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).