Nanogap-Resolved Adsorption-Coupled Electron Transfer by Scanning Electrochemical Microscopy: Implications for...

Area of Science:

• Electrochemistry
• Surface Science
• Chemical Kinetics

Background:

• Adsorption-coupled electron-transfer (ACET) reactions involve coupled electron transfer and adsorption of redox-active molecules.
• These reactions are crucial in electrocatalysis, often involving reversibly adsorbed reductants as intermediates.
• ACET mechanisms can be concerted (simultaneous reduction and adsorption) or non-concerted (separate steps).

Purpose of the Study:

• To experimentally identify the mechanism of ACET reactions.
• To differentiate between concerted, non-concerted, and mixed ACET mechanisms.
• To investigate the role of reductant adsorption in ACET reaction kinetics.

Main Methods:

• Utilized nanoscale scanning electrochemical microscopy (SECM) in a transient voltammetric mode.
• Employed finite element simulation to compare experimental data with theoretical mechanisms.
• Studied ferrocene derivatives adsorbed on highly oriented pyrolytic graphite as model systems.

Main Results:

• Demonstrated experimental identification of ACET reaction mechanisms for the first time.
• Revealed a non-concerted mechanism for ferrocene derivatives adsorbed on graphite.
• Showed that the ACET step is intrinsically slower than outer-sphere electron transfer by at least four orders of magnitude.

Conclusions:

• Adsorption can thermodynamically facilitate ACET but does not necessarily accelerate the reaction.
• The non-concerted mechanism implies adsorption and electron transfer occur sequentially.
• SECM-based transient voltammetry is a powerful tool for dissecting elementary steps in electrocatalysis.