Alkyl groups as synthetic vehicles in gold-mediated oxidative coupling reactions

Article abstract of DOI:10.1039/c3cp43956a

The use of surface-bound alkyl and phenyl groups as synthetic vehicles in coupling reactions on oxygen-activated Au(111) is demonstrated by the formation of ethers via alkyl and phenyl iodides. Ethers are formed by successive additions of surface-bound alkyl groups to adsorbed atomic oxygen to form first the alkoxy and then the ether. The addition of the ethyl group to adsorbed oxygen on Au(111) is the rate-limiting step leading to diethyl ether formation. Alkyl groups also add to adsorbed alkoxy groups formed from alcohols. An unusual feature of the alkyl iodide reactions on Au is that oxygen is not required for the activation step; hence, opening new potential reactive pathways on metallic Au. This journal is

Full text of DOI:10.1039/c3cp43956a

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Alkyl groups as synthetic vehicles in gold-mediated
oxidative coupling reactions†
Bingjun Xu,a Robert J. Madixb and Cynthia M. Friend*ab
Cite this: Phys. Chem. Chem. Phys., 2013,
15, 3179
The use of surface-bound alkyl and phenyl groups as synthetic vehicles in coupling reactions on oxygen-
activated Au(111) is demonstrated by the formation of ethers via alkyl and phenyl iodides. Ethers are
formed by successive additions of surface-bound alkyl groups to adsorbed atomic oxygen to form first
the alkoxy and then the ether. The addition of the ethyl group to adsorbed oxygen on Au(111) is the
rate-limiting step leading to diethyl ether formation. Alkyl groups also add to adsorbed alkoxy groups
formed from alcohols. An unusual feature of the alkyl iodide reactions on Au is that oxygen is not
required for the activation step; hence, opening new potential reactive pathways on metallic Au.
Received 6th November 2012,
Accepted 11th January 2013
DOI: 10.1039/c3cp43956a
www.rsc.org/pccp
(2) surface-assisted partial dehydrogenation to form alkenes with
the same number of carbon atoms as the alkyl groups,3,10,15,16
Introduction
or complete dehydrogenation to deposit carbon;12,17 and (3)
C–C coupling of two alkyl groups, forming alkanes with twice
the number of carbon atoms in the original alkyl3 (Scheme 1).
The relative contribution of these three pathways depends
largely on the nature of the surface because of variability in
the activity for C–H bond activation and in the strength of
carbon–metal bonds.
Indeed, the chemistry of alkyl groups on transition metal
surfaces varies widely, reflecting the activity of the metal for
C–H and C–C bond activation. Complete dehydrogenation of
alkyl groups to deposit surface carbon is favored on VIB metal
surfaces, such as W(100),17 reflecting the strong metal–carbon
and metal–hydrogen bonds. On the other hand, alkyl groups,
e.g. ethyl, partially dehydrogenate to yield olefins, e.g. ethene
(Scheme 1, path 2), in competition with hydrogenation to form
alkanes, e.g. ethane, on VIIIB metal surfaces, e.g. Fe,18 Ru,19
Rh,20 Ni,21 Pd22 and Pt(111),23 reflecting their activity for reversible
Alkyl iodides, especially methyl iodide, are classic substrates in
organic synthesis. Likewise alkyl iodides are promising sub-
strates for surface-mediated coupling reactions, because they
are readily activated by metal surfaces, forming the corre-
sponding surface-bound alkyl group even on metallic gold.
Furthermore, surface-bound alkyl groups are thought to be
key intermediates in a range of heterogeneous processes, most
notably in the selective oxidation of alkanes, a key reaction in
the conversion of natural gas to useful products.
Once alkyl groups are present on the surface, there is the
potential for a host of reactions, including addition to electron-
rich species, such as adsorbed oxygen atoms.1–7 Indeed, the
addition of alkyl groups to oxygen to form dialkyl ethers has been
reported previously on Ag(110).8,9 While there are many similar-
ities between the reactions of Au and Ag, there are also several
distinct differences in coupling processes involving oxygenates.
Herein, we further explore differences in the behavior of gold and
silver involving reactions of alkyl iodides so as to provide a basis
for understanding and predicting their surface reactivity.
In the absence of other surface species, there are three
general categories of reactions involving surface-bound alkyl groups
on metal1–7,10–15 and semiconductor surfaces:16 (1) hydrogenation by
adsorbed atomic hydrogen, to form the corresponding alkanes;3,10,13
a Department of Chemistry and Chemical Biology, Harvard University, Cambridge,
MA 02138, USA. E-mail: cfriend@seas.harvard.edu; Fax: +1 617 496 8410;
Tel: +1 617 495 4052
b School of Engineering and Applied Sciences, Harvard University, Cambridge,
MA 02138, USA
Scheme 1 Possible reaction pathways for the adsorbed alkyl groups on metal
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cp43956a
and semiconductor surfaces.
c
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C–H bond activation. The lack of C–C coupling reactions for 0.1 ML O (O/Au(111)) by introducing an appropriate amount of
these surfaces is likely due to their relatively strong C–metal ozone at 200 K.37 The oxygen atom coverage was calibrated by
bonds24 and the facile competing hydrogenation and dehydro- comparing the amount of O2 evolution in temperature-
genation pathways.25,26
programmed desorption to that formed from saturation cover-
Coinage metals (Cu, Ag, and Au) behave differently; they have age of oxygen atoms, which is 1.1 ML.37 An error of Æ15% in
relatively low activity for C–H activation and possess considerably oxygen coverage is expected due to day-to-day variation in O3
weaker carbon–metal bonds.24 These factors, as well as the struc- concentration. Oxidation of the surface in this manner leads to
tural factors, play a role in determining the relative contributions of release of Au atoms to form nanostructures containing Au
these pathways. First, the weaker C–metal bonds will lead to greater and O, most of which are smaller than 2 nm in diameter, as
mobility for adsorbed alkyl groups at a given temperature relative to evidenced by our previous STM studies.36 We refer to the oxygen-
the early and mid-transition elements, opening a channel for C–C covered Au nanoparticles on Au(111) as O/Au(111).36
coupling on all three coinage metal surfaces. This coupling process
Temperature programmed reaction (TPR) was conducted
is also structure-sensitive. There is more coupling on Cu(110) according to well-established protocols, which are described
compared to Cu(111) and Cu(100), which both yield products of in detail elsewhere.36 Regardless of the dosing temperature of
the dehydrogenation and hydrogenation pathways.3 C–C coupling reactants, the surface was cooled to 120 K before starting TPR.
is the dominant reaction for alkyl groups adsorbed on Ag(111), The heating rate for all temperature-programmed experiments was
Ag(110)9,27 and for the (111) and (100) surfaces of Au.5,7 The nearly constant at 5 K sÀ1. The reaction products were identified by
selectivity for C–C coupling on Au also depends on the molecular quantitative mass spectrometry (Hiden HAL/3F) using fragmenta-
structure of the alkyl group. For example, C–C coupling occurs tion patterns obtained from neat samples for reference; these
exclusively for methyl iodide on Au(100),5 whereas products from patterns were found to be in good agreement to NIST reference
all three pathways (Scheme 1) contribute for higher-molecular data (Tables S1 and S2, ESI†).38,39 All traces in the temperature-
weight alkyls, such as ethyl, on both Au(100) and Au(111).5,7 Since programmed reaction spectra shown here are parent ions of the
the molar ratio for the hydrogenation and partial dehydrogenation species, with contributions from cracking fragments of other species
products is 1 : 1,5,7 disproportionation is likely; the hydrogen with the same m/z subtracted for clarity. The selectivity towards
released by dehydrogenation rapidly reacts with neighboring alkyls different products is calculated on a molar basis. Procedure for
groups to form the alkane.
calculating selectivities is described in the ESI.†
Oxidation of alkyl groups has also been investigated widely
Iodocompounds were introduced to the surface at 150 K.
since this is a key step in the oxidation of alkanes.6,8,9,28,29 The Exposures corrected for enhancement due to direct dosing are
reactivity depends on the O–metal and C–metal bond strengths given here in terms of Langmuir (L) (1 Langmuir = 1 Â 10À6 Torr
as well as the capability of the surface to activate C–H bonds. seconds). Note that this calculation does not take into relative
Ethoxy forms on Ag(110) and Rh(111) via the addition of ethyl cracking efficiencies in the ion gauge or possible errors due to
to adsorbed oxygen at relatively low temperatures (250 and 355 K, adsorption on, reaction on, or displacement from chamber
respectively),6,9 with oxygenates such as aldehyde and ether being walls. Unless otherwise noted, 6 L is the typical dose for organic
the major products. In contrast, CO is the only product derived molecules.
from alkoxy on oxygen-covered Ni(100) surface, in part due to the
X-ray photoelectron (XP) spectra were acquired with an
strength of the O–Ni bond. Further, methyl will add directly to analyzer pass energy of 17.9 eV and a multiplier voltage of 3 kV
adsorbed oxygen to form alkoxy on Rh and Ag,29 whereas on using MgKa X-rays (300 W) as the excitation source. The binding
Mo(100) it prefers to bind directly to the metal below 0.4 ML energy (BE) calibration was referenced to the Au(4f7/2) peak at
oxygen coverage.30,31 This behavior is consistent with the hypo- 83.9 eV. The I(3d) and O(1s) spectra were accumulated with 50
thesis that a stronger metal–oxygen bond inhibits the reaction and 300 scans, respectively, to enhance the signal-to-noise ratio.
of adsorbed oxygen with alkyl groups, the energies for the metal–
The I(3d5/2) XP spectra were used to identify and quantify the
amount of molecularly adsorbed alkyl iodides and adsorbed
iodine. The position and width of phenyl iodide I(3d5/2) peak
oxygen bonds being Ag (11.1 kcal molÀ1 32
)
o Rh (52 kcal molÀ1 33
o
)
Ni (130 kcal molÀ1 34
)
o Mo (145 kcal molÀ1).35
Herein we illustrate that ether formation on oxygen covered were obtained by using a single Gaussian peak to fit the trace
metallic gold surface shares critical mechanistic steps with corresponding to adsorbed phenyl iodide at 150 K, since the
those on Ag(110),8,9 forming via two successive alkyl transfers to C–I bond breaking does not occur until 250 K.2 Subsequently,
adsorbed oxygen atoms. Differences in the activation energies of the XP spectrum of phenyl iodide on Au(111) annealed to 200 K
competing reaction steps in the common mechanism lead to was fitted with two Gaussian peaks, one of which used the same
substantial differences in product distributions on the two metals. parameters for molecularly adsorbed phenyl iodide; the other
one is attributed to adsorbed iodine. These parameters were
then used to fit the spectrum of phenyl iodide on Au(111)
Experimental
annealed to 300 K, adjusting peak areas to reach best fitting.
All experiments were performed in an ultrahigh vacuum (UHV) A similar procedure was applied to fit the spectra of other alkyl
chamber with a base pressure below 2 Â 10À10 Torr. Established iodides, with the assumption that the position and width of the
procedures were followed in the preparation of the clean, I(3d5/2) peak corresponding to adsorbed iodine are the same for
ordered Au(111) surface.36 The surface was first populated with alkyl and phenyl iodides.
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The O(1s) spectra were decomposed as follows: first, the peak
Paper
Table 1 Bond dissociation energies for methyl, ethyl and phenyl iodides
position and width characteristic of atomic oxygen (formed from
ozone exposure) were obtained by using a single Gaussian peak.
Subsequently, peak parameters for ethoxy were obtained by reacting
ethanol with preadsorbed oxygen and fitting the O(1s) spectrum
with two Gaussian peaks, one of which used the same parameters as
for adsorbed oxygen; the other peak is attributed to methoxy. The
peak for molecularly adsorbed ethanol was obtained from excess
ethanol adsorbed onto an oxygen-covered surface. The peak assign-
ments and areas are summarized in Tables S3–S5 (ESI†).
Percentage of
Maximum molar
Bond disso-
molecule decom- amount of decomposed
ciation energya posed on Au(111) molecule (referenced to
Molecule (kcal molÀ1
)
at 150 K
ethyl iodide)
CH3CH2–I 55.8 Æ 1.5
B30%
B5%
None
100%
67%
31%
CH3–I
C6H5–I
57.1 Æ 0.5
65.0 Æ 1
a
Data adopted from ref. 41.
The extent of the C–I bond dissociation of methyl, ethyl and
phenyl iodide on clean Au(111) was quantified on the basis of the
magnitudes of the I(3d5/2) peaks for adsorbed I (619.0 eV) and
molecular iodide (620.5 eV).13 Typically, the alkyl iodides adsorb
molecularly at 150 K. The extent of dissociation correlates with the
respective C–I bond strengths (Table 1 and Tables S3 and S4,
ESI†).41 Upon adsorption at 150 K ethyl iodide (B30%) decom-
poses more than methyl iodide (B5%), and no scission of the C–I
bond in phenyl iodide occurs. With heating of the surface to 300 K
the most dissociation occurs for ethyl iodide, followed by methyl
and then phenyl iodide, exhibiting a molar ratio of 2 : 3 : 1. Based
on the absolute coverage of phenyl groups formed from phenyl
iodide reported previously (0.027 ML)2 the coverages of methyl
and ethyl formed here are calculated to be 0.054 ML and
Results
Alkyl Iodide reactions on clean Au(111)
The reactions of methyl, ethyl, and phenyl iodides were inves-
tigated first to determine conditions for producing alkyl species
on Au(111). C–C coupling to yield ethane, butane, and biphenyl
from methyl, ethyl and phenyl iodides, respectively, was
observed (Fig. 1). These results are generally in agreement with
those published previously.7 The rate of formation of ethane
from methyl iodide peaks at B350 K, whereas butane formation
from ethyl iodide is a maximum at 280 K. In contrast to the
previous results of Paul et al.7 only small amounts ethane and
ethene (B15% each) were observed from ethyl iodide, the major
product being butane (B80–90%). Beta-H scission in the adsorbed
ethyl is clearly unfavorable on the gold surface. Since both ethane
and butane weakly interact with Au(111), their evolution must be
limited by reaction.40 In contrast, biphenyl evolution at B410 K
is limited in rate by desorption.2 In the case of ethyl no
molecular hydrogen formation was detected, suggesting that
disproportionation of ethyl groups gives rise the C2 products, as
proposed previously.7 No molecular hydrogen evolution was
observed in decomposition of alkyl or phenyl iodides, nor was
there any detectable carbon left on the surface after reaction.
Iodine (I2) desorbs in a broad desorption feature between 600 and
900 K (data not shown). These results are generally consistent
with previous reports in the literature.2,5,7
0.081 ML, respectively, using the integrated peak areas of I(3d5/2
)
normalized to Au(4f7/2).
Reaction of alkyl iodides on O-covered Au(111)
Alkyl transfer to adsorbed oxygen occurs with exposure of
methyl and ethyl iodides to oxygen-covered Au(111), leading
to significant formation of dialkyl ethers (Fig. 2a and b). When
CD3I was introduced to O/Au(111) (yO = 0.1 ML), dimethyl ether-d6,
evolves at 170 K. Only a small amount of ethane-d6 (about 5% of
the total product yield) evolves near 350 K. No other gaseous
products were observed in the range of 2–200 amu. Specifically, no
methanol-d4 (m/z 36), formaldehyde-d2 (m/z 32), methyl formate-d4
(m/z 64) or D2 (m/z 4) was observed. Furthermore, no residual
carbon was detected after reaction based on the absence of CO2
evolution in the subsequent oxidation by atomic oxygen.
Similarly, diethyl ether is one of the main products in the
oxidation of ethyl iodide on O/Au(111) (yO = 0.1 ML), evolving at
B215 K and accounting for B20% of the products. A signifi-
cant amount of acetaldehyde (B55%) is also formed at B240 K,
similar to the temperature of its formation in the partial
oxidation of ethanol on O/Au(111).42,43 Water desorption occurs
at the same temperature as acetaldehyde, indicating they are
likely formed from the same reaction. Only a small amount
(B2%) of ethyl acetate was detected.
Exposure of O/Au(111) to phenyl iodide leads primarily to
combustion (Fig. 2c). Neither phenyl radicals nor phenol was
formed. There was a substantial amount of carbon deposition
after reaction, as measured by the residual carbon burned
off with subsequent dosing of ozone followed by heating.
The residual carbon is B10 times greater than the CO2 peak
area in the initial temperature programmed reaction (Fig. 2c).
Fig. 1 Reaction of methyl (top), ethyl (middle) and phenyl (bottom) iodides on
Au(111). 6 L of alkyl or phenyl iodides were introduced at 150 K in each reaction.
The heating rate is 5 K sÀ1
.
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The mechanism for ethyl iodide oxidation was further
probed by reaction of CH3CH2I with preadsorbed CD3CD2O
formed from reaction of CD3CD2OD with O/Au(111) (yO
=
0.1 ML). Addition of the ethyl group from the iodide to the
preadsorbed ethoxy was indicated by the dominant product
CD3CD2OCH2CH3, formed at B190 K (Fig. 4a). Notably, neither
ethyl acetate nor acetaldehyde was detected. The ether formed
in this manner (Fig. 4a) evolves B25 K lower than diethyl ether
from CH3CH2I alone (Fig. 4b). The amount of diethyl ether
from the coupling of ethanol and ethyl iodide is B25 times
larger than that from the self-coupling reaction of ethyl iodide.
Decreasing the amount of CD3CD2OD dosed, while fixing all other
parameters, results in a reduced amount of CD3CD2OCH2CH3 and
increased amounts of products from oxidation of ethyl iodide,
i.e. CH3CH2OCH2CH3, butane and acetaldehyde (data not shown)
as the concentration of adsorbed CD3CD2O decreases. The
analogous coupling reaction between ethyl iodide and methanol
also occurs to form methyl ethyl ether selectively (ESI,† Fig. S1).
The product distribution of the oxidation of ethyl iodide
depends strongly on the initial oxygen coverage (Fig. 5). All
selectivities were calculated on a molecular basis (ESI†). How-
ever, it is important to recognize that the efficiency for the
conversion of ethyl group into C4 products (butane, diethyl
ether and ethyl acetate) is twice as much as that of the C2
product, i.e. acetone, and 4 times that of CO2. In the absence of
atomic oxygen butane is the dominant product (B70%), but it
decreases precipitously as the initial oxygen coverage is increased.
Partial oxidation products, i.e. diethyl ether and acetaldehyde,
dominate when the oxygen coverage is 0.1 ML. As the initial
oxygen coverage increases above 0.1 ML, the selectivity for
acetaldehyde rises at the expense of diethyl ether. The peak
temperatures of the products remains the same over the entire
range of oxygen coverage studied. The maximum selectivity for
acetaldehyde reaches approximately 60% at an initial oxygen
coverage of 0.1 ML. A small amount of ethyl acetate (o5%) is
Fig. 2 Reactions of alkyl iodides on O/Au(111) (yO = 0.1 ML): (a) CD3I, (b)
CH3CH2I and (c) C6H5I. In all cases, 6 L of the respective alkyl iodides were
introduced at 150 K. The heating rate is 5 K sÀ1. Surface oxygen was prepared by
ozone exposure at 200 K. The CD3I was used instead of CH3I to avoid the
potential overlap of mass fragments for dimethyl ether, methyl formate (not
observed) and formic acid (not observed).
Water evolution commences at B185 K, signifying the com-
mensurate activation of C–H bonds by O and C–I bond activa-
tion (150–200 K). The broad tail of the water peak (200–400 K)
indicates that the dehydrogenation of the phenyl ring occurs
over a wide range of temperature as the surface is heated.
Evidence for formation of adsorbed ethoxy from addition of
ethyl derived from ethyl iodide to adsorbed O is obtained from
analysis of O(1s) X-ray photoemission (Fig. 3a), suggesting that the
diethyl ether is formed from two successive ethyl transfer reac-
tions. A single O(1s) peak centered at 529.5 eV characteristic of
adsorbed O was observed after exposure of the surface to ozone.42
After introduction of CH3CH2I to the O-covered surface at 150 K
B80% of the O(1s) signal shifts from peak characteristic of
adsorbed O at 529.5 eV to 531.5 eV, indicative of ethoxy bound
to Au (Fig. 3).42,43 The remaining B20% of the O(1s) signal
remains unchanged (Tables S3–S5, ESI†). There were no visible
features in the O(1s) region after heating to 300 K, indicating that
all adsorbed oxygen is consumed in the production of the gaseous
diethyl ether (Fig. 2b). The amount of ethyl iodide decomposed in
the presence of adsorbed oxygen (yO = 0.1 ML) is measured to be
within 10% of the amount on clean Au(111). The total amount of
ethyl distributed in the products matches the measured amount
of I deposited by reaction within 20% (ESI†).
Fig. 4 (a) Coupling of ethyl iodide with ethanol-d6 on O/Au(111) (yO = 0.1 ML)
forming diethyl ether. 6 L of CD3CD2OD and CH3CH2I were sequentially intro-
duced at 150 K to O/Au(111). (b) Desorption temperatures of diethyl ethers
formed from different pathways. The heating rate is 5 K sÀ1. Surface oxygen was
prepared by ozone exposure at 200 K.
Fig. 3 (a) O(1s) and (b) I(3d) X-ray photoemission spectra of CH3CH2I on
O/Au(111) (yO = 0.1 ML) at different temperatures. The top trace in (a) is the
O(1s) spectrum for ethanol introduced on O/Au(111) at 150 K (yO = 0.1 ML). 6 L
of CH3CH2I was introduced to O/Au(111) at 150 K.
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Scheme 2 Reaction mechanism for the decomposition of ethyl iodide on
O/Au(111).
The C–C coupling of alkyl groups is likely to be reaction-
limited rather than diffusion- or desorption-limited. The tempera-
tures for C–C coupling of adsorbed methyl and ethyl groups to
form ethane and butane correspond to activation energies of 89
and 80 kJ molÀ1 (based on the Redhead analyses with a pre-
exponential factor of 1013). Both are much higher than the
desorption energies of the alkanes from Au(111) (ethane,
5.7 kcal molÀ1; n-butane, 13.4 kcal molÀ1).44 Biphenyl is reported
to desorb from Au(111) around 405 K at low coverages, indicating
that the evolution of biphenyl from phenyl iodide is a desorption-
limited process owing to the strong ring-surface interaction.2
Further, the temperatures for alkane formation lie well above
those necessary for C–I bond activation. The rate-limiting step
would therefore appear to be recombination of the alkyl groups.
Although ether formation on Ag(110) and Au(111) shares
critical mechanistic steps, the differences in activation energies
for specific reactions, including reactions competing with ether
formation, result in substantial differences in product distribu-
tions and, therefore, selectivity. The temperatures of formation
of acetaldehyde on Ag(110) and Au(111) are consistent with
those in the partial oxidation of ethanol (via ethoxy) on the
respective surfaces,43,45 supportive of ethoxy as a reaction
intermediate in the formation of the ethers. Oxidative coupling
occurs at a significantly lower temperature than alkyl recombi-
nation on Au(111), whereas the opposite is observed on Ag(110)
(Table 2).8,9 Thus a higher selectivity toward diethyl ether vs.
n-butane is observed on Au(111) at similar initial oxygen
coverages. Competing formation of ethylene via b-H elimina-
tion is also completely suppressed on O/Au(111) since it occurs
30–100 K above that of C–C coupling or oxidative coupling; it is,
however, a competitive reaction on Ag(110).
Fig. 5 Product distribution of oxidation of ethyl iodide as a function of initial
oxygen coverage. Minor products (ethyl acetate, ethane and ethylene) are not
shown for clarity.
formed with initial oxygen coverages above 0.05 ML, and it stays
roughly constant. In contrast, ethyl acetate is the main product
(>80%) when ethanol is introduced to Au(111) at initial oxygen
coverages below 0.2 ML.43 CO2 appears above 0.1 ML oxygen
coverage at 300–500 K (Fig. S2, ESI†) and reaches B70% of the
yield of all products at an initial oxygen coverage of 1 ML.
Discussion
The alkyl groups are excellent synthetic vehicles on gold surface
for two main reasons: (1) they form at low temperature (o200 K)
and are stable over a wide temperature window before being
consumed by the C–C coupling reaction to form alkanes. For
example, methyl iodide dissociates to methyl and iodine below
200 K, and the resulting methyl groups are stable until ethane
formation near 350 K. Similarly, the surface-bound ethyl group is
stable on Au(111) between 150 and 315 K. These temperatures
reflect activation energies for recombination between 80 and
90 kJ molÀ1. The alkyl groups are thus accessible for reactions
with other coadsorbed substrates, and the selectivity of the
competing reactions will be dictated by kinetic competition. (2)
They react readily with electron-rich surface intermediates, such
as oxygen and alkoxys, reacting with adsorbed oxygen on Au(111)
at 150 K to form the corresponding alkoxys (Fig. 3a) and with
alkoxys to form ethers at B200 K (Fig. 2).
It has been reported that ‘‘spectator’’ atomic oxygen is
necessary for ether formation from ethyl iodide on Ag(110).8
It is not clear if this also applies to Au(111) since there is no
Ether formation from alkyl iodides on O/Au(111) proceeds via
successive alkyl transfers, first to adsorbed oxygen and then to
the alkoxy (Scheme 2), similar to the pathway suggested for
Ag(110).8,9 The first step is demonstrated by the XPS studies
(Fig. 3a) and the second is confirmed by the selective formation
of d5-diethyl ether from coadsorbed ethanol-d6 and d0-ethyl
iodide on O/Au(111) at 150 K (Scheme 3; Fig. 4).43 The alkyl
transfer steps are consistent with the general mechanistic frame-
work established for oxidative-coupling reactions on Au.42
Scheme 3 Mechanism for coupling of ethanol-d6 and ethyl iodide.
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Table 2 Comparison of evolution temperatures of products in the oxidation of
ethyl iodide on atomic oxygen covered Au(111) (yO = 0.2 ML) and Ag(110)
(yO = 0.33 ML)
transfer to surface oxygen (Fig. 4b). This deduction is also
consistent with the higher yield of ether (B25 : 1) via the
preadsorbed alkoxy. Furthermore, the selectivity for diethyl
ether formation on Au(111) (yO = 0.1 ML) is 50% higher in
the coupling of ethanol and ethyl iodide than for self-coupling
of ethyl iodide.
Evolution temperature (K)
Product
Au(111)
Ag(110)a
Diethyl ether
Acetaldehyde
Ethyl acetate
Ethanol
Butane
Ethylene
CO2
215
240
235
—
285
—
235
268
—
268
227
245
627
Conclusions
Adsorbed oxygen promotes the coupling of alkyl groups formed
via the activation of C–I bonds of alkyl iodides to form ethers on
metallic gold surface. Similar to metallic silver, ether is formed
via successive additions of the alkyl groups to adsorbed oxygen.
Ethyl transfer to adsorbed oxygen on Au(111) appears to be rate-
limiting in the ether formation. Differences in the activation
barriers for the ether formation with respect to other competing
pathways result in different selectivities on silver and gold surfaces.
315
a
Data adopted from ref. 8 and 9.
reliable way to prepare isolated alkoxys on Au(111) (alcohols
introduced to O/Au(111) at 150 K do not completely remove
atomic oxygen to form the corresponding alkoxys).42 Moreover,
the instability of alkoxys on O/Au(111) above 200 K with respect
to b-H elimination precludes the their isolation by annealing
O/Au(111) in the presence of alcohols. The fact that ether
formation on Au(111) always occurred in the presence of atomic
oxygen in this work does not prove the necessity of atomic
oxygen for the reaction.
Acknowledgements
We gratefully acknowledge the support of the U.S. Department
of Energy, Basic Energy Sciences, under Grant No. FG02-84-
ER13289 (CMF). RJM acknowledges the support of the National
Science Foundation (NSF CHE 0952790). BX also acknowledges the
Harvard University Center for the Environment for support through
the Graduate Consortium in Energy and the Environment.
The dependence of the product distributions of oxidation of
ethyl iodide on the initial oxygen coverage differs significantly
for Ag(110) and Au(111). At the lowest oxygen coverages C–C
coupling is favored on both surfaces due to the excess of
adsorbed alkyl groups, but alkane formation decreases with
rising oxygen coverage on both surfaces, since adsorbed oxygen
increasingly reacts with the ethyl groups. On Ag(110) when the
oxygen coverage is increased at a constant ethyl iodide dose, the
selectivity for diethyl ether initially increases and then plateaus
at 0.12 ML O because oxidative coupling competes favorably
with alkyl recombination (Table 2).8 In contrast, the selectivity
for ether formation on Au(111) decreases sharply from its
highest value at 0.1 ML O (Fig. 5) due to competing partial
and total oxidation pathways which may originate in the facile
b-hydride elimination from the alkoxy by attack of adsorbed
oxygen;46 this reaction may not occur on Ag(110). Another source
of the difference is that esterification to form ethyl acetate via the
self-coupling of ethoxy occurs on Au(111),43 but is absent on
Ag(110). We did not attempt to optimize the yield of diethyl ether
on Au(111), but the fact that both acetaldehyde and ethyl acetate
form at higher temperatures (have higher apparent activation
energies) than the ether suggest that higher selectivity for
ether formation should be possible on gold surfaces by better
optimization of the reaction conditions. Therefore, the low
activation energy for oxidative coupling reactions relative to
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