2 research outputs found
Revisiting the Redox Properties of Hydrous Iridium Oxide Films in the Context of Oxygen Evolution
The
electrochemistry of hydrous iridium oxide films (HIROF) is
revisited. Cyclic voltammograms of HIROFs display two reversible redox
couples commonly assigned to the IrĀ(III)/IrĀ(IV) and IrĀ(IV)/IrĀ(V) transitions,
respectively. However, compared to the first, the second redox couple
has significantly less charge associated with it. This effect is interpreted
as partial oxidation of IrĀ(IV) as limited by nearest neighbor repulsion
of resulting IrĀ(V) sites. Thus, the redox process is divided into
two steps: one preceding and one overlapping the oxygen evolution
reaction (OER). Here, the āsuper-nernstianā pH dependence
of the redox processes in the HIROF is used to expose how pH controls
the overpotential for oxygen evolution, as evidenced by the complementary
increased formation of IrĀ(V) oxide. A recently formulated binuclear
mechanism for the OER is employed to illustrate how hydrogen bonding
may suppress the OER, thus implicitly favoring IrĀ(V) oxide formation
above the thermodynamic onset potential for the OER at low pH
Potential-Dependent Structural Memory Effects in AuāPd Nanoalloys
Alloying of metals offers great opportunities for directing
reactivity
of catalytic reactions. For nanoalloys, this is critically dependent
on near-surface composition, which is determined by the segregation
energies of alloy components. Here AuāPd surface composition
and distribution of Pd within a Au<sub>0.7</sub>Pd<sub>0.3</sub> nanoalloy
were investigated by monitoring the electrocatalytic
behavior for the oxygen reduction reaction used as a sensitive surface
ensemble probe. A time-dependent selectivity toward the formation
of H<sub>2</sub>O<sub>2</sub> as the main oxygen reduction
product has been observed, demonstrating that the applied potential
history determines surface composition. DFT modeling suggests that
these changes can result both from Pd surface diffusion and from exchange
of Pd between the shell and the core. Importantly, it is shown that
these reorganizations are controlled by surface adsorbate population,
which results in a potential-dependent AuāPd surface composition
and in remarkable structural memory effects