Ultrathin platinum films for methanol and formic acid oxidation: activity as a function of film thickness and coverage
Thomas P. Moffat
Self-terminating electrodeposition was used to grow ultrathin Pt overlayers on 111 textured Au thin films. The Pt thickness was digitally controlled by pulsed potential deposition enabling the influence of overlayer thickness on electrocataytic reactions, such as methanol and formic acid oxidation, to be examined. Bimetallic and bifunctional effects associated with a sub-monolayer coverage of Pt on Au yield enhanced catalysis. For films grown using one deposition pulse the peak rate of CH3OH oxidation was enhanced by a factor of 4 relative to bulk Pt. The overlayer consisted of 2 nm diameter monolayer Pt islands that cover 80 % of the surface. However, voltammetric cycling resulted in a loss of the enhanced activity associated with the as-deposited sub-monolayer films. For thicker Pt films the electrocatalytic activity decreased monotonically until bulk Pt behavior was obtained beyond three monolayers. For HCOOH oxidation improvements in activity in excess of a 100-fold were observed for sub-monolayer Pt films. The best performance was associated with a fractional surface coverage of Pt close to 0.2 that was produced by voltammetric cycling of a film originally grown using one deposition pulse. Bulk Pt behavior was observed for formic acid oxidation on Pt films greater than three monolayers in thickness.