Reaction of ethylene with clean and carbide-modified Mo(110): Converting surface reactivities of molybdenum to Pt-group metals

Article abstract of DOI:10.1021/ja960656l

A comparative investigation of the surface reaction of ethylene with clean Mo(110) and carbide-modified Mo(110) has been carried out using high-resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD). As typically observed for early transition metals, the clean Mo(110) surface interacts very strongly with ethylene, as indicated by the decomposition of ethylene to produce C₂H₂ surface species at temperatures as low as 80 K. The surface acetylene species further decompose to atomic carbon and hydrogen at higher temperatures. The strong reactivity of the Mo(110) surface can be modified by the formation of carbide. The surface reactivity is modified in such a way that the reaction mechanism of ethylene on C/Mo(110) is very similar to those typically observed on Pt-group metal surfaces: At 80 K, ethylene molecules bond to the C/Mo(110) surface in the di-σ bonded configuration; a new surface reaction intermediate, which can be best described as ethylidyne species, is detected in the temperature range of 260-350 K. In addition, the interaction of ethylene with oxygen-modified Mo(110) is also compared to reveal the different modification effects of carbon and oxygen adatoms on the reactivities of Mo(110). The oxygen-modified Mo(110) surface is found to be inert toward the decomposition of ethylene, as indicated by the formation of weakly adsorbed π-bonded ethylene species at 80 K and by the reversible molecular desorption at higher temperatures.

Full text of DOI:10.1021/ja960656l

J. Am. Chem. Soc. 1996, 118, 11599-11609
11599
Reaction of Ethylene with Clean and Carbide-Modified
Mo(110): Converting Surface Reactivities of Molybdenum to
Pt-Group Metals
B. Fru1hberger and J. G. Chen*
Contribution from the Corporate Research Laboratories, Exxon Research and Engineering
Company, Annandale, New Jersey 08801
ReceiVed February 28, 1996X
Abstract: A comparative investigation of the surface reaction of ethylene with clean Mo(110) and carbide-modified
Mo(110) has been carried out using high-resolution electron energy loss spectroscopy (HREELS) and temperature
programmed desorption (TPD). As typically observed for early transition metals, the clean Mo(110) surface interacts
very strongly with ethylene, as indicated by the decomposition of ethylene to produce C2H2 surface species at
temperatures as low as 80 K. The surface acetylene species further decompose to atomic carbon and hydrogen at
higher temperatures. The strong reactivity of the Mo(110) surface can be modified by the formation of carbide.
The surface reactivity is modified in such a way that the reaction mechanism of ethylene on C/Mo(110) is very
similar to those typically observed on Pt-group metal surfaces: At 80 K, ethylene molecules bond to the C/Mo(110)
surface in the di-σ bonded configuration; a new surface reaction intermediate, which can be best described as ethylidyne
species, is detected in the temperature range of 260-350 K. In addition, the interaction of ethylene with oxygen-
modified Mo(110) is also compared to reveal the different modification effects of carbon and oxygen adatoms on
the reactivities of Mo(110). The oxygen-modified Mo(110) surface is found to be inert toward the decomposition
of ethylene, as indicated by the formation of weakly adsorbed π-bonded ethylene species at 80 K and by the reversible
molecular desorption at higher temperatures.
I. Introduction
those of Pt-group metals. These conclusions were based
primarily on catalytic reactor studies using amorphous powder
carbide materials as catalysts.5
One of the most fundamental questions in chemistry is how
to convert the reactivity of one group of elements to another.
In general, transition metals on the left side of the Periodic Table
are much more reactive than their counterparts on the right side.
For example, early transition metals (Groups IVB-VIB) are
known to be highly reactive toward the decomposition of
unsaturated hydrocarbon molecules.1-3 Such a high reactivity
often leads to the cleavage of carbon-carbon bonds, which is
not desired in many industrial reactions. On the other hand,
the Group VIII metals, especially the Pt-group noble metals (Pt,
Pd, Ir, Rh, Ru, Os), interact with unsaturated hydrocarbon
molecules in a less reactive manner.4,5 For example, these
metals are excellent catalysts for hydrogenation reactions, during
which a -CH2dCH2- moiety is converted to -CH3-CH3- with
the carbon-carbon bond remaining intact. Because early
transition metals are much less expensive than the Pt-group
metals, it would be of significant importance if the reactivities
of early transition metals could be converted to those of Pt-
group metals. It has been demonstrated in catalysis literature5
that the catalytic properties of early transition metals could be
significantly modified by the formation of interstitial carbides,
and that the modified reactivities often showed similarities to
In order to understand the fundamental aspects of how the
formation of interstitial carbides would convert the chemical
properties of early transition metals to those of Pt-group metals,
we have carried out a series of experiments to directly compare
the reactivities of atomically clean Mo(110) and carbide-
modified Mo(110) surfaces. The main objective of this paper
is to provide spectroscopic evidence, by using ethylene as a
probing molecule, to demonstrate that the surface reactivity of
an early transition metal, such as molybdenum, can indeed be
converted to those characteristic of Pt-group metals.
The advantage of using ethylene as a probing molecule is
that the adsorption and decomposition of ethylene on transition
metal surfaces have been investigated extensively in both
experimental and theoretical studies. The interaction of ethylene
with transition metals is usually described in terms of the Dewar-
Chatt-Duncanson theory,1 which involves primarily the dona-
tion-back-donation of π-electrons of ethylene and d-electrons
of metals. In this mechanism, electrons in the highest filled
π-orbital of ethylene are partially donated to an empty σ-orbital
of the metal atom. Such a donation weakens the π-bond of
ethylene and lowers the energy level of the lowest-lying
antibonding π*-orbital, which in turn allows a more efficient
back-donation of electrons from the metal atom, further
weakening the CdC bond. Recent calculations1 have demon-
strated a general trend in the bonding of ethylene across the
second-row transition metal series. On the left side of the
Periodic Table ethylene was found to form purely covalent
bonds, resulting in a metallacyclopropane with a carbon-carbon
single bond. On the right side of the Periodic Table the bonding
was described as an optimize mixing between purely covalent
bonding and donation-back-donation bonding. In the later case,
X Abstract published in AdVance ACS Abstracts, November 1, 1996.
(1) Blomberg, M. R. A.; Siegbahn, P. E. M.; Svensson, M. J. Phys. Chem.
1992, 96, 9794.
(2) Zaera, F. Chem. ReViews, 1995, 95, 2651; and references therein.
(3) Bent, B. E. Chem. ReViews, 1996, 96, 1361; and references therein.
(4) See for example: (a) Steininger, H.; Ibach, H.; Lehwald, S. Surf.
Sci. 1982, 117, 685. (b) Hills, M. M.; Parmeter, J. E.; Mullins, C. B.;
Weinberg, W. H. J. Am. Chem. Soc. 1986, 108, 3554. (c) Gates, J. A.;
Kesmodel, L. L. Surf. Sci. 1983, 124, 68. (d) Nascente, P. A. P.; Van Hove,
M. A.; Somorjai, G. A. Surf. Sci. 1991, 253, 167. (e) Marinova, T. S.;
Kostov, K. L. Surf. Sci. 1987, 181, 573.
(5) (a) Oyama, S. T. Catal. Today 1992, 15, 179; and references therein.
(b) Oyama, S. T. The Chemistry of Transition Metal Carbides and Nitrides,
Blackie Academic and Professional: Glasgow, 1996.
S0002-7863(96)00656-7 CCC: $12.00 © 1996 American Chemical Society

11600 J. Am. Chem. Soc., Vol. 118, No. 46, 1996
Fru¨hberger and Chen
the carbon-carbon bond distance is between that of a double
and a single bond, indicating a weaker interaction of the CdC
bond with the second-row group VIII metals. Accordingly, in
experimental studies the reaction pathway of ethylene is often
distinctly different from one group of metals to another.2,3 For
example, early transition metals are generally found to be highly
reactive, leading to the formation of deep dehydrogenation
products, C2Hx (x e 2), with both carbon atoms bonding to the
surface. These intermediates undergo further thermal decom-
position to produce atomic carbon and hydrogen. The interac-
tion of ethylene with the group VIII noble metals, in contrast,
generally occurs via different mechanisms. It is now well
established that the reaction of ethylene with group VIII noble
metals, in particular the close-packed faces of Pt, Pd, Ir, Rh,
and Ru, proceeds via an ethylidyne (CCH3) intermediate, which
has been discussed in detail in several recent review articles.2,3
High- resolution electron energy loss spectroscopy (HREELS)
is among the most widely used diagnostic techniques for
identification of this surface reaction intermediate.4
II. Experimental Methods
The experiments were carried out in a three level stainless
steel ultrahigh vacuum (UHV) chamber previously described
elsewhere.7 Briefly, the chamber is equipped with a double-
pass cylindrical mirror electron energy analyzer (CMA) for
Auger- and photoelectron spectroscopy measurements, a Mg/
Al dual anode X-ray source for X-ray photoelectron spectros-
copy, a random flux shielded quadrupole mass spectrometer
(QMS) for temperature programmed desorption studies and low-
energy electron diffraction (LEED) optics for surface structure
determination. The bottom level of the chamber is equipped
with an LK3000 double-pass high-resolution electron energy
loss spectrometer (HREELS) for vibrational analysis. The
HREELS spectra reported here were acquired with a primary
beam energy of 6 eV. Angles of incidence and reflection were
60° with respect to the surface normal in the specular direction.
Count rates in the elastic peak were typically 5 × 105 to 1 ×
106 cps, and the spectral resolution was between 30-35 cm-1
fwhm (full-width at half maximum). For TPD experiments the
Mo(110) sample was heated with a linear heating rate of 3 K/s.
The single crystal sample was a [110] oriented, 1.5 mm thick
molybdenum disk (99.999%), 12 mm in diameter, and was
purchased from Metal Crystals and Oxides, Ltd., Cambridge,
England. The Mo(110) surface was cleaned using standard
procedures, as described in detail previously.7 After cleaning
the Mo(110) surface displayed a sharp characteristic bcc(110)
LEED pattern. Sample cleanliness was verified with Auger
spectroscopy and HREELS. All gases were of research purity
and were used without further purification. Exposures are
estimated from uncorrected ion gauge readings, expressed in
units of Langmuir (L), with 1 L ) 1.33 × 10-6 mbar.s.
Figure 1. HREELS spectra recorded after exposing Mo(110) to 1 L
and 10 L of C2H4 at 80 K. The spectra were recorded at on- and 15°
off-specular directions.
contaminant CO7 and N2,8 respectively, which adsorb onto the
surface during dosing and data acquisition (∼40 min). From
the vibrational intensities of these two features in the specular
HREELS spectra and from the TPD peak areas of CO and N2,
we estimate the coverages for both of these contaminants to be
less than 0.04 monolayers (ML).
The low frequency feature at 250 cm-1 is also present in the
spectrum of the clean Mo(110) surface, indicating that it is
related to the vibrational motion of the Mo(110) substrate.
Previous surface phonon studies indicate that, for a metal
substrate with a C2V symmetry, at least one dipole-active surface
phonon mode is expected in this frequency range.9 We therefore
assign this feature to the surface phonon mode of the Mo(110)
substrate.
One important observation in Figure 1a,b is the absence of
any vibrational feature in the frequency range of 1300-1450
cm-1, which is the spectroscopic region for the relatively intense
scissor modes of CH2 groups. A comparison of the on- and
off- specular HREELS spectra of the 1 L C2H4/Mo(110) surface
also reveals that the relative intensities of the vibrational features
change significantly. The intensities of the 250, 390, 740 and
1130 cm-1 features decrease substantially in the off-specular
measurements (notice the different expansion factors), suggest-
ing a dipolar excitation mechanism for these vibrational modes.9
The HREELS spectra displayed significant changes at
exposures above 1 L, as demonstrated in Figure 1c,d. Vibra-
tional features were detected at 365, 620, 920, 1095, 1365 and
III. Results
∼2915 cm-1
.
The feature at 1875 cm-1 is again due to
3.1. Adsorption and Reaction of Ethylene on Clean Mo-
(110). 3.1.1. Low Temperature Adsorption. Figure 1 shows
vibrational spectra, recorded in the on- and off-specular direc-
tions, after the clean Mo(110) surface was exposed to ethylene
at a surface temperature of 80 K. At low exposures (up to ∼1
L exposure, Figure 1a,b), vibrational features are observed at
250, 390, 740, 905, 1130 and ∼2900 cm-1. The two additional
features at 1875 and ∼2200 cm-1 are due to trace amounts of
contaminant CO. The most significant spectroscopic difference
between the low and high ethylene exposures is the onset of
two new vibrational features at higher exposures at 620 and
1365 cm-1, respectively. A comparison of Figure 1c,d indicates
that the relative intensities of the vibrational features are nearly
the same in the on- and off-specular measurements.
To assist the assignment of the vibrational features, spectra
for corresponding low and high exposures were also recorded
(6) (a) Chen, J. G. Chem. ReV. 1996, 96, 1477; and references therein.
(b) Chen, J. G. J. Catal. 1995, 154, 80. (c) Chen, J. G.; Fru¨hberger, B.;
Weisel, M. D.; Baumgartner, J. E.; DeVries, B. D. in Ref. 5b, p 439.
(7) Fru¨hberger, B.; Chen, J. G. Surf. Sci. 1995, 342, 38.
(8) Fru¨hberger, B.; Colaianni, M. L.; Chen, J. G. in preparation.
(9) Ibach, H.; Mills, D. L. Electron Energy Loss Spectroscopy and
Surface Vibrations; Academic Press: New York, 1982.

Reaction of Ethylene with Clean and Carbide-Modified Mo(110)
J. Am. Chem. Soc., Vol. 118, No. 46, 1996 11601
Figure 2. HREELS spectra recorded after exposing Mo(110) to 1 L
and 10 L of C2D4 at 80 K.
after exposing the sample to the deuterated compound, C2D4.
These spectra are shown in Figure 2. After an exposure of 1 L
C2D4 (Figure 2a) at 80 K, vibrational features were observed at
255, 400, 640, 710, 1095 and 2170 cm-1. Similar to that
observed for C2H4, significant spectroscopic changes occurred
at C2D4 exposures greater than 1 L. As shown in Figure 2b,
vibrational features at 455, 695, 860, 1000, 1125 and 2190 cm-1
were detected after an exposure of 10 L C2D4.
Figure 3. TPD spectra of hydrogen following the thermal decomposi-
tion of ethylene on Mo(110). The inset shows the AES peak-to-peak
height ratios of C(KLL)/Mo(MNN) as a function of ethylene exposures
at 80 K after subsequently heating to 1200 K.
3.1.2. Thermal Decomposition of Ethylene on Mo(110).
Figure 3 shows the desorption of hydrogen after exposing Mo-
(110) to 1 L and 10 L of ethylene at 80 K. The only desorption
product, resulting from the decomposition of adsorbed ethylene,
is hydrogen at all ethylene exposures. At ethylene exposures
up to 1 L, hydrogen desopriton is characterized by one peak
centered at approximately 432 K. After exposing the surface
to a saturation dose of 10 L at 80 K, a second hydrogen
desorption feature is detected at approximately 301 K. In
addition, at ethylene exposures g1 L, molecularly desorbed
ethylene was also detected in the TPD measurements at
approximately 150 K (spectra not shown). The amount of
ethylene decomposition as a function of exposure is shown in
the inset of Figure 3. The AES measurements were carried out
after the ethylene-exposed surfaces were heated to 1200 K. At
exposures up to 1 L, the amount of atomic carbon on the surface,
as approximated by the AES peak-to-peak height ratio of the
C(KLL)/Mo(MNN) features, increases almost linearly with
ethylene exposure at 80 K. The AES ratio increases only
slightly at higher exposures, from 0.12 at 1 L to 0.15 at 10 L.
Based on the relative sensitivity factors for the C(KLL) and
Mo(MNN) transitions,10 these two AES ratios correspond to
atomic C/Mo ratios of 0.24 and 0.30, respectively.
Figure 4 shows a comparison of hydrogen TPD spectra from
C2H4/Mo(110) and H/Mo(110). These two surfaces were
prepared by exposing Mo(110), at 80 K, to 10 L ethylene and
hydrogen, respectively. This exposure results in saturation
coverages for both molecules at 80 K. The TPD spectrum of
hydrogen/Mo(110) is characterized by a desorption peak at 415
K, which has an asymmetric shoulder on the high temperature
side. More details about the TPD and vibrational investigations
of the dissociative adsorption of hydrogen on various surface
sites of Mo(110) will be published elsewhere.11 The most
important difference in Figure 4 is the absence of the 301 K
desorption peak from the hydrogen/Mo(110) surface. In fact,
Figure 4. Comparison of TPD spectra of hydrogen from Mo(110)
surfaces exposed to 10 L C2H4 and 10 L H2 at 80 K.
TPD measurements following the dissociation of hydrogen on
Mo(110) never reveal any desorption features at T < 350 K at
all hydrogen coverages.11
In order to further investigate the origin of the hydrogen
desorption feature at 301 K, we have carried out coadsorption
experiments with C2D4 and C2H4. Figure 5 shows the TPD
spectra following the desorption of D2, HD and H2. The clean
Mo(110) surface was first exposed to 1 L C2D4 and subsequently
to 9 L C2H4 at 80 K. The TPD spectra of all three hydrogen
isotope molecules contain a low temperature desorption peak
at 301 K and a high temperature peak at 417-441 K. However,
the relative peak areas of these two TPD features are very
different for the three molecules. The peak area ratios of the
low and high temperature features change from e0.02 for D2
to 0.25 for H2.
HREELS results following the thermal decomposition of
ethylene on the clean Mo(110) surface are shown in Figure 6.
Figure 6a-c show the thermal behavior of the C2H4/Mo(110)
surface, prepared by exposing Mo(110) to 1 L of ethylene at
80 K. The sample was heated for 30 s to the indicated
temperatures, which were chosen on the basis of the TPD results.
(10) Davis, L. E.; MacDonald, N. C.; Palmberg, P. W.; Riach, G. E.;
Weber, R. E. Handbook of Auger Electron Spectroscopy; Perkin Elmer:
Eden Prairie, Minnesota, 1976; p 13.
(11) Colaianni, M. L.; Fru¨hberger, B.; Chen, J. G. in preparation.

11602 J. Am. Chem. Soc., Vol. 118, No. 46, 1996
Fru¨hberger and Chen
Figure 5. Comparison of TPD spectra of D2, HD and H2 following
the thermal decomposition of ethylene on clean Mo(110). The adsorbed
layer was prepared at 80 K by first dosing 1 L C2D4 followed by dosing
9 L C2H4.
Figure 7. Comparison of TPD spectra of hydrogen following the
decomposition of ethylene on clean and chemically modified surfaces.
See text for the preparation of chemically modified Mo(110) surfaces.
for the reaction of ethylene with chemically modified Mo(110)
surfaces. Our primary focus was on carbide-modified Mo(110)
surfaces. The reaction of ethylene on oxygen-modified Mo-
(110) was used as a chemically inert model system to demon-
strate the different modification effects by carbon and oxygen.
The preparation of carbide-modified Mo(110) surfaces have
been discussed in detail previously7,12,13 In brief, carbide-
modified surfaces were prepared by exposing Mo(110) to
ethylene at 600 K followed by annealing to 1200 K. During
annealing, carbon diffuses into the sample and fills interstitial
lattice sites. The accumulation of carbon in this manner during
repetitive dosing/annealing cycles can be followed by AES;
C/Mo(110) surfaces with different AES C/Mo ratios can be
obtained by controlling the number of dosing/annealing cycles.13
The C/Mo(110) surfaces used in the current study, with an AES
C(KLL)/Mo(MNN) ratio in the range of 0.13-0.32, are
characterized by a p(4 × 4) LEED pattern. The exact structures
of the p(4 × 4)-C/Mo(110) surfaces are not known at present.12,13
Oxygen-modified surfaces were prepared by direct dissociation
of molecular oxygen. All oxygen modified-surfaces used in
this study were characterized by a p(2 × 2) LEED pattern, which
corresponded to an oxygen coverage of θo ) 0.25 ML.14
Figure 7 compares thermal desorption spectra of hydrogen
from the decomposition of ethylene on clean and chemically-
modified Mo(110) surfaces. The bottom spectrum was obtained
from the clean Mo(110) surface, followed by carbide-modified
Mo(110) surfaces with different C/Mo ratios as determined by
AES, and an oxygen-modified surface with 0.25 ML of atomic
oxygen. All surfaces were exposed to 1 L of ethylene at 80 K.
The desorption of hydrogen, resulting from the thermal decom-
position of ethylene, is observed from the clean Mo(110) surface
as well as C/Mo(110) surfaces. The hydrogen peak areas from
all C/Mo(110) surfaces are g80% of that from clean Mo(110),
indicating that the C/Mo(110) surfaces remain reactive toward
the decomposition of ethylene. In contrast, the peak area of
hydrogen from oxygen-modified Mo(110) is < 7% of that from
Mo(110), demonstrating that the decomposition of ethylene is
inhibited by the presence of as little as 0.25 ML of chemisorbed
oxygen on Mo(110).
Figure 6. HREELS results following the thermal decomposition of 1
L C2H4 on Mo(110). A HREELS spectrum, obtained after heating a
10 L C2H4/Mo(110) surface to 350 K, is also compared. All HREELS
spectra were recorded at 80 K.
As shown in Figure 6a-c, the number of vibrational features
and their frequencies remain essentially the same at temperatures
up to 350 K. Minor spectroscopic changes in this temperature
range are due to the desorption and decomposition of CO and
N2 impurities.7,8 The occurrence of a vibrational feature at 560
cm-1 in Figure 6c, related to the ν(Mo-O) mode, is a result of
contaminant CO decomposition.7 For comparison, Figure 6d
shows the HREELS spectrum after heating a 10 L C2H4/Mo-
(110) surface to 350 K. This spectrum is very similar to Figure
6c, suggesting the formation of a similar surface intermediate
at 350 K, independent of the initial ethylene exposures at 80
K. After further heating the C2H4/Mo(110) surfaces to 450 K,
which is above the TPD desorption temperature of hydrogen,
all hydrocarbon related vibrational features disappear in the
HREELS spectra (not shown); the only intense feature is the
ν(Mo-C) mode of the atomic carbon on Mo(110).
(12) Young, A. B.; Slavin, A. J. Surf. Sci. 1991, 245, 56.
(13) Fru¨hberger, B.; Chen, J. G.; Eng, Jr. J.; Bent, B. E. J. Vac. Sci.
Technol. 1996, A14, 1475.
3.2. Adsorption and Reaction of Ethylene on Chemically-
Modified Mo(110). In this section, we will describe results
(14) Bauer, E.; Poppa H. Surf. Sci. 1983, 127, 243.

Reaction of Ethylene with Clean and Carbide-Modified Mo(110)
J. Am. Chem. Soc., Vol. 118, No. 46, 1996 11603
Figure 9. On- and off-specular HREELS spectra of the reaction
intermediate on C/Mo(110). The adsorbed layer was prepared by dosing
C/Mo(110) to 5 L C2H4 at 80 K followed by annealing to 300 K for 30
s.
Figure 8. HREELS results following the thermal decomposition of 1
L C2H4 on C/Mo(110). All HREELS spectra were recorded at 80 K.
HREELS spectra following the thermal decomposition of
ethylene on a carbide-modified Mo(110) surface are reproduced
in Figure 8. The C/Mo(110) surface used in Figure 8 was
characterized by a C/Mo AES ratio of ∼0.20. Spectra obtained
from C/Mo(110) surfaces with higher C/Mo AES ratios show
similar sequence of spectroscopic evolutions as those depicted
in Figure 8. Before exposure to ethylene the p(4 × 4)-C/Mo-
(110) surface exhibited a single C-Mo stretching vibration at
∼380 cm-1 (Figure 8a). After exposure to 1 L ethylene at 80
K (Figure 8b), vibrational features were found at 635, 905, 1035,
1180, 1395 2935 and 3010 cm-1. It is important to point out
that this spectrum is qualitatively different from corresponding
experiment on clean Mo(110) (Figure 1a). In addition, signifi-
cant spectroscopic changes were observed after heating the
surface to 260 K for 30 s. As shown in Figure 8c, a new set of
vibrational features was found at 525, 920, 1075, 1345, 1430
and 2915 cm-1. Further heating to 350 K (Figure 8d) did not
strongly alter the spectra in terms of the number of vibrational
features and their frequencies, although the intensities of these
features decreased after the heating.
Figure 9 shows a comparison of on- and off-specular
HREELS spectra of the thermal decomposition intermediate on
the carbide-modified surface. The surface was prepared by
exposing the C/Mo(110) surface to 5 L of ethylene at 80 K
followed by heating to 300 K for 30 s. The on-specular
spectrum (Figure 9a) is very similar to the 260 K spectrum in
Figure 8 in terms of the number of vibrational features as well
as their frequencies. Figure 9 shows that the relative intensities
of all vibrational features alter only slightly in the on- and off-
specular measurement.
Figure 10. HREELS results following the thermal decomposition of
1 L C2D4 on C/Mo(110). All HREELS spectra were recorded at 80 K.
oxygen coverage of 0.25 ML. Figure 11 shows a comparison
of HREELS spectra recorded after exposing the surface to C2H4
(Figure 11a) and C2D4 (Figure 11b) at 80 K, and after heating
the adsorbed layer to 220 K for 30 s (Figure 11c). The two
low frequency features, at 390 and 595 cm-1, are due to the
deformation and stretching Mo-O motions of the O/Mo(110)
substrate, respectively.15 After exposing the O/Mo(110) to
ethylene at 80 K, the HREELS spectrum shows one intense
feature at 965 cm-1, and other relatively weak features at 1170,
1335, 1420, 1595, 2975 and 3065 cm-1
. Similarly, the
HREELS spectrum of the C2D4/O/Mo(110) surface shows one
intense feature at 720 cm-1, and other relatively weak features
at 900, 980, 1130, 1270, 1500, 2240 and 2300 cm-1. Vibra-
tional features related to ethylene molecules almost completely
disappeared after the adsorbed layer was heated to 220 K. This
surface was again characterized by a p(2 × 2)-O LEED pattern
and AES measurements revealed that the C(KLL)/Mo(MNN)
ratio of this surface was less than 0.02. These observations
suggest that, in agreement with the TPD results from the O/Mo-
(110) surface (Figure 7), the majority of adsorbed ethylene
species desorbs from the O/Mo(110) surface without undergoing
decomposition.
To assist the vibrational assignments, HREELS spectra were
also recorded following the thermal decomposition of C2D4 on
the carbide-modified surface, as shown in Figure 10. After
exposure to 1 L C2D4 at 80 K (Figure 10a), vibrational features
were found at 405, 670, 870, 980, 1145, 2185 and 2255 cm-1
.
Heating to 260 K (Figure 10b) again led to significant
spectroscopic changes. The spectra displayed features at 560,
670, 870, 1015, 1105 and 2180 cm-1. The relatively weak
feature around 2920 cm-1 is due to adsorption and decomposi-
tion of a small fraction of non-deuterated ethylene molecules.
Finally, the thermal behavior of ethylene on an oxygen-
modified surface was studied using HREELS. The O/Mo(110)
surface was characterized by a p(2 × 2) LEED pattern with an
(15) Colaianni, M. L.; Chen, J. G.; Weinberg, W. H.; Yates, J. T., Jr.
Surf. Sci. 1992, 279, 211.

11604 J. Am. Chem. Soc., Vol. 118, No. 46, 1996
Fru¨hberger and Chen
Table 1. Vibrational Assignment of π-Bonded Ethylene Species
symmetry
C2H4 (C2D4)
(gas-phase)20
C2H4 (C2D4)
(2 × 2)-O/Mo(110)
vibrational assignment
D2h
C2V
ωH/ωD
ωH/ωD
F(CH2) (CH2-rocking) (ν10)
ω(CH2) (CH2-wagging) (ν7)
τ(CH2) (CH2-twisting) (ν6)
δ(CH2) (CH2-scissor) (ν3)
δ(CH2) (CH2-scissor) (ν12)
ν(CC) (ν2)
B2u
B1u
B1g
Ag
B3u
Ag
B2
A1
A2
A1
B1
A1
B1
B2
826 (593)
949 (720)
1.39
1.32
1.38
1.37
1.34
1.07
1.36
1.32
965 (720)
1170 (900)
1335 (980)
1420 (1130)
1595 (1500)
2975 (2240)
3065 (2300)
1.34
1.30
1.36
1.26
1.06
1.33
1.33
1222 (883)
1342 (981)
1444 (1078)
1623 (1515)
2989 (2200)
3106 (2345)
νs(CH2) (ν11)
B3u
B2u
νa-CH2 (ν9)
π-bonded ethylene is given in Table 1. The similar vibrational
frequencies between ethylene molecules in the gas-phase20 and
on O/Mo(110) again confirm a very weak interaction between
ethylene and the oxygen-modified Mo(110) surface. In addition
to the ethylene vibrational modes, two relatively weak features
are also observed in the C2D4 spectrum at 1270 cm-1 and 2155
cm-1, respectively. We are unable to assign the 1270 cm-1
feature because this frequency is not expected for any funda-
mental vibrational mode for C2D4; its frequency also differs
from expected values of the overtone and/or combination modes
of the more intense, low-frequency modes in Figure 11b. The
2155 cm-1 is most likely due to the adsorption of residue CO
on the terminal sites, as has been previously observed on the
O/Mo(110) surface.21
The observation of a very intense symmetric CH2 wag mode,
ω(CH2), at 965 cm-1 (and the ω(CD2) mode at 720 cm-1) is
another indication of the very weak interaction between ethylene
and the O/Mo(110) substrate. The concept of correlating the
adsorption strength to the intensity of the ω(CH2) mode is first
proposed by Ibach and Mills in the comparative studies of
ethylene on Ag(110) and Pt(111).9 For example, on the weakly
adsorbed C2H4/Ag(110) surface, where C2H4 is π-bonded with
its molecular plane parallel to the surface, the ω(CH2) mode is
by far the most intense feature.9 A simple comparison with
gas-phase C2H4 (D2h symmetry) reveals that the ω(CH2) mode
is the only vibrational motion with a dipole moment perpen-
dicular to the plane of molecules . Based on these observations,
Ibach and Mills propose a general rule which states that for
weakly chemisorbed molecules, the strongest dipole losses are
those infrared active modes of the molecule in the gas phase
which produce a perpendicular dipole moment when the
molecule is adsorbed on the surface. They further state that
the relative strength of dipole losses resulting from the
breakdown of the symmetry is a measure of the adsorption
strength. This is because a strong interaction of ethylene with
the substrate would reduce the molecular symmetry from D2h
to symmetry groups of C2V or lower, which would allow the
detection of additional dipole-active modes with comparable
intensities to that of the ω(CH2) mode.9,22 This observation
will be demonstrated later for the strongly adsorbed di-σ
ethylene on carbide-modified Mo(110).
Figure 11. Comparison of HREELS spectra of C2H4 and C2D4 on
O/Mo(110). A HREELS spectrum, obtained after heating the C2H4/O/
Mo(110) surface to 220 K, is also compared.
IV. Discussion
4.1. Weak Interaction of Ethylene on O/Mo(110): For-
mation of π-Bonded Species. A simple qualitative comparison
of the vibrational spectra indicates that the adsorption of ethylene
and the subsequent reaction pathways are distinct for the three
types of surfaces investigated in this study. Most straightfor-
ward is the interpretation of the results for ethylene interaction
with the oxygen-modified Mo(110) surface. Ethylene molecules
adsorb weakly at 80 K and desorb without significant amount
of decomposition, as suggested by the hydrogen TPD spectrum
(Figure 7e) and by the very low AES C/Mo ratio (e0.02)
following the TPD experiment.
The HREELS spectra of ethylene on O/Mo(110) at 80 K
(Figure 11) can be readily assigned to weakly adsorbed
π-bonded molecular ethylene. Such a bonding configuration
constitutes one extreme of the synergetic bond described by
the Dewar-Chatt-Duncanson theory, namely the one which is
dominated by the donation from the filled π-orbital of ethylene
into a vacant metal orbital without any subsequent backdonation
from the metal substrate. Ethylene bonding of this type is
typical for the less reactive Group IB metals16-18 and has also
been observed on oxygen-modified early transition metal.19 The
assignment of the vibrational modes in Figure 11 to the
The observed weak interaction of ethylene with oxygen-
modified Mo(110) is likely due to an electronic modification
of the Mo(110) substrate upon oxygen chemisorption, although
we cannot exclude steric effects on the basis of our data. One
possible explanation is that the electronegative oxygen atoms
attract/localize the electron density of the substrate, reducing
the degree of backdonation into the empty π*-orbital of ethylene.
(16) Nyberg, C.; Tengstal, C. G.; Andersson, S.; Holmes, M. W. Chem.
Phys. Lett. 1982, 87, 87.
(17) McCash, E. M. Vacuum 1990, 40, 423.
(18) Backx, C.; de Groot, C. P. M.; Biloen, P. Appl. Surf. Sci. 1980, 6,
256.
(19) (a) Chen, J. G.; Weisel, M. D.; Liu, Z. M.; White, J. M. J. Am.
Chem. Soc. 1993, 115, 8875. (b) Chen, J. G.; DeVries, B. D.; Fru¨hberger,
B.; Kim, C. M.; Liu, Z.-M. J. Vac. Sci. Technol. A 1995, 13, 1600.
(20) Herzberg, G. Molecular Spectra and Molecular Structure Vol. II;
Krieger Publishing Company: Malabar, Florida, 1991; p 326.
(21) (a) Chen, J. G.; Colaianni, M. L.; Weinberg,W. H.; Yates, Jr., J. T.
Chem. Phys. Lett. 1991, 177, 113. (b) Colaianni, M. L.; Chen, J. G.;
Weinberg, W. H.; Yates, J. T., Jr. J. Am. Chem. Soc. 1992, 114, 3735.
(22) Sheppard, N. Ann. ReV. Phys. Chem. 1988, 39, 589; and references
therein.

Reaction of Ethylene with Clean and Carbide-Modified Mo(110)
J. Am. Chem. Soc., Vol. 118, No. 46, 1996 11605
Table 2. Vibrational Assignment of Chemisorbed Acetylene Species
1 L C2H4 (C2D4)
on Mo(110) at 80 K
(this work)
vibrational
assignment
C2H2 (C2D2)
on Fe(110)23
C2H2 (C2D2)
on Fe(111)24
C2H2 (C2D2)
ωH/ωD
ωH/ωD
on Ru(001)25
ωH/ωD
ωH/ωD
ν-CM
Fs-CH
Fas-CH
δs-CH
δas-CH
ν-CC
460 (420)
700 (550)
870 (680)
1150 (935)
1415 (1140)
1240 (1235)
2940 (2210)
1.10
1.27
1.28
1.23
1.24
1.00
1.33
470 ( )
660 ( )
375 (350)
520 ( )
765 (585)
980 (715)
1.07
390 (400)
740 ( )
(640)
905 (710)
( )
1130 (1090)
2900 (2170)
0.98
( )
1.35
1.37
935 (710)
(905)
1145 (1130)
2850 (2130)
1.32
1.27
1.01
1.34
1135 (1085)
2940 (2210)
1.05
1.33
1.04
1.34
ν-CH
The formation of π-bonded ethylene species, as a result of
oxygen-modification, appears to be true for all transition metal
surfaces so far investigated.2,22
In HREELS experiments vibrational modes can be excited
either via the dipolar mechanism or the nondipolar mechanisms,
such as the impact and resonance scattering processes.9 The
former mechanism obeys the surface dipole selection rule. It
states that only those vibrations which belong to the totally
symmetric representations, A, A1 and A′, of the point groups
describing the symmetry of the adsorbed complex will be excited
by the dipole scattering mechanism. Experimentally, vibrational
losses that are the result of dipole scattering appear in a
scattering lobe that is sharply peaked in the specular direction.
Comparisons of the on- and off-specular HREELS spectra
therefore allow one to determine the dipolar or nondipolar nature
for each individual vibrational mode, which in turn enable one
to determine the symmetry of the adsorption complex. Upon
adsorption on surfaces, the symmetry group of acetylene is
reduced from D∞h in the gas phase to several possible lower
symmetry groups, such as C2V, C2, Cs (with symmetry plane
either parallel or perpendicular to the carbon-carbon axis) and
C1 (no symmetry). The correlation tables for these symmetry
groups have been provided previously.23,24 As shown in Table
2, there are four C-H bending modes for adsorbed acetylene
species. The number of dipole active C-H bending modes
changes from one in C2V to four in C1 symmetry groups. Based
on the number of dipole active C-H bending modes, several
attempts have been made to derive the adsorption symmetry of
chemisorbed acetylene on various surfaces.22-26
4.2. Strong Interaction of Ethylene on Mo(110): Forma-
tion of Acetylene at 80 K. The HREELS results indicate that
the interaction of ethylene with clean Mo(110) is very strong.
The vibrational features, obtained after exposing Mo(110) to
up to 1 L of ethylene at 80 K (Figures 1a and 2a), are
qualitatively different from those of gas phase values20 and from
those typically observed for chemisorbed ethylene molecules.2,3
In particular, the HREELS spectra up to 1 L of C2H4 exposure
do not contain any features in the frequency range of 1300-
1450 cm-1 in either the on- or off-specular directions (Figure
1a,b). This is consistent with the complete absence of a CH2
group. Upon comparison with available literature data we find
our spectra to be most consistent with the vibrational data for
chemisorbed acetylene species on metal surfaces.22 Table 2
shows a comparison of the vibrational features in Figures 1a
and 2a to those of chemisorbed acetylene on Fe(110),23 Fe-
(111),24 and Ru(001).25 The relatively intense features in
Figures 1a and 2a can be readily assigned to the characteristic
C2H2 (C2D2) surface species: the ν(C-H) mode at 2900 (2710)
cm-1, the ν(C-C) mode at 1130 (1095) cm-1, the symmetric
in-plane δs(C-H) mode at 905 (710) cm-1, the symmetric out-
of-plane Fs(C-H) mode at 740 (535) cm-1, the ν(Mo-C) mode
at 390 (400) cm-1 and the surface phonon mode at 250 (255)
cm-1
. The symmetric out-of-plane Fas(C-H) mode, which
The comparison of the on- and off-specular HREELS spectra
in Figure 1 reveals that the intensities of the ν(Mo-C) mode at
390 cm-1, the Fs(C-H) mode at 740 cm-1 and the ν(C-C)
mode at 1130 cm-1 decrease substantially in the off-specular
measurement (notice the different expansion factors in Figure
1a,b), indicating they are excited primarily by the dipolar
mechanism.9 In contrast, the intensities of the δs(C-H) mode
at 905 cm-1 and the ν(C-H) mode at 2900 cm-1 remain nearly
constant in the on- and off-specular measurements, suggesting
that these two modes are excited by the nondipolar mechanism.9
These observations are similar to those reported for chemisorbed
acetylene on Ru(001) by Parmeter et al.,25 who demonstrated a
nondipolar excitation mechanism for the δs(C-H) and ν(C-
H) modes. Previous off-specular angular dependence studies
of acetylene on Fe(110)23 and Ni(111)26 also indicated that the
vibrational intensities of these two features are primarily
contributed by the nondipolar scattering mechanism. Because
of the different excitation mechanisms for the in-plane Fs(C-
H) mode at 740 cm-1 and the out-of-plane δs(C-H) mode at
905 cm-1 in Figure 1, it is tempting to derive the symmetry of
the acetylene species on the Mo(110) surface. However, as
pointed out by Parmeter et al. for acetylene on Ru(001),25
without detailed knowledge of the surface binding site of
acetylene, assignment of these modes to “in-plane” and “out-
of-plane” is not possible. At present, we would only like to
point out that, because not all four C-H bending modes are
would be expected near 860-880 cm-1 for non-deuterated
acetylene,23 is resolved in the deuterated spectrum (Figure 2a)
at 640 cm-1. The detection of acetylene species indicates that
the decomposition of ethylene occurs at temperatures as low as
80 K on clean Mo(110). The strong reactivity of Mo(110)
toward ethylene is typical for early transition metals. For
example, the low temperature decomposition of ethylene has
been reported on other early transition metal surfaces, in
particular on W(100) and W(110) surfaces.22
It should be pointed out that, for strongly chemisorbed
hydrocarbon molecules, the normal modes in the frequency
range of 800-1200 cm-1 are often composed of substantial
amounts of both the CC stretch and CH (CD) bend internal
coordinates.20 The relative contributions vary for deuterated
and non-deuterated hydrocarbons. As a result, the relative
intensities of these vibrational features often differ substantially
for the deuterated and non-deuterated compounds, as revealed
by the comparison of the vibrational intensities in Figures 1a
and 2a. It is also important to point out that the vibrational
frequencies for the stretching motions of carbon-carbon and
C-H bonds in Table 2 are significantly lower than their gas-
phase values. This is related to the rehybridization of acetylene
from sp (gas-phase) to sp3 after adsorption on surfaces.23-25
(23) Erley, W.; Baro, A. M.; Ibach, H. Surf. Sci. 1982, 120, 273.
(24) Seip, U.; Tsai, M.-C.; Ku¨ppers, J.; Ertl, G. Surf. Sci. 1984, 147, 65.
(25) (a) Parmeter, J. E.; Hills, M. M.; Weinberg, W. H. J. Am. Chem.
Soc. 1986, 108, 3563. (b) Parmeter, J. E.; Hills, M. M.; Weinberg, W. H.
J. Am. Chem. Soc. 1987, 109, 72.
(26) Ibach, H.; Lehwald, S. J. Vac. Sci. Technol. 1981, 18, 625

11606 J. Am. Chem. Soc., Vol. 118, No. 46, 1996
Fru¨hberger and Chen
detected in Figure 1, the symmetry group of the acetylene
species on Mo(110) is Cs or higher.
spectrum, obtained after heating the 10 L surface to 350 K
(Figure 6d), indicates that acetylene is the only surface species
after heating to 350 K. In addition, the intensities of the
acetylene features are slightly higher than heating the 1 L surface
to 350 K (Figure 6c), qualitatively supporting the TPD and AES
observations of an increase in the amount of decomposition as
the ethylene exposure increases from 1 L to 10 L.
The additional hydrogen desorption peak, at 301 K, occurs
at a much lower temperature than that for chemisorbed atomic
hydrogen from Mo(110) (Figure 4). This observation suggests
that this desorption peak is likely related to the recombination
of hydrogen atoms that are not thermally accommodated with
the Mo(110) surface. One possible mechanism is that a
hydrogen atom, once produced by the decomposition of the
C-H bond of ethylene, recombines with another hydrogen atom
without adsorbing on the surface first. The origin of the other
hydrogen atom could be either a pre-adsorbed hydrogen atom
due to the decomposition of ethylene to acetylene at exposures
e1 L or a hydrogen atom of an adjacent ethylene or acetylene
molecule. Similar thermally unaccommodated reactions have
been reported previously. For example, Sun and Weinberg have
reported reactions involving unaccommodated atoms in the
decomposition of formate on Ru(001).27
The subsequent thermal decomposition of the acetylene
species, at ethylene exposures up to 1 L, is straightforward, as
indicated by the TPD spectrum in Figure 3 and the HREELS
data in Figure 6a-6c. The HREELS spectra show the char-
acteristic vibrational features of acetylene up to 350 K, at which
temperature the desorption of hydrogen starts to become
significant in the TPD spectrum. HREELS spectrum recorded
after heating the adsorbed layer to 375 K for 30 s (not shown)
revealed an overall decrease in the vibrational intensities of
acetylene without producing any new vibrational features.
These observations indicate that the acetylene species, once
produced by the reaction of ethylene with Mo(110) at 80 K,
remains to be the only stable intermediate until its eventual
thermal decomposition to produce atomic carbon and atomic
hydrogen, with the latter recombining to desorb as hydrogen
gas. It is interesting to note that the desorption of hydrogen
and the decomposition of acetylene species fall into a similar
temperature range. One possible explanation is that hydrogen
desorption is necessary to free up additional surface sites to
allow the further decomposition of acetylene species.
At ethylene exposures >1 L at 80 K, the HREELS spectra
reveal the presence of additional surface species, as shown in
Figures 1c,d and 2b. After exposing the surface to 10 L
ethylene, additional vibrational features are observed at 620 and
1365 cm-1 for C2H4, and at 455, 860 and 1000 cm-1 for C2D4.
The vibrational frequencies of the pre-existing acetylene species
are also shifted by up to (35 cm-1; the relative intensities of
these features, in the on-specular HREELS spectra, are also
significantly modified. By comparing with literature data,4,22
the new vibrational features can be related to chemisorbed
ethylene molecules. They can be assigned as the CH2 (CD2)
rock mode at 620 (455) cm-1 and the CH2 (CD2) scissor mode
at 1365 (1000) cm-1. The 860 cm-1 feature in Figure 2b can
be assigned as the CD2 wag mode of C2D4; the corresponding
CH2 wag mode of C2H4 is not resolved in Figure 1c from the
1090 cm-1 feature. As will be discussed later for the interaction
of ethylene with carbide-modified Mo(110), additional vibra-
tional features of chemisorbed ethylene would appear in a similar
spectroscopic range as that of the pre-existing acetylene, which
might account for the frequency shifts in the acetylene vibra-
tional features in Figures 1c and 2b.
The TPD data following the coadsorption of C2D4 and C2H4
(Figure 5) also support the desorption mechanism involving an
unaccommodated hydrogen atom. As shown in Figure 5, the
low temperature desorption peak is nearly absent in the TPD
spectrum of D2. This is expected since all C2D4 molecules (at
1 L) decompose to produce chemisorbed C2D2 and D atoms at
80 K, and the chemisorbed D atoms recombine to desorb as D2
only at higher temperatures. On the other hand, the TPD
spectrum of H2 shows a relatively strong low temperature peak
at approximately 301 K. This is again expected because of the
decomposition of molecularly adsorbed C2H4, which is present
on the surface after the additional dose of 9 L C2H4. The
presence of the low temperature peak in the TPD spectrum of
HD further supports the assumption that the unaccommodated
H atom recombines with either a pre-adsorbed D atom or with
a D atom of a nearby C2D2 molecule. More detailed coadsorp-
tion experiments are needed to differentiate between these two
possible mechanisms.
The TPD spectra in Figure 5 further confirms that the
compositions of the adsorbed species are different from 1 L
and 10 L exposures. The ratios of the low and high temperature
peaks change from e0.02 for D2 to 0.25 for H2. This trend is
reversed when the dosing sequence is reversed, i.e., by adsorbing
1 L of C2H4 followed by 9 L of C2D4 (spectra not shown). If a
single chemical species, such as either ethylene or acetylene, is
produced at both low and high ethylene exposures, one would
expect to obtain very similar peak ratios of the low and high
temperature desorption features for both H2 and D2, independent
of the dosing sequence. Finally, the difference in the desorption
temperatures of D2 in Figure 5 (441 K) and H2 in Figure 3 (427
K) can be readily explained by the kinetic isotope effect in either
the cleavage of the C-H and C-D bonds of the surface species
or the recombination of hydrogen and deuterium atoms.
4.3. Decomposition of Ethylene on C/Mo(110): Forma-
tion of Ethylidyne Species. The reaction mechanisms of
ethylene on C/Mo(110) are distinctly different from either Mo-
(110) or O/Mo(110). As revealed by the hydrogen TPD spectra
in Figure 7, the C/Mo(110) surface remains reactive toward the
decomposition of ethylene. The hydrogen desorption peak at
423 K broadens and a low temperature peak at 335 K becomes
more pronounced on C/Mo(110) surfaces with higher C/Mo
Figures 1c and 1d show very similar on- and off-specular
spectra for the 10 L surface, i.e., the surface with a mixture of
acetylene and ethylene. This would suggest that the local
symmetries for both acetylene and ethylene are C1 (no sym-
metry). Since the acetylene/Mo(110) (prepared at e1L ethyl-
ene) overlayer demonstrates an angular dependence that is
expected for a symmetry group of Cs or higher, one can argue
that the adsorption of additional ethylene at higher exposures
destroys the local symmetry of acetylene. More structure
sensitive tools will be required to provide a more detailed
description of this overlayer.
The TPD data shown in Figures 3-5 suggest that some of
the molecularly adsorbed ethylene molecules, produced after
10 L exposure at 80 K, undergo further decomposition at higher
temperatures. As shown in the TPD spectra in Figure 3, an
additional hydrogen desorption peak is observed for the 10 L
surface at 301 K. A comparison of the total hydrogen peak
areas in Figure 3 shows an increase of approximately 25% when
the ethylene exposure increases from 1 L to 10 L. This is in
good agreement with the AES measurements (inset of Figure
3), which suggest that the C/Mo atomic ratios, after the TPD
experiments, increase from 0.24 to 0.30. The HREELS
(27) Sun, Y.-K.; Weinberg, W. H. J. Chem. Phys. 1991, 94 , 4587.

Reaction of Ethylene with Clean and Carbide-Modified Mo(110)
J. Am. Chem. Soc., Vol. 118, No. 46, 1996 11607
Table 3. Vibrational Assignment of di-σ Bonded Ethylene Species
C2H4 (C2D4)
vibrational
assignment
C2H4 (C2D4)
on Pt(111)4a
C2H4 (C2D4)
on Ru(001)4b
on (4 × 4)-C/Mo(110)
ωH/ωD
ωH/ωD
(this work)
ωH/ωD
νs-MC
470 (450)
660 ( )
1.04
460 (420)
775 ( )
900 (700)
1145 (900)
1040 ( )
1450 (1210)
2940 (2210)
3050 (2295)
1.10
380 (405)
635 ( )
905 (670)
1180 (870)
1035 (980)
1395 (1145)
2935 (2185)
3010 (2255)
0.94
CH2-rock
CH2-twist (s)
CH2-wag (s)
ν-CC
790 (600)
980 (740)
1050 (900)
1430 (1150)
2920 (2150)
3000 (2250)
1.32
1.32
1.17
1.24
1.36
1.34
1.29
1.27
1.35
1.36
1.06
1.22
1.34
1.35
CH2-scissor (s)
νs-CH2
νas-CH2
1.20
1.33
1.33
Table 4. Vibrational Assignment of Ethylidyne Surface Intermediates
C2H4 (C2D4)
∆
C2H4 (C2D4)
C2H4 (C2D4)
(CH3C)Co3(CO)9
on (4 × 4)-C/Mo(110) f
∆
∆
vibrational
assignment
on Pt(111) f
on Ru(001) f
((CD3C)Co3(CO)9)
260-300 K
415 K4a
ωH/ωD
280 K4b
ωH/ωD
(IR; Cal.)21
ωH/ωD
(this work)
ωH/ωD
νs-MC
νas-MC
F-CH3
δs-CH3
δas-CH3
ν-CC
430 (410)
600 (600)
980 (790)
1350 (990)
1420 (1030)
1130 (1160)
2890 (2080)
2950 (2220)
1.05
1.00
1.24
1.36
1.38
0.98
1.39
1.33
480 (480)
( )
1000 (800)
1370 (1000)
1450 ( )
1140 (1150)
2945 (2190)
3045 (2280)
1.00
401 (393)
555 (536)
1004 (828)
1356 (993)
1420 (1031)
1163 (1182)
2888 ( )
1.02
1.04
1.21
1.37
1.38
0.98
380 ( )
525 (560)
920 (670)
1345 (1015)
1430 ( )
1075 (1105)
2915 (2180)
( )
0.94
1.37
1.33
1.25
1.37
0.99
1.34
1.34
0.97
1.34
νs-CH3
νas-CH3
2930 (2192)
1.34
ratios. The overall areas of the hydrogen desorption peaks from
carbide-modified surfaces remain to be g80% of that from the
clean Mo(110) surface, suggesting a similar decomposition
probability of ethylene on clean and carbide-modified surfaces.
However, the HREELS spectra, obtained after exposing
C/Mo(110) to 1 L of C2H4 or C2D4 at 80 K (Figures 8b and
10a), are clearly different from the corresponding spectra of
ethylene on either clean (Figures 1 and 2) or oxygen-modified
(Figure 11) surfaces. Unlike the dissociative adsorption on the
clean Mo(110) surface, ethylene adsorbs on the C/Mo(110)
surface molecularly intact. Furthermore, unlike the weakly
adsorbed π-bonded species on O/Mo(110), the vibrational
frequencies and relative intensities of ethylene species on C/Mo-
(110) are substantially different from those of gas phase ethylene
molecules. By comparing to literature data,2-4,22 the vibrational
features in Figures 8b and 10a can be assigned to strongly
adsorbed, di-σ bonded C2H4 or C2D4, respectively. A vibrational
assignment of the observed HREELS features is given in Table
3, where it is compared with those of di-σ bonded ethylene
molecules adsorbed on Pt(111)4a and Ru(001).4b As shown in
Table 3, the vibrational features at 3010 (2255) and 2935 (2185)
cm-1 can be readily assigned to the asymmetric and symmetric
CH2 (CD2) stretching modes, respectively. These frequencies
are lower than the corresponding values for gas phase C2H4
(C2D4), which are observed at 3106 (2345) and 2989 (2200)
cm-1, respectively.20 The observation of lower CH2 stretching
frequencies for the di-σ bonded ethylene has been reported on
various metal surfaces2,22 and has been attributed to the
rehybridization of carbon from sp2 in the gas phase to sp3 in
the strongly bonded di-σ configuration on metal surfaces.
As compared in Table 3, other vibrational features of ethylene
on C/Mo(110) also appear in the characteristic vibrational
frequency range of the di-σ bonded ethylene species. The
vibrational feature at 1395 (1145) can be assigned to the scissor
mode of the CH2 (CD2) groups. The isotopic ratio (ωH/ωD) of
this pair of features, 1.22, is much smaller than the expected
value of 1.41. A low (ωH/ωD) ratio seems to be rather common
for the di-σ bonded ethylene species on most transition metal
surfaces.28 For example, the (ωH/ωD) ratios of the CH2 scissor
modes have been reported to be 1.24 on Pt(111),4a 1.20 on Ru-
(001),4b 1.15 on Ni(110)28a and 1.19 on Ni(111).28b For strongly
adsorbed ethylene, the normal modes in the frequency range of
800-1200 cm-1 are often composed of substantial amounts of
the CC stretch, CH2 wag and CH2 twist internal coordinates.9,20
As a result, the vibrational assignment in this frequency range
is often somewhat ambiguous. The vibrational frequencies of
the CC stretch, CH2 wag and CH2 twist normal modes also vary
significantly from one substrate to another,4,22,28 as shown in
Table 3 by the large variation of the vibrational frequencies of
these three modes on the surfaces of Pt(111), Ru(001) and
C/Mo(110).
A new surface intermediate is detected after heating the
ethylene exposed C/Mo(110) surface to the temperature range
of 260-300 K (Figures 8c and 10b). An inspection of the
frequencies and relative intensities of the vibrational features
clearly reveals that the intermediate is neither di-σ bonded
ethylene nor acetylene. By comparing with literature data, the
vibrational features in Figures 8c and 10b can be best described
as being due to an ethylidyne species, CCH3 (CCD3).4,22 Table
4 shows a comparison of the vibrational assignment of ethyli-
dyne species on Pt(111),4a Ru(001)4b and on C/Mo(110).
Vibrational frequencies of a well characterized molecular cluster
compound, (CH3C)Co3(CO)9, are also compared in Table 4.29
The formation of ethylidyne species on C/Mo(110) is suggested
by the observation of the ν(C-C) mode at 1075 (1105) cm-1
and the characteristic group frequencies related to the CH3
group, which are the intense νs(CH3) feature at 2915 (2180)
cm-1, the δas(CH3) and δs(CH3) modes at 1430 and 1345 (1015)
cm-1, respectively, and the F(CH3) mode at 920 (670) cm-1
.
Although all vibrational features in Figures 8c and 10b can
be assigned to the ethylidyne species, the relative intensities of
our spectra are somewhat different from the literature data.22
For example, ethylidyne species were found to be on the 3-fold
sites of Pt(111) with the C-C bond perpendicular to the surface,
resulting in a C3V symmetry for the adsorbate complex.4a As a
result, the on-specular HREELS spectrum is characterized by
(28) (a) Stroscio, J. A.; Bare, S. R.; Ho, W. Surf. Sci. 1984, 148, 499.
(b) Lehwald, S.; Ibach, H. Surf. Sci. 1979, 84, 315.
(29) Skinner, P.; Howard, M. W.; Oxton, I. A.; Kettle, S. F. A.; Powell,
D. B.; Sheppard, N. J. Chem. Soc. Faraday II 1981, 77, 685 and 1203.

11608 J. Am. Chem. Soc., Vol. 118, No. 46, 1996
Fru¨hberger and Chen
the dipole active ν(Pt-C), ν(C-C) and δs(CH3) modes; these
features are at least five times more intense than the other
vibrational features.4a Furthermore, as expected for the per-
pendicularly adsorbed species, the relative intensities of eth-
ylidyne on Pt(111) change substantially in the off-specular
HREELS measurements4a However, as shown in Figure 9, the
ethylidyne species on C/Mo(110) show very minor differences
between the on- and off-specular measurements. Except that
the ν(Mo-C) mode is better resolved in the off-specular
spectrum, the relative intensities of the ethylidyne features
remain nearly the same in the on- and off-specular measure-
ments. By comparing Figure 9 with Table 4, it is clear that all
vibrational features of the ethylidyne species on C/Mo(110) are
observed in the on-specular HREELS. This observation sug-
gests that all these vibrational modes are dipole-active, which
can only be achieved when the symmetry group of the surface
complex is C1 (no symmetry). There are several ways to explain
the C1 symmetry of the ethylidyne surface species. One is that
the ethylidyne species are adsorbed in a random and non-
perpendicular adsorption geometry with respect to the C/Mo-
(110) substrate. Another possibility is that the C/Mo(110)
surface itself is not flat on the length of atomic scale.
Considering the fact that the p(4 × 4) LEED pattern exists over
a large range of C/Mo stoichiometries (AES ratios of C(KLL)/
Mo(MNN) from 0.13 to 0.32 in the current study), it is rather
difficult to describe the C/Mo(110) surface by a simple and non-
reconstructed overlayer structure with a 4 × 4 surface periodic-
ity. A recent scanning tunneling microscopy (STM) study of
C/W(110) surfaces indicates that the carbide-modified W(110)
surface is reconstructed on the atomic scale, resulting in a
vertical buckling of up to 0.5 Å with a horizontal periodicity
of 3 - 4 Å.30 If similar atomic scale reconstruction also occurs
on the C/Mo(110) surface, it might account for the lack of
symmetry for the ethylidyne surface complex on this surface.
Again, studies with more precise structural tools, such as the
atomic resolution STM30 or tensor LEED31 techniques, would
provide a more definitive description of the adsorbate and
substrate structures of ethylidyne on C/Mo(110).
4.4. Similar Surface Reaction Mechanisms of C/Mo(110)
and Pt-Group Metals. The results shown above clearly
indicate that the surface chemistry of Mo(110) can be signifi-
cantly modified by the formation of carbide. The very strong
interaction between ethylene and Mo(110), characteristic for
early transition metals, can be significantly “tamed” by the
formation of carbide. Furthermore, unlike the oxygen-modified
Mo(110) surface that becomes inert toward the decomposition
of ethylene, the carbide-modified Mo(110) surface remains
active toward the decomposition of ethylene, although the
decomposition temperature and reaction intermediates are
qualitatively different from the clean Mo(110) surface. More
importantly, the modified reaction pathway on C/Mo(110) is
very similar to those typically observed for the decomposition
of ethylene on close-packed faces of Pt-group metals.3,22 As
observed on C/Mo(110) in this study, the reaction mechanisms
of ethylene on Pt-group metals surfaces are characterized by
the formation of strongly adsorbed di-σ species at 80 K and by
the production of ethylidyne species as the decomposition
intermediate at higher temperatures.
in the TPD measurements (Figure 7) and by the decrease of
vibrational intensities in the HREELS data (Figure 8d). In
comparison, ethylidyne is thermally stable in the temperature
range of approximately 340-415 K on Pt(111),4a 280-360 K
on Ru(001)4b and 180-300 K on Ir(111).4e
The formation of ethylidyne species on Pt-group metal
surfaces, and the lack of this intermediate in the decomposition
of ethylene on early transition metal surfaces, demonstrate an
important difference in the formation and/or decomposition of
metal-carbon bonds on the two types of surfaces. On Pt-group
metal surfaces, the production of ethylidyne indicates that one
of the metal-carbon bonds, produced at low temperatures by
the formation of di-σ ethylene, can be broken at higher
temperatures with the carbon-carbon bond still intact. For
example, a recent vibrational study following the thermal
decomposition of ethylene on Pt(111) demonstrates a gradual
transformation of di-σ ethylene (Pt-CH2-CH2-Pt) to ethylidene
(Pt2-CH-CH3) and to ethylidyne (Pt3-C-CH3).32 In contrast,
on early transition metal surfaces the metal-carbon bonds
remain intact once they are produced. The subsequent thermal
decomposition of the adsorbate/substrate complex occurs at the
carbon-carbon bonds instead of the metal-carbon bonds, as
observed on clean Mo(110). Such a difference in the sequence
of bond-cleavage could be one of the primary reasons why Pt-
group metals are superior catalysts than early transition metals
for dehydrogenation/hydrogenation reactions, which require that
the catalyst facilitates the C-H bond cleavage/formation without
breaking the carbon-carbon bonds. The observation of eth-
ylidyne species on the C/Mo(110) surface indicates that the
bond-cleavage sequence of the carbon-carbon and metal-
carbon bonds of the adsorbate/substrate complex resembles those
on Pt-group metals. This observation could explain the very
similar catalytic activities of molybdenum carbides and Pt-group
metals for the dehydrogenation and hydrogenation of hydro-
carbon molecules.5
The most important conclusion from the current study is that
the surface reactivity of Mo(110) can be converted to that of
Pt-group metals. This observation implies that the C/Mo(110)
surface can be used as a reliable model system to understand
the fundamental aspects of how the formation of interstitial
carbides would convert the chemical properties of early transi-
tion metals to those of Pt-group metals. One obvious advantage
of using carbide-modified surfaces as model systems is that they
allow one to utilize powerful surface science spectroscopies,6
which would be otherwise impossible for powder materials. For
example, we have recently applied the combination of Near-
Edge X-ray Absorption Fine-Structure (NEXAFS), X-ray Pho-
toelectron Spectroscopy (XPS), LEED and TPD to investigate
the relationship between the location of carbon and the reactivity
of the carbide-modified Mo(110) model surfaces.13 These
results indicate that the Pt-like properties can only be achieved
when carbon atoms are occupying the interstitial sites instead
of the surface sites. In addition, carbide-modified Mo(110)
surfaces can also be used as a model system for future surface
science measurements of the density of states (DOS) near the
Fermi level, which has been postulated by band structure
calculations to be the main reason for the similar catalytic
properties of early transition metal carbides and Pt-group
metals.33
In addition, the thermal stability of the ethylidyne species
on C/Mo(110) is also similar to those on Pt-group metals. On
C/Mo(110), the ethylidyne species are found to be stable in the
temperature range of 260-300 K; the decomposition becomes
significant at 350 K, as indicated by the evolution of hydrogen
V. Conclusions
From the results and discussion given above, we summarize
the following important observations for the reaction of ethylene
with clean and chemically modified Mo(110) surfaces:
(30) Bode, M.; Pascal, R.; Wiesendanger, R. Surf. Sci. 1995, 344, 185.
(31) Jentz, D.; Rizzi, S.; Barbieri, A.; Kelly, D. G.; Van Hove, M. A.;
Somorjai, G. A. Surf. Sci. 1995, 329, 14.
(32) Cremer, P.; Stanners, C.; Niemantsverdriet, J. W.; Shen, Y. R.;
Somorjai, G. Surf. Sci. 1995, 328, 111.

Reaction of Ethylene with Clean and Carbide-Modified Mo(110)
J. Am. Chem. Soc., Vol. 118, No. 46, 1996 11609
(1) The clean Mo(110) surface interacts very strongly with
ethylene, as indicated by the decomposition of ethylene to
acetylene at temperatures as low as 80 K. The acetylene species
decompose to produce atomic carbon and gas phase hydrogen
at T > 350 K. No other thermal decomposition intermediates
are observed on the clean Mo(110) surface.
(2) The strong reactivity of Mo(110) can be inhibited by
chemisorbed oxygen with a coverage as low as 0.25 ML.
Ethylene molecules only weakly interacting with the O/Mo-
(110) surface to produce π-bonded ethylene species at 80 K,
which desorb molecularly at T < 150 K without decomposition.
(3) The surface reactivity of C/Mo(110) is qualitatively
different from either clean or oxygen-modified Mo(110) sur-
faces. While the carbide-modified Mo(110) remains reactive
toward the decomposition of ethylene, the decomposition occurs
via a different reaction pathway. Ethylene adsorbs on C/Mo-
(110) in the strongly bonded di-σ configuration at 80 K;
ethylidyne species are produced as reaction intermediates in the
temperature range of 260-300 K. This reaction pathway is
characteristic for the decomposition of ethylene on closed-
packed Pt-group metals. These results indicate that the Mo-
(110) surface can be modified by carbon in such a way that it
converts the reactivity of Mo to that of Pt-group metals. This
observation is in excellent agreement with catalysis data that
molybdenum carbides often show catalytic reactivities that are
unique for Pt-group metals, especially for dehydrogenation and
hydrogenation reactions.5
(33) See, for example, Eberhart, M. E.; MacLaren, J. M. in Ref. 5b, p
107.
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