10
Investigación Revista Mexicana de Física 41, No. :3 (1995) :386~:395 The Frumkin model applied to the adsorption process of benzene on a Pt electro de from sulfuric acid solutions M.A. QUIROZ, F. CORDOVA Instituto de Estudios Avanzados, Universidad de las Américas-Puebla Apartado postal 100, Sta. Catal'ina Mártir, 72820 Cholala, Puebla, México L. SALGADO, M. VINIEGRA Departamento de Química. Universidad Autónoma Metropolitana-Iztapalapa Apartado postal 55-534, Og340 México, D.F., México Y. MEAS Centro de Investigación y Desarrollo Tecnológico en Electroquímica, S. C. Apartado postal 064, Pedro Escobedo 76700 Querétaro, México Al\'D G. DÍAZ Instituto de Física, Universidad Nacional A utónoma de México Apartado postal 20-364, 1000 México, D.F., México Recibido el 29 de noviembre de 1993; aceptado el 8 de diciembre de 1994 ABSTRACT. The electroadsorption of benzene on a polycrystalline Pt electrode from 0.5 M H,SO, + x MC 6 H 6 solutions was studied as a function of the uenzene concentration,time and potential of adsorption. It was observed that the maximum adsorption takes place in the double layer regioIl, even though the adsorption process can also occur in the hydrogcn rcgion. At the potential of max- imum adsorption the experimental results showed that benzene Illolecules are strongly adsorbed on the Pt surface l each one occupying about 7.6:i: 1 accessible Pt sites and an average surface of 43 Á. The dependence of the coverage degree, e b , OIl the adsorption time and benzene COllcen- tration was analyzed by mean s of the Frumkin adsorption model. Howeverl in arder to explain both the adsorption process and the value of the Frumkin interaction factor al the quasichemical approximation developed by Gnidelli for the Frnmkin isothenn was nsed. RESUMEN. En este trabajo se presenta un estudio electroquímico del proceso de adsorción de benceno sobre nn electrodo de Pt policristalino, a partir de solnciones de H,SO, 0.5~! + e.H. x ~I, usando como variables de análisis la concentración de benceno ell solución, el tiempo de contacto y el potencial de adsorción. Se observó que la adsorción ocurre en un amplio intervalo de potencial, incluida una porción de la región de hidrógeno del electrodo de Pt, con un máximo en la región de la doble capa. Los resultados experimentales mostraron que al potencial de máxima adsorción (0.39 V /erh), las molécnlas de benceno son fuertemente arlsorbidas (prom. - t.G~d,= 35.4 kJ Illol- 1 L ocupando en promedio, cada una de ellas, 7.6:i: 1 sitios accesibles de Pt y una superficie de 43 Á. La dependencia del grado de recubrimiento ele la superficie de Pt por benceno eh, con la.. " variables de análisis fue tratada mediante el uso del modelo de adsorción de Frumkin. Sin embargo, tanto el proceso de adsorción C0l110el factor de interacción calculado (ii = -4.3) fueron explicados aplicando la aproximación cuasiquímica desarrollada por Guidelli a la isoterma de Frumkin.

The Frumkin model applied to the adsorption process ... · ofbenzene on aPt electro de from sulfuric acid solutions ... variables de análisis fue tratada mediante el uso del modelo

Embed Size (px)

Citation preview

Page 1: The Frumkin model applied to the adsorption process ... · ofbenzene on aPt electro de from sulfuric acid solutions ... variables de análisis fue tratada mediante el uso del modelo

Investigación Revista Mexicana de Física 41, No. :3 (1995) :386~:395

The Frumkin model applied to the adsorption processof benzene on a Pt electro de from sulfuric acid solutions

M.A. QUIROZ, F. CORDOVA

Instituto de Estudios Avanzados, Universidad de las Américas-PueblaApartado postal 100, Sta. Catal'ina Mártir, 72820 Cholala, Puebla, México

L. SALGADO, M. VINIEGRA

Departamento de Química. Universidad Autónoma Metropolitana-IztapalapaApartado postal 55-534, Og340 México, D.F., México

Y. MEASCentro de Investigación y Desarrollo Tecnológico en Electroquímica, S. C.

Apartado postal 064, Pedro Escobedo 76700 Querétaro, México

Al\'D

G. DÍAZInstituto de Física, Universidad Nacional A utónoma de México

Apartado postal 20-364, 1000 México, D.F., MéxicoRecibido el 29 de noviembre de 1993; aceptado el 8 de diciembre de 1994

ABSTRACT. The electroadsorption of benzene on a polycrystalline Pt electrode from 0.5 M H,SO, +x M C6H6 solutions was studied as a function of the uenzene concentration,time and potential ofadsorption. It was observed that the maximum adsorption takes place in the double layer regioIl,even though the adsorption process can also occur in the hydrogcn rcgion. At the potential of max-imum adsorption the experimental results showed that benzene Illolecules are strongly adsorbedon the Pt surfacel each one occupying about 7.6:i: 1 accessible Pt sites and an average surfaceof 43 Á. The dependence of the coverage degree, eb, OIl the adsorption time and benzene COllcen-tration was analyzed by mean s of the Frumkin adsorption model. Howeverl in arder to explainboth the adsorption process and the value of the Frumkin interaction factor al the quasichemicalapproximation developed by Gnidelli for the Frnmkin isothenn was nsed.

RESUMEN. En este trabajo se presenta un estudio electroquímico del proceso de adsorción debenceno sobre nn electrodo de Pt policristalino, a partir de solnciones de H,SO, 0.5~! + e.H. x ~I,usando como variables de análisis la concentración de benceno ell solución, el tiempo de contacto yel potencial de adsorción. Se observó que la adsorción ocurre en un amplio intervalo de potencial,incluida una porción de la región de hidrógeno del electrodo de Pt, con un máximo en la regiónde la doble capa. Los resultados experimentales mostraron que al potencial de máxima adsorción(0.39 V/erh), las molécnlas de benceno son fuertemente arlsorbidas (prom. - t.G~d,= 35.4 kJIllol-1 L ocupando en promedio, cada una de ellas, 7.6:i: 1 sitios accesibles de Pt y una superficiede 43 Á. La dependencia del grado de recubrimiento ele la superficie de Pt por benceno eh, con la.."variables de análisis fue tratada mediante el uso del modelo de adsorción de Frumkin. Sin embargo,tanto el proceso de adsorción C0l110el factor de interacción calculado (ii = -4.3) fueron explicadosaplicando la aproximación cuasiquímica desarrollada por Guidelli a la isoterma de Frumkin.

Page 2: The Frumkin model applied to the adsorption process ... · ofbenzene on aPt electro de from sulfuric acid solutions ... variables de análisis fue tratada mediante el uso del modelo

TIIE FRUWm,¡ MOllEL APPUEll TO TIIE AllSOHPTION PROCESS OF BENZENE. . . 387

l. INTHOllUCTION

The adsorption of organie eompounds over metallie eleetrodes, from aqueous solutions, hasbeen a subjeet of special attention in eleetroeatalysis. This proeess frequently determinesthe aetivity, seleetivity and reaetion rate of a metal 11--7).Owing to the solvated nature ofthe interfacial region, the eleetroadsorption can be eonsidered as a replaeement reaetionof solvent moleeules in the adsorbed layer by organie moleeules eoming from the bulksolution [8-14]. It is well known that the surfaee eoverage degree depends on the solubilityof the organie eompounds, the adsorption time aud, mainly, on the imposed potential atthe eleetrode [8J.Thercfore, the eleetroadsorption can be properly defined as the ehemisorption of organie

species from the aqueons solntion over metallie eleetrodes at the potential where theproeess is being eontrolled.It has been widely demonstrated that aromatie eomponnds are spontaneously and

irreversibly adsorbed on Pt eleetrodes [15,17]. Important ex-"il" and in-"il" experimentalevidenee strongly suggests that the aromatie 7r-e]eetrons are extensively involved in theinteraetion of the unsaturated moleeules \Vith the metal surfaee [18-21). Moreover, thearomatie-metal interaetions are unimpeded by the particular kind of ehemiea] species ofthe supporting eleetrolyte used 122).From the great number of aromatie eompounds studied, it is important to note the

searee informal ion reported untilno\V about the eleetrosorption eharaeteristies of benzene,eYen Ihough it is the simplest aromatie eompound. The adsorption of benzene labeled \VithC-14 \Vas studied on platinum-plated gold eleetrodes in 1N 112504 and 1I3P04 by lIeilandel al. [23). They found, using this radiotraeer method, that the benzene moleeules \Vereehemisorbed \Vithout dissociation on the surfaee, \Vith the probable loss of the aromatieeharaeler. This subjeet \Vas further investigated by Gileadi el al. [24J using eleetroehemiealmethods. The eorrespondiug adsorption results \Vere eompal'ed \Vith those previously ob-tained by the radiotraeer method, iu order to establish the extent to \Vhich these methodseould give eonsistent results. Although all potentiometrie teehniques agreed \Vith theHlcasufed values of the co\'crage dcgrec of benzcne, it was Bot the case whcn the rf:'sllltsobtained from these teehniques \Vere eompared \Vith those obtained through the radio-traeer technique. The diserepaney \Vas attributed by the authors to the existence on thesurfaee of signifieant concentrations of intel'lnediate radicals, formed dnl'Íng the oxidationof benzene at CO2 at higher positive potentials. It is important to point ont that, in thiscase, the eleclrochemieal methods appear to offer distinct advantages over the radioaeti"emethods. The operational methodology, essentially based on eyclie voltammetry for thestudy of benzene eleetroadsorption on platinum, has been considered in the experimen-tal ehapters of the advanced \Vork in eleetrochemistry published by Gileadi el al. 125].lIoweYer, in this \\'ork no particular treatment \Vas developed with respeet to the partia]oxidation of benzene, it \Vas assumed that under established experimental conditions, theoxidation to CO2 and water is complete. Indeed, this assnmption in addition to thoserequired to validate lhe use of eleelrochemical lechniques for lhe meaSllJ'ement of elee-lrosorption have been confirmed recenlly by l3altrusehat el al. 126-28]. In lhis \\'ork theseauthors ha,'e sho\\'n that lhe preadsorbed benzene can be desorbed \\'ithont modificationin the hydrogen regio n if the negative potential limit is 2 0.1 V, 01' as hydrogenated

Page 3: The Frumkin model applied to the adsorption process ... · ofbenzene on aPt electro de from sulfuric acid solutions ... variables de análisis fue tratada mediante el uso del modelo

388 M.A. QUIROZ ET AL.

product (cyc!ohexane) if the negative potential limit is :s 0.1 Y; more important yet,the desorption process can also occur by benzene oxidation yielding COz and water asthe only detectable products. Even though in these works the adsorption process itselfwas scantly emphasized, the importance of the results to be based on the fact that thebenzene oxidation on polycrystalline platinum electrodes in the oxygen regio n adsorptionled to COz as the final product. This last conc!usion provides the experimental supportrequired to calculate the charge associated with the benzene oxidation and, hence, thesurface concentration of benzene from which a detailed study of the electrosorption processcan be developed.

Until recently, the values of the adsorption parameters had been interpreted on a strictlyphenomenological basis, using the Frumkin adsorption mode!. According to this modelthe dependence of the coverage degree on the electrode potential is due to the change inthe energy of the double-Iayer capacitar which results when water is replaced by organicmolecules having a lower dielectric constant. Furthermore, the conditions under which thebehavior of Frumkin is obtained have been examinated on a non-thermodynamic basis ofmolecular character proposed by Guidelli [14]. This molecular model altempts to predictthe behavior of the Frumkin isotherm at the greater possible range of IJ, ¡.e., the fractionof surface covered, taking into account specifically the infiuence, nature of the interactionsand orientation of the solvent molecules in the metal-solution interface.

Thus, it is to be expected that the molecular interpretation of the experimental dataof adsorption leads to more reliable conc!usions about the interactions at the monolayerwhen molecular models representing nearly an acceptable reality at the interface are used.

Therefore, the aim of this work has been to evaluate the experimental isotherm of thebenzene adsorption on platinum in acidic media by means of a thermodynamic treatmentof the resuits (Frumkin) which is subsequently complemented with non.thermodynamicarguments of molecular character (Guidelli) in order to interpret the corresponding ad-sorption parameters.

2. EXPERIMENTAL

All the experiments were carried out at room temperature in 0.5 M HZS04 solutions assupporting electrol)'te, prepared from sulfuric acid (96% Suprapur Merck) and threefolddistilled water (16 r-Irl water system 1). These solutions were deaerated by bubbling UHPnitrogen gas (99.999% Infra) befare preparing the benzene (99+% Aldrich) solutions, inarder to avoid the organic evaporation. The benzene concentration in solution was variedin the 10-7_10-4 M range.

The working electrode was a polycrystalline Pt wire (99.999% Aldrich), 0.134 cmz

geometrical are a sealed at Pyrex glass, the counter electrode was a graphite rod (EG &G PARC). Hg/HgzS04/KZS04 (satd.) was used as the reference electrode, in the textaH the potentials are reported on the reversible hydrogen electrode (rhe) seale at roomtemperature.

All measurements were made by using an EG & G PARC mode! 273 potentiostat-galvanostat and the voltammograms recorded by means of a GRAPHTEC model \VXllOOX-Y-t plotter.

Page 4: The Frumkin model applied to the adsorption process ... · ofbenzene on aPt electro de from sulfuric acid solutions ... variables de análisis fue tratada mediante el uso del modelo

TIIE FnUMKIN MODEL ArrLlED TO TIIE ADSOnrTION rnOCESS OF BENZENE.. . 389

3. REsul:rs AND DISCUSSION

The adsorption and oxidatioll processes of benzelle were carried out as a function ofbenzene concentration, time and potential of adsorption. A typical voltammogram isobserved in Fig. 1, which is similar to those previously reported [29). The quantity of elec-tricity illvolved in these processes was measured from the correspondillg potentiodynamicprofiles, but some considerations about the method of calculation were taken into accountin each case. The preadsorbed benzene is oxidized at potentials greater than 0.60 V butit was observed that more than one potelltial cyele was required for a complete oxidation,hence, the charge associated with the benzelle oxidatioll, Qo, was calculated from

"(1)

where ,Q?,tO is the charge required for the Pt oxidation in a solution free of benzenealld QPtO the charge involved ill the mutual oxidation of both, benzene and Pt durillgthe ¡-th cyele. Jt was observed, in al! of the experiments, that the total oxidation ofbellzene was achieved in the first five cyeles, ¡.e., n ::;5. On the other hand, the decreasein hydrogen adsorption originated by the benzene adsorption, c.QH, was used in thetraditional way [3,30-33) to measure the coverage degree of the Pt surface by bellzene(lIb) at a given set of experimental conditions

c.QH- --0-'QH (2)

Jn this case, the integration negative limit illto the hydrogell regio n was restricted toO.IOV in arder to avoid the reduction process of preadsorbed benzene [26].The adsorption of benzene from an aqueous solution can be considered to take place

though a replacement reaction

(3)

lIlass transport from the bulk solutions to the metal surface may be expected to be therate-control!ing step if the strncture of the adsorbate does not change in the adsorptionprocess and/or its concentration in solution is very low. Therefore, the benzene coveragedegree wil! vary with the adsorption time as [34]

(4)

where lIeq and IIb(t) are the equilibrium and instantaneous values of fractional coverage ata given benzene concentration, D is the diffusion coefficient (6.6 x 10-6 cm2 S-I [23]), f(is the adsorption coefficient, and rm the maximum surface coverage. Figure 2 shows thisdiffusional control for the mass transport of benzene to the Pt surface at short adsorptiontimes and low concentrations.

Page 5: The Frumkin model applied to the adsorption process ... · ofbenzene on aPt electro de from sulfuric acid solutions ... variables de análisis fue tratada mediante el uso del modelo

390 },t.A.QUIROZ ET AL.

i/t'A

JO

20

10

O

-10

-20

-JO

o

~""" ,I \ lOI \ ••i', - ,

f OOH\, '"

-'"--- ----

IO_----l4--/ EtVlrhe)I

III,

I,,,I ,, IL.J

FIGURE 1. (a) Elfeet oí the benzene adsorption on the hydrogen electrosorptioll. (b) Electroxida-tion process oíbellzene. Dellzene preadsorbed at Ed = 0.30 Vfrhe durillg 600 s írom a 0.5 M 11250,+ 1.0 x 10-2 M G6lf6 solutioll. Dashed curve: bare Pt electrode.

-"""..•- .•...•...- +--.

1.0

eb0.8

0.6

0.4

0.2

0.0O 10 20 30

112t .do

40112/B

FIGURE 2. Adsorption oí benzcllc al 0.39 V/rhe a.."i a functioll oC lhe adsorptioll time al varioushulk concentrationB oí hellzelle: (6) 5 x 10-7 M; (_) 1 X 10-6 M; (x) 1 X 10-5 ~I; (O) 5 x 10-5 M.

Now, the benzene electroadsorption on Pt ill 0.5 i\11l2S04 solution contailling variousamounts of benzcne \Vas analyzetl as a function oí t}¡c adsorption potcntial. For thcsc fUOS

the adsorption time ",as fixed at 600 s, it roughly correspond to the time at which theamount oí adsorbed benzelle remains constant in aH cases. The results oí these experimentsare shown in Fig. 3, where the amount oí adsorbed benzene r, defined as r = Obrm [141and being rm = 3.9 x 10-10 mol cm-2, is plotted against the adsorption potential. The

Page 6: The Frumkin model applied to the adsorption process ... · ofbenzene on aPt electro de from sulfuric acid solutions ... variables de análisis fue tratada mediante el uso del modelo

TIIE FRUMKINMODELAPPLIEDTO TIIE ADSORPTIONPROCESSOF BENZENE... 391

0.50.40.40.3

1.2

~e 1.0"Oe-"-~ 0.8o~

0.6

0.6 0.7E (V/rhe)

FIGURE3. Adsorption of benzene on a Pt electrode fram 0.5MH2SO, + xMe6H6 solutions. Bulkconcentrations of benzene, x: (.) 1 x 10-6 M; (+) 5 x 10-6 M; (xlix 10-5 M; (O) 5 X 10-5 M;(o) 1 X 10-' M. Adsorplion time oC 000 s.

r-E curves have the typical dome-like shape expected for the adsorption of neutral organiccompounds on Pt surfaces [12,14]. The maximum adsorption takes place in the doublelayer region at a potential of 0.39 V which is somewhat negative to the potential of zerocharge (pzc) value of Pt (0.5-0.6 V) in acidic media [231. Therefore, this shift of pzc shouldbe due to the displacement of surface water by the adsorption of benzene molecules ratherthan to the influence of the hydrogen adsorption.From the charge associated with the hydrogen monolayer, the initial number of surface

Pt sites per s<¡.cm was calculated: 110 = 2.5 X 1015 Pt sites cm-2• This value allows us toestimate the number of surface Pt si tes occupied by one molecule of adsorbed benzene,NI'I, from the relation: NI'I = naO Ir. The experimental data aboye analyzed lead to Npt =6.5-8.6. This result indicates that one molecule of adsorbed benzene occupies about 7.6:1:1accessible Pt sites, which is in accordance to the data of Heilant et al. [23] and Stickneyet al. [35). In addition, the average surface occupied by one adsorbed molecule of benzenemay be calculated with the aid of the following equation [151:a = 101616.02 x 1023r, whichwith a r value of 3.9 x 10-10 mol cm-2 results in a a value of 43 A in good agreementwith the values reported from I3ET measurements [36) and theoretical methods [15]. Thea and rm values support the assumption of a ,,-d-bonded benzene molecule Iying flat onthe Pt surface [19,20,36,37].On the other hand, it is well known that the adsorption fram solutions is a relatively

complex phenornenon, which essentially depends on the nature of the interfacial region.The adsorption of organic compoullds on the Pt group metal surfaces has been oftenviewed as a highly irreversible process,i.e., with a very large J( value. Thus, when thesolution from which the adsorption takes place is exchanged for pure solvent, then littleor no desorption occurs. Hence, it is expected that the adsorbed species are preferablydesorbed trough reduction and/or oxidation reactiollS. This is the case cornrnonly found

Page 7: The Frumkin model applied to the adsorption process ... · ofbenzene on aPt electro de from sulfuric acid solutions ... variables de análisis fue tratada mediante el uso del modelo

392 M.A. QUIROZ ET AL.

.o-8(l) o- .~~E!

~.o •(l)

~ -10 ' ..~~g: . -.

•j •

-12 -~...:..- --14

-160.0 0.2 0.4 0.6 0.8 1.0

FIGURE 4. Calculaled ln[C(1 - Bb)/Bb] vs. Bb plol al lhe same pOlenlial adsorplioll and bulkconcentrations oC benzene as in Fig. 2.

in the adsorption studies of aromatic compounds from aqueous solutions, where due tothe strong interaction between the surface electrode and the 11'-€lectrons of the aromaticring, the absorbed layer remains confined to a monolayer.Owing to this fact as well as to the good concentration.coverage correlations found in

many aromatic.metal systems studied [3,14,23,30,31], the adsorption data has often beenfit to an equation of a particular adsorption isotherm. Even though this manner to handlethe experimental data is a controversial subject yet, important experimental and theoricalevidences point out that the electrosorption behaviour can be analyzed by means of theadsorption model of Frumkin [141:

G (~G~s) ae55.5 exp - ----¡¡;¡;- = e exp (1 _ e) , (5)

where (G/55.5) is the mol fraction of the organic compound in the bulk, ~G~s thestandar Gibbs energy of adsorption at zero coverage and a the Frumkin interaction factor.Rearranging Eq. (5) one has

G(l - e) ~GoIn e = ln(55.5) + R:js + ae. (6)

Substitution oC the benzcne adsorption data, obtaincd at the potential oC maximum ad-sorption (0.39 V) as a function of the adsorplion time and iu the concentration range of5 x 10-7-5 X 10-5 M, into Eq. (6) generales the curves of Fig. 4.By least.squares fitting of the plots in Fig. 4 to a straight line, values of ~G~s and

Frumkin interaction factor a are immediately obtained from the corresponding inlercepts

Page 8: The Frumkin model applied to the adsorption process ... · ofbenzene on aPt electro de from sulfuric acid solutions ... variables de análisis fue tratada mediante el uso del modelo

TIIE FIlUM'ON MODEL AI'I'L1ED TO TlIE ADSOIlPTION 1'1l0CESS OF BENZENE. .. 393

TABLE 1. Frumkin parameters obtained as a function oC the bulk concentration oí benzene al thepolenlial of maximulIl adsorpliou (0.39 V).

C/moll-Ia

-l>G~d./kJ 1001-1

5 X 10-7

-4.640.2

1 X lO-o-4.239.0

1 x 10-'-4.333.1

5 x 10-'-4.2 (ñ = -4.3)29.3

aud slopes. The values of a aud l>G~s for various bulk coueenlrations of benzene in lhesolulion are given iu Table 1.

For lhe adsorpliou of aliphalie eompounds on Hg, eonsidered as a physisorplion, usuallylhe a parameler lakes negalives values in the range from -1 lo -3 [12,14,38), bul for lheadsorplion of aromalie eompounds on lhe PI group surfaees lhe information is laeking.Howe\'er, from lhe quasiehemieal approximalion devcloped by Cuidelli for lhe Frumkinisolherm [14]' an attempt lo explain the average value of the a parameter will be madI'as follows. Aeeording lo lhe Cuidelli's approximalion, lhe allraelive water-waler inlerae-lions, IVww, depends on lhe eoverage degree of lhe metal surfaee by organic moleeules,0b' Al low 0b values lhe waler displaeemenl should be restrieted due to the predominaneeof lhe IVww, bul al medium 0b values lhe number of interaeling water-waler pairs hasdeereased suffieienlly lo makc lh~ removal of waler moleeules from the metal surfaee bylhe adsorbing organie moleeules a favourable proeess. This adsorplion model prediels anegalive slope for lhe InfC(1 - 0)/01 vs. ° plOl wilh a minimum vaIue at lhe pOlential ofmaximum eoverage. Therefore, sinee lhis adsorption behaviour is clearly observed in the0.2 :;; Ob :;; 0.8 range of the InlC(1 - 0)/0] vs. ° plOls of Fig. 4, whieh were eonstrueledfrom the 0b values ealculated at the potential of maximum adsorption (0.39 V), then theaverage value of Frumkin iuteraetion factor, ii = -4.3 (Table 1), may be eonsidered asa satisfaetory result. Furthemore, an important feature of this adsorption model is thatit permits a reasonable molecular interpretation for the experimental observation thatthe adsorption of aromatie eompounds, from an aqueous solution, takes place through areplaeement reaction as showed in Eq. (3).It is also known thal reaetiou (3) occurs spontaucously on PI eleclrodes [15]' and

this eonfirmed by the l>G~s values reporled iu Table 1. In addition, when the values ofÓG~ls are plotted as function of In(C/55.5) (Fig. 5), a linear relationship with a slope of2.4 kJ mol-I is observed in the 5 x 10-7-5 x 10-5 ~l range of benzene concentration insolution.

This resull is in agreement with the Frumkin isolherm, [Eq. (5)]' for which a sJope of2.5 kJ mol-1 is predieled.

4. CO:"CLUSIO:"S

On lhe basis of lhe resulls oblaiued we may eonclude that the adsorption of benzene fromsulfuric acid solutions is a spoutaneous process, being its extension strongly dependent onboth the time and the potential of adsorption. At the pOlential of maximum adsorption,0.39 V, this proeess adequately satislies the adsorption model of Frumkin.The Frumkin isotherm is perhaps lhe adsorplion isolherm based on molecular models

Page 9: The Frumkin model applied to the adsorption process ... · ofbenzene on aPt electro de from sulfuric acid solutions ... variables de análisis fue tratada mediante el uso del modelo

394 M.A. QUIROZ ET AL .

•25

-45-20 -18 -16 -14 -12

Ln (C/55.5)

FIGURE5. The dependen ce of the Gibbs energy of adsorption on the mol fraetion of benzene inthe bulk using the equation of the Frumkin isotherm.

more developed nowdays. Our experimental results are finely justified by the Frumkinisotherm whieh also provides a clear idea of the reemplacement proeess of water moleculesby the adsorbing aromatic molecules. The Cuidelli's theoretical treatment for the Frumkinisotherm [14] together with Hubbard's work on the adsorption of aromatic compounds [15-16], are at the present time the most important theorical and experimental support toanalyse and to interpret the adsorption data of aromatic-metal systems that present ahigh irreversibility degree.

ACKNOWLEDGMENTS

This work was financially supported by the Consejo Nacional de Ciencia y Tecnología(0544/E9108 CONACyT) and by Instituto de Estudios Avanzados of the Universidad delas Américas-Puebla, México.

REFERENCES

1. H. Wroblowa and M. Green, Elcetroehimica Acta 8 (1963) 679.2. L.L. and L. Christensen, J. Org. Chcm. 43 (1978) 2059.3. K. Sasaki, A. Kumai, J. lIarada and S. Nak.~hnri, E/cetroehimiea Acta 28 (1983) 671.4. T. Iwasita, W. Vielstich and E. Santos, J. E/cetroana/. Chcm. 229 (1978) 367.5. J. Zhu, Th. lIartung, D. Tegtmeyer, H. Baltrusehat and J. Heitbaum, J. E/cetroana/. Chcm.

24 (1988) 273.6. K. Shimazu and H. Kita, J. Catal. 83 (1983) 407.7. Yu. B. Vassiliev, V.S. Bagotzky, O.A. Khazova, V.V. Cherny and A.M. Meretsky, J. E/ce-

troana/. Chcm. 98 (1979) 253, 273.8. E. Gileadi, J. E/cetroanal. Chcm. 11 (1966) 137.

Page 10: The Frumkin model applied to the adsorption process ... · ofbenzene on aPt electro de from sulfuric acid solutions ... variables de análisis fue tratada mediante el uso del modelo

TIlE FRUMKIN MODEL APPLlEIl TO TIIE ADSOHPTION PHOCESS OF BENZENE. .. 395

9. J.O'M. Dockris, E. Cilcadi and K. Müller, Eleelroehimiea Acta 12 (1967) 1301.10. J.O'M. Doekris, M.A.V. Dcvanathan and K. Müllcr, Proe. R. Soe. London Servo A 274 (1963)

55.11. D.H. Evcrctt, Trans. Faraday Soe. 60 (1964) 1803; 61 (1965) 2478.12. D.D. Damaskin, O.A. Petrii and V.V. Datrokov, Adsorplion oJ Organie Compounds on

Eleetrodes, Plenum Prcss, New York (1971) p. 86.13. S. Trasatti, J. Eleetroanal. Chem. 53 (1974) 335.14. R. Cuidclli, Adsorption oJ Molecttles at Metal Eleetrodes, J. Lip Kowski and 1'.1". Ross (cds.),

VCH Publishers, luc., Ncw York (1992) p. 1.15. ~I.P. Soriaga, 1'.11. \Vilson and A.T. Hubbard, J. Am. Chem. Soe. 104 (1992) 2735, 2742,

3937.16. M.P. Soriaga, J.II. \Vhitc, D. Song aud A.T. Jluhhard, J. Eleetroana!. Chem. 171 (1984) 359.17. C. lIorányi amI E. Rizmaycr, Elcetroehimica Acta 31 (1986) 401.18. P.\\'. Sclwood, J. Am. Chem. Soe. 79 (1957) 4637.19. J.L. Cland and C.A. Somorjai, Adv. Colloid InterJaee Sei. 5 (1976) 205.20. D.~1. llaaland, Surfaee Sei. 111 (1981) 555.21. A.\V. Adamson, Physieal Chemistry oJ SurJaee, \Vilcy Iutcrscicncc, Ncw York (1990) p. 421.22. M.P. Soriaga, J.H. \Vhitc, D. Song and A.T. Hubbard, J. Phys. Chem. 88 (1984) 2284.23. \V. Hciland, E. Cilcadi and J.O'M. Dockris, J. Phys. Chem. 70 (1966) 1207.24. E. Cilcadi, L. Duic and J.O'~1. Dockris, Eleetroehimiea Acta 13 (1968) 1915.25. E. Gileadi, E. Kirowa-Eisller alld J. Penciner, lnlcrfaeial E/cctrochemistTy: An Experimental

Appmaeh, Addison- \Vesley Publishing COlllpany, lnc., Advanccd Dook Program, Reading,Massachusetts (1975) p. 444.

26. U. Schlllielllann and H. DalIrnschat, Eleetroana/. Chem. (1993) (in press).27. T. Hartung and 11. Daltruschat, Langmuir 6 (1990) 953.28. T. Hartnng, V. Schmiemann, 1. Kalllphauscn and H. DalIruschat, Ana/. Chem. 63 (1991) 44.29. M.A. Quiroz, 1.. Salgado, ~1. Viniegra, C. Diaz, Y. ~leas and F. Córdova, Poster prcsentado

at the 43rd ISE Conference, Argentina (1992).30. V.S. Dagotsky and Yu. D. Vassilicv, Eleetroehimiea Acta 11 (1966) 1439.31. P. Zclenay and J. Sohkowski, E/eetroehimica Acta 29 (1984) 1715.32. E. Lammy-Pitara, L. Dcncharif and J. Darbicr, A1'1'1.Catal. 18 (1985) 117.33. D. Dittins-Cattanco, E. Santos, \V. Vielstieh and V. Linke, Eleetroehimica Acta 33 (1988)

1499.34. E. Cileadi, D.T. Rubin and J.O'M. Dockris, J. Phys. Chem. 10 (1965) 3335.35. J.L. Stickney, ~I.P. Soriaga, A.T. Hubbard and S.A. Anderson, J. Eleetroana!. Chcm. 25

(1981) 73.36. A.1.. ~lcC¡cllan and JI.F. Jlarnsbcrger, J. Colloid Sei. 23 (1967) 577.37. J.C. Dertolini amI J. ~lassardier, The Chemieal !'hysies oJ Solid SurJaees and Heterogeneosus

Catalysis Vol. 3, part D., D.A. King and D.P. \Voodruff, eds. Elscvier, New York (1984) p.107.

38. ~1. Carlá, C. Abisi, ~I.R. Moncelli and ~I.L. Foresti, J. Colloid InterJaee Sei. 132 (1989) 72.