Thickness Extension Mode Resonance Oscillation Research Articles

Effects of Thickness Extension Mode Resonance Oscillation of Acoustic Waves on Catalytic and Surface Properties. VII. Catalytic Activity of AgAu Binary Alloy for Ethanol Decomposition

The effects of thickness extension mode resonance oscillation (TERO) of acoustic waves on ethanol decomposition over thin AgAu binary alloy film catalysts were studied. TERO enhanced the activity of the alloy catalysts for both ethylene and acetaldehyde production. The activation for ethylene production by TERO was nearly independent of alloy composition, whereas that for acetaldehyde production exhibited a maximum at around 80% surface Au content. The effects of TERO on the alloy were intrinsically different from those on an Ag metal catalyst, in which TERO-induced activation occurred only for ethylene production. Doppler measurements showed that vertical lattice displacement caused by the TERO became the smallest at an alloy composition of 80% surface Au content and demonstrated that the TERO effects on acetaldehyde production over the AgAu binary alloy catalysts increased significantly in the case where Ag-Au atom interactions became strong. A mechanism of the alloy activation by TERO is discussed.

Promotion of partial oxidation of methanol over thin Pt and Pd film catalysts by resonance oscillation of acoustic waves

The thickness extension mode resonance oscillation (TERO) of bulk acoustic waves generated on z-cut LiNbO3 by rf electric power was employed to the catalytic oxidation of methanol over thin Pt and Pd film catalysts deposited. Both the catalysts produced formaldehyde, methyl formate, and carbon dioxide as carbon-containing products in the gas phase. The TERO considerably increased the selectivity for formaldehyde production on Pt at a moderate rf power, indicating the ability of promoting the partial oxidation of methanol, whereas a small and monotonous decrease in the selectivity of the reaction on Pd occurred with power. The different TERO effects on the selectivity are discussed.

Effects of Thickness Extension Mode Resonance Oscillation of Acoustic Waves on Catalytic and Surface Properties. 5. Effects of Ferroelectric Crystal Thickness on the Catalytic Behavior of Ag for Ethanol Decomposition

The five kinds of ferroelectric z-cut LiNbO3 crystals with different thicknesses of 0.3, 0.5, 1.0, 2.0, and 2.2 mm were employed to reveal the effects of frequency in thickness extension mode resonance oscillation (TERO) of acoustic waves on the catalytic properties of a 100 nm thick Ag film deposited onto the crystals. The different thicknesses of the crystals provided 11.2, 7.3, 3.6, 1.8, and 1.6 MHz as the primary resonance frequency, respectively. In ethanol decomposition producing ethylene and acetaldehyde, the characteristics of the TERO that caused significant increases in catalytic activity for ethylene production were observed for the five frequencies, but the extent of activity increments depended on the frequency: a maximum appeared at a medium frequency of 7.3 MHz. Remarkably large decreases in the activation energy for ethylene production occurred with 1.6−1.8 MHz, compared to those of the rest of the frequencies. Laser Doppler measurements showed the different distributions of lattice displ...

Different acoustic wave effects of thickness extension and thickness shear mode resonance oscillation on ethanol decomposition over Pd catalysts deposited on poled ferroelectric LiNbO 3 single crystals

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.

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

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 ( 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...

Effects of Thickness Extension Mode Resonance Oscillation of Acoustic Waves on Catalytic and Surface Properties. III. Ethanol Decomposition on a Thin Ag Film Catalyst Deposited on Negatively Polarized z-cut LiNbO3

Effects of Thickness Extension Mode Resonance Oscillation of Acoustic Waves on Catalytic and Surface Properties. III. Ethanol Decomposition on a Thin Ag Film Catalyst Deposited on Negatively Polarized <i>z</i>-cut LiNbO₃

Effects of Thickness Extension Mode Resonance Oscillation of Acoustic Waves on Catalytic and Surface Properties. II. Ethanol Decomposition on a Thin Pd Film Catalyst Deposited on Positively Polarized z-Cut LiNbO3

Effects of Thickness Extension Mode Resonance Oscillation of Acoustic Waves on Catalytic and Surface Properties. II. Ethanol Decomposition on a Thin Pd Film Catalyst Deposited on Positively Polarized <i>z</i>-Cut LiNbO₃

Extraordinary Lattice Displacement by Pulse Thickness Extension Mode Resonance Oscillation of Acoustic Wave and Its Effect on the Activity for Ethanol Oxidation of a Thin Pd Film Catalyst

Pulse radio frequency (rf) electric power was applied to a ferroelectric single crystal of z-cut LiNbO3 to generate pulse thickness extension mode resonance oscillation (p-TERO) of bulk acoustic wave, and the effects of p-TERO on the catalytic activity for ethanol oxidation of a Pd catalyst deposited on the crystal were studied. The employment of the pulse method made it possible to introduce several times larger rf power to the catalyst, compared to conventional continuous rf power introduction. The p-TERO remarkably increased catalytic activity for acetaldehyde production, and its effects were particularly great in a high rf power region. A laser Doppler method was developed to monitor lattice displacement caused by p-TERO. The patterns of lattice displacement became nonuniform in a high rf power region by the appearance of extraordinary lattice displacement. Very large activity increases in the region were associated with the lattice displacement. The mechanism of catalyst activation by p-TERO is discu...

Effects of Thickness Extension Mode Resonance Oscillation of Acoustic Waves on Catalytic and Surface Properties I. Ethanol Decomposition on a Thin Ag Film Catalyst Deposited on Positively Polarized Z-Cut LiNbO3

A 100-nm thick Ag film was deposited on a positively polarized z-cut LiNbO3 ferroelectric substrate, and the effects of thickness extension mode resonance oscillation (TERO) of bulk acoustic waves generated by rf electric power on the catalytic properties were studied. In ethanol decomposition, the TERO caused a remarkable increase in ethylene production without changes in acetaldehyde production, and the activation energy for ethylene production decreased considerably. The activity for ethylene production increased in a nonlinear manner with increasing rf power, and its selectivity resulted in a marked enhancement. A laser Doppler method demonstrated that the TERO produced randomly distributed standing waves with large lattice displacement vertical to the surface. In photoelectron emission spectra, the TERO caused a positive shift of the threshold energy in photoelectron emission from the Ag surface, which was indicative of an increase in a work function-related parameter in the presence of TERO. The vertical dynamic lattice displacement is proposed to be responsible for positive work function shifts. Correlation existed between activity enhancement and the work function shift, which indicated that activity enhancements with TERO are associated with increases in the work function. A mechanism of TERO effects on selectivity changes is discussed.

Acoustic surface lattice vibration by resonance oscillation and its effects on the selectivity of ethanol decomposition over a thin Pd film catalyst surface

The effects of acoustic surface lattice vibration on the reaction selectivity of ethanol decomposition over a thin Pd film deposited on a positive polar ferroelectric LiNbO 3 substrate were studied. Ethanol decomposition on the Pd surface produced ethylene as dehydration and acetaldehyde as dehydrogenation. The artificially forced acoustic lattice vibration due to thickness-extensional mode resonance oscillation increased ethylene production remarkably but little enhancement of acetaldehyde production: the selectivity for ethylene production was increased significantly. Three-dimensional laser Doppler measurements showed standing wave patterns, indicating that acoustic lattice vibration had large lattice displacement vertical to the surface: its amplitude reached 43 nm at 2 W. In photoelectron emission spectra, acoustic lattice vibration increased the threshold energy of electron emission from the Pd surface, which indicated an increase in a work function-relating factor. A model for the effects of acoustic surface lattice vibration is proposed.

Acoustic wave effects of thickness-extensional mode resonance oscillation on catalytic activity for CO oxidation of thin TiO 2 and NiO films deposited on a z-cut LiNbO 3 single crystal

The effects of thickness-extensional mode resonance oscillation (TERO) on the catalytic activity for CO oxidation of thin TiO 2 and NiO films deposited on a z-cut LiNbO 3 ( z-LN) single crystal were investigated. TERO at 3 W caused 4 – 7-fold increases in the catalytic activity of TiO 2/ z-LN and NiO/ z-LN and remarkable decreases in the activation energy of the reaction. Vertical lattice displacement by TERO was decreased remarkably by the presence of the metal oxides. The catalyst activation is due to the interactions of TERO with the carriers in the semiconductor metal oxides.

Effects of acoustic waves generated on a positively polarized lead strontium zirconium titanate substrate upon catalytic activity of a deposited Ag thin film

A positively polarized lead strontium zirconium titanate (PSZT) substrate was employed for the generation of thickness-extensional mode resonance oscillation (TERO), and the effects of TERO on the catalytic activity and the surface properties of a 100-nm Ag film catalyst deposited on the substrate were investigated. The catalytic activity for ethanol oxidation increased 18-fold with TERO at 3 W. In low energy photoelectron spectroscopy, a threshold energy for photoelectric emission from the Ag surface shifted linearly to the lower energy side with increasing power of TERO, thus indicating a decrease in the work function of the Ag surface. On the basis of the behavior of lattice displacement measured by a laser Doppler method, a model for changes in catalytic activity and work function is proposed.

Design of heterogeneous catalysts with artificially controllable functions using effects of acoustic wave excitation

The effects of surface acoustic waves (SAWs) and resonance oscillation (RO) of bulk acoustic waves on catalytic activity and selectivity were studied in an attempt to design a heterogeneous catalyst which has artificially controllable functions for chemical reactions. The propagation of Rayleigh SAW and shear horizontal leaky SAW through thin Pd catalyst films deposited on poled ferroelectric LiNbO3 and LiTaO3 crystals caused considerable activity increases in ethanol oxidation: the enhancement was much larger for the oxidized than the reduced Pd. Similar effects were observed for a Ni catalyst. The RO effects on catalyst activation for ethanol oxidation were associated with the polarized (positive and negative) surfaces and the vibration modes of the ferroelectric substrates. The thickness extension mode resonance oscillation (TERO) of z-cut LiNbO3 caused different changes in the activation energy and reaction order between the positive and negative plane of the LiNbO3 substrate. Different catalyst activation was induced between TERO and the radial extension mode resonance oscillation (RERO) of a Pb0.95Sr0.05Zr0.53Ti0.47O3 ferroelectric substrate. For ethanol decomposition on a Ag catalyst, the TERO of z-cut LiNbO3 increased ethylene production without significant enhancement of acetaldehyde production, thus demonstrating that TERO has the ability to change the selectivity in the catalytic reaction. Large dynamic lattice displacement, surface potential changes, and work function shifts were observed with TERO. A mechanism of acoustic wave excitation effect on the catalyst activation is discussed.

Artificial control of selectivity for ethanol decomposition on a Ag catalyst by thickness-extensional mode resonance oscillation of z-cut LiNbO 3

Thickness-extensional mode resonance oscillation (TERO), generated by radio frequency power on a thin Ag film catalyst deposited on a ferroelectric z-cut LiNbO 3 single crystal, caused a marked enhancement of ethylene production without significant changes in acetaldehyde production in ethanol decomposition: selectivity for ethylene production increased from 58% without TERO to 96% at 5 W. The activation energy of ethylene production decreased remarkably with TERO-on. Laser Doppler measurements showed that the TERO produced an irregular pattern of lattice displacement up to 60 nm which is proposed to induce strong Ag metal–oxygen atom interactions, thus promoting oxygen abstraction from the adsorbed ethanol.

Different effects of acoustic resonance oscillation on activation of a thin Pd film catalyst deposited on an oppositely polarized ferroelectric LiNbO 3 single crystal

A 100 nm thick Pd catalyst film was deposited on either positively or negatively polarized surfaces of a poled ferroelectric z-cut LiNbO 3 single crystal which generates a thickness-extensional mode resonance oscillation (RO) by r.f. electric power. The effects of 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.