Adsorption and Electrooxidation of Ethylene on Au Surfaces

O comportamento eletroquímico na oxidação do etileno em superfícies de Au preferencialmente orientado é investigado em solução aquosa 1 M H2SO4 a 25 °C. Uma onda quadrada de potencial simétrica e periódica é aplicada a eletrodos policristalinos de Au para obter superfícies orientadas dos tipos (111), (110) e (100), as quais são caracterizadas por voltamogramas cíclicos e micrografias SEM. É comprovado que o etileno se adsorve em potenciais ligeiramente inferiores ao potencial de carga zero da in ter face Au/1 M H2SO4, e dois resíduos são detectados num experimento de “strip ping” anódico. A eletro-oxidação voltamétrica do etileno adsorvido e as curvas de polarização anódicas do etileno no meio da solução são medidas nas diferentes superfícies orientadas do Au.


In tro duc tion
Elec tro chem i cal ox i da tion of dou ble and tri ple bonded or ganic sub stances on no ble met als has been ex ten sively stud ied in the last de cades [1][2][3] .It is well known that Au polycrystalline elec trodes have low electrocatalytic ac tivity for the ox i da tion of sat u rated or ganic sub stances in acid so lu tions, but higher ac tiv ity for un sat u rated or ganic species 4,5 .
It has not been clearly es tab lished whether eth yl ene adsorbs on Au be fore its electrooxidation.On the one hand, Weaver et al. 6 have pre sented ex per i men tal data ob tained by SERS (Sur face En hance ment Raman Spec tros copy) cor re spond ing to C=C stretch ing and sym met ri cal =CH2 bend ing bands for eth yl ene ad sorp tion on rough Au electrodes.Ad sorbed eth yl ene prob a bly in volves a C=C bond par al lel to the sur face, with CH bonds tilt ing away from the sur face.On the other hand, Pas tor et al . 7have claimed that ad sorbed eth yl ene at po ten tials from 0.2 to 0.8 V is rinsed away from the elec trode by the base elec tro lyte in flux cell ex per i ments.On-line DEMS (Dif fer en tial Elec tro chem ical Mass Spec trom e try), to gether with in-situ FTIRS (Fourier Trans form In fra red Spec tros copy) have been used to iden tify the eth yl ene electrooxidation prod ucts (eth a nol, acethaldehyde, ace tic acid, and car bon di ox ide) at po tentials higher than 0.8 V vs RHE.
The sur face prop er ties of Au upon ad sorp tion strongly de pend on the metal crys tal lo graphic ori en ta tion 8 .Structural sen si tiv ity to acet y lene ad sorbed spe cies on Au sin gle crys tal sur faces has been found in acid me dia 9 , how ever there is no ex per i men tal data avail able for eth yl ene ad sorbates on Au ori ented sur faces.The aim of this work is to study the eth yl ene ad sorp tion on Au and the in flu ence on the sur face mor phol ogy on its ad sorp tion and electrooxidation in acid me dia.

Ex per i men tal
Polycrystalline ( pc ) and pref er en tially ori ented Au ( c.a. 0.2 cm 2 real area) as work ing elec trodes and a Au foil (4 cm 2 geo met ric area) as the coun ter elec trode were used.
All the po ten tials were re ferred to a re vers ible hy dro gen elec trode (RHE).
Pref er en tially ori ented Au elec trodes were pre pared using a sym met ric square wave po ten tial sig nal (SSWPS) at fre quen cies cov er ing the 2 to 12 kHz range.The lower poten tial limit of the SSWPS was var ied in the -0.2 to 0.5 V range, while the up per limit was held at 1.6 V.In this way, (100)-, (110)-, and (111)-type pref er en tially ori ented Au sur faces were ob tained, de pend ing on the ex per i men tal con di tions 10 .
Cy clic voltammetric re sponses of the O-adatom electrosorption pro cess in the 0.0-1.7 V po ten tial range in 1.0 M H 2 SO 4 were used as fin ger prints for the pref er entially ori ented Au sur faces.The char ac ter iza tion of the elec tro chem i cally ori ented Au elec trodes was com plemented by Scanning Elec tron Mi cros copy (SEM) im ages.
Voltammetric and cur rent tran sient mea sure ments were per formed in the work ing so lu tion, i.e. eth yl ene sat u rated (ul tra high pu rity, Mathe son) 1.0 M H 2 SO 4 .Ad di tionally, microflux cell ad sorp tion ex per i ments were made by holding the po ten tial within the -0.3 to 0.5 V range for 10 min in the same so lu tion.The cur rent tran sient as so ci ated with eth yl ene ad sorp tion was mea sured at each ad sorp tion poten tial, and the an odic strip ping voltammogram at 0.1 V/s was run af ter re plac ing the so lu tion with the base elec trolyte.All the ex per i ments were con ducted at 25 °C.

Re sults
The potentiodynamic I/E pro file of pc Au run in 1 M H 2 SO 4 be tween 0 and 1.7 V at 0.1 V/s is de picted in Fig. 1a.The on set po ten tial of the Au ox ide be gins at 1.35 V, and the an odic pro file shows three over lap ping cur rent peaks be fore the ox y gen evo lu tion re ac tion.A sharp ox ide electroreduction peak is ob served at c.a. 1.2 V. On the right of Fig. 1a, the SEM micrography for pc Au shows nonfaceted sur face do mains

Elec trode prep a ra tion
Af ter ap ply ing the SSWPS to a pc Au wire be tween 0.1 and 1.6 V at 4 kHz for 10 min, a (100)-type Au ori en ta tion is de vel oped (Fig. 1b).The cor re spond ing cy clic voltammogram shows an an odic peak lo cated at 1.43 V and a small sec ond an odic peak at 1.52 V.The cor re spond ing SEM micrography shows spike-like stepped do mains with square def i ni tion pat terns.
A (111)-type Au sur face re sults af ter ap ply ing the SSWPS to a pc Au wire be tween -0.2 and 1.6 V at 12 kHz dur ing 15 min.A cy clic voltammogram re corded sub sequently af ter wards shows three an odic peaks, at 1.42 V, 1.52 V, and a no ta bly high peak lo cated at 1.65 V (Fig. 1c).It is worth not ing that the re sult ing pref er en tially ori ented Au sur face ap pears to be rather un sta ble upon po ten tial cycling till 1.7 V.The cor re spond ing SEM micrography shows small crys tal lites with spike-like ar range ments involv ing ori ented tet ra he dra.
Finally, when the SSWPS is ap plied to a pc Au wire between 0.5 and 1.6 V at 2 kHz for 10 min, a fac eted (110)-type Au sur face is ob tained.The cor re spond ing cyclic voltammogram re corded at 0.1 V/s (Fig. 1d) shows an an odic spike at c.a. 1.38 V and two dis tin guish able an odic peaks, one at 1.43 V and the other at 1.48 V.The sur face mor phol ogy ex am ined by SEM ex hib its a com plex dis tribu tion of crys tal lites formed by rect an gu lar steps.

Eth yl ene ad sorp tion and strip ping
Eth yl ene res i dues are de tected af ter ad sorp tion from eth yl ene sat u rated 1.0 M H 2 SO 4 elec tro lytes at po ten tials lower than 0.2 V, and no tran sient cur rents are re corded dur ing eth yl ene ad sorp tion.Fig ure 2 shows the an odic strip ping voltammogram of the eth yl ene res i due ad sorbed for 10 min at -0.1 V on a (110)-type Au elec trode run at The voltammetric electrooxidation of eth yl ene ad sorbates on all of the Au sur faces at ad sorp tion po ten tials lower than 0.2 V, shows that two res i dues are formed namely RI and RII .The an odic strip ping of RI is lo cated in the dou ble layer re gion just be fore the Au ox ide for ma tion, and the cor re spond ing strip ping for RII is in the en tire poten tial do main of the sta bil ity of the Au ox ide.
The charge den si ties as so ci ated with the an odic stripping of RI and RII show a quasi -par a bolic po ten tial depend ence on all Au sur faces (Fig. 3).For ad sorp tion po ten tial val ues higher than 0.2 V noadsorbate is formed on Au.It is worth not ing that this value is placed in the neigborhood of the po ten tial of zero charge (E pzc ) of pc Au.The E pzc of pc Au in sul fate-containing so lu tions is ca.0.18 V 11 .Ox i da tion charge den si ties for eth yl ene ad sorbed spe cies are rather small, RII be ing al most twice as high as RI .The (111)-type Au sur face ex hib its the lower sur face cov er age val ues for the RII res i due.
The independences of both res i dues is tested through a par tial an odic strip ping ex per i ment.In this re spect, RI is ox i dized by an an odic scan to 1.3 V; the sub se quent an odic strip ping till 1.7 V shows no RI res i due and the same amount of charge for RII as that found in a to tal an odic strip ping ex per i ment.Thus, the ad sorp tion of eth yl ene involves two dif fer ent, in de pend ent ad sor bates.
The ex is tence of eth yl ene ad sor bates on Au elec trodes in neu tral so lu tions has also been tested, and two types of ad sor bates have been found 12 .

The ef fect of the pref er en tially ori ented Au sur face on ethyl ene ad sorp tion and electrooxidation
The ef fect of the sur face mor phol ogy was ap proached by em ploy ing pref er en tially ori ented Au sur faces.Fig. 3 shows a com par i son be tween the amount of ad sorbed species (ox i da tion voltammetric charges), RI and RII , as a func tion of the ad sorp tion po ten tial on the dif fer ent Au surfaces.It seems that trigonal plane atom ar range ments do not fa vor eth yl ene ad sor bates as ei ther RI or RII .
Voltammetric bulk eth yl ene electrooxidation run at 0.1 V/s on the dif fer ent pc and ori ented sur faces shows a broad an odic peak ex tend ing from 0.9 V to the an odic evo lu tion of ox y gen.A max i mum at ca. 1.3 V is ob served just be fore the on set po ten tial of the Au ox ide.The split ting of the com plex an odic pro file can be achieved for lower scan rates.
Steady-state po lar iza tion ex per i ments per formed for eth yl ene electrooxidation on pref er en tially ori ented Au sur faces lead to Tafel slope val ues of ca.0.06 V/de cade, inde pend ent of the sur face mor phol ogy, though the cur rent val ues are higher for (111)-type Au sur faces.It seems that he amount of ad sorbed eth yl ene acts in hib it ing the bulk eth yl ene electrooxidation pro cess.

Dis cus sion
The ad sorp tion and ox i da tion of eth yl ene on metal substrates has been ex per i men tally 13,14 and the o ret i cally 15,16 stud ied, due to its ap pli ca tion in electrosynthesis pro cesses.
At the gas/solid in ter face, eth yl ene ad sorp tion can be de scribed by two dif fer ent types of ad sor bates, namely, a di-σ-bonded state where the car bon at oms are rehybridized to wards the sp 3 sin gle C-C bond ly ing per pen dic u lar to the sur face, and a π-bonded state where the mol e cule re tains the sp 2 hy brid iza tion with the dou ble C=C bond ly ing paral lel to the sur face 17 .Dif fer ent pa ram e ters can af fect the Fig ure 3. The po ten tial de pend ence of the an odic charge den sity for ad sorbed res i dues ox i dized in RI and RII on the dif fer ently ori ented Au sur faces in aque ous 1 M H 2 SO 4.     ad sorp tion of eth yl ene on sin gle crys tal and pc sur faces.Thus, the coadsorption of al kali at oms, such as po tas sium and ce sium, at rel a tively low tem per a tures can change di-σ-spe cies into π-bonded spe cies 14,18 .This ef fect has been ex plained as an overpromotion of charge to ad ja cent metal at oms pre vent ing the for ma tion of more tightly bonded spe cies such as di-σ-states.The re sponse of the surface to al kali at oms is un der stood as an elec tron trans fer from the al kali to the metal va lence bands.This is combined with the elec tro static screen ing of the re sul tant pos itive charge by metal elec trons.The rel a tive amount of π-bonded spe cies (weak ad sor bates) and di-σ-spe cies (strong ad sor bates) in the pres ence of these al kali at oms has been de tected by TDS (Ther mal Desorption Spec troscopy) 13 and UPS (Ul tra vi o let Pho to elec tron Spec troscopy) 17 .Fur ther more, the coadsorption of ox y gen at oms also in hib its the for ma tion of di-σ-spe cies due to the withdrawal of charge from metal at oms 13 .
There is a lack of in for ma tion about eth yl ene ad sorption at the elec tro chem i cal in ter face, how ever, in-situ SERS pro vides ev i dence of eth yl ene ad sorp tion.The analy sis of the vi bra tional spec tra shows bands at 1545 cm -1 and 1278 cm -1 , re spec tively cor re spond ing to the C=C stretch ing and =CH2 bend ing modes on rough Au in acid me dia.The split ting of both sig nals is ap par ently due to adsorp tion at two en er get i cally dif fer ent sur face sites.These bands, at trib uted to ad sorbed eth yl ene, ap pear and dis appear upon purg ing the so lu tion with eth yl ene and ar gon, respec tively 6 , show ing a weak in ter ac tion with the Au sur face.
How ever, it has to be em pha sized that the for ma tion of eth yl ene ad sor bates was not found when flux cell ex per iments were per formed, and only physisorption ac counts for the in ter ac tion of the mol e cule with the Au sur face in acid me dia at po ten tials higher than 0.2 V 7 .
Our re sults clearly show two ad sorbed res i dues, RI and RII, that can be voltammetrically fol lowed in an an odic strip ping ex per i ment.These res i dues are in de pend ently oxi dized and the charge ox i da tion den si ties for both spe cies have dif fer ent con tri bu tions in the whole ad sorp tion po tential range.It is likely that eth yl ene ad sorbed as σ and π species can be formed on the Au elec trode, the in flu ence of the elec trode po ten tial value be ing like that pro moted by the coadsorption of al kali at oms in the gas / solid in ter face.
It is known that π orbitals of an or ganic un sat u rated adsor bate can do nate a neg a tive charge ex cess to metal d orbitals, and a back do na tion from the metal orbitals to the π antibonding orbitals of the mol e cule is ac com plished.
How ever, the neg a tive charges given by the eth yl ene mol ecule to the Au at oms are not enough to achieve a chemisorption pro cess at po ten tials higher than E pzc .Never the less, po ten tials lower than E pzc ren der a neg a tive charge ex cess on the Au sur face that can be do nated to the C at oms of eth yl ene, pref er en tially yield ing ei ther di-σ-bonded-type ad sor bates for ad sorp tion po ten tials much lower than E pzc , or π-bonded-type ad sor bates for adsorp tion po ten tials slightly lower than E pzc .On the other hand, the in hi bi tion of eth yl ene ad sorp tion at po ten tials higher than E pzc by spe cific an ion ad sorp tion can not be neglected.
It is worth not ing that ad sorp tion of acet y lene on Au 9 gives sim i lar an odic strip ping pro files for res i dues at adsorp tion po ten tials higher than E pzc .In this re spect, the neg a tive charge ex cess do nated by the acet y lene mol e cule to the Au sur face leads to a stron ger bounded ad sor bate than eth yl ene, but it seems that the na ture of acetylenic res idues is not very dif fer ent from that of eth yl ene.

Con clu sions
1) The ad sorp tion of eth yl ene on pc and pref er en tially ori ented Au elec trodes in aque ous 1 M H 2 SO 4 takes place at po ten tials neg a tive to the E pzc of the Au/1 M H 2 SO 4 .
2) Eth yl ene ad sorbed res i dues, RI and RII , are electrooxidized in the dou ble layer re gion and in the Au oxide po ten tial do main, re spec tively.
The an odic charge den sity in volved in RII electrooxidation is twice as large as that in RI .
3) Two types of eth yl ene ad sor bates are pos tu lated on Au sur faces, that is, di-σ-bonded and π-bonded spe cies, the for mer be ing fa vored at neg a tive po ten tials far from the Epzc of the Au/1 M H 2 SO 4 .
4) Au (111)-type pref er en tially ori ented elec trodes exhibit a higher electrocatalytic ac tiv ity for eth yl ene electrooxidation and lower amounts of RII ad sor bates.

Ac knowl edg ments
This work was fi nan cially sup ported by the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) of Ar gen tina.A.M.C.L. is mem ber of the research ca reer at the Comisión de Investigaciones Científicas de la Provincia de Bue nos Ai res, (CIC).C.F.Z. is a mem ber of the Programa de Desarrollo de Ciencias Básicas (PEDECIBA), Uru guay.

Fig ure 2 .
Fig ure2.An odic voltammetric strip ping run at 0.1 V/s of ad sorbed eth yl ene on a (110)-type Au elec trode in aque ous 1 M H 2 SO 4; ad sorption po ten tial -0.1 V and ad sorp tion time = 10 min.