:
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ELSEVIER
2 3 M a y 1997
CHEMICAL
PHYSICS
LETTERS
Chemical Physics Letters 270 (1997) 345-350
02 dissociation on Ag(001)" the role of kink sites
F. Buatier de Mongeot, A. Cupolillo, U. Valbusa, M. Rocca
Centro di Fisica deUe Superfici e deUe Basse Temperature del C,N.R and INFM,
Dipartimento di Fisica, via Dodecaneso 33, 16146 Genova, Italy
Received 14 October 1996; in final form 13 March 1997
Abstract
The dynamics of the dissociative adsorption of 02 on Ag(001 ) were investigated with a supersonic molecular beam source
and electron energy loss spectroscopy versus surface temperature. Contrary to the case of Ag(110) where dissociation occurs
at the atomic terraces and has a high probability, for Ag(001 ) we find that only 0.44% of the adsorbed molecules dissociate
at room temperature. An Arrhenius analysis indicates that the process is thermally activated and the activation energy
coincides with the energy for generating kinks, which are thus identified as the active sites. The interplay between a local
geometry similar to a (110) site and enhanced charge transfer to the antibonding molecular orbitals is responsible for the
pronounced reactivity of such sites. Molecules adsorbed at (001) terrace sites instead desorb with a high probability. ©
1997 Published by Elsevier Science B.V.
"Why and how do molecules break at surfaces?"
This apparently simple problem has kept surface scientists busy for decades and lies at the heart of much
of the chemical industry [ 1 ]. This question is particularly intriguing for the case of dissociative oxygen adsorption on Ag surfaces, a reaction which is at the basis
of the epoxidation process of ethylene for which silver catalysts show a unique selectivity. The reactivity
of Ag with 02 is known to be strongly face dependent
[2-6], with the sticking coefficient, so, for molecular adsorption being near unity at large impact energy
(Ei ~0.8 eV) on Ag(110) and Ag(001 ); so is smaller
by many orders of magnitude for A g ( l l l ) [6-8].
Dissociation was demonstrated to involve the molecular precursor for A g ( l l 0 ) [3,4] and Ag(001) [5];
in both cases no direct dissociation channel is active
at least for Ei up to 0.8 eV. Chemisorbed dioxygen is
thereby stable below 130 K and shows a strong weakening of the oxygen-oxygen bond. while for molecular adsorption the weakening of the O - O bond and
s are similar on the two surfaces, the initial dissociation probability on Ag(110) is 0.63, which is orders
of magnitude bigger than the value found on Ag (001 )
where this figure is 4.4 x 10 -3 [4,5].
In accord with the precursor model, which implies a
competition between desorption and dissociation, So
diss
decreases on A g ( l l 0 ) with the surface temperature
T [4]. The large initial dissociation probability, and
the STM observations on the formation of the n x 1
A g - O adrows [9-11 ] suggest that 02 dissociation on
Ag(110) takes place on fiat terraces.
In this Letter we will show that, contrary to the case
of Ag(110), dissociation on Ag(001) is induced by
defects, which we identify with kink sites at steps.
While the stabilization of chemisorbates at steps and
kinks is a well known phenomenon [ 12-17], the selective disruption of the molecular bond by the kink
site is demonstrated for the first time to our knowledge. The activity of the kinks on Ag(001 ) is thereby
connected to their geometry, which resembles that of
0009-2614/97/$17.00 ~) 1997 Published by Elsevier Science B,V. All rights reserved.
PH S 0 0 0 9 - 2 6 1 4 ( 9 7 ) 0 0 3 8 1 - 3
346
E Buatier de Mongeot et al.IChemical Physics Letters 270 (1997) 345-350
sites.
The present experiment was performed with the
same apparatus used for previous investigations of 0 2 Ag(001) [5,18], O2-Ag(111) [6] and O2-Ag(110)
[3,4]. Details are reported elsewhere [20]. For the
present work let us recall that we have at our disposal a
home-built high resolution electron energy loss spectrometer (HREELS) [ 4 ] to characterize the adsorbed
species and a supersonic molecular beam to dose the
molecular oxygen. The HREEL-spectra were recorded
in-specular at an impact energy Ee = 3.36 eV and
Oe = 63 °, i.e. conditions for which dipole scattering is
dominant. The supersonic molecular beam allows one
to overcme the adsorption barrier and thus to have a
high sticking coefficient in the molecular well (s0
0.6) thanks to the high impact energy Ei (up to 0.8
eV) of the impinging molecules [5]. High coverages
of molecular oxygen are then achieved in a short time
and exposure takes place at a background pressure of
10-1° mbar, thus minimizing contamination. The coverage ((9) will be expressed in monolayer units relative to the surface density of Ag atoms on the (001)
plane (1ML = 1.2 x 1015cm-2). The Ag(001) single crystal, with a nominal miscut of less than 0.25 °
was prepared by sputtering with 500 eV Ne + ions,
followed by annealing to 800 K and oxidation. The
sample temperature T was measured by a ChromelAlumel thermocouple with an accuracy of 5 K.
A typical EEL spectrum is shown in the inset of
Fig. 1. It was recorded after an exposure of 30 ML of
O2 at a molecular beam impact energy Ei = 0.8 eV
and normal incidence. The substrate temperature was
200 K, i.e. slightly above the dissociation temperature [ 18 ]. In all cases the spectra were recorded after
having cooled the sample to 100 K. The peak at 33
meV indicates, in accord with the literature [21,22],
that oxygen has dissociated and adatoms have formed.
The conversion of the loss intensity into oxygen coverage is not straightforward as no calibration of the
dynamical dipole moment is possible: no LEED superstructure is formed, and TDS is not a valuable indication as thermally activated oxygen migration to
the bulk is known to take place at higher temperatures
[23,24]. We therefore assumed for the conversion the
same dynamical charge q* = 0.43e- as for oxygen on
Ag(110) [25] and that depolarization was negligible
at these low coverages. While the latter approximation
is reasonable, the former is questionable. The uncer(110)
500 400
I I
1E-1
300
I
T(K)
250
I
"e
200
I
e 30 ML F_xposure]
l,_1ML Exm e l
•
-
E= 66 meV
~
1E-2
~
"
76meV
1E-3 -~
,
,~,
,
I ..'..Vi:x,,o
1E-4
0
:
p
2
25
~
50 75
Loss (meV)
I
,
3
4
l r r (1/K xlO3)
I
,
_I
5
,_
6
Fig. 1. Atomic oxygen uptake at different crystal temperatures
plotted in Arrhenius form. The two sets of data correspond to
exposures of 30 ML ( O ) and 1 ML (I-1); Inset: HREELS spectrum recorded after exposing 30 ML at 200 K. the spectrum
was recorded after cooling the sample to 100 K (Ee = 3.36 eV
ei = 8s = 63°).
tainty in the absolute value of the coverage should not
exceed a factor 2. Our conclusions are however independent of the absolute coverage determination.
In Fig. 1 the so derived oxygen coverage is reported
in Arrhenius form for exposures of 1 and 30 ML. As
one can see the intensity of the 33 meV mode increases
by approximately one order of magnitude between 180
K and 400 K. The lower set of data ( 1 ML exposure)
corresponds to the region where O depends linearly
on exposure and is therefore proportional to the initial dissociative sticking coefficient, sdiss. Such behavior is at variance with O2-Ag(110), where a fourfold
decrease of s0diss (T) is observed over the same temperature range [4], due to the competition between
desorption and dissociation from the molecular well.
Thermal desorption measurements for O2-Ag(001)
[ 18] indicate, on the other hand, that the desorption
parameters of Ag(001) are comparable to those of
Ag(110) so that an analogous trend with temperature would be expected for the dissociation probability, unless another mechanism is active for Ag(001 ).
An Arrhenius analysis of the data, yields an activa-
E Buatier de Mongeot et aL/Chemical Physics Letters 270 (1997) 345-350
I
I
I
I
25
I
6.0
20
'
1E+01E.i+m+
1 ~ ' ' r1
I
f
I
r-
76.9=
-'T",
'1
"," '
x6102
",,.,. ,08 :" | .
._,.-7" L_ ",.,...
10
3.0
'
317
"
15
)
I
(b):1802
(a):is 02
x6103~
4.0
i
I
(02) = 0.034ME
e
5.0
'
347
/
81.5
/
t
2.0
1E-3
1.0
O.C
0.0
I
0.0 0.1
I
=
5--
I/
o.2 o.3 o.4
02 Coverage(ME)
I
0.1
I
I
0.2
0.3
02 Coverage(ME)
I
0.4
Fig. 2. Atomic oxygen coverage measured after annealing up to
236 K the O2 covered surface versus molecular oxygen coverage.
Molecular oxygen is dosed with the molecular beam at a substrate
temperature T = 100 K for which 02 is stable on the surface. A
small fraction of the molecules dissociates while most of them
desorb intact. Inset: The dissociation branching ratio (nr. of dissociated molecules/total hr. of chemisorbed molecules - crosses)
is plotted versus the initial O2 coverage. The empty square and
diamonds represent the defect/terrace population ratio of molecular oxygen, as deduced from the EELS spectra (see discussion
in the description of Fig. 3).
tion energy E = 76 + 5 meV for small exposures and
E = 66 -4- 5 meV for 30 ML. The difference is possibly related to the decrease in the uptake rate due to
incipient saturation of the surface.
In Fig. 2 the result of a different experiment is reported: dioxygen is adsorbed at 100 K, a temperature
at which 02 molecules are stable on Ag(001); the
sample is heated eventually to T = 236 K, where either
dissociation or desorption has taken place. We the find
that the atomic oxygen coverage depends only weakly
on the initial 02 coverage. For initial 02 coverages as
low as 0.03 ML, the dissociated oxygen coverage is
already 70% of the value reached when annealing the
surface saturated with 0.48 ML of 02. The branching
ratio for dissociation is plotted versus initial O2 coverage in the inset of Fig. 2 (crosses). As one can see
less than 1% of the molecules dissociate for saturation coverages, while ~ 30% do so in the low coverage limit. This indicates that the 02 admolecules are
already mobile at low temperature and they are able
to seek active defect sites which promote dissociation
OIj
• ~ x4103
• .
L
0
32.4
.,
J
I
20
,
I
T
I
,
ix,,o=
I
40
60
80
F_nergyLoss(meV)
,
~
100
Fig. 3. EELS spectra of molecularly chemisorbed oxygen in the
low coverage limit ( 0 ( 0 2 ) = 0.034 ML, T = 100 K): (a) 1602
and (b) 1802. The isotopic shift indicates that both losses are
due to oxygen. The main loss around 80 meV is due the O - O
stretching motion of molecules chemisorbed on the flat terraces,
the weak loss around 30 meV is due to the molecule-surface
vibration, while the loss around 64 meV arises from the stretching
vibration of 02 bound to defects. The line through spectrum 3a
is the result of a gaussian fit.
[ 13,12]. Such sites must be few in number as they are
readily saturated at low coverage. Molecules adsorbed
on the fiat terraces, on the contrary, are desorbed. In
accord with this mechanism we find that when annealing the 02 covered surface the dissociation branching
ratio is larger than the value estimated from the initial sticking probability at room temperature, which is
Pdiss (300 K) = 4.4 x 10 -3 [ 5 ]. The two experiments
differ with respect to the surface residence time of the
molecules, which at 100 K is larger by orders of magnitude.
Further evidence for the presence of special adsorption sites is given by the EELS spectra shown in Fig. 3.
As one can see, apart from the dominant loss at ~ 80
meV due to the O - O stretching mode of the "normal"
admolecules (i.e. chemisorbed at terrace sites), and
the molecule-surface vibration around 30 meV, a further peak is present at ~ 64 meV. When dosing with
1802 (Fig. 3b) this loss shows the expected isotope
shift indicating that it is not due to an impurity and
disappears upon heating the crystal to 150 K indicating that it is not due to silver oxide or to O adatoms.
348
E Buatier de Mongeot et al./Chemical Physics Letters 270 (1997) 345-350
It is, moreover, present for all 02 impact energies.
We assign it, therefore, to the stretching mode of 02
molecules adsorbed at defect sites. The frequency of
such stretch vibrations is remarkably low, indicating
that a significant bond weakening has occurred. Such
molecules are therefore the most reasonable candidates as precursors to dissociation. The ratio of the
area of the 64 to 80 meV losses, obtained by a gaussan fit (Fig. 3a), is 0.058. As the total coverage of
02 in Fig. 3a is 0.034 ML as estimated by TDS, the
coverage of the molecules responsible for the 64 meV
vibration must be of the order of 0.002 ML, and even
less if the dynamical dipole moment is larger than for
molecules at terrace sites. This ratio (empty square)
compares well with the dissociation branching ratio
found at the same coverage (0.057 - inset Fig. 2). We
have plotted in the inset of Fig. 2 (empty diamonds)
the ratio of the area of the 64 and 80 meV peaks for
a set of EELS spectra recorded at higher resolution in
order to better resolve the weak 64 meV feature. As
one can see, the dissociation probability scales with
coverage in the same way as the defect/terrace population ratio of molecular oxygen. This observation
implies that the only molecules which undergo dissociation on Ag(001) are those bound to defect sites,
while most of the molecules chemisorbed on terraces
desorb. The present conclusions would not be affected
if, contrary to evidence [5,19] the loss at 32.4 meV
(spectrum 3a) were due to atomic oxygen dissociated
in a direct process, as the intensity of this loss prior to
thermal dissociation is more than one order of magnitude weaker than the atomic oxygen signal after thermal dissociation has occurred.
Another argument supports the defect-induced dissociation mechanism: after deliberately damaging the
surface with 500 eV Ne + ions at room temperature ( 1
/zA/cm 2 x 10 min) we find a tenfold increase in the
dissociation probability compared to the case of the
well annealed surface.
In order to identify the active sites responsible for
02 dissociation, we report in Table 1 the energies for
creating various possible surface defects on Ag(001 ),
calculated within the effective medium theory [26].
If we compare these values with the activation energy
we find for the dissociation process ( E = 76 -t- 5 meV
Fig. 2b) ; it is evident that the only possible surface
defect with an energy compatible with our measurement is the kink ( E = 82 meV). The theoretical value
-
Table 1
Energies for the production of surface defects on Ag(001 ) calculated within the effective medium theory by Stoltze [26]
Defect
Energy (meV)
kink
adatom-vacancy
adatom from steps
adatom from kink
vacancy diffusion
dissociation of islands
82
694
502
364
417
182-382
was also confirmed by an STM study of Ag( 111 ) by
Poensgen et al. [27] which gave E = 73 meV. Slightly
larger values were found in an STM study ofAg(115)
( E = 117 meV) [28] and in another theoretical calculation using the embedded atom method ( E = 102
meV) [29], while the trend of the various defect energies was confirmed. A few words are necessary in
order to clarify the term "kink energy" in the present
context. The dissociation probability at the temperature T is proportional to the equilibrium density of
kinks. The kink density in turn (if thermodynamical
equilibrium is reached) is proportional to the Boltzmann factor e x p ( - E / K T ) where E is the energy difference between the ground state (step with no kinks)
and the excited state (step with a kink). E is therefore
the energy cost for the formation of a kink. Between
the fundamental and excited states an activation barrier could exist, which would then determine the rate
of kink production, but not the kink density in thermodynamical equilibrium (care must be taken because
sometimes in the literature the kink energy E is called
the activation energy). Kink formation and diffusion
are also responsible for the "frizziness" of steps when
observed with scanning tunneling microscopy. Such
an effect was indeed demonstrated to be present on
Ag( 111 ) above 150 K, while for lower temperatures
the steps edges became sharp [30]. Under our experimental conditions (T in a range from 200 up to 450
K) we can thus safely assume that the kink site concentration is in thermodynamic equilibrium.
But why do oxygen molecules dissociate easily at
kink sites? Energetical considerations indicate that
steps are preferentially oriented along [ 110] thus exposing ( 111)-like microfacets, while the occurrence
of steps along [001 ] is strongly unfavoured [29]. The
( 111 ) plane of Ag is extremely inert towards oxygen
E Buatier de Mongeot et al./Chemical Physics Letters 270 (1997) 345-350
adsorption, so that we do not expect ( 111)-like steps
to play a particular role in the dissociation process.
On the contrary, a kink along the [ 110] step produces
a local environment for which the dissociation probability is known to be large. Apart from structural
arguments, the enhancement in reactivity at the kinks
has to do with an increase in charge donation to the
antibonding molecular orbitals, as demonstrated by
the 20% downshift of the 02 stretching frequency
compared to the terrace sites. It is indeed well known
that at the lower step edge an increase in electronic
density occurs [ 31 ], and calculations for CO/Pt and
C O / C u suggest that at kinks a significant bond weakening takes place due to hybridization between the
metal d-electrons and the antibonding orbitals of CO
[32,33].
A step-site density of ~ 0.006 ML can be estimated
assuming that the average terrace size (460 A,) is determined by the nominal miscut of the surface (less
than 0.25°). The observed density of molecules vibrating at 64 meV (0.002 ML) at T = 100 K reflects
the number of static kink sites, due to the pinning of
steps at defects. The number of static kinks turns out
to be of the order of one every third step atom. Such a
density increases with T due to the thermal activation
of kink production. E.g. for the case of A g ( l l l ) at
room temperature Poensgen et al. [27] found an average density of 0.1 kinks per step site. Assuming the
thermal production rate to be the same on the two surfaces, we would thus expect a density of (0.4) kinks
per step site at 300 K. We must point out that this
is the instantaneous density. New kink configurations
are continuously generated by kink diffusion, production and annihilation. The estimated lifetime of a kink
configuration is: tkink ~ 0.01 S [30]. In our experiment the exposure time necessary to reach saturation
is of the order of 103 s; in that time approximately
105 new kink configurations are produced, so that all
step sites (0.006 ML) are repeatedly visited by kinks
during oxygen dosing.
At room temperature we find saturation coverages
Osat around 0.1 ML of atomic oxygen (500 ML exposure); given the estimated step density this in turn
means that on average 8 oxygen molecules are dissociated per step site. This implies that the oxygen
atoms after dissociation have to diffuse away from the
step edge, so that further molecules can react at kinks.
Considering that the above given step density is surely
349
underestimated as (i) the steps try to run along [ 110]
and are thus longer if the miscut is not parallel to this
direction, and (ii) more steps are present due to pinning at dislocations and imperfections, the estimated
number of molecules dissociated per step site could
be considerably lower. At higher substrate temperatures, on the other hand, thermal diffusion of oxygen
adatoms away from the steps is expected to be important and, as a matter of fact, a threefold increase
in Osat is found when increasing the surface temperature from 200 to 375 K. Moreover for O2-Ag(001)
an energy release of about 0.8 eV/atom occurs upon
dissociation [2] so that some hyperthermal diffusion
of the oxygen fragments is also expected to take place
[34,35].
As a last observation we remark that, as the
molecules bound at kinks have their axis tilted, with
one O atom pointing downwards, an easy pathway for
subsurface incorporation of the "hot" oxygen atoms
is open [24].
In conclusion, we have shown that unlike Ag(110)
where dissociation of O2 takes place at terrace sites,
on Ag(001) the rupture of the molecular bond occurs
selectively at kinks. The interplay between a local geometry similar to a (110) site and enhanced charge
transfer to the antibonding molecular orbitals is responsible for the pronounced reactivity of such sites.
Molecules adsorbed on (001) terraces instead desorb
with a high probability.
Acknowledgements
The authors wish to acknowledge the financial support of the EEC-HCM contracts ERBCHRXCT930104 and ERBCHRXCT930326 and
helpful discussions with R. Ferrando and K.W. Jacobsen.
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