Surface-oxide growth at platinum electrodes in aqueous H2SO4: Reexamination of its mechanism through combined cyclic-voltammetry, electrochemical quartz-crystal nanobalance, and Auger electron spectroscopy measurements

doi:10.1016/j.electacta.2003.11.008

Electrochimica Acta

Volume 49, Issues 9–10, 15 April 2004, Pages 1451-1459

https://doi.org/10.1016/j.electacta.2003.11.008 Get rights and content

Abstract

The mechanism of platinum surface electro-oxidation is examined by combined cyclic-voltammetry (CV), in situ electrochemical quartz-crystal nanobalance (EQCN) and ex situ Auger electron spectroscopy (AES) measurements. The CV, EQCN and AES data show that the charge density, interfacial mass variation and intensity of the O-to-Pt AES signal ratio increase in a continuous, almost linear manner as the potential is raised from 0.85 to 1.40 V. In addition, the charge density, mass variation and O-to-Pt signal ratio profiles follow each other, thus indicating that the surface oxidation proceeds by a progressive coordination of O-containing species to the Pt substrate. The coupled CV and EQCN measurements lead to in situ determination of the molecular weight of the interfacial species; these were identified as chemisorbed O (O_chem) at 0.85≤E≤1.10 V and as O²⁻ in the form of a surface PtO at 1.20≤E≤1.40 V. The AES results reveal that the first half-monolayer of O_chem is formed through discharge of H₂O molecules and such formed O_chem resides on the Pt surface. Subsequent discharge of H₂O molecules leads to formation of the second half-monolayer of O_chem that is accompanied by the interfacial place exchange of O_chem and surface Pt atoms; this process results in the development of a quasi-3D surface PtO lattice comprising Pt²⁺ and O²⁻. AES data demonstrate that the place-exchange process occurs in the 1.10–1.20 V potential range. The experimentally determined molecular weight of the species added to the surface is 15.8 g mol⁻¹, which points to O and to anhydrous PtO as the surface oxide formed.

Introduction

The oxide growth at noble-metal electrodes attracted a lot of attention in the initial stages of the development of surface electrochemistry, when it became apparent that cyclic-voltammetry (CV) profiles recorded in high-purity electrolytes allowed one to distinguish the potential regions corresponding to the double-layer charging, oxide growth, and if applicable to the under-potential deposition of hydrogen (UPD H) [1], [2]. Platinum has always been of particular interest owing to the electrocatalytic properties of the metal and the platinum-oxide system has always been treated as a prototype in the comprehension of oxide development and growth at other, non-noble transition metals of industrial importance such as Fe, Ni, or Co [3]. At present, it is well established that an oxide film present on the noble metal affects the mechanism and kinetics of various anodic processes or anodic redox reactions at the surface by: (i) affecting the thermodynamics of the reaction at the double layer; (ii) changing the electronic properties of the metal surface; (iii) imposing a barrier to charge transfer across the surface-oxide film; and (iv) influencing the adsorption behavior of reaction intermediates and/or products at the catalytic surface, principally through effect (ii) and a related site-blocking effect [4], [5]. The surface oxide varies from the bulk one and often has different physico-chemical properties such as the electronic and magnetic properties or the crystallographic structure and lattice parameter.

The oxide growth at Pt electrodes achieved by potential cycling or potentiostatic polarization has been a subject of intense scientific discussion [5], [6], [7], [8], [9], [10], [11], [12], [13]. At present, it is well known that the surface crystallographic orientation and the electrolyte composition result in a unique interface that gives rise to metal-anion interactions which, consequently, affect the potential at which the oxide growth commences. In aqueous H₂SO₄, which is of interest to the work reported here, the oxide formation at Pt starts at 0.85 V. The following oxide-formation mechanism at Pt electrodes was proposed [11], [14]: $Pt + H_{2} O → Pt  OH_{ads} + H^{+} + e^{−}; 0.85–1.10 V$ $Pt  OH_{ads} → place exchange (OH  Pt)_{quasi-3Dlattice}$ $(OH  Pt)_{quasi-3Dlattice} →(Pt  O)_{quasi-3Dlattice} + H^{+} + e^{−}; 1.10–1.40 V$ where Eq. (1) refers to the oxidation of H₂O molecules with formation of adsorbed hydroxyl group (OH_ads), Eq. (2) represents the interfacial place exchange between OH_ads and surface Pt atoms that leads to formation of a quasi-3D lattice, and Eq. (3) stands for the subsequent oxidation of OH within the quasi-3D lattice resulting in the departure of H⁺. This mechanism was proposed on the basis of the reversibility of the Pt oxidation within its initial stage, i.e. for the surface coverage (θ) by OH_ads being θ_OH≤0.20. The CV peaks in the 0.85–1.10 V potential region were assigned to different surface compounds (such as Pt₄OH, Pt₃OH, and Pt₂OH) and were not considered to originate from the polycrystalline nature of the electrode. In subsequent years, it was revealed that the chloride ion (Cl⁻) suppresses the initial oxide formation, the process taking place up to ∼1.10 V [15]. The Pt oxide growth was observed to proceed beyond 1.10 V and the CV profile was shown to recover its characteristic features. Thus, it was not clear how the second part of the CV profile (corresponding to (OHPt)_{quasi-3Dlattice}→(PtO)_{quasi-3Dlattice} could be observed if the initial place-exchanged surface lattice comprising Pt atoms and OH groups had been eliminated by the competitively adsorbed chloride. The oxidation mechanism shown in , , was questioned by Birss et al. [16], who, on the basis of electrochemical quartz-crystal microbalance (EQCM) data, correctly concluded that the above mechanism [11] would require two distinct regions of mass response, one corresponding to the addition of 17 g mol⁻¹ in the first step (Eq. (1)) and the other corresponding to a mass change of −1 g mol⁻¹ upon H departure (Eq. (3)). Birss et al. observed that the anodic mass-response profile in the 0.85–1.40 V potential region involves a continuous mass increase [16], thus contradicting the mechanism depicted by , , . A two-electron mechanism of the Pt surface oxidation, that excluded OH_ads as an intermediate state, was proposed by Harrington based on his simulation of ac voltammetry results recorded over the region of Pt oxide-film formation [17]. Thus, the early mechanism of the initial Pt oxide development [11], [14] was not supported by EQCM or ac voltammetry results [16], [17]. Burke [18] suggested that the Pt surface oxide is a hydrated species having a complex lattice structure comprising Pt cations, O²⁻ or OH⁻ (depending on the electrolyte’s pH) and hydrating water molecules. However, the acceptance of this proposal requires in situ molecular weight determination before it can be accepted and such results are not available.

In this paper, we re-examine the mechanism of initial Pt oxidation using complementary cyclic-voltammetry (CV), Auger electron spectroscopy (AES), and electrochemical quartz-crystal nanobalance (EQCN) measurements. EQCN allows us to measure nanogram interfacial mass variations and, when coupled with CV, leads to in situ molecular weight determination. AES results shed light on the surface stability of adsorbed O-containing species present at the Pt electrode surface and allow us to examine if the interfacial place exchange takes place, and if so in which potential range. We related CV results to the EQCN ones and subsequently determine the molecular weight of the surface oxide formed. This allows us to distinguish between hydrated and unhydrous oxide species.

Section snippets

Electrochemical and EQCN Measurements

CV and EQCN measurements were conducted using a Seiko/PAR Quartz Crystal Analyzer (model QCM 917) connected to a PAR (model 263A) potentiostat controlled by PAR M270 Electrochemical Software. A planar AT-cut quartz crystal was operated as the oscillator at a resonant frequency of 8.9 MHz in air (it drops to ∼8.8 MHz when the crystal is in contact with an aqueous medium). A change of the vibrational frequency of the quartz-crystal resonator was converted to an interfacial mass variation using the

CV and EQCN results

In Fig. 1, we show a CV transient (i versus E) and a mass-response profile (Δm versus E) for a Pt electrode in 0.5 M aqueous H₂SO₄ recorded at s=50 mV s⁻¹. It clearly shows the onset potential of surface oxidation (at E≅0.85 V) in the positive-going scan and the onset of oxide reduction (at E≅1.10 V) in the negative-going scan; the potential regions where the Pt surface oxide is formed and subsequently reduced upon the scan reversal are indicated (Fig. 1A). This CV transient is a standard one [2],

Conclusions

1.
The electrochemical quartz-crystal nanobalance when combined with other experimental methodologies is a powerful tool in research on electro-oxidation of noble-metal electrodes and provides an insight into the interfacial mass balance.
2.
The experimentally determined molecular weight of the Pt surface oxide is 15.8 g mol⁻¹, thus pointing to the addition of O to the surface and PtO as the oxide species formed. The surface oxide is an anhydrous species.
3.
Platinum electro-oxidation does not involve OH_ads

Acknowledgements

Grateful acknowledgements are made to the NSERC of Canada and MRN du Québec (Electrocatalysis Research Chair) for support of this research project. G. Jerkiewicz acknowledges a Research Initiation Grant from Queen’s University. M.P. Soriaga wishes to acknowledge the National Science Foundation (CHE-9703521) and the A. Robert. Welch Foundation for support of this research. Some of the results presented in this paper were produced by G. Jerkiewicz during his sabbatical-leave stay (1999) and a

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^☆: These results were first presented at the 197th Electrochemical Society Meeting, Toronto, Canada, May 2000, Abstract no. 175 authored by G. Jerkiewicz, G. Vatankhah, J. Lessard, M.P. Soriaga.

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Surface-oxide growth at platinum electrodes in aqueous H2SO4: Reexamination of its mechanism through combined cyclic-voltammetry, electrochemical quartz-crystal nanobalance, and Auger electron spectroscopy measurements☆

Abstract

Introduction

Section snippets

Electrochemical and EQCN Measurements

CV and EQCN results

Conclusions

Acknowledgements

J. Electroanal. Chem.

J. Electroanal. Chem.

Surf. Sci.

J. Electroanal. Chem.

J. Electroanal. Chem.

Progr. Surf. Sci.

J. Electroanal. Chem.

J. Electroanal. Chem.

Electrochim. Acta

J. Electroanal. Chem.

J. Electroanal. Chem.

Surf. Sci. Rep.

J. Electroanal. Chem.

J. Electrochem. Soc.

Bull. Chem. Soc. Jpn.

Can. J. Chem.

J. Electrochem. Soc.

J. Am. Chem. Soc.

Collect. Czech. Chem. Commun.

Surface-oxide growth at platinum electrodes in aqueous H₂SO₄: Reexamination of its mechanism through combined cyclic-voltammetry, electrochemical quartz-crystal nanobalance, and Auger electron spectroscopy measurements☆