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oxide interfaces by isotope-labeling technique
Solid State Ionics 172 (2004) 93 – 99 www.elsevier.com/locate/ssi
Comparison of catalytic activity for CH4 decomposition at the metal/oxide interfaces by isotope-labeling technique Teruhisa Horita *, Katsuhiko Yamaji, Tohru Kato, Natsuko Sakai, Harumi Yokokawa National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, Japan Received 9 November 2003; received in revised form 26 January 2004; accepted 5 February 2004
Abstract The catalytic activity was compared between Ni and Au electrodes on an Y2O3-stabilized ZrO2 (YSZ) substrate. Secondary ion mass spectrometry (SIMS) imaging analysis was applied to visualize the distribution of active parts for the decomposition of CH4 and/or adsorption of reformed gases (H2, O2, CO, CO2, and C) around the metal/oxide interfaces. To analyze the metal/oxide interfaces in a submicrometer level, stripe-shaped metals (Ni and Au) were prepared. Annealing in isotope-labeled gases was conducted at 973 K for 300 s in the mixture of CH4, 18O2, and D2O. SIMS images and line analyses clearly showed the difference of catalytic activity for CH4 decomposition between Ni and Au; a significant carbon deposition was observed at the Ni-stripe surface, whereas almost no C at the Austripe. Isotope oxygen exchange (16O/18O exchange) occurred on the Ni-stripe surface, whereas no reaction at the Au-stripe surface. Microstructure change of the stripe metals (both Ni and Au) after label annealing was observed by scanning probe microscope (SPM). D 2004 Elsevier B.V. All rights reserved. PACS: 81.65.Mq; 66.30.D; 66.30.H; 68.49.Sf; 82.47.Ed Keywords: Isotope labeling; Metal/oxide interfaces; CH4 decomposition; SIMS; SOFC
1. Introduction In high-temperature electrochemical devices, such as solid oxide fuel cells (SOFCs) and sensors, the metal electrode/oxide electrolyte interfaces are thought to be active parts for the electrochemical reaction as well as catalytic reaction [1]. Reactant gases are ionized with exchanging the electrons, and they are transported into solid materials around the metal/oxide interfaces. Hence, it is important to optimize the metal/oxide interface structures for improving the performance of these devices. In order to fabricate an optimum metal/oxide interfaces, the nature of metal and oxide should be clarified in terms of reactivity with gases and diffusivity of reacted species. The reactivity of metal/oxide interfaces with gases has been normally investigated by the electrochemical methods with changing the applied voltages, atmospheres, and temperatures. Although many possible reaction models have been suggested by the electrochemical methods, there remain uncertainties regarding the surface information of solid materials around the metal/oxide interfaces. This study aims to show the * Corresponding author. Tel.: +81-29-861-4542; fax: +81-29-861-4540. E-mail address: [email protected] (T. Horita). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.02.033
reactivity of metals and oxides with atmospheric gases by the labeling method. We have already reported the application of secondary ion mass spectrometry (SIMS) technique to analyze the electrochemical reaction active parts at the electrode/oxide interfaces in a labeled oxygen (18O2) atmosphere [2– 4]. By using stable isotope oxygen (18O2), the active parts for the electrochemical reaction were clearly visualized around the metal/oxide interfaces in a ‘‘frozen’’ state, with a resolution of submicrometer level. In this paper, we will report the application of this technique to observe the catalytic activity for CH4 decomposition and adsorption of the reformed gases [5]. When CH4 is introduced to the metal/oxide interfaces at high temperature (873 –1073 K), CH4 will be decomposed, and some of CH4 will be reformed with H2O in the following reactions: CH4 ! C þ 2H2
ð1Þ
CH4 þ H2 O ¼ CO þ 3H2
ð2Þ
CO þ H2 O ¼ CO2 þ H2
ð3Þ
We want to clarify the position in which the above reactions are active around the metal/oxide interfaces. To
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T. Horita et al. / Solid State Ionics 172 (2004) 93–99
observe the movements of H and O, stable isotopes D2O and 18 O were adopted to label the reaction parts for the gases. The samples were annealed in the mixture of CH4, 18O2, and D2O to simulate the steam reforming with low H2O content. In order to determine the metal/oxide interface in submicrometer level, stripe-shaped metal electrodes were adopted on the oxide electrolytes. Also, when the above reaction occurs, the metal surface may be changed significantly by the reaction with atmospheric gases to form oxides and carbides. Thus, the surface microstructure change was investigated precisely by the surface probe microscopy (SPM) before and after the CH4 – 18O –D2O treatments. In this report, the catalytic activity of CH4 decomposition was compared between Ni and Au on the Y2O3-stabilized ZrO2 substrate.
2. Experiments 2.1. Samples Fig. 1 shows the schematic diagram (top) and optical microstructure images (bottom) of stripe electrode/Y2O3stabilized ZrO2 (YSZ) samples. The substrate was a sintered polycrystal YSZ, whose density up to 99% of theoretical. The YSZ surface was polished to obtain the smooth and flat surface by a diamond paste (up to 1/4 Am grade). Ni- and Au-stripe electrodes were prepared by the RF-sputtering techniques on the sintered oxide substrates. We have chosen
nickel (Ni) and gold (Au) as test electrodes, because Ni is a typical metal for anode materials for SOFCs [6 – 8], and Au is a typical standard electrode material for electrochemical analysis [9]. The stripe size is 0.5 mm wide and 3 mm long. The thickness is about 1 Am. In the bottom part of Fig. 1, the brighter parts are the stripe electrode and dark parts indicate the YSZ substrate. As shown in the figure, the stripe electrodes/YSZ interfaces are clear and distinct boundaries without any voids. 2.2. Annealing in CH4 – D2O – 18O2 (isotope labeling) The gas mixture of CH4, D2O, and 18O2 was examined to simulate the steam reforming and direct introduction of CH4 in SOFCs. The total gas pressure was 0.5 bar, and the partial pressure of the mixed gases was as follows: p(CH4)/p(D2O)/ p(18O2) = 0.45/0.02/0.03 bar. D2O and 18O2 were used to label the movements of hydrogen and oxygen, respectively. The labeled gases introduction was conducted at 973 K for 300 s. The thermodynamic equilibrium oxygen partial pressure is around 10 25 bar, and the reformed gas composition is calculated by the equilibrium calculation, as follows: p(O2) = 5.86 10 25 bar, p(H2) = 4.32 10 1 bar, p(H2O) =8.8210 3 bar, p(CO) = 3.2310 2 bar, p(CO2) = 1.06 10 3 bar, p(CH4) = 2.58 10 2 bar. This oxygen partial pressure is low enough to deposit carbon in the rector. 2.3. SIMS imaging analysis and scanning probe microscope (SPM) analysis After annealing in CH4 – D2O – 18O2 atmosphere, the samples were quenched to room temperature within 30 s. This technique enables us to observe a ‘‘frozen state’’ of labeled elements distribution in the samples by secondary ion mass spectrometry (SIMS, CAMECA ims-5f ) in its imaging mode. The primary ion beam was Cs+ (acceleration voltage: 10 kV; primary beam current: less than 1 nA; beam diameter: 0.2 Am), and the distribution of negative secondary ions (1H, 2D, 12C, 16O, 18O, Au, and Ni16O) was measured in the area of 100 100 Am. After the SIMS analysis, the depths of the SIMS craters were measured by a surface profiler system (Dektak3, Veeco/Sloan Technology). Precise surface microstructures were examined by the scanning probe microscope (SPM) in its dynamic force mode (DFM; SEIKO Instruments SPA-3000HV). The optically smooth surface of Ni and Au was observed before and after the annealing treatments.
3. Results and discussion 3.1. SIMS images around the Ni/YSZ interfaces at different depths Fig. 1. Schematic diagram of stripe-shaped Ni and Au electrodes on Y2O3stabilized ZrO2 (top) and optical microscope images around the Ni-stripe/ YSZ and Au-stripe/YSZ interfaces (bottom).
Fig. 2 shows SIMS images around the Ni-stripe/YSZ interfaces after the isotope labeling at different depths (at
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Fig. 2. SIMS images around the Ni-stripe/YSZ interfaces at different depths (each image at top: at 0 nm in depth, bottom: at 200 nm in depth).
0 and 200 nm in depth). The brighter parts in the images indicate the higher signal counts of each secondary ion. High concentrations of 1H and 12C are observed on the Ni surface, whereas low concentration on the YSZ surface at the depth of 0 nm (top surface of the sample). The concentration of 2D is so low that we cannot observe the distribution clearly. For the distribution of oxygen, the images of 16O and 18O show relatively high concentration at the Ni as well as YSZ. This suggests that the activity for isotope oxygen exchange (16O/18O exchange) is similar between Ni and YSZ surfaces. At the deeper position (bottom of each image, 200 nm in depth), the images of 1H, 2D, and 12C
show almost no distribution. Therefore, the adsorption of H and C are only at the surface region of Ni less than 200 nm in depth. In order to analyze the SIMS image quantitatively, line analysis was examined around the Ni/YSZ interface at different depths (Fig. 3). At the surface (Fig. 3a), the signals of 1H and 12C are high at the Ni part, whereas those of 16 O and 18O are constant on all the surfaces. The signal counts of 1H at the Ni part are about one order of magnitude higher than those at the YSZ. The signal counts of 12C are about 700 –800 cps at the Ni part, whereas those are 300 – 400 cps at the YSZ. Therefore, the signal counts of 12 C are about two times higher at the Ni than the YSZ.
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Fig. 3. Line analysis of SIMS images around the Ni-stripe/YSZ interfaces at different depths. (a) At the depth of 0 nm, (b): at the depth of 200 nm (.: 1H; n: 2D; D: 12C; x: 16O; j: 18O; : Ni16O).
From the line analysis, Ni surface is active for the reaction with H and C than YSZ. At the depth of 200 nm (Fig. 3b), the signals of 16O, 18O, and 1H show almost homogeneous distribution around the interface. For the distribution of 12C, the higher signal counts are observed at the YSZ than at the Ni. However, the signal counts of 12C are lower than the observed value at surface in Fig. 3a (less than 2 102 cps). 3.2. SIMS images around the Au/YSZ interfaces at different depths To compare the catalytic activity of metals, the Au/YSZ interfaces were analyzed with a similar method. Fig. 4 shows SIMS images around the Au-stripe/YSZ interfaces at
different depths. At the surface (top of each image, at 0 nm in depth), high signal counts of 12C and 18O are observed at the YSZ parts, whereas those are low at the Au part. Thus, the Au surface is low activity for CH4 decomposition and/or the adsorption of the reformed gases. The 16O/18O exchange reaction did not occur on the Au surface because of the lower 18O concentration. At the deeper position (bottom of each image, at 200 nm in depth), the distribution of the above secondary ions is almost the same with the top surface (at 0 nm in depth). Thus, the diffusion of the above elements into solids is considered to be very small. For the distribution of 1H, the SIMS image at the surface (depth = 0 nm) shows homogeneous distribution. Thus, the concentration of adsorbed H is considered to be same level between Au and YSZ surfaces at the depth of 0 nm. To analyze the images quantitatively, line analysis was also examined at the Au/YSZ interfaces (Fig. 5). All the signals, except Au, show the higher intensities at the YSZ surface than the Au surface (Fig. 5a). This suggests the low catalytic activity of Au for CH4 decomposition and low activity for adsorption of H, C, and O. The signal of 2D is too low to detect the distribution. In the line analysis at the deeper position (Fig. 5b), the distribution of elements is essentially the same with the images at the surface (at 0 nm in depth). A significant difference of surface catalytic activity between Ni and Au was observed for the decomposition of CH4 and adsorption of the reformed gases; Ni was active for CH4 decomposition and carbon deposition, whereas Au was inactive for those reactions. With respect to the existing form of carbon, Ni carbides or Ni carbonates can be formed on the surface, although we need more data to confirm this assumption. Also, the distribution of oxygen (16O and 18O) was considerably different between Ni and Au; Ni surface was oxidized and 16O/18O exchange occurred on the surface, whereas almost no reaction on Au surface. The metal/oxide interfaces are very clear in the SIMS images, and the surface diffusion in the lateral direction of oxygen or hydrogen was not detected in the SIMS imaging analysis (almost no diffusion in lateral direction). From these results, the reaction zones at the metal/oxide interfaces are thought to be smaller than the SIMS primary beam diameter (less than 200– 300 nm in diameter). 3.3. Surface microstructure change by SPM The SIMS images clearly show the difference of the catalytic activity of metals. During the reaction with atmospheric gases, the metal surface can be changed by the formation of oxides, carbonates, and carbides. In order to clarify the change of microstructures on the metal surface, surface probe microscope (SPM) images were taken before and after the CH4 – D2O – 18O2 treatments. Fig. 6 shows surface microstructure change of Ni
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Fig. 4. SIMS images around the Au-stripe/YSZ interfaces at different depths (each image at top: at 0 nm in depth, bottom: at 200 nm in depth).
and Au before and after the annealing treatments. Both the metal surfaces were optically smooth and glossy after the annealing. However, the SPM analysis indicates the growth of metal particles and the increase in height difference of the metals. The roughness of mean square surface ‘‘rms’’ value can indicate the degree of surface change in SPM images. For the Ni surface, the rms value increases from 3.4 to 9.2 nm, which suggests the increase of height difference of Ni surface. This increase can be related to the gas adsorption on Ni surface and/or the reaction with gases to form oxide, carbonates, and carbide. For the Au surface, the rms value increases from 1.5 to 50 nm. A significant increase of height
difference is due to the sintering of Au particles by annealing.
4. Conclusion Catalytic activities for CH4 decomposition and reformed gases reaction were compared at the stripe-shaped Ni and Au on YSZ by the SIMS imaging analysis. The isotopelabeling technique has been applied to visualize the distribution of reacted species, such as H, C, and O. In the mixtures of CH4 + D2O + 18O2, the samples were annealed at 973 K, and SIMS analysis was examined for the quenched
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Fig. 5. Line analysis of SIMS images around the Au-stripe/YSZ interfaces at different depths. (a) At the depth of 0 nm, (b): at the depth of 200 nm (.: 1H; n: 2 D ; D: 12C; x: 16O; j: 18O; : Au).
Fig. 6. Scanning probe microscope images for Ni and Au before and after the annealing treatments.
T. Horita et al. / Solid State Ionics 172 (2004) 93–99
samples. Ni was active for CH4 decomposition and reaction with the reformed gases, whereas Au was inactive for those reactions. Carbon deposition was significant on the Ni surface as well as 16O/18O exchange. The Au surface was inactive for the reaction with C and O. The microstructure change was examined by SPM. Ni surface was changed with the reaction of atmospheric gases, presumably by the formation of carbide and oxides. The Au surface was changed by sintering during annealing.
Acknowledgements A part of this study was supported by the Industrial Technology Research Grant Program in 2002 – 2003 from New Energy and Industrial Technology Development Organization (NEDO) of Japan. The corresponding author is grateful to the organization.
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References [1] N.Q. Minh, J. Am. Ceram. Soc. 76 (1993) 563. [2] T. Horita, K. Yamaji, N. Sakai, M. Ishikawa, H. Yokokawa, T. Kawada, T. Kato, J. Electrochem. Soc. 145 (9) (1998) 3196 – 3202. [3] T. Horita, K. Yamaji, N. Sakai, H. Yokokawa, T. Kawada, T. Kato, Solid State Ionics 127 (2000) 55. [4] T. Horita, K. Yamaji, N. Sakai, Y. Xiong, T. Kato, H. Yokokawa, T. Kawada, J. Power Sources 106 (2002) 224. [5] T. Horita, K. Yamaji, T. Kato, N. Sakai, H. Yokokawa, Solid State Ionics 169 (2004) 105 – 113. [6] H. Itoh, T. Yamamoto, M. Mori, T. Horita, N. Sakai, H. Yokokawa, M. Dokiya, J. Electrochem. Soc. 144 (1997) 641. [7] Y. Jiang, A.V. Virkar, J. Electrochem. Soc. 148 (7) (2001) A706. [8] S.P. Jiang, Y. Duan, J.G. Love, J. Electrochem. Soc. 149 (9) (2002) A1175. [9] B.A. van Hassel, B.A. Boukamp, A.J. Burggraaf, Solid State Ionics 48 (1991) 155.
Comparison of catalytic activity for CH4 decomposition at the metal/oxide interfaces by isotope-labeling technique Teruhisa Horita *, Katsuhiko Yamaji, Tohru Kato, Natsuko Sakai, Harumi Yokokawa National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, Japan Received 9 November 2003; received in revised form 26 January 2004; accepted 5 February 2004
Abstract The catalytic activity was compared between Ni and Au electrodes on an Y2O3-stabilized ZrO2 (YSZ) substrate. Secondary ion mass spectrometry (SIMS) imaging analysis was applied to visualize the distribution of active parts for the decomposition of CH4 and/or adsorption of reformed gases (H2, O2, CO, CO2, and C) around the metal/oxide interfaces. To analyze the metal/oxide interfaces in a submicrometer level, stripe-shaped metals (Ni and Au) were prepared. Annealing in isotope-labeled gases was conducted at 973 K for 300 s in the mixture of CH4, 18O2, and D2O. SIMS images and line analyses clearly showed the difference of catalytic activity for CH4 decomposition between Ni and Au; a significant carbon deposition was observed at the Ni-stripe surface, whereas almost no C at the Austripe. Isotope oxygen exchange (16O/18O exchange) occurred on the Ni-stripe surface, whereas no reaction at the Au-stripe surface. Microstructure change of the stripe metals (both Ni and Au) after label annealing was observed by scanning probe microscope (SPM). D 2004 Elsevier B.V. All rights reserved. PACS: 81.65.Mq; 66.30.D; 66.30.H; 68.49.Sf; 82.47.Ed Keywords: Isotope labeling; Metal/oxide interfaces; CH4 decomposition; SIMS; SOFC
1. Introduction In high-temperature electrochemical devices, such as solid oxide fuel cells (SOFCs) and sensors, the metal electrode/oxide electrolyte interfaces are thought to be active parts for the electrochemical reaction as well as catalytic reaction [1]. Reactant gases are ionized with exchanging the electrons, and they are transported into solid materials around the metal/oxide interfaces. Hence, it is important to optimize the metal/oxide interface structures for improving the performance of these devices. In order to fabricate an optimum metal/oxide interfaces, the nature of metal and oxide should be clarified in terms of reactivity with gases and diffusivity of reacted species. The reactivity of metal/oxide interfaces with gases has been normally investigated by the electrochemical methods with changing the applied voltages, atmospheres, and temperatures. Although many possible reaction models have been suggested by the electrochemical methods, there remain uncertainties regarding the surface information of solid materials around the metal/oxide interfaces. This study aims to show the * Corresponding author. Tel.: +81-29-861-4542; fax: +81-29-861-4540. E-mail address: [email protected] (T. Horita). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.02.033
reactivity of metals and oxides with atmospheric gases by the labeling method. We have already reported the application of secondary ion mass spectrometry (SIMS) technique to analyze the electrochemical reaction active parts at the electrode/oxide interfaces in a labeled oxygen (18O2) atmosphere [2– 4]. By using stable isotope oxygen (18O2), the active parts for the electrochemical reaction were clearly visualized around the metal/oxide interfaces in a ‘‘frozen’’ state, with a resolution of submicrometer level. In this paper, we will report the application of this technique to observe the catalytic activity for CH4 decomposition and adsorption of the reformed gases [5]. When CH4 is introduced to the metal/oxide interfaces at high temperature (873 –1073 K), CH4 will be decomposed, and some of CH4 will be reformed with H2O in the following reactions: CH4 ! C þ 2H2
ð1Þ
CH4 þ H2 O ¼ CO þ 3H2
ð2Þ
CO þ H2 O ¼ CO2 þ H2
ð3Þ
We want to clarify the position in which the above reactions are active around the metal/oxide interfaces. To
94
T. Horita et al. / Solid State Ionics 172 (2004) 93–99
observe the movements of H and O, stable isotopes D2O and 18 O were adopted to label the reaction parts for the gases. The samples were annealed in the mixture of CH4, 18O2, and D2O to simulate the steam reforming with low H2O content. In order to determine the metal/oxide interface in submicrometer level, stripe-shaped metal electrodes were adopted on the oxide electrolytes. Also, when the above reaction occurs, the metal surface may be changed significantly by the reaction with atmospheric gases to form oxides and carbides. Thus, the surface microstructure change was investigated precisely by the surface probe microscopy (SPM) before and after the CH4 – 18O –D2O treatments. In this report, the catalytic activity of CH4 decomposition was compared between Ni and Au on the Y2O3-stabilized ZrO2 substrate.
2. Experiments 2.1. Samples Fig. 1 shows the schematic diagram (top) and optical microstructure images (bottom) of stripe electrode/Y2O3stabilized ZrO2 (YSZ) samples. The substrate was a sintered polycrystal YSZ, whose density up to 99% of theoretical. The YSZ surface was polished to obtain the smooth and flat surface by a diamond paste (up to 1/4 Am grade). Ni- and Au-stripe electrodes were prepared by the RF-sputtering techniques on the sintered oxide substrates. We have chosen
nickel (Ni) and gold (Au) as test electrodes, because Ni is a typical metal for anode materials for SOFCs [6 – 8], and Au is a typical standard electrode material for electrochemical analysis [9]. The stripe size is 0.5 mm wide and 3 mm long. The thickness is about 1 Am. In the bottom part of Fig. 1, the brighter parts are the stripe electrode and dark parts indicate the YSZ substrate. As shown in the figure, the stripe electrodes/YSZ interfaces are clear and distinct boundaries without any voids. 2.2. Annealing in CH4 – D2O – 18O2 (isotope labeling) The gas mixture of CH4, D2O, and 18O2 was examined to simulate the steam reforming and direct introduction of CH4 in SOFCs. The total gas pressure was 0.5 bar, and the partial pressure of the mixed gases was as follows: p(CH4)/p(D2O)/ p(18O2) = 0.45/0.02/0.03 bar. D2O and 18O2 were used to label the movements of hydrogen and oxygen, respectively. The labeled gases introduction was conducted at 973 K for 300 s. The thermodynamic equilibrium oxygen partial pressure is around 10 25 bar, and the reformed gas composition is calculated by the equilibrium calculation, as follows: p(O2) = 5.86 10 25 bar, p(H2) = 4.32 10 1 bar, p(H2O) =8.8210 3 bar, p(CO) = 3.2310 2 bar, p(CO2) = 1.06 10 3 bar, p(CH4) = 2.58 10 2 bar. This oxygen partial pressure is low enough to deposit carbon in the rector. 2.3. SIMS imaging analysis and scanning probe microscope (SPM) analysis After annealing in CH4 – D2O – 18O2 atmosphere, the samples were quenched to room temperature within 30 s. This technique enables us to observe a ‘‘frozen state’’ of labeled elements distribution in the samples by secondary ion mass spectrometry (SIMS, CAMECA ims-5f ) in its imaging mode. The primary ion beam was Cs+ (acceleration voltage: 10 kV; primary beam current: less than 1 nA; beam diameter: 0.2 Am), and the distribution of negative secondary ions (1H, 2D, 12C, 16O, 18O, Au, and Ni16O) was measured in the area of 100 100 Am. After the SIMS analysis, the depths of the SIMS craters were measured by a surface profiler system (Dektak3, Veeco/Sloan Technology). Precise surface microstructures were examined by the scanning probe microscope (SPM) in its dynamic force mode (DFM; SEIKO Instruments SPA-3000HV). The optically smooth surface of Ni and Au was observed before and after the annealing treatments.
3. Results and discussion 3.1. SIMS images around the Ni/YSZ interfaces at different depths Fig. 1. Schematic diagram of stripe-shaped Ni and Au electrodes on Y2O3stabilized ZrO2 (top) and optical microscope images around the Ni-stripe/ YSZ and Au-stripe/YSZ interfaces (bottom).
Fig. 2 shows SIMS images around the Ni-stripe/YSZ interfaces after the isotope labeling at different depths (at
T. Horita et al. / Solid State Ionics 172 (2004) 93–99
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Fig. 2. SIMS images around the Ni-stripe/YSZ interfaces at different depths (each image at top: at 0 nm in depth, bottom: at 200 nm in depth).
0 and 200 nm in depth). The brighter parts in the images indicate the higher signal counts of each secondary ion. High concentrations of 1H and 12C are observed on the Ni surface, whereas low concentration on the YSZ surface at the depth of 0 nm (top surface of the sample). The concentration of 2D is so low that we cannot observe the distribution clearly. For the distribution of oxygen, the images of 16O and 18O show relatively high concentration at the Ni as well as YSZ. This suggests that the activity for isotope oxygen exchange (16O/18O exchange) is similar between Ni and YSZ surfaces. At the deeper position (bottom of each image, 200 nm in depth), the images of 1H, 2D, and 12C
show almost no distribution. Therefore, the adsorption of H and C are only at the surface region of Ni less than 200 nm in depth. In order to analyze the SIMS image quantitatively, line analysis was examined around the Ni/YSZ interface at different depths (Fig. 3). At the surface (Fig. 3a), the signals of 1H and 12C are high at the Ni part, whereas those of 16 O and 18O are constant on all the surfaces. The signal counts of 1H at the Ni part are about one order of magnitude higher than those at the YSZ. The signal counts of 12C are about 700 –800 cps at the Ni part, whereas those are 300 – 400 cps at the YSZ. Therefore, the signal counts of 12 C are about two times higher at the Ni than the YSZ.
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Fig. 3. Line analysis of SIMS images around the Ni-stripe/YSZ interfaces at different depths. (a) At the depth of 0 nm, (b): at the depth of 200 nm (.: 1H; n: 2D; D: 12C; x: 16O; j: 18O; : Ni16O).
From the line analysis, Ni surface is active for the reaction with H and C than YSZ. At the depth of 200 nm (Fig. 3b), the signals of 16O, 18O, and 1H show almost homogeneous distribution around the interface. For the distribution of 12C, the higher signal counts are observed at the YSZ than at the Ni. However, the signal counts of 12C are lower than the observed value at surface in Fig. 3a (less than 2 102 cps). 3.2. SIMS images around the Au/YSZ interfaces at different depths To compare the catalytic activity of metals, the Au/YSZ interfaces were analyzed with a similar method. Fig. 4 shows SIMS images around the Au-stripe/YSZ interfaces at
different depths. At the surface (top of each image, at 0 nm in depth), high signal counts of 12C and 18O are observed at the YSZ parts, whereas those are low at the Au part. Thus, the Au surface is low activity for CH4 decomposition and/or the adsorption of the reformed gases. The 16O/18O exchange reaction did not occur on the Au surface because of the lower 18O concentration. At the deeper position (bottom of each image, at 200 nm in depth), the distribution of the above secondary ions is almost the same with the top surface (at 0 nm in depth). Thus, the diffusion of the above elements into solids is considered to be very small. For the distribution of 1H, the SIMS image at the surface (depth = 0 nm) shows homogeneous distribution. Thus, the concentration of adsorbed H is considered to be same level between Au and YSZ surfaces at the depth of 0 nm. To analyze the images quantitatively, line analysis was also examined at the Au/YSZ interfaces (Fig. 5). All the signals, except Au, show the higher intensities at the YSZ surface than the Au surface (Fig. 5a). This suggests the low catalytic activity of Au for CH4 decomposition and low activity for adsorption of H, C, and O. The signal of 2D is too low to detect the distribution. In the line analysis at the deeper position (Fig. 5b), the distribution of elements is essentially the same with the images at the surface (at 0 nm in depth). A significant difference of surface catalytic activity between Ni and Au was observed for the decomposition of CH4 and adsorption of the reformed gases; Ni was active for CH4 decomposition and carbon deposition, whereas Au was inactive for those reactions. With respect to the existing form of carbon, Ni carbides or Ni carbonates can be formed on the surface, although we need more data to confirm this assumption. Also, the distribution of oxygen (16O and 18O) was considerably different between Ni and Au; Ni surface was oxidized and 16O/18O exchange occurred on the surface, whereas almost no reaction on Au surface. The metal/oxide interfaces are very clear in the SIMS images, and the surface diffusion in the lateral direction of oxygen or hydrogen was not detected in the SIMS imaging analysis (almost no diffusion in lateral direction). From these results, the reaction zones at the metal/oxide interfaces are thought to be smaller than the SIMS primary beam diameter (less than 200– 300 nm in diameter). 3.3. Surface microstructure change by SPM The SIMS images clearly show the difference of the catalytic activity of metals. During the reaction with atmospheric gases, the metal surface can be changed by the formation of oxides, carbonates, and carbides. In order to clarify the change of microstructures on the metal surface, surface probe microscope (SPM) images were taken before and after the CH4 – D2O – 18O2 treatments. Fig. 6 shows surface microstructure change of Ni
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Fig. 4. SIMS images around the Au-stripe/YSZ interfaces at different depths (each image at top: at 0 nm in depth, bottom: at 200 nm in depth).
and Au before and after the annealing treatments. Both the metal surfaces were optically smooth and glossy after the annealing. However, the SPM analysis indicates the growth of metal particles and the increase in height difference of the metals. The roughness of mean square surface ‘‘rms’’ value can indicate the degree of surface change in SPM images. For the Ni surface, the rms value increases from 3.4 to 9.2 nm, which suggests the increase of height difference of Ni surface. This increase can be related to the gas adsorption on Ni surface and/or the reaction with gases to form oxide, carbonates, and carbide. For the Au surface, the rms value increases from 1.5 to 50 nm. A significant increase of height
difference is due to the sintering of Au particles by annealing.
4. Conclusion Catalytic activities for CH4 decomposition and reformed gases reaction were compared at the stripe-shaped Ni and Au on YSZ by the SIMS imaging analysis. The isotopelabeling technique has been applied to visualize the distribution of reacted species, such as H, C, and O. In the mixtures of CH4 + D2O + 18O2, the samples were annealed at 973 K, and SIMS analysis was examined for the quenched
98
T. Horita et al. / Solid State Ionics 172 (2004) 93–99
Fig. 5. Line analysis of SIMS images around the Au-stripe/YSZ interfaces at different depths. (a) At the depth of 0 nm, (b): at the depth of 200 nm (.: 1H; n: 2 D ; D: 12C; x: 16O; j: 18O; : Au).
Fig. 6. Scanning probe microscope images for Ni and Au before and after the annealing treatments.
T. Horita et al. / Solid State Ionics 172 (2004) 93–99
samples. Ni was active for CH4 decomposition and reaction with the reformed gases, whereas Au was inactive for those reactions. Carbon deposition was significant on the Ni surface as well as 16O/18O exchange. The Au surface was inactive for the reaction with C and O. The microstructure change was examined by SPM. Ni surface was changed with the reaction of atmospheric gases, presumably by the formation of carbide and oxides. The Au surface was changed by sintering during annealing.
Acknowledgements A part of this study was supported by the Industrial Technology Research Grant Program in 2002 – 2003 from New Energy and Industrial Technology Development Organization (NEDO) of Japan. The corresponding author is grateful to the organization.
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