Molecular-beam study of the chemisorption of CO on well shaped palladium
particles epitaxially oriented on MgO(100)
Surf. Sci., (1991)DOI: 10.1016/0039-6028(91)90592-G
Abstract
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.