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 ${10}^{-7\mathrm{}}$ 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 the structure of the particles and their temperature have a smaller influence.

• Received 22 February 2000

DOI:https://doi.org/10.1103/PhysRevLett.85.776