Figure 1. Atomically resolved Cu/Pd alloy surface used in hydrogen
purification. The brighter circles are individual Pd atoms and the smaller
hexagonally arranged circles are the Cu atoms of the Cu(111) surface.[10]
Metal Alloy Catalysts
Palladium alloy catalysts play a central role in a wide variety of industrially important applications such as hydrogenation reactions, separations, storage devices, and fuel cell components. We have explored the structural and electronic properties of both palladium/copper and palladium/gold alloys. Our imaging capabilities have allowed us to determine the atomic composition and hydrogen dissociation activity of these important alloys.
Figure 1. Atomically resolved Cu/Pd alloy surface used in hydrogen
purification. The brighter circles are individual Pd atoms and the smaller
hexagonally arranged circles are the Cu atoms of the Cu(111) surface.[10]
Alloy Catalysts for a Cleaner Future
Low-temperature STM and spectroscopy is being used to investigate the atomic-scale structure of Pd/Au and Pd/Cu near-surface alloys, created by depositing Pd on both Au(111) and Cu(111) at a variety of temperatures.[1, 2] The choice of supporting metal allowed the study of Pd particles as a function of size; from individual atoms to islands ~8 nm in diameter.
Our work has revealed that the electronic structure of isolated Pd atoms in both Au and Cu hosts is very different from bulk Pd. This data is the first to show how the electron density in alloys varies locally at the nanoscale, as other techniques average across the whole surface. Our results allow us to differentiate geometric effects (how the atoms are rearranged) from ligand effects (how the electron density is distributed) and thus further the understanding about how alloys operate as such good catalysts and separation membranes.[1, 2] We are currently studying the uptake of hydrogen on these alloys. By varying the surface composition of Pd we are able to isolate the best species for hydrogen dissociation. Understanding the atomic nature of these catalytically active sites will allow the design of better alloys for hydrogen related applications.
While experiments performed on our low-temperature STM provide great insight into many chemical systems, the operating conditions are not necessarily practical for “real world” applications. For this reason we perform parallel experiments on an electrochemical STM (EC-STM) that operates in an ambient environment (i.e. room temp. and 1 atm.).
1
. Baber, A. E.; Tierney, H. L.; Sykes, E. C. H. Composition and Electronic Properties of Catalytically Important Pd/Au Alloys. In preparation 2009.
2. Tierney, H. L.; Baber, A. E.; Sykes, E. C. H. Atomic-Scale Electronic Structure of Catalytic Sites on Pd/Cu Near Surface Alloys. JOURNAL OF PHYSICAL CHEMISTRY C 113, (2009), 7246-7250