Specific anion adsorption in the course of upd of Zn2+ ions on platinum

Journal of Electroanalytical Chemisu3' 434 (1~)7) 201 - 207

ELSEVIER

Specific anion adsorption in the course of upd of G. HorAnyi

Zn 2+

ions on platinum

Akiko Aramata"

Centred Rexean'h hi ~lil~lt~,'.l'c~rChemist~3, ¢!fll,e Hungarian Academy ~ St'ient'ex. P.O. BoX 17. 1525 Budapest. Hungt~r~, ~*Catah'.~'is Research Centre, Hokkaido l/ni~:¢rs~ty, Sappon~ (1¢~),Japan

Received 31 tam~ry 1997; ~¢vised 25 April 1997

Ah~I~ICt

The Sl~clfi¢ ¢imon ~idsorplion ai'¢onipallyin~ lhe upd ~I' Zn ~' ions on ph,|immi el:cl|T~dt~n lilts bcel'l .~ludied by the radi¢~Iraccr lecbnique~ using ~S la!~'Jh:d Sail]hale ,,,pcclcs and t~*{l hil~lled Cl ion.~ II has Ix~en l'omid ll|al slilphale hms ad~orb Icudily o~ lhe lop el Zn ~daloms while lhe a~'4oq)lioll sli~11[~.I11el' CI ion,4 ~]n file Zill adlayer in Si~lli[iC|llilly lower lh~n lhal of IiS(.)~ ( S 0 4 ) ions. Indirect radio|l~=cer evidence ohlained from ~:Oml~lilive adsorplion studies is prcsenled concerning lhe relaliw:ly high adsolbabilily of H :, PO~ ions, II follows |h~m Ihe e,~perimenlal tesult,~ obtained that the calculation of the mass and charge balance (involving ~hc detcnninatiem ¢~f the so-called eleclrost~q~tion valence) requires the knowledge of the exlenl of Ihe anion adsorption on the adatom layer. @ 1997 Elsevier

Science S~A. g'e~wm~ds: tlpd; Zn ~' itms; I~, elecirt~le: Induced anion adsorplion; Radiotracer techni |ne

1. Introduction The underpotential deposition of Zn ~'= ions on plato inure, palladium and gold electrodes has been studied at various ptl values m a series of previous communications The effect of anions on the underpotcnlial deposition of Zu "~~ ions was considered in a recent paper [6], It is well known from the literature thal upd of several metal ions is accompanied by the coadsorption of anions. (See, for instance, literature cited in [1 ~6] and some recent results on the CulPllhalide system obtained with different lech° niques [7], induced adsorption of sulphate, bisulphate anions by Cu adatoms on stepped PI surfaces [8] and bisulo phate ad,qorption TI adlayers on PIt I I 1) [9]). The clarification of such secondary effects as anion adsorption accompanying the upd of Zn 2~ ions is of high importance as without the knowledge of these phenomena neither the charge and mass balance nor the questions connected with the stnlcture of the adlayer can be treated adequately. it is evident thai in the case of the occurrence of anion adsorption any stoichiometric calculation based only on the charge value determined, for instance, on the basis of " Corresponding author. [email protected].

Fax:

+36°1-2603899;

e-m,'til:

0022-0728/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. Pil S0022-0728(97 )00264°7

cyclic w~llamme~fic measurements could be very mislead ing. In the present Mage of the inveMigalions it is imporo tant to estimate which cases, studied previously..,.hould b~; revisited in the light of |he result,4 oblal~lcd frc,m anion adsorption sludies. The first informalion on lhe anion coadsorpli,,m, (aui~n adsorpUon induced by metal adatoms) was furnished by the radiotracer lechnique in the seventies and early eighties On the b~.gi., of these results ~ ~imilar study can hc suggested also in the case of upd of Zn "~* ions. Polycrystalline platinum as support and ~~S In,lied HSO~ (SO~-~) and U'CI labelled C! ions seem to constio. lute t!~e ~implest system~ fer lhe first plelimina~y study ~i~ get information on whether the simultaneous adsorption of anions could play some role in th(~ upd of Zn ~' lon,~. 2. Experimenlal The electr(ruchemical cell alld the ni,,asurin;!~ lec°hniqttc des crib~,l in previous adsorption studies were u~ed [I 2=~14L '~S labelled su!pburic acid (specific ~tctivity i~5 MBq mmol ~l) and ~'CI la!~lled CI iolls (.~p~cific activity 26 MBq mmol ~i) were us~:d in lhe radio~raccr ~d~rplio, studies. Platinized ek,ctrodes were prepared by the tlsual eleco trodeposition process [I i.12] on the gold-plated plaslic toil

G, Hor6ny~ A. Arama~n/ Journal oj" Electroanalytical Chemist~' 434 (1997) 201-207

~q)2

forming the bottom of the cell. Roughness factor values of the electrodes u~ed (determined by hydrogen adsorption) were about It~. HCIO4 and H,SO~ solutions in various c~ontrations were u ~ d as supporting electrolytes. Potential values quoted are given on RHE scale. Zn' + ions were in the form of ~lts by dissolving ZnO in the supporting el~trolyte used.

30 2

I

3

I0

3. Results

3, I, Study of the mait~fec~tures ch¢zrewterisittg ~he sorption ~~f ~S labelled ,s'ulphote species in the presea,ce of Z~ ~' ictus Fi~, 1 ~how~ Ihe voltamme|6c behaviol;r of a platinized e l e ~ t t ~ in the pre~nee and absence of Zn ~' tons u~ing H~SO, t ~upporting electrolyte, Similar behaviour was re= ported for ~nt~th polycrys!alline PI ele.ctr~gles [2,3]~ 'll~e changes in the vollamme!ric bchavk~tr of Pt caused by the ~ i ~ z e of Zn ~+ ion~ are the very signs of the elec° tmso~p~ton of Zn ~* ions interpreted in terms of the upd of Zn ~* ions in S e previous communications [I-6], Cover~ ~ e values with r e s e t to Zn adatoms of about 0.2 we~ foam in acid medium [2-4] for low potentials (0 mV on RHE ~ale). The calculation is ba~d on the assumption that complete discharge of Zn ~+ ions takes place, It is assumed that gn adatoms displace adsorbe~l hydrogen: however, there is an overlapping of the two Ul~ processes in acid | ~ d i u m HC!O~ supportiug elcctt~lyt¢ is used for the study of the specific adsocption of labelled sulphate species in the p~¢¢ and absence of gn ~' ions.

0

211

I0

30

I/mitt

Fig, 2. I-itlhal~¢ll~m of the ad:~t~-plhmo1"h i . l i e d sulphah~ ~l~C,i¢~by Ztl

at file o~m~ol i~d~lcd hy tll~ ~ll~:tw. S¢~:Ooll 3: A~klilion O| ~ ~ IO r: I reel dm ~ nonola~ll,~d It ~St)..,

Direct experimental evidence pkwing the enhancement of 1be adsorp0on of la~lled sulphale sixties is demonsmoed in Fig, 2, After the formation of a Zn adatom layer in HCIO~ ~uppomng ele~ Jyte a| 0 mY, labelled H ~SO~ was added to tbe solution phase. 11~e rapid increase in the coum rate {above the background) attests the enhanced adsorption. The mobility of the a d s o ~ d species can be shown easily by adding a great excess of non-labelled H ~SO4 to the solution phase. Tbe rapid decfca~ of the ¢ounbrat¢ is the sign of the exchange processes i.e. of the high mobilily of the a d s o ~ d species (Seclion 3 in Fig, 2). Fig~ 3 shows 1he potential dependence of the adsorption of labelled HSO~ (SO~-) species at various Zn ~+ and at fixed HCIO4 and H~SO4 ~mconw~ions. The curves pr¢= ~nted in Fig. 3 demonstrate that using HCIO4 supporting

0,~

! 2

0~

!/ -1,$

0 E~V

Fig L C3~ti~ ' ~ ~ c ' ~ ¢ s in ~ ab~ac,e (d~leO line) and of Z~ "~+ ions { s ~ i n g etec~iyte: I n ~ l din- ~ H,SO~:

'

200

400

600

~mV Fig, 3, Polonti~ dependence of the specific adsorption of labelled sulphate ,speciesat various Zn :+ concentrations in Ihe pre~nce of I x 10 -4 tool dm -~ labelled H,SO~ in 5 x 10-: tool din -~ HCIO~. Cz.~+: (I) 0; (2) 5 x 10-4; (3) !.5 × lO- 3; (4) 4.5 × !0- ~ tool din- ~.

G. Hordnyi. A. Aramata / Journal {~fElectrc~malyt~cal Chemislrx' 434 { 19971 201-207

electrolyte, the enhancement of the specific adsorption of sulphate species can be observed in the potential range where the upd of Z n 2 + ions takes place. The existence of two different types of adsorbed sulphate species can be demonstrated by simple potential step experiments as shown in Fig. 4. In these experiments the potential of the electrode was switched from a value ( E = 700 mV) where no upd of Zn ~÷ ions takes place to a value where th,~=. coverage with respect to Zn adatoms is high (0 mV). The experiments were carried out in the presence ¢,f 5 × 10 -~4 mol dm ~:~ labelled H,SO.t and Zn "~* ions in various concentrations. The dotted line in Fig. 4 represents the desorption of sulphate species following a potential switch in the abe :~ence of Zn ~+ ions, The rapid decrease of the countorate (proportional to the extent of adsoq~tion) is the behaviour expected on the basis of previous studies and is in asia'e,, meat with the very t~et that no specific adsorption of sulphate ions takes phlce at low l~)tcntials (see curve I in Fig, 3), In the presence of Zn ~' ions the coon{orate vs, lime cutwe obtained Ibllowing a potential switch goes througl~ a minimmn. The position and the magnitude of the minimum depends on the Zn 2+ concentration. The lower the concentration of the Zn 2+ ions the more pronounced will be the minimum. The phenomena observed can be explained easily by accepting the existence of two different kinds of adsorbed labelled species. The potential switch from 700 mV to 0 mV should be IOIIowed by a rapid desorption of sulphate species adsorbed on platinum surface sites as expected (see dotted line in Fig. 4). In the presence of Zn ~" ions at 0 mV their

700 ..............

40

0 mV

.

~,===~ ~ . . , , a ~ ~ ' , ~ ' ~ ' = ~ ` ~ t ~

o 2a

0

5

l0 thnin

Fig. 4. Count-rate vs. time curves obtained following i~)tential switches from 700 mV. to 0 m V in the absence (dotted line) and presence of Zn 2~ ions (curves I, 2 and 3) in a solution of 2 x I0 ~2 reel d m ° ~ HCIO,, and 5 x 10 -4 reel d m -~ labelled H . S O a. Concentration of Zn z* ions: (I) I . S x 10-4; (2) 1 . 5 x 10-3; (3) 7 . 5 x 10 -3 reel dot -~,

203

40

|

2

3

:

30

4,

i

t

!/

!\

,

o 10

0

I0

20

30

40

ttmm Fig. 5, Efttwt of CI: tOilS 011 the adsorption of I n . l i e d sulphate ~l~ies m Ihtr presence o1" Zn: * ions, Solution: 5 × 10" ~ reel d m - ~ HCI(), ,) 5 ×° ! 0 ~ real dm ~ hd~,:lled I t : S O ) + 12 x 1 0 ~ reel dm ~ ~ Zn :~ ; t.5 × in ') reel d m ~ IICI w~ls added |o the solution phase at the moment i|tdiculed hy the affow0 Section I: Counl-r~le w, Ih|v¢ c o | r e {,it 7(N) inV, 2: Ch~tnge~ tollowlng, ,~)lential swilch to 0 mV, 3: ~wilch In 7{10 nlV, 4: ~Mdllion ot C I ions ,, 711UmV0 fir A poleultM ~witch in 0 m V

upd take,~place and the rate of the formalion of the adlayer should depend on the concentration of the Zn ~* ions in the solution phase, if sulphate species adsorb on the top of Zn adatoms the rate of the increase in the ad.~orption of the labelled species will be influenced by the rate of the formation of the Zn adlayer i.e. by the concentration of Zn '~ ions. The higher the Zn 2' concentration the Inure rapid will be the apparent adsorption of the anions on the top of the adlayer. An interesting observation, the different influence of C I ions on the two adsorption stales, allows us to give a direct demonstration of the validity of the a~sumptions outlined al~we, It is a well known fact the CI ions displace adsorb,;d sulphate species ahnost completely from Pt surfaces even at a very low concentration of the CI ions in the solution phase. In contrast to this we found that the sulphate species adsorbed on the top of Zn adlayer are not sensitive to the presence of CI ~ ions. This pt~enomenon allows us to achieve the radiotracer study of the adsorption processes on the top of Zn ado{ores at any potential without the disturbing effect of the adsorpo {ion occurring on free Pt sites not covered by Zn adau)m~, Fig. 5 demonstrates the effect of C I ions on the adsorption of sulphate species. This figure clearly ~hows that while CI~ ions displace the adsorbed labelled molecules from a Pt surface not occupied by Zn adatoms (E = 700 mV) the adsorption o1' the same labelled ~pecie,~ on the Zn ad!ayer (at E ~=0 mV) does not change in the presence of CI ions (~t least at the concentration ratio~, used in the experiment). In Fig, 6 an attempt is made to ~how at lea~ in ti~e first nmgh approximation how the curve~ predated in Fig, 4 can be constructed from simple curves representing the desorption of sulphate species from a Pt surface following a potential switch from 700 mV to 0 mV (see dotted line

G+ Ho,rdnvi. A, Aramata / Journal *$ E/ectrnanalyticat Chemistry, 434 t 19971 201-207

2C~

9

[:;ig,

(L A ~¢h¢o~1i¢ t¢:lYee~nt~:liOlAo | the ¢oftgl~lliOfl of ¢otlll|:la|e ,,~,

|il~h~ etlf++eP+{3) ~howll il~ |:'~g, 4+1~,+++,0++;++l~'+neo,n+N~m,;tin + t,~,deP,~+¢ptlo4+l ff~m+ Pt mt~ffm¢o~ile+ {I) m+d the ~_+¢~1+o1~:1~+¢ton the Zn mdlmyer (2), See det~+l~ explmm~lio¢~ in the le~l, { I + ~md t in

The simultaneously taken voltammetric and voltmdio+ metric curves give information on how the specific adsorption of sulphate species changes in the course of the voltammetric measurements. In Fig. 8A a voltammetric curve obtained at low sweep rate in the presence of labelled H~SO4 and Zn ~+ (how* ever, in the ab~nce of CI ~- ions) is presented+ The corresponding voltradiometric curves, i.e. the count-rate vs. time curves during c o n ~ u t i v e cycles are shown in Fig. 8B. The shoul~r on the initial section of the positive sweep in Fig+ 8A and the differences in the position of am~tlic and catl~;~dic ~ a k s prove that the origin of the hysteresis observed i+~ lhe c a ~ of Fig. 7A should be connected with the beha~iour of the Zn adlayer formed ou the suiTace with relatively high real surface area. The complex behaved:mrduring cycles i~fl¢cted by Fig. 8B should ~ ascribed in the existence of the 1we |yl~,s of ad,~orbed species. R+:ally+ in the plesenc¢ of CI ions

m'bimwyunit~)~

A curve in Fig. 4)+ on the one hand. and from the F (or count+rate) vs, time curve obtained for the adsorption of labelled s ~ i e s at 0 mV in the simultaneous premnce of e l and Zn 2' ions, on the other hand (see Fig, 5), In the pre.~:nce of CI ~ ions the~ is a possibility of determining the potemia! de~ndence of the 'ad~+rption of lab¢ll~ sulphate st~ies in the I~tenti_a! .qmge where the u n d e ~ e n t i a l deposition of Ztr~+ ions tokes place withou! t ~ disturbing role of adsorption pcocesms occnmnp~ on PI sites free fr~m Zn M~toms. Trw~ I" vs, E and the cot~sponding A I ' / A E vs. E curves are p~sented in Fig~ 7A and B+ rcspecli~/ely+ The !" ~ E eur~,e was taken starting ft~m ~ ) mV whex¢ no M,~q~t~ of laMIled sulphate sp~cie:s can he o i ~ e ~ l M the pre~nc-e of e l - i~tms, The t~tential was ,hii~d to | ~ ~g~ti~¢ vM~s in 25 mV ~eps waidng at cash ~ n t i a l for 10 to 15 mix. Thus the finM ~ n t i a l value, 0 mV+ was a t t a i ~ after several hours, Following ~s ~iure the direr|kin of the potential shift was D ~ t e the slowne,~s of the measurement a ~markable hyst¢~,ds ea~ be ob~p+,,~,xl as demonstrated by Fig, 7A. Thi~ situation is well r~flected by the corresponding A l~/A E vs. E curves, it may h¢ sccn frtm~ Fig. 7B thai m ' ~ g h two well dcfi~d peaks ¢~n M o h m r v ~ similarly t~ t ~ v ~ i ~ t d ¢ p~ks ( ~ Fig, 1) the "'positive going" "'e,e g ~ v e g ~ ' " experiments do no~ furnish strictly ~eves, A closer esamination of the vellum+ carves, taken at low s ~ e p rues leads to the ¢~c~as~n ~ the ~ lack of symmetry can be ohin the c~+¢ of ~,~ol~mmel~ measu~ments, as well. fi~s ~ n o n could be connected with the structure at" the platinum black layer i.e. with the slowness of the ~,;,lmmm of the equilibrium slate in such systems.)

®

0

2O0

4t~

I~~mV

B

4O0 E,~V

+!

Fig+ 7, (A) Potential dependence of the adsorption of labelled sulphate species induced by Zn ado|ores in the presence of CI- ions. 5 × I 0-2 reel dm -~ HCIO4; 5 × I0 -3 reel dm -~ Zn(CIO4),; 5 × lO -4 reel dm -~ labelled H_,SO+ and 4 × 1 0 -4 reel dm -~ CI-. (B) The A F / A E (in arbitrary units) vs. E curve corresponding to the data presented in (A).

205

G, iiordnyi. A. Amnu~ta / Joun~al of Elec~rvanah,gca! Chemisu3' 434 ¢! 997~ 201-207

A

20

0,2

0.!

~0.5

<

0.25

"

-0~I "'" ,,

-0.2

0

~ ' ~

r ......... ~

0

~

5

~ IO

15

|/rain

-0,,~ 0

2(~)

4OO

Fig. I0 S|ndy of the cnh+mcelnen| of the ad~o~pli~nof I+d~ll~dC I ions mdIJ¢¢d by Zn ad~l|om~, Tb+ I' v~. timt~ curve tbllowiug a p4'){en[isI r-.v,,'it~'h fi'offl ~(X) |I~V to 0 n | V in the ahs~nce (d~ll¢'d liBe} l.tn,:l pl'~el~g¢ (2) of Zu ~' io,~ ( 5 × In '4 ;I|O1 din "° i} |n ,.~ lO ;' Iiii)I d i l l "~ HCIO~

B

~md 2~; I0 ~ tool dm ~ labelled CI

ion~

!0

\/i\ / 0

/' 40

20

¢~0

3.2. Stud), qf the adsorption of labelled CI

80

t/rain Fig. 8. (A) Voltammetric curve obtained in the presence of labelled H : S O ~ ( 5 × I O ~ tool dm ' ) a m l Z n :+ i o n s ( 4 x l O

~ moldm-~)in

I × I0-~ mol dnf ~ HCIO4 Sweep i,~c: 0.4 mV ~-~l. (B) The corresl~,mding voltradiometliC curves dul'iug I;OIiSC+BIiVe cycles.

where only one t y ~ of labelled adsorbed species exists the shape of the consecutive voltradiometrie curves becomes simpler as sl~own in Fig. 9.

°

ions

On the basis of the results presented above there is no doubt that the adsorption strength of C I - ions on the Zn adatoms is significantly lower than that of the sulphate species. This evidence, however, does not exclude the possibil~ ity of the specific adsorption of CI -~ ions on the Zn adatoms in the absence of sulphate species, using only HCIO 4 supporting electrolyte, Fig, lO shows the results of potential step exD~riments in the presence of labelled CI ~ that were carried out similarly to those presented in Fig. 4, It follows from the curve~ presented in Fig. I 0 that there is some increase in the adsorption of C I ions. induced by Zn adatoms; however, the extent o f this increase is less than 5% of the

()

20 ,~

10

$ ;

i\

0 ~,~,,,,,~

~,,~

l0

20

;'00

~ 0

mV

5

~0

4{)

t/min

0

25

50

75

t/rain Fig. 9. The shape of the consecutive voltradiometric curves in the presence of CI- ions. Dala as in Fig. 7A. Sweep rate: 0.4 mV s- ;.

Fig, II, Eff¢¢l of addi|io|| of H~P04 off lfi¢ i~d~oq~lJ~;lot ~ulphale ,~p¢cio,~ ad~ort~cd on Zn ad.,'gom~,in Ih0 pff~scllge ~l Ci ~ ion~, So|uli~,|!~: 5× I0 ~ reel dm ~ HCJO,; + 5 × 10 °4 reel din= ~ labelled H~50,; 4=g x l 0 '~4 reel dm ~ Zn ~ + 5 × 1 0 4 reel 0m ~ CI- ion~, L 6 × l U ~ reel dm -~ H~F04 was added (o the ~olutiorJ pha~ at 0 mV ~; ~h¢ moment indicated by the arrow.

206

G. Hordmyt. A. Aramam / Journal of Electroanalytical Chemisto, 434 (1997) 201-207

10

} ¢:t

0

tO

s tllool

~i~ l~. C,mnt~tofe w, time ¢utve~ (ollowit~ potential ~witeh¢~ from 000 •V IO 0 nlV ill IM pt~llC¢ Of I X tO' ~ 11101dm ~ llIMlled H~SO,~o 4 × 1 0 " * u~l d|TV ~ go ~* and ~ x l O '~ tool dm '~ HCt in 5 × 1 0 dm ~ lICtOr. ~t ~ar~oo~H~PO,,cemcentrations.(I) 0: (2) 4× I0~: ()) L$ × 1 0 ~ ; (4) 4.S × 10 ~: (5) 2 × 10" ~ mot dirt ~ H ~t~)~.

~ n c c m e n t observed in the ca.~ of sulphate ions. Addle lion of I × 10 ~ reel dm "~ H:SO~ eliminates completely the ad~rptinn of CI - ions on the top Zn adatoms.

700 mV (on RHE scale) and that taking place on the top of Zn adatoms (at 0 mV), however, a significant difference can be found in the adsorption strength of the two adsorption states as demonstrated by Fig. 5 in the case of sulphate ions. it is an important observation that the extent of the specific ad~rption of Ci ions on the Zn adlayer is very low at least in the l~e~nce of HCIO4 supporting electrolyte. Analysing the sorption behaviour of sulphate ions in the presence of Zn ~4 ions it can be assumed that in the potential range between 0 and 700 mV (up to tile beginning of the oxygen adsorption or platinum oxide fiwmation) the potential dependence of the apparent adsorption of the anion is determined by the combined potential decadence of the two adsorption types, In the first approximation we could assume that the potential decadence of the anion adsorption induced hy Zn adatoms is determin¢;d by t ~ potential dependence of tile coverage with t~spect to the inducing agent, Zn adatoms (Oz.), On the other hand. the surface sites occupied by Zn adaloms are no more available for the Pt~anion surlace m|eractions, Thus the potential dependence of the adsoq~tion ot sulphate species under fixed concentration values may he given by the following equation:

Yo (i

ozo) i',,(E) + + 0to(/;

3.3. 6fleet

~ H~PO,~ ions

In contrast to CI .... ions H~P()~ ions are able to dis. place sulphate s ~ i e s a d ~ b e d on the top of Zn adatoms, This is shown by Fig. ! I where the effect of addition of H~PO,t on the 0Msorption of labelled sulphate ion~ is ~monstr~ted, ~ n d i n g on the concentration of H ~P(')~ (~t a fixed c~meentrctioa of lahel~ sulplmte species) a or ie~ p c ~ o u n c ~ decrease in t ~ extem of ~h,: ~ o ~ ' i o n of s u i t e ~ i e s can be ob~rved as demon°

by vh , IZ

Y.

(i)

- -

where /~,(E) is the potential dependence of the anion admrption in Ihe absence of Zal:" ions, I'l is the extent of the a d ~ t i o n at 0 mV at a fixed Zn 2* concentration. 0~, is the potential dependent coverage with respect to Zn adatoms changing from an initial value at O mV to zero al potentials about 3$0 mV. At low potentials and high Zn ~* concentratitms ( E < 300) I"1 :~ |ix(E) tile simplified

(2)

E,(L') + equation can he obtained.

Th¢~ rc~hs show that~ significants~ific adsorption H~PO~- ions on the Zn adl~er should be taken into cons~tiorl, Thos it would he of some interest to carry out a direct study of the adsoq~tion phosphoric acid using ~:P l ~ l l ~ phosphate species, This aim will he c~mside ~ in a continuation of the pm~ent work.

¢.~pe~i~nt~l results p t ~ a t ~ d do ma leave any tkmbt ~ t ~ n e c t a r i n e o|" the spo~-ific adsowaion H$O~-($O~ ) and H~ PO~: ions on the top of Zn adatoms, It i~ important to stres~s that the p~enon~na ~ e d be ascribed to e ~ t a t i c forces as they occur in the presence of a great excess of HCIO~ supporting elecIn this m,~pect the~e is no difference in the character of ~ occurring on free Pt sites, for in~ance, at

F

E Fig. 13, Schematic representation of the composition of the potential dependence of sulphate adsorption in the presence of Zn '÷ ions (3). (I) Adsorplion on Pt surface sites: (2) Adsorption on Zn adatoms ( ~ e explanation in the text,)

G. Horcbo'i. A. Aranmm / dourmd ef Electro,malytic~d Chemist O. 434 ~tgO7J 201-2G7

The second member of Eq. (2)(O:~ F t ) is furnished by Fig. 7A while the FA(E) function is given in Fig. 3. Thus the curve corresponding to Eq. (2) can be constructed easily as shown by Fig. 13. These results prove that in the presence of sulphate ions the formation of the Zn adatom layer involves simultaneous anion adsorption thus neither the amount of Zn adatoms nor the so-called "electrosorption valency" can be calculated without the knowledge of (he extem of anion adsorption. Although the experimental evidence found and presented here is characteristic for polycrystalline surfaces should be derived from the behaviour of various crystal faces fanning the surface of the elcctrtxle studied. This means that tile possible role of the phenomena observed should also be considered in single crystal studies.

Acknow|edgemenls Financial support ftx~m Hungarian Science Foanda|ion (OTKA. Grants T014448 and T023056) is acknowledged.

207

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