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Figure 4.11 Optimized structures of CH
x
O species, as indicated, over aqueous-solvated
Pt(111) as determined by DFT in Cao et al. [2005]. Horizontal and vertical arrows indicate
C 2H and O 2H cleavage steps, respectively. Reaction energies are included for the aqueous
phase [Cao et al., 2005] and the vapor phase (in parentheses) [Desai et al., 2002]. The thermo-
dynamically preferred aqueous phase pathway is indicated by bold arrows (in blue). (See color
insert.)
energies and the reaction mechanisms for methanol decomposition. Plotted in the
upper portions of each panel in Fig. 4.12 are the free energies for the various methanol
dehydrogenation intermediates over the aqueous phase Pt(111) surface for a range of
external potentials (tuned by the application of an external charge q). The filled sym-
bols in Fig. 4.12 are for the q ¼ 0 system; the open symbols are for the q=0 systems.
The reaction energies for different elementary steps are plotted in the lower portion of
each panel as a function of potential. The reaction energies were determined by sub-
tracting the free energy versus potential curves shown in the upper panels: DE
free
¼
E
free( products)
-E
free(reactants)
. Similar to the aqueous phase mechanism, the initial dehy-
drogenation step shows that C22H cleavage is heterolytic and thermodynamically
favored over heterolytic O22H cleavage in the potential range of 20.5 to
þ
1.0 V
with respect to an NHE. However, the free energy of the methoxy species has a greater
dependence on the potential than does the hydroxymethyl species, indicating that
O22H cleavage becomes a competing path at higher potentials. This behavior is in
agreement with experimental findings by Cao and co-workers [Cao et al., 2005],
which show that methanol dehydrogenation involves a dual-path mechanism on
Pt(111) at potentials greater than
þ
0.35 V with respect to an NHE. Following the
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