Model catalysts for high-pressure spectroscopic investigations
Date
2009
Authors
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Publisher
University of Delaware
Abstract
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. 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 polarization modulation infrared reflection absorption spectroscopy (PM-IRAS). Likewise, a “materials gap” exists between single crystal surfaces and the supported nanoparticle catalysts used industrially. This work also includes preliminary investigations exploring the use of the reverse micelle technique for catalyst synthesis in order to produce nanoparticles with well-controlled dispersion.
Adsorption of CO on Pt(100) was investigated at elevated pressures using PM-IRAS measurements. At sample temperatures 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 up to 6.3 cm-1. The full width half maximum of the IR band was observed to decrease by 27% 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 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 ~0.95 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 molecules. Above certain sample temperatures, the absorption band associated with adsorbed CO is no longer observed in the measured spectra. This is attributed to CO dissociation at higher sample temperatures, resulting in carbon contamination of 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. Temperature programmed desorption (TPD) measurements also corroborate this finding by excluding desorption as a likely cause of the loss of the CO band. Adsorbed CO measured in a CO oxidation reaction environment exhibit reversible adsorption/desorption processes around CO desorption temperatures.
In parallel to the above high-pressure studies, initial steps toward overcoming the materials gap were taken through the synthesis of Ru nanoparticles using the reverse micelle synthesis technique. Reverse micelles were created using a microemulsion consisting of butyl ammonium laurate-water-hexanes. Dynamic Light Scattering was used to characterize the reverse micelles, which exhibited tunable sizes ranging from 5 to 30 nm depending on the molar water-to-butyl ammonium laurate ratio used. Reverse micelles having a diameter of 6 nm were then used to produce Ru nanoparticles from a RuCl3 precursor solution. The resulting Ru particles had a diameter of ~3 nm, as determined from Small-Angle X-ray Scattering measurements. These results are a promising first step towards the goal of examining well-controlled nanoparticle catalysts over a wide range of pressure conditions using PM-IRAS. This approach has the potential to bridge both the pressure and materials gaps, allowing for significant improvements in the comparisons of model catalytic investigations with industrial catalytic performance.