a comparative study on the adsorption of benzyl alcohol, toluene and benzene on platinum

Electrochimica Acta 45 (2000) 4279–4289

A comparative study on the adsorption of benzyl alcohol,toluene and benzene on platinum

Jose Luis Rodrıguez, Elena Pastor *Departamento de Quımica Fısica, Uni6ersidad de La Laguna, Calle Astrofisico F.S. s/n, Campus de Anchicta, 38071, La Laguna,

Tenerife, Spain

Received 19 November 1999; received in revised form 27 March 2000

Abstract

The adsorption of benzyl alcohol, toluene and benzene on platinum was studied using cyclic voltammetry combinedwith on-line mass spectrometry (DEMS). Flow cell procedures allow the detection of volatile products formed duringadsorption of these molecules and their displacement with CO, as well as during the oxidative and reductive strippingof the adsorbed layer. Three adsorption potentials were chosen: 0.20, 0.35 and 0.50 V versus reversible hydrogenelectrode (rhe). Toluene and benzene adsorb without dissociation. Total hydrogenation of the ring with formation ofmethyl-cyclohexane and cyclohexane, respectively, was observed suggesting that the aromatic ring of the adsorbedspecies lies on the surface. For benzyl alcohol, the presence of the �OH group favours the dissociative adsorption: therupture of the C�C bond between the ring and the �CH2OH group produces CO and benzene. Hydrogenolysis ofbenzyl alcohol also occurs in the Pt(H) region with formation of toluene. Both adsorbed toluene and benzene frombenzyl alcohol react with H2 producing methyl-cyclohexane and cyclohexane, respectively. Thus, benzene and tolueneformed from benzyl alcohol also seem to adsorb with the aromatic ring parallel-oriented on the surface. © 2000Elsevier Science Ltd. All rights reserved.

Keywords: Aromatic compounds; Platinum electrodes; Adsorption; On-line mass spectrometry

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1. Introduction

The irreversible adsorption of aromatic molecules atthe electrolyte/solid metal interface has been the subjectof many studies in the past few years. Different motiva-tions can be found for this research. Thus, for example,benzene is considered as a model system for the studyof aromatic compounds [1–3], whereas the interest inbenzoic acid is related to its corrosion inhibition prop-erties [4,5]. For the adsorption of these aromatic com-pounds, as well as for more complicated molecules such

as hydroquinone, other quinones or diphenols [6–8], atleast two different orientations are possible at the sur-face: flat-oriented intermediates when all C-atoms ofthe ring are involved in the adsorption (h6); and verti-cally-oriented species interacting only through a doublebond (h2).

The present work is devoted to studying the adsorp-tion of benzyl alcohol (BA) at polycrystalline platinumin acid media. This molecule presents two active centresfor the interaction with the surface: the p-system of thearomatic ring and the alcoholic group. In order toestablish the influence of the latter, toluene (with a�CH3 group instead of the �CH2OH group, thus avoid-ing the interaction through the �OH) and benzene(without the �CH2OH group) have also beeninvestigated.

* Corresponding author. Tel.: +34-922-318028; fax: +34-922-318002.

E-mail address: [email protected] (E. Pastor).

0013-4686/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.

PII: S 0013 -4686 (00 )00561 -2

J.L. Rodrıguez, E. Pastor / Electrochimica Acta 45 (2000) 4279–42894280

The adsorption of benzyl alcohol at Pt(111) has beenstudied by EELS and Auger spectroscopies, showingthat a stable residue was formed which oxidises com-pletely to CO2 [9]. Recently, the influence of the natureof the electrode on the composition of the adsorbedlayer formed from benzyl alcohol has been establishedat platinum and palladium electrodes [10]. For theformer, CO2 is the sole electrooxidation product,whereas for the latter benzene was also detected duringthe anodic stripping of the adlayer. On the other hand,a dissociative reaction leading to the formation ofbenzene and the hydrogenolysis producing toluene wereobserved at both platinum and palladium when benzylalcohol is present in the bulk of the solution [11].However, some differences were apparent between thehydrogenation at these electrodes: complete hydrogena-tion of the aromatic ring was achieved at platinum,whereas only small amounts of fully- and partially-hy-drogenated compounds were detected at the palladiumelectrode [11].

This paper describes the results on the hydrogenationof the adsorbed layer in the absence of BA in the bulkof the solution, obtained by means of differential elec-trochemical mass spectrometry (DEMS) using a flowcell procedure. This technique allows the detection ofgaseous and volatile species formed during the electro-chemical processes. Moreover, the influence of the ad-sorption potential on the nature of the adsorbed layer isalso established and compared with the results fortoluene and benzene. In this way, the combined effect ofthe aromatic ring and the alcoholic group on the ad-sorption reactions can be discussed.

2. Experimental

Solutions were prepared with Millipore-MilliQ pluswater and analytical grade chemicals; 2 mM benzylalcohol, 5 mM toluene and 5 mM benzene solutions inthe base electrolyte (0.1 M HClO4) were used. Fordisplacement experiments, CO was driven through thepure electrolyte for 20 min to obtain a CO-saturatedsolution.

During DEMS experiments, volatile species generatedat the electrode evaporate at the pores of the membraneinto the vacuum, and are detected by the mass spec-trometer with a time constant of ca. 1 s. This timeconstant is small enough to allow mass spectrometriccyclic voltammograms (MSCVs) for selected masses tobe recorded in parallel to cyclic voltammograms (CVs)at a scan rate of 0.01 V s−1. Appropriate mass to chargeratios (m/z) have to be selected for this purpose.

A small flow cell containing approx. 2 cm3 electrolytesolution was used. In this cell, solution exchange wascarried out holding the potential control on the workingelectrode. The latter was prepared by sputtering a thin

platinum layer onto a hydrophobic membrane (ScimatLtd., average thickness 60 mm, porosity 50%, mean poresize 0.17 mm). This membrane acts as the interfacebetween the solution and the vacuum chamber. Thegeometric area of the electrode was 0.64 cm2 and thereal area varied between 10 and 20 cm2. The referenceelectrode was a reversible hydrogen electrode (rhe) inthe electrolyte solution, and a platinum wire served ascounter electrode. More details about the DEMS tech-nique have been given elsewhere [12,13].

All experiments were carried out at room tempera-ture. Solutions were deoxygenated with purified argon(99.998%).

2.1. Experimental procedure

Each adsorption experiment consisted of the follow-ing steps: (1) after activation of the electrode, thepotential was set to the adsorption potential Ead; (2) thesupporting electrolyte was replaced by the organic-con-taining solution, and the corresponding current andmass transients for selected m/z ratios were recorded; (3)after 5 min, the solution was completely replaced bypure base electrolyte; (4) positive- or negative-goingpotential scans were recorded at 0.01 V s−1, and theelectrooxidation or reduction products from the ad-sorbed residues were identified with the MSCVs.

In those experiments in which displacement of previ-ously adsorbed benzyl alcohol, benzene or toluene wasattempted with CO, the procedure described above forthe adsorption of the organic was repeated for carbonmonoxide. That is, first the adsorption of the organiccompound was performed following steps (1)–(3). Thenthe CO-containing solution was admitted into the cellfor 5 min. Simultaneously, faradic current and ioncurrent transients for appropriate m/z values wererecorded. Finally, the CO-containing solution was re-placed again by several rinses with the base electrolytesolution and the stripping of the residues was carriedout.

Mass charge densities involved in the adsorption,oxidation and reduction processes can be calculated byintegration of the corresponding mass transients orMSCVs.

Mass to charge ratios chosen for investigated com-pounds represent the most intense fragment (mainpeak) according to the expected fragmentations [14,15].Thus, the following values were selected: m/z=91 fortoluene ([C6H5CH2]+), m/z=78 for benzene ([C6H6]�+),m/z=83 for methyl-cyclohexane ([C6H11]+), m/z=56for cyclohexane ([C4H8]+), m/z=81 for methyl-cyclo-hexene ([C6H9]+) and m/z=67 for cyclohexene([C5H7]+). For the case of cyclohexane, the signal form/z=84, which corresponds to its molecular peak([C6H12]�+), can also be used due to its high intensity inthe mass spectrometer.

.L. Rodrıguez, E. Pastor / Electrochimica Acta 45 (2000) 4279–4289 4281

Fig. 1. Mass transients recorded during adsorption of benzylalcohol at different Ead. (A) Production of benzene (m/z=78).(B) Production of toluene (m/z=91).

3. Results and discussion

3.1. Interaction of benzyl alcohol with the platinumsurface. Benzyl alcohol adsorbates

3.1.1. Adsorption and direct electrooxidation of theresidues

Fig. 1 shows the time dependence for the signalsrelated to benzene (m/z=78, [C6H6]�+) and toluene(m/z=91, [C6H5CH2]+) recorded during admission ofthe 2 mM benzyl alcohol solution at different Ead (step(2) in the experimental procedure). No other com-pounds were detected. Toluene is formed only when thesurface is covered by Had (this is the situation at 0.20V), whereas benzene is produced at Ead=0.20 and 0.35V, its yield decreasing as the potential is set to morepositive values. From these results, two different pro-cesses can be distinguished:1. The interaction of the �CH2OH group with Pt(H),

producing the hydrogenolysis of the molecule withthe formation of toluene:

C6H5CH2OH+2Pt(H)�C6H5CH3+2Pt+H2O(1)

2. The dissociation of BA with the production ofbenzene:

C6H5CH2OH+Pt(H)�C6H6+ (C1)Pt (2)

C1 species was not detected by DEMS, and therefore,it has to stay adsorbed at the Pt surface. These equa-tions try to describe the global processes but do notrepresent the detailed mechanism.

However, transients in Fig. 1 do not give informationabout the adsorbed species, assuming that the masssignals are related with bulk processes (for adsorptionreactions, the responses for selected m/z ratios areexpected to decrease to the ground signal level afterseveral seconds, but in this case they extend until theorganic-containing solution is replaced by the support-ing electrolyte).

Direct oxidation of the adsorbates remaining on thesurface after electrolyte exchange produces only CO2

([CO2]�+, m/z=44) (see Fig. 2, solid and dashed linesfor Ead=0.50 and 0.20 V, respectively). Five cycles upto 1.50 V are necessary for the complete stripping of theadsorbed layer [10], although only the first one is givenin Fig. 2 for the sake of clarity. The CVs (Fig. 2A, solidand dashed lines) show that the residues are mainlyoxidised in the platinum oxide region with a contribu-tion centred at 1.30 V. However, three features can beclearly distinguished in the MSCVs (Fig. 2B, solid anddashed lines), two of them in the double layer region ofplatinum: one in the 0.50–0.90 V potential range dur-ing the positive-going potential scan and the second

Fig. 2. Electrooxidation of the residues obtained from BAafter adsorption from a 5 mM solution in 0.1 M HClO4; solidline, direct oxidation after adsorption at Ead=0.50 V; dashedline, direct oxidation after adsorption at Ead=0.20 V; anddotted–dashed line, oxidation of the adsorbates formed atEad=0.20 V after four cycles in the Pt(H) region. (A) CVs and(B) MSCVs for CO2 (m/z=44); dotted line, CV in puresupporting electrolyte.


Fig. 3. Time dependence for the mass signal detected duringdisplacement of BA residues with CO at different adsorptionpotentials. (A) Mass signal for toluene (m/z=91) and (B)mass signal for benzene (m/z=78).

potentials. Mass transients in Fig. 3 confirm that tolu-ene and benzene are formed not only from bulk BAmolecules as shown in Fig. 1, but also remain adsorbedat the surface after solution exchange. Then, we canwrite the adsorption of these compounds:

C6H5CH3+Pt� (C6H5CH3)Pt (3)

C6H6+Pt� (C6H6)Pt (4)

and the displacement with CO as:

(C6H5CH3)Pt+CO� (CO)Pt+C6H5CH3 (5)

(C6H6)Pt+CO� (CO)Pt+C6H6 (6)

The electrooxidation of the species at the surfaceafter adsorption of CO (not shown) suggests that at0.20 V most of the residues are displaced. Increasingthe adsorption potential, a contribution in the PtOregion is apparent confirming that species from BAremain coadsorbed with CO (see Fig. 8 in Ref. [14]).Therefore, it is possible that the amount of displacedbenzene diminishes with the potential only because itcannot be replaced by CO at more positive Ead, i.e. Eq.(6) is not completely shifted to the right. This result willbe compared later with the adsorption experimentsdirectly performed with benzene.

3.1.3. Electroreduction of the residuesElectroreduction of the residues formed from BA was

obtained by potential cycling in the hydrogen region.C7- and C6-hydrocarbons have been detected as de-picted in Figs. 4 and 5, respectively. At least four cyclesin the range Ead–0.0 V are necessary for the totaldesorption of reducible adsorbates. Only the first po-tential cycle is shown in the figures. Toluene (m/z=91,Fig. 4B) is desorbed at EB0.20 V and its productiondiminishes from Ead=0.20 to 0.50 V. The detection oftoluene at the latter adsorption potential cannot berelated with the desorption of adsorbed toluene (inverseof Eq. (3)), taking into account that no hydrogen ispresent on the platinum surface at this Ead. Moreover,no toluene was displaced with CO at Ead=0.50 V (Fig.2). Therefore, this production should be originatedfrom other molecule, probably from intact benzyl alco-hol molecules adsorbed at Ead=0.50 V which sufferhydrogenolysis during the potential scan down to 0.0 V:

(C6H5CH2OH)Pt+Pt(H)�C6H5CH3+2Pt+H2O(7)

The potential of maximum intensity for the signal form/z=91 coincides with the onset for other mass tocharge ratios. Thus, hydrogenation of toluene tomethyl-cyclohexene (m/z=81, [C6H9]+) and methyl-cy-clohexane (m/z=83, [C6H11]+) is established for EB0.10 V just in the potential region for H2 evolution(Fig. 4C and D, respectively). The global process can bewritten as follows:

centred around 0.75 V during the negative sweep. Themain contribution to the CO2 signal is observed duringthe positive run, attaining the maximum mass intensitysimultaneously with the maximum current in the CVs.The amount of CO2 evolved, calculated by integratingthe MSCVs for the five potential cycles, is almostconstant for the different Ead. However, the contribu-tion in the double layer region during the first positive-going scan diminishes at Ead=0.50 V, probablybecause at this potential oxidation to CO2 of the grouprelated with this feature partially occurs.

3.1.2. Displacement with COIn order to get some additional information about

the nature of the adsorbed species, the adsorption ofCO was performed at the same Ead after adsorption ofbenzyl alcohol and electrolyte replacement. During theintroduction of the CO-saturated solution, differentm/z values were followed in order to identify thoseadsorbed species which can be displaced. Thus, toluene(m/z=91) and benzene (m/z=78) were detected. Thesemass transients decay to the background intensity inapprox. 90 s. No other time-dependent m/z ratios wereobtained. Toluene is produced only at Ead=0.20 V(Fig. 3A), whereas the yield of benzene (Fig. 3B) de-creases as the adsorption potential is set to more posi-tive values, detecting a very small signal at Ead=0.50V. The absence of a time-dependent response for m/z=91 at Ead=0.35 and 0.50 V, is easily explained takinginto account the lack of adsorbed hydrogen at these

J.L. Rodrıguez, E. Pastor / Electrochimica Acta 45 (2000) 4279–4289 4283

Fig. 4. Electroreduction of the residues obtained from BAafter adsorption from a 5 mM solution in 0.1 M HClO4 atEad=0.20 V (solid line) and Ead=0.50 V (dashed line). (A)CVs and (B), (C) and (D) MSCVs for toluene (m/z=91),methyl-cyclohexene (m/z=81) and methyl-cyclohexane (m/z=83), respectively.

C6H5CH3+2H2�C6H9CH3+H2�C6H11CH3 (8)

The formation of methyl-cyclohexane was confirmedthrough the signal for m/z=98 ([C6H11CH3]�+, notshown in the figure). It has to be noticed that the signalfor methyl-cyclohexene is a factor of 10 smaller thanfor methyl-cyclohexane suggesting that only smallamounts of the partially hydrogenated product isformed.

Benzene is desorbed (inverse of Eq. (4)) for EB0.35V (Fig. 5B). The reduction of benzene leads to cyclo-hexane (m/z=56, [C4H8]+, Fig. 5D) and traces ofcyclohexene (m/z=67, [C5H7]+, Fig. 5C) in the hydro-gen evolution region:

C6H6+2H2�C6H10+H2�C6H12 (9)

The signal for m/z=56 could also be considered as acontribution of a methyl-cyclohexane fragment. How-ever, its intensity is higher than that expected from thefragmentation of this compound. Furthermore, the for-mation of cyclohexane was confirmed through the sig-nal for m/z=84 ([C6H12]�+, not shown).

From Figs. 4 and 5, it should be noticed that theamount of C7-hydrocarbons decreases changing Ead

from 0.20 to 0.50 V, opposite to the behaviour observedfor C6-hydrocarbons.

The electrooxidation of the adsorbates remaining atthe surface after potential cycling in the Pt(H) region(Fig. 2, dotted–dashed line), evidences an increase inthe signal for CO2 in the 0.50–0.90 V potential rangeand a drastic decrease for the contribution in the PtOregion. Thus, it can be assumed that the �CH2OHgroup is retained at the surface, at least in part, duringreduction of the adlayer, i.e. fragmentation of initiallyadsorbed C7-species also occurs during potential cyclingdown to 0.0 V. Comparing the integrated mass signalsfor m/z=44 from Fig. 2B dashed and dotted–dashedcurves, it is established that 55–60% of CO2 is lostduring the reduction process.

Charge densities related to the formation of tolueneand benzene during displacement with CO and duringpotential cycling in the hydrogen region are given inTable 1. From this table, it can be deduced that tolueneis present at the surface only at Ead=0.20 V, but it canbe formed from other adsorbates during reduction assuggested before. On the other hand, the amount ofdisplaced benzene decreases as the potential is set tomore positive values, opposite to the trend observedduring the reduction. Previously, it has been describedthat at Ead=0.20 V, most aromatic species are des-orbed during CO-admission. The yield of benzene dis-placed at this Ead is five times the amount desorbedduring potential cycling in the 0.20–0.0 V region of theresidues formed at the same potential. According tothese results, it can be established that benzenemolecules present on the surface at Ead=0.20 V cannot

Fig. 5. Electroreduction of the residues obtained from BAafter adsorption from a 5 mM solution in 0.1 M HClO4 atEad=0.20 V (solid line) and Ead=0.50 V (dashed line). (A)CVs and (B), (C) and (D) MSCVs for benzene (m/z=78),cyclohexene (m/z=67) and cyclohexane (m/z=56), respec-tively.


Table 1Charge density corresponding to different m/z values detected during potential cycling in the hydrogen region (red.) anddisplacement (displ.) of benzyl alcohol adsorbatesa

Charge density (C cm−2 1014)Ead/V

m/z=91 (red.) toluene m/z=78 (displ.) benzene m/z=78 (red.) benzenem/z=91 (displ.) toluene

3.20.20 10.35.3 2.31.7 3.6 8.50.35 0.01.5 0.6 10.00.00.50

a For details see text.

be completely stripped during potential scans down to0.0 V. The increasing charge density of benzene pro-duced during potential cycling in the Pt(H) region withEad, contrasts with the decrease of the mass chargeinvolved in the transients for m/z=78 in Fig. 3B. Thesedifferences will be discussed later.

3.2. Interaction of toluene with the platinum surface.Toluene adsorbates

3.2.1. Direct electrooxidation of the residuesSimilar experiments were performed using a 5 mM

toluene solution in 0.1 M HClO4. CVs and MSCVs forthe direct electrooxidation of toluene adsorbates (Fig.6, solid and dashed lines for Ead=0.50 and 0.20 V,respectively), display similar features to those observedfor BA. The main difference is related with the firstcontribution to the CO2 signal in the double layerregion of Pt, which increases with the adsorption poten-tial being absent at Ead=0.20 V. This observationcould be related to a partial oxidation of the �CH3

group during adsorption as Ead is fixed more positive,allowing the oxidation of this group to CO2 to occureasily, i.e. at lower potentials during the positive-goingpotential scan. Also for toluene adsorbates, five cyclesare necessary for the complete oxidation of the residues.

3.2.2. Displacement with CODuring displacement experiments with CO, only tolu-

ene (m/z=91) was detected, the intensity of the masssignal decreasing abruptly as the potential is increased(Fig. 7). Then, intact toluene molecules seem to be themain adsorbed species (Eq. (5)). However, the strippingof the residues formed during this process (not shown)indicates that even at Ead=0.20 V, the adsorbateswhich oxidise in the platinum oxide region are notcompletely displaced. For Ead=0.20 V, this amount issmall, but for Ead=0.50 V, the same anodic currentand CO2 intensity as in Fig. 6 were observed in thepotential region 0.90–1.50 V after interaction with CO.Therefore, the adsorbate formed at this Ead cannot bedisplaced with carbon monoxide.

3.2.3. Electroreduction of the residuesFinally, the reduction of the adlayer has been per-

formed (Figs. 8 and 9). C7- (Fig. 8) and C6-hydrocar-bons (Fig. 9) were detected as for BA. The differenceswith the latter are the absence of benzene (m/z=78,Fig. 9B) and the low ion current for cyclohexane (m/z=84, Fig. 9C). This fact and the time-independentsignal obtained for benzene during the admission ofCO, indicate that toluene has a low trend for thedissociation of the C�C bond between the aromatic ringand the �CH3 group when adsorbed at the platinumelectrode. Methyl-cyclohexane (m/z=83, Fig. 8D) isthe main reduction product, although traces of methyl-

Fig. 6. Electrooxidation of the residues obtained from tolueneafter adsorption from a 5 mM solution in 0.1 M HClO4; solidline, direct oxidation after adsorption at Ead=0.50 V; dashedline, direct oxidation after adsorption at Ead=0.20 V; anddotted–dashed line, oxidation of the adsorbates formed atEad=0.20 V after four cycles in the Pt(H) region. (A) CVs and(B) MSCVs for CO2 (m/z=44); dotted line, CV in puresupporting electrolyte.


Fig. 7. Time dependence for the mass signal for toluene(m/z=91) detected during displacement of toluene residueswith CO at different adsorption potentials.

signal for m/z=91 is shown at two Ead in Figs. 8 and9. Fig. 8B shows that desorption of toluene commencesat approx. 0.35 V for Ead=0.50 V, similar to the onsetpotential for benzene from BA (m/z=78, Fig. 5B).Comparing the shape of the curves in Fig. 8B and Fig.4B, the difference in the onset potential for the signalm/z=91 confirms that toluene is a reduction productfrom BA residues formed at Ead=0.50 V and it is notinitially adsorbed at this adsorption potential. Theamount of desorbed toluene during electroreductiondecreases to half at Ead=0.20 V in Fig. 8B. However,the opposite trend is observed for the amount of CO-displaced toluene (Table 2). Similar behaviour was alsodescribed for benzene formed during the adsorption ofBA (Table 1). These observations accord with the for-mation of partially oxidised toluene during adsorptionat Ead=0.35 and 0.50 V, which can be desorbed duringa potential excursion down to 0.0 V, but cannot bedisplaced with CO.

The anodic stripping of the residues remaining at thesurface after potential cycling in the Pt(H) region showsno contribution for the mass signal m/z=44 in thedouble-layer range, and results in a decrease of 65–70%in the CO2 signal with respect to the direct oxidation inSection 3.2.1 (compare dashed and dotted–dashedcurves in Fig. 6B).

cyclohexene (m/z=81, Fig. 8C) were also detected. Theintegrated mass signals for these compounds are practi-cally constant in the whole potential range, as can beobserved in Table 2. All these species are detected atEB0.1 V simultaneously with H2 evolution.

Dependence of the mass charge density on Ead is onlyobserved for toluene. For this reason, only the mass

Fig. 8. Electroreduction of the residues obtained from tolueneafter adsorption from a 5 mM solution in 0.1 M HClO4 atEad=0.20 V (solid line) and Ead=0.50 V (dashed line). (A)CVs and (B), (C) and (D) MSCVs for toluene (m/z=91),methyl-cyclohexene (m/z=81) and methyl-cyclohexane (m/z=83), respectively.

Fig. 9. Electroreduction of the residues obtained from tolueneafter adsorption from a 5 mM solution in 0.1 M HClO4 atEad=0.20 V (solid line). (A) CVs and (B), (C) and (D)MSCVs for benzene (m/z=78), cyclohexene (m/z=67) andcyclohexane (m/z=84), respectively.


Table 2Charge density corresponding to different m/z values detected during potential cycling in the hydrogen region (red.) anddisplacement (displ.) of toluene adsorbatesa


m/z=91 (red.) toluene m/z=83 (red.) methyl-cyclohexane m/z=84 (red.) cyclohexanem/z=91 (displ.) toluene

3.70.20 6.08.6 0.37.1 7.2 0.40.35 3.66.8 7.5 0.60.00.50


3.3. Interaction of benzene with the platinum surface.Benzene adsorbates

3.3.1. Direct electrooxidation of the residuesThe anodic stripping of the residues obtained after

adsorption from a 5 mM benzene+0.1 M HClO4

solution can be seen in Fig. 10, solid and dashed curves.The CO2 signal develops two features, the most intensein the positive-going potential scan attaining a broadpeak at 1.28 V, and a small contribution in the nega-tive-going sweep centred at 0.75 V (Fig. 10B). Compar-ing these MSCVs with those in Fig. 2B and Fig. 6B forBA and toluene adsorbates, respectively, the main dif-ference concerns the absence of a contribution to thesignal for m/z=44 in the 0.50–0.90 V potential rangeduring the positive run (the onset potential for m/z=44in Fig. 10B is around 0.75 V). Then, the production ofCO2 in this potential range can be undoubtedly as-signed to the oxidation of the �CH2OH or �CH3 groupsof BA and toluene, respectively.

The oxidation of benzene adsorbates also needs sev-eral (five) cycles. The aromatic ring is quite difficult tooxidise, and therefore, it is responsible for the similarbehaviour obtained during the electrooxidation of theadlayer formed from the three organic molecules.

3.3.2. Displacement with COContacting the platinum electrode covered with a

monolayer of benzene residues, with a CO-saturatedelectrolyte solution, results in the replacement of theadsorbates. Simultaneously, the signal for m/z=78sharply increases indicating the desorption of benzene(Fig. 11). The integration of the mass transients showsthat the amount of benzene evolved decreases as Ead isset to more positive values. Conversely, potential excur-sions up to 1.50 V show that the contribution in theplatinum oxide potential region (due to the aromaticresidue), increases with Ead (not shown, see Fig. 5 inRef. [16]). This result suggests that the interaction ofthe aromatic ring with the platinum surface is strength-ened as Ead is increased. In this way, CO cannotdisplace benzene molecules adsorbed at Ead=0.50 V,i.e. Eq. (6) is not favoured to occur to the right. The

same behaviour has been previously described for BAand toluene residues.

3.3.3. Electroreduction of the residuesPotential cycling in the Pt(H) region produces both

the desorption of intact benzene molecules (Fig. 12B)and the hydrogenation of the aromatic ring with theformation of cyclohexene (m/z=67, Fig. 12C) andcyclohexane (m/z=84, Fig. 12D). The latter reactiontakes place in the H2 evolution region. The integrationof the mass signals for m/z=67 and 84 (Table 3),shows that the corresponding charge density is indepen-

Fig. 10. Electrooxidation of the residues obtained from ben-zene after adsorption from a 5 mM solution in 0.1 M HClO4;solid line, direct oxidation after adsorption at Ead=0.50 V;dashed line, direct oxidation after adsorption at Ead=0.20 V;and dotted–dashed line, oxidation of the adsorbates formed atEad=0.20 V after four cycles in the Pt(H) region. (A) CVs and(B) MSCVs for CO2 (m/z=44); dotted line, CV in puresupporting electrolyte.


Fig. 11. Time dependence for the mass signal for benzene(m/z=78) detected during displacement of benzene residueswith CO at different adsorption potentials.

Ead=0.20 V is smaller (approx. 0.5) than at Ead=0.35and 0.50 V. Comparing these data with those fordisplaced benzene, it can be established that theamount of benzene displaced with CO is not propor-tional to the amount of benzene on the surface. It ispossible that a strengthening of the p-interaction as Ead

is shifted in the positive direction, i.e. an increase in theelectron donation from the ring to the Pt surface, is thecause for the decrease in the intensity of the masstransients in Fig. 10. On the other hand, benzenedesorbs during potential cycling in the hydrogen regionbecause the donation of p-electrons probably dimin-ishes around the pzc of the Pt surface and even more atthose potentials negative to the pzc. The same explana-tion can be assumed for the behaviour observed fortoluene adsorbates in the previous section.

It has to be mentioned that the anodic stripping ofthe residues after the electroreduction processes (Fig.10, dotted–dashed line), demonstrates that a small partof the residue remains at the surface (about 15%) evenif the number of potential cycles in the hydrogen regionis increased.

3.4. Comparati6e discussion

The comparative analysis of the results for the ad-sorption of BA, toluene and benzene let us establishsome general trends as well as some specific characteris-tics of the interaction of these aromatic compoundswith platinum.

It is observed that residues formed at Ead=0.20 Vare completely displaced with CO. Thus, from theseexperiments, it is possible to establish, at least in part,the nature of the adsorbates. The intact molecule isdesorbed from benzene and toluene residues. From BA,both toluene and benzene are detected indicating thattwo different reactions take place during adsorption:hydrogenolysis with production of toluene, and frag-mentation of the C�C bond between the aromatic ringand the alcoholic group for the formation of benzene.The absence of the latter from toluene adsorbates dur-ing admission of CO confirms that the fragmentation ofthe C7-molecule is related to the presence of the �OHgroup in BA.

Interaction of CO with the adlayers formed at Ead=0.35 and 0.50 V shows that carbon monoxide displacesonly part of the aromatic residues at these potentials. Apossible explanation for this fact is that the aromaticring is partially oxidised during adsorption as Ead isincreased. Thus, it can be assumed that the aromaticring donates electrons to the platinum surface, thedonation increasing with the potential. In this way, thestrength of the interaction is enhanced and CO cannotcompletely displace these molecules. This is the case forbenzene residues. For toluene and BA adsorbates, par-tial oxidation of the �CH3 or �CH2OH groups seems to

dent of the adsorption potential. Therefore, the ioncurrents for m/z=67 and 84 at only one Ead arenecessary to be shown in Fig. 12.

On the other hand, benzene desorbs during the nega-tive potential excursion at EB0.35 V. According tothis fact, the integrated mass signal for m/z=78 for

Fig. 12. Electroreduction of the residues obtained from ben-zene after adsorption from a 5 mM solution in 0.1 M HClO4

at Ead=0.20 V (solid line) and Ead=0.50 V (dashed line). (A)CVs and (B), (C) and (D) MSCVs for benzene (m/z=78),cyclohexene (m/z=67) and cyclohexane (m/z=84), respec-tively.


occur during adsorption. Accordingly, the yield of dis-placed species with CO in Tables 1–3 diminishes withEad because of the enhancement of the interaction ofthe residues with the Pt surface, rather than a decreasein the amount of species present on the surface.

The oxidation of the adsorbed layer yields CO2 in allcases. The contribution for the signal m/z=44 in thePtO region is associated with the oxidation of thearomatic ring as shown from benzene experiments. Thering is not oxidised in one potential cycle up to 1.50 V,and five potential cycles are required for the threeorganic substances. The production of CO2 during thefirst potential cycle in the region 0.50–0.90 V is relatedto the oxidation of the �CH3 or �CH2OH groups fortoluene and BA, respectively (no potential-dependentsignal for m/z=44 is detected in this potential rangefor benzene). It should be mentioned that this is just thepotential region for the oxidation of COad andmethanol residues, which suggests that, at least for BAat Ead=0.20 V, CO is one of the adsorbates. Thepresence of COad at Pt in a BA-containing solution hasbeen established by FTIR [17].

Further information on the adsorbed species is ob-tained from potential cycling of the residues in thePt(H) region. The main reduction products from tolu-ene and benzene adsorbates were methyl-cyclohexaneand cyclohexane, respectively. Therefore, total hydro-genation of benzene and toluene seems to occur quiteeasily at a platinum electrode. This result suggests thatboth toluene and benzene adsorb through the aromaticring, i.e. involving a h6 interaction. With this orienta-tion, total hydrogenation should be favoured. However,small amounts of partially hydrogenated compounds(methyl-cyclohexene and cyclohexene, respectively)were detected. Soriaga et al. have suggested an edgewise(h2) orientation in order to explain the partial hydro-genation of hydroquinone to cyclohexanol [7]. This wasbased on the argument that the double bond which ismost distant from the Pt surface, is not accessible forhydrogenation. This explanation is also valid for ourexperiments.

Previous studies on the adsorption of benzene atplatinum surfaces reported that no partially hydro-

genated hydrocarbons were detected during benzenehydrogenation [1–3]. In these papers smooth polycrys-talline or monocrystalline Pt electrodes were used,whereas in the present work a porous layer was em-ployed. However, the effect of the surface roughness,which is higher for the porous electrode, should be theinhibition of the adsorption in the vertical (h2) orienta-tion [8]. That is, the amount of partially hydrogenatedcompounds should be higher for smooth polycrystallineplatinum, but this is not the case. Then, the differenceobserved has to be mainly related to a problem ofsensitivity of the DEMS instruments used in Refs [1–3],considering that only traces have been detected in thepresent paper.

Another explanation for the formation of partialhydrogenated species is that the hydrogenation takesplace in a stepwise mechanism, methyl-cyclohexene andcyclohexene being the intermediates during the forma-tion of the total hydrogenated compounds methyl-cy-clohexane and cyclohexane, respectively.

For the case of BA adsorbates, a series of reductioncompounds were detected during the cathodic scanfrom Ead down to 0.0 V: products which conserve theC7-structure (toluene, methyl-cyclohexene and methyl-cyclohexane), and compounds which have lost the alco-holic group, and therefore, present only the C6-ring(benzene, cyclohexene and cyclohexane). No BA isdesorbed during potential cycling in the hydrogen re-gion. The same procedure applied to the adsorbatesresulting from the interaction with toluene has shownthat no benzene is formed and only traces of cyclohex-ene and cyclohexane were detected. Moreover, displace-ment with CO has demonstrated that toluene does notsuffer fragmentation during adsorption. From theseresults, it can be established that the fragmentation ofthe molecule during potential cycling in the hydrogenregion is favoured for BA residues opposite to tolueneadsorbates.

Partial (cyclohexene and methyl-cyclohexene) or total(cyclohexane or methyl-cyclohexane) hydrogenation ofBA adsorbates can be explained in terms of the samearguments discussed before for toluene and benzeneadsorbed species. Then, similar orientations as for the

Table 3Charge density corresponding to different m/z values detected during potential cycling in the hydrogen region (red.) anddisplacement (displ.) of benzene adsorbatesa


m/z=78 (displ.) benzene m/z=78 (red.) benzene m/z=84 (red.) cyclohexane m/z=67 (red.) cyclohexene

0.61.7 0.70.20 0.071.4 1.3 0.9 0.090.350.70.50 0.81.2 0.07



latter compounds should be expected for the adspeciesfrom BA.

Finally, the desorption of benzene and toluene fromBA adlayers formed at Ead=0.50 V has to be consid-ered. Benzene is detected from BA during the negativesweep for EB0.35 V, as observed also from benzeneadsorbates, but at Ead=0.50 V the amount of benzenedisplaced with CO is small because of the strong inter-action with a positively charged Pt surface. However,during the negative scan this interaction decreases asthe potential approximates to the pzc and initiallyadsorbed benzene desorbs. For the case of toluenedetected from BA in these experiments, the explanationis different. At Ead=0.50 V no (H)ad is present at thePt surface, and therefore, toluene cannot be formed atthis adsorption potential. Accordingly, no toluene isdisplaced during CO admission. Interaction of a differ-ent adsorbate with Pt(H) during the excursion down to0.0 V, results in the production of toluene for EB0.20V (prior to the evolution of H2). The onset for thedesorption of toluene adsorbed from toluene at Ead=0.50 V is 0.35 V, much more positive than the 0.20 Vobserved from BA. This new adsorbate could be intactBA molecules which suffer hydrogenolysis at EB0.20V.

4. Conclusions

Differential electrochemical mass spectrometry(DEMS) has been shown to be an especially suitabletechnique for the straightforward detection of volatilespecies produced during electrochemical reactions ofadlayers at monolayer levels. Adsorbates formed frombenzyl alcohol, toluene and benzene oxidise to CO2 andcan be reduced to fully-hydrogenated hydrocarbons.Thus, cyclohexane and methyl-cyclohexane are themain reduction products from benzene and toluene,respectively. For the case of benzyl alcohol, hy-drogenolysis and dissociation of the molecule occur,and therefore, toluene and benzene as well as the totalhydrogenated hydrocarbons, cyclohexane and methyl-cyclohexane, were detected. Only small traces of par-tially hydrogenated compounds were observed frombenzene, toluene and BA adlayers.

Displacement with CO allows the characterisation ofthe adsorbed species. Benzene and toluene adsorbmainly without dissociation. However, for benzyl alco-hol, the presence of the OH in the molecule favours therupture of the C�C bond between the aromatic and thealcoholic groups. Thus, benzene, CO and toluene seem

to be the main adsorbed species. Also intact benzylalcohol molecules could be present for Ead]0.35 V.

In all cases, a h6 interaction with the aromatic ringparallel to the surface is suggested, thus allowing thetotal hydrogenation of the ring to take place. However,small amounts of h2-adsorbates cannot be disregardedand could be responsible for the production of traces ofpartially hydrogenated hydrocarbons.

Acknowledgements

The authors gratefully acknowledge the Gobierno deCanarias for financial support of this work (project PI1999/071).

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a comparative study on the adsorption of benzyl alcohol, toluene and benzene on platinum

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