Fine Tuning of Activity for Nanoscale Catalysts

Ability to tune the electronic and structural properties of nanocatalysts can potentially lead towards the superior catalytic enhancement that was reported for the Pt{sub 3}Ni(111)-skin surface. Here we report investigation of the extended well-defined surfaces of Pt and PtM alloys (M=Ni,Co,Fe,V,Ti,Re) as well as Pt(hkl) single crystalline surfaces for various catalytic reaction. The electrode surfaces were initially characterized in ultra-high vacuum by AES, LEIS and UPS before controlled transfer into electrochemical environment. Catalytic properties have been determined in three compartment electrochemical cell for oxygen reduction reaction (ORR) by rotational disk electrode technique. The single crystalline surfaces of Pt electrodes have been used to benchmark the activity range that could be expected on pure Pt electrodes. We have proposed that surface modifications induced by the second metal, and consequent catalytic enhancements could occur through the following effects: (1) Electronic effect, due to changes in the metallic d-band center position vs. Fermi level; and (2) Structural effect, which reflects relationship between atomic geometry, and/or surface chemistry, i.e., dissolution - surface roughening. It has been reported that Pt bimetallic alloys, could form surfaces with two different compositions. Due to surface segregation annealed surfaces can form the outermost Pt-skin surface layer, while the lightlymore » sputtered surfaces have the bulk ratio of alloying components and form Pt-skeleton outermost layers as a result of dissolution of non-precious atoms. In principle, different near-surface compositions (Pt-skin, Pt-skeleton and polycrystalline Pt) have been found to have different electronic structures. Modification in Pt electronic properties alters adsorption/catalytic properties of corresponding materials. The most active systems for the electrochemical oxygen reduction reaction (ORR) are established to be the Pt-skin near-surface formation. The similar levels of catalytic enhancement have been established for corresponding nanoscale materials. In addition to electronic properties we have found how catalytic activity could be affected by the arrangement of surface defects on nanoscale surfaces. Ability to control surface and near surface catalyst properties enables fine tuning of catalytic activity and stability of nanoscale surfaces.« less