Reaction of Various Reductants with Oxide Films on Pt Electrodes As Studied by the Surface Interrogation Mode of Scanning Electrochemical Microscopy (SI-SECM): Possible Validity of a Marcus Relationship

Abstract

The surface interrogation mode of scanning electrochemical microscopy (SI-SECM), an in situ method for the quantification of adsorbed intermediates at electrodes at open circuit, was used to evaluate the rate of reaction of four redox mediators of different reducing power (methyl viologen+ > Ru(NH3)6 2+ > Fe(II)[EDTA]2− > Fe(CN)6 4−) with electrogenerated platinum oxides (PtO x ) at different pHs. The rate constant determined by SI-SECM was sensitive to the nature of the Pt oxide formed at different potentials. A simplified model with finite element method simulation was used to fit the SI-SECM experimental response. The extraction of kinetic constants and their comparison to the potential (reducing power) of the mediator yielded a Marcus-theory type relationship. Although these are clearly not outer sphere reactions, if subjected to the Marcus theory treatment, one can extract an estimated value of λ = 1.3 ± 0.3 eV for the reduction of “place exchanged” oxide, which forms with oxide coverage values larger than 0.5 (as PtO). Steady state feedback experiments were also conducted in which the reduced form of the mediators was oxidized at Pt with different oxide coverage, i.e., over a wide range of potentials. A comparison of SI-SECM kinetic analysis with the Pt substrate at open circuit to the case where an electrode bias is applied indicates that for Fe(II)[EDTA]2−, the feedback current follows the oxide coverage at the Pt substrate. We suggest that thin layers of oxide provide a chemical route in which the oxide mediates oxidation of the reduced electrogenerated species; this route is governed by the reactivity of the different species of oxide formed at the electrode and not necessarily by other effects, such as direct electron tunneling through the oxide layer.

Pseudo-Single-Crystal Electrochemistry on Polycrystalline Electrodes: Visualizing Activity at Grains and Grain Boundaries on Platinum for the Fe2+/Fe3+ Redox Reaction

“…The differences in peak currents for the single-crystal measurements may be attributed to the variations in oxide formation on different facets which inhibit the oxidation of Fe 2+ . 47 The important point here, however, is that The differences between the activity of grains and grain boundaries can be seen in the I-E plots in Figure 8, constructed for these different areas from analysis of images at various potentials, from the onset potential to approaching the mass transport limited potential. As in the case of perchlorate, the trend of surface current increase with the increase of the potential (driving force) is clearly seen for both grain boundary areas and areas within grains, but grain boundary areas show higher activity at all potentials.…”

“…45,46 Fe 2+ oxidation on platinum is further complicated by the fact that it takes place at potentials at which oxidation of the platinum surface also occurs. 47 Platinum surface oxidation consists of a number of steps whose significance are timescale and potential dependent. 47,48 Initially, surface oxidation occurs through the fast formation of Pt-OH.…”

“…47 Platinum surface oxidation consists of a number of steps whose significance are timescale and potential dependent. 47,48 Initially, surface oxidation occurs through the fast formation of Pt-OH. 49 Further oxidation to form PtO and PtO 2 occurs slowly over the course of seconds to tens of seconds.…”

“…This is particularly true for the Fe 2+/3+ redox process. 47,50 Based on our recent work, 21,[24][25][26][27][28][29]33,51 SECCM provides a powerful method to investigate whether structural effects hold for the oxidation of Fe 2+ on polycrystalline platinum.…”

Review—Advances in Scanning Electrochemical Microscopy (SECM)

“…Reactivity of platinum oxides with different reductants.-In a related SI-SECM study, the effect of electrode surface oxidation on the rate of apparently outer sphere electrode reactions (e.g., R → O + ne) was investigated by considering (1) the reaction of the oxide chemically with R, and (2) tunneling of electrons from R through the oxide. 79 SI-SECM was used to evaluate the chemical rate constants of (1) for the reduced form of four redox mediators of different reducing power 6 4− ) with platinum oxides formed under mildly anodic conditions at different pH to form < 1.5 monolayers of adsorbed O as (PtO x ). This follows the scheme shown in Figure 3 where A represents PtO x , and SI-SECM was used to evaluate the open circuit chemical reactivity of PtO x .…”