Clarifying the role of Ru in methanol oxidation at Rucore@Ptshell nanoparticles

Abstract

The catalytic activity of Ru_core@Pt_shell nanoparticles (NPs) towards CO oxidation, a strongly adsorbed intermediate that compromises the performance of direct methanol fuel cells, is known to be significantly better than at Pt alone. However, a systematic study aimed at understanding the beneficial effect of Ru on Pt during the methanol oxidation reaction (MOR) has not been carried out as yet. Here, Ru_core@Pt_shell NPs, having a controlled Pt_shell coverage of zero to two monolayers and two different Ru_core sizes (2 and 3 nm), were synthesized using the simple polyol method to determine the precise role and impact of Ru on the MOR in 0.5 M H₂SO₄ + 1 M methanol at RT and 60 °C. Because the structure of our Ru_core@Pt_shell NPs is known with such certainty, we were able to show here that the rate of methanol adsorption/dehydrogenation can be accelerated either by compression of the Pt_shell (by making the Ru_core larger) when it is less than one monolayer in thickness, or by decreasing the electronic effect of the Ru_core on the Pt_shell (achieved by adding a second Pt layer to the Pt_shell). At low overpotentials, decreasing the Pt_shell thickness also helps in increasing the rate of the MOR by enhancing the rate of oxidation of adsorbed CO. Finally, it is shown that the bi-functional effect of Ru on the Pt_shell plays only a minor role in the catalysis of the MOR, especially at large particles where CO surface diffusion is facilitated.