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desorption on Pt(S)-[9(111) × (100)]
Surface Science I 15 (1982) 524-540 North-Holland Publishing Company
524
THE EFFECT OF SULFUR ON CO ADSORI’TION,‘DESORPTION I-[9(111) x (loo)] G.E. GDOWSKI
and R.J. MADIX
Deportmentof Chemical Received
20 October
ON
Engineerrng,
1981: accepted
Stunford
University.
for publication
Stunford,
4 December
Califoronriu 94.10.5, USA
198 1
CO adsorption/desorption on clean and sulfur covered Pt(S)-[9( I I I) X (IOO)] surfaces was studied using AES. TPD, and modulated beam experiments. CO desorption occurred from two states on the clean surface - a low temperature state associated with the (1 I I) terraces and a high temperature state associated with the steps/defects. Thermal desorption results indicated that above small CO coverages conversion from the low temperature state into the high temperature state was activated and that back conversion was slow. Sulfur preferentially adsorbed at step/defect sites and decreased the population of the high temperature desorption state. Modulated beam experiments were performed in order to determine CO adsorption/desorption parameters as a function of sulfur coverage on the Pt crystal. The sticking coefficient and binding energy of CO decreased as the sulfur concentration increased. Sulfur adsorption at step/defect sites decreased the CO sticking coefficient only slightly but increased the effective rate constant for CO desorption signific~tly. Sulfur adsorption on the terraces affected CO adosrption more than sulfur at step sites. On the clean surface the effective rate constant for CO desorption was lX10’5s-‘exp
i
Desorption occurred from both terrace and step/defect sites, but the kinetics were characteristic the step/defect sites. For the surface on which step/defect sites were blocked by sulfur effective desorption rate constant was k err =1X10’3s-‘exp
i
of the
-
indicating an appreciable interactions had probably (111) planes. The results exponential factor.
decrease in CO binding on the terraces, though sulfur-CO repulsive from clean made kc,, larger than the true rate constant for desorption showed clearly a compensation effect in activation energy and pre-
1. Introduction While many previous studies have dealt with the adsorption/desorption of CO on clean platinum surfaces [l-26], few have examined the effect of poisons, in particular sulfur, on this phenomenon [27-301. Briefly, CO desorption from clean, stepped Pt( 111) occurs in two states: a broad, low temperature state which has been associated with two binding configurations on the flat (111) terraces and a high temperature state which has been associated with
~39-602g/82/~-~/$02.75
0 1982 North-Holland
GE.
Gdowski, R.J. Mudix / Effect
of sulfur
on CO adsorption on Pt
525
step/defect sites. The former state is believed to be due to linear and bridge bonded CO species [ 13-18,20,21] whose desorption energies decrease with increasing coverage [21]. A modulated molecular beam study [lo] has determined the rate constant for CO desorption from flat Pt(ll1) in the low coverage limit to be 2.9 X lOI s-’ exp A thermal desorption study [29] has shown the desorption activation energy for the low temperature state to decrease from 31 to 24 kcal/mole with increasing CO coverage. Recently, Winicur et al. [12] have determined an average adsorption energy of 31.1 kcal/mole, which decreases with coverage, for CO on Pt( 111) using Low-Energy Molecular Beam Scattering. Recent modulated molecular beam studies (9- 11) have also probed the high temperature state. Activation energies for desorption were 34.9 [9], 33.6 [lo], and 36.2 kcal/mole [ 111. These studies will be discussed in more detail below. Recent results for CO desorption from sulfur covered Pt(ll1) surfaces [28,29] have shown that sulfur decreased the CO binding energy. Kelemen et al. [29] determined for a (2 X 2) sulfur overlayer that the activation energy for desorption decreased from 18 to 12 kcal/mole with increasing CO coverage. In this study the effect of sulfur on the desorption of CO was examined using temperature programmed desorption (TPD) and modulated beam experiments. The modulated beam experiments allowed the determination of desorption parameters in the low coverage limit, eliminating the effect of repulsive interactions among adsorbed CO molecules.
2. Experimental The experimental apparatus used in this study has been described in detail previously [31]. Briefly, the stainless steel vacuum chamber was equipped with a quadrupole mass spectrometer, four-grid LEED-Auger optics, a beam modulation system, and separate dosing lines for the introduction of H,S and CO into the chamber. The modulated molecular beam was generated in an array of fused capillaries of 0.005 cm diameter and was collimated by a 2 mm hole located in a solid copper gasket that separated the beam source from the scattering chamber. The chamber was pumped by a 220 l/s vat-ion pump and a titanium sublimation pump. Base pressures were in the low 10 -lo Torr range. The modulated beam experiments were run in the phase-sensitive mode of detection. Two identical lock-in amplifiers were used simultaneously to monitor the amplitudes of the product signals in-phase and 90” out-of-phase with the primary beam. These signals are designated I, and &,, respectively. The beam center line and mass spectrometer were each oriented 45” to the surface normal. The beam was incident on the open face of the steps and approxi-
526
C. E. Gdowski, R.J. Madix / Effeecr of s&fur on CO crdsorprion on Pt
Table 1 Surface
Pt and S
Clean Sulfur coverage just necessary (2 X 2)s (l/4 ML) (6 X fi)R30”S (l/3 ML)
to eliminate
CO( nH)
(150eV)/Pt
(235 eV)
1.2 3.4 1.3 10.5
mately perpendicular to the step rows. CO fluxes of 1014 molecules/cm2. s were used. The modulation frequency was 112 Hz. The crystal was heated by radiation from a tungsten filament located closely behind the sample. Pt/Pt-lO%Rh thermocouples spotwelded to the back of the crystal were used for temperature measurements. The previously cleaned platinum crystal was treated by heating in 5 X 10 -’ Torr of oxygen at 1300 K for a few minutes and then heating to 1500 K for a few seconds to ensure cleanliness, which was monitored by AES. Sulfur was deposited on the Pt(S)-[9( 111) X (lOO)] crystal surface by adsprbing H,S at 300 K and heating to 600 K to desorb H,. Following Berthier et al. [32] and Bonzel and Ku [2’7], the amount of surface sulfur deposited was determined by the ratio, R,, of the peak-to-peak heights of the S and Pt (150 eV) and the Pt (235 eV) Auger transitions. Two well defined sulfur overlayers on Pt(ll1) have been formerly observed [32,33]: the (fix fi)R30°S surface (l/3 ML) resulting from saturation coverage of sulfur deposited by H,S at 300 K and the (2 X 2)s overlayer (l/4 ML). Table 1 lists the R, values obtained experimentally for the well defined sulfur surfaces.
3. Results 3.1. temperature
~r~~ramrned desorprion
3. I. 1. Clean surface A temperature programmed desorption spectrum for a saturation exposure of CO on the clean Pt(S)-[9(111) X (IOO)] surface is shown in fig. 1. The spectrum shows distinct high (on) and low (or) temperature peaks similar to other spectra reported on flat and stepped Pt(ll1) single crystal surfaces [3,5,6,15,16,22,28]. The high temperature state at 540 K has been observed on most “flat” surfaces, but is more pronounced on the stepped surfaces. Previous studies have attributed this peak to CO desorption from defect sites on the-flat surfaces.[3,6,16]. In order to determine the amount of CO desorbing in the an state, attempts were made to isolate the state by exposing the crystal to small doses of CO. These attempts failed, except at extremely small CO doses, as
527
CO/CO
&
t
200
L
I
t
300
I
a
CLEAN
b
R,=334
f
L
400 500 TEMPERATURE
1
t
600
70’
(K)
Fig. 1. Saturation CO desorption spectrum on Pt(S)-[9( 111)X( IOO)] from {a) the clean surface and (b) a sulfur covered surface with R, =3.4. Tbe adsorption temperature was 205 K and the heating rate was 20 K/s.
there was always some of the or_ state preserrt. A typical result is shown in fig, 2, curve (a)_ An attempt to isolate the an state was then made by preflashing to 500 K before completing the desorption. At a low CO coverage the preflash resulted in a conversion of the at_ to the o(n state (fig. 2, curve (b)), but at a slightly higher CO coverage (though not enough to saturate the oFi state), the at state was still present (fig. 2, curve (c)). The peak temperature, TP,
24 c z 2 -
co/co
90 /A,,
a
NO PRE-FLASH
b
PRE-FLASHED
TO 5ODK
c
PRE-FLASHED
TO 5OOK
(CURVE
al
I.0
t” ;r” 8 z fi 5: z!l
I
L 400
s 500
TEMPERATURE
600 (K1
F’ig. 2. CO desorption spectra on the clean Pt(S)-[(9( I I 1) X (MO)]. The percentage of CO saturation coverage is (a) 2.9, (b) 3.8, and (c) 5.7. Curves (b) and (c) were obtained after preflashing the crystal to 500 K. CO was adsorbed at 205 K. The heating rate was 32 K/s. A,, is the area under the desorption curve.
528
G.E. Gdowski, R.J. M&ix / Effeci of sulfur on CO dsorption on Pt
of the (in state also decreased with increasing CO coverage. To check if the initial filling of the (Ye state was due to non-uniform dosing of the crystal surface, background doses of CO were made. This procedure did show a slight increase in the (in state over the LY,_ state at low coverages, but saturation of the (in state still did not result without substantial filling of the (Ye state. After dosing and preflashing to 485 K, the crystal was redosed at 205 K to saturation to see if an increase in total CO coverage could be obtained. No increase in total CO concentration in either state was obtained with this procedure. 3. I, 2. Sulfur covered surfaces Fig. 1 shows the desorption spectrum for CO at R, = 3.4. This sulfur coverage was the minimum necessary to eliminate the CO(a,) peak. The spectrum also shows a small decrease in (Yestate and a shift of the (or peak to a lower temperature. Elimination of the (mu state was not possible without a slight decrease in the (Ye state. By comparing the areas of the respective desorption spectra, the elimination of the (mu state decreased the total CO concentration at saturation by 11 * 2%. Therefore the LYEstate contributed to slightly less than this value at saturation on the clean surface. For CO adsorption on Pt( 111) saturation coverage was found to be 0.68 ML at 170 K [3] and 0.50 ML at room temperature [8]. Assuming one CO adsorbed per two platinum atoms at the steps, the percentage decrease in CO adsorption should be between 8 and 11% of the saturation coverage for this surface at 205 K. Therefore saturation coverage on the steps appears to be one CO per two Pt atoms. (For the (0 X fi)R30°S structure, there are two S atoms per three Pt step atoms and for the (2 X 2)s structure, one S atom per two Pt step atoms. Assuming a linear relationship between R, and sulfur coverage, R, values calculated for step saturation were 3.3 and 2.8 for the (6 X fi)R30° and (2 X 2) structures, respectively. The R, value at S saturation was used in the calculation since it was easily reproduced. The experimentally obtained value of 3.4 is consistent with an arrangement of sulfur atoms in a (6X fi)R30° structure along the step atoms, with sulfur atoms on both terraces adjacent to the step.) With increasing sulfur coverage (at S coverages below that necessary to saturate the step sites) the (in peak temperature decreased 49 K from the clean surface to the surface with the CO(Q) peak almost completely suppressed (R, = 2.8). This decrease represented a factor of 16 increase in the rate constant at 550 K on the sulfur covered surface, assuming the applicability of Redhead’s formula [34]. It should be mentioned that increasing the initial CO coverage decreased the peak temperature, probably due to CO-CO repulsive lateral interactions. To eliminate this effect, peak temperatures were taken at approximately the same CO coverages of 2 X lOI molecules cm-‘.
G. E. Gdowski, R.J. Mudix / Effect of sulfur on CO adsorption on Pt
529
3.2. Modulated molecular beam
Since the modulated beam results indicate that the CO desorption process was first order (there was no change in the phase lag as the CO beam flux was varied) only the details pertinent to first order adsorption/desorption for modulated beam experiments will be discussed. The reader is referred to the literature for further discussion of the technique [35-371. In phase sensitive modulated beam experiments a reaction product vector, e eei@, is obtained. It is defined 1351as the ratio of the first Fourier component of the desorption rate k,A, to the first Fourier component of the primary beam F,:
(1)
ce -i* = kdA,/F,;
where k, is the first order desorption rate constant, A, is the first Fourier component of the surface concentration and e and cp are the amplitude and phase angle of the reaction product vector. The desorption rate constant and modulation frequency are related to L and (p by
where S is the sticking probability and o is the modulation frequency. Assuming the usual form for the desorption rate constant, exp( -Z$,/RT), the following expression for Ed was obtained
k, = u
E,/RT=In(tan#)-ln(w/p). A plot of In(tan#) versus l/T yields a straight line with slope Ed/R. Fig. 3 illustrates a typical plot for a partially sulfur covered surface (R, = 2.2). Table2 lists values of v and Ed obtained for CO on various sulfur covered platinum surfaces. The desorption energy decreased smoothly from 36.2 to 22.3
Table 2 R,
S (atoms/cm’)
E (kcal/mole)
v (s-1)
1.2 2.2 3.0 3.4 4.5 6.1 6.7 7.4 8.2
0.0 0.5x 1.0x 1.2x 1.8X 2.6X 3.0x 3.3x 3.7x
36.2 32.7 27.4 27.5 25.7 24.2 23.3 24.8 22.3
3.9x 10’4 5.9X 10’2 I x1013 4.8X IO’* 2.1 x IO’2 1.9x lOI 5.5 x IO’2 2.1 x 1o12
10’4 10’4 10’4 lOI lOI 10’4 10’4 10’4
I
x1o’5
G. E. Gdowski, R.J. Mudix / Effect of sulftr on CO udsorptmn on Pt
530
08$ 0.6 a ._ _I tt: 0.44 ”
0.3-
5 c ; J
Gpp=3.9
SEC -I
X lOi
0.2-
0.1,
’ I.5
1.6
1.7 X IO3
I/T
1.8
1.9
(K-I1
Fig. 3. Plot of ln(tan+) versus inverse Pt(S)-[9(111)X(100)] with R,=2.2.
temperature
for CO desorption
from
sulfur
covered
I / 1 I
700l
650-
l
l. 600 -
1.0
14s
I I**
e*
.
I so-
.’
* .
I I
I I I
500 -
45OI 00
.
.
I.0
I
II
I
I
I
I
I
2.0
3.0
4.0
5.0
60
7.0
80
1
90
I
10x
Rs Fig. 4. Plot of the variation of T4s for CO desorption with sulfur coverage on Pt(SW( 111)X f ~@31. The dashed line denotes the sulfur coverage just necessary to eliminate the CO(aH) peak from the TPD spectrum.
GE. Gdowski, R.J. Madix / Effect of sulfur on CO ahorption
on Pt
531
kcal/mole, while Y decreased from lOI to 2 X lOI s-‘. It is interesting to note that none of the results obtained indicated a branch process. At a phase lag of 45” the desorption rate constant for a first order reaction is equal to the modulation frequency. The temperature at which the phase lag is 45”, Td5, is then a good indicator of relative desorption rates (see fig. 4). A decrease in Td5 with sulfur coverage indicates that the rate constant (at fixed T) increases. At low coverages of sulfur there was an initial rapid decrease in Td5 until the step/defect sites were occupied. Further increase in sulfur coverage resulted in a more gradual decrease in Td5.This observation, which is independent of sticking probability, clearly separates the desorption process from steps and terrace regions. If the sticking probability is less than unity, the detected signal consists of two components: those molecules which desorb from the chemisorption potential well, having a probability S and those which are reflected without entering the chemisorption well, having a reflection probability 1 - S. The sum of the two components gives the measured amplitude, E= (1 +&J)“‘+
(1+
where k; is the desorption
(:;$J”‘7
rate constant
for those molecules
not chemisorbed;
R, = 2.2 s, =0.53
I
I 500
I
I
TEMPERATURE
Fig. 5. Illustration versus temperature
I
600
I
I
700 (K)
of CO sticking coefficient determination from the normalized on sulfur covered Pt(S)-[9( 111) X (IOO)] with R, =2.2.
signal amplitude
G.E. Gdowski, R.J. Mudix / Effect
532
of sulfur on
CO udsorption on Pt
I.
00
1.0
20
3.0
4.0
50
60
70
8.0
9.0
100
RS Fig. 6. Plot of the variation of the CO sticking coefficient with sulfur coverage on Pt(S)-[9( 100) X (IOO)]. The dashed line denotes the sulfur coverage Just necessary to eliminate the CO( aH) peak from the TPD spectrum.
k; is always larger than the modulation frequency, w, therefore the second term simplifies to 1 - S. For those molecules which chemisorbed there exist two limiting cases. First, at high temperatures k, B w and the first term simplifies to S and z = 1. Second, at low temperatures w B k, and the first term is very small (i.e., its contribution to the amplitude is completely demodulated). The amplitude, 4, then becomes 1 - S. The difference between the low and high temperature limits of e is then the sticking probability for the chemisorbed molecules. A typical result is illustrated in fig. 5 for R, = 2.2. The effect of sulfur coverage on sticking probability of CO is illustrated in fig. 6. The sticking probability decreased only slightly with saturation of the step sites (0.56 to 0.52), after saturation of step/defect sites there was a more rapid decrease. For both the step/defect sites and terrace sites the plot exhibits a linear dependence of the sticking coefficient on unblocked sites, i.e., S a a( 1 0). For the lines drawn in fig. 6, the ratio of a for the steps/defects to a for the terraces is 0.32. The stronger dependence of the sticking coefficient on the state of the terraces is evident.
4. Discussion 4.1. Temperature programmed
desorption
The results on the clean surface indicated an activated conversion from the terrace sites to the step sites. This activated conversion may result from
C. E. Gdowski, RJ. Mudix / Egecr of su@r on CO adsorpfion on Pt
533
repulsive lateral interaction between CO molecules, yet the conversion for low CO coverages appears to be irreversible at or below 500 K. Such interactions occurred in the au state, as shown by the decrease in Tp with increasing CO concentration at the steps. Further, without preflashing the on state could only be isolated at extremely low CO coverages. It should also be mentioned that even after a preflash to isolate au the peak widths at half maximum increased with increasing CO coverage. At extremely small coverage the FWHM was only slightly larger (- 3 K) than expected for first order desorption, but at higher coverages the FWHM was about 30 K too large (> 75 K) to be due to a simple first order desorption. This could result either from repulsive lateral interactions within the adsorbed state or from some interference due to desorption from the low temperature state, Unfortunately, these effects could not be separated. Since preflashing to 485 K and redosing to saturation gave no increase in total CO coverage, it must be concluded that all the step sites were accessible to the CO molecules at 205 K. The higher coverages apparently forced CO to saturate the step sites. The barrier to conversion must therefore be the order of the CO-CO repulsion energy near saturation. It is interesting to compare the value of the peak temperature for (in found in this study with that for the (100) plane. For desorption from the highest temperature CO state on Pt( 100) McCabe and Schmidt [4] found Tp = 550 K, a value close to that found in this study for on. It should be mentioned, though, that they also found that CO desorption from the high temperature state on Pt(S)-[3(111) X (NO)] (Pt(211)) occurred at 610 K, 60” higher than on the Pt(lO0) surface, thus indicating a higher CO binding energy on the stepped surface. The CO desorption spectra for low sulfur coverage indicated that adsorbed sulfur migrated to step/defect sites. This was in agreement with Berthier et al’s results 1321,which showed that adsorption of sulfur on Pt( 111) occurred with localization of the sulfur atoms at sites of m~mum coordination of the metal. Further, the peak temperature for CO(cu,) decreased with increasing sulfur coverage at the step sites. This indicated, as shown at higher sulfur coverages [29,30], that there existed repulsive lateral interactions between coadsorbed CO and sulfur.
The modulated beam results indicate the same trend in desorption kinetics at low sulfur coverages as observed in thermal desorption. Both Td5 in the modulated beam experiments and Tp for (in in thermal desorption decreased with increasing sulfur coverage. The large initial decrease in Te5 was followed by a more gradual decrease with increasing sulfur coverage. The transition region occurred in the vicinity of the sulfur coverage just necessary to suppress the (mu peak in the TPD experiments. The initial rapid decrease can be attributed to repulsive interactions between the adsorbed CO and S atoms
534
G.E. Gdowski. R.J. Mu&x / E//eecr of sulfur on CO odsorprim
on PI
along the steps. The activation energy for desorption decreased 9 kcal/mole as the steps were saturated with sulfur. Tripling the total surface sulfur concentration reduced Ed only an additional 5 kcal/mole. The change in sticking coefficient with sulfur buildup showed a trend opposite to k,. Initially, at low sulfur coverages, the sticking coefficient decreased slowly until the step sites were saturated with sulfur. As the terraces became populated with sulfur, it decreased more rapidly. The results here indicate that steps do not significantly increase the sticking probability of CO at least via any bonding interactions, as blockage of the steps by sulfur caused only a small decrease in the adsorption probability. This is in agreement with Lin and Somorjai [lo], who determined the same sticking coefficient (S = 0.74) for CO on Pt( 111) and Pt(S)-[6(111) X (loo)] (Pt(557)) and with McCabe and Schmidt [4], who found only a slight difference in S for CO on Pt(ll1) (S - 0.34) and Pt(S)-[3( 111) X (loo)] (Pt(211)) (S - 0.24). The fact that adsorption of sulfur onto step sites did not change the sticking probability shows that conversion of precursor CO into the chemisorbed state is not enhanced by the clean surface even though the binding energy at the steps is substantially greater than on the terraces. It must be concluded that collisional processes on the terraces dominate the primary adsorption event. This conclusion is supported by the strong decrease of the sticking probability as the terraces become populated with sulfur. Since it has been established that there exist at least two main adsorption sites for CO on the clean stepped platinum surfaces, the following reaction sequence was analyzed for the modulated beam results:
(6)
where P is the probability of chemisorption adsorption directly onto the terraces, and the indicesg, t, and s refer to gas, terrace, and step, respectively. It has been suggested [9] that step (7) may be in equilibrium. If this were the situation, it could be shown (see the appendix) that the mechanism reduces to a branched process (the same result would also occur if there were no conversion between the steps and terraces). This mechanism is consistent with the results observed here only if the probability of adsorption onto the step sites dominates. If that is the case, k,, is measured directly.
G. E. Gdowski, R.J. Mudix
/
Effect of sulfur on CO adsorption on Pt
535
I
o.o’G; lO.O-
d 2
: 20.0v Y & 30.0(z w 5 40.0 ADSORPTION
- DESORPTION
PATHWAY
J
Fig. 7. A simple potential energy diagram for CO adsorbed on a stepped surface.
The reaction product vector which results when no simplifying assumptions are made about the above mechanism was derived and is shown in the appendix. The expression for the phase lag is seen to be very complex. Simplifications of the general expression are instructive. Although few data exist on the energetics of surface migration it is believed that the barrier for migration is approximately 10 to 20% of that for desorption [38]. Although k, and k, are rate constants for conversion between different types of sites, it is likely that the activation energies for these steps are small compared to the desorption energies; k, and k, should then be larger than kdt and k,,. If they were also assumed to be larger than w*, the following expression for the phase lag results:
dk, + k,) tan9=k,k,,+k,k,,=<’
(10)
From the TPD results, it is reasonable to assume that conversion from the terrace sites to the step sites is faster than the reverse process, that is, k, B k,. The effective rate constant can then either be k,, or k2kd,/k,. In either case the apparent activation energy is that of desorption from the steps, as shown by a potential energy diagram (see fig. 7). Further, if the transition state is the same for desorption from the terraces and the steps, then k,, = k, kd,/k, and k,, = 2k,,. Therefore, desorption from step sites on the clean surface occurs with Eds = 36.2 kcal/mole and Y= 5 X lOi s-r. Two other modulated beam studies of CO adsorption/desorption on clean Pt( 111) surfaces with steps [lo] or defects [9] have just been reported. In both
536
studies
G.E. Gdowskr, R.J. Mudix / Effect o/sulfur
the kinetic
parameters
keff = 7.9 X lOI s-’
exp
keff = 1.25 X lOI s-’ exp
on CO udsorptim
on Pt
determined (ref. [lo]), (ref. 193))
are in good agreement with those found here, but the mechanistic interpretations differ. By assuming that migration of CO could only occur from the terraces to the steps, Lin and Somorjai [lo] concluded that the desorption parameters determined were characteristic of the step sites. While we agree with their conclusion, we question the assumption that CO migration from the steps to the terraces does not occur especially at the temperatures (above 500 K) at which the study was performed. Campbell et al. [9] allowed for conversion between COtt, and CO,,, and concluded that since the lifetime of the molecule at a step site is significantly longer than at a terrace site, the parameters determined were appropriate for desorption from the step/defect sites. They further stated that it could not be determined if desorption occurred preferentially from a step site ( keff = kds), or by diffusion to a terrace site first and then desorption keff = k,k,,/k,. In fact, as shown above, the energetics of either process are equivalent. A more precise significance of the desorption rate constant can be inferred from microscopic reversibility [39,40]. The independence of the CO adsorption probability on sulfur concentration at the step sites shows that adsorption occurs via the terraces. Correspondingly, if microscopic reversibility applies, desorption must occur preferentially from the terraces as well. The preexponential factor characteristic of terrace desorption, v~,, is then increased by nearly two orders of magnitude by the equilibrium constant, k, /k,. The origin of this difference is apparently a more restricted mobility of CO bound to step sites with a corresponding decrease in entropy. This loss of mobility is consistent with the higher binding energy at the steps. High pre-exponential factors for CO desorption have been discussed previously for Pt( 110) [7] and Ru(OO1) [41]. Upper limits for the pre-exponential at low coverages were estimated to be 9 X 10” [7] and 1.2 X 10’s s-’ [41]. Both of these examples illustrate that large pre-exponential factors for immobile adsorbed states are reasonable. As sulfur was added to the surface, the rate constant for CO desorption increased, as expected for the repulsive interactions between CO and S. In the context of the model discussed for the clean surface the parameters determined are for the tightest binding state. With preferential adsorption of sulfur at step/defect sites, the parameters determined on the low sulfur coverages surfaces (R, -e 3.4) reflect repulsive interactions between CO and S at the step sites. As the sulfur coverage was increased past the point where all the step/defect sites were covered (R, > 3.4) the desorption parameters were believed to be due to the terraces. At the very least the rate constant for
G. E. Gdowski, R.J. Mudix / Ejj ec t oj su lj ur on CO udsorption on Pt
537
desorption from the terraces cannot exceed the value measured under these conditions. The activation energy reported here for desorption from the terraces (27.5 kcal/mole) is smaller than that found by Lin and Somorjai [lo] (29.9 kcal/mole) and Winicur et al. [ 121 (31.1 kcal/mole). This discrepancy may be attributable to the adsorbed sulfur, which besides blocking sites decreases the CO interaction with the platinum surface, as shown by the shift to lower temperatures of the CO thermal desorption spectrum with adsorbed sulfur (see fig. 1).
5. Conclusion The energetics of desorption of CO from Pt(S)-[9(111) X (loo)] at low coverages is determined by binding to step sites. Sulfur selectively poisons these sites until they become saturated. Over this range of sulfur coverage the sticking probability is only slightly affected, whereas the rate constant for CO desorptoin increases sharply. This increase is due to a 9 kcal/gmole decrease in the desorption energy. At higher sulfur coverages, k, increases slowly and the sticking probability decreases in proportion to the sulfur concentration. The dominant mode of adsorption occurs via the terraces.
Acknowledgement The authors AT03-79ER104)
gratefully acknowledge for this work.
the support
of the DOE
(Grant
DE-
Appendix Determination surface
of reaction product vectors for first order desorption from a stepped
For the purpose of illustration CO was used as the reactant. In the model developed, it was assumed that: (1) CO had two adsorption sites available to it, step and terrace; (2) desorption from both sites was first order; (3) migration of CO between the sites was allowed. The following nomenclature was used: S, sticking coefficient; I, intensity of the unmodulated beam; g(t), gating function of the beam chopper; w, frequency of modulation; c exp( - i+), reaction product vector with # the phase lag and z the apparent reaction probability. The reaction product vector has been shown to be equal to the ratio of the first Fourier coefficients of the rate of product formation to the reactant forcing function [35].
538
Gdowski, R.J. Mudix / Effecr of m&r OH CO adsorpiiow on PI
GE.
The following reaction sequence was considered:
co(t)
2q,
(A-2)
7
k,
kti
co(t)+ co(g)f
(A-3)
CO@,kds + COB, ’
(A.4)
where P is the probability of adsorption on the terraces and g, t, and s denote gas, terrace, and step. The surface mass balances for this mechanism were d(CO~~~)/d~=~~~~(~)
- k(CO,,,> +k,(CO,,,)
-k&O&
d(CO,,,),‘dt = (1 -P>
S@(t) - k,(CO,,,) + k,(CO(,,) -k&O&
(A-5) (A.61
For the linear mechanism the gating function, g(t), may be written as g 2”‘; Cot9 as A eiWf,and Cots, as x eiw’. If it was assumed that the step and terrace concentrations were in equilibrium, i.e., UCOJ
= k2(COWL
(A.7)
then
(A-8) (A.91 The reaction product vector was k
rexp(-i+)
A
=dt+Q!
k,,a
(A.lO)
zg ’
with t: =
S(A*
(A.1 1)
+ I?*)“‘,
tan # = A/B,
(A.12)
where A=
’ 1 + ( q’kdt)’ I-P
P
B= I+
(#/kit
)” +
1 + h/kd2
’
of sulfur
GE. Gdowski. R.J. Ma&x / Effect
This result is the same as that for a branch 1351. When tions A=
the surface mass balance
PSIg+
on CO adsorption on Pt
mechanism
equations
derived
by Jones et al.
were solved without
any assump-
k,a ((k,
(k, +k,,)*+a*
+kd
539
(A.13)
-id,
and X=
(1 -P) (k,
SIg+
k,A ((k2 +k,,)
+kds)*+u2
The reaction
product
Eexp(-i+)
(A.14)
-id.
vector was
=+$+y,
(A.15)
with S( (Y*+ p*)“* C=
(~.i6) (k,k,,
+ k,,k,,
+ k,k,,
- a’)’
+ a*( k, + k,, + k, + k,,)*
’ (A.17)
tan+=a/p, where
a=~ [ (4 +k, +kdt +kdWds -Pkdt(
k,,k,
+Pkdtk,,(k, P=(k,k,,
- LO’) - (1 + kcd + (I-
+k,,k,)(k,k,s
+kdskd*W*kdt +~*(Pkdt(h
-
+ +kdt
+k&,t)
P) kds( k,,k,
- w’)
P) k,,k,,(k,
+ kd]
+k,,k, kdsk, +k2)
+
+k,,k,,
7
-a’)
kdskdt) +
(1
-P)
kdsb
+k2
+kds%
if k,,k,
Bkdtr kdsr a*,
then
tan+=
dk, k k 2
+k,) +k k dt
I
. ds
References [I] [2] [3] [4]
L.A. West and G.A. Somojai, J. Chem. Phys. 57 (1972) 5143. R.L. Palmer and J.N. Smith, J. Chem. Phys. 66 (1974) 1453. D.M..Collins and W.E. Spicer, Surface Sci. 69 (1977) 85. R.W. McCabe and L.D. Schmidt, Surface Sci. 66 (1977) 101.
(~.18)
540
[S) [6] [7] [8) [9] [IO] f I I] [ 121 [ 131 [ 141 [ 151 [I61 [17]
G. E. Glowski,
R.J. MCI& / Eflect of sutfur on CO a&x-prim
on Pr
F.P. Netzer and R.A. Wille, Surface Sci. 74 (1978) 547. R.W. McCabe and L.D. Schmidt, Surface Sci. 65 (1977) 189. J. Fair and R.J. Madix, J. Chem. Phys. 73 (I 980) 3480. M.A. Barteau, E.I. Ko and R.J. Madix, Surface Sci. 102 (1981) 99. C.T. Campbell, G. Ertl, H. Kuipers and J. Segner, Surface Sci. 107 (I 98 I) 207. T.H. Lin and G.A. Somorjai, Surface Sci. 107 (1981) 573. J.A. Fair and R.J. Madix, unpubIished results. D.H. Winicur, J. Hurst, CA. Becker and L. Wharton, Surface Sci. 109 (1981) 263. HI. Krebs and H. Ltith, Appl. Phys. 14 (1977) 337. A.M. Baro and H. Ibach, J. Chem. Phys. 71 (1979) 4812. R.A. Shigeishi and D.A. King, Surface Sci. 58 (1976) 379. H. Hopster and H. Ibach, Surface Sci. 77 (1978) 109. H. Froitzheim, H. Hopster, H. Ibach and S. Lehwald, Appl. Phys. 13 (1977) 147. [ 1R] K. Horn and J. Pritchard, J. Physique 38 (1977) C4- 164. [ 191 A. Crossley and D.A. King, Surface Sci. 95 (I 980) 131. [20] A. Crossley and D.A. King, Surface Sci. 68 (1977) $28. [21] P.R. Norton, J.W. Goodale and E.B. Selkirk, Surface Sci. 83 (1979) 189. [22] G. Ertl, M. Neumann and K.M. Streit, Surface Sci. 64 (1977) 393. [23] D.M. Collins and W.E. Spicer, SurfaFe Sci. 69 (197’7) 114. [24] D.M. Collins, J.B. Lee and WE. Spicer, Surface Sci. 55 (1976) 389. 1251 P.R. Norton and P.J. Richards, Surface Sci. 49 (1975)567. [26] G. Kneringer and F.P. Netzer, Surface Sci. 49 (1975) 125. [27] H.P. Bonzel and R. Ku, J. Chem. Phys. 58 (1973) 4617. [28] T.E. Fischer and S.R. Kelemen, J. Catalysis 53 (1978) 24. [29] S.R. Kelemen, T.E. Fischer and J.A. Schwan, Surface Sci. 8 I (I 979) 440. (301 N.M. Abbas and R.J. Madix, to be published. [3 l] J. McCarty, J. Falconer and R.J. Madix, J. Catalysis 30 (1973) 235. [32] Y. Berthier, M. Perdereau and J. Oudar, Surface Sci. 36 (19733 225. 1331 W. Heegemann, K.H. Meister, E, Bechtold and K. Hayek, Surface Sci. 49 (1975) 161. [34] P.A. Redhead, Vacuum I2 (1962) 203. [35] R.H. Jones, D.R. Olander, W.J. Siekhaus and J.A. Schwarz, J. Vacuum Sci. Technol. 9 (1972) 1424. [36] J.A. Schwarz and R.J. Madix, Surface Sci. 46 (1974) 3 17. [37] D.R. Olander and A. Ullman, Intern. J. Chem. Kinet. 8 (1976) 625. [38] L.D. Schmidt, in: Interactions on Metal Surfaces, Ed. R. Gomer (Springer. New York, 1975). [39] M.J. Cardillo, M. Balooch and R.E. Stickney. Surface Sci. 50 (I 975) 263. [40] G. Comsa, in: Proc. 7th Intern. Vacuum Congr. and 3rd Intern. Conf. on Solid Surfaces. Vienna, 1977, p. 1317. [4l] H. Pfnur, P. Feulner, HA. Engelhardt and D. Menzel, Chem. Phys. Letters 59 (1978) 481.
524
THE EFFECT OF SULFUR ON CO ADSORI’TION,‘DESORPTION I-[9(111) x (loo)] G.E. GDOWSKI
and R.J. MADIX
Deportmentof Chemical Received
20 October
ON
Engineerrng,
1981: accepted
Stunford
University.
for publication
Stunford,
4 December
Califoronriu 94.10.5, USA
198 1
CO adsorption/desorption on clean and sulfur covered Pt(S)-[9( I I I) X (IOO)] surfaces was studied using AES. TPD, and modulated beam experiments. CO desorption occurred from two states on the clean surface - a low temperature state associated with the (1 I I) terraces and a high temperature state associated with the steps/defects. Thermal desorption results indicated that above small CO coverages conversion from the low temperature state into the high temperature state was activated and that back conversion was slow. Sulfur preferentially adsorbed at step/defect sites and decreased the population of the high temperature desorption state. Modulated beam experiments were performed in order to determine CO adsorption/desorption parameters as a function of sulfur coverage on the Pt crystal. The sticking coefficient and binding energy of CO decreased as the sulfur concentration increased. Sulfur adsorption at step/defect sites decreased the CO sticking coefficient only slightly but increased the effective rate constant for CO desorption signific~tly. Sulfur adsorption on the terraces affected CO adosrption more than sulfur at step sites. On the clean surface the effective rate constant for CO desorption was lX10’5s-‘exp
i
Desorption occurred from both terrace and step/defect sites, but the kinetics were characteristic the step/defect sites. For the surface on which step/defect sites were blocked by sulfur effective desorption rate constant was k err =1X10’3s-‘exp
i
of the
-
indicating an appreciable interactions had probably (111) planes. The results exponential factor.
decrease in CO binding on the terraces, though sulfur-CO repulsive from clean made kc,, larger than the true rate constant for desorption showed clearly a compensation effect in activation energy and pre-
1. Introduction While many previous studies have dealt with the adsorption/desorption of CO on clean platinum surfaces [l-26], few have examined the effect of poisons, in particular sulfur, on this phenomenon [27-301. Briefly, CO desorption from clean, stepped Pt( 111) occurs in two states: a broad, low temperature state which has been associated with two binding configurations on the flat (111) terraces and a high temperature state which has been associated with
~39-602g/82/~-~/$02.75
0 1982 North-Holland
GE.
Gdowski, R.J. Mudix / Effect
of sulfur
on CO adsorption on Pt
525
step/defect sites. The former state is believed to be due to linear and bridge bonded CO species [ 13-18,20,21] whose desorption energies decrease with increasing coverage [21]. A modulated molecular beam study [lo] has determined the rate constant for CO desorption from flat Pt(ll1) in the low coverage limit to be 2.9 X lOI s-’ exp A thermal desorption study [29] has shown the desorption activation energy for the low temperature state to decrease from 31 to 24 kcal/mole with increasing CO coverage. Recently, Winicur et al. [12] have determined an average adsorption energy of 31.1 kcal/mole, which decreases with coverage, for CO on Pt( 111) using Low-Energy Molecular Beam Scattering. Recent modulated molecular beam studies (9- 11) have also probed the high temperature state. Activation energies for desorption were 34.9 [9], 33.6 [lo], and 36.2 kcal/mole [ 111. These studies will be discussed in more detail below. Recent results for CO desorption from sulfur covered Pt(ll1) surfaces [28,29] have shown that sulfur decreased the CO binding energy. Kelemen et al. [29] determined for a (2 X 2) sulfur overlayer that the activation energy for desorption decreased from 18 to 12 kcal/mole with increasing CO coverage. In this study the effect of sulfur on the desorption of CO was examined using temperature programmed desorption (TPD) and modulated beam experiments. The modulated beam experiments allowed the determination of desorption parameters in the low coverage limit, eliminating the effect of repulsive interactions among adsorbed CO molecules.
2. Experimental The experimental apparatus used in this study has been described in detail previously [31]. Briefly, the stainless steel vacuum chamber was equipped with a quadrupole mass spectrometer, four-grid LEED-Auger optics, a beam modulation system, and separate dosing lines for the introduction of H,S and CO into the chamber. The modulated molecular beam was generated in an array of fused capillaries of 0.005 cm diameter and was collimated by a 2 mm hole located in a solid copper gasket that separated the beam source from the scattering chamber. The chamber was pumped by a 220 l/s vat-ion pump and a titanium sublimation pump. Base pressures were in the low 10 -lo Torr range. The modulated beam experiments were run in the phase-sensitive mode of detection. Two identical lock-in amplifiers were used simultaneously to monitor the amplitudes of the product signals in-phase and 90” out-of-phase with the primary beam. These signals are designated I, and &,, respectively. The beam center line and mass spectrometer were each oriented 45” to the surface normal. The beam was incident on the open face of the steps and approxi-
526
C. E. Gdowski, R.J. Madix / Effeecr of s&fur on CO crdsorprion on Pt
Table 1 Surface
Pt and S
Clean Sulfur coverage just necessary (2 X 2)s (l/4 ML) (6 X fi)R30”S (l/3 ML)
to eliminate
CO( nH)
(150eV)/Pt
(235 eV)
1.2 3.4 1.3 10.5
mately perpendicular to the step rows. CO fluxes of 1014 molecules/cm2. s were used. The modulation frequency was 112 Hz. The crystal was heated by radiation from a tungsten filament located closely behind the sample. Pt/Pt-lO%Rh thermocouples spotwelded to the back of the crystal were used for temperature measurements. The previously cleaned platinum crystal was treated by heating in 5 X 10 -’ Torr of oxygen at 1300 K for a few minutes and then heating to 1500 K for a few seconds to ensure cleanliness, which was monitored by AES. Sulfur was deposited on the Pt(S)-[9( 111) X (lOO)] crystal surface by adsprbing H,S at 300 K and heating to 600 K to desorb H,. Following Berthier et al. [32] and Bonzel and Ku [2’7], the amount of surface sulfur deposited was determined by the ratio, R,, of the peak-to-peak heights of the S and Pt (150 eV) and the Pt (235 eV) Auger transitions. Two well defined sulfur overlayers on Pt(ll1) have been formerly observed [32,33]: the (fix fi)R30°S surface (l/3 ML) resulting from saturation coverage of sulfur deposited by H,S at 300 K and the (2 X 2)s overlayer (l/4 ML). Table 1 lists the R, values obtained experimentally for the well defined sulfur surfaces.
3. Results 3.1. temperature
~r~~ramrned desorprion
3. I. 1. Clean surface A temperature programmed desorption spectrum for a saturation exposure of CO on the clean Pt(S)-[9(111) X (IOO)] surface is shown in fig. 1. The spectrum shows distinct high (on) and low (or) temperature peaks similar to other spectra reported on flat and stepped Pt(ll1) single crystal surfaces [3,5,6,15,16,22,28]. The high temperature state at 540 K has been observed on most “flat” surfaces, but is more pronounced on the stepped surfaces. Previous studies have attributed this peak to CO desorption from defect sites on the-flat surfaces.[3,6,16]. In order to determine the amount of CO desorbing in the an state, attempts were made to isolate the state by exposing the crystal to small doses of CO. These attempts failed, except at extremely small CO doses, as
527
CO/CO
&
t
200
L
I
t
300
I
a
CLEAN
b
R,=334
f
L
400 500 TEMPERATURE
1
t
600
70’
(K)
Fig. 1. Saturation CO desorption spectrum on Pt(S)-[9( 111)X( IOO)] from {a) the clean surface and (b) a sulfur covered surface with R, =3.4. Tbe adsorption temperature was 205 K and the heating rate was 20 K/s.
there was always some of the or_ state preserrt. A typical result is shown in fig, 2, curve (a)_ An attempt to isolate the an state was then made by preflashing to 500 K before completing the desorption. At a low CO coverage the preflash resulted in a conversion of the at_ to the o(n state (fig. 2, curve (b)), but at a slightly higher CO coverage (though not enough to saturate the oFi state), the at state was still present (fig. 2, curve (c)). The peak temperature, TP,
24 c z 2 -
co/co
90 /A,,
a
NO PRE-FLASH
b
PRE-FLASHED
TO 5ODK
c
PRE-FLASHED
TO 5OOK
(CURVE
al
I.0
t” ;r” 8 z fi 5: z!l
I
L 400
s 500
TEMPERATURE
600 (K1
F’ig. 2. CO desorption spectra on the clean Pt(S)-[(9( I I 1) X (MO)]. The percentage of CO saturation coverage is (a) 2.9, (b) 3.8, and (c) 5.7. Curves (b) and (c) were obtained after preflashing the crystal to 500 K. CO was adsorbed at 205 K. The heating rate was 32 K/s. A,, is the area under the desorption curve.
528
G.E. Gdowski, R.J. M&ix / Effeci of sulfur on CO dsorption on Pt
of the (in state also decreased with increasing CO coverage. To check if the initial filling of the (Ye state was due to non-uniform dosing of the crystal surface, background doses of CO were made. This procedure did show a slight increase in the (in state over the LY,_ state at low coverages, but saturation of the (in state still did not result without substantial filling of the (Ye state. After dosing and preflashing to 485 K, the crystal was redosed at 205 K to saturation to see if an increase in total CO coverage could be obtained. No increase in total CO concentration in either state was obtained with this procedure. 3. I, 2. Sulfur covered surfaces Fig. 1 shows the desorption spectrum for CO at R, = 3.4. This sulfur coverage was the minimum necessary to eliminate the CO(a,) peak. The spectrum also shows a small decrease in (Yestate and a shift of the (or peak to a lower temperature. Elimination of the (mu state was not possible without a slight decrease in the (Ye state. By comparing the areas of the respective desorption spectra, the elimination of the (mu state decreased the total CO concentration at saturation by 11 * 2%. Therefore the LYEstate contributed to slightly less than this value at saturation on the clean surface. For CO adsorption on Pt( 111) saturation coverage was found to be 0.68 ML at 170 K [3] and 0.50 ML at room temperature [8]. Assuming one CO adsorbed per two platinum atoms at the steps, the percentage decrease in CO adsorption should be between 8 and 11% of the saturation coverage for this surface at 205 K. Therefore saturation coverage on the steps appears to be one CO per two Pt atoms. (For the (0 X fi)R30°S structure, there are two S atoms per three Pt step atoms and for the (2 X 2)s structure, one S atom per two Pt step atoms. Assuming a linear relationship between R, and sulfur coverage, R, values calculated for step saturation were 3.3 and 2.8 for the (6 X fi)R30° and (2 X 2) structures, respectively. The R, value at S saturation was used in the calculation since it was easily reproduced. The experimentally obtained value of 3.4 is consistent with an arrangement of sulfur atoms in a (6X fi)R30° structure along the step atoms, with sulfur atoms on both terraces adjacent to the step.) With increasing sulfur coverage (at S coverages below that necessary to saturate the step sites) the (in peak temperature decreased 49 K from the clean surface to the surface with the CO(Q) peak almost completely suppressed (R, = 2.8). This decrease represented a factor of 16 increase in the rate constant at 550 K on the sulfur covered surface, assuming the applicability of Redhead’s formula [34]. It should be mentioned that increasing the initial CO coverage decreased the peak temperature, probably due to CO-CO repulsive lateral interactions. To eliminate this effect, peak temperatures were taken at approximately the same CO coverages of 2 X lOI molecules cm-‘.
G. E. Gdowski, R.J. Mudix / Effect of sulfur on CO adsorption on Pt
529
3.2. Modulated molecular beam
Since the modulated beam results indicate that the CO desorption process was first order (there was no change in the phase lag as the CO beam flux was varied) only the details pertinent to first order adsorption/desorption for modulated beam experiments will be discussed. The reader is referred to the literature for further discussion of the technique [35-371. In phase sensitive modulated beam experiments a reaction product vector, e eei@, is obtained. It is defined 1351as the ratio of the first Fourier component of the desorption rate k,A, to the first Fourier component of the primary beam F,:
(1)
ce -i* = kdA,/F,;
where k, is the first order desorption rate constant, A, is the first Fourier component of the surface concentration and e and cp are the amplitude and phase angle of the reaction product vector. The desorption rate constant and modulation frequency are related to L and (p by
where S is the sticking probability and o is the modulation frequency. Assuming the usual form for the desorption rate constant, exp( -Z$,/RT), the following expression for Ed was obtained
k, = u
E,/RT=In(tan#)-ln(w/p). A plot of In(tan#) versus l/T yields a straight line with slope Ed/R. Fig. 3 illustrates a typical plot for a partially sulfur covered surface (R, = 2.2). Table2 lists values of v and Ed obtained for CO on various sulfur covered platinum surfaces. The desorption energy decreased smoothly from 36.2 to 22.3
Table 2 R,
S (atoms/cm’)
E (kcal/mole)
v (s-1)
1.2 2.2 3.0 3.4 4.5 6.1 6.7 7.4 8.2
0.0 0.5x 1.0x 1.2x 1.8X 2.6X 3.0x 3.3x 3.7x
36.2 32.7 27.4 27.5 25.7 24.2 23.3 24.8 22.3
3.9x 10’4 5.9X 10’2 I x1013 4.8X IO’* 2.1 x IO’2 1.9x lOI 5.5 x IO’2 2.1 x 1o12
10’4 10’4 10’4 lOI lOI 10’4 10’4 10’4
I
x1o’5
G. E. Gdowski, R.J. Mudix / Effect of sulftr on CO udsorptmn on Pt
530
08$ 0.6 a ._ _I tt: 0.44 ”
0.3-
5 c ; J
Gpp=3.9
SEC -I
X lOi
0.2-
0.1,
’ I.5
1.6
1.7 X IO3
I/T
1.8
1.9
(K-I1
Fig. 3. Plot of ln(tan+) versus inverse Pt(S)-[9(111)X(100)] with R,=2.2.
temperature
for CO desorption
from
sulfur
covered
I / 1 I
700l
650-
l
l. 600 -
1.0
14s
I I**
e*
.
I so-
.’
* .
I I
I I I
500 -
45OI 00
.
.
I.0
I
II
I
I
I
I
I
2.0
3.0
4.0
5.0
60
7.0
80
1
90
I
10x
Rs Fig. 4. Plot of the variation of T4s for CO desorption with sulfur coverage on Pt(SW( 111)X f ~@31. The dashed line denotes the sulfur coverage just necessary to eliminate the CO(aH) peak from the TPD spectrum.
GE. Gdowski, R.J. Madix / Effect of sulfur on CO ahorption
on Pt
531
kcal/mole, while Y decreased from lOI to 2 X lOI s-‘. It is interesting to note that none of the results obtained indicated a branch process. At a phase lag of 45” the desorption rate constant for a first order reaction is equal to the modulation frequency. The temperature at which the phase lag is 45”, Td5, is then a good indicator of relative desorption rates (see fig. 4). A decrease in Td5 with sulfur coverage indicates that the rate constant (at fixed T) increases. At low coverages of sulfur there was an initial rapid decrease in Td5 until the step/defect sites were occupied. Further increase in sulfur coverage resulted in a more gradual decrease in Td5.This observation, which is independent of sticking probability, clearly separates the desorption process from steps and terrace regions. If the sticking probability is less than unity, the detected signal consists of two components: those molecules which desorb from the chemisorption potential well, having a probability S and those which are reflected without entering the chemisorption well, having a reflection probability 1 - S. The sum of the two components gives the measured amplitude, E= (1 +&J)“‘+
(1+
where k; is the desorption
(:;$J”‘7
rate constant
for those molecules
not chemisorbed;
R, = 2.2 s, =0.53
I
I 500
I
I
TEMPERATURE
Fig. 5. Illustration versus temperature
I
600
I
I
700 (K)
of CO sticking coefficient determination from the normalized on sulfur covered Pt(S)-[9( 111) X (IOO)] with R, =2.2.
signal amplitude
G.E. Gdowski, R.J. Mudix / Effect
532
of sulfur on
CO udsorption on Pt
I.
00
1.0
20
3.0
4.0
50
60
70
8.0
9.0
100
RS Fig. 6. Plot of the variation of the CO sticking coefficient with sulfur coverage on Pt(S)-[9( 100) X (IOO)]. The dashed line denotes the sulfur coverage Just necessary to eliminate the CO( aH) peak from the TPD spectrum.
k; is always larger than the modulation frequency, w, therefore the second term simplifies to 1 - S. For those molecules which chemisorbed there exist two limiting cases. First, at high temperatures k, B w and the first term simplifies to S and z = 1. Second, at low temperatures w B k, and the first term is very small (i.e., its contribution to the amplitude is completely demodulated). The amplitude, 4, then becomes 1 - S. The difference between the low and high temperature limits of e is then the sticking probability for the chemisorbed molecules. A typical result is illustrated in fig. 5 for R, = 2.2. The effect of sulfur coverage on sticking probability of CO is illustrated in fig. 6. The sticking probability decreased only slightly with saturation of the step sites (0.56 to 0.52), after saturation of step/defect sites there was a more rapid decrease. For both the step/defect sites and terrace sites the plot exhibits a linear dependence of the sticking coefficient on unblocked sites, i.e., S a a( 1 0). For the lines drawn in fig. 6, the ratio of a for the steps/defects to a for the terraces is 0.32. The stronger dependence of the sticking coefficient on the state of the terraces is evident.
4. Discussion 4.1. Temperature programmed
desorption
The results on the clean surface indicated an activated conversion from the terrace sites to the step sites. This activated conversion may result from
C. E. Gdowski, RJ. Mudix / Egecr of su@r on CO adsorpfion on Pt
533
repulsive lateral interaction between CO molecules, yet the conversion for low CO coverages appears to be irreversible at or below 500 K. Such interactions occurred in the au state, as shown by the decrease in Tp with increasing CO concentration at the steps. Further, without preflashing the on state could only be isolated at extremely low CO coverages. It should also be mentioned that even after a preflash to isolate au the peak widths at half maximum increased with increasing CO coverage. At extremely small coverage the FWHM was only slightly larger (- 3 K) than expected for first order desorption, but at higher coverages the FWHM was about 30 K too large (> 75 K) to be due to a simple first order desorption. This could result either from repulsive lateral interactions within the adsorbed state or from some interference due to desorption from the low temperature state, Unfortunately, these effects could not be separated. Since preflashing to 485 K and redosing to saturation gave no increase in total CO coverage, it must be concluded that all the step sites were accessible to the CO molecules at 205 K. The higher coverages apparently forced CO to saturate the step sites. The barrier to conversion must therefore be the order of the CO-CO repulsion energy near saturation. It is interesting to compare the value of the peak temperature for (in found in this study with that for the (100) plane. For desorption from the highest temperature CO state on Pt( 100) McCabe and Schmidt [4] found Tp = 550 K, a value close to that found in this study for on. It should be mentioned, though, that they also found that CO desorption from the high temperature state on Pt(S)-[3(111) X (NO)] (Pt(211)) occurred at 610 K, 60” higher than on the Pt(lO0) surface, thus indicating a higher CO binding energy on the stepped surface. The CO desorption spectra for low sulfur coverage indicated that adsorbed sulfur migrated to step/defect sites. This was in agreement with Berthier et al’s results 1321,which showed that adsorption of sulfur on Pt( 111) occurred with localization of the sulfur atoms at sites of m~mum coordination of the metal. Further, the peak temperature for CO(cu,) decreased with increasing sulfur coverage at the step sites. This indicated, as shown at higher sulfur coverages [29,30], that there existed repulsive lateral interactions between coadsorbed CO and sulfur.
The modulated beam results indicate the same trend in desorption kinetics at low sulfur coverages as observed in thermal desorption. Both Td5 in the modulated beam experiments and Tp for (in in thermal desorption decreased with increasing sulfur coverage. The large initial decrease in Te5 was followed by a more gradual decrease with increasing sulfur coverage. The transition region occurred in the vicinity of the sulfur coverage just necessary to suppress the (mu peak in the TPD experiments. The initial rapid decrease can be attributed to repulsive interactions between the adsorbed CO and S atoms
534
G.E. Gdowski. R.J. Mu&x / E//eecr of sulfur on CO odsorprim
on PI
along the steps. The activation energy for desorption decreased 9 kcal/mole as the steps were saturated with sulfur. Tripling the total surface sulfur concentration reduced Ed only an additional 5 kcal/mole. The change in sticking coefficient with sulfur buildup showed a trend opposite to k,. Initially, at low sulfur coverages, the sticking coefficient decreased slowly until the step sites were saturated with sulfur. As the terraces became populated with sulfur, it decreased more rapidly. The results here indicate that steps do not significantly increase the sticking probability of CO at least via any bonding interactions, as blockage of the steps by sulfur caused only a small decrease in the adsorption probability. This is in agreement with Lin and Somorjai [lo], who determined the same sticking coefficient (S = 0.74) for CO on Pt( 111) and Pt(S)-[6(111) X (loo)] (Pt(557)) and with McCabe and Schmidt [4], who found only a slight difference in S for CO on Pt(ll1) (S - 0.34) and Pt(S)-[3( 111) X (loo)] (Pt(211)) (S - 0.24). The fact that adsorption of sulfur onto step sites did not change the sticking probability shows that conversion of precursor CO into the chemisorbed state is not enhanced by the clean surface even though the binding energy at the steps is substantially greater than on the terraces. It must be concluded that collisional processes on the terraces dominate the primary adsorption event. This conclusion is supported by the strong decrease of the sticking probability as the terraces become populated with sulfur. Since it has been established that there exist at least two main adsorption sites for CO on the clean stepped platinum surfaces, the following reaction sequence was analyzed for the modulated beam results:
(6)
where P is the probability of chemisorption adsorption directly onto the terraces, and the indicesg, t, and s refer to gas, terrace, and step, respectively. It has been suggested [9] that step (7) may be in equilibrium. If this were the situation, it could be shown (see the appendix) that the mechanism reduces to a branched process (the same result would also occur if there were no conversion between the steps and terraces). This mechanism is consistent with the results observed here only if the probability of adsorption onto the step sites dominates. If that is the case, k,, is measured directly.
G. E. Gdowski, R.J. Mudix
/
Effect of sulfur on CO adsorption on Pt
535
I
o.o’G; lO.O-
d 2
: 20.0v Y & 30.0(z w 5 40.0 ADSORPTION
- DESORPTION
PATHWAY
J
Fig. 7. A simple potential energy diagram for CO adsorbed on a stepped surface.
The reaction product vector which results when no simplifying assumptions are made about the above mechanism was derived and is shown in the appendix. The expression for the phase lag is seen to be very complex. Simplifications of the general expression are instructive. Although few data exist on the energetics of surface migration it is believed that the barrier for migration is approximately 10 to 20% of that for desorption [38]. Although k, and k, are rate constants for conversion between different types of sites, it is likely that the activation energies for these steps are small compared to the desorption energies; k, and k, should then be larger than kdt and k,,. If they were also assumed to be larger than w*, the following expression for the phase lag results:
dk, + k,) tan9=k,k,,+k,k,,=<’
(10)
From the TPD results, it is reasonable to assume that conversion from the terrace sites to the step sites is faster than the reverse process, that is, k, B k,. The effective rate constant can then either be k,, or k2kd,/k,. In either case the apparent activation energy is that of desorption from the steps, as shown by a potential energy diagram (see fig. 7). Further, if the transition state is the same for desorption from the terraces and the steps, then k,, = k, kd,/k, and k,, = 2k,,. Therefore, desorption from step sites on the clean surface occurs with Eds = 36.2 kcal/mole and Y= 5 X lOi s-r. Two other modulated beam studies of CO adsorption/desorption on clean Pt( 111) surfaces with steps [lo] or defects [9] have just been reported. In both
536
studies
G.E. Gdowskr, R.J. Mudix / Effect o/sulfur
the kinetic
parameters
keff = 7.9 X lOI s-’
exp
keff = 1.25 X lOI s-’ exp
on CO udsorptim
on Pt
determined (ref. [lo]), (ref. 193))
are in good agreement with those found here, but the mechanistic interpretations differ. By assuming that migration of CO could only occur from the terraces to the steps, Lin and Somorjai [lo] concluded that the desorption parameters determined were characteristic of the step sites. While we agree with their conclusion, we question the assumption that CO migration from the steps to the terraces does not occur especially at the temperatures (above 500 K) at which the study was performed. Campbell et al. [9] allowed for conversion between COtt, and CO,,, and concluded that since the lifetime of the molecule at a step site is significantly longer than at a terrace site, the parameters determined were appropriate for desorption from the step/defect sites. They further stated that it could not be determined if desorption occurred preferentially from a step site ( keff = kds), or by diffusion to a terrace site first and then desorption keff = k,k,,/k,. In fact, as shown above, the energetics of either process are equivalent. A more precise significance of the desorption rate constant can be inferred from microscopic reversibility [39,40]. The independence of the CO adsorption probability on sulfur concentration at the step sites shows that adsorption occurs via the terraces. Correspondingly, if microscopic reversibility applies, desorption must occur preferentially from the terraces as well. The preexponential factor characteristic of terrace desorption, v~,, is then increased by nearly two orders of magnitude by the equilibrium constant, k, /k,. The origin of this difference is apparently a more restricted mobility of CO bound to step sites with a corresponding decrease in entropy. This loss of mobility is consistent with the higher binding energy at the steps. High pre-exponential factors for CO desorption have been discussed previously for Pt( 110) [7] and Ru(OO1) [41]. Upper limits for the pre-exponential at low coverages were estimated to be 9 X 10” [7] and 1.2 X 10’s s-’ [41]. Both of these examples illustrate that large pre-exponential factors for immobile adsorbed states are reasonable. As sulfur was added to the surface, the rate constant for CO desorption increased, as expected for the repulsive interactions between CO and S. In the context of the model discussed for the clean surface the parameters determined are for the tightest binding state. With preferential adsorption of sulfur at step/defect sites, the parameters determined on the low sulfur coverages surfaces (R, -e 3.4) reflect repulsive interactions between CO and S at the step sites. As the sulfur coverage was increased past the point where all the step/defect sites were covered (R, > 3.4) the desorption parameters were believed to be due to the terraces. At the very least the rate constant for
G. E. Gdowski, R.J. Mudix / Ejj ec t oj su lj ur on CO udsorption on Pt
537
desorption from the terraces cannot exceed the value measured under these conditions. The activation energy reported here for desorption from the terraces (27.5 kcal/mole) is smaller than that found by Lin and Somorjai [lo] (29.9 kcal/mole) and Winicur et al. [ 121 (31.1 kcal/mole). This discrepancy may be attributable to the adsorbed sulfur, which besides blocking sites decreases the CO interaction with the platinum surface, as shown by the shift to lower temperatures of the CO thermal desorption spectrum with adsorbed sulfur (see fig. 1).
5. Conclusion The energetics of desorption of CO from Pt(S)-[9(111) X (loo)] at low coverages is determined by binding to step sites. Sulfur selectively poisons these sites until they become saturated. Over this range of sulfur coverage the sticking probability is only slightly affected, whereas the rate constant for CO desorptoin increases sharply. This increase is due to a 9 kcal/gmole decrease in the desorption energy. At higher sulfur coverages, k, increases slowly and the sticking probability decreases in proportion to the sulfur concentration. The dominant mode of adsorption occurs via the terraces.
Acknowledgement The authors AT03-79ER104)
gratefully acknowledge for this work.
the support
of the DOE
(Grant
DE-
Appendix Determination surface
of reaction product vectors for first order desorption from a stepped
For the purpose of illustration CO was used as the reactant. In the model developed, it was assumed that: (1) CO had two adsorption sites available to it, step and terrace; (2) desorption from both sites was first order; (3) migration of CO between the sites was allowed. The following nomenclature was used: S, sticking coefficient; I, intensity of the unmodulated beam; g(t), gating function of the beam chopper; w, frequency of modulation; c exp( - i+), reaction product vector with # the phase lag and z the apparent reaction probability. The reaction product vector has been shown to be equal to the ratio of the first Fourier coefficients of the rate of product formation to the reactant forcing function [35].
538
Gdowski, R.J. Mudix / Effecr of m&r OH CO adsorpiiow on PI
GE.
The following reaction sequence was considered:
co(t)
2q,
(A-2)
7
k,
kti
co(t)+ co(g)f
(A-3)
CO@,kds + COB, ’
(A.4)
where P is the probability of adsorption on the terraces and g, t, and s denote gas, terrace, and step. The surface mass balances for this mechanism were d(CO~~~)/d~=~~~~(~)
- k(CO,,,> +k,(CO,,,)
-k&O&
d(CO,,,),‘dt = (1 -P>
S@(t) - k,(CO,,,) + k,(CO(,,) -k&O&
(A-5) (A.61
For the linear mechanism the gating function, g(t), may be written as g 2”‘; Cot9 as A eiWf,and Cots, as x eiw’. If it was assumed that the step and terrace concentrations were in equilibrium, i.e., UCOJ
= k2(COWL
(A.7)
then
(A-8) (A.91 The reaction product vector was k
rexp(-i+)
A
=dt+Q!
k,,a
(A.lO)
zg ’
with t: =
S(A*
(A.1 1)
+ I?*)“‘,
tan # = A/B,
(A.12)
where A=
’ 1 + ( q’kdt)’ I-P
P
B= I+
(#/kit
)” +
1 + h/kd2
’
of sulfur
GE. Gdowski. R.J. Ma&x / Effect
This result is the same as that for a branch 1351. When tions A=
the surface mass balance
PSIg+
on CO adsorption on Pt
mechanism
equations
derived
by Jones et al.
were solved without
any assump-
k,a ((k,
(k, +k,,)*+a*
+kd
539
(A.13)
-id,
and X=
(1 -P) (k,
SIg+
k,A ((k2 +k,,)
+kds)*+u2
The reaction
product
Eexp(-i+)
(A.14)
-id.
vector was
=+$+y,
(A.15)
with S( (Y*+ p*)“* C=
(~.i6) (k,k,,
+ k,,k,,
+ k,k,,
- a’)’
+ a*( k, + k,, + k, + k,,)*
’ (A.17)
tan+=a/p, where
a=~ [ (4 +k, +kdt +kdWds -Pkdt(
k,,k,
+Pkdtk,,(k, P=(k,k,,
- LO’) - (1 + kcd + (I-
+k,,k,)(k,k,s
+kdskd*W*kdt +~*(Pkdt(h
-
+ +kdt
+k&,t)
P) kds( k,,k,
- w’)
P) k,,k,,(k,
+ kd]
+k,,k, kdsk, +k2)
+
+k,,k,,
7
-a’)
kdskdt) +
(1
-P)
kdsb
+k2
+kds%
if k,,k,
Bkdtr kdsr a*,
then
tan+=
dk, k k 2
+k,) +k k dt
I
. ds
References [I] [2] [3] [4]
L.A. West and G.A. Somojai, J. Chem. Phys. 57 (1972) 5143. R.L. Palmer and J.N. Smith, J. Chem. Phys. 66 (1974) 1453. D.M..Collins and W.E. Spicer, Surface Sci. 69 (1977) 85. R.W. McCabe and L.D. Schmidt, Surface Sci. 66 (1977) 101.
(~.18)
540
[S) [6] [7] [8) [9] [IO] f I I] [ 121 [ 131 [ 141 [ 151 [I61 [17]
G. E. Glowski,
R.J. MCI& / Eflect of sutfur on CO a&x-prim
on Pr
F.P. Netzer and R.A. Wille, Surface Sci. 74 (1978) 547. R.W. McCabe and L.D. Schmidt, Surface Sci. 65 (1977) 189. J. Fair and R.J. Madix, J. Chem. Phys. 73 (I 980) 3480. M.A. Barteau, E.I. Ko and R.J. Madix, Surface Sci. 102 (1981) 99. C.T. Campbell, G. Ertl, H. Kuipers and J. Segner, Surface Sci. 107 (I 98 I) 207. T.H. Lin and G.A. Somorjai, Surface Sci. 107 (1981) 573. J.A. Fair and R.J. Madix, unpubIished results. D.H. Winicur, J. Hurst, CA. Becker and L. Wharton, Surface Sci. 109 (1981) 263. HI. Krebs and H. Ltith, Appl. Phys. 14 (1977) 337. A.M. Baro and H. Ibach, J. Chem. Phys. 71 (1979) 4812. R.A. Shigeishi and D.A. King, Surface Sci. 58 (1976) 379. H. Hopster and H. Ibach, Surface Sci. 77 (1978) 109. H. Froitzheim, H. Hopster, H. Ibach and S. Lehwald, Appl. Phys. 13 (1977) 147. [ 1R] K. Horn and J. Pritchard, J. Physique 38 (1977) C4- 164. [ 191 A. Crossley and D.A. King, Surface Sci. 95 (I 980) 131. [20] A. Crossley and D.A. King, Surface Sci. 68 (1977) $28. [21] P.R. Norton, J.W. Goodale and E.B. Selkirk, Surface Sci. 83 (1979) 189. [22] G. Ertl, M. Neumann and K.M. Streit, Surface Sci. 64 (1977) 393. [23] D.M. Collins and W.E. Spicer, SurfaFe Sci. 69 (197’7) 114. [24] D.M. Collins, J.B. Lee and WE. Spicer, Surface Sci. 55 (1976) 389. 1251 P.R. Norton and P.J. Richards, Surface Sci. 49 (1975)567. [26] G. Kneringer and F.P. Netzer, Surface Sci. 49 (1975) 125. [27] H.P. Bonzel and R. Ku, J. Chem. Phys. 58 (1973) 4617. [28] T.E. Fischer and S.R. Kelemen, J. Catalysis 53 (1978) 24. [29] S.R. Kelemen, T.E. Fischer and J.A. Schwan, Surface Sci. 8 I (I 979) 440. (301 N.M. Abbas and R.J. Madix, to be published. [3 l] J. McCarty, J. Falconer and R.J. Madix, J. Catalysis 30 (1973) 235. [32] Y. Berthier, M. Perdereau and J. Oudar, Surface Sci. 36 (19733 225. 1331 W. Heegemann, K.H. Meister, E, Bechtold and K. Hayek, Surface Sci. 49 (1975) 161. [34] P.A. Redhead, Vacuum I2 (1962) 203. [35] R.H. Jones, D.R. Olander, W.J. Siekhaus and J.A. Schwarz, J. Vacuum Sci. Technol. 9 (1972) 1424. [36] J.A. Schwarz and R.J. Madix, Surface Sci. 46 (1974) 3 17. [37] D.R. Olander and A. Ullman, Intern. J. Chem. Kinet. 8 (1976) 625. [38] L.D. Schmidt, in: Interactions on Metal Surfaces, Ed. R. Gomer (Springer. New York, 1975). [39] M.J. Cardillo, M. Balooch and R.E. Stickney. Surface Sci. 50 (I 975) 263. [40] G. Comsa, in: Proc. 7th Intern. Vacuum Congr. and 3rd Intern. Conf. on Solid Surfaces. Vienna, 1977, p. 1317. [4l] H. Pfnur, P. Feulner, HA. Engelhardt and D. Menzel, Chem. Phys. Letters 59 (1978) 481.