Oscillatory behavior in the electrochemical oxidation of formic acid on Pt(100) rotation and...

7/21/2019 Oscillatory behavior in the electrochemical oxidation of formic acid on Pt(100) rotation and temperature effects.pdf

1/8

Journal of Electroanalytical Chemistry 500 (2001) 3643www.elsevier.nl/locate/jelechem

Oscillatory behavior in the electrochemical oxidation of formicacid on Pt(100): rotation and temperature effects

T.J. Schmidt *, B.N. Grgur 1, N.M. Markovic, P.N. Ross, Jr.

Materials Science Diision, Lawrence Berkeley National Laboratory, Uniersity of California, Berkeley, CA 94720, USA

Received 8 May 2000; received in revised form 27 June 2000; accepted 1 August 2000

Dedicated to Professor R. Parsons on the occasion of his retirement from the position of the Editor in Chief of the Journal of

Electroanalytical Chemistry and in recognition of many contributions to electrochemistry

Abstract

We investigated the oscillatory behavior in the kinetics of formic acid electrooxidation on Pt(100) in 1 mM HClO4 solution. We

studied the effect of different experimental parameters on the oscillatory behavior, viz. defined HCOOH mass-transport to the

electrode surface by using the rotating disk electrode technique, the temperature of the supporting electrolyte, and the nature of

anions. We suggest that the interdependence of the reaction steps during HCOOH oxidation, the adsorption of anions and the

competition for adsorption sites among the reaction partners and intermediates lead to complex non-linear kinetics. It was evident

that once the individual reactions in the dual path mechanism reach steady state the oscillations vanish. These conditions can be

reached either by enhanced formic acid reaction rates induced by electrode rotation or by increased temperature. Under specific

conditions of anion and formic acid concentration, relaxational oscillations can be transformed into mixed-mode oscillations. 2001 Elsevier Science B.V. All rights reserved.

Keywords: Oscillatory electrochemical reaction; Formic acid; Pt(100) electrode

1. Introduction

Oscillation of current or potential in electrochemical

reactions is a well known phenomenon, e.g. in metal

dissolution [1,2], peroxide reduction [3], the hydrogen

oxidation reaction [1,4], or in the oxidation of small

organic molecules like ethylene [5], formaldehyde [6,7],methanol [8] or formate/formic acid [5,9,10]. In the past

decade oscillatory instabilities during formic acid oxida-

tion have been extensively studied on low-index Pt(hkl)

electrodes [11 15]. Although general agreement exists

that current oscillations during formic acid oxidation

arise from oscillating surface coverages by adsorbed

formic acid, adsorbed poisoning intermediates, oxygen

containing species and anions from the supporting elec-

trolytes, the true nature of the oscillatory behavior is

still controversial. For example, one explanation for

sustained oscillations under potentiostatic control was

the local variation of the pH close to the electrode

during HCOOH oxidation on Pt(100), resulting in a

sudden change of the surface coverage by electroactive

oxygenated species (OHad), therefore changing the reac-

tion rate for removing the poisoning intermediate (e.g.adsorbed carbon monoxide, COad) which is formed by

HCOOH decomposition [11,14]. Additionally to the pH

effect, Tripkovic et al. proposed in their study that the

number of surface sites occupied by bridge-bonded

COad plays an important role in the oscillatory behav-

ior of formic acid oxidation on stepped Pt(hkl) elec-

trodes [12]. In contrast to the pH hypothesis, Markovic

and Ross proposed that the periodicity, the amplitude,

and the absolute magnitude of the current oscillations

during HCOOH oxidation on Pt(100) depend on at

least three parameters, viz. electrode potential and po-tential history, formic acid concentration, and the con-

centration of the anion of the supporting electrolyte

[13]. Closely following a concept described by Degn [1],

* Corresponding author. Tel. +1-510-4864793; fax: +1-510-

4865530.

E-mail address: [email protected] (T.J. Schmidt).1 Current address: Faculty of Technology and Metallurgy, Univer-

sity of Belgrade, Karnegijeva 4, 11000 Belgrade, Yugoslavia.

0022-0728/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 0 7 2 8 ( 0 0 ) 0 0 3 4 2 - 9


2/8

T.J. Schmidt et al./ Journal of Electroanalytical Chemistry 500 (2001) 3643 37

Strasser et al. pointed out that the presence of a suffi-

ciently large ohmic resistance is crucial to develop

oscillatory instabilities in formic acid oxidation on all

three low-index Pt single crystals [15]. For details about

the theory of electrochemical oscillators we refer to

Refs. [16,17]. All results for formic acid oxidation on

the Pt(hkl) electrodes were obtained on stationary elec-

trodes, i.e. the oscillatory behavior was not investigatedunder well defined hydrodynamic conditions. In order

to simulate enhanced mass transport conditions, the

effect of electrolyte stirring on the oscillatory behavior

during formic acid oxidation was studied and discussed

in Refs. [11,1315]. The main findings from the applied

turbulent mass-transport induced by magnetic stirring

of the electrolyte is that under certain experimental

conditions the enhanced mass transfer of HCOOH

caused the oscillations either to cease [11,14] or to be

sustained with different shapes and periodicity [13,15].

It is important to note that Schell and coworkers [6,10]

found that sustained current oscillations can be devel-

oped during the galvanostatic oxidation of formic acid

on a rotating polycrystalline platinum disk electrode. In

addition, these studies showed that oscillatory behavior

on polycrystalline Pt was not substantially affected by

the temperature of the electrolyte [6,10]. To the best of

our knowledge there are no similar studies of the

oscillatory behavior on Pt(hkl) electrodes under (i) well

defined mass-transport conditions and (ii) variable

temperature.

In this paper we present results on oscillations devel-

oped during formic acid oxidation on Pt(100) in 1 mMHClO4 solution. Our aim is to demonstrate the influ-

ence ofdefinedmass-transport to the electrode surface

by utilizing the rotating disk electrode (RDE) tech-

nique. Additionally, the effect of temperature was stud-

ied under specific experimental conditions in order to

isolate the effect of surface reaction kinetics on the

oscillations. In the last part, we report on so-called

mixed-mode oscillations obtained under specific condi-

tions concerning anion and formic acid concentration.

2. Experimental

The results presented here were conducted using twodifferent Pt(100) single crystals (0.283 cm2, Figs. 14;

0.572 cm2, Fig. 5). The single crystals were flame an-

nealed in a hydrogen flame and cooled in a mild stream

of 2% H2+Ar. One crystal (0.283 cm2) was mounted

into the disk position of an insertable ring disk elec-

trode assembly (Pine Instruments), fully described pre-

viously [18]. Both electrodes were transferred into a

thermostated standard three compartment electrochem-

ical cell and immersed into the Ar-purged electrolyte

under potential control at 0.2 V (Ar, Matheson

Research Purity; 1 mM HClO4, EM Science suprapure;

or 5 mM H2SO4, J.T. Baker ultrex, both prepared with

triply pyrodistilled water). The reference electrode was

a saturated calomel electrode (SCE) separated by a

closed bridge from the working electrode compartment

in order to avoid chloride contamination. A circulating

constant temperature bath (Fischer Isotemp Circulator)

maintained the temperature of the electrolyte within

0.5C. The measurements were carried out either at

298 K or at 318 K. All measurements were conducted

non-isothermally, i.e. keeping the temperature of the

reference electrode constant (298 K) while that of the

working electrode was varied. After recording the basevoltammetry of the Pt(100) electrode in order to check

the cleanliness of the electrode preparation, the elec-

trode was held at 0.2 V and formic acid (Aldrich

ACS Reagent) was injected to obtain a 12 or 35 mM

HCOOH solution, respectively. All experiments were

carried out on the unreconstructed Pt(100)(11) sur-

face present in acid and alkaline electrolyte [19].

3. Results

3.1. Oscillation characteristics on stationary electrode

The general shape of the polarization curve for for-

mic acid oxidation on Pt(100) in 1 mM HClO4 (12 mM

HCOOH, 10 mV s1, 298 K) between hydrogen ad-

sorption (referred to as Hupd) and oxide formation

(referred to as OHad) is shown in Fig. 1. This curve

reproduces the behavior we reported previously [13].

Since the oxidation currents for formic acid oxidation

under potentiodynamic conditions overlap in the posi-

tive and negative sweeps, respectively, the voltam-

mograms, for clarity, are unfolded with respect to thepotential axis. Starting from the negative potential

limit, the oxidation of HCOOH is suppressed up to ca.

Fig. 1. (a) Unfolded potentiodynamic curve for formic acid oxidation

on Pt(100) (10 mV s1). Sweep reversed at 0.7 V. (b) Potentiostatic

transient at 0.63 V in a sweep-and-hold experiment after sweepingfrom 0.7 V negatively. 1 mM HClO4+12 mM HCOOH, 0 rpm, 298

K.


3/8

T.J. Schmidt et al./ Journal of Electroanalytical Chemistry 500 (2001) 364338

Fig. 2. Potentiodynamic curves for formic acid oxidation on Pt(100)

(10 mV s1) at 0 rpm, 1600 rpm and 3600 rpm, respectively (only the

negative sweep is shown). 1 mM HClO4+12 mM HCOOH, 298 K.

static current oscillations are typical for so-called relax-

ation oscillations, where the system remains in either

one of two quasi-stationary states, followed by a sharp

transition from one state to another. Hence, the time

the system stays in the active and inactive state is

defined as tA and tI, respectively (see insert in Fig. 1).

The total charge passed in a single oscillation can be

referred to as QA and QI flowing during tA and tI. It isnoteworthy that the sustained potentiostatic oscillations

at 0.63 V (insert Fig. 1) are relatively stable in ampli-

tude and frequency over a long period of time (i.e.

15 min) before significant changes in the shape are

observed.

3.2. Influence of electrode rotation on oscillatory

behaior

3.2.1. Potentiodynamic conditions

The effects of rotation rate on the oscillatory behav-

ior during the first positive and consecutive negative

sweeps, respectively, are shown in Fig. 2. There are two

characteristics that represent the effect of rotation: (i) in

the positive sweep the oscillations cease immediately

under electrode rotation; and (ii) the number of spikes

recorded in the negative sweep direction decreases on

increasing the rotation rate. In the latter case, the

number of oscillations is decreased from eight at the

stationary electrode to four when rotating at 1600 rpm,

and finally to two when the rotation rate is increased up

to 3600 rpm. At rotation rates above 3600 rpm, we

were not able to develop oscillations in these potentio-dynamic experiments. Fig. 2 also shows that with in-

creasing rotation rate the system stays longer in both

the deactivated (tI increases) and activated (tA in-

creases) states. As a consequence the frequency of the

oscillations goes down with increasing rotation rate.

The effect of the rotation rate on the amplitude of the

oscillations is not as pronounced, although a detailed

inspection of Fig. 2 reveals a small decrease with in-

creasing electrode rotation. Finally, we note that in the

consecutive two following sweeps, the number of oscil-

lations was always smaller than in the first sweep or no

oscillations were observed anymore (especially at 3600

rpm, the polarization curve presented was a first sweep

phenomenon).

3.2.2. Potentiostatic conditions

Fig. 3 shows the influence of rotation on potentio-

statically developed oscillations at 0.63 V. In these

experiments the potential was swept (10 mV s1) from

the negative potential limit to 0.7 V, and then after the

reversal of the sweep direction the potential was held at

0.63 V. At this potential we were recording the sus-

tained oscillations shown on the left-hand side of Fig.3a, which qualitatively resemble the current transient

from the insert in Fig. 1. We then started to rotate the

0.2 V due to the presence of adsorbed CO containing

poisoning intermediates formed by formic acid decom-

position (for simplicity, hereafter denoted as COad,

although other organic intermediates might be present

[20,21]). The concomitant removal of COad and the

beginning of formic acid oxidation starts at ca. 0.2 V,resulting in the increase in the current. Eight subse-

quent anodic current oscillations are observed between

ca. 0.55 and 0.7 V. After reversal of the potential at 0.7

V, 12 additional oscillations appear up to 0.4 V, fol-

lowed by a monotonic decrease of the current when

approaching the initial potential. We note that when

sweeping the electrode consecutively over a longer pe-

riod of time, however, the number of oscillations in

both positive and negative sweeps was reduced before

the oscillations ceased completely, indicating that the

oscillatory behavior seen in potentiodynamic experi-

ments is of time-dependent nature. The potentiody-namic data in Fig. 1 are in general agreement with data

presented in Refs. [13,14], which were recorded under

similar conditions.

The insert in Fig. 1 is representative for current

oscillations recorded under potentiostatic conditions. In

this experiment, the potential was swept positively from

0.2 to 0.7 V and stopped at 0.63 V upon the reversal

point (sweep-and-hold experiment). This procedure

produced almost the same current transients compared

to potential stepping experiments from 0.2 V to the

region where current oscillations occur. We, therefore,focused in this study on sweep-and-hold experiments.

As already mentioned previously [11,13] the potentio-


4/8


RDE at 400 rpm as indicated in Fig. 3a for ca. 90 s

before stopping the electrode rotation again. Obviously,

the oxidation current for formic acid oxidation in-

creased when the rotation started, indicating higher

reaction rates induced by the enhanced mass-transport

to the electrode surface. However, as in the sweep

experiments (Fig. 2), the amplitude decreases slightly

once rotation is applied but stays relatively stable

within the duration of the experiment. An inspection of

Fig. 2 also shows that, at 400 rpm, the current peaks

are sharper than under stationary conditions, whereas

the induction period from the deactivated to the acti-

vated state (tI) increases with time. As a consequence,

the frequency of oscillations decreases continuously

(Fig. 3a). However, when the rotation was stopped

after 80 s, the reverse behavior was observed, and over

an additional 40 s the shape of the oscillations, the

frequency and amplitude slowly recover to the state

recorded before rotation. Similar observations were

made by Strasser et al. on Pt(100) using magnetic

stirring of the electrolyte [15]. Fig. 3b shows that the

increase of the rotation rate from 400 to 1600 rpm leads

to an increase of the reaction rate, a continuous de-

crease of the amplitude of the current spikes and the

frequency of oscillations. Fig. 3b also demonstratesthat, in contrast to Fig. 3a, the current peaks broaden

and the poisoning period increases, so after 250 s the

oscillations are annihilated completely. It is noteworthy

that we were unable to restore oscillations over the next

10 min, as observed in Ref. [15], or when we stopped

the rotation. We also found that the time required for

the oscillations to cease completely decreases with in-

creasing rotation rate due to the enhanced formic acid

concentration on the surface at higher rotation rates.

3.3. Influence of temperature on oscillatory behaior

Another possibility to enhance the reaction rate for

formic acid is the increase in temperature of the elec-

trolyte (Fig. 4). Note that due to the inverse propor-

tionality of the diffusivity, D, and viscosity of the

electrolyte, with increasing temperature, T, (DT/

), mass transport to the electrode is not affected by the

temperature change. Since we were not able to develop

current oscillations when we started the experiment at

temperatures above 303 K (in contrast to galvanostatic

oscillations on polycrystalline Pt reported in Ref. [10]),

we carried out the following experiment. We started the

experiment at 298 K and recorded potentiostatic cur-

rent oscillations at 0.63 V under sweep-and-hold condi-

tions. The resulting current transients are shown on the

left side in Fig. 4. In order to demonstrate the regular

shape of the oscillations, we magnified the time scale

for the first 4 s of the experiment. After an additional

60 s we started to increase the temperature of the

electrolyte to 318 K and recorded the response of the

system. The temperature increase results instanta-

neously in higher HCOOH oxidation rates and a slight

decrease in the frequency. The oscillations themselves,however, stay in the same regular shape as shown in the

time-magnified part of Fig. 4. Only after ca. 145 s,

when the desired temperature of 318 K equilibrated in

the electrolyte, do the current spikes start to lose their

regular shape, resulting in a more or less chaotic

development of current oscillations with irreproducible

frequency and amplitude (note that for 145t330 s

the current transients are shown on an expanded time

scale). The last current spike is observed at ca. 310 s

before the oscillations cease completely. We also note

that we were unable to revive the oscillatory behaviorafter this time through rotation or through a change of

the potential.

Fig. 3. Influence of rotation rate on potentiostatic transients at 0.63

V: (a) after switching from 0 rpm to 400 rpm and vice versa; (b) after

switching from 0 rpm to 1600 rpm. 1 mM HClO4+12 mM HCOOH,

298 K.

Fig. 4. Influence of temperature on potentiostatic transients at 0.63 V.As indicated, the temperature was increased from 298 to 318 K. 1

mM HClO4, 12 mM HCOOH, 0 rpm.


5/8


Fig. 5. Evaluation of so-called mixed-mode oscillations on Pt(100):

potentiostatic transient at 0.47 V. The insert shows the section

betweent=100 s and 130 s. 5 mM H2SO4+35 mM HCOOH, 0 rpm,

298 K.

tors. For a discussion of this issue, we refer to Section

4.1.

4. Discussion

In Section 3, we presented results which illustrate

that in addition to bulk concentration of formic acid,electrode potential and potential history, the concentra-

tion of specifically adsorbing anions [13], or the pres-

ence of a high ohmic resistance [15], two other

parameters can be added as important factors in deter-

mining the oscillatory behavior of formic acid. We

demonstrated the strong influence of (i) enhanced and

defined mass-transport to the electrode surface induced

by electrode rotation and (ii) the electrolyte tempera-

ture on the oscillatory instabilities. In what follows, our

aim is neither to propose any detailed mechanism for

oscillations during formic acid oxidation nor to present

any model which reproduces the experimental results,

but rather to report how the enhanced rate of formic

acid oxidation, induced by an increase of both mass-

transport of formic acid and temperature of the elec-

trolyte, may effect the interdependence of the

experimental parameters responsible for complex non-

linear kinetics.

4.1. Formic acid oxidation mechanism and oscillating

surface coerages

It is now well established by various electrochemicaland spectroscopical studies that HCOOH electrooxida-

tion on Pt proceeds via the so-called dual-path mecha-

nism proposed by Capon and Parsons [22] which is

summarized in the following reaction scheme:

According to this mechanism the direct oxidation

path can be ascribed to dehydrogenation of HCOOH to

CO2 via an active intermediate (often referred to as

COOHad, reaction 1) [22]. The CO2 production in the

parallel path can be assigned to a (non-faradaic) dehy-

dration of HCOOH to adsorbed CO (representing one

of several possible products; reaction 2) in a first step

which is followed by the COadoxidation reaction (reac-tion 4). OH adsorption, which is generally assumed to

be necessary for the CO-oxidation reaction, occurs in

3.4. Influence of bisulfate anion concentration on

oscillatory behaior

Fig. 5 shows some potentiostatic current transients

recorded in 35 mM HCOOH containing 5 mM H2SO4electrolyte at 0.47 V (sweep-and-hold experiment in the

negative sweep, 0 rpm). In our previous publication (see

figure 11 in Ref. [13]) no oscillations were observed

with the same amount of sulfuric acid but at slightly

higher potentials (0.53 V) and formic acid concentra-

tion (50 mM HCOOH), indicating the strong influence

of both parameters on the oscillatory behavior. Under

our present experimental conditions, we developed ini-

tially periodic oscillations with very low frequency (ca.

0.1 s1) and a long deactivation period (Fig. 5). In the

third oscillatory wave, however, first instabilities on the

deactivation branch occurred, followed by a sudden

frequency increase in the next current waves. Addition-

ally, the aforementioned current instabilities in the de-

activation period developed to periodically returning

current spikes. The overall current transient behavior

illustrated in Fig. 5 represents so-called mixed-modeoscillations. These are characterized by the appearance

of an oscillation with a large amplitude which is fol-

lowed periodically by a small current spike on the

deactivating branch of the main oscillatory wave (see

insert in Fig. 5). Similar sequences of mixed-mode

oscillations were found for galvanostatic HCOOH oxi-

dation on polycrystalline Pt (2 M HCOONa+0.5 M

H2SO4) [6] and in the potentiostatic oxidation of

HCOOH on Pt(100) (1 M HCOONa+0.5 M H2SO4,

applying a large external ohmic resistance) [15]. In the

latter work the mixed-mode oscillations were explainedby the presence of a chemical species as a second

feed-back variable giving rise to two interacting oscilla-


6/8


acid electrolyte by decomposition of water (reaction

3). In what follows, we propose that the interdepen-

dence of the reaction steps in reactions 1 4 and the

competition for adsorption sites among the reaction

partners and intermediates lead to complex non-linear

kinetics. For example, if the reaction rate kpkox,

then COad accumulates on the surface and poisons

reactions 1, 3 and 4. For these conditions, therefore,the rate of oxidation of formic acid is mostly deter-

mined by the rate of both the formation of OHad [23]

and the oxidative removal of COad through a Lang-

muirHinshelwood (L-H) type reaction [24]. It is im-

portant to note that the rate of the L-H reaction is

strongly affected by the delicate balance between the

coverage of COad, OHad and anions from the support-

ing electrolytes (reaction 5, kA). While anions have

negligible effects on the interaction of COad with the

surface atoms, they indeed have a strong effect on

OHad adsorption [25] and adsorption of formic acid

[13]. Hence, in order to develop a more realistic pic-

ture about the competition of reactants, intermediates

and products for bare Pt sites the anion adsorption/

desorption from the supporting electrolyte must be

incorporated in the reaction scheme proposed above

for the oxidation of HCOOH. In our previous study,

we emphasized that anions play a very important role

in the development of the oscillatory behavior during

the oxidation of HCOOH on the Pt(100) surface [13].

The most important observation from this work is

that by adding small amounts of sulfuric acid (up to 1

mM HSO4

) to the base electrolyte (1 mM HClO4) weobserved an increase in the induction time, tI, and a

decrease of the time in the activated state, tA. AtcH2SO41 mM, no oscillation could be observed any-

more. The same effects were observed by adding small

amounts of the more strongly adsorbing Cl, but at

concentrations three orders of magnitude smaller. The

effects of anions on tI can be interpreted by the afore-

mentioned competition between anions and OHad for

the same adsorption sites (initially starting at defect

sites of the crystal), which in terms of our reaction

scheme implies a decrease of kOH. The time in the

activated state, tA, decreases for the same reasons,

namely the competition of formic acid and anions for

the same Pt surface sites. At this point one may em-

phasize that in order to produce sustained current

oscillations it is necessary to create surface conditions

which allow the system to reach the specific balance of

the individual reaction rates presented in our scheme.

To see this balance in action, we created the sustained

mixed-mode oscillations under specific conditions of

anion and formic acid concentrations (5 mM H2SO4,

35 mM HCOOH), Fig. 5. For similar types of oscilla-

tions on Pt(100) Strasser et al. [15] suggested that theyare triggered and controlled by an additional negative

feed back variable, which gives rise to two interacting

oscillators and consequently to mixed-mode oscilla-

tions. Most importantly, however, they ascribed the

additional negative feed back variable as the result of

the presence of some (unknown) chemical species.

From our experiments, it turned out that these chemi-

cal species may be specifically adsorbing anions, which

can control the adsorption of both OHad and formic

acid molecules.At this point, it is appropriate to note that recently

Strasser et al. pointed out that the presence of a large

ohmic resistance, R, is a crucial parameter to develop

oscillations during formic acid oxidation [15,26]. The

mechanism of action of a large ohmic resistance is

rather simple. The potential drop from the output of

the potentiostat/galvanostat to a point in the bulk

electrolyte in front of the reference electrode, E, is

split into the potential drop across the double-layer

(double-layer potential, DL) and the ohmic potential

drop (E=DL+IR). The latter includes all ohmicdrops, including those from resistors added in series in

the external circuit [27]. Under current flow, at con-

stant E, DL changes when the current changes. In

terms of the dual path mechanism for HCOOH oxida-

tion, the changing double layer potential leads to the

following scenario under potentiostatic conditions

(E=const., R=const). When the current is high as in

the activated state (direct path, reaction 1) the poten-

tial drop across the double-layer is much lower than

E, leading to slow OHad formation rates (reaction 3)

and, as a consequence to COad accumulation on thesurface (reaction 2). Due to the poisoning of the sur-

face by COad the direct path becomes inhibited and

the current decreases. Lower currents, however, in-

crease DL and, consequently, the OHad formation

rate is increased, which, in turn, leads to freeing of

surface sites for the direct path, and a concomitant

rise in the current to complete the cycle. In a simple

picture, the formation of OHad can be switched on

and off periodically like the light in a room by inser-

tion of a large ohmic resistance. It is, however, not

clear whether the presence of an ohmic resistance issufficient to create the oscillatory behavior during for-

mic acid oxidation. Our experiment clearly showed

that simply by varying the concentration of anions one

can control the mode of oscillation. Unfortunately, in

the model proposed by Strasser et al. anion adsorption

is not taken into account [26]. In addition, this model

also cannot explain the oscillatory behavior found un-

der conditions of rather high electrolyte/anion concen-

tration [5,6,9,12,28], i.e. under conditions where the IR

drop seems to be negligible. However, closely follow-

ing Koper and Sluyters [16], a minimum value of theIR drop is necessary to observe oscillations, but this

parameter does not seem to represent a sufficient one.


7/8


4.2. Current oscillations at defined mass -transport

As mentioned in Section 1, all previous results for the

oscillatory behavior during formic acid oxidation on

platinum single crystal electrodes were obtained on a

stationary electrode. A common shortcoming of this

electrode configuration is that the mass transport of

reacting species is not well controlled, and for lowconcentrations of electroactive species one usually mea-

sures pure transient currents. Transient behavior during

the oxidation of formic acid on stationary electrodes

arises from a slow diffusion of HCOOH through a

relatively thick diffusion layer (200500 m). The best

way to overcome this limitation is to use the RDE

method, where a known pattern of hydrodynamic flow

is imposed on the solution through the Nernst diffusion

layer with thickness of typically 50 m. Conse-

quently, the RDE offers (pseudo) steady state currents

for the oxidation of formic acid even when small con-

centrations of HCOOH are used and relatively fast

sweep rates are applied (Fig. 2). With a more effective

supply of HCOOH to the Pt(100) electrode in RDE

measurements, we can nicely control the surface con-

centration of HCOOH, c0. Thus, RDE experiments

may create very similar conditions to those we achieved

by monitoring the effects of the formic acid concentra-

tion on the oscillatory behavior but on a stationary

electrode [13]. For the latter system, we found that with

increasing formic acid concentration (i) tI increases, (ii)

tAdecreases and (iii) the amplitude of the current spikes

rises. Due to the higher HCOOH bulk concentration,cb, the HCOOH surface concentration, c0, is increased.

Assuming positive order reaction kinetics for formic

acid dehydration (reaction 2), this results in higher

COad formation rates and at constant kOH in slower

rates for the oxidative removal of COad, kox (increasedtI), in agreement with the negative reaction order kinet-

ics for oxidation of solution phase CO [24]. The same is

valid for decreased tA: the higher poisoning rate forces

the system to leave the activated state faster due to a

faster inhibition of reaction 1. The higher amplitude

can be explained by the higher HCOOH surface con-

centration, c0, assuming positive order kinetics for for-

mic acid oxidation (see e.g. Ref. [29] and references

therein). Note, at formic acid concentration above 100

mM, no oscillations could be found anymore [13],

consistent with our supposition that if the rate of

reaction 4 (oxidative removal of COad) becomes con-

stant and is in equilibrium with kp and kOH, the oscilla-

tions will cease and the overall reaction will reach its

steady state.

In RDE measurements, the formation of OHad is a

rotation rate independent process, and consequently the

major effect of the rotation on the oscillatory behaviorshould be discussed in the light of the variation of the

surface concentration of formic acid. Figs. 2 and 3

clearly reveal that the rate of formic acid oxidation and

the activated/deactivated time is increased by increasing

the mass transport of HCOOH (rotation of electrode)

to the electrode. On the other hand, the amplitude and

the frequency are decreased by increasing the rotation

rate, and at 1600 rpm the oscillations cease after ca. 250

s. Interestingly, the steady state current observed after

the oscillatory behavior vanishes represents the deacti-vated state, indicating that the formic acid dehydration

step (reaction 2) is becoming the dominant process with

constant reaction rates while kox, kp and kOH are in

equilibrium. We recall that the same happens on the

stationary electrode, but for a much higher bulk con-

centration of formic acid. Additionally, these experi-

ments may shed some light on the influence of an

ohmic resistance on the oscillatory behavior. Since the

magnitude of an eventually present IR drop in the

electrolyte should not be influenced by electrode rota-

tion, we conclude that the presence of a large IR drop

is not a sufficient parameter to induce the development

of sustained current oscillations during formic acid

oxidation. However, we want to note that the theoreti-

cal model presented in the work by Strasser et al. [26]

does fit their experimental results under their experi-

mental conditions [15], which, in turn, are different

from ours.

4.3. Temperature effects on oscillatory instabilities

The effects of temperature on the oscillatory behav-

ior in the electrochemical oxidation of formic acid onPt(100) are in nature very similar to the rotation effects

discussed above. Most notably, the reaction steps 14

were affected differently by the temperature variation.

As a consequence, the periodicity, the amplitude and

the absolute magnitude of the current oscillations

changed continuously with an increase of temperature.

Recently, we found that the energetics of OH formation

on a platinum single crystal are only slightly affected

over the temperature range of 278 333 K [25]. This

indicates that the temperature has only a minor effect

on the reaction of OH formation (reaction 3 in our

scheme). On the other hand, we showed that the rates

of both the direct and the poison oxidation pathways

of formic acid are enhanced at higher temperatures.

The latter process increases the formation of COadwhich, in turn, can effectively block the sufficient ad-

sorption of OHad required to trigger the transition of

the system from an inactive to an active state. It

appears, therefore, that the temperature has an indirect

effect on the surface concentration of OHad. Due to a

lack of OHad at higher temperature, smaller current

amplitudes are obtained at 318 K than at 298 K, and

the system stays longer in the deactivated, i.e. COadpoisoned state. It is interesting that at 318 K after ca.

150 s the stable relaxational oscillations were slowly


8/8


transformed into a chaotic-like type of oscillation

(Fig. 5). Although the information is extremely limited,

it is reasonable to propose that the development of the

chaotic current spikes after ca. 145 s may tentatively

be ascribed to arise due to some fluctuations of the

thickness of the diffusion layer by thermal motion of

the electrolyte. Nevertheless, regardless of the true na-

ture of the chaotic behavior, our experiments showunambiguously that the main effect of the temperature

on the oscillatory behavior is to convert the complex

non-linear kinetics to steady state oxidation of formic

acid. Fig. 5 shows that at 318 K after ca 300 s the

oscillations ceased completely, indicating that the over-

all rate of reaction is under steady state conditions.

5. Conclusions

We investigated the oscillatory behavior in the kinet-

ics of formic acid electrooxidation on Pt(100) in 1 mM

HClO4 solution. We studied the effect of different

parameters on the oscillatory behavior, viz. defined

HCOOH mass-transport to the electrode surface by

using the rotating disk electrode technique, the temper-

ature of the supporting electrolyte, and the nature of

anions. We suggest that the interdependence of the

reaction steps during HCOOH oxidation, the adsorp-

tion of anions and the competition for adsorption sites

among the reaction partners and intermediates lead to

complex non-linear kinetics. It was evident that once

the individual reactions in the dual path mechanismreach steady state, the oscillations vanish. These condi-

tions can be reached either by enhanced formic acid

reaction rates induced by electrode rotation or by in-

creased temperature. Under specific conditions of anion

and formic acid concentration, relaxational oscillations

can be transformed into mixed-mode oscillations.

Acknowledgements

We want to acknowledge H.A. Gasteiger for helpful

discussions. This work was supported by the Assistant

Secretary for Conservation and Renewable Energy,

Office of Transportation Technologies, Electric and

Hybrid Propulsion Division of the U.S. Department of

Energy under Contract No. DE-AC03-76SF00098.

References

[1] H. Degn, J. Chem. Soc., Faraday Trans. 64 (1968) 1348.

[2] P. Russel, J.S. Newman, J. Electrochem. Soc. 133 (1986)

2093.

[3] T.G.J. Van Venrooij, M.T.M. Koper, Electrochim. Acta 40

(1995) 1689.

[4] K. Krischer, M. Luebke, W. Wolf, M. Eiswirth, G. Ertl, Elec-

trochim. Acta 40 (1995) 69.[5] J. Wojtowicz, N. Marincic, B.E. Conway, J. Chem. Phys. 48

(1968) 4333.

[6] M. Schell, F.N. Albahadily, J. Safar, Y. Xu, J. Phys. Chem.

93 (1989) 4806.

[7] M. Hachkar, M. Choy de Martinez, A. Rakotondrainibe, B.

Beden, C. Lamy, J. Electroanal. Chem. 302 (1991) 173.

[8] M. Krausa, W. Vielstich, J. Electroanal. Chem. 399 (1995) 7.

[9] N.A. Anastasijevic, H. Baltruschat, J. Heitbaum, J. Elec-

troanal. Chem. 272 (1989) 89.

[10] F.N. Albahadily, M. Schell, J. Electroanal. Chem. 308 (1991)

151.

[11] F. Raspel, R.J. Nichols, D.M. Kolb, J. Electroanal. Chem.

286 (1990) 279.[12] A.V. Tripkovic, K. Popovic, R.R. Adzic, J. Chim. Phys. 88

(1991) 1635.

[13] N. Markovic, P.N. Ross, Jr., J. Phys. Chem. 97 (1993) 9771.

[14] F. Raspel, M. Eiswirth, J. Phys. Chem. 98 (1994) 7613.

[15] P. Strasser, M. Lubke, F. Raspel, M. Eiswirth, G. Ertl, J.

Chem. Phys. 107 (1997) 979.

[16] M.T.M. Koper, J.H. Sluyters, J. Electroanal. Chem. 371 (1994)

149.

[17] P. Strasser, M. Eiswirth, M.T.M. Koper, J. Electroanal. Chem.

478 (1999) 66.

[18] N.M. Markovic, H.A. Gasteiger, P.N. Ross, J. Phys. Chem.

99 (1995) 3411.

[19] I.M. Tidswell, N.M. Markovic, P.N. Ross, Phys. Rev. Lett. 71

(1993) 1601.

[20] O. Wolter, J. Willsau, J. Heitbaum, J. Electrochem. Soc. 132

(1985) 1635.

[21] S.G. Sun, J. Clavilier, A. Bewick, J. Electroanal. Chem. 240

(1988) 147.

[22] A. Capon, R. Parsons, J. Electroanal. Chem. 45 (1973) 205.

[23] T.J. Schmidt, R.J. Behm, B.N. Grgur, N.M. Markovic, P.N.

Ross Jr., Langmuir 16 (2000) 8169.

[24] N.M. Markovic, B.N. Grgur, C.A. Lucas, P.N. Ross, J. Phys.

Chem. B 103 (1999) 487.

[25] N.M. Markovic, T.J. Schmidt, B.N. Grgur, H.A. Gasteiger,

P.N. Ross, Jr., R.J. Behm, J. Phys. Chem. B 103 (1999) 8568.

[26] P. Strasser, M. Eiswirth, G. Ertl, J. Chem. Phys. 107 (1997)

991.[27] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Wiley,

New York, 1980.

[28] H. Okamoto, Electrochim. Acta 37 (1992) 37.

[29] V.S. Bagotzky, Y.B. Vassilyev, Electrochim. Acta 12 (1967)

1323.

.

Oscillatory behavior in the electrochemical oxidation of formic acid on Pt(100) rotation and...

Documents

Transcript of Oscillatory behavior in the electrochemical oxidation of formic acid on Pt(100) rotation and...