- Email: [email protected]
Different effects of acoustic resonance oscillation on activation of a thin Pd film catalyst deposited on an oppositely polarized ferroelectric LiNbO3 single crystal
Surface Science 417 (1998) 384–389
Different effects of acoustic resonance oscillation on activation of a thin Pd film catalyst deposited on an oppositely polarized ferroelectric LiNbO single crystal 3 N. Saito, K. Sato, Y. Inoue * Department of Chemistry, Nagaoka University of Technology, Nagaoka, Niigata 940-2188, Japan Received 22 April 1998; accepted for publication 27 August 1998
Abstract A 100 nm thick Pd catalyst film was deposited on either positively or negatively polarized surfaces of a poled ferroelectric z-cut LiNbO single crystal which generates a thickness-extensional mode resonance oscillation (RO) by r.f. electric power. The effects of 3 the RO on the kinetic behavior of ethanol oxidation over a Pd catalyst were studied. A 3.4 MHz RO caused a dramatic decrease in activation energy from 156 kJ mol−1 to 36 kJ mol−1 for Pd on a positively polarized surface [(+)Pd ] and to 0 kJ mol−1 for Pd on a negatively polarized surface [(−)Pd ]. Reaction orders in ethanol and oxygen pressures showed that the RO changed ethanol to be weakly adsorbed on both (+)Pd and (−)Pd and oxygen to be very strongly adsorbed on (+)Pd. The RO caused opposite changes in the surface potential of the catalyst with respect to the polarized surfaces: a negative surface potential was generated for (+)Pd, and vice versa. These surface potential changes associated with an acoustoelectric effect and an electric field due to the ferroelectric substrate are responsible for the different kinetic behavior. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Acoustic wave; Catalyst activation; Ethanol; Pd; Polarized surface
1. Introduction It is of particular interest to design a catalyst surface with an artificially controllable function for chemical reactions, but only a few studies have been performed so far [1–7]. In a previous study, using a poled ferroelectric single crystal of z-cut LiNbO as substrate, we have fabricated a catalyst 3 in which both the front and back planes of the crystal were covered with a thin Pd film so as to generate a 3.4 MHz resonance oscillation (RO) by applying radio frequency electric power [8]. It has been demonstrated that the RO is able to cause * Corresponding author.
an anomalous increase in catalytic activity for ethanol oxidation over the Pd thin film, thus indicating that the dynamic lattice displacement of RO plays an important role in catalyst activation. The ferroelectric substrate employed is a single domain structure, whose spontaneous polarization is perpendicular to the surface, and exposes a positively polarized surface at one plane and a negatively polarized surface at the other. Since the Pd film was deposited on both negative and positive polar surfaces in the previous study, however, it was difficult to distinguish differences in catalytic behavior between the oppositely polarized surfaces [8]. The present study was undertaken to examine
0039-6028/98/$ – see front matter © 1998 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 98 ) 0 06 9 2 -X
N. Saito et al. / Surface Science 417 (1998) 384–389
the effects of the polarized surfaces on RO-induced catalyst activation. Two kinds of catalysts were prepared: one had a structure in which the negative polar surface of z-cut LiNbO was covered with 3 an active Pd thin film and the opposite positive polar surface with a catalytically inactive Al thin film, whereas the other had the opposite combination of the polar surface and metals. Ethanol oxidation over the two catalysts was compared in the absence and presence of RO. We have discovered that the positively and negatively polarized surfaces have significantly different RO effects on catalytic kinetic behavior. The activation energy and reaction orders of the catalytic oxidation vary with the RO in significantly different manners between the oppositely polarized substrates. The surface potential of the catalysts with the RO was measured to investigate differences in surface characteristics between the positively and negatively polarized surfaces, and it has been demonstrated that the RO caused a negative potential shift with respect to the positively polarized surface, and vice versa. Such polarization-dependent surface potential changes with the RO were well reflected by those in the kinetic parameters of the catalytic reaction.
2. Experimental A ferroelectric single crystal of z-cut LiNbO 3 (referred to as z-LN ) with a single domain structure was employed as a substrate which had a rectangular shape with 1 mm thickness [8]. The structures of fabricated catalysts are shown in Fig. 1. One catalyst has a structure of a catalytically active Pd thin film (which works as both electrodes and catalysts) on the positively polarized
385
surface and a catalytically inactive Al (which is used as an electrode only) on the negatively polarized surface of z-LN. This catalyst is denoted here as (+)Pd. The other has just the opposite combination: Pd is on the negative polar surface and Al on the positive polar surface [denoted as (−)Pd ]. The notation (±)Pd is given for a sample for which both the positive and negative surfaces are covered with Pd. Pd film was deposited at a thickness of 100 nm by resistance heating of a pure Pd metal in vacuum, and Al with electron beam deposition at the same thickness. Radio frequency (r.f.) power generated from a network analyzer (Anritsu MS3606B) was amplified by an amplifier ( Kalmus, 250FC ) and applied to the sample after impedance adjustment. The resonance characteristics were measured by the network analyzer. The catalytic ethanol oxidation was carried out in a gas circulating vacuum apparatus, and the reactants and products were analyzed by a gas chromatograph directly connected to the apparatus. The temperature of the catalyst surface was measured through a BaF window of the 2 reaction cell by a non-contacting method using a radiation thermometer and was controlled using an outer electric furnace. A small CA thermocouple connected catalyst was used as a standard sample for temperature calibration, and the emissivity was evaluated to be 0.6 by comparing temperatures measured by the thermometer and thermocouple. Changes in surface potential of the (+)Pd and (−)Pd with RO were measured in air at room temperature by a dynamic condenser electrometer, Andou Electric Co AA-2404. Either a (+)Pd or (−)Pd sample was placed in parallel with a probe of the electrometer at a distance of 3 mm. A voltage generated on the Pd surface was monitored in the absence and presence of RO.
3. Results
Fig. 1. Structures of Pd thin film catalysts deposited on a positively and a negatively polarized ferroelectric z-LiNbO sub3 strate: shaded box, Pd; empty box, Al.
At room temperature, resonance lines appeared at a frequency of 3.4 (first), 10.5 (second) and 17.4 MHz (third ), which was nearly proportional to 2k−1 where k=1, 2, 3. In the present work, the primary resonance frequency of 3.4 MHz (k=
386
N. Saito et al. / Surface Science 417 (1998) 384–389
1) was used for catalyst activation, unless otherwise specified. The frequency dependence of the resonance showed that a z-LN crystal has a thickness mode vibration [8]. Upon the introduction of 3 W r.f. power, a dramatic increase in acetaldehyde production occurred immediately for both the (+)Pd and (−)Pd catalysts, and the enhanced activity was maintained until the power was turned off. Interestingly, there was a considerable difference in activity enhancement between (+)Pd and (−)Pd. Fig. 2 shows the Arrhenius plots of the reaction over the temperature range 358–408 K. Without RO, the slope of the plot yielded an activation energy of 156 kJ mol−1. With RO on, a remarkable but different decrease in activation energy occurred for both (+)Pd and (−)Pd: it decreased to 36 kJ mol−1 for (+)Pd and 0 kJ mol−1 for (−)Pd. A decrease in the activation energy was larger for (−)Pd than for (+)Pd. Fig. 3 shows the partial pressure dependence of ethanol oxidation on (−)Pd. Without RO, the reaction order, m, with respect to oxygen pressure, P , was 0.5, and the order, n, with respect to o ethanol pressure, P , was 0.2. With RO on at 3 W, e the value of m remained unchanged, whereas that of n increased to 1.0. Fig. 4 shows the results of
Fig. 2. Temperature dependence of ethanol on (+)Pd and (−)Pd. R.f. power=3 W, frequency=3.4 MHz. & RO off, # RO on, (−)Pd; 6 RO on, (+)Pd.
Fig. 3. Partial pressure dependence of the reaction on (−)Pd. R.f. power=3 W, frequency=3.4 MHz, T=363 K. & RO off, $ RO on.
Fig. 4. Partial pressure dependence of the reaction on (+)Pd. R.f. power=3 W, frequency=3.4 MHz, T=363 K. & RO off, $ RO on.
387
N. Saito et al. / Surface Science 417 (1998) 384–389 Table 1 Changes in kinetic parameters of ethanol oxidation with RO Resonance oscillation
(−)Pd (+)Pd
off on off on
Reaction orders
Activation energy (kJ mol−1)
Oxygen, m
Ethanol, n
0.5 0.5 0.5 −0.1
0.2 1.0 0.2 1.0
reaction orders on (+)Pd. With RO off, the m and n values were respectively 0.5 and 0.2. With RO on, the m value dramatically decreased to −0.1, whereas n increased to 1.0. All the kinetic parameters are summarized in Table 1. In surface potential measurements, the introduction of r.f. power at non-resonance frequencies caused no significant changes. Fig. 5 shows changes in the surface potential with RO on at a resonance frequency of 3.4 MHz at 3 W. For (+)Pd, a negative shift occurred with RO, increased with time, and reached a nearly constant level. With turning the power off, the shift returned to an original zero level. A nearly the same but opposite positive shift was induced for (−)Pd. Fig. 6 shows surface potentials at constant levels as a function of r.f. power. For (+)Pd, a negative shift increased monotonically with power and attained a level of −16 V at 3 W. A similar shift
Fig. 5. Changes in surface potential with RO. R.f. power=3 W, frequency=3.4 MHz, T=RT.
156 0 156 36
except for an opposite direction was observed for (−)Pd.
4. Discussion Dramatic increases in the catalytic activity with RO on occurred for both (+)Pd and (−)Pd, as those observed for (±)Pd catalysts in a previous study [8]. The interesting feature is that decreases in the activation energy with RO on were significantly different between the oppositely polarized surfaces: (+)Pd provided 36 kJ mol−1 and (−)Pd 0 kJ mol−1. Note that a previously obtained value of 12 kJ mol−1 for (±)Pd was close to an intermediate between (+)Pd and (−)Pd. As shown in Table 1, the reaction orders with respect to P and e
Fig. 6. Surface potential changes as a function of r.f. power. Frequency=3.4 MHz, T=RT. + Non-resonance frequencies, & (−)Pd, $ (+)Pd.
388
N. Saito et al. / Surface Science 417 (1998) 384–389
P without RO were 0.2 and 0.5, respectively. It o has been shown that the rate determining step in ethanol oxidation on Pd surfaces without RO is the removal of a hydrogen atom from a CH 2 group of an adsorbed ethanol molecule [2]. Thus, by assuming the surface reaction of adsorbed ethanol and dissociatively adsorbed oxygen atom, the following rate equation can be derived: (1) V=k앀K P K P /(1+앀K P +K P )2 o o e e o o e e where k is a rate constant, and K and K are o e equilibrium constants of ethanol and oxygen, respectively. This equation can be transformed to: 앀k앀K P K P /V=1+앀K P +K P (2) o o e e o o e e The plot of 앀P /V against P showed a good e e linear relationship, which indicates that the mechanism proposed above holds in the present oxidation. In the presence of RO, the reaction orders varied in a different way depending on the oppositely polarized surfaces. For (−)Pd, the reaction order with respect to P increased from 0.2 to 1.0, e and that with respect to P remained unchanged. o The change is explained by assuming that ethanol adsorption becomes weaker with RO on: the condition 1&앀K P +K P leads to V=k앀K P K P o o e e o o e e in Eq. (1). This simplified equation has a reaction order, 0.5, with respect to P which was the same o as the observed reaction order with RO on. On the other hand, for (+)Pd, the order with respect to P showed the same change (from 0.2 e to 1.0) as observed for (−)Pd, whereas the reaction order with P decreased dramatically from 0.5 to o −0.1. The assumption of 1+앀K P &K P in o o e e Eq. (1) leads to the first order with respect to P . e This assumption also transforms the P -related o terms in Eq. (1) to 앀K P /(1+앀K P )2 which is o o o o simplified either to 앀K P for 1&앀K P or o o o o (K P )−1/2 for 1%앀K P . Namely, depending on o o o o a relation in magnitude between 1 and 앀K P , o o the reaction order varies from 0.5 to −0.5. The observed value of −0.1 falls within this range, and it is more likely that the condition of 앀K P >1 o o is held, because of a negative n value. These kinetic features clearly indicate that the RO functions to produce not only weaker adsorption of ethanol but also stronger adsorption of oxygen for (+)Pd.
Opposite surface potential changes occurred with RO according to the polarization axis direction of z-cut LiNbO : (+)Pd produced a negative 3 potential and vice versa. These potential changes are obviously induced by an increase and a decrease in electron density at the surface, which are considered to be associated with an acoustoelectric effect and an electric field due to the spontaneous polarization axis. The former facilitates the electron movements, whereas the latter controls their direction. From the polarization field, it is evident that the movement of electrons toward the (+) surface is promoted, but that toward the (−) surface is depressed. This results in electron enrichment on a (+)Pd surface and electron deficiency on a (−)Pd surface. It is rationally accepted that these electron density changes affect the adsorption of oxygen and ethanol on the oppositely polarized Pd surfaces. In a very recent study using Ag thin film deposited on polycrystalline lead strontium zirconate titanate (Pb Sr Zr Ti O ) with a thickness exten0.95 0.05 0.53 0.47 3 sional resonance oscillation, it has been demonstrated that the RO caused opposite surface potential changes similar to those observed in the present study between (+)Ag and (−)Ag and, more interestingly, a decrease by 0.12 eV in threshold energy to evolve photoelectrons from (+)Ag surface with UV irradiation, compared to that without RO, which indicates that an electric double layer structure due to spill-out electrons at a Ag surface is affected by RO [9]. It is likely that a similar situation holds for the (+)Pd surface in view of an analogy observed in the surface potential changes. A previous study on the effect of a shear horizontal leaky surface acoustic wave on ethanol oxidation over a Ni thin film deposited on LiTaO showed that the adsorbed oxygen has a 3 significant effect on the removal of a hydrogen from the adsorbed ethanol [2]. In a thermal desorption study on single crystals [10], Madix and coworker showed that the state of oxygen is associated with efficient removal of a hydrogen from adsorbed ethanol, which indicates that the negatively charged surface oxygens promote the process [10]. Since it is evident that the negative surface potential shift of (+)Pd caused by the RO facilitates the formation of negatively charged sur-
N. Saito et al. / Surface Science 417 (1998) 384–389
face oxygens, it is rationally accepted that a strongly adsorbed oxygen is formed on (+)Pd with the RO, which explains the changes in reaction order. In conclusion, the thickness extensional mode RO has polarization-dependent effects on the activation of catalyst surfaces. A model can be proposed in which large dynamic lattice displacement affects not only the arrangement of surface atoms but also the density of electrons at the catalyst surfaces.
Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research (B) from the Ministry of Education, Science, Sports and Culture.
389
References [1] Y. Inoue, M. Matsukawa, K. Sato, J. Am. Chem. Soc. 111 (1989) 8965. [2] Y. Inoue, Y. Watanabe, T. Noguchi, J. Phys. Chem. 99 (1995) 9898. [3] Y. Inoue, J. Chem. Soc., Faraday Trans. 90 (1994) 815. [4] Y. Ohkawara, N. Saito, Y. Inoue, Surf. Sci. 357/358 (1996) 777. [5] H. Nishiyama, N. Saito, M. Shima, Y. Watanabe, Y. Inoue, Faraday Disc. 107 (1997) 425. [6 ] M. Gruyters, T. Mitrelias, D.A. King, Appl. Phys. 61 (1995) 243. [7] S. Kelling, T. Mitrelias, Y. Matsumoto, V.P. Ostanin, D.A. King, Faraday Disc. 107 (1997) 435. [8] N. Saito, Y. Ohkakwara, Y. Watanabe, K. Sato, Y. Inoue, Appl. Surf. Sci. 121/122 (1997) 343. [9] Y. Ohkawara, N. Saito, K. Sato, Y. Inoue, Chem. Phys. Lett. 286 (1998) 502. [10] M.A. Barteau, R.J. Madix, Surf. Sci. 120 (1982) 262.
Different effects of acoustic resonance oscillation on activation of a thin Pd film catalyst deposited on an oppositely polarized ferroelectric LiNbO single crystal 3 N. Saito, K. Sato, Y. Inoue * Department of Chemistry, Nagaoka University of Technology, Nagaoka, Niigata 940-2188, Japan Received 22 April 1998; accepted for publication 27 August 1998
Abstract A 100 nm thick Pd catalyst film was deposited on either positively or negatively polarized surfaces of a poled ferroelectric z-cut LiNbO single crystal which generates a thickness-extensional mode resonance oscillation (RO) by r.f. electric power. The effects of 3 the RO on the kinetic behavior of ethanol oxidation over a Pd catalyst were studied. A 3.4 MHz RO caused a dramatic decrease in activation energy from 156 kJ mol−1 to 36 kJ mol−1 for Pd on a positively polarized surface [(+)Pd ] and to 0 kJ mol−1 for Pd on a negatively polarized surface [(−)Pd ]. Reaction orders in ethanol and oxygen pressures showed that the RO changed ethanol to be weakly adsorbed on both (+)Pd and (−)Pd and oxygen to be very strongly adsorbed on (+)Pd. The RO caused opposite changes in the surface potential of the catalyst with respect to the polarized surfaces: a negative surface potential was generated for (+)Pd, and vice versa. These surface potential changes associated with an acoustoelectric effect and an electric field due to the ferroelectric substrate are responsible for the different kinetic behavior. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Acoustic wave; Catalyst activation; Ethanol; Pd; Polarized surface
1. Introduction It is of particular interest to design a catalyst surface with an artificially controllable function for chemical reactions, but only a few studies have been performed so far [1–7]. In a previous study, using a poled ferroelectric single crystal of z-cut LiNbO as substrate, we have fabricated a catalyst 3 in which both the front and back planes of the crystal were covered with a thin Pd film so as to generate a 3.4 MHz resonance oscillation (RO) by applying radio frequency electric power [8]. It has been demonstrated that the RO is able to cause * Corresponding author.
an anomalous increase in catalytic activity for ethanol oxidation over the Pd thin film, thus indicating that the dynamic lattice displacement of RO plays an important role in catalyst activation. The ferroelectric substrate employed is a single domain structure, whose spontaneous polarization is perpendicular to the surface, and exposes a positively polarized surface at one plane and a negatively polarized surface at the other. Since the Pd film was deposited on both negative and positive polar surfaces in the previous study, however, it was difficult to distinguish differences in catalytic behavior between the oppositely polarized surfaces [8]. The present study was undertaken to examine
0039-6028/98/$ – see front matter © 1998 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 98 ) 0 06 9 2 -X
N. Saito et al. / Surface Science 417 (1998) 384–389
the effects of the polarized surfaces on RO-induced catalyst activation. Two kinds of catalysts were prepared: one had a structure in which the negative polar surface of z-cut LiNbO was covered with 3 an active Pd thin film and the opposite positive polar surface with a catalytically inactive Al thin film, whereas the other had the opposite combination of the polar surface and metals. Ethanol oxidation over the two catalysts was compared in the absence and presence of RO. We have discovered that the positively and negatively polarized surfaces have significantly different RO effects on catalytic kinetic behavior. The activation energy and reaction orders of the catalytic oxidation vary with the RO in significantly different manners between the oppositely polarized substrates. The surface potential of the catalysts with the RO was measured to investigate differences in surface characteristics between the positively and negatively polarized surfaces, and it has been demonstrated that the RO caused a negative potential shift with respect to the positively polarized surface, and vice versa. Such polarization-dependent surface potential changes with the RO were well reflected by those in the kinetic parameters of the catalytic reaction.
2. Experimental A ferroelectric single crystal of z-cut LiNbO 3 (referred to as z-LN ) with a single domain structure was employed as a substrate which had a rectangular shape with 1 mm thickness [8]. The structures of fabricated catalysts are shown in Fig. 1. One catalyst has a structure of a catalytically active Pd thin film (which works as both electrodes and catalysts) on the positively polarized
385
surface and a catalytically inactive Al (which is used as an electrode only) on the negatively polarized surface of z-LN. This catalyst is denoted here as (+)Pd. The other has just the opposite combination: Pd is on the negative polar surface and Al on the positive polar surface [denoted as (−)Pd ]. The notation (±)Pd is given for a sample for which both the positive and negative surfaces are covered with Pd. Pd film was deposited at a thickness of 100 nm by resistance heating of a pure Pd metal in vacuum, and Al with electron beam deposition at the same thickness. Radio frequency (r.f.) power generated from a network analyzer (Anritsu MS3606B) was amplified by an amplifier ( Kalmus, 250FC ) and applied to the sample after impedance adjustment. The resonance characteristics were measured by the network analyzer. The catalytic ethanol oxidation was carried out in a gas circulating vacuum apparatus, and the reactants and products were analyzed by a gas chromatograph directly connected to the apparatus. The temperature of the catalyst surface was measured through a BaF window of the 2 reaction cell by a non-contacting method using a radiation thermometer and was controlled using an outer electric furnace. A small CA thermocouple connected catalyst was used as a standard sample for temperature calibration, and the emissivity was evaluated to be 0.6 by comparing temperatures measured by the thermometer and thermocouple. Changes in surface potential of the (+)Pd and (−)Pd with RO were measured in air at room temperature by a dynamic condenser electrometer, Andou Electric Co AA-2404. Either a (+)Pd or (−)Pd sample was placed in parallel with a probe of the electrometer at a distance of 3 mm. A voltage generated on the Pd surface was monitored in the absence and presence of RO.
3. Results
Fig. 1. Structures of Pd thin film catalysts deposited on a positively and a negatively polarized ferroelectric z-LiNbO sub3 strate: shaded box, Pd; empty box, Al.
At room temperature, resonance lines appeared at a frequency of 3.4 (first), 10.5 (second) and 17.4 MHz (third ), which was nearly proportional to 2k−1 where k=1, 2, 3. In the present work, the primary resonance frequency of 3.4 MHz (k=
386
N. Saito et al. / Surface Science 417 (1998) 384–389
1) was used for catalyst activation, unless otherwise specified. The frequency dependence of the resonance showed that a z-LN crystal has a thickness mode vibration [8]. Upon the introduction of 3 W r.f. power, a dramatic increase in acetaldehyde production occurred immediately for both the (+)Pd and (−)Pd catalysts, and the enhanced activity was maintained until the power was turned off. Interestingly, there was a considerable difference in activity enhancement between (+)Pd and (−)Pd. Fig. 2 shows the Arrhenius plots of the reaction over the temperature range 358–408 K. Without RO, the slope of the plot yielded an activation energy of 156 kJ mol−1. With RO on, a remarkable but different decrease in activation energy occurred for both (+)Pd and (−)Pd: it decreased to 36 kJ mol−1 for (+)Pd and 0 kJ mol−1 for (−)Pd. A decrease in the activation energy was larger for (−)Pd than for (+)Pd. Fig. 3 shows the partial pressure dependence of ethanol oxidation on (−)Pd. Without RO, the reaction order, m, with respect to oxygen pressure, P , was 0.5, and the order, n, with respect to o ethanol pressure, P , was 0.2. With RO on at 3 W, e the value of m remained unchanged, whereas that of n increased to 1.0. Fig. 4 shows the results of
Fig. 2. Temperature dependence of ethanol on (+)Pd and (−)Pd. R.f. power=3 W, frequency=3.4 MHz. & RO off, # RO on, (−)Pd; 6 RO on, (+)Pd.
Fig. 3. Partial pressure dependence of the reaction on (−)Pd. R.f. power=3 W, frequency=3.4 MHz, T=363 K. & RO off, $ RO on.
Fig. 4. Partial pressure dependence of the reaction on (+)Pd. R.f. power=3 W, frequency=3.4 MHz, T=363 K. & RO off, $ RO on.
387
N. Saito et al. / Surface Science 417 (1998) 384–389 Table 1 Changes in kinetic parameters of ethanol oxidation with RO Resonance oscillation
(−)Pd (+)Pd
off on off on
Reaction orders
Activation energy (kJ mol−1)
Oxygen, m
Ethanol, n
0.5 0.5 0.5 −0.1
0.2 1.0 0.2 1.0
reaction orders on (+)Pd. With RO off, the m and n values were respectively 0.5 and 0.2. With RO on, the m value dramatically decreased to −0.1, whereas n increased to 1.0. All the kinetic parameters are summarized in Table 1. In surface potential measurements, the introduction of r.f. power at non-resonance frequencies caused no significant changes. Fig. 5 shows changes in the surface potential with RO on at a resonance frequency of 3.4 MHz at 3 W. For (+)Pd, a negative shift occurred with RO, increased with time, and reached a nearly constant level. With turning the power off, the shift returned to an original zero level. A nearly the same but opposite positive shift was induced for (−)Pd. Fig. 6 shows surface potentials at constant levels as a function of r.f. power. For (+)Pd, a negative shift increased monotonically with power and attained a level of −16 V at 3 W. A similar shift
Fig. 5. Changes in surface potential with RO. R.f. power=3 W, frequency=3.4 MHz, T=RT.
156 0 156 36
except for an opposite direction was observed for (−)Pd.
4. Discussion Dramatic increases in the catalytic activity with RO on occurred for both (+)Pd and (−)Pd, as those observed for (±)Pd catalysts in a previous study [8]. The interesting feature is that decreases in the activation energy with RO on were significantly different between the oppositely polarized surfaces: (+)Pd provided 36 kJ mol−1 and (−)Pd 0 kJ mol−1. Note that a previously obtained value of 12 kJ mol−1 for (±)Pd was close to an intermediate between (+)Pd and (−)Pd. As shown in Table 1, the reaction orders with respect to P and e
Fig. 6. Surface potential changes as a function of r.f. power. Frequency=3.4 MHz, T=RT. + Non-resonance frequencies, & (−)Pd, $ (+)Pd.
388
N. Saito et al. / Surface Science 417 (1998) 384–389
P without RO were 0.2 and 0.5, respectively. It o has been shown that the rate determining step in ethanol oxidation on Pd surfaces without RO is the removal of a hydrogen atom from a CH 2 group of an adsorbed ethanol molecule [2]. Thus, by assuming the surface reaction of adsorbed ethanol and dissociatively adsorbed oxygen atom, the following rate equation can be derived: (1) V=k앀K P K P /(1+앀K P +K P )2 o o e e o o e e where k is a rate constant, and K and K are o e equilibrium constants of ethanol and oxygen, respectively. This equation can be transformed to: 앀k앀K P K P /V=1+앀K P +K P (2) o o e e o o e e The plot of 앀P /V against P showed a good e e linear relationship, which indicates that the mechanism proposed above holds in the present oxidation. In the presence of RO, the reaction orders varied in a different way depending on the oppositely polarized surfaces. For (−)Pd, the reaction order with respect to P increased from 0.2 to 1.0, e and that with respect to P remained unchanged. o The change is explained by assuming that ethanol adsorption becomes weaker with RO on: the condition 1&앀K P +K P leads to V=k앀K P K P o o e e o o e e in Eq. (1). This simplified equation has a reaction order, 0.5, with respect to P which was the same o as the observed reaction order with RO on. On the other hand, for (+)Pd, the order with respect to P showed the same change (from 0.2 e to 1.0) as observed for (−)Pd, whereas the reaction order with P decreased dramatically from 0.5 to o −0.1. The assumption of 1+앀K P &K P in o o e e Eq. (1) leads to the first order with respect to P . e This assumption also transforms the P -related o terms in Eq. (1) to 앀K P /(1+앀K P )2 which is o o o o simplified either to 앀K P for 1&앀K P or o o o o (K P )−1/2 for 1%앀K P . Namely, depending on o o o o a relation in magnitude between 1 and 앀K P , o o the reaction order varies from 0.5 to −0.5. The observed value of −0.1 falls within this range, and it is more likely that the condition of 앀K P >1 o o is held, because of a negative n value. These kinetic features clearly indicate that the RO functions to produce not only weaker adsorption of ethanol but also stronger adsorption of oxygen for (+)Pd.
Opposite surface potential changes occurred with RO according to the polarization axis direction of z-cut LiNbO : (+)Pd produced a negative 3 potential and vice versa. These potential changes are obviously induced by an increase and a decrease in electron density at the surface, which are considered to be associated with an acoustoelectric effect and an electric field due to the spontaneous polarization axis. The former facilitates the electron movements, whereas the latter controls their direction. From the polarization field, it is evident that the movement of electrons toward the (+) surface is promoted, but that toward the (−) surface is depressed. This results in electron enrichment on a (+)Pd surface and electron deficiency on a (−)Pd surface. It is rationally accepted that these electron density changes affect the adsorption of oxygen and ethanol on the oppositely polarized Pd surfaces. In a very recent study using Ag thin film deposited on polycrystalline lead strontium zirconate titanate (Pb Sr Zr Ti O ) with a thickness exten0.95 0.05 0.53 0.47 3 sional resonance oscillation, it has been demonstrated that the RO caused opposite surface potential changes similar to those observed in the present study between (+)Ag and (−)Ag and, more interestingly, a decrease by 0.12 eV in threshold energy to evolve photoelectrons from (+)Ag surface with UV irradiation, compared to that without RO, which indicates that an electric double layer structure due to spill-out electrons at a Ag surface is affected by RO [9]. It is likely that a similar situation holds for the (+)Pd surface in view of an analogy observed in the surface potential changes. A previous study on the effect of a shear horizontal leaky surface acoustic wave on ethanol oxidation over a Ni thin film deposited on LiTaO showed that the adsorbed oxygen has a 3 significant effect on the removal of a hydrogen from the adsorbed ethanol [2]. In a thermal desorption study on single crystals [10], Madix and coworker showed that the state of oxygen is associated with efficient removal of a hydrogen from adsorbed ethanol, which indicates that the negatively charged surface oxygens promote the process [10]. Since it is evident that the negative surface potential shift of (+)Pd caused by the RO facilitates the formation of negatively charged sur-
N. Saito et al. / Surface Science 417 (1998) 384–389
face oxygens, it is rationally accepted that a strongly adsorbed oxygen is formed on (+)Pd with the RO, which explains the changes in reaction order. In conclusion, the thickness extensional mode RO has polarization-dependent effects on the activation of catalyst surfaces. A model can be proposed in which large dynamic lattice displacement affects not only the arrangement of surface atoms but also the density of electrons at the catalyst surfaces.
Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research (B) from the Ministry of Education, Science, Sports and Culture.
389
References [1] Y. Inoue, M. Matsukawa, K. Sato, J. Am. Chem. Soc. 111 (1989) 8965. [2] Y. Inoue, Y. Watanabe, T. Noguchi, J. Phys. Chem. 99 (1995) 9898. [3] Y. Inoue, J. Chem. Soc., Faraday Trans. 90 (1994) 815. [4] Y. Ohkawara, N. Saito, Y. Inoue, Surf. Sci. 357/358 (1996) 777. [5] H. Nishiyama, N. Saito, M. Shima, Y. Watanabe, Y. Inoue, Faraday Disc. 107 (1997) 425. [6 ] M. Gruyters, T. Mitrelias, D.A. King, Appl. Phys. 61 (1995) 243. [7] S. Kelling, T. Mitrelias, Y. Matsumoto, V.P. Ostanin, D.A. King, Faraday Disc. 107 (1997) 435. [8] N. Saito, Y. Ohkakwara, Y. Watanabe, K. Sato, Y. Inoue, Appl. Surf. Sci. 121/122 (1997) 343. [9] Y. Ohkawara, N. Saito, K. Sato, Y. Inoue, Chem. Phys. Lett. 286 (1998) 502. [10] M.A. Barteau, R.J. Madix, Surf. Sci. 120 (1982) 262.