The controlled growth of thin metallic films on a foreign support is a powerful approach for tuning their catalytic activity.The reactivity of these systems can be rationalized in terms of the so-called electronic and geometric (strain) effects.Pd-overlayers on Au substrates have received significant attention over the years, particularly in the context of the hydrogen evolution/oxidation reactions and formic acid (HCOOH) oxidation.In this contribution, we shall correlate the effective lattice strain of Pd nanoshells grown on Au nanoparticles with their electrochemical reactivity towards CO and HCOOH electro-oxidation, as well as CO2 reduction.