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J. Phys. Chem. 1992, 96, 2222-2225
Effect of Surface Acoustic Wave Propagated on Ferroelectric LiNb03 on Catalytic Activity of a Deposited Pd Thin Film Yasunobu Inoue,* Masahiko Matsukawa, and Kazunori Sato Analysis Center, Nagaoka University of Technology, Nagaoka, Niigata 940-21, Japan (Received: December 28, 1990)
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The effect of a surface acoustic wave (SAW) generated on a LiNb03 ferroelectric crystal on the catalytic activity of a Pd thin film deposited on SAW propagation path was studied. The catalytic activity for ethanol oxidation immediately increased and became several times larger than the activity before the SAW was turned on. With the SAW off, the activity decreased to the original level. The activation energy of the reaction decreased from 42 to 29 kJ mol-l with the SAW on. A half reaction order in the oxygen pressure and the first order in ethanol pressure were obtained, both of which remained nearly unchanged with the SAW on. It is proposed that the SAW gives rise to a decrease in the true activation energy. For an electroluminescent Cu-doped ZnS layer deposited instead of the catalyst, a pulsed SAW caused acoustoluminescence, the intensity of which increased with increasing SAW power. It was shown that an electric field as high as 104 V cm-’ was generated on the SAW propagating surface. The SAW effect on the catalytic activity was discussed on the basis of the electric field and geometric effects.
Introduction The artificial control of heterogeneous catalysis by solid surfaces is an interesting topic, since it might lead to the construction of catalysts having self-control functions, that is, an intelligent-type catalyst. An essential required for such artificial control is that the activity of the catalyst be controlled by an external signal. In an attempt to establish functional catalysts, we have focused on the surface properties of ferroelectrics having spontaneous polarization. The arrangement of the polarization axis along the direction perpendicular to the surface permits formation of both positively and negatively polar surfaces. We have employed the oppositely polarized ferroelectric LiNb03 as a catalyst support on which catalytically active metals (Pd,Cu) or transition-metal oxides (NiO, Ti02) were deposited.’-s It has been demonstrated that the positively and negatively polarized LiNb03 supports are able to give rise to considerably different effects on not only the adsorption of H2and O2on Ti024but on the catalytic oxidation of CO on Pd’ and Ni0.2 The roles of the polarized supports in the modification of catalytic properties are propared to be a “static” effect of the ferroelectric support. One of the more interesting features of poled ferroelectrics is their capability for supporting the propagation of surface acoustic waves (SAW). This wave is related to piezoelectricity of crystals having no center symmetry and is characterized by the forced displacement of lattice atoms in the vicinity of the surface. It would be expected that such lattice atom displacement positively influences the catalytically active phases deposited on the surface. In comparison with the above-mentioned static effect, the effect of a SAW is thought to be a “dynamic” effect on the catalytic phases deposited, since the catalytic phase undergoes time-dependent distortion associated with SAW frequency. Figure 1 compares schematically both concepts for the roles of the ferroelectric supports. There have been so far few studies on the SAW effect on chemisorption and catalysis-related surface phenomena. In previous communications, we briefly reported that the SAW generated on a LiNb03 crystal has the ability to enhance the catalytic activity of Pd and Cu thin films deposited on the propagation The observed catalysis-related SAW effect (1)Inoue, Y.; Yoshioka, 1.; Sato, K. J . Phys. Chem. 1984,88, 1148. (2)Inoue, Y.; Sato, K.; Suzuki, S.; Yoshioka, I. Proc. I n t . Congr. Catal., 8th, 1984,299. (3) Inoue, Y.; Sato, K.; Suzuki, S. J. Phys. Chem. 1985,89, 2827. (4) Inoue, Y.; Sato, K.; Hayashi, 0.J. Chem. Soc., Faraday Tram. 1 1987, 83, 3061. ( 5 ) Inoue, Y.; Matsuo, J.; Sato, K . J . Chem. Soc., Faraday Tram. 1 1990, 86,261 1. (6)Inoue, Y.; Matsukawa, M.; Sato, K . J . Am. Chem. SOC.1989,1 1 1 , 8965.
0022-365419212096-2222$03.00/0
is a new surface phenomenon and needs further investigation. The present work was undertaken to elucidate the SAW effects on the kinetic behavior of catalytic ethanol oxidation and to clarify the mechanism of the catalytic activation. A thin film of palladium was used as the catalytically active phase. A kinetic study was carried out to determine the reaction mechanism of the oxidation in the absence and presence of the SAW. The catalytic activity as a function of radio frequency (rf) power was investigated. One of the surface phenomena occurring during the SAW propagation is considered to be the generation of an electric field, because of nonsymmetric displacement of the constituent atoms. In order to confirm this, measurements of electroluminescence were carried out. Instead of the catalyst, a Cu-doped ZnS electroluminescent material was deposited on a ferroelectric surface on which the SAW was propagated. A relationship between the intensity of luminescence and the applied SAW power was obtained.
Experimental Section A single crystal of 128O rotated Y-cut LiNb03, which is able to generate a Rayleigh type SAW, was used as a substrate. At each end of the substrate, the interdigital transducer (IDT) electrodes for the SAW generation and detection were fabricated photolithographically in the form of 20 and 14 pairs of a double finger with a spacing of 25 pm. Catalytically inactive metals used for the electrodes were either 100 nm thick Au covering 10 nm Cr or 100 nm Al. Between the SAW generation and detection electrodes, spaced 16 mm apart, a Pd film was deposited in the shape of a square (10 X 10 mm) by vapor deposition from an electrically heated pure Pd foil. The 10 nm thick Pd catalyst was prepared with an evaporation rate of 0.02-1 nm s-l. Thickness of the film was controlled during evaporation by a thickness monitor (Sloan DTM 200). In addition, an Ar ion sputtering method was used for the preparation of the Pd catalysts: a Pd metal target was sputtered in an Ar atmosphere at a rate of 0.1 nm s-I. A 40 nm thick Pd film was prepared. This thickness was measured with a surface profdometer (ULVAC Dektak IIA). The 10 nm thick Pd cataIyst prepared by the evaporation method was employed, unless otherwise specified. Figure 2 shows an SAW-related electric circuit employed for the catalytic reaction. Radio frequency power was generated from a network analyzer (Anritsu MS620J), then amplified by an amplifier (Kalmus Engineering 105C), and applied to the catalyst device. The output signal through the catalyst phase was retumed to the network analyzer to monitor the level of the SAW. The attenuators were added to the circuit for protection from reflected ( 7 ) Inoue, Y.; Matsukawa, M. J . Chem. Soc.,Chem. Commun. 1990,296.
0 1992 American Chemical Society
Effect of a Surface Acoustic Wave Propagated on LiNb03
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2223
Reaction Reaction
Figure 1. Schematic representation of models for static (a) and dynamic (b) effects caused by ferroelectric substrates: (a) oppositely polarized surfaces; (b) SAW surface.
Figure 3. Bandwidth characteristics of the SAW device used in the catalysis experiment.
NWA
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la] Figure 2. An electriccircuit for SAW generation and catalytic reactions: NWA, network analyzer; A, attenuator; M, amplifier; C, thin film Pd catalyst; D, SAW device. tlh
waves. Radio frequency power up to 1 W was applied for the catalytic reaction. The catalytic oxidation of ethanol was carried out in a gas circulation apparatus with a high vacuum system. The background pressure was 3 X 10" Torr. The connection between the SAW catalyst in a glass cell and the outside electric circuit was made as short as possible to minimize loss in the transfer of rf power. Prior to the catalytic reaction, the thin film Pd catalyst was subjected to oxidation at 523 K in 50 Torr of oxygen (1 Torr = 133.3 Pa) and then to reduction at 373 K in 50 Torr of hydrogen. The ethanol oxidation was investigated in the pressure range of 10-80 Torr and in the temperature range 343-375 K. The reactants and products were analyzed by a gas chromatograph connected to the reaction system. The temperature of the catalysts was monitored by a small thermocouple in direct contact with the back face of the LiNbO, substrate; the temperature was controlled by an outside furnace surrounding the reaction cell. Care was taken in measurements of catalyst temperature, since the direct contact of the thermocouple to the Pd catalyst surface negatively influenced the propagation of the SAW. Thus, the temperature was measured at the back face of the LiNbO, substrate. A test showed that changes in the temperature of the front face (Pd surface) can be detected in the back face with a delay of less than 10 s and within an accuracy of 1 K. These conditions permitted accurate control of the reaction temperatures in the absence and presence of the SAW. For measurements of acoustoluminescence, from a Nikolet Model 7059,electroluminescent Cu-doped ZnS (referred to as ZnS:Cu) powder was suspended in butyl acetate and then deposited homogeneously as a thin film (a few pm thick and a size of 15 X 15 mm) on the center of the SAW-propagating path. In order to obtain the frequency of electric field suitable for the electroluminescenceof ZnS:Cu, the SAW from the network analyzer was combined through a mixer with a short rectangular pulse. The pulse having a frequency of 0.1-100kHz and a width of 10 ps-1 ms was generated from a pulse generator (Hewlett Packard 214A). The modulation was monitored with a digitizing oscilloscope (Hewlett Packard HP54502A). The modulated SAW was amplified and then introduced to the ZnSCu-deposited SAW device, while electroluminescence was detected by a photomultiplier (Hamamatsu R955C) placed closely above the SAW device. RMult8 Figure 3 shows the bandwidth characteristics of the SAW generated. The wave generated at one end is able to reach the
Figure 4. Effect of SAW on ethanol oxidation over Pd catalyst. A 10 nm thick Pd film prepared by evaporation with resistance heating (Po = 30 Torr, P, = 30 Torr): 0,reaction at 375 K, A, at 353 K. A 40 nm thick Pd film prepared by Ar ion sputtering (Po = 40 TOK,P, = 40 Torr): 0,reaction at 353 K. Applied power for all reactions was J = 1 W.
1
l a
10
9 I Torr
50 100
[b
10
p0 /Torr
50
100
Figure 5. Dependence of the reaction rate upon ethanol (Po = 30 Torr) (a) and oxygen (P,= 30 Torr) (b) pressures in the presence and absence of SAW (T = 353 K,J = 1 W): 0, SAW on; 0 , SAW off.
other end through the catalytic phase. The center frequency of the SAW, F,is given by
F = V/X (1) where Vis the propagating velocity of SAW and X is the wavelength which is equivalent to one unit length of the IDT electrode (8 times finger width in this case).* Since V = 3980 m s-l and X = 200 pm, it follows that F is 19.9 MHz. The observed frequency of 19.4 MHz was almost the same as this calculated value. The attenuation of the SAW power was below -8 mdB at the center frequency. The bandwidth was ca. 1.0MHz at attenuation of -10 mdB and ca. 1.5 MHz at -20 mdB. For the catalytic reaction, the SAW was scanned between 19.15 and 19.65 MHz at an interval of 1.4 s. Figure 4 shows changes in the catalytic activity for ethanol oxidation with the SAW on and off at different reaction temperatures. Turning the SAW on caused an immediate rise in the activity by a factor of 1.6at 315 K and 2.6 at 353 K. With the SAW off, the activities decreased to their original levels. The same results were obtained on a Pd film prepared by Ar ion sputtering. The activity before the SAW was turned on was considerably higher than that of the Pd film prepared by evap(8)
Wohltjen, H.;Dessy, R. Anol. Cbem. 1979, 51, 1458.
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The Journal of Physical Chemistry, Vol. 96, No. 5, 1992
Inoue et al. 40 -
h2C 3
n 201
I
0
0.2
0.4 0.6 OB J I W
Figure 6. Change in the catalytic activity with SAW power. V/Vois the ratio of activity in the SAW on to the S A W off ( T = 353 K,Po = P, = 30 Torr).
Downloaded by UNIV OF GEORGIA on September 12, 2015 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/j100184a036
'Ot
I
1.0
2.0
1.5
25
JIW Figure 8. Change in luminescence intensity with S A W power (pulse width = 100 p , pulse frequency = 1 kHz).
'
7 1
0
°
1
Figure 9. A linear relationship between In B and P I 4 .
Le., a surface temperature increase by SAW. This possibility, however, is ruled out for two reasons. First, the activity increase by the SAW is dependent upon the kinds of the catalytic reactions; it is prominent for the ethanol oxidation on Pd6 and the dimerization of formaldehyde on C U ,whereas ~ it is scarcely observed for the hydrogenation of ethylene6 and 1,3-butadiene on Pd. Second, the activation energy of the SAW-activated reaction differs from that of the unactivated reaction, indicating that the SAW gives rise to an intrinsic effect on the kinetic behavior of the catalytic reaction. The observed rate equation was approximated to first order with respect to P, and to a half-order with Po. In the oxidation reactions on Pd catalysts, oxygen is chemisorbed dissociatively and the above-mentioned rate equation can be derived in two ways depending on the contribution of ethanol. On the assumption that the rate-determining step is a reaction of a dissociatively chemisorbed oxygen with ethanol, the rate equation is given by vg
= kgP,(K$,)'/2/O
+ (K$,)'/21
(2)
where kgis the rate constant of the reaction and KOis equilibrium constant of oxygen adsorption. This equation is reduced to the observed one
Vg = k , P c ( K ~ , , ) 1 / 2
(3)
under the condition of 1 >> (K,,PO)II2.Another possibility is that the rate-determining step is a surface reaction between the dissociativeiy chemisorbed oxygen and the adsorbed ethanol! which leads to V, = k,(K30)1/2Kd'c/11+ ( K 3 0 ) ' / z+ KJ'c12
(4)
where k, is a rate constant of the surface reaction and K, is equilibrium constant of ethanol adsorption. This equation is simplified to V, = k , ( K ~ o ) ' / 2 K , P , (5) under the conditions of 1 >> (&Po)'/*+ K Z C . From these (9) Kunugi, T.; Kono, T.; Yanagisawa, M.; Arai, H . Nippon Kuguku Kaishi 1912, 2211.
Effect of a Surface Acoustic Wave Propagated on LiNb03
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equations, it follows that the observed preexponential factors involve both contributions from the preexponential factor of the rate constant of the rate-determining step and of the equilibrium constants of adsorption. As for activation energy, eqs 3 and 5 give respectively the following relationship E, = E, - Q , / 2 and E, = E , - Q , / 2 - Q, (6) where E , and E, are respectively apparent activation energy and true activation energy of ethanol oxidation and Q, and Q, are respectively the heat of oxygen and ethanol adsorption. From these relationships, it is suggested that the decrease in E, with the SAW on is associated with increases in Q,,and/or Q,. For instance, since the heat of O2adsorption on Pd filmslOJlis reported to be 280 kJ mol-', its change (e&, by 26 kJ mol-I) would be enough to cause the decrease in E,, 13 kJ mol-'. However, no significant changes were observed in the reaction orders in Powith the SAW on, which means that there were negligibly small differences in Q, between the SAW on and the SAW off. A similar situation holds for Q,. Thus eq 6 leads to AE, = AE, for the difference between the SAW on and the SAW off, and it is proposed that the SAW has more effect on lowering the true activation energy barrier than on modifying the strength of adsorption bonds. There are two factors to be taken into account for the SAW effects on the catalytic activity. One is an electric field effect, and the other is a geometric effect. The former results from the displacement of lattice atoms having no center of symmetry. Lakin'* derived the following equation for the electric field on piezoelectric substrates on the basis of perturbation theory for electromagnetic to elastic surface waves. The electric field, E,, is given by
E, = { ~ T F E ' J / ~ F ' % ~+( ~e)]1/2
(7)
where J is the average power, d is the acoustic beam width, V is the velocity of the wave, t is the permittivity of the substrate, to is the vacuum permittivity, and Kz is the electromechanical coupling coefficient. On the other hand, the following relationship between the luminescence intensity of ZnS:Cu, B, and the applied voltage, 4, has been experimentally well establi~hed'~ and is given by B = Bo exp(-A/4-'/2)
(8)
where A and Boare constants. By combining eq I with eq 8 and from E, = (27r/X)4, one is able to obtain the following equation In B = -CJ-'14 + In Bo (9) where C = A{2E'/rFdto(l E))-'/~. For the present results (Figure 8), a good relationship is found between the left side term, In B, and J-'/4,as shown in Figure 9. These results give experimental evidence that eq 7 can be used for the calculation of strength of an electric field generated at the surface. The electric field is calculated to be as high as 1 X lo4V cm-I at a power of 1 W. It is to be noted that this value falls within an electric field range of 104-105 V cm-I, previously determined for the electro-
+
Somoriai. G.A. Aneew. Chem.. Int. Ed. E n d . 1977. 16. 92. Toyosiima, I.; SomGjai, G. A. datal. Ren-ici. Eng: 1979,19, 105. Lakin, K. M. J . Appl. Phys. 1971, 42, 899. Kirton, J. Panel Electroluminescence; Handbook on Semiconductors; Moss, T. S., Ed.; North-Holland: Amsterdam, 1981; Vol. 4, p 672. (10) (1 1j (12) (1 3)
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2225 luminescence of ZnS:Cu 1 a ~ e r s . I ~ Among the catalytic reactions examined, the SAW effects such as the changes in reaction rate and activation energy appeared in the oxidation of ethanol and the dimerization of formaldehyde, whereas the effects were negligibly small for the hydrogenation of ethylene and butadiene.' The difference suggeststhat the SAW effects is prominent for the reaction involving oxygen and/or polar molecules. This is in line with the consideration that polar adsorbed species more readily undergo the influence of the field than nonpolar species. In the previous study on the CO oxidation,' the positively and negatively polarized LiNb03 crystals were used as a catalyst substrate for the Pd thin films, and it was shown that the activation energy of the reaction differed between the oppositely polarized substrates. These results evidently indicate the effect of electric fields on the catalytic reaction. However, there is a marked difference between the effect of the polarized substrates and the SAW effect (cf. Figure 1). In the latter case, both the positive and negative fields simultaneously exist on the SAW surface. Thus, this is likely to lead to the situation that the opposite field is immediately compensated for in the case that the catalyst on the SAW propagation is a pure metal. With respect to the present SAW-induced activity increase with a Pd catalyst, it might be argued that the electronic states of the thin Pd film are different from those of usual Pd metals or that the Pd film is oxidized at a surface region during an oxidation reaction to form an oxide layer which has surface characteristics different from that of the metal. In the latter case, it is likely that the SAW affects the density of charges in the oxide. The geometric effect is due to the displacement of the surface lattice atoms during the SAW propagation, which results in dynamic changes in a bond length and angle between the lattice atoms. Fifty wavelengths of the SAW were formed in the Pd film. From parameters such as the reaction rate increased by SAW, the number of Pd atoms exposed at the surface, No, and the center frequency, F, we are able to calculate the probability, p, that an adsorbed species is allowed to react and desorbs from a surface Pd atom per SAW. By assuming that No is 1.3 X lOI5 cm-2, the value of p at 363 K is estimated as 5 X lo4. This extremely small value suggests either that only certain Pd atoms are sites activated by the SAW or that the efficiency of activation due to the SAW is very small. From the geometric point of view, the most highly displaced atoms are located at the top and the bottom position of the wave. Provided that these atoms are responsible for the catalytic activation by SAW, the value of p increases to an order of At present, we have no evidence showing that these atoms play an active role in the SAW effect. Nevertheless, it is interesting to note that the feature of these atoms are analogous to that of atoms at a "step" position in a crystal surface. The small value of p suggests that much larger increases in the catalytic activity would be possible if the SAW effect was transferred more efficiently to catalytically active phases. In this respect, there is a need to apply the SAW for various kinds of catalysts and catalytic reactions. Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research from The Ministry of Education, Science and Culture and by The Japan Securities Scholarship Foundation. Registry No. Pd, 7440-05-3; LiNbO,, 12031-63-9; EtOH, 64-17-5; Cu, 7440-50-8; ZnS, 1314-98-3.
