Decomposition of Cyclooctene on Pt(111)
J. Phys. Chem., Vol. 100, No. 20, 1996 8407
Surface geometry seems favorable for the proposed C8H6 inter-
mediate: the third-nearest-neighbor distance on the Pt(111)
surface is 5.54 Å, approximately what might be expected for
the distance between two Pt atoms σ-bonded to carbon atoms
in the 1- and 4-positions on the COT ring. (The distance
between the H atoms in these two positions is about 5.3 Å.)
The presence of two additional Pt-C σ-bonds also could explain
the species’ resistance to removal by side reactions in BPTDS.
consisted solely of benzene. However, m/z ) 78 also tracks
desorption of COD and COT, and the former is produced in
significant amounts over the same temperature range as benzene
in BPTDS, so the calculated yields of benzene from COE
surface decomposition clearly must be taken only as upper
limits. Given these considerations, the total amount of benzene
produced by decomposition of COE is less than 2% of a
monolayer.
Comparing the H2 TDS curves from COE with that from
benzene in Figure 2 further suggests that the total amount of
benzene produced by decomposition of COE is small. The H2
TDS peaks from cyclooctene above 300 K in no way resemble
those characteristic of benzene dehydrogenation, whether the
benzene is directly adsorbed, as in Figure 2a, or produced by
Incorporating all of these implications, a proposed mechanism
for the decomposition of COE on Pt(111) is shown in Figure
1
. Molecular COE is the only hydrocarbon species we ever
observed with BPTDS in large quantities after dosing COE,
either before or after annealing it. The large magnitude of the
COE BPTDS signal after annealing to 250 K or less reveals
that it is still mostly molecularly adsorbed. Between 250 and
1
3,14
dehydrogenation of larger hydrocarbons.
The complete
dissimilarity of the hydrogen evolution from the two molecules,
especially the conspicuous absence of the characteristic H2 TDS
350 K, it loses an allylic hydrogen to form a species that cannot
be desorbed directly with BPTDS. By 350 K this allyl adsorbate
loses another hydrogen at the γ-carbon, yielding a C8H12 species
which again cannot be directly desorbed in BPTDS. This
species begins to dehydrogenate at ∼360 K. It may transiently
convert to an adsorbed COT species, as suggested by the low
1
for benzene at about 500 K, indicates that if any adsorbed
benzene is produced, it is present in very low quantities.
Conclusion
4
yield of COT in both TDS at 440 K and in BPTDS after the
Using BPTDS and quantitative TDS, it has been shown that
when a monolayer of cyclooctene is dosed to Pt(111), about
4
30 K flash. This must quickly dehydrogenate further to form
the C8H6 intermediate pictured in Figure 1, with two extra Pt-C
σ-bonds stabilizing the adsorbate. This species remains stable
from ∼440 to 560 K. Heating it above 560 K causes it to
dehydrogenate further. Dehydrogenation is complete by 850
K, at which point only graphitic carbon remains on the surface.
1
0% of the original cyclooctene monolayer desorbs below 350
K, and the remainder undergoes successive dehydrogenation to
produce graphitic carbon. COE is converted to benzene with
less than 2% yield. In the dehydrogenation of COE, a species
of stoichiometry C8H12 is stable at 350 K. Between 430 and
560 K, an adsorbed intermediate species with stoichiometry
C8H6 (determined by TDS) is present. Neither of these species
gives significant low-temperature BPTDS yields, indicating that
they are not composed of molecularly adsorbed versions of
stable gaseous molecules. Low yields of COT in both TDS
and BPTDS suggest that adsorbed COT may be a transient
intermediate at 430 K in the conversion of the C8H12 species to
the C8H6 species. Above 560 K, the C8H6 intermediate simply
decomposes to yield hydrogen gas and surface carbon by 850
K.
The high temperature of the BPTDS peaks for benzene
indicates that it probably is produced during BPTDS by reaction
of some unobserved surface species. The small amount of
benzene observed in both TDS and BPTDS supports the
assumption that the reaction pathway that produces benzene is
only a minor side reaction, as shown in Figure 1. The most
intense TDS peak for benzene occurs at about 390 K, suggesting
that it is produced during the conversion from the C8H12 to the
C8H6 species.
The amount of benzene formed by decomposition of COE
can be quantified with TDS by comparing the area of the m/z
Acknowledgment. The authors wish to thank the National
Science Foundation for sponsoring this research.
)
78 curve obtained when a monolayer of benzene is dosed
directly to a Pt(111) surface (Figure 2a) with the area of the
m/z ) 78 curve from the COE TDS experiment (Figure 2b).
Assuming that all of the intensity in the m/z ) 78 curve in the
COE TDS arises from benzene, the area of this curve (above
References and Notes
(1) Campbell, J. M.; Seimanides, S.; Campbell, C. T. J. Phys. Chem.
1
989, 93, 815.
2) Campbell, C. T. Crit. ReV. Surf. Chem. 1994, 4, 49 and references
therein.
(3) Hostetler, M. J.; Dubois, L. H.; Nuzzo, R. G.; Girolami, G. S. J.
3
2
50 K) is only 1.7% of the area of the m/z ) 78 trace (above
(
30 K) in the benzene TDS. Only 45% of the molecules in a
monolayer of benzene adsorbed on Pt(111) will desorb in a TDS
experiment (the other 55% dehydrogenate to leave surface
Am. Chem. Soc. 1993, 115, 2044.
(4) Hostetler, M. J.; Nuzzo, R. G.; Girolami, G. S.; Dubois, L. H. J.
Phys. Chem. 1994, 98, 2952.
1
carbon). When this factor is taken into account, the benzene
that desorbs during a COE TDS experiment corresponds to only
(
5) Henn, F. C.; Diaz, A. L.; Bussell, M. E.; Hugenschmidt, M. B.;
Domagala, M. E.; Campbell, C. T. J. Phys. Chem. 1992, 96, 5965.
6) Henn, F. C.; Bussell, M. E.; Campbell, C. T. J. Vac. Sci. Technol.
991, A9, 10.
7) (a) McLafferty, F. W.; Stauffer, D. B. The Wiley/NBS Registry of
0
.8% of a monolayer of benzene.
(
For comparison, the amount of adsorbed benzene formed
1
2
during the COE TDS experiment can be measured with BPTDS
and related to the BPTDS signal from a monolayer of benzene
dosed directly to a Pt(111) surface. The BPTDS peak area of
the m/z ) 78 signal after a 350 K flash (which gave the largest
m/z ) 78 signal in the COE BPTDS experiment) is compared
with the BPTDS peak area of the m/z ) 78 signal from a
monolayer of adsorbed benzene at 100 K. This comparison
indicates that decomposition of COE on Pt(111) yields <1.1%
of a monolayer of adsorbed benzene. (In this comparison, it is
useful to remember that adsorbed benzene at 100 K is
quantitatively desorbed as molecular benzene in BPTDS, even
(
Mass Spectral Data; John Wiley & Sons: New York, 1989. (b) Eight Peak
Index of Mass Spectra, 3rd ed.; The Royal Society of Chemistry:
Nottingham, UK, 1983.
(8) Newton, M. A.; Campbell, C. T. Catal. Lett., in press.
(9) Bartmess, J. E.; Georgiadis, R. M. Vacuum 1983, 33, 149. (Note
that the correlation of ion gauge sensitivity factor with the number of
electrons shown in this reference was used for species not specifically
studied in this work.)
(10) Paffet, M. T.; Campbell, C. T.; Taylor, T. N. J. Chem. Phys. 1986,
85, 6176.
(
(
11) Domagala, M. E.; Campbell, C. T. Surf. Sci. 1994, 301, 151.
12) Xu, C.; Koel, B. E.; Newton, M. A.; Frei, N. A.; Campbell, C. T.
J. Phys. Chem., submitted.
(13) Rodriguez, J. A.; Campbell, C. T. J. Phys. Chem. 1989, 93, 826.
2,5
after it has been preflashed to 380 K. )
(14) Rodriguez, J. A.; Campbell, C. T. J. Catal. 1989, 115, 500.
In both of the previous calculations, it was assumed that the
m/z ) 78 signal in the COE decomposition experiments
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