Photocatalytic Water Splitting to Hydrogen over a Visible Light-Driven LaTaON2 Catalyst

CHINESE JOURNAL OF CATALYSIS Volume 27, Issue 7, July 2006 Online English edition of the Chinese language journal Cite this article as: Chin J Catal, 2006, 27(7): 556–558.

SHORT COMMUNICATION

Photocatalytic Water Splitting to Hydrogen over a Visible Light-Driven LaTaON2 Catalyst LIU Meiying1, YOU Wansheng1, LEI Zhibin1, Tuyoshi TAKATA2, Kazunari DOMEN2,#, LI Can1,* 1

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China

2

Department of Chemical System Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

Abstract: The LaTaON2 catalyst with a perovskite structure has been successfully prepared by a high-temperature ammonolysis technique using NH4Cl as the mineralizer. The optical absorption edge of LaTaON2 is significantly red-shifted compared with that of LaTaO4. Under visible light irradiation, LaTaON2 with Pt or Ru as a co-catalyst shows high activity for reduction of water to H2 in the presence of ethanol as a sacrificial electron reagent. A remarkable improvement in production efficiency was observed when both Pt and Ru were present. Key Words: high-temperature ammonolysis; lanthanum; tantalum; oxynitride; platinum; ruthenium; visible light; photocatalysis; water splitting; hydrogen

Photocatalytic splitting of water into hydrogen and oxygen using sunlight has received considerable attention because of its potential applications in conversion of solar energy into chemical energy. Most investigations so far have focused on photocatalysts of transition metal oxides containing d0 metal ions [1–3] such as Ti4+，Zr4+, Nb5+, and Ta5+ and metal oxides such as Ga3+, In3+, Ge4+, Sn4+, and Sb5+ with d10 electronic configuration [4–6]. These photocatalysts, under UV light irradiation, show relatively high activity and excellent chemical stability for water splitting to produce hydrogen. However, the band gap energies of these oxides are too large to allow the efficient absorption of most of the photons in the solar spectrum. Although non-oxide photocatalysts such as CdS and CdSe have narrower band gaps that match the spectral distribution of the sunlight reaching the Earth surface, the main drawback of these materials is their instability under irradiation with sunlight. Hence, the development of photocatalysts with high activity under visible light (λ > 400 nm) is a very challenging but important subject of research for the efficient utilization of solar energy. In contrast to the oxide semiconductors, a range of metal oxynitrides, such as TaON, LaTiO2N, and SrTaO2N [7–9]

formed by introducing nitrogen atoms into the lattice of the parent oxides, possess narrower band gaps due to a mixing of N 2p states with O 2p states. Although these oxynitrides have aroused great interest in the water splitting reaction driven by visible light, the development of oxynitride photocatalysts with high activity remains limited. The authors have synthesized many transitional metal oxynitrides under a flow of ammonia and have investigated their photocatalytic behaviors for water reduction and oxidation under visible light irradiation from aqueous solutions containing appropriate sacrificial reagents [10]. Pt has thus been demonstrated to be an excellent co-catalyst for H2 production from water due to its low overpotential [11]. However, in the case of the oxynitride photocatalysts, the activity for water reduction to H2 under visible light irradiation can be increased by the photodeposition of Ru, which is attributed to an improved contact between Ru and oxynitride semiconductors [12]. In a previous study [13], the authors of this study have reported the synthesis of a novel pyrochlore-type Y2Ta2O5N2 catalyst with a band gap of 2.2 eV. Under visible light irradiation, Y2Ta2O5N2 shows high activity for water reduction and

Received date: 2006-03-30. * Corresponding author. Tel: +86-411-84379070; Fax: +86-411-84694447; E-mail: [email protected] # Corresponding author. Tel: +81-3-5841-1148; Fax: +81-3-5841-8838; E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (20403018, 20503034), the National Key Basic Research and Development Program (2003CB214500), and the Solution-Oriented Research for Science and Technology (SORST) Program of Japan Science and Technology Corporation (JST). Copyright © 2006, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.

LIU Meiying et al. / Chinese Journal of Catalysis, 2006, 27(7): 556–558

oxidation in aqueous solutions containing suitable sacrificial reagents, with the potential for overall water splitting into H2 and O2. In this study, a perovskite-type LaTaON2 using a high-temperature ammonolysis technique with the aid of a halide mineralizer has been successfully synthesized, and its photocatalytic properties for water decomposition under visible light (λ > 420 nm) in the presence of various noble metals as co-catalysts have been reported. A significant enhancement in H2 evolution was found when both Pt and Ru were present. The starting oxide LaTaO4 was synthesized using a conventional solid-state route by calcining a stoichiometric mixture of La2O3 and Ta2O5 powders at 800oC for 2 h and then at 1200oC for 10 h in air. The LaTaON2 catalyst was prepared by nitriding LaTaO4 under a flow (200 ml/min) of NH3 gas at 900oC for 24 h with the aid of a mineralizer NH4Cl in a quartz tube. The resulting LaTaON2 catalyst sample was bright red in color. X-ray powder diffraction (XRD) patterns of LaTaO4 and LaTaON2 are shown in Fig. 1. The patterns of both LaTaO4 and LaTaON2 are consistent with those reported in the literature [14,15]. The BET surface area of LaTaON2 powder was estimated to be 1.43 m2/g based on the N2 adsorption. The scanning electron microscopy (SEM) image (not shown) revealed that the LaTaON2 sample was irregular in shape and was estimated to be 1–5 µm in diameter. The formation of such large particles and the low surface area of LaTaON2 powder are attributed to the sintering at high temperature during the synthesis.

Fig. 1 XRD patterns of LaTaO4 (1) and LaTaON2 (2)

Fig. 2 shows the UV–visible diffuse reflectance spectra of LaTaO4 and LaTaON2. LaTaON2 exhibits strong absorption in the visible region, and the absorption edge of LaTaON2 is approximately at 600 nm, with a significant red shift by about 270 nm compared to that of LaTaO4. The band gap, which is estimated from the onset of the absorption edge, is decreased from 3.9 eV for LaTaO4 to 2.1 eV for LaTaON2. The signifi-

cant narrowing of the band gap is associated with the substitution of O2− with N3− in the host matrix of LaTaO4, in which the N 2p orbital is at a higher potential level than the O 2p orbital, and the predominant component of the valence band of LaTaON2 is a mixing state of N 2p with O 2p.

Fig. 2 UV-visible diffuse reflectance spectra of LaTaO4 (1) and LaTaON2 (2)

The photocatalytic reaction was conducted in a top-irradiation-type Pyrex glass cell connected to a closed gas circulation and evacuation system under a 300 W Xe lamp equipped with a cut-off filter (λ > 420 nm), together with a water jacket to remove infrared irradiation. The photocatalyst powder (0.3 g) was dispersed by magnetic stirring in a 20% (v/v) aqueous ethanol solution (200 ml) for H2 evolution. Pt and/or Ru were photodeposited on the LaTaON2 catalyst in situ using H2PtCl6·6H2O, (NH4)2RuCl6, and RuCl3·3H2O as precursors, and were examined as potential H2 evolution promoters. The evolved gas was analyzed online by gas chromatography using a thermal conductivity detector, molecular sieve 5A column, and Ar carrier. In the case of the LaTaON2 catalyst, no O2 evolution was detected in the aqueous solution even in the presence of AgNO3 as a sacrificial electron acceptor. Table 1 shows the dependence of the initial rate of H2 evolution on LaTaON2 modified by Pt or/and Ru. The rate of H2 evolution was examined after reaction for 1 h. As shown in Table 1, no H2 evolution was detected in the absence of the loaded metals even after extended irradiation, indicating that the activity of the LaTaON2 catalyst for water decomposition is extremely low in the absence of a noble metal co-catalyst. However, H2 is produced after loading a noble metal Pt or Ru as a co-catalyst onto the surface of LaTaON2. This phenomenon is related to the decrease in the overpotential for water decomposition. Although the tested noble metals (Pt and Ru) can act as promoters for water photoreduction to H2, the resulting activity for the formation of H2 (3–8 µmol/h) is poor when modified by Pt or Ru alone. It is noteworthy that the activity for H2 evolution is substan-

LIU Meiying et al. / Chinese Journal of Catalysis, 2006, 27(7): 556–558

tially enhanced by the simultaneous deposition of Pt and Ru. The optimum activity for H2 evolution was obtained with the deposition of 0.15% Pt–0.25% Ru, with the H2 evolution rates of 20 and 38 µmol/h using (NH4)2RuCl6 and RuCl3·3H2O as the Ru precursors, respectively.

References [1] Domen K, Kudo A, Onishi T. J Catal, 1986, 102(1): 92 [2] Kudo A, Tanaka A, Domen K, Maruya K, Aika K, Onishi T. J Catal, 1988, 111(1): 67 [3] Kato H, Asakura K, Kudo A. J Am Chem Soc, 2003, 125(10):

Table 1 Dependence of the rate of H2 evolution from an aqueous ethanol solution on Pt and/or Ru co-catalysts over LaTaON2 under visible light irradiation Co-catalyst

Precursor

Rate of H2 evolution (µmol/h)

–

–

0.15% Pt

H2PtCl6·6H2O

0 3

0.25% Ru

(NH4)2RuCl6

8

0.15% Pt–0.25% Ru

H2PtCl6·6H2O and

20

H2PtCl6·6H2O and

[4] Sato J, Saito S, Nishiyama H, Inoue Y. J Phys Chem B, 2001, 105(26): 6061 [5] Ikarashi K, Sato J, Kobayashi H, Saito N, Nishiyama H, Inoue Y. J Phys Chem B, 2002, 106(35): 9048 [6] Sato J, Kobayashi H, Ikarashi K, Saito N, Nishiyama H, Inoue Y. J Phys Chem B, 2004, 108(14): 4369 [7] Hitoki G, Takata T, Kondo J N, Hara M, Kobayashi H, Domen K. Chem Commun, 2002, (16): 1698 [8] Kasahara A, Nukumizu K, Hitoki G, Takata T, Kondo J N, Hara

(NH4)2RuCl6 0.15% Pt–0.25% Ru

3082

38

RuCl3·3H2O Reaction conditions: catalyst, 0.3 g; 20% (v/v) ethanol aqueous solution, 200 ml; 300-W Xenon lamp (λ > 420 nm).

M, Kobayashi H, Domen K. J Phys Chem A, 2002, 106(29): 6750 [9] Yamasita D, Takata T, Hara M, Kondo J N, Domen K. Solid State Ionics, 2004, 172(1–4): 591 [10] Liu M Y. [PhD Dissertation]. Dalian, China: Dalian Institute of

The synergistic enhancement effect of Pt and Ru on the hydrogen evolution versus that of Pt or Ru alone is possibly related to a more facilitated electron transfer from the conduction band of the LaTaON2 catalyst to the Pt–Ru co-catalysts compared to Pt or Ru alone. In conclusion, LaTaON2 has been successfully synthesized by the mineralizer-assisted high-temperature ammonolysis technique and has been found to be active for photocatalytic water reduction to produce H2 under visible light irradiation when a noble metal is deposited on it. The activity of Pt/Ru-loaded LaTaON2 for H2 evolution is much superior to that of Pt-LaTaON2 and Ru-LaTaON2.

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