20 the Influence of Nitromethane Adsorption on the Oxidation of Formic Acid at Platinum Electrodes

8/11/2019 20 the Influence of Nitromethane Adsorption on the Oxidation of Formic Acid at Platinum Electrodes

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THE INFLUENCE OF NITROIvfETHANE AIMORFTION ONTHE OXIDATION OF FORMIC ACID AT PLATINUM

ELECTRODES*

A. M. BARUZZI, V. Sotis and M. C. GIORDANO

Pepertamnto de F~uimii, Fact&ad de Cicncias uunuas, Universidad National de Chdoba So00c &dotq AZ &La

(Rec.&& 3 March 1981

Aktrrt43a-dsorption of nit- at Pt clectr~ and its iIlauencc on HCOOH elatrocatalytkoxidation were uvestigated y eyelie eltamm etry nd pettntiDstatie uasi-steady tate o larizatioa wve s.Potearti&namic /E culycs n the pmaana of nitromethane show a co&deraMecurmnt -in tbalowpattntial re&ul (ct.245 v). This &act is, trtaiatainrd uttdar QWJ i-steady atate ccltitious. Electroaotption

studicspcrC~~Nithnitro~iadrcucdthatpartiollyisducedspecicsareadsorbsdwtheekaradtsurfaenThercsPesiescouldbe mspensibtc or the observed promotinE ef%4Z.The ~WU&S &aincd with nitrometbane additian are compued with *cctonitrik and dimcthylaulfoixide.

C~mpetitiun for adsorption sitea and mod&cations of au&cc adsorption energy produced by additiveadsorption, account for their specific intlucnce on the HCO OH oxidation rate.

INTRODUCllON

Cogdsarption of several substances on Pt electrodeshave b&n repoti to produce different e&&s onelectrocatalytic processes. An increase in current forthe first oxidation peak on the electr~talytic oxid-

ation of HCOOH is produced by acetonitrile(M-1, 23, I-[3], suifur[4] and foreign metalssuch as Hg, I%, Bi, Cd, Tl, Ag an d Cu[l, 5-J. Theprnsencc of these substances influences the HCOO Heltztrn-oxidation either preventing the formation ofthe strongly ,bound intermediate[l, 2, 51 or throughsome ape&R type of interaction between the adsorbedspecies and Pt su&tcc[24]. In tbis context t would beinteresting o investigate he intluence of other organicmolecules which strongly interact with the Pt sur-face[6, 9) on the kinetics of the HCOO H electro-oxidation.

In this work the Wluence of nitromethane co-adsorption on Pt in HCOO H electro-oxidation is

studied as a function of its bulk concentration, adsorp-tion time and adsorption potential. Nitromethaneexhibits physicochemical properties between that of asubstance having a high dipolar mom ent, nucleophyhcand H-bon d acceptor properties as dimethylsulfoxide(DMSO ) and acetonitrile MeCN ) with a lower dipolarmom ent. Both sub stances, DMS O[9] and MeCN[ lo]strongly interact with Pt in aqueous acidic solutionsbut their intluence on HCOO H oxidation seems o bedifferent[ 1 ]_

This work attempts to correlate some specitic pro-

* This work wm presmted at the International Society ofBkcnochemistry 1 st meeting, Venice, Italy. 22-26 Septem-her 1980.

+ prsscmt address Department of Chemistry, SoutlIam-ptou University Southampton, U.K. British Couocil post-doctral fellowship.

perties of the foreign organic molecule and theirin&ence on the formic acid oxidation rate.

EXPERIMENTAL

Nitromethane (Fischer certitied) was pnriSed aspreviously described[ 121. ts quality was veriBed by irspectrometry, mass spectrometry and gas chmm ato-graphy. Dimethylam ine (CH&NH was prepared bydneomposing its chtorohydrate by adding N&RI Thegas evolved was conden sed at low temperature andlater purified by se veral distillations.

Formic acid (Mallinckrodt) was used withoutfurther purification. The concentration of CHaN0 2was changed from lo- to 1 M and the concentrationsof the formic acid solutions were lo- .0.25 and 1 hi.

The supporting electrolyte, 0.5 M H&C?+ was pre-pared from or grade chem ical (AnalaR) and triplydistilled water. Solutions were d eoxygenated by bub-bling purified nitrogen through them

Small sealed 99.999 % Pt w ire.working electrodes, aPtpt foil counterelectrode and a hydrogen referenceelectrode in the same solution were employed. Thegeometrical area of the Pt wires ranged from 0.1 to0.2 cm. Pt foils of 2 and 9 cm were used as workioaelectrodes in some experiments. The real electrode a&was determined by standard procedu~133 . The sur-face coverage degree by the adaorbed species (Borg)was determined either from the decrease of the anodiccharge related to the oxidation of adsorbed hydrogen(Q; - Q~*/Qd, m G l fmm th infxaseofchargein the O-electrosorpnon region due to the anodicoxidation of the adaorbed species, (Qz- QuQz- QE).CycBc vos were obtained at room tem-

perature using a conventionai set up. I/E profiles wererecorded eitber with a X-Y recorder photographed


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274 A. M. BARUZZI, V. SO& AND h4. c. GIORD~

from an oscilllecope screen. Also current-time trans- nitrometh ne is enhaacsd, WhCItthCpOtBIitiBliShCidients were recorded after addition of nitromcthane at within the H-adatom potential region. This &cct isdifferent potentials. An anodic or catholic potentio- shown in Fig. 2 where the positive going potentialdynamic I /E profile was rtazorded immediately folfow- sweep is kcp at 0.1 V/s (02) and the negative potentialing the tepnsicnt to determine e&her oxidation charge sweep is w from u2 to oI = a01 V/s at twoof elcctrosorbed species or change of coverage of diff~t potastinls within the H-&atom potentialelectrosorbed H. Potentiost&c polariiW .ion curves for region. Q ualitatively the same IL&& is observed when

the HCO OH oxidation both in the presence and the the slow o is employed on the positive going potentialabsence of CH,N02 were also made. sweepThe above observations are opposite to those shown

in the ehztrocatalvtic oxidation of HCO OH atPt[l, 141 in the abskcc of the added suhatanas.ESULTS

General features observed on for mic acid oxidation inthe pr esence o nitromethane

The cyclic voltammetric I /E profile For HCO OHoxidation (Fig. 1) in the absence of CH ,NO, exhibitsthree &e&y known oxidation currentpeaksPaf.Pa2,and Pa3during the positive going potential sweep. Themain at-on will be paid on peaks Pal and Pa2 andhow their relative heights change with the CH sNOtconcentration in the bulk of the solution through theapplication of different potential time perturbationprograms.

The addition of nitromethane causes the mostsign&ant increase of current in the potential range ofcurrent peak Pal. I t also influeRces the current in thepotential range of Pa2 and Pa3 by producing adecrease in current amxiated with these oxidationproceSseS.

The maximum current increase OfPalisrcachedat[CH SNOJ z 1 x lo- M, butevenat 1 M theanodiccurrent in the low potential region related to Pal isgreater than in the absence of nitromethane. Thepromoting influence of CH ,N02 on the HCO OHelectro-oxidation rate is maintained at poteatial sweeprates as low as 0.001 V/s. However, as the potentialsweep rate exceeds 0.S V/s, the predominant effect isthe dcc*casc of the anodic current in the potentialrange of Pa2

The increase in current on Pal by addition of

The enhancement of the promotiiry e&ct on F byincrestsing the time spent in the H-a&tom region inthe presence of CH SNOl is also shown when thecurrent for the first anodic peak (Fig. 3) is recorded atdifferent v both in the presence and in the absence ofCH ,NOI after scanning the 0.05-0.4 V range at0.01 v/s.

Potentiostatic stepwise quasi-steady state Z/E curvesare shown in Fig. 4. Each current value was taken after

an anodicckaning procedure was employed and thecurrent/time decay at each potential W a verySdl BDd BhOSt CoflBtant SlOpE2. Tht t.iMC da@ WYBS

about 3 min.In the presence of CH XNOt there is an increase in

current between w 0.1 and 0.4 V. As the oxidationprocess associated with Pa2 is inhibited in the presenceof CH SNOI, the current obtained over 0.4 V is lowerthan in its absence.

Eiectrosorption o nitromethane and relaed species

It has been previously shown[6] that CHsN02 iselectroactive at Pt electrodes in acidic aqueoussolutions undergoing a partial ekctrorcdction tonitrosomethane,-CH,NOi and methylhydroxylamincCH ,NH OH. The total reduction to m&Mamine.CH BN Hl is only accomplished either at high tempera:tures or by increasing the pH[lS].

The possibilities for nitroalkane electroreductioncan be SUmmariXA as follows[16]:

E/V, VI RHE

Fig 1 Cyclic wltamm~,gxaxm for HCO QH oxidation in m and AXeWe of CW,NOA . - c-0 25 M without additive. --- CHcoon 0.25 M + CcH,N9 1.6x tW2 M. Bme ekctroty&e H,SO, 0.5 M.

sweep rmtc .1 v /s.


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The inauemr of nitromcthane adsorption

RCHNO,H++ [RCH~NO)] H+ ~[RcH(~H+)=[RC(:NH:)]

I

CRC

RCE

(1) WI

+H,O

NOHJW+ RCHNHOH

(Iv) (W

4e-

4H+

w

rJH:) - 2H+ + 2e-(W

275

--I

The direct clectroreduction of species (H) to (III) isvery difficult in acidic media Moreover, the yield on

As hy rogen ions are depleted from the interface the

spanics (1II) at low pH is hindered by the catalyticRNHl formation at low potentials becomes more

hydrogen evtilution reaction through an ion radicallikely.

mf4mnism~l5JThe potentiodynamic I B behavior of CHjNOz at

various concentrations in comparison to the base

RNHOH + H+ -+ RNH,+OH 5 RfiHIOH ,z RNH, + OH- I

Fig. 2. Effect of lim spent on W regiou u Pal and Pal in prince ofCHJNQ1 I CHCooH 1 x IO- M,CCH,NO, 1 x 10-s M . sweep rate u* 0.1 v/s; vs 0.01 v/s.

Is? ?-

>

_

mI lovil

0 --.--1

0f 4 _,,--4

k _--- _u---C--

4

E.

B -,/

,I--

.,a-0

o __ ________-__D-----

Fk. 3. DcpGndcnaofi,,onswap~~v,,~sofH region at 0.1 V/s. 0 C,,

+ CC,NO,0.25 M, cz,,, a25 M,

10-Z M.

electrolyte is shown in Fig. 5. In the presence ofC&NO 2 an oxidation peak (1) arises in the d.1. redonand simultaneously H adwcpticm is gm daally

hindered . Th e initiation of O-electrosolorption on P t isdisplaced towards more positive potentials and twooxidation peaks (II) and (IIr) arc seen before the O3evolution potential is reached. The total charge for tbcsurface oxide electroredudion does not changeappreciably, but the process appears to be moreirreversible, and an increas e of cathodic current in theo.cbo.3 v range is observed.

This behavior depends on whether the potentialsweep has covered totally or parGaIly tbc O-elec-trosorption potential range (Fig. 6). The extendedI Ep r o f i l e obtained after cycling in the H-&atom and61. potential rang e (Fig. 7) show s that Hadsorption-desorption is almost Comp IeteEy nhibited.On the folIowiug positive going p otential sweep* onlyoxidation peak (III) is o&ii&. As the H elec-trosorption potential region iai xcl& the I/E profile


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276 A, M. BARUZZI, . Soris~~D ht. c. ~IORDAIWJ

04

t

YLT2 0.3-

has been injected at 0.4V, indicating ,thatadsorption at this potential is fotlowed by the eke-troreduction of the molecu le. This redox process isalmost complete in the d.1. region since the O-electrosorption potential region remain s almostunaffected. On the other hand , after a n appreciablecathodic transient at 0.2 V, a considerable incrms e inoxidation charge was found and oxidation peaks IIand III are evident.

Fig. 5. Modifications of cyclic vol tammograms of 0.5 MH2S04 by CH,NOZ addition. 0 C,, ,.+ O.OM ; 1,1

x 1O-4 M; 2.4 Y lo-* M; 3.2.5 x lo- ti; 4,l x IO- M.

Fig 6. Modikation of I / E petentiodynamicproziht s thenegative going potential sweep is sterted at less pcsitive

potentiab. Cm,NO, 0.1 M.

Form ally, the kinetics of the adsorption process ofchemisorbed particles in nltromethane solutions wasdetermined through tkbz xidation charge ofthe adsor-bed particles as a function of the adsorption time f, atdiifermt adsorptions Eud. TIE dep endenct of B_ onlog t follows a linear dependence, which corresponds

to the kinetic behav ior of the majority of organicsubstances on P t electrodeCl7-j and reproduces pre-vious results[6].

Fii 7. Voltannnolpam obtakd after step (c) in Fig. 6 - - --.secondq&.


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Tbc itdim of nitromethme iadsorption 27

Also,auappsuentdepeudenceof surfaacoverage byadsorbed particles in nitrom ethane solutions, e,, onSod was found for low CH.NOa concentration. The0 Us Ead plot shows a maximum around Ct20-0.25 V.

The 0xidaGon &urge for particles adsorbed at 2 VatWt~urs~s~on value at [CH,NO,] ca 1Q- _M.

duxease m the H &ctro-oxldatloncharge at a2V is proportional to the logarithm of thenitrometbane concentration (Fig. 8) in the 0 rangebetweeu 0.2 and 0.6. A Limiting surtke coverage ofabout 0.6 is found for CH3NO r concentration be-tween 10s2 and lo- 1 M.

DIStXJSSION

The experimental data would indicate that differentefectrosorbed species exist on Pt in nitromethaaesolutions. These species would arise from the gradualelectroreduction stage of the chemisorbed CHoN 02molecule.

From the reduction charge of the transient obtainedat 0.2 V and the related hydrogen coverage recoveredafter the transient a number of 2 ap.s. was o&&cd.T&i&g into account the possibility of two adsorptionsites on the pt surface for each CH ,N02 molecule, theexchange of four electrons on the electroreductionprocess would account for the possibility of the NH-intermediate on tbe electrode surface. Species such as

Fii 8. Dcpcod- of ste dy slate covetage on nitrometbanebulk conantration at gad 0 2

(V)and (Vi) in reaction scheme (l), should he stabilizedby adsorption on the electrode surf.. preventing itsfurther hydrolysis. The ii interaction with Pt shouldaccount for the promoting influence of CH3N 02 onHCOOH electro-oxidation. Further experimentsmade with a substance containing -NH group givesupport to this interpretation. Figure 9(a) shows thepotentiodyn amic I/E profile of a Pt electrode in the


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278 A M. BMUZZL V Sods AND M. C. G~ORDANO

E/V, v WE

Fig. 10. Voltammogram of HCOOH oxidation in pnscncc of (C&),NH. Crmooir 0.25M. activatedekctrodc in base elec~oiyte --- After dipping in @ZE I,),NH.

base electrolyte immediately after it has been dippedinto liquid (CH,),NH, for few seconds. The chemi-sorptz~n of (CH&NH on Pt influences both thestrongly adsorbed intermediate oxidation [Fig. 9(b)]and the potentiodynamic i / E profile of HCOOHoxidation (Fig. JO).

These effects are qualitatively very similar to thatdue to the adsorbed particles electroformed in theCHJNOs containing solutions. The enhancement ofthe promoting effect after holding the potential in theH adatom potential region is in agreement with theabove m entioned results. Therefore, the intluence ofCHBNOl in the HCOOH electraoxidation on Pt isqualitatively similar to that produced through com -petitive adsorp tion by other s ubstances such asCH,CN [l, 23 and (CH,),SO[lO]. The relative ef-ficiency of the i&uence of the diRerent substances canbe evaluated in terms of the current density read at thepotential of current peak Pal with respect to thatfound for th e electrolyte in the absence of foreignsubstances. For the three substances aiready men-tioned the following order results

(iCH3CN/iH10) > (iCZ&N$,/iH,O) > i(CH&SO/2 .The slope of the line r dependence 0 vs log

[CH3N0,] at the maximum adsorption potentialunder stationary state conditions, yields an apparentsurface heterogeneity parameter ari&ng from ad-sorbed spe&s in CHJNO s containing solutions ofcu 5.4 kcal/mo l. Und er practically the sam e experi-men tal conditions the corresponding apparent he.tero-gene&y faders in DMSO containing solutions[9]and in MeCN containing solutions[iO] are 2 and9 kcai/mol, respectively.

The differences in surf= h&erogen&y pammeterbyadsorptionofpartickscomingfromdi@etent addedsubstances should selectively I-&e& through th 0 d-sorption energy of hydroge n atoms[ 181 and, con-sequently on the electro-oxidation rate of HCOO H

q CH,CN

z

y CH,NO,.

\

,/.ey

\,

Fig. Il. Dcpend rpce of relative current values or Pal onadditive molar mthalpy ef vaporization.

through the weakely adsorbed -COOH inter-mcdiate[l4]. The strongly adsorbed intermed iate for-mation, which inhibits HCOOH electro-oxidation inthe low potential region, must also be modified. In

addition, considering the molar cnthalpy of vapor-ization (AH,,) of each added substance as an ap-proximate measurement of the interaction energy w iththe surface, a good correlation is found (Fig. 11)between AH, and the oxidation rate of HCOOH at lowpotentials in the presence of co-adsorbed organicspaGs. Thus, it appears that molecuks stronglyiotera&ng with platinum surface influence theHCOOH electro-oxidation through competition foradsorption sites and modification of surf&x adsorp-t n energy for rcadants and products. This leadseither to activation or to inhibition of the clec-trocatalytic process,

_~&~DwW ~ ~JW-T~M work was supp&tal in put withtM th~ from the t&tm cjo N aeioaal de In~oncs- y T- of Arpentine. Oae of WI (A. M. B.)tbmttks ONKET of Argentina or the fc&wship granted.


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The innucncef Ditrot#mtmnc adsorption 279

l-l30 authors also wish to thmk Professor w. viilstich for his Rocmniki Chemii, Ann. Sot. Chim Pohmorum X4 2113suggcstioo s duriog prepa ration of this paper. (1976).

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20 the Influence of Nitromethane Adsorption on the Oxidation of Formic Acid at Platinum Electrodes