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Absorption of D(g) atoms in ultrathin Mg films and Pd-catalyzed decomposition of MgD2
Journal of Industrial and Engineering Chemistry 15 (2009) 253–256
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Absorption of D(g) atoms in ultrathin Mg films and Pd-catalyzed decomposition of MgD2 Tae Seung Kim, Jun Woo Song, Seung June Lee, Jihwa Lee * School of Chemical and Biological Engineering, Seoul National University, Silim san 56-1, Gwanak-gu, Seoul 151-744, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 August 2008 Accepted 8 September 2008
We have investigated the interaction of the gas-phase D atoms with ultrathin Mg films thermally evaporated on SiO2 by temperature programmed desorption. The gas-phase D atoms impinging upon a Mg (100 A˚) film directly penetrate into the bulk with a constant probability of 0.22 until the top 16 A˚ is deuterated, whereafter the MgD2(s) growth quickly slows down. Upon heating the partially deuterated Mg film, Mg atoms diffuse out to the surface to desorb at the same temperature as a pure Mg film. On the other hand, MgD2(s) decomposes to result in simultaneous desorption of D2 and Mg at 590 K with an activation energy of 133 5 kJ/mol. A Pd layer of 20 A˚ thickness deposited on top of MgD2(s) acts as a catalyst to greatly reduce the decomposition temperature from 590 to 350 K. We interpret the catalytic effect of Pd in terms of reverse spill-over, in which the surface of Pd clusters in contact with MgD2(s) provides an intermediate adsorption sites with a low activation energy for the recombinative desorption of D atoms. ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: D atom beam MgD2(s) Temperature programmed desorption Zeroth-order desorption Pd catalyst
1. Introduction High prospects for hydrogen fuel-cell-powered vehicles have stimulated much research on the hydrogen storage problem in recent years [1]. Extensive investigations devoted to a variety of metal hydrides for many decades have made much progress [2]. Among many metals, Mg is still attracting much attention because its theoretical gravimetric storage density of 7.8% is large enough to be a practical storage medium and also Mg is an abundant element in the earth crust. However, Mg has two major drawbacks; the heat of formation of MgH2 is large (72.0 kJ/mol) [3] and therefore the dehydriding process usually requires a high temperature of 350 8C. Furthermore, magnesium hydride acts as a diffusion barrier for hydrogen atom into the interior of Mg powder, which also requires a high pressure. In order to overcome these problems, various approaches have been investigated. Notably, Mg nanoparticles [4] prepared by ball milling together with various catalysts such as transition metals [5] and metal oxides [6] have been studied to show much improvements of the hydriding as well as dehydriding kinetics. Most studies made so far were mainly concerned with preparation and characterization of the materials along with the kinetics measurements. Therefore, the catalytic mechanisms are still poorly understood.
* Corresponding author. Tel.: +82 2 880 7076. E-mail address: [email protected] (J. Lee).
In this paper, we report results of experimental studies on the absorption of gas-phase D atom in ultrathin magnesium films and the decomposition properties of the MgD2(s) by temperature programmed desorption (TPD). The use of a hydrogen atom beam allows one to easily hydrogenate a Mg film due to its high reactivity towards Mg so that one can study the dehydriding process using UHV techniques. The effects of a very thin Pd layer deposited on top of MgD2(s) on the decomposition of MgD2(s) have also been studied. We will show that the Pd layer greatly reduces the decomposition temperature of MgD2(s) from 590 to 350 K, which we interpret in terms of reversible spill-over. 2. Experiment The UHV system (Fig. 1) employed for the current study has been described elsewhere [7]. Briefly, it is a 50 l stainless steel chamber pumped by a 520 l turbomolecular pump, in which the base pressure is 1 1010 Torr. It is equipped with AES, LEED, and TPD capabilities. It is also equipped with a differentially pumped hydrogen atom beam source, a W capillary tube of 1 mm in diameter heated to 1900 K by electron bombardment. The sticking probability of thermal D atom at 1900 K on Si(1 0 0) is not known but Hausen and Vogl [8] have estimated it to be 0.4 by molecular dynamics calculations. Taking this value, the D atom flux of at the sample surface was estimated to be 4.0 1013 atoms/cm2 s based on the uptake rate of D atoms on a clean Si(1 0 0) surface separately measured by TPD.
1226-086X/$ – see front matter ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2008.09.008
T.S. Kim et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 253–256
254
Fig. 2. TPD spectrum of 38 ML Mg deposited on SiO2.
Fig. 1. Schematic diagram of the experimental ultrahigh vacuum system.
Si(1 0 0) wafers with 1000 A˚ SiO2 layer on top grown by a standard wet oxidation process and cut into rectangular pieces of 1.2 cm 1.8 cm were used as samples. Pt(1 1 1) was also used as a sample to study its catalytic effect on the decomposition of MgD2. The sample mounted on a rotatable XYZ manipulator could be cooled with liquid nitrogen down to 90 K and resistively heated to 1300 K. Pd was evaporated in situ by e-beam evaporation and Mg by thermal evaporation of Mg ribbon mounted on a pair of W wires at rates of 0.48 and 0.115 A˚/s, respectively. The deposition rate of Pd and Mg were monitored with a quartz crystal microbalance. In TPD the sample temperature was linearly ramped at a heating rate of 2 K/s using a PID controller and a programmable dc power supply. The QMS was differentially pumped using a 50 l/s turbomolecular pump. It has a protruding cylindrical aperture of 4.5 mm in diameter. All the TPD measurements were made with the sample very close to the aperture of the QMS so as to detect molecules desorbing only from the front surface of the sample. D2 desorption signals were calibrated against that from saturated (2 1) H: Si(1 0 0) surface. 3. Results and discussion 3.1. Absorption of D(g) atom in Mg films and decomposition of MgD2 In Fig. 2 is shown the TPD spectrum of 100 A˚ thick Mg film deposited at 300 K on SiO2. A 100 A˚ Mg film corresponds to 38 monolayers (ML) (1 ML = 1.12 1015 atoms/cm2) of hexagonally close-packed Mg (0 0 0 1) planes. The Mg multilayer desorbs to result in an asymmetric peak at 517 K. The exponentially increasing leading edge followed by a sharp decrease in the signal indicates a zeroth-order desorption as expected for a multilayer Mg film. In contrast to a pure Mg film, the TPD spectra of 100 A˚ Mg films exposed to the D atom beam for various times show a new Mg peak at 590 K in addition to the one at 517 K (Fig. 3a). D2 desorption peak is also observed (Fig. 3b), and its peak temperature of 590 K exactly coincides with the new Mg peak in Fig. 3a. Furthermore, as the D atom exposure time increases, the intensity of the Mg peak at 517 K decreases, while those of D2 as well as Mg peaks at 590 K concomitantly increase. This correlation clearly indicates that the simultaneous desorption of D2 and Mg at 590 K occurs by the decomposition of MgD2(s). A small shoulder peak of Mg at 650 K is
Fig. 3. TPD spectra of (a) D2 and (b) Mg desorbing from a Mg(100 A˚) film on SiO2 for various D atom beam dosing times.
also observed, which is probably due to the decomposition of magnesium silicate formed at the Mg–SiO2 interface [9]. The D2 desorption peaks for various exposure times have a common leading edge, suggesting that the decomposition of MgD2(s) obeys a zeroth-order kinetics. The coverage-independent zeroth-order desorption rate can be expressed as R = n exp(Ed/ RT) with the pre-exponential factor n and the activation energy of desorption Ed. The Arrhenius plot of the leading edge shown in Fig. 4 is quite linear, and from the slope of the plot we estimate the
T.S. Kim et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 253–256
255
Fig. 6. Proposed morphology of partially deuterated Mg(100 A˚) film showing a nonuniform polycrystalline film and enhanced deuteration along the grain boundaries.
Fig. 4. Arrhenius plot of the leading edge of the D2 TPD spectra shown in Fig. 3b.
activation energy of Ea = 133 5 kJ/mol for decomposition of MgD2(s). This is in agreement with the previously reported values ranging from 120 to 160 kJ/mol [10]. By comparing the integrated D2 desorption signals with that from a (2 1) H/Si(1 0 0) surface at saturation, we can tell how much MgD2(s) has formed. In Fig. 5 is plotted the average thickness of the deuterated Mg layer as a function of the D atom fluence. The growth rate of MgD2 is fairly linear until on average 6 ML of Mg (equivalently 16 A˚) is deuterated and thereafter it quickly slows down. The reaction probability of D atom in the linear growth regime can be estimated to be 0.22. The probability smaller than unity can be attributed to the reflection of impinging D(g) atoms. 3.2. The initial growth mode of MgD2(s) Hydrogenation of a bulk Mg under a high pressure proceeds by nucleation followed by grain growth, in which a sigmoidal uptake curve with an induction period is usually observed [11]. The linear uptake of D atoms up to the deuteration of 6 ML of Mg (Fig. 5) suggests that MgD2 formation using a D atom beam does not seem to follow the same kinetics at least in the initial stage. Rather, we believe that the entire surface layer of Mg are deuterated because D(g) atom readily adsorbs on Mg in contrast to D2 molecules for which the sticking probability is very small due to an activation barrier for dissociation [12]. In addition, direct penetration of the
incident D atoms is expected based on the following argument. Hydrogen atom is known to penetrate into the subsurface region of Cu (1 1 1) [13]. The nearest neighbor distance in Mg metal is 3.21 A˚, while that in Cu is 2.56 A˚. This being considered, direct penetration of the impinging D(g) atoms into a Mg film would occur more efficiently than in Cu. Thus, the constant initial MgD2 growth rate observed here can be ascribed to a finite penetration depth of the impinging D(g) atoms, which is approximately about 15 A˚. Once the thickness of the MgD2(s) becomes comparable to the penetration depth of the D(g) atom, further growth will be controlled by the diffusion of D atoms, a slow process at the sample temperature of 300 K employed [14]. 6 ML of Mg layer corresponds to a thickness of 16 A˚. Then, it is difficult to imagine how the impinging D(g) atoms can penetrate so deeply into the Mg layer. Other factors which can influence the growth rate such as the morphology of the Mg layer have to be considered. The Mg film deposited on SiO2 is expected to grow by Volmer–Weber mode, namely 3D island formation followed by grain growth, to result in a polycrystalline film with non-uniform thickness and many grain boundaries. Penetration depth of the impinging D(g) atoms as well as the diffusion rate of the absorbed D atoms will be enhanced through the grain boundaries. A possible morphology of the deuterated Mg film is schematically depicted in Fig. 6. In Fig. 3 we have shown that metallic Mg desorbs at 517 K from a partially deuterated Mg film while MgD2(s) decomposes later at 590 K to result in simultaneous desorption of Mg and D2. The growth mode of MgD2(s) we proposed above seems to be in contradiction to this observation, for the growth mode implies that metallic Mg underneath the MgD2(s) layer has to diffuse out to the surface before desorption. Nevertheless, we think the out-diffusion of Mg is possible at 500 K mainly through the grain boundaries as explained above. 3.3. Catalytic decomposition of MgD2 by Pd
Fig. 5. The thickness variation of the deuterated Mg layers with increasing D atom exposure.
As seen in Fig. 3, MgD2 decomposes at fairly high temperate of 590 K, which is one of the major disadvantages for using Mg as a practical hydrogen storage medium. In order to study the effect of Pd on the decomposition of MgD2, a 100 A˚ Mg/SiO2 was first deuterated with the D atom beam for 20 min and then a 20 A˚ thick Pd layer was evaporated on top. According to the D atom uptake curve in Fig. 5, the average thickness of the MgD2 layer formed before Pd deposition is 18 A˚. The D2 TPD spectrum (Fig. 7) now shows a sharp peak at 350 K, which is lower by 240 K than that from a Pd-free MgD2. On the other hand, Mg desorbs in a single peak at somewhat higher temperature of 538 K. The large decrease in the D2 desorption temperature is apparently due to the catalytic effect of the Pd deposited on top of the MgD2(s) layer. The catalytic effect could be due to the weakening of the Mg–D bond by direct electronic
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T.S. Kim et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 253–256
As D2 desorbs at 350 K, a metallic Mg layer is left behind and the Mg atoms in contact with Pd now desorb at 538 K (Fig. 6), 20 K higher than that of a Mg multilayer on SiO2. This is another effect of Pd, indicating an attractive interaction between Mg ad Pd. In the early stage of MgD2(s) decomposition metallic Mg layer formed between Pd and MgD2(s) does not desorb until 538 K is reached. This Mg layer may cause the catalytic effect of Pd to diminish because of the loss of contact between Pd and MgD2(s). In fact, this is not the case because all D2 desorb in a single peak at 350 K. Pd does not form an alloy with Mg. Thus, it seems that the Pd-MgD2(s) contact is somehow maintained until all MgD2(s) decompose. This could be due to concurrent out-diffusion of D atom and in-diffusion of Mg to minimize the total surface free energy. However, we feel that more detailed investigation with better characterization of the film morphologies is needed to clarify these interesting points. 4. Summary Fig. 7. TPD spectra of D2 and Mg from a Mg (100 A˚) film on SiO2 after 20 min. D atom beam dosing followed by deposition of Pd (20 A˚) at room temperature as shown in the inset.
The TPD spectrum of a multilayer Mg film deposited on SiO2 shows a zeroth-order desorption peak at 517 K. The gas-phase D atoms impinging upon this Mg film at room temperature directly penetrate into the bulk with a constant probability of 0.22 until 16 A˚ of Mg is deuterated, whereafter the MgD2(s) growth quickly slows down. Upon heating the partially deuterated Mg film, Mg atoms diffuse out to the surface through the top MgD2(s) layer to desorb at the same temperature as a pure Mg film. On the other hand, MgD2(s) decomposes to result in simultaneous desorption of D2 and Mg at 590 K with an activation energy of 133 5 kJ/mol. A Pd layer of 20 A˚ thickness deposited on top of a deuterated Mg layer acts as a catalyst to greatly reduce the decomposition temperature from 590 to 350 K. We interpret the catalytic effect of Pd in terms of reverse spill-over, in which the surface of Pd clusters in contact with MgD2(s) provides an intermediate adsorption sites with a low activation energy for the recombinative desorption of D atoms. References
Fig. 8. Schematic potential energy diagrams showing Pd-catalyzed decomposition of MgDx(s) in two steps, where M represents Pd or Mg.
interactions with Pd in contact. However, we note that hydrogen atoms adsorbed on the Pd (1 1 0) surface desorb at 300 K [15], which is close to the peak temperature of 350 K observed for Pdcapped MgD2. This suggests that D2 desorption occurs in two steps via an intermediate state as schematically shown by the potential diagrams in Fig. 8. D atoms first migrate onto the Pd surface by the reaction MgD2 + Pd ! Mg + D(ad)/Pd, whereby the recombinative desorption of D2, 2D(ad) ! D2(g), subsequently occurs at the Pd surface. One can think the Pd-catalyzed desorption of D2 as a reverse spill-over, in which Pd acts as an intermediate site with a low activation energy for D2 desorption.
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L. Schlapbach, A. Zuettel, Nature 414 (2001) 353. G. Sandrock, J. Alloy Compd. 293 (1999) 877. Y. Song, Z.X. Guo, R. Yang, Phys. Rev. B 69 (2004) 094205. A. Zaluska, L. Zaluski, J.O. Stro¨m–Olsen, J. Alloy Compd. 288 (1999) 217. G. Liang, J. Huot, S. Boily, A. Van Neste, R. Schulz, J. Alloy Compd. 292 (1999) 247. K.-F. Aguey-Zinsou, J.R. Ares Fernandez, T. Klassen, R. Bormann, MRS Bull. 41 (2006) 1118. J.Y. Kim, J. Lee, J. Chem. Phys. 113 (2000) 2856. Hansen, Vogl, Phys. Rev. B 57 (1998) 13295. W. Oelerich, T. Klassen, R. Bormann, J. Alloy Compd. 315 (2001) 237. J.F. Ferna´ndez, C.R. Sa´nchez, J. Alloy Compd. 356 (2003) 348. V.V. Boldyrev, M. Bulens, B. Delmon, Studies in Surface Science and Catalysis, Elsevier, 1979. J.K. Nørskov, A. Houmøller, P.K. Johansson, B.I. Lundgvist, Phys. Rev. Lett. 46 (1981) 257. Th. Kammler, J. Ku¨ppers, J. Chem. Phys. 111 (1999) 8115. V.P. Zhdanov, A. Krozer, B. Kasemo, Phys. Rev. B 47 (1993) 11044. R.J. Behm, V. Penka, M.G. Cattania, K. Christmann, G. Ertl, J. Chem. Phys. 78 (1983) 7486.
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Absorption of D(g) atoms in ultrathin Mg films and Pd-catalyzed decomposition of MgD2 Tae Seung Kim, Jun Woo Song, Seung June Lee, Jihwa Lee * School of Chemical and Biological Engineering, Seoul National University, Silim san 56-1, Gwanak-gu, Seoul 151-744, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 August 2008 Accepted 8 September 2008
We have investigated the interaction of the gas-phase D atoms with ultrathin Mg films thermally evaporated on SiO2 by temperature programmed desorption. The gas-phase D atoms impinging upon a Mg (100 A˚) film directly penetrate into the bulk with a constant probability of 0.22 until the top 16 A˚ is deuterated, whereafter the MgD2(s) growth quickly slows down. Upon heating the partially deuterated Mg film, Mg atoms diffuse out to the surface to desorb at the same temperature as a pure Mg film. On the other hand, MgD2(s) decomposes to result in simultaneous desorption of D2 and Mg at 590 K with an activation energy of 133 5 kJ/mol. A Pd layer of 20 A˚ thickness deposited on top of MgD2(s) acts as a catalyst to greatly reduce the decomposition temperature from 590 to 350 K. We interpret the catalytic effect of Pd in terms of reverse spill-over, in which the surface of Pd clusters in contact with MgD2(s) provides an intermediate adsorption sites with a low activation energy for the recombinative desorption of D atoms. ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: D atom beam MgD2(s) Temperature programmed desorption Zeroth-order desorption Pd catalyst
1. Introduction High prospects for hydrogen fuel-cell-powered vehicles have stimulated much research on the hydrogen storage problem in recent years [1]. Extensive investigations devoted to a variety of metal hydrides for many decades have made much progress [2]. Among many metals, Mg is still attracting much attention because its theoretical gravimetric storage density of 7.8% is large enough to be a practical storage medium and also Mg is an abundant element in the earth crust. However, Mg has two major drawbacks; the heat of formation of MgH2 is large (72.0 kJ/mol) [3] and therefore the dehydriding process usually requires a high temperature of 350 8C. Furthermore, magnesium hydride acts as a diffusion barrier for hydrogen atom into the interior of Mg powder, which also requires a high pressure. In order to overcome these problems, various approaches have been investigated. Notably, Mg nanoparticles [4] prepared by ball milling together with various catalysts such as transition metals [5] and metal oxides [6] have been studied to show much improvements of the hydriding as well as dehydriding kinetics. Most studies made so far were mainly concerned with preparation and characterization of the materials along with the kinetics measurements. Therefore, the catalytic mechanisms are still poorly understood.
* Corresponding author. Tel.: +82 2 880 7076. E-mail address: [email protected] (J. Lee).
In this paper, we report results of experimental studies on the absorption of gas-phase D atom in ultrathin magnesium films and the decomposition properties of the MgD2(s) by temperature programmed desorption (TPD). The use of a hydrogen atom beam allows one to easily hydrogenate a Mg film due to its high reactivity towards Mg so that one can study the dehydriding process using UHV techniques. The effects of a very thin Pd layer deposited on top of MgD2(s) on the decomposition of MgD2(s) have also been studied. We will show that the Pd layer greatly reduces the decomposition temperature of MgD2(s) from 590 to 350 K, which we interpret in terms of reversible spill-over. 2. Experiment The UHV system (Fig. 1) employed for the current study has been described elsewhere [7]. Briefly, it is a 50 l stainless steel chamber pumped by a 520 l turbomolecular pump, in which the base pressure is 1 1010 Torr. It is equipped with AES, LEED, and TPD capabilities. It is also equipped with a differentially pumped hydrogen atom beam source, a W capillary tube of 1 mm in diameter heated to 1900 K by electron bombardment. The sticking probability of thermal D atom at 1900 K on Si(1 0 0) is not known but Hausen and Vogl [8] have estimated it to be 0.4 by molecular dynamics calculations. Taking this value, the D atom flux of at the sample surface was estimated to be 4.0 1013 atoms/cm2 s based on the uptake rate of D atoms on a clean Si(1 0 0) surface separately measured by TPD.
1226-086X/$ – see front matter ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2008.09.008
T.S. Kim et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 253–256
254
Fig. 2. TPD spectrum of 38 ML Mg deposited on SiO2.
Fig. 1. Schematic diagram of the experimental ultrahigh vacuum system.
Si(1 0 0) wafers with 1000 A˚ SiO2 layer on top grown by a standard wet oxidation process and cut into rectangular pieces of 1.2 cm 1.8 cm were used as samples. Pt(1 1 1) was also used as a sample to study its catalytic effect on the decomposition of MgD2. The sample mounted on a rotatable XYZ manipulator could be cooled with liquid nitrogen down to 90 K and resistively heated to 1300 K. Pd was evaporated in situ by e-beam evaporation and Mg by thermal evaporation of Mg ribbon mounted on a pair of W wires at rates of 0.48 and 0.115 A˚/s, respectively. The deposition rate of Pd and Mg were monitored with a quartz crystal microbalance. In TPD the sample temperature was linearly ramped at a heating rate of 2 K/s using a PID controller and a programmable dc power supply. The QMS was differentially pumped using a 50 l/s turbomolecular pump. It has a protruding cylindrical aperture of 4.5 mm in diameter. All the TPD measurements were made with the sample very close to the aperture of the QMS so as to detect molecules desorbing only from the front surface of the sample. D2 desorption signals were calibrated against that from saturated (2 1) H: Si(1 0 0) surface. 3. Results and discussion 3.1. Absorption of D(g) atom in Mg films and decomposition of MgD2 In Fig. 2 is shown the TPD spectrum of 100 A˚ thick Mg film deposited at 300 K on SiO2. A 100 A˚ Mg film corresponds to 38 monolayers (ML) (1 ML = 1.12 1015 atoms/cm2) of hexagonally close-packed Mg (0 0 0 1) planes. The Mg multilayer desorbs to result in an asymmetric peak at 517 K. The exponentially increasing leading edge followed by a sharp decrease in the signal indicates a zeroth-order desorption as expected for a multilayer Mg film. In contrast to a pure Mg film, the TPD spectra of 100 A˚ Mg films exposed to the D atom beam for various times show a new Mg peak at 590 K in addition to the one at 517 K (Fig. 3a). D2 desorption peak is also observed (Fig. 3b), and its peak temperature of 590 K exactly coincides with the new Mg peak in Fig. 3a. Furthermore, as the D atom exposure time increases, the intensity of the Mg peak at 517 K decreases, while those of D2 as well as Mg peaks at 590 K concomitantly increase. This correlation clearly indicates that the simultaneous desorption of D2 and Mg at 590 K occurs by the decomposition of MgD2(s). A small shoulder peak of Mg at 650 K is
Fig. 3. TPD spectra of (a) D2 and (b) Mg desorbing from a Mg(100 A˚) film on SiO2 for various D atom beam dosing times.
also observed, which is probably due to the decomposition of magnesium silicate formed at the Mg–SiO2 interface [9]. The D2 desorption peaks for various exposure times have a common leading edge, suggesting that the decomposition of MgD2(s) obeys a zeroth-order kinetics. The coverage-independent zeroth-order desorption rate can be expressed as R = n exp(Ed/ RT) with the pre-exponential factor n and the activation energy of desorption Ed. The Arrhenius plot of the leading edge shown in Fig. 4 is quite linear, and from the slope of the plot we estimate the
T.S. Kim et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 253–256
255
Fig. 6. Proposed morphology of partially deuterated Mg(100 A˚) film showing a nonuniform polycrystalline film and enhanced deuteration along the grain boundaries.
Fig. 4. Arrhenius plot of the leading edge of the D2 TPD spectra shown in Fig. 3b.
activation energy of Ea = 133 5 kJ/mol for decomposition of MgD2(s). This is in agreement with the previously reported values ranging from 120 to 160 kJ/mol [10]. By comparing the integrated D2 desorption signals with that from a (2 1) H/Si(1 0 0) surface at saturation, we can tell how much MgD2(s) has formed. In Fig. 5 is plotted the average thickness of the deuterated Mg layer as a function of the D atom fluence. The growth rate of MgD2 is fairly linear until on average 6 ML of Mg (equivalently 16 A˚) is deuterated and thereafter it quickly slows down. The reaction probability of D atom in the linear growth regime can be estimated to be 0.22. The probability smaller than unity can be attributed to the reflection of impinging D(g) atoms. 3.2. The initial growth mode of MgD2(s) Hydrogenation of a bulk Mg under a high pressure proceeds by nucleation followed by grain growth, in which a sigmoidal uptake curve with an induction period is usually observed [11]. The linear uptake of D atoms up to the deuteration of 6 ML of Mg (Fig. 5) suggests that MgD2 formation using a D atom beam does not seem to follow the same kinetics at least in the initial stage. Rather, we believe that the entire surface layer of Mg are deuterated because D(g) atom readily adsorbs on Mg in contrast to D2 molecules for which the sticking probability is very small due to an activation barrier for dissociation [12]. In addition, direct penetration of the
incident D atoms is expected based on the following argument. Hydrogen atom is known to penetrate into the subsurface region of Cu (1 1 1) [13]. The nearest neighbor distance in Mg metal is 3.21 A˚, while that in Cu is 2.56 A˚. This being considered, direct penetration of the impinging D(g) atoms into a Mg film would occur more efficiently than in Cu. Thus, the constant initial MgD2 growth rate observed here can be ascribed to a finite penetration depth of the impinging D(g) atoms, which is approximately about 15 A˚. Once the thickness of the MgD2(s) becomes comparable to the penetration depth of the D(g) atom, further growth will be controlled by the diffusion of D atoms, a slow process at the sample temperature of 300 K employed [14]. 6 ML of Mg layer corresponds to a thickness of 16 A˚. Then, it is difficult to imagine how the impinging D(g) atoms can penetrate so deeply into the Mg layer. Other factors which can influence the growth rate such as the morphology of the Mg layer have to be considered. The Mg film deposited on SiO2 is expected to grow by Volmer–Weber mode, namely 3D island formation followed by grain growth, to result in a polycrystalline film with non-uniform thickness and many grain boundaries. Penetration depth of the impinging D(g) atoms as well as the diffusion rate of the absorbed D atoms will be enhanced through the grain boundaries. A possible morphology of the deuterated Mg film is schematically depicted in Fig. 6. In Fig. 3 we have shown that metallic Mg desorbs at 517 K from a partially deuterated Mg film while MgD2(s) decomposes later at 590 K to result in simultaneous desorption of Mg and D2. The growth mode of MgD2(s) we proposed above seems to be in contradiction to this observation, for the growth mode implies that metallic Mg underneath the MgD2(s) layer has to diffuse out to the surface before desorption. Nevertheless, we think the out-diffusion of Mg is possible at 500 K mainly through the grain boundaries as explained above. 3.3. Catalytic decomposition of MgD2 by Pd
Fig. 5. The thickness variation of the deuterated Mg layers with increasing D atom exposure.
As seen in Fig. 3, MgD2 decomposes at fairly high temperate of 590 K, which is one of the major disadvantages for using Mg as a practical hydrogen storage medium. In order to study the effect of Pd on the decomposition of MgD2, a 100 A˚ Mg/SiO2 was first deuterated with the D atom beam for 20 min and then a 20 A˚ thick Pd layer was evaporated on top. According to the D atom uptake curve in Fig. 5, the average thickness of the MgD2 layer formed before Pd deposition is 18 A˚. The D2 TPD spectrum (Fig. 7) now shows a sharp peak at 350 K, which is lower by 240 K than that from a Pd-free MgD2. On the other hand, Mg desorbs in a single peak at somewhat higher temperature of 538 K. The large decrease in the D2 desorption temperature is apparently due to the catalytic effect of the Pd deposited on top of the MgD2(s) layer. The catalytic effect could be due to the weakening of the Mg–D bond by direct electronic
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T.S. Kim et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 253–256
As D2 desorbs at 350 K, a metallic Mg layer is left behind and the Mg atoms in contact with Pd now desorb at 538 K (Fig. 6), 20 K higher than that of a Mg multilayer on SiO2. This is another effect of Pd, indicating an attractive interaction between Mg ad Pd. In the early stage of MgD2(s) decomposition metallic Mg layer formed between Pd and MgD2(s) does not desorb until 538 K is reached. This Mg layer may cause the catalytic effect of Pd to diminish because of the loss of contact between Pd and MgD2(s). In fact, this is not the case because all D2 desorb in a single peak at 350 K. Pd does not form an alloy with Mg. Thus, it seems that the Pd-MgD2(s) contact is somehow maintained until all MgD2(s) decompose. This could be due to concurrent out-diffusion of D atom and in-diffusion of Mg to minimize the total surface free energy. However, we feel that more detailed investigation with better characterization of the film morphologies is needed to clarify these interesting points. 4. Summary Fig. 7. TPD spectra of D2 and Mg from a Mg (100 A˚) film on SiO2 after 20 min. D atom beam dosing followed by deposition of Pd (20 A˚) at room temperature as shown in the inset.
The TPD spectrum of a multilayer Mg film deposited on SiO2 shows a zeroth-order desorption peak at 517 K. The gas-phase D atoms impinging upon this Mg film at room temperature directly penetrate into the bulk with a constant probability of 0.22 until 16 A˚ of Mg is deuterated, whereafter the MgD2(s) growth quickly slows down. Upon heating the partially deuterated Mg film, Mg atoms diffuse out to the surface through the top MgD2(s) layer to desorb at the same temperature as a pure Mg film. On the other hand, MgD2(s) decomposes to result in simultaneous desorption of D2 and Mg at 590 K with an activation energy of 133 5 kJ/mol. A Pd layer of 20 A˚ thickness deposited on top of a deuterated Mg layer acts as a catalyst to greatly reduce the decomposition temperature from 590 to 350 K. We interpret the catalytic effect of Pd in terms of reverse spill-over, in which the surface of Pd clusters in contact with MgD2(s) provides an intermediate adsorption sites with a low activation energy for the recombinative desorption of D atoms. References
Fig. 8. Schematic potential energy diagrams showing Pd-catalyzed decomposition of MgDx(s) in two steps, where M represents Pd or Mg.
interactions with Pd in contact. However, we note that hydrogen atoms adsorbed on the Pd (1 1 0) surface desorb at 300 K [15], which is close to the peak temperature of 350 K observed for Pdcapped MgD2. This suggests that D2 desorption occurs in two steps via an intermediate state as schematically shown by the potential diagrams in Fig. 8. D atoms first migrate onto the Pd surface by the reaction MgD2 + Pd ! Mg + D(ad)/Pd, whereby the recombinative desorption of D2, 2D(ad) ! D2(g), subsequently occurs at the Pd surface. One can think the Pd-catalyzed desorption of D2 as a reverse spill-over, in which Pd acts as an intermediate site with a low activation energy for D2 desorption.
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