Formic acid catalytic electrooxidation on Pt covered by Au adstructures – role of electronic surface properties

Abstract

In numerous reports present in scientific literature it has been shown that addition of gold to platinum enhances the electrocatalytic activity towards formic acid oxidation, a reaction of importance to low temperature direct fuel cells. While various Pt-Au systems exhibit synergistic effect towards formic acid oxidation, there is no consensus, if the enhanced catalytic activity is due to changes in electronic properties of the surface, or due to the geometric effect. One of the possible reason of this situation is the fact that the electronic properties of Pt-Au systems, especially related to valence band, are seldom measured directly, and in lieu of experimental data authors present the results of DFT calculation. To better understand the role of both: electronic and geometric factors, here we present some insight on the experimentally observable electronic and catalytic properties of Pt-Au surface where the changes of electronic properties were minimized by purposefully depositing Au on Pt to form a separate phase. To study the electronic properties we used an unique approach, which allowed us to obtain the valence band spectra of the Pt-Au surfaces, where contribution from bulk was minimized. While the synergy between Pt and Au towards catalytic formic acid electrooxidation was clearly visible, the changes in valence band spectra for Pt-Au systems, observed by us, can be explained as a superposition of properties of pure Pt and Au elements. Thus we conclude that the synergistic effect in formic acid oxidation in Pt-Au systems, where Pt and Au form separate phases, is most probably not due to changes of the electronic properties beyond superposition of properties of pure metals, but rather caused by other factors, such as geometric effect.