Origin of the selectivity in the gold-mediated oxidation of benzyl alcohol

Article abstract of DOI:10.1016/j.susc.2012.03.013

Benzyl alcohol has received substantial attention as a probe molecule to test the selectivity and efficiency of novel metallic gold catalysts. Herein, the mechanisms of benzyl alcohol oxidation on a gold surface covered with atomic oxygen are elucidated; the results show direct correspondence to the reaction on gold-based catalysts. The selective, partial oxidation of benzyl alcohol to benzaldehyde is achieved with low oxygen surface concentrations and takes place through dehydrogenation of the alcohol to form benzaldehyde via a benzyloxy (C₆H₅-CH₂O) intermediate. While in this case atomic oxygen plays solely a dehydrogenating role, at higher concentrations it leads to the formation of intermediates from benzaldehyde, producing benzoic acid and CO₂. Facile ester (benzyl benzoate) formation also occurs at low oxygen concentrations, which indicates that benzoic acid is not a precursor of further oxidation of the ester; instead, the ester is produced by the coupling of adsorbed benzyloxy and benzaldehyde. Key to the high selectivity seen at low oxygen concentrations is the fact that the production of the aldehyde (and esters) is kinetically favored over the production of benzoic acid.

Full text of DOI:10.1016/j.susc.2012.03.013

Surface Science 606 (2012) 1129–1134
Contents lists available at SciVerse ScienceDirect
Surface Science
journal homepage: www.elsevier.com/locate/susc
Origin of the selectivity in the gold-mediated oxidation of benzyl alcohol
a
a
b,
Juan Carlos F. Rodríguez-Reyes , Cynthia M. Friend , Robert J. Madix ⁎
a
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, United States
School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, United States
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Benzyl alcohol has received substantial attention as a probe molecule to test the selectivity and efficiency of
novel metallic gold catalysts. Herein, the mechanisms of benzyl alcohol oxidation on a gold surface covered
with atomic oxygen are elucidated; the results show direct correspondence to the reaction on gold-based
catalysts. The selective, partial oxidation of benzyl alcohol to benzaldehyde is achieved with low oxygen
surface concentrations and takes place through dehydrogenation of the alcohol to form benzaldehyde via a
Received 28 November 2011
Accepted 19 March 2012
Available online 30 March 2012
Keywords:
Gold
Catalysis
Oxidation
Alcohols
benzyloxy (C
at higher concentrations it leads to the formation of intermediates from benzaldehyde, producing benzoic
acid and CO . Facile ester (benzyl benzoate) formation also occurs at low oxygen concentrations, which
6 5 2
H –CH O) intermediate. While in this case atomic oxygen plays solely a dehydrogenating role,
2
indicates that benzoic acid is not a precursor of further oxidation of the ester; instead, the ester is produced
by the coupling of adsorbed benzyloxy and benzaldehyde. Key to the high selectivity seen at low oxygen
concentrations is the fact that the production of the aldehyde (and esters) is kinetically favored over the
production of benzoic acid.
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
We have elucidated the mechanism of several gold-mediated
oxidation reactions, using gold surfaces pre-covered with atomic oxygen
under ultra-high vacuum (UHV) conditions [9–22]. The generation of
atomic oxygen on the surface provides active oxidant species which
activate many substrates containing acidic hydrogen. Under these
conditions the atomic oxygen also induces the formation of gold
nanoparticles on the originally ordered Au(111) surface [15,22–25]. The
use of UHV conditions allows for a controlled environment in which the
concentration of reactants and the nature of the surface can be monitored
with high precision. As noted above these studies correlate well with the
results of other investigations conducted under more practical working
conditions where direct comparisons are possible [7–10,26,27].
The oxidation of benzyl alcohol is a test reaction for probing the
properties of novel gold-based catalysts [28–70]. The work reported here
focuses on the mechanism for this reaction. Although the use of benzyl
alcohol as a probe is partially motivated by the importance of its oxidation
products (benzaldehyde, benzoic acid and benzyl esters) in the fragrance
and food industries, it possesses unique physical and chemical properties
attractive for practical studies. Since this alcohol does not form allotropes
with water (as it is the case for most aliphatic alcohols), its purity is
advantageous for studying solvent-free catalysis [29–36,44–49,58,59].
Further, due to its relatively high boiling point, the reaction can be studied
over a broad temperature span without phase changes. Chemically, a
consistent pattern of reactivity over a wide variety of gold-based catalysts
is observed, characterized by an extremely selective conversion to
benzaldehyde (~90%) [31–40,44–46,54–62]. Limited amounts of benzyl
benzoate are usually detected, and benzoic acid is often undetected. This
fact has been explained by assuming that the acid readily undergoes a
Gold-based heterogeneous catalysis has been an intense area of
research in the recent years [1–5]. The propitious effort in this area is
related to the search for environmentally friendly synthetic processes,
where organic solvents are not necessary, innocuous oxidants (e.g.
oxygen, air, hydrogen peroxide) are employed, and renewable energy
sources can be used (e.g. through photocatalysis). Remarkably, two
desired characteristics in modern catalysis, cleaner reactions and less
expensive processes, are combined exceptionally for gold-based
catalysis.
Although the underlying principles behind the oxygen-assisted
activity of gold are emerging [1,5,6], the complexity of the supported
catalyst still presents challenges for complete understanding. In
particular, the details regarding the activation of molecular oxygen
2
(O ) remain unclear for both supported and unsupported gold catalysts.
Recent experiments with nanoporous (unsupported) gold show high
activity and selectivity for alcohol esterification and suggest that the
catalytic activity, in particular the activation of molecular oxygen, may
be closely linked to under-coordinated gold surface atoms [7,8]. The
reactions observed were entirely predicted from our previous studies
under controlled conditions (using ultra high vacuum methods) [9] and
parallel the results obtained in the liquid phase at practical catalytic
conditions as well [10].
⁎
Corresponding author.
E-mail address: rmadix@seas.harvard.edu (R.J. Madix).
0039-6028/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2012.03.013

1130
J.C.F. Rodríguez-Reyes et al. / Surface Science 606 (2012) 1129–1134
condensation reaction with the alcohol, forming the ester and eliminating
water, even in solvent-free conditions [30–33,45–47,59]. While conden-
sation can be expected in solution, the present work demonstrates that in
a solvent environment the production of ester and acid take place through
different mechanisms, with the latter being kinetically more demanding.
of 1015 [73–75]. The use of other factors (e.g., 1013) leads to a difference
from the values reported here of about 10 kJ/mol.
3
. Results and discussion
3.1. The interaction of benzyl alcohol with clean Au(111)
2
. Experimental section
Benzyl alcohol is relatively strongly bound to the gold surface,
exhibiting binding energies of 90 and 105 kJ/mol, as deduced from a
Redhead analysis of the desorption temperatures of a saturated layer
Experiments were performed in a chamber with a base pressure
−
10
below 2×10
Torr, equipped with a mass spectrometer (Hiden HAL
(
310 and 360 K, Fig. 1). Strong interactions between aromatic
3
F) and an electron-energy loss spectrometer (LK 2000). In addition,
the chamber is outfitted with an Auger electron spectrometer (PHI
5–155) and low-energy electron diffraction optics (PHI 15–120),
which are employed to determine the cleanliness and morphology of
the Au(111) surface.
compounds and metal surfaces are common and are usually associated
with adsorption geometries where the aromatic ring lies nearly parallel
to the surface [76–82]. In the case of benzyl alcohol, analysis of the
observed EELS vibrational modes according to the surface selection
rules (Fig. 2 and Table 1) supports this binding geometry. While the
multilayer of the alcohol features strong signals corresponding to
modes whose dipole moments lay parallel to the ring (e.g. νC-O, νC-H),
they are attenuated nearly ten-fold with respect to out-of-plane modes
1
The gold surface was routinely cleaned by cycles of ozone (O
exposure at 200 K followed by an annealing to 600 K; the temperature-
programmed desorption of clean surface treated with is
characterized by the sole evolution of O at 540 K [23]. Once this is
3
)
a
O
3
2
(
e.g. ϕC-C and γC-H) at monolayer coverage [78]. The presence of two
verified, the clean surface is annealed to 900 K for 10 min to achieve the
herringbone reconstruction characteristic of a flat Au(111) surface
peaks is attributed to lateral, coverage dependent interactions in the
adsorbed layer, but this effect was not studied in detail.
[
13,23]. When the surface contamination is significant (appearing
The strong interaction of benzyl alcohol with the gold surface
apparently leads to thermal decomposition in the course of heating.
Indeed, ~30% of the initial amount of adsorbed benzyl alcohol
undergoes thermal decomposition on the gold surface, based on post-
oxidation TPRS (Table 2). It is noteworthy that aliphatic alcohols have
been shown to desorb cleanly from gold during temperature pro-
grammed desorption [9,11,18–20]. Thus, this fact indicates that benzyl
alcohol can potentially poison a gold catalyst more easily than other
alcohols, especially when high temperatures are employed [38].
+
clearly in AES or distorting the LEED pattern), Ar sputtering/annealing
cycles were employed.
Oxidation experiments were done by first populating the surface
with atomic O through the introduction of an appropriate amount of O
3
at 200 K. Briefly, ozone was dosed from a trap maintained near 190 K by
an alcohol/dry ice cooling bath [13,71]. The coverage of atomic O is
determined by referencing the amount of O evolving during the TPRS
2
of the oxygen-covered surface (hereafter denoted as O/Au), to that
reported for the oxygen-saturated surface (1.1 ML O/Au) [71]. This
procedure leads to the formation of surface nanostructures (~2 nm
diameter) [13,15,23,27]. Since the morphology (and oxidative chemis-
3.2. Quantitative analysis of the products of benzyl alcohol oxidation
try) of the O/Au surface varies with the temperature chosen for O
3
dosing [12,13], this temperature was set to 200 K. An 1 O/Au surface is
8
As the initial amount of surface oxygen preadsorbed is increased,
the overall reaction behavior shifts from partial oxidation toward
combustion (Fig. 1). This dependence is quantitatively assessed in
Table 2, where relative product yields (based on the total amount of
product) are estimated from the corrected integrated intensities from
TPRS experiments (the plotted data can be found in Fig. S1). Similarly
to the behavior observed during condensation (Section 3.1), carbon
deposition takes place during oxidation. This process is apparently
inherent to the gold surface under the conditions of our experiments.
At lower oxygen precoverages partial oxidation to benzaldehyde
and coupling to benzylbenzoate dominate the products. With higher
prepared to perform isotopic labeling experiments by exposing the
1
8
−6
O/Au surface to 2 L of H
2
O (1 L=10 Torr·s) at 150 K and annealing
16
16 18
to 220 K. By subsequently measuring the amount of
O ,
2
O
O and
(m/z=32, 34 and 36, respectively) during the TPD of the oxygen
covered surface, it was determined that ~80% of the surface oxygen is
18
O
2
18
O.
Benzyl alcohol (Sigma-Aldrich, 99.8%) was dosed at 150 K. While
during condensation experiments doses sufficient to guarantee multi-
layer formation were employed, oxidation experiments were carried
out using amounts corresponding to a saturated monolayer. However,
when a second reactant was introduced (e.g. benzaldehyde, 99.5%;
paraformaldehyde, 95%; methanol, 99.8%; all supplied by Sigma-
Aldrich), a dose of benzyl alcohol sufficient to reach only half of a
saturated monolayer of benzyl alcohol was employed. Compounds
employed to react with benzyl alcohol were used in excess to favor the
coupling reactions. An error of ~5% is estimated for the reported
coverages of both benzyl alcohol and atomic O.
For TPRS experiments a heating rate of 5 K/s was employed.
Reaction products were identified by comparing their cracking patterns
to those of pure samples (Supporting Information, Table SI). The yield of
each compound was quantified from the mass spectrometer intensity of
its parent ion, which was corrected scaled by the ionization efficiency of
the molecule, the relative intensity of the parent ion and instrumental
2
oxygen precoverages benzoic acid and CO become the major products.
High selectivities for partial oxidation are obtained at oxygen
precoverages below 0.05 ML. Similarly, it has been reported that an
increase in the concentration of oxidizing species under practical
conditions (e.g. by increasing the amount of oxygen employed during
the reaction) leads to a decrease in aldehyde selectivity [36,60,69].
These results are consistent with the inverse relationship between
aldehyde selectivity and alcohol conversion observed in the liquid
phase [29–32,35,45,59,69].
3.3. Reaction mechanism during the oxidation of benzyl alcohol
Previous investigations using isotopically labeled alcohols have
unequivocally demonstrated that the hydroxyl (OH) group is dehy-
drogenated; [9,83,84] accordingly, benzyl alcohol is expected to react
(
quadrupole transmission and multiplier gain) deviations [20,22].
Residual carbon after each experiment was quantified by exposing the
surface to O and performing a TPRS experiment following oxidized
carbon (as CO or CO ). Hereafter this procedure will be referred to as
6 5 2
with O to form benzyloxy (C H CH –O) species:
3
2
2
C H –CH –OH þ O→2C H CH –O þ H O:
ð1Þ
This facile dehydrogenation process yield water as a product near
200 K during TPRS experiments (Fig. 1). In addition, the initial
6
5
2
6
5
2
2
post-oxidation TPRS. Binding (adsorption) energies are estimated from
desorption temperatures in TPRS data using the Redhead method [72],
assuming in all cases a first-order reaction and a pre-exponential factor

J.C.F. Rodríguez-Reyes et al. / Surface Science 606 (2012) 1129–1134
1131
Fig. 1. Temperature-programmed desorption of benzyl alcohol and its oxidation products from a gold surface with various oxygen precoverages from 0.0 (i.e. desorption of BnOH from clean
gold, bottom set of spectra) to 0.15 ML O (top set of spectra). In all cases the amount of alcohol adsorbed corresponds to that necessary to guarantee a saturated monolayer on clean Au.
changes in the HREEL spectra are consistent with this reaction (Fig. 2,
1440 cm−1, have disappeared, consistent with benzaldehyde formation
near this temperature:
Table 1). The characteristic νAu-O on the oxygen-covered surface
−
1
(
360 cm ) significantly decreases in intensity on exposure to benzyl
2
C H –CH –O þ O→2C H CHO þ H O:
ð2Þ
alcohol at 140 K and disappears after heating to 190 K.
6
5
2
6
5
2
As the surface is annealed, benzyloxy species are further dehydroge-
nated. Between 190 and 240 K, the vibrational modes associated with the
Together with the disappearance of the feature at 1440 cm−1, a
−1
weak feature emerges at 1360 cm , which is assigned to benzoate (νO-
C-O) species, based on the formation of similar carboxylates during gold-
mediated alcohol oxidation [9,11]. Corresponding to the removal of the
aldehydic H atom to form benzoate, a second water feature appears in
the TPRS spectra at 245 K when high O coverages are employed (Fig. 1).
Indeed, TPRS of the oxidation of benzaldehyde shows that water
evolves at 240 K (Fig. S2), indicating that benzoate formation occurs as
follows:
−
1
2
methylene (CH ) group in the alcohol, νC-H at 2880 cm
and δC-H at
2
C H –CHO þ 3O→2C H COO þ H O:
ð3Þ
6
5
6
5
2
Based on previous studies [11,13], reaction (3) can be explained as
the consequence of two reactions:
C H –CHO þ O→C H CHOO
ð3aÞ
6
5
6
5
2
C H –CHOO þ O→C H COO þ H O:
ð3bÞ
6
5
6
5
2
Benzoic acid evolves at 350 K without CO
2
evolution at an oxygen
coverage of 0.08 ML. This minor pathway is restricted to a narrow span
of O coverages, and we suggest it occurs when the oxygen concentra-
6 5
tion is sufficient to promote the formation of C H CHOO (Eq. (3a)) but
6 5
not that of benzoate (Eq. (3b)), and that C H CHOO rearranges at 350 K
to yield benzoic acid.
In agreement with Eq. (3), HREELS experiments following the
oxidation of benzaldehyde show the formation of the νO-C-O mode upon
annealing to temperatures as low as 300 K (Fig. S3). It is then clear that
benzoate species are surface products of benzaldehyde oxidation,
2
which later evolve as benzoic acid and CO :
2
C H –COO→C H COOH þ CO þ residual carbon:
ð4Þ
6
5
6
5
2
(
Neither steps 3b or 4 are considered to be elementary steps, but
the others are.)
This reaction was confirmed by performing oxidations with 18O and
following the labeled products by TPRS (Fig. 3). Both benzoic acid and
CO
the products is the same in both cases, which strongly suggest that
benzoic acid and CO are formed through the benzoate intermediate.
Oxidation with 18O show that benzyl benzoate does not incorporate
2
incorporate 18O atoms in their structure and the ratio of O/ O in
18
16
Fig. 2. Vibrational (HREEL) spectra of benzyl alcohol on clean and oxygen-covered gold
surfaces. Benzyl alcohol multilayers were obtained by dosing the alcohol at 150 K;
annealing to 230 K is sufficient to desorb the multilayers and leave a saturated monolayer.
Oxidation experiments were carried out by adsorbing the alcohol on a 0.1 ML O/Au
surface at 140 K, followed by annealing to the indicated temperatures.
2
18
O into its structure, as evidenced from the absence of m/z=214 and

1132
J.C.F. Rodríguez-Reyes et al. / Surface Science 606 (2012) 1129–1134
Table 1
Selected electron energy losses during the condensation/oxidation of benzyl alcohol on Au(111). The assignment has been based on the indicated references.
Assigned
vibration
Multilayer on
clean Au
Monolayer on
clean Au
Monolayer on
O/Au, 140 K
Monolayer on
O/Au, 240 K
Monolayer on
O/Au, 300 K
Monolayer on
O/Au, 400 K
References
a
ϕ(C–C)
γ(C–H)
ν(C–O)
ν(O–C–O)
δ(CH
o
420
720
1020
–
420
720
1020
–
420
720
1020
–
420
720
–
420
720
–
420
700
–
[74]
[74]
[9,18]
[9,18]
[18]
o
i
i
–
1360
–
1360
–
2
)
o
1440
2850
3010
1440
2840
3010
1440
2840
3010
1440
–
ν(C–H)o methylene
ν(C–H)i ring
–
–
[18]
[74]
3010
3020
3020
a
Depending on the orientation of the dipole moment with respect to the aromatic ring, vibrational modes are catalogued as in-plane or out-of-plane by subscripts i and o, respectively.
m/z=216 (Fig. 3). This result indicates that the ester is not formed
through benzoate or benzoic acid. Instead, benzyl benzoate is formed
through the coupling of two unlabeled species. Following the pathway
elucidated for lower molecular weight alcohols [9,11], this coupling
reaction is
Pre-annealing temperatures below 170 K result in low yields for
benzaldehyde and benzyl benzoate and in significant amounts of benzyl
formate. However, at higher temperatures benzaldehyde and benzyl
benzoate are produced in higher yields, clearly indicating that they are
formed prior to the introduction of formaldehyde. The yield of benzyl
formate progressively decreases and is negligible upon annealing above
220 K (data not shown). From the inflexion points of the sigmoidal curves
in Fig. 5, we determine that the formation of benzaldehyde and benzyl
benzoate takes place at 175 K and 185 K, respectively. This temperature is
approximately 65 K below the formation of benzoate species, corre-
sponding to a difference of 20 kJ/mol in the kinetic barriers (~50 kJ/mol
for aldehyde/ester formation, ~70 kJ/mol for benzoate formation). This
difference in kinetic barriers is rather significant, especially when
compared to other alcohols. For example, the gold-mediated oxidation
of methanol, previously investigated in our group, indicates that the
corresponding aldehyde and ester (formaldehyde and methyl formate)
are produced at 220 K, temperature very close to that at which formate
1
=
C H –CH –O þ C H –CHO þ
2
O→C H CH –O–CðOÞ–C H
6
5
1
2
6
5
6
5
2
6
5
=
þ
2
H O
ð5Þ
ð5aÞ
ð5bÞ
ð5cÞ
2
which is comprised of the following elementary steps:
C H –CH –O þ C H –CHO→C H CH –O–CHð¼ OÞ–C H
6
5
2
6
5
6
5
2
6
5
C H CH –O–CH–O–C H þ O→C H CH –O–CðOÞ–C H þ OH
6
5
2
6
5
6
5
2
6
5
(precursor to formic acid) is observed spectroscopically (225 K). Thus,
2
OH→O þ H O:
2
compared to methanol (and, presumably, to other aliphatic alcohols),
benzyl alcohol can be selectively oxidized to benzaldehyde/benzyl
benzoate due to the fact that further oxidation is kinetically hindered.
Since the formation of aldehyde/ester requires lower temperatures
for benzyl alcohol (175 K) than for methanol (220 K), it would be
expected that the oxidation of a mixture of these two alcohols will lead
mostly to coupling of benzaldehyde with methoxy and benzyloxy
(benzyl- and methyl- benzoate), in detriment of those involving the
formation of formaldehyde (methyl- and benzyl- formate), as shown in
Scheme 1a and b. Indeed, the coupling products detected from the
oxidation of these mixtures are exclusively benzoates (Table 3 and Fig.
S5), regardless of the order in which these alcohols are dosed. The
formation of methyl formate observed when only methanol is dosed is
completely suppressed in the presence of benzyl alcohol. Conversely,
the formation of benzyl benzoate is not affected by the presence of
methanol. The only cross-coupling product detected is methyl benzoate
Ester formation can be viewed as the result of the nucleophilic attack
of benzyloxy species onto a benzaldehyde molecule to form the
hemiacetal (Eq. (5a)), which β-hydride eliminates to liberate the ester
Eq. (5b)).
When benzaldehyde was added to a benzyloxy-covered surface [19],
obtained by exposure of an oxygen-covered surface to benzyl alcohol at
50 K, the amount of benzyl benzoate produced doubled, confirming
reaction (5) (Fig. S4). Similarly, the addition of formaldehyde to the
benzyloxy-covered surface resulted in the formation of benzyl formate
Fig. S4), following the coupling reaction shown in Scheme 1 (a). These
(
1
(
results suggest the synthesis of targeted benzyl esters by introducing any
aldehyde into the reactor during benzyl alcohol oxidation.
3
.4. Kinetics of benzyl alcohol oxidation
(
although both benzyl formate and methyl benzoate feature a parent
In order to determine the temperature at which the benzyloxy
ion m/z=136, they can be distinguished by their different desorption
temperatures, as shown in Fig. S5). Thus, the formation of formaldehyde
and its coupling products is intercepted by the formation of benzalde-
hyde and its coupling products, benzoate esters. This preference has
been previously seen under working conditions [43,52,70], providing
further support to the applicability of our mechanism to vastly different
reaction conditions.
dehydrogenates to benzaldehyde and subsequent reaction occurs to
form the ester (Eq. (5)), surfaces pre-covered with 0.05 ML of O were
dosed with benzyl alcohol at 150 K and flash-annealed to increasingly
higher temperatures. Subsequent dosing with formaldehyde was then
done to probe for unreacted benzyloxy species by following benzyl
formate formation (Scheme 1 (a)) by TPRS (Fig. 4).
Table 2
Relative yields during the oxidation of benzyl alcohol on a gold surface covered with various amounts of oxygen. Mass spectrometer integrated areas have been quantified according to the
procedures described in Section 2.
O coverage (ML)
Benzyl alcohol
Benzaldehyde
Benzoic acid
Benzyl benzoate
Combustion gases (CO+CO
2
)
Residual carbon
0
0
0
0
0
0
.00
.02
.05
.08
.12
.15
73
54
38
20
0
0
7
15
18
5
0
0
1
2
1
2
0
2
6
6
1
0
0
0
0
13
46
65
27
37
40
41
47
29
0
4

J.C.F. Rodríguez-Reyes et al. / Surface Science 606 (2012) 1129–1134
1133
Scheme 1. Esters formed through the coupling of a) benzyl alcohol and formaldehyde,
b) methanol and benzaldehyde, and c) methanol and formaldehyde.
Table 3
Relative integrated intensities of characteristic traces associated with the gold-mediated
coupling of methanol, benzyl alcohol, and a mixture of them. In the latter case, two
sequences for dosing were considered. Alcohols were dosed at 150 K on a 0.05 ML O/Au
surface, using doses close to ½ saturated monolayer.
Substrates
(m/z=60)
(m/z=212)
(m/z=136)
Methyl formate
Benzyl benzoate
Methyl benzoate
Methanol
100
0
0
100
0
60
1
1
st methanol
nd benzyl alcohol
st benzyl alcohol
nd methanol
2
0
0
100
100
100
0
2
Benzyl alcohol
Fig. 3. Temperature-programmed desorption of selected oxidation products from the Au
18
16
surface covered with a mixture of O and O at two different coverages. The insertion of
one and two O in the resulting products is represented with one and two asterisks,
respectively.
4
. Conclusions
1
8
The remarkable selectivity achieved during the gold-mediated
oxidation of benzyl alcohol is associated with conditions in which
benzyl alcohol is in excess with respect to surface atomic oxygen. The
kinetic barrier for aldehyde formation is found to be ~20 kJ/mol lower
than that for benzoate formation (which ultimately leads to benzoic
acid and combustion gases). This difference is sufficiently large to
ensure a selective oxidation to benzaldehyde.
Acknowledgments
R.J.M. and J.C.F.R.-R. gratefully acknowledge the support of the
National Science Foundation for this research (NSF CHE 0952790).
C.M.F. gratefully acknowledges support of the U.S. Department of
Energy, Basic Energy Sciences, grant no. FG02-84-ER13289. Dr. B. Xu is
acknowledged for valuable discussions.
Appendix A. Supplementary data
Supplementary data to this article can be found online at doi:10.
1
016/j.susc.2012.03.013.
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