Metal surfaces in electrochemical environments are important in satisfying future energy and remediation needs. One focus of recent activity is the four electron electroreduction of O₂ to H₂O. Despite intensive effort, little is understood about this reaction, which complicates design of new catalysts.  There is little or no insight into the initial steps in this reaction, the identity of the rate determining steps, or even the way in which O₂ associates initially with the active surface.

Figure 1: 5 nm x 5 nm AFM image (left) and schematic (right) of the (2´2) overlayer formed by Bi on a Au(111) surface.  The open adlayer structure is active for both oxygen and peroxide reduction.

We are using probe microscopic and spectroscopic means on well-defined catalyst surfaces along with computational methods to interrogate intermediates and understand the mechanism of this reaction.  As an example, we have a long standing interest in the (2´2) adlattice formed during the underpotential deposition of Bi on a Au(111) surface, which exhibits reactivity for both oxygen and peroxide electroreduction as shown in Figure 1.  We showed that reduction of peroxide on a Au surface modified with Bi adatoms proceeded via the spontaneous cleavage of peroxide to form two Bi hydroxides; a schematic reaction diagram is shown in Figure 2.

Figure 2: Proposed hydrogen peroxide reduction mechanism on the (2´2) Bi/Au(111) surface. The structure enclosed in red brackets is an intermediate.  The orange, pink, blue and white circles represent the Au, Bi, O and H atoms, respectively.

More recently, we are using Surface Enhanced Raman Spectroscopy (SERS) and Surface Enhanced Infrared Adsorption Spectroscopy (SEIRAS) to examine oxygen reduction activity on other surfaces, such as Pt and Pt-alloys. The electroreduction of nitrate on Au, Cd, and Cu surfaces is also being investigated with AFM and the surface enhanced spectroscopies to determine the active catalyst structure and mechanism.  An in situ AFM image of the Cd adlattice found to exhibit maximal catalytic activity for nitrate electroreduction is shown in Figure 3.

Figure 3:  6 X 6 nm in situ STM image of the Cd adlattice during nitrate reduction.  The Moiré pattern results as a consequence of the interaction between Cd, nitrate, and the underlying Au surface.

The insight we obtain from these studies is used to design materials that may exhibit enhanced activity.  We emphasize coupling inorganic materials, such as polyoxometalates, with electrochemical activity.  For example, we showed in a series of papers that these polyoxometalates form ordered adlayers on Ag surfaces, as shown in Figure 4.  We are developing ways to utilize these polyoxometalate-modifed electrodes in fuel cells and other energy-related applications.

Figure 4:  75 nm ´ 75 nm in situ STM image of H₄SiW₁₂O₄₀ on Ag(111).