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

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

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.

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surface science

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

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

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.

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-