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J. Chem. Phys., Vol. 109, No. 14, 8 October 1998
K. T. Queeney and C. M. Friend
2
4
high-coordination sites. We propose that this methoxy inter-
mediate binds at high-coordination sites as well, based not
only on the deposition of oxygen in those sites but also on
previous studies of the reverse reaction, methyl radical addi-
tion to highly coordinated oxygen on Mo͑110͒.
Our results suggest that the distinct reactivity of metha-
nol on oxygen-modified Mo͑110͒ relative to the clean sur-
face arises from changes in the ability of the surface to pro-
mote C–H bond weakening, rather than from significant
changes in the C–O bond potential due to surface oxidation.
Competition between several decomposition pathways for a
single methoxy on oxygen-modified Mo͑110͒ suggests that
the chemistry of this intermediate is very sensitive to local
electronic structure.
M. L. Colaianni, J. G. Chen, W. H. Weinberg, and J. T. Yates, Surf. Sci.
279, 211 ͑1992͒.
2
2
5
6
K. T. Queeney and C. M. Friend ͑unpublished results͒.
This high-coordination site was previously proposed to be the quasi-
threefold site ͑Ref. 24͒; however, recent density functional calculations
indicate that sulfur on Mo͑110͒ resides in a low-symmetry site slightly
off-axis from the long bridge site ͑Ref. 56͒. Since an exact structural
characterization of oxygen overlayers on Mo͑110͒ has not yet been carried
out, it is possible that the oxygen in low-symmetry, high-coordination
sites resides in a similar off-axis site.
Directed dosing through a stainless steel tube ϳ1 cm in diameter, with the
surface held ϳ5 mm from the end of the tube, was used for dosing of all
reagents in this study. Pressures reported correspond to the rise in back-
ground pressure and hence do not account for enhanced pressure at the
crystal face.
27
2
2
8
9
D. A. Chen and C. M. Friend, J. Am. Chem. Soc. ͑in press͒.
The lack of concomitant intensity in m/eϭ29 and m/eϭ27 confirms that
this product is CO and not a hydrocarbon ͑i.e., ethane or ethylene͒.
30
16
16
5
0% of the ␣ water peak contains O; less than 1% O contamination
ACKNOWLEDGMENTS
18
was detected in the O sample.
2
3
3
1
2
M. K. Weldon and C. M. Friend, Rev. Sci. Instrum. 66, 5192 ͑1995͒.
S. R. Heller and G. W. A. Milne, EPA/NIH Mass Spectral Data Base, Vol.
The authors gratefully acknowledge the Department of
Energy for support of this work under Grant No. DE-FG02-
1, Molecular Weights 30–186 ͑U.S. Government Printing Office, Wash-
84-ER13289, and Dr. Per Uvdal and Dr. Marcus Weldon for
ington, D.C., 1978͒.
3
3
3
3
4
5
helpful discussions.
K. T. Queeney, C. R. Arumainayagam, and C. M. Friend ͑in preparation͒.
D. A. Chen and C. M. Friend, J. Phys. Chem. 101, 5712 ͑1997͒.
Methane is the only hydrocarbon product detected from reaction of
1
H.-F. Liu, R.-S. Liu, K. Y. Liew, R. E. Johnson, and J. H. Lunsford, J.
CH OH on clean Mo͑110͒ ͑Ref. 16͒.
3
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Ϫ1
3
There is a small peak at ϳ1200 cm which is assigned to a double loss
Ϫ1
4
from ͑Mo–O͒ at 605 cm ; this feature is observed consistently in our
5
spectra when there is high intensity in the quasi-threefold oxygen stretch.
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1
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8
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Preliminary scanning tunneling microscopy ͑STM͒ results indicate some
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47
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1
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When conditions permitted, 1000 scans each for sample and background
were collected in one experiment, which is analogous to co-adding two
sets of 500 scans.
53
2
3
All oxygen coverages reported herein are based on comparison of Auger
O/Mo ratios with the O/Mo ratio obtained after reaction of a saturated
5
5
5
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5
4
5
6
7
8
N. Kruse, G. Abend, and J. H. Block, Surf. Sci. 211/212, 1038 ͑1989͒.
P. G. Clark and C. M. Friend, unpublished results.
M. Chen and C. M. Friend ͑to be published͒.
K. T. Queeney and C. M. Friend ͑in preparation͒.
D. A. Chen and C. M. Friend, unpublished results.
overlayer of CH OH on clean Mo͑110͒. The absolute oxygen coverage
3
from methanol reaction on Mo͑110͒ was previously determined via x-ray
photoelectron spectroscopy to be ϳ0.35 ML ͑Ref. 34͒ in a procedure
outlined in detail elsewhere ͑Ref. 14͒.
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