The Correlation of Strain and Catalytic Activity of Au, Pt, and Pd on Graphene

Abstract

The catalytic activity of transition metals is highly influenced by the shape of their d-bands. This can be influenced by adjacent materials through two mechanisms: the strain effect, where the degree of orbital overlap affects their electronic structure; and the ligand effect, where the orbital overlap of two different atoms convolves their electronic structures, altering the energy distribution of their bands and transferring charge between them. Benefits derived from manipulating these effects are usually obtained through alloying or the use of core-shell structures, but a support substrate will produce these as well, although they are more difficult to attribute to a substrate as these effects only have an effective range of a few atoms. In a previous study looking at a few monolayer equivalents of Pt on graphene, it was observed that the two effects dominated at different amounts of deposition, with detrimental ligand effects dominating at less than three monolayer equivalents of deposition while beneficial strain effects dominated at higher deposition amounts. Normally, these effects are inseparable, but these effects could be decoupled due to the way that Pt bonds with graphene allowing it to alleviate strain at low dimensions. This seeks to expand on that study by characterizing the interactions between Au and Pd in addition to Pt by depositing up to five monolayer equivalents of each metal and observing how the structure and catalytic activity of each change with the increasing self-influence of each metal.The average atomic strain in Au and Pt were measured using extended X-ray absorption fine structure analysis, while the atomic structure of Pd was observed through scanning transmission electron microscopy. It was found that Pt and Au both show a compressive strain while graphene imposes a tensile strain on Pd, but this strain gradually lessened with deposition amount for Au while it remained relatively constant for Pt and Pd. This demonstrated that the catalysts were highly influenced by their strain as their catalytic activity showed similar trends, with the Pt and Pd catalysts being relatively invariant in catalytic activity over the deposition range observed while the Au samples showed a gradual decline in activity with deposition amount. In comparison to reference samples, all three catalysts are improved by their interactions with graphene, which is supported by computational analysis, but the nature of its influence is different on each.