Palladium Nanoparticles Supported on Titanium-Doped Graphitic Carbon Nitride for Formic Acid Dehydrogenation

Article abstract of DOI:10.1002/asia.201700041

Pd nanoparticles (NPs) supported on Ti-doped graphitic carbon nitride (g-C₃N₄) were synthesized by a deposition–precipitation route and a subsequent reduction with NaBH₄. The features of the NPs were studied by XRD, TEM, FTIR, XPS, EXAFS and N₂-physisorption measurements. It was found that the NPs had an average size of 2.9 nm and presented a high dispersion on the surface of Ti-doped g-C₃N₄. Compared to Pd loaded on pristine g-C₃N₄, the Pd NPs supported on Ti-doped g-C₃N₄ exhibited a high catalytic activity in formic acid dehydrogenation in water at room temperature. The enhanced activity could be attributed to the small Pd NPs size, as well as the strong interaction between Pd NPs and Ti-doped g-C₃N₄.

Full text of DOI:10.1002/asia.201700041

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Palladium nanoparticles supported on titanium doped graphitic
carbon nitride for formic acid dehydrogenation
Authors: Yongmei Wu, Meicheng Wen, Miriam Navlani-García,
Yasutaka Kuwahara, Kohsuke Mori, and Hiromi Yamashita
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Chemistry - An Asian Journal
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Palladium nanoparticles supported on titanium doped graphitic
carbon nitride for formic acid dehydrogenation
[
a,b]
[a]
[a]
[a,c]
[a,c,d]
Yongmei Wu , Meicheng Wen , Miriam Navlani-García , Yasutaka Kuwahara , Kohsuke Mori
,
[a,c]
and Hiromi Yamashita*
Abstract: Pd nanoparticles (NPs) supported on Ti-doped graphitic
carbon nitride (g-C were synthetised by deposition-
precipitation route and a subsequent reduction with NaBH . The
features of Pd supported Ti-doped g-C were studied by XRD,
TEM, FT-IR, XPS, EXAFS and N physisorption measurements. It
was found that the NPs had an average size of 2.9 nm and
presented a high dispersion on the surface of Ti-doped g-C
Compared with Pd loaded on pristine g-C , Pd NPs supported Ti-
doped g-C catalyst exhibited a high activity in formic acid
catalytic ability at mild conditions, but it should be also selective
toward the FA dehydrogenation pathway. It was reported that
the active systems are mainly based on precious metals such as
3 4
N )
a
4
[
4]
[5]
[6,7]
[8]
[9]
3
N
4
Ir , Ru, Au,
Pt, and Pd. Among these precious metal
2
catalysts, Pd-based heterogeneous catalysts have been found
to be the most active single-component nanocatalysts and have
provided notable activities in the additive-free FA
N .
3 4
[
3]
3
N
4
dehydrogenation at low temperature. Recently, many attempts
to improve the catalytic activity for the dehydrogenation of FA
have been considered. Several studies found in the literature
claimed that Pd-based bimetallic and trimetallic systems with
3 4
N
dehydrogenation in water at room temperature. The enhanced
activity can be attributed to the small Pd NPs size as well as the
[
10-12]
strong interaction between Pd NPs and Ti-doped g-C
3 4
N .
controlled sizes or core-shell structure (such as AuPd/C,
[
12-14]
[15]
[16]
PdAg/C,
CoAuPd/C,
PdCu/resin,
Pd-MnO
x 2
/SiO -
[
17]
[18]
NH
2
and CrAuPd/SiO -NH
2
2
catalysts) exhibited higher
turnover frequencies (TOFs) at low temperature than
monometallic Pd catalysts, while other investigations are
focused on the effect of the support features in enhancing the
Introduction
Formic acid (HCOOH, FA), has emerged as a promising H
storage material, as it is a major product in biomass processes,
2
final catalytic performance of Pd NPs. Typical catalyst supports,
[19]
including activated carbon materials,
macroreticular basic
[
1]
and shows interesting intrinsic properties (it is nontoxic, it is
liquid at room temperature and has high energy density, and it is
[20]
[21]
[22]
resin,
graphene oxide,
MOF–graphene
have been reported to be effective to
and pyridinic-N-
[23]
doped carbon hybrids
[
2]
suitability for easy transportation, handling, and safe storage).
FA catalytic decomposition can proceed via dehydrogenation (1)
and/or dehydration (2) pathways.
prevent the aggregation of Pd NPs as well as to enhance
catalytic performances due to the synergism between active Pd
species and support surface.
[
3]
3 4
Graphitic carbon nitride (g-C N ) has attained increasing
interest by the research community and it is considered a
HCOOH → H
2
2
+ CO
2
; ΔG°298K = −35.0 kJ/mol (1)
promising alternative to conventional carbon materials in
HCOOH → H
O + CO ; ΔG°298K = −14.9 kJ/mol (2)
[24]
numerous research fields.
When g-C
3
N
4
is used as
heterogeneous catalyst support, the high nitrogen content can
provide numerous sites for metallic NPs anchorage. Moreover,
delocalised electron states can be developed. The work
functions of some noble metals such as Pd or Pt are situated
In order to obtain ultrapure H
and, therefore, the optimum catalyst should not only display high
2
, FA dehydration must be avoided
3 4
between the conduction and valence bands of g-C N and
[
a]
Dr. Y. Wu, Dr. M. Wen, Dr. M. Navlani-García, Dr. Y. Kuwahara,
Dr. K. Mori, Prof. H. Yamashita
Graduate School of Engineering
Osaka University
Schottky barrier can be created by metal particles with higher
work function, enhancing therefore the charge separation at the
[25-27]
metal-semiconductor interface,
which would be ultimately
reflected in the final catalytic behaviour. According to this, g-
could be considered as an “active support” that promotes
the final activity and/or selectivity of the supported metal NPs. In
Osaka 565-0871 (Japan)
E-mail: yamashita@mat.eng.osaka-u.ac.jp
Dr. Y. Wu
Department of Chemical and Environmental Engineering,
Xingjiang Institute of Engineering
Urumqi, 83009 (PR China)
Dr. Y. Kuwahara, Dr. K. Mori, Prof. H. Yamashita
Unit of Elements Strategy Initiative for Catalysts & Batteries (ESICB)
Kyoto University
Katsura, Kyoto 615-8520 (Japan)
Dr. K. Mori
JST, PRESTO, 4-1-8 Honcho, Kawaguchi,
Saitama 332-0012 (Japan)
3 4
C N
[b]
[c]
[d]
[28]
terms of FA dehydrogenation, Cai et al.
prepared 8% Pd NPs
loading on mesoporous g-C (Pd@CN) named Mott-Schottky
3
N
4
catalyst and applied it in the room temperature dehydrogenation
of FA in water. They found that the catalytic performance over
Pd@CN was higher than that of similar Pd@carbon catalysts.
[
29]
Yoon and coworkers
polymeric g-C loading Pd NPs exhibited a TOF value of 144
in FA dehydrogenation, which was higher than that of the
multimetallic catalyst CoAuPd/C and AgPd/C. However,
mesoporous g-C materials have typically been prepared by
using SiO as a hard-template, which involves quite intricate de-
also reported that mesoporous
3 4
N
−1
h
Electronic Supplementary Information (ESI) available: N
physisorption data, chemical analysis data for nitrogen and reaction
kinetics data.
2
3
N
4
2
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templating procedures using HF or NH
4
HF
2
. These synthetic
peaks located at 2θ = 27.4° and 13.1°, are present in the
diffractograms. The main peak (at 2θ = 27.4°) is ascribed to the
(002) planes (d=0.325 nm), typical interlayer stacking reflection
procedures always show important shortcomings, not only in
terms of numerous synthetic steps and economic issues, but
also related to environmental considerations. In addition, the
[
30]
of conjugated aromatic systems.
The minor peaks (2θ =13.1°),
versatility of g-C
3
N
4
-based catalytic systems also lies in the fact
correspond to a distance of 0.675 nm and was indexed as (100)
planes in-planar tris-s-triazine structural packing motifs. With
that pivotal aspects, such as their electronic features, as well as
their textural and surface chemical characteristics can be tuned
by doping with heteroatoms. Along these lines, some studies
where this approach has been considered by using various
respect to xTi-g-C
TiO phase were observed. XRD patterns of the xTi-g-C
not differ significantly from pristine g-C except for
3 4
N samples, no significant diffraction peaks of
2
3
N
4
did
a
3
4
N ,
[30]
[31]
[32]
[33]
[34]
[35]
elements, such as B,
F,
S,
Fe,
Zn,
or K
are well
broadening observed in the (100) peak and an important
decrease of the (002) peak intensity with increasing Ti content,
revealing the presence of a host–guest interaction and titanium
species role in inhibiting the polymeric condensation. Similar
documented, which were mainly applied for photocatalytic
reaction such as photodegradation of organic pollutants under
visible light. On based of first-principles calculations, Pan et al.
[33]
predicted that g-C
3
N
4
nanotubes slightly functionalized with Ti
due to possibly inducing
Encouraged by their work, herein, we
report the preparation of Pd NPs supported on Ti-doped g-C
by a facile deposition–precipitation route followed by reduction
using NaBH . It was found that the as-synthesized Pd/Ti-g-C
results were observed in the Fe-doped g-C
3
N
4
,
Zn-doped g-
Additionally, Pd/xTi-g-
samples did not exhibit diffraction peaks of Pd species
[34]
2-
[37]
may improve the storage ability of H
2
C
C
3
N
N
4
4 3 4
and [WO ] -doped g-C N .
[
36]
hydrogen spillover.
3
4
3
N
4
(Figure 1(B)), which indicated that Pd NPs were highly dispersed
on the surface of xTi-g-C . Compared with the pure g-C
xTi-g-C and Pd/xTi-g-C samples showed no obvious shift
of (002) and (100) lines, demonstrating that the graphitic
stacking of g-C remained after incorporation of Ti and Pd in
the original material.
3
N
4
3 4
N ,
4
3
N
4
3
N
4
3 4
N
catalyst exhibited high activity in the FA dehydrogenation
reaction in water at room temperature. The strong interaction
between Pd NPs and titanium species in graphitic carbon nitride
might be responsible for the outstanding performance displayed
by the present catalytic system.
3
N
4
Results and Discussion
The textural properties of all samples were characterised by N
2
adsorption–desorption experiments. The isotherms of all
samples are shown in Figure S1 (A) and (B) (in the Supporting
Information). All samples had similar N adsorption-desorption
2
isotherm profiles. Specific surface area determined by
Brunauer–Emmett–Teller (BET) method and pore volumes are
listed in Table 1. In the case of xTi-g-C
surface areas slightly increased with the increase of Ti contents.
As listed in Table 1, the amounts of Pd in the Pd/xTi-g-C and
Pd/g-C were found to be around 0.9 wt%, while Ti contents in
3 4
N samples, the BET
3
N
4
3
N
4
3 4
these Pd/xTi-g- C N samples ranged from 0.98 wt% to 1.65
wt%.
Table 1. Textural properties and metal contents in xTi-g-C
g-C
3
N
4
and Pd/xTi-
N .
3 4
Pore
S
m
BET
-1
Pd content
(wt%)
Ti content
(wt%)
Sample
2
Volume
(
g )
-1
(
cm
3
g )
g-C
3
N
4
2.3
8.2
13.5
14.8
5.6
12.2
12.0
11.7
0.063
0.111
0.121
0.138
0.069
0.109
0.108
0.108
-
-
-
-
-
-
-
0
1
1
.5Ti-g-C
.0Ti-g-C
.5Ti-g-C
3
3
3
N
N
N
4
4
4
-
Pd/g-C
3
N
4
0.86
0.89
0.89
0.88
-
Pd/0.5Ti-g-C
Pd/1.0Ti-g-C
Pd/1.5Ti-g-C
3
N
N
N
4
4
4
0.98
1.47
1.65
3 4 3 4
Figure 1. XRD patterns of (A) xTi-g-C N and (B) Pd/xTi-g- C N .
3
3
XRD patterns of g-C
3
N
4
and xTi-g- C
3
N
4
materials are
Figure 2 (A) and (B) show the FT-IR spectra of xTi-g-C
3 4
N
displayed in Figure 1(A). As it can be observed, two diffraction
and Pd/xTi-g-C samples, respectively. For pure g-C , the
3
N
4
3 4
N
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−
1
broad bands in the 3500–2800 cm region can be assigned to
N-H stretching vibration modes. On their side, the marked bands
−
1
located at 1000-1650 cm are due to the stretching vibration
[
32]
modes of C=N and C-N heterocycles.
Additionally, the
−
1
significant band appearing at 803 cm is attributed to the ring-
sextant out-of-plane bending vibration of triazine units.
[
30, 33]
It
should be noticed that the incorporation of Ti species and
subsequent loading of Pd NPs did not cause considerable
changes in the original graphitic structure of g-C N , as
3 4
noticeable changes in the main characteristic bands of g- C
3 4
N
were not observed.
The structure and morphology of Pd/g- C
were analysed by TEM microscopy (See Figure 3). TEM
micrographs revealed that g- C and Ti doped g- C showed
typical platelet-like morphology with several layers. The average
Pd NPs size Pd/g- C (determined by counting 100 NPs), was
determined to be about 4.6 nm (Figure 3(A)). Interestingly, Pd
NPs in Pd/0.5Ti-g-C and Pd/1.0Ti-g-C showed an uniform
dispersion on the Ti doped g- C support surface with
theaverage diameter of 2.6 nm and 2.9 nm, respectively (Figure
(B) and 3(C)), while the size increased to 4.4 nm for Pd/1.5Ti-
g-C (Figure 3(D)). It was reported that the strong interactions
between Pd NPs and g-C could induce the formation of
small-size Pd NPs and then stabilise the resulted nanoparticles.
3 4
N and Pd/xTi-g-
3 4
C N
3
N
4
3 4
N
3
N
4
3
N
4
3 4
N
3
N
4
3
3
N
4
3
N
4
[
29]
Our results suggest that the suitable amount of Ti species
would benefit the formation and stabilization of small Pd NPs.
3 4 3 4
Figure 2. FT-IR spectra of (A) xTi-g-C N and (B) Pd/xTi-g- C N .
3 4 3 4 3 4
Figure 3. TEM images and the corresponding particle size distribution histograms of (A) Pd/g-C N , (B) Pd/0.5Ti-g- C N , (C) Pd/1.0Ti-g- C N and (D) Pd/1.5Ti-g-
C N .
3 4
XANES spectra of Pd K-edge for Pd/g-C
, together with PdCl and Pd foil (reference samples) are
shown in Figure 4(A). The shapes of the XANES spectra and the
3
N
4
, Pd/1.0Ti-g-
edge positions of Pd/g-C
those of PdCl and Pd foil, suggesting that Pd species are in the
mixture of metal and 2+ oxidation state. FT-EXAFS data are
3 4 3 4
N and Pd/1.0Ti-g-C N are between
C
3
N
4
2
2
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3 4 3 4
shown in Figure 4(B). For Pd/g- C N and Pd/1.0Ti-g-C N
samples, a main peak at 2.5 Å assignable to Pd−Pd bonds is
observed, together with an additional small peak at 1.5 Å
ascribed to Pd−O bond. This indicates that Pd species in Pd/g-
C
3
N
4
and Pd/1.0Ti-g-C
3 4
N are present as the mixture of metallic
2+
Pd and PdO. Even after the NaBH
4
reduction treatment, Pd
species were partially retained as a consequence of their
interaction with nitrogen-containing groups on the support
[
38]
surface.
It should be noticed that this was also observed in
[
29, 39]
the case of the Pd/g-C
3
4
N catalyst.
Figure 5. (A) Ti K-edge XANES and (B) FT-EXAFS spectra of 1.0Ti-g-C
Pd/1.0Ti-g-C and reference Ti sample (anatase TiO ).
3 4
N ,
3
N
4
2
To characterize the chemical composition and electronic
structure of g-C , Pd/g-C , 1.0Ti-g-C and Pd/1.0Ti-g-
, XPS analysis was carried out. As shown in Figure 6(A), C,
N, O, Pd and Ti elements were detected in these samples. From
Figure 6(B), the typical g-C XPS N 1s spectrum can be
3
N
4
3
N
4
3 4
N
3 4
C N
3
N
4
deconvoluted into three peaks located at 398.3, 399.0 and 400.8
2
eV, caused by the sp hybridised aromatic N atoms bonded to
Figure 4. (A) Pd K-edge XANES and (B) FT-EXAF spectra of Pd/g-C
Pd/1.0Ti-g-C N4 and reference Pd samples (Pd foil and PdCl ).
3 4
N ,
3
2
carbon atoms (C-N=C), the tertiary nitrogen (N-(C)
3
) and the
In comparison with pure
sample showed a shift of the three peaks
toward higher binding energy, revealing the strong interaction
[
30]
uncondensed amino groups (N-H).
g-C , 1.0Ti-g-C
3
N
4
3 4
N
Figure 5(A) shows Ti K-edge XANES spectra of 1.0Ti-g-
, Pd/1.0Ti-g–C and reference compounds (anatase TiO
powder). Anatase TiO reference showed well-defined pre-edge
4
-
C
3
N
4
3
N
4
2
between [TiO4] and the in-plane nitrogen in the s-triazine rings
and in the amine (-NH) group present on the g-C N surface.
2
3
4
4
-
peaks from 4960 to 4975 eV, which are due to the transitions
from the 1s core level of the Ti atom to three different kinds of
This result again confirms that [TiO4] are inserted into the s-
triazine rings of g-C
electron density of g-C
3
N
4
and a substantial modification of the
4
. After Pd NPs loading, important shifts
molecular orbitals (1t1g, 2t2g, and 3eg) of TiO
2
having octahedral
Meanwhile, in the case of 1.0Ti-g-C and
, the 1s to 3d transition of isolated titanium
atoms surrounded by four oxygen atoms originated a marked
3
N
[
40]
geometry.
Pd/1.0Ti-g–C
3 4
N
to higher binding energies were evidenced in N 1s region of
Pd/g-C compared to that of g-C , also indicating a strong
interaction between Pd NPs and g-C support. With respect to
Pd/1.0Ti-g-C sample, the peak of the tertiary nitrogen (N-
C) ) (400.2 eV) is found to appear at higher binding energy than
that in 1.0Ti-g-C (399.6 eV), implying a stronger interaction
between Pd NPs and [TiO
well as the electron structure change for Pd/1.0Ti-g-C
(C) displays the XPS Ti 2p spectra of 1.0Ti-g-C
Pd/1.0Ti-g-C samples. For 1.0Ti-g-C , two peaks at 458.8
and 464.5eV were observed in the Ti 2p spectra, assigned to Ti
3
N
4
3
N
4
3 4
N
3
N
4
[
40-42]
and single pre-edge peak at around 4670 eV.
Similar
3
N
4
results were reported by our group on single-site Ti containing
(
3
[43]
hierarchical mesoporous silica.
The Fourier transforms of the
3
N
4
EXAFS spectra are shown in Figure 5(B). The anatase TiO
powder displayed peaks ascribed to the existence of oxygen
neighbors (Ti–O) and Ti neighbors (Ti–O–Ti) at ca. 1.8 and 2.0-
2
4-
4
]
located in the s-triazine rings as
. Figure
and
3
N
4
6
3 4
N
3
.0 Å, respectively. The observation of the latter peak indicated
the presence of aggregated TiO species with octahedral
coordination. The 1.0Ti-g-C and Pd/1.0Ti-g-C showed one
single peak at ca. 1.8 Å, indicating the existence of [TiO
incorporated within g-C without the formation of aggregated
TiO . According to the results of XRD, FT-IR and Ti K-edge
EXAFS, it was conclusively indicated that titanium species are
intercalated into the tri-s-zine units of g-C in the form of
3
N
4
3 4
N
2
3
N
4
3 4
N
2
p
3/2 and Ti 2p1/2, respectively, which are in good agreement with
4-
4
]
4+ [44]
those of Ti .
3 4
With respect to Pd/1.0Ti-g- C N sample, the
3
N
4
binding energy of Ti 2p3/2 and 2p1/2 located at around 458.6 and
64.3 eV, respectively, are positively shifted by 0.2 eV. This
result might suggest the interaction between Pd NPs and Ti
species. Figure 6(D) shows the Pd 3d XPS of Pd/g-C and
Pd/1.0Ti-g-C . For Pd/g-C , the peaks at 341.3 and 336.0
eV are ascribed to Pd 3d3/2 and Pd 3d5/2, while peaks
2
4
3
N
4
4
-
3 4
N
[
4
TiO ] moieties.
3
N
4
3 4
N
0
0
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Figure 6. XPS spectra of (A) the survey spectra, (B) high resolution of N1s, (C) Ti 2p and (D) high-resolution of Pd 3d.
2
+
2+
at 343.7 and 338.1 eV are attributed to Pd 3d3/2 and Pd 3d5/2
,
0
2+
respectively, showing the co-existence of Pd and Pd species
and being thus consistent with the previously discussed EXAFS
3 4
results. The position of Pd peak in Pd/1.0Ti-g-C N shifted to
higher binding energies as compared to the peak showed by the
Ti-free material, which can be attributed to the enhanced
3 4
attraction force between Pd atoms and 1.0Ti-g-C N support.
0
2+
The contribution of Pd and Pd species to the total Pd was
determined from the relative areas calculated for their peaks. It
0
2+
was found that Pd and Pd accounted for 90 and 10 %,
3 4
respectively, in Pd/g-C and 86 and 14% in Pd/1.0Ti-g-C N ,
3
N
4
0
indicating that Pd is the main specie on the surface of the as-
prepared catalyst.
The catalytic activity of Pd/g-C
FA dehydrogenation was evaluated under ambient conditions
30 °C) in the absence of any bases. Control experiments
performed with g-C or xTi-g-C without Pd NPs did not
show H gas generation. H evolution profiles over Pd/g-C
and Pd/xTi-g- C catalysts are shown in Figure 7(A). It was
found that the activities of Pd/xTi-g-C catalysts are far better
than that of Pd/g-C catalyst. The turnover frequency (TOF,
mol Pd h ) achieved by Pd/xTi-g-C catalysts are
shown in Figure 7(B). Pd/1.0Ti-g-C exhibited the highest
activity (77.0 h ) among the samples examined, which was
almost two times higher than that of Pd/g-C . It was also
superior to those values previously reported by our group, such
3 4 3 4
N and Pd/xTi-g-C N for
(
3
N
4
3 4
N
2
2
3 4
N
3
N
4
3
N
4
3
1
N
4
−
mol H
2
3 4
N
3
N
4
-1
3
N
4
-1
as Pd/H-BETA (59.2 h , at 30 °C) with a similar Pd content and
3 4
Figure 7. (A) Comparison of catalytic performances of Pd/xTi-g-C N catalysts
[
45]
-1
[46]
for FA dehydrogenation. (B) TOF values plotted against the Ti content in
Pd/xTi-g-C
under the same reaction conditions
or Pd/C (26 h , 30 °C)
N .
3 4
and those addressed in the literature for different Pd/C-based
-1
[47]
systems, such as commercial Pd/C (50 h , 30 °C),
in situ
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-
1
[48]
synthesized Pd/C (64 h , 25°C)
with citric acid, PdAu@Au/C
-1
[49]
-1
[13]
-1
(
9
21.4 h , 92 °C),
Ag@Pd/C (49 h , 40 °C), PdAu/C (27 h ,
2 °C), CoAuPd/C (37 h , 25 °C)
[
11]
-1
[15]
-1
and CoAuPd/GO (45 h ,
[
50]
25 °C).
Additionally, CO was not detected (detection limit
about 10 ppm) in the gas mixture generated from FA catalysed
by Pd/1.0Ti-g-C , suggesting that Pd/1.0Ti-g-C catalyst
has ~100% selectivity for dehydrogenation of FA into H and
CO . In the case of Pd/mpg-C catalyst previously reported,
the existence of the basic nitrogen atoms (-NH) located at g-
provided a basic environment around Pd NPs, which
3
N
4
3 4
N
2
2
3 4
N
3 4
C N
promoted the O–H dissociation, subsequently associated with
the C–H bond cleavage from the Pd–formate intermediate to
[
28, 51]
generate H
of Pd in the Pd/1.0Ti-g-C
Pd NPs are highly dispersed on the surface of 1.0Ti-g-C
2
.
Our TEM result showed that the particle size
3
N
4
sample was around 2.9 nm and that
3
N
4
without any aggregation. Therefore, the enhanced activity
displayed by this catalyst can primarily be related to the
combined effect of improved dispersion and the presence of Pd
NPs of small size. In order to evaluate the influence of basic
Scheme 1. Illustration of possible reaction pathway of FA dehydrogenation
over Pd/Ti-g-C N .
3 4
nitrogen atoms (-NH) located at g-C
N species in the g- C , Pd/g-C
g-C were calculated from XPS results and are listed in Table
3
N
4
, atomic concentrations of
3
N
4
3
N
4
, 1.0Ti-g-C and Pd/1.0Ti-
3 4
N
4
-
3
N
4
4
suggested that the synergic effect between [TiO ] moieties and
S1 (in the Supporting Information). According to Table S1,
however, the basic nitrogen atoms (-NH) concentration in Pd/g-
Pd NPs plays a pivotal role in enhancing the catalytic activity in
FA dehydrogenation. Tsang et al. reported that the electron
density enrichment of Pd surface facilitates the rate determining
C
3
N
4
is a little higher than that of Pd/1.0Ti-g-C
that amine group in Pd/1.0Ti-g-C are not the important factor
for attaining a high catalytic activity.
3 4
N , suggesting
N
4
C–H activation of the Pd-formate intermediate to produce H
2
and
which is opposite to our results. According to result of
XPS, in the case of Pd/1.0Ti-g-C , the binding energy of Pd
3
[
13]
2
CO ,
As is known, the metallic Pd is the effective component for
the FA dehydrogenation reaction in water at room temperature.
3 4
N
4+
shifts positively, while that of Ti shifts to lower binding energy,
proving a substantial electron transfer between Pd NPs and
However, the activities of Pd/1.5Ti-g-C
than that of Pd/0.5Ti-g-C . Although they have similar Pd
loading amount, Pd particles size of Pd/1.5Ti-g-C is slightly
larger than that of Pd/0.5Ti-g-C . According to XPS results, Pd
and PdO contents in Pd/xTi-g-C were calculated and are
3 4
N catalysts are better
4
-
N
4
[TiO
4
]
moieties. It is assumed that such change in electronic
3
N
4
state influences the chemisorption energy of intermediates
adsorbed on the catalysts, eventually leading to the improved
catalytic activity. Upon consideration of our results, we deduced
3
3
N
4
3
N
4
listed in Table S2 (in the Supporting Information). It can be seen
from Table S2 that there is not distinct difference in Pd and PdO
a possible mechanism for FA decomposition over Pd/xTi-g-C
as schematically shown in Scheme 1, in which the [TiO
moieties cooperatively participate in the reaction pathway; first,
3 4
N
4
-
4
]
proportions among Pd/0.5Ti-g-C
Pd/1.5Ti-g-C samples. According to Pan’s studies, compared
to g-C nanotube, slightly Ti functionalized g-C nanotube
may enhance the storage capacity of H , which may be possible
Some previous reports also
3
N
4
,
Pd/1.0Ti-g-C
3
N
4
and
4
-
N
4
4
chemisorption of FA onto [TiO ] site and Pd NPs takes place
3
N
4
3
N
4
via the interaction between the hydroxyl H atom of HCOOH and
the O atom of Ti-O to form an intermediate formate specie (Step
1). Secondly, the formed intermediate formate species undergo
3
2
[
36]
to induce hydrogen spillover.
describe that the chemisorption of FA over titanium oxides takes
place by the hydroxyl H atom of HCOOH interacting with the O
elimination to afford CO
2
and a metal-hydride, Pd-[H] species
(Step 2). Finally, the reaction of the hydride species with H atom
[
52]
4-
atom of Ti-O to form an intermediate formate species. It may
explain that Pd/1.5Ti-g-C has a higher activity than Pd/0.5Ti-
g-C , although it has a larger Pd particle size.
To further evaluate the effect of Ti species on the activity
of FA dehydrogenation, Pd supported on anatase TiO
was prepared by a similar procedure and same Pd content. In
addition, the physical mixture of Pd/g-C and anatase TiO
power (0.20 g of Pd/g-C + 0.005 g of anatase TiO ) was also
interacting with [TiO
4
]
2
produces molecular H , accompanied
N
4
with the regeneration of the original metal species (Step 3).
3
3
N
4
2
catalyst Conclusions
3
N
4
2
FA dehydrogenation using Pd NPs supported on Ti-doped
graphitic C N4 was reported. Pd/Ti-g-C N4 with optimum Ti
3
N
4
2
3
3
subjected to the reaction under identical conditions. The catalytic
performances in the FA dehydrogenation over these catalysts
are shown in Figure S2 (in the Supporting Information). The
content having small Pd particles size showed a superior
catalytic ability for H production from FA at room temperature
2
and without additives, which far outperformed Pd/g-C N . The
3
4
mechanically mixed sample gave
displayed by Pd/g-C . For Pd/TiO
much lower than that of the Pd/1.0Ti-g-C
a
similar result to that
catalyst, the activity was
. These results
obtained excellent activity might be originated from the
interaction between Pd NPs and [TiO4] moieties incorporated in
4
-
3
N
4
2
3
N
4
g-C N4 structure. The protocol presented in this study might
3
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provide a very facile strategy to design cost-effective and highly
efficient heterogeneous catalysts for the H production from FA.
2
the China Scholarship Council and National Nature Science
Foundation of China (No. 21667029).
Keywords: Dehydrogenation
•
Formic acid
•
Hydrogen
Experimental Section
production • Nanoparticles • Graphitic carbon nitride
Materials: Palladium chloride (PdCl
2
,
>99%), sodium borohydride
>99%) and formic acid
HCOOH, >98%) were purchased from Nacalai Tesque. Urea (CH O,
99%) was purchased from Tokyo Chemical Industry Co. Ltd.
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(NaBH , >99%), dicyandiamide (C H N ,
4
(
>
4 2
N
8
Titanium(IV) oxysulfate sulfuric acid hydrate (TiOSO
was acquired from Sigma-Aldrich.
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3 4 3 4
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· xH SO ·yH O)
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8911.
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This work was financially supported by Grant-in-Aid for Scientific
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
FULL PAPER
Hydrogen production from formic acid
Yongmei Wu, Meicheng Wen, Miriam
Navlani-García, Yasutaka Kuwahara,
Kohsuke Mori, and Hiromi Yamashita*
dehydrogenation
reaction
was
successfully achieved over Pd-
supported Ti-doped graphitic carbon
nitride.
Page No. – Page No.
Palladium nanoparticles supported
on titanium doped graphitic carbon
nitride for formic acid
dehydrogenation
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