Photocatalytic Dehalogenation of Aromatic Halides on Ta2O5-Supported Pt-Pd Bimetallic Alloy Nanoparticles Activated by Visible Light

Article abstract of DOI:10.1021/acscatal.7b01735

Dehalogenation of aromatic halides is one important reaction for detoxification and organic synthesis. Photocatalytic dehalogenation with alcohol, a safe hydrogen source, is one promising method; however, systems reported earlier need UV irradiation. We found that Pt-Pd bimetallic alloy nanoparticles (ca. 4 nm) supported on Ta₂O₅ (PtPd/Ta₂O₅), on absorption of visible light (λ > 450 nm), efficiently promote dehalogenation with 2-PrOH as a hydrogen source. Catalytic dehydrogenation of 2-PrOH on the alloy in the dark produces hydrogen atoms (H) on the particles. Photoexcitation of d electrons on the alloy particles by absorbing visible light produces hot electrons (e_hot^-). They efficiently reduce the adsorbed H atoms and produce hydride species (H^-) active for dehalogenation. The catalytic activity depends on the Pt/Pd mole ratio; alloy particles consisting of 70 mol % of Pt and 30 mol % of Pd exhibit the highest activity for dehalogenation.

Full text of DOI:10.1021/acscatal.7b01735

Article
Photocatalytic Dehalogenation of Aromatic Halides on Ta2O5-
Supported Pt-Pd Bimetallic Alloy Nanoparticles Activated by Visible Light
Hirokatsu Sakamoto, Jun Imai, Yasuhiro Shiraishi, Shunsuke Tanaka, Satoshi Ichikawa, and Takayuki Hirai
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01735 • Publication Date (Web): 28 Jun 2017
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Photocatalytic Dehalogenation of Aromatic Halides on Ta2O5-
Supported Pt–Pd Bimetallic Alloy Nanoparticles Activated by Visible
Light
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Hirokatsu Sakamoto,† Jun Imai,† Yasuhiro Shiraishi,*,†,‡ Shunsuke Tanaka,‖ Satoshi Ichikawa,§ and
Takayuki Hirai†
† Research Center for Solar Energy Chemistry, and Division of Chemical Engineering, Graduate School of Engineering
Science, Osaka University, Toyonaka 560-8531, Japan
‡ Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST),
Saitama 332-0012, Japan
‖ Department of Chemical, Energy and Environmental Engineering, Kansai University, Suita 564-8680, Japan
§ Institute for NanoScience Design, Osaka University, Toyonaka 560-8531, Japan
ABSTRACT: Dehalogenation of aromatic halides is one important reaction for detoxification and organic synthesis. Photocatalytic
dehalogenation with alcohol, a safe hydrogen source, is one promising method; however, early-reported systems need UV irradiation.
We found that Pt–Pd bimetallic alloy nanoparticles (ca. 4 nm) supported on Ta2O5 (PtPd/Ta2O5), when absorbing visible light (λ >450
nm), efficiently promotes dehalogenation with 2-PrOH as a hydrogen source. Catalytic dehydrogenation of 2-PrOH on the alloy in
the dark produces hydrogen atoms (H) on the particles. Photoexcitation of d electrons on the alloy particles by absorbing visible light
produces hot electrons (ehot–). They efficiently reduce the adsorbed H atoms and produce hydride species (H–) active for
dehalogenation. The catalytic activity depends on the Pt/Pd mole ratio; alloy particles consisting of 70 mol % of Pt and 30 mol % Pd
exhibit the highest activity for dehalogenation.
KEYWORDS: Photocatalysis· Visible light · Alloy nanoparticle · Hot electron · Dehalogenation
INTRODUCTION
removed Cl– forms NaCl by the reaction with H+ and NaOH (eq.
5). The overall reaction can be expressed as eq. 6.
Catalytic dehalogenation of aromatic halides is one of the
very important reactions for detoxification and organic
synthesis.1,2 It is usually carried out on heterogeneous catalysts
loaded with Pd or Rh nanoparticles with H2 as a hydrogen
source,3–5 which holds potential explosion risks. Alternative
method using a safe hydrogen source such as alcohols is
RCH2OH + 2h+ → RCHO + 2H+
H+ + e– + M → H–M
(1)
(2)
(3)
(4)
(5)
desirable. Pd or Rh nanoparticles supported on carbon6–8 or
H–M + e– → H––M
9,10
Fe2O3
promote the reaction with alcohol as a hydrogen
Ar–Cl + H––M → Ar–H + Cl– + M
Cl– + H+ + NaOH → H2O + NaCl↓
source, but need relatively high temperature (>340 K). Design
of catalytic system that efficiently promotes dehalogenation
with alcohol at room temperature is desired.
Photocatalysis has also been studied extensively since it
promotes dehalogenation with alcohols at room temperature
under light irradiation.11–14 Semiconductor TiO2 loaded with Pd
or Rh nanoparticles has been used as a catalyst with base such
as NaOH.15–20 Photoexcitation of the catalysts produces
conduction band electrons (eCB–) and valence band holes (hVB+).
Ar–Cl + RCH2OH + NaOH → Ar–H + RCHO + H2O + NaCl↓
(6)
These reactions proceed at room temperature with alcohol, and
they enable safe, green, and sustainable dehalogenation. These
early-reported systems, however, need UV light for catalyst
photoexcitation. A new catalyst that promotes dehalogenation
by visible light is therefore the most desired.
+
The hVB oxidize alcohols and produce aldehydes (or ketones)
and protons (H+) (eq. 1). The H+ are reduced by the e–
accumulated on the metal, producing hydrogen atoms (H–M)
(eq. 2). They are further reduced with e– (eq. 3), producing
hydride species (H––M) active for dehalogenation (eq. 4). The
A metal nanoparticles/semiconductor system driven by light
absorption of metal particles21 is one promising system for
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visible-light-driven photocatalysis, where Au22–28 or Pt29–32
Preparation and characterization of catalysts. The Pt1–
particles supported on semiconductors such as TiO2 have been
used. In these systems, metal particles absorb light and produce
hot electrons (ehot–) by the activation of their electrons.33,34 The
ehot– are injected into the semiconductor CB and the eCB– reduce
substrates, while the positively-charged hot holes (δ+) on the
metal oxidize substrates, thus promoting photocatalytic
xPdx/Ta2O5 catalysts were prepared by two step procedures
consisting of simultaneous impregnation of H2PtCl6·6H2O and
Pd(NO3)2 onto Ta2O5 [average diameter, 1.3 μm; surface area,
2.6 m2 g–1] followed by H2 reduction (see Experimental
Section).16,37 Total metal loadings on Ta2O5 were set at 8.8
mol % [= (Pt + Pd) / Ta2O5 × 100], and x denotes the mole
fraction of Pd in the metal particles [x = Pd / (Pt + Pd)].
Transmission electron microscopy (TEM) observation of
Pt0.7Pd0.3/Ta2O5 (Figure 1a and b) exhibits round and spherical
metal particles with average diameter 3.8 nm. Figure 1c and d
indicate that these particles can be indexed as face-centered
cubic (fcc) structures. As shown in Figure S1 (Supporting
Information), Pt1/Ta2O5, Pt0.5Pd0.5/Ta2O5, and Pd1/Ta2O5 contain
metal particles with similar diameters (3.8, 3.5 and 4.0 nm,
respectively), indicating that Pd alloying scarcely affects the
particle diameter.
reactions. Recently, we designed
a
new metal
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nanoparticles/semiconductor system; Pt particles supported on
semiconductor Ta2O5 promote aerobic oxidation under visible
light with significantly high quantum yield ~25 % (550 nm),35
which is the highest yield among the systems reported earlier.
As shown in Scheme 1, the Pt particles absorb visible light and
–
produce ehot– via interband transition of d electrons (ed ). In this
case, a high Schottky barrier (ϕB) is created at the Pt–Ta2O5
interface due to their large Fermi level difference.36 The ehot– on
Pt are therefore scarcely injected into the Ta2O5 CB. In contrast,
strong Pt–Ta2O5 interaction leads to donation of Ta2O5 CB
electrons to Pt particles and increases d electron density of the
particles. This enhances interband transition and produces a
40
b
a
3.8 ± 0.9 nm
30
20
10
0
–
large number of ehot–. Efficient ehot donation to O2 produces
peroxide species active for oxidation, significantly enhancing
aerobic oxidation.
Scheme 1. Aerobic oxidation on Pt/Ta2O5 under visible light.a
0
1 2 3 4 5 6 7 8
Size / nm
Ta2O5
Pt
Pt
–
c
d
ehot
sp
B
ECB
EF
O2
–
ed
d
LUMO
(π*)
HOMO
(π)
O2
sp
EVB
Figure 1. (a) TEM image of Pt0.7Pd0.3/Ta2O5 and (b) size
distribution of alloy particles. (c, d) High-resolution images.
a EF, Fermi level; ϕB [(eV) = W – ], height of Schottky barrier;
W, work function of Pt (eV); , electron affinity of Ta2O5 (eV).
X-ray photoelectron spectroscopy (XPS) of the alloy catalysts
(Figure S2, Supporting Information) reveals that Pt 4f, 4d, and
Pd 3d peaks appear at the positions similar to those of Pt- or Pd-
only catalysts because electronegativity of Pt (2.28) and Pd
(2.22) are similar.38,39 The Pt/Pd ratio of Pt0.7Pd0.3/Ta2O5
determined by XPS analysis is 2.38 mol/mol. This is similar to
the mole ratio of total Pt and Pd amounts determined by the
inductively coupled plasma (ICP) analysis (2.25 mol/mol) after
dissolution of the catalyst with aqua regia. In addition, these
ratios are close to the mole ratio of the Pt and Pd precursors
(2.43 mol/mol). As shown in Figure 1c, d, the lattice spacing of
the alloy particles (111) determined by TEM (0.226 nm) is in
between that for standard Pt(111) (JCPDS 04-0802, 0.227 nm)
and Pd(111) (JCPDS 05-0681, 0.225 nm), and is similar to the
value calculated based on the Vegards law (0.226 nm). These
The present work aims at application of the Pt/Ta2O5 catalyst
to photocatalytic dehalogenation with alcohol as a hydrogen
source. We loaded Pt-Pd bimetallic alloy particles (ca. 4 nm) on
Ta2O5. Visible light irradiation of the PtPd/Ta2O5 catalyst in 2-
PrOH with NaOH efficiently promotes dehalogenation of
aromatic halides. The photoreaction results and spectroscopic
evidence revealed that catalytic dehydrogenation of 2-PrOH on
the alloy particles in the dark produces H atoms. They are
–
efficiently reduced by a large number of ehot formed by
interband transition of the alloy particles by absorbing visible
light. This produces a large number of H– species and enables
highly efficient dehalogenation.
RESULTS AND DISCUSSION
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findings clearly indicate that Pt and Pd components are mixed
Photocatalytic activity. Photocatalytic dehalogenation of p-
homogeneously. As shown in Figure 2a, diffuse-reflectance
(DR) spectrum of Pt1/Ta2O5 shows a broad absorption at λ >350
nm, assigned to interband transition of Pt.40 Pd1/Ta2O5 shows
similar absorption assigned to light scattering by Pd particles.16
The alloy catalysts therefore exhibit similar absorption
regardless of their Pt/Pd ratios.
chlorotoluene was performed. 2-PrOH (5 mL) containing p-
chlorotoluene (0.1 mmol), catalyst (20 mg), and NaOH (0.3
mmol) was stirred under Ar atmosphere in the dark or under
irradiation of visible light (λ >450 nm). The temperature of the
solutions was maintained at 298 ± 0.5 K. Figure 3 summarizes
the amount of toluene formed by 6 h reaction. In that, GC
analysis of the solution for all systems detected only toluene and
acetone as products. As shown by black bars, in the dark
condition, Pt1/Ta2O5 produces very small amount of toluene (9
μmol). Increasing Pd amount in the alloy increases the activity;
Pt0.8Pd0.2 and Pt0.7Pd0.3/Ta2O5 catalysts produce relatively large
amounts of toluene (ca. 40 μmol). Further Pd alloying, however,
decreases the activity, where Pd1/Ta2O5 produces very small
amount of toluene (11 μmol).
10
b
0.56
0.52
0.48
0.44
0.4
Pt0.7Pd0.3/Ta2O5
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4.8 %
5
a
2
1
0
0
Pt1
400
500
600
/ nm
700
As shown by orange bars (Figure 3), Pt1/Ta2O5 shows almost
no activity enhancement even under visible light irradiation. In
contrast, the alloy catalysts enhance the activity, where
Pt0.7Pd0.3/Ta2O5 produces the largest amount of toluene (93
μmol), which is twice that obtained in the dark. Further Pd
alloying, however, decreases the activity. In addition, a physical
mixture of Pt0.7/Ta2O5 and Pd0.3/Ta2O5 shows very low activity.
These findings indicate that the Pt-Pd alloy containing small
amount of Pd promotes dehalogenation efficiently under visible
light. It must be noted that, as indicated by ″reuse 1 and 2″ in
Figure 3, the Pt0.7Pd0.3/Ta2O5 catalyst recovered after the
reaction, when reused for further reaction, produces 91 mol
and 92 mol toluene, respectively, which are almost the same
amounts obtained by the fresh catalyst (93 mol). This indicates
that the catalyst is reusable without loss of catalytic activity. It
is also noted that ICP analysis of the solution recovered after
the photoreaction does not detect Pt or Pd component. This
indicates that leaching of these components from the alloy
particles does not occur.
Pt0.7Pd0.3
Pt0.5Pd0.5
Ta2O5
400
Pd1
600
nm
800
Figure 2. (a) DR UV-vis spectra of the catalysts and (b) action
spectrum for photocatalytic dehalogenation of p-chlorotoluene on
Pt0.7Pd0.3/Ta2O5. Apparent quantum yield for toluene formation
(ΦAQY) was determined with the equation: ΦAQY (%) = [{(Yvis
–
Ydark) ×2} / (photon number entered into the reaction vessel)] 
100, where Yvis and Ydark are the toluene formed (μmol) under light
irradiation and in the dark, respectively.
It is also noted that, as shown in Figure 3, Pt1/Ta2O5 produces
large amount of H2 during the reaction, although
dehalogenation scarcely occurs. This is because, as shown in eq.
7, Pt has a low over potential for H2 generation;41 the H atoms
produced on the particles are easily removed as H2:
a
dark
λ >450 nm
100
H–M + H–M → H2↑ + 2M
(7)
50
In contrast, the alloy catalysts scarcely produce H2 during the
reaction. This indicates that they promote dehalogenation
preferentially, while suppressing H2 formation.
toluene
H2
Figure 4 shows the time-dependent change in the amounts of
substrate and products on Pt0.7Pd0.3/Ta2O5 during reaction of p-
chlorotoluene under visible light. After 6 h photoreaction, p-
chlorotoluene (100 mol) is transformed to toluene (100 mol)
almost completely together with a formation of acetone (100
mol). This indicates that, as shown by eq. 6, its –Cl group is
removed stoichiometrically via the formation of toluene,
acetone, and NaCl.16 It is also noted that, after complete
transformation of p-chlorotoluene to toluene (>6 h), H2 starts to
generate. This suggests that the alloy catalysts preferentially
promote dehalogenation, while suppressing H2 formation. As
shown in Figure S3 (Supporting Information), similar tendency
0
Figure 3. Amount of toluene and H2 formed during the reaction of
p-chlorotoluene on the respective catalysts for 6 h, (black) in the
dark or (orange) under visible light irradiation (λ >450 nm; light
intensity at 450–800 nm, 16.8 mW cm–2). The reuse 1 and reuse 2
show the data obtained with Pt0.7Pd0.3/Ta2O5 catalyst when reused
for further reactions. Catalysts were reused after simple washing
with 2-PrOH followed by drying in vacuo.
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is observed in the dark condition, although the dehalogenation
rate is slower.
H–PtPd + ehot– → H––PtPd
(10)
(11)
1/2RCH2OH + δ+ → 1/2RCHO + H+
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Nucleophilic addition of H– to aromatic halide (e) promotes
dehalogenation (eq. 4). The removed Cl– are solidified by the
reaction with H+ and NaOH (eq. 5).
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50
0
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Scheme 2. Possible mechanism for dehalogenation on PtPd/Ta2O5.
H2
10
NaOH
0
2
4
6
8
12
t / h
Figure 4. Time course for the amounts of substrates and products
during reaction of p-chlorotoluene on Pt0.7Pd0.3/Ta2O5 under λ >450
nm irradiation.
e
a
dark
b
Action spectrum for dehalogenation on Pt0.7Pd0.3/Ta2O5
obtained by monochromatic light irradiation is shown in Figure
2b. A good correlation is observed between the apparent
quantum yields for toluene formation (ΦAQY) and absorption
spectrum of the catalyst. This clearly indicates that interband
transition of d electrons on the alloy by absorbing visible light
promotes dehalogenation. The Pt0.7Pd0.3 alloy particles loaded
on TiO2 (Pt0.7Pd0.3/TiO2), when used for photocatalytic
dehalogenation under visible light irradiation, produces only
8.1 mol toluene, which is much lower than that of
Pt0.7Pd0.3/Ta2O5 (93 mol; Figure 3). This indicates that, as
described in our Pt/Ta2O5 system,35 d electron density of the
alloy particles increased by strong interaction with Ta2O5
support enhances the interband transition of d electrons by
visible light and produces a large number of ehot–, leading to
efficient dehalogenation.
VIS
c
1
2
d
1
2
Dehydrogenation of 2-PrOH. As shown in Scheme 2a, the
first step of dehalogenation is the dehydrogenation of 2-PrOH,
producing H atoms on the metal surface. The reaction occurs ″in
the dark″. This is confirmed by the reaction of 2-PrOH without
p-chlorotoluene. Figure 5 shows the amount of acetone formed
in the dark or under visible light. In both cases, Pt1/TiO2 shows
the highest activity, and the activity decreases by the Pd
alloying. The rate-determining step for this reaction is the H
abstraction of alcohol on the metal,42,43 and its activity depends
on the position of metal d-band center relative to their Fermi
levels.44 As shown in Scheme 3a, adsorption of alcohol onto the
metal creates bonding () and antibonding (*) states between
the d-band center and HOMO and LUMO of alcohol,
respectively.45,46 The HOMO electrons of alcohol are donated
to the bonding state and undergoes dehydrogenation.47 The d-
band center of Pt and Pd lies at –2.2 and –1.8 eV, respectively,48
indicating that Pd creates upper-shifted bonding state. This
suppresses electron donation from HOMO of 2-PrOH to the
bonding state and shows lower dehydrogenation activity than Pt
(Figure 5). Increasing amount of Pd alloying leads to upper-
shift of the d-band center48 and decreases the dehydrogenation
activity. It is note that, as shown in Figure 5, the amount of
acetone formed under visible light (orange) is similar to that
formed in the dark (black). This means that ehot– formed on the
metal by light absorption scarcely enhances dehydrogenation of
2-PrOH. These results clearly suggest that, during
Dehalogenation mechanism. The dehalogenation proceeds
via the three steps consisting of (i) formation of H atoms on the
metal surface by dehydrogenation of 2-PrOH, (ii) formation of
H– species by the reduction of H atoms, and (iii) dehalogenation
by the H– species. The mechanism on the PtPd/Ta2O5 catalyst
can be explained by Scheme 2. In the first step (a), 2-PrOH is
adsorbed on the metal and undergoes dehydrogenation in the
dark condition, producing acetone and H atoms. In the dark, the
–
adsorbed H atoms are reduced by the d electrons (ed ) of metal
and produce H– species (b):
H–PtPd + ed– → H––PtPd
(8)
Upon visible light irradiation, interband transition of ed– on the
metal produces a large number of ehot– and δ+ pairs (c).
H–PtPd + hv →H–PtPd (ehot– + δ+)
(9)
The ehot– reduce the adsorbed H atoms and create a large number
of H– (d), where the δ+ on the metal are consumed by oxidation
of 2-PrOH.
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dehalogenation, the H atoms are mainly produced by the dark
and produces H– species. However, as shown in Figure 3,
reaction. It is also noted that Pd1/Ta2O5 produces very small
amount of acetone (Figure 5). This suggests that very low
dehalogenation activity of Pd1/Ta2O5 (Figure 3) is due to the
low activity of dehydrogenation of 2-PrOH, producing very
small amount of H atoms on the surface.
Pd1/Ta2O5 shows very low dehalogenation activity in the dark.
This is because it is less active for dehydrogenation of 2-PrOH
and scarcely produces H atoms on the surface. The PtPd/Ta2O5
alloy catalysts promote dehydrogenation of 2-PrOH (formation
of H atoms) relatively efficiently and stabilizes the H atoms by
relatively strong H–M binding. This reduces the H atoms
efficiently (formation of H–) and promotes efficient
dehalogenation in the dark.
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Scheme 3. Energy diagram for the electronic states of (a) metal–
alcohol and (b) metal–H bonding.
60
30
0
100
50
0
a
antibonding state
dark
 >450 nm
(σ*)
Pd
EF
PtPd
LUMO
(π*)
Pt
e–
HOMO
(π)
d-band center
alcohol
bonding state
(σ)
–
b
ehot
Figure 5. Formation of acetone by dehydrogenation of 2-PrOH on
Pt0.7Pd0.3/Ta2O5 at 298 K. The selectivity for H2 formation defined
by eq. 12 was also shown in the figure.
antibonding state
(σ*)
Pd
EF
–
ed
Formation of H– in the dark. The second step for
dehalogenation is the reduction of H atoms on the metal,
producing H– species active for dehalogenation. In the dark, as
shown in Figure 3, Pt1/Ta2O5 shows very low dehalogenation
activity, although H atoms are produced efficiently by the
enhanced dehydrogenation of 2-PrOH (Figure 5). This is
because the formed H atoms are readily removed as H2 gas due
to the weak H–M interaction. Figure 5 (right) shows the H2
selectivity defined as the ratio of the amount of formed H2 to
that of formed acetone, as follows:
PtPd
H 1s
Pt
d-band center
bonding state
(σ)
Formation of H– under visible light. Upon visible light
irradiation, PtPd/Ta2O5 shows much enhanced dehalogenation
activity than that in the dark (Figure 3). This is because the H
atoms on the metal particles are efficiently reduced by the
photogenerated ehot–, producing a large number of H– that
promotes efficient dehalogenation.15–20 As shown in Figure 6,
diffuse-reflectance infrared Fourier transform (DRIFT) analysis
of H2-adsorbed Pt0.7Pd0.3/Ta2O5 confirms this. H2 adsorbed in
the dark (black) shows a band at 1963 cm–1 for the H–M
species.52 Visible light irradiation of the sample (blue), however,
shows a red-shifted band at 1948 cm–1. This suggests that
negatively-charged H– species, which are adsorbed on the metal
surface more strongly than H atoms,53 are indeed generated by
visible light irradiation. This indicates that, as shown in Scheme
3b, the donation of ehot– on the alloy to the bonding state of the
adsorbed H atoms indeed produces H– efficiently. As a result of
this, a large number of H– formed by visible light irradiation
enhances dehalogenation of aromatic halides (Figure 3).
H2 selectivity (%) = [H2] / [acetone]  100
(12)
The H2 selectivity on Pt1/Ta2O5 is almost 100%, indicating that
the formed H atoms are readily removed as H2. This can be
explained by the lower d-band center of Pt. As shown in
Scheme 3b, the H atoms adsorbed on the Pt surface creates
antibonding state at a level relatively lower than the Fermi level;
therefore, this state is occupied by the electrons. This weakens
H–Pt interaction and allows H2 generation (eq. 7), which is
related to the lower over potential for H2 generation on Pt.49 In
contrast, as shown in Figure 5, the H2 selectivity decreases with
an increase in the Pd amount within the alloy. This means that
the H atoms formed on the metal surface remain without H2
formation. This is because, as shown in Scheme 3b, the Pd d-
band center lies at upper level. This strengthens H–M
interaction and suppresses H2 generation.50,51 This therefore
promotes efficient donation of d electrons to the bonding state
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Table 1. Dehalogenation results on Pt0.7Pd0.3/Ta2O5.a
 >450 nm irradiation
dark
 >450 nm irradiation
4
5
6
7
8
9
time
/ h
Yield
/ % b
No.
1
substrate
light
product
+
–
+
–
+
–
+
–
+
–
+
–
+
–
+
–
+
–
+
–
>99
66
12
6
without H2
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F(R) 0.0002
43
>99
55
6
2200
2100
2000
 / cm–1
1900
>99
43
4
6
98
Figure 6. DRIFT spectra of H2-adsorbed Pt0.7Pd0.3/Ta2O5 measured
at 303 K (black) in the dark and (blue) after 450 nm light irradiation.
Catalyst (20 mg) was evacuated (6.8 × 10–3 Torr) at 423 K for 3 h.
H2 (15 Torr) was added and left for 1 h in the dark or under
photoirradiation. Green line shows the spectrum obtained at 303 K
after photoirradiation without H2.
5
8
63
98
6
5
66
>99
74
7
96
12
24
24
Dehalogenation of aromatic halides. The Pt0.7Pd0.3/Ta2O5
catalyst was used for dehalogenation of substituted aromatic
halides. As listed in Table 1, several aryl chlorides and
bromides are transformed to the dehalogenated products with
more than 89 % yields, while maintaining the substituents. As
shown in Scheme 4, the dehalogenation step involves two
consecutive reactions consisting of (i) nucleophilic addition of
H– to aromatic halide, and (ii) abstraction of Cl– from the
intermediate. To clarify its kinetics, Hammett plot analysis54
was carried out. Dehalogenation of chlorobenzene (1) and some
p-substituted ones (2, 6, and 7) was performed on
Pt0.7Pd0.3/Ta2O5 for 1 h in the dark or light irradiation condition.
The obtained first-order rate constants, k (mM h–1), are shown
in Table S1 (Supporting Information). Figure 7 plots the logk
versus the substituent constant, σ.55 Almost identical
relationship is observed in both conditions with the slope
(Hammett constant, ) –0.96. The results clearly indicate that
dehalogenation in the both conditions proceeds via the same
mechanism consisting of (i) nucleophilic H– addition and (ii)
abstraction of Cl–, as summarized in Scheme 4.
89
8
59
91
9
55
89
10
63
+
–
+
–
91
68
89
62
11
12
12
24
a Reaction conditions: 2-PrOH (5 mL), substrate (0.1 mmol), NaOH
(0.3 mmol), catalyst (20 mg), Ar (1 atm), in the dark or under
visible light irradiation, temperature (298 K). b = (product formed)
/ (initial amount of substrate) × 100.
Scheme 4. Mechanism for dehalogenation by H– species.
The negative  indicates that electron-donating substituents
accelerate dehalogenation, although nucleophilic H– addition
(step i) is accelerated by electron-withdrawing substituents.56
This is because the abstraction of Cl– (step ii) is the rate-
determining step; this occurs slowly in the present system
operated at room temperature. The Cl– abstraction is accelerated
by the electron-donating substituents because they stabilize the
negative charge of intermediate.57 This therefore results in
negative  for the present systems (Figure 7). Visible light
i
ii
H+ + NaOH
irradiation enhances the H– formation by photogenerated ehot
–
and efficiently promotes the first step (nucleophilic addition of
H– to aromatic halides). This produces a large number of the H-
adduct intermediates and accelerates the second step (Cl–
abstraction) accordingly.
Effect of the alloy amount. The activity of Pt0.7Pd0.3/Ta2O5
catalyst depends on the loaded amount of alloy particles. This
is confirmed by the dehalogenation reactions on the
Pt0.7Pd0.3/Ta2O5 catalysts with different loadings of Pt0.7Pd0.3
alloy [(Pt + Pd) / Ta2O5 × 100 = 2.2, 4.4, 8.8, and 13 mol %].
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As shown in Figure 8 (blue), the sizes of alloy particles on the
CONCLUSION
respective catalysts are similar (3.7–4.0 nm). The black and
orange bars show the amounts of toluene formed in the dark and
under photoirradiation, respectively. The dark activity increases
with the alloy loadings because the increased surface area of
alloy produces larger number of H–. Photoirradiation further
enhances the reaction, but the enhancement depends on the
alloy amount; low-alloy-loading catalysts show small
enhancement, but high-alloy-loading catalysts shows large
enhancement. As shown in Figure S4 (Supporting Information),
absorbance of alloy particles in the visible region decreases with
a decrease in the alloy loadings. This indicates that efficient
light absorption by the alloy particles is the important factor for
photocatalytic dehalogenation.
We found that Pt-Pd bimetallic alloy particles supported on
Ta2O5 efficiently promote photocatalytic dehalogenation under
visible light at room temperature with 2-PrOH as a hydrogen
source. The alloy particles absorbing visible light promote
interband transition of d electrons and produces large number
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5
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7
8
9
–
of ehot–. The ehot efficiently reduce the H atoms produced by
dehydrogenation of 2-PrOH and produce H– species active for
dehalogenation. The activity depends on the Pt/Pd ratio; the
alloy particles consisting of 70 mol % Pt and 30 mol % Pd
exhibit the highest activity. The basic concept presented here
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–
based on efficient formation of H– by the photogenerated ehot
on the metal under visible light may contribute to the creation
of more efficient photocatalytic system for dehalogenation and
to the design of new system for sunlight-driven selective
organic transformations.
0.5
2
6
EXPERIMENTAL SECTION
1
Catalyst preparation. Ta2O5 particles were supplied from
Wako. Pt1-xPdx/Ta2O5 catalysts with metal loadings 8.8 mol %
[x = Pd/(Pt + Pd); x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, and 1]
were prepared as follows: Ta2O5 (1 g) was added to water (40
mL) containing H2PtCl6·6H2O and Pd(NO3)2 [57/0, 50/2, 44/5,
39/7, 33/10, 11/20, and 0/25 mg/mg, respectively]. Water was
removed by evaporation at 353 K with vigorous stirring for 12
h. The obtained powders were reduced under H2 flow at 673 K
(heating rate, 2 K min–1; holding time at 673 K, 2 h).
Pt0.7Pd0.3/TiO2 catalyst was prepared using P25 TiO2 (JRC-TIO-
4), kindly supplied from the Catalyst Society of Japan, in a
manner similar to that of Pt0.7Pd0.3/Ta2O5.
Photoreaction. 2-PrOH (5 mL) containing catalyst (20 mg),
p-chlorotoluene (0.1 mmol), and NaOH (0.3 mmol) were added
to a Pyrex glass tube (φ 12 mm; capacity, 20 mL). The tube was
sealed with a rubber cap and purged with Ar gas (5 min). The
tube was set in a water bath, whose temperature was kept
digitally at 298  0.5 K.22 The tube was irradiated at λ >450 nm
under magnetic stirring by a 2 kW Xe lamp (USHIO Inc.) in a
manner similar to that of our Pt/Ta2O5 system.35 After the
reaction, the gas phase was analyzed by GC-BID (Shimadzu,
GC-2010 plus). The catalyst was recovered by centrifugation,
and the resulting solution was analyzed by GC-FID (Shimadzu,
GC-2010). The substrate and product amounts were calibrated
with authentic samples. All reactions and analysis were
performed twice, where the errors were less than 1%. Action
spectrum was obtained by the reaction in 2-PrOH (2 mL)
containing catalyst (8 mg), p-chlorotoluene (0.2 mmol), and
NaOH (0.5 mmol) with monochromated light.58
0

= - 0.96
- 0.5
- 1
R2 = 0.997
7
- 0.5
0
0.5
1

Figure 7. Hammett plot for dehalogenation of some p-substituted
chlorobenzenes on Pt0.7Pd0.3/Ta2O5 (black) in the dark or (orange)
under photoirradiation. The k denotes the first-order rate constant
for dehalogenation (mM h–1) determined by 1 h reaction. The
reaction conditions are identical to those in Figure 3.
150
100
50
5
4
3
2
1
0
0
2.2
4.4
8.8
13
Alloy loading / mol%
Analysis. DRIFT spectra were measured on a FT/IR-610
system (JASCP Corp.),59 where 450 nm light was irradiated by
a Xe lamp (300 W; Asahi Spectra Co. Ltd.; Max-302; light
intensity, 68 W cm–2). Spectral irradiance of the light sources
are shown in Figure S5 (Supporting Information). TEM
observations,60 XRD, XPS, and DR UV-vis measurements were
performed according to the procedure described previously.61
Figure 8. Effect of the loaded amount of Pt0.7Pd0.3 alloy on the
toluene formation by dehalogenation of p-chlorotoluene (6 h
reaction). Circles show average sizes of Pt0.7Pd0.3 alloy on the
catalysts determined by TEM (Figure S1, Supporting Information).
Reaction conditions: 2-PrOH (5 mL), substrate (0.2 mmol), NaOH
(0.5 mmol), catalyst (20 mg), Ar (1 atm), in the dark or under
visible light, temperature (298 K). In all runs, H2 was scarcely
detected, indicating that H atoms of 2-PrOH are selectively used
for dehalogenation.
ASSOCIATED CONTENT
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(23) Sugano, Y.; Shiraishi, Y.; Tsukamoto, D.; Ichikawa, S.; Tanaka,
Parameter for Hammett analysis (Table S1), TEM images and size
distribution of alloy particles (Figure S1), XPS charts (Figure S2),
Time profiles for the dehalogenation reaction in the dark (Figure
S3), DR UV-vis spectra (Figure S4), Spectral irradiance of light
sources (Figure S5). This material is available free of charge via the
Internet at http://pubs.acs.org.
S.; Hirai, T. Angew. Chem., Int. Ed. 2013, 52, 5295–5299.
(24) Primo, A.; Corma, A.; García, H. Phys. Chem. Chem. Phys.
2011, 13, 886–910
(25) Jia, H.; Zhu, X.-M.; Jiang, R.; Wang, J. ACS Appl. Mater.
Interfaces 2017, 9, 2560–2571.
(26) Tanaka, A.; Hashimoto, K.; Kominami, H. J. Am. Chem. Soc.
2012, 134, 14526–14533.
(27) Kumar, D.; Lee, A.; Lee, T.; Lim, M.; Lim, D.-K. Nano Lett.
2016, 16, 1760–1767.
AUTHOR INFORMATION
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Corresponding Author
(28) Zhang, Z.; Li, A.; Cao, S.-W.; Bosman, M.; Li, S. Nanoscale
2014, 6, 5217–5222.
shiraish@cheng.es.osaka-u.ac.jp
(29) Shiraishi, Y.; Tsukamoto, D.; Sugano, Y.; Shiro, A.; Ichikawa,
S.; Tanaka, S.; Hirai, T. ACS Catal. 2012, 2, 1984–1992.
(30) Zai, W.; Xue, S.; Zhu, A.; Luo, Y.; Tian, Y. ChemCatChem
2011, 3, 127–130.
(31) Shiraishi, Y.; Sakamoto, H.; Fujiwara, K.; Ichikawa, S.; Hirai,
T. ACS Catal. 2014, 4, 2418–2425.
(32) Shiraishi, Y.; Sakamoto, H.; Sugano, Y.; Ichikawa, S.; Hirai, T.
ACS Nano 2013, 7, 9287–9297.
(33) Singh, J.; Singh, N. Phys. Rev. B 1978, 18, 2954–2960.
(34) Bobyrev, Y. V.; Petnikova, V. M.; Rudenko, K. V.; Shuvalov,
V. V. Quantum Electron. 2002, 32, 789–792.
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(36) Chun, W.-J.; Ishikawa, A.; Fujisawa, H.; Takata, T.; Kondo, J.;
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by the Grant-in-Aid for Scientific
Research (No. 26289296) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan (MEXT).
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