Efficient hydrogen production from formic acid using TiO2-supported AgPd@Pd nanocatalysts

Article abstract of DOI:10.1039/c4ta06988a

We report here the significant enhancement of catalytic activity of Ag-Pd bimetallic nanocatalysts with the formation of Ag-Pd catalysts having an average diameter of 4.2 ± 1.5 nm on TiO₂ nanoparticles using a two-step microwave (MW)-polyol method. Data obtained using XRD and STEM-EDS indicated that Ag-Pd bimetallic nanocatalysts consisted of Ag₈₂Pd₁₈ alloy core and about 0.5 nm thick Pd shell, denoted as AgPd@Pd. The hydrogen production rate of AgPd@Pd/TiO₂ from formic acid, 16.00 ± 0.89 L g^-1 h^-1, was 23 times higher than that of bare AgPd@Pd prepared under MW heating at 27 °C. It was even higher by 2-4 times than the best Ag@Pd and CoAuPd catalysts at 20-35 °C reported thus far. The apparent activation energy of the formic acid decomposition reaction using AgPd@Pd catalyst decreased from 22.8 to 7.2 kJ mol^-1 in the presence of TiO₂. Based on negative chemical shifts of the Pd peaks in the XPS data and the measured activation energies, the enhancement of catalytic activity in the presence of TiO₂ was explained by the lowered energy barrier in the reaction pathways because of the strong electron-donating effects of TiO₂ to Pd shells, which enhance the adsorption of formate to the catalyst and dehydrogenation from formate.

Full text of DOI:10.1039/c4ta06988a

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Efficient hydrogen production from formic acid
using TiO2-supported AgPd@Pd nanocatalysts†
Cite this: J. Mater. Chem. A, 2015, 3,
4453
a
b
c
a
*
Masashi Hattori, Hisahiro Einaga, Takeshi Daio and Masaharu Tsuji
We report here the significant enhancement of catalytic activity of Ag–Pd bimetallic nanocatalysts with the
formation of Ag–Pd catalysts having an average diameter of 4.2 ꢀ 1.5 nm on TiO2 nanoparticles using a
two-step microwave (MW)–polyol method. Data obtained using XRD and STEM-EDS indicated that Ag-
Pd bimetallic nanocatalysts consisted of Ag82Pd18 alloy core and about 0.5 nm thick Pd shell, denoted as
AgPd@Pd. The hydrogen production rate of AgPd@Pd/TiO2 from formic acid, 16.00 ꢀ 0.89 L gꢁ1 hꢁ1
,
was 23 times higher than that of bare AgPd@Pd prepared under MW heating at 27 ꢂC. It was even higher
by 2–4 times than the best Ag@Pd and CoAuPd catalysts at 20–35 ꢂC reported thus far. The apparent
activation energy of the formic acid decomposition reaction using AgPd@Pd catalyst decreased from
22.8 to 7.2 kJ molꢁ1 in the presence of TiO2. Based on negative chemical shifts of the Pd peaks in the
XPS data and the measured activation energies, the enhancement of catalytic activity in the presence of
TiO2 was explained by the lowered energy barrier in the reaction pathways because of the strong
electron-donating effects of TiO2 to Pd shells, which enhance the adsorption of formate to the catalyst
and dehydrogenation from formate.
Received 18th December 2014
Accepted 5th January 2015
DOI: 10.1039/c4ta06988a
www.rsc.org/MaterialsA
reported recently using Ag@Pd core–shell and CoAuPd alloy
catalysts.4,5 Although hydrogen production rates of about 4 and
8 L gꢁ1 hꢁ1 have been achieved, higher catalytic activity without
Introduction
Hydrogen gas (H2) has been anticipated as a clean energy
resource that can replace fossil fuels. However, at ordinary
temperatures and normal pressures, H2 is in a gaseous state
that is not compatible with storage in a small space. This
constraint limits miniaturization of efficient hydrogen produc-
tion systems for mobile applications. To resolve this issue, a
technique must be developed to extract H2 instantly from liquid
fuels on a small scale. As such a liquid fuel, formic acid has
attracted great attention because it is produced by a combina-
tion of CO2 and H2O with irradiation by sunlight and because it
is generated as a primary product in articial photosynthesis.1
Some reports have described hydrogen production from the
decomposition of formic acid using solid catalysts such as core–
shell Au@Pd/C catalysts.2,3 However, for formic acid dehydro-
genation, such catalysts necessitate elevated temperatures
(>80 ꢂC). Moreover, the generation of CO gas greatly reduces
catalytic activity. However, hydrogen production without CO
production from formic acid at room temperature has been
CO production is anticipated for practical application.
Reportedly, the catalytic activity of Pd nanoparticles for for-
mic acid electrooxidation is enhanced in the presence of tita-
nium dioxide (TiO2).6 Therefore, it is expected that the catalytic
activity of Ag@Pd core–shell nanocatalysts for formic acid
decomposition in an aqueous solution can also be promoted by
loading Ag@Pd core–shell nanocatalysts on TiO2. Usually,
nanoparticles are loaded on an oxide semiconductor by sinter-
ing, which requires high-temperature treatment. It is difficult to
load core–shell nanometals on an oxide semiconductor without
alloying the core and shell metals. Based on previous results
obtained for the catalytic activity of the Ag–Pd bimetallic system
for formic acid decomposition,4 Ag@Pd core–shell nano-
catalysts should not be alloyed because the catalytic property of
the core–shell nanocatalyst is lost through alloying at high
temperatures. To overcome this shortcoming, a new method
must be used to synthesize core–shell nanoparticles directly on
an oxide semiconductor.
In this paper, we describe a two-step microwave (MW)–polyol
method used to prepare Ag–Pd bimetallic core–shell nano-
catalysts directly on TiO2 nanoparticles. Actually, TiO2 catalysts
have been widely used as photocatalysts7,8 and as photo-
electrodes for dye-sensitized solar cells.9,10 For such applica-
tions, TiO2 nanoporous substrates such as nanoparticle lms
and TiO2 nanotube arrays have been developed.11,12 Therefore,
the TiO2 nanoporous substrates and techniques described in
aInstitute for Materials Chemistry and Engineering, Kyushu University, Kasuga
816-8580, Japan. E-mail: tsuji@cm.kyushu-u.ac.jp
bDepartment of Energy and Material Sciences, Faculty of Engineering Sciences, Kyushu
University, Kasuga 816-8580, Japan
cInternational Research Center for Hydrogen Energy, Kyushu University, Motooka,
Fukuoka, 819-0395, Japan
† Electronic supplementary information (ESI) available: Temperature proles of
reagent solutions in each experiment and STEM and STEM-EDS images of bare
AgPd@Pd nanoparticles. See DOI: 10.1039/c4ta06988a
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this work are expected to be useful for developing a practical 250 W or using conventional oil-bath heating for 10 min under
hydrogen generation device from formic acid. For these Ar bubbling. The temperature proles of the solutions under
reasons, the trial undertaken for this study represents a MW heating and oil-bath heating are shown in Fig. S3 (ESI†).
reasonable and practical challenge. Results show that MW Finally, the prepared samples were separated from EG solution
heating is more effective than conventional oil-bath heating for by centrifuging the obtained colloidal solution at 22 200g for 60
preparing Ag–Pd/TiO2 nanoparticles. The hydrogen production min. They were dispersed in distilled water. Then, PVP was
rate of Ag–Pd/TiO2 nanoparticles from formic acid without CO removed from the Ag–Pd/TiO2 surface by sonication for 30 min
emission was higher than reported values4,5 at room tempera- and PVP and Ag–Pd/TiO2 were separated by centrifuging at
ture. To clarify the enhancement of catalytic activity, nano- 22 200g for 60 min. These processes were repeated six times for
catalysts were characterized using X-ray diffraction (XRD), removing PVP adequately. Finally, the Ag–Pd/TiO2 particles
transmission electron microscopy (TEM), TEM-energy were dispersed in distilled water.
dispersed X-ray spectroscopy (EDS), spherical-aberration-cor-
rected scanning TEM (STEM), STEM-EDS, and X-ray photo-
electron spectroscopy (XPS). A much higher catalytic activity
with a lower apparent activation energy than that in previous
reports4,5 has been obtained for hydrogen production from
formic acid without CO emission at room temperature. The
effects of the TiO2 support on the enhancement of the catalytic
activity are discussed on the basis of XPS data and the activation
energies of formic acid decomposition.
Preparation of bare Ag–Pd nanoparticles
To examine the effects of TiO2, bare Ag–Pd bimetallic nano-
catalysts were also prepared using MW heating. 15 ml of EG
solution containing 120 mg PVP and 12.26 mg AgNO3 was
heated with MW irradiation at 50 W for 20 min. Then, 2 ml of
EG containing 16.5 mg Pd(NO3)2 was added to this solution and
heated with MW irradiation at 250 W for 10 min. The obtained
Ag–Pd nanoparticles (called bare Ag–Pd (MW)) were precipi-
tated in acetone, washed by sonication for 30 min, and then
dried. These processes were repeated twice. Finally, they were
dispersed in distilled water. The temperature prole of the
solution under MW heating was the same as that in the prep-
aration of Ag–Pd/TiO2 under MW heating.
Experimental
Preparation of TiO2 nanoparticles
TiO2 nanoparticles were prepared by the hydrolysis of titanium
tetraisopropoxide (Ti(Oi-Pr)4) (Wako Pure Chemical Industries
Ltd) in 1,5-pentanediol (Tokyo Chemical Industry Co. Ltd)
under MW heating.13 3 mmol Ti(Oi-Pr)4 was added to 50 ml of
1,5-pentanediol. The solution was irradiated in the MW oven
(Shikoku Keisoku Kogyo K.K.; m type, maximum output power
750 W) at 200 W for 3 min. Then 2 ml distilled water was added
to this solution and irradiated at 700 W for 1 h. The temperature
prole of the reagent solution is shown in Fig. S1 (ESI†). The
solution temperature increases to 175 ꢂC aer 3 min heating at
Characterization of product particles
For TEM, TEM-EDS (JEM-2100F; JEOL), STEM and STEM-EDS
(JEM-ARM200F; JEOL), samples were prepared by dropping
colloidal solutions of the products onto Cu grids. The average
sizes of product particles were determined by measuring more
than 100 particles in STEM images, although much larger
particles than average ones were used for the EDS analyses of
single particles to obtain clear images. The XRD patterns of the
samples were measured using Cu Ka radiation (SmartLab;
Rigaku Corp.). The XPS data of the product were measured
using Al Ka radiation (AXIS-165; Shimadzu Corp.). The extinc-
tion spectra of the product solutions were measured using a
spectrometer (UV-3600; Shimadzu Corp.) in the ultraviolet (UV)-
visible (Vis)-near infrared (NIR) region.
ꢂ
200 W. It drops to 150 C aer injection of distilled water and
increases again to about 195 ꢂC aer 6 min at 700 W and
becomes nearly constant aer that. For the use of TiO2 as a
support for metallic nanoparticles in the next step, products
were obtained by centrifuging the colloidal solution at 22 200g
for 30 min. Then they were redispersed in ethylene glycol (EG)
(Kishida Chemical Co. Ltd).
Hydrogen generation activity of Ag–Pd and Ag–Pd/TiO2
nanocatalysts
Preparation of Ag–Pd/TiO2 nanoparticles
Ag–Pd bimetallic nanocatalysts were formed on TiO2 using a
two-step heating method. First, Ag core nanoparticles were
formed in the presence of TiO2 nanoparticles. 15 ml of EG
solution containing 850 mg polyvinylpyrrolidone (PVP: average
molecular weight of 40k in terms of monomer units) (Wako
Pure Chemical Industries Ltd) and 12.26 mg AgNO3 (Kishida
Chemical Co. Ltd) were mixed with the colloidal solution of
17.25 mg TiO2 nanoparticles. The mixed solution was heated
with MW irradiation at 50 W for 20 min under Ar bubbling.
Temperature proles of the reagent solution under MW irra-
diation are shown in Fig. S2 (ESI†). In the second step, 2 ml of
The hydrogen production activity of the prepared samples was
examined using the following method: the total gas volume
from a stirred glass tube containing 20 ml of 0.25 M aqueous
formic acid and the prepared sample (metallic catalyst weight of
5.8 mg) was measured using a gas burette. A schematic view of
the measurement system is shown in Fig. S4 (ESI†). The
hydrogen gas volume as production per gram of Ag–Pd catalyst
per hour was calculated using eqn (1) and (2).5
xa ¼ PatmVgas/RTnFA
,
(1)
EG containing 16.5 mg Pd(NO3)2 (Kishida Chemical Co. Ltd) where xa is the conversion, Patm stands for the atmospheric
was added to this solution and heated with MW irradiation at pressure, Vgas represents the generated volume of gas, R denotes
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the universal gas constant, T is the room temperature (300 K), with a large specic surface area (Fig. S2, ESI†). In the second
and nFA is the mole number of formic acid.
step, Pd(NO3)2 was further reduced in EG to obtain Ag–Pd
bimetallic particles on TiO2 under MW heating at 250 W or oil-
bath heating. The reaction temperature was 176–178 ꢂC (Fig. S3,
ESI†), which was higher than that in the rst step to make
Ag–Pd particles adhere to a TiO2 support strongly. Fig. 2a and
2b–d respectively portray STEM and STEM-EDS images of
nanoparticles obtained from the reduction of AgNO3 in the
presence of TiO2 nanoparticles aer rst MW heating. Spherical
Ag nanoparticles with an average diameter of 2.6 ꢀ 1.1 nm were
formed. It is noteworthy that Ag nanoparticles were not loaded
on TiO2, but were instead formed in EG solution. This fact was
conrmed not only from TEM images but also from the colour
of the solution, which was yellow aer centrifugal separation
because of the formation of spherical Ag particles.
Fig. 3a–g show STEM and STEM-EDS images of products
aer second MW-heating for Pd shell formation. Fig. 3h shows
line analysis data along the red line depicted in Fig. 3g. These
images show that Ag–Pd bimetallic nanocatalysts with an
average diameter of 4.2 ꢀ 1.5 nm were loaded uniformly on TiO2
nanoparticles and 0.5 nm-thick pure Pd shell was formed on Ag
or Ag–Pd alloy core metal. The Pd/Ag atomic ratio in whole
Ag–Pd bimetallic nanocatalysts was determined as 0.25 ꢀ 0.03
from STEM-EDS analysis. Fig. 4a–g show STEM and STEM-EDS
images of products using oil-bath heating for Pd shell forma-
tion. Fig. 4h shows line analysis data along the red line depicted
in Fig. 4g. These images illustrate that the shape, size, and
composition of the products using oil-bath heating were almost
the same as that of the products of MW-heating. The average
size was 4.4 ꢀ 1.2 nm, the thickness of the pure Pd shell was
Rhydrogen ¼ Vgas/2mmetalt,
(2)
where Rhydrogen represents the initial rate of hydrogen genera-
tion when xa reaches 20%, mmetal is the weight of the metallic
catalyst, and t is the reaction time when xa reaches 20%.
The H2, CO2, and CO gases were measured using a gas
chromatograph (GC7100; J-Science): 10 ml of 1 M aqueous for-
mic acid and the prepared sample (metallic catalyst weight of
0.97 mg) mixture were stirred for 30 min in a 110 ml glass tube
lled with Ar gas. Then the atmosphere in the glass tube was
measured. When the H2, CO2, and CO concentrations were
determined using GC, data were corrected using the standard
gas, which was N2 gas containing 50 000 ppm H2, CO2, and CO
gas.
Results and discussion
Structural characterization of TiO2 nanoparticles
Fig. 1a shows a typical TEM image of TiO2 particles, where
monodispersed nanoparticles with an average diameter of 10 ꢀ
2 nm are formed. Fig. 1b shows XRD patterns of the powder
sample for which peaks are observed at 2q ¼ 25.3ꢂ, 37.8ꢂ, 48.1ꢂ,
53.9ꢂ, and 55.1ꢂ. These peaks can be indexed to {101}, {004},
{200}, {105}, and {211} facets of anatase-type of TiO2 (PDF 00-
021-1272). No peak of rutile-type TiO2 is observed (PDF 01-076-
0325). Apparently, the products are composed solely of anatase-
type of TiO2.
Structural characterization of Ag–Pd/TiO2 nanoparticles
Ag–Pd/TiO2 nanoparticles were prepared by two-step heating. In
the rst step, AgNO3 was reduced in EG in the presence of TiO2
ꢂ
at a relatively low reaction temperature of about 130 C under
MW heating at a low power of 50 W to obtain small Ag particles
Fig. 2 STEM and STEM-EDS image of Ag nanoparticles synthesized
using microwave heating in the presence of TiO2 nanoparticles. (a)
Fig. 1 TEM and XRD pattern of TiO2 nanoparticles. (a) TEM and (b) STEM image, (b) Ag component, (c) Ti component and (d) all
XRD.
components.
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Fig. 3 STEM and STEM-EDS images of AgPd@Pd/TiO2 (MW) nanocatalysts. (a) STEM image, (b) Ag and Pd components, (c) all components, (d)
STEM image, (e) Ag component, (f) Pd component, (g) Ag and Pd components and (h) line analysis data along the red line shown in (g).
about 0.5 nm and the Pd/Ag atomic ratio in whole Ag–Pd These peaks can be indexed to {111}, {200}, {220}, and {311}
bimetallic nanocatalysts was 0.25 ꢀ 0.02. On the other hand, facets of the Ag component of fcc Ag–Pd bimetallic particles. All
there was a difference in the loading amount of catalysts on the of these peaks shi to larger 2q by 0.2–0.7ꢂ from those of pure
TiO2 support for the two heating methods. In the case of oil- fcc Ag crystals (PDF 01-087-0720: 2q ¼ 38.20ꢂ, 44.40ꢂ, 64.60ꢂ,
bath heating, there were some unloaded bare-Ag–Pd catalysts and 77.60ꢂ for {111}, {200}, {220}, and {311}, respectively)
(see Fig. 4a–c) which were not observed in the products using because of alloying that occurs between Ag and Pd. According to
MW-heating. Based on the data presented above, high-power Vegard's law,15 which is known to be applicable to Ag–Pd
MW heating at 250 W was required for strong adhesion of systems,16 about 18 ꢀ 1% of Pd atoms are dissolved in Ag–Pd
Ag–Pd nanoparticles to TiO2. Under MW irradiation at high particles by alloying. Weak shoulder peaks are observed at 2q ¼
power, it is likely that Ag–Pd nanoparticles are heated locally 40.1ꢂ, 46.7ꢂ, 68.1ꢂ, and 82.1ꢂ, corresponding to the {111}, {200},
because of skin effects of nanoparticles under MW14 so that {220}, and {311} facets of the Pd component of Ag–Pd bimetallic
Ag–Pd particles are strongly adhered to TiO2 nanoparticles.
particles. Peak positions of the Pd component are close to those
To examine crystal structures and composition of products of pure Pd atoms (PDF 01-005-0681: 2q ¼ 40.12ꢂ, 46.66ꢂ, 68.09ꢂ
in greater detail, we measured XRD patterns of bare Ag–Pd and 82.10ꢂ, respectively, for {111}, {200}, {220}, and {311}),
bimetallic nanoparticles prepared without the addition of TiO2 indicating that a major Pd component exists as pure Pd shells
support (Fig. 5a) and TiO2-supported Ag–Pd bimetallic nano- over Ag-rich Ag82Pd18 alloy cores. Based on XRD data, it was
particles (Fig. 5b). The observed XRD patterns of Ag and Pd inferred that both Ag–Pd/TiO2 and Ag–Pd bimetallic particles
components in bare Ag–Pd bimetallic and TiO2-supported consist of Ag82Pd18@Pd/TiO2 and Ag82Pd18@Pd core–shell
Ag–Pd nanoparticles are similar, indicating that the presence of particles and that the Ag : Pd atomic ratio in AgPd cores is
TiO2 does not affect the crystal structure of Ag–Pd bimetallic independent of the presence of TiO2 nanoparticles.
particles. Aside from TiO2 anatase-peaks, major peaks are Consequently, when we examined the catalytic activity of
observed at 2q ¼ 38.3ꢂ, 44.7ꢂ, 65.1ꢂ, and 78.1ꢂ in both patterns. Ag82Pd18@Pd/TiO2 and Ag82Pd18@Pd core–shell particles,
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Fig. 4 STEM and STEM-EDS images of the AgPd@Pd nanocatalysts reduced by high-temperature (176 ꢂC) oil-bath heating with TiO2 nano-
particles. (a) STEM image, (b) Ag and Pd components, (c) all components, (d) STEM image, (e) Ag component, (f) Pd component, (g) Ag and Pd
components, and (h) line analysis data along the red line shown in (g).
The UV-Vis-NIR spectra of the products in each stage were
measured (Fig. 6). Anatase-type TiO2 nanoparticles showed
extinction below about 350 nm and a weak tail band from
Fig. 5 XRD patterns of (a) bare AgPd@Pd and (b) AgPd@Pd/TiO2
prepared using the MW–polyol method.
only the effects of TiO2 were found through comparison with
the two data. Hereinaer, for the sake of clarity, we respectively
Fig. 6 UV-Vis-NIR extinction spectra of TiO2 nanoparticles, mixture of
designate Ag82Pd18@Pd/TiO2 and Ag82Pd18@Pd particles as
AgPd@Pd/TiO2 and AgPd@Pd.
Ag and TiO2 nanoparticles and AgPd@Pd/TiO2 nanocatalysts prepared
using MW heating.
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approximately 350 to about 600 nm. The SPR band of spherical
silver particles is observed at around 400 nm,17 whereas a long
tail band without a prominent peak is found for Pd nano-
particles.18 When Ag core particles are prepared in the presence
of TiO2 particles under MW heating at 50 W, aside from the TiO2
peak, a typical SPR band of spherical Ag nanoparticles was
observed in the 350–490 nm region with a peak at about 412 nm
because of the formation of free spherical Ag particles in the
supernatant in this stage. The extinction spectrum of AgPd@Pd/
TiO2 particles comprises a long tail band from 200 nm to 1000
nm with a shoulder peak at about 400 nm. Generally speaking,
the surface plasmon resonance (SPR) band of core–shell parti-
cles reects optical properties of shell metals.19,20 Consequently,
the long tail band and the peak at about 400 nm can be
attributed respectively to the SPR bands of Pd and Ag compo-
nents of AgPd@Pd/TiO2 particles.
Hydrogen generation activity of AgPd@Pd and AgPd@Pd/TiO2
nanoparticles
Fig. 8 presents the catalytic activities of AgPd@Pd/TiO2 (MW),
bare AgPd@Pd (MW), a mixture of bare AgPd@Pd (MW) and
TiO2, and AgPd@Pd/TiO2 (oil) nanoparticles as counterparts for
H2 generation from formic acid decomposition. The H2 gener-
ation rates, obtained from relationships (1) and (2), are
presented in Table 1. Corresponding reported data of Ag@Pd
and CoAuPd catalysts4,5 are given for comparison. The
hydrogen production rate of AgPd@Pd/TiO2 (MW) sample was
16.00 L gꢁ1 hꢁ1 at room temperature (27 ꢂC), which is about four
times higher than that of Ag@Pd catalysts at 20 ꢂC (3.67 L gꢁ1 hꢁ1
)
4
and about two times higher than that of CoAuPd catalysts at
25 ꢂC (7.9 L gꢁ1 hꢁ1).5 It was about 23 times higher than that of
bare AgPd@Pd (MW). The average size of bare AgPd@Pd (MW)
To obtain chemical states of bare AgPd@Pd and AgPd@Pd/
TiO2 nanocatalysts, XPS spectra were measured (Fig. 7). For
comparison, binding energies of pure Pd and Ag metals are
presented as vertical dotted lines. The XPS spectra, which
provide information related to chemical states within several
nanometer depth from the surface of nanoparticles, show that
binding energies for Pd (3d5/2 and 3d3/2) peaks in bare
AgPd@Pd shi to lower values by about 0.6 eV compared to
those of pure Pd (3d5/2 ¼ 335.1 eV, 3d3/2 ¼ 340.3 eV). Taking
account of the fact that AgPd@Pd particles are covered by pure
Pd shells (Fig. S5, ESI†), these binding energy shis originate
from electron transfer from Ag to Pd because of a difference of
work functions between Ag (4.7 eV) and Pd (5.1 eV). The binding
energies of Ag 3d5/2 and Ag 3d3/2 in bare AgPd@Pd respectively
shi to lower values by about 1.2 eV compared with those of
pure Ag 3d (3d5/2 ¼ 368.2 eV, 3d3/2 ¼ 374.2 eV). The binding
energies of Ag 3d peaks in Pd rich (>90%) Ag–Pd alloys are
known to shi to lower values by about 1 eV.21 It is therefore
reasonable to assume that the peak shis arise from the
formation of the Ag–Pd alloy around the interface of the Ag-core
and Pd-shell. It is noteworthy that the binding energies of Pd
(3d5/2 and 3d3/2) and Ag (3d5/2 and 3d3/2) in AgPd@Pd/TiO2
(MW) both shi to lower values by about 0.9 and 0.5 eV,
respectively, compared with those of bare AgPd@Pd (MW).
These shis suggest that some electrons are transferred from
TiO2 to Pd and Ag because of the large difference of the work
functions between Ag (4.7 eV) or Pd (5.1 eV) and TiO2 (4.0 eV).
Fig. 8 Gas generation by decomposition of formic acid (0.25 M, 20
ml) versus time in the presence of (a) AgPd@Pd/TiO2 (MW) nano-
particles, (b) bare AgPd@Pd (MW) nanoparticles, (c) the mixture of bare
AgPd@Pd (MW) and TiO2 nanoparticles, and (d) AgPd@Pd/TiO2 (oil)
nanoparticles at 27 ꢂC.
Table 1 Hydrogen production rates from the catalytic decomposition
of formic acid in water at different temperatures
Temperature
(ꢂC)
H2 gas volume
(L gꢁ1 hꢁ1
Catalyst
)
AgPd@Pd/TiO2 (MW)
AgPd@Pd/TiO2 (MW)
AgPd@Pd/TiO2 (MW)
AgPd@Pd/TiO2 (MW)
AgPd@Pd/TiO2 (MW)
Bare AgPd@Pd (MW)
Bare AgPd@Pd (MW)
Bare AgPd@Pd (MW)
Bare AgPd@Pd (MW)
Bare AgPd@Pd (MW)
Mixture of AgPd@Pd
(MW) and TiO2
27
40
60
70
90
27
40
60
70
90
27
16.00 ꢀ 0.89
16.80 ꢀ 0.52
17.89 ꢀ 0.38
21.26 ꢀ 0.77
26.73 ꢀ 0.53
0.71 ꢀ 0.06
1.19 ꢀ 0.09
1.98 ꢀ 0.18
2.54 ꢀ 0.30
3.34 ꢀ 0.22
2.14 ꢀ 0.21
AgPd@Pd/TiO2 (oil)
Ag@Pd (ref. 4)
Ag@Pd (ref. 4)
27
20
35
25
11.76 ꢀ 0.32
3.67
4.58
Fig. 7 XPS spectra for AgPd@Pd/TiO2 (MW) nanoparticles and bare
CoAuPd (ref. 5)
7.9
AgPd@Pd (MW) nanoparticles, (a) Pd 3d3/2,5/2 and (b) Ag 3d3/2,5/2
.
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was 4.3 ꢀ 1.2 nm (Fig. S5, ESI†). These sizes were nearly the
same as those of AgPd@Pd/TiO2 (MW) nanocatalysts (4.2 ꢀ 1.5
nm). Therefore, the much higher catalytic activity of AgPd@Pd/
TiO2 (MW) cannot be explained simply by the increase in the
specic surface area of nanocatalysts. It can, however, be
attributed to enhancement by the assistance of the TiO2
support. To examine the catalytic activity of pure TiO2 nano-
particles in formic acid decomposition, formic acid was mixed
with TiO2 nanoparticles without the addition of AgPd@Pd (MW)
nanocatalysts. Then the gas emissions were observed. No H2 or
CO2 gas emission was observed using only TiO2 nanoparticles.
This result led us to conclude that TiO2 does not act as a catalyst
in formic acid decomposition in the absence of AgPd@Pd
nanocatalysts.
Fig. 9 Plots of ln(Rhydrogen) vs. 1000/T for AgPd@Pd/TiO2 (MW) and
bare AgPd@Pd (MW) nanocatalysts.
The hydrogen production rate of AgPd@Pd/TiO2 (MW) was
eight times higher than that of the mixture of bare AgPd@Pd
(MW) and TiO2 nanoparticles. This result indicates that strong
adhesion of AgPd@Pd (MW) and TiO2 nanoparticles using MW
heating is necessary to enhance the synergy effects between
AgPd@Pd and TiO2. Moreover, the hydrogen production rate of
AgPd@Pd/TiO2 (MW) was 1.4 times higher than that of
AgPd@Pd/TiO2 (oil). The average size of AgPd@Pd/TiO2 (oil)
was 4.4 ꢀ 1.2 nm (see Fig. 4a–c). These sizes were nearly the
same as those of AgPd@Pd/TiO2 (MW) nanocatalysts (4.2 ꢀ 1.5
nm). This fact demonstrates that MW heating is more effective
than conventional oil-bath heating for strong adhesion between
AgPd@Pd nanoparticles and TiO2 nanoparticles.
Hydrogen gas production rates were measured at various
temperatures using AgPd@Pd/TiO2 (MW) and bare AgPd@Pd
(MW) catalysts (Table 1). The results show that the hydrogen
production rate of bare AgPd@Pd (MW) increases more with
increasing temperature from 27 to 90 ꢂC than that of AgPd@Pd/
TiO2 (MW).
catalysts (see Fig. 9). The Ea value of bare AgPd@Pd (MW)
catalyst is lower than those of Pd/C (53.7 kJ molꢁ1 22 and Ag@Pd
)
(30 kJ molꢁ1).4 It is noteworthy that the Ea value of AgPd@Pd/
TiO2 (MW) is lower by a factor of three than that of bare
AgPd@Pd (MW) catalyst. It is the lowest value ever reported in
the literature.
Reaction mechanism of dehydrogenation of formic acid over
AuPd@Pd/TiO2
Based on previous DFT calculations on intermediate species,23,24
formate is an important intermediate in formic acid decom-
position on metallic catalysts. A possible decomposition
scheme of formate on AgPd@Pd/TiO2 is shown in Fig. 10.
Decomposition starts by activating the C–H bond of formate
adsorbed on the catalytic surface. Under our conditions, mon-
odentate formate (species A) transforms efficiently to more
stable bidentate formate (species B), in which both oxygen
atoms bind to the catalyst surface. Otherwise CO is formed
through decomposition of monodentate formate (species A).4
It is known that the chemical decomposition of formic acid
proceeds via two main pathways, i.e., the dehydrogenation
reaction to form CO2 + H2 and the dehydration reaction to form
CO + H2O
*
Bidentate formate (species B) decomposes into CO2 + H*
HCOOH / CO2 + H2: DG ¼ ꢁ48.4 kJ molꢁ1
HCOOH / CO + H2O: DG ¼ ꢁ28.5 kJ molꢁ1
(3)
(species C), where X* denotes intermediates adsorbed onto the
surface. Recombination of two H* and CO2 leaving from the
*
(4) surface results in the formation of CO2 + H2 gases (species D).
Based on theoretical calculations on potential energy surfaces
(PESs) of the formic acid decomposition reaction, the energy
barrier from species B to species C is highest. It is the rate-
determining step. Hu et al.24 calculated energy barriers between
species B and C for Pt, Pd, Rh, and Au catalysts using {111}
facets of metallic surfaces and found that a low energy barrier
height for Pd is qualitatively consistent with the high hydrogen
Although H2 and CO2 emissions from reaction (3) were
observed, no CO emission from reaction (4) was detected using
GC for AgPd@Pd/TiO2 at 27–90 ꢂC. Based on GC detection, the
concentration of CO, which reduces the catalytic activity of
AgPd@Pd/TiO2, was estimated as <10 ppm. The apparent acti-
vation energies of reaction (3) using AgPd@Pd/TiO2 (MW) and
bare AgPd@Pd (MW) catalysts were estimated from the
following relationship.
ln(Rhydrogen) ¼ ꢁEa/RT + C
(5)
In this equation, Rhydrogen is the initial rate of hydrogen
generation, Ea is the apparent activation energy, and C is a
constant. The Ea values were estimated as 7.2 and 22.8 kJ molꢁ1
,
respectively, for AgPd@Pd/TiO2 (MW) and bare AgPd@Pd (MW) Fig. 10 The schematic diagram of formic acid decomposition path-
ways on the Pd surface of AgPd@Pd/TiO2.
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production rate from formic acid. Moreover, when they calcu- observed, larger negative chemical shis were observed for Pd
lated PESs of Pd–Ag bilayer systems, the energy barrier of the peaks of AgPd@Pd/TiO2 in comparison with those of bare
last process becomes lower than that of pure Pd metal because AgPd@Pd because of electron-donating effects of TiO2 to Pd
the migration energy of H* to the top surface of Pd decreases in shells. Based on the ndings presented above, the marked
the presence of Ag cores.
enhancement of catalytic activity of AgPd@Pd/TiO2 was attrib-
The current results show that the hydrogen production rate uted to electron transfer from TiO2 to AgPd@Pd catalysts,
is greatly enhanced in the presence of TiO2 for the AgPd@Pd promoting C–H cleavage of formic acid over the surface of
system. Large negative chemical shis were observed for XPS AgPd@Pd/TiO2 nanocatalysts. In fact, CO emission, which
peaks of Pd (3d5/2,3/2) atoms for AgPd@Pd/TiO2 compared with reduces catalytic activity of AgPd@Pd/TiO2, was not observed at
those for AgPd@Pd. The work function has been regarded as an 27–90 ꢂC. Our method, which provides a novel preparation
important parameter of the catalytic system for the formic acid route for core–shell AgPd@Pd nanocatalysts on TiO2 particles
decomposition system over M@Pd (M ¼ Ag, Rh, Au, Ru, Pt) with high catalytic activity, is applicable for efficient hydrogen
core–shell catalysts. The catalytic activity increases concomi- production systems intended for mobile applications.
tantly with decrease of the work function.4 Therefore, the best
activity has been obtained for Ag@Pd. We found here that the
catalytic activity of AgPd@Pd particles is greatly enhanced in
Acknowledgements
the presence of TiO2 having a further lower work function. We thank Mrs Keiko Uto of our institute for her advice related to
Larger negative chemical shis in XPS peaks for AgPd@Pd/TiO2 the preparation of TiO2 nanoparticles using MW heating. This
than those of bare AgPd@Pd show that more electrons were work was supported by JSPS KAKENHI (grant nos 25286003 and
transferred from TiO2 to Pd shells in the presence of TiO2. 25550056) and by the Management Expenses Grants for
Consequently, the high catalytic activity of AgPd@Pd/TiO2 National University Corporations from MEXT.
originates from a larger amount of electron transfer not only
from Ag cores but also from the TiO2 support to Pd shells. The
current results show that the apparent activation energy of
Notes and references
reaction (3) using AgPd@Pd catalyst decreases by a factor of
three in the presence of TiO2. The greater degree of electron
transfer from the Pd layer to intermediate species A–C in the
presence of TiO2 strengthens the adsorption of formates and
decreases the energy barriers of hydrogen formation and proton
diffusion over catalysts.
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J. Mater. Chem. A, 2015, 3, 4453–4461 | 4461