Ž .Applied Surface Science 144–145 1999 385–389

Effects of resonance oscillation with a thickness-extensionalmode on activation of thin film metal and metal oxide catalysts

deposited on poled ferroelectric substrates

N. Saito, H. Nishiyama, K. Sato, Y. Inoue )

Department of Chemistry, Nagaoka UniÕersity of Technology, Nagaoka, Niigata 940-2188, Japan

Abstract

Ž .The effects of resonance oscillation RO with a thickness-extensional mode generated in a poled ferroelectric z-cutLiNbO single crystal by rf power were studied on reaction selectivity for the ethanol decomposition of thin film Au, Ag and3

WO catalysts deposited. For a 100 nm Ag catalyst, a 3.5 MHz RO at 3 W caused an increase in ethylene production at 5733

K by a factor of 16 without any change in acetaldehyde production. The selective promotion of ethylene production by ROwas also observed at 573 K for a Au catalyst in which the activity for ethylene production increased by a factor of 5. For aWO catalyst the RO increased both the ethylene and acetaldehyde productions, but the former was more largely promoted3

than the latter. The RO caused decreases in respective activation energy. The selective production of ethylene is associatedŽ .with the RO-induced strong oxygen–metal and metal oxide surface interactions which are produced by dynamic lattice

displacement. q 1999 Elsevier Science B.V. All rights reserved.

PACS: 82.65.J

Keywords: Resonance oscillation; LiNbO ; Ag catalyst3

1. Introduction

In heterogeneous catalysis by metals and metaloxides, the design of catalyst surfaces with artifi-cially controllable functions is very important andchallenging. Recently, we have demonstrated that

Ž .resonance oscillation RO generated on a poledferroelectric crystal by rf electric power has thefeature of dynamic lattice displacement which is

w xeffective for the activation of metal catalysts 1–3 .

) Corresponding author. Tel.: q81-258-47-9832; Fax: q81-258-47-9830; E-mail: inoue@analysis.nagaokaut.ac.jp

A thickness-extensional mode RO generated at 3 Won a z-cut LiNbO single crystal was found to3

increase the catalytic activity for ethanol oxidation ofw xa deposited Pd catalyst by a factor of 1880 4 .

More recently, for the same reaction on a Ag cata-lyst deposited on polycrystalline Pb Sr -0.95 0.05

Ž .Zr Ti O PSZT crystal, it has been shown that0.57 0.43 3

a thickness-extensional mode RO caused unique po-larized surface-dependent changes in the reactionbehavior, which was completely different from a

w xradial-extensional mode RO 5 .From a viewpoint of high catalyst performance, it

is strongly desirable to artificially control the selec-

0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-4332 98 00828-9

( )N. Saito et al.rApplied Surface Science 144–145 1999 385–389386

tivity of catalytic reactions. Thus, the present studywas undertaken to reveal the effects of a thickness-extensional mode RO on catalyst selectivity. Thethin film catalysts of Au and Ag metals and of aWO metal oxide were employed for ethanol decom-3

position to produce ethylene and acetaldehyde. ForAu and Ag metal catalysts, RO was found to in-crease ethylene production without any change inacetaldehyde production which indicated the selec-tive promotion of ethylene production. For a WO3

catalyst, the activity for both the products wereaccelerated, but the ethylene production was moresignificantly promoted than the acetaldehyde produc-tion. The RO effects are concluded to be useful forthe design of metal and metal oxide surfaces withartificially controllable functions for reaction selec-tivity.

2. Experimental

A ferroelectric single crystal of z-cut LiNbO3Ž .referred to as z-LN whose polarization axis isperpendicular to the surface was cut into a rectangle

w xshape and used as substrate for RO generation 4 .Both of the front and back crystal planes werecovered with a 100 nm thick Ag or Au film byresistance-heating of a respective pure metal in highvacuum. Au wires were attached to the thin films forrf power introduction. These catalysts are referred toas Agrz-LN and Aurz-LN, respectively. For a

Fig. 1. RO effects on ethanol decomposition on a Agrz-LNcatalyst. Reaction temperature T s573 K, RO power Js3 W.Ethanol pressure Pes4.0 kPa. B; ethylene, v; acetaldehyde.

Fig. 2. RO effects on ethanol decomposition on a Aurz-LNcatalyst. T s573 K, Js3 W, Pes4.0 kPa. B; ethylene, v;acetaldehyde.

WO rz-LN catalyst, z-LN crystal planes were first3

covered with Al electrodes and then with a 100 nmWO film by reactive sputtering using a WO target3 3

w xin an ArrO atmosphere 6 .2

For the RO generation, rf power from a networkŽ .analyzer Anritsu MS3606B was amplified by anŽ .amplifier Kalmus, 250FC and then applied to the

sample after impedance adjustment. The catalystswere placed in a glass cell equipped with BNC-junc-tions for rf-power introduction. The catalytic ethanoldecomposition was carried out in a conventional gascirculating vacuum apparatus, and the reactant andproducts were analyzed by a gas chromatographdirectly connected to the reaction system. Measure-ments of catalyst temperatures were performed by a

Fig. 3. Arrhenius plots of ethylene production rate, V , in ethanole

decomposition on a WO rz-LN catalyst. Js3 W, Pes1.3 kPa.3

B: RO-off, I: RO-on.

( )N. Saito et al.rApplied Surface Science 144–145 1999 385–389 387

Fig. 4. Arrhenius plots of acetaldehyde production rate, V , ina

ethanol decomposition on a WO rz-LN catalyst. Js3 W, Pes3

1.3 kPa. v: RO-off, `: RO-on.

non-contacting method using a radiation thermome-ter and by a shift in resonance frequency. Thesetemperatures were calibrated with those measured bya thermocouple directly attached to a sample surface.The temperature of catalyst surface was controlledusing an outer electric furnace.

3. Results

As demonstrated previously, a z-LN crystal has aw xthickness mode vibration 4,7 . A sample depositing

100 nm Au thin film showed the same resonanceŽ . Ž .lines at a frequency of 3.5 First , 10.5 second and

Ž .17.5 third MHz at room temperature as observedw xpreviously 4 . These frequencies were in accordance

with a series of 2ky1 where ks1, 2, 3. In thepresent work, the primary resonance frequency of

Ž .3.5 MHz ks1 was used for catalyst activation,unless otherwise specified.

Fig. 1 shows ethanol decomposition at 573 K on aAgrz-LN catalyst. Without RO, the reaction pro-duced both ethylene and acetaldehyde with constantrates from an initial stage in which the former wasslightly largely produced. When RO was generatedat 3 W, an immediate increase in the ethylene pro-duction occurred, and the activity increased by afactor of 16. The increased activity was maintainedas far as RO was turned on and decreased to theoriginal level with RO-off. On the other hand, foracetaldehyde production, no significant change wasobserved with RO-on and RO-off. The selectivity forethylene production is defined as the ratio of activityfor ethylene production to that for total production ofethylene and acetaldehyde, i.e.,

Ss100=V r V qVŽ .e e a

where V and V are ethylene and acetaldehydee a

production activity, respectively. The value of S was58% with RO-off and increased to 90% with RO-onat 3 W.

Fig. 2 shows a result of ethanol decomposition ona Au catalyst in which acetaldehyde is a majorproduct. Turning the RO on resulted in an immediateincrease in ethylene production activity by a factorof 5, whereas the acetaldehyde production remainedwithout changes. The value of S increased from 6%to 10% by RO. It is to be noted that the selective ROeffect on the ethylene production occurs for both Agand Au in which the major product is ethylene andacetaldehyde, respectively. From the Arrhenius plotsof ethylene and acetaldehyde production rate overthe temperature range 573–653 K, activation energywas compared in the presence and absence of RO.The activation energy for ethylene production de-creased from 155 to 145 kJ moly1 and that for

Table 1Changes in activation energy and ethylene selectivity with RO

y1Ž . Ž .Catalyst Activation energy kJ mol S % at 573 K

Ethylene Acetaldehyde

RO-off RO-on RO-off RO-on RO-off RO-on

Agrz-LN 156 115 95 95 58 90Aurz-LN 155 145 91 91 6 10WO rz-LN 115 64 46 21 13 303

( )N. Saito et al.rApplied Surface Science 144–145 1999 385–389388

acetaldehyde production, 91 kJ moly1, remained un-changed.

Fig. 3 shows the Arrhenius plots of the ethyleneproduction on a WO rz-LN catalyst. Without RO,3

the slope of the plot yielded an activation energy forethylene production of 115 kJ moly1. With RO-on, itdecreased to 64 kJ moly1. Fig. 4 shows the Arrhe-nius plots for acetaldehyde production. The activa-tion energy decreased from 46 kJ moly1 with RO-offto 21 kJ moly1 with RO-on. The RO effects on theethanol decomposition over the three catalysts aresummarized in Table 1.

4. Discussion

The interesting features of RO effects on theethanol decomposition on the Ag and Au metals arethat only the ethylene production was enhancedwithout giving rise to significant changes in theacetaldehyde production, irrespective of which prod-uct is a major one. On the other hand, the RO effecton the same reaction over the WO metal oxide was3

preferable enhancement of ethylene production, al-though acetaldehyde activity was also increased to alesser extent.

In measurements of temperature dependence ofresonance frequency, it was shown that little temper-ature rise occurred with a z-LN crystal when 3 WRO was applied at higher temperatures than 520 K.Since the present reactions were carried out abovethis temperature, a temperature rise during RO-on isnegligible. The fact that only ethylene productionwas accelerated without any increase in acetaldehydeproduction in spite of as high activation energy as91-95 kJ moly1 of acetaldehyde production on Agand Au catalysts gives evidence to little rise inreaction temperatures with RO-on. Therefore, we canconclude that no thermal effect contributes to theselectivity changes.

In a previous kinetic study of ethanol oxidation ona positively polarized Pdrz-LN catalyst, the reactionorders with respect to oxygen pressure decreased

w xfrom 0.5 to y0.1 with RO-on 8 . This change hasbeen related to the RO-induced stronger bond forma-tion of adsorbed oxygen, thus suggesting that the ROaffects the electronic and geometric factors of the Pdsurface so as to produce strong oxygen-Pd surface

atom interactions. In previous measurements of pho-toemitted electrons from a Ag thin film deposited on

w xa positively polarized PSZT substrate 5 , a thick-ness-extensional mode RO caused a negative shift ofphotoemission threshold energy by 0.12 eV, which isassociated with a decrease in work functions of theAg metal. This result is indicative of RO effects onthe electronic or geometric properties of the thin filmmetal surfaces. It is likely that similar changes takeplace with the Agrz-LN catalyst. Thus, an increasein ethylene selectivity with RO is associated with aresult that RO induces strong oxygen–Ag surfaceinteractions which promote the abstraction of H O2

from adsorbed ethanol and hence accelerate dehydra-tion reaction to ethylene.

Thin film metal oxides deposited on the propaga-Ž .tion path of surface acoustic waves SAWs have

been shown to cause not only significant SAW prop-agation loss, but also decreases in the amplitude of

w xSAW-induced lattice displacement 9 . We have re-cently showed that the thin films of NiO and ZnOchanges their adsorptive properties in the presence of

w xSAWs 10 . These changes are associated with inter-actions between charge carriers in metal oxides and

w xSAWs 11 . Since RO has a common feature withSAWs in dynamic lattice displacement, it is likelythat charge carriers in WO are influenced by RO3

which affects the density of electrons responsible forthe adsorption of ethanol. An increase in ethyleneselectivity of the reaction on WO indicates that the3

strong interactions with oxygen occur, as demon-strated in the metal catalysts, although their extent israther weak, compared to that of the metals.

In conclusion, RO is effective for the control ofcatalyst selectivity and promising to the design ofmetal and metal oxide catalyst surfaces which permitthe artificial control of selectivity.

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