Engineering of highly active Au/Pd supported on hydrogenated urchin-like yolk@shell TiO2 for visible light photocatalytic Suzuki coupling

Article abstract of DOI:10.1039/c9cy00798a

Efficient smart photocatalysts and their surface engineering are necessary for the effective conversion of light energy into chemical energy in photocatalyzed organic reactions. Herein, we designed a hydrogenated urchin-like yolk@shell TiO₂ structure decorated with Au and Pd nanoparticles (HUY@S-TOH@AuPd) as a robust photocatalyst for C-C coupling reactions. The resulting architecture exhibits considerable photocatalytic performance in Suzuki coupling reactions under visible light irradiation with a turnover frequency (TOF) value as high as 7095 h^-1. The beauty of this engineered structure lies on the following four points: (I) the urchin-like structure provided a large accessible surface area for high light harvesting as well as high noble metal anchoring; (II) the yolk@shell mesoporous architecture improved the absorption of light by multiple scattering; (III) the presence of Ti³⁺ species on the surface of TiO₂ decreased the band gap of the structure to the visible region; (IV) the enriched electron density of Pd through the injection of hot electrons from Au as well as the flow of electrons from the titanium dioxide semiconductor to the metals accelerated the rate-determining step. The merging of bimetallic plasmonic nanoparticles and urchin like yolk-shell hydrogenated titanium dioxide architecture can open an avenue for designing photocatalysts with high stability and promising activity as well as high direct harvesting of visible light for a broad range of photocatalytic organic reactions.

Full text of DOI:10.1039/c9cy00798a

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PAPER
Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
nanotubes produced by atomic layer deposition for hydrogen
generation from hydrolysis of ammonia borane
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DOI: 10.1039/C9CY00798A
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ARTICLE
Engineering of Highly Active Au/Pd Supported on Hydrogenated
Urchin-like Yolk@Shell TiO2 for Visible Light Photocatalytic Suzuki
Coupling
Received 00th January 20xx,
Accepted 00th January 20xx
Sahar Rohani, a,b Abolfazl Ziarati, b,c Ghodsi Mohammadi Ziarani, a Alireza Badiei, c Thomas Burgi b,*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Efficient smart photocatalysts and their surface engineering are necessary for the effective conversion of light energy into
chemical energy in photo-catalyzed organic reactions. Herein, we designed a hydrogenated urchin-like yolk@shell TiO2
structure decorated with Au and Pd nanoparticles (HU@S-TOH@AuPd) as a robust photocatalyst for C-C coupling
reactions. The resulting architecture exhibits a considerable photocatalytic performance in Suzuki couplings under visible
light irradiation with a turnover frequency (TOF) value as high as 7095 h-1. The beauty of this engineered structure is lying
on the following four points; (I) the urchin-like structure provided large accessible surface area for high light harvesting as
well as high noble metals anchoring; (II) the yolk@shell mesoporous architecture improved the absorption of light by
multiple scattering; (III) the presence of Ti3+ species on the surface of TiO2 decreased the band gap of the structure to the
visible region; and (IV) the enriched electron density of Pd through the injection of hot electrons from Au as well as the
flow of electrons from the titanium dioxide semi-conductor to the metals, accelerated the rate-determining step. The
merging of bimetallic plasmonic nanoparticles and urchin like yolk-shell hydrogenated titanium dioxide architecture can
open an avenue for designing photocatalysts with high stability and promising activity as well as high direct harvesting of
visible light for a broad range of photocatalytic organic reactions.
bond formation.7-9 Photocatalytic Suzuki reaction driven by
visible light is an ideal procedure, due to its clean and
abundant energy source, benign environmental impact and
sustainability.10-12
TiO2 nanomaterials are extensively considered to be state of
the art photocatalysts for energy conversion and
environmental protection.13-15 Nevertheless, the low
photocatalytic performance caused by large band gap, rapid
recombination rate of photo-generated carriers and low
Introduction
Development of environmentally friendly photocatalytic
materials for organic molecular transformations with high
activity and stability in mild conditions is of great importance
for both fundamental research and practical applications.1, 2
One special focus is on the use of the photo-induced
electrons/holes for reduction/oxidation of organic molecules.3,
4
Despite the fact that the simultaneous application of photo-
excited electrons and holes for organic reactions offer a novel
and promising catalytic strategy for the economical and green
chemical synthesis, it is rarely reported and still a challenge for
the scientific community.5, 6 Suzuki−Miyaura coupling is one of
the most powerful tools in modern organic synthesis for C−C
specific surface area are challenging issues which need to be
17
addressed to enable useful applications.16,
Structure
engineering is a highly promising approach for improving the
optical and electronic properties of TiO2 in the visible light
region.18 For this aim, one way is deposition of noble metal
particles on the surface of titania.19 This noble metal grafting
can efficiently increase the interfacial charge transfer by
formation of a ‘‘Schottky’’ barrier between titania and metal,
which can be indexed to the various Fermi levels of metal and
a. Department of Chemistry, Faculty of Science, University of Alzahra, Tehran, Iran
b. Department of Physical Chemistry, University of Geneva, 30 Quai Ernest-
Ansermet, 1211, Geneva 4, Switzerland
20
TiO2.
Under visible light illumination, photo-induced
electrons in the conductive band (CB) of titania can be
transferred to noble metals as the Fermi level of these metal
particles are located below the CB of the n-type TiO2
semiconductor.21
c. School of Chemistry, College of Science, University of Tehran, Tehran, Iran
E-mail: Thomas.Buergi@unige.ch
Tel: +41(0)22 379 65 52
Electronic
Supplementary
Information
(ESI)
available.
See
DOI: 10.1039/x0xx00000x
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On the other hand, Pd nanoparticles (NPs) play an important for photocatalytic Suzuki-Miyaura coupling reactions at room
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DOI: 10.1039/C9CY00798A
role in C-C bond-forming reactions due to their unique ability temperature under visible light irradiation.
to activate reactants via the formation of metal–carbon
bonds.22 Although, Pd NPs can strongly absorb the UV and
visible light through interband electronic transitions, their
weak plasmonic properties inhibit the application of Pd in the
plasmonic field.12, 23, 24 With the help of the strong localized
Results and discussion
Preparation and characterization of HUY@S-TOH/AuPd
architecture
surface plasmon resonance (LSPR) of Au under light irradiation
Fig. 1 illustrates the multi-step procedure for the preparation
Au/Pd bimetallic nanoparticles have been proven efficient in
of HUY@S-TOH/AuPd nanoarchitectures. In step I, the Y@S-TO
light harvesting and as catalyst for Suzuki coupling reaction.25-
spheres are formed by one-pot solvothermal method using
27
In this regards, several efforts have been devoted to
polyethylene glycol as soft template.40 Next, in step II,
improve the activity of heterogeneous AuPd-based TiO2
dissolution–recrystallization
process
in
an
alkaline
catalysts for visible-light-induced Suzuki coupling reaction. Yan
et al. reported a well-designed Au–TiOx–Pd photocatalyst that
indicated high performance towards Suzuki−Miyaura reactions
at ambient conditions under solar irradiation.28 Zhang and co-
workers fabricated Au-Pd/TiO2 catalyst, which exhibited high
photocatalytic activity for coupling reactions of aryl boronic
acids with substituted aryl halides under visible light.24 Inspired
by these reports, we tried to design an engineered Au-Pd/TiO2
architecture to improve the photocatalytic activity in Suzuki
reaction.
water/alcohol media was used to produce hierarchical urchin
like shell (HUY@S-TO). In step III, the 3D HUY@S-TOH
Among various morphologies of TiO2, yolk–shell (Y@S)
structures, composed of a movable core inside a hollow cavity
enclosed by a porous outer shell, have gained much research
30
attention due to their unique properties.29,
These smart
materials are conducive for photocatalysis due to relatively
high surface area, high light-harvesting, low diffusion
resistance and multiple light-scattering within the sphere
interior voids.30, 31 Up to date, it has been demonstrated that
nanoarchitectures with hierarchical surface have enhanced
features compared to those with smooth surface.32 Among
them, readily accessible urchin-like structures with properties
of interconnected porous framework and high specific surface
area, can increase the efficiency of light-harvesting as well as
facilitate the accessibility of reactants to the active sites.33 On
the other hand, surface structure engineering has afforded
many breakthroughs in enhancing the photocatalytic activity
of titania through creating a defect-rich or disordered surface
layer with black colour therefore improving its optical
absorption in the visible region.34, 35 The presence of oxygen
vacancies on the surface of black TiO2, provides excess
electrons at the defect sites, decreases the band gap of TiO2 to
Fig. 1. Overall flowchart for the fabrication of HUY@S-TOH/AuPd architecture.
architectures were obtained by using of hydrogen treatment
method.42 Finally, the as-prepared Au/Pd core/shell NPs were
decorated on the surface of HUY@S-TOH to yield HUY@S-
TOH/AuPd architecture (step IV).
The SEM analysis was used to investigate the surface
morphology of designed architecture. The SEM image of
HUY@S-TOH/AuPd (Fig. 2a) shows the presence of urchin-like
monodisperse microspheres with a diameter of ~ 3 µm. The
broken sphere (yellow arrow) implies the Y@S configuration in
which a movable solid core was located inside the urchin-like
the visible region and decreases the charge recombination.24,
36, 37
Based on the above considerations, the design of noble metal
NPs decorated yolk–shell TiO2 structure with hydrogenated
urchin-like surface is an effective method to solve the low
light-harvesting efficiency of titania which can lead to the next
generation of advanced photocatalytic materials for coupling
reactions. Therefore, following research endeavours from our
groups,38-41 herein we wish to propose the hierarchical urchin-
like yolk@shell TiO2 architecture decorated with plasmonic
Au/Pd NPs (HU@S-TOH@AuPd) as an advanced architecture
Fig. 2. SEM images of HUY@S-TOH/AuPd architecture. The scale bars are 1 µm (a, b)
and 200 nm (c).
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shell. From SEM images of HUY@S-TOH/AuPd in higher
magnifications (Fig. 2b, c), the well-developed hairy needles
assembled on the surface of microspheres are clearly seen,
resulting in urchin like structures.
View Article Online
DOI: 10.1039/C9CY00798A
More detailed morphological information of the designed
structures were obtained by TEM analysis in comparison with
the Y@S-TO structure. The typical TEM images of Y@S-TO (Fig.
3 a-c) in different magnifications, confirmed the Y@S
configuration in this structure. Moreover, the relatively
smooth surface in this structure is clearly obvious (Fig. 3c).
Besides, Fig. 3 (f-h) revealed hierarchical urchin-like structure
Fig. 4. HRTEM images of Y@S-TO (a, b) and HUY@S-TOH/AuPd (c, d). The scale bars
are 50 nm (a, c) and 5 nm (b, d).
X-ray photoelectron spectroscopy (XPS) was used to analyze
the valence states of Ti, Pd and Au in the modified HUY@S-
TOH/AuPd architecture (Fig. S3). As can be seen in Fig. S3a,
two bands located at binding energy (BE) of 459.0 eV and
464.9 eV were attributable to the Ti 2p3/2 and Ti 2p1/2
photoelectrons in Ti4+ chemical state, while two peaks at 456.9
eV and 463.2 eV were indexed to the 2p3/2 and 2p1/2 core levels
of Ti3+ in the black TiO2. Besides, the Au 4f and Pd 3d spectra
demonstrated two contributions resulting from spin–orbit
Fig. 3. TEM images of Y@S-TO (a-c) and HUY@S-TOH/AuPd (f-h). The scale bars are 1
μm (a, b, f, g), and 200 nm (c, h).
of HUY@S-TOH that was created by dissolution–
recrystallization process in water/alcohol media. As can be
seen in these images, the urchin-like needles were successfully
assembled on the surface of Y@S structure.
splitting. The BE peak at 83.2 eV correspond to Au 4f7/2
,
High-resolution TEM (HRTEM) images of Y@S-TO (Fig. 4 a,b)
and HUY@S-TOH/AuPd (Fig. 4 c, d), clearly indicate that the
two structures have good crystallinity with an interplanar
distance of 0.35 nm between (101) TiO2 planes (red arrows).
Meanwhile, in the case of HUY@S-TOH/AuPd architecture, a
thin disordered surface layer (white arrows) owning to the
effect of hydrogen treatment could be observed.43
Additionally, in Fig. 4 (c, d) spherical AuPd NPs are evident on
the surface needles of HUY@S-TOH architecture with the
average diameter of ~5 nm and the lattice spacing of about
0.24 nm, which matched well with the (111) crystalline plane
of Au (yellow arrows). A narrow shell on the surface of Au NPs
(blue line, ca. 0.7 nm) is obvious that can be attributed to the
Pd deposition and preparation of Au-Pd core/shell NPs, as later
on confirmed by XPS analysis. Moreover, to better distinguish
the structure of Au and Pd, the HRTEM analysis was done for
pristine AuPd nanoparticles. As is clear from Fig. S1, the Au
core (darker part) is covered by the Pd shell (lighter part).
Energy Dispersive X-ray Spectrometry (EDS) elemental
mapping analysis was also performed to identify the elemental
composition of the architecture (Fig. S2). The results reveal
that the HUY@S-TOH/AuPd structure mainly contains uniform
distribution of Ti, O and Au as well as Pd elements.
suggesting the presence of Au (0) species on the surface of
nanoarchitecture.44 Meanwhile, the main BE peak of Pd 3d5/2
was located at 333.7 eV, suggesting that the Pd species are in a
metallic state which has been proven efficient for attaining
high catalytic performance toward Suzuki coupling reaction.
Remarkably, a slight positive shift relative to the typical BE of
Ti 2p3/2 at 459.0 eV and also a negative shift of the typical BE
for Au (0) and Pd (0) at 84.0 eV and 335.0 eV, respectively, can
be attributed to the transfer of electrons from the titania to
the Au/Pd core/shell nanospheres.24, 45, 46 These electron flows
can enhance the electron density around Pd particles which
can accelerate the Suzuki reaction. Besides, the weight amount
of the Au and Pd species in the engineered HUY@S-TOH/AuPd
structure, determined by XPS analysis, are ~ 2.3 and 0.6 wt%
respectively.
The XRD and DRS analysis of Y@S-TO, HUY@S-TOH and
HUY@S-TOH/AuPd were also performed (Fig. S4). The XRD
patterns confirmed that all architectures possess a good
crystallinity with the well-defined tetragonal anatase structure
of TiO2 (JCPDS No. 21-1272). After hydrogenation, a slight shift
to higher diffraction angles was observed in HUY@S-TOH
which can be attributed to the reduction of the interplanar
spacing of crystalline titania phase.47 In the engineered
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HUY@S-TOH/AuPd structure, all reflections are preserved, temperature (Table S1, entry 8); however, a mixture of protic
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DOI: 10.1039/C9CY00798A
implying that the decoration with Au–Pd NPs has no effect on EtOH and H2O led to a profound increase in catalytic activity
the crystal phase of the TiO2 support. Moreover, two small (Table S1, entry 13). This result is very different from that
reflections could be observed at ~38˚ and 44˚, for (111) and obtained in conventional Suzuki coupling reactions catalyzed
(200) planes of Au, respectively (JCPDS: 10-0784). No distinct by heterogeneous catalysts. Generally, polar DMSO and DMF
diffraction peaks ascribed to Pd are distinguished, because of are considered to be good solvents for Suzuki coupling, leading
the size of Pd shell deposited on the surface of Au core is lower to the generation of the corresponding products in high yields.
than the detection limit of XRD analysis (<4 nm). Evidence for Nevertheless, this is not the case for photo-catalyzed Suzuki
the formation of Ti+3 and Au–Pd core-shell NPs on the coupling reactions. Additionally, another protic organic solvent
engineered structure was also seen by UV-vis diffuse mixture involving MeOH gave moderate yields of the desired
reflectance spectra (DRS) in which the HUY@S-TOH/AuPd product in this photocatalyzed coupling reaction. These results
architecture clearly demonstrated different features from confirmed that protic solvents play a crucial role in
HUY@S-TOH and Y@S-TO structures. As shown in Fig. S5, Y@S- photocatalyzed Suzuki reactions over HUY@S-TOH/AuPd
TO displayed great light adsorption at wavelengths above 400 nanocatalyst. It is guessed that protic solvents may be oxidized
nm. In contrast, the spectrum of HUY@S-TOH demonstrates more easily through photo-generated holes transferred from
significant absorption of visible light that may be due to the Y@S-TO. In contrast, aprotic solvents, like DMSO and DMF
presence of Ti3+ ions on the surface of structure. After could not be oxidized by this hole transfer since they have
decorating with Au-Pd, even stronger absorption in visible higher oxidation potentials.48 Subsequently, the effects of
region was observed for the modified HUY@S-TOH/AuPd various bases in the mixture of EtOH:H2O were investigated
architecture. The strong peak at ~532 nm is owning to the and the best result was obtained using K2CO3 (Table S1, entry
LSPR effect of the core/shell Au-Pd NPs.46
13). Moreover, the reaction did not proceed without the base
N2 adsorption-desorption measurement was utilized to (Table S1, entry 15), as it is necessary to activate the
investigate the porous structure and pore size distribution of phenylboronic acid and it is facilitating the trans-metalation
HUY@S-TOH/AuPd sample (Fig. S6). Unlike P25, which is not step.49 Besides, different loadings of Au/Pd nanoparticles on
mesoporous,
for
Y@S-TO
and
HUY@S-TOH/AuPd the HUY@S-TOH architecture were investigated in the Suzuki
architectures, typical IV isotherms are observed, indicating the model reaction (Fig. S8). As emerges from the data, the best
presence of mesopores in these structures. Thanks to the result was obtain using 3 wt% of Au/Pd. By increasing the
advantage of urchin-like surface as well as Y@S structure, the metal loading beyond 3 wt%, no considerable improvements in
BET surface area of HUY@S-TOH/AuPd (306 m2 .g–1) is nearly product yields were observed. To obtain better insights of
two times larger than that of Y@S-TO (181 m2 .g–1) and much HUY@S-TOH/AuPd
nanoarchitecture
efficiency,
its
larger than P25 (41 m2 .g–1). This high surface area is desirable photocatalytic activity in the model reaction under optimized
for great adsorption of organic substrates and efficient conditions was compared with P-25, P-25/AuPd, Y@S-
anchoring of AuPd NPs leads to improved photocatalytic TO/AuPd, HUY@S-TOH/Au, HUY@S-TOH/Pd and HUY@S-
coupling reaction. According to Barrett-Joyner-Halenda (BJH) TOH/AuPd structures. As can be seen in Fig. S9, by using of P-
analysis, the HUY@S-TOH/AuPd structure presented a narrow 25, the reaction did not proceed at all probably due to the lack
pore-size distribution in the range of 2−8 nm (Fig. S7).
3.2. Photocatalytic investigations
of visible light activity as well as the absence of metal NPs in
this structure. However, the activity of catalyst is significantly
The influence of some critical reaction conditions in the Suzuki improved with decoration of Au-Pd nanoparticles on the
coupling, has been investigated by using of HUY@S-TOH/AuPd structures, so that the yield of reaction increased to 69% and
photocatalyst under a 300 W Xenon lamp irradiation, as 78% after 1h using P-25/AuPd and Y@S-TO/AuPd, respectively.
simulated sunlight, at room temperature. The Suzuki reaction The higher photocatalytic activity of Y@S-TO/AuPd compared
between 4-iodotoluene with phenylboronic acid, as model to P25/AuPd can be ascribed to the higher surface area and
reaction, did not proceed in the dark (Table S1, entry 1) and at better light harvesting ability, provided by yolk/shell structure
80 °C (in the dark) a moderate yield (68%) of corresponding in Y@S-TO/AuPd. By using monometallic structures, HUY@S-
product was observed, (Table S1, entry 2), indicating the TOH/Au gave only little product (17%) and HUY@S-TOH/Pd
significant influence of irradiation on the catalytic demonstrated inferior activity compared with HUY@S-
performance. Among several solvents that were used for the TOH/AuPd, as the yield of product was only 43%. This implies
reaction, the polar aprotic solvents, such as DMF and DMSO, that the presence of both Au and Pd plays an important role in
gave low yield of the product and the nonpolar solvent, this photocatalytic reaction. Interestingly, the highest
toluene, also afforded a trace yield (Table S1, entries 3-5). In photocatalytic activity was observed using HUY@S-TOH/AuPd
contrast, higher yield of product was observed in the polar architecture with a product yield of 94% in 1h. This
protic solvents like ethanol and methanol (Table S1, entries 6, considerable activity can be attributed to the hydrogenated
7). In addition, the reactions in pure H2O hardly proceeded visible light active structure containing Ti3+ species on the
because of the poor solubility of 4-iodotoluene at room surface of TiO2. The latter enhance the light harvesting
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Table 1. Scope of Suzuki coupling reaction catalyzed by HUY@S-TOH/AuPd under
efficiency of photocatalysts through inhibition of the
recombination of photo-generated electrons and holes by
decreasing the band gap of TiO2 into the visible region.
Furthermore, scattering of light on the hierarchical surface and
between inner core and outer shell in urchin-like Y@S
structure can enhance the light harvesting efficiency.
Moreover, the results indicate the strong interaction between
noble metals and TiO2-x. Electron transfer between Ti3+ and
metal particles, as previously verified by XPS analysis, leads to
an electron-rich Pd surface, which finally increases the
catalytic performance of this architecture.50 These results
clearly demonstrate the high activity of the used urchin-like
hydrogenated HUY@S-TOH/AuPd nanoarchitecture for
photocatalytic Suzuki coupling reaction.The reaction protocol
was further extended in the coupling of different aryl halides
and phenylboronic acids to confirm the scope of catalytic
system by HUY@S-TOH/AuPd under visible light irradiation
(Table 1). As expected, in the case of aryl iodides, the
corresponding biaryls were obtained in higher yield than aryl
bromides and chlorides due to the weaker C−I bond.
Moreover, the electronic nature of the substituents on the
phenyl rings can affect the yield of desired products. Aryl
halides with electron withdrawing substituents exhibited
higher product yields as compared to those with electron
donating ones. This result is reasonable, since the electron
withdrawing substituents result in electron deficient aromatic
ring which facilitates the nucleophilic attack via the photo-
induced electrons on electron-rich Pd NPs. To better visualize
the effect of different aryl halides on the reaction rate of
Suzuki coupling, the yield of some bromobenzenes vs
Hammett’s parameter are shown in Fig. S10. To avoid
complications by steric hindrance effects, Hammett plot is
done for substituents in the para-position. As expected, in the
presence of electron-withdrawing groups at para-substituted
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visible-light irradiation. a
DOI: 10.1039/C9CY00798A
HUY@S-TOH/AuPd
X
B(OH)2
R1
K2CO3, EtOH:H2O (2:1)
Visible light, r.t.
R2
R2
R1
1
2
3a-r
MeO
Me
3a
3b
3c
X= I, 0.5h, 95%, TOF= 6740
X= Br, 3h, 88%, TOF= 1040
X=Cl, 8h, 61%, TOF= 239
X= I, 1h, 94%, TOF= 3335
X= Br, 4h, 75%, TOF= 665
X= I, 0.5h,97%, TOFb= 6882
O2N
MeOC
OHC
3d
3e
3f
X= I, 0.5h, 98%, TOF= 6953
X= Br, 2h, 94%, TOF= 1667
X= Br, 3h,85%, TOF= 1005
X= Br, 3h, 80%, TOF= 946
NC
CN
NC
3g
3h
3i
X=Br, 3h, 82%, TOF= 970
X= Br, 4h, 78%, TOF= 692
X= Br, 5h, 72%, TOF= 511
H2N
NC
Me
Cl
3j
3k
3l
X= Br, 2h, 87%, TOF= 1543
X= I, 3h,90%, TOF= 1064
X= Br, 4h, 82%, TOF= 727
X= I, 0.5h, 98%, TOF= 6953
X= Br, 2h, 90%,TOF= 1596
Me
Me
MeO
Me
Me
3m
3n
3o
X= I, 1h, 98%, TOF= 3476
X= I, 0.5h, 96%, TOF= 6811
X= Br, 2.5h, 90%, TOF= 1277
X=Cl, 7h, 65%, TOF= 330
X= I, 0.5h, >99%, TOF= 7095
Cl
Me
OHC
Me
MeOC
Me
3p
3q
X= Br, 3h, 88%, TOF= 1040
3r
X= Br, 3h, 83%, TOF= 981
X= I, 0.5h, >99%, TOF= 7095
X= Br, 2h, 92%,TOF= 1632
a
Reaction conditions: arylhalide (1.0 mmol), phenylboronic acids (1.5 mmol),
HUY@S-TOH/AuPd catalyst (5 mg), K2CO3 (2 mmol) and EtOH:H2O (6 mL), Xe
lamp, water bath, room temperature. Yields are referred to isolated pure
compounds.
b TOF (h-1) is calculated based on the yield of products and mole of Pd used.
51, 52
bromobenzenes, the yield of biphenyls were enhanced.
These results are concurrent when correlated with the
53
Hammett parameter values.
Fig. 5. Plausible mechanism for the visible light induced photocatalytic Suzuki–Miyaura
coupling reaction using HUY@S-TOH/AuPd.
Furthermore, meta-substituted arylhalides showed lower
activity than those with para- and ortho-substituents, probably
a consequence of both steric hindrance and electronic effect
(Table 1, 3i-k).
To get insight into the mechanism responsible for
photocatalyzed Suzuki reaction using HUY@S-TOH/AuPd
architecture, the coupling of 4-iodotoluene with phenylboronic
acid in the presence of electron and hole scavengers were
studied. 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was used as
the electron scavenger to trap the electrons from Pd, which
can activate the 4-iodotoluene.54 By addition of DMPO, no
desired product was observed, suggesting that the coupling
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Fig. 6. Reusability study of HUY@S-TOH/AuPd in the photocatalytic Suzuki coupling
reaction could not proceed without the reduction by the
electrons from Pd. Next, the role of photo-generated holes
from nanoarchitecture was investigated. Phenylboronic acid
can combine with OH− ions in basic solution and in principle
adsorb on TiO2 via electrostatic interaction. The photo-induced
holes can diffuse to the adsorption sites of phenylboronic acid,
the adsorbed molecules could be oxidized with the cleavage of
C−B bond.28 Therefore, phenylboronic acid can be activated via
photo-generated holes under visible light illumination. To
demonstrate this, triethanolamine (TEOA) as the hole
scavenger, was used in the same reaction. It was observed that
the reaction was quenched. Even though these results
demonstrate the importance of the electron–hole pair in the
photocatalytic Suzuki reaction, the exact role of holes is not
yet clear.55, 56
View Article Online
reaction of 4-iodoanisole and phenylboronic acid.
DOI: 10.1039/C9CY00798A
A comparison reveals the highest TOF for the engineered
HUY@S-TOH/AuPd nanoarchitecture in the photocatalytic
Suzuki reaction with respect to other reports. As can be seen
in Fig. 7 and Table S1, 24, 28, 54-57, 59-64 our catalyst remained the
best performer reported yet (TOF = 7095 h-1). The good
performance of hydrogenated visible light active architecture
can be attributed to the urchin-like surface structure that
provide more accessible surface area for higher Au-Pd
anchoring, prevent the agglomeration of Pd species during
catalytic cycles.
Conclusions
In conclusion, we report an engineered design for the
synthesis
of
HUY@S-TOH/AuPd
architecture
that
Based on the above experimental results and mechanistic
studies, a plausible photocatalytic mechanism can be proposed
for the Suzuki–Miyaura coupling over the HUY@S-TOH/AuPd
photocatalyst (Fig. 5). Under the irradiation of visible light,
hydrogenated titania can absorb part of the visible light and
produce electron-hole pairs. The photo-induced electrons can
be injected into Au/Pd nanoparticles.57 In parallel, the
produced hot electrons, resulting from strong LSPR effect of
Au, could be injected into Pd shell to increase the electron
density of Pd.58 Subsequently, the electron-rich Pd NPs can
activate C–X bond of aryl halide and facilitate the oxidative
addition step that seems to be the rate-determining step of
the Suzuki reaction. On the other hand, the photo-generated
holes can transfer to protic organic solvent, such as EtOH, or
can activate the phenylboronic acid by cleaving the C−B
bonds.56, 59 While the oxidized phenyl boronic acids transfer to
the activated aryl halides, the cross-coupling reaction occurs to
produce the desired products via reductive elimination.
The photo-stability and reusability tests, as important
characteristics in photocatalytic reactions, were performed to
explore the stability of designed architecture. After
photocatalytic Suzuki coupling of 4-iodoanisole and
phenylboronic acid, the HUY@S-TOH/AuPd photocatalyst was
separated from the reaction solution by centrifugation,
washed with hot EtOH, and then reused for the same reaction.
As becomes clear from Fig. 6, the nanoarchitecture indicated
high catalytic performance for six cycles without significant
loss of activity (every run is complete during 30 min).
demonstrated excellent photocatalytic activity in the visible-
light-promoted Suzuki coupling reactions under ambient
conditions. The improved activity of catalyst can be attributed
to the hierarchical structure with high accessible surface area,
high anchoring sites for Au/Pd, efficient light harvesting in
urchin-like Y@S structure, and considerable photon absorption
efficiency of visible light due to existence of oxygen vacancy as
well as noble metals NPs. Moreover, the cooperative
promoting effects between plasmonic Au and Pd NPs is an
effective way for transferring energetic electrons from gold to
palladium, by LSPR effect. A mechanistic investigation of
photocatalytic Suzuki reaction suggested that photo-induced
holes are transferred into EtOH or phenylboronic acid,
whereas photo-generated electrons are injected to Pd NPs
which can accelerate the coupling reaction. This synthetic
approach for structuring Au/Pd NPs on the surface of
hydrogenated titania in advanced architectures can be
extended to rational design of structures with greatly
improved photocatalytic activity for various energy
applications and also other cross-coupling reactions using
visible light.
Fig. 7. Comparison of the maximum TOF value of HUY@S-TOH/AuPd with those of
previously published for visible-light Suzuki reactions.
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
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Catalysis Science & Technology
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ARTICLE
37.
38.
X. Chen, L. Liu and F. Huang, Chem. Soc. Rev., 2015, 44, 1861-1885.
Conflicts of interest
There are no conflicts to declare.
View Article Online
S. Rohani, G. Mohammadi Ziarani, A. Badiei, A. Ziarati, M. Jafari and A.
DOI: 10.1039/C9CY00798A
Shayesteh, Appl. Organomet. Chem., 2018, e4397.
39.
40.
A. Ziarati, A. Badiei, R. Grillo and T. Burgi, ACS Appl. Mater. Interfaces,
2019, 11, 5903-5910
A. Ziarati, A. Badiei, R. Luque and W. Ouyang, J. Mater. Chem. A, 2018, 6,
8962-8968.
Acknowledgements
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A. Ziarati, A. Badiei and R. Luque, Appl. Catal. B: Environ., 2019, 240, 72-78.
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The financial support from the research council of Alzahra
University, University of Geneva, and the University of Tehran
are gratefully acknowledged. SR would like to thank Ani
Baghdasaryan for her help in NMR measurements.
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DOI: 10.1039/C9CY00798A
TOC entry
Engineering of Highly Active Au/Pd Supported on Hydrogenated Urchin-like
Yolk@Shell TiO2 for Visible Light Photocatalytic Suzuki Coupling
Sahar Rohani, a,b Abolfazl Ziarati, b,c Ghodsi Mohammadi Ziarani, a Alireza Badiei, c Thomas Burgi b,*
Engineered hydrogenated urchin-like yolk@shell TiO2 decorated with Au/Pd nanoparticles were
designed via sequential steps and employed in visible light photocatalytic Suzuki coupling.