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Ethylidyne Species on Palladium(111) Surface: Infrared Spectroscopy Study, Papers of Chemistry

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
surface science
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. University of Wisconsin Milwaukee, Mihraukee, W153211, USA
Received 17 February 1997: accepted for publication 2 June 1997
Abstract
The adsorption of ethylene adsorbed on Pd(lll) at -300 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
It has been suggested previously that transition-
metal catalyzed hydrocarbon reactions proceed in
the presence of a hydrocarbon layer. The initial
suggestion was based on a post-mortem analysis
of a platinum single crystal model catalyst
following ethylene hydrogenation where an
ordered ethylidyne layer was found [1]. Thus,
information on the nature of a catalytic surface
during
reaction under an external pressure of sev-
eral tort is crucial in determining how the catalytic
reaction proceeds. This nature of the catalytically
active surface has been probed in the presence of
a high external pressure ( ~ 1 tort) in a number of
* Corresponding author. Present address: University of
Wisconsin Milwaukee, Department of Chemistry, PO Box 213,
Milwaukee, WI 53201 0413, USA. Fax: (+1)414 229.5036:
e-mail: wtt@csd.uwm.edu
0039-6028/97/'$17 00 <~ 1997 Elsevier Science B.V. All rights reserved.
Pll
S0039-6028 (97)00475-5
ways; for example, using sum-frequency genera-
tion (SFG) [2] which is inherently surface sensitive,
fluorescence yield near-edge spectroscopy
(FYNES) [3] and infrared (IR) spectroscopy [4].
SFG has revealed the presence of ethylidyne
species on the surface during ethylene hydrogena-
tion. Unfortunately, this technique requires
extremely sophisticated equipment and is difficult
and time consuming. Over the last few years,
reflection-absorption IR spectroscopy has devel-
oped into a technique which, while not entirely
routine, has evolved to the point that it is relatively
accessible to most surface analysis laboratories. It
also has the advantage that the physical principles
and data analysis are extremely well developed.
Unfortunately, it is not inherently surface sensitive
so that interference from the gas phase can become
a problem at higher gas pressures.
pf3
pf4
pf5

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ELSEVIER Surface Science 391 (1997) 145 149

surface science

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

It has been suggested previously that transition-

metal catalyzed hydrocarbon reactions proceed in

the presence of a hydrocarbon layer. The initial

suggestion was based on a post-mortem analysis

of a platinum single crystal model catalyst

following ethylene hydrogenation where an

ordered ethylidyne layer was found [1]. Thus,

information on the nature of a catalytic surface

during reaction under an external pressure of sev-

eral tort is crucial in determining how the catalytic

reaction proceeds. This nature of the catalytically

active surface has been probed in the presence of

a high external pressure ( ~ 1 tort) in a number of

  • Corresponding author. Present address: University of Wisconsin Milwaukee, Department of Chemistry, PO Box 213, Milwaukee, WI 53201 0413, USA. Fax: ( + 1 ) 4 1 4 229.5036: e-mail: wtt@csd.uwm.edu

0039-6028/97/'$17 00 <~ 1997 Elsevier Science B.V. All rights reserved. Pll S0039-6028 ( 9 7 ) 0 0 4 7 5 - 5

ways; for example, using sum-frequency genera-

tion (SFG) [2] which is inherently surface sensitive,

fluorescence yield near-edge spectroscopy

(FYNES) [3] and infrared (IR) spectroscopy [4].

SFG has revealed the presence of ethylidyne

species on the surface during ethylene hydrogena-

tion. Unfortunately, this technique requires

extremely sophisticated equipment and is difficult

and time consuming. Over the last few years,

reflection-absorption IR spectroscopy has devel-

oped into a technique which, while not entirely

routine, has evolved to the point that it is relatively

accessible to most surface analysis laboratories. It

also has the advantage that the physical principles

and data analysis are extremely well developed.

Unfortunately, it is not inherently surface sensitive

so that interference from the gas phase can become

a problem at higher gas pressures.

146 M. Kaltchev et ell : Sur/ace Science 39l (1997J 145 149

There are several ways in which the above

problems can be overcome. In cases where the

adsorbate desorbs to leave a clean surface, merely

heating the sample in the high-pressure ambient

forms a clean surface to provide a background

spectrum in the presence of a high pressure of gas

[4]. This is then used to collect a background

spectrum. The sample is then allowed to cool to

the experimental temperature, the spectrum of

an adsorbate-covered surface is collected, and

rationing the two allows the adsorbate spectrum

to be collected. This strategy has been used to

advantage to follow the chemistry of CO and NO

on Pd( 111 ) [4]. Alumina-supported, model palla-

dium catalysts with relatively high surface areas

have been used [5]; these increase the surface

sensitivity by a factor of ~20 relative to a planar

single crystal sample, which allows IR spectra of

surface species to be easily collected in the presence

of up to ~ 1 tort of ethylene. Unfortunately, the

homogeneity of the model single crystal catalyst is

lost in this case.

Ethylidyne, and both 7r and di-a-bonded ethy-

lene, have been detected on Pt( 111 ) in the presence

of pressures of up to ~ 5 x 10-s torr of ethylene

[6]. Unfortunately, this pressure is several orders

of magnitude below that which could be considered

a catalytic regime. We have, however, extended

this approach by studying a system where the

surface species are sufficiently different from the

gas-phase precursor that their peaks can be iden-

tified in the windows between the gas-phase fea-

tures and which therefore allows their spectra to

be collected in the presence of an external gas-

pressure. We have used this strategy to examine

the nature of the species present on the surface of

a palladium single crystal in the presence of a high

pressure (up to ~ 1 torr) of ethylene much higher

pressures than have been attained previously on a

model single crystal catalyst. Unfortunately, the

reactivity of the surface species with high pressures

of hydrogen could not be probed using the Pd( 111 )

single crystal used for these experiments because

of the tendency of palladium to absorb hydrogen.

We are currently exploring the possibility of carry-

ing out these reactions using foils.

The IR sample cell used for these experiments

is constructed from a "~" flange,-4 six-way cross,

which had been modified by moving one flange by

~20 ~' to allow IR radiation to impinge on the

sample with the optimal 80'-' IR incidence angle.

The cell is attached to the main chamber via a

gate valve, which, when closed, completely isolates

the IR cell from the ultrahigh vacuum ( U H V )

chamber and, when open, allows sample transfer

into it.

The IR optical train is mounted onto a Y'-thick

optical table that is mounted to the same frame as

the UHV chamber. Light from a Midac M2000,

Fourier-transform IR spectrometer is steered to an

off-axis parabola mirror placed ~ 10 cm from the

sample. Test experiments using visible radiation

shows an image size of approximately 0.Scm.

Plane-polarized radiation is obtained by passing

the light through a Harrick polarizer made by

placing chevrons of germanium at the Brewster

angle. The whole of this optical path is enclosed

in plexiglass boxes which are purged with dry air

from a Whatman air drier or using nitrogen boil

off" from a MVE Cryogenics Dewar. Light that is

reflected from the sample is steered via two concave

mirrors to a liquid-nitrogen-cooled, mercury cad-

mium telluride ( M C T ) IR detector (Graseby

Infrared, USA).

The IR spectrometer is controlled using

SpectraCalc software running on a microcomputer.

This collected and transformed the signal and has

capabilities for smoothing and plotting the data.

The background and adsorbate-covered spectra

were collected for ~ 1000 scans depending of the

desired level of signal:noise ratio with a spectral

resolution of 4 c m - 1. The IR radiation is furnished

by an air-cooled Globar source incorporated into

the Midac spectrometer.

The palladium single crystal sample was cleaned

using standard procedure which consisted primar-

ily of heating in oxygen (1 x 10 -6 tort. 1000 K) to

remove carbon and annealing to 1150 K to desorb

any remaining oxygen [7]. This resulted in further

segregation of carbon to the surface of the sample,

which could then be removed by repeating the

above procedure. Since the carbon KLL Auger

feature is obscured by an intense palladium peak,

a more effective method for gaging sample cleanli-

ness is to dose the sample with oxygen and perform

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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