Current Oscillations in the Electrochemical Oxidation of Formic Acid at

J. Phys. Chem. 1994,98, 7613-7618

7613

Current Oscillations in the Electrochemical Oxidation of Formic Acid at Pt Single Crystal Surfaces F. Raspel and M. Eiswirth’ Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 0-141 95 Berlin, Germany Received: February 23, 1994; In Final Form: May 24, 1994’

The electrochemical oxidation of formic acid under potentiostatic conditions was found to exhibit transient oscillations on all three low-index plane Pt surfaces in an unstirred electrolyte. Depending on structure, substantial differences occurred concerning induction time, oscillation period, and shape as well as number of oscillations. Stirring could stop the oscillations, but also restart them when they had faded spontaneously. It is shown that a potentiostatic model, based on the branched oxidation path of formic acid and taking the pH in the double layer into account, can qualitatively explain the observed phenomena.

1. Introduction

CE

Numerous studies have been dedicated to oscillating electrochemical reactions. Oscillations were obtained with the electrodissolution of metals (see ref 1 for a review) as well as with several electrocatalytic reactions, such as the oxidation of H2 on platinum in the presence of certain cations,” the oxidation of small organic molecules, e.g., methanol, formaldehyde,and formic acid? and the reduction of inorganic molecules and ions, among them nitric acid: hydrogen per~xide,~ and peroxodisulfate.lOJ1 It is well-known that the current/voltage ( I / y )curves of a given reaction (e.g., formic acid oxidation) can differ significantly for different surface orientations,12-15but up to now the dynamics of electrochemical oscillators has been studied mostly on polycrystalline electrodes; see for instance the detailed work on the galvanostatic oxidation of formic acid on Pt.l”l* Recently, it was shown that with this reaction (transient) oscillations can also be obtained potentiostatically on a Pt( 100) single crystal surface.lg.20 Here we compare the behavior of three low-index plane surfaces, viz. Pt(100), Pt(llO), and F’t(lll), and polycrystalline material. Although oscillations were obtained on all single crystalelectrodes,their behavior was found to differ strongly from one another. On polycrystalline electrodesoscillations turned out to be difficult to detect. Electrochemical oscillators are often referred to as ”potentiostatic”,if the voltage Vbetween working and referenceelectrode is held constant while the current oscillates. However, in most cases the potential cp of the working relative to the reference electrode will also oscillate because the IR drop does (and V = cp IR is fixed). A number of such systems have been modeled successfully.11~21.22 The use of a Luggin capillary can (to some extent) suppress this effect so that changes of cp (and the resulting nonlinearities) may be negligible for qualitative mechanistic considerations. Such an approach is used in section 4. It is shown that for the present system the Occurrence of oscillations can be explained even under the assumption of truly potentiostatic conditions. Stoichiometric network analysis (SNA)23 is used there, so we briefly sketch the procedure. SNA allows the use of complete parameter sets (Le., arbitrary nonnegative rate constants) avoiding specific assumptions. This is particularly helpful, if (as with formic acid oxidation) the rates are not wellknown. More important, using SNA the set of all stationary states can be written down in closed form (as nonnegative linear combinations of extreme subnetwork@), allowing one to determine whether any of them can undergo a local bifurcation. For an existence proof of oscillations, it suffices to find any parameter combination satisfyinga sufficientcondition for a Hopf bifurcation

+

~~~

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~~

*Abstract published in Advunce ACS Abstrucrs, July 1, 1994.

0022-3654/94/2098-76 13304.50/0

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Ngure 1. Electrochemical cell: (a) electrochemical glasscell,(b) working electrode (WE), (c) Luggin capillary, distance to W E on the order of a millimeter. (d) valve (closed), (e) saturated calomel electrode (SCE)as reference electrode (RE), (0 fritt, (g) counter electrode (CE)Pt wire, (h) stirring magnet.

(which implies the existence of a limit cycle).” This can be achieved by just considering the signs of certain principal minors of the matrix obtained in the linear stability problem of the stationary states.25 2. Experimental Section

The conventionalthreecompartmentelectrochemicalcell used is shown in Figure 1. A standard calomel reference electrode was used; all potentials below are given vs SCE. The working electrode was dipped into the electrolyte and then pulled out as far as possible in order to avoid effects from the edges.26 The cell was kept under nitrogen about 10 min before and during the experiment. The solutions were prepared with tridistilled water, p.A. formic acid, and suprapure HC104 and NaOH (Merck). The cylindrical single crystals were cut from a platinum rod, oriented with a Laue technique S O S O , and polished mechanically and subsequently electrochemically by application of 10 V for 3-5 s in a solution of 10% HC1 and 90%ethanol with a graphite counter electrode. Before the measurements the crystal was annealed at 1500 K in a Bunsen flame for 1 min, allowed to cool, and rinsed with tridistilled water for 30 s, in such a way that a drop remained to cover the oriented surface, which could thus be introduced into the electrolyte without having air contact. The quality of the surface was checked by recording cyclovoltammograms in 0.5 M H2S04 (suprapure, Merck) and comparing them to literature data.27 0 1994 American Chemical Society

7614

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The Journal of Physical Chemistry, Vol. 98, No. 31, 1994 I

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Figure 3. I / V curves of low-index plane Pt surfaces in M HC104 0.05 M HCOOH (50 mV/s). No spikes were observed for Pt( 111).

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Figure 2. I / V curves of low-index plane Pt surfaces in 0.5 M HClOd + 0.26 M HCOOH with scan rate of 50 mV/s.

The results had been found to depend on the formic acid concentration as well as on the pH of the electrolyte.19 The latter was varied without changing the total ion concentration by preparation of the solution with different ratios of HC104 and NaC104. It turned out that pH and HCOOH concentrations were crucial parameters, but the results did not change significantly with total ion concentration for constant pH and HCOOH concentration, so that in later experiments it was not necessary to keep the ion strength of the solution strictly constant.

3. Results Cyclovoltammograms of the three low-index plane surfaces at high HCOOHconcentration and low pH are reproduced in Figure 2. In accordance with ref 12, there are significant quantitative differences between the different surfaceorientations. They have in common that they exhibit a hysteresis, and the turnover is very low for both small and large potentials; Le., inhibition of formic acid oxidation occurs on both ends of the hysteresis. At higher pH and lower HCOOH concentration pronounced sharp spikes with strongly enhanced current were obtained in the cathodic sweep for Pt(100) and small spikes in the anodic direction for Pt(llO),whilenonewereobservedonPt(ll l),asshowninFigure 3. Figure 4 compares Z/Vcurves in 0.1 M HC104 to a solution with the same amount of NaC104. Obviously, the pH effect (vanishing of spikes) cannot be attributed to C1- impurities in the suprapure perchloric acid (as suggested in ref 20). The occurrence of such spikes may serve as a hint to where to look for oscillations, but there is no reliable correlation: On Pt(100) transient potentiostatic oscillations were obtained when stopping an Z/V scan in either direction (whereas spikes were only observed in the cathodic sweeps), although the potential range of oscillatory

0.4 0.6 0.8 kE/V Figure 4. I / Vcurves of Pt( 110) in (a) 0.1 M HClO4 + 0.05 M HCOOH and (b) 0.1 M NaC104 0.05 M HCOOH.

-0.2

0

0.2

+

behavior was somewhat larger in the anodic than in the cathodic direction. The optimum concentration of formic acid (Le., the highest amplitudes) turned out to be around 0.05 M, a t which molarity oscillations were still observed a t a pH of 2 (after adding HC104 up to M). Addition of the same amount of NaC104 did not noticeably alter the oscillatory behavior. Figure 5 shows typical current oscillations on Pt( 100) obtained with 0.05 M HCOOH at 500 mV. Such oscillations occurred either when stopping the voltage sweep at a suitable potential or after applyingjumps of potential from both high or low potentials to an appropriate value. The peak forms were always typical of relaxation behavior, but the interval between the current maxima increased with time until none occurred any more, and the system remained in a low-current state. For Pt(ll0) it should first be mentioned that the Z/Vcurve

The Journal of Physical Chemistry, Vol. 98, No. 31, 1994 7615

Electrochemical Oxidation of Formic Acid I

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cathodic direction (around 500 mV, both sweep and jump) and after a comparatively long induction period of nearly 4 min; the oscillations, too, were fairly slow (CO.1 Hz). They exhibited a characteristic relaxation shape, and the period increased until the system settled down on the low-current level (Figure 8).Similar to Pt(1 lo), the potential had to be lowered to about 0 mV before oscillations could be obtained again. The main differencesin the oscillations on the different surface orientations are summarized in Table 1. The influenceof stirring has been examined exemplarily with Pt(lOO), because at this surface the most prolonged oscillations have been found. Figure 9a shows the influence of stirring on the I/Ycurves. In the case of stirring just one of the formerly seven spikes was left. Concerning the current oscillations, we found two different kinds of behavior depending on the initial situation: (a) Starting the stirrer during oscillations made them fade away, and the current settled to a steady value closer to the maximum than to the minimum of the oscillations. Hence, the system did not go toa passivated state (Figure 9b). (b) After the oscillations died spontaneously, the current took on the minimum of the last oscillatory cycle. Stirring for a few seconds reinvoked oscillations quite similar to those obtained before. Only a few new starts (mostly just two) have been observed. Afterward, oscillations could be reestablished by flame-annealing of the crystal. Several polycrystalline electrodes were examined under the same and similar conditions, namely, a rotating Pt disk, a Pt sheet, and a thick and a thin Pt wire (1- and 0.1-mm diameter, respectively). The wires were annealed to compare directly to the single crystals; the other electrodes were cleaned in boiling concentrated sulfuric acid followed by anodic treatment. The ZIVdiagrams of the thick wire, sheet, and disk exhibited just one sharp spike in the cathodic direction (Figure lo), but we failed to detect oscillations after potential jumps into this region from anodic or cathodic values. In contrast, the I/ Vdiagram on the thin wire showed up to four spikes in the anodic scan (Figure lob), and oscillations did occur, as reproduced in Figure 11. 4. Mechanism and Discussion

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Figure 6. I/Vcurves of Pt(ll0) in 10-3 M HClO4 + 0.05 M HCOOH (20 mV/s). The curves change with time: (a) right after flame-annealing, (b) sixth scan, (c) final form after 10 scans.

obtained right after annealing the crystal in the flame changed with time (Figure 6). The small spikes between 600 and 700 mV vs SCE in the anodic sweep only developed after a few cycles and then slightly shifted in the cathodic direction before a stable (asymptotic) I/V curve was obtained. Oscillations were only obtained after potentialchanges into the region of spikes in anodic direction. The oscillatory behavior was not noticeably influenced whether the potential was adjusted via a sweep or a jump. However, it was crucial to establish a sufficiently low potential prior to oscillation measurements. This becomes evident from Figure 7, where the behavior after jumps to +650 mV from increasing initial potential are shown. For starting points above about 100 mV no oscillations were observed any more. Incontrast toPt(100), thepeakformofoscillationsonPt(ll0) changed with time (Figure 7). After an inductionperiod of about 2 s, fairly harmonic period-1 oscillations developed, which then changed into compound behavior consistingof a large and small peak. Shortly afterward the interval between large peaks increased, while several small oscillations with increasing amplitude occurred in between (Figure 7a,b). Although no spikes were seen in the I / V curves on Pt( 11 1) (Figure 3), oscillations could still be obtained, but only in the

It is well-known that the oxidation of formic acid proceeds via a branched reaction pathway, either through adsorbed CO or a 'COOH radica1.28.m For a discussion of the experimental findings, we include the diffusion of protons and formic acid between the bulk of the solution and the double layer as well as the dissociation of water. This gives rise to the following reaction scheme:

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(3) (4) (5)

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(8) Here denotesan empty site, 'COOH, CO, and OH are adsorbed, and HCOOH, H+, and OH- refer to species in the double layer. The bracketed species are assumed nones~ential.~~ Since most

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7616 The Journal of Physical Chemistry, Vol. 98, No. 31, 1994

Raspel and Eiswirth

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TABLE 1: Comparison of Typical Oscillatory Behavior on Different Pt Electrodes oscillation range in surface orientation spikes in I / Y curve anodic/cathodicdirection (mV) . . 400-550/400-500 100 several in cathodic direction 110 several in anodic direction 600-800)-

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CO- and OH-covered states in the SNA formalism, one should actually use two more extreme currents in order to avoid prediction of unphysical states, e.g., by introducing (however small) desorptionof CO and OH, leading to E4 and E5 in Figure 12. This is, however, of no concern for an existence proof of oscillations.] El is unstable and best thought of as autocatalytic removal of a passivating CO adlayer. All others are stable. E2 represents a high-current state, while E3 establishes a connection beween El and Ez. The steady states are given by nonnegative linear combinations of these extreme currents. For simplicity (since it is sufficient to find any parameter set), we choose the simple sum El + E2 E3; the resulting current is also shown in Figure 12. Following Clarke,23we call the stoichiometric matrix Y and the kinetic one K ; the principal minors of order i of S = - Y K ~ are referred to as 0,'s. All 0s which include species are positive or zero, but at least one On, n < 5 , depending on becomes negative. This is a special case of the conditions described in ref 25; therefore, it guarantees the existence of a Hopf bifurcation. (Here in particular 03 (*, HCOOH, OH) and 03 (*, CO, OH-) are negative.) The mechanism above is actually very similar to the 1CW systems described in ref 24, though in the present case a conservationconstraint occurs (constant electrode area, i.e., + CO OH = constant), because of which the network has to be somewhat more complicated to give rise to oscillations. In practice, one expects a finite IR drop (even when the Luggin capillary is close to the surface) and consequently finite changes of cp during oscillations (which can often cause oscillations in connection with fairly simple purely chemical networks21*22).

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period transient oscillations are observed.

rate constants for the above scheme are not known, we use SNA to show that the suggested mechanism is actually oscillatory. We first eliminate 'COOH adiabatically, Le., assume that reaction 5 is fast. The resulting network diagram is shown in Figure 12; it can be broken down into the extreme currents (subnetworks)El, E2, and E3. [For an inclusion of the passivated

+

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The Journal of Physical Chemistry, Vol. 98, No. 31, 1994 7617

Electrochemical Oxidation of Formic Acid

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Pt disc without rotation; (d) Pt disc with 500 rpm.

Nevertheless, the observed phenomena can be qualitatively explained even when potential changes are neglected.

Figure 12. Network digram (DN), extreme current diagrams (EJ,and the currentdiagram (Dc)used to show the existenceof a Hopf bifurcation of a slightly simplifiedversion of the branched-chain mechanismof formic acid oxidation (nonessentialspecies are not included).

In physicochemical terms the occurrence of oscillations can be explained as follows: For low H+ concentration in the double layer, a high current can flow because CO poisoning is prevented by reaction 8 as long as OH adsorption (7) is fast enough. However, the protons generated in (4), ( 5 ) , and (8) will lower the pH, and therefore OH adsorption decreases, eventually favoring the CO passivated state. H+production stops; the protons can dissipate into the bulk until OH adsorption can again overcompensate CO formation. The CO is then reacted away autocatalytically, and the cycle repeats. Despite the fairly high diffusion coefficient of H+, complete homogenization of the solution cannot occur on the relevant time scale. Therefore, the pH near the electrode will be slightly lowered in each oscillatory cycle so that H+ will diffuse away less and less effectively. Thus, the system will spend more and more time in the CO-covered passive state until it does not recover at all any more (Figure 5). However, the original concentration distributions can (approximately) be reestablished by stirring the solution for a while; after a transient time, oscillations should set in again, as was indeed observed experimentally. Conversely, turning on the magnetic stirrer during (unstirred) oscillations tends to suppress the accumulation of H+ near the surface; the CO passivated state can therefore not form. While this also causes the oscillations to die, they do so on a level of fairly high current, also in agreement with experiment (Figure 9). Thus, the above mechanism can readily explain the (at first glance seemingly contradictory) effects of stirring. Nevertheless, the effects that only transient oscillations were observed and

7618 The Journal of Physical Chemistry, Vol. 98, No. 31, 1994 stirring could only restart them for a few times (after which the crystal had to be repurified) may, in addition to concentration changes, also be due to some slow change of surface properties (e.g., because of impurities or surface roughening). In contrast to the present work, Markovic and RossZorecently reported oscillations with much lower amplitude which were restricted to Pt(100), extended down to pH 1, and not sensitive to stirring. The authors concludedthat the pH near the electrode should not be an essential variable, which would contradict the above mechanism. However, the pronounced quantitative and, even more importantly, the striking qualitative differences in the experimental findings make comparison of mechanistic considerations very difficult, in particular because of the large number of control parameters (such as trace impurities,20pretreatment of the electrode, exact turning potential, sweeping direction (and possibly velocity), and even the electrode size). Some of these are not easy to compare for different studies. We shall therefore refrain from a more detailed mechanistic discussion before more experimental results are available. In any case the complete mechanism is expected to be more complex, although already the simple scheme discussed above gives a consistent (qualitative) rationalization for the effects described here. Despite significant differences between the low-index planes of Pt, there is-for the results of section 3-110 convincing reason to assume that qualitatively different mechanisms operate, since all were found to undergo hysteresis and oscillations (in contrast to earlier st~dies*~,20). The rates of the surface processes 4-8 are expected to be very structure-sensitive, which may be sufficient to explain the differences in shape, amplitude, and period. The failure to detect oscillatory behavior on most polycrystalline electrodes is somewhat surprising since all low-index planes do oscillate. The result may be due to the absence of a sufficiently effective synchronizingmechanism. Thus, individualcrystallites on the surface should of course have the tendency to oscillate (and may too), but random phase relationships may actually prevent the observation of macroscopic oscillations. However, oscillatory behavior did occur using thin wires. Perhaps no effective synchronizing process is required, if the size of the crystallites becomes comparableto the dimension of the electrode.

5. Conclusions In contrast to earlier reports, formic acid oxidation exhibits fixed-potential oscillations on all three low-index Pt surfaces. Already a simple strictly potentiostatic model can qualitatively

Raspel and Eiswirth explain the oscillatory instability and the effects of stirring. Oscillatory behavior does occur on polycrystallineelectrodes but remains difficult to detect.

Acknowledgment. The authors are indebted to B. Beran for preparation of the single crystals, M. Liibke and P. A. Thiel for some experimentswith polycrystallineelectrodes, and D. M. Kolb for his support and helpful discussions. References and Notes (1) Hudson, J. L.; Bassett, M. R.Rev. Chem.Eng. 1991,7,109. Hudson, J. L.; Tsotsis, T. T. Chem. Eng. Sci. 1994.49, 1493. (2) Butler, J. A. V.; Armstrong, G. Nuture 1932, 129, 613. (3) Horanyi, G.; Visy, C. J. Electrounul. Chem. 1979, 103, 353. (4) Kodera, T.; Yamazaki, T.; Kubota, N. Electrochim. Actu 1986,31, 1477. (5) Krischer,K.;Liibke,M.; Wolf, W.;Eiswirth,M.;Ertl,G. Eer.EunsenGes. Phys. Chem. 1991,95, 820. (6) Krischer,K.;Liibke,M.;Eiswirth,M.;Wolf, W.;Hudson, J.L.;Ertl, G . Physicu D 1993,62, 123. (7) Hachkar, M.; Bedcn, B.; Lamy, C. J . Electroam/. Chem. 1990,287, 81. (8) Horanyi, G.; Rizmayer, E. M. J. Electroual. Chem. 1983,143,323. (9) Fetner, N.; Hudson, J. L. J. Phys. Chem. 1990,94, 6506. (10) Frumkin, A. Z . Elektrochem. 1955,59,807. (11) Wolf, W.; Ye, J.; Purgand, M.; Eiswirth, M.; Doblhofer, K. Eer. Bunsen-Ges. Phys. Chem. 1992,96, 1797. (12) Clavilier, J.; Parsons, R.;Durand, R.; Lamy, C.; Leger, J. M. J. Electround. Chem. 1981, 124, 321. (13) Adzic, R. R.;Tripkovic, A. V.; O'Grady, W. E. Nuture 1982,296, 137. (14) Lamy, C.; Leger, J. M.; Clavilier, J.; Parsons, R. J. Electround. Chem. 1983, 150, 71. (15) Motoo. S.; Furuva, N. J. Elecrrounul. Chem. 1985, 184. 303. (16) Miiller, E.; Tan&, S . Z . Elektrochem. 1928,5,24. (17) Okamoto, H. Electrochim. Actu 1992, 37, 37. (18) Schell, M.; Albahadily, N.; Safar, J.; Xu,Y . J . Phys. Chem. 1989, 93, 4806. (19) Raspel, F.; Nichols, R.J.; Kolb, D. M. J. Electrounul. Chem. 1990, 286, 279. (20) Markovic, N.; R m ,P. N. J. Phys. Chem. 1993, 97,9771. (21) Koper, M. T. M. Electrochim. Actu 1992, 37, 1171. (22) Koper, M. T. M.; Sluyter, J. H. J. Electrounul.Chem. 1993,352,51. (23) Clarke, B. L. Adv. Chem. Phys. 1980, 42, 1. (24) Eiswirth, M.; Freund, A.; Ross, J. J. Phys. Chem. 1991,95, 1294; Adv. Chem. Phys. 1991,80, 127. (25) Biirger, J.; Eiswirth, M., to be published. (26) Dickertmann, D.; Koppitz, F. D.; Schultze, J. W. Electrochim. Acru 1979, 21, 967. (27) Yamamoto, K.; Kolb, D. M.; Kbtz,R.;Lehmpfuhl,G. J . Electround. Chem. 1919,96, 233. (28) Capon, A,; Parsons, R. J. Electrounul. Chem. 1973,44, 239. (29) Sun, S. 0.;Clavilier, J.; Bewick,A. J . Electrounul.Chem. 1988,240, 147. ~~