Structural, compositional and electrochemical characterization of Pt-Co oxygen-reduction catalysts.

Pt-Co thin-film electrocatalysts have been characterized using low-energy ion-scattering spectroscopy (LEISS), X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), temperature-programmed desorption (TPD) and electrochemistry (EC). For comparative purposes, LEISS and EC were also carried out on a bulk Pt(3)Co(111) single crystal. The extensive experimental work resulted in the establishment of the surface phase diagram of the alloy film marked by a substantial divergence between the composition at the interface and that in the interior. When a dual-layer deposit of Pt and Co was annealed at high temperatures, alloy formation transpired in which the outermost layer became single-crystalline and enriched in Pt. The preferential surface segregation of Pt, however, was not sufficient to generate a platinum-only overlayer or "skin". Invariably, Co was found to co-exist with Pt, independent of the substrate [Mo(110) or Ru(0001)] employed; Pt(3)Co was the most favored composition. The same result, the absence of a Pt skin, was likewise indicated at the post-thermally-annealed surface of the bulk Pt(3)Co(111) monocrystal. For alloy-film surfaces more enriched in Pt than Pt(3)Co, the topmost layer was constituted primarily, but not exclusively, of Pt(111) domains. The proclivities of the alloys towards enhanced catalysis of the oxygen-reduction reaction were assessed in terms of their voltage efficiencies, as manifested by the open-circuit potential (OCP) in O(2)-saturated sulfuric acid electrolyte. The Pt(3)Co surface, whether from the thin film or the bulk single crystal, exhibited the highest OCP, a significant improvement over pure Pt but still appreciably lower than the thermodynamic limit. The degradation of the Pt(3)Co thin-film surface was predominantly due to Co corrosion. A minimal amount was spontaneously dissolved upon simple immersion in solution; slightly higher dissolution occurred at potentials above the OCP. The fraction that was not immediately corroded proved to be stable even after prolonged periods at potentials more positive than the OCP.

Electrocatalytic hydrogenation and oxidation of aromatic compounds studied by DEMS: Benzene and p-dihydroxybenzene at ultrathin Pd films electrodeposited on Au(hkl) surfaces.

Differential electrochemical mass spectrometry (DEMS) was used to investigate the electrocatalytic hydrogenation and oxidation of benzene and p-dihydroxybenzene (hydroquinone, H(2)Q) chemisorbed on ultrathin Pd films electrodeposited at Au(332) and Au(111) surfaces. At low sub-monolayer coverages on Au(332), the Pd ad-atoms preferentially adsorb on the step sites. At higher (but still sub-monolayer) coverages, Pd also occupies terrace sites; on Au(111), only terrace sites are available. The electrochemical reactivities of the subject compounds at the Pd-decorated steps and terraces were then compared. No hydrogenation products were detected by DEMS for either benzene or H(2)Q, but a considerable degree of electrodesorption of (intact) benzene occurred near the hydrogen-evolution region (HER). Anodic oxidation of both compounds yielded only CO(2) as the volatile (DEMS-detectable) product, although it took up to three anodic cycles to attain exhaustive (complete) oxidation. The anodic oxidation of benzene was also accompanied by potential-induced desorption of (unreacted) benzene particularly in the case of the stepped surface. Electrodesorption at the HER was more facile at the terrace sites than from the step sites; the opposite was true for electrodesorption at the more positive potential. Contrary to hydrogen, which does not adsorb at such monoatomic rows, both aromatic species adsorb when the nominal Pd coverage just corresponds to step decoration.