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Surface Science 261 (1992) 164-170
North-Holland
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science
CO adsorption on ultrathin MgO films grown on a Mo( 100) surface:
an IRAS study
Jian-Wei He, Cesar A. Estrada, Jason S. Corneille, Ming-Cheng Wu and D. Wayne Goodman
*
Department of Chemistry, Texas A&M Unicersity, College Station, TX 77843-3255, USA
Received 10 June 1991; accepted for publication 23 August 1991
CO adsorption on MgO ultrathin films grown on a Mo(100) surface is studied using infrared reflection absorption spectroscopy
GRAS). CO adsorbed on 7 ML of MgO shows a stretch frequency at 2178 cm-’ which is blue-shifted relative to that of gas-phase
CO (2143 cm-‘). This blue-shift is believed to arise from electron charge donation from the CO% orbitals to the MgO surface.
The heat of adsorption of CO on 7 ML of MgO is estimated to be 9.9 kcal/mol using an isothermal adsorption method. CO
adsorbed on the MgO thin films desorbs between 100 and 180 K, as indicated by temperature-programmed
desorption.
1. Introduction
Due to the importance of magnesium oxide
(MgO) in heterogeneous catalysis [1,2], particularly in the oxidative coupling of methane [3], the
interaction of CO with MgO surfaces has received considerable attention both experimentally
and theoretically
[4-61. In addition,
since
chemisorption and reaction of CO on oxide surfaces occur with the participation of oxygen, surface hydroxyl groups and coordinatively unsaturated surface cations, CO has been extensively
used as a probe molecule to elucidate the nature
of active sites on oxide surfaces (ref. [21 and
references therein).
In an early study using infrared spectros~py,
Guglielminotti et al. [4] reported that CO, adsorbed on MgO powder at room temperature,
yielded several absorption bands in the 1000-2200
cm-’ range. These bands have been assigned to
correspond to CO molecules weakly bound to
cationic sites and to negatively charged polymeric
CO clusters. Plater0 et al. [5] reported that CO
adsorbed onto polycrystalline MgO at 77 K ex-
* To whom correspondence
~39-6028/92/$05.00
should be addressed.
hibits IR peaks around 2150 cm-i. In addition,
these CO adsorption states show a red-shift in
the CO stretch frequency as the CO coverage is
increased. Using an ab initio theoretical method,
Colbourn and Mackrodt f6] found that CO binds
to a defect-free Mg~lOO) surface only at or near
a Mg*+ ion with a binding energy of _ 9
kcal/mol. The preferable configuration is with
the CO perpendicular to the surface with the C
atom attached to a Mg2+ ion. This calculation
further predicted that the charge transfer between CO and the surface is small (O.O07e-I, and
involves transfer of electrons from the CO.%
orbitals to the oxide surface. An improved calculation by Pope et al. [61 computes the binding
energy of CO to Mg2+ to be 8.6 kcal/mol for
binding through C (Mg2+-CO) and IO.6 kcal/mol
for binding through oxygen (Mg’+-00.
Recently, we have studied CO adsorption onto
MgO ultrathin films grown on a Mo(100) substrate using infrared reflection absorption spectroscopy (IRAS). The results are presented and
discussed in this paper. Compared with a bulk
single-crystal oxide, the study of thin oxide films
grown on a metal substrate offers several advantages. For example, (a) it enables one to circumvent surface charging in the use of electron spec-
0 1992 - Elsevier Science Publishers B.V. Ail rights reserved
troscopies to characterize the thin film oxides,
and (b) the properties of the thin film oxides,
such as the density of surface defects and the size
of clusters, can be tailored by varying the film
thickness, the oxidation conditions, etc.
2. Experimental
The experiments were performed in an ultrahigh vacuum chamber, described previously 171,
equipped with infrared spectroscopy, Auger electron spectroscopy (AES), and low-energy electron
diffraction (LEED). The sample was spot-welded
to two Ta wires on the backface of the crystal
which allowed resistive heating of the sample to
1500 K and cooling to 80 K. In addition, the
sample could be heated to 2300 K via e--beam
heating. The Mo(100) sample was cleaned using
cycles of oxidation in oxygen (2 X 10s7 Torr, and
a sample temperature, T,, of 1600 K) and annealing in vacuum CT, = 18~-20~
Kj. After this
procedure, AES indicated a clean surface, with
C, 0 and S impurities less than 1 at%, and
7 MLMgO/MO(iOO)
I
VA
1
/
MO
Ms
10
130
KINETIC
250
370
ENERGY
490
610
(elf)
Fig. 1. Auger electron spectra of clean, covered with 0.9 ML
of Mg, and covered with 7 ML of MgO Mo(100) surfaces.
LEED exhibited a sharp (1 x 1) substrate pattern. The infrared spectra were obtained in the
single-reflection mode at an 85” incident angle
with 4 cm-’ resolution. The spectra shown are
raw data, corrected only for the baseline.
High-purity Mg ribbon wrapped around a
tungsten filament was used for the Mg deposition. Before each dose, the Mg source was extensively degassed. As illustrated in the AES spectra
in fig. 1, no impurities accumulated on the surface during the Mg deposition and oxidation.
3. Results and discussion
3.1. Preparation of MgO thin films
The MgO thin films were prepared using two
methods: In the first method, Mg was deposited
onto the Mo(100) surface in vacuum at a sample
temperature of 90 K. The Mg coverage was then
determined using the established relationship of
the Mg(44 eV)/M~l86
eV) AES ratio versus Mg
coverage [81. Next, the Mg/Mo(l~)
surface was
oxidized at 400 K in 1 X lo-’ Torr of 0, for 15
min. In the second method, Mg was deposited in
an 0, background of 1 x lo-’ Torr at 400 K.
The deposition rate was approximately 1 ML per
minute (1 ML = 1.0 X 1015atoms/cm2, the atomic
density of the Mo(100) surface). The MgO films
synthesized with these two procedures displayed
identical properties regarding CO adsorption.
Fig. 1 shows the AES spectra of a clean
Mo~lOO) surface, 0.9 ML of Mg and 7 ML of
oxidized Mg on a Mo(l~) surface. These spectra
indicate that no detectable carbon is present on
any of these surfaces. Metallic Mg and Mg2” in
magnesium oxide are characterized by AES peaks
at 44 eV (MgL,,VV)
and 32 eV (MgL,,,OL,,,
OL,,), respectively [9,10]. The films prepared
using these procedures have also been characterized by low-energy electron diffraction (LEED),
electron energy loss spectroscopy (ELS), and
high-resolution electron energy loss spectroscopy
(HREELS) IS]. The detailed results regarding
synthesis and characterization of ultrathin MgO
films on MO (100) are presented elsewhere [8]. It
has been shown that the HREELS spectra of the
J.-W He et ccl. / CO adsorption on
166
MgOfilms an Mo(lOO)
magnesium oxide thin films are nearly identical
to those of single-c~stal Mg~lOO). The MgO
films exhibit a one-to-one stoichiomet~ and exhibit the characteristic electronic properties of
bulk MgO. LEED studies also have shown that
the MgO films grow epitaxially with the (100)
face parallel to the Mo(100) surface. The growth
mode (Frank-van
de Merwe or StranskiKrastanov) and the morphology of the MgO films
are not well understood at present. However,
studies regarding the morphology and microstructure of the MgO/Mo(lOO) system are currently
underway in this laboratory.
.
.
-.
3.2. CO IR spectra at w6m.s CO insures
Fig. 2 presents the IR spectra of CO adsorbed
onto 7 ML of MgO supported on a MO (100)
surface as a function of CO exposure. CO adsorption and IR spectral collection were carried
out at a sample temperature of 90 K. Fig. 3 shows
the peak frequency and integrated intensity versus the logarithm of the CO exposure. Two points
in figs. 2 and 3 are noteworthy:
ho.02%
I h
CO/MgO/Mo(lOO)
6~~0
=7ML
1
2170
CO EXPOSURE
6L
2200
2100
FREQUENCY
2000
(cm-‘)
Fig. 2. IR spectra of CO on 7 ML of MgO supported on a
Mo(100) surface at the indicated CO exposures. The sample
temperature during the CO exposure and IR spectral coliection was 90 K.
L_
0
i
1
LOG [EXF&“RE
(L;]
Fig. 3. The frequency and integrated intensity of the CO IR
peak in fig. 2 as a function of log(C0 exposure).
(1) The CO adsorbed onto the MgO films
shows a stretch frequency at 2178 cm-’ which is
35 cm-’ higher than that of gas-phase CO (2143
cm-‘). This blue-shift in the CO stretch frequency implies that the bonding between CO and
the MgO surface arises mainly from electron donation from the CO5u orbitals to the MgO.
Upon CO adsorption onto a solid surface, electrons in the C05a orbitals are donated to the
substrate, and electrons from the substrate backdonated into the CO 2~* orbitals. On a metal
surface, the electron charge transfer primarily
consists of the backdonation. Because the 2~r*
orbitals of CO are antibonding, this backdonation
causes a weakening of the C-O bond, and, in
turn, leads to a red-shift in the CO stretch frequency. Upon CO adsorption onto a Mg2+ cation
site on a MgO surface, the charge transfer has
been predicted to be from the CO5a orbitals to
the Mg2+, rather than from the MgO surface to
CO [6]. This 5a donation consequently strengthens the C-O bond, due to a stabilization of the
.I.- W. He et al. / CO ahorption
CO molecular orbitals that arises from the formation of a slightly positively charged CO molecule
(CO”). Theoretical calculations have shown that
the CO 5a orbital consists primarily of a lone pair
of electrons on the carbon atom and essentially is
nonbonding in character ill]. The removal of
electron charge from the 5a orbital increases the
electron affinity of the C atom, consequently stabilizing the CO molecular orbitals, and thereby
strengthening the C-O bond [ill. It is, therefore,
concluded that the blue-shift in the stretch frequency of CO on MgO arises primarily from the
C05a donation, the essential component of the
CO-MgO bond. A blue-shifted CO stretch frequency has been also reported for CO adsorbed
onto other oxides such as TiO,, ZnO, and NiO
La.
(2) In fig. 2, as the CO coverage increases, the
peak frequency remains unchanged. CO adsorbed
onto a metal surface usually displays a blue-shift
in the stretch frequency as the CO coverage increases. The blue-shift is due to CO-CO interactions (vibrational
coupling) and competition
among CO molecules for backdonation from the
surface (electrostatic effect) [12,131. On a transition metal surface, both effects lead to a blue-shift
of the CO stretch frequency. On an oxide surface
(NiO), however, it has been demonstrated that
the dipole-dipole
coupling blue-shifts the CO
frequency (27 cm-‘), whereas the electrostatic
effect red-shifts the CO frequency (43 cm-‘) [5].
Therefore, the invariant frequency in fig. 2 (for
low CO surface coverages) may arise because of
the compensation between a blue-shift CO due to
dipole-dipole coupling and a red-shift CO due to
electrostatic effects. For CO on a transition metal
surface, as discussed above, electron charge
transfers from the metal to the C02~* orbitals.
As the CO coverage increases, more CO
molecules compete for the electron charge. Consequently, less charge is transferred to each CO
molecule, and the CO stretch frequency blueshifts accordingly. However, for the CO-MgO
system, an increase in the CO surface coverage
increases the number of electron donors, and
thus decreases the overall electron charge transferred from each CO molecule to the substrate.
As a result, the CO stretch frequency red-shifts
on MgOfilms
167
on Mo(100)
t&,o
=7ML
2170
CO PRESSURE
FREQUENCY
Fig. 4. IR spectra of CO on 7 ML of
Mo(100) surface as a function of the
sure. The sample temperature during
spectral collection was
(cm-‘)
MgO supported on a
background CO presCO exposure and IR
90 K.
and approaches that of gas-phase CO. This redshift is clearly evident in the isothermal adsorption experiment, as shown in the following section.
3.3. Isothermal adsorption of CO
In order to estimate the adsorption energy of
CO on the MgO thin films, isothermal adsorption
measurements were carried out. Fig. 4 shows the
IR spectra of CO on 7 ML of MgO in background CO at the indicated pressure. The sample
temperature during the spectral collection was 90
K. In fig. 4, as the CO pressure is increased, the
CO IR intensity increases continuously, and the
peak frequency red-shifts slightly. This experiment clearly indicates that only a portion of the
MgO surface sites are populated with CO by a
direct CO exposure at 90 K. The remaining adsorption sites apparently have a smaller activation
energy for desorption and can only be populated
either in background CO or at substrate temperatures < 90 K. The red-shift of the CO frequency
(6 cm-‘) in fig. 4 is d ue to an electrostatic effect,
i.e., an increase in the CO surface coverage leads
J.-W.. He et al. / CO adsorption on MgO films on Mo(lO0)
168
0.0 -
’
-5.0
This value agrees well with the theoretical one of
9.0 kcal/mol for CO on a MgO(100) surface 161,
and the desorption activation energy of 10.6
kcal/mol estimated for CO on MgO thin films
from temperature-programmed
desorption measurements (see next section).
0
94K
c
98K
”
102K
3.4. CO temperature-programmed
I
-6.0
-7.0
-8.0
LOG [CO PRESSURE
(TORR)]
Fig. 5. Frequency
and integrated
intensities of the IR peak of
CO on 7 ML of MgO supported
on a MoflOO) surface as a
function of Iog(C0 pressure).
to a decrease in the donated charge from each
CO molecule to the MgO substrate.
Isothermal adsorptions were conducted at several temperatures, and the integrated IR intensities acquired as a function of CO exposure (fig.
5). Clausius-Clapyron plots of the CO coverages
as indicated by the integrated IR intensities versus l/T are shown in fig. 6. The slopes correspond to a CO heat of adsorption of 9.9 kcal/mol.
r
z
INTEGRATED
-3 1
5
g
2
-5 1
.
w
g
-6.
.
B
o
.,I
desorption
Fig. 7 shows the temperature
programmed
desorption spectra of CO from N 10 ML of MgO
on a Mo(100) surface following CO exposure at
the indicated levels at 100 K; the heating ramp
rate was 7 K/s. The TPD spectra exhibit a desorption feature with a peak temperature of 180
K. This peak temperature remains constant with
an increase in CO coverage, indicating first-order
desorption kinetics. This peak clearly does not
saturate at CO exposures as high as 50 L, consistent with the IR results that showed a continuous
increase in the CO IR intensity in the 2-1000 L
CO exposure range (fig. 3). Assuming a frequency
/
I
CO/MgO/Mo(lOO)
A
g,,,go= -10
ML
INT.
.
0.005 1
.
0.004
.
0.003
:
s
-a
t
-4
-9 tLYId
0.9
100
1.0
1.1
1 /T (K-’ )x1 O-2
Fig. 6. Clausius-Clapyron
plots for CO adsorption
on 7 ML of
MgO supported
on a MO (1001 surface. The data were taken
from fig. 5.
260
420
560
TEMPERATURE
740
900
(K)
Fig. 7. Temperature-programmed
desorption
spectra of CO
on 10 ML of MgO supported
on a MotlOO) surface. The MgO
thin film was dosed with CO at 100 K: (al 0.2 L. (bl 0.4 L, (c)
1.2 L, (d) 5 L, (e) 12 L, (fl 25 L and (g) 50 L. The heating
ramp rate was 7 K/s.
169
J.-W He et al. / CO adsorption on MgO films on Mo(lO0)
factor of 1013, the desorption activation energy is
estimated to be 10.6 kcal/mol using the Redhead
approximation [14]. This value is in good agreement with the adsorption energy of 9.9 kcal/mol
obtained in the isothermal adsorption experiments described in the previous section. The integrated area of spectrum (g) in fig. 7 is equal to
that of the “(Y” peak in the TPD spectrum of CO
from a Mo(100) surface. Previous work suggests
that this “(Y” peak at saturation corresponds to
- 0.5 ML of CO [El. The spectrum (g) in fig. 7,
therefore, corresponds to approximately 0.5 ML
of CO on the MgO thin films.
I
0
1
1
2
3
4
5
eMgO
(ML)
6
7
Fig. 9. The integrated
intensity of the CO IR peak in fig. 8 as
a function of MgO coverage.
3.5. CO IR spectra at various A4gO coverages
Fig. 8 shows the IR spectra of CO adsorbed
onto MgO thin films supported on a Mo(100)
surface at several MgO coverages. The CO exposure (- 1000 L) and IR spectral collection were
carried out with the sample at 90 K. For the IR
spectrum acquired at zero coverage of MgO, the
Mo(100) surface was exposed at 400 K to lo-’
Torr of oxygen for 15 min (the same temperature
0
!,L._,. ,.i,,::,+&+&j
2200
2100
FREQUENCY
2000
(cm-‘)
Fig. 8. IR spectra of CO adsorbed
on various coverages of
MgO on Mo(100). CO exposure ( - 1000 L) and IR spectral
collection were carried out at a sample temperature
of 90 K.
and oxygen background pressure used for the
preparation of the MgO thin films). For this
oxidized Mg(100) surface, a peak at 2201 cm-i
following CO adsorption is evident. As the MgO
coverage increases above 0.9 ML, a CO stretch
frequency at 2178 cm-’ is evident and remains
essentially unchanged up to 7 ML. A plot of the
CO IR intensity as a function of MgO coverage is
presented in fig. 9. Initially, the integrated CO
intensity increases from 0 to - 1 ML. A further
increase in the MgO coverage from l-7 ML leads
to a decrease in the integrated intensity. In the
absence of significant changes in the IR crosssection of CO between a MgO coverage of 1 and
7 ML, integrated intensity should merely reflect
the overall capacity of the surface to adsorb CO.
These results then suggest that the MgO films
formed at the l-2 ML level are relatively rough
and apparently become considerably smoother as
the coverage increases above 2 ML. It is also
possible that an increase in the MgO film thickness results in an increase in the number of steps
(enhanced film roughness). CO could then adsorb
on the steps with the C-O axis parallel to the
substrate surface and thus will not have a component of the vibrational dipole in the direction
perpendicular to the surface. Consequently, the
population of CO assuming the perpendicular
bonding geometry on the MgO films is decreased
leading to the decrease in the CO IR intensity, as
observed in fig. 9.
170
J.-W. He et al. / CO adsorption on MgO films on Mo(100)
4. Summary and conclusions
(1) CO adsorbed onto 7 ML of MgO ultrathin
films grown on a Mo(100) surface shows a stretch
frequency at 2178 cm-’ which is 35 cm-’ higher
than the stretch frequency of gas-phase CO (2143
cm-i). This blue-shift in CO stretch frequency is
explained as due to the formation of the COMgO bond that predominantly consists of electron donation from the Co50
orbital to the
MgO substrate.
(2) The heat of CO adsorption on 7 ML of
MgO thin films is deduced to be 9.9 kcal/mol
utilizing an isothermal adsorption method.
(3) CO adsorbed onto 10 ML of MgO thin
films exhibits a peak at N 180 K in the temperature-programmed desorption spectra with a corresponding desorption activation energy of 10.6
kcal/mol.
(4) As the CO coverage increases, the CO
stretch frequency red-shifts and is interpreted to
arise from an electrostatic effect. An increase in
the CO coverage increases the number of electron donors, and thus decreases the charge donated from each CO molecule to the substrate.
The CO stretch frequency thereby red-shifts toward that of gas-phase CO.
(5) The IR integrated intensity of CO on the
MgO thin films decreases in the 1-7 ML MgO
coverage range, suggesting that the MgO films
become significantly smoother as the coverage is
increased above the first monolayer.
Acknowledgement
We acknowledge with pleasure the support of
this work by the Department of Energy, Office of
Basic Science, Division of Chemical Sciences and
the Gas Research Institute.
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