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The adsorption and decomposition of ethylene on Pt(210), (1 · 1)Pt(110) and (2 · 1)Pt(110)
Vacuum/volume41/numbers 1-3/pages 57 to 59/1990
0042-207X/9053.00 + .00 © 1990 Pergamon Press plc
Printed in Great Britain
The adsorption and decomposition of ethylene on Pt(210), (1 x 1)Pt(110) and (2 x 1)Pt(110) E Yagasaki, A L Backman
and R I Masel,
University of Illinois, 1209 W. California St, Urbana, IL 61801,
USA
This paper summarizes the results of a series of previous papers where the adsorption and decomposition of ethylene were examined using mainly TPD and EELS. The results show that at 110 K the ethylene adsorbs into a type 2 rt-bound complex on Pt(210). Some of the ~-bound ethylene desorbs upon annealing to 250 K, and a small fraction is converted into ethane, and adsorbed methyl groups. However, the majority of the ~-bound ethylene is stable to almost 300 K. A t 330 K, the ~-bound ethylene is converted to ethylylidyne (P4 - C C H 2 - ) . Upon further heating, the ethylylidyne and methyl groups undergo a series of dehydrogenations and carbon-carbon bond scission processes. Only adsorbed carbon atoms and C2 species ( - [ C - C]x-)are seen at 700 K. Ethylene also adsorbs into a rt-bound complex on a 93 K (1 x 1)Pt(110) sample. However, the 7t-bound ethylene is partially converted to a di-cr species upon heating to 160 K. Some of the di-¢ and ~-bound ethylene reacts to form to methane and carbon atoms between 270 and 330 K, while the remainder reacts to form ethylylidyne. The ethylylidyne quickly dehydrogenates upon further heating so that only adsorbed carbon atoms and C2 species ( - [ C - C]x-) are seen at 450 K. The chemistry is more complex on (2 x 1)Pt(110). A mixture of di-a and rt-bound ethylene forms when ethylene adsorbs on (2 x 1) Pt(110) at 93 K. At low coverage, the mixture is converted to ethylidyne upon heating to 330 K. However, at high coverages, mixture of ethylidyne (P3 = C C H 3 ) and ethylylidyne is seen. Again there are a series of dehydrogenations and carbon-carbon bond scission processes upon further heating. Only adsorbed carbon atoms and C2 species ( - [ C - C]x-) are seen at 750 K.
Introduction Over the past several years, the adsorption and decomposition of ethylene on platinum single crystals has been the subject of significant investigation. There have been many papers on ethylene adsorption on P t ( l l l ) as reviewed in ref 1. There also has been some work on ethylene adsorption on Pt(100) *-6. However, the adsorption of ethylene on stepped platinum surfaces has not been extensively studied. Nevertheless, calculations 6'7 indicate that different intermediates should form on stepped crystals than on closed packed planes. In the previous literature, Wesner e t al 8 examined ethylene adsorption at 300 K on P t ( l l 0 ) and P t ( l l l ) with angular resolved XPS. They found that the carbon-carbon bond axis in the adsorbed ethylene was oriented differently on Pt(110) than on Pt(l 11). However, it was unclear from this study whether different intermediates were forming on P t ( l l l ) and P t ( l l 0 ) or whether instead the same intermediates were forming on the two surfaces, and only the geometry of the two intermediates changed. In recent papers 9-11 we examined ethylene adsorption on Pt(210), (1 × 1) Pt(ll0), and (2 x I) P t ( l l 0 ) with Temperature Programmed Desorption (TPD) and Electron Energy Loss Spectroscopy (EELS) and found that different intermediates form on (I x l)Pt(ll0), (2 x 2)Pt(ll0), and Pt(210) than on P t ( l l 1) or Pt(100). Here, we will summarize our findings. One should refer to reference 9-t~ for additional details.
Experimental The experiments presented here were done using standard surface spectroscopic techniques as described in refs 9-12. The
apparatus was of standard design with a base pressure of 5 x 10 -I1 torr. Pt(210) and P t ( l l 0 ) samples were cut from a Metron single crystal rod. Each sample was polished with diamond paste, and then mounted in the vacuum system. The sample was then oxidized, sputtered and annealed until no impurities could be detected by AES and a sharp (1 x 1)Pt(210) or (2 x 1)Pt(110) LEED pattern was seen. The sample was then exposed to a measured amount of ethylene through a capillary array doser. Subsequently, the surface was examined with TPD, EELS, and LEED. The clean (2 x 1)Pt(110) sample was also converted to a (1 x 1) reconstruction following the procedure of Ferrer and Bonze113. One is referred to ref 10 for details.
Results Figure 1 shows composite T P D spectra for ethylene adsorption on (1 x 1)Pt(ll0), (2 x 1)Pt(ll0), and Pt(210). Four desorption products were detected during ethylene decomposition on (1 x 1) P t ( l l 0 ) : ethylene (27AMU), ethane (30AMU), hydrogen (2 AMU) and methane (15 and 16 AMU). Integration of the peak areas and correcting for the relative sensitivities of our mass spectrometer and relative pumping speeds indicates that the ethylene, methane, and hydrogen desorb in the ratio 1: 1.2: 2.5. There was some variation in the amount of ethane which desorbed, which we associate with variations in the amount of background hydrogen in the vacuum system. In contrast, only three species were observed during ethylene adsorption on Pt(210) ethylene, ethane, and hydrogen. Integration of the peak areas and correcting for the relative sensitivities of our mass spectrometer and relative pumping speeds indicates 57
E Yagasaki et al: Ethylene on Pt(210), (1 x 1 ) Pt(11 O) and (2 x 1 ) Pt(11 O)
(lxl) Pt(llO)
(2xl) Pt(llO)
Figure 2 compares a series of EELS spectra of ethylene adsorbed on (i x 1) Pt(ll0), (2 x l) P t ( l l 0 ) and Pt(210). They are all very similar. We find that the EELS spectrum of ethylene on a Pt(210) sample at 1 I0 K, has peaks in the same positions as those in Zeises salt. Zeises salt is a prototype of a type 2 n - b o u n d intermediate ~. Therefore we propose that on Pt(210) at 110 K, the ethylene adsorbs into a type 2 n - b o u n d complex. Some of the n - b o u n d ethylene desorbs upon annealing to 250 K, and a small fraction is converted into ethane. There is also a new peak at 1370 cm-~ in Figure 2, which we associate with methyl groups. However, the majority of the n - b o u n d ethylene is stable to almost 300 K. At 300-330 K, the n - b o u n d ethylene is converted to a new species. The stoichiometry of the new species is CCH=, and it has an EELS spectrum very similar to the ir spectrum of 1, 1, l, 2 tetrachloroethane. Therefore, we assign the species to ethylylidyne (/a,~ - C C H 2 - ). U p o n further heating, the ethylylidyne and methyls undergo a series of dehydrogenations and c a r b o n - c a r b o n bond scission processes. There are intermediates such as ethylyl (/% - C C H = ), methylyne (l~3 = CH), C 2 species, and carbon atoms at temperatures between 350 and 600 K. However, only adsorbed carbon atoms and Cz species ( - [ C - C ] , - ) are seen at 700 K. Figure 2 also shows a series of EELS spectra taken during ethylene adsorption on (1 x l)Pt(110). As with Pt(210), when ethylene adsorbs on a 93 K (1 x 1)Pt(ll0) sample, a type 2nb o u n d complex forms. U p o n annealing to 160K, there are changes in the relative intensities of the modes, and growth of a
I Pt(210)
METHANE
j~_ROGEN
J
~ E N
.~OGEN
100 300 500 700 I00 300 500 700 100 300 500 700 Temperature, K
Figure I. A series of composite TPD spectra taken by exposing a clean (I x l)Pt(ll0), (2 x l)Pt(ll0) or Pt(210) sample to 1, 1.1, and 1.6 L of ethylene, respectively, then heating at 14 K s- ~.
that the ethylene, hydrogen and ethane desorb in the ratio I: 0.65: 0.16. The same three species were detected during ethylene decomposition on (2 x 1)Pt(ll0). Integration of the peak areas and correcting for the relative sensitivities of our mass spectrometer and relative pumping speeds indicates that the ethylene, hydrogen, and ethane desorb in the ratio 1 : 1.3: 0.07.
(lxl)Pt(110)
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. 3000
4000
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1000
2000
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Figure 2., A series of EELS spectra taken by exposing a clean (1 x l)Pt(110), (2 x 1)Pt(110) or Pt(210) sample to ethylene then sequentially annealing to the temperatures indicated. The exposures are 0.24 L on Pt(210) and 1.0 L on (1 x 1)Pt(110) and (2 x 1)Pt(110). 58
E Yagasaki et al: Ethylene on Pt(210), (1 x 1 )Pt(110) and (2 x 1 )Pt(110)
new mode at 1050 cm-~. These changes are indicative of the 7t-bound ethylene being partially converted to a di-a species upon heating to 160 K ]°. At higher temperatures, TPD shows that some of the di-a and 7t-bound ethylene reacts to form to methane (and carbon atoms) between 270 and 330 K, while EELS shows that the remainder reacts to form the species with an EELS spectrum which we associate with ethylylidyne. Both the EELS and TPD results show that the ethylylidyne quickly dehydrogenates upon further heating so that only adsorbed carbon atoms and C 2 species (-[C = C]x-) are seen at 450 K. The chemistry is more complex on (2 x 1)Pt(1 I0). We observe many more modes during ethylene adsorption on (2 x 1)Pt(110) than we do on (1 x 1)Pt(110) or Pt(210). At 100 K, there are two peaks which do not shift greatly upon deuteration, a 1210 c m peak which we associate with the carbon-carbon mode in rebound ethylene, and a 1050 cm- l peak which we associate with the carbon-carbon stretch in di-tr ethylene. Therefore, we conclude that both di-a and type 2 It-bound ethylene form when ethylene adsorbs onto a (2 x 1)Pt(110) sample at 93 K. Upon heating, we again see a mixture of species. The TPD data shows that at low coverage, the ethylene is converted to CCH 2 intermediate upon heating to 330 K. However, Figure 2 shows that at high coverages, the adsorbed ethylene is converted to a mixture ofCCH 2 and CCH 3 species at 330 K. The EELS spectra are also coverage dependent. We have identified the peaks that are prominent at low coverage as being associated with the CCH2 species and the peaks which grow more quickly with coverage than the 1420 c m - ~ peak as being associated with the CCH 3 species. The peak positions associated with the CCH 3 species are in good agreement with those measured for ethylidyne (1~3 -= CCH3) on P t ( l l l ) by Ibach et al 2"3, while the CCH 2 species has the EELS spectrum which we associate with ethylylidyne (p,~ = CCH2- ). Therefore, we conclude that there is a mixture of ethylidyne and ethylylidyne on the (2 x 1)Pt(ll0) surface at 330 K. As with Pt(210) there are a series of dehydrogenations and carbon-carbon bond scission processes upon further heating. Only adsorbed carbon atoms and C 2 species (-[C --- C]~-) are seen at 700 K.
Discussion The results here show that ethylene adsorption on stepped platinum surfaces is quite different from that on the closed packed planes of platinum examined previously. At 100 K we observe type 2n-bound ethylene on Pt(210) and (1 x 1)Pt(110), and a mixture of di-a and type 2n-bound ethylene on (2 x 1) Pt(110). By comparison, at 100 K only di-a ethylene is seen on
P t ( l l l ) 2.a and Pt(100) 6. At 300 K, we observe ethylylidyne on Pt(210) and (1 x 1)Pt(ll0), and a mixture of ethylylidyne and ethylidyne on (2 x l)Pt(110). By comparison, ethylidyne is seen at the same conditions on Pt(11 I) 2. a and (5 x 20)Pt(100) 6 while di-tr vinylidene is seen at 290K on (1 x l)Pt(100). We also observe significant methane formation on (1 x 1)Pt(ll0) and traces of methane formation on Pt(210). No methane formation is observed during ethylene decomposition on P t ( l l l ) , (5 x 20)Pt(100), or (1 x 1)Pt(100). Thus, it seems that the chemistry of ethylene decomposition on (1 x 1)Pt(ll0), (2 x 1)Pt(ll0), and Pt(210) is quite different than that on P t ( l l l ) , (5 x 20)Pt(100), and (1 x 1)Pt(100).
Conclusions In summary then, we find that ethylene decomposition on Pt(210), (1 x 1)Pt(110), and (2 x l)Pt(110) is very different than ethylene decomposition on P t ( l l l ) or Pt(100). We observe rebound ethylene, methyls, and ethylylidyne on (1 x 1)Pt(l I0) and Pt(210), and n-bound ethylene, And ethylylidyne on (2 x l)Pt(ll0). None of these species is observed during ethylene decomposition on Pt(111) or Pt(100). We also observe methane desorption is detected during ethylene decomposition on Pt(111), Pt(100), or (2 x l)Pt(110). Thus, the decomposition of ethylene on stepped surfaces is quite different than ethylene decomposition on the fiat surfaces of platinum.
Acknowledgements 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 Mircoanalysis of Materials which is supported as a national facility, under National Science Foundation Grant DMR 86-12860. Equipment was provided by NSF grants CPE 83-51648 and CBT 87-04667.
References i N Sheppard, Ann Rev Phys Chem, 39, 589 (1988). 2 H lbach and S Lehwald, J Vac Sci Technol, 15, 407 (1978). 3 H Steininger, H Ibaeh and S Lehwald, Surface Sci, 117, 685 (1982). 4 T E Fischer and S R Kelemen, Surface Sci, 69, 485 (1977). s G H Hatzikos and R I Masel, Surface Sci, 185, 479 (1987). 6 G H Hatzikos and R I Masel, In Catalysis 1987 (Edited by J W Ward), p883. Elsevier, Amsterdam (1988). 7 G H Hatzikos and R ! Masel, Unpublished work. 8 D A Wesner, F P Coenen and H P Bonzel, J Vac Sci Technol, A5, 927 (1987). 9 E Yagasaki, A L Backman and R I Masel, J Phys Chem, To appear. 1o E Yagasaki and R I Masel, Surface Sci, To appear. 11 A L Baekman and R I Masel, Submitted. 12 A L Backman, PhD Dissertation, University of Illinois (1989). 13 S Ferrer and H P Bonzel, Surface Sci, 119, 234 (1982).
59
0042-207X/9053.00 + .00 © 1990 Pergamon Press plc
Printed in Great Britain
The adsorption and decomposition of ethylene on Pt(210), (1 x 1)Pt(110) and (2 x 1)Pt(110) E Yagasaki, A L Backman
and R I Masel,
University of Illinois, 1209 W. California St, Urbana, IL 61801,
USA
This paper summarizes the results of a series of previous papers where the adsorption and decomposition of ethylene were examined using mainly TPD and EELS. The results show that at 110 K the ethylene adsorbs into a type 2 rt-bound complex on Pt(210). Some of the ~-bound ethylene desorbs upon annealing to 250 K, and a small fraction is converted into ethane, and adsorbed methyl groups. However, the majority of the ~-bound ethylene is stable to almost 300 K. A t 330 K, the ~-bound ethylene is converted to ethylylidyne (P4 - C C H 2 - ) . Upon further heating, the ethylylidyne and methyl groups undergo a series of dehydrogenations and carbon-carbon bond scission processes. Only adsorbed carbon atoms and C2 species ( - [ C - C]x-)are seen at 700 K. Ethylene also adsorbs into a rt-bound complex on a 93 K (1 x 1)Pt(110) sample. However, the 7t-bound ethylene is partially converted to a di-cr species upon heating to 160 K. Some of the di-¢ and ~-bound ethylene reacts to form to methane and carbon atoms between 270 and 330 K, while the remainder reacts to form ethylylidyne. The ethylylidyne quickly dehydrogenates upon further heating so that only adsorbed carbon atoms and C2 species ( - [ C - C]x-) are seen at 450 K. The chemistry is more complex on (2 x 1)Pt(110). A mixture of di-a and rt-bound ethylene forms when ethylene adsorbs on (2 x 1) Pt(110) at 93 K. At low coverage, the mixture is converted to ethylidyne upon heating to 330 K. However, at high coverages, mixture of ethylidyne (P3 = C C H 3 ) and ethylylidyne is seen. Again there are a series of dehydrogenations and carbon-carbon bond scission processes upon further heating. Only adsorbed carbon atoms and C2 species ( - [ C - C]x-) are seen at 750 K.
Introduction Over the past several years, the adsorption and decomposition of ethylene on platinum single crystals has been the subject of significant investigation. There have been many papers on ethylene adsorption on P t ( l l l ) as reviewed in ref 1. There also has been some work on ethylene adsorption on Pt(100) *-6. However, the adsorption of ethylene on stepped platinum surfaces has not been extensively studied. Nevertheless, calculations 6'7 indicate that different intermediates should form on stepped crystals than on closed packed planes. In the previous literature, Wesner e t al 8 examined ethylene adsorption at 300 K on P t ( l l 0 ) and P t ( l l l ) with angular resolved XPS. They found that the carbon-carbon bond axis in the adsorbed ethylene was oriented differently on Pt(110) than on Pt(l 11). However, it was unclear from this study whether different intermediates were forming on P t ( l l l ) and P t ( l l 0 ) or whether instead the same intermediates were forming on the two surfaces, and only the geometry of the two intermediates changed. In recent papers 9-11 we examined ethylene adsorption on Pt(210), (1 × 1) Pt(ll0), and (2 x I) P t ( l l 0 ) with Temperature Programmed Desorption (TPD) and Electron Energy Loss Spectroscopy (EELS) and found that different intermediates form on (I x l)Pt(ll0), (2 x 2)Pt(ll0), and Pt(210) than on P t ( l l 1) or Pt(100). Here, we will summarize our findings. One should refer to reference 9-t~ for additional details.
Experimental The experiments presented here were done using standard surface spectroscopic techniques as described in refs 9-12. The
apparatus was of standard design with a base pressure of 5 x 10 -I1 torr. Pt(210) and P t ( l l 0 ) samples were cut from a Metron single crystal rod. Each sample was polished with diamond paste, and then mounted in the vacuum system. The sample was then oxidized, sputtered and annealed until no impurities could be detected by AES and a sharp (1 x 1)Pt(210) or (2 x 1)Pt(110) LEED pattern was seen. The sample was then exposed to a measured amount of ethylene through a capillary array doser. Subsequently, the surface was examined with TPD, EELS, and LEED. The clean (2 x 1)Pt(110) sample was also converted to a (1 x 1) reconstruction following the procedure of Ferrer and Bonze113. One is referred to ref 10 for details.
Results Figure 1 shows composite T P D spectra for ethylene adsorption on (1 x 1)Pt(ll0), (2 x 1)Pt(ll0), and Pt(210). Four desorption products were detected during ethylene decomposition on (1 x 1) P t ( l l 0 ) : ethylene (27AMU), ethane (30AMU), hydrogen (2 AMU) and methane (15 and 16 AMU). Integration of the peak areas and correcting for the relative sensitivities of our mass spectrometer and relative pumping speeds indicates that the ethylene, methane, and hydrogen desorb in the ratio 1: 1.2: 2.5. There was some variation in the amount of ethane which desorbed, which we associate with variations in the amount of background hydrogen in the vacuum system. In contrast, only three species were observed during ethylene adsorption on Pt(210) ethylene, ethane, and hydrogen. Integration of the peak areas and correcting for the relative sensitivities of our mass spectrometer and relative pumping speeds indicates 57
E Yagasaki et al: Ethylene on Pt(210), (1 x 1 ) Pt(11 O) and (2 x 1 ) Pt(11 O)
(lxl) Pt(llO)
(2xl) Pt(llO)
Figure 2 compares a series of EELS spectra of ethylene adsorbed on (i x 1) Pt(ll0), (2 x l) P t ( l l 0 ) and Pt(210). They are all very similar. We find that the EELS spectrum of ethylene on a Pt(210) sample at 1 I0 K, has peaks in the same positions as those in Zeises salt. Zeises salt is a prototype of a type 2 n - b o u n d intermediate ~. Therefore we propose that on Pt(210) at 110 K, the ethylene adsorbs into a type 2 n - b o u n d complex. Some of the n - b o u n d ethylene desorbs upon annealing to 250 K, and a small fraction is converted into ethane. There is also a new peak at 1370 cm-~ in Figure 2, which we associate with methyl groups. However, the majority of the n - b o u n d ethylene is stable to almost 300 K. At 300-330 K, the n - b o u n d ethylene is converted to a new species. The stoichiometry of the new species is CCH=, and it has an EELS spectrum very similar to the ir spectrum of 1, 1, l, 2 tetrachloroethane. Therefore, we assign the species to ethylylidyne (/a,~ - C C H 2 - ). U p o n further heating, the ethylylidyne and methyls undergo a series of dehydrogenations and c a r b o n - c a r b o n bond scission processes. There are intermediates such as ethylyl (/% - C C H = ), methylyne (l~3 = CH), C 2 species, and carbon atoms at temperatures between 350 and 600 K. However, only adsorbed carbon atoms and Cz species ( - [ C - C ] , - ) are seen at 700 K. Figure 2 also shows a series of EELS spectra taken during ethylene adsorption on (1 x l)Pt(110). As with Pt(210), when ethylene adsorbs on a 93 K (1 x 1)Pt(ll0) sample, a type 2nb o u n d complex forms. U p o n annealing to 160K, there are changes in the relative intensities of the modes, and growth of a
I Pt(210)
METHANE
j~_ROGEN
J
~ E N
.~OGEN
100 300 500 700 I00 300 500 700 100 300 500 700 Temperature, K
Figure I. A series of composite TPD spectra taken by exposing a clean (I x l)Pt(ll0), (2 x l)Pt(ll0) or Pt(210) sample to 1, 1.1, and 1.6 L of ethylene, respectively, then heating at 14 K s- ~.
that the ethylene, hydrogen and ethane desorb in the ratio I: 0.65: 0.16. The same three species were detected during ethylene decomposition on (2 x 1)Pt(ll0). Integration of the peak areas and correcting for the relative sensitivities of our mass spectrometer and relative pumping speeds indicates that the ethylene, hydrogen, and ethane desorb in the ratio 1 : 1.3: 0.07.
(lxl)Pt(110)
\ 4a0,e0o looo
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4 ~ 7%0 w ~ . J 4
/~
~L~_J~--31oK
9s
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~
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Pt(210)
vll'~,°,,o
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~
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.... .__ = ~
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2950
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,,
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2000
E n e r g y Loss, c m
~ . ~J _t _~_, .~; j ~ _ , o o ~
. 3000
4000
0
1000
2000
3000
4000
-1
Figure 2., A series of EELS spectra taken by exposing a clean (1 x l)Pt(110), (2 x 1)Pt(110) or Pt(210) sample to ethylene then sequentially annealing to the temperatures indicated. The exposures are 0.24 L on Pt(210) and 1.0 L on (1 x 1)Pt(110) and (2 x 1)Pt(110). 58
E Yagasaki et al: Ethylene on Pt(210), (1 x 1 )Pt(110) and (2 x 1 )Pt(110)
new mode at 1050 cm-~. These changes are indicative of the 7t-bound ethylene being partially converted to a di-a species upon heating to 160 K ]°. At higher temperatures, TPD shows that some of the di-a and 7t-bound ethylene reacts to form to methane (and carbon atoms) between 270 and 330 K, while EELS shows that the remainder reacts to form the species with an EELS spectrum which we associate with ethylylidyne. Both the EELS and TPD results show that the ethylylidyne quickly dehydrogenates upon further heating so that only adsorbed carbon atoms and C 2 species (-[C = C]x-) are seen at 450 K. The chemistry is more complex on (2 x 1)Pt(1 I0). We observe many more modes during ethylene adsorption on (2 x 1)Pt(110) than we do on (1 x 1)Pt(110) or Pt(210). At 100 K, there are two peaks which do not shift greatly upon deuteration, a 1210 c m peak which we associate with the carbon-carbon mode in rebound ethylene, and a 1050 cm- l peak which we associate with the carbon-carbon stretch in di-tr ethylene. Therefore, we conclude that both di-a and type 2 It-bound ethylene form when ethylene adsorbs onto a (2 x 1)Pt(110) sample at 93 K. Upon heating, we again see a mixture of species. The TPD data shows that at low coverage, the ethylene is converted to CCH 2 intermediate upon heating to 330 K. However, Figure 2 shows that at high coverages, the adsorbed ethylene is converted to a mixture ofCCH 2 and CCH 3 species at 330 K. The EELS spectra are also coverage dependent. We have identified the peaks that are prominent at low coverage as being associated with the CCH2 species and the peaks which grow more quickly with coverage than the 1420 c m - ~ peak as being associated with the CCH 3 species. The peak positions associated with the CCH 3 species are in good agreement with those measured for ethylidyne (1~3 -= CCH3) on P t ( l l l ) by Ibach et al 2"3, while the CCH 2 species has the EELS spectrum which we associate with ethylylidyne (p,~ = CCH2- ). Therefore, we conclude that there is a mixture of ethylidyne and ethylylidyne on the (2 x 1)Pt(ll0) surface at 330 K. As with Pt(210) there are a series of dehydrogenations and carbon-carbon bond scission processes upon further heating. Only adsorbed carbon atoms and C 2 species (-[C --- C]~-) are seen at 700 K.
Discussion The results here show that ethylene adsorption on stepped platinum surfaces is quite different from that on the closed packed planes of platinum examined previously. At 100 K we observe type 2n-bound ethylene on Pt(210) and (1 x 1)Pt(110), and a mixture of di-a and type 2n-bound ethylene on (2 x 1) Pt(110). By comparison, at 100 K only di-a ethylene is seen on
P t ( l l l ) 2.a and Pt(100) 6. At 300 K, we observe ethylylidyne on Pt(210) and (1 x 1)Pt(ll0), and a mixture of ethylylidyne and ethylidyne on (2 x l)Pt(110). By comparison, ethylidyne is seen at the same conditions on Pt(11 I) 2. a and (5 x 20)Pt(100) 6 while di-tr vinylidene is seen at 290K on (1 x l)Pt(100). We also observe significant methane formation on (1 x 1)Pt(ll0) and traces of methane formation on Pt(210). No methane formation is observed during ethylene decomposition on P t ( l l l ) , (5 x 20)Pt(100), or (1 x 1)Pt(100). Thus, it seems that the chemistry of ethylene decomposition on (1 x 1)Pt(ll0), (2 x 1)Pt(ll0), and Pt(210) is quite different than that on P t ( l l l ) , (5 x 20)Pt(100), and (1 x 1)Pt(100).
Conclusions In summary then, we find that ethylene decomposition on Pt(210), (1 x 1)Pt(110), and (2 x l)Pt(110) is very different than ethylene decomposition on P t ( l l l ) or Pt(100). We observe rebound ethylene, methyls, and ethylylidyne on (1 x 1)Pt(l I0) and Pt(210), and n-bound ethylene, And ethylylidyne on (2 x l)Pt(ll0). None of these species is observed during ethylene decomposition on Pt(111) or Pt(100). We also observe methane desorption is detected during ethylene decomposition on Pt(111), Pt(100), or (2 x l)Pt(110). Thus, the decomposition of ethylene on stepped surfaces is quite different than ethylene decomposition on the fiat surfaces of platinum.
Acknowledgements 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 Mircoanalysis of Materials which is supported as a national facility, under National Science Foundation Grant DMR 86-12860. Equipment was provided by NSF grants CPE 83-51648 and CBT 87-04667.
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