Enhancement of catalytic activity of AgPd@Pd/TiO2 nanoparticles under UV and visible photoirradiation

Article abstract of DOI:10.1039/c6ta05699g

The effects of photoirradiation for the production of hydrogen from the decomposition of formic acid (FA) were studied using Ag₉₃Pd₇@Pd/TiO₂ (anatase (A) or P25 (P)) nanocatalysts. The catalytic activity was enhanced by a factor of 1.5-1.6 under UV and visible (vis) photoirradiation at room temperature for both TiO₂ supports. It was explained by the formation of an electron-rich Pd shell because of the migration of photogenerated electrons from the TiO₂ surface. The catalytic activity of Ag₉₃Pd₇@Pd/TiO₂ (A) was 1.7-7.3 times higher than that of Ag₉₃Pd₇@Pd/TiO₂ (P) without and under photoirradiation at 27-90 °C. The catalytic activity of Ag₉₃Pd₇@Pd/TiO₂ (A) under photoirradiation at 27 °C with 468 mmol H₂ g per catalyst per h, is the best value ever reported out of all of the heterogeneous catalysts using TiO₂ (A) as a photocatalyst at room temperature.

Full text of DOI:10.1039/c6ta05699g

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Title
Enhancement of catalytic activity of AgPd@Pd/TiO nanoparticles
2
under UV & visible photoirradiation†
a
b
c
c
Masaharu Tsuji,* Daisuke Shimamoto, Keiko Uto, Masashi Hattori, and
b,c,d
Hiroki Ago
a
International Research and Education Center of Carbon Resources, Kyushu
University, Kasuga 816-8580, Japan. E-mail: tsuji@cm.kyushu-u.ac.jp
b
Department of Applied Science for Electronics and Materials, Graduate School
of Engineering Sciences, Kyushu University, Kasuga 816-8580, Japan
c
Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga
816-8580, Japan
ｄ
Art, Science, and Technology Center for Cooperative Research (KASTEC),
Kyushu University, Kasuga 816-8580, Japan
Abstract
Effects of photoirradiation for the hydrogen production from formic acid (FA)
decomposition were studied using Ag93Pd
nanocatalysts. The catalytic activity was enhanced by a factor of 1.5–1.6 under UV
visible (Vis) photoirradiation at room temperature for both TiO supports. It was
explained by the formation of electronꢀrich Pd shell because of migration of
photogenerated electrons from the TiO surface. The catalytic activity of
7 2
@Pd/TiO (anatase (A) or P25 (P))
&
2
2
Ag93Pd
7
@Pd/TiO
2
(A) was 1.7
−
7 2
7.3 times higher than that of Ag93Pd @Pd/TiO (P)
o
without and under photoirradiation at 27–90 C. The catalytic activity of
o
ꢀ1 ꢀ1
Ag93Pd
7
@Pd/TiO
2
(A) under photoirradiation at 27 C, 468 mmol H
2
g catalyst h ,
is the best value ever reported in the all heterogeneous catalysts using TiO
photocatalysts at room temperature.
2
(A) as
1

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Introduction
Hydrogen is a promising energy carrier that can be used to generate electricity in a
fuel cell, with water being the only waste product. The search for effective
techniques of hydrogen gas generation from liquid fuels has remained a difficult
challenge for mobile hydrogen energy systems. Formic acid (FA) attracts great
attention as such a liquid fuel because it has high energy density, nontoxicity, and
excellent stability at room temperature. It is known that Pdꢀbased catalysts involving
coreꢀshell and alloy types of catalysts are effective for the hydrogen production from
1
ꢀ6
FA decomposition at room temperature. According to a systematic study on M@Pd
(M = Ru, Rh, Pt, Ag, Au) coreꢀshell nanocatalysts for hydrogen production from FA
7
decomposition, which gave higher catalytic activity than pure Pd/C catalysts, the
best catalytic activity has been obtained from Ag@Pd catalysts for which a high
ꢀ1
ꢀ1
initial hydrogen rate of about 4 L g h was achieved at room temperature. The
highest catalytic activity of Ag@Pd core–shell nanocatalysts was explained by the
largest electron transfer from Ag core to Pd shell because of the lowest work
function of Ag (4.7 eV) among M compared with that of Pd (5.1 eV).
To enhance the catalytic activity, we have recently studied the preparation of
Ag@Pd nanocatalysts loaded on TiO
2
nanoparticles using a microwave (MW)ꢀpolyol
8
9
method, where ethylene glycol (EG) was used as both solvent and reductant. Based
on sphericalꢀaberrationꢀcorrected scanning transmission electron microscopy
(
Csꢀcorrected STEM), STEMꢀenergy dispersed Xꢀray spectroscopy (EDS), Xꢀray
diffraction (XRD), and Xꢀray photoelectron spectroscopy (XPS) data, product
nanocatalysts consisted of Ag–Pd alloy core and Pd shell nanocrystals loaded on
anatase type of TiO
2
nanoparticles. Ag and Pd atoms are partially alloyed with each
o
other under heating at 176–178 C. Therefore, Ag82Pd18 alloy and Pd shell
2

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2
nanocatalysts were formed. Using TiO support, about 4 times higher hydrogen
ꢀ1
ꢀ1
8
2
production rate of 16.00±0.89 L g h than that without TiO support was obtained.
The significant enhancement of catalytic activity of AgPd@Pd in the presence of
TiO was explained by further electron transfer from TiO to Pd because the work
function of TiO (4.0 eV) is lower than those of Ag (4.7 eV) and Pd (5.1 eV).
For the practical application of AgPd@Pd/TiO
2
2
2
2
catalysts, even higher activity is
7
required. Based on previous work on Ag–Pd bimetallic catalysts, the catalytic
activity of AgPd alloy catalyst was much lower than that of Ag@Pd core–shell
catalysts. It is therefore expected that the catalytic activity of AgPd@Pd/TiO
enhanced by dealloying AgPd core. For this purpose, we prepared
@Pd/TiO (x = 7, 10, 15) catalysts having lower Pd content in AgPd alloy
core in an aqueous solution under MW heating at 100 C. Results show that
@Pd/TiO (x = 7, 10, 15) catalysts obtained after heating for 30 min, 1 h,
2
can be
Ag100ꢀxPd
x
2
o
10
Ag100ꢀxPd
x
2
and 2 h, respectively, have much higher catalytic activity than that of
8
Ag82Pd18@Pd/TiO
activity obtained for Ag93Pd
three times higher than that of Ag82Pd18@Pd/TiO
TiO has widely been used as photocatalysts for which charge separation due to
the formation of excited electrons and hole pairs occurs on the TiO
this study effects of photoirradiation on the hydrogen production from FA
decomposition was studied using Ag93Pd @Pd/TiO catalyst to further enhance the
catalytic activity. Hereafter, Ag93Pd @Pd/TiO catalyst is denoted as AgPd@Pd/TiO
for the sake of clarity. To ascertain effects of TiO
TiO , denoted as TiO (A), prepared by MWꢀpolyol method,
commercially obtained P25 type TiO
2
catalyst obtained in EG at higher temperature. The best catalytic
ꢀ1
ꢀ1
7
@Pd/TiO
2
catalyst, 46.03±2.27 L g h , was about
2
prepared in EG.
2
11ꢀ14
2
surfaces.
In
7
2
7
2
2
2
support, not only pure anatase
8,10
2
2
but also
2
, denoted as TiO
2
(P), was used. This is the first
3

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report on effects of photoirradiation and TiO
2
(A or P) support for AgPd@Pd/TiO
2
catalysts in the hydrogen production from FA decomposition. TiO
2
has been used as
photocatalysts for
We demonstrate here that the
under UV & visible (Vis) light irradiation is
photocatalyst or supports of AuPd, Au, Pd, Pt, Rh, Cu, CdSꢀTiO
2
1
5ꢀ19
the hydrogen production from FA decomposition.
catalytic activity of AgPd@Pd/TiO
2
2
higher than the above photocatalysts loaded on TiO .
Experimental
AgPd@Pd/TiO
2
(A) was prepared using the same procedure as that reported
previously. In brief, TiO nanoparticles were prepared by the hydrolysis of titanium
tetraisopropoxide in 1,5ꢀpentanediol under MW heating at 700 W for 60 min. In the
first step, Ag core nanoparticles were formed in the presence of TiO nanoparticles.
5 ml of distilled water containing 300 mg polyvinylpyrrolidone (PVP) and 12.26
mg AgNO was mixed with the colloidal solution of 71.88 mg TiO nanoparticles.
10
2
2
1
3
2
The mixed solution was heated at 95 °C with MW irradiation at 120 W for 40 min
under Ar bubbling. In the second step, 2 ml of distilled water containing 8.3 mg
Pd(NO
min under Ar bubbling. The solution temperature increased to about 100 °C under
MW irradiation after about 1 min. When AgPd@Pd/TiO (P) was prepared, TiO (A)
3 2
) was added to this solution and heated with MW irradiation at 400 W for 30
2
2
was replaced with commercially obtained TiO (P: Nippon Aerosil Co. Ltd.) and then
2
the same procedure was used for the synthesis of catalysts.
Morphologies, composition, and surface analyses of product particles were
conducted using TEM, TEM–EDS, Csꢀcorrected STEM, STEM–EDS, atomic
absorption spectrometry (AAS), XRD, and XPS data. The hydrogen production rate
was determined using a gas burette. When effects of UV irradiation were studied, Xe
4

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lamp (Asahi Spectra Co.,Ltd., MAX 303 type: 250−385 nm mirror module) with a
light guide was used (Fig. S1†). Irradiance of UV light in a reaction cell was
2
measured as 580 W/m . Further details of experimental and characterization methods
of catalysts are described in electronic supplementary information†.
Results and discussion
TEM images and XRD patterns of TiO
2
(A or P) particles are shown in Fig. S2† and
S3†. Results show that TiO support prepared by MW heating was solely
2
anataseꢀtype with an average diameter of 10±2 nm, whereas P25 type one is a 92 : 8
mixture of anatase and rutile types with an average diameter of 27±8 nm. Since
2 2
particle sizes of TiO (A) are smaller than those of TiO (P), XRD patterns give
broader peaks.
TEM and TEM–EDS images of AgPd@Pd/TiO
Fig. S4†. AgPd@Pd/TiO catalysts with an average diameter of 4.6±0.9 nm were
loaded uniformly on both TiO (A or P) nanoparticles. This implies that little
difference exists in the average diameter of AgPd@Pd catalysts loaded on TiO (A)
and TiO (P) supports. To obtain further detailed information on crystal structures of
2
(A or P) are shown in Fig. 1 and
2
2
2
products, Csꢀcorrected STEM and STEMꢀEDS images and their line analysis data
are measured (Fig. 2A and 2B). Approximately 0.8 nmꢀthick Pd shells were formed
on the AgPd core particles for both TiO
2
supports. The Pd/Ag ratios of
AgPd@Pd/TiO (A) and AgPd@Pd/TiO (P) were estimated to be 0.34±0.02 from
2
2
EDS data, which were in good agreement with those measured by AAS (0.35±0.02).
Almost the same sizes, morphologies, and composition of AgPd@Pd nanocatalysts
2 2 2
were formed on TiO (A) and TiO (P) supports, indicating that TiO (P) does not
affect the nucleation and growth of AgPd@Pd nanoparticles.
5

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Fig. 3 shows XRD patterns of AgPd@Pd/TiO
rutile types of TiO peaks, weak peaks derived from {200}, {220}, and {311} facets
of fcc AgPd particles are observed. These peaks shift to larger 2θ from those of pure
2
(A or P). Aside from anatase and
2
8
,10
fcc Ag crystals (red dotted lines in Fig. 3) because of alloying of Ag core and Pd.
A weak peak was observed around 2θ = 34° in two samples indexed to {101} facets
of PdO (PDF 01ꢀ088ꢀ2434). This result means that a small amount of PdO was
formed under MW heating in aqueous solution.
To characterize chemical states of AgPd@Pd/TiO
were measured. The full XPS spectra are shown in Fig. S5†, where O 1s, Ti 2p, Ag
d, and Pd 3d peaks are identified in both samples. In the expanded XPS spectra of
the 330–345 eV range (Fig. 4a), besides Pd 3d peaks, weak PdO 3d peaks were
observed. The binding energies of Pd 3d5/2 and 3d3/2 peaks in AgPd@Pd/TiO (A or
2
(A or P) samples, XPS spectra
3
2
P) were almost identical and shift to lower values by about 1.0 eV compared to those
of pure Pd (3d5/2 = 335.1 eV, 3d3/2 = 340.3 eV) shown by red dotted lines because of
electron transfer from Ag and TiO
among Ag (4.7 eV), TiO (4.0 eV), and Pd (5.1 eV). Similarly, the binding energies
of Ag 3d5/2 and 3d3/2 peaks in AgPd@Pd/TiO (A or P) shift to lower values by about
.2 eV compared with those of pure Ag 3d (3d5/2 = 368.2 eV, 3d3/2 = 374.2 eV)
shown by red dotted lines (Fig. 4b). These negative chemical shifts suggest that some
electrons are transferred from TiO to Ag because of difference of the work functions
between Ag and TiO
It is known that hydrogen is produced from H
2
to Pd arising from differences of work functions
2
2
1
2
2
.
2
O in the presence of metal/TiO
photocatalysts under photoirradiation. It may also be produced from decomposition
of FA in the presence of TiO photocatalysts without the presence of metallic
2
12
2
15
catalysts. In order to examine contribution of the above reactions, hydrogen
6

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production was examined under photoirradiation into AgPd@Pd/TiO
2
catalysts in an
+ HCOOH
catalysts. In the former condition, effects of
O decomposition on AgPd@Pd@TiO catalysts under
UV & Vis photoirradiation was examined, whereas those from FA decomposition on
TiO photocatalysts was examined in the latter case. No H + CO gas emission was
observed without FA or AgPd@Pd/TiO catalysts, indicating that the contribution of
aqueous solution without FA and into an aqueous solution of a TiO
mixture without AgPd@Pd/TiO
hydrogen generation from H
2
2
2
2
2
2
2
2
above background processes is negligible under photoirradiation in our conditions.
We also examine preꢀirradiation effects of UV & Vis light. Before catalytic reaction,
we irradiated UV & Vis light for 30 min, and then the hydrogen production from FA
decomposition was measured. No significant difference was obtained between
preꢀirradiated sample and that without photoirradiated one. It was therefore
concluded that preꢀirradiation effects, such as UV cleaning on the surface of catalysts,
are unimportant.
2 2
The initial H production rate (Rinitial) of AgPd@Pd/TiO (A or P) samples was
measured using the following method: total gas volume from a stirred glass tube
containing 20 ml of 0.25 M aqueous FA and the prepared sample (metallic catalyst
weight of 5.1 mg and TiO
2
support weight of 23.0 mg) was measured using a gas
, H , and CO were performed on
burette (see Fig. S1†). Detailed gas analyses for CO
2
2
a gas chromatograph and no CO emission was detected using GC for all samples at
○
2
7−90 C. Temporal variation of total gas (H
2
+ CO
(A or P) at various temperatures from 27 °C
to 90 °C was measured without or under photoirradiation (Fig. 5). Table 1 shows the
2
) generation by decomposition
of FA in the presence of AgPd@Pd/TiO
2
R
initial values obtained from equation (S1) of ESI† along with reported data for AgꢀPd
7,8
bimetallic systems.
2
The Rinitial values of AgPd@Pd/TiO (A or P) without
7

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o
photoirradiation at 27 C were larger than those of Ag@Pd and AgPd@Pd without
o
7,8
catalyst support and Ag@Pd/C at 20−35 C. The Rinitial value of AgPd@Pd/TiO
2
ꢀ1
ꢀ1
○
ꢀ1 ꢀ1
(
A) increases from 44.1 L g h at 27 C to 382 L g h at 90 °C without
ꢀ1
ꢀ1
ꢀ1
photoirradiation. Under UV irradiation, it increases from 70.1 L g h to 404 L g
ꢀ1
h at 90 °C. This indicates that the catalytic activity is enhanced by a factor of 1.6 at
○
2
7 C under photoirradiation, however, the degree of enhancement decreases with
○
increasing the reaction temperature from 1.6 to 1.1 at 27−90 C. The catalytic
activity of AgPd@Pd/TiO
2
2
(P) was smaller than that of AgPd@Pd/TiO (A) by
○
factors of 1.7−7.1 at 27−90 C without photoirradiation. It should be noted that a
2
similar enhancement of the catalytic activity is observed for AgPd@Pd/TiO (P) by
photoirradiation. Its Rinitial value under photoirradiation is larger than that without
○
photoirradiation by a factor of 1.5 at 27 C and the enhancement degree decreases to
○
a factor of 1.2 at 90 C. On the basis of above findings, the degree of enhancement of
the catalytic activity by photoirradiation is similar (1.5−1.6) between AgPd@Pd/TiO
2
○
(
A) and AgPd@Pd/TiO (P) at 27 C and decreases to 1.1−1.2 for both TiO (A and
2
2
○
P) supports at 90 C.
The catalytic activity has often been given by initial turnover frequency
(
TOFinitial) given in equation (S2) of ESI†. The TOFinitial values obtained in this study
o
at 27−90 C are given in Table 1. In Table S1† are compared TOFinitial values
o
o
6,7,20ꢀ30
obtained in this study at 27 C with reported data at 20−30 C.
values of AgPd@Pd/TiO (A) and AgPd@Pd/TiO (A) + light are larger than those of
AgPd@Pd/TiO (P) and AgPd@Pd/TiO
The TOFinitial
2
2
2
2
(P) + light by factors of 1.6 and 1.5,
o
respectively. The TOFinitial value of AgPd@Pd/TiO
2
(A) at 27 C is larger than that of
o
7
Ag50Pd50 alloy at 20 C by a factor of 1.34, whereas it is comparable to that of
o
6
2
Ag42Pd58 alloy at 25 C. The highest TOFinitial value of AgPd@Pd/TiO (A) + light,
8

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ꢀ1
ꢀ1
306 mol H
2
mol metal catalyst h , is larger than most of previous data. However, it
is by 8−20% smaller than those of AuPdꢀMnO
x
/ZIFꢀ8ꢀrGO (382.1 mol H
2
mol metal
ꢀ
1
ꢀ1
ꢀ1 ꢀ1 21,24
catalyst h ) and AgPdꢀHs/G (333 mol H
that the AgPd@Pd/TiO
hydrogen production from FA decomposition at room temperature.
The catalytic activity is also estimated by H production rate per the entire
photocatalyst including support using equation (S3) of ESI†. Results obtained at room
2
mol metal catalyst h ).
This shows
2
(A) + light catalyst is one of best performance catalysts for the
2
temperature are listed in Table S2† along with other reported data using TiO
2
14ꢀ19
photocatalysts.
Until now AuPd/TiO
2
nanowire (NW) was the best performance
catalyst using TiO (A) photocatalyst with a H production rate of 17.7 mmol H g
2
2
2
ꢀ1
ꢀ1 15
catalyst h . It should be noteworthy that the H
2
production rate of AgPd@Pd/TiO
2
ꢀ1
ꢀ1
(
A or P) under photoirradiation, 468 or 64.0 g catalyst h , are 26 or 3.6 times larger
than that of AuPd/TiO
2
NW. Present results show that AgPd@Pd (A or P)
support is the best combination between metals and TiO
photocatalyst is used for hydrogen generation from FA
nanoparticles loaded on TiO
2
2
support when metals/TiO
2
decomposition at room temperature.
The apparent activation energies were estimated from the following relation
using Rinitial data given in Table 1.
ln(Rinitial) = −E
a
/RT + C
(1)
In that equation, Rinitial is the initial rate of hydrogen generation, E
activation energy, and C is a constant. The E values were estimated respectively as
(A), AgPd@Pd/TiO (A) + UV & Vis,
(P) + UV & Vis (Fig. S6†). The E values
a
is the apparent
a
ꢀ1
3
0, 25, 47, and 43 kJ mol for AgPd@Pd/TiO
2
2
AgPd@Pd/TiO (P), and AgPd@Pd/TiO
2
2
a
9

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of AgPd@Pd/TiO
2
(A) and AgPd@Pd/TiO
2
(A) + UV & Vis catalysts are smaller
than those of AgPd@Pd/TiO
2
2
(P) and AgPd@Pd/TiO (P) + UV & Vis one by factors
of 1.6−1.7. The lower E value obtained using AgPd@Pd/TiO (A) than that using
a
2
AgPd@Pd/TiO
2
(P) agrees with the higher catalytic activity of the former catalyst.
ꢀ1
The E values decrease by 4−5 kJ mol under photoirradiation for AgPd@Pd/TiO
a
2
(A or P). The enhancement of catalytic activity under photoirradiation also agrees
with the decrease in the activation energy.
The active site of FA decomposition in this study is not on TiO
2
photocatalysts but
it is on the Pd shell of AgPd@Pd, because no FA decomposition occurs under
photoirradiation to FA/TiO photocatalysts without loading AgPd@Pd nanoparticles.
Based on XPS measurements (Fig. 4), electronic states of Pd component in
AgPd@Pd/TiO (A) and AgPd@Pd/TiO (P) catalysts without photoirradiation were
almost identical. According to our previous studies, the higher catalytic activity of
AgPd@Pd/TiO (A) catalysts than that of AgPd@Pd (A) ones without
photoirradiation has been explained by electron donating effects of TiO (A) support
to Pd shell in AgPd@Pd (A). Photocatalytic behavior of TiO under photoirradiation
According to such results, electronꢀhole pairs are
under UV & Vis photoirradiation. As shown in Fig. 6,
photogenerated electrons in the TiO conduction band can transfer to the Pd shell of
AgPd@Pd/TiO catalysts because of larger work function of Pd (5.1 eV) than those
of TiO (4.0 eV) and Ag (4.7 eV), whereas the photogenerated holes remain in the
2
2
2
8
,10
2
2
2
11ꢀ14
has extensively been studied.
formed on the surface of TiO
2
2
2
2
15
valence band due to the electronꢀsink effect. The formation of further electronꢀrich
Pd shell under photoirradiation will be a main reason for the enhancement of
catalytic activity by UV & Vis photoirradiation. We found that effects of
photoirradiation decrease with increasing the reaction temperature. With increasing
1
0

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2
the reaction temperature, the migration rate of photogenerated electrons from TiO to
Pd will increase. However, the recombination rate between electronꢀhole pairs may
also increase with increasing the reaction temperature, so that total numbers of
electrons which can arrive at the surface of Pd shell may decrease. Thus the degree of
enhancement of catalytic activity decreases with increasing the reaction temperature.
We found that the catalytic activity of AgPd@Pd particles depends strongly on the
TiO
AgPd@Pd/TiO
content of TiO
amount of TiO
Photocatalytic activity of TiO
hole recombination for TiO (A) and TiO
found to be greater for rutile in all cases, varying between one and over three orders
of magnitude depending on the method used. The E value of AgPd@Pd/TiO (A) is
about half of that of AgPd@Pd/TiO (P). The larger catalytic activity of smaller E
value of AgPd@Pd/TiO (A) may be linked to the difference in slower electron–hole
recombination rate on TiO (A) surface than that on TiO (R) one. According to
previous studies on the catalytic activity of Pd/TiO Pd/TiO (A) demonstrated a
greater activity than Pd/TiO (R) in selective hydrogenation reactions due to the
superior metal supporting behavior of TiO (R). The higher catalytic activity has
been explained by its high specific surface area and strong interaction with metal
2
support. The catalytic activity of AgPd@Pd/TiO
(P) both without and under photoirradiation, even though the
(rutile: R) in TiO (P) is small (8%). This implies that a small
(R) greatly affects the catalytic activity of AgPd@Pd particles.
2
(A) is higher than that of
2
2
2
2
31ꢀ33
2
(A or R) has been studied.
The rate of electron–
2
2
(R) particles has been determined and was
a
2
2
a
2
2
2
34ꢀ38
2
,
2
2
2
3
9,40
nanoparticles.
AgPd@Pd/TiO
area and strong interaction of TiO
Thus additional reasons for the higher catalytic activity of
(A) than that of AgPd@Pd/TiO (P) may be high specific surface
(A) with AgPd@Pd.
2
2
2
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Conclusions
Effects of UV & Vis photoirradiation were studied for the hydrogen production from
FA decomposition using Ag93Pd @Pd/TiO nanocatalysts. Both anatase and
commercially obtained P25 types of TiO were examined as supports. The catalytic
activity of AgPd@Pd/TiO (A or P) was enhanced by a factor of 1.5–1.6 under UV
Vis photoirradiation at room temperature. It was explained by the formation of
electronꢀrich Pd shell owing to migration of photogenerated electrons from the TiO
7
2
2
2
&
2
(A or P) surface to Pd shell. The degree of enhancement decreased from 1.5–1.6 to
o
o
1
.1–1.2 with increasing reaction temperature from 27 C to 90 C probably because
of increase in the recombination rate between electronꢀhole pairs. The catalytic
activity of AgPd@Pd/TiO
2
(A) under photoirradiation was 1.8−7.3 times higher than
o
that of Ag93Pd @Pd/TiO
7
2
(P) at 27–90 C. Its catalytic activity at room temperature,
ꢀ1
ꢀ1
4
68 mmol H g catalyst h , is the best value ever reported in the all heterogeneous
2
catalysts using TiO
2
(A) as photocatalysts. Therefore AgPd@Pd/TiO
2
(A)
photocatalyst is applicable to efficient hydrogen production systems intended for
mobile applications.
Acknowledgements
This work was supported by JSPS KAKENHI Grant number 25286003.
Notes and references
a
International Research and Education Center of Carbon Resources, Kyushu University,
Kasuga 816-8580, Japan. E-mail: tsuji@cm.kyushu-u.ac.jp
b
Department of Applied Science for Electronics and Materials, Graduate School of
Engineering Sciences, Kyushu University, Kasuga 816-8580, Japan
1
2

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c
Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga 816-8580,
Japan
ｄ
Art, Science, and Technology Center for Cooperative Research (KASTEC), Kyushu
University, Kasuga 816-8580, Japan
Electronic Supplementary Information (ESI) available. Experimental details, TEM, and
lists of hydrogen production rates from FA decomposition. See DOI: 10.1039/b000000x/
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Table 1 Hydrogen production rates from catalytic decomposition of formic acid
in water at different temperatures without and under UV & Vis photoirradiation.
Temperature
H gas volume
TOF
2
Catalyst
○
ꢀ1 ꢀ1
-
1
(
C)
(L g h )
(h )
193
306
414
509
642
695
1031
1307
1669
1765
27
AgPd@Pd/TiO
AgPd@Pd/TiO (A) + light
AgPd@Pd/TiO (A)
2
(A)
27
44.1 ± 1.3
70.1 ± 2.1
94.8 ± 2.8
117 ± 4
2
27
40
40
60
60
70
70
90
90
27
27
40
40
60
60
70
70
90
90
20
35
20
27
2
AgPd@Pd/TiO
AgPd@Pd/TiO
AgPd@Pd/TiO (A) + light
AgPd@Pd/TiO (A)
AgPd@Pd/TiO (A) + light
2
(A) + light
2
(A)
147 ± 7
2
159 ± 8
2
236 ± 12
299 ± 15
382 ± 19
404 ± 20
6.2 ± 0.2
9.6 ± 0.3
32.8 ± 1.0
40.2 ± 1.2
65.6 ±3.3
75.7 ± 3.8
141 ± 7
2
2
AgPd@Pd/TiO (A)
AgPd@Pd/TiO (A) + light
2
AgPd@Pd/TiO
AgPd@Pd/TiO (A) + light
AgPd@Pd/TiO (P)
AgPd@Pd/TiO (A) + light
2
(P)
2
42
2
143
176
287
331
614
624
768
955
2
2
AgPd@Pd/TiO (P)
AgPd@Pd/TiO (A) + light
2
AgPd@Pd/TiO
AgPd@Pd/TiO (A) + light
AgPd@Pd/TiO (P)
2
(P)
2
143 ± 7
2
176 ± 9
AgPd@Pd/TiO
2
(A) + light
219 ± 11
3.67 ± 0.05
4.58 ± 0.05
4.23 ± 0.05
0.71 ± 0.06
a
Ag@Pd
a
Ag@Pd
a
Ag@Pd/C
b
AgPd@Pd
a
Ref. 7.
Ref. 8.
b
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Figure Captions
Fig. 1 TEM and TEM–EDS images of the (A) AgPd@Pd/TiO
P) nanocatalysts: (a) TEM image, (b) Ag component, (c) Pd component, (d) Ti component,
and (e) AgꢀPdꢀTi components, (f) EDS data.
2 2
(A) and (B) AgPd@Pd/TiO
(
Fig. 2 Csꢀcorrected STEM, STEMꢀEDS images, and line analyses along blue lines in 2e1
and 2e2 of the (A) AgPd@Pd/TiO (A) and (B) AgPd@Pd/TiO (P) nanocatalysts.
2
2
2 2
Fig. 3 The XRD patterns of (A) AgPd@Pd/TiO (A) and (B) AgPd@Pd/TiO (P)
nanocatalysts. Red dots are anatase, whereas green dots are rutile.
2 2
Fig. 4 XPS spectra for (A) AgPd@Pd/TiO (A) and (B) AgPd@Pd/TiO (P) nanocatalysts:
(
a) Pd 3d3/2,5/2 and (b) Ag 3d3/2,5/2.
Fig. 5 (a) Gas generation by decomposition of formic acid (0.25 M, 20 mL) vs. time in the
○
presence of (A) AgPd@Pd/TiO
2
(A) and (B) AgPd@Pd/TiO
2
(P) nanocatalysts at 27−90 C
without and under photoirradation.
Fig. 6 Enhancement mechanism of catalytic activity of AgPd@Pd/TiO
2
under UV
irradiation.
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Table of contents
2
Hydrogen production rate of AgPd@Pd/TiO nanocatalysts from formic acid
decomposition was enhanced by 50–60% at room temperature under
photoirradiation.
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Fig. 1 TEM and TEM–EDS images of the (A) AgPd@Pd/TiO2 (A) and (B) AgPd@Pd/TiO2 (P) nanocatalysts:
(
a) TEM image, (b) Ag component, (c) Pd component, (d) Ti component, and (e) Ag-Pd-Ti components, (f)
EDS data.
2
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Fig. 2 Cs-corrected STEM, STEM-EDS images, and line analyses along blue lines in 2e1 and 2e2 of the (A)
AgPd@Pd/TiO2 (A) and (B) AgPd@Pd/TiO2 (P) nanocatalysts.
2
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Fig. 3 The XRD patterns of (A) AgPd@Pd/TiO2 (A) and (B) AgPd@Pd/TiO2 (P) nanocatalysts. Red dots are
anatase, whereas green dots are rutile.
2
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Fig. 4 XPS spectra for (A) AgPd@Pd/TiO2 (A) and (B) AgPd@Pd/TiO2 (P) nanocatalysts: (a) Pd 3d3/2,5/2
and (b) Ag 3d3/2,5/2.
2
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Fig. 5 (a) Gas generation by decomposition of formic acid (0.25 M, 20 mL) vs. time in the presence of (A)
AgPd@Pd/TiO2 (A) and (B) AgPd@Pd/TiO2 (P) nanocatalysts at 27−90 ○C without and under
photoirradation.
2
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Fig. 6 Enhancement mechanism of catalytic activity of AgPd@Pd/TiO2 under UV irradiation.
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6x40mm (150 x 150 DPI)