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Interactions in co-adsorbed CO + O2Pd(111) layers
surface science ELSEVIER
Surface Science 334 (1995) 19-28
Interactions in co-adsorbed CO + O2/Pd(111)layers Kurt W. Kotasifiski 1, Franz Cemi~, Arne de Meijere 2, Eckart Hasselbrink * Fritz-Haber-Institut der Max-Planck-Gesellsehafl, Faradayweg 4-6, D-14195 Berlin, Germany Received 2 January 1995; accepted for publication 29 March 1995
Abstract
The interactions of saturated layers of molecularly adsorbed 0 2 on Pd(lll) with post-dosed CO have been studied with temperature programmed desorption (TPD) and electron energy loss spectroscopy (EELS) in the surface temperature range 100-230 K. It has been found that CO does not react chemically with O2(a) to form CO 2. CO does react with adsorbed O atoms at higher surfaces temperatures resulting in the production of CO 2. Introducing CO to a saturated 02 layer decreases the total O2(a) coverage as well as changing the relative populations of the three chemisorbed 0 2 species. Keywords: Adsorption kinetics; Carbon monoxide; Chemisorption; Electron energy loss spectroscopy; Low index single crystal surfaces; Oxygen; Palladium; Solid-gas interfaces; Vibrations of adsorbed molecules
1. Introduction
Chemical systems tend to minimize their free energy in an attempt to achieve thermodynamic equilibrium. This proposal was first formulated by van't Hoff [1] and its relationship to the second law of thermodynamics has been discussed in the literature [2-7]. This is not the case if energetic or entropic barriers make the rates of the forward and reverse reactions too slow for equilibrium to be reached within practical time scales. Similarly, mass transport constraints, such as non-uniform mixing or - in the case of surface reactions - site blocking, can hinder equilibration. The pathway to chemical equi-
* Corresponding author. 1 Present address: B248 Chemistry, National Institute of Standards and Technology, Gaithersburg, MD 20899 USA. 2 Present address: Laboratorie Kastler Brossel de l'l~cole Normale Sup6rieure, 24, rue Lhomout, F-75231 Paris Cedex 05, France.
librium, that is, the reaction dynamics can, however, be less than straightforward and lead to some surprising results as the system attempts to equilibrate. Thus, even though the thermochemistry predicts what the equilibrium state would be when achieved, it gives us little insight into the rate of equilibration and the mechanism by which equilibrium will be attained. The oxidation of CO to CO 2 on Pd and Pt has received great attention in the literature [8]. On both metals, CO reacts facilely with adsorbed O atoms at around 400 K. On P t ( l l l ) surfaces at roughly 150 K [9], CO also reacts to form CO 2 in the presence of O 2 ( a ) . This reaction channel concurs with the thermally activated dissociation of O2(a) into O(a). Hence it has been interpreted to be due to reaction with nascent O atoms immediately after O 2 ( a ) dissociation, in the spirit of a hot atom reaction. In this work, we show that on P d ( l l l ) neither molecular oxygen nor nascent O atoms react directly with CO. The oxidation of CO on Pd(111) occurs only after
0039-6028/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSDI 0 0 3 9 - 6 0 2 8 ( 9 5 ) 0 0 4 8 7 - 4
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K.W. Kotasihski et al. / Surface Science 334 (1995) 19-28
complete dissociation of adsorbed O z to the atomic form at surface temperatures far above those that support O2(a). Molecular oxygen adsorbed on P d ( l l l ) is peculiar in that it exhibits three chemisorbed states. We have recently investigated [10] how surface temperature and oxygen coverage affect the populations of the three chemisorbed states, denoted o91, o92 and °)3- We have found that the to 2 state is formed spontaneously upon adsorption at all coverages, that the o93 state is populated only at higher coverages, and that the o92 species can convert to the o91 species by means of a thermally activated process. The molecular O 2 chemisorbed species were first directly observed on Pd(111) by Imbihl and Demuth [11], who observed, in addition to a physisorbed species, three vibrational bands in electron energy loss (EEL) spectra which they assigned to three chemisorbed molecular species. The progression of vibrational frequencies was interpreted in terms of different degrees of electron transfer from the surface into the molecule. As has been shown in theoretical studies [12], electron transfer leads to a weakening of the O - O bond and a strengthening of the O2-surface bond. We introduced the nomenclature o91, o92 and 093 (in order of decreasing chemisorption binding energy but increasing vibrational frequency) to denote these three binding states and the corresponding EEL peaks. Consistent with our later observations described in Ref. [10], Imbihl and Demuth noted that the o91 band, i.e. the most strongly chemisorbed species, is not spontaneously formed upon exposure at Ts = 30 K and that it appears only after warming the crystal. The O2/Pd(111) system has also been studied by Guo, Hoffman and Yates [13] who reported thermal desorption (TD) spectra similar to those reported by us as well as to those reported by Matsushima in an earlier study [14]. Guo, Hoffman and Yates characterized the dissociation of O2(a) into O(a). The onset of dissociation of O2(a) into O(a) occurs at T~ _> 180 K. They determined that the saturation coverage of O2(a) corresponds to 0.62 O / P d and that 0.17 O / P d remain on the surface as O(a) after thermal desorption of O2(a). During thermal desorption from saturated layers of 02 on P d ( l l l ) , three features, labeled a l , a 2 and a 3 by Matsushima [14], are observed at roughly 130,
170 and 210 K. The position and size of these peaks depends on coverage, heating rate, adsorption temperature, presence of co-adsorbed O(a) and annealing of the chemisorbed layer. We have recently demonstrated that the lowest temperature TD feature corresponds to the o93 state, whereas the two higher temperature TD features are correlated with desorption from the o92 state. While we cannot rule out some molecular thermal desorption from the oga state, we have proposed that the predominant fate of the o91 species is dissociation into adsorbed atomic oxygen. The appearance of two TD features arising from the °)2 state is likely due to strong lateral interactions between chemisorbed 02 and between O2(a) and O(a) at T~ > 180 K. CO has been shown to adsorb initially into threefold hollow sites on P d ( l l l ) . As the coverage is increased, CO eventually occupies only bridge sites. At coverages over 0.60 ML (CO/Pd), both bridge and atop sites are occupied. CO is bound more strongly to P d ( l l l ) than is 02, with the onset of thermal desorption occurring above 400 K at low coverage [15]. We have determined, by means of the retarding potential method in EELS, that both CO and 02 lead to an increase of the work function of roughly 1 eV. These work function changes are consistent with the values reported in the literature [16,17]. The results of the investigations into the sensitivity of 02 layers to the presence of co-adsorbed CO are presented here. We find that as the result of competitive adsorption at 100 K, CO decreases the saturation coverage of 02 on P d ( l l l ) . The brunt of this effect is borne by the o)2-02 species. The coverage of o92-O2 decreases monotonically with increasing CO exposure. The displaced o92-O2 partially converts to the o91- and o93-O2 species; however, an even greater amount of the displaced o92-O2 desorbs from the surface. The effects of CO co-adsorption on the EEL and TD spectra corroborate our previous assignments [10] of the chemical fates of the three chemisorbed 02 species.
2. Experimental Experiments were performed in a turbo-pumped UHV chamber with a base pressure of ~ 2 × 10 -1°
21
K.W. Kotasihski et a l . / Surface Science 334 (1995) 19-28
mbar. The P d ( l l l ) crystal is cooled conductively with a liquid-N 2 reservoir and heated radiatively by a W filament. Surface preparation has been described in previous publications [18,19]. TD and EEL spectra provide checks for surface cleanliness and order. The crystal temperature was monitored with a NiCr-Ni thermocouple spot welded to the back of the crystal. The EEL spectrometer has been described in detail elsewhere [20]. Resolution of 12 meV (100 cm -1) and count rates of I X l0 s s-a were routinely achieved. All spectra displayed here were recorded along the specular direction, for an incident angle of 70 ° , and at an incident energy of 2.2 eV (corrected for work function changes). Thermal desorption spectra were recorded with an unshielded quadrupole mass spectrometer. Both TD and EEL spectra are digitized and stored directly on a microcomputer. The thermal desorption results were reproduced in a second vacuum chamber with a different Pd(111) crystal. The crystal is exposed to 02 through a capillary array doser with the gas flow controlled by a precision leak valve. Reproducible exposures are obtained by monitoring the time of exposure and pressure rise in the chamber. The crystal was exposed to 02 at T~ = 100 K. In all experiments presented below the Pd(111) surface was first saturated with O 2 and then exposed to varying amounts of 13CO. The CO was admitted to the chamber through a precision leak valve to establish a uniform background pressure. The reported exposure values for CO are approximate because on one hand the pressure-time product does not include the contribution of the CO pressure rise and fall times. These times are on the order of a few seconds; therefore, they only seriously affect dosing times of < 60 s. On the other hand CO dosing was done with the crystal within the EELS spectrometer, where the pumping speed may be larger than at the preparation level, where the pressure was read. All CO exposures were carried out at ~ 1 X 10 -8 mbar.
3. Results Exposure of saturated 0 2 layers to CO leads to significant changes in 0 2 thermal desorption spectra. A typical multiplexed thermal desorption spectrum
/
Saturated 02/Pd(111) layer -6 F e x p o s e d to 1.1x10 m b a r s C O / ~, Ts=lO0 K
fl J. i; !
ii
;
il
:
i
ii;:=
! !
;-
i ~. ..
o
i
,...-=.
!
;
;
~
.
~coi
i
pi ~ 0 2-i
100
;
.;
i 200
C 300
,;
' 400
Surface Temperature
' 500
600
[K]
Fig. 1. Multiplexed temperature programmeddesorption (/3 = 3.3 K s -z) of masses 32 (O2), 29 (13CO) and 45 (t3CO2) resulting from a saturated 02/Pd(111) layer which has been exposed to ll0X 10-8 mbar.s 13CO.
13C0 and 13C02 desorbing from a mixed 13COq-O2/Pd(lll) layer is displayed in Fig. 1.
of 02,
This layer was produced by exposing a saturated O 2 / P d ( l l l ) layer to 110 X 10 -8 m b a r . s of CO. Fig. 1 demonstrates that no CO 2 product evolves from the surface in the temperature regime where O2(a) is present on the surface, i.e. for T~ < 2 2 0 K. A selection of TD spectra from CO + O 2 layers on P d ( l l l ) is represented in Fig. 2. From this series of spectra, it is immediately obvious that progressive exposure to CO reduces the magnitude of molecular 0 2 desorption. The a s feature is the least effected by CO coadsorption. After an initial shift to lower temperature caused by the lowest CO exposure, this feature remains virtually constant up to exposures of 130 X 10 -8 mbar. s. This CO exposure leads to the adsorption of 0.19 ML CO. At exposures > 130 X 10 -8 mbar. s, the of3 feature is reduced in intensity without significant changes in its form. The higher temperature TD features exhibit immediate and continuous shifts and reduction in intensities with increasing CO exposure. The EEL spectra corresponding to the CO + 0 2 layers which gave rise to the TD spectra of Fig. 2-are
22
K.W. Kotasihski et al. / Surface Science 334 (1995) 19-28
02+CO/Pd(111) T s = 100 K
~'
o~ ='
~
m rsCO
¢ c
/
~ - - ~ ,
13X10-8
U)
~,,'//~,~.._~
O
J
34x10"s
~
72xlO'S
=Z 195x10 "s I
150
100
i
200
250
S u r f a c e T e m p e r a t u r e [K]
Fig. 2. Temperature programmed desorption (/3 = 3.3 K s - ] ) of O 2 from saturated O 2 / P d ( l l l ) layers which have been exposed to various amounts of 13CO. The CO exposure is indicated in the figure. Ts = 100 K for all exposures.
shown in Fig. 3. We observe that the majority to2 species displays a monotonic decrease in EEL intensity with increasing CO exposure. The intensities of both the to1 and to3 transitions exhibit an initial increase in intensity, reach a relatively stable value, then fall again at very high ( > 130 × 10 -s mbar. s) CO exposures. Interestingly, the vibrational frequencies of all three 0 2 states shift very little with
increasing CO coverage. Both the 601 and to2 peaks shift down in frequency by roughly 1 - 2 meV ( 8 - 1 6 c m - t ) . The FWHM of these EEL transitions is also found to increase - the to1 transition broadens from 20 to 25 meV (160 to 200 cm -a) whereas the toe peak increases in width from 13 to 15 meV (105 to 120 cm-1). The to3 peak remains essentially unchanged in peak position and width. The to3 FWHM is ~ 14 meV (113 cm-1), slightly larger than the instrumental resolution of 12 meV (100 cm-1). To quantify these data, the TPD and EELS signals must be calibrated to the coverage. We have done this in the following manner. The TPD signal can be easily calibrated since both the saturation coverage for an 0 2 layer (0.62 O / P d ) and the amount of O(a) remaining after the completion of molecular desorption (0.17 O / P d ) are known [13]. The total desorption signal from a CO-free saturated 0 2 layer thus corresponds to an amount of desorbed 0 2 equivalent to a coverage of 0.45 O/Pd. For the calibration of the EEL signal, we assumed that the oscillator strengths of the three transitions are the same and independent of coverage. Since there is no a priori proof of this assumption, this method of calibration must be tested. In Fig. 4 are presented data for two methods of determining the total 02 coverage on the surface resulting from dosing a saturated layer of chemisorbed 0 2 with various amounts of CO.
O2+CO/Pd(111) Ts= 100 K :. x2O
mbar s CO
" "..,,,.%_
,4
i
0
m2 o~~~
i
i
0.6
13x10"s
~
I+
0,5
TPD-+ O(a)J
~ 0.4 ~"
-
~
-"l I " - 12 meV
It
J
x=o
~_"'~"'"
E z
34x10" 8
.: .":
0
"~ "~-"~
x20 "'. ,'~,",-,,,,,~.'-~.,",,,.,,._ _ CO-Pd ~- -..".....,.< x20 I 0
I
50
:'...
72X10"s ",,.,_,.
". ¢"
..." ".
.
"""f'"P~ ~
100
150
0.2
~ "c-o ~'-" .J
200
0.3
195x10 "e
''P'~"
I
250
300
Energy Loss [meV]
Fig. 3. Electron energy loss spectra recorded from coadsorbed layers of CO + O 2 corresponding to the same layers as those used in Fig. 2.
0
0.1
0.0
0
I
50
I
1O0
t
150
200
CO Exposure [×10"8 mbar s]
Fig. 4. (a) Total initial (O from O2)/Pd coverage as determined from the T P D + O ( a ) EELS method described in the text as a function of CO exposure. (b) Total (O from O2)/Pd coverage as determined by the EEL intensities of the molecular 0 2 vibrational losses as described in the text.
23
K. W. Kotasihski et al. / Surface Science 334 (1995) 19-28
The 0 2 coverage values displayed by the filled symbols in Fig. 4 were determined by a combined TPD and EELS method. Heating the coadsorbed layer to 255 K results in desorption o f O z and dissociation of O2(a) into O(a). However, as has been shown above, O 2 does not react upon thermal dissociation with CO and the onset of the O ( a ) + C O ( a ) ~ CO a reaction occurs at roughly 350 K. This onset is consistent with that reported by Ertl and Koch [16]. Hence, the amount o f chemisorbed 0 2 in the layer prior to heating the surface to 255 K can be determined b y measuring the amount of desorbed O 2 and the amount o f O ( a ) left on the surface as the result of dissociation. The amount of desorbed O 2 is obtained from t h e calibrated O a TD signal. The amount of O ( a ) is determined from the O ( a ) E E L signal, calibrated to the 0.17 O / P d which arises from dissociation in a pure 0 2 layer heated to 255 K. The sum o f the calibrated 0 2 TD and the O ( a ) E E L signals is displayed by" filled symbols in Fig. 4.
0.2
0.1
r
~
Ja)
0.4
>0 0 0
0.2
0(4
t o 255
I
KI
_
0 0')
~
IAfter h e a t i n g
0 -I
i
i
i
I
I
I - ~
i
(d) ~
~"1
I
i
i
i
I
I
I t"""~l
50
100
!1. 0.2
0.1
0-1 0
100
1 0
2 0
O'
1 0
200
C O Exposure [xlO "s mbar s]
Fig. 5. Signal dependence as a function of CO exposure. Coverage of the (a) to1-O 2 state as determined by EELS; (b) to2-O 2 state as determined by EELS; (c) co3-O z state as determined by EELS; and (d) O(a) state as determined by EELS. Magnitude of (e) ot12 thermal desorption (160 < T~< 240 K) reported in O/Pd equivalents and (f) o~3 thermal desorption (100 < T~< 160 K) reported in O/Pd equivalents.
/". 0.3 o.
ov eJ oi
0.2
o 0 0 0
0.1
.m
oe e
i 50 CO Exposure
i 100
i 150
200
[ x l O "8 m b a r s ]
Fig. 6. CO coverage determined from EELS versus CO exposure for CO dosed onto Oz(a)/Pd(lll) surfaces held at 100 K.
The open symbols in Fig. 4 represent the O2(a) coverage as a function of CO e x p o s u r e as determined from the sum o f the o9a, o9a and o93 EEL intensities. The surface temperature throughout these experiments was 100 K. For these data, the total coverage was assumed to be given by the sum o f the c°l, o92 and o93 E E L integrated peak areas normalized to the total area of the o91, °-)2 and 603 peaks arising from the saturated pure O2(a) layer, for which the coverage is known to be 0.62 ML. Comparison of the two data sets in Fig. 4 demonstrates that these two methods of determining the O2(a) coverage agree on average to within 8% for all CO exposures. This gives us confidence that we can use the i n t e g r a t e d E E L peak areas as a measure of the coverage o f the o91, o92 and °-)3 species. F r o m both figures it is clear that the total O z ( a ) coverage decreases monotonically with increasing CO exposure. Re-exposure of these co-adsorbed layers to an amount o f 0 2 that would normally saturate the surface at T~ = 100 K does not lead to an increase of the O2(a) coverage. In the left-hand column o f Fig. 5 are displayed the coverages o f the to 1, o92 and o93 species determined from the integrated EEL l~eak areas as a function of CO exposure. The CO coyerage resulting from these exposures is displayed in Fig. 6. The CO coverage was estimated by assuming that the saturation intensity o f the stretch transition of three-fold co-ordinated CO at ~ 229 m e V (1840 cm - 1 ) corresponds to 0.33 C O / P d [15]. Fig. 6 indicates that the sticking coefficient of CO is constant and close to unity throughout
24
K. W. Kotasihski et al. / Surface Science,334 (1995) 19-28
this coverage range, as has been observed for CO adsorption on oxygen-free P d ( l l l ) surfaces [16]. The coverage of the 092 species is fotmd to decrease monotonically with increasing CO exposure. T h e coverage of the 091 species is initially increased by the adsorption of small amounts of CO ( < 0.1 ML); thereafter, its coverage drops very slowly with increasing CO exposure until exposures of > 130 × 10 -8 mbar- s ( > 0.2 ML CO) lead to a rapid depopulation of this state. The coverage of the o93 species also experiences an initial increase after short exposures to CO and then remains roughly constant• until CO exposures > 130 X 107~8 m b a r s cause a rapid decline in the 093 population. The right-hand column of Fig. 5 presents the results of heating the co-adsorbed CO + 0 2 layers. Fig. 5d displays the O( a) coverage as estimated from the EEL intensity of the O - P d stretch. Adsorbed oxygen results f r o m the dissociation of O2(a) induced by heating the chemisorbed layers to 255 K. Fig, 5 e displays the integrated amount of 0 2 which desorbs in the surface temperature region 160 _< Ts _<230 K. We denote this al, 2, i.e., the amount of O2(a) which desorbs through the ceI and a 2 TD features. Fig, 5f displays the integrated amount of 0 2 which desorbs in the surface temperature region 100 _< Ts _< 160 K. We denote this a3, i.e., the amount of O2(a) which desorbs through the a 3 TD feature. :Justification for setting the boundary between the a 3 and at, 2 features to T s = 160 K is drawn from the general shapes of the TD features, the disappearance of the09 3 EEL peak for surfaces heated to T~ >_ 160 K, the,observation of a roughly constant 092 intensity for surfaces heated to T~ _< 160 K, and a posteriori from correlations of the a 3 feature with 093 and the al, 2 feature with 092' While placing this boundary at T~ = 140 or 150 K changes the magnitudes, this choice does not change the shape of the ot12 and a 3 curves, What is clear from inspection o f the left- and right-hand columns of Fig. 5 is that, in general, the CO exposure dependence of the O(a) signal follows the CO exposure dependence of the :to1 state; the CO exposure dependence o f the al, 2 TD feature follows that of the 09~ state; and the CO exposure dependence of the a 3 TD feature follows that of the 093 s~te: ,The only deviation from these trends is observed~ for the initial points i n the w 1 and O(a)
curves. These deviations will be discussed in the next section.
4. Discussion 4.1. Effects of CO exposure on 02(a) at 100 K
Inspection of the EELS-determined coverage versus CO exposure curves in Figs. 4 and 5 shows that O2(a) is forced to adjust in order to accommodate co-adsorbed CO(a). The presence of co-adsorbed CO(a) dramatically reduces the coverage of 092-02. Some of the 092-02 is forced to desorb from the surface, as is indicated by the decrease in the total O2(a) coverage displayed in Fig. 4, while some of the o)2-0 2 is converted into the 091- and 093-0 2 chemisorption states. We have previously demonstrated [10] that an activation barrier separates the 09a and to 2 states such that the o)1 state is not populated directly from the gas phase, but rather, by way of a thermally activated process out of the w 2 state. As a consequence, the 091 state is only observed in a pure Oe(a) layer when this layer has been annealed for some length of time which depends on the annealing temperature. We propose here that co-adsorbed CO lowers the barrier between the to 1 and 092 states; thus facilitating the rapid population of the 091 and rapid equilibration of the populations in the w I and 092 states. The accelerated equilibration may be the result of a reduced barrier height caused by lateral interactions and the effects of CO adsorption on the substrate a n d / o r cooperative effects arising during CO adsorption (see below). This would explain why the 091 coverage first increases with CO exposure. Apparently, however, the to I coverage is limited to some saturation value. This saturation value is probably determined by two factors: (i) the availability of surface sites which can transfer sufficient charge to the 0 2 moiety and (ii) competitive adsorption with CO as the CO exposure is increased. Oxygen adsorbed in the 091 state, which is the most strongly bound O2(a) species, can compete more successfully with CO for adsorption sites than the more weakly bound 092-0 2 species. This argumentation - a lowering o f the barrier between the 09a and 092 states combined with competitive adsorption between 0 2
K.W. Kotasihski et aL / Surface Science 334 (1995) 19-28
and CO - can adequately explain the trends observed in the 601 and to 2 coverage versus CO exposure curves. Since competitive adsorption between CO and 0 2 must be invoked to explain the decrease in total O2(a) coverage with increasing CO exposure, it seems surprising that the to 3 state - the most weakly bound O2(a) state, which is only observed at high coverages - is able not only to hold its own versus CO, but also, that the coverage in this state is found to increase at low CO exposure. Certainly simple rationalization of the competitive adsorption process, that is, that the most weakly bound species is the most susceptible to attack from a more strongly bound species, does not suffice to explain this behavior. Two possible contributing factors for the surprising stability of the to3 state in the face of CO co-adsorption can be found in its electronic structure and its assumed adsorption geometry. Because t % - O 2 is only observed at high 0 2 coverage, we can presume that the t% species is not stably formed on the P d ( l l l ) surface as long as the coverage allows for the creation of the to I and to 2 states. In other words, a barrier between the to2 and to 3 states is extremely small to nonexistent; thus, the equilibrium between o92- and t03-O 2 lies essentially completely on the side of the more strongly bound w 2 state up to the point where this state saturates. Saturation will presumably arise from increasing lateral repulsion between the negatively charged 0 2 molecules. Formation of the w 3 species involves a smaller amount of charge transfer from the substrate. Hence, the lateral repulsion it experiences will also be smaller, allowing it to be adsorbed within otherwise densely packed 0 2 layers. It may be worth noting that t03-O 2 is also least affected by coadsorbed O(a), which points in the same direction. The adsorption geometry of the to 3 state has not been unambiguously determined; however, comparison to analogous organometallic complexes and information obtained on the angular distribution of photodesorbed t % - O 2 [21] suggest that the adsorbed 0 2 molecules are bound through one end of the molecule with the molecular axis inclined somewhat with respect to the surface normal. This configuration contrasts with the lying-down (molecular axis parallel to the surface) geometries determined for the o~1 and to 2 species [11,22]. The inclined geometry of
25
the to 3 state could facilitate the packing in of 0-)3--0 2 into dense O2(a) and O2(a) ~- CO(a) layers. Our suggestion, therefore, as to why competitive O 2 - C O adsorption proceeds at the expense of to202, whereas tO1 and to 3 survive, is that to a can more effectively compete with CO on the basis of its stronger (with respect to the w 2 state) adsorbatesurface bond, while the to3 state can survive on weak binding sites which do not appear to be strongly affected by the presence of co-adsorbed CO until the CO coverage exceeds ~ 0.2 ML. We note that our work function change measttrements indicate that CO and O 2 are electron withdrawing groups of similar strength; thus, it is perhaps not too surprising that ~o3-O 2 can co-exist with C O ( a ) s i n c e it has already been demonstrated that it can co-exist with the strongly electron withdrawing w 2 and to 1 species, as well at O(a). Note that, on the one hand, we should n o t be surprised by the displacement of 0 2 by CO - the system is merely trying to minimize its free energy by means of the exchange reaction (1) listed below. Since the binding energy of CO(a) (1.5 eV [15]) is more than twice that of O2(a) (0.65 e g roughly estimated from thermal desorption), the exchange reaction O2(a ) + CO(g) ~ O2(g ) + CO(a)
(1)
is highly exothermic and at equilibrium for a temperature where O2(a) is stable, we should expect a Pd surface exposed to an equimolar mixture of 0 2 and CO to be completely covered by CO(a) with very little O2(a) present on the surface. Nonetheless, the efficiency of this process is strikingly high. Fig. 7 displays the dependence of the total O2(a) coverage on the CO coverage determined, in both cases, by EELS. The solid line in the figure represents the O2(a) coverage dependence that would be expected if 0.8 0 2 molecules w e r e to desorb for every one CO molecule that adsorbs. The error margin on this number is largely determined by the uncertainty in the 0 2 saturation coverage. The coverage versus exposure data also indicates that the CO sticking coefficient remains near the yalue for an O2(a)-free surface. Hence, virtually every CO collir sion with the surface dislodges an 0 2 molecule. Recent studies [23] of CO + 0 2 on Pt(111) have
26
K. W. Kotasihski et al. / Surface Science 334 (1995) 19-28
0.6 ~"0 ~ & ~ 0 ~ 0
0.5
"•
O2(a) coverages do not follow the energetic ordering of the o91, o92 and o93 species, i.e., we have observed that the o92 species is displaced before the more weakly bound o93 species. This is related to a breakdown in the assumption that all adsorption sites are equivalent. Nonetheless, at sufficiently high CO exposures; virtually all of the O2(a) is displaced from the surface.
CO + 02/Pd(111) T s = 100 K
0.4 11.3
0
~
~
0
0.2 0.1 L
I ~ I 1).1 0.2 CO Coverage [CO/Pd]
,
I O.3
Fig. 7. Total O2(a) coverage versus CO exposure for CO postdosed onto saturated 02 layers. The solid line indicates the behavior expected if each adsorbing CO molecule removes on average ~ 0.8 O2(a) from the surface.
reported much lower 0 2 removal efficiencies than that reported here. The results of our experiments show that O2(a) is not effective at blocking the adsorption of CO. This is not surprising since CO adsorbs by means of a precursor-mediated mechanism on CO-covered P d ( l l l ) [16]. Evidently O2(a) does not disrupt the CO precursor state and CO also adsorbs via a precursor-mediated pathway on CO + O z coadsorbed layers. While the data in Fig. 6 exhibit some small deviations from a unit CO sticking coefficient independent of coverage over this exposure regime, a more exacting calibration of the CO exposure and direct comparison to CO sticking on pure CO layers would be required to reveal whether co-adsorbed O 2 subtly influences the sticking dynamics of CO. Since 0 2 is not effective at blocking the adsorption of CO and since, as demonstrated by our EELS results, the 0 2 saturation coverage is reduced by coadsorbed CO, the net result of CO post-dosing is to effectively accelerate the rate ofoO 2 desorption. This acceleration, as discussed by Akerlund et al. [23], may result from repulsive interactions, a transfer of the CO heat of adsorption to O2(a) or a combination of both, or some other process upon which we shy from speculating about. Thus, given a sufficient exposure of CO, we should expect that virtually all O2(a) should be removed and that CO will saturate the surface. On the other hand, our experiments demonstrate that along the path to equilibration, the decay in the
4.2. Effects o f C O on 02(a) thermal chemistry
We have found that no CO 2 production occurs in the temperature regime in which O2(a) is present on the Pd(111) surface. This demonstrates that the chemical behavior of a CO + O2/Pd(111) co-adsorbed layer is different than that observed on Pt(111) surfaces where a reaction producing CO 2 at 150 K has been observed [9]. Our own investigations of the CO + O 2 / P t ( l l l ) system confirm the occurrence of this low T~ CO + O 2 reaction. On P d ( l l l ) we find that CO 2 is first formed above 350 K; that is, CO 2 production is the result of a reaction between CO(a) and O(a). No CO 2 producing reaction between CO(a) and O2(a) or CO(a) and so-called hot atoms created in the dissociation of O2(a) ~ 2 0 ( a ) is observed on Pd(lll). CO co-adsorption has a marked effect on O 2 TD spectra. The a 1 feature is severely suppressed and shifted to lower Ts. T h e a z feature becomes less well-resolved but does not appear to shift strongly in temperature. The a 3 feature is initially shifted to lower T~, increasing first in intensity, then declining at high CO exposures. As shown in Fig. 4b, the total amount of molecularly desorbed O 2 decreases monotonically with increasing CO exposure. From Fig. 5e it can be seen that the total amount of O2(a) desorbing through the 0(1 and ce2 features also decreases monotonically with increasing CO. We have previously proposed [10] that both the cq and the c~2 desorption features result predominantly from desorption out of the o92 state whereas the a 3 feature results from desorption out of the o93 state. If this were true, there should be strong correlations between (i) the o92 coverage and the amount of 02 which desorbs by way of the a 1 and a 2 channels and (ii) the o93 coverage and the intensity of the c~3 feature. By comparing the CO exposure dependence of the o92 coverage displayed in Fig. 5b
K. W. Kolasihski et aL / Surface Science 334 (1995) 19-28
o.s
Q
Saturated O2/Pd(111) layer T s = 100 K
0.5
o o
0.3
~
o.~'
....... -q
0.1 0
.~. s O ~ l O ~ T n ~ r ~ i ~ n o v e r a I 0
I 50
I 1oo
150
I 200
CO Exposure [ x l 0 a mbar s]
Fig. 8. Comparison of the oxygen coverage thermally desorbed from the surface with the coverage contained in the to 2 and oJ3 states prior to the initiation of thermal desorption.
with the CO exposure dependence of the magnitude of the O/1,2 desorption signal (Fig. 5e), we find that not only the trend of these two signals is the same, but also that the agreement in absolute values is nearly one to one. Similarly good agreement is found between the 0)3 and the a 3 signals displayed in Figs. 5c and 5f, respectively. As a further test of whether thermal desorption of O2(a) proceeds predominantly through the 0)2 and 0)3 states, we display in Fig. 8 the CO exposure dependence of the total amount of thermally desorbed O 2 as well as the sum of the 0)2 and 0)3 coverages determined from EELS. The agreement is very good (within 5%) except for the one point from the CO-free surface. The reason for this deviation is the effect of CO on the 0)1 to 0)2 population ratio. On the CO-free surface, the barrier separating the 0)1 and 0)2 states is substantial. Equilibration between these states occurs very slowly at 100 K (the temperature at which exposures were made and EEL spectra were recorded). Therefore, 0)1 production is a process which competes with desorption from the 0)2 state during the temperature programmed desorption experiment from the CO-free surface. The data seem to indicate that CO, even in low concentrations, lowers the barrier between the 0)1 and 0)2 states
27
sufficiently such that equilibrium is attained rapidly (within ~ 5 min). In addition, analogous to the effect mentioned above regarding 02 desorption, some of the heat released during the adsorption of CO may aid the 0)2 species in overcoming the activation barrier toward entering the 0)1 state. This conclusion is corroborated by EELS experiments on co-adsorbed layers. For the CO + 02 system, we observed none of the time-dependent coverage changes, e.g. 0)2 ~ 0)a conversion, reported previously [10] for pure O 2 / P d ( l l l ) layers. At this point it should come as no surprise that the 0)1 coverage displayed in Fig. 5a correlates strongly with the O(a) signal reported in Fig. 5d with the only notable exception corresponding to the data point for the CO-free surface. This behavior is entirely consistent with our previous assertion that the 0)1 state acts as the precursor to molecular oxygen dissociation, that the 0)1 state is produced during heating of the pure O2(a) layer, and that little if any desorption transpires out of this state. The behavior of the O(a) versus CO exposure curve also indicates that the co-adsorbed CO does not induce the dissociation of O2(a). The reduction of the overall O2(a) coverage observed in EELS and TPD is the result of desorption of 02 during CO exposure. With respect to the physical reason for the nonobservation of the hot atom reaction yielding CO 2 in coincidence with O2(a) dissociation, it can only be speculated which of the subtle difference may cause this surprising result. 0 2 dissociation seems to start at higher temperatures on Pd(111) than on Pt(111) and proceeds preferentially via the strongly bound 0)l-state. On the other hand CO coadsorption on Pd(111) shift rapidly the population into the 0)a-State, which is of distinct different chemical character and not observed on Pt(111).
5. Conclusions
The results of this investigation can be summarized as follows: (a) CO decreases the saturation coverage of O2(a) on P d ( l l l ) at 100 K. (b) The initial decrease in coverage is due to the suppression of chemisorption in the 0)2-02 state.
28
K. W. Kotasi~ski et aL / Surface Science 334 (1995) 19-28
(c) CO co-adsorption sufficiently lowers the barrier between the (.01- and o92-O2 states such that equilibration of the o91 to o92 population ratio within the mixed layer occurs rapidly. (d) We have confirmed our previous assignment of the a 3 desorption feature to the o93 state, the a 1 and a 2 desorption features to the o92 state, and the o91 state as the precursor to dissociation into the atomic state. (e) The o91-O2 state competes with CO.for adsorption sites more effectively than does the geometrically :similar o92 state, most likely because of its stronger interaction with the surface than that of the to2 state. ( f ) The weakly bound o93 state also competes more effectively for adsorption sites than o92-02. This is most likely related to o93-O2 seeking a much different adsorption site/geometry which involves less charge transfer than that occupied by either t%-O2, o92-02 or CO. This lends credence to the suggestion that o93-O2 binds in a tilted atop configuration rather than in a flat-lying geometry as do the o91 and to e species. ( g ) In contrast to P t ( l l l ) no CO 2 is produced from the coadsorption system in conjunction with 02 dissociation at around 180 K.
Acknowledgements We thank G. Ertl for continuous support throughout these studies. K.W.K. acknowledges the Alexander von Humboldt-Stiftung for the granting of a post-doctoral fellowship.
References [1] J.H. van't Hoff, Vorlesungen fiber theoretische und physikalische Chemie (Vieweg, Braunschweig, 1898). [2] J.W. Gibbs, Trans. Connecticut Acad. 3 (1876) 108. [3] H. von Helmholtz, Berl. Sitz.-Ber. 23 (1882) 825. [4] H. Le Chatelier, Ann. Mines 13 (1888) 157. [5] L. Boltzmann, Ann. Phys. 22 (1884) 39. [6] M. Planck, Vorlesungen fiber Thermodynamik (Veit, Leipzig, 1897). [7] F. Haber, Thermodynamik Technischer Gasreaktionen (Oldenbourg, Berlin, 1905). [8] T. Engel and G. Ertl, Adv. Catal. 28 (1979) 1. [9] W.D. Mieher and W. Ho, J. Chem. Phys. 99 (1993) 9279. [10] K.W. Kotasifiski, F. Cemi~ and E. Hasselbrink, Chem. Phys. Lett. 219 (1994) 113. [11] R. Imbihl and J.E. Demuth, Surf. Sci. 173 (1986) 395. [12] I. Panas and P. Siegbahn, Chem. Phys. Lett. 153 (1988) 458. [13] X. Guo, A. Hoffman and J.T. Yates, Jr., J. Chem. Phys. 90 (1989) 5787. [14] T. Matsushima, Surf. Sci. 157 (1985) 297. [15] X. Guo and J.T. Yates, Jr., J. Chem. Phys. 90 (1989) 6761. [16] G. Ertl and J. Koch, in: Adsorption-Desorption Phenomena, Ed. F. Ricca (Academic Press, New York, 1972) p. 345. [17] L. Hanley, X. Guo and J.T. Yates, Jr., J. Chem. Phys. 91 (1989) 7220. [18] E. Hasselbrink, H. Hirayama, A. de Meijere, F. Weik, M. Wolf and G. Ertl, Surf. Sci. 269/270 (1992) 235. [19] F. Weik, A. de Meijere and E. Hasselbrink, J. Chem. Phys. 99 (1993) 682. [20] R. Unwin, W. Stenzel, A. Garbout, H. Conrad and F.M. Hoffmann, Rev. Sci. Instrum. 55 (1984) 1809. [21] A. de Meijere, H. Hirayama and E. Hasselbrink, Phys. Rev. Lett. 70 (1992) 1147. [22] W. Wurth, J. StShr, P. Feulner, X. Pan, K.R. Bauchspiess, Y. Baba, E. Hudel, G. Rocker and D. Menzel, Phys. Rev. Lett. 65 (1990) 2426. [23] C./~,kerlund, I. Zoric, J. Hall and B. Kasemo, Surf. Sci. 316 (1994) L1099.
Surface Science 334 (1995) 19-28
Interactions in co-adsorbed CO + O2/Pd(111)layers Kurt W. Kotasifiski 1, Franz Cemi~, Arne de Meijere 2, Eckart Hasselbrink * Fritz-Haber-Institut der Max-Planck-Gesellsehafl, Faradayweg 4-6, D-14195 Berlin, Germany Received 2 January 1995; accepted for publication 29 March 1995
Abstract
The interactions of saturated layers of molecularly adsorbed 0 2 on Pd(lll) with post-dosed CO have been studied with temperature programmed desorption (TPD) and electron energy loss spectroscopy (EELS) in the surface temperature range 100-230 K. It has been found that CO does not react chemically with O2(a) to form CO 2. CO does react with adsorbed O atoms at higher surfaces temperatures resulting in the production of CO 2. Introducing CO to a saturated 02 layer decreases the total O2(a) coverage as well as changing the relative populations of the three chemisorbed 0 2 species. Keywords: Adsorption kinetics; Carbon monoxide; Chemisorption; Electron energy loss spectroscopy; Low index single crystal surfaces; Oxygen; Palladium; Solid-gas interfaces; Vibrations of adsorbed molecules
1. Introduction
Chemical systems tend to minimize their free energy in an attempt to achieve thermodynamic equilibrium. This proposal was first formulated by van't Hoff [1] and its relationship to the second law of thermodynamics has been discussed in the literature [2-7]. This is not the case if energetic or entropic barriers make the rates of the forward and reverse reactions too slow for equilibrium to be reached within practical time scales. Similarly, mass transport constraints, such as non-uniform mixing or - in the case of surface reactions - site blocking, can hinder equilibration. The pathway to chemical equi-
* Corresponding author. 1 Present address: B248 Chemistry, National Institute of Standards and Technology, Gaithersburg, MD 20899 USA. 2 Present address: Laboratorie Kastler Brossel de l'l~cole Normale Sup6rieure, 24, rue Lhomout, F-75231 Paris Cedex 05, France.
librium, that is, the reaction dynamics can, however, be less than straightforward and lead to some surprising results as the system attempts to equilibrate. Thus, even though the thermochemistry predicts what the equilibrium state would be when achieved, it gives us little insight into the rate of equilibration and the mechanism by which equilibrium will be attained. The oxidation of CO to CO 2 on Pd and Pt has received great attention in the literature [8]. On both metals, CO reacts facilely with adsorbed O atoms at around 400 K. On P t ( l l l ) surfaces at roughly 150 K [9], CO also reacts to form CO 2 in the presence of O 2 ( a ) . This reaction channel concurs with the thermally activated dissociation of O2(a) into O(a). Hence it has been interpreted to be due to reaction with nascent O atoms immediately after O 2 ( a ) dissociation, in the spirit of a hot atom reaction. In this work, we show that on P d ( l l l ) neither molecular oxygen nor nascent O atoms react directly with CO. The oxidation of CO on Pd(111) occurs only after
0039-6028/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSDI 0 0 3 9 - 6 0 2 8 ( 9 5 ) 0 0 4 8 7 - 4
20
K.W. Kotasihski et al. / Surface Science 334 (1995) 19-28
complete dissociation of adsorbed O z to the atomic form at surface temperatures far above those that support O2(a). Molecular oxygen adsorbed on P d ( l l l ) is peculiar in that it exhibits three chemisorbed states. We have recently investigated [10] how surface temperature and oxygen coverage affect the populations of the three chemisorbed states, denoted o91, o92 and °)3- We have found that the to 2 state is formed spontaneously upon adsorption at all coverages, that the o93 state is populated only at higher coverages, and that the o92 species can convert to the o91 species by means of a thermally activated process. The molecular O 2 chemisorbed species were first directly observed on Pd(111) by Imbihl and Demuth [11], who observed, in addition to a physisorbed species, three vibrational bands in electron energy loss (EEL) spectra which they assigned to three chemisorbed molecular species. The progression of vibrational frequencies was interpreted in terms of different degrees of electron transfer from the surface into the molecule. As has been shown in theoretical studies [12], electron transfer leads to a weakening of the O - O bond and a strengthening of the O2-surface bond. We introduced the nomenclature o91, o92 and 093 (in order of decreasing chemisorption binding energy but increasing vibrational frequency) to denote these three binding states and the corresponding EEL peaks. Consistent with our later observations described in Ref. [10], Imbihl and Demuth noted that the o91 band, i.e. the most strongly chemisorbed species, is not spontaneously formed upon exposure at Ts = 30 K and that it appears only after warming the crystal. The O2/Pd(111) system has also been studied by Guo, Hoffman and Yates [13] who reported thermal desorption (TD) spectra similar to those reported by us as well as to those reported by Matsushima in an earlier study [14]. Guo, Hoffman and Yates characterized the dissociation of O2(a) into O(a). The onset of dissociation of O2(a) into O(a) occurs at T~ _> 180 K. They determined that the saturation coverage of O2(a) corresponds to 0.62 O / P d and that 0.17 O / P d remain on the surface as O(a) after thermal desorption of O2(a). During thermal desorption from saturated layers of 02 on P d ( l l l ) , three features, labeled a l , a 2 and a 3 by Matsushima [14], are observed at roughly 130,
170 and 210 K. The position and size of these peaks depends on coverage, heating rate, adsorption temperature, presence of co-adsorbed O(a) and annealing of the chemisorbed layer. We have recently demonstrated that the lowest temperature TD feature corresponds to the o93 state, whereas the two higher temperature TD features are correlated with desorption from the o92 state. While we cannot rule out some molecular thermal desorption from the oga state, we have proposed that the predominant fate of the o91 species is dissociation into adsorbed atomic oxygen. The appearance of two TD features arising from the °)2 state is likely due to strong lateral interactions between chemisorbed 02 and between O2(a) and O(a) at T~ > 180 K. CO has been shown to adsorb initially into threefold hollow sites on P d ( l l l ) . As the coverage is increased, CO eventually occupies only bridge sites. At coverages over 0.60 ML (CO/Pd), both bridge and atop sites are occupied. CO is bound more strongly to P d ( l l l ) than is 02, with the onset of thermal desorption occurring above 400 K at low coverage [15]. We have determined, by means of the retarding potential method in EELS, that both CO and 02 lead to an increase of the work function of roughly 1 eV. These work function changes are consistent with the values reported in the literature [16,17]. The results of the investigations into the sensitivity of 02 layers to the presence of co-adsorbed CO are presented here. We find that as the result of competitive adsorption at 100 K, CO decreases the saturation coverage of 02 on P d ( l l l ) . The brunt of this effect is borne by the o)2-02 species. The coverage of o92-O2 decreases monotonically with increasing CO exposure. The displaced o92-O2 partially converts to the o91- and o93-O2 species; however, an even greater amount of the displaced o92-O2 desorbs from the surface. The effects of CO co-adsorption on the EEL and TD spectra corroborate our previous assignments [10] of the chemical fates of the three chemisorbed 02 species.
2. Experimental Experiments were performed in a turbo-pumped UHV chamber with a base pressure of ~ 2 × 10 -1°
21
K.W. Kotasihski et a l . / Surface Science 334 (1995) 19-28
mbar. The P d ( l l l ) crystal is cooled conductively with a liquid-N 2 reservoir and heated radiatively by a W filament. Surface preparation has been described in previous publications [18,19]. TD and EEL spectra provide checks for surface cleanliness and order. The crystal temperature was monitored with a NiCr-Ni thermocouple spot welded to the back of the crystal. The EEL spectrometer has been described in detail elsewhere [20]. Resolution of 12 meV (100 cm -1) and count rates of I X l0 s s-a were routinely achieved. All spectra displayed here were recorded along the specular direction, for an incident angle of 70 ° , and at an incident energy of 2.2 eV (corrected for work function changes). Thermal desorption spectra were recorded with an unshielded quadrupole mass spectrometer. Both TD and EEL spectra are digitized and stored directly on a microcomputer. The thermal desorption results were reproduced in a second vacuum chamber with a different Pd(111) crystal. The crystal is exposed to 02 through a capillary array doser with the gas flow controlled by a precision leak valve. Reproducible exposures are obtained by monitoring the time of exposure and pressure rise in the chamber. The crystal was exposed to 02 at T~ = 100 K. In all experiments presented below the Pd(111) surface was first saturated with O 2 and then exposed to varying amounts of 13CO. The CO was admitted to the chamber through a precision leak valve to establish a uniform background pressure. The reported exposure values for CO are approximate because on one hand the pressure-time product does not include the contribution of the CO pressure rise and fall times. These times are on the order of a few seconds; therefore, they only seriously affect dosing times of < 60 s. On the other hand CO dosing was done with the crystal within the EELS spectrometer, where the pumping speed may be larger than at the preparation level, where the pressure was read. All CO exposures were carried out at ~ 1 X 10 -8 mbar.
3. Results Exposure of saturated 0 2 layers to CO leads to significant changes in 0 2 thermal desorption spectra. A typical multiplexed thermal desorption spectrum
/
Saturated 02/Pd(111) layer -6 F e x p o s e d to 1.1x10 m b a r s C O / ~, Ts=lO0 K
fl J. i; !
ii
;
il
:
i
ii;:=
! !
;-
i ~. ..
o
i
,...-=.
!
;
;
~
.
~coi
i
pi ~ 0 2-i
100
;
.;
i 200
C 300
,;
' 400
Surface Temperature
' 500
600
[K]
Fig. 1. Multiplexed temperature programmeddesorption (/3 = 3.3 K s -z) of masses 32 (O2), 29 (13CO) and 45 (t3CO2) resulting from a saturated 02/Pd(111) layer which has been exposed to ll0X 10-8 mbar.s 13CO.
13C0 and 13C02 desorbing from a mixed 13COq-O2/Pd(lll) layer is displayed in Fig. 1.
of 02,
This layer was produced by exposing a saturated O 2 / P d ( l l l ) layer to 110 X 10 -8 m b a r . s of CO. Fig. 1 demonstrates that no CO 2 product evolves from the surface in the temperature regime where O2(a) is present on the surface, i.e. for T~ < 2 2 0 K. A selection of TD spectra from CO + O 2 layers on P d ( l l l ) is represented in Fig. 2. From this series of spectra, it is immediately obvious that progressive exposure to CO reduces the magnitude of molecular 0 2 desorption. The a s feature is the least effected by CO coadsorption. After an initial shift to lower temperature caused by the lowest CO exposure, this feature remains virtually constant up to exposures of 130 X 10 -8 mbar. s. This CO exposure leads to the adsorption of 0.19 ML CO. At exposures > 130 X 10 -8 mbar. s, the of3 feature is reduced in intensity without significant changes in its form. The higher temperature TD features exhibit immediate and continuous shifts and reduction in intensities with increasing CO exposure. The EEL spectra corresponding to the CO + 0 2 layers which gave rise to the TD spectra of Fig. 2-are
22
K.W. Kotasihski et al. / Surface Science 334 (1995) 19-28
02+CO/Pd(111) T s = 100 K
~'
o~ ='
~
m rsCO
¢ c
/
~ - - ~ ,
13X10-8
U)
~,,'//~,~.._~
O
J
34x10"s
~
72xlO'S
=Z 195x10 "s I
150
100
i
200
250
S u r f a c e T e m p e r a t u r e [K]
Fig. 2. Temperature programmed desorption (/3 = 3.3 K s - ] ) of O 2 from saturated O 2 / P d ( l l l ) layers which have been exposed to various amounts of 13CO. The CO exposure is indicated in the figure. Ts = 100 K for all exposures.
shown in Fig. 3. We observe that the majority to2 species displays a monotonic decrease in EEL intensity with increasing CO exposure. The intensities of both the to1 and to3 transitions exhibit an initial increase in intensity, reach a relatively stable value, then fall again at very high ( > 130 × 10 -s mbar. s) CO exposures. Interestingly, the vibrational frequencies of all three 0 2 states shift very little with
increasing CO coverage. Both the 601 and to2 peaks shift down in frequency by roughly 1 - 2 meV ( 8 - 1 6 c m - t ) . The FWHM of these EEL transitions is also found to increase - the to1 transition broadens from 20 to 25 meV (160 to 200 cm -a) whereas the toe peak increases in width from 13 to 15 meV (105 to 120 cm-1). The to3 peak remains essentially unchanged in peak position and width. The to3 FWHM is ~ 14 meV (113 cm-1), slightly larger than the instrumental resolution of 12 meV (100 cm-1). To quantify these data, the TPD and EELS signals must be calibrated to the coverage. We have done this in the following manner. The TPD signal can be easily calibrated since both the saturation coverage for an 0 2 layer (0.62 O / P d ) and the amount of O(a) remaining after the completion of molecular desorption (0.17 O / P d ) are known [13]. The total desorption signal from a CO-free saturated 0 2 layer thus corresponds to an amount of desorbed 0 2 equivalent to a coverage of 0.45 O/Pd. For the calibration of the EEL signal, we assumed that the oscillator strengths of the three transitions are the same and independent of coverage. Since there is no a priori proof of this assumption, this method of calibration must be tested. In Fig. 4 are presented data for two methods of determining the total 02 coverage on the surface resulting from dosing a saturated layer of chemisorbed 0 2 with various amounts of CO.
O2+CO/Pd(111) Ts= 100 K :. x2O
mbar s CO
" "..,,,.%_
,4
i
0
m2 o~~~
i
i
0.6
13x10"s
~
I+
0,5
TPD-+ O(a)J
~ 0.4 ~"
-
~
-"l I " - 12 meV
It
J
x=o
~_"'~"'"
E z
34x10" 8
.: .":
0
"~ "~-"~
x20 "'. ,'~,",-,,,,,~.'-~.,",,,.,,._ _ CO-Pd ~- -..".....,.< x20 I 0
I
50
:'...
72X10"s ",,.,_,.
". ¢"
..." ".
.
"""f'"P~ ~
100
150
0.2
~ "c-o ~'-" .J
200
0.3
195x10 "e
''P'~"
I
250
300
Energy Loss [meV]
Fig. 3. Electron energy loss spectra recorded from coadsorbed layers of CO + O 2 corresponding to the same layers as those used in Fig. 2.
0
0.1
0.0
0
I
50
I
1O0
t
150
200
CO Exposure [×10"8 mbar s]
Fig. 4. (a) Total initial (O from O2)/Pd coverage as determined from the T P D + O ( a ) EELS method described in the text as a function of CO exposure. (b) Total (O from O2)/Pd coverage as determined by the EEL intensities of the molecular 0 2 vibrational losses as described in the text.
23
K. W. Kotasihski et al. / Surface Science 334 (1995) 19-28
The 0 2 coverage values displayed by the filled symbols in Fig. 4 were determined by a combined TPD and EELS method. Heating the coadsorbed layer to 255 K results in desorption o f O z and dissociation of O2(a) into O(a). However, as has been shown above, O 2 does not react upon thermal dissociation with CO and the onset of the O ( a ) + C O ( a ) ~ CO a reaction occurs at roughly 350 K. This onset is consistent with that reported by Ertl and Koch [16]. Hence, the amount o f chemisorbed 0 2 in the layer prior to heating the surface to 255 K can be determined b y measuring the amount of desorbed O 2 and the amount o f O ( a ) left on the surface as the result of dissociation. The amount of desorbed O 2 is obtained from t h e calibrated O a TD signal. The amount of O ( a ) is determined from the O ( a ) E E L signal, calibrated to the 0.17 O / P d which arises from dissociation in a pure 0 2 layer heated to 255 K. The sum o f the calibrated 0 2 TD and the O ( a ) E E L signals is displayed by" filled symbols in Fig. 4.
0.2
0.1
r
~
Ja)
0.4
>0 0 0
0.2
0(4
t o 255
I
KI
_
0 0')
~
IAfter h e a t i n g
0 -I
i
i
i
I
I
I - ~
i
(d) ~
~"1
I
i
i
i
I
I
I t"""~l
50
100
!1. 0.2
0.1
0-1 0
100
1 0
2 0
O'
1 0
200
C O Exposure [xlO "s mbar s]
Fig. 5. Signal dependence as a function of CO exposure. Coverage of the (a) to1-O 2 state as determined by EELS; (b) to2-O 2 state as determined by EELS; (c) co3-O z state as determined by EELS; and (d) O(a) state as determined by EELS. Magnitude of (e) ot12 thermal desorption (160 < T~< 240 K) reported in O/Pd equivalents and (f) o~3 thermal desorption (100 < T~< 160 K) reported in O/Pd equivalents.
/". 0.3 o.
ov eJ oi
0.2
o 0 0 0
0.1
.m
oe e
i 50 CO Exposure
i 100
i 150
200
[ x l O "8 m b a r s ]
Fig. 6. CO coverage determined from EELS versus CO exposure for CO dosed onto Oz(a)/Pd(lll) surfaces held at 100 K.
The open symbols in Fig. 4 represent the O2(a) coverage as a function of CO e x p o s u r e as determined from the sum o f the o9a, o9a and o93 EEL intensities. The surface temperature throughout these experiments was 100 K. For these data, the total coverage was assumed to be given by the sum o f the c°l, o92 and o93 E E L integrated peak areas normalized to the total area of the o91, °-)2 and 603 peaks arising from the saturated pure O2(a) layer, for which the coverage is known to be 0.62 ML. Comparison of the two data sets in Fig. 4 demonstrates that these two methods of determining the O2(a) coverage agree on average to within 8% for all CO exposures. This gives us confidence that we can use the i n t e g r a t e d E E L peak areas as a measure of the coverage o f the o91, o92 and °-)3 species. F r o m both figures it is clear that the total O z ( a ) coverage decreases monotonically with increasing CO exposure. Re-exposure of these co-adsorbed layers to an amount o f 0 2 that would normally saturate the surface at T~ = 100 K does not lead to an increase of the O2(a) coverage. In the left-hand column o f Fig. 5 are displayed the coverages o f the to 1, o92 and o93 species determined from the integrated EEL l~eak areas as a function of CO exposure. The CO coyerage resulting from these exposures is displayed in Fig. 6. The CO coverage was estimated by assuming that the saturation intensity o f the stretch transition of three-fold co-ordinated CO at ~ 229 m e V (1840 cm - 1 ) corresponds to 0.33 C O / P d [15]. Fig. 6 indicates that the sticking coefficient of CO is constant and close to unity throughout
24
K. W. Kotasihski et al. / Surface Science,334 (1995) 19-28
this coverage range, as has been observed for CO adsorption on oxygen-free P d ( l l l ) surfaces [16]. The coverage of the 092 species is fotmd to decrease monotonically with increasing CO exposure. T h e coverage of the 091 species is initially increased by the adsorption of small amounts of CO ( < 0.1 ML); thereafter, its coverage drops very slowly with increasing CO exposure until exposures of > 130 × 10 -8 mbar- s ( > 0.2 ML CO) lead to a rapid depopulation of this state. The coverage of the o93 species also experiences an initial increase after short exposures to CO and then remains roughly constant• until CO exposures > 130 X 107~8 m b a r s cause a rapid decline in the 093 population. The right-hand column of Fig. 5 presents the results of heating the co-adsorbed CO + 0 2 layers. Fig. 5d displays the O( a) coverage as estimated from the EEL intensity of the O - P d stretch. Adsorbed oxygen results f r o m the dissociation of O2(a) induced by heating the chemisorbed layers to 255 K. Fig, 5 e displays the integrated amount of 0 2 which desorbs in the surface temperature region 160 _< Ts _<230 K. We denote this al, 2, i.e., the amount of O2(a) which desorbs through the ceI and a 2 TD features. Fig, 5f displays the integrated amount of 0 2 which desorbs in the surface temperature region 100 _< Ts _< 160 K. We denote this a3, i.e., the amount of O2(a) which desorbs through the a 3 TD feature. :Justification for setting the boundary between the a 3 and at, 2 features to T s = 160 K is drawn from the general shapes of the TD features, the disappearance of the09 3 EEL peak for surfaces heated to T~ >_ 160 K, the,observation of a roughly constant 092 intensity for surfaces heated to T~ _< 160 K, and a posteriori from correlations of the a 3 feature with 093 and the al, 2 feature with 092' While placing this boundary at T~ = 140 or 150 K changes the magnitudes, this choice does not change the shape of the ot12 and a 3 curves, What is clear from inspection o f the left- and right-hand columns of Fig. 5 is that, in general, the CO exposure dependence of the O(a) signal follows the CO exposure dependence of the :to1 state; the CO exposure dependence o f the al, 2 TD feature follows that of the 09~ state; and the CO exposure dependence of the a 3 TD feature follows that of the 093 s~te: ,The only deviation from these trends is observed~ for the initial points i n the w 1 and O(a)
curves. These deviations will be discussed in the next section.
4. Discussion 4.1. Effects of CO exposure on 02(a) at 100 K
Inspection of the EELS-determined coverage versus CO exposure curves in Figs. 4 and 5 shows that O2(a) is forced to adjust in order to accommodate co-adsorbed CO(a). The presence of co-adsorbed CO(a) dramatically reduces the coverage of 092-02. Some of the 092-02 is forced to desorb from the surface, as is indicated by the decrease in the total O2(a) coverage displayed in Fig. 4, while some of the o)2-0 2 is converted into the 091- and 093-0 2 chemisorption states. We have previously demonstrated [10] that an activation barrier separates the 09a and to 2 states such that the o)1 state is not populated directly from the gas phase, but rather, by way of a thermally activated process out of the w 2 state. As a consequence, the 091 state is only observed in a pure Oe(a) layer when this layer has been annealed for some length of time which depends on the annealing temperature. We propose here that co-adsorbed CO lowers the barrier between the to 1 and 092 states; thus facilitating the rapid population of the 091 and rapid equilibration of the populations in the w I and 092 states. The accelerated equilibration may be the result of a reduced barrier height caused by lateral interactions and the effects of CO adsorption on the substrate a n d / o r cooperative effects arising during CO adsorption (see below). This would explain why the 091 coverage first increases with CO exposure. Apparently, however, the to I coverage is limited to some saturation value. This saturation value is probably determined by two factors: (i) the availability of surface sites which can transfer sufficient charge to the 0 2 moiety and (ii) competitive adsorption with CO as the CO exposure is increased. Oxygen adsorbed in the 091 state, which is the most strongly bound O2(a) species, can compete more successfully with CO for adsorption sites than the more weakly bound 092-0 2 species. This argumentation - a lowering o f the barrier between the 09a and 092 states combined with competitive adsorption between 0 2
K.W. Kotasihski et aL / Surface Science 334 (1995) 19-28
and CO - can adequately explain the trends observed in the 601 and to 2 coverage versus CO exposure curves. Since competitive adsorption between CO and 0 2 must be invoked to explain the decrease in total O2(a) coverage with increasing CO exposure, it seems surprising that the to 3 state - the most weakly bound O2(a) state, which is only observed at high coverages - is able not only to hold its own versus CO, but also, that the coverage in this state is found to increase at low CO exposure. Certainly simple rationalization of the competitive adsorption process, that is, that the most weakly bound species is the most susceptible to attack from a more strongly bound species, does not suffice to explain this behavior. Two possible contributing factors for the surprising stability of the to3 state in the face of CO co-adsorption can be found in its electronic structure and its assumed adsorption geometry. Because t % - O 2 is only observed at high 0 2 coverage, we can presume that the t% species is not stably formed on the P d ( l l l ) surface as long as the coverage allows for the creation of the to I and to 2 states. In other words, a barrier between the to2 and to 3 states is extremely small to nonexistent; thus, the equilibrium between o92- and t03-O 2 lies essentially completely on the side of the more strongly bound w 2 state up to the point where this state saturates. Saturation will presumably arise from increasing lateral repulsion between the negatively charged 0 2 molecules. Formation of the w 3 species involves a smaller amount of charge transfer from the substrate. Hence, the lateral repulsion it experiences will also be smaller, allowing it to be adsorbed within otherwise densely packed 0 2 layers. It may be worth noting that t03-O 2 is also least affected by coadsorbed O(a), which points in the same direction. The adsorption geometry of the to 3 state has not been unambiguously determined; however, comparison to analogous organometallic complexes and information obtained on the angular distribution of photodesorbed t % - O 2 [21] suggest that the adsorbed 0 2 molecules are bound through one end of the molecule with the molecular axis inclined somewhat with respect to the surface normal. This configuration contrasts with the lying-down (molecular axis parallel to the surface) geometries determined for the o~1 and to 2 species [11,22]. The inclined geometry of
25
the to 3 state could facilitate the packing in of 0-)3--0 2 into dense O2(a) and O2(a) ~- CO(a) layers. Our suggestion, therefore, as to why competitive O 2 - C O adsorption proceeds at the expense of to202, whereas tO1 and to 3 survive, is that to a can more effectively compete with CO on the basis of its stronger (with respect to the w 2 state) adsorbatesurface bond, while the to3 state can survive on weak binding sites which do not appear to be strongly affected by the presence of co-adsorbed CO until the CO coverage exceeds ~ 0.2 ML. We note that our work function change measttrements indicate that CO and O 2 are electron withdrawing groups of similar strength; thus, it is perhaps not too surprising that ~o3-O 2 can co-exist with C O ( a ) s i n c e it has already been demonstrated that it can co-exist with the strongly electron withdrawing w 2 and to 1 species, as well at O(a). Note that, on the one hand, we should n o t be surprised by the displacement of 0 2 by CO - the system is merely trying to minimize its free energy by means of the exchange reaction (1) listed below. Since the binding energy of CO(a) (1.5 eV [15]) is more than twice that of O2(a) (0.65 e g roughly estimated from thermal desorption), the exchange reaction O2(a ) + CO(g) ~ O2(g ) + CO(a)
(1)
is highly exothermic and at equilibrium for a temperature where O2(a) is stable, we should expect a Pd surface exposed to an equimolar mixture of 0 2 and CO to be completely covered by CO(a) with very little O2(a) present on the surface. Nonetheless, the efficiency of this process is strikingly high. Fig. 7 displays the dependence of the total O2(a) coverage on the CO coverage determined, in both cases, by EELS. The solid line in the figure represents the O2(a) coverage dependence that would be expected if 0.8 0 2 molecules w e r e to desorb for every one CO molecule that adsorbs. The error margin on this number is largely determined by the uncertainty in the 0 2 saturation coverage. The coverage versus exposure data also indicates that the CO sticking coefficient remains near the yalue for an O2(a)-free surface. Hence, virtually every CO collir sion with the surface dislodges an 0 2 molecule. Recent studies [23] of CO + 0 2 on Pt(111) have
26
K. W. Kotasihski et al. / Surface Science 334 (1995) 19-28
0.6 ~"0 ~ & ~ 0 ~ 0
0.5
"•
O2(a) coverages do not follow the energetic ordering of the o91, o92 and o93 species, i.e., we have observed that the o92 species is displaced before the more weakly bound o93 species. This is related to a breakdown in the assumption that all adsorption sites are equivalent. Nonetheless, at sufficiently high CO exposures; virtually all of the O2(a) is displaced from the surface.
CO + 02/Pd(111) T s = 100 K
0.4 11.3
0
~
~
0
0.2 0.1 L
I ~ I 1).1 0.2 CO Coverage [CO/Pd]
,
I O.3
Fig. 7. Total O2(a) coverage versus CO exposure for CO postdosed onto saturated 02 layers. The solid line indicates the behavior expected if each adsorbing CO molecule removes on average ~ 0.8 O2(a) from the surface.
reported much lower 0 2 removal efficiencies than that reported here. The results of our experiments show that O2(a) is not effective at blocking the adsorption of CO. This is not surprising since CO adsorbs by means of a precursor-mediated mechanism on CO-covered P d ( l l l ) [16]. Evidently O2(a) does not disrupt the CO precursor state and CO also adsorbs via a precursor-mediated pathway on CO + O z coadsorbed layers. While the data in Fig. 6 exhibit some small deviations from a unit CO sticking coefficient independent of coverage over this exposure regime, a more exacting calibration of the CO exposure and direct comparison to CO sticking on pure CO layers would be required to reveal whether co-adsorbed O 2 subtly influences the sticking dynamics of CO. Since 0 2 is not effective at blocking the adsorption of CO and since, as demonstrated by our EELS results, the 0 2 saturation coverage is reduced by coadsorbed CO, the net result of CO post-dosing is to effectively accelerate the rate ofoO 2 desorption. This acceleration, as discussed by Akerlund et al. [23], may result from repulsive interactions, a transfer of the CO heat of adsorption to O2(a) or a combination of both, or some other process upon which we shy from speculating about. Thus, given a sufficient exposure of CO, we should expect that virtually all O2(a) should be removed and that CO will saturate the surface. On the other hand, our experiments demonstrate that along the path to equilibration, the decay in the
4.2. Effects o f C O on 02(a) thermal chemistry
We have found that no CO 2 production occurs in the temperature regime in which O2(a) is present on the Pd(111) surface. This demonstrates that the chemical behavior of a CO + O2/Pd(111) co-adsorbed layer is different than that observed on Pt(111) surfaces where a reaction producing CO 2 at 150 K has been observed [9]. Our own investigations of the CO + O 2 / P t ( l l l ) system confirm the occurrence of this low T~ CO + O 2 reaction. On P d ( l l l ) we find that CO 2 is first formed above 350 K; that is, CO 2 production is the result of a reaction between CO(a) and O(a). No CO 2 producing reaction between CO(a) and O2(a) or CO(a) and so-called hot atoms created in the dissociation of O2(a) ~ 2 0 ( a ) is observed on Pd(lll). CO co-adsorption has a marked effect on O 2 TD spectra. The a 1 feature is severely suppressed and shifted to lower Ts. T h e a z feature becomes less well-resolved but does not appear to shift strongly in temperature. The a 3 feature is initially shifted to lower T~, increasing first in intensity, then declining at high CO exposures. As shown in Fig. 4b, the total amount of molecularly desorbed O 2 decreases monotonically with increasing CO exposure. From Fig. 5e it can be seen that the total amount of O2(a) desorbing through the 0(1 and ce2 features also decreases monotonically with increasing CO. We have previously proposed [10] that both the cq and the c~2 desorption features result predominantly from desorption out of the o92 state whereas the a 3 feature results from desorption out of the o93 state. If this were true, there should be strong correlations between (i) the o92 coverage and the amount of 02 which desorbs by way of the a 1 and a 2 channels and (ii) the o93 coverage and the intensity of the c~3 feature. By comparing the CO exposure dependence of the o92 coverage displayed in Fig. 5b
K. W. Kolasihski et aL / Surface Science 334 (1995) 19-28
o.s
Q
Saturated O2/Pd(111) layer T s = 100 K
0.5
o o
0.3
~
o.~'
....... -q
0.1 0
.~. s O ~ l O ~ T n ~ r ~ i ~ n o v e r a I 0
I 50
I 1oo
150
I 200
CO Exposure [ x l 0 a mbar s]
Fig. 8. Comparison of the oxygen coverage thermally desorbed from the surface with the coverage contained in the to 2 and oJ3 states prior to the initiation of thermal desorption.
with the CO exposure dependence of the magnitude of the O/1,2 desorption signal (Fig. 5e), we find that not only the trend of these two signals is the same, but also that the agreement in absolute values is nearly one to one. Similarly good agreement is found between the 0)3 and the a 3 signals displayed in Figs. 5c and 5f, respectively. As a further test of whether thermal desorption of O2(a) proceeds predominantly through the 0)2 and 0)3 states, we display in Fig. 8 the CO exposure dependence of the total amount of thermally desorbed O 2 as well as the sum of the 0)2 and 0)3 coverages determined from EELS. The agreement is very good (within 5%) except for the one point from the CO-free surface. The reason for this deviation is the effect of CO on the 0)1 to 0)2 population ratio. On the CO-free surface, the barrier separating the 0)1 and 0)2 states is substantial. Equilibration between these states occurs very slowly at 100 K (the temperature at which exposures were made and EEL spectra were recorded). Therefore, 0)1 production is a process which competes with desorption from the 0)2 state during the temperature programmed desorption experiment from the CO-free surface. The data seem to indicate that CO, even in low concentrations, lowers the barrier between the 0)1 and 0)2 states
27
sufficiently such that equilibrium is attained rapidly (within ~ 5 min). In addition, analogous to the effect mentioned above regarding 02 desorption, some of the heat released during the adsorption of CO may aid the 0)2 species in overcoming the activation barrier toward entering the 0)1 state. This conclusion is corroborated by EELS experiments on co-adsorbed layers. For the CO + 02 system, we observed none of the time-dependent coverage changes, e.g. 0)2 ~ 0)a conversion, reported previously [10] for pure O 2 / P d ( l l l ) layers. At this point it should come as no surprise that the 0)1 coverage displayed in Fig. 5a correlates strongly with the O(a) signal reported in Fig. 5d with the only notable exception corresponding to the data point for the CO-free surface. This behavior is entirely consistent with our previous assertion that the 0)1 state acts as the precursor to molecular oxygen dissociation, that the 0)1 state is produced during heating of the pure O2(a) layer, and that little if any desorption transpires out of this state. The behavior of the O(a) versus CO exposure curve also indicates that the co-adsorbed CO does not induce the dissociation of O2(a). The reduction of the overall O2(a) coverage observed in EELS and TPD is the result of desorption of 02 during CO exposure. With respect to the physical reason for the nonobservation of the hot atom reaction yielding CO 2 in coincidence with O2(a) dissociation, it can only be speculated which of the subtle difference may cause this surprising result. 0 2 dissociation seems to start at higher temperatures on Pd(111) than on Pt(111) and proceeds preferentially via the strongly bound 0)l-state. On the other hand CO coadsorption on Pd(111) shift rapidly the population into the 0)a-State, which is of distinct different chemical character and not observed on Pt(111).
5. Conclusions
The results of this investigation can be summarized as follows: (a) CO decreases the saturation coverage of O2(a) on P d ( l l l ) at 100 K. (b) The initial decrease in coverage is due to the suppression of chemisorption in the 0)2-02 state.
28
K. W. Kotasi~ski et aL / Surface Science 334 (1995) 19-28
(c) CO co-adsorption sufficiently lowers the barrier between the (.01- and o92-O2 states such that equilibration of the o91 to o92 population ratio within the mixed layer occurs rapidly. (d) We have confirmed our previous assignment of the a 3 desorption feature to the o93 state, the a 1 and a 2 desorption features to the o92 state, and the o91 state as the precursor to dissociation into the atomic state. (e) The o91-O2 state competes with CO.for adsorption sites more effectively than does the geometrically :similar o92 state, most likely because of its stronger interaction with the surface than that of the to2 state. ( f ) The weakly bound o93 state also competes more effectively for adsorption sites than o92-02. This is most likely related to o93-O2 seeking a much different adsorption site/geometry which involves less charge transfer than that occupied by either t%-O2, o92-02 or CO. This lends credence to the suggestion that o93-O2 binds in a tilted atop configuration rather than in a flat-lying geometry as do the o91 and to e species. ( g ) In contrast to P t ( l l l ) no CO 2 is produced from the coadsorption system in conjunction with 02 dissociation at around 180 K.
Acknowledgements We thank G. Ertl for continuous support throughout these studies. K.W.K. acknowledges the Alexander von Humboldt-Stiftung for the granting of a post-doctoral fellowship.
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