Heterogeneous redox catalysis with lead dioxide anodes

355

J Elecrroanal Chem, 182 (1985) 355-366 Elsevler

Sequoia

S.A .

Lausanne - Pnnied m The Netherlands

HETEROGENEOUS

FRITZ

BECK

REDOX

and WOLFGANG

CATALYSIS

WITH

LEAD

DIOXKDE

ANODES

GABRIEL

Vnruersrry-GH- Dursburg, FB6- Eiehfrochemre, D 4100 Duuburg 1, Lorharstr 63 (F RG) (Recened

8th June

1984.

m revlsed form 15th August

Pb02 anodes were -used to oxlcbze cyclohevanone HCIO, m a two electron reaction

1984)

oxlme

to mtroso

compounds

m 1 AI HISOJ

or 1 M

NO NOH

_I- Nu-

-

+

H+

i-

2

e-

NU

Nu- are nucleoptiles hke OH-, NO? and ClThe electrode malenal reacts chenucally v.101 the o~lme and it IS oxldlzed electrochenucally at potentials very close 10 the Pb”/Jf redox potential The chenucal step has been stu&ed separately by corrosion expenments III aqueous HCIO, An upper ht of clurrent density IS observed, mdependent of convection, due 10 the formation of a film composed of Pb(II) products on the electrode A rcacuon hxmlation was not attamed under our condluons Contrary to thus. the PbOz electrode behaves mertly m the case of lower ahphauc alcohols (C2-C,) UI 1 M H2S0, Potenuak are about 0 5 V more posluve The mert anode generates the radical catlon m the rate determuung srep R-OH

+ (R-OH)’

+ + e-

Corronon expenmenls &close a rate, wfuch IS lower by four orders of magmtude (lcorr = 2-3 pA cm-‘) wvlth reference to the oxune expenment and only slrghtly above the corrosion rate m pure 1 M HCIO,

(I) INTRODUCTION

Lead loxldz is known to be a strong chemical oxidant, especially in acid soluttons. In the postttve plate of the lead actd accumulator, beneftt IS derived from the very posttive redox potential of the PbOJPbSO, couple. On the other hand, the lead dioxide anode as an oxygen electrode or as a synthesis anode ts commonly regarded as an “mert” electrode matenal. Thts contradicts more or less the overall chemical expenence, and the question arises as to whether at least in some cases, tlus vrew should be reconsidered m terms of heterogeneous redox catalyysis [I], where m a chemical step (1) at the surface of the electrode, the substrate S IS oxldrzed to intermediate, Z, and in an electrochenucal OOZZ-0728/85/SO3

20

0 1985 Elsewer

Sequoia

.%A

356

follow-up

reaction

(2), the btvalent

S,,+Pb$+Z+PbO+P

lead is reoxrdlzed

to PbO,. (1)

?__ ___--__, PbO+HzO+Pb6,+2H++2e-

(2)

It is the atm of thts paper to contrtbute to thrs question, whtch has not yet been dtscussed wrdely m spite of the fact that PbO, IS a very rmportant electrode mater& Lead dtoxrde 1s a complicated electrode. The electrochemtcal behaviour IS not only Influenced by the usual parameters, but nonstoichtometry, porosity, texture and nonconductmg reduction products ltke PbO, Pb(OH)2 or PbSO, may play undefined roles Thts is one of the reasons why this electrode material 1s much less subject to fundamental mvestigations as 1s the case for other electrodes.

(II) EXPERIhlENTAL

Acid electrolytes were used throughout. mostly 1 M H,SO, or 1 M HCIO,, made from analyttcal grade 98% H,SO, or 70% HCIO, and btdistrlled water. Lead dloxlde electrodes were apphed 111three verstons. (I) A cyhndncal Pt electrode with 0.1 cm diameter and 1 8 cm length was plated with a 100 pm PbO, layer by pohn-tzing WI~~J = 23 mA cm-’ (I= 13 mA) for 1 h at 65°C tn a magnetically stirred bath of 200 g dmm3 Pb(NO,), and 5 g dme3 concentrated HNO,, (Type I) (n) A rotattng Pt dtsc (A = 0.2 cm’) was coated with PbOz m an analogous manner, (Type II) (m) As well as these Pt/PbO, compostte electrodes, a classic “lead anode” was fabricated by snodlzmg a Pb sheet (n = 0 1 cm), 99.95% punty for 1 h tn 1 M H2S04 at 20 mr~4cm-‘, (Type III) Counter electrodes were made of Pt or graphrte filled polypropylene. The reference electrode was Hg/Hg,SO, m 1 M H2SOa, which 1s 674 mV postttve vs. SHE Potenttals measured vs thrs electrode are denoted as U,. The temperature was 25 OC. The soluhons were deaerated pnor to the begmnmg of measurement, they were shghtly strrred magnetically. Organic starttng mater& were cyclohexanone oxrme, 95 % (5 % H,O) from BASF and ahphattc alcohols “chenucally pure” from Merck. Voltammetnc measurements were performed with a combmatron of a Wenking potenttostat Bank (LT Zj, an Hewlett Packard XY-recorder (HP Moseley 7030 AM) and a function generator Bank (VSG 72) The rotating disc electrode equtpment was an Analytical Ro:ator ASRP from Pme Instruments (Grove City, PA), m combmatton wtth a Metrohm platmum drsc electrode EA 289/2 Corroston potenttals wei e recorded wtth a Lmseis xt-recorder.

3.57

(111) RESULTS

(III

(IlI1

I)

Anodrc oxrdatron of cyciohexanone o_xrnre I) Current voltage curves

Figure 1 shows stationary current voltage curves for 0 1 M cyclohexanone oxlme In 1 M H,SO, and 1 M HCIO,, respectively. Both curves are obtamed at a rotating Pt/PbO?-disc electrode (Type II) and are relatively steep through the rest potential; cathodic reduction of PbO, therefore starts unmedlately at potentials negative to tius pomt The hysteresis for the H,SO, curve IS much more pronounced than for the HClO, curves Basic tunes (wIthout oxlme) are reproduced as well for both electrolytes Cychc current voltage curves for the Pb/PbSO, electrode (Type III) are gven m Fig 2 for 1 M H,SO, (a) and for the same electrolyte with mcreasmg concentrations of cyclohexanone oxlme (b, c, d) Some addmonal formation of the electrode has been accomplished m the course of the uutlal cycles

I

’I

I I I I I

I I I I

I

Fig. 1 Anolc

current

II

voltage curves at a rotatmg

Pt/PbO,

(100 pm)-dsc

electrode

(Type II) at 25 “C

Start at the rest potenhal Electrolyte compos:tton Rotatlon speed 400 rpm Voltage rate 1 mV s-’ ) 1 M H,SO., wth (1) 0 M cyclobewnone o-e. (7) 0 1 M cyclohexanone oxlme, (- - -) 1 M (HCIO, wth (1) 0 M cyclohexanone o-e, (2) 0 1 M cyclohexanone oxune

w 5

0.6 -5

.O

-10

t

FIN 2 Cychc current voltage curves at a Pb/Pb& electrode (T_ype 111) wth 10 mV s-’ scan rate at 25 “C 7th cycles are shown The composlbons of the magnetIcally stn-red electrolytes were (a) 1 M H2SOJ, (b) 1 AI HzSO, +0 03 M cyclohexanone oxlme, (c) 1 M H2S0, +O 1 Al cyclohexanone oxme, (d) 1 M H,SO_, + 0 3 M cyclohewnone owe (I/I

I -3) Rotatrng

dsc expervnents

The lmumg cut-rent denslty measured at an anodlcally actwated Pt/Pb02-rotatmg disc electrode (Type Ii) rn 1 M H,SO1/O.l M cyclohexanone oxme accordmg to Fig 1 was evaluated at U, = 0 95 V as a function of rotation speed n of the electrode over a wide range The results are shown as a,,&&plot m Fig 3. Two sections can be recogmzed, namely a part hnearly dependent on 6, and a part Independent

15

jllm /rnA-cm

-*

1

25

Fig 3 Lmutmg current denslues at a rotaung Pt/PbO, dsc electrode (Type II) III 1 M H2S04/0 1 M cyclohexanone oxlme at various temperatures as a funcuon of rOfaf.IOnSpeeds. Us = 0 95 V (0) 0 "C, (8) 25 “C. (e) 50 “C 0 (- - -) For comparison or; 0 1 AI K,[FeJCN)6J/l M H,SO, at 25 “C fg

4 The same as

Fig 3. but wth 1 M HCIO,/O

1 M cyclohexanone

as electrolyte

at 25 “C

359

of it. Measurements were made at three temperatures and with 0 1 M K,[Fe(CN),] for comparison at 25 “C. The same sequence of measurements was performed in 1 M HClOJO.1 M cyclohexanone oxrme The results are represented m Frg 4, showing the same charactenstics as m Fig 3, but wrth an appreciably lower hmrtatron of maxtmum lirmtmg current density. (I/1.1.3)

Corrosion

cfthe

PbO,

la)ler

The cyhndncal Pt/PbOz electrode (Type I) wr*th a 100 pm PbO, layer was exposed to aqueous sol&tons of HClO, and cyclohexanone oxune of various composrttons The perchlonc acrd electrolyte was chosen due to the htgh solubility of the correspondmg lead salt The electrode was located at the periphery of a cyhndncal cell with a constantly steed electrolyte m order to have defined convection at the electrode surface. The temperature was held constant at 25 OC. The rest potential of the electrode was contmuously recorded. After a distinct penod of constant positive potentral, T, the potentral dropped down sharply to values about 0.8 V more negatrve, cf. Fig. 5. At thts point, the PbO, layer had dsappeared totally from the platmun~ electrode The reciprocal transition tune, l/7, denved from these potennal/ttme curves, ts used as a measure for the corroston rate m the followmg two plots. Figure 6 shows, that l/r mcreases pnmanly wtth mcreasmg concentratron of the oxime, but then attams a maxmmm value, correspondmg to the rotating disc electrode expenments of Fig. 4. Indeed, the maximum 0.8 h-’ corresponds to T = 1.33 h, which 1s J = 17 mA cm-’ m terms of cd, companng well wrth j,, = 12 mA cm-” in Fig 4. Thrs compares also to the cds for PbO, layer formatton, cf. sectron II.

03 ‘+,, 12 c

-7 3

4 r,

5

>

w-

4

8

12

16

20

Fig 5 Potenud ume curves 111 the course of corrosion of a cyhndncal 100 pm PbOl layer on a Pt electrode (Type I) m stu~ed 1 M HCIU, with vanol?s amounts of cyclohexanone oxlme ai 25 “C (1) 0 2 M,(2)01M.(3)005M,(4)002M.(51001M

360

O.75 15

•

o

•

0.5"

1 °0"

0,25-

O5

OHClO/+/tool I"I

COx1rn//rno~ 1-1 ]

I

1

i

I

0.5

10

1,5

Z0

25

,

,

0

FI 8 6 Reciprocal c o r r o s i o n u m e v e r s u s c o n c e n t r a t i o n o f c y c l o h e x a n o n e o r a m e for e l e c t r o d e t y p e I, a c ) h n d n c a l P b O 2 layer (100 # m ) , m stirred s o l u t i o n s at 25 ° C , ( O ) 1 M HCIO4, (®) 2 M HCIO,: Fig 7 Rectprocal c o r r o s i o n t i m e v e r s u s c o n c e n t r a t i o n o f p e r e h l o n e acid t y p e I a c y l m d n c a l P b 0 2 layer ( 1 0 0 / ~ m ) m surreal s o l u u o n s at 25 ° C

(Coxjm ¢ =

0 5 M ) for e l e c t r o d e

If the molar concentration of the oxtme exceeds that of the rmneral acid, the rate o f c o r r o s i o n a g m n d e c r e a s e s T h a s e f f e c t Is c o n f t r r n e d w i t h a n e x p e r i m e n t w i t h v a r i a b l e H C I O 4 c o n c e n t r a t , o n a c c o r d i n g t o F i g 7, s h o w a n g c l e a r l y a l o w c o r r o s i o n r a t e ff a c i d c o n c e n t r a t i o n ts l o w a n d a s t e e p r i s e t o h a g h e r r a t e s o f c o r r o s i o n i f t h e acid concentrataon exceeds the oxtme concentration.

( I I I 2) Anodtc oxidation of lower ahphattc alcohols Lower ahphauc alcohols, C2-C4, have been oxadlzed at a Pt/PbO2 (100 #rn)lotatmg disc electrode, n = 400 rpm (const). The electrolyte throughout was 1 M H a S O 4, t h e a l c o h o l c o n c e n t r a t i o n s w e r e v a n e d m t h e r a n g e 0 . 0 1 - 5 M . S t a t i o n a r y c u r r e n t v o l t a g e c u r v e s w e r e m e a s u r e d a t 25 ° C w~th a s c a n r a t e o f 1 m V s - l F i g u r e 8 s h o w s a r e p l o t o f t h e s e d a t a a s T a f e l f i n e s f o r n - p r o p a n o l in t h e u p p e r r a n g e o f a l c o h o l c o n c e n t r a t a o n s . T h e c u r v e s a r e s h i f t e d b y a b o u t 0.5 V m t h e pos~tave d L r e c t l o n m c o m p a r i s o n t o t h e o x a m e c u r v e s , cf, F i g 1. A d o u b l e l o g a r i t h m a e p l o t o f c u r r e n t d e n s m e s a t Us = 1.3 V v s a l c o h o l c o n c e n t r a tions reveals quasi-zero order behavaour at low concentrations and quasl-ftrst order a t hagh a l c o h o l c o n c e n t r a t i o n s , cf. F i g 9.

361

Fig 8 Tafel ~10:s for the anod~c oxldauon of n-propanol III 1 M H,SO, at a rolatmg Pt/Pb02-disc eklrode (Tvpe II), 400 rpm 25 OC, 1 mV 5-l (x) 0 M. (0) 0 1 M, (3 fi 5 M. (A) 1 M. (0) 2 5 M. ts) 5 M n-propanol

Fig 9 Double loganttumc plots for the evaluation of the alcohol reactIon order m the anodvz oxldatlon of alcohols m 1 M H,SO, at a Pt/PbO, (100 pm) rotatmg dsc electrode at 25 OC (0) EtOH (4) II-PI-OH, (A) I-PrOH

(IV) DISCUSSION

(I7

1) Anodjc oxldatron of cyclohexanone oxme

The anodlc oxidation of oxunes has not yet been studed extenwely. Only half a page 1s devoted to this topic in a well known compendmm [2] Schrmdt was the fmt to ox.Aze a ketone, acetone oxme, m 2 % H,SO, at a platmum anode [3]. He “propylpseudonitrole” = 2-nitroso-2-mtro propane as a - charactenstic obtained

362

product Chenucal oxrdatron leads to analogous mttoso compounds. Chlonnatton of cyclohevane oxune yielded I-rutrojo-1-chloro-cyclohexane [4] Our preparatrve work, reported elsewhere [5], confirmed the formatron of a carbomum mtermedtate m an ECE-reactron, wluch must also be operative in the examples mentioned above.

o=

N-OH

-

-k

H+

+

2e-

Thts mtermedrate reacts wrth nucleophrles hke NO; and ClIt is predommantly Hz0 that acts m aqueous electrolytes as a nucleophrle, leadmg to cyclohexanone and HNO. The anodrc oxtdatton at a PbO, electrode occurs at a potential quite close to the redox potential of PbOJPbO, cf Figs 1 and 2 This IS hrghIy mdrcative of a mecharnsm accordtng to a redox catalysrs, eqns. (1) and (2). PbO, has already been used as a chermcal oxrdant for oxtme oxrdation [6] In 1 M H,SO, as an electrolyte, the electrode wrll passivate m the course of the chenucal step (1) due to the formatton of msoluble PbSO, It JS only m the course of pnmary anodic polanzatron m the posttrve duectJon, that this passrvating layer is electrochemicaily removed and the chenucal step can proceed smoothly. After the acttvatton tn the forward scan drrection, the reaction occurs wrth lower overvoltage m the reverse scan drrectron, cf. Frg 1. The dtstance to the basic curve (oxygen evolutron) IS now about 0 7 V. A drffusron hmitatron IS observed finally. Lurnting current densttres are proportional prunanly to k’;;, where n IS the rotation speed (Frg 3). Thus, the Levrch equation IS valtd From the slope, the drffuslon coefficknt of the oxune molecule 1s found to be D = 5.1 x 10e5 cm’ s-l (25 “C). A control expenment wth the same molar concentratton of K,[Fe(CN),] leads to a hne with only 50 % of the slope m the presence of the oxtme. From tlus it follows that two electrons are mvolved m the overall process according to eqn. (3) However, at higher rotation speeds,/,, becomes mdependent of n, cf Frg. 3. At ftrst glance, thrs should be understandable as a kmetrc Iinntatton due to the chen-ucal step eqn (l), which JS gtven by [1,7] J max= zFk,Kc,

[ A]

(4)

where k, IS the chemcal rate constant, K IS the adsorptron constant, [S],, = Kc,, cs IS the substrate concentratton and [A] 1s the maxtmum concentratron of oxidized surface states attamed at positive potentials. J mu values are, Indeed, lower by about one order of magmtude m comparison drffusion hnuted cds at the relatively high oxlme concentrations m thrs region, cf. Figs 6 and 7. However, rf an actrvatton energy 1s calculated from the temperature dependence of J,,.,~, rather a low value of I7 kJ mol-’ 1s found. TIus indrcates another drffuston process as the hmiting factor. We assume therefore the formatton of a sohd layer of reduction product at the surface of PbO, at higher rates. The

363

substrate molecule D, is the drffusron

must chffuse through thrs layer, uluch may be porous PbSO,. coefftctent in tlus film and 6, its thtckness, the current density

If IS

grven by [8,9] J’ll-ll= zFD,/(

6, + SD,/D)

When 6, z+ SD,/D, attamed. Jm;u = z FD F/S ,z

a maxrmum

(5) current

density

Independent

of rotation

speed

is

(6)

Its temperature dependency reflects the temperature coeffrctent of Dr. Lunitmg current densrty 1s much lower m the case of 1 M HClO, as an electrolyte. cf. Frgs 1 and 4. Nearly no hysteresis is observed in the current voltage curve We interpret thts m the same way as above However, the composttron of the frIm has changed, it 1s presumably more dense and does not vary much with polansatton As already mentioned m section (III 1.1) both curves go relauvely steeply through the rest potentral, underlmmg the reverstble character of the redox couple mvolved The Pb/PbO, electrode, also useful as a syntheses anode, has been characterized by slow cychc voltammetry, cf. Frg 2 In the absence of oxtme, a PbO, layer is generated durmg the course of the whole anodrc process, not only at the anodrc peak, but also m the regton posrhve to it, where oxygen begms to be produced tn paraIle1 In the reverse scan, a reductron peak 1s observed Strmlar curves have been reported tn the battery hterature [lo-131 In the presence of cyclohexanone oxtme, the anodrc part IS apprecrably amphfred, at 0 3 M oxune the ratro of charges Q+/Q_ goes from about 1 m the absence of oxnne to about 100 Tlus current amphftcatton in the regron of redox transttion IS highly mdicatrve of redox catalysts [1,7,14,15] Once agam, an actrvatton of the electrode 1s clearly exhrbtted. The anodrc hump, which is due to PbSO, oxrdatron, must be traced 111order to estabhsh a clean PbOz surface. Our corroston experrments m perchlonc actd further support the concept of redox catalysrs. They show clearly, that the chemical step may proceed contmuously, tf PbO, 1s avarIable and reduction products do not passrvate the electrode Accordmg to Frg 6. l/7 as a measure of corroston rate mcreases wrth mcreasmg o.xrme concentration Thrs mdicates drffuston lmuted oxrdatron of oxtme molecules. However, a ma_~um value of l/7 = 0 8 h-’ is attamed at 0.15 M oxune. The transltron trmes T can be expressed as equivalent current densttres J_,~, obeymg the cylmdnc symmetry of the electrode, wtth the followmg formula [16]Jcorr=

[/~-@+

(7)

where r IS the radius of the Pt base electrode wrth length h, A4 and p are the molecular wetght and density of PbO,, Q rs the charge eqtuvalent of PbO, and z = 2. l/7 = 0.8 h-r corresponds then wrth~ EOll-._X= 17 mA cmm2, wluch IS very close to = 12 mA cm-* measured at the rotating chsc electrode, cf. Ftg. 4 The explanaJ max

364

tion for thrs hnutatton m terms of a film of Pb(OI-I), as a dtffusron barner at the electrode surface IS given above. The corrosion potential IS attained by superposltlon of the anodx process eqn (3) and the cathodic reduction of PbO, [17], wluch can be wntten m two parts: PbOz + 2 H+ Pb(OH)?

Pb(OH)2

+ 2 H+ -

(8a)

Pb*+ + 2 H,O

t8b)

The cathodtc curve m HClO, has been pubhshed by Harnpson et al [18]. From these mdrvtdual curves, nnxed potent& follow wluch are very close to the corrosion potenttals measured, cf Ftg 5 If o,xnne concentratton exceeds that of the acid, the corroston rate decreases. The oxune 1s part&y protonated, and the hydrolyses product, hydroxylamine, which 1s in equtbbnum accordmg to

o=

NOH

+

Hz0

+

HzSOa

e

0

+

NH20H

.

H2S0,,

absorbs addrttonal amounts of acrd. However, protons are needed growth of the @a) and (8b) If there IS a low H’ concentratron, occur [19], and the drffuston controlled overall reaction rate wrll bly Thrs potnt of vrew IS clearly supported by the expenment concentratron described u-r Fig. 7 (IV

(9)

c-

2) Anodlc

oxldatlon

of lower

for both reactions Pb(OH), layer wtll decrease apprectawith vanable acrd

airphatrc alcohols

If ahphatrc alcohols are oxrdlzed at a PbOz anode, the oxrdatron potential is much more postttve than 111 the case of oxrme, cf Ftg 8. The large distance from the Pb’+r-+ redox potentral means that a catalyttc redox mechamsm IS impossible. The electrode must be regarded as inert. From the Tafel slopes of 120 mV per current decade and from the reactton order of umty at htgher alcohol concentratrons (Fig. 9), we conclude the pnmary oxidatton to the radical catron to be the rate determming step: R-OH

-

[R-OH]*+

+ e-

(IO)

Smce the classtcal work of Elbs and Brunner [20], ahphattc alcohols have been SubJect several tunes to anolc oxldattons [21-231. Mechamsttc conclusions have been drawn for Pt by several authors [24-261, takmg mto consideration a catalyttc dehydrogenauon at the nonoxldrzed surface and a chemical reaction wtth PtO, at the oxtdlzed surface In no case was met-t behavrour of the Pt-electrode found. Inertness of PbO, versus ahphatic alcohols in spite of tbetr low oxidation potentrals has been confumed by corrosion expenxnents. A PbO, electrode (Type I) wtth a 100 pm thrck PbO, layer was exposed to the stu-red corrosion medmm defmed rn Table 1. From the weight dtfference after 43 h, corrosion rates have been

365

TABLE

1

Corrosion Orgamc

0 5 M

of a 100 pm PbO, substrate

o=

2 5 M 2SM

2 5 M 25M a Calculated b Calculated

NOH n-PrOH n-PrOH +PrOH I-PI-OH

layer m a surred

solwon

of 1 M HCIO,

IpI

u,,,rr/v

25

090

-

25

0 75

1.25

25 80 25 80

0 0 0 0

90 90 90 90

myth various

i,,rr/ pA cm-’

T/h

7100” 90 llooob 111

11

substrates

Remark After ref 27

i7000 30 236 19 191

EA=

69 kJ mol-’ EA=

74 id mol-’

from welgbt loss (0 6 Se) after 43 h from weight loss (0 4 %) after 43 h

calculated at 25 “C to be of the same order as in the blank the PbOz layer was totally consumed after about 1GO h. (V) CONCLUDING

Anodrc

orgamc

solutton

[21]

At 80 “C,

REVARKS

oxrdation

of cyclohexanone

proceed wa redox-catalysis as follows:

oxune at PbO, electrodes was shown to Our arguments for ths mterpretatlon of our results are

(1) Near comctdence of the current voltage curve m the presence of oxtme wtth the reversible curve of formatton/reduction of lead droxtde (2) Large cd amphfrcatton at a Pb/PbO, electrode in the presence of oxtme (3) Drffusion hnuted corrosion of PbO,-layer m aqueous HClO, m the presence of oxime, mdrcating the mdependent performance of a chemical step. A fourth strong argument, namely reaction hmrtatron of the overall current voltage curve could not be observed under our expertmental condrttons due to the fact that formatron of a layer of Pb(I1) products on the electrode at lugher cds prevented expenmental venficauon On the other hand, ah these arguments do not hold m the case of anodrc oxtdation of ahphatic alcohols on PbO,. It is for tlus reason that we exclude tlus mterpretatron m tins case. The only other example of heterogeneous redox catalysts on PbO, in the course of electroorgantc oxidation of benzene and some aromatrcs to qumone was demonstrated by Kuhn et al [28]. Some other oxtde anodes hke NtO, and AgO, [29,30] and Tl/Cr203 [7] show tins mechamsm for various substrates Many other surface ftxed redox systems may behave in this way, cf. examples cited mater-ml, no generahzations are m ref. 7. Concemmg PbOz, a classic electrode possible at this tune 131,321. Our fmdings with alcohols as negative examples underline tlus oprmon

366 ACKNOWLEDGEMENT

Generous provlslon of cyclohexanone oxlme Lbdwlgshafen/tiem, IS gratefully acknowledged

by

BASF

Akhengesellschaft;

REFERENCES i C P Andneux. J hi: Dumas-Bouchtat and J hl Saleant, J Electroanal Chem , 123 (1981) 171 2 N L Welnbeg III N L Wetnberg (Ed ), Techruques of Electroorgamc Synthesis. Part 1, Wiley, New York, 1974. p 448 3 J Schrmdt, Ber Dtsch Chem Ges, 33 (1900) 871 4 D L. Hamrmck and h4 W Llster, J Chem Sot, (1937) 489 5 F Beth, W Gabnel and W Kmer. Ber Dtsch Chem Ges, to be pubhshed 6 ‘4 Fed&e and H hfittemacht, Z Chem ,4 (1964) 389 7 F Beck and H Schulz. Ehxtrochvn Acta rn press 8 h4 Delamar, h4 C Pham, PC Lar-azz and J E Dubols J Eiectroanal Chem , 108 (1980) 1. 9 PC Lacaze, h4 C Pham, M Delamar and J E Dubors, J Electroanal Chem , 108 (1980) 9 10 H S Panesar m K-H Colhns (Ed ), PoHer Sources 3. One1 Press, Newcastle-upon-Tyne, 1971, p 79 21 Th S Sharpe, J Electrochem Sot, 122 (1975) 845 12 J G Sunderland, J Elec~oanal Chem ,71 (1976) 341 13 K R Bullocb and D H McClelland, J Electrochem Sot , 124 (1977) 168 14 H Lund and J Stmonet, J Electraanal Chem , 65 (1975) 205 15 P Mart~gny, G Mabon. J %monl:t and G Mousset. J Ehxtroanal, Chem . 121 (1981) 349 16 W Gabnel, Dlplomarbelt UruvenltBt Dmsburg 1981 17 C Wagner and W Traud, Z Elektrochem, 44 (1938) 391 18 N A Hampson, PC Jones and RF Ph~bps, Can J Chem . 46 (1968) 1325 19 F Beth, J Electroanal Chem, 65 (1975) 231 20 K Elbs and 0 Bnmner, Z Eiehtrochem ,6 (1900) 608 21 E h4iXler and A bus. Z Elektro:hem . 27 (1921) 55. E Muller, rbld, 27 (1921) 565 22 G Zeller, Tram Electrochem Sex , 92 (1947) 335 23 K I Krqschenko, M Ya foschtn, N G Bahhchsatratsyan and GA Koharow, Khtm Prompt MOSCOH, 45 (1959) 496, Chem Abstr , 1 (1969) 101253f 24 V S Bagotslw and Ju B Vasll’ev. Electrochun Acta, 9 (1964) 869, 11 (1966) 1439 25 S S Ecskorovamaya. Ju B VasIl’efl and V S Bagotsku. Elehtrokhmuya, 2 (1966) 167. 1439. 4 (1968) 315, 8 (1972) 376 26 E Sokolova, Electrochm~. Acta, 20 (1975) 323 27 F Beck, Z Naturforsch , 32A (1977) 1042 28 J S Clarke. RE Eh~gamusoe and AT Kuhn, J Electroanai Chem , 70 (1976) 33 29 hl Flelschmann, K. Konneh and D Pletcher, J Electroanal Chem, 31 (1971) 39 30 M Flelschmann, K Konnek and D Pletcher, J Chem Sot Perkm Trans. 2 (1972) 1396 31 T H Randle and A T Kuhn m A T Kuhn (Ed ), Electrochenustry of Lead, Acadenuc Press, London, 1979. p 217 32 E E Conway m S Trasartl (Ed ), Electrode.sof Conductive Metal Oxides, Usewer, Amsterdam, 1981, p 492