Transition state for alkyl group hydrogenation on Pt(111)

Article abstract of DOI:10.1021/ja075292j

Substituent effects have been used to probe the characteristics of the transition state to hydrogenation of alkyl groups on the Pt(111) surface. Eight different alkyl and fluoroalkyl groups have been formed on the Pt(111) surface by dissociative adsorption of their respective alkyl and fluoroalkyl iodides. Coadsorption of hydrogen and alkyl groups, followed by heating of the surface, results in hydrogenation of the alkyl groups to form alkanes, which then desorb into the gas phase. Temperature-programmed reaction spectroscopy was used to measure the barriers to hydrogenation, ΔE_H^≠, which are dependent on the size of the alkyl group (polarizability) and the degree of fluorination (field effect). This example is one of only two surface reactions for which the influence of the substituents on ΔE _H^? has been correlated with both the field and the polarizability substituent constants of the alkyl groups in the form of a linear free energy relationship. Increasing both the field and the polarizability constants of the alkyl groups increases the value of ΔE_H ^?. The substituent effects are quantified by a field reaction constant of ρ_F = 27 ± 4 kJ/mol and a polarizability reaction constant of ρ_α = 19 ± 3 kJ/mol. These suggest that the transition state for hydrogenation is slightly cationic with respect to the alkyl group on the Pt(111) surface, RC + H ←{RC ^δ+ ...H)^?.

Full text of DOI:10.1021/ja075292j

Published on Web 06/04/2008
Transition State for Alkyl Group Hydrogenation on Pt(111)
Pingping Ye† and Andrew J. Gellman*,†,‡,§
Departments of Chemistry and Chemical Engineering, Carnegie Mellon UniVersity, Pittsburgh,
PennsylVania 15213, and National Energy Technology Laboratory, U.S. Department of Energy,
P.O. Box 10940, Pittsburgh, PennsylVania 15236
Received August 25, 2007; E-mail: gellman@cmu.edu
Abstract: Substituent effects have been used to probe the characteristics of the transition state to
hydrogenation of alkyl groups on the Pt(111) surface. Eight different alkyl and fluoroalkyl groups have
been formed on the Pt(111) surface by dissociative adsorption of their respective alkyl and fluoroalkyl
iodides. Coadsorption of hydrogen and alkyl groups, followed by heating of the surface, results in
hydrogenation of the alkyl groups to form alkanes, which then desorb into the gas phase. Temperature-
programmed reaction spectroscopy was used to measure the barriers to hydrogenation, ∆EqH, which are
dependent on the size of the alkyl group (polarizability) and the degree of fluorination (field effect). This
example is one of only two surface reactions for which the influence of the substituents on ∆EqH has been
correlated with both the field and the polarizability substituent constants of the alkyl groups in the form of
a linear free energy relationship. Increasing both the field and the polarizability constants of the alkyl groups
increases the value of ∆EqH. The substituent effects are quantified by a field reaction constant of FF ) 27
( 4 kJ/mol and a polarizability reaction constant of FR ) 19 ( 3 kJ/mol. These suggest that the transition
state for hydrogenation is slightly cationic with respect to the alkyl group on the Pt(111) surface, RC + H
T {RCδ+ · · ·H}q.
on Pd(111) and Ag(111) surfaces.6–9 Studies of the deiodination
of a series of alkyl iodides and the dechlorination of dichloro-
ethanes have shown that the transition states for both C-I and
C-Cl bond cleavage are homolytic and that substituents have
almost no effect on the relative energies of the transition state
and the initial state. The use of substituent effects offers a unique
approach to probing the transition states for heterogeneous
catalytic reactions and, ultimately, understanding the influence
of catalysts on the kinetics of each elementary step.1
1. Introduction
Heterogeneous catalysis is fundamentally a kinetic phenom-
enon. A deep understanding of this phenomenon requires a
combined understanding of both catalytic reaction mechanisms
and the influence of catalysts on elementary surface-reaction
kinetics. The latter implies that it is important to understand
the nature of the transition states to elementary catalytic surface
reactions.1 Unfortunately, there are very few experimental
studies of the transition states to elementary catalytic reaction
steps because of the lack of experimental methods for character-
izing transition states on surfaces. The use of substituent effects
is one successful approach that can be used for experimental
study of the transition states for a number of elementary surface
reactions.2,3 One example is a study of the ꢀ-hydride elimination
of alkoxides on the Cu(111) surface, which has shown that the
transition state is cationic with respect to the alkoxy reactant,
{RCδ+ · · ·H}q.4 Another example is carboxylic acid deproto-
nation on Cu(100).5 Substituent effects revealed that the
transition state for deprotonation is anionic with respect to the
initial-state acid, {RCO2δ– · · ·H}q and occurs early in the reaction
coordinate. A third example is dehalogenation of alkyl halides
Hydrogenation of alkyl groups is one of the most common
reduction reactions occurring on transition metal surfaces. For
example, methane formation from methyl moieties has been
observed and studied on single crystals of Pt(111), Ni(100),
Pd(100), Ru(001), Cu(110), and Cu(111) and on films of iron,
nickel, palladium, lead, gold, and copper.10 Alkyl group hydro-
genation can be studied on metal surfaces by first generating
isolated alkyl groups from alkyl iodides. It is well-known that
propyl groups can be formed on the Pt(111) surface by
adsorption of propyl iodide at low temperature, followed by
heating to dissociate the C-I bond and generate adsorbed propyl
groups and iodine atoms. During heating in the presence of
preadsorbed hydrogen, propyl groups on the Pt(111) surface
can be hydrogenated to form propane, which desorbs at about
† Department of Chemistry, Carnegie Mellon University.
‡ Department of Chemical Engineering, Carnegie Mellon University.
§ U.S. Department of Energy.
(6) Buelow, M. T.; Gellman, A. J. J. Am. Chem. Soc. 2001, 123, 1440–
1448.
(1) Gellman, A. J. J. Phys. Chem. B 2002, 106, 10509–10517.
(2) Gellman, A. J. Curr. Opin. Solid State Mater. Sci. 2001, 5, 85–90.
(3) Gellman, A. J.; Buelow, M. T.; Street, S. C.; Morton, T. H. J. Phys.
Chem. A 2000, 104, 2476–2485.
(7) Buelow, M. T.; Immaraporn, B.; Gellman, A. J. J. Catal. 2001, 203,
41–50.
(8) Buelow, M. T.; Zhou, G.; Gellman, A. J.; Immaraporn, B. Catal. Lett.
1999, 59, 9–13.
(4) Gellman, A. J.; Dai, Q. J. Am. Chem. Soc. 1993, 115, 714–722.
(5) Immaraporn, B.; Ye, P. P.; Gellman, A. J. J. Phys. Chem. B 2004,
108, 3504–3511.
(9) Immaraporn, B.; Ye, P. P.; Gellman, A. J. J. Catal. 2004, 223, 98–
105.
(10) Zaera, F. J. Mol. Catal. 1994, 86, 221–242.
9
8518 J. AM. CHEM. SOC. 2008, 130, 8518–8526
10.1021/ja075292j CCC: $40.75
2008 American Chemical Society

Transition State for Alkyl Group Hydrogenation on Pt(111)
A R T I C L E S
250 K.11,12 The rate limiting step in the appearance of the
propane is the hydrogenation of the propyl groups.12
2. Experimental Section
All experiments were performed in an ultrahigh-vacuum chamber
evacuated with a cryopump to a base pressure of 2 × 10-10 Torr.
This chamber is equipped with a quadrupole mass spectrometer. It
is also equipped with an Ar+ ion gun for cleaning the surface and
several leak valves for introduction of gases and vapors of the alkyl
and fluoroalkyl iodides used in this study. In addition, X-ray
photoelectron spectroscopy (XPS) was used to monitor the cleanli-
ness of the surface and to measure the initial coverage of iodides.
The Pt(111) sample purchased from Monocrystals Co. was
mounted by spot-welding it between two Ta wires connecting it to
a manipulator, which allowed translation in the x, y, and z directions
and rotation about the z-axis by 360°. The Pt(111) sample could
be cooled to temperatures less than 90 K and resistively heated to
temperatures greater than 1200 K. The temperature was measured
by a chromel-alumel thermocouple spot-welded to the Pt(111)
sample. The surface was first cleaned by cycles of Ar+ sputtering,
followed by annealing to 1000 K. Surface cleanliness was deter-
mined by using XPS. Between experiments, the surface was cleaned
by annealing at 1000 K to desorb iodine and by annealing at 1000
K in 2 × 10-7 Torr of O2 to remove any residual carbon from the
sample surface.
The temperature-programmed reaction (TPR) spectra were
obtained by using a Dycor M200M quadrupole mass spectrometer.
The adsorption of alkyl iodides was performed with the Pt(111)
surface cooled to 95 K. Exposures of the hydrogen and the alkyl
iodides to the Pt(111) surface were recorded in units of Langmuirs
(1 L ) 10-6 Torr·s), with the pressure measured by an ion gauge
and left uncorrected for ion-gauge sensitivity. After adsorption of
the alkyl iodides, the sample was positioned in front of the aperture
to the mass spectrometer. The Pt(111) sample was then heated at
a rate of ꢀ ) 2 K/s while using the mass spectrometer to monitor
signals of desorbing species at up to five m/q ratios.
The relative initial coverages of the alkyl iodides and hydrogen
reported in this paper were determined as follows. Atomic iodine
was deposited onto the Pt(111) surface by adsorbing ethyl iodide
onto a clean surface at approximately 95 K and annealing at 700
K for 60 s. This left atomic iodine on the surface without significant
amounts of carbon. Higher total iodine coverages were achieved
by repeated cycles of exposing the surface to 2 L of ethyl iodide
and then heating to 700 K. The cumulative iodine coverage on the
surface following each ethyl iodide adsorption and thermal decom-
position cycle was measured by using XPS and is shown in Figure
1. It can be seen from Figure 1 that a cumulative ethyl iodide
exposure of about 4 L saturated the Pt(111) surface with iodine.
An ethyl iodide exposure of 1 L produced ∼30% of the saturation
coverage of iodine. Because one ethyl iodide molecule produces
one iodine atom on the surface after annealing to 700 K, the iodine
coverage can be used as a measure of the coverage of ethyl groups
generated by ethyl iodide exposure. Except when indicated, the alkyl
iodide exposures used in this work were controlled to give alkyl
group coverages that corresponded to ∼30% of the saturation
coverage of iodine on the Pt(111) surface.
Thermal decomposition of alkyl iodides on the Pt(111) surface
can also lead to the formation of alkanes in the absence of
preadsorbed hydrogen. This must occur via a mechanism that
uses some source of hydrogen other than adsorption from the
gas phase. Immediately following cleavage of the C-I bond in
ethyl iodide, the resulting ethyl groups dehydrogenate at
temperatures as low as 170 K to yield ethylene. During heating,
the chemisorbed ethylene is then hydrogenated at a higher
temperatures to reform ethyl groups, which can then be
hydrogenated to form ethane.13 In this mechanism, the rate-
limiting step in the appearance of ethane in the gas phase is the
hydrogenation of the adsorbed ethylene rather than the hydro-
genation of the ethyl groups.
The hydrogenation of olefins such as ethylene and propylene
has been studied on the Pt(111) surface with and without
preadsorbed hydrogen.11,12,14,15 On Pt(111), ethylene undergoes
self-hydrogenation via an initial C-H bond breaking step to
generate adsorbed hydrogen atoms which are consumed by
subsequent hydrogenation of the remaining ethylene. The C-H
bond breaking in ethylene is the rate-limiting step for self-
hydrogenation of ethylene on Pt(111). Coadsorption of hydrogen
with ethylene increases the ethane yield and lowers the apparent
activation energy for the hydrogenation reaction. Ethylene
hydrogenation occurs by a stepwise mechanism in which ethyl
groups are formed as intermediates. The ethyl groups can either
acquire a second hydrogen atom at the R-carbon to form ethane,
which desorbs, or lose a ꢀ-hydrogen atom by ꢀ-hydride
elimination to form ethylene.14 Propylene is hydrogenated by a
similar mechanism in which hydrogenation to a propyl group
is rate-limiting and is followed by fast hydrogenation of the
propyl group to form propane, which desorbs.15
In this work, the activation barrier to the hydrogenation of
alkyl groups, ∆EqH, has been measured by using a set of
substituted alkyl and fluoroalkyl groups coadsorbed on Pt(111)
with an excess of H atoms. The alkyl and fluoroalkyl groups
were prepared by thermal dissociation of the corresponding
iodides. The alkyl and fluoroalkyl substituent groups were
bonded to the R-C and influenced the value of ∆EqH. One
important feature of this work has been the fact that it has used
substituents varying in both their field and their polarizability
substituent constants and has used enough different substituents
to be able to correlate the impact of both field and polarizability
effects on the nature of the transition state. This provides insight
into the character of charge distribution both in the initial state
and in the transition state, rather than just the change in charge
distribution between the two states. The only other such study of
both field and polarizability effects in an elementary surface reaction
step has been a study of C-I bond cleavage on the Ag(111)
surface.6 In the case of alkyl group hydrogenation on Pt(111)
studied in this report, the correlation of the substituents’ field and
polarizability constants, σF and σR, with ∆EqH suggests that the
initial-state alkyl groups are slightly anionic with respect to the
transition state for hydrogenation, RC + H T {RCδ+ · · ·H}q.
Hydrogen atoms were deposited onto the Pt(111) surface by
exposing the clean surface at a temperature of 120 K to H2. The
exposures of H2 used in this study were 0, 1, 3, and 10 L. The
coverage of hydrogen was determined by temperature-programmed
desorption (TPD) measurements in which the Pt(111) surface was
heated at a rate of 2 K/s while monitoring the desorption of
hydrogen by using the mass spectrometer. A saturation coverage
of atomic hydrogen was achieved after a 10 L exposure of the
Pt(111) surface to H2. Exposures of 1 and 3 L produced hydrogen
coverages of 40 and 70% of saturation, respectively.
The alkyl and fluoroalkyl iodides were purchased commercially
from Aldrich Chemical Co. and SynQuest Laboratories, Inc.,
respectively. They were purified by cycles of freeze-pump-thawing
before use. The purity of the gases and vapors introduced into the
vacuum chamber were verified by using the mass spectrometer.
(11) Chrysostomou, D.; French, C.; Zaera, F. Catal. Lett. 2000, 69, 117–
128.
(12) Scoggins, T. B.; Ihm, H.; White, J. M. Isr. J. Chem. 1998, 38, 353–
363.
(13) Zaera, F. J. Phys. Chem. 1990, 94, 8350–8355.
(14) Zaera, F. J. Phys. Chem. A 1990, 94, 5090–5095.
(15) Zaera, F.; Chrysostomou, D. Surf. Sci. 2000, 457, 71–88.
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A R T I C L E S
Ye and Gellman
binding energy of EB ) 620.8 eV). The I 3d5/2 binding energy
remained unchanged after annealing to a temperature of 160
K. Upon heating the sample to 200 K, however, the I 3d5/2
binding energy decreased by 0.7 eV to EB ) 620.1 eV. This
shift to a lower binding energy is indicative of dissociation of
the C-I bond and the deposition of the iodine atom on the
Pt(111) surface and, as a consequence, the formation of an
adsorbed CF3CH2CH2- group. The I 3d5/2 peak areas in the XP
spectra shown in Figure 2 remained constant until the sample
temperature reached approximately 800 K, indicating that
adsorbed iodine atoms are stable on the Pt(111) surface up to
this temperature. In conclusion, CF3CH2CH2I decomposes on
the Pt(111) surface at temperatures below 200 K, leaving
CF3CH2CH2- and iodine on the surface.
On the Pt(111) surface, C-I bonds in other fluoroalkyl iodides
are expected to break at temperatures below 200 K, the
temperature by which CF3CH2CH2I has decomposed to form
CF3CH2CH2- and iodine on the surface. It has been reported
that the transition state for dehalogenation is homolytic on
Ag(111) and Pd(111) surfaces and that the energy barriers to
the cleavage of C-I bonds in different fluorine-substituted alkyl
iodides on the Ag(111) and Pd(111) surfaces are relatively
insensitive to the fluorination of the alkyl group.6,7 It has been
reported also that the barriers to dehalogenation are relatively
insensitive to the nature of the surface and that the range of
these barriers is relatively small.1 Accordingly, we expect that
the barriers to C-I bond cleavage of all the alkyl and fluoroalkyl
iodides that we have studied on the Pt(111) surface should be
similar to that of CF3CH2CH2I and that the C-I bonds are
cleaved in all these species by annealing to ∼200 K.
Figure 1. I 3d XPS signal on the Pt(111) surface during repeated exposures
to ethyl iodide at 95 K, followed by thermal decomposition by heating to
700 K. Saturation of the Pt(111) surface with iodine occurs after a
cumulative exposure of ∼4 L.
3. Results
In this study, the hydrogenation kinetics of eight different
alkyl and fluoroalkyl groups (Table 1) were measured on the
Pt(111) surface. The alkyl and fluoroalkyl groups were prepared
from the corresponding alkyl iodides by thermal dissociation
of their C-I bonds on the Pt(111) surface. In the presence of
preadsorbed hydrogen, heating resulted in hydrogenation of the
alkyl groups to alkanes. This section is divided into three
subsections describing the thermal decomposition of alkyl
iodides, the hydrogenation of alkyl groups, and the hydrogena-
tion of fluoroalkyl groups.
3.1. Formation of Alkyl and Fluoroalkyl Groups on Pt(111).
Thermal decomposition of alkyl iodides adsorbed on single-
crystal metal surfaces is a well-established method for producing
alkyl groups. In many studies, it has been shown that low-
temperature adsorption of alkyl iodides, followed by heating,
results in the dissociation of the C-I bond.16,17 Provided that
the coverage of the atomic iodine coadsorbed with the alkyl
groups does not approach saturation, the iodine acts as a passive
spectator to subsequent reactions of the alkyl groups. On the
Pt(111) surface, C-I bond cleavage in 1-iodopropane occurs
at temperatures between 160 and 220 K and results in the
formation of adsorbed n-propyl groups, CH3CH2CH2-, and
atomic iodine. On Pt(111) and Cu(111) surfaces, C-I bonds in
ethyl iodide dissociate when heated to approximately 170 K.6,18
As in the cases of alkyl iodides, C-I bonds in fluoroalkyl
iodides have been shown to cleave at temperatures below 200
K on Ag(111) and Pd(111) surfaces to produce adsorbed
fluoroalkyl groups.6,7
3.2. Hydrogenation of Alkyl Groups on Pt(111). Heating of
the alkyl and fluoroalkyl groups generated by alkyl iodide
dissociation on the clean Pt(111) surface results in thermal
decomposition in the temperature range 300-500 K. Figure 3
shows the TPR spectra obtained after exposure of the Pt(111)
surface at 95 K to 1.5 L of 1-iodopropane. This exposure
resulted in a coverage of 1-iodopropane that was ∼30% of the
saturation coverage of iodine on the Pt(111) surface. The
desorption traces shown in Figure 3 were obtained by monitoring
the signal of ionization fragments at m/q ) 2 (hydrogen), 27
(propylene and propane), 29 (propane), and 127 (1-iodopropane).
Heating this low coverage of 1-iodopropane leads to C-I bond
cleavage but not to the desorption of the alkyl iodide or propane
(the hydrogenation product). Although Zeara et al. did observe
some formation of propane during the decomposition of propyl
iodide on the clean Pt(111) surface, their experiment was
performed with the propyl iodide at close to saturation coverage,
whereas this experiment was performed at roughly one-third
of the saturation coverage.11 This coverage dependence of the
self-hydrogenation is also consistent with the observations of
White et al.12 In our work, we observe that at low temperature,
some propylene desorbs with a peak desorption temperature of
approximately 150 K. This low-temperature feature indicates
that there is some C-I bond cleavage on the surface at these
temperatures. However, rather than indicating a low-temperature
ꢀ-hydride elimination step leading to propylene desorption, it
is quite possible that the signal at m/q ) 27 is due to the direct
desorption of a propyl group and its subsequent fragmentation
in the ionizer of the mass spectrometer. This low-temperature
formation of alkyl radicals during C-I cleavage has been
observed before on Ni(100) and Cu(111) surfaces.19,20 Molecular
In this study, the C-I bonds in alkyl and fluoroalkyl iodides
have been observed to break at temperatures below 200 K on
the Pt(111) surface. C-I bond cleavage in CF3CH2CH2I on
Pt(111) is revealed by the I 3d5/2 XP spectra shown in Figure
2. CF3CH2CH2I was adsorbed molecularly at 95 K on the
Pt(111) surface with the C-I bond intact (note the I 3d5/2 peak
(16) Bent, B. E. Chem. ReV. 1996, 96, 1361–1390.
(17) Zaera, F. Chem. ReV. 1995, 95, 2651–2693.
(18) Zaera, F. Surf. Sci. 1989, 219, 453–466.
(19) Zaera, F.; Tjandra, S. J. Phys. Chem. 1994, 98, 3044–3049.
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Transition State for Alkyl Group Hydrogenation on Pt(111)
A R T I C L E S
Table 1. Field Substituent Constants (ΣσF), Polarizability Substituent Constants (ΣσR), Temperatures of Maximum Hydrogenation Rate
(Tmax), and Barriers to Hydrogenation (∆EqH) of the Alkyl and Fluoroalkyl Groups Studied on the Pt(111) Surface
alkyl group
substituents
field constants
ΣσF
polarizability constants
ΣσR
Tmax
∆EqH (kJ/mol)
CH3CH2-
CH3CH2CH2-
(CH3)2C(H)-
CH3
CH3CH2
H
CH3
CH3
CH3
CH3
CH3
CF3CH2CH2
CF3CH2
CF3CF2CH2
(CF3)2CH
0
0
0
0
0
0
0
0
0.12
0.23
0.24
0.44
0
0
0
-0.35
-0.49
0
-0.35
-0.49
-0.70
234
232
211
60.8
60.3
54.6
-0.35
-0.35
-0.35
-0.35
-0.35
-0.55
-0.46
-0.53a
-0.57
(CH3)3C-
0
-1.05
189
48.8
CF3CH2CH2CH2-
CF3CH2CH2-
CF3CF2CH2CH2-
(CF3)2CHCH2-
0.12
0.23
0.24
0.44
-0.55
-0.46
-0.53
-0.57
245
258
258
263
63.7
67.2
67.2
68.6
a Estimated on the basis of substituent-group additivity and a fall-off factor of 2.3 with the number of carbon atoms from reaction center.26,27
Figure 2. I 3d XPS of the Pt(111) surface after exposure to 2 L of
CF3CH2CH2I at 95 K and subsequent annealing to the temperatures indicated
on the right. The I 3d5/2 binding energies are 620.8 eV for iodine in the
adsorbed CF3CH2CH2I and 620.1 eV for iodine atoms on the Pt(111) surface.
Figure 3. TPR spectra of 1.5 L CH3CH2CH2I on clean Pt(111). The surface
was exposed to CH3CH2CH2I at 95 K. The heating rate was 2 K/s. Signals
were monitored at m/q ) 2, 27, 29, and 127. No production of propane is
observed.
hydrogen desorption occurred with two peaks centered at
approximately 315 and 415 K. These results are similar to those
reported by White et al.12 This behavior is typical of n-propyl
groups on transition-metal surfaces where they undergo ꢀ-hy-
dride elimination to form di-σ-bonded propylene.12 Subse-
quently, the di-σ-bonded propylene dehydrogenates to form
propylidyne, CH3CH2CtPt, which decomposes to graphitic
carbon and atomic hydrogen at temperatures above 400 K. The
hydrogen desorption peak at 315 K is attributed to the
recombination of hydrogen atoms released by the ꢀ-hydride
elimination of propyl groups and the decomposition of the di-
σ-bonded propylene to propylidyne. The hydrogen desorption
peak at 415 K is due to the decomposition of propylidyne.11
The assignment of the peak at 315 K to recombination of
hydrogen atoms on the Pt(111) surface is supported by the fact
that the peak desorption temperature of hydrogen from the clean
Pt(111) surface is observed in the range 270-320 K. The broad
hydrogen desorption peaks at higher temperatures arise from
the decomposition of hydrocarbon fragments remaining on the
surface. The important observation from the point of view of
this work is that there is no propane formed during decomposi-
tion of propyl groups on the clean Pt(111) surface.
TPR spectra of coadsorbed hydrogen and propyl groups on
the Pt(111) surface reveal the formation and desorption of
propane. Figure 4 shows the TPR spectra obtained from the
Pt(111) surface by using hydrogen pre-exposures of 0, 1, 3,
and 10 L at 120 K followed by exposures of 1.5 L of
1-iodopropane at 95 K. An exposure of 10 L of H2 produced a
monolayer (saturation coverage) of hydrogen atoms on the
Pt(111) surface, whereas exposures of 1 and 3 L of H2 produced
approximately 0.4 and 0.7 ML of hydrogen, respectively. Figure
4 clearly reveals the reaction between adsorbed hydrogen atoms
and propyl groups, resulting in the formation and desorption of
propane at approximately 235 K. Figure 4 also reveals that the
propane desorption-peak temperature is independent of the
adsorbed hydrogen coverage.
The coadsorption of hydrogen and other alkyl iodides on the
Pt(111) results in the formation of alkanes during subsequent
heating. In TPR of propyl groups coadsorbed with high
hydrogen coverages, no propylene desorption is observed. TPR
spectra of iodoethane, 2-iodopropane, and 2-iodo-2-methyl
propane coadsorbed with hydrogen atoms on Pt(111) surfaces
(20) Lin, J. L.; Bent, B. E. J. Phys. Chem. 1993, 97, 9713–9718.
9
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A R T I C L E S
Ye and Gellman
Figure 5. TPR spectra of 2 L of CF3CH2CH2I on clean Pt(111). The surface
was exposed to CF3CH2CH2I at 95 K. The heating rate was 2 K/s. Signals
were monitored at m/q ) 2 and 29. Heating of adsorbed CF3CH2CH2-
does lead to the formation and desorption of trifluoropropane.
Figure 4. TPR spectra of 1.5 L CH3CH2CH2I on Pt(111) with different
pre-exposures to H2. The surface was first exposed to H2 at 120 K and then
to 1.5 L of CH3CH2CH2I at 95 K. The heating rate was 2 K/s. The desorption
of propane was monitored by collecting the signal at m/q ) 29 and is
observed to occur once hydrogen has been preadsorbed on the surface.
propane desorption occurred at 270 K. These results indicate
that hydrogenation of the alkyl groups is kinetically favored
over hydrogenation of the olefin. Furthermore, a previous study
of the TPR of propyl groups coadsorbed with deuterium has
shown that the product is dominated by d1-propane, again
suggesting that there is little contribution from an olefin
intermediate.11 Thus, during TPR of coadsorbed alkyl iodides
and hydrogen on the Pt(111) surface, alkane desorption is rate-
limited by alkyl group hydrogenation rather than olefin
hydrogenation.
that were first exposed to 0, 1, 3, and 10 L of H2 are similar to
those obtained by using 1-iodopropane. In the absence of
preadsorbed hydrogen, there is no alkane formation. As shown
in Table 1, the alkanes produced by the hydrogenation of the
alkyl groups by preadsorbed hydrogen all form at different
temperatures. For the alkyl iodides studied in this work, the
alkane desorption temperature is independent of the hydrogen
coverage on the Pt(111) surface.
In order to prove that the evolution of alkanes from the
Pt(111) surface during alkyl hydrogenation is not rate-limited
by desorption of the alkane product, TPD spectra of butane on
the Pt(111) surface were obtained (data not shown). During
heating, submonolayer coverages of butane desorbed at ap-
proximately 175 K, and it is reasonable to expect that sub-
monolayer coverages of ethane, propane, and iso-butane would
desorb at even lower temperatures. The peak temperatures for
alkane desorption during alkyl hydrogenation are all higher than
175 K, indicating that the evolution of the alkanes from the
Pt(111) surface is rate-limited by the hydrogenation step rather
than by the desorption of the alkane product. Thus, the peak
temperatures for alkane desorption during TPRS of coadsorbed
hydrogen and alkyl groups can be used to estimate the activation
barriers for hydrogenation, ∆EqH.
In order to confirm that the alkanes formed during TPR of
coadsorbed hydrogen and alkyl groups were the product of alkyl
hydrogenation rather than olefin hydrogenation, TPR experi-
ments were performed by using hydrogen and propylene
coadsorbed on a Pt(111) surface. TPR spectra were obtained
from the Pt(111) surface exposed to 1 L of H2 followed by 0.35
L of propylene. XPS of the C 1s level showed that the coverage
of propylene was approximately the same as the coverage of
propyl groups used in the TPR experiments described above.
On the Pt(111) surface prepared with coadsorbed hydrogen and
propyl groups, propane desorption occurred at approximately
235 K (Figure 3), whereas with propylene as the precursor,
3.3. Hydrogenation of Fluoroalkyl Groups on Pt(111). Ad-
sorption of fluoroalkyl iodides on the Pt(111) surface leads to
the formation and desorption of fluoroalkanes during heating.
Figure 5 shows the TPR spectra obtained after a 2 L exposure
of CF3CH2CH2I to the clean Pt(111) surface at 95 K. As in the
case of CH3CH2CH2I, this exposure resulted in a coverage of
CF3CH2CH2I that was equal to ∼30% of the saturation coverage
of iodine on the Pt(111) surface. In contrast to the TPR spectra
obtained from the alkyl iodides on the clean Pt(111) surface,
which showed no alkane production, a peak attributed to
desorption of trifluoropropane was observed at 285 K during
heating of adsorbed CF3CH2CH2I. The desorption of trifluoro-
propane was confirmed by monitoring the signals of ionization
fragments at m/q ) 29 (CH2CH3+), 59 (CFCHCH3+), 69
(CF3+), and 79 (CF2CH2CH3+). The source of hydrogen for
this self-hydrogenation must be the decomposition of
CF3CH2CH2- groups. One of the most likely mechanisms of
hydrogen release is ꢀ-hydride elimination by the CF3CH2CH2-
groups on the Pt(111) surface.21 Hydrogen then reacts with the
remaining CF3CH2CH2- groups to produce CF3CH2CH3. As
has been shown in previous work, fluorination of the propyl
group increases the barrier to ꢀ-hydride elimination.21,22 As a
result, ꢀ-hydride elimination occurs at a higher temperature in
CF3CH2CH2- than in CH3CH2CH2-. At a higher temperature,
the rate constant for hydrogenation is higher, and thus, the
(21) Ye, P. P.; Gellman, A. J. J. Phys. Chem. B 2006, 110, 9660–9666.
(22) Forbes, J. G.; Gellman, A. J. J. Am. Chem. Soc. 1993, 115, 6277–
6283.
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Transition State for Alkyl Group Hydrogenation on Pt(111)
A R T I C L E S
Figure 6. TPR spectra of 2 L of CF3CH2CH2I on Pt(111) with different
pre-exposures to H2. The surface was first exposed to H2 at 120 K and then
to 2 L of CF3CH2CH2I at 95 K. The heating rate was 2 K/s. The desorption
of trifluoropropane was monitored by collecting the signal at m/q ) 29. In
the presence of preadsorbed hydrogen, the peak temperature for trifluoro-
propane desorption decreases, indicating that the rate of hydrogenation by
preadsorbed hydrogen is faster than that by self-hydrogenation.
Figure 7. TPR spectra of fluoroalkyl iodides coadsorbed with hydrogen
on Pt(111). The surface was first exposed to 10 L of H2 at 120 K and then
to fluoroalkyl iodides at 95 K. The exposure to fluoroalkyl iodides was
controlled to give a coverage roughly equal to 30% of the saturation
coverage of iodine on the Pt(111) surface. The heating rate was 2 K/s.
ature increases. On the basis of this trend for the hydrogenation
of fluoroalkyl groups, one would expected that the hydrogena-
tion temperature of (CF3)2CHCH2-, the most highly fluorinated
of the reactants used, would be the highest among those of these
four fluoroalkyl groups; however, Figure 7 shows that (CF3)2-
CHCH2- seems to behave anomalously. The hydrogenation
temperature of (CF3)2CHCH2- was the lowest among those of
the four fluoroalkyl groups. This anomaly might arise from steric
effects of (CF3)2CHCH2, which differ from those of the other
straight-chain fluoroalkyl groups because (CF3)2CHCH2 is
branched. To demonstrate the impact of steric effects on the
hydrogenation of (CF3)2CHCH2-, Figure 8 shows a comparison
of TPR spectra obtained by exposing the Pt(111) surface to 10
L of H2, followed by 2.4 and 0.5 L exposures to (CF3)2CHCH2I.
The hydrogenation temperature for (CF3)2CHCH2 was observed
to increase from 235 to 263 K as a result of decreasing the
exposure of (CF3)2CHCH2I. At the lower exposure (0.5 L), the
surface is not crowded with (CF3)2CHCH2-, and the interactions
between (CF3)2CHCH2- molecules are negligible. The peak
hydrogenation temperature of 263 K observed after the 0.5 L
exposure to (CF3)2CHCH2I was the highest among those of the
four fluoroalkyl groups used in this study. The possible role of
steric effects on the hydrogenation kinetics of the straight-chain
fluoroalkyl groups were explored by using CF3CF2CH2CH2I,
the longest of the straight-chain fluoroalkanes. TPR spectra were
obtained from Pt(111) surfaces pre-exposure to 10 L H2 and
then exposure to 0.5 and 2.3 L of CF3CF2CH2CH2I (data not
shown). The hydrogenation temperatures were the same for the
two exposure conditions, suggesting that exposures to the
straight-chain fluoroalkyl iodides of e2.3 L were low enough
to limit steric effects.
CF3CH2CH2- groups self-hydrogenate, whereas the CH3CH2-
CH2- groups do not. Figure 5 also shows that hydrogen
desorption occurs over the temperature range from 275 to 350
K, with a peak centered at approximately 310 K. Hydrogen is
clearly being released onto the Pt(111) surface at the temper-
atures at which trifluoropropane is desorbing into the gas phase.
In the absence of preadsorbed hydrogen, the desorption of
trifluoropropane during CF3CH2CH2- decomposition is prob-
ably rate-limited by the kinetics of ꢀ-hydride elimination rather
than hydrogenation.
In the presence of preadsorbed hydrogen on the Pt(111)
surface, fluoroalkyl groups undergo hydrogenation without prior
ꢀ-hydride elimination. TPR spectra of CF3CH2CH2- groups
(Figure 6) reveal that the CF3CH2CH3 desorption temperature
decreases as the hydrogen pre-exposure increases from 0 to 3
L and then remains constant at 258 K as the hydrogen pre-
exposure increases from 3 to 10 L. This temperature is signi-
ficantly lower than the temperature of 285 K observed for
CF3CH2CH3 desorption during CF3CH2CH2- decomposition on
Pt(111) in the absence of preadsorbed hydrogen. This shift in
the peak hydrogenation temperature suggests that the rate-
limiting step for hydrogenation switches from the ꢀ-hydride
elimination step, in the absence of preadsorbed hydrogen, to
the C-H bond formation step, in the presence of preadsorbed
hydrogen. The peak temperature of 258 K was used to estimate
the activation barrier for hydrogenation of CF3CH2CH2- groups
on the Pt(111) surface.
The hydrogenation kinetics of three other fluoroalkyl groups
(CF3CF2CH2CH2-, CF3CH2CH2-, and (CF3)2CHCH2-) on the
Pt(111) surface are similar to those of CF3CH2CH2-. The TPR
spectra of these four fluoroalkyl iodides coadsorbed with
hydrogen on the Pt(111) surface are shown in Figure 7. The
hydrogenation temperatures for CF3CH2CH2CH2-, CF3CF2CH2-
CH2-, and CF3CH2CH2- were approximately 245, 258, and
258 K, respectively. As the extent of fluorination increases, the
∆EqH increases, and thus, the observed hydrogenation temper-
As in the case of the hydrogenation of alkyl groups, it is
also important to demonstrate that evolution of fluoroalkanes
from the Pt(111) surface is rate-limited by hydrogenation and
not by the desorption of fluoroalkanes. Not all the fluoroalkanes
produced are commercially available; hence, it was not possible
to experimentally measure the molecular desorption kinetics of
9
J. AM. CHEM. SOC. VOL. 130, NO. 26, 2008 8523

A R T I C L E S
Ye and Gellman
barriers to hydrogenation of a set of alkyl and fluoroalkyl groups
with varying degrees of fluorine substitution and alkyl group
size. On the hydrogen saturated Pt(111) surface, the hydrogena-
tion mechanism is assumed to occur via the elementary steps
shown below.
RCH2CH2-I f RCH2CH2 + I
RCH2CH2 + H f RCH2CH3
RCH2CH3 f RCH2CH3,(g)
(kC-I
(kH)
)
(1)
(2)
(3)
(kdes
)
Here, RCH2CH2-I is chosen to represent the alkyl iodides with
different substituents, R. Prior to the reaction, H2 is adsorbed
dissociatively to produce hydrogen atoms, and the alkyl iodide is
adsorbed at low temperature. During heating, the C-I bond of the
alkyl iodide dissociates at temperatures below 200 K to produce
an alkyl group and an iodine atom. Cleavage of the C-I bond is
facile and occurs at lower temperatures than the subsequent
reaction steps. The second reaction step is the hydrogenation
of the alkyl group by preadsorbed hydrogen to form an adsorbed
alkane. Preadsorption of deuterium rather than hydrogen results
predominantly in the formation of d1-propane in the case of
propyl groups on Pt(111).11 The final reaction step is the
desorption of the alkane. Our independent measurements of the
alkane desorption kinetics show that the rate constant for
desorption of the alkane is much higher than the rate constant
for hydrogenation of the alkyl group, kdes > kH. Therefore, the
hydrogenation of the alkyl group is the rate-limiting step in the
production and desorption of the alkane.
Dehydrogenation (ꢀ-hydride elimination) of the alkyl group
on the Pt(111) surface must be considered as a possible source
of hydrogen for hydrogenation of alkyl groups to alkanes. The
alkyl group could lose a hydrogen atom by ꢀ-hydride elimination
to form an olefin. The olefin could then either desorb from the
surface or react further to form an alkylidyne.11,12 Previous study
of the thermal activation of propyl (isopropyl and n-propyl)
groups on the Pt(111) surface has demonstrated that the presence
of preadsorbed hydrogen greatly increases the propane yield at
the expense of propene production; in the case of n-propyl
groups, no propene desorption was detected in the absence of
preadsorbed hydrogen.11 Under our experimental conditions,
with a monolayer coverage of preadsorbed hydrogen, it is
reasonable to assume that hydrogenation by adsorbed hydrogen
atoms is the predominant reaction path leading to the formation
of alkanes. Thus, the alkane desorption kinetics can be used to
measure the kinetics of the hydrogenation of alkyl groups.
Analysis of the kinetics for the formation of alkanes by
hydrogenation of alkyl groups are discussed in the Supporting
Information. Because the measurement is performed with an
excess of hydrogen initially adsorbed on the surface, the overall
hydrogenation and alkane desorption kinetics are pseudo-first-
order in the alkyl group coverage, and the desorption kinetics
can be analyzed by using the first-order Redhead equation and
an estimated pre-exponential factor of V ) 1013 s-1 to obtain
estimates of the barriers to hydrogenation, ∆EqH. This is
consistent with the analysis by Zaera et al. of the kinetics of
ethyl group hydrogenation to ethane on the Pt(111) surface.13
There have been no careful measurements of the pre-exponential
factors for this type of two-component second-order surface
reaction. It should be pointed out that a systematic error in the
value of the pre-exponent used in this work would change the
estimated barriers to hydrogenation, ∆EqH, systematically. It
would not change the trend observed for the substituent effects
Figure 8. TPR spectra of (CF3)2CHCH2I coadsorbed with hydrogen on
Pt(111). The surface was first exposed to 10 L of H2 at 120 K and then to
(CF3)2CHCH2I at 95 K (2.4 or 0.5 L). The heating rate was 2 K/s. The
desorption of (CF3)2CHCH3 was monitored by collecting the signal at m/q
) 69. Decreasing the exposure to (CF3)2CHCH2I increases the hydrogena-
tion temperature.
all four fluoroalkane products. Instead, however, it is sufficient
to consider the previously mentioned results for the kinetics of
butane desorption from the clean Pt(111) surface. The peak
desorption temperature of submonolayer butane coverages on
the Pt(111) surface was approximately 175 K, well below the
fluoroalkane desorption temperatures observed after hydrogena-
tion of the fluoroalkyl groups. The desorption energy and thus
the desorption-peak temperature of fluorinated alkyl ethers have
been found to be lower than those of their hydrocarbon ether
counterparts on the Cu(111), Al(110), and Pt(111) surfaces.23–25
It is reasonable to assume that the desorption energies and
desorption-peak temperatures of fluorinated alkanes will also
be lower than those of their hydrocarbon counterparts. TPD
showed that desorption of butane from the Pt(111) surface
occurs at approximately 175 K, suggesting that CF3CH2CH2CH3,
CF3CF2CH2CH3, CF3CH2CH3, and (CF3)2CHCH3 should desorb
at even lower temperatures. The fact that the peak temperatures
for fluoroalkane desorption following hydrogenation of the four
fluoroalkyl groups are all >200 K indicates that the appearance
of the fluoroalkane products in the gas phase is rate-limited by
hydrogenation rather than by desorption. Thus, the peak
temperatures for fluoroalkane desorption observed in TPR
spectra of fluoroalkyl groups and hydrogen coadsorbed on the
Pt(111) surface can be used to estimate the activation barriers,
∆EqH, for hydrogenation of the fluoroalkyl groups.
4. Discussion
4.1. Analysis of Alkyl Hydrogenation Kinetics on Pt(111).
The TPR spectra of alkyl groups and hydrogen coadsorbed on
the Pt(111) surface have been obtained in order to measure the
(23) Meyers, J. M.; Gellman, A. J. Surf. Sci. 11997, 372, 71–178.
(24) Meyers, J. M.; Street, S. C.; Thompson, S.; Gellman, A. J. Langmuir
1996, 12, 1511–1519.
(25) Takeuchi, K.; Salmeron, M.; Somorjai, G. A. Surf. Sci. 1992, 279,
328–340.
(26) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165–195.
(27) Taft, R. W.; Topsom, R. D. Prog. Phys. Org. Chem. 1987, 1.
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8524 J. AM. CHEM. SOC. VOL. 130, NO. 26, 2008

Transition State for Alkyl Group Hydrogenation on Pt(111)
A R T I C L E S
reaction constant is FF ) 27 ( 4 kJ/mol, and the value of the
polarizability reaction constant is FR ) 19 ( 3 kJ/mol.
The reaction constants provide some insight into the nature
of the transition state for alkyl group hydrogenation on the
Pt(111) surface. The field effect substituent constant is an
empirical measure of the dipole moment of the substituent. The
field substituent constant increases with increasing fluorination,
which generates a dipole with its positive end oriented close to
the hydrogenation reaction center. The fact that FF is positive
indicates that increasing the dipole moment of the substituent
increases the reaction barrier or, in other words, destabilizes
the transition state with respect to the initial state. This is
consistent with a reaction in which the reaction center, the
R-carbon atom, is more electropositive in the transition state
than in the initial state alkyl group. To put the magnitude of FF
) 27 ( 4 kJ/mol into perspective, it can be compared to the
values of FF for other surface reactions. An example of a
heterolytic process is the ꢀ-hydride elimination reaction of
alkoxides to form aldehydes on the Cu(111) surface. In that
reaction, the value of the field reaction constant is FF ≈ 150
kJ/mol, and the ꢀ-carbon atom is cationic or electropositive in
the transition state with respect to the initial state,
{RCδ+ · · ·H}q.5 An example of a homolytic process is the C-I
cleavage in the alkyl and fluoroalkyl iodides on the Ag(111)
surface, where the reaction constant has a value of FF ) -17
kJ/mol. Because the reaction constant for C-I cleavage is quite
small, the transition state can be described as homolytic,
although the fact that FF < 0 indicates that there is a slight
increase in electron density in the transition state relative to
the initial state.6 The value of FF for hydrogenation of alkyl
groups on the Pt(111) surface is higher than that for C-I
cleavage on the Ag(111) surface but with the opposite sign.
The positive but relatively low value of FF indicates that the
transition state for hydrogenation of alkyl groups is slightly
cationic with respect to the initial state or, in other words, that
there is a slight decrease in electron density at the reaction center
in the transition state leading to the formation of the C-H bond,
{RCδ+ · · ·H}q.
Figure 9. Linear free energy relationship for the barriers to hydrogenation,
∆EqH, of alkyl and fluoroalkyl groups on Pt(111) by using a two-variable
linear fit to the field (σF) and polarizability (σR) substituent constants. The
field effect is plotted by using the top- and left-hand axes. The polarizability
effect is plotted by using the bottom- and right-hand axes. The reaction
constants are FF) 27 ( 4 kJ/mol and FR) 19 ( 3 kJ/mol.
or the qualitative insights into the nature of the transition states
obtained from our ultimate interpretation of these trends.
4.2. Transition state for Hydrogenation of Alkyl Groups. The
goal of this study has been to probe the nature of the transition
state for alkyl group hydrogenation by hydrogen atoms on the
Pt(111) surface by measuring the activation barriers for hydro-
genation, ∆EqH, of a set of different alkyl and fluoroalkyl groups.
The alkyl and fluoroalkyl groups used in this study have
substituents, R, with a range of field substituent constants, σRF ,
and polarizability substituent constants, σRR, found in the
literature.26,27 As can be seen from Table 1, the values of ∆EHq
are influenced by the substituents on the alkyl groups. The values
of ∆EqH can be correlated with the field and polarizability
substituent constants by using a simple linear free energy
relationship.
The value of the polarizability reaction constant for alkyl
group hydrogenation on the Pt(111) surface is FR ) 19 ( 3
kJ/mol. Polarizability always stabilizes nearby charges by
screening. If the charge density (positive or negative) is greater
in the transition state than in the reactant, highly polarizable
substituents will stabilize the energy of the transition state and
thus lower the reaction barrier. If the charge density (positive
or negative) is higher in the reactant than in the transition state,
highly polarizable substituents will stabilize the energy of the
reactant and thus increase the reaction barrier. Note that the
convention used for polarizability constants is that σR takes on
increasingly negative values as the polarizability of the sub-
stituent group increases.26,27 Thus, a substituent with greater
polarizability (more negative value of σR) energetically stabilizes
(lowers) the energy of nearby charges. The value of FR ) 19 (
3 kJ/mol for hydrogenation of alkyl groups on the Pt(111)
surface indicates that the polarizability of the substituent is
stabilizing the transition state with respect to the initial state.
Thus, the charge density on the R-carbon must be slightly greater
in the transition state than in the reactant. Our final picture of
the transition state for alkyl group hydrogenation on the Pt(111)
surface is shown in Figure 10.
∆EqH(R) ) FRσRR + FFσFR + ∆EqH(H)
(4)
The reaction constants, FF and FR, are measures of the
sensitivity of the barrier to the field and polarizability effects
of the substituents, respectively. The quantity ∆EqH(H) is the
barrier to hydrogenation of an alkyl group with hydrogen as a
substituent, which in this study would be the case for an
adsorbed methyl group, H3C-. The values of FF and FR have
been determined by fitting eq 4 to the data in Table 1, and the
results are illustrated in Figure 9. The field effect has been
isolated by plotting ∆EqH(R) - FRσRR versus σFR by using the
top- and the left-hand axes, whereas the polarizability effect
has been isolated by plotting ∆EqH(R) - FFσRF versus σRR by using
the bottom- and the right-hand axes. The value of the field
9
J. AM. CHEM. SOC. VOL. 130, NO. 26, 2008 8525

A R T I C L E S
Ye and Gellman
change in charge distribution between the two states. The only
other such study of both field and polarizability effects in a
surface reaction has been a study of C-I bond cleavage on the
Ag(111) surface.6 In the case of alkyl group hydrogenation on
Pt(111), both the field effect, FF ) 27 ( 4 kJ/mol, and the
polarizability effect, FR ) 19 ( 3 kJ/mol, cause the barrier to
hydrogenation to increase. The magnitude of the reaction
constants is small but suggests that the R-carbon in the transition
state for hydrogenation of alkyl groups on Pt(111) is slightly
cationic with respect to the R-carbon atom in an adsorbed alkyl
group and that the charge density on the R-carbon in the
transition state is slightly higher than in the reactant.
Figure 10. Transition state for alkyl group hydrogenation on the Pt(111)
surface. The substituent effects reveal that the transition state is relatively
cationic with respect to the initial state and that the trasition state holds a
higher charge density.
5. Conclusion
The activation barriers to hydrogenation, ∆EqH, of substituted
alkyl groups adsorbed on the Pt(111) surface have been shown
to be influenced by the field effects and polarizability of the
substituents. One important feature of this work has been the
fact that it has used substituents varying in both their field and
polarizability substituent constants and has used enough different
substituents to be able to correlate the impact of both field and
polarizability effects on the nature of the transition state. This
provides insight into the character of charge distribution in both
the initial state and in the transition state, rather than just the
Acknowledgment. This work has been funded by NSF grant
number CBET-0651182.
Supporting Information Available: Details of the kinetic
analysis of the TPD spectra to yield estimates of the activation
energies for alkyl group hydrogenation, ∆EqH. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA075292J
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8526 J. AM. CHEM. SOC. VOL. 130, NO. 26, 2008