Pt Flecks on Colloidal Au (Pt∧Au) as Nanostructured Anode Catalysts for Electrooxidation of Formic Acid

Abstract

Nanostructured Pt-on-Au electrocatalysts (coded as Ptm∧Au, m being the atomic Pt/Au ratio), prepared by Pt deposition on Au colloids in two size ranges (Au-I, 10.0 ± 1.2 nm; Au-II, 3.0 ± 0.6 nm) (Zhao and Xu, Phys. Chem. Chem. Phys. 2006, 8, 5106), were employed for the electrooxidation of formic acid (HCOOH) at concentrations of 0.2−3.2 M by cyclic voltammetry. The HCOOH electrooxidation over a Pt shell fully covering Au-I colloids (Ptm∧Au-I at m > 0.2, Pt dispersion < 20%) occurred mainly in the high potential range (0.6−1.0 V vs SCE). The lowering of m in Ptm∧Au-I samples resulted in a remarkable increase in the current of HCOOH electrooxidation in the lower potential range (−0.2 to 0.6 V) due to a continued enhancement in the Pt utilization associated with the changes in the Pt-dispersion state. The areal activity (intrinsic activity) of Pt flecks (Pt dispersion > 50%) was 5 times and their mass-specific activity 25 times higher than those of the conventional Pt/C and core@shell structured Pt∧Au-I catalysts. The use of Ptm∧Au-II for HCOOH electrooxidation produced qualitatively similar results, thus demonstrating a dramatic enhancement of the electrocatalytic activity of Ptm∧Au/C by reducing the domain size of Pt deposits on Au surfaces. Moreover, the intrinsic activity of Pt flecks in Ptm∧Au-II was found to be 4 times higher than those in Ptm∧Au-I, which uncovers that smaller Au particles can serve as a kind of “activity promoter” to their carrying Pt flecks. The current of HCOOH electrooxidation over the Ptm∧Au catalysts also varied significantly, according to the HCOOH concentration. The highest current was obtained only in an appropriate HCOOH concentration window for a given Pt∧Au/C catalyst.