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Effects of acoustic waves generated on a positively polarized lead strontium zirconium titanate substrate upon catalytic activity of a deposited Ag thin film
Solid State Ionics 136–137 (2000) 819–823 www.elsevier.com / locate / ssi
Effects of acoustic waves generated on a positively polarized lead strontium zirconium titanate substrate upon catalytic activity of a deposited Ag thin film N. Saito, Y. Ohkawara, K. Sato, Y. Inoue* Department of Chemistry, Nagaoka University of Technology, Nagaoka, Niigata 940 -2188, Japan
Abstract A positively polarized lead strontium zirconium titanate (PSZT) substrate was employed for the generation of thickness-extensional mode resonance oscillation (TERO), and the effects of TERO on the catalytic activity and the surface properties of a 100-nm Ag film catalyst deposited on the substrate were investigated. The catalytic activity for ethanol oxidation increased 18-fold with TERO at 3 W. In low energy photoelectron spectroscopy, a threshold energy for photoelectric emission from the Ag surface shifted linearly to the lower energy side with increasing power of TERO, thus indicating a decrease in the work function of the Ag surface. On the basis of the behavior of lattice displacement measured by a laser Doppler method, a model for changes in catalytic activity and work function is proposed. 2000 Elsevier Science B.V. All rights reserved. Keywords: Thickness-extensional mode resonance oscillation; Ag thin film; Catalytic activity
1. Introduction Surface acoustic waves and resonance oscillations generated by a piezoelectric effect have been applied to thin film catalysts deposited on ferroelectric substrates in an attempt to design heterogeneous catalysts which have artificially controllable functions for chemical reactions [1–8]. A single crystal of z-cut lithium niobate and a polycrystalline crystal of lead strontium zirconium titanates (PSZT) have been employed as ferroelectric substrate for the generation of resonance oscillation. The catalytic *Corresponding author. Tel.: 1 81-258-479-832; fax: 1 81258-479-830. E-mail address: [email protected] (Y. Inoue).
activity of Pd thin films for ethanol oxidation increased 1900-fold by thickness-extensional mode resonance oscillation (TERO) of z-cut LiNbO 3 [9]. Furthermore, TERO caused an increase in the activity for ethylene production without enhancement of the activity for acetaldehyde production in ethanol decomposition on a Ag thin film, thus indicating that TERO has the capability to change reaction selectivity [10]. A ferroelectric PSZT crystal has the advantage of offering two different vibrations of TERO and radialextensional mode resonance oscillation (RERO). An interesting feature is that there are clear differences in catalyst activation between the two modes of resonance oscillation [11]. We have investigated changes in the surface properties of Ag with RERO
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00506-3
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N. Saito et al. / Solid State Ionics 136 – 137 (2000) 819 – 823
and TERO [11]. In low energy photoelectron spectroscopy (LEPS), RERO caused no changes in the photoelectron emission pattern, whereas TERO induced a negative shift of photoelectron emission pattern for Ag deposited on a positively polarized PSZT. This phenomenon is very important to understand the acoustic wave excitation mechanism, since the shift is related to a change in the work function of the Ag surface. Thus, in the present study, in order to reveal the TERO effect upon a Ag surface deposited on a positively polarized PSZT substrate, detailed LEPS measurements have been performed, together with laser Doppler measurements to evaluate lattice displacement caused by TERO.
spectrometer (Riken Keiki, AC-1) was composed of a deuterium lamp, a controller, and a gridded air counter which can quench the counter discharge and suppress positive ion bombardment. The Ag catalyst sample was irradiated with monochromatized light of the deuterium lamp scanned in the range 200–360 nm, and emitted electrons were counted. Measurements were done in air, dried air or oxygen atmosphere. The excitation energy is nearly proportional to (CPS)a where CPS represents the yield of photoelectron emission (count per second). Since the sample is metal, a was taken as 0.5.
3. Results 2. Experimental A poled ferroelectric polycrystalline Pb 0.95 Sr 0.05 Zr 0.53 Ti 0.47 O 3 (referred to here as PSZT) with a spontaneous polarization of 28 mC cm 22 and a Curie temperature of 593 K was employed as substrate. The sample had the form of a disc (25 mm in diameter and 0.2 mm in thickness) whose polarization axis was normal to the surface, thus exposing a positively polarized surface at one plane and a negatively polarized surface at the other. Both planes of the disk were first covered with catalytically inactive Ag paste electrodes for input of high frequency electric power. Then the positively polarized plane was covered with an active Ag film catalyst at a thickness of 100 nm by an evaporation method with resistance heating of a pure Ag metal in high vacuum, whereas the negatively polarized plane was left intact. High frequency electric power was generated from a network analyzer, amplified, and introduced to a sample. Catalyst temperature was monitored by a radiation thermometer through a BaF 2 window and controlled by an outer electric furnace. Catalytic oxidation of ethanol was carried out in a gas-circulating apparatus, and the products were analyzed by a gas chromatograph connected to the reaction system. Electrons in metals are emitted from surfaces when irradiated by light with energy higher than the work function. The photoelectron emission was measured by a LEPS method whose principle and apparatus were reported elsewhere [12]. Briefly, a
Fig. 1 shows ethanol oxidation at 383 K over Ag deposited on a positively polarized PSZT substrate. When acetaldehyde production proceeded steadily, an electric power of 3 W was introduced to generate TERO. An immediate increase in acetaldehyde production occurred, and the enhanced production of acetaldehyde continued until power was turned off. The increased activity returned to an original level with power-off. The ratio of activity with TERO-on
Fig. 1. TERO effects on ethanol oxidation over Ag deposited on a positively polarized PSZT. Ethanol pressure Pe 5 4.0 kPa, oxygen pressure Po 5 4.0 kPa, reaction temperature T 5 383 K, applied power J 5 3 W.
N. Saito et al. / Solid State Ionics 136 – 137 (2000) 819 – 823
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to that with TERO-off showed that the activity increased 18-fold. Fig. 2 shows the photoelectron emission patterns of LEPS with TERO-off and TERO-on. With TEROoff, photoelectrons emitted from the surface were negligible below 4.5 eV, but began to appear at 4.57 eV, above which emitted electrons increased remarkably with increasing photon energy. This result indicates that threshold energy for electron emission was 4.57 eV without TERO. With TERO-on at 1 W, electron emission pattern shifted to a lower photon energy side, and the threshold energy decreased to
Fig. 3. A decrease in threshold energy with TERO power.
Fig. 2. Photoelectron emission pattern at different TERO power. h, power-on; j, power-off. Measurements were carried out in air at room temperature.
4.55 eV. Further negative shifts of the emission pattern occurred with increases in TERO power to 2 and 3 W. Fig. 3 shows the shift of threshold energy as a function of power. The threshold energy remained nearly unchanged at a power of lower than 0.5 W and decreased nearly in proportion to TERO power in the range 0.5–3 W. The threshold energy shift amounted to 2 0.15 eV at 3 W. Fig. 4 shows threshold energy as a function of frequency applied to a sample. No shift of threshold energy occurred at around 10 MHz. With increasing frequency, threshold energy began to gradually decrease, followed by a dramatic decrease at 10.1 MHz, passed through a minimum at 10.2 MHz, and increased sharply with increasing frequency. This PSZT sample had a resonance frequency of 10.2 MHz, and it is to be noted that a frequency corresponding to the appearance of the minimum threshold energy was exactly the same as that of the resonance frequency to generate TERO. Lattice displacement was measured by a laser Doppler method in a direction vertical to the surface. Fig. 5 shows changes in lattice displacement with frequency applied to a sample. Small lattice displacement occurred at around 10 MHz, gradually increased with increasing frequency, passed through a maximum at 10.2 MHz and then decreased steeply.
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4. Discussion
Fig. 4. Changes in threshold energy with TERO frequency. J 5 3 W.
Fig. 5. Lattice displacement vertical to the surface with TERO. J 5 3 W. Measurements were carried out in air at room temperature.
Note that the frequency for maximum displacement was in a good agreement with that observed for the largest threshold energy shift.
As shown in Fig. 3, the LEPS results demonstrated that the threshold energy of electron emission patterns shifted to the lower energy side with increasing power for the generation of TERO. Since the threshold energy corresponds to a parameter related to the work function of the Ag surface, this shift indicates that the work function of Ag decreases linearly with increasing power in the higher power range. The interesting feature is that the work function of the thin film metal surface can be controlled in an artificial manner through TERO. In a previous study on measurements of contact potential difference, TERO produced negative voltages on the Ag surface deposited on a positively polarized PSZT, whereas no changes occurred with RERO [11]. Similar changes in surface potential were observed for Pd deposited on a positively polarized z-cut LiNbO 3 substrate [13]. These results suggest that dynamic lattice movement vertical to the surface is useful for the generation of negative voltages. A mechanism that the combined effects of sonic wave-electron interactions and the direction of the spontaneous polarization axis in PSZT are involved has been proposed [11]. Namely, the interactions of the sonic wave with electrons facilitate the movement, while a strong field due to polarization axis controls the direction of the electron movement. Thus, the positively polarized PSZT surface accumulates electrons at the surface, and hence the Ag film in contact with this plane turns out to produce a negatively charged surface. The negative voltages generated might be pointed out to be responsible for changes in the work function. However, the extent of its effect on work function was calculated to be 0.008 eV or less. This value was too small to explain the observed shift of 0.15 eV. Thus, the observed work function shift is not explained in terms of merely static electric field effects at surface. Laser Doppler measurements showed that the largest lattice displacement vertical to the surface was induced at a frequency at which TERO was generated. Thus, it is suggested that the changes in work functions with TERO are associated with dynamic behavior of lattice displacement at a negatively charged surface. Work function of metals is controlled by two factors: cohesive energy of metal
N. Saito et al. / Solid State Ionics 136 – 137 (2000) 819 – 823
atoms in bulk and a double electric layer formed at a surface [14]. In particular, the latter electric layer plays an important role in transition metals and is determined by the extent of ‘spill-out’ electrons. A model is proposed that the dynamic lattice displacement vertical to the surface has an influence on the density of the ‘spill-out’ electrons so as to reduce a barrier of the double electric layer. Another possibility of work function decrease would be the desorption of a negatively-charged adsorbed species such as oxygen or water with TERO, since LEPS measurements were not performed in vacuum. However, the repetition of measurements produced very good reproducibility, and exactly the same results were obtained when the atmosphere in the measurements was changed to air, dry air or pure oxygen. These results indicate that the contribution of the desorption is small. A rate-determining step of ethanol oxidation on metal catalysts such as Pd and Ag is proposed to be the abstraction of a hydrogen atom from an adsorbed ethanol [4,13]. The previous study using surface acoustic waves has demonstrated that a strongly adsorbed reactant is predominantly influenced by the acoustic wave excitations [6] and that negatively charged surface oxygen, which is produced through strong adsorption, is important in the enhancement of the reaction rate. The kinetic behavior of ethanol oxidation over Pd deposited on a positively polarized LiNbO 3 substrate has shown that the reaction order with respect to oxygen pressure decreases dramatically from 0.5 to 2 0.1 with TERO, thus indicating that TERO induces the strong adsorption of oxygen on the Pd surface [13]. It is likely that a similar situation holds for a Ag surface. Since a decrease in work function facilitates the electron transfer from the metal surface to the adsorbed species [15], TERO permits the formation of strongly adsorbed and negatively charged oxygen at the Ag surface. This rationally explains the catalytic activity enhancement of Ag on a positively polarized PSZT with TERO. As for geometric effects on catalyst activation, it is difficult to consider that the lattice displacement has a direct influence on the bond distance and arrangement of Ag surface atoms, since the displacement was the order of a few nanometres at maxi-
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mum. However, the Ag thin films employed are polycrystalline whose surfaces have many imperfections such as grain boundaries, dislocations, and vacancies. Thus, the possibility of the geometric effects still remains in case that the acoustic wave is concentrated on imperfect sites to such an extent that it influences the arrangement of specific local structures. In conclusion, the TERO effects are characterized by dynamic lattice displacement vertical to the surface and by changes in the work function-related electronic states which are proposed to be responsible for the catalytic activity enhancement.
Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture.
References [1] Y. Inoue, M. Matsukawa, K. Sato, J. Am. Chem. Soc. 111 (1989) 8965. [2] Y. Inoue, M. Matsukawa, K. Sato, J. Phys. Chem. 96 (1992) 2222. [3] Y. Inoue, J. Chem. Soc. Faraday Trans. 90 (1994) 815. [4] Y. Inoue, Y. Watanabe, T. Noguchi, J. Phys. Chem. 99 (1995) 9898. [5] Y. Ohkawara, N. Saito, Y. Inoue, Surf. Sci. 357–358 (1996) 777. [6] H. Nishiyama, M. Shima, N. Saito, Y. Watanabe, Y. Inoue, Faraday Dis. 107 (1997) 425. [7] S. Kelling, T. Mitrelias, Y. Matsumoto, V.P. Ostanin, D.A. King, J. Chem. Phys. 107 (1997) 5609. [8] S. Kelling, S. Cerasari, H.H. Rotermund, G. Ertl, D.A. King, Chem. Phys. Lett. 293 (1998) 325. [9] N. Saito, Y. Ohkawara, Y. Watanabe, Y. Inoue, Appl. Surf. Sci. 121–122 (1997) 343. [10] N. Saito, H. Nishiyama, K. Sato, Y. Inoue, Chem. Phys. Lett. 297 (1998) 72. [11] Y. Ohkawara, N. Saito, K. Sato, Y. Inoue, Chem. Phys. Lett. 286 (1998) 502. [12] H. Kirihata, M. Uda, Rev. Sci. Instrum. 52 (1981) 68. [13] N. Saito, K. Sato, Y. Inoue, Surf. Sci. 417 (1998) 384. [14] N.D. Lang, W. Kohn, Phys. Rev. B 3 (1971) 1215. [15] R.I. Masel, in: Principles of Adsorption and Reaction on Solid Surfaces, John Wiley, New York, 1996, p. 181.
Effects of acoustic waves generated on a positively polarized lead strontium zirconium titanate substrate upon catalytic activity of a deposited Ag thin film N. Saito, Y. Ohkawara, K. Sato, Y. Inoue* Department of Chemistry, Nagaoka University of Technology, Nagaoka, Niigata 940 -2188, Japan
Abstract A positively polarized lead strontium zirconium titanate (PSZT) substrate was employed for the generation of thickness-extensional mode resonance oscillation (TERO), and the effects of TERO on the catalytic activity and the surface properties of a 100-nm Ag film catalyst deposited on the substrate were investigated. The catalytic activity for ethanol oxidation increased 18-fold with TERO at 3 W. In low energy photoelectron spectroscopy, a threshold energy for photoelectric emission from the Ag surface shifted linearly to the lower energy side with increasing power of TERO, thus indicating a decrease in the work function of the Ag surface. On the basis of the behavior of lattice displacement measured by a laser Doppler method, a model for changes in catalytic activity and work function is proposed. 2000 Elsevier Science B.V. All rights reserved. Keywords: Thickness-extensional mode resonance oscillation; Ag thin film; Catalytic activity
1. Introduction Surface acoustic waves and resonance oscillations generated by a piezoelectric effect have been applied to thin film catalysts deposited on ferroelectric substrates in an attempt to design heterogeneous catalysts which have artificially controllable functions for chemical reactions [1–8]. A single crystal of z-cut lithium niobate and a polycrystalline crystal of lead strontium zirconium titanates (PSZT) have been employed as ferroelectric substrate for the generation of resonance oscillation. The catalytic *Corresponding author. Tel.: 1 81-258-479-832; fax: 1 81258-479-830. E-mail address: [email protected] (Y. Inoue).
activity of Pd thin films for ethanol oxidation increased 1900-fold by thickness-extensional mode resonance oscillation (TERO) of z-cut LiNbO 3 [9]. Furthermore, TERO caused an increase in the activity for ethylene production without enhancement of the activity for acetaldehyde production in ethanol decomposition on a Ag thin film, thus indicating that TERO has the capability to change reaction selectivity [10]. A ferroelectric PSZT crystal has the advantage of offering two different vibrations of TERO and radialextensional mode resonance oscillation (RERO). An interesting feature is that there are clear differences in catalyst activation between the two modes of resonance oscillation [11]. We have investigated changes in the surface properties of Ag with RERO
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00506-3
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N. Saito et al. / Solid State Ionics 136 – 137 (2000) 819 – 823
and TERO [11]. In low energy photoelectron spectroscopy (LEPS), RERO caused no changes in the photoelectron emission pattern, whereas TERO induced a negative shift of photoelectron emission pattern for Ag deposited on a positively polarized PSZT. This phenomenon is very important to understand the acoustic wave excitation mechanism, since the shift is related to a change in the work function of the Ag surface. Thus, in the present study, in order to reveal the TERO effect upon a Ag surface deposited on a positively polarized PSZT substrate, detailed LEPS measurements have been performed, together with laser Doppler measurements to evaluate lattice displacement caused by TERO.
spectrometer (Riken Keiki, AC-1) was composed of a deuterium lamp, a controller, and a gridded air counter which can quench the counter discharge and suppress positive ion bombardment. The Ag catalyst sample was irradiated with monochromatized light of the deuterium lamp scanned in the range 200–360 nm, and emitted electrons were counted. Measurements were done in air, dried air or oxygen atmosphere. The excitation energy is nearly proportional to (CPS)a where CPS represents the yield of photoelectron emission (count per second). Since the sample is metal, a was taken as 0.5.
3. Results 2. Experimental A poled ferroelectric polycrystalline Pb 0.95 Sr 0.05 Zr 0.53 Ti 0.47 O 3 (referred to here as PSZT) with a spontaneous polarization of 28 mC cm 22 and a Curie temperature of 593 K was employed as substrate. The sample had the form of a disc (25 mm in diameter and 0.2 mm in thickness) whose polarization axis was normal to the surface, thus exposing a positively polarized surface at one plane and a negatively polarized surface at the other. Both planes of the disk were first covered with catalytically inactive Ag paste electrodes for input of high frequency electric power. Then the positively polarized plane was covered with an active Ag film catalyst at a thickness of 100 nm by an evaporation method with resistance heating of a pure Ag metal in high vacuum, whereas the negatively polarized plane was left intact. High frequency electric power was generated from a network analyzer, amplified, and introduced to a sample. Catalyst temperature was monitored by a radiation thermometer through a BaF 2 window and controlled by an outer electric furnace. Catalytic oxidation of ethanol was carried out in a gas-circulating apparatus, and the products were analyzed by a gas chromatograph connected to the reaction system. Electrons in metals are emitted from surfaces when irradiated by light with energy higher than the work function. The photoelectron emission was measured by a LEPS method whose principle and apparatus were reported elsewhere [12]. Briefly, a
Fig. 1 shows ethanol oxidation at 383 K over Ag deposited on a positively polarized PSZT substrate. When acetaldehyde production proceeded steadily, an electric power of 3 W was introduced to generate TERO. An immediate increase in acetaldehyde production occurred, and the enhanced production of acetaldehyde continued until power was turned off. The increased activity returned to an original level with power-off. The ratio of activity with TERO-on
Fig. 1. TERO effects on ethanol oxidation over Ag deposited on a positively polarized PSZT. Ethanol pressure Pe 5 4.0 kPa, oxygen pressure Po 5 4.0 kPa, reaction temperature T 5 383 K, applied power J 5 3 W.
N. Saito et al. / Solid State Ionics 136 – 137 (2000) 819 – 823
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to that with TERO-off showed that the activity increased 18-fold. Fig. 2 shows the photoelectron emission patterns of LEPS with TERO-off and TERO-on. With TEROoff, photoelectrons emitted from the surface were negligible below 4.5 eV, but began to appear at 4.57 eV, above which emitted electrons increased remarkably with increasing photon energy. This result indicates that threshold energy for electron emission was 4.57 eV without TERO. With TERO-on at 1 W, electron emission pattern shifted to a lower photon energy side, and the threshold energy decreased to
Fig. 3. A decrease in threshold energy with TERO power.
Fig. 2. Photoelectron emission pattern at different TERO power. h, power-on; j, power-off. Measurements were carried out in air at room temperature.
4.55 eV. Further negative shifts of the emission pattern occurred with increases in TERO power to 2 and 3 W. Fig. 3 shows the shift of threshold energy as a function of power. The threshold energy remained nearly unchanged at a power of lower than 0.5 W and decreased nearly in proportion to TERO power in the range 0.5–3 W. The threshold energy shift amounted to 2 0.15 eV at 3 W. Fig. 4 shows threshold energy as a function of frequency applied to a sample. No shift of threshold energy occurred at around 10 MHz. With increasing frequency, threshold energy began to gradually decrease, followed by a dramatic decrease at 10.1 MHz, passed through a minimum at 10.2 MHz, and increased sharply with increasing frequency. This PSZT sample had a resonance frequency of 10.2 MHz, and it is to be noted that a frequency corresponding to the appearance of the minimum threshold energy was exactly the same as that of the resonance frequency to generate TERO. Lattice displacement was measured by a laser Doppler method in a direction vertical to the surface. Fig. 5 shows changes in lattice displacement with frequency applied to a sample. Small lattice displacement occurred at around 10 MHz, gradually increased with increasing frequency, passed through a maximum at 10.2 MHz and then decreased steeply.
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N. Saito et al. / Solid State Ionics 136 – 137 (2000) 819 – 823
4. Discussion
Fig. 4. Changes in threshold energy with TERO frequency. J 5 3 W.
Fig. 5. Lattice displacement vertical to the surface with TERO. J 5 3 W. Measurements were carried out in air at room temperature.
Note that the frequency for maximum displacement was in a good agreement with that observed for the largest threshold energy shift.
As shown in Fig. 3, the LEPS results demonstrated that the threshold energy of electron emission patterns shifted to the lower energy side with increasing power for the generation of TERO. Since the threshold energy corresponds to a parameter related to the work function of the Ag surface, this shift indicates that the work function of Ag decreases linearly with increasing power in the higher power range. The interesting feature is that the work function of the thin film metal surface can be controlled in an artificial manner through TERO. In a previous study on measurements of contact potential difference, TERO produced negative voltages on the Ag surface deposited on a positively polarized PSZT, whereas no changes occurred with RERO [11]. Similar changes in surface potential were observed for Pd deposited on a positively polarized z-cut LiNbO 3 substrate [13]. These results suggest that dynamic lattice movement vertical to the surface is useful for the generation of negative voltages. A mechanism that the combined effects of sonic wave-electron interactions and the direction of the spontaneous polarization axis in PSZT are involved has been proposed [11]. Namely, the interactions of the sonic wave with electrons facilitate the movement, while a strong field due to polarization axis controls the direction of the electron movement. Thus, the positively polarized PSZT surface accumulates electrons at the surface, and hence the Ag film in contact with this plane turns out to produce a negatively charged surface. The negative voltages generated might be pointed out to be responsible for changes in the work function. However, the extent of its effect on work function was calculated to be 0.008 eV or less. This value was too small to explain the observed shift of 0.15 eV. Thus, the observed work function shift is not explained in terms of merely static electric field effects at surface. Laser Doppler measurements showed that the largest lattice displacement vertical to the surface was induced at a frequency at which TERO was generated. Thus, it is suggested that the changes in work functions with TERO are associated with dynamic behavior of lattice displacement at a negatively charged surface. Work function of metals is controlled by two factors: cohesive energy of metal
N. Saito et al. / Solid State Ionics 136 – 137 (2000) 819 – 823
atoms in bulk and a double electric layer formed at a surface [14]. In particular, the latter electric layer plays an important role in transition metals and is determined by the extent of ‘spill-out’ electrons. A model is proposed that the dynamic lattice displacement vertical to the surface has an influence on the density of the ‘spill-out’ electrons so as to reduce a barrier of the double electric layer. Another possibility of work function decrease would be the desorption of a negatively-charged adsorbed species such as oxygen or water with TERO, since LEPS measurements were not performed in vacuum. However, the repetition of measurements produced very good reproducibility, and exactly the same results were obtained when the atmosphere in the measurements was changed to air, dry air or pure oxygen. These results indicate that the contribution of the desorption is small. A rate-determining step of ethanol oxidation on metal catalysts such as Pd and Ag is proposed to be the abstraction of a hydrogen atom from an adsorbed ethanol [4,13]. The previous study using surface acoustic waves has demonstrated that a strongly adsorbed reactant is predominantly influenced by the acoustic wave excitations [6] and that negatively charged surface oxygen, which is produced through strong adsorption, is important in the enhancement of the reaction rate. The kinetic behavior of ethanol oxidation over Pd deposited on a positively polarized LiNbO 3 substrate has shown that the reaction order with respect to oxygen pressure decreases dramatically from 0.5 to 2 0.1 with TERO, thus indicating that TERO induces the strong adsorption of oxygen on the Pd surface [13]. It is likely that a similar situation holds for a Ag surface. Since a decrease in work function facilitates the electron transfer from the metal surface to the adsorbed species [15], TERO permits the formation of strongly adsorbed and negatively charged oxygen at the Ag surface. This rationally explains the catalytic activity enhancement of Ag on a positively polarized PSZT with TERO. As for geometric effects on catalyst activation, it is difficult to consider that the lattice displacement has a direct influence on the bond distance and arrangement of Ag surface atoms, since the displacement was the order of a few nanometres at maxi-
823
mum. However, the Ag thin films employed are polycrystalline whose surfaces have many imperfections such as grain boundaries, dislocations, and vacancies. Thus, the possibility of the geometric effects still remains in case that the acoustic wave is concentrated on imperfect sites to such an extent that it influences the arrangement of specific local structures. In conclusion, the TERO effects are characterized by dynamic lattice displacement vertical to the surface and by changes in the work function-related electronic states which are proposed to be responsible for the catalytic activity enhancement.
Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture.
References [1] Y. Inoue, M. Matsukawa, K. Sato, J. Am. Chem. Soc. 111 (1989) 8965. [2] Y. Inoue, M. Matsukawa, K. Sato, J. Phys. Chem. 96 (1992) 2222. [3] Y. Inoue, J. Chem. Soc. Faraday Trans. 90 (1994) 815. [4] Y. Inoue, Y. Watanabe, T. Noguchi, J. Phys. Chem. 99 (1995) 9898. [5] Y. Ohkawara, N. Saito, Y. Inoue, Surf. Sci. 357–358 (1996) 777. [6] H. Nishiyama, M. Shima, N. Saito, Y. Watanabe, Y. Inoue, Faraday Dis. 107 (1997) 425. [7] S. Kelling, T. Mitrelias, Y. Matsumoto, V.P. Ostanin, D.A. King, J. Chem. Phys. 107 (1997) 5609. [8] S. Kelling, S. Cerasari, H.H. Rotermund, G. Ertl, D.A. King, Chem. Phys. Lett. 293 (1998) 325. [9] N. Saito, Y. Ohkawara, Y. Watanabe, Y. Inoue, Appl. Surf. Sci. 121–122 (1997) 343. [10] N. Saito, H. Nishiyama, K. Sato, Y. Inoue, Chem. Phys. Lett. 297 (1998) 72. [11] Y. Ohkawara, N. Saito, K. Sato, Y. Inoue, Chem. Phys. Lett. 286 (1998) 502. [12] H. Kirihata, M. Uda, Rev. Sci. Instrum. 52 (1981) 68. [13] N. Saito, K. Sato, Y. Inoue, Surf. Sci. 417 (1998) 384. [14] N.D. Lang, W. Kohn, Phys. Rev. B 3 (1971) 1215. [15] R.I. Masel, in: Principles of Adsorption and Reaction on Solid Surfaces, John Wiley, New York, 1996, p. 181.