STRUCTURE OF THE CO BOND ON SUPPORTED PD PARTICLES - INFLUENCE OF SIZE AND SURFACE-STATE

We have investigated the surface of supported palladium particles by static secondary ion mass spectrometry (SSIMS). Pd particles were grown in situ on alumina (oxide layer and sapphire surfaces) and stabilized by heating treatment. The particle size, density and crystallographic structure were determined in previous studies by transmission electron microscopy and diffraction (TEM and TED). Various ionic species are detected by SSIMS analysis which makes it possible to characterize the CO absorbed layer: Pd(n)CO+ (n = 1,2) for molecular adsorption and Pd(n)C+ for CO dissociation. The size dependence of the bonding state of CO (linear, bridge, ...) was monitored by: PdCO+/SIGMA-(n)Pd(n)CO+ signal ratio over various size particles (mean diameter in the 2-9 nm range). Investigations were performed as a function of CO coverage (adsorption at room temperature) and also under CO dissociation conditions: heating under CO atmosphere or CO + O2 (catalysis). The data analysis shows that on clean Pd particles smaller than 3 nm the CO molecules give rise mainly to PdCO+ species. We have interpreted this result by the adsorption of CO on two palladium atoms, the carbon end being tightly bonded to a low coordination Pd atom and the oxygen end weakly bonded to a neighbour Pd atom. These couples of Pd atoms form the specific sites for CO dissociation, the density of which depends on the roughness of the particle surface.