Palladium Copper Chromium Ternary Nanoparticles Constructed In situ within a Basic Resin: Enhanced Activity in the Dehydrogenation of Formic Acid

Article abstract of DOI:10.1002/cctc.201700595

PdCuCr ternary nanoparticles (NPs) responsible for the efficient production of H₂ from formic acid are constructed in situ during the initial catalytic reaction within a macroreticular basic resin that possesses ?N(CH₃)₂ functional groups. A high H₂ production rate of 174 000 mL h^?1 g_pd ^?1 with a high turnover frequency of 830 h^?1 based on Pd can be achieved, which is substantially higher than that obtained with bimetallic PdCu and PdCr and monometallic Pd NPs. Physicochemical characterization was performed by using X-ray absorption fine structure analysis and high-angle annular dark-field scanning transmission electron microscopy. The kinetic isotope effect revealed not only the stabilization of highly dispersed NPs but also the boosting of the C?H bond dissociation step by a synergistic alloying effect induced by the introduction of Cr atoms, which plays an important role to achieve efficient catalytic activity. Moreover, this heterogeneous catalytic system exhibits high durability without agglomeration and leaching and has potential for applications in high-pressure gas-evolution systems.

Full text of DOI:10.1002/cctc.201700595

Accepted Article
Title: PdCuCr Ternary Nanoparticles Constructed in Situ within a Basic
Resin: Enhanced Activity for the Dehydrogenation of Formic Acid
Authors: Kohsuke Mori, Kohei Naka, Shinya Masuda, Kohei Miyawaki,
and Hiromi Yamashita
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10.1002/cctc.201700595
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PdCuCr Ternary Nanoparticles Constructed in Situ within a Basic
Resin: Enhanced Activity for the Dehydrogenation of Formic Acid
Kohsuke Mori,*[a][b][c] Kohei Naka,[a] Shinya Masuda,[a] Kohei Miyawaki,[a] and Hiromi Yamashita*[a][c]
Dedication ((optional))
Abstract: PdCuCr ternary nanoparticles (NPs) responsible for the
efficient production of hydrogen from formic acid (FA; HCOOH) are
constructed in situ during the initial catalytic reaction within a
macroreticular basic resin possessing –N(CH3)2 functional groups. A
could further realize economical CO2-mediated hydrogen storage
energy cycling by the regeneration of FA through the
hydrogenation of CO2 with H2.[4]
The decomposition of FA by the selective dehydrogenation
pathway is thermodynamically favored (∆G = −48.4 kJ·mol−1) and
is requisite for the production of pure H2 without the formation of
CO and H2O by the competitive dehydration pathway (∆G = −28.5
kJ·mol−1),[5] because the CO byproduct is toxic to Pt fuel cell
catalysts.[6] The selective dehydrogenation of FA under
homogeneous reactions with soluble organometallic complexes
at ambient temperatures has been intensively investigated to
date.[7] In spite of outstanding progress, current studies have been
focused on the development of efficient heterogeneous catalysts
due to practical reasons for on-board applications. Au, Pt, and
especially Pd nanoparticles (NPs)-supported catalysts have been
reported to exhibit outstanding catalytic activity.[8] The
development of bimetallic NPs, which constitute the architectural
configuration of two metals as an alloy or with a core–shell
structure, would be the other promising strategy, and would
enable further tailoring of the electronic properties and geometric
structure of Pd NPs due to the interplay of the different adjacent
metals, which is expected to ultimately enhance the catalytic
activity and selectivity.[9] For example, Pd-based alloys and core–
shell structures with Ag, Au, Ni, and Cu have been fabricated.[10]
The active Pd species is electronically promoted by charge
transfer originated from the difference in the work functions of two
metals, which plays a pivotal role in achieving high catalytic
activity. Ternary particles, such as CrAuPd, CoAuPd, and
CoAgPd, have more recently proven to have even further
improved catalytic activities.[11]In addition to unique synergistic
effects, the partial replacement of expensive noble metal NPs with
low-cost metals is important from a perspective of atomic
economy. However, the detailed effect of the introduction of third
metals, such as Cr and Co, on the enhancement of catalytic
activity has not yet been elucidated.
high H2 production rate of 174,000 mL h-1 gpd with a high turnover
-1
frequency (TOF) of 830 h-1 based on Pd can be attained, which is
substantially larger than those with bimetallic PdCu, PdCr, and
monometallic Pd NPs. Physicochemical characterization using X-ray
absorption fine structure (XAFS), high-angle annular dark-field
scanning transmission electron microscopy (HAADF-STEM), and the
kinetic isotope effect revealed that not only the stabilization of highly
dispersed NPs but also the boosting of the C–H bond dissociation
step by a synergistic alloying effect induced by the introduction of Cr
atoms plays an important role to achieve efficient catalytic activity.
Moreover, this heterogeneous catalytic system exhibits high durability
without agglomeration and leaching, and has further potential for
application to high-pressure gas evolution systems.
Introduction
Hydrogen (H2) has received significant attention from industry
as an ultimate ideal clean energy carrier to address environmental
and socioeconomic issues, in consideration of its high energy
density, non-toxicity, and efficient energy conversion by
combination with polymer electrolyte membrane fuel cell
(PEMFC) technology.[1] However, the efficient and safe storage,
handling, and controllable liberation of hydrogen are still key
challenges. A practical strategy is the storage of hydrogen in a
liquid phase under ambient conditions, which could be used to
release hydrogen for direct use in fuel cells by appropriate
catalysts.[2] Formic acid (FA; HCOOH) has emerged as one of the
most promising hydrogen storage materials,[3] due to its high
hydrogen content (4.4 wt%; 53.4 g/L), nontoxicity, and being a
nonflammable liquid under ambient conditions, and its application
We have recently reported a promising strategy for the design
of efficient heterogeneous catalysts toward hydrogen production
from FA by utilizing a basic ion exchange resin as an organic
support,[10e, 10f, 12] in which the accommodated NPs, such as Pd,
PdAg, and PdCu, are active centers responsible for the
production of high-quality H2. In the course of our ongoing studies
to explore practical catalysts for targeted reactions, we report here
that the PdCuCr ternary NPs within a macroreticular basic resin
bearing –N(CH3)2 functional groups exhibit significant catalytic
enhancement over that of bimetallic PdCu, PdCr, and
monometallic Pd NPs. Such improvement can be attributed to not
only the stabilization of highly dispersed NPs but also the boosting
of the C–H bond dissociation step by the positive alloying effect
induced by the introduction of Cr atoms. In addition, interesting
[a]
Prof. Dr. K. Mori, Mr. K. Naka, S. Masuda, K. Miyawaki, Prof. Dr. H.
Yamashita
Graduate School of Engineering, Osaka University
1-2 Yamadaoka, Suita, Osaka 565-0871 (Japan)
E-mail: mori@mat.eng.osaka-u.ac.jp, yamashita@mat.eng.osaka-
u.ac.jp
Prof. Dr. K. Mori
JST, PRESTO
4-1-8 Hon-Cho, Kawaguchi, Saitama, 332-0012 (Japan)
Prof. Dr. K. Mori, Prof. Dr. H. Yamashita
Unit of Elements Strategy Initiative for Catalysts & Batteries
Kyoto University, ESICB, Kyoto Univ. (Japan)
[b]
[c]
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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aspects such as the in situ generation of PdCuCr NPs during the
induction period, their characterization by means of
physicochemical methods, and their potential application to a
high-pressure gas evolution system are discussed.
construction of ternary NPs. Although there have been some trials
for the application of ternary catalysts to the dehydrogenation of
FA,[11] this is, to the best of our knowledge, the first report that
demonstrates improved catalytic activity through the use of the
PdCuCr ternary catalyst. The addition of HCOONa ratio also
influenced on the catalytic activity, and the maximum activity was
obtained at HCOONa:HCOOH = 9:1 (Figure S1).
Results and Discussion
Effect of metals
The commercially available IRA96SB resin (SBET = 45 m2 g1)
with weakly basic –N(CH3)2 groups was utilized as a support for
the fabrication of PdCuM (Pd 3wt%; molar ratio of Pd:Cu:M =
1:0.5:0.5, M = Cr, Co, Mn, Ni, Zn, Fe) ternary catalysts by the
addition of first-row transition metals. The deposition of metals
within the basic resin proceeded from an aqueous solution of
PdCl2, CuCl2·2H2O, and the nitrate of the third metal. The samples
were then pre-reduced with NaBH4 aqueous solution.
Monometallic Pd, Cu, and Cr, and bimetallic PdCu and PdCr
(molar ratio of Pd:M = 1:0.5) catalysts supported on resin were
also synthesized by the same method, followed by reduction with
NaBH4. No peaks corresponding to the metals could be confirmed
by X-ray diffraction (XRD) measurements for all of the samples.
Figure 2. Effect of the composition of catalyst in the dehydrogenation
of FA. (A) The mole fraction of Cu for Pd1Cr0.5 and (B) mole fraction of
Cr for Pd1Cu0.5
.
The composition of the PdCuCr ternary catalysts was
successfully tuned by simply varying the initial molar ratio of the
metal precursors. Figure 2A shows the effect of the Cu mole
fraction for Pd1Cr0.5 on the turnover frequency (TOF) based on the
Pd, in which the activity was maximized with Cu=0.5. Such
volcano type activity suggests the formation of a uniform PdCuCr
structure within the macroreticular resin. In contrast, the plot of
TOF based on the Pd with respect to the mole fraction of Cr for
Pd1Cu0.5 revealed that the addition of Cr =0.1 was sufficient to
enhance the catalytic activity, and further increase in the amount
of Cr had no influence on the catalytic activity (Figure 2B).
The catalytic activity for the dehydrogenation of FA was also
affected by the chemical properties of the resin: PdCuCr catalysts
prepared with other resins possessing various functional groups,
such as strongly acidic –SO3H, weakly acidic –COOH, and
strongly basic NMe3Cl, showed considerably low activity (Figure
S2). Notably, the catalytic activity of PdCuCr/resin with weakly
basic –N(CH3)2 group was significantly higher by a factor of 5-12
than those PdCuCr catalysts with the same metal contents
prepared with conventional inorganic supports, such as CeO2,
MgO, ZrO2, Al2O3, TiO2, and SiO2 (Figure S3). The prominent role
of the macroreticular structure in combination with the weakly
basic –N(CH3)2 groups has already been elucidated by our group
for the Pd and PdCu bimetallic systems.[10e, 10f] Experimental and
density functional theory (DFT) calculations revealed that the
weakly basic –N(CH3)2 group functions as a co-catalyst for the
acceleration O−H bond cleavage in FA, as well as facilitating
stabilization of the formate intermediate.
Figure 1. Effect of first-row transition metal addition to Pd-based
catalysts for the dehydrogenation of FA.
The dehydrogenation of FA was conducted using the prepared
Pd-based catalysts using HCOOH/HCOONa = 9/1 aqueous
solution, as summarized in Figure 1. The reaction proceeded with
high reproducibility for all catalysts and the ratios of produced H2
to CO2 were almost 1 during the course of the reaction, which
represents complete dehydrogenation. PdCuCr catalysts
disclosed the highest activity among those investigated, which
was substantially higher than those with monometallic Pd and the
bimetallic PdCu and PdCr catalysts. Under optimized conditions,
turnover frequency (TOF) of 830 h-1 based on the Pd employed
was attained, in which the H2 production rate approached to
174,000 mL h-1 gPd-1. The comparison of catalytic activity with
previously reported ternary systems are summarized in Table S1.
The reaction over pure Cu and Cr did not proceed to any
significant extent (data not shown). The addition of Co, Mn, and
Ni also had a moderately positive effect on the catalytic activity,
while the contributions of Zn and Fe were negligible or negative.
These results indicate a prominent synergistic effect by the
It should be noted that the present PdCuCr/resin catalyst
almost maintained its catalytic activity when another portion of FA
was added, where the volume of evolved gas linearly increased
with the reaction time (Figure S4). The reaction using the filtrate
after the removal of the catalyst did not proceed to any significant
extent and inductively coupled plasma spectroscopy (ICP)
analysis revealed no metal leaching.
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Recently, a simple and continuous high-pressure H2 production
system that would efficiently suppress a large amount of energy
during the compression process has been desired because of its
potential application for future hydrogen fuel filling stations.[13] The
PdCuCr catalyst can provide 2.0 MPa H2 at 353 K within 4 h
(Figure S5). This catalytic system may thus offer a simple and
efficient hydrogen storage/release vector using a heterogeneous
catalyst that could have industrial application for PEMFCs.
thus conclusively the NPs constructed in situ during the initial
course of the reaction. A similar phenomenon has been confirmed
in the case of the PdCuCo/resin catalytic system, in which fresh
catalyst had agglomerated particles with a mean diameter of 10.8
nm, while highly dispersed NPs with a mean diameter of 2.4 nm
can be observed in the isolated catalyst after the induction period
of approximately 2 min (Figure S6).
Characterization of active PdCuCr/resin catalyst
The periodically monitored time course during the initial stage
of FA dehydrogenation is shown in Figure 3A. The PdCuCr/resin
catalyst exhibited an induction period of approximately 2 min, in
which no noticeable gas evolution was detected and a slight color
change of the catalyst from light gray to dark gray was observed.
This observation indicates that the putative catalyst is
transformed into the actual active species in the reaction medium,
accompanied by structural changes.
Figure 3. (A) Time profile for the dehydrogenation of FA over the
Pd1Cu0.5Cr0.5 resin catalyst. (B) TEM image of the NaBH4-pretreated
fresh Pd1Cu0.5Cr0.5 resin catalyst. (C) TEM image of the isolated
Pd1Cu0.5Cr0.5 resin catalyst after reaction for 5 min.
Figure 4. TEM images of the initial catalysts (A, B, C) and recovered
catalysts after reaction for 1 h (D, E, F); Pd1Cu0.5Cr0.5 resin (A, D),
Pd1Cu0.5 resin (B, E), and Pd resin (C, F).
On the contrary, no color change or significant induction period
were observed for the monometallic Pd and bimetallic PdCu
catalysts. The average mean diameters of the catalysts reduced
by NaBH4 prior to the catalytic reaction were determined to be 2.0
and 2.4 nm, respectively, by TEM analysis (Figure 4B and 4C).
These results suggest that the addition of first-row transition
metals, such as Cr and Co, to the PdCu catalyst plays an
important role in the formation of ternary particles. One possible
trigger may be ascribed to the change of pH in the reaction
medium; the initial large NPs were formed by the reduction with
NaBH4 aqueous solution under basic conditions, which produce
To clarify the structure of the actual active species in the
dehydrogenation of FA, the NaBH4-pretreated fresh PdCuCr/resin
catalyst (Pd:Cu:Cr = 1:0.5:0.5) and the isolated catalyst after 5
min reaction were characterized. Transmission electron
microscopy (TEM) observation indicated the fresh catalyst had
irregular sized agglomerated particles with a mean diameter of
8.1 nm (Figure 3B). Interestingly, highly dispersed NPs with an
average diameter of 2.3 nm can be observed throughout the
observation area in the isolated PdCuCr/resin catalyst after the
induction period (Figure 3C). The catalytically active species is
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the PdCu NPs accompanied with the agglomerated Cr3+ species.
This putative NPs were transformed into the actual active species
by the further reduction with FA under acidic conditions. Since the
Pd2+ and Cu2+ are completely reduced by the initial treatment by
NaBH4 aqueous solution under basic conditions, such
transformation did not occur for Pd and PdCu NPs.
The in situ constructed PdCuCr NPs during the induction period
act as a key active species to achieve high catalytic activity;
therefore, the structure−activity relationship of the catalyst was
elucidated by performing detailed characterization of the isolated
catalyst. XRD measurements revealed no diffraction peaks,
presumably due to the smaller size of the particles.
Figure 5A shows a high-angle annular dark-field scanning
transmission electron microscopy (HAADF-STEM) image and
energy dispersive X-ray spectroscopy (EDS) mapping. The
elemental distribution along a single NP gave strong evidence for
the formation of random ternary alloy structures in the
PdCuCr/resin catalyst, without the formation of a core−shell
structure, where all Pd, Cu, and Cr atoms were homogeneously
distributed throughout the NPs. Trace amount of un-doped Cr
species can also be detected in the image area, which confirms
that additional Cr doping is unnecessary, as demonstrated in
Figure 2B.
ray absorption fine structure (FT-EXAFS) spectra, a single peak
for contiguous metallic Pd−Pd bonds was observed at
approximately 2.2-2.5 Å (Figure 6B). However, the Pd−Pd
distances for alloy samples were slightly shifted to a shorter
interatomic distance compared with that for monometallic Pd and
Pd foil, and a more noticeable shift was observed in the case of
the PdCuCr samples. This indicates the formation of shorter
heteroatomic Pd−Cu and/or Pd−Cr bonds in the catalyst.
A similar tendency can be observed in the Cu K-edge XAFS
spectra. The first arising edge peak at 8982 eV in the XANES
spectra of the Cu foil is attributed to the 1s → 4p transition (Figure
7A).[15] A characteristic single absorption peak was observed in
the range of 8983-8985 eV for Cu2O, which is due to the electronic
dipole-allowed transition of CuI. The spectra for resin-supported
PdCu and PdCuCr were similar to that for Cu foil but were
different from Cu2O, which confirms its metallic form. Minor
differences from the spectrum for Cu foil are presumably
explained by the alloying with Pd and/or Cr. In the FT-EXAFS
spectra (Figure 7B), both PdCu and PdCuCr, as with the Cu foil,
exhibited a strong singlet peak centered at ca. 2.4 Å due to
neighboring metal–metal bonding without the formation of Cu−O
and Cu−O−Cu bonds, which was detectable in the spectra for
CuO and Cu2O. The peak position of PdCu was slightly shifted to
a longer interatomic distance compared to that for Cu foil by the
alloying with Pd atoms. In contrast, the broadening of the main
peak can be observed for the PdCuCr sample, which clearly
indicates the formation of shorter Cu−Cr bonds and longer Cu−Pd
bonds.
The Cr K-edge XANES spectrum for CrO3 had an intense pre-
edge peak at 5992 eV, which is characteristic of terminal Cr6+=O
species with a tetrahedral coordination (Figure 8A).[16] The shape
of the XANES spectrum for PdCuCr/resin is quite different from
that of CrO3 due to the absence of the characteristic pre-edge
peak, but is similar to those of Cr3+-type spectra, such as Cr2O3
and Cr(NO3)3. The edge energy for the PdCuCr sample is higher
than that for Cr foil, which suggests that the Cr species is in a +3
oxidation state. In the FT-EXAFS spectra, the bond length of the
first Cr−O bond in PdCuCr resembles those in Cr2O3 and Cr(NO3)3,
but is longer than that in CrO3, which reveals the lack of a Cr=O
double bond in the PdCuCr/resin sample (Figure 8B). In
comparison with the reference Cr2O3, the intensity of the second
peak for PdCuCr/resin is significantly weakened. This peak in
PdCuCr is attributable to the contiguous Cr−O−M (M: Cr, Cu, Pd)
bonds. The reduced intensity in this region can be explained by
the formation of structurally disordered and extremely small
NPs.[17] No metallic bonds, detectable at around 2.3 Å for Cr foil,
were observed. Since the reduction potentials of Cr3+ ions
(E◦(Cr3+/Cr2+) = −0.42 V, E◦(Cr2+/Cr0) = −0.90 V vs. NHE) are more
negative than Pd2+ and Cu2+ ions (E◦(Pd2+/Pt0) = +0.99 V,
E◦(Cu2+/Cu0) = +0.34 V vs. NHE), the reduction of Cr3+ ions is
retarded more than those of Pd2+ and Cu2+ ions, and preferentially
constitute extremely small clusters on the surface of NPs by
interaction with Pd0 and Cu0, as illustrated in Figure 5B. This
assumption was supported by the durability test, which is
discussed later.
Figure 5. (A) HAADF-STEM image and EDS elemental mapping
results, and (B) the proposed structure for the Pd1Cu0.5Cr0.5 resin
catalyst, in which Pd, Cu, and Cr atoms are represented in yellow, red,
and blue, respectively.
The shapes of the Pd K-edge X-ray absorption near edge
structure (XANES) spectra of the PdCuCr/resin, Pd, and PdCu
samples resembled that of Pd foil with distinct two peaks
attributed to the allowed 1s → 5p transition,[14] but differed from
that for PdCl2 (Figure 6A). In the Fourier transform-extended X-
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Effect of Cr doping
Based on our previous report on reaction mechanisms for the
dehydrogenation of FA over basic resin-supported Pd and Pd
alloy catalysts,[10e, 10f] it was deduced that the present PdCuCr
catalytic system also follows a similar reaction pathway. First,
O−H bond dissociation in FA is assisted by the weakly basic –
N(CH3)2 group as a co-catalyst, which produces a metal–formate
intermediate accompanied with a –+HN(CH3)2 group. The metal–
formate intermediate undergoes C−H bond dissociation to afford
CO2 and a metal-hydride. The reaction of the hydride species with
–+HN(CH3)2 produces molecular hydrogen, along with the
regeneration of the active center. The cooperative action by
adjacent active NPs and basic –N(CH3)2 groups of the resin
support has been well evidenced by both experimental and DFT
studies in our previous reports.[10f, 18]
Figure 6. Pd K-edge (A) XANES and (B) FT-EXAFS spectra.
Table 1. KIE in the dehydrogenation of FA for various Pd-based catalysts.
Formic
Acid
Reaction rate
(mmol·min-1)
Entry
1
Catalyst
kH/kD
HCOOH
DCOOH
HCOOH
DCOOH
HCOOH
0.41
0.30
0.22
0.14
0.10
-
Pd1Cu0.5Cr0.5/resin
1.37
-
2
3
Pd1Cu0.5/resin
Pd/resin
1.59
DCOOH
0.06
1.73
Table 2. Average diameters determined by TEM and the concentration of
CO impurity in the dehydrogenation of FA for various Pd-based catalysts.
Average diameter / nm
CO
concentration
(60 min)
/ ppm
Catalyst
After
reaction
(60 min)
Before
reaction
2.3
2.0
2.4
2.3
3.7
4.6
1.4
2.0
7.0
Pd1Cu0.5Cr0.5/resin
Pd1Cu0.5/resin
Pd/resin
Figure 7. Cu K-edge (A) XANES and (B) FT-EXAFS spectra.
In an effort to elucidate the role of Cr doping, investigation of
the kinetic isotope effect (KIE) was performed using HCOOH and
DCOOH. The results are summarized in Table 1. The kH/kD
values obtained using the PdCuCr/resin catalyst was 1.37, which
was smaller than that observed with monometallic Pd (1.73) and
bimetallic PdCu (1.59). This result is consistent with the order of
catalytic activity and indicates that the PdCuCr catalyst facilitates
C−H bond dissociation of the metal–formate intermediate. One
possible reason for the improved catalytic activity can be
attributed to the formation of an electron-rich Pd species by the
energy transfer from Cr and/or Cu atoms to Pd atoms, due to the
net difference in the ionization potentials (Pd: 8.34 eV, Cu: 7.72
eV, Cr: 6.77 eV), which equilibrates their Fermi levels. The Pd 3d
peak in the X-ray photoelectron spectroscopy (XPS) spectra are
shifted to lower binding energy for both the PdCu (335.4 eV) and
PdCuCr (335.4 eV) in comparison with that for the monometallic
Pd catalyst (335.8 eV) (Figure S7).
Figure 8. Cr K-edge (A) XANES and (B) FT-EXAFS spectra.
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Another important role of Cr species on the catalytic
performance may be ascribed to a stabilization effect toward the
undesired agglomeration of NPs, which enhances the durability of
the catalyst. A linear increase in the time profile without a
substantial deactivation was observed for the dehydrogenation of
FA using the PdCuCr/resin catalyst (Figure 1A). During the initial
stage, a relatively high reaction rate can be observed for
PdCu/resin, and even in the presence of the monometallic
Pd/resin catalyst. However, the catalytic activity gradually
decreased as the reaction time advanced. Thus, each catalyst
was isolated after catalytic reaction for 1 h and subjected to TEM
analysis for comparison of the average particle sizes with the
initial particle sizes (Figure 4 and Table 2). Particle size
distributions were also shown in Figure S8. A TEM image of the
PdCuCr/resin catalyst showed that the particle sizes remained
essentially unchanged after the reaction, and without
agglomeration of the NPs; the mean particle diameter after the
reaction was determined to be 2.3 nm, which was consistent with
that of the initial catalyst particles. In contrast, the average
diameter of the isolated PdCu catalyst particles was 3.7 nm, which
was slightly larger than the 2.4 nm diameter of the initial PdCu
catalyst particles. More significant enlargement can be observed
in the case of pure Pd, where the average diameter of the isolated
catalyst was determined to be 4.6 nm, which was almost twice
that of the initial particle diameter.
The high durability of the PdCuCr catalyst may also be ascribed
to the efficient inhibition of poisoning by CO produced via the
dehydration pathway, which effectively prolongs the lifetime of the
catalyst. The PdCuCr catalytic system efficiently suppresses the
formation of unfavorable CO impurity, whereby the concentration
was 1.4 ppm after 1 h. This concentration can meet the criteria of
the PEMFC standard, which requires a CO concentration lower
than 10 ppm.[19] The PdCu catalyst also suppressed the formation
of CO, by which the CO concentration was less than 2.0 ppm. In
contrast, 7.0 ppm of CO was detected when using the
monometallic Pd catalyst, by which the adsorption of FA is
prevented by CO poisoning, thereby shortening its durability. The
selectivity for FA decomposition can be explained by the
adsorption mode of FA on the surface of NPs. The bridging
(bidentate) formate intermediate mainly produces H2 and CO2 by
the dehydrogenation pathway, whereas the linear (monodentate)
and multilinear (multimonodentate) modes predominantly give
rise to CO and H2O via the dehydration pathway.[10a] The electron-
rich Pd species predominantly stabilizes the bridging formate
species due to strong back-donation from the metal to formate.
Thus, the PdCu and PdCuCr catalysts, of which the Pd species
are in the electron-rich state in comparison with that of Pd, as
evidenced by XPS analysis, can minimize CO production.
monometallic Pd NPs. Not only is the agglomeration of NPs
inhibited by the stabilization effect of surface Cr cluster as anchors,
but also the C–H bond dissociation step is boosted by a
synergistic effect to achieve efficient catalytic performance. The
notable characteristics of the present catalytic system, such as
the facile preparation method, high durability, and the efficient
suppression of toxic CO formation, are prerequisites that meet an
ideal hydrogen vector in terms of potential industrial application
for PEMFCs.
Experimental Section
Materials: PdCl2, CuCl2·2H2O and Cr(NO3)3·9H2O were
purchased from Nacarai tesque. All resins were Amberlite,
supplied by Rohm and Hass.
Synthesis of PdCuCr/resin catalyst: Prior to the deposition,
resins were crushed by a ball mill (600 rpm for 10 min). The resin
(1.0 g) was mixed with 80 mL of aqueous solution containing 10
mL of PdCl2 (29.7 mM), 5 mL of CuCl2·2H2O (29.7 mM), and 5 mL
of Cr(NO3)3·9H2O (29.7 mM) and stirred at room temperature for
1 h. The suspension was evaporated under vacuum and the
obtained powder was dried overnight. Subsequently, the samples
were pre-reduced with NaBH4, giving PdCuCr/resin (Pd 3wt%;
molar ratio of Pd : Cu : Cr = 1 : 0.5 : 0.5). Monometallic (Pd, Cu,
and Cr), bimetallic (PdCu and PdCr; Pd: M = 1: 0.5), and ternary
(PdCuM, M=Co, Mn, Ni, Zn, Fe) supported on resin were also
prepared by same method followed by the reduction with NaBH4.
ICP analysis clearly indicated that the desired amount of metal
species are successfully loaded onto the each supports.
Characterization: BET surface area measurements were
performed using a BEL-SORP max (Bel Japan, Inc.) at 77 K. The
sample was degassed in vacuum at 373 K for 2 h prior to data
collection. Inductively coupled plasma optical emission
spectrometry (ICP-OES) measurements were performed using a
Nippon Jarrell-Ash ICAP-575 Mark II. TEM micrographs were
obtained with a Hitachi Hf-2000 FE-TEM operated at 200 kV.
STEM images and elemental mapping were obtained using JEOL
ARM200F equipped with a Kevex energy-dispersive X-ray
detector (JED-2300T) operated at 200 kV. XPS spectra were
measured by Shimadzu AXIS-ULTRA DLD. Pd, Cu, and Cr K-
edge XAFS spectra were recorded using a fluorescence-yield
collection technique at the beam line 01B1 station with an
attached Si (111) monochromator at SPring-8, JASRI, Harima,
Japan (prop. No. 2015B1083, 2016A1095). The data reduction
was performed by the REX2000 program (Rigaku).
Dehydrogenation of formic acid: The catalyst was placed into
a reaction vessel (30 mL) with a reflux condenser and equipped
with gas burette. After the purging with Ar, HCOOH/HCOONa =
9/1 aqueous solution (1M, 10 mL) was added into the reaction
vessel and reacted at 348 K with with magnetic stirring. The
evolved CO gas was monitored using a Shimadzu GC14B gas
chromatograph equipped with Methanizer. TOFs (h-1) were
determined according to the following equation.
Conclusions
PdCuCr ternary NPs constructed in situ during the initial catalytic
reaction within a macroreticular basic resin bearing –N(CH3)2
functional groups offers a simple and efficient catalytic system for
the production of H2 from FA with substantially high activity and
selectivity compared to that for bimetallic PdCu, PdCr, and
ꢁ
ꢅ
/ꢈꢉ
ꢂꢃꢄ ꢆꢇ
TOF ꢀ
ꢊ
∙ꢍ
ꢋꢌ
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10.1002/cctc.201700595
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FULL PAPER
Chem. Soc. 2015, 137, 11743-11748; c) N. Wang, Q. Sun, R. Bai, X. Li,
G. Guo, J. Yu, J. Am. Chem. Soc. 2016, 138, 7484-7487; d) M. Ojeda, E.
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Where Patm is the atmospheric pressure, VH2 is the generated
volume of H2, R is the gas constant, T is reaction temperature,
N
Pd is the mol number of Pd, and t is the reaction time.
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The present work was supported by JST-PRESTO
(JPMJPR1544). The part of this work was supported by Grants-
in-Aid for Scientific Research (Nos. 26220911, 25289289, and
26630409, 26620194) from the Japan Society for the Promotion
of Science (JSPS) and MEXT.
Keywords: ternary nanoparticle • PdCuCr • formic acid •
hydrogen carrier • ion exchange resin
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Entry for the Table of Contents (Please choose one layout)
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PdCuCr ternary NPs constructed in
situ within a macroreticular basic resin
acts as a simple and efficient
K. Mori,* K. Naka, S. Masuda, K.
Miyawaki, and H. Yamashita*
Page No. – Page No.
heterogeneous catalyt for the
dehydrogenation of formic acid
PdCuCr Ternary Nanoparticles
Constructed in Situ within a Basic
Resin: Enhanced Activity for the
Dehydrogenation of Formic Acid
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