AES and TDS studies of electrochemically oxidized Pt(100)

Abstract

Anodic films formed on Pt(lOO) in 0.3M HF using a quasi thin-layer electrochemical cell within a vacuum envelope were transferred to ultra-high vacuum for study by AtS and TDS. Films generated at potentials above 1.lV (RHE) survived emersion and pumpdown in a hydrated state. As the emersion potential increased, the lntegrated H 2 0 and 02 thermal desorption signals , also using stripping studies, concluded that, after surface adsorption started at D.8V, dermasorption (sorption into the first few layers of the metal) commenced at 1.DV. Dermasorption and surface adsorption continued together .up to 1.6V, where a monolayer of PtD was thought to be present. At higher potentials some of the PtD was further oxidized.Vetter and Schultze [5] also analyzed charging and stripping curves and concluded that the coverage of adsorbed oxygen did not exceed a few percent, with a rate-determining, field-dependent place exchange leading to a phase oxide that represented most of the anodic charge. Fleischmann et al. [6] proposed the formation of four different "oxides" on Pt anodes. AngersteinKozlowska et ale [7] have given the most detailed interpretation of voltammetric and ellipsometric data to date. They have proposed a series of oxidation steps which occur as a polycrystalline Pt electrode is swept to 3 progressively higher potentials in acidic electrolyte. The initial oxidation, in the potential region 0.8-0.93V(RHE), was ascribed to reversible hydroxylation of the surface. Above 0.93V place exchange between adsorbed hydroxyl species and Pt occurs, and electrochemical irreversibility sets in. The place exchange is a rather slow process, and leads to complex aging behavior. In the potential range of ca. 1.0 to 1.2V (depending upon time factors) the peak potential of the subsequent reduction wave is independent of the total oxidation charge. This invariance was ascribed to constant Pt-O geometry, and the charge passed in this region was considered due to oxidative deprotonation of the hydroxyls previously incorporated into the surface. The above conclusions were based upon analysis of details of cyclic voltammetry curves. Optical studies showed that for oxidation up to a charge of 0.6e-/Pt the same value of the ellipsometric parameter ~ was observed for a given surface charge on both the anodic-going and cathodicgoing sweeps. For more complex oxidations the ~/Q curves for the anodic and cathodic sweeps diverged, suggesting that the oxidized species first formed was also the first reduced. This species was suggested to be the reversible hydroxyl groups residing at the surface. A change in the ~/Q slope at 0.5e-/Pt (anodic direction) was ascribed to the onset of formation of the deprotonated oxide. At 1.0 e-/Pt (1.13V) a further change was seen in the ~/Q slope, which then continued unchanged out to at least 2 e-/Pt. The identification of the individual chemical species in this complex scheme was indirect and has awaited confirmation by direct spectroscopic and structural studies. and Pt (+1V). Long polarization at 3.2...

Long-range structural effects in the anomalous voltammetry on ultra-high vacuum prepared Pt (111)

“…SUch a reversible oxygen species a s a s could have exciting implications for electrocatalysis. Although no oxygen was seen by Auger or thermal desorption on surfaces emersed at potentials just above this couple, this does not necessarily militate against an oxygen-type species since highly reversible species can spontaneously discharge in the electrolyte layer which remains on the surface upon emersion [14].…”

“…However, the electrode could be cycled through this peak many times with no discernible change in the vo1tam-metry. The lack of oxygen at lower potentials does not show that the 730 mV peak is not due to electrosorption of oxygen, since work on Pt(100) has shown that low coverages of electrosorbed oxygen do not survive emersion, evacuation, and transfer [14].…”

“…The potential range of the anomalous reversible couple on Pt(lll) partially overlaps the onset of the surface oxidation of either Pt(100) or polycrystalline Pt electrodes [14]. It is thus useful to consider whether the charge passed in this region is due to some form of highly reversible electrosorbed oxygen related to 0 d or OH d in some way.…”

“…Recent studies by electron spectroscopy and thermal desorption spectroscopy [14,27] have established that the oxygen-type species forming at more anodic potentials (>1 V RHE) is place-exchanged OH, and that the change that occurs with increasing potential of formation is the coordination of the OH group(s) around the Pt surface atoms. In this potential range, ex-situ spectroscopic analyses have not indicated that the oxygen-type species forming on the Pt(lll) surface is any different in molecular configuration from that forming at the Pt(100) (or polycrystalline Pt) surface.…”

Catalytic effects on methanol oxidation produced by cathodization of platinum electrodes

“…Using Auger spectroscopy [91], it has been observed that the anodic peak at ca. 1.2 V (peak (V)) originates from the oxidation of Pt (111), but the surface species corresponding to the highly reversible peaks at 0.75 V on Pt(111) could not be identified by this ex situ technique.…”