Three distinctive doping effects to modify the electronic and geometric properties of Pd nanocrystals for HCOOH decomposition to H2/CO2 are presented: Bi atoms take preferable residence on higher index sites, which leads to a reduction in HCOOH dehydration; Te atoms dwell favourably on terrace sites, which reduces the rate of dehydrogenation; Ag atoms, without site specificity, induce strong electronic effects to promote the activity on the dwindling number of surface Pd sites at high coverage.