Alcohol Chemistry as a Probe of Mixed-Metal Phases: Reactions of 2-Propanol on Cobalt-Covered Mo(110)

Article abstract of DOI:10.1021/jp9621989

The reactions of 2-propanol on cobalt-covered Mo(110) have been investigated in an effort to test for special reactivity associated with mixed Co-Mo phases. 2-Propanol reacts via 2-propoxide on a continuous Co overlayer supported on Mo(110), as shown by X-ray photoelectron and infrared spectroscopies.Carbon monoxide, acetone, propene, and hydrogen are the only gaseous products, while carbon and oxygen remain on the surface after temperature-programmed reaction up to 760 K.The acetone and CO production increase whereas propene and propane production decrease as the Co coverage is increased from 0 to 1.5 monolayers (ML).This is consistent with independent reactions occurring on both Co and Mo, since 2-propanol is known to produce propene and propane on Mo(110).The C-O bond retention products (acetone and CO) are attributed to reaction on the Co overlayer.Infrared and temperature-programmed reaction studies confirm that the evolution of CO is desorption limited and that decomposition to CO competes with acetone formation at 330 K.Acetone is produced via selective cleavage of the β C-H bond (adjacent to the oxygen), as shown by selective isotopic labeling experiments.The observed C-O bond retention on Co is consistent with the intermediate strength of the Co-O bond compared to other M-O bonds for transition metals.There is no evidence for aggregation of the Co overlayer induced by 2-propoxide, and therefore the observed reactivity is that of the dispersed Co overlayer.

Full text of DOI:10.1021/jp9621989

17640
J. Phys. Chem. 1996, 100, 17640-17647
Alcohol Chemistry as a Probe of Mixed-Metal Phases: Reactions of 2-Propanol on
Cobalt-Covered Mo(110)
D. A. Chen and C. M. Friend*
Department of Chemistry, HarVard UniVersity, 12 Oxford Street, Cambridge, Massachusetts 02138
ReceiVed: July 22, 1996; In Final Form: September 9, 1996X
The reactions of 2-propanol on cobalt-covered Mo(110) have been investigated in an effort to test for special
reactivity associated with mixed Co-Mo phases. 2-Propanol reacts via 2-propoxide on a continuous Co
overlayer supported on Mo(110), as shown by X-ray photoelectron and infrared spectroscopies. Carbon
monoxide, acetone, propene, and hydrogen are the only gaseous products, while carbon and oxygen remain
on the surface after temperature-programmed reaction up to 760 K. The acetone and CO production increase
whereas propene and propane production decrease as the Co coverage is increased from 0 to 1.5 monolayers
(ML). This is consistent with independent reactions occurring on both Co and Mo, since 2-propanol is known
to produce propene and propane on Mo(110). The C-O bond retention products (acetone and CO) are
attributed to reaction on the Co overlayer. Infrared and temperature-programmed reaction studies confirm
that the evolution of CO is desorption limited and that decomposition to CO competes with acetone formation
at 330 K. Acetone is produced via selective cleavage of the â C-H bond (adjacent to the oxygen), as shown
by selective isotopic labeling experiments. The observed C-O bond retention on Co is consistent with the
intermediate strength of the Co-O bond compared to other M-O bonds for transition metals. There is no
evidence for aggregation of the Co overlayer induced by 2-propoxide, and therefore the observed reactivity
is that of the dispersed Co overlayer.
Introduction
lattice constant for Co(0001) (2.51 Å) compared to Mo(110)
(3.15 × 2.73 Å), and the atoms are 27% less densely packed
than for Co(0001).12 At a coverage of 1 ML, the overlayer
undergoes a phase transition, which results in an (8 × 2) LEED
pattern as the Co atoms compress along the [001] direction to
form a surface structure that is only 2% less densely packed
than Co(0001). Cobalt islands form on top of the first uniform
layer upon annealing overlayers with coverages greater than 1.3
ML.11 Hence, the Co-on-Mo(110) system provides an excellent
means for studying structural effects, as well as a means for
probing reactivity associated with mixed Co-Mo sites, which
are formed at low Co coverages.
The reactions of alcohols on transition metal surfaces are of
interest because oxygen-containing molecules are present in
liquid fuelstocks and in adhesives used for metal bonding.
Furthermore, the reaction of alcohols on transition metal surfaces
serves as a model for the catalytic oxidation of alcohols to
ketones and aldehydes as well as the microscopic reverse of
the Fischer-Tropsch synthesis of hydrocarbons from CO and
hydrogen (syngas). Cobalt in particular is used as Fischer-
Tropsch catalyst to convert syngas into primary alcohols and
aldehydes.1 Thus, the identification and characterization of
intermediates formed from the reaction of alcohols is important
for understanding the mechanism of oxygenate synthesis from
CO and hydrogen.2 Mixed-metal materials are of particular
interest because they offer the potential for reactivity different
from the individual constituents, either via alteration in geo-
metric structure due to interactions between the metals or via
electronic effects that may lead to synergy between the two
metals. Indeed, synergistic effects between Co and Mo have
been proposed based on theoretical studies to explain Co
promotion in hydrodesulfurization catalysis.3 Furthermore,
substantial differences in the adsorption properties of carbon
monoxide have been reported for thin metal films, such as Pd
on Nb(110),4 which have larger lattice constants than the bulk
materials. Similar results have been reported for CO adsorption
on several other thin metal films supported on refractory
metals.4-10
Cobalt overlayers were chosen for these studies because of
both the proposed synergistic effects between Co and Mo in
heteroatom removal catalysis and the anticipated qualitative
differences in alcohol reactivity on Co versus Mo. Qualitative
differences in Co and Mo reactivity would facilitate deconvo-
lution of chemical processes occurring on the two metals, thus
making the Co-on-Mo(110) system ideal for investigations of
reactivity as a function of overlayer structure and composition.
Previous work has shown that alkoxides, including 2-propoxide,
undergo C-O bond scission on Mo(110).16,17 Specifically,
2-propanol reacts via 2-propoxide to form propene, propane,
and hydrogen on Mo(110), along with surface carbon and
oxygen from decomposition.16,17 The C-O bond scission in
2-propoxide is driven by the strong Mo-O bond. In contrast,
alkoxides react with C-O bond retention on mid- and late-
transition metals, which have intermediate metal-oxygen bond
strengths.2,17-22 For example, CO is evolved from the reactions
of alcohols on Rh(111),21 Fe(100),22 and Ni(100).18 Accord-
ingly, C-O bond retention is anticipated for alkoxide reaction
on Co, based on the similarity of the Co-O (91. ( 3.2 kcal/
mol), Fe-O (93.3 ( 4.1 kcal/mol), and Rh-O (96.8 ( 10 kcal/
mol) bond strengths.24-26 Indeed, this study shows that acetone
and CO are the primary gaseous products of 2-propanol reaction
on uniform Co layers (θCo g 1 ML).27 Furthermore, the
Herein, we discuss the reactions of 2-propanol on Co-covered
Mo(110). Cobalt overlayers on Mo(110) have been previously
investigated in detail, and the lattice spacing was found to
depend on Co coverage.5,11-15 For coverages less than 1 ML,
the Co overlayer grows pseudomorphically, and a (1 × 1) low-
energy electron diffraction (LEED) pattern is observed. This
overlayer is strained due to the higher symmetry and smaller
X Abstract published in AdVance ACS Abstracts, October 15, 1996.
S0022-3654(96)02198-3 CCC: $12.00 © 1996 American Chemical Society

Reactions of 2-Propanol on Co-Covered Mo(110)
J. Phys. Chem., Vol. 100, No. 44, 1996 17641
dependence of the product distribution on Co coverage indicates
that the reactivities of Co and Mo are additive, not synergistic,
and that there are no significant differences in the chemistry of
the Co layers with different lattice constants.
Experimental Section
Experiments were performed in two ultrahigh-vacuum cham-
bers, which have been described previously,28-31 each with a
base pressure of e2 × 10-10 Torr. Both chambers were
equipped with a UTI-100C quadrupole mass spectrometer and
electron optics for low-energy electron diffraction. The mass
spectrometer was interfaced with a Compaq 486 computer that
allowed data collection for up to 10 masses in a single
experiment.32 One chamber was equipped with a Physical
Electronics 5300 X-ray photoelectron system. X-ray photo-
electron data were analyzed and fit on an Apollo DN 3500
workstation using the software provided. The O(1s), C(1s), Co-
(2p), and Mo(3d) spectra were collected in consecutive scans
over 10, 10, 6, and 2 min intervals, respectively, using a band-
pass energy of 17.9 eV. The binding energies were referenced
to the Mo(3d5/2) peak at 227.8 eV. For the X-ray photoelectron
spectra of 2-propanol on 1 ML of Co, background spectra for
1 ML of Co were subtracted from the O(1s) and C(1s) regions.
The second chamber was equipped with a single-beam, clean-
air-purged Fourier transform infrared spectrometer (Nicolet,
Series 800). All FT-infrared reflection absorption spectra were
averaged over 1000 scans using an MCT A detector at 2 cm-1
resolution. Sample spectra were ratioed against a background
spectrum after heating the crystal to 760 K and allowing it to
cool to 150 K.
Cobalt was deposited on the surface by passing current
through three braided tungsten wires that were wrapped around
a Co wire (99.999% purity), as described elsewhere.14,15 Cobalt
was deposited on the crystal at 100 K, and the overlayer was
annealed to 760 K for 60 s before 2-propanol exposure. The
Co coverage was calibrated using LEED since there is a sharp
phase transition at 1 ML, which causes a change from a (1 ×
1) to an (8 × 2) pattern for coverages e1 ML; the Co(2p) X-ray
photoelectron signals increased linearly with dosing time and
were reproducible to within 4%. There is no evidence for Co-
Mo alloy formation based on LEED, X-ray photoelectron, and
temperature-programmed reaction data.14,15,33
2-Propanol (Fischer Scientific, 99+%), (CH3)2CDOH, (CD3)2-
CHOH, (CD3)(CH3)CHOH, and (CD3)2CDOD (99% D, Cam-
bridge Isotope Laboratories) were used as received. The
samples were stored in glass bottles and subjected to several
freeze-pump-thaw cycles prior to use. The purities of the
samples were confirmed by mass spectrometry.
Figure 1. Temperature-programmed reaction of 2-propanol on (a) 1
ML of Co on Mo(110) and (b) clean Mo(110). The representative
masses for each product are uncorrected for the cracking contributions
from other products. The shaded region in the 28 and 41 amu signal
represents the contribution from acetone fragmentation.
For high exposures of 2-propanol on Co overlayers (θCo g1
ML), two low-temperature desorption states at 160 and 210 K
are also observed (Figure 1a). The 160 K peak is attributed to
sublimation of condensed 2-propanol since its intensity increases
without bound as the 2-propanol exposure increases. The peak
at 210 K is observed at exposures lower than those required
for multilayer condensation and reaches maximum intensity at
the minimum exposure required for appearance of the multilayer
peak. The 210 K peak is attributed to molecular desorption
since no deuterium is incorporated into 2-propanol-d0 when
deuterium is present on the surface. Specifically, the 46:45 amu
signal is 3:200, the same as for the 2-propanol-d0 sample.
Isotopic labeling studies indicate that the â C-H(D) bond
(adjacent to the oxygen) is selectively cleaved during acetone
formation (Scheme 1). Only acetone-d0 (43 and 58 amu) is
produced from (CH3)2CDOH whereas only acetone-d6 (46 and
Results
Temperature-Programmed Reaction Data. 2-Propanol
forms acetone, CO, H2, and a small amount of propene during
temperature-programmed reaction on an ordered 1 ML Co
overlayer (Figure 1a).34 Nonselective decomposition to ad-
sorbed carbon, oxygen, and gaseous hydrogen is a competing
process, as determined by X-ray photoelectron spectroscopy.
No other reaction products were detected in a comprehensive
search of all masses up to 80 amu. Specifically, no propane is
formed since the 29 amu signal is insignificant, and no ethane,
ethylene, acetylene, or methane is evolved, based on the absence
of 30, 26, 25, and 16 amu signals, respectively.35,36 The
temperature-programmed reaction data also indicate that CO
evolution is desorption-limited, given that CO evolves from both
Co(0001)37 and the Co overlayers at 425 K.

17642 J. Phys. Chem., Vol. 100, No. 44, 1996
Chen and Friend
SCHEME 1
64 amu) results from (CD3)2CHOH. Furthermore, there is a
kinetic isotope effect when deuterium is substituted for hydrogen
at the â position, as in (CH3)2CDOH and (CD3)2CDOD. For
example, the peak temperature of acetone evolution is ∼20 K
higher than for (CH3)2CHOH and (CD3)2CHOH.
The kinetic isotope effect further indicates that acetone
evolution at 330 K is reaction-limited. Acetone itself mainly
decomposes on the Co overlayers although there is some
desorption below 200 K. Furthermore, in the reaction of a 1:5
mixture acetone-d6 and 2-propanol, only acteone-d0 is evolved
at 330 K, while all of the acetone-d6 desorbs below 200 K.
Isotopic labeling studies also show that the small amount of
propene formed on the 1 ML Co film occurs via selective
dehydrogenation of one of the methyl groups, by a mechanism
similar to that reported for 2-propanol reaction on Mo(110).17
The reaction of (CH3)2CDOH produces propene-d1 as the major
propene product, and the 42:41 amu ratio is the same as the
41:40 ratio for an authentic sample of propene-d0. The absence
of propene-d0 from (CH3)2CDOH reaction clearly shows that
the C-H(D) bonds at the γ position are selectively cleaved.
Similarly, the reaction of (CD3)2CHOH yields propene-d5, not
propene-d6, ruling out an isotope effect.38 Deuterium and
2-propanol coadsorption experiments further demonstrate that
there is no H-D exchange in the intermediate that gives rise to
propene.39
Figure 2. Relative yields of various products as a function of Co
coverage for the reaction of 2-propanol on Co-covered Mo(110).
Integrated signals were divided by the largest value in the data set so
that the vertical range is from zero to unity in all cases. The
representative signals for each product were corrected for the cracking
contribution from other products.36 The error was calculated from the
standard deviation determined by five different experiments. Data for
all Co coverages were collected in a single day.
The product distribution for 2-propanol reaction depends on
the Co coverage in a manner that indicates additive, not
synergistic, reactivity of Co and Mo. As reported previously,
C-O bond cleavage is promoted on Mo(110) (Figure 1b).
When Co is deposited on the Mo(110), the yields of products
associated with Mo(110) (propane and propene) monotonically
decrease, and those associated with the Co overlayer (CO and
acetone) increase (Figure 2). Furthermore, there are no
significant shifts in the evolution temperatures of the carbon-
containing products, indicating that there is no significant
perturbation of the chemistry of one metal by the other.
Careful scrutiny of the propene formation curves as a function
of Co coverage suggests that a minor amount of propene is
formed from reaction on mixed Co-Mo sites (Figure 3).
Specifically, for Co coverages below 0.5 ML, there are two
propene peaks (â1, â2) at 410 and 455 K. The â1 peak (410 K)
is attributed to reaction on Mo, based on the correspondence in
peak temperature to propene formed on Mo(110). The intensity
of the â1 peak and the total propene yield decrease with
increasing Co coverage, whereas the â2 peak intensity reaches
a maximum at 0.25 ML. The decrease in total intensity of the
propene signal with increasing Co coverage reflects the com-
petition between propene evolution and the formation of
products containing intact C-O bonds. A minor amount of â2
propene (∼6% of the propene signal from reaction on Mo(110))
is formed at 1.5 ML of Co. This suggests that the â2 peak
might arise from reaction at a mixed Co-Mo sites and that a
small fraction of 2-propanol is still able to access the Mo surface
on the continuous Co overlayer, perhaps via defect sites.
X-ray Photoelectron Studies. The X-ray photoelectron
spectrum of 2-propanol on 1 ML of Co clearly shows that the
C-O bond remains intact after heating to 250 K and is
consistent with a 2-propoxide intermediate (Figure 4). The
Figure 3. Temperature-programmed reaction of 2-propanol on Co
overlayers as a function of Co coverage for propene (41 amu). The
shaded regions represent cracking contributions from acetone (not
shown).
O(1s) binding energy of 531.8 eV is characteristic of intact C-O
bonds. The lack of intensity at 530.3 eV, which is the binding
energy of atomic oxygen on 1 ML of Co, indicates that the
C-O bond is not broken. Intact O-H bonds are expected to
have a higher O(1s) binding energy. For example, condensed
2-propanol has an O(1s) binding energy of 533.4 eV. The C(1s)
spectra are likewise consistent with formation of 2-propoxide
on the 1 ML Co overlayer below 250 K. Specifically, there
are two C(1s) states at 286.3 and 284.9 eV with a relative area
of 1:1.9. The 286.3 eV peak is attributed to the oxygen-bound

Reactions of 2-Propanol on Co-Covered Mo(110)
J. Phys. Chem., Vol. 100, No. 44, 1996 17643
CO, which has a binding energy at 285.7 eV on 1 ML of Co
but may also have a contribution from the C-O bond in
2-propoxide. The additional peak at 284.4 eV is consistent with
intact C-C bonds but is too low in energy to be from
2-propoxide.
X-ray photoelectron spectra obtained after heating to 550 K
indicate that only atomic carbon and oxygen are present (data
not shown). The O(1s) state at 530.3 eV and the C(1s) peak at
282.8 eV are characteristic of atomic oxygen and carbon species,
respectively. No further changes occur upon heating to 760 K
(Figure 4).
The amount of 2-propoxide on the surface at 250 K decreases
with increasing Co coverage. At a Co coverage of 1.5 ML,
the total amount of reaction is ∼73% of that on Mo(110), as
calculated from the C(1s) signal at 250 K.41,42 The estimated
coverage of 2-propoxide on 1 ML of Co is ∼0.2 ML based on
a comparison of the C(1s) signal intensity to that of 2-propoxide
on Mo(110), which has a coverage of 0.3 ML.43 This suggests
that molecular desorption and 2-propoxide formation are
competing processes.
X-ray photoelectron studies also indicate that 2-propanol-
induced aggregation of the 1 ML Co overlayer is not significant
below 500 K. There is no measurable shift in the Co(2p3/2
)
binding energy upon adsorption of 2-propoxide on the 1 ML
Co overlayer at 250 K. The peak position and intensity of the
Mo signal do not change in either case. The Co(2p3/2) binding
energy shifts by +0.2-0.3 eV upon heating 2-propanol on the
1 ML Co overlayer to 375 K, consistent with the deposition of
atomic oxygen at this temperature; the Co(2p3/2) peak to shifts
+0.3 eV relative to the pure Co overlayer after depositing ∼0.2
ML of oxygen on 1 ML of Co at 100 K. Although large oxygen
exposures (∼1 ML) previously have been shown to cause
aggregation of the Co overlayer upon heating,14 the attenuation
of the Co(2p) signal resulting from Co aggregation is minimal
below 500 K.
Infrared Studies. The infrared spectrum of 2-propanol
heated to 250 K on 1 ML of Co is consistent with 2-propoxide
formation, based on its close resemblance to that of 2-propoxide
on Mo(110) (Figure 5). Increasing the Co coverage from 1.0
to 1.5 ML results in only minor differences in the spectra of
both 2-propanol and (CD3)2CHOH at 250 K. This suggests that
there are no dramatic changes in the bonding caused by
variations in the Co overlayer lattice constant. The 2-propoxide
intermediate on Mo(110) has been observed previously using
electron energy loss and X-ray photoelectron spectroscopies.16
The infrared data suggest that the O-H bond of 2-propanol is
broken by the Co monolayer as well, since the ν(O-H) mode
is not observed. While it is possible that the ν(O-H) mode
would be absent in infrared data due to orientation effects, we
have recently shown that the ν(O-H) mode of HOCH2CH2O-
Mo(110) can be detected with both infrared and electron energy
loss spectroscopies.44 The correspondence between infrared
spectra on the Co monolayer and Mo(110) also indicates that
the molecular framework remains intact after O-H bond
scission. The spectra of (CD3)(CH3)CHOH, (CD3)2CHOH, and
(CD3)2CDOD heated to 250 K on 1 ML of Co correspond to
those of the respective 2-propoxide on Mo(110).
Figure 4. X-ray photoelectron data for the (a) O(1s) region and (b)
C(1s) region after heating 2-propanol condensed on 1 ML of Co to (i)
250, (ii) 400, and (iii) 760 K.
carbon, and the 284.9 eV peak is attributed to the methyl
carbons.40 The ratio of approximately 1:2 substantiates these
assignments; 2-propoxide on Mo(110) has C(1s) binding ener-
gies at 286.5 and 285.1 eV also with a relative area of 1:2. No
atomic carbon is observed after heating 2-propanol condensed
on 1 ML of Co to 250 K.
At 400 K, the appearance of an O(1s) peak at 530.2 eV,
characteristic of atomic oxygen, indicates that C-O bond
cleavage has occurred (Figure 4). However, the persistence of
the 531.8 eV peak shows that there are still intact C-O bonds.
The 531.8 eV peak could be from CO, given that the O(1s)
binding energy of CO on 1 ML of Co is ∼532 eV. The major
peak in the C(1s) region after heating to 400 K is from atomic
carbon at 282.9 eV. The 285.8 eV peak is most likely due to
Based on the close correspondence in the infrared spectra of
the various isotopic forms of 2-propoxide on Mo(110) and 1
ML of Co, the mode assignments are assumed to be similar to
the Mo(110) case. Model calculations are planned to establish
detailed mode assignments.45 The major difference in the
spectra for 2-propoxide on the Co overlayer and Mo(110) is
the increase in the full width at half-maximum (fwhm) of the
ν(C-O) mode. In particular, the 935-925 cm-1 mode is

17644 J. Phys. Chem., Vol. 100, No. 44, 1996
Chen and Friend
Figure 6. Infrared reflection-absorption spectra showing the ν(CtO)
region. Data were collected after heating a saturation exposure of
2-propanol to 375 K on (a) Mo(110) and (b) 1 ML of Co. Spectrum c
is for a subsaturation exposure of CO on 1 ML of Co at 100 K.
1 ML of Co heated to 375 K is 1968 cm-1 46
The intensities
.
of the ν(CtO) modes observed from the decomposition of
2-propanol depend on Co coverage. For example, the relative
intensity of the ν(CtO) mode is greater on 1.5 ML Co than on
1 ML of Co although the frequencies are virtually identical
(1978 and 1975 cm-1, respectively). These results are generally
consistent with the increase in CO yield as a function of Co
coverage and indicate that there is not a substantial change in
mechanism upon varying the amount of Co deposited.
The infrared data further suggest that some intact 2-propoxide
remains on the surface as CO is formed. The spectrum at 375
K is nearly identical to that of a low coverage of 2-propanol on
1 ML of Co heated to 250 K, with the addition of the ν(CtO)
peak. Subsaturation exposures of 2-propoxide-d0 and (CD3)-
(CH3)CHO- on 1 ML of Co show that the C-O stretching
modes shift to lower frequency by as much as 10-15 cm-1 as
a function of 2-propanol coverage. The same behavior is
observed for 2-propoxide-d0 and (CD3)(CH3)CHO- on Mo-
(110).47,48 This dependence of ν(C-O) on coverage has also
been reported for methoxide on Mo(110)30 and is attributed to
decreased dipole-dipole coupling at lower coverages.
The infrared spectrum of 2-propanol on 1 ML of Co, heated
to 400 K, demonstrates that most products have left the surface
because the intensities of all the modes are decreased to near
the noise level. These results are consistent with temperature-
programmed reaction data which show that all products except
â2-propene are evolved below 400 K. At 425 K, only small
peaks at 1090 and 2970 cm-1 are observed. The low intensities
preclude identification of the remaining intermediate(s) and
further indicate that the intermediate giving rise to â2 propene
at 450 K is minor.
Figure 5. Infrared reflection-absorption spectra for a saturation
exposure of 2-propanol after heating to 250 K on (i) Mo(110) and (ii)
1 ML of Co for the (a) low-frequency region and (b) C-H stretching
region.
broader and splits into two peaks, both of which are slightly
higher in energy than on Mo(110) (919 cm-1). This change is
not simply due to a coverage effect; saturation coverage on 1
ML of Co is reduced by ∼25% compared to Mo(110), and a
25% reduction in coverage on Mo(110) caused a decrease in
fwhm and a shift to lower energy. Hence, we attribute the
broadening to heterogeneity of the surface. Similarly, the ν-
(C-O) for (CD3)(CH3)CHO-, (CD3)2CHO-, and (CD3)2-
CDO- are broadened on the 1 ML Co overlayer compared to
on Mo(110).
The infrared data also confirm that CO is formed from
2-propoxide reaction on Co-covered Mo(110) at temperatures
below 375 K and remains on the surface up to ∼400 K. CO is
identified based on the development of a new peak in the
infrared spectrum at 1975 cm-1 upon heating 2-propanol or
(CD3)(CH3)CHOH condensed on 1 ML of Co to 375 K (Figure
6); the absence of a significant isotopic shift confirms the
assignment as ν(CtO). Furthermore, the stretching frequency
for CO adsorbed on 1 ML of Co was independently measured
as 1975 cm-1 for a subsaturation coverage, and the frequency
of CO coadsorbed with 2-propanol (2-propanol:CO ) 5:1) on
Discussion
The reactions of 2-propanol on Co monolayers supported on
Mo(110) are qualitatively different than on Mo(110) and similar
to those on other mid-transition metal surfaces, in that most
C-O bonds are retained on uniform Co overlayers. Specifi-
cally, the reaction of 2-propanol on 1 ML of Co yields acetone
and CO, whereas reaction on Mo(110) produces propene and
propane. Hydrogen and surface carbon and oxygen are

Reactions of 2-Propanol on Co-Covered Mo(110)
J. Phys. Chem., Vol. 100, No. 44, 1996 17645
Nonselective dehydrogenation and C-C bond scission, yield-
ing CO, competes with acetone elimination. Although the
detailed mechanistic steps are not identified by our experiments,
our infrared data show that adsorbed CO is formed by 375 K,
and acetone evolution is complete at this temperature. The CO
remains on the surface until approximately 425 K. There is
also some C-O bond dissociation competing with acetone and
CO production, based on the detection of residual carbon and
oxygen following temperature-programmed reaction to 375 K
using X-ray photoelectron spectroscopy. The residual carbon
is partly a by-product of CO production, but the oxygen must
be due to competing C-O bond dissociation. Carbon monoxide
is known to dissociate on stepped CO surfaces, polycrystalline
Co,61-63 and Co overlayers on Mo(110),5 W(110), and W(100).6
In contrast, no CO dissociates on Co(0001).37,64,65 These results
suggest that the C-O dissociation might occur on defects in
the Co overlayer and that the dissociation proceeds, at least in
part, via adsorbed CO.
Co overlayers on Mo(110) show predominantly an additive
rather than synergistic effect in the reactions of 2-propanol since
the product distributions are mainly due to independent reaction
on the two metals. Because 2-propanol forms different products
on the Co overlayer (acetone and CO) and the Mo(110) surface
(propene and propane), the contributions of the two metals can
be easily distinguished. The smooth increase in the yields of
products with intact C-O bonds is consistent with an indepen-
dent and simultaneous decrease in C-O bond cleavage products
(Figure 2). Notably, there are no significant differences in the
reactivity of 2-propanol as the overlayer changes from a
pseudomorphic to the close-packed structure in the Co coverage
range 1.0-1.5 ML.
The only evidence for a reactivity different than those of the
two constituent metals is the evolution of â2-propene at 455 K
(Figure 3). The â2 state cannot be attributed to pure Co sites
even though it persists up to 1.5 ML of Co, since it decreases
with increasing Co coverage above 0.5 ML. Neither is it due
to pure Mo, since there is not a â2 feature in the temperature-
programmed reaction spectrum of 2-propanol on Mo(110).
Furthermore, â2-propene production is greatest in the range
0.25-0.5 ML of Co, a coverage at which the population of
mixed Co-Mo sites is at a maximum. For example, the yield
of â1-propene, which is associated with the Mo(110) surface,
is virtually zero at the highest Co coverage (1.5 ML). The yield
of â2-propene is ∼5% that of the â1-propene signal from clean
Mo(110). At 0.5 ML, the â1 and â2 propene peak heights are
nearly the same, although the total yield of propene is reduced
to approximately half that on Mo(110). As the Co coverage is
increased, â1 production decreases more rapidly than â2,
suggesting that the â2-propene is related to the presence of Co.
The persistence of propene production at coverages above 1
ML of Co is explained in terms of accessibility of 2-propanol
to the Mo atoms. At a Co coverage of 1 ML, the â2-propene
production is ∼15% of the total propene production on Mo-
(110) itself, and the packing density of this pseudomorphic layer
is ∼23% lower than for bulk Co.66 As the Co coverage is
increased, the packing density is also increased to nearly that
of Co(0001) for coverages above 1.3 ML. The â2-propene
production is lower on the more densely packed Co overlayers
with relative yields of 6% and 4% compared to Mo(110) for
reaction on 1.5 and 2.5 ML of Co, respectively. We tentatively
attribute the small amount of propene production on the densely
packed Co films to defects in the overlayer that render mixed
Co-Mo sites accessible to the 2-propanol. This assertion is
supported by the splitting in the C-O stretch in 2-propoxide
on 1 ML of Co, which suggests that the Co overlayers are
Figure 7. Proposed reaction scheme for 2-propanol on a uniform Co
overlayer.
produced on both surfaces. Infrared and X-ray photoelectron
studies both demonstrate that reaction proceeds via 2-propoxide
on both the Co overlayers and Mo(110). Importantly, Co
aggregation does not play a role in the reactions of 2-propanol
on the Co overlayer. Studies of the Co overlayer on Mo(110)
have shown that the oxygen (0.25 ML) and Co segregate, but
only above 500 K.
The qualitative difference in chemistry on the Co overlayer
versus the Mo substrate is explained by periodic trends in the
metal-oxygen bond strengths. All of the metal-oxygen bond
energies for the group 8 transition metals are between 91 and
97 kcal/mol.24,26 Accordingly, 2-propanol reaction on the Co
overlayer is very similar to that on Ni(100), Fe(100), and Pd-
(111).18,19,23 In all cases, the C-O bond is retained, yielding
acetone and CO in conjunction with H2. The C-O bond of
2-propanol is similarly retained on Rh(111) and Pt(111), on
which CO and H2 are formed.21,49 In contrast, C-O bond
cleavage is promoted on Mo(110);16,17 the greater Mo-oxygen
bond strength (133.9 ( 5 kcal/mol)24,26 provides a greater
thermodynamic driving force for C-O bond cleavage.
The proposed mechanism for 2-propanol reaction (Figure 7)
is consistent with all of our data. The O-H bond of 2-propanol
is cleaved below 250 K on the Co overlayer, affording
2-propoxide, based on X-ray photoelectron and infrared reflec-
tion absorption studies. There is also competing molecular
desorption at 210 K for high 2-propanol exposures. 2-Propoxide
has also been identified on other surfaces investigated, including
Mo(110),16 Rh(111)50 and Pd(111).49 On Co-covered Mo(110),
selective â C-H bond cleavage is the rate-limiting step for
acetone formation, as shown by selective isotopic labeling
experiments. A similar mechanism is operative for acetone
formation from 2-propanol on O-covered Rh(111) and is
attributed to the lower homolytic C-H bond energy due to
electron withdrawal by the neighboring oxygen atom.21 The
weaker C-H bonds â to the metal center also lead to preferential
â C-H bond breaking in other alkoxides bound to both surfaces
and metal clusters, provided that the C-O bond remains
intact.17,19,51-60

17646 J. Phys. Chem., Vol. 100, No. 44, 1996
Chen and Friend
(13) Kuhn, W. K.; He, J.-W.; Goodman, D. W. J. Phys. Chem. 1994,
98, 264-269.
heterogeneous. Furthermore, the dependence of the propene
evolution yields and peak shapes on degree of order in the Co
overlayers is consistent with our proposal that â2-propene
formation is related to defect sites.
(14) Chen, D. A.; Friend, C. M. Surf. Sci., in press.
(15) Chen, D. A.; Friend, C. M.; Xu, H. Langmuir 1996, 12, 1528-
1534.
(16) Uvdal, P.; Wiegand, B. C.; Serafin, J. G.; Friend, C. M. J. Chem.
Phys. 1992, 97, 8727-8735.
(17) Wiegand, B. C.; Uvdal, P. E.; Serafin, J. G.; Friend, C. M. J. Am.
Chem. Soc. 1991, 113, 6686-6687.
(18) Johnson, S. W.; Madix, R. J. Surf. Sci. 1982, 115, 61-78.
(19) Davis, J. L.; Barteau, M. A. Surf. Sci. 1987, 187, 387-406.
(20) Wiegand, B. C.; Uvdal, P.; Serafin, J. G.; Friend, C. M. J. Phys.
Chem. 1992, 96, 5063-5069.
(21) Xu, X.; Friend, C. M. Surf. Sci. 1992, 260, 14-22.
(22) Benziger, J. B.; Madix, R. J. J. Catal. 1980, 65, 36-48.
(23) Sault, A. G.; Madix, R. J. J. Phys. Chem. 1988, 92, 6025-6028.
(24) Bond strengths are reported for diatomic molecules in the gas phase.
(25) The reactivity of bulk Co has not been studied, but it is expected
to behave similarly to Rh, Ni, and Fe, which are its neighbors in the periodic
table. The reactions of alcohols of these surfaces result in products with
intact C-O bonds.18,21,22
(26) CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton,
FL, 1989.
(27) A monolayer of Co corresponds to the same number of atoms as
in the underlying Mo(110) surface.
(28) Xu, X.; Friend, C. M. J. Am. Chem. Soc. 1991, 113, 6779-6785.
(29) Roberts, J. T.; Friend, C. M. J. Am. Chem. Soc. 1986, 108, 7204-
7210.
(30) Weldon, M. K.; Uvdal, P.; Friend, C. M.; Serafin, J. G. J. Chem.
Phys. 1995, 103, 5075.
(31) Weldon, M. K.; Friend, C. M. Surf. Sci. 1994, 310, 95-102.
(32) Liu, A. C.; Friend, C. M. ReV. Sci. Instrum. 1986, 57, 1519-1522.
(33) We did not observed the LEED pattern associated with Co-Mo
alloying that was reported by Bauer et al. for higher Co coverages.11
Furthermore, the Co(2p) and Mo(3d) peaks in the X-ray photoelectron
spectra did not shift in energy compared to the pure Co and Mo binding
energies. Finally, the Co desorption peak shape in the temperature-
programmed desorption spectra for Co on Mo(110) is characteristic of first-
order kinetics, which is the expected behavior for metal desorption from a
metal substrate. Thus, we do not observe any evidence for Co-Mo alloying,
which is consistent with the results reported by Goodman et al.5
(34) The heating was not exactly linear but was highly reproducible. It
was 7.8-13.1 K/s for temperatures between 250 and 500 K.
(35) Isobutane is ruled out as a product of 2-propanol reaction on Co-
covered Mo(110) based on studies of (CD3)2CHOH on 1 ML of Co. There
was no 49 amu signal, which is the major cracking fragment of isobutane-
d6 in the reaction of (CD3)2CHOH. Similarly, no 50 amu signal above 200
K was observed in the reaction of (CD3)2CDOD, which would result from
isobutane-d7.
The lack of a significant synergistic effect in the reactions of
2-propanol on Co-covered Mo(110) is different from previous
work on other bimetallic systems, but similar to our previous
study of methanethiol on the same Co overlayers.15 Although
X-ray photoelectron studies of bimetallic surfaces indicate that
the overlayer metal is electronically perturbed,67 2-propanol
reaction on Co-covered Mo(110) is apparently not sensitive to
these electronic effects. Theoretical modeling is necessary to
fully address this point by specifically determining the types of
orbital interactions that are important in bonding and reactivity
of 2-propanol. The chemistry of 2-propanol is not sensitive to
the lattice spacing of the Co overlayer. Previous studies of other
metal overlayers deposited on metal substrates have shown that
molecules like CO, H2, NO, and ethylene have desorption
characteristics on the monolayer films that are different than
on the bulk metals.4,6-10,68,69 It is interesting to note that the
molecules for which a sensitivity to surface structure has been
reported have π bonds. These results suggest that the degree
of chemical sensitivity to the surface geometric structure depends
strongly on the reactant bound to the surface. However, the
degree of synergy might also depend on the nature of the two
metals involved as well as on the geometric structure of the
surface. Recent studies of methanethiol on Ni overlayers
supported on W(100) indicate a high degree of sensitivity to
the geometric structure.70 Hence, more open faces of the
supporting metal may lead to a larger perturbation in the
chemical properties.
Conclusions
2-Propanol reaction on the Co overlayers occurs through a
2-propoxide intermediate. Acetone and CO are evolved from
reaction of 2-propanol on Co, while propene and propane are
evolved from reaction on Mo. The â2 propene peak in the
temperature-programmed reaction data may be from a mixed
Co-Mo site. Acetone is formed via selective â C-H bond
scission, and decomposition to CO is a competing process.
Aggregation of the Co overlayer is not induced by 2-propoxide
below 500 K and therefore should not influence the chemistry
of 2-propanol.
(36) The mass spectrometer signals were corrected for mass fragmenta-
tion of the various other products by using measured ion ratios. Acetone is
the only source of 58 amu signal, and its contribution to the 43 amu signal
is derived from the measured 43:58 amu of 4.44. The resulting peak area
for 43 amu was entirely accounted for by propane (in the Mo(110) case).
The 41 amu peak, which was used to monitor propene, was then corrected
for the contribution from acetone (41:58 amu ) 0.18) and from propane
(41:43 amu ) 0.66). The 29 amu signal was corrected for propene
fragmentation (29:41 amu ) 0.01) and acetone fragmentation (29:58 amu
) 0.20). The 28 amu signal was used to monitor CO production, after
subtracting the rising base line due to background CO. Corrections for
propane (28:29 amu ) 0.68), propene (28:41 amu ) 0.025), and acetone
(28:58 amu ) 0.76) were subsequently made.
Acknowledgment. We gratefully acknowledge the support
of this work by the U.S. Department of Energy, Office of Basic
Energy Sciences, Grant DE-FG02-84-ER13289.
(37) Bridge, M. E.; Comrie, C. M.; Lambert, R. M. Surf. Sci. 1995, 67,
393-404.
References and Notes
(38) The reaction of (CD3)2CHOH does not yield propene-d6. The major
product is probably propene-d5, but it is difficult to determine whether any
other isotopes of propene are present because acetone-d6 has a major
cracking fragment at 46 amu, and the exact cracking pattern of propene-d5
is not known.
(39) No propene-d1 was produced since the product 41:42 amu ratio
was 1.5, which is the same as an authentic sample of propene in our
chamber. Also, the 43:58 amu ratio was 5.5, which is correct for an authentic
sample of acetone. Note that deuterium incorporation may not occur simply
because deuterium desorbs around 300 K, while propene is not formed until
450 K.
(40) Gelius, U.; Heden, P. F.; Hedman, J.; Lindberg, B. J.; Manne, R.;
Nordberg, R.; Nordling, C.; Siegbahn, K. Phys. Scr. 1970, 2, 70-80.
(41) The O(1s) region was difficult to integrate reliably due to a rising
baseline at ∼535 eV from the Co L3M23M45 Auger transition at 538 eV.
(42) This calculation assumes that there are no significant changes in
the morphology of the Co overlayer occur at 250 K.
(1) Sachtler, W. M. H.; Ichikawa, M. J. Phys. Chem. 1986, 90, 4752-
4758.
(2) Brown, N. F.; Barteau, M. A. Langmuir 1992, 8, 862-869.
(3) Harris, S.; Chianelli, R. R. J. Catal. 1986, 98, 17-31.
(4) Neiman, D. L.; Koel, B. E. Mater. Res. Soc. Symp. Proc. 1987,
83, 143-153.
(5) He, J.; Goodman, D. W. Surf. Sci. 1991, 245, 29-40.
(6) Johnson, B. G.; Berlowitz, P. J.; Goodman, D. W. Surf. Sci. 1989,
217, 13-37.
(7) Heitzinger, J. M.; Gebhard, S. C.; Koel, B. E. Surf. Sci. 1992, 275,
209-222.
(8) Berlowitz, P. J.; Goodman, D. W. Langmuir 1988, 4, 1091-1095.
(9) Heitzinger, J. M.; Gebhard, S. C.; Koel, B. E. Chem. Phys. Lett.
1992, 200, 65-70.
(10) Heitzinger, J. M.; Gebhard, S. C.; Koel, B. E. J. Phys. Chem. 1993,
97, 5327-5332.
(11) Tikhov, M.; Bauer, E. Surf. Sci. 1990, 232, 73-91.
(12) He, J.; Kuhn, W. K.; Goodman, D. W. J. Am. Chem. Soc. 1991,
113, 6416-6419.
(43) The 2-propoxide coverage calculated here is in agreement with
previous work.16
(44) Queeney, K. T.; Arumainayagam, C. R.; Weldon, M. K.; Friend,
C. M.; Blumberg, M. Q. J. Am. Chem. Soc. 1996, 118, 3896-3904.

Reactions of 2-Propanol on Co-Covered Mo(110)
J. Phys. Chem., Vol. 100, No. 44, 1996 17647
(45) Detailed assignments of the modes of 2-propoxide require theoretical
modeling due to intramolecular coupling, which changes upon perturbation
of one or more modes.16,20 Specifically, the extensive coupling between
the ν(C-O) and other modes such as the δ(CH3) and νa(C-C) stretch is
different for 2-propoxide-d0 and 2-propanol-d0 because of differences in
their C-O force constants. Isotopic substitution also alters the degree of
coupling by shifting the vibrational energies. Both ab initio calculations
and experimental studies of isotopic forms of 2-propoxide on Mo(110) show
that there is less intramolecular coupling in (CD3)(CH3)CHO- than in
(CH3)2CHO-, (CD3)CHO-, or (CD3)2CDO- on Mo(110).16,71
(46) Note that the C-O stretching frequency for CO depends strongly
on CO coverage.
(55) Davis, J. L.; Barteau, M. A. Surf. Sci. 1988, 197, 123-152.
(56) Davis, J. L.; Barteau, M. A. Surf. Sci. 1990, 235, 235-248.
(57) Bryndza, H. E.; Calabrese, J. C.; Marsi, M.; Roe, D. C.; Tam, W.;
Bercaw, J. E. J. Am. Chem. Soc. 1986, 108, 4805-4813.
(58) van Asselt, A.; Burger, B. J.; Gibson, V. C.; Bercaw, J. E. J. Am.
Chem. Soc. 1986, 108, 5347-5349.
(59) Chisholm, M. H.; Tatz, R. J. Organometallics 1986, 5, 1590-1598.
(60) Chisholm, M. H.; Eichhorn, B. W.; Folting, K.; Huffman, J. C.;
Tatz, R. J. Organometallics 1986, 5, 1599-1606.
(61) Papp, H. Surf. Sci. 1985, 149, 460-470.
(62) Prior, K. A.; Schwaha, K.; Lambert, R. M. Surf. Sci. 1978, 77,
193-208.
(63) Jagannathan, K.; Srinivasan, A.; Hedge, M. S.; Rao, C. N. R. Surf.
Sci. 1980, 99, 309-319.
(47) The C-O stretch for 2-propoxide-d0 on Mo(110) shifts from 919
cm-1 at saturation coverage to 903 cm-1 at 75% saturation coverage;
likewise, the C-O stretch for (CD3)2(CH3)2CHO- shifts from 894 to 887
cm-1
.
(64) Bridge, M. E.; Comrie, C. M.; Lambert, R. M. J. Catal. 1979, 58,
28-33.
(48) For 2-propoxide-d0, the 1107 cm-1 peak, which also has significant
intensity, shifts to 1098 cm-1 for a subsaturation exposure on 1 ML of Co.
However, the frequencies of the other less intense modes do not change
with coverage. This is expected since the extent of dipole-dipole coupling
scales with the intensity of the dipole moment.
(65) Papp, H. Surf. Sci. 1983, 129, 205-218.
(66) Hydrocarbon formation accounts for ∼67% of the total reaction,17
and we are assuming that the selectivity for propene evolution on the mixed
Co-Mo sites is not significantly lower.
(67) Rodriguez, J. A.; Goodman, D. W. Science 1992, 257, 897-903.
(68) Koel, B. E.; Smith, R. J.; Berlowitz, P. J. Surf. Sci. 1990, 231,
325-332.
(69) Berlowitz, P. J.; Goodman, D. W. Surf. Sci. 1987, 187, 463-480.
(70) Mullins, D. R. Private communication, 1996.
(71) Uvdal, P.; MacKerell Jr., A. D.; Wiegand, B. C.; Friend, C. M.
Phys. ReV. B 1995, 51, 7844-7848.
(49) Sexton, B. A.; Rendulic, K. D.; Hughes, A. E. Surf. Sci. 1995, 121,
181-198.
(50) Bol, C. W. J. Partial Oxidation of Hydrocarbons on Rh(111). Ph.D.
Thesis, Harvard University; 1995.
(51) Brainard, R. L.; Madix, R. J. Surf. Sci. 1989, 214, 396-406.
(52) Bowker, M.; Madix, R. J. Surf. Sci. 1982, 116, 549-572.
(53) Bowker, M.; Madix, R. J. Surf. Sci. 1980, 95, 190-206.
(54) Brainard, R. L.; Peterson, C. G.; Madix, R. J. J. Am. Chem. Soc.
1989, 111, 4553-4561.
JP9621989