Paul Ulrich Biedermann

The impact of spectator species on the interaction of H2O2 with platinum – implications for the oxygen reduction reaction pathways

The impact of electrolyte constituents on the interaction of hydrogen peroxide with polycrystalline platinum is decisive for the understanding of the selectivity of the oxygen reduction reaction (ORR). Hydrodynamic voltammetry measurements show that while the hydrogen peroxide reduction (PRR) is diffusion-limited in perchlorate- or fluoride-containing solutions, kinetic limitations are introduced by the addition of more strongly adsorbing anions. The strength of the inhibition of the PRR increases in the order ClO4(-)≈ F(-) < HSO4(-) < Cl(-) < Br(-) < I(-) as well as with the increase of the concentration of the strongly adsorbing anions. Electronic structure calculations indicate that the dissociation of H2O2 on Pt(111) is always possible, regardless of the coverage of spectator species. However, the adsorption of H2O2 becomes strongly endothermic at high coverage with adsorbing anions. A comparison of our observations on the inhibition of the PRR by spectators with previous studies on the selectivity of the ORR shows that oxygen is reduced to H2O2 only under conditions at which the PRR kinetics is significantly limited, while the ORR proceeds with a complete four-electron reduction only when the PRR is sufficiently fast. Therefore, only a H2O2-mediated pathway that includes a competition between the dissociation and the spectator coverage-dependent desorption of the H2O2 intermediate is enough to explain and unify all the observations that have been made so far on the selectivity of the ORR.

Formation of Highly Ordered and Orientated Gold Islands: Effect of Immersion Time on the Molecular Adlayer Structure of Pentafluorobenzenethiols (PFBT) SAMs on Au(111)

Self-assembled monolayers (SAMs) of pentafluorobenzenethiol (PFBT) on Au(111) substrates, prepared with different immersion times (ITs) at room temperature, were studied using scanning tunneling microscopy (STM) and infrared reflection-absorption spectroscopy (IRRAS). In the present study, the focus was on several important points of interest in the field of SAMs. First, the gold islands formed upon adsorption of PFBT molecules on the gold surface were monitored at different ITs in terms of their size, density, and shape. After short ITs (5 to 30 min), small gold islands with rounded shape were formed. These gold islands were arranged in a rather regular fashion and found to be quite mobile under the influence of the STM-tip during the scanning. When the IT was increased to 16 h, the results revealed the formation of highly ordered and orientated gold islands with very unusual shapes with straight edges meeting at 60° or 120° running preferentially along the [11(-)0] substrate directions. The density of the gold islands was found to decrease with increasing IT until they almost disappeared from the SAMs prepared after 190 h of IT. On top of the gold islands, the PFBT molecules were found to adopt the closely packed (10√3 × 2) structure. Second, a number of structural defects such as disordered regions at the domain boundaries and dark row(s) of molecules within the ordered domains of the PFBT SAMs were observed at different ITs. The SAMs prepared after 190 h of IT were found to be free of these defects. Third, at low and moderate ITs, a variation in the PFBT molecular contrast was observed. This contrast variation was found to depend mainly on the tunneling parameters. Finally, our results revealed that the organization process of PFBT SAMs is IT-dependent. Consequently, a series of structural phases, namely, α, β, γ, δ, and ε were found. The α-, β-, γ-, and δ-phases were typically accompanied by the ε-phase that appeared on top of gold islands. With increasing IT, the α→β→ γ→δ→ε phase transitions took place. The resulting ε-phase, which covered the entire gold surface after 190 h of IT, yielded well-ordered self-assembled monolayers with large domains having a (10√3 × 2) superlattice structure.

Self‐Assembled Monolayers: Star‐Shaped Crystallographic Cracking of Localized Nanoporous Defects (Adv. Mater. 33/2015)

On page 4877, F. U. Renner, A. Bashir, M. Valtiner, and co-workers describe a star-like dealloying corrosion morphology that appears during the localized attack of smooth well-prepared Cu-Au surfaces. The surfaces are initially protected by thiol or selenol inhibitior films. Localized dealloying of Cu-Au produces nanoporous gold under stress and crystallographic cracks - thereby opening a new approach combining surface science with nanoscale mechanical testing.