2040
J. Phys. Chem. B 2003, 107, 2040-2045
Effects of Thickness Extension Mode Resonance Oscillation of Acoustic Waves on Catalytic
and Surface Properties. IV. Activation of a Ag Catalyst for Ethanol Decomposition by
Overtone Resonance Frequencies
N. Saito and Y. Inoue*
Department of Chemistry, Nagaoka UniVersity of Technology, Nagaoka, 940-2188, Japan
ReceiVed: September 25, 2002; In Final Form: December 7, 2002
The effects of resonance frequencies of acoustic waves on catalytic and surface properties were studied. The
overtone resonance frequencies of 3.5, 10.8, and 17.9 MHz were applied to a 100 nm thick Ag catalyst
deposited on a ferroelectric z-cut LiNbO3 crystal which generated thickness extension mode resonance
oscillation (TERO). For ethanol decomposition, the TERO enhanced ethylene production without significant
changes in acetaldehyde production for all the frequencies. The extent of catalyst activation strongly depended
on the resonance frequency. In a low power region (<0.8 W), the TERO effect had a maximum at the middle
of frequency, whereas in a high power region (>1.0 W), it increased in the order 3.5 > 10.8 > 17.9 MHz.
The activation energy for ethylene production decreased remarkably in the presence of TERO, the extent of
which strongly depended on the frequency. Laser Doppler measurements showed that with increasing resonance
frequency, the number of standing waves increased markedly, whereas the amplitudes of the wave decreased
considerably. The specific catalytic activity, defined as the activity enhancement per the density of wave,
increased in a nonlinear manner with lattice displacement. The resonance frequency effects of TERO on
catalyst activation are discussed.
Introduction
catalysts and for better understanding of the mechanism, it is
of particular importance to clarify the effects of resonance
frequency, that is, to see how different resonance frequencies
affect the catalytic and surface properties.
In a series of the studies of acoustic wave resonance
oscillations in heterogeneous catalysis,1-11 we have demon-
strated the effects of lattice vibration modes and the polarization
fields on the catalytic activity and reaction selectivity. The
thickness extension mode resonance oscillation (TERO) of bulk
acoustic waves, generated by applying radio frequency power
to a ferroelectric z-cut LiNbO3 crystal, increased ethylene
production markedly with a little influence on acetaldehyde
production in ethanol decomposition over thin Ag and Pd film
catalysts. Interestingly, the thickness shear mode resonance
oscillation (TSRO) generated on an x-cut LiNbO3 crystal had
no ability to activate ethylene and acetaldehyde production.12
Based on the lattice vibration and photoemission spectroscopic
analyses,13 it was clearly demonstrated that TERO induced large
vertical lattice displacement that caused positive work function
shifts, which were responsible for activity enhancement and
selectivity changes. Furthermore, the TERO effects were shown
to be strongly dependent on the polarized surfaces of the
ferroelectric crystal: the catalyst activation and selectivity
changes were different between a Ag catalyst deposited on a
positively polarized surface (denoted here to as (+)Ag) and that
deposited on a negatively polarized surface ((-)Ag).14 The
differences were associated with the findings that the positive
work function shifts caused by the TERO were larger for (+)-
Ag than for (-)Ag. The polarization-dependent TERO effects
on catalytic and surface properties also existed for (+)Pd and
(-)Pd catalysts.7,15
There are two methods to obtain the different resonance
frequencies of TERO. One is to employ overtone frequencies
that appear as a series of resonance frequencies such as the first,
the second, the third, and so on. The other is to change the
crystal thickness of a ferroelectric crystal, since the frequency
of TERO is inversely proportional to the thickness of a
ferroelectric crystal. In the present study, the former method
was chosen as a first step of research. In a preliminary
investigation, we have found that the number and amplitudes
of standing waves generated by TERO vary with resonance
frequencies. In order to accurately examine the resonance
frequency effects, it is considered beneficial to employ a
ferroelectric crystal with a square shape, rather than that with a
previously employed rectangular shape, since uniform distribu-
tions of standing waves would be more expected, because of a
symmetric structure. The TERO effects with the first frequency
of 3.5 MHz have already been reported for a ferroelectric crystal
with a rectangular shape. However, because of the different
crystal shape, it would be more convenient to show the results
obtained for the first resonance frequency of a square shape
ferroelectric crystal here, together with those for the second and
third frequencies for better comparison of resonance frequency
effects.
Experimental Section
In all the studies of TERO effects, we have used the first
primary resonance frequency of 3.5 MHz only. For further
development of acoustic wave excitation effects on metal
A z-cut LiNbO3 single crystal was used as a ferroelectric
substrate. The crystal (1 mm in thickness) was cut into a square
shape of 14 mm × 14 mm. By assuming that the vibration mode
of resonance oscillation is a thickness extension mode, fre-
* Corresponding author.
10.1021/jp022136k CCC: $25.00 © 2003 American Chemical Society
Published on Web 02/08/2003