Active Sites for the Oxygen Reduction Reaction on the Low and High Index Planes of Palladium

Active sites for the oxygen reduction reaction (ORR) have been studied on the low index planes, n(100)-(111) and n(100)-(110) series of Pd with the use of hanging meniscus rotating disk electrode (RDE) in 0.1 M HClO4 saturated with O2. The Kotekey−Levich plots give a linear line with a slope of about 4, showing four electron reduction for the ORR on Pd electrodes, as is the case of Pt. Activity of ORR, which is estimated by the reduction current density at 0.90 V (RHE) j ORR,0.90V, gives the following order on the low index planes: Pd(110) < Pd(111) < Pd(100). This order is completely opposite to that of Pt in 0.1 M HClO4: Pt(100) < Pt(111) < Pt(110). Specific activity of Pd(100) is nearly three times as high as that of Pt(110) at 0.90 V (RHE). The values of j ORR, 0.90 V increase linearly with the increase of the terrace atom density on n(100)-(111) and n(100)-(110) series of Pd. This fact supports that (100) structure is the active site for ORR on Pd. ORR activity does not depend on the step structure of n(100)-(111) and n(100)-(110) series.