Nanoporous CeO2 supports prepared from amorphous alloys: Enhanced catalytic performance for hydrogen generation from formic acid

Full text of DOI:10.1246/bcsj.20180214

Advance Publication Cover Page
Nanoporous CeO supports prepared from amorphous alloys: Enhanced catalytic
2
performance for hydrogen generation from formic acid
Ai Nozaki,* Ayane Yamashita, Ryosuke Fujiwara, Chiyako Ueda,
Hiroaki Yamamoto, and Masao Morishita
Advance Publication on the web September 26, 2018
doi:10.1246/bcsj.20180214
©
2018 The Chemical Society of Japan
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Nanoporous CeO
2
supports prepared from amorphous alloys: Enhanced catalytic
performance for hydrogen generation from formic acid
1
1
1
1
1
1
Ai Nozaki,* Ayane Yamashita, Ryosuke Fujiwara, Chiyako Ueda, Hiroaki Yamamoto, and Masao Morishita
1
Department of Chemical Engineering and Materials Science Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji,
Hyogo 671-2201
E-mail: nozaki@eng.u-hyogo.ac.jp
Masao Morishita received his Ph. D degree from Osaka University in 1986. He joined Kobe Steel, Ltd. as a research engineer.
In 1992, he moved to University of Hyogo as a research associate and was promoted to Professor at the Department of
Chemical Engineering and Materials Science in 2005. He has also served as a committeeman of the project of OECD for
the nuclear fuel waste safety management since 2010. He was awarded the Spriggs Phase Equilibria Award (2005) from the
American Ceramic Society. His research scope is chemical thermodynamics of materials related to energy conversion.
Ai Nozaki received her Ph. D degree in engineering from Osaka University in 2016. Then she was a postdoctoral researcher
at Osaka University (2016). Currently, she is working at University of Hyogo as an assistant professor. Her present research
focuses on catalytic performance of nanoporous material prepared from amorphous alloy.
Abstract
Nanoporous CeO
chemical dealloying of the Ce-Al amorphous alloy, followed by
synthesis of the Au-Pd/CeO catalysts. The synthesized Au-
Pd/CeO catalysts showed higher catalytic activity for hydrogen
generates electric energy when hydrogen reacts with oxygen in
the air; thus, it is a source of clean environmentally friendly
energy. However, many problems must be addressed before the
use of hydrogen fuel cells can be expanded, especially
concerning the distribution of fuel remotely. From the viewpoint
of energy and cost, it is difficult to store and transport hydrogen
fuel. As a solution to this problem, a chemical hydrogen storage
2
samples as supports were prepared by
2
2
generation from formic acid than catalysts using supports
prepared from a crystalline alloy precursor.
/
generation system that can utilize a high hydrogen-containing
Keywords: nanoporous metal oxide, amorphous alloy,
hydrogen generation.
compound for storing and transporting hydrogen in a stable state
2
0-23
is attractive
.
We focused on hydrogen production from formic acid,
which is an energy carrier that can effectively utilize carbon
dioxide. Formic acid has a hydrogen content of 4.4 wt%, and
1
. Introduction
The properties and structures of the catalytic support
greatly affect catalytic performance. CeO
2
1-3
is a simple basic
. Multiple methods
CO
2
, a by-product from the formic acid decomposition reaction,
+
20, 23
oxide for promoting the dissociation of H
exist for preparing CeO
hydroxide or cerium oxalate. In the present study, we focused on
can be recycled back to formic acid by hydrogen reduction
.
2
, such as calcination of cerium
Development of a high-performance catalyst is the key to
efficient hydrogen generation from formic acid. Pd is one of the
most selective toward the hydrogen production as well as
catalytically active for formic acid decomposition. However,
monometallic Pd catalysts have low reusability due to the CO
poisoning. Various bimetallic Pd-based catalysts are already
4
-6
nanoporous CeO
2
prepared by the dealloying method .
Nanoporous materials were prepared via selectively extracting a
7
, 87, 8
specific component from a mother alloy
. The dealloying
method possesses the following advantages: 1. The pore size can
be controlled from nano to micro scales by controlling the
extraction time and temperature; 2. Samples with high surface
area and a co-continuous porous structure can be obtained; 3.
Low coordination atoms tend to be exposed on the surface. We
have already reported that using an amorphous alloy as the
nanoporous material precursor results in a material with higher
2
3
24
25
26-29
reported in the literature (PdCu , PdCo, PdNi , PdAu
).
Among them, Pd−Au bimetallic catalysts have received
attention due to their higher resistance to CO poisoning and
excellent catalytic performance in selective formic acid
dehydrogenation.
In this study, we investigated the effect of the precursor’s
9-
catalytic activity than can be obtained using a crystalline alloy
2
atomic arrangement on the resulting CeO structure and the
1
2
.
catalytic performance of the prepared catalysts for hydrogen
generation from formic acid. We developed catalysts that
demonstrated excellent performance for hydrogen generation
Amorphous alloys are disordered in the atomic
arrangement, and many researchers across a wide range of fields
have been attracted to study such materials because of their
2
from formic acid: Au-Pd particles supported on CeO .
1
3-18
unique properties
. By using an amorphous alloy as the CeO
2
precursor, the resulting material was expected to exhibit much
better catalytic performances than if a crystalline alloy precursor
was employed. Al can form an amorphous structure with Ce
2. Experimental
2.1 Catalyst preparation.
Scheme 1 presents the procedure for sample preparation.
According to the literature, amorphous Ce-Al alloys can be
prepared within a Ce range of 7%–10%. A master ingot of
1
9
(composition range of Ce: 7 to 10 atom%) , and Al can be
selectively extracted by NaOH treatment. Thus, a CeO
was prepared from an amorphous Al-Ce alloy.
2
support
Ce
8
Al92 was prepared from a mixture of pure Ce and Al lumps
The hydrogen fuel cell, a next-generation energy candidate,
using the arc-melt technique in a highly purified Ar atmosphere.

reaction. During the reaction, the mixture was magnetically
stirred at 333 K and no air was allowed into the system. After
appropriate intervals, the generated gas was measured by reading
the scale on the gas burette. To investigate catalyst reusability,
the supernatant was removed after the reaction was completed
and more reaction mixture was added using a syringe to start the
next reaction cycle in the reusability test.
3
. Results and Discussion
.1 Characterization of the CeO supports
To investigate the crystallinities of a-CeAl, c-CeAl, a-CeO
c-CeO , XRD measurements were carried out (Figure 1). Sharp
3
2
2
, and
2
peaks indicating the presence of crystalline phases were not
observed in a-CeAl (Figure 1a), and broad peaks derived from
the amorphous states were observed. It was confirmed that a-
CeAl, prepared by the rapid quenching method, mainly existed
in an amorphous state. The XRD pattern of c-CeAl showed sharp
2
Scheme 1. Procedure for the preparation of Au-Pd/CeO .
peaks, assigned to Al and Ce
existed in a crystalline state (Figure 1c). The peaks assigned to
the CeO crystal were observed from a-CeO and c-CeO , and
3
Al11, indicating that c-CeAl mainly
8
Amorphous Ce Al92 (a-CeAl) in the form of ribbons (1 mm wide,
2
2
2
1
0–20 μm thick) were produced from master ingots through
Al could be selectively extracted from the Ce-Al alloy by the
dealloying treatment (Figure 1b and d). After drying processes
liquid quenching by a single roller melt-spinning method using
a copper wheel. The Al moieties were selectively extracted from
a-CeAl by immersion in 1 M aqueous NaOH at 343 K for 10 h
followed by washing with distilled water and drying, to fabricate
of a-CeO
color which originates from CeO
2
and c-CeO
2
, the color of the samples turned yellow in
crystals. This result suggests
2
that Ce species were easily oxidized during the drying process
under air.
FE-SEM was used to observe the morphology of the sample
before and after the dealloying treatment (Fig. 2). No pores and
cracks were observed in a-CeAl and c-CeAl, while many pores
nanoporous CeO
Ce-Al alloys (c-CeAl), a-CeAl was heated at 573 K under
vacuum. The nanoporous CeO samples prepared from NaOH
treatment of c-CeAl was denoted c-CeO
Au-Pd/a-CeO and Au-Pd/c-CeO
wt%, Pd loading: 3 wt%) were synthesized by depositing Au-Pd
on the as-synthesized a-CeO and c-CeO .The appropriate
quantities of HAuCl (11.8 mmol/L) and Pd(NH )Cl (9.7
mmol/L) were dissolved in 20 mL water. After adding CeO
2 2
(a-CeO ). For the preparation of crystalline
2
2
.
2
2
(Au loading: 4 and 7
and CeO
2 2 2
ligaments were observed in a-CeO and c-CeO . The
porous structure was expected to form from the selective
2
2
4
3
2
2
support to the solution, the mixture was stirred at room
temperature for 18 h. The mixture was filtered and washed
several times. Then, the solids were air-dried overnight and pre-
4
reduced with NaBH . For comparison, Au-Pd particles (Au
loading: 1, 2, 3, 4, and 5 wt%, Pd loading: 3 wt%) were deposited
on JRC-CEO-2 (reference catalyst, The Catalysis Society of
Japan) via the same method to yield Au-Pd/JRC-CEO-2 as a
standard reference catalyst. Au/Pd/JRC-CEO-2 and Pd/Au/JRC-
CEO-2 were prepared by depositing Au on Pd/JRC-CEO-2 and
depositing Pd on Au/JRC-CEO-2, respectively.
2
.2 Catalyst characterization.
Figure 1. XRD patterns of (a) a-CeAl, (b) a-CeO
2
, (c) c-CeAl,
X-ray diffraction (XRD; Rigaku, Ultima IV) was used to
and (d) c-CeO
2
.
analyze the crystallinity of the samples. Field emission-scanning
electron microscopy (FE-SEM; Jeol, JSM-7001) was used to
observe the surface morphology of the samples. The sample
surfaces were coated with carbon using an ion-sputtering
instrument. The atomic ratio on the surface was analyzed by
energy dispersive X-ray spectrometry (EDX, EDAX Ltd. DX-4).
The surface areas of the samples were estimated by the
Brunauer-Emmett-Teller (BET) method using nitrogen
physisorption isotherms obtained at 77 K (MicrotracBEL Corp.
BEL-SORP mini). X-ray photoelectron spectroscopy (XPS) was
performed to evaluate the Au oxidation state on the sample
surface.
2
.3 Catalytic reactions.
2
The prepared Au-Pd/CeO catalysts (100 mg) were placed
into a rubber septum-sealed quartz reaction vessel. The reaction
vessel was connected to a gas burette and purged with N . A
mixture of formic acid (0.23 mL), triethanolamine (NEt : 0.286
mL) and distilled water (2.48 mL) was injected to initiate the
2
Figure 2. FE-SEM images of (a) a-CeAl, (b) c-CeAl, (c) a-CeO
2
,
3
and (d) c-CeO
2
.

Table 1. Atomic ratios, surface areas, pore volume and pore
According to the XPS measurement results (Table S1), the
Au/Pd atomic ratio of Au-Pd/JRC-CEO2 (Au: 4 wt%, Pd: 3
wt%) was about 1. Yu et al. reported that Pd atoms which lack
adjacent Au atoms favor dehydration of HCOOH, whereas Pd
atoms that possess Au atoms as nearest neighbors favor
dehydrogenation of HCOOH. It is assumed that the Au atoms
on Au-Pd/JRC-CEO2 (Au: 4 wt%, Pd: 3 wt%) contributes to the
selective hydrogen production via formic acid decomposition..
To investigate the effect of catalyst support on the catalytic
properties, the prepared porous CeO (a-CeO and c-CeO )
2 2 2
substrates were used as catalyst supports. The activity of
Au(4wt%)-Pd/JRC-CEO2 was higher than that of the prepared
2
Au(4wt%)-Pd/a-CeO catalyst (Figure 3 and S4). XPS results
showed that the surface composition ratios of Au and Pd were
different and much of the Pd was exposed on the Au(4wt%)-
diameter of the prepared alloys and CeO
2
samples.
Atomic ratioa
bPore
Pore
Surface area
Sample
/ at％
volume diameter
2
6
m2g-1
/
3
-1
Ce
Al
/cm g
/ nm
a-CeAl
c-CeAl
9
9
91
91
9
<1
<1
-
-
-
-
a-CeO
2
2
91
95
100
244
117
123
0.52
0.31
0.23
7.7
12.4
7.1
c-CeO
5
JRC-CEO2
0
2
Pd/a-CeO surface (Table 2). The surface atomic ratios were
different depending on the support material. Despite the same
amount of Au (4wt%) and Pd (3wt%) loaded, the surface of the
a
Atomic ratio determined by EDX
Surface area estimated by the BET method.
b
CeO
2
support did not have the ideal 1:1 Au:Pd ratio; therefore,
as a support
the differences in surface area and structure of CeO
2
extraction of Al moieties from a-CeAl and c-CeAl by the
dealloying treatment. However, some pores in c-CeO collapsed,
resulting in aggregated CeO ligaments. The extraction of Al
moieties from the crystalline alloys proceeded rapidly near the
grain boundary, which tends to cause disaggregation of CeO
On the contrary, in the case of amorphous alloys, the Al moieties
were extracted uniformly and slowly from the surface. Thus, the
dissolution behavior of Al strongly affected the surface
greatly influenced the structure of the supported metal. To make
the surface composition ratio of Au and Pd approximately 1:1,
the amount of Au supported was increased to 7 wt%. The
2
2
catalytic activity of Au(7wt%)-Pd/a-CeO
as high as that of Au(4wt%)-Pd/a-CeO
performances of Au-Pd/a-CeO and Au-Pd/c-CeO
liquid phase reduction were also investigated (Figure 3).
Compared with Au-Pd/c-CeO , a higher catalytic activity was
observed for Au-Pd/a-CeO
that Au(7 wt%)-Pd/a-CeO
2
showed about 2 times
. The catalytic
prepared by
2
.
2
2
2
2
2
morphology of the resulting CeO supports.
2
. In the reusability test, it was found
showed the highest activity among
The composition ratios of Ce and Al in the samples before
and after dealloying treatment were measured by EDX (Table 1).
The composition ratios of the a-CeAl and c-CeAl were ca. 9
atom% Ce and 91 atom% Al. After the de-alloying treatment, the
ratio of Al moieties decreased, indicating Al moieties were
selectively extracted.
2
the prepared samples. (Figure S4).
The BET surface areas of the samples were estimated using
nitrogen physisorption isotherms (Table 1). The surface areas of
2
−1
a-CeAl and c-CeAl were quite low (<1 m g ) due to their
smooth surface morphologies (Figure 2a and b). a-CeO had a
high surface area compared to JRC-CEO2. a-CeO prepared
from the Ce-Al amorphous alloy had a higher surface area (244
2
2
2
−1
m g ) than that prepared from the crystalline alloy (c-CeO
2
, 117
m g ). The collapse of pores and the aggregation of CeO
ligaments might have caused the low surface area for c-CeO
2
−1
2
2
.
3
.2 Catalytic performance of Au-Pd/CeO
To investigate the loading method, metal species, and
2
amounts of metal loaded, a decomposition reaction of formic
acid was carried out by loading metals over JRC-CEO2 instead
of a-CeO . The time course for the hydrogen production from
2
Figure 3. Gas evolution from formic acid over Au-Pd/CeO
catalysts.
2
formic acid decomposition reaction using various catalysts was
monitored periodically, as shown in Figure S1. The reactions
proceed over Pd/JRC-CEO2, while, in the reusability test, the
catalytic activity decreased significantly. Pd surface poisoning
from by-product CO generation was thought to be the source of
this decreased activity. Pd is typically alloyed with Au to
suppress this deactivation. Au-Pd/JRC-CEO2 showed excellent
catalytic properties at 333 K and also performed well in the
reusability test with excellent reuse activity. To improve the
catalytic activity, the relationship between Au loading and gas
production over the Au-Pd/JRC-CEO2 catalyst was investigated.
As shown in Figure S2, Au(4wt%)-Pd/JRC-CEO2 showed high
activity and reusability. The effects of loading order on the
catalytic activity and reusability were examined (Figure S3). It
was found that the Au-Pd/JRC-CEO2 catalyst supporting Au and
Pd at the same time exhibited the highest activity and reusability.
Table 2. Loading amounts of Au and atomic ratios of Au and Pd
2
over Au-Pd/CeO samples.
Atomic ratioa
Au
Sample
/
at％
/
wt%
Au
Pd
Au-Pd/JRC-CEO2
4
4
7
7
51.7
30.0
52.9
59.9
48.3
70.0
47.1
40.1
Au-Pd/a-CeO
2
Au-Pd/c-CeO
2
a
Atomic ratio determined by XPS.

4
. Conclusion
In the Au 4f XPS spectra of the Au-Pd/CeO
Fig.4), the binding energies of Au-Pd/CeO samples were lower
compared to that of Au/CeO . The Pd 3d XPS spectra of the Au-
2
Pd/CeO samples showed lower binding energies than Pd/CeO .
2
samples
The sample supporting Au-Pd on nanoporous ceria
prepared from an amorphous alloy exhibited high catalytic
activities during the formic acid decomposition reaction.
Although the appropriate amount of Au differed depending on
the support, the catalyst activity was found to be improved by
controlling the Au:Pd composition ratio to approximately 1:1.
(
2
2
2
The shifts in binding energy indicated the changes in the charge
states of Au and Pd, verifying the formation of the alloy
3
0
structures.
Figure 5 shows the XRD patterns of the Au-Pd/CeO
samples (Au(7 wt%)-Pd/a-CeO , Au(7 wt%)-Pd/c-CeO and
Au(4 wt%)-Pd/JRC-CEO2). The peaks of Au-Pd/CeO samples
2
The CeO supports prepared from the amorphous alloys had fine
2
structures and appear to be effectively used for improving the
catalytic activity compared with catalysts prepared from a
crystalline alloy.
2
2
2
were located between two dotted dash lines which are assigned
to Au(111) (2θ=38.3º) and Pd(111) (2θ=40.1º) reflections. The
peak position in the XRD patterns suggest that the prepared Au-
Pd structures were alloy. The crystallite diameters of Au-Pd were
Acknowledgement
This study was partially supported by a Grant-in-Aid for
Young Scientists (B) (No. 17K14835) from the Ministry of
Education, Culture, Sports, Science, and Technology (MEXT),
Japan. A. N. also acknowledges Iketani Science and Technology
Foundation (No. 0291063), Izumi Science and Technology
Foundation (No. H29J035), and Japan Association for Chemical
Innovation, Japan.
estimated to be 4.7, 8.0, and 5.1 nm for Au-Pd/a-CeO
CeO and Au-Pd/JRC-CEO2, respectively, which were
calculated from the half height widths of the peaks. The
crystallite size of Au-Pd particle on a-CeO is small, which
suggests that Au-Pd/a-CeO may have small particle sizes. It is
assumed that smaller Au-Pd alloy particles can be prepared by
dispersing Au-Pd particles on a-CeO having the large surface
area and leads to the high catalytic activity for hydrogen
generation from formic acid. a-CeO showed excellent support
2
, Au-Pd/c-
2
,
2
2
2
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