The voltammetry of surfaces vicinal to Pt{110}: Structural complexity simplified by CO cooling

Abstract

By flame-annealing and cooling a series of Pt n{110} × {111} and Pt n{110} × {100} single crystal electrodes in a CO ambient, new insights into the nature of the electrosorption processes associated with Pt{110} voltammetry in aqueous acidic media are elucidated. For Pt n{110} × {111} electrodes, a systematic change in the intensities of so-called hydrogen underpotential (Hupd) and oxide adsorption voltammetric peaks (for two dimensionally ordered (1 × 1) terraces and linear {111} × {111}step sites) point to a lack of surface reconstruction with all surfaces adopting a (1 × 1) configuration. This is in contrast to hydrogen cooled analogues which give rise to significant residual surface disorder, probably associated with the excess 50% of atoms remaining atop of the surface upon deconstruction of the {110} − (1 × 2) terrace phase. In contrast, Pt n{110} × {100} stepped electrodes, when cooled in gaseous CO following flame-annealing, show a marked tendency towards surface reconstruction, even at low step densities. Variations in potential of the Pt{110}-(1 × 1) Hupd electrosorption peaks as a function of specific ion adsorption strength and pH point to weak specific adsorption for both anions (including perchlorate and fluoride) and cations (including Na + and K +). CO charge-displacement measurements of the potential of zero total charge (PZTC) allow inferences to be made concerning the nature of the electrosorbed species in the hydrogen underpotential deposition (Hupd) region. Hence, a coherent interpretation of the complex voltammetric phenomena often displayed by platinum surfaces vicinal to the {110} plane is proposed.

Effects of the Interfacial Structure on the Methanol Oxidation on Platinum Single Crystal Electrodes

“…For the Pt(111) and Pt(100), this procedure leads to the formation of a well-ordered surface with the nominal orientation [20,21]. Due to the reconstruction phenomena undergone by the Pt(110) surfaces, which is strongly affected by the treatment, the surface presented here corresponds to the (1 × 2) or (1 × 1)/(1 × 2) structure with major (1 × 2) contribution [22,23]. Thus, the observed behavior should correspond with that expected for the (1 × 2) surface, although contributions from the (1 × 1) cannot be discarded.…”

“…It should be noted that the voltammograms for this surface are very sensitive to the actual structure of the electrode, likely related to the fraction of the surface having the (1 × 2) reconstruction. This ratio depends, in turn, on the annealing conditions [23], so that the measured currents for the methanol oxidation reaction for this electrode show large variability. For the Pt(110) electrode, the voltammogram for the oxidation of methanol shows a wave with a complex shape in the positive scan, whose onset is located at ca.…”

“…The major difference between the behavior of the Pt(111) electrode and the other two basal planes is the presence/absence of a pure double layer region where no specific adsorption occurs. For the Pt(110) and Pt(100) electrodes, OH is readily adsorbed at low potentials when hydrogen is being desorbed [23,[42][43][44][45], and thus there is no pure double layer region. Then, any molecule that interacts with the surface should either compete with hydrogen or OH for the adsorption sites.…”

Nitrate anion reduction in aqueous perchloric acid as an electrochemical probe of Pt{1 1 0}-(1 × 1) terrace sites

“…As with many studies involving Pt{110} single crystals (including reference [11]), preparation of the well-defined surface was achieved using Clavilier's flame-annealing method [24] with subsequent cooling of the crystal in a hydrogen atmosphere [25]. In two recent publications, it has been shown that such treatment will lead to a Pt{110} surface that is not optimal so far as long range order is concerned [26,27]. Rather, in order to generate the largest, defect-free Pt{110}-(1x1) planes, it is necessary to cool a flame-annealed Pt{110} electrode in a stream of CO in order to preclude the formation of Pt{110}-(1x2) reconstructed domains which may subsequently result in surface disorder associated with the lifting of the (1x2) phase to the (1x1) phase [26][27][28][29][30][31].…”

“…In two recent publications, it has been shown that such treatment will lead to a Pt{110} surface that is not optimal so far as long range order is concerned [26,27]. Rather, in order to generate the largest, defect-free Pt{110}-(1x1) planes, it is necessary to cool a flame-annealed Pt{110} electrode in a stream of CO in order to preclude the formation of Pt{110}-(1x2) reconstructed domains which may subsequently result in surface disorder associated with the lifting of the (1x2) phase to the (1x1) phase [26][27][28][29][30][31]. Because the surface density of platinum atoms in the (1x2) phase is 50% higher than for the (1x1) phase, lifting of the surface reconstruction will result in an excess of Pt{110}-(1x1) islands on-top of the underlying, flat nominal Pt{110}-(1x1) plane [32].…”