The key scientific and technological challenge of the future hydrogen economy is to develop new materials with low-content of Pt and Pt-group noble-metals that exhibit high activity, improved selectivity and stability by controlling their structure and composition. Design of Pt-bi and tri-metallic catalysts of different shapes, sizes and configurations (i.e. core-shell nanoparticles, nanoalloys, nanoclusters) are the most promising approaches to resolve challenges of fuel cells commercialisation, particularly in automotive industry and integrated power system technologies.

The extensive studies have been conducted on various Pt-Pd bimetallic structures over the past few decades. Epitaxial ultrathin films of Pd (1-10 ML) on single crystal surfaces of Pt(hkl) and Au(hkl) have been shown to exhibit higher electrocatalytic activity for formic acid oxidation (FAO) than the bulk Pd and bulk Pt ^{1, 2}. Pd and PdAu-alloy nanoparticles covered by electrodeposited Pt monolayer shell have been shown to be excellent catalysts for oxygen reduction reaction with much higher activity and stability than the bulk Pt ^{3, 4} . In a three component system Pt-nanoclusters (Pt-nc) on Au@Pd (core@shell) particles showed higher activity for FAO dependent on the thickness of Pd shell ⁵.

In this work, we explored the effect of sub-ML Pt - nc on the electrocatalytic activity and stability behaviour of Pd thin films electrodeposited on Au. Uniform distribution of Pt-nc was achieved by reduction of the spontaneously adsorbed [PtCl₄]^2- complex on the surfaces known as spontaneous deposition⁶ and by the surface limited redox replacement reaction (SLRR) of adsorbed hydrogen ⁷. The study was focused on the deposition of sub-ML Pt-nc on epitaxial Pd films on Au of different thicknesses between 2ML and 10 ML. It has been shown that 2ML Pd films are pseudomorphic and adsorbs hydrogen, while for higher thicknesses (3-10 ML) Pd films maintain their epitaxial structure with the underlying Au and feature energetically separated H adsorption and absorption processes ⁸. The comparison of Pd films of different thickness allowed us to explore the effect of sub ML Pt-nc on H-sorption behaviour and FAO activity. The study showed that presence of Pt-nc contributed to the significant changes in the kinetics of H-sorption and increased the stability of Pd toward dissolution during potential cycling in 0.1 M H₂SO₄ and in particular during FAO. The FAO on Pd films modified with Pt-nc exhibited significant changes compared to the pure Pd/Au surfaces featuring increased CO_ads poisoning. However, the stability of these surface toward dissolution was significantly better than that of pure Pd/Au. For example, 200 potential cycles of FAO with a scan rate of 50 mV/s on 4 ML Pd/Au resulted in a complete dissolution of Pd film. However, deposition of ~ 0.5 ML of Pt via SLRR of H on the 4 ML Pd/Au improved the surface stability such that the same number of potential cycles resulted in a loss of ~ 1 ML of Pd (equivalent thickness measured by the changes of hydrogen absorption).

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