First-Principles Simulation of the Active Sites and Reaction Environment in Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 113

interface for the Pt/Pt 2 Co(111) overlayer model was found to be 0.54, which is greater
than the value of 0.44 calculated for Pt(111) (M.J. Janik and M. Neurock, unpublished
work, 2007). These results show qualitatively that DFT calculations, together with the
double-reference method to consider the potential-dependent energy of surface-bound
species, can begin to examine the competition between species for surface sites in an
electrochemical half-cell. This competition becomes more complex in an actual fuel
cell environment, where, at the cathode for example, water, oxygen, and electrolyte
anionic groups may compete for surface adsorption sites, and this competition will
depend on the local concentrations and structures available to these components.

4.4.4 Elementary Reaction Thermodynamics at the Aqueous,
Electrified Interface: Methanol Dehydrogenation

The presence of solution at a metal surface, as has been discussed, can significantly
influence the pathways and energetics of a variety of catalytic reactions, especially
electrocatalytic reactions that have the additional complexity of electrode potential.
We describe here how the presence of a solution and an electrochemical potential
influence the reaction pathways and the reaction mechanism for methanol dehydro-
genation over ideal single-crystal surfaces.
The dehydrogenation of adsorbed methanol initially proceeds via one of two poss-
ible paths. The first involves the activation of the C22H bond to form an adsorbed
hydroxymethyl (CH 2 OH (ad) ) intermediate. The second path proceeds via the acti-
vation of the O22H bond of methanol to form an adsorbed methoxy (CH 3 O (ad) ) inter-
mediate. The resulting hydrogen atom can either be adsorbed to the metal surface or
solvated by the aqueous phase. Periodic DFT investigations [Desai et al., 2002;
Greeley and Mavrikakis, 2002, 2004] of vapor phase methanol dehydrogenation (in
the absence of co-adsorbed water) over Pt(111) show that the initial dehydrogenation
step involves the homolytic activation of the C22H bond to form hydroxymethyl
and hydride surface intermediates, with an overall reaction energy of 216 kJ/mol.
The activation of the O22H bond of adsorbed methanol results in the formation
of a methoxy and hydride surface intermediate with an overall reaction energy of
þ 64 kJ/mol (80 kJ/mol less favorable). The energetics for the subsequent

CH 2 OH (ad) dehydrogenation steps indicate that the methanol dehydrogenation

proceeds via the following reaction path:

CH 3 OH (ad) ! CH 2 OH (ad) þ H (ad) ! CHOH (ad) þ 2H (ad)
! COH (ad) þ 3H (ad) ! CO (ad) þ 4H (ad) (4 : 12)

These results refer to dehydrogenation of methanol on the ideal close-packed Pt(111)
terrace. It is well established that step edges can be present on Pt surfaces and influence
the surface chemistry. The resulting paths at the coordinatively unsaturated step sites
lead to significantly enhanced binding energies. We showed previously that the overall
reaction energy for the methanol to form methoxy in the vapor phase at a model
Pt(211) step site now becomes exothermic, with reaction energy that is 78 kJ/mol
more exothermic than at the (111) terrace. The reaction of methanol to hydroxymethyl,