First-Principles Simulation of the Active Sites and Reaction Environment in Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 105

(PEMFC)/direct methanol fuel cell (DMFC) cathode limit the available sites for
reduction of molecular oxygen. Alternatively, at the anode of a PEMFC or DMFC,
the oxidation of water is necessary to produce hydroxyl or oxygen species that partici-
pate in oxidation of strongly bound carbon monoxide species. Taylor and co-workers
[Taylor et al., 2007b] have recently reported on a systematic study that examined the
potential dependence of water redox reactions over a series of different metal electrode
surfaces. For comparison purposes, we will start with a brief discussion of electronic
structure studies of water activity with consideration of UHV model systems.
The coverage of water influences the mechanism for water dissociation. At low
coverage, a single H 2 O molecule binds atop a single metal atom and dissociates
homolytically to form adsorbed OH and H species:

H 2 O (ad) ! OH (ad) þ H (ad) (4 : 5)

The mechanism involves a metal atom insertion into the O22H bond, thus resulting in
the formation of an adsorbed metal22OH species (at the same or similar binding site)
and a new metal22H bond. This is a classic bond activation process, which involves a
significant stretch of the O22H bond in order to lower the antibonding s OH orbital to
enable it to accept electron density from the metal. The reaction has been calculated by
DFT to be endothermic by þ 90 kJ/mol over Pt(111) surfaces with an activation
barrier of þ 130 kJ/mol [Desai et al., 2003b].
Higher water coverages and the presence of solution both act to lower the barriers to
activate water. The intermolecular interactions that result from hydrogen bonding with
other water molecules stabilize the activated HO22H complex over the entire dis-
sociation reaction coordinate. For metals with high workfunctions, the aqueous
phase can enable heterolytic water activation

H 2 O (ad) ! OH (ad) þ H (aq)

(4 : 6)

in which hydrogen bonding within the aqueous phase stabilizes the formation of a
solvated proton as well as the OH (ad) surface intermediate. In this mechanism, the
hydroxide remains adsorbed to the metal surface at the same binding site (as
before), and the high workfunction of the metal surface enables it to favorably
accept the anionic hydroxide charge. For Pt(111), the heterolytic pathway has been
calculated to be endothermic by þ 75 kJ/mol, with an activation barrier of þ 90 kJ/
mol [Desai et al., 2003b]. The heterolytic barrier is 40 kJ/mol lower than the homo-
lytic barrier. The heterolytic barrier is significantly lower because the aqueous phase
significantly stabilizes the charged transition state by enabling O22H cleavage without
the formation of a new metal22H bond. Instead, the hydrogen-bonded water network
directly accepts and shuttles the proton into solution, thus forming H 3 O (aq) or H 5 O 2 þ (aq)
species; the transition state is stabilized by the formation of the HO (ad) 22H þ 22OH 2(aq)
state, without significant perturbation of H 2 O over the metal surface. It is well
established that polar solvents can significantly stabilize reactions involving charged
transition states. From the chemistry discussed here, the water can be considered as a
co-catalyst, and thus changes the mechanism.