Water reduction and oxidation on Pt-Ru/Y2Ta2O 5N2 catalyst under visible light irradiation

Article abstract of DOI:10.1039/b407892f

Y₂Ta₂O₅N₂ is presented as a novel photocatalyst with high activity for water splitting under visible-light irradiation in the presence of appropriate sacrificial reagents; the activity for reduction to H₂ is increased by the incorporation of Pt or Ru as a co-catalyst, with a significant increase in production efficiency when both Pt and Ru are present.

Full text of DOI:10.1039/b407892f

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Water reduction and oxidation on Pt–Ru/Y2Ta2O5N2 catalyst under
visible light irradiation
Meiying Liu,a Wansheng You,a Zhibin Lei,a Guohua Zhou,a Jianjun Yang,a Guopeng Wu,a Guijun Ma,a
Guoyou Luan,a Tuyoshi Takata,b Michikazu Hara,b Kazunari Domen*b,c and Can Li*a
a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
Dalian, Liaoning, 116023, China. E-mail: canli@dicp.ac.cn
b Chemical Resource Laboratory, Tokyo Institute of Technology, Nagatsuta 4259, Midoni-ku, Yokohama
226-8503, Japan. E-mail: kdomen@res.titech.ac.jp
c Core Research for Evolutional Science and Technology, Japan Science and Technology Corporation
(CREST, JST), 2-1-13 Higashiueno, Taito-ku, Tokyo 110-0015, Japan
Received (in Cambridge, UK) 27th May 2004, Accepted 13th July 2004
First published as an Advance Article on the web 19th August 2004
Y2Ta2O5N2 is presented as a novel photocatalyst with high
color and had an X-ray diffraction pattern consistent with that of
pyrochlore-type Ln2Ta2O5N2.15
activity for water splitting under visible-light irradiation in the
presence of appropriate sacrificial reagents; the activity for
reduction to H2 is increased by the incorporation of Pt or Ru
as a co-catalyst, with a significant increase in production
efficiency when both Pt and Ru are present.
Energy-dispersive X-ray (EDX) spectroscopy combined with
combustion analysis and thermogravimetric oxidation of
Y2Ta2O5N2 samples gave a powder composition of Y : Ta : O :
N = 1 : 1 : 2.47 : 1.09 (atomic ratio), indicating that the
as-synthesized Y2Ta2O5N2 particles were in fact non-stoichiometric
and defective. The BET surface area of the Y2Ta2O5N2 powder
was 0.42 m2 g21, as determined from the N2 adsorption–desorption
isotherm. Scanning electron microscopy (SEM) revealed that
primary particles of about 20–100 nm in diameter had aggregated
into large secondary particles of 0.5–4 mm. The formation of such
large particles and the low surface area of the Y2Ta2O5N2 powder
are attributed to the lengthy high-temperature sintering adopted in
the synthesis process.
The photocatalytic reaction was carried out in a closed gas
circulation and evacuation system under a 300 W Xe lamp
equipped with a cut-off filter (l w 420 nm). Photocatalytic water
reduction and oxidation were performed using an aqueous solution
of 20% (v/v) ethanol and 0.01 M AgNO3. Pt and Ru were
photodeposited on the Y2Ta2O5N2 catalyst in situ from precursors
of H2PtCl6?6H2O and RuCl3?3H2O to evaluate the potential of
these elements as H2 evolution promoters. La2O3 (0.2 g) was added
to maintain the pH of the solution at 8.5. H2 and O2 evolution
was measured by on-line gas chromatography with thermal con-
ductivity detector. UV-vis diffuse reflectance spectra were recorded
using a Jasco V-550 spectrophotometer.
Photocatalytic splitting of water using solar energy is a challenging
and interesting topic of research with the potential to provide clean
and renewable H2 as an energy resource. To date, many mixed-
metal oxides such as K2La2Ti3O10,1 NaTaO3,2 and LnTaO4 (Ln =
La, Pr, Ce, Nd, Sm)3 have been studied for water splitting as a
means of producing both H2 and O2, as such oxides have excellent
stability and high activity. However, these catalysts are only active
under ultraviolet (UV) light, which accounts for only a small
fraction (5%) of the solar spectrum at the earth’s surface. Therefore,
the development of visible-light responsive photocatalysts for
overall water splitting is currently attracting much attention as a
potentially efficient utilization of solar energy.4–6
Considerable effort has been made to extend the absorption edge
of semiconductors with wide bandgaps into the visible-light
region.7,8 The incorporation of non-metal atoms (C, N, F, P and
S) into metal-oxide lattices is one way to narrow the bandgap of the
parent oxide because of their p states mixing with O 2p states.9–11 A
number of non-oxide photocatalysts have recently been reported to
be promising candidates for overall water splitting under visible-
light irradiation.6,12,13 Oxynitrides such as TaON and LaTiO2N
have been shown to exhibit absorption in the visible region and
stable photocatalytic activity for water reduction and oxidation
using appropriate sacrificial reagents. However, the range of
oxynitride photocatalysts synthesized to date remains limited.
Although platinum is already generally considered to be the best
metal promoter for H2 production from water due to its low
overpotential, it has been recently shown to exhibit remarkably
enhanced efficiency for H2 evolution under visible light when Ru
was deposited on TaON.14 This increased efficiency has been
attributed to improved contact between the TaON and Ru.
However, the simultaneous use of Pt and Ru as promoters of
photocatalytic water reduction has yet to be reported.
Figure 1 shows the UV-vis diffuse reflectance spectra of YTaO4
and Y2Ta2O5N2. YTaO4 exhibits strong absorption in the UV
region with an absorption edge at about 330 nm, corresponding to
a bandgap energy of 3.8 eV. The absorption band threshold of
Y2Ta2O5N2 occurs at 560 nm, shifted about 230 nm from that
of YTaO4, and corresponds to an estimated bandgap energy of
2.2 eV. The smaller bandgap energy of Y2Ta2O5N2 is attributed to
the partial replacement of O22 by N32 in the crystalline matrix of
This communication reports Y2Ta2O5N2 as a novel visible light-
driven photocatalyst with high activity for oxidation of water in the
presence of a sacrificial electron acceptor (Ag1). This catalyst
also reduces water to H2 in the presence of a sacrificial electron
donor (ethanol) and Pt or Ru as a co-catalyst, with a significant
enhancement of H2 production efficiency in the presence of both Pt
and Ru.
YTaO4 was synthesized by a solid-state reaction method. Y2O3
and Ta2O5 powder were mechanically mixed in stoichiometric
quantities, and then calcined at 1073 K for 2 h and 1473 K for 10 h
in air. Y2Ta2O5N2 was obtained by nitriding YTaO4 powder under
a flow of ammonia gas (flow rate: 40 ml min21) at 1123 K for 15 h
in a quartz tube. The Y2Ta2O5N2 powder was yellowish-green in
Fig. 1 UV-vis diffuse reflectance spectra of YTaO4 and Y2Ta2O5N2.
2 1 9 2
C h e m . C o m m u n . , 2 0 0 4 , 2 1 9 2 – 2 1 9 3
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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Fig. 2 Dependence of the rate of H2 evolution from an aqueous ethanol
solution on the atomic ratio of Pt to Ru on Y2Ta2O5N2 catalyst under
visible-light irradiation. Catalyst, 0.3 g; ethanol solution (20% v/v), 200 ml;
light source, 300 W Xe lamp (l w 420 nm).
Fig. 3 O2 production from an aqueous AgNO3 solution over Y2Ta2O5N2
catalyst under visible-light irradiation. Catalyst, 0.3 g; La2O3, 0.2 g; 0.01 M
AgNO3 solution, 200 ml; light source, 300 W Xe lamp (l w 420 nm).
YTaO4, where the energy level of the N2p orbital is higher than
that of the O2p orbital, and the predominant population of the
valence band of Y2Ta2O5N2 by a hybrid orbital between N2p and
O2p.
0
peak attributable to the metallic Ag deposited on the Y2Ta2O5N2,
further confirming that the photocatalyst is essentially stable.
In summary, Y2Ta2O5N2 catalyst synthesized by nitriding
YTaO4 under ammonia flow has a small optical bandgap energy
of 2.2 eV and functions as an efficient photocatalyst for water
oxidation to O2 in the presence of a sacrificial electron acceptor
(Ag1). This catalyst also exhibits activity for water reduction to H2
in the presence of a sacrificial electron donor (ethanol) and Pt or Ru
as a co-catalyst. This activity is significantly enhanced by the
presence of both Pt and Ru.
Figure 2 correlates the rate of H2 evolution over Y2Ta2O5N2
with the ratio of photodeposited Pt to Ru in the reaction solutions.
In the initial stage of the reaction, H2PtCl6 and RuCl3 were reduced
to Pt and Ru, which serve as H2-evolution promoters. It was
confirmed by X-ray photoelectron spectroscopy (XPS) that
0
H2PtCl6 and RuCl3 in the solution were deposited as metallic Pt
0
and Ru particles, with no oxidized noble metals present. The rate
of H2 evolution was examined after reaction for 1 h. Although Pt
generally functions as an excellent promoter for photocatalytic
water reduction to H2, 0.15 wt% Pt/Y2Ta2O5N2 exhibited very low
H2-evolution activity (37 mmol h21 g21). In contrast, H2 reduction
was remarkably enhanced by the addition of 0.25 wt% Ru to
Y2Ta2O5N2, reaching 170 mmol h21 g21. The activity of H2 evolu-
tion was further increased by co-depositing both Pt and Ru on
the Y2Ta2O5N2 catalyst, increasing with the amount of Pt to a
maximum of 833 mmol h21 g21 over a catalyst of Y2Ta2O5N2 with
0.15 wt% Pt and 0.25 wt% Ru. This rate is 22 times greater than that
for 0.15 wt% Pt/Y2Ta2O5N2. Deposition of more than 0.15 wt% Pt
lowered the activity from this peak.
This work was financially supported by the National Natural
Science Foundation of China (Grant Nos. 90210036, 20273070),
the National Key Basic Research and Development Program
(Grant No. 2003CB214500), the Core Research for Evolutional
Science and Technology (CREST) Program of the Japan Science
and Technology Corporation (JST), and the Innovation Program
of the Chinese Academy of Sciences (Grant No. DICP K 2002 F1).
The authors also gratefully acknowledge the China Petroleum &
Chemical Corporation (Grant No. X503019) for financial support.
Notes and references
The remarkably high activity for photocatalytic water reduction
(H2 evolution) achieved by the presence of both Pt and Ru as noble
metal co-catalysts is attributable to facile electron migration from
the conduction band of Y2Ta2O5N2 to the Pt–Ru co-catalysts,
thereby retarding the possibility of electrons recombining with
holes in the valence band and improving the charge separation
efficiency. This promoting effect is much greater when both Pt and
Ru are present as co-catalysts compared to one or the other alone.
Figure 3 shows the results for photocatalytic O2 evolution from
0.01 M AgNO3 solution over Y2Ta2O5N2. No reaction took place
in the absence of light. Upon irradiation, O2 was produced at an
initial rate of 140 mmol h21. With prolonged irradiation, the rate of
O2 evolution decreased, due to a decrease in the Ag1 concentration
1 T. Takata, Y. Furumi, K. Shinohara, A. Tanaka, M. Hara, J. N. Kondo
and K. Domen, Chem. Mater., 1997, 9, 1063.
2 H. Kato, K. Asakura and A. Kudo, J. Am. Chem. Soc., 2003, 125, 3082.
3 M. Machida, S. Murakami and T. Kijima, J. Phys. Chem., 2001, 105,
3289.
4 Z. Zou, J. Ye, K. Syama and H. Arakawa, Nature, 2001, 414, 625.
5 Z. Lei, W. You, M. Liu, G. Zhou, T. Takata, M. Hara, K. Domen and
C. Li, Chem. Commun., 2003, 2142.
6 A. Ishikawa, T. Takata, J. N. Kondo, M. Hara, H. Kobayashi and
K. Domen, J. Am. Chem. Soc., 2002, 124, 13547.
7 M. Anpo, Catal. Surv. Jpn., 1997, 1, 169.
8 D. C. Cronemeyer, Phys. Rev., 1959, 113, 1222.
9 R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science,
2001, 293, 269.
10 T. Ohno, T. Mitsui and M. Matsumura, Chem. Lett., 2003, 32, 364.
11 S. Sakthivel and H. Kisch, Angew. Chem., Int. Ed., 2003, 42, 4908.
12 G. Hitoki, T. Takata, J. N. Kondo, M. Hara, H. Kobayashi and
K. Domen, Chem. Commun., 2002, 1698.
13 A. Kasahara, K. Nukumizu, G. Hitoki, T. Takata, J. N. Kondo,
M. Hara, H. Kobayashi and K. Domen, J. Phys. Chem., 2002, 106, 6750.
14 M. Hara, J. Nunoshige, T. Takata, J. N. Kondo and K. Domen, Chem.
Commun., 2003, 3000.
15 F. Pors, R. Marchand and Y. Laurent, J. Solid State Chem., 1993,
107, 39.
0
in solution and the deposition of reduced metallic Ag on the
catalyst surface, shielding the catalyst from irradiation. During the
first 10 h of reaction, Ag1 ions in the solution (2000 mmol) were
almost entirely consumed to produce 470 mmol of O2. Only a small
amount of N2 (v 2% of the evolved O2) was detected in the early
stage of the reaction (1–3 h), demonstrating that photodegradation
of the catalyst is negligible in this reaction. No noticeable diffe-
rences were observed in the X-ray diffraction patterns of the catalyst
before and after reaction, except for the emergence of a diffraction
C h e m . C o m m u n . , 2 0 0 4 , 2 1 9 2 – 2 1 9 3
2 1 9 3