Reduction of nitroaromatic compounds on supported gold nanoparticles by visible and ultraviolet light

Article abstract of DOI:10.1002/anie.201003908

Shedding light: Nitroaromatic compounds on gold nanoparticles (3 wt %) supported on ZrO₂ can be reduced directly to the corresponding azo compounds when illuminated with visible light or ultraviolet light at 40 °C (see picture). The process occurs with high selectivity and at ambient temperature and pressure, and enables the selection of intermediates that are unstable in thermal reactions. Copyright

Full text of DOI:10.1002/anie.201003908

Angewandte
Chemie
DOI: 10.1002/anie.201003908
Photochemistry
Reduction of Nitroaromatic Compounds on Supported Gold
Nanoparticles by Visible and Ultraviolet Light**
Huaiyong Zhu,* Xuebin Ke, Xuzhuang Yang, Sarina Sarina, and Hongwei Liu
Photocatalytic processes have attracted significant interest
since the 1970s and are still very active research areas
today.[1,2] The degradation of organic pollutants and the
production of hydrogen and oxygen from water by using
semiconductor photocatalysts such as TiO2, ZnO, and CdS
have been extensively studied. However, few studies on
photocatalytic reactions under visible light with catalysts such
as gold nanoparticles (AuNPs) have been reported in the area
of organic chemistry. Although fine gold particles have been
used for centuries in stained glass windows, researchers have
only recently discovered two of the most significant proper-
ties of AuNPs.[3] Firstly, AuNPs can catalyze many reactions of
organic compounds at elevated temperatures, including the
oxidation of various substrates and the reduction of nitro-
benzene.[4–6] Secondly, AuNPs can strongly absorb visible light
because of the surface plasmon resonance (SPR) effect.[7–11]
This effect is the collective oscillation of conduction electrons,
which resonate with the electromagnetic field of the incident
light to result in a significant enhancement of the local
electromagnetic fields near the rough surfaces of the AuNPs.
As a result of the SPR effect, light absorption for typical
spherical AuNPs is generally observed between 520 and
550 nm,[5] which can generate excited electrons in the AuNPs
and also cause rapid heating.[12] Together, these two proper-
ties offer an interesting hypothesis: the molecules on the
AuNPs can be activated for reaction by their proximity to the
heated AuNPs and their interaction with the oscillating
electrons in the AuNPs. Thus, we may be able to drive
reactions on the AuNPs by using visible light at ambient
temperature. The AuNPs also exhibit significant ultraviolet
UV absorption that causes an interband excitation of
electrons from 5d to 6sp;[10,13,14] this excitation may also be
used to drive chemical reactions. Given that visible light
(wavelength 400–700 nm) and UV irradiation (wavelength <
400 nm) constitute around 43% and 4% of the solar energy
emitted by the sun, respectively,[15] a photocatalytic process
with supported AuNP photocatalysts has the potential to use
sunlight to efficiently drive chemical reactions. Light absorp-
tion by AuNPs is also a local effect that is limited to the gold
particles, so that the light heats only the AuNPs, which
generally account for a few percent of the overall catalyst
mass.[4–6,16,17] Therefore, such a process, if successfully real-
ized, can be conducted at ambient temperature; thus this
photocatalytic process can produce compounds that would
have been unstable intermediates in a thermal reaction at
high temperature.
Aromatic azo compounds are widely used in the produc-
tion of dyes, food additives, and pharmaceutical products.[5,18]
Currently, the synthesis of these compounds is often con-
ducted under high pressures and at high temperatures using
transition-metal reducing agents.[5,18] The by-products formed
from the reducing agent are harmful to the environment.[18]
Recently, it was reported that aromatic azo compounds could
be synthesized from the corresponding nitroaromatic com-
pounds through a two-step, one-pot reaction with catalysts
comprising AuNPs on TiO2 or CeO2 at 1008C or higher.[6]
Firstly, nitrobenzene was over-reduced to aniline on the gold
catalysts by hydrogen (9 bar). The reaction was continued by
flushing out the H2 and introducing O2 at 5 bar and 1008C to
oxidize aniline to azobenzene. Azobenzene was formed as an
intermediate in the thermal hydrogenation of nitrobenzene to
form aniline, but, in this case, the azobenzene was unstable
under the reaction conditions and was rapidly reduced to
aniline.[6,17] A subsequent oxidation step was thus required to
produce the target azobenzene. As photocatalytic reactions
are mostly conducted at ambient temperature and atmos-
pheric pressure,[1,2] many intermediates are stable under such
conditions and would not react further. If the direct reduction
of nitroaromatic compounds to their corresponding azo
aromatic compounds can be realized by a photocatalytic
process, the synthesis of aromatic azo compounds would be a
much more controlled, simplified, and greener process.
To test the possibility of driving the reduction of nitro-
benzene with light, AuNPs were supported on zirconia
powder by reducing HAuCl4 with NaBH4 in the presence of
ZrO2 powder (full details of the synthesis and character-
ization are given in the Experimental Section). Photocatalysts
with three gold loadings (1.5, 3.0, and 5.0wt% of the overall
catalyst mass) on the ZrO2 powder were prepared. TEM
images of these samples showed that the gold existed on ZrO2
[*] Prof. Dr. H. Y. Zhu, Dr. X. B. Ke, Prof. Dr. X. Z. Yang, S. Sarina,
Dr. H. W. Liu
Chemistry Discipline
Queensland University of Technology
Brisbane, Qld 4001 (Australia)
Fax: (+61)731-381-804
E-mail: hy.zhu@qut.edu.au
Prof. Dr. X. Z. Yang
College of Chemistry and Chemical Engineering
Inner Mongolia University
Huhhot, 010020 (P.R. China)
[**] We thank Prof. Jincai Zhao, Prof. Cheng Guo, and Dr. Xi Chen for
constructive discussions and Yi-Cheng Huang for her careful
revision of the manuscript. X.K. is indebted to the QUT for a Vice-
Chancellor’s Fellowship and X.Y. to the Australian Government and
the Cheung Kong Group for an Endeavour Research Fellowship. The
authors also gratefully acknowledge financial support from the
National Natural Science foundation of China (project no.
20920102034).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003908.
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as nanoparticles of about 6 nm in size (Figure 1). These
the reaction system was 408C because of the irradiation, and a
control experiment showed that a negligible reduction of
nitrobenzene occurred in the dark at 408C. No reaction was
observed in a blank experiment conducted in with ZrO2
powder instead of AuNPs under otherwise identical condi-
tions. Furthermore, when the light intensity was reduced from
0.30 WcmÀ2 to 0.15 WcmÀ2, 0.03 WcmÀ2, and 0.017 WcmÀ2
but all the other experimental conditions remained
unchanged, the conversion activity of nitrobenzene decreased
from 100% to 76.5%, 50%, and 30%, respectively. It was
thus evident that the reaction was driven by visible light.
Furthermore, the new photocatalyst also showed high con-
version activities in the highly selective reduction of other
nitroaromatic compounds to produce the corresponding azo
compounds (Table 1).
catalysts were used directly for photocatalytic reactions. The
catalystsꢀ performances in the reduction of nitroaromatic
compounds under incandescent light illumination (mainly
composed of visible light (see Figure S1 in the Supporting
Information) are shown in Table 1.
The photocatalytic reduction of nitrobenzene resulted in a
high conversion and a high selectivity for azobenzene when
light of wavelengths above 400 nm and an intensity of
0.30 WcmÀ2 (measured at the reaction system) were used.
The photonic efficiency of the reduction reaction could be
estimated from the radiation absorbed by the photocatalyst
(details of the measurement and calculation of the photonic
efficiency of the current study are given in Figure S1 in the
Supporting Information).[19] The reduction of nitrobenzene
using 3wt% AuNPs supported on ZrO2 proceeded with an
efficiency of 6.9%, which exceeded that of the well-known
photocatalytic reaction that uses a TiO2 catalyst under UV
irradiation (efficiency < 1.5%). The AuNP photocatalysts are
able to catalyze the reduction of 0.1molLÀ1 nitrobenzene (the
molar ratio of reacted nitrobenzene to gold in the photo-
catalyst is 197).
Figure 1. Characterization of the catalysts. a) TEM image of 3wt% Au/
ZrO2 (3wt% of gold nanoparticles on ZrO2); b) X-ray photoelectron
spectra (XPS) of 3 wt% Au/ZrO2; and c) UV/Vis diffuse reflectance
spectra of the ZrO2 support and 3wt% Au/ZrO2 photocatalysts.
The X-ray photoelectron spectroscopy (XPS) spectra of
the supported gold photocatalysts with different gold contents
all showed the same pattern. The binding energies of Au 4f7/2
and Au 4f5/2 electrons are 84.0 eV and 87.7 eV, respectively
(Figure 1b ). These values are identical to those for metallic
gold, and suggest that the photocatalysts exists in the metallic
state. Therefore, the absorption peak at approximately
520 nm in the UV/Vis spectrum of the gold supported on
ZrO2 (Figure 1c) can be attributed to a SPR absorption by the
AuNPs, as ZrO2 exhibited little absorption of light with
wavelengths longer than 330 nm. The band gap of ZrO2 is
about 5 eV.[20] These results further confirm the conclusion
that the AuNPs supported on ZrO2 were the photocatalysts
and the reduction of nitroaromatic compounds was driven by
visible light (further details on the choice of oxide support are
given in the Supporting Information).
Table 1: Reduction of nitroaromatic compounds in the presence of
supported metal nanoparticles.[a]
Reactant
Main product
Conv.
[%]
Sel.
[%]
Photonic
efficiency [%]
100[b]
100[c]
100[d]
58[d]
>99
95
6.9
7.3
3.6
2.1
3.6
56
>99
82
100[d]
The significant UV absorption of AuNPs (Figure 1c) can
also drive the photocatalytic reduction of nitrobenzene. The
conversion of nitrobenzene under UV irradiation with an
intensity of 0.017 WcmÀ2 was found to be 36%, which is
higher than that under visible light of the same intensity
(30%). This result demonstrates that the wavelength of the
irradiation can influence the reaction rate of the photo-
catalytic reduction.
As oxygen gas was released as an unforeseen by-product,
a deflated gas expansion bag was connected to the sealed
round-bottom flask in order to collect the gas released
throughout the photocatalytic reduction. The gas in the bag
was analyzed by using GC. The quantity of the released
oxygen was estimated from its volume, which was equivalent
to about (82 Æ 5)% of the amount derived from the quantity
of the reacted nitrobenzene. The release of oxygen was not
[a] Reduction reaction was conducted in an argon atmosphere at 408C
using 30 mL of isopropyl alcohol mixed with 0.3 mmol KOH, 3 mmol
nitrobenzene, and 100 mg catalyst. [b] Reaction time 5 h. [c] Reaction
time 3 h. [d] Reaction time 6 h.
After 5 hours, 100% of the nitrobenzene was reduced and the
product was found to contain more than 99% azobenzene.
The photocatalyst with a gold content of 3 wt% was found to
have the highest activity. Photocatalysts with a lower or
higher gold content (1.5 wt% or 5 wt%) clearly showed a
poorer performance (see Table in the Supporting Informa-
tion). TEM observations showed that a gold loading higher
than 3wt% led to aggregation of the NPs (data not shown).
This aggregation reduces the area of the AuNP surface on
which the catalytic reaction takes place. The temperature of
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observed in reduction reactions under thermal reaction
conditions.[6] The photocatalytic reduction evidently pro-
ceeded by a mechanism different to that of conventional
reduction or catalytic reduction in a thermal reaction process.
We also found that the choice of solvent substantially
affected the reduction activity of the AuNPs. Solvents such as
ethanol, isopropyl alcohol, benzyl alcohol, and hexyl alcohol
were used, but the best performance was achieved when
isopropyl alcohol was used. For example, when the reduction
reaction was carried out in isopropyl alcohol, only 30%
conversion and 60% selectivity were achieved with the 3wt%
Au/ZrO2 photocatalyst after 5 hours, when all the other
experimental conditions remained unchanged. It has been
reported that isopropyl alcohol can be oxidized to acetone in
a catalytic reaction with heating, thereby effecting the
simultaneous reduction of the compound that is dissolved in
the isopropyl alcohol.[21] If isopropyl alcohol did act as a
reducing agent, the reaction would be:
isopropyl alcohol was oxidized. In the second process (the
reaction after the first hour), the production of aromatic azo
compounds proceeded with species formed in the first
process, but did not rely on the oxidation of isopropyl alcohol.
The presence of KOH enhances the abstraction of a
hydrogen atom from isopropyl alcohol, which is a hydrogen-
donor solvent.[22] Indeed, hydrogen atoms bound to the AuNP
surface (H–AuNP) have been detected from supported
AuNPs in the presence of isopropyl alcohol.[23] The Au H
À
bond is relatively stable, hence, H–AuNP species can form on
the surface of AuNPs in the reaction system of the present
study. These H–AuNP species are capable of combining with
À
the oxygen atoms of N O bonds to yield HO–AuNP species.
The electrons that are excited when the AUNPs absorb light
can provide the activation energy that is required for the
À
cleavage of the N O bond. The HO–AuNP species, which
have been previously reported,[24] can release oxygen gas and
transform into H–AuNP species that can be recycled in the
subsequent reaction process (see Figure 3). We therefore
tentatively propose the following elementary reactions that
may be involved in this photocatalytic process:
Careful analysis of the product by GC confirmed the
formation of acetone during the reaction. However, the
amount of acetone was only one eighth of the quantity
calculated from Equation (1). It was also noted that most of
the acetone in the reaction system was formed in the first hour
of the reaction. Thereafter, the amount of acetone remained
almost constant even though the reaction proceeded further
to produce more azobenzene (Figure 2). Hence, isopropyl
alcohol acted only as an initiator and did not participate in
any of the subsequent reactions. Clearly, the photocatalytic
reaction, especially after the first hour, did not proceed
according to Equation (1). This reaction should involve two
processes; in the first process (the reaction in the first hour),
As shown in Figure 2, the azoxybenzene/azobenzene ratio
was approximately 2:1 after the first hour of reaction. Thus,
the overall reaction of the first process can be derived by
summing the above elementary reactions [(2 ꢁ Eq. (2)) + (4 ꢁ
Eq. (3)) + (3 ꢁ Eq. (4)) + Eq. (5)] to give:
Figure 2. Time-conversion plot for nitrobenzene reduction and acetone
formation with 3wt% Au/ZrO2 and irradiation intensity of 0.3 WcmÀ2
The reduction reaction was conducted in an argon atmosphere at
408C using 30 mL isopropyl alcohol with 0.3 mmol KOH, 3 mmol
substrate, and 100 mg catalyst. The axis on the right-hand side refers
to the acetone content, which is below 1%.
.
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saturated with H–AuNP and HO–AuNP species, and the
acetone content of remains unchanged. It follows that the
amount of acetone, which is the product of the first reaction
process, should be directly proportional to the quantity of
AuNPs in the reaction system. Indeed, when the photocatalyst
quantity, which is proportional to the surface area of AuNPs,
was increased from 100 mg to 200 mg, the content of acetone
in the reaction system doubled after one hour of reaction.
According to Figure 3, the H–AuNP species play a pivotal
role in this photocatalytic reaction. It is, therefore, expected
that the removal of the species would prohibit the reaction.
2,2’,6,6’-tetramethylpiperidine N-oxyl (TEMPO) is a hydro-
gen abstractor and can abstract hydrogen atoms from the
surface of gold to form hydroxylamine.[26] When 200 mg of
TEMPO was added to the reaction system with 100 mg of
3wt% Au/ZrO2 photocatalyst, almost no reduction of nitro-
benzene occurred. We did not detect any products or
intermediates of the reactions between TEMPO and reactant.
This result confirms the existence and role of H–AuNP
species in the systems in the present study. To further verify
the important role of the H–AuNP and HO–AuNP species,
after 5 hours of reaction, 3 mmol of fresh nitrobenzene was
injected into a sealed reaction system in which 3 mmol
nitrobenzene had already been converted into azobenzene.
Two hours later, more azobenzene and azoxybenzene formed,
with a negligible increase in the amount of acetone. Evidently,
the conversion of the injected nitrobenzene was mainly due to
the H–AuNP and HO–AuNP species.
Figure 3. Mechanism for the photocatalytic reduction of nitroaromatic
À
compounds. H–AuNP reacts with the N O bonds to produce HO–
AuNP species, which subsequently decompose to produce oxygen
molecules and H–AuNP species.
In the second process, the oxidation of isopropyl alcohol
was negligible and a large amount of azoxybenzene formed in
the first process (Figure 2) was reduced. Hence, we exclude
the reaction in Equation (2) and sum up the elementary
reactions [(2 ꢁ Eq. (3)) + Eq. (4) + (2 ꢁ Eq. (5))] to give:
The above equation suggests that an intramolecular
oxidation/reduction reaction of nitrobenzene and azoxyben-
zene took place in the second stage of the reaction; the
oxygen atoms were oxidized from an oxidation state of À2 to
oxygen molecules (oxidation state of 0), and the nitrogen
atoms were reduced from an oxidation state of + 3 to a state
of À1. By summing together the reactions in Equations (6)
and (7), the reaction for the overall photocatalytic process is:
The role of the H–AuNP and HO–AuNP surface species
provides an explanation for the lower activity of the sample
with larger AuNPs (obtained by heating the 3wt% AuNPs on
ZrO2). When the surface area was reduced, the number of
surface species also reduced, thus resulting in poorer perfor-
mance. It is also known that AuNPs smaller than 2 nm exhibit
a very weak SPR effect.[27] The mean size of the AuNPs
(3wt%) on ZrO2 was approximately 6 nm; the AuNPs thus
have a large surface to accommodate the surface species and
good SPR light absorption. Hence the photocatalyst exhibited
relatively high activity.
À
The key step of the reduction is to break the N O bonds.
À
Electrophilic N O bond cleavage was realized by the H–
AuNP species, and occurred preferentially in the presence of
excited electrons provided by the illuminated AuNPs. The
absorption of visible light by AuNPs through the SPR effect
caused changes in the electron distribution over the energy
levels. The 6sp electrons of gold gained energy through SPR
absorption and migrated to higher energy levels[9,11,13,14] (left
panel, Scheme in the Supporting Information). UV irradi-
ation can also generate energetic electrons by an interband
excitation of electrons from 5d to 6sp (right panel, Scheme in
the Supporting Information).
The azobenzene/acetone ratio (2:1) and the amount of
oxygen released (87.5% of amount of the reacted nitro-
benzene) in this equation are consistent with the experimen-
tal results ((82 Æ 5)%). Azoxybenzene, which is produced by
the removal of a single oxygen atom from the dimeric
structure of nitrosobenzene, was also detected in our experi-
ments (the azoxybenzene content during the reaction is
shown in Figure 2). Azobenzene was produced by loss of the
À
oxygen atom of the N O bond in azoxybenzene [Eq. (5)].
The oscillating electrons interact strongly with the elec-
trophilic nitro groups of the nitrobenzene molecules, and
The product of reactions [Eqs. (2) and (3)] and the dimeric
structure of nitrosobenzene have also been previously
reported.[25]
À
assist the cleavage of the N O bonds by H–AuNP species on
the AuNPs (Figure 3). Increasing the irradiation intensity
produces more oscillating electrons. To verify the important
role of the oscillating electrons further, the light irradiation
was turned off after 1.5 hours of reaction time. The reaction
did not proceed further as the H–AuNP species could not
According to Equation (2), the oxidation of isopropyl
alcohol results in the H–AuNP species. After the initial
reaction stage, the oxidation of isopropanol should cease
when the surface of AuNPs in the reaction system becomes
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À
break the N O bonds without the oscillating electrons (which
are not produced in the dark), although there were sufficient
H–AuNP species.
Keywords: azo compounds · nanoparticles · photochemistry ·
reduction · surface plasmon resonance
.
The most outstanding feature of the new photocatalysts is
their moderate ability in catalyzing redox reactions. The well-
known TiO2 semiconductor photocatalyst has a high oxida-
tion potential (2.7 V).[4] These catalysts generally exhibit poor
selectivity, thus making them a poor choice for catalytic
synthesis of fine chemicals. It is also difficult to adjust the
oxidation potential of the semiconductor photocatalysts in
order to tune their selectivity. In contrast, the moderate
catalyzing properties of the supported AuNP photocatalysts,
combined with the ability to conduct photocatalytic reduc-
tions at lower temperature and pressure than those for
thermal reactions, have enabled us to select an unstable
intermediate of a thermal reaction as the product, such as in
the case of aromatic azo compounds. This discovery reveals a
new class of useful catalytic processes for fine chemical
production. These processes are greener than thermal pro-
cesses, have the potential to utilize solar energy, and can be
applied in temperature-sensitive synthesis.
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Experimental Section
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mixture under vigorous stirring for 30 min. An aqueous solution of
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Activity test: The reduction of nitrobenzene was conducted in an
argon atmosphere at 408C unless otherwise specified. Nitrobenzene
(3 mmol) was dissolved in isopropyl alcohol (30 mL), and a solution
of KOH in isopropyl alcohol (0.1m, 3 mL) and the catalyst (100 mg)
were added. The mixture was stirred magnetically during reaction and
illuminated with incandescent light or UV light. The gaseous samples
were analyzed using a Shimadzu GC-2014 GC with 5 A molecular
sieve column and the liquid products were analyzed using an Agilent
HP-6890 GC with a DB-Wax column.
Characterization: TEM images were recorded with a Philips
CM200 transmission electron microscope employing an accelerating
voltage of 200 kV. The specimens were fine powders deposited onto a
copper microgrid coated with a holey carbon film. The composition of
some samples was determined by using the energy-dispersive X-ray
spectroscopy attachment of an FEI Quanta 200 scanning electron
microscope. The diffuse reflectance UV/Vis spectra of the samples
were recorded on a Cary 5000 UV–Vis–NIR Spectrophotometer.
Received: June 28, 2010
Revised: September 21, 2010
Published online: November 4, 2010
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