(Invited) The Effect of Size and Thickness of Pt Nanostructures on Au on Formic Aid Oxidation

Abstract

The design of highly active Pt bimetallic nanostructures (monolayers, alloys, nano-clusters) with the atomic scale control has been an intense area of research. The low-content Pt nanostructures often exhibit enhanced electrocatalytic activity due to a combination of bifunctional, ligand and/or ensemble effects. Many variables play a role in the control of the Pt activity such as size, thickness, distribution and composition. Therefore more research is needed to gain an understanding of each of these aspects during a particular electrocatalytic reaction. Studies of sub-ML and ML Pt films grown by surface limited redox replacement (SLRR) on Au showed a strong correlation between the surface activity and the structure of Pt1-5. The studies of ethanol oxidation and hydrogen oxidation reaction on Pt sub-ML and ML films demonstrated the effect of the convolution of the tensile strain, due to the misfit with Au, and compressive strain due to the finite size Pt nanoclusters characteristic for SLRR grown films morphology.1, 3 Also, studies on multilayered Pt nanoclusters have shown a correlation between the activity and the coverage of Pt on the catalyst surface.6, 7 The Pt nanoclusters with coverage as low as 15-25% of the Au substrate area, showed very high activity and improved selectivity toward direct pathway of FAO.7, 8 Here in this work we studied the effect of Pt coverage and morphology on formic acid oxidation (FAO) and CO adsorption on two-dimensional thin films. We compared the activity of Pt films and Pt nanoclusters toward FAO using two electrochemical approaches: i) the surface limited redox replacement (SLRR) method based on the galvanic replacement of underpotentially deposited metals Cu and Pb and ii) spontaneous deposition (SD) i.e. reduction of the spontaneously adsorbed PtCl4 2-complex.Epitaxial Pt films of varied thicknesses (up to 10 ML), that completely cover Au, grown by SLRR of Cu UPD and Pb UPD showed very slight differences during FAO which can be attributed to their roughness and size of Pt nanoclusters. Also, the CO stripping measurements in 0.1 M H2SO4 solution showed a larger range of the positive potential shift for films of higher roughness (grown by SLRR of Cu UPD). Compared to the continuous Pt films, a significant enhancement of FAO has been observed on Pt sub-ML. The activity measured by the current of FAO forward scan shows an increase for lower Pt sub-ML coverage with the most active surfaces of 20-25% fractional coverage of Au substrate. The correlation between the coverage and a positive shift of CO stripping potential has been observed and it is in agreement with already reported results.9 Furthermore Pt nanoclusters of the same coverage (~25 %) grown via SLRR of Pb and 1SD protocol, showed significant differences in FAO activity. The results indicate ~5 times the higher activity of smaller clusters (developed by SD) demonstrating a role of Pt clusters distribution (clusters size) on the FAO activity.