J . Phys. Chem. 1990, 94, 1066-1072
1066
Ethylene Adsorption and Decompositlon on (2X 1) Pt( 110 ) E. Yagasaki, A. L. Backman, and R. 1. Masel* Chemical Engineering Department, University of Illinois, Urbana. Illinois 61801 (Received: April 3, 1989)
The adsorption and decomposition of ethylene on the (2x1) Pt(ll0) surface is examined with TPD and EELS. It is found that ethylene adsorption on (2X 1) Pt( 110) shows unique properties and is not simply a combination of the chemistry seen on the closed packed faces of platinum. The ethylene adsorbs molecularly on (2x1) Pt(ll0) to yield a mixture of di-o and r-bound ethylene. Some of the adsorbed ethylene desorbs upon heating, some reacts to form ethane, and some decomposes to yield a mixture of ethylidyne (q3 =C-CH,), ethan- 1-yl-Zylidyne ( q 4 zCCHz-), and hydrogen. Additional hydrogen is liberated upon further heating, to produce a mixture of C2 species ([-CsC-I,), methylidyne ( q s =CH),multiply bound CCH complexes, and adsorbed carbon. The chemistry on (2x1) Pt(ll0) is different than that occurring on other faces of platinum, which provides further evidence that the mechanism of ethylene decomposition on platinum is controlled by the symmetry of the platinum surface.
Introduction
Experimental Section
Over the years, there have been many papers on ethylene adThe experiments reported in this paper were done using the sorption on p l a t i n ~ m . ~ - ~However, * it has been only recently apparatus and procedures described p r e v i ~ u s l y . * A ~ ~Pt( ~ ~110) realized that the intermediates which form when ethylene adsorbs single-crystal sample was cut from a Metron single-crystal rod. on platinum vary with the structure of the platinum ~ u r f a c e . ~ ~ - ~ ’The sample was polished with diamond paste and then mounted So far ethylene adsorption and decomposition have been studied in one of two vacuum systems. The sample was then oxidized, in detail on a limited number of surfaces. Several have sputtered, and annealed until no impurities could be detected by examined the mechanism of ethylene decomposition on Pt( 111). AES. A sharp (2x1) LEED pattern was seen at this stage. Next, We have done work on the mechanism of ethylene decomposition the sample was exposed to a measured amount of ethylene through on (5x20) and (1x1) Pt(100).28-29However, there has been little a capillary array doser. Subsequently, the sample was examined work on the mechanism of ethylene decomposition on any other with TPD, EELS, and LEED. face of platinum. The object of this study is to determine the mechanism of ethylene decomposition on (2x1) Pt(ll0). The (2x1) P t ( l l 0 ) (1) Bond, G. C. Discuss. Faraday SOC.1966.41, 200. surface has the washboard structure shown in Figure 1.32 There (2) Morgan, A. E.; Somorjai, G. A. J . Chem. Phys. 1969, 51, 3309. are (1 1 1) terraces and (1 11) steps. The symmetry at the steps (3) Smith, D. L.; Merrill, R. P. J . Chem. Phys. 1970, 52, 5861. (4) Lang, B.; Joyner, R. W.; Somorjai, G. A. Surf. Sci. 1972, 30, 454. on the (2x1) Pt(1 IO) surface is different from the symmetry at (5) Demuth, J. E. Chem. Phys. Lett. 1977, 45, 12. all of the sites on all of the other faces of platinum where ethylene (6) Kesmodel, L. L.; Baetzold, R. C.; Somorjai, G. A. Surf. Sci. 1977, 66, decomposition has been studied previously. It is believed that the 299. symmetry of the surface can affect the ethylene adsorption (7) Stair, P. C.; Somorjai, G. A. J . Chem. Phys. 1977, 66, 2036. (8) Ibach, H.; Lewald, S. J . Vac. Sci. Technol. 1978, 15, 407 chemistry.2q Hence, there is the possibility that (2x1) Pt(1 IO) (9) Stair, P. C.; Somorjai, G. A. Chem. Phys. Lett. 1976, 41, 391. will show unique ethylene decomposition chemistry. (10) Weinberg, W. H.; Deans, H. A.; Merrill, R. P. Surf. Sci. 1974, 41, In previous work Wesner et aL2’ examined ethylene adsorption 312. on Pt(l IO) using X-ray photoelectron diffraction. They found (11) Netzer, F. P.; Wille, R. A. Surf. Sci. 1978, 74, 547. that when ethylene decomposes on P t ( l l 0 ) the carbon-carbon (12) Kesmodel, L. L.; Dubois, L. H.; Somorjai, G. A. Chem. Phys. Lett. 1978, 56, 267. bond in the ethylene remains parallel to the surface. In contrast, (13) Kesmodel, L. L.; Dubois, L. H.; Somorjai, G. A. J. Chem. Phys. 1979, the carbon-carbon is perpendicular to the surface at 300 K on 70, 2180. Pt( 11 1). Wesner et al. suggest that the intermediates which form (14) Demuth, J. E. Surf. Sci. 1979, 84, 3 15. when ethylene interacts with P t ( l l 0 ) are different than those (15) Demuth, J. E. Surf. Sci. 1979, 80, 367. (16) Baro. A. M.: Ibach. H. J . Chem. Phws. 1981. 74. 4194 which are produced when ethylene interacts with P t ( l l 1 ) . (17j Demuth, J. E. S u r - Sci. 1980, 93, c82. However, without additional spectroscopy, this suggestion cannot (18 ) Steininger, H., Ibach, H.; Lehwald, S.SUI$ Scr. 1982, 117, 685. be considered to be verified. (19) Albert, M. R.; Sneddon, L. G.; Eberhardt, W.; Greuter, F.; GusWe have done calculations to examine the interaction of tafsson. T.; Plummer. E. W. Surf. Sci. 1982, 120. 19. (20) Skinner, P.; Howard, M.-W.; Oxton, I. A.; Kettle, S.F. A.; Powell, ethylene with (2x1) Pt( 1 According to our calculations, D. B.; Sheppard, N. J . Chem. SOC.,Faraday Trans. 2 1981, 77, 1203. the Cq atoms on the (1 11) terraces and steps in (2x1) P t ( l l 0 ) (21) Salmeron, M.; Somorjai, G. A. J . Phys. Chem. 1982, 86, 341. should behave much like the C9 atoms on extended Pt( 1 1 1) planes. (22) Creighton, J. R.; White, J. M. Surf.Sci. 1983, 129, 327. However, there is the possibility of extra chemistry near the C, (23) Ogle, K. M.; Creighton, J. R.; Akhter, S . ; White, J. M. Surf. Sci. 1986, 169, 246. and C , , atoms on the (2x1) P t ( l l 0 ) surface. For example, (24) Berlowitz, P.; Megiris, C.; Butt, J. B.; Kung, H. H. Langmuir 1986, a-bound ethylene is favored at the C7atoms,” while a butterfly 1, 206. complex can form at the bridge between two C l l atoms.31 Dia(25) Van Strien, A. J.; Nieuwenhuys, B. E. Surf. Sci. 1979, 80, 226. grams of some of the possible intermediates are shown in Figure (26) Fischer, T. E.; Kelemen, S. R. Surf.Sci. 1977, 69, 485. 2. There are also some interesting steric hindrances near the (27) Wesner, D. A.; Coenen, F. P.; Bonzel, H. P. J . Vac. Sci. Technol. A 1981, 5, 927. bottom of the steps, which would inhibit ethylidyne formation, (28) Hatzikos, G. H.; Masel, R. I. Surf. Sci. 1987, 185, 479. and thereby possibly create new chemistry. (29) Hatzikos, G. H.; Masel, R. 1. In Catalysis Ward, J . W., Ed.; Elsevier: In this paper we have examined the adsorption and decompoAmsterdam, 1988; p 883. sition of ethylene on (2x1) P t ( l l 0 ) using TPD and EELS to try (30) Backman, A. L.; Masel, R. I. Unpublished work. (31) Hatzikos, G. H.; Masel, R. I. Unpublished work. to see how the decomposition of ethylene is different on (2x1) (32) Adams, D. L.; Nielsen, H. B.; VanHove, M. A.; Ignatiev, A. Surf. Pt( 1 10) than on other faces of platinum. Sci. 1981, 104, 47. 1
*Send correspondence to this author.
0022-3654/90/2094-1066$02.50/0
,
(33) Lee, F.; Backman, A. L.; Lin, R.; Gow, T. R.; Masel, R. I. Surf. Sci. 1989, 216, 173.
0 1990 American Chemical Society
Ethylene Adsorption and Decomposition on Pt( 110)
The Journal of Physical Chemistry, Vol. 94, No. 3, 1990 1067
100 200 300 400 500 600 700 800 900 1000 TEMPERATURE / K
Figure 3. A series of 2 amu TPD spectra taken by exposing a clean 100 K (2x1) P t ( l l 0 ) sample to varying amounts of ethylene then heating at 14 K/s.
P
3
\
V
Figure 1. Surface structure of (2x1) Pt( 110).
E
v) v)
HHH
9
Ethylidyne
Di- Gethylene
%bound ethylene
Butterfly complex
n h
Ethylidene
Vinylidene
Methylene
Methylidyne
Ethylylidyne
0.20 L 0.16L0.10 L 0-06 L background
Quad - G Acetylene
Figure 2. Some of the intermediates that have been postulated to form during ethylene decomposition on various faces of platinum.
The TPD work was done in the same apparatus used for our previous studies of ethylene adsorption on Pt(100).28 The UHV system was of standard design with a working base pressure of 1 X Torr. The system was equipped with a PHI 4-161 sputter gun, a PHI 15-120 LEED/AES system, and a Balzers QMA 112 mass spectrometer. Initially, AES was done with a PHI 10-155 CMA. However, during the course of the work the CMA was moved to another apparatus. Subsequently, the AES measurements were done with the LEED screens. During a TPD run, the sample was cleaned until no impurities could be detected by AES. The sample was then dosed with a measured amount of ethylene through a capillary array. The sample was rotated so it faced an opening in a shield over the mass spectrometer. The geometry was such that only the front face of the crystal was in line of sight with the mass spectrometer. Next, the sample was heated at a fixed rate of 14 K/s under computer control. Everything else was standard. One should refer to our previous work28for more details. The EELS work has done in a second apparatus. This apparatus consisted of a ultrahigh vacuum (UHV) system with a working base pressure of 5 X Torr. The UHV system was equipped with a PHI 4- 161 sputter gun, a PHI 15-120 LEED/ AES unit, a Riber QX mass spectrometer, and an LK Technologies LK-2000-DAC EELS spectrometer. During a EELS run, the sample was sputtered, oxidized, and annealed until no impurities could be detected by AES or EELS and a sharp (2x1) LEED pattern was seen. The sample was then exposed to a measured amount of ethylene at 93 K. The sample was then briefly annealed to 100 K and cooled back to 93 K. Then an EELS spectrum was recorded. Subsequent spectra were taken by se-
I
Id0 200 300 400 500 600 700 800 900 1000 TEMPERATURE / K
Figure 5. A series of 30 amu TPD spectra taken by exposing a clean 100 K (2x1) Pt( 110) sample to varying amounts of ethylene then heating at 14 K/s.
quentially annealing to a higher temperature for 2 min, cooling back to 93 K, and then recording an EELS spectrum. There was some buildup of impurities on the sample at 93 K. As a result, we were only able to take about five spectra before we had to reprepare the sample and start over.
Results Figures 3-5 show a series of TPD spectra taken by exposing a clean (2X 1) Pt( 110) sample to varying amounts of ethylene and then heating as indicated. Only three desorption products were detected during ethylene decomposition on (2X 1) Pt( 110): ethylene, ethane, and hydrogen. At low exposures, the hydrogen desorbs in two peaks at 290 and 350 K. The 350 K peak shifts to higher temperatures with increasing exposure. Simultaneously, a new peak grows into the spectrum at 430 K. There is also a high-temperature tail in the hydrogen spectrum which extends up to 700 K. In addition, there is a peak at 230 K at ethylene exposures above 0.2 langmuir. The 230 K peak is of the size expected for background hydrogen, and the peak grows when the sample is predosed with hydr0gen.4~Thus, the 230 K peak might be an artifact. However, the remaining peaks do not change in the presence of background hydrogen, suggesting that they are associated with ethylene decomposition. Integration of the peak areas indicates that, at low coverages, approximately 45% of the
Yagasaki et al.
1068 The Journal of Physical Chemistry, Vol. 94, No. 3, 1990 I
270
x50
500 K
300 ... K
250 K
I
I l l
I Ii Ii
Ii
- -
in30
0
1000
2000
3000
4000
Energy Loss (cm-') 0
1000
2000
3000
4000
Energy Loss (cm-l) Figure 6. A series of EELS spectra taken by exposing a clean 93 K (2x1) Pt(ll0) sample to 1 langmuir of ethylene and then sequentially annealing to the temperatures indicated for 2 min. The scale factors in the figures are calculated relative to the elastic peak. As noted in the text, the elastic peaks were much smaller in the 400 and 450 K spectra than in the rest of the spectra. As a result we adjusted the size of the 400 and 450 K spectra in the figure so that the peaks heights are as they would have been if there had been no reduction in the intensity of the elastic peak.
hydrogen desorbs in the 290 K peak. The fraction which desorbs in the 290 K peak varies with exposure, so that with a 1-langmuir exposure, about a third of the hydrogen desorbs in the 290 K peak, another 40% desorbs between 320 and 450 K, and the remainder of the hydrogen desorbs above 450 K. Figures 4 and 5 show the ethylene and ethane desorption spectra. Ethylene desorbs in a sharp peak at 280 K and a series of smaller peaks centered at about 220 and 415 K independent of coverage. Ethane desorbs in a small peak at 280 K. Integration of the peak areas indicates that at a 2-langmuir exposure, the ethane peak is about 6% of the 280 K ethylene peak. Figure 6 shows a series of EELS spectra taken by exposing a clean, 93 K, (2x1) Pt( 110) sample to 1 langmuir of ethylene and then annealing as indicated. At 100 K one observes a complex spectrum with distinct peaks at 475, 670, 805,960, 1210, 1430, and 2950 cm-' and shoulders about 1020 and 3070 cm-I. The intensities of all of the modes decrease upon annealing to 250 K, but there is little change in peak position. However, the 960-, 1020-, 1210-, and 3070-cm-' peaks disappear upon annealing to 300 K, while new peaks grow into the spectrum at 460,990, and 1 130 cm-I. There is also evidence for a distinct shoulder at 1340 cm-' and a small feature at 830 cm-' which is about the same size as the noise. The 830-cm-' feature is reproducible, however. All of the peaks shrink upon annealing to 350 K. However, there is little change in peak position. The peak at 2950 cm-' shifts to higher frequencies upon annealing to 400 K, while the peaks between 800 and 1500 cm-' become less distinct. In addition, there is an unusual effect in that ratio of the intensity of all of the inelastically scattered peaks to the intensity of the elastic beam increases by a factor of 6 upon annealing to 400 K. Note, however, that the number of counts in the inelastic channels does not change significantly upon annealing from 350 to 400 K. Instead the number of counts in the elastically scattered beam decreases by about a factor of 6 during the annealing process. The decrease in the elastic peak is reproducible, suggesting that the result is not an experimental artifact. However, it is difficult to determine why the elastic peak
Figure 7. A series of EELS spectra taken by exposing a clean 93 K (2x1) Pt(1 IO) sample to 1 langmuir of deuterated ethylene and then sequentially annealing to the temperatures indicated.
is changing so drastically in this temperature range. Fortunately, the elastic mode recovers upon annealing to 500 K. At that temperature, distinct peaks at 270, 520, 840, 2230, and 3045 cm-' are observed. There is also a broad peak centered at 1290 cm-I. Further heating causes the 840-, 1290-, and 3045-cm-I peaks to disappear. However, the 270-, 520-, and 2230-cm-' peaks are stable to high temperature. We have also taken EELS spectra a t ethylene exposures of 0.024,0.044, and 0.1 1 langmuir. At 93 K, the peak positions and relative intensities were essentially identical with those in Figure 6. At 300 K the spectra were very similar to those in Figure 6. However, the shoulder at 1340 cm-' was not clearly distinguished in the 0.024-langmuir spectra. Further, there were some changes in the relative intensities of the peaks with coverage. At 500 K, the 840-, 1290-, and 3045-cm-I peaks were strongly attenuated in the 0.024-langmuir spectra, and the 2230-cm-I peak was proportionally larger than in the 500 K spectrum in Figure 6. We also detected new modes at about 700 and 1280 cm-'. These two modes are also seen in our low-exposure deuterated spectra. Thus, it appears that there are some changes in the nature of the adsorbed species with coverage. EELS spectra of deuterated ethylene adsorbed on (2x1) Pt(1 10) were taken to supplement the EELS results above. Figure 7 shows a series of spectra taken by exposing a clean (2x1) Pt(l10) sample to 1 langmuir of C2D4at 93 K and then annealing as indicated. At 93 K, one observes distinct peaks at 480,713,940, 1145, 1290, and 2200 cm-' and shoulders at 590 and 2130 cm-I. All of the peaks shrink upon heating to 250 K. However, there is little change in peak position. In contrast, there is a sudden change when the sample is heated to 300 K. The peaks at 590, 710,940, and 1290 cm-' disappear, while new peaks grow into the spectrum at 780 and 1020 c d . There also appears to be small peaks at about 550 and 670 cm-I. Further heating causes a reduction in the elastic peak as before, and then a reappearance of elastic peak at higher temperatures. At 500 K, distinct features are seen at 270,570,680,940, 1270, and 2248 cm-I. The 680-, 940-, and 1270-cm-' peaks disappear upon further heating. However peaks at 270, 523, and 2240 cm-l are stable to high temperatures. We have also done LEED measurements under the conditions of the experiments above. The clean Pt( 110) sample showed a sharp (2x1) LEED pattern with little intensity between the spots. The intensity between the spots got much brighter when ethylene adsorbed. However, the LEED spots remained sharp, and no new LEED patterns were seen at any coverage. There were several
Ethylene Adsorption and Decomposition on Pt( 110)
The Journal of Physical Chemistry, Vol. 94, No. 3, 1990 1069
TABLE I: A Comoarison of the Vibrational Freauencies of Etbvlene Adsorbed on a 100 K (2x1) Pt(ll0) Crvstal to Those Observed Previouslv C2H4on C2H4 on tentative C2H, on Zeise’s C2H4 on C2H4Br2 C2H4 on C2H4 on assignment (2x1) Pt(ll0) Pt(l1 1)8*18 Pt(100)28 (gauge)34 salt35 Pd(110)3638 Pd(l1 1)39 Pt(210)30 ~~~~~
M-C stretch CH2 rock CHI twist C H I wag u(C-C) CH2 scissors CH2 stretch tentative assignment M-C stretch CD2 rock CD2 twist CD2 wag u(C-C) CD2 scissors CD2 stretch
475 670 805 960*‘ 1020,O 1210 1430 2950, 3070
470, 560 660 790 980 (sh) 1050 1430 2920, 3000 (sh)
450 984 984 1580 2936
369 67 1 966 1217 1453, 1551 2974, 3032
C2H4 on (2x1) Pt(1 IO)
C2D4on Pt(l11)8
C2D4on Pt(2t0)30
480
450
36 1 479
590 713 940, 1290 1145 2130 (sh), 2220
600 740 900 (sh) 1150 2150, 2250
708 1313 951, 1059 2184, 2302
898 1 I04 1278 1019 1420 2953, 3000
C2D4Br2 (gauge)34 712 79 1 947 1014 1141 2174, 2271
369 72 1 975 1243 1426, 1515 3013, 3079 D4 Zeise’s
525 757 1353 1059, 962 2193, 2331
? 690 915 -1200 1400 2990 C2D4 on Pd(l ? 530
700 1274 2245
343,533 691 91 1 1078, 1129, 1145 1502 1418 2996 C2D4 on Pd( 11 367 534 673 778, 840, 953 355 1058 2246
’The assignments on these two bands may be reversed. bsh = shoulder.
subtle changes in the intensity of the areas between the LEED spots upon annealing to various temperature up to 700 K. Nevertheless, the LEED pattern remained sharp. No new spots or LEED patterns were seen at any coverage. We looked, in particular, for changes in the LEED intensities between 350 and 400 K, since there is a factor of 6 change in the intensity of the elastic mode in EELS over that temperature range. However, no distinct changes in the LEED results were seen upon annealing from 350 to 400 K. There were some new LEED patterns upon annealing to temperatures above 700 K. However, we did not examine them in detail.
Discussion The results here show that ethylene adsorption and decomposition on (2x1) P t ( l l 0 ) is rather complicated. At 100 K, there appears to be two different kinds of adsorbed ethylene on the surface. Table I summarizes our EELS results for ethylene and deuterated ethylene adsorption on (2x1) Pt(l10). Notice that the peaks at 1020 and 1210 cm-l in the 100 K EELS spectrum shown in Figure 6 do not shift greatly upon deuteration. Thus, it appears that both peaks are associated with carbon-carbon stretches. In other work, we find that when we partially reconstruct the surface into a (1 X 1) reconstruction, the intensities of the two peaks change independently. The presence of two independent carbon-carbon stretches is strongly suggestive of there being two species on the (2x1) P t ( l l 0 ) surface at 100 K. We associate these two species with di-a and a-bound ethylene. Table I compares the peak positions in our 100 K EELS spectrum to those reported p r e v i o u ~ l y . ~ Previous ~ ~ ~ , ~work . ~ ~has ~ ~shown (34) Neu, J. T.; Gwinn, W. D. J . Chem. Phys. 1950, 18, 1642. (35) Hiraishi, J. Spectrochim. Acta 1969, 25A, 749. (36) Powell, D. B.; Scott, J. G.V.; Sheppard, N. Spectrochim. Acta 1972, 28A, 327. (37) Bandy, B. J.; Chesters, M. A.; Mcdougall, G.S.; Sheppard, N. Surf. Sci. 1984, 139, 87. (38) Chesters, M. A.; Mcdougall, G.S.;Pemble, M. E.; Sheppard, N. Appl. Surf.Sei. 1985, 22/23, 369. (39) Gates, J. A.; Kesmcdel, L. L. SurJ Sci. 1982, 120, L461. (40) Lehwald, S.; Ibach, H. Surf.Sci. 1979, 89, 425. (4 I ) Pouchert, C. J., Ed. The Aldrich Library of Infrared Spectra, 3rd ed.; Aldrich Chemical Co.: Milwaukee, WI, 1981; p 53. (42) Stuve, E. M.; Madix, R. J. J. Phys. Chem. 1985, 89, 105. (43) Yagasaki, E.; Backman, A. L.; Masel, R. 1. Unpublished work. (44) Ibach, H.; Mills, D. L. Electron Energy Loss Specrroscopy and Surface Vibrations; Academic Press: New York, 1982. (45) Lehwald, S.;Erley, W.; Ibach, H.;Wagner, W. Chem. Phys. Lett. 1979, 62, 360. (46) Gervasio, G.; Rossetti, R.; Stanghellini, P. L. Organometallics 1985, 4 , 1612. (47) Demuth, J. E.; Ibach, H. Surf.Sei. 1978, 78, L238. (48) Zaera, F.J. Am. Chem. SOC.1989, I l l , 4240. (49) de la Cruz, C.; Sheppard, N. J . Chem. SOC.,Chem. Commun. 1987, 1854.
that, when ethylene adsorbs on P t ( l l 1 ) and (5x20) or (1x1) Pt( loo), a di-a complex forms.*#” The di-a complex shows peaks at 470, 660, 790,980, 1050, and 2920 cm-I as indicated in Table I. We observe peaks at all of these frequencies, suggesting that there is some di-a ethylene on the (2x1) Pt(l10) surface at 100 K. However, the 1210- and 3070-cm-’ peaks seen in the 100 K spectrum in Figure 6 and the 1290- and 2130-cm-’ peaks in the 93 K spectrum in Figure 7 are missing. Further, the 960-cm-l peak in the 100 K spectrum in Figure 6 and the 940-cm-l peak in the 93 K spectrum in Figure 7 are much larger than expected from the spectrum of di-a ethylene adsorbed on Pt( 11 1) or Pt(100). Thus, it appears that something in addition to a di-a complex forms at 100 K on (2x1) Pt(ll0). A likely possibility is a-bound ethylene. In other work, we have found that a a-bound intermediate forms when ethylene adsorbs on Pt(210).30 A *-bound intermediate is also formed when ethylene adsorbs on Pd(1 10).36J7The *-bound intermediate on Pt(210) shows distinct peaks at 671, 966, 1217, 1453, 2974, and 3032 cm-’. Notice that again we observe peaks at all of these frequencies in our 100 K EELS spectrum. We also observe peaks at 1020 cm-I in Figure 6 and 940 cm-l in Figure 7. These peaks are not expected for a-bound ethylene. Further, the intensity of the 2950-cm-I mode in Figure 6 is larger than one would expect for a-bound ethylene alone. However, all of the peaks in our 100 K EELS spectra are seen in either di-a or a-bound ethylene, or both. Thus it appears that there is a mixture of di-a and *-bound ethylene on the surface at 100 K. Such a result is not unexpected. Both di-a and a-bound ethylene ~ * ~on~ ~ ~ ~ have been observed on other faces of p l a t i n ~ m ’ and supported platinum ~atalysts.4~ The (2x1) P t ( l l 0 ) surface is complicated, with many different kinds of inequivalent sites, as shown in Figure 1. There are hollow sites with a symmetry much like those on P t ( l l 1 ) and step sites with a symmetry much like those on Pt(210). Di-a ethylene forms at the hollow sites on Pt( 11l).8312313Thus, one would expect di-a ethylene to also form at the hollow sites on (2x1) Pt(ll0). In contrast, *-bound ethylene forms at the step sites on Pt(210).30 Thus, a-bound ethylene could form at the step sites on (2x1) Pt( 110). As a result, it is not surprising that both di-a and a-bound ethylene are formed when ethylene adsorbs on (2x1) Pt(ll0). The presence of two species makes peak assignments difficult. Further, there is some controversy about the peak assignments in one of our reference compounds, Zeise’s salt.35,36Thus, while we have indicated peak assignments in Table I, we must emphasize that they are only tentative assignments. We do want to note, however, that we view the “a-bound“ ethylene observed here as having a/*-binding much like that in Zeise’s salt. Under other conditions, we have also observed a different *-bound form of (50) Sheppard, N. Annu. Reu. Phys. Chem. 1988, 39, 589.
1070 The Journal of Physical Chemistry, Vol. 94, No. 3, 1990
Yagasaki et al.
TABLE iI: A Comparison of the Spectrum of Etbylene Adsorbed on a 300 K (2x1) Pt(ll0) Sample to tbe Peak Positions in Spectra Reported Previously C2H4 on (2X I)Pt( 1 IO) ethylidyne vinylidene di-a ethylene ethylidyne at 300 K tentative on 300 K on 300 K on 150 K di-a ethylene complex 1, I ,1,2-tetraassignment C2H2" C2H: Pt( 11l)8,'8i20 (1x1)Pt( Pt( 11 l ) g on Pd(100)42 (CH,C)CO,(CO)~~~ chloroethane4' M-C-C bend 310 220 M-C stretch 460" 460b 430,600 452 470, 560 390 385 715 CH,, CHI rock 670 (w)" 900 815 660 1004 715 CH2twist 830 (w) 830 (w) 790 (sh)C 815 CH2 wag 990" 1048 960 920 952, 1055 C-C stretch 1130" 1 I3Ob 1130 1586 1050 1135 1163 1280 CH, bend 1340b 1350 1356 CHI scissors 1420" 1420 1455 1430 CH, stretch 2950' 2950b 2950, 2890 (sh) 2920 2920 2980 2888,2930 2940,3050 d3-ethylid yne
tentative assignment
M-C-C bend M-C stretch CD,, CD, rock CD, twist CD, wag C-C stretch CD, bend CD2 scissors CD, stretch
C2D4 on (2x1) Pt(ll0) at 300 K 430 550 (w) 670 (w) 780 1150
q3CCD3on Pt(l1l) T~C=CD, on (1x1) C2D4 on Pt(ll1) C2D40nPd(100) complex at 300 K Pt(100) at 300 K at I50 K at 80 K (CD,C)Co,(C0)9 300 410 790 1I60
427
450
384
709 1583
740 900
600 660 1220
990 1020 2208
2080,2220
1 I30 2218
1150 2150,2250
920 2215
210 387 828 1128 1260 2192
"Peaks associated with the CCH2 species. bPeaks associated with the CCH, species. Csh = shoulder; w = weak. ethylene with a carbon-carbon bond at 1620 cm-'. The next section of the paper will consider what happens as the surface is heated and the ethylene decomposes. First consider the temperature range from 100 to 280 K. Figure 4 shows that, when the (2x1) P t ( l l 0 ) surface is heated from 100 to 280 K, ethylene desorbs. Simultaneously, there is a reduction of all of the peaks in the EELS spectrum. Nevertheless, there is little change in the positions or relative intensities of the peaks in EELS. One would have thought that, with two forms of ethylene sitting on different sites on the surface, one of the forms would have desorbed preferentially. However, apparently that does not occur. The major ethylene TPD peak is sharp. There is a low-temperature tail on the ethylene TPD spectra. However, the EELS spectra do not show any evidence of a change in the ratio of the di-u and s-bound states upon annealing to 250 K. EELS also shows that the ratio of the di-u and a-bound states is independent of coverage. As a result, it appears either that some sort of equilibrium is established on the surface so that the ratio of the states does not change when the surface is annealed to 250 K and then cooled back to 93 K, or that for some unknown reason the rate of desorption of di-a ethylene is identical with that of a-bound ethylene. From our EELS results, we conclude that, except for some ethylene loss, there is little change in the layer upon annealing from 100 to 270 K. In contrast, a major change occurs when the layer is annealed to 300 K. Between a third and 45% of the hydrogen in the ethylene desorbs. Simultaneously, there is a large change in the EELS spectrum. We propose that, again, there is a mixture of species on the surface. Notice that the fraction of the hydrogen desorbing in the 290 K peak varies significantly with coverage. The TPD data suggests that the average stoichiometry of the layer changes from C,H,,, at low coverage to C2H2,, at high coverage. One could get such a change in stoichiometry if some of the hydrogen remained behind on the surface at high coverage. However, in other we have found that, when we coadsorb D2 and C2H4, the D2 desorbs in a single peak at 200 K. Thus, one would not expect the extra hydrogen seen at high coverage to remain behind on the surface unless the extra hydrogen was bound to carbon. Thus, the TPD results indicate that, on average, there are proportionally more hydrogen carbon bonds at high coverage than at low coverage. Such a result, is suggestive of there being more than one species on the surface at 300 K.
Further evidence for multiple species is given in the EELS data. The results described above indicate that there is a change in the CH, deformation region of the EELS spectrum of the overlayer with coverage at 300 K. At low coverage, we only observe a 1420-cm-' peak. Such a peak is normally associated with a CHI scissors mode.44 However, at high coverages we also observe a 1340-cm-' shoulder. CH, deformations at 1340 cm-' are usually associated with a CH3 bending mode.@ Thus, it appears that there is a change in the species on the surface with coverage at 300 K. At low coverage, the overlayer mainly contains C H 2 groups suggestive of CCHz species. However, a CH3 group grows in with increasing coverage. We associate the CH3 group with a CCH3 species. From the stoichiometry, we calculate that we have mostly CCH, species at low coverages. We also calculate that the proportion of CCH3 groups increases with coverage. The calculations suggest that there are about twice as many CCH, as CCH2 species when the surface is saturated with ethylene at 93 K and annealed to 300 K . One can use the EELS spectra to identify the CCH2and CCH3 species. Table I1 compares the peak positions in our 300 K EELS spectra to those from ref 8, 12, 13, 18, 20, 28, and 37-42. We have labeled the peaks that are prominent in our 0.024-langmuir spectrum as being associated with the CCH, species and the peaks which grow more quickly with coverage than the 1420-cm-' peak as being associated with the CCH3 species. Notice that the peak positions associated with the CCH, species are in good agreement with those measured for ethylidyne on Pt( 11 1) by Ibach et aL8J8 We do not resolve the 900-cm-' CH, rocking mode expected for ethylidyne. However, the 900-cm-' peak is small in Ibach's spectrum. Thus, it is unlikely that we would be able to resolve the 900-cm-' peak under the 990-cm-' peak from the CCH, species. As a result, we suggest that there is ethylidyne (v3 sCCH3) on the (2x1) P t ( l l 0 ) surface at 300 K. As noted above, we also have a CCH, species on the surface at 300 K. The CCHz species is harder to identify. The only previous report of a CCH, species on platinum is Hatzikos et al.'sZ8 report of a di-u bound vinylidene (7, =C=CH2) species on (1 X 1) Pt( 100). However, Table I1 shows that there is little agreement between the peak positions for the CCH, species in Figure 6 and the peak positions for di-u vinylidene. For example the carboncarbon bond is at 1130 cm-' in the CCH, species in Figure 6 and I586 cm-' in di-cr vinylidene. Thus, it does not appear that there is di-u vinylidene on the (2x1) Pt(l10) surface at 300 K.
Ethylene Adsorption and Decomposition on Pt( 110) Sheppardsonotes that a di-a7r form of vinylidene can also form under certain circumstances. The di-a7r form of vinylidene has an 1R spectrum with many features in common with the CCH, species observed here. However, no intense 1130-cm-' peak is expected for di-a7r vinylidene. Further, the EELS spectrum of deuterated di-a7r vinylidene should show an intense C-C mode at around 1450 cm-l. No such peak is observed. Thus it does not appear that there is di-a7r vinylidene on the surface at 300 K. The CCH, species on (2x1) Pt(ll0) does have many features in common with Ibach's spectrum of di-a ethylene on Pt( 11 1).l8 All of the carbon-hydrogen modes line up as indicated in Table 11. The carbon-carbon in the CCH, species is at 1130 cm-I and not the 1050 cm-' expected for di-a ethylene on platinum. However, a carbon-carbon stretch at about 1130 cm-' is expected for ethylidyne. As a result we propose that the CCH, species has some similarities to di-a ethylene on the CH, end of the molecule and some similarities to ethylidyne on the CC end of the molecule. A likely species is ethan-1-yl-2-ylidyne (q4 =CCH,-) which we will refer to as "ethylylidyne". A diagram of ethylylidyne is shown in Figure 2. We do not have a reference spectrum for ethylylidyne. However, spectra are available for a related compound 1, l ,1,2tetrachloroethane. Table I1 compares the peak positions for the CCH, species on (2x1) Pt(l10) to those for 1,1,1,2-tetrachloroethane. Notice that there is good agreement between the EELS peaks in our CCH, spectrum and those for 1,l ,1,2-tetrachloroethane. Thus, we suggest that there is some ethylylidyne on the surface (2x1) Pt(110) surface at 300 K. The presence of ethylylidyne would also explain why Wesner et were not able to observe tilting of the carbon-carbon bond at 300 K durng ethylene adsorption on Pt(ll0). It is not unreasonable that a mixture of ethylidyne and ethylylidyne would form at 300 K on (2x1) Pt(ll0). On P t ( l l 1 ) di-a ethylene is converted to ethylidyne at 300 K.18 The details of the ethylidyne formation from di-a ethylene are not well es? ~ ' the ethylidyne forms tablished. However, our p r o p o ~ a l ~is~that via a mechanism such as that in Figure 8a: the di-a ethylene loses a hydrogen to form a highly strained q3 =CHCH2- intermediate; then the strain is relieved by an intramolecular hydrogen-transfer process yielding ethylidyne. The sites on (2x1) Pt(ll0) have the same symmetry as the sites on Pt( 1 1 1). See Figure 1. Thus, one would also expect the highly strained q3 =CHCH2- intermediate to form on (2x1) Pt(ll0). The q3 =CHCH,- intermediate could react to form ethylidyne at the tops of the hills on (2x1) Pt( 110). However, there is not sufficient room for ethylidyne to form down in the valleys on (2x1) Pt(ll0). Thus, the q3 =CHCH2- intermediate has to find some other way to relieve its strain. Figure 8b shows that one way for the q3 =CHCH,- intermediate to relieve its strain is for it to bond to the step and dehydrogenate to yield ethylylidyne. Figure 8b shows the case of a di-a ethylene with a C-C bond oriented perpendicular to the step. However, a similar argument also applies to a di-a ethylene oriented parallel to the step. Again, ethylidyne cannot form down in the valley, but ethylylidyne can. In contrast, ethylidyne can form up on the hills on (2x1) Pt( 1 IO). Thus, it is not unreasonable that some of the adsorbed ethylene would be converted to ethylylidyne and some would be converted to ethylidyne on (2x1) Pt(ll0). The mechanism in Figure 8b is of course, hypothetical. However, there is one interesting piece of evidence which supports it. Notice that the 290 K H2 TPD is sharp in the 1-langmuir TPD spectrum in Figure 3. D, desorbs in a single peak at 200 K43 from a C2H4-covered(2X 1 ) Pt( 110) surface. Therefore, the 290 K peak in Figure 3 must be reaction rate limited at high coverages. Yet one observes a sharp peak at 290 K even though both ethylidyne and ethylylidyne are forming simultaneously. One would not expect to observe a sharp TPD peak when two different reaction pathways are occurring on the surface. However, the implication of Figure 8 is that ethylidyne and ethylylidyne both form via a common q3 =CHCH2- intermediate. If the rate-determining step in both processes were the formation of this common intermediate, then the H, peak should be sharp as is observed. Admittedly, one could also get a sharp TPD peak via some unusual coincidence.
The Journal of Physical Chemistry, Vol. 94, No. 3, 1990 1071
Figure 8. (a) Mechanism of ethylidyne formation on R(1 1 1) as proposed in ref 29, 31, and 48. (b) A speculative diagram showing what would happen if a similar process occurred down in the valleys on (2x1) Pt(1 10).
However, it is reasonable to assert that ethylylidyne and ethylidyne both form via a common intermediate on the (2x1) P t ( l l 0 ) surface at 300 K. It is unclear, from our data, what happens upon heating to between 350 and 450 K. For reasons that we do not understand, the elastic peak in our EELS spectrum is strongly attenuated between 350 and 450 K. In addition, the resolution of our instrument degrades to about 11 meV even though we do not change any of the settings in our EELS spectrometer. We were not successful in adjusting the EELS spectrometer to account for these changes. The observed loss in elastic intensity is reproducible. Further, the peaks associated with our various loss features are not attenuated. As a result, it appears that the observed decresae in the elastic intensity is real. A loss of elastic intensity is expected if the surface were disordering4 Howver, there was no evidence of disordering in LEED. It is difficult to explain the decrease in the elastic intensity and the broadening of the elastic peak in the absence of evidence of an increase in disorder from LEED. Hence, no explanation will be offered. We do note that the poor spectral resolution makes it difficult to interpret our EELS spectra between 350 and 450 K. As a result, we do not want to speculate about what happens in this temperature range. Fortunately, the elastic intensity and resolution recovers upon annealing to 500 K. The 500 K EELS spectrum in Figure 6 shows peaks at 840, 2230, and 3045 cm-'. There is also a broad peak centered at 1290 cm-'. A 2220-cm-' peak has been observed previously by Lehwald et al.45 during acetylene adsorption on Ni(7 9 11). It is associated with a carbonaceous species which Lehwald et al. call "C,". The exact nature of the C2 species is unclear from our work. It could simply be two carbon atoms strung together or it could be some other form of carbonaceous deposit with carbon-carbon triple bonds i.e. [-C=C-],. In any case, the C2 species is observed on (2x1) Pt(ll0) and on another stepped surface Ni(7 9 11). However, the C2 species is not seen on Pt( 11 1),l8Pt( or Ni( 111).40 Thus, it seems likely that the C2 species is associated with the steps on the Pt( 1 10) surface. The C2species is more prominent at low coverages than at high coverages. Ethylylidyne is too. Thus, we propose that the C2 species forms from ethylylidyne on the (2X 1) Pt( 1 10) surface. At 300 K there is also ethylidyne on the surface at high coverages. The ethylidyne decomposes via a different mechanism than the ethylyldiyne, since at 500 K there are peaks at 840, 1290, and 3045 cm-' which are strongly attenuated at low coverages. Very similar EELS peaks are also seen during ethylidyne decomposition at high temperatures on many surfaces including Pt(l1 l), as indicated in Table 111. At present, the mechanism of ethylidyne decomposition has not been conclusively established. However, the best evidence is that, on Pt( 1 1 l ) , the ethylidynedecomposes to yield a mixture of methylidynes, multiply bound CCH species, and adsorbed carbon.,' We propose that very similar chemistry also occurs on the terraces of (2x1) Pt( 110). Comparison to the reference spectra given in Table I11 indicates the possibility of there also being various acetylenic species on the surface. Thus, we propose that the decomposition of ethylidyne is very similar on (2x1) Pt(ll0) to the decomposition chemistry
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1072 The Journal of Physical Chemistry, Vol. 94, No. 3, 1990
Yagasaki et al.
TABLE 111: A Comparison of the Peak Positions Observed at 500 K during Ethylene Adsorption on (2x1) Pt(ll0) to the Peak Positions in Spectra Reported Previously tentative C2H4 on (2x1) "C2" on Ni C2H4 on 470 K butterfly methylidyne on assignment Pt( 110) at 500 K (1 1 9 7)40*4',47 Pt( 1 11)'6 complex46 Ni( 11 1)47 M-C stretch CH bend CH def C-C stretch CH, stretch tentative assignment M-C stretch CD bend CD def C-C stretch CD, stretch
270," 520 840 1290' 2230, 1290' 3045 C2D4on (2x1) Pt( 110) at 500 K 270," 570 680 940 1270, 2250 2250
350
- -
420 830 1430" 1 170" 3000
2220 C2 on Ni (1 1 9 7)45,47 350
590 837 1120 1199 3020 d4-butterfly complex46
2980 CD on Ni( 1 1
550
630 910 1185 2250
2200
780
2160
"It is unclear whether this mode is associated with a carbon-metal stretch, a change in the phonon spectrum of the surface, or an impurity. 'Center of broadband. "Small peaks on broad band. Pt(l11) and (5~20)Pt(l00)
100 K Y
Y
1
300 K
500 K
(1xl )Pt(lOO) HPH
3
Qq -g=J? 500 K
700 K
Figure 9. Mechanism of ethylene decomposition on (2x1) Pt(l10).
on Pt( 1 1 1). However, we cannot exclude extra chemistry at the steps. The high-temperature spectra in Figures 3 and 6 show that the methylidynes and multiply bound CCH species decompose above 500 K. However, the C2 species are stable to high temperature. In summary then, we propose that the decomposition of ethylene on (2x1) Pt(1 10) follows the mechanism shown in Figure 9. The ethylene adsorbs molecularly on the (2x1) Pt(ll0) surface to yield a mixture of di-a and x-bound ethylene. Some of the adsorbed ethylene desorbs upon heating, some reacts to form ethane, and some decomposes to yield a mixture of ethylidyne, ethylylidyne, and hydrogen. Additional hydrogen is liberated upon further heating, to produce a mixture of C2 species, methylidynes, multiply bound CCH species, acetylenic complexes, and adsorbed carbon. It is useful to compare the mechanism in Figure 9 to the mechanism of ethylene decomposition on other faces of platinum. Previous investigators have examined ethylene adsorption and decomposition on Pt(l1 1),'-24 (5x20) Pt(100), and (1x1) Pt( 100).26*28Figure 10 summarizes the chemistry on these other faces. The ethylene adsorbs molecularly to form a di-a complex on all three faces. On P t ( l l 1 ) and (5x20) Pt(100), the di-o complex decomposes to form ethylidyne at 320 K and a mixture of CH, and CCH, (x = 1, 2) species above 400 K. In contrast, on (1 X 1) Pt( loo), the di-a species reacts to form vinylidene at 300 K and a quad-a acetylene at 400 K. So far, there has not been any evidence for the presence of the C2 complex or ethylylidyne on any face of platinum except (2x1) Pt(ll0). Thus, it appears that the (2x1) Pt( 110) surface shows unique ethylene decomposition chemistry as expected t h e ~ r e t i c a l l y . ~ ~ * ~ ' Conclusion In summary, we find that ethylene decomposition on (2x1) Pt( 1 10) is considerably different than that seen on the closed packed faces of platinum. We propose that, when ethylene adsorbs
100 K
300 K
400 K
Figure 10. A summary of the mechanism of ethylene decomposition on platinum as proposed in ref 13, 16, 18, 28, and 30: (a) Pt( 11 1) and (5x20) Pt(100), (b) (1x1) Pt(100).
on (2X 1) Pt( 1lo), it follows the decomposition pathway shown in Figure 9. Initially, there is a mixture of di-a and x-bound species on the surface. Some of the di-r and x-bound species desorb upon heating to 300 K, while others react to form a mixture of ethylylidyne, ethylidyne, and hydrogen. The ethylylidyne then decomposes to yield C2 species and carbon, while the ethylidyne decomposes to yield a mixture of multiply bound CCH species, methylidyne, and adsorbed carbon. The C2 and ethylylidyne intermediates are unique to (2x1) Pt(l10); they are not seen during ethylene decomposition on Pt( 11 l), (5x20) Pt( loo), or (1 X 1) Pt( 100). Thus, it seems that the intermediates which form when ethylene adsorbs on platinum depend strongly on the surface structure and are not simply a combination of the chemistry seen on the closed packed faces of this metal. Acknowledgment. It is a pleasure to dedicate this paper to Professor Harry Drickamer. While Professor Drickamer did not contribute to this work directly, it is his leadership in the department and in the profession that made the work possible. Professor Drickamer was one of the first chemical engineers to apply modern physical chemical methods to engineering problems. He has been the intellectual leader in the Chemical Engineering Department at Illinois for many years. It was Professor Drickamer's influence that created the environment which allowed work like this to succeed. Therefore, it is a pleasure to dedicate this paper to him. This work was supported by the National Science Foundation under Grant CBT 86- 13258. Sample preparation was done using the facilities of the University of Illinois Center for Microanalysis of Materials which is supported as a national facility, under National Science Foundation Grant DMR 86- 12860. Equipment was provided by N S F grants CPE 83-51648 and CBT 87-04667. The experimental assistance of Ms. Bin Chen and Dr. Won Ho Lee is greatly appreciated. Registry No. Pt, 7440-06-4; ethylene, 74-85-1; ethylene-d4, 683-73-8; ethylidyne, 67624-57- 1; ethan- 1-yl-2-ylidyne, 6732 1-66-8; methylidyne, 33 15-37-5.
