Co atom) and two N atoms. This site is likely more favourable than site A as the difference in ionic radius creates a gap between the Co and Ta atom that Cu can partially occupy, as shown in Figure and. On Co 50-S , there are two Co atoms at site B and a surface rearrangement occurs. This rearrangement leaves a gap for Cu to occupy (Figure ). The result is a strong adsorption energy of -3.82 eV/atom. However site A on this surface forms a larger gap that allows Cu to fully incorporate into the surface layer leading to a particularly large adsorption energy of -4.61 eV/atom, as shown in Figure .