First-Principles Simulation of the Active Sites and Reaction Environment in Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 117

most preferred pathway, methanol dehydrogenation involves the sequence

CH 3 OH (ad) ! CH 2 OH (ad) þ H (aq) ! CHOH (ad) þ 2H (aq)
! CHO (ad) þ 3H (aq) ! CO þ 4H (aq)

(4 : 14)

which is identical to the most favorable path determined in the aqueous phase (i.e, at a
constant charge of q ¼ 0 and not at constant potential). At more anodic potentials,
however, a parallel path involving the initial O22H cleavage to form methoxy
followed by C22H cleavage to form formaldehyde (CH 2 O (ad) ) can also become
favorable. The potential that results at the electrified aqueous/metal interface in this
system has a strong influence on the reaction chemistry as well as the reaction mech-
anism. This again stresses the point that the extrinsic environment, which here includes
the presence of a solution and an applied potential, can significantly influence the
catalytic properties of the system.

4.4.5 Potential Dependence of Elementary Chemical Reactions
in Electrocatalysis: Coupling of Carbon Monoxide and
Hydroxyl Species

In the previous section, we described how the double-reference method could be used
to follow the influence of water and potential on elucidating the elementary pathways
and their thermodynamic reaction energies. The simulation of electrocatalytic kinetics
is difficult owing to difficulty in appropriately tracking electron transfer kinetics over
complex metal surfaces. The double-reference method was recently used to determine
the elementary reaction activation barriers at the electrified interface as a function of
potential for the oxidation of carbon monoxide over Pt(111) [Janik and Neurock,
2006] and Pt/Ru alloy [Janik et al., 2007] surfaces. The electro-oxidation of carbon
monoxide from the Pt or alloy surface requires both the activation of water to form
oxygen or hydroxyl species and the subsequent coupling of these species with
adsorbed carbon monoxide to form the oxidized product (CO 2 or COOH, which is
rapidly oxidized to CO 2 ). In this section, we concentrate on the coupling reaction
between CO and OH, as both experimental studies [Gasteiger et al., 1994] and DFT
calculations [Janik and Neurock, 2006; Liu et al., 2003] have identified OH as the
likely oxidizing species. In addition, we have already discussed the activation of
water over Pt in Section 3.2. Here, we compare the calculated activation barriers for
the oxidation of CO for UHV, micro-solvated, and double-reference (solvated, electri-
fied interface) DFT model systems.
DFT estimates in the literature report barriers for the coupling of CO and OH at
UHV conditions over the Pt(111) surface of 0.89 eV at low coverage ( 9 ML) [Desai
and Neurock, 2003a] and 0.58 eV [Shubina et al., 2004] and 0.44 eV for higher
1

4 ML coverages [Gong et al., 2003]. Desai and Neurock examined the CO þ OH
coupling step at the aqueous - metal interface by filling the vacuum region of the
unit cell with explicit water molecules to simulate an aqueous solution. They found
that the barrier for CO oxidation at 9 ML was reduced from 0.89 eV at UHV conditions