C O M M U N I C A T I O N S
that the 43 amu trace in Figure 2 is consistent with the desorption
of vinyl acetate. Some CO2 (44 amu) also desorbs due to acetate
decomposition. The very weak 45 amu signal indicates that
essentially no acetic acid desorbs from the surface, further confirm-
ing that the 43 amu signal is due to vinyl acetate formation, in
accord with eq 1.
These results indicate that ethylene can react rapidly with surface
acetate species adsorbed on oxygen-covered Pd(111) to form vinyl
acetate, strongly suggesting that the catalytic synthesis of vinyl
acetate proceeds via a surface η2-acetate intermediate. It should be
emphasized that this does not preclude the participation of other
surface intermediates in the catalytic reaction under realistic
conditions but indicates that a surface η2-acetate species is a likely
candidate for the surface acetic-acid-derived reactant in the synthesis
of vinyl acetate. The data presented here do not allow us to establish
which of the proposed mechanisms mentioned above, either
ethylene or vinyl reaction with acetate, operates. However, the
ability to measure the rate of an elementary step in this reaction
under ultrahigh vacuum conditions will allow this question to be
addressed.
It is evident from Figure 1 that ethylidyne is formed on the
surface as it becomes depleted of acetate species, and is most likely
not involved in the formation of VAM. Ethylidyne species thermally
decompose to yield hydrogen and carbon at ∼450 K on clean
Pd(111), which will oxidize to form water and carbon dioxide,
respectively.17 It has recently been suggested that ethylene is the
source of total combustion products,17 and the detection of
ethylidyne species suggests a possible route for this reaction. The
TPR data of Figure 2 also suggest that CO2 can form from the
decomposition of the acetate, and presumably the relative contribu-
tion of ethylene combustion and acetate decomposition to the
formation of CO2 will depend on the rate at which the acetate
species react with ethylene compared to their decomposition rate.
Figure 2. TPR data collected at 44 (CO2), 43 (vinyl acetate), and 45 (acetic
acid) amu for a Pd(111)-(2×2)O surface saturated with acetate species
with a beam of ethylene impinging onto the surface with an effective
pressure of ∼1 × 10-5 Torr.
by ethylene at a beam pressure of 1 × 10-4 Torr, since neither
lowering the ethylene pressure to 2 × 10-5 Torr nor increasing it
to 2 × 10-3 Torr changed the shape of the curves shown in Figure
1.
To model the removal kinetics, the reaction between ethylene
and an acetate species is taken to be first order in the coverage of
each reactant and therefore second-order overall, giving a rate of
acetate removal as
dΘ
dt
-
) kΘΘe
(2)
where Θ is the relative acetate coverage and k the reaction rate
constant. This reaction can, in principle, proceed via the initial
formation of a vinyl species, which reacts directly with the acetate,18
or alternatively by reaction between ethylene and the acetate where
VAM is formed by a final hydrogen elimination step.19 Θe therefore
refers to the relative coverage of ethylene-derived species. It is
assumed that adsorbed acetate species block ethylene adsorption
and, with the saturation acetate coverage given by Θ0, eq 2
becomes
Acknowledgment. We gratefully acknowledge support of this
work by the U.S. Department of Energy, Division of Chemical
Sciences, Office of Basic Energy Sciences, under Grant No. DE-
FG02-92ER14289.
References
dΘ
dt
-
) kΘΘe0(Θ0 - Θ)
(3)
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Θ )
(4)
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A TPR experiment was carried out in which a beam of ethylene
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(2×2)O surface while the sample temperature was ramped at 1.7
K/s. The results are displayed in Figure 2, monitoring fragments at
44, 43, and 45 amu with a quadrupole mass spectrometer. The mass
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