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F. Calaza et al.
(H = 1/9 ML), while at a higher, acetate-saturation cov-
erage (H = 1/3 ML), it decreases to 73 kJ/mol. Further-
more, the calculated activation barrier for the subsequent
b-hydride elimination step increased from ?43 kJ/mol at
low coverages (H = 1/9 ML) to ?61 kJ/mol at the higher,
saturation coverage (H = 1/3 ML). The calculated acti-
vation energies at high coverages, where the experiments
are performed, were in good agreement with the experi-
mental values. As noted above, the complete acetate
removal kinetics could be modeled using a single rate
constant [2, 3]. This appears to contradict the DFT results
as the surface is depleted of acetate during the titration,
which at first glance would be thought to increase the
activation barrier. Spectroscopic results, however, show
that the surface acetate species are predominantly replaced
by ethylidyne species towards the end of the titration
reaction. Thus, while the acetate coverage on the surface
decreases during a titration experiment, which, according
to the above arguments would affect the reaction activation
energy, and thus the reaction rate constant, the sum of the
coverages of all of the reactants and products remains high.
Thus, the environment around each of the acetate ? eth-
ylene reaction centers evolves in a relatively complex way
as the titration reaction proceeds. The fact that the acetate
titration curves can be fit by a single rate constant suggests
that these different environments result in reaction activa-
tion energies that do not change drastically during the
course of the reaction. This correspondingly implies that
the influence of the local environments and lateral inter-
actions created by adsorbed acetate and ethylidyne inter-
mediates on the reaction center are quite similar. Thus,
while, in principle, any kinetic model should explicitly take
into account modifications to the reaction activation energy
due to variations in the reaction environment, it may be
that, in the case of VAM synthesis, at least, changing the
environment from acetates to ethylidyne species, or a
combination of these, does not strongly affect the reaction
activation energy as long as the surface remains crowded.
In order to determine the acetate titration kinetics
explicitly, and to provide a more stringent comparison
between experiment and theory, the sequential kinetic
equations for both the insertion and b-hydride elimination
steps in the Samanos pathway, and VAM desorption are
solved numerically to model the time-dependent coverage
of the acetoxyethyl intermediate, the vinyl acetate product
and the ethylidyne species. The more rigorous kinetic
analysis carried out here allows the rate constants for the
formation of the acetoxyethyl species from acetate and
adsorbed ethylene, acetoxyethyl dissociation, b-hydride
elimination, and product desorption to be obtained as a
function of reaction temperature. The resulting activation
energies are compared with theoretical predictions, where
good agreement is obtained.
2 Experimental Methods
Infrared data were collected using a system that has been
described previously [13]. Briefly, the sample could be
resistively heated to 1200 K, or cooled to 80 K by thermal
contact with a liquid-nitrogen filled reservoir. Infrared
spectra were collected using a Bruker Equinox infrared
spectrometer and a liquid-nitrogen-cooled, mercury cad-
mium telluride detector. The complete light path was
enclosed and purged with dry, CO2-free air. The C2H4
(Matheson, Research Grade) and acetic acid (Aldrich,
99.99?%), were transferred to glass bottles, which were
attached to the gas-handling line for introduction into the
vacuum chamber. Kinetic measurements were carried out
by initially saturating the Pd(111) surface with acetate
species by exposure to acetic acid. A flux of ethylene
impinged onto the sample from a collimated dosing source
to obtain an enhanced flux at the Pd(111) single crystal
surface while minimizing the background pressure by using
an ethylene background pressure of 1 9 10-4 Torr [2, 3].
This ethylene pressure was selected to be sufficiently high
that no variation in acetate reaction rate was observed as a
function of ethylene pressure. The acetate removal kinetics
were measured by monitoring the acetate asymmetric OCO
vibrational mode at *1414 cm-1 [2], the vinyl acetate
mode (at *1788 cm-1 [3]) and the ethylidyne mode (at
*1330 cm-1 [14–17]). The acetoxyethyl intermediate
(which would occur at *1718 cm-1 [3]) was not detected
in experiments carried out using C2H4.
The temperature-programmed desorption (TPD) exper-
iments were carried out in a separate ultrahigh vacuum
chamber that has been described in detail elsewhere [18].
The Pd(111) single crystal was cleaned using a standard
protocol and its cleanliness monitored using Auger spec-
troscopy and TPD collected following oxygen adsorption.
3 Theoretical Methods
3.1 Kinetic Analysis
The important elementary C–O bond forming and C–H
bond breaking steps associated with the Samanos pathway
are depicted in Scheme 1 [4–7]. The rate constant for the
CH3
kdes
kh
ki
kr
C
VAM(gas)
VAM(ads)
VAMH(ads)
C2H4
O
O
Scheme 1 Depiction of the reactions occurring on the surface during
titration of acetate species by ethylene in the Samanos pathway and
the rate constants used in the fit to the reaction kinetics
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