Au-NPs with different sizes and distributions
on CeO2 NPs. The length of the scale bars in the images is 10 nm. The
mean size of Au-NPs in image (a) is 2 nm; in image (b) is 5 nm; and in
image (c) is 8 nm. Panel (d) is the XPS spectra of 3% Au/CeO2. Panel
(e) is the UV-vis diffuse reflectance spectra of Au-NPs with different
sizes on CeO2.
65
85.3
27
41
(22%) decreased. This was similarly observed in the hydro-
genation of acetophenone where the conversion decreased to
9%. However, the reduction of styrene oxide could not take
place under such irradiation. Furthermore, when the cut-off
wavelength was below 600 nm, the reduction of azobenzene
still proceeded (20% of conversion), but acetophenone and
styrene oxide could not be reduced.
13
1.5
54
40
78
A relationship between the cut-off wavelength of the irradiation
and the reduction potentials of the reactants should exist. The
reduction potentials of azobenzene, acetophenone and styrene
oxide are À1.1 eV, À1.9 eV and À2.4 eV, respectively.1,9 Upon
correlating these reduction potentials with the reduction results
acquired under irradiation of different cut-off wavelengths, it can
be seen that a shorter cut-off wavelength is required to reduce a
compound with a more negative reduction potential. Conduction
electrons in the Au-NPs gain energy from the light absorption
and their distribution over the energy levels shifts to higher levels
in the 6sp band.4 The cut-off wavelength determines the energy of
the excited electrons as well as their distribution over the high
energy levels (Fig. 2). The shorter the wavelength is, the higher the
energy of the excited electrons is (Table S1, ESIw).
The electrons excited by the light with wavelength longer
than 600 nm are able to induce the reduction of azobenzene
(reduction potential À1.1 eV), but cannot induce the reduction of
more negative reduction potentials. Obviously the wavelength of
the light determines the reduction power of the photocatalysts.
This is rational from the view of thermodynamics that reductions
with more negative reductive potential could not proceed because
the reactant molecules could not gain the photo-excited electrons
at lower energy levels. This may imply that the reactant molecules
in the reduction with more negative reduction potentials can only
gain the excited electrons of Au-NPs at the higher energy levels
(Fig. 2), which are excited by light of shorter wavelength. As the
SPR absorption of the Au-NPs is very weak in the range longer
than 650 nm (Fig. 1e), the conversion of all the three reactants is
negligible (Table S1, ESIw). This confirms again that the SPR
absorption of Au-NPs is crucial for the catalytic activity.
2.4
129
62
21
80
78
0.2a
43.7
a
Reaction conditions: 0.05 g catalysts, 0.4 mmol substrate in 24 ml of
b
isopropanol. It stands for turnover number and was calculated by
the mole conversion of the reactant and mole gold content (2.84%,
c
determined by the energy dispersive X-ray spectrum). The reactant
decomposed to CO2 and H2O.
different sizes of Au-NPs—and thus contents of gold—were
also prepared. The light absorption by the sample with larger
Au-NPs shifts to a longer wavelength and has higher intensity
(Fig. 1e), indicating a stronger SPR effect.5 The strong light
absorption is important for the photocatalytic performance,
but the performance is also dependent on other factors,
particularly the specific surface area of the Au-NPs. The larger
the surface area is, the more active sites there are. The sample
with smaller Au-NPs has a larger specific surface area but less
intensive light absorption.8 The sample with a mean gold
particle size of 5 nm on CeO2 exhibits the highest photocatalytic
activity in reducing styrene oxide (Fig. S1, ESIw).
A very interesting finding is the correlation between the
irradiation wavelength and the reduction ability of the supported
Au-NPs (Table S1, ESIw). In experiments, glass filters were
employed to block the irradiation below a certain wavelength
(e.g. o550 nm), this wavelength is called cut-off wavelength.
When light with wavelength longer than 550 nm was used—by
filtering out wavelength shorter than 550 nm—the reduction
of azobenzene still proceeded even though the conversion
c
3510 Chem. Commun., 2012, 48, 3509–3511
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Given that the reductions were conducted in the solvent of
isopropanol that is a hydrogen-donor and the presence of KOH
enhances the abstraction of hydrogen from isopropanol,6c it is
possible that species of the hydrogen atom bonded to the
Au-NP surface form and react with the reactant molecules on
the Au-NPs,4 meanwhile the reactant molecules may gain
excited electrons from Au-NPs. The formation of Au-H species
is confirmed (ESIw). The electrons remain in the excited state for
up to 0.5–1 ps.10 Hence, it is possible that a small number
of excited electrons in Au-NPs gain sufficient energy to be
captured by reactant molecules adsorbed on the Au-NPs under
visible light irradiation, resulting in the photocatalysis.11
In summary, this study demonstrates that the Au-NP on
CeO2 is an effective visible light photocatalyst for the reductions
of organic compounds and its reduction power can be tuned by
manipulating the irradiation wavelength. Such photocatalytic
syntheses are sustainable and clean as production processes
utilize sunlight for chemical synthesis.
Fig. 2 Reduction potential of reduction reactions and schematic
band structure of supported Au-NPs with glass filters of different
cut-off wavelengths: (a) 420 nm; (b) 550 nm; (c) 600 nm.
In an attempt to clarify the influence of light irradiation
intensity, the irradiation intensity was reduced from 0.4 to 0.3
and 0.15
W , while other experiment conditions
cmÀ2
Notes and references
unchanged. We found that the conversion of styrene oxide
decreased from 19.7% to 9.6% and 5.2%, respectively. Similar
changes were observed for the reduction of the azoaromatics
and the ketones (Table S2, ESIw). These results further confirm
that the reduction reactions were driven by visible light.
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The Arrhenius equation was applied to derive the apparent
activation energies of the reductions via photocatalytic and
thermal catalytic processes. For one reduction reaction, the
difference between the activation energies of the two processes
indicates the contribution of light irradiation in reducing the
apparent activation energy. The results of three reactions are
shown in Fig. S2 (ESIw). Take acetophenone as an example,
the activation energy is 43.7 kJ molÀ1 for the photocatalytic
reduction and 68 kJ molÀ1 for thermal reduction. The difference
between the two activation energies (24.3 kJ molÀ1) is the
activation energy reduction by irradiation (Fig. S2, ESIw).
The relative reduction in the apparent activation energy due
to the light irradiation, expressed in percentage of the apparent
activation energy of the corresponding thermal process,
DEa/Ea (%), is also calculated for each reaction (Fig. S2, ESIw).
We found that DEa/Ea (%) is correlated with the values of the
reduction potentials in the order of styrene oxide, acetophenone
and azobenzene (Fig. 2). It means that only the energetic
electrons beyond the reduction potential (excited by light with
shorter wavelength) play a major role in activating reduction
reaction. These observations reveal that the excited electrons at
high energy levels in Au-NPs play a key role in the reactions.
Au-NPs were also loaded on TiO2 and ZrO2 and we found
that for reduction of styrene oxide the selectivity to produce
styrene is moderately lower than that of the Au-NP on CeO2
(Table S3, ESIw). It appears that the support materials have
limited impact on the conversion of the reductions.
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c
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Chem. Commun., 2012, 48, 3509–3511 3511