First-Principles Simulation of the Active Sites and Reaction Environment in Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 94

4.2 THE ELECTRIFIED AQUEOUS METAL INTERFACE
While experiment and theory have made tremendous advances over the past few
decades in elucidating the molecular processes and transformations that occur over
ideal single-crystal surfaces, the application to aqueous phase catalytic systems has
been quite limited owing to the challenges associated with following the structure and
dynamics of the solution phase over metal substrates. Even in the case of a submersed
ideal single-crystal surface, there are a number of important issues that have obscured
our ability to elucidate the important surface intermediates and follow the elementary
physicochemical surface processes. The ability to spectroscopically isolate and resolve
reaction intermediates at the aqueous/metal interface has made it difficult to experimen-
tally establish the surface chemistry. In addition, theoretical advances and CPU limit-
ations have restricted ab initio efforts to very small and idealized model systems.
A number of significant experimental advances in spectroscopy, however, have
occurred over the past decade that make it possible to interrogate and resolve surface
reaction intermediates at complex solution/metal interfaces. This includes the
development and application of broadband sum frequency generation (BB-SFG), atte-
nuated total reflection (ATR), and surface enhanced infrared spectroscopy (SEIRS),
which allow for the vibrational signatures of adsorbed intermediates to be separated
from the background solution or solvent. In addition to these advances in spec-
troscopy, the continued increase in computational power coupled with novel algorithm
developments have enabled the simulation of much larger and complex systems,
including model aqueous/metal interfaces and, more generally, liquid/solid inter-
faces. It should be carefully noted, though, that these simulations have only recently
become possible, and thus the size of the systems, as well as the ability to
simulate their dynamics, is still limiting. Despite these issues, theory and spectroscopy
have become invaluable partners in elucidating the chemistry at these complex
interfaces.
As might be expected, the results from both theory and experiment suggest that the
solution is more than a simple spectator, and can participate in the surface physico-
chemical processes in a number of important ways [Cao et al., 2005]. It is well estab-
lished from physical organic chemistry that the presence of a protic or polar solvent
can act to stabilize charged intermediates and transition states. Most C22H, O22H,
C22O, and C22C bond breaking processes that occur at the vapor/metal interface
are carried out homolytically, whereas, in the presence of aqueous media, the hetero-
lytic pathways tend to become more prevalent. Aqueous systems also present the
opportunity for rapid proton transfer through the solution phase, which opens up
other options in terms of reaction and diffusion.
The presence of an (applied) potential at the aqueous/metal interface can, in
addition, result in significant differences in the reaction thermodynamics, mechan-
isms, and structural topologies compared with those found in the absence of a poten-
tial. Modeling the potential has been a challenge, since most of today's ab initio
methods treat chemical systems in a canonical form whereby the number of electrons
are held constant, rather than in the grand canonical form whereby the potential is held
constant. Recent advances have been made by mimicking the electrochemical model