Adsorption and electrooxidation of ethylene on Au surfaces

ARTICLE

Adsorption and electrooxidation of ethylene on Au surfaces

C.F. Zinola^I; A.M. Castro Luna^II

^ILaboratorio de Electroquímica, Facultad de Ciencias, Tristan Narvaja 1674, P.O.Box 10773, Montevideo, Uruguay

^IIInstituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA) Sucursal4, Casilla de Correo 16 (1900) La Plata, Argentina

ABSTRACT

The electrochemical behavior of ethylene electrooxidation on preferentially oriented Au surfaces is investigated in 1 M H₂ SO₄ aqueous solution at 25°C. A symmetric square wave periodic potential signal is applied to polycrystalline Au electrodes to obtain (111)-, (110)-, and (100)-type oriented surfaces, which are characterized by cyclic voltammograms and SEM micrographs. It is found that ethylene adsorbs at potentials just below the potential of zero charge of the Au/1 M H₂ SO₄ interface, and two residues are detected in an anodic stripping experiment. The adsorbed ethylene voltammetric electrooxidation, and the bulk ethylene anodic polarization curves are measured at the differently oriented Au surfaces.

Keywords: ethylene, Au, adsorption, oriented surfaces, oxidation

RESUMO

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 interface Au/1 M H₂SO₄, e dois resíduos são detectados num experimento de "stripping" 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.

Introduction

Electrochemical oxidation of double and triple bonded organic substances on noble metals has been extensively studied in the last decades^1-3. It is well known that Au polycrystalline electrodes have low electrocatalytic activity for the oxidation of saturated organic substances in acid solutions, but higher activity for unsaturated organic species^4,5.

It has not been clearly established whether ethylene adsorbs on Au before its electrooxidation. On the one hand, Weaver et al.⁶ have presented experimental data obtained by SERS (Surface Enhancement Raman Spectroscopy) corresponding to C=C stretching and symmetrical =CH2 bending bands for ethylene adsorption on rough Au electrodes. Adsorbed ethylene probably involves a C=C bond parallel to the surface, with CH bonds tilting away from the surface. On the other hand, Pastor et al.⁷ have claimed that adsorbed ethylene at potentials from 0.2 to 0.8 V is rinsed away from the electrode by the base electrolyte in flux cell experiments. On-line DEMS (Differential Electrochemical Mass Spectrometry), together with in-situ FTIRS (Fourier Transform Infrared Spectroscopy) have been used to identify the ethylene electrooxidation products (ethanol, acethaldehyde, acetic acid, and carbon dioxide) at potentials higher than 0.8 V vs RHE.

The surface properties of Au upon adsorption strongly depend on the metal crystallographic orientation⁸. Structural sensitivity to acetylene adsorbed species on Au single crystal surfaces has been found in acid media⁹, however there is no experimental data available for ethylene adsorbates on Au oriented surfaces.

The aim of this work is to study the ethylene adsorption on Au and the influence on the surface morphology on its adsorption and electrooxidation in acid media.

Experimental

Polycrystalline (pc) and preferentially oriented Au (c.a. 0.2 cm² real area) as working electrodes and a Au foil (4 cm² geometric area) as the counter electrode were used.

All the potentials were referred to a reversible hydrogen electrode (RHE).

Preferentially oriented Au electrodes were prepared using a symmetric square wave potential signal (SSWPS) at frequencies covering the 2 to 12 kHz range. The lower potential limit of the SSWPS was varied in the -0.2 to 0.5 V range, while the upper limit was held at 1.6 V. In this way, (100)-, (110)-, and (111)-type preferentially oriented Au surfaces were obtained, depending on the experimental conditions¹⁰.

Cyclic voltammetric responses of the O-adatom electrosorption process in the 0.0-1.7 V potential range in 1.0 M H₂SO₄ were used as fingerprints for the preferentially oriented Au surfaces. The characterization of the electrochemically oriented Au electrodes was complemented by Scanning Electron Microscopy (SEM) images.

Voltammetric and current transient measurements were performed in the working solution, i.e. ethylene saturated (ultra high purity, Matheson) 1.0 M H₂SO₄. Additionally, microflux cell adsorption experiments were made by holding the potential within the -0.3 to 0.5 V range for 10 min in the same solution. The current transient associated with ethylene adsorption was measured at each adsorption potential, and the anodic stripping voltammogram at 0.1 V/s was run after replacing the solution with the base electrolyte. All the experiments were conducted at 25°C.

Results

The potentiodynamic I/E profile of pc Au run in 1 M H₂ SO₄ between 0 and 1.7 V at 0.1 V/s is depicted in Fig. 1a. The onset potential of the Au oxide begins at 1.35 V, and the anodic profile shows three overlapping current peaks before the oxygen evolution reaction. A sharp oxide electroreduction peak is observed at c.a. 1.2 V. On the right of Fig. 1a, the SEM micrography for pc Au shows nonfaceted surface domains

Electrode preparation

After applying the SSWPS to a pc Au wire between 0.1 and 1.6 V at 4 kHz for 10 min, a (100)-type Au orientation is developed (Fig. 1b). The corresponding cyclic voltammogram shows an anodic peak located at 1.43 V and a small second anodic peak at 1.52 V. The corresponding SEM micrography shows spike-like stepped domains with square definition patterns.

A (111)-type Au surface results after applying the SSWPS to a pc Au wire between -0.2 and 1.6 V at 12 kHz during 15 min. A cyclic voltammogram recorded subsequently afterwards shows three anodic peaks, at 1.42 V, 1.52 V, and a notably high peak located at 1.65 V (Fig. 1c). It is worth noting that the resulting preferentially oriented Au surface appears to be rather unstable upon potential cycling till 1.7 V. The corresponding SEM micrography shows small crystallites with spike-like arrangements involving oriented tetrahedra.

Finally, when the SSWPS is applied to a pc Au wire between 0.5 and 1.6 V at 2 kHz for 10 min, a faceted (110)-type Au surface is obtained. The corresponding cyclic voltammogram recorded at 0.1 V/s (Fig. 1d) shows an anodic spike at c.a. 1.38 V and two distinguishable anodic peaks, one at 1.43 V and the other at 1.48 V. The surface morphology examined by SEM exhibits a complex distribution of crystallites formed by rectangular steps.

Ethylene adsorption and stripping

Ethylene residues are detected after adsorption from ethylene saturated 1.0 M H₂ SO₄ electrolytes at potentials lower than 0.2 V, and no transient currents are recorded during ethylene adsorption. Figure 2 shows the anodic stripping voltammogram of the ethylene residue adsorbed for 10 min at -0.1 V on a (110)-type Au electrode run at 0.1 V/s. The same adsorption experiment run in the base electrolyte shows negligible difference with respect to the repetitive voltammogram. The anodic stripping voltammograms of the ethylene residues are similar for the rest of the oriented Au surfaces examined.

The voltammetric electrooxidation of ethylene adsorbates on all of the Au surfaces at adsorption potentials lower than 0.2 V, shows that two residues are formed namely RI and RII. The anodic stripping of RI islocated in the double layer region just before the Au oxide formation, and the corresponding stripping for RII is in the entire potential domain of the stability of the Au oxide.

The charge densities associated with the anodic stripping of RI and RII show a quasi-parabolic potential dependence on all Au surfaces (Fig. 3). For adsorption potential values higher than 0.2 V noadsorbate is formed on Au. It is worth noting that this value is placed in the neigborhood of the potential of zero charge (Epzc) of pc Au. The Epzc of pc Au in sulfate-containing solutions is ca. 0.18 V¹¹. Oxidation charge densities for ethylene adsorbed species are rather small, RII being almost twice as high as RI. The (111)-type Au surface exhibits the lower surface coverage values for the RII residue.

The independences of both residues is tested through a partial anodic stripping experiment. In this respect, RI is oxidized by an anodic scan to 1.3 V; the subsequent anodic stripping till 1.7 V shows no RI residue and the same amount of charge for RII as that found in a total anodic stripping experiment. Thus, the adsorption of ethylene involves two different, independent adsorbates.

The existence of ethylene adsorbates on Au electrodes in neutral solutions has also been tested, and two types of adsorbates have been found¹².

The effect of the preferentially oriented Au surface on ethylene adsorption and electrooxidation

The effect of the surface morphology was approached by employing preferentially oriented Au surfaces. Fig. 3 shows a comparison between the amount of adsorbed species (oxidation voltammetric charges), RI and RII, as a function of the adsorption potential on the different Au surfaces. It seems that trigonal plane atom arrangements do not favor ethylene adsorbates as either RI or RII.

Voltammetric bulk ethylene electrooxidation run at 0.1 V/s on the different pc and oriented surfaces shows a broad anodic peak extending from 0.9 V to the anodic evolution of oxygen. A maximum at ca. 1.3 V is observed just before the onset potential of the Au oxide. The splitting of the complex anodic profile can be achieved for lower scan rates.

Steady-state polarization experiments performed for ethylene electrooxidation on preferentially oriented Au surfaces lead to Tafel slope values of ca. 0.06 V/decade, independent of the surface morphology, though the current values are higher for (111)-type Au surfaces. It seems that the amount of adsorbed ethylene acts inhibiting the bulk ethylene electrooxidation process.

Discussion

The adsorption and oxidation of ethylene on metal substrates has been experimentally^13,14 and theoretically^15,16 studied, due to its application in electrosynthesis processes.

At the gas/solid interface, ethylene adsorption can be described by two different types of adsorbates, namely, a di-σ-bonded state where the carbon atoms are rehybridized towards the sp³ single C-C bond lying perpendicular to the surface, and a π-bonded state where the molecule retains the sp² hybridization with the double C=C bond lying parallel to the surface¹⁷. Different parameters can affect the adsorption of ethylene on single crystal and pc surfaces. Thus, the coadsorption of alkali atoms, such as potassium and cesium, at relatively low temperatures can change di-σ-species into π-bonded species^14,18. This effect has been explained as an overpromotion of charge to adjacent metal atoms preventing the formation of more tightly bonded species such as di-σ-states. The response of the surface to alkali atoms is understood as an electron transfer from the alkali to the metal valence bands. This is combined with the electrostatic screening of the resultant positive charge by metal electrons. The relative amount of π-bonded species (weak adsorbates) and di-σ-species (strong adsorbates) in the presence of these alkali atoms has been detected by TDS (Thermal Desorption Spectroscopy)¹³ and UPS (Ultraviolet Photoelectron Spectroscopy)¹⁷. Furthermore, the coadsorption of oxygen atoms also inhibits the formation of di-σ-species due to the withdrawal of charge from metal atoms¹³.

There is a lack of information about ethylene adsorption at the electrochemical interface, however, in-situ SERS provides evidence of ethylene adsorption. The analysis of the vibrational spectra shows bands at 1545 cm^-1 and 1278 cm^-1, respectively corresponding to the C=C stretching and =CH2 bending modes on rough Au in acid media. The splitting of both signals is apparently due to adsorption at two energetically different surface sites. These bands, attributed to adsorbed ethylene, appear and disappear upon purging the solution with ethylene and argon, respectively⁶, showing a weak interaction with the Au surface.

However, it has to be emphasized that the formation of ethylene adsorbates was not found when flux cell experiments were performed, and only physisorption accounts for the interaction of the molecule with the Au surface in acid media at potentials higher than 0.2 V⁷.

Our results clearly show two adsorbed residues, RI and RII, that can be voltammetrically followed in an anodic stripping experiment. These residues are independently oxidized and the charge oxidation densities for both species have different contributions in the whole adsorption potential range. It is likely that ethylene adsorbed as σ and π species can be formed on the Au electrode, the influence of the electrode potential value being like that promoted by the coadsorption of alkali atoms in the gas / solid interface.

It is known that π orbitals of an organic unsaturated adsorbate can donate a negative charge excess to metal d orbitals, and a back donation from the metal orbitals to the π antibonding orbitals of the molecule is accomplished. However, the negative charges given by the ethylene molecule to the Au atoms are not enough to achieve a chemisorption process at potentials higher than Epzc. Nevertheless, potentials lower than Epzc render a negative charge excess on the Au surface that can be donated to the C atoms of ethylene, preferentially yielding either di-σ-bonded-type adsorbates for adsorption potentials much lower than Epzc, or π-bonded-type adsorbates for adsorption potentials slightly lower than Epzc. On the other hand, the inhibition of ethylene adsorption at potentials higher than Epzc by specific anion adsorption cannot be neglected.

It is worth noting that adsorption of acetylene on Au⁹ gives similar anodic stripping profiles for residues at adsorption potentials higher than Epzc. In this respect, the negative charge excess donated by the acetylene molecule to the Au surface leads to a stronger bounded adsorbate than ethylene, but it seems that the nature of acetylenic residues is not very different from that of ethylene.

Conclusions

1) The adsorption of ethylene on pc and preferentially oriented Au electrodes in aqueous 1 M H₂ SO₄ takes place at potentials negative to the Epzc of the Au/1 M H₂ SO₄.

2) Ethylene adsorbed residues, RI and RII, are electrooxidized in the double layer region and in the Au oxide potential domain, respectively.

The anodic charge density involved in RII electrooxidation is twice as large as that in RI.

3) Two types of ethylene adsorbates are postulated on Au surfaces, that is, di-σ-bonded and π-bonded species, the former being favored at negative potentials far from the Epzc of the Au/1 M H₂ SO₄.

4) Au (111)-type preferentially oriented electrodes exhibit a higher electrocatalytic activity for ethylene electrooxidation and lower amounts of RII adsorbates.

Acknowledgments

This work was financially supported by the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) of Argentina. A.M.C.L. is member of the research career at the Comisión de Investigaciones Científicas de la Provincia de Buenos Aires, (CIC). C.F.Z. is a member of the Programa de Desarrollo de Ciencias Básicas (PEDECIBA), Uruguay.

Received: June 30, 1996; November 19, 1996

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Publication Dates

• Publication in this collection
  30 Oct 2012
• Date of issue
  1997

History

• Received
  19 Nov 1996