Toluene formation from coadsorbed methanethiol and benzenethiol on the Ni(111) surface

Full text of DOI:10.1021/ja953926t

J. Am. Chem. Soc. 1996, 118, 3781
3781
Toluene Formation from Coadsorbed Methanethiol
and Benzenethiol on the Ni(111) Surface
†
,‡
Sean M. Kane, Deborah R. Huntley,* and
John L. Gland*,
†
Department of Chemistry, UniVersity of Michigan
Ann Arbor, Michigan 48109
Oak Ridge National Laboratory
Oak Ridge, Tennessee 37381
ReceiVed NoVember 22, 1995
We report our observation of interspecies carbon-carbon
bond formation during the reaction of coadsorbed methanethiol
and benzenethiol on the Ni(111) surface. Toluene formation
has been detected between 250 and 320 K in addition to methane
and benzene, the hydrogenolysis products. Increased concentra-
tions of benzenethiolate and methanethiolate, the surface
intermediates, increase the amount of toluene formed. As
observed previously during methanethiol decomposition, a small
Figure 1. Thermal desorption spectra taken after methanethiol exposure
sufficient to produce a saturation coverage (0.25 ML) followed by an
equivalent exposure of benzenethiol on the Ni(111) surface.
1
amount of ethane was produced in addition to the cross-
Adsorbed thiols, such as benzenethiol or methanethiol, have
coupling product; however, no biphenyl was observed by
temperature-programmed desorption. On adsorption at 100 K,
dissociation of the sulfur-hydrogen bond in both thiols results
in the formation of a stoichiometric amount of the adsorbed
thiolate and adsorbed hydrogen. Addition of external hydrogen
decreases the amount of toluene produced up to 25% from the
untreated surface yield, despite the presence of hydrogen from
thiol dissociation. Oxygen pretreatment of the Ni surface results
in increased toluene production for a wide range of coadsorbed
thiolate coverages. Water formation below the toluene forma-
tion temperature decreases surface hydrogen, causing the toluene
yield to increase substantially compared to methane and benzene
yield. Toluene increases up to a factor of 20 were observed
for high coadsorbed coverages. Together these results clearly
indicate that competition between hydrogen addition and
alkylation controls toluene formation.
been characterized mainly for understanding carbon-sulfur bond
1
,18-22
23
activation,
lubrication, and the properties of self-
2
4
assembled monolayers. On most metal surfaces, the sulfur-
hydrogen bond dissociates on adsorption, leaving surface
hydrogen and the thiolate bound to the surface. Thermal
reaction of the thiolate typically leads to hydrogenolysis to form
the corresponding hydrocarbon or nonselective surface decom-
position. Carbon-carbon bond formation has not previously
been observed for two coadsorbed organothiols.
The equipment used in this experiment has been described
1
9
in detail previously. Products are studied by temperature-
programmed reaction, following adsorption on a clean or
predosed Ni surface. The crystal was heated at 5 K/s and
temperature-intensity profiles were collected for several masses.
Adsorbed thiol coverages were determined by measuring final
sulfur coverages with Auger electron spectroscopy after an-
nealing. No sulfur-containing species except the physisorbed
thiols desorb from the Ni surface in the temperature range under
observation, so final sulfur coverages accurately reflect initial
total thiolate coverages. The thiols were purified by several
freeze-pump-thaw cycles and applied through separate direc-
tional dosers, with the crystal positioned within 1 mm of each
doser. H and O were dosed by back-filling the chamber to a
Alkylation is typically performed in industry by the reaction
of alkanes and olefins in HF or H2SO4 reactors.2 Benzene
alkylation usually involves a Friedel-Crafts reaction of the
phenyl ring with alkyl halides over an AlCl3 catalyst.3 Studies
have also examined carbon-carbon bond formation with solid
acids,5 zeolites,7 molten salts,8 and transition metal com-
plexes.9 Carbon bond formation reactions have been studied
on several metal surfaces, including organohalide reactions on
,4
,6
,10
2
2
1
1-13
14,15
16
17
Cu,
Au,
and Pd and methyl radical reactions on Pt.
specified pressure for a set period of time.
†
‡
The coadsorption of a selected range of methanethiol and
benzenethiol exposures was examined. In addition to the
amounts of thiols dosed, the effect of dosing order was
investigated. Extensive data for each thiol separately have been
University of Michigan.
Oak Ridge National Laboratory.
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1
,19
reported previously for the Ni(111) surface.
The primary
1
955; Vol. 2.
products of the reactions of the individual thiols are hydrocar-
bons formed from hydrogenolysis of the sulfur-carbon bond.
Experiments have shown that adsorbed alone on a clean surface,
(
(
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(
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methanethiol produces methane at 273 K and benzenethiol
38.
19
forms a series of benzene desorption peaks above 265 K.
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002-7863/96/1518-3781$12.00/0 © 1996 American Chemical Society

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782 J. Am. Chem. Soc., Vol. 118, No. 15, 1996
Communications to the Editor
Figure 3. Thermal desorption spectra of toluene desorption from the
Ni(111) surface predosed with oxygen, hydrogen, and a clean surface
followed by approximately half-saturation coverages each of meth-
anethiol and benzenethiol. Inset shows water desorption prior to toluene
formation for the oxygen-predosed case.
Figure 2. Thermal desorption spectra of toluene from (a) saturation
exposure of methanethiol (0.25 ML) followed by equivalent exposure
to benzenethiol, (b) low exposure (0.08 ML) of methanethiol followed
by saturation exposure of benzenethiol, and (c) benzenethiol adsorbed
in the absence of predosed methanethiol. Final sulfur coverages resulting
from the coadsorption experiments are indicated.
factor. Toluene formation decreases up to 25% have been
observed. Substantial amounts of hydrogen are available on
the surface from low-temperature thiol decomposition prior to
external hydrogen addition. The small decrease in toluene
produced is consistent with a small increase in availability. The
decrease is largest for intermediate and small coverages of
coadsorbed methanethiol followed by benzenethiol. The de-
crease was measured relative to the total amount of thiol
adsorbed to compensate for any variations in initial thiol
coverage caused by hydrogen predosing. Avenues for further
exploring the effect of surface hydrogen require the elimination
of hydrogen formed from sulfur-hydrogen bond dissociation.
exposure sufficient to give 0.25 ML (approximately saturation)
of methanethiol followed by an equivalent exposure of ben-
zenethiol. Toluene is formed and desorption begins at 250 K
and is complete by approximately 320 K. A smaller toluene
formation peak is centered at 380 K. Submonolayer coverages
of toluene have been observed to undergo complete decomposi-
tion on the Ni(111) surface.25 Low-temperature (100 K)
adsorption of toluene performed for this study also leads to
complete decomposition for submonolayer coverages. For
coadsorbed thiols, hydrogen, methane, and benzene are the other
major products which desorb from the surface, while trace
amounts of ethane have been detected, similar to previous
methanethiol decomposition experiments.1 The methane and
benzene desorption peaks are very similar to the peaks formed
during decomposition of each thiol individually on a clean Ni-
2
6
27,28
On Pt and Ni
surfaces, water is formed from the reaction
of H S with preadsorbed oxygen. Addition of 2 L (1 L ) 1 ×
2
6
-
10 Torr s) of preadsorbed oxygen, annealed to 600 K, causes
low-temperature water formation at 190 K which decreases the
amount of hydrogen available at coupling temperature (see inset,
Figure 3). Thus, oxygen addition is expected to increase the
formation of toluene if competition with hydrogenolysis is a
primary controlling factor. Increases in toluene formation up
to a factor of 20 have been observed with preadsorbed oxygen
for a wide range of initial coadsorbed coverages and for two
different coverages of preadsorbed oxygen. Toluene yield
accounts for approximately 22% of the total desorbing products,
equivalent to 0.06 mL, demonstrating that the reaction selectivity
is indeed controlled by competition with hydrogenolysis.
We have established both that carbon-carbon bond formation
occurs from coadsorbed methanethiol and benzenethiol on the
Ni(111) surface and that the selectivity is enhanced when the
competing hydrogenolysis mechanism is hindered. This pro-
vides significant insight into a novel and catalytically important
type of surface reaction.
(
111) surface. The correlation between toluene desorption and
thiolate hydrogenolysis indicates that toluene formation is
limited by carbon-sulfur bond activation. A crude estimate
of the relative yields can be found by correcting the observed
peak intensities for the ionization efficiencies and mass spec-
trometer transmission function for each molecule. This infor-
mation can also be related to absolute coverage through
comparison to monolayers of each thiol alone on the Ni(111)
surface. Maximum yields of toluene from the initially clean
surface were found to be slightly less than 1% of the total
products desorbing from the surface, which accounts for
approximately 0.003 ML. Figure 2 indicates that the amount
of toluene formed increases with increasing methanethiol
predosed exposure and a constant benzenethiol exposure
equivalent to saturation of a clean surface. A similar pattern
was observed when the order of adsorption was reversed. No
additional carbon-carbon bond formation products were de-
tected from reaction of coadsorbed methanethiol and ben-
zenethiol.
Acknowledgment. This work is supported by the Division of
Chemical Sciences of BES-DOE under Grant DE-FG02-91 ER14190
at the University of Michigan and under Contract DE-AC05-
96OR22464 at Oak Ridge National Laboratory managed by Lockheed-
Martin Energy Research Corporation.
The selectivity of the coupling reaction relative to hydro-
genolysis has been probed by increasing and decreasing the
availability of surface hydrogen at coupling temperatures as
illustrated in Figure 3. Addition of external hydrogen to increase
the availability of surface hydrogen should decrease the amount
of coupling observed if competition is a primary controlling
JA953926T
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