AgPd@Pd/TiO2 nanocatalyst synthesis by microwave heating in aqueous solution for efficient hydrogen production from formic acid

Article abstract of DOI:10.1039/c5ta01434d

Ag_100-xPd_x/TiO₂ (x = 7, 10, and 15) catalysts for hydrogen production from formic acid were synthesized in aqueous solution using MW heating. The hydrogen production rate of Ag_100-xPd_x@Pd/TiO₂

Full text of DOI:10.1039/c5ta01434d

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AgPd@Pd/TiO2 nanocatalyst synthesis by
microwave heating in aqueous solution for efficient
hydrogen production from formic acid†
Cite this: DOI: 10.1039/c5ta01434d
a
b
a
a
Received 24th February 2015
Accepted 14th April 2015
*
Masashi Hattori, Daisuke Shimamoto, Hiroki Ago and Masaharu Tsuji
DOI: 10.1039/c5ta01434d
www.rsc.org/MaterialsA
Ag100ꢀxPdx/TiO2 (x ¼ 7, 10, and 15) catalysts for hydrogen production and a reductant. In the rst step, small Ag core particles were
from formic acid were synthesized in aqueous solution using MW prepared in the presence of TiO2 particles. Then Pd shells were
heating. The hydrogen production rate of Ag100ꢀxPdx@Pd/TiO2 synthesized in the second step. Based on spherical-aberration-
increased concomitantly with decreasing x. The best catalytic activity corrected scanning transmission electron microscopy (STEM),
ever reported was obtained for Ag93Pd7@Pd/TiO2 among all hetero- STEM-energy dispersed X-ray spectroscopy (EDS), X-ray diffrac-
geneous catalysts.
tion (XRD), and X-ray photoelectron spectroscopy (XPS) data, we
demonstrated the preparation of Ag–Pd alloy core and Pd shell
nanocrystals loaded on anatase-type of TiO2 nanoparticles
(denoted as AgPd@Pd/TiO2). Ag and Pd atoms are partially
The search for effective techniques of hydrogen gas (H2)
generation from liquid fuels has remained a difficult challenge
for mobile hydrogen energy systems. Formic acid (FA) attracts
great attention as a liquid fuel because of its high energy
density, nontoxicity, and excellent stability at room tempera-
ture. Moreover, FA is producible by a combination of H2O and
CO2 by irradiation with sunlight as a primary product in arti-
cial photosynthesis,1–3 which makes FA more attractive for use
in a sustainable and reversible energy storage cycle.
Some reports have described hydrogen production from the
decomposition of formic acid using solid catalysts such as core–
shell Au@Pd/C catalysts.4,5 Shortcomings of most such catalysts
are high operating temperature (>80 ꢁC) for efficient FA
decomposition and reduction of catalytic activity because of CO
coproduction. These shortcomings were overcome using an
Ag@Pd core–shell nanocatalyst, for which a high initial
hydrogen rate of about 4 L gꢀ1 hꢀ1 was achieved at room
temperature without CO coproduction.6 The high catalytic
activity of Ag@Pd core–shell nanocatalysts was explained by
electron transfer from the Ag core to the Pd shell because of the
larger work function of Pd (5.1 eV) than that of Ag (4.7 eV).6
We recently studied the preparation of Ag@Pd nanocatalysts
loaded on TiO2 nanoparticles using a two-step microwave-polyol
method, where ethylene glycol (EG) was used as both a solvent
ꢁ
alloyed with each other under heating above 170 C. Therefore,
Ag82Pd18 alloy and Pd shell nanocatalysts were formed. Using
TiO2 support, a higher hydrogen production rate of 16.00 ꢂ 0.89
L gꢀ1 hꢀ1 than that in the previous report6 was obtained. When
we compared the effects of TiO2 support using AgPd@Pd cata-
lysts without TiO2 support (denoted as bare AgPd@Pd), the initial
hydrogen production rate of AgPd@Pd/TiO2 was 23 times higher
than that of bare AgPd@Pd. Signicant enhancement of catalytic
activity of AgPd@Pd in the presence of TiO2 was explained by
further electron transfer from TiO2 to Pd because the work
function of TiO2 (4.0 eV) is lower than that of Pd (5.1 eV).7
For the practical application of AgPd@Pd/TiO2 catalysts,
even higher activity is required. Based on previous work on
catalytic activity of the Ag–Pd bimetallic system,6 the catalytic
activity of the AgPd alloy catalyst was much lower than that of
core–shell catalysts. These facts suggest that the catalytic
activity of Ag@Pd core–shell catalysts decreases greatly by
alloying between the Ag core and the Pd shell. Consequently, it
is expected that the catalytic activity of AgPd@Pd/TiO2 can be
greatly enhanced by dealloying the AgPd core. For this purpose,
a new simple method for preparing AgPd@Pd/TiO2 catalysts
with a low Pd content in the AgPd core must be developed.
In our previous study, AgPd@Pd/TiO2 catalysts were synthe-
sized in EG by MW heating at 176–178 ꢁC for about 10 min.7 The
Ag core and Pd shell were partially alloyed under heating at such
a high temperature. This communication describes our attempt
to prepare AgPd@Pd/TiO2 catalysts having a lower Pd content in
AgPd at much lower temperature under MW heating. Here we
use an aqueous solution as the solvent. The reagent solution was
aInstitute for Materials Chemistry and Engineering, Kyushu University, Kasuga
816-8580, Japan. E-mail: tsuji@cm.kyushu-u.ac.jp
bDepartment of Applied Science for Electronics and Materials, Graduate School of
Engineering Sciences, Kyushu University, Kasuga 816-8580, Japan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ta01434d
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ꢁ
kept at about 100 C. The effect of alloying in the AgPd core on nanocatalysts with an average diameter of 4.6 ꢂ 0.9 nm were
the catalytic activity was examined by changing the heating time loaded uniformly on TiO2 nanoparticles and an approximately
used for the preparation of AgPd@Pd/TiO2 catalysts. Results 0.8 nm-thick pure Pd shell was formed on the Ag or the Ag–Pd
show that catalytic activity depends strongly on the extent of alloy core metal. The Pd/Ag atomic ratio in whole Ag–Pd bime-
alloying of the AgPd core. We have succeeded in the preparation tallic nanocatalysts was determined to be 0.32 ꢂ 0.02 using
of Ag100ꢀxPdx@Pd/TiO2 (x ¼ 7, 10, and 15) catalysts having much STEM-EDS analysis. The Pd/Ag atomic ratio was also analyzed
higher catalytic activity than that of Ag82Pd18@Pd/TiO2 catalysts using AAS (see ESI†). The result obtained was in reasonable
obtained in EG.7
agreement with that estimated from the STEM-EDS analysis.
AgPd@Pd/TiO2 was prepared using the following method. STEM and STEM-EDS images of AgPd@Pd/TiO2 (1 h) and
First, Ag core nanoparticles were formed in the presence of AgPd@Pd/TiO2 (2 h) were also observed (see Fig. S4 and S5,
anatase-type of TiO2 nanoparticles with 10 nm average diam- ESI†). These results show that AgPd@Pd nanocatalysts of
eter, synthesized by MW heating8 (see STEM and XRD data in AgPd@Pd/TiO2 (1 h) and AgPd@Pd/TiO2 (2 h) respectively had
Fig. S1, ESI†). Then 15 mL of distilled water containing 300 mg an average diameter of 4.4 ꢂ 0.7 nm and 4.5 ꢂ 1.1 nm and about
polyvinylpyrrolidone (PVP) and 12.26 mg AgNO3 was mixed with 0.6 nm-thick and 0.5 nm-thick pure Pd shell. The Pd/Ag atomic
the colloidal solution of 71.88 mg TiO2 nanoparticles. The ratio in whole AgPd@Pd nanocatalysts of AgPd@Pd/TiO2 (1 h)
mixed solution was heated at 95 ꢁC with MW irradiation at and AgPd@Pd/TiO2 (2 h) was determined to be 0.33 ꢂ 0.03 and
120 W for 40 min under Ar bubbling. In the second step, 2 mL of 0.31 ꢂ 0.01 from STEM-EDS analysis. These results show that all
distilled water containing 8.3 mg Pd(NO3)2 was added to this samples had nearly the same morphology, size, and composi-
solution and heated with MW irradiation at 400 W for 30 min, tion, which indicates that added Pd(NO3)2 was reduced
1 h, or 2 h under Ar bubbling. Typical temperature proles of completely and that AgPd@Pd nanoparticles were formed by
MW heating when Ag nanoparticles and Pd shell were formed MW heating in 30 min.
are shown in Fig. S2 (ESI†). These products are respectively
Fig. 2 shows XRD patterns of AgPd@Pd/TiO2 (30 min),
called AgPd@Pd/TiO2 (30 min), AgPd@Pd/TiO2 (1 h), and AgPd@Pd/TiO2 (1 h), and AgPd@Pd/TiO2 (2 h) in the 2q ¼
AgPd@Pd/TiO2 (2 h) hereinaer. The solution temperature 20–90ꢁ range. An expanded XRD pattern in the 2q ¼ 37–40ꢁ
increased to about 100 ꢁC under MW irradiation. Character- range is shown in Fig. S6 (ESI†), where a major peak of the Ag
ization of product particles was conducted using STEM, component is observed. Aside from TiO2 anatase-peaks
STEM–EDS, XRD, XPS, and atomic absorption spectrometry observed at 2q ¼ 25.2ꢁ, 47.9ꢁ, 53.5ꢁ, 54.9ꢁ, and 62.4ꢁ indexed to
(AAS). The hydrogen production rate was determined using a {101}, {200}, {105}, {211} and {204} facets (PDF 01-071-1168),
gas burette. Further details related to experimental methods are major peaks derived from {111}, {200}, {220}, and {311} facets of
described in the ESI.† Fig. S3 (ESI†) shows a typical STEM image the Ag component of fcc Ag–Pd bimetallic particles were
of TiO2 and Ag particles, where some strong white contrast Ag observed. However, these peaks slightly shi to larger 2q from
particles are not loaded on light contrast TiO2 particles.
those of pure fcc Ag crystals (PDF 01-071-3762: 2q ¼ 38.12ꢁ,
Fig. 1a–e show STEM and STEM-EDS images of AgPd@Pd/ 44.31ꢁ, 64.45ꢁ, and 77.41ꢁ for {111}, {200}, {220}, and {311},
TiO2 (30 min). Fig. 1f shows line analysis data along the red line respectively). These peak shis occur by alloying of the Ag core
depicted in Fig. 1d. Results show that Ag–Pd bimetallic and Pd. According to Vegard's law,9 which is known to be
Fig. 1 STEM and STEM-EDS images of the AgPd@Pd/TiO2 (30 min) nanocatalysts: (a) STEM image, (b) Ag component, (c) Pd component,
(d) Ag and Pd components, (e) all components, and (f) line analysis data along the red line shown in panel (e).
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AgPd@Pd/TiO2 (1 h) > AgPd@Pd/TiO2 (30 min) z AgPd@Pd/
TiO2 (2 h), indicating that the relative amount of PdO to Ag is
independent of the reaction time.
To characterize the chemical states of AgPd@Pd/TiO2
samples, XPS spectra were measured (Fig. 3a and b). For
comparison, bare AgPd@Pd nanoparticles prepared using the
same process as that of AgPd@Pd/TiO2 (30 min) were also
measured. In the XPS spectra of the 330–346 eV range (Fig. 3a),
Pd 3d and PdO 3d peaks were observed in all samples. The
intensity ratio of the PdO peak to the Pd peak is independent of
the reaction time. These results also show that PdO was formed
under MW heating in aqueous solution and the amount of PdO
was not correlated with the reaction time. The Pd 3d5/2 and 3d3/2
peaks in bare AgPd@Pd shi to lower values by about 1.0 eV
Fig. 2 XRD patterns of all AgPd@Pd/TiO2 nanocatalysts.
compared to those of pure Pd (3d5/2 ¼ 335.1 eV, 3d5/2
¼
340.3 eV) because of electron transfer from Ag to Pd arising
from a difference of work functions between Ag and Pd. Simi-
larly, the binding energies of Ag 3d5/2 and 3d3/2 peaks in bare
AgPd@Pd 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). These
binding energies were similar to those of Ag 3d peaks in Pd rich
(>90%) Ag–Pd alloys.11 Therefore, it is reasonable to assume that
the peak shis originate from the formation of Ag–Pd alloys
around the interface of the Ag-core and the Pd-shell.
The binding energies of Pd 3d5/2 and 3d3/2 peaks of all three
AgPd@Pd/TiO2 samples were almost identical and shied to
lower values by about 0.9 eV compared to those of bare
AgPd@Pd nanoparticles (3d5/2 ¼ 334.2 eV, 3d3/2 ¼ 339.4 eV). At
the same time, the binding energies for Ag 3d5/2 and 3d3/2 peaks
of all AgPd@Pd/TiO2 samples were also almost identical and
shied to lower values by about 0.9 eV compared with those of
applicable to Ag–Pd systems,10 about 7.4%, 10.1%, and 15.3% of
Pd atoms are dissolved respectively in AgPd alloy core particles
of AgPd@Pd/TiO2 (30 min), AgPd@Pd/TiO2 (1 h), and
AgPd@Pd/TiO2 (2 h). Here, a weak peak was observed at 2q ¼
34.4ꢁ in all samples indexed to {101} facets of PdO (PDF 01-088-
2434), which was not observed in Ag–Pd particles of AgPd@Pd/
TiO2 prepared in EG solution (denoted as AgPd@Pd/TiO2 (EG)).7
This result means that a small amount of PdO was formed under
MW heating in aqueous solution. Moreover, the fact that the
intensity ratio of the PdO {101} peak to the Ag {111} peak was
bare AgPd@Pd nanoparticles (3d5/2 ¼ 367.1 eV, 3d3/2
¼
373.0 eV). These shis suggest that some electrons are trans-
ferred from TiO2 to Pd and Ag because of large differences of the
work functions between Ag or Pd and TiO2. Taking account of
the fact that the binding energies for Ag and Pd are almost
identical in all Ag100ꢀxPdx@Pd/TiO2 (x ¼ 7, 10, and 15) samples,
AgPd@Pd nanoparticles were sufficiently adhered onto TiO2 by
MW heating for 30 min.
The initial H2 production rate (Rhydrogen) of AgPd@Pd/TiO2
samples was measured using the following method: 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 5.1 mg) was measured using a gas burette (see Fig. S7,
ESI†). Detailed gas analyses for CO2, H2, and CO were performed
on a gas chromatograph and no CO emission was detected using
GC for all samples at 27–90 ꢁC (see Fig. S8, ESI†). Temporal
variation of total gas generation by decomposition of formic acid
in the presence of Ag93Pd7@Pd/TiO2 (30 min), Ag90Pd10@Pd/
TiO2 (1 h), and Ag85Pd15@Pd/TiO2 (2 h) at room temperature
ꢁ
(27 C) is shown in Fig. 4a. The Rhydrogen values obtained from
eqn (S1) and (S2) of ESI† are presented in Table 1. For compar-
ison, the corresponding data for Ag82Pd18@Pd/TiO2,7 bare
Ag@Pd,6 and CoAuPd alloy nanoparticles12 are given. The
Rhydrogen value of the Ag93Pd7@Pd/TiO2 (30 min) sample
ꢀ1
increases greatly from 46.03 L gꢀ1 hꢀ1 at 27 C to 371.79 L g
ꢁ
Fig. 3 XPS spectra of all AgPd@Pd/TiO2 nanocatalysts and bare
AgPd@Pd nanocatalyst: (a) Pd 3d3/2,5/2 and (b) Ag 3d3/2,5/2
.
hꢀ1 at 90 C. The Rhydrogen value at 27 C is about three times
ꢁ
ꢁ
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ln(Rhydrogen) ¼ ꢀEa/RT + C
(1)
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 respectively to be 29.8,
32.5, and 37.8 kJ molꢀ1 for Ag93Pd7@Pd/TiO2 (30 min),
Ag90Pd10@Pd/TiO2 (1 h), and Ag85Pd15@Pd/TiO2 (2 h) (see Fig. S9,
ESI†). These values were higher than Ea of Ag82Pd18@Pd/TiO2
prepared in EG solution (7.2 kJ molꢀ1).7 The formation of PdO
on the surface, which interferes with the catalytic activity, might
be one reason for the increase in Ea values.
Fig. 4 (a) Gas generation by decomposition of formic acid (0.25 M,
20 mL) vs. time in the presence of AgPd@Pd/TiO2 (30 min) nano-
catalysts, AgPd@Pd/TiO2 (1 h) nanocatalysts and AgPd@Pd/TiO2 (2 h)
In the last section we discuss on the origin of decrease in the
catalytic activity with increasing x in Ag100ꢀxPdx@Pd/TiO2 nano-
nanocatalysts at 27 ꢁC. (b) The hydrogen generation rate variation of catalysts. The atomic ratios of the Pd shell to the AgPd core in
AgPd@Pd/TiO2 samples follows the alloying rate of Ag100ꢀxPdx at
Ag100ꢀxPdx(x ¼ 7, 10, and 15)@Pd particles were estimated from
27 ꢁC.
STEM-EDS and XRD data to be 0.25, 0.21 and 0.15, respectively.
This result suggests that the Pd/AgPd atomic ratio decreases with
an increasing x. On the other hand, we found that the total Pd/Ag
atomic ratio of Ag100ꢀxPdx@Pd/TiO2 was nearly constant between
30 min and 2 h of heating. In addition, the particle sizes of
Table 1 Hydrogen production rates from catalytic decomposition of
formic acid in water at different temperatures
Ag100ꢀxPdx@Pd of all samples were almost the same (z4.6 nm).
On the basis of the above ndings, reduction of Pd2+ on the Ag or
AgPd core is completed within 30 min and alloying between the
Ag core and the Pd shell occurs between 30 min and 2 h. Tsang
et al.6 reported that Ag@Pd nanocatalysts showed their highest
catalytic activity when the Ag core was completely covered by a
thin Pd shell. The catalytic activity rapidly dropped off both when
the surface area of the Pd shell became low or the Pd shell
became thicker. It is therefore reasonable to assume that the
surface area of the Pd shell of Ag100ꢀxPdx@Pd nanoparticles
prepared in this study decreases with an increasing x because Pd
atoms in the Pd shell decrease owing to alloying between the Ag
core and the Pd shell at longer MW heating.
Temperature
(ꢁC)
H2 gas volume
(L gꢀ1 hꢀ1
Catalyst
)
Ag93Pd7@Pd/TiO2 (30 min)
Ag93Pd7@Pd/TiO2 (30 min)
Ag93Pd7@Pd/TiO2 (30 min)
Ag93Pd7@Pd/TiO2 (30 min)
Ag93Pd7@Pd/TiO2 (30 min)
Ag90Pd10@Pd/TiO2 (1 h)
Ag90Pd10@Pd/TiO2 (1 h)
Ag90Pd10@Pd/TiO2 (1 h)
Ag90Pd10@Pd/TiO2 (1 h)
Ag90Pd10@Pd/TiO2 (1 h)
Ag85Pd15@Pd/TiO2 (2 h)
Ag85Pd15@Pd/TiO2 (2 h)
Ag85Pd15@Pd/TiO2 (2 h)
Ag85Pd15@Pd/TiO2 (2 h)
Ag85Pd15@Pd/TiO2 (2 h)
Ag82Pd18@Pd/TiO2 (EG)7
Ag@Pd6
27
40
60
70
90
27
40
60
70
90
27
40
60
70
90
27
20
25
46.03 ꢂ 2.27
91.06 ꢂ 4.80
143.77 ꢂ 3.33
230.40 ꢂ 5.69
371.79 ꢂ 9.86
31.74 ꢂ 1.64
57.96 ꢂ 3.08
128.52 ꢂ 5.85
149.06 ꢂ 6.38
285.04 ꢂ 12.01
19.17 ꢂ 1.49
33.68 ꢂ 0.32
75.85 ꢂ 1.32
128.05 ꢂ 8.22
253.21 ꢂ 10.54
16.00 ꢂ 0.89
3.67
Conclusions
For this study, we prepared Ag100ꢀxPdx@Pd/TiO2 (x ¼ 7, 10,
and 15) nanocatalysts under MW heating in aqueous solution for
suppressing the alloying of the Ag core and Pd. The hydrogen
generation rates of Ag100ꢀxPdx@Pd/TiO2 (x ¼ 7, 10, and 15)
catalysts depend strongly on the degree of alloying in
Ag100ꢀxPdx@Pd/TiO2 catalysts. They increase by 2.4 times with a
decrease in the Pd content in the AgPd core from 15% to 7%. The
initial hydrogen formation rate of Ag93Pd7@Pd/TiO2 (30 min)
from formic acid, 46.03 L gꢀ1 hꢀ1, was about three times higher
than that of Ag82Pd18@Pd/TiO2 (EG).7 To the best of our knowl-
edge, this is the best value ever reported among all heteroge-
neous catalysts.6,12 Our novel method for producing high catalytic
activity of core–shell AgPd@Pd nanocatalysts on TiO2 particles
under low temperature conditions is applicable for efficient
hydrogen production systems intended for mobile applications.
CoAuPd12
7.9
higher than that of Ag82Pd18@Pd/TiO2 (EG) at 27 ꢁC (16.00 L gꢀ1
hꢀ1)7 and about 13 and 6 times higher than those of Ag@Pd
catalysts at 20 ꢁC (3.67 L gꢀ1 hꢀ1)6 and CoAuPd alloy catalysts at
25 ꢁC (7.9 L gꢀ1 hꢀ1).12 On the other hand, the Rhydrogen values of
Ag90Pd10@Pd/TiO2 (1 h) and Ag85Pd15@Pd/TiO2 (2 h) samples
were, respectively, 31.74 L gꢀ1 hꢀ1 and 19.17 L gꢀ1 hꢀ1 at 27 ꢁC.
Fig. 4b shows the dependence of Rhydrogen on Pd contents in the
AgPd core. It is noteworthy that the Rhydrogen value increases
greatly with decreasing Pd content in the AgPd core and that the
catalytic activity is independent of the amount of PbO which was
largest in AgPd@Pd/TiO2 (1 h). These results indicate that the
catalytic activity depends on the extent of alloying between Ag
and Pd in the AgPd core under our experimental conditions.
Hydrogen gas production rates were measured at various
Acknowledgements
temperatures using AgPd@Pd/TiO2 catalysts (Table 1). The This work was supported by JSPS KAKENHI Grant numbers
apparent activation energies were estimated from the following 25286003 and 25550056, and by a Management Expenses Grant
relationship.
for National University Corporations from MEXT.
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