(630a) Tuning Surface Reactivity Through Electronic Structure Changes In Alloys

Introducing a guest metal into a host can potentially improve the surface reactivity of the host metal. This is accomplished via electronic structure changes, or by creating a second type of surface site that participates in the reaction ? the bifunctional mechanism. While alloys exhibiting superior performance may be discovered using a trial and error, it is a clumsy and inefficient approach. Rational alloy design is a more desirable approach, but is still not practical. Identifying metals that participate in the bifunctional mechanism may be suggested in part by the behavior of the adsorbates on the pure metal surfaces. Predicting electronic structure changes, however, is still beyond current understanding.

Alloys modify the electronic structure through lattice strain and electronic ligand effects. Controlling these effects experimentally is very difficult, while separating them is impossible. Separating and controlling the two effects using theoretical surface science techniques such as density functional theory (DFT) is a practical means of study, making DFT a powerful tool in advancing the understanding of the influence of alloys on surface reactivity.

One application of alloys that has received attention is the development of Pd-alloy hydrogen separation membranes. Hydrogen dissociatively adsorbs on Pd surfaces and migrates into the subsurface. In the presence of a sufficient driving force, the hydrogen diffused through the bulk to the opposing surface, where it associates and desorbs. Sulfur, however, adsorbs strongly on Pd surfaces, effectively poisoning the surface even at low concentrations. Pd-alloys have demonstrated an increased tolerance to sulfur, which make them promising alternatives to pure Pd membranes. Since both hydrogen and sulfur, in the form of H2S, adsorb weakly on Cu, Ag, and Au ? metals used in Pd-alloy membranes ? enhanced sulfur tolerance appears to be purely electronic in nature. How the secondary metals alter the electronic structure of Pd has previously not received much attention. We have examined H2S decomposition on model Pd-alloy surfaces using DFT. In this talk, we present these results and relate them to alloy induced electronic structure changes of the surface.