Gas phase atomic hydrogen-induced hydrogenation of cyclohexene on the Ni(100) surface

Article abstract of DOI:10.1021/jp9637789

Gas phase hydrogen atoms add to adsorbed cyclohexene at 100 K on the Ni(100) surface, resulting in cyclohexane formation during subsequent TPR (temperature-programmed reaction) experiments. No C-C bond activation is observed after exposure to gas phase atomic hydrogen. Vibrational and isotope studies indicate that a cyclohexyl intermediate is formed by the addition of gas phase atomic hydrogen to adsorbed cyclohexene. This adsorbed cyclohexyl is hydrogenated primarily by surface hydrogen to form cyclohexane during subsequent heating. Cyclohexane yields increase with increasing atomic hydrogen exposure, and yields in the 90% range have been observed with large atomic hydrogen exposures. In the presence of only coadsorbed hydrogen, no significant hydrogen addition to adsorbed cyclohexene is observed, and cyclohexene desorption dominates during subsequent TPR experiments. Vibrational spectroscopy indicates that π-bonded cyclohexene with the C=C bond parallel to the surface is the dominant surface species in the presence of coadsorbed surface hydrogen. In contrast, di-σ-bonded cyclohexene is the dominant species in the absence of coadsorbed hydrogen. In the absence of coadsorbed hydrogen, dehydrogenation of the adsorbed cyclohexene results in benzene formation with increasing temperature. Adsorbed benzene formed by dehydrogenation has been identified after heating to 390 K in the absence of hydrogen using vibrational spectroscopy.

Full text of DOI:10.1021/jp9637789

3540
J. Phys. Chem. B 1997, 101, 3540-3546
Gas Phase Atomic Hydrogen-Induced Hydrogenation of Cyclohexene on the Ni(100) Surface
Kyung-Ah Son† and John L. Gland*
Department of Chemistry, UniVersity of Michigan, Ann Arbor, Michigan 48109
ReceiVed: NoVember 13, 1996; In Final Form: February 11, 1997X
Gas phase hydrogen atoms add to adsorbed cyclohexene at 100 K on the Ni(100) surface, resulting in
cyclohexane formation during subsequent TPR (temperature-programmed reaction) experiments. No C-C
bond activation is observed after exposure to gas phase atomic hydrogen. Vibrational and isotope studies
indicate that a cyclohexyl intermediate is formed by the addition of gas phase atomic hydrogen to adsorbed
cyclohexene. This adsorbed cyclohexyl is hydrogenated primarily by surface hydrogen to form cyclohexane
during subsequent heating. Cyclohexane yields increase with increasing atomic hydrogen exposure, and yields
in the 90% range have been observed with large atomic hydrogen exposures. In the presence of only coadsorbed
hydrogen, no significant hydrogen addition to adsorbed cyclohexene is observed, and cyclohexene desorption
dominates during subsequent TPR experiments. Vibrational spectroscopy indicates that π-bonded cyclohexene
with the CdC bond parallel to the surface is the dominant surface species in the presence of coadsorbed
surface hydrogen. In contrast, di-σ-bonded cyclohexene is the dominant species in the absence of coadsorbed
hydrogen. In the absence of coadsorbed hydrogen, dehydrogenation of the adsorbed cyclohexene results in
benzene formation with increasing temperature. Adsorbed benzene formed by dehydrogenation has been
identified after heating to 390 K in the absence of hydrogen using vibrational spectroscopy.
Introduction
considerable reactivity when diluted with inerts which enhance
mobility.5 The estimated activation energy for hydrogen atom
addition to solid olefins is approximately 1.5 kcal/mol.3 Dis-
proportionation of the alkyl radicals produced appears to proceed
without activation energy.
This study of the reactions between gas phase atomic
hydrogen and adsorbed cyclohexene is part of our program to
explore the role of hydrogen in C-C bond activation processes.
Cyclohexene is a particularly interesting reactant because the
complete range of surface reactions including isomerization,
hydrogenolysis, hydrogenation, and dehydrogenation can all
occur with this cyclic olefin.
Reactions of atomic hydrogen with olefins have been studied
extensively in the gas phase.1 Atomic hydrogen primarily adds
to the π bonds forming a vibrationally excited alkyl radical.
These excited radicals can undergo either dissociation or
collisional stabilization. Limited hydrogen abstraction also
occurs during reactions between olefins and gas phase atomic
hydrogen. In most cases, the ratio of hydrogen abstraction to
hydrogen addition is below 0.1.1 The activation energy for
hydrogen atom addition to alkenes in the gas phase is generally
well below 7 kcal/mol. For example, the activation energy for
hydrogen atom addition to gas phase propylene is 1.5 kcal/mol
in the 77-90 K temperature range.1
Reactions of gas phase atomic hydrogen with solid olefins
at low temperatures (<150 K) have also been investigated, and
hydrogen addition to the double bond to form an alkyl radical
is the primary process observed.2-5 The alkyl radicals produced
either react with other alkyl radicals or react with hydrogen
atoms depending on the mobility of alkyl radicals in the solid.5
Reactions between alkyl radicals predominate in solids where
diffusion of alkyl radicals is rapid, while reactions with atomic
hydrogen dominate when high-viscosity matrices are employed.
In both cases, disproportionation and combination of the
intermediates result in the formation of the final stable products.
The immobilizing effect of the matrix also affects the overall
reaction rate.5 For instance, undiluted solid 1-hexene does not
appear to react with atomic hydrogen at 77 K but shows
In contrast to H atom reactions with gas phase olefins or solid
olefins, reactions of atomic hydrogen with olefinic hydrocarbons
adsorbed on the surface have been investigated only recently.
Bent and co-workers observed that gas phase atomic hydrogen
adds to adsorbed ethylene to form ethyl and to adsorbed benzene
to form partially hydrogenated benzene at 110 K on the Cu-
(111) surface.6a Similar reactions have been also observed for
adsorbed ethylene on the Cu(100) surface.6b Isotope studies
have shown that the resulting ethyl species dissipate energy and
accommodate to the surface before decomposition or rearrange-
ment occurs.6b The estimated cross section for hydrogen
addition to ethylene on the Cu(111) surface is 18 Å2, which is
an order of magnitude larger than the cross section for hydrogen
abstraction from cyclohexane on the same surface.7,8
We have found that small strained cycloalkanes react readily
with gas phase atomic hydrogen on the Ni(100) surface.9
Hydrogen addition leading to ring opening is dominant for
three-, four-, and five-membered cycloalkanes on the Ni(100)
surface. This reactivity may be associated with the high p
character of the strained C-C bond in these small cycloalkanes.
On the other hand, atomic hydrogen addition is not observed
for cyclohexane, which has unstrained C-C bonds similar to
saturated n-alkane’s. Instead, hydrogen abstraction dominates
in the adsorbed cyclohexane, leading to cyclohexene and
cyclohexadiene formation during exposure to gas phase atomic
hydrogen.9
Cyclohexene is an interesting reactant since the π and π*
orbitals are the primary interaction centers, resulting in nearly
parallel adsorption of the unsaturated carbon-carbon bond on
the Ni surface.10,11 As a result, some C-H bonds in CH2 groups
are very close to the surface and susceptible to activation. In
fact, dehydrogenation is the dominant thermal process observed
for cyclohexene adsorbed on Ni surfaces.10,11 On the other hand,
† Present address: Science and Technology Center for Synthesis, Growth
and Analysis of Electronic Materials, Department of Chemistry and
Biochemistry, University of Texas at Austin, Austin, TX 78712.
X Abstract published in AdVance ACS Abstracts, March 15, 1997.
S1089-5647(96)03778-9 CCC: $14.00 © 1997 American Chemical Society

Hydrogenation of Cyclohexene on the Ni(100) Surface
J. Phys. Chem. B, Vol. 101, No. 18, 1997 3541
cyclohexane adsorbs reversibly, and no dehydrogenation is
observed on the low-index Ni surfaces.10 Cyclohexane adsorbs
with the carbon ring parallel to the metal surface, and HREELS
spectra show a C-H soft mode due to C-H‚‚‚M interactions
involving the axial hydrogens.12-16
Recently, we have demonstrated that adsorbed cyclohexene
can be hydrogenated to cyclohexane on the Ni(100) surface by
both gas phase atomic hydrogen and desorbing bulk hydrogen
at a low temperature and pressure.17 Gas phase atomic hydrogen
addition to cyclohexene on the less reactive Cu(100) surface
has also been recently characterized.18 Cyclohexyl formation
followed by subsequent â-hydrogen elimination to form cyclo-
hexene is the main reaction observed for cyclohexene adsorbed
on this surface. No cyclohexane formation was observed. To
the best of our knowledge, the hydrogenation of cyclohexene
to cyclohexane has been observed previously only with high
pressures of both cyclohexene and hydrogen. Cyclohexene
hydrogenation over the Pt(223) stepped surface requires a high
pressure with excess hydrogen (H2/cyclohexene ∼ 10).19 At
room temperature, cyclohexene dehydrogenation is dominant
in the 10-7 Torr pressure range on the Pt(223) surface.
Hydrogenation only begins to dominate in the 100 torr pressure
range on this rough surface.19 The activation energy estimated
for hydrogenation is 5 kcal/mol at 77 Torr. Disproportionation
of cyclohexene is not observed under these high-pressure
conditions so no benzene is observed. Catalytic hydrogenation
of cyclohexene over supported Ni, Pt, and Pd catalysts also
requires atmospheric pressures of hydrogen and temperatures
above 300 K.20-23
Figure 1. TPR spectra of submonolayer cyclohexene adsorbed on the
Ni(100) surface. Dehydrogenation is dominant, and only a small amount
of cyclohexene desorbs at 181 K. Benzene produced from dehydro-
genation desorbs around 440 K.
controls sample temperature and monitors a range of masses or
up to 26 specified masses.24
Vibrational spectra have been taken using an HREELS
spectrometer with a 127° cylindrical deflection monochromator
and an identical analyzer. The count rate for the elastic peak
was 104-106 counts/s with ∼10 meV resolution. The incident
beam energy was ∼4.5 eV at a pass energy of 0.29 eV. All
the spectra were collected in the specular direction at 60˚ from
the surface normal. Spectra were collected with a PC interfaced
to the HREELS electronics. The signal was averaged over 25
scans of the chosen energy region, using 1.5 meV steps and a
0.5 s dwell time at each step.
In this work, a detailed mechanistic study of gas phase atomic
hydrogen-induced cyclohexene hydrogenation has been per-
formed using vibrational spectroscopy (HREELS) and isotope
experiments. The present work clearly shows that cyclohexene
hydrogenation by gas phase atomic hydrogen occurs at 100 K
on the Ni(100) surface.
Results and Discussion
Cyclohexene on the Ni(100) Surface. Desorption and
dehydrogenation are the main thermal pathways observed for
cyclohexene adsorbed alone on the Ni(100) surface. Dehydro-
genation is dominant for small coverages, while molecular
desorption of cyclohexene increases with increasing surface
coverage. The TPR spectra for a submonolayer of cyclohexene
formed using an apparent 0.008 langmuir cyclohexene exposure
at 100 K on the Ni(100) surface is shown in Figure 1. Only a
small amount of cyclohexene desorbs molecularly at 181 K.
Dehydrogenation is dominant, resulting in both hydrogen and
benzene desorption with increasing temperature. A fraction of
the benzene (78 amu) produced by dehydrogenation desorbs
around 440 K. The remaining adsorbed benzene dehydrogenates
around 460 K, resulting in the high-temperature hydrogen peak.
The 78 amu peaks below 200 K are caused by the fragmentation
of cyclohexene in the ionizer.17 The lack of significant
cyclohexene disproportionation is clearly indicated by the
absence of cyclohexane desorption. Cyclohexene desorption
spectra taken for a series of increasing cyclohexene exposures
are shown in Figure 2. With increasing cyclohexene exposure,
molecular cyclohexene desorption at 181 K increases. For
apparent exposures above 0.012 langmuir, desorption from
cyclohexene multilayers increases at 140 K.
Experimental Section
The experiments were performed in an ultrahigh-vacuum
chamber which has been described previously in detail.9 The
background pressure of the system during the experiments was
∼5 × 10-11 Torr. The Ni(100) crystal was cleaned by Ar+
sputtering followed by annealing to 1000 K, oxygen treatment,
and hydrogen treatment. The cleanliness of the surface was
verified by AES.
Microchannel array-directed beam dosers and leak valves
were used for reagent dosing. Cyclohexene (Aldrich, 99%) was
used after being purified by several freeze-pump-thaw cycles.
Hydrogen (Matheson, 99.9999%) and deuterium (Matheson,
99.5%) were used without further purification both for adsorp-
tion and for the production of atomic hydrogen. The arbitrary
exposure units reported have not been corrected for large
preferential dosing fluxes and ion gauge sensitivity factors.
Exposures are presented in langmuirs (1 langmuir ) 1 × 10-6
Torr s) based on the background pressure reading from ion
gauge and are intended to be used as a basis for internal
comparisons.
Gas phase atomic hydrogen was created on a ∼1800 K
tungsten filament by the dissociation of molecular hydrogen at
9.0 × 10-7 Torr. Since an accurate dissociation coefficient is
not available, exposures of atomic hydrogen are given in terms
of exposures of molecular hydrogen to the W filament as
measured by an ion gauge.
TPR spectra were taken using a quadrupole mass spectrometer
(QMS) and a linear heating rate of 3 or 5 K/s. The desorption
spectra were collected with Hunt Scientific software, which
Benzene adsorption/desorption from this Ni(100) sample is
shown in Figure 3 for use as a reference. Molecular benzene
desorption is observed at 462 K. Dehydrogenation of benzene
results in hydrogen desorption over the 350-500 K temperature
range on this surface, as previously reported.25
Adsorbed benzene formed by cyclohexene dehydrogenation
has also been characterized using vibrational spectra in Figure

3542 J. Phys. Chem. B, Vol. 101, No. 18, 1997
Son and Gland
Figure 4. HREELS spectra of (a) multilayer cyclohexene on the
Ni(100) surface at 100 K, (b) monolayer cyclohexene on the Ni(100)
surface at 100 K, and (c) cyclohexene heated to 390 K on the Ni(100)
surface. These spectra were taken at the specular angle.
Figure 2. Thermal desorption of cyclohexene from the Ni(100) surface.
Molecular desorption of cyclohexene becomes significant as the surface
coverage is increased. Desorption of monolayer cyclohexene occurs at
181 K. Desorption of multilayers appears at 140 K as the exposure is
increased.
TABLE 1: Assignments of HREELS Peaks for Cyclohexene
Adsorbed on the Ni(100) Surface at 100 Ka
cyclohexene cyclohexene cyclohexene
(ad)/mono
(ad)/multi
(liq)
description
415
662
909
452
720
878
917
1038
C-C-C + C-C-H bend
C-C-H + CdC-C bend
C-C stretch
860
C-C-H bend
C-C stretch + C-C-H bend
+ CH2 rock
1083
1082
1138
1264
1321
1392
1438
1447
1456
1653
2840
2860
2882
2898
2929
2940
2960
2993
3022
C-C-H bend
1243
1305
1280
1341
C-C-H bend + CH2 twist
C-C-H bend
C-C-H bend + CH2 wag
CH2 scissor
1441
1415
1600
CdC stretch
CH2 asym stretch
Figure 3. TPR spectra of benzene adsorbed on the Ni(100) at 100 K.
Desorption and dehydrogenation of monolayer benzene occur in the
same temperature range.
2937
2909
CH2 sym stretch
4. Spectrum a corresponds to a multilayer of cyclohexene at
100 K. Spectrum b corresponds to a monolayer of cyclohexene
adsorbed on the Ni(100) surface at 100 K. Spectrum c was
taken after heating this cyclohexene monolayer to 390 K.
The vibrational spectrum of multilayer cyclohexene (a) is
similar to the vibrational spectrum of liquid cyclohexene.
Vibrational assignments for multilayer cyclohexene are indicated
in Table 1. These assignments are based on those for the
vibrational spectra of both liquid phase cyclohexene and
cyclohexene adsorbed on the Pt(111) surface.26 The monolayer
HREELS spectrum (b) is very similar to the spectrum of a
monolayer formed by annealing a cyclohexene multilayer to
165 K (data not shown).
Compared to the multilayer spectrum (a), the peaks at 662
and ∼1600 cm-1 are absent in the monolayer spectrum (b). The
disappearance of the 1600 cm-1 CdC stretching mode in the
monolayer spectrum suggests that cyclohexene’s double bond
is parallel to the surface based on the surface dipole selection
rule. Parallel adsorption of cyclohexene’s unsaturated carbon-
carbon bond on Ni and Pt surfaces has been observed previ-
ously.10,11 In a parallel configuration the double bond may
interact with the surface either by forming a π bond or by
forming a η2-di-σ bond. The peak at 662 cm-1 is caused by a
a Energies of corresponding IR peaks for liquid cyclohexene26 are
included for comparison. Peak energies are given in cm-1
.
CdC-C bending mode.26,27 Therefore, the absence of this peak
in the monolayer spectrum suggests that η2-di-σ bonding is
dominant for monolayer cyclohexene. Similar changes in the
intensity of the bending modes have been observed for cyclo-
hexene on the Pt(111) surface where the η2-di-σ bonding
configuration is thought to dominate.26a The 662 cm-1 bending
mode reappears in the presence of coadsorbed hydrogen,
suggesting that π bonding becomes dominant due to the
decreased interaction observed in the presence of coadsorbed
hydrogen.
The peak around 860 cm-1 in Figure 4b is caused by a C-C
stretching mode. The appearance of the C-C stretching mode
for adsorbed cyclohexene indicates that some of the C-C bonds
in adsorbed cyclohexene are tilted with respect to the surface,
although the CdC bond is paralleled to the surface. A tilted
geometry would result in axial hydrogens which are not pointed
toward the surface.10,11 This tilted geometry is consistent with
the absence of C-H soft modes for adsorbed cyclohexene.

Hydrogenation of Cyclohexene on the Ni(100) Surface
J. Phys. Chem. B, Vol. 101, No. 18, 1997 3543
Figure 5. TPR spectra of cyclohexene coadsorbed with surface
hydrogen on the Ni(100) surface. Most cyclohexene desorbs below 200
K without reaction.
Figure 6. TPR spectra of cyclohexene coadsorbed with surface
deuterium on the Ni(100) surface. Isotope exchange between adsorbed
cyclohexene and surface deuterium has been observed.
TABLE 2: Comparison of Vibrational Modes between
Adsorbed Cyclohexene Annealed at 385 K and Adsorbed
Benzene;28 Strongest IR Modes of Gas Phase Benzene35,36
Are Also Included (Peak Energies in cm-1
)
cyclohexene(ad) benzene(ad)28 benzene
at 385 K
at room temp (g)35,36 mode
description
792
766
673
υ4
C-H(out-of-plane)
bend
1090
1008
1038
υ14
C-H(in-plane)
bend
1398
2980
1452
3024
1486
3063
υ13
υ12
C-C stretch
C-H stretch
Spectrum c in Figure 4 taken after flashing adsorbed
cyclohexene to 390 K is very similar to the spectrum of benzene
adsorbed on the Ni(100) surface.28 Peak assignments for
spectrum c are indicated in Table 2. With heating the most
significant changes observed are the appearance of a strong peak
at 792 cm-1 and the increase of C-H stretching energy to 2980
cm-1. The 792 cm-1 peak corresponds to benzene’s C-H out-
of-plane bending mode.28 The increase of the C-H stretching
energy up to 2980 cm-1 is caused by the sp2 hybridization of
the benzene formed. As expected, this C-H stretching energy
is smaller than the 3024 cm-1 energy reported for a complete
monolayer of benzene.28 A similar energy increase with
increasing surface coverage has been previously reported for
benzene on the Pt(111) surface.29
Cyclohexene Coadsorbed with Surface Hydrogen. Mo-
lecular cyclohexene desorption is dominant for cyclohexene
coadsorbed with hydrogen on the Ni(100) surface (Figure 5).
No significant amount of hydrogenation by surface hydrogen
has been observed. The small 56 amu hydrogenation peak near
250 K is likely to be associated with reactive surface defects.30
Desorption of dehydrogenated products is not observed in the
presence of coadsorbed surface hydrogen/deuterium. The 78
amu peak at 181 K is caused by the fragmentation of cyclo-
hexene. However, the hydrogen peak around 460 K clearly
indicates that dehydrogenation of residual adsorbed organic
occurs at an elevated temperature. Apparently a small fraction
of the adsorbed cyclohexene remains on the surface and
dehydrogenates with increasing temperature even in the presence
of coadsorbed hydrogen/deuterium. The sharp low-temperature
hydrogen peak around 260 K clearly indicates that coadsorbed
cyclohexene affects the desorption energetics of surface hydro-
gen.
Figure 7. HREELS spectra of (a) monolayer cyclohexene on the
Ni(100) surface at 100 K and (b) cyclohexene coadsorbed with surface
hydrogen. The CdC-C bending mode appears at 673 cm-1 in the
presence of coadsorbed hydrogen. These spectra were taken at the
specular angle.
Comparisons between the hydrogen (2 amu) and deuterium (4
amu) peaks indicate that the sharp hydrogen peak at 260 K is
associated primarily with hydrogen exchanged out of cyclo-
hexene. The 83 amu peak at 181 K represents 17% of the 82
amu peak at the same temperature and is larger than natural
abundance of [
13C]cyclohexene. Therefore, cyclohexene-d1
must be the main cause of the 83 amu peak at 181 K. The 83
amu peak at 140 K represents ∼2% of the corresponding 82
amu peak and is consistent with natural abundance of [13C]-
cyclohexene. The amount of hydrogen produced by exchange
cannot be determined directly because of large cross section
variations between hydrogen and deuterium. With our mass
spectrometer, the ion yield for deuterium (4 amu) was ap-
proximately 40% relative to hydrogen (2 amu).
Cyclohexene interaction with the surface is modified sub-
stantially in the presence of coadsorbed hydrogen as highlighted
by the vibrational spectra in Figure 7. In the presence of
coadsorbed hydrogen, a new peak appears at 673 cm-1, and
the intensities of the peaks in the region of 1100-1500 cm-1
are increased. On the basis of the comparisons with the IR
spectrum of liquid cyclohexene,27 we believe that the peak at
673 cm-1 is caused by a CdC-C deformation mode. This peak
suggests that the CdC bond in cyclohexene is not disturbed
significantly by adsorption in the presence of coadsorbed
Isotope exchange occurs below 181 K for cyclohexene
coadsorbed with deuterium on the Ni(100) surface (Figure 6).

3544 J. Phys. Chem. B, Vol. 101, No. 18, 1997
Son and Gland
Figure 8. TPR spectra of monolayer cyclohexene after exposure to
95 langmuirs of atomic hydrogen at 100 K. Cyclohexane is produced
and desorbs at 178 K. No benzene formation is observed.
Figure 9. TPR spectra of multilayer cyclohexene on a hydrogen-
precovered Ni(100) surface taken after exposure to 141 langmuirs of
atomic hydrogen at 100 K. Cyclohexane is produced and desorbs above
122 K. Benzene formation caused by hydrogen abstraction is observed
above 354 K.
hydrogen. The increased intensities of the peaks appearing
between 1100 and 1500 cm-1 due to the C-C-H and CH2
bending modes indicate that the cyclohexene’s hydrogens
become less strongly bound to the surface in the presence of
coadsorbed hydrogen.
Both hydrogenation and hydrogen abstraction are observed
following reaction of multilayer cyclohexene with gas phase
atomic hydrogen. The TPR spectra in Figure 9 were taken for
a multilayer of cyclohexene adsorbed on the hydrogen-preex-
posed Ni(100) surface after exposure to 141 langmuirs of atomic
hydrogen. A hydrogen-preexposed surface was used to inhibit
thermal dehydrogenation of cyclohexene. In this figure, hy-
drogen abstraction is clearly indicated by both benzene de-
sorption above 355 K and residual hydrocarbon dehydrogenation
above 400 K. Hydrogenation of multilayer cyclohexene is
indicated by cyclohexane desorption at the multilayer desorption
temperature of 141 K. Cyclohexane produced by hydrogenation
desorbs at 141 and 175 K. With increased atomic hydrogen
exposures, the total cyclohexane yield increases (data not
shown). The amount of benzene produced does not show any
strong correlation with cyclohexane yield. Therefore, we
propose that benzene formation results from hydrogen abstrac-
tion by gas phase atomic hydrogen rather than from dispropor-
tionation of cyclohexene.
Cyclohexene Exposed to Gas Phase Atomic Hydrogen.
Gas phase atomic hydrogen hydrogenates adsorbed cyclohexene
at 100 K, and cyclohexane is produced during subsequent TPR
experiments. The TPR spectra in Figure 8 indicates that
cyclohexane is formed at 178 K with a yield of approximately
90% after adsorbed cyclohexene was exposed to 95 apparent
langmuirs of gas phase atomic hydrogen.31 A small amount of
unreacted cyclohexene desorbs at the same temperature. The
observed cyclohexane formation is mainly caused by cyclo-
hexene hydrogenation initiated by gas phase atomic hydrogen.
Hydrogenation of cyclohexene by bulk hydrogen occurs at the
desorption temperature of bulk hydrogen and can easily be
distinguished from addition of gas phase atomic hydrogen.17
The small high-temperature hydrogen peak above 400 K
indicates that a small amount of residual hydrocarbon dehy-
drogenation is occurring at an elevated temperature after
exposure to gas phase atomic hydrogen.
The changes in the vibrational spectra with atomic hydrogen
exposure at 100 K (Figure 10) clearly indicate that cyclohexene
is hydrogenated by gas phase atomic hydrogen. Spectrum a in
Figure 10 was taken from a monolayer of cyclohexene at 100
K. Spectrum b was obtained after exposing a monolayer of
cyclohexene to 95 apparent langmuirs of atomic hydrogen. A
reference vibrational spectrum c of monolayer cyclohexane is
shown in Figure 10 for comparison. The dashed inset in
spectrum c shows that cyclohexane’s soft modes at 2600 cm-1
are not observed in the presence of coadsorbed hydrogen. After
adsorbed cyclohexene is exposed to gas phase atomic hydrogen,
the energies and relative intensities of several vibrational modes
are modified, and new low-energy vibrational modes appear as
indicated in Table 3. These new modes at 399 and 460 cm-1
correspond closely to C-C-C bending and torsional modes in
monolayer cyclohexane12 and clearly indicate saturation of
carbon-carbon bonds in cyclohexene during atomic hydrogen
exposure. The intensity of the peak at 460 cm-1 appears to
include a contribution from Ni-C stretching mode, suggesting
that adsorbed hydrogenated species may be interacting directly
with the Ni surface. The C-H stretching mode is also shifted
to a lower energy (2895 cm-1) after exposure to gas phase
atomic hydrogen, indicating a higher degree of carbon saturation.
The intensities of the peaks corresponding to the CH2 scissoring
The fact that no noncyclic alkanes desorb with increasing
temperature indicates that hydrogen-induced C-C bond activa-
tion cannot be observed with adsorbed cyclohexene (Figure 8).
No benzene desorption has been observed after reaction of
monolayer cyclohexene with gas phase atomic hydrogen,
suggesting that hydrogen abstraction is not a primary reaction
pathway of monolayer cyclohexene (Figure 8). Abstraction
from adsorbed cyclohexene could also result in the formation
of dehydrogenated species which would be dehydrogenated
above 400 K.25 No increase in the hydrogen peaks above 400
K is observed after reaction with gas phase atomic hydrogen as
indicated by direct comparison of Figures 8 and 5, which were
both run with 30 eV ionization energy. We propose that
competition between adsorption of gas phase atomic hydrogen
on the Ni surface and abstraction from the adsorbed organic
appears to play an important role in controlling the reaction
selectivity. Adsorption of gas phase atomic hydrogen on the
Ni surface is a nonactiVated exothermic process (∆H ∼ -64
kcal/mol).9 However, hydrogen abstraction from cyclohexene
is activated by more than 9 kcal/mol of activation energy32 and
is less exothermic (∆H ∼ 23 kcal/mol).33 These energetics
which strongly favor adsorption apparently preclude abstraction
as a dominant reaction pathway for the adsorbed monolayer.

Hydrogenation of Cyclohexene on the Ni(100) Surface
J. Phys. Chem. B, Vol. 101, No. 18, 1997 3545
Figure 11. TPR spectra of multilayer cyclohexene adsorbed on a
hydrogen-presaturated Ni(100) surface taken after exposure to 139
langmuirs of atomic deuterium. Singly deuterated cyclohexane-d1 is
the main hydrogenated product observed.
Figure 10. HREELS spectra of (a) monolayer cyclohexene on the Ni-
(100) surface at 100 K, (b) monolayer cyclohexene on the Ni(100)
surface taken after exposure to 95 langmuirs of atomic hydrogen at
100 K, and (c) monolayer cyclohexane on the Ni(100) surface at 100
K. The dashed line attached to spectrum c is a part of HREELS
spectrum for cyclohexane coadsorbed with hydrogen on the Ni(100)
surface. These spectra were taken at the specular angle.
TABLE 3: Comparison of Vibrational Modes of Adsorbed
Cyclohexene Exposed to Gas Phase Atomic Hydrogen and
Cyclohexane Adsorbed on the Ni(100) Surface at 100 K;
Vibrational Modes (IR and Raman) of Gas Phase
Cyclohexane35,37 Are Also Listeda
H +
gas
phase
cyclohexene(ad) cyclohexane(ad) mode
description
399
393
υ6
383
C-C-C bend,
torsion
460
843
480
850
υ16
υ5
υ23
υ31
523 (w) C-C-C bend
Figure 12. A schematic diagram for the proposed reaction mechanism
of cyclohexene hydrogenation on the Ni(100) surface. Gas phase atomic
hydrogen adds to cyclohexene at 100 K to form cyclohexyl. This
intermediate is hydrogenated preferentially by surface hydrogen to form
cyclohexane.
802
785
863
C-C stretch
CH2 rock
C-C stretch
1041
1276
1374
1424
1023
1257
1331
1431
υ15 1030 (m) CH2 rock
υ22 1027
υ21 1266
υ29 1261
υ20 1347
υ28 1355
C-C stretch
CH2 twist
CH2 twist
CH2 wag
CH2 wag
CH2 scissor
for cyclohexyl compared to adsorbed cyclohexane. The C-H
soft mode are not observed for cyclohexane coadsorbed with
surface hydrogen (dashed line in spectrum c). The decreased
intensities of the peaks corresponding to C-C stretching mode
at ∼843 and ∼1041 cm-1 in spectrum b suggest that the
cyclohexyl group may be oriented more nearly parallel to the
surface than the cyclohexene ring.
υ3
1465
υ14 1437 (m) CH2 scissor
υ19 1443
υ27 1457
CH2 scissor
CH2 scissor
softened C-H
stretch
2653
2888
2895
υ1
2930
CH2 asym stretch
Mechanistic studies with isotopic hydrogen clearly show that
the surface intermediate formed by addition of gas phase atomic
hydrogen primarily adds a single surface hydrogen to form
cyclohexane. Figure 11 shows the TPR spectra taken from
cyclohexene coadsorbed with hydrogen after exposure to 139
langmuirs of atomic deuterium. Singly deuterated cyclohexane-
d1 (m/e ) 85), not doubly deuterated cyclohexane-d2 (m/e )
86), is the main hydrogenation product. Therefore, surface
hydrogen must be involved in the hydrogenation process,
although surface hydrogen alone cannot hydrogenate cyclohex-
ene. Cyclohexene-d1 (m/e ) 83), cyclohexane (m/e ) 84), and
cyclohexane-d2 (m/e ) 86) are also formed in addition to the
primary cyclohexane-d1 product, indicating that some exchange
also occurs. The intensity of the cyclohexane-d2 peak at 170
K suggests that a small fraction of the deuterated intermediate
may react with desorbing bulk deuterium.17
υ12 2915 (m) CH2 asym stretch
υ25 2933
υ2 2852
CH2 asym stretch
CH2 sym stretch
υ13 2860 (m) CH2 sym stretch
υ18 2897
υ26 2863
CH2 sym stretch
CH2 sym stretch
a IR-active modes are given with their intensities (vs ) very strong,
m ) medium, and w ) weak). Peak energies are given in cm-1
.
mode (1424 cm-1) are also increased after atomic hydrogen
exposure. All these observations are consistent with cyclohex-
ene hydrogenation by gas phase atomic hydrogen at 100 K. The
hydrogenated product is believed to be adsorbed cyclohexyl
based on both these spectroscopic results and isotope experi-
ments described in the next paragraph. An adsorbed cyclohexyl
intermediate is not likely to be further hydrogenated by gas
phase atomic hydrogen since hydrogen abstraction rather than
hydrogen addition dominates in sp3-hybridized carbon during
atomic hydrogen exposure.34 The absence of the C-H soft
mode in spectrum b may be caused by the presence of
coadsorbed hydrogen or may be caused by a different orientation
On the basis of these results, we propose a two-step
mechanism for cyclohexane formation from adsorbed cyclo-
hexene (Figure 12): (i) gas phase atomic hydrogen adds to
adsorbed cyclohexene to form adsorbed cyclohexyl at 100 K;

3546 J. Phys. Chem. B, Vol. 101, No. 18, 1997
Son and Gland
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Summary and Conclusions
Hydrogenation of cyclohexene is observed on the Ni(100)
surface at 100 K during the exposure to gas phase atomic
hydrogen. The formation of a cyclohexyl intermediate is
indicated by both vibrational spectroscopy and isotope labeling
experiments. The appearance of ring deformation modes and
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hydrogenated to cyclohexane primarily by the surface hydrogen
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significant with only multilayer cyclohexene. Surface hydrogen
does not react with coadsorbed cyclohexene on the Ni(100)
surface. Molecular desorption is the dominant thermal process
for cyclohexene in the presence of coadsorbed hydrogen.
Vibrational spectroscopy indicates that π-bonded cyclohexene
with the CdC bond parallel to the surface is the dominant
surface species in the presence of coadsorbed surface hydrogen.
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in the absence of coadsorbed hydrogen. In the absence of
coadsorbed hydrogen, dehydrogenation of the adsorbed cyclo-
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fied using vibrational spectroscopy after heating to 390 K in
the absence of hydrogen. A fraction of this benzene desorbs at
440 K, with the remainder being dehydrogenated with
increasing temperature.
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References and Notes
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