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This document reports a study on the adsorption of ethylene on pd(111) surface using reflection-absorption infrared spectroscopy. The researchers confirm the presence of ethylidyne species on the surface due to the presence of a vibrational mode at 1329 cm-1. The study reveals that ethylidyne is present on the surface in the presence of gas-phase ethylene and that there may be a slight increase in coverage.
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ELSEVIER Surface Science 391 (1997) 145 149
Reflection-absorption infrared spectroscopy of ethylene on
palladium( 111 ) at high pressure
M. Kaltchev, A.W. Thompson, W.T. Tysoe * Deparmwnt g/'Chendstry and Laborator3'ji~r Smjace Studies. Universityof Wisconsin Milwaukee, Mihraukee, W153211, USA Received 17 February 1997: accepted for publication 2 June 1997
Abstract
The adsorption of ethylene adsorbed on P d ( l l l ) at - 3 0 0 K is studied using reflection absorption infrared spectroscopy which confirms the formation of an ethylidyne species because of the presence of vibrational mode at 1329 cm 1 with a less-intense peak at 1089 cm ~. The 1329cm 1 methyl mode is well away from any vibrational modes of gas-phase ethylene, which allows the spectrum of the surface species to be collected in the presence of high pressures (up to ~ 1 tort) of ethylene. These results reveal that ethylidyne is present on the surface in the presence of gas-phase ethylene and that there may be a slight increase in coverage. The width of the line, however, increases substantially by 5.3±0.4cm ~torr ~. This effect is ascribed to a loss of order in the ethylidyne layer probably caused by co-adsorption of ethylene. <, 1997 Elsevier Science B.V.
Keywords: Alkenes: Chemisorption: Infrared absorption spectroscopy: Low index single crystal surfaces: Palladium: Reflection spectroscopy; Single crystal surfaces
during reaction under an external pressure of sev-
0039-6028/97/'$17 00 <~ 1997 Elsevier Science B.V. All rights reserved. Pll S0039-6028 ( 9 7 ) 0 0 4 7 5 - 5
146 M. Kaltchev et ell : Sur/ace Science 39l (1997J 145 149
148 M. Kaltchev et al. ," Surlhce Science 391 (1997) 145 149
data up to 1.0 torr indicating the continued pres- ence of ethylidyne under these conditions. The 1089 cm i feature is not shown in these data since it becomes obscured by gas-phase features as the pressure increases. It is clear, however, that the shape of the spectrum changes as the pressure increases so that the peak intensity decreases as the pressure increases and correspondingly the width a half maximum increases. These changes are documented in Figs. 3 and 4, which plot the integrated peak area of the 1329cm -1 peak (Fig. 3) and its variation in full-width at half maximum (Fig. 4), respectively, as a function of ethylene pressure. There is a slight increase in integrated peak area as the ethylene pressure increases by approximately 7% as the pressure changes to 1.0 torr. Note, however, that the error in the area measurement is sufficiently large that it is not clear whether this is statistically significant although there does seem to be a tendency to accommodate slightly more ethylidyne on the sur-
= 6.
a. 5.
3.5 I I I I i I 0.0 0.2 0,4 0.6 0.8 1. P r e s s u r e / T o r r Fig. 3. A plot of the integrated intensity of the 1329 cm ~ fea- ture in the RAIRS spectrum of P d ( l l l ) in the presence of ethylene plotted as a function of ethylene pressure.
E I> < .,t- ii
14
12
10
Slope = 5.3 cm-l.Torr q
2 I I I I I I 0.0 0.2 0.4 0.6 0.8 1. P r e s s u r e / t o r t Fig. 4. A plot of the Full width at half maximum of the 1329cm ~ feature in the RAIRS spectrum of Pd(lll)in the presence of ethylene plotted as a function of ethylene pressure.
face as the pressure increases. This is not too difficult to understand since any defect sites not occupied during adsorption in U H V are likely to become saturated under the influence of higher pressures. A linear regression fit to the data of Fig. 4 line shows that the line width varies substantially with pressure by 5.3_+0.4cm-~torr 1. It is easy to show that pressure broadening caused by collision with the gas-phase cannot account for this effect and the most generous estimate yields a maximum value of pressure broadening of ~ 1 0 - 4 c m l tort -1. An alternative explanation is that it is caused by a loss of order in the ethylidyne over- layer. It is not clear whether this is formed by the apparently extra ethylidyne accommodated onto the surface suggested by the data in Fig. 3. It is likely that this would result in an increase in order of ethylidyne species adsorbed on identical sites and a corresponding sharpening of the peaks. An alternative possible explanation is that defect struc- tures are formed on the surface in the presence of
M. Kaltchev et al. /Sulface Science 391 (1997) 145 -149 149
an external pressure o f ethylene due to the a d s o r p - tion of ethylene between a d s o r b e d ethylidyne species. N o t e that a d s o r b e d ethylene has been detected under high external ethylene pressures using sum frequency generation on P t ( l l l ) [2] and will p r o b a b l y also occur on P d ( l l l ). U n f o r t u n a t e l y , any possible ethylenic species are obscured in our experiment by the p r e p o n d e r a n c e of gas-phase ethylene. As noted above, experiments are now u n d e r w a y in which an a n n e a l e d foil is substituted for the single crystal which will allow high pressures of h y d r o g e n ( ~ s e v e r a l torr) to be a d d e d to the mixture. Since this is a h o m o n u c l e a r molecule, it is IR invisible and will allow the surface to be examined during a catalytic reaction.
Acknowledgements
We gratefully a c k n o w l e d g e s u p p o r t o f this work by the US D e p a r t m e n t o f Energy, Division o f
Chemical Sciences, Office o f Basic Energy Sciences, under grant no. D E - F G 0 2 - 9 2 E R 1 4 2 8 9.
References
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