CO adsorption and desorption on size-selected Pd _n/TiO _2(110) model catalysts: Size dependence of binding sites and energies, and support-mediated adsorption

摘要

The nature of CO adsorption on Pd _nTiO _2(110) (n 1, 2, 7, 20) has been examined using temperature-programmed desorption (TPD), temperature-dependent helium ion scattering (TD-ISS), and X-ray photoelectron spectroscopy (XPS). All samples contain the same number of Pd atoms (0.10 ML-equivalent) deposited as different size clusters. The TPD and TD-ISS show that CO binds in two types of sites associated with the Pd clusters. The most stable sites are on top of the Pd clusters (on-top sites), however, there are also less stable sites, in which CO is bound in association with, but not on top of the Pd (peripheral sites). For saturation CO coverage over a fixed atomic concentration of Pd (present in the form of Pd _n clusters of varying size), the population of CO in peripheral sites decreases with increasing cluster size, while the on-top site population is size-independent. This is consistent with what geometric considerations would predict for the density of the two types of sites, provided the clusters adsorb predominantly as 2D islands, which ISS results suggest to be the case. The XPS analysis indicates that CO-Pd binding is dominated by π-backbonding to the Pd _n clusters. The results also show evidence for efficient support-mediated adsorption (reverse-spillover) of CO initially impinging on TiO _2 to binding sites associated with the Pd clusters.