ABSTRACT
Mediators and Its Implications in Electrocatalysis ...................................313 4.6 Detection and Quanti†cation of Adsorbed Hydrogen at Platinum:
Application to the Catalytic Decomposition of Formic Acid .................. 326 4.7 A Catalytic Interrogation: The Reaction of Electrogenerated
Bromine with Adsorbed Carbon Monoxide on Platinum ........................ 334 4.8 Outlook for SI-SECM .............................................................................. 340 References ......................................................................................................... 341
The electrochemical investigation of outer sphere electron transfer reactions coupled to complicated homogeneous reactions has become highly developed over the last decades because of the utilization of powerful techniques, like cyclic and ultramicroelectrode (UME) voltammetry and digital simulations [1-2]. However, understanding the inner sphere electrocatalytic reactions is more dif†cult because of their greater complexity and the dif†culty in obtaining reliable quantitative data about intermediates and products on electrode surfaces. A variety of electrochemical and surface spectroscopic methods that can be employed with electrodes have been developed for this purpose [3]. For instance, mass sensitive devices such as the electrochemical quartz crystal microbalance allow the detection of small changes in the mass of an electrode during an electrochemical process. Infrared (IR) based techniques have been very successful at detecting carbonyl-based adsorbed species in electrocatalytic systems, second harmonic generation studies provide rich molecular information of chemisorbed species, and radiotracer methods have been used to distinguish mechanistic pathways in selected catalytic systems. However, it has been dif†cult to obtain the necessary versatility and sensitivity to address complex electrochemical processes in a direct quantitative manner. We review here an in-situ electrochemical technique based on the scanning electrochemical microscope (SECM) [4] for the detection and quanti†cation of adsorbed species at electrodes that may be useful in this area [5]. This technique is based on the feedback mode of SECM, which is one of the †rst developed [6] and perhaps one of the most distinctive of this technique, because it can correlate an electrochemical signal at the SECM tip to topographical, chemical, and electrochemical features [7] in a variety of systems (e.g., imaging interdigitated arrays [8] or a leaf [9], detecting an immobilized enzyme [10] and measuring the heterogeneous kinetics on semiconductors [11]). The SECM tip can detect changes in the electrochemical signal produced by the concurrent production and collection of the species of a mediator pair (O/R, where only one of the species is present initially in solution); variations in the diffusive «ux of these species caused by interaction with the substrate allow the current measured at the SECM tip to respond accordingly. One of the most useful features of SECM is the ability to calculate the tip current as a function of different solution and interfacial phenomena, i.e., electrochemical kinetics, by digital simulation [4]. We can thus consider the mediator to be an interrogation agent that reports to the SECM tip the state of the surface being examined.
