(654b) Spectroscopic Investigation of CO Adsorption On Pt(100) At near-Atmospheric Pressures Using PM-IRAS

In molecular-level catalytic
investigations, discrepancies that exist between surface science observations
under ultra-high vacuum (UHV) conditions and industrial catalytic performance
at higher pressures are referred to as the ?pressure gap.?  For example,
changes in the population of adsorption sites and variation in reaction
mechanisms on catalyst surfaces have been observed as pressure increases above
UHV conditions (1-2).  This work addresses this pressure issue through
the investigation of surface adsorption behavior of model catalytic systems
over the range from UHV to atmospheric pressures using a polarization
modulation infrared reflection absorption spectroscopy (PM-IRAS) system
constructed in our laboratory.  In order to study catalytic systems above UHV
conditions, investigative techniques capable of functioning in higher pressure
environments without undue interference from gas phase molecules are required. 
This PM-IRAS system is capable of removing contributions from gas phase
molecules to yield surface vibrational spectra.

Adsorption of CO on Pt(100) was
investigated at near-atmospheric pressures using PM-IRAS measurements.  At a
sample temperature of 325 K, a linear C-O stretch (~2090 cm^-1) was
observed.  Peak sharpening and a frequency shift were observed for this CO
adsorption band at higher pressures.  At 325 K, the frequency shift increased
with exposures between 1 and 200 Torr CO by up to ~6 cm^-1.
 A corresponding decrease in the linewidth of the IR band was observed over the
same range.  These results suggest that dipole-dipole coupling effects play an
important role in understanding the surface adsorption behavior in this system
at higher pressures.

A dipole-coupling model (3)
was applied to these experimental results.  The model predicted CO surface
coverages on the Pt(100) surface increasing from ~0.7 at 1 Torr CO to >0.9
at 200 Torr CO.  These results indicate that at higher pressures the CO surface
coverage on Pt(100) is much greater than similar measurements obtained under
UHV conditions.  The predicted increases in surface coverage at higher
pressures were verified through analysis of the integrated peak areas of the
measured absorption bands.  The calculated areas were observed to increase by
up to 50% in magnitude over the pressure range from 1 Torr CO to 200 Torr CO.  Measurements
obtained during reduction from a high-pressure environment indicate that
high-pressure adsorption behavior is a mix of reversible and irreversible
processes.

Measured PM-IRAS spectra exhibit
significant broadening and decreasing frequency with increasing sample
temperature.  These effects are consistent with phonon dephasing models for
adsorbed CO (4).  Disappearance of the IR band at higher sample
temperatures is attributed to CO dissociation, resulting in carbon deposition on
the Pt(100) surface.  Subsequent spectra obtained after exposure of the system
to oxidative conditions reveal the return of the absorption band corresponding
to adsorbed CO.  Adsorbed CO measured in a CO oxidation reaction environment
exhibit reversible adsorption/desorption processes below CO desorption
temperatures, in contrast to the experiments conducted in a pure CO
environment.

1.      G. Rupprechter, C. Weilach, Nano Today 2, Issue 4, 20-29
(August 2007)

2.      D. Stacchiola, A.W. Thompson, M. Kaltchev, W.T. Tysoe, J. Vac. Sci.
Technol. A 20 (6), 2101-2105 (2002).

3.      J. Lauterbach, R.W. Boyle, M. Schick, W.J. Mitchell, B. Meng,
W.H. Weinberg, Surface Science 350, 32-44 (1996).

4.      B.N.J. Persson, F.M. Hoffmann, R. Ryberg, Phys. Rev. B 34,
2266 (1986).