Chemisorbed Oxygen at Pt(111): a DFT Study of Structural and Electronic Surface Properties

Simulations based on density functional theory are used to study the electronic and electrostatic properties of a Pt(111) surface covered by a layer of chemisorbed atomic oxygen. The impact of the oxygen surface coverage and orientationally ordered interfacial water layers is explored. The oxygen adsorption energy decreases as a function of oxygen coverage due to the lateral adsorbate repulsion. The surficial dipole moment density induced by the layer of chemisorbed oxygen causes a positive shift of the work function. In simulations with interfacial water layers, ordering and orientation of water molecules strongly affect the work function. It is found that the surficial dipole moment density and charge density are roughly linearly dependent on the oxygen surface coverage. Moreover, we found that water layers exert only a small impact on the surface charging behavior of the surface

Time-evolution of structures of quenched Pt(111) electrode surfaces upon potential cycling

Interest has arisen in the electrochemical surface science of quenched Pt(111) electrode surfaces characterized by cyclic-voltammetry (C-V), as in the work of Clavilier. Experiments on variation of annealing temperature and quenching conditions show that the H UPD profile for Pt(111) depends on the heating temperature (500, 800 and 1000°C) with quenching at 25°C. An important result of the present work is that progressive increase of the positive potential limit in C-V from 1.0 to 1.4V, RHE, leads to continuous and irreversible sequential changes of the i vs E profile which eventually does not change further, even with cycling for 12 h between +0.05 and +1.4V; then the i vs E profile has become similar to that of Hubbard for Pt(111). The results suggest that the heating and, in particular, the quenching procedure may lead to a metastable surface structure derived from the supposedly initially well formed (111) surface, and this metastable structure is "electrochemically annealed" through reconstruction (by cycling) back to a stable structure on which the H accommodation is still near that having the "theoretical" equivalent charge of 240 μC cm-2. On the structure produced by quenching, it is evidently more difficult to electrosorb OH or O species than on the stable (111) or polycrystalline Pt surfaces; this effect is not due to adsorbed impurities

Morphology, chemical composition, and electrochemical characteristics of colored titanium passive layers

Brightly and uniformly colored passive layers on Ti are formed by application of ac polarization in aqueous NH4BF4. A wide spectrum of well-defined colors is accomplished by varying the ac voltage. The passive films are stable in the ambient and in aqueous chloride, perchlorate, and sulfate solutions. Optical microscopy and scanning electron microscopy analyses indicate that the passive layers are compact and do not show fractures or cracks. An X-ray photoelectron spectroscopy (XPS) characterization of the colored passive layers demonstrates that their surface-chemical composition depends on the ac polarization voltage. The main constituents of the passive layers are Tiz+, O2-, and F- (z varies from 4 to 2 depending on the film's depth). Fluoride in the film originates from decomposition of NH4BF4, and it accumulates at the inner metal/passive-film interface. XPS depth profiling shows that the higher the ac voltage applied, the thicker the passive film formed. Electrochemical properties of the colored Ti passive layers are determined by recording polarization curves in the 120.8 to 3.2 V range as well as Tafel plots in the hydrogen evolution reaction (HER) region in 1.0 M aqueous H2SO4. The polarization curves show that the corrosion potential of the colored passive layers shifts toward less-negative potentials indicating that they are more stable than Ti under the same conditions. The passive region for the colored layers resembles that for Ti. The Tafel plots for the HER demonstrate that the passive layers have higher activity toward the HER than Ti. The Tafel relations reveal new features that can be associated with the partial breakdown/decomposition of the passive layers, H absorption, and the onset of Ti hydride formation.Peer reviewed: YesNRC publication: N

Comparative effects of adsorbed S-species on H sorption into Pd from UPD and OPD H : a kinetic analysis

Peer reviewed: YesNRC publication: Ye

Evidence of an Eley-Rideal mechanism in the stripping of a saturation layer of chemisorbed CO on platinum nanoparticles.

International audienceThe oxidative stripping of a saturation layer of CO(chem) was studied on platinum nanoparticles of high shape selectivity and narrow size distribution. Nanospheres, nanocubes, and nano-octahedrons were synthesized using the water-in-oil microemulsion or polyacrylate methods. The three shapes allowed examination of the CO(chem) stripping in relation to the geometry of the nanoparticles and presence of specific nanoscopic surface domains. Electrochemical quartz crystal nanobalance (EQCN) measurements provided evidence for the existence of more than one mechanism in the CO(chem) stripping. This was corroborated by chronoamperometry transient for a CO(chem) saturation layer at stripping potentials of E(strip) = 0.40, 0.50, 0.60, and 0.70 V. The first mechanism is operational in the case of CO(chem) stripping at lower E(strip) values; it proceeds without adsorption of anions or H(2)O molecules and corresponds to desorption of a fraction of CO(chem) in the form of a prepeak in voltammograms or in the form of an exponential decay in chrono-amperometry (CA) transients. The second mechanism is operational in the desorption of the remaining CO(chem) at higher E(strip) values and gives rise to at least two voltammetric peaks or two CA peaks. Analysis of the experimental data and modeling of the CA transients lead to the conclusion that the stripping of a saturation layer of CO(chem) first follows an Eley-Rideal mechanism in the early stage of the process and then a Langmuir-Hinshelwood mechanism