Crucial Role of Low Coordination Sites in Oxygen Reduction, CO Stripping, and Size Effect for Nano-sized Pt Particles

Abstract

A series of nano-sized Pt particles with good crystallinity and small crystallite sizes were made with a new synthetic method. Analysis of cyclic voltammograms of them suggests that the weak hydrogen adsorption peak at -0.56 v can be assigned to defect sites in addition to edge/corner sites. A size effect theory consisting of two ORAs (oxygen reduction activity) size effects are proposed: the primary effect that becomes important only when the percentage of corner/edge/defect sites is dominant as in particles with sizes less than ~18 A. The secondary effect is associated with (111) terrace size. Experimental results are supplied to support the two-effect hypothesis. Several voltammetric peaks observed in H2SO4 and HClO4 for nano-sized Pt/C are assigned to Pt-OH formation/reduction on low coordination sites. These peaks are separated form main Pt-OH peaks by 180-280 mv and they are hypothesized to be related to primary ORA size effect. CO stripping on nano-sized Pt particles exhibits an onset potential close to the potential of hypothesized Pt-OH formation on low coordination sites. This supports the theory: adsorbed OH sites for CO stripping are related to low coordination Pt sites. Two distinct CO stripping peaks are observed for Pt particles with XRD sizes larger than 3.3 nm. They are assigned to different sites, terrace sites vs. other sites. The details of the CO stripping curves are discussed.