MOLECULAR-BEAM STUDY OF THE CHEMISORPTION OF CO ON WELL SHAPED PALLADIUM PARTICLES EPITAXIALLY ORIENTED ON MGO(100)

The CO adsorption-desorption transients have been measured on supported Pd particles by a molecular beam technique. The Pd particles were epitaxially grown, in UHV, on clean MgO(100) surfaces. The particles, fully characterized by TEM in the preceding paper, have a top-truncated half octahedron shape. For particles larger than 4 nm the desorption is first order and the zero coverage desorption energy is nearly the same as for a continuous film (approximately 30 kcal/mol). At high coverage the transients display two desorption regimes explained by a fast and a low desorption from the (100) and (111) facets, respectively. For particles smaller than 3 nm, the desorption kinetics does not follow a simple first-order law, even at low coverage, and the desorption rate is considerably reduced in comparison with larger particles. The origin of this size effect, corresponding to an increase of the desorption energy, is discussed in terms of the modification of the electronic structure of the particles and of the presence of additional high-energy adsorption sites.