J. CHEM. SOC. FARADAY TRANS., 1994. 9@5), 815-81h FARADAY COMMUNICATIONS Acoustic Effects on Catalytic Activities of Cu and Pd Thin Films combined with Piezoelectric Lead Strontium Zirconium Titanate Activated by Low-frequency Voltage Yasunobu lnoue Department of Chemistry, Nagaoka University of Technology, Nagaoka , Niigata 940-21,Japan A piezoelectric material with a dynamic radial-extensional lattice displacement has been used as a substrate for thin film Cu and Pd catalysts. Acoustic effects were found to bring about a marked increase in activity at a resonance frequency. Recently, we have demonstrated that the surface acoustic waves (SAWs) of Rayleigh and shear horizontal leaky types which are produced on ferroelectric single crystals of LiNbO, and LiTaO, have marked effects on the enhancement of activity of thin film Pd and Cu catalysts deposited on the SAW propagation path.'--6 The activity increases by the SAWs are closely associated with the forced lattice displace- ment of the catalyst surfaces and the generation of high elec- tric fields.Another method for producing forced lattice displacement of crystals is piezoelectric bulk lattice distortion which is caused by low-frequency (LF) voltage applied to electrodes deposited on the front and back planes of poled piezoelectric crystals. An interesting feature is that the lattice displacement takes place to a remarkable extent at a resonance frequent!,. On the basis of our preliminary work, we have predicted that there exist acoustic effects in which the application of LF voltage to the piezoelectric crystal as a catalyst substrate leads to the activation of a thin film catalyst deposited thereon.' The present study aims to verify the presence of the acoustic effects which are considered to permit in situ control of reaction rates while catalytic reactions proceed.A lead strontium zirconium titanate sample, Pbc,,,, Sro,05Zro,53Tio~4,03(referred to as PSZT). was prepared in the form of a disc 20 mm in diameter and 0.2 mm in thick- ness. Silver electrodes were deposited on the front and back planes of the disc. Copper or palladium, as catalyst. was deposited by an evaporation method at a thickness of SO nm. sufficient to cover the Ag electrodes completely.LF voltage was introduced to the electrodes in the range 0-15 V (peak-to-peak voltage) with a frequency of 60--110 kHz. In order to find a resonant frequency, admittance of the samples was measured with a network analyser at reaction temperature. The catalytic oxidation of ethanol was examined with a gas-circulating reaction system, and the products were analysed with a gas chromotograph. A small CA thermocouple was attached to the catalyst surface to monitor the surface tem- peratures. Fig. 1 shows the formation of acetaldehyde by oxidation of ethanol on Cu at 383 K and on Pd at 353 K. After the reac- tion rate reached a stationary level, LF voltage was applied at a frequency of 86.1 kHz. The reaction rate on Cu imme-diately increased by a factor of 4.7.The increased rate was maintained as long as LF voltage was imposed and returned to the original rate with turn-off. The figure also shows the result when a constant voltage was applied instead of LF voltage. In this case, there was little increase in the reaction rate. For the Pd catalyst, an increase in the reaction rate occurred with LF voltage on. but the increase was 7O(Il less lhan that with the Cu catalyst. As shown in Fig. 1, the tem- perature of the catalyst surface was raised with LF voltage m, but decreased to the original level after 10 min, since the :atalyst temperature was accurately controlled by an electric "urnace Note that the increased catalytic activity was main- rained independently of the short-period fluctuation of the ,u r face temper at ure.Fig. 2 shows an increase in the activity of the Cu catalyst with increasing LF voltage. No saturation of the activity was lbserved over the voltage range used. Fig. 3 shows Arrhenius plots of the reaction rate on the Cu catalyst. With LF voltage jn the temperature dependence varied and the activation :nergy decreased from 55 to 29 kJ mol-', indicating that mhancement of the catalytic activity is related to intrinsic ,urface pheomena rather than to a thermal effect. Fig. 4 shows the ratio of activity changes with the Cu cata- :>st as a function of frequency. There was little change below off v cu 2 4 t 'h Fig. 1 Acetaldehyde formation from ethanol oxidation on applica- tion of LF and dc voltage.(0)Cu catalyst,f= 86.1 kHz, LF voltage 1 l,F)= 15 V, T = 383 K; (0) 1 = 353 K; (a) Pd catalyst, f= 86.1 kHz. V,, = 15 V, Cu catalyst, dc voltage CV,,) = 15 V, T = 383 K. 816 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I 0 5 10 15 20 VLF/v Fig. 2 Relative activity, ron/roff,as a function of V,,. Cu catalyst, T = 383 K,f= 86.1 kHz. r,, , activity with V,, = 15 V; roff,activity without LF voltage (V,, = 0). 65 kHz. With increasing frequency, the activity increased markedly, particularly above 80 kHz, reaching a maximum at 86.1 kHz and then decreasing sharply with higher fre- quencies. Measurements of admittance of the catalyst-depositing PSZT sample showed that the resonance frequency was 86.0 kHz which is in good agreement with the frequency at which the catalytic activity increased.Since the resonance causes an extraordinary radial-extensional dis- placement, it is evident that the displacement of lattice atoms of the catalyst at the resonance mode may be associated with the activity increase. Recently, Boronin and co-workers' showed that the work function of a Pt(100) surface combined with piezoelectric quartz varies at the eigenfrequency. In X-ray photoelectron sputter depth profile analysis of the Cu catalyst used for the catalytic reaction at 393 K, an X-ray induced L,M,,,M,,, Auger peak due to Cu' was observed not only at the surface but also in the bulk, indicating that the total layer of the Cu catalyst was oxidized to form Cu,O 2.0 1.o c k 0.5-0 E,s 0.1 'Q 2.4 2.6 2.8 103 KIT Fig.3 Arrhenius plots for the reaction on a Cu catalyst. (a)VLF = V,,15 V,f= 86.1 kHz, (0)= 0. 5 c I I I I I I 60 80 100 120 frequency/kHz Fig. 4 r,,,/rOff,as a function of frequency. Cu catalyst, T = 383 K, V,, = 15 V. A broken line shows admittance. r,, ,activity with V,, = 15 V, roff,activity with V,, = 0. during the catalytic reaction. Thus, it is likely that differences between the semiconducting and metallic characters are associated with a larger increase in the catalytic activity with LF voltage for Cu than for Pd (Fig. l), since the lattice dis- placement is considered to influence the electron density at the surface of semiconducting oxide catalysts, which is similar to that observed in the SAW Application of dc voltage in place of LF voltage had little effect on activation of the Cu catalyst.From these results, it is evident that the res- onance mode of lattice displacement plays an important role in the activation of surfaces, the efficiency of which is strongly associated with the arrangements of atoms and electronic structures at the catalyst surface. This work was supported by the Sumitomo Foundation. The author thanks Mr. T. Kamoshida for measurements of the catalytic activity. References 1 Y. Inoue, M. Matsukawa and K. Sato, J. Am. Chem. SOC., 1989, 111,8965. 2 Y. Inoue and M. Matsukawa, J. Chem. SOC., Chem. Commun., 1990,296. 3 Y. Inoue, M. Matsukawa and K. Sato, J. Phys. Chem., 1992, 96, 2222. 4 Y. Inoue, M. Matsukawa and H. Kawaguchi, J. Chem. SOC., Faraday Trans., 1992,88,2923. 5 Y.Inoue and M. Matsukawa, Chem. Phys. Lett., 1992,198,246. 6 Y. Inoue and Y.Watanabe, Catal. Today, 1993. 7 T. Kamoshida, K. Sat0 and Y. Inoue, Proc. 11th Surface Science Conference, The Surface Science Society of Japan, Tokyo, 1991, p. 49. 8 V. N. Brezhnev, A. I. Boronin, V. P. Ostanin, V. S. Tupikov and A. N. Belyaev, Chem. Phys. Lett., 1992, 191, 379. Communication 3/07479J; Received 21st December, 1993