Bridging the Pressure and Materials Gaps: High Pressure Sum Frequency Generation Study on Supported Pd Nanoparticles

T. Dellwig, G. Rupprechter, H. Unterhalt, H.-J. Freund
2000 Physical Review Letters  
Infrared-visible sum frequency generation vibrational spectroscopy is applied for the first time to monitor CO stretching vibrations on alumina supported Pd nanoparticles in a pressure range from 10 27 to 200 mbar. The adsorption behavior of Pd aggregates with 3 and 6 nm mean size is dominated by surface defects and two different adsorption sites (twofold bridging and on-top) were identified. The CO adsorption site occupancy on Pd nanocrystals is mainly governed by the gas phase pressure while
more » ... ase pressure while the structure of the particles and their temperature have a smaller influence. PACS numbers: 61.46. + w, 42.62.Fi, 82.65.Jv To close the gaps between fundamental research and applied heterogeneous catalysis is one of the major challenges in catalysis science. The "pressure gap" originates from the need of most surface sensitive techniques of a vacuum environment, in contrast to catalyzed reactions that are carried out at atmospheric pressure or higher. The "materials gap" is due to the fact that many fundamental studies use model systems (e.g., single crystals) that cannot represent the complex structure of a supported catalyst. In this Letter we demonstrate that both gaps can be simultaneously bridged by applying optical sum frequency generation (SFG) vibrational spectroscopy on a well-defined model catalyst of Pd nanoparticles grown on an ordered alumina film. As a photon-based inherently surface sensitive technique, infrared-visible sum frequency generation spectroscopy can be used from ultrahigh vacuum (UHV) to ambient conditions. The effectiveness of SFG spectroscopy as an in situ diagnostic tool to monitor adsorbates on single crystal metal and oxide surfaces at low and high pressure has been demonstrated in a number of studies [1] [2] [3] [4] . SFG has also been carried out on thin oxide films grown on metal single crystals [5] and on polycrystalline metal foil [6], but to our knowledge no SFG spectra of adsorbates on nanometer-sized metal aggregates were published. The present study shows the ability of SFG spectroscopy to study gas adsorption on supported Pd nanoparticles over a wide pressure (10 27 to 200 mbar) and temperature (190-300 K) range. The CO adsorption site occupancy on Pd nanocrystals was found to be substantially different from single crystal surfaces and it is mainly governed by the gas phase pressure while the structure and the temperature of the particles have a smaller influence. The SFG process has been reviewed in the literature [1] [2] [3] [4] [5] [6] [7] . SFG is a second-order nonlinear optical process which involves the mixing of tunable infrared (v IR ) and visible light (v vis ) to produce a sum frequency output (v SFG v IR 1 v vis ) (Fig. 1) . The process is allowed only in a medium without inversion symmetry (in the electric dipole approximation), e.g., at surfaces (or interfaces) where the inversion symmetry is broken. The dominant SFG signal is hence generated by the adsorbate. The applicability of SFG spectroscopy to nanostructured supported catalysts has been questioned for several reasons. Rough surfaces may considerably scatter the laser beams, and the high dispersion and structural heterogeneity of a supported catalyst, possibly leading to disordered adsorbates FIG. 1. Schematic representation of the sum frequency generation process and the detection of the SFG signal utilizing spatial, spectral, and temporal filtering (MC: monochromator; PM: photomultiplier; disc.: discriminator). The STM images show an overview of Pd nanoparticles grown on aluminum oxide at 300 K (left: 500 3 500 Å 2 ) and atomically resolved facets on a Pd nanocrystal. 776 0031-9007͞00͞85(4)͞776(4)$15.00
doi:10.1103/physrevlett.85.776 pmid:10991396 fatcat:hpz326emtvh6lghiaicdbgs67i