J. Phys. Chem. 1995,99, 6167-6175
6167
Sticking Probabilities for CO Adsorption on Pt(ll1) Surfaces Revisited J. Liu,? M. Xu, T. Nordmeyer, and F. Zaera* Department of Chemistry, University of Califomia, Riverside, Califomia 92521 Received: January 10, 1995@
The coverage dependence of the sticking probaiblity for carbon monoxide on R(111) surfaces was investigated by using the dynamic method originally devised by King and Wells. The CO uptake was studied as a function of CO beam flux, surface-to-doser distance, and surface temperature. The sticking probability on the clean surface was found to be quite high in all cases, about 0.8, and to remain approximately constant at low ) layer forms temperatures up to coverages close to 0.50 monolayers (ML), at which point a ~ ( 4 x 2 ordered on the surface. This behavior is explained by a model originally proposed by Kisliuk where the molecules adsorb on a highly mobile extrinsic precursor state before migrating to their final chemisorbed state. Above 0.50 ML the sticking probability then drops suddenly, presumably because the chemisorption energy drops as compressed CO layers start to form on the surface. The saturation coverage varies at low temperatures with CO beam flux because of the induced changes in the adsorption-desorption dynamics at the high coverages by the changes in CO impinging rates. The effect of inhomogeneities in the spatial distribution of the CO beam across the surface was studied by changing the distance between the sample and the doser, which was found to affect the overall shape of the CO uptake curves. Finally, the adsorption kinetics was determined as a function of surface temperature: the uptake was found to change from precursor mediated below 200 K to a more Langmuir type behavior around room temperature.
1. Introduction The adsorption of carbon monoxide on Pt( 111) is one of the most thoroughly studied systems in the field of surface science.'-9 Besides the basic knowledge that may be derived from examination of seemingly simple systems such as this, understanding CO adsorption on transition metals is also important in connection with catalytic processes such as Fischer-Tropsch synthesisIn-l2and CO oxidation in automobile catalytic converter^.'^-'^ Moreover, kinetic studies of catalytic reactions have demonstrated that the adsorption of the reactants often control the rate of the overall p r o c e ~ s e s , ' ~so~ 'several ~ research groups have set out to investigate that step in some detail. In the case of CO, quite a few different techniques have been used to study the effect of parameters such as coverage, temperature, and incident CO energy on the kinetics of the adsorption. -9 In spite of the number of studies already published in this area, there are still a few unanswered questions with respect to the dynamics of the CO adsorption. The purpose of the research reported here was 2-fold: (1) to characterize the kinetics of the adsorption of carbon monoxide on platinum surfaces under conditions resembling those encountered in high-pressure catalytic reactions, and (2) to determine the effect of the most relevant experimental parameters on the data obtained by using the dynamic method originally developed by King and Wells for measuring sticking coefficients.'8-2n In this paper we report results from measurements on the coverage dependence of the CO sticking probability on clean Pt(ll1) surfaces. We have focused on a couple of poorly studied aspects of the adsorption process, namely, on the reversibility of the CO chemisorption at high coverages and on the temperature dependence of the sticking rate of CO on the surface. Our studies indicate that CO adsorption on Pt( 111) at low temperatures ((200 K) is mediated by a mobile extrinsic Permanent address: Department of Chemistry, Tsinghua University, Beijing, People's Republic of China 100084. Abstract published in Advance ACS Abstracts, April 1, 1995. @
0022-365419512099-6167$09.00/0
precursor state that makes, in effect, the CO sticking probability constant, about 0.8, up to coverages around 40% of a monolayer. A dynamic adsorption-desorption equilibrium is established at high coverages, where the heat of adsorption is low, causing the saturation coverage to be affected by the incident CO beam flux: the higher the flux, the higher the equilibrium coverage. Finally, at higher surface temperatures the sticking probability decreases monotonically with increasing surface CO coverage, in a fashion that may be described by the Langmuir model of adsorption. With respect to the experimental details of the method devised by King and Wells, the changes in the CO uptake were characterized as a function of CO incident beam flux and surface-to-doser distance. As mentioned above, the beam flux was shown to directly affect the equilibrium saturation coverage on the surface when the desorption rates are not negligible. The homogeneity of the gas flux across the crystal surface, which depends on parameters such as the surface-to-doser distance, also modifies the observed dependence of the sticking probability on surface coverage. Even though it would seem as if it would be desirable to set up the experiment so the singlecrystal surface intercepts as much of the adsorbate's beam as possible, it was found that the inhomogeneity of the profile of the beam brings about some artifacts in the results obtained under such arrangements. Consequently, the ideal doser-tosurface distance has to be chosen as a compromise between the fraction of the beam being intercepted and the spatial homogeneity of that beam. Finally, the contribution of the background gases to the total CO uptake was estimated to amount to about 5- 10% under most experimental conditions.
2. Experimental Details All the experiments reported here were performed in a 6.0 L stainless steel ultrahigh-vacuum (UHV)chamber evacuated with a 300 Us turbo-molecular pump to a base pressure of about 3 x 10-ln Torr. This chamber is equipped with a UTI lOOC 0 1995 American Chemical Society
Liu et al.
6168 J. Phys. Chem., Vol. 99, No. 16, 1995
COIPt(ll1) Uptake at 300 K L = 1.00 cm Q = 9.7~10'~molec s"
Turbo
Molecular
r
I
Spectrometer Chamber
Flag down
Flag i
0
20
40
60
Time I s Figure 2. Typical uptake curve for CO on Pt(ll1) at 300 K obtained by using the setup shown in Figure 1. The surface-to-doserdistance was L = 1.00 cm, and the CO beam effusive rate was Q = 9.7 x l O I 3 molecules/s. Figure 1. Schematic representation of the UHV system used in the
experiments reported here (top view). quadrupole mass spectrometer, a sputtering ion gun, a molecular beam doser, and a sample holder capable of three-dimensional translation and on-axis rotation (Figure 1). The collimated beam doser consists of a 1.2 cm diameter multichannel array made up of microcapillary glass tubes 2 mm in length and 10 pm in diameter each. The gas flux is set by setting both leak valve 1 and the backing gas pressure in volume A, which is measured by a MKS Baratron pressure gauge initially calibrated against the equilibrium vapor pressure of water at different temperatures. A movable stainless steel flag was placed between the doser and the sample in order to be able to intercept the beam at will. The sample, a 0.9 cm diameter platinum single crystal cut in the (1 11) direction and polished using standard procedures, was mounted by spot-welding two tantalum wires to the back of the crystal. Most of the holding parts in the manipulator were placed behind the sample so that adsorption of the gas from the doser onto the manipulator could be minimized. The sample could be either cooled to liquid nitrogen temperatures or heated resistively to 1300 K; the temperature was measured using a chromel-alumel thermocouple spot-welded to the back of the crystal. The platinum crystal was cleaned in situ by a combination of oxygen treatments at 1100 K and Ar+ ion sputtering-annealing cycles before each experiment. The carbon monoxide (99.9% purity) was obtained from Liquid Carbonic Specialty Corp. and used as supplied. The CO pressures in the vacuum chamber were measured by using a Bayard-Alpert nude ion gauge and corrected by the correspondN 0.90).21 ~ ing sensitivity factor ( S C ~ / S= The CO beam flux was set before each group of experiments by using the following procedure: (1) The three-way valve 3 was initially tumed so that region A was evacuated; (2) region B was filled with CO to a specified backing pressure by using leak valve 2 ; (3) valve 3 was then switched to allow for the expansion of the CO in region B to region A; and (4)the beam flux into the UHV chamber was set by adjusting leak valve 1. The beam effusive rate was monitored by measuring the UHV equilibrium pressure, Peq,taking advantage of the fact that the two parameters are proportional to each other. After the initial setting of the flux, steps 1-3 were repeated for each experiment without altering the position of valve 1, ensuring the same beam effusive rate as in the calibration experiment. The effusive rates could then be modified by changing the backing pressure PA+B,
making use of the fact that again those two parameters are linearly related. In any given dosing experiment the CO partial pressure was recorded as a function of time by using the quadrupole mass spectrometer located in the back of the UHV chamber. Figure 2 shows a typical uptake trace for CO on Pt( 111) at 300 K. At time zero valve 3 is switched to the filling position (doser on, step 3 above) to initiate the leaking of the CO into the UHV chamber. With the flag blocking the direct beam, the CO partial pressure in the UHV chamber is allowed to rise to a steady value, a process that takes a few seconds. At time t = ti the flag is then removed, allowing the beam to strike the surface directly and causing the CO partial pressure to drop instantly because of the removal of some of the CO molecules from the gas phase upon adsorption on the surface. After some time the platinum surface becomes saturated, and therefore incapable of adsorbing CO any longer, at which point the CO partial pressure in the vacuum chamber retums to the steady value; after that the flag is retumed to the blocking position (t = te). Finally, valve 3 is switched back (doser off) to clear region A, causing the CO partial pressure to drop to the original base pressure.
3. Sticking Probability and Coverage Calculations In this section the method used to analyze the experimental data obtained using the setup described above will be discussed. Previous methods have been extended here in order to include the contribution of adsorption from background gases to the sticking coefficient measurements. First, the time dependence of the sticking probability, s(t), is calculated by using the formalism originally developed by Madey.I9 According to his derivation
where, as indicated in Figure 3a, P(t) denotes the CO partial pressure measured experimentally, Peq(t) represents the CO partial pressure expected if the direct beam were blocked (calculated by a least squares method using a combination of an exponential and a linear function), Pbase(t) is the original base pressure (best fit using a linear function), andf is the fraction of impinging molecules intercepted by the crystal surface (determined experimentally, as described later). Figure 3b displays the time dependence of the sticking probability for CO on Pt(ll1) at 300 K calculated by applying eq 1 to the raw data displayed in Figure 3a.
CO Adsorption on Pt(111) Surfaces
CO/Pt(l 11) Uptake at 300 K 1.51
1.0 1
,
h
a x 0.8 -5 3 0.6 -
J. Phys. Chem., Vol. 99, No. 16, 1995 6169
COPt(111) Uptake at 300 K L 1.OO cm Q = 9 . 7 ~013 1 molec s" ,.d--w I
n
0.8
1
n e 0.4 ul
"'w *%
-
x,. \
(b) Stlcking Probablllty
0.0
0.2
0.4
0.6
0.8
Coverage, e / ML 0.0 0.6
-
Figure 4. Sticking probability vs coverage for CO adsorption on Pt(111) at 300 K, as calculated from the data in Figure 3.
(c) co\
The time dependence of the total CO adsorbed on the surface, N(t), is obtained by adding both the direct and background adsorption terms,
(direct beam)
0.0
N(r) = Nd(t)
(background)
+ Nb(t)
(6)
and the absolute adsorbate coverage at time t, O(t), is thus given by
1 e(?)= N(t) -= Jr[aAP(t) + P s ( t ) P ( t ) ]dt A A 0
where a is a constant that does not depend on beam flux, sample-to-doser distance, or surface temperature. The total number of molecules adsorbed on the surface from the direct incident beam, Nd(t), is then given by Nd(t) = a Jr[Pe,(t) - P ( t ) ]dt = a Jr@(t) dt
(3)
The rate for background adsorption dNb(t)/dt, on the other hand, is proportional to the CO partial pressure in the vacuum chamber, P(t) and to the sticking probability at the corresponding coverage:
(4) where B is another constant, also independent of beam flux, sample-to-surface distance, and surface temperature. The number of molecules that adsorb on the surface from the background gases, Nb(t), is then
The initial sticking probability SO was used in this expression for times before t = q because ~ ( tcan ) only be evaluated after time ti, when the flag is removed. This approximation has only a small effect on the calculation of surface coverages, since the total adsorption before flag removal amounts to less than 10% of saturation in all cases.
(7)
where A is the area of the crystal surface (measured in our case to be 0.64 cmz, or 9.6 x l O I 4 platinum surface atoms). The values of a and /?in this equation are obtained by independent calibration experiments (see below). Figure 3c shows the time dependence of the direct, background, and total CO coverages for the data given in Figure 3a, as calculated by using eqs 3, 5 , and 7, respectively, and Figure 4 shows the corresponding dependence of the sticking probability on surface coverage obtained by combining the data from parts b and c of Figure 3. Coverages in these figures are reported in monolayers, ML, which are defined as the number of adsorbed molecules per platinum surface atom.
4. Calibration of the Apparatus In order to be able to convert the raw data obtained with the King and Wells technique into uptake curves such as that shown in Figure 4,the values forf, a,and B in eqs 1 and 7 need to be determined. The fraction of the beam intercepted by the crystal, was determined by measuring the cyclohexane partial pressure drop associted with the condensation of cyclohexane molecules on the Pt surface at 90 K. The raw data from these experiments are shown in Figure Sa for several surface-to-doser distances L, and the corresponding f values calculated by using eq 1 (and by assuming a unity sticking probabilityz2)are plotted as circles in Figure 5b. A small adjustment to the initial calculated values was required in order to take into account the adsorption that occurs on the sample holder. Average values calculated by Campbell et al.23and Winkler et al.24 are displayed in the same graph for comparison (solid curve). Our experimental values tumed out to be somewhat higher than the theoretical ones, perhaps because while the calculations were done for capillary length-to-diameter ratios between 40 and 50,23,24 the value for that ratio in our doser is 200. A larger ratio is expected to yield a better collimated beam and to thus result in a larger value for
.f
Next we describe the procedure used to estimte the value for
a. This parameter is in principle equivalent to the pumping speed of the system for CO, which can be determined by initially setting a given steady CO flux into the chamber (by using the
6170 J. Phys. Chem., Vol. 99, No. 16, 1995
Liu et al.
CO/Pt(lll) at 90 K (a) Raw Data
.-E u)
c
(a) Uptake
(b) TPD 28amu
L.4.75 cm Q=8.5x1d3molec s
e
"e
J
p.
=9.6
$
u) u)
g0
.
O 50
0
. 100
I
-at P
l
150
I /
Unblocked Beam
J
i
I
5 n
Time I s
'
0
0
.c
e 0.8-
\\It-
E
f
0.6-
0
0.4-
.-c 0.20 c 0
5
10
15
300
400
500
600
700
Time I s Temperature / K Figure 6. (a) CO uptake spectra from experiments with blocked (PbJ and unblocked ( P u b ) beams under identical conditions (L = 0.75 cm and Q = 8.5 x I O l 3 moleculesls). (b) Corresponding temperatureprogrammed desorption spectra for the experiments in (a).
Experimental
-
.c
E
Blocked Beam
(b) Intercepted Beam Fraction
c-
Calculated
LIcm Figure 5. (a) Cyclohexane uptake on Pt(ll1) at 90 K for various surface-to-doserdistances L. (b) Fraction of beam intercepted by the sample, as a function of surface-to-doser distance L. The circles represent the experimental values calculated from the data in (a), while the solid curve is an approximate interpolation of the data reported in refs 22 and 23.
eq 7 for the uptake data at 300 K assuming that the CO saturation coverage on Pt( 111) at that temperature is Os,, = 0.50 ML. The final value obtained this way was a = 610 f.40 Ws = (1.44 f 0.10) x lo7 ML cm*/(Torr s). The determination of the beam effusive rates also allows for the development of an altemative procedure for calculating the contribution of the direct beam to the buildup of the surface coverage. This is done by first estimating the average beam flux, F, impinging onto the surface by using the formula
CO doser) and then following the time evolution of the CO partial pressure, PTJHV, after suddenly shutting off the CO source. The pumping speed S, is calculated from these data by using the formula
and then calculating the absolute surface coverages by integrating the sticking probabilities over time:
L
"."
I
I I
I
I
I
I
I
0.0
0.5
1.0
1.5
2.0
2.5
where VUHV is the volume of the vacuum chamber. A value S, = 300 f 20 L/s was obtained this way. Alternatively, S, can be also (more accurately) calculated by first determining the total effusive rate of the beam doser, Q. This can be done by following the changes in CO pressure in the regions A and B behind the doser, PA+B,as a function of time. Q is given by
(9) where VA+Bis the combined volume of regions A and B, and S, is given by
Q sp= PUHV
A value S, = 260 It 5 L/s was estimated by this method. As it can be seen, the calculations from both methods agree within 15%. It could then be assumed that a = S, = 260 Ws, but the use of this value when analyzing the CO uptake data yields surface coverages much lower than expected. One possible reason for this discrepancy could be the fact that since some parts of the manipulator may act as getters, the local pumping speed around the sample may in effect be larger than that measured in our experiments. It was therefore decided that the value of a needed to be recalibrated. This was done by using
Finally, the value of /3 should be related to the impinging frequency of gases on surfaces, which can be directly estimated by using the kinetic theory of gases:
where vav is the average speed of the gas molecules and M is their molecular weight ( M = 4.65 x glmolecule). For CO at 300 K the value calculated this way comes out to be /3 = 1.63 x lo5 ML cm2/(Torr s). However, as in the case of a, the true value of /3 depends on the actual pressure of CO around the sample, which can easily be different than that measured by the ion gauge. The alp ratio was therefore independently estimated by measuring the relative contributions from the direct beam and the background gases in a given experiment to the overall CO adsorption. Figure 6 shows a typical example of the data used in these calculations. The left panel (Figure 6a) displays uptake Curves obtained with the beam both blocked (Pbl) and unblocked (Pub) under the same set of dosing conditions, Le., at fixed surface temperature (T = 90 K), beam effusive rate (Q = 8.5 x lOI3 molecules), and surface-to-doser distance (L = 0.75 cm). The right panel (Figure 6b) shows the corresponding CO TPD spectra obtained after stopping the uptake at tl and waiting until t 2 . Only background adsorption is possible when the beam is blocked, while both the direct beam and the background contribute to the uptake when the beam is
J. Phys. Chem., Vol. 99, No. 16, 1995 6171
CO Adsorption on Pt(111) Surfaces
L = 0.75 cm
CI
9 1.0
8
E
p 0.8
1
I
0.6
8
0.6
0.4 z3 0.2
.8
0.0
0.0
0.2
0.4
0.6
0.8
0
1.0
Coverage, e I ML
Figure 7. Coverage dependence of the CO sticking probability on Pt(1 11) at T = 90 K and L = 0.75 cm as a function of beam effusive
50
100
M L ~ / F
0.0
I
I
I
I
rate Q.
allowed to impinge directly on the surface. By applying eq 7 to both cases, the following expression is derived:
&=--'unblocked 'blocked
If the CO dosing time t2 is chosen to be short enough so that the sticking probability is rendered approximately constant (SO x 0.85), eq 14 can be reduced to
with
The apparent small rise in the sticking probability during the initial stages of the CO uptake seems to be related to a transient induced by the removal of the flag. This rise is more noticeable at high CO beam fluxes, and its behavior over time is the same regardless of the effusive rate (it extends to higher coverages at higher fluxes). It could be thought that this effect is the result of an experimental artifact, except that the same behavior was not seen in the cyclohexane conciensation experiment (Figure 5). Also, the initial changes reported here have been reported previously by other researcher^.^^,^^ One possible explanation for this behavior could be that the presence of a small number of CO molecules on the surface could help accommodate the kinetic energy of the new molecules arriving from the gas phase. This type of argument has been previously used to explain the similar behavior reported for the uptake of nitrogen on Ru(001)
surface^.^' from which
-a_@
s-
Re-Rp
Ol-R,
Application of eq 17 to the data from Figure 6 yielded a value for the alp ratio of 18, and a more careful averaging of numbers from several experiments obtained by using eq 14 resulted in a value of 22 f 4. /3 is therefore equal to (6.55 zk 0.80) x lo5 ML cm2/(Torr s). 5. Results
In this section the results from our studies on the effects of both the characteristics of the CO gas beam and the surface temperature on the CO uptake over Pt( 1 11) will be presented. The CO coverage dependent sticking probabilities at 90 K are shown in Figure 7 as a function of effusive rate Q for a fixed surface-to-doser distance L = 0.75 cm. After an apparent initial small rise at the very low coverages, the sticking probability s(8) reaches a high value of about 0.8 and then remains constant over a wide range of coverages, up to about 8 = 0.40. Above 8 = 0.40 s(6) drops rapidly to 0. Even though the drop-off starts at approximately the same surface coverage for all effisive rates, it extends to higher final saturation coverages at the larger CO fluxes (Figure 8).
The changes in final coverages with effusive rate observed in the low-temperature CO uptake experiments are related to the dynamic equilibrium that is established between the absorption and desorption steps at high coverages because of the drastic drop in CO adsorption energy in that This is corroborated by the fact that blocking of the incident beam after the surface reaches saturation causes the CO partial pressure to instantly rise and to then gradually return to the equilibrium value (Figure 9a). Similarly, re-exposure of the surface to the incident beam (re-removal of the flag) leads first to a sudden drop in the CO partial pressure and then to its subsequent slow recovery. The pressure changes detected after blocking of the beam are due solely to the desorption of the extra CO, while those seen after the flag is removed are the result of a combined effect from both adsorption and desorption. The initial pressure jumps induced by the placing and removing of the flag in front of the beam are proportional to the beam effusive rate, and the rate of desorption after blocking the beam is roughly proportional to the weakly adsorbed CO held on the surface above 0.50 ML by the dynamic equilibrium (Figure 9b). These pressure changes induced by the blocking and unblocking of the beam are reversible and can be repeated many times. The coverage dependence of the CO sticking probabilities at 90 K as a function of surface-to-doser distance L is shown in Figure 10 for three different CO effusive rates. Changing the sample-to-doser distance changes not only the effective beam flux intercepted by the surface but also the beam profile.
6172 J. Phys. Chem., Vol. 99, No. 16, 1995
Liu et al.
COlR(111)
CO/Pt(lll) at 90 K
Y
L = 1.OO cm Q 9.7~10'~molec s"
L = 0.75 cm Q = 1.3x1Ol4molec 5' -.-----
=tn A
1.0
& - 0.8 E
2
0.6
2
1
I
80
120
1
0
5
8.0 -
40
Time I s
B
0.0
0.0
CO/R(111) at 90 K
(b)
E 0.4 5c 0.2 0.2
0.4
0.6
0.8
1.0
Coverage, e / ML
Figure 11. Coverage dependence of the CO sticking probability as a function of surface temperature, T. The experimental data is shown as dots, while the fits to Kisliuk's equation are displayed as solid curves.
0.bO
'
0.bl
'
0.b2
0.Q3
'
'
0.Q4
A01 ML Figure 9. (a) CO uptake raw data for T = 90 K, L = 0.75 cm, and Q = 1.3 x lOI4 molecules/s. The beam was unblocked at t = 10, 70, and 100 s and blocked back again at t = 55 and 85 s to demonstrate the reversibility of the adsorption at high coverages. (b) CO adsorption and desorption rates calculated from the data in (a). A 8 is referenced to the saturaiton coverage when the beam is blocked.
C O / R ( l l l ) at 90 K
doser, the surface intercepts a large fraction of the whole beam, but it also samples more of the flux gradient, so the impinging rate varies significantly across the surface. When the crystal is placed farther away, on the other hand, the fraction of the beam intercepted is smaller, but the beam distribution becomes more uniform. It can be seen from the data in Figure 10 that, at a given constant beam effusive rate, shorter surface-to-doser distances result in larger saturation coverages (because of the higher effective flux) and lower coverages for the breaking point at which the sticking probability starts to decrease (because of the inhomogeneity of the beam). Moreover, at the higher beam fluxes and shorter surface-to-doser distances, the sticking probability rises steadily before reaching its maximum value and also drops off more gradually toward saturation than when the sample is farther away. All these anomalies can be related to the change in the beam profile (see below); in order to obtain CO uptake data free from these problems, an intermediate distance of 1.00 cm was chosen for the other measurements reported here. Finally, the changes observed in the CO uptake kinetics with surface temperature are illustrated in Figure 11. At 160 K or below the sticking probability stays nearly constant around 0.80 & 0.05 up to 8 = 0.40 ML and then drops almost linearly to 0 at about 0.65 ML. Between 200 and 300 K, on the other hand, the sticking probability starts at a value of 0.65 and decreases gradually with increasing coverage until surface saturation is reached, this time at 8 = 0.50 ML. Above 350 K the observed (dynamic) saturation coverage decreases even further, until becoming effectively zero above 550 K, because of the significant increase in the rate of thermal desorption that occurs at high temperatures. 6. Discussion
0.0
0.2
0.4
0.6
0.8
1.0
Coverage, 0 / ML Figure 10. Coverage dependence of the CO sticking probability at T = 90 K as a function of surface-to-doser distance L for various beam effusive rates Q: (a) 1.6 x lOI3, (b) 3.7 x and (c) 1.6 x lOI4 molecules/s. Typical molecular beams generated by capillary arrays such as the one used in our experiments have flux distributions that peak at the center of the doser and decrease slowly in the radial direction. As a consequence, when the crystal is close to the
The adsorption of CO on Pt( 111) surfaces has already been studied extensively by many research groups over the last couple of decades.'-9,25,26.28-36Regarding the structure of CO on Pt(11l), LEED experiments have shown that room temperature adsorption yields a (hxJ?;)R3Oo ordered layer for coverage up to 8 = 0.33 and then a second ~ ( 4 x 2 structure ) around 0.50 ML.2,30,31Vibrational studies have indicated that up to 8 = 0.33 ML CO adsorbs exclusively on on-top sites, while at higher coverages it occupies both on-top and bridge site^.^,^ At low tempetatures, below 160 K, an additional series of compressed layers form on the surface as the coverage increases to about 0.70 ML.'s2 ESDIAD, LEED, and vibrational studies have all led to the conclusion that the structure of the compressed layers
CO Adsorption on R(111) Surfaces involves fault lines separating antiphase domains of ~ ( 4 x 2 CO ) p a t c h e ~ ' * ~ ,and ~ l Jthat ~ the terminal CO species in those lines are tilted at an angle of up to 6" from the normal.2 With respect to the energetics for CO adsorption, the initial activation energy for desorption has been reported to be somewhere between 27 and 35 kcallmol and the preexponential factor to range from 1 x 1013 to 4 x lOI5 Since the adsorption is nonactivated, the desorption energy barrier equals the heat of adsorption. This energy decreases with surface coverage, slowly at first (by 10-20% at 0.50 ML), and more drastically close to saturati~n.~*~~~ The CO sticking probabilities on R ( l l 1 ) have also been measured by a few investigator^.^,^^,^^,^^,^^,^^ Values for the initial room temperature sticking probability of 0.84, 0.74, and 0.79 have been reported by Steininger et al.,26Campbell et al.,33 and Lin and S ~ m o r j a irespectively. ,~~ At low temperatures the sticking probability has also been shown to be constant (about 0.9) up to close to 0.20 ML and to then decrease to 0 at saturation;z5at room temperature it has been reported to decrease monotonically in a Langmuir fashion i n ~ t e a d . ~ ~ . ~ ~ In this paper we address the effects that both the beam flux and the surface temperature have on the CO sticking probability evolution with coverage. We start by focusing on the effect of the beam flux on the saturation coverage. Figure 8 shows that the saturation coverage for CO on Pt( 111) at 90 K depends on the beam flux of CO gas molecules impinging on the surface. Such coverage displays a constant value of about 0.50 f 0.05 ML at low CO fluxes (below about 1.5 x l O I 3 molecules/(cm2 s), or 0.01 M L / s ) , but then increases at higher fluxes. The extra CO adsorbed at the large impinging rates is only weakly held on the surface, so the increase seen in the final saturation coverage is the result of a dynamic equilibrium that is estblished between the CO adsorption and its desorption. The data in Figure 9 prove this last point, for it shows that when the incident CO beam is intercepted by the flag, the extra CO desorbs readily into the gas phase. The kinetic behavior discussed above can be modeled by using a simple Langmuir m0de1.~'%~* First, a distinction needs to be made between the CO strongly held on the surface (the only type up to 80 = 0.50 ML) and that weakly and reversibly adsorbed at the high coverges; we define the coverage of the second type as 8 h c = 8 - 80. The rates for adsorption and desorption at high coverages can then be calculated by the following expressions:
J. Phys. Chem., Vol. 99,No. 16, 1995 6173 when Rads
= Rdes
(22)
Substitution of eqs 21 and 22 into eq 20 leads to the well-known expression
s-1.26333-36
ehc ,axbF 'hc
sat
=
'hc
max
+ bF
where
Finally, expression 23 can be rearranged into a more convenient form:
'hc
1 - 1 sat bF
+'hc
1 max
The inset in Figure 8 shows a least squares fit of our experimental data to eq 25. The following values were obtained: 8 h c = 0.52 f 0.05 ML and b = 7.3 f 1.0 s (when obtained in these F is in units of ML/s). The value of 8 h c calculations corresponds to a total saturation coverage in the high-pressurehigh-flux limit of about one CO molecule per platinum surface atom (8 = 80 8 h c max = 1.02 f 0.05 ML), which may seem somewhat high in view of the surface science UHV experimental results reported so far, but which is in line with those used routinely for characterization of the metal dispersion in supported c a t a l y ~ t . ~In~ ,any ~ ~ case, since this number was obtained by experiments in the low-pressure limit, it should be used with caution. Analysis of the adsorption and desorption data shown in Figure 9 provides an independent way of determining both S and kd. Taking the derivative of eqs 18, 19, and 20 with respect to the surface coverage 8hc, we get.
+
and dRdes
kd = -
and
where S(8) is the sticking probability s(0) normalized by the number of empty sites on the surface, h ( 8 ) the rate constant for desorption, and 8 h c max the highest possible coverage for the weakly adsorbed CO. Both S and kd may, in principle, be coverage dependent, but the approximately linear nature of both the s vs 8 plots at high coverages and the data in Figure 9b suggests that, at least to a fxst approximation, this may not be the case. Thus, under the steady state conditions reached at saturation, 'hc
= 'hc
sat
Using the values from Figure 9b, F = 0.047 MUS, d R / d 8 h c = -0.24 S-', and d R d e s / d 8 h c = 0.16 S-', and taking o h c = 0.52 ML from above, we get k d = 0.16 f 0.04 s-l and S = 0.9 f 0.5. Equation 24 then yields a value for b of about 5.6 f 4.0 s, which is within the same order of magnitude of that estimted by fitting the data in Figure 8. Assuming a preexponential factor of 10'3-10'5 s-l for the desorption, the activation energy E d for the extra CO layer is estimated to be approximately 6.8 f 1.0 kcallmol. Another characteristic of the CO beam that must be taken into consideration when analyzing kinetic measurements like the ones reported here is its spatial profile. One way that the flux density profile of the beam across the surface can be altered is by changing the distance between the beam source and the solid sample. Multichannel microcapillary dosers usually deliver a reasonably homogeneous beam over a large cross section, but the flux profile of the beam impinging on a surface depends not only on the characteristic of the doser but also on the relative sizes of the doser (diameter 6) and the surface (diameter D)and on the surface-to-doser distance (L). Campbell
Liu et al.
6174 J. Phys. Chem., Vol. 99, No. 16, 1995 and Va10ne~~ have reported calculations for the flux distribution and the fraction of the beam intercepted by the sample for various doser designs, which they characterized by the quantity ymax= arctan(Dl21). They found that for dlD = 1 the beam flux intensity at the rim of the crystal surface was about 75% of that of the center for ymax= 20" but only about 50% for Ymax = 60". They also concluded that, for a constant ymax, smaller values of dlD yield larger beam flux gradients. In our experiments, where dlD was about 1.O, the surface-to-doser distance L was varied between 0.40 and 1.90 cm, so Ymax changed from 53" to 15", which corresponds to beam flux differences between the center and the edges that go from roughly 40% to less than 20%. Our results indicate that the coverage dependence of the sticking probabilities measured under non-uniform beam conditions (closer distances) does deviate from the true behavior, presumably because the concentration gradient that builds up over time across the surface has no chance to be evened out by surface diffusion. At very close surface-to-doser distances and high fluxes the sticking probability seems to increase with coverage, and the break point in the curve occurs at lower coverages than those seen for the other cases. In order to avoid this problem, larger distances need to be chosen for these experiments even though that represents a drop in the value off(the fraction of beam intercept) and therefore a reduction in the signal-to-noise ratio. The third point we want to address in this paper refers to the observation that the CO uptake curves change in a qualitative way when the surface temperature is increased. At low temperatures the sticking probability remains approximately constant within a wide range of surface coverages; only around 0.40 ML does the uptake curves display the sharp break believed to be associated with the sudden change in CO binding energy. At higher temperatures, between 200 and 350 K, a gradual decrease in the sticking probability with coverage and a constant saturation coverage of 0.50 ML were observed instead. These changes can, in principle, be explained by a model initially developed by Kisliuk, who invoked the participation of an extrinsic mobile precursor state to derive the equation a
TABLE 1: Parameters Obtained by Fitting the Temperature Dependent Sticking Probability Curves Shown in Figure 11 to Kisliuk's Equation surface tempK 130 SO,
160
205
255
300
350
initial sticking probability 0.83
K,Kisliuk's parameter
0.80 0.66 0.66 0.65 0.65 0.016 0.031 0.184 0.242 0.260 0.258
around 0.65 above 200 K. According to Kisliuk,
so the changes in SO seen here could be associated with a reduction in either the trapping probability into the precursor state (5) andor the probability from the molecules in the precursor state to transfer to a chemisorbed site (pa). On the basis of the high initial value of SO and on other published data we are inclined to believe that the trapping probability is the one most affected by the temperature of the surface. Poelsema et al. observed a 14% decrease in the sticking probability for CO on Pt( 111) in going from a surface temperature of 90 to 300 K.28 Jiang et al., on the other hand, reported initial sticking probabilities that were independent of t e m p e r a t ~ r e . ~ ~ Regarding the results for K , the value obtained at 300 K, 0.26, is in good agreement with that reported by Campbell et al.33 The lower value observed at the lower temperatures can at first glance be associated with an increase in the desorption probability of the precursor above an occupied site. On the basis of Kisliuk's model, K can be represented by4I
an expression that can be rearranged into
from which
-a
(32) where K is a constant that reflects probability for chemisorption from a precursor state above an occupied site?' However, the behavior observed in the CO uptake data on Pt( 111) at low temperatures, namely, the flat region seen up to 0.40 ML and the gradual decrease above 0.50 ML, cannot be mimicked by this model. The best fit, which would be obtained with small values of K, would require the sticking probability to remain almost constant up to surface coverages close to saturation. As mentioned before, the difficulty in fitting the data to the model can be explained by the fact that some extra CO adsorbs weakly on the surface after saturation of the layer corrsponding to the strongly bonded species (80 = 0.50 ML). In view of all this, the complete uptake curve was again divided into two regimes for the purpose of the data analysis, one that encompasses coverages up to 0.50 ML and that is described by the precursormediated adsorption model and a second above 0.50 ML related to the weakly adsorbed state. The temperature dependent s(0) curves shown in Figure 11 were then fit to Kisliuk's equation (solid curves) by using Omax = 0.50 ML (to include only the strongly bonded CO); the values obtained for SO and K from these fits are reported in Table 1. Our analysis shows that the value of the initial sticking probability for CO, SO, goes from about 0.80 below 160 K to
Here kd, ki, E d , and km, kk, E m are the rate contants, preexponential factors, and activation energies for desorption and for diffusion from the extrinsic precursor state, respectively. Plots of K against temperature, T, and of ln[(llK) - 11 against 1lRT are shown in Figure 12. In spite of the fact that no good linear relationship between ln[(llK) - 11 and 1lRT was obtained, an upper limit of 2.5 f 0.5 kcal/mol for Ed - Em could still be estimated from the data. It is interesting to note that the CO molecules occupy only on-top sites at 8 < 0.33 ML but both on-top and bridge sites at 8 > 0.33 ML, which could be the reason why no true Langmuir behavior is observed in the uptake at room temperature (which would correspond to K = 1). Nevertheless, the diffusion between bridge site and on-top sites is believed to be much faster than the impinging rate; quite low barriers have been reported in the literature for site interconversion.6 The temperature dependence of the CO uptake was previously studied by Jiang et aLZ5Their results agree qualitatively with ours, except for the fact that the break point in their lowtemperature uptake curves appears at lower coverges than in ours. This discrepancy may be explained by the differences in the experimental setups used in both cases, since in their apparatus dlD = 0.5 and ymaxKZ 60". Their arrangement
CO Adsorption on Pt( 111) Surfaces
4.0 -
0.0
100
200
300
400
J. Phys. Chem., Vol. 99, No. 16, 1995 6175
- 0.4
KvsT h ( l / K - I ) vs 1/RT
/-
/
I
1.o
I
I
2.0
1
1
I
3.0
1
I
t 0.3
0.0
4.0
/ kcal'! mol
Figure 12. Kisliuk's parameter, K,as a function of surface temperature. Also shown in ln[(l/K) - 11 against IIRT.
produces a fairly inhomogeneous beam across the surface, which, as we discussed above, leads to uptake curves with break points shifted to lower coverages. Finally, it is worthwhile noting that even though the model used in this discussion works quite well at a semiquantitative level, it does present a few limitations. For one, no lateral interactions among the adsorbed molecules are included in the derivation of eq 28. This may be a reasonable approximation at low surface concentrations, but repulsi'on among the adsorbed molecules, which is believed to induce significant changes in the adsorption energy at higher coverages, modifies the uptake behavior. Zhdanov42 has indeed shown that nearest neighbor repulsive interactions reduce the sticking probabilities from those estimated by Kisliuk's equation at high coverages. Furthermore, Kisliuk's model does not include the effects of either islanding or path crossing during migration of molecules in the precursor state; both parameters have been shown to also reduce the value of s(0) at high coverage^.^^,^ Finally, Kisliuk's model assumes an almost unpopulated precursor state and therefore applies better in the low gas flux limit.
7. Summary The coverage dependence of the sticking probability for CO adsorption on Pt(111) surfaces has been measured as a function of CO beam effusive rate, surface-to-doser distance, and surface temperature. Changes in the flux of CO molecules on the surface result in changes in the saturation surface coverage at low temperatures. Those changes are explained by the competition between CO adsorption and desorption that takes place at high coverages as a consequence of the reduction in the CO adsorption energy. Changes in the surface-to-doser distance were also found to alter the beam flux profile across the crystal surface, which in tum modifies the shape of the measured CO uptake curves. The use of beams with a homogeneous profile was found to be critical in obtaining accurate measurements of the coverage dependence of the sticking probabilities. The surface temperature also affects the sticking probabilities: higher temperatures lead to a reduction in the initial sticking probability, an increase in the relative rate for desorption from the precursor state compared to that for chemisorption, and a reduction in the saturation coverage (because of the elimination of the compressed layers that form above 0.50 ML at low temperatures). The uptake behavior was analyzed to a first-order approximation by using the precursor adsorption model developed by Kisliuk.
Acknowledgment. Financial support for this research was provided in part by a grant from the National Science Founda-
tion (CHE-9222164). Additional funds were provided by Los Alamos National Laboratory through the Los Alamos-University of California Collaborative Research (LACOR) Program.
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