- Email: [email protected]
H2 reactivity on the surface of LaNi4.7Sn0.3
Journal of Alloys and Compounds 402 (2005) 219–223
H2 reactivity on the surface of LaNi4.7Sn0.3 Masashi Sato a,b , Hirohisa Uchida c , Marit Stange a , Volodymyr A. Yartys a,∗ , Shunsuke Kato c , Yusuke Ishibashi c , Masahiro Terashima c , Rui Yamakawa c , Haru-Hisa Uchida d a
d
Institute for Energy Technology, Department of Physics, Instituttveien 18, P.O. Box 40, N-2027 Kjeller, Norway b Department of Chemistry, Faculty of Mathematics and Natural Science, University of Oslo, N-0315 Oslo, Norway c Department of Applied Science, School of Engineering, Tokai University, 1117 Kita-Kaname, Hiratsuka, Kanagawa 259-1292, Japan Department of Human Development, Environmental and Resources, School of Humanities and Culture, Tokai University, 1117 Kita-Kaname, Hiratsuka, Kanagawa 259-1292, Japan Received 18 January 2005; accepted 9 February 2005 Available online 31 May 2005
Abstract The influence of O2 and H2 O on the hydrogen reactivity with the surface of LaNi4.7 Sn0.3 was studied at 298 K by a volumetric Wagener method. A significant enhancement of the H2 reactivity by precovering the surface by both O2 and H2 O was observed. A formation of the oxygen deficient SnO2−x in the surface layer acting as a catalyser of the H2 dissociation stands as possible reason for such an improvement. The present data explain the reasons for the improvement of hydrogen cycling properties by the Sn-doped LaNi5 alloys. © 2005 Elsevier B.V. All rights reserved. Keywords: Intermetallic compound; Hydrogen absorbing materials; Gas–solid reaction; Catalysis
1. Introduction LaNi5 intermetallic compound and chemically related systems are intensively studied as hydrogen storage materials [1–3]. Partial substitution of La and Ni by chemically related elements is an efficient way in modifying and controlling hydrogen absorption and desorption behaviours [4,5]. Partial substitution on Ni sites in the LaNi5 compound by such elements as Al, Co and Mn induces a significant surface modification and influences hydrogen dissociation on the surface [6]. Presence of oxide or hydroxide in the surface layer is mainly responsible for the initial activation process, hydrogen absorption behaviour and improvement of the corrosion resistance. On the surface of the alloys modified by substitutions, the dissociation mechanism becomes different compared to the binary alloy LaNi5 . ∗
Corresponding author. Tel.: +47 63 80 64 53; fax: +47 63 81 29 05. E-mail address: [email protected] (V.A. Yartys).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.02.066
Tin (Sn) substitutions for Ni in LaNi5 have a distinct effect on a number of properties of the hydrides including their stability, cycling behaviour (both for thermal and electrochemical processes) and hydrogen absorption/desorption kinetics [7–9]. The beneficial improvement of these characteristics motivates further studies in the area and is important for the applications of the metal hydrides. Some of the modifications by Sn have a significant impact on the surface properties [10,11]. However, in spite of numerous studies aimed on better understanding of the role of Sn in the Sn-modified LaNi5 compounds, fundamental processes taking place on the surface still remain unclear. The quantitative information on the surface processes influenced by Sn substitutions in LaNi5 is required to probe the crucial role of Sn. The aim of present work was to determine the quantitative reactivity of H2 , O2 and H2 O with a pure surface of LaNi4.7 Sn0.3 and effects of O2 /H2 O precoverage on the H2 reactivity under ultrahigh vacuum conditions.
220
M. Sato et al. / Journal of Alloys and Compounds 402 (2005) 219–223
2. Experimental 2.1. Sample preparation The LaNi4.7 Sn0.3 compound was produced by Ar arc furnace melting from the constituent elements of high purity La (99.9%), Ni (99.9%) and Sn (99.9%). In order to ensure homogeneity, the sample was remelted several times and annealed in a sealed evacuated quartz ampoule for one week at 1223 K. The sample was subsequently quenched into a mixture of ice and water. Powder X-ray diffraction study with Cu K␣1 radiation indicated the formation of purely single phase LaNi4.7 Sn0.3 with hexagonal CaCu5 type structure. ˚ and The refined unit cell lattice parameters, a = 5.0638 (1) A ˚ are in a good agreement with the reference c = 4.0350 (1) A, data [12]. For the studies of the interaction with gases, a rectangular block with a specific surface area of 0.29 cm2 has been cut from the ingot. 2.2. Reactivity measurement The reaction probability, r, defined by the ratio of the rate of reacted gas molecules to the rate of impinging of the gases on the surface, was volumetrically determined by means of Wagener method [13,14]. A block sample was carefully degassed in vacuum by increasing its temperature in steps until reaching finally 1300 K; at each temperature point the overall pressure achieved was less than 1 × 10−7 Pa. H2 gas with a grade of 99.99999% or O2 gas with a purity of 99.99% were supplied through a hot Pd or a hot Ag diffusion cell, respectively, to purify the gases. An ultra high purity H2 O with resistivity 18 M cm was used. The reaction probability was measured as a function of the amount of coverage, N, on a monolayer (ML) scale. More detailed information concerning the method applied is described elsewhere [14,15].
3. Results and discussion 3.1. Reactivity of H2 , O2 , H2 O on LaNi4.7 Sn0.3 The reaction probabilities of gases, rH2 , rO2 and rH2 O , with a pure surface of the LaNi4.7 Sn0.3 as a function of the amount of absorbed at 298 K hydrogen (NH2 ), oxygen (NO2 ) or water vapour (NH2 O ) are shown in Fig. 1. At initial stage, the highest reaction probability, r = 1, was observed for O2 and H2 O. With increasing coverage of the reacted O2 gas, rO2 decreases gradually down to rO2 ∼ 2.1 × 10−1 and then stabilises forming a plateau. The plateau value is slightly higher than the corresponding value observed for LaNi5 [16]. On the other hand, rH2 O decreases rather slowly. In contrast, the reaction probability of hydrogen at initial stage equals to rH2 O ∼ 10−2 . This is nearly one order of magnitude lower than for the LaNi5 surface under similar vacuum conditions (at P = 10−7 Pa) [15]. Increase of the amount of
Fig. 1. Change in the reaction probabilities of H2 , O2 and H2 on LaNi4.7 Sn0.3 as a function of the amount of reacted gas at 298 K.
H2 gas reacted with the surface rapidly decreases rH2 down to around 2 × 10−4 . Previously, the influence of the equilibrium pressure of hydrogen on the reaction probability was pointed out [17]. With decreasing hydrogen equilibrium pressures, the reaction probability on a clean metal surface becomes higher since the influence of the backward reaction of H2 release from the metal is smaller [17]. However, it is clear that a significant decrease of rH2 for the Sn-doped LaNi4.7 Sn0.3 compared to the undoped LaNi5 [15,16,18] could not be related to the different stability of the hydrides. Indeed, in case of LaNi5 -type compounds, partial substitution on Ni sites by Sn decreases the plateau pressure [7]. Despite of no literature available for pressure-composition isotherms of the Sn doped LaNi5 –H system at very low hydrogen concentrations measured in this study, it is reasonable to assume that the equilibrium pressure for the LaNi4.7 Sn0.3 –H system is lower than for the system LaNi5 –H given in the present work conditions. However, again, rH2 for LaNi4.7 Sn0.3 has the value lower than for LaNi5 , showing that the contribution of the backward reaction from the surface of LaNi4.7 Sn0.3 is rather insignificant. All these data indicate a substantial difference in the mechanism of the surface processes in LaNi4.7 Sn0.3 compared to LaNi5 . 3.2. Effect of O2 precoverage on the reactivity of H2 The measurements of the H2 reactivity with LaNi4.7 Sn0.3 were performed for the clean surface and for the precovered by oxygen alloy (see Fig. 2(a)). The effect of O2 preadsorption on rH2 was examined for preoxidised surface conditions at three levels of rO2 (see Fig. 1): (i) rO2 = 1.9 × 10−1 , (ii) rO2 = 2.1 × 10−1 and (iii) rO2 = 9.1 × 10−1 . The inset of Fig. 2(a) presents an enlarged first part of the dependences. From the data of Fig. 2(a), it is clear that oxygen precoverage
M. Sato et al. / Journal of Alloys and Compounds 402 (2005) 219–223
221
For such high reactivity of H2 on the O2 precovered LaNi4.7 Sn0.3 surface, two possible mechanisms of the reaction can be proposed as (a) the formation of H2 O gas by a reaction of the dissociated H atoms with the O atoms on the surface, or (b) the hydrogen dissolution onto and penetration through the oxygen precovered surface.
Fig. 2. (a) Effect of O2 precoverage on the H2 reactivity with LaNi4.7 Sn0.3 surface at 298 K. Inset: The dependencies at small level of NH2 shown at larger scale. (b) The changes in the partial pressure of H2 O are shown as a function of the amount of reacted H2 . The levels of rO2 are: (A) rO2 = 1.9 × 10−1 ; (B) rO2 = 2.1 × 10−1 ; (C) rO2 = 9.1 × 10−1 ; (D): without O2 precoverage.
causes a significant enhancement of the reactivity of H2 . Indeed, the lowest reactivity is observed for the pure surface–H2 system (curve D). The dependences from O2 content show a highest reactivity for rO2 = 2.1 × 10−1 (curve A). However, even at the lowest level of O2 in the surface layer, rO2 = 9.1 × 10−1 (curve C), hydrogen reactivity slightly but noticeably increases. It is interesting to note that the observed for the LaNi4.7 Sn0.3 features of the H2 reactivity with the precovered by O2 surface are different from the findings for LaNi5 . Indeed, after poisoning by O2 of the surface of LaNi5 the H2 reactivity drastically decreases [15]. The main reason for that could be caused by the behaviour of nickel. In particular, despite pure Ni is very active in interaction with H2 , its precoverage by O2 dramatically reduces the H2 reactivity (by about four orders of magnitude) [15,16]. In spite of La the effect is opposite and oxygen-deficient La2 O3 shows higher H2 reactivity [19], in total the behaviour of LaNi5 is dominated by Ni.
Fig. 2(b) shows the changes in the partial pressures of H2 O gas during the reaction of H2 with the surface. From these data, it is clear that the fastest rate of water formation is characteristic for the lowest O2 precoverage (rO2 = 9.1 × 10−1 ; curve (C)). However, at the same time, a sharp decrease of rH2 takes place during the H2 exposure for this sample. This means that the first mechanism plays a more important role in this case and that the contribution of H2 O gas formation into the enhancement of H2 reactivity with preoxidized LaNi4.7 Sn0.3 surface seems to be negligibly small. On the other hand, for the higher O2 coverage (rO2 = 2.1 × 10−1 ; curve (A)) the partial pressure of H2 O increases rather slowly. Such behaviour can result from a contribution from both (a) and (b) processes. However, rH2 remains higher level of reactivity with the surface, which makes evident that the process (b) plays more a important role on the preoxidized surface at a higher level of the O2 precoverage. Despite the overall effect of the H2 O formation from the reaction of H2 with the preoxidized layer on LaNi4.7 Sn0.3 is rather small, previous reports showed a distinctly different behaviour for the surfaces not containing Sn. Indeed, on the preoxidised surfaces of La and Ni during the H2 exposure water is not formatted at all [16,19]. On the other hand, it is well recognised that the SnO2 can be reduced by H2 even at moderate temperatures [20,21]. These suggest that the increases of H2 O partial pressure during H2 exposure may be mostly attributed to the reduction of SnO2 with H2 taking place already at 298 K. Furthermore, SnO2−x can play a catalytic role in splitting on the surface of H2 molecules which, in turn, will facilitate hydrogen adsorption by bulk material. Available reference data further support an idea that Sn presence on the surface of LaNi4.7 Sn0.3 is responsible for the enhancement of H2 reactivity after its precoverage by O2 . Lim et al. by XPS studies have observed the formation of tin oxide SnO2 on the surface of Sn-doped LaNi5 and suggested that its presence improves corrosion resistance [11]. Tin oxide is normally formed as a nonstoichiometric, oxygen deficient SnO2−x and behaves as an n-type semiconductor [20]. Because of O deficiency, SnO2−x can exhibit enhanced activity in the dissociation of the H2 molecules [22–24]. That is why a partial replacement of Ni by Sn may assist in the H2 dissociation on the surface even at a small amount of Sn doping. The optimisation of the H2 reactivity via Sn doping and O2 precoverage requires further systematic studies to better understand the role of Sn both on the surface and in the bulk material.
222
M. Sato et al. / Journal of Alloys and Compounds 402 (2005) 219–223
ent from the behaviours of the undoped LaNi5 . H2 reactivity on a pure surface has been reduced by Sn substitution for Ni. In contrast, a great enhancement of the H2 reactivity on poisoned by O2 and H2 O surface takes place, with H2 reactivity increasing from LaNi4.7 Sn0.3 –H2 to LaNi4.7 Sn0.3 –H2 O–H2 and, further, to the LaNi4.7 Sn0.3 –O2 –H2 system. The beneficial advantages of the partial replacement of Ni by Sn are to modify the surface process and to assist the H2 reaction with the poisoned surface. This effect is observed even at small Sn substitutions, which for LaNi4.7 Sn0.3 are equal to only 6 at.%.
Acknowledgments
Fig. 3. Effect of H2 O precoverage (rH2 O = 9.1 × 10−1 ) on H2 reactivity with LaNi4.7 Sn0.3 surface at 298 K.
3.3. Effect of H2 O precoverage on H2 reactivity Fig. 3 shows the effect of H2 O precoverage on the H2 reactivity. Water slightly reduces hydrogen reactivity at the initial stage. However, dependence for the H2 O-precovered alloy rather quickly crosses the line for the pure H2 because of a rapid drop of the reactivity for the pure H2 contrasting to a rather slow decrease of the reactivity in case of the H2 O precoverage. These results agree with the observations of beneficial improvements of the kinetic behaviours during the electrochemical H charging for the Sn-doped alloys in aqueous solutions [9]. We note in addition that for the similar level of precoverage the activity of H2 is higher for O2 compared to H2 O (see Figs. 2 and 3). However, in general the behaviour of the system simultaneously containing hydrogen, water and oxygen is more complex. Adzic et al. reported a remarkable improvement of the life cycle on the gaseous absorption and desorption process caused by Sn substitution, but less significant changes observed for the electrochemical charge–discharge [10]. The reference data and the results of this study show that the mechanism of Sn influence on the processes of H absorption can have roots in: (a) formation in the surface layer of the Sn oxide which catalyses H uptake and is formed during the activation process leading to segregation of the oxides on the surface; (b) Sn-caused changes of the properties of the surface layer which affect the H2 dissociation, penetration into the bulk material of the atomic H and formation of the hydride phase.
4. Conclusions The reaction probabilities of hydrogen for the modified by gases surface layers of LaNi4.7 Sn0.3 are significantly differ-
This study was conducted in the frame of an international joint research project “Advanced Metal Hydrides with Record High Volume Density of Hydrogen and Catalytic Surface Properties” of the New Energy and Industrial Technology Development Organization (NEDO), Japan. The authors are grateful to NEDO for financial support. M. Sato and M. Stange gratefully acknowledge the financial support of the Scandinavia-Japan Sasakawa foundation for travelling to Japan. M. Sato also thanks an international research scientist exchange program between University of Oslo, Norway and Tokai University, Japan.
References [1] K.H.J. Buschow, P.C.P. Bouten, A.R. Miedema, Rep. Prog. Phys. 45 (1982) 937. [2] J.J. Reilly, G.D. Adzic, J.R. Johnson, T. Voigt, S. Mukerjee, J. McBreen, J. Alloys Compd. 293–295 (1999) 569. [3] G. Sandrock, J. Alloys Compd. 293–295 (1999) 877. [4] T. Sakai, K. Oguro, H. Miyamura, N. Kuriyama, A. Kato, H. Ishikawa, J. Less-Common Met. 161 (1990) 193. [5] H.H. Uchida, M. Tada, Y.C. Huang, J. Less-Common Met. 88 (1982) 81. [6] H. Uchida, Int. J. Hydrogen Energy 24 (1999) 861. [7] M.H. Mendelsohn, D.M. Gruen, A.E. Dwight, Mater. Res. Bull. 13 (1978) 1221. [8] R.C. Bowman Jr., C.H. Luo, C.C. Ahn, C.K. Witham, B. Fultz, J. Alloys Compd. 217 (1995) 185. [9] B.V. Ratnakumar, C. Witham, R.C. Bowman Jr., A. Hightower, B. Fultz, J. Electrochem. Soc. 143 (8) (1996) 2578. [10] G.D. Adzic, J.R. Johnson, J.J. Reilly, H.S. Lim, J. Electrochem. Soc. 142 (10) (1995) 3424. [11] Q. Lim, S. Zhao, D.J. Zhu, B. Song, Z. Mei, J. Alloys Compd. 351 (2003) 91. [12] O.Yu. Khyzhum, M.V. Lototsky, A.B. Riabov, C. Rosenkilde, V.A. Yartys, S. Jørgensen, R.V. Denys, J. Alloys Compd. 356–357 (2003) 773. [13] S. Wagener, Br. J. Appl. Phys. 1 (1950) 225. [14] E. Fromm, H.G. Wulz, J. Less-Common Met. 101 (1983) 469. [15] H. Uchida, Y. Ohtani, M. Ozawa, T. Kawahata, T. Suzuki, J. LessCommon Met. 172–174 (1991) 983. [16] H. Uchida, S. Nakazawa, K. Ishikawa, T. Irisawa, J. Alloys Compd. 231 (1995) 684. [17] E. Fromm, H. Uchida, J. Less-Common Met. 66 (1979) 77. [18] H. Uchida, T. Suzuki, S. Nakazawa, S. Kashima, Z. Phys. Chem. 181 (1993) 429.
M. Sato et al. / Journal of Alloys and Compounds 402 (2005) 219–223 [19] N. Hosoda, H. Uchida, Y. Ohtani, T. Takahashi, E. Fromm, Z. Phys. Chem. NF 164 (1989) 1129. [20] J. Szuber, G. Gzempik, R. Larciprete, D. Koziej, B. Adamowicz, Thin Solid Films 391 (2001) 198. [21] N. Yamazoe, J. Fuchigami, M. Kishikawa, T. Seiyama, Surf. Sci. 86 (1979) 335.
223
[22] S.G. Ansari, P. Boroojerdian, S.R. Sainkar, R.N. Karekar, R.C. Aiyer, S.K. Kulkarni, Thin Solid Films 295 (1997) 271. [23] Y. Hayakawa, M. Komoda, A. Ono, Sens. Actuators B 42 (1997) 131. [24] T. Kanashima, A. Fujimoto, M. Okuyama, Sens. Actuators B 64 (2000) 189.
H2 reactivity on the surface of LaNi4.7Sn0.3 Masashi Sato a,b , Hirohisa Uchida c , Marit Stange a , Volodymyr A. Yartys a,∗ , Shunsuke Kato c , Yusuke Ishibashi c , Masahiro Terashima c , Rui Yamakawa c , Haru-Hisa Uchida d a
d
Institute for Energy Technology, Department of Physics, Instituttveien 18, P.O. Box 40, N-2027 Kjeller, Norway b Department of Chemistry, Faculty of Mathematics and Natural Science, University of Oslo, N-0315 Oslo, Norway c Department of Applied Science, School of Engineering, Tokai University, 1117 Kita-Kaname, Hiratsuka, Kanagawa 259-1292, Japan Department of Human Development, Environmental and Resources, School of Humanities and Culture, Tokai University, 1117 Kita-Kaname, Hiratsuka, Kanagawa 259-1292, Japan Received 18 January 2005; accepted 9 February 2005 Available online 31 May 2005
Abstract The influence of O2 and H2 O on the hydrogen reactivity with the surface of LaNi4.7 Sn0.3 was studied at 298 K by a volumetric Wagener method. A significant enhancement of the H2 reactivity by precovering the surface by both O2 and H2 O was observed. A formation of the oxygen deficient SnO2−x in the surface layer acting as a catalyser of the H2 dissociation stands as possible reason for such an improvement. The present data explain the reasons for the improvement of hydrogen cycling properties by the Sn-doped LaNi5 alloys. © 2005 Elsevier B.V. All rights reserved. Keywords: Intermetallic compound; Hydrogen absorbing materials; Gas–solid reaction; Catalysis
1. Introduction LaNi5 intermetallic compound and chemically related systems are intensively studied as hydrogen storage materials [1–3]. Partial substitution of La and Ni by chemically related elements is an efficient way in modifying and controlling hydrogen absorption and desorption behaviours [4,5]. Partial substitution on Ni sites in the LaNi5 compound by such elements as Al, Co and Mn induces a significant surface modification and influences hydrogen dissociation on the surface [6]. Presence of oxide or hydroxide in the surface layer is mainly responsible for the initial activation process, hydrogen absorption behaviour and improvement of the corrosion resistance. On the surface of the alloys modified by substitutions, the dissociation mechanism becomes different compared to the binary alloy LaNi5 . ∗
Corresponding author. Tel.: +47 63 80 64 53; fax: +47 63 81 29 05. E-mail address: [email protected] (V.A. Yartys).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.02.066
Tin (Sn) substitutions for Ni in LaNi5 have a distinct effect on a number of properties of the hydrides including their stability, cycling behaviour (both for thermal and electrochemical processes) and hydrogen absorption/desorption kinetics [7–9]. The beneficial improvement of these characteristics motivates further studies in the area and is important for the applications of the metal hydrides. Some of the modifications by Sn have a significant impact on the surface properties [10,11]. However, in spite of numerous studies aimed on better understanding of the role of Sn in the Sn-modified LaNi5 compounds, fundamental processes taking place on the surface still remain unclear. The quantitative information on the surface processes influenced by Sn substitutions in LaNi5 is required to probe the crucial role of Sn. The aim of present work was to determine the quantitative reactivity of H2 , O2 and H2 O with a pure surface of LaNi4.7 Sn0.3 and effects of O2 /H2 O precoverage on the H2 reactivity under ultrahigh vacuum conditions.
220
M. Sato et al. / Journal of Alloys and Compounds 402 (2005) 219–223
2. Experimental 2.1. Sample preparation The LaNi4.7 Sn0.3 compound was produced by Ar arc furnace melting from the constituent elements of high purity La (99.9%), Ni (99.9%) and Sn (99.9%). In order to ensure homogeneity, the sample was remelted several times and annealed in a sealed evacuated quartz ampoule for one week at 1223 K. The sample was subsequently quenched into a mixture of ice and water. Powder X-ray diffraction study with Cu K␣1 radiation indicated the formation of purely single phase LaNi4.7 Sn0.3 with hexagonal CaCu5 type structure. ˚ and The refined unit cell lattice parameters, a = 5.0638 (1) A ˚ are in a good agreement with the reference c = 4.0350 (1) A, data [12]. For the studies of the interaction with gases, a rectangular block with a specific surface area of 0.29 cm2 has been cut from the ingot. 2.2. Reactivity measurement The reaction probability, r, defined by the ratio of the rate of reacted gas molecules to the rate of impinging of the gases on the surface, was volumetrically determined by means of Wagener method [13,14]. A block sample was carefully degassed in vacuum by increasing its temperature in steps until reaching finally 1300 K; at each temperature point the overall pressure achieved was less than 1 × 10−7 Pa. H2 gas with a grade of 99.99999% or O2 gas with a purity of 99.99% were supplied through a hot Pd or a hot Ag diffusion cell, respectively, to purify the gases. An ultra high purity H2 O with resistivity 18 M cm was used. The reaction probability was measured as a function of the amount of coverage, N, on a monolayer (ML) scale. More detailed information concerning the method applied is described elsewhere [14,15].
3. Results and discussion 3.1. Reactivity of H2 , O2 , H2 O on LaNi4.7 Sn0.3 The reaction probabilities of gases, rH2 , rO2 and rH2 O , with a pure surface of the LaNi4.7 Sn0.3 as a function of the amount of absorbed at 298 K hydrogen (NH2 ), oxygen (NO2 ) or water vapour (NH2 O ) are shown in Fig. 1. At initial stage, the highest reaction probability, r = 1, was observed for O2 and H2 O. With increasing coverage of the reacted O2 gas, rO2 decreases gradually down to rO2 ∼ 2.1 × 10−1 and then stabilises forming a plateau. The plateau value is slightly higher than the corresponding value observed for LaNi5 [16]. On the other hand, rH2 O decreases rather slowly. In contrast, the reaction probability of hydrogen at initial stage equals to rH2 O ∼ 10−2 . This is nearly one order of magnitude lower than for the LaNi5 surface under similar vacuum conditions (at P = 10−7 Pa) [15]. Increase of the amount of
Fig. 1. Change in the reaction probabilities of H2 , O2 and H2 on LaNi4.7 Sn0.3 as a function of the amount of reacted gas at 298 K.
H2 gas reacted with the surface rapidly decreases rH2 down to around 2 × 10−4 . Previously, the influence of the equilibrium pressure of hydrogen on the reaction probability was pointed out [17]. With decreasing hydrogen equilibrium pressures, the reaction probability on a clean metal surface becomes higher since the influence of the backward reaction of H2 release from the metal is smaller [17]. However, it is clear that a significant decrease of rH2 for the Sn-doped LaNi4.7 Sn0.3 compared to the undoped LaNi5 [15,16,18] could not be related to the different stability of the hydrides. Indeed, in case of LaNi5 -type compounds, partial substitution on Ni sites by Sn decreases the plateau pressure [7]. Despite of no literature available for pressure-composition isotherms of the Sn doped LaNi5 –H system at very low hydrogen concentrations measured in this study, it is reasonable to assume that the equilibrium pressure for the LaNi4.7 Sn0.3 –H system is lower than for the system LaNi5 –H given in the present work conditions. However, again, rH2 for LaNi4.7 Sn0.3 has the value lower than for LaNi5 , showing that the contribution of the backward reaction from the surface of LaNi4.7 Sn0.3 is rather insignificant. All these data indicate a substantial difference in the mechanism of the surface processes in LaNi4.7 Sn0.3 compared to LaNi5 . 3.2. Effect of O2 precoverage on the reactivity of H2 The measurements of the H2 reactivity with LaNi4.7 Sn0.3 were performed for the clean surface and for the precovered by oxygen alloy (see Fig. 2(a)). The effect of O2 preadsorption on rH2 was examined for preoxidised surface conditions at three levels of rO2 (see Fig. 1): (i) rO2 = 1.9 × 10−1 , (ii) rO2 = 2.1 × 10−1 and (iii) rO2 = 9.1 × 10−1 . The inset of Fig. 2(a) presents an enlarged first part of the dependences. From the data of Fig. 2(a), it is clear that oxygen precoverage
M. Sato et al. / Journal of Alloys and Compounds 402 (2005) 219–223
221
For such high reactivity of H2 on the O2 precovered LaNi4.7 Sn0.3 surface, two possible mechanisms of the reaction can be proposed as (a) the formation of H2 O gas by a reaction of the dissociated H atoms with the O atoms on the surface, or (b) the hydrogen dissolution onto and penetration through the oxygen precovered surface.
Fig. 2. (a) Effect of O2 precoverage on the H2 reactivity with LaNi4.7 Sn0.3 surface at 298 K. Inset: The dependencies at small level of NH2 shown at larger scale. (b) The changes in the partial pressure of H2 O are shown as a function of the amount of reacted H2 . The levels of rO2 are: (A) rO2 = 1.9 × 10−1 ; (B) rO2 = 2.1 × 10−1 ; (C) rO2 = 9.1 × 10−1 ; (D): without O2 precoverage.
causes a significant enhancement of the reactivity of H2 . Indeed, the lowest reactivity is observed for the pure surface–H2 system (curve D). The dependences from O2 content show a highest reactivity for rO2 = 2.1 × 10−1 (curve A). However, even at the lowest level of O2 in the surface layer, rO2 = 9.1 × 10−1 (curve C), hydrogen reactivity slightly but noticeably increases. It is interesting to note that the observed for the LaNi4.7 Sn0.3 features of the H2 reactivity with the precovered by O2 surface are different from the findings for LaNi5 . Indeed, after poisoning by O2 of the surface of LaNi5 the H2 reactivity drastically decreases [15]. The main reason for that could be caused by the behaviour of nickel. In particular, despite pure Ni is very active in interaction with H2 , its precoverage by O2 dramatically reduces the H2 reactivity (by about four orders of magnitude) [15,16]. In spite of La the effect is opposite and oxygen-deficient La2 O3 shows higher H2 reactivity [19], in total the behaviour of LaNi5 is dominated by Ni.
Fig. 2(b) shows the changes in the partial pressures of H2 O gas during the reaction of H2 with the surface. From these data, it is clear that the fastest rate of water formation is characteristic for the lowest O2 precoverage (rO2 = 9.1 × 10−1 ; curve (C)). However, at the same time, a sharp decrease of rH2 takes place during the H2 exposure for this sample. This means that the first mechanism plays a more important role in this case and that the contribution of H2 O gas formation into the enhancement of H2 reactivity with preoxidized LaNi4.7 Sn0.3 surface seems to be negligibly small. On the other hand, for the higher O2 coverage (rO2 = 2.1 × 10−1 ; curve (A)) the partial pressure of H2 O increases rather slowly. Such behaviour can result from a contribution from both (a) and (b) processes. However, rH2 remains higher level of reactivity with the surface, which makes evident that the process (b) plays more a important role on the preoxidized surface at a higher level of the O2 precoverage. Despite the overall effect of the H2 O formation from the reaction of H2 with the preoxidized layer on LaNi4.7 Sn0.3 is rather small, previous reports showed a distinctly different behaviour for the surfaces not containing Sn. Indeed, on the preoxidised surfaces of La and Ni during the H2 exposure water is not formatted at all [16,19]. On the other hand, it is well recognised that the SnO2 can be reduced by H2 even at moderate temperatures [20,21]. These suggest that the increases of H2 O partial pressure during H2 exposure may be mostly attributed to the reduction of SnO2 with H2 taking place already at 298 K. Furthermore, SnO2−x can play a catalytic role in splitting on the surface of H2 molecules which, in turn, will facilitate hydrogen adsorption by bulk material. Available reference data further support an idea that Sn presence on the surface of LaNi4.7 Sn0.3 is responsible for the enhancement of H2 reactivity after its precoverage by O2 . Lim et al. by XPS studies have observed the formation of tin oxide SnO2 on the surface of Sn-doped LaNi5 and suggested that its presence improves corrosion resistance [11]. Tin oxide is normally formed as a nonstoichiometric, oxygen deficient SnO2−x and behaves as an n-type semiconductor [20]. Because of O deficiency, SnO2−x can exhibit enhanced activity in the dissociation of the H2 molecules [22–24]. That is why a partial replacement of Ni by Sn may assist in the H2 dissociation on the surface even at a small amount of Sn doping. The optimisation of the H2 reactivity via Sn doping and O2 precoverage requires further systematic studies to better understand the role of Sn both on the surface and in the bulk material.
222
M. Sato et al. / Journal of Alloys and Compounds 402 (2005) 219–223
ent from the behaviours of the undoped LaNi5 . H2 reactivity on a pure surface has been reduced by Sn substitution for Ni. In contrast, a great enhancement of the H2 reactivity on poisoned by O2 and H2 O surface takes place, with H2 reactivity increasing from LaNi4.7 Sn0.3 –H2 to LaNi4.7 Sn0.3 –H2 O–H2 and, further, to the LaNi4.7 Sn0.3 –O2 –H2 system. The beneficial advantages of the partial replacement of Ni by Sn are to modify the surface process and to assist the H2 reaction with the poisoned surface. This effect is observed even at small Sn substitutions, which for LaNi4.7 Sn0.3 are equal to only 6 at.%.
Acknowledgments
Fig. 3. Effect of H2 O precoverage (rH2 O = 9.1 × 10−1 ) on H2 reactivity with LaNi4.7 Sn0.3 surface at 298 K.
3.3. Effect of H2 O precoverage on H2 reactivity Fig. 3 shows the effect of H2 O precoverage on the H2 reactivity. Water slightly reduces hydrogen reactivity at the initial stage. However, dependence for the H2 O-precovered alloy rather quickly crosses the line for the pure H2 because of a rapid drop of the reactivity for the pure H2 contrasting to a rather slow decrease of the reactivity in case of the H2 O precoverage. These results agree with the observations of beneficial improvements of the kinetic behaviours during the electrochemical H charging for the Sn-doped alloys in aqueous solutions [9]. We note in addition that for the similar level of precoverage the activity of H2 is higher for O2 compared to H2 O (see Figs. 2 and 3). However, in general the behaviour of the system simultaneously containing hydrogen, water and oxygen is more complex. Adzic et al. reported a remarkable improvement of the life cycle on the gaseous absorption and desorption process caused by Sn substitution, but less significant changes observed for the electrochemical charge–discharge [10]. The reference data and the results of this study show that the mechanism of Sn influence on the processes of H absorption can have roots in: (a) formation in the surface layer of the Sn oxide which catalyses H uptake and is formed during the activation process leading to segregation of the oxides on the surface; (b) Sn-caused changes of the properties of the surface layer which affect the H2 dissociation, penetration into the bulk material of the atomic H and formation of the hydride phase.
4. Conclusions The reaction probabilities of hydrogen for the modified by gases surface layers of LaNi4.7 Sn0.3 are significantly differ-
This study was conducted in the frame of an international joint research project “Advanced Metal Hydrides with Record High Volume Density of Hydrogen and Catalytic Surface Properties” of the New Energy and Industrial Technology Development Organization (NEDO), Japan. The authors are grateful to NEDO for financial support. M. Sato and M. Stange gratefully acknowledge the financial support of the Scandinavia-Japan Sasakawa foundation for travelling to Japan. M. Sato also thanks an international research scientist exchange program between University of Oslo, Norway and Tokai University, Japan.
References [1] K.H.J. Buschow, P.C.P. Bouten, A.R. Miedema, Rep. Prog. Phys. 45 (1982) 937. [2] J.J. Reilly, G.D. Adzic, J.R. Johnson, T. Voigt, S. Mukerjee, J. McBreen, J. Alloys Compd. 293–295 (1999) 569. [3] G. Sandrock, J. Alloys Compd. 293–295 (1999) 877. [4] T. Sakai, K. Oguro, H. Miyamura, N. Kuriyama, A. Kato, H. Ishikawa, J. Less-Common Met. 161 (1990) 193. [5] H.H. Uchida, M. Tada, Y.C. Huang, J. Less-Common Met. 88 (1982) 81. [6] H. Uchida, Int. J. Hydrogen Energy 24 (1999) 861. [7] M.H. Mendelsohn, D.M. Gruen, A.E. Dwight, Mater. Res. Bull. 13 (1978) 1221. [8] R.C. Bowman Jr., C.H. Luo, C.C. Ahn, C.K. Witham, B. Fultz, J. Alloys Compd. 217 (1995) 185. [9] B.V. Ratnakumar, C. Witham, R.C. Bowman Jr., A. Hightower, B. Fultz, J. Electrochem. Soc. 143 (8) (1996) 2578. [10] G.D. Adzic, J.R. Johnson, J.J. Reilly, H.S. Lim, J. Electrochem. Soc. 142 (10) (1995) 3424. [11] Q. Lim, S. Zhao, D.J. Zhu, B. Song, Z. Mei, J. Alloys Compd. 351 (2003) 91. [12] O.Yu. Khyzhum, M.V. Lototsky, A.B. Riabov, C. Rosenkilde, V.A. Yartys, S. Jørgensen, R.V. Denys, J. Alloys Compd. 356–357 (2003) 773. [13] S. Wagener, Br. J. Appl. Phys. 1 (1950) 225. [14] E. Fromm, H.G. Wulz, J. Less-Common Met. 101 (1983) 469. [15] H. Uchida, Y. Ohtani, M. Ozawa, T. Kawahata, T. Suzuki, J. LessCommon Met. 172–174 (1991) 983. [16] H. Uchida, S. Nakazawa, K. Ishikawa, T. Irisawa, J. Alloys Compd. 231 (1995) 684. [17] E. Fromm, H. Uchida, J. Less-Common Met. 66 (1979) 77. [18] H. Uchida, T. Suzuki, S. Nakazawa, S. Kashima, Z. Phys. Chem. 181 (1993) 429.
M. Sato et al. / Journal of Alloys and Compounds 402 (2005) 219–223 [19] N. Hosoda, H. Uchida, Y. Ohtani, T. Takahashi, E. Fromm, Z. Phys. Chem. NF 164 (1989) 1129. [20] J. Szuber, G. Gzempik, R. Larciprete, D. Koziej, B. Adamowicz, Thin Solid Films 391 (2001) 198. [21] N. Yamazoe, J. Fuchigami, M. Kishikawa, T. Seiyama, Surf. Sci. 86 (1979) 335.
223
[22] S.G. Ansari, P. Boroojerdian, S.R. Sainkar, R.N. Karekar, R.C. Aiyer, S.K. Kulkarni, Thin Solid Films 295 (1997) 271. [23] Y. Hayakawa, M. Komoda, A. Ono, Sens. Actuators B 42 (1997) 131. [24] T. Kanashima, A. Fujimoto, M. Okuyama, Sens. Actuators B 64 (2000) 189.