Single Pd(0) atom encapsulated in multiporphyrin arrays as a highly efficient heterogeneous catalyst

Article abstract of DOI:10.1021/jp034029b

The well-defined 'single' Pd atoms' encapsulation in Pd-mediated multiporphyrin arrays was discussed. The encapsulated Pd atoms showed much higher catalytic activity for photoinduced hydrogen evolution in comparison to Pd nanoparticles prepared from dilute K₂PdCl₄ solution. It was found that as the encapsulated Pd atoms were immobilized on the glass surface, the catalyst separation was much easier.

Full text of DOI:10.1021/jp034029b

©
Copyright 2003 by the American Chemical Society
VOLUME 107, NUMBER 15, APRIL 17, 2003
LETTERS
“
Single” Pd(0) Atom Encapsulated in Multiporphyrin Arrays as a Highly Efficient
Heterogeneous Catalyst
,†
Dong-Jin Qian,* Tatsuki Wakayama, Chikashi Nakamura, and Jun Miyake
Tissue Engineering Research Center, National Institute of AdVanced Industrial Science and Technology,
Higashi 1-1-1, Tsukuba, Ibaraki 305-8562, Japan
ReceiVed: January 7, 2003
Well-defined “single” Pd(0) atoms were encapsulated in Pd-mediated multiporphyrin arrays and used as a
catalyst. Compared with the Pd(0) nanoparticles prepared from dilute K
2 4
PdCl solution, the encapsulated
Pd(0) atoms showed much higher catalytic activity for photoinduced hydrogen evolution.
Palladium-catalyzed reactions such as hydrogen production,
hydrogenation, and the Heck reaction have attracted much
attention because of their importance to clear energy develop-
ment and organic synthesis applications.1,2 Although many
Pd(0)-based catalysts are well established, catalyst separation
and enhancement of catalytic activity are still large challenges
and continue to be the focus of intense research. Over the last
three decades, several elegant approaches have been explored
to overcome the limitation of catalyst separation, for example,
the multiporphyrin arrays, especially those directly assembled
on the solid supports, showed a well-defined chemical structure
and high thermal and chemical stability and were expected to
a possibility for practical applications. In the present work, the
2+
linkage, the Pd ion, in the multiporphyrin arrays was reduced
by H2 in water, which results in well-separated Pd(0) atoms in
the organized ultrathin films (Scheme 1). The porphyrin that
was used for the assembly was zinc-5,10,15,20-tetra(4-pyridyl)-
21H,23H-porphine (ZnTPyP). Experimental details on the
3
8d
aqueous and fluorous biphase catalysis, reactions in supercritical
preparation of multilayers can be found in our previous paper.
media,4 and catalyst immobilization onto solid supports.
However, for biphase catalysis including immobilized Pd(0)
particles, particle size often plays an important role in catalytic
activity since hydrogen or organic molecules interacting with
5
The encapsulated Pd(0) atoms were used as a catalyst for
photoinduced hydrogen evolution with zinc-5,10,15,20-tetra(4-
methylpyridyl)-21H,23H-porphine tetrakis(methochloride) (ZnT-
MPyP) as a photosensitizer, ethylenediamine-N,N,N′,N′-tet-
raacetic acid, disodium salt (EDTA) as an electron donor, and
6
small palladium clusters is a strong function of cluster size.
2
+
9
To achieve a highly active palladium catalyst, recently many
palladium nanoparticles have been prepared and used as an
methyl viologen (MV ) as an electron carrier. To evaluate
the catalytic activity of the present Pd(0) atoms, we compared
the hydrogen evolution rate with the use of Pd(0) nanoparticles
under the same experimental conditions. These Pd(0) nanopar-
ticles were prepared by reducing 0.02 mM and 0.2 mM K2-
7
efficient catalyst.
We present here a novel methodology to prepare an encap-
sulated Pd(0) single atom in multiporphyrin arrays to be used
in catalyzing hydrogen production. We have previously reported
the preparation of Pd-mediated multiporphyrin arrays at the air-
1
0
PdCl4 aqueous solutions with H2 gas. Figure 1 shows two
photographs obtained from field emission scan electron mi-
croscopy (FESEM) that indicate that the average particle sizes
are about 20-30 and 40-50 nm, respectively.
8
water interface and directly on solid supports. Multilayers of
*
Corresponding author. E-mail: dongjin-sen@aist.go.jp. Tel: 81-298-
For the H2 evolution measurement with encapsulated Pd(0)
atoms as a catalyst, the glass plates with one to four layers of
Pd-ZnTPyP multiporphyrin arrays were assembled and reduced
6
1-3013. Fax: 81-298-61-2692.
†
Present address: Institute of Advanced Materials and Technology,
Fudan University, China. E-mail: djqian@fudan.edu.cn.
1
0.1021/jp034029b CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/22/2003

3334 J. Phys. Chem. B, Vol. 107, No. 15, 2003
Letters
SCHEME 1: Schematic Illustration of Encapsulated Pd(0) Atoms in the Multiporphyrin Arrays
by H2 gas in water. A hydrogen electrode was used to detect
the hydrogen concentration produced. As an example, Figure 2
shows a plot of hydrogen concentration versus illumination time
with a piece of Pd(0)-encapsulated multiporphyrin layer-coated
glass as a catalyst. An almost linear increase in the hydrogen
evolution was recorded under illumination. The hydrogen
evolution rates were calculated according to the hydrogen
concentration 20 min after illumination, and the obtained data
are summarized in Tables 1 to 3.
To obtain the hydrogen evolution rate per nanogram of Pd(0),
the surface coverage of Pd(0) was calculated according to the
TABLE 1: Hydrogen Evolution Rate with the Encapsulated
Pd(0) Atoms as a Catalyst
quantity of Pd(0)
atoms on glass
H2 evolution rate
layer
H2 evolution rate
per nanogram of Pd(0)
numbers surface (ng)a (nmol mL min ) (pmol mL min ng
-1
-1
-1
-1
-1
)
1
2
3
4
18.4
1.97
5.18
6.04
4.01
9.00
12.2
4.70
9.25
20.4
7.40
17.3
32.7
110
100
91
90
102
75
74
64
84
98
4
6
9.8
6.7
44.4
8.0
8
1
63
63.8
45
44
75.1
75
1
2
1
3
99
88
72
a
Estimated from the glass area used and the surface density of Pd
in the multiporphyrin arrays.
TABLE 2: Hydrogen Evolution Rate with Pd(0)
Nanoparticles Prepared from 0.02 mM K
2
PdCl
4
as a
Figure 1. FESEM images of Pd(0) nanoparticles prepared from (a)
Catalyst
0
2 4
.02 mM and (b) 0.2 mM K PdCl solution.
H
2
evolution rate per
quantity of
Pd(0) (ng)
H
2
evolution rate
nanogram of Pd(0)
-
1
-1
(pmol mL min ng-1)
-1
-1
(nmol mL min
)
6
95
50
5
1.30
1.42
2.34
3.89
20
7.3
3.6
3
1
6
1
300
TABLE 3: Hydrogen Evolution Rate with Pd(0)
Nanoparticles Prepared from 0.2 mM K PdCl as a Catalyst
2
4
2
H evolution rate per
quantity of
Pd(0) (ng)
H
2
evolution rate
nanogram of Pd(0)
-
1
-1
(pmol mL min ng-1)
-1
-1
(nmol mL min
)
Figure 2. Plot of hydrogen concentration vs illumination time. The
650
1950
6500
2.16
2.90
4.93
9.25
3.3
1.5
0.76
0.71
reaction was preformed in a 10 mM tris-HCl buffer (pH 7.4) containing
2
+
0
.2 mM ZnTMPyP, 15 mM EDTA, 1 mM MV , and a piece of glass
coated with encapsulated Pd(0) atoms.
13000

Letters
J. Phys. Chem. B, Vol. 107, No. 15, 2003 3335
surface coverage of ZnTPyP in the multiporphyrin monolayer,
Acknowledgment. This work was supported by NEDO’s
International Joint Research Grant Program. D.-J.Q. acknowl-
edges NEDO’s ITTF program.
-
10
2,8d
2.4 × 10
mol/cm , and the composition of Pd-ZnTPyP
multiporphyrin arrays (molar ratio of Pd to ZnTPyP, 2:1). The
data are summarized in Table 1. The hydrogen evolution rate
per nanogram of Pd(0) nanoparticles was also calculated, and
the data are listed in Tables 2 and 3.
References and Notes
A comparison of the data in Table 1 with those in Tables 2
and 3 reveals the following important features. First, the
encapsulated Pd(0) atoms show that the average hydrogen
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