Abstract
Oxygen reduction reaction (ORR) limits the efficiency of fuel
cells, and thus it is of great interest to discover nanocatalysts with
an ORR activity superior to that of Pt.1 Recent research efforts
have been focused on understanding surface chemistry and electronic
structures on Pt alloy model surfaces that exhibit an ORR
activity higher than that of Pt.2 On bulk Pt alloy surfaces, Pt can
segregate in the outermost layer to form a “sandwich-segregation”3
or “Pt-skin structure”,2e which is shown to weaken the metal-oxygen
bond strength relative to Pt4 and increase specific ORR activity
(based on Pt surface area). In addition, pure Pt on the outermost
surface layer can be obtained on Pt alloys by acid removal of
transition metals,2d,e,5 and such surfaces also exhibit enhanced ORR
activity relative to Pt. It is essential to examine if the ORR
mechanisms established for bulk Pt alloy surfaces are applicable
to tailoring the activity of Pt alloy nanoparticles. Pt alloy nanoparticles6
are known to exhibit specific ORR activity 2-5 times
higher than that of Pt. However, the origin in the enhancement of
ORR activity of Pt alloy nanoparticles is not understood, as little
is known about chemical compositions of near-surface regions of
nanoparticles
