Understanding XANES of Supported Pd Particles: Size, Adsorbate and Alloy Effects

An improved understanding of the properties of transition metals would enable us to predict the reactivity of their surfaces and therefore lead us to the potential dream of a priori design of heterogeneous catalysts. To this end, understanding the effects of structure, alloying and chemisorption are imperative for modeling. In this work density functional theory based simulations of x-ray absorption near edge structure (XANES) using a CASTEP code are utilized to evaluate the effect of adsorbates, particle size and alloying on Pd catalysts. Good agreement with experimental results has been achieved showing a significant decrease at the L3 edge (2p3/2→d transition) with a decreasing particle size. In addition, exposure of the catalysts to CO and H2 results in significant increases in intensity as well as shifts in the edge position. In the case of PdPt alloys, an increase in the leading edge intensity was observed with Pd whereas the leading edge of Pt decreased upon alloying. Density functional theory calculations allow calculation of the band structure of the metals which can be represented as density of states (DOS). It has been shown that the particle size effects originate from the changes in hybridization as the coordination and lattice constant change. CO and H adsorption create a depletion of available states at the Fermi level and increase in intensity beyond the edge due to the formation of bonding and antibonding adsorbate-metal orbitals. OH adsorption reversely correlates with the d-band center wherein the adsorption strength increases as the d-band center moves away from the Fermi level due to the domination of Pauli repulsion. Alloying effects range from those dominated by strain effects (PdNi) to PdPt which is dominated by the orbital extent. Changes to the leading edges of the XANES spectra upon alloying Pd with metals containing a filled ¬d-band (Zn, Ga and Ge) are described as the results of the charge transfer effects.