Physical and chemical properties of high density atomic oxygen overlayers under ultrahigh vacuum conditions: (1 x 1)-O/Rh(111)

Article abstract of DOI:10.1063/1.480890

In this paper, we elaborate on our previous communication of high coverages of oxygen on Rh( 111) [J. Chem. Phys. 110, 2757 (1999)]. When dosing with O₂, half of a monolayer of O is adsorbed. Higher coverages can be achieved when exposing the surface to O atoms. As the quantity of adsorbed O increases from a half to a full monolayer, the overlayer structure undergoes several distinct phase changes. At a full monolayer, the (1 x 1)-O structure is stable at surface temperatures less than ～400 K. Continued dosing with O atoms results in the rapid migration of O into the bulk. We also report on the chemical reactivity of this densely oxygen-covered surface with CO, H₂, and propene.

Full text of DOI:10.1063/1.480890

Physical and chemical properties of high density atomic oxygen overlayers under
ultrahigh vacuum conditions: (1×1)- O/Rh(111)
K. D. Gibson, Mark Viste, Errol Sanchez, and S. J. Sibener
Citation: The Journal of Chemical Physics 112, 2470 (2000); doi: 10.1063/1.480890
View online: http://dx.doi.org/10.1063/1.480890
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 112, NUMBER 5
1 FEBRUARY 2000
Physical and chemical properties of high density atomic oxygen
overlayers under ultrahigh vacuum conditions: „1؋1…-O/Rh„111…
K. D. Gibson, Mark Viste, Errol Sanchez, and S. J. Sibener
The James Franck Institute and The Department of Chemistry, The University of Chicago,
Chicago, Illinois 60637
͑
Received 8 July 1999; accepted 26 October 1999͒
In this paper, we elaborate on our previous communication of high coverages of oxygen on Rh͑111͒
J. Chem. Phys. 110, 2757 ͑1999͔͒. When dosing with O , half of a monolayer of O is adsorbed.
͓
2
Higher coverages can be achieved when exposing the surface to O atoms. As the quantity of
adsorbed O increases from a half to a full monolayer, the overlayer structure undergoes several
distinct phase changes. At a full monolayer, the (1ϫ1)-O structure is stable at surface temperatures
less than ϳ400 K. Continued dosing with O atoms results in the rapid migration of O into the bulk.
We also report on the chemical reactivity of this densely oxygen-covered surface with CO, H , and
2
propene. © 2000 American Institute of Physics. ͓S0021-9606͑00͒70404-1͔
I. INTRODUCTION
produce O atoms. The narrow widths of the He diffraction
features indicate that the O atoms are well-ordered. Using
temperature programmed desorption ͑TPD͒ measurements,
where the temperature of the crystal is ramped while moni-
toring any reaction products evolved, it was shown that there
is twice as much O deposited on the Rh for the (1ϫ1)
Knowledge of the interaction of O with metal surfaces is
critical for understanding corrosion and catalytic oxidation.
Working under ultrahigh-vacuum ͑UHV͒ conditions allows
for the preparation of well-characterized surfaces, and af-
fords the use of many tools to investigate any surface pro-
cesses. However, this involves working at gas fluxes much
smaller than would exist under the real-world conditions of
high pressure where these reactions normally occur. This of-
ten causes kinetically slow or improbable reactions, which
may have significant effects when the reactant pressure is
several Torr or greater and the impingement rate is high, to
occur too slowly to be easily observed under UHV condi-
pattern as for the ⌰ ϭ0.5 ML (2ϫ2) structure. Recent the-
O
oretical investigations have concluded that 1 ML of adsorbed
10,11
10
O should be stable.
Walter et al. concluded that there is
a kinetic constraint to growing a full monolayer with O2
dosing because, once the coverage reaches 0.5 ML, the sites
where O dissociation is energetically favorable are blocked.
2
For surface temperatures below ϳ400 K, this overlayer
is stable for many minutes. At much higher temperatures, it
is evident from diffraction measurements that the O coverage
begins decreasing, due to some oxygen migration into the
bulk. For surface temperatures below 400 K, it is possible to
add more O with continued O dosing. Most, if not all of this
additional O is absorbed into the bulk. In this paper, we will
present detailed experiments which examine the growth, sta-
bility, and reactivity of the high-density (1ϫ1) oxygen over-
layer on the Rh͑111͒ surface.
tions, where the pressure at the surface is typically less than
Ϫ6
1
ϫ10
Torr. A possible example of this has been sug-
1
–3
gested by studies of CO oxidation on Ru͑0001͒.
Under
UHV conditions, the O coverage is less than 1.0 ML ͑mono-
layer͒, and the oxidation proceeds by a Langmuir–
Hinshelwood ͑L–H͒ mechanism. Ru exhibits the lowest ac-
tivity of the transition metals studied. However, at pressures
of several Torr and under oxidizing conditions, the rate of
CO production on Ru at 500 K becomes higher than for
2
There may be other methods for growing the dense over-
layer than using an atom source. It has been shown that using
NO , instead of O can produce higher coverages of O under
other transition metal surfaces. It has been proposed that a
full monolayer of O adsorbs on Ru͑0001͒ at these pressures,
and that the reaction then may proceed by an efficient Eley–
2
2
Rideal ͑E–R͒ mechanism.1–3 If this were the case, then the
UHV conditions on several transition metal surfaces;
Pt͑111͒,12 Pd͑111͒, and Ru͑0001͒.
13
14–16
In the case of
rate-limiting step might be the dissociation of molecular oxy-
gen on the surface. To study such high-coverage reactions
under UHV conditions, it should be possible to circumvent
the problem of low fluxes by using atomic O to overcome the
Pd͑111͒, there is evidence that at least 1 ML of O is ad-
sorbed, but it is not clear how well this overlayer is ordered.
Using a stepped Ru͑0001͒ crystal, Parrott et al.17 were able
to grow 1 ML of adsorbed O after long exposure of the
rate-limiting O dissociation step, and adsorb the higher cov-
2
Ϫ5
crystal to 10
2
Torr of O at a surface temperature Ts
erage necessary for the E–R reaction.
Much is already known about the interaction of O with
rhodium. Under UHV conditions, dosing a Rh͑111͒ surface
ϭ300 K. For Ru͑0001͒, a (1ϫ1) phase with ⌰Oϭ1.0 ML
has recently been produced under UHV conditions at Ts
ϭ600 K.16 In this paper, we will discuss our attempts to use
4
with O leads to a saturation coverage of ⌰ ϭ0.5 ML
2
O
1
5
2
5–8
(
MLϵmonolayer is 1.6ϫ10 /cm ).
we outlined how we prepared a (1ϫ1)-O/Rh͑111͒ surface.
In a previous paper,
NO to grow a monolayer of O on the Rh͑111͒ surface. An-
2
9
other possible route, which we briefly explore, is to use O2
with high translational energies. If there is an activation bar-
To do this, we used a radio-frequency nozzle beam source to
0021-9606/2000/112(5)/2470/9/$17.00
2470
© 2000 American Institute of Physics
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1

J. Chem. Phys., Vol. 112, No. 5, 1 February 2000
Atomic oxygen overlayers on Rh(111)
2471
III. RESULTS
A. Surface structures at 0.5 and 1.0 ML coverage
rier for the dissociative chemisorption of O on the half-
2
coverage (2ϫ2)-O surface, the higher translational energy
could overcome the barrier and lead to a higher coverage of
oxygen.
Figures 1 and 2 show He diffraction scans taken along
two principal symmetry directions of the surface at Ts
In our lab, the reaction of CO with adsorbed O has been
extensively studied.1
8,19
ϭ325 K. For the scans taken after O exposure, the surface
2
For these experiments, the O was
was prepared by dosing for 20 min at T ϭ325 K, and then
s
deposited by the decomposition of O , and the coverage was
2
briefly cooling to 200 K. When dosing with O, the surface
always less than 0.5 ML. We can now extend these studies to
a much higher oxygen coverage regime made possible by the
efficient deposition afforded by the atom beam. As already
mentioned, it has been suggested that the change in the re-
action kinetics for the oxidation of CO on Ru͑0001͒ under
was at T ϭ325 K when first exposed to the beam, and the
s
dosing was continued while the surface was cooled to Ts
ϭ200 K. Starting at the higher temperature prevents H ad-
sorption, and continuing as the surface cools minimizes the
amount of O absorption. For ⌰ ϭ0.5 ML, the surface is
O
high pressures of O is due to the formation of a full mono-
2
1
–3
20
covered by (2ϫ1) domains with their axes oriented along
different symmetry directions, giving a (2ϫ2) diffraction
pattern. Thus, there are half-order peaks, indicated by the
¯
layer of adsorbed O.
Stampl and Scheffler have calcu-
lated that a (1ϫ1)-O/Ru͑0001͒ overlayer should be stable,
and it has recently been observed under UHV conditions.16
͗ ͘
arrows. For the 112 direction, Fig. 1, depositing twice as
Stampfl and Scheffler also theoretically investigated the oxi-
dation of CO on the (1ϫ1)-O/Ru͑0001͒ surface.2
1,22
Their
much O clearly results in the disappearance of the half-order
peaks. The angular positions of the diffraction features are
consistent with a commensurate overlayer ͑Rh–Rh distance
¯
conclusion was that there could be some direct E–R reaction
of incoming CO molecules, but only for those having ener-
gies well in excess of 1 eV, and with a reaction rate much
lower than that experimentally observed. They speculate that
the initial E–R reaction creates some vacancies in the over-
layer, and these can be more readily occupied by CO than an
͗ ͘
of 2.69 Å͒. For the 101 direction, Fig. 2, the half-order
peaks are still present after O atom dosing, but are greatly
attenuated. This apparently small residual corrugation is con-
sistent with a slight adsorbate-induced buckling of the
Rh͑111͒ surface, a common occurrence for close-packed
metal surfaces.24 ͑There is no evidence of this in the elec-
tronic structure calculations of Ganduglia-Pirovano and
O atom from O decomposition. These CO molecules then
2
react with neighboring O atoms via a L–H mechanism, ac-
counting for most of the CO produced.
2
1
1
The behavior of Rh is different than that of Ru. Under
the high pressure, oxidizing conditions at which the Ru
Scheffler. ͒ The diffraction peaks also have distinct satellite
peaks in this direction. The position of the principle peaks
indicates a commensurate overlayer, and the position of the
satellite peaks near specular are consistent with a superlattice
structure with a repeat distance of ϳ40 Å. The near disap-
pearance of the half-order diffraction features in both prin-
ciple symmetry directions with the presence of twice as
much O, as measured using TPD, is consistent with the
growth of 1 ML after dosing with atomic O.
shows a much enhanced rate of CO production, the rate of
2
1
reaction on Rh͑111͒ is reduced. However, with the atom
beam, we can grow a full monolayer of oxygen under UHV
conditions and at low temperatures, where the (1
ϫ1)-O/Rh͑111͒ surface is stable. This temperature is below
that where the surface temperature dependent L–H reaction
rate is fast, but a direct process should be relatively insensi-
tive to the surface temperature. This situation suggests a re-
gime for examining possible E–R behavior on Rh.
The narrow diffraction peaks indicate a well-ordered
overlayer. By comparing the width of the specular reflection
with that of the instrument function, it is possible to estimate
the size of ordered domains, or coherence length, of ad-
II. EXPERIMENT
25
sorbed O. The result for several incident angles gives a
coherence length of between 100 and 200 Å. This is consis-
tent with the fact that the Rh crystal is miscut by ϳ1 ,
roughly along this azimuth, and that the surface has terraces
that are ϳ130 Å.
These experiments were performed in a three-molecular-
beam scattering machine which has been described in detail
o
1
9,23
elsewhere.
We have also previously described the forma-
9
tion of (1ϫ1)-O/Rh͑111͒ using a beam of atomic oxygen.
Accordingly, only procedures specific to this paper will be
described.
B. Adsorbed oxygen ordering at coverages between
0.5 and 1.0 ML
For investigating the reaction of high energy CO with
the O overlayer, a 1% mixture of 13CO in H was used. It
2
was possible to achieve a mean kinetic energy of 1180 meV
when expanded through a nozzle heated to 725 K. At this
temperature, disproportionation was just becoming evident,
We also investigated the surface structure as a function
of ⌰ for coverages greater than 0.5 ML when dosing with
O
the O atom beam at T ϭ325 K. With continued dosing, the
s
as shown by the CO signal when measuring the beam di-
half-order diffraction peaks present at ⌰ ϭ0.5 ML decrease
2
O
rectly. H was chosen as the carrier gas rather than inert He
in size. However, they grew in again at ⌰ ϭ0.7Ϯ0.05 ML.
2
O
because of the much higher energies achievable. Though H2
dissociatively adsorbs on the Rh͑111͒ surface, and also re-
acts with low coverages of O at elevated surface tempera-
tures, there was little interference with the experiments re-
ported here. More details will be given with the results.
We took diffraction spectra at O coverages where the half-
order intensity exhibited extrema. The results are shown in
Figs. 3 and 4. As expected, there is a (2ϫ2) pattern at ⌰O
ϭ0.5 ML and a (1ϫ1) at ⌰ ϭ1.0 ML. For intermediate
O
coverages, the narrow diffraction peaks that are present indi-
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2472
J. Chem. Phys., Vol. 112, No. 5, 1 February 2000
Gibson et al.
¯
FIG. 1. He diffraction spectra taken along the
͗
112
͘
azimuth and at three different incident angles; E ϭ20 meV and T ϭ325 K. ͑a͒, ͑b͒, and ͑c͒ are for
i
s
⌰Oϭ0.5 ML, deposited by O dosing. Arrows indicate the positions of the half-order peaks. ͑d͒, ͑e͒, and ͑f͒ are for ⌰ ϭ1 ML, deposited by O atom dosing.
2
O
cate a well-ordered surface. The diffraction scans indicate
the growth of different ordered structures for coverages be-
tween 0.5 and 1.0 ML. These phase changes are not kineti-
cally limited by the dosing rate, as the same results are seen
at different O atom fluxes. For the experiments shown here,
the O atom flux was sufficient to reach a coverage of ⌰O
ϭ0.7Ϯ0.05 ML in less than 7 min. By reducing the oxygen
backing pressure, we also dosed at a much slower rate, and it
even when the time at this temperature is less than 30 s. By
400 K, there is a definite attenuation of the diffraction peaks
after 5 min. Between 400 K and 425 K, small half-order
peaks become clear after the 5 min time period. Oxygen does
not desorb at these temperatures, implicating migration into
the bulk of the Rh crystal.
D. Oxygen absorption
took 28 min to reach ⌰ ϭ0.7Ϯ0.05 ML; the intensities of
O
The saturation coverage of O when dosing with O is 0.5
the half-order peaks showed the same progression.
2
ML. Further O can be absorbed, but only slowly at Ts
ϭ325 K. This has been measured for temperatures between
C. Stability of the „1؋1…-O/R1„111… overlayer
4
00 K and 600 K using a room temperature supersonic beam
2
6
To investigate the stability of the (1ϫ1)-O overlayer as
with a flux of 60 ML/min. These results were extrapolated
to produce the solid line in Fig. 6. As mentioned in the first
paper, we were unable to measure any additional O uptake
a function of T , we grew 1.0 ML overlayers at 325 K.
s
9
These were heated to various temperatures for 5 min and
then cooled back to 325 K and the diffraction spectrum taken
and compared with the spectrum taken before heating. The
normalized results are plotted in Fig. 5 for the specular and
at T ϭ325 K with as much as 80 min of continued O expo-
s
2
sure after completion of 0.5 ML. That elevated temperatures
are needed to adsorb O while the crystal is exposed to O is
2
1
5
st-order diffraction peaks. When the surface was heated to
25 K, half-order peaks are quite evident in the spectrum,
consistent with the work of Wider et al.27 At T ϭ325 K. O
s
does not absorb from the (1ϫ1) overlayer, unless there is
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J. Chem. Phys., Vol. 112, No. 5, 1 February 2000
Atomic oxygen overlayers on Rh(111)
2473
¯
FIG. 2. He diffraction spectra taken along the
͗
101
͘
azimuth and at three different incident angles; E ϭ20.1 meV and T ϭ325 K. ͑a͒, ͑b͒, and ͑c͒ are for
i
s
⌰Oϭ0.5 ML, deposited by O dosing. Arrows indicate the positions of the half-order peaks. ͑d͒, ͑e͒, and ͑f͒ are for ⌰ ϭ1 ML, deposited by O atom dosing.
2
O
continued exposure of the surface to O atoms. The rate of
absorption is dependent both on the surface temperature and
the O atom flux. The results for three different fluxes are
shown in Fig. 6. The surface was first dosed with O atoms at
Tsϭ325 K to produce a monolayer coverage. The surface
was then exposed for an additional period of time at the
indicated surface temperature. The amount of O in the TPD
spectra greater than 1.0 ML was then assigned to the ab-
sorbed state.
much higher surface temperature than the (1ϫ1) stability
regime for Rh͑111͒. Using a neat, room temperature NO2
beam, we did manage to grow a structure that had a (1
ϫ1) diffraction pattern and showed the equivalent of ϳ1
ML of O in the TPD spectrum after exposure of the 400 K
2
surface for ϳ10 min. The result is shown in Fig. 7, compared
with a monolayer grown at T ϭ325 K with the O atom
s
beam. Since O from the (1ϫ1) overlayer still does not mi-
grate into the bulk at a rapid rate when T ϭ400 K, the exact
s
We also have evidence that the absorption rate is not
constant; after about 0.5 ML has been absorbed, the absorp-
tion rate starts to decrease. This is similar to what Peterlinz
time is not critical; unlike with the O atom beam, the NO2
beam does not load up the bulk very fast at this temperature.
Presumably, the NO does not dissociate as well on an
2
2
6
et al. saw when using NO dosing; we did not continue the
oxygen-covered surface as on the bare Rh͑111͒. With Ts
ϭ325 K or lower, we were unable to deposit a monolayer of
2
dosing to see if it reached some asymptotic value.
O with NO even after much longer exposures.
2
E. Investigations of alternate procedures to produce
high-density O overlayers
2
. O beams with high translational energies
2
1
. NO2
With a room temperature O beam ( E Ϸ90 meV) we
͗
͘
2
It has been recently shown that NO can be used to grow
can only achieve ⌰ ϭ0.5 ML on the Rh͑111͒ surface. We
also dosed the surface using an oxygen beam with higher
2
O
a monolayer of O on Ru͑0001͒.16 This was done at 600 K, a
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2474
J. Chem. Phys., Vol. 112, No. 5, 1 February 2000
Gibson et al.
¯
͗ ͘
112 azimuth, Ei
FIG. 4. He diffraction spectra taken along the
͗
01
¯
1
͘
azimuth, Ei
FIG. 3. He diffraction spectra taken along the
ϭ19.5 meV, ␪ ϭ45°, and T ϭ325 K, after progressively longer exposures
ϭ19.6 meV, ⌰iϭ55°, and Tsϭ325 K, after progressively longer exposures
to the O atom beam at Tsϭ325 K. The tick marks below the spectrum
i
s
to the O atom beam at T ϭ325 K.
s
where ⌰ ϭ0.7 ML are the expected positions of 1/6-order diffraction
O
peaks.
translational energy. This was done by making a mixture of
1
% O in He, and expanding at high pressure through a
2
nozzle heated to 723 K. The mean energy of the O mol-
ecules in this beam was 530 meV. We dosed the surface at
2
there was a fraction of a monolayer of absorbed O. Then,
while still dosing with O atoms, the surface was also exposed
to the mechanically-chopped CO beam. The total waveform
of any scattered or product molecules could then be col-
T ϭ250 K and 300 K, and at incident angles of ␪ ϭ30°,
s
i
4
5°, and 60°. Assuming 2nd-order Langmuir adsorption
2
8
29
kinetics and an initial sticking coefficient of 0.5, the ini-
tial increase in coverage was roughly consistent with the es-
timated O flux, but in no case did we achieve oxygen cover-
ages greater than 0.5 ML.
lected. For T р400 K, we were unable to detect any CO2
s
product ͑a high mass 44 background in our detector makes
detecting small quantities difficult͒. However, some reaction
must have occurred because postexposure TPD spectra
showed that very little O was left. The extent of the O deple-
tion in any period of time was dependent on two factors; the
relative fluxes of the CO and O atoms, and the surface tem-
perature, with the lower temperatures resulting in the least O
F. Chemical reactivity of „1؋1…-O/Rh„111…
1. CO oxidation
To begin our investigation of the reaction of CO with a
monolayer of O, we used a neat beam of normal abundance
depletion. At T ϭ325 K and with a relatively intense CO
beam, we used He diffraction to investigate the surface or-
s
1
2 16
CO͑ C O͒,
͗
E
͘
Ϸ90 meV. The experiments were started
by growing a (1ϫ1)-O overlayer, dosing long enough that
dering. The (1ϫ1) pattern of the well-ordered O overlayer
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J. Chem. Phys., Vol. 112, No. 5, 1 February 2000
Atomic oxygen overlayers on Rh(111)
2475
FIG. 5. Comparison of the specular and 1st-order diffraction features for the
Rh͑111͒ surface initially covered with 1.0 ML of adsorbed O, and then
heated to the indicated temperature. IB is the intensity of the feature at Ts
ϭ325 K, immediately after growing the overlayer with the O atom beam. IA
is the intensity of the feature at T ϭ325 K, after heating the surface to the
s
¯
indicated temperature for 5 min. Spectra were taken along the
͗
112
͘
azi-
muth, ␪ ϭ45°, and E ϭ19.7 meV.
i
i
quickly disappeared once the surface was exposed to CO,
with a weaker but still narrow specular feature remaining.
When the CO beam was stopped and exposure to the O atom
beam continued, the diffraction pattern at least partially re-
covered, though with small half-order features.
¯
FIG. 7. He diffraction spectra taken along the
͗
112
͘
azimuth, Ei
ϭ20.4 meV, ␪ ϭ45°, and T ϭ200 K. The spectrum shown in ͑a͒ was taken
after exposure of the surface to an NO beam at T ϭ400 K. The spectrum
shown in ͑b͒ was taken after growing a monolayer with the O atom beam at
i
s
2
s
The conclusion is that the (1ϫ1)-O is reactive with CO,
T ϭ325 K.
s
presumably slowly producing CO , which we are unable to
2
measure above the detector background. It is important to
note that this was a room temperature beam with an average
rate directly related to both the relative O atom and CO flux,
and the surface temperature. Another problem is that CO
sticks to clean Rh͑111͒ at the surface temperature where the
translational energy of
͗ ͘
E Ϸ90 meV, without molecules
having translational energies close to 1 eV, which is what
Stampfl and Scheffler2
1,22
predict is necessary for the reac-
(
1ϫ1)-O overlayer is stable. It is possible that as oxygen is
tion on the densely oxygen-covered Ru. We were unable to
maintain the oxygen coverage under our experimental con-
ditions. One possibility was that the flux of CO was too great
relative to the rather weak O beam. We tried using a room
temperature beam of 20% CO in He. This gives a faster, and
more dilute, beam of CO. The principle findings were essen-
consumed, the vacated site is occupied by a CO molecule
which blocks the adsorption of further oxygen.
Though we were unable to adjust conditions to do the
continuous dosing experiments originally envisaged, it was
still possible to investigate the role of CO translational en-
ergy. We can measure the initial rate of reaction by measur-
ing the quantity of adsorbed oxygen after a short exposure to
CO molecules of varying translational energies. We were
also interested in detecting any small amount of reaction
product. Because of the significant detector background at
tially the same; no detected CO product, but an O depletion
2
masses 28 and 44, we used 13CO seeded with H . At the
2
surface temperatures of our experiments, the H does not
2
interfere with CO oxidation; water formation was not ob-
served, nor was there any decrease in the quantity of ad- and
absorbed oxygen after exposure to the H2.
The results for CO exposure are qualitatively summa-
rized in Fig. 8, which shows postreaction TPD spectra for
O , CO, and CO for two different beam energies, 590 and
2
2
1
180 meV, and surface temperatures, 200 and 325 K. In each
case, the surface was dosed with O at T ϭ325 K until 1.1
s
ML had been deposited. The surface temperature was then
changed to that indicated, and the surface exposed for a fur-
FIG. 6. The measured uptake rate of O for a surface with an initial mono-
layer coverage, as a function of surface temperature and O atom flux. Ex-
perimental data is shown with symbols, and are labeled with the O flux in
ther 30 min to the O and CO/H beams. There was some
2
centerline enrichment of the heavier gas so the beams were
ϳ3%–4% CO. The flux of CO at the crystal was ϳ0.02
ML/s for the room temperature beam and ϳ0.01 ML/s for
ML/s. The solid line is for the adsorption of O using O dosing, extrapolated
2
from the results of Peterlinz ͑Ref. 26͒.
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J. Chem. Phys., Vol. 112, No. 5, 1 February 2000
Gibson et al.
reactivity at both translational energies increases with sur-
face temperature; the rate of an E–R reaction should be
rather insensitive to the surface temperature. There is no evi-
dence of a kinetic barrier; the reaction rate is roughly similar
for the two very different translational energies. As men-
tioned earlier, even a CO beam with an average kinetic en-
ergy of 90 meV reacted readily. For surface temperatures
below 400 K, some CO can stick to the Rh͑111͒ surface.3
That we see clear evidence for adsorbed CO at the surface
temperatures studied is confirmation of the previously men-
tioned surmise that CO was blocking sites for oxygen ad-
sorption, thus preventing us from doing continuous dosing
0,31
experiments. On the clean Rh͑111͒ surface, CO does not
2
stick except at cryogenic temperatures. Its presence in the
postreaction TPD spectra at 350–400 K is likely due to its
formation while the surface temperature is ramped. One ex-
planation for the presence of CO and CO peaks in the TPD
2
spectra is that small islands of CO are created within the O
overlayer; the CO is produced at the edges, where CO and
2
O are in close proximity, and the CO evolves from the center
of these islands. We have also seen that the O TPD spectra
2
are altered after reaction, so there is possibly some further
change in the O overlayer. The question we cannot yet an-
swer is how the initial CO is deposited. One possibility is
that some CO is trapped in a shallow adsorption well on top
of the O overlayer, from which it could then react. However,
one would expect that this mechanism would favor an in-
creased reaction rate at lower surface temperatures. Another
possibility is that the initial CO could be adsorbed at defects
in the overlayer. Of course, there is also the explanation
mentioned in the Introduction, where the initial holes in the
O overlayer are created by high translational energy (E
Ͼ1 eV) CO molecules via an E–R mechanism. Our results
are that there is not an appreciable barrier, and the initial
reaction is very sensitive to surface temperature, indicative
of a L–H mechanism. Once some of the O in the overlayer is
removed, CO can be adsorbed and block the further adsorp-
tion of O. Since CO bonds carbon end down, it would not be
surprising that it would be unreactive with incoming O at-
oms.
FIG. 8. Postreaction TPD spectra for O ͑a͒, CO ͑b͒, and CO ͑c͒ taken after
2
2
13 16
exposing the (1ϫ1)-O overlayer simultaneously to O-atoms and C O for
0 min. Curves are labeled by the mean translational energy of the incident
3
CO, and the surface temperature at which the exposure took place.
the higher energy beam. The O atom flux was ϳ0.05 ML/s.
When the relative fluxes are factored in, the apparent
reaction rate is the same for both translational energies at
Tsϭ325 K. The most O is left on the surface when Ts
ϭ200 K with the results for the two beam energies being
indistinguishable. For both surface temperatures some ad-
sorbed CO is observed, but only the 325 K surface exposed
2
. H oxidation
2
to the room temperature beam evolved much CO while the
For the reaction with H , O was adsorbed on the surface
2
2
surface temperature was ramped. In no case did we observe
at T ϭ325 K, and then the sample was rapidly heated to 575
s
any CO product while the surfaces were exposed to the CO
K, where the rate of water formation is rapid. When ⌰O
2
beam.
Ͻ0.5 ML, exposure of the surface to H leads to the imme-
2
A crude estimate of the upper bound for the reaction
probability under the most reactive conditions was estimated
using experiments as outlined before, where the quantity of
diate evolution of H O, as observed in the mass spectrom-
2
eter. At ⌰ ϭ0.5 ML, the water evolution is not as rapid;
O
there is an induction period wherein the rate of H O produc-
2
O in the TPD spectra taken after exposure of the sample to
2
tion builds to a peak, before rapidly falling off as the O is
O atoms with and without CO exposure was compared. As-
suming that the difference is due solely to the OϩCO reac-
tion, the probability for CO reacting with a preadsorbed
overlayer of (1ϫ1) oxygen is on the order of 2%. We also
depleted. When ⌰ Ͼ0.5 ML, this induction period becomes
progressively longer, and there is less signal at the position
O
of the maximum desorption rate. When ⌰ Ϸ1 ML, the H O
O
2
evolution is so slow that it is difficult to see the signal above
the background in the mass spectrometer. This behavior is
reasonable in all cases if the reaction proceeds by a L–H
mechanism, where the first step is the dissociative adsorption
did an experiment at T ϭ400 K with 1180 meV CO. The
s
only difference between this experiment and those shown in
Fig. 8 was that there was no CO in the postreaction TPD.
2
These results argue against a direct ͑E–R͒ reaction and
of H , and the dissociative sticking coefficient is much
2
more for a L–H reaction between coadsorbed reactants. The
higher on the bare Rh surface than on an O overlayer. For
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J. Chem. Phys., Vol. 112, No. 5, 1 February 2000
Atomic oxygen overlayers on Rh(111)
2477
The diffraction measurements do not show any evidence of a
compressed phase, where the surface coverage of O is
greater than 1.0 ML. However, since the absorption is depen-
dent on continued dosing, a reasonable supposition is that
extra O atoms are transiently incorporated into the (1ϫ1)
overlayer, thus changing the energetics to favor the migra-
tion of O into the bulk. Theoretical calculations of
Ganduglia-Pirovano and Scheffler11 predict that O will be
readily incorporated into the bulk only when the (1ϫ1)-O
overlayer is essentially complete.
We also found that it was possible to grow the (1ϫ1)
overlayer with NO but, unlike O atom exposure, only at an
2
elevated surface temperature. The growth is not as fast as
when using O atoms, and leads to a much lower rate of
subsurface O deposition. This is probably due a requirement
FIG. 9. Comparison of the evolution of the acetone signal produced after the
exposure of the surface with ⌰ ϭ0.5 and 1.0 ML to propene. The arrow
O
that NO decomposition take place on the Rh surface, and
indicates the time that propene exposure was begun.
2
that all of the sites are effectively blocked as the O coverage
reaches 1.0 ML. We did not succeed in growing a dense
oxygen overlayer using high translational energy O2 mol-
ecules.
low coverages of O, there are many bare sites on the Rh
surface for the H to dissociatively adsorb before reacting.
2
As the quantity of adsorbed O increases, the number of
empty Rh sites decreases, and the reactivity is low, but
builds as the number of empty Rh sites increases. When the
O coverage is high, there is still some reactivity. At this
surface temperature, the absorption rate for adsorbed O is
becoming appreciable, and this can also open up surface
sites.
At surface temperatures where it is stable, the (1
ϫ1)-O/Rh͑111͒ surface is not inert to CO. The reaction
takes place much more rapidly at higher surface tempera-
tures, indicative of primarily a L–H reaction between ad-
sorbed reactants, rather than the direct E–R mechanism. Us-
ing CO beams of varying translational energy, we also found
no evidence of a kinetic barrier to the reaction. It would be
interesting to perform these experiments with Ru, on which
the (1ϫ1)-O overlayer is apparently stable at much higher
surface temperatures. If experiments could be done at tem-
peratures where CO does not coadsorb, it would be possible
to definitively measure both the kinetics and reaction dynam-
3. Partial oxidation of propene
When a Rh͑111͒ surface has 0.5 ML of adsorbed O, the
5
surface produces acetone from propene. These temperature
programmed reaction experiments were done by heating the
coadsorbed reactants, starting from a low surface tempera-
ture. The experiments shown in Fig. 9 were done in a differ-
ent manner, by first adsorbing the O, and then exposing the
ics CO production.
2
The densely oxygen covered surface modifies the rates at
which other surface reactions take place. When exposed to
H , low coverages of oxygen readily produce water, but on
2
3
25 K surface to a propene beam. When ⌰ ϭ0.5 ML, ac-
O
the densely oxygen-covered surface, there is no apparent re-
action. This is likely due to the necessity of having bare Rh
etone is promptly evolved. When ⌰ ϭ1.0 ML, there is a
O
long induction period, though from the area under the curve,
approximately twice as much acetone is made. From the
sites for the dissociative chemisorption of the H . When a
2
half monolayer of adsorbed oxygen is exposed to a propene
beam, there is nearly instantaneous production of acetone. A
full monolayer of oxygen also produces acetone, but only
after a long induction period. Again, for the maximum reac-
tion rate, some bare Rh sites are apparently necessary.
5
model of Xu and Friend, the reaction requires that the me-
thylene carbon of the propene be bonded to the Rh, while the
2-carbon reacts with the O. The very gradually occurring
increase in the reaction rate for ⌰ ϭ1 ML could be due to
O
the initially very low concentration of exposed surface Rh
sites, which rapidly increase as the reaction proceeds.
ACKNOWLEDGMENTS
This work was supported by the Air Force Office of
Scientific Research and, in part, by the NSF-MRSEC Pro-
gram at The University of Chicago Award Nos. DMR-
IV. DISCUSSION AND CONCLUSIONS
In this paper, we have investigated the growth and sta-
bility of the (1ϫ1)-O/Rh͑111͒ overlayer. When using O to
2
9
400379 and DMR-9808595.
dose, only a (2ϫ2) structure, with ⌰ ϭ0.5 ML, can be pro-
O
duced. At T ϭ325 K, O atom dosing rapidly produces the
s
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