| dc.description.abstract | The current study employed cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM) to probe the redox chemistry and spatial structure of an iron oxide thin film adsorbed on an ordered Pt(111) electrode in pH3 sulfate solution (1 mM H2SO4 + 0.1 M K2SO4) containing 10 mM FeSO4. We used the conventional annealing-and-quenching method to treat the Pt(111) electrode. The subsequently produced sub-monolayer native oxide on this Pt sample enabled the formation of an ordered FeO thin film. In situ STM imaging yielded a unique spoke – wheel (SW) structure between 0.1 and -0.1 V (vs. Ag/AgCl) in pH3 sulfate solution. As opposed to the hexagonal FeO(111) bilayer structure observed on Pt(111) in vacuum, the SW structure was distorted hexagonal. While the potential was shifted to -0.4 V, the spoke-wheel (SW) structure transformed into an ordered spot-like morphology with Pt(111) - (6 39) oblique structure. On the other hand, when the potential was shifted from -0.1 V towards negative values, the surface transformed into a disordered moiré pattern, representing an irreversible reaction. As the potential continued to decrease, a second layer of iron oxide and a third layer of metallic iron gradually formed on the surface. The growth of metallic iron followed a three-dimensional mode known as the Volmer-Weber mode.
In order to investigate the possible coadsorption of anion and borate with Fe2+ on the Pt electrode, we conducted similar STM experiment with a Pt(111) electrode in pH3 chloride medium (1 mM HCl + 0.1 M KCl + 10 mM FeCl2). At 0.1 V, the surface exhibited varying sizes of micro triangular loops (TL) morphology, which was consistent with the results in sulfuric acid, showing a distorted hexagonal lattice. Shifting the potential from 0.1 to 0 V resulted in a transformation of the TL to a SW – like structure. It has the same characteristics as the SW pattern seen in pH3 sulfate medium, except it is apparently less ordered. The different surface morphologies indicated the co-adsorption of anions from the solution with iron ions on the electrode surface. In the presence of borate in the solution, a unique trapezoidal morphology was observed at negative potentials, confirming the co-adsorption of borate and iron ions on the electrode surface.
The catalytic activity of Pt(111) modified with iron oxide toward the oxidation of carbon monoxide (CO), oxygen reduction reaction (ORR), oxidation of formic acid (FAO), and hydrogen evolution reaction (HER) are examined. The CO oxidation reaction has been a model system to gain insight of heterogenous catalytic reaction. Voltammetric results show that the Fe(OH)x modifier causes CO admolecule to be oxidized at more negative potential than that of bare Pt(111). In situ STM is used to probe the state of Fe(OH)x – modified Pt(111) electrode immersed in CO – saturated perchloric acid, revealing Fe(OH)x and CO are adsorbed in segregated domains and compete for surface sites on the Pt electrode. It is likely that the domain boundies between Fe(OH)x and CO domains act as the active sites for CO oxidation.
The Fe(OH)x modified Pt(111) also exhibits higher activity toward ORR and FAO. The half wave potential for ORR shifts from -0.29 to -0.19 V and oxygen molecule is reduced to water via the 4e- process in 0.1 M KOH. The FAO activity is manifested by the increase of peak current by three times. These merits can stem from an increase of oxygen binding strength of oxygen entities to the Fe(OH)x modified Pt electrode. However, the Fe(OH)x modifier does not show a positive effect of Pt electrode on the reduction reaction. For example, Fe(OH)x/Pt(111) electrode cannot promote water splitting in alkaline media. | en_US |