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Different acoustic wave effects of thickness extension and thickness shear mode resonance oscillation on ethanol decomposition over Pd catalysts deposited on poled ferroelectric LiNbO3 single crystals
Surface Science 532–535 (2003) 359–363 www.elsevier.com/locate/susc
Different acoustic wave effects of thickness extension and thickness shear mode resonance oscillation on ethanol decomposition over Pd catalysts deposited on poled ferroelectric LiNbO3 single crystals Y. Yukawa, N. Saito, H. Nishiyama, Y. Inoue
*
Department of Chemistry, Nagaoka University of Technology, Nagaoka, Niigata 940-2188, Japan
Abstract The vibration mode effects of resonance oscillation (RO) on ethanol decomposition over thin Pd film catalysts were studied using thickness shear mode RO (TSRO) and thickness extension mode RO (TERO). The TSRO accelerated neither ethylene nor acetaldehyde productions, whereas the TERO increased selectivity for ethylene production dramatically. Laser Doppler method showed that the TSRO caused small vertical lattice displacement, which contrasted to large vertical lattice displacement of the TERO. Photoelectron emission spectra showed clear differences in threshold energy shifts between TSRO and TERO: the TSRO induced little threshold energy shift, but the TERO caused marked positive shifts. A mechanism of catalyst activation due to the lattice vibration modes is discussed. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Single crystal surfaces; Catalysis; Lead; Acoustic waves
1. Introduction In aiming at designing a heterogeneous catalyst that has artificially controllable functions, we have employed the resonance oscillation (RO) of bulk acoustic waves generated by applying radio frequency electric power to ferroelectric materials. There are various kinds of the lattice vibration modes in the RO which are determined by the crystal structures and the direction of polarization axis. In previous studies, we have used the thick-
*
Corresponding author. Tel.: +81-258-47-9832; fax: +81258-47-9830. E-mail address: [email protected] (Y. Inoue).
ness extension mode RO (TERO) generated on a ferroelectric z-cut LiNbO3 single crystal and found that the TERO significantly enhanced the catalytic activities for ethanol and CO oxidation of thin metals (Ag, Pd and Ni) as well as metal oxide (NiO and TiO2 ) films deposited on the crystal [1–5]. More recently, we have demonstrated that the TERO has the useful ability to change the selectivity of catalytic reactions: it increased ethylene production markedly without affecting acetaldehyde production in ethanol decomposition on a thin Ag film catalyst. We have also applied thickness shear mode RO (TSRO) to the same catalytic system, but no significant activation of the Ag catalyst was observed [6]. For the different effects of vibration modes on catalyst activation, a question
0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00195-X
360
Y. Yukawa et al. / Surface Science 532–535 (2003) 359–363
arises whether or not differences between TERO and TSRO are intrinsically specific for Ag metal catalyst only. To clarify this, it is important to extend research to other metal catalysts. In the present study, the TSRO and TERO were applied to a thin Pd film catalyst for ethanol decomposition. The lattice displacements of the Pd catalysts in different vibration modes were investigated by a three-dimensional laser Doppler method. The surface properties were examined by photoelectron emission spectroscopy in the presence of the TSRO, and compared with previous results for the TERO.
a stage movable toward x and y direction. A sample surface was irradiated by a He–Ne laser beam, and a reflected Doppler signal was monitored by a vibrometer (Ono sokki LV-1300). Lattice displacement was transferred to three-dimensional x–y–z images. Photoelectron emission was measured in air by a low-energy photoelectron spectroscopic method [7]. A sample surface was irradiated with monochromatized UV light from a deuterium lamp. The wavelength of light was scanned over the range of 230–300 nm, and the numbers of emitted photoelectron were counted against photon energy. The threshold energy for photoelectron emission was compared in the absence and presence of RO.
2. Experimental sections 3. Results In ethanol decomposition on a Pd catalyst, both ethylene and acetaldehyde were produced as major
Activity / µmol h-1
1
(a)
(b)
0.75
0.5
0.25
0 Off 1 Activity / µmol h-1
Two kinds of the poled ferroelectric single crystals were used: one was x-cut LiNbO3 (x-LN) with 0.5 mm in thickness and the other was z-cut LiNbO3 (z-LN) with 1 mm. The two crystals were cut to have a rectangular shape of 44 14 mm. Pd metal was deposited at a thickness of 100 nm on a positively polarized surface of the ferroelectric substrates, and the back plane of a negatively polarized surface was covered with a catalytically inactive Au film (which worked as an electrode only) of the same thickness. The two metal films were prepared by evaporation in vacuum with resistance heating of pure Pd and Au metals, respectively. The prepared catalyst was placed on a quartz glass cell equipped with the BNC terminal for rf electric power introduction. Radio frequency (rf) electric power was generated from a network analyzer (Anritsu MS3606B), amplified, and introduced to the catalyst after impedance adjustment. The primary resonance frequency of 3.6 MHz was employed. Catalytic ethanol decomposition was performed in a gas circulating vacuum apparatus, and reactant and products were analyzed by an on-line gas chromatograph. Catalyst temperature was monitored by the frequency shifts of resonance lines, which were very sensitive to temperature, and controlled by an outer electric furnace within an accuracy of 0.2 K. Lattice displacement was measured by a laser Doppler method. A catalyst sample was placed on
On
Off
(c)
Off
On
Off
On
Off
(d)
0.75
0.5
0.25
0 Off
On
Off
Off
Fig. 1. Effects of TSRO on activity for ethylene (a) and acetaldehyde (b) production and of TERO on activity for ethylene (c) and acetaldehyde (d) production. Tr (for reaction temperature) ¼ 583 K, Pr (for rf power) ¼ 3 W, Pe (for pressure of ethanol) ¼ 4.0 kPa.
Y. Yukawa et al. / Surface Science 532–535 (2003) 359–363
products and their amounts increased in proportion to reaction time. Fig. 1 shows the effects of TSRO and TERO on activity for ethylene and acetaldehyde productions. When the TSRO was generated at 3 W, and the activities for ethylene and acetaldehyde production remained nearly unchanged, indicating that the TSRO has no activation effects. On the other hand, TERO-on at the same power caused a significant increase in the ethylene production: the activity, Ve , increased from 0.04 to 0.94 lmol h1 . The high activity was maintained as long as the TERO was turned on, and lowered to an original level with TERO-off. Interestingly, the enhancement of activity, Va , for acetaldehyde production with TERO-on was quite small (from 0.11 to 0.15 lmol h1 ). To compare the activation effects of the TSRO and TERO, activation coefficient, R (¼ Von =Voff ), was defined as the ratio of catalytic activity (Von ) with power-on
361
to that (Voff ) with power-off. The TERO effects were R ¼ 24 for ethylene production and R ¼ 1:4 for acetaldehyde production. Ethylene selectivity, defined as 100 Ve =ðVe þ Va Þ, increased from 27% to 86% with TERO-on at 3 W. Fig. 2 shows the laser Doppler patterns of Pd when TSRO and TERO were generated at 2 W. For TSRO, no marked changes in the surface morphology were observed over the x–y plane. On the other hand, the pattern of TERO provided very large standing waves distributed randomly over the plane. The waves were normal to the surface, and their amplitudes corresponded to the magnitude of lattice displacement. Fig. 3 shows the distributions of lattice displacement with TSRO and TERO. For the TSRO, the maximum lattice displacement, Lmax , was 6 nm, and the average
(a)
4000
TSRO
N
3000
2000 1000
0 0
10
20
30
40
50
Lattice displacement / nm 200
(b) TERO
N
150
100
50
0 0 Fig. 2. Three-dimensional lattice displacement patterns with TSRO-on (a) and TERO-on (b) obtained by a laser Doppler method. Tm (for temperature of measurements) ¼ RT, Pr ¼ 2 W.
10
20
30
40
50
Lattice displacement / nm Fig. 3. Distributions of lattice displacement with the TSRO (a) and TERO (b). Pr ¼ 2 W.
362
Y. Yukawa et al. / Surface Science 532–535 (2003) 359–363
lattice displacement, Lav , was 2 nm. The value of full width at half maximum (FWHM) was 3 nm. For the TERO, Lmax attained at 43 nm, and Lav was 16 nm. The FWHM was as broad as 20 nm. These results indicate that the TSRO has little component of lattice vibration toward vertical direction, whereas the TERO was characterized by very large vertical lattice displacement. In photoelectron emission measurements, the wavelength of light to irradiate a Pd surface was scanned, and threshold energy at which electrons were evolved from the Pd surface was detected. The threshold energy for the Pd surface was 4.83 eV in the absence of TSRO and TERO. Fig. 4 shows threshold energy shifts as a function of rf power. The TSRO showed the invariance of threshold energy even up to 3 W, although the TERO caused a shift toward
Threshold energy shift ∆Φ / eV
0.15
TERO 0.1
0.05
TSRO
0
Rf Power / W
Fig. 4. Threshold energy shifts of photoelectron emission as a function of rf power with TSRO ( ) and TERO ( ). Tm ¼ RT. The results for TERO were taken from Ref. [11].
higher threshold energy linearly with power as demonstrated previously [11]. Table 1 summarizes changes in the surface properties of Pd and kinetic behavior of ethanol decomposition with TSRO and TERO.
4. Discussion For ethanol decomposition on Pd catalysts, the different vibration modes of RO exhibited contrary results. The TSRO had no effects on the activation of a Pd surface for both ethylene and acetaldehyde productions. On the other hand, the TERO increased the activity for ethylene production markedly but that for acetaldehyde production to a small extent. In a previous study [6], we have examined the TERO and TSRO effects for ethanol decomposition on Ag catalysts and found that the TERO activated the Ag catalysts to be dominantly selective for ethylene production. The temperature dependence of the reaction became smaller with TERO-on, and hence the activation energy for ethylene production decreased from 156 to 115 kJ mol1 , whereas the TSRO had no effects on the activation of the Ag catalysts [12]. Thus, there is a similarity in a trend of catalyst activation between Pd and Ag catalysts with respect to the vibration modes of RO, although the extent of catalyst activation was different. This indicates that the TERO effects are useful for different kinds of the metal catalysts. Laser Doppler measurements showed that the amplitudes of standing waves vertical to the surface were intrinsically different between TSRO and TERO. Since the amplitudes reflect the magnitudes of lattice displacement, it turns out that the TERO has very large vertical lattice displacement:
Table 1 TSRO and TERO effects on Pd catalysts 2W
3W
Lattice displacement/nm
Threshold energy shift/eV
6 43
0.00 þ0.05
Activation coefficient (R) Ethylene
TSRO TERO
1 24
Acetaldehyde 1 1.4
Y. Yukawa et al. / Surface Science 532–535 (2003) 359–363
the largest lattice displacement Lmax reached 43 nm at 2 W, as shown in the distributions of lattice displacement (Fig. 3). Contrary to this, the TSRO retained very small vertical component for the lattice displacement in which Lmax was as small as 6 nm. The photoelectron emission spectra showed that the characteristics of photoelectron emission were also different between TSRO and TERO. The TSRO had little influence on the threshold energy for photoelectron emission, but the TERO caused positive shifts significantly. The threshold energy of photoelectron emission spectra of metal surfaces corresponds to the work function. Table 1 compares changes in surface properties and catalytic activity with TSRO and TERO, and it is evident that the three factors such as catalyst activation, vertical lattice displacement and work function shifts are closely related each other. There is a possibility that vertical lattice displacement induces the distortion of catalyst surface, affecting atom-atom distance and the arrangements of surface metal atoms. However, if one assumes uniform deformation for the lattice displacement of a flat surface, a change in atom-atom distance is roughly calculated to be an extent of order of 105 %, which indicates that the deformation effects are extremely small. According to a jellium model, the work function of transition metals is mainly determined by the density of electrons that spill out from the outermost layer of the metal surface, since the density is responsible for an energy barrier resulting from a surface electric dipole layer [8,9]. To explain increases in work function with TERO-on, a model has been proposed in a previous study based on the assumption that the large vertical lattice displacement has significant influences on the density of ‘‘spill out’’ electrons [10]. The present results that the TSRO with lattice displacement parallel to the surface has negligible effects on work function are in line with the proposed model. It is apparent that the activity and selectivity increases for ethylene production with the TERO are associated with the enhancement of work
363
function, which has strongly weakened interactions between the adsorbed ethanol and the Pd metal surface. This promotes the formation of weakly adsorbed ethanol that favors the dehydration reaction, as shown previously [10]. In a comparison of TERO effects on ethanol decomposition between Pd and Ag film catalysts, the extent of activation energy decrease was smaller for Pd than for Ag, which was associated with smaller lattice displacement and work function shifts for Pd than for Ag [11]. It appears that the electronic structure differences of the metal surfaces are responsible for the differences in TERO effects. In conclusion, the TSRO and TERO effects on the reaction selectivity were different for ethanol decomposition on Pd, and it has been shown that the large vertical lattice displacement of the TERO is useful for catalyst activation of various kinds of metal catalysts. Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research (B) from The Ministry of Education, Science, Sports and Culture. References [1] N. Saito, Y. Ohkawara, Y. Watanabe, Y. Inoue, Appl. Surf. Sci. 121/122 (1997) 343. [2] N. Saito, K. Sato, Y. Inoue, Surf. Sci. 417 (1998) 384. [3] N. Saito, Y. Ohkawara, K. Sato, Y. Inoue, MRS proceedings, 497 (1998) 215. [4] N. Saito, H. Nishiyama, K. Sato, Y. Inoue, Appl. Surf. Sci. 144/145 (1999) 385. [5] N. Saito, M. Sakamoto, H. Nishiyama, Y. Inoue, Chem. Phys. Lett. 341 (2001) 232. [6] N. Saito, Y. Inoue, J. Chem. Phys. 113 (2000) 469. [7] Y. Ohkawara, N. Saito, K. Sato, Y. Inoue, Chem. Phys. Lett. 286 (1998) 502. [8] N.D. Lang, W. Kohn, Phys. Rev. B 1 (1970) 4555. [9] J.R. Smith, Phys. Rev. 181 (1969) 522. [10] N. Saito, Y. Inoue, J. Phys. Chem. B 106 (2002) 5011. [11] Y. Yukawa, N. Saito, H. Nishiyama, Y. Inoue, J. Phys. Chem. B 106 (2002) 10174. [12] N. Saito, H. Nishiyama, K. Sato, Y. Inoue, Chem. Phys. Lett. 297 (2002) 72.
Different acoustic wave effects of thickness extension and thickness shear mode resonance oscillation on ethanol decomposition over Pd catalysts deposited on poled ferroelectric LiNbO3 single crystals Y. Yukawa, N. Saito, H. Nishiyama, Y. Inoue
*
Department of Chemistry, Nagaoka University of Technology, Nagaoka, Niigata 940-2188, Japan
Abstract The vibration mode effects of resonance oscillation (RO) on ethanol decomposition over thin Pd film catalysts were studied using thickness shear mode RO (TSRO) and thickness extension mode RO (TERO). The TSRO accelerated neither ethylene nor acetaldehyde productions, whereas the TERO increased selectivity for ethylene production dramatically. Laser Doppler method showed that the TSRO caused small vertical lattice displacement, which contrasted to large vertical lattice displacement of the TERO. Photoelectron emission spectra showed clear differences in threshold energy shifts between TSRO and TERO: the TSRO induced little threshold energy shift, but the TERO caused marked positive shifts. A mechanism of catalyst activation due to the lattice vibration modes is discussed. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Single crystal surfaces; Catalysis; Lead; Acoustic waves
1. Introduction In aiming at designing a heterogeneous catalyst that has artificially controllable functions, we have employed the resonance oscillation (RO) of bulk acoustic waves generated by applying radio frequency electric power to ferroelectric materials. There are various kinds of the lattice vibration modes in the RO which are determined by the crystal structures and the direction of polarization axis. In previous studies, we have used the thick-
*
Corresponding author. Tel.: +81-258-47-9832; fax: +81258-47-9830. E-mail address: [email protected] (Y. Inoue).
ness extension mode RO (TERO) generated on a ferroelectric z-cut LiNbO3 single crystal and found that the TERO significantly enhanced the catalytic activities for ethanol and CO oxidation of thin metals (Ag, Pd and Ni) as well as metal oxide (NiO and TiO2 ) films deposited on the crystal [1–5]. More recently, we have demonstrated that the TERO has the useful ability to change the selectivity of catalytic reactions: it increased ethylene production markedly without affecting acetaldehyde production in ethanol decomposition on a thin Ag film catalyst. We have also applied thickness shear mode RO (TSRO) to the same catalytic system, but no significant activation of the Ag catalyst was observed [6]. For the different effects of vibration modes on catalyst activation, a question
0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00195-X
360
Y. Yukawa et al. / Surface Science 532–535 (2003) 359–363
arises whether or not differences between TERO and TSRO are intrinsically specific for Ag metal catalyst only. To clarify this, it is important to extend research to other metal catalysts. In the present study, the TSRO and TERO were applied to a thin Pd film catalyst for ethanol decomposition. The lattice displacements of the Pd catalysts in different vibration modes were investigated by a three-dimensional laser Doppler method. The surface properties were examined by photoelectron emission spectroscopy in the presence of the TSRO, and compared with previous results for the TERO.
a stage movable toward x and y direction. A sample surface was irradiated by a He–Ne laser beam, and a reflected Doppler signal was monitored by a vibrometer (Ono sokki LV-1300). Lattice displacement was transferred to three-dimensional x–y–z images. Photoelectron emission was measured in air by a low-energy photoelectron spectroscopic method [7]. A sample surface was irradiated with monochromatized UV light from a deuterium lamp. The wavelength of light was scanned over the range of 230–300 nm, and the numbers of emitted photoelectron were counted against photon energy. The threshold energy for photoelectron emission was compared in the absence and presence of RO.
2. Experimental sections 3. Results In ethanol decomposition on a Pd catalyst, both ethylene and acetaldehyde were produced as major
Activity / µmol h-1
1
(a)
(b)
0.75
0.5
0.25
0 Off 1 Activity / µmol h-1
Two kinds of the poled ferroelectric single crystals were used: one was x-cut LiNbO3 (x-LN) with 0.5 mm in thickness and the other was z-cut LiNbO3 (z-LN) with 1 mm. The two crystals were cut to have a rectangular shape of 44 14 mm. Pd metal was deposited at a thickness of 100 nm on a positively polarized surface of the ferroelectric substrates, and the back plane of a negatively polarized surface was covered with a catalytically inactive Au film (which worked as an electrode only) of the same thickness. The two metal films were prepared by evaporation in vacuum with resistance heating of pure Pd and Au metals, respectively. The prepared catalyst was placed on a quartz glass cell equipped with the BNC terminal for rf electric power introduction. Radio frequency (rf) electric power was generated from a network analyzer (Anritsu MS3606B), amplified, and introduced to the catalyst after impedance adjustment. The primary resonance frequency of 3.6 MHz was employed. Catalytic ethanol decomposition was performed in a gas circulating vacuum apparatus, and reactant and products were analyzed by an on-line gas chromatograph. Catalyst temperature was monitored by the frequency shifts of resonance lines, which were very sensitive to temperature, and controlled by an outer electric furnace within an accuracy of 0.2 K. Lattice displacement was measured by a laser Doppler method. A catalyst sample was placed on
On
Off
(c)
Off
On
Off
On
Off
(d)
0.75
0.5
0.25
0 Off
On
Off
Off
Fig. 1. Effects of TSRO on activity for ethylene (a) and acetaldehyde (b) production and of TERO on activity for ethylene (c) and acetaldehyde (d) production. Tr (for reaction temperature) ¼ 583 K, Pr (for rf power) ¼ 3 W, Pe (for pressure of ethanol) ¼ 4.0 kPa.
Y. Yukawa et al. / Surface Science 532–535 (2003) 359–363
products and their amounts increased in proportion to reaction time. Fig. 1 shows the effects of TSRO and TERO on activity for ethylene and acetaldehyde productions. When the TSRO was generated at 3 W, and the activities for ethylene and acetaldehyde production remained nearly unchanged, indicating that the TSRO has no activation effects. On the other hand, TERO-on at the same power caused a significant increase in the ethylene production: the activity, Ve , increased from 0.04 to 0.94 lmol h1 . The high activity was maintained as long as the TERO was turned on, and lowered to an original level with TERO-off. Interestingly, the enhancement of activity, Va , for acetaldehyde production with TERO-on was quite small (from 0.11 to 0.15 lmol h1 ). To compare the activation effects of the TSRO and TERO, activation coefficient, R (¼ Von =Voff ), was defined as the ratio of catalytic activity (Von ) with power-on
361
to that (Voff ) with power-off. The TERO effects were R ¼ 24 for ethylene production and R ¼ 1:4 for acetaldehyde production. Ethylene selectivity, defined as 100 Ve =ðVe þ Va Þ, increased from 27% to 86% with TERO-on at 3 W. Fig. 2 shows the laser Doppler patterns of Pd when TSRO and TERO were generated at 2 W. For TSRO, no marked changes in the surface morphology were observed over the x–y plane. On the other hand, the pattern of TERO provided very large standing waves distributed randomly over the plane. The waves were normal to the surface, and their amplitudes corresponded to the magnitude of lattice displacement. Fig. 3 shows the distributions of lattice displacement with TSRO and TERO. For the TSRO, the maximum lattice displacement, Lmax , was 6 nm, and the average
(a)
4000
TSRO
N
3000
2000 1000
0 0
10
20
30
40
50
Lattice displacement / nm 200
(b) TERO
N
150
100
50
0 0 Fig. 2. Three-dimensional lattice displacement patterns with TSRO-on (a) and TERO-on (b) obtained by a laser Doppler method. Tm (for temperature of measurements) ¼ RT, Pr ¼ 2 W.
10
20
30
40
50
Lattice displacement / nm Fig. 3. Distributions of lattice displacement with the TSRO (a) and TERO (b). Pr ¼ 2 W.
362
Y. Yukawa et al. / Surface Science 532–535 (2003) 359–363
lattice displacement, Lav , was 2 nm. The value of full width at half maximum (FWHM) was 3 nm. For the TERO, Lmax attained at 43 nm, and Lav was 16 nm. The FWHM was as broad as 20 nm. These results indicate that the TSRO has little component of lattice vibration toward vertical direction, whereas the TERO was characterized by very large vertical lattice displacement. In photoelectron emission measurements, the wavelength of light to irradiate a Pd surface was scanned, and threshold energy at which electrons were evolved from the Pd surface was detected. The threshold energy for the Pd surface was 4.83 eV in the absence of TSRO and TERO. Fig. 4 shows threshold energy shifts as a function of rf power. The TSRO showed the invariance of threshold energy even up to 3 W, although the TERO caused a shift toward
Threshold energy shift ∆Φ / eV
0.15
TERO 0.1
0.05
TSRO
0
Rf Power / W
Fig. 4. Threshold energy shifts of photoelectron emission as a function of rf power with TSRO ( ) and TERO ( ). Tm ¼ RT. The results for TERO were taken from Ref. [11].
higher threshold energy linearly with power as demonstrated previously [11]. Table 1 summarizes changes in the surface properties of Pd and kinetic behavior of ethanol decomposition with TSRO and TERO.
4. Discussion For ethanol decomposition on Pd catalysts, the different vibration modes of RO exhibited contrary results. The TSRO had no effects on the activation of a Pd surface for both ethylene and acetaldehyde productions. On the other hand, the TERO increased the activity for ethylene production markedly but that for acetaldehyde production to a small extent. In a previous study [6], we have examined the TERO and TSRO effects for ethanol decomposition on Ag catalysts and found that the TERO activated the Ag catalysts to be dominantly selective for ethylene production. The temperature dependence of the reaction became smaller with TERO-on, and hence the activation energy for ethylene production decreased from 156 to 115 kJ mol1 , whereas the TSRO had no effects on the activation of the Ag catalysts [12]. Thus, there is a similarity in a trend of catalyst activation between Pd and Ag catalysts with respect to the vibration modes of RO, although the extent of catalyst activation was different. This indicates that the TERO effects are useful for different kinds of the metal catalysts. Laser Doppler measurements showed that the amplitudes of standing waves vertical to the surface were intrinsically different between TSRO and TERO. Since the amplitudes reflect the magnitudes of lattice displacement, it turns out that the TERO has very large vertical lattice displacement:
Table 1 TSRO and TERO effects on Pd catalysts 2W
3W
Lattice displacement/nm
Threshold energy shift/eV
6 43
0.00 þ0.05
Activation coefficient (R) Ethylene
TSRO TERO
1 24
Acetaldehyde 1 1.4
Y. Yukawa et al. / Surface Science 532–535 (2003) 359–363
the largest lattice displacement Lmax reached 43 nm at 2 W, as shown in the distributions of lattice displacement (Fig. 3). Contrary to this, the TSRO retained very small vertical component for the lattice displacement in which Lmax was as small as 6 nm. The photoelectron emission spectra showed that the characteristics of photoelectron emission were also different between TSRO and TERO. The TSRO had little influence on the threshold energy for photoelectron emission, but the TERO caused positive shifts significantly. The threshold energy of photoelectron emission spectra of metal surfaces corresponds to the work function. Table 1 compares changes in surface properties and catalytic activity with TSRO and TERO, and it is evident that the three factors such as catalyst activation, vertical lattice displacement and work function shifts are closely related each other. There is a possibility that vertical lattice displacement induces the distortion of catalyst surface, affecting atom-atom distance and the arrangements of surface metal atoms. However, if one assumes uniform deformation for the lattice displacement of a flat surface, a change in atom-atom distance is roughly calculated to be an extent of order of 105 %, which indicates that the deformation effects are extremely small. According to a jellium model, the work function of transition metals is mainly determined by the density of electrons that spill out from the outermost layer of the metal surface, since the density is responsible for an energy barrier resulting from a surface electric dipole layer [8,9]. To explain increases in work function with TERO-on, a model has been proposed in a previous study based on the assumption that the large vertical lattice displacement has significant influences on the density of ‘‘spill out’’ electrons [10]. The present results that the TSRO with lattice displacement parallel to the surface has negligible effects on work function are in line with the proposed model. It is apparent that the activity and selectivity increases for ethylene production with the TERO are associated with the enhancement of work
363
function, which has strongly weakened interactions between the adsorbed ethanol and the Pd metal surface. This promotes the formation of weakly adsorbed ethanol that favors the dehydration reaction, as shown previously [10]. In a comparison of TERO effects on ethanol decomposition between Pd and Ag film catalysts, the extent of activation energy decrease was smaller for Pd than for Ag, which was associated with smaller lattice displacement and work function shifts for Pd than for Ag [11]. It appears that the electronic structure differences of the metal surfaces are responsible for the differences in TERO effects. In conclusion, the TSRO and TERO effects on the reaction selectivity were different for ethanol decomposition on Pd, and it has been shown that the large vertical lattice displacement of the TERO is useful for catalyst activation of various kinds of metal catalysts. Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research (B) from The Ministry of Education, Science, Sports and Culture. References [1] N. Saito, Y. Ohkawara, Y. Watanabe, Y. Inoue, Appl. Surf. Sci. 121/122 (1997) 343. [2] N. Saito, K. Sato, Y. Inoue, Surf. Sci. 417 (1998) 384. [3] N. Saito, Y. Ohkawara, K. Sato, Y. Inoue, MRS proceedings, 497 (1998) 215. [4] N. Saito, H. Nishiyama, K. Sato, Y. Inoue, Appl. Surf. Sci. 144/145 (1999) 385. [5] N. Saito, M. Sakamoto, H. Nishiyama, Y. Inoue, Chem. Phys. Lett. 341 (2001) 232. [6] N. Saito, Y. Inoue, J. Chem. Phys. 113 (2000) 469. [7] Y. Ohkawara, N. Saito, K. Sato, Y. Inoue, Chem. Phys. Lett. 286 (1998) 502. [8] N.D. Lang, W. Kohn, Phys. Rev. B 1 (1970) 4555. [9] J.R. Smith, Phys. Rev. 181 (1969) 522. [10] N. Saito, Y. Inoue, J. Phys. Chem. B 106 (2002) 5011. [11] Y. Yukawa, N. Saito, H. Nishiyama, Y. Inoue, J. Phys. Chem. B 106 (2002) 10174. [12] N. Saito, H. Nishiyama, K. Sato, Y. Inoue, Chem. Phys. Lett. 297 (2002) 72.