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on the other hand, was also found to be more exothermic, but was only improved by
26 kJ/mol. Experimental studies in UHV also show that defects on the (111) surface
play an important role in methanol decomposition, and favor O22H cleavage through
the methoxy intermediate [Ehlers et al., 1985]. O22H cleavage is also enhanced
at
(111)
steps
in
electrocatalytic
systems,
favoring
formation
of
formic
acid
[Housmans et al., 2006].
DFT investigations of methanol dehydrogenation over Pt(111) for the aqueous
phase system show the influence of water on the methanol dehydrogenation mechan-
ism. Okamoto and co-workers [Okamoto et al., 2003] investigated homolytic aqueous
phase methanol dehydrogenation. A direct comparison of the homolytic and hetero-
lytic paths at the aqueous/Pt(111) interface, however, showed that the heterolytic
cleavage of the initial methanol C22H dehydrogenation step was preferred over homo-
lytic cleavage by over 43 kJ/mol [Cao et al., 2005]. The differences between aqueous
phase and vapor phase mechanisms parallels those discussed above for water acti-
vation, where the H
þ
product is stabilized by the aqueous media and the high work-
function of the Pt(111) surface allows for the enhanced bonding with the charged
CH
x
O
(ad)
species. In addition, hydrogen bonding with the aqueous phase stabilizes
both
the
adsorbed
methanol
(CH
3
OH
(ad)
)
and
the
dehydrogenated
species
(CH
x
O
(ad)
), further influencing the energetics from the vapor phase.
Overall, the aqueous phase dehydrogenation mechanism [Cao et al., 2005] was
found to follow a similar ordering of C22H and O22H cleavage steps to the vapor
phase case [Desai et al., 2002; Greeley and Mavrikakis, 2002, 2004]. The calculated
reaction energies for the aqueous phase and vapor phase systems are shown in
Fig. 4.11 (Plate 4.1) (the vapor phase values are in parentheses), along with the opti-
mum geometries for the adsorbed aqueous phase intermediates (showing the hydrogen
bonds between H
2
O and CH
x
O species). The preferred aqueous phase pathway for
heterolytic dehydrogenation over the ideal Pt(111) surface follows the order
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
:
13)
However, the activation of the C22H bond of CHOH
(ad)
to form COH followed by
O22H cleavage to CO
(ad)
is also a viable pathway (Fig. 4.11).
The results here clearly demonstrate some of the important differences between
reactions in the vapor phase and those in the aqueous phase. Water solvates the
ions that form and thus enhances the heterolytic bond activation processes. This
leads to more significant stabilization of the charged transition and product states
over the neutral reactant state. The changes that result in the overall energies and
the activation barriers of particular elementary steps can also act to alter the reaction
selectivity and change the mechanism.
The potential that develops in an electrochemical system such as a fuel cell can also
act to significantly influence the energies, kinetics, pathways, and reaction mechan-
isms. The double-reference potential DFT method [Cao et al., 2005] described earlier
was used to follow the influence of an external surface potential on the reaction
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