Visible light-driven selective hydrogenation of unsaturated aromatics in an aqueous solution by direct photocatalysis of Au nanoparticles

Article abstract of DOI:10.1039/c7cy02291c

Selective hydrogenation of various chemical bonds, such as CC, CC, CO, NO, and CN, is efficiently driven by visible light over a supported gold nanoparticle (AuNP) photocatalyst under mild reaction conditions. The reaction system exhibits high substituent tolerance and tunable selectivity by light wavelength. Density functional theory (DFT) calculations demonstrated a strong chemisorption between the reactant molecule and metal resulting in hybridized orbitals. It is proposed that direct photoexcitation between hybridized orbitals is the main driving force of the hydrogenation reaction. The hydrogenation pathway is investigated by the isotope tracking technique. We revealed the cooperation of water and formic acid (FA) as a hydrogen source and the hydrogenation route through Au-H species on the AuNP surface.

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Visible light driven selective hydrogenation of unsaturated
aromatics in aqueous solution by direct photocatalysis of Au
nanoparticle
Received 00th January 20xx,
Accepted 00th January 20xx
Yiming Huang,a Zhe Liu,a Guoping Gao,a Qi Xiao,a, b Wayde Martens,a Aijun Du,a Sarina Sarina,*a
Cheng Guo,c and Huaiyong Zhua
DOI: 10.1039/x0xx00000x
www.rsc.org/
Abstract: Selective hydrogenation of various chemical bonds, C=C, C≡C, C=O, N=O and C=N are efficiently driven by visible
light over supported gold nanoparticle (AuNP) photocatalyst under mild reaction conditions. The reaction system exhibits
high substituent tolerance and tunable selectivity by light wavelength. Density functional theory (DFT) calculation
demonstrated a strong chemisorption between reactant molecule and metal resulting in a hybridized orbitals. It is
proposed that direct photoexcitation between the hybridized orbitals is the main driving force of the hydrogenation
reaction. The hydrogenation pathway is investigated with isotope tracking technique. We reveal the cooperation of water
and formic acid (FA) as hydrogen source and its hydrogenation route through Au-H species on the AuNP surface.
100oC).15 Elevated temperature in traditional hydrogenation
may accelerate undesired side reactions, such as
hydrogenation of the aromatic rings of the reactants, wasting
Introduction
Photocatalysis using plasmonic metal (Au, Ag, Cu)
nanoparticles (NPs) has attracted much attention because it
directly utilize solar power and usually associate with mild
reaction conditions.1-8 The NPs of these metal strongly absorb
visible light via localized surface plasmson resonance (LSPR)
effect which is the light-induced collective oscillation of the
metal conduction electrons, established when the incident
light frequency is resonant with the frequency of metal
electron oscillation in response to the restoring force of the
positive nuclei, it results in photoexcitation of electrons.9-11
Chemical transformations can thusly be mediated due to the
direct charge excitation within the metal-adsorbed organic
molecular system.12 Linic et al proposed the detailed theory
that the strong chemisorption between metal NPs and organic
reactant leads to the hybridized orbitals. The light irradiation
can direct excite electrons between those orbitals to trigger
chemical transformations.13,14
the reduction agent resulting in poor economic viability.16,17 In
respect to moderate reaction condition, various homogeneous
or heterogeneous transition metal based catalysts including
gold, nickel, ruthenium, cobalt, copper or palladium are widely
employed in the hydrogenation of unsaturated bonds of C=C,
C≡C, C=O, N=O and C=N toward benign processes.18-21 Yet,
challenges remain in energy efficiency, product selectivity and
catalyst recyclability.22 Thus, the hydrogenation reactions can
be greatly promoted by visible light driven mild photocatalysis.
Meanwhile, the high corrosivity of hydrogen gas at high
pressure and temperature causes hydrogen embrittlement of
pressurized metal reactors resulting high safety risks.23,24 In
contrast, with other reducing agent such as formic acid (FA),
the necessity of high pressure vessels as well as corresponding
risks can be avoided.25 FA is an environmentally green liquid
reducing agent readily produced from biomass, being a
convenient hydrogen donor, it only generates carbon dioxide
and water as oxidized products.26 Commercially supplied FA is
always an aqueous solution. In organic synthesis, however,
water is not a widely used solvent despite its low cost and low
environmental toxicity.27,28 Hence, there have been limited
reports involving aqueous solution of FA in organic
hydrogenation reactions to the best of our knowledge.29-31
In this work, we developed an environmental friendly
selective hydrogenation system for unsaturated aromatics, the
reaction is driven by a renewable energy source visible light
irradiation, FA aqueous solution is employed as a green
hydrogen source and the reaction is taken place in water
solvent. The supported AuNP photocatalyst is prepared by
impregnation-reduction method. Five representative types of
The selective hydrogenation of unsaturated organics is a
fundamental reaction in organic synthesis that traditionally
attained with the assistance of pressurized hydrogen gas under
high pressures, and elevated reaction temperatures (over
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hydrogenation reactions were investigated under mild photocatalytic system is demonstrated in Table 1. The
temperature in aqueous FA solution without other additives. irradiation promotes the hydrogenation with excellent
The reaction mechanism is investigated by isotope study tolerance to various functional groups, especially reducible
where we found water plays more role than solvent but a substituents such as ketones, aldehydes and alkenes.
hydrogen source cooperated with FA.
Table 1. Performance of Au@ZrO2 catalyst for five hydrogenation reactions. The red
numbers are the product yield under visible light irradiation, and the black numbers in
the parentheses are the product yield for the control reaction in the dark.
Results and Discussion
Hydrogenation of Aromatic Olefins[a]
DFT calculation is conducted using nitrobenzene as modelling
molecule to simulate the adsorption of reactant onto AuNP
surface, as shown in Figure 1a, the possible bonding between
nitrobenzene and an isolated Au cluster is 3.836 Å which
suggests
a low affinity. On the other hand, a strong
chemisorption was observed between nitrobenzene molecule
and ZrO2 supported AuNP as indicated in Figure 1b. The
nitrobenzene molecule bond with Au through two O-Au bonds
with around 2.1 Å bond length, the adsorption energy was
calculated to be -0.802 eV. Such strong bonding results in an
organic-metal complex on the AuNP surface and hybridization
of their orbitals to for bonding and antibonding states. In this
study, the HOMO-LUMO gap of gas-phase nitrobenzene
molecule is calculated to be 3.25 eV and that of isolated AuNP
is 0.36 eV, however the HOMO-LUMO gap for nitrobenzene-Au
complexes was found remarkably reduced to be 0.18 eV which
is lower than both isolated AuNP and nitrobenzene.
Consequently, the direct resonant photoexcitation of electron
take place between the hybridized bonding and antibonding
states.13 This direct photoexcitation is the main reason for the
enhanced reaction activity under light irradiation.
Hydrogenation of Aromatic Nitro-compounds
Hydrogenation of Aromatic Aldehydes[a]
Figure 1. Chemisorption of nitrobenzene onto Au cluster (a) and ZrO2 supported Au NP
(b) simulated using DFT methods.
The photocatalytic hydrogenation reaction system was
tested with five types of unsaturated aromatics, the catalytic
activities of model reactions as well as control reactions,
presented with product yields, are shown in Table 1. For
nitrobenzene hydrogenation, the reaction under 0.5 W/cm2
light irradiation results in 98% yield whereas 13% in the dark.
Similarly results were observed with hydrogenation of styrene,
benzaldehyde, phenylacetylene and benzalaniline. The
remarkable difference in the yields between the reactions
under irradiation and in the dark demonstrates the
contribution of photocatalysis. Such contribution is
quantitatively reflected by the 73% decrease of apparent
activation energy between light irradiated styrene
hydrogenation and that of reaction in dark, 35.3% decrease
was observed for the hydrogenation of nitrobenzene (Figure
S1). The broad functional group tolerance of the new
Hydrogenation of Aromatic Alkyne[d]
Hydrogenation of Aromatic Imine[a]
Reaction conditions: 50 mg catalysts, 1 mmol reactant, 2 mL of 85% FA mixed
with 2 mL deionized water as solvent, light intensity 0.5 W/cm2, 40°C, 1 atm
argon, reaction time 8 h. [a] reaction time 16 h. [b] 80°C, reaction time 24 h. [c]
UV light with peak wavelength at 365 nm, 24 h. [d] 60°C, 16 h. [e] combined yield
of both styrene and ethylbenzene. The products were identified by mass
spectrometry (MS) and yields were measured by gas chromatography (GC) using
external standard.
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It is worth noting that FA itself, having a carbonyl group, reaction system, phenylacetylene was reduced to styrene and
should be active for the condensation with amines and alkenes ethylbenzene along with side product of acetophenone, which
at elevated temperatures (over 70 °C).32 The low operating is attributed to the hydration of phenylacetylene in the
temperature allows our photocatalytic reaction system to presence of a hydroxyl group.34 As shown in Table 2, increasing
effectively avoid such side reactions and achieving water/FA ratio can significantly enhance the yields of reductive
extraordinary chemoselectivity.
products, while the production of acetophenone is inhibited.
Compared to aromatic aldehyde (Table 1, entry 21), we In the cases of benzaldehyde and styrene hydrogenation
found a poor yield (17%) for aliphatic aldehyde at 40 °C (Table S2), increasing water ratio can remarkably promote the
whereas negligible activity was observed in the dark (Table 1, reaction rate. These results imply a role more than solvent for
entry 27). The hydrogenation of aliphatic aldehyde demands water. Prior to the mechanism investigation, the Au@ZrO2
greater reduction power since the reduction potential of photocatalyst was characterized. Figure 2a indicates 3 wt%
aliphatic aldehydes is generally more negative than that of AuNPs, with a mean size of 7 nm (Figure 2d), are well
aromatic aldehydes. For example, the reduction potential of dispersed on the ZrO2 surface and predominantly exhibit
acetaldehyde is -1.7 eV and that of benzaldehyde is -1.36 eV crystal face (111) (Figure 2b). Light absorption peak of
(Table S1). When the hydrogenation of aliphatic aldehyde was Au@ZrO2 at 520 nm (Figure 2c) is attributed to the LSPR effect
conducted under UV irradiation at 40 °C a much higher yield of of AuNPs as ZrO2 support exhibits negligible visible light
53% was achieved. The energy of UV photons is greater than absorption. AuNPs also absorb UV light because electrons of
that of visible photons, meaning the UV photons are able to metal can be directly excited by photons from ground state to
generate photoexcited electrons with higher energies than the high energy states.
ones generated by LSPR absorption of Au NPs in the visible
range. These higher energy electrons are capable of reducing
compounds with more negative reduction potentials.33 In
addition, when the reaction temperature was raised to 80 °C
and the reaction was prolonged to 24 h, the yields for visible
light irradiated reaction and reaction in the dark were 42% and
4%, respectively. Therefore, we conclude that AuNPs can
combine photonic energy (light) and thermal energy (heat or
IR irradiation) because metals have a continuous electron
energy level. Above results reveal an important feature that
the reduction power of the reported system is tuneable by
regulating the irradiation wavelength and reaction
temperature.
Table 2. Impact of water/FA ratio on the reductive performance of phenylacetylene
hydrogenation
Figure 2. Characterizations of Au@ZrO2 photocatalyst. (a) TEM image and (b) High-
resolution TEM image of Au@ZrO2, (c) Diffuse reflectance UV-vis spectra of Au@ZrO2
and ZrO2 after Kubelka-Munk transformation; (d) Particle size distribution of Au@ZrO2.
To clarify the reaction pathway, isotope tracking was
applied by using D2O and H218O. We found the H-D ratio in the
product ethylbenzene was 28% to 72%, which is close to 1:2.5.
Selectivity
H2O/FA
Conversion
%
Such an H-D ratio in the product cannot be rationalized by
cation exchange between FA and water (the excessive amount
of water in FA solution should causing H-D ratio 1:4 in a typical
reaction). Pure water was found no activity in the
hydrogenation. Hence, we hypothesis that hydrogen atoms are
transferred from water to the reactant in the assistance of FA.
The role of water could be overlooked although FA aqueous
solution was previously reported as hydrogen donor.35 Among
gaseous products, C18O2 was detected with a 16O:18O ratio
approximately 2:1 when H218O was used. A rational deduction
is that FA reacts with H218O producing an intermediate
subsequently provides hydrogen to the styrene hydrogenation
and itself is oxidized to C18O2 and H2O. The possible
intermediate is orthoformic acid which has the formula
HC(OH)3,36 being a unstable hydrate has not been isolated to
date which consists of one water and one FA molecule.
Entry
(Volume ratio)
a
0
b
c
100
90
15
5
1
2
3
4
1.5:8.5
3.5:6.5
6:4
100
100
100
100
0
0
10
59
73
25
22
8:2
Reaction conditions: 50 mg Au@ZrO2 photocatalyst, 0.3 mmol phenylethyne as
reactant, formic acid mixed with H2O as solvent, 0.5 W/cm2 irradiance, 60 °C, 1
atm argon gas and 16 h.
An unexpected finding is that the ratio of water to FA in the
FA aqueous solution has significant impact on the reductive
activity of the photocatalytic system. For example, in our
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The proposed complete route is illustrated in Scheme 1. the H-Au surface species (product of step 2). Therefore, the
The hydration of FA (step 1,) yields orthoformic acid, which is ratio of H to D in the product ethylbenzene should be 1:2,
then oxidized on the surface of AuNPs (step 2), yielding H-Au which is very close to our observation mentioned above.
surface species.37 The orthoformic acid is oxidized possibly Similarly, when H218O was used, the content of 18O atoms in
following the same principle as the oxidation of a primary the product CO2, is agreed with the experimental observation.
alcohol to carboxylic acid and eventually to carbon dioxide and
water.38
The proposed mechanism is verified with the
hydrogenation of nitrobenzene. Similar results were received,
In the next step, firstly the reactant molecular is adsorbed shown in Table 3, except pure 16O2 was observed in gaseous
on the surface of Au nanoparticles following the adsorption product suggesting a source differ from H218O. We investigated
model shown in Figure 1b. In order to further verify the the compositional evolution of the reaction (Figure 3a). In the
chemisorption, taking nitrobenzene adsorbed on Au@ZrO2 as first two hours, the nitrobenzene was consumed rapidly and
an example, FTIR was employed to analyse the adsorbed aniline formed correspondingly. A notable side-product
nitrobenzene, the spectrum of adsorbed nitrobenzene shown formanilide was detected after 2 hours, resulting from the
in Figure S2 shows characteristic peaks from nitrobenzene with condensation of aniline and FA and it was confirmed by a
~8 cm-1 shifting, it indicates the existence of the control experiment where aniline was used as initial reactant.
a
chemisorption that is in agreement with the model from To our surprise, no azo or azoxy compounds were detected,
Figure 1b. suggesting that our reaction system does not strictly follow the
Thus, the H-Au surface species are capable of reducing Haber’s mechanism. In the Haber’s mechanism, nitro group is
chemisorbed reactant, in styrene’s case, two hydrogen atoms reduced stepwise first to nitrosobenzene and then the
are added into the C=C double bond basically following hydroxylamine, hydroxylamine can easily react with
Horiuti-Polanyi Mechanism (step 3 and 4,),39 the reaction nitrosobenzene yielding azoxybenzene and then further
equation as follow:
reduce to azobenzene and eventually aniline, details see
Figure S3. However, when nitrosobenzene was employed as
reactant in our system, azobenzene was produced
predominantly along with the consumption of substrate in the
first 5 hours, it was gradually converted to aniline in the rest of
reaction period without detection of other intermediates
(Figure 3b), and such process is in agreement with Haber’s
mechanism. Additionally, we received similar result in the
reduction of hydroxylamine and azobenzene as shown in Table
S3. Thus, we can conclude that the reduction of nitrobenzene
directly yield aniline instead of flowing a stepwise reduction
These two reduction steps consume 2 H-Au species and
restore the AuNP surface for subsequent catalytic cycles (step
5,). This inference is supported by the fact that the reduction is
inhibited when removing these surface species by adding 0.1
equiv. of 2, 2’, 6, 6’-tetra-methylpiperidine N-oxyl (TEMPO) to
the photocatalytic reaction system, which can abstract
hydrogen atoms from the AuNP surface,40 and found only
trace amounts of ethylbenzene in the product.
process, accordingly
a
tentative mechanism for the
photocatalytic reduction of nitrobenzene is proposed in Figure
S4.
Table 3. Isotope abundance of deuterium and 18O in the products of nitrobenzene
hydrogenation.
Entry
1
Water
D2O
Conversion (%)
87
Product
PhNH2
Abundance (%)
H
D
34
66
2
3
16O
63
18O
37
0
Scheme 1. Mechanism for the photocatalytic hydrogenation of styrene.
CO2
O2
H218O
74
The proposed reaction mechanism is supported by step to
step isotope study. According to step 1 in Scheme 1, the H-Au
(or D-Au) surface species form on the AuNP surface with the H
(or D) atoms from the orthoformic acid and the H or D atom
should be eventually found in the product ethylbenzene.
When D2O was used, the orthoformic acid formed in the
reaction has two D atoms and one H atom available to yield
100
Reaction conditions: 50 mg Au@ZrO2, 1 mmol nitrobenzene, 2 ml Formic acid
mixed with 2 ml water as solvent, light intensity 0.5 W/cm2, 40 ℃, 1 atm Argon, 8
h. The products were identified by mass spectrometry. MS and yields were
measured by GC using external standard.
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The influence of light wavelength on photocatalysis was the reduction of styrene and benzaldehyde. More
studied by investigating action spectra of hydrogenations of photoexcited electrons generated by LSPR absorption have
styrene, nitrobenzene and benzaldehyde. In an action sufficient energy to drive the benzaldehyde reduction
spectrum, the photocatalytic efficiency is plotted against light compared with those in the styrene reduction as the reduction
wavelength. Quantum yield (Φ) which was converted from potential of benzaldehyde is higher than that of styrene.
reaction rate is used to present the photocatalytic efficiency. It
was calculated as followed:
The dependence of the reaction yield on the irradiance was
investigated and results are shown in Figure 5, the yield of
Φ = [the number of converted reactant molecules×100]/[the hydrogenation increases with increasing irradiance for all four
number of incident photons]. reactions. It is direct evidence that irradiation can remarkably
Two types of action spectrum are observed as shown in Figure promote the hydrogenation reactions. In addition, the relative
4. The result indicates that AuNPs can most efficiently drive contribution of the irradiation in comparison to the thermal
the hydrogenation of nitrobenzene by the LSPR effect as the contribution for the hydrogenation reactions is clarified.
highest Φ value was observed at 530 nm (Figure 4c). For light
spectrum other than LSPR wavelength, the photocatalytic
activity is attributed to the electrons directly excited by
photons.
Figure 3. Time-conversion plot. (a) nitrobenzene hydrogenation and (b) nitrosobenzene
hydrogenation. Reaction condition: 200 mg Au@ZrO2 dispersed into a solution of 15 ml
of 85% formic acid and 15 ml deionized water, light intensity 0.5 W/cm2, 40 ℃, 1 atm
argon, reaction time 8 h, 0.5 ml specimen was taken every hour and analysed by mass
spectrometry (identifying the species) and GC (determining the concentration of the
species using external standard).
These photoexcited produced by the photons of shorter
Figure 4. Action spectra of selected hydrogenations. Hydrogenation of styrene (a),
wavelength possess higher energy levels but less population. It
benzaldehyde (b) and nitrobenzene (c) catalysed by Au@ZrO2. The purple line
explains the relatively high quantum yield of 400 nm in
represents the diffuse reflectance UV-vis spectrum of Au@ZrO2, blue marks represent
hydrogenation of nitrobenzene. The light of 590 nm and 620
nm are neither triggering LSPR effect nor delivering photons
with sufficient energy, therefore resulting low reaction rate. In
the styrene and benzaldehyde’s case (Figure 4a-b), the
reaction demands more reduction power, meaning hot-
electrons with higher energy, due to their much more negative
reduction potential (Table S1). Although strong LSPR
the quantum yield of each wavelength.
The column is the yield of light reaction while the white
part is the yield of the reaction in the dark at the same
reaction temperature which represents the contribution of
thermal effect. When irradiation was applied, the yield
increased significantly and almost linearly. It is also noted that
at higher irradiance, the contribution of the irradiation to the
overall reaction rate is greater. For example, when the
irradiance was 0.1 W/cm2 the contribution of light to the yield
of nitrobenzene hydrogenation was merely 25.6%, while the
contribution was 79% when irradiance was raised up to 0.5
absorption can generate
a large population of excited
electrons,20 many of them do not have sufficient energy to
enable absorbed styrene to cross the activation energy barrier.
On the other hand, photons of 400 nm wavelength can
generate photoexcited electrons with sufficient energy to drive
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W/cm2. These results indicate that the irradiation is the
predominating force driving the hydrogenation reactions.
Generally, higher irradiance can produce more photoexcited
electrons resulting higher reaction rate. The photoexcited
electrons that have insufficient energy to induce the reduction
will relax and release their energy to heat the lattice of the
AuNP. Such a photo-thermal effect also accelerates the
reaction. Therefore, higher reaction rates are always observed
at higher irradiance.
green column represents the contribution of light. Hydrogenation of nitrobenzene (a),
styrene (b), benzyl aldehyde (c) and imine (d).
Lastly, the reusability of the Au@ZrO2 photocatalyst have
been investigated. In the hydrogenation of nitrobenzene,
Au@ZrO2 is recovered by centrifugation after each
photocatalytic reaction round, the recovered photocatalyst is
washed with water and ethanol and then directly applied in
next cycle reaction without further treatment or regeneration.
The conversion of nitrobenzene hydrogenation over Au@ZrO2
remain 97% after six cycle runs as shown in Figure 6. In
addition, the TEM images of Au@ZrO2 after one photocatalytic
reaction are collected and shown in Figure S5. The
photocatalytic reaction round does not have notable impact
on the morphology of Au@ZrO2. The particle dispersion and
size distribution remain unchanged. In addition, surface of
sample has not been covered by organics. Thus, the Au@ZrO2
photocatalyst exhibits good reusability, the reason could be
the resistance of Au metal to oxidative and acidic
environment, meanwhile the moderate reaction condition is in
favour of the reusability of photocatalyst.
100
80
60
40
20
0
1
2
3
4
5
6
Cycle Runs
Figure 6. Reusability of Au@ZrO2 in hydrogenation of nitrobenzene. Reaction
conditions: 50 mg Au@ZrO2 dispersed into a solution of 2 ml of 85% formic acid and 2
ml deionized water containing 0.5 mmol nitrobenzene, light intensity 0.5 W/cm2, 40 ℃,
1
atm argon, reaction time 4 h, specimen was analysed by GC (determining the
concentration of the species using external standard).
Conclusions
In summary, this work reports a photocatalytic hydrogenation
reaction system for the hydrogenation of five types of
unsaturated aromatics using aqueous solutions of FA as the
reductive agent over a visible or UV illuminated Au@ZrO2
photocatalyst at ambient temperatures and pressures. The
photocatalytic system exhibits high catalytic efficiency and
broad substituent tolerance. One can tune the reduction
power of this system over a wide range by regulating the
irradiation wavelength, and moderating the reaction
temperature. We have found that water plays an active role of
enhancing the reductive efficiency of FA, it reacts with FA to
yield orthoformic acid that provides the hydrogen to yield H-
Au surface species. These H-Au species react with the groups
Figure 5. Dependence of the photocatalytic activity of Au@ZrO2 for hydrogenation on
irradiance. The grey part of a column represents the contribution of thermal effect and
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to be reduced in the hydrogenation. This finding will renew the nm). The irradiance was 0.5 W/cm2 unless otherwise specified.
o
understanding of the selective reductions and inspire future The reaction temperature was carefully controlled at 40 C by
application of FA as hydrogen source. Another important using an air conditioner unless otherwise specified. The
finding is that the hydrogenation of nitro aromatic compounds reaction system in the dark was conducted using a water bath
follows a mechanism distinct from the well-known Haber placed above a magnetic stirrer, and the reaction flask was
mechanism, yielding corresponding anilines directly due to the wrapped with aluminium foil to isolate the contents from the
strong reduction power of the new photocatalytic system. This influence of light. The reaction temperature was maintained at
photocatalytic process is green, in terms of reduction agent, the same temperature as the corresponding reaction under
and reaction conditions, is chemoselective, and is feasible for a irradiation. At the end of reaction time, the product was firstly
wide range of molecules.
extracted with equivalent amount of dichloromethane, and
then 2 mL aliquots were collected and filtered through a
Millipore filter (pore size of 0.45 µm) to remove particulate
matter. The clear liquid-phase products were analysed with an
Agilent 6980 gas chromatography (GC) using a HP-5 column to
analyse the change in the concentrations of reactants and
products. An Agilent HP5973 mass spectrometer was used to
identify the products. In the analysis of gaseous products, prior
to the photocatalytic reaction, the reaction tube was purged
with argon gas and sealed to isolate the reaction from air.
After reaction, a 1 mL gas sample was taken from the
atmosphere above the reaction suspension and analysed using
mass spectrometry.
Experimental Section
Materials. Zirconium(IV) oxide (ZrO2, particle size <100 nm,
TEM), gold (III) chloride trihydrate (HAuCl4·3H2O, ≥99.9% trace
metal basis), silver (III) chloride (Ag(NO3)3, ≥99.9%), sodium
borohydride powder (NaBH4, ≥98.0%), Deuterium oxide (D2O,
99.9 atom% D), Nitrosobenzene (C6H5NO, ≥97%), azobenzene
(C12H10N2,
≥98%),
styrene
(C8H8,
≥99.9%),
N-
(phenylmethylene)- Benzenamine (C13H11N, ≥96%) and L-lysine
(2,6-diaminocaproic acid, ≥97%), were purchased from Sigma-
Aldrich (Australia). H218O (≥96 atom% 18O) was purchased from
HuaYi Isotopes Co. (China). Formic acid solution (HCOOH,
85%), ethanol (CH3OH, ≥95%) and dichloromethane (CH2Cl2)
were of analytical grade. All chemicals were used as received
without further purification unless otherwise noted. The water
used in all experiments was prepared by an Ultrapure Water
System from Merck Millipore Co.
Action Spectrum Test. An action spectrum indicates the
dependence of reaction rate on the wavelength of irradiation,
which provides evidence for the mechanism of how the
photocatalyst responses to different light wavelengths and
activates the reactant molecules. Action spectrum
experiments were conducted with light emitting diode (LED)
lamps (Tongyifang, Shenzhen, China) with wavelengths of 400
± 5 nm, 470 ± 5 nm, 530 ± 5 nm, 590 ± 5 nm, and 620 ± 5 nm.
The light intensity was measured to be 0.50 W/cm2 using an
energy meter (CEL-NP2000) from AULTT Company and other
reaction conditions were maintained identical to those of
typical reaction procedures.
Synthesis of Au@ZrO2 photocatalyst. ZrO2 supported AuNPs
(Au@ZrO2) was prepared by the impregnation-reduction
method. For example, to prepare 3 wt% Au@ZrO2, ZrO2
powder (2.0 g) was dispersed into an aqueous solution of
HAuCl4 (32.7 mL, 0.01 M) under magnetic stirring at room
temperature, followed by addition of a lysine aqueous solution
(10 mL, 0.53 M) while it was vigorously stirred for 30 min. The
pH value of the mixture was 8-9. To this suspension a freshly
prepared aqueous NaBH4 (10 mL, 0.35 M) was added
dropwise. The mixture was aged overnight, and then the solid
was separated by centrifugation, washed with water (three
times), ethanol (once), and was dried at 60 °C in a vacuum
oven for 24 h.
DFT Calculation Methods. Geometry optimization and
electronic structure calculations were carried out using density
function theory plus long range dispersion correction under
the TS method of DFT-D as implemented in the Dmol3
software41,42 with Effective Core Potentials. Exchange-
correlation interaction is treated as generalized gradient
approximation (GGA) with the Perdew, Burke, and Ernzerhof
(PBE) functional and electronic Eigen functions are expanded
in terms of DND with a real-space cut-off of 4.4 Å. The
convergence criteria for energy change, force, and
displacement during geometry optimization were set to be 2.0
Characterisation. The morphology and elemental composition
of photocatalysts were studied using a JEOL 2100 transmission
electron microscopy (TEM) coupled with an energy dispersion
X-ray (EDX) spectrometer (X-MAXN 80TLE, OXFORD
Instruments). The accelerating voltage of TEM was 200 KV.
Diffuse reflectance UV-visible spectra of the catalysts were
collected with a Varian Cary 5000 spectrometer with BaSO4 as
a reference. The FTIR spectra were measured with a Nicolet
380 instrument.
−3 Å, respectively.
× 10−5 Ha, 4.0 × 10−3 Ha/Å, and 5.0 × 10
The vacuum space was more than 20 Å, which was enough to
avoid the interaction between periodical images.
Conflicts of interest
General procedure for photocatalytic reactions. In a typical
activity test, a 25 mL Pyrex round-bottom flask was used as
container, and after 1 mmol reactants and 50 mg catalyst had
been added, the flask was filled with 1 atm argon gas and
sealed in order to isolate the reaction from air. The flask was
then stirred magnetically and irradiated with a halogen lamp
(from Nelson, 500W, and wavelength in the range of 400-750
There are no conflicts to declare.
Acknowledgements
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DOI: 10.1039/C7CY02291C
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