Solid State Ionics 136–137 (2000) 693–697www.elsevier.com/ locate / ssi

Solid electrolyte aided studies of NO–CO reaction on Pd

a bSoonho Kim , Gary L. HalleraElectrochemistry Laboratory, Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, South Korea

bDepartment of Chemical Engineering, P.O. Box 208286, Yale University, New Haven, CT 06520-8286, USA

Abstract

The Pd-catalyzed NO–CO reaction at temperatures between 320 and 4808C exhibited electrochemical promotion. Thisreaction was performed in a fuel-cell configuration where Pd is coated on Y O -stabilized ZrO . The electrochemical2 3 2

promotion of the catalytic activity and modification of the selectivity to N O is reversible and this system showed both2

electrophilicity and electrophobicity, depending on the direction of oxygen ion pumping. In a typical experiment, theenhancement of reaction rate is 100 times greater than the rate of oxygen ion removal from the catalyst electrode. Rateenhancement, which is defined as the ratio of the reaction rate under electrochemical oxygen removal to the rate under opencircuit, of r 5 2.2, r 5 2.2 and r 5 4.2 were measured for V 521.8 V at 3708C. 2000 Elsevier Science B.V.CO N O N WR2 2 2

All rights reserved.

Keywords: Electrochemical promotion; Non-Faradaic; NO reduction

Materials: Palladium; Yttria-stabilized zirconia; Nitric oxide; Carbon monoxide

1. Introduction b0-Al O [2] and an aqueous alkaline electrolyte2 3

solution [3] were found to induce a similar effect.The catalytic properties of solid catalysts can be The rate of the catalytic reaction can be up to 100

modified by doping the active phase or by enhancing times greater than the open-circuit catalytic rate [4]5metal–support interactions. It is frequently accepted and up to 3310 times larger than the rate of ion

that such modifications of catalyst properties result supply to the catalyst through the solid electrolytefrom changes in the binding energies of chemisorbed [5]. This is why this novel effect has been termedspecies although geometric considerations often have non-Faradaic electrochemical promotion of catalytican important role. It has been reported that the activity (NEMCA). The terms electrochemical pro-catalytic activity and selectivity of metals can be motion (EP) [6] and in situ continued promotionaltered dramatically and reversibly by supplying or (ICP) [7] have been also proposed for the NEMCAremoving oxide anions at the metal surface by effect by other investigators.

22interfacing the catalyst with an O -conducting solid The possibility of employing solid electrolytes toelectrolyte [1]. Likewise, the sodium ion conductor monitor surface activities of adsorbed species was

first suggested by Wagner [8] in 1970. Subsequently,it has been shown that this electrochemical approach*Corresponding author. Fax: 11-203-432-0761.

E-mail address: gary.haller@yale.edu (G.L. Haller). to catalysis can be employed not just to monitor, but

0167-2738/00/$ – see front matter 2000 Elsevier Science B.V. All rights reserved.PI I : S0167-2738( 00 )00514-2

694 S. Kim, G.L. Haller / Solid State Ionics 136 –137 (2000) 693 –697

also to control the chemical activities of surface tion is considered to be the rds for the NO–COspecies. In principle, NEMCA is an in situ tool to reaction over precious metals, so we have deliberate-influence the electronic properties of the catalyst and ly chosen the reaction conditions to render the othermonitor the kinetics at the same time. It can also be reaction steps at equilibrium.used to monitor electronic properties of catalysts atdifferent kinetic conditions.

In 1975 Pancharatnam et al. [9] applied electro- 2. Experimental methodschemical oxygen pumping making use of the oxideionic conductivity of zirconia-based electrolytes, to These experiments were performed in a fuel-cellthe decomposition of nitric oxide in the presence of configuration reactor as shown in Fig. 1. Yttria-oxygen. The rate of nitric oxide decomposition over stabilized zirconia tubes (10 mm o.d., 9 mm i.d. andboth gold and platinum electrodes was found to be 305 mm in length) were purchased from Zircoa Inc.increased by up to two orders of magnitude on and washed with concentrated HNO and acetone to3

pumping oxygen away from the catalyst. eliminate possible contaminates. The volume of the2When employed as an active catalyst support, the reactor calculated by pr times the length of the

3solid electrolyte generates spillover ions and the working electrode is ca. 10 cm and the apparent2spillover ions are accompanied by their compensat- surface area of the working electrode is 43 cm .

ing charges in the metal, thus forming spillover A porous but continuous thin Pd film was placeddipoles. The presence of spillover dipoles and the both inside and outside of a YSZ tube using a thinconcomitant change in work function eF alter the coating of Engelhard Pd A2985 paste. In order tostrength of the chemisorptive bond of covalently improve the wettability of the paste, Pd A2985 isbonded reactants and intermediates, thus changing further diluted in an equal amount of acetone. Thisactivation energies and reaction rates. The effect is film was dried at 1508C and calcined in air at 4508C.expected to increase as the ionic character of the This procedure was repeated until conductivity of the

23 21covalent bond increases. film became greater than 10 V cm. This filmThe first objective of NEMCA experiments is to acted as both catalyst and electrode. Thin Ag wires

achieve a higher catalytic activity by the application were attached to the electrode film and connected toof electric potential at the catalyst electrode. In order the galvanostat–potentiostat (EG&G Princeton Ap-to enhance the overall reaction rate, one should plied Research Versastat 273-81).understand the reaction mechanism and rate-deter- Hay eSep Q (80–100 mesh) packed in a 1/8-inchmining step (rds) of the reaction. The NO dissocia- o.d. 8 ft stainless steel and molecular sieve 5A

Fig. 1. Schematic diagram of fuel-cell configuration NEMCA study cell: working electrode, Pd; counter electrode, Pd; reference electrode,Ag.

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(45–60 mesh) column were used to separate the ible and no permanent change of the Pd catalyst hasreaction product. The Hay eSep Q partly separated taken place.N O and CO from other components of the gas The observed dramatic increase in reaction rate2 2

22mixture and the molecular sieve 5A accomplished upon O removal from the catalyst leads to rates ofthe remaining separation. CO oxidation more than 100% higher than the

maximum rate obtainable on the same catalyst and atthe same temperature for any gaseous composition

3. Results and discussion under open-circuit conditions. The enhancement ofthe reaction rate can be explained by simple changes

3.1. Transient response and enhancement factor A in the surface coverage of chemisorbed CO andatomic oxygen or even complete reduction of the Pd

Both galvanostatic and potentiostatic operations surface resulting from the continuous removal of22were explored in the course of the experiments and O , but it is more likely that the change of surface

both modes of operation were found to produce work function plays a significant role because thesimilar results. Galvanostatic transients were ob- enhancement of reaction rate is orders of magnitudetained and analyzed as described previously by greater than the rate of oxygen ion removal.Vayenas et al. [5].

Fig. 2 shows the effect of applying constant3.2. Effect of electrochemical promotioncurrents to the catalyst electrode while monitoring

the reactor effluent CO concentration and thus the2The electrochemical promotion, that is, the en-rate of the reaction under gas inlet conditions of

hancement of the rate of reaction by electrochemicalP 50.61 kPa, P 50.65 kPa at 4008C. At the startNO COoxygen ion pumping for the Pd catalyzed CO–NOof the experiment the circuit is open and the catalystreaction is observed at temperatures between 320 andis at a steady-state activity. This catalyst system4808C. The atomic balance on oxygen in the gasexhibited electrophilic behavior (L , 0) at negativephase is not closed during the electrochemical re-current (I , 0) and electrophobic behavior (L . 0) ataction in the fuel-cell configuration because oxygenpositive current (I . 0). The phenomenon is revers-ions can be added or removed via the solid elec-trolyte.

The rate of reduction of NO increases more than22100 times greater than the rate of removal of O

from the catalyst via negative current application. InFig. 3a, the steady-state rates for CO , N and N O2 2 2

production, r , r , r respectively, and theCO N N O2 2 2

selectivity to N O, S , are presented as a function2 N O2

of V at 3708C for a constant inlet pressure ofWR

reactants P 50.61 kPa and P 50.65 kPa. De-NO CO

pending upon the direction of oxygen pumping thePduYSZ fuel cell configuration showed both elec-trophilicity and electrophobicity at the same tempera-ture. In an electrophilic regime, a negative currentenhances the reaction rate and a positive currentreduces the reaction rate. In contrast, the reactionrate increased in proportion to the oxygen ioniccurrent in the electrophobic regime.

Fig. 2. Galvanostatic transient showing the effect of step changes The rate enhancement ratio r is defined as:of the applied current on the rate of CO consumption, r as aCO

function of time: T54008C, P 50.61 kPa, P 50.65 kPa,NO CO oF 5120 ml /min; (—) I; (m) r . r 5 r /r0 CO

696 S. Kim, G.L. Haller / Solid State Ionics 136 –137 (2000) 693 –697

various gas phase compositions and temperature.Most of the experimental results in the electrophilicregion lie on the theoretical curve derived fromequations,

2r 1 2r rr N N O N ONO 2 2 2] ]]]] ]]]]5 5 1 1 (1)r 2r 1 r 2r 1 rCO N N O N N O2 2 2 2

S Sr N O N ONO 2 2] ]]]]]] ]]]5 1 1 5 1 1 (2)r 2 2 S2(1 2 S ) 1 SCO N ON O N O 22 2

when the enhancement of reaction rate is an order ofmagnitude greater than the rate of oxygen transport,which confirms both the validity of the aboveequation and the reliability of our gas chromatog-raphy analysis. Therefore the electrophilicity of thesystem is a result of NEMCA. In contrast, ex-perimental results in the electrophobic regime de-viated from the theoretical curve because the en-hancement of reaction rate is of the same order ofmagnitude of the rate of oxygen transport.

In the NEMCA regime, the change in S wasN O2

purely induced by means of electrochemical oxygenpumping and the mass balance is closed, withinexperimental error, as a result of slow oxygen iontransport through the YSZ comparing to the NO–COreaction rate at this temperature. In this particularexperiment, it was possible to reduce S to zero byN O2

NEMCA alone.

Fig. 3. Effect of electrochemical promotion on catalytic activityand selectivity: (a) T53708C, P 50.61 kPa, P 50.65 kPa,NO CO

F 5120 ml/min; (n), r ; (s), r ; (h), r ; (3), S ; (b)0 CO N O N N O2 2 2 4. ConclusionsT54108C, P 50.61 kPa, P 50.65 kPa, F 530 ml/min.NO CO 0

Our experiments have shown that a fuel cellconfiguration of PduYSZ exhibits an electrochemicalwhere r is reaction rate under applied potential while

o promotion for the NO–CO reaction in the tempera-r is reaction rate under open-circuit condition. Rateture range of 320–4808C. The electrochemical pro-enhancement of r 52.2, r 52.2 and r 54.2CO N O N2 2 2motion of the catalytic activity, r and modificationare measured for V 521.8 V. One should also COWRof the selectivity, S are reversible and showednotice that S is also dependent on V . The N ON O WR 22

both electrophilicity at I , 0 and electrophobicity atdependence of S on V has been observed byN O WR2I . 0. In a typical experiment, the enhancement ofother researchers [10,11].reaction rate is 100 times greater than the rate ofIn order to investigate the effectiveness of V onWRoxygen ion transport. Rate enhancement of r 5the S , various experimental conditions were CON O 22

2.2, r 52.2 and r 54.2 were measured fortested. Fig. 3b shows the experimental observations N O N2 2

at 4108C, but similar behavior was observed for V 521.8 V at 3708C.WR

S. Kim, G.L. Haller / Solid State Ionics 136 –137 (2000) 693 –697 697

[3] S.G. Neophytides, D. Tsiplakides, P. Stonehart, M. Jaksic,AcknowledgementsV.C.G. Vayenas, J. Phys. Chem. 100 (1996) 14803.

[4] C. Pliangos, I.V. Yentekakis, X.E. Verykios, C.G. Vayenas, J.We thank the National Science Foundation for Catal. 154 (1995) 124.

partial support of this research. S. Kim wishes also to [5] S. Bebelis, C.G. Vayenas, J. Catal. 118 (1989) 125.acknowledge Pohang University of Science and [6] R.M. Lambert, M.S. Tikhov, A. Plarermo, I.V. Yentekakis,

ACS Abstract 015 (1996).Technology for contributions to this stipend and[7] I.V. Yentekakis, G. Moggridge, C.G. Vayenas, R.M. Lambert,tuition at Yale.

J. Catal. 146 (1994) 292.[8] C. Wagner, Adv. Catal. 21 (1970) 323.[9] S. Pancharatnam, P.A. Huggins, D.M. Mason, J. Electro-

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