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Photocatalytic activities of TiO2 loaded with NiO
Volume 133, number 6
CHEMICAL PHYSICS LETTERS
6 February 1987
PHOTOCATALYTIC ACTIVITIES OF TiOa LOADED WITH NiO Akihiko KUDO, Kazunari DOMEN, Ken-ichi MARUYA and Takaharu ONISHI ResearchLaboratoryof Resources Utibzation,TokyoInstituteof Technology, 4259 Nagatsuta.Midori-ku, Yokohama227, Japan Received 18 June 1986; in final form 5 November 1986
A pretreated NiO-Ti02 powder system is an active catalyst for photocatalytic decomposition of Hz0 into H2 and O2 in aqueous alkaline solution (3 N NaOH) as well as under NaOH coating conditions.
1. Introduction Although many studies on photocatalytic reactions over semiconductor powders have been reported, very few systems are recognized to accomplish sustained energy conversion reactions with reasonable reaction rates. Among them, the photo -catalytic decomposition of HZ0 into H2 and O2 is one of the systems examined most extensively. SrTiO, powder systems loaded with transition metal oxides have been confirmed to perform catalytic reactions in liquid and gaseous water [ l-31. Recently the present authors have reported another system, NiO-K$lbsO r,, which is also active in the stable photocatalytic decomposition of Hz0 [ 41. On the other hand, TiOz is one of the best known photocatalysts, especially since the works of Honda and Fujishima [ $61 and Bard et al. [ 7,8]. It has been well established that photoelectrochemical decomposition of HZ0 proceeds on a TiOz photoanode with a small applied voltage. However, in the case of a TiOz powder system, there seems to be some controversy on the capability of decomposition of HZ0 into H, and OZ. Recent results of several workers revealed that Pt-TiO, powder cannot decompose HZ0 in a stoichiometric amount, but evolves only H2 into the gas phase in aqueous solution [9-l 11. Furthermore, the activity of Hz evolution decreased rapidly with irradiation time. The lack of O2 evolution was explained by the formation of peroxide adsorbed on the catalyst surface [ 121. It should be noted that
0 009-2614/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Pt-TiO, coated with NaOH, as has been reported by Sato et al. [ 131, can decompose water vapor into H2 and O2 with a high efficiency, although the role of NaOH has not been well understood. In this paper the activities of TiO,? catalyst loaded with NiO for photodecomposition of HZ0 in aqueous NaOH solution and under NaOH coating conditions and H2 evolution from aqueous methanol solution were examined.
2. Experimental Ti02 (Anatase) was purchased from MCB. TiOz loaded with NiO (1.5 wt%) was prepared by the impregnation method from aqueous Ni(N03)2 solution and calcined at 573 K in air. The TiOz and NiO( 1.5 wt%)-TiO, catalysts were reduced at 773 K by H2 ( z 40 kPa) for 2 h and then reoxidized at 473 K by O2 ( x 16 kPa) for 1 h as pretreatments; hereafter they are referred to as R773-0473 TiOz and R773-0473 NiO( 1.5 wt%)-TiOZ, respectively. These pretreatments were carried out in a closed gas circulation system with a vacuum line in a manner similar to the pretreatments for NiO-SrTi03 [2] and NiO-K_,Nb601, [ 41. The Hz evolution reaction from aqueous methanol solution ( H20:CH,0H = 30: 1 by volume, 300 ml) and photodecomposition of distilled water and aqueous NaOH solution (300 ml) over TiOz and NiO( 1.5 wtO/o)-TiOz (1 g) were carried out in an inner irradiation reaction cell using a 517
Volume 133, number 6
6 February 1987
CHEMICAL PHYSICS LETTERS
high-pressure mercury lamp (USHIO, UM-452,450 W). The photodecomposition of water on TiOz and NiO( 1.5 wtO/b)-TiO, coated with NaOH (12 wt”h) was studied in a flat bottom reaction cell made of quartz using the same lamp. NaOH was coated on TiOz and NiO( 1.5 wt%)-TiOz following the method described by Sato et al. [ 131. The amounts of evolved Hz and O2 were measured using gas chromatography (Ar carrier, MS-5A column).
3. Results and discussion The H2 evolution reactions from aqueous methanol solution on TiOz and NiO( 1.5 wt%)-TiO, are shown in fig. 1. The R773-0473 NiO( 1.5 wtOh)-Ti02 showed a much higher activity than non-treated TiOz, R773-0473 TiOz and non-treated NiO( 1.5 wt%)-Ti02. The present authors reported in a previous paper [ 141 on the SrTi03 photocatalyst sys-
tern that Ni metal exists at the interface of NiO and SrTi03 after R773-0473 pretreatment, and the surface of NiO was covered with hydroxyl groups during the reaction. The same structure is expected for R773-0473 NiO(1.5 wtOh)-TiO* catalyst. In distilled water, however, a small amount of HZ evolution (1.1 nmol/h), but no O2 evolution, was observed on R773-0473 NiO(1.5 wtO/o)-TiO,. This behavior is different from that of NiO-SrTi03 [ 1,2] and NiO-1(4Nb601, [ 41. The activities of nontreated Ti02, R773-0473 TiOl and non-treated NiO(1.5 wt%)-TiOz were negligible. When R773 -0473 NiO( 1.5 wt%)-TiO, was irradiated in aqueous NaOH solution (3 N), the rate of Hz evolution increased by almost five times as shown in fig. 2. Furthermore, O2 evolution was also observed after an induction period of 2-3 h, although the evolved H2 and O2 were not in the stoichiometric ratio even after 20 h. Some amount of oxidized species might be adsorbed on the TiOz surface as a peroxide and/or a superoxide. In any event, the observation of stationary evolution of O2 from aqueous solution should be emphasized while no O2 evolution on Pt-TiOz was observed [9-l 11. When R773-0473 NiO(1.5 wt%)-TiO, was coated with NaOH(12 wt%), the photocatalytic
t/h Fig. I. Effects of pretreatment and supporting of NiO on Ti02 photocatalyst for H2 evolution from aqueous methanol solution (H20:CHjOH=30:l in volume). 0: pretreated NiO(l.5 wt%)-Ti02; A: non-treated NiO( 1.5 wt%)-TiO,; 0: pretreated TiOl; o: non-treated TiO?. Catalyst: 1 g; solution: 300 ml; cell: inner irradiation reaction cell; light source: high-pressure mercury lamp (450 W).
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t/h Fig. 2. Photodecomposition of aqueous NaOH solution (3 N) over pretreated NiO(l.5 wt%)-TiO,. 0: Hz; 0: Oz. Catalyst: 1 g; solution: 300 ml; cell: inner irradiation reaction cell; light source: high-pressure mercury lamp (450 W)
Volume 133, number 6
CHEMICAL PHYSICS LETTERS
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(1) The NiO-Ti02 catalyst can decompose Hz0 into Hz and O2 in aqueous NaOH solution ( 3 N) but cannot do so in distilled water. The latter fact is markedly different from the cases of pretreated NiO-SrTiOs and NiO-IL,Nb60L ,. (2) One of the roles of NiO is to act as a site of Hz evolution and, furthermore, NiO may assist the evolution of O2 into the gas phase. ( 3 ) The role of NaOH coating is not merely to prevent the reverse reaction ( 2H2 + O2-, 2H20), because the reverse reaction is negligible on NiO-TiOz. Thus, the coated NaOH seems to make the surface condition more favorable for decomposition of H20. However, the reason for the large difference in activity in aqueous NaOH solution (3 N) from that in NaOH coating condition is still not clear.
References
t/h Fig. 3. Photocatalytic decomposition of water on TiO, and NiO(1.5 wt%)-Ti02 coated with NaOH(12wt%). 0: Hz; 0: 0, over pretreated NiO(1.5 wt%)-TiO,, A: H2 over non-treated NiO(1.5 wt%)-Ti02; 0: H2 over pretreated TiOl: 0: H2 over non-treated Ti4. Catalyst: 1 g; cell: flat bottom reaction cell; light source: high-pressure mercury lamp (450 W).
decomposition of Hz0 proceeded as shown in fig. 3. Both Hz (32 pmoYh) and O2 (14 umol/h) were evolved in almost the stoichiometric ratio, although the activity depended strongly on the condition of the catalyst surface, i.e. the wetness of coated NaOH. A similar effect was studied in detail on Pt-TiOz by Sato et al. [ 131. The NaOH (12 wt%) -coated nontreated NiO(1.5 wt%)-TiO, showed a low activity of Hz evolution (2.3 PmoVh), as shown in fig. 3, and a small amount of O2 evolution (0.2 umol/h) was also observed. The NaOH(12 wtOh)-coated R7730473 Ti02 and non-treated TiOz showed very low activities. From these results, the following conclusions are derived:
[ 1] K. Domen, S. Naito, M. Soma, T. Onishi and T. Tamaru, J. Chem. Sot. Chem. Commun. (1980) 543. [2] K. Domen, S. Naito, T. Onishi, K. Tamaru and M. Soma, J. Phys. Chem. 86 (1982) 3657. [ 31 J.-M. Lehn, J.-P. Sauvage, R. Ziessel and L. Hilaire, Israel J. Chem. 22 (1982) 168. [4] K. Domen, A. Kudo, A. Shinozaki, A. Tanaka, K. Maruya and T. Onishi, J. Chem. Sot. Chem. Commun. (1986) 356. [5] A. Fujishima and K. Honda, Bull. Chem. Sot. Japan 44 (1971) 1148. [6] A. Fujishima and K. Honda, Nature 238 (1972) 37. [7] B. Kraeutler and A.J. Bard, J. Am. Chem. Sot. 100 (1978) 2239. [S] B. Kraeutler and A.J. Bard, J. Am. Chem. Sot. 100 (1978) 5985. (91 A. Mills and G. Porter, J. Chem. Sot. Faraday Trans. I 78 (1982) 3659. [ lo] J. Kiwi and M. Griltzel, J. Phys. Chem. 88 (1984) 1302. [ 111 J. Kiwi and C. Morrison, J. Phys. Chem. 88 ( 1984) 6 146. [ 121 B. Gu, J. Kiwi and M. Grgtzel, Nouv. J. Chim. 9 (1985) 539. [ 131 K. Yamaguchi and S. Sato, J. Chem. Sot. Faraday Trans. I 81 (1985) 1237. [ 141 K. Domen, A. Kudo, T. Onishi, N. Kosugi and H. Kuroda, J. Phys. Chem. 90 (1986) 292.
519
CHEMICAL PHYSICS LETTERS
6 February 1987
PHOTOCATALYTIC ACTIVITIES OF TiOa LOADED WITH NiO Akihiko KUDO, Kazunari DOMEN, Ken-ichi MARUYA and Takaharu ONISHI ResearchLaboratoryof Resources Utibzation,TokyoInstituteof Technology, 4259 Nagatsuta.Midori-ku, Yokohama227, Japan Received 18 June 1986; in final form 5 November 1986
A pretreated NiO-Ti02 powder system is an active catalyst for photocatalytic decomposition of Hz0 into H2 and O2 in aqueous alkaline solution (3 N NaOH) as well as under NaOH coating conditions.
1. Introduction Although many studies on photocatalytic reactions over semiconductor powders have been reported, very few systems are recognized to accomplish sustained energy conversion reactions with reasonable reaction rates. Among them, the photo -catalytic decomposition of HZ0 into H2 and O2 is one of the systems examined most extensively. SrTiO, powder systems loaded with transition metal oxides have been confirmed to perform catalytic reactions in liquid and gaseous water [ l-31. Recently the present authors have reported another system, NiO-K$lbsO r,, which is also active in the stable photocatalytic decomposition of Hz0 [ 41. On the other hand, TiOz is one of the best known photocatalysts, especially since the works of Honda and Fujishima [ $61 and Bard et al. [ 7,8]. It has been well established that photoelectrochemical decomposition of HZ0 proceeds on a TiOz photoanode with a small applied voltage. However, in the case of a TiOz powder system, there seems to be some controversy on the capability of decomposition of HZ0 into H, and OZ. Recent results of several workers revealed that Pt-TiO, powder cannot decompose HZ0 in a stoichiometric amount, but evolves only H2 into the gas phase in aqueous solution [9-l 11. Furthermore, the activity of Hz evolution decreased rapidly with irradiation time. The lack of O2 evolution was explained by the formation of peroxide adsorbed on the catalyst surface [ 121. It should be noted that
0 009-2614/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Pt-TiO, coated with NaOH, as has been reported by Sato et al. [ 131, can decompose water vapor into H2 and O2 with a high efficiency, although the role of NaOH has not been well understood. In this paper the activities of TiO,? catalyst loaded with NiO for photodecomposition of HZ0 in aqueous NaOH solution and under NaOH coating conditions and H2 evolution from aqueous methanol solution were examined.
2. Experimental Ti02 (Anatase) was purchased from MCB. TiOz loaded with NiO (1.5 wt%) was prepared by the impregnation method from aqueous Ni(N03)2 solution and calcined at 573 K in air. The TiOz and NiO( 1.5 wt%)-TiO, catalysts were reduced at 773 K by H2 ( z 40 kPa) for 2 h and then reoxidized at 473 K by O2 ( x 16 kPa) for 1 h as pretreatments; hereafter they are referred to as R773-0473 TiOz and R773-0473 NiO( 1.5 wt%)-TiOZ, respectively. These pretreatments were carried out in a closed gas circulation system with a vacuum line in a manner similar to the pretreatments for NiO-SrTi03 [2] and NiO-K_,Nb601, [ 41. The Hz evolution reaction from aqueous methanol solution ( H20:CH,0H = 30: 1 by volume, 300 ml) and photodecomposition of distilled water and aqueous NaOH solution (300 ml) over TiOz and NiO( 1.5 wtO/o)-TiOz (1 g) were carried out in an inner irradiation reaction cell using a 517
Volume 133, number 6
6 February 1987
CHEMICAL PHYSICS LETTERS
high-pressure mercury lamp (USHIO, UM-452,450 W). The photodecomposition of water on TiOz and NiO( 1.5 wtO/b)-TiO, coated with NaOH (12 wt”h) was studied in a flat bottom reaction cell made of quartz using the same lamp. NaOH was coated on TiOz and NiO( 1.5 wt%)-TiOz following the method described by Sato et al. [ 131. The amounts of evolved Hz and O2 were measured using gas chromatography (Ar carrier, MS-5A column).
3. Results and discussion The H2 evolution reactions from aqueous methanol solution on TiOz and NiO( 1.5 wt%)-TiO, are shown in fig. 1. The R773-0473 NiO( 1.5 wtOh)-Ti02 showed a much higher activity than non-treated TiOz, R773-0473 TiOz and non-treated NiO( 1.5 wt%)-Ti02. The present authors reported in a previous paper [ 141 on the SrTi03 photocatalyst sys-
tern that Ni metal exists at the interface of NiO and SrTi03 after R773-0473 pretreatment, and the surface of NiO was covered with hydroxyl groups during the reaction. The same structure is expected for R773-0473 NiO(1.5 wtOh)-TiO* catalyst. In distilled water, however, a small amount of HZ evolution (1.1 nmol/h), but no O2 evolution, was observed on R773-0473 NiO(1.5 wtO/o)-TiO,. This behavior is different from that of NiO-SrTi03 [ 1,2] and NiO-1(4Nb601, [ 41. The activities of nontreated Ti02, R773-0473 TiOl and non-treated NiO(1.5 wt%)-TiOz were negligible. When R773 -0473 NiO( 1.5 wt%)-TiO, was irradiated in aqueous NaOH solution (3 N), the rate of Hz evolution increased by almost five times as shown in fig. 2. Furthermore, O2 evolution was also observed after an induction period of 2-3 h, although the evolved H2 and O2 were not in the stoichiometric ratio even after 20 h. Some amount of oxidized species might be adsorbed on the TiOz surface as a peroxide and/or a superoxide. In any event, the observation of stationary evolution of O2 from aqueous solution should be emphasized while no O2 evolution on Pt-TiOz was observed [9-l 11. When R773-0473 NiO(1.5 wt%)-TiO, was coated with NaOH(12 wt%), the photocatalytic
t/h Fig. I. Effects of pretreatment and supporting of NiO on Ti02 photocatalyst for H2 evolution from aqueous methanol solution (H20:CHjOH=30:l in volume). 0: pretreated NiO(l.5 wt%)-Ti02; A: non-treated NiO( 1.5 wt%)-TiO,; 0: pretreated TiOl; o: non-treated TiO?. Catalyst: 1 g; solution: 300 ml; cell: inner irradiation reaction cell; light source: high-pressure mercury lamp (450 W).
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t/h Fig. 2. Photodecomposition of aqueous NaOH solution (3 N) over pretreated NiO(l.5 wt%)-TiO,. 0: Hz; 0: Oz. Catalyst: 1 g; solution: 300 ml; cell: inner irradiation reaction cell; light source: high-pressure mercury lamp (450 W)
Volume 133, number 6
CHEMICAL PHYSICS LETTERS
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6 February 1987
(1) The NiO-Ti02 catalyst can decompose Hz0 into Hz and O2 in aqueous NaOH solution ( 3 N) but cannot do so in distilled water. The latter fact is markedly different from the cases of pretreated NiO-SrTiOs and NiO-IL,Nb60L ,. (2) One of the roles of NiO is to act as a site of Hz evolution and, furthermore, NiO may assist the evolution of O2 into the gas phase. ( 3 ) The role of NaOH coating is not merely to prevent the reverse reaction ( 2H2 + O2-, 2H20), because the reverse reaction is negligible on NiO-TiOz. Thus, the coated NaOH seems to make the surface condition more favorable for decomposition of H20. However, the reason for the large difference in activity in aqueous NaOH solution (3 N) from that in NaOH coating condition is still not clear.
References
t/h Fig. 3. Photocatalytic decomposition of water on TiO, and NiO(1.5 wt%)-Ti02 coated with NaOH(12wt%). 0: Hz; 0: 0, over pretreated NiO(1.5 wt%)-TiO,, A: H2 over non-treated NiO(1.5 wt%)-Ti02; 0: H2 over pretreated TiOl: 0: H2 over non-treated Ti4. Catalyst: 1 g; cell: flat bottom reaction cell; light source: high-pressure mercury lamp (450 W).
decomposition of Hz0 proceeded as shown in fig. 3. Both Hz (32 pmoYh) and O2 (14 umol/h) were evolved in almost the stoichiometric ratio, although the activity depended strongly on the condition of the catalyst surface, i.e. the wetness of coated NaOH. A similar effect was studied in detail on Pt-TiOz by Sato et al. [ 131. The NaOH (12 wt%) -coated nontreated NiO(1.5 wt%)-TiO, showed a low activity of Hz evolution (2.3 PmoVh), as shown in fig. 3, and a small amount of O2 evolution (0.2 umol/h) was also observed. The NaOH(12 wtOh)-coated R7730473 Ti02 and non-treated TiOz showed very low activities. From these results, the following conclusions are derived:
[ 1] K. Domen, S. Naito, M. Soma, T. Onishi and T. Tamaru, J. Chem. Sot. Chem. Commun. (1980) 543. [2] K. Domen, S. Naito, T. Onishi, K. Tamaru and M. Soma, J. Phys. Chem. 86 (1982) 3657. [ 31 J.-M. Lehn, J.-P. Sauvage, R. Ziessel and L. Hilaire, Israel J. Chem. 22 (1982) 168. [4] K. Domen, A. Kudo, A. Shinozaki, A. Tanaka, K. Maruya and T. Onishi, J. Chem. Sot. Chem. Commun. (1986) 356. [5] A. Fujishima and K. Honda, Bull. Chem. Sot. Japan 44 (1971) 1148. [6] A. Fujishima and K. Honda, Nature 238 (1972) 37. [7] B. Kraeutler and A.J. Bard, J. Am. Chem. Sot. 100 (1978) 2239. [S] B. Kraeutler and A.J. Bard, J. Am. Chem. Sot. 100 (1978) 5985. (91 A. Mills and G. Porter, J. Chem. Sot. Faraday Trans. I 78 (1982) 3659. [ lo] J. Kiwi and M. Griltzel, J. Phys. Chem. 88 (1984) 1302. [ 111 J. Kiwi and C. Morrison, J. Phys. Chem. 88 ( 1984) 6 146. [ 121 B. Gu, J. Kiwi and M. Grgtzel, Nouv. J. Chim. 9 (1985) 539. [ 131 K. Yamaguchi and S. Sato, J. Chem. Sot. Faraday Trans. I 81 (1985) 1237. [ 141 K. Domen, A. Kudo, T. Onishi, N. Kosugi and H. Kuroda, J. Phys. Chem. 90 (1986) 292.
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