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OBSERVATION OF BISTABILITY IN CYCLIC VOLTAMMETRIC EXPERIMENTS ON ETHANOL, PROPANOL, BUTANOL AND FORMIC ACID/FORMATE XIAORONGCAI and MARK SCHELL Department of Chenustry and the Center for Noneqmhbnum Structures, Southern MethodM Umverslty, Dallas, TX 75275, U S A (Recewed 8 May 1991, UI rewedform
3 July 1991)
Abatrac-The effects of varymg the lower potential lmut were studied m cycbc voltammetnc expenments conducted on several electrocatalysed oxidation processes the oxidation of fornuc acld/formate, ethanol, propanol and butanol In an aclchc medium, each oxidation process, SubJectedto a cychng potential, exhlbl&zda b&able response over a range of values m the lower potential hnut Wltiun the region of blstablhty, two different cycbc voltammograms, one urltha relatively large ampbtude and one mth a small amphtude, were found to coexist Experunents conducted on the oxtdatlon of butanol revealed that the in size untb respect to increases in the alkaluuty of the solution, a result region of blstablbty dm that ISconsistent ~ntb the descnptlon of blstablbty provrded by the theory of catastrophes The cause of the cycbc voltammetnc blstabtity IS explamed m terms of a nonlinear feedback mecbamsm mvolvmg a reaction between strongly bound carbon monoxide and oxtdes The reamon couples an mduect path of the oxldatlon processes to tbe process by whch an oxide layer IS formed Key words voltammograms, nonhear, hfurcations, catastrophe, coexistence
1. INTRODUCTION The electrocatalysed
oxidation of small orgamc molecules at a platinum electrode under condltrons of constant current IS often accompanied by oscdlatlons m the electrode potentlal[ l-61 Oscillatory hehavlour lmphes that these oxidation processes possess nonhnear feedback mechamsms[7] It nught he expected that vanations of these feedback mechamsms are ative under potentiostatlc conditions However, often only stable steady states, which fit a single valued
curve, are realized under con&tions of constant potential even when oscillations are exhibited under galvanostatlc conditions In some cases, cychc voltammetry reveals a large change on a short time scale&111 This change rmght be interpreted as a nonlinear effect but, such an effect cannot be described as a drastic change of behavlour m the same sense as transltlons to osclllatlons On the other hand, a brstable response lmphes the presence of nonhnear feedback mechamsms Recently, it was demonstrated m a cychc voltammetnc study on the oxldatlon of fornuc acid that, under certain expenmental conQtions, the system relaxes to one of two cyclic voltammograms that repeat[l l] Initial conditions determme which one 1s eventually observed It was also shown that a &an&on from one cychc voltammogram to the other could be. mduced by apphcatlon of perturbations In this paper, we report results from apphcatlons of cychc voltammetry m which the effects of varying the lower potential hnut were exammed These results demonstrate that cychc voltammetnc brstabdlty occurs m three additional oxidation processes, the oxldatlon of three pnmary alcohols ethanol,
propanol and butanol For purposes of companson, the cychc voltammetnc behavlour of the oxidation of forrmc and/formate was re-exammed but m a solution w&h a sulfunc actd concentration that was an order of magmtude greater than that used m Ref [ 1l] In the expenments, the lower potential hnut (Ipl) was employed as a “btiurcation parameter ” Plots of the Ipi agamst peak currents from cychc voltammograms reveal that, m a solution contammg 10m3M H,SO,, each oxidation process exhlhts a hstable response over a substantial range of expenmental conditions In the case of butanol, results are also presented from voltammetnc expenments conducted m alkaline solutions These results show that regrons of hstabihty decrease m size, and eventually Qsappear, ~th respect to increases m alkahmty These changes m behavlour are consistent wth the cusp catastrophe plcture[l2] of hstable systems
2. EXPERIMENTAL A Pme Instrument, Model RDE4, potenbostat was used to control the potential and measure the current Cyclic voltammograms were recorded with an X-Y recorder (Houston Instrument, 2000 recorder) A three-neck, SOOml flask, which was immersed m a bath held at 25 0 f 0.2”C, served as the electrochemical cell The cell contamed a 4QOml solution cons& mg of 1.0 M orgamc solute and lo-’ M H,SO,. The choice of the md concentra~on 1s somewhat artnWary: prehminary cychc voltammetnc expenments inthe existence of blstabbty over a substantial M of condrhons 111both acid and alkahne medmms At large acid concentrations (10 M 673
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H2S0,) and large base concentrations (1 0 M NaOH), blstahhty was not detected It was decided that the detailed study would be conducted m approximately the middle of the pH range m which a blstable response was observed Cychc voltammetnc expenments were also conducted using solutions conslstmg of 1 0 M butanol and several different concentrations of NaOH Dntilled water, which was also processed through a m&pore delomzatlon unit, was used m all solutions The sodium formate was obtained from Malhnckrodt Inc (Pans, Kentucky, U S A), l-butanol (spectrophotometnc grade) and l-propanol (99 92%, 0 07% water) from Aldnch Chemical Co Inc (Milwaukee, Wlsconsm, U S A ) and the Ethyl Alcohol (Absolute-200 proof) was obtained from AAPER Alcohol and Chemical Co (Shelbyvllle, Kentucky, U S A ) Aad solutions were made with reagent grade sulfunc acid (EM Science, Cherry Hi, NJ) and alkaline solutions were made usmg sodmm hydroxide pellets A saturated calomel electrode, see (Beckman) and a platinum wire served as the reference electrode and counter electrode, respectively All potentials are reported with respect to the reference electrode A rotating disk, 18 0 mm m diameter, 7 5 mm polycrystallme platmum, 10 5 mm Teflon (PAR model No AFD11580, senal No 4619), was employed as the working electrode This disk was attached to a Pme Instrument ASR-type rotator and rotated at 3000 rpm The working electrode was cleaned electrochemltally before each set of expenments m a solution of
0 5 M H2S0, The cleaning procedure consisted of passing the workmg electrode through the followmg steps the potential was held at a value of 2 0 V (see) for 4Os, then at OOV for 5s, -04V for 10s and then finally the potential was held at 0 0 V for 5 s Durmg these steps the electrode was rotated at the rate of 1000 rpm This procedure was usually repeated 30 times Next, mth the rotation rate set at zero, the potential was cycled between -220 and 1200 mV, until a cychc voltammogram corresponding to a clean platinum surface was obtained The electrode was then washed urlth dlst&d water and transferred to one of the solutions under study 3. RESULTS 3 1 Voltammetrrc behavtour tn an acldtc medwn 3 l(a) Fortnrc acrd/formate Results from apphcations of cychc voltammetry to the oxldatlon of fonmc acld/formate are presented first Begmmng Hnth a sufficiently small value for the lpi ( < 120 mV) and cycling the working electrode potential between that value and the value 1000 mV, a large amplitude cyclic voltammogram, with charactenstlcs like those of the cychc voltammogram (CV) shown m Fig la, was obtained The lpi was then increased m small increments Each value of the Ipl was held constant long enough that transient behavlour disappeared Followmg this procedure, the system passed through &fferent members of a family of large amplitude CVs, examples are presented m Fig 1b and c Upon changmg the lpl to a value greater than some cntical
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BtstabPty m cychc vohannnetry value, the system relaxed to a completely different type of CV An example of this type of CV IS shown m Fig Id Thts latter CV belongs to a separate family, a family of small amphtude CVs The value of the ipl at which the- drasttc change m the response was observed was shghtly greater than the value of the lpl belongmg to the measured cychc voltammogram shown m Fig lc It can be deduced from Fig lc that the transitton from the large amplitude CV to the small amplitude CV occurred for a value of the lpl that 1s close to the value of the potenhal at which the system exhthted a sudden nse m current on the reverse sweep of the large amphtude CVs After the transitton to a small amplitude CV was made, the lpl was increased and then decreased m small decrements The system was allowed to relax at each value of the Ipl Decreasing the Ipl caused the system to pass through other members of the family of small amplitude CVs Eventually, another cnttcal value was passed at which point the system relaxed to a large amplitude CV Between the two cnttcal values, two CVs, one from each family, were found to coexist for each value of the lpi For example, the CVs m Fig lb and e were found under the same expenmental condtttons In Fig 2, peak current values from measured cychc voltammograms are plotted against the lpl The plot reveals the regton of btstabthty m whtch both large and small amplitude CVs were found to coexist 3 l(b) Alcohols Measurements of changes m the voltammetnc response with respect to vanattons m the Ipl were also conducted on the oxtdatton of ethanol, propanol and butanol SubJectmg the oxidation processes to a cycling potential revealed m each case two different families of CVs, (large and small amplitude CVs), and a regon of coextstence The upper potential hmrt used m these three sets of expenments was 1200mV (see) (Butanol did not exhibit a btstable response when an upper potential hmtt of 1000 mV was used ) A set of CVs, measured dunng the oxtdatton of ethanol, 1s presented m Rg 3 Peak currents are approxtmately an order of magnitude less than those found m the oxldatton of formic actd/formate However, though shifted to a potential higher than that of formic acld/formate, the charactenstm sharp nse m current on the reverse part of the sweep IS clearly seen m the large amphtude CVs, see Fig 3a-c The CV m Fig 3a was measured at a value of the lpi that was lower than any value wtthm the regton of coexntence, the CV m Ag 3b was found wtthm the region of coexistence, and the CV m Fig 3c was the last large amplitude CV that was measured m the dnecuon of increasing lpl values After increasing the ipl to a value greater than that used m Fig 3c, the system relaxed to a small amplitude CV Smnlar to the case of fonmc actd/formate, the transitton to the small amplitude CV takes place for a value of the lpi that 1s m the vtcrmty of the value of the potential at which the sudden nse m current occurs on the reverse sweep The small amphtude CV shown m Fig. 3d was measured under the same condmons as those of the large amphtude CV shown m Fig 3b Large amphtude CVs, one measured for a value of the Ipl outside the regton of coextstence and the last
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Frg 2 Peak current from measured cychc voltammograms ISplotted agamst the lpf Condtuons are the same as m Fig I Opencwckscorrespond to measurements mrded whde increasing the lpl in small increments Closed circles correspond to measurements taken whde decreasing the ipl m small decrements To the ngbt of the transition down, measured values were approxunately the same and hence, only closed cxcles are shown Between the two transition pomts the system 1s hstable
large amplitude CV that was measured on Increasing the lpi, for both propanol and butanol, are shown m Ftg 4 The small amphtude CVs look smular to the one m Fig 3d and are not shown The large amphtude CVs agam show charactertsttc sudden rises m the current on the reverse sweep However, m expertments on butanol, a quantttattve difference was Qscovered with regards to the transtton from the large amplitude CV to the small amplitude CV Tlus transitton occurred at a value of the lpl that was slightly greater than the value used m Fig 4d Notice that, unlike the other oxtdatton processes, the transitron occurred at a value of the lower hnut that IS substantially less than the value of the potenttal m the large amplitude CV at whrch the sudden increase m current occurs This quahtattve difference IS attnbuted to the underlymg stab&y properties of the butanol system The regton of hstahhty for butanol IS smaller than the others The small size of the hstable regton implies the system IS close to regtons m parameter space where brstabrhty disappears As the system approaches these parts of parameter space, the potential at which the sharp nse m current occurs durmg the reverse sweep becomes less mdtcattve of the value of the lpi at which the large amplitude CV disappears Plots of peak current us lpl are presented for each process m Ag 5, the plot correspondmg to the oxtdatton of ethanol IS shown m Ftg 5a, propanol m Fig 5b and butanol m Fig 5c These plots reveal the Interval of values for the lpi m which the system exhthts a hstable response. Nottce that the regton of coexistence 1s small m the case of the oxnlatton of butanol Also note that, m the case of butanol, the peak current was not as large after the tram&on from the small to large amplitude CV as it was in the ongmal family of large amplitude 0%. Thts result mdicates that substanttal surface changes can occur whtle movmg the system along the lower branch of states. For instance, place exchange reacttons wtll change the surface.
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Rg 3 Cychc voltammograms measured durmg the ondatlon of ethanol after transient behavlour dlsappeared Upper potential hmlt = 1200 mV, sweep rate = SOmV s-‘, solution 10 M ethanol, low3 M H,SO, (a) lpi = 100 mV ‘l&s value of the lpl corresponds to a value that IS less than any value of the lpi m the repon of coexistence (b) lp/ = 400mV (c) lpf = 680mV (d) Ipl = 400 mV This CV was measured under the same conditions as the CV m (b) The CVs m (a) (b) and (c) are large amplitude CVs and the one m (d) IS a small amphtude CV Note the charactenstlc sudden nse dunng the reverse sweep m the large amplitude CV Increasing the fpl to a value slightly greater than that m (c) causes a transition to the small amplitude CV The value of the lpi m (c) 1s m the vlcuuty of the value of the potenhal at which the sudden nse m current occurs (for all three large amplitude CVs) dunng the reverse sweep
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POTENTIAL (mv) POTENTIAL @IV) Fig 4 (a) Cychc voltammogram measured durmg the oxldatlon of propanol Solution 1 0 M propanol, 1O-3M H2S0, lpl= 200 mV Other conditions the same as m Fig (3) This was the only repeating cychc voltammogram found for tis value of the lpl (b) Condltlons the same as (a) except IpI= 620 mV (c) Cyclic voltammogram measured durmg the oxulation of butanol Solution 10 M butanol, lo-’ M H,SO, lpi = 0 0 mV Other conditions the same as (a) No other repeating cychc voltammogram was found for this value of the lpf (d) Conditions the same as (c) except lpi = 400 mV
Bzstabzhtyzn cycltc voltammetry 3 2 Voltammetm medra
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Cychc voltammetnc
behavlour of butanol m allcahne solutions was also stud& A set of CVs 1s &splayed m Ftg 6 Each cychc voltammogram was recorded after transient behavzour disappeared m solutions contammg lo-)M NaOH and usmg an upper potential hrmt (upl) of 800mV Though the large amplitude CVs, Fig 6a, b and c, are different from those shown m Fig 4c and d, zt 1s apparent that at least some components of the mechamsm causing hstabzhty m aczd solution are active m alkaline solutzons, the charactenstzc sharp increase m current 1s still present m the reverse sweep A plot of peak currest us lpl, shown m Fig 7a, reveals the regzon of coexzstence We also note that a hstable response was not obtained m alkaline solutions using a up1 of 12OOmV The results of several expenments indicate the followmg trend if a gzven pH and a fixed up1 the oxzdatlon of butanol exhibits a voltammetnc regon of coexistence ~nth respect to changes m the 1~1,then this repon of coexistence shrmks and eventually vamshes wzth respect to increases m the alkahmty of the solution It was found that once the regzon of coexistence vanished, a new one could be obtained by changmg the up1 to a lower value On increasing the alkahmty wzth thus new fixed value of the upl, the regzon of coexistence eventually vanishes agam, and so on (For [NaOH] > 10-l M, a bzstable response was not found ) For a value of the concentration of NaOH shghtly greater than the value at which a regzon of coexzstence vanished, a plot of the peak current us the lpi produces a single value relatzonshzp Such a plot 1s shown m Fig 7b Now instead of a drastic change ze a transition from a large amplitude CV to a small amplitude CV, one CV smoothly deforms mto the other as one slowly decreases or increases the lpi The constructzon of the same plot under the same conditions except wzth a small decrease m the sodium hydroxzde concentration reveals a regzon of coexls-
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LOWER POTENTIAL LIMIT (I Fzg 5 Peak current from measured cychc voltammograms is plotted against the lower potenhal lmut ConQtions are the same as m Fzgs 3 and 4 (a) Oxidation of ethanol (b) oxldatlon of propanol (c) Oxniat~on of butanol Approxlmate values of the lpiat wlch the large amplitude CV ather became unstable or disappeared are (a) 690, (b) 630 and (c) 402 mV
tence %s result 1s consistent wzth the cusp-catastrophe picture of bzstable systems]121 Gven that a system possesses a bzstable response over an interval of values of a specdied expenmental constraint,
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Rg 6 Cyclic voltammograms measured durmg the oxulatmn of butanol Solution 10 M butanol, 10e3 M
NaOH
Upper potential hnut = &l@mV, (a) lpf = -500 mV, (b) fpl = -300 mV, (c) I@=-2OOmV,(d)lpl= -300mV
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catastrophe theory predtcts that tt 1s possible to choose a second constraint that can be varied m such a way that the mterval m which bistabihty occurs decreases and then disappears Finally, we wish to pomt out that m addition to a btstable response, more complicated behavtour was observed durmg cychc voltammetnc expenments conducted on the oxtdatton of ethanol m alkaline soluttons Cychc voltammograms with penods greater than the period of the potential, as well as apenodtc voltammograms, were obtained Such behavtours are beyond the scope of the study considered here and are reported elsewhere[l3] 4. RELATIONSHIP WITH ELECTROCHEMICAL MECHANISMS In order to provide a possible explanation for the occurrence of cychc voltammetnc bistabihty, tt is necessary to review several features of the mechanisms for the oxidation processes as well as the process by which an “oxtde layer” 1s formed We begn wtth the description of the large amplitude CVs corresponding to the oxtdatton of forrnate/fornnc actd[l] The mtttal nse m current that occurs m the forward duectton of the CVs shown m Fig la-c 1s attnbuted to the direct oxtdatton of the fuel A postulated mechanism for the oxidation of formtc acid 1s[S]
(There haviour aad[l4]) directly strongly
HCOOH + HCOOH(a)
(1)
HCOOH(a) + COOH + I-J,
(2)
COOH+CO,+H++ee-,
(3)
H-H++e-
(4)
exists expenmental evidence that the beof formate is similar to that of fornnc In addition to reaction paths that lead to CO,, other paths involve the formation of bonded carbon monoxtde, eg[8], HCOOH(a) + CO + Hz0
(5)
Other mechanisms for the direct oxtdatton of the fuel and formatton of both bridge and linear bonded carbon monoxide are listed m the hterature[ll, 14,151 At larger potentials, but preceding the locations of maxima m Ftg la-c, the oxtdatton of carbon monoxide begins to stgmficantly contnbute to the current QH+CO-,H++C02+e-
(6)
Thts reaction (6), is preceded by the chemrsorption of hydroxyl radicals, H,O+QH+H+
+e-
(7)
There exists at least two possible alternative descnptions for the oxtdatton of carbon monoxtde instead of hydroxyl radicals, reactton of carbon monoxtde with a closely related species 1s plausible, eg[9], HzO+~O+2H++C0,+2e-
(8)
Another alternative to equation (6) is the reaction[l6] QH + CO + GOOH,
(9)
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Fig 7 Peak current from cychc voltammograms measured durmg the oxldatlon of butanol IS plotted against the lower potential hmlt (a) Condltlons are the same as those m Fig 6, (b) as for (a), except [NaOH] = 10-l M and upper potential limit = 500 mV
which would then be followed by the reaction wntten m equation (3) Wtthm the potential range m which CO is oxtdtzed, the reaction m equation (3) 1s expected to be fast Consequently, it 1s dtfficult to Qfferenttate between the two reactions written m equations (6) and (9) After passing through the current maximum m the forward sweep (see Fig la), the ensuing decrease m current 1s usually attnbuted to the continued formation of oxtdes[l, 17,181 On the reverse sweep, the reduction of oxides and oxidation reactions combme to produce a small anodlc current until a value of the potential 1s reached at which point most of the oxides are removed In the vtcmtty of thts value of the potential, the current, due to the resumptton of direct oxidation of the fuel at bare sites, suddenly increases to a large value The underlying chemistry that 1sresponstble for the large amplitude CVs measured durmg the oxtdation of the alcohols 1s substanttally different from the chemistry that takes place m the oxidation of formic aad For alcohols, the appearance of appreciable anodic currents during the forward sweeps takes place at larger values of the potenttals (compare Fig 1 wrth Figs 3 and 4) Tlus nse m current occurs simultaneously with production of CO* [ 10, 191 From the results presented m Ref [ 191,we conclude that, m the case of ethanol solutions[20] a substantial amount of the CO2 produced 1s from oxtdation of carbon monoxide (The study m Ref [ 191was conducted using concentrations different from these used here The apphcahhty of those results to these presented here 1s an assumption ) Surface bound carbon monoxide
Bstabhty IU cycb voltammetry
can form from dtssocmuve chemtsorptton of ethanol[9] Reactton of acetaldehyde and hydrated acetaldehyde, which, at least to some extent, are expected to be intermediates of some of the reactton pathways, also leads to surface bound carbon monoxtde[lO] In addition to carbon dtoxide, acettc acid IS an end product m the omdation of ethanol The first peak m the ethanol CV was determmed to be caused by a decrease m productton of CO,, and the second mcrease 1s attributed to an mcrease m the productton of aceuc actd (a stgmficant amount of ace& actd IS also produced at potentials under the first peak) [ 191 Although Qfferences extst among the oxtdatton of the alcohols and stgmficant Qfferences exist between the oxtdatton of the alcohols and the oxidation of fornuc actd, substanual evidence supports the idea that each oxldauon process possesses at least one reactton pathway that mvolves the formation of surface carbon monoxtde as an mtermedlate[9, 10, 15, 19-211 A reasonable hypothesis is that the oxtdauon of carbon monoxtde mvolves the same chemistry, eg equation (6), m each of the processes In all oxidation processes, the small amplitude CVs exhibit a mostly anodtc but relauvely small current Thus the electrode surface most hkely retains an oxide layer throughout the cychng process It is therefore necessary to revtew some classtcal results on the formatton of oxide layers It was revealed m the study by Angerstem-Kozlowskt et a1[22] that the process m which an oxide layer 1s formed m actd solutton (without other solutes present) takes place in several stages Many of the stages of oxide-layer formation, as well as reduction of the layer, occur at potentials well below that at which oxygen evolution takes place The first three stages of oxide formatton correspond to chemtsorptton of hydroxyl radicals At higher potentials PtO forms and the rearrangement of part of the oxtde layer also occurs, which mvolves place exchange reacttons Reduction of oxide species is also complicated For example, reductton of PtO can require a lower reduction potenual then PtOH An important result wtth regards to the cause of hstabthty 1s that the further the system advances along the stages of oxtde formauon the lower the potenttal necessary to reduce the oxtdes Even without constdenng the coupling through chemical reactions, the presence of orgamcs has a constderable effect on oxide formatron For example, the three stages of chemtsorptton of hydroxyl radicals correspond to successive formattons of three dtfferent sublattice arrangements[22] However, if the surface ISalready parttally occupied by, say, CO, the mechantsm for chemtsorption of hydroxyl radicals wtll change On companng cychc voltammograms and chargtng curves, tt was deduced some time ago that, m the presence of orgamcs, oxtdes form on the surface of the electrodes at rather low potential values, substanttally lower than the value at whtch hydroxyl radtcals are chemlsorbed m acid solutions wtthout other solutes present Nevertheless, when oxides dominate the surface of the electrode, the stages of oxide formatton that occur m actd-only solutton are expected to kick m An important coupling wtth regards to the occurrence of btstabihty 1s hypothesized to be the chemical reaction mvolvmg oxides and carbon monoxtde, eg
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equatton (6) Evtdence suggests that thus reactton IS explosive In spectroscoptc studtes it was observed that the surface concentratton of carbon monoxtde rapidly decreased after potenttal values were achteved at which carbon monoxide 18 oxtdtzed@,9]. Furthermore, the vahres of peaks m the waveforms of potential osctllattons measured under condtttons of constant current are conststent wtth the occurrence of a reactton mvolvmg carbon monoxtde and oxides[6] These peaks are followed by a sudden drop m the potenttal which 1s consistent wtth an explonve reaction A proposed rate law descrtbmg an explosive reaction, whtch was employed to both descnbe and simulate osctllatory behavtour[2,5,6] 1s wrttten as rate = L(OH)gO)S,
(10)
where round brackets denote surface concentratton, S represents the surface concentratton of vacant sttes and k IS a potenual dependent rate coeffictent. The above descnpuons can be combined to provide an explanatton of btstahhty If uuttal condtttons are such that oxtdes dominate the surface of the electrode under condtttons of btstabthty, then the reactions whtch occur in the process by whtch an oxtde layer 1s formed wtll be the predommant reactions durmg the forward sweep In this case, the system relaxes to a dynamical state m which oxtde-layer formatton develops to an advanced stage dunng the forward sweep Consequently, oxtdes cannot be completely removed on the reverse sweep However, tt follows from practtcal reasons that, for a fixed value of the upper potenual hrmt, a cnucal value of the fpl extsts such that, for values of the lpi less than that cnttcal value, the system wtll relax to a state m which all oxides can be removed on the reverse sweep regardless of uuhal condmons On the other hand, tf at the begmnmg of the sweeping process, condmons are such that enough surface CO is formed, then the reacuon between carbon monoxtde and oxldes[equatton (6)] will dommate much of the forward sweep Since this reacuon removes oxides, development of an oxide layer is delayed Oxides that do form are removed dunng the reverse sweep A strong nonhneanty m the rate of the reaction that mvolves strongly bonded CO and oxtdes as reactants, such as that contained m equatton (lo), IS cructal A rate law for the reaction in equation (6) that is proporttonal to the concentrations of carbon monoxide and hydroxyl radicals 1s weakly nonhnear m the sense that the rate would decrease as the amount of CO decreases Since the amount of water 1s much greater than the amount of CO, the CO would be rapidly replaced by hydroxyl radtcals through the reaction written m equauon (7) Hence, the system would relax to a state correspondmg to a small amplitude CV m whtch reactions that occur m oxtde layer formatton dominate the forward sweep However, tf the rate of reactton wntten m equation (6) IS proportional to the denwty of vacant sites, then the reaction IS autocatalytic, ae the reaction produces at least two vacant sites for every molecule of CO, produced Such a reaction can remove QH faster than tt IS chemtsorbed through reacuon (8), even as the concentrauon of CO decreases Consequently, the reacuon can prevent the system from relaxing to a small amphtude CV
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Among the pnmary alcohols, the values of the fpl at which the system was observed to make a transition from a large amplitude CV to a small amphtude CV follow a trend consistent with the above explanation These values, measured under the same conditions (see Fig 5 ) are 690mV for ethanol, 630mV for propanol and 402 mV for butanol It follows from the above explanation of hstablhty that the more carbon monoxide adsorbed durmg the cycling process, the larger the value of the lpl at which transition to the small-amplitude CV occurs Thus, it is concluded that the amount of CO adsorbed decreases with respect to increases m the length of the carbon chain of the pnmary alcohols This conclusion, that there 1s a decrease m the amount of CO adsorbed with respect to an increase m the length of the carbon chain, 1s precisely what was determined m a spectroscopic study on the oxtdatlon of the same pnmary alcohols reported m Ref [lo]
SCHELL
the value of the lpf at whch the large amplitude CV ceased to exist increased with respect to decreases m the number of carbon atoms Tins result implies that the amount of carbon monoxide adsorbed also increases v&h respect to decreases m the length of the carbon chain of the pnmary alcohols, which 1s consistent Hnth the results of the spectroscopic study reported m Ref [lo] alcohols,
Acknowledgement-This
REFERENCES
5. SUMMARY Blstablhty was found m four different electrocatalysed oxldatlon processes subjected to cychc voltammetry, the oxldatlon of forrmc acld/formate, ethanol, propanol, and the oxldatlon of butanol These results demonstrate that cyclic voltammetnc blstable responses can be common m those electrochemical processes m which strongly bound intermediates form We emphasize that cychc voltammetnc blstablhty 1s not the same as blstablhty mvolvmg the coexistence of stationary states Rather, It IS bistablhty mvolvmg the coexistence of limit cycles[ 111 The cause of the cychc voltammetnc blstablhty was attnbuted to the competltlon between oxide layer formation and the oxldatlon of the organic molecules An important role IS played by a nonlinear reaction between the strongly bound intermediate, carbon monoxide and either hydroxyl radicals or closely related species The reaction mhlblts the process by which an oxide layer 1sformed Consequently, if lmtlal condltlons are such that a sufficient amount of the strongly adsorbed intermediate forms on the surface of the electrode, then the oxides that do form can be reduced on the reverse sweep This removal of oxides allows the oxldatlon of orgamcs to take place at vacant sites and m this case, a large amplitude CV IS obtained as hmltmg behavlour On the other hand, if mltlal condltlons are such that the surface of the electrode 1s covered by a sufficient amount of oxide, the system relaxes to a state m which oxides cannot be completely removed, this state corresponds to a small amplitude CV Expenmental results revealed the length of the interval of values m the lower potential limit within which the system was hstable Among the pnmary
research was supported by the
Robert A Welch Foundation, Grant No N-1096
5 6 I 8 9
10 11 12 13 14
15
R P Buck and L R Gnffith, .I elecrrochem Sot 109, 1005 (1962) J WoJtowuz, N Marmclc and B E Conway, J them Phys 48, 4333 (1968) M Schell, F N Albabaddy, J Safar, and Y Xu, J phys Chem 93,4806 (1989) N A Anastasgevlc, H Baltruschat and J Hatbaum, J electroanal Chem 212, 89 (1989), N A Anastaqevlc, G Hambltzcr, T Hartung, J -M Zhu, H Baltruschat and J Heltbaum, De&ma-Monographs 120, 255 (1990) Y Xu and M Schell, J phys Chem 94, 7137 (1990) F N Albahaddy and M Schell, J electroanal Chem 3og, 151 (1991) U F Franck. m Temooral Order. Ed&d bv L Rennna and N I Jaeger) pp’ 2-12 Spnnger, Be& (1985) ” S G Sun and J Clawher and A Bewck, J electroanal Chem 240, 147 (1988) D S Corngan and M J Weaver, J electroanal Chem 241, 143 (1988) L-W H Leung and M Weaver, J Lungmus 6, 323 (1990) G R Panda and M Schell, J phys Chem 95, 2356 (1991) R Thorn, Structural Stabrlrty and Morphogenesls (Translated by D H Fowler) BcnJamm, Rcadmg, MA (1975) M Schell and X R Cal. J them Sot . Faraaizv Tram 87, 2255 (1991) H K~ta, T Katagm, and K Kummatsu, J electroanal Chem 220, 125 (1987). K Kummatsu and H K~ta. J electroial Chem ii8, 155 (1987) R Parsons and T VanderNoot, J electroanal Chem 257, 9 (1988)
16 K Chandrasekaran, J C Wass and J O’M Bockns, J electrochem Sot 137, 518 (1990) 17 W Kelstlch, Fuel Cells (Translated by D J G Ives)
Wiley, New York (1970) 18 V S Bagoyzky and YU B Vasdyev, Electrochun Acia 9, 869 (1964) 19 L-W H Leung and M J Weaver, J phys Chem 92,
4019 (1988) 20 B Beden, M-C Mann, F Hahn and C Lamy, _ J electroanal Chem 229; 353 (1987) 21 J M Perez. B Beden. F Hahn. A Aldaz and C Lamv. <, J electroanal Chem ‘262, 251’(1989) 22 H Angerstem-Kozlowskl, B E Conway and W B A Sharp, J electroanal Chem 43, 9 (1973)
3 July 1991)
Abatrac-The effects of varymg the lower potential lmut were studied m cycbc voltammetnc expenments conducted on several electrocatalysed oxidation processes the oxidation of fornuc acld/formate, ethanol, propanol and butanol In an aclchc medium, each oxidation process, SubJectedto a cychng potential, exhlbl&zda b&able response over a range of values m the lower potential hnut Wltiun the region of blstablhty, two different cycbc voltammograms, one urltha relatively large ampbtude and one mth a small amphtude, were found to coexist Experunents conducted on the oxtdatlon of butanol revealed that the in size untb respect to increases in the alkaluuty of the solution, a result region of blstablbty dm that ISconsistent ~ntb the descnptlon of blstablbty provrded by the theory of catastrophes The cause of the cycbc voltammetnc blstabtity IS explamed m terms of a nonlinear feedback mecbamsm mvolvmg a reaction between strongly bound carbon monoxide and oxtdes The reamon couples an mduect path of the oxldatlon processes to tbe process by whch an oxide layer IS formed Key words voltammograms, nonhear, hfurcations, catastrophe, coexistence
1. INTRODUCTION The electrocatalysed
oxidation of small orgamc molecules at a platinum electrode under condltrons of constant current IS often accompanied by oscdlatlons m the electrode potentlal[ l-61 Oscillatory hehavlour lmphes that these oxidation processes possess nonhnear feedback mechamsms[7] It nught he expected that vanations of these feedback mechamsms are ative under potentiostatlc conditions However, often only stable steady states, which fit a single valued
curve, are realized under con&tions of constant potential even when oscillations are exhibited under galvanostatlc conditions In some cases, cychc voltammetry reveals a large change on a short time scale&111 This change rmght be interpreted as a nonlinear effect but, such an effect cannot be described as a drastic change of behavlour m the same sense as transltlons to osclllatlons On the other hand, a brstable response lmphes the presence of nonhnear feedback mechamsms Recently, it was demonstrated m a cychc voltammetnc study on the oxldatlon of fornuc acid that, under certain expenmental conQtions, the system relaxes to one of two cyclic voltammograms that repeat[l l] Initial conditions determme which one 1s eventually observed It was also shown that a &an&on from one cychc voltammogram to the other could be. mduced by apphcatlon of perturbations In this paper, we report results from apphcatlons of cychc voltammetry m which the effects of varying the lower potential hnut were exammed These results demonstrate that cychc voltammetnc brstabdlty occurs m three additional oxidation processes, the oxldatlon of three pnmary alcohols ethanol,
propanol and butanol For purposes of companson, the cychc voltammetnc behavlour of the oxidation of forrmc and/formate was re-exammed but m a solution w&h a sulfunc actd concentration that was an order of magmtude greater than that used m Ref [ 1l] In the expenments, the lower potential hnut (Ipl) was employed as a “btiurcation parameter ” Plots of the Ipi agamst peak currents from cychc voltammograms reveal that, m a solution contammg 10m3M H,SO,, each oxidation process exhlhts a hstable response over a substantial range of expenmental conditions In the case of butanol, results are also presented from voltammetnc expenments conducted m alkaline solutions These results show that regrons of hstabihty decrease m size, and eventually Qsappear, ~th respect to increases m alkahmty These changes m behavlour are consistent wth the cusp catastrophe plcture[l2] of hstable systems
2. EXPERIMENTAL A Pme Instrument, Model RDE4, potenbostat was used to control the potential and measure the current Cyclic voltammograms were recorded with an X-Y recorder (Houston Instrument, 2000 recorder) A three-neck, SOOml flask, which was immersed m a bath held at 25 0 f 0.2”C, served as the electrochemical cell The cell contamed a 4QOml solution cons& mg of 1.0 M orgamc solute and lo-’ M H,SO,. The choice of the md concentra~on 1s somewhat artnWary: prehminary cychc voltammetnc expenments inthe existence of blstabbty over a substantial M of condrhons 111both acid and alkahne medmms At large acid concentrations (10 M 673
614
X CAI and M SCHELL
H2S0,) and large base concentrations (1 0 M NaOH), blstahhty was not detected It was decided that the detailed study would be conducted m approximately the middle of the pH range m which a blstable response was observed Cychc voltammetnc expenments were also conducted using solutions conslstmg of 1 0 M butanol and several different concentrations of NaOH Dntilled water, which was also processed through a m&pore delomzatlon unit, was used m all solutions The sodium formate was obtained from Malhnckrodt Inc (Pans, Kentucky, U S A), l-butanol (spectrophotometnc grade) and l-propanol (99 92%, 0 07% water) from Aldnch Chemical Co Inc (Milwaukee, Wlsconsm, U S A ) and the Ethyl Alcohol (Absolute-200 proof) was obtained from AAPER Alcohol and Chemical Co (Shelbyvllle, Kentucky, U S A ) Aad solutions were made with reagent grade sulfunc acid (EM Science, Cherry Hi, NJ) and alkaline solutions were made usmg sodmm hydroxide pellets A saturated calomel electrode, see (Beckman) and a platinum wire served as the reference electrode and counter electrode, respectively All potentials are reported with respect to the reference electrode A rotating disk, 18 0 mm m diameter, 7 5 mm polycrystallme platmum, 10 5 mm Teflon (PAR model No AFD11580, senal No 4619), was employed as the working electrode This disk was attached to a Pme Instrument ASR-type rotator and rotated at 3000 rpm The working electrode was cleaned electrochemltally before each set of expenments m a solution of
0 5 M H2S0, The cleaning procedure consisted of passing the workmg electrode through the followmg steps the potential was held at a value of 2 0 V (see) for 4Os, then at OOV for 5s, -04V for 10s and then finally the potential was held at 0 0 V for 5 s Durmg these steps the electrode was rotated at the rate of 1000 rpm This procedure was usually repeated 30 times Next, mth the rotation rate set at zero, the potential was cycled between -220 and 1200 mV, until a cychc voltammogram corresponding to a clean platinum surface was obtained The electrode was then washed urlth dlst&d water and transferred to one of the solutions under study 3. RESULTS 3 1 Voltammetrrc behavtour tn an acldtc medwn 3 l(a) Fortnrc acrd/formate Results from apphcations of cychc voltammetry to the oxldatlon of fonmc acld/formate are presented first Begmmng Hnth a sufficiently small value for the lpi ( < 120 mV) and cycling the working electrode potential between that value and the value 1000 mV, a large amplitude cyclic voltammogram, with charactenstlcs like those of the cychc voltammogram (CV) shown m Fig la, was obtained The lpi was then increased m small increments Each value of the Ipl was held constant long enough that transient behavlour disappeared Followmg this procedure, the system passed through &fferent members of a family of large amplitude CVs, examples are presented m Fig 1b and c Upon changmg the lpl to a value greater than some cntical
100
550
lOtI0
POTENTIAL 0nV) l3g I Cychc voltammograms measured durmg the oxldatlon of fomuc acld/formate after transient behavlour dlsappeared Current ISplotted agamst potential (see) Sweep rate = 50 mV s-‘, Upper potential hmlt = 1OOOmV(see), solution 10 M sodmm formate, IO-) M H,SO, (a) Lower potential hmlt (@l) = 100 mV Only one repeatmg cychc voltammogram was observed at th15 value of the Ipl (b) /pl = 200 mV (c) lpi = 240 mV (d) fpl = 242 mV Tlus small amphtude CV was the only cychc voltammogram found that repeated at 011svalue of the lpi (e) fpl= 200 mV Thus small amphtude CV coexuts with the one shown 111(b)
BtstabPty m cychc vohannnetry value, the system relaxed to a completely different type of CV An example of this type of CV IS shown m Fig Id Thts latter CV belongs to a separate family, a family of small amphtude CVs The value of the ipl at which the- drasttc change m the response was observed was shghtly greater than the value of the lpl belongmg to the measured cychc voltammogram shown m Fig lc It can be deduced from Fig lc that the transitton from the large amplitude CV to the small amplitude CV occurred for a value of the lpl that 1s close to the value of the potenhal at which the system exhthted a sudden nse m current on the reverse sweep of the large amphtude CVs After the transitton to a small amplitude CV was made, the lpl was increased and then decreased m small decrements The system was allowed to relax at each value of the Ipl Decreasing the Ipl caused the system to pass through other members of the family of small amplitude CVs Eventually, another cnttcal value was passed at which point the system relaxed to a large amplitude CV Between the two cnttcal values, two CVs, one from each family, were found to coexist for each value of the lpi For example, the CVs m Fig lb and e were found under the same expenmental condtttons In Fig 2, peak current values from measured cychc voltammograms are plotted against the lpl The plot reveals the regton of btstabthty m whtch both large and small amplitude CVs were found to coexist 3 l(b) Alcohols Measurements of changes m the voltammetnc response with respect to vanattons m the Ipl were also conducted on the oxtdatton of ethanol, propanol and butanol SubJectmg the oxidation processes to a cycling potential revealed m each case two different families of CVs, (large and small amplitude CVs), and a regon of coextstence The upper potential hmrt used m these three sets of expenments was 1200mV (see) (Butanol did not exhibit a btstable response when an upper potential hmtt of 1000 mV was used ) A set of CVs, measured dunng the oxtdatton of ethanol, 1s presented m Rg 3 Peak currents are approxtmately an order of magnitude less than those found m the oxldatton of formic actd/formate However, though shifted to a potential higher than that of formic acld/formate, the charactenstm sharp nse m current on the reverse part of the sweep IS clearly seen m the large amphtude CVs, see Fig 3a-c The CV m Fig 3a was measured at a value of the lpi that was lower than any value wtthm the regton of coexntence, the CV m Ag 3b was found wtthm the region of coexistence, and the CV m Fig 3c was the last large amplitude CV that was measured m the dnecuon of increasing lpl values After increasing the ipl to a value greater than that used m Fig 3c, the system relaxed to a small amplitude CV Smnlar to the case of fonmc actd/formate, the transitton to the small amplitude CV takes place for a value of the lpi that 1s m the vtcrmty of the value of the potential at which the sudden nse m current occurs on the reverse sweep The small amphtude CV shown m Fig. 3d was measured under the same condmons as those of the large amphtude CV shown m Fig 3b Large amphtude CVs, one measured for a value of the Ipl outside the regton of coextstence and the last
I0
675
I
I 200
LOWER POTENTUL
400
LIMIT (mV)
Frg 2 Peak current from measured cychc voltammograms ISplotted agamst the lpf Condtuons are the same as m Fig I Opencwckscorrespond to measurements mrded whde increasing the lpl in small increments Closed circles correspond to measurements taken whde decreasing the ipl m small decrements To the ngbt of the transition down, measured values were approxunately the same and hence, only closed cxcles are shown Between the two transition pomts the system 1s hstable
large amplitude CV that was measured on Increasing the lpi, for both propanol and butanol, are shown m Ftg 4 The small amphtude CVs look smular to the one m Fig 3d and are not shown The large amphtude CVs agam show charactertsttc sudden rises m the current on the reverse sweep However, m expertments on butanol, a quantttattve difference was Qscovered with regards to the transtton from the large amplitude CV to the small amplitude CV Tlus transitton occurred at a value of the lpl that was slightly greater than the value used m Fig 4d Notice that, unlike the other oxtdatton processes, the transitron occurred at a value of the lower hnut that IS substantially less than the value of the potenttal m the large amplitude CV at whrch the sudden increase m current occurs This quahtattve difference IS attnbuted to the underlymg stab&y properties of the butanol system The regton of hstahhty for butanol IS smaller than the others The small size of the hstable regton implies the system IS close to regtons m parameter space where brstabrhty disappears As the system approaches these parts of parameter space, the potential at which the sharp nse m current occurs durmg the reverse sweep becomes less mdtcattve of the value of the lpi at which the large amplitude CV disappears Plots of peak current us lpl are presented for each process m Ag 5, the plot correspondmg to the oxtdatton of ethanol IS shown m Ftg 5a, propanol m Fig 5b and butanol m Fig 5c These plots reveal the Interval of values for the lpi m which the system exhthts a hstable response. Nottce that the regton of coexistence 1s small m the case of the oxnlatton of butanol Also note that, m the case of butanol, the peak current was not as large after the tram&on from the small to large amplitude CV as it was in the ongmal family of large amplitude 0%. Thts result mdicates that substanttal surface changes can occur whtle movmg the system along the lower branch of states. For instance, place exchange reacttons wtll change the surface.
676
X CAI and M ScHeLL I
0)
00
1200
600
800
400
1200
.-I
600
900
1200
800
406
POTENTIAL (mv)
1200
POTENTIAL (rnv)
Rg 3 Cychc voltammograms measured durmg the ondatlon of ethanol after transient behavlour dlsappeared Upper potential hmlt = 1200 mV, sweep rate = SOmV s-‘, solution 10 M ethanol, low3 M H,SO, (a) lpi = 100 mV ‘l&s value of the lpl corresponds to a value that IS less than any value of the lpi m the repon of coexistence (b) lp/ = 400mV (c) lpf = 680mV (d) Ipl = 400 mV This CV was measured under the same conditions as the CV m (b) The CVs m (a) (b) and (c) are large amplitude CVs and the one m (d) IS a small amphtude CV Note the charactenstlc sudden nse dunng the reverse sweep m the large amplitude CV Increasing the fpl to a value slightly greater than that m (c) causes a transition to the small amplitude CV The value of the lpi m (c) 1s m the vlcuuty of the value of the potenhal at which the sudden nse m current occurs (for all three large amplitude CVs) dunng the reverse sweep
’ (b) 1 0
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200
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POTENTIAL (mv) POTENTIAL @IV) Fig 4 (a) Cychc voltammogram measured durmg the oxldatlon of propanol Solution 1 0 M propanol, 1O-3M H2S0, lpl= 200 mV Other conditions the same as m Fig (3) This was the only repeating cychc voltammogram found for tis value of the lpl (b) Condltlons the same as (a) except IpI= 620 mV (c) Cyclic voltammogram measured durmg the oxulation of butanol Solution 10 M butanol, lo-’ M H,SO, lpi = 0 0 mV Other conditions the same as (a) No other repeating cychc voltammogram was found for this value of the lpf (d) Conditions the same as (c) except lpi = 400 mV
Bzstabzhtyzn cycltc voltammetry 3 2 Voltammetm medra
? 2.0
behavrour of butanol m alkalane
s 9
Cychc voltammetnc
behavlour of butanol m allcahne solutions was also stud& A set of CVs 1s &splayed m Ftg 6 Each cychc voltammogram was recorded after transient behavzour disappeared m solutions contammg lo-)M NaOH and usmg an upper potential hrmt (upl) of 800mV Though the large amplitude CVs, Fig 6a, b and c, are different from those shown m Fig 4c and d, zt 1s apparent that at least some components of the mechamsm causing hstabzhty m aczd solution are active m alkaline solutzons, the charactenstzc sharp increase m current 1s still present m the reverse sweep A plot of peak currest us lpl, shown m Fig 7a, reveals the regzon of coexzstence We also note that a hstable response was not obtained m alkaline solutions using a up1 of 12OOmV The results of several expenments indicate the followmg trend if a gzven pH and a fixed up1 the oxzdatlon of butanol exhibits a voltammetnc regon of coexistence ~nth respect to changes m the 1~1,then this repon of coexistence shrmks and eventually vamshes wzth respect to increases m the alkahmty of the solution It was found that once the regzon of coexistence vanished, a new one could be obtained by changmg the up1 to a lower value On increasing the alkahmty wzth thus new fixed value of the upl, the regzon of coexistence eventually vanishes agam, and so on (For [NaOH] > 10-l M, a bzstable response was not found ) For a value of the concentration of NaOH shghtly greater than the value at which a regzon of coexzstence vanished, a plot of the peak current us the lpi produces a single value relatzonshzp Such a plot 1s shown m Fig 7b Now instead of a drastic change ze a transition from a large amplitude CV to a small amplitude CV, one CV smoothly deforms mto the other as one slowly decreases or increases the lpi The constructzon of the same plot under the same conditions except wzth a small decrease m the sodium hydroxzde concentration reveals a regzon of coexls-
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LOWER POTENTIAL LIMIT (I Fzg 5 Peak current from measured cychc voltammograms is plotted against the lower potenhal lmut ConQtions are the same as m Fzgs 3 and 4 (a) Oxidation of ethanol (b) oxldatlon of propanol (c) Oxniat~on of butanol Approxlmate values of the lpiat wlch the large amplitude CV ather became unstable or disappeared are (a) 690, (b) 630 and (c) 402 mV
tence %s result 1s consistent wzth the cusp-catastrophe picture of bzstable systems]121 Gven that a system possesses a bzstable response over an interval of values of a specdied expenmental constraint,
fg+yE/ 200 400
100
800
~(fqyJO0~~~
400 POTENTIAL
(mv)
-400
200
800
POTENTIAL (mV)
Rg 6 Cyclic voltammograms measured durmg the oxulatmn of butanol Solution 10 M butanol, 10e3 M
NaOH
Upper potential hnut = &l@mV, (a) lpf = -500 mV, (b) fpl = -300 mV, (c) I@=-2OOmV,(d)lpl= -300mV
X
618
CAI
and M
SCHELL
catastrophe theory predtcts that tt 1s possible to choose a second constraint that can be varied m such a way that the mterval m which bistabihty occurs decreases and then disappears Finally, we wish to pomt out that m addition to a btstable response, more complicated behavtour was observed durmg cychc voltammetnc expenments conducted on the oxtdatton of ethanol m alkaline soluttons Cychc voltammograms with penods greater than the period of the potential, as well as apenodtc voltammograms, were obtained Such behavtours are beyond the scope of the study considered here and are reported elsewhere[l3] 4. RELATIONSHIP WITH ELECTROCHEMICAL MECHANISMS In order to provide a possible explanation for the occurrence of cychc voltammetnc bistabihty, tt is necessary to review several features of the mechanisms for the oxidation processes as well as the process by which an “oxtde layer” 1s formed We begn wtth the description of the large amplitude CVs corresponding to the oxtdatton of forrnate/fornnc actd[l] The mtttal nse m current that occurs m the forward duectton of the CVs shown m Fig la-c 1s attnbuted to the direct oxtdatton of the fuel A postulated mechanism for the oxidation of formtc acid 1s[S]
(There haviour aad[l4]) directly strongly
HCOOH + HCOOH(a)
(1)
HCOOH(a) + COOH + I-J,
(2)
COOH+CO,+H++ee-,
(3)
H-H++e-
(4)
exists expenmental evidence that the beof formate is similar to that of fornnc In addition to reaction paths that lead to CO,, other paths involve the formation of bonded carbon monoxtde, eg[8], HCOOH(a) + CO + Hz0
(5)
Other mechanisms for the direct oxtdatton of the fuel and formatton of both bridge and linear bonded carbon monoxide are listed m the hterature[ll, 14,151 At larger potentials, but preceding the locations of maxima m Ftg la-c, the oxtdatton of carbon monoxide begins to stgmficantly contnbute to the current QH+CO-,H++C02+e-
(6)
Thts reaction (6), is preceded by the chemrsorption of hydroxyl radicals, H,O+QH+H+
+e-
(7)
There exists at least two possible alternative descnptions for the oxtdatton of carbon monoxtde instead of hydroxyl radicals, reactton of carbon monoxtde with a closely related species 1s plausible, eg[9], HzO+~O+2H++C0,+2e-
(8)
Another alternative to equation (6) is the reaction[l6] QH + CO + GOOH,
(9)
I
I
0
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LOWER
POTENTIAL
LIMIT (mV)
Fig 7 Peak current from cychc voltammograms measured durmg the oxldatlon of butanol IS plotted against the lower potential hmlt (a) Condltlons are the same as those m Fig 6, (b) as for (a), except [NaOH] = 10-l M and upper potential limit = 500 mV
which would then be followed by the reaction wntten m equation (3) Wtthm the potential range m which CO is oxtdtzed, the reaction m equation (3) 1s expected to be fast Consequently, it 1s dtfficult to Qfferenttate between the two reactions written m equations (6) and (9) After passing through the current maximum m the forward sweep (see Fig la), the ensuing decrease m current 1s usually attnbuted to the continued formation of oxtdes[l, 17,181 On the reverse sweep, the reduction of oxides and oxidation reactions combme to produce a small anodlc current until a value of the potential 1s reached at which point most of the oxides are removed In the vtcmtty of thts value of the potential, the current, due to the resumptton of direct oxidation of the fuel at bare sites, suddenly increases to a large value The underlying chemistry that 1sresponstble for the large amplitude CVs measured durmg the oxtdation of the alcohols 1s substanttally different from the chemistry that takes place m the oxidation of formic aad For alcohols, the appearance of appreciable anodic currents during the forward sweeps takes place at larger values of the potenttals (compare Fig 1 wrth Figs 3 and 4) Tlus nse m current occurs simultaneously with production of CO* [ 10, 191 From the results presented m Ref [ 191,we conclude that, m the case of ethanol solutions[20] a substantial amount of the CO2 produced 1s from oxtdation of carbon monoxide (The study m Ref [ 191was conducted using concentrations different from these used here The apphcahhty of those results to these presented here 1s an assumption ) Surface bound carbon monoxide
Bstabhty IU cycb voltammetry
can form from dtssocmuve chemtsorptton of ethanol[9] Reactton of acetaldehyde and hydrated acetaldehyde, which, at least to some extent, are expected to be intermediates of some of the reactton pathways, also leads to surface bound carbon monoxtde[lO] In addition to carbon dtoxide, acettc acid IS an end product m the omdation of ethanol The first peak m the ethanol CV was determmed to be caused by a decrease m productton of CO,, and the second mcrease 1s attributed to an mcrease m the productton of aceuc actd (a stgmficant amount of ace& actd IS also produced at potentials under the first peak) [ 191 Although Qfferences extst among the oxtdatton of the alcohols and stgmficant Qfferences exist between the oxtdatton of the alcohols and the oxidation of fornuc actd, substanual evidence supports the idea that each oxldauon process possesses at least one reactton pathway that mvolves the formation of surface carbon monoxtde as an mtermedlate[9, 10, 15, 19-211 A reasonable hypothesis is that the oxtdauon of carbon monoxtde mvolves the same chemistry, eg equation (6), m each of the processes In all oxidation processes, the small amplitude CVs exhibit a mostly anodtc but relauvely small current Thus the electrode surface most hkely retains an oxide layer throughout the cychng process It is therefore necessary to revtew some classtcal results on the formatton of oxide layers It was revealed m the study by Angerstem-Kozlowskt et a1[22] that the process m which an oxide layer 1s formed m actd solutton (without other solutes present) takes place in several stages Many of the stages of oxide-layer formation, as well as reduction of the layer, occur at potentials well below that at which oxygen evolution takes place The first three stages of oxide formatton correspond to chemtsorptton of hydroxyl radicals At higher potentials PtO forms and the rearrangement of part of the oxtde layer also occurs, which mvolves place exchange reacttons Reduction of oxide species is also complicated For example, reductton of PtO can require a lower reduction potenual then PtOH An important result wtth regards to the cause of hstabthty 1s that the further the system advances along the stages of oxtde formauon the lower the potenttal necessary to reduce the oxtdes Even without constdenng the coupling through chemical reactions, the presence of orgamcs has a constderable effect on oxide formatron For example, the three stages of chemtsorptton of hydroxyl radicals correspond to successive formattons of three dtfferent sublattice arrangements[22] However, if the surface ISalready parttally occupied by, say, CO, the mechantsm for chemtsorption of hydroxyl radicals wtll change On companng cychc voltammograms and chargtng curves, tt was deduced some time ago that, m the presence of orgamcs, oxtdes form on the surface of the electrodes at rather low potential values, substanttally lower than the value at whtch hydroxyl radtcals are chemlsorbed m acid solutions wtthout other solutes present Nevertheless, when oxides dominate the surface of the electrode, the stages of oxide formatton that occur m actd-only solutton are expected to kick m An important coupling wtth regards to the occurrence of btstabihty 1s hypothesized to be the chemical reaction mvolvmg oxides and carbon monoxtde, eg
679
equatton (6) Evtdence suggests that thus reactton IS explosive In spectroscoptc studtes it was observed that the surface concentratton of carbon monoxtde rapidly decreased after potenttal values were achteved at which carbon monoxide 18 oxtdtzed@,9]. Furthermore, the vahres of peaks m the waveforms of potential osctllattons measured under condtttons of constant current are conststent wtth the occurrence of a reactton mvolvmg carbon monoxtde and oxides[6] These peaks are followed by a sudden drop m the potenttal which 1s consistent wtth an explonve reaction A proposed rate law descrtbmg an explosive reaction, whtch was employed to both descnbe and simulate osctllatory behavtour[2,5,6] 1s wrttten as rate = L(OH)gO)S,
(10)
where round brackets denote surface concentratton, S represents the surface concentratton of vacant sttes and k IS a potenual dependent rate coeffictent. The above descnpuons can be combined to provide an explanatton of btstahhty If uuttal condtttons are such that oxtdes dominate the surface of the electrode under condtttons of btstabthty, then the reactions whtch occur in the process by whtch an oxtde layer 1s formed wtll be the predommant reactions durmg the forward sweep In this case, the system relaxes to a dynamical state m which oxtde-layer formatton develops to an advanced stage dunng the forward sweep Consequently, oxtdes cannot be completely removed on the reverse sweep However, tt follows from practtcal reasons that, for a fixed value of the upper potenual hrmt, a cnucal value of the fpl extsts such that, for values of the lpi less than that cnttcal value, the system wtll relax to a state m which all oxides can be removed on the reverse sweep regardless of uuhal condmons On the other hand, tf at the begmnmg of the sweeping process, condmons are such that enough surface CO is formed, then the reacuon between carbon monoxtde and oxldes[equatton (6)] will dommate much of the forward sweep Since this reacuon removes oxides, development of an oxide layer is delayed Oxides that do form are removed dunng the reverse sweep A strong nonhneanty m the rate of the reaction that mvolves strongly bonded CO and oxtdes as reactants, such as that contained m equatton (lo), IS cructal A rate law for the reaction in equation (6) that is proporttonal to the concentrations of carbon monoxide and hydroxyl radicals 1s weakly nonhnear m the sense that the rate would decrease as the amount of CO decreases Since the amount of water 1s much greater than the amount of CO, the CO would be rapidly replaced by hydroxyl radtcals through the reaction written m equauon (7) Hence, the system would relax to a state correspondmg to a small amplitude CV m whtch reactions that occur m oxtde layer formatton dominate the forward sweep However, tf the rate of reactton wntten m equation (6) IS proportional to the denwty of vacant sites, then the reaction IS autocatalytic, ae the reaction produces at least two vacant sites for every molecule of CO, produced Such a reaction can remove QH faster than tt IS chemtsorbed through reacuon (8), even as the concentrauon of CO decreases Consequently, the reacuon can prevent the system from relaxing to a small amphtude CV
X CM and M
680
Among the pnmary alcohols, the values of the fpl at which the system was observed to make a transition from a large amplitude CV to a small amphtude CV follow a trend consistent with the above explanation These values, measured under the same conditions (see Fig 5 ) are 690mV for ethanol, 630mV for propanol and 402 mV for butanol It follows from the above explanation of hstablhty that the more carbon monoxide adsorbed durmg the cycling process, the larger the value of the lpl at which transition to the small-amplitude CV occurs Thus, it is concluded that the amount of CO adsorbed decreases with respect to increases m the length of the carbon chain of the pnmary alcohols This conclusion, that there 1s a decrease m the amount of CO adsorbed with respect to an increase m the length of the carbon chain, 1s precisely what was determined m a spectroscopic study on the oxtdatlon of the same pnmary alcohols reported m Ref [lo]
SCHELL
the value of the lpf at whch the large amplitude CV ceased to exist increased with respect to decreases m the number of carbon atoms Tins result implies that the amount of carbon monoxide adsorbed also increases v&h respect to decreases m the length of the carbon chain of the pnmary alcohols, which 1s consistent Hnth the results of the spectroscopic study reported m Ref [lo] alcohols,
Acknowledgement-This
REFERENCES
5. SUMMARY Blstablhty was found m four different electrocatalysed oxldatlon processes subjected to cychc voltammetry, the oxldatlon of forrmc acld/formate, ethanol, propanol, and the oxldatlon of butanol These results demonstrate that cyclic voltammetnc blstable responses can be common m those electrochemical processes m which strongly bound intermediates form We emphasize that cychc voltammetnc blstablhty 1s not the same as blstablhty mvolvmg the coexistence of stationary states Rather, It IS bistablhty mvolvmg the coexistence of limit cycles[ 111 The cause of the cychc voltammetnc blstablhty was attnbuted to the competltlon between oxide layer formation and the oxldatlon of the organic molecules An important role IS played by a nonlinear reaction between the strongly bound intermediate, carbon monoxide and either hydroxyl radicals or closely related species The reaction mhlblts the process by which an oxide layer 1sformed Consequently, if lmtlal condltlons are such that a sufficient amount of the strongly adsorbed intermediate forms on the surface of the electrode, then the oxides that do form can be reduced on the reverse sweep This removal of oxides allows the oxldatlon of orgamcs to take place at vacant sites and m this case, a large amplitude CV IS obtained as hmltmg behavlour On the other hand, if mltlal condltlons are such that the surface of the electrode 1s covered by a sufficient amount of oxide, the system relaxes to a state m which oxides cannot be completely removed, this state corresponds to a small amplitude CV Expenmental results revealed the length of the interval of values m the lower potential limit within which the system was hstable Among the pnmary
research was supported by the
Robert A Welch Foundation, Grant No N-1096
5 6 I 8 9
10 11 12 13 14
15
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