Reduction of Pd2+ pre-adsorbed on cyanide-modified Pt(111) electrodes: Adlayer metallization vs. metal-on-metal deposition

We have studied the reduction of Pd2+ irreversibly and non-covalently immobilized on a cyanide-modified Pt(111) electrode using cyclic voltammetry (CV) and in-situ scanning tunnelling microscopy (STM). Contrary to Cu2+, the reduction of which results in the metallisation of the cyanide adlayer (J. Solid State Electrochem. 20 (2015) 1087-1094), reduction of Pd2+ results on the deposition on the Pt(111) substrate of Pd triangular nanoislands containing approximately 30 atoms in average and evenly distributed over the electrode surface. A potential excursion below 0.20 V vs. RHE results in the islands swelling and increasing their surface. This swelling effect is size-dependent, and while the smaller islands increase their area by as much as 60-75% within the hydrogen region, bigger islands swell by just 12%. We attribute this effect to hydrogen absorption within the palladium islands, which also increases the mobility of the Pd atoms and leads to the formation of larger islands. Cyclic voltammetry suggests that repeated cycling into the hydrogen evolution region results in an increase in the size of epitaxial Pd bidimensional islands on Pt(111), which is consistent with the evolution observed using STM.