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Adsorption and interaction of CO and NO on Pt(410)
341
Surface Science 155 (1985) 341-365 North-Holland, Amsterdam
ADSORF’TION AND INTJZRACTION OF CO AND NO ON Pt(410) I. TPD studies Y.O. PARK,
W.F. BANHOLZER
* and R.I. MASEL
Department of Chemical Engineering, Roger A&m California Street, Urbana, Illinois 61801, USA
Received 8 March 1984; accepted for publication
Laboratory,
31 December
** University
of Illinois, 1209 W.
1984
NO and CO adsorption and the NO/CO reaction on Pt(410) are studied by TPD. NO is found to dissociate on Pt(410) at 120 K, but it reacts to form N,O at higher exposures. The N,O desorbs in two peaks at 135 and 150 K. CO adsorbs molecularly, and desorbs in 5 peaks at 550, 500,450, 380 and about 130 K. CO is also found to dissociate upon heating, leaving a carbon residue on the surface which changes the TPD spectra. The NO/CO reaction shows a surface explosion at 360 K. These results provide additional evidence that Pt(410) has unusual reactivity, as predicted by Banbolzer, Park, Mak and Masel, Surface Sci. 128 (1983) 176.
1. Intmduction The NO/CO reaction on platinum has been studied by many previous investigators [l-11]. It shows a wealth of unusual behavior including surface explosions [2], and oscillations [4]. The rate of the reaction has been found to vary significantly with crystal face; Pt(ll1) shows almost no activity, [8] while Pt(lOO) is quite active [9]. Here, we are considering the NO/CO reaction on a face for which there has been little previous work, Pt(410). Pt(410) is an unusual surface from an orbital symmetry standpoint. The model of Banholzer et al. [12] predicts that the step sites on Pt(410) have the right symmetry to break NO and CO bonds. Previous X-Ray Photoemission Spectroscopy (XPS) data [13] suggest that NO partially dissociates upon adsorption on Pt(410) at 300 K. All of the close-packed faces of platinum and all of the other stepped platinum surfaces considered previously show molecular adsorption of NO under similar conditions. Flash of an NO saturated layer yields a N, peak which shifts from 480 to 440 K with increasing coverage, and an oxygen peak above 800 K. NO desorption is not * Present address: General Electric Company, CSEB 410, P.O. Box 8, Schenectady, 12301, USA. ** Correspondence should be addressed to this author.
0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
New York
342
Y. 0. Park et al. / CO and NO on Pt (410). J
detected when dosing is done so that little gas impinges on the sides and back of the crystal. XPS results [13] show that CO molecularly adsorbs, but there are changes in the XPS data upon heating that are consistent with dissociation. No similar changes are seen with the other platinum faces considered previously. However, enhanced CO dissociation has also been observed [14] on Ni(310) and (510), i.e., surfaces for which similar symmetry arguments apply. These results show that CO and NO behave differently on Pt(410) than on other platinum faces. Therefore, one might expect Pt(410) to have unusual properties for the NO/CO reaction. The purpose of this paper is to explore CO adsorption and the NO/CO reaction on Pt(410) using Temperature Programmed Desorption (TPD). 2. Experimental
method
The TPD data presented here were taken using the chamber and Metron crystal described previously [13,15]. The vacuum system was of standard design, but it had an exceptionally large set of pumps so the system could be routinely evacuated to below the xray limit of the ion gauge. The crystal was aligned using Laue back reflection and cut by electrical discharge. It was polished on both sides and cleaned using standard techniques. The sample was mounted by spotwelding two 0.25 mm tantalum wires to the crystal. The manipulator allowed heating to be accomplished by either resistance heating the support wires and conductive heat transfer to the crystal, or by electron bombardment on the back of the crystal. All of the data presented here used resistance heating. In later experiments, a liquid nitrogen cooled block was added to the system. It allowed the sample to be cooled to 120 I(. The temperature was monitored by a 0.07 mm Chromel-Alumel thermocouple spotwelded to the crystal’s edge. Regulation of the heating and data acquisition were under the control of an LSI-11 minicomputer. Heating rates of 8 to 10 K/s, reproducible to 0.5 K/s were possible. Partial pressures were measured by a Riber QXlOO quadrapole mass spectrometer. Dosing of the sample was accomplished using a Varian leak valve and increasing the ambient pressure and waiting until the desired exposure was reached, or by rotating the crystal so that it was 40 mm away from a dosing tube which was fitted with a glass capillary array. Calibration of the capillary array doser was accomplished by comparing desorption spectra obtained by using the two techniques. In the coadsorption experiments, the crystal was given the desired exposure to the first gas and, after the pressure had fallen below 7 x 10-i’ Torr, which usually took ca. 5 min, the second gas was introduced. Two different dosing valves were used. In this way, it was possible to precisely control the amount of each gas exposed to the crystal, and to dose with gas mixtures. AES work was done with a PHI lo-155 CMA.
Y. 0. Park et al. / CO and NO on PI (410). I
343
8
m
z
IO
s
20
30
Time,
sec.
Time
, sec.
Fig. 1. Some raw TPD traces for CO and NO desorption from Pt(410).
There is one other experimental detail of note. When these experiments were first started the low temperature peaks could not be resolved due to interference with desorption from the support rods. However, the support rods were insulated, so that only the last cm was exposed to reactive gases. (The exposed surface area of the rods is only 0.15 cm*, which should be compared to a total surface area of 1.5 cm* for our crystal.) The heating routine was then modified so that the peak from the support rods could be separated from those on the crystal. A capillary array doser was also used to eliminate support effects. Fig. 1 shows the results of these efforts. Notice that there is a sharp peak which we attribute to desorption from the support rods, and then separate peaks, which AES shows to be largely associated with desorption from the crystal. There is a tail under the peaks which might also be associated with desorption from the support rods. Experimentally the sharp peaks from the support rods were always small, and well resolved from those from our sample, although the tail could be a problem. In the spectra to follow, the sharp peak from the support rods was suppressed. The data in fig. 1 also allow one to estimate a response time of the system. Notice that the sharpest peaks in fig. 1 have a half width of less than 0.2 s. In other experiments, peaks as narrow as 0.05 s have been seen. In the experiments to follow the narrowest peaks will have a half width of 0.8 s or more. Thus, it does not appear that the broadening of the system has an important influence on the width of the peaks.
3. Results 3.1. co Fig. 2 shows a series of TPD spectra taken by exposing a Pt(410) sample to CO at room temperature then flashing. One observes a series of three overlap-
344
Y.O. Park et al. / CO and NO on Pi j410). 1
Mass 28 CO/Rf410)
300
400
500
600
Temperature,
700
800
K
Fig. 2. A series of mass 28 TPD spectra taken after adsorption of CO on a clean Pt(410) sample at 310 K, p = 25 K/s.
ping peaks at 550, 500, and 450 K labeled &, &, and & respectively. The & peak fills first. It is seen even at low coverages, and it saturates at an exposure of 4 L. The & fills next. It appears as a shoulder on the & at low coverages. However, at exposures above 0.5 L it is resolved into a separate peak. The & fills last. It first appears as a shoulder on the & peak. However, it is resolved into a separate peak at coverages above 5 L. Ail three peaks seem to have positions which are independent of coverage, which is suggestive of a first order desorption process. Fig. 3 shows the results of a similar experiment done by exposing a Pt(410) sample to CO at 120 K. One observes five peaks labeled (Y,,, LYE,p,, &, and & respectively. The three p peaks grow non-sequentially, just as was described above, but there is some blurring of the peaks at higher coverages. The (Ye and CY~ are new. The ~yacomes off just as we start the heating process. It looks like peaks commonly attributed to desorption from the support rods. However, it did not disappear when we used a capillary array and AES revealed that CO is desorbing at low temperatures. Thus, the (Ye, must come in part from the sample. The a0 is large feature, and it continues to grow with increasing
Y.O. Park ei al. / CO and NO on Pt (410). I
345
coi Pt (410)
II 100
I
200
I
I
300
400
Temperature
I
500
I
600
K
Fig. 3. A series of mass 28 TPD spectra 120-150 K, /3 = 17 K/s.
taken after adsorption
of CO on a clean Pt(410) sample at
exposure up to the highest exposures shown (30 L). Its size is very dependent on the adsorption temperature and there possibly could be some support rod contributions. We did not have good control of the adsorption temperatures below 150 K and could not sort out the support rod effects. Hence, we did not explore the (~a in detail. The a,, comes off in the temperature range of 370-400 K, i.e. above room temperature even though it is not seen upon room temperature adsorption. The peak is small, and its position decreases with increasing exposure, suggesting either a second order desorption process, or a decrease in the activation energy of desorption with increasing coverage. The peak intensity increases with increasing exposure saturating at 30 L. During the course of the experiments, it was observed that the desorption spectra obtained depended on the history of the sample. If the sample was cleaned by heating briefly in oxygen or ntiric oxide and then flashed to 1500 K in a vacuum of better than 5 X lo-” Torr, the TPD spectra produced were similar to those in fig. 2. However, if repetitive exposures were done without cleaning the surface in oxygen between exposures, the CO desorption spectra would evolve into those shown in fig. 4. As is evident from fig. 4, the & is reduced by 50% and shifts upward by ca. 50°C. The size of the & and & peaks appear largely unaffected but they shift down by ca. 10°C. If the surface producing the spectra in fig. 4 was exposed to 18 L of oxygen at 1100 K and
346
Y. 0. Park et al. / CO and NO on Pt (410). I
Mass 28
350
450
550 Temperature
co/c-
650
P1(410
750
K
Fig. 4. A series of mass 28 TPD spectra taken after adsorption of CO on a 310 K Pt(410) sample which was contaminated by ca. 25% of a monolayer or carbon, p = 25 K/s. The carbon was a residue formed by repeatedly saturating the sample with CO and flashing to 1500 K.
then flashed to 1500 K, the spectra obtained were undistinguishable from those in fig. 2. Auger Electron Spectroscopy (AES) was done to examine the changes in the surface that accompany the changes in the desorption spectra. It was found that about 10% of the carbon from the CO was left on the surface in the first flash. Additional carbon is deposited in subsequent flashes, until eventually about 25% of a monolayer of carbon builds up on the surface. It appears that the carbon buildup produces the changes in the flash desorption spectra that are observed. Several attempts were made to resolve the & and & peaks in fig. 2. Selective annealing was tried first, but the resultant spectra were very similar to those in fig. 2. Another method was to try to vary the heating rate in an attempt to get increased resolution. At a heating rate of 27 K/s, the various peaks were very close together and were only slightly resolved. At heating rates below 10 K/s or above 35 K/s, the & and & peaks merge to form one broad peak. The resolution of the vacuum system, as determined from the sharp peaks in figs. 1 and 12 is such that the peaks could have been resolved at higher heating rates if normal kinetics applied. Since they are not, something unusual must be happening on this surface that makes it difficult to resolve the & and & peaks. The difficulty in resolving the two peaks made it impossible to use variation in heating rate to determine detailed kinetics of the desorption
Y. 0. Park et al. / CO and NO on Pt (410). I
I
2
3
4 Exposure
5
6
7
6
9
34-l
IO
(L)
Fig. 5. The relative CO coverage, calculated from flash desorption peak areas, as a function of the calibrated CO exposure at room temperature.
process. Fig. 5 is a plot of relative coverage (calculated from the CO peak areas) versus exposure at room temperature. As is evident from this curve, the sticking probability is not constant but decreases with coverage. The surface population at saturation can be determined from the initial slope in fig. 5 via
( UP/P) no (dtY/dL),
so ’
where no is the surface population at saturation (molecules/cm’), u is the molecular velocity, p is the gas density at the dosing pressure P, 8 is the relative coverage, L is the exposure, and So is the initial sticking probability. Lin and Somojai report an initial sticking probability for CO on Pt(5(111) x (111)) of 0.74 [16], while McGabe and Schmidt determined a sticking probability for CO on Pt((lll)~(lOO)) to be 0.34 [17], and Gdowski and madix report a value of 0.55 for Pt(9(111) x (100)) [18]. If we assume So = 0.55, we calculate a saturation coverage of CO of 1.0 X 1Ol5 molecules/cm’; several previous investigators have measured the saturation coverage of CO on Pt(lOO) and get values near 1 X 1Ol5 molecules/cm2, so our value is reasonable. Admittedly, our method is not as accurate as those of previous investigators. At very high exposures (> 100 L, i.e. long exposure times) at room temperature we observe another effect. It appears that the CO peak intensity decreases slowly with increasing exposure. No similar reduction was seen when the crystal was held at 120 K or when the sample was covered by ca. 25% of a monolayer of carbon. It is not seen with other faces mounted on the same manipulator. At first the reduction was considered to be an artifact. However,
348
Y.O. Park et ul. / CO and NO on Pt (410). I
we did several tests and were not able to isolate a possible source of the artifact. One test was to leave the sample in the vacuum for up to an hour and then flash the surface. No comparabte loss in CO intensity was seen. Another test was to valve off the ion pump, and turn off each of the filaments in the chamber one at a time. That had no effect on the reduction in intensity. Finally, we tried to look for a likely contaminant, but were not able to detect anything. There is still the possibility of the CO reacting with something in the chamber to produce a low level contaminant. However, in order to explain the data the ~nta~nant would have to not stick at low temperatures or on a surface which is contaminated with 25% of a monolayer of carbon. The plausibility of this occurring needs to be judged by the reader. We note, however, that there is nothing so far which excludes the possibility of there being some real chemistry, which causes the reduction in CO peak intensity. In fact we believe that to be the most likely possibility. 3.2. NO Studies of NO adsorption on Pt(410) were also done. The results of a study of NO adsorption on Pt(410) at room temperature were discussed previously (151. When a 300 K Pt(410) sample is saturated with NO and then flashed, a single N, peak which shifts from 490 to 440 with increasing coverage is seen. There is also an 0, peak above 900 K, but no NO was detected when the crystal was dosed in a way that little NO impinged on the sides of the crystal or the support rods. The maximum NO coverage at room temperature corresponded to 5 x 1014 molecules/cm*, i.e. about one dissociated NO for every two surface atoms. When the sample is dosed with NO at 120 K, the desorption spectrum changes substantially. Figs. 6 and 7 show a series of mass 28 and 44 spectra taken after adsorption of NO at 120 K. At low exposures, the mass 28 spectrum shows two peaks at 420-450 and 350-390 K, labeled /Ii and flz respectively. Both peaks shift to lower temperatures with increasing exposure. The peaks appear to fill nonsequentially. The higher temperature peak is very similar to the peak seen in the room temperature experiments, but the lower peak is unique to the 120 K adsorption experiments. At higher exposures, the intensity of both peaks decrease with increasing exposure. Simultaneously, two mass 44 peaks grow into the spectrum. Flashes of an N”O saturated sample shown in fig. 7 shows that the mass 44 peaks are due to N,O, and AES was used to confirm that the mass 44 peaks were coming from the sample. Annealing experiments were also done. It was found that annealing at 300 K produces a sharp reduction in the & peak and an increase in the J$. This suggests that there is a inner conversion occuring on the surface which complicates the interpretation of the TPD data in fig. 6. Fig. 7 also shows the development of the mass 44 (N,O) TPD spectrum with
Y.O. Park et al. / CO and NO on Pt (410). I
loo
I 200
I
x)0
I 400
I 500
I 600
349
I 700
Temperature K Fig. 6. A series of mass 28 TPD spectra taken after adsorption of NO on Pt(410) at 120 K, /3 = 17 K/s.
increasing exposure. One observes two peaks at 135 and about 155 K. The peaks grow simultaneously with increasing coverage. The position of the lower temperature peak is almost independent of coverage, suggesting a first order desorption process. The higher temperature peak moves to lower temperatures with increasing exposure, suggesting either a second order desorption process or a coverage dependent activation energy of desorption. Both peaks saturate at 1OOL exposure. At that point, the 440 K N, peak is reduced to the point where it is barely distinguishable from the noise. There are some low temperature mass 28 and mass 30 features, but their size was small enough, that most of their intensity could be attributed to cracking of N,O in the mass spectrometer. Fig. 8 is a plot of the relative coverages of NCnd) and NZOCad)as a function of exposure determined from 3, = &.qq;.
(21
8 N,O
(3)
=
spN20/2pt$
peak areas where PN2 and PNzO are the N, and N,O flash desorption respectively, P FJ is the N, peak area produced by a platinum (410) sample which had been saturated with NO at room temperature, and 6 is a correction factor for the sensitivity of the system for N,O relative to that of N, determined from comparing AES peak intensities. 8, reaches a maximum of increases with increased exposure. At saturation, 0.9 at 3 L exposure. 8,,, e N,o is approximately 1.
Y.O. Park er al. / CO and NO on PI (410).
350
I
100 L
3OL
15 L
IOL
7L
2L
100
200
300
400 temperature
500
600
700
K
Fig. 7. A series of N,O TPD spectra taken after adsorption of NO on Pt(410) at 120 K, [j = 20 K/s. The 30 L spectrum is a mass 46 spectrum from a sample which had been exposed to N’50. The rest of the scans are mass 44 spectra
from samples
exposed
to N140.
One can estimate absolute coverages of NO from the data in fig. X, assuming that the saturation density of CO at room temperature represents a coverage of 1 X 1O’j molecules/cm2. B = 1 corresponds to about 5 X 1014 atoms or molecules/cm2, so that the maximum t?, corresponds to about 4.5 x 1Or4 atoms/cm2 and the maximum in eNzo corresponds to about 5 X 1014 atoms/cm’. The saturation coverage of nitrogen atoms at room temperature is
Y.O. Park et al. / CO and NO on Pt (410). I
351
Exposure, Longmuirs
Fig. 8. The coverages of N, and N,O as a function of exposure at 120 K as determined from flash desorption peak areas.
also about 5 x lOi atoms/cm’. by adsorbing NO at 120 K, but to 200 K. The results in figs. 6, 7 and that N,O is being formed from the N,O desorbs below 200 K. 3.3. CO+
One can get about 1 x 1015 oxygen atoms/cm’ half of them desorb when the surface is heated 8 combined with the AES results show clearly NO on Pt(410) at low temperatures, and that
NO
Coadsorption of NO and CO generally resulted in the formation of CO, and N,. Fig. 9 shows the mass 28 desorption spectra obtained when the crystal was exposed to 1 L of NO followed by varying amounts of CO. When Pt(410) is exposed to 1 L of NO and then flashed, a single mass 28 peak is seen at 430 K. Two additional langmuirs of CO produces a broadening and shifting of the peak, and the appearance of a new peak at 550 K. A small CO, peak is also seen. If the surface is exposed to 1 L of NO then 10 L of CO, the spectrum looks qualitatively like the spectrum taken when the surface is exposed exclusively to CO, but the peak intensities vary, and the features are less distinct. Again CO, was formed. No NO desorption was detected in any of the runs above. 0, desorption was seen in the runs where the surface was exposed to just NO, but not when it was exposed to 1 L NO then 2 L or more of CO. CO, formation was also observed during the dosing procedure. The mass 28 signal in the spectra above is somewhat ambiguous since N, and CO have the same mass. In order to resolve this ambiguity, experiments were done with Cl30 and N150. Fig. 10 shows the spectra obtained after a Pt(410) sample was exposed to 2 L of NO and 2 L of NO followed by 10 L of Ci30. Notice that the N, peak is strongly attenuated when CO is added to the
Y. 0. Park et al. / CO and NO on PI (410).
352
I
I
/
Pt (410)
I 350
I
I
I
450
550
650
Temperature
Fig. 9. A series of TPD spectra
K
taken by sequentially
dosing a 300 K Pt(410) simple
with NO then
co.
layer. Evidently, CO displaces the nitrogen containing species from the surface. The N, peak is also substantially broadened and its peak maximum is shifted down to 370 K. This suggests that there is apparently some interaction between the nitrogen containing species and the CO on the surface. However. the CO
I
2L NO + IO L d30
I 350
I 450
I 550 Temperature
Fig. 10. TPD spectra C”0. /I = 24 K/s.
I 650 K
taken after exposing
a 300 K Pt(410) to 2 L of N140
followed
by 10 L of
Y.O. Park et al. / CO and NO on PI (410). I
353
!
2L CO+ 10LN’50/Pt(410)
z;;;;;;; I 350
I 450
I
I
550
650
Temperoiure,
K
Fig. 11. A series of TPD spectra taken followed by 10 L of N150, /S = 25 K/s.
after exposing
a 305 K Pt(410)
sample
to 2 L of Cl20
peak looks very similar to the one seen when the crystal is dosed with pure CO, so apparently, the interaction has less effect on the CO peak. Note that CO is the major species on the surface, so this is not a surprising result. Fig. 11 shows a series of spectra produced when the sample is exposed to 2 L of CO followed by 10 L of N150. A comparison of the spectra in fig. 12 with those in figs. 2 and 3 above shows that pi- and &-CO are displaced by NO, but the &-CO is largely unaffected. Note that originally the /I2 corresponded to only about 20% of the total CO. There is also a new CO feature at 370 K, which may be q-CO. The N, peak also looks very different from the peaks seen above or the peaks seen when the surface is exposed to pure NO. There is a low temperature, 380 K, peak, and a higher temperature 460 K peak. Neither of these peaks can be easily identified with the peaks seen when the surface is exposed to just NO. No NO, O,, or N,O desorption was detected, but CO, was seen. The CO, peak is very broad with peaks at 400 and 480 K. It looks very similar to the CO, peaks seen on other surfaces. However, there is one important difference: the peak is much smaller than it would have been if most of the CO, was formed during the flash. Of course, we also observed CO, formation during the dosing process, so it is not surprising that the CO, peak is too small. One can estimate the fraction, F, of the CO, which formed during the dosing process from
where AN, and A,, are the flashed desorption peak areas for N, and CO, respectively, and R i!s the ratio of the sensitivity of the mass spectrometer for N, to that of CO, weighted by the relative pumping speeds for the two gases. A value of 1.6 was estimated for R, from the slow flash of a polycrystalline platinum sample, assuming F = 0 for a case where negligible 0, desorption was seen (no CO, formation was observed during dosing). F works out to be
Y. 0. Park et al. / CO and NO on PI (410).
30 L 50/50
I
NO CC
Pt (410)
J 100
I
I
I
I
I
I
200
300
400
500
600
700
Temperature
K
Fig. 12. Mass 2X and mass 44 TPD spectra taken after exposure of a 120 K Pt(410) sample to 30 L of a 50/50 mixture of CO and NO. Note that the mass 28 spectrum has been magnified by 2 X.
2/3 for the data in fig. 12, which suggests that the crystal is quite active for the NO/CO reaction at room temperature. A set of NO/CO coadsorption experiments was also done with an adsorption temperature of 120 K. The runs where the surface was saturated with NO then exposed to CO or vice versa were uninteresting because the displacement of NO by CO or CO by NO is a slow process at 120-150 K. However. simultaneous coadsorption of CO and NO at 120 K yields some rather interesting results. Fig. 13 shows a series of TPD spectra taken after a 120 K Pt(410) sample was exposed to 30 L of a 50/50 mixture of CO and NO, then flashed. One observes three peaks at mass 44, two broad peaks below 200 K and a very sharp feature at 360 K. The two low temperature peaks are very similar to the peaks seen when the surface is exposed only to NO. Results of
Y.O. Park et al. / CO and NO on Pi (410). I
s
x3
1
IO0
I
t
I
200
300
400
Temperalure,
I
500
355
Mass28
I
600
K
Fig. 13. Mass 28,44 and 46 TPD spectra taken after exposure of a 120 K Pt(410) sample to 30 L of a SO/50 mixture of C”O and N150. Note that the mass 28 and 30 spectra have been magnified by 3x.
similar experiments using N150 are given in fig. 13. One observes that the N,O comes out in two peaks at 135 and 150 K, while the CO, comes off in two peaks at 150 and 360 K. Note, CO, is not seen when the sample is dosed with only NO, so the CO, must be formed via a reaction between NO and CO. The 360 K peak is very sharp; its half width is only 10 K. Thus, it is indicative of a so called “surface explosion”. If one blindly fits first order rate parameters using the method of Edwards [30] one finds an apparent exponential of 103’/s and an apparent activation energy of 62 kcal/mol for the peak. Clearly, no quantitative interpretation of these numbers is meaningful, but they do suggest that a simple analysis is not suitable. Figs. 12 and 13 also show the mass 28 and 30 spectrum taken under the same conditions as the mass 44 and 46 spectra discussed above. One observed small mass 28 features at 135, 155, and 360 K, and some even smaller features in the range of 400-500 K. The structure of the peaks is very similar to that in the mass 44 peak. Note however, that the cracking pattern [31] of N,O and CO, contain major contributions at mass 28. When those contributions are subtracted from the mass 28 spectrum in fig. 12, one finds that the peaks at 360 and 155 K are strongly attenuated. There remains a small cont~bution below 140 K, which can be attributed to cu,-CO. The peaks in the range of 400-500 K remain, but these peaks are relatively small. Thus, it is suggested that when the crystal is saturated with a SO/SO mixture of CO and NO, almost all of the CO remaining on the surface desorbs as CO, during the surface explosion. Significant CO desorption above 400 K was observed when excess CO was used. Fig. 13 also shows the mass 30 (15N,) peak taken with an excess of NO. We observe a small mass 30 peak below 200 K, which can be at least partially attributed to cracking of N,O. However, there does not appear to be any nitrogen desorption above 200 K. Thus, it seems that most of the nitrogen has desorbed before the surface explosion takes place.
356
Y.O. Park et ul. / CO und NO on Pt (410). I
4. Discussion 4.1. NO
The NO spectra here provide additional evidence of the unusual reactivity of Pt(410). In previous work it was found that NO partially dissociates upon adsorption on Pt(410) at room temperature [13]. Flash of a Pt(410) sample which had been saturated with NO at room temperature yields N, and 0,. but no detectable NO when the dosing is done so that little NO impinges on the sides and back of the crystal [15]. Here, similar experiments were done at 120 K. We observe startling, but different chemistry. At low exposures, only N, and 0, desorption was seen, but as the coverage was raised, the nitrogen peak disappears, and an N,O peak forms. Thus, it appears that much of the nitrogen was converted into N,O. The N,O desorbs below 200 K. Very little nitrogen was detected in AES above 200 K. Thus, it is clear that some unique chemistry is occurring on Pt(410) below 200 K. Previous workers [1,31] have suggested that the N,O forms via reaction between NO and adsorbed nitrogen. Thus it is suggested that NO probably dissociates on Pt(410) at 120 K! By comparison, no N,O formation has been observed on any other clean platinum sample below 400 K. Clearly. Pt(410) has unusual properties. It is not immediately obvious why the N,O forms at low temperatures but it is not seen at higher temperatures. N,O could conceivably form by three different mechanisms: a Rideal-Eley mechanism (5)
N@d) + NO&, + NZO,,,, 3 a Langmuir-Hinshelwood NCadj+ NO&l, --) N*Oc,,, ’ or a precursor
mechanism (6)
mechanism
NC,,, + NO,,,, + N,Oc,,, 3
T(7)
where NcadJ and N,O(,,, are adsorbed nitrogen and nitrous oxide, and NO,,,, and strongly adsorbed NOC”,,, and NO(,,, are gas phase, weakly adsorbed, nitric oxide. The first mechanism can be easily excluded. XPS results [13] have shown that NO partially dissociates on Pt(410) at room temperature. Thus, if reaction(3) has an appreciable rate, then N,O formation should be seen when the sample is reexposed to NO at room temperature. Experimentally no N,O could be detected in such an experiment. Reaction (4) is more difficult to exclude. It was previously found [13] that most of the NO which adsorbs on Pt(410) dissociates at room temperature. However, infrared studies [32] have indicated
Y.O. Park et al. / CO and NO on Pt (410). I
357
that there is a small amount of undissociated NO on the surface. This NO is probably held somewhere on the terraces or near impurities, so it should be less reactive than NO adsorbed near the steps. Still it is curious that no N,O formation is detected even though Ncadj and NO(,,, are present on the surface. Thus it is unlikely that N,O forms by reaction (4). This leaves reaction (5). Reaction (5) assumes that N,O forms by a reaction between adsorbed nitrogen (from dissociated NO) and a weakly bound NO species. One would only expect there to be a reasonable concentration of weakly bound species at low temperatures and high exposures. Thus, appreciable N,O formation should only be observed under such conditions, in agreement with the data. 4.2. CO The CO data also show unusual features. The simplest interpretation of the five peaks in the carbon monoxide desorption spectra would suggest that there were five different CO binding sites on the surface corresponding to the five different peaks in the flash desorption spectra. If one assumes that the TPD follow a first order rate law with a preexponential of 10t3/s one calculates binding energies of 32, 29, 26,22 and approximately 9 kcal/mol for the p3, &, uncertainty in P,Y al* and (Ye states respectively. Clearly, there is considerable these energies, but the orders of magnitude are useful. Evidently the first four sites have a similar binding energy for CO. This explanation of multiple sites initially seemed reasonable, given the heterogeneous nature of the surface. It is possible that /3i, and & resulted from CO adsorbed from different sites on the (100) terraces and p3 was caused by CO adsorbed at the (110) steps, with the (Ye and (Yecoming from the second monolayer.. Other combinations of sites also need to be considered. These possibilities will be explored below. It is useful to compare the spectra here to those for CO adsorption on other faces of platinum. McCabe and Schmidt find that CO desorption from Pt(ll0) yields a main TPD peak at 460 K which shifts to lower temperatures with increasing coverage [17] and a smaller peak at 350 K. They find that Pt(ll1) gives a peak which moves from 525 to 450 K with coverage, and a small shoulder at 400 K. The TPD spectrum for CO desorption from Pt(lOO) are quite complicated [17,20,21] with surface reconstructions occurring during adsorption and desorption. McCabe and Schmidt [17] report a four peak TPD spectrum for CO desorption from a (5 X 20)-(100) surface. At low coverages the largest feature is a high temperature state with a peak temperature near 550 K. The peak shifts to lower temperatures with increasing coverage. There are also three smaller lower temperature states, one at about 525 K, one at 450 K, and a third at about 400 K. Barteau, Ko and Madix find similar behavior [20], except that their peak temperatures are slightly lower due to their lower heating rates and the lowest temperature peak is absent. Crossley and King
358
Y.O. Park et al. / CO and NO on Pt (410). I
[21] also report a three peak structure, with peaks at 550, 470 and 400 K. They also studied desorption of CO from a (1 X 1) Pt(lOO) and argue that the 550 K peak is caused by desorption from a (5 x 20) reconstructed (100) surface. Stepped surfaces have been studied to a lesser extent. McCabe and Schmidt [17] report two peaks for CO desorption from Pt(210), a large peak at 625 K, and a smaller peak at - 475 K. In published work, we have observed similar spectra for CO desorption from Pt(210), although the high temperature peak was only at 615 K. McFealey and coworkers [22] report two peaks for CO desorption from Pt(321), a peak at 550 K, and a second peak at 440 K. Collins and Spicer [23] report two peaks for CO desorption from PtS[6(111) x (loo)], a large peak at 450 K and a smaller peak at 550 K. One should notice that the TPD spectra for CO desorption from these stepped surfaces have similar features. There is a peak with a maximum which corresponds roughly to the temperature of the main peak for CO desorption from the terraces, and another higher temperature peak, which is unique to the stepped surfaces, and therefore has been associated with CO desorption from the steps. Surprisingly though CO desorption from Pt(410) does not at first glance seem to follow the same general trends. The high temperature peak which is seen at ca. 615 K on Pt(210) is absent here. Further, the peak at 550 K is smaller than it should be based on the available data for CO desorption from the Pt(lOO) terraces. Admittedly, Pt(410) does not undergo the same reconstructions as Pt(lOO), so it may be incorrect to compare the spectra directly. Most stepped surfaces give a peak at 550 K, so it is possible that the 550 K peak contains a major contribution from the steps. Still, the behavior observed here is unusual; the highest temperature peak is too small which suggests that something unusual is happening on the (410) that does not happen on other stepped surfaces. There is other evidence that something unusual is occurring on this surface. Note that it was found that the & and & peaks merge at high heating rates. Flash desorption peak widths are generally inversely related to heating rate. At large heating rates, one would expect increased resolution provided the system responds quickly enough. The sharp peaks in fig. 1 show that our system response time is below 0.1 s; this response time of the system would not prevent us from resolving individual peaks. Thus, if the system behaved normally, the /3, and & states should be resolved into seperate peaks at high heating rates. CO desorption from other stepped platinum surfaces gives the expected behavior [22,23]. However, here, it is found that as the heating rate is increased above 30 K/s and & merges with the &. This suggests that some additional processes are taking place on the surface besides desorption. The data in fig. 5 provide some insight into the additional process occurring on the surface. Notice that the &-CO peak shifts up to 605 K, when 25% of a monolayer of carbon is deposited on the surface. Yet, previous investigators found that the CO/carbon interaction is repulsive on platinum so the peak
Y.O. Park et al. / CO and NO on Pt (410). I
359
should shift down. The & and & show the expected behavior but &-CO does not. Unpublished data indicate that the 615 K peak on a Pt(210) surface shifts down to 605 K when the Pt(210) sample was contaminated with Ca 25% of a monolayer of carbon. Thus, it appears that the peak at ca. 615 K which was missing on a clean surface, reappears once the surface is contaminated by 25% of a monolayer of carbon. Simultaneously, the deposition of carbon on the surface, as indicated by AES, stops. This suggests that the process which deposits carbon is related to the process which attenuated the 615 K peak. X-ray Photoemission Spectroscopy (XPS) was done in an attempt to clarify the unusual behavior seen here. Fig. 14 shows a series of XPS spectra taken using the apparatus and procedures described in Park et al. [13]. The XPS spectrum of a CO saturated Pt(410) crystal ,shows C,, and Or, peaks at 285 and 530 eV respectively. Heating the layer to 407 K, results in a broadening and shift in both the C,, and O,, peaks. The shift continues upon further heating until at 537 K, the C,, and O,, peaks lie at 282.5 and 528.8 eV
.-..I yl8L COExposure
290
Fig. 14. A series of C,, heating as indicated.
BINDING and O,,
285
280
275
ENERGY -
XPS spectra
taken after exposing
a Pt(410) sample
to CO and
Y.O. Park et a/. / CO and NO on Pr (410). I
360
*IS
I .c.*:=* ......_ ..*_P ‘+=** _,,,*.:-.,. .._.*.:I !
,.,:.* -:_. :..“.<
IL02
$po&b
/”
then heot
..j -* * to 500°C .......-.w%- 1 _.G: . . . . /Adsorb ’ .- “2....:-*.* 9L co ._*
535 530 525 Binding Energy, eV
54GddeBinding Energy, eV Fig. 15. Additional C,, and O,, XPS spectra taken after exposing a Pt(410) sample heating as indicated.
to CO and
respectively. Fig. 15 shows similar results at other coverages, and again similar shifts are seen. These kinds of shifts are often seen in XPS, and imply that the chemical nature of the species on the surface changes during the heating process. Park et al. [13] analysed the shifts in detail. They found that adsorption of NO or 0, at room temperature yields an O,, XPS peak at 528.8 eV. Oxygen dissociates on most platinum faces, and so it is likely that the 528.8 eV peak can be associated with a dissociated species. The 530 eV peak corresponds very closely to that seen for molecular species on Pt(lOO). Further the peak shifts in the C,, and O,, regions seen here, are very close to those which one observes during dissociation of CO on other metals, for example W(110) [24) and NO on Pt(lOO) [34]. Therefore, one can argue that the 530 eV peak probably corresponds to a molecular species (see Park et al. [13] for additional arguments). The shift, then can be attributed to a dissociation process (i.e. CO,, -+ C,, + O,, ). The AES results of Park et al. [19] also provide evidence for CO dissociation. When CO adsorbs on Pt(410), one observes 0-KLL and C-KLL Auger peaks with the right relative intensities for molecular CO. When the layer is heated to 500 K the 0-KLL peak disappears, while the C-KLL peak shifts
Y.O. Park et al. / CO and NO on Pt (410). I
361
slightly. Temperature cycling in a CO atmosphere produces a buildup of carbon on the surface; the buildup process has been from the AES Gun. It is difficult to find a mechanism for carbon buildup that does not involve CO dissociation. Hence, it is concluded that CO dissociates on Pt(410). The observation that CO dissociates on Pt(410) is not completely surprising. Vannice et al. [25] showed that a properly prepared platinum catalyst has significant initial methanation activity. CO bond breaking is a critical step in the methanation process, so apparently some form of platinum is active for CO dissociation. Pt(410) is a reasonable candidate. Pt(410) has previously been found to be unusually active for NO bond breaking [13,15]. Analysis of this data [12] indicates that Pt(410) is unusually active, because the symmetry of the highest occupied orbitals at the step happens to match those of lowest unoccupied (i.e. ?r* and a*) orbitals in the NO. NO and CO have the identical symmetry and similar electronic structures. Thus, one would expect faces that are unusually active for NO bond breaking to be also unusually active for CO bond breaking. Ni(310) and Ni(510), which have similar electronic structures to Pt(410), have already been demonstrated to be unusually active for CO bond breaking [14]. Thus, if any face of platinum is active for CO bond breaking, Pt(410) should. The only question is whether the energetics are favorable, and evidently they are. It is interesting to speculate how the dissociation process would affect the results of a TPD experiment. For the sake of argument, assume that the activation energy for dissociation is larger than that of desorption. If so, desorption should be the dominant reaction at low temperatures. The amount of dissociation will depend on the difference in the magnitude of the two barriers and the availability of active (stepped) sites. At low temperatures, dissociation may occur but the products will only slowly diffuse out of the active sites. As the temperature is increased, the rate of dissociation should increase faster than that of desorption because the differences in the activation energies will become less important. Further, at higher temperatures diffusion will be faster, increasing the turnover at the active sites. Thus, one would not expect the dissociation process to have a major effect on the lower temperature TPD peaks, but the higher temperature TPD peaks should be attenuated. Here, we were not able to observe the peak expected at ca. 615 K for desorption from the steps. Further, the 550 K peak was considerably smaller than it should have been for desorption from the terraces. We also observed decreases in the CO intensity at very long exposure times, or by heating the surface in CO. Surprisingly, a peak at 605 K is seen when enough carbon is deposited on the surface to poison the dissociation process. Clearly, something is occurring to attenuate the higher temperature peaks, just as one would expect if dissociation were occurring at high temperatures. The dissociation process could also cause the & and & peaks to appear to merge at higher heating rates. Increasing heating rates generally cause TPD
362
Y.O. Park et al. / CO and NO on Pt (410). I
peak positions to increase, and peak intensities to rise. Thus, the & peak, which is not strongly attenuated by the dissociation process, will move to higher temperatures and grow, with increasing heating rate. The &, however, comes at temperatures where the dissociation process is important. An increase in the heating rate will produce an increase in the attenuation. Further, the shift in the peak temperature will be less than it would be otherwise, because the attenuation increases with increasing temperature. Thus the & and & peaks should appear to merge at higher heating rates, just as was observed. The & and & peaks do not merge on a sample covered by ca. 25% of a monolayer of carbon, i.e. one inactive for CO dissociation, and in fact we find normal behavior, where we get increased resolution with increased heating rates. Thus, it appears that the dissociation process is related to the process that causes the & and & peaks to merge. Of course, it was also found that the & (500 K) and & (550 K) peaks fill non-sequentially. This is rather unusual behavior and it suggests that the & and & peaks are associated with different binding sites on the surface. For example, the & might be associated with desorption from the terraces, and the & might be associated with desorption from the steps. Surface reconstructions could also play an important role in the two peaks. Pt(lOO) shows peaks at both 550 and 500 K; so we cannot say that the & occurs just from desorption from the steps. However, perhaps the peak from the steps overlays the peak from the terraces. The observation (fig. 5) that the & peak is attenuated by dissociated CO to a much greater extent than the & suggests that much of the & comes from desorption from the steps. 4.3. NO/
CO coadsorption
The coadsorption experiments also indicate that Pt(410) has unusual properties. As with many other faces of platinum, when a Pt(410) sample is exposed to a mixture of CO and NO, CO, formation is seen. However, unlike all of the other faces studied previously, some CO, formation was observed during adsorption at room temperature. The remaining CO, comes out in a broad peak that is much smaller than the N, peak. CO, formation was also observed at 150 K in the low temperature experiments shown in fig. 14. By comparison the most active face studied previously, Pt(lOO), shows negligible CO, formation below 400 K. Thus, we conclude that Pt(410) has unusual properties for the NO/CO reaction. A detailed examination of fig. 11 reveals that the CO, peak behaves differently on Pt(410) than on other faces. Notice that in fig. 11, CO, fosrnation continued even after all of the nitrogen containing species desorbed. AES reveals no nitrogen on the surface. Some of the CO, may come from disproportionation of CO. However, we did not observe as large of a CO, peak when a CO saturated Pt(410) sample was flashed. Thus, it is likely that the bulk of the CO, forms via a reaction between NO and CO.
Y.O. Park et al. / CO and NO on Pt (410). I
363
We can speculate why CO, desorption continues after the nitrogen containing species desorb. Previous studies [l-lo] have indicated that the NO/CO reaction on platinum occurs in two steps: NO@d, + N@d) + O,, 3
(8)
owl + qd,
(9)
+ co2 3
with step (8) rate controlling on the faces considered previously. Here, it could also be suggested that the NO/CO reaction occurs in the same two steps. However, since CO, formation continues even after the reaction (8) is complete, reaction (9) must be rate controlling at higher temperatures. At lower temperatures, something else must occur. Notice that in fig. 11, the CO, first starts to desorb at about 350 K. CO, desorption continues up to about 600 K. It is difficult to fit a peak this broad with a simple kinetic rate law, and reasonable parameters. Thus, it is suggested that there is more than one kinetic process occurring on the surface. The data in fig. 11 provide some insight into the two processes. Notice, that the pi- and &-CO peaks are missing in fig. 11, but the & CO peak is largely unaffected. Thus, one could suggest that there is a difference in the reactivity of &- and &-CO. Admittedly, we do not know whether the main CO peak in fig. 11 is truely the & peak; it could be a peak due to a new species formed via an interaction between adsorbed NO and CO. However, the data clearly suggest that there are at least two different forms of CO on the surface with different reactivities. The low temperature experiments show radically different behavior. We observe two low temperature mass 44 peaks at 136 and 150 K, which seem to be similar to the two peaks seen above for adsorption of just NO. There is also a CO, peak overlaying the 150 K N,O peak and a sharp CO, peak at 360 K reaction indicative of a “surface explosion”. Previous studies of the NO/CO on Pt(lOO) [2,28] also reveal a CO, peak indicative of a surface explosion, but that peak is at a somewhat higher temperature, i.e. 410 K. On Pt(lOO), the explosion occurs when there is still a significant amount of NO on the surface, and it might be associated with an interaction between NO and CO [ll]. However, clearly that is not occurring here. Notice that the surface explosion occurs at 360 K, while most of the nitrogen containing species on the surface desorb below 200 K. No surface nitrogen is detected in AES above 200 K. The explosion cannot be associated with an interaction with the NO, because there is little NO left. Thus, some other mechanism must cause the explosion. Barteau et al. [20] observed that 0, desorption from Pt(100) was autocatalytic at 670 K due to a surface reconstruction. Perhaps the explosion reported here is caused by a similar mechanism. Another possibility is an attractive interaction of 0, and CO, although the data does not reproduce the peak shifts predicted by such a model. Chemical Autocatalysis can also be considered. At the moment the data do not seem to fully support any of these models, so some additional work needs to be done to explain the explosion.
364
Y.O. Park et al. / CO and NO on Pt (410). I
The CO, peak at 150 K is also quite unusual. Previous studies of the CO/NO reaction on platinum have found little CO, formation below 400 K. Here a CO, peak is seen at 150 K. Experimentally the peak is only seen when the crystal is dosed with a mixture of CO and NO; CO, is not detected when just NO or CO is used provided we are careful to pretreat the chamber. Thus, it is clear that the CO, is formed via a reaction between NO and CO, and not, for example, by a reaction between NO or N,O and carbon on the filaments. We do not observe CO, during dosing, so it probably forms during heating. We do not know, for sure, whether the CO, forms on the crystal; gas phase CO, was detected when the AUGER was turned on, so AES could not be used to check. However, there are no previous reports of a NO/CO reaction at 150 K on tantalium leads or alumina insulation so it is likely that the CO, is formed on the crystal. If so, this observation would provide additional evidence that the NO was dissociating below 200 K on Pt(410).
5. Conclusions The work here provides additional evidence that Pt(410) has unusual properties. We observe evidence for NO decomposition and N,O formation on Pt(410) at 120 K, CO, formation at 150 K, CO dissociation and carbon deposition at 480 K, and an surface explosion during the NO/CO reaction at 360 K. These results are radically different from those on most other platinum surfaces which suggests that Pt(410) is especially reactive for NO and CO bond breaking as suggested by Banholzer et al. [12].
Acknowledgements This work was supported by the National Science Foundation under grant CPE 83-11791. This work made use of the University of Illinois, Center for the Microanalysis of Materials, which is supported as a national facility by the National Science Foundation.
References [l] [2] [3] [4] [5] [6] [7]
D. Lorimer and A.T. Bell, J. Catalysis 59 (1979) 223. M. Lesley and L.D. Schmidt, Chem. Phys. Letters. 102 (1983) 459. R.M. Lambert and C.M. Comrie, Surface Sci. 46 (1974) 61. W. Adlhoch, H.G. Lintz and T. Weisker, Ber. Bunsenges. Physik. Chem. 84 (2980) 410. W. Adlhoch and H.G. Lintz, Surface Sci. 78 (1978) 58. G.L. Bauerle, G.R. Service and K. Nobe, I&EC Process 11 (1972) 54. S.P. Singh-Bopari and D.A. King, in: Proc. 4th Intern. Conf. on Solid Surfaces, Cannes, 1980, p. 403.
Y.O. Park et al. / CO and NO on Pt (410). I [8] [9] [lo] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] (231 [24] (251 [26] [27] [28] [29] [30] [31] [32] [33] [34]
365
R.J. Gorte and L.D. Schmidt, Surface Sci. 111 (1981) 260. T.E. Fischer and S.R. Keleman, J. Catalysis 53 (1978) 24. M. Shelef, Catalysis Rev. Sci. Eng. 11 (1975) 1. W.F. Banholzer and R.I. Masel, Surface Sci 137 (1984) 339. W.F. Banholzer, Y.O. Park, K.M. Mak and R.I. Masel, Surface Sci. 128 (1983) 176. Y.O. Park, R.I. Masel and K. Stolt, Surface Sci. 13 (1983) 385. 2. Murayama, I. Kojima, E. Miyazaki and I. Yasumari, Surface Sci. 118 (1982) 281. W.F. Banholzer and R.I. Masel, J. Catalysis 85 (1984) 127. T.H. Lin and G.A. Somorjai, Surface Sci. 107 (1981) 573. R. McCabe and L.D. Schmidt, Surface Sci. 86 (1977) 101. G.E. Gdowski and R.J. Madix, Surface Sci. 115 (1982) 524. Y.O. Park, W.F. Banholzer and R.I. Masel, Appl. Surface Sci. 19 (1984) 145. M.A. Barteau, E.I. Ko and R.J. Madix, Surface Sci. 102 (1981) 99. A. Crossley and D.A. King, Surface Sci. 95 (1980) 131. M.R. McClelland, J.L. Gland and F.R. McFealey, Surface Sci. 112 (1981) 63. D.M. Collins and W.E. Spicer, Surface Sci. 69 (1977) 85. E. Umbach, J.C. Fuggel and D. Menzel, J. Electron Spectrosc. Related Phenomena 10 (1977) 5. M.A. Vannice, J. Catalysis 40 (1975) 129. Y. Iwasawa, R. Mason, M. Textor and G.A. Somorjai, Chem. Phys. Letters 44 (1976) 468. H. Hopster and H. Ibach, Surface Sci. 77 (1978) 109. S.R. Kelemen, T.E. Fischer and J.A. Schwarz, Surface Sci. 81 (1979) 440. K. Schwaha and E. Bechtold, Surface Sci. 66 (1977) 383. D. Edwards, Surface Sci. 54 (1976) 1. Mass Spectroscopy Data Centre, Eight Peak Index of Mass Spectra (Pendragon House, Palo Alto, 1974). R.I. Masel, E. Umbach, J.C. Fuggel and D. Menzel, Surface Sci. 79 (1979) 26. W.F. Banholzer, R. Parise and R.I. Masel, Surface Sci., to be published. H.P. Bonzel and G. Pirug, Surface Sci. 62 (1977) 45.
Surface Science 155 (1985) 341-365 North-Holland, Amsterdam
ADSORF’TION AND INTJZRACTION OF CO AND NO ON Pt(410) I. TPD studies Y.O. PARK,
W.F. BANHOLZER
* and R.I. MASEL
Department of Chemical Engineering, Roger A&m California Street, Urbana, Illinois 61801, USA
Received 8 March 1984; accepted for publication
Laboratory,
31 December
** University
of Illinois, 1209 W.
1984
NO and CO adsorption and the NO/CO reaction on Pt(410) are studied by TPD. NO is found to dissociate on Pt(410) at 120 K, but it reacts to form N,O at higher exposures. The N,O desorbs in two peaks at 135 and 150 K. CO adsorbs molecularly, and desorbs in 5 peaks at 550, 500,450, 380 and about 130 K. CO is also found to dissociate upon heating, leaving a carbon residue on the surface which changes the TPD spectra. The NO/CO reaction shows a surface explosion at 360 K. These results provide additional evidence that Pt(410) has unusual reactivity, as predicted by Banbolzer, Park, Mak and Masel, Surface Sci. 128 (1983) 176.
1. Intmduction The NO/CO reaction on platinum has been studied by many previous investigators [l-11]. It shows a wealth of unusual behavior including surface explosions [2], and oscillations [4]. The rate of the reaction has been found to vary significantly with crystal face; Pt(ll1) shows almost no activity, [8] while Pt(lOO) is quite active [9]. Here, we are considering the NO/CO reaction on a face for which there has been little previous work, Pt(410). Pt(410) is an unusual surface from an orbital symmetry standpoint. The model of Banholzer et al. [12] predicts that the step sites on Pt(410) have the right symmetry to break NO and CO bonds. Previous X-Ray Photoemission Spectroscopy (XPS) data [13] suggest that NO partially dissociates upon adsorption on Pt(410) at 300 K. All of the close-packed faces of platinum and all of the other stepped platinum surfaces considered previously show molecular adsorption of NO under similar conditions. Flash of an NO saturated layer yields a N, peak which shifts from 480 to 440 K with increasing coverage, and an oxygen peak above 800 K. NO desorption is not * Present address: General Electric Company, CSEB 410, P.O. Box 8, Schenectady, 12301, USA. ** Correspondence should be addressed to this author.
0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
New York
342
Y. 0. Park et al. / CO and NO on Pt (410). J
detected when dosing is done so that little gas impinges on the sides and back of the crystal. XPS results [13] show that CO molecularly adsorbs, but there are changes in the XPS data upon heating that are consistent with dissociation. No similar changes are seen with the other platinum faces considered previously. However, enhanced CO dissociation has also been observed [14] on Ni(310) and (510), i.e., surfaces for which similar symmetry arguments apply. These results show that CO and NO behave differently on Pt(410) than on other platinum faces. Therefore, one might expect Pt(410) to have unusual properties for the NO/CO reaction. The purpose of this paper is to explore CO adsorption and the NO/CO reaction on Pt(410) using Temperature Programmed Desorption (TPD). 2. Experimental
method
The TPD data presented here were taken using the chamber and Metron crystal described previously [13,15]. The vacuum system was of standard design, but it had an exceptionally large set of pumps so the system could be routinely evacuated to below the xray limit of the ion gauge. The crystal was aligned using Laue back reflection and cut by electrical discharge. It was polished on both sides and cleaned using standard techniques. The sample was mounted by spotwelding two 0.25 mm tantalum wires to the crystal. The manipulator allowed heating to be accomplished by either resistance heating the support wires and conductive heat transfer to the crystal, or by electron bombardment on the back of the crystal. All of the data presented here used resistance heating. In later experiments, a liquid nitrogen cooled block was added to the system. It allowed the sample to be cooled to 120 I(. The temperature was monitored by a 0.07 mm Chromel-Alumel thermocouple spotwelded to the crystal’s edge. Regulation of the heating and data acquisition were under the control of an LSI-11 minicomputer. Heating rates of 8 to 10 K/s, reproducible to 0.5 K/s were possible. Partial pressures were measured by a Riber QXlOO quadrapole mass spectrometer. Dosing of the sample was accomplished using a Varian leak valve and increasing the ambient pressure and waiting until the desired exposure was reached, or by rotating the crystal so that it was 40 mm away from a dosing tube which was fitted with a glass capillary array. Calibration of the capillary array doser was accomplished by comparing desorption spectra obtained by using the two techniques. In the coadsorption experiments, the crystal was given the desired exposure to the first gas and, after the pressure had fallen below 7 x 10-i’ Torr, which usually took ca. 5 min, the second gas was introduced. Two different dosing valves were used. In this way, it was possible to precisely control the amount of each gas exposed to the crystal, and to dose with gas mixtures. AES work was done with a PHI lo-155 CMA.
Y. 0. Park et al. / CO and NO on PI (410). I
343
8
m
z
IO
s
20
30
Time,
sec.
Time
, sec.
Fig. 1. Some raw TPD traces for CO and NO desorption from Pt(410).
There is one other experimental detail of note. When these experiments were first started the low temperature peaks could not be resolved due to interference with desorption from the support rods. However, the support rods were insulated, so that only the last cm was exposed to reactive gases. (The exposed surface area of the rods is only 0.15 cm*, which should be compared to a total surface area of 1.5 cm* for our crystal.) The heating routine was then modified so that the peak from the support rods could be separated from those on the crystal. A capillary array doser was also used to eliminate support effects. Fig. 1 shows the results of these efforts. Notice that there is a sharp peak which we attribute to desorption from the support rods, and then separate peaks, which AES shows to be largely associated with desorption from the crystal. There is a tail under the peaks which might also be associated with desorption from the support rods. Experimentally the sharp peaks from the support rods were always small, and well resolved from those from our sample, although the tail could be a problem. In the spectra to follow, the sharp peak from the support rods was suppressed. The data in fig. 1 also allow one to estimate a response time of the system. Notice that the sharpest peaks in fig. 1 have a half width of less than 0.2 s. In other experiments, peaks as narrow as 0.05 s have been seen. In the experiments to follow the narrowest peaks will have a half width of 0.8 s or more. Thus, it does not appear that the broadening of the system has an important influence on the width of the peaks.
3. Results 3.1. co Fig. 2 shows a series of TPD spectra taken by exposing a Pt(410) sample to CO at room temperature then flashing. One observes a series of three overlap-
344
Y.O. Park et al. / CO and NO on Pi j410). 1
Mass 28 CO/Rf410)
300
400
500
600
Temperature,
700
800
K
Fig. 2. A series of mass 28 TPD spectra taken after adsorption of CO on a clean Pt(410) sample at 310 K, p = 25 K/s.
ping peaks at 550, 500, and 450 K labeled &, &, and & respectively. The & peak fills first. It is seen even at low coverages, and it saturates at an exposure of 4 L. The & fills next. It appears as a shoulder on the & at low coverages. However, at exposures above 0.5 L it is resolved into a separate peak. The & fills last. It first appears as a shoulder on the & peak. However, it is resolved into a separate peak at coverages above 5 L. Ail three peaks seem to have positions which are independent of coverage, which is suggestive of a first order desorption process. Fig. 3 shows the results of a similar experiment done by exposing a Pt(410) sample to CO at 120 K. One observes five peaks labeled (Y,,, LYE,p,, &, and & respectively. The three p peaks grow non-sequentially, just as was described above, but there is some blurring of the peaks at higher coverages. The (Ye and CY~ are new. The ~yacomes off just as we start the heating process. It looks like peaks commonly attributed to desorption from the support rods. However, it did not disappear when we used a capillary array and AES revealed that CO is desorbing at low temperatures. Thus, the (Ye, must come in part from the sample. The a0 is large feature, and it continues to grow with increasing
Y.O. Park ei al. / CO and NO on Pt (410). I
345
coi Pt (410)
II 100
I
200
I
I
300
400
Temperature
I
500
I
600
K
Fig. 3. A series of mass 28 TPD spectra 120-150 K, /3 = 17 K/s.
taken after adsorption
of CO on a clean Pt(410) sample at
exposure up to the highest exposures shown (30 L). Its size is very dependent on the adsorption temperature and there possibly could be some support rod contributions. We did not have good control of the adsorption temperatures below 150 K and could not sort out the support rod effects. Hence, we did not explore the (~a in detail. The a,, comes off in the temperature range of 370-400 K, i.e. above room temperature even though it is not seen upon room temperature adsorption. The peak is small, and its position decreases with increasing exposure, suggesting either a second order desorption process, or a decrease in the activation energy of desorption with increasing coverage. The peak intensity increases with increasing exposure saturating at 30 L. During the course of the experiments, it was observed that the desorption spectra obtained depended on the history of the sample. If the sample was cleaned by heating briefly in oxygen or ntiric oxide and then flashed to 1500 K in a vacuum of better than 5 X lo-” Torr, the TPD spectra produced were similar to those in fig. 2. However, if repetitive exposures were done without cleaning the surface in oxygen between exposures, the CO desorption spectra would evolve into those shown in fig. 4. As is evident from fig. 4, the & is reduced by 50% and shifts upward by ca. 50°C. The size of the & and & peaks appear largely unaffected but they shift down by ca. 10°C. If the surface producing the spectra in fig. 4 was exposed to 18 L of oxygen at 1100 K and
346
Y. 0. Park et al. / CO and NO on Pt (410). I
Mass 28
350
450
550 Temperature
co/c-
650
P1(410
750
K
Fig. 4. A series of mass 28 TPD spectra taken after adsorption of CO on a 310 K Pt(410) sample which was contaminated by ca. 25% of a monolayer or carbon, p = 25 K/s. The carbon was a residue formed by repeatedly saturating the sample with CO and flashing to 1500 K.
then flashed to 1500 K, the spectra obtained were undistinguishable from those in fig. 2. Auger Electron Spectroscopy (AES) was done to examine the changes in the surface that accompany the changes in the desorption spectra. It was found that about 10% of the carbon from the CO was left on the surface in the first flash. Additional carbon is deposited in subsequent flashes, until eventually about 25% of a monolayer of carbon builds up on the surface. It appears that the carbon buildup produces the changes in the flash desorption spectra that are observed. Several attempts were made to resolve the & and & peaks in fig. 2. Selective annealing was tried first, but the resultant spectra were very similar to those in fig. 2. Another method was to try to vary the heating rate in an attempt to get increased resolution. At a heating rate of 27 K/s, the various peaks were very close together and were only slightly resolved. At heating rates below 10 K/s or above 35 K/s, the & and & peaks merge to form one broad peak. The resolution of the vacuum system, as determined from the sharp peaks in figs. 1 and 12 is such that the peaks could have been resolved at higher heating rates if normal kinetics applied. Since they are not, something unusual must be happening on this surface that makes it difficult to resolve the & and & peaks. The difficulty in resolving the two peaks made it impossible to use variation in heating rate to determine detailed kinetics of the desorption
Y. 0. Park et al. / CO and NO on Pt (410). I
I
2
3
4 Exposure
5
6
7
6
9
34-l
IO
(L)
Fig. 5. The relative CO coverage, calculated from flash desorption peak areas, as a function of the calibrated CO exposure at room temperature.
process. Fig. 5 is a plot of relative coverage (calculated from the CO peak areas) versus exposure at room temperature. As is evident from this curve, the sticking probability is not constant but decreases with coverage. The surface population at saturation can be determined from the initial slope in fig. 5 via
( UP/P) no (dtY/dL),
so ’
where no is the surface population at saturation (molecules/cm’), u is the molecular velocity, p is the gas density at the dosing pressure P, 8 is the relative coverage, L is the exposure, and So is the initial sticking probability. Lin and Somojai report an initial sticking probability for CO on Pt(5(111) x (111)) of 0.74 [16], while McGabe and Schmidt determined a sticking probability for CO on Pt((lll)~(lOO)) to be 0.34 [17], and Gdowski and madix report a value of 0.55 for Pt(9(111) x (100)) [18]. If we assume So = 0.55, we calculate a saturation coverage of CO of 1.0 X 1Ol5 molecules/cm’; several previous investigators have measured the saturation coverage of CO on Pt(lOO) and get values near 1 X 1Ol5 molecules/cm2, so our value is reasonable. Admittedly, our method is not as accurate as those of previous investigators. At very high exposures (> 100 L, i.e. long exposure times) at room temperature we observe another effect. It appears that the CO peak intensity decreases slowly with increasing exposure. No similar reduction was seen when the crystal was held at 120 K or when the sample was covered by ca. 25% of a monolayer of carbon. It is not seen with other faces mounted on the same manipulator. At first the reduction was considered to be an artifact. However,
348
Y.O. Park et ul. / CO and NO on Pt (410). I
we did several tests and were not able to isolate a possible source of the artifact. One test was to leave the sample in the vacuum for up to an hour and then flash the surface. No comparabte loss in CO intensity was seen. Another test was to valve off the ion pump, and turn off each of the filaments in the chamber one at a time. That had no effect on the reduction in intensity. Finally, we tried to look for a likely contaminant, but were not able to detect anything. There is still the possibility of the CO reacting with something in the chamber to produce a low level contaminant. However, in order to explain the data the ~nta~nant would have to not stick at low temperatures or on a surface which is contaminated with 25% of a monolayer of carbon. The plausibility of this occurring needs to be judged by the reader. We note, however, that there is nothing so far which excludes the possibility of there being some real chemistry, which causes the reduction in CO peak intensity. In fact we believe that to be the most likely possibility. 3.2. NO Studies of NO adsorption on Pt(410) were also done. The results of a study of NO adsorption on Pt(410) at room temperature were discussed previously (151. When a 300 K Pt(410) sample is saturated with NO and then flashed, a single N, peak which shifts from 490 to 440 with increasing coverage is seen. There is also an 0, peak above 900 K, but no NO was detected when the crystal was dosed in a way that little NO impinged on the sides of the crystal or the support rods. The maximum NO coverage at room temperature corresponded to 5 x 1014 molecules/cm*, i.e. about one dissociated NO for every two surface atoms. When the sample is dosed with NO at 120 K, the desorption spectrum changes substantially. Figs. 6 and 7 show a series of mass 28 and 44 spectra taken after adsorption of NO at 120 K. At low exposures, the mass 28 spectrum shows two peaks at 420-450 and 350-390 K, labeled /Ii and flz respectively. Both peaks shift to lower temperatures with increasing exposure. The peaks appear to fill nonsequentially. The higher temperature peak is very similar to the peak seen in the room temperature experiments, but the lower peak is unique to the 120 K adsorption experiments. At higher exposures, the intensity of both peaks decrease with increasing exposure. Simultaneously, two mass 44 peaks grow into the spectrum. Flashes of an N”O saturated sample shown in fig. 7 shows that the mass 44 peaks are due to N,O, and AES was used to confirm that the mass 44 peaks were coming from the sample. Annealing experiments were also done. It was found that annealing at 300 K produces a sharp reduction in the & peak and an increase in the J$. This suggests that there is a inner conversion occuring on the surface which complicates the interpretation of the TPD data in fig. 6. Fig. 7 also shows the development of the mass 44 (N,O) TPD spectrum with
Y.O. Park et al. / CO and NO on Pt (410). I
loo
I 200
I
x)0
I 400
I 500
I 600
349
I 700
Temperature K Fig. 6. A series of mass 28 TPD spectra taken after adsorption of NO on Pt(410) at 120 K, /3 = 17 K/s.
increasing exposure. One observes two peaks at 135 and about 155 K. The peaks grow simultaneously with increasing coverage. The position of the lower temperature peak is almost independent of coverage, suggesting a first order desorption process. The higher temperature peak moves to lower temperatures with increasing exposure, suggesting either a second order desorption process or a coverage dependent activation energy of desorption. Both peaks saturate at 1OOL exposure. At that point, the 440 K N, peak is reduced to the point where it is barely distinguishable from the noise. There are some low temperature mass 28 and mass 30 features, but their size was small enough, that most of their intensity could be attributed to cracking of N,O in the mass spectrometer. Fig. 8 is a plot of the relative coverages of NCnd) and NZOCad)as a function of exposure determined from 3, = &.qq;.
(21
8 N,O
(3)
=
spN20/2pt$
peak areas where PN2 and PNzO are the N, and N,O flash desorption respectively, P FJ is the N, peak area produced by a platinum (410) sample which had been saturated with NO at room temperature, and 6 is a correction factor for the sensitivity of the system for N,O relative to that of N, determined from comparing AES peak intensities. 8, reaches a maximum of increases with increased exposure. At saturation, 0.9 at 3 L exposure. 8,,, e N,o is approximately 1.
Y.O. Park er al. / CO and NO on PI (410).
350
I
100 L
3OL
15 L
IOL
7L
2L
100
200
300
400 temperature
500
600
700
K
Fig. 7. A series of N,O TPD spectra taken after adsorption of NO on Pt(410) at 120 K, [j = 20 K/s. The 30 L spectrum is a mass 46 spectrum from a sample which had been exposed to N’50. The rest of the scans are mass 44 spectra
from samples
exposed
to N140.
One can estimate absolute coverages of NO from the data in fig. X, assuming that the saturation density of CO at room temperature represents a coverage of 1 X 1O’j molecules/cm2. B = 1 corresponds to about 5 X 1014 atoms or molecules/cm2, so that the maximum t?, corresponds to about 4.5 x 1Or4 atoms/cm2 and the maximum in eNzo corresponds to about 5 X 1014 atoms/cm’. The saturation coverage of nitrogen atoms at room temperature is
Y.O. Park et al. / CO and NO on Pt (410). I
351
Exposure, Longmuirs
Fig. 8. The coverages of N, and N,O as a function of exposure at 120 K as determined from flash desorption peak areas.
also about 5 x lOi atoms/cm’. by adsorbing NO at 120 K, but to 200 K. The results in figs. 6, 7 and that N,O is being formed from the N,O desorbs below 200 K. 3.3. CO+
One can get about 1 x 1015 oxygen atoms/cm’ half of them desorb when the surface is heated 8 combined with the AES results show clearly NO on Pt(410) at low temperatures, and that
NO
Coadsorption of NO and CO generally resulted in the formation of CO, and N,. Fig. 9 shows the mass 28 desorption spectra obtained when the crystal was exposed to 1 L of NO followed by varying amounts of CO. When Pt(410) is exposed to 1 L of NO and then flashed, a single mass 28 peak is seen at 430 K. Two additional langmuirs of CO produces a broadening and shifting of the peak, and the appearance of a new peak at 550 K. A small CO, peak is also seen. If the surface is exposed to 1 L of NO then 10 L of CO, the spectrum looks qualitatively like the spectrum taken when the surface is exposed exclusively to CO, but the peak intensities vary, and the features are less distinct. Again CO, was formed. No NO desorption was detected in any of the runs above. 0, desorption was seen in the runs where the surface was exposed to just NO, but not when it was exposed to 1 L NO then 2 L or more of CO. CO, formation was also observed during the dosing procedure. The mass 28 signal in the spectra above is somewhat ambiguous since N, and CO have the same mass. In order to resolve this ambiguity, experiments were done with Cl30 and N150. Fig. 10 shows the spectra obtained after a Pt(410) sample was exposed to 2 L of NO and 2 L of NO followed by 10 L of Ci30. Notice that the N, peak is strongly attenuated when CO is added to the
Y. 0. Park et al. / CO and NO on PI (410).
352
I
I
/
Pt (410)
I 350
I
I
I
450
550
650
Temperature
Fig. 9. A series of TPD spectra
K
taken by sequentially
dosing a 300 K Pt(410) simple
with NO then
co.
layer. Evidently, CO displaces the nitrogen containing species from the surface. The N, peak is also substantially broadened and its peak maximum is shifted down to 370 K. This suggests that there is apparently some interaction between the nitrogen containing species and the CO on the surface. However. the CO
I
2L NO + IO L d30
I 350
I 450
I 550 Temperature
Fig. 10. TPD spectra C”0. /I = 24 K/s.
I 650 K
taken after exposing
a 300 K Pt(410) to 2 L of N140
followed
by 10 L of
Y.O. Park et al. / CO and NO on PI (410). I
353
!
2L CO+ 10LN’50/Pt(410)
z;;;;;;; I 350
I 450
I
I
550
650
Temperoiure,
K
Fig. 11. A series of TPD spectra taken followed by 10 L of N150, /S = 25 K/s.
after exposing
a 305 K Pt(410)
sample
to 2 L of Cl20
peak looks very similar to the one seen when the crystal is dosed with pure CO, so apparently, the interaction has less effect on the CO peak. Note that CO is the major species on the surface, so this is not a surprising result. Fig. 11 shows a series of spectra produced when the sample is exposed to 2 L of CO followed by 10 L of N150. A comparison of the spectra in fig. 12 with those in figs. 2 and 3 above shows that pi- and &-CO are displaced by NO, but the &-CO is largely unaffected. Note that originally the /I2 corresponded to only about 20% of the total CO. There is also a new CO feature at 370 K, which may be q-CO. The N, peak also looks very different from the peaks seen above or the peaks seen when the surface is exposed to pure NO. There is a low temperature, 380 K, peak, and a higher temperature 460 K peak. Neither of these peaks can be easily identified with the peaks seen when the surface is exposed to just NO. No NO, O,, or N,O desorption was detected, but CO, was seen. The CO, peak is very broad with peaks at 400 and 480 K. It looks very similar to the CO, peaks seen on other surfaces. However, there is one important difference: the peak is much smaller than it would have been if most of the CO, was formed during the flash. Of course, we also observed CO, formation during the dosing process, so it is not surprising that the CO, peak is too small. One can estimate the fraction, F, of the CO, which formed during the dosing process from
where AN, and A,, are the flashed desorption peak areas for N, and CO, respectively, and R i!s the ratio of the sensitivity of the mass spectrometer for N, to that of CO, weighted by the relative pumping speeds for the two gases. A value of 1.6 was estimated for R, from the slow flash of a polycrystalline platinum sample, assuming F = 0 for a case where negligible 0, desorption was seen (no CO, formation was observed during dosing). F works out to be
Y. 0. Park et al. / CO and NO on PI (410).
30 L 50/50
I
NO CC
Pt (410)
J 100
I
I
I
I
I
I
200
300
400
500
600
700
Temperature
K
Fig. 12. Mass 2X and mass 44 TPD spectra taken after exposure of a 120 K Pt(410) sample to 30 L of a 50/50 mixture of CO and NO. Note that the mass 28 spectrum has been magnified by 2 X.
2/3 for the data in fig. 12, which suggests that the crystal is quite active for the NO/CO reaction at room temperature. A set of NO/CO coadsorption experiments was also done with an adsorption temperature of 120 K. The runs where the surface was saturated with NO then exposed to CO or vice versa were uninteresting because the displacement of NO by CO or CO by NO is a slow process at 120-150 K. However. simultaneous coadsorption of CO and NO at 120 K yields some rather interesting results. Fig. 13 shows a series of TPD spectra taken after a 120 K Pt(410) sample was exposed to 30 L of a 50/50 mixture of CO and NO, then flashed. One observes three peaks at mass 44, two broad peaks below 200 K and a very sharp feature at 360 K. The two low temperature peaks are very similar to the peaks seen when the surface is exposed only to NO. Results of
Y.O. Park et al. / CO and NO on Pi (410). I
s
x3
1
IO0
I
t
I
200
300
400
Temperalure,
I
500
355
Mass28
I
600
K
Fig. 13. Mass 28,44 and 46 TPD spectra taken after exposure of a 120 K Pt(410) sample to 30 L of a SO/50 mixture of C”O and N150. Note that the mass 28 and 30 spectra have been magnified by 3x.
similar experiments using N150 are given in fig. 13. One observes that the N,O comes out in two peaks at 135 and 150 K, while the CO, comes off in two peaks at 150 and 360 K. Note, CO, is not seen when the sample is dosed with only NO, so the CO, must be formed via a reaction between NO and CO. The 360 K peak is very sharp; its half width is only 10 K. Thus, it is indicative of a so called “surface explosion”. If one blindly fits first order rate parameters using the method of Edwards [30] one finds an apparent exponential of 103’/s and an apparent activation energy of 62 kcal/mol for the peak. Clearly, no quantitative interpretation of these numbers is meaningful, but they do suggest that a simple analysis is not suitable. Figs. 12 and 13 also show the mass 28 and 30 spectrum taken under the same conditions as the mass 44 and 46 spectra discussed above. One observed small mass 28 features at 135, 155, and 360 K, and some even smaller features in the range of 400-500 K. The structure of the peaks is very similar to that in the mass 44 peak. Note however, that the cracking pattern [31] of N,O and CO, contain major contributions at mass 28. When those contributions are subtracted from the mass 28 spectrum in fig. 12, one finds that the peaks at 360 and 155 K are strongly attenuated. There remains a small cont~bution below 140 K, which can be attributed to cu,-CO. The peaks in the range of 400-500 K remain, but these peaks are relatively small. Thus, it is suggested that when the crystal is saturated with a SO/SO mixture of CO and NO, almost all of the CO remaining on the surface desorbs as CO, during the surface explosion. Significant CO desorption above 400 K was observed when excess CO was used. Fig. 13 also shows the mass 30 (15N,) peak taken with an excess of NO. We observe a small mass 30 peak below 200 K, which can be at least partially attributed to cracking of N,O. However, there does not appear to be any nitrogen desorption above 200 K. Thus, it seems that most of the nitrogen has desorbed before the surface explosion takes place.
356
Y.O. Park et ul. / CO und NO on Pt (410). I
4. Discussion 4.1. NO
The NO spectra here provide additional evidence of the unusual reactivity of Pt(410). In previous work it was found that NO partially dissociates upon adsorption on Pt(410) at room temperature [13]. Flash of a Pt(410) sample which had been saturated with NO at room temperature yields N, and 0,. but no detectable NO when the dosing is done so that little NO impinges on the sides and back of the crystal [15]. Here, similar experiments were done at 120 K. We observe startling, but different chemistry. At low exposures, only N, and 0, desorption was seen, but as the coverage was raised, the nitrogen peak disappears, and an N,O peak forms. Thus, it appears that much of the nitrogen was converted into N,O. The N,O desorbs below 200 K. Very little nitrogen was detected in AES above 200 K. Thus, it is clear that some unique chemistry is occurring on Pt(410) below 200 K. Previous workers [1,31] have suggested that the N,O forms via reaction between NO and adsorbed nitrogen. Thus it is suggested that NO probably dissociates on Pt(410) at 120 K! By comparison, no N,O formation has been observed on any other clean platinum sample below 400 K. Clearly. Pt(410) has unusual properties. It is not immediately obvious why the N,O forms at low temperatures but it is not seen at higher temperatures. N,O could conceivably form by three different mechanisms: a Rideal-Eley mechanism (5)
N@d) + NO&, + NZO,,,, 3 a Langmuir-Hinshelwood NCadj+ NO&l, --) N*Oc,,, ’ or a precursor
mechanism (6)
mechanism
NC,,, + NO,,,, + N,Oc,,, 3
T(7)
where NcadJ and N,O(,,, are adsorbed nitrogen and nitrous oxide, and NO,,,, and strongly adsorbed NOC”,,, and NO(,,, are gas phase, weakly adsorbed, nitric oxide. The first mechanism can be easily excluded. XPS results [13] have shown that NO partially dissociates on Pt(410) at room temperature. Thus, if reaction(3) has an appreciable rate, then N,O formation should be seen when the sample is reexposed to NO at room temperature. Experimentally no N,O could be detected in such an experiment. Reaction (4) is more difficult to exclude. It was previously found [13] that most of the NO which adsorbs on Pt(410) dissociates at room temperature. However, infrared studies [32] have indicated
Y.O. Park et al. / CO and NO on Pt (410). I
357
that there is a small amount of undissociated NO on the surface. This NO is probably held somewhere on the terraces or near impurities, so it should be less reactive than NO adsorbed near the steps. Still it is curious that no N,O formation is detected even though Ncadj and NO(,,, are present on the surface. Thus it is unlikely that N,O forms by reaction (4). This leaves reaction (5). Reaction (5) assumes that N,O forms by a reaction between adsorbed nitrogen (from dissociated NO) and a weakly bound NO species. One would only expect there to be a reasonable concentration of weakly bound species at low temperatures and high exposures. Thus, appreciable N,O formation should only be observed under such conditions, in agreement with the data. 4.2. CO The CO data also show unusual features. The simplest interpretation of the five peaks in the carbon monoxide desorption spectra would suggest that there were five different CO binding sites on the surface corresponding to the five different peaks in the flash desorption spectra. If one assumes that the TPD follow a first order rate law with a preexponential of 10t3/s one calculates binding energies of 32, 29, 26,22 and approximately 9 kcal/mol for the p3, &, uncertainty in P,Y al* and (Ye states respectively. Clearly, there is considerable these energies, but the orders of magnitude are useful. Evidently the first four sites have a similar binding energy for CO. This explanation of multiple sites initially seemed reasonable, given the heterogeneous nature of the surface. It is possible that /3i, and & resulted from CO adsorbed from different sites on the (100) terraces and p3 was caused by CO adsorbed at the (110) steps, with the (Ye and (Yecoming from the second monolayer.. Other combinations of sites also need to be considered. These possibilities will be explored below. It is useful to compare the spectra here to those for CO adsorption on other faces of platinum. McCabe and Schmidt find that CO desorption from Pt(ll0) yields a main TPD peak at 460 K which shifts to lower temperatures with increasing coverage [17] and a smaller peak at 350 K. They find that Pt(ll1) gives a peak which moves from 525 to 450 K with coverage, and a small shoulder at 400 K. The TPD spectrum for CO desorption from Pt(lOO) are quite complicated [17,20,21] with surface reconstructions occurring during adsorption and desorption. McCabe and Schmidt [17] report a four peak TPD spectrum for CO desorption from a (5 X 20)-(100) surface. At low coverages the largest feature is a high temperature state with a peak temperature near 550 K. The peak shifts to lower temperatures with increasing coverage. There are also three smaller lower temperature states, one at about 525 K, one at 450 K, and a third at about 400 K. Barteau, Ko and Madix find similar behavior [20], except that their peak temperatures are slightly lower due to their lower heating rates and the lowest temperature peak is absent. Crossley and King
358
Y.O. Park et al. / CO and NO on Pt (410). I
[21] also report a three peak structure, with peaks at 550, 470 and 400 K. They also studied desorption of CO from a (1 X 1) Pt(lOO) and argue that the 550 K peak is caused by desorption from a (5 x 20) reconstructed (100) surface. Stepped surfaces have been studied to a lesser extent. McCabe and Schmidt [17] report two peaks for CO desorption from Pt(210), a large peak at 625 K, and a smaller peak at - 475 K. In published work, we have observed similar spectra for CO desorption from Pt(210), although the high temperature peak was only at 615 K. McFealey and coworkers [22] report two peaks for CO desorption from Pt(321), a peak at 550 K, and a second peak at 440 K. Collins and Spicer [23] report two peaks for CO desorption from PtS[6(111) x (loo)], a large peak at 450 K and a smaller peak at 550 K. One should notice that the TPD spectra for CO desorption from these stepped surfaces have similar features. There is a peak with a maximum which corresponds roughly to the temperature of the main peak for CO desorption from the terraces, and another higher temperature peak, which is unique to the stepped surfaces, and therefore has been associated with CO desorption from the steps. Surprisingly though CO desorption from Pt(410) does not at first glance seem to follow the same general trends. The high temperature peak which is seen at ca. 615 K on Pt(210) is absent here. Further, the peak at 550 K is smaller than it should be based on the available data for CO desorption from the Pt(lOO) terraces. Admittedly, Pt(410) does not undergo the same reconstructions as Pt(lOO), so it may be incorrect to compare the spectra directly. Most stepped surfaces give a peak at 550 K, so it is possible that the 550 K peak contains a major contribution from the steps. Still, the behavior observed here is unusual; the highest temperature peak is too small which suggests that something unusual is happening on the (410) that does not happen on other stepped surfaces. There is other evidence that something unusual is occurring on this surface. Note that it was found that the & and & peaks merge at high heating rates. Flash desorption peak widths are generally inversely related to heating rate. At large heating rates, one would expect increased resolution provided the system responds quickly enough. The sharp peaks in fig. 1 show that our system response time is below 0.1 s; this response time of the system would not prevent us from resolving individual peaks. Thus, if the system behaved normally, the /3, and & states should be resolved into seperate peaks at high heating rates. CO desorption from other stepped platinum surfaces gives the expected behavior [22,23]. However, here, it is found that as the heating rate is increased above 30 K/s and & merges with the &. This suggests that some additional processes are taking place on the surface besides desorption. The data in fig. 5 provide some insight into the additional process occurring on the surface. Notice that the &-CO peak shifts up to 605 K, when 25% of a monolayer of carbon is deposited on the surface. Yet, previous investigators found that the CO/carbon interaction is repulsive on platinum so the peak
Y.O. Park et al. / CO and NO on Pt (410). I
359
should shift down. The & and & show the expected behavior but &-CO does not. Unpublished data indicate that the 615 K peak on a Pt(210) surface shifts down to 605 K when the Pt(210) sample was contaminated with Ca 25% of a monolayer of carbon. Thus, it appears that the peak at ca. 615 K which was missing on a clean surface, reappears once the surface is contaminated by 25% of a monolayer of carbon. Simultaneously, the deposition of carbon on the surface, as indicated by AES, stops. This suggests that the process which deposits carbon is related to the process which attenuated the 615 K peak. X-ray Photoemission Spectroscopy (XPS) was done in an attempt to clarify the unusual behavior seen here. Fig. 14 shows a series of XPS spectra taken using the apparatus and procedures described in Park et al. [13]. The XPS spectrum of a CO saturated Pt(410) crystal ,shows C,, and Or, peaks at 285 and 530 eV respectively. Heating the layer to 407 K, results in a broadening and shift in both the C,, and O,, peaks. The shift continues upon further heating until at 537 K, the C,, and O,, peaks lie at 282.5 and 528.8 eV
.-..I yl8L COExposure
290
Fig. 14. A series of C,, heating as indicated.
BINDING and O,,
285
280
275
ENERGY -
XPS spectra
taken after exposing
a Pt(410) sample
to CO and
Y.O. Park et a/. / CO and NO on Pr (410). I
360
*IS
I .c.*:=* ......_ ..*_P ‘+=** _,,,*.:-.,. .._.*.:I !
,.,:.* -:_. :..“.<
IL02
$po&b
/”
then heot
..j -* * to 500°C .......-.w%- 1 _.G: . . . . /Adsorb ’ .- “2....:-*.* 9L co ._*
535 530 525 Binding Energy, eV
54GddeBinding Energy, eV Fig. 15. Additional C,, and O,, XPS spectra taken after exposing a Pt(410) sample heating as indicated.
to CO and
respectively. Fig. 15 shows similar results at other coverages, and again similar shifts are seen. These kinds of shifts are often seen in XPS, and imply that the chemical nature of the species on the surface changes during the heating process. Park et al. [13] analysed the shifts in detail. They found that adsorption of NO or 0, at room temperature yields an O,, XPS peak at 528.8 eV. Oxygen dissociates on most platinum faces, and so it is likely that the 528.8 eV peak can be associated with a dissociated species. The 530 eV peak corresponds very closely to that seen for molecular species on Pt(lOO). Further the peak shifts in the C,, and O,, regions seen here, are very close to those which one observes during dissociation of CO on other metals, for example W(110) [24) and NO on Pt(lOO) [34]. Therefore, one can argue that the 530 eV peak probably corresponds to a molecular species (see Park et al. [13] for additional arguments). The shift, then can be attributed to a dissociation process (i.e. CO,, -+ C,, + O,, ). The AES results of Park et al. [19] also provide evidence for CO dissociation. When CO adsorbs on Pt(410), one observes 0-KLL and C-KLL Auger peaks with the right relative intensities for molecular CO. When the layer is heated to 500 K the 0-KLL peak disappears, while the C-KLL peak shifts
Y.O. Park et al. / CO and NO on Pt (410). I
361
slightly. Temperature cycling in a CO atmosphere produces a buildup of carbon on the surface; the buildup process has been from the AES Gun. It is difficult to find a mechanism for carbon buildup that does not involve CO dissociation. Hence, it is concluded that CO dissociates on Pt(410). The observation that CO dissociates on Pt(410) is not completely surprising. Vannice et al. [25] showed that a properly prepared platinum catalyst has significant initial methanation activity. CO bond breaking is a critical step in the methanation process, so apparently some form of platinum is active for CO dissociation. Pt(410) is a reasonable candidate. Pt(410) has previously been found to be unusually active for NO bond breaking [13,15]. Analysis of this data [12] indicates that Pt(410) is unusually active, because the symmetry of the highest occupied orbitals at the step happens to match those of lowest unoccupied (i.e. ?r* and a*) orbitals in the NO. NO and CO have the identical symmetry and similar electronic structures. Thus, one would expect faces that are unusually active for NO bond breaking to be also unusually active for CO bond breaking. Ni(310) and Ni(510), which have similar electronic structures to Pt(410), have already been demonstrated to be unusually active for CO bond breaking [14]. Thus, if any face of platinum is active for CO bond breaking, Pt(410) should. The only question is whether the energetics are favorable, and evidently they are. It is interesting to speculate how the dissociation process would affect the results of a TPD experiment. For the sake of argument, assume that the activation energy for dissociation is larger than that of desorption. If so, desorption should be the dominant reaction at low temperatures. The amount of dissociation will depend on the difference in the magnitude of the two barriers and the availability of active (stepped) sites. At low temperatures, dissociation may occur but the products will only slowly diffuse out of the active sites. As the temperature is increased, the rate of dissociation should increase faster than that of desorption because the differences in the activation energies will become less important. Further, at higher temperatures diffusion will be faster, increasing the turnover at the active sites. Thus, one would not expect the dissociation process to have a major effect on the lower temperature TPD peaks, but the higher temperature TPD peaks should be attenuated. Here, we were not able to observe the peak expected at ca. 615 K for desorption from the steps. Further, the 550 K peak was considerably smaller than it should have been for desorption from the terraces. We also observed decreases in the CO intensity at very long exposure times, or by heating the surface in CO. Surprisingly, a peak at 605 K is seen when enough carbon is deposited on the surface to poison the dissociation process. Clearly, something is occurring to attenuate the higher temperature peaks, just as one would expect if dissociation were occurring at high temperatures. The dissociation process could also cause the & and & peaks to appear to merge at higher heating rates. Increasing heating rates generally cause TPD
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peak positions to increase, and peak intensities to rise. Thus, the & peak, which is not strongly attenuated by the dissociation process, will move to higher temperatures and grow, with increasing heating rate. The &, however, comes at temperatures where the dissociation process is important. An increase in the heating rate will produce an increase in the attenuation. Further, the shift in the peak temperature will be less than it would be otherwise, because the attenuation increases with increasing temperature. Thus the & and & peaks should appear to merge at higher heating rates, just as was observed. The & and & peaks do not merge on a sample covered by ca. 25% of a monolayer of carbon, i.e. one inactive for CO dissociation, and in fact we find normal behavior, where we get increased resolution with increased heating rates. Thus, it appears that the dissociation process is related to the process that causes the & and & peaks to merge. Of course, it was also found that the & (500 K) and & (550 K) peaks fill non-sequentially. This is rather unusual behavior and it suggests that the & and & peaks are associated with different binding sites on the surface. For example, the & might be associated with desorption from the terraces, and the & might be associated with desorption from the steps. Surface reconstructions could also play an important role in the two peaks. Pt(lOO) shows peaks at both 550 and 500 K; so we cannot say that the & occurs just from desorption from the steps. However, perhaps the peak from the steps overlays the peak from the terraces. The observation (fig. 5) that the & peak is attenuated by dissociated CO to a much greater extent than the & suggests that much of the & comes from desorption from the steps. 4.3. NO/
CO coadsorption
The coadsorption experiments also indicate that Pt(410) has unusual properties. As with many other faces of platinum, when a Pt(410) sample is exposed to a mixture of CO and NO, CO, formation is seen. However, unlike all of the other faces studied previously, some CO, formation was observed during adsorption at room temperature. The remaining CO, comes out in a broad peak that is much smaller than the N, peak. CO, formation was also observed at 150 K in the low temperature experiments shown in fig. 14. By comparison the most active face studied previously, Pt(lOO), shows negligible CO, formation below 400 K. Thus, we conclude that Pt(410) has unusual properties for the NO/CO reaction. A detailed examination of fig. 11 reveals that the CO, peak behaves differently on Pt(410) than on other faces. Notice that in fig. 11, CO, fosrnation continued even after all of the nitrogen containing species desorbed. AES reveals no nitrogen on the surface. Some of the CO, may come from disproportionation of CO. However, we did not observe as large of a CO, peak when a CO saturated Pt(410) sample was flashed. Thus, it is likely that the bulk of the CO, forms via a reaction between NO and CO.
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We can speculate why CO, desorption continues after the nitrogen containing species desorb. Previous studies [l-lo] have indicated that the NO/CO reaction on platinum occurs in two steps: NO@d, + N@d) + O,, 3
(8)
owl + qd,
(9)
+ co2 3
with step (8) rate controlling on the faces considered previously. Here, it could also be suggested that the NO/CO reaction occurs in the same two steps. However, since CO, formation continues even after the reaction (8) is complete, reaction (9) must be rate controlling at higher temperatures. At lower temperatures, something else must occur. Notice that in fig. 11, the CO, first starts to desorb at about 350 K. CO, desorption continues up to about 600 K. It is difficult to fit a peak this broad with a simple kinetic rate law, and reasonable parameters. Thus, it is suggested that there is more than one kinetic process occurring on the surface. The data in fig. 11 provide some insight into the two processes. Notice, that the pi- and &-CO peaks are missing in fig. 11, but the & CO peak is largely unaffected. Thus, one could suggest that there is a difference in the reactivity of &- and &-CO. Admittedly, we do not know whether the main CO peak in fig. 11 is truely the & peak; it could be a peak due to a new species formed via an interaction between adsorbed NO and CO. However, the data clearly suggest that there are at least two different forms of CO on the surface with different reactivities. The low temperature experiments show radically different behavior. We observe two low temperature mass 44 peaks at 136 and 150 K, which seem to be similar to the two peaks seen above for adsorption of just NO. There is also a CO, peak overlaying the 150 K N,O peak and a sharp CO, peak at 360 K reaction indicative of a “surface explosion”. Previous studies of the NO/CO on Pt(lOO) [2,28] also reveal a CO, peak indicative of a surface explosion, but that peak is at a somewhat higher temperature, i.e. 410 K. On Pt(lOO), the explosion occurs when there is still a significant amount of NO on the surface, and it might be associated with an interaction between NO and CO [ll]. However, clearly that is not occurring here. Notice that the surface explosion occurs at 360 K, while most of the nitrogen containing species on the surface desorb below 200 K. No surface nitrogen is detected in AES above 200 K. The explosion cannot be associated with an interaction with the NO, because there is little NO left. Thus, some other mechanism must cause the explosion. Barteau et al. [20] observed that 0, desorption from Pt(100) was autocatalytic at 670 K due to a surface reconstruction. Perhaps the explosion reported here is caused by a similar mechanism. Another possibility is an attractive interaction of 0, and CO, although the data does not reproduce the peak shifts predicted by such a model. Chemical Autocatalysis can also be considered. At the moment the data do not seem to fully support any of these models, so some additional work needs to be done to explain the explosion.
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The CO, peak at 150 K is also quite unusual. Previous studies of the CO/NO reaction on platinum have found little CO, formation below 400 K. Here a CO, peak is seen at 150 K. Experimentally the peak is only seen when the crystal is dosed with a mixture of CO and NO; CO, is not detected when just NO or CO is used provided we are careful to pretreat the chamber. Thus, it is clear that the CO, is formed via a reaction between NO and CO, and not, for example, by a reaction between NO or N,O and carbon on the filaments. We do not observe CO, during dosing, so it probably forms during heating. We do not know, for sure, whether the CO, forms on the crystal; gas phase CO, was detected when the AUGER was turned on, so AES could not be used to check. However, there are no previous reports of a NO/CO reaction at 150 K on tantalium leads or alumina insulation so it is likely that the CO, is formed on the crystal. If so, this observation would provide additional evidence that the NO was dissociating below 200 K on Pt(410).
5. Conclusions The work here provides additional evidence that Pt(410) has unusual properties. We observe evidence for NO decomposition and N,O formation on Pt(410) at 120 K, CO, formation at 150 K, CO dissociation and carbon deposition at 480 K, and an surface explosion during the NO/CO reaction at 360 K. These results are radically different from those on most other platinum surfaces which suggests that Pt(410) is especially reactive for NO and CO bond breaking as suggested by Banholzer et al. [12].
Acknowledgements This work was supported by the National Science Foundation under grant CPE 83-11791. This work made use of the University of Illinois, Center for the Microanalysis of Materials, which is supported as a national facility by the National Science Foundation.
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