title: Adsorption energetics of CO on supported Pd nanoparticles as a function of particle size by single crystal microcalorimetry.
abstract: The heat of adsorption and sticking probability of CO on well-defined Pd nanoparticles were measured as a function of particle size using single crystal adsorption microcalorimetry. Pd particles of different average sizes ranging from 120 to 4900 atoms per particle (or from 1.8 to 8 nm) and Pd(111) were used that were supported on a model in situ grown Fe(3)O(4)/Pt(111) oxide film. To precisely quantify the adsorption energies, the reflectivities of the investigated model surfaces were measured as a function of the thickness of the Fe(3)O(4) oxide layer and the amount of deposited Pd. A substantial decrease of the binding energy of CO was found with decreasing particle size. Initial heat of adsorption obtained on the virtually adsorbate-free surface was observed to be reduced by about 20-40 kJ mol(-1) on the smallest 1.8 nm sized Pd particles as compared to the larger Pd clusters and the extended Pd(111) single crystal surface. This effect is discussed in terms of the size-dependent properties of the Pd nanoparticles. The CO adsorption kinetics indicates a strong enhancement of the adsorbate flux onto the metal particles due to a capture zone effect, which involves trapping of adsorbates on the support and diffusion to metal clusters. The CO adsorption rate was found to be enhanced by a factor of ∼8 for the smallest 1.8 nm sized particles and by ∼1.4 for the particles of 7-8 nm size.
authors.affiliation: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany.
authors.forename: J M
authors.initials: JM
authors.lastname: Flores-Camacho
authors.forename: J-H
authors.initials: JH
authors.lastname: Fischer-Wolfarth
authors.forename: M
authors.initials: M
authors.lastname: Peter
authors.forename: C T
authors.initials: CT
authors.lastname: Campbell
authors.forename: S
authors.initials: S
authors.lastname: Schauermann
authors.forename: H-J
authors.initials: HJ
authors.lastname: Freund